Final Report Summary - ICOEUR (Intelligent coordination of operation and emergency control of EU and Russian power grids)
Executive summary:
Bulk power grids may encounter major blackouts, due to increasing complication in operation and control of interconnected power grids, as well as lack of exchange of information. Therefore, advanced methods for monitoring, control and protection of large scale systems are essential. These methods are particularly important for a possible future interconnection between the European and Russian electricity transmission systems. The ICOEUR project is a three-year collaborative European Union (EU)-Russian project led by the TU Dortmund University (Prof. Dr C. Rehtanz) (on the EU side) and the Energy Systems Institute (Prof. Dr N. Voropai) (on the Russian side). The project is co-funded by the European Commission (EC)'s Seventh Framework Programme (FP7) and the Russian Federal Agency on Science and Innovations. In the project, a new generation of state estimators was developed. It provides European and Russian Transmission system operator (TSO)s with an accurate snapshot and robust indicators of the system state even during emergency situations. The key technologies on which this new state estimators are based is the Wide area monitoring system (WAMS) technology, where time-synchronised measurements are combined with a central data concentrator and distributed state estimators. In the context of the ICOEUR project this newly developed estimator system was tested by simulations in a large scale network as well as in a large scale implementation of the WAMS system.
Significant progress has been made in fast stability assessment methods aiming to identify instable modes of large scale power systems due to improper voltage or angle deviations as well as inter-area oscillations. Realisation of such a system is possible under combined utilisation of strategic placed Phasor measurement unit (PMU)s, centralised data collection and efficient stability assessment algorithms.
The ICOEUR project has delivered tools to support the cooperation of TSOs in maintaining individual and general system stability. Local protection systems are activated selectively, systematically and in a coordinated way. These tools address individual and general system stability issues in order to maintain stability of the interconnected wide area system as large as possible.
As common basis for the evaluation of all tools and methods the ICOEUR consortium has developed a reduced dynamic network model of the European and Russian transmission grids. This model is based only on public available data and can be used for network extension planning, time domain simulation and stability analysis.
For dissemination of the project results three stakeholder workshops and one final workshop were organised by the ICOEUR consortium in Europe and in Russia. Furthermore, more than 30 scientific publications were placed in national and international journals and conferences. To further disseminate the findings, ICOEUR will publish a book in the second half of the year 2012 containing all the main project results.
In addition, an exploitation plan has been developed, in order to assure that the ICOEUR tools which have reached a nearly-industrial level will be implemented the potential users.
Project context and objectives:
The interstate integration of power grids provides multiple advantages concerning operation security, integration of renewable energy as well as energy trading. Due to this fact the European Network of Transmission System Operators for Electricity (ENTSO-E) interconnection expands continually since its establishment. Consideration is given to different scenarios of joint operation of ENTSO-E with power grids on the territory of the former Union of Soviet Socialist Republics (USSR). Due to the fact that such an interconnection is second to none in the World in terms of the scale and distance of the interconnection and number of countries involved, strong research and development (R&D) and innovations are urgently required along with the recent development of technologies. Bulk power grids may encounter major blackouts, which originate in increasing complication in monitoring, operation and control of interconnected power grids as well as in limited knowledge of the total system state. Therefore the possible future interconnection between the European and Russian electricity transmission systems requires elaborating methods for monitoring, control and protection of large scale systems and especially for the support of their interconnections. The development and prototypically implementation of these new methods and tools was the major goal of the ICOEUR project. New technologies like Wide area monitoring, control and protection as well as advanced network controllers (FACTS) and High voltage direct current (HVDC) systems were considered.
The ICOEUR project has focused among others on the following urgent, high-impact functional needs, which can be regarded as improvements of the current state of the art:
- delivery of clear concepts of optimal interconnection of large-scale power systems as of EU and Russia;
- concepts for future oriented and sustainable grid expansion and grid enhancement;
- methods to increase the observability of large scale power system interconnections;
- better sensing, monitoring, understanding and predictability of the power system state;
- advanced simulation tools: transmission devices, smart equipment, protection systems etc;
- novel control methods of large scale interconnected transmission systems, as European and Russian;
- innovative concepts for cooperation of TSOs in interconnected power systems with regard to stability control issues.
This leads to the following main project objectives per work package (WP) (only scientific WPs):
The main objectives of WP1 are were to identify and specify the needs and requirements for extension and interconnection of large-scale power systems and to create realistic models of ENTSO-E and Russian networks as basis for the following WPs.
The main objective of WP2 was to identify the best technology for the extension and interconnection of EU and Russian systems.
The main objective of WP3 was the development of methods and tools for monitoring and control of power systems in EU and Russia.
The main objective of WP4 was definition of efficient protection functions for secure operation of large scale interconnected networks.
Project results:
The interstate integration of power grids provides multiple advantages concerning operation security as well as energy trading. Due to these facts the Central European Power System (ENTSO-E-CE, formerly UCTE) expands continually since its establishment and the ties to other interconnected systems like NORDEL grow stronger and further new ties with neighbouring countries are either being constructed or planned. The recent interconnection of Turkey underlines this trend.
Similar developments are valid in the Intrusion prevention system / Uninterruptible power supply (IPS / UPS) system of Russia and its neighbouring countries. Particular consideration has to be given to different scenarios of joint operation of the ENTSO-E- and IPS / UPS systems.
With large scale deployment of renewable generation throughout Europe, in particular large scale wind farms and future solar power plants, interstate interconnections are of growing importance to secure energy supply. They optimise the utilisation of energy sources within larger areas, promote electricity trading between different regions, and meet the requirements of economic development.
The major benefits that motivate TSOs to build up interconnections to neighbouring transmission systems are:
- optimisation of the use of installed capacities;
- reliability improvements reducing the economic cost of power outages;
- improved control of system frequency to minimise major disturbances;
- sharing reserve capacities and reducing the level of reserves required;
- providing mutual support for the interconnected systems in case of emergency;
- improved energy market conditions in better integrated large scale systems;
- facilitating large scale integration of renewable energies due to higher flexibility of the interstate network operations.
Due to the fact that both systems of European and Russian alone and especially with interconnections are second to none in the world in terms of the scale and distance of the interconnection and number of countries involved, strong R&D and innovations are urgently required along with the recent development of technologies.
Presently, there are numerous enlargement projects of ENTSO-E and IPS/UPS under consideration and investigation:
- interconnection of Turkey was recently established;
- interconnection to northern Africa (Tunisia, Libya, Morocco, etc.);
- interconnection to P.R. China;
- and, most significantly, the interconnection of the two largest systems ENTSO-E and IPS / UPS.
It has to be mentioned that there are actually several system bottlenecks identified within the networks of EU and Russia. These congestions have to be considered as well and need to be relieved with the right technologies strengthening the interconnected power systems. The realisation of an interconnection of bulk power systems, which differ in their technical characteristics, is not trivial and its technical and economic efficiency depends on the chosen technology as well as its impact on system operation security. Currently there are multiple transmission technologies with miscellaneous technical properties available: i.e. cost efficient and well proven HVAC technologies, with the disadvantage of direct disturbance extension between interconnected systems or more sophisticated HVDC transmission systems with better controllability but higher investments. In order to improve system stability, to control load flow, to facilitate electricity trading and to optimise the utilisation of energy resources in interconnected power systems Flexible alternating current transmission systems (FACTS) and HVDC as well as other innovative compensation or control devices can be used. Due to that complexity as a first step the technically and economically optimal realisation of future large scale interconnected power systems have to be investigated regarding interconnection technologies. The beneficial integration of appropriately selected technologies is a precondition for the future development of large scale interconnected power systems.
However, bulk power grids may encounter major blackouts, often with catastrophic consequences for system and consumers. Some of such severe blackouts occurred for instance in Europe and Russia in 2003, 2005 and 2006, respectively. Among the main factors leading to occurrence and development of such emergencies, researchers call complication in operating conditions of the power grids and their control in a market environment as well as insufficient coordination of control at an interstate level. The latter particularly manifested itself during the 2006 European blackout. Therefore the possible future extension of power system interconnections requires elaborating methods for monitoring, control and operation of large scale systems and especially for the support of their interconnections.
In the following sections of this report, the main results of the ICOEUR project are described. Section 1 introduces the models created by the consortium form simulating a large scale interconnection. In section 2 deals with monitoring of interconnected power systems. This is basis for the control and protection technologies introduced in section 3 and 4 respectively.
1. Power system static and dynamic models
For investigation of large scale interconnected systems and to verify the operability of the ICOEUR results, models of the respective networks are required. Such models for static and transient investigations are introduced in the present section. These models are based purely on public available sources and can be used as reference cases for research purposes. It has to be stated clearly that the focus is both on the internal needs of the network development as well as their extensions and interconnections with surrounding systems. Furthermore, improvements of the simulations tools used by the project participants are described in this section.
Network model
Case studies in the ICOEUR project are carried out on a common network model, which was developed by the project partners based on publicly available data. This network model consists of 545 nodes and includes a reduced dynamic model of an interconnected transmission system of ENTSO-E and IPS / UPS. In the following the general modelling of the grid, the steady state data and the dynamic data are introduced.
The nodes of the reduced grid model are chosen according to the grid maps of ENTSO-E and IPS / UPS based on the following criteria:
- regions with important generation clusters are represented by one node;
- regions with important load clusters are represented by one node;
- important junctions between transmission lines are represented by one node.
The voltage level, the length and the number of transmission lines between these nodes are estimated according to the grid maps. All other transmission lines are neglected. It is assumed that the specific reactance and the nominal current of the transmission lines are modelled with common values according to the literature. As only public available data is used, breakdown to further transmission line types is not possible. All transmission lines are converted to a uniform voltage level of 380 kV and transformers are neglected.
Every node of the reduced network model is equipped with a load and a generation cluster representing the total load and generation of the replaced area. The distribution of load within each country is allocated to the nodes according to the population density. The amount of generation of each node cluster on the ENTSO-E part of the model is obtained by market simulation. Input data for each market simulation scenario are the values for load and non-controllable generation at each node of the reduced network model (which are defined in different scenarios), the grid topology (for consideration of congestions) and power plant data (location, fuel costs, start-up costs). The outcome of the market simulation is the unit commitment of the power plants for each scenario, which balances the residual load and allows for transit flows according to the available cross-border capacities based on a market coupling algorithm. To each scenario of the market simulation there belongs a dynamic model, which is based on the load situation, the unit commitment and the in-feed from non-controllable generation.
EUROSTAG API
EUROSTAG is the standard simulation tool used by the partners of the project. In order to comply with the needs of the project partners of interfaces between EUROSTAG and other external programs, the EUROSTAG API has been developed to support the use of an external program to drive the simulation of EUROSTAG.
The EUROSTAG API completes the EUROSTAG batch mode and can support the user to interface the EUROSTAG computational to an external program to control the execution of EUROSTAG and to calculate non-standard control feedback signals. It allows and easy the modelling of complex control strategies that cannot be expressed in the form of control blocks:
- optimisation;
- multi-agent Systems;
- iterative methods;
- artificial intelligence / fuzzy Logic;
- discrete event dynamical system.
This EUROSTAG API consists of a dll library (with the associated header) and a python module that implements some wrapping functions for easier use. The library can be called by any other scripting or programming language supporting the dynamic linked libraries (e.g. Matlab, C++, Java). The main functionalities provided by the EUROSTAG API are:
- performing a load flow;
- performing a dynamical simulation;
- pausing at particular times;
- recovering some information;
- adding new events.
This tool is particularly useful for advanced research purpose where non-conventional operation must be implemented such as PMU modelling, Special protection schemes (SPS) and Wide Area measurement protection and control (WAMPAC).
2. Monitoring of interconnected power systems
A basic requirement for future large scale power systems is the most modern monitoring technology beyond the nowadays state of the art. Nowadays, regional control centres get steady state information of their own system only. With increasing interconnections and size of the system, system wide information of the entire interconnected system as well as dynamic information is needed for reliable power system operation.
Therefore the development of methods and tools for monitoring of large scale power systems is a key requirement and objective. The communication and data exchange between control centres as well as system state estimation based on WAMS are innovative approaches. Use will be made of the latest developments in synchronised wide area measurements, in information and communication technology, and in system identification. New developments need to privilege the interconnection concepts and technical solutions that offer flexibility and minimise the impact on the power system operational organisation to permit a progressive and modular extension of the electrical system to be interconnected with the pan-European system.
This section deals with proposals for innovative solutions in this area. The implementation of a large scale WAMS is presented. This is followed by distributed state estimation, which is promising to significantly improve the quality of system information. Finally, the tool for carrying out a topological vulnerability analysis of the transmission grid will be introduced.
Large-scale WAMS implementation
For the implementation of control and protection functions time-synchronised data about the power system state in large scale is of highest importance. The partners of the ICOEUR project have implemented a Large Scale WAMS (LS-WAMS) with one Phasor data concentrator (PDC) WAProtector system of ELPROS in Ljubljana and four PMUs in Europe. In Russia one PDC and two further PMUs have been installed in Novosibirsk and Irkutsk. All PMUs are connected to the 230V and are solely used to examine frequency, voltage magnitudes and phase angle deviations. Several system wide events have been detected by the 230V LS-WAMS. Regarding the experiences obtained by the WAMS pilot project we can conclude that a central WAMS inside ENTSO-E CE can be established, as can be observed from:
- Technical view: technology is available.
- Sensitivity of data: the basic data exchanged between TSOs will be frequency and voltage phasors. The pilot project has proven that data measured on the 230V LSWAMS has strong correlation with data from high voltage measurements and since low voltage data is public these data cannot be treated as confidential.
- Costs: PMU devices are available for a reasonable price and also compared to the importance of the power network system a central PDC is not a big investment.
We propose a central WAMS inside ENTSO-E CE with the following technical specifications:
- integration of one PMU inside each TSO region;
- central PDC with 20 ms database resolution;
- oscillation detection;
- over-/under-frequency detection;
- voltage angle difference monitoring;
- islanding detection;
- oscillation source detection;
- automatic event recording;
- direct connection with TSO control centres for event informing by standard protocol IEC 60870-5-104;
- remote access to data visualisation for each TSO region.
Distributed state estimation for large power interconnections
This tool is intended for solution of state estimation problems of large power interconnections in distributed management and control systems consisting of computers of lower layer, placed in national (or local) control centres, and computer of upper layer (server), placed in one of these centres.
Formulation of state estimation problem in the algorithm, realised in this tool, is based on the classical weighted least square estimation criterion.
Computers of lower layer in this distributed software tool are charged with solution of state estimation problems of power systems, considered as subsystems of a whole interconnection. Computer of upper layer has to solve the problem of calculation the values of state variables (magnitudes and angles of voltages) in boundary nodes of subsystems. The set of computation modules of this complex includes the modules of lower level, performing computation functions of computers of lower layer and the modules of upper level, performing computation functions of computer of upper layer.
Topological vulnerability analysis of power grid
This tool applies complex network theory to analyse the topological vulnerability of power grids. This tool is based on traditional topological method but extended to introduce some electrical engineering specificities. Three indices have been derived which allows to characterise the topological vulnerability of power grid:
- the entropy degree metric which allows to examine the structural features of power grids;
- the Electrical betweeness extends the notion of betweenness centrality by introducing electrical engineering specificities;
- the net-ability introduces a global metric to measure the performance of a power grid.
The main advantage of topological network methods on traditional operational-status-based methods is its simplicity to understand and apply.
3. Control of interconnected power systems
Beyond the better monitoring, new control approaches are required. This section starts with an overview about the most promising transmission technologies, including the corresponding device models developed in this project. Then, new control approaches are described which have been developed in the frame of the ICOEUR project. New control technologies and control schemes are presented in the present section. The focus is on coordinated power flow control, optimisation of interconnections and inter-area oscillations. Finally, an evaluation of the different interconnection concepts is presented.
Transmission technologies
Worldwide, transmission of electricity mostly operates in AC, including extra-high-voltage and ultra-high voltage AC systems. Nowadays, however, it is possible (due to increasing progress of technologies development), also with the scope to prevent the cascading effect of emergencies, to consider the options of electric power system interconnections in Direct current (DC) and / or hybrid (when AC and DC are used in couple) modes. The following technologies have been considered in the ICOEUR project: FACTS technologies. In accordance with the objectives of ICOEUR, the scope of this investigation has been limited to the most promising and broadly used types of FACTS. Namely, three main typologies have been considered: Shunt (especially SVC and STATCOM), Series (mainly TCSC, DPFC and SSSC), Combined shunt and series (UPFC). FACTS devices enhance system controllability allowing a more flexible system operation and improving system performances in terms of power transfer capacity, power flow control, stability and so on. Concerning the inter-area power oscillations that could affect the large interconnection with possible outages and vast black-outs, the utilisation of FACTS devices with additional stabilising controls seems to be the most suitable countermeasure, in addition to conventional PSS. FACTS application, improving power flow control, reduces the need for construction of new transmission lines avoiding higher asset costs.
Conventional HVDC technologies
Non-synchronous connection technologies are considered as an alternative solution for system connection between ENTSO-E RG CE and IPS / UPS. DC technologies have preferences over AC technologies for several applications, such as long distance overhead transmission, asynchronous systems interconnections via back-to-back (BTB) installations, longer submarine / underground cable links. DC technologies in large transmission systems may also result in a more cost-effective and thus 'easier to realise' perspective for merging the electricity markets of ENTSO-E RG CE and IPS / UPS. In addition, due to the advanced progress of state-of-the-art power electronics nowadays, DC technologies are becoming absolutely necessary for large off-shore wind farms connection, for so-called smart grids systems, and for reactive compensation of AC systems. One of the major advantages of HVDC solutions is the decoupling character of the link which is very important in case of system disturbances. In addition, HVDC links allow the control of the power flow which discharges the grid under overload and emergency situations. An HVDC tie linking two AC systems could also be used as FACTS device to improve the coupled AC system performance by means of additional control features. Possible control functions are for example: frequency control, redistribution of the power flow in the AC network, power oscillations damping.
VSC-HVDC technologies
The high-power semiconductors allow developing self-commutated VSCs (based on gate turn-off and integrated-gate commutated thyristors, and on insulated-gate bipolar transistors) converter technologies. VSC-HVDC technology is now emerging as a robust and economical alternative for future transmission grid expansion. In particular, embedded VSC-HVDC applications, together with the WAMS, in meshed AC grids could significantly improve overall system performance, enabling smart operation of transmission grids with increased security and efficiency. Most common implementations of VSC-HVDC technologies are: network interconnections, bottleneck mitigations, integration of renewable energy sources, DC in-feed to large urban areas, DC segmented grid. Technology advantages of VSC-HVDC incorporated in meshed AC grids are: power flow control flexibility, fast response to disturbances, multi-terminal configurations. VSC-HVDC transmission also offers a superior solution for many challenging technical issues associated with integration of large-scale renewable energy sources such as offshore wind power.
Dynamic models of the above mentioned devices were needed by the project participants for the simulation software EUROSTAG. Most devices are already available in the standard library of EUROSTAG. The devices DPFC and VSC-HVDC had to be developed by the consortium. These models are described below.
DPFC model
This tool consist of a detailed DPFC model allowing to perform static and dynamic simulation with this new kind of FACTS device in the EUROSTAG simulation software. A Dynamic Power Flow Controller (DPFC) is used to control the active power flow. It consists of a Phase-shift transformer (PST) and a Thyristor switched-series capacitor (TSSC). In EUROSTAG dynamic models of an Inter-Phase-Controller (IPC) and Thyristor controlled series capacitor (TCSC)s are already available. These models are combined and enhanced for the model of the DPFC. The TSSC part allows only small but fast changes of the transferred active power. In contrast large, but slow changes of the active power are possible with the PST part. Due to this, the PST part only changes the tap position if the TSSC part has reached the upper or lower border of the possible admittance. In contrast to the TCSC the PST allows only a stepwise change of the reactance.
The DPFC is represented in EUROSTAG by two current injectors which are coupled by new macro blocks. In addition to the normal branches the injectors are connected to the environmental network with branches of a reactance of 1 000 p.u. to avoid a shutdown of the injectors in case of a line tripping.
VSC-HVDC model
A static and dynamic model VSC-HVDC for the EUROSTAG simulation software was developed in the frame of the ICOEUR project. The dynamical model has been compared to a manufacture-provided model trying to reproduce the same results. This has been done by adapting of few parameters of the developed model to match the behaviour of the reference one. The results obtained are excellent for the DC Voltage and Reactive power controllers. For which regards the active power control loop the results are not exactly the same but the dynamics are represented accurately enough. The model is rich enough and it can represent actual VSC HVDC systems within the domain of validity of the reference model. In the following we introduce several tools which have been developed in ICOEUR for controlling the power system with FACST and HVDC devices.
Coordinated control of Power flow controlling (PFC) devices
Energy market activities together with integration of renewable energies are currently causing an increase of distance between generation and load as well as an increase of volatility of power flows. As a consequence, the TSOs install PFC devices to increase transmission capacity and controllability of their grids. In the ICOEUR project a new real-time coordination system for PFC devices is developed, using a distributed approach. Agents exchange local information about the status and loading of power system devices. By evaluation of these messages, controlling agents perform a distributed topology and sensitivity analysis to be used in a weighting function to determine the next control actions. The coordination system is verified by comparing it with optimally coordinated set-points provided by an optimal power flow tool from the PEGASE project for several test cases. As a further verification the dynamic control behaviour of the multi-agent coordination system is tested on the ICOEUR network model with several PFCs having mutual impact on each other. These test cases show that the coordination system reacts in an appropriate and robust way on contingency events. The impact of communication delays and different sensor reporting rates on the PFC control is negligible for PSTs, while the control behaviour of fast PFC-devices is significantly influenced.
Distributed optimal operation problems for large power interconnections: This tool solves the static active power optimisation problem of large power interconnection, i. e the problem of economic dispatch for power systems within interconnection.
This software tool is intended for solution of optimal operation problems of large power interconnections in distributed management and control systems consisting of computers of lower layer, placed in national (or local) control centres, and computer of upper layer (server), placed in one of these centres.
Computers of lower layer in this distributed software tool are charged with solution of optimisation problems of power systems, considered as subsystems of a whole interconnection. Computer of upper layer has to solve the problem of calculation the values of state variables (magnitudes and angles of voltages) in boundary nodes of subsystems. The set of computation modules of this complex includes the modules of lower level, performing computation functions of computers of lower layer and the modules of upper level, performing computation functions of computer of upper layer.
Input data for solution of this problem in each of power systems within an interconnection are data on cost characteristics of power stations, loads and electric network parameters of power systems within interconnection. In distributed management and control system these data should be delivered to lower layer computer, located in control centre of this power system.
Large scale power system inter-area oscillation analysis
The purpose of the tool is to assess the small-signal oscillatory stability of a very large power system and to help finding the best countermeasures to prevent oscillatory instability. The tool methodology consists of: linearisation of algebraic-differential equations that describe the oscillating behaviour of the power systems, build-up of the state matrix (reduced matrix of coefficients of algebraic-differential system) and 'modal calculation' of eigenvalues and eigenvectors that represent the electromechanical oscillation modes. The last step of the eigenanalysis is to calculate the eigenvalues / eigenvectors to detect the most critical electromechanical oscillation modes, generally corresponding to interarea modes. Due to the high required computational resources, a selective calculation methodology has been developed: the needed Central processing unit (CPU) time is reduced very much preserving the accuracy of most critical modes. The method used for this selective calculation is based on the Arnoldi method.
This tool has been validated by comparison with time-domain simulations. Taking into consideration an oscillation mode detected by modal analysis, the correlated time-domain simulation has to confirm the mode characteristics in terms of frequency, damping and geographical shape. This validation work has been successfully carried out on the CE+Turkey+IPS/UPS network.
Interconnection concepts
This last part of this section reports about a comparison of the various interconnection concepts. Based on the above described technologies and developed tools investigations have been performed to compare an interconnection between ENTSO-E CE and IPS/UPS with the following technologies:
- only HVAC;
- combined HVAC and HVDC;
- only HVDC.
Concerning interarea oscillations a modal analysis was carried out to compare the different interconnection scenarios. In case of synchronous interconnection realised by eleven AC lines, the modal analysis pointed out that the slowest global modes (equal to 0.1 Hz), spanning all over the two merged systems, are much damped; the IPS/UPS internal modes are characterised by a smaller but yet good damping. Some lower damping modes have been revealed within CE system. The application in Alytus (LT) of TCSC or SVC or BTB (hybrid AC DC) equipped with PSS, has indicated to be not much effective in enhancing the oscillatory system behaviour, especially for modes not crossing the interconnection. In case of synchronous (hybrid AC-DC) interconnection with three BTBs and eight AC lines, the DC links involve a significantly higher reactance of the AC interconnection between the two systems. As a consequence, the modes crossing the interface are characterised by lower frequencies; moreover a general damping worsening of the most critical modes has been observed. As in the synchronous interconnection scenarios, the BTBs PSSs based on local frequency appeared completely ineffective for the damping improvement. The full asynchronous DC interconnection, cutting the power oscillations between the two systems, is substantially equivalent, from the oscillations point of view, to the present situation with the two separated systems. With respect to the synchronous interconnection, the asynchronous one generally amplifies the swings of generators close to the interconnection. Again the BTB PSSs based on local frequency can give a very small contribution to the power oscillation damping improvement; however this contribution is lightly higher than the one obtained by the BTB PSS for synchronous interconnection, due to the larger frequency deviations observed close to the converter stations. The use of remote signals, especially a properly chosen power flow, can significantly improve the oscillation damping. The IPS / UPS modes damping appeared to be substantially independent on the power exchange whereas some CE modes present a certain dependence, positive or negative, on the power exchange.
Synchronous coupling between ENTSO-E RG CE and IPS / UPS is feasible upon implementing several technical, operational and organisational measures and establishing the legal framework. As the implementation phase for those measures and conditions is recognised as a long process, a synchronous coupling can be considered only as a long term perspective. In order to achieve a joint, world-largest electricity market platform between ENTSO-E RG CE and IPS / UPS synchronous areas the construction of HVDC links between the interface countries may be considered as a medium-term alternative solution for system coupling. The major advantage of HVDC solutions is the decoupling character of the link which is very important in case of system disturbances. In addition HVDC links allow the control of the power flow which discharges the grid under overload and emergency situations.
Based on the project results, it is clear that purely HVDC asynchronous interconnection of the European and Russian power systems represents a very interesting and effective solution. All the advantages of the HVDC technologies combined with the proper control strategies should insure that the coupling of these two systems results in benefits greatly surpassing all the necessary efforts and problems. The hybrid interconnection solution combined with the advanced control strategies might also represent the plausible solution, but in certain scenarios disadvantages of this type of interconnection in comparison to pure asynchronous solution are evident. The pure AC synchronous interconnection represents the simplest, but also from the aspect of system stability the least satisfactory solution.
4. Protection of interconnected power systems
Another required innovation for interconnected power systems are new protection schemes. Efficient protection functions for secure operation of large scale systems of EU and Russia in both isolated operation mode and interconnected mode are needed. In this context methods for quantifying the operational risk and for assessing the stability of a large transmission system were developed.
The analysis of black-out risks shows that IPS / UPS, ENTSO-E CE and the interconnection of both grids are heterogeneous networks whose structure is vulnerable to selective failures of critical buses or lines but robust to random failures. Therefore, it is necessary to protect these critical buses or lines identified by various metrics (i.e. entropic degree, electrical betweenness and net-ability drop). Moreover, the analysis indicates that the interconnection of ENTSO-E CE and IPS / UPS power grids enhance the network performance of IPS / UPS; comparatively, interconnected power grid is the most robust among three power grids while IPS / UPS power grid is the most vulnerable. Hence, from structural perspective, ENTSO-E CE and IPS / UPS power grids are beneficial from the interconnection since the interconnected power grid can not only improve overall network performance of IPS / UPS power grid but also structural robustness of ENTSO-E and IPS / UPS power grids. The present section focuses on stability and protection techniques, which were developed in ICOEUR.
Out-of-step protection system
With the future possibility of an interconnection between ENTSO-E CE and IPS / UPS power systems by use of AC tie lines the occurrence of out-of-step operation at the interface between these power systems can be dangerous for both of them and result in undesirable consequences for the systems and consumers. Within the framework of the ICOEUR project the new modified Selective out-of-step protection and prevention system (SOSPPS) with the use of PMU measurements was proposed.
The novelty of this approach is its interdependence on having a complete observability of the system. A defense plan analyses voltage phasors measured at different network locations to determine the security state of the transmission corridor. If a disturbance is detected, the protection system separates the network parts connected by the corridor to avoid synchronous machines falling out of step due to the divagation of the voltage phasors. Simulations were carried out to compare the defense plan with conventional protection systems based on distance protection relays. It was shown that the proposed defense plan can avoid loss of synchronism in cases of severe system disturbances by controlled separation of transmission corridors.
Soft sensor for stability loss discovery
The purpose of this tool is to compute the system gramians for power systems. Computing these gramians is needed for the development of soft sensor for power system stability loss discovery by means of Lyapunov-based stability order assessment.
A new general approach is suggested for the solution of differential or algebraic Lyapunov and Sylvester equations to calculate power system static stability degree. New forms are obtained for controllability and observability Gramians and also for finite and infinite time Cross-Gramians. Asymptotic expansions for finite time controllability and observability Gramians are found to analyse the stability of dynamic system in using control modes at times, when a system is reaching the stability boundary.
The formulation proposed allows to introduce distributed computation system since dynamic matrix spectrum computation without considering functional characteristics can be realised in local subsystems, and computation results with functional characteristics matrices could be sent to Central system to compute power system dynamic matrix spectrum with considering functional characteristics.
The models of static stability calculation is fulfilled in state space form taking into account algebraic balance equations 'online'. The gramians with finite and infinite time are calculated by means of local subsystems models data usage. Frequency spectrums controllability gramians allow to point out static stability indexes for local subsystems and central system in distributed computation scheme. The degree of static stability for local subsystems may be used in optimal adaptive coordination algorithms on the high level power control system.
Under frequency load-shedding model
When system disturbances are so severe that they result in fast frequency drops which lead to governor and boiler responses not fast enough then automatic load shedding is activated in order to restore the production-load balance. This load shedding is characterised by a number of triggering frequencies and associated load quantities to be shed at each step. In case of joint operation of Eastern and Western power systems any disturbance will affect the whole joint system. The diversity of the number of steps, frequency settings, and the amount of load to be shed at each step can deteriorate the frequency transient process and even endanger interconnected system's stability. It is therefore very important to be able to simulate accurately different load shedding strategies.
This tool allows simulating in the EUROSTAG simulation software two underfrequency load shedding strategies (ENTSO-E and IPS / UPS). It allows analysing dynamics of frequency behavior for different levels of active power deficiency and underfrequency load shedding structure.
Potential impact:
mThe main potential impact of the ICOEUR project can be summarised as follows:
- ICOEUR has prepared the way for the use of synergies between western and eastern approaches for power system operation and interconnection.
- ICOEUR has delivered marketable tools to be used by TSOs and other users of power system analysis software (please compare section about exploitation of results).
- ICOEUR has provided solutions to improve coordination between TSOs in an interconnected power system for monitoring and data sharing.
- ICOEUR provides solutions for dynamic network monitoring taking into account instantaneous values for faster dynamic changes in power systems.
- ICOEUR has stimulated international cooperation and communication. Several new international partnerships have been initiated and are also expected to persist after the end of the project (as for example on WAMS development by TUDO and ELPROS).
Dissemination activities
The ICOEUR consortium has carried out the following dissemination activities:
- Homepage: The ICOEUR homepage is reachable at the following address: http://www.icoeur.eu/ The homepage includes broad information about the ICOEUR project as well as individual activities and results. Downloads of useful documentations, connatural links and references is provided. This includes all public available deliverables of the ICOEUR project. The website announces all workshops and meetings and offers proceedings of project workshops and conferences.
- Stakeholder workshops: A first interaction of the ICOEUR partners with the stakeholders was organised as a workshop in Ljubljana (Slovenia) on 21 January 2010. The topic of the workshop was 'Method and tools for monitoring and control of large power systems', which covers the results of WP3. The invited stakeholders were exclusively from TSOs and particularly those who are specialist in the presented topic. A substantial report was prepared and submitted to the stakeholders.
The second ICOEUR stakeholder workshop took place in Lausanne (Switzerland) on 13 September 2011. The workshop topic was 'Flexible technologies to enhance the transfer capacity and the stability in large interconnected power systems'. It is related mainly to WP2. It was attended by 35 participants whose 10 stakeholders were from TSO companies and universities.
The third ICOEUR stakeholder workshop took place in Moscow (Russia) on 28 March 2012. The workshop topic was 'Protection functions for enhancement of stability and security of large interconnected power systems'. It is related to WP4. With these three stakeholder workshops we have organised one event for each of the main scientific WPs.
- Paper / Conferences: The consortium has published more than 30 scientific papers about the ICOEUR research results at national and international journals and conferences.
- Final workshop: At the end of the ICOEUR project a workshop was organised in Brussels (Belgium) on 24 Mai 2012. The workshop topic was 'Monitoring, control and protection of large interconnected power systems'. This workshop was used to present the final results of all WPs to stakeholders and other interested participants.
- ICOEUR book: In order to make the results of the ICOEUR project available to a broad society and in a sustainable way, the consortium is writing a book, containing all main results of the project. This book will be published in the second half of the year 2012. The title of the book is: 'Monitoring, control and protection of interconnected power systems' and it is structured as follows:
1. Requirements for monitoring, control and operation.
2. System model and dynamic phenomena:
a. load flow and dynamic model;
b. dynamic phenomena.
3. Monitoring of interconnected power systems:
a. monitoring technologies;
b. wide area monitoring;
c. distributed state estimation;
d. dynamic state estimation;
e. inter-TSO solutions for monitoring and state estimation.
4. Control of interconnected power systems:
a. control technologies;
b. coordinated power flow control;
c. control of interconnected networks;
d. optimisation of interconnections.
5. Stability and Protection Techniques in Interconnected Power Systems:
a. protection technologies;
b. dynamic security assessment and risk estimation;
c. containment of disturbances;
d. out-of-step protection;
e. load shedding and power plant protection.
6. Combined operation of monitoring, control and protection.
Exploitation of results
Many tools have been developed in the framework of the ICOEUR project. There is a large heterogeneity in the maturity-level of these tools, both in terms of the technical aspects and of the considered exploitation plans.
Some tools are still not mature enough for industrialisation and will need additional research. Examples of tools at this stage are the development of soft sensors for power system stability loss discovery and the optimal tuning of adaptive robust controllers. Some other tools have reached a nearly-industrial level but have not yet well defined exploitation plan even if the prospects are well identified. Example are the LINEAR tool and the distributed software complex for optimal operation/state estimation in large power interconnections.
Different tools are based on the same software and some synergies can therefore be envisaged. For example the new models of the DPFC and the underfrequency load sheeding systems are both using the EUROSTAG software. These new models are currently considered for integration in the EUROSTAG standard library.
Finally, some tools have already reached a nearly-industrial level and the corresponding exploitation plans are well defined. Examples are the EUROSTAG API which will be integrated in the next industrial version of EUROSTAG and the LDM-TG tool which will be used for solving development task for local TSOs.
To summarise the currently well-defined exploitation plans:
- The EUROSTAG API and the DPFC model will be integrated in the commercial EUROSTAG versions.
- The LINEAR tool could be used by the Italian TSO
- The LDM-TG tool is planned to be used for very specific development tasks for local TSOs and for various scientific projects.
- The distributed software complex for state estimation and optimal operation problems in large power interconnections will be integrated into large commercial software of the Distributed management and control system.
Project website: http://www.icoeur.eu
Bulk power grids may encounter major blackouts, due to increasing complication in operation and control of interconnected power grids, as well as lack of exchange of information. Therefore, advanced methods for monitoring, control and protection of large scale systems are essential. These methods are particularly important for a possible future interconnection between the European and Russian electricity transmission systems. The ICOEUR project is a three-year collaborative European Union (EU)-Russian project led by the TU Dortmund University (Prof. Dr C. Rehtanz) (on the EU side) and the Energy Systems Institute (Prof. Dr N. Voropai) (on the Russian side). The project is co-funded by the European Commission (EC)'s Seventh Framework Programme (FP7) and the Russian Federal Agency on Science and Innovations. In the project, a new generation of state estimators was developed. It provides European and Russian Transmission system operator (TSO)s with an accurate snapshot and robust indicators of the system state even during emergency situations. The key technologies on which this new state estimators are based is the Wide area monitoring system (WAMS) technology, where time-synchronised measurements are combined with a central data concentrator and distributed state estimators. In the context of the ICOEUR project this newly developed estimator system was tested by simulations in a large scale network as well as in a large scale implementation of the WAMS system.
Significant progress has been made in fast stability assessment methods aiming to identify instable modes of large scale power systems due to improper voltage or angle deviations as well as inter-area oscillations. Realisation of such a system is possible under combined utilisation of strategic placed Phasor measurement unit (PMU)s, centralised data collection and efficient stability assessment algorithms.
The ICOEUR project has delivered tools to support the cooperation of TSOs in maintaining individual and general system stability. Local protection systems are activated selectively, systematically and in a coordinated way. These tools address individual and general system stability issues in order to maintain stability of the interconnected wide area system as large as possible.
As common basis for the evaluation of all tools and methods the ICOEUR consortium has developed a reduced dynamic network model of the European and Russian transmission grids. This model is based only on public available data and can be used for network extension planning, time domain simulation and stability analysis.
For dissemination of the project results three stakeholder workshops and one final workshop were organised by the ICOEUR consortium in Europe and in Russia. Furthermore, more than 30 scientific publications were placed in national and international journals and conferences. To further disseminate the findings, ICOEUR will publish a book in the second half of the year 2012 containing all the main project results.
In addition, an exploitation plan has been developed, in order to assure that the ICOEUR tools which have reached a nearly-industrial level will be implemented the potential users.
Project context and objectives:
The interstate integration of power grids provides multiple advantages concerning operation security, integration of renewable energy as well as energy trading. Due to this fact the European Network of Transmission System Operators for Electricity (ENTSO-E) interconnection expands continually since its establishment. Consideration is given to different scenarios of joint operation of ENTSO-E with power grids on the territory of the former Union of Soviet Socialist Republics (USSR). Due to the fact that such an interconnection is second to none in the World in terms of the scale and distance of the interconnection and number of countries involved, strong research and development (R&D) and innovations are urgently required along with the recent development of technologies. Bulk power grids may encounter major blackouts, which originate in increasing complication in monitoring, operation and control of interconnected power grids as well as in limited knowledge of the total system state. Therefore the possible future interconnection between the European and Russian electricity transmission systems requires elaborating methods for monitoring, control and protection of large scale systems and especially for the support of their interconnections. The development and prototypically implementation of these new methods and tools was the major goal of the ICOEUR project. New technologies like Wide area monitoring, control and protection as well as advanced network controllers (FACTS) and High voltage direct current (HVDC) systems were considered.
The ICOEUR project has focused among others on the following urgent, high-impact functional needs, which can be regarded as improvements of the current state of the art:
- delivery of clear concepts of optimal interconnection of large-scale power systems as of EU and Russia;
- concepts for future oriented and sustainable grid expansion and grid enhancement;
- methods to increase the observability of large scale power system interconnections;
- better sensing, monitoring, understanding and predictability of the power system state;
- advanced simulation tools: transmission devices, smart equipment, protection systems etc;
- novel control methods of large scale interconnected transmission systems, as European and Russian;
- innovative concepts for cooperation of TSOs in interconnected power systems with regard to stability control issues.
This leads to the following main project objectives per work package (WP) (only scientific WPs):
The main objectives of WP1 are were to identify and specify the needs and requirements for extension and interconnection of large-scale power systems and to create realistic models of ENTSO-E and Russian networks as basis for the following WPs.
The main objective of WP2 was to identify the best technology for the extension and interconnection of EU and Russian systems.
The main objective of WP3 was the development of methods and tools for monitoring and control of power systems in EU and Russia.
The main objective of WP4 was definition of efficient protection functions for secure operation of large scale interconnected networks.
Project results:
The interstate integration of power grids provides multiple advantages concerning operation security as well as energy trading. Due to these facts the Central European Power System (ENTSO-E-CE, formerly UCTE) expands continually since its establishment and the ties to other interconnected systems like NORDEL grow stronger and further new ties with neighbouring countries are either being constructed or planned. The recent interconnection of Turkey underlines this trend.
Similar developments are valid in the Intrusion prevention system / Uninterruptible power supply (IPS / UPS) system of Russia and its neighbouring countries. Particular consideration has to be given to different scenarios of joint operation of the ENTSO-E- and IPS / UPS systems.
With large scale deployment of renewable generation throughout Europe, in particular large scale wind farms and future solar power plants, interstate interconnections are of growing importance to secure energy supply. They optimise the utilisation of energy sources within larger areas, promote electricity trading between different regions, and meet the requirements of economic development.
The major benefits that motivate TSOs to build up interconnections to neighbouring transmission systems are:
- optimisation of the use of installed capacities;
- reliability improvements reducing the economic cost of power outages;
- improved control of system frequency to minimise major disturbances;
- sharing reserve capacities and reducing the level of reserves required;
- providing mutual support for the interconnected systems in case of emergency;
- improved energy market conditions in better integrated large scale systems;
- facilitating large scale integration of renewable energies due to higher flexibility of the interstate network operations.
Due to the fact that both systems of European and Russian alone and especially with interconnections are second to none in the world in terms of the scale and distance of the interconnection and number of countries involved, strong R&D and innovations are urgently required along with the recent development of technologies.
Presently, there are numerous enlargement projects of ENTSO-E and IPS/UPS under consideration and investigation:
- interconnection of Turkey was recently established;
- interconnection to northern Africa (Tunisia, Libya, Morocco, etc.);
- interconnection to P.R. China;
- and, most significantly, the interconnection of the two largest systems ENTSO-E and IPS / UPS.
It has to be mentioned that there are actually several system bottlenecks identified within the networks of EU and Russia. These congestions have to be considered as well and need to be relieved with the right technologies strengthening the interconnected power systems. The realisation of an interconnection of bulk power systems, which differ in their technical characteristics, is not trivial and its technical and economic efficiency depends on the chosen technology as well as its impact on system operation security. Currently there are multiple transmission technologies with miscellaneous technical properties available: i.e. cost efficient and well proven HVAC technologies, with the disadvantage of direct disturbance extension between interconnected systems or more sophisticated HVDC transmission systems with better controllability but higher investments. In order to improve system stability, to control load flow, to facilitate electricity trading and to optimise the utilisation of energy resources in interconnected power systems Flexible alternating current transmission systems (FACTS) and HVDC as well as other innovative compensation or control devices can be used. Due to that complexity as a first step the technically and economically optimal realisation of future large scale interconnected power systems have to be investigated regarding interconnection technologies. The beneficial integration of appropriately selected technologies is a precondition for the future development of large scale interconnected power systems.
However, bulk power grids may encounter major blackouts, often with catastrophic consequences for system and consumers. Some of such severe blackouts occurred for instance in Europe and Russia in 2003, 2005 and 2006, respectively. Among the main factors leading to occurrence and development of such emergencies, researchers call complication in operating conditions of the power grids and their control in a market environment as well as insufficient coordination of control at an interstate level. The latter particularly manifested itself during the 2006 European blackout. Therefore the possible future extension of power system interconnections requires elaborating methods for monitoring, control and operation of large scale systems and especially for the support of their interconnections.
In the following sections of this report, the main results of the ICOEUR project are described. Section 1 introduces the models created by the consortium form simulating a large scale interconnection. In section 2 deals with monitoring of interconnected power systems. This is basis for the control and protection technologies introduced in section 3 and 4 respectively.
1. Power system static and dynamic models
For investigation of large scale interconnected systems and to verify the operability of the ICOEUR results, models of the respective networks are required. Such models for static and transient investigations are introduced in the present section. These models are based purely on public available sources and can be used as reference cases for research purposes. It has to be stated clearly that the focus is both on the internal needs of the network development as well as their extensions and interconnections with surrounding systems. Furthermore, improvements of the simulations tools used by the project participants are described in this section.
Network model
Case studies in the ICOEUR project are carried out on a common network model, which was developed by the project partners based on publicly available data. This network model consists of 545 nodes and includes a reduced dynamic model of an interconnected transmission system of ENTSO-E and IPS / UPS. In the following the general modelling of the grid, the steady state data and the dynamic data are introduced.
The nodes of the reduced grid model are chosen according to the grid maps of ENTSO-E and IPS / UPS based on the following criteria:
- regions with important generation clusters are represented by one node;
- regions with important load clusters are represented by one node;
- important junctions between transmission lines are represented by one node.
The voltage level, the length and the number of transmission lines between these nodes are estimated according to the grid maps. All other transmission lines are neglected. It is assumed that the specific reactance and the nominal current of the transmission lines are modelled with common values according to the literature. As only public available data is used, breakdown to further transmission line types is not possible. All transmission lines are converted to a uniform voltage level of 380 kV and transformers are neglected.
Every node of the reduced network model is equipped with a load and a generation cluster representing the total load and generation of the replaced area. The distribution of load within each country is allocated to the nodes according to the population density. The amount of generation of each node cluster on the ENTSO-E part of the model is obtained by market simulation. Input data for each market simulation scenario are the values for load and non-controllable generation at each node of the reduced network model (which are defined in different scenarios), the grid topology (for consideration of congestions) and power plant data (location, fuel costs, start-up costs). The outcome of the market simulation is the unit commitment of the power plants for each scenario, which balances the residual load and allows for transit flows according to the available cross-border capacities based on a market coupling algorithm. To each scenario of the market simulation there belongs a dynamic model, which is based on the load situation, the unit commitment and the in-feed from non-controllable generation.
EUROSTAG API
EUROSTAG is the standard simulation tool used by the partners of the project. In order to comply with the needs of the project partners of interfaces between EUROSTAG and other external programs, the EUROSTAG API has been developed to support the use of an external program to drive the simulation of EUROSTAG.
The EUROSTAG API completes the EUROSTAG batch mode and can support the user to interface the EUROSTAG computational to an external program to control the execution of EUROSTAG and to calculate non-standard control feedback signals. It allows and easy the modelling of complex control strategies that cannot be expressed in the form of control blocks:
- optimisation;
- multi-agent Systems;
- iterative methods;
- artificial intelligence / fuzzy Logic;
- discrete event dynamical system.
This EUROSTAG API consists of a dll library (with the associated header) and a python module that implements some wrapping functions for easier use. The library can be called by any other scripting or programming language supporting the dynamic linked libraries (e.g. Matlab, C++, Java). The main functionalities provided by the EUROSTAG API are:
- performing a load flow;
- performing a dynamical simulation;
- pausing at particular times;
- recovering some information;
- adding new events.
This tool is particularly useful for advanced research purpose where non-conventional operation must be implemented such as PMU modelling, Special protection schemes (SPS) and Wide Area measurement protection and control (WAMPAC).
2. Monitoring of interconnected power systems
A basic requirement for future large scale power systems is the most modern monitoring technology beyond the nowadays state of the art. Nowadays, regional control centres get steady state information of their own system only. With increasing interconnections and size of the system, system wide information of the entire interconnected system as well as dynamic information is needed for reliable power system operation.
Therefore the development of methods and tools for monitoring of large scale power systems is a key requirement and objective. The communication and data exchange between control centres as well as system state estimation based on WAMS are innovative approaches. Use will be made of the latest developments in synchronised wide area measurements, in information and communication technology, and in system identification. New developments need to privilege the interconnection concepts and technical solutions that offer flexibility and minimise the impact on the power system operational organisation to permit a progressive and modular extension of the electrical system to be interconnected with the pan-European system.
This section deals with proposals for innovative solutions in this area. The implementation of a large scale WAMS is presented. This is followed by distributed state estimation, which is promising to significantly improve the quality of system information. Finally, the tool for carrying out a topological vulnerability analysis of the transmission grid will be introduced.
Large-scale WAMS implementation
For the implementation of control and protection functions time-synchronised data about the power system state in large scale is of highest importance. The partners of the ICOEUR project have implemented a Large Scale WAMS (LS-WAMS) with one Phasor data concentrator (PDC) WAProtector system of ELPROS in Ljubljana and four PMUs in Europe. In Russia one PDC and two further PMUs have been installed in Novosibirsk and Irkutsk. All PMUs are connected to the 230V and are solely used to examine frequency, voltage magnitudes and phase angle deviations. Several system wide events have been detected by the 230V LS-WAMS. Regarding the experiences obtained by the WAMS pilot project we can conclude that a central WAMS inside ENTSO-E CE can be established, as can be observed from:
- Technical view: technology is available.
- Sensitivity of data: the basic data exchanged between TSOs will be frequency and voltage phasors. The pilot project has proven that data measured on the 230V LSWAMS has strong correlation with data from high voltage measurements and since low voltage data is public these data cannot be treated as confidential.
- Costs: PMU devices are available for a reasonable price and also compared to the importance of the power network system a central PDC is not a big investment.
We propose a central WAMS inside ENTSO-E CE with the following technical specifications:
- integration of one PMU inside each TSO region;
- central PDC with 20 ms database resolution;
- oscillation detection;
- over-/under-frequency detection;
- voltage angle difference monitoring;
- islanding detection;
- oscillation source detection;
- automatic event recording;
- direct connection with TSO control centres for event informing by standard protocol IEC 60870-5-104;
- remote access to data visualisation for each TSO region.
Distributed state estimation for large power interconnections
This tool is intended for solution of state estimation problems of large power interconnections in distributed management and control systems consisting of computers of lower layer, placed in national (or local) control centres, and computer of upper layer (server), placed in one of these centres.
Formulation of state estimation problem in the algorithm, realised in this tool, is based on the classical weighted least square estimation criterion.
Computers of lower layer in this distributed software tool are charged with solution of state estimation problems of power systems, considered as subsystems of a whole interconnection. Computer of upper layer has to solve the problem of calculation the values of state variables (magnitudes and angles of voltages) in boundary nodes of subsystems. The set of computation modules of this complex includes the modules of lower level, performing computation functions of computers of lower layer and the modules of upper level, performing computation functions of computer of upper layer.
Topological vulnerability analysis of power grid
This tool applies complex network theory to analyse the topological vulnerability of power grids. This tool is based on traditional topological method but extended to introduce some electrical engineering specificities. Three indices have been derived which allows to characterise the topological vulnerability of power grid:
- the entropy degree metric which allows to examine the structural features of power grids;
- the Electrical betweeness extends the notion of betweenness centrality by introducing electrical engineering specificities;
- the net-ability introduces a global metric to measure the performance of a power grid.
The main advantage of topological network methods on traditional operational-status-based methods is its simplicity to understand and apply.
3. Control of interconnected power systems
Beyond the better monitoring, new control approaches are required. This section starts with an overview about the most promising transmission technologies, including the corresponding device models developed in this project. Then, new control approaches are described which have been developed in the frame of the ICOEUR project. New control technologies and control schemes are presented in the present section. The focus is on coordinated power flow control, optimisation of interconnections and inter-area oscillations. Finally, an evaluation of the different interconnection concepts is presented.
Transmission technologies
Worldwide, transmission of electricity mostly operates in AC, including extra-high-voltage and ultra-high voltage AC systems. Nowadays, however, it is possible (due to increasing progress of technologies development), also with the scope to prevent the cascading effect of emergencies, to consider the options of electric power system interconnections in Direct current (DC) and / or hybrid (when AC and DC are used in couple) modes. The following technologies have been considered in the ICOEUR project: FACTS technologies. In accordance with the objectives of ICOEUR, the scope of this investigation has been limited to the most promising and broadly used types of FACTS. Namely, three main typologies have been considered: Shunt (especially SVC and STATCOM), Series (mainly TCSC, DPFC and SSSC), Combined shunt and series (UPFC). FACTS devices enhance system controllability allowing a more flexible system operation and improving system performances in terms of power transfer capacity, power flow control, stability and so on. Concerning the inter-area power oscillations that could affect the large interconnection with possible outages and vast black-outs, the utilisation of FACTS devices with additional stabilising controls seems to be the most suitable countermeasure, in addition to conventional PSS. FACTS application, improving power flow control, reduces the need for construction of new transmission lines avoiding higher asset costs.
Conventional HVDC technologies
Non-synchronous connection technologies are considered as an alternative solution for system connection between ENTSO-E RG CE and IPS / UPS. DC technologies have preferences over AC technologies for several applications, such as long distance overhead transmission, asynchronous systems interconnections via back-to-back (BTB) installations, longer submarine / underground cable links. DC technologies in large transmission systems may also result in a more cost-effective and thus 'easier to realise' perspective for merging the electricity markets of ENTSO-E RG CE and IPS / UPS. In addition, due to the advanced progress of state-of-the-art power electronics nowadays, DC technologies are becoming absolutely necessary for large off-shore wind farms connection, for so-called smart grids systems, and for reactive compensation of AC systems. One of the major advantages of HVDC solutions is the decoupling character of the link which is very important in case of system disturbances. In addition, HVDC links allow the control of the power flow which discharges the grid under overload and emergency situations. An HVDC tie linking two AC systems could also be used as FACTS device to improve the coupled AC system performance by means of additional control features. Possible control functions are for example: frequency control, redistribution of the power flow in the AC network, power oscillations damping.
VSC-HVDC technologies
The high-power semiconductors allow developing self-commutated VSCs (based on gate turn-off and integrated-gate commutated thyristors, and on insulated-gate bipolar transistors) converter technologies. VSC-HVDC technology is now emerging as a robust and economical alternative for future transmission grid expansion. In particular, embedded VSC-HVDC applications, together with the WAMS, in meshed AC grids could significantly improve overall system performance, enabling smart operation of transmission grids with increased security and efficiency. Most common implementations of VSC-HVDC technologies are: network interconnections, bottleneck mitigations, integration of renewable energy sources, DC in-feed to large urban areas, DC segmented grid. Technology advantages of VSC-HVDC incorporated in meshed AC grids are: power flow control flexibility, fast response to disturbances, multi-terminal configurations. VSC-HVDC transmission also offers a superior solution for many challenging technical issues associated with integration of large-scale renewable energy sources such as offshore wind power.
Dynamic models of the above mentioned devices were needed by the project participants for the simulation software EUROSTAG. Most devices are already available in the standard library of EUROSTAG. The devices DPFC and VSC-HVDC had to be developed by the consortium. These models are described below.
DPFC model
This tool consist of a detailed DPFC model allowing to perform static and dynamic simulation with this new kind of FACTS device in the EUROSTAG simulation software. A Dynamic Power Flow Controller (DPFC) is used to control the active power flow. It consists of a Phase-shift transformer (PST) and a Thyristor switched-series capacitor (TSSC). In EUROSTAG dynamic models of an Inter-Phase-Controller (IPC) and Thyristor controlled series capacitor (TCSC)s are already available. These models are combined and enhanced for the model of the DPFC. The TSSC part allows only small but fast changes of the transferred active power. In contrast large, but slow changes of the active power are possible with the PST part. Due to this, the PST part only changes the tap position if the TSSC part has reached the upper or lower border of the possible admittance. In contrast to the TCSC the PST allows only a stepwise change of the reactance.
The DPFC is represented in EUROSTAG by two current injectors which are coupled by new macro blocks. In addition to the normal branches the injectors are connected to the environmental network with branches of a reactance of 1 000 p.u. to avoid a shutdown of the injectors in case of a line tripping.
VSC-HVDC model
A static and dynamic model VSC-HVDC for the EUROSTAG simulation software was developed in the frame of the ICOEUR project. The dynamical model has been compared to a manufacture-provided model trying to reproduce the same results. This has been done by adapting of few parameters of the developed model to match the behaviour of the reference one. The results obtained are excellent for the DC Voltage and Reactive power controllers. For which regards the active power control loop the results are not exactly the same but the dynamics are represented accurately enough. The model is rich enough and it can represent actual VSC HVDC systems within the domain of validity of the reference model. In the following we introduce several tools which have been developed in ICOEUR for controlling the power system with FACST and HVDC devices.
Coordinated control of Power flow controlling (PFC) devices
Energy market activities together with integration of renewable energies are currently causing an increase of distance between generation and load as well as an increase of volatility of power flows. As a consequence, the TSOs install PFC devices to increase transmission capacity and controllability of their grids. In the ICOEUR project a new real-time coordination system for PFC devices is developed, using a distributed approach. Agents exchange local information about the status and loading of power system devices. By evaluation of these messages, controlling agents perform a distributed topology and sensitivity analysis to be used in a weighting function to determine the next control actions. The coordination system is verified by comparing it with optimally coordinated set-points provided by an optimal power flow tool from the PEGASE project for several test cases. As a further verification the dynamic control behaviour of the multi-agent coordination system is tested on the ICOEUR network model with several PFCs having mutual impact on each other. These test cases show that the coordination system reacts in an appropriate and robust way on contingency events. The impact of communication delays and different sensor reporting rates on the PFC control is negligible for PSTs, while the control behaviour of fast PFC-devices is significantly influenced.
Distributed optimal operation problems for large power interconnections: This tool solves the static active power optimisation problem of large power interconnection, i. e the problem of economic dispatch for power systems within interconnection.
This software tool is intended for solution of optimal operation problems of large power interconnections in distributed management and control systems consisting of computers of lower layer, placed in national (or local) control centres, and computer of upper layer (server), placed in one of these centres.
Computers of lower layer in this distributed software tool are charged with solution of optimisation problems of power systems, considered as subsystems of a whole interconnection. Computer of upper layer has to solve the problem of calculation the values of state variables (magnitudes and angles of voltages) in boundary nodes of subsystems. The set of computation modules of this complex includes the modules of lower level, performing computation functions of computers of lower layer and the modules of upper level, performing computation functions of computer of upper layer.
Input data for solution of this problem in each of power systems within an interconnection are data on cost characteristics of power stations, loads and electric network parameters of power systems within interconnection. In distributed management and control system these data should be delivered to lower layer computer, located in control centre of this power system.
Large scale power system inter-area oscillation analysis
The purpose of the tool is to assess the small-signal oscillatory stability of a very large power system and to help finding the best countermeasures to prevent oscillatory instability. The tool methodology consists of: linearisation of algebraic-differential equations that describe the oscillating behaviour of the power systems, build-up of the state matrix (reduced matrix of coefficients of algebraic-differential system) and 'modal calculation' of eigenvalues and eigenvectors that represent the electromechanical oscillation modes. The last step of the eigenanalysis is to calculate the eigenvalues / eigenvectors to detect the most critical electromechanical oscillation modes, generally corresponding to interarea modes. Due to the high required computational resources, a selective calculation methodology has been developed: the needed Central processing unit (CPU) time is reduced very much preserving the accuracy of most critical modes. The method used for this selective calculation is based on the Arnoldi method.
This tool has been validated by comparison with time-domain simulations. Taking into consideration an oscillation mode detected by modal analysis, the correlated time-domain simulation has to confirm the mode characteristics in terms of frequency, damping and geographical shape. This validation work has been successfully carried out on the CE+Turkey+IPS/UPS network.
Interconnection concepts
This last part of this section reports about a comparison of the various interconnection concepts. Based on the above described technologies and developed tools investigations have been performed to compare an interconnection between ENTSO-E CE and IPS/UPS with the following technologies:
- only HVAC;
- combined HVAC and HVDC;
- only HVDC.
Concerning interarea oscillations a modal analysis was carried out to compare the different interconnection scenarios. In case of synchronous interconnection realised by eleven AC lines, the modal analysis pointed out that the slowest global modes (equal to 0.1 Hz), spanning all over the two merged systems, are much damped; the IPS/UPS internal modes are characterised by a smaller but yet good damping. Some lower damping modes have been revealed within CE system. The application in Alytus (LT) of TCSC or SVC or BTB (hybrid AC DC) equipped with PSS, has indicated to be not much effective in enhancing the oscillatory system behaviour, especially for modes not crossing the interconnection. In case of synchronous (hybrid AC-DC) interconnection with three BTBs and eight AC lines, the DC links involve a significantly higher reactance of the AC interconnection between the two systems. As a consequence, the modes crossing the interface are characterised by lower frequencies; moreover a general damping worsening of the most critical modes has been observed. As in the synchronous interconnection scenarios, the BTBs PSSs based on local frequency appeared completely ineffective for the damping improvement. The full asynchronous DC interconnection, cutting the power oscillations between the two systems, is substantially equivalent, from the oscillations point of view, to the present situation with the two separated systems. With respect to the synchronous interconnection, the asynchronous one generally amplifies the swings of generators close to the interconnection. Again the BTB PSSs based on local frequency can give a very small contribution to the power oscillation damping improvement; however this contribution is lightly higher than the one obtained by the BTB PSS for synchronous interconnection, due to the larger frequency deviations observed close to the converter stations. The use of remote signals, especially a properly chosen power flow, can significantly improve the oscillation damping. The IPS / UPS modes damping appeared to be substantially independent on the power exchange whereas some CE modes present a certain dependence, positive or negative, on the power exchange.
Synchronous coupling between ENTSO-E RG CE and IPS / UPS is feasible upon implementing several technical, operational and organisational measures and establishing the legal framework. As the implementation phase for those measures and conditions is recognised as a long process, a synchronous coupling can be considered only as a long term perspective. In order to achieve a joint, world-largest electricity market platform between ENTSO-E RG CE and IPS / UPS synchronous areas the construction of HVDC links between the interface countries may be considered as a medium-term alternative solution for system coupling. The major advantage of HVDC solutions is the decoupling character of the link which is very important in case of system disturbances. In addition HVDC links allow the control of the power flow which discharges the grid under overload and emergency situations.
Based on the project results, it is clear that purely HVDC asynchronous interconnection of the European and Russian power systems represents a very interesting and effective solution. All the advantages of the HVDC technologies combined with the proper control strategies should insure that the coupling of these two systems results in benefits greatly surpassing all the necessary efforts and problems. The hybrid interconnection solution combined with the advanced control strategies might also represent the plausible solution, but in certain scenarios disadvantages of this type of interconnection in comparison to pure asynchronous solution are evident. The pure AC synchronous interconnection represents the simplest, but also from the aspect of system stability the least satisfactory solution.
4. Protection of interconnected power systems
Another required innovation for interconnected power systems are new protection schemes. Efficient protection functions for secure operation of large scale systems of EU and Russia in both isolated operation mode and interconnected mode are needed. In this context methods for quantifying the operational risk and for assessing the stability of a large transmission system were developed.
The analysis of black-out risks shows that IPS / UPS, ENTSO-E CE and the interconnection of both grids are heterogeneous networks whose structure is vulnerable to selective failures of critical buses or lines but robust to random failures. Therefore, it is necessary to protect these critical buses or lines identified by various metrics (i.e. entropic degree, electrical betweenness and net-ability drop). Moreover, the analysis indicates that the interconnection of ENTSO-E CE and IPS / UPS power grids enhance the network performance of IPS / UPS; comparatively, interconnected power grid is the most robust among three power grids while IPS / UPS power grid is the most vulnerable. Hence, from structural perspective, ENTSO-E CE and IPS / UPS power grids are beneficial from the interconnection since the interconnected power grid can not only improve overall network performance of IPS / UPS power grid but also structural robustness of ENTSO-E and IPS / UPS power grids. The present section focuses on stability and protection techniques, which were developed in ICOEUR.
Out-of-step protection system
With the future possibility of an interconnection between ENTSO-E CE and IPS / UPS power systems by use of AC tie lines the occurrence of out-of-step operation at the interface between these power systems can be dangerous for both of them and result in undesirable consequences for the systems and consumers. Within the framework of the ICOEUR project the new modified Selective out-of-step protection and prevention system (SOSPPS) with the use of PMU measurements was proposed.
The novelty of this approach is its interdependence on having a complete observability of the system. A defense plan analyses voltage phasors measured at different network locations to determine the security state of the transmission corridor. If a disturbance is detected, the protection system separates the network parts connected by the corridor to avoid synchronous machines falling out of step due to the divagation of the voltage phasors. Simulations were carried out to compare the defense plan with conventional protection systems based on distance protection relays. It was shown that the proposed defense plan can avoid loss of synchronism in cases of severe system disturbances by controlled separation of transmission corridors.
Soft sensor for stability loss discovery
The purpose of this tool is to compute the system gramians for power systems. Computing these gramians is needed for the development of soft sensor for power system stability loss discovery by means of Lyapunov-based stability order assessment.
A new general approach is suggested for the solution of differential or algebraic Lyapunov and Sylvester equations to calculate power system static stability degree. New forms are obtained for controllability and observability Gramians and also for finite and infinite time Cross-Gramians. Asymptotic expansions for finite time controllability and observability Gramians are found to analyse the stability of dynamic system in using control modes at times, when a system is reaching the stability boundary.
The formulation proposed allows to introduce distributed computation system since dynamic matrix spectrum computation without considering functional characteristics can be realised in local subsystems, and computation results with functional characteristics matrices could be sent to Central system to compute power system dynamic matrix spectrum with considering functional characteristics.
The models of static stability calculation is fulfilled in state space form taking into account algebraic balance equations 'online'. The gramians with finite and infinite time are calculated by means of local subsystems models data usage. Frequency spectrums controllability gramians allow to point out static stability indexes for local subsystems and central system in distributed computation scheme. The degree of static stability for local subsystems may be used in optimal adaptive coordination algorithms on the high level power control system.
Under frequency load-shedding model
When system disturbances are so severe that they result in fast frequency drops which lead to governor and boiler responses not fast enough then automatic load shedding is activated in order to restore the production-load balance. This load shedding is characterised by a number of triggering frequencies and associated load quantities to be shed at each step. In case of joint operation of Eastern and Western power systems any disturbance will affect the whole joint system. The diversity of the number of steps, frequency settings, and the amount of load to be shed at each step can deteriorate the frequency transient process and even endanger interconnected system's stability. It is therefore very important to be able to simulate accurately different load shedding strategies.
This tool allows simulating in the EUROSTAG simulation software two underfrequency load shedding strategies (ENTSO-E and IPS / UPS). It allows analysing dynamics of frequency behavior for different levels of active power deficiency and underfrequency load shedding structure.
Potential impact:
mThe main potential impact of the ICOEUR project can be summarised as follows:
- ICOEUR has prepared the way for the use of synergies between western and eastern approaches for power system operation and interconnection.
- ICOEUR has delivered marketable tools to be used by TSOs and other users of power system analysis software (please compare section about exploitation of results).
- ICOEUR has provided solutions to improve coordination between TSOs in an interconnected power system for monitoring and data sharing.
- ICOEUR provides solutions for dynamic network monitoring taking into account instantaneous values for faster dynamic changes in power systems.
- ICOEUR has stimulated international cooperation and communication. Several new international partnerships have been initiated and are also expected to persist after the end of the project (as for example on WAMS development by TUDO and ELPROS).
Dissemination activities
The ICOEUR consortium has carried out the following dissemination activities:
- Homepage: The ICOEUR homepage is reachable at the following address: http://www.icoeur.eu/ The homepage includes broad information about the ICOEUR project as well as individual activities and results. Downloads of useful documentations, connatural links and references is provided. This includes all public available deliverables of the ICOEUR project. The website announces all workshops and meetings and offers proceedings of project workshops and conferences.
- Stakeholder workshops: A first interaction of the ICOEUR partners with the stakeholders was organised as a workshop in Ljubljana (Slovenia) on 21 January 2010. The topic of the workshop was 'Method and tools for monitoring and control of large power systems', which covers the results of WP3. The invited stakeholders were exclusively from TSOs and particularly those who are specialist in the presented topic. A substantial report was prepared and submitted to the stakeholders.
The second ICOEUR stakeholder workshop took place in Lausanne (Switzerland) on 13 September 2011. The workshop topic was 'Flexible technologies to enhance the transfer capacity and the stability in large interconnected power systems'. It is related mainly to WP2. It was attended by 35 participants whose 10 stakeholders were from TSO companies and universities.
The third ICOEUR stakeholder workshop took place in Moscow (Russia) on 28 March 2012. The workshop topic was 'Protection functions for enhancement of stability and security of large interconnected power systems'. It is related to WP4. With these three stakeholder workshops we have organised one event for each of the main scientific WPs.
- Paper / Conferences: The consortium has published more than 30 scientific papers about the ICOEUR research results at national and international journals and conferences.
- Final workshop: At the end of the ICOEUR project a workshop was organised in Brussels (Belgium) on 24 Mai 2012. The workshop topic was 'Monitoring, control and protection of large interconnected power systems'. This workshop was used to present the final results of all WPs to stakeholders and other interested participants.
- ICOEUR book: In order to make the results of the ICOEUR project available to a broad society and in a sustainable way, the consortium is writing a book, containing all main results of the project. This book will be published in the second half of the year 2012. The title of the book is: 'Monitoring, control and protection of interconnected power systems' and it is structured as follows:
1. Requirements for monitoring, control and operation.
2. System model and dynamic phenomena:
a. load flow and dynamic model;
b. dynamic phenomena.
3. Monitoring of interconnected power systems:
a. monitoring technologies;
b. wide area monitoring;
c. distributed state estimation;
d. dynamic state estimation;
e. inter-TSO solutions for monitoring and state estimation.
4. Control of interconnected power systems:
a. control technologies;
b. coordinated power flow control;
c. control of interconnected networks;
d. optimisation of interconnections.
5. Stability and Protection Techniques in Interconnected Power Systems:
a. protection technologies;
b. dynamic security assessment and risk estimation;
c. containment of disturbances;
d. out-of-step protection;
e. load shedding and power plant protection.
6. Combined operation of monitoring, control and protection.
Exploitation of results
Many tools have been developed in the framework of the ICOEUR project. There is a large heterogeneity in the maturity-level of these tools, both in terms of the technical aspects and of the considered exploitation plans.
Some tools are still not mature enough for industrialisation and will need additional research. Examples of tools at this stage are the development of soft sensors for power system stability loss discovery and the optimal tuning of adaptive robust controllers. Some other tools have reached a nearly-industrial level but have not yet well defined exploitation plan even if the prospects are well identified. Example are the LINEAR tool and the distributed software complex for optimal operation/state estimation in large power interconnections.
Different tools are based on the same software and some synergies can therefore be envisaged. For example the new models of the DPFC and the underfrequency load sheeding systems are both using the EUROSTAG software. These new models are currently considered for integration in the EUROSTAG standard library.
Finally, some tools have already reached a nearly-industrial level and the corresponding exploitation plans are well defined. Examples are the EUROSTAG API which will be integrated in the next industrial version of EUROSTAG and the LDM-TG tool which will be used for solving development task for local TSOs.
To summarise the currently well-defined exploitation plans:
- The EUROSTAG API and the DPFC model will be integrated in the commercial EUROSTAG versions.
- The LINEAR tool could be used by the Italian TSO
- The LDM-TG tool is planned to be used for very specific development tasks for local TSOs and for various scientific projects.
- The distributed software complex for state estimation and optimal operation problems in large power interconnections will be integrated into large commercial software of the Distributed management and control system.
Project website: http://www.icoeur.eu