Wind power plant control in power system primary frequency regulation based on reference model

Proizvodnja električne energije iz energije vjetra predstavlja danas najprominentniji obnovljivi izvor u suvremenim elektroenergetskim sustavima (EES-ima) i općenito se može reći da energija vjetra zauzima značajan udjel u „globalnoj energetskoj matrici“, odnosno proizvodnom portfoliju. Predviđeni scenarij razvoja EES-a podrazumijeva visok stupanj integracije vjetroelektrana do 2050. godine, od 12% do 50%. S velikim udjelom vjetroelektrana suvremene izvedbe u proizvodnom portfoliju, odnosno proizvodnih jedinica na temelju OIE, smanjuje se inercija cjelokupnog EES-a, čime se pogoršavaju njegove dinamičke i regulacijske karakteristike. Općenito, inercija sinkronih generatora konvencionalnih proizvodnih jedinica ima izuzetno značajan utjecaj u EES-ima na stabilnost frekvencije sustava tijekom poremećaja. Zbog toga pitanje sudjelovanja vjetroelektrana u funkcijama pogona sustava, pri značajnoj integraciji OIE, postaje od velikog značaja. U radu je istražena mogućnost sudjelovanja vjetroelektrana za povećanje stabilnosti frekvencije sustava u trenucima neposredno nakon pojave poremećaja u EES-u, odnosno sudjelovanja vjetroelektrana u inercijskom odzivu sustava ili primarnoj regulaciji. Na osnovi referentnog modela termoelektrane bez međupregrijanja pare (engl. Generating Unit with Non-Reheat Steam Turbine – GUNRST) predložen je i razrađen sustav upravljanja inercijskim odzivom vjetroelektrane. Predloženi pristup je zasnovan na ideji da, neovisno o iznosu proizvodnje vjetrogeneratora, inercijski odziv EES-a ima slične karakteristike kao sa sinkronim generatorima termoelektrana koje vjetroelektrane „potiskuju“ prilikom optimalne raspodjele opterećenja. Programski paket MATLAB/Simulink je korišten za implementaciju i analizu predloženih rješenja uz odgovarajuće vrednovanje rezultata simulacija osnovnog modela vjetroelektrane korištenjem licenciranog programskog paketa PSS®E v33.0. Analizom odziva sustava u cjelini, preko frekvencije centra inercije, pokazano je da primjena referentnog modela upravljanja vjetroelektranom (engl. Model Reference Control – MRC) poboljšava odziv sustava u inercijskoj, odnosno početnoj fazi odziva na poremećaje. Karakteristike referentnog modela upravljanja su analizirane, te je pokazano da njegovom primjenom EES zadržava slične dinamičke karakteristike odziva promjene frekvencije kao i s konvencionalnim elektranama sa sinkronim generatorima neovisno o stupnju integracije vjetroelektrana. Today electricity generation from wind energy represents the most prominent renewable energy source (RES) in modern power systems. Generally it can be said that wind energy represents a large share of the „global energy matrix” or generating portfolio. The projected scenario of the power system development predicts a large scale integration of generation capacities based on wind energy by 2050, from 12% to 50% in different countries. With a large share of modern variable speed wind power plants in generating portfolio, and other generating units based on RES, the inertia of the whole electricity system is reduced, so dynamic and regulating characteristics of a power system are deteriorated. System operators are required to perform impact analysis on existing power system structure in order to provide stable and reliable power system operations in case of large-scale integration of wind power. Gradually conventional generation units will be replaced by wind power plants and the available reserves of conventional power plants will also be reduced. Therefore, the management of reserve power in conventional generating units will be difficult, especially in isolated power system. Consequently, changes in wind speed can cause a significant imbalance of power in such systems. In addition, modern wind turbines based on a variable speed wind generators (VSWG), which are connected to the network via the power electronics (frequency converters) and have the rotating speed of the wind turbine partially or completely „separated" from the frequency of the network. Therefore, the inertia of the entire system will be reduced and higher rates of frequency changes will be present during the power system disturbances so that the primary regulators of conventional power plants will have less time to react in order to maintain a stable frequency. In conditions when the integration rate of wind power exceeds a certain value (over 30-40% of the installed capacity), it can become difficult to keep the existing strategy of frequency regulation, primarily due to the significant financial cost, but also due to technical and complex requirements. Therefore, the inclusion of wind power in the frequency control and active power control for maintaining frequency stability will become a necessity of modern power systems. In general, inertia of synchronous generators of conventional generating units has an extremely important role in the stabilization of the system frequency during the power system disturbances. Therefore, the participation of wind power in the power system ancillary service of frequency support becomes of major importance and represents an important research area for power system operators. The possibility of wind power plants participation in frequency stabilization immediately after the occurrence of power system disturbances or participation of wind power plants in the inertial system response and primary frequency control is analyzed in this work. The following issues are identified as research objectives of this thesis: Feasibility and possibility analyses of wind power plant participation in power system frequency operation and control; Proposal of power system frequency control algorithm for variable speed wind power plants; Evaluation of the characteristics of the proposed control algorithm (reference model control) in power system frequency control functions on the selected test models of power systems by using appropriate software tools. The research part of this thesis was carried out in accordance with the scientific methods of power system analysis and control system analysis in order to achieve the desired dynamic behavior of observed physical systems. General cognitive process method, normative and the methods of analysis and synthesis were used during the research and evaluation. During the preparation of the research, many latest publicly available scientific and technical literature and the results of several studies related to the dynamic regime of power systems regarding integration of wind power plants were analyzed. The MATLAB/Simulink software is used for the simulation and analysis of the proposed solutions. An adequate validation of the basic model of power system, developed using MATLAB/Simulink platform is performed with a licensed software PSS®E v33.0. The defined research hypothesis related to the possibility of provision of wind power plant support to frequency stability of power system is confirmed. By using the proposed model reference control algorithm it is confirmed that wind power generator responds on similar manner as a synchronous generator of conventional thermal power plant during the initial phase of the response on active power disturbance. Thus, the effect of „temporary primary frequency control support” is realized. The thesis is organized in the following chapters: In the Introduction part the brief presentation of the research problem and used methodology is given. The issues related to the analyzed problem are emphasized. Brief review of integration issues of the generating units based on renewable energy sources, mainly wind power plants, in traditional power systems is presented. The integration of wind power plants in existing power systems represents an extensive challenge for power system operators since it involves the changes in power system operational characteristics. Thus the dynamic analysis of power systems with large-scale integration of wind power plants is very interesting area for research and analysis. Second chapter gives the review of the theory of wind power integration in power systems and important definitions and terms related to the impact analysis of wind power on power system operation. Also, basic characteristics of different wind power technologies are presented in this chapter. Main issues related to the power system frequency stability and power system dynamics are presented in the third chapter. Basic issues and terms related to active power and frequency regulation in power systems are described. Primary frequency control and power system characteristics in case of wind power plant integration are analyzed in more detail. The fourth chapter contains description of the used power system models for power system analysis. Also, a detailed representation and description of the wind power turbine-generator used for simulation and analysis is given in this chapter. In the fifth chapter, detailed structure of wind power plant model is described and used test model of power system is presented as modeled in MATLAB/Simulink software. The used model of wind power plant is a generic wind turbine generator model based on General Electric double-fed induction generator (DFIG). All analyses and simulations were performed on the relevant test models of power systems by using the appropriate methods for evaluation and with the software tools PSS®E and MATLAB/Simulink. Participation of the wind power plants in dynamic control functions of power system is enabled by using the appropriate control algorithms that are described in details in the sixth chapter. This chapter contains the description of inertial response control algorithm (df/dt), the „governor“ or „droop“ control algorithm (∆f) and their combination. The combined control algorithm is used for the synthesis and analysis of the proposed model reference control. This chapter contains the presentation and critical analysis of the control algorithms presented in the reviewed literature. Chapter seven gives a detail mathematical derivations of the proposed model reference control algorithm which is used for the realization of the „temporary synthesized inertial response“ which enables the wind power generators to respond on similar manner as conventional synchronous generators. Reference model of GUNRST – Generating Unit with Non-Reheat Steam Turbine is used due to its superior performances of dynamic response on power system disturbances. Based on the reference model of GUNRST unit, a new control algorithm for inertial response of wind power plants was synthesized. The proposed algorithm is based on the idea that the inertial response of power systems has similar characteristics, independently from the production of wind power plants, which suppress conventional thermal units with synchronous generators during power system dispatching. Typical requirements for model reference control algorithm are: Definition of a „preferred” time diagram of a supplied active power of wind turbines after disturbance in order to mimic the response of conventional GUNRST units Provision of primary frequency control support independent of power production (wind speed), or emulation of behavior of synchronous generators of conventional power plants Provision of setting the time to return to the optimal speed value of the generator in accordance with the MPPT characteristic. An equal amount of kinetic energy that is delivered to the system relatively quickly should be „returned" to the wind turbine. By observing the power system frequency response using frequency of the centre of inertia as an indicator it is demonstrated that the proposed MRC – Model Reference Control of wind power plants improves the power system response in inertial or initial phase of power system disturbance response. The characteristics of the MRC algorithm are analyzed and evaluated in Chapter eight and it is demonstrated that dynamic characteristics of power system frequency response remain similar independently from wind power plant integration rate. The important characteristic of the MRC algorithm, which is confirmed by simulation analysis results, is its independence from wind speed. In the final chapter the conclusions are given in accordance with the obtained results of analysis: The synthesis of the control algorithm for primary frequency regulation in accordance with the requirement to mimic the referent GUNRST model is performed; Wind power plant response emulating the referent GUNRST unit is confirmed by using the proposed MRC algorithm; By using the proposed MRC algorithm it is accomplished that the „injected“ active power is independent of power system steady-state parameters, i.e. initial wind speed, which is an inherent characteristic of conventional synchronous generators; Accordingly, the proposed control algorithm ensures that the frequency response of the system is independent on the integration level of wind power, which represents the main contribution of this study. In the final chapter the proposals for further analysis are also given: Setting of the parameters of the applied MRC algorithm and its coordination analysis with the existing control system in accordance with the requirements of specific grid codes; Analysis of the possibility of application of modern signal processing techniques i.e. Discrete Wavelet Transform for the assessment of frequency of the center of inertia with the required upgrade of the proposed MRC algorithm; Analysis of MRC algorithm performances on off-shore wind power plants.