Si Chen. Network reduction in power system analyses

Transcription

1 Si Chen Network reduction in power system analyses Master project Thesis, March 2009

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3 Si Chen Network reduction in power system analyses Master project Thesis, March 2009

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5 Network reduction in power system analyses Author: Si Chen Supervisor(s): Arne Hejde Nielsen, Associate Professor in DTU Electrical Engineering Zhao Xu, Associate Professor in DTU Electrical Engineering Ole Holmstrøm, Specialist in DONG Energy Power Holding A/S Department of Electrical Engineering Centre for Electric Technology (CET) Technical University of Denmark Elektrovej 325 DK-2800 Kgs. Lyngby Denmark Tel: (+45) Fax: (+45) Release date: Class: 1 (public) Edition: First Comments: This report is a part of the requirements to achieve Master of Science in Electrical Engineer at Technical University of Denmark. The report represents 30 ECTS points. Rights: Si Chen, 2009

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7 PREFACE I would like to thank my parents for supporting me all the time. The project was proposed by DONG Energy Power Holding A/S and accomplished at Centre for Electrical Technology (CET)/Technical University of Denmark (DTU) as a part of requirements to achieve the Master of Science in Electrical Engineer. I would like to thank my supervisors Associate Professor Arne Hejde Nielsen in DTU Electrical Engineering, Associate Professor Zhao Xu in DTU Electrical Engineering and Ole Holmstrøm, Specialist in DONG Energy Power Holding A/S for their valuable guidance, data supporting and all the valuable advices. I also want to thank Professor Vladislav Akhmatov in DTU Electrical Engineering for some detailed advices. At last, I would like to appreciate my fellow Nan Qin study together with me over the years. And, those technical and philosophical discussions in power system filed impel me to accomplish the project. Si Chen iv

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9 ABSTRACT The future integration of large off-shore wind farm requires a more advanced external grid for testing the transient behavior of the wind farm. A reduced entire power network system was proposed replace the present external grid which calculated by Thevenin equivalent. Based on the literature study the different technique of the dynamic network reduction, the coherent based equivalent method will be implemented in a real power system model. The model established in the power system analysis tool DIgSILENT PowerFactory. While, the Network reduction module inside the software will be tested by some study cases. The systemized procedure has been introduced and realized in the model simulation. An automatic network reduced program is developed. vi

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11 Contents Preface... iv Abstract... vi List of tables... xii List of figure... xiv 1 Introduction Background State of the art Goals Delimitation Power system transient time frame Transient stability Method Dynamic power network reduction theory Introduction Dynamic power network reduction techniques Modal equivalent technique Estimate equivalent technique Coherency equivalent technique Summary of multiple power network reduction techniques Coherency equivalent method System model establish Typical disturbances Indentify the coherent generators Generators aggregation Aggregate the generator controller Power network topology reduction Evaluation Network reduction function in DIgSILENT Powerfactory Introduction of the Network Reduction function Network Reduction process Reduced system definition Network Reduction command viii

12 3.2.3 Technical Background of Network Reduction Study Case Case Case Modeling West Denmark Power System (WDKPS) Background of the WDKPS model Model layout The mixed power unit Modeling of local CHP unit Modeling of local wind energy Modeling of consumption centre Modeling of conventional power plant Modeling of large wind farm Aggregated model for a large wind farm Modeling of the ongoing offshore wind farm- Anholt Time Domain Simulation of WDKPS Predefined short circuit event Set up Time domain simulation Set up calculation of initial condition Result object Run a simulation Identify coherent generator group Aggregate coherent generators Aggregate Busbars Reduction result evaluation Extra result Conclusions Summary Future Work Bibliography A. Appendix Modeling nine-bus system in DIgSILENT PowerFacotry A. 1 IEEE 9 bus bars system A. 2 Reduce No.2 generator for nine-bus system (Case 1) A.2.1 Using Load Equivalent A.2.2 Using Ward Equivalent A.2.3 Using Extended Ward Equivalent ix

13 A. 3 Reduce Load C for nine-bus system (Case 2) A.3.1 Using Load Equivalent A.3.2 Using Ward Equivalent A.3.3 Using Extended Ward Equivalent B. Appendix WDKPS Simulation B. 1 Original system model B. 2 Reduced system model B. 3 DPL script B.3.1 Fault definition B.3.2 Identify coherent generator group B.3.3 Aggregate coherent groups B.3.4 Reduce Busbar... 5 x

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15 LIST OF TABLES Table 2.1: Summary of multiple power network reduction techniques Table 4.1: Mixed power unit data for 150kV Busbars of WDKPS Table 4.2: The generator type data in pu for modeling the local CHP unit Table 4.3: Key parameters using in modeling large off-shore wind farm in WDKPS Table 4.4: The synchronous generator parameter for simulating Anholt offshore wind farm Table 5.1: List of coherent group of WDKPS Table 5.2: Busbars s Number look up table for aggregate script Table A.1: Generator Data of nine-bus system (100MVA base) Table A.2: Preliminary calculation of nine-bus system (100MVA base) Table A.3: Collection of common impedance parameters when eliminated No.2 generator in nine-bus system using Network Reduction function Table A.4: Equivalent loads parameters for nine-bus system when eliminated No.2 generator in nine-bus system using Network Reduction function Table A.5: Equivalent voltage sources parameters for nine-bus system when eliminated No.2 generator in nine-bus system using Network Reduction function Table A.6: Equivalent extended voltage sources parameters for nine-bus system when eliminated No.2 generator in nine-bus system using Network Reduction function Table A.7: Collection of common impedance parameters when eliminated Load C in nine-bus system using Network Reduction function Table A.8: Equivalent loads parameters for nine-bus system when eliminated Load C in nine-bus system using Network Reduction function Table A.9: Equivalent voltage sources parameters for nine-bus system when eliminated Load C in nine-bus system using Network Reduction function Table A.10: Equivalent extended voltage sources parameters for nine-bus system when eliminated Load C in nine-bus system using Network Reduction function xii

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17 LIST OF FIGURE Figure 1.1: Typical time frames for a full range of power system transient [2]... 4 Figure 2.1: Divide the entire power network system into two parts... 7 Figure 2.2: Divide the power system as several circles by electrical distance Figure 3.1: Network reduction button in the DIgSILENT PowerFactory tools Icon bar Figure 3.2: Swing curve of No.3 generator when eliminate No.2 generator of nine-bus system Figure 3.3: Swing curve of No.3 generator when eliminate Load C of nine-bus system Figure 4.1: Sketch map of WDKPS respecting the geographical location Figure 4.2: Local mixed power production and consumption unit using in WDKPS model Figure 4.3: Sketch map of showing the location of Anholt wind farm integration point Figure 5.1: The flow chart of predefine the short circuit even in WDKPS Figure 5.2: Fault location selected dialog Figure 5.3: Look up table corresponding Busbar name and number Figure 5.4: The flow chart of identify the coherent group for WDKPS model Figure 5.5: Outer loop of aggregate coherent generator Figure 5.6: First inner loop for calculate the equivalent generators Figure 5.7: Second inner loop for Create the new generators Figure 5.8: The flow chart of Busbars aggregation Figure 5.9: Boundary definition of the WDKPS model Figure 5.10: Swing curve of Anholt offshore wind farm equivalent generator Figure 5.11: Terminal voltage measured in per unit for Anholt offshore wind farm equivalent generator Figure 6.1: Swing curve of Anholt offshore wind farm equivalent generator Figure A.1: Nine-bus system model layout in DIgSILENT Power factory xiv

18 Figure A.2: Layout of Nine Bus System with reduced No.2 generator using Load Equivalent Figure A.3: Layout of Nine Bus System with reduced No.2 generator using Ward Equivalent Figure A.4: Layout of Nine Bus System with reduced No.2 generator using Extended Ward Equivalent Figure A.5: Layout of Nine Bus System with reduced Load C using Load Equivalent Figure A.6: Layout of Nine Bus System with reduced Load C using Ward Equivalent Figure A.7: Layout of Nine Bus System with reduced Load C using Extended Ward Equivalent Figure B.1: Original 400 kv part WDKPS model in DIgSILENT PowerFactory Figure B.2: Original 150 kv part WDKPS model in DIgSILENT PowerFactory Figure B.3: Reduced 400 kv part WDKPS model in DIgSILENT PowerFactory Figure B.4: Reduced 150 kv part WDKPS model in DIgSILENT PowerFactory xv

19 1 INTRODUCTION 1.1 Background The wind energy penetration in the power grid is dramatic increasing in the last decade worldwide. The Danish government also releases the energy political about increasing the share of renewable energy to at least 30 per cent of the energy consumption in [1] In order to achieve the goal, one aspect is erection of the new wind energy generation unit. On 21 February 2008, the Danish government agreed to erection of new offshore wind turbine with an installed capacity of 400MW by The other aspect is1 future expansion of the Danish electricity transmission grid. The interaction between the transmission system and the new large wind farms is the priority research area nowadays. Several grid impacts of the transient stability studies need to be approved on the security of supply before the integration. The studies of wind farm involve harmonic impact, reactive power, voltage stability, frequency stability and so on. Many stability studies concern only a local area normally the point of common coupling (PCC).[2] The surrounding area is then regards as the small or large parts of external network. The ultimate aggregation is the simple Thevenin equivalent which only contents the steady state information of the external grid. This, however, is not appropriate to the transient simulation. Thus, the more advanced external grid which contents the dynamic information is required. Electrical power system is one of the largest systems because its components tend to have a high order of complexity.[15] The simulation time of the wind farm studies which involved such large amount entire grid data will be significantly slowing down even with the powerful computing capabilities available today. Therefore, a method to reduce the original complex power network model both respecting the steady state and dynamic characteristics has been proposed. Especially, the method will be realized for a DIgSILENT PowerFactory model of a large power system. 1

20 1.2 State of the art Based on the two system concept, one of the systems considering as the internal study area where the wind farm integrated. The rest of the power system considering as the external area is then aggregated to one or several equivalent generators. Although many dynamic reduction techniques such as the modal approach, the estimation approach and coherency approach are developed, the widely practiced technique is the last approach because of its visualized result elements and other advantages. So far, the network reduction has been practiced in many grid cases with various platforms. DIgSILENT PowerFactory is an electromagnetic transients program (EMPT) which is almost standard industry practice software widely used in power system analyses especially including the wind energy.[2] Furthermore, the DIgSILENT Programming Language (DPL) offers an interface for automating tasks in such a processed program. The new calculation functions of the program script could used for mathematical expressions, input/output routines, Powerfactory object procedure calls, subroutines calls and PowerFactory commands. These will make the automatic power system reduction with different study area definition possible. 1.3 Goals Besides the variation network reduction techniques investigation in theoretically, this master project has two objectives: Testing the Network reduction module both in DIgSILENT PowerFactory 13.2 and 14.0 with IEEE nine-bus system. Investigate whether the function can be used in the dynamic reduction purpose or not, or how much it can be used in dynamic reduction. Develop a systemization procedure for generic dynamic network reduction in DIgSILENT PowerFactory using DPL script. Perform and evaluate the reduction result for West Denmark power system (WDPS) model. The DIgSILENT PowerFactory Network Reduction module doesn't intent to use in dynamic reduction. However, by the exploration of the network reduction object and result characteristics in different precondition, the module is useful for one of the steps in systemization dynamic reduction. The dynamic reduction is performed respecting the coherent approach network reduction technique. The scripts were programmed in 2

21 several steps with independent function that can be checked in the middle of the reduction process. This project does not aim at developing a new technique of network reduction but at developing a systemization procedure respecting one of the network reduction techniques and valid it in DIgSILENT PowerFactory. The procedure should in generic level that also fit to other power system model only with modifying some key input parameters. 1.4 Delimitation Reduction techniques of power system are different for various studies. Thus, different power system stability studies need to be classified. And the relevant transient stability is the specific research area for this master project Power system transient time frame Electric power system is one of the most complex systems which need to be maintained the electric quantity and quality continuously. However, in large scale power system, some elements, such as generators and controllers, have their own dynamic behaviours when receive a specific disturbance. The dynamic behaviour of modern power system has been experiencing significant changes due to restructuring of power industry, especially for the dramatically increased wind energy penetration. Thus, the power system dynamic stability issue become the essential problem of power system analysis.[6] Power system dynamic stability defined as whether or not the system can maintain the stable operation after various disturbances. The transitional period of the power system is called power system transient. Figure 1.1 shows typical time frames for a full range of power system transient. 3

22 Figure 1.1: Typical time frames for a full range of power system transient [2] Classify the transients by the time range of the study is an appropriate investigation method from the modelling point of view. The time frame for lightning and switching is in micro to milliseconds. The system recovery from this disturbance is either negligible or depending on the information sought. Thus, in this master project, the lightning and switching transient were not included. The main researched power transient was transient stability period which estimate the power system dynamic stability during large disturbance, such as fault. The first swing stability is about 1 second and the second swing stability is about 3 to 10 seconds. The Longer term dynamic period could last minutes or hour. For different research period, the analysed element model and computing algorithm were quite different. Another wide used classify method is concerning various disturbances and system simulation mathematics method. According to the definition formulated by the Institute of Electrical and Electronics Engineers (IEEE), the stability problem can be classified as transient stability and steady state stability. If the duration of disturbance was very short that could be considered as the small disturbance, and the system recovery tend to the 4

23 same operation condition before the small disturbance occurring, the system is call small signal stable. In small signal analysis reality, the nonlinear differential equations which represent the power system were replaced by linear differential equation generally. Furthermore, the linear equations reduced following such assumptions as neglect the dynamic behaviours of the system elements and controllers. Consequently, the system stability estimation which could be calculated by algebraic functions is called steady state stability. Obviously, the method concerning the element s dynamic behaviours was the strictly small signal stability, but the method neglect the element dynamic behaviours was steady state stability also as the reduced small signal stability. Contrarily, the large disturbance, such as fault, knowing as the transient stability analysis is mainly discussed direction which will be given in detail in the following chapter Transient stability The previous section classified the power system dynamic stability, and investigated different periods of transient behaviour. The way in which a system responds to a large disturbance such as short circuit or line tripping is basic evaluation method of a reduced system according to this thesis project aim. It is the problem of large disturbance stability, and the effect of such a disturbance has on the system behaviour, that is called transient stability analysis. Assume that before the fault occurs the power system is operating at some initial stable steady state operating condition. The power system transient stability problem is defined as that of assessing whether or not the system will reach an acceptable steady state operating state. Obviously, in order to verify the system transient stability, in accordance with different initial conditions and various disturbances, plenty of calculation must be carried out. During these processes, the power system presents as nonlinear system which using nonlinear differential equations represent. There are two methods estimate the transient stability problem, the time simulation technique and the transient energy function technique. Firstly, the time domain simulation technique can be taken up in 5 major steps: The elements modelling and topologic between elements were represent as simultaneous equations which were composed of differential equations and algebraic functions. Calculate load flow or steady state operation as the initial operation. Setting a valued fault as the large disturbance. 5

24 Calculate the state variable following the time serious step by step, the time domain curve consequently made. (In general, the state variable was generator rotor angle.) Estimated the transient stability by assessing whether or not the system will reach the acceptable steady state operating state. Secondly, there is another technique estimated transient stability by assessing the transient energy function which is not discussed in this project. 1.5 Method In order to achieve the project goals illustrated above, a suitable dynamic network reduction technique that can be practised in DIgSILENT PowerFactroy will start off with review of the candidate reduction techniques. A most feasible technique will be picked up and studied in detail after introduced and compared with other techniques. The DIgSLENT PowerFactory simulation environment will be investigated. The systemization reduction procedures performed in the platform will respected the selected reduction technique. So, the flow chart is necessary connected between the theory and the practice. A cleared and feasible procedure is translated in a programmable form. This is also useful when the script applying to other power system models. Then, it will be convenient to find the part needed to be modified. 6

25 2 DYNAMIC POWER NETWORK REDUCTION THEORY 2.1 Introduction Electrical power system is one of the largest scale systems which contents the components tend to have high order of complexity. It is impossible or not economical to build detail models even with the powerful computing capabilities available nowadays. Therefore, it becomes necessary to develop the equivalent models that properly represent both steady state and dynamic characteristics of the full order models. When dealing the stability analysis, in general, the system can be divided into an internal system and an external system. The internal system used to treat as the researched system which is a specific utility owns or a particular power market defined. The rest part of the power system called the external system. The external system was often a large system which has certain electrical distant or geographical extent. The external system is the one which need spend considerable effort to reduce. That is because the interesting research is the effect of the internal system from the external system. In historic experiments, the external system which may include several generators and loads has been reduced to an equivalent generator and a load even an infinite Busbar. The power network reduction technique nowadays has better systematization and feasibility. The requirement was fixed: the equivalent system need to have similar response as the original system. After the reduction process the equivalency system can save a lot of simulation time and manpower still remaining the required accuracy. External system Internal system Figure 2.1: Divide the entire power network system into two parts 7

26 The equivalent model of power system is desirable for many applications.[4] Generally, there are three kind of dynamic analysis applications: 1. Large scale power system off-line transient stability analysis with large disturbance. 2. Large scale power system off -line dynamic stability analysis with small disturbance. 3. Large scale power system on-line security assessment. The first application for transient stability needs have the detail model include the system structure and element parameters. During this situation, the physical power system which is a non-linear system can be represented as a linear system. The requirement of the equivalent system is the similar rotor oscillation trace as the original system during a large disturbance. For the second application, the system was described in linear differential equations. The requirement of the equivalent system is the approached mode and mode shape of the original system. The third application is used when have variable system operation situations, variable system parameters and a large number of measured positions. The requirement is fast responded in order to have an equivalent system having similar security assessment result. The main research filed of this thesis project was focused on the first application. The transient stability analysis can evaluate the power network reduction result in dynamic level. In virtue of DIgSILENT PowerFactory, the physical power network can be simulated as a linear model. Then, the network reduction work is based on this linear model. 2.2 Dynamic power network reduction techniques The literature presents a number of dynamic power network reduction techniques, basically, there are three different methods used transient RMS-simulation. The methods are: 1. The modal equivalent 2. The estimation equivalent 3. The coherency equivalent 8

27 2.2.1 Modal equivalent technique The modal equivalent method is based on the linear power network model and the eigenvalue of the external system which reduced as reducing the order of the system. The original system can be represented with a complete set of differential equations and a reduced set of differential equations. It is mostly used for the large scale power system off-line transient stability analysis with small disturbance. The result described as the linear state space formulation by depression the order. Thus, it is not based on the real electrical power system element. That is means the results are amount of equations which can t be visualized as a simplified system. Assuming the original system can be separated as an internal system and an external system. And the disturbance in the internal system doesn t affect the external system a lot. With respecting a particular mode of oscillation, according to the modal equivalent theory, some nodes were in non-important oscillation mode for external area dynamic response that can be eliminates. The dynamic equivalent is the determination of the modal generators parameters Estimate equivalent technique The estimate equivalent technique is normally used in large scaled power system on-line security assessment. The estimate equivalent technique doesn t need the detail information about the entire structure and parameters of the external system. The procedure of the estimate equivalent is: 1. Perform a disturbance. 2. Record the system responds on the boundary between internal and external system. 3. Estamat the parameters of the external system base on the recorded signal. The last step used several optimal parameter identification methods. Because of this equivalent technique used for on-line analyses, the reduce speed was one of the important requirements. Also, the reduced system was estimated by the record of the original system. The record accuracy will influence the accuracy of the result. 9

28 2.2.3 Coherency equivalent technique The coherency equivalent technique is based on identifying the coherent generators by comparison of the generator s rotor angle deviation during a off-line large disturbance condition. If the generator rotor angle deviation curves have highly similarity, the generators can be aggregated as one by modified the generator parameters in theoretically. This means the reduction degree is mainly dependents on how many coherent generators can be found. Then, when the generators aggregated in a dynamic way, the Busbars and loads will be aggregated accordingly by equivalent impedance. Consequently, the full order power system which contents many Busbars can be represented as a reduced power system with less Busbar number. The reduction method is based on the full model have already been built, and the fault test can be performed in the internal researched area. Then the coherent generators can be accordingly found. The result of the reduced system is a number of aggregated real elements with modified parameters Summary of multiple power network reduction techniques The network reduction techniques are based on the system which divided as internal and external. With compare the three power network reduction techniques, the advantages and disadvantages will be list in table below. Then the coherency equivalent as the real elements based reduced method will be discussed in detail and finally practised in the real system. Table 2.1: Summary of multiple power network reduction techniques Reduction techniques Modal equivalent Estimate equivalent Coherent equivalent Precondition Application Reduce speed Detailed Off-line small Acceptable model disturbance Boundary information only Detail model On-line disturbance Off-line large disturbance Fast Acceptable Reduction result form Modal elements Experiential equivalent elements Aggregated equivalent elements 10

29 2.3 Coherency equivalent method The coherency approach method is a widely used technique. Coherency means that, upon a remote disturbance, some groups of generators swing together and can be represented by a single aggregated machine finally. Power network reduction researches based on coherency technique can be taken up in 5 procedures. 1. Divide the system into internal and external system. Remain the internal system and reduce the external system. 2. Assume a large disturbance inside the internal system and identify the coherency generators in external system. 3. Aggregate those coherency generators as one. 4. Reduce the topologic of the network system. 5. Obtain the reduced system and calculated the parameter. Besides the reduce steps, the equivalent model need to be evaluated. If there is a similar rotor oscillation trace measured at one of the generator terminal inside the internal system during a large disturbance, the equivalent model was successful. The criterion of the evaluation will be discussed in the simulation result chapter System model establish Dynamic equivalent model is for electromechanical dynamic analysis. Considering a power network system which has N generators, the system based on the classical second-order electromechanical model, the well-known swing equations for generator i could be represented as: ddωω ii MM ii dddd = PP mm ii PP eeii DD ii ωω ii (ii = 1,2,, NN) ddδδ ii dddd = ωω ii (2.1) Where, 11

30 MM ii : Inertia ωω ii : Rotor speed of the ii generator PP mmii : Mechanical power PP eeii : Electrical power DD ii : Damping coefficient When the generator worked at an operation point, the generator s rotor motion equation can be reduced as a linear differential equation around that operation point. As the preconditions, assuming the mechanical power as zero and all the coefficients combined as one coefficient KK. MM ii dd 2 δδ ii dddd + KK δδ ii + DD ii ddδδ ii dddd = 0 (2.2) δδ ii : The rotor angel increment of generator i Then, the eigen equation of the this differential equation is: MM ii pp 2 + DD ii pp + KK = 0 (2.3) When the damping is zero, the root eigen value pp can be represented as: pp 1,2 = ±jj KK MM ii = ±jjωω nn (2.4) These pairs of root eigen values indicate that after a disturbance, the generator rotor angle will oscillate with a rotor angle frequency ωω nn. With tracking this rotor angle frequency change, the system sensitivity mode approach can be found. The specific method will be illustrated below. In the power network system which contents N generators and n nodes, can be represented as 2N differential equations and N+n algebraic equations. There are 4N+2n variables. They are: ωω : The generator rotor speed increment δδ : The generator rotor angle increment 12

31 PP GG : The generator electrical power increment injected to the grid PP mm : The generator mechanical power increment PP LL : Load increment θθ : The generator mechanical angle increment If the simultaneous equations system has N+n boundary conditions, with using the implicit trapezoidal integrator method, the system differential equations will be solved with steady numerical answers. Furthermore, the DIgSILENT PowerFactory indicates that this method is used in its real time simulation to obtain the calculated parameters. In this case, we care about the generator rotor oscillation curve. The boundary conditions will be simulated as the mechanical power increment and load increment. Under steady state condition, these two variables are all equal to zero. And, during the large disturbance, they have another simulation profile. This will be illustrated in the next subchapter. Most of coherent power network reduction techniques were based on the second-order generator s equivalent model because of its simplified and can explain the dynamic electrical characteristics of the system Typical disturbances In this subsection, the 4 typical disturbances are discussed and simulated. They are: Three phase short circuit Load shedding Generator shedding Transmission line shedding By investigated the 4 typical disturbance, the generator rotor oscillation curve can be obtained. Take the three phase short circuit as an example, the boundary conditions can be represented as: PP mm = PP aaaaaa tt = 0 + PP LL = 0 (2.5) The three phase short circuit normally occurs in a short time, about 0.1 seconds. Thus, the electrical power increment and load increment simulate as a constant equaling to the 13

32 value at tt = 0. And, the mechanical power increment simulate as an accelerated increment. The reduction result will be different because of the location and size of the disturbance. The coherent reduction technique based on assumption that the considering fault on a certain Busbar and observing that the coherency behavior of the generators are not change much as the fault clearing time increasing or decreasing Indentify the coherent generators It is well known that the essential system parameter for determination of coherency behavior is the electrical distance between generators. The electrical distance concept is considered both geographic distance and the electrical element. And, the electrical distance is measured starting at the fault location in the internal system. Three difference circles will be explained for the external system. Outside the inner circle represent the region has short distance with the fault location. In this area, the dynamic behaviors of the generators affect the internal system a lot. Therefore, more groups of coherent generators will be found. Outside the middle circle, the region contents more generators. However, the long electrical distance between this region and the fault location made the generators less affect to the internal system. Thus, in this region, generators are easier to be reduced than the inner circle. The outer circle defines the boundary of the system beyond which no generator representation of any kind is necessary. Figure 2.2: Divide the power system as several circles by electrical distance 14

33 This coherency region identification is useful when considered the new facility adding to the power network system is offshore wind farm. If treat the offshore wind farm as the internal system, the other generators all have certain distance from the wind farm. That is made more generators in the outside middle circle or outer circle, and fewer generators in the middle circle. Consequently, fewer groups of coherency generators will be found if research on the offshore wind farm as the internal system. In order to indentify the coherent generators fast and accurately, there are four basic assumptions for simplify the elements. 1. The coherent group identification is independent with the magnitude of the disturbance. With a certain disturbance the system can be divided into several groups by linearization the system. 2. The coherent group identification is independent with detailed model of each generator unit model. Thus, the generator model is expressed by the secondorder electromechanical model. The excitation system, driven motor and governor system are neglected. 3. The coherent group identification has little effect with load. The dynamic equivalent is composed of a generator and an electric load in parallel. 4. On the assumption that the X/R ratio is big enough. The active and reactive load flow will calculate independently. Based on the four assumptions written above, the coherent generators identification will be calculated fast and met the accuracy requirements. There are several methods that can be used for coherency identification: (1) linear time simulation, (2) weak-link method, (3) two time scale method, and (4) tolerance based method. The tolerance based method is wildly used, and the simulation will be carried out in time domain. It is also possible to identify the coherency in frequency domain based on FFT (Fast Fourier Transform). In power network system, the generator s rotor angles have relative oscillation when facing a disturbance. This low frequency oscillation (Electromagnetic oscillation) normally is within 0.25~2.5 Hz. The Since the model using in this thesis is simplified power network system model of west Danish system, the generator rotor oscillation frequency during the disturbance of each generator is approximate 0.6 Hz. And, because of the computer precision it is difficult to identify the coherent group. Then, the coherency efficiency was decided to be used for the identification. The strictly coherency identification criterion is: 15

34 mmmmmm δδ ii (tt) δδ jj (tt) εε ttεε(0, ττ] (2.6) Where, ττ = 1~3ss is the recorded simulation time. εε = 5 ~10 is rotor angle deviation between two generators. δδ ii, δδ jj are rotor angles. The equation means within a simulation time ττ, after a large disturbance, the difference of two generator rotor angle deviations are not larger than a very small criterion εε in every sampling points. The generators rotor angle deviations during the simulation time are defined as the deviations of a reference generator rotor angle. This kind of coherent area always in a closed region where has close electrical distance from one generator to another. Practically, there is another definition: δδ ii (tt) δδ jj (tt) = cccccccccc (2.7) This equation gives another option that the two coherent generators may have lager difference of rotor angle deviation than εε. But εε is always approximate equals to a constant angle within the simulation time. Consequently, the two generators are coherent. This kind of coherent area will like a cycle around the fault location. The electrical distances measured from recorded generators to the fault location are similar. In this thesis work, from the DIgSILENT PowerFactory simulation platform, because the tracing of all generator rotor angles can be obtained, there is a numerical method that calculated the variables correlation coefficient. A coherent generators identify program will be developed and executed in a real model. Further method used in the program will be discussed in the simulation chapter Generators aggregation After identify the coherent generator group, the generators in the same group can be aggregated as a single equivalent generator. The criterion of aggregation is the 16

35 mechanical power and electromagnetic power of the equivalent generator equal to the summary of each generator in group. In this thesis, the generator using the classical second order model, accordingly only considering aggregate the mechanical parameters and transient impedance. The equivalent motion equation of the generator rotor represented as: nn nn ( MM ii ) dddd dddd = PP mm ii PP eeii ( ii=1 ii=1 nn ii=1 nn DD ii ii=1 )ωω (2.8) Thus, the parameters of the equivalent generator are listed below. The inertia of the equivalent generator is: nn MM = MM ii ii=1 (2.9) The damping coefficient of the equivalent generator is: nn DD = DD ii ii=1 (2.10) In the motion equation of generator rotor, the electrical power also can be represented as: PP eeee = EE ii UU ii XX ii sin δδ ii (2.11) Where, EE ii : Transient electromotive force UU ii : Generator terminal voltage δδ ii : The power factor 17

36 The XX ii is the summary of the generator impedance which contents the transient impedance XX dd. Thus, the transient impedance of the equivalent generator is: XX dd = 1 1 nn ii=1 XX dd (2.12) Finally, the three main parameters inertia, damping coefficient and transient impedance are aggregated to establish the equivalent generator Aggregate the generator controller Since the mainly pending reduced Busbars of the power system in this thesis project are the 150kV Busbars which contents the local CHP generators and local wind generators. Those generation units do not constrain the reactive power and voltage control. The generators just assumed as directly feed into the local network. The generator controller in this thesis project mentions the automatic voltage regulator (AVR) and power system stabilizer used to control the excitation of the central generators. The additional state variables of the AVR are the measured value of the terminal voltage, the internal state in the AVR and the output from the AVR (the excitation voltage). While, the additional state variables of the PSS are the internal and output electrical torque signals of PSS. All the generator excitation voltage control effect dependent on the time constant of the generator. Generally, AVR as well as speed governor are not action in the first two seconds. Hence, identify and aggregation the coherent generators don t consider the effect of the AVR. If concerning aggregation the control chain of the central power plane which is not include in this project, the theoretical method is aggregating the transfer functions of the control chain. In reality, the function will have high complex order that can t be reduced as the equivalent parameters. Hence, firstly, the centre power plants don t recommend to be reduced because of its playing an important role in the power system. Secondly, when reduce the control chain of the centre power plant the empirical method has been 18

37 used. Several equivalent parameters for the equivalent controller have been tested to approach the original control effect Power network topology reduction After aggregate the coherent generators, the remained Busbars and transmission lines connected between also need to be reduced. The topological reduction method was required. Then, the external system can be divided as two parts, the remained part and the pending eliminated part. Gauss elimination method has been used for eliminating the part which doesn t contents the equivalent generators. II RR II EE = YY RRRR YY EEEE YY RRRR VV RR YY EEEE VV EE (2.13) Where, II RR : The injection current vector of the remained node II EE : The injection current vector of the eliminated node VV RR : The voltage vector of the remain node VV EE : The voltage vector of the eliminated node YY RRRR : The self-admittance of the remained system YY EEEE : The self-admittance of the eliminated system YY RRRR : The mutual-admittance of the remained system YY EEEE : Then mutual-admittance of the eliminated system The injection current vector of the remained node can be represented as: II RR = YY RRRR YY RRRR YY EEEE 1 YY EEEE VV RR + YY EEEE YY EEEE 1 II EE (2.14) The first item of representing the remained node current is either admittance elements or the remained node voltage. The second item contents the eliminated node current which can be represented as the injection power of the remained node. Thus, the equivalence injection power can be obtained. This aggregated method is based on all the elements can be represented as constant impedance. Obviously, the constant load also can be aggregated in this step. 19

38 2.3.7 Evaluation With the same remote fault, compare the measuring swing curve and power flow of the generator on the fault Busbar between the reduced system and original system. The power network reduction result should achieve the transient stability model without introducing a significantly differences between the reduced system and the original system. Firstly, the load-flow calculation in the reduced system should give the same result with acceptable error for the internal researched area as for the original system. Secondly, the swing curve for the testing generator inside the internal system also should give the acceptable similar result. In this project, one of the numerical compared criterions will be given as the form of the correlation coefficient between two systems. A coefficient value close to +1.0 and the acceptable error at magnitude of the curve will prove the reduction method is approved. 20

39 3 NETWORK REDUCTION FUNCTION IN DIGSILENT POWERFACTORY 3.1 Introduction of the Network Reduction function DIgSILENT PowerFactory s network reduction algorithm produces an equivalent representation of the reduced part of the network.[28][29] The equivalent representation contains the interface nodes (connection point) that may be connected by equivalent common impedances and voltage sources. After the reduction process, the new elements are created in the gird folder which can be found in Data Manager. But, they are not drawn in the diagram automatically. The user needs to draw existing net elements to compare the load flow information on the connection point between original system and reduced system. For testing the network reduction function both in DIgSILENT PowerFactory Version 13.2 and 14.0, the example configuration used is classical nine-bus system. The network reduction function is independent from the number of bus bars which user defined to remain. That means if there is only one connection between external system which is expected to be reduced and the internal system, the network reduction module will work as well. The reduction result will just be an equivalent voltage source on that bus bar. Also, if there are three Busbars expected remained, the equivalent result will be three voltage sources on each Busbar and three common impedances interconnected between each of Busbar. In PowerFactory Version 13.2 the slack element should be inside the grid which will be reduced. If the slack element has been selected as the expected remained system, the reduction function can t be processed. The error comes as There is no coupling Busbars. In classical nine-bus system, the slack element is defined as No.1 generator. Comparing with Version 13.2, Version 14.0 has improvements at this point. In Version 14.0, if the slack element is located in the grid which is to be reduced, a neighbourhood around this slack element will be automatically defined. That means the neighbourhood around this slack element will automatically remain after Network Reduction procedure. The neighbourhood is defined based on a search from the slack element towards each 21

40 connection, until a non-zero impedance element is passed. This means that the neighbourhood contains all elements encountered up to and including the non-zero impedance element. This improvement from Version 13.2 to 14.0 is reasonable because the slack bus which contents the slack element is vital information for the original power network. The slack bus is an infinite bus where the voltage magnitude is predetermined and which is at the same time reference node and balancing bus. The technique of solving a classical transient stability problem is illustrated by conducting a study of the classical nine-bus system. The basic data of the classical ninebus system for preliminary calculation is given in Appendix A. 1 in detail. The disturbance initiating the transient is a three-phase fault occurring on one of a remained Busbar after the Network Reduction procedure. The fault is clear in around five cycles (0.1 s). 3.2 Network Reduction process The Network Reduction process will be emphasized in Version 14.0, also compared to the Version 13.2 when it is a difference Reduced system definition First of all, the nine-bus system shall be divided into an internal system and an external system. The internal system is the study area expected to be remained. In Version 13.2, the system needed to be divided into two grids, a distribution system and a transmission system. While, the Version 14.0 has more flexible definition: boundary element definition. The grid boundary divides the power network into two systems. One of the parts shall be remained as its original detailed representation and the other part that is to be reduced. The method for define a new Boundary is as below: Freeze the network diagram. Choose the desired cubicles, right click them. (The desired cubicles orientate the interior system which shall be remained.) Choose in the context sensitive menu Define...-->Boundary... The dialogue of the new Boundary will pop up. Press OK. Chosen the alternative interior system was by defined different boundary elements. 22

41 3.2.2 Network Reduction command In version 14.0, there is a Network Reduction button in the additional tools Icon bar. Figure 3.1: Network reduction button in the DIgSILENT PowerFactory tools Icon bar. This is one of an improvement from Version 13.2 to In Version 13.2, the process is executed by a command in the input window. And, the network reduction command provides only one option: the original grid data which defined to be reduced will be destroyed. So, to avoid loss of data, it is needed to create a copy version before execute the Network Reduction. In Version 14.0, the Network Reduction command provides four main setting options: Basic Options Output Options Advanced Options Verification Options It is worth to mention the Output option. The Output tab is used to specify whether the reduced grid parameter shall be reported only, or whether the reduced grid shall be stored in a new variation. This function is useful when original data must be remained and the equivalent parameter required. Further information about process the Network Reduction shall be found in [29] Technical Background of Network Reduction The Network Reduction function in DIgSILENT PowerFactory is applied to load flow, short circuit and stability calculations. The stability analysis is in the steady state level until the present version. The more advanced transient stability will be discussed. The algorithm in Version 14.0 based on the sensitivity matrices. The basic idea is that the 23

42 sensitivities of the equivalent grid, measures at the connection point in the retained grid, must equal to the sensitivities of the gird that has been reduced.[13] 3.3 Study Case The load flow equivalent result which obtained from Network Reduction module is consist of common impedance between each two boundary buses and apparent power injection at boundary bus bars. In DIgSILENT PowerFactory 14.0, the apparent power injection can be represented by different equivalent elements. Take load flow equivalent result as the example, there are three optional models available. Load Equivalent: a load demand. Ward Equivalent: an AC voltage source which is configured as a Ward Equivalent. Extended Ward Equivalent: an AC voltage source which is configured as an Extended Ward Equivalent. The last two equivalent models were based on the same element (AC voltage source) but different element type definition.[11] The three equivalent models all can give the same load flow result as the original system. However, different equivalent model and different area selected as the reduced system will affect the result of classical transient stability test. The disturbance initiating the transient is a three-phase short circuit occurring on Busbar No.3 at 0.0 second. The fault is cleared at 0.1 second. The layout of the nine-bus system is in Appendix A. 1. Two study case results will be compared. They reduce the original nine-bus system to eight buses system. In the case 1, the bus which contents a generator will be selected as the external system to be reduced. While, in Case 2, the reduced bus selected will be a bus contents a load. The loads in this nine-bus system are treated as constant impedance. During analyze each study case, all the three optional equivalent models will be tested and compared Case 1 The boundary definition of case 1 split the No.7 Busbar and No.2 Busbar as the external system to be reduced, and the rest Busbars as the internal system. All three result model options have been selected one by one. From collecting and comparing the result 24

43 elements parameter, it is not difficult to find their common impedances have the same result during the three alternative models. All the collected results are listed in Appendix A. 2. It also contents the two loads, two ward equivalent model voltage sources and two extended ward model voltage sources parameters. In order to track the dynamic performance of the No.3 generator, the Run Simulation command will be initialized and executed. The simulation time is 5 seconds and represented as RMS calculations. Further information for set up the initialize and run simulation command will be given in the chapter 5.2. The No. 3 generator has been selected as the result object, and its rotor angle selected as the tracking signal in rad. Figure 3.2: Swing curve of No.3 generator when eliminate No.2 generator of nine-bus system In Figure 3.2, the red curve is from tracking the No.3 generator s rotor angle change respecting the time before reduced the system. Then, the green, blue and golden curves represent the rotor angle change after reduce the Busbar No.2 and Busbar No.7. From this selection of external system, the No.2 generator will be deleted and replaced by the common impedance, general load or voltage source elements. The rotor angles starting points of all the curves are equal. They are proving the reduction result from Network Reduction module was right in load flow calculation. Although none of reduced tracking curves is identical as the original system s which testified the Network Reduction module can t be used in Dynamic simulation. The differential of the three 25

44 curves after the reduction process can still exhibit the characteristics of three different equivalent elements. The two equivalent model content voltage sources are showing the same track representing as blue and golden curve. And, the two curves are generally narrow than the green curve which represent the equivalent load reduced model. That is because the voltage source can dominate the voltage on the connection node. Thus, during the time domain simulation, the voltage source makes the so connected generator faster to achieve the next stead state operating condition. However, the load equivalent model s curve in green represents the natural damping. To sum up, when the reduced system contents the elements which have dynamic performances, like generators, the Network Reduction equivalent system can t represent the original system in dynamic analyses Case 2 In case 2, it is aim to test the Network Reduction module when the reduced part defined as a constant load. Furthermore, the constant load represents a kind of elements in power system which doesn t have dynamic characteristics like cables, lines, Busbars. It is defined the Load C and its neighborhood elements until another Busbar has been found. The results collection in the Appendix A. 3.With the same simulation and selection procedure as in Case 1, the swing curve of No.3 generator when eliminate Load C of nine-bus system shows below. 26

45 Figure 3.3: Swing curve of No.3 generator when eliminate Load C of nine-bus system In Figure 3.3, the red curve also represents the rotor angle change of No.3 generator before reduce the system. If reduced system has the same tracking rotor angle change of No.3 generator, the reduced system can fully represent the original system in dynamic analyses. In the study case 2, the No.8 Busbar (A. 3) was replaced by common impedance between two neighborhood bus bars and two general loads on the two buses. However, if the equivalent model selected as voltage source, the equivalent impedance wasn t the same with the original system. From the result curve from Case 1 and Case 2, two conclusions jumped up. Firstly, when the reduced Busbar contents the elements don t have the dynamic characteristics, like cable and load, the load equivalent model of Network Reduction result can be used in dynamic analyses. Secondly, the other two equivalent models have the same dynamic behavior for the transient stability analyses. This testing result is useful for the network topology reduction after reduce the generator kind elements which have dynamic characteristics. 27

46 4 MODELING WEST DENMARK POWER SYSTEM (WDKPS) 4.1 Background of the WDKPS model As the purpose of the project, the WDKPS model will be as the original system model for developing and testing the coherent based network reduction algorithms in DIgSILENT PowerFactory. The model in is provided by DONG Energy Power Holding A/S.[25] As a preparatory work, it is important to investigate the object of study. Also, some preliminary definition will be made as a reduction conditions. The simulation process and result in chapter 5will be bases on these assumptions. Against the physical power network of west Denmark, the model used in this project concern the transmission power system at the voltage of 150kV and 400kV. The simplified model of WDKPS established based on this two voltage level. Also, there are many wind turbine site and CHP units feeding into the local distribution networks at the voltage level 60kV. In this study case, the 60kV consumptions have been translated into 150kV Busbars. Hence, all the generation and consumption unites are equivalented at the 150kV Busbars. It is well known that Danish power network as hub connected between the European electrical transmission grid and the Nordic electrical transmission grid. This WDKPS model represent three connection point indicate the model connecting with Sweden at Vester Hassing, connecting with Norway at Tjele, and the European grid as the external grid connecting at Kassø. For better understanding the geographical location is shown in Figure 4.1. And, the real location names and the corresponding Busbar symbols are shown in Table

47 Figure 4.1: Sketch map of WDKPS respecting the geographical location 4.2 Model layout From the WDKPS model layout in DIgSILENT PowerFactory shown in Figure B.1 and Figure B.2, we can see the system can be separated as two transmission circle according to the voltage level. Nearly all the 400kV Busbars connect with the 150kV Busbars via 400kV/150kV transformer. The model contents totally 45 Busbars. 30 Busbars is at 150kV level and 15 Busbars is at 400kV level. It is of which kV Busbar and kV Busbar connected with transformers. At 150kV network system, each Busbar 29

48 contents a mixed power unit: local combined heat power (CHP) plant, local Wind power generation and local consumption. Besides, there are two large off-shore wind farm involved in this model: Horn Rev I and II. Several conventional large power plants also take into account. The model is a simplified model which represents some basic characteristics of the entire WDKPS in a specific operation scenario in the condition of The detail information will be given in the chapter 4.3 to chapter The mixed power unit The wind energy penetration in the Danish power grid is dramatic increasing in the last decade. The traditional power system was built of the large power plants supported by oil and gas feeding into the transmission power networks. Because of the increasing wind energy inject to the power grid, the mainly dominated traditional power plant has been changed into a mixed unit dominated form. The contents of the mixed dominated unit for WDKPS are the different kinds of concept amount of wind energy and the local combined heat power (CHP). Additionally, the large consumption centre will be an important relevant element considering in the unit. These mixed units are the basic cell concept about modeling the 150kV network of WDKPS. The Figure 4.2 below shows the mixed unit which consist of the three elements. The two concept of power production under different operation represented as two generators with two system load flow operation characteristics. The operation characteristics are several trigger element used to define the production of the generators by percentage. Figure 4.2: Local mixed power production and consumption unit using in WDKPS model The Table 4.1 shows the Busbars, generators and loads data at 150kV network. They will be used for aggregating the coherent generators in chapter

49 Table 4.1: Mixed power unit data for 150kV Busbars of WDKPS Location Bus CHP WDG Load Nominal Rated Nominal Rated Peak Abildskov ADL Bredebro BBR Bilstrup BIL Endrup END Ferslev FER Frøstrup FRT Hatting HAT Herning HER Hornbæk HNB Idomlund IDU Karlsgårde KAE Kassø KAS Kingstrup KIN Klim KLM Knabberup KNA Landerupgård LAG Loldrup LOL Lykkegård LYK Malling MAL Modelund MLU Mosbæk MOS Nordjyllandsværket NEV Rev REV Ribe RIB Endstedværket SHE Skærbækværket SKV Studstrupværket SSV Tange TAN Tinghøj THØ Tjele TJE Trige TRI Vester Hassing VHA 31

50 4.3.1 Modeling of local CHP unit With the considering of the simplified WDKPS modeling, the local CHP unit represented by standardized synchronous generator model inside the DIgSILENT PowerFactory. The parameters of the generators are user defined. The significant effective parameters are synchronous reactance, inertia, transient time constant, transient reactance and so on. As the power network reduction research issue, all the generators can be simplified represented as the same basic generator type but with different nominal power and load flow demand. The nominal power data come from the size of the real aggregated generator at predefined location. The load flow dependents on the central operator for all local CHP unit. In this situation, the central CHP operator commission 70% of rated active power production for each of the local CHP unit, and require none reactive power from them. It indicates the local CHP unit import or export reactive without control, and it doesn t ability to control the Busbar voltage. The basic parameters of each local CHP generator type represent in the Table 4.2 below: Table 4.2: The generator type data in pu for modeling the local CHP unit Parameters Data 2 pu 2 pu 2 pu 2 pu 0.1 pu s XX dd XX dd XX qq XX qq XX ll TT dddd HH 10 s XX dd : The synchronous reactance in dd axis XX qq : The synchronous reactance in qq axis XX dd : The transient reactance in dd axis XX qq : The transient reactance in qq axis XX ll : The leakage reactance TT dd0 : Transient time constant HH : Acceleration time constant 32

51 All the data defined as the per unit value. The SI values of each local CHP unit also dependent on the nominal apparent power. The local CHP unit data defined in the WDKPS model listed in the Table Modeling of local wind energy The local wind energy also known as the distribution wind generators (DWG) inject in the 150kV Busbars considering in the present model. The wind speed variations doesn t taking into account in this project. Thus, the entire model represents a specific operation state. Under these assumptions, the DWG also modeled as the standardized synchronous generator model inside the DIgSILENT PowerFactory. Whereas it operated by a centre distribution model as 100% production. It is of cause a best assumption of the wind energy injection. Actually, the theory of modeling the local CHP and the local Wind energy are the same. However, it is worth to deal the two units separately for the better representation of the real power system. Also, the reduced result of the model will base on the equivalent mixed unit Modeling of consumption centre The consumption centre represented as a general load model inside the DIgSILENT PowerFactory with constant active power commissioned the 100% rated situation and constant reactive power as none requirement. Those active power and reactive power are independent from the dynamic voltage behavior. 4.4 Modeling of conventional power plant The conventional power plant of a power system is the centre unit which has to be 100% under control. They all have large capacity may be hundreds of MW and located in few points along the power system. So, it is difficult to find two conventional power plants operated coherently. Furthermore, the modeling of conventional power plant in the simplified model doesn t content its controller information. For the coherent based network reduction purpose, the conventional power plant defined as the slack element that remained in the reduced system. 4.5 Modeling of large wind farm 33

52 4.5.1 Aggregated model for a large wind farm Modeling the large wind farm depends on the purpose of the investigation. In the investigation of power network reduction, the large wind farm can be represented as a reduced model. Because the attentive point is the response of the wind farm to a disturbance event occurring in the entire power system. In the WDKPS model, the large wind farm reduced as one wind turbine model. Because the result from the wind farm internal network that does no matter to the collective response of the wind farm can be neglected in this case.[24] The size of the offshore wind farms tend to above 100 MW. In this WDKPS model, there are two large scale offshore wind farm, Horns Rev I and Horns Rev II.Horns Rev I has 160MW capacity and Horns Rev II has 200MW capacity. Therefore, integration such amount of power has to be restricted by the Transmission system operator (TSO). The necessary voltage control and frequency control of such large scale offshore wind farm are also utilized for stable and reliable of the Danish transmission system. According to the grid, the active power shall be regulated upon a forecast desired power production or a required production under the desired value. Also, the wind farm shall be equipped with reactive power as more or less neutral value. However, with such amount of power produced and temporarily replacement of central power plant, the wind farm will be oblige to provide more ancillary system service. The reduced wind farm model can be represented as a synchronous generator modeling in the DIgSILENT PowerFactory In the version 14.0, the wind turbine can be represented as a synchronous generator with an option wind generator. Because of this WDKPS model was a modified simplified model with 46 Busbars, the two large scaled offshore wind farms modeling on the same Busbar END 150, and they share the same generator type. The simplified model of large wind farm still shall achieve the required accuracy. Table 4.3: Main parameters using in modeling large off-shore wind farm in WDKPS Name of WF Horns Rev I Horns Rev II Connected Busbar END 150 END 150 Nominal Apparent Power 220 MVA 220 MVA Active Power set point 160 MW 200 MW Power Factor 1 1 The two large scaled wind farms operated under the rated production in this model. 34

53 4.5.2 Modeling of the ongoing offshore wind farm- Anholt With the Danish Energy Agency agreement of 21 February 2008, it was decided to build largest offshore wind farm of Denmark at Djursland Anholt.[1] Anholt offshore wind farm will have an installed capacity of 400MW. Thus, it will twice larger than the resent largest offshore wind farm, Horns Rev2, 200MW capacity. It is appropriate to consider alternative technical solutions for design Anholt, and most importantly to take the external grid dynamic behavior into account. Various examined technical solutions for grid connection have been done by Energinet.dk, a TSO company of Denmark. The studies have shown that 400kV power station Trige is the optimal wind farm connection point to land. Because the surrounding power networks are sufficiently developed to consume production from offshore wind farm. Similarly, a large share of wind energy could be consumed locally in the Århus area or efficiently transferred to the other major centers in East Jutland. Anholt wind farm planned by Danish government, and it expected online in Until 2008, there are three alternative solutions recommended used in connection the wind farm to the 400kV transmission grid. There are: Two parallel 150kV cables connected to 400kV network through a single 400/150kV transformer. A 220kV cable connected to 400kV network through a separate 400/200kV transformer. A VSC-HVDC solution which created a special platform in the vicinity of offshore wind farm. Figure 4.3: Sketch map of showing the location of Anholt wind farm integration point 35

54 In this thesis work, based on the WDKPS model in the condition in 2005, the Anholt will simulated as a new wind farm adding to the model. And, the wind farm will be performed as a synchronous generator. The generator parameter given in the table below: Table 4.4: The synchronous generator parameter for simulating Anholt offshore wind farm Generator name Anholt Rated MVA 400MVA Rated Voltage 400kV Power factor 0 Type Offshore Wind Farm XX dd 2 pu XX dd 2 pu XX qq 2 pu XX qq 2 pu XX ll 0.1 pu TT dddd s HH 10 s For entire WDKPS steady state and transient stability study, there are several reasons why the Anholt offshore wind farm can be simulated as a single generator. 36

55 5 TIME DOMAIN SIMULATION OF WDKPS 5.1 Predefined short circuit event Various fault events can be introduced to the transient stability study. Furthermore, it also dependents on the critical fault clearing time. Because of the symmetrical network representation, the basic simulation function only allows the insertion of symmetrical fault. In this case, the symmetrical short circuit event will perform at t=0.0s and clear at t=0.1s according to the time axis built in the DIgSILENT PowerFactory starting from The location was at 150 kv Busbar Trige where the tested new large scale offshore wind farm will be integrated. The algorithm of predefine the short circuit and short circuit clear events shows in the flow chart below. With input a Busbar number, the location of the short circuit event can be automatically selected. It is worth to mention that the default general selection of a DPL script only contents the element object. Thus, a general set selection for short circuit event objects have to be made before perform the script. 37

56 Input short circuit Busbar N0 N=0 First Busbar obus Next Busbar N+=1 No N=N0? Yes Break Record Busbar as the short circuit location object Figure 5.1: The flow chart of predefine the short circuit even in WDKPS After execute the script, in the upcoming dialogue of the selection fault location a number represented a Busbar was required. For example, No.42 indicates 150 kv Busbar Trige. Figure 5.2: Fault location selected dialog 38