Wind Farms in Weak Grids Compensated with STATCOM

Transcription

1 Nordic PhD course on Wind Power (June 5-11, 2005) Final Examination Report Ph.D. Candidate: Akarin Suwannarat Supervisors: Birgitte Bak-Jensen Institute of Energy Technology, Aalborg University Pontoppidantstraede 101, 9220 Aalborg East, Denmark

2 Contents 1. Problem statement Project solutions Wind farm model STATCOM model Model simulation Conclusion. 10 References. 10 i

3 1. Problem Statement Topics involved: Wind Farms in Weak Grids Compensated with STATCOM. Contact person: Marta Molinas Simulation model of a wind farm based on induction generators connected through a weak grid and compensated with a STATCOM Develop a simulation model (using Matlab, EMTDC or similar) for a 100 MW wind farm using squirrel cage induction generators magnetized with fixed capacitor banks, and connected to the grid through a weak grid (low short circuit current: strong grid usually gives a short circuit ratio >20, use this assumption). 1. Model the STATCOM as a voltage source converter. 2. Use decoupled control of active and reactive power for controlling the STATCOM. 3. Simulate a short circuit and compare the transient response of the system with and without the STATCOM. See how much the transient stability margin is improved with the STATCOM and see what other beneficial effects the STATCOM has on the system stability. 4. Give a generalized expression for the rating of the STATCOM as a function of the system characteristics and contingency level, such as short circuit ratio and voltage sag. 1

4 2. Project Solutions In this project work, the power system simulation program, Power Factory (DIgSILENT), is used as a tool for models development and power system simulation. This project work is done by the following procedures: 1) Wind farm model An aggregated 100 MW squirrel cage induction generator (SCIG) wind farm model is implemented. 2) STATCOM model The STATCOM model is developed as a voltage source converter including the decoupled control of active and reactive power for controlling the STATCOM. 3) Transient stability simulation (Short circuit simulation) A short circuit is simulated and the transient response of the system with and without the STATCOM is simulated. 4) Transient stability analysis The transient stability margin with the STATCOM is analyzed and other beneficial effect the STATCOM on the system stability is discussed. 2

5 3. Wind Farm Model A simulation model for a 100 MW wind farm using squirrel cage induction generator magnetised with fixed capacitor bank is modelled by the power system simulation program, Power Factory (DIgSILENT). Figure 1. The 100 MW SCIG Wind Farm system, developed in Power Factory (DIgSILENT). As far as steady-state and transient studies are concerned, a wind farm is similar to a conventional thermal power plant composed of several identical generating units. To assess its behaviour it is normal practice to use aggregated models in which the units are replaced by a single equivalent unit. The same methodology was applied to the case of the wind parks, thus allowing the development of an aggregated wind park model. The aggregated model is actually a dynamic equivalent in the classic sense: the concept of equivalent wind unit comes as a result of the aggregation of the characteristics of the individual wind units in a single unit with equivalent characteristics [1], [2]. The aggregated model should be one entity in power system dynamic simulations, similar to other entities, such as generators, controllers, loads and lines. A constant speed wind turbine has a unique algebraic relation between wind speed, mechanical power and electrical output power. This mechanical power can be summed and then be applied to a squirrel cage induction generator with a rating of the individual machines that are aggregated. Therefore, the characteristic response of a squirrel cage induction generator to disturbances can be kept. The 100 MW wind farm with SCIG, developed in Power Factory, is depicted in Figure 1. 3

6 The structure of an aggregated wind farm model with squirrel cage induction generator is depicted in figure 2. The mechanical power of turbines are aggregated and applied to one equivalent induction generator with a rating power equal to a whole wind farm. It has to be emphasized that the overview given in the figure 2 is only valid for the aggregation approach used in this report. Wind speed Model Overall wind speed Rotor Model Individual mechanical power Power Aggregation Aggregated mechanical power Generator Model P, Q V, f Grid Model Wind parameter Turbine characteristics Generator characteristics Figure 2. Structure of an aggregated wind farm model with constant speed wind turbine (SCIG). In this paper, it is assumed that the wind farm can be represented with one single constant wind turbine. The characteristics of this wind turbine can be calculated by the following equations: S eq = ⁿ S i ; i=1 C eq = ⁿ C i ; i=1 P m,eq = ⁿ P m,i ; i=1 where n is the total number of wind turbines composing the wind farm, the superscript eq refers to the single equivalent wind turbine and the meaning of the variables is as follows: S - Nominal power (MVA); C - Capacitance of the reactive power compensation system; P m - Mechanical power. An aggregated 100 MW wind farm with squirrel cage induction generator (SCIG) model development is explained in this section. The wind farm model is connected to the grid through a week grid (short circuit ratio < 20). The short circuit ratio (SCR) of the interconnection is given by: SCR = SCC/S base Where SCC is the short circuit power delivered from the grid for a three-phase fault [3]. Figure 1 show the network under investigation where the wind farm is connected at 60 kv to a system with short circuit ration of 14. The impedances of cables within the park are neglected and only the transformer is taken into account, because the impedance of the relative short cables with in the wind-farm will be small when compares to the impedance of the grid connection [4]. 4

7 4. STATCOM Model The STATCOM model as a voltage source converter, including the decoupled control of active and reactive power for controlling the STATCOM is developed. 4.1 System configuration and modelling A STATCOM applied to the power system is depicted in Figure 3. Its components are the shunt inverter, the transformer and the connection filter. The shunt inverter is modelled as a voltage control source. The basic electronic block of a STATCOM is a voltage source converter (VSC), with rapidly controllable amplitude and phase angle. In addition to this, the controller has a coupling transformer and a dc capacitor. The control system can design to maintain the magnitude of the bus voltage constant by controlling the magnitude and/or phase shift of the VSC output voltage [5]. In this paper, STATCOM control system is modelled by DIgSILENT Simulation Language (DSL). The control system proposed for the STATCOM is depicted in figure 3. The instantaneous three phase variables of the STATCOM can be described in state-space form by dq components using Park s transformation. The active and reactive powers injected into the electricity system by the STATCOM are: P = v d i d +v q i q = v d i d Q = -v d i q +v q i q = -v d i q Therefore, p and q are proportional to i d and i q, respectively and the control of the power injected into the power system reduce to the control of i d and i q [6]. The detailed study can be found in [6]. Figure 3. Structure of a STATCOM model as a voltage source converter, including the decoupled control of active and reactive power. 5

8 4.2 Control system The control system proposed for the STATCOM is depicted in figure 4. The desired reactive power exchange is achieved by the i q controller. The capacitor voltage is kept constant to ensure proper inverter control. The voltage control amplifies the voltage error and generates the reference for the active power to be exchanged by the inverter so that the capacitor voltage remains constant [6]. Figure 4. Structure of a STATCOM control model. 6

9 5. Model Simulation In this section, the 2-phase short circuit (110 ms) is introduced in the system and transient stability is simulated. The transient response of the system with and without the STATCOM is analyzed. The rating of the STATCOM as a function of the system characteristics and contingency level, such as short circuit ratio and voltage sag is expressed. The aggregated 100 MW Wind Farm with SCIG, installed with the STATCOM, is depicted in figure 5. Figure 5. The aggregated 100 MW Wind Farm with SCIG, installed with the STATCOM, developed in Power Factory (DIgSILENT). 5.1 Transient stability with and without STATCOM The 2-phase short circuit (110 ms) is introduced to the system. The response of the ac voltage magnitude, mechanical torque and rotor speed of SCIG with and without the STATCOM are provided in the figure 6, 7 and 8 respectively. The fast controllable reactive power provided by the additional STATCOM improves the speed of response of the voltage magnitude in the case of SCIG wind turbines. 7

10 Figure 6. The transient responses of the system: RMS voltage at TT_terminal in p.u. (a) with the STATCOM (read line) and (b) without the STATCOM (blue line). Mechanical torque and rotor speed responses of the SCIG wind turbines are also benefited as the rapid response to system disturbances of the STATCOM. (a) (b) Figure 7. The transient responses of the system: Mechanical torque in p.u. (a) with the STATCOM and (b) without the STATCOM. Figure 8. The transient responses of the system: Rotor speed of SCIG wind farm in p.u. (a) with the STATCOM (above) and (b) without the STATCOM (below). 8

11 5.2 Voltage Sag and Short circuit ratio Voltage sag has been defined as a reduction in the voltage magnitude from its nominal value for a duration ranging from a few milliseconds to one minute. The STATCOM is expected to offer limited capability for reducing the intensity of the sag as shown in figure 9. The STATCOM is expected to offer limited capability for increased short circuit ratio. Fast control of reactive power, provided by STATCOM is possible to maintain ac voltage within desired limits. The increased short circuit ratio could not be found in this simulation by the additional STATCOM. Figure 9. The transient responses of the system: Phase voltage in p.u. (a) with the STATCOM (above) and (b) without the STATCOM (below). 9

12 6. Conclusion This paper has described a comprehensive study of the application of a STATCOM to a wind farm. The detailed transient model for the fixed speed wind turbines technologies is developed. An aggregated 100 MW wind farm with squirrel cage induction generator (SCIG) model is generated. The STATCOM model as a voltage source converter, including the decoupled control of active and reactive power for controlling the STATCOM is modelled. A proposal for the control system of a PWM-based STATCOM connected to the power system is investigated. The results of the simulations showed that the use of a STATCOM improves the transient stability of the system. The STATCOM is expected to offer limited capability for improving mechanical torque and rotor speed responses of the SCIG wind turbine and reducing the intensity of the sag. The results show that as long as the fixed speed machine is complemented with sufficient reactive power control, such as STATCOM, the transient performance is improved. References [1] J.G. Slootweg, W.L. Kling; Aggregated Modelling of Wind Parks in Power system Dynamic Simulations, Delft University of Technology, the Netherlands [2] Rui M.G. Castro, J.M. Ferreira de Jesus; An Aggregated wind Park Model, Technical University of Lisbon, Portugal [3] C. Abbey, B. Khodabakhchian, F. Zhou; Transient modelling and comparison of wind generator topologies, International conference on power system transients (IPST 05), Canada [4] J.G. Slootweg, W.L. Kling; Modelling of Large Wind Farms in Power System Simulation, Delft University of Technology, the Netherlands [5] N. Mithulananthan, Claudio A. Canizares; Comparison of PSS, SVC, and STATCOM controllers for damping power system oscillation, IEEE Trans. Power Systems, October 2002 [6] P. Garcia-Gonzalez, A. Garcia C.; Control system for a PWM-based STATCOM, Universidad Pontificia Comillas de Mardrid, Spain [7] P.S. Sensarma, K.R. Padiyar; Analysis and performance Evaluation of a Distribution STATCOM for Compensating Voltage Fluctuations, The Department of Electrical Engineering, Indian Institute of Science, India 10