Facts based voltage sag mitigation and stability improvement in induction generator based wind farms

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Facts based voltage sag mitigation and stability improvement in induction generator based wind farms

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Abstract

In recent years generation of electricity using directly connected asynchronous generator (induction generator) based Wind Energy Conversion Systems (WECS) has received considerable attention world wide. The connection of these Fixed Speed Induction Generators (FSIG) to the Point of Common Coupling (PCC) in the electrical grid network is one of the major concerns of the transmission system planners. Reactive power system and ride through capability are the primary issues to meet the grid code requirements for the wind. In this paper the impact of a Static Synchronous Compensator (STATCOM) with FSIGs for dynamic and steady state operation in the presence of voltage fluctuations is proposed. The simulation results demonstrated the effectiveness of the STATCOM device in stabilizing the Fixed Speed Wind Energy Conversion Systems (FSWECS) using dynamic reactive power compensation. The simulation analysis is performed using Power System Computer Aided Design (PSCAD)/Electromagnetic Transient Program with DC Analysis (EMTDC) software simulation package.

Keywords : Wind Energy Conversion Systems (WECS), Fixed Speed Induction Generators (FSIG), Point of Common Coupling (PCC), Flexible AC Transmission Systems (FACTS), Reactive Power compensation (RPC), Voltage stability.

Abstract
Introduction
Electrical modeling and requirements of fixed speed induction generators
Modeling and control of statcom
1. Equivalent circuit of Statcom
Modeling of statcom
1. Transient stability model of statcom
2. PWM control
Simulation results
Conclusions
References

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[...] When compared to the conventional voltage control practices, FACTS controllers like STATCOM offers dynamic reactive power compensation and provides superior voltage support capability Electrical Modeling and Requirements of fixed speed induction generators Wind farms with induction generators generate real power and consume reactive power. Fig2 illustrates the single line diagram of a wind power generation unit, connected to a power system network. Fig 2 Single line diagram of a WECS The dynamic representation of single cage FSIG is based on 5th order model. [...]

[...] The VSC switching strategy is based on a sinusoidal PWM technique which offers simplicity and good response. The controller input is an error signal obtained from the reference voltage and the value rms of the terminal voltage measured. Such error is processed by a PI controller the output is the angle δ, which is provided to the PWM signal generator. An error signal is obtained by comparing the reference voltage with the rms voltage measured at the PCC. The PI controller processes the error signal and generates the required angle to drive the error to zero. [...]

[...] Fig 12 STATCOM Connection at Mid-point of an AC Power System TRANSIENT STABILITY MODEL OF STATCOM In this model, angel order based on voltage error will be generated. The measured reactive power of the STATCOM and the measured voltage of the power system at the mid-point will be the inputs to a comparator as shown in Fig 13. The setting value in terms of the desired voltage for STATCOM is 1.0 pu. The output of subtracting of reference voltage minus measured voltage will be as input of PI controller and the output of PI controller is the angle order δ. [...]

[...] This overshoot is characteristic of PI controllers and can be ameliorated with integrator anti-windup Based on the VI characteristics as shown in Fig when compared to Static Var Compensator STATCOM proves to be preferable for stability analysis of FSIGs. Fig 9 VI Characteristics of STATCOM 2.1 Equivalent circuit of Statcom The STATCOM behaves as a solid state synchronous voltage source that is analogous to an ideal synchronous machine. It is connected in shunt to the AC system as shown in Fig 10 and generates or absorbs balanced set of currents at the fundamental frequency, with rapidly controllable amplitude and phase angle. [...]

[...] With the wind turbine acting as the prime mover, the mechanical power extracted from wind and the corresponding mechanical torque is given as Pm e ArVw3 Cp ( ) Tm = K ( s - Fig 4 Speed / Slip Torque Characteristics of an Induction machine In terms of tip speed ratio τ ,the equivalent electrical torque produced in the generator is Te = (eCP R3 vw2 The electromagnetic torque and the rotor speed of an induction generator at startup are shown in Fig 5 and Fig 6.According to Swing equation which describes the dynamic behavior of single rotating mass wind mill drive train J The over speed of the generator depends on the inertia and duration of the disturbance. [...]

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