Power Quality Issues of Induction Generators in Different Operating Modes

Abstract Fast wind energy developments and their integration in existing power network may result in power quality problems. Therefore, in this paper, an attempt has been made to tackle the major power quality issues related to induction generators operating as grid connected induction (GCIG) and self excited induction generator (SEIG). MATLAB SIMULINK models as developed for GCIG and SEIG operation are found to be useful to analyse these effects. Simulated results as obtained on these induction generators have been used to identify the machine parameters affecting the power quality issues such as active & reactive power, total harmonic d istortion etc. Further analysis of simulated results for two operating modes i.e. GCIG and SEIG seems to be useful fo r the design of such generators for overall power quality improvements.

R r = per unit rotor resistance R s = per unit stator resistance L s = per unit stator leakage inductance L r = per unit rotor leakage inductance L m = per un it magnetizing inductance THD (I) = total harmon ics distortion of current THD (V) = total harmonics distortion of voltage f = frequency in Hz P = active power in kW Q = reactive power in kVA R V = Vo ltage in volts p = pole pairs

Introduction
Wind energy has been harnessed since the first wind mill was developed by the ancient Persians in 7th century AD. However it has been ignored as majo r source of electrical power for long time. In recent years wind power generation has experienced a very fast develop ment in the who le world [1]. As the wind power share is rapidly increasing in grid, the in fluence of wind turbines & induction generators on power quality is becoming an important issue. It is observed that the power generated by wind turbine fluctuates much mo re than produced by conventional generators. The voltage and frequency variations becomes more p re dominant when it is operating in isolated mode i.e. SEIG. Singh et al [2] presented a review of g rid code requirements for wind power interconnection in various countries. Grantham et al [3] observed that in case of SEIG operation the final steady-state self-excitation voltages and frequencies were found varying with the load demand and causing poor quality of supply. Melicio et al [4] presented the harmonic behaviour of currents injected in the electrical network by wind generation using fast Fourier transform technique. Yin Lee et al [5] given a case study choosing different connected location and installed capacity of wind generators and adopting fixed capacitors, SVC and STATCOM, respectively, for co mpensation to observe the influence on the power quality. The study was conducted using MATLAB/ SIMULINK as analysing tool. Sharma et al [6][7][8][9][10] in their papers reviewed various power quality issues and discussed power quality issues arising due to induction generators. Excessive reactive power import fro m the power network is found to be one of the reasons behind the power quality problems. Ch icco et al [11] presented an experimental analysis of two Italian wind farms for power quality requirements in frequency and voltage variations, harmonics etc. in h igh voltage grids. Grunbaum [12] given some salient design features of SVC and STATCOM and highlighted three cases of application in distribution systems where Static Var Co mpensator (SVC) and STATCOM were found suitable devices for voltage stability and power quality improvement in grids connected with wind generation. Singh [13] presented an analysis of the three-phase self-excited induction generator with static compensator (STATCOM ) as a voltage regulator. Current controlled voltage source inverter was used as STATCOM, to provide fast dynamic response to maintain constant voltage at SEIG terminals during severe load perturbations. It also acts as a source and sink of reactive power. Sharaf et al [14] presented a novel FACTS-modu lated power filter co mpensator for voltage stabilization and power quality enhancement using a PWM switching dynamic tri-loop error driven PID controller. With such costly and complex controls, induction generators may loose their basic attraction of simp le construction and cost. This may be avoided by proper design of such mach ines fro m power quality point of view. [15][16][17] are found to be helpful to develop the MATLAB Simulink models for present analysis.
This paper presents new proposals to improve the power quality by analysing the simulated results on induction generators. An attempt has been made to analyse the effect of some of the mach ine parameters effecting power quality issues for the machines operating in grid and isolated mode. Reco mmendations as proposed for the proper design of mach ine may be helpful to control the reactive and active power, thus resulting in to an overall improvement of power quality.

System Modelling
Capacitor model is shown in Fig. 1(a) and Fig. 1(b) [15] may be used at stator terminals of induction mach ine during self-excitation along with resistive load.
MATLAB/ SIM ULINK schematic models as developed for the analysis of grid connected machines and for self excited mode are shown in fig 2 &

Gri d Connec te d Oper ation
As induction generator is connected to a strong grid hence no variations / fluctuations are observed in voltage and frequency. Fig 4 to 17 shows the effect of machine parameters as observed on the power quality issues listed in section 2.
The effect of rotor resistance on active power for d ifferent values of speed is shown in Fig 4 to    It is observed that with the increase in L s real power output is found decreasing. Nature of variat ions is found to be dependent upon the operating speeds and size of the mach ine.
Analysis  • There is hardly any effect on active power in case of low rating machines. It is significant for large rat ing machines.
• Low value of stator leakage reactance is desirable to deliver maximu m power output in case of large machines.
The variations in active power at different speeds with change in stator resistance are shown in Fig 11 to Fig 12. Decrease in real power is observed with an increase in the value of R s (pu) due to additional power loss at any operating speed. Therefore machine is to be designed with low value of stator resistance.  Magnetizing reactance "L m " is found to be the most effective parameter effecting the reactive power consumption of GCIG. The effect of L m on reactive power is shown in Fig 13 to 16.
It is observed that that with increase in L m there is reduction in reactive power requirement of the machine. This variation is due to the shifting of operating point on the magnetization curve of induction generator due to any change in L m . Therefore the value of L m should be high which indicates the low saturation level in the magnetic parts of the mach ine Rotor resistance is found to be effective to control the total harmonic distortion THD (I).  Fro m the figures for any operating speed it is observed that • THD (I) increases with increase in rotor resistance for every mach ine.
• Change is more effective in bigger machines in contrast to mediu m and small rating mach ines.  Therefore in order to reduce THD (I) lo w value of rotor resistance is found to be desirable for wind energy conversion.

Self Excited Operati on
As the induction generator is operating in SEIG mode the variations in terminal voltage and frequency are observed. Fig 19 to 35 shows simu lated results on 160 kW machine to look into the effect of machine parameters on power quality issues. The effect of rotor resistance on active power for t wo different values of operating speed is shown in Fig 19. Active output power decreases with an increase in rotor resistance and it is observed that the drop is uniform. Therefore in order to deliver more output similar to GCIG here also low values of rotor resistance are desirable. The stator resistance is required to be varied within the limits as increasing the stator resistance beyond certain limits the IG output active power drops sharply. Low values of stator and rotor leakage reactance are desirable to deliver maximu m power output. With the increase in speed the active power output of the IG also increases.
The variation in reactive power demand of the SEIG with mach ine parameters is shown in Fig 23 -26.  The reactive power requirement of SEIG is decreases with increase in rotor resistance, stator resistance, stator leakage reactance and rotor leakage reactance. The effects are mo re predominant than GCIG as in this case there is no grid present to maintain the voltage and to supply reactive power. For a fixed value of excitation capacitor, its reactive power requirement suddenly falls to a lower level due to sudden fall in terminal vo ltage. Fig 27-30 shows the variation in terminal voltage when mach ine parameters are varied The terminal voltage of the SEIG drops with the increase in rotor as well as stator resistance. Terminal voltage falls to very low value when stator and rotor leakage reactance are increased beyond certain limits. Therefore terminal voltage seems to be very sensitive in terms of machine parameters.    • For Inject ing more act ive power to the grid mach ine may be designed with low value of rotor resistance but on the other hand it results into excessive reactive power demand. Therefore one machine may be opted as slip ring induction mach ine i.e. master controller for active and reactive power control.
• Any increase in stator resistance results into a drop in active power output of the machine due to increased power loss in the stator. Lo w value of stator and rotor leakage reactance is desirable to deliver maximu m power output in case of large machines.
• Magnetizing reactance "L m " is found to be the most effective parameter effecting the reactive power consumption of GCIG. Increase in L m reduces the reactive power requirement of the machine. Therefore the value of L m should be high which indicates the reduction in magnetizing current for the same value of air gap flu x. This may be achieved during the selection of core material for the construction of the machine.
• %THD (I) increases with increase in rotor resistance for every machine. Therefore in order to reduce %THD (I) low value of rotor resistance is found to be desirable. II) Self excited induction generators: • Active output power decreases with an increase in rotor resistance & stator resistance. Therefore in order to deliver more output similar to GCIG here also low values of rotor & stator resistance are desirable.
• Generated frequency is found to be dependent upon rotor resistance only. It is due to the reduction of speed of rotating air gap flu x to meet the additional generation at constant speed operation.
• All the mach ine parameters are found more effect ive to control the power quality problems in SEIG in contrast to grid connected operation. The reactive power requirement of SEIG decreases with increase in rotor & stator resistance. For a fixed value of excitation capacitor, its reactive power requirement suddenly falls to a lower level due to sudden fall in terminal vo ltage.
• Terminal voltage is found to be very sensitive to all mach ine parameters. Hence p roper design of SEIG can lead to batter voltage regulation.
• Rotor parameters are found to be most effect ive in controlling the %THD, its value Increases with increase in R r & L r . Stator resistance effect on %THD is co mparat ively less.

Conclusions
As per discussions in section 4 it may be concluded that the machine parameters affect the power quality when induction generators are operating in grid connected or self excited modes. For better quality supply in terms of active power and THD most of the machine parameters (such as stator and rotor parameters) must be on the lower side for two operations i.e. grid and self excited. Whereas a high value of L m i.e. magnetizing reactance is reco mmended for the reduction of magnetizing current or corresponding reactive power import fro m power network. As far as possible the core construction should be such that the operation is always near to knee point on the saturation curve of the mach ine i.e. in a low saturation zone. Proposals as made above may be useful to control the power quality issues by proper design of induction generators.