DFIG Turbine With Reactive Power
Control System

DFIG wind turbines are largely deployed due to their
variable speed feature and hence influencing system dynamics.


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ABSTRACT


This paper describes active power control in a grid connected variable speed wind electric generation system (WEG) using squirrel cage induction machine. The chosen variable speed WEG system consists of a wind turbine, squirrel-cage induction generator. The stator winding is connected directly to the 50 Hz grid and the rotor is driven by a variable-pitch wind turbine. The pitch angle is controlled in order to limit the generator output power at its nominal value for winds exceeding the nominal speed (9 m/s). In order to generate power, the Induction generator speed must be slightly above the synchronous speed. Speed varies approximately between 1 pu at no load and 1.005 pu at full load. Each wind turbine has a protection system monitoring voltage, current and machine speed. Reactive power absorbed by the Induction generator is partly compensated by capacitor banks connected at each wind turbine low voltage bus. The rest of reactive power required to maintain the voltage at bus is provided by a STATCOM with a 3% droop setting. The WEG scheme is simulated using system models in MATLAB-Simulink and the performance is studied.

INTRODUCTION


Nowadays alternative energy sources are being widely used for both grid connected systems Among the alternative sources in recent years’ wind energy stands and assumes one of the most important rules in power systems of several countries. In addition to the environmental issues, wind energy can be used to supply stand alone or grid connected loads with good energy quality and safety. The variable speed wind turbines (VSWT) are more efficient in regarding to the fixed speed ones (FSWT) and also present a better dynamic response. In this system Reactive power absorbed by the Induction generator is partly compensated by capacitor banks connected at each wind turbine low voltage bus. The rest of reactive power required to maintain the voltage at bus is provided by a STATCOM.

DESCRIPTION


System consists of 20 MVA type III variable speed wind turbines, 10 MVA type I fixed speed wind turbine generators and ±3 MVAR STATCOM connected to 25 kV medium voltage collection bus. This test system is investigated for reactive power management with the 230 kV power grid. An algorithm is written in the master controller titled” reactive management". In this algorithm, the reactive power references are computed using the voltage and net reactive power at the point of interconnection and their values are proportional to each of the individual quadrature axis current components. The voltage dips are programmed by creating a voltage event at t=5 sec and by changing the wind speed of fixed speed wind turbine from 8 m/sec to 10 m/sec at t=11 sec.

WIND ENERGY CONVERSION SYSTEM


To date, there have been a variety of WECS’ designs; however by far the most popular and widely used is the horizontal axis wind turbine (HAWT). As made clear in the Introduction, the design of interest is the low-cost, low-power HAWT design common in rural and urban applications. Such systems are becoming increasingly popular and consist of following 4 main components: -

Rotor Assembly


This consists of the blades of the turbine, along with the hub; upon which the blades are mounted. The performance of a wind turbine is greatly affected by blade geometry, and in many designs, this component is also the most expensive part of the turbine unit.

Drive Train


Connecting the rotor to the generator is the drive train. In larger wind turbine systems, the drive train includes gearing to increase the speed of rotation from the rotor into the generator. Small turbines do not have this feature; the drive train for these systems is simply a connecting shaft.

Generator


The generator converts the mechanical rotation of the drive train into electricity. Small turbine generators are commonly of the 3- phase, permanent magnet type; however other generator types have been used.

Controller


To protect the system, in addition to converting the output of the generator to domestic voltages, a power electronic interface converter is necessary.

Turbine Modeling


There are two ways in which we can divide the complete control strategies of the machine, one is the scalar control and the other one is the vector control. The limited use of scalar control makes way for the vector control. Although it is easy in executing the scalar control strategies, but the inherent coupling effects present give very slow response. This problem is overcome by the vector control. An Induction machine can be executed like a dc machines with the help of vector control strategy. Vector control is employed for achieving a decoupled control for both active and reactive powers. The base on which the vector control theory is based is d-q axis theory.

D-q axis transformation (reference frame theory)


Direct-quadrature zero conversion is a mathematical conversion employed to make easy the analysis of a three phase circuits, where three AC quantities are converted to two DC quantities. Various mathematical calculations are performed on the imaginary DC quantities and the AC quantities are again recovered by performing an inverse transformation on the DC quantities. It is very similar to Park’s transformation, and it solves the problem of AC parameters that are varying with time. Employed to simplify the analysis of three phase circuits, where three AC quantities are converted to two DC quantities. Owing a smooth air-gap in the induction machine, the self-inductances of both the stator and rotor coil is constant, but the mutual inductances vary with the rotor movement with respect to that of the stator. Therefore the analysis of the induction machine in real time becomes very complex because of varying mutual inductances, as the voltage is nonlinear. Change of variables are therefore employed for the stator and rotor parameter to remove the effect of varying mutual inductances. This conversion leads to imaginary magnetically decoupled two phase machine. The orthogonally placed balanced windings are called d and q windings that can be considered as stationary or rotating relative to that of the stator. In the stationary reference frame, the ds and sq. axes are fixed on the stator, with either ds or qs axis coinciding the phase axis of the stator. In the rotating frame, the rotating d-q axes may be either fixed on the rotor or made to move with synchronous speed.

CONCLUSION


This paper describes the model of variable speed wind turbines with 3-phase induction motor using back-to back PWM voltages source converters and the corresponding control schemes. The variable speed wind turbine is capable of controlling the output active and reactive power independently. To control the active and reactive power we use STATCOM using PWM switching. The proposed wind farm controller using STATCOM is simulated.
Hence STATCOM was used to inject reactive power to maintain voltage level within limits and also eliminates power fluctuations and this confirms the excellent performance of the proposed system for power quality improvement.