Simulation and Analysis of a Prototype Rotor Fed Induction generator for Wind Power

Parameter Controlling using MATLAB Samiya Zafar

Department of Electrical Engineering NED University of Engineering and Technology

Karachi, Pakistan samiyazafar@neduet.edu.pk

Abstract— With wind as a source of power, induction generators for remote and windy areas are suitable. Especially for remote areas where wind is available in bulk, small scale wind farms are suitable for which a complete set up is analyzed in this paper. The rotor windings are energized through a 3 phase AC of low frequency and the stator supplies 3 phase AC to the load. The method of pitch control is employed for constant frequency operation. The simulation results are also presented.

constant. DFIG based wind turbines can supply reactive power even at zero active power [1]. The similar approach is used for generation but with an exception; the output of the generators is not sent to the grid, instead it is used to feed the load for the nearby locations, such that the induction generator no more remains doubly excited. The only voltage source converter used is on the rotor.

Keywords-wind power; induction generator; rotor frequency;

reactive power; pitch control; PWM inverter

I. INTRODUCTION

A. Rotor voltage

II. IMPLEMENTATION

Generating electrical power is never easier. With the

advancements in this area, more than many methods are available now for power generation. Environment friendly methods are always the preferred choice, such as those with wind and solar energy.

According to [1], balancing reactive power within a grid is one of the fundamental tasks of transmission system operators. With increasing portion of wind power, wind turbines have to contribute to reactive power generation during steady state as well as during transient conditions. Possible sources of reactive power in an offshore wind farm are capacitor banks as well as switchable reactors, flexible AC transmission devices (FACTS) and the wind turbines themselves. Beyond these, it is possible to influence the reactive power of a wind farm indirectly by using on-load tap changers. The requirements have to be fulfilled at the PCC (Point Of Common Coupling), which offers several options to provide the desired power factor.

Variable speed wind turbines are equipped with voltage source converters (VSC) and doubly-fed induction machines (DFIM). The stator of a DFIM is directly connected to the grid while the rotor winding is supplied through VSC [1]. By varying frequency and magnitude of the rotor voltage the generated active and reactive power can be controlled independently of each other. The rotor side converter (RSC) usually provides active and reactive power control of the machine while the line-side converter (LSC) keeps the voltage of the DC-link

Flux in the machine is d irectly proportional to the rotor voltage.

Ф α Vr (1)

As we increase Vr, we expect induced EMF to increase;

e= k Ф ω (2) But at the same time, electrical torque also increases as in (3) that opposes the torque provided by the wind.

τ = k Ф IA (3) This decreases the overall speed ω, and hence induced EMF also goes down. So an increase in rotor voltage decreases output voltage and frequency.

B. Rotor frequency The overall speed is the sum of the speed due to wind and speed due to supplied rotor frequency. Decreasing rotor speed will directly reduce the speed (and hence frequency);

ω = ωw + ωrf (4)

Also the flux in the machine is inversely proportional to the rotor frequency.

Ф α (1 / fr) (5)

978-1-4244-8782-0/11/$26.00 ©2011 IEEE

That is, a decrease in rotor frequency increases flux hence induced torque also increases, as in (3) which opposes wind torque, hence speed goes down drastically and also the terminal voltage as in (2).

C. Speed control for induction generator It is seen that changes in one or both of the rotor excitation parameters change both of the output terminal characteristics. Even if rotor frequency is kept constant then changes alone in rotor voltage will give precise control over stator voltages but to keep the speed of the rotation constant (and hence the output frequency constant) when either wind varies or load varies (or the rotor voltage varies)requires all wind turbines to be designed with some sort of speed control.

Pitch Controlled Wind Turbines: On a pitch controlled wind turbine the turbine's electronic controller checks the power output of the turbine several times per second. When the power output becomes too high, it sends an order to the blade pitch mechanism which immediately pitches (turns) the rotor blades slightly out of the wind. Conversely, the blades are turned back into the wind whenever the wind drops again [2].

The rotor blades thus have to be able to turn around their longitudinal axis (to pitch). During normal operation the blades will pitch a fraction of a degree at a time - and the rotor will be turning at the same time [2]. The pitch mechanism is usually operated using hydraulics or electric actuators [3].

D. Block Diagram

Figure 1. Block Diagram for Prototype Implementation

The block diagram in Fig. 1 suggests the required setup for the proposed method of generation in laboratory. Starting from the right is a conventional single phase source available, at a frequency of 50 Hz and 220V. The Induction generator’s rotor is to be supplied with a variable voltage and a constant but low frequency. The transformer merely steps down the voltage to serve as input for Uncontrolled AC to DC converter. This is employed because to convert a high frequency into a low frequency, it is necessary to convert the high frequency into

DC as an intermediate step in conversion. This DC can then be inverted into AC of any frequency and any number of phases. The next block is a three phase inverter. For the variable voltage output, PWM inverter is the best choice available. The output is a three phase variable voltage at a low frequency, which is used to energize Induction generator’s rotor. The output of Induction generator feeds a three phase load.

1) Dc Motor: The upper two blocks in the diagram needs an extra explanation. For the laboratory analysis of this set up, it is required that there be a replication of wind, which rotates the rotor of the induction generation, as would the wind turbine do if connected to the rotor A small DC motor coupled to the shaft of the induction generator serves this purpose well. It is obvious that a DC motor would require a DC power supply at its terminal as well as on the field. As stated earlier, the only source available is AC at 220 V and 50 Hz. Again a rectifier is needed. It is required that the rectifier be a controlled one so that the voltage fed to the DC motor is variable. It is assumed that the speed is maintained using the pitch control on a wind farm. But for the laboratory analysis purpose, speed is maintained by varying the speed of the DC motor every time a change in load occurs. That is, if load increases and the speed drops, the speed of the DC motor is increased (and hence of the shaft coupled to it) so that speed is brought back to its previous value and the frequency supplied to the load is constant. Similarly, DC motor is slowed down when load decreases to avoid over speeding. To implement what’s suggested above, it is required that speed control of the DC motor be achieved.

a) Armature voltage control method: In the armature voltage control method, the voltage applied to the armature circuit, Va is varied without changing the voltage applied to the field circuit of the motor. Therefore, the motor must be separately excited to use armature voltage control. When the armature voltage is increased, the no-load speed of the motor increases while the slope of the torque-speed curve remains unchanged since the flux is kept constant[4]. It is discovered that this method is easy to implement. It is efficient and simple. All that is left to do is to manage to get variable voltage at the armature. As described earlier, this is easily achievable using a controlled rectifier placed in between the single phase supply and the DC motor.

III. SIMULATION MATLAB simulation for the induction generator, voltage source converter and a 3 phase load set up is analyzed. Since constant speed control is decided to be achieved using pitch control, the model uses constant speed generator. The voltage

APPARENT

POWER (VA)

POWER FACTOR

ACTIVE POWER

(W)

RE- ACTIVE POWER

(VAR)

ROTOR VOLT.

(V)

STATOR / LOAD VOLT

(V)

FREQU ENCY (Hz)

3750 0.8 lag 3000 2250 26.5 460 60

3750 0.7 lag 2625 2678 26.3 460 60

3750 0.9 lag 3375 1635 26.55 460 60

3000 0.8 lag 2400 1800 24.6 460 60

3429 0.7 lag 2400 2449 25.5 460 60

2667 0.9 lag 2400 1163 23.8 460 60

2667 0.8 lag 2133 1600 23.76 460 60

2240 0.7 lag 1568 1600 22.57 460 60

3671 0.9 lag 3304 1600 26.33 460 60

is variable, but the frequency fed to the rotor is held constant, depending on one’s choice and depending upon the wind factor for the location.

With a 3750 KVA induction generator on which,

Stator resistance: Rs = 1.115 Ω Stator Inductance: Ls = 0.005974 H Rotor Resistance: Rr = 0.1 Ω Rotor Inductance: Lr = 0.005974 H Mutual Inductance: Lm = 0.2037 H

Rotor speed due to wind: Nw = 1740 rpm Rotor frequency: fr = 2Hz No of poles in generator: P = 4 Speed due to rotor frequency: Nrf = 120*fr/P = 60 rpm Combined Speed: N = 1740 + 60 = 1800 rpm Stator frequency: fs = N*P/120 = 60 Hz Rated voltage: 460 V Rated frequency: 60 Hz

Different Load conditions, i.e. several possible ratios of active and reactive power are studied. The injected rotor voltage is required to be increased whenever power factor goes down, while the frequency is maintained at 60 Hz.

The simulation results are presented in the table I.

TABLE. I

The simulation result for the generated stator terminal voltage (phase to phase) is presented in Fig. 2. Rotor and stator current waveforms are shown in Fig. 3. The rotor speed curve is given as in Fig. 4.

Figure 2. Stator Phase to Phase Voltage

Figure 3. Rotor Current and Stator Current

Figure 4. Rotor Speed in rpm

An important feature to note is the amount of active and reactive power drawn by the rotor for the amount of power produced. This is shown in the simulation result for instantaneous active and reactive power flow into the rotor (Refer to Fig. 5). Note that the amounts are negligible with respect to the output power of the generator.

Figure 5. Instantaneous active and reactive power at rotor

Here it may also be important to note that the shape of the voltage waveform supplied to the rotor by Voltage Source Converter (i.e. a back to back rectifier and inverter) is not exactly sinusoidal. The PWM inverter produces a waveform which is as shown in simulation result in Fig. 6.

Figure 7. Armature Current and Voltage for DC motor

IV. HARDWARE Based on the derived approach and simulations, an experimental set up is organized. The DC motor rotates the shaft of the Induction generator. Rotor windings are excited through a set of diode rectifier and IGBT inverter. The thyristors used in the rectifier for controlling the armature input voltage to the dc motor and the IGBTs in the inverter are provided with gate pulses using MATLAB pulse generator for the former and MATLAB PWM generator for the later, using parallel port. The maximum voltage and current to occur during the operation as dictated by simulation results were considered in the selection of diodes, thyristors and IGBTs. The operation is satisfactory and matches the simulation results, except for the system losses that were not accounted for in the simulation. The oscilloscope view for the generated stator voltage (scaled down to a lower value) is shown in Fig. 8.

Figure 6. Voltage waveform injected to the rotor This result is obtained without a filter. It is obvious that when a filter is used to smooth out this input voltage to rotor, the generated voltage at stator has fewer harmonic in it. The DC motor with its converter supplying the armature voltage is also simulated. The results for armature voltage, armature current, field current and shaft speed are noted, and are shown in Fig. 7.

Figure8. Stator generated voltage

V. CONCLUSION

REFERENCES

[1] Erlich, M. Wilch and C. Feltes “Reactive Power Generation by DFIG Based Wind Farms with AC Grid Connection” UNIVERSITY DUISBURG-ESSEN. URL: http://www.uni-duisburg.de/fb9/ean. EPE 2007 - Aalborg

[2] Muljadi, E. and Butterfield, C.P. 2000.

“Pitch-Controlled Variable-Speed Wind Turbine Generation”. NREL/CP-500-27143. Conference paper, presented at the 1999 IEEE Industry Applications Society Annual Meeting, Phoenix, Arizona, October3-7,1999. http://www.nrel.gov/docs/fy00osti/27143.pdf. Retrieved August 21, 2007.

[3] Marian P. Kaźmierkowski, Ramu Krishnan, Frede Blaabjerg,

Control in power electronics, 2002.

[4] S. J. Chapman, Electric machinery fundamentals, 3rd ed., WCB/McGraw-Hill, New York, 1998

This paper performed an analysis and simulation of the operating characteristics of a special case of double-fed induction generator using MATLAB simulation approach. The simulation study shows that the characteristics are affected by its injected rotor-voltage and frequency, yet one of them can be kept constant and the other varied to get the desired results. A comparison between DFIG stator and rotor real power shows that the rotor power is normally obviously smaller than the stator power. The hardware implemented based on the simulation study produced the results as were expected.