Investigation of the Performance of Induction Wind Generator by Using Space Vector Representation

Mustafa S.Bakr School of Electronic and Electrical Engineering

University of Leeds Leeds, United Kingdom

E-mail: m7baker.ms@gmail.com

Mohammed Moanes Ezzaldean Electrical Engineering Department

University of Technology Baghdad, Iraq

E-mail: dr.mohammed.moanes@uotechnology.edu.iq

Abstract— Wind generators are considered as one of the most important renewable energy sources due to its low impact on environment. Wind energy technology coverts the wind kinetic energy into mechanical energy in which it rotates the generator rotor to produce electrical energy feds into a grid or supplied a stand-alone load. In this paper, a constant speed constant frequency (CSCF) induction generator is simulated; the main advantage of using induction generator is due to its simple and robust structure. MATLAB program is developed to simulate the operation of CSCF asynchronous generator including its dynamic behavior by using the state space representation and Runge-Kutta numerical method. Generator characteristics are obtained under various mode of operation such as: motoring mode and generating mode, by running the generator from zero speed. It is observed that CSCF generating system has several stability shortfalls when connected to the grid because the generator frequency and speed cannot be softly adjusted. Speed can be adjusted within narrow range by regulating the blade attack and pitch angle. Nowadays, CSCF is not widely used. VSCF, in particular DFIG, is commonly used because of the system ability to operate at with different levels of wind speed and, dissimilar to CSCF, it accepts start-up and regenerative braking mode of operation because of the existence of AC-DC-AC converter.

Keywords-DFIG; IG; CSCF; VSCF; wind turbine; AC-DC-AC converter.

I. INTRODUCTION

Nowadays, wind energy is one of the most promising and important renewable energy source because of its low impact on environment and the advanced development in wind energy equipment and power electronic technology [2, 6]. It is expected that by 2020, wind turbine technology may supply 20 percent of Europe power consumption [3].

Wind energy system converts the wind energy into rotation mechanical energy that drives a generator to produce electrical energy which supplies the load. The main components of the wind energy system are the wind turbine itself, a driving shaft, a gearbox (if exist), the generator and a control system [4].

Studies of turbine-generator system dynamic behavior is necessary for system stability due to different technologies and conventional generators used for wind turbine-generator system which increases the wind share in power generation

[1]. Modelling of wind turbine-generator system consists of the following items: the aerodynamic, the drive train system, the generator, and the power converter [1, 6].

The doubly fed asynchronous generator (DFIG) is common to use in wind energy technology due to its ability to de-coupling the control of the active and reactive power by controlling the terminal voltage. In addition, switching losses and harmonic are lower with better overall efficiency and performance [5, 8]. In this paper, induction generator is considered due to its robust, rigid and simple construction [4]. In this study, the induction generator is represented by using space vector representation and simplified by using Clark transformation converting the three-phase equivalent circuit into two-phase equivalent circuit for the purpose of analysis [4].

II. INDUCTION WIND GENERATOR

Various generation systems can be used for the grid interfacing such as constant speed constant frequency (CSCF), which consists of a squirrel-cage asynchronous machine connected directly to the grid, and variable speed constant frequency (VSCF), which consists of either induction or synchronous machine connected though AC-DC-AC converter to the grid [4]. In this paper, a CSCF induction generator is simulated, the main advantage of using asynchronous generator is due to its robust and simple structure [5]. On the other hand, CSCF permits the machine to operate over a limited speed range, one or two speed range. Therefore, it is generally used for low rating wind turbine [1]. Conversely, VSCF can operate with a wide speed range and permits start-up and regenerative mode [2].

In the induction generator, the rotor is driven by the wind turbine when wind speed is within the required range. In order to generate power, the speed of the rotor (nr) needs to exceed the stator field synchronous speed (ns) [4]. The generated power is then injected into the grid through the stator winding or both the stator and rotor winding, through AC-DC-AC converter, (DFIG) [1] where the air-gab flux can be excited from the grid either by the stator winding or the rotor winding via a controller [4].

The three-phase induction generator performance under steady-state and transient conditions has to be studied in order to design a proper control system to insure the system

2015 17th UKSIM-AMSS International Conference on Modelling and Simulation

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DOI 10.1109/UKSim.2015.87

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stability. A set of equations that describes the instantaneous variations is used which is based on the machine equivalent three-phase circuit concept. The following assumptions are relied for developing the model [5]:

• The stator and rotor windings are symmetrically distributed, so the three phase resistances, leakage inductances and magnetizing are equal.

• The system is harmonic-free as the produced magnet motive force is distributed sinusoidally across the stator.

• Skin effect, saturation and hysteresis effects are neglected.

• The stator and rotor iron have very high permeability.

III. DEVELOPED PROGRAM

According the mathematical model, a MATLAB program is developed; the Algorithm of the program is illustrated here: • Mechanical and electrical characteristics of the induction

generator are defined. • Initialise arrays to store the generator variables, state

vector equation values, time and the solutions of fourth order Runge-Kutta.

• Set the simulation and step change time. • Firstly, the program will run at mechanical torque (Tm

=10 N.m). Later on, the program simulation time will be divided into three intervals and Tm will change from -1 to -10 and then -20 N.m. This process simulates the wind turbine generator system as the generator will start from stand-still and then accelerates to rated value for sufficient wind speed supplying power to the grid.

• Calculate the rotor and stator voltages in abc form and then transform them into d-q form for simplification.

• Apply fourth order runge-kutta method to solve the state vector to update the change of rotor speed and flux linkage in d-q form.

• Obtain the stator and rotor currents from the flux linkage values by the fourth runge kutta method.

• Evaluate the electromagnetic torque (Te) then use the inverse of Clark transformation to obtain the machine voltage and current in abc form.

• Formulate the obtained voltage, current, torque, speed and time in arrays.

• Return to step 3 repeating the aforementioned procedure until the time is reach to the simulation time.

• Plot the induction machine parameters versus time (stator voltage, stator and rotor current, rotation speed and the electromagnetic torque).

• Stop the program.

IV. SIMULATION AND RESULTS

The generated power fed into the grid is affected by the induction machine and wind-turbine performances. The induction generator converts the harvest mechanical power

obtained from the wind into usable electrical power fed into the grid.

However, the conversion efficiency affects significantly by the wind speed by which the induction machine can operate in either the super synchronous (generation) mode or the sub-synchronous (motoring) mode or the synchronous mode (Ns=Nr). Therefore, that would affect dramatically on the grid stability without proper control system in both the wind turbine and the conversion system.

Figure 1, Induction Machine 3-phase Stator Voltages

Figure 2, Induction Machine 3-phase Stator Currents

Figure 3, Induction Machine 3-phase Rotor Currents

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Firstly, when electromagnetic torque set to 10 N.m, the induction machine works at the sub-synchronous mode in which, Slip>0 and rotor speed < synchronous speed as shown in figure 4. This mode occurs when the wind velocity is lower than the cut-in (very low wind speed). In this work, CSCF system is used, for this generating system, the stator winding of the induction machine is connected directly to the grid [7]. The stator voltage is supplied from the grid in the motoring mode as shown in figure 1. In this generating system, a voltage regulation circuit is required when the machine operating in the generation mode to prevent large inrush current and a capacitor bank must be provided for power factor correction. In addition, power factor can’t be controlled in this generating system as frequency and slip can’t be controlled. Figure 2 & 3 show the rotor and stator currents. It takes about 200 ms to reach the steady state. The large inrush current in the transient state can cause stability issues in the grid such as producing harmonics. This have to be limited by using filters between the induction generator and the grid. Moreover, the stator and rotor frequency and the phase-shift between the voltage and current waveforms must be maintained at constant value for synchronisation purpose with the grid.

Figure 4, Rotor speed of Induction Machine

Figure 5, Electromagnetic Torque of Induction Machine at TM=10 N.M

The asynchronous machine might be switch off from the grid when it works in the motoring mode, otherwise, active stall or pitch angle control must be used to adjust the blades pitch or attack angle to earn the maximum possible wind power and accelerate the asynchronous machine to work in

the generating mode. Active stall control is proper for this work because the power rate of the used wind turbine-generator is limited to low rating values and that could be included from the mechanical and electrometrical torque of the machine when works in the motoring mode at the rated rotor speed (see figure 5).

Figure 6, Induction Generator Three Phase Stator Voltage

Now, when Tm set to -1, -10 and then -20 N.m, the synchronous machine is working from stand-still to super-synchronous mode (rated value when slip<0 and Nr>Ns). A control system can be used to alter the turbine and, thus, the generator from one operational state to another. The wind-turbine generator system is brought from stand-still (where Tm=-1 N.m and Nr accelerates from stand still to 200 r.p.m in 500 ms as shown in figure 8 (a) and figure 9) to partial load (where Tm=-10 N.m & Nr accelerate approximately to rated speed in 0.7 second as shown in figure 8 (b) and figure 9). Finally, the induction machine is brought to full load mode (where Tm=-20 N.m and Nr accelerated to rated value, 1552 r.p.m as shown in figure 8 (c) and figure 9. The induction machine is connected directly to the grid supplying power into it, the supplied voltage into the grid is shown in figure 6 and it is 187 Volt peak.

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(a) Induction Generator Three Phase Rotor Current at

Tm= -1 & -10 &-20 N.m

(b) Induction Generator Three Phase Rotor Current at Tm= -10 N.m

(c) Induction Generator Three Phase Rotor Current at Tm= -20 N.m

Figure 7.

The ripple of stator and rotor current is high during the transient-state, therefore, filters have to be used between the grid and the induction generator to minimise harmonic components and reduce the inrush current to the grid during the transient and steady state. As well as this, a proper controller can be used to connect the generator to the grid when it operates in the steady state and a voltage regulator is required to prevent voltage fluctuation and hence inrush current during steady state.

(a) Rotor Speed of Induction Generator at Tm= -1 & -10 & -20 N.m

(b) Rotor Speed of Induction Generator at Tm= -10 N.m

(c) Rotor Speed of Induction Generator at Tm= -20 N.m Figure 8.

In case of the mechanical torque increases due to rise in

wind power, the stator and rotor current and flux increases as shown in figure 7 and figure 8. As well as this, slip value will be smaller and thus the system conversion efficiency will be reduced because the generator operating point will move closer to the stall-toque point. When the slip gets smaller than the maximum slip, the generator speed will keep accelerating while the output power and torque reduce significantly and eventually the induction machine will be crashed. To prevent this, active or pitch stall control is

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required to control the wind power and keep it within the rated value and hence the mechanical torque.

Figure 9, Induction Generator Electromagnetic Torque at Tm= -1, -10 & -20 N.m

Nowadays, CSCF is not common to use. VSCF, in

particular DFIG, is more common because the system can operate with a wind range of wind speed and, dissimilar CSCF, it permits start-up and regenerative braking mode because of the existence of AC-DC-AC converter.

V. CONCLUSION

Space vector and Clark transformation are used to develop a mathematical model for the asynchronous machine. Machine characteristics are obtained under different operating mode such as: motoring mode (where Tm=10 N.m) and running the machine from stand-still to generating mode (Tm= -1,-10 and -20 N.m). It is observed that CSCF generating system has several stability issues when connected directly to the grid because machine frequency and speed (slip) cannot be smoothly adjusted. Speed can be adjusted within small range by varying the blade attack and pitch angle. Therefore, VSCF system is looked to be more desirable where speed is adjustable and frequency is controlled by AC-DC-AC converter between the machine rotor and grid. This system is more stable and

has fewer harmonic components. In addition, it permits start-up and regenerative mode and can operate with wide range of speed. All in all, machine modelling is essential for proper controller design and, therefore, operates the wind turbine-generator system close to the optimum point.

ACKNOWLEDGMENT

This material is based on work supported by Dr. Li Zhang, School of Electronic and Electrical Engineering, Leeds University. It is also funded by The Higher Committee for Education Development in Iraq (HCED).

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