Control Of Grid Connected Induction Generator Using Naturally Com Mutated Ac Voltage Controller

CONTROL OF GRID CONNECTED INDUCTION GENERATOR USING NATURALLY COMMUTATED AC VOLTAGE CONTROLLER

Elec. Power and Machines Dept.-College of Eng.

A. F. Almarshoud College of Technology P.O.Box 42826

Cairo University Giza, Egypt

Riyadh 1155 1 Saudi Arabia

M. A. Abdel-halim

ABSTRACT The paper presents complete analysis of induction generator linked to the network through ac voltage controller utilizing anti-parallel thynstors. The performance characteristics regarding the harmonic contents, active power, reactive power, power factor and efficiency have been computed. These characteristics have been determined with the help of a novel abc-dq circuit model. The model posses the advantages of both the dq and direct phase models .

1. INTRODUCTION Induction generators have two states of operation. They are either autonomous units or grid-connected units. The power factor of the grid-connected induction generator is fixed by its slip and its equivalent circuit parameters and not affected by the load. The quadrature component of the current output is nearly constant for any fixed terminal voltage and frequency and leads the voltage. It is necessary therefore to operate such generators in parallel with synchronous machines. These synchronous machines do not only supply the quadrature lagging current demanded by the load but also supply sufficient quadrature lagging current to neutralize the quadrature leading component of the current delivered by the induction generator. Thus, the synchronous machines in parallel with an induction generator determine its voltage and frequency, while its output is fixed by its slip [ 11. Induction generators when driven by wind turbines are liable to run near its synchronous speed when the wind speed is low. This results in operation at bad power factor and low efficiency. To improve the

A. I. Alolah EE Dept.-College of Eng. King Saud University P.O.Box 800, Riyadh 11421

Saudi Arabia

power factor and efficiency of the generator at such loads, it is recommended to lower its terminal voltage. Thus an ac voltage controller used as an interface between the network and the generator is useful in this concern [2,3]. In previous work by one of the authors [4]an ac voltage controller utilizing power transistors as electronic switches has been used to link the generator to the grid. Forced commutation has been applied, and the converter has been provided with switched free-wheeling paths. In the present paper, a rather simple ac voltage controller is used to control the active and reactive power of an induction generator connected to an infinite bus bar. The ac voltage controller utilizes a set of anti-parallel thyristors. The performance of the induction generator is studied through modeling it by a novel equivalent circuit in a pseudo-stationary abc-dq reference frame. Based on the circuit model a state space mathematical model is develop ed. The model is capable of dealing with the nonlinearities introduced by the used electronic solid-state switches. The performance characteristics have been computed for a wide range of operating conditions through a simulating computer program.

2. PROPOSED CONTROL CIRCUIT The proposed circuit is shown in Fig. 1. Each stator phase has a control circuit that consists of two antiparallel thyristors. This control circuit links the induction generator to the network. The terminal voltage of the generator is controlled by controlling the triggering angle, a of the thyristors.

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The current begins to flow at this angle and the thyristor is naturally commutated when the current falls down to zero. The current, power factor, active and reactive power of the generator are controlled through the variation of the triggering angle of the thyristors.

ihase bus bar

3- SYSTEM MODELLING The stator of the induction machine is modeled in the direct phase reference frame, abc while its rotor is modeled as two pseudo-stationary coils in the dq reference frame (Fig. 2). This new model has the following advantages: i-

There is no need to transform the stator voltage and current quantities as the stator is modeled keeping its original physical arrangement. ii- Stator direct phase modeling allows unbalanced conditions and nonlinearities arising from the use of electronic switches and other reasons to be easily represented. iii- Using pseudo-stationary coils for the rotor results in time-independent mutual inductances between the stator and rotor coils. Thus the advantages of the dqo models are reserved.

Fig. 1: Generator System

The five coil currents and the rotorspeedare chosen to be the state variables. Based on the previous circuit model the voltage matrix equation of the machine could be formulated as follows:

where [R].[i] is the resistive voltage drop matrix [x].p[i] is the transformer voltage matrix w[G].[i] is the rotational voltage matrix.

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Fig.2: Circuit model of the Induction Generator

If the rotor d-axis is chosen along the magnetic axis of the stator phase “a”, and neglecting the space harmonic fluxes and the saturation effects, The [XI and [GI matrices are given by:

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stator-phase (rms) currents, harmonic factor, power factor, active and reactive power, and the generator efficiency.

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A three-phase, 11 kW squirrel cage induction generator having the specifications and parameters given in table 1 has been used for computations. The performance characteristics have been computed at different speeds and different triggering angles.

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Table 1: parameters for induction generator

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Figure 3 shows the generator fundamental current. As the triggering angle increases, the fundamental current remains nearly constant for a certain range, and then drops rapidly approaching the zero value. The constant current range gets wider at lower speeds. Figure 4 shows the generator harmonic factor. The harmonic factor increases as the triggering angle increases. The phase shift between the fundamental current component and the phase voltage (displacement angle) is shown in Fig. 5. The fundamental current component leads the pha se voltage. The angle remains more or less constant as the firing angle increases, and then increases rapidly and exceeds the 90 degree. The average torque (Fig. 6) behaves in a manner similar to that of the current. The variation of the delivered active power of the generator with the firing angle at different speeds is shown in Fig. 7. The pattern of the active power is in general similar to those of the current and torque. The induction generator generally consumes reactive power (Fig. 8). This reactive power slightly increases as the triggering angle increases over a certain range, then it decreases rapidly approaching zero.

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Fig. 7: The Active Power

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The power factor remains high for triggering angle less than 160 degree especially at high speeds, But when the triggering angle increases, power factor decreases rapidly to zero and may go to

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Fig. 8: The Reactive Power

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negative values at higher triggering angles as shown in Fig. 9. The efficiency behaves in a manner similar to that of the power factor as shown in Fig. 10. At high triggering angles the values of the power factor and efficiency are negative, this happens because at high triggering angles the generated power is not enough to cover the stator and core losses. So, the generator will absorb active power from the grid.

5- CONCLUSIONS The paper has presented novel circuit and mathematical model capable of representing the steady state and transient conditions of induction generator when electronic switches are connected to its stator lines. The model has been used to analyze an induction generator connected to the network through an ac voltage controller. The use of solidstate devices as electronic switches enables the control of the active power and reactive power delivered by the induction generator to the network.

[3] S. Suresh Babu, G.J. Mariappan and S. Palanichamy, “ A novel Grid Interface for Wind-Driven Grid-Connected Induction Generators” , Proc. of the IEEE/IAS Intemational Conference on Industrial Automation and Control, 1995, pp 373-376. [4] M. A. Abdel-halim, “ Solid state control of a grid connected induction generator”, Electric Power Components and Systems, 29: 163-178,2000. [5] B. Adkins and R.G. Harley, “The General Theory of Alternating Current Machines: Application to Practical Problems”, Chapman and Hall, London, 1975.

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From the computed performance characteristics, the followings are concluded: i- The efficiency is negative at high triggering angle. This is because the generated power at these triggering angles does not cover the stator copper and core losses especially at low speeds. Consequently, the induction machine takes an electrical power from the network. But at a large super synchronous speed, electric power is delivered to the network by the induction generator. ii-The regions where the power factor or the efficiency is negative are considered impractical operation regions for the induction generator. iiiThe existence of solid-state devices is associated with existence of current harmonic contents.

[I] Stephen J. Chapman “Electric Machinery Fundamentals”, 2”dedition, McGraw-hill, 1985.

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Fir1ng angle (a)

Fig. 9: The Power Factor

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[2] V. Subbiah and K. Geetha, “Certain Investigations on a Grid Connected Induction Generator with Voltage Control”, Proc. Of the IEEE Intemational Conference on Power Electronics, Drives and Energy Systems, 1996, pp

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Fig. 10: The Efficiency

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6. REFERNCES

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