Abstract – This paper presents a topology for direct online starting of Induction Motors (I.M.s) using Thyristor Binary Switched Capacitor (TBSC) compensator operating in closed loop. TBSC is based on a chain of Thyristor Switched Capacitor (TSC) banks arranged in binary sequential manner. A transient free switching of TBSCs is carried out.
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Proposed topology allows switching in/out of capacitor banks according to the reactive power requirement of induction motors in very fast responding closed loop. Simulation results show that the proposed scheme can achieve reactive power compensation in cycle to cycle basis. Proposed scheme can be used for direct online starting of I.M.s with voltage sag mitigation at starting, which helps improving stability of the system and Power Factor (P.F.) improvement in steady state. Keywords – Reactive power compensation, TBSC, transient free switching, voltage sag, Power Factor
Induction motors (I.M.) constitute a large portion of power system. Three-phase induction motors represent the most significant load in the industrial plants, over the half of the delivered electrical energy . Starting of induction motor may cause a problem of voltage sag in the power system. The IEEE defines voltage sag as: A decrease to between 0.1 and 0.9 p.u. in rms voltage or current at the power frequency for durations of 0.5 cycle to 1 min . An induction motor at rest can be modeled as a transformer with the secondary winding short circuited. Thus when full voltage is applied, a heavy inrush current (of 6 to 10 times the rated value) is drawn from the power system that causes voltage sag. As the motor accelerates and attains the rated speed, the inrush current decays and the system voltage recovers . Voltage sag can cause mal-operation of voltage sensitive devices such as computers, relays, programmable logic controllers etc. [chetan]. Also because of the highly inductive nature of the motor circuit at rest, the power factor is very low, usually of the order of 10 to 20 percent . Thus reactive power demand at the starting of I.M. is very high and it reduces as motor picks up the speed. There are several solutions to minimize this problem, the most common are : reactor start, auto transformer start, delta-wye start, capacitor start, soft starter, frequency variable driver (FVD) etc. All these methods except capacitor start are based on a motor terminal voltage reduction to decrease the rotor current, reducing the line voltage drop . Problem with this method of starting is that the motor torque is directly proportional to the square of the supply voltage hence decrease in the motor terminal voltage will cause the motor torque to decrease, which may be insufficient for driving the required load . Soft starter and frequency variable driver methods are the most expensive and complex,
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