STEADY STATE AND TRANSIENT ANALYSIS OF A STATIC SLIP-ENERGY-RECOVERY DRIVE

Abstract

With the progress in automation, there is a growing demand for reliable step-less variable-speed drives which have the ability to respond quickly and accurately to exter nal speed and torque demands. They should also have the capability of giving long term stability and good transient performance. With rapid developments In the technology of thyristors and solid state devices, electric drives with solid state control are becoming increasingly popular and are replacing all types of conventional drives. The induction motor seems to be an attractive proposition as a drive, be cause of its unmatched ruggedness, simple construction and tow Capital Cost. The speed control of induction motor may be accomplished by several methods, such as pole changing, pole amplitude modulation, cascading, stator voltage control, frequency control and rotor resistance control. All of these methods suffer from such disadvantages, as excessive power loss in the control, complicated control circuitry, lack of elegance, higher capital cost, lack of continuous variation in speed etc. Compared to these, the static slip-energy-recovery scheme provides superior advantages such as higher efficiency, low co-m/o<dckvt. capital cost, better stability, simple control circuitry and a wide range of speed control. In static slip-energy-recovery scheme, slip-frequency rotor voltages are rectified by a diode bridge and applied to the line commutated controlled thristor bridge, which operating in its inverting mode returns energy to the supply. The energy fed back depends upon the supply voltage, the direct input voltage and the inverter firing angle. The direct feed back of slip energy into the supply results in a fairly good variable speed drive with firing angle as the speed control parameter. The experimental and theoretical investigations of the steady-state behaviour have been carried out by many authors* The theoretical studies are mainly based on the equivalent circuit concept, giving approximate performance characteris tics. But hardly any literature is available on the stability analysis and transient behaviour of this system. The aim of the present study is to develop a rigorous mathematical model for analyzing the steady state and transient behaviour of static-slip energy recovery drive systems. Accord ingly an idealised model describing the behaviour of the system has been developed based on the concept of coupled circuit approach. The system is analyzed in a synchronously rotating reference frame. The generalised two-axes non-linear differen tial equations of the system are established from the equations of the symmetrical induction machine and from the equations which express the relationship between the converter variables. The parameters involved in these equations are such that they can be easily measured experimentally at the terminals of the induction machine. The steady state equations of the system are obtained by setting the time rate of change of all currents to zero in xne generalised system equations. Expressions have been developed for torque, supply current, system power factor and efficiency, using which the steady state performance character istics are predetermined for different firing angles. The speed control system has been fabricated and tested. A good correlation obtained between the computed and experimental results establishes the validity and correctness of the steady state expressions developed for the system. The development of wide speed range static drives has led to an increased interest in the stability considerations. Induction machines which are perfectly stable on an infinitebus may become unstable with controlled thyristors in the circuit. In order to investigate the regions of instability the perturbation equations are derived by linearising the dynamic equations of the system about an operating point. The characteristic equation, which is the most essential require ment for studying the stability behaviour of any system, has been developed directly from the perturbation equations. Sta bility studies have been carried out using Routh Hurwitz's Criterion. The firing angle being the main speed controlling para_ meter,regions of instability have been plotted in the torquefiring angle plane. The effects of system Inertia, applied voltage, filter inductance and resistance on the unstable -Vregion3 are investigated. The stability studies indicate that the system remains stable for higher values of system inertia, where as lower values of inertia produce instability over a certain range of firing angles and torques. The region of instability expands with an increase in the applied voltage and the filter time constant. It is established that from the stability considerations the slip-energy recovery drive is superior to variable frequency induction motor drive. The investigation of transient behaviour of the system is of great practical importance, because of severe instan taneous torque generation during the transition period impos ing undue strain on the mechanical parts of the drive. In order to investigate the transient behaviour, the non-linear differential equations describing the dynamics of the system have been simulated on a digital computer and solved by the application of Runga-Kutta method. As the most important transients from electromechanical considerations are those of electromagnetic torques and speed, these are investigated in detail when the system is started from rest under no load and loaded conditions. Transient behaviour has also been studied when the drive is reconnected to the supply while the rotor is still rotating. The study has been carried out for different initial speeds of rotation and for various firing angle settings. The effects of system inertia and the filter time constant on the torque and speed transients following a switching operation are investigated* Quite -VIsevere transient torques are observed, the largest encountered during the investigation being about four times the full load torque. With increased firing angle setting of the inverter, the amplitude of torque peaks are considerably reduced where as torque oscillations and run-up time are very much increased. Higher values of system inertia cause more oscillations in the transient torque characteristics with an appreciable increase in the run-up time. The analysis of the transient behaviour of the system has been carried out for (i) sudden decrease and increase in the applied voltage (ii) sudden application of load sudden decrease and increase in the firing angle of the inver ter. The effects of system inertia and filter time constant on the torque and speed transients have also been investigated. With a sudden change in the supply voltage, transient torques are more oscillatory for increased system inertia. A similar effect is observed with a decrease in the filter time constant. Quite severe negative torques are observed when the firing angle is suddenly increased. Severity of negative torque is considerably increased with higher values of system inertia and lower values of filter time constant. The theoretical studies on stability and transient performance have been verified experimentally.