Short Circuit Modelling And Parameter Extraction Of A Turbo Generator

ABSTRACT Short circuit modelling and parameter extraction of a synchronous generator entails development of a mathematical model of three phase short circuit of a turbo generator and the study of its characteristics or envelops at short circuit under constant speed and saturated conditions.. The transient parameters have a significant impact on the magnitude of the fault currents generated within the machine during an event such as a 3 phase short-circuit. Power system designers routinely use the generator transient parameters as a key variables to aid in the design of the complete power generation system. The method used, was keeping the generator in an offline mode. Two separately excited direct current motors were used as prime movers, a special spring mechanism and a gear system of the ratio 1:5, as the drive system to maintain constant speed. The ratio is because the direct current motors have a speed of 600 rpm and the generator has a speed of 3000 rpm. A resistor was inserted connected to terminal B of the motor to reduce the start up current. The field voltage was measured between terminals A and B. The generator was fully excited and the sudden short circuit current was initiated through a short circuit generator connected to the turbo generator which was monitored under saturated conditions and constant speed. At no load, rated voltage stands at the terminals of the armature winding. In the process, the three phase short circuit model was developed using the fundamental machine equations of d-q-0 sequence. The machine was simulated at constant speed as well as variable speed condition using mathlab and embedded functions of simulink. The simulink is a computer based software. The parameters were extracted during the short circuit and saturated conditions initiated at constant speed and variable speed conditions. The influence of speed departures on shaft torques and current waveforms were also studied. The result showed that at constant speed, the subtransient current gave 5.200p.u, transient current gave 4.600p.u and steady state gave 0.4548p.u; the subtransient reactance, transient reactance and steady state reactance have the values of 0.1923pu, 0.217pu and 2.1978 pu respectively. The time constants of sub transient and transient condition of the generator at constant speed are 0.021 and 0.1923 seconds respectively from the extracted values obtained. The result showed ninety nine percent (99%) accuracy when compared with the method of standard synchronous equation. The case of variable speed was also studied in the same procedure. An interesting scheme for reducing the length of time the high short circuit current and torque persisted, was the inclusion of a designed scheme to demagnetize the field winding immediately the short circuit occurred in nano seconds using Mosfet.

Keywords: synchronous generator, constant speed, saturated condition, direct current motors, parameters.