Effect of Static Power Supply in Alternator Used for Short-Circuit Testing-Observation of Shaft Voltage

All electrical power equipment, before putting into service, have to pass through numbers of tests to prove their functional performance. In this context, short-circuit test is one of the vital performance tests. A high power alternator used in short-circuit test laboratory is taken as a study object in this paper. Normally, a motor is used as the prime mover for this type of alternator. In this system, the driving motor is eliminated and a thyristor-based static drive is placed to run the machine. The benefit with the static drive is that the single machine can be operated in dual mode, either as a motor or as an alternator. But the negative side of this system is the generation of voltages in the machine shaft. Machine bearings are susceptible to failure if these unwanted shaft voltages cause a high level of bearing currents and persist for a prolonged period. Very little studies were carried out on the power generators related to shaft voltages with perspective to static power supply. This paper explains the details of the static drives applied on this machine for different operational modes. Various causes of generation of shaft voltages are analyzed in view of this special alternator. The outcome of this paper can also be applicable to other large synchronous machines.