Real time digital simulation and testing of generator protection elements.

Abstract

Power system protection is designed to identify and isolate the system from any type of fault or abnormal condition which may endanger the equipment and operation of the system as a whole. Ground faults are the most common types of faults in generators and can damage the stator winding severely. Stator winding protection therefore becomes one of the crucial protection functions in generator protection. The grounding method used plays an imperative role in determining which protection functions are to be employed on the generator. This thesis reviews different types of stator winding faults that occur for a generator and how the generator is protected against these faults using different types of protection system. It also presents how the different types of generator grounding affect generator protection schemes, focusing on high and low impedance grounding. The development of real time digital simulators has greatly improved the simulation and testing of protection studies. In the past, mathematical models were not fully compatible for the representation of the complete synchronous generator stator. The Real Time Digital Simulator (RTDS) has developed a synchronous generator phase domain model which allows for simulation of generator stator internal faults. This thesis illustrates the suitability of the third harmonic voltage protection scheme against stator internal faults. An overview of abnormal conditions that occur on a generator was also reviewed, how they affect the generator and their protection systems. The thesis focused on reverse power, over-excitation, and differential and current unbalance protection. The loss of field excitation in synchronous generators also largely contributes to voltage instability. The large consumption of reactive power and rapid changes in the system components leads to severe damage of the generator and jeopardizes system stability. This thesis looks into loss of field excitation events and how their impacts can be reduced by using the R-X protection scheme. It also illustrates results based on closed loop testing conducted using hardware generator protection relay and the models developed on the RTDS. The simulation and testing of generator protection functions were proved to be theoretically and practically correct which could be used as a guideline for improvements in protection studies.