Generator excitation control and coordination for protection.

Abstract

The operation of a generator may easily be affected by faults within the machine itself as opposed to external disturbances occurring on the network to which it is connected. Generator protection must therefore be designed to react efficiently in both conditions. This thesis is a research based modeling, simulation and testing of generator excitation system control and protection. Real time digital simulator is employed in the simulation studies conducted as well as a commercial generator protection relay for hardware in-loop testing. The generator model used allows the user to model an excitation system separately for testing of internal faults; this enables excitation system limiters to be embedded in the automatic voltage regulator system model. Under and over-excitation limiters have been modeled and tested under different case studies conducted to study the system behavior and to ensure proper coordination with protection relays. These case studies are done using a software generator relay and the results are verified through hardware in-loop testing. The excitation limiters modeled and tested include straight-line characteristic under-excitation limiter which allows under-excitation operation case studies and two over-excitation limiters which are tested in the over-excitation region. Detailed representation of results based on different case studies conducted is entailed in the thesis. The results demonstrated confirm theoretical aspects based on different literatures reviewed as well as methodologies employed in testing excitation limiters with protection of generators for coordination. The generator protective functions studied are the ones closely coordinated with respect to excitation limiters and are employed to protect the generator. From the set of results obtained, it has been shown that excitation limiters prevent the generator from operating in operation zones that are thermally dangerous to the machine. When a major disturbance occurs in a power system, it is expected that generators help the system to return to a stable condition. However, for this to happen generators must remain synchronized and be operated within their capability limits, this has hence encouraged the coordination and control studies conducted in the dissertation.