Impact of STATCOM on the Loss of Excitation Protection

Introduction

The protection of generator is very complex and challenging as the generator in the system is connected to many auxiliary equipment’s like prime mover, excitation system and grid as shown in the Figure 1. In case of a fault on a generator it is not enough to trip the main breaker and isolate generator from grid because the generator is connected to auxiliary equipment’s. If one of the auxiliary equipment connected to a generator develops a fault then it has serious impact on generator even though there is no actual fault in the generator. Each and every auxiliary equipment’s connected to the generator makes the protection of generator challenging.

A fault or abnormality in any auxiliary equipment may lead to abnormal operating condition of generator. It is obvious that all abnormal operating condition needs to be identify as early as possible to prevent the mal-operation of generator.

Figure 1.

Generator connection in power system

The electrical faults that occur on a generator are phase fault, ground fault and inter-turn fault on the stator winding and rotor short circuit fault. In addition to electrical faults there are several abnormal conditions like unbalanced loading of stator winding, loss of excitation, loss of prime mover and over speeding of prime mover that may occur in generator. Among all fault/abnormal condition loss of excitation is the frequent fault that accounts for 70% of all generator failure. Hence it is crucial to identify the excitation loss as early as possible.

While the generation and transmission line capacity are constrained by scarce resources and external causes, the system's demand is growing quickly. Recent advancement in power electronic devices led to development of FACTS devices by using such devices it is capable to utilize the existing transmission network and improve stability of the network. The FACTS devices provide numerous benefits like increase in transmission line compatibility, reactive power support, control of voltage and improves stability of the network.

The protection relays are impacted by FACTS devices like STATCOM that are connected in transmission lines to provide reactive power or regulate voltage. The transient response of STATCOM (Static Synchronous Compensator) during LOE condition alters the impedance seen by relay which causes reach problem/delay in operating time. The STATCOM in transmission line threatens the generator and system stability so it is necessary to modify the traditional generator loss of excitation protection scheme to avoid mal-operation of relay.