Enhancing generator differential performance for black-start conditions

The power system is designed to operate over long periods of time without an interruption in service. However, it is inevitable that at some point, some portion of the system will be shut down, whether for maintenance or because of a disturbance. In the second case, it is essential that the system be restored quickly and orderly. This may require one or more generators with black-start capability. During a black start, the generator is brought up to nominal speed and voltage. If the step-up transformer is directly connected to the generator (unit connection), there will not be appreciable inrush current circulating through the generator windings due to the gradual increase of the generator terminal voltage. In the case where a circuit breaker is located between the generator and the step-up transformer, voltage is suddenly applied to the transformer and inrush current circulates through the generator windings. The resulting transformer inrush current can often cause the generator differential relay to misoperate because of poor or uneven current transformer performance caused by the dc component found in the transformer inrush current. It is critical that these misoperations be prevented to ensure the reliability of the system. This paper includes an overview of differential protection and its application to synchronous generators. Focus is given to black-start conditions and their effect on generator differential protection performance. Real-world event data are used to analyze the effectiveness of various methods in detecting black-start conditions and preventing differential element misoperation. This paper discusses limitations and considerations for each method and an improvement to an existing differential element.