A review of high-impedance and low-impedance differential relaying for bus protection

Differential protection is usually applied on bus protection because of its high selectivity since it does not need to coordinate with other relays. Bus differential protection in power system networks operates on a principle defined by Kirchoff´s current law. The law states that the vector sum of all currents entering and leaving a node or bus is equal to zero. It is this principle that is used in all bus differential protection regardless of the relay type used. This paper discusses the fundamentals of bus protection with a focus on the two common methods typically used: high-impedance and low-impedance bus differential relaying. Included are the basic theories of high impedance and low-impedance differential relaying and their operational concepts. A comparison between the two methods which points out the benefits, drawbacks, concerns and considerations is also given. The importance of current transformer selection and performance are considered in this paper as part of the consideration in bus differential protection scheme design.