In an effort to ensure N-1 network security, grid operators keep looking for equipment that helps them fulfil this requirement. Within the medium voltage power grids of the world, millions of medium voltage circuit breakers are in use. Due to their large number and their low criticality compared to high voltage circuit breakers, medium voltage circuit breakers must combine a high degree of reliability and economic viability, i.e. they must operate reliably while be economically justifiable for the operator. This combination of the need for reliability and for economic justification has led equipment manufacturers to implement effective low-cost functions, such as the Overcurrent release function and the Under-voltage release function.
This article describes the methods that can be used in order to test the Overcurrent release and the Under-voltage release function. A new test method for testing circuit breakers without close coils is also explained. When performed together with other measurements, Under-voltage release test results or Overcurrent release test results can be used to assess the circuit breakers condition more extensively, helping grid operators to ensure a high level of electrical power system reliability.