Performance of vacuum interrupters in electrical power systems with an effectively earthed neutral

Abstract

Circuit breakers using vacuum interrupters (VI) are often tested according to the IEC standard 62271-100. This standard specifies a series of current interruption tests to verify the operation of the circuit breaker under a variety of different failure conditions. The IEC 62271-100 standard will be updated at the end of 2016 to include additional new tests for medium voltage circuit breakers, designed to verify successful interruption in electrical power systems with an effectively earthed neutral. In the current version of the standard, three-phase tests are performed in the non-effectively earthed neutral case. This situation is referred to as kpp = 1.5, where kpp is the first-pole-to-clear factor. In the updated standard, additional three-phase tests are performed with an effectively earthed neutral, referred to as kpp = 1.3. This setup leads to higher stress on the second phase to clear. In order to evaluate the effect of the new tests, two different VI designs, in regular commercial use which previously passed the 62271-100 standard, were tested according to the new requirements. One VI style was an axial magnetic field (AMF) design for 40.5kV/31.5kA, and the other a radial magnetic field (RMF) design for 17.5kV/31.5kA. Both VI designs were first tested with symmetric and asymmetric close/open operations for kpp = 1.5. The VI's were then tested with symmetric operations (T100s) and a large number of asymmetric operations (T100a) with kpp = 1.3. Both VI types passed these test duties, which greatly exceeded the requirements of the updated IEC 62271-100. This performance indicates that well-designed VI's that pass the current version of IEC 62271-100 will also be able to pass the new kpp = 1.3 tests for systems with an effectively earthed neutral.