Investigation of Parallel Generator Unit-Connection effects on Third Harmonic Stator Earth Fault Protection

Third harmonic protection is one of the methods used for preventing damage to generator stator winding due to earth fault. Its effects have been investigated for single generator unit connection. However, there is a dearth of information on its effects on parallel generator unit connection which is a more practical situation in generating stations. In this work, the effect of parallel generator unit-connection on third harmonic earth fault protection is investigated. Ordinary non-linear algebraic equations obtained from non-fault and earth fault circuit models of two generators connected in parallel using Kirchhoff’s voltage and current laws for different load conditions were solved generators connected in parallel using Kirchhoff’s voltage and current laws for different load conditions were solved terminal were estimated for the different load conditions. Third Harmonic Voltage (THV) at neutral and terminals of the parallel generators for no-load, light load and full load conditions were 115.8 V and 60 V, 66.8 V and 34.7 V, 231.6 V and 120.1 V respectively for non-fault condition. With an earth fault at the neutral of generator one, THV at its neutral and terminal became 0 V and 160 V, 0 V and 92.5 V, 0 V and 320 V for no-load, light load and full load conditions respectively while generator two has 140.4 V and 56.5 V, 75.5 V and 23.5 V, 280.8 V and 113 V at neutral and terminal respectively. In conclusion, for an earth fault on one generator of a parallel unit connected pair, values of third harmonic voltage at its neutral and terminal agrees with that of a single unit-connected generator. As a result, third harmonic protection will operate effectively if applied to a parallel generator unit connection.