Third harmonic ground-fault protection system of unit-connected generator with two parallel branches per phase

The operating conditions of a ground-fault protection reacting to a difference in 3rd harmonic voltages at the star point and at the output terminals of a unit-connected generator with a dipole stator winding formed by two parallel branches in phase are analysed. The voltage feeding the measuring element of the ground-fault protection in the generator is determined for different ways in which its star point is grounded. The paper, analyse the operating conditions of the above protection systems in the unit-connected generator with two parallel branches in stator windings phase and one pair of poles, on the basis of simulation studies. The simulations were run for real generator of 120 MW equipped with generator breakers with additional capacitances to ground. Realistic values of the breakdown channel in the main isolation of the generator stator windings were taken into account. This resistance was determined in experimental tests conducted in real conditions during arcing ground faults.