Generator split-phase protection

The stator winding of a hydrogenerator is often made up of coils with multiple turns in the same slot. It is therefore possible for faults to develop between adjacent turns on the same phase (turn-toturn faults). These faults cannot be detected by the stator differential protection because there is no difference between the neutral- and terminal-side currents. Split-phase protection, an overcurrent element responding to the difference between the currents in the winding parallel branches, is typically provided to detect these faults. Ideally, the split-phase element should be sensitive enough to detect a single shorted turn. Despite the fact that the current in this turn can be six to seven times the machine nominal current, the current seen by the split-phase protection can be quite small, in the order of one-twentieth of the generator full-load current. In addition, a spurious split-phase current can be measured due to current transformer (CT) errors, saturation during external faults in particular. Therefore, primary considerations in the application of split-phase protection are the method of measuring the difference in the currents between the parallel branches and the proper selection of the CT used for this purpose.