Current injection device for earth-fault protection of a synchronous machine rotor
The purpose of the rotor earth-fault protection is to detect earth faults in the excitation circuit of synchronous machines.
The excitation field circuit is isolated during normal operating conditions. The field circuit can be exposed to abnormal mechanical or thermal stress due to vibrations, overcurrent, choked cooling medium flow, etc. This may result in the breakdown of the insulation between the field winding and the rotor iron at a point exposed to excessive stress. A single earth fault is not very dangerous and does not cause immediate damage, because the fault current is small due to the low voltage. More dangerous is a second earth fault that appears as a rotor winding interturn fault and causes severe magnetic unbalance and heavy rotor vibrations leading to severe damage.
Therefore, it is essential that any occurrence of an insulation failure is detected and that the machine is disconnected as soon as possible. Normally, the machine is tripped after a short time delay.
For generators with slip rings the rotor insulation resistance is sometimes reduced due to the accumulated coal dust layer produced by the coal brushes.