Woodward
Manual MRR1 GB
8
DOK-TD-MRR1 Rev.A
4. Working principle
4.1 Rotor earth fault protection
A single earth fault on the field windings of a synchronous generator produces no immediate dam-
aging effect. It must be detected and removed be-cause of the possibility of a second earth fault
that could short a part of the field winding and cause severe damage.
MRR1 provides an alarm and a trip element for the rotor earth fault protection.
The insulation resistance measurement system of the MRR1 can be used for synchronous genera-
tors of both types, with and without brushes.
It works with a low frequency alternating test voltage (0,51 Hz, ±24V) which is connected via slip
rings to the main excitor winding and to rotor earth (ref. to connection diagram).
The low frequency of the test voltage is used to avoid malfunction due to the field-to-earth
capacitance (CE) which can reach values up to 1 µF on large generators. The test voltage is fed
via two current limiting resistors (RV) to both ends of the field winding. If only one end of the field
winding can be connected, the terminals A3 and A4 should be paralleled.
The earth current is measured via shunt resistor RM.
Figure 4.1: Measuring principle of the earth current
The insulation resistance measurement is based on the subsequent measurement of the shunt
resistor voltage of two consecutive half cycles, i.e. positive and negative test voltage.
From the differential voltage between the positive and negative measuring voltage at the end of
each half cycle the insulation resistance is calculated. Thus avoiding wrong measurement due to
the earth capacitance and possible voltage offsets.
