Restricted Ground Differential Element (87R)(Migii Q) - GE Multilin MIGII Instruction Manual

2 PRODUCT DESCRIPTION

2.10 RESTRICTED GROUND DIFFERENTIAL ELEMENT (87R)(MIGII Q)

2.10RESTRICTED GROUND DIFFERENTIAL ELEMENT (87R)(MIGII Q)
This unit detects ground faults in solidly grounded generators, through resistance and high impedance reactance.
This unit calculates 3I current from the measured phase currents, and measures the generator ground 3I
O terminal Oneutral
current from the generator, present in the corresponding input. The difference between both values is the differential current
I , a value that must exceed a user-programmable value (setting) in order to activate the protection unit.
diff
2
This partial output is supervised by a percentage function, whose purpose is to avoid undesired operation caused by CT
saturation in a severe external fault situation, or by a mistake in the CTs in stable load condition.
This supervision was usually performed using an external stabilization resistor, where that the unbalance must exceed a
certain level to produce a trip. This solution has the inconvenience of reducing the protection sensitivity.
This is why the 87R element incorporates this supervision function, which compares the I magnitude with the maximum
diff
phase current I , present all times. This slope defines the protection unit sensitivity, and is determined by a setting K.
ýmax
Therefore:
If the calculated differential current is higher than the sensitivity setting plus the K * I value, this protection element 87R
j max
will issue an internal output, which after the set time delay will activate the trip contact enabled for that purpose.
Figure 2–4: CHARACTERISTIC CURVE FOR ELEMENT 87R
The sensitivity and resolution of this unit allow fault detection in high impedance systems, that is, where the fault currents
may not exceed 10 A (primary).
The basic settings of this unit are the I level, the percentage slope K, and the tripping timer.
diff
Additionally, settings are available for the CT ratios on the phase side and on the neutral side, which allow this Protection
element to operate in applications where these ratios have different values. These settings are located in the General
Settings group.
The scheme connection for this unit is as follows:
GEK-113188A MIGII Digital Protection for Electrical Machines 2-13