GEK-113188A
MIGII Digital Protection for Electrical Machines
2-13
2 PRODUCT DESCRIPTION
2.10 RESTRICTED GROUND DIFFERENTIAL ELEMENT (87R)(MIGII Q)
2
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
O terminal
current from the measured phase currents, and measures the generator ground 3I
Oneutral
current from the generator, present in the corresponding input. The difference between both values is the differential current
I
diff
, a value that must exceed a user-programmable value (setting) in order to activate the protection unit.
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
diff
magnitude with the maximum
phase current I
ýmax
, present all times. This slope defines the protection unit sensitivity, and is determined by a setting K.
Therefore:
If the calculated differential current is higher than the sensitivity setting plus the K * I
j max
value, this protection element 87R
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
diff
level, the percentage slope K, and the tripping timer.
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:
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