Current Inputs; Phase Sequence And Transformer Polarity - GE 750 Instruction Manual

750/760
Feeder Management Relay
Phase Sequence and
Transformer Polarity

Current Inputs

CAUTION
CAUTION
Ground and Sensitive
Ground CT Inputs
NOTE
NOTE
3–8
For the correct operation of many relay features, the instrument transformer
polarities shown above in the Typical Wiring Diagram on page 3–7 must be followed.
Note the solid square markings shown with all instrument transformer connections.
When the connections adhere to this drawing, the arrow shows the direction of
power flow for positive watts and the positive direction of lagging vars. The phase
sequence is user programmable to be either ABC or ACB rotation.
The 750/760 relays have five (5) channels for AC current inputs, each with an
isolating transformer and an automatic shorting mechanism that acts when the
relay is withdrawn from its case. There are no internal ground connections on the
current inputs. Current transformers with 1 to 50000 A primaries may be used.
Verify that the relay's nominal input current of 1 A or 5 A matches the
secondary rating of the connected CTs. Unmatched CTs may result in
equipment damage or inadequate protection.
IMPORTANT: The phase and ground current inputs will correctly measure
to 20 times the current input's nominal rating. Time overcurrent curves
become horizontal lines for currents above the 20 × CT rating. This
becomes apparent if the pickup level is set above the nominal CT rating.
There are two dedicated ground inputs referred throughout this manual as the
Ground Current and the Sensitive Ground Current inputs. Before making ground
connections, consider that the relay automatically calculates the neutral (residual)
current from the sum of the three phase current phasors. The following figures show
three possible ground connections using the ground current input (Terminals G10
and H10) and three possible sensitive ground connections using the sensitive
ground current input (Terminals G3 and H3).
The ground input (Terminals G10 and H10) is used in conjunction with a Zero
Sequence CT as source, or in the neutral of wye-connected source CTs. The ground
current input can be used to polarize both the neutral and sensitive ground
directional elements. When using the residual connection set the
setpoint to a value equal to the
The sensitive ground current input is intended for use either with a CT in a source
neutral of a high-impedance grounded system, or on ungrounded systems. On
ungrounded systems it is connected residually with the phase current inputs. In this
case, the
SENSTV GND CT PRIMARY
the setpoint. The sensitive ground current input can be connected
PHASE CT PRIMARY
to a Zero Sequence CT for increased sensitivity and accuracy when physically
possible in the system.
Units that do not have the Sensitive Ground input (such as older units which have
been upgraded with new firmware) use the G3 and H3 terminals as the polarizing
input. The G10 and H10 terminals are used for the Ground input. These connections
will be shown on the terminal assignment label on the back of the relay's case.
The Sensitive Ground input (G3 and H3 terminals) must only be used on systems
where the maximum ground current does not exceed 500 A.
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setpoint.
PHASE CT PRIMARY
setpoint should be programmed to a value equal to
Electrical Installation
GROUND CT PRIMARY
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