Phase Sequence And Transformer Polarity; Ac Current Transformer Inputs; Ground And Sensitive Ground Ct Inputs - GE 750 Instruction Manual

3.2 TYPICAL WIRING
For the correct operation of many relay features, the instrument transformer polarities shown above in Figure
3–9: TYPICAL WIRING DIAGRAM 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 and 760 have five 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shorting mechanism that acts when the relay is withdrawn from its case.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 connec-
3
tions 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.
CAUTION
IMPORTANT: The phase and ground current inputs will correctly measure to 20 times the cur-
rent 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
CAUTION
CT rating.
There are two dedicated ground inputs referred throughout this manual as the 'Ground Current' and the 'Sensi-
tive Ground Current' inputs. Before making ground connections, consider that the relay automatically calcu-
lates 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, H10) and three possible sen-
sitive ground connections using the sensitive ground current input (terminals G3, H3).
The ground input (terminals G10, 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 Ground CT Primary setpoint
to a value equal to the Phase CT Primary setpoint.
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 setpoint should be programmed to a value
equal to the Phase CT Primary setpoint. The sensitive ground current input can be connected 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) will use the G3, H3 terminals as the polarizing input. Terminals G10, H10 are
used for the Ground input. These connections will be shown on the terminal assignment label on the
NOTE
back of the relay's case.
The Sensitive Ground input (G3, H3 terminals) must only be used on systems where the maximum
ground current does not exceed 500 A.
NOTE
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Courtesy of NationalSwitchgear.com

3.2.4 PHASE SEQUENCE AND TRANSFORMER POLARITY

3.2.5 AC CURRENT TRANSFORMER INPUTS

3.2.6 GROUND AND SENSITIVE GROUND CT INPUTS

750/760 Feeder Management Relay
3 INSTALLATION
GE Power Management