C Urrent I Nputs - GE 750 Instruction Manual

CHAPTER 3: INSTALLATION
3.2.1 Phase
Sequence and
Transformer
Polarity
3.2.2 Current Inputs
3.2.3 Ground and
Sensitive
Ground CT
Inputs
Note
NOTE
750 FEEDER MANAGEMENT RELAY – INSTRUCTION MANUAL
For the correct operation of many relay features, the instrument transformer polarities
shown above in the Typical Wiring Diagram on page 3–10 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transformer and an automatic shorting mechanism that acts when the relay is withdrawn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.
CT PRIMARY
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
setpoint should be programmed to a value equal to the
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) 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.
setpoint to a value equal to the
GROUND CT PRIMARY
ELECTRICAL INSTALLATION
PHASE
SENSTV GND CT
PHASE CT PRIMARY
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