IEEE standard C57.13.3 serves as the
ANSI guide to standardize instrument transformer grounding practices.
The grounding of CTs is important to both safety and the correct
operation of protective relays.
To assure safe and reliable operation,
the neutral of the CT secondary should have a single ground location
for each circuit. The single ground is irrespective of the number of
CTs or the chosen grounding location.
Utilizing a single ground eliminates
the risk of redundant ground loops and associated problems.
During normal operation more than one
ground on a CT circuit is not an obvious problem, other than the
difficulties it may cause during testing. However, during a fault
condition, multiple grounds allow a different ground potential rise
for each CT.
The result is a significant current
flow through the CT circuit that is not representative of the primary
current. This ground loop typically creates a potential across the
operating coil of the differential relay, causing the relay to pick
up as though a fault exists in the relay’s protective zone.
Tripping a differential relay due to a
fault external to the zone of protection is one of the more frequent
nuisance trips. These nuisance trips may not only shut down the load
but may require a maintenance crew to spend days testing to determine
that no real problem exists in the differential zone.
Further, the actual problem may go
undiscovered until the system is re-energized into the original
fault.
To demonstrate what happens with a
second ground on the CT circuit, refer to Figure 1. Figure 1 shows a
typical differential relay with two CTs per phase. The recommended
method of grounding is to install a single ground point at the first
point of application (switchboard or relay panel) of the CT secondary
circuit.
In the case of a fault internal to the
protective zone, the voltage developed by the CTs is of the same
polarity. The magnitude of voltage drop across the operating coil is
sufficient to operate the relay.
A
second ground is on a CT mounted near where a ground fault occurs. If
the fault creates a ground potential rise of 100 volts, the
protective relay will experience sufficient voltage across the
operating coil to cause the relay to nuisance trip even though the
fault was outside the fault zone
.
Just as with any other event, there is
an exception to this standard. Many of the new multifunction relays
(ABB, Schweitzer, GE/Multilin, and Basler) are designed to connect
all CTs coming into the relay in a wye connection. Each wye has to be
grounded.
The most desirable way to do this is to
bus the wye points together at the relay panel and have a single
conductor to ground to make certain the relay has but one ground
potential.
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