DET-882
SecoGear Medium-voltage Switchgear Application and Technical Guide
System and Equipment Protection
©2017 General Electric All Rights Reserved
29
Ground Overcurrent Protection
Ground-overcurrent protection is provided by either time-
overcurrent or instantaneous overcurrent relays. To
minimize damage to circuit equipment and circuit
conductors, sensitive ground-fault protection is desirable.
The three most commonly used connections for ground-
overcurrent relays are the residual connection (51N), the
ground-sensor (balanced-flux or zero-sequence)
connection (50GS or 51GS), and the neutral current
transformer connection (51G).
Residually connected ground-overcurrent relays (51N) are
wired in the ground (neutral)-return current transformer
lead of three current transformers connected in wye. The
relay detects the current of a ground fault by measuring
the current remaining in the secondary of the three phases
of the circuit as transformed by the current transformers.
The minimum pickup of the relay is determined by the
current transformer ratio. On systems with line-to-neutral
connected loads, the ground-overcurrent relay (51N)
pickup must be set above any expected maximum single-
phase unbalanced load. If an instantaneous ground-
overcurrent element (50N) is used, it must be set above any
expected unbalance due to unequal current transformer
saturation on phase faults or transformer inrush currents.
Residually connected ground-overcurrent relays are
usually applied on solidly grounded systems.
Ground-sensor (GSCT) relaying schemes use an
instantaneous (50GS) or time-delay (51GS) overcurrent
relay or relay element connected to the secondary of a
window-type current transformer through which all load
current-carrying conductors pass. The relay detects the
ground current directly from this current transformer,
provided the equipment ground conductor and cable
shielding bypass the current transformer. Ground faults
15 A (or less) in the primary circuit can be detected with
this scheme. Ground-sensor relaying schemes are usually
applied on low-resistance or solidly grounded systems.
Neutral ground relaying typically uses a time-delay
overcurrent relay (51G) connected in the secondary of the
current transformer, located in the neutral of a wye-
connected transformer, wye-connected generator, or the
neutral of a neutral-deriving transformer bank.
Some systems are designed with no intentional grounds.
To detect the first ground on this type of system, a
sensitive directional ground overcurrent device may be
employed. Optionally, a set of potential transformers wired
wye-wye or wye-broken delta with indicating lights or
voltmeters can be used to indicate the presence of a
ground fault on an otherwise ungrounded system.
Incoming Lines
Incoming line ground-over-current relay protection
consists of either a residually connected relay (51N) or a
relay (51G) connected to a current transformer in the
transformer neutral ground connection. Ground-sensor
relaying (51GS) on incoming lines is not recommended
because of the size, number, and construction of the
incoming line conductors.
For solidly grounded systems with source transformers
located remote from the switchgear, residually connected
ground-overcurrent relays (without instantaneous) are
most often applied. Some utility users omit all incoming line
ground relays on solidly grounded systems and rely on
three phase-overcurrent relays to provide complete phase-
and ground-fault protection.
For impedance or resistance grounded systems with local
source transformers, a ground relay (51G) connected to a
current transformer in the transformer neutral connection
is most applicable. A typical current transformer ratio for
the neutral current transformer is one-half to one-quarter
the maximum ground-fault current (e.g., a 200:5 CT ratio is
appropriate for the neutral CT in series with a 400 A, 10 s
neutral grounding resistor). This ratio permits sensitive
settings of the ground relay and selective operation with
downstream ground-sensor relays. The ground relay is the
system backup relay for the medium voltage system. It
also provides ground-fault protection for the transformer
and its secondary conductors. If a transformer primary
circuit breaker is used, the secondary ground-overcurrent
relay (51G) in the transformer neutral connection should
trip both the transformer primary and secondary circuit
breaker.
Feeders
Ground-sensor (zero-sequence) relay arrangements use
instantaneous-overcurrent relays (50GS) or time-
overcurrent relays (51GS) and are appropriate for both
resistance grounded and solidly grounded systems. These
arrangements provide sensitive ground-fault protection for
both branch circuits and feeder-distribution circuits.
Good selectivity can be obtained for a distribution system
incorporating this type of relaying on all branch and feeder
distribution circuits; however, a feeder breaker with
ground-sensor relaying usually cannot be made selective
with downstream feeders using residual ground relaying.
In addition, ground-sensor relaying is not applicable to
circuits with metal-enclosed conductors (non-seg Bus Duct)
because of the impracticability of passing the phase
conductors through a single current transformer.
Ground-sensor relaying is rarely applied to circuits
terminated with potheads because of the special mounting
installation procedures required.
