GE AKD-10 Application Manual page 31

Table 30.1 Operation with alarm relay
Option Operational description
An overvoltage relay coil Operation with the alarm relay is the same as
rating of 199 to 208V, pickup described in Table 29.1, although the connections
range of 16-64V or 70-140V. are different. Assuming rated system voltage on
the potential transformers' primary, the three
Potential transformers with secondary voltages add up vectorially to zero.
broken delta connected Thus, there is normally no voltage on the relay. If
secondaries. one phase of the system becomes grounded, the
potential transformer on the grounded phase
Note that either of the above would be short-circuited and the voltages on the
mentioned indicating lights other two transformers would rise to full phase-
or voltmeters can be used to-phase voltage. The secondary voltages would
as ground indicators with also rise to the phase-to-phase values (120 volts).
this option. Because these two voltages are in series at an
angle of 60 degrees under ground fault conditions,
the voltage imposed on the relay is three times the
voltage on each potential transformer secondary
under normal conditions (208 volts).
General note
A combination of ground indication and metering or relaying on the same set of
potential transformers is not recommended. Metering not only may require dif-
ferent primary and/or secondary connections; it also increases the probability of
faults in the secondary circuits with consequent false indications of grounds on
the primary system.
Table 30.2 Operation with test switch
Option Operational description
Test switch. (For either lamp The lamp test feature is performed using the
test or test-for-ground.) normally closed contact of the test switch. The
test-for-ground feature is performed using the
normally open contact. The user must specify
which test feature is to be furnished.
Ground fault protection on solidly grounded systems
The preferred method of providing ground fault protection
™
on WavePro power circuit breakers is using the ground
fault function on the Power+
™
MicroVersaTrip PM trip unit. This is referred to as integral
ground fault protection and requires no external relaying or
control power. Integral ground fault is applicable to 3-phase,
3-wire or 4-wire systems with single or multiple sources.
When multiple source systems are encountered, each
source can be grounded upstream of the main secondary
breakers in accordance with the NEC, eliminating the need
for complex ground and neutral bus systems required for
single-point grounding of the source neutrals.
Ground fault protection for large power systems can be very
complex when there are multiple line-ups, each with multiple
sources and tie breakers that interconnect the switchgear
line-ups. Some of the complexity can be eliminated if the
power system is designed as a 3-phase, 3-wire grounded wye
system. This removes the interconnected neutral between
switchgear line-ups and, as a result, eliminates the need for
the interconnections between neutral sensors on the source
and tie breakers. In many cases, there are few actual 4-wire
loads and these can be served by small delta-wye lighting
transformers located close to the 4-wire distribution panel.
Special consideration must be given to power systems having
continuously paralleled sources or operating as networks. In
some of these cases, ground fault protection is best accom-
plished by using the Ground Break system. This system
consists of current sensors for each phase and/or neutral
conductors, a relay with separate current pick-up and time
delay settings, and a fault indicator/reset device. As an option,
the fault indicator/reset device can be replaced by a Monitor
Panel. In addition to providing the fault indication and reset
functions, the Monitor Panel provides a feature that allows
tripping or no tripping of the circuit breaker(s) in the ground
fault scheme during testing. The Ground Break system requires
a control power source and shunt trips on the circuit breakers.
™ ™
, MicroVersaTrip Plus or