|
Why
Your Capacitor Bank Should be Left Ungrounded
Introduction
Should
medium voltage capacitor banks on industrial and commercial
power systems be grounded? This question often arises,
and the answer is usually no for the following reasons:
-
Grounded capacitor banks can interfere with a facilities
ground fault protection system and cause the entire
facility to lose power (main breaker trip).
- Harmonic
currents in the ground path can cause harmonic interference
with control and communication systems.
- Capacitor
discharge currents may damage nearby surge arresters.
Interference
with a facilities ground fault protection system is
the primary reason for not grounding a capacitor bank
or harmonic filter bank. Although this interference
can be reduced or eliminated through system modification,
it may require protective coordination analysis, relay
changes and/or grounding resistor changes. This adds
cost and complexity to the installation and may degrade
the sensitivity of the existing ground fault protection
system. This bulletin describes how a grounded capacitor
bank can interfere with a facilities ground fault protection
system and suggest that all banks applied on industrial
and commercial power systems be left ungrounded.
Figure
1
- Typical industrial facility showing ground fault
protection system, failed capacitor, and fault current
path for a failed capacitor.
Background
In
general, most industrial facilities with 2.4kV through
13.8 kV distribution are resistive grounded through
a grounding resistor as shown in Figure 1. The systems
are resistive grounded to reduce the fault current and
arc damage during line to ground faults. This is particularly
true for systems with directly connected motors. The
resistor typically has a 10 second ampere rating (continuous
and 60 second ratings are also available, but are not
typical) that is approximately equal to the ground current
that would flow if a line to ground fault should occur.
The continuous current rating of a 10 second resistor
is much less than the 10 second rating. For example,
a 200 amp 10 second resistor may have a continuous rating
of 50 amps. For this reason, the ground fault relay,
device 51G, is typically set to pick up at approximately
10% of the resistor rating or 20 amps. This protects
the resistor from un-cleared feeder faults and other
system abnormalities that may damage the resistor.
Figure
2
- Time current coordination plot of system shown in
Figure 1
Resistive
Grounded
Systems
Figure
1 is used to illustrate how a grounded capacitor bank
can interfere with the ground fault protection system
of a resistive grounded system. The main concern arises
when a capacitor fails as shown in Figure 1 by the "X".
Since medium voltage capacitors fail shorted, a faulted
capacitor is like applying a line to ground fault on
the facilities power system. Since the system is grounded
through a 200 amp resistor, approximately 200 amps of
fault current will flow though the resistor. This 200
amps of current in the ground resistor will be detected
by the ground fault relay (51G) and will cause it to
pickup since its pickup is set at approximately 10%
of the resistor rating. This will cause the main breaker
of the facility to trip, causing a plant shut down and
possibly a major economic loss.
One
might think the 40 amp current limiting capacitor fuse
would blow before the main breaker, but this is not
the case as shown in Figure 2. In fact, a resistor on
the order of 500 amps would be required before coordination
would be achieved between the 40 amp capacitor fuse
and the facilities main ground fault protection relay.
For larger capacitors with larger fuses, the limit of
coordination (or resistor size) would be even higher,
on the order of 1400 amps for a 500 kvar capacitor.
To
make the situation worse, the cause of the main breaker
tripping would not be known unless the bank is equipped
with a fast enough unbalance detection scheme (main
breaker trips and clears before the capacitor fuse blows)
that has a mechanical flag or latching contact. Upon
re-energization of the system, the main breaker would
trip again. This would repeat itself until the capacitor
fuse blew.
Even
after the capacitor fuse blows, there is still a strong
possibility that the system could not be brought on-line.
This would be due to the unbalance on the capacitor
bank. For the system in figure one, the unbalance current
flow for a blown fuse would be approximately 26 amps,
enough to trip the main ground fault relay.
If
the bank is equipped with an unbalance detection scheme,
the cause of the main breaker tripping may be discovered,
but there may still be a problem of determining which
capacitor has failed. This is because the main breaker
trips before the fuse is allowed to melt and clear (see
Figure 2). Therefore, blown fuse indicators would not
be actuated, and an ohm meter or capacitance tester
would be necessary. This would be a time and cost consuming
task.
Solidly
Grounded
Systems
Although most commercial and industrial power systems
are resistive grounded, there are situations where they
may be solidly grounded. This may be due to the design
engineers preference or because the facility is directly
connected to the utility distribution system. The same
possibility as describe above exist for systems that
are connected to the utility through a solidly grounded
step down transformer, but is less likely because the
ground fault currents are much higher, and the current
limiting fuse is much quicker at the higher current
levels. This can be seen in Figure 2. Typically ground
faults would be on the order of 7000 to 20,000 amps
on these systems, and coordination could be achieved.
Coordination, however should be checked, since a plant
shut down can be quite costly.
Systems
Connected Directly To A
Utility Distribution System
Facilities that are fed directly from their utilities
distribution system (no step down transformer) as shown
in Figure 3 are grounded in accordance with the utilities
grounding practices. Typically this would be a multi-grounded
4-Wire system.
Figure
3
- Ground fault current path when industrial is connected
directly to utility distribution feeder.
The
facility may or may not have ground fault relays on
the service entrance conductors. If relays are present,
the ground fault relay would be a residually connected
overcurrent relay or a 51N device. These relays are
typically not set as low as the 51G device due to error
currents in the phase relays. Facilities strive for
minimum allowable pickup settings, and therefore coordination
should still be checked as was done in Figure 2.
For
utilities that utilize a uni-grounded system or an ungrounded
system, the utility should be consulted to determine
if the capacitor bank grounding will interfere with
their system. Ground relays on Uni-grounded utility
distribution system may be set low enough to pickup.
Ungrounded system raise an unrelated concern with overvoltages
from uncleared ground faults.
Conclusion
With the forgoing discussion it
should be clear that the installation of grounded capacitor
banks to industrial and commercial power system can
interfere with a facilities ground fault protection
system. Proper engineering may be done to overcome these
problems, but this adds to the cost and complexity of
the installation.
The
negative issue associated with ungrounded capacitor
banks are as follows:
•
Fuse coordination is difficult on smaller banks because
a failed capacitor will only draw three times the banks
nominal current rating.
•
Overvoltage conditions can occur on the unfaulted capacitors
on unbalance banks due to a fuse blowing. This condition
can be eliminated by changing the number of capacitors
, isolating neutral connections, and/or by adding a
neutral unbalance detection scheme to the bank.
The
above issues can be engineered out of the design by
the manufacturer without any regard to the system for
which it is being applied to. This puts the responsibility
with the manufacturer and not the facilities engineer
or design engineer working on the capacitor bank installation.
NEPSI’s
bulletin on Neutral Voltage Unbalance
Protection provides further information on the above
material.
Northeast
Power Systems, Inc.
66
Carey Road
Queensbury, New York 12804
Phone: 518-792-4776
Fax: 518-792-5767
E-mail:
sales@nepsi.com
Website:
www.nepsi.com
|
