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Harmonic
Filter Bank
Protection
Introduction
The
protection of a harmonic filter is different than that
of a shunt capacitor bank. Typically shunt capacitor
banks are protected for case rupture by expulsion or
current limiting fuses. For metal-enclosed capacitor
banks, full-range current limiting fuses are used since
they expel no gas during interruption and they limit
fault damage. In addition to case rupture protection,
ungrounded capacitor banks are typically protected by
a sensitive unbalance protection system that protects
the remaining good capacitors from overvoltages (due
to neutral voltage shift) that result when a capacitor
fuse blows. The above capacitor protection concerns
also apply to harmonic filters, but filters have additional
overcurrent protection concerns for the tuning reactor(s).
The following discussion provides background information
on harmonic filter bank protection
The
figure below shows a comprehensive harmonic filter bank
protection system. The figure shows a filter bank feeder
breaker connected to an ungrounded-wye connected harmonic
filter bank. Several different relays are shown with
their ANSI number designation. Current transformers
provide the current signal required for these relays.
A main disconnect switch and main fuses are shown just
ahead of a set of vacuum switches that are used to automatically
switch the bank on and off. In some cases the feeder
breaker may form an integral part of the metal-enclosed
harmonic filter bank. In which case, the main-fuses
and disconnect switch may become unnecessary.
Figure
1 -
Typical harmonic filter
protection system
The
following points can be made in regards to figure 1.
The
protection of a filter bank can be divided into two
parts, capacitor protection, and reactor protection.
Reactor
Protection
The tuning reactor should be protected
for both faults and overloads. Overloads may occur for
any of the following reasons:
The
overload protection is best achieved by a thermal overcurrent
relay (Device 49) mounted in each phase of the filter
bank. This relay is sensitive to RMS current and ambient
temperature and can be set to trip at RMS current values
that will cause damage to the reactor. This sensitive
protection is not achievable with fuses and is more
sensitive than a regular 51 relay. The thermal relay(s)
should be wired to trip the filter bank by either the
internal filter bank vacuum switches switches (for multi-stage
banks with independent stage protection) or the upstream
circuit breaker.
In
addition to overload protection, the reactors should
also be protected for phase and ground faults. Protection
against phase faults can be achieved with current limiting
fuses on the main disconnect switch or by 50/51 relays
on the filter bank supply. If relays are to be used,
they should be wired to trip the feeder breaker or the
next upstream device since most filter bank capacitor
switches (either oil switches, vacuum breakers, and
vacuum contactors) are not rated to interrupt fault
currents.
If
the reactor is iron-core (enclosed filter banks usually
are), consideration should be given to installing the
current limiting fuses on the main disconnect even if
a feeder breaker or integral breaker exist. The primary
concern is the bracing of the iron-core reactor. During
a load side reactor fault, full voltage is applied across
the iron-core reactor, and it saturates to its low,
air-core reactance. If this saturation affect is not
accounted for in the original design (usually it is
not due to cost), current limiting fuses should be considered.
Ground
fault protection is not required on solidly grounded
systems since they can be detected by the phase fault
protective devices. For resistive grounded systems,
however, a 50/51N or 50GS device is usually required.
These devices can detect low level ground faults and
can be wired to trip either the upstream breaker or
the filter bank switches if the resistor rating is low
enough.
Capacitor protection
The filter capacitors should be
protected against case rupture (due to internal capacitor
faults) and overvoltages from blown capacitor fuses.
In addition, the filter bank capacitors should be taken
off line if one of the main disconnect fuses blow. This
protection function is achieved with single ungrounded
wye-connected capacitor banks with neutral unbalance
protection systems. The figure below shows the details
of a typical neutral unbalance protection system. The
figure shows "individual capacitor fusing"
(versus "group fusing") and an unbalance detection
system.
"Individual
capacitor fusing" is almost always necessary in
ungrounded filter banks since a failed capacitor only
draws three times the banks rating. In addition, high
rated full-range current limiting fuses are expensive.
The fuses should have a voltage rating equal to the
line-line voltage unless they are tested for voltages
that exceed their normal rating. These capacitor fuses
should be coordinated with the upstream disconnect fuses,
thermal relays, and over current relays.
Figure
2 -
Unbalance protection system by
neutral voltage sensing
The
unbalance protection system consists of a neutral voltage
sensing device as shown in figure 2. This device is
rated for full line potential since it may be subjected
to full line-voltage during abnormal system conditions
or when capacitor fuses open. The neutral voltage sensor
has a very high impedance since it is desirable to keep
the filter bank ungrounded. The relay (device 59) is
normally set to trip the bank off-line for neutral voltages
(resulting from blown capacitor fuses) that can cause
damage to the remaining healthy capacitors due to detuning
or neutral voltage shift. NEPSI's standard protection
system has two tripping points. The "level 1"
trip will alarm plant personnel of a blown fuse while
the "level 2" trip will trip either the filter
bank switches or the upstream breaker. It is important
that this relay be coordinate with the upstream ground
fault relays so that it does not trip during upstream
ground faults.
In
addition to the above protection system, it may be desirable
to have overvoltage protection on the filter bank if
the electrical system is not equipped with one. The
main concern here is over voltages that can occur during
light load and ground fault conditions.
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
Copyright © 1999
- 2009 Northeast Power Systems, Inc.
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