PARALLELLING MEDIUM VOLTAGE VACUUM BREAKERS
– A HIGHER CONTINUOUS CURRENT SOLUTION
James Benke
Chand Tailor
Eaton Corporation
Eaton Corporation
June 2006
ABSTRACT
For systems that require continuous current
ratings higher than the standard ANSI ratings,
three possible solutions are available for
consideration. (1) Cooling fans can be added to
circuit breaker cell to allow increased current
through the breaker (2) Two or more standard
circuit breakers can be connected in parallel in
an appropriate switchgear assembly (3) Two or
more circuit breakers can be connected in
parallel inside an appropriate fan-cooled
switchgear assembly. These methods of forcedair cooling with additional fans or paralleling
circuit breakers are not new in the industry.
Today’s smaller, lighter circuit breakers and
modern design practices, however, have refined
the art of building switchgear assemblies to
make it easier and more economical to connect
circuit breakers in parallel. Regardless of which
circuit breaker method is selected to meet the
required continuous current requirement, special
attention must be given to a number of
switchgear assembly design areas.
INTRODUCTION
The focus of this paper will be on the technique
of achieving higher continuous current ratings
by connecting medium voltage vacuum circuit
breakers in parallel. Depending upon the
continuous current level desired, it may or may
not be necessary to utilize additional fan
cooling. For example, two standard 3000 A
circuit breakers applied in a properly designed
parallel connection can meet a 5000 A
continuous current requirement without
additional fan cooling. The same
configuration can be provided with fan-cooling
to enable it to carry continuous currents up to
6300 A. Some common parallel switchgear
connection schemes include:
a) Two 3000 A circuit breakers rated 5000 A,
without fan cooling
b) Two 3000 A circuit breakers rated 6300 A,
with fan cooling.
c) Three 3000 A circuit breakers rated 6300 A,
without fan cooling
d) Two 4000 A circuit breakers rated 6300 A,
without fan cooling
e) Two 4000 A circuit breakers rated 8000 A,
with fan cooling
f) Three 4000 A circuit breakers rated 8000 A,
without fan cooling
NOTICE: Each manufacturer must test his
design to prove the performance meets the
rating assigned.
Customers have discovered that parallel
connection schemes can be viable solutions to
higher continuous current requirements.
Although this subject can be complicated from a
design and testing standpoint, this paper
addresses many of the basic factors to consider
when looking for solutions for applications
which require continuous current carrying
capabilities which are higher than the preferred
ratings specified in C37.06-2000. Standards do,
allow for just such solutions when designs are
proven through appropriate testing in
accordance with applicable standards.
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COOLING FANS ADDED TO CIRCUIT
BREAEKR COMPARTMENT
Let’s first consider a standard 3000 A vacuum
circuit breaker. With cooling fans this circuit
breaker can easily carry 4000 A without
overheating (Figure 1). The standard 36-inch
[914.4 mm] wide switchgear assembly units
accommodate the necessary airflow
requirements to enable the equipment to carry
the 4000 A current without exceeding the
temperature limits specified in the standards.
All necessary control equipment is included to
start the fans when the temperature or current
indicate the fans are needed, and to operate
them until the temperature or current indicate
the fans are not needed. Alarm circuitry is
provided to give warning if the flow of cooling
air becomes insufficient or is lost entirely. If
required by the customer, a redundant fan
system can also be provided.
Figure 1 – Typical Switchgear Assembly Unit
With 3000A VCP-W Circuit Breaker Fan
Cooled to Allow 4000A Continuous
PARALLELING CIRCUIT BREAKERS
A second method for providing higher
continuous ratings is to connect circuit breakers
in a parallel switchgear arrangement. This
approach may or may not also utilize a fancooled configuration as just discussed. It
depends upon the desired continuous current
rating. When circuit breakers are connected in
parallel, it is important that circuit breakers with
identical ratings be used in the paralleling
scheme. It should be recognized that the
interrupting rating of the individual circuit
breaker used in the scheme is neither increased
nor decreased. Therefore, interrupting rating of
the circuit breaker(s) should be selected based
on maximum fault current that it will be
required to interrupt. With identical breakers,
precise timing during the closing or opening
operations is not critical. The circuit breaker
that closes first must be able to carry either fault
current or the continuous current for the short
period of time until the other circuit breaker
closes. Similarly, the last circuit breaker to
open is capable of interrupting the full fault
current. It is important that the closing circuit
be set up to ensure automatic closing of the
second breaker within a minute following the
closing of the first breaker unless the first circuit
breaker has closed onto a fault, of course, in this
situation, the second CB shall not close. An
incomplete sequence control circuit must also be
included in the design that would open the first
breaker if the second breaker did not close
within pre-set time limit, to prevent overheating
of the first breaker from carrying increased
circuit current. Additional electrical interlocks
should be provided to ensure that both circuit
breakers are racked-in and fully connected in
the normal operating position within their
respective compartments before either circuit
breaker can be closed. Further, provision should
be made to ensure that if one circuit breaker
opens for any reason, the second breaker is
automatically opened within a few seconds.
Addition of visual or audible alarms should also
be considered to indicate when either circuit
breaker is withdrawn from its normal operating
position.
Adherence to this philosophy of circuit breaker
selection and control reduces safety and
reliability concerns. A parallel approach does
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require that steps be taken to balance the
primary current flowing through each circuit
breaker. Over-current protection should be
provided for individual circuit breakers as well
as for the current in the entire circuit. Main bus
construction must also be configured to ensure
balanced impedances in each phase of the
circuit. Current transformers connected in
series may be required to maintain balanced the
primary current through each parallel circuit.
Finally, the design must be verified by
continuous current thermal tests. An example of
such proven design with all of the above
consideration is given in Figure 2
interrupted the current. Typical overall
interrupting times can be in range of 69 to 117
milliseconds when using parallel breakers. The
increase in the overall interrupting time should
be taken into consideration when devising a
specific relaying and protection scheme.
CONCLUSIONS
Regardless of which solution is chosen for
increasing the continuous current rating, special
attention must be given to: (1) selection of
circuit breakers with required interrupting
capability (2) design of electrical controls and
protection scheme (3) design of switchgear
assembly, including safety interlocks and
primary circuit arrangement and layout for
proper impedance balance (4) documented
testing, and (5) economics.
Connecting circuit breakers in parallel is not a
new concept and has become an acceptable
practice for some customers. When a circuit
breaker with the preferred continuous current
ratings does not meet the continuous current
requirement for an intended application,
solutions may be available in the form of fan
cooling, paralleling or an appropriate
combination these methods.
AUTHORS
Figure 2 – Switchgear Assembly Rated to
Carry 5000 A Continuous Current, by
Connecting two 3000 A Circuit Breakers in
Parallel
ADDITIONAL CONSIERATIONS
In the parallel circuit configuration, opening the
first circuit breaker commutates the current into
the second circuit breaker. Then the second
circuit breaker opens to interrupt the current.
Therefore, it should be noted that the overall
interrupting time from initiation of trip signal to
first breaker until the second breaker interrupts
the current is longer than if a single breaker
James J. Benke is an Engineering Manager at
the Eaton Technical Center in Pittsburgh, PA.
Jim has more than 12 years of experience in the
design, analysis and testing of medium-voltage
circuit breakers. He has a Mechanical
Engineering degree from University of
Pittsburgh and an MBA from the University of
Phoenix.
Chand Tailor is a Product Applications
Specialist at the Eaton Medium Voltage
Assembly plant in Greenwood, SC. Chand has
more than 24 years of experience in customer
order engineering, marketing, and applications
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of medium voltage switchgear. He has a
Bachelor of Engineering degree in Electrical
from M S University, Baroda, India, and a
Masters of Science in Electrical Engineering
from University of Pittsburgh, PA. He is also a
registered Professional Engineer in State of
South Carolina.
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