Download Application of 60 Hz rated medium voltage vacuum circuit

White Paper WP131002EN
Effective March 2015 
Application of 60 Hz rated medium
voltage vacuum circuit breaker at 50 Hz
Introduction
Continuous current ratings
Although ANSI rated medium voltage vacuum
circuit breakers are typically applied to voltages
and currents with a frequency of 60 Hz, the
growing global market increasingly requires circuit
breakers to operate reliably in 50 Hz applications
as well. Figure 1 illustrates how the voltage and
current waves differ for the two frequencies over
time (milliseconds).
One of the most important factors that influence
the rated continuous current is the resistance
of the current carrying conductors. As effective
resistance increases, the heat produced by the
current also increases. Similarly, decreasing
the effective resistance of the current carrying
conductors reduces the heat produced.
For a given amount of time, circuit breakers in
a 60 Hz application will experience more frequent
zero crossings than will circuit breakers in a
50 Hz application.
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Figure 1. Frequency comparison
60 Hz
For a circuit breaker of a specific continuous
current rating, the rated value was primarily
established by measuring the temperature rise of
the current carrying components with the rated
current applied. The stabilized temperature must
have been less than or equal to the maximum
values stated in the relevant test standard.
The material properties of the current carrying
conductors determine the resistance in this
testing. Obviously, the composition of the material
is important, but additionally the surface area of
the current carrying components contributes to
a factor known as the Skin Effect. The electrons
of the continuous current, being of the same
electromagnetic charge, tend to repel each other.
This causes the distribution of the current passing
through the conductors to be concentrated more at
the outer edges (or “skin”) of the conductors than
at the middle of the conductors.
As the frequency of the current increases, the
resistance due to the Skin Effect increases. By
operating a circuit breaker rated for 60 Hz at
50 Hz, the reduced Skin Effect resistance allows
the circuit breaker to run slightly cooler. Therefore,
continuous current performance for a 60 Hz circuit
breaker will be better when operating the circuit
breaker at 50 Hz.
Short circuit ratings
As can be seen in Figure 1, the amount of time
between current zero events is longer for 50 Hz
than it is for 60 Hz. Additionally, the rate of change
of the current is higher for 60 Hz than it is for
50 Hz. These factors must be considered when
evaluating short circuit performances.
The maximum arcing time that is possible during
a short circuit fault at 50 Hz may exceed the 60
Hz tested values by up to 1.7 milliseconds. As a
result, before applying a circuit breaker that has
been rated for 60 Hz into a 50 Hz application,
consideration should be given to the equipment
being protected to ensure that the potential
additional 1.7 milliseconds of arcing time will
not result in unacceptable increased equipment
damage. In most practical applications, this
increase in time is inconsequential.
White Paper WP131002EN
Application of 60 Hz rated medium
voltage vacuum circuit breaker at 50 Hz

Effective March 2015
The vacuum interrupters used on Eaton medium voltage circuit
breakers have extremely good performance characteristics that
are not significantly affected by the rate of change of the current
being interrupted. The IEEET standard for testing of medium voltage
circuit breakers (C37.09-1999) allows testing at 60 Hz to be used
to demonstrate the performance at 50 Hz for circuit breakers not
significantly affected by the rate of change of the current.
Short time withstand ratings
The short time withstand rating of a circuit breaker verifies that
the structure of the circuit breaker is capable of withstanding the
magnetic forces associated with a high current over a 2-second
time span when the components heat up. As previously discussed,
the heat generated from 50 Hz current is slightly lower than the
heat generated at 60 Hz. The magnetic forces are proportional to
the RMS current, which does not change for different frequencies.
Therefore, the short time withstand performance would be equal
to or better at 50 Hz than at the rated 60 Hz.
Dielectric ratings
Power frequency withstand voltage ratings are based on a 1-minute
test, with an allowable variation in the power frequency of ±20%.
Therefore, testing at either 60 Hz or 50 Hz demonstrates the
performance of the circuit breaker at both frequencies.
Full wave lightning impulse withstand voltage ratings are based on
a specific voltage waveform, and therefore are independent of the
power frequency of the circuit breaker.
Mechanical endurance ratings
Mechanical endurance ratings are independent of the power
frequency of the circuit breaker. However, charging motors could
operate slightly slower at 50 Hz control voltage than they operate at
60 Hz. Eaton’s charging motors on medium voltage vacuum circuit
breakers are designed to charge the circuit breaker in less than
7 seconds at 60 Hz nominal voltage. Operation at 50 Hz control
voltage may result in a charging time increase of up to 1.4 seconds,
which will have no effect at all on circuit breaker endurance. Charging
time will remain well within the allotted time of the IEEE standard
(15 seconds).
Capacitive current switching ratings
Tests demonstrating capacitance current switching capabilities
of circuit breakers made at the rated power frequency of 60 Hz
+5% may be considered to prove the breaking characteristics at
50 Hz. If tests are made outside this frequency range (for example,
50 Hz) these may be considered to prove the characteristics
at 60 Hz provided that the instantaneous recovery voltage across
the current interrupting contacts of the circuit breaker, during the
first 8.33 milliseconds, is not less than that which would occur
for a 60 Hz test.
Summary
For most applications, using a 60 Hz rated Eaton medium voltage
vacuum circuit breaker on a 50 Hz circuit is acceptable. Some
analysis of the short circuit requirements of the system are
recommended, because arcing time could potentially be slightly
longer. If the rated short circuit performance of the circuit breaker
is very close to the maximum possible short circuit condition of
the circuit, i.e., if there is very little margin between the circuit
requirements and the circuit breaker ratings, then applicationspecific high-power laboratory testing is recommended.
References
C37.04-1999—IEEE Standard Rating Structure for AC High-Voltage
Circuit Breakers, Institute of Electrical and Electronics Engineers,
3 Park Avenue, New York, NY 10016-5997, USA.
C37.09-1999—IEEE Standard Test Procedure for AC High-Voltage
Circuit Breakers Rated on a Symmetrical Current Basis, Institute of
Electrical and Electronics Engineers, 3 Park Avenue, New York, NY
10016-5997, USA.
C37.013-1997—IEEE Standard for AC High-Voltage Generator
Circuit Breakers Rated on a Symmetrical Current Basis, Institute of
Electrical and Electronics Engineers, 3 Park Avenue, New York, NY
10016-5997, USA.
C37.013a-2007—IEEE Standard for AC High-Voltage Generator Circuit
Breakers Rated on a Symmetrical Current Basis Amendment 1:
Supplement for Use with Generators Rated 10–100 MVA, Institute
of Electrical and Electronics Engineers, 3 Park Avenue, New York, NY
10016-5997, USA.
62271-100—IEC International Standard for high-voltage switchgear
and controlgear—alternating-current circuit breakers.
Authors
Anthony T. Ricciuti is a Specialist Engineer at Eaton in Pittsburgh, PA.
Anthony has a bachelor’s degree in Mechanical Engineering from the
University of Pittsburgh, and has more than 20 years of experience
in the design, testing, and analysis of circuit breakers.
Brad Leccia is an Engineering Manager at Eaton in Pittsburgh,
PA. Brad has a bachelor’s degree in Mechanical Engineering from
Pennsylvania State University, and has more than 20 years of
experience in the design, testing, and analysis of circuit breakers.
About Eaton
Eaton is a power management company with 2014 sales of
$22.6 billion. Eaton provides energy-efficient solutions that
help our customers effectively manage electrical, hydraulic and
mechanical power more efficiently, safely and sustainably. Eaton has
approximately 100,000 employees and sells products to customers
in more than 175 countries. For more information, visit Eaton.com.
Eaton
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Cleveland, OH 44122
United States
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Publication No. WP131002EN / Z16335
March 2015
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