Page 32 - Energize February 2021

TECHNICAL

Medium voltage circuit breaker

condition assessment

Information from Omicron

Modern industry cannot operate without electricity, and circuit breakers are indispensable in any

electrical power system. They are the only pieces of equipment which can switch under normal load,
as well as under fault conditions and must be able to reliably disconnect a faulty section from the
power network as quickly as possible.

ircuit breakers must be able for the opening and the closing operation to be stored. To release the energy stored in
to meet this requirement even the springs, two coils are needed to control the springs remotely. The opening spring is
after they have been in- charged during the closing operation of the breaker, and the closing spring is charged by
Cservice for a long time. Their a motor.
reliability is essential for the prevention
of consequential power system failures Testing of medium voltage circuit breakers
and the safety of the electrical power The following is a brief overview of the most important medium voltage circuit breaker
supply. Typical tests include measurement methods.
measurements of operating times and
the static contact resistance. Off-service measurement methods
A more extensive condition • Timing: Timing measurements according to the IEC 62271-100 standard are used to
assessment is possible when further determine operation time and belong to the most common tests. The timing test uses a
measurements are performed, such as resistance or voltage threshold to determine the state of the main contacts.
motion/contact travel measurements, • Static contact resistance: This test verifies whether the resistance of the main contacts
or the measurement of the motor allows the current to flow with minimal losses.
current signature. • Dynamic contact resistance: This records the contact resistance during the operation of
For circuit breakers that are equipped the breaker and delivers information about wear-related problems with main and arcing
with an under-voltage release or an over-
current release, testing these features
further increases the available information
that can be used during the condition
assessment of the circuit breaker.

Medium voltage circuit breakers
While old medium voltage (MV) circuit
breakers often used oil as an interrupting
medium, more modern types almost
exclusively use vacuum. Essential
elements of a breaker include the
interrupter unit, the mechanical linkage,
and the operating mechanism with an
energy storage system. The energy
needed to operate a circuit breaker is
high, and it must be made available within
a few milliseconds, i.e., almost
instantaneously.
Springs are used in most cases
because they are simple and reliable.
Two separate springs allow the energy Figure 1: Under-voltage release test signal

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