Page 34 - Energize February 2021

TECHNICAL

Figure 4: Closing time with the motion/contract travel as start reference

Overcurrent release
Purpose of overcurrent releases
Current transformer releases are used on circuit breakers in substations where no grid-
independent supply voltage is available. These are low-cost stations with basic
functionality. Quite often they do not have any remote-control feature and the breakers do
not have a close coil. Such substations are common in distribution grids where the
downstream infrastructure is not critical, like in residential areas.
Overcurrent releases are activated by a current. The current comes from the tripping
transformer which usually has a nominal value of 0,5, 1,0 or 5 A AC. An overcurrent relay
feeds the current flow from the tripping transformer to the circuit breaker. The tripping
transformer and the overcurrent relay (self-powered overcurrent relay) are usually powered
by the secondary side of the CTs.
In the event of an overcurrent, the relay switches the current of the tripping transformer
to the circuit breaker and thus causes it to open the main contacts and isolate the faulty
grid part.

Overcurrent release test
With the breaker in closed position, a current is ramped up in steps until the breaker trips.
This is the trip current. Then the ramp ends.

Other designations
• IEC 62271-100: Indirect current release
• ABB: Transformer-operated release
• Siemens: Current transformer operated release
• Also in use: Indirect over-current release

Closing time calculation for circuit breakers without a close coil
All breakers should have at least a trip coil so that faults can be isolated. Some old breakers
or breakers with overcurrent release do not have any close coil. These breakers are closed
manually. According to IEC 62271-100, the closing time is the time from the moment the
close coil is energised until the contacts touch in all poles.
For circuit breakers without a close coil the closing time calculation according to IEC
standard cannot be applied. Therefore, an alternative approach must be used. The closing
time can be the time when the circuit breaker main contacts begin to move until the
contacts touch in all poles. n

Contact Alexander Dierks, Alectrix, Tel 082 658-6938, alex@alectrix.co.za

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