two circuits to be staggered, e.g. one at 5 seconds and the other at 10 seconds, so that the two circuit breakers do
not reclose simultaneously following a fault affecting both cir
cuits.
For multi-shot Autoreclose cycles, the second shot and subsequent shot dead times are usually longer than the
first shot, to allow time for semi-permanent faults to burn clear, and for the CB to recharge. Typical second and
third shot dead time settings are 30 seconds and 60 seconds respectively.
8.2.3 LOAD REQUIREMENTS
Some types of electrical load might have specific requirements for minimum and/or maximum dead time, to
pr
ev
ent damage and minimise disruption. For example, synchronous motors are only capable of tolerating
extremely short supply interruptions without losing synchronism. In practise it is desirable to disconnect the motor
from the supply in the event of a fault; the dead time would normally be sufficient to allow a controlled shutdown.
Induction motors, on the other hand, can withstand supply interruptions up to typically 0.5 seconds and re-
accelerate successfully.
8.2.4 CIRCUIT BREAKER
For HSAR, the minimum dead time of the power system will depend on the minimum time delays imposed by the
cir
cuit br
eaker during a tripping and reclose operation.
After tripping, time must be allowed for the mechanism to reset before applying a closing pulse, otherwise the
circuit breaker might fail to close correctly. This resetting time will vary depending on the circuit breaker, but is
typically 0.1 seconds.
Once the mechanism has reset, a CB Close signal can be applied. The time interval between energising the closing
mechanism and making the contacts is called the closing time. A solenoid closing mechanism may take up to 0.3
seconds. A spring-operated breaker, on the other hand, can close in less than 0.1 seconds.
Where HSAR is required, for the majority of medium voltage applications, the circuit breaker mechanism reset time
itself dictates the minimum dead time. This would be the mechanism reset time plus the CB closing time. A
solenoid mechanism is not suitable for high speed Autoreclose as the closing time is generally too long.
For most circuit breakers, after one reclosure, it is necessary to recharge the closing mechanism energy source
before a further reclosure can take place. Therefore the dead time for second and subsequent shots in a multi-
shot sequence must be set longer than the spring or gas pressure recharge time.
8.2.5 FAULT DE-IONISATION TIME
For HSAR, the fault deionising time may be the most important factor when considering the dead time. This is the
time r
equir
ed for ionised air to disperse around the fault position so that the insulation level of the air is restored.
You cannot accurately predict this, but you can obtain an approximation from the following formula:
Deionising time = (10.5 + ((system voltage in kV)/34.5))/frequency
Examples:
At 66 kV 50 Hz, the deionising time is approximately 0.25 s
At 132 kV 60 Hz, the deionising time is approximately 0.29 s
8.2.6 PROTECTION RESET TIME
It is essential that any time-graded protection fully resets during the dead time, so that correct time discrimination
will be maintained after reclosing on to a fault
. For HSAR, instantaneous reset of protection is required. However at
distribution level, where the protection is predominantly made up of overcurrent and earth fault devices, the
protection reset time may not be instantaneous. In the event that the circuit breaker recloses on to a fault and the
protection has not fully reset, discrimination may be lost with the downstream protection. To avoid this condition
the dead time must be set in excess of the slowest reset time of either the local device or any downstream
protection.
Typical 11/33 kV dead time settings are as follows:
Chapter 13 - Autoreclose P14x
306 P14xEd1-TM-EN-1
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