What Happens if DC supply fails in an electrical substation?

This article describes the importance of DC supply in substations. Circuit breakers are the main power-controlling elements that are switched on and off during normal operation, and they isolate the faulty section in the event of faults. Circuit breaker operation can not be performed if the DC supply is not available in the substation. DC is the emergency power that is always available in the case of blackouts.

Importance of DC supply in an electrical substation

We use circuit breakers and switch gears for power supply distribution to ensure electrical power’s safe and systematic utilization. The breaker is mainly used to turn the electrical supply on and off. The circuit breaker also carries out protection functions.


During an electrical fault, the protection system trips the circuit breaker to cut off the supply to the faulty section. Thus, a breaker is used in an electrical network for;

  • Closing 
  • Cut off 
  • Tripping of the breaker in the case of a fault

The circuit breaker performs an on /off and trip function when we feed 110 VDC supply to the closing and tripping coil. The protection relay issues a tripping command to the breaker in the case of fault. The tripping coil of the breaker has a rating of 110 Volts. If the 110-volt supply fails, the breaker can not trip, and the heavy current flows through the electrical network. In this case, the upstream breaker will not get tripped without a DC supply. The fault will not be cleared if the breaker does not trip, and the heavy current flowing in the electrical network may cause heavy damage to electrical switch gears and equipment. In this case, the fire hazard can not be negated.

Thus, the 110-volt DC supply system is of prime importance for protecting an electrical network in substations.

What additional protection system is required for 110 VDC  supply continuous monitoring? 

Given the importance of the DC power supply in substations, the trip circuit supervision relay monitors its healthiness during breaker closing and tripping. The breaker will not close if the 110 DC circuit is not healthy. In normal running conditions, if there is a problem in the 110 VDC supply system, the trip circuit supervision relay outputs an alarm signal to facilitate early troubleshooting of the supply system. 

To make the 110 VDC supply more reliable, we can have two trip circuits. In the case of one trip circuit’s failure, another trip circuit will function. 

Thus, it is crucial to ensure the healthiness of the tripping circuit at all times to safeguard an electrical network.

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