The Selective DC Scheme in circuit breaker control circuits is a specialized design used to enhance reliability and ensure fail-proof operation in power system protection and control.
In a Selective DC Scheme, multiple (normally two) DC supplies are taken to ensure that control circuit of CB remains functional even if one of the supplies fails. This is particularly used for 132KV and above circuit breakers where high reliability of circuit breaker operation is essential.
A Basic Circuit Diagram of Selective DC Scheme
The above diagram illustrates a basic Selective DC Scheme used in a circuit breaker control circuit to ensure DC supply continuity in case of any of the dc supplies failure. This scheme uses two independent DC power sources to maintain continuous supply.
Here, DC Supply 1 & DC Supply 2 are two independent DC sources. The system can operate using either supply if one fails.
There is K12 Relay (Automatic Changeover Relay) placed across the DC source 1. It is energized when DC Supply 1 is available. If DC Supply 1 fails, the relay de-energizes,
K12 contacts (Normally Open & Normally Closed Contacts) determine the active DC source for the circuit breaker control. When K12 is energized, it connects DC Supply 1 to the control circuit.
When K12 is de-energized, the circuit automatically switches to DC Supply 2.
Working Principle of Selective DC Circuit
- In normal condition DC Supply 1 is providing power. K12 Relay is energized. K12 NO contact closes, allowing DC Supply 1 to feed the breaker control circuit. The NC contact remains open, isolating DC Supply 2.
- If DC Supply 1 fails (e.g., battery depletion or fuse blown), the K12 relay de-energizes. K12 NO contact opens, disconnecting DC Supply 1. Instantly, K12 NC contact closes, allowing DC Supply 2 to take over.The breaker control circuit continues functioning using DC Supply 2.
Advantages of Selective DC Scheme
- ✔ Redundancy: Ensures the breaker control circuit remains operational even if one DC supply fails. If not the entire, atleast some essential part of the circuit should be connected with Selective DC.
- ✔ Automatic Switchover: No manual intervention is required when switching between DC sources when one of the supplies fails.
- ✔ Prevents Power Interruptions: Seamlessly shifts to backup supply to prevent control failures due to the absence of DC.
- ✔ Enhanced Reliability: Used in critical applications like substations to prevent loss of control power.