Relay protection secondary circuit wiring

The secondary wiring process for relay protection involves connecting CTs/VTs to the relay, using test blocks for safe testing, and verifying relay logic through secondary injection tests.

Overview of Secondary Wiring

Secondary wiring in relay protection refers to the low-voltage connections between current transformers (CTs), voltage transformers (VTs), and the protective relay. Unlike primary wiring, which carries high currents and voltages, secondary wiring deals with signals that represent system conditions. Proper secondary wiring ensures accurate relay operation, correct tripping, and safe testing without energizing the full system .

Key Components

• CT and VT secondary terminals: Provide scaled-down current and voltage signals to the relay.
• Relay terminals: Accept inputs from CTs/VTs and output trip/alarm signals.
• Test blocks or sockets: Allow safe insertion of test equipment without disturbing the main wiring.
• Shorting links: Prevent open-circuiting of CT secondaries during testing, which could cause dangerous voltages .

Wiring Procedure

1. Preparation
   • Verify relay type, ratings, and wiring diagrams.
   • Ensure CTs and VTs are correctly installed and secondary terminals are accessible.
   • Fit CT shorting links across live terminals before connecting test equipment .
2. Connection
   • Connect CT secondary leads to the relay input terminals, maintaining correct polarity.
   • Connect VT secondary leads to voltage input terminals.
   • Route trip and alarm outputs to the breaker or annunciation system.
   • Use test blocks to facilitate future testing and maintenance .
3. Verification
   • Check all connections against wiring diagrams.
   • Ensure insulation and mechanical integrity.
   • Confirm that test plugs or sockets isolate relay circuits from switchgear wiring when inserted .

Secondary Injection Testing

Secondary injection testing is performed before primary injection tests to verify relay logic and trip functionality without energizing the high-voltage system . The process involves:

• Injecting simulated current or voltage signals directly into the relay.
• Using a PC-based test set with amplifiers to generate precise signals for CT and VT inputs.
• Monitoring relay outputs (trip/alarm) through digital inputs on the test set.
• Adjusting phase angles and magnitudes to simulate fault conditions.
• Recording results for verification and commissioning reports .

Safety Considerations

• Always short CT secondaries before disconnecting or inserting test plugs.
• Ensure test equipment is rated for the relay type and voltage.
• Verify that the relay is isolated from the primary system to prevent accidental tripping or damage .

Commissioning and Maintenance

After wiring and testing:

• Configure relay settings using manufacturer software (e.g., ABB PCM600 for Relion relays).
• Perform functional tests to ensure correct operation under simulated fault conditions.
• Document wiring, test results, and relay settings for future maintenance . Proper secondary wiring and testing ensure reliable relay operation, system protection, and safety for personnel and equipment. Following these steps minimizes the risk of misoperation and allows efficient troubleshooting during maintenance or commissioning.