Electromechanical Protective Relays are usually attached to medium voltage circuit breakers to detect abnormalities in the current flowing within the electrical system.
The mechanical assembly of relays may become defective due to thermal and/or electrical stress(s). In industrial settings, the accumulation of contaminants like corrosion and dirt can potentially contribute to the mis-operation of the associated circuit breaker. For this reason, preventive maintenance should be conducted on a regular basis to ensure reliable operation.
Protective Relay Testing Procedure
The proper cleaning and testing of protective relays is usually found in the manufacturer's instruction leaflet. However, if this type of documentation is not available, follow the procedure below to test medium voltage electromechanical devices.
1. Visual Inspection
- Remove the relay cover
- Inspect the gasket of the cover
- Inspect for cracks or frame tightness
- Clean the covers and glass thoroughly
- Remove the relay assembly from the case
- Short circuit the CT terminals for safety
- Open all of the trip circuits
- Foreign objects like metal bits and dust should be remove from the case and the relay. These may cause problems on the mechanical parts and erratic operation of the relay.
- Blow dust by blowing air gently using a hand syringe.
- Metal bits or corrosion should be remove from the magnet poles or disc using a brush or magnet cleaner.
- Hold the relay up to the light to ensure that the gap has good clearance and that the disc does not rub.
- Check for moisture problems. If you see rust spots on the relay, it is important to check if the relay is in proper operational environment. Moisture can cause severe corrosion and problems in the mechanical components.
- Check for loose connections. Taps, screws, bolts, nuts and pivotal joints should be tight.
- The bearings should be smooth. To check, the disc is rotated manually to close the contacts and letting the action of the spiral spring to the relay disc to its de-energized position. You should observe for smoothness and should not be sluggish. Clean and put oil on the mechanism. However, if cleaning and oiling fails, the relay must be reconditioned or replaced.
- The operation of targets should be manually checked. This is done by fitting the armatures and checking if there is a showing target.
- The relay coil must be inspected to ensure that it is not subjected to high currents for a long time.
- The components that touch together during a relay's normal de-energized position must be cleaned. This is to prevent the relay from getting stuck or operate erroneously especially on low current faults.
2. Electrical Testing
- Disconnect the relay from the trip and power circuits for testing.
- Secondary injection testing.
- This allows you to check the operation of the circuit breaker, relay connections and the relay assembly.
- This testing method is conducted by injecting current directly into the relay.
3. Other Tests
- Insulation resistance measurement - This test ensures that the insulation of the relay is within the acceptable operating limits. Ideally, the relay insulation should have a resistance of a few MΩ to TΩ.
- Seal-in operation and target testing - Most protective relays installed on electrical facilities are the seal-in and target combination. The test verifies that the contacts will seal-in to its close position with the minimum DC current.
- Zero Check - This test is conducted on relays that has a time dial. This test is done to determine the time dial reading when the moving contacts are closed by turning the time dial to zero position and the relay is fixed.
- Testing Relay Pick-up - This will test the min and max frequency, voltage, current to close the relay.
- Instantaneous Testing - There are some protective relays that operate instantaneously These types usually have a separate instantaneous component. This type of relay doesn't have any intentional delay. An instantaneous components at twice the pick-up must operate between 0.016 and 0.30 seconds.
Related electrical test equipment available for rent from Protec:
Editor’s Note: This post was originally published on October, 2012 and has been updated freshness, accuracy and comprehensiveness.