Protective relays usually attached to medium voltage circuit breakers to detect abnormalities in the current flowing in the electrical system. When a protective relay detects a problem, it isolates the fault with minimum disturbance to the electrical facility. Protective relays are designed to respond quickly on power events like abnormal influx of current in the electrical circuit.
The common type of protective relay is the electromechanical one. The mechanical assembly may become defective after a while due to thermal/electrical stress and the accumulation of contaminants like corrosion or dirt. In an industrial setting, regular preventive maintenance should be conducted once a year to ensure reliable operation.
Protective Relay Testing Procedure
The proper cleaning and testing of protective relays is usually found on the manufacturer's user manual. However, if you do not have the manual of the device, you can do the procedure below to test medium voltage electromechanical devices. The information was taken from various manuals and testing publications to ensure consistency with the acceptable testing standards.
1. Visual Inspection
1a. Remove the relay cover
Inspect the gasket of the cover
Inspect for cracks or frame tightness
Clean the covers and glass thoroughly
1b. Remove the relay assembly from the case
You should short circuit the CT terminals for safety
Open all of the trip circuits
1c. 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 cause erratic operation of the relay.
1d. Blow dust by blowing air gently using a hand syringe.
1e. Metal bits or corrosion should be remove from the magnet poles or disc using a brush or magnet cleaner
1f. Hold the relay up to the light to ensure that the gap has good clearance and that the disc does not rub.
1g. 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.
1h. Check for loose connections. Taps, screws, bolts, nuts and pivotal joints should be tight.
1i. 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 deenergized position. You should observe for smoothness and should not be sluggish. You can clean and put oil on the mechanism. However, if cleaning and oiling fails, the relay must be reconditioned or replaced.
1j. The operation of targets should be manually checked. This is done by fitting the armatures and checking if there is a showing target.
1k. The relay coil must be inspected to ensure that it is not subjected to high currents for a long time.
1l. The components that touch together during a relay's normal deenergized position must be cleaned. This is to prevent the relay from getting stuck or operate erroneously especially on low current faults.
2. Electrical Testing
2a. Disconnect the relay from the trip and power circuits for testing.
2b. 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 to the secondary terminals of the device transformer.
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.
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.
This will test the min and max frequency, voltage, current to close the relay.
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.
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