A protective relay is a piece of switchgear that recognizes a failure and starts the circuit breaker’s action to isolate the problematic component from the rest of the system. They are small, self-contained, and capable of spotting anomalies. Protective relays continuously measure the electrical values that differ between normal and fault circumstances in order to identify abnormal conditions in electrical circuits. Voltage, current, frequency, and phase angle are the electrical quantities that could alter in the presence of a defect. The protective relays receive signals from the faults indicating their existence, type, and location by modifying one or more of these parameters. The relay activates to close the breaker’s trip circuit after detecting the malfunction. As a result, the breaker is opened and the defective circuit is disconnected.

In electrical substations, protective relaying is used to sound an alarm or prompt the removal of any problematic power system component from service. The rest of the system’s ability to function effectively may be harmed or interfered with by an element’s irregular behavior. When an electrical failure occurs, the protective relaying reduces damage to the machinery and service interruptions. Relays, together with other machinery, aid in reducing damage and enhancing service.

RELAY MAINTENANCE AND TESTING

To guarantee that your protection system continues to deliver satisfactory results for numerous years after installation, regular maintenance and testing are required. Although the built-in, self-testing characteristics of microprocessor relays can be expected to detect the majority of defects, this does not suffice to meet regulatory standards or cover the other parts of the protection scheme. Therefore, it is advised that a protection system undergo regular testing and inspection. Depending on the relay technology, tests could consist of:

• Mechanical and visual inspection
• Insulation resistance test
• Tests for further injections
• Checking the relay settings
• Functional testing of all auxiliary devices, current and voltage transformers, and wire connections
• Control validation.

Mechanical And Visual Inspection

A comprehensive visual and mechanical inspection comes first in all protective relay testing and maintenance procedures. To avoid completely disabling the protection if the circuit to be tested is already in operation, only one relay (if any) should be removed at a time. Whether a relay is solid-state, electromechanical, or microprocessor-based, what needs to be checked differs depending on the kind. 

Insulation Resistance Test

Conduct circuit-to-frame and ground insulation-resistance testing on each electromechanical relay. The relay instruction manual should be used to identify the procedures for conducting insulation-resistance testing on solid-state and microprocessor relays. High-voltage insulation tests might not be advised by all relay manufacturers.

Tests For Further Injections

• To demonstrate stability for external faults and to establish the effective current setting for internal faults, each protective relay must undergo primary injection tests.
• Each protective relay will undergo a secondary injection test to show that it is operational at one or more setting values. 

Checking The Relay Settings

The as-left relay settings should be in accordance with the most updated coordination and arc-flash research or engineering setting files. Check to make sure that every setting is in line with the most recent protective device coordination study or setting sheet provided by the equipment owner. This data is frequently provided on a time-current graph of the coordination study showing the relay’s properties.

Functional Testing

In addition to visually inspecting and testing the protective relays, system functional tests may be used to demonstrate the proper operation of all sensing, processing, and action devices as a whole. All interlock safety devices should be checked for fail-safe functions in addition to their particular design functions while conducting system functional tests. It’s also important to confirm that all signaling, alarm, and sensing equipment are functioning properly. Circuits for the lock-out relay, block close, relay self-test, power supply failure, and trip coil monitor alerts back to SCADA should all be tested. Switches for bus restoration and/or transfer should be tested for functionality. Check the metering on safety relays and metres against a calibrated source, make sure communication lines are open for both local and remote devices, and make sure control annunciation systems are not alarm-free. Investigate any alarms that are present.

Control Validation

Control circuits and current transfer circuits need to be put back into regular operation after testing is completed. Check to make sure all systems are in their default mode of operation or position, transfer and restoration plans are activated, and monitoring and protection tools are working.

RELAY TEST SET

The testing apparatus for testing protection relays with various functionalities is called a protection relay test set. All types of relays, including electromagnetic, high-burden, microcomputer, digital, and other types, may be tested using one reliable protective relay test set. The protection relay test set may be used with the comprehensive software programme to test the function, logic, traits, actions, and all other types of features in the protective relays so that we can confirm the relays are effectively functioning.

IMPORTANCE OF RELAY TEST SET

A relay test set is a useful tool for finding defects that cannot be seen by visual inspection alone. Faults can cause significant equipment damage or even failure if they are not discovered in time. Relay testing is a cost-effective way to safeguard crucial electrical equipment systems and prevent the cost of downtime or repairs brought on by equipment failure. Relays are tested to make sure they are working properly and are in good condition by replicating typical relay operating situations. By using these diagnostic techniques, you can find out if a relay’s coil is compressing when switching positions. If this happens while the system is running, contacts frequently arc and carbonize with continued use. In that situation, the transient voltage may circulate throughout the system and harm equipment. Snubber circuits are frequently employed to prevent this kind of damage, but if they malfunction, it’s possible that the contacts will sustain damage that will eventually cause the relay to fail.

CONCLUSION

Protective relays should undergo acceptance testing both before they are put into service and on a regular basis afterward to guarantee reliable performance due to their crucial function in the power system. Periodic testing should be carried out in a typical industrial application at least every two years.