Introduction to Relays

Relays are indispensable components within secondary control circuits, serving as the fundamental elements of protection and automation in electrical systems. Their operational principle is elegantly simple: when an input parameter—voltage, current, or power—reaches a preset threshold, electromagnetic force actuates the relay’s contact mechanism, causing it to open or close. Despite their simplicity, relays play a decisive role in safeguarding both primary equipment and personnel.

In modern power systems, several categories of relays are widely applied. Alternating current (AC) voltage relays monitor voltage levels within secondary circuits of voltage transformers. These can be subdivided into overvoltage and undervoltage relays. An overvoltage relay typically detects zero-sequence voltage; when a three-phase imbalance elevates this voltage beyond its setting, the relay issues a signal. Conversely, undervoltage relays supervise either phase-to-phase or phase-to-ground voltages. Upon a system fault causing voltage to sag beneath the defined limit, the relay responds immediately with a signal.

AC current relays are used extensively within transformer cooling arrangements, initiating forced-air cooling systems when the load surpasses normal thresholds. They are also employed to interlock tap-changing mechanisms under overload conditions.

The direct-current (DC) auxiliary, or intermediate relay, is perhaps the most ubiquitous type, often utilized to amplify control signals and ensure reliable circuit logic execution. Due to its versatility, it remains an irreplaceable component of nearly every secondary circuit.

High-power relays are employed in specialized protective schemes where the risk of electromagnetic interference is heightened, such as in gas relay tripping circuits or direct trip schemes of extra-high-voltage busbars. Their design ensures reliable operation even when control cables extend over long distances.

Finally, time-delay relays are critical in coordinating sequential operations. Within high-voltage substations—typically 220 kV and above—they form part of three-phase discrepancy protections, where precise timing ensures stable and selective fault clearance.

Commissioning Conditions

Before any commissioning or calibration activity, several prerequisites must be meticulously observed:

1. A detailed relay calibration work instruction must be prepared in advance, clearly stipulating procedures, safety measures, and acceptance criteria.

2. All required test instruments and tools must be assembled and verified for validity. These include portable DC power supplies, relay protection test sets, multimeters, screwdrivers, and insulation testers. Calibration instruments must be within their certification period to guarantee accurate results.

Commissioning Tasks

Relay commissioning encompasses multiple verification tasks, each tailored to the characteristics of the device under test. The principal categories include:

1. AC voltage relay calibration.

2. AC current relay calibration.

3. DC intermediate relay calibration.

4. High-power relay calibration.

5. Time-delay relay calibration.

Commissioning Methods

AC Voltage Relay Calibration

• Inspection: The relay must be visually examined for cracks, deformation, or obstructed mechanical movement. Contacts should move freely without sticking.

• Insulation: Using a 1000 V megohmmeter, the coil and contacts must exhibit insulation resistance no less than 1 MΩ relative to the base.

• Test Procedure: Connect the AC test set output to the relay coil terminals (A1, A2), and link the relay’s normally open contact to the test set input. Gradually increase the test voltage until the contact closes. Record the operating voltage. Then, slowly decrease the voltage until the contact reopens, recording the reset voltage.

• Verification: The measured values must align with the specified settings—typically, reliable operation above 1.05 times the setpoint, and reliable non-operation below 0.95 times the setpoint.

DC Intermediate Relay Calibration

• Inspection: External condition and free movement of components are confirmed.

• Insulation: Insulation resistance must exceed 1 MΩ at 1000 V.

• Operating Value: Apply DC voltage gradually to the coil until closure occurs; record the pickup voltage. Reduce the voltage until the contact reopens, noting the dropout voltage.

• Operating Time: Using the test set’s timing function, apply rated voltage to the coil as a start signal, and measure the time until the contact closes. For relays directly linked to tripping circuits, this time must be below 10 ms.

• Verification: The operating voltage range should be between 55% and 70% of rated voltage.

High-Power Relay Calibration

• Inspection and Insulation: Conducted as per standard practice.

• Operating Value: Apply DC voltage to the coil and observe contact closure.

• Operating Power: Measure current at the moment of closure, then calculate power using voltage and current. The operating voltage must remain within 55–70% of rated, while operating power must not fall below 5 W.

Time-Delay Relay Calibration

• Inspection and Insulation: As above, ensuring no mechanical hindrance and insulation ≥ 1 MΩ.

• Time Verification: Apply rated DC voltage to the coil to initiate timing, and use the closure of the normally open contact as the stop signal. Record the elapsed time and compare with manufacturer specifications.

Precautions

1. The type and characteristics of each relay must be confirmed before testing, avoiding inadvertent damage from incorrect procedures.

2. Relay testing is not a substitute for complete functional circuit testing.

3. AC and DC relays are not interchangeable; the rated voltage must match that of the secondary circuit.

4. Each relay must undergo no fewer than three test repetitions to ensure statistical reliability.

5. Testing should preferably be carried out at the installed location, as mounting conditions may affect performance in certain designs.

Regulatory Requirements

Relay commissioning and testing must adhere strictly to established standards and safety guidelines, including but not limited to:

• Eighteen Major Anti-Accident Measures of the State Grid Corporation of China (2018 Revision).

• Verification Regulations for Relay Protection and Power System Security Automatic Devices (DL/T 995—2016).