Testing of REF Protection Relay: Pickup Test, Operating Time Test, Functional Checks with Real Examples

Restricted Earth Fault (REF) protection is one of the most sensitive protection schemes used for detecting ground faults within power transformers, generators, and reactor windings. The REF relay operates based on the principle of current balance and can detect low magnitude earth faults that other protection schemes might miss. Testing of REF protection relay is a mandatory activity during commissioning and periodic maintenance to verify that the relay will correctly operate for internal earth faults while remaining stable during external faults.

In this technical guide, we will discuss everything you need to know about testing REF protection relays. We will discuss the operating principle, test procedures, pickup current verification, operating time measurement, and real examples from actual commissioning reports.

1. What is REF Protection Relay?

Restricted Earth Fault protection is a unit type protection scheme that protects a specific zone against earth faults. The term “restricted” indicates that the protection is limited to a defined zone between the CTs on the line side and the neutral CT. Unlike standby earth fault protection which covers the entire system, REF protection operates only for faults within its protected zone.

The REF relay receives current inputs from phase CTs and the neutral CT of the protected equipment. Under normal conditions and external fault conditions, the vector sum of these currents is zero. When an earth fault occurs within the protected zone, the fault current returns through the neutral and creates an imbalance. The relay detects this imbalance and initiates tripping.

REF protection is particularly useful for detecting earth faults near the neutral point of star-connected transformer windings. In this region, the fault current magnitude is low due to reduced voltage. Overcurrent protection may not be sensitive enough to detect such faults, but REF protection can operate even for faults very close to the neutral point.

Modern numerical relays like the Siemens 7SJ66 series provide REF protection function along with other protection elements. These relays use advanced algorithms to calculate the differential current and compare it with the pickup setting. The relay designation 64T indicates transformer restricted earth fault protection.

2. Why Testing of REF Protection Relay is Necessary

Testing of REF protection relay serves multiple purposes in power system protection. First, it verifies that the relay settings match the protection study requirements. Second, it confirms that the relay correctly measures the neutral current through the CT inputs. Third, it validates that the relay operates at the correct pickup value with acceptable operating time.

During commissioning of a new transformer or substation, REF relay testing is performed before the equipment is energized. This pre-commissioning test identifies any wiring errors, CT polarity mistakes, or setting errors that could cause protection failure or unwanted tripping. A wrong CT connection can cause the relay to see spill current during normal load conditions and trip the transformer unnecessarily.

REF protection is often the only protection that can detect low current earth faults near the transformer neutral. If this protection fails to operate during an actual fault, the fault may continue for an extended period causing severe damage to the transformer winding insulation. This makes testing of REF protection relay extremely important for transformer safety.

3. Equipment Required for REF Relay Testing

The testing of REF protection relay requires a single-phase or three-phase relay test set capable of injecting current into the neutral CT input is the primary requirement. Unlike differential relay testing which needs multiple current channels, REF testing can be performed with a simpler test setup.

The test set must be capable of producing:

• Current injection up to 10A or higher depending on CT ratio
• Fine resolution current adjustment (0.001A steps) for accurate pickup determination
• Accurate timing measurement for operating time verification
• Stable output for extended duration tests

Additional equipment includes a laptop computer with relay configuration software for reading and verifying relay settings, appropriate connecting cables with proper current ratings, and isolation equipment to prevent backfeed during testing.

4. Pre-Test Preparations and System Parameters

Before starting the actual REF tests, engineers must verify system parameters and relay configuration. The following example uses actual data from a REF relay testing project I completed in 2023 to demonstrate the testing procedure and provide practical context for understanding the process.

System Information (from test example):

• Project: 132/33 kV Substation
• Location: GAMPHOJOL
• Feeder: BAY 103 (IR3B)
• Protection Designation: 64T HV (Restricted Earth Fault for HV winding)

Relay Information:

• Relay Model: Siemens 7SJ6612-5DB90-1FB0/BB
• Relay Serial Number: GF1809514853
• Auxiliary Supply: 120 V DC

Protection Settings:

• REF Pickup (64T): 0.040 A (secondary)
• REF Trip Timer: 0 (instantaneous operation)

CT Ratio:

• Neutral CT Ratio: 200/1 A

These parameters determine the pickup values and expected operating characteristics for the test.

5. Current Measurement Verification Test

The first step in REF relay testing is verifying that the relay correctly measures the injected neutral current. This test confirms proper wiring connections and validates the CT ratio settings in the relay.

5.1 Current Measurement Test Procedure

1. Connect the test set current output to the neutral current input terminals of the relay
2. Inject a known current value (typically 1A secondary)
3. Observe the current reading on the relay display or through the configuration software
4. Verify that the displayed primary current matches the expected value based on CT ratio

5.2 Current Measurement Test Results (from actual test report)

The relay converts 1A secondary injection to 200A primary using the 200/1 neutral CT ratio. This test confirms that the neutral CT input is correctly wired and the CT ratio is properly configured in the relay. If the displayed value does not match the expected value, there may be a CT ratio setting error or wiring problem that must be corrected before proceeding with pickup tests.

6. REF Protection Pickup Test Procedure

The pickup test verifies that the relay operates at the correct current setting. The relay should operate when the injected current reaches or exceeds the pickup setting, and it should not operate below the pickup setting.

6.1 Pickup Test Procedure

1. Connect the test set to the neutral current input of the relay
2. Set the test equipment to inject current starting from zero
3. Slowly increase the current while monitoring relay operation
4. Record the current value at which the relay operates (pickup current)
5. Compare the actual pickup with the relay setting
6. Repeat the test to verify consistency

6.2 Pickup Calculation

For the test report example, the REF pickup setting is 0.044A secondary. This represents the minimum current at which the relay should operate. Testing at values slightly below and at the pickup setting verifies the relay’s sensitivity.

6.3 REF Protection Settings (from test report):

The trip timer setting of 0 indicates that the relay operates instantaneously without any intentional time delay once the pickup threshold is exceeded.

7. REF Protection Test Results with Examples

The test report provides actual test results that show how REF pickup testing is performed and documented. Two test points are taken to verify relay operation near the pickup threshold.

7.1 REF Test Results (from actual test report)

7.2 Analysis of Test Results

Test Point 1 – At Pickup Setting (0.040 A):
The first test applies at the pickup setting of 0.040A. The relay operated in 124 ms at this current level. The longer operating time of 124 ms at this current level shows that the relay is operating at the boundary of its operating characteristic, which is expected behaviour at the exact pickup threshold.

Test Point 2 – At Pickup Setting (0.044 A):
The second test applies 0.044A which is above the pickup setting of 0.040A. The relay operated in 33 ms at this current level. The faster operating time of 33 ms indicates that the relay is operating well within its characteristic, as the applied current exceeds the set pickup value by 10%.

8. Stability Test for REF Protection

Stability testing is an important part of REF relay testing. The stability test verifies that the relay does not operate during external faults or normal load conditions.

8.1 Stability Test Procedure:

For a complete REF protection scheme with phase CTs and neutral CT:

1. Inject balanced three-phase currents through the phase CT inputs
2. Inject the sum of phase currents through the neutral CT input with opposite polarity
3. The vector sum should be zero, representing an external fault or through current condition
4. Verify that the relay does not operate

9. Additional Functional Tests

Several other tests are performed to verify complete relay functionality.

• Check of LED Indicators: All front panel LED indicators are to be tested to verify they illuminate correctly for different alarm and trip conditions. The test verifies that the REF trip LED lights up when the relay operates.
• Check of Trip Relays: Trip output contacts are to be tested for proper operation when tripping conditions are met. The test verifies that the trip contact closes when REF protection operates.
• Check of Signal Output Relays: Alarm and indication output relays are to be verified for correct operation. These may include REF alarm, REF trip indication, and protection operated signals.
• Check of Binary Inputs: All digital inputs for CB status, blocking signals, and other functions are to be tested. Binary inputs may be used for REF blocking during certain conditions.
• Alarm and Trip Functions: The complete logic from fault detection to output operation is to be verified. This includes any time delays, blocking logic, and trip circuit supervision.

10. Conclusion

Testing of REF protection relay is a mandatory activity that every protection engineer must perform with precision. The test procedures covered in this guide show how systematic testing verifies relay performance for detecting restricted earth faults within the protected zone.

The real commissioning test results presented in this article show that modern numerical REF relays like the Siemens 7SJ66 operate with high accuracy. The relay operated at 0.040 A with 124 ms operating time and at 0.044 A with 33 ms operating time. These results confirm that the relay provides sensitive and fast protection for earth faults.

11. Frequently Asked Questions (FAQs)