Testing of Distance Protection Relay: Zone 1, 2, 3 Testing, SOTF, Power Swing Blocking, and VT Fuse Fail

Distance protection relays are the primary protective devices used in high voltage transmission lines across power systems worldwide. These relays measure the impedance between the relay location and fault point to determine whether a fault exists within their protected zone. Testing of distance protection relay is a mandatory activity performed during commissioning and periodic maintenance to verify that the relay will operate correctly during actual fault conditions.

In this technical guide, we will discuss everything you need to know about testing distance protection relays. We will discuss the test procedures, required equipment, zone testing methods, power swing blocking, and real examples from actual commissioning reports.

1. What is a Distance Protection Relay?

A distance protection relay operates based on the measurement of impedance at the relay location. During normal conditions, the relay sees high impedance. When a fault occurs on the protected line, the impedance measured by the relay drops. The relay compares this measured impedance with pre-set impedance values corresponding to different zones of protection.

The relay calculates impedance using the voltage and current signals received from voltage transformers (VT) and current transformers (CT). Based on the calculated impedance and its angle, the relay determines the fault location and initiates tripping if the fault is within the protected zone.

Modern numerical distance relays like the Siemens 7SA522 series use quadrilateral characteristics or mho characteristics for fault detection. These relays can detect phase-to-phase faults and phase-to-ground faults with high accuracy and selectivity.

2. Why Testing of Distance Protection Relay is Necessary

Testing of distance protection relay serves multiple purposes in power system protection. First, it verifies that the relay settings match the protection coordination study requirements. Second, it confirms that the relay hardware is functioning properly without any manufacturing defects. Third, it validates the CT and VT connections are correct and the relay is receiving accurate voltage and current signals.

During commissioning of a new substation or transmission line, distance relay testing is performed before the line is energized. This pre-commissioning test identifies any wiring errors or setting mistakes that could cause protection failure or unwanted tripping during normal operation.

Periodic testing during maintenance outages helps detect relay degradation over time. Component aging, environmental factors, and firmware issues can affect relay performance. Regular testing catches these problems before they cause protection failures during actual faults.

3. Equipment Required for Distance Relay Testing

The testing of distance protection relay requires specialized test equipment. A relay test set capable of injecting three-phase currents and voltages simultaneously is the primary requirement. Modern relay test sets from manufacturers like Omicron, ISA, Megger, and Doble are commonly used for this purpose.

The test set must be capable of producing:

• Three-phase voltage injection up to 120V per phase
• Three-phase current injection up to 30A per phase
• Precise control of phase angles between voltage and current
• Accurate timing measurement for trip time verification
• Multiple fault simulation modes including phase faults and ground faults

Additional equipment includes a laptop computer with relay configuration software, appropriate connecting cables, and isolation equipment to prevent backfeed during testing.

4. Pre-Test Preparations and System Parameters

Before starting the actual impedance tests, engineers must verify system parameters and relay configuration. The following information is required from the protection settings document:

CT and VT Ratios: For a 132 kV transmission line, typical values include:

• Primary Voltage: 132 kV
• Secondary Voltage: 110 V
• Primary Current: 600 A
• Secondary Current: 1 A

Line Parameters (from an actual test report):

• Line Length: 29.3 km
• Line Reactance per km: 0.193 Ohm/km
• Line Angle: 67 degrees
• Fault Characteristic: Quadrilateral

These parameters determine the zone reach settings and timing delays programmed into the relay.

5. Voltage and Current Measurement Verification

The first step in distance relay testing is verifying that the relay correctly measures injected voltages and currents. This test confirms proper wiring connections and validates the relay analog input circuits.

5.1 Voltage Measurement Test

Individual phase voltages are injected one at a time and the relay display is checked. The following example shows actual test results:

The relay converts secondary voltage to primary values using the VT ratio. With 63.51V secondary injection and 132kV/110V VT ratio, the relay should display approximately 76 kV for single-phase injection.

5.2 Current Measurement Test

Similar verification is performed for current inputs:

The relay converts 1A secondary current to 600A primary using the 600/1 CT ratio. Minor differences like 599A instead of 600A are acceptable within measurement tolerances.

6. Zone Impedance Testing Procedure

Zone impedance testing is the core of distance protection relay testing. This test verifies that the relay trips within the correct time for faults at various points along the protected line and backup zones.

Distance relays have multiple zones with different reach and timing settings:

• Zone 1: Covers 80-85% of the protected line with instantaneous tripping
• Zone 2: Covers 100% of protected line plus 50% of next line with time delay (typically 300-500 ms)
• Zone 3: Provides remote backup protection with longer time delay (typically 800-1200 ms)
• Reverse Zone: Detects faults behind the relay location

Test Procedure:

1. Calculate the impedance values for different percentage reaches of each zone
2. Set the test equipment to inject voltage and current that produce the desired impedance at specific angles
3. Apply the fault condition and measure the actual trip time
4. Compare actual trip time with expected trip time
5. Record pass or fail result based on acceptable deviation

6.1 R-Phase Impedance Test Results Example

The following actual test results demonstrate how impedance testing is performed and documented. These results are from a Siemens 7SA522 relay tested on a 132/33 kV substation feeder.

6.1.1 Zone 1 Tests (Instantaneous Tripping)

Zone 1 tests show trip times between 35 to 55 milliseconds. These are instantaneous trips where the relay operates as fast as its measuring algorithm allows. The 0.000 s nominal time indicates no intentional delay is programmed.

6.1.2 Zone 2 Tests (500 ms Time Delay)

Zone 2 tests verify the 500 ms time delay operation. Actual trip times of 520-526 ms show deviations of 4-5% from nominal, which is within acceptable limits for numerical relays.

6.1.3 Zone 3 Tests (800 ms Time Delay)

Zone 3 provides backup protection with 800 ms delay. The higher impedance values represent faults further from the relay location.

6.2 Phase-to-Phase Fault Testing

Distance protection relays must detect both phase-to-ground and phase-to-phase faults. Testing is performed for all fault combinations including RY, YB, BR, and three-phase faults.

6.2.1 RY Phase Fault Test Results

6.2.2 YB Phase Fault Test Results

6.2.3 Three-Phase Fault Test Results

All phase fault combinations must pass the impedance tests before the relay is considered ready for service.

6.3 Reverse Zone Testing

The reverse zone protects against faults behind the relay location. This zone uses negative phase angles to simulate faults in the reverse direction. The test results below show reverse zone operation:

Negative phase angles indicate the fault current is flowing in the opposite direction compared to forward faults. The relay correctly identifies these as reverse zone faults and trips with the appropriate time delay.

7. Power Swing Blocking Test

Power swings occur during system disturbances when large power transfers cause impedance oscillations. These oscillations can appear as faults to distance relays and cause unwanted tripping. Power swing blocking (PSB) prevents the relay from operating during stable power swings while allowing operation for actual faults.

Power Swing Detection Settings:

• Operating Mode: All zones blocked
• Power Swing Trip: No

The test verifies that distance protection zones are blocked when power swing condition is detected. The relay uses the rate of change of impedance to distinguish between faults (fast impedance change) and power swings (slow impedance change).

8. Switch Onto Fault (SOTF) Testing

Switch Onto Fault protection provides instantaneous tripping when a circuit breaker closes onto a faulted line. This situation occurs when maintenance crews fail to remove temporary grounds before energizing a line or when a fault develops during the dead time of auto-reclosing.

SOTF Test Procedure:

1. CB manual close input is made high
2. A fault is applied within SOTF activation time
3. Relay should trip instantaneously by SOTF function

SOTF Test Result:

The 23.30 ms trip time demonstrates that SOTF provides faster protection compared to normal Zone 1 operation (35-55 ms) since no fault detection algorithm delay is involved.

9. Fuse Fail Monitoring Test

Voltage transformer secondary fuse failure can cause incorrect impedance measurement and lead to relay maloperation. The fuse fail monitoring function detects loss of VT secondary voltage and blocks distance protection to prevent unwanted tripping.

Fuse Fail Test Procedure:

Each phase voltage is removed individually while maintaining the other two phases at normal voltage:

The relay should detect the missing phase voltage and block distance protection from operating. This prevents incorrect operation due to unbalanced voltage measurement.

10. Additional Functional Tests

Several other tests are performed to verify complete relay functionality:

• Check of LED Indicators: All front panel LED indicators are tested to verify they illuminate correctly for different alarm and trip conditions.
• Check of Trip Relays: Trip output contacts are tested for proper operation when tripping conditions are met.
• Check of Signal Output Relays: Alarm and indication output relays are verified for correct operation.
• Check of Binary Inputs: All digital inputs for CB status, blocking signals, and other functions are tested.
• Alarm and Trip Functions: The complete logic from fault detection to output operation is verified.

11. Acceptance Criteria for Test Results

The following criteria determine whether test results are acceptable:

Trip Time Deviation:

• Zone 1: Allows ±10% or ±20 ms deviation
• Zone 2 and Zone 3: Allows ±10% deviation from nominal time
• The test results show deviations of 4-8% which are within acceptable limits

Impedance Accuracy:

• Zone reach accuracy of ±5% is acceptable
• The test points are selected at boundaries and mid-points of each zone

Operating Sequence:

• Zone 1 must operate faster than Zone 2
• Zone 2 must operate faster than Zone 3
• Reverse zone must not operate for forward faults

12. Conclusion

Testing of distance protection relay is a mandatory activity that every protection engineer must master. The test procedures covered in this guide demonstrate how systematic testing verifies relay performance across all protection zones and fault types. From basic voltage and current measurement verification to advanced impedance testing at various phase angles, each test serves a specific purpose in validating relay operation.

13. Frequently Asked Questions