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Ensuring Reliability: A Guide to Medium Voltage Switchgear Testing

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Ensuring Reliability: A Guide to Medium Voltage Switchgear Testing


Medium voltage (MV) switchgear is critical for the safe and efficient distribution of electrical power. To ensure its reliability and proper functioning, rigorous testing procedures are essential. This post will explore the key tests performed on MV switchgear, providing insights into their objectives and methodologies.




1. Current Transformer (CT) Testing

CT testing is a fundamental aspect of switchgear maintenance. The primary objectives of these tests are to verify the CT's ratio, polarity, burden, knee point, winding resistance, and magnetization curve, as well as to conduct insulation tests.

  • Procedure:
    • A CT analyzer is used to measure the CT ratio, polarity, burden, knee point, and winding resistance.
    • The results are compared with the factory test report to ensure they are within acceptable tolerances.
    • Insulation tests are performed using a megger to assess the insulation between the primary and secondary windings, as well as between the windings and ground.





2. Voltage Transformer (VT) Testing


VT testing is equally important to guarantee the accuracy of voltage measurement and the proper operation of protection devices. These tests aim to verify the VT's ratio, polarity, winding resistance, and insulation.

  • Procedure:
    • A CPC100 device is used to measure the VT ratio and winding resistance.
    • Insulation tests are carried out using a megger.







3. Circuit Breaker Timing and Insulation Testing


Testing the circuit breaker's timing is crucial to ensure it operates within the specified timeframes, and insulation tests verify its dielectric strength. 

  • Procedure:
    • A breaker analyzer is used to measure the closing and opening times of the circuit breaker.
    • Insulation tests are performed using a megger.
    • High potential AC tests are conducted to assess the breaker's ability to withstand high voltages.






4. Bus Bar Testing


Bus bar testing involves contact resistance measurement and high potential testing.

  • Procedure:
    • A micro-ohm meter is used to measure contact resistance.
    • High potential AC tests are performed to evaluate the insulation of the bus bars.


5. Circuit Breaker High Potential Testing


This test is specifically designed to assess the circuit breaker's ability to withstand high voltages after installation and to check the integrity of the vacuum bottle. 

  • Procedure:
    • AC high-voltage injection is used to test the circuit breaker's withstand voltage. 
    • Insulation is measured before and after the high voltage test. 









6. Miniature Circuit Breaker (MCB) Testing


MCB testing focuses on verifying the trip time characteristic and checking the auxiliary contact.

  • Procedure:
    • A Sverker is used as a current source to test the MCB's trip time.
    • The auxiliary contact is checked for proper operation.

7. Auxiliary Relay Testing

Auxiliary relays are tested to verify their output contacts, pick-up and drop-off voltage, and operating and resetting times.

  • Procedure:
    • A Sverker is used as a voltage source, and a multimeter as an ammeter.
    • The relay's output contacts, pick-up and drop-off voltage, and operating and resetting times are measured.



8. Ammeter and Voltmeter Testing

This test aims to calibrate and adjust the readings of ammeters and voltmeters.

  • Procedure:
    • A FREJA 300 is used to inject current and voltage values.
    • The injected values are compared with the meter readings, and the error is calculated.

9. Current Transformer Loop Testing

This test verifies the integrity of the entire current transformer loop within the switchgear.

  • Procedure:
    • A FREJA is used to inject current, and a mini-clamp is used to measure the current at various points in the loop.


10. Voltage Transformer Loop Testing


Similar to CT loop testing, this test checks the integrity of the entire VT loop.

  • Procedure:
    • A FREJA is used to inject voltage, and a multimeter is used to measure the voltage at different points in the loop.


11. Lockout Relay (86) Testing


Lockout relays are crucial for tripping circuit breakers in response to protective relay commands. These tests verify their contact operation, pick-up and drop-off characteristics, and timing.

  • Procedure:
    • A Sverker is used as a voltage source, and a Fluke as an ammeter.
    • The relay's contacts, pick-up and drop-off values, and operating and resetting times are measured.

12. Lockout Supervision (74-86) Testing


This test verifies the functionality of the lockout supervision relay, which monitors the DC supply to the lockout relay.

  • Procedure:
    • A Sverker is used as a voltage source and a Fluke as an ammeter.
    • The relay's contacts, pick-up and drop-off values, and operating and resetting times are measured.

13. DC Supervision Relay Testing


This test checks the DC supervision relay, which monitors critical DC loops within the panel.

  • Procedure:
    • A Sverker is used as a voltage source and a multimeter as an ammeter.
    • The relay's contacts, pick-up and drop-off values, and operating and resetting times are measured.


14. Trip Circuit Supervision Relays (TCS) Testing


TCS relays monitor the trip circuit to ensure its health. This test verifies their functionality.

  • Procedure:
    • A Sverker is used as a voltage source and a Fluke as an ammeter.
    • The relay's contacts, pick-up and drop-off values, and resetting time are measured.


15. Primary Injection Testing

This test verifies the overall CT ratio and relay settings.

  • Procedure:
    • A primary injection tester is used to inject current into the bus bar.
    • Current is measured using a power clamp and mini-clamp.

16. Bus Bar Stability Testing


This test checks the CT ratio, relay settings, and CT polarity, and verifies that the summation of current is zero under normal conditions.

  • Procedure:
    • A primary injection tester is used to inject current.
    • Current is measured using a mini-clamp.


17. Medium Voltage Switchgear Function Test


The Medium Voltage Switchgear Function Test is a crucial, comprehensive evaluation that combines various individual tests to validate the overall operation of the switchgear system. It represents a final verification phase, ensuring all components work together as designed after installation.

  • Procedure:

    • DC Loop Segregation: The DC control circuits within the panel are systematically checked by energizing each loop individually to ensure proper isolation and operation.
    • Closing Command Circuit: The circuit that initiates the closing of the circuit breaker is tested to confirm its functionality.
    • Opening Command Circuit: The circuit that initiates the opening of the circuit breaker is tested to confirm its functionality.
    • Automatic Transfer Switch (ATS) Logic and Functionality: The automatic transfer switch logic is tested to ensure it operates correctly in transferring power between sources.
    • Anti-parallel Scheme Check: The anti-parallel scheme, if present, is verified.
    • Mechanical Interlock Check: Mechanical interlocks are inspected to ensure they prevent unsafe operations.
    • Circuit Breaker Interlock Check: Interlocks between the circuit breaker, close switch (CS), and bay control unit (BCU) with the earth switch are verified.
    • Alarm Signal Check: Alarm signals are traced from the source to the relay to ensure proper indication.
    • Inter-trip Check: Inter-trip functions between circuit breakers at different voltage levels are tested.
    • Relay Logic Confirmation: The overall relay logic is confirmed.
    • Circuit Breaker Functionality Check: The circuit breaker's functionality is thoroughly checked, including operations such as traveling contact operation, manual charging, manual close and open operations, racking in and out, and counter operation. The trip coil (TOC), MOC, trip coil, and close coil are also verified.

This expanded explanation offers a more detailed breakdown of the functional tests performed on medium voltage switchgear.



18. Protection Relay (Main Relay) Testing


Protection relays are vital components of MV switchgear, responsible for detecting faults and initiating tripping commands to circuit breakers. This test ensures the relay functions according to its approved final settings.

  • Procedure:
    • Burden test: The burden of the relay is measured under both normal and abnormal operating conditions.
    • Binary input/output check: The functionality of the relay's binary inputs and outputs is verified.
    • Measurements: The CT and VT ratios are set, and the relay's readings are compared with injected values.
    • High Impedance Differential Protection for Bus Bar circuit (87B): The relay's pickup and drop-off characteristics and timing are tested.
    • Transformer differential (87T): The relay's pickup and drop-off characteristics and timing are tested for transformer differential protection.
    • Instantaneous over-current (50): The relay's pickup and drop-off values and timing are tested.
    • Instantaneous earth fault (50N): The relay's pickup and drop-off values and timing are tested.
    • Time delayed earth fault (51N): The relay's pickup and drop-off values and timing are tested.
    • Thermal overload: The relay's response to thermal overload conditions is verified.


This expanded explanation provides a more comprehensive overview of the tests performed on main protection relays in MV switchgear.
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