Ring Main Unit (RMU) Test Procedure

Introduction

It is recommended that routine tests be carried out in compliance with the requirements established by IEC 60298.

In a ring main unit distribution system, the feeder encompasses the entire supply area and concludes at the substation from where it originated. The unit makes a closed loop, and the bicycles form a ring. 

The advantages of RMU include the need for less conductor material because each component of the wing has a lower carrying capacity than the radial system. Another benefit is reduced voltage fluctuations. Designing a radial system might be difficult.

By following this complete RMU testing method statement, you can confidently check the integrity and functionality of your RMU system, allowing for seamless operation and reducing potential hazards.

Safety Measures 

The following safety measures must be considered before to, during, and after the test measurements.

• Safety labeling will be implemented.
• Isolate the area using Safety Warning Tape.
• Maintain a safe distance from the equipment being tested.
• Wear necessary personal protective equipment (PPE) before starting any type of testing activity.
• Implementation of appropriate grounding.

Tools Required

The following is a list of the essential tools for ring main unit (RMU) testing should be prepared prior to start the testing:

• Contact Resistance Kit
• 5.0 KV Megger
• High Voltage Testing Kit
• Primary Injection Kit
• Multimeters (Fluke meters)

Relevant Codes & Standards

• NFPA 70
• NEMA
• IEC 60282
• IEC 60265

HV Breaker

Visual Inspection

• Check the RMU for visible evidence of damage, wear, (or) loose connections.
• Ensure that the RMU is correctly grounded.
• Check for correct labelling and signage.

Contact Resistance Test

The HV Breaker consists of three components: 

• Incomer, 
• Transformer, and 
• Outgoing feeders. 

Each of these items must be tested for contact resistance before being subjected to the high voltage test.

The steps listed below must be followed for all three phases.

1). Incomer to Ground

The above procedure is carried out by turning on the earth switch in the incomer breaker and then conducting the contact resistance test by connecting 100 A to 

• R phase to ground, 
• Y phase to ground, and 
• B phase to ground.

2). Outgoing to Ground

The procedure mentioned above is carried out by turning on the earth switch in the incoming breaker and then performing the contact resistance test by providing 100 A to

• R phase to ground, 
• Y phase to ground, and 
• B phase to ground.   

3). Connect the Transformer (or) Fuse unit to Ground

The procedure mentioned above is carried out by turning on the earth switch in the fuse unit breaker and then performing the contact resistance test by delivering 100A to

• R phase to ground, 
• Y phase to ground, and 
• B phase to ground.   

4). Connect the Transformer (Fuse unit) to the Incomer Unit

To do the above sequence test, turn on the breaker for the transformer and incomer units. To test the fuse unit, we need a dummy fuse. The dummy fuse should have a rating of more than 100 A & no resistance. 

We should not utilize the rated fuse provided by the manufacturer. 

After arranging the zero resistance dummy fuses in the transformer feeder (fuse unit), apply 100 amps via contact resistance equipment across the R phase of the fuse unit (transformer) and the incomer unit. 

Repeat for the other Y & B phases.

5). Transformer Unit (Fuse unit) to the Outgoing Unit

To do the above sequence test, turn on the breaker for the transformer unit and the outgoing unit. 

To test the fuse unit, we need a dummy fuse. The dummy fuse must have a rating of more than 100 amps and no resistance. 

We should not utilize the rated fuse provided by the manufacturer. 

After arranging the zero resistance dummy fuses within the transformer feeder (fuse unit), apply 100 amps via contact resistance equipment across the R phase of the fuse unit (transformer) and the incomer unit. 

Repeat for all remaining Y & B phases.

Insulation Resistance Test            

Before injecting the voltage, we must take safety precautions, such as removing the power wires from the breaker to the transformer and removing the fuse from the fuse unit. The SF6 gas pressure needs to be positive.

The bushings on the breakers should be cleaned & tightened. All breakers must be turned ON.

1). Insulation Test before High Voltage

• Measure the insulation resistance test across each phase and ground by delivering 5KV DC through a megger for 1 minute. 
• Before applying 5 KV, ensure that all three units’ breakers are closed.                                  

2). Insulation Test after High Voltage

• Measure the insulation resistance test across each phase and ground by delivering 5KV DC through a megger for 1 minute. 
• Before applying 5 KV, ensure that all three units’ breakers are closed.       

High Voltage Test (High Volt Test or HV Test)

• Establish Hi-volt equipment for injecting the needed voltage to ensure the system’s ability to endure. 
• Before injecting the voltage, we must take safety precautions, such as removing the power wires from the breaker to the transformer and removing the fuse from the fuse unit.
• The bushings on the breakers need to be cleaned & tightened. All breakers must be turned ON.

The following equation must be calculated to inject the high voltage. 

High Voltage Test = [(Rated Voltage X 2) / (√3)]

• The above-mentioned high voltage must be injected for one minute and must withstand. 
• Note the leakage current at time of injection.

Functional Checks

Functional checks include testing the following mechanical interlocks.

• Opening/Closing switch disconnectors.
• Opening/Closing the transformer tee. 
• Opening/Closing the transformer tee’s earth switch.
• Opening/Closing for switch disconnectors.
• E-Circuit Breaker – Earth Switch Interlock.

Earth Fault Indication Test

To perform the Earth Fault Indication test, provide rated CT current to the Earth leakage CT primary & monitor the light blinking at the detector relay.

Micro-ohmmeter Test

• Use a micro-ohmmeter to determine the resistance of important connections.
• Verify that the resistance values are inside acceptable limits.

Earth Resistance Test 

• Conduct an earth resistance test to confirm correct grounding.
• Confirm that the earth resistance values meet the standards.

Thermal Imaging

• Utilize a thermal imaging camera to detect any hotspots or anomalies in the RMU.
• Pay significant attention to connectors, busbars, and any load-bearing components.

Protection Settings Verification 

• Verify protection relay settings.
• During fault simulations, verify protection system operation.

Documentation

• Document all test outcomes, including measurements & observations.
• Check the results against manufacturer specs.
• Record deviations from specifications & corrective actions.

De-Energization

• De-energize RMU safely.
• Reinstall safety devices and obstacles that removed for testing.

Report

• Write a detailed test report describing the methodology, results, and recommendations.
• Include deviations from requirements and corrective actions.