High Voltage Switchgear Installations: Key Planning Considerations

Planning high-voltage switchgear means balancing safety, reliability, cost and ease of maintenance. Most Nigerian factories, estates and commercial buildings take supply at 11 kV or 33 kV from their DisCo, while transmission substations operate at 132 kV and 330 kV. This guide covers the decisions that matter at each level.

1. Choose the right voltage level

The supply voltage depends on the size of the load, the distance from the network and what the DisCo can offer at your location:

• 11 kV: common for commercial buildings, estates and smaller factories.
• 33 kV: used for larger industrial loads, or where 11 kV capacity is limited.
• 132 kV and 330 kV: transmission level, used for very large industrial consumers and grid substations.

A load assessment comes first, because it sets the transformer size and the switchgear ratings. See why load assessment matters.

2. Choose the substation arrangement

The single-line diagram decides how reliable the supply is when equipment fails or needs maintenance.

For 11 kV and 33 kV customer substations

• Ring main unit (RMU): compact, sealed switchgear that connects a transformer to a ring network and lets the ring stay live while one section is isolated. Very common in estates and commercial buildings.
• Indoor vacuum circuit breaker (VCB) panels: used where there are several transformers or feeders, with protection relays on each circuit.
• Two incoming supplies: critical sites can take two feeders with interlocked incomers, so a fault on one feeder does not black out the site.

For transmission substations

• Single busbar: cheapest, but a busbar fault shuts the whole station down.
• Single busbar with bypass: breakers can be maintained without an outage, but a busbar fault still affects everything.
• Double busbar: flexible and easy to maintain.
• Ring bus: economical and flexible, but a fault opens the ring.
• Breaker-and-a-half: very high availability, at the cost of more breakers and more complex protection.

3. Size the equipment correctly

• Normal load current for every busbar, breaker and cable
• Short-circuit fault level, which the switchgear must be rated to withstand and break
• Insulation level and safe clearances
• Protection scheme and relay coordination, so the nearest breaker trips first
• Earthing of the substation and equipment

4. Choose the switchgear type

• Air-insulated switchgear (AIS): lower equipment cost and easy to inspect, but needs more space.
• Gas-insulated switchgear (GIS) and SF6 RMUs: compact and well sealed against dust and humidity, which suits many Nigerian sites. SF6 gas must be handled by trained personnel.
• Vacuum switchgear: widely used at 11 kV and 33 kV, with low maintenance.

5. Commission before energising

No HV switchgear should be energised until it has been tested. A typical commissioning programme includes:

• Insulation resistance testing
• AC or DC pressure (hipot) testing, and VLF testing of cables
• Contact resistance of breakers and busbar joints
• Primary and secondary injection of protection relays
• Breaker timing and functional tests, including interlocks
• Earth resistance testing of the substation earthing

Read the safety rules for working in high-voltage areas before any site work.

Talk to Corustar

Corustar Technological Services is an electrical engineering contractor based in Ikeja, Lagos, working across Nigeria. See our HV/LV Switchgear Maintenance & Commissioning service, request a quote, or call +234 816 976 9672. For outages and faults, call our 24/7 emergency line on 0705 739 9297.