22 kV substation design.

Why 22 kV is the Singapore standard

Singapore's distribution network is built around three voltage tiers: 230/400 V (low tension), 6.6 kV (legacy, being phased out), and 22 kV (the modern distribution standard). Above 22 kV is sub-transmission at 66 kV and 230 kV — territory occupied by rail depots, very large industrial sites and the grid itself.

The reason a 22 kV intake is preferred for medium-to-large demand: at LT, the cables become impractically large above a few hundred amps. Stepping up to 22 kV brings the current down by a factor of about 55 and lets you serve megawatts of demand with sensibly-sized cables. The trade-off is the customer takes on a substation, switchgear, transformer and all the associated regulatory weight.

What's actually inside a 22 kV customer substation

A typical 22 kV / 0.4 kV customer substation includes the following components:

• SP incoming cable termination. The HT cable from SP arrives at the customer's cable end-box.
• 22 kV Ring Main Unit (RMU) or HT switchgear panel — typically SF6-insulated. Provides incoming switching, isolation and earthing, and outgoing switching to the transformer.
• Protection & metering. SP's metering on its side; customer's protection relays (overcurrent and earth fault) on the customer side.
• 22 kV / 0.4 kV transformer. Typically dry-type cast resin for indoor installation, vector group Dyn11, impedance around 6% for a typical distribution transformer. Rating sized to the diversified load with appropriate margin.
• LV switchboard. Main switchboard immediately downstream of the transformer. Houses the LV main incomer, busbar, outgoing feeders, capacitor bank if used.
• Earthing system. Separate or combined earthing electrodes for HT, LV neutral, equipment earth.
• Auxiliary supplies. Substation lighting, ventilation fans, control supplies, small power.
• Control panel. Annunciator, alarm interfaces, SCADA tie-ins if applicable.
• Cable trays and trunking. Segregated HT, LV and control routes.

The substation room — what it has to provide

The substation room is part of the building works and must satisfy a long list of requirements from SP and from BCA / SCDF:

• Floor area — sized to the equipment with clear maintenance access on all required sides. Typical small 22 kV substations are 40–60 m²; larger or dual-transformer rooms can be 100 m² or more.
• Floor loading — capable of supporting the transformer (multi-tonne) and the switchgear.
• Ceiling height — typically 3 m clear minimum to accommodate equipment and overhead cable management.
• Doors — large enough to admit the largest piece of equipment (usually the transformer) plus access doors for personnel; specified opening direction; with rated fire performance where required.
• Ventilation — sized to dissipate transformer heat losses; natural where possible, mechanical with fail-safe interlocks where required.
• Fire performance — fire-rated enclosure as per SCDF requirements; sometimes with smoke detection and CO₂ or alternative fire suppression.
• Flood resilience — raised floor / kerb to keep the substation above local flood level.
• SP access — independent access from a public road or fire engine access; security arrangements as specified.
• No services overhead — no water pipes, drainage or non-essential services routed above the substation room.

Protection & co-ordination

The 22 kV / LV protection scheme has to do three things simultaneously: (1) protect the equipment from fault damage; (2) protect personnel from arc flash and shock; and (3) co-ordinate with SP's upstream protection so that customer faults don't take out SP's network.

Typical protection scheme:

• 22 kV incomer protection — overcurrent (50/51) and earth fault (51N), often with sensitive earth fault.
• Transformer protection — primary side overcurrent, restricted earth fault, optionally differential protection on larger transformers, plus mechanical protections (Buchholz, oil temperature, winding temperature on oil-filled designs).
• LV main protection — main breaker with electronic trip unit, settings co-ordinated upstream and downstream.
• Selectivity study — time-current curves plotted; verifies that faults at any point trip only the nearest upstream device.

Submission cycle (where the design meets the regulator)

1. SP application with single-line diagram, load schedule, substation room sketch.
2. SP issues Conditions of Service Connection.
3. PE-endorsed detailed design aligned to the Conditions.
4. BCA submission of the substation room as part of the building works approval.
5. SCDF submission for fire safety aspects of the substation room.
6. Procurement, installation, T&C.
7. SP energisation.

Common pitfalls

• Substation room positioned underneath habitable space. Acoustic and safety concerns; sometimes forces redesign.
• Insufficient transformer offloading space. Transformer maintenance requires room to wheel it out.
• Ventilation undersized. Transformer overheats; thermal trips during summer.
• Earthing electrodes installed late. Site congestion makes earth electrode work expensive and uncertain in fall-of-potential testing.
• Protection settings copied from another project. Co-ordination has to be specific to your network — there is no "standard" set of settings.
• LV switchboard built before the protection co-ordination is finalised. Forces switchboard modifications late.

Frequently asked questions

Can a 22 kV substation be located on an upper floor?

Yes, although ground floor or basement is more common for transformer access. Upper-floor substations need careful structural design (floor loading), transformer access route (lift or removable wall panel) and fire/flood considerations.

How big does the transformer need to be?

Sized to the diversified maximum demand with engineering margin — typical sizing factors range from 1.1× to 1.4× of MD depending on growth allowance and harmonic content. Avoid significantly oversizing — it costs more capital and runs at low efficiency.

Oil-filled or dry-type transformer?

For Singapore indoor distribution, cast-resin dry-type is the modern default — no fire/spill concerns, low maintenance. Oil-filled is still used for larger ratings or where supplied directly by SP.

Can we connect a generator in parallel?

Parallel-to-grid operation requires explicit SP approval, anti-islanding protection, and additional licensing under EMA's generation licence regime if above threshold. Most customer generators are emergency-only (closed transition or open transition, not parallel).