Box-type substations (KTP): what is inside and how to specify

A box-type substation (KTP) is a factory-assembled 10/0.4 kV substation in which the HV switchgear, the transformer and the LV distribution board share one weather-proof enclosure. For industrial parks, logistics centres and residential districts in Uzbekistan it replaces a brick-built substation with a unit that arrives tested and is connected within days of being set on its foundation. This guide covers what is inside a box-type substation, how the American-type ZGS and the European-type YB differ, and what a complete specification should contain.

What is inside a box-type substation

Every KTP has three functional blocks:

• HV compartment (10 kV, occasionally 6 kV). Receives the incoming cable(s) and protects the transformer with a load-break switch and HRC fuses, a vacuum circuit breaker (VCB) with protection relay, or a ring main unit with loop-in/loop-out panels. Cable terminations, earthing switch, voltage indicators and interlocks belong here.
• Transformer compartment. Houses an oil-immersed transformer (S13, amorphous-alloy SH15 or energy-efficiency grade 1 S22) or a cast-resin dry-type transformer (SC(B)13, SC(B)H15 or SCB18) of 50–2500 kVA. Oil-immersed units normally require oil containment in the foundation; dry-type units require adequate ventilation, usually forced, and a winding temperature controller with fan control.
• LV board (0.4 kV, NKU). An air circuit breaker (ACB) as incomer, MCCB outgoing feeders, metering, a capacitor bank for reactive-power compensation and LV surge arresters where specified, plus the auxiliary circuits: heating, lighting and fans.

The enclosure – galvanised steel, aluminium-zinc sheet or stainless steel with anti-corrosion painting, ventilated roof, lifting lugs and base frame – is designed for −40 °C to +45 °C.

American-type ZGS vs European-type YB

The two types answer different questions: ZGS minimises footprint and cost, YB maximises access and configuration freedom.

The ZGS combined transformer substation is built to JB/T 10217-2013 and DL/T 1267-2013 as a fully enclosed, fully insulated unit: no live 10 kV part is exposed to the operator. It suits districts that need many small supply points, typically 250–630 kVA.

The YB prefabricated substation is the choice when the LV board must carry a dozen feeders, when a dry-type transformer is required for fire-safety reasons, or when the HV switchgear must be serviced from inside the enclosure. A type-test report for the YB European-type substation is listed on the certificates page.

Single-line examples

Three schemes cover most enquiries from Uzbekistan.

1. Radial (terminal) feed, one transformer. 10 kV cable → load-break switch with HRC fuses (or VCB) → 630 kVA transformer → ACB 1000 A → 8 MCCB feeders + capacitor bank (for example 150 kvar). The standard ZGS-Z configuration and the smallest YB; suitable for a warehouse or a residential block fed from one source.

2. Loop-in/loop-out (ring network). Two cable panels with load-break switches + one transformer protection panel → 1000 kVA transformer → LV board. Both ring cables pass through the substation, so supply is restored by switching if one cable fails. Implemented as ZGS-H or as a YB with a three-panel XGN15-12 ring main unit.

3. Two transformers with LV bus-tie. Two incoming panels → 2 × 1000 kVA → two ACB incomers and an ACB bus-tie with automatic transfer, sized so that one transformer carries the critical load while the other is out. Typical for a logistics centre with refrigerated storage or a continuous production process; normally a YB or a prefabricated cabin substation.

How to specify a KTP

A quotation is only accurate if the following points are fixed in the enquiry, in roughly this order:

1. Incoming scheme. Radial or ring; load-break switch with fuses or VCB; number of HV cable panels; cable type and cross-section; whether the network operator requires an HV metering panel with VT and CT.
2. Transformer. Rated power, 10 or 6 kV primary, vector group (Dyn11 or Yyn0; Dyn11 is the usual choice for unbalanced LV loads, but confirm it with the network operator), oil-immersed or dry-type, loss class. Example: with an assumed 800 kW peak load at cos φ 0.9 the demand is about 890 kVA, so 1000 kVA leaves roughly 10 % reserve. At that rating the S13 oil-immersed transformer (S13-M-1000) has 830 W no-load loss, 10 300 W load loss and 4.5 % short-circuit impedance; the amorphous-alloy SH15 (SH15-M-1000) reduces no-load loss to 450 W at the same load loss.
3. LV feeders. ACB rating (1600 A for 1000 kVA at 0.4 kV), number and rating of MCCB feeders, spare ways, fixed or withdrawable execution, short-circuit rating of the board.
4. Metering. Commercial metering on the HV or LV side, meter class, CT ratios, space for the operator's meter and a communication interface.
5. Reactive-power compensation. Target cos φ (0.92–0.95 is a common target; confirm the value in your connection conditions), bank size in kvar, number of steps, detuned reactors if drives or LED lighting dominate the load.
6. Heating, ventilation and enclosure. Thermostatically controlled anti-condensation heaters in the HV and LV compartments, filtered louvres, forced fans and a temperature controller for dry-type transformers, IP54, −40 °C to +45 °C; sheet material, colour, door orientation and cable entry.

Installation and foundation

The foundation drawing is issued with the order; the general requirements are:

• A reinforced-concrete plinth or a cable basement of about 0.8–1.2 m depth (European type) with openings for HV and LV cables; the box is bolted to an embedded steel frame.
• An oil sump or gravel bed under an oil-immersed transformer, sized for the full oil volume.
• An earthing grid bonded to the enclosure, transformer neutral and HV earth bar; the target resistance is set by the project.
• Clearance for door swing, free space in front of the louvres, and access for a truck-mounted crane: a 1000 kVA YB weighs several tonnes and is lifted by its lugs in one piece.
• Drainage away from the base and level, compacted ground with the bearing capacity stated in the drawing.

On delivery day the unit is placed, cables terminated, insulation-resistance and interlock tests performed, and the transformer energised at no load before feeders are switched in.

Delivery timeline

The stages below are planning assumptions for a standard 10/0.4 kV unit; actual dates are fixed in the quotation.

• Technical clarification, typically 1–2 weeks. Single-line diagram, foundation drawing and equipment list are approved by the customer.
• Manufacturing and assembly. Transformer, HV panels and LV board are built and installed in the enclosure at the Xinhong plant in Pizhou, Jiangsu, including secondary wiring.
• Factory acceptance test. Transformer routine tests and a functional test of the assembled substation; the customer may attend or receive the report.
• Transport. By rail or road from Jiangsu to Uzbekistan; allow several weeks for transit and customs clearance.
• Site installation. Placement and connection take days, provided the foundation and cables are ready.

Because the unit arrives assembled and tested, the critical path on site is the foundation and cable works, not the electrical installation.

Specification checklist

• ZGS for small, low-cost supply points up to 1600 kVA with few LV feeders; YB for 6–16 feeders, dry-type transformers or two-transformer schemes up to 2500 kVA.
• Fix the incoming scheme (radial or ring) and the metering side first.
• Size the transformer against a stated load assumption and compare no-load and load loss values (S13, SH15 or grade 1 models).
• Define the ACB, MCCB feeders, compensation and spare ways of the LV board.
• Specify heaters, ventilation, IP54 and the −40 °C to +45 °C range.
• Prepare foundation, earthing and crane access before the unit arrives.

Send us the single-line diagram, load list and site conditions and we will return a configured KTP proposal with dimensions, foundation drawing and loss data. Request a quotation through the contact page.