PV step-up substations for solar plants in Uzbekistan

Utility-scale solar plants in Uzbekistan are built on open, flat, hot and dusty ground. PV step-up substations for solar plants sit between the inverters and the 10 kV or 35 kV collector network, carrying peak load, peak ambient temperature and wind-blown sand at the same hour of the day. This article sets out what to define when specifying these units, with the ZGS-PV step-up box substation, the CHT1-PWS booster station and the ESS integrated cabin as reference designs.

From inverter output to the collector network

Central and large string inverters deliver three-phase AC at a voltage set by the inverter manufacturer, typically between 0.6 and 0.8 kV; the reference designs below are rated for 0.8 kV. This is well above the 0.4 kV of standard distribution equipment, so each inverter block needs its own step-up transformer to the collector voltage: 10 kV for small plants on a distribution network, 35 kV for utility-scale plants feeding a main substation, at 50 Hz. A step-up unit combines three parts in one factory-assembled enclosure:

• LV chamber: terminals for one or two inverters, circuit breakers, auxiliary power and communication equipment.
• Transformer: oil-immersed in most outdoor designs, 0.8 kV to 10 or 35 kV, one LV winding per inverter.
• HV chamber: vacuum circuit breaker or load switch with fuses and cable terminations, optionally as a ring network so that several units share one feeder.

The ZGS-PV step-up box substation follows this layout with capacities up to 6300 kVA and a choice of terminal (Z) or ring-network (H) HV connection in the model code.

Split-winding transformers for two inverters

Most central-inverter manufacturers do not permit two inverters to be paralleled on one LV busbar, because of circulating currents between the inverters and the fault contribution each would see from the other. The standard answer is a split-winding transformer: one HV winding and two separate 0.8 kV windings, each feeding its own inverter.

• The high impedance between the two LV windings limits the interaction and the fault contribution from one inverter to the other.
• One transformer, one HV switch and one foundation serve two inverter blocks, halving the HV connection points on the feeder.

In the ZGS-PV model code the letter F designates the split-winding version and the suffix G a photovoltaic application. Sizing follows the inverters and must include the power factor. A worked example with assumed figures: two inverters of 1250 kW, required to operate down to power factor 0.9 at their terminals, need 2 x 1389 kVA = 2778 kVA, which calls for a 3150 kVA unit rather than 2500 kVA.

Box-type PV step-up substations for solar plants

The ZGS-PV places the HV switchgear and protective fuses inside the transformer tank, using the oil as insulation for the 10 kV or 35 kV side, which reduces clearances and makes the unit compact. Features that matter on site:

• Hermetically sealed tank with the oil isolated from the atmosphere (no conservator, no breather) and external plate radiators.
• IP54 for the HV and LV chambers, IP68 for the transformer body; enclosure surfaces shot-peened before coating for resistance to corrosion, UV and sand.
• Optional pressure gauge, thermometer and load-switch position signals with communication interface for the plant SCADA.

In the alternative Chinese-type (Huashi) layout the transformer tank stands outside the enclosure, cooled directly by ambient air, and the enclosure holds only the HV and LV chambers. The HV side uses either a vacuum circuit breaker or a load switch with fuses; in the fuse combination an interlock trips the switch when any fuse operates, and the switch can only be closed again after the fuse is replaced. Enclosure protection is not less than IP54, transformer body IP68.

PCS booster stations and integrated cabins for battery storage

Storage is increasingly specified alongside solar plants to shift energy into the evening peak. Between the batteries and the collector network sits a bidirectional PCS with its own step-up transformer, which carries power in both directions and may be loaded at night; the daylight-only load profile of a PV transformer does not apply.

The CHT1-PWS booster station is a skid-mounted unit combining the PCS, a busbar bridge, an LV chamber for communication and auxiliary distribution, an oil-immersed transformer and an HV chamber with a vacuum load switch or circuit breaker. The number in the model code is the rated power in kW. Assembly, wiring and testing are completed in the factory and the unit is lifted into place as one piece. Where fire-safety requirements rule out oil, the European-type booster station uses a dry-type transformer in the same configuration.

The ESS integrated cabin is a standard 10 ft or 20 ft prefabricated cabin containing the HV/LV unit, bidirectional PCS, local monitoring, cooling unit, access control, lighting and a fire-protection unit with PACK-level early warning of thermal runaway; it is designed for desert, high-altitude, cold and coastal sites.

Desert design: heat, dust and cooling

IEC 60076-1 (GB/T 1094) and GOST practice base the rated temperature rise on normal service conditions of 40 °C maximum ambient, with a 30 °C monthly average for the hottest month. Summer air temperatures on Uzbek solar sites exceed the 40 °C limit, and the transformer reaches peak load at the same hour as the enclosure roof is under full sun. Points to define in the specification:

• Ambient temperatures: maximum and average values on the datasheet, with a reduced temperature-rise design or a derating rule where 40 °C is exceeded; minimum winter temperature for oil selection.
• Cooling: sealed tank with external plate radiators; louvres with sand filters and forced ventilation for the LV chamber; air-conditioning for chambers housing the PCS or communication equipment.
• Dust, sand and sun: IP54 chambers, IP68 tank, sealed cable entries from below, a ventilated double roof or sunshade, and a coating resistant to abrasion and UV.
• Monitoring: oil temperature and tank pressure signalled to SCADA.

No-load loss matters more than on a distribution transformer, because the unit normally stays energised from the grid for all 8760 hours of the year while it generates only in daylight. For orientation only: the S13-M-2500 10/0.4 kV distribution transformer in our range has a no-load loss of 1830 W, about 16 000 kWh per year. A 35 kV split-winding PV unit is a different design, so request guaranteed losses at tender stage and evaluate them with your own tariff assumptions.

Grid-code and protection considerations

The network operator's connection conditions for each project define reactive-power range, ride-through behaviour and protection settings at the point of connection. Most are inverter functions; the step-up unit must not restrict them. In general terms:

• Apparent power: rate the transformer for the inverter output at the required power factor, not at unity.
• Impedance and vector group: follow the inverter manufacturer's transformer specification, which normally defines the permissible impedance range, vector group (commonly Dy11y11 or Yd11d11, depending on the inverter's earthing concept) and LV neutral treatment; impedance sets both the fault level at the inverter terminals and the voltage drop at full output.
• Protection: a vacuum circuit breaker with overcurrent and earth-fault relay where the feeder needs selective tripping, or a load switch with fuses for simpler schemes; surge arresters at the HV terminals; pressure-relief device and oil temperature indication on the tank; one circuit breaker per inverter winding.

Each unit is routine-tested at the factory, and a type-test report from a CNAS-accredited laboratory is available for the YBM(YHB)-40.5/1.14-6300 Chinese-type (Huashi) PV/wind prefabricated substation; see the certificates page.

Specification checklist for PV step-up substations

1. Inverter AC voltage, number of inverters per unit and their maximum continuous current.
2. Required kVA at the specified power factor; split-winding (F) version for two inverters.
3. Collector voltage (10 or 35 kV) and HV connection scheme: terminal or ring network.
4. HV switching device and relay protection requirements.
5. Site ambient maximum and minimum, altitude and dust load; derating rule and IP class.
6. Guaranteed losses, impedance, vector group and LV neutral treatment from the inverter specification.
7. Foundation, cable entries, transport dimensions, factory tests and documentation language (EN/RU).

All units are manufactured by Jiangsu Xinhong Electrical Equipment Co., Ltd. and configured to each plant. Send the inverter datasheet, single-line diagram and site design conditions through our contact page to receive a quotation with transformer parameters, enclosure layout and delivery time for Uzbekistan.