WEG SKW Solar Skid: Electrocenter for String Inverters

What it is and where it is used

The WEG SKW Solar Skid is the transformer station from WEG, a Brazilian manufacturer of motors, transformers and electrical equipment, for solar plants with string inverters. In Brazil it is known as an eletrocentro solar or “WEG solar skid”, and WEG manufactures it in that country with its own engineering, which according to the catalog lets it adapt the design to the particular conditions of the project.

The SKW, a transformer station, does not carry the inverter inside (an architecture with string inverters). It receives in its low-voltage panel the 800 V AC outputs of the string inverters. It steps up the voltage with a transformer and delivers to the collector network through the medium-voltage switchgear. WEG offers it in two lines, one for distributed generation and another for centralized plants. From the same manufacturer, the site has the data sheet of the WEG transformer.

Variants SKW1000, SKW2500, SKW5000 and SKW3000 to SKW9000

Characteristics of the distributed generation line

According to the SKID Solar WEG - SKW catalog:

• Low voltage: 800 V inputs, main protection with a breaker, class I+II surge arresters, 20 kVA auxiliary transformer; 50 and 60 Hz.
• Transformer: oil-sealed or dry-type IP54 for outdoor use, primary of 13.8, 23.1 or 34.5 kV, secondary of 0.8 kV (two secondaries on the SKW5000), aluminum windings, taps of ±2 × 2.5%, Dyn1d0 vector group and K-factor = 1. Oil containment pit not included.
• Metering and protection switchgear: air-insulated, with a vacuum breaker, 17.5, 24 or 36 kV class, 630 A, IP3X or IP5XW for outdoor use, and overcurrent protection with a Siemens 7SR line relay.
• Installation: on a masonry base.

Characteristics of the centralized generation line

• Low voltage: 10 to 30 inputs of 800 VAC, 50 or 60 Hz depending on the model, main protection with 1250 or 2000 A three-pole breakers and class I+II surge arresters.
• Transformer: oil-sealed with conservator, primary of 13.8, 23.1 or 34.5 kV depending on the model, secondaries according to the inverter, Dy11y11 vector group, aluminum windings and a metal oil pit integrated into the skid.
• Own protections: Buchholz relay, pressure valve with contacts and oil level indicator.
• Protection and switching panel: SF6-insulated, with a disconnector and vacuum breaker, 630 A. Short-circuit rating of 16 to 25 kA for 1 s depending on the model. 50/51, 50/51N protection (other functions on request).
• Construction: 3150 × 3720 × 7471 mm, 20 to 25 t depending on the model, IP54, for outdoor use, C3 or C4 anti-corrosion protection according to ISO 12944-6; UPS and 10 or 20 kVA auxiliary transformer as options.
• Standards listed in the catalog: EN 61000 (parts 4-2 to 4-6), CISPR 11, EN 55011, EN 50178, IEC 62116, NBR IEC 62116, IEC 62109-1 and -2, NBR 16149 and NBR 16150. The catalog does not list the switchgear or transformer standards; it is advisable to request them in the order documentation.

Protection of the transformer switchgear

In both lines the transformer is protected with a vacuum breaker and overcurrent relay: a Siemens 7SR relay in the distributed line and 50/51, 50/51N functions in the centralized one. In the centralized line, the Buchholz and the pressure valve are wired to the trip of that switchgear, as set out in trips of a solar station. The relay coordinates with the collector network feeder. Anti-islanding and the utility requirements are resolved at the point of connection.

Communications and integration

The centralized line declares Modbus RTU (RS485, USB), Modbus TCP (Ethernet) and IEC 61850. With IEC 61850 the relay and the station can be integrated into the substation SCADA with the same data model as the rest of the IEDs. Plant power setpoints are distributed by the plant controller (PPC) to the inverters.

What is checked at commissioning

At a general level: inspection and grounding of the skid. Transformer tests (oil-filled or dry-type). Verification that the Buchholz, pressure and level trip the breaker. Secondary injection to the switchgear relay. Interlocks and SF6 gas of the switchgear. Low-voltage panel and auxiliaries. Communications. Energization from the collector network and inverter start-up. The guide on commissioning a solar skid details each stage.

Frequent technical doubts

• Catalog: WEG publishes the SKID Solar WEG - SKW catalog in Portuguese (code 50114048, revision 03, February 2024). The values are declared as reference values and other ratings and voltages are available on request.
• Voltages in Brazil and elsewhere: the catalog voltages (13.8, 23.1 and 34.5 kV) are those of the Brazilian grid; for another country the collector voltage is defined in the order.
• Siemens 7SR relay: the exact relay reference and its manual are taken from the documentation of the delivered equipment.

How we help

At Tiza Group we carry out the testing and commissioning of transformer stations and of the complete plant: transformer, switchgear and relay, own protections, communications and energization, in Colombia or elsewhere on request. See the scope in testing and commissioning of solar plants. For the coordination of the station relay and the engineering, see engineering design and studies, done remotely worldwide. To request a quotation, write to us through contact.

Related guides

• Solar transformer station (skid): what it is and parts
• Central or string inverter in solar plants: architecture
• WEG transformer
• Step-up substation of a solar plant
• Protections of a solar plant
• Solar plant commissioning tests
• Huawei JUPITER-9000K-H1 and STS-6000K-H1: solar station
• Sungrow MVS6400-LV, MVS8960-LV and 1+X SG6600UD-MV

References

• WEG, catalog SKID Solar WEG - SKW (code 50114048, rev. 03, 02/2024).
• ISO 12944-6, corrosion protection of steel structures by protective paint systems.
• IEC 61850, communication networks and systems for power utility automation.

Frequently asked questions