TMG-250 250 kVA 10/0.4 kV transformer (S13-M-250)

TMG-250 (S13-M-250) technical data

Rated current and permissible load

Losses, efficiency and comparison with amorphous SH15

The amorphous-alloy core of the SH15 cuts the no-load loss by 52 % compared with the S13. For one 250 kVA transformer this saves about 1314 kWh per year, because no-load loss is present the whole time the transformer is energised, regardless of the load.

Where a 250 kVA transformer is used

A 250 kVA unit is typically chosen to supply a single mid-sized building or site from its own substation: an apartment building, a school or clinic, a small production unit or a wholesale warehouse. It feeds lighting, ventilation, lifts, pumps and process equipment, with headroom for seasonal load peaks. The hermetically sealed corrugated tank with vacuum oil filling keeps the oil out of contact with the atmosphere, which suits substations that are not permanently staffed.

• Apartment building with lifts and pumps
• School, kindergarten or clinic
• Small production unit or workshop
• Wholesale warehouse with cold rooms
• Office building or retail complex
• Pad-mounted or prefabricated compact substation

How to choose a TMG-250

Start from the calculated load. For continuous operation it is sensible to load a transformer to 70–80%, which for the TMG-250 means 158–180 kW at cos φ 0.9. That margin absorbs the starting of lift, pump and compressor motors and several years of load growth. If the actual load is well below 158 kW and no growth is expected, the TMG-160 is more economical, with lower no-load loss and capital cost. If the design load already exceeds 180 kW, or a production extension is planned, take the TMG-400 now rather than replacing the transformer in a few years.

Choose the vector group by the nature of the load. Apartment buildings, schools and clinics have many single-phase loads, so the phases are loaded unevenly and third-harmonic currents flow in the neutral. Dyn11 is preferred here: it copes better with unbalanced loading, circulates third harmonics in the delta HV winding and gives a larger single-phase fault current, helping protection operate reliably on long 0.4 kV feeders. Yyn0 remains the traditional choice where the load is mainly three-phase and the neutral lightly loaded, as in a pump station or a workshop with electric motors. The TMG-250 load loss is 3200 W with Dyn11 and 3050 W with Yyn0.

Then choose the loss level. No-load loss is incurred all 8760 hours a year regardless of load, so at low average loading the amorphous SH15-M-250 is the better choice: 140 W instead of 290 W for the S13, 52% less. This is typical of schools, offices and warehouses without refrigeration, where the transformer is almost idle at night, at weekends and during holidays. The SH15 is built with Dyn11 as standard; with a three-phase three-limb core, Yyn0 can also be supplied on request, so state it in your inquiry if you need it. With a high, steady load, such as a two-shift workshop or a warehouse with cold rooms, the standard S13 remains a sound choice, and projects requiring energy-efficiency grade 1 to GB 20052-2020 can use the S-M-NX1.

Compact substation with a 250 kVA transformer

The 250 kVA transformer is also supplied as part of a compact substation: ZGS American-type pad-mounted substation (KTPN), up to 1600 kVA, or YB European-type substation (BKTP / GKTP), up to 2500 kVA. The substation is assembled and tested at the factory, so the site work mainly comes down to setting it in place and connecting the cables.

Other TMG (S13) ratings