1 – 20 MVA
Power Transformers
Power transformers from 1 MVA to 20 MVA in the 33/11 kV class — ONAN, ONAF, ester-filled (KNAN) or dry type (AN), with CRGO or amorphous core, Dyn11 or the vector group your system needs, with on-load or off-circuit tap changing, built to IS 2026 and IEC 60076.
Power transformer under assembly
Overview
Fully active and operational power-class output.
33/11 kV substation and heavy industrial ratings.
Rated output published at each cooling stage.
Power-class capability, in-house.
The power transformer line closed the last gap in the chain. Between distribution and power class we now cover 16 kVA right through to 20 MVA — a span very few regional manufacturers hold on a single site.
The standard substation build is 33/11 kV, Dyn11, ONAN, with an off-circuit tap changer at +5% to −10%. Where the network needs voltage regulation under load we fit an on-load tap changer at +5% to −15%, and where the load profile justifies forced cooling we supply ONAF with a rated output at each cooling stage.
Technical data
Specification.
Built to the purchaser's schedule. Fields marked On enquiry are issued against a specific enquiry. Delivery as per the client's requirement, confirmed with the quotation. Open the rows below for construction details, reference tables and fittings.
General
| Parameter | Specification |
|---|---|
| Rated power | 1 MVA to 20 MVA |
| Typical ratings | 1 · 1.6 · 2 · 2.5 · 3.15 · 5 · 6.3 · 8 · 10 · 12.5 · 16 · 20 MVA |
| Voltage class | 33 / 11 kV; highest system voltage 36 kV |
| Phases | Three-phase |
| Type | Oil-immersed, outdoor, core type — plain or corrugated tank; dry type to order |
| Frequency | 50 Hz standard; 60 Hz for export |
| Cooling | ONAN or ONAF; KNAN / KNAF with natural ester fluid; AN / AF for dry type — rated output stated at each stage |
| Applicable standards | IS 2026 (Parts 1–5), IS 3639, IEC 60076 series |
Electrical
| Parameter | Specification |
|---|---|
| Vector group | Dyn11 standard; YNd11, YNyn0, Dd0 and others to order |
| Impedance | Per rating — see the utility reference table below; tolerance ±10% |
| Tap changer | Off-circuit +5% to −10% in 2.5% steps (7 positions), or on-load +5% to −15% in 1.25% steps (17 positions) |
| Neutral earthing | Solidly earthed; other arrangements to order |
| Insulation level (BIL) | 170 kVp at 33 kV; 75 kVp at 11 kV |
| Temperature rise | 45 K winding / 40 K top oil over 50 °C ambient, or 55 K / 50 K where the specification allows |
| Short-circuit withstand | Per IS 2026 (Part 5) — 2 seconds at the specified fault level |
| Guaranteed losses | As per technical specification — no-load and load losses guaranteed per rating against the purchaser's schedule |
Construction and further particularsMaterials, tank, paint and mechanical details
Construction
| Parameter | Specification |
|---|---|
| Core | CRGO silicon steel lamination to IS 3024, step-lap mitred, bolt-free clamping; amorphous metal core where the loss schedule calls for it |
| Windings | Electrolytic copper, disc or helical by rating |
| Insulating fluid | Mineral transformer oil to IS 335:2018, or natural ester (K-class) fluid to IEC 62770 |
| Oil preservation | Conservator with diaphragm sealing, at least 10% of total oil volume |
| Radiators | Detachable pressed-steel radiators with shut-off valves, air release and drain plug |
| Tank | Fabricated plain or corrugated, vacuum and pressure tested |
| Painting | Four coats, 52–60 µm total dry film, shade 632 Admiral Grey per IS 5 |
| Marshalling box | IP55, weather and vermin proof, with space heater and MCB |
Reference data — what the standards requireTables from the Indian Standard, for checking any quotation against it
Impedance as specified by Indian utilities
| Rating | Impedance at 75 °C, principal tap | Specified by |
|---|---|---|
| 3.15 MVA | 6.25% | OPTCL |
| 5.0 MVA | 7.15% | OPTCL, APDCL, MSEDCL |
| 8.0 MVA | 8.35% | OPTCL |
| 10.0 MVA | 8.35% | APDCL, MSEDCL |
Reproduced so you can see the basis we work to. Your own specification governs; we build to it.
| Class | Tap positions | Range |
|---|---|---|
| Three-phase up to 200 kVA | 4 | +2.5% to −5% of HV in 2.5% steps |
| Three-phase above 200 up to 2500 kVA | 7 | +5% to −10% of HV in 2.5% steps |
| Power transformer, off-circuit (utility practice) | 7 | +5% to −10% in 2.5% steps |
| Power transformer, on-load (utility practice) | 17 | +5% to −15% in 1.25% steps |
| Class | Winding rise (by resistance) | Top-oil rise (by thermometer) | Ambient basis |
|---|---|---|---|
| Three-phase up to 200 kVA (IS 1180) | 40 K | 35 K | 50 °C |
| Three-phase above 200 up to 2500 kVA (IS 1180) | 45 K | 40 K | 50 °C |
| Single-phase up to 25 kVA (IS 1180) | 40 K | 35 K | 50 °C |
| IEC 60076-2 baseline | 65 K (average) | 60 K | 40 °C |
Standard and optional fittingsWhat is supplied as standard, and what is available to order
Standard Fittings
Supplied as standard
- Conservator with diaphragm oil sealing and isolating valves
- Magnetic oil level gauge, 150 mm dial, with low-level alarm contact
- Double-float Buchholz relay with isolating valves, bleed valve and test cock
- Spring-loaded pressure relief device with alarm and trip contacts
- Oil temperature indicator and winding temperature indicator, 150 mm dials
- Silica gel breather with oil seal
- Detachable radiators with top and bottom shut-off valves
- Two 50 mm filter valves on diagonally opposite corners
- Bottom drain-cum-sampling valve, padlockable
- HV and LV bushings with arcing horns and bimetallic connectors
- Marshalling box to IP55 with space heater
- Lifting lugs, jacking pads, haulage lugs and bidirectional flanged rollers
- Rating, diagram, details, guarantee and danger plates
Testing
Routine tests before dispatch
- Winding resistance at each tap
- Ratio, polarity and phase relationship
- Impedance voltage at minimum, normal and maximum tap
- Load loss and no-load loss at 100% and 110% voltage
- Insulation resistance and polarisation index
- Harmonic level in no-load current
- Separate-source and induced overvoltage withstand
- OLTC duty cycle through all positions
- Core insulation test at 2000 V for 1 minute
- Capacitance and tan delta
- Tank vacuum test and pressure test
- Oil leakage and gas collection
Compliance
Standards & testing.
Routine tests are performed on every unit before dispatch. Type tests are certified from CPRI, ERDA or NABL-accredited laboratories on request.
| Standard | Scope |
|---|---|
| IS 1180 (Part 1):2014 | Outdoor oil-immersed distribution transformers up to 2500 kVA, 33 kV — including the energy-efficiency levels |
| IS 2026 (Parts 1–5) | Power transformers — general, temperature rise, insulation levels, terminal markings, short-circuit withstand |
| IEC 60076 series | International power transformer standard — the export equivalent |
| IS 3024 | Cold-rolled grain-oriented (CRGO) electrical steel sheet and strip |
| IS 335:2018 | Insulating mineral oil for transformers and switchgear |
| IS 3639:1966 | Fittings and accessories for power transformers |
| IS 2099 / IS 7421 | HV and LV porcelain bushings |
| IS 3637 | Gas-and-oil-operated (Buchholz) relays |
| IS 5:2007 | Colours for ready-mixed paints — shade 631 for distribution, 632 Admiral Grey for power |
| IS 6600:1978 | Guide for loading of oil-immersed transformers |
Type & special tests
Available on request
- Lightning impulse with chopped wave, all three phases HV and LV — IS 2026 (Part 3)
- Temperature-rise test on the extreme minus tap — IS 2026 (Part 2)
- Thermal and dynamic short-circuit withstand — IS 2026 (Part 5)
- Noise level measurement — IS 2026 (Part 10)
- Post-short-circuit reactance change limited to 2%
- Frequency response analysis (FRA) as a special test
- Dissolved gas analysis as a special test
Purchaser and third-party pre-dispatch inspection is accommodated on request, including for export consignments. Type-test certificates are issued from CPRI, ERDA or NABL-accredited laboratories — copies available on request.
Inside the works
Tank and paint, in our own works.
Power-class tanks are fabricated, blasted and painted on the same lines as the distribution range, under the same inspection regime.
Walk the whole works
Applications
Where it goes to work.
Distribution substations
33/11 kV substation duty across utility networks.
Heavy industrial plant
Process plant, rolling mills and large manufacturing loads.
Industrial parks
Bulk supply into planned industrial and warehousing estates.
Generation evacuation
Step-up duty at solar and wind plants — see our solar transformer page.
Questions
Frequently asked.
What is the largest power transformer you manufacture?
Do you supply on-load tap changers?
What is the difference between ONAN and ONAF on your units?
Can you meet a DISCOM's guaranteed technical particulars schedule?
Often supplied together
Discuss a power transformer requirement.
Send the rating, voltage class, vector group, impedance and tap range you need, and we will confirm feasibility and delivery.