In short. Transformer sizing for a generator-fed system has two parts. The steady-state part is simple: the transformer kVA must carry the demand after diversity, with margin for growth and site temperature. The harder part is energisation. A transformer draws magnetising inrush of several times rated current for a few cycles, and a diesel generator with high sub-transient reactance sees that as a severe voltage dip. The generator, not the transformer, is usually the limiting item.
Where transformers appear in a generator installation
Most generator sets sold in India below 1,000 kVA produce 415 V directly and feed the switchboard without a transformer. A transformer enters the picture in four common situations: a step-up from a 415 V generator plant to an 11 kV campus ring main, a medium-voltage generator feeding step-down substations, an isolation transformer on a sensitive load, and a rental set back-feeding an existing 11 kV intake during a long DISCOM outage. Each case asks two questions: can the transformer carry the load, and can the generator energise it.
The steady-state calculation
S = (√3 × VLL × I) ÷ 1000
Here S is the three-phase apparent power in kVA, VLL is the line-to-line voltage in volts and I is the line current in amperes. Rearranged, it gives the full-load current on either side: a 1,000 kVA transformer at 11 kV draws 52.5 A on the primary and delivers 1,391 A at 415 V (illustrative). Those currents fix cable sizes, breaker ratings and CT ratios, which is why the kVA must be settled before switchgear is ordered.
The kVA itself should come from a load study rather than the sum of nameplates. Apply diversity, add known expansion, then derate for site conditions. Transformer ratings under IS 1180 and IS 2026 assume a reference ambient; a transformer in a poorly ventilated substation in Hyderabad in May runs hotter and should not sit at 100 percent for long periods. We normally look for 60 to 80 percent loading under the maximum demand the generator will actually carry. The generator sizing guide and the transformer capacity and loading calculator cover the demand side.
Why energisation is the real problem
When a transformer is switched onto a live source the core may saturate on the first half cycle, and the magnetising current can reach eight to twelve times rated current before decaying over several cycles to a few seconds. The peak depends on the point on the voltage wave at which the breaker closes and on the residual flux in the core, so it is partly chance. On a stiff DISCOM supply this is a nuisance. On a diesel generator it is a different event. The alternator’s sub-transient reactance, typically 0.10 to 0.20 per unit, limits the current it can push into a near short circuit, and the inrush is drawn through that reactance. Terminal voltage collapses, the AVR forces field current to recover, and the controller may trip on under-voltage or over-current. The engine is hardly involved, because magnetising current is almost entirely reactive.
Three outcomes are possible: the set rides through a deep but short dip, the set trips, or the transformer’s primary protection operates on the inrush. Which one occurs depends on the ratio of transformer kVA to generator kVA, the alternator reactances, the excitation system and the protection settings. The transformer energisation and inrush study page describes how this is modelled.
How much generator does a transformer need
No single multiplier is correct for every case, but field experience and manufacturer application notes converge on a screening rule. A generator with rated kVA roughly equal to or greater than the transformer kVA, with a PMG-excited alternator and a sub-transient reactance at the low end of the range, will usually energise an unloaded transformer with a dip under 30 percent. A much smaller generator sees a deeper dip and tripping becomes likely. Treat the table as a screening guide; the final answer needs the alternator datasheet and a transient calculation.
| Transformer kVA relative to generator kVA | Typical energisation behaviour | What to do |
|---|---|---|
| Up to 50 percent of generator rating | Modest dip; energisation routine | Check protection settings only |
| 50 to 100 percent of generator rating | Noticeable dip; depends on X”d, excitation and residual flux | Transient check with alternator data |
| Larger than the generator | Deep dip, under-voltage trips, possible primary protection operation | Energise on voltage ramp, or split into smaller transformers |
| Several transformers closed together | Inrush adds up like one very large transformer | Stagger closing with time delays |
The soft energisation trick
The cleanest way to avoid inrush is not to switch the transformer onto a live bus at all. If the transformer breaker is closed while the generator is at rest and the set is started with the AVR ramping voltage from zero over a few seconds, the core flux builds gradually and there is no saturation transient. Many controllers offer a voltage ramp for this purpose. It only helps the first transformer at start-up; any transformer added later to a live bus sees full inrush.
Impedance, voltage drop and fault level
Transformer percentage impedance, usually 4 to 6 percent for distribution sizes, cuts both ways. Higher impedance limits secondary fault current and may allow lower-rated switchgear, but it increases regulation, so secondary voltage falls further as load rises. On a generator-fed system the generator’s own reactance is already large, so a high-impedance transformer compounds the dip under motor starting. The voltage drop guide covers the cable side of the same problem, and the fault current guide shows how generator and transformer impedances combine to set the short-circuit duty of the downstream board. Vector group matters too: a Dyn11 transformer gives a secondary neutral for earthing and blocks zero-sequence current between windings.
A worked example (illustrative)
A pharmaceutical unit near Hyderabad has an 11 kV intake and a 750 kVA Dyn11 transformer feeding a 415 V board with a measured maximum demand of 480 kVA. The DISCOM feeder will be out for ten days and a 415 V rental set will back-feed the transformer. A 500 kVA set would run at 96 percent, too high for ten days, so a 625 kVA set is chosen to keep loading near 77 percent. Energisation is the issue: the transformer is 120 percent of the generator rating, and switching it onto a live set would produce a dip the under-voltage protection is unlikely to tolerate. The sequence adopted is to close the transformer breaker with the set stopped, start on voltage ramp, then close the 11 kV and 415 V feeders in turn, after confirming the primary protection will not operate on normal magnetising current.
Related photographs and videos
Common mistakes
- Sizing the generator from the transformer kVA alone, as if the transformer were a load; the real load is behind it and the rating only sets an upper limit.
- Ignoring inrush and discovering at commissioning that the generator trips every time the transformer breaker is closed.
- Specifying a transformer at 100 percent of the expected demand with no allowance for ambient temperature, which shortens insulation life in a hot substation.
- Energising several campus transformers simultaneously from one generator plant, which multiplies the inrush and collapses the bus voltage.
What to send us
Send a photograph of the transformer nameplate showing kVA, voltages, vector group and percentage impedance, and a photograph of the alternator nameplate so we can look up its reactances. Add the single-line diagram, the measured or estimated maximum demand on the secondary, and whether the transformer will be energised at start-up or switched onto a live bus. Use the application engineering enquiry and we will return a sizing and energisation recommendation.
Questions engineers ask
Can a 500 kVA generator energise a 1,000 kVA transformer?
Not reliably when the transformer is switched onto a live bus; the inrush approaches the alternator’s short-circuit capability and the dip will often trip the set. If the transformer is already connected when the generator starts and the AVR ramps voltage gradually, the same generator usually energises it without difficulty.
Does transformer inrush load the engine?
Very little, because magnetising current is almost entirely reactive. The stress falls on the alternator and the AVR, which is why alternator reactance and excitation type matter more than engine rating for this event.
What transformer loading is sensible on a generator-fed system?
Aim for 60 to 80 percent of transformer rating under the demand the generator will actually carry. That leaves margin for high ambient temperature, starting currents and the growth that almost always arrives.
Do I need a transient study or is a rule of thumb enough?
For a single transformer well below the generator rating a rule of thumb is adequate. When the transformer approaches or exceeds the generator rating, when several transformers are involved, or when the installation is critical, a study using the alternator reactances is cheap compared with a failed commissioning.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.