In short. A rental transformer is needed when the temporary generator produces one voltage and the site or load needs another, typically 415 V generation feeding an 11 kV or 6.6 kV network, or a medium-voltage rental plant stepping down to 415 V. Select it on kVA, voltages, vector group, impedance and protection, and check that the generator can energise it, because transformer inrush can exceed the generator’s transient capability even when the running load is small.
When a temporary power system needs a transformer
Most rental generators in India generate 415 V three-phase at 50 Hz, which matches the majority of factory, commercial and construction loads, so no transformer is needed. The exceptions should be recognised early, because a transformer changes the layout, cable sizes, protection and mobilisation plan.
- The outage work is on the incoming 11 kV or 33 kV transformer or HT switchgear, and the plant must be fed through its HT network rather than directly at the LT board.
- The load is spread over a large area. Sending several hundred kVA a kilometre at 415 V needs heavy parallel cables and still suffers voltage drop; stepping up to 11 kV for the long run uses far less copper.
- The load runs at a non-standard voltage, such as 690 V drives, 3.3 kV or 6.6 kV motors, or 110 V tools on an imported package.
- The rental plant is above roughly 2 to 3 MVA, where generators feed a medium-voltage bus through step-up transformers. See multi-megawatt rental power.
In each case the transformer is part of the temporary plant and has to be rated together with the generator, switchgear and cables.
The selection data that matters
These parameters have to be fixed before a unit can be matched to a project.
| Parameter | Why it matters on a rental job | Typical check |
|---|---|---|
| Rated kVA | Sets the continuous load at site ambient | Cover peak temporary load with margin for 45 degree Celsius summers |
| Primary and secondary voltage, tap range | Wrong tap means the load sees high or low voltage all day | Confirm the actual site voltage, not the nominal, and the direction of power flow |
| Vector group | Decides phase shift and whether a neutral is available | Dyn11 is the common Indian distribution group; paralleling with an existing transformer needs the same group |
| Percentage impedance | Sets fault level on the secondary and voltage regulation under load | Lower impedance gives better regulation but higher fault current for the switchgear to clear |
| Protection | HT fuse or breaker, Buchholz, winding temperature, earth fault | Protection must suit the generator’s lower fault current, not only the utility’s |
Energisation: the generator must survive the inrush
A transformer draws a magnetising inrush when it is switched on. The peak depends on the point on the voltage wave at closing and on residual flux in the core, and it is commonly several times rated current for a few cycles, decaying over a second or more. A grid absorbs this easily. A rental generator has a much higher source impedance, so the inrush produces a deep voltage dip that the AVR and governor must recover from. A generator rated comfortably above the running load can still trip on under-voltage, see the inrush as a fault, or sag long enough to drop contactors already connected. There are three ways to manage this.
- Size the generator for the energisation event. A generator kVA of 1.5 to 2 times the transformer kVA is a common starting point for dead energisation, but the confirmed answer comes from the generator’s transient data against the transformer’s inrush figure.
- Energise the transformer while the generator builds up voltage, so flux rises gradually. This needs a controller and breaker arrangement that support soft-start energisation.
- Energise the transformer from the utility before transfer, where the sequence allows, so the generator picks up a magnetised core under load.
Where the transformer is large relative to the generator, an energisation study is worthwhile. See transformer energisation and inrush studies for how that is done.
Protection and earthing change when the source is a generator
Utility fault current at an industrial 11 kV bus can be 10 kA or more. A rental generator feeding the same bus through a step-up transformer typically delivers around three times its rated current into a fault, decaying quickly unless the alternator has permanent-magnet excitation. Relays set for utility fault levels may not operate, or may operate too slowly, on generator supply, so settings on both sides of the rental transformer should be reviewed for the generator case before energising.
Neutral earthing also needs a decision. A Dyn11 step-down transformer creates a new neutral on its secondary, which must be earthed at the transformer with the electrode resistance measured, while the generator’s own neutral on the delta side is treated separately. The rules are covered in earthing and neutral grounding, and the switching and interlocking in ATS and changeover switchgear engineering.
Cables and layout around a rental transformer
A transformer adds two cable systems. The LV side carries the full current, around 1,390 A for 1,000 kVA at 415 V, which means three or four single-core cables per phase and attention to current sharing. The HT side carries roughly 52 A for the same 1,000 kVA at 11 kV through one three-core XLPE cable with heat-shrink terminations made by an HT-competent jointer, because a failed termination under load is a serious arc hazard. See temporary power cables for sizing practice.
An oil-filled unit needs a level pad, a spill bund, clearances from buildings and exhaust, and a barricade where HT terminals are exposed. Crane access and ground bearing for a 3 to 5 tonne unit should be confirmed at the site survey, not on delivery day.
A worked illustration
A pharmaceutical unit near Hyderabad must take its 1,000 kVA, 11/0.415 kV transformer out for a ten-day overhaul (illustrative). The shutdown load is about 450 kVA. Because the LT board can be fed directly at 415 V with the transformer isolated, no rental transformer is needed and a 625 kVA set through a temporary changeover carries the load with starting margin. Had the work been on the LT board, with other substations fed through the HT ring, a step-up transformer, HT breaker and a larger generator sized for energisation would have been in scope. The asset being isolated, not the load, decides whether a transformer is required.
Common mistakes
- Rating the generator on transformer running load only. The generator trips on the first energisation and the outage starts with a dead plant.
- Assuming the vector group does not matter for a temporary job. A phase shift against an existing transformer makes paralleling or closed-transition transfer impossible and can cause a short circuit at the tie.
- Leaving utility relay settings unchanged on generator supply. A fault on the temporary system is not cleared quickly, and the damage and arc energy are far higher than they should be.
- Treating HT terminations as a quick field job. A poorly made termination fails under load, usually at the worst moment of the shutdown.
- Forgetting the secondary neutral earth. Earth-fault protection on the temporary LV system has no return path and touch voltages are uncontrolled.
What to send us
Send the single-line diagram showing the incoming supply, the asset being isolated and the proposed temporary connection point, nameplate photographs of the existing transformer, and the load list or maximum demand records for the outage period. Add site photographs of the proposed generator and transformer positions and the crane access. With these we can confirm whether a transformer is needed at all and size the generator for energisation rather than running load. Send a generator rental enquiry or see transformer sizing for the calculation background.
Questions engineers ask
Can a 415 V rental generator feed my 11 kV plant?
Yes, through a step-up transformer with its own HT breaker and protection. The generator must be rated for the transformer inrush, the relay settings must be reviewed for generator fault levels, and the DISCOM incomer must be isolated and locked out so there is no possibility of back-feeding the grid.
How much bigger than the transformer should the generator be?
It depends on the transformer design and whether the core is energised cold. A generator kVA of 1.5 to 2 times the transformer kVA is a reasonable planning assumption for dead energisation, but the confirmed answer comes from the generator’s transient data and the transformer’s inrush figure, or from soft-start energisation that avoids the problem.
Oil-filled or dry-type for a temporary job?
Dry-type units are lighter, need no bund and suit short urban jobs. Oil-filled units suit longer industrial deployments where a bund and clearances can be provided.
Do I need to change the plant’s relay settings for the rental period?
Usually yes. Generator fault current is a fraction of utility fault current, so overcurrent elements set for the grid may not pick up. Temporary settings are recorded, applied for the rental period and restored at handback.
Generators by rating
- Under 15 kVA generators
- 15–50 kVA generators
- 50–125 kVA generators
- 125–250 kVA generators
- 250–500 kVA generators
- 500–1000 kVA generators
- 1–2 MVA generators
- 2 MVA and above generators
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.