In short. A medium-voltage generator system generates at 3.3 kV, 6.6 kV or 11 kV instead of 415 V, so that large blocks of power move at modest currents, connect to an existing 11 kV network and are protected by relays. It is worth considering for single sets above about 2,000 kVA, plants above about 4 MVA, or sites where the generator sits far from the loads. It is engineered from the single-line diagram outward, never bought from a catalogue.
When medium voltage is the right architecture
Most generator installations in Telangana and Andhra Pradesh are correctly built at 415 V, as the low-voltage generator systems page explains. Medium voltage earns its extra cost in four situations. The first is scale: a 3,000 kVA set at 415 V would deliver over 4,000 A, which no practical cable or breaker arrangement handles economically, while at 11 kV it delivers 157 A. The second is distance: a captive plant feeding loads spread across a 500 metre site cannot run 415 V feeders that far without unacceptable voltage drop, but an 11 kV ring with step-down transformers at each block does so comfortably. The third is integration: a campus that already receives DISCOM supply at 11 kV or 33 kV and distributes at 11 kV can connect generators to that network directly and pick up every substation with one scheme. The fourth is paralleling with the grid, where the point of connection and the protection philosophy are already at medium voltage.
| Factor | 415 V generation | 11 kV generation |
|---|---|---|
| Current for 2,500 kVA | About 3,478 A | About 131 A |
| Conductor to switchboard | Busduct or many parallel cables | Single 3 core XLPE cable per set |
| Switchgear | Air circuit breakers, 415 V board | Vacuum circuit breakers in metal-clad panels |
| Protection | Breaker releases, some relays | Numerical relays with CTs and VTs |
| Transformers | None unless feeding an MV network | Step-down at each load centre |
| Installation licence | LT contractor | HT contractor and CEIG approval |
| Typical economic range | Up to about 1,500 to 2,000 kVA per set | Above about 2,000 kVA or multi-MVA plants |
What changes in the equipment
The engine is the same. The alternator is wound for the higher voltage with form-wound coils, heavier insulation and a larger frame, and it is usually a 4 pole machine at 1,500 rpm like its low-voltage equivalent. The alternator terminals feed a generator circuit breaker, normally a vacuum breaker in a metal-clad panel, through current transformers for protection and metering and voltage transformers for sensing and synchronising. The neutral is brought out separately and earthed through a resistor or a transformer rather than solidly, so that an earth fault inside the stator is limited to a current the winding can survive. From the generator breaker the power goes to an 11 kV bus, and from there to step-down transformers that produce the 415 V the loads actually use. Each of those elements is engineered for the specific project, and the guide to transformer sizing covers the step-down units and their energisation from a generator source.
Protection is the heart of the design
At 415 V a generator is protected by its breaker releases and the controller’s voltage and frequency limits. At 11 kV a numerical relay takes over, and the protection scheme is where most of the engineering hours go. A typical generator relay provides overcurrent with voltage restraint, stator earth fault, differential protection on larger machines, reverse power, loss of excitation, negative phase sequence, under and over voltage and frequency, and a synchronism check for paralleling. A relay can offer dozens of functions, and enabling all of them is as poor a practice as enabling none; each setting must be derived from the alternator reactances, the earthing method and the system fault levels.
The instrument transformers deserve as much attention as the relay. A voltage transformer ratio entered wrongly corrupts every voltage-based function at once. A current transformer secondary earthed at two points creates a circulating loop that produces false differential operation. The generator protection relay architecture page describes the functions and the protection coordination page shows how they are graded with the downstream feeders.
Earthing the neutral
How the generator neutral is earthed decides the magnitude of an earth fault current, the overvoltage that appears on the healthy phases during a fault, and whether the plant can continue to run with a single earth fault present. Solid earthing, which is universal at 415 V, is rarely used on medium-voltage generators because the resulting earth fault current would burn the stator iron. Low-resistance earthing limiting the current to a few hundred amperes is the common choice for industrial plants, giving enough current for selective protection while limiting damage. High-resistance earthing limiting the current to a few amperes allows continued operation during a first fault but needs a different protection approach. Where several generators share a bus, only one neutral is normally earthed at a time, or a separate earthing transformer is used, to avoid circulating third harmonic currents. The earthing and neutral page sets out the options.
Approvals, people and lifecycle
A medium-voltage installation falls under the Central Electricity Authority regulations for high-voltage installations and must be inspected and approved by the Chief Electrical Inspector before energisation. The installation must be carried out by a licensed HT contractor, the operating staff must hold the appropriate authorisation, and switching must follow a written permit system. Those obligations continue for the life of the plant, together with periodic relay testing, insulation resistance and tan delta checks on the alternator and cables, and maintenance of the breaker mechanisms. Our position is straightforward: medium-voltage generator systems are sourced and engineered on request for a defined project, beginning with the single-line diagram, a load study and a fault level and protection study, rather than offered as standard packages.
Common mistakes
- Choosing 11 kV generation for a 1,000 kVA set because the DISCOM intake is at 11 kV; a 415 V set with a step-up transformer does the same job at far lower cost.
- Solidly earthing a medium-voltage alternator neutral as if it were a 415 V machine; the first stator earth fault destroys the core.
- Enabling every function on the numerical relay with default settings; nuisance trips follow and the real protection is never proven.
- Energising all the step-down transformers together from a newly started generator; the combined inrush collapses the voltage and the set trips.
- Treating CEIG approval as a formality at the end; drawings rejected at inspection delay energisation by months.
What to send us
Send the existing single-line diagram, the DISCOM supply voltage and sanctioned demand, the load list by substation or block with the largest motors, the site layout showing distances between the proposed generator position and the load centres, and whether grid paralleling is intended. Photographs of the existing 11 kV switchgear nameplates help us judge compatibility. Use the application engineering enquiry and we will advise whether medium-voltage generation is justified and what studies the project needs.
Questions engineers ask
At what size does a generator need to be medium voltage?
There is no fixed threshold, but above about 2,000 kVA for a single set, or above about 4 MVA for a multi-set plant, the current at 415 V becomes expensive to switch and distribute and 11 kV generation is usually cheaper overall. Below that, 415 V generation with a step-up transformer where needed is normally the better choice.
Which medium voltages are used for generators in India?
The usual choices are 3.3 kV, 6.6 kV and 11 kV, with 11 kV the most common because it matches the standard distribution voltage of most industrial sites and DISCOM networks. The alternator is wound for the chosen voltage at the factory and cannot be changed afterwards.
Can a medium-voltage generator be rented?
Yes, 11 kV rental sets and containerised rental plants with step-up transformers exist for shutdowns and bridging power, but they need the same switchgear, protection, earthing and CEIG approvals as a permanent installation. The lead time and engineering effort are therefore much greater than for a 415 V rental.
Does a medium-voltage installation need Electrical Inspector approval?
Yes. Any installation above 650 V falls under the high-voltage provisions of the CEA regulations and must be approved by the Chief Electrical Inspector to Government before it is energised, with drawings, test reports and the HT contractor’s completion certificate submitted in advance.
Is the alternator efficiency different at 11 kV?
Marginally, and not in a way that drives the decision. The savings of a medium-voltage scheme come from lower distribution losses over long runs and from smaller conductors and switchgear, while the step-down transformers add their own losses. The comparison must be done on the whole system, not the alternator alone.
Looking for a specific rating? See the generator full-load current chart for every common size from 7.5 to 3,000 kVA at 415 V and 11 kV.
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.