In short. Running a diesel generator in parallel with the DISCOM grid makes it part of the public network for as long as the breaker is closed. That requires synchronising before connection, control of real and reactive power after it, interface protection that disconnects the set on any grid abnormality or loss of mains, and written approval from the utility. In India the boundary is set by CEA regulations and the DISCOM, and must be settled before equipment is bought.
Two fundamentally different modes
A standby generator in island mode is the only source on its bus. Its governor sets the frequency, its AVR sets the voltage, and the load takes whatever it needs. The same generator paralleled to the grid is a tiny machine connected to a very large one. The grid now dictates frequency and voltage; the generator cannot change them. What the governor setpoint now controls is how many kW the set pushes into the bus, and what the AVR setpoint now controls is how many kVAr. Everything about the control philosophy, the protection and the responsibility changes at the moment the breaker closes, and the two modes have to be engineered as separate cases.
A controller that is excellent in island mode and has no grid-parallel capability is not a shortcoming; it is a different product. The functions needed for the parallel case are described below and in utility paralleling.
Before the breaker closes: synchronising
The generator can only be connected to a live grid when its voltage waveform matches the grid’s. Phase sequence must be identical, voltage magnitude within a few percent, frequency within a fraction of a hertz, and phase angle within a few electrical degrees at the instant of closing. The generator controller achieves this by trimming speed and excitation until the window is met, then issues the close. A separate synchronism-check relay supervises the close circuit independently so that a controller fault cannot close the breaker out of phase. The mechanics of matching are covered under generator synchronisation and apply equally to generator-to-generator and generator-to-grid cases; the consequence of getting it wrong against the grid is more severe, because the grid’s fault contribution is far larger than another generator’s.
After the breaker closes: controlling kW and kVAr
Once in parallel, the control system must give the generator a target. Common targets are a fixed kW output (base load), a fixed import from the grid (import control or peak shaving), zero export at the point of connection, or a load ramp used to move load between grid and generator before opening one of them. On the reactive side the target is usually a fixed power factor at the generator terminals, typically 0.8 to 0.9 lagging.
| Control objective | What the controller holds constant | Typical application |
|---|---|---|
| Base load | Generator kW at a setpoint | Planned output on site fuel |
| Import control (peak shaving) | Grid import kW at or below a limit | Keeping demand under the contract demand |
| Zero export | Power at the point of common coupling never negative | Where the DISCOM prohibits export |
| Soft transfer | Ramp rate of kW between sources | Closed-transition transfers and planned load moves |
| Power factor | Generator kVAr proportional to kW | All of the above |
Import control and zero export depend on a metering CT and VT set at the point of common coupling feeding the controller. Without that measurement the controller knows only its own output, not the site’s net position, and cannot hold an import limit. The economics are treated under peak shaving.
Interface protection
The grid-connected generator must disconnect itself whenever the grid is abnormal and, above all, whenever the DISCOM feeder it is connected to opens. If it does not, it keeps energising a section of public network the DISCOM believes is dead, a condition called islanding. The protection package that prevents this is called interface protection and its functions are reasonably standard.
- Under and over voltage (27/59) and under and over frequency (81U/81O), with settings that distinguish a grid disturbance from normal fluctuation.
- Loss-of-mains detection, usually rate of change of frequency (81R) and vector shift (78), acting within a few hundred milliseconds when the generator suddenly finds itself alone with the local load.
- Reverse power (32), where export is not permitted and in any case to detect motoring.
- Synchronism check (25) supervising the paralleling breaker.
- Earth-fault or neutral voltage displacement protection appropriate to the connection voltage and transformer winding arrangement.
Where the connection is at 11 kV or above, the DISCOM will usually require the protection on the HV side or duplicated, in a numerical relay with event recording. Setting philosophy is covered under reverse power and generator protection and protection coordination.
The regulatory boundary in India
A captive generator may not be connected in parallel with the distribution network without the licensee’s consent. The technical conditions come from the CEA regulations on connectivity of distributed generation and on safety and electric supply, and from the DISCOM’s own procedures, which in Telangana and Andhra Pradesh typically require an application with the single-line diagram, the protection scheme and settings, generator and relay data, and a witnessed test before the parallel is permitted. Export from diesel is generally neither permitted nor paid for, so most approved schemes are import-control or soft-transfer only, with reverse power set to trip. The consumer remains responsible for the safety of DISCOM staff on the network, which is why loss-of-mains protection is non-negotiable. Grid paralleling is therefore never a commissioning setting switched on by an installer; it is a project with a design, a protection study, an application and a witnessed test.
Engineering sequence we recommend
Start with a written objective and the hours per year the set will run in parallel. Obtain the DISCOM’s requirements before design. Produce a single-line diagram and a short-circuit study covering the combined fault contribution of grid and generator, because switchgear rated for the grid alone may now be under-rated. Design the controls and interface protection with settings calculated against the grid parameters the DISCOM supplies. Then commission in stages: synchroniser check with the close coil disconnected, first parallel at minimum load with recording, protection injection, and the witnessed loss-of-mains test. Our synchronising and switchgear service covers the equipment side.
Common mistakes
- Enabling the parallel function in a controller because it is available, without DISCOM consent or interface protection; the site is islanding a public feeder in the first disturbance.
- Omitting loss-of-mains protection on the grounds that reverse power will catch it; when the local load matches the generator output, reverse power never operates and the island persists.
- Not re-rating the site switchgear for the combined fault level; the generator’s contribution plus the grid’s exceeds the breaker’s rating.
- Running import control without a measurement at the point of common coupling; the controller holds generator kW steady while the site demand still exceeds the limit.
What to send us
Send the generator nameplate and controller type, the site single-line diagram including the DISCOM connection voltage and the transformer rating, your contract demand and a recent demand profile if the objective is peak shaving, a plain statement of what you want the parallel to achieve, and any communication you have had with the DISCOM. We will tell you what the scheme needs in control, protection and approval, and whether the objective is economic. Use the application engineering enquiry.
Questions engineers ask
Is it legal to parallel a diesel generator with the grid in India?
Yes, with the written consent of the distribution licensee and compliance with CEA regulations and the DISCOM’s technical conditions. Without consent it is a violation that can lead to disconnection, and it endangers utility staff working on what they believe is a dead line.
Can I export surplus power from my diesel generator to the DISCOM?
In almost all cases no. Export from diesel is not permitted or not paid for by Indian DISCOMs, so approved schemes are configured for import control or zero export, with reverse-power protection set to trip the generator if it ever pushes power into the grid.
What is islanding and why is it dangerous?
Islanding is when the DISCOM feeder opens and the generator continues to supply the section of network it is connected to, including the public line. Linesmen expecting a dead circuit may be electrocuted, and when the feeder is reclosed the generator is connected out of phase. Loss-of-mains protection exists to disconnect the generator within a fraction of a second.
What extra equipment does grid paralleling need compared with standby?
An electronic governor and an AVR that accept bias signals, a controller with synchronising and kW/kVAr control, a synchronism-check relay, an interface protection relay with voltage, frequency, loss-of-mains and reverse-power functions, a motorised breaker rated for the combined fault level, and metering at the point of common coupling if import control is wanted.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.