Solar DG Synchronisation in a Factory: Minimum DG Loading and What Goes Wrong

A grid-tied solar plant can run alongside a DG set only if a controller holds solar output low enough that the DG stays above its minimum load and never receives power back. Makers typically ask for about 30% of rated load as the floor. Without that control, the DG trips on reverse power or runs badly under-loaded.

Why solar and a DG do not mix by default

A grid-tied inverter is a follower. It needs a stable voltage and frequency from somewhere else and pushes current into it. When the grid fails, the inverter must stop within seconds, which is what the CEA connectivity standards require: the plant must cease energising an unintended island within two seconds, and reconnect only after supply has been stable for at least sixty seconds. The same behaviour is described in our guide to solar, DG and battery integration.

When the DG starts, it gives the inverters a voltage to follow, and after the reconnection delay they start exporting into the DG bus. A DG is a poor receiver. Its engine cannot absorb power, so if solar supplies more than the factory is drawing, the surplus flows backwards into the alternator, which tries to motor the engine. A diesel engine also dislikes running lightly loaded: the usual consequences are poor combustion, carbon build-up and, over time, higher maintenance. That is the reason for the minimum-loading rule. The percentage comes from the engine and alternator maker, so take it from the DG's manual or datasheet and confirm it in writing. About 30% is the figure commonly quoted.

The sizing rule that falls out of it

Take the load on the DG bus at the time you expect solar to run, subtract the DG's minimum load, and that is the most solar the DG bus can accept at that moment. If the DG is rated at 500 kW and the maker's floor is 30%, the engine wants at least 150 kW of load at all times. When the factory is drawing 250 kW at midday, solar can supply at most about 100 kW of it, because the DG must still carry the other 150 kW, however much the roof could produce. Load changes through the day, so the permitted solar output moves with it.

That is why the controller matters more than the inverter count. The arithmetic is easy; the difficulty is doing it fast enough that a cloud passing over the roof or a big motor tripping cannot move the DG outside its limits.

The control scheme: how the plant is held in check

A PV curtailment controller, sometimes called a DG-solar controller, does three jobs. It measures the DG's output with a CT-based meter, it calculates the permitted solar output as above, and it sends a power limit to the inverters over a communication link, usually Modbus over RS-485 or Ethernet. The inverters must support external power limiting, which most commercial string inverters do, and you need to check each make and model.

A separate reverse-power relay on the DG breaker is the backstop. The controller reduces solar so the DG stays loaded; the relay trips the DG feeder or the solar feeder if it sees power flowing back despite the controller. Keep the two functions distinct in the design: control is the first line, protection is the second, and the protection setting must sit outside the control band so that normal control action never trips it.

What goes wrong in practice

• No fail-safe on lost communication. If the link drops and the inverters keep their last setting, solar can run with no limit. Test this by pulling the cable.
• A controller that reads the wrong place. The CT must measure the DG's output, not the feeder after a mixed bus. Measure where the DG's own load is defined.
• Slow response. A scheme that updates once every ten seconds may allow a swing the engine cannot take. Ask for the response time in the specification and confirm it on site.
• Mixed inverter makes. Different protocols and ramp behaviours on one controller multiply commissioning time.
• Several DG sets, one controller. With paralleled sets, the minimum load applies to the running set or sets, which changes as sets start and stop.
• Changeover surprises. The transfer scheme may open the neutral or switch four poles; the earthing arrangement under DG supply needs checking for the inverters.
• Treating it as a solar-only problem. The DG panel builder, inverter supplier and electrical contractor each test their own box. Appoint one party to own the sequence of operation.

Commissioning tests to insist on

Write these into the contract so they are priced: an outage simulation with solar running; a DG start and the timed return of solar after the reconnection delay; a load throw-off test with solar at its permitted limit, watching DG load and the controller's response; a loss-of-communication test; a reverse-power relay trip test by injection; and a low-load test with the DG at its minimum. Record DG load, solar output and relay status through each test. Our testing and commissioning scope lists how we witness these.

Does it still pay?

Solar with a DG saves diesel only while the DG would otherwise be running in daylight, and only up to the limit above. A factory with long daytime outages benefits most; one whose DG runs for an hour a week gains little from the control scheme and should treat solar as a grid-only decision. Run your own hours and diesel cost through the DG vs solar calculator, and see the transformers and DG sets page for how the electrical side is designed. A battery changes the sums by letting solar charge instead of being curtailed; the battery energy storage page covers when that is worth it.

How Secured Engineers helps

We design the electrical system and the solar plant in-house, so the DG panel, changeover and inverter control are specified as one sequence rather than three suppliers' boxes. Fire NOC, CEIG and DISCOM net-metering are filed in-house, and MEPF and solar can sit under one contract. We work from offices in Noida (Delhi NCR) and Ludhiana, and the company has completed 535+ projects across 18+ states, with 89.77% repeat clients.

To plan a solar plant that works with your DG, send the single-line diagram, DG nameplates and a month of DG run hours. See the transformers and DG sets page, message us on WhatsApp or call +91 70099 85817, or ask for a free solar proposal.