By Vvon Engineering Team | Published | Updated
Compare PV-generator interlocks and coordinated controls, with minimum loading, reserve, protection and commissioning requirements.
Solar EPC & street lighting RFQ checklist (TXT)
PV and generator supplies need a coordinated operating scheme if they share a bus. Unplanned interaction can cause trips, unstable operation or reverse power. Check compatibility, protection and controls before permitting parallel operation.
A standard grid-tied inverter measures the voltage and frequency in front of it and injects current in step with what it finds. It does not set frequency and it cannot form a voltage by itself. It also carries anti-islanding protection, required by the utility and tested to IEC 62116, whose whole purpose is to detect that the source behind it has gone and to disconnect quickly.
Most anti-islanding schemes work by injecting a small deliberate disturbance and watching how the source responds. A stiff utility grid absorbs the disturbance and nothing moves. A single diesel set is a small source with limited inertia, so the disturbance actually moves the frequency, and the inverter reads its own perturbation as evidence that the grid has gone. It disconnects. That is the protection working correctly rather than failing.
Add to that the ordinary behaviour of an engine governor. As PV picks up load, the engine backs off, and a lightly loaded governor holds frequency less tightly. The inverter sees frequency wandering outside its window and trips on that instead.
On the grid, surplus generation flows out through the meter and becomes an export credit. On generator supply there is nowhere for surplus to go. If PV output exceeds the site load, the excess flows back into the alternator, which cannot absorb real power. It motors the engine instead, and the engine is not designed to be driven.
Reverse-power protection can disconnect a generator when power flows in the wrong direction. The actual trip, engine response and damage risk depend on the set and controls. Coordinate protection, minimum loading and PV curtailment with the generator manufacturer's limits.
The reason factories want solar running through a generator period is fuel. The catch is that a diesel engine run lightly loaded for hours suffers incomplete combustion, unburnt fuel and oil past the rings, glazed bores and carbon in the turbocharger. Generator makers publish a minimum recommended continuous loading for exactly this reason. Read the manual for your own set rather than accepting a contractor's assumption, and treat that figure as a floor.
So the target is not maximum solar. It is as much solar as possible while keeping the engine above its minimum loading and well clear of reverse power. That is a narrower operating band than most owners imagine, and on a large array during a light shift it can mean curtailing a good deal of available generation. Better to know that before the project than after.
On a clear February morning the control problem is slow and easy. Through the pre-monsoon period, when cloud builds through the afternoon, irradiance across a large roof can fall by a large fraction in seconds. The engine has to make that up, and an engine's ability to accept a sudden load step is what limits how much PV you can safely run in parallel with it.
Two settings deal with this: a cap on PV penetration while in generator mode, and a ramp rate limit on the inverters so that recovery after a cloud does not arrive as a step. Both belong in the specification, with figures agreed against the engine's load acceptance data rather than assumed.
A properly designed and tested interlock can prevent PV operation during generator supply. Confirm its electrical implementation and fail-safe behaviour; an auxiliary contact alone is not proof that the arrangement is safe.
A compatible meter and control loop may limit PV during generator operation. Confirm the measurement location, response, reserve margin and safe response to communication loss with both manufacturers. Do not assume that an ordinary utility export limiter is a complete generator-control system.
A programmable controller reads generator load, the engine minimum load setpoint and PV output, then writes an active power limit to each inverter over Modbus. This is the arrangement that genuinely saves fuel while protecting the engine. It needs inverters that accept a remote active power setpoint, it needs the register map from the inverter maker, and it needs proper commissioning time rather than an afternoon.
Storage can absorb or supply power within its charge state, power rating and control response. It may reduce generator transients, but it does not eliminate reserve and protection requirements. Compare it with the other options on the actual operating duty and commercial case.
Ask for those results in the commissioning file, signed and dated. On a site with a generator, that file is what tells the next engineer why the plant behaves the way it does at ten in the morning on a load-shedding day.
Consult the relevant edition and the documents for the exact equipment and site: SMA: PV-diesel control and minimum generator loading; SREDA: Net Metering Guidelines and current application documents.
Share your application, equipment requirements and procurement timeline. Our engineers can help clarify the configuration and scope for your enquiry.