In short. A sugar mill generates most of its own power from bagasse during the crushing season, so the diesel generator is not the main source. Its roles are black-starting the boiler and turbine auxiliaries, carrying essential loads when the turbo-alternator trips or the grid tie opens, and supplying off-season maintenance power. Sizing follows the black-start sequence and the essential bus, and the set must be ready each season after months standing idle.
How power works in a sugar mill
During the season, which in Telangana and coastal Andhra runs for roughly 150 to 180 days, bagasse from the mills fires the boilers, the steam drives turbo-alternators and often the mill turbines, and the plant exports surplus power to the DISCOM under its cogeneration agreement. Outside the season the plant imports power for maintenance. This architecture changes what a diesel generator is for. It is not sized to crush cane. It is sized to get the cogeneration plant running from a dead bus, to protect the process when the in-house generation trips, and to keep the plant alive during the off-season. The operating philosophy, the single-line diagram and the grid interconnection agreement define those roles. The planning approach is laid out in black start.
| Event | What the diesel set must supply | Duration | Design driver |
|---|---|---|---|
| Season start or total blackout | Boiler feed pump, ID and FD fans, fuel feeders, turbine lube oil and jacking pumps, DCS, lighting | Until the first turbo-alternator is synchronised, typically one to a few hours | Black-start load build-up, largest motor start |
| Turbo-alternator or boiler trip during crushing | Lube oil pumps, mill and cane carrier controls, massecuite and juice pumps to prevent setting, centrifugal brakes, lighting | Minutes to hours | Essential bus, fast transfer |
| Grid tie opens with export running | Usually nothing, if islanding and load shedding work; otherwise the essential bus | Seconds to minutes | Islanding scheme and under-frequency shedding |
| Off-season maintenance | Workshops, pumps, cranes, lighting, offices | Weeks, when import tariff or reliability is poor | Fuel cost and prime rating |
Black start: building load in the right order
From a dead bus, the diesel set energises the essential switchboard and the plant brings up the boiler in steps: DCS and instrumentation on UPS, lube and jacking oil pumps for the turbine, the boiler feed pump, the forced and induced draught fans, bagasse or alternate fuel feeders, then the turbine auxiliaries until steam is available and the turbo-alternator can be synchronised. Each step is a motor start on a small source, and the ID fan and the boiler feed pump are often the largest. The order and timers come from the boiler supplier’s start procedure, and the diesel set is sized to accept each step with the load already running.
kVA_required = max over steps of (kVA_running_before + kVA_start_of_step)
kVA_running_before is the load on the set when a step is called and kVA_start_of_step is the starting demand of the motor or group in that step. A worked example (illustrative): a mill brings up 250 kVA of pumps, feeders and controls, then starts a 200 kW ID fan on a VFD, which adds about 260 kVA at the current limit, for a peak near 510 kVA; it then starts a 160 kW boiler feed pump on a soft starter at 3.5 times, adding about 680 kVA on top of roughly 470 kVA running, for a peak near 1,150 kVA. The second step governs, and a set of around 1,250 to 1,500 kVA, or two paralleled sets, is the practical range for this sequence. Fitting the feed pump with a VFD or starting it before the fan changes the answer, which is why the sequence is engineered. See large motor starting on diesel generators.
Protecting the process when the turbo-alternator trips
When in-house generation trips mid-crush, the cane carrier and mills stop and the massecuite in pans and crystallisers starts to cool and set. The essential loads are therefore the ones that prevent damage during the stoppage: turbine and mill lube oil, pumps that keep massecuite and juice moving, stirrers in the crystallisers, brakes and controls for the batch centrifugals, boiler safety systems, and lighting. The transfer from the plant bus to the diesel set must be fast enough to keep the lube oil pumps alive, which argues for a DC or UPS-backed emergency lube pump in addition to the generator. Batch centrifugals are high-inertia machines with heavy starting demand and should not be restarted on the diesel set unless it is sized for them. See restoration and restart sequencing.
Islanding, grid export and plant voltages
Mills that export to the DISCOM operate in parallel with the grid, and when the grid tie opens on a disturbance the cogeneration plant must either island successfully, shedding load to match the turbo-alternator output, or trip. Where it islands cleanly the diesel set is a backup to the backup. Where it trips, the diesel set picks up the essential bus and the black-start sequence begins. Any arrangement in which the diesel set itself parallels with the plant generation or the grid requires synchronising equipment, protection studies and the utility’s agreement; grid export from a diesel set is not assumed to be permitted. Plant distribution is normally at 11 kV or 3.3 kV with 415 V motor control centres, so the diesel set is either a medium-voltage machine on the essential MV bus or a low-voltage set on a dedicated essential board, and transformer inrush on a small source must be checked in the first case. See generator and grid paralleling and medium voltage generators.
Readiness after the off-season
A sugar mill’s diesel set spends months doing little and is then expected to black-start the plant on the first day of crushing. Fuel that has stood through the monsoon grows sludge and absorbs water, batteries sulphate, hoses age, and rodents find the control panel. A pre-season readiness programme should test the fuel and polish or replace it, load test the set to rating, exercise every breaker in the black-start path, verify the UPS batteries for the DCS, and run the black-start sequence as a drill with the boiler team present. During the season the set is hard to remove from service, so the overhaul belongs in the off-season with a rental set connected if maintenance power is still needed. See fuel cleaning and polishing and load bank testing.
Common mistakes
- Sizing the diesel set as a share of the mill’s crushing load. The number is enormous and irrelevant, and the black-start duty that matters is never engineered.
- Letting the boiler team start the feed pump and ID fan in whichever order is convenient. The second start exceeds the set’s acceptance and the black start fails with the boiler half warm.
- Relying on the diesel set alone for turbine lube oil during a trip. The transfer interval is long enough to damage bearings, and an emergency DC or UPS pump was the correct answer.
- Leaving season-old fuel in the tank. The set starts, runs for twenty minutes and stops on blocked filters during the first blackout of the season.
- Paralleling the diesel set with the plant bus without a protection study. A grid disturbance back-feeds the set, or the DISCOM finds an unapproved connection.
What to send us
Send the plant single-line diagram with the cogeneration plant, grid tie and essential bus, the boiler supplier’s start-up sequence with motor ratings and starting methods, the essential load list for a turbo-alternator trip, and the off-season maintenance load. For an existing set, add the nameplate, the last load test record, the fuel storage details and the age of the batteries. Send your application details and we will reply with a step-by-step black-start load profile for your review.
Questions engineers ask
Does a sugar mill with cogeneration still need a diesel generator?
Yes, for black start, for essential loads when the turbo-alternator or boiler trips, and for off-season maintenance power. The cogeneration plant cannot start itself from a dead bus without an independent source for its auxiliaries.
How big should the black-start generator be?
Large enough to accept the biggest step in the boiler start sequence with the earlier loads already running, which is usually the boiler feed pump or the induced draught fan. For a typical mill this falls between several hundred kVA and a few megawatts, depending on motor sizes and starting methods.
Can the diesel set run in parallel with the turbo-alternator?
Only with synchronising controls, load sharing and a protection coordination study, and never in a way that exports to the grid without the DISCOM’s approval. Most mills keep the diesel set on a separate essential bus with a transfer scheme instead.
How do we keep the set ready through the off-season?
Run it under load monthly, keep the batteries on a maintained charger, test and polish the fuel before the season, and rehearse the black-start sequence with the boiler team a few weeks before crushing begins.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.