Short answer
Deviation charges apply when a plant injects more or less power than it scheduled. A plant reduces them in three ways: a more accurate forecast, timely revision of the schedule during the day, and a battery that fills the gap between schedule and actual generation in real time.
What are deviation (DSM) charges?
Grid-connected generators in India tell the load despatch centre in advance how much power they will inject in each 15-minute time block. This is the schedule. The Deviation Settlement Mechanism (DSM) compares the schedule with what the plant actually injected, block by block.
When actual injection differs from the schedule by more than the permitted tolerance, the plant pays a deviation charge. The tolerance bands and the rates are set by the applicable central or state regulation and have been revised several times, so always check the order currently in force for your plant.
Why do solar and wind plants deviate?
Solar and wind output depends on weather, and weather forecasts are never exact. Three situations cause most of the deviation:
- Forecast error. Cloud cover or wind speed turns out different from the day-ahead forecast.
- Fast changes inside a time block. A passing cloud or a wind ramp moves output faster than any schedule revision can follow.
- Plant events. An inverter trips or a feeder is taken out, and the available capacity is suddenly lower than scheduled.
What are the three levers to reduce deviation?
1. Forecast better
The schedule is only as good as the forecast behind it. Plant-specific forecasting that learns from the site’s own generation history, and that is updated through the day with live data, removes much of the error before the day begins.
2. Revise the schedule in time
Regulations allow schedules to be revised with notice. A plant that watches the gap between forecast and schedule, and submits revisions as soon as the gap opens, pays for fewer blocks of deviation. This is a process problem as much as a technical one: someone, or something, has to be watching.
3. Use storage to close the gap
A battery can act inside the time block. When generation falls short of the schedule it discharges; when generation exceeds the schedule it charges. The battery does not need to be large to help, because it only has to cover the error, not the whole output. The limit is its state of charge, which is why dispatch has to plan ahead.
What does the EMS do about it?
An energy management system joins the three levers into one loop:
- It compares the latest forecast with the committed schedule for the coming blocks and shows the expected deviation and its cost.
- It alerts the operator when a schedule revision would be worthwhile.
- It dispatches the battery within the current block to hold injection close to schedule, while keeping enough state of charge for the blocks ahead.
- It records schedule, actual and deviation for every block, so that settlement statements can be checked.
In Loopgain EMS these functions sit together under forecasting and DSM risk, next to dispatch and operating modes.
Key takeaways
- Deviation is measured per 15-minute block against the plant’s own schedule.
- Bands and rates change; confirm the regulation currently in force.
- Forecast accuracy, timely revisions and storage each remove a different part of the error.
- The value comes from running all three in one control loop.
