Captive Generation Plant

..."A captive power plant is a facility that provides a localized source of power to an energy user. These are typically industrial facilities, large offices or data centers. The plants may operate in grid parallel mode with the ability to export surplus power to the local electricity distribution network."...

https://www.clarke-energy.com/applications/captive-power-plants/#:~:text=A%20captive%20power%20plant%20is,the%20local%20electricity%20distribution%20network.

You need to calculate the load required and the generating capacity present, as well as a failsafe system of isolation for your plant...the generator is properly loaded at around 50-60%, you need a power use profile for the plant, so a monitoring and recording history of plant electrical usage is required to determine duty suitability..

https://www.ceew.in/cef/masterclass/explains/captive-power-generation

EDG can be used as Prime power generator as well. However, following points to be reviewed:

1) Same DG can be used for Emergency duty as well as for Prime power duty. When used as Prime power generator, the generator may need to be derated. Cooling arrangement also may need review. Manufacturer may advise the derating factor / cooling arrangement modifications if necessary.

2) Warranty for the machine may be different when used for Prime duty.

3) Emergency DG set is provided with what is called 'Day Tank' for fuel supply. This may not suit when the DG is used as Prime power generator. Bigger fuel storage and pumping facilities may have to be planned.

4) Emissions allowed are easy when it comes to EDG set, considering it runs only occassionally. When used for supplying Prime power, the location as well as height of the stack may need to be reviewed to meet the Pollution regulations applicable.

If you google 'Prime power vs Emergency power DG set' you may get more details.

...and open a dialogue with several <...Captive Generation Plant...> suppliers if none is installed, or the actual supplier if one is installed. By telephone, that is.

Those questions can really only be answered by the utility or legal authority serving your operation, as they are the ones who actually wrote the rules, and will enforce them as necessary.

If you feel they are being applied incorrectly, your only real recourse lies in approaching that authority and challenging them legally.

That being said, to charge you duty on electricity that you provide to your load when the utility supply has failed is patently unfair, and a court should fix that with little legal effort. However, if you are running your emergency generator to peak shave in parallel with the utility, that may be difficult to defend against the intent of the duty.

Years ago in the US this was a common situation, and no utility would challenge this, and would not imagine they could recover imagined damages from such operation. However, your country’s situation and laws are possibly quite different.