Operating voltage for V V F D

Actually, I don,t think there is any VSD Drive for 6.6KV. So far we have used VSD for normal 690 Volt for 800 KW motor. You never told us what is the input voltage to your 800KW motor?

Is your 800 KW motor operating at 6.6KV or your 6.6KV, supply to your transformer primary side in which the secondary of the transformer is supply to your motor...???

The required power devices for a 6.6 kV drive would be very expensive. That's an unusually high input voltage for a drive in that power range.

what is the spec for your motor

There are a few suppliers of true 6.6kV VFDs, Siemens / Robicon is one of them, Cegelec used to, but I don't know what happened to them after being broken up by the French Gov't. Most of the others, i.e. ABB, Toshiba, GE, Rockwell and Eaton offer Medium Voltage VFDs, but as far as I know do not truely go to 6.6kV because of the extreme cost to develop the product ( I may be wrong however because it has been a few years since I had to do this). So they too will use transformers both up and down with lower voltage VFDs, usually 3300V or 4160V f you insist on using a 6.6kV motor (more on this later though).

You are absolutely correct to be concerned about the efficiency losses in a Transformer based system. For motors that large, those losses will repesent a very significant permenent operating cost. But you actually need only concern yourself with the load side transformer losses because you will most likely end up with line side transformers anyway. Large MV VFDs such as Robicon's will have at least 12 pulse, and usually 18 pulse front-ends to help mitigate harmonics. They do that by using custom wound Detal-Star transformers to create phase shifts. So the transforemers on the line side will be there anyway and you can therefore use a reduction ratio as well without much additional cost and no appreciable additional losses. Assume then that some transformer losses on the line side are a necessary evil no matter what you do.

But the load side transformer to step the voltage back up again is another matter. Assumeing 3-5% losses in the transformer, when both drives are running you will be wasting 48-80kW in load side transformer losses alone! Then add the additional heat rejection that 48-80kW represents to your climate control system and you have a significant permanent drop in efficiency to consider, probably in the neighborhoor of around 100-160kW of additional losses.

If using a low voltage VFD such as 690V, you also have to consider the swithing losses inside of the VFD because those losses are akin to I²R losses, meaning the higher the current, the higher the losses. The devices then will be handling a lot more current at that low voltage so they will reject a lot more heat which again, is an efficiency loss plus a heat removal loss. Try to stick with options that use a true Medium Voltage output, as high as possible.

To keep your initial purchase cost as low as possible then, consider using a lower MV motor voltage that is standard in your part of the world, i.e. 4160V if you are in North America or 3300V if you are anywhere else. That way, you will have more potential bidders on the drive system and since as I said, you will end up with line side transformers anyway, this will be a more reasonable solution.