HT to HT Voltage Regulator

My suggestion is to find the original consultant/installer who designed your system and force him to correct this glaring error, if not then your "fix" (if one exists) is going to cost you many years of your "free" solar energy output.

Your situation illustrates the problem of buying on price alone and not having competent engineers, who have done this before, guide you. Reminds me of the saying "How come there's never enough money to do it right, but always enough money to do it over?"

I agree, all circuitry to deal with the varying incoming line voltage, and the varying output voltage should be at the 220V level. The 1MW inverter designs should be such that they regulate their output currents to find the maximum-power-point of draw from the solar panels, rather than to establish a preset output voltage. This activity would be at the solar current-input and 220V level. If the controllers in his 1MW inverters are unable to do this, pstanwar9229 has a problem. If he proposes five 220V add-on-regulator stages, these each have to contain a 1MW dc rectifier plus a second new 1MW inverter output, which is not an inexpensive matter. He might as well scrap his existing inverters and purchase new appropriate ones which can handle the task.

Get 5 new transformers with built in HT voltage regulation. Here's a link of a supplier who is in India. http://jindaltransformers.com/manufacturer/distribution-transformers/jindal/automatic_voltage_regulator.html

Typical source of power variations in HT supplies are due to un-expected load demands from an improperly designed distribution system! The most common contributing factors of voltage fluctuations are due to voltage drops along the grid, triggered or caused by under-sized transmission lines, (improper wire gauge either for aluminum or copper).

The solution of using of voltage regulator is temporary and limited. Most of the time will only be applicable if the problem is mostly over-voltage in nature? A longer term solution is to increase power plant capacity and/or the use of properly sized transmission lines, (proper AWG, aluminum or copper), in the network. Demand factor needs to be considered in the design supply!

Improperly designed, what you mean by that......

rashavarek -

Part of good engineering practices is to ensure that transmission lines in the supply grid are properly sized to meet the designed capabilities of the power source... Considerations for present and future demand should also be incorporated and considered as a good business practice.

Adding new power source(s) just to supplement and meet increased power demand without corresponding changes in its transmission mode will be an issue, problematic as the OP is now encountering!

Read and digest OP's comment /response...

Thanks all all for your valuable suggestions.

Our Plant has 10 Inverters each 500KW, 5 Transformer with dual LT side winding.

When our plant was commissioned there was not such kind of situations like Under or Over Voltage. These issues are increased now a days because of Lots of Wind Farms installed nearby us. I agree with @vsar that here are under-sized transmission lines. The Grid is quite old and can't hold too much generation.

Is it SAFE to install a voltage regulator between Inverter and Transformer?

Having a voltage regulator although not a complete protection may make your investment a bit safer... You still need to install an automatic system disconnect for your protection from extreme grid fluctuations beyond the limits of your regulators?