Why such a big drop transformer?

I done a such a program: 380VAC to the inverter, the inverter output to the transformer, the transformer output voltage levels are 7, 220V 230V 400V 415V 440V 475V 575V 220V in use for testing, but when the transformer output voltage with load after down to 200V 37KW with the load of air compressor to ask you why there is such a big prawn drop

PWM inverter output waveform mostly, which contains a  certain amount of switching frequency harmonic (ripple current)

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Before receiving the transformer is usually required filter reactor, or that part of the harmonics will cause the transformer core loss and coil large eddy current loss

It was also part of the inductor built into the transformer, also called sub-circuit inductance transformer or built-in transformer, this transformer short-circuit impedance can often achieve more than 15%

Your problem description is not very clear, there may be two situations

1) ordinary transformer, the inverter output,  directly to the transformer, causing excessive wear and tear resistance. Ordinary transformer (small power 1kW or less), the sine wave input conditions, the voltage change rate of 3% to 5%, the greater the transformer capacity, the voltage rate of change is smaller. If the direct input PWM waveform, the harmonics can also cause changes in the rate of voltage rise.

2) built-in inductance of the transformer, short-circuit impedance is high, simple to understand, the equivalent of an inductor and a transformer, the inductor, there will be pressure drop, which means that the actual input voltage drop across the inductor to lose, output voltage will lower the natural. As the load increases, the primary current increases, the voltage drop across the inductor increases, the input voltage drops, the output voltage will be reduced.
(No load when the primary current is very small, very small drop on the inductor, so the no-load voltage is biased towards the normal.)