Three windings transformer

I am studying a SLD (single line drawing) wherein the designer is using a transformer (1000KVA) with dual primary windings (3-phase both delta connected) and single secondary winding (3-phase star connected) to step up voltage to 33KV. Both primaries are fed with 3-phase, 1000V, AC , 50Hz separately the outputs of two inverters. I shall appreciate if output (33KV) from this transformer is technically and briefly explained to me. Nothing else is mentioned on the SLD about this transformer.

• Turns ratio is important. Dec 19, 2016 at 18:32
• What's to explain? Dec 19, 2016 at 18:33
• I don't see a single question mark. What is your question? May 7, 2017 at 7:31

2 Answers

If the outputs of two inverters need to be combined and stepped up to a higher voltage, that is probably a good way to do it. The frequency, voltage and phase relationship of the two inverters still needs to be controlled. The frequencies and voltages need to match fairly closely. The two inverters need to be in phase with each other or held to a specific phase difference. The phase difference could be used to reduce the harmonic content of the output waveform. If the transformer secondary is has a smaller windings of each phase connected in series with the main winding of another phase to form a "zig-zag" secondary, that can contribute to the harmonic reduction.

It seems like this must be part of a frequency changing scheme or a scheme for tying to grids together in a situation where the grids can not be coordinated with each other. Perhaps it is part of a power-outage ride-thru scheme. Whatever the case, two inverters may be used because there is something preventing one larger inverter from being used. However two inverters may be used just for the purpose of improving the final waveform.

The two primaries will not increase voltage in the secondary; they will act in parallel to increase the current available in the secondary.

If primary A and primary B both have 1000 turns, and the secondary has 33000 turns, then yes, if either primary A or B are given 1000V, the secondary will be 33000V and the other primary will be 1000V. Transformers can backfeed. If both primary A and B are given 1000V separately or in parallel, the secondary will still be 33000V.