I am designing an inverter to push back power from a DC grid into AC grid using an existing transformer. So do I need to have a specific ratio transformer or any ratio can work. The nominal voltage of the DC grid is 3 kV. and the transformer is 1.073 kV AC
Dear Nanda Kishor Panda , please, could you provide a schematic or clarify better which grids you have, voltage levels and so on? The transformer, for example, must have two levels, one for the primary and one for the secondary. It will be easier for the others to reply and you will get better and more specific answers.
Andrea Mariscotti Please find the attached schematic. The transformer is a three winding transformer and I want to attach two VSC to the two secondaries.
So you have a transformer with two secondaries. You said that "the transformer is 1.073 kV AC", but what is this voltage? the secondary phase-to-neutral? pls, consider that for the delta winding you have sqrt(3) higher voltage and you do not have neutral point. So, the connection of these 2 inverters will be troublesome, besides that you drew other loads downstream the transformer: maybe a substation rectifier, not a load, but you cannot connect the VSC like that.
Just to anticipate with a short piece of info.
If you have Vdc that is the converter dc link voltage, the output ac side is ideally a sinusoidal waveform that can barely "touch" the Vdc limit, so it's rms is 0.707*Vdc. If you deform the waveform with the known 3rd harmonic injection (up to 15-16% if I remember well) you increase thee rms, ad you reach the maximum of 0.78 ideally for a square wave inverter.
If you have a 3 kV line, then the two dc links must be in series. If you have a 1.5 kV line they can be connected in parallel.
Since your transformer is sized for a 6 pulse rectifier (ok 2, two secondaries....) the ac output of the inverter and the nominal secondary voltage will coincide more or less (the transformation factor of a 6 pulse bridge is 1.35*Vacrms for the dc output).
Now, you have some problems:
- to inject power back into the grid you need a slightly larger voltage, and you will have it when trains are braking; however you inject directly onto the secondary and the rectifier and not with a separate transformer, that I do not see as a lucky move ...);
- the two inverters "see" each other and are decoupled by the reactance of the transformer, but see also the rectifiers downstream: so, control and stability can be troublesome, especially if when they are in series you must balance the two Vdc of the dc links.