AC reversible converter, single phase 2

[xj25]

Hi everybody,

I got this reversible inverter topology from a possible supplier, but don´t understand the AC input side converter structure (single phase, two converters in parallel).

Any help or reference where to check it?

Thanks!

 

[LiteYear]

I'm not quite sure which bit is giving you trouble, but the whole thing is not too complicated (once you know it!) so perhaps this will help:

On the left is the three phase rectifier. For reference, read:

http://services.eng.uts.edu.au/~venkat/pe_html/ch05s1/ch05s1p1.htm

In the middle is a DC bus.

On the right are two, three-level, single phase inverters in parallel - one is drawn at the top and the other is drawn underneath. If you're unfamiliar with a (two-level) H-bridge inverter, start there:

http://en.wikipedia.org/wiki/H-bridge

To then understand a three-level inverter is not a big step. See here, but note that in the single phase case, only two legs are used, not three:

http://www.pwrx.com/pwrx/app/TLI%20Series%20Application%20Note.pdf

Is there anything more specific that is not clear? I'm confused about your mention of an "AC input side converter". The converter (on the right) is typically thought of as the output. Although, since this is a "reversible" inverter (made possible by controlling the rectifier FETs rather than just using diodes) so power flow can go back the other way.

 

[xj25]

Thanks for the reply Lite,

the link is missing, I found this that suppose that is what you were triying to reference
[link]http://www.pwrx.com/pwrx/app/TLI%20Series%20Application%20Note.pdf[/url]

The right side converter in this case is connected to supply through a 1-phase transformer (Railway supply network of 25kV) and is used as reversible, the load is the 3phase side.

One thing that I miss; I get than the converter can "modulate" an AC signal from DC bus to the input side but there I am used to the concept of syncronous generators, and the power injected being function of power angle between the electromotive force and the supply voltage. Here there is not such difference, the converter just generate the AC at the input. is it the transformer impedance that makes the difference between the angles?
Adjoint block diagram to clearify a little the question...

 

[LiteYear]

If I understand you correctly, your assumption is spot on - inverters with an AC supply on their output dictate power flow by controlling the angle across the effective impedance of the transformer. If the transformer were not present, the impedance would be supplied by an output LC filter. Either way, if the inverter output leads the AC supply voltage, power flow is from inverter to AC supply and vice-versa.

There are variations on this concept with more complicated power flow control methodologies, but this phase angle one is the most common.

 

[LionelHutz]

The load dictates the power flow if the inverter is simply modulating to produce an AC output.

 

[ScottyUK]

For an inverter synchronised to a large system where 'the load' is significantly greater than the capability of the individual source it isn't true to say that the load determines the power flow. In this case the infinite bus model is appropriate, and the inverter switching pattern determines the load flow through the coupling impedance which, as previously noted, behaves in a similar manner to the synchronous reactance of a generator whereby the source will operate at a phase displacement relative to the system it is connected to.


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If we learn from our mistakes I'm getting a great education!

 

[xj25]

Thanks! I had though exactly in the transformer as "substituing" in some way the "syncronous reactance" in a sync generator, so it seems to make sense...