Power Electronics Development Center

Showing results for 
Search instead for 
Did you mean: 

How and where is the per unit portion of the vi's defined?

I noticed that most of the GPIC example vi's have many of the outputs defined in terms of per unit. How and where is this unit defined?

0 Kudos
Message 1 of 6

Regarding per unit scaling, I've asked a colleague, Lakshmi, at Oak Ridge National Labs to recommend some papers for you that have a clear explaination with additional background information. Please see the links below:

I hope this helps. Thank you very much Lakshmi!

0 Kudos
Message 2 of 6

Thanks for the links to those papers, it looks like there's a lot of good information there.

It still doesn't quite answer my original question, although now I think I can rephrase it better.

When dealing with per unit scaling, there's always a base unit needed to define the scale. What is the default base unit for the examples and where is it defined?

0 Kudos
Message 3 of 6

The base unit for output voltage and current are typically set at the rated continous power rating of the converter.

For example, if the rated output phase current is 223 Amps, then then 223 Amps has a per unit current value of 1.

For DC link, it's typically set at the nominal DC link voltage. For example, if the nominal DC Link voltage is 850 Volts, then 850 Volts has a per unit value of 1.

One advantage of this approach is that over-voltage and over-current trip limits often do not need to change when the control code is run on converters of different power levels. Only the scaling to per unit in the analog input loop changes based on the rated power . Even the control loop tuning doesn't change very much.

0 Kudos
Message 4 of 6

I think I understand how per unit works pretty well now, but my original question still hasn't really been answered.

I've opened NI GPIC Grid Active Front End.lvproj. Say that I want to change the voltage and current ratings to match the physical system that I have. Where are these ratings defined in the project?

I'm attempting to run [RT] NI GPIC Grid Active Front End.vi and [FPGA] Grid Tied AFE Control with FRF Stability Analysis.vi.

0 Kudos
Message 5 of 6

The analog input scaling gains and offsets are set in the analog input loop of the FPGA application via front panel control registers. The scaling formulas are listed there as well. The actual value you need depends on the type of voltage and current sensors you use. Be sure to select the appropriate protection trip limits, which are defined in your per unit scaling.

0 Kudos
Message 6 of 6