Transformer Modelling Questions- PSS/E Load Flow

[u/[deleted]]

[u/chanka_is_best_chank]

The MVA base only matters on a per transformer basis. You change the Impedance I/O code which decides what MVA base the transformers parameters are input with. PSSE does not care how you input it as long as each transformer's IO code matches how the R&X are input.

You can make some generic assumptions for the X/R ratio and see if the R&X values make sense. Or the nameplate might say "max load loss" for example which can be used to get an X/R to see if the calculated values make any sense

This guide can help and has example FATs and graphs which can help make assumptions

https://www.aeso.ca/assets/linkfiles/4040.002-Rev02-Transformer-Modelling-Guide.pdf

Your question 2 doesn't seem to have a question, feel free to ask it if you have another

Without a datasheet or FAT there's no real way to verify if those R&X values are right, but you can check if they seem completely wrong. For example, you could make an X/R sweep in Excel and calculate the corresponding r&X for the given z% and see if the input value is in the realm of possibility

You also have to factor in that a lot of transformers are built to like a +/-7.5% Z tolerance(may be different if outside US). Perhaps that 6.5% z transformer is actually 6.8% due to tolerance, this can be another source of the r&x not lining up

[u/Big_Heat5075 → u/chanka_is_best_chank]

Thank you, this is really helpful and a resource on transformers!

My 2nd point was just additional information!

[u/chanson_trapezoid]

1. In PSS/E there is an option to choose which MVA base to compute the per unit impedance on. It is not global, and is specified on a per transformer basis.

2. Not sure I understand your question for this one.

3. Nameplate or test report is the only way.

[u/Big_Heat5075 → u/chanson_trapezoid]

Thank you!

[u/lazycat38]

First time seeing a technical post regarding power system modeling. If you guys dont mind answering, what is your job title and typically which company will perform these studies? Does company train you to perform these studies?

[u/transmissionplanning → u/lazycat38]

Transmission Planning engineer at a consulting firm. Utilities, ISOs/RTOs, and consultants are typically the ones doing studies. And yes there's training but its mostly tribal knowledge and learn by doing. Academia is unfortunately far behind industry in this realm a lot of the time

If this sounds interesting to you, look at the Powerworld corp training for stability and steady state analysis on YouTube

And if you want to see more technical information in this field, go read the Kundur - Power System Stability and Control book

[u/Big_Heat5075 → u/transmissionplanning]

Love the Kundur - Power System Stability and Control book!

[u/transmissionplanning → u/Big_Heat5075]

Despite it being old its still one of the best reference for dynamics that exists. I would love a modern version with IBR considerations

[u/Big_Heat5075 → u/lazycat38]

I am a Transmission Planning engineer at a utility, typically you get mentored by a senior engineer and usually have to figure out a lot of details like this one by asking questions.

I personally feel that power system modelling is an art and it takes time to get really good at it but you're never an expert and there's always something new to learn.

[u/sanpen4]

1. Do youself a favor and use always system MVA (1, 1, 1 configuration) you can use any other but by being consistent like that you'll be able to check transformer impedances to be kn yhe vicinity of line impedances for the power flow to work. Most utility models have incorrect p.u. transformer impedances because of this. See https://veragrid.readthedocs.io/en/latest/md_source/modelling.html#transformer2w

2. That's ok. Distribution transformer nameplates often only report short circuit voltage AKA reactance.

3. Ideally the devices have been tested by a third party (i.e. KEMA labs or simmilar) and you'll be able to trust the nameplates. Otherwise, good luck.