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Ohmic Method of Fault Calculation 1
- Thread starter Cerkit
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1
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HamburgerHelper
Electrical
Why would anyone want to do it ohmically when you can do it in PU and just convert it at the end to ohmic values? I don't want to have to track how impedances are reflected from one side to the other on transformers.
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If you can't explain it to a six year old, you don't understand it yourself.
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If you can't explain it to a six year old, you don't understand it yourself.
HamburgerHelper
Electrical
The calculations are for all intensive purposes the same. The only thing that I can think of that is different is that 1.0 PU voltage references 1.0 Line to Line and 1.0 L to Neutral. In per unit calculations, the square root of three disappears. You'll put in PU equipment data in ASPEN One-Liner. The transformer data in ETAP looks like it is in PU ( It doesn't matter really though. It is 24 for one and two dozen for the other.
If your program is doing the fault current calculations, it is doing all the transferring impedances from one side or the other or converting it to PU. What you do get if you are looking at things in PU is that certain equipment has common PU ranges. A large transformer's impedance is going to be between 7-12% PU. I couldn't tell you off the top of my head what that is ohmically for a given transformer size and voltage. I couldn't look at a data sheet with PU values and tell you without pulling out a calculator whether a large 400 MVA transformer should have 200 ohms of impedance or 20 ohms or 2 ohms. I could tell you what its PU should be. When you specify a transformer, you specify it with a certain PU impedance, never in ohms. Transformer inrush will be between 6-12 PU of the transformer. Locked rotor current will be similar. So, what you get with using PU units even if you are not doing the fault current calculations is that you know what the range of values should be for different types of equipment. If you know the PU ranges, it becomes much easier to spot mistakes or do rough calcs.
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If you can't explain it to a six year old, you don't understand it yourself.
If your program is doing the fault current calculations, it is doing all the transferring impedances from one side or the other or converting it to PU. What you do get if you are looking at things in PU is that certain equipment has common PU ranges. A large transformer's impedance is going to be between 7-12% PU. I couldn't tell you off the top of my head what that is ohmically for a given transformer size and voltage. I couldn't look at a data sheet with PU values and tell you without pulling out a calculator whether a large 400 MVA transformer should have 200 ohms of impedance or 20 ohms or 2 ohms. I could tell you what its PU should be. When you specify a transformer, you specify it with a certain PU impedance, never in ohms. Transformer inrush will be between 6-12 PU of the transformer. Locked rotor current will be similar. So, what you get with using PU units even if you are not doing the fault current calculations is that you know what the range of values should be for different types of equipment. If you know the PU ranges, it becomes much easier to spot mistakes or do rough calcs.
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If you can't explain it to a six year old, you don't understand it yourself.
rcw retired EE
Electrical
Use the MVA method for the same reasons HH mentioned. It gives you an intuitive feel for short circuits, motor starting voltage drop, and impact of larger/ smaller transformers. Ohmic method has too many zeros to get lost. MVA method is not as exact but it is close enough for most industrial applications.
"Short Circuit ABC- Learn it in an Hour, Use it Anywhere, Memorize No Formula" is the title of Moon Yuen's IEEE paper that HH referenced recently in another thread. (I'm partial to this method, since I worked with Moon).
ETAP seems to use a combination for input data. Some input forms require % values, some per unit and on others ohmic values. I'm currently working on a large ETAP study (>1500 busses) and am spot checking the data with the MVA method. It helps pickup input data errors.
"Short Circuit ABC- Learn it in an Hour, Use it Anywhere, Memorize No Formula" is the title of Moon Yuen's IEEE paper that HH referenced recently in another thread. (I'm partial to this method, since I worked with Moon).
ETAP seems to use a combination for input data. Some input forms require % values, some per unit and on others ohmic values. I'm currently working on a large ETAP study (>1500 busses) and am spot checking the data with the MVA method. It helps pickup input data errors.
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