Generator Negative and Zero Sequence Resistance 2

[corvalan]

I am interested to know more about the negative and zero sequence RESISTANCE for small (i.e. 1 MVA) low voltage (i.e. 480V) STANDBY GENERATORS.

Is the negative sequence resistance equal or similar to the positive sequence resistance?

Is the negative sequence resistance typically a percentage of the positive sequence resistance which I do know?

How about the zero sequence resistance? Can I estimated it based on the positive sequence resistance?

The reason for this question is that I want to model this generator as accurately as possible in the SKM software. In the generator component editor in the second subview named ANSI Contribution, you enter the positive sequence reactance and then its corresponding X/R ratio. You also need to enter the negative sequence reactance and the corresponding X/R ratio. Finally you need to enter the zero sequence reactance and its corresponding X/R ratio. I know all the information for the positive sequence section. I also know the negative sequence reactance and the zero sequence reactance. To enter the negative and zero sequence X/R I will their corresponding resistance.

Thanks.

 

[davidbeach]

The resistance is the resistance, the same in all sequences.

 

[Skogsgurra]

You will have better luck if you google zero sequence impedance etcetera.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[HamburgerHelper]

I don't think it is the same in all sequences.

For rotating machines, the negative sequence resistance will be different than the positive sequence resistance due to the negative sequence circling the rotor at twice the rotating frequency so it will have a different resistance due to the skin depth being different.

I think that a generator connected in delta instead of wye would change things,too.

 

[dpc]

What type of analysis are you wanting to do? Generator resistance will be much lower than the reactance and used to be commonly neglected. You can probably find typical resistance or X/R data from the manufacturer. I believe negative sequence resistance will not necessarily be equal to positive sequence, but I'm doubting that it is going to make much difference for any type of typical calculations for a 1000 kVA generator.

It's also fairly common just to set negative sequence quantities equal to positive sequence for generators.

Good luck.

Dave

 

[waross]

You are confusing resistance with impedance.
You should be asking about impedance.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[corvalan]

Actually not. I am interested in the negative sequence RESISTANCE because that is a parameter I have to enter in the SKM software to model the generator contribution data (ANSI FORMAT).

 

[waross]

David Beach has answered your question.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[HamburgerHelper]

Here is a statement that I found from a negative sequence motor protection document. I believe it should be applicable to generators since generators are just driven motors.

3.4 The frequency of the negative sequence current induced in the rotor will be equal to (supply frequency) x (2-slip) Hz. This is due to the fact that it is revolving in the opposite direction . This frequency will be typically around 99 Hz during normal motor operation . Due to skin effect, high frequency negative sequence currents encounter high rotor resistance. This inturn results in over heating. The increased resistance is typically 5 times the normal positive sequence resistance.

http://www.lntebg.com/mps/neg.html

 

[Skogsgurra]

@ corvalan
OK, I see now. Most of us didn't think so far. Skin effect makes a lot of difference. Thanks for being so persistent.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Thanks for the information HH.

The increased resistance is typically 5 times the normal positive sequence resistance.

I would try values from 3 times to 7 times and compare the results. This will give you some indication as to whether it is safe to make an assumption in the absence of hard data.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Interesting. Any of the generator test data I've seen only provides a single stator resistance value.