Line impedance calculations

[sjvanhecke]

I was wondering if anyone can give me a formula or somewhere I can find it to get positive and zero sequence line impedances and angles for a transmission line. Thanks

 

[Cooda76]

For what application, you are looking for this calculation, normally you can get this thing from the manufaturer and the angle you are talking about is the angle between the Sending end and receiving end voltage. if this is what you are looking for, you have to calculated it using basic modelling technique , if it is a short, medium or long transmission line.
cheers !

 

[sjvanhecke]

The situation is that I am replacing existing KD-10 relays with SEL-311A relays and the customer has no line data. Getting the reach off of these relays is not really an issue, but according to the KD-10 manual the reach setting (if desired) can be affected by the impedance angle. I have nothing from the original setting of this relay.

 

[davidbeach]

The only way to get the information you are looking for is to know the conductor type (and quantity) and line geometry.

 

[cuky2000]

Probably if you provide basic information there will better responses. For instance:

1- Is the transmission line underground or overhead?

2- What is the phase conductor size and physical configuration?

3- Any shield wire(s) and size.

4- Any information on soil resistivity?

 

[sjvanhecke]

1- overhead

2- 795 mcm ACSR and vertical configuration with all 3 phases on one side of a pole. It is a 46 kV circuit with a second circuit on the other side of the pole.

3- static line on top of pole

4- no info on soil resistivity

 

[vtpower]

Unfortunately, it doesn't sound like you have any software to model this with. We use 795 ACSR commonly for our subtransmission.

You will have to dig into either a power engineering handbook, or a wire company's electrical data (such as southwire) to get the positive, and zero sequence for a given length (usually per thousand feet). Find out how many feet of line you have and you should be able to calculate it out.

For your phase distance elements you can find the angle by converting from rectangular to polar form after you have the total line impedance. For your ground elements, the same applies, but I believe the 311A also uses a Ko factor which you will need to find the calculation for.

 

[Deansharafi]

There are formulas for calculating positive and zero sequence impedances, however if you don't know the values (positive sequence impedance is provided by the manufacturer and does not change zero sequence depends on soil quality, season etc), the best thing to do is to carry out a Line Parameter Testing. To do that, you need to de-energise the line and carry out injections to the next substation. If there is another line on the same tower, you will need the zero sequence mutual impedance between the two lines as well, because in case of one line out of service and earthed at both ends, your zero sequece impedance can change considerably.
If injection is not possible find what type of conductors are used and refer to manufacturer information. For overhead lines the angle is around 75 Deg. If you can not find any information on conductors, use 0.4 ohm/km as a safe approximation.

 

[jghrist]

Equations for calculating line impedance (with angles) and typical values for transmission lines can be found in the Westinghouse T&D Reference Book.