IEEE 835 Cable Voltages

[Ahmed Abdelfattah]

Hello,
I'm about confused about the cable voltages mentioned in IEEE 835. Do they refer to the cable rated voltage or its operating voltage?
I have a cable rated for 145kV and is used in 138kV network. Which voltage should I be used when I want to size the cable using IEEE 835?
Thanks

 

[paulengr]

Hello,
I'm about confused about the cable voltages mentioned in IEEE 835. Do they refer to the cable rated voltage or its operating voltage?
I have a cable rated for 145kV and is used in 138kV network. Which voltage should I be used when I want to size the cable using IEEE 835?
Thanks

Cable voltages are rated voltages.

Current drives ampacity because the thermal heat source is I-squared x R. It doesn’t matter if the cable is carrying 120 V or 145 kV...the thermal effect is the same and the cables ability to dissipate heat in air or while buried determines ampacity in insulated cables. Thus is in contrast to overhead cable design where either temperature (hardware or metallurgical limitations) or sag is the limiting factor.

 

[Julius Right]

In chapter Introduction to the Power Cable Ampacity Tables

3.1 Parameters

3.1.1 Voltage

It is only 138 kV and no 145 [up to 230 kV]

Nevertheless AEIC CS7/1993 [for 69-138 kV] recommends other insulation thickness [720 mils]

However the temperature drop on insulation has a minimum influence.

 

[Ahmed Abdelfattah]

Thanks Paul and Julius.

The reason I'm asking this question is because I know that IEEE 835 tables are based on typical cable specifications such as insulation and jacket thicknesses, dissipation factor, and SIC. These differ with the rated cable voltage.

So, the question now is whether the tables for 138kV cables are based on typical construction of 138kV rated cable? And if I have 145kV cable, which is not in these tables, do I have to resort to Neher McGrath method to calculate the ampacity?

 

[paulengr]

Thanks Paul and Julius.

The reason I'm asking this question is because I know that IEEE 835 tables are based on typical cable specifications such as insulation and jacket thicknesses, dissipation factor, and SIC. These differ with the rated cable voltage.

So, the question now is whether the tables for 138kV cables are based on typical construction of 138kV rated cable? And if I have 145kV cable, which is not in these tables, do I have to resort to Neher McGrath method to calculate the ampacity?

Hard to say how close these are. The fact is you are planning a 138 kV run, I would say that for estimation purposes those factors are going to be trivial. Burial depth and grouping matter far more. But it isn’t a standard size so it’s made to order. So if it was my project I would plan via the tables with at least some design margin. Then get the actual cable design details (made to order) and pay for a Neher-McGrath run (or buy outright) to establish final operating limits. Since it is pretty much made to order and/or you can adjust TOF with the resistors you might be able to push limits a bit. As far as I know IEEE 836 tables are fairly conservative and N-M yields higher limits, “typical” being conservative. I’m struggling to find all the data for IEEE 835. N-M does the work for you.

 

[Ahmed Abdelfattah]

Thank you Paul.

I think I will go with N-M. I have all cable information from the manufacturer. I can build the cable model on ETAP and run the N-M method since the cable in direct buried.

 

[Julius Right]

Do you mean Nelder and Mead or Neher & McGrath method?

In my opinion there are some example in IEEE 835 it self also.

 

[Ahmed Abdelfattah]

I meant Neher & McGrath. I believe that the examples provided in IEEE 835 are based on this method. Also, ETAP is using the same method for underground cable ampacity calculations.

 

[paulengr]

I meant Neher & McGrath. I believe that the examples provided in IEEE 835 are based on this method. Also, ETAP is using the same method for underground cable ampacity calculations.

The original four Neher-McGrath papers from the 1950s if memory serves are some of the few that IEEE has basically made public. The ampacity tables in NEC are straight from those papers. Many of the factors in IEEE 835 are beyond the original work...extensions to the original work. N-M kind of held back some things so you had to go to them for instance if your direct burial case is outside the handful they supply.

Prior to those original four papers ampacity was pretty much a guess. N-M converted the thermal problem into electrical units to aid in solving the problem similar to the mechstronics methods of converting load torques and springs into RLC equivalents so we can load it into Spice. IEEE 835 just extended it beyond that, but of course N-M (the company) did, too.

 

[Ahmed Abdelfattah]

I didn't know that Neher & McGrath have more than one published paper. I have the one published in October 1957 and I thought this is the only one. I tried to apply the principles of this paper on 15kV cable sizing but it didn't go well. I had to resort back to ETAP.

Currently (or whenever I have time) I'm reading two books; Electrical Power Cable Engineering (W. A. Thue) and Rating of Electrical Power Cables (G. J. Anders). I know it is not a good idea to scatter my mind on two references at the same time but I find that these two books complement each other. I'm expecting that when I'm done I will have good handle on N-M paper and IEEE 835 extended work.

 

[paulengr]

I didn't know that Neher & McGrath have more than one published paper. I have the one published in October 1957 and I thought this is the only one. I tried to apply the principles of this paper on 15kV cable sizing but it didn't go well. I had to resort back to ETAP.

Currently (or whenever I have time) I'm reading two books; Electrical Power Cable Engineering (W. A. Thue) and Rating of Electrical Power Cables (G. J. Anders). I know it is not a good idea to scatter my mind on two references at the same time but I find that these two books complement each other. I'm expecting that when I'm done I will have good handle on N-M paper and IEEE 835 extended work.

It should be just Neher but McGrath did a lot of lab work.

A simplified mathematical procedure for determining the transient temperature rise of cable systems [includes discussion]

The transient temperature rise of a cable system under a constant load, that is, the manner in which the temperature of the conductor rises with time after the constant load is applied, is a matter of considerable interest to cable engineers, since it may serve as a basis for short time ratings...

ieeexplore.ieee.org

Procedures for calculating the temperature rise of pipe cable and buried cables for sinusoidal and rectangular loss cycles [includes discussion]

It has been customary in calculating the temperature rise of cables in duct to take account of the nature of the load cycle, since this has the effect of reducing the temperature rise from that which would result if the load were maintained continuously at a constant value. With the advent of...

ieeexplore.ieee.org

The Phase Sequence Impedance of Pipe-Type Cables

The positive or negative phase sequence impedance of a pipe-type cable may be calculated by conventional formulas, suitably modified to include the effect of magnetic properties of the pipe. Calculation of the zero phase sequence impedance, however, is considerably more complicated, involving...

ieeexplore.ieee.org

At the time they were hung up on analog component equivalents because they were using analog computers (op amps) to run the calculations. Neher talks about this in a 1950 paper.

Looking back the four papers I was thinking of were the four shock papers.

 

[Julius Right]

Neher,McGrath-Temperature and Load Capability of Cable Systems October 1957

The IEC replica of this publication is IEC 287/1969 [ a single volume]

Calculation of the continuous current rating of cables (100% load factor)

However it is in continuous development under the name IEC 60287-6 volumes.

 

[Ahmed Abdelfattah]

Paul / Julius,

Thanks for the information. I think I have more digging to do on this subject.