cable calc 2

[shocker9]

Hi guys
Can somebody help me with cable calc.
Our company tends to use the Amtech software which I'm trained in but it does not help with any HV cable calcs. I need a formula for this. The voltages that we use are generally 6.6,11,33 and 132kv.
So a common formula would help alot.

Thanks

 

[wareagle]

What kind of calculations do you need?

 

[ppedUK]

Amtech is for calculating cable sizes etc. for LV systems to BS7671.

HV cable calculations are a completely different ball game, especially if you are carrying out hand calculations. There is no one magic formula, it depends on the types of cable, the installation (OHL or underground system), the system fault levels etc. etc. If you haven't done HV calcs before, employ someone who has.

 

[7anoter4]

As ppedUK said "if you are carrying out hand calculations" it is more complicate.
You have to keep in your mind the fact there is not a cable "rated current" but "a rated conductor insulation temperature ".
For XLPE, for instance, the maximum temperature for short-time is 250 oC and for indefinite time 90 oC.
First of all check the "short-circuit thermal stability".
You need to know: maximum short-circuit current [from the Utility in your connection point on the Grid). For short-circuit current calculation see IEC 60909 [if you have to state the current by yourself]. You need also the short-circuit protection clearing time [sec].
See:

http://www.okonite.com/engineering/short-circuit-currents.html

Second, you have to check the steady state load current [current-carrying capacity].
You may calculate it following [for instance] IEC 60287, but better to take it from standards as [for instance] DIN-VDE 0298 or IEEE Std 835-1994.
There are many way to run a cable: underground, in duct banks, through cable trays-open or close, exposed to sun shine or not and so on.
It is important if there are other power cables or other heat sources as hot pipes in the vicinity.
For cable laid in the ground you have to now the specific soil thermal resistance, the depth and the load factor, any way.
Usually you don't need to check the voltage drop. But, for medium voltage system, if there are big induction motor [starting DOL for example] supplied through this cable, you may have
to check the voltage drop also and you have to start from the source transformer [at least].

 

[Samuel2500]

I have previously worked for a cable systems company (EHV and HV) and the calculations, as the other gentlemen have stated, are not straight forward, there are a lot of factors you need to consider. We had in-house programs to do these for us. However, i constructed my own MathCAD program to do this calculation for me (which you could also).

Refer to IEC 60840. This pretty much spells out how to calculate it. (from memory, or it will refer to one that will help). This is for XLPE cables..

Hope that helps.

 

[7anoter4]

Welcome Samuel2500,
Is good to encounter somebody who worked for a cable systems company and namely of EH voltage cables. I, myself, worked in this field- mainly in design and technology of low and medium voltage cables [and control and telecommunication cable, also].
I think IEC 60840 is a standard for testing cable and accessories from 36 to 170 kV. From 6 to 30 kV there is another standard: IEC 60502-2[IEC 60502-4].
But I don't think this standard can help to calculate the rated current, any way. May be one could extract from the "requirements" the dielectric losses limits [or shield losses and build-up voltage limits].
The IEC 60605-1 could help in order to "rebuild" the cable and so facilitate the calculation [in the case of lack of manufacturer cable data].
If you know the ampacity produced by manufacturer [and if it is from manufacturer test and not extract from a standard] you may adapt this for your conditions.

 

[xxjohnh]

If you are buying the cable 33/132/220 kV, it is a good idea to include the worst case configuration in your quote and have the vendor produce their own calculations to verify yours. Most have inhouse programs and consider such variables as shield bonding. ETAP is a good start to get the size, then verify it with the vendor calcs. So many people use IEC and NEC tables which do not match the design case.

 

[7anoter4]

CORRECTION: In my post of 17 Sep 5:11, I indicated a wrong IEC standard no. Instead of:
IEC 60605-1:1978, "Reliability of systems, equipment and components. Guide to reliability testing."
Has to be: IEC 60502-2 "Power cables with extruded insulation and their accessories for rated voltages from 6 kV (Um = 7.2 kV) up to 30 kV (Um = 36 kV)"

 

[cgrodzinski]

Gentlemen, the thread started with the post asking for cable calculation not testing and we should come back to it.

While calculation of the cable ampacity is not a new problem its modelling is IMHO the biggest obstacle as well as understanding the cable working conditions. The MV calculations should not pose a problem with exception if the system goes unbalanced. The main standard for cable calculation is IEC60287 for continuous ampacity ratings and IEC60949 for short circuit calculations as two main documents. While there are many programs for these calculations not all of them are applicable to EHV cables or if the circuit is influenced by other utilities. All depends on quality of inputs and software algorithm.