115kV cable overloading

[leoliu]

Hi, we are an utility. Someone is concerning 5% overloading of a 115kV underground transmission cable with the overload time duration of 30 min. I know it shouldn't be a concern, as we normally talk about hours in discussing cable's short time overload capacity. Can anyone point to me any references or standards which help to explain the 5% cable overloading for 30 min is not an issue?

 

[crshears]

Not all 115 kV cables are created equal . . .

Direct buried? If so, three separate phases appropriately spaced, or one bundled cable? What type of backfill? How moist is the backfill?

Or all three phases in a pressurized [low pressure or high pressure?] pipe with static insulating oil? Or with oil being circulated? If circulated, only to level out hot spots or actually directed through some sort of cooler or heat exchanger?

I encountered some of each type in my career.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[davidbeach]

It would also matter what the loading was pre-overload. If you’ve been at 98% loading for hours and hours and then jump to 105% you have a lot less margin than if you’d been at 60% load and then jump to 105%.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[7anoter4]

Referring the post no.3 of davidbeach, in my opinion ,you may consult IEEE Standard 242/2001-this is the edition I have with me -chapter 9.5.2 Overload capacity.
However, here are some remarks about this standard-first the cables which are about here are only up to 15 kV. Second: in the formula from chapter 9.5.2.4 Development of intermediate characteristics, the definition of the variable θ is missing-it has to be the time in hours- and the exponent -θ.K must be -θ/K.

 

[FPelec]

Hi leoliu; definitely, not an issue. You can use IEC 60853-2.

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[7anoter4]

According to IEEE 242/2001 chapter 9.5.2.4 [formula] if the initial current was 999.99 A, the overloaded current 1051 A, then the conductor temperature -in 1800 sec-was only 90.43oC -instead of 105oC- the maximum emergency overload permissible.

 

[waross]

A question;
Are the rules for the reduction in life of an overheated transformer applicable here, albeit somewhat modified?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[7anoter4]

In connection with cable aging -estimating cable life expectancy see

https://elek.com/articles/estimatin...t=Most power cables have a,of up to 50 years.

 

[bacon4life]

A back in 2015 I was considering how adding a section of 2000 kcmil solid dielectric 115 kV into an overhead line would impact our line ratings. Rather than try to calculate the allowable overload myself, then vendor was quite willing to provide ampacity calculations for each of our emergency rating conditions. Now that NERC/FERC is pushing for utilities to actually implement ambient adjusted ratings and short term ratings, I assume it is even more common for cable vendors to provide this info.

The cable vendor provided me a 4 hour rating for their cable based on the following assumptions:
1) Preloading at 50 percent of the steady state rating prior to the emergency (525 amps).
2) A 4 hour overload period.
3) Post emergency loading at the steady state rating (1052 amps).

The allowable short term loading was 175 percent (1850 amps).

 

[7anoter4]

It is very interesting, bacon4life, however, I am not sure, but I think your cable manufacturer used the same formula in order to state the overload.
I found in Southwire catalogue for 2000 kcmil copper,115 kV, the rated 1230 A [and not 1052]
So, calculating, using IEEE 242 formula, for only 105 oC- underground buried K=6- for 4 hrs. overload of 149% [with respect of 1230] but 175% with respect to 1052 A.

 

[bacon4life]

Wow, with typoes in IEEE 242 I am glad I asked the manufacturer to do the calculations.

The Southwire catalog lists the rating includes a 75% load factor, but it doesn't state anything about how they define the period for calculating the load factor. In general, I have found it very hard to translate load factor based ratings into actionable guidance to use for real-time control.

 

[7anoter4]

You are right, bacon4life, Southwire catalogue does not clarify what is the Fig.1 and Fig.2. But, using the cable details attached and following the Neher and McGrath theory-with IEEE 835 help- I deducted that it is about underground- simply buried 36” deep- in 90 RHO, 25oC earth, 3 single core cables in flat arrangement 7.5” apart.