100/133% INSULATION LEVEL

[raadhatab]

Can anybody explain, what is the idea behind having 100% insulation level and 133% insulation level in testing XLPE, EPR medium voltage cables?

Thank you

 

[rbulsara]

133% insulation level cables are intended to help cope with transient overvoltages than can occur during arcing or restriking ground faults on ungrounded or high resistance grounded systems, which MV systems often are.

100% insulation is perfectly suitable for solidly grounded systems.

Rafiq Bulsara

http://www.srengineersct.com

 

[rcw retired EE]

Say the testing specification requires an overvoltage test at some multiple of the rated voltage. The cable is rated for 5 kV 133% or 8 kV 100%. Same cable, different voltage ratings depending on if it is applied on a grounded or ungrounded ssytem. The testing spec probably has a differnt multiplier for the 100% 8kV rating than the 133% 5kV rating.

 

[cuky2000]

Cable dielectric withstand is a function of insulation thickness (100%, 133% or 173%). Cable are tested to withstand overvoltage for 1 min, 1 hr or longer resembling the effect that a SLG fault could produce in the unfaulted phase.

Therefore, depending of the system grounding (solidly, impedance grounded or ungrounded) the stresses in the insulation with respect to ground should be designed accordingly for 100%, 133% or larger. For larger cable insulation, it is common to use the next standard voltage level.

 

[raadhatab]

Thank you for your replies, however the cable spec is :
Single conductor 5-15KV, 100% &133% insulation level.

My questions is
1- Do i have to order the insulation level depending on the status of grounding? Any standard for that?
2- Any explanations Why 33% and 73% more insulation, any calculations?

Thanks again

 

[electricpete]

Solid line to ground fault on ungrounded system => max steady state overvoltage to ground (ignoring transients) is 1.73 = 1 + 73%.
Hopefully that is obvious.

For high-resistance ground, I think the neutral grounding resistor is sized at Xc0 / 3. Solid line to ground fault on then leads to max steady state overvoltage to ground (ignoring transients) of 1.33 = 1 + 33%. That's not as obvious to me before my morning coffee, but I know it's true.

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(2B)+(2B)' ?

 

[electricpete]

Actually Xc0/3 is max size of resistor and max size will lead to max overvoltage during sustainted/steady-state line-to-ground fault.

And I'm not so sure where the 133% comes from. Maybe someone can explain it.

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(2B)+(2B)' ?

 

[electricpete]

I was clearly wrong in my initial statement about the reason for 133%. I don't think it can be calculated. It is not associated with sustained overvoltage on high-R ground (since that would still be 1.73 even on HRG system). However 133% is associated with HRG systems that may see sustained overvoltage per IEEE141 and there is a lot of discussion of various practices in the thread here:

thread238-232606

"There are three levels of conductor insulation for MEDIUM-VOLTAGE CABLES: 100, 133, and 173% levels. The solidly grounded system permits the use of 100% insulation level. When the fault on the other system will raise the system voltage above normal during the time of the fault, 133% insulation level should be specified if the fault is cleared within one hour. When the fault will remain on the system for an indefinite time, 173% voltage level insulation

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(2B)+(2B)' ?

 

[cuky2000]

1- Do I have to order the insulation level depending on the status of grounding? Yes. Engineering is responssible of this determination. See the file enclosed to help guiding the engineering judgement
2- Any standard for that? IEEE Std 141 (7.2.5) & IEEE Std C62.92.1
3- Any explanations :
a)Why 33% and 73% more insulation: During SLG fault, the faulted phase voltage approaches to zero while the unfaulted phase voltage rise. (See the enclosed file for additional information)

CALC. GUIDELINE:
[sub]In the absence of the sequence impedance parameters (reactances & resistances)at the cable connection, the following general assumptions may be used:
• Effectively grounded system: the phase-to ground voltage (V[sub]Lg[/sub]) typically could rise up to 80% of the max V[sub]LL[/sub] (phase-to-phase maximum operating nominal voltage): V[sub]cable[/sub] ~ 0.8(max V[sub]LL[/sub])
• Ungrounded system: V[sub]cable[/sub] ~ V[sub]LL[/sub]
• Impedance Grounded: 0.8(max V[sub]LL[/sub])< V[sub]cable[/sub] < max V[sub]LL[/sub] = 1.05xV[sub]LL[/sub] [/sub]

b) Any calculations?: The above calculation guideline could be sufficient for most MV applications. However, if more accurate information is required, the withstand cable temporary overvoltage (TOV) could be determined using EMTP or similar transient software. An acceptable practical approach to determine the TOV could be as follows:

Min Cable voltage withstand rating > 1.05xCOGxV[sub]LL[/sub][sub]

NOTE: The COG (Coefficient of grounding) could be estimated considering the sequence impedance parameters. For details about estimate the COG, refer to IEEE Std C62.92.1 or the enclosed link.

http://files.engineering.com/getfil...6&file=Sample_of_Coefficient_of_Grounding.jpg

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