medium voltage cable insulation rating?

[bfield]

Does anyone have information on the selection of insulation rating for medium voltage cable. The procedure or calculations needed to select 100% insulation or 133% insulation.

 

[busbar]

Reference NEC Article 310 and IEEE Red Book.

 

[gordonl]

In Canada (and the US I beleive) there are three grades of cable used with low resistance grounded systems as we call them.

100% insulation - ground faults clear within 1 minute
133% insulation - ground faults clear within 60 minutes
173% insulation - ground faults can persist indefinitely

 

[fluxcap]

See Okonite for a more long winded explanation:

http://www.okonite.com/engineering/electrical-formulas.html

 

[cuky2000]

Beware that some MV cable with same insulation thickness could have dual rating. For example, 5kV cable, 133% insulation level (ungrounded systems) could be rated for 8kV, 100% insulation level (grounded systems).

 

[peebee]

Interesting. . . Until I read this thread, I'd always thought of 133% as being suitable for ungrounded operation. The Okonite site posted by fluxcap explains why: "133 Percent Level - This insulation level corresponds to that *formerly* designated for ungrounded systems."

Does anyone know when this change occurred?

 

[peebee]

No-one knows when this change happened? I'm thinking it can't be more than maybe 2 or 5 years. . . .

 

[Modula2]

Anyone know what does it mean in the above okonite document, the caveat on the X0/X+ ratio of -1 to -40? Is there an implied X/R ratio, or maximum peak voltage in the voltage ratings?

 

[peebee]

Modula2 -- I can't find your referenced caveat on the site posted by fluxcap -- is that on a different page in the Okonite site somewhere?

 

[Modula2]

It's directly after the 173 Percent Level, with the two asterics (the word "caveat" was mine). I change my question to ask about implied maximum peak voltage related to rms percent level in relation to X0/X+ ratio.

 

[RRaghunath]

Electrical systems with Xo/X1 of 3 or less / Ro/X1 of 1 or less at all points and under all operating conditions are known as effectively grounded systems. This is as per IEC (also as per IEEE, I think).
In other words, the line-to-ground fault current (3Io)is equal to or greater than 0.6 times the three phase fault current ("Applied Protective Relaying" by Westinghouse).
However, I too cannot understand the statement in Okonite document as indicated by Modula2 above.

 

[peebee]

I think they are referring to the capacitance of the distribution system(?). Implying that if you have an ungrounded system with a high cable capacitance (or high capacitance from any source I guess) that an arcing ground fault could lead to excessively high voltages from capacitive charging?

 

[cuky2000]

QUESTIONS: Anyone know what does it mean ……the ratio of the zero to positive phase reactance of the system at the point of cable application lies between -1 and -40 since excessively high voltages may be encountered in the case of ground faults.

ANSWER: This statement translated in numeric terms could be indicated as follow:

-1< Xo/X1< -40

This equate to a Coefficient Of Grounding (COG = 110%) per ANSI (Former ASA Std. C84.1-1954).
_{For our colleagues in the IEC marketplace, the COG=110% represent an Earth Fault Factor (EFF=0.191).}

The above electrical parameters indicates that this is an ungrounded (unearthed) system that in the event of phase to ground fault the overvoltage on a sound phase could rise up to 10% above the line-to-line voltage. This is obvious that a 100% rated insulation cable do not met the requirement in term of insulation strength to withstand the overvoltage creating dielectric overstresses on the cables that will induce a premature insulation failure.

Please remember that Cable is rated related with the voltage phase to ground (Vo).
- Wyes grounded system Vo/Vll=0.58.This is applicable only to systems where the normal voltage between the cable conductor and the insulation shielding tape, or metal sheath, will not exceed 58% of the phase to phase voltage.
Ungrounded systems Vo/Vll>0.58 and beside that the system may operate for a limited period of time with one phase accidentally grounded. To protect the cable against this faulted condition, it has arbitrarily been decided to reinforce the insulation thickness increasing the cable rated voltage by 33% resulting that for 133% insulation level Vo/Vll=0.77