Insulation Level

WHAT DO THEY MEAN WHEN THEY TALK ABOUT

INSULATION LEVEL ?

Insulation level applies to what’s referred to as shielded cables or cables with an insulation shield. Generally cables rated at 5 kV and higher are shielded but some at 5 kV, depending on their construction, may be unshielded.

When choosing a cable’s voltage class it’s based on the phase to phase voltage of the system on which the cable is to be used. Cable voltage classes are standardized in North America and it’s standard practice to use the next higher cable class for a certain system voltage class. For example a 13.8 kV system uses a 15 kV rated cable and a 27.6 kV system uses a 28 kV rated cable and so on.

The insulation thickness and minimum conductor size for each voltage class is designed to limit the maximum voltage stress at the conductor shield surface and the average stress in the insulation wall. These stresses are calculated based on insulation thickness and on the phase to ground voltage that the insulation is subjected to. The voltage the insulation is subjected to during normal operation is the phase to phase voltage divided by Ö 3 or by 1.73 . This is the voltage across the insulation wall during normal operation all day long every day and this is the same for both wye and delta three phase systems.

If there is a fault somewhere on a three phase system the voltage in the system is disturbed because voltage on the faulted phase may go to zero or close to it and voltage on the other two phases rises. Theoretically this voltage rise can take the phase to ground voltage the cables on the unfaulted phases see up to what is normally the phase to phase voltage. This situation can persist until a fault is cleared or the system is de-energized by breakers or fuses protecting it. We choose a cable’s insulation level based on the fault clearing time of the system in which it’s used.

If fault is cleared one minute or less you can use 100% insulation level cable.

If fault clearing time is from one minute to one hour you should use 133% insulation level cable.

There is also a 173% insulation level but this is very uncommon. 173 % is used on systems where the fault clearing time is indefinite.

As the insulation level increases so does the insulation thickness for a given voltage class. For example, a 15kV 100%, XLPE insulated cable would have an insulation thickness of 0.175" ( 4.44 mm ). The same cable with 133% insulation level would have an insulation that’s 0.220" ( 5.59 mm ) thick. The 173% insulation level thickness is not specified by any recognized standard so we generally use the thickness for the 100% level for the next higher standard voltage class of cable. In this example we would go to 25 kV, 100% or a cable with 0.260" ( 6.60 mm ) of insulation.

The only exception to 100% vs 133% is the 5 kV cable class. In this case most North American standards specify 0.090" ( 2.29 mm ) for both insulation levels.

As you can see a higher insulation level cable generally has a thicker insulation. Insulation thickness for these levels is not a direct ratio of 100% to 133% but is related to limiting the electrical stress that the insulation sees before a fault is cleared. Controlling this stress will help prevent cables on the unfaulted phases from failing due to overvoltage while a fault may be present. If fault clearing time is longer, a chance of cable failure is increased so a higher insulation level is required.

Since 133% insulation level cables have an increased insulation thickness, they can be used in place of 100%, but the reverse is not true. Also electrical stress during fault and normal operating conditions is less in 133% insulated cable, therefore expected cable life will be increased. Because of this some users, especially utilities, specify 133% cable even though fault clearing time would allow 100%.

Generally 100% insulation level cable is used on grounded wye or star systems and 133% is used on delta systems. This is because generally a fault is cleared very quickly on a wye ( ie: within a few cycles) but could persist for longer periods on a delta system. The real governing factor is fault clearing time. Up to one minute use 100%! Up to one hour use 133%!

By : D.S. Reith, C.E.T. – Applications Specialist

July 31, 2001