Sheath Standing Voltage at Fault Condition of HV 69kV cu / XLPE Cable 6

[leur2011]

Our 69kV underground direct buried cables installation is having a single point bonding system, 3 x 1core-1000MCM cu/XLPE/CuW/Lead/HDPE per phase laid in trefoil formation with associated 1 x 1core - 4/0 cu AWG ground continuity conductor per phase for a span of 2 kilometers, link box with sheath voltage limiter (SVL) is provided at the mid-point of the circuit (1000m). Solidly grounded at both ends.

The cable manufacturer induced voltage calculation at normal condition indicates 0.02 V/m at load current 371 A. During fault conditions we consider 40 kA short circuit fault current. The sheath standing voltage for 3-phase fault is 1.24 V/m which is below our tolerable 5 kVrms, but for single phase to ground fault the manufacturer consider 40 kA fault current and the result is 11.2 V/m that is 11.2 kV at mid-point.

My question: In case of single point bonding system, having 3 cables per phase (trefoil formation), and where system is operated in abnormal condition with only one cable per phase; do we need that the sheath voltage calculation shall take into account the division of any ground fault return current between the three ground continuity conductors?

 

[7anoter4]

Correction: case 3 :
The return short-circuit current flows through all 9 copper tape shields and 9 lead sheaths in parallel with the 3 grounding conductors.

 

[leur2011]

7anoter4, excellent illustration for study cases of different method of single point grounding system for conductor phase-to-ground fault within inside and outside of the HV cable.

The IEEE-575 does an excellent guide for bonding the sheath of HV cable, and it does mentioned the safe limit for induced sheath voltage at normal condition as 65 Volts. But at fault condition, the IEEE-575 didn't recommend any safe limit value of induced sheath voltage e.g. 1000 Volts, 5000 Volts etc. at fault condition, any idea?

I've found one thread below discussing similar topic but can't find standard acceptable value of induced sheath voltage at fault condition. Thanks.

thread238-399727Link

 

[7anoter4]

In my opinion, it is not a permissible limit of the voltage built-up in short-circuit conditions. You have to calculate the voltage drop and to choose the suitable SVL in order to protect the HDPE sheath from breakdown. See for instance:

http://www.akpowersolutions.com/Med...re__aeb01351-0f0d-4c7e-b417-ae3810c56047..pdf

 

[jghrist]

I think there is some confusion. My understanding is that the cable sheaths are grounded at the both ends but are open in the middle, connected to sheath voltage limiters (SVLs). So, as long as the induced voltage is below the SVL flashover value, there will be no sheath current.

During a Ø-grd fault, there will be current in only the faulted phase, the earth, and the ground continuity conductors. The level of current in the phase is not 1/3 of the maximum design current, but is dependent on the source and cable positive- and zero-sequence impedance. There will be voltage induced in the shield from the phase current. There will also be a ground potential rise at the mid-point because of currents in the earth and ground continuity conductors. The total voltage between the shield and ground at the middle depends on both the shield induced voltage and the ground potential rise. Calculation will be complex. Perhaps a program such as SEL MALZ and SPLITS could be used. See

http://www.sestech.com/Products/SoftModule/Splits.htm

 

[7anoter4]

Thank you,jghrist.

 

[leur2011]

Thanks jghrist for sharing your expert clarification.

But I want to find out when you mentioned that during phase to ground fault, e.g. single conductor to ground fault (trefoil formation), the level of current in the phase is not 1/3 of the maximum design current (40kA), but is dependent on the source and cable positive- and zero-sequence impedance.

In my understanding, during phase to ground fault, each cable within the phase (say phase A) of trefoil formation, the current through each cable will be 40kA/3, assuming equal impedance due to the source is a 3 winding auto XFR, the tertiary winding will act as balancer for short circuit current flowing back to the source.

 

[jghrist]

In my understanding, during phase to ground fault, each cable within the phase (say phase A) of trefoil formation, the current through each cable will be 40kA/3, assuming equal impedance due to the source is a 3 winding auto XFR, the tertiary winding will act as balancer for short circuit current flowing back to the source.

In a phase-to-ground fault with a source at only one end of the cable, all of the current will flow in the faulted phase. No current will flow in the unfaulted phases.