The sheaths or shields of HV underground cables are tightly coupled with the core. If both ends of the sheath are grounded, there would be sheath circulating currents in the order of 60-80% of the main conductor currents which would severely lower the thermal rating of the cable. It would be very expensive to do so.

If both ends of the sheath are open, there would be sheath to ground flashovers during transients such as energization and faults.

Common practice in North America is bonding. There are two main types of bonding schemes: 1) single point bonding and 2) cross-bonding.

Single-point bonding is used when the cable is short (around 1 km). You ground one end and leave the other end open but protected with a surge arrester. The length of the cable section is limited by the permissible standing voltage at the open end, arrester rating, and effective sheath BIL. Different jurisdictions have different limits.

Cross-bonding is used in longer cables where you use several major sections in series. A major section consists of 3 minor sections of equal length where the sheaths are transposed so that circulating currents are cancelled during steady-state operation. At sending and receiving ends of a major section, the sheaths are grounded. The sheaths at the cross-bonding points are protected with surge arresters. Each minor section is still limited to 1 km in length.

There are variants of these bonding schemes, but the principle is roughly the same: you do not want steady-state sheath circulating currents and you want to protect the thin sheath insulation against transient overvoltages.

The attached excerpt from my lecture notes may help.

Pros: If the XLPE cables is grounded on both side you can reduce grounding resistance on main TS for example 220/110 kV, since grounding resistance of 110/X kV are parallel connected to the main TS. In some cities all grounding grid of TS consists of shields of cables if there is no place fro grounding grid.

Cons: You can transfer potential of main grounding system to the lower TS (110 kV) in the case of both ends of cable shield are grounded so you can make risk for personnel in those TS-s

Cons: If on side of cable is ungrounded also on that side you can have maybe high transfer potential from main grounding TS at the end of cable so personnel working on the cable shield can be in risk situation,

For me the better solution is grounding on both sides and if cables are long phase shields can be transpositioned.

Respectful Srete Nikolovski,

Thank you a lot for sharing this answer. I asked this question because some standards do not give any difference in specification ( The Indian standard IS 7098 part 2 1985 for example) which is confusing for me.

In addition to what you have answered, I think that the size of cable will be different too since higher insulation level is required for ungrounded cable, is it true ?

Dear Kemari

I am not a cable expert but I think that the isolation of cable do not depend of metalic shield of cable. But for the use the same XLPE cable cross section the level of ampacity of cable in the air and in the ground is not the same because the thermal disipation is not the same.

Ok, thank you Professor Srete Nikolovski for all of your answers.

The sheaths or shields of HV underground cables are tightly coupled with the core. If both ends of the sheath are grounded, there would be sheath circulating currents in the order of 60-80% of the main conductor currents which would severely lower the thermal rating of the cable. It would be very expensive to do so.

If both ends of the sheath are open, there would be sheath to ground flashovers during transients such as energization and faults.

Common practice in North America is bonding. There are two main types of bonding schemes: 1) single point bonding and 2) cross-bonding.

Single-point bonding is used when the cable is short (around 1 km). You ground one end and leave the other end open but protected with a surge arrester. The length of the cable section is limited by the permissible standing voltage at the open end, arrester rating, and effective sheath BIL. Different jurisdictions have different limits.

Cross-bonding is used in longer cables where you use several major sections in series. A major section consists of 3 minor sections of equal length where the sheaths are transposed so that circulating currents are cancelled during steady-state operation. At sending and receiving ends of a major section, the sheaths are grounded. The sheaths at the cross-bonding points are protected with surge arresters. Each minor section is still limited to 1 km in length.

There are variants of these bonding schemes, but the principle is roughly the same: you do not want steady-state sheath circulating currents and you want to protect the thin sheath insulation against transient overvoltages.

The attached excerpt from my lecture notes may help.

Respectful Luis Marti,

Many thanks for your detailed answer and your lecture notes. Indeed, I've more clear idea now.

Although, I have two questions if you kindly respond :

Best regards.

1. Can single-point bonding be employed for long cables too, if yes, what are the conditions ? .

You can extend the use to 2-3 km if you use center-point bonding. Grounded in the middle and surge arresters at the other end. More than that you run into steady-state standing voltage limits and/or surge arrester limits. Sheath insulation BIL is not very high and it degrades over time. As a rule of thumb, I de-rate it to about ¼ of as-new strength over the life of the cable

2. Since that circulating currents are cancelled during steady-state operation, could the parallel ground continuity conductor be removed in cross-bonding where the sheaths are transposed ?

In cross-bonded cables one or more ground conductors are normally added for short circuit current carrying purposes. As soon as there is a fault, there is no longer cancellation.

Thank you again Dr Luis Marti for clarification.

Sincerely.

I want also to thanks to prof. Marti who is the leader in the transients in cables and cable modeling

Srete

As said from the previous correspondence the bonding of high voltage cable systems shall be taken into consideration and it is under the responsibility of the system design engineer. Typically, the current circulating in the conductor induces a voltage in the metallic screen, this voltage is depending on the cable arrangement, for cables laid trefoil or very close this voltage is at the minimum value, increasing the cable spacing the voltage will increase. If the metallic screen is short circuited both ends (solid bonded) this induced voltage will cause a circulation current in the screen that is function of the cross-sectional area of the screen. For medium voltage cables till 30 kV the metallic screen is generally not too big say 25 mm2 and then the circulating current for cables laid trefoil is not too high and the losses in the screen are very low and generally for these kinds of systems the solid bonding of the screen at both the extremities is recommended and used. For high voltage cables >60 kV the metallic screen cross sectional area is generally very high because it shall be capable to sustain the fault current of the system to ground that is normally much higher than that of the medium voltage cables. It is then understandable that the circulating current in the screen is very high and consequently also the losses, in this case the so called (special bonding) system is to be used. When the length is limited to one or two lengths (indicatively

Dear Ernesto Zaccone,

Many thanks to you for all the detailed explication.

Is it the same with DC cables ?

for DC cables there are no induced voltages in the screen and the cable system is always solid bonded