MV cable shields and bonding at both ends?

[Dale001289]

On the primary side of the MV transformer they bonded the cable shields to earth along with the EGC. At the switchgear side where the cables terminate the shields are also bonded to the switchgear grounding bus. (The shields are routed through a ZCT on the incoming)
The engineering company says the shields must be floating at the transformer end in order prevent circulating current via the cable shield. But I’ve seen shields bonded in both cable ends on previous jobs and had no issues. Thoughts?


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[Ziyad Alahrbi]

Hi, this is my first time here. I have done some projects related MV cable replacements. I will talk from my country and company standard. I will give you a general idea but you have to refer to the applicable standards in your company/country.

We have four scenarios:
1. three core cable - shield single point grounded.
In this installation, the shield will induce voltage if you have unbalanced current in the phases. Usually in three core cable, the mutual inductance between phase conductors are the same and thus you will not face unbalance voltage drop through cable if the load is balanced.

2. three core cable - shield both end grounded.
same as "1" but the unbalanced load will introduce a current circulating in the shield. If your design will work to supply unbalanced loads you might have to derate your cable and circulating current wont be an issue.

3. single core cables (three phases) - shield single end grounded
mutual inductance between cables will produced unbalanced voltage drop through the cables thus unbalanced current supplied to the load (even if the load is balanced). shield will produce a voltage at the open terminal side and this voltage might high (~0-1000V). This high shield voltage might damage the insulation at the open end. You can mitigate this by doing calculation to the shield and selecting the right approach to decrease this induced voltage. there multiple methods in the industry you might refer to it like cross linking and termination with overvoltage limiters.

4. Single core cables(thee phases) - shield both end grounded.
In this case, there will be no voltage in the shield because of the circulating current in it. However, this circulating current will worsen the case of mutual inductance unbalance, typically worse than case "3".

Important note, induced voltage in the shield depends on current and length.

Regarding ZCT and as per my knowledge, three phase only shall going inside. So, when you insert the three phase with shields in side in the CT and complete the termination, the shields shall be routed back through the CT, going in the CT- then - coming back CT.

Best Regards.
Ziyad Alharbi

 

[Dale001289]

Hi, this is my first time here. I have done some projects related MV cable replacements. I will talk from my country and company standard. I will give you a general idea but you have to refer to the applicable standards in your company/country.

We have four scenarios:
1. three core cable - shield single point grounded.
In this installation, the shield will induce voltage if you have unbalanced current in the phases. Usually in three core cable, the mutual inductance between phase conductors are the same and thus you will not face unbalance voltage drop through cable if the load is balanced.

2. three core cable - shield both end grounded.
same as "1" but the unbalanced load will introduce a current circulating in the shield. If your design will work to supply unbalanced loads you might have to derate your cable and circulating current wont be an issue.

3. single core cables (three phases) - shield single end grounded
mutual inductance between cables will produced unbalanced voltage drop through the cables thus unbalanced current supplied to the load (even if the load is balanced). shield will produce a voltage at the open terminal side and this voltage might high (~0-1000V). This high shield voltage might damage the insulation at the open end. You can mitigate this by doing calculation to the shield and selecting the right approach to decrease this induced voltage. there multiple methods in the industry you might refer to it like cross linking and termination with overvoltage limiters.

4. Single core cables(thee phases) - shield both end grounded.
In this case, there will be no voltage in the shield because of the circulating current in it. However, this circulating current will worsen the case of mutual inductance unbalance, typically worse than case "3".

Important note, induced voltage in the shield depends on current and length.

Regarding ZCT and as per my knowledge, three phase only shall going inside. So, when you insert the three phase with shields in side in the CT and complete the termination, the shields shall be routed back through the CT, going in the CT- then - coming back CT.

Best Regards.
Ziyad Alharbi

I really appreciate your response.
We have case 4. The length varies from a couple thousand feet to less 50 feet. It appears the better methodology is to ground at a single end as per case 3.

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[Dale001289]

Hi, this is my first time here. I have done some projects related MV cable replacements. I will talk from my country and company standard. I will give you a general idea but you have to refer to the applicable standards in your company/country.

We have four scenarios:
1. three core cable - shield single point grounded.
In this installation, the shield will induce voltage if you have unbalanced current in the phases. Usually in three core cable, the mutual inductance between phase conductors are the same and thus you will not face unbalance voltage drop through cable if the load is balanced.

2. three core cable - shield both end grounded.
same as "1" but the unbalanced load will introduce a current circulating in the shield. If your design will work to supply unbalanced loads you might have to derate your cable and circulating current wont be an issue.

3. single core cables (three phases) - shield single end grounded
mutual inductance between cables will produced unbalanced voltage drop through the cables thus unbalanced current supplied to the load (even if the load is balanced). shield will produce a voltage at the open terminal side and this voltage might high (~0-1000V). This high shield voltage might damage the insulation at the open end. You can mitigate this by doing calculation to the shield and selecting the right approach to decrease this induced voltage. there multiple methods in the industry you might refer to it like cross linking and termination with overvoltage limiters.

4. Single core cables(thee phases) - shield both end grounded.
In this case, there will be no voltage in the shield because of the circulating current in it. However, this circulating current will worsen the case of mutual inductance unbalance, typically worse than case "3".

Important note, induced voltage in the shield depends on current and length.

Regarding ZCT and as per my knowledge, three phase only shall going inside. So, when you insert the three phase with shields in side in the CT and complete the termination, the shields shall be routed back through the CT, going in the CT- then - coming back CT.

Best Regards.
Ziyad Alharbi

The transformers came equipped with surge protectors (ie voltage limiters) installed on the HV primary side but the lead engineer had them removed. From your feedback it seems these should be utilized for single point grounding.


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[Julius Right]

Cable shield connections are recommended in IEEE-575 IEEE Guide for the Application of Sheath-Bonding Methods for Single-Conductor Cables and the Calculation of Induced Voltages and Currents in Cable Sheaths
5.4 Single-Point Bonding
5.4.1 Sheath Standing Voltages
It depends on phase arrangement: trefoil, flat and other and the ratio S/d -distance on shield diameter, main conductor current and length.
Current practice in the United States appears to permit a steady-state sheath voltage of 65 V-90 V, although there is not much evidence to substantiate this. The calculation way is in Appendix D Calculation of Induced Voltages.
There are some methods used to mitigate this voltage build-up. One recommended is phase cross-bonding.
If the cable sheath[shields) are bonding and grounding both ends the induced voltage is reduced but the losses of circulating current through shield may reduce the cable ampacity.
In order to calculate the losses the Table 5-3 from Okonite Engineering Handbook ,IEC 60287-1-1 ,IEEE 835 or Neher and McGrath theory may be used.

 

[Dale001289]

Cable shield connections are recommended in IEEE-575 IEEE Guide for the Application of Sheath-Bonding Methods for Single-Conductor Cables and the Calculation of Induced Voltages and Currents in Cable Sheaths
5.4 Single-Point Bonding
5.4.1 Sheath Standing Voltages
It depends on phase arrangement: trefoil, flat and other and the ratio S/d -distance on shield diameter, main conductor current and length.
Current practice in the United States appears to permit a steady-state sheath voltage of 65 V-90 V, although there is not much evidence to substantiate this. The calculation way is in Appendix D Calculation of Induced Voltages.
There are some methods used to mitigate this voltage build-up. One recommended is phase cross-bonding.
If the cable sheath[shields) are bonding and grounding both ends the induced voltage is reduced but the losses of circulating current through shield may reduce the cable ampacity.
In order to calculate the losses the Table 5-3 from Okonite Engineering Handbook ,IEC 60287-1-1 ,IEEE 835 or Neher and McGrath theory may be used.

Thank you very much Julius


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