ThePunisher
Electrical
- Nov 7, 2009
- 384
Hi all,
I have surfed into this topic and learned a great deal of discussions on the above topic (almost beaten to death). I have an installation at site which we are currently debating and would like to respectfully obtain the opinion of experts here. I hope this post won't be ignored.
PLEASE SEE ATTACHED Sketch for this posting.
1. INSTALLATION 1 (Cable Bus from transformer secondary to switchgear)
In this installation, the single core cables' individual shields are bonded together and passed through the ZCT at the 13.8 kV switchgear incoming section and eventually connected to the switchgear ground bus. This is because the individual single core cable shields are passed through the ZCT and in such case, we have to pull back the same shields back to the ZCT and then to ground bus, so that the induced currents would cancel out in the event of a ground fault and the ZCT would be able to see the phase currents only (and component of the fault in the phase). Here is the catch. Our EPC design contractor advised us that the shields at the transformer secondary end will be unbonded, and their braids will be coiled and taped (single point shield bonding with the open shields at the source). A concern was raised about the cumulative shield voltage at the transformer secondary terminations as the EPC did not conduct any estimation. However, my take is that since the voltage is 13.8 kV, circuit length of cables is only 30 meters and the secondary is connected to a 75A NGR. I would assume for now that the shield voltage at the open end would not be of any concern during normal operations. I can ask the pre-commissioning contractor to perform shield voltage measurement to document it by doing a primary current injection on the cable using the transformer(no idea how but will ask them the details). There is an equipment bonding conductor running inside the cable bus which interconnects the switchgear ground bus and transformer secondary temrination box...so I would think this will take care of the ground return. Any comments, in case I miss something?
2. INSTALLATION 2 (3-Cable from switchgear feeder breaker to VFD Input Section)
This is the area where we get some discussion with my construction team. The three core cable at the source feeder circuit breaker section is stripped out before the ZCT and then bonded to the switchgear ground bus. Hence, there is no shield that passed through the ZCT like that of Item 1 above.
a. My take is that, since there is no cable shield passing through the ZCT towards the termination, there is no need to pull the shields into the ZCT. Would everyone agree with me?
b. The construction team has bonded the shields of the 3 core cable at the switchgear feeder breaker section (see 2.a) but left the shields unbonded and open/coiled'taped at the VFD input section, thinking that they will do single point bonding philosophy too. I believe I can bond both cable shields at both the switchgear feeder section and in the VFD input section (both ends) and should not worry about shield circulating currents as the shields are part of a three core cable and hence, the circulating currents should be very minimal as they tend to cancel out (nearly) in a balanced three phase current flow.
There is also an EGC within the three core cable which they also bonded on both ends at ground buses and hence, this will serve as the path of least resistance for the GF....which is only 75A by the way.
Any comments on my understanding in case I missed something.
Thank you very much in advance for any comments I will get. STARS waiting.
I have surfed into this topic and learned a great deal of discussions on the above topic (almost beaten to death). I have an installation at site which we are currently debating and would like to respectfully obtain the opinion of experts here. I hope this post won't be ignored.
PLEASE SEE ATTACHED Sketch for this posting.
1. INSTALLATION 1 (Cable Bus from transformer secondary to switchgear)
In this installation, the single core cables' individual shields are bonded together and passed through the ZCT at the 13.8 kV switchgear incoming section and eventually connected to the switchgear ground bus. This is because the individual single core cable shields are passed through the ZCT and in such case, we have to pull back the same shields back to the ZCT and then to ground bus, so that the induced currents would cancel out in the event of a ground fault and the ZCT would be able to see the phase currents only (and component of the fault in the phase). Here is the catch. Our EPC design contractor advised us that the shields at the transformer secondary end will be unbonded, and their braids will be coiled and taped (single point shield bonding with the open shields at the source). A concern was raised about the cumulative shield voltage at the transformer secondary terminations as the EPC did not conduct any estimation. However, my take is that since the voltage is 13.8 kV, circuit length of cables is only 30 meters and the secondary is connected to a 75A NGR. I would assume for now that the shield voltage at the open end would not be of any concern during normal operations. I can ask the pre-commissioning contractor to perform shield voltage measurement to document it by doing a primary current injection on the cable using the transformer(no idea how but will ask them the details). There is an equipment bonding conductor running inside the cable bus which interconnects the switchgear ground bus and transformer secondary temrination box...so I would think this will take care of the ground return. Any comments, in case I miss something?
2. INSTALLATION 2 (3-Cable from switchgear feeder breaker to VFD Input Section)
This is the area where we get some discussion with my construction team. The three core cable at the source feeder circuit breaker section is stripped out before the ZCT and then bonded to the switchgear ground bus. Hence, there is no shield that passed through the ZCT like that of Item 1 above.
a. My take is that, since there is no cable shield passing through the ZCT towards the termination, there is no need to pull the shields into the ZCT. Would everyone agree with me?
b. The construction team has bonded the shields of the 3 core cable at the switchgear feeder breaker section (see 2.a) but left the shields unbonded and open/coiled'taped at the VFD input section, thinking that they will do single point bonding philosophy too. I believe I can bond both cable shields at both the switchgear feeder section and in the VFD input section (both ends) and should not worry about shield circulating currents as the shields are part of a three core cable and hence, the circulating currents should be very minimal as they tend to cancel out (nearly) in a balanced three phase current flow.
There is also an EGC within the three core cable which they also bonded on both ends at ground buses and hence, this will serve as the path of least resistance for the GF....which is only 75A by the way.
Any comments on my understanding in case I missed something.
Thank you very much in advance for any comments I will get. STARS waiting.