Parallel cables routed in tray-200 amp ground currents
Parallel cables routed in tray-200 amp ground currents
(OP)
We are measuring 200 amps in the ground conductors from the 480 volt substation ground bus to the MCC ground buses. The 480 volt sub is a double-ended substation with two 3000 kVA 12.5kV-480 volt transformers. Each half of the substation powers three 1600 bus MCCs, each with 1000 amps of load. The MCC feeder cables are four 1/C-500MCM cables/phase routed in cable trays for 300'. The MCC feeder cables have been installed in the cable trays with the A phase cables grouped together (in one plane), then the B phase cables grouped together, then C phase cables grouped together with the single 1/C-4/0 ground conductor laying on top. I realize this is not the proper method for installing three phase cables in tray, but I inherited this problem. The motors powered from the six MCCs are connected to the MCC starters via 3/C with ground tray cable with approximately 15 amps measured in each ground conductor to the motor. The ground current to the motors is almost identical for each motor on the various MCCs.
The transformer neutrals are connected to the substation ground bus via busbar. The substation is located 150' behind the plant with elevated cable trays routing the 480 volt TC cables to the MCCs inside the plant. The six MCCs are located close together inside the plant electrical room with cable tray distribution from the MCCs to the motors. The cable trays are bonded to the MCC ground buses and each other.
I suspect the MCC parallel feeder cable routing is the problem, but I find it hard to believe it would be to this magnitude. Any ideas?
The transformer neutrals are connected to the substation ground bus via busbar. The substation is located 150' behind the plant with elevated cable trays routing the 480 volt TC cables to the MCCs inside the plant. The six MCCs are located close together inside the plant electrical room with cable tray distribution from the MCCs to the motors. The cable trays are bonded to the MCC ground buses and each other.
I suspect the MCC parallel feeder cable routing is the problem, but I find it hard to believe it would be to this magnitude. Any ideas?






RE: Parallel cables routed in tray-200 amp ground currents
William
RE: Parallel cables routed in tray-200 amp ground currents
RE: Parallel cables routed in tray-200 amp ground currents
Answer to rbulsara: Tie breaker is normally open
RE: Parallel cables routed in tray-200 amp ground currents
OK. Open tie eliminates suspicion of circulating currents.
Next question:
Does the 15A ground current in individual motor grounding conductor flow only when the motor is running or the ground current is also there when the motor is off? Please check a few motors.
RE: Parallel cables routed in tray-200 amp ground currents
If my hypothesis is correct, bonding the MCC ground bus to building steel may eliminate the motor circuit ground currents.
RE: Parallel cables routed in tray-200 amp ground currents
Are these motors fed via adjustable frequency drives?
I agree with jghrist that this is likely some type of circulating current not directly related to the motor current.
RE: Parallel cables routed in tray-200 amp ground currents
What is probably happening is that all the grounded tray is and ground conductors are forming a very solid shorted turn where current is induced by the considerable magnetic assymetry of the cable installation. An easy way to verify this is to plot how your ground current changes with current through the main conductors as their should be a linear relationship.
In a theoretical sense fixing it is easy - remove the assymetry in the cable installation or open the ground loop. Unfortunately practically I don't think that you can avoid reinstalling the cables!
RE: Parallel cables routed in tray-200 amp ground currents
RE: Parallel cables routed in tray-200 amp ground currents
Is there anything that is trying to use the equipment ground as a neutral? If so, you need to put neutral conductors into the cable trays and separate the load neutrals at the MCCs from the equipment ground. Accidental or purposeful regrounding of a neutral and using the EGC as a neutral are 2 common violations. You have to remember that 70% is a passing score on licensing exams which means that the registered EE, the electrcians, and the electrical inspectors could all be only 70% correct or 75% correct.
If the phase wires are not quadruplexed around each other you can rent a high reach, cut the nylon cable ties, and rearrange the wires ABCABCABCABC. If not, you would need to shut down 1 MCC at a time and rearrange the conductors.
Are the 500 KCM phase conductors operating as single conductors in free air? If so, you can cut voltage drop and improve system efficiency by doubling the number of conductors and still be Code on your cable tray fill provided that the original conductors have 1 conductor diameter of space between them. Some of the extra conductors would need to be neutrals plus the equipment ground resulting in six 500 KCM per phase instead of 4 and three 500 KCM for the neutral instead of zero.
Explain to your boss that when you heat up the wires he is paying for electricity and not getting it.
RE: Parallel cables routed in tray-200 amp ground currents
not quit sure if I understand the problem but you should not have each phase grouped ie aaa, bbb,ccc the group should be abc this way inductances will cancell refer to nec section one conductors in mulyiple conduits
scooter
RE: Parallel cables routed in tray-200 amp ground currents
RE: Parallel cables routed in tray-200 amp ground currents
A bonding jumper (and I refer to the Main Bonding Jumper or MBJ) is the connection between the equipment grounding bus and the grounded bus (neutral) at the service entrance (your MCC). This jumper ensures that fault currents will reach a physically grounded point (the grounded bus).
Is your MCC mains section labeled as service equipment?
William
RE: Parallel cables routed in tray-200 amp ground currents
For example our fiberline building has six 30kV,725V transformers each feeding an MCC.
All distribution is with trefoil cables.
Currents up to 5 amps are measured in the motor feeder grounding conductors.
These motor currents seem to add up and up to 51 amps is measured on the switchgear main grounding conductor to building ground.
The grounding conductors for trays also have measurable currents in the order of around 10 amps.
I believe the currents measured in the motor grounding conductors (yellow-green) are caused by the non cancllation of the magnetic field in the cable.
The local authorities do not accept such currents although I could not get any value from them. The NEC & CEC describe do not give magnitudes for the allowable grounding currents, anybody has any idea?
RE: Parallel cables routed in tray-200 amp ground currents
These currents sound very high. I'm not in your part of the world so can't give any Standards to refer to. A helfpul comparison would be to look at sensitive earth fault protection units to see what tends to work i.e. the Merlin Gerin NS250 Vigi units have a 3 A maximum setting for a 250A breaker.
Other questions that come to mind are -
* What power rating and winding types are the transformers?
* Can you describe the neutral to earth connection
arrangement for the transformers and MCC's are?
RE: Parallel cables routed in tray-200 amp ground currents
The IEC and VDE standards that I have seen so far do not mention any values.
In the building I refered to there are nine transformers 30kV/0.725V delta-wye (Dy5)
3.125 MVA dry type transformers.
Also in same building there is also one 30kV/400V, 3.125 MVA delta-wye (Dyn5)arrangement, TN-S system, that is 3 phase 4 wire with separate neutral and ground conductor. I have not yet made any measurements on this MCC.
The last transformer is a 30kV/6.3kV, 6.3 MVA delta-wye (Dyn5) feeding an O2 plant located about 100m away.
The distribution system for the 725 V MCC's is an IT sytem, that is neutral not grounded. Ground fault detection is made with Bender relays that measure zero sequence voltage as part of their scheme to evaluate ground faults.
On the MCC's where I measured those continous ground currents all phase voltages to ground are balanced meaning no real ground faults.
I also intentionally made measurents on MCC's that have no VFD's, there are no significant harmonics in the currents measured so far.
I was thinking of possible interaction between the two systems IT and TN-S.
RE: Parallel cables routed in tray-200 amp ground currents
RE: Parallel cables routed in tray-200 amp ground currents
RE: Parallel cables routed in tray-200 amp ground currents
Each of the two 12.5kV-480V substation transformers have a bus connection between the transformer neutral and the substation ground bus; in addition, there are two MCCs with neutral buses which are cable connected to the substation ground bus. From what we can tell, the "300' long current transformer" is causing current flow from the substation ground bus thru the ground conductor from the sub ground bus to the MCC ground bus, then thru the motor ground conductors (using TC cable) to building ground, then back to the sub ground via the underground ground grid.