Utility System Grounded to DC Transit System

[BPrescott]

Our utility has been asked by the public transit system to bond our grounded system neutral, which is an AC system, to their grounding grid with serves as a ground for both of their AC and DC equipment. What are the concerns for stray DC current flowing though our AC system?

 

[davidbeach]

Where would it flow and what would drive it? Current flows in a complete loop, and with only one connection there wouldn't be current flow. If both systems are grounded in the same vicinity, they are tied together to some degree, so a solid connection shouldn't be a problem.

 

[waross]

We need more information;
Does the DC system use a ground return?
Does the DC system ground one line of the DC or do they create a line equipotential from both power lines for grounding?
Do they use multiple grounds on the DC system?
Do they use high resistance grounding on the DC system?
Do you use multiple neutral grounds on your utility system?
What does a map of the area look like with both your grounding points and the DC system grounding points overlaid?
What are the ground conditions, and do the various ground electrodes make good contact with ground?
Are the transit grounds in good condition or does the transit company want to avoid the expense of repairs by using your grounds?

DC currents over your system may be a non event, or they may be a problem. It depends on a lot of factors.

 

[davidbeach]

What I've seen for light rail systems is that the rails are isolated from ground with an impedance ground at the traction substation. Ground faults are detected by monitoring the voltage across the grounding resistance.

While isolated from ground, the rails have to remain essentially at ground potential to allow safe crossing. For a while after the opening of the west side line of Portland's MAX system, there was a problem at one or two of the stations were there was enough potential between the rails and the surrounding ground that guide dogs became reluctant to cross the track.

 

[BPrescott]

I will post the details after I have the meeting with the transit authority. Thanks for the replies!

 

[Skogsgurra]

waros's list is something to keep in mind.

I had a case where the rail DC system more or less eroded the bottom part of a control cubicle for an opening bridge away. The two systems had nothing in common, except the ground connection. There were probably many other parasitic ground connections as well. You always get them in "elongated" systems - wanted or not.

Gunnar Englund

http://www.gke.org

 

[BPrescott]

After meeting with the transit authority here is the information that we have. The DC line and rails are isolated from ground, essentially "floating". The only DC connection to ground is on 3 driven ground rods that are bonded to DC equipment housings. The AC system has it own grounding grid (designed to 5 ohms) that will be separated from the DC ground rods by 50 feet of earth. And our utility neutral would be bonded to this. Therefore, DC return current should not travel though our neutral since the DC negative line and rails are bonded together and isoloated from ground. Though there is a small posibility of Dc leakage current and we were told this was on the order of milli-amps. Would bonding our neutral to a lower impedance ground redirect line to ground fault current through that grounding grid and create step and touch potential that the transit people did not anticipate?



Does the DC system use a ground return?
No, the return is through the DC negative line and the rail.

Does the DC system ground one line of the DC or do they create a line equipotential from both power lines for grounding?
The Dc lines are not grounded, it is a "floating" system.

Do they use multiple grounds on the DC system?
No, there will be 3 separate DC groung rods driven at the inverting station to ground DC equipment housings.

Do they use high resistance grounding on the DC system?
No, the ground rods are to be 25 ohms or less.

Do you use multiple neutral grounds on your utility system?
Yes, our netural has many connections to driven ground rods.

What are the ground conditions, and do the various ground electrodes make good contact with ground?
Our grounds are assumed to be within spec of 25 ohm or less per rod.

Are the transit grounds in good condition or does the transit company want to avoid the expense of repairs by using your grounds?
Transit grounds will be new and installed to spec.