# Control voltage between two substations

#### mshields

##### Senior Member
I've got two substations 34.5kV to 4160V about 1500 feet away from each other. The protective relaying scheme I'd like to employ involves communications between the two. Control is 125VDC. Running between the two presently is a single 34.5kV ductbank and a series of manholes. There are plenty of spares in the duct bank. I know better than to run the control voltage through the same manhole but a colleague is challenging me that it can be done if you barrier the low voltage in the manhole.

I've seen this done with lower albeit disparate voltages in handholes where one doesn't have to crawl into the thing but I see no practical way of accomplishing this in a manhole. Has anyone ever done this? Know how to do this if feasible?

By the way, this duct bank actually has a mix of 4160 and 34.5kV. It's an existing condition. I'd have thought this a violation. I am told it is not and that I could add more of either voltage if need be. Is that true?

Thanks,

Mike

#### templdl

##### Senior Member
I've got two substations 34.5kV to 4160V about 1500 feet away from each other. The protective relaying scheme I'd like to employ involves communications between the two. Control is 125VDC. Running between the two presently is a single 34.5kV ductbank and a series of manholes. There are plenty of spares in the duct bank. I know better than to run the control voltage through the same manhole but a colleague is challenging me that it can be done if you barrier the low voltage i
n the manhole.

I've seen this done with lower albeit disparate voltages in handholes where one doesn't have to crawl into the thing but I see no practical way of accomplishing this in a manhole. Has anyone ever done this? Know how to do this if feasible?

By the way, this duct bank actually has a mix of 4160 and 34.5kV. It's an existing condition. I'd have thought this a violation. I am told it is not and that I could add more of either voltage if need be. Is that true?

Thanks,

Mike
Greetings Mike,
1500' with a 125vdc control voltage? Before you start beating youself up to see if you could run the line as you would like to do my concern would be what voltage you would have left after a 1500' run. Remember that you really have 1500' in one direction and 1500' back.

#### augie47

##### Moderator
Staff member
You might take a look at 300.4(C)(2)(e) which seems to allow such.
If you still have concern, you can always use a fiber-optic cable.

#### petersonra

##### Senior Member
1500 feet is a long ways.

It will probably work OK at 125VDC, and if you can use some existing wires I can understand why it might be attractive to do so.

#### mshields

##### Senior Member
1500 ft

1500 ft

I'll certainly calc it out but thought since we're dealing in mA's it would be workable. Fiber Optics is a great idea.

I don't see that 300.4(C)(2)(e) and it wasn't immediately apparent which paragraph in that neighborhood might apply. i.e. 300.4(C) has no sub paragraphs.

Thank you very much gentlemen

#### mshields

##### Senior Member
Ah hah!

Ah hah!

augie47 - found it.

You meant 300.3(C)(2)e and yes that does give me an out.

THANKS!

#### augie47

##### Moderator
Staff member
sorry typo or old age unsure which....
POCOs here often pull FO cable in the same conduits as the high voltage cables.

#### GoldDigger

##### Moderator
Staff member
sorry typo or old age unsure which....
POCOs here often pull FO cable in the same conduits as the high voltage cables.
So do they teach HV line technians how to handle the fiber or do they teach HV safety to the fiber techs?

#### augie47

##### Moderator
Staff member
So do they teach HV line technians how to handle the fiber or do they teach HV safety to the fiber techs?
Now that you mention it, they do seem to have a high rate of job openings in the FO tech dept

#### JoeStillman

##### Senior Member
I think FO is the way to go. Digital protective relays should be available with a FO protectionn information interface.