How to select DC voltage level for Substations

[loganath4880]

Can u pls explain the basics for selection of DC voltage for substation used for control and protection applications, i came across 30V, 110V, 125V and 220 volts level, breaker manufacturer will supply closing and tripping coils according to our request, and most of the numerical protection relays are also capable of operating from 48-220 V ac or Dc.

 

[Zogzog]

In the US I think 125VDC is the most common and you will have an easier time finding parts and accessories.

 

[TurbineGen]

It depends on your application. 125 is very common in the U.S. as Zogzog suggested. I have seen several 48 VDC installations as well. Usually 48 VDC was used when the power requirements on the DC was low. I can't remember ever seeing 220 VDC though.

Is your breaker solenoid operated or is it mechanical energy storage? If it is solenoid operated, I would suggest 125 VDC to reduce the operating current. If it is mechanical energy (ie. spring charged) it really would make no difference.

If I were you, I'd specify 125VDC if for no other reason than it's the most common and easiest to find replacment parts.


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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[ScottyUK]

240V DC is out there in the UK in some of the bigger / older transmission substations and some of the larger power plants. Nasty if you make a mistake. It tends to result in one of those days you remember for all the wrong reasons.

Agree 110V DC or 125V DC is common. A nominal 110V battery (55x 2V cells) floats at 124V DC; beware when specc'ing equipment.


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If we learn from our mistakes I'm getting a great education!

 

[slavag]

Good Q.
Many countries: 110/125V DC is common.
Biggest companies with air breakears/first GIS for the HV is 220V DC ( reason voltage drop and peak of current).
Today for my pinion, preferable 110/125 V DC, if possible of course 48V DC, for medium voltage level 48DC.
Best Regards.
Slava

 

[oldfieldguy]

Most common with the electrical utilities in my area is 125 VDC (60 lead-acid cells). I had one client who have a 1940's vintage powerplant that used 250 VDC.

Some of my industrial clients used 48 VDC on older equipment but all the newer stuff was going in at 125 volts.

Much of the newer equipment doesn't have the current requirements of the older equipment and voltage drop is less of a problem.

old field guy

 

[dpc]

I'd go with 125 V dc or 48 V dc UNGROUNDED unless you have a strong reason to use something else. 250 V dc is something to stay away from. If you go lower, such as 24 V, you risk not being able to get properly rated equipment for utility-grade substation equipment. For a small substation or switchgear lineup, 48 V dc is fine. For larger substations, use 125 V dc. If you have existing substations, it can be advantageous to use the same voltage everywhere.

 

[cranky108]

The problem with 24v is the dirt that develops on the relay contacts is just enough that it can cause a high impedance condition. Where the higher voltages will tend to punch through the dirt on the contacts.

If the station is a very small distribution sub, then 48v will work. other wise use 125vdc.

If you look on the SEL web site there is a paper on balanced ungrounded, vers unbalanced ungrounded battery systems. (By balanced and unbalanced I am meaning that the natural tend is for the capacatance to balance the voltage to neutral, where if a high impedance resistor is used to tie each rail to ground you can modify that balance).

 

[loganath4880]

Thanks to all for their valuable reply