Substation and Power Plant DC, for control and protection 4

Never seen anything but floating. Mostly 110 V DC.

Suspect issues with galvanic corrosion if grounded.

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

For US utilities, the dc control power is always ungrounded (at least that is the goal)in my experience. For large stations, battery voltage is normally a nominal 125 V dc (60 or 59 cells), but will float higher. Smaller distribution substations are sometimes 48 V dc.

David Castor

Gunnar and Dave.
Thanks a lot for your answers.
Gunnar, a try found some information about corrosion issue.
Are you have some documents?

Best Regards.
Slava

Hi Slava,
We here also use floated DC batteries with control of insulation of poles. Typically batteries are 220 V DC. Only for small substations for private owners we try to push 110 V DC, which according to me is sufficient with numerical relay protections.
But I met one exception. After rehabilitation of industrial power plant new owner of the plant - Belgian company Solvay - insisted that negative pole to be permanently grounded. Their project manager didn't accept any discussions stating that in all installations of Solvay around the world it is the same practise.

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

Hi Plamen.
Thanks a lot!!!!
Its really intresting information.
We try found those exceptions and understand what is a reasons for it, what is advantages and disadvantages, what type of monitoring is used, are used two pole MCB's or single pole MCB's, etc..
Best Regards.
Slava

Telecoms battery chargers (almost) always earth the positive pole which avoids the corrosion problems alluded to above.

I have seen both floating and resistive centre-point earthed systems in substations but never a scheme with a solidly grounded pole.


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

Hi Slava,

As far as I remember (this project was some 3-4 years ago) they used two-pole MCB's.
Main reason to use grounded pole was that in case of damaged insulation on positive pole MCB will trip and it will be clear on which circuit the leakage is. According to me this is not so good idea, but it was not my design...
Of course they used control only from positive pole, but for 110 kV and 6,3 kV level it is still sufficient.

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

Around here it is pretty common to ground the center point of a battery at the the mid-point (between cells 30 & 31 on a 60-cell bank) through a resistor. Then, indicator lamps (120 volt) or relays (125 VDC) are connected from the bank positive and negative terminals to ground. A ground on the DC system will light up a lamp or pick up a relay for annunciation.

Not everybody does this. My present employer's "engineering" staff just gets glassy-eyed thinking about it when I mention it as a way to stave off DC ground problems.

old field guy

Background information. The reason batteries that are larger than 48vdc float is personal safty. You grab ahold of a floating 250vdc battery, it will hurt like hell, but you will survive. The added benifit of a floating battery is detecting battery grounds from when the racoons start eating the insulation on your cables.
Communication sites where they do not have many raccons, and 48vdc or less often ground the positive terminal, so it will blow the fuse.