125VDC in MV SWGR

[eanton22]

Hello guys,

I am currently reviewing bids on MV Swgr. I am new at this and I wanted to know where is the 125VDC power source in the SWGR come from? Also, what does the 125VDC service inside the SWGR? I know this might sound vague but does 125VDC source usually provided by the vendor (who provides the SWGR) or is it coming from some place else outside the SWGR, then fed into the SWGR?

Also, what does the 125VDC power? Circuit breakers, CT's,etc?

Thanks

 

[jim dungar]

Hello guys,

I am currently reviewing bids on MV Swgr. I am new at this and I wanted to know where is the 125VDC power source in the SWGR come from? Also, what does the 125VDC service inside the SWGR? I know this might sound vague but does 125VDC source usually provided by the vendor (who provides the SWGR) or is it coming from some place else outside the SWGR, then fed into the SWGR?

Also, what does the 125VDC power? Circuit breakers, CT's,etc?

Thanks

The opening and closing circuits of the breakers are normally 125VDC.
Usually a battery bank and charging system is part of every MV switchgear room. Every switchgear manufacturer has the ability to provide the 125VDC system as an option/adder.

 

[eanton22]

What powers the CT's??


Thank you

 

[72.5kv]

The cts are used for metering and protection, thier source is the current flowing in and out of the breakers. Just be very careful when working with CT's

 

[RUWired]

The power transformers (PT'S) reduce down to the control voltage needed for metering and control operation.

 

[djohns6]

The power transformers (PT'S) reduce down to the control voltage needed for metering and control operation.

Not always the case . In the utility industry , PT's ( Potential transformers )are mainly used for potential sources for relaying and metering . As stated , the control functions are supplied by the station DC source and by the station AC supply .

 

[mdshunk]

Also, what does the 125VDC power?

I wonder how many customers have had switchgear installed, and only asked themselves that question after the fact.

Short answer, a big rack of batteries.

 

[brian john]

I wonder how many customers have had switchgear installed, and only asked themselves that question after the fact.

Short answer, a big rack of batteries.

And how many skip maintenance and testing of this critical item. MOST in our are and I can state for a FACT.

 

[djohns6]

And how many skip maintenance and testing of this critical item. MOST in our are and I can state for a FACT.

Some years ago I did a battery capacity test on a set of substation batteries ( 130VDC ) that had failed during a fault in a 13.8 kv bay . The batteries failed instantly when a load was applied . No surprise , but they needed documentation . Anyway , the distribution bay literally burned to the ground . All of the control cables , relay panels , etc . were toast . Dammndest sight I ever laid eyes on .

 

[jim dungar]

And how many skip maintenance and testing of this critical item. MOST in our are and I can state for a FACT.

One of my customers found out someone had turned off (opened the output disconnect) on battery banks in two different rooms. There was no way to open any of their breakers if a overcurrent or fault had occurred. They now have UV alarming on the banks.

 

[zog]

One of my customers found out someone had turned off (opened the output disconnect) on battery banks in two different rooms. There was no way to open any of their breakers if a overcurrent or fault had occurred. They now have UV alarming on the banks.

I have seen that several times. When I was teaching substation maintenence courses one of the test question I had asked what the batteries did, by far the most common answer was they were a back up power system for the switchgear control power. Very few knew what they really did (And that includes many engineers)

 

[zog]

And how many skip maintenance and testing of this critical item. MOST in our are and I can state for a FACT.

I have seen some neglected systems too, you could argue that this is the most important part of the power system, if they dont work, nothing else will.

 

[beanland]

DC Systems

DC Systems

Utilities use 48VDC, 125VDC and 250VDC for control power. I agree that maintenance of these battery banks is often very poor. New FERC rules mandate testing of battery systems by POCOs.

It would be a poor design that burned down due to battery failure. That is what backup devices and protective device coordination are all about. However, $&!# happens!

 

[jghrist]

The power transformers (PT'S) reduce down to the control voltage needed for metering and control operation.

Power transformers transform the transmission voltage (HV or EHV) to the distribution voltage (MV).

Station power (or auxiliary power) transformers transform the distribution voltage to low voltage for powering the lights, battery chargers, and other stuff in the switchgear room.

Voltage transformers (VT's), formerly called potential transformers (PT's) transform the distribution voltage down to low voltage for relaying and metering. VT's are of lower capacity than station power transformers and have a very accurate voltage ratio.

 

[jim dungar]

Voltage transformers (VT's), formerly called potential transformers (PT's) ...

VT instead of PT, is another hard habit to break. I have been trying to do it for 30+ years.

The utilities world moved to VT, the industrial world is still >50% PT.

 

[zog]

At least 50%

 

[eanton22]

Thank you, I learned alot through your threads.

 

[brian john]

Utilities use 48VDC, 125VDC and 250VDC for control power. I agree that maintenance of these battery banks is often very poor. New FERC rules mandate testing of battery systems by POCOs.

It would be a poor design that burned down due to battery failure. That is what backup devices and protective device coordination are all about. However, $&!# happens!

I know of a few cases. Also had Emergency calls to replace bateries ASAP as the utility was going to cut power, due to visual condition of batteries.

 

[noonan]

Me and my colleagues are still using PT.

And I've also seen control power ckts. with no
maintenance. Battery jumpers corroded completely
through so there was no connection. Very scary!!

 

[hockeyoligist2]

And how many skip maintenance and testing of this critical item. MOST in our are and I can state for a FACT.

We have 16, most are 130V some are 48V. The first Wednesday of each month, I or a coworker checks every one of ours. But, we are are a rare breed! Or I should say the boss is! He worries a lot!