DC power supply for Switchgears

[rajendrarohra1978]

We have plant commissioned in 1999, this has switchgear breakers which have interlock for DC supply i.e in case DC power is off the breaker will be open. Is this a Standard requirement.
Please let me know any standard which has this requirements, we need to remove this interlocks.

Thanks in advance

 

[DTR2011]

I saw such a scheme some years ago at a steel mill in the US. Most of the plant started its life in Italy and had been moved to its new location. Maybe 1980’s design.

During battery testing, a bad battery was found. Whoever changed the battery was not aware of how to jumper around bad battery to replace on line. It was presumed that the battery charger had battery eliminator filter. It did not.

When I arrived, several SEL relays and most ABB circuit shield relays were smoked. Plant was down. Major headache.

All of this because the existing ABB combiflex scheme had a DC under voltage trip, perhaps similar to what you have.

Since relays had to be replaced, we utilized a new SEL relay that has DC voltage monitoring, including ground fault detection. The analog DC voltage is now available to plant DCS. The ground fault alarm is as well. We set 2 alarms for low DC voltage.

The battery charger was also upgraded to include the battery eliminator filter caps.

I think this is a better solution. We also trained the staff on how to use jumper cables to jump around a bad battery on line for replacement.

 

[RRaghunath]

The requirement comes from process safety side. When the DC is down, the process safety is compromised as no protections would work and no breaker operation is possible.
I have seen undervoltage release being specified for circuit breakers that control gas compressor motors, so that the breaker would automatically open in case of DC control supply failure.

 

[bacon4life]

We had a customer that jumpered around a bad battery. Unfortunately, the customer never bother replacing the battery and we never got a SCADA warning because the charger continued supplying DC. At least DTR2011's smoking a few relays is better than actual fault that cannot be cleared because of loss of DC. In order to remove the undervoltage interlocks, you would need to re-engineer the entire system to ensure that means are in place to address all the failure modes this change would introduce.

 

[itsmoked]

Can someone explain how you "jumper around a battery" in a string without shorting it?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[JG2828]

Keith,
You generally remove the bad battery first. Although if the battery really is completely dead then it wouldn't hurt to short it out.

 

[itsmoked]

JG; I get that totally dead aspect, sure. Is that the only "jumpered" case? As otherwise I can't see a safe way of doing it without creating possibly a large arc or conversely having to drop the entire bank causing mayhem.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Compositepro]

A battery is by definition a series string of cells. Normally one would jumper around a bad cell in a battery. You could have a series string of batteries and jumper around one bad battery. But you cannot allow the overall voltage to change significantly.

 

[Parchie]

itsmoked (Electrical)
Can someone explain how you "jumper around a battery" in a string without shorting it?

Keith,
I get what you are thinking! That thing is done almost all of the time in power plants. The solution is to break the jumper of the battery string and pull out the defective cell and then returning the jumpers, shorting-out the connection previously occupied by the dead cell. Say you operate a 60-cell string (120VDC), the remaining terminal voltage will now be 118V, which is not that bad--> but you have to replace the defective cell! Since it it takes an amount of time to initially charge and make a ready replacement cell, you have to temporarily "short-out" the cell location for the duration. All those actions are based on the assumption that your battery charger has the feature like that of a battery eliminator (doesn't drop the string voltage appreciably when you are in the process of replacing the dead cell)!

Another way of doing it is to tie-up with another battery string (if available) and then break the jumpers on the string with a defective cell, letting it open until you can replace the dead cell with an identical/same model new cell. Hope that clears you mind there!

 

[Mbrooke]

But how/why would it cause relays to smoke?

 

[Parchie]

Mbrooke,
Haven't you heard about relays drawing more current when not fully sealed-in?

 

[itsmoked]

Hope that clears you mind there!

Thanks Parchie.

But not really.
I can see a completely flat cell - 0.0 volts across it. Jumper it, no problem. System still functions.

You have a new cell. You can not jumper it!

I'm guessing you set some selector switch to "CHARGER RUNNING SYSTEM" taking the battery stack out of service. You quickly pull the jumper cable, pull the bad cell, stuff in the new cell reconnecting it and then switch the selector back to battery bank AND charge.

Is that how it's done?


As for DTR's smoking habit. If the relays are motor run you could toast the motors by under-volting them. DC solenoids don't usually smoke on not operating, only AC ones. I guess you could have crazy aggressive DC solenoids that are high current short duty cycle, 1~2 seconds, that disconnect themselves at the end of the stroke. If they can't reach the end of the stroke they smoke.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Hi Keith - high speed tripping relays draw a fair bit of current then have a cut-throat contact to either kill the coil or insert an economy resistor in the coil circuit. If they don't travel fully and open the cut-throat contact the coil will be toast in a few seconds.

 

[DTR2011]

The relays went up in smoke because the batteries were removed from the DC system, leaving the system running only on the charger. The charger didn’t have the battery eliminator filter installed as an option.

Removal of the batteries, via DC breaker, caused the DC voltage to have a very high level of ripple and an over voltage.

The DC under voltage relay did not have a cut out or disconnect switch installed. The DC under voltage relay is/was wired to trip all 4 138kV breakers, hence the unsuccessful attempt to work around the relay for a battery change out. If memory serves correctly, the DC UV was set to 95V for a 125V nom system.

 

[itsmoked]

Thanks Scotty! Confirmation of a guess.

Thanks for the detail there DTR.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ppedUK]

@ Rajendrarohra1978........you don't state what type of plant you are referring to, but circuit breaker DC No-Volt trip coils are often used on embedded generation (usually up to a few tens of MW), and as a matter of fact, some genset manufacturer's will not warranty the set unless these are fitted. I have had several instances of this in the last few years.

 

[rajendrarohra1978]

The Plant I am referring in Air Separation plant with Compressor Motors. The design is available in only one of three plants. This design where unavailability of DC power in Substation will cause total power failure is also not helping in case we need to change the DC Charger, which is a future project. I was searching for any standards which are requiring DC power supply for MV Breakers interlock, otherwise I need to remove this interlock.

 

[ppedUK]

In motor circuits there is often an Emergency Stop button, or other safety interlocks (system over-pressure etc) which may wired in "fail safe" configuration, such that the opening of a normally energised (held closed) safety contact, or an open circuit in the wiring removes the DC control supply from the DC No Volt coil in order to trip the breaker. Some equipment control systems / control panels often cannot accommodate the standard "Energise to trip" opening release (shunt trip) coil on a circuit breaker. Could this be the case in your application?



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[waross]

Has anyone used a jumper that incorporates a large diode to change out cells on the fly?

Bill
--------------------
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Jimmy Carter