Substation Loads

[Scottq68]

Hello,

My question concerns substations feeding main switch gear which in turn powers various parts of our facility.

Outside our plant adjacent to the incoming power & associated transformers we have four (4) large disconnects which then feed four different switchgear in our power house. The switch gear distribute the incoming power to busways and powerpanels throughout the plant. Two of the switchgear are crosstied so that one switchgear could handle the entire load.

Almost two weeks ago, we began to have problems with one of the disconnects outside tripping due to a single phase detector malfunction even though there was no single phase condition present. The current load on this switchgear was on the order of 1800 amps. We operate 24/7, so the decision was made to utilize the crosstie and combine the load of the two switchgear onto the other which had a load of around 2200 amps. Both of these switchgear are nameplate rated at 4000 amps and the ammeter upper limit is 4000 amps. Since we switched, we have noticed a continuous load of 4000 amps which pegs at an approximate value of 4200 amps regularly throughout the day.

The Engineering Director has informed those subordinate to his position that in his experience, we could run these switchgear at 110% capacity and he had done so at other facilities for years.

I personally do not have much experience with this larger switchgear, but due to the smell/discoloration in our powerhouse since the crosstie switch, I thought I would submit this question for your thoughtful response. We have also noticed a voltage drop from 480v to around 455v in the powerhouse with lower voltage readings taken at various points across the plant.

Thanks for your time.

 

[kingpb]

I assume that when the tie breaker is closed, one of the main breakers is open. If not you will have circulating current that will cause some additional heating, and possibly require you to derate the bus.

If the bus is rated for 4000A, then the main breaker must be set at a maximum of 4000A. If you are operating above 4000A, then something is wrong with your over current protection scheme. You could only run at 110% of the bus rating if the main breaker was oversized for the bus.

The drop in voltage is possible indication that the 480V transformer that is now carrying all the load, may be slightly undersized. As the voltage goes down, the load is going to draw more current. I would check the nameplate on the transformer and see what the top end rating is. It may be necessary to adjust the taps on the transformer. If on the nominal tap, then you can go the full 5% and get back to 480V.

Smell + Discoloration = Disaster at the most inopportune time not to mention possible severe injury or death

 

[zog]

Everything kingpb stated is right on the money. Additionally you need to find out what is causing the "single phase trip". If you can provide additional info about this equipment, (I assume it is a MVVCB or MVACB not a disconnect) maybe we can get you going in the right direction.

Kingpb was also correct about the lower voltages, keep in mind that your loads are going to draw the same power no matter what voltage, so at a lower voltage they are going to draw more current, which may lead to many other problems such as overheating and coordination issues. Also, I know this is a given, but if you decide to change the tap, make sure the transformer is denergized!!!!

Tell your engineering director that doing something in the past does not make it right. Slavery comes to mind of a good example.

 

[charlie b]

I like to use the following analogy, whenever someone tells me, “It’s never been a problem before.” Feel free to pass it along to your Engineering Manager.

• Suppose that just before you back your car out of the driveway each morning, you put on blinders and earmuffs.

• Suppose that you wait for a random amount of time, and then just back into the street.

• When you get into the street, you can take off the blinders and the earmuffs, and drive to work.

• Question: If you do this ten days in a row, and if you don’t hit anything during those ten days, would you conclude that this is a safe driving habit? Or would it take 20 consecutive days without incident, to convince you it was safe? 30? How many?

Everyone is welcome to steal this analogy shamelessly. But give me authorship credit, if you wish to use the following aphorism:

"An accident waiting for a place to happen will, given time, find that place.” Charlie Beck

 

[GH_Vegas]

Outside our plant adjacent to the incoming power & associated transformers we have four (4) large disconnects which then feed four different switchgear in our power house.

Scottq68,
Perhaps I am not clear but you neglected to mention if the Power Company/Utility (POCO) makes connection to these 4 large disconnects. I will assume so, and with this the disconnects may be attached to different 'circuits' from the utility. By closing your cross-tie, the switchgear may be seeing your plant load and the additional utility circuit load (all the downstream customers connected to that circuit).

I have seen this in the past, where a water pumping plant had the bus-tie closed, connecting 2 separate utility circuits. Voltages were 'off', currents high, phase angles just plain weird. This had been going on for years until the utility (actually the meter dept while doing a routine inspection) finally noticed and made the customer open the cross-tie. Everything got 'normal after that. It turns out that the customer had disabled the interlock that prevented the bus-tie operation if the main circuit feed disconnects were closed. No one at the water plant remembers why they did it.

I am not saying any interlocks were disabled on your part, but you may have inadvertantly tied two separate utility circuits together. Some utilities are better at tracking this than others.

More simply, can the faulty equipment giving the single phase indication be replaced? All was well before the cross-tie, so I wouldn't recommend adjusting taps, etc...There is something more going on here. And I suspect there is more than 4200 amps actually crossing the bus-tie. Depends on where your ammeters are located.

Respectfully submitted,
GH_Vegas

 

[augie47]

tie switch

tie switch

am I not correct that if his arrangement has feeds from two different transformers and the tie switch is closed in addition to the "mains" that if the transformers are of different impedance, one will draw from the other thus causing an increased looad on one transformer ?

 

[dbuckley]

Normally the buss tie breaker is mechanically interlocked to the buss supply breakers to prevent all three breakers being closed simultaneously, which of course means you cant do a no-break changeover, so it is not unheard of that the interlocking is "defeated" (often requiring no more than the aqusition of a key) to enable no-break changeover.

The question is at this very moment are all three breakers closed, or just two, the tie and one supply...? If all three are closed then all manner of odd behavior could be expected, some quite nasty, possibly the worst being if the utility is supplying through two different routes with a few degrees of phase shift betwixt the supplies...

 

[Scottq68]

Substation loads continued...

Substation loads continued...

First, I would like to thank you guys for the quick responses and then offer some further information as well as see what you guys think about the direction the Engineering Director has decided to pursue.

The disconnects adjacent to the incoming power transformer are S&C switchgear all fed from the same incoming power. The switchboards in the powerhouse are Western-Electric rated at 4000 amps each. Only two are crosstied and the defective unit was locked out both at the outside disonnect and the main throw on the Western-Electric switchboard.

The S&C switchgear are all protected by 'Open-Phase Detection' circuits, one of which was defective and false tripping which led to the cross-tie I mentioned previously.

Yesterday, during the heat of the day, the main switchgear finally tripped on the unit with the 4000 amp load. To explain the fluctuating load, we use a lot of injection molding equipment with hydraulic presses that cycle roughly every few seconds (on the order of 60 presses) that also utilize mold heaters that cycle off & on in barrel heaters on the injection units. We knew this caused the fluctuation we were seeing but I hadn't mentioned in the previous post.

When the switchgear tripped out, the defective open phase detection was removed and bypassed on the S&C switchgear outside and the crosstie eliminated and everything put back as it was (with the exception of the open phase detection system) which brings me to my final question.

This weekend some down time has been scheduled to remove the remaining open phase detection systems on the remaining S&C switchgear.

Could anyone shed any light on the wisdom of this decision? I recall horror stories of open phase conditions ruining equipment/motors etc...

 

[zog]

Removing protection is never a good idea, you seem to have a better sense of what should be done than your engineering director.

 

[kingpb]

Removing protection is never a good idea, you seem to have a better sense of what should be done than your engineering director.

I would make a call to the plant safety director, and have a little off the record discussion. They are not usually the most technical lads, but can swing a mighty big sword when it comes to inappropriate safety issues.

Based on what has been said, understanding that there is always two sides to every story, this all sounds risky at best.

 

[jim dungar]

I recall horror stories of open phase conditions ruining equipment/motors etc...

Some options are:
Put your phase loss protection on the 480v side of your substations, in particular at the critical loads. These will be less expensive and easier to service than the ones in the MV gear.

Have your phase loss devices alarm and annuciate rather than trip. This way you will still have the protection without experiencing uncontrolled shut downs.