Primary cut-outs

[ryan_618]

I did an inspection on Saturday and had a chance to talk with one of the POCO guys. He was telling me that with some of the newer transformers he's seen that it nearly takes an act of God to open the primary cut-outs to clear a line to case fault.

Do any of you high voltage guys know anything about what he was saying?

 

[charlie]

Re: Primary cut-outs

Assume we are serving a 200 ampere service with a pole mounted transformer. We would use a 25 kVA transformer with a 15T fuse. This fuse will carry 150% continuously and is a slow blowing fuse. Our system is 13.2 kV so the Primary is 7.62 kV for the transformer. 7620 volts/22.5 amperes = 338.3 kVA or 1300% load on the transformer to blow the fuse. Remember, we are protecting the primary line from downstream trouble, we are not protecting the customer's service equipment.

 

[c-h]

Re: Primary cut-outs

Charlie,

you made me curious: Are you saying that you would use a 15A fuse that in reality carries 22.5A indefinitely? And that this translates to no protection whatsoever for the transformer? As smaller MV fuses exist: What is the reason for using one which is many times larger than the protected load? I'm sure there is a good reason, I just can't figure it out.

It seems that a smaller fuse would blow faster, thereby reducing the voltage sag experienced by the rest of the grid in case of a short circuit.

Is there any protection on the secondary side?

 

[don_resqcapt19]

Re: Primary cut-outs

Ryan,
At a fire I watched a 4" conduit with 500kcmil burn and melt down for over 20 minutes. About 40' of conduit and wire were vaporized. Quite a fireworks show!! This was the service for a commercial building and fed from a 750 KVA transformer. The primary fuses did not open and the wiring at the underground utility valut was starting to smoke when the power company got there. He took one look and said "I'm not going down there to pull the fuses". He drove to a substation and dumped half of the downtown area to kill the primary. After he cleared the secondary to the fire building he put the transformer back in service to restore power to other buildings in the area.
Don

 

[ryan_618]

Re: Primary cut-outs

With this in mind, how does a line to ccase fault on the supply side get cleared? Also, why do we bond on the line side of service, and why does the NEC require that that bonding jumper be larger than any other bonding conductor in the whole building?

 

[hurk27]

Re: Primary cut-outs

I know this does not seems right. but it has been this way for as long as I can remember. It is one of main reason's for keeping the SEC's as close to the point of entrance as possible. We had this discussion a while back when there was a post on California. allowing the SEU to run inside of the wall to the 3-R panel on the outside of a building. I was in shock that they allowed this in California. But as we know many transformers might have 3 or 4 houses on it and all the POCO tries to do is to protect there conductors feeding the transformers not the secondary from the transformer. I had a house that the main service shorted out and as the service entrance wire arced across the basement concrete floor, the floor was exploding from the heat of the arc. Luckily it stopped at the meter lugs and the home owner had stucco outside walls and it didn't go up in flames but it would have if it was wood. It seems to me they should have a simple in-line fuse at the transformer for every drop to a house.

 

[charlie]

Re: Primary cut-outs

If you have a bolted fault or a fault inside the transformer, the cutout would open and protect the primary line (that is what we are protecting). The smaller the fuse, the more damage they will take with lightning strokes and will be more likely to fail. Therefore we use a 15T for all of our transformers from a 50 kVA on down on our 13.2 kV system. Remember, our goal is to keep the maximum number of customers in service and for our faults to burn open, voltage sag is still service. We will repair outage when we are notified that power has been interrupted.

Maybe if the NEC required a cable limiter at the service point, that would take care of everyone's concerns. If that were done, why not permit unprotected SE conductors inside a building since there would be some protection? This would also put some extra change in your jeans.

 

[paul32]

Re: Primary cut-outs

A 25kVA transformer at 240V is 104A. A 200A service at 80% rating is 160A continuous. Shouldn't the transformer be able to supply the service? Why would that be a 200A service instead of 150A with that transformer?

 

[iwire]

Re: Primary cut-outs

Paul Because the power company knows from historical data that the 200 amp service calculated by the NEC articles will not draw 160 amps.

If they provided everyone with a transformer to match the service size, a lot of electricity would be wasted just from running over sized transformers.

How did I do Charlie?

 

[don_resqcapt19]

Re: Primary cut-outs

I have a 200 amp service. There are 5 other homes connected to the same 25 KVA transforemr as my service is connected to. One of these other homes has a 200 amp service, one a 60 amp and the other 3 have 100 amp services.
Don

 

[charlie]

Re: Primary cut-outs

You are on it Bob.

 

[Ed MacLaren]

Re: Primary cut-outs

An insurance investigator asked me for my opinion about a house fire that occured recently.

It was discovered that the service grounding electrode conductor had become disconnected (or burned off) at the first ground rod, just below and a bit to one side of the meter location. It wasn?t fastened and was dangling in the wind.
At every point where it had touched the concrete foundation a spall of concrete was blown off, and at every point where it had touched the wood steps there was a burnt area, and at one point had set the steps afire.

The earth was dried out (baked, he called it) surrounding the ground rod. A fire started at the oil-fired water heater, which was connected to the outdoor (grounded) oil tank by a 3/8 inch copper tubing oil line.

The investigator believes that a winding-to-winding fault developed in the PoCo transformer. Because the secondary neutral is normally tied to the grounded primary neutral, that should blow the high-voltage fuse in the primary cutout.

If the ground tie between the primary and secondary neutrals on the pole was open/broken/missing, the fault current would try to return to the primary neutral through the service grounding conductors, through the earth to the pole, and up the "butt-plate" ground wire to the primary neutral.

They believe that the service ground wire at the house burned off at the first ground rod. The fault current then tried to get to ground through the oil line to the grounded outdoor oil tank, burned off the 3/8 inch copper oil line and ignited the oil.

I didn't know enough about this kind of thing to advise him.

Does this sound plausible? Perhaps Charlie, the utility guy, would comment?

Ed

[ March 11, 2004, 09:29 PM: Message edited by: Ed MacLaren ]

 

[bennie]

Re: Primary cut-outs

Ed: The situation you have described is why the utility neutral should be disconnected when system is on generator power.

A transfer switch will see the high to low fault and transfer neutrals.

 

[charlie]

Re: Primary cut-outs

Sorry Ed, due to the legal implications, I can't comment. However, if you take a look at my previous posts . . .

Bennie, now you are on it.

 

[bennie]

Re: Primary cut-outs

Charlie: We seem to forget the primary reason for grounding and using the MGN system is; to prevent voltage feed through on a winding to winding fault.

The path is usually on the service neutral, after a few lamps are wiped out.

Should the fault burn the MGN connection, the service neutral will be whatever the voltage is of the primary.

Please correct me if I have the wrong slant on this.

[ March 12, 2004, 09:17 AM: Message edited by: bennie ]

 

[ryan_618]

Re: Primary cut-outs

Sorry guys, but could you define "MGN" please?

 

[charlie]

Re: Primary cut-outs

You are correct Bennie. That is why 250.4(A)(1) states in part, "or unintentional contact with higher-voltage lines". The MGN (Multi-Grounded Neutral) works very well to clear our primary phase to ground faults even when they are through a transformer.

As much as I want everyone to be clear about the way our fuses work, you need to also understand that a 25 kVA transformer will put out 4000 to 7000 amperes of fault current at the secondary bushings. That will take out the primary fuse. The problem occurs when the fault is too far from the transformer and it sees it as additional load.

The system actually works well most of the time under fault conditions. Once in a great while, things do not work well and then things get dangerous.

Ryan, the MGN is our neutral conductor and is required to be grounded four times per mile by the NESC, not counting any customer grounds. We install lightning arrestors every 1200 feet to accomplish this. Also, every switch, transformer, capacitor bank, riser pole, and primary dead end gets a lightning arrester installation (usually on all phases). The MGN is grounded to the pole ground at every lightning arrester location.

 

[bennie]

Re: Primary cut-outs

The most common distribution system is the delta wye setup. The star point of the wye is common through out the system, from the generator to the last device in a home.

This common conductor is grounded to earth at numerous locations. (MGN) Multi ground neutral, is the technical name for the conductor.

The MGN electrically connects all delta/wye primaries and secondaries. This is my reason for all my yelling about the definition of a separately derived system.

There is many wireman, who do not venture past the service switch, to see the entire picture.

 

[don_resqcapt19]

Re: Primary cut-outs

Bennie,

This is my reason for all my yelling about the definition of a separately derived system.

It is still a seperate system even with a MGN. The primary system current can only return to the primary system source and the secondary system current can on return to the secondary system source. Grounded conductor current from the secondary cannot return to the primary grounded conductor. The system current is exclusive to the system source.
Don

 

[charlie]

Re: Primary cut-outs

Bennie, you are correct but I don't think everyone understands what you said.

Our transformers are all delta primary / wye secondary. We use the wye secondary to feed the transformers in the downstream substations in a delta configuration.

This doesn't always hold up for all electric utilities and all installations but it does hold up for most substations.