My POCO transformer failed

[MD Automation]

Good morning all. Just a quick story that I thought might interest some of you.

After 35 years of flawless service, the transformer on my POCO pole (BGE) got tired of working for me on Friday and decided to retire I am in a fairly rural location and we lose power every now and then, so I did not think much of it when the “lights” went out. It certainly was odd though - ~9AM, 70F, not a lick of wind, absolutely beautiful Spring day – no heat or AC running. I don’t normally jump to phone the outage in because my neighbors usually beat me to it. After maybe 15 minutes I did pick up the phone and dialed BGE and was surprised to hear the robot voice say that there was no outage reported yet in my area. As it is asking me if I still want to “Press 1” to report the outage, I look out the front window to the pole and notice this…

Never have seen that cutout open in 30+ years. I am the only house on this transformer, that’s a 7.6 kV primary in case anyone was curious. No squirrels run around on these wires, and I did not see any charbroiled bird on the ground. I assume that even if I did not call, BGE would have eventually figured this out when the meter did not answer the next ping.

Long story short, they got a bucket truck out and that first lineman looked the transformer over quickly for obvious dead shorts. Finding none, he put a new fuse in, put on his earmuffs, and closed it. The fuse seemed to hold but the transformer did not have any secondary output. And he was suspicious after seeing the center tap bushing on the secondary leaking oil.

So he radioed in for a replacement 25 kVA and a second crew got it installed around 3 PM.

They also replaced the old cutout while working up there. The new cutout frame/insulator seemed to be plastic or polymer instead of the old porcelain? And the new fuse holder looks plastic whereas the old one looked like fiberglass? That second crew told me there is some type of fusible link that likely opened up inside the can as it failed, and as it went it over-pressurizes it and forces oil out.

Fired up the new 25 kVA and everything was back to normal. There seems to have been no ill effect on any appliances from whatever fault eventually opened up inside that can. Maybe lucky for me it went out with a whimper rather than a bang?

Anyway… just a simple story. I am an engineer, so this stuff always fascinates me. I thought it was odd after so many years of answering the call of AC/Backup Heat/Stove/Water Heaters coming in and out, that the poor little transformer decides to kick the bucket with almost no load at all.

 

[augie47]

septuagenarians don't care for these stories

 

[mbrooke]

Thats how they size those things. Over-load them so they fail about every 30 years. Lightning and water ingress can also help them fail.

Yes the new insulators tends to be polymer.

 

[hillbilly1]

Thought I might have a transformer or protector failing, LED lights would blink off for about 1/2 a second. Three different circuits. Sometimes four or five times a minute, then go for hours. It started again last night, put my meter on record, but was only showing a 4 volt drop at the switch. Threw full load on the service, A/C, lights, pump, stove, etc. No blinking. Replacing the old res grade switches today to see if that makes a difference. Strange that it’s happening on different circuits, but they are the original switches from 92 when I built the house. Panel is a mlo Square D QOB, no fop on any of the breakers or the main.

 

[synchro]

Thought I might have a transformer or protector failing, LED lights would blink off for about 1/2 a second. Three different circuits. Sometimes four or five times a minute, then go for hours. It started again last night, put my meter on record, but was only showing a 4 volt drop at the switch. Threw full load on the service, A/C, lights, pump, stove, etc. No blinking. Replacing the old res grade switches today to see if that makes a difference. Strange that it’s happening on different circuits, but they are the original switches from 92 when I built the house. Panel is a mlo Square D QOB, no fop on any of the breakers or the main.

Can you tell if the LED lights on different circuits are blinking off at the same time, or are they separated enough so that you can't observe their light output at the same time?

 

[hillbilly1]

Can you tell if the LED lights on different circuits are blinking off at the same time, or are they separated enough so that you can't observe their light output at the same time?

Three different rooms, three different circuits. Master bath, living room and kitchen. Happens so fast, it’s hard to tell. Was sitting in the living room, and tried to watch if the kitchen did it at the same time, didn’t look like it. If it goes away after I change the switches, then I will know that was it. Switches made in the 90’s were when I think the Chinese imports started. Just replaced the LED’s in the kitchen with a different K, and the the fixtures in bathroom and living room are a couple of months old. It’s a pretty quick blip, because my Fluke 89 is not catching it.

 

[MD Automation]

septuagenarians don't care for these stories

Augie… 1000 pardons if I added stress to your weekend with a dumb story

I am only a handful of years from joining your septuagenarian party and I meant to imply nothing about any possible connection between us and that stupid can.

If it makes you feel any better, the second crew assured me that the old pot would get lovingly rewound, rebuilt and repainted and they all get shipped to a tropical beach retirement. They only need to work a few hours a week serving some small lighting loads (LED of course). I am sure it will enjoy a splendid retirement.

Thats how they size those things. Over-load them so they fail about every 30 years. Lightning and water ingress can also help them fail.

Yes the new insulators tends to be polymer.

MBrooke… I have read that over the years but it’s the first example I’ve seen at the end of my driveway. As an engineer, it’s counterintuitive for sure.

What’s the biggest load to lift?

200 lbs.

OK, get me a 100 lb. rope.

Sure can, but what happens when it breaks?

Silly, you just get another 100 lb. rope!

I also thought something like that would fail in a dramatic thunder and lightning storm or some sweltering 107F August afternoon. Or when I switched on my Electric Arc Furnace to make some steel! But I did run the bathroom fan with my shower, so it might have been my (Catholic guilt) fault after all.

 

[augie47]

If it makes you feel any better, the second crew assured me that the old pot would get lovingly rewound, rebuilt and repainted and they all get shipped to a tropical beach retirement. They only need to work a few hours a week serving some small lighting loads (LED of course). I am sure it will enjoy a splendid retirement.

 

[ptonsparky]

The 30 year old transformer failed at the Farm some years ago. 1:30 AM. The utility crew had an audience of 100 or so that cheered them when the lights came back on. We offered them a beer and barbecue. The beer was declined.

 

[mikeames]

Thats how they size those things. Over-load them so they fail about every 30 years. Lightning and water ingress can also help them fail.

Yes the new insulators tends to be polymer.

25kva for one house, I don't think thats overloaded at all. I owned a 3500 sqft house with about 90 recessed lights 5 tons of AC double oven etc.. I turned on as much as I could and was pulling 40 amps on the service. Most would be hard pressed to draw 25kva on an average house and if they do it will be for a short time. Beside why idle a bigger transformer definitely when your could "overload" a smaller one for 30 years?

 

[mbrooke]

25kva for one house, I don't think thats overloaded at all. I owned a 3500 sqft house with about 90 recessed lights 5 tons of AC double oven etc.. I turned on as much as I could and was pulling 40 amps on the service. Most would be hard pressed to draw 25kva on an average house and if they do it will be for a short time. Beside why idle a bigger transformer definitely when your could "overload" a smaller one for 30 years?

Depends on the home though. For a gas house a 5kva would do. If the home is all electric with a tankless I've seen them hang a 50kva.

 

[mbrooke]

Augie… 1000 pardons if I added stress to your weekend with a dumb story

I am only a handful of years from joining your septuagenarian party and I meant to imply nothing about any possible connection between us and that stupid can.

If it makes you feel any better, the second crew assured me that the old pot would get lovingly rewound, rebuilt and repainted and they all get shipped to a tropical beach retirement. They only need to work a few hours a week serving some small lighting loads (LED of course). I am sure it will enjoy a splendid retirement.




MBrooke… I have read that over the years but it’s the first example I’ve seen at the end of my driveway. As an engineer, it’s counterintuitive for sure.

What’s the biggest load to lift?

200 lbs.

OK, get me a 100 lb. rope.

Sure can, but what happens when it breaks?

Silly, you just get another 100 lb. rope!

I also thought something like that would fail in a dramatic thunder and lightning storm or some sweltering 107F August afternoon. Or when I switched on my Electric Arc Furnace to make some steel! But I did run the bathroom fan with my shower, so it might have been my (Catholic guilt) fault after all.

Unlike a rope, they don't fail the second you overheat one. Just like a new Incandescent light bulb will not burn out the second its hit with 150 volts or even 240 volts.

 

[cpickett]

MBrooke… I have read that over the years but it’s the first example I’ve seen at the end of my driveway. As an engineer, it’s counterintuitive for sure.

It seems they are sized big enough to meet the average or at least the base demand, then are considered 'overloaded' during peak demand times. With the utility not getting paid for losses before the meter, it makes sense they don't want to install bigger transformers than necessary. I'd argue that this is still engineering, just engineering for a different goal than what we are used to.

 

[Hv&Lv]

Depends on the home though. For a gas house a 5kva would do. If the home is all electric with a tankless I've seen them hang a 50kva.

That’s where demand charges should be implemented...

 

[Frank DuVal]

I don’t normally jump to phone the outage in because my neighbors usually beat me to it.

Twice I was at home about 8 AM, and the power went out. Since most neighbors work in larger city nearby, they are gone by 6AM or so. I called to report the outage to the computer/recording. The I get a phone call back asking if I had checked my breakers as no one else had called in. "Yes I did, no power to house". Both cases were line damage that affected at least 20 houses! Lots of trees.... with dead limbs... mostly cut back now, so power is usually on.

 

[mbrooke]

It seems they are sized big enough to meet the average or at least the base demand, then are considered 'overloaded' during peak demand times. With the utility not getting paid for losses before the meter, it makes sense they don't want to install bigger transformers than necessary. I'd argue that this is still engineering, just engineering for a different goal than what we are used to.

Yes and true

 

[mbrooke]

That’s where demand charges should be implemented...

Should...Could... remember those 3 500kva on a pole?

 

[MD Automation]

I'd argue that this is still engineering, just engineering for a different goal than what we are used to.

CPickett (and MBrooke) – well said. I understand completely and, as you indicated, it is just not the engineering I am used to. They have clearly figured it out so that it works well enough for everybody.

A last picture that I took as they were finishing up…

That’s the nice new cutout assembly, now mostly plastic. The first lineman that came out for initial troubleshooting said the old fuse holders were fiberglass and he hated handling ones that had been up for years, even with gloves. And the second crew indicated the old porcelain insulator frames had some minor cracking problems over time, I guess with water seeping into any tiny flaw in the glazing and then freezing?

I did notice the new fuse inserted was labeled “10D” on the business end under the top cap. I wonder if that’s 10 amps and the “D” is some speed/time rating?

btw – both these crews were hard working and were great to watch. Careful and deliberate while working with that live primary. I did offer water and soda to them (and the flaggers) but I had no beer and bbq – so ptonsparky wins that game!

 

[kwired]

If it is 10 amps and 7.2 kV, that lets it pull 72 kVA before it is even thinking about tripping.

Many single phase transformers have an internal protector in them and they usually don't install a fuse on them around here. But maybe they do anyway where you are and it is a higher fuse than the transformer and is only there to cut out when the thing fails yet not take the whole line down at the same time.

 

[mbrooke]

I know of "DR" speed fuses... HV&LV would know what the "D" stands for. Often times there are metal letters near the bottom of the pole indicating the amps and speed of the fuses, ie 15T or 25K.

Typically pole transformers are fused at 2x their primary FLA, with fuse link time current curves generally starting at about 200% of the listed rating. Higher fusing, such as 3x or more is not unheard of. If the transformer is completely self protected (not sure if the new one is CSP- old doesn't appear to be CSP) they may fuse it as high as they want too since its already protected internally with a primary fuse and secondary "circuit breaker".