Danger of generator backfeeding utilities

[powerplay]

With so many power outages this time of year, I get asked and hear about backfeeding panels for homes.

a) I have installed a generator panel that disconnects all lines and neutral before reconnecting engaging generator power. I had heard that the neutral from the utility is the best source of ground... are there occassions where the neutral does not disconnect or that is a dangerous path to backfeed the utilites grounded conductor?

b) Someone had argued that when a farmer had used a generator that fed back into the utility and they received a cheque for the surplus power generated. In the case where people use solar, stream, or generator power to feed back to the utility, during a power outage would the power continue to backfeed the utility or does it get interrupted during a power outage? I hear of illegal double male ends backfeeding panels with the Main Breaker off. If someone left the Main breaker closed and backfeed while there is a power outage, would the 120/240 volt backfeed transform to 14.4KV through the transformers that the Line Crew be working on. How dangerous is this?...It sounds like alot of people know how to backfeed a panel during a power outage, but the neutral is still connected.

 

[ActionDave]

With so many power outages this time of year, I get asked and hear about backfeeding panels for homes.

a) ... are there occassions where the neutral does not disconnect or that is a dangerous path to backfeed the utilites grounded conductor?

Yes, it is common. There is no danger of backfeeding as long as the ungrounded/hots are isolated with a transfer switch or panel interlock.

b).... In the case where people use solar, stream, or generator power to feed back to the utility, during a power outage would the power continue to backfeed the utility or does it get interrupted during a power outage?

A grid tied solar system has a contactor that is held in by the utility power. If utility power goes out the contactor opens and isolates the solar system from the power grid. I have never seen a grid tie generator, but it would need a similar connection.

I hear of illegal double male ends backfeeding panels with the Main Breaker off. If someone left the Main breaker closed and backfeed while there is a power outage, would the 120/240 volt backfeed transform to 14.4KV through the transformers that the Line Crew be working on.

Yes.

How dangerous is this?...It sounds like alot of people know how to backfeed a panel during a power outage, but the neutral is still connected.

Very.

 

[Hv&Lv]

I know of several people that have a double male plug that backfeed the whole panel. If they forget to trip the main breaker and they backfeed the line, If the line is down and crossed up, it will generally trip the generator overload. If they are trying to supply all their neighbors with power by backfeeding a line, they better have a pretty big generator, or the overload will trip again. I have been to places where the generator WAS backfeeding a line. Grounding the line fixed that problem quick.

I had another line that was still up, the fuse had blown when a tree brushed the line and slapped the hot and the neutral together. When I refused the line, I watched to see what would happen to the generator that was backfeeding the line. I heard the generator shut down, and thought it was on a transfer switch, although I thought the transfer was way to fast. I went to the house, and was told that I had burnt the generator up by not informing the owners that I was going to turn the power back on. These people actually had the nerve to ask if we were going to pay for the replacement. They couldn't understand why I thought that question was funny...


In answer to the question, It is extremely dangerous, not only for lineman, but also for the kids and neighbors trying to be helpful that find a fallen line before we do.

 

[Jraef]

You are confusing the term "backfeeding" with being inherently dangerous. It is not. When you are generating, you are putting voltage INTO the grid. Nothing at all inherently dangerous about that. It's called "Distributed Generation"; everyone connected to the grid can pump energy into it and make it available for all to use.

However when the grid goes down, like in a storm, and you are STILL generating into the grid, now you have created a dangerous situation. Because when the grid goes down and the linemen need to work on it, they disconnect the SOURCE of the voltage potential from the nearest substation. But of you are still pumping potential INTO the grid when they think it is going to be dead, you can kill them. Or if not that and they detect live voltage potential on the line after they have opened the substation connection, how are they going to know where the voltage is coming from? They can find it, EVENTUALLY, but it takes time to trace it back and isolate it. In the mean time thousands of people are screaming at them to get their power back.

So utilities deal with this in two ways:
1) They do not allow anyone to generate into the grid. Safe, but not popular with the solar / wind crowd.
2) They require that anyone who wants to feed into the grid and get paid for the energy, to have a system that AUTOMATICALLY disconnects it from the grid if the grid goes down.

If you have a temporary backup generator, you are also required to have an isolating transfer switch of some sort, meaning one that allows you to feed your house from the generator but ONLY after it has isolated itself off of the grid. Failure to obey this rule is subject to hefty fines and is actually a crime in some areas because of the risk to linemen.

 

[topgone]

I guess not everyone can backfeed to the grid. There is a formal document required as well as the corresponding degree of protection that the utility in the area requires of DG owners in order not to endanger people as well as not to get your unit damaged should untoward events happen, like faults.

 

[T.M.Haja Sahib]

In India,only 4 pole transfer switches are permitted.But in US,I think both 3pole and 4 pole transfer switches are permitted depending on the application.I think it has to do with the system of earthing also.

 

[Besoeker]

I've done a little work with generators that have been connected in parallel with the grid.
The earliest I recall, about 1965, was a Paxman Diesel 1,500 kVA unit. It had a reverse power trip to prevent it trying to power the whole of Scotland and electrocute power company workers in the event of a power outage.

 

[T.M.Haja Sahib]

..........The earliest I recall, about 1965................

(Mods may excuse for OT)

You are one of the most respectable electrical engineering veterans this forum has..........

 

[Besoeker]

(Mods may excuse for OT)

You are one of the most respectable electrical engineering veterans this forum has..........

Kind words thank you.

Not sure what the Old Testament needs to be excused for though...........

 

[infinity]

Kind words thank you.

Not sure what the Old Testament needs to be excused for though...........

OT=off topic.

 

[Besoeker]

OT=off topic.

Yes, I know.
Sometimes my wit is wasted..........

Back to the topic.
Another generator system we did was about 10 years later.
It was a disused water turbine with a DC generator in a paper mill. It was a relic from the days when most of the machinery was DC. A lot of the drive motors still are but they now use individual variable speed DC drives that get their input power from the AC supply.
So, what we did was build a 1600A mains commutated inverter* to convert the DC to AC and feed it into the supply. This allowed the water turbine to be put back into service. And it still is.
It's a free resource and electricity generated repaid the capital cost of the equipment in about eight months. And it goes on giving to this day.


*It's a fairly big inverter even by today's standards, but nearly 40 years ago was in the early days of hockey puck type SCRs.