120/240 closed delta backfeed

[jackk]

I would like to hear the thoughts around the dangers of closed 120/240 systems. We know that the NEC does not allow a generator to back feed into a utility's system. What about the situation where a closed delta can feed back into a system if one of the primary phases is lost?

Here is a real situation. A transmission substation, owned by a transmission utility, receives it’s station service from the local utility. The service is a 12.47kV to 120/240V closed delta with one phase center tapped. This is done to achieve 120V. The three-phase 240V power is used mainly for large transformer pumps and fans.

These transformers can be 3 single phase pole top, or padmount transformers. The smallest ones would be (3) 50 KVA transformers. There is no three phase switching on the high side of these transformers, just typical fuses.

Closed delta systems can be seen in older parts of towns. However, in these cases it would seem that most of the switching is three phase, and if one phase was lost, there is so much connected load, that a fuse would quickly blow on the bank that would be back feeding attempting to pick up the load.

This transmission substation is in a rural location and is likely the only 3 phase load on the line. If one phase is lost, due to the local utility's single phase recloser, or a downed line, the transmission’s station service will feed back on the open phase. Isn’t this similar to a generator feeding back on the line? Is there something in the NEC that prohibits this arrangement?

 

[LarryFine]

Welcome to the forum.

No, it's not the same thing. It's not expected for a dead circuit to be energized.

It is expected that a live circuit with one line open would still be energized.

 

[tortuga]

I would like to hear the thoughts around the dangers of closed 120/240 systems. We know that the NEC does not allow a generator to back feed into a utility's system. What about the situation where a closed delta can feed back into a system if one of the primary phases is lost?

By backfeed you mean it will energize the open primary wire?
If a single primary is lost it (or opens) just becomes an open delta bank, and 3 phase motors will continue to work.
I think from the perspective of the plant maintenance guys this seen as an advantage rather than a liability.

 

[jaggedben]

For sure it's beyond the scope of the NEC.

 

[jackk]

Here is the scenario. A tree falls on one conductor and it breaks. The line towards the load is hanging down the pole or has landed on the ground with a low fault current. The line on the ground from the substation is dead as the local utility's single phase recoloser has opened. The 120/240 volt delta is now back feeding into the down phase on the 12.47 KV line. Thus, energizing a line that would otherwise be dead. Another aspect that could happen, is that it will also be powering all the homes on this circuit if it is able to. This just depends on the fuses that feed the 12.47/240 volt transformer and the load on the line that has been separated from the substation. This has happened (though I don't know how long the high side cut out fuse lasted), and I can imagine a situation where it could be dangerous, just like when someone back feeds a generator into a utility line when the power is out. I worked at one utility that did not allow delta secondary systems on its system due this issue. Utilities have warnings against illegally hooking up generators, and I'm wondering if the NEC addresses this situation which can occur anytime there is a closed low side delta.

 

[jaggedben]

The NEC doesn't address it because everything you've mentioned belongs to the utility. It's outside of the scope of the NEC. To a resi electrician like me the single-phase recloser sounds like a bad idea, but go look in the NESC, or whatever saftey code your state's utilities are bound by, to see if there's anything about that. The NEC is the wrong code to be asking about.

 

[jackk]

Thanks Jaggedven, The single phase recloser is often used for rural systems. Most of the load is single phase, and this can save 2/3 of the customers from experiencing the outage. This arrangement also occurs anytime a 3 phase tap off of the main line has cut out fuses for protection. The NESC is likely silent on this issue, as the issue is occurring at a low voltage, though I will check. I was hoping it was covered by the NEC, as the local utility could be feeding a load by primary metering and this same condition would apply. A small wood mill for example.

 

[LarryFine]

The 120/240 volt delta is now back feeding into the down phase on the 12.47 KV line. Thus, energizing a line that would otherwise be dead.

The POCO guys are all too familiar with this scenario, and know to treat every line as energized.

 

[tortuga]

Here is the scenario. A tree falls on one conductor and it breaks. The line towards the load is hanging down the pole or has landed on the ground with a low fault current. The line on the ground from the substation is dead as the local utility's single phase recoloser has opened. The 120/240 volt delta is now back feeding into the down phase on the 12.47 KV line.

Can an advanced recloser control like a SEL 651R be programed / networked to stay open and open the other two lines based on some inputs?

https://selinc.com/products/651R/

Is that problem unique to delta banks? Seems like if you had a 2 bushing single phase you'd get a similar issue?

 

[jackk]

I think it is an issue, even though every line is treated as energized. Why does the NEC prevent back feeding? Why do utility companies run adds not to plug in a generator? What if the line is back fed and someone from the public contacts it? If I Google plugging a generator into an outlet a typical result is: Backfeeding is extremely dangerous and could cause injury or death to yourself, a utility worker, or your neighbors.

This is not a problem for 120/208 or 277/480 as these voltages can only be achieved with transformers with a wye secondary. This is only an issue when 120/240 three phase is needed. To be clear, it is only an issue with full delta low side banks. My knowledge is that 120/240 systems were not industry standard after the 1960's. These delta systems can still be seen, but mostly in old urban installations. However, the utility industry can be slow to change and station service for a substation can still be 120/240 three phase. Sometimes these are fed with an open delta, which then solves the back feed issue. However, my concern, as I've stated above, is the use of closed delta systems in rural areas. I don't work for the local utility; I'm assisting the transmission company that has a standard of closed delta systems for station service. I'm looking to see if the transmission company is violating a code by back feeding onto the local utility's system.

 

[jaggedben]

Nobody is saying it's not an issue. But it's not the same as 'backfeeding' a separate source like a generator. It's the same power source energizing the circuit it normally energizes. Perhaps it's energized from a direction you don't expect, but I'd argue you will expect it and not make assumptions if you are a properly trained utility lineman. Regardless, the particular situation you're describing would not be addressed by the NEC. It would be addressed by utility safety codes or practices and lineman trainining.

That all said, I think the issue is broadly similar to why the NEC requires handle ties on a multi-wire branch circuit. And it speaks to why safety training and some level of electrical theory is important for electricians. As a general rule of thumb, all ungrounded conductors of a multi-wire circuit must be disconnected to safely work on that circuit.

...

This is not a problem for 120/208 or 277/480 as these voltages can only be achieved with transformers with a wye secondary. This is only an issue when 120/240 three phase is needed. To be clear, it is only an issue with full delta low side banks. ...

Broadly speaking I think that is incorrect. Under the right circumstances voltage could 'backfeed' through a wye winding or a wye connected load, too. See above comment about multi-wire circuits.

 

[tortuga]

The back-feed is unlikely to come up on the NEC side of things becasue most NEC regulated transformers are on a common trip breaker. Regardless the NEC is silent on the issue.

This is only an issue when 120/240 three phase is needed.

If I am understanding correctly you'd have the same issue with any 'delta' style primary distribution where two bushing single phase transformers are used?
I'd be surprised if advanced reclosers were not able to address the issue, as utilities are upgrading those things like crazy, due to wild fire dangers and the huge liability the have if they spark a fire.
In some areas here utilities are just converting entire networks of 12 - 13.5kv distribution feeders to underground and rebuilding / replacing substations with more advanced controls. Its quite impressive they worked along a highway I frequent all fall and winter converting about 30 miles of lines. Its really kinda funny to see a underground pvc go up the side of a house or pole only to feed a weatherhead then back down to the meter. but thats how they are doing it.

My knowledge is that 120/240 systems were not industry standard after the 1960's.

240 Delta is still 'standard' as in the current ANSI C84, and I think most if not all public utility commissions or grid regulators adopt it.

I could see 220Y/127 working for your application, if your lighting is all LED now 127 would not be a issue and 220 is more within spec for a 240V motor than 208.

 

[jackk]

Here is the transformer configuration I'm talking about. Let's just assume an ideal system and an ideal transformer. It doesn't matter which H bushing, so I'll just use bushing H1 on transformer D. Let's also assume that the high side is 12.47 kV or 7200 from phase to ground. The voltage at H1 to ground is 7200 Volts. These are all common assumptions for a common system. If I disconnect bushing H1 so that it is open, the voltage on H1, disconnected from the system, will remain at 7200 Volts. This is due to the 2 transformers (E and F) that are energized form the delta on the low side, which back-feeds transformer D. Now let's consider the load/source/open. If there is an open as drawn, this transformer will feed the load. If the load is large enough and the fuses are small enough, a fuse might blow, but it might not if the fuse is large enough to hold the load. This can and has happened.

 

[jackk]

In the case I'm applying this, the transmission company is receiving power from the local transmission utility. The connection is primary metered and the transformer is owned by the transmission company. Which puts the transmission company behind the utility meter, and hence under the NEC code. The transmission companies standards state that the NEC is to be followed. In addition, the NESC doesn't typically address low voltage connections.

 

[jackk]

This practice can cause a wire on the ground to be energized from the load side of a system and other conditions. Looking at the NEC code, would this fall under 705.40? Loss of a Primary Source. Only if one considers the back feeding transformer "production equipment". It seems that this condition, back feeding voltage into a power system, is the same electrically as back feeding with a generator or a solar panel, I just don't know what part of the NEC would apply.

 

[jim dungar]

The NEC does not address the issue of back feeding into a power company distribution grid.

 

[jaggedben]

This has absolutely nothing to do with article 705 or multiple power sources. All your stuff is still energized from once source.

The NEC addresses this everywhere it requires handle ties or simultaneous disconnection for ungrounded conductors of the same circuit. If I'm troubleshooting an appliance and I discover that a conductor has come unspliced in a junction box, I would be foolish to assume that no voltage is backfeeding through the appliance I haven't switched off all ungrounded conductors of the circuit. This is no different. It really doesn't matter that it's a transformer and not a load, and it's not a particularly special case.

If the 'transmission company' is really taking high voltage service under the NEC (funny how that seemed to change from the orginal post) then I believe they need a service disconnect on the supply side of the transformer to disconnect all ungrounded conductors simultaneously.

 

[Hv&Lv]

Here is the transformer configuration I'm talking about. Let's just assume an ideal system and an ideal transformer. It doesn't matter which H bushing, so I'll just use bushing H1 on transformer D. Let's also assume that the high side is 12.47 kV or 7200 from phase to ground. The voltage at H1 to ground is 7200 Volts. These are all common assumptions for a common system. If I disconnect bushing H1 so that it is open, the voltage on H1, disconnected from the system, will remain at 7200 Volts. This is due to the 2 transformers (E and F) that are energized form the delta on the low side, which back-feeds transformer D. Now let's consider the load/source/open. If there is an open as drawn, this transformer will feed the load. If the load is large enough and the fuses are small enough, a fuse might blow, but it might not if the fuse is large enough to hold the load. This can and has happened.

Might have ferroresonance that could feed back, but won’t feed a load.
Ferroresonance can put probably about 26,000V on the open wire.

But it won’t feed a load.

You say a transformer is backfeeding and that section of line is feeding loads.
There is a fuse above the bank that won’t allow but so much current through, plus you have all that current going through the coil of the transformers.
I’ve had that happen to me before. The loads had as one member told me “just a little bit of power.

 

[Hv&Lv]

By backfeed you mean it will energize the open primary wire?
If a single primary is lost it (or opens) just becomes an open delta bank, and 3 phase motors will continue to work.
I think from the perspective of the plant maintenance guys this seen as an advantage rather than a liability.

Beg to differ..
Deltas with a floating ground won’t magically become an open delta when a fuse blows.
The high side bushings have to be grounded for an open delta

 

[tortuga]

Beg to differ..
Deltas with a floating ground won’t magically become an open delta when a fuse blows.
The high side bushings have to be grounded for an open delta

I might be a bit foggy care to give a refresher?
We're talking about the diagram in post#13 right?
I do remember a plant that had seeming lost a primary phase and the 3 phase motors were still running.
It was old 240 D.