US Electrician qualifications for a Brit



I guess there aren't any utility engineers reading this thread. I was holding off hoping one of them would comment.
L-L or delta transformers are _REALLY_ discouraged for new installations. Most new installations are Y-Y for three phase or L-N for single phase. L-L can cause all sorts of problems when a phase is lost. First off there is ferroresonance. That's when the magnetizing inductance of the transformer resonates with the line capacitance of the open phase. It's a real problem for underground service because of the higher capacitance of underground lines. It can still be a problem for open overhead, particularly at higher voltages, particularly 34.5kV. I have heard of lots of problems when utilities upgrade to 34.5kV primary and still have delta connected services they can't change, for example a customer with 240V center tapped delta. The other problem is with back feed to a faulted phase. Yes, motors can back feed and add to fault current. But usually the motors can't blow the line fuses on the transformer. But a delta connected bank will. It's not uncommon for every delta bank to blow one or two fuses when one of the primary phases has a hard fault (like crossed over conductors) and the source is single phase tripped. After the line is repaired and the feed restored, the line crew must then go around and refuse a bunch of transformer banks. Three phase tripping eliminates this, but many utilities like the single phase fault protection out on long lines.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
|
| |> |> Single phase is cheaper in rural areas. But maybe a usable compromise |> |> is to rule out ground return systems, which means you have to have at |> |> least 2 wires, and use 2 line wires out of the 3 for single phase. |> |> Then you can at least get the same ratio between L-N and L-L as you |> |> get with real three phase. But then, three phase is just one more |> |> wire, and allows the 3 wires to be a bit thinner to serve the same |> |> area. |> |> |> | |> | Y connected transformers are preferred on distribution systems. No |> problems |> | with ferroresonance or backfeed. |> |> I would definitely have three phase transformer secondaries configured |> as Y or star. The primary would be delta. The loads would be L-L or |> L-L-L. |> | | I guess there aren't any utility engineers reading this thread. I was | holding off hoping one of them would comment. | | L-L or delta transformers are _REALLY_ discouraged for new installations. | Most new installations are Y-Y for three phase or L-N for single phase. L-L
I still see L-L, which I assume to be D-Y, all over the place. And in dry transformers, it seems to be the only way it is done.
| can cause all sorts of problems when a phase is lost. First off there is | ferroresonance. That's when the magnetizing inductance of the transformer | resonates with the line capacitance of the open phase. It's a real problem | for underground service because of the higher capacitance of underground | lines. It can still be a problem for open overhead, particularly at higher | voltages, particularly 34.5kV. I have heard of lots of problems when | utilities upgrade to 34.5kV primary and still have delta connected services | they can't change, for example a customer with 240V center tapped delta. | The other problem is with back feed to a faulted phase. Yes, motors can | back feed and add to fault current. But usually the motors can't blow the | line fuses on the transformer. But a delta connected bank will. It's not | uncommon for every delta bank to blow one or two fuses when one of the | primary phases has a hard fault (like crossed over conductors) and the | source is single phase tripped. After the line is repaired and the feed | restored, the line crew must then go around and refuse a bunch of | transformer banks. Three phase tripping eliminates this, but many utilities | like the single phase fault protection out on long lines.
How does a D-Y backfeed (not the motor backfeeds)?
Anyway, if you really really want all the harmonics issues you'll get with a Y-Y transformer configuration, then you can have your Y primary. The design of the L-L and L-L-L loads doesn't depend on the transformer primary that I am aware of.
What do you suggest as a means to prevent primary ground return through the secondary service drop ground?
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
--snipped a bunch--

It depends on where you are. Some utilities still have substations set up for delta connected primaries, hence they continue to add new services delta or L-L. But for a brand new substation, or for primary voltage upgrades, it's overwhelmingly set up for Y connected primaries.

Unless special ordered. But as you note, most dry types are are D-Y. Ferroresonance isn't a problem there since they are pretty much only used where the primary is feed from a three phase breaker.
--snipped more--

If a phase is opened on the primary, the opened phase wants to center between the other two phases. It can backfeed enough current to blow the primary fuses.

Zero sequence harmonics. The positive and negative phase sequence harmonics go through the D-Y connection also. But yep, more harmonics. The zero sequence harmonics go back to the substation, and usually there is a delta or zig-zag winding somewhere that will eat up them up.

Nothing is done. You trade that off with the voltage rise on a primary to secondary fault. In a delta system, there is no primary ground present, so it relies entirely on the secondary ground system. Since that often can be 10 + ohms, it's possible to get a LOT of voltage rise when that happens. One good thing is that the ground relays can be much more sensitive to trip quicker, but then there is a loss of selectivity.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
|> How does a D-Y backfeed (not the motor backfeeds)? | | If a phase is opened on the primary, the opened phase wants to center | between the other two phases. It can backfeed enough current to blow the | primary fuses.
What are the phase angles of this current with respect to the remaining phases?
|> What do you suggest as a means to prevent primary ground return through |> the |> secondary service drop ground? |> | | Nothing is done. You trade that off with the voltage rise on a primary to | secondary fault. In a delta system, there is no primary ground present, so | it relies entirely on the secondary ground system. Since that often can be | 10 + ohms, it's possible to get a LOT of voltage rise when that happens. | One good thing is that the ground relays can be much more sensitive to trip | quicker, but then there is a loss of selectivity.
But the delta system won't have a primary neutral to secondary neutral path because there is no primary neutral to have a path with. So all you have is the middle L-L voltage relative to ground (e.g. half of the L-L voltage) as capacitive coupling. The ground can eat that easily as the current is just small charging current. But when you have L-N, the primary N is connected to the secondary N since both are grounded there. Bad neutral connections don't result in a system failure, but rather, in ground currents that get picked up elsewhere at better connections. The end result is part of the primary return current is by way of the LV service drop neutral and the customer grounding electrodes.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Somewhere close to in phase, unless a motor is pulling it out.

It's a trade off of the all the time problematic neutral current with Y connected vs. the potential high voltage on the ground under failure with delta. The ground on a service may not be low enough in impedance to trip the primary protection, so it could just sit and cook.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
|
| wrote: |> |> |> How does a D-Y backfeed (not the motor backfeeds)? |> | |> | If a phase is opened on the primary, the opened phase wants to center |> | between the other two phases. It can backfeed enough current to blow |> the |> | primary fuses. |> |> What are the phase angles of this current with respect to the remaining |> phases? | | Somewhere close to in phase, unless a motor is pulling it out.
In phase to which of the 2 remaining phases?
If you have 3 lines running to a transformer primary that is a delta, where the secondary is wye, and one of those 3 lines loses power, what is the phase of the voltage being fed back into the dead line by that delta primary?
Suppose phase B goes out and phases A and C remain powered. The A-C winding in the delta would be powered at full voltage (minus any drops due to the extended loading effects). Windings A-B and B-C would be in series relative to phases A and C, resulting in the corresponding secondary windings having half voltage. Phase B coming it is tapped in the center of that A-B and B-C series. I would think that the voltage being applied to line B would be however far offset from the center point that the middle of A-C would be at, which would be 28.9% (1/(2*sqrt(3)) of the L-L voltage (half of the L-G voltage), and at a 90 degree angle from A-C.
Is this the backfeed you are referring to?
| It's a trade off of the all the time problematic neutral current with Y | connected vs. the potential high voltage on the ground under failure with | delta. The ground on a service may not be low enough in impedance to trip | the primary protection, so it could just sit and cook.
Are you referring to a fault directly to earth? I don't see how that would be any different between delta and wye connected loads where the source is a wye secondary and the ground is carried on the distribution but does not normally carry current. The choice here is how to load the circuit, not how to supply the circuit. The circuit is the same (source is a secondary in wye configuration). What differs is whether the load put on it to feed a LV customer is connected D-Y or Y-Y or for single phase, L-L or L-N.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload


Yep. It's enough to blow fuses on the primary when one phase is grounded in some cases, guaranteed to blow fuses in a phase crossover (phase lines looped over each other, quite a common occurrence in overhead).

As you note in other posts, the utilities don't run separate grounds. When they have L-L or L-L-L connections, no neutral / ground is run. If they ran the ground, voltage rise on the secondary would be less of a problem. But it's the absence of a connection to the much lower impedance ground present in a Y connected system where the utility neutral is run along pole to pole.
I have an anecdotal story about just this kind of failure. One of my in laws farm irrigation systems suffered a primary to case or secondary failure in one of the three transformers feeding the service. The primary voltage is 12.5/7.2kV, with most services connected Y. The irrigation service is connected Y to center tapped delta. The Y center point is not connected to anything else. The branch line to the irrigation service does not have a neutral run along with it - just the three line conductors. Two irrigation services are powered from the tank. The transformer pole has one ground rod, one of the irrigation services has one ground rod, and my in laws service has two ground rods. The second ground rod was added when we replaced the panel to upgrade to a larger pump. A owl tangled up in the phase conductors several poles away. We know it was a owl because his smoking carcass was on the ground right aftarwards. He tangled two of the phase conductors around each other. One fuse at the lateral dropped right away. After a couple of minutes, one of the transformer cans on the pole popped and one more fuse dropped. When the can blew, the whole irrigation system went hot. There was arcing, hissing and sizzling along the irrigation pipe throughout the farm. I'm sure anyone in the river near the suction line was would have been fried. This continued for about 15 minutes until the third fuse finally dropped out. The only damage to my inlaws equipment was to a float control that ran a small pump to keep a cattle trough full. There pump motor was definetly saved by proper ground.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
| As you note in other posts, the utilities don't run separate grounds. When | they have L-L or L-L-L connections, no neutral / ground is run. If they ran | the ground, voltage rise on the secondary would be less of a problem. But | it's the absence of a connection to the much lower impedance ground present | in a Y connected system where the utility neutral is run along pole to pole.
When no neutral / ground is run, then what all does the LV secondary get connected to? Just the winding center and the electrode at the pole?
| I have an anecdotal story about just this kind of failure. One of my in | laws farm irrigation systems suffered a primary to case or secondary failure | in one of the three transformers feeding the service. The primary voltage | is 12.5/7.2kV, with most services connected Y. The irrigation service is | connected Y to center tapped delta. The Y center point is not connected to | anything else. The branch line to the irrigation service does not have a | neutral run along with it - just the three line conductors. Two irrigation | services are powered from the tank. The transformer pole has one ground | rod, one of the irrigation services has one ground rod, and my in laws | service has two ground rods. The second ground rod was added when we | replaced the panel to upgrade to a larger pump. A owl tangled up in the | phase conductors several poles away. We know it was a owl because his | smoking carcass was on the ground right aftarwards. He tangled two of the | phase conductors around each other. One fuse at the lateral dropped right | away. After a couple of minutes, one of the transformer cans on the pole | popped and one more fuse dropped. When the can blew, the whole irrigation | system went hot. There was arcing, hissing and sizzling along the | irrigation pipe throughout the farm. I'm sure anyone in the river near the | suction line was would have been fried. This continued for about 15 minutes | until the third fuse finally dropped out. The only damage to my inlaws | equipment was to a float control that ran a small pump to keep a cattle | trough full. There pump motor was definetly saved by proper ground.
Would this have happened if the system was designed as I suggested, where:
1. The distribution supply secondary is WYE, with the center point solidly earthed.
2. A grounding-only wire, NOT used as a neutral runs along the poles of the distribution, originating at the WYE center point, and is also earthed at periodic intervals.
3. An optional neutral may be run, which may also be earthed only at poles where the grounding-only wire is NOT earthed.
4. Each LV customer tap primary is connected L-L for single phase or delta for three phase, if there is no neutral.
5. If there is a neutral, LV customer tap primary may be connected L-N for single phase and WYE for three phase, but may also be connected L-L or delta. The neutral is NOT earthed at this pole or within 10 meters of any ground wire earthing.
6. All three phase LV customer tap secondaries are always WYE with the center point connected to the grounding-only wire described in #2 and is also solidly earthed with one or more electrodes at that pole.
7. All single phase LV customer tap secondaries are always center tapped with that tap connected to the grounding-only wire described in #2 and is also solidly earthed with one or more electrodes at that pole.
8. No center tapped delta.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Yes.
It would need to have at least arresters so that it couldn't develop high voltage with respect to local ground.

For both 4&5, there still is the problem of ferroresonance & backfeed.

#6 & #7 You still have secondary neutral currents going through the ground. This will eliminate some of the problems, but not all.

What's the problem with delta secondaries if grounded?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
|> When no neutral / ground is run, then what all does the LV secondary |> get connected to? Just the winding center and the electrode at the |> pole? | | Yes.
Sounds like what I want ... in single phase.
|> Would this have happened if the system was designed as I suggested, where: |> |> 1. The distribution supply secondary is WYE, with the center point |> solidly earthed. |> |> 2. A grounding-only wire, NOT used as a neutral runs along the poles |> of the distribution, originating at the WYE center point, and is |> also earthed at periodic intervals. |> |> 3. An optional neutral may be run, which may also be earthed only at |> poles where the grounding-only wire is NOT earthed. | | It would need to have at least arresters so that it couldn't develop high | voltage with respect to local ground.
That's why it is earthed. Would it be better if it were not earthed?
|> 4. Each LV customer tap primary is connected L-L for single phase or |> delta for three phase, if there is no neutral. |> |> 5. If there is a neutral, LV customer tap primary may be connected L-N |> for single phase and WYE for three phase, but may also be connected |> L-L or delta. The neutral is NOT earthed at this pole or within |> 10 meters of any ground wire earthing. | | For both 4&5, there still is the problem of ferroresonance & backfeed.
These are common for dry transformers. Why wouldn't it be a problem with them, too?
Sounds like it will be necessary to always have a 5 wire distribution to be safe.
And how would single phase backfeed?
And what if, instead of one of the phases going out, the neutral does?
|> 6. All three phase LV customer tap secondaries are always WYE with the |> center point connected to the grounding-only wire described in #2 |> and is also solidly earthed with one or more electrodes at that pole. |> |> 7. All single phase LV customer tap secondaries are always center tapped |> with that tap connected to the grounding-only wire described in #2 |> and is also solidly earthed with one or more electrodes at that pole. | | #6 & #7 You still have secondary neutral currents going through the ground. | This will eliminate some of the problems, but not all.
But not very much at all feeding into the customer ground.
|> 8. No center tapped delta. |> | | What's the problem with delta secondaries if grounded?
Perhaps none if D-D. Y-D would have backfeed. In any case, it's not part of the design I gave because the intent of it is to have a smaller L-G voltage.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

If the neutral is grounded only back at the substation and lightning hits the line, it would develop high voltage with respect to local ground. Even induced voltage from a nearby strike could result in equipment failure. By placing an arrester neutral to ground, it would limit the neutral to ground voltage rise.

Usually they are fed with three phase protection. Phase loss between the source and transformer is rare. Single phase protection is prefered by many utilities for distribution, and loss of a single phase - either open or grounded is more common.

The impedance of backfeed on single phase would be small, just the load current. In single phase the major danger would be ferroresonance.

With the current multipoint grounded wye systems, it becomes SWER (Single Wire Earth Return). With no grounding except back at the substation, the neutral could be subject to ferroresonance.

In most cases, but not always. The voltages are lower but currents higher in secondary systems. Neutral drop can be significant if there is significant neutral current. In a properly designed customer's system there should be, but that's not always the case.

D-D and Y-D with no connection to center point are about the same. I absolutely agree on the standardization point - the delta services just add to the number of "standard" voltages.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
|
| wrote: |> |> 3. An optional neutral may be run, which may also be earthed only at |> |> poles where the grounding-only wire is NOT earthed. |> | |> | It would need to have at least arresters so that it couldn't develop |> high |> | voltage with respect to local ground. |> |> That's why it is earthed. Would it be better if it were not earthed? | | If the neutral is grounded only back at the substation and lightning hits | the line, it would develop high voltage with respect to local ground. Even | induced voltage from a nearby strike could result in equipment failure. By | placing an arrester neutral to ground, it would limit the neutral to ground | voltage rise.
If the neutral is not grounded, I can believe that. But consider two points:
1. In my design I do suggest grounding the neutral to earth at as many as every other pole. The other poles would be where the non-conducting groundING wire is earthed.
2. What happens to the line wires that are not earthed? They are not grounded anywhere. Would they not develop that high voltage?
|> |> 4. Each LV customer tap primary is connected L-L for single phase or |> |> delta for three phase, if there is no neutral. |> |> |> |> 5. If there is a neutral, LV customer tap primary may be connected L-N |> |> for single phase and WYE for three phase, but may also be connected |> |> L-L or delta. The neutral is NOT earthed at this pole or within |> |> 10 meters of any ground wire earthing. |> | |> | For both 4&5, there still is the problem of ferroresonance & backfeed. |> |> These are common for dry transformers. Why wouldn't it be a problem |> with them, too? | | Usually they are fed with three phase protection. Phase loss between the | source and transformer is rare. Single phase protection is prefered by many | utilities for distribution, and loss of a single phase - either open or | grounded is more common.
But that loss of a single phase does not propogate into the premise dry transformer?
|> |> 6. All three phase LV customer tap secondaries are always WYE with the |> |> center point connected to the grounding-only wire described in #2 |> |> and is also solidly earthed with one or more electrodes at that |> pole. |> |> |> |> 7. All single phase LV customer tap secondaries are always center |> tapped |> |> with that tap connected to the grounding-only wire described in #2 |> |> and is also solidly earthed with one or more electrodes at that |> pole. |> | |> | #6 & #7 You still have secondary neutral currents going through the |> ground. |> | This will eliminate some of the problems, but not all. |> |> But not very much at all feeding into the customer ground. | | In most cases, but not always. The voltages are lower but currents higher | in secondary systems. Neutral drop can be significant if there is | significant neutral current. In a properly designed customer's system there | should be, but that's not always the case.
What about this setup:
The single phase customer drop has 2 phase lines at 240 or 480 volts. It reaches the service meter placed on a pole just a short distance away from the customer building, then runs over to the building where it enters through a disconnect and immediately to a dry transformer inside. The secondary of the dry transformer derives 120/240 and its center tap is earthed by 2 paths going out either side of the point where the power comes in.
The service drop neutral comes to that pole with the meter, and is earthed there. But it is not extended across to the building.
What I've been told before is that the transformer in the building would be subject to lightning induced ground gradient voltages by a nearby strike. But I would think that to be small if the distance between the pole and building is not too great (for example 3 meters).
|> |> 8. No center tapped delta. |> |> |> | |> | What's the problem with delta secondaries if grounded? |> |> Perhaps none if D-D. Y-D would have backfeed. In any case, it's not |> part of the design I gave because the intent of it is to have a smaller |> L-G voltage. |> | | D-D and Y-D with no connection to center point are about the same. I | absolutely agree on the standardization point - the delta services just add | to the number of "standard" voltages.
It seems all the dispute with my design was how I described it could be connected at the MV distribution side. But I think my design still has merit (other than for the fact that there is no chance in hell the world would make any changes today). Light (as in incandescent illumination _and_ small appliances) loads would be connected to 24 volts wired L-N (maybe 24-0-24) and everything else would be connected to 288 volts wired L-L coming from single phase 144-0-144 or three phase 288Y/166. The 24 volt (24-0 or 24-0-24) system would be separately derived from a 288 volt branch circuit. There might be more than one such system if there is a lot more incadescent lighting. I think I would go with 72 Hz.
I do have a few different designs for the ultimate receptacle.
Now I just need to get my time machine working :-)
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Delta delta was used long ago- capacitance problems could mean that appreciable fault current could flow in the case of a phase to ground fault- without being detected. This can occur even where resonance or ferroresonance is not a factor. In addition, the problem with ungrounded overhead lines is that the line to ground potential is at the mercy of any overhead charged cloud- a cause of such phase to ground faults (this was discovered about 70 years agoso Delta at the HV level went out) . Hence the idea of a system which is Y grounded. Y-Y transformers have problems with third harmonics in the voltages while Y-Delta do not. Part of the problem can be eliminated by Y-Y delta for larger units such as autotransformers or grounding both Y s to the same point so that triplen harmonic currents flow through but voltages are normal. Where single phase supplies are considered there are advantages to a (grounded) Y secondary.
--

Don Kelly snipped-for-privacy@shawcross.ca
remove the X to answer
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
| Delta delta was used long ago- capacitance problems could mean that | appreciable fault current could flow in the case of a phase to ground | fault- without being detected. This can occur even where resonance or | ferroresonance is not a factor. In addition, the problem with ungrounded | overhead lines is that the line to ground potential is at the mercy of any | overhead charged cloud- a cause of such phase to ground faults (this was | discovered about 70 years agoso Delta at the HV level went out) . | Hence the idea of a system which is Y grounded. Y-Y transformers have | problems with third harmonics in the voltages while Y-Delta do not. Part of | the problem can be eliminated by Y-Y delta for larger units such as | autotransformers or grounding both Y s to the same point so that triplen | harmonic currents flow through but voltages are normal. Where single phase | supplies are considered there are advantages to a (grounded) Y secondary.
So what about every step along the way being a D-Y transformer, with the star point grounded AND carried along (and earthed at various intervals), but all loads connect only L-L-L (e.g. a D-Y for each LV service) or L-L?
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
----------------------------
remove the X to answer
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
| But, with the grounded neutral carried along, you also have the option of | loads from line to ground(L-N). This is the usual practice. In many rural | situations a line and the grounded neutral are carried down one road | supplying single phase transformers. while on other roads the other phases | and neutral are used in order to get a rough load balance. This is cheaper | than running either 3 phase or 2 of the 3 phases. If you check your service | in the back alley in a residential area in a town or city, the same thing is | done. For example, in my block, one phase (7200V to ground) and neutral are | brought across and run underground to distribution transformers to get | 240/120V single phase. The neutral is common to both the HV and LV sides. | Even if 3 phase is present on the local line in the alley (if overhead), the | typical procedure is as above, using L-N rather than L-L to supply local | distribution transformers. A transformer designed for line to grounded | neutral operation is cheaper than one designed for line to line operation at | the same voltage.
However, I still would not do it that way. If cost were the exclusive reason for all decisions, we could have a much lower cost electrical system than we have today. But it would also be much less safe.
The inverse of this is that we can have a safer system, but it will cost some more. And that is how I would design it: safer
Having the same metal wire used for a return of MV circuits _and_ being connected to the LV customer service drop and the premise electrodes and EGC does have dangers. Many of those can be avoided, and many technical problems I believe may be linked to this bad practice, could be avoided by having a separate grounding wire on the MV distribution that is not used for return current whatsoever, which can be used for grounding the secondary side of the MV->LV transformers (along with a grounding electrode).
| There may be areas where single phase loads are connected line to line but | I haven't lived in any of those. | | In the past many small town supplies were D-D with local single phase | transformers connected line to line and when the lines were getting near | capacity, the main substation transformer was either reconnected D-Y or more | likely replaced while the single phase transformers were then connected line | to neutral. This meant that a small town's could be changed with a minimal | disturbance and cost as no new distribution transformers or larger wires | were needed (insulation was more than adequate).
How were the MV->LV transformers grounded before ... and after? I bet they were grounded with a wire that was NOT carrying current before, and afterwards, they were grounded with a wire that was carrying current.
They had a safer ground wire before. They took a step backwards in safety for economic reasons.
So, tell me, how can I wire up an isolation at the end of the service drop which isolates my grounding wires from the distribution neutral?
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
remove the X to answer ----------------------------
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
| In the situation that I mentioned, a previous step down delta-delta | transformer was changed to a delta-wye transformer. Then single phase loads | referenced to ground could be take from each phase. Increased capacity. | Certainly there will be neutral current in the case of unbalanced loads. Is | this a major factor for safety? Not really, provided that the neutral is | properly grounded and of adequate size. Note that the heaviest currents in | the neutral (about 97-100%) were those due to the 120/240 Edison system | customer loads -these would not change but account for 95-100% of the total | neutral current. In fact, any primary neutral current will actually reduce | the current in the neutrals (admittedly not by much). In your home you are | dealing with 120V/240V loads which are rarely balanced so the neutral | carries current- does this bother you? Yes, equipment is tied to a seperate | ground for good reasons. Note that the current carrying neutrals of a MV LV | or HV-MV system are also very well grounded and the fact that they may carry | current in the case of unbalanced loads is recognised and accounted for.
Having the neutral carry current does NOT bother me when there is a separate grounding conductor. Even in cases where that is not quite true, such as poor connetions, a fraction of LV is not nearly the same level of issue as a fraction of MV. There is no separate ground in MV distribution circuits. That, combined with connection between the current carrying MV neutral and the customer service drop neutral, are what I have issue with. Add the 4th (for L-L transformers) or 5th (for L-N transformers) wire and use it correctly, then I do not have an issue with a solid metallic path from customer to distribution.
| Now consider the delta with a neutral tap on one side. Will this mean that | the neutral is not carrying current- ideally so but ??? Suppose also that | it was 12.5KV line to line. That means that 2 legs are at 6.25KV with | respect to ground and the other is at 14KV with respect to ground. Is this | better than having all 3 legs at 7.2KV to ground? Zig- zag grounding | transformers were often used to get a neutral point which was equidistant | electrically from all phases. The center tapped leg of a delta is a cheap, | but poorer alternative to this.
I'm not suggesting a center tapped delta.
| Seeing that the user with a single phase supply sees no difference from the | situation where the distribution transformer is connected l-l vs l-n on the | primary- your last question is meaningless. Run a separate ground wire if | you want.
Please clarify what you mean by "Run a separate ground wire if you want." There are a number of different ways to accomplish that. But given that power company practice is to connect the secondary of the transformer to the primary current carrying conductors, then the first step to running a separate ground wire is to have another transformer with its primary wired L-L (240 volts) and its secondary not connected to the primary at all ... not even to the service drop neutral (which also must not be grounded to earth anywhere near the points the new separate ground is earthed).
| If you are looking at an industrial system taking 3 phase from a delta with | a neutral on one side and single phase to neutral loads on the tapped side- | where while neutral current can't flow, unbalanced voltages can result -then | I would prefer a Grounded wye system. A separate safety ground wire to the | frames of equipment is just as feasible there as with the household single | phase system. | Note also that ground fault protection is a hell of a lot easier with a Y.
I still think you are misunderstanding me. It seems you are assuming that when the loads (transformers at customer taps) are L-L or L-L-L, then the source of the circuit they connect to must have a delta secondary. I do realize you described the case where a town switched from delta secondary (for example at 7200 volts) feeding L-L and L-L-L loads, to a wye secondary (for example at 12470 volts) feedling L-N and L-N*3 loads, to achieve a 73% boost in system capacity.
But this was all in repsonse to my description of how things should be from the beginning, which would have precluded that down from having the starting point they had in the first place.
The substation transformer secondary should be WYE. The center point is earthed at the substation. Now there are two different ways to run that circuit:
1. Run 5 wires, identified as A,B,C for the phases, N for the neutral, and G for the grounding wire. Loads can then be connected to any combination of A,B,C,N as needed.
2. Run 4 wires, identified as A,B,C for the phases and G for the grounding wire. Loads can then be connected to any combination of A,B,C as needed.
With design #1 you can have L-N taps for customer service transformers. With design #2 you are limited to L-L taps. In all cases the secondary would be WYE.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

------------ The present practice is to use (2) with taps being line to neutral. This means that 3 phase services don't have to be run everywhere and there is a cost savings. Transformers are also cheaper. As for safety- it is questionable whether there is any real safety penalty compared to (1). Now using (1), in effect, you are running a ground wire back to the substation and grounding it there. This wire will be coupled to the primary and since loads can be quite unbalanced, there can be appreciable voltages induced in the wire. These can be much higher than those due to neutral current. This can be avoided by use of multiple ground points but in the case of a poor ground at a customer entrance there is the possibility (however rare) that induced voltages from hundreds to thousands can occur- particularly in the case of a fault on the primary. Such voltages could exceed the voltages produced by a high common neutral current (which is actually much less of a problem) . There are safety concerns with both systems but how many safety problems have been caused by this considering the extensive use of the common HV-LV neutral point/ conductor throughout North America for the last 80 years? There are the "stray-voltage" situations met in some dairy barns but, the cause claimed, has, as far as I know not been proven technically (although proof in court is easier).
--

Don Kelly snipped-for-privacy@shawcross.ca
remove the X to answer
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.