autotransformer sanity check

[Besoeker]

It has been established that jraef missed that it was an auto not an isolation. That has been settled.

And my wit was wasted............

 

[electrofelon]

And my wit was wasted............

And I am a third that needs glasses

 

[winnie]

And I agree with you too. It would have made you a good 480 neutral, but seems more robust tying. Great question.

A wye autotransformer does not do a good job of deriving a neutral. If you only connected X1, X2, and X3 then H0 would vary quite a bit due to any imbalance on H1, H2, H3.

It could be made to work by using an unconnected 'secondary' set of delta windings. These would not have any external connection, but circulating currents on the delta would act to stabilize H0.

It could also be made to work if you had some sort of 'zig-zag' winding, where each leg of the core has two phases on it. Difficult to do with all those different voltage connections, but possible.

All in all the easiest thing to do is simply connect X0 to the supply neutral.

-Jon

 

[mike_kilroy]

A wye autotransformer does not do a good job of deriving a neutral. If you only connected X1, X2, and X3 then H0 would vary quite a bit due to any imbalance on H1, H2, H3.

It could be made to work by using an unconnected 'secondary' set of delta windings. These would not have any external connection, but circulating currents on the delta would act to stabilize H0.

It could also be made to work if you had some sort of 'zig-zag' winding, where each leg of the core has two phases on it. Difficult to do with all those different voltage connections, but possible.

All in all the easiest thing to do is simply connect X0 to the supply neutral.

-Jon

Thank you for the info.

 

[electrofelon]

Yes thank you Jon

 

[Bugman1400]

I don't see how you can separate the X0 & H0 on an auto since they share the same wire. Its not like the H wdg and X wdg are on two separate bobbins around the core. Seems like the only way to stabilize the neutral is to tie it (X0/H0) to a solid ground. I thought a zig-zag provided a medium to high impedance ground.

 

[Ingenieur]

I don't see how you can separate the X0 & H0 on an auto since they share the same wire. Its not like the H wdg and X wdg are on two separate bobbins around the core. Seems like the only way to stabilize the neutral is to tie it (X0/H0) to a solid ground. I thought a zig-zag provided a medium to high impedance ground.

a zig-zag provides a low Z only to 0 seq current(s) when present
otherwise higher
typically used to derive a gnd on delta systems for fault return

 

[mike_kilroy]

I don't see how you can separate the X0 & H0 on an auto since they share the same wire. Its not like the H wdg and X wdg are on two separate bobbins around the core. Seems like the only way to stabilize the neutral is to tie it (X0/H0) to a solid ground. I thought a zig-zag provided a medium to high impedance ground.

Very interesting comment/discussion! "stabilize???" the neutral???

Do we need to start a new thread to discuss this or can we morph this into that discussion since the OP was fully answered?

In either case I would like to pursue it some more! Just because it seems to me that even with imbalances on the H1,2,3 with respect to Ho, that would just manifest itself as a slightly off true '0 volts' for Ho...

so minuscule it would be immeasurable, not important, inconsequential, and thus tying it to Xo would not be required although that seems it would bring this inconsequential tidbit offset to same level...

WHERE can we further discuss this fascinating topic? Do we have administrator approval to continue here, or?

 

[mike_kilroy]

]

I don't see how you can separate the X0 & H0 on an auto since they share the same wire. Its not like the H wdg and X wdg are on two separate bobbins around the core. Seems like the only way to stabilize the neutral is to tie it (X0/H0) to a solid ground. I thought a zig-zag provided a medium to high impedance ground.

Bugman, for discussion clarity, no one suggested separating Xo and Ho IN THE TRANSFORMER.

The discussion was whether OP should bring his present Xo into this xfmr's Xo/Ho or not (and thus tie HIS neutral to output neutral or not)...

 

[kwired]

I don't see how you can separate the X0 & H0 on an auto since they share the same wire. Its not like the H wdg and X wdg are on two separate bobbins around the core. Seems like the only way to stabilize the neutral is to tie it (X0/H0) to a solid ground. I thought a zig-zag provided a medium to high impedance ground.

If you were supplying only balanced three phase loads and no line to neutral loads, I don't see that you need to tie anything to the wye point.

]

Bugman, for discussion clarity, no one suggested separating Xo and Ho IN THE TRANSFORMER.

The discussion was whether OP should bring his present Xo into this xfmr's Xo/Ho or not (and thus tie HIS neutral to output neutral or not)...

"Neutral" in this case is a common point to both the high and low voltage leads, there is no true input and output neutral like there could be on a separately derived system. Same concept as the common lead on a single phase autotransformer - it is simply one point in that system - it is very likely also the grounded conductor of the supply circuit, but there isn't really a secondary circuit, just taps on a coil(s) that give you different voltages based on how many turns there are between each point being measured.

 

[electrofelon]

]

Bugman, for discussion clarity, no one suggested separating Xo and Ho IN THE TRANSFORMER.

The discussion was whether OP should bring his present Xo into this xfmr's Xo/Ho or not (and thus tie HIS neutral to output neutral or not)...

Right. The transformer would - in true autotransformer fashion - derive the neutral with the three X phases, then you could use it for the H/load. That was the thinking.

If you were supplying only balanced three phase loads and no line to neutral loads, I don't see that you need to tie anything to the wye point.

In my case, although I didnt state it, The "load" is a three phase PV inverter. I am 98% sure that the neutral is just used for phase monitoring and carries little to no current and that the output is very closely balanced. Thus perhaps we didntt need to bring the supply neutral from the 208 source for this application.

 

[mike_kilroy]

In my case, although I didnt state it, The "load" is a three phase PV inverter. I am 98% sure that the neutral is just used for phase monitoring and carries little to no current and that the output is very closely balanced. Thus perhaps we didntt need to bring the supply neutral from the 208 source for this application.

Well now that you say THAT, there is a VERY GOOD reason to tie the power sources neutral and the output neutral together!!!!

If you do NOT tie the PV inverter's drive thru the Xo/Ho to the power source, your pv inverter will be FLOATING with respect to some ground reference! The result WILL be blown power devices earlier than if you had tied them!!

Think for a minute: not tied, your pv up there in the lightening zone will take a hit and not have a low impedance path to ground on the 460v input side! ALL lines will jump high with respect to ground. The result will be blown pcb traces, blown IGBTs etc. IF on the other hand you tie thru to the input's neutral, those 3pc 46ov will have low Z to ground and will not jump so high in voltage, thus surviving until the next BIG one.

In addition, if your pv unit was built outside the USA, it is likely built with inferior PRV rated semiconductors in the power path if they have not been in the business for 10+ years yet. History shows that it takes many years of selling into non WYE countries like ours before they wise up and learn to built with higher PRV devices. This is why many EU inverters REQUIRE WYE connection yet today or no warranty.

TIE it. Not because it may balance some small offset voltage, but to make your installation many times more robust!

 

[winnie]

Right. The transformer would - in true autotransformer fashion - derive the neutral with the three X phases, then you could use it for the H/load. That was the thinking.

Unfortunately this is not correct for a wye-wye 3 phase autotransformer. It would work for a single phase autotransfomer, and it would work for a zig-zag transformer, but in a wye-wye autotransformer the neutral simply is not stable. If you lightly load one phase then reduced current would flow on that leg, and the leg to X0/H0 voltage would climb, in the limit to almost the line-neutral voltage.

In my case, although I didnt state it, The "load" is a three phase PV inverter. I am 98% sure that the neutral is just used for phase monitoring and carries little to no current and that the output is very closely balanced. Thus perhaps we didntt need to bring the supply neutral from the 208 source for this application.

In this case, with a well balanced load and little neutral current, the system might work, but I would not trust it.

-Jon

 

[electrofelon]

Unfortunately this is not correct for a wye-wye 3 phase autotransformer. It would work for a single phase autotransfomer, and it would work for a zig-zag transformer, but in a wye-wye autotransformer the neutral simply is not stable. If you lightly load one phase then reduced current would flow on that leg, and the leg to X0/H0 voltage would climb, in the limit to almost the line-neutral voltage.



In this case, with a well balanced load and little neutral current, the system might work, but I would not trust it.

-Jon

Ok thanks for the explanation. So why is it that the neutral of a wye secondary on a two winding transformer is stable? Of course I understand the construction differences in the two, just not seeing what makes one have a stable neutral and the other not.

BTW this is academic, they have been installed with supply and load neutrals both common on the XO/HO terminal and are working great.

 

[kwired]

Unfortunately this is not correct for a wye-wye 3 phase autotransformer. It would work for a single phase autotransfomer, and it would work for a zig-zag transformer, but in a wye-wye autotransformer the neutral simply is not stable. If you lightly load one phase then reduced current would flow on that leg, and the leg to X0/H0 voltage would climb, in the limit to almost the line-neutral voltage.




In this case, with a well balanced load and little neutral current, the system might work, but I would not trust it.

-Jon

No experience with such a beast, but sure seems to me that attaching the 480/277 supply neutral to the wye point and the ungrounded supply lines to the outer ends will force 277 volts across each coil, less any voltage drop in the supply circuit. Neutral can't "drift" when you put such a supply conductor there, unless there is unbalance severe enough to throw the supply voltage off balance.

Add: if you don't connect supply neutral to the wye point - then yes voltage to neutral will vary depeding on load balance conditions.

 

[GoldDigger]

It seems to me that if you have a wye-wye *autotransformer* the H0 and X0 are exactly the same point!
How can that be unstable?

Sent from my XT1585 using Tapatalk

 

[MAC702]

It seems to me that if you have a wye-wye *autotransformer* the H0 and X0 are exactly the same point!
How can that be unstable?...

That's what I thought from the very beginning, and I'm still hoping to learn something, but it makes me feel good to think we are coming back to that.

I am NOT an xformer or motor guy.

 

[electrofelon]

It seems to me that if you have a wye-wye *autotransformer* the H0 and X0 are exactly the same point!
How can that be unstable?

Sent from my XT1585 using Tapatalk

That's what I thought from the very beginning, and I'm still hoping to learn something, but it makes me feel good to think we are coming back to that.

.

Yes they are the same point, no question about that. The discussion is about running a neutral with the supply, or not and letting the transformer derive the neutral for the load. We have three possibilities (#3 is new, I dont think it has been discussed yet):

1. supply neutral and load neutral both get landed to the common XO/HO terminal
2. Only the load neutral gets landed on the XO/HO terminal and no neutral is run with the supply.
3. Run the supply neutral to the load without hitting the XO/HO terminal.

What would happen for #3?

 

[ggunn]

Assuming the inverter needs a neutral, with an autotransformer you run a neutral from the inverter to the transformer and from the transformer back to the service. With an isolation transformer you run a neutral from the inverter to the transformer but not from the transformer back to the service. Easy peasy.

 

[kwired]

Yes they are the same point, no question about that. The discussion is about running a neutral with the supply, or not and letting the transformer derive the neutral for the load. We have three possibilities (#3 is new, I dont think it has been discussed yet):

1. supply neutral and load neutral both get landed to the common XO/HO terminal
2. Only the load neutral gets landed on the XO/HO terminal and no neutral is run with the supply.
3. Run the supply neutral to the load without hitting the XO/HO terminal.

What would happen for #3?

They are not the same point if not bonded together. Your conditions 2 and 3 each leave one point or the other "floating". When things are balanced they will be at/very near same potential. Throw some unbalance in there and one or the other will drift potential wise from the other. Bond them together and you force them to be same potential regardless of balance condition.