# Corner-Grounded Delta System Question



## PEExam123 (Nov 28, 2020)

Hello All,

                What would be the VAG for Corner grounded Delta system with Neutral *grounded* at WYE connected source ?  Please see attached picture.

If the WYE Source Neutral is *ungrounded*, VAG would be Vline-line = 13.2 kV for this example.


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## jd5191 (Nov 29, 2020)

Is this a hypothetical question that you made up or a question based on a practice problem you found somewhere?

If the wye is the source and delta is the load, and you corner ground the delta, you also ground a phase of the source, I would think you need to do a fault analysis for a single phase to ground fault and need impedances to calculate currents and then voltages across those impedances.

Unless you're asking the question in a no-load condition, if that's the case then the delta on the right doesn't matter at all.


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## akyip (Dec 17, 2020)

I tried to take a shot at this question, because I'm also curious myself and I haven't really seen many examples for a corner-grounded delta.

The only 2 practice problems I remember regarding corner-grounded delta are from the NCEES practice exam and PPI practice exam. Both questions had the corner-grounded delta connected to an ungrounded source, which is simpler. Here, all the corner ground is establish a 0 V reference at one phase of the delta. The line-to-line voltages (difference between phases voltages, Van - Vbn for example) remain unchanged.

I'm not fully sure about what happens when a corner-grounded delta is connected to a grounded Y source, but I tried drawing it out. I THINK Vag is still 13.2 KV because Vbg = 0 when corner B of the delta is grounded... So Vag = Vab - Vbg = 13.2 KV - 0 V = 13.2 KV... but I'm not sure if the typical relationships between phase voltage and line voltage change when a grounded Y source feeds a corner-grounded delta load...

@Zach Stone, P.E., @Cram For The PE, @justin-hawaii: do you guys have any input on corner-grounded delta examples like this?


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## jd5191 (Dec 17, 2020)

@akyip what software are you using to draw out these things


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## akyip (Dec 17, 2020)

jd5191 said:


> @akyip what software are you using to draw out these things


OneNote.

I've been using a tablet (Surface) and a tablet pen to do practice exams. Saves a lot of paper!


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## Dothracki PE (Dec 17, 2020)

akyip said:


> OneNote.
> 
> I've been using a tablet (Surface) and a tablet pen to do practice exams. Saves a lot of paper!


Wish I thought of that! Although I have been hoarding notepads I got from vendors at work so I used some of those.


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## Byk (Dec 17, 2020)

My biggest question here is what is happening to the neutral on wye side.

You grounded one point (the neutral) from the WYE and a different point (B phase) at the load, which created a direct B-to-N short ?

Btw, @akyip I agree with you that Vag=13.2


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## akyip (Dec 17, 2020)

Byk said:


> My biggest question here is what is happening to the neutral on wye side.
> 
> You grounded one point (the neutral) from the WYE and a different point (B phase) at the load, which created a direct B-to-N short ?
> 
> Btw, @akyip I agree with you that Vag=13.2


Not sure about that question either. I'm not sure how the current flows between the source and load in the case of a Y grounded source feeding a corner-grounded delta.

I definitely see your point about the neutral being a short-circuit... Maybe that's why we don't see many problems about a grounded Y source feeding a corner-grounded delta?

I just haven't seen that many examples with a corner-grounded delta at all...


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## DarkLegion PE (Dec 17, 2020)

Will be following this.. was pretty confused seeing this pop up on the NCEES practice


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## akyip (Dec 17, 2020)

DarkLegion said:


> Will be following this.. was pretty confused seeing this pop up on the NCEES practice


The NCEES practice one was an ungrounded source feeding a corner-grounded load. That situation is much simpler: all the corner ground does is make the grounded phase a 0-V reference point. It does not impact the voltages and voltage differences at other points. Line-to-line voltages will still be the same values in that situation (ungrounded source feeding corner-grounded delta), whether or not a delta load is corner-grounded.

It is the case of a grounded source feeding a corner-grounded delta, that I do not quite understand.


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## akyip (Dec 17, 2020)

Byk said:


> My biggest question here is what is happening to the neutral on wye side.
> 
> You grounded one point (the neutral) from the WYE and a different point (B phase) at the load, which created a direct B-to-N short ?
> 
> Btw, @akyip I agree with you that Vag=13.2


There's one other thing I thought about that further makes me think that Vag will still be 13.2 KV for the case of a grounded source feeding a corner-grounded delta.

When a delta becomes an open-delta, its delta phase voltages (which are line-to-line voltages) do not change. So corner-grounding a delta will certainly not change the delta phase voltages (the line-to-line voltages), and I don't think the ground being extended back to the source would also change the delta phase voltages.

Just my two cents.


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## Byk (Dec 17, 2020)

akyip said:


> Not sure about that question either. I'm not sure how the current flows between the source and load in the case of a Y grounded source feeding a corner-grounded delta.
> 
> I definitely see your point about the neutral being a short-circuit... Maybe that's why we don't see many problems about a grounded Y source feeding a corner-grounded delta?
> 
> I just haven't seen that many examples with a corner-grounded delta at all...


I see a fair amount of corner grounded delta in the old buildings. The biggest benefit is that you make other two phases very stable.

One thing to note tho is that one shouldn't assume that you can simply connect neutral to the grounded corner to get lower voltage i.e. to get 120 from 240. 



akyip said:


> There's one other thing I thought about that further makes me think that Vag will still be 13.2 KV for the case of a grounded source feeding a corner-grounded delta.
> 
> When a delta becomes an open-delta, its delta phase voltages (which are line-to-line voltages) do not change. So corner-grounding a delta will certainly not change the delta phase voltages (the line-to-line voltages), and I don't think the ground being extended back to the source would also change the delta phase voltages.
> 
> Just my two cents.


@akyip you are absolutely right on the open delta, voltages are the same. In fact most of the buildings that are fed from overhead (at least in my state) are using open delta. Utility loves it because it's a lot cheaper and easier to install.


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## akyip (Dec 17, 2020)

Byk said:


> I see a fair amount of corner grounded delta in the old buildings. The biggest benefit is that you make other two phases very stable.
> 
> One thing to note tho is that one shouldn't assume that you can simply connect neutral to the grounded corner to get lower voltage i.e. to get 120 from 240.
> 
> ...


That's nice to know. I didn't think they could actually use a center-tapped open delta. I'm guessing the 57.7% rule for open-delta still applies to center-tapped open delta: the power drawn is only 57.7% of what is normally supplied from a full delta. And that the currents flowing in the phases are delta current values instead, because of the open-delta...


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## Zach Stone P.E. (Dec 18, 2020)

I think this question may be a bit of a misnomer.

Corner grounding for a delta connection is used at the power supply (i.e. the secondary of the transformer feeding the system). It is just a way to ground a delta system since there is no center neutral point like what is readily available on a wye secondary.

Corner grounded deltas are not used for a load, your load would be grounded through the equipment ground to ground any metal exterior, like the outer metal case of a motor.


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## akyip (Dec 18, 2020)

Zach Stone said:


> I think this question may be a bit of a misnomer.
> 
> Corner grounding for a delta connection is used at the power supply (i.e. the secondary of the transformer feeding the system). It is just a way to ground a delta system since there is no center neutral point like what is readily available on a wye secondary.
> 
> Corner grounded deltas are not used for a load, your load would be grounded through the equipment ground to ground any metal exterior, like the outer metal case of a motor.


Thanks for the FYI.

This makes the only 2 practice problems I know with corner-grounded deltas a bit misleading, since they had corner-grounded deltas as loads.

It also sounds like I shouldn't worry too much about corner-grounded deltas on the exam...


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## Cram For The PE (Dec 30, 2020)

PEExam123 said:


> Hello All,
> 
> What would be the VAG for Corner grounded Delta system with Neutral *grounded* at WYE connected source ?  Please see attached picture.
> 
> ...


In this sketch you have a grounded wye. So:
Vag=Vcg=7.62kV

if the wye source was un-grounded

Vag=Vcg=13.2kV

Was this the question being asked? I was not sure if this was a question or a statement.


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## akyip (Dec 30, 2020)

Cram For The PE said:


> In this sketch you have a grounded wye. So:
> Vag=Vcg=7.62kV
> 
> if the wye source was un-grounded
> ...


So then, does that mean for the case of a grounded Y feeding a corner-grounded delta, the corner-grounded delta's phase voltages are Vab = Vbc = Vca = 7.62 KV? (Magnitudes only)

So then does that mean that for a grounded Y feeding a corner-grounded delta, the corner-grounded delta's delta phase voltages equal the line-to-neutral voltages of the grounded Y source? (Neglecting any voltage drop on the line conductors...)


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## Cram For The PE (Dec 30, 2020)

akyip said:


> So then, does that mean for the case of a grounded Y feeding a corner-grounded delta, the corner-grounded delta's phase voltages are Vab = Vbc = Vca = 7.62 KV? (Magnitudes only)
> 
> So then does that mean that for a grounded Y feeding a corner-grounded delta, the corner-grounded delta's delta phase voltages equal the line-to-neutral voltages of the grounded Y source? (Neglecting any voltage drop on the line conductors...)


See below


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## akyip (Dec 30, 2020)

Cram For The PE said:


> See below
> 
> View attachment 20459


That is VERY enlightening! I wish I thought of that. Thank you, @Cram For The PE!


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## Cram For The PE (Dec 30, 2020)

akyip said:


> That is VERY enlightening! I wish I thought of that. Thank you, @Cram For The PE!


It is really just by observation you can see. Good question to ask for cram for the Pe Vol V!
But now think about the line currents. Anyone can guess what they look like?


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## Byk (Dec 31, 2020)

Cram For The PE said:


> See below
> 
> View attachment 20460


Shouldn't it e 13.2KV or am I missing something?

Also, it might be helpful to add letters so it's easer to understand what voltages are shown in the graph.


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## DarkLegion PE (Dec 31, 2020)

Cram For The PE said:


> It is really just by observation you can see. Good question to ask for cram for the Pe Vol V!
> But now think about the line currents. Anyone can guess what they look like?


Are we going to have line current through the 2 grounded phases and a phase current through the delta phase ungrounded?


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## Cram For The PE (Dec 31, 2020)

Byk said:


> Shouldn't it e 13.2KV or am I missing something?
> 
> Also, it might be helpful to add letters so it's easer to understand what voltages are shown in the graph.


Yes you are correct. I put 13.8 instead of 13.2. I just fixed the typo.


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## Cram For The PE (Dec 31, 2020)

DarkLegion said:


> Are we going to have line current through the 2 grounded phases and a phase current through the delta phase ungrounded?


Can you could draw a sketch of what you mean?


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## DarkLegion PE (Jan 1, 2021)

Cram For The PE said:


> Can you could draw a sketch of what you mean?


Hmm I'm thinking something like this? (Sorry I reused your diagram lol)


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## Cram For The PE (Jan 1, 2021)

DarkLegion said:


> Hmm I'm thinking something like this? (Sorry I reused your diagram lol)
> 
> View attachment 20486


That is incorrect.


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## kris7o2 (Jun 28, 2021)

Hello, 

I am trying to analyze the above figure from Cram for the PE, which was very helpful, but I am trying to apply formulas provided from the first post,

Vag = Vab - Vbg (etc)

If I were to apply the formula to the above problem I am struggling to see how to plug in the numbers to match the number provided here,


If there is any further explanation or drawing I would greatly appreciate the thelp.


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## kris7o2 (Jul 1, 2021)

kris7o2 said:


> Hello,
> 
> I am trying to analyze the above figure from Cram for the PE, which was very helpful, but I am trying to apply formulas provided from the first post,
> 
> ...


Actually spent more time reviewing this figure. I understand it now. Thanks so much for making this!


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