Corner-Grounded Delta System Question

[PEExam123]

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.  

 

[jd5191]

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.

 

[akyip]

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?

 

[jd5191]

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

 

[akyip]

@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!

 

[Dothracki PE]

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. 

 

[Byk]

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

 

[akyip]

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...

 

[DarkLegion PE]

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

 

[akyip]

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.

 

[akyip]

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.

 

[Byk]

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. 

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. 

 

[akyip]

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 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. 

View attachment 20228

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...

 

[Zach Stone P.E.]

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. 

 

[akyip]

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...

 

[Cram For The PE]

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.  

View attachment 19635

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.

 

[akyip]

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.

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...)

 

[Cram For The PE]

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

 

[akyip]

See below

View attachment 20459

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

 

[Cram For The PE]

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?