Corner Grounded Delta

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mull982

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It is my understanding that a corner grounded delta system if for ground fault detection. Is this the only reason for corner grounding a system? How does a corner gounded system work for ground fault detection and OCPD's?

Are all of the line-line voltage in a corner grounded system still the same? I'm assuming that the grounded phase to ground voltage is zero and the other two phase to ground voltages are normal phase go ground readings?
 
mull982 said:
It is my understanding that a corner grounded delta system if for ground fault detection. Is this the only reason for corner grounding a system? How does a corner grounded system work for ground fault detection and OCPD's?

Are all of the line-line voltage in a corner grounded system still the same? I'm assuming that the grounded phase to ground voltage is zero and the other two phase to ground voltages are normal phase go ground readings?
Click to expand...
Using a grounded system limits the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation. {See 250.4(A)(1)} The grounding provides a path for ground fault currents to return to the source on the first ground fault and operate the overcurrent protective device.

The line to line voltages are still the same; however, the line to ground will be stabilized and will be the same as line to line for the ungrounded phases. :D
 
mull982 said:
It is my understanding that a corner grounded delta system if for ground fault detection. Is this the only reason for corner grounding a system? How does a corner gounded system work for ground fault detection and OCPD's?

Are all of the line-line voltage in a corner grounded system still the same? I'm assuming that the grounded phase to ground voltage is zero and the other two phase to ground voltages are normal phase go ground readings?
Click to expand...

Corner grounded systems are used for more than just ground fault detection. While they are not necessarily common, they are not rare as power distribution systems, especially at 240V.

Yes, you are correct about the voltages.
 
I have been reading up on the different transformer types and just wanted to sumarize and make sure I understood the principles of the following types:

Ungrounded Delta:

- No phase is intentioally grounded.
- All Line to ground voltages read (L-L/1.73)
- Upon ground fault, the faulted phase L-G reading is zero and the other two
L-G phase readings are L-L V.

Corner Grounded Delta

- One phase is intentionally grounded
- All phase L-L voltages are normal and the same
- The grounded phase L-G reading is zero and the other two
L-G phase readings are L-L V.

Open Delta

- Has one phase winding removed
- All three phase L-L voltages are the same and normal
- If there is no intentional ground then it acts like an ungrounded xfmr as
described above.
- If on phase faults to ground or is intentionally grounded ("Corner
Grounded") then the the transformer acts as described above.
- Transformer has 58% capacity.

Are these summaries correct?

If I'm in the field and am taking voltage readings how can I tell if I have a delta system or an open delta system without looking at the transformer arrangement?
 
080411-1030 EST USA

mull982:

An ungrounded anything can float to whatever is determined by leakage impedances and external current and/or voltage sources thru these impedances and the resultant effect.

Your 1.732 constant assumes equal leakage from each line to ground and no other leakages, for example, primary to secondary leakage.

.
 
What is the difference between a wye and delta motor winding? I'm assuming these refer to the way that the stator is wired. I understand that a delta connection will have more current in the windings than it will on the line, but what does this mean as far as motor performance?

Is there any particular reason to wire a motor one way as opposed to another. I've heard of doing this for starting reasons but other than that I am not to familiar with this subject.
 
mull982 said:
What is the difference between a wye and delta motor winding? I'm assuming these refer to the way that the stator is wired.
Click to expand...
Yes it only refers to the internal motor windings.
I understand that a delta connection will have more current in the windings than it will on the line, but what does this mean as far as motor performance?
Click to expand...
It does not have more current in the windings than in the supply.
[/quote]Is there any particular reason to wire a motor one way as opposed to another. [/quote]
Not that I know of.
I've heard of doing this for starting reasons but other than that I am not to familiar with this subject.
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Yes, a delta motor can, if it has 12 leads (for a dual voltage motor), be connected in wye for start and delta for run. The will reduce the current required to start the motor, but it also reduces the starting torque and cannot be used for all loads. If the motor is wired for 480, the coils will only see 277 when connected in wye.
 
mull982 said:
I have been reading up on the different transformer types and just wanted to sumarize and make sure I understood the principles of the following types:

Ungrounded Delta:

- No phase is intentioally grounded.
- All Line to ground voltages read (L-L/1.73)
- Upon ground fault, the faulted phase L-G reading is zero and the other two
L-G phase readings are L-L V.
............
Click to expand...


- No phase is intentionally grounded. TRUE
- All Line to ground voltages read (L-L/1.73) NOT TRUE
- Upon ground fault, the faulted phase L-G reading is zero and the other two
L-G phase readings are L-L V. TRUE

gar said:
An ungrounded anything can float to whatever is determined by leakage impedances and external current and/or voltage sources thru these impedances and the resultant effect.

Your 1.732 constant assumes equal leakage from each line to ground and no other leakages, for example, primary to secondary leakage.
Click to expand...

Correct. Sort of. To expand: an ungrounded delta system is merely "capacitively coupled" to the earth. What you read with your voltmeter will be pretty arbitrary.

I used to work at a university campus that had several of these systems in buildings built in the '60's. Voltage to ground would vary with humidity and day to day and hour to hour. I have even had readings GREATER than 300V to ground on a 240V ungrounded delta system, but it usually would vary between maybe 60 or 70V and the low 200's. You basically have a large capacitor. The earth as one conductor, the 240V source as the other and the air inbetween as the dielectric. No way to predict what you are going to measure from conductor to conductor.

This type of service usually has 3 ground fault indicator lights at the service disconnecting means. All three lights burn dim with no faults present. These bulbs will usually have a higher than 240V rating and will be wired line on one side, ground or equipment enclosure on the other. When a fault on one phase is present that light will go out (0V protential difference) and the other two lamps will burn brighter than before the fault since the voltage across them has stabilized at 240V.
 
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charlie said:
Very cool, dad!
:grin: :grin: :grin: :grin: :grin: :grin:​
Click to expand...

Thanks!

Some good shooters in Kokomo last month. Very intense. Big smiles all around that day! He worked hard though shooting nearly every day before school for about 2 months prior. He normally shoots a few times a week all year.
 
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