transformer delta connection line to ground voltage 4

[jaime.A]

Hello

We have a delta to wye transformer 13.2kV to 480/277V.
I was looking at the working distance from the transformer front to a planned switchgear front. An NEC screen shot of 110.34(A)is provided, so my question is: what would be considered voltage to ground on the delta side of the transformer?

 

[waross]

1> If the delta has a grounding transformer then the voltage to ground will be line to line voltage divided by root three.
2> If the delta is floating, then the normal line to ground voltage will be line to line divided by root three, but in the event that one phase becomes grounded, the voltage to ground of the other two phases will rise to line to line voltage. Most AHJs will require you to use line to line voltage as the maximum line to ground voltage.
3> If the delta is corner grounded then the voltage to ground of the ungrounded phases will be line to line voltage.
4> If the delta is center tap grounded then the voltage to ground of the high phase will be 1/2 root three times line to line voltage. That is the voltage to ground from the high phase will be 86.6% of line to line voltage.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[davidbeach]

Dollars to donuts, that 13.2kV comes from a utility transformer with a grounded wye on the 13.2kV side. The distribution feeder may be uni-grounded or it may be multi-grounded (ask the utility). It will fall somewhere in the 2501-9000V range. But the cost of complying with the 9001-25000V ranges is incrementally small, use that one and then add some margin just to be nice to the folks that will need to service it over the years. Always said that if the inspector gets out the tape measure you don't have enough clearance regardless of the actual measurement.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[prc]

The 13.2 kV primary voltage can go up by 10 % continuously and then the voltage to ground will be 8.38 kV. Some neutral shift of delta cannot be ruled out taking the margin to 9 KV very small. As Waross said, in the case of any LG fault on the 13.2 kV line, the voltage to ground will rise to 13.2 kV.

 

[jaime.A]

Thanks for the explanation; I just had to draw it out.

 

[wcaseyharman]

I’m not sure this is relevant to you, but many utilities in the US at least design their systems to be “effectively grounded.” If I remember right to be effectively grounded Xo<3 *X1. If a system is effectively grounded the phase voltage will rise no more than 138% of nominal during ground fault conditions.
The Westinghouse T&D reference book (at least in my 1964 edition) has a very good section on this topic.
Surge arrestors and other electrical devices are often designed and installed considering this requirement. My point: quite possibly the utility is effectively grounded and a utility document probably states this somewhere.
I think for your purposes, that unless the utility uses a less common system neutral configuration (at least in the US) nominal voltage should be VLL/1.73 so roughly 7.6kV.

 

[jaime.A]

Thanks for the replies. I will go with 13.2kV as the voltage to ground.

I also found this in the NEC handbook.
"ungrounded system is used, the voltage to ground (by definition) is the
greatest voltage between the given conductor and any other conductor
of the circuit. For example, the voltage to ground for a 480-volt
ungrounded delta system is 480 volts"

It is a challenge to find room for the switchgear in such a small room.

 

[stevenal]

There is no ground reference in the diagram above. Did you mean to ground the star point of the source?

 

[jghrist]

You need to find out if it is an ungrounded system. As davidbeach points out, most medium voltage utility systems in the US are solidly grounded. In this case, the voltage to ground is 7.6 kV. It doesn't matter how the transformer primary is connected. What matters is the voltage feeding the transformer.

Are 13.2 kV parts on the transformer front or the switchgear front exposed live parts?

 

[Bronzeado]

Hi folks,

If the electrical system is floating, how can the voltage (to "ground") will normally be measured?


Cheers,

Herivelto S. Bronzeado
Brasília, Brazil

https://www.linkedin.com/in/heriveltobronzeado/

https://www.researchgate.net/profile/Herivelto_Bronzeado

 

[davidbeach]

Where does floating, in this instance, come from? There's nothing to suggest that this isn't a hoe-hum run-of-the-mill utility primary service from a grounded 13.2kV system to a customer supplied delta high-side transformer. Pretty basic stuff.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[Bronzeado]

Hi davidbeach,

In the company I worked for, there is a electrical system on 69 kV that are supplied by a transformer wye-delta 230/69 kV. A zig-zag transformer is used to give a ground reference to the delta system (69 kV).
When the zig-zag transformer is out of service, the delta system becomes floating with the PTs phase to ground measuring "virtual voltages" as there is no reference to the ground.
In this case, a "neutral" reference will be provided by the system line capacitances and the PTs will measure something like "phase-to-neutral" voltages which is not that voltage when the system is complete. Normally, a 3V0 relay is applied to detect a ground fault.


Cheers,

Herivelto S. Bronzeado
Brasília, Brazil

https://www.linkedin.com/in/heriveltobronzeado/

https://www.researchgate.net/profile/Herivelto_Bronzeado

 

[wcaseyharman]

On our 55kV systems it was common to use a broken delta PT set with a resistor bridging the delta and a overvoltage relay (in parallel with the resistor) for both detecting ground faults and suppressing ferroresonace. I assume
You had something similar.

 

[prc]

Bronzeado,
1) The case Jaime presented was a Dyn distribution transformer getting supplied from the yn connected secondary of a step-down transformer. So, star neutral is grounded at the sending end of the line.
2) In the case you mentioned, there is a delta connection on transformers at both ends of the line with grounding transformers at both ends. I have seen such a system in Syria. Then without grounding transformers the type of phenomenon ( termed Neutral Inversion) that you
described can happen. This neutral inversion will disappear if (a) you have high capacitors connected to the line, e.g. Surge absorbers connected to tertiary of EHV auto-transformers, or (b) an electromagnetic PT (star-connected primary winding with neutral grounded) is connected to the line.