PT L-L vs L-G Ratings 2

[rockman7892]

Can someone explain the difference in the Line-Line and Line-Ground ratings for the MV ABB PT in attachment. This datasheet gives model numbers for Line-Line and Line-Ground connected units and I was wondering what the difference is for PT with same primary voltage ratings?

For instance the 2400/4160Y unit is shown under both the L-L and L-G units with the same with the same winding ratio and thus the same 2400V primary voltage rating. Is the only difference here based on the insulation or bushing ratings at the terminals?

I know for pole mounted transformers you can get them for L-G connections where neutral connection is bonded internally to transformer enclosure and thus there is only (1) bushing. Or you can order for L-L connection where transformer has two bushings to be able to connect L-L.

Does anyone know what other manufacturer makes a similar PT as attached besides GE?

Thanks for the help.

 

[RRaghunath]

When the VT is connected L-G, the 2400V / 69.3V will apply.
4160V/120V is the system voltage rating on primary and secondary and refers to L-L voltages, as always. The same will be primary and secondary winding voltages if the VT is meant for connection L-L.
The turns (or winding) ratio remains same in both cases (L-L and L-G connections).
Rated voltage factor gives the design overvoltage withstand capability of the VT windings and is in multiples of rated winding voltage (2400V if meant for L-G connection and 4160V if meant for L-L connection).
Hope that clarifies.

 

[waross]

L-L Two high voltage bushings.
L-N One high voltage bushing. For higher voltages may have graduated insulation.

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

 

[rockman7892]

RRaghunath

Thanks for the clarification. Still missing something here on my end as its not 100% clear. Let me try to clarify my understanding of each.

First looking at the L-L rated PT with 2400/4160Y rating and winding ratio of 20. I get that the 2400V in this case refers to a L-L connection on a 2400V system and thus gives a 120V secondary. But what is meant by the 4160Y rating for the L-L rated PT? Is this referring to the PT being connected to L-G/L-N on a 4160V system which would give 2400V across PT and 120V secondary? If so this is not truly a L-L connection, correct? This PT would have a rated voltage of 2400V.

Second for the L-G rated PT with 2400V/4160GY rating and winding ratio of 20. If this PT is connected L-G on a 2400V system then the secondary will be the 69.3V you referenced above. If this PT is connected L-G on a 4160V system then the PT will have 2400V on primary and as secondary voltage of 120V. This PT would also appear that it has a 2400V rated primary winding?

Would both of these PT's not have the same 2400V rated primary winding?

Perhaps the thing that is throwing me off is the 4160Y nomenclature on the L-L rated PT and confusion on weather this is referencing 4160V L-L or 2400V connected L-N on a 4160V system.

 

[bacon4life]

Although your attachment did not open, I see your confusion when looking at the ABB catalog. Like you, I do not understand how a PT with a ratio of 20:1 can be connected Line-to-Line with a 120 volt secondary.

https://library.e.abb.com/public/e1...3def26/1VAP429551-DB_VIZ-75,11_April 2021.pdf

 

[waross]

That confusion reminds me of a 333KVA transformer issue.
We were using 3 x 333 KVA, 480V:13.8kV transformers in a delta wye step-up configuration.
We needed a spare transformer.
The manager bought a 277/480V:13.8kV transformer.
At that time and place, given the cost of a return via sea transport, mistakes were never returned.
That transformer is probably still in the storage shed.



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

 

[stevenal]

After downloading the file provided, I found I could append .pdf to it and have a usable file. In line with waross, the diagrams on page 2 show one unit with a ground lug at H2, and another with an insulated terminal. I share the confusion regarding the voltage and ratio.

 

[waross]

The moral of this:
"Don't trust a sales engineer without verification."
If you haven't been "Got by a misinformed sales engineer" yet, just wait.
It's coming.

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

 

[RRaghunath]

L-L PT primary is 4160V and Secondary is 120V.
L-G PT is 2400V (4160/sqrt3) primary and 69.3V (120/sqrt3) secondary.
This is what I mean.

 

[waross]

L-L PT primary is 4160V and Secondary is 120V. Ratio 4160V/120V = 34.67:1,
Ratio given, 20:1
L-G PT is 2400V (4160/sqrt3) primary and 69.3V (120/sqrt3) secondary. Ratio 2400V/69.3V = 34.6:1,
Ratio given, 20:1
This is what we mean.
The confusion generated by the numbers in the ABB catalogue.
Your numbers are what we would expect, RRaghunath.

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

 

[scottf]

2400/4160Y means that the VT is designed to be connected line-to-neutral on a 4160V system OR line-to-line on a 2400V system. 2400V is the rated voltage. 2400V/120V = 20:1 ratio

2400/4160GY means that the VT is designed to only be connected line-to-ground on a 4-wire grounded wye 4160V system. 2400V is the rated voltage. 2400V/120V = 20:1 ratio

So a VT with a rating of 2400/4160Y is a 2-bushing VT and a VT with a rating of 2400/4160GY is a 1-bushing VT.

There are some special designs (under Group 4B of IEEE C57.13) that have a rating like 4160/4160GY and these units are meant for operation at 58% of rated voltage. Frankly, I've never understood why such ratings are needed now with modern relays.

 

[waross]

Thanks for the clear explanation, Scott.

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

 

[rockman7892]

Scottf - Thanks for the explanation. Makes sense.

I’m curious on a 4.16kv application when connecting PT L-G what is the advantage of choosing a 2-bushing Y version vs the single bushing YG version? Assuming the 2 bushing version costs more.

 

[scottf]

You have to use a 2_buahing unit if you connect line to neutral on a non-rigidly grounded system because the neutral could float up to the line voltage during certain fault conditions.