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Uquresh1 (Electrical) (OP)
4 May 11 10:00

which Voltage should I use to divide by 115 to get my VT Ratio?
ScottyUK (Electrical)
4 May 11 10:20
20125 is the line - earth voltage. 34500 is the line-line voltage. Use 20125V to calculate the turns ratio.

If we learn from our mistakes I'm getting a great education!

rbulsara (Electrical)
4 May 11 10:58
If 3 VTS are used in a 3 phase configuration, the VTR will be the same for Y or delta but the actual voltage rating will be different.

Not sure where you got the information from and what are you trying to do, specify a VT or use VTR in a relay programming or do some calcs?.  

You still need to know the VT connections, Y or delta and number of VTS to specify a VT correctly. For all I know you can have a single phase VT on a 3 phase system, connected either L-L or L-N.

As a side note, the rating 20125/34500Y does not exactly match up to 1.732 multiplier so once you select a VT, you need to use the actual VTR on the VT nameplate, and not what you might have calculated.

Rafiq Bulsara

Uquresh1 (Electrical) (OP)
4 May 11 11:05
Its a 3-Phase VT connected grounded Wye - grounded Wye. I just need the VT ratio for my relay settings and wasn't sure whether to do 20125/115 or 34500/115? The customer should have provided me with the VTR since they ordered the damn things, but they just told me thts all the info they have.
FYI...the system voltage is 34.5KV.
rbulsara (Electrical)
4 May 11 11:29
For accurate relay settings, you should use the VTR on the nameplate. However for most practical purposes, it would be 300 in this case, assuming 115V is the line-line voltage on the secondary of the VT.

115/(sqrt 3) = 66
34500/115= 300 or 20125/66=304.

For a 34.5 kV, L-N voltage would be 19919. 19919/66=302.

I would still ask for the relay and VT schematics.


Rafiq Bulsara

stevenal (Electrical)
4 May 11 18:10
From IEEE C57.13 for outdoor bushings:
20125/34500 Grd Y 175/300:1

From IEEE C57.13 for indoor bushings:
20125/34500 Grd Y 175:1

ScottyUK is correct, use the smaller number. 175:1 corresponds to a 115 V secondary and 300:1 gives 67 V.

scottf (Electrical)
4 May 11 18:24
rbulsara, I must disagree with your posts.

The ratio is 20125:115V, which is 175:1. It doesn't matter how the VT is connected. The rated primary voltage is 20125V and the rated secondary voltage is 115 it's written.

Also, because the rating lists "GY" it means it's a single-bushing unit meant for line to ground connection only.

You're correct that 34500/sqrt3 is not 20125, but that doesn't mean the ratio is not still 175:1, but rather means that the secondary voltage is not exactly 115V when the system voltage is 34500V.

rbulsara (Electrical)
4 May 11 19:07
Yes, if 115 is the L-N voltage on the VT secondary on a wye connected 3 phase VT. That is why I would ask for a wiring diagram.


Rafiq Bulsara

rbulsara (Electrical)
4 May 11 19:13
I asked first where he is reading the data from. I thought they are talking about system voltage not the PT rating when they said 20125/34500Y.

If that is what is on the PT, yes it is 175.


Rafiq Bulsara

davidbeach (Electrical)
4 May 11 19:15
As scottf said, it doesn't matter how it is connected.  A 20125/34500GY primary can ONLY be connected line-neutral/ground at a voltage not exceeding 20125.  The VTR is/can only be 175.  Since the primary of the VT can never be connected in delta, there is no worry about an open delta connection, but even then the VTR would still be 175.
rbulsara (Electrical)
4 May 11 20:38
This particular VT cannot be connected in delta in 34.5 kV system. I did not pick that he has the VT ratings in his hand. (or why would he ask the question). I erroneously thought he was trying to pick or specify a VT for a 34.5kV grounded wye system.

You can certainly have delta connected VTs or single phase line to line VTs.  

Rafiq Bulsara

scottf (Electrical)
4 May 11 21:03
Yes, you can connect VTs, in general, in many configurations, but the ratio never changes.  
davidbeach (Electrical)
4 May 11 21:10
But single bushing VTs can only be connected in wye, it takes a two bushing VT if connecting in delta.
rbulsara (Electrical)
4 May 11 22:18
What did I say in my very first line in my very first post?

Rafiq Bulsara

scottf (Electrical)
5 May 11 8:42
The voltage rating doesn't change with how it's connected either (which was my main point).

The rated voltage is the voltage that the primary winding is rated to have applied across it. How that voltage gets there (line-to-line, or line-to-neutral/ground) doesn't really matter to the VT. All other considerations are insulation-based, etc...

rbulsara (Electrical)
5 May 11 9:14
You are talking about a given VT in hand, with a fixed voltage rating. I was talking about a given system voltage and possible VTs.


Rafiq Bulsara

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