Transformer Relay VT inputs

[HamidEle]

We have an application as described below:

3MVA transformer 13.8KV-4160V. CTs are located on the primary side of the transformer and VT signals come from the secondary side of the transformer (4160V SWGR). We need the program the relay with phase VT ratio. But not quite sure what ratio we should use: 13.8KV/120V or 4160V-120V?

Any inputs will be appreciated.

 

[davidbeach]

Use the PTR of the actual, real, instrument transformers.

 

[HamidEle]

David, you meant using 4160V/120V?

 

[davidbeach]

Probably, but what are you doing with them? What is the relay? A PTR of 34.67 will give you the actual voltage at the location of the VT. Using a PTR of 115 will give you an approximation of the voltage somewhere other than where the VT is located. Without any sort of voltage drop compensation, you'll never actually have the high-side voltage.

 

[HamidEle]

It is GE F35 relay. The client wants to save money VT with higher voltage. There existing Relay uses 34.67 instead of 115, which could be wrong.

 

[stevenal]

Just giving the relay model does not indicate how the voltages are being used in its programming. Metering? Under/over voltage protection? Over/under frequency?

 

[iamrip]

The VT input from the secondary of the transformer is used to monitor the voltage at the primary side?

 

[HamidEle]

Yes, indeed. It was set up that way. No one wants to change it.

 

[ShooterBoter]

What is the transformer vector group?
What is your aim with the relay, i.e. at what fault scenario should the relay pick up?

 

[HamidEle]

Shooterbooster, The transformer vector group is Delta ( Primary) and secondary ( Y). I understand the metering will be wrong. Since it is for plant internal measurement, it is not a big concern from client perspective. They implemented this way and they keep doing this from day one.
The purpose of the relay is to protect the primary of the transformer. The relay will pick up at Phase and ground overcurrent ( TOC and IOC).

 

[waross]

See Thread
vector group of transformer
thread238-125306


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ShooterBoter]

Well, if you (or your client) don't care about the measuring being wrong then I don't understand your issue.
Assuming of course you don't have any directional protection (pure over current IOC and TOC; these protection functions don't care about the voltage/angle)

But not quite sure what ratio we should use: 13.8KV/120V or 4160V-120V?
What are you measuring U, I, P, Q, pf...?

 

[HamidEle]

We are using 4200/120V ratio. We are not using any directional protection. I think this issue can be closed. Thanks for all the inputs.