HV Potential Transformer. 2

[Engineer1916]

Eng Tips Family,
I hope all of you are keeping safe from the current pandemic. I have a question about the turn ratio of HV potential transformer. In west coast I have noticed that the most PT specified are Y connected with center tap on secondary side. For example for a 230kV system, PT would be (3) 1 phase 133kV - 115V/69V with 115V taps of secondaries of PT connected in Y. Which will produce 115V Line to neutral and 199V Line to line, while the 69V tap is left unused. I have seen this configuration on multiple jobs. My question is why specify 69V tap if we are not going to use it?

 

[Palletjack]

What would the 69V tap be L-L?

 

[davidbeach]

Line relays almost always use the 69V taps and metering uses the 115V taps.

 

[Engineer1916]

davidbeach,
Thank you for your answer. Is there a reason for using 69V tap for relaying? Why not make it simple and use same voltage for both metering and relay. I am just trying to understand the logic.

 

[Engineer1916]

Palletjack,
69V is L-N.

 

[Palletjack]

69V is L-N

Yes, I know. But line to line is 119V.
Some relays can use the L-L 120V.

 

[davidbeach]

Because that’s the way it’s always been. Look at the voltage requirements of a KD or an HZ. Look at the standard ratings of modern relays, many are 150V. Do you want the relay flat topping the voltage? If you have relays that are good for +/- 300V, go for the lower ratio tap. It don’t complain when your event report isn’t what you want it to be.

 

[scottf]

important to keep in mind that for IEEE units, the rated voltagea are designed to give nice/even ratios.

For example. a 230kV VT would have rated voltages of 230,000GY/138,000 and standard ratio of 1200/2000:1:1, which gives rated secondary voltages of 115/69V. Of course, for 230kV systems, the actual nominal line-to-ground voltage is ~132,800 V which results in secondary voltages of 110.7/66.4V.

 

[bacon4life]

Historically metering was was on a separate PT winding from relaying. We had one large station that used to have enough relaying voltge burden that the relaying winding on the PT read about one percent lower than the metering winding. For stations with low burden microprocessor based relaying, we switched to using one PT winding for group 1 protection and the other PT winding for group 2 protection, with the metering coming from the protective relays.

David- What kind of modern relays have a 150 V limit for event records? I assumed the 150 Vac limit in the blues ones was steady state, and that they could record up to 365 Vac for 10 seconds.

 

[davidbeach]

Modernish relays; starts with a ‘3’. Not going to take the time at the moment to dig out the event reports, but relays with the 150V option flat topped at about 162V while presumably relays with the 300V option wouldn’t have. Impedance grounded transformer, so the voltage would have gotten to about 208V. Line relays applied in a distribution application where they got the 120V tap instead of the 69V tap.

Either spec’ed at 300V or connected to the 69V winding would have been fine. It, as it was, didn’t work... WPRC 2017 Events Session.

 

[bacon4life]

We keep having to change out 3xx relays for 4xx relays... suppose this is one more factor to consider.