X and Y Windings on VTs

[111R]

Most voltage transformers (or PTs) have secondary X and Y windings with a 115V secondary (X1-X3) and a 67V secondary (X2-X3). I have a few questions in regards to this:

[ul]
[li]Have you ever seen a winding used at both the 67V tap and the 115V tap? I've come across some drawings where the 67V is going to a protective relay and the 115V is going to a metering panel. The neutral (X3) is shared between the two. So, a section of the winding may have a different burden that the remaining winding. Are there any issues with this?[/li]
[li]I've noticed that 67V is often used for relaying and 115V is used for metering even when supplied from separate windings. What is the reason behind this? Is 67V recommended for relaying?[/li]
[li]To expand on the first question, what would occur if a CT was used at multiple taps? For example, X1-X3 was used for one load and X1-X5 for another?[/li]
[/ul]

Thanks

 

[davidbeach]

[ul]
[li]Yes, not often but it happens.[/li]
[li]In the days of electromechanical distance relays 67 was the voltage, no other options available. Newer relays tended to follow that; even through the 311 family, SEL specified 67V nominal; the 400 series allow higher voltages. Really nice on our 115kV system since that means a 1000:1 VT ratio. On the other hand, all the distribution feeder relays that have voltage have always been connected to the 115V tap.[/li]
[li]The current would be divided and neither instrument would see a true replica of the primary current.[/li]
[/ul]

 

[cranky108]

As it turns out, 67V phase to neutral is about 115V phase to phase, so the 1000:1 ratio works out well for relaying (on a 115 kV line). On some PT's and CCVT's the full winding voltage can be 577:1 or 600:1, which is confusing (on a 115 kV line).
Since most electromechinical distance relays were employed looking for phase to phase faults, the potentials were connected, phase to phase. So the thing about 67 volts did not apply, because the relays actually saw 115 Volts. And in fact, when switching from electromechinical relays to microprocessors, it is common to find that there is no PT neutral with the relaying potentials.

In the case of directional over current electromechinical relays, the relays typically would have about 200 Volts applied, as this was the phase to phase voltage of the full winding from the PT's. And in fact in the older substations they would have phase to phase PT's, and no phase to ground voltage available (single ratio PT's). So the distribution relays of old were of a higher voltage.

In the case of the CT, the CT would produce the full current, but as you are taping at different terminals, the voltage would divide between the terminals, and in some ratio of the burden placed on it. In other words, you would get nothing meaningful from it.

 

[DTR2011]

Having the relaying and metering voltages separate (amd different magnitudes) also makes troubleshooting systems that have not been well documented (and modified) over the years a bit easier.