Voltage Factor of PTs for Extended Periods

[Kiribanda]

Gentlemen,

On a 15 kV ungrounded system,it is normal to use STAR GROUNDED-OPEN DELTA VTs to obtain 59N function.During a ground fault the STAR winding is subjected to full line to line voltage (in the worst case) and continue to operate till the fault is found and cleared. The time taken to clear the fault by the maintenance crew may be 2-3 hours or even 2-3 days especially in remote unmanned subs.

Voltage factor of most VTs are 2xCONT/1.73x60sec. As such, is it possible to buy VTs with a voltage factor of 1.73xCONT to cater for such extended periods of fault clearing? Could anybody please mention about a known manufacturer's web site or contact?

Kiri

 

[RRaghunath]

IEC 60044-2 specifies voltage factor for VTs in system that doesn't have automatic isolation on earth faults:
1.2 continuous and 1.9 for 8hrs. I think this is good enough for isolated systems.
If the fault is allowed to stay longer than 8hrs, it could cause many other problems including failure of major equipment due to arcing grounds or the earth fault turning in to a phase-to-phase fault and then automatic tripping taking place.

 

[davidbeach]

Applying VTs on an ungrounded system, I'd always use VTs with a primary voltage rating equal to phase to phase voltage rather than phase to neutral.

There is no point connecting a 59N relay to an open delta set of VTs, and no particular means of creating an open delta on the secondary of three wye connected VTs. What you probably meant is a broken delta secondary. Open delta and broken delta are two distinct and quite different connections. Broken delta is very commonly used for 59N and open delta has no V0 information.

With modern numeric relays there is no need for the broken delta, the relay can calculate the V0 value.

 

[Runsor]

For example ABB, at least here in Scandinavia, is using the term "open delta" instead of the correct term "broken delta". While distribution transformers here are always three phase transformers, there has not been much confusion.

When two voltage transformers are used in open delta, it's then called "V-connection".

Sum summarum: When somebody is measuring residual voltage using "open delta", he actually means "broken delta".

 

[davidbeach]

The problem with "V-connection" is that it could be either an open delta or an open wye. Why confuse things? There are two perfectly good, concise, descriptive terms for two different connections; one is a broken delta and the other is an open delta. How difficult can it be to use the proper term? Isn't precession something we should be striving for as engineers?