PT Configuration 1

[charz]

Have any body come across the PT configuration as shown in the attachment. I haven't seen this type of PT configuration. What configuration is this? Why is it used? Why "B" Phase is earthed?

 

[davidbeach]

Probably used on a generator application. Not sure about all of the details, but that grounded B phase is common as it reduces the possible secondary faults that can show up as ground faults on the primary and trip the unit. If the wye point is grounded, any phase to neutral on the secondary will show up as a primary ground fault. With the B grounded it would only be a B to neutral fault. Besides that, the grounded B is the same as what you'd have had with a open delta set of VTs. Add the third VTs, wire them in wye, ground B and the old instruments don't see any difference.

 

[stevenal]

I see dual grounds on the B phase. Multiple grounds have been known to cause problems.

 

[ScottyUK]

This configuration is very common on older British gear and still fairly common today throughout the distribution industry, perhaps for consistency with the legacy installations. It's used in general distribution as well as for generator VTs.

 

[charz]

"but that grounded B phase is common as it reduces the possible secondary faults that can show up as ground faults on the primary and trip the unit."

Could you elaborate that sentence. I failed to understand. The basic principle of PT is that it should not be short circuited to
earth. Just for Imagination, if the neutral of the secondary is not grounded and the phase is grounded, Can I imagine it like a single line to ground fault on ungrounded system.

 

[davidbeach]

Grounded wye-grounded wye VT connected to an ungrounded system with a secondary fault between phase and neutral in the control wiring. That will produce a certain level of 3V0 that can be detected and may result in the unit being tripped. That can happen for an A-neutral fault, a B-neutral fault, or a C-neutral fault. The secondary must be grounded, exactly once, for safety; so if a grounded neutral carries risk for any phase-neutral fault is there a better approach? That better approach is to ground one phase instead. With all four wires brought to the relay, it can't tell any difference between the two grounding methods for anything that happens on the primary. But on the secondary, the only control wiring fault that can produce 3V0 is a fault between B and neutral. A more secure design without reducing dependability. The biggest potential downside is someone unfamiliar with the scheme coming along later and "fixing" it in the field.

 

[charz]

"But on the secondary, the only control wiring fault that can produce 3V0 is a fault between B and neutral."

This concept is new to me and pricking me hard. Would be grateful if you could clarify.
Under normal conditions at PT secondary , Van + Vbn + Vcn = (1∠0 + 1∠-120 + 1∠120 ) = 0 Volts,
Under control wiring fault between phase "a" and "Neutral", Van + Vbn + Vcn = (0 + 1∠-120 + 1∠120 ) = 1.732?-150.

If secondary "a" to neutral fault occurs - It means secondary "a" Phase is completely short circuited. I suppose this will prompt the primary "a" phase fuse to blow. On the other hand If the secondary "a" to ground fault occurs, this will blow the secondary "a" Phase
fuse to blow. So if any of the fuse blows, it will result in 3V0 operating 59G element.