Phase-Ground VTs on Delta Transformer

[SSLA]

Folks:

We have a project at substation where the transformer have delta secondary windings. The windings supply metal clad switchgear through U/G cable (PILC would be my guess by observation). The metal clad supplies a 13.8 kV distribution system. There is no sign of a grounding reactor at the substation but there may be one in the metal clad (I have been unable to find a single line for the metal clad but one is on the way).

The transformers will be upgraded in two years and may be changed from delta to wye (requiring all of the buswork and protections to be re-done, I would guess).

We want to meter on the LV side of the power transformers using standard three element metering. This will measure properly and allow use of easily available off-the-shelf VTs.

But...the VTs would be connected phase to ground. Every thing else is connected phase-phase. There is a ground detection system, indicating voltmeter and annunciation. No trace of automatic protection that I can find as yet.

The maximum voltage on the system is 13.8kV + 6% for ULTC = 14.63kV. The VTS are rated 15.96kV for 30 seconds, so they will stand up to full phase-phase voltage ... for a little while.

Would anyone have any experience hooking VTs phase-to-ground on a delta system. The VTs will supply revenue metering only about 6 VA max.

Is it just too risky since grounds may hang on longer than 30s?

Any advice?

 

[scottf]

SSLA-


The configuration you describe lends itself to ferroresonance problems, depending on the overall system configuration.

I can't think of any reason not to use 2-bushing VTs with the primaries connected line-to-line, i.e. in a full delta. The cost difference is negligible and they are just as available as L-G VTs.

 

[RRaghunath]

The 13.2kV system described seems to be an ungrounded system. Most probably, the existing VTs for earth fault detection and annunciation are also connected in Star on primary and either star or broken delta on secondary side.
The disadvantage if you go with Y/y connected VTs with star points on primary as well as secondary earthed (in ungrounded system) is that a L-G fault on secondary may be seen by the earth fault detection system as 13.2kV earth fault and cause alarm. I don't see any other issue with the proposed arrangement.
There is a practice to earth v-phase on VT secondary that solves this issue.
IEEE 242 (Buff book) includes discussion on this.

 

[scottf]

raghun....I understood from the original post that the VTs were to be used for metering. Most modern metering schemes will be grounded-wye connected secondaries.

 

[slavag]

Hi.
Scottf are right, phase to ground VT is ferroresonance issue or some another kind of problem.
please see one our case.
Bushing line to line VT is good solution or VT with broken delta winding with dumping resistor.

 

[pwrtran]

The system is a 3-wire delta. If you make PT's connected in ground Wye fashion what could happen is when there is a L-G fault on 13.8 downstream, there should be no fault current before but since you make the PT as a grounding bank and it will blow up your PT'S.

So you may better use 2-element metering or add a grounding transformer at the station.