Potential transformer open delta measurements 2

[breaker8]

We have two existing PT's connected in open delta (connected line to line (A with B) and (C with B)) to a transmission line but we need to measure the line to ground voltage of all three phases. We were thinking of using 3 auxiliary transformers connected to the secondary of the PT's. If we connect the auxiliary transformers in a Delta - Y configuration, are we going to get the line-to-ground voltages of the transmission line?

 

[PWR]

You won't get meaningful results unless you connect your PTs (you need three) line to neutral on the primary side. Even if you had three PTs connected line to line on the primary, they cannot measure what the line to neutral voltage is. Under balanced conditions, the ratio will be 1.732 of course, but if you want to actually measure the value accurately the PTs must be connected appropriately.

 

[busbar]

Can't do it. Primary zero-sequence voltage is not reflected in the secondaries.

Or, there will be no voltage shift on the PT secondary terminals with respect to ground for a primary ground fault or other imbalance. [Believe me, I learned the hard/embarrassing way.]

 

[jbartos]

Suggestion: Reference:
Donald Beeman "Industrial Power System Handbook," 1st Edition, McGraw-Hill Book Company, Inc., 1955
Section: Y-Delta Grounding Transformers
A Y-Delta transformer can be utilized as a grounding transformer. In this case the delta must be closed to provide a path for the zero sequence current, but the delta can be made up at any convenient voltage level. It may or may not be used to serve other loads. The Y winding must be of the same voltage rating as the circuit which is to be grounded. The connection of the transformer is shown in Fig. 6.8.

Over the Grounded Y transformer winding, a phase voltage to ground can be measured for all three phases.

 

[busbar]

May I suggest that a grounded-wye—broken-delta transformer configuration will only indicate deviation from balanced phase-to-ground voltage?

Application of an auxiliary grounded–wye—broken-delta transformer on the secondary of a open-delta PT set will not reflect any primary-side phase-to-ground imbalance [zero-sequence voltage.] There is no argument that negative-sequence voltage [phase-to-phase voltage imbalance] can be sensed through an open-delta PT set.

An interesting, but very beefy 3ø auxiliary VT with a broken-delta winding is on page 22 of

http://www.geindustrial.com/products/buyersguide/GEP-9186-2.pdf

 

[jbartos]

Suggestion: Normally, three PTs with broken delta secondary are required.
Reference:
J. Lewis Blackburn "Protective Relaying Principles and Applications," Second Edition, Marcel Dekker, Inc., 1998,
Page 196
7.4 Ground-Detection Methods for Ungrounded Systems
Figure 7.5 Voltage Ground-Fault Detection Using Three Voltage Transformers Connected Wye-Ground-Broken-Delta

 

[scottf]

breaker8-

I concur that you will need 3 VTs connected line-to-ground to get a true measurement.

As a side note, what is your system voltage. I assume since you have 2-bushing VTs now and you mention it's a transmission voltage, it would be 46 kV or 69 kV, if you're in the US.

For future consideration...most folks using 2 VTs connected in broken delta, do so to save the expense from purchasing 3 VTs, with the thought that 2 pcs of 2-bushing VTs are cheaper than 3 pcs of 1-bushing VTs. I suggest that folks faced with that decision take a good look at both options. Today, at 46 and 69 kV, most will find that 3 pcs of 1-bushing units will be about the same price, if not cheaper, than buying 2pcs of 2-bushing units.

 

[breaker8]

Thanks to everyone for your help!!!