Open Delta PT vs Wye PT 1

[TxPowerEE]

Hello all,
My first post here, so if I leave anything out let me know. I am dealing with a utility that is demanding we install wye-wye pts for protective relaying in our gear. We are connecting to the utlity with 5 2.2MW gens at 13.2kV. We have capacity for up to 12. The system will be paralleled for several minutes but will never export power.

The utility is demanding the y-y PT because of a requirement in IEEE 1547. Their argument is that if a phase upstream of my facility (on their grid) goes to ground and the gens are online the y-y pt is required to capture a voltage sag due to the ground fault. The protective relay is a GE750. I get the sense they are demanding this only because its in 1547, they can't explain to me why the y-y is superior in this instance to the open delta.

My questions
1) Is there any advantage of the y-y over the open delta?
2) Looking for a voltage drop to clear a ground fault seems like a rather round about way of clearing a ground fault. Wouldn't is just be better to use the 51g setting in the relay to clear the ground fault? Or possibly the residual ground fault settings?

 

[TxPowerEE]

A pdf of the system is attached with identifying info redacted. The utility told us about their requirements or new requirements nearly 2 years after the system was installed. As stated earlier they reviewed and approved the drawings before the system was manufactured. TX PUC does not specify requirements, the PUC only states for a system this size a mutually agreeable interconnect design is required.
REM76 - we are using the 67N but the utlity is still requiring(AKA dictating) phase to ground sensing.

Also I know we could probably adjust for the phase shift but the aux PT's are so inepensive and it's a cleaner solution, especially for commissioning, maintenance and troublshooting in the future.

To all thanks for the input!

 

[Marmite]

Bearing in mind the short duration of the parallel and lack of export it does seem heavy handed by the utility. How about suggesting a timed interlock that breaks the parallel after a set time, say 5 mins? The risk of backfeeding into a fault on their system in that case would be minimal.
Regards
Marmite

 

[rcw retired EE]

I recall that Y-Y PT connections can be susceptible to ferro-resonance and that open-delta PT's are not. Is my memory fuzzy or is this true?

We usually apply a Y-Y set of VT's with dual secondary windings, one connected in grounded wye and one wired broken delta with a loading resistor. The broken delta with resistor minimizes ferro-resonance and gives a 3Vo voltage for ground detection.

 

[rbulsara]

Txpoweree:

Just went thru this with SR 750 on a gen plant similar to yours. Read page 5-46 of the manual. Y-Y is necessary to use the voltage as the polarization quantity for directional ground fault function, 67N, which most utiltiy co. would require. Current polarization is usually not possible for most configuraiton with generator plants ).(it may works for a solidly grounded transformer source if within reach). Impedance grounded system are even worse with delta PTs. Y-Y solves all these head-aches at very minimal price, if implemented initially.

We resolved the 67N issue by using very sensitive non-directional GF setting for the duration of closed transition, instead of 67N.

 

[slavag]

Hi.
RCWilson:
"We usually apply a Y-Y set of VT's with dual secondary windings, one connected in grounded wye and one wired broken delta with a loading resistor. The broken delta with resistor minimizes ferro-resonance and gives a 3Vo voltage for ground detection".
Yes, it's common practic, but newer relays are include option build broken delta internally on Y connection base and 3I0 too.
As far as I know, SEL also have option of NPS voltage detection ( maybe GE too) 32Q ( I hope, it's right ANSI code) protection for the GF. Actually possible build internally U2 on the open delta connection base. This functionality recommended by SEL for the solidly and low impedance systems.
What do you think?
Best Regards.
Slava

 

[waross]

How about using the existing PTs and setting the protection relays for 69 volts?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[scottf]

waross...as long as not using for revenue metering, that should be perfectly acceptable.

 

[TxPowerEE]

waross
I must be tired, you've lost me on this one??

 

[waross]

If you connect the existing PTs in wye instead of delta, the nominal output voltage will be 69 volts.
See if your relays will accept a 69 volt input.
You may be able to use the existing PTs at 69 volts.
Can you comment n this scheme davidbeach?
Thanks.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

waross:

If exiting arrangement is open delta as OP said, there are only two PTs. You can't just add a third PT on the 15kV trunion. It needs to be replaced with a 3-pT trunion which would also require rewiring of intruments.

It is not as easy at it sounds, plus all the political issues it creates.

 

[rbulsara]

I am also not sure why so many PTs needs to be replaced??? Is there a oneline for this?

Is there a common gen bus which then parallels with the utility via utiltiy mains or each gen is connected to individual utility service?

The utiltiy co. is only interested at the intertie connection which usually is the utility main breaker.

Plus answer to OP's original question is on Page 5-46 of the manual for SR-750 relay.

 

[scottf]

A 2-bushing VT with a "line-to-line" ratio with a 120 V rated secondary can be connected line-to-ground and will have a 69 V rating on the secondary. As is mentioned, 3 VTs per point would need to be used.

 

[slavag]

Hi.
RBulsara:
"I am also not sure why so many PTs needs to be replaced??? Is there a oneline for this?
Is there a common gen bus which then parallels with the utility via utiltiy mains or each gen is connected to individual utility service?
The utiltiy co. is only interested at the intertie connection which usually is the utility main breaker."
TxPowerEE :
From your SLD not so clear , are you have VT's on the BB or not.
If not, I see two options for you (of course, if your utilities will request 3xphase to ground VT's mandatory).
1. Replaced 4 sets of utilities VT's and add aux VT's for the phase siffting and synchronazing purpose.
2. More problematic option,but save for you many changes in the secondary wiring. add additional set of 3xVT's for the protection only. Of course it's depend on the place.

Next option, poor option, but also possible use ( from time to time we used in old application).
If your 4 new relays installed on the same bus with generators, you can add one common set 3xVT's on the BB and connect it in parallel between all 4 utilities CB's, original VT you will use for all other porpuses.
Best Regards.
Slava

 

[rbulsara]

I think most of the posts are going on a tangent here. It is not the question of voltage, etc.

The original question is why the utiltiy co is insisting on Wye PT. The answer to that is that the 67N fucntion is the GE SR-750 relay would only work with Wye PT using voltage as the polarization source or an independent current polarization source for detecting revesal of ground fault current. There is a very expensive way to get the current polariztion source but PT is much easier and preferred by the utilties.

There are ways around meeting utiltiy's concern but they have to be worked out with them, not here. See my first post in this thread.

 

[busbar]

It may be helpful to distinguish , in ANSI parlance, the difference between “open delta” and “broken delta” (“Broken delta is the equivalent of a 3V0 filter where PT primaries are configured in grounded wye.) From IEEE C57.13.3,

 

[davidbeach]

Nice diagram busbar.

 

[JJHorak]

I strongly concur with the previous statements that phase to ground VTs are very important for ground fault monitoring. Even if you have a system where it is possible to sense ground faults via current measuring schemes, it is great to have a backup scheme.

Four more reason to avoid open delta VTs that I want to point out:
1) In directional (67) relays, the phase angle between Vo and Io is a means used for for direction to fault analyis, With phase to phase VTs, you loose Vo and a method of sensing direction to fault. Even if you have other schemes, such as negative sequence directional logic, there are some cases the other schemes do not work as well as Vo vs Io.
2) If you have any Io flowing in your load, then you loose the power component associated with Io in your metering. i.e., an alternate equation of 3 phase power is
P = 3*(V1xI1 + V2*I2 + V0*Io). If there is no Vo measurement, you loose the third element of the equation.
3) You loose accurate per-phase power mesurement. You can't do a good job of Ia*Van when all you have is Vph-ph.
4) Event reports are harder to understand with open delta VTs. If you have an A phase to ground fault, and all you have is Vab and Vca, you are guestimating what happend to voltage on phase A.

My experience with phase to phase VTs has generally been a bad one relative to phase to ground VTs.