Open Delta PT vs Wye PT
Open Delta PT vs Wye PT
(OP)
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?
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?






RE: Open Delta PT vs Wye PT
RE: Open Delta PT vs Wye PT
Is it just Y-Y PT or earthed through neutral?? Explain how your generator neutral earthing and the grid earthing was carried out?
RE: Open Delta PT vs Wye PT
RE: Open Delta PT vs Wye PT
Very shortly.
100%+.. agree with David.
Best Regards.
Slava
RE: Open Delta PT vs Wye PT
I'm not sure I fully agree that open-delta PTs are a relic of electro-mechanical relays. This arrangement is commonly used in metalclad switchgear mainly because two PTs are cheaper than three. The open delta works OK for metering which is what they were generally used for.
But I agree that for protection in distributed generation applications, three PTs are generally going to be necessary to get useful relaying information for the reasons David Beach gave.
RE: Open Delta PT vs Wye PT
"This arrangement is commonly used in metalclad switchgear mainly because two PTs are cheaper than three. The open delta works OK for metering which is what they were generally used for."
But we can agree, for the 3xPT per BB, price isn't important . And today many designers would like used voltage information not only for metering , also for the NPS and PPS voltage meas.
Regards.
Slava
RE: Open Delta PT vs Wye PT
DPC is 100% on the money the utility is going to shove this down my throat, but their protection engineers could not tell me why they wanted Y PT's. The just kept saying "because IEEE 1547 says its required." Incidentally, IEEE 1547 isn't required by the Utility commission, whose interconnect standard does not mention PT configuration.
DB - the system is grounded, but the utility configuration is unknown. I'll know more after a meeting today.
RE: Open Delta PT vs Wye PT
Maybe I'm not understanding the configuration.
RE: Open Delta PT vs Wye PT
RE: Open Delta PT vs Wye PT
DB substantial now if you had to replace 26 PT's after the fact, parts and materials not to mention lengthy outages while the PT's are replaced. If it was done up front the cost would be minimal.
RE: Open Delta PT vs Wye PT
I'm still not clear on why 26 PTs have to be replaced?
RE: Open Delta PT vs Wye PT
If we didn't install the aux pt's the utility signal would be shifted 30 degrees due to the Y-Y vs Open Delta and all of the generators, Utility board buses and Utility breaker PT's feed the synchronizing controls. The synch controls ,possibley, could be adjusted to account for this shift but it is not the optimal solution and could create maintenance/functionality issues down the road.
RE: Open Delta PT vs Wye PT
But it sounds like you already have a solution.
RE: Open Delta PT vs Wye PT
Intresting thread.
From technical point of view.
Y-Y connection is more better, BUT NOT MUST for the genrator protection, synchronazing and metering.
As I understand, you have open delta connection on the BB too and sinchronazed genset to BB.
I think you need aux, PT for the BB PT only ( I don't know how sections you have).
voltage connection before auv PT you will use for the genset sinchro, after aux PT for the utilities infeeds.
Regards.
Slava
RE: Open Delta PT vs Wye PT
RE: Open Delta PT vs Wye PT
If TxPowerEE have " synchronizers and sync check relays can be set to expect a 30° phase difference at synchronism (actually any multiple of 30°) " it's solve all problems ( I'm hope).
Good Luck.
Slava.
P.S. In your new projects, please try use Y-Y.
RE: Open Delta PT vs Wye PT
RE: Open Delta PT vs Wye PT
Open delta gives you accurate V1 and V2, but not V0 as stated above. If it is important to know V0, such as for ground detection, then YY is required. Do you have utility ground detection presently and how is it accomplished? Does the utility have a standard for this? It is unusual for a utility distribution circuit in the U.S. to be anything but solidly grounded (there are exceptions).
Since this is a retrofit, it would be preferable to minimize the cost and retrofit YY only where required. I think it is unreasonable for the utility to say you must upgrade every PT in this existing system unless there is demonstrated a need.
I wouldn't go so far as to say open delta is a relic, there are many still being installed and they work fine for many purposes. YY is a minimal cost add for a new systems and probably worthwhile for the added flexibility even if there is not an immediate need.
RE: Open Delta PT vs Wye PT
RE: Open Delta PT vs Wye PT
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!
RE: Open Delta PT vs Wye PT
Regards
Marmite
RE: Open Delta PT vs Wye PT
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.
RE: Open Delta PT vs Wye PT
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.
RE: Open Delta PT vs Wye PT
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
RE: Open Delta PT vs Wye PT
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Open Delta PT vs Wye PT
RE: Open Delta PT vs Wye PT
I must be tired, you've lost me on this one??
RE: Open Delta PT vs Wye PT
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
RE: Open Delta PT vs Wye PT
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.
RE: Open Delta PT vs Wye PT
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.
RE: Open Delta PT vs Wye PT
RE: Open Delta PT vs Wye PT
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
RE: Open Delta PT vs Wye PT
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.
RE: Open Delta PT vs Wye PT
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,
RE: Open Delta PT vs Wye PT
RE: Open Delta PT vs Wye PT
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.