PT connection for 47 relay 1

[alehman]

Is there any advantage of (3) PT's connected wye-wye versus (2) PT's open delta - open delta for use to detect phase imbalance or open an open phase utilizing a negative phase sequence overvoltage (47) relay? A client contends that if the center phase at the PT primary is open, the relay will not respond.

There is an article on Basler's web site that touches on this and seems to indicate the OD-OD configuration would be fine. Also some relay mfg's show examples with OD-OD. I was hoping to find a detailed discussion or mathematical treatment of the subject.

 

[busbar]

 
The open-delta PT connection will correctly reflect negative-sequence voltage {ø-ø} but not zero-sequence voltage {neutral shift.} Negative-sequence overvoltage is the measured quantity for a 47 device, and not zero-sequence voltage. Id est, it is a “3-wire” device and not a “4-wire” device. The client’s concern seems misguided. Don’t know of a negative-sequence voltage relay the uses/requires a wye-neutral connection to correctly operate.

http://www.abbus.com/abbrelays/pdf/ILsIBs/Allentwn/74172e.pdf

pg 5

http://www.geindustrial.com/products/manuals/icr/geh1783.pdf

pg 6
Neither has a 4th-wire Y-neutral connection and both show typical open-delta PTs

 

[alehman]

Busbar: Thanks for the info. The 47 relay is the ABB device in your link. It responds to undervoltage of the difference |V1| - |V2| (scalar quantities only as best as I can discern). In the case of a totally open phase, |V1| should =|V2| so the relay would see zero volts and actuate. As you stated, the relay theoretically ignores V0.

I guess the question is how can I show the OD PT's will correctly reproduce the negative (and positive) sequence voltages? I'm going to start reading C57.13 unless there is a better reference. Blackburn doesn't really get into PT connections in much detail.

 

[busbar]

 
PTs connected in a symmetrical arrangement like delta—delta, open-delta—open-delta, ungrounded-wye—ungrounded-wye or grounded-wye—grounded-wye should all reflect the ratio of primary phase-to-phase voltages into the secondary windings scaled by PT ratio—that is, the ratio of positive-to-negative-sequence voltage. [Grounded-wye—grounded-wye also reflects zero-sequence voltage, but is of no concern here.]

Doesn’t a zero-volt condition on one phase equate to zero positive-sequence volts and one per-unit negative-sequence volts, such that the difference is one per-unit?

In the ABB 47 instruction leaflet, the acceptance-test setup uses two 120V sources with a variable angle between them, to simulate typical conditions on PT primaries.

There is some discussion of open-delta PTs in chapter 2 of

http://www.geindustrial.com/products/applications/GET-97D.pdf

 

[busbar]

C57.13-1993 figures 6a and 6c show delta/open-delta PT connections for Group I and Group 2 PTs, intended for phase-to-phase-connected primaries. There is some discussion in §2 of ANSI/IEEE C57.13.3-1983 Guide for the Grounding of Instrument Transformer Secondary Circuits and Cases

Note that the ABB I-T-E 47 instruction leaflet recommends open-delta PTs for use with their negative-sequence voltage relay, as does GE for their ICR relay.

Also, the links in my 11-Aug post are broken.

http://www.abbus.com/abbrelays/pdf/ILsIBs/Allentwn/74172e.pdf

http://www.geindustrial.com/products/manuals/icr/geh1783.pdf

 

[alehman]

I can't match your results for V1=0, V2=1, but the condition was for one phase open, not 0 volts.

Taking Va as 1.0<0,

With C phase open for example, Vc=Vb=1.0<-120 and V1=V2=0.577<30.
If it is B phase, then Vb=0.5<60 and Vc=1.0<120 and V1=0.5<0, V2=0.5<-60.

 

[busbar]

Yes, I agree. My apologies.