PT Phase Shifting Problem

[eglaude]

Hello All,
The project is to refurbish a large 5500HP,4kV Brushless Sync motor and convert the motor protection controls from the induction discs to our standard relay protection scheme for MV sync motors using the Multilin SR469/SPM relays.
The 4kV motor is fed directly from a captive stepdown transformer, DELTA/Y, stepping down a 13.2kV feeder to 4.16kV. For motors of this size, it is typical for us to use a 13.2 feeder breaker as the "starter".
The problem that I am trying to avoid is installing a 4.16kV/120V PT on the 4kV bus to control power factor of the sync motor. I have no choice but to install CTs on the bus phases, A&B per the SPM, but I was hoping to use an existing PT that is located in the swtichgear where the 469/SPM will be located. The file attaced is a schematic of the OPEN DELTA PT configuration. It is an old peice of gear as you can tell.
Since the power transfomer will be phase shifitng by 30deg on the secondary side, and the phase current CTs for the motor will be mounted on the secondary bus, is there any way I can use the existing 13.2k/120 PT voltage and make the SPM think that it is monitoring the 4kV side?
Note the polarity marks on the PT diagram are wired somewhat different that a typical OPEN DELTA. I'm not quite sure if the resultant is from this PT is 120 Va & Vb or Va & Vc?
Thank you for insight and help.
-Eric

 

[dpc]

Without getting into all the messy details of this particular design, you can purchase auxiliary PTs (1:1 ratio) and use them to correct the phase shift difference created by the power transformer. The aux PTs are not too expensive. I have done this before.

Also, the PTs are not even really necessary for the basic protective features in the 469. You lose the voltage functions, obviously as well as power related functions, but the critical functions are strictly current-based.

 

[Skogsgurra]

I have a similar problem with an installation where the PF controller is connected to Y connected PTs while the cos(phi) transducer expects D connected PTs. The result is that the motor can only be run with a capacitive PF, which is not normal. As soon as mains voltage goes up and the PF hence shifts to inductive, the cos(phi) transducer gets saturated and the whole loop PF drifts into low excitation so that the machine trips. Happens every eight minute. Guess what happens to the 12 MW motor..

Anyhow, trying to keep things short, I don't tell about the repairs and the discussions we have had. I have told these guys that they have to change the PT connection - which isn't easy since there are other things connected as well. So, my next way out would be to put a couple of intermediate PTs and connect them to correct for the false 30 degrees that we have now. It will be a small cost compared to another motor repair. Had three of them already - totally unnecessary.

So, a set of PTs on the 110 V side connected to turn phase back to true angle is what I would try.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Skogsgurra]

We seem to agree here, dpc!

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[dpc]

Great minds think alike.....