PT Configuraiton on solidly grounded System
PT Configuraiton on solidly grounded System
(OP)
For MV Switchgear PT's on a solidly grounded 4.16kV system it looks like the most common two options are Open-Delta/Open-Delta or wye-gnd/wye-gnd.
I've always heard that the two winding open delta configuration is cheaper and is therefore the more commonly used configuration (I'm curious how much cheaper these typically are).
In terms of diagnostics however it sounds like the wye-gnd PT's are a better option as you look at L-N voltages and all of the sequence voltages and currents when looking at relay event reports or PQ metering.
So is this usually a decision made based on cost or the available switchgear features such as relays, metering types, etc...?
What about resistance grounded systems? with a wye-gnd PT the windings would need to be rated for L-L voltage so I'm not sure if these are commonly used on LRG systems? I have seen "Broken Delta" PT's used on a few of these systems.
Does anyone know of a good reference that discusses the various PT configurations for different system applications?






RE: PT Configuraiton on solidly grounded System
I am unsure of why anyone wouldn't want wye-gnd wye-gnd relays with microprocessor relays. The relay can filter out any sequence component or harmonic. We use deltas to filter out the zero sequence for transformer differential. Inside of plants with high resistive grounding, I have only seen YYgnd PTs. We have broken deltas in places that might lose their ground source and be fed by generation but that is only because that voltage was being fed to a mechanical relay. For event analysis, I would like to see LN instead of LL values. I think I have only seen something other than YYgnd when you wanted to filter out or feed a sequence component to a mechanical relay.
RE: PT Configuraiton on solidly grounded System
Just because something is solidly grounded, does not mean it will stay that way.
RE: PT Configuraiton on solidly grounded System
www.eng-tips.com/viewthread.cfm?qid=216248
Also a nice explanation here,
https://www.linkedin.com/pulse/tech-brief-2-proble...
Hope that helps, MikeL.
RE: PT Configuraiton on solidly grounded System
RE: PT Configuraiton on solidly grounded System
Open delta can be done with a single PT while wye has to be done with 3 PT's, This means the the wye option takes up about 4X to 5X as much physical real estate and costs about 3X as much. These reasons make open delta fairly popular for metal enclosed switchgear. Metal clad generally had a PT drawer so using wye just costs the extra for two more PT's.
Any time I've done wye PT's they were L-L rated, regardless of the system being solidly or resistively grounded.
RE: PT Configuraiton on solidly grounded System
Can you please explain how you can get an open-delta connection with a single PT?
RE: PT Configuraiton on solidly grounded System
Even though the relay can do the 3V0 calculation, if any one of the three wye-ground signals gets affected by an electrician or relay tech, the generator trips. The two wires on the broken delta windings only go to the one multifunction relay input. The three VT wires for the wye voltages go to several relays, meters, synchronizers, regulators and fault recorders with many opportunities for a spurious dip in voltage that creates a false 3V0 signal.
For the price of two wires, we reduce false trips during commissioning and performance testing.
But a loose VT primary fuse connection still can trip the unit.
RE: PT Configuraiton on solidly grounded System
RE: PT Configuraiton on solidly grounded System
RE: PT Configuraiton on solidly grounded System
With modern relays, wire everything into the relay WYE. Modern relays wired up like this can still access and use all 3V0 or delta or open delta quantities like David mentioned.
With older relays I've often used the rule of thumb that "the secondary cct is opposite the primary" Meaning if you have a delta primary system, the secondary ccts should be WYE. If you have A WYE primary system, the secondary ccts should be delta. I'm not sure if this is 100% always true, but it's been true for all the 'older' systems I've worked on.
RE: PT Configuraiton on solidly grounded System
RE: PT Configuraiton on solidly grounded System
RE: PT Configuraiton on solidly grounded System
Why do you not want to ground the wye on the primary PT on an LRG system?
RE: PT Configuraiton on solidly grounded System
RE: PT Configuraiton on solidly grounded System
So I'm curious why it was suggested above to not ground the primary wye point on an impedance grounded system?
RE: PT Configuraiton on solidly grounded System
RE: PT Configuraiton on solidly grounded System
Do wye/wye PT's help mitigate potential ferroresonance issues with PT's or is this not as much an issue as it is with large distribution transformers?
RE: PT Configuraiton on solidly grounded System
Besides that 3PT3 which is 350/350VA we also use a 50/50VA PT.