PT Configuraiton on solidly grounded System

[rockman7892]

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?

 

[HamburgerHelper]

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.

 

[cranky108]

Having PT's in delta on what can become an ungrounded system, with the secondary in broken delta is a good method of detecting a ground fault, by connecting a voltage relay across the open.

Just because something is solidly grounded, does not mean it will stay that way.

 

[catserveng]

This thread from a few years ago seemed to discuss this issue pretty well,

http://www.eng-tips.com/viewthread.cfm?qid=216248

Also a nice explanation here,

https://www.linkedin.com/pulse/tech-brief-2-problem-open-delta-potential-ben-standish-pe-cxa

Hope that helps, MikeL.

 

[davidbeach]

Don't need the broken delta connection when using decent relays; they can calculate 3V0 if given grounded wye inputs.

 

[LionelHutz]

Talking 4.16kV and end-use switchgear,

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.

 

[scottf]

Lionel-

Can you please explain how you can get an open-delta connection with a single PT?

 

[rcw retired EE]

David - We try to use the broken delta connection for bus fault detection on generator bus to minimize false trips.

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.

 

[LionelHutz]

scottf - it's an open delta PT

 

[scottf]

Interesting...those are not really around anymore in the North American market (that I'm aware of) and I don't think my company makes one for the European market. What area are you in and could you share the manufacturer/type of this type PT?

 

[marks1080]

Open delta with a single PT is a bit misleading if I am properly following you. You can achieve the same (similar) functionality using a single PT knowing that faults will either create an overvoltage or undervoltage condition for any kind of fault. This information can be used like an open corner delta. When I hear the term 'open corner delta' I assume there are three PT's available, but wired with an open corner. I consider performing this function with a single PT to be more like a 'poor man's open corner delta.' Kinda like using a ground switch for 'teleprotection' and calling it 'poor man's transfer trip.'

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.

 

[LionelHutz]

North America. GE/ITI model 3PT3 is readily available. I think there is someone else, but I can't recall who.

 

[scottf]

3PT3 is still available, which is a 5kV unit. I was thinking 15kV when I wrote the post. I don't believe any such design is available above 5kV, at least that I've ever seen. Thanks

 

[rockman7892]

So with an LRG system it looks like you can still use wye/wye-gnd PT as long as they are rated for L-L voltage.

Why do you not want to ground the wye on the primary PT on an LRG system?

 

[rockman7892]

Also will a broken delta (3 transformers) provide all of the phase-phase voltages or is it only good for providing a 3V0 voltage?

 

[rockman7892]

I just realized after I asked my question about the wye/wye-gnd PT that if the primary of the wye is not grounded then there will be no zero sequence voltage component in the secondary, nor can the relay accurately calculate this zero sequence component.

So I'm curious why it was suggested above to not ground the primary wye point on an impedance grounded system?

 

[HamburgerHelper]

If that was me, I meant wye-gnd-wye-gnd.

 

[rockman7892]

OK so even on LRG system wye-gnd/wye-gnd CT's are the preferred configuration and the PT's must be rated for L-L voltage.

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?

 

[LionelHutz]

scottf - that's why I put "Talking 4.16kV and end-use switchgear"

Besides that 3PT3 which is 350/350VA we also use a 50/50VA PT.