PT Ferroresonance vs Modern SEL relay

[Coco_HueHueHue]

Hello,

I am working on modifications onto an old protection panel.
There are 80 ohms resistors in parallel (Y-G) of the Y-G / Y-G PTs secondary on each phase (primary is 3ph/3wire 25kV and directly connected to the utility). This is mentioned as ferroresonnance protection and it makes sense. The old mechanical voltage relays are being replaced with a new SEL-751 relay and I have seen that setup in the past. Does someone know if the resistors are still needed? Can the digital relay compensate for that and prevent ferroresonance or if the relay technology has zero effect and the resistors are still required?

I didn't work much with MV in the past (more LV only) and I don't want to remove the resistors if they are necessary, but I sure want to throw them in the bin if they are not!

Thank you!
Coco​

 

[thermionic1]

We have several thousand distribution subs in our system. Feeder protection is either CO's or SEL. We started putting SEL 251's in the early/mid 90's, now we use 351A's. I don't recall every seeing resistors on any of our PT circuits, nor do I recall any issues with our Y-Y PT's. Nearly all of our stations have breakers or reclosers. In the field we may have single phase switches. We have 25kV, 13kV and some lower voltages here and there.

 

[Coco_HueHueHue]

Yeah, thanks for you input.
I will see if other people have different view of this. But, in SEL-751 instruction manual, I found zero mention of the word "resistor" in regards to PT connection and the word "ferroresonance" isn't mentioned once. I am starting to be convinced that those resistors might not be necessary anymore.

Coco

 

[RRaghunath]

Star/Broken Delta connection of PTs is used for sensing Ground faults by measurement zero sequence voltage.
In such case, a resistor is placed across broken delta secondary terminals of the PT.
I haven't seen resistors across secondaries of individual phase PTs.

R Raghunath

 

[cranky108]

Are these wound PT's, or CCVT's? Or is there a condition where there is significant capacitance to interact with the PT's? In either case the resistors maybe required.
It sounds like what you are doing is reducing the burden on the PT's, so they may need to be larger, if the issue is still present.
Then again, the broken delta PT windings would still need a resistor. But do you still need this? Maybe not if all your relays are being changed to SEL's.

 

[waross]

Do I understand the question.
Are you asking if ferro-resonance may be mitigated by replacing relatively low rsistance devices (resistors) with very high impedance relays.
Does the question suggest an answer?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Coco_HueHueHue]

Hi Waross,

Sorry if my question isn't clear.
My question is mainly: Should I keep the resistor previously installed in parallel of the PTs secondary when replacing the old electromechanical relays by the new SEL-751 relay. Can the SEL-751 relay prevent the ferroresonance phenomenon?

Thanks!
Coco

 

[dpc]

I'd suggest checking with SEL. They are generally pretty helpful. I don't recall ever seeing resistors on the secondary of Y-Y PTs. They are commonly required for BROKEN DELTA ground detection schemes.

 

[scottf]

I've never seen resistors placed in parallel with any kind of burden.

The only thing I can think of is if the resistors are there in case the unit is energized with the relay isolated out of the circuit (Open test switch for example) and the resistors would be left to give the VT some load. Unloaded VTs are more prone to ferroresonance.

 

[Coco_HueHueHue]

This comes from a Schneider Cahier Technique.
The exemple in figure 11 is exactly how its done in the panel I am working on. The cahier technique even has explanations on how this setup prevents or damps the phenomenon of ferroresonance. However, I understand the setup at figure 12 is said better than the one at figure 11.

I guess I will keep the resistors. They will not be any kind of problem for the new SEL-751 and at worst they will stay there for nothing. At best, they will keep preventing any kind of ferroresonance by keeping a constant load on the VTs.

Thanks for you input all of you!
Coco

 

[scottf]

Do you have the previous page available? Would like to see what it says leading into the paragraph at the top of the page you posted.

 

[Coco_HueHueHue]

Sure, here it is.

 

[Coco_HueHueHue]

For everyone's interest (who are interested on the subject). SEL wrote to me to keep any ferroresonance protection if there was any as the relay does not prevent it in any way.

 

[wcaseyharman]

I’ve installed resistors that way exactly once.
Using aux PTs and a broken delta connection seems much preferable as it doesn’t load the PT under normal operation and possibly reduce the measurement accuracy - the PTs would only be loaded under ground fault conditions.

Edit - as an aside I plan to do exactly that next week for a small gen - we have some spare aux PTs that we’ll wire up to a single 120 ohms resistor. Right now it has YY PTs and I’m going to add a set of aux PTs in broken delta.

 

[3I0]

I have encountered two different installations with Yg-Yg PTs and no damping resistance connected to the secondary that went into ferroresonance while they were running ungrounded on the high side (normally they were connected to high impedance grounded systems) and a transient occurred to start it. In both cases protections tripped on volts-per-hertz (24) elements; looking at high sampling frequency secondary waveforms it was probably for the best though as the voltage was significantly elevated (>2x nominal peak voltages).

The resistors are there to protect the PTs and have no bearing on the relay, provided your PTs have sufficient burden capability to drive them without causing a significant steady state voltage drop. Agree with wcaseyharman that aux PT's with resistance in the broken corner delta secondary is the preferred way to go though versus introducing steady state loading and heat generation.