Ferroresonance on 6.6kV, Y0Y0 connected Voltage Transformers 2

[rex2704]

I work for a utility in Canada. I will first give some background on our network in question…

We have 9MVA, 66kV/6.6kV, YND1 transformer. The HV side has a transmission line with the circuit breaker about 100kM away. The breaker probably performs only 3-pole operation. The LV side of this transformer is connected to 6.6kV switchboard with about 50ft cable. On this LV side we have 3 single phase voltage transformers (VT) connected Y0Y0. These VTs have single secondary winding and are very lightly loaded (1500VA VTs with burden of <5VA connected on them). The VTs have primary fuses mounted on top. These are indoor type VTs with metering accuracy (0.3WXYZ) and rated for 7.2kV. I do not have their knee point voltage information.

My questions are…

Should we be concerned about ferroresonance issues with these VTs when energizing the main transformer from the HV side breaker? I am asking this question specifically for the VTs and not the main transformer. Main transformer, by the way, is energized without any load connected to it.

If there is a likelyhood of ferroresonance, then would it suffice to have VTs rated for line-to-line voltage with a knee point of about 2p.u.? or do we have to replace these VTs with an additional open delta winding and damping resistors?

There are two 6.6kV generators connected to the same switchboard. They have high impedance grounding on them. So when any of the two generators is running, the 6.6kV side is not completely ungrounded.

I have read quite a few papers published on this subject (I found was Schneider's Cahier Technique document to be the best) and I am getting an impression that ferroresonance should not be an issue at 6.6kV voltage levels, especially with such a small ungrounded system.

I have always found this forum to be very helpful and there are some real experts here who are willing to share their knowledge. I am looking forward to some good responses.

Thankyou everyone in advance!

 

[slavag]

Hi Rex2704.
Before two days, we had ferrorsonance on the 11kV.
We had intresting thread on the issue in the Forum.
Topicstarter was Herivelto Bronzeado, try found here.

You have a "good chance" for the ferroresonance with this short cable from trafo to switchboard, a low value of capacitance. As I understood , your VT before CB ( for the synchronazing issue).
But after connection to load it will work fine.
We have a same problem now, VT w/o broken delta connection.
If you have option for the replace VT to VT with broken delta connection and add dumping resistor, it will added value, but..possible will not help in all cases.

Best Regards and Good Luck.
Slava

 

[slavag]

Hi.
Please find attached DR of "fault"

 

[rex2704]

I would appreciate opinions from a few other experts too.

Thanks!

 

[scottf]

rex2704-

Best option would be to buy a 2-bushing VT rated for line-to-line voltage. This will significantly reduce the chance for ferro-resonance. This will be more cost-effective (and in reality possibly more technically effective) than trying to buy a PT with 2 secondary windings.

 

[slavag]

YES, Scottf is 100% right.
A best solution is used phase to phase voltages.
Best Regards.
Slava

 

[busbar]

 
A good general reference on ferroresonance is IEEE C57.105, Guide for Application of Transformer Connections in Three-Phase Distribution Systems. §8 lists grdY-grdY and grdY-Y as some of the configurations least susceptible to ferroresonance.

 

[rex2704]

Thank you all for your responses.

I also felt that line-to-line rated VT should be sufficient, if at all there is a risk of ferroresonance. From your comments (scottf and slavag) it appears to me that this solution is even better than having a dual secondary winding VT. I believe that the point you are trying to make is about the fact that line-to-line VT will, on most occassions, not saturate and ferroresonance cannot occur if VT does not saturate. Is this what you meant?

Scottf, you have also mentioned about buying '2-bushing VT'. My switchgear is indoor and so I was considering an indoor VT, which typically does not have bushings. I am assuming that this should not be an issue.

Thanks, once again!

 

[scottf]

rex2704-

"2-bushing" can still apply to indoor VTs, as it means that both the H1 and H2 terminals are insulated for line voltage. Typically, 2 bushing VTs will have 2 primary fuses, but you can get them with only 1 primary fuse, which is what I suggest you do. You do not want a fuse on the H2 bushing/terminal, which will be connected to neutral.

Also, the use of 2-bushing, line-to-line rated VTs is recommended because if there is a line to neutral fault, the highest that the neutral can float up to is 1.73 times the line-to-neutral voltage. Using a line-to-line rated VT will be rated to handle this on a continuous basis. But yes, it is to ensure that the VT does not saturate. In the case of VTs, ferro-resonance is started when the VT is driven up to saturation, which turns it from a high impedance across the primary winding, to a low impedance across the primary winding.

 

[rex2704]

Thanks Scottf.

I agree with all your comments. I am looking at having only one primary fuse per VT.

When I read the term 'bushing' I thought that you meant a VT with those porcelain or epoxy insulators on top (which you get on outdoor VTs). At 6.6kV, indoor VTs have a bolted terminal right on top of the VT without any additional insulator bushing. That was my confusion.

 

[Bronzeado]

rex2704,

I would say that, technically, line-to-line rated VT connected phase-to-ground, with the knee at 1.2 pu is a good choice.

I wonder if you could post later the additional cost (%) for the new VT. Thank you!

Regards,

Herivelto Bronzeado

 

[scottf]

2-bushing VT with 1 fuse should be roughly the same cost as a 1-bushing VT. $10-$15 at the most difference.

 

[Bronzeado]

scottf,

Thank you for the information on price.

Remember that we may loose some informatiom on phenomena related to sequence zero by using VTs connected phase-to-phase.

Regards,

Herivelto

 

[scottf]

Bronzeado-

In this case, no one is suggesting to connect the VTs line-to-line, but rather use VTs rated to handle line-to-line voltage connected line-to-neutral.

 

[Bronzeado]

scottf,

Sorry! I misanderstood.

I thought 2-bushing VTs were those that have 2 equal bushing connected between phases. We have bought them in the past. What really is a 2-bushing VT?

By the way, we have the same problem as rex2504 has. Our VTs in the 69kV delta system are rated as 69/root3. This system (69kV) has a zig-zag reactor to create a grounding reference.

Sometimes it is necessary to take out the zig-zag reactor for maintenance and, then, a 3V0 protection scheme is switched on. In this case, if a ground-fault occurs, the VT may be submitted to a phase-to-phase voltage. Thats is why I think the VT should be rated for phase-to-phase voltage.

Best regards,

Herivelto Bronzeado

 

[rex2704]

Bronzeado, I am in a process of procuring these VTs. I am looking at ABB, SADTEM and GE as my options. I will post the p.u. cost when I get it. By the way, I am considering VT with a voltage factor of 1.9UN, 8hrs. The VT is 7200/120V. So 1.9UN (UN=4157V) will be about 7898V which is roughly 1.2 times my system voltage of 6.6kV.

Scottf, I have found three different types of VTs. First type is rated for line-to-neutral voltage only. So this VT cannot be subjected to line-to-line voltages. Second type is rated for line-to-line voltages. It is fully fluxed to line-to-line value and can be connected either line-to-line or line-to-neutral. But it will withstand line-to-line voltage. This type can be purchased with one primary fuse or two fuses depending upon how you want to connect it. The third type is rated line-to-line but it MUST BE CONNECTED LINE-TO-NEUTRAL ONLY. This VT is rated for line-to-line voltage but one of its terminal is not fully insulated and this terminal MUST be connected to ground. So this VT, although rated for line-to-line value, cannot be connected line-to-line. I am not sure how this third type will behave in case of ground faults on an ungrounded network.

So, in your case, your VT is rated for line-to-neutral voltage which is why you enable 3V0 scheme when the zig-zag reactor is out of service. But you are right, your VT may be at risk during the time it takes for your 3V0 scheme to clear a ground fault (because it will be subject to line-to-line voltage and it is not rated to withstand that voltage).

 

[rex2704]

To add to my earlier note on the types of VT...

The third type that I have mentioned is CSA group 3 VT (rated for line-to-line but one terminal must be connected to ground). The voltage factor of 1.9UN, 8Hrs on a group 3 VT is designated as group 3C and CSA approves its use on ungrounded networks. I do not know if CSA talks about the difference in insulation of one terminal or not. I got the information about the insulation (for neutral terminal) from a manufacturer's website.

 

[trosepe]

There is are very good article on this in the IEEE Grid - Industrial Application Society Magazine that deals with ferroresonance on HV systems using VT's. My wife accidently discarded the issue but it was an issue in late 08-early 09.

 

[rex2704]

Thank you Trosepe for that info.

 

[Rodmcm]

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