Switching surge? 3

[egstasea]

I have a 13.2kV double ended switch gear, with a primary 13.2kV feed from the utility. At the other end a backup feed from a generator.
I am seeing what I believe to be a switching transient when I switch the loads to and from the generator. The generator is 2000 kW 480V wye and stepped up to 13.3kV delta. The generator breaker is on the 480V side. It is an air breaker - Cutler Hammer SPB100 series rated for 4,000 Amps. The generator breaker is interlocked with the incoming 13.2kV motorized air break switch. The transfer from generator to utility is open transition. I am seeing 2 distinct problems that both manifest themselves as excessive arcing across the arcing contacts on the generator breaker.
Scenario #1: plant is running on generator and the sequence to transfer to utility is initiated, when the generator breaker breaks the load we have seen excessive arcing across the arcing contacts on two phases of the generator breaker.
Scenario #2: plant is running on generator and the sequence to transfer to utility is initiated, the generator breaker breaks the load, and 3 seconds later the incoming 13.2 kV air break switch is closed. At that point in time we have seen arcing across the OPEN arcing contacts in the generator breaker.

The loads are mostly inductive, but there is a 1 mile of 13.2kV underground cable feeding the several unit substations.

I have been looking at surge supressor/arrestor circuits, but I do not know where to apply them, at the generator breaker? on the 13.2k V side of the step up transfomer. I alos have questions on sizing.
Am I on th right track? any sugestions welcomed.

 

[jbartos]

Suggestion: The 13.2kV transmission lines may have capacitive charging, i.e. capacitance that is interacting with transformer and generator inductances. Therefore, some oscillations and transients materialize causing the arcing. The surge arrestors/suppressors are usually on the higher voltage side; although it may be on low voltage side near expensive item such as motor, generator, etc. to protect it.

 

[Elecme]

I agree with Jbartos that the reason may be the capacitive charging in the transmission line but this is for scenario #2. As for scenario #1. For scenario #2, I belive you should check the step up scheme(transformer and circuit breaker) and take some measurements.

Elecme

 

[egstasea]

JBARTOS, ELECME thank you. Any suggested guidlines on sizing, or manufacturers.

 

[rbulsara]

I suspect your air break 'switch', it is not closing all the three phases simultaneouosly. Normally a circuit breaker should be used to make and break the load, a swtich should only be used for isolation (no load use). I find the use of air break 'switch' as making or breaking loads unusual, specially as motorized.

I would consider providing 13.8 kV 'breakers' for both the tuitltiy and generator incoming to the 13.8 kV swichgear and operate them to transfer the loads.

Also long cables and transfromer may be causing ferroresonnant conditions. A long shot but worth looking into. If arcing is caused by overvoltages (which could happen in a ferro resonnant conditons when all phases are not making or breakig at the same time), application of surge suppressors will not help, they will simply operate undersurge and open fuses/breaker on overcurrent.

 

[rbulsara]

Just curious what is the load current while making and breaking the circuit? Do you have soft loading and off loading capalibilties?

 

[rbulsara]

More I think of it, it is customary to operate the generator and step-up trasnformer as a Unit and therefore only HV side breaker is used as disconnect for the gen-xfmrs unit, not the lv side breaker between the transformer and the generator.

Also the grounding scheme may also have to be reviewed. You did not provide any information on it.

You need to get some good consultants involved.

 

[jbartos]

Suggestion: Visit

http://www.erico.com/erico_public/default.asp

http://www.erico.com/erico_public/general_info/CRITECTNCR017.asp

for:
Medium Voltage Distribution Class Arresters

http://www.erico.com/erico_public/general_info/CRITECTNCR015.asp#jumpCommonMode

for:
Common Mode Voltage
etc. for more info

 

[alehman]

I agree with jb's thoughts.

As for scenario #1, It is normal to see a fairly good flash when a large LV breaker operates under load. The SPB breaker is an insulated case type, not really intended for heavy-duty switching application such as this. Is the transformer normally connected to the bus and energized from the utility source? How are the generator neutral and transformer neutral grounded?

Scenario #2 - The cable has a lot of capacitance. I think the real problem is the combination of cable and the unloaded (purely inductive) transformer. Analysis with EMTP or similar software may help determine the true cause. Surge supression would probably help. You should consider intermediate or station class arrestors at the 13.2kV bus. Also, arrestors on the LV side, probably on the upstream side of the breaker, would be advisable.

If possible, consider an arrangement to automatically disconnect the long cable from the bus while you switch between generator and utility. You could easily test this before implementing the controls.

 

[egstasea]

Thanks again.

Rbulsara - the 13.2 kV switch is normally opened under no load conditions, when the utility is out. It does however make load when the utility comes back. I do not have any way to stage on the load. The load can be up to 2000 amps.
I did not understand the last comment on your first post. Are you saying that arrestors wouldn't help at all?, or the wouldn't help if the closing times between phases are off? or they wouldn't help if I have a resonnant condition? please elaborate

Rbulsar and Alehman asked about grounding - The 13.2kV from the utility is wye, there are 10 kV arrestors in the receiving section. The neutral is not carried through the gear or out to the unit substations. The generator transformer is wired 13.2 delta/480v wye. The generator is also wired wye. The neutral and ground are bonded at the generator breaker. The neutral is not switched at the breaker.

Alehman - The generator transformer is normally hot.

Jbartos - do your comments re: common mode change now that I supplied a better expaination of the system grounding. Thanks for the link.

 

[rbulsara]

egsstasea:

1. Breaking 2000A load will cause some arcing. It would help to find some way fo stage the loads.

2. What I meant that applying surge arrestors, in a case of ferroresonant conditon, will not help alleviate the conditon. However the operation of surge arrestors will confirmm the overvoltage condition if it exists.

3. I am not sure what bearing it may have on the issue you have, but for the step-up tranformer the norm would be to keep the lv side(generator side) 'delta' and the HV (load side) a grounded 'Wye'. The grounded wye generator winding is very common. I am not sure applying a voltage from a different source, on a ungroudned winding (delta) of a transformer, within 3 seconds of deenergization from the secondary side is a very good idea. Some tranfromer experts may shed some light on this issue.

It seems you may have a few different things combining to create the problem you have. With all due respect the installation does not seem to be designed very well.

I still think you should be using the MV 'breakers' as switching devices. This will in fact remove the voltage from the gen breaker and also eliminate unnecessary energinztion of the transformer in your scenario#2.

If I could be of further assistance, let me know.

 

[jbartos]

Suggestions to egstasea (Electrical) Dec 21, 2003 marked ///\\The neutral is not carried through the gear or out to the unit substations. The generator transformer is wired 13.2 delta/480v wye. The generator is also wired wye.
///Normally, when the generator is connected in wye, the generator transformer will be connected into delta. this is to break the zero sequence current path by the transformer delta winding. It is not clear why the transformer is 13.2kV delta / 480V wye connected.
If the present generator (2MW) has solidly grounded neutral system grounding, there should be at least ground fault alarm to prevent damages due to arcing.\\ The neutral and ground are bonded at the generator breaker.
///This would indicate the solidly grounded neutral system grounding.\\The neutral is not switched at the breaker.
///This will pose a problem if the circuit breaker has a ground fault sensing and trip since there are two different paths in existence for the phase to equipment ground fault, which are dividing the magnitude of the ground fault current. Try to obtain Reference:
AscoFacts, Volume 2 Number 3 "Ground Fault Protection Considerations in Systems with Multiple Neutral-To-Ground Connections" from Automatic Switch Company.\\

 

[hvcad]

The arcing transients you have described are consistient with an transient oscillation taking place between the system inductances and capacitances. Surge arrester devices could be specified which would limit the overvoltages by absorbing some of the energy in the transient. A full EMT type analysis would have to be undertaken to understand the true nature of the transients, and to assess the effectiveness and required energy rating of any surge arrester devices deployed to limit overvoltages. Such an anlysis would include all a representation of the transformer non-linear magnetising characteristic and system earthing arrangements and would therefore address all the concerns raised by others.

Dr K S Smith
Mott-MacDonald, Power Systems Division
Glasgow, Scotland.