Switching surge?
Switching surge?
(OP)
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.
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.






RE: Switching surge?
RE: Switching surge?
Elecme
RE: Switching surge?
RE: Switching surge?
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.
RE: Switching surge?
RE: Switching surge?
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.
RE: Switching surge?
http://www.erico.com/erico_public/default.asp
http://www.erico.com/erico_public/general_info/CRITECTN...
for:
Medium Voltage Distribution Class Arresters
http://www.erico.com/erico_public/general_info/CRITECTN...
for:
Common Mode Voltage
etc. for more info
RE: Switching surge?
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.
RE: Switching surge?
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.
RE: Switching surge?
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.
RE: Switching surge?
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.\\\
RE: Switching surge?
Dr K S Smith
Mott-MacDonald, Power Systems Division
Glasgow, Scotland.