Voltage spikes experienced with a vacuum bottle breaker 3

[relayman1000]

Comparing the voltage spikes produced with interruption of a medimum voltage vacuum bottle with those produce by an air blast breaker. Are they the same in new vacuum gear? What about in older vacuum gear? I know that the early ventage vacuum gear had surge capacitors but I have gotten the more i am believing that the modern vacuum contact designs did not need a surge a capacitor due to improved contacts. Bottom line is can we use vacuum medium voltage breakers on motors just as we can air blast breakers with no surge capacitors because they both have about the same voltage surges and spikes?

 

[Skogsgurra]

I would not install vacuum breakers on old equipment without making sure the transients stay within limits. Not on new equipment either. Had a recent 6 kV generator with very bad damages caused by vacuum breaker overvoltage.

On inspection, it was revealed that the switchgear guys had "forgotten" surge arresters.

Not so sure that capacitors alone will work. You will probably need a damping series resistor, too. Or use surge arresters.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[relayman1000]

Thank you for taking the time ot give me some comments . This is valuable. What do you think about using surge arrestors or limiters at the motor as opposed to capacitors?

 

[Skogsgurra]

As I said, surge arresters or snubbers are better. Capacitors usually ring heavily and can cause very high overvoltages if not dimesioned correctly.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[relayman1000]

Thanks for your input.

 

[alehman]

There is an IEEE Joint Transformer/Switchgear working group that is attempting to address this problem as relates to transformer failures. The proposed C57.142 application guide remains in draft form.

I saw a presentation by an engineer from Eaton recently that suggested there has a been a significant increase in failure rate of dry-type transformers fed from vacuum breakers, particularly where cable between is fairly short. He was advocating use of snubbers where justified by computer modeling.

 

[RRaghunath]

There are no. of papers on the overvoltages with use of Vacuum breakers / contactors.
Many users prefer to blindly specify surge arrestors with vacuum contactors / circuit breakers, when used for motors, dry type transformers or generators (the key word being dry type insulation). This is not an issue with oil filled transformers that traditionally have higher BIL.

If precise engineering is called for, it is a practice to assess the overvoltage risk individually, by taking the motor inductance and the power cable capacitance. Generally, large motors with short cables are considered to be at risk.

 

[Gianoli]

relayman1000,

I am going to assume when you use the term air blast circuit breaker you really mean air-magnetic circuit breaker. There are air blast circuit breakers but they were usually applied at the transmission circuit voltage levels. The ones used in medium voltage circuits were usually generator breakers. I don’t think I have ever seen an air blast circuit breaker switch a motor circuit.

I think it would be advisable for a number of reasons to use a contactor rather than a power circuit breaker to switch medium voltage motor circuits were possible. 800 amp contactors are commonly available from a number of manufactures at voltage levels up to 7200 volts. This should take care of motors up to about 9000 HP. If the motors you are switching are larger or are feed at 13.2 kV you may have to use a power circuit breaker.

When using vacuum contactors or vacuum circuit breaker to switch motor circuits you definitely need to protect the motor against harmful switching surges. Typically this protection will consist of surge capacitor and a surge arrestor. The surge capacitor is used to slope the front of the surges and the surge arrestor is used to reduce the magnitude of harmful surges. Both of these functions are often packaged as one unit, which should be installed as close as possible to the motor. The motor junction box is a good location if there is room.

The requirement for surge suppression apparatus with vacuum switchgear should not viewed as a deterrent. The advantages that vacuum switchgear brings to the table more than out weighs the costs of the surge suppression requirement.

 

[wolf39]

The harmful voltage spikes have their origin in the extreme effectiveness of vacuum bottles to extinguish currents. Its the high delta I/delta t figure which is causing the problems. Modern vacuum bottles therefore are not "better" in this respect than the older types.

Wolf

http://www.hydropower-consult.com

 

[EngCarlos]

We worked recently in old Toshiba vacuum breakers (1982) and we find very high contact-resistance (2000 microohms).
Somebody knows the life of this botles?

 

[oldfieldguy]

Gianoli--

Air blast circuit breakers are not uncommon in 1940's and 1950's vintage powerhouses at the 15-kV level. The ones I had to dea with are fixed, not draw-out, and had high interrupting capacity. A few users are trying to retrofit back to high capacity vacuum or SF6 breakers to replace these air blast breakers since parts are impossible to find and maintenance is becoming a nightmare.

That's one place where the vacuum breaker's voltage spike issues are surfacing.

EngCarlos--

Make sure that the high contact resistance is actually in the bottle itself and not the connections elsewhere in the power circuit. There are multiple connections to deal with and any of them could be a problem. Usually contact erosion gets a vacuum bottle under heavy use before contact resistance surfaces.

old field guy

 

[alehman]

Gianoli - Why is a contactor better than a circuit breaker in this regard?


The cut-off current of the arc is a primary determinant of the resulting overvoltage. There is some variation in cut-off between manufacturers of vacuum bottles. I've been told that manufacturers did make some attempt to reduce cut-off current in their designs. SF6 and true air-blast interrupters have this problem, but not to the same level.

The IEEE working group was formed because of the apparent recent increase in transformer failures. This has been noticed particularly in large data centers where vacuum breakers are commonly used as transfer switches feeding dry-type transformers. Cable runs are typically very short in these facilities and they are exercised frequently. There have been a few failures of oil insulated transformers also attributed to this.

 

[Gianoli]

*Gianoli - Why is a contactor better than a circuit breaker in this regard?*

I mentioned this in my previous post for a few reasons:

A. The alloys used for vacuum interrupter contacts are always a compromise. They can’t weld together. They should have good electrical conductivity. They should have good thermal conductivity. They need good mechanical stability so they don’t deform or fracture from repeated impact. Have to be able to be machined. Should not erode excessively when exposed to high current metal vapor arc’s and they should not produce high values of current chopping. On and on it goes… If I know I am selecting an alloy for breaker contacts that may be required to interrupt 63kA fault currents and close into faulted circuits regularly I probably select a slightly different alloy than I would for a contactor or switch. Small changes in the alloy can have big implications with respect to current chopping. If I know I am selecting a material for a motor contactor vacuum interrupter current chopping is higher on the list than it would be for a power circuit breaker.


B. The operating mechanism of a contactor is rated for more operations that the operating mechanism of a power circuit breaker.

C. Contactor is typically more economical than a power circuit breaker.


Dry type transformers (low BIL, high surge impedance), short cables (low circuit capacitance) and a vacuum circuit breaker (perhaps higher chopping levels than a contactor) and frequent switching operations are a bad combination. However, I don’t think such situations are unmanageable. One needs to be very diligent in the application and location of surge arresters in such circuits.

To answer relayman1000’s original question:

*Bottom line is can we use vacuum medium voltage breakers on motors just as we can air blast breakers with no surge capacitors because they both have about the same voltage surges and spikes? *


If you really are using an air-blast circuit breaker (not a air magnetic breaker) and have no surge arrestors in place now and have not be experiencing any problems you have probably been real lucky.

Air-blast circuit breaker is notorious for current chopping especially the transmission voltage level breakers. Overvoltage control resistors are mandatory in most cases. The medium voltage air-blast circuit breakers that I am familiar with (General Electric type ARA/ARH, Westinghouse type CA, and Brown Boveri type DB) were known as “station-type” circuit breakers. As oldfieldguy mentioned they had a higher interrupting capacity than standard medium voltage power circuit breakers and were fixed mounted in big metal enclosures. The top end GE and Westinghouse breakers were rated at 2500 MVA with continuous current rating up to 7000 Amps. We used them as generator breakers in hydroelectric plants and always had surge arrestors installed at the generator terminals.

I would not apply vacuum switchgear to any medium voltage motor circuit without surge arrestors. They are not overly expensive therefore provide cheap insurance.

 

[alehman]

Thanks Gianoli. Sounds like we should be taking a closer look at contactors.

 

[mirilovic]

So, what is criterion for use of surge arresters? When they are needed? Do I have to place them every time when feeding motor/dry type transformer?

 

[wolf39]

We all agree: The solution to protect electric components from harmful spikes caused by vacuum breakers is the application of surge arresters. This obviously adds costs.

Can someone tell us how much we have to pay for a threephase vacuum breaker of, lets say 36 kV and 2500 Amps, and how much we have to add for the surge arresters? Of course, the additional surge arresters require extra space and extra erection costs, but just let us compare the bare components.

If we can protect, for instance, the drive motor responsible for a critical production process, we should do it. The extra costs involved are negligible compared with the loss of revenue the surge arresters can prevent.

Wolf

http://www.hydropower-consult.com

 

[relayman1000]

Gianoli, Thanks for your commments . You are right I should have said air magnetic breaker because I am talking about switchgear. Do you or anyone else know of a device that will cut down on the voltage wave slope but that is not a capacitor. An arrestor just cuts down on the magnitued of the spike. Is there anything non-capacitive that will cut down on the slope of a wave front that you know of? We had a 3 phase capacitor on the terminals of a motor with a vacuum breaker and when it closed the capacitor which was liquid filled blew up and caused a fire. Now the plant wants to take then at the motor all out and we aren't sure it is wise with a vacuum breaker. Comments?

 

[wolf39]

As I said before: We have to expect higher voltage spikes the higher the (delta I/delta t) figure is at the moment when the arc is being extinguished. If the arcs are extinguished during the natural current zero crossing of each phase we can expect moderate voltage spikes. However, vacuum bottles are extremely effective and the phase currents extinguish even BEFORE current zero crossing, thus generating excessive voltage spikes. The same, but to a lesser extent, happens in an air blast circuit breaker, which also has powerful arc quenching qualities.

I assume that voltage spikes are generated in a falling sequence as follows:

1. Vacuum circuit breaker (highest voltage spikes)

2. Air blast circuit breaker

3. SF6 circuit breaker

4. Oil circuit breaker (lowest voltage spikes)

Its possible that we have to swop the air blast and the SF6 sequence but I believe that a 20 bar air pressure is more efficient (violent) in the arc quenching than the gas of an SF6 breaker. The heavy SF6 gas cannot be pressurized to high levels as it liquifies early.

Maybe a breaker designer can enlighten us further.

Wolf

http://www.hydropower-consult.com

 

[Gianoli]

Wolf39:

It is going to be hard to rank interrupting technologies in order of which tend to chop the most current. In some cases it is going to depend on the voltage level and specific breaker. Here are some general thoughts:

1. Air-Blast – For sure the worst. Keep in mind that all the breaker interrupting technologies mentioned in your list in some manner employ the thermal or magnetic properties of the arc to assist with interruption; even vacuum. Therefore there is a relationship between the current being interrupted and the strength or speed of the interrupting force. The only exception is air-blast. The constant pressure, constant interrupting force is always present. Some of the high speed, two cycle transmission voltage level breakers are horrendous. It’s nothing to chop hundreds of Amps. The implication of this can be disastrous if not accounted for. Air-Blast breakers (above say 138kV) are almost always fitted with overvoltage control resistors as standard equipment.

2. The older double pressure SF6. – Not made any more but some still around.

3. Vacuum.

4. Live Tank Oil. – Some had an oil injection feature that could chop current.

5. Single Pressure SF6. – These are really the only breakers still being manufactured above 138kV. The manufactures have the technology down to a science and chopping current is not a major concern.

6. Dead Tank Oil. – Old technology but thousands still in service.

7. Air-Magnetic. –Very soft

 

[alehman]

The Eaton presentation (not to sound too commercial) suggested they use Electromagnetics Transient Program (EMTP) to model systems and estimate the overvoltage and ring frequencies. From that a judgment can be made as to whether suppression is necessary. RC snubber circuits are thought to be a better option for suppression.

Factors that influence the risk:
1. Chop current
2. Cable length between breaker and motor or xfmr
3. For transformers - load on the secondary
4. Motor or xfmr BIL
5. Frequency of breaker operation