Circuit Breakers: Air vs. SF6 Insulated 3

[jabou]

We are investigating replacement breakers (dead tank) for a 25kV distribution substation. I have compiled much information with respect to vacuum vs. SF6 interupters. However, the big boss has expressed concern using air insulated breakers (ones with air at atmosphere in the high voltage compartment).

I have found very little information about non-air insulated breakers. I know they exist because we put in SF6 insulated breakers last year, but they have leakage issues.

Q1. Are SF6 insulated breakers generally considered reliable? Is leakage a common/consistent problem? Who are some vendors (for installation in Canada)?

Q2. Is there such a thing as vacuum insulated breakers?

Q3. Any articles comparing the two technologies - air vs. SF6 insulated?

Q4. Boss's concern is during spring, with high humidity and fluctuating temperatures around freezing, ice will build up on the metal components in the tank and cause a flash over between phases. Of course, heaters come installed but they generally break down before the breaker. I see that it can theoretically happen. Does it happen in reality? Mind you, it only has to happen once at the wrong time...

 

[RRaghunath]

I see there are two things here. One the switchgear bus bars and the other the circuit breakers.

Circuit breakers can be SF6 or Vacuum type i.e. the interrupting chamber has Sf6 gas (under pressure)in SF6 CBs and the Vacuum CBs have vacuum bottles as interrupting chambers. Air circuit breakers (air as interrupting medium) or popular at lower voltages and I haven't come across for higher voltages such as the present one 25kV.

The switchgear bus bars are normally air insulated irrespective of whether the breakers are SF6 type or Vacuum type or air type. The other option is SF6 insulated bus bars and such switchgear is called GIS or Gas Insulated Switchgear (where-in Sf6 gas is employed for busbar insulation and also as interrupting medium in circuit breakers (different pressures). This switchgear is compact and is normally used where the space is a constraint. This is specified even in other cases where the required fault levels are beyond the range of conventional switchgear.

When it comes to makes Siemens, ABB make Vacuum switchgear and ABB, Merlin Gerin can supply SF6 switchgear. All of them can supply GIS. GE, Fuji, Alstom and many other makes must be there but not much popular in India (where I live).

When it comes to reliability, I prefer SF6 sealed type circuit breakers and VCBs. I have come across gas leakages with the SF6 breakers that are not sealed and are a real problem.

With regard to flash over in the switchgear, I didn't understand the apprehension as the air in the bus bar chambers is supposed to be maintained dry and the interrupting chambers are either sealed or under pressure.

Trust the above is helpful.

 

[stevenal]

With regard to question 4, we had several vacuum in air breakers delivered with their thermostats set at zero, ensuring they would never switch on in our climate. The stats were located in the high voltage compartment, away from easy access. The vents were all low, so little natural convection could take place. One warning sign we ignored was the ozone odor. Eventually one flashed over and we discovered the problem and dried/fixed the others. We now spec our breakers with the thermostats located in the low voltage compartment and with vents high and low. Monthly substation checks include checking for heat. If you know where the heater is located inside, you can place your hand over this spot outside and know whether it's on.

The heaters themselves are very rugged. Thermostats and circuit breakers are more likely to be failure points. And some manufacturers try to get away with ordinary wire at the heater's terminals which will eventually get brittle and break from the heat. Heater wire is required.

 

[busbar]

Stevenal raises good points. Some specify equipment heaters operated at ¼ of nameplate watt density (½ voltage.) An ammeter in the neutral lead is also specified, with periodic reading as part of routine logs.

To me, a thermostat of the grade stocked at an “orange box” store mounted on a handy box is a substandard arrangement, considering the potential duty expected. There are far more reliable controls that have a small price compared to that of equipment failure and associated outage loss.

Retrofit may offer some switchgear life extension prior to wholesale replacement.

 

[candu6]

In eastern Canada we have gone to SF6, switchyard breakers for all new and replacement equipment. We have standardized with ABB breakers but we have also used GE and Mitsubishi.

 

[thekingpin]

Here in the Australian Coal Mining Industry SF6 is widely used and is considered very reliable. The only issue that may be worth considering in cold climates is the temperature at which sulphur hexafluoride liquifies. I am not sure of what temperature exactly but it is not that low.

 

[sitelock]

It has been my experience in the US that air, SF6, and vacuum bottle breakers are used. Air insulated and interrupting breakers have a variety of problems associated with them in Substation rated equipment. I am most familiar with air blast interrupting breaker design that uses a higher pressure blast of air directed at the gap when the contacts open to extinquish the arc. It is my experience that this technology is not the typically favored technology in the US and other countries. There are inherent problems with design, time to extinquish the arc, and transient voltages generated by the arc. And the size of the device can also be a restraint if you installing a new substation in areas in view of residential areas.

Companies produce fully SF6 insulated substations to provide some of the smallest substations for certain applications.

At 25Kv, I would think SF6 insulated vacuum bottle interrupting breakers, and SF6 interrupting breakers would be the dominant technologies. SF6 insulated tanks with vacuum bottles interruption might be a typical design for your application. Although SF6 insulated "puffer" breakers exist also. All SF6 insulated equipment is subject to leaks. I have never purchased any brand of SF6 equipment where at least one or more devices have developed leaks. It is a good rule to assume such pressurized equipment will leak. Regular inspection for leaks is essential. Halogen hand held leak detectors should be equipment for field personnel that maintain this equipment. I have seen high pressure SF6 breakers built with supplemental SF6 gas tanks integrated in design to compensate for inherent leaks. Common problems with SF6 insulated equipments can exist with location and temperature range of operation. SF6 will liquidfy at certain colder temperatures and is related to the gas pressure used in the equipment. The liquidfication temperatue goes up with increase of tank pressure. Manufacturers overcome this problem with tank heaters. Lower pressure interrupters (10 PSI) liquidfy close to 32 degrees below zero.

Another issue of pressurized equipment from a factory is the concern of the pressure gauge design. Equipment with relative pressure gauges built at lower altitudes will register higher pressures when delivered to your site at a higher altitude. Some manufacturers will simply reduce the pressure of the gas at the factory to ensure the appropriate pressure at your site. The inherent flaw here is that you have less SF6 in the tank and a reduction of total insulation capabilities. Another concern is the pressure in the tank will vary with temperature. It is a critical concern when picking a device and manufacturer. Some equipment will not be suitable if you experience wide temperature extremes with your seasons.

SF6 is considered an ozone depleting gas and its use can present other issues depending on the environmental rules and laws that affect you. In the US you cannot simply vent into the atmosphere to make internal repairs. You have to invest in an SF6 "CART" that removes, filters, dries and stores the gas removed from the equipment.

One often overlooked issue with any SF6 medium to high voltage switchgear or breakers are concerns with by- products produced when a catastrophic failure occurs. High pressure interrupters can produce significant amount of by-products contained in the white powder found at failure sites. Many of the by-products are listed by the manufacturer. Again your environmental laws may dictate how you handle these by-products. Many of the by-products are considered hazardous materials and require haz-mat procedures for clean-up. The types of by-products generated will vary with the materials used for internal parts of the breaker, gas pressure, presence of moisture and the voltage and the level of current generated during the fault. One problem I encountered was transportation of the failed breaker. Again environmental laws apply. I found it almost impossible to find shippers that would transport a failed SF6 device to the manufacturer for failure analysis. Because you cannot accurately access the by-products in the equipment, shippers are reluctant to ship it for you. I often included specification clauses that the manufactuer had responsible of arranging return shipment and disposal of these failed devices. And if you don't return it to the manufacturer, you are faced with the issue of disposal. In the US, the equipment was considered hazardous material and required special handling and disposal at specified hazardous material disposal sites. It was becoming a major issue when I last was involved in applying this equipment. I know that more specific rules of handling failed equipment and by products were being considered by various industry associations at the time .......so more specific information is likely in place now.

Despite these problems SF6 breakers would still be my preference. Failure of these devices are often contained within the tank and do not present the safety risks associated with catastrophic failures of other devices using other mediums of interruption.

And despite what manufacturers will tell you, fault interrupting vacuum bottles can also develop leaks that allow the insulating medium to enter the bottle and cause a violent failure during the next interruption. There are means to detect this in regular maintenance.

And one last point.......solid state breakers were being developed when I last investigated breaker technologies. They had many advantages for substation application. The fault detection devices could detect rate of rise of a fault and initiate interruption of the fault very quickly to the extent you could control fault current levels on the protected system. I know such technology is commercially available in distribution class equipment. S&C is the only company that comes to mind. They may produce substation class equipment. It is perhaps an ideal breaker device, but cost would be a critical factor here.

I would not use air breakers.

I have seen some companies use Cooper electronically controlled reclosers as substation breakers depending on your application. They are considered well designed devices and use vacuum interrpupting breaker bottles that are often used in substation class equipment. I recall 12 KA rated bottles were available at 35kV rated reclosers......and they may have 16 KA rated bottles now. You could specify substation grade and rated porcelain insulators for the recloser to obtain full substation ratings. Electronic control of these reclosers can be adapted to a wide range of equipment already in place in a system.

You would find much written material concerning comparison of these technologies and your equipment representative would be a good first source, as his information would direct you to other current industry accepted white papers and other applicable information. Calls to substation engineers in utilities in your geographic location might also give you some idea of commonly accepted technologies and preferred devices and manufacturers.

I hope this gives you some help with your equipment selection.

 

[mersin33]

dear all,

reading all these, I have to admit that I was a little bit lost. please correct me if there is anything wrong with the below:

-when somebody talks about "air insulated breaker", I understand 600V breakers. I have not heard of an air insulated breaker at 25-kV level. it is the "air-blast", breaker which is (or used to be) used at 25kV level.

-breakers are named with their interrupting media: vacuum, SF6, air-blast, oil, air.

-heaters supplied in the station breakers are mostly used to prevent internal condensation. however, SF6 brakers are supplied with tank heaters to prevent liquidification of SF6 gas under extreme cold temperatures.

-air-blast technology is very old for this voltage, and not offered by many companies.

-both vacuum and SF6 (pretty much the only two options you have got at 25kV for new units) breakers are both reliable and proven technologies. both interrupting media may fail; i.e. vacuum can be lost, SF6 may leak. SF6 leak is much more likely to happen than the other. however it is very easy to detect SF6 leak, and can be automated to give signals. you don't have the same chance with vacuum. vacuum bottle needs no maintenance on the other hand.

-my personal preference would be vacuum due to their low maintenance. however, because of very high speed interrupting capability, they tend to create TRV problems depending on the feeder parameters (cable, etc). in some applications, surge arrestors may be needed.

-the design of the outdoor tank would not (should not) allow water accumulation between phases, therefore icing of the accumulated water should not be a problem.

hope this helps,

regards,

 

[stevenal]

We are talking about the secondary insulation medium. Vacuum in air is common in these voltages, which means the vacuum interrupters are housed in an air filled cabinet. Some have vacuum in oil. SF6 requires no secondary insulation since interruption takes place within a grounded metal tank.

 

[KeorcZhang]

i agree Saltan's opinion.

It is popular to use VCB in medium voltage system (=<35kV) in china. meanwhile SF6-insulated breaker is for high voltage system.
Oil-insulated breaker is outdated since it is difficult to maintain.

There is no manufactory to supply MV or HV Air-insulated breaker,which sounds impossible to extinguish arc.


Regards!

 

[bonesa]

http://www.powersystems.ca

is mnf's of sf6 disconectors and MINISUB

 

[dmpavey]

As you mentioned Canada, I thought I would refer you to BC Hydro who have extensive experience of SF6 switchgear for outdoor use. This includes both those that leak and those that do not.

They currently purchase a SF6 / vacuum circuit breaker from FKI Switchgear in the UK

 

[ScottyUK]

dmpavey,

Would that be a Hawker Siddeley design? They were FKI's switchgear division last I heard.


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If we learn from our mistakes,
I'm getting a great education!

 

[dmpavey]

Yes, it is Hawker Siddeley. The circuit breaker is the dead tank type with magnetic actuator designed by Whipp & Bourne following their success with the pole mounted Recloser

 

[PowerfulStuff]

Hmm, a very interesting discussion. Our site (22kV) has recently changed to VCB's as standard as the Chief Engineer believed that SF6 was no longer the best technology at this voltage. He was probably right but I liked the SF6 as, even when there is a leak, many breakers are able to break (once) the arc as long as SF6 remains in the tank at atmospheric pressure. We have had some SF6 leaks on outdoor switchgear, although the manufacturer assured us that this had never happened before (just unlucky, I guess ).

On another point standards in this country have higher maintenance requirements for vacuum breakers (Hi Pot across an open breaker) every year as there is no external way of determining if a leak has occurred.