MV Motor Starters vs Circuit Breakers

[trat1208]

Can anyone give me some guidance or point me to some technical papers regarding the use of medium voltage breakers vs starters based on starts/stops a day?

All of my company's power plants were built in the 1950s and 1960s and were originally baseload. All 4kV motors were supplied from switchgear breakers and for the most part were started when the unit was brought on and stopped when the unit came off. In the past few years, due to deregulation and the competitive market, we have been forced to cycle our units to follow load and as a result see more starts/stops on our 4kV motors. For instance, for our pulverized coal units, mills might typically get started stopped twice a day with load changes.

Recently we had a breaker failure lead to a switchgear fire. Approximately 30 minutes after one of our more frequently cycled motors was started (by closing breaker) there was a bus overload trip and operators found a switchgear fire. The fire originated from the aforementioned breaker cubicle. The breaker was just about completely destroyed, so visually there was nothing of value that could be gained from inspecting it postmortem. Based on this sequence, our assumption is that the breaker main contacts made up poorly due to eroded surfaces or poor alignment and heated up and started on fire. The breaker had been rebuilt within the past two years by a qualified shop, so lack of maintenance is not believed to have been an issue.

I am trying to put together recommendations as a follow up to this event. I believe that several manufacturers can offer motor starter retrofits into original switchgear lineups. Is such a retrofit worthwhile if we are talking about two starts/stops per day? Also, under the assumption that contact wear due to number of operations was a contributor and if two starts/stops a day is not excessive, would switching to a vacuum retrofit breaker make sense as a recommendation? My assumption being that contact wear is less of a problem in vacuum breakers vs air breakers.

Thanks.

 

[DTR2011]

I would check with Cutler Hammer and see what they have to say. It sounds like you are in the US. I would guess that the original Switchgear is either GE or Westinghouse. CH now owns the West Ampguard starter line.

Contact wear is one thing, but I believe operations on the mechanism is also a consideration. A MV starter has a much simpler mechanical design and as such, can handle the repeated starts / stops. The contactor drops out and latches in magnetically. A poorly adjusted mech have also caused a problem. As you said, there is not much to examine to make that determination.

 

[Zogzog]

We have customers that do the same thing, using a MV breaker as a starter puts a lot of wear on the breakers and we have witnessed fatuige failures of some parts such as metal rods snapping in half, stuff even a proper refirbishment can't really prevent unless you went as far as checking each part with an X-ray or something looking for internal cracks or fatuige in the metal. Some of these issues can be found by inspecting each part on a light table after cleaning (Standard procedure for us), keep in mind not all breaker shops have the same QA procedures, many in fact don't even do a complete disassembly of the mech or replace wearable parts (Bearings, pins, etc...) as SOP.

The metals used back in the 50's and 60's was not as high quality or as stong of alloys as are used today, we have had some success in manufacturing new parts using better alloys for commonly failed parts. What type of breaker was this? Perhaps I have some type specific suggestions.

 

[Zogzog]

Forgot about the rest of your question. Yes, you can put a starter retrofit into most breaker cubicals and we get this request often, but in many cases they do not hold up to the use as well as most well built air breakers of your vintage.

Recommendations all depend on the specific breakers you have now.

 

[dpc]

When I did power plant design, we used breakers for all MV motors. But, these were base load units and the motors were probably stopped & started a dozen times a year, on average.

When I started doing other types of design work, I was surprised to find the MV contactors much more common for motor control.

The starters are rated for more operations than the breakers. But the starters (all of them) look like junk compared to a vacuum circuit breaker, at least to me. The newer vacuum contactors might be better, but the old air contactors always scared me.

But two starts/day is probably an application for a starter and not a breaker, at least per the specs.

 

[trat1208]

Thanks to all of you for your input.

The switchgear involved in this particular failure was ITE HV. The breaker was a 1200A.

DPC:
When you said that two starts/day is probably an application for a starter and not a breaker per the specs, what are the specs that you are referring to? Breaker manuals that I have looked at don't really address equipment limitations based on operation frequency. Sometimes a note might be in there about the need for more frequent maintenance if used for motor starter application, but I've got motors that get a start every three months and others that get a start once a day.

 

[dpc]

IIRC, breakers have a repetitive duty rating for number of operations as well as a much lower number of rated fault interruptions at rated duty.

I recall a figure of 10,000 operations before servicing for metal-clad vacuum breakers.

So at two starts/day, you'd get there in about 13 years.

For vacuum contactors, the rated electrical life is claimed to be 250,000 based on specs I've seen in the past, but I haven't looked at this information in several years.

I have a preference for breakers over contactors, but at some level of frequent starts, the contactor approach becomes more cost effective, at least on paper.

 

[oldfieldguy]

I had a client who used GE PowerVac 15 kV breakers to control some 8000-horsepower pumps that started twice a day. His routine was to perform contract wear measurements quarterly. As the measured wear began to approach the maximum allowable, that breaker was swapped out for one in transformer serve that had previously only operated on rare occasions.

In another case, low-voltage power circuit breakers were used to control 480-volt motors. After a couple of decades of operation, they started failing in various wear-related fashions. These have since been replaced by contactors.

Bottom line: Breakers will work, but you need to keep an eye on things.

old field guy

 

[Zogzog]

HV's are difficult breakers to get the adjustments correct and also use lubricants not normally stocked by most shops.

Also HV's have very high maintenence intervals, every 6 months or 2000 operations, whichever comes first.