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BREAKER MAINTENANCE
Circuit Breaker Maintenance, Sentinels On Guard
By: Fred Tanguay
From a maintenance perspective, a power system is a very simple thing to break down into its basic component parts. Most power apparatus have very few moving parts. The essential ingredient to reliable operation is insulation health. Unfortunately, when things go wrong it usually happens in some type of spectacular fashion.
The guardians of the power system are the circuit breaker and the protective relay system that supervises it. When a fault or overload occurs, the protection system responds by activating circuit breakers to isolate the defective components. Equipment can withstand the tremendous heat, force and energy of a fault for only a limited time. In fact this time can be as short as cycles in extreme cases. The circuit breaker is a workhorse. It is there to stop the fault current flow before unaffected components are damaged or destroyed.
The hard part of its job is breaking the current. Fault currents can be huge, typically, in the order of 20 times the full load rating of the breaker. This extreme current creates massive amounts of heat and electromagnetic force that can literally destroy cables, bus ducts, transformers and switchgear. This is why it is absolutely critical that the circuit breakers used in your power system are kept in top operating condition at all times. It is essential to remember the important areas of circuit breaker maintenance so that you can develop an effective maintenance program, one that keeps your equipment in good working order and reduces those nasty unplanned outages.
The circuit breaker comes in many forms and types. To better describe these distinctions we can start by looking at different voltage ratings such as high, medium and low voltage. We can separate breaker types again by the method used to interrupt the arc, such as sulfur hexaflouride gas (SF6), vacuum, oil and air. Yet another distinction is made by their operating mechanism such as stored energy, pneumatic and solenoid. Finally their type and location of installation is defined as simply indoor or outdoor, rack in or draw out. By taking a detailed look at all of these permutations and combinations we come up with five common areas where you have to focus your maintenance efforts. The five key maintainable areas are:
· The Operating Mechanism: designed to move the contacts at a high rate of speed in either the open or close direction and latch the contacts closed.
· The Contact Assembly: designed to carry the full load current with minimal temperature rise and to assist in breaking the arc when the contacts open.
· The Arc Extinguishing System: designed to quench and cool the arc as the contacts open and to eventually extinguish the arc as the contacts come fully open.
· Power and Control Connections: power connection points connect the circuit breaker to the power system. They can be bolted or spring tensioned connections. Control connections tie the electrical control system of the breaker to the power system control network.
· The Insulation System: is the main electrical insulation that separates the live electrical components from ground and from phase to phase. The insulation system also helps to contain the arc when the contacts open.
COMMON FAILURES
As with any piece of equipment, breakers are not failure proof and no circuit breaker is maintenance free. In fact, breakers require more maintenance than any other piece of equipment solely because of their mechanical complexity.
Before we discuss various maintenance practices, it is important to understand what some of the most common circuit breaker failures are. Once you know what the common problems are, then you can devise a scope of work that will detect them.
Any failure is a very serious situation. A breaker not working under a fault condition could be the reason for massive system destruction and huge financial loss to your company.
TRIPPING SYSTEM TROUBLES
Trip system problems come from the failure of the trip coil or protective relay. These problems are easily found by simple calibration and function testing methods on the protective equipment. However, it is sometimes the practice to calibrate a relay without actually causing the breaker to operate. This may be the case because the calibration process is being undertaken while the system is still in operation. If this has been done, make sure that the breaker is entirely operated through the protective system as soon as possible after the relay calibration is completed. In all too many cases, I have found breakers where trip coils, mechanisms or control wiring have become inoperative, and the breaker would never trip regardless of the signal it receives.
The other extreme exists as well. We go from the breaker that refuses to trip to the one that is constantly tripping - better known as the nuisance trip. Personally, I think this is more than just a nuisance when you consider the down time and frustration that these cause. A breaker nuisance trips for one of five reasons: defective relay; the protection settings are wrong; the settings were changed by someone; the proper settings were never applied; or the connected load has changed. Sometimes these operations are misinterpreted as false trips instead of real trips. A real trip is a symptom of a problem, not a problem. Often though, the action taken to eliminate the breaker trip symptom is to turn up the settings so that the breaker doesn't trip any more. This can be disastrous because the fault will eventually erupt into something much larger later on.
Reliable settings are the first step for proper circuit breaker performance. With an accurate and up-to-date co-ordination study on file for your power system, you have the ability to make an educated decision about changing settings without putting your entire operation at risk.
TRUE STORY!
A plant maintenance electrician on night shift gets a call that a main 600-volt, 3000 amp breaker has tripped. This has happened before and he has reset it without trouble. He goes to the substation and does what he usually does, closes the breaker. He has a quick look around and sees no problem, so he returns to his tool crib.
About 20 minutes later another call comes in. The same breaker has tripped. Now he's mad. He goes to the substation and this time turns up the overcurrent relay settings figuring there is an overload somewhere. Again he closes the breaker and returns back to the crib.
About 30 minutes pass and a third call comes to him. Now it's the production shift manager. After a brief, colourful description of how the product can't flow without power, the poor electrician returns to the substation to fix the breaker once and for all. This time his repair involves a 2x4 piece of lumber. The breaker is now permanently closed, fixed for good!
Thirty minutes later, our friend is in the cafeteria having coffee, on break, telling his pals how he saved the shift and was thanked by the night shift manager. Coffee is suddenly interrupted by a loud bang (explosion), the lights go out, the plant goes very quiet and in the distance he can hear fire trucks.
Three days of down time and $400,000 in repair bills later, the plant was back in service. The electrician was reminded that 2x4s are not part of his tool kit. The start of the problem was a burning ground fault in an outdoor bus duct. The poor breaker was doing what it was supposed to do. No one took the time to look at what was causing the symptoms (see Figure 7).
INSULATION SYSTEM DECAY
Insulation system failures are attributed to the operating environment and the frequency and severity of operation. You must be aware of the environment. Substations that are unattended for long periods of time can develop problems without notice. This is particularly the case in outdoor, medium-voltage stations. Here, moisture from a leaky roof or excess humidity can cause the formation of corona (a faint glow adjacent to the surface of an electrical conductor at high voltage) very quickly. This can eventually lead to serious tracking and insulation failure.
Obviously, any water leaking into switchgear is very serious and may cause an arc well before corona even begins. A dirty environment contaminated with conductive or corrosive material will also lead to premature insulation decay and eventual failure.
Breakers that frequently operate under heavy loads and faults, will suffer insulation damage caused by the extreme heat of the arcs drawn. It is common to see inter-phase barriers, arc chutes and pole supporting insulation burned. This will also lead to tracking and eventual failure.
MECHANICAL SYSTEM STIFFNESS
General mechanical failure of the trip, close and the racking mechanism is caused by wear and tear, contamination from a dirty environment and finally, excessive force (the human kind!). Lubrication is essential to keep the linkages, rollers and bearings in the mechanism working freely. Dirt buildup inside the mechanism can dry out or gum up the mechanism, causing the bearings and rollers to seize. You are eventually left with a breaker that is, for all intents and purposes, stuck shut or stuck open. On the other side, the breaker could be very sluggish in operation. Both of these cases would allow fault currents to flow for a much greater time than your equipment can withstand, leading to a destructive failure.
Many circuit breakers are physically abused. This comes from a complete lack of knowledge on how to safely operate a breaker. Any breaker that requires some type of lever, several people or a pry bar to make it work, has a problem. The application of an excessive amount of force is only making the problem worse.
POWER AND CONTROL PROBLEMS
Power and control connection problems result from a misalignment in the switchgear cell (again, usually caused by using too much force to remove and insert draw-out type breakers).
If a bus finger fails under load, the result would be disastrous. The bus fingers are not designed to break any type of current what so ever. If they were to fail under full load, the ensuing arc would destroy the breaker and cause serious damage to the switchgear. The failure of control fingers is just as important since this may disconnect the trip and close controls from the protection system.
CONTACT WEAR
Rarely do you see contact wear, unless the breaker is really over-worked because of the operation frequency, switching heavy loads, multiple operations under fault, under-rating or insufficient interrupting rating. When you find excessive contact wear take a very close look at the application of the breaker. This could be a very dangerous situation waiting to get a lot worse.
LOSS OF INSULATING MEDIUM
Gas, vacuum and oil breakers require an additional extinguishing medium to break the arc when the contacts part. Most manufacturers will tell you that their breakers will survive an operation under fault if the medium disappears. I would not want to be around to witness this! Oil is simple because there is always a sight glass. And a large puddle under the breaker is sure sign of problems. Vacuum breakers are another story. Most are fitted with pressure switches to monitor the vacuum level. It amazes me to see so many of these switches not wired up to some type of warning or annunciation system. SF6 breakers have the same type of level detection system as vacuum breakers. But, again, the system is rarely wire to a warning system. This gas can be detected by using a simple Freon gas detector. If there are arc by-products in the gas you will also notice the smell of rotten eggs in the substation.
PLANNING MAINTENANCE
Now that we understand some of the most common problems and the huge impact they can have on your operation, it's time to devise a preventive maintenance program. To start, you must look at the contributing factors that will cause wear and tear on your equipment. Essentially, these factors are common for all electrical equipment so you can apply the same school of thought to your whole system. The key factors are: loading; frequency and severity of operation; environment; age and type.
LOADING
Loading is important because it determines the rising temperature of the breaker (above ambient temperature). High operating temperatures will cause loss of lubrication, oxidation of the contacts and slow decay of the main insulation. Your system operating philosophy should dictate that you use a typical 70 to 80 per cent load factor. The load itself is a very important consideration. Ask yourself: how important is the load and what affect does it have on my operation if it was suddenly stopped? A breaker feeding a cooling plant for office air conditioning is not as important as compared to one supplying an air traffic control radar system.
FREQUENCY OF OPERATION
There are two extremes for breaker operation. First, the only time most breakers are ever operated is during maintenance. This is particularly true for commercial, institutional, healthcare and medium-sized industrial sectors. The second is that your breakers operate on a regular basis. Automated controls will cause the breakers to switch heavy loads and operations under faulted conditions. This is typically the case in the utility and large industrial sectors. Unfortunately, when breakers operate they do so for a reason, and this reason is the deciding factor in determining the maintenance cycle. Operation types are very simple to categorize. The simplest is normal switching operations. All of us do this from time to time to isolate a section of our system to allow some type of work to be done. Good switching plans keep the wear and tear to a minimum because the load is shed gradually during shutdown and slowly reapplied during startup. The next level of operation is automatic switching, which is typically used in source and emergency transfer systems. These are very common systems where the load can never be shutdown (such as data centres and hospitals). In this application, full loads will be switched without warning on a regular basis. The final level of operation is created by the protection and control system. These fall into the two classes of overload and short circuit. These are the types of operations that cause the greatest wear and tear. Most modern breakers are designed to operate under full fault current for only a limited number of operations.
ENVIRONMENT
Start by simply taking a good look around. Ask yourself if the environment is clean, dry, wet, dirty, corrosive, well ventilated or sealed. Harsh operating environments are detrimental to sensitive insulation components. This is a large contributing factor to insulation failure. Further, the arc interrupting system can be greatly degraded to the extent that the breaker will loose its ability to break an arc even under normal loads.
AGE
Obviously age is a factor. Older breakers suffer from insulation decay and mechanical wear and tear depending on their operating frequency. If the operating frequency is low, you may well find a few mechanical concerns because of the simplicity of the older operating mechanisms. Unfortunately, breakers are not like fine wine - they don't get better with age. The simple fact is that older equipment has greater maintenance needs. Keep in mind that there is a point where maintenance costs versus replacement costs turns into a negative economy. Sometimes it becomes necessary to replace rather than maintain. You may find this to be the case regardless of the age factor. As utility fault levels increase, the fault interrupting capability of your senior equipment is no longer sufficient.
CIRCUIT BREAKER TYPE
Circuit breakers are like cars, pure and simple. Some are excellent and some are not. Most have quirks that require special attention and all have unique special problems. Experience dictates which ones are better quality. The final word on maintenance frequency is given to the manufacturer. Every breaker comes with an instruction book that contains a wealth of information about how to use and maintain a breaker safely. Consult this book as part of the planning stage. From experience, I have seen maintenance cycles range over a wide spectrum - from six months to four years. It is your operation that determines this cycle. Every facility has different operating environments and thus, some equipment may have different maintenance cycles.
Fred Tanguay is Field Service Division Manager with Black & McDonald Ltd.
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