ABB Power Services

Our aim is to increase systems reliability and extend the life of their equipment with concomitant gains for our customers’ competitiveness. We support our customers with high-quality services in preventive and corrective maintenance to improve risk mitigation and improved safety. ABB's service portfolio allows utilities and industrials to maximizing the return on all assets by ensuring a high reliability, reducing life cycle costs and ensuring optimized performance while lowering environmental impact.

Tuesday, November 20, 2012

Are Your Circuit Breakers Healthy?



By Jeff Spoljarick, ABB High Voltage Service

Read the full article online as printed in the September issue of Electricity Today:
http://online.electricity-today.com/doc/electricity-today/et-sep2012-digital/2012092201/#54


Utilities are faced with the same problem they have always had: Demand cycles. But now there’s more of it. It’s no longer enough to keep your fleet running when you need it – you must have a healthy fleet to ensure long-term reliability.

During early medical practice, doctors could not understand how or why patients developed infections after even simple surgery. Interestingly though, doctors  hypothesized  that something that they could not see did create  and spread infection. It wasn’t until Louis Pasteur discovered the germ theory in 1861 that this mystery was solved. Today, we know contaminates, some of which we cannot see, can create serious performance risks for SF6 (sulfur hexafluoride) gas circuit breakers.

To prevent contaminates from causing these serious performance risks, SF6 gas used as a dielectric medium in electrical equipment must meet minimum International Electrotechnical Commission (IEC) or ASTM International industry standards, whether that standard is for new or recycled gas.

North American electrical utilities use SF6 gas as an arc quenching agent for two reasons: (1) its high-performing dielectrical properties and (2) its relatively low cost. However, over the course of time and use, this gas can become tainted due to operational use or poor maintenance practices of untrained personnel.

We know that current technology has the capabilities to remove most contaminants that may mix with this gas; understanding these contaminants and how they are formed allows our industry to recycle this gas for continuous, multiple uses. However, first you need to know and understand what contaminates have entered the switchgear. This is done by performing integrity tests regularly. Some of the more common contaminants that can form within or enter the SF6 gas zones will be further explored. 

CONDUCTING PARTICLES
Metal or carbonaceous material particles when present, have a tendency to bounce around in the SF6 gas between the inner conductor and the grounded enclosure. They are electrically charged and repelled as they touch an electrode. Depending on their mass, the applied voltage, the distance between electrodes, and other factors, they may or may not reach the other electrode before reversing polarity in a 60-Hz (hertz) system.

In any case, each time the particle reaches either electrode, it produces a small spark. At normal operating voltage these sparks will cause some local ionization of the SF6 gas. However, internal flashover from this is rare because SF6 has the unique characteristic of absorbing free electrons as quickly as they are generated. However, if the voltage gradient in volts per millimeter is high enough, electrons are generated so rapidly that ionization by collision proceeds faster than the ions absorbed by the SF6 gas and the ion avalanche produces an internal flashover.

PARTICLES IN THE SYSTEM
Despite all precautions in design, quality improvements, packing, handling, and installation, free conducting particles are occasionally introduced into the system from one or more of the following sources.

1. Mistakes in handling at the factory after the tests are completed and sealing the circuit breaker for shipment.
2. Vibration in shipment
3. Carelessness in installation, not following the proper cleanliness procedures given in the instruction manual
4. Particles generated at moving parts, such as sliding contacts  

MOISTURE
Moisture is an impurity that may cause concern in gas-insulated systems. New or recycled SF6 traditionally has a very low moisture level, typically less than 50 parts per million by volume (ppmv).

The SF6 gas compartment must be thoroughly cleaned of any moisture before filling with SF6 gas to avoid the water molecules, which stick to the solid surfaces inside the equipment, from later mixing with the SF6 gas. SF6 gas-insulated equipment should be evacuated to about (1 mmHg) and held for a period of time—dependent on compartment size—which greatly diminishes the chance for having too much moisture.

An SF6 circuit breaker, although dry after all factory tests are completed, may be exposed to excessive moisture during installation. This makes it necessary to verify the moisture level before placing the circuit breaker into service. New commercial SF6 gas may have a moisture level ranging from 30 to 50 ppmv. The allowable commercial limit is 71 ppmv as per ASTM D2472: “Standard Specification for Sulfur Hexafluoride”, a standard published by ASTM International.

It should be noted that moisture is absorbed on the interior component surfaces and that it is released from the surfaces as the temperature rises. So, the best time for drying out a circuit breaker is when the temperature is above 32°F. In other words, evacuation and drying should not be attempted when the temperature is below freezing (32°F, 0°C) unless heat is applied to the tank or cylinders using a blanket. Direct flame heat is not acceptable. The correct method for drying the moisture from a circuit breaker is with an SF6 gas cart using a desiccant tower and additionally, replacing the desiccant in each SF6 gas zone (tank) of the equipment.

ABB recommends all SF6 leaks be corrected to eliminate the migration of moisture into the switchgear. Until this required maintenance occurs, moisture will enter the circuit breaker through the leak point due to the fact that the higher concentrated moisture in the surrounding atmosphere will migrate to the lower moisture content in the SF6 switchgear.

POWER TIP: Remember when it was common practice to check for moisture in gas cylinders by opening the valves and releasing the “snow?” This practice is no longer tolerated. Today, there are a myriad of tools that easily allow you to check and capture moisture in the field. It’s just as important to regularly check your circuit breaker for moisture content during the initial commissioning process and then as required thereafter as per the manufacturer’s instruction book recommendations.

VACUUM PUMPS
Vacuum pump selection and setup is critical. See the “Sizing A Vacuum Pump” and our Dos and Don’ts for some recommendations.

When evacuating, always start the vacuum pump before opening the valves leading to the circuit breaker, and close the valves before shutting off the vacuum pump.
The vacuum gauge for reading system vacuum should be positioned separate from the vacuum pump connection. If the vacuum gauge must be connected in the exhaust line, there should be a valve between the vacuum pump and the vacuum gauge that must be closed when a reading is taken. This will eliminate a false low reading that is a result of a pressure drop in the hose.

Evacuation should continue until the maximum evacuation pressure level is reached with a target rise of <0.05mm/hour (millimeters per hour) to eliminate any moisture vapors. Failure to reach this level is an indicator of a major leak, excessive amounts of water in the circuit breaker, or a defective vacuum pump. This must be investigated before proceeding. If a large amount of water was accidently admitted into the system during installation, the vacuum pumping may be necessary for several more hours. After successfully evacuating the system, the vacuum pump should be shut off via a valve from the circuit breaker then shut down, in that order.

SIZING A VACUUM PUMP
Vacuum pump sizes as listed below are recommended.

Compartment Size

Vacuum Pump Size
Up to 500 ft3 (15 m3)
30 to 50 cfm (50 to 80 m3/h)
500 to 1000 ft3
50 to 100 cfm (80 to 160 m3/h)
1000 to 2000 ft3
100 to 200 cfm (160 to 320 m3/h)

 Do’s and Don’ts

DO’S

Vacuum Pump Type
1. Avoid belt driven vacuum pumps
2. Use dual vane, direct drive or scroll pumps
3. Consider a back flow prevention valve as a must
4. Use a gas ballast
5. Insert a manual shut off valve between pump and gas compartment

Hose Diameter and length
1. Use large hose diameters
2. Use a minimum number of connectors to eliminate potential vacuum leaks
3. Use only vacuum tight shut-off valves
4. Recommend stainless steel hoses when available
5. Ensure hoses remain clean when not in use

Vacuum Metering
1. Attach vacuum gauge at opposite end of vacuum pump.
2. Valve off pump when reading and pulling from the same fitting. Valve should be between pump and sensor.
3. Pull vacuum down to < 1 mmHg.
4. Valve off pump and check vacuum rise.
5. Target rise of < 0.05 mm/hour

DON’T

Vacuum Pump Type
1. Do not use a piston vacuum pump
2. Do not exceed 0.02 mmHg for your pump base pressure
3. Do not use a visibly damaged unit.

Hose Diameter and length
1.     Do not use long hoses; keep hoses as short as possible
2.     Do not use wet rubber hoses; dry before using
3.     Do not loosely or partially connected hoses

SF6 GAS BY-PRODUCT CONTAMINATION
How it happens
Gas contamination in SF6 equipment arises from at least the following three sources.
I.
Impurities already present when gas is received from the supplier.
However, it is highly unlikely that any measurable amount of impurities are in new SF6 cylinders, therefore, a chemical analysis of cylinders is not necessary.

II.
Impurities introduced during filling or operation of circuit breaker.
For breakers that require evacuation prior to filling, when a vacuum is pulled to 200 microns, 99.98 percent of the air will be removed from the system, and at typical operating pressures for SF6-insulated equipment, air will be less than 0.007 percent of the SF6-air mixture.

III.
Contamination introduced during arc decomposition.
Koichi Hirooka and other experts in the chemical analysis of arc decomposition, detected SF4 and S2F2 as the primary gaseous decomposition products of arcing in dry SF6.

KNOW THE MATH

According to calculations performed by Koichi Hirooka and other researchers, the following reaction can  occurs when the temperature is above 2000C:
 2S2F2 = SF4 + 3S

·       When moisture is present, hydrolysis of the SF4 can occur as follows:
SF4 + H2O = SOF2 + 2HF

·       A further possible reaction yields sulphur dioxide:
SOF2 + H2O = SO2 + 2HF

·       The HF reacts with all silicon compounds. With silica the reaction is:
SiO2 + 4HF = SiF4 + 2H2O

If the gases mentioned in the “Know Your Math” sidebar are mild in nature, a rotten egg smell will be present. Strong concentrations will cause burning to the eyes, nostrils, and lung tissues. Safety must always be your first priority. Have the proper ventilation gear with you before you begin working. If any physical irritation or mental distortion is evident, seek medical attention immediately. Even after all the SF6 gas has been reclaimed from a breaker, if it has undergone heavy or repeated arcing, great care must be used in breathing air inside the breaker.

Another byproduct of SF6 arc decomposition is the metal vapors. The vaporized metal instantly conjoins with fluorine to form metal fluorides. The metal fluorides are powders which fall on all surrounding surfaces including the bottom of the circuit breaker.

Aluminum fluoride is white, copper fluoride is tan. Both are particularly hygroscopic—if touched immediately after opening the arced area they will absorb moisture from the skin so quickly it will instantly cause severe burns. The recommended action is to remove the metal fluoride with a vacuum cleaner with HEPA filters immediately after opening the circuit breaker—the fluorides are not removed during the evacuation process. Gloves, goggles, and a respirator are required to do this safely.

After the contaminates have been exposed to the air for a long period of time, they will glean moisture from the atmosphere making them safe to touch; however, the fluorides stick to solid surfaces and are extremely difficult to remove.

Evidently conducting particles, moisture and SF6 decomposition by-products all can have an effect on the performance and safety of your SF6 gas circuit breakers. Each installed application varies from the next so there is no absolute answer to say when a breaker is “infected”. Only by conscientious testing can one realize what impurities are present in each circuit breaker.

ABB strongly advocates the testing of SF6 switchgear’s SF6 gas integrity on an annual basis. Additionally, equipment that uses only the smallest sample needed to perform the test such as infrared (IR) technology will supply consistent results that also help limit the emissions of this greenhouse gas during the testing process.

Jeff Spoljarick, ABB’s SF6 gas management Business Development Manager, Mt. Pleasant, PA, is leading the business efforts which offer complete SF6 gas management services to the company’s customers.

3 comments:

emma99 said...

It is truly a great and helpful information on ur articles.I am satisfied that you simply shared this useful information with us.Please stay us informed like this. Thanks for sharing.

circuit breaker

Peter Knauf said...
This comment has been removed by a blog administrator.
Roel Bobis said...

I wonder is circuit breaker here in the philippines is good?

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