Repair of Capacitors

by - Mr D M Tagare, MD, Madhav Capacitors Pvt Ltd , Sachin V Shelar, S Prabhu

1.0 BACKGROUND:

Usually state electricity boards float tenders to repair power transformers, instrument transformers, circuit breakers, panels etc. They have fixed the repairable/replaceable items and the cost for each and thus have standardised the process. This is feasible because many parts of transformers, circuit breakers etc. are recoverable.

Naturally following the same logic, state electricity boards are floating tenders for repair of capacitors. However, this has to be studied. In case of capacitors, lot of material is not recoverable, also the recovery value of failed material is nil. In many cases, capacitors are not repairable and repair is not economical from the point of view of cost and life.

One of the most critical locations for satisfactory operation of capacitors is a very thin gap between two aluminium foils of opposite polarity. This gap is made up of solid dielectric i.e. polypropylene and liquid dielectric. There should be absolutely no air bubble or any type of impurity in this gap. Thus when capacitors are made the moisture and air is removed under continuos evacuation lasting 4 to 5 days. They are impregnated with liquid under vacuum and sealed.

If carbonised oil with acidic impurity gets into this gap or if the oil is drained out while repairing from this gap and air gets in, then the element is beyond repair. This is a serious handicap as far as repairs of capacitors are concerned. With due diligence capacitors can be repaired but only partially.

1.1 INTRODUCTION :

Capacitors are operated at various locations throughout our country. Their application ranges from substations of electrical utilities like state electricity boards, process industries (cement, steel etc.), and water supply schemes. While the normal life of capacitors is substantial, premature failure of capacitor can be averted by proper selection and following installation and maintenance procedures stipulated by manufacturer. Inspite of these, failures in the field have been observed. Some of these failures can be rectified at site or at works. Bulk of these capacitors is operational at state electricity board. Unlike other equipments (like transformer etc.) repair of capacitors involves substantial amount, when repairs are major and may be equivalent to the cost of a new unit in some extreme cases. Thus, a conclusion as to which units are feasible for repairs have to be arrived at based on cost benefit ratio. Units manufactured prior to 1977 contain PCB impregnant, which have been banned due to their hazardous nature. Disposal of such units has to be done carefully. Repair of these is not advisable.

1.2. MODES OF CAPACITOR FAILURE:

The capacitors are liable to fail during testing at manufacturers works. This will indicate any defects in material or workmanship. Their failures can be rectified while the units are still at the manufacturers works.

After the units have been installed at the site, failures may occur due to improper selection of units or accessories or improper handling of equipments. These can be rectified only in certain cases where rectification at site is possible. In other instances, it has to be transported to the manufacturers works for further investigation and rectification.

Many consumers are still using the equipments, which have become obsolete with time or due to technological up-gradation. A typical case is in the selection of switching devices. Many consumers continue to use oil circuit breakers (BOCB/MOCB) for switching capacitor banks. These are not suitable for capacitor switching due to their poor arc quenching properties.

The other causes for failures may be due to non-utilisation of series reactors wherever they are required. Here it may be noted that wherever parallel switching of capacitors is carried out in steps, the second step and succeeding steps call for utilisation of series reactors to limit the inrush current. Failure may be compounded if the unit is weak or defective.

More often than not, capacitor units are designed at standard rating, without taking in view the exact application and the rigorous duty cycle of the capacitor banks. It is mainly so in collieries and rolling mills where the duty cycle is very stringent and supply contains harmonics. There are also locations where continuous power factor improvement is required and interruptions are not warranted. Failure of units in such locations causes heavy penalty to the user. Therefore proper selection should be made. Necessary deratings should be done with respect to voltage level, viz. 6.6 kV loads should be compensated with 7.2 kV rated capacitors, similarly 11 kV load should have 12 kV compensation to allow for compensation against harmonic overvoltage and voltage rise due to series reactor.

1.3 IMPROPER SELECTION OF ACCESSORIES.

The accessories going with the capacitor are Isolators, Circuit breakers, Lightning arrestors, Series reactors, Instrument transformers etc. Either improper rating or improper operation may cause subsequent problems, for example an isolator with improper latching of the moving contacts with the stationary mechanism causes sparking and subsequent blowing of main fuses and in its absence may cause the other protective equipment to trip. As discussed in above, selection of BOCB and MOCB, which are not suitable for capacitor switching, may, cause serious damages to the equipment or the capacitor banks themselves. There is also a serious operational hazard as it may injure operating personnel.

TABLE 1. FAILURE ANALYSIS OF CAPACITORS.

TYPES OF CHECK	ABNORMALITY OBSERVED	CAUSES	REMEDIES/ CORRECTIVE ACTIONS.
Physical.	1. Bushing broken 2. Bushing cracked, chipped or broken. 3. Leakage at the soldered joint of bushing to the tank.	Holding or shoving of units by the bushing. Breakage in transit.	Replace the broken or leaky bushings. Never hold/lift by bushing, adopt proper packaging and give necessary instructions for handling with care to transporter and user.
	4. Leakage from tank.	Improper welding.	Plug the leakages by welding. Proper care to be taken so as to form uniform welding and tank not to get too hot during spot welding of leaks.
	5. Tank bulge.	Forces caused due to short circuit conditions.	Use material of proper thickness for the tank. Match tank rupture characteristics with fuse characteristics. Unit not repairable.
	6. Loss of oil.	Improper welding, improper handling.	Leakages should be made up and immediately sealed in dust free environment.
Electrical.	1. Capacitance has increased.	One or more elements or sections in series are shorted.	Replace the elements. Units to be sent to manufacturer for proper correction.
	2. Open circuit observed on checking capacitance between terminals.	One series sections in the stack disconnected or a series connection may be removed due to bad soldering or crimping of conductors.	Connection should be made tight by soldering or crimping of conductor. This should be done properly and meticulously
	3. Insulation resistance to body is either low or zero.	Failure of wrapping (outer) insulation.	Open the unit and put new outer insulation, and reimpregnate the unit.
Testing at works. (Upon receipt of capacitors from the field.)	1. Break down on application of DC voltage.	Polypropylene/inter stack insulation of material punctured.	Open the unit, find out the faulty element and replace it.
	2. Internal noise on DC voltage.	Impurity in polypropylene.	Check the individual elements. Replace the faulty one.
	3. Break down on application of AC voltage between terminals and container.	Failure of outer insulation.	Replace the outer insulation.
	4. Discharge time not within limits. (This is a rare case.)	Resistance removed from their joints or few resistances have open circuited or high value resistance connected.	Open the unit and replace resistors of proper rating.
	5. Tan delta high.	Contamination of oil or due to elements aging.	Repairs not recommended.
Upon opening the units	1. Oil is blackened.	Due to short circuit.	Replace black oil only if black oil has not percolated into the elements.
	2. Oil is blackened on top and on sides.	Due to partial discharge at leads / through resistors, etc.	Change the oil.
	3. Oil is blackened and has intruded into the elements.	Failure of elements due to loose winding internal short circuit or moisture ingress.	Repairs not recommended.
	4. Insulating paper has charred.	Bad quality of insulating paper.	Replace insulating paper.
	5. Insulating polypropylene has melted.	Over heating and over stressing. (Short circuit)	Replace faulty element.
	6. Discharge resistance opened out.	Solder joints not proper, loose caps of resistors.	Replace the resistors.
	7. Discharge resistance shorted.	Bad quality of resistors.	Replace the resistors.
	8. Connecting leads between stacks have come out .	Improper connection during manufacture.	Properly connect.
	9. Solder connections have came out	Dry solder, non-uniform solder thickness, bad soldering.	Re-solder properly.
	10.Mechanical connection removed.	Mechanical stress due to short circuit current.	Do proper connections.
	11.Tabs, if inserted, have been torn out.	Improper handling during manufacturing.	Elements with such abnormalities be replaced with fresh ones for proper performance.
	12.Dirt in the capacitor.	Improper handling during manufacture, holes in joints.	Repairs not recommended
	13.Water traces inside capacitor.	Improper handling during manufacture, holes in joints.	Repairs not recommended
	14.Dust and contamination on inside surface of tank.	Improper chemical treatment of tank.	Clean the tank properly and do the necessary chemical treatment. (Phospatising). Reassemble and impregnate.
Inspection of stacks and elements.	1. Elements are punctured.	Improper selection of voltage stress level (Improper selection of polypropylene thickness.)	Repairs not recommended.
	2. Punctured across aluminium foil.	Abnormalities, sharp and usually slit edges of aluminum foil.	Replace by new elements.
	3. Punctured at the beginning of turns of elements.	Due to partial discharge, improper insulation provided on both ends.	Provide proper insulation in case dielectric thickness is insufficient replace elements.
	4. It has punctured between elements.	Voids in dielectric or overstressing of capacitor due to overvoltage transient and harmonics. Dust between elements.	Replace elements with fresh one and reimpregnate.
	5. Punctured randomly.	Stress on elements.	Replace elements with fresh one and reimpregnate.
	6. Elements not impregnated uniformly, some portion is dry.	Elements are pressed hard, too tight winding.	Replace elements with fresh ones and reimpregnate.
	7. Elements have spots around which oil deposition has formed a ring.	Impurities in oil and / or polypropylene forming a weak spot , partial discharge across the ring may cause the ring.	Replace elements with fresh ones and reimpregnate.
	8. Elements have solid and waxy deposit.	Impurities in impregnating oil.	Replace oil and faulty elements and impregnate.
	9. Element is as hard as rock.	Excessive compression during winding.	Replace elements.
	10. Folded aluminum edge has pressed and indented polypropylene.	Excessive compression during winding.	Replace elements.
	11. Inserted tabs have indented polypropylene.	Excessive compression during winding.	Replace elements.
	12. Aluminum foil is at uneven distances from oil gap.	Improper winding.	Replace elements.

While in certain cases above it is possible to carry out repair , in many case it may not be advisable to carry out repairs on the units as the life of such units would be uncertain and hence replacement of old unit with fresh unit is recommended.

1.4. ECONOMIC FACTORS :

If the entire dielectric of capacitor is not repairable, one ends up with building a new capacitor inside an old container. This is costlier than making a new capacitor. Only when minor repairs are involved such as internal reconnections, topping up of oil under vacuum etc. it is economical to undertake repairs. However, this can not be predicted from superficial examination of unit in the field.

1.5 RECOMMENDATIONS ON HANDLING OF OLD UNITS WITH PCB :

Studies have shown that chlorinated diphenyl (PCB) is considered as environmental contaminant and possibly injurious to certain aquatic, bird and animal life.

Following care should be taken while handling the capacitor containing PCB.

1. Avoid inhaling of vapours of PCB.
2. Avoid contact of PCB with food or pharmaceuticals.
3. Avoid contact with eyes or prolonged contact with skin.
4. Do not reuse contaminated clothing.
5. Wash skin with soap and water and eyes by flushing with water.
6. Stop leakage of the capacitor with the help of araldite, solder or any mechanical means. If leakage or spill occurs, PCB must be collected and disposed off as stated below.

1.6 DISPOSAL OF WASTE POLYCHLORINATED DIPHENYL (PCB) :

All PCB, failed capacitors rags contaminated with PCB, etc should be disposed off in an appropriate incineration plant (temperature 10000C) or some land-fill sites away from populated area. While selecting land-fill sites care should be taken to see that the PCB is not carried into natures water system by rainfall, drainage etc.

CONCLUSION:

Thus we can conclude as follows.

1. When tanks have bulged and multiple elements have been ruptured due to heavy short circuit or any other system abnormality, economically it may not be feasible to repair such a unit as the cost of repair is equivalent to the cost of fresh units and life of a repaired unit is uncertain.
2. When only a couple of elements have been damaged due to any system abnormality or defects in manufacturing it is possible to replace the same.
3. When ingress of blackened oil has taken place in multiple elements, it may not be proper to replace only oil. The unit should be discarded.
4. If a capacitor is more than 10 years old then it is better to go for a new capacitor instead of repairing an old capacitor
5. Capacitor containing PCB, can not be repaired and should be disposed off properly.

Consistent Improvements through Field Experience & Continuous Innovations for Customer Satisfaction !!

Madhav Capacitors Pvt Ltd