Introduction

Our modern civilization; often called the machine age is more truly the energy age. Through the ages man has successfully devised more and more complex machines to make things and perform services. Man could succeed with this program only as he discovered the sources of energy to power his machines.

There are basically two sources of energy :

1) Renewable sources

2) Non-renewable resources.

The renewable sources are wind, sun, waves and energy from water. These sources are continuously replaced by nature and therefore they are inexhaustible.

The nonrenewable sources of energy are also called conventional sources of energy and these are fossil fuels such as coal, gas, oil and nuclear fuels. These sources are limited in nature and would exhaust one day.

As the conventional sources are likely to be exhausted one day, the generation of electrical energy from non-conventional sources assumes a greater significance .

However, the development of technology will take considerable time as it has many basic problems, technical as well as economical. Thus immediate solution lies in the conservation of energy.

Power Capacitors have occupied unique position in LT distribution network and their roll as energy conservation device is undisputed

How capacitors conserve Energy

Electrical energy is most convenient form of energy as it can be transmitted , distributed and utilized over a very large area. Electrical loads further convert electrical energy into other forms of desired output such as mechanical motion, heat, light etc. Electrical loads which operate by virtue of magnetic field induced by an alternating current power source are called inductive loads. These include equipment such as induction motors, electric furnaces, fluorescent lamps and transformers. These machines draw from supply "apparent power" in terms of "KVA" which is in excess of "useful power" measured in KW required by the machine. The ratio KW/KVA = Power Factor. The system will be more efficient when KVA=KW i.e. reactive power (apparent power) - KVA is zero and power factor is unity.

Power factor can be alternatively defined as the cosine of the phase displacement angle by which the total circuit current leads or lags the circuit voltage. These relationships are shown in Fig.1.

The power factor improvement is therefore equivalent to reducing the lag between total current I and voltage V. In an inductive circuits (loads) the curent lags behind the voltage and in purely capacitve circuit (in capacitor), current leads voltage by 900.

Therefore, the introduction of capacitive elements to inductive loads can compensate for the lagging currents associated with these loads. Thus we can consider capacitor as a ôreactive power generatorö (KVAr generator). This effect can be explained as follows :

"A capacitor may be considered a kilovar generator because it supplies the magnetizing requirement (kilovars) to induction devices. This action may be explained in terms of the energy stored in capacitors and induction devices. As the voltage in AC circuits varies sinusoidally, it alternatively passes through zero-voltage and starts towards maximum voltage, the capacitor stores energy in its electrostatic, and the induction device gives up energy from its electromagnetic field. As the voltage passes through a maximum point and starts to decrease, the capacitor gives up energy and the induction device stores energy. Thus, when a capacitor and an induction device are installed on the same circuit, there will be an exchange of magnetizing current between them, that is, the leading current taken by the capacitor neutralizes the supply lagging magnetizing current to the induction device. The capacitor may be considered to be a kilovar generator since it actually supplies magnetizing requirements to the "induction device".

Manufacturing of LT capacitors (Review of processes then & now)

As capacitor basically consists of two elements - dielectric (which stores the charge) and electrodes (which carries current), the manufacturing process revolves around these two basic materials.

Traditionally paper and oil (as impregnant) have been used as dielectric and aluminium foil as electrodes.

In the mid 1950s synthetic fluids classified under the collective term askarel were introduced for use of impregnants. These fluids became generally known by the initials "PCB" (polychlorinated bipheny) and they were supplied under trade names , such as Aroclor, Biclor, Pyrclor, Clophen, etc. They had the advantages of being non-flammable and having a high permittivity, and high dielectric strength. PCB has to be used with paper made from wood pulp, called kraft paper after the process by which it is made, as shortened life resulted if rag- based paper was used and impregnated with PCB.

Because of initial viscosity of PCB coupled with its relatively high cost compared with mineral oil, the amount of impregnating fluid contained within the capacitor had to be drastically reduced. The original cylindrical windings were, therefore, replaced by coreless windings compressed to a flat overall shape and assembled in a rectangular pack before being inserted into a tight-fitting container with the free dielectric fluid kept to a minimum. Inspite of many improvements in kraft paper and polychlorinated biphenyl impregnants, by the mid nineteen-sixties it came to be accepted that 100 KVAr presented the maximum high-voltage unit rating obtainable with impregnated paper dielectric.Developments in low-voltage unit type capacitors were also proceeding and 50KVAr was found to be the maximum unit rating obtainable with PCB impregnated paper dielectric design in the 380 to 660 voltage range.

The reduced rating was due to larger volume of material per KVAr associated with low-voltage capacitor design, and in any event was not serious as the low-voltage market demands a closely stepped range of ratings from 1KVAr upwards.

The end of 1960s saw the advent of the mixed dielectric capacitor with ratings upto 225 KVAr. The mixed dielectric consists of oriented polypropylene (OPP) interleaved with paper. The paper acts as a wick, permitting the impregnating fluid penetrate between the layers of film.

In 1975 virtually all the capacitors produced were impregnated with PCB. However objections were beginning to be raised in respect of its use. It was claimed with PCB which is not bio-degradable, constituted both a health and an environment hazard. The objections were so strong that the manufacture of PCB in UK and USA had stopped by 1978. Japan banned the use of PCB completely, America restricted its use and many food industries, paint manufacturers and water authorities in the UK also objected to its use.

Therefore, capacitor manufacturers were completed to look for alternative impregnants for both low-voltage and high-voltage capacitors. It was difficult initially to find a satisfactory replacement.

In the case of low-voltage capacitors the necessity of finding a new impregnant coincided to some extent with developments already in progress to substitute polypropylene film for paper as dielectric.

It is obvious that drastic changes in capacitor design have occurred over the past 25 years and the impact of these in reducing the weight per KVAr.

Numerous alternative impregnants to PCB have been, and are being, investigated. But so far none has possessed the quality of non-flammability, although they have properties similar to those of PCB without being non-bio-degradable. The capacitor technology was revolutionized by introduction of biaxially oriented polypropylene film. The film is a remarkably good dielectric and it displaced paper from its position of principal dielectric in the manufacture of capacitors.

The polypropylene film has a very smooth finish and therefore it does not provide a good wetting surface for spread of impregnating oil.

The capacitor manufacturers has to take recourse to a thin layer of paper juxtaposed with polypropylene film so that
the natural absorption of paper provided an uniform spread of oil across the polypropylene film when the capacitor cell was wound.

This construction got considerable acceptance primarily because it was a strong design to sustain high stresses which are present at HT voltages. The same design found use in LT systems as well because high dielectric strength of polypropylene resulted in considerable reduction in content of dielectric and oil compared to earlier design of capacitors which has only paper as dielectric. Apart from low dielectric strength, the paper also produced high temperature rise in continuous operation which required large quantity of oil for removing heat and also larger radiating area of the container.

Modern Process

With developments in insulation technology and other efficient raw materials, the sizes of all capital electrical equipments like transformers , motors etc. got reduced significantly. The design of capacitor came under close scrunity because combination of polypropylene with paper with large oil content began to be looked upon as bulky and inefficient.

A radical change in manufacture of capacitors was brought about by a new technique of integrating the conducting medium with the dielectric. The separate aluminium foils were replaced by a coat of metallisation on the polypropylene film itself.

This construction reduced the size of capacitor by nearly half. Paper was dispensed with and the dielectric strength of polypropylene film was put to very effective use to produce a very compact and the light weight construction of capacitors. These capacitors are known as metallised polypropylene film capacitors or MPP capacitors.

Significant gains of MPP Technology

MPP technology proved to be of great relevance from operational view point. The capacitor performance was greatly enhanced because of very low internal loss. This occurred owing to absence of paper and facilitated energy conservation.

MPP technology has brought forth several features of convenience :

Use of oil was also considerably curtailed and totally dispensed within some designs. The dimensions became very compact and the weight was also reduced leading to ease of handling.

Hermetic sealing can be limited to individual cells because of facility of modular construction. The down time for repairs is reduced considerably and any general technician can attend to repairs at site.

Costwise an MPP capacitor is about 40% to 50% cheaper than mixed dielectric capacitors.

The design of MPP capacitor permits individual canning of single phase cells into special long drawn aluminium containers. Fault location becomes easier because it it localised to any one cell.

MPP capacitors have a natural property of self healing. This relates to localised fault current which may flow for a very short duration at a small point where some impurity may break down the dielectric. The current ceases because the metallisation evaporates under the heat of fault current. This property limits the damage due to localised fault.

MPP Technology - Apprehensions

MPP Technology, through modern, is often criticised on various grounds, i.e. some users have apprehensions about it’s basic suitability for power capacitors. The apprehensions and their clarifications are as follows :

A) Single layer dielectric design is introduced to stand in price competition war!

MPP technology is a breakthrough in capacitor manufacturing and is a result of several years of dedicated efforts. As it is an advanced technology it offers better product at a lesser cost. Technology always aims at simplification of process, better understanding of materials i.e.optimum use of materials. MPP technology has achieved both, cost reduction is a by product. Newer technologies making better and more cost effective products is a world wide phenomenon as is evident in electronic products.

B) MPP technology gives short life to capacitors due to

a) Thin metal coating

b) Trapped air in winding

c) Critically low contact area !

a) In the traditional method of manufacturing capacitors, the minimum thickness of aluminum foil is limited both by foil manufacturing as well as due to winding process capability as it is a separate element in winding. Both these limitations are not applicable to MPP film, as electrode becomes an integral part of dielectric system. Winding process requirements are met by MPP film.

The thickness of electrode is decided by purely electrical considerations such as :

1) Resistance of electrodes

2) Di/dt considerations.

Once these factors are taken care of long life is assured. Thus higher metal thickness beyond the optimum does not decide the life of capacitors.

b) In traditional method the trapped air is removed by vacuum impregnation.

Upgradation of technology requires alround upgradation of manufacturing process. MPP film manufactured on a state of the art plant but capacitor coil wound from this film on outdated winding machines, is bound to give inferior products. In such case MPP technology is not at fault. The right technology would remove this fault . Capacitors wound on ultra modern winding machines running at high speed with accurate tension control creates "near vacuum" between film layers. After undergoing endspray process, and making lead connections, coil ends are ‘sealed’ by epoxy to prevent the contact of air with electrode system.

Thus the process does not require vacuum impregnation and oil to keep the air out. Absence of air prevents carona effect and partial discharge.

c) The process of metallisation gives "heavy edge" deposition to the electrode and it is these heavy edges which make contact with endspray metal. Thus contact area is adequate . Further, capacitors are designed and checked to meet dv/dt and di/dt parameters for the required duty. Manufacturers not having adequate technical backup and testing facilities are likely to give products not meeting the specifications which can give a bad name to MPP technology.

MPP capacitors are only suitable for small ratings upto 300V AC

MPP film technology is dynamic technology and it has made in roads in all spheres of capacitor manufacturing . HV capacitors and HV high energy pulse discharge capacitors (6.6 KV) are being made using MPP film. With these applications, L.V. power capacitors seems a small field and 300V limit an insignificant border.

Self healing property of MPP film ultimately kills the capacitor !

Self healing property of MPP film capacitors is a unique property, not present in any other type of capacitor construction. This offers several advantages

* Small internal faults are cleared automatically and damage is restricted to local area.

* The loss of electrode area is so small that it does not affect total area and hence rating of the capacitor . Many million self healings are required to reduce the effective area.

In service, the loss of capacitance is observed in all designs of capacitors. In some designs the failure is sudden and with self healing this failure is soft . Soft mode allows capacitor to be monitored and replaced in due course of time. All capacitors have limited life.

For the capacitor to maintain the soft failure mode, throughout its life, it must be properly designed for the application, operated within the designed limit, maintained during use, and retired before secondary faults can occur. System conditions and wrong application of capacitor are more likely to hurt the capacitor than the self healing property.

IS 2834 which governs the specifications for shunt power capacitors did not include MPP designs in its earlier editions but since 1986 it has recognized MPP as fully compatible for use in different temperature categories right upto super tropical category of 50 deg. C ambient. There is no distinction between different designs as far as performance and other criteria laid down by the IS are concerned.

Factor affecting design of capacitors

As noted earlier capacitors basically consist of two elements- dielectric and electrode - factors affecting these elements influence the design of capacitors.

These factors are :

a) Dielectric strength and Mechanical properties

b) Loss factor ( Tan d)

c) Partial discharge inside capacitor

d) System conditions - voltage and voltage variation , presence of harmonic frequencies, switching frequency and duty of capacitor

e) Environmental condition - temperature humidity etc.

a) Dielectric Strength and mechanical properties

Plastic films - Polypropylene or polyester has already replaced paper in western countries as dielectric in capacitor. Polypropylene has excellent dielectric strength (380 KV/mm for 8 mic. film) and low loss factor (2 x 10-4 at 1 Khz ) against 15 KV/mm and Tan d 3 x 10-3 for paper . Further it has superior mechanical properties in both machine and transverse direction and it is more suitable for coil winding on high speed automatic machines as compared to paper. These properties allow designer to use higher stress values resulting into compact and efficient design. Stress values of 60 V / mic. are common in European design. Further MPP film has positive attributes due to inherent self Healing property of film.

b) Loss factor - Tan d

The loss factor of the capacitor is based on a inherent value measured on the metallised film. However this factor is much more influenced by the capacitor construction and the healing behaviour . Complete healing and good contact between endspray and metal coating of the film leads to the capacitor with a low loss factor. Coating with a surface resistance of 6-9 ohms cm2 are found to give minimum loss factor. This factor is also influenced by the surface structure of the coating of the film. Rough surface structures generally lead to greater capacity losses than smooth ones.

c) Partial Discharge inside capacitor

Partial discharge can occur on the film under influence of alternating current and presence of air. The resulting ions influence the insulating properties of plastic film. Partial Discharge can occur above a specific voltage. Beyond this so called partial discharge inception voltage, the film is damaged, depending on voltage and length of exposure. For a.c. application, there is a particular point at which the material losses itÆs insulation characteristics this point is also influenced by frequency, higher the frequency lower is inception voltage.

d) System conditions

Electrical system conditions such as voltage, presence of harmonics, affect the design of capacitors . The equipment should be operated as close to rated voltage as possible. Systems where high voltage - over voltage - could occur cause electrical breakdown - whenever harmonics - which are generated by non-linear loads such as rectifiers, solid state drives, inverters etc. - are present, current drawn by capacitor at higher frequencies will be more and it may damage the capacitor. The duty of capacitor - i.e. fluctuating loads, impact loading on motors, and switching frequency of such loads will require special considerations in design of capacitor.

e) Environmental conditions

Operating temperature effects the performance of all electrical equipment’s. As the dielectric properties are dependent upon temperature as well as humidity, due care has to be taken while designing capacitors. Continuous operation at higher temperature reduces the life of capacitor.

National & International Standards for L.T. power capacitors

There are three National and one International standards for these devices (for shunt capacitor for LT Power system application). Indian Standards are :

IS:2834 - 1986 (Specification for shunt capacitors for power system)

IS13340-1993 (Power capacitors of self healing type for A.C. power systems having rated voltage upto 650V - specification)

IS 13341-1992 (Requirements and methods for ageing test, self healing test, and destruction test on shunt capacitors of the self healing types for AC power system - 650V)

International Standard :

IEC Pub.831-1 (1988) - Shunt capacitors of the self healing type for A.C.power systems having a rated voltage upto and including 660V.

These standards laydown the basic requirements for the capacitor applications & system conditions. They also give the methods for testing to meet the requirements.

Important parameters influencing the performance of capacitors

Maximum permissible voltage

The system voltage fluctuates due to transients , change of load conditions, presence of harmonics, etc. The amplitude of the over voltages that may be tolerated without significant deterioration of capacitor depends on its duration, the number of applications and the capacitor temperature. Normally it is assumed that the capacitor will not be subjected to higher voltages than 1.15 UN for more than 200 applications (UN = rated voltage).

Switching voltages having a first peak of 2 2 x applied voltages with a maximum duration of 1/2 cycle can occur when capacitor banks are switched On. Such 5000 switching operations per year are acceptable under such conditions. The maximum peak transient current may reach a value of 100 x IN.If the frequency of the switching increases or the magnitude of peak current exceeds , the values have to be limited to lower values by using current limiting devices.

Maximum Permissible Current

Generally Power capacitors are required to sustain a continuous current (rms value) of 1.3 IN at rated sinusoidal voltage and rated frequency, excluding transients, if we take capacitance tolerance of +10%, the maximum permissible current can be upto 1.43 IN.

The overcurrent are to take care of over voltages upto & including 1.1 UN as discussed in 7.1 above. Also the combined effects of harmonics have to be considered.

Effect of Harmonics

As the reactance of the capacitance is inversely proportional to frequency :

It follows that if the applied voltage contains components having higher frequencies than fundamental frequencies, the current will be greater than would be produced by the same voltage at fundamental frequency.

In practice the higher frequency components are invariably the higher odd number harmonics of fundamental supply frequency such as third, fifth, seventh etc.

If V1 = r.m.s. value of voltage at fundamental frequency,

I 1 = r.m.s. value of capacitor current due to V1, h3, h5 , h7 etc. are the rms values of the third, fifth, seventh etc. harmonics expressed as a percentage of V1.

Then total capacitor current in I= 0.01 I1 (1002 + 9 h32 + 25 h52 + 49 h72 etc.)

The effect of harmonics on power capacitors therefore needs proper understanding, as all modern installations have solid state drives, converters, inverters etc. which produce harmonics. The harmonics can also be present due to rectifiers and saturated transformers. Harmonic analysis should be carried out and data made known to capacitor manufacturer for proper design of capacitor.

Testing of Power Capacitors

The standards referred above give the details of test requirements, test conditions etc for testing power capacitor. The tests have been classified as :

* Routine Tests : Tests to be carried out on every capacitor by the manufacturer.

*Type Tests: These tests are carried out to ascertain that the capacitor meets all the requirements as regards design, size, material construction, specified characteristics and operational requirements.

*Acceptance Tests : These tests form the basis of acceptance between manufacturer and purchaser. These tests could be routine tests and/or type tests.

Type tests which help to predict life and long term performance of the capacitors are discussed.

Ageing/Endurance Tests

The importance of this test is evident as IS13341 deals, exclusively with this test. Even though this is classified as type test, it should be routinely carried out on a predetermined batch frequency. This will help to ascertain long term performance of capacitors whenever identical material is purchased from different sources or material specifications are changed due to unavoidable circumstances which is a reality in India .

During the Ageing Test, the capacitor is subjected to the following stresses which enhance failure :

1) Thermal Stress (Stressing of dielectric)

2) Voltage Stress (Stressing of dielectric)

3) Current Stress (Stressing of electrode).

The sequence of the testing is as follows :

a) The capacitor case temperature is taken to highest mean temperature specified, either in enclosure wherein heated air is circulated or in liquid bath and this temperature is maintained constant during test.

b) The capacitance is measured in temperature stabilized condition.

c) The capacitor is energized at a voltage equal to 1.25 UN for 750 hrs. (voltage stress).

d) After 750 hrs, capacitor is subjected to 1000 discharge cycles consisting of :

i) charging capacitor to a d.c. voltage of 2 UN and

ii) discharging the capacitor through an inductance of

L = 1000 Mic. Henry tolerance + 20%

C where C is the measured capacitance in mfd.

(This creates voltage and current stress.)

e) The capacitor is energised at a voltage equal to 1.25 UN for 750 hrs. (voltage stress).

Total duration of test is 1500 hours. During the test the temperature of case is to be maintained constant and no permanent breakdown, interruption or flashover is allowed . At the end of the test , the capacitor is allowed to cooldown freely to ambient temperature and the capacitance is measured under the same condition as before the test. The maximum permitted variation of capacitance values are 5 % on one phase and 3 % average over three phases. It can be shown that endurance follows an inverse law EN where E is the stress and value of n varies between 9 to 12. Higher values of n indicate better reliability.

Verification of fire properties

Fires in electrical installation have always posed problems and it is essential to verify that failure inside capacitor does not start fire. Risk of fire is more prevalent in oil filled capacitors. Certain impregnant have shown 30-40% improvement in fire retarding properties of capacitors. It is therefore essential to use fire retardant materials in capacitor construction. In case of hazardous condition capacitor filled with SF6 are used.

Power capacitors are normally fitted with safety devices such as pressure sensitive devices which disconnect the capacitors in event of internal faults and avoid explosion and prevention of fire.

These tests are covered by IS 12672-1989 and the destruction test described in IS13341-Clause 7 also aims at proving this test. Failure inside capacitor is promoted by application of DC voltage and its behaviour is checked when a.c. is applied.

Conclusion

MPP Technology

1) MPP technology is a milestone in the manufacture of capacitors.

2) Apprehensions about this technology are cleared by proper technical explanations/data.

3) Manufacturers who had criticised this technology have adopted this technology either by front door or back door methods. Those who have not yet adopted, are likely to do so in near future.

4) This is the technology of today and not of yesterday.

Capacitor is a highly stressed equipment in electrical system compared to other devices. Unless it is properly specified and correctly applied it is bound to fail. The testing , particularly endurance/ageing tests and tests to prove the safety of the device play important part in design and manufacturing of capacitors.

As some of the type tests decide the long term performance of capacitors, they should be carried out on regular basis by the manufacturers.

Capacitors will continue to play the roll of energy conservation device considering the present power scenario.