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APP and MD Capacitors (Low Tension Type)
APP and MD Capacitors:-
This group of capacitors has a distinct feature of having separate aluminium foil as an electrode. The solid dielectric can be two or more layers of polypropylene as in APP Capacitors or a combination of condenser tissue paper and polypropylene as in MD or Mixed Dielectric Capacitors.
Another distinguishing aspect is the introduction of a suitable oil as a liquid electric.
The Building Blocks :-
1) The polypropylene Film is specified as BOPP with hazy surface on one or both the sides. The inherent strength is very high (480 - 600 Volts/micron DC.). The molecules are brittle though. The film is stretched in both directions during manufacture, thus orienting the crystals along the line of stretch i.e. biaxially. This improves impregnation by oil and increases it's strength.
The haziness, about 0.2-0.3 microns average, is actually roughening of a smooth surface by creating multiple, cross-connecting, microscopic channels. This helps the impregnating oil to rise through wick action and gravity and fill up all possible empty cavities.
Very thin films are costly and comparatively failure prone, since the haziness comes at the cost of overall thickness.
2) Condenser Tissue Paper is actually a mass of thin pulp, rolled to desired thickness and dried. The fibres or micelle, mesh into one another. This gives large cavities inside - from which trapped air and moisture must be meticulously removed and substituted with oil.
The paper molecules are flexible compared to PP molecules. They can withstand sudden electrical pulses much better and are ideal as dielectrics on networks which produce all types of surges continuously. Thus Mixed Dielectric capacitors are suitable for a very rugged and exacting service. However, losses in paper are high. The voltage stresses are low. The size and cost of MD capacitors are high.
3) Aluminium foil plays the part of a conducting electrode. It does not play any part as a dielectric material. Hence its thickness can be conveniently reduced - the common thickness available today being 5 microns.
The foil edges are cut mechanically. If examined under a microscope they have irregular and sharp points jutting out as shown. The voltage stresses on these sharp points rise very high and cause partial discharges into the edge gap.
This is taken care of by - Folding the edge on itself by a few Mms or
- by laser cutting-which is ideal-but very costly.
4) Oil replaces air and moisture in the voids within the dielectric portion. It gives strength and increases the life of a capacitors and as such is a very critical component of the entire system. It itself must be filtered to very fine degree and degassed. It is reinforced with anti-oxidants and scavengers. The scavengers lock out acids and broken chain lengths of oil molecules arising out of partial discharges.
5) Discharge Resistors : Normally externally fitted on L.T. Capacitors, they discharge the residual voltage from the peak level to 50 volts or less, within one minute. Burnt out resistors will not perform and present a risk to human life as well as to capacitors.
These resistors form a sizeable - portion of the total heat loss defined for a capacitor - although this portion of the loss does not reflect the dielectric quality.
6) Internal fuses helps to isolate a faulty element and keep the capacitor going. In L.T. capacitors, the elements in a phase are all in parallel. Thus isolation of an element may cause phase unbalance - but no harmful increase in voltage on remaining elements. Quite often, in a well constructed unit, these fuses become redundant.
Advantages of APP/MD L.T. Capacitors :-
1) Unlike MPP Capacitors (in some cases), there is no deterioration of output current with passage of time.
2) Losses in APP Capacitors gather around 1.0 to 1.5 Watts/KVAR.
in MD Capacitors gather around 1.5 to 2.0 Watts/KVAR.
in MPP Capacitors gather around 2.0 to 2.5 Watts/KVAR.
3) They are more rugged and can withstand severe voltage surges. They can also withstand upto certain amount of harmonic loading.
Disadvantages of APP Capacitors :-
1) They are more bulky and heavier than MPP Capacitors.
2) Their costs are in multiples of two and more, than the costs of equivalent MPP Capacitors.
3) They are prone to developing oil leakage’s - particularly under higher temperatures. This reduces their actual life.
4) Replacement of a faulty unit in a bank is clumsy.
"Madhav" Capacitors are manufactured with full quality control at each stage. The basic blocks are individually tested with full understanding of the weakness of each constituent. The subassemblies and final products are tested for compliance with in-house, as well as BIS Standards. Their field performance over a period of 45 years is outstanding. (Earlier these were all paper capacitors).
Where it is advantages to go for APP Capacitors :-
1) They are preferable on networks where the voltage fluctuations are wide and night time voltages rise considerably.
2) They are preferable in installations where current and voltage surges are present due mostly to the nature of machinery used.
3) They are preferable where moderate harmonics are suspected.
4) They are preferable near generators, bus ducts, hazardous areas etc.
5) They are ideal - when they are not expected to be obsolete in a short time, where longer trouble - free, least maintenance operation, is expected and where of-course, the budgeting is liberal and open-minded.
Consistent Improvements through Field Experience & Continuous Innovations for Customer Satisfaction !!
Madhav Capacitors Pvt Ltd
