How Does Your Capacitor Works ?
Self Healing MPP Capacitors (Low Tension Type)
Self - Healing Process :-
In the traditional capacitors, two or more layers of insulating, solid dielectric were wound between individual layers of thin aluminium foil- to form a capacitor. Now, no layer can have an absolutely uniform thickness at every spot, nor can have zero pinholes & conducting particles - over several Sq. meters of a surface that goes into forming today’s power capacitors. If a single layer of a solid dielectric were to be used, it will fail at several points. A second layer can cover the defects in the first layer - since defects on both of them are likely to overlap at very few locations. This possibility further increases by a quantum jump if three layers are used between conducting layers of aluminium foil.
However one can vacuum deposit a conducting metal of low evaporation temperature like zinc or aluminium & do away with a separate thick aluminium foil altogether. When a short occurs across a defects, the short circuit current can instantly evaporate this deposit & form aluminium or zinc oxide - both of which are non-conducting. Thus the area around the defect is isolated & the capacitor can rework. This process is called Self-Healing. The thickness of the aluminium deposit has to be accurately controlled so that the film definitely evaporates & does not require too high a temperature. If it fails to evaporate & isolate the defective spot, a permanent short circuit will form & the capacitor will go out of service. This thickness is measured in resistance per unit area. It is 3-4 Ohms for aluminium & Ohms for zinc.
Advantages of Self Healing Capacitors :
Since all the defects in a single layer of Metallised Polypropylene (MPP) can be healed at the manufacturing stage only, a single layer capacitor can be formed quite comfortably at higher dielectric operating voltage stresses. This gives a capacitor that replaces a thick aluminium foil(5-6 micron) with a thin deposit (0.2 to 0.3 micron) & allows single layer of thinner polypropylene in place of two or three layers of thicker polypropylene, the size & the costs go down drastically. It has replaced the traditional capacitors at a very fast rate.
The Dry Capacitors :
A Metallised film has an edge clearance at one end - usually 2.5 mm for 440 Volts capacitors. Metallising reaches the edge at other end. Alternate layers are so formed that the Metallising on one set comes at one end A & Metallising on following set comes at end B. Round coils are wound & the ends are sprayed with zinc. A conducting lead is soldered on to these surfaces. There is a microscopic layer of air between these layers. The coils are wound tight. They are further shrunk under heat treatment. This reduces the air thickness between layers very significantly.
The Short Falls of S-H Capacitors :
a) Moisture getting in between layers oxidises the thin deposits in it thickness fully.The oxidising boundary detaches a healthy section of a deposit. This results in rapid or continuous fall of capacitor current. This happens mainly in badly & loosely wound capacitors. The coil ends are normally sealed with an epoxy or the coils are immersed in an insulating liquid to prevent this.
b) The zinc-spray & the aluminium deposit form a bi- metallic physical joint- which corrodes aluminium preferentially in the presence of moisture. This cuts off the entire healthy metallisation below from the conducting edge, resulting in rapid fall of capacitor current. To prevent this, the metallisation thickness just at the edge conducting edge is increased by what is called a heavy edge deposit. Another method that helps is - to deposit zinc on the lower deposit of aluminium - in what is called an aluminium -alloy deposit.
c) Consider a large air gap between layers & an irregularity in the form of a sharp point. As the voltage increases across the dielectric, at same point there will be electron streamers originating from this sharp point & cutting through the air path. This is the beginning of a partial discharge. It will create hot spots & eventually fail the coil. Air has a breakdown voltage of 4 KV/mm & can easily produce & sustain partial discharges.
However if the air path is microscopic, air will breakdown and establish a short circuit path, rather than sustain a partial discharge. This will increase the leakage current. Leakage current is Ohmic. It makes the coil hot.
d) The most critical portion of the S - H Capacitor is the edge gap. The full coil voltage applies across this gap. It is spread on a very thin base 0.2 / 0.3 micron thick as against 5-6 micron thick in traditional capacitors. The voltage stress is very high- leading to instant or even sustained partial discharges, should the voltages cross the air gap strength.
Normally a 2.5 mm gap across a 0.3 micron base can sutain A.C. voltages upto 440 volts + 10%. This makes these capacitors unsuitable where there are steady high voltages or sudden & continuous voltage fluctuations.
Please note that European networks with distribution at 380 volts are quite comfortable with S-H Capacitors.
A way to over come these defects would be to fill up these gaps with a suitable oil under vacuum. The oil with breakdown values of 60-80 V/micron increases the gap strength considerably. Japan is carrying out field trails with S-H Capacitors, filled with SF-6 gas, under pressure, on networks rated at 3300 & 6600 volts AC.
e) In S-H Capacitor, current flows from one end of the coil to the other end axially along the cross-section determined by the full length of the wound foil. By contrast, in a traditional capacitor, it flows circularly along the length of the winding with a cross-section determined by the width of the wound foil. This gives a very low self inductance to S-H coils as compared to traditional coils. These self inductance’s are inadequate to inherently limit starting or paralleling currents between two capacitors as compared to traditional capacitors. These unrestricted current flows, create instant high voltages, puncture a dielectric & blow up capacitors. Capacitor bursting is more common with S-H Capacitors than with traditional capacitors.
A choke coil in S-H capacitor takes care of this problem & is a must.
In conclusion one can say that S-H Capacitors are highly economical & could be used successfully if we understand what their limitations are & under which circumstances - not to use them.
Madhav S-H Capacitors are not tightly wound. They are dried and impregnated under high vacuums with capacitor oils. Further each unit has internal or external choke coil. Besides, we study strictly where they are being applied.
Where it is advantages to go for MPP Type Capacitors ?
1) Distribution lines where voltage and load variations over a 24 hour period are moderate. Typical example -Mofusil areas with a large spread of various loads served by substations with automatic on load tap changes. One can down scale this to suit.
2) Automatically controlled capacitor banks with built - in over voltage, under voltage, over current & p.f. correction controls & with current limiting chokes on each step.
3) Rural distribution lines - heavily overloaded and supplying power at perennially low voltages. To some extent, overloaded zones of other distribution lines also.
4) And of - course where one's budget for capacitors is rather tight, but with attention to (1) & (3) above.
Where MPP Capacitors are not to be recommended :
1) On load with widely fluctuating currents such as strip mills, arc furnaces, workshops with heavy presses and similar impulse type energy drawing machines, welding machines, etc.
2) Locations where higher incidence of harmonics are expected.
3) Hazardous areas [ oil installation, new power generators or generator bus ducts] where explosions are not allowed. Generally MPP Capacitors are more explosion - prone than other types of capacitors.
4) Areas with high short circuit level for distribution networks. (This is likely to affect self healing).
5) Supply systems with wide daily voltage fluctuations - where the night time voltages shoot up beyond the guaranteed limits.
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