Baldev Raj Narang, in this technical article, explains the various types of low-tension (LT) capacitors.The three types of LT power capacitors have their distinctive features. The purpose of this article is to present a clear technical comparison of the different types to enable users to make an informed selection, after taking into account the superiority or otherwise of various performance parameters of the three types. Oil filled MPP (metalized polypropylene) construction is not discussed because it is very uncommon. Oil causes erosion of zinc layer over a period of time due to chemical reaction, thus most of the manufacturers employ resin-filled MPP construction.
The various features of this basic type of MPP capacitors are:
Self Healing: MPP film gained widespread acceptance due to its path-breaking feature of self healing. It takes care of sudden heavy inrush currents caused due to any internal or external factors without disconnecting the capacitor from the supply by localized self-healing process at the locations which experience overheating.
Overpressure Interrupter Mechanism: It is a three-phase mechanical fuse incorporated in each capacitor. If there is excessive internal pressure development due to overloading or at the end of service life of capacitor, the mechanical fuse safely isolates all the poles from supply thus avoiding any unsafe condition during failure.
Segmented Metalized Film: This special design film is being used increasingly in place of single continuous type construction. If there are several occurrences of self-healing process in a small metalized film area due to heavy currents, self-healing action alone may not be able to cope up with the situation due to the enormous amount of energy produced. This may result in failure of the capacitor.
This is prevented by segmented film construction, which restricts the fault within the affected segment area and the affected segment is isolated. Hence, the fault is not allowed to spread throughout the film, enhancing the capacitor’s life. However, the use of segmented film is rarely resorted to by manufacturers for economical considerations. Once the users and consultants start insisting on their preference for segmented film construction, the popularity of this type will increase.
The above-mentioned features make the capacitor inherently self protecting thus eliminating provision of any external safety measures.
The other advantages of MPP capacitors are: lower dielectric losses, lower size, lower tolerances and ease of installation.
The basic MPP capacitor element, which is the active part, is the same as that for resin-filled MPP capacitor, thus lending it the all important self healing feature.
Features that separate gas-filled MPP from the resin-filled type are:
Absence of flammable impregnating agents: The use of biodegradable non-toxic inert gas as filling agent lends it increased protection as the gases emitted in the event of a failure do not encounter resin.
No carbon footprint: Use of inert gas as filling medium makes the construction environment friendly and renders zero carbon footprint.
Lower operating temperature: The presence of inert gas acts as a cooling agent and lowers the operating temperature
The benefits of MPP design provide lower dielectric losses, lower size, lower tolerances and ease of installation as available for resin filled MPP type.
Features that place gas-filled MPP at a disadvantage over resin filled MPP are:
Insulation of the active part to ground: The filling material provides ground insulation. Oil and resin have a much higher dielectric strength of 10kV per mm as compared to 3 kV per mm dielectric strength of inert gas nitrogen. However this does not create any issue because bare winding is not used and additional earthing barrier is available due to presence of insulating liner used during manufacturing.
Heat dissipation: A better thermal conductivity of the filling medium provides better heat dissipation leading to increased capacitor life. Thermal conductivity of nitrogen (0.026 W/m°K,) is much lower than that of resin (0.15 W/m°K). Thus, heat dissipation is poor for gas-filled design. However the cooling effect produced by inert gas compensates this.
Risk of undetected gas leakage: The possibility of potential undetected gas leakage exists in case of any cracks in the casing leading to safety hazard due to operation of capacitor without a filling medium.
Possible oxidation of MPP film: The construction process of gas-filled design is very critical and requires moisture and oxygen to be removed under vacuum in an autoclave. There is a possibility of traces of moisture and oxygen being left inside leading to oxidation of metallization on MPP film. The deterioration of MPP film causes multiple issues like internal discharges, decreased capacitance value and increased losses.
Inefficient working of over-pressure interrupter: The over-pressure interrupter mechanism works best with resin-filled construction as the winding is solidly held in the can and a well defined localized pressure area is created for accurate, faster and effective operation.. The precision and perfection needed for operation of over-pressure interrupter mechanism is not feasible in gas filled construction.
Dislodging of zinc layer: The arcing between winding terminals during normal operation exerts reverse pressure on the zinc layer away from the winding surface. Over a period of time this causes loss of zinc metallization, resulting in faster failure of capacitor. This drawback is absent in epoxy-filled construction as the encapsulation with resin gives mechanical strength to hold zinc metallization in place.
APP (All Polypropylene) capacitor is a forerunner of MPP design. It is considered more robust due to use of higher thickness film along with aluminum foil and operation of active part while being fully submerged in oil, which is the filling medium. After the introduction of superior MPP technology with self-protecting features and with possibility of equally or more robust construction, low-voltage APP capacitors have been phased out almost everywhere in the world except in India where they still enjoy some popularity due to non availability of good quality MPP capacitors.
Features that place APP capacitors at a disadvantage over MPP capacitors are:
Absence of inherent safety features: The absence of any inbuilt safety mechanism to clear internal faults leads to continued operation under abnormal conditions of over-temperature, over-pressure and degradation of oil. This leads ultimately to complete failure under unsafe conditions. The undesirable mode of failure under heavy loads, or at the end of service-life has made APP capacitors unusable for LT applications, worldwide.
High dissipation: The dielectric losses 0.5 W/KVAR of APP film capacitors are higher as compared to 0.2W/KVAR for MPP film capacitors.
High manufacturing tolerances: Manufacturing of APP capacitors is done under slow-speed manual process. Thus the manufacturing tolerances are higher compared to those for MPP capacitors manufactured by high-speed winding machines.
Poor end-connections: The end connection quality is poor due to bimetallic nature of aluminum-to-copper end connections.
High operating temperatures: Operating temperatures are higher due to higher dielectric losses.
Poor space optimization: As the active part needs to be completely submerged in oil it leads to higher space requirement and lower power density.
Risk of unsafe operation: The removal of moisture and air under vacuum is a critical process. Any lapses or poor oil quality can lead to unsafe operation.
There seems no justification in continuing with APP design. A properly manufactured gas-filled MPP capacitor is largely at par with resin-filled MPP capacitor. Gas-filled capacitors have technical superiority but the manufacturing process is both critical and capital intensive. Resin-filled MPP capacitor manufacturing employs simplified automated manufacturing process, perfected over the past thirty years. Achieving desired capacitor quality is easier, economical and more feasible for resin-filled construction but the same cannot be said about Gas Filled construction. Resin-filled MPP capacitors employing segmented film seem to be the ideal choice.
Baldev Raj Narang is an Electrical Engineering Graduate from Delhi College of Engineering, University of Delhi and is currently CEO of Pune-based Clariant Power System Ltd, a company, which in association with FRAKO Germany, is engaged in providing solutions in reactive power systems and power quality management to Indian and overseas customers in diverse fields. Narang has previously worked for Indian Oil Corporation and Century Enka Ltd, among others. He can be reached via email: email@example.com or handphone: +91-77-199-96868.