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Hermetically sealed, filled capacitors with an overpressure contact breaker should always be used in critical ambient conditions (high humidity, aggressive atmospheres, high temperatures), if the workload and power supply conditions are unknown as well as in situations that demand increased attention to safety.
VS MPP capacitors feature a self-healing dielectric. In the event of a dielectric breakdown in the coil (short circuit), the metal coating vaporises around the breakdown site owing to the high temperature of the transient arc that is produced. Owing to the excess pressure generated during such a breakdown, the metal vapour is pushed outwards away from the centre of the site within the space of just a few microseconds. This creates a coating-free corona around the breakdown site that completely isolates it and means the capacitor remains fully functional during a dielectric breakdown.
The self-healing properties of a capacitor can decrease with time and with constant overloading. This bears the risk of a non-healing breakdown with a permanent short circuit. Therefore self-healing must not be confused with failsafe.
Compensation capacitors are divided into two type families (A and B) in accordance with IEC 61048 A2.
• Type A capacitors defined:
"Self-healing parallel capacitors; without an (overpressure) contact breaker in the event of failure".
They are referred to as unsecured capacitors.
• Type B capacitors defined:
"Self-healing capacitors for series connection in lighting circuits or self-healing parallel
capacitors; with an (overpressure) contact breaker in the event of failure".
These are referred to as hermetically sealed, secured capacitors.
In accordance with the standard, the discharge resistor of both capacitor families must be capable of reducing capacitor voltage to a value of under 50 V in the space of 60 seconds after disconnection from the mains.
Capacitors without a contact breaker, unsecured,
Type A capacitors in accordance with IEC 61048 A2
IEC 61048 A2-compliant Type A capacitors are self-healing and require no short-circuit protection for normal operation.
Type A capacitors are not fitted with a specific failsafe mechanism as prescribed by the standards for Type B capacitors. Nevertheless, the requirements laid down in the standard for Type A capacitors, especially with regard to temperature and service life tests, are designed to ensure a sufficient degree of device safety and availability provided the device was correctly installed and operated under calculable and known ambient operating conditions.
Even so, in very rare cases these capacitors can still develop erratic behaviour due to overloading or at the end of the device´s service life.
For that reason, Type A capacitors should only be integrated into luminaires for operation in ambient conditions that are uncritical with regard to flammable materials. Luminaires should feature protection against secondary damage inside and outside the luminaire in the event of a defect.
Temperature-protected capacitors are a further development of Type A capacitors and are fitted with a thermal fuse that is triggered by overheating as a result of electrical or thermal overloading. They are tested in accordance with IEC 61048 A2 and comply with Type A requirements. Excess temperatures cause the two wire ends of the element inside the fuse to melt into bead shapes that are fully isolated from each other by special insulation.
In 99% of all the rare cases of critical capacitor failure, this failure is preceded by a gradual increase in the loss factor, which leads to an increase in the winding temperature and thus triggers the thermal fuse.
Vossloh-Schwabe recommends that preference be given to Type A capacitors with a thermal fuse as a matter of course for reasons of safety.
Type A capacitors predominantly feature a plastic casing.
Capacitors with a contact breaker,
secured Type B capacitors in accordance with IEC 61048 A2
Self-healing capacitors do not require short-circuit protection for normal operation as they automatically regenerate after a dielectric breakdown. However, as a result of frequent self-healing caused by overloading (voltage, current, temperature) or towards the end of the capacitor´s service life, overpressure can build up inside the capacitor (due to the decomposition products of the vaporised polypropylene).
In order to prevent the capacitor casing from exploding in such cases, hermetically sealed capacitors in accordance with IEC 61048 A2 (Type B capacitors) are fitted with an overpressure contact breaker. If excess pressure builds up within these capacitors, e.g. due to undue thermal loading or excessive voltages or at the end of the capacitor´s service life, a concertina section opens out that causes the casing to expand lengthways. As a result, the wire contacts rupture at a predetermined breaking point, which irreversibly interrupts the current (contact breaker).
This type of overpressure-protected capacitor with a contact breaker is also referred to as a flame- and explosion-proof capacitor with a break-action mechanism.
Type B capacitors with a contact breaker are available in an aluminum casing.
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Compensation of Idle Current
When using magnetic ballasts a phase shift occurs between the mains voltage and the current drawn. This phase shift is expressed by the power factor l, which generally ranges between a value of 0.3 and 0.7 with inductive circuits.
As a result of this phase shift, idle current, which does not boost the efficiency of the lighting unit, is also taken up from the power supply network in addition to real power. Power utility companies therefore require an increase of the power factor to values of over 0.85 for systems exceeding a certain rating (usually upwards of 250 W per external conductor).
Compensation capacitors are used to counteract idle current (by increasing the power factor) and can be connected either in parallel or in series.
Thanks to a power factor of approx. 0.95, electronic ballasts do not need to be operated with compensation capacitors.
Compensation using Series Capacitors
Series compensation employs a so-called dual circuit (two fluorescent lamp circuits connected in parallel), whereby the capacitor, which is connected in a branch of the circuit, over compensates the inductive idle current to such an extent that it covers the idle current of both ballasts. This type of circuit is only used with fluorescent lamps. As series capacitors are dimensioned for nominal-voltage and ballast tolerances, the lamp in the capacitor branch of the dual circuit operates with a higher current and thus also with a higher rating. Apart from differences in lamp brightness, the power loss in the circuit branch with the capacitor will also be greater.
An advantage of the dual circuit is that it prevents the radiated light from flickering.
The higher current in the so-called capacitive lamp circuit causes an up to 14% increase in lamp rating and a reduction of the lamp service life by as much as 20%. This goes hand in hand with substantial technical, ecological and economic disadvantages.
Series capacitors have to meet very high technical requirements to suit various aspects like temperature, nominal voltage, tolerances of the capacitance values, etc.
As defined by EC directive 2000/55/EC (European Standard EN 50294 governing the measurement of total power consumption), a series capacitor is considered to be a part of the ballast. If the system rating of the capacitive circuit containing the lamps and ballasts is then determined in line with the above definition, rating increases of up to 14% will become apparent in comparison to operation without a series capacitor. Experience has shown that this increased power consumption often means devices fall in the directive´s "banned" category. It is therefore strongly advised that due consideration be given to the elevated power consumption values common to using series capacitors for compensation purposes.
Parallel Compensation
During parallel compensation, each lamp circuit is assigned to a capacitor connected in parallel to the mains. Only one capacitor providing sufficient capacitance is needed for luminaires with several lamps. Parallel compensation does not affect current flow through a discharge lamp. The requirements placed on parallel capacitors are clearly lower than those for series capacitors.
However, parallel compensation can be subject to limitations when using audio-frequency ripple control pulses if the system operates with a connected rating of over 5 kVA and ripple control frequencies of over 300 Hz are used. The respective power utility company should be consulted for advice in such cases.
Parallel compensation is used in fluorescent lamp and high-pressure discharge lamp circuits.
As parallel compensation offers substantial advantages, this has become the accepted method in the last few years.
Metallised Polypropylene Film Capacitors
Metallised polypropylene film capacitors are designed to compensate the inductive idle current drawn by discharge lamps (fluorescent lamps, high-pressure mercury vapour lamps, high-pressure sodium vapour lamps and metal halide lamps with a ceramic discharge tube) in 50 Hz/60 Hz grids. All Vossloh-Schwabe compensation capacitors for luminaires feature a metallised polypropylene film dielectric. Compensation capacitors help to increase the power factor to values of over 0.85 l as required by power utility companies.
Construction of Metallised Polypropylene Film Capacitors
VS MPP capacitors contain a low-loss metallised polypropylene film dielectric, which is produced by depositing a thin layer of zinc and aluminum or pure aluminum vapour onto one side of the polypropylene film. The contacts at either end of the capacitor coil are created by spraying on a layer of metal and thus guarantee a high current-carrying capacity as well as a low-inductive connection between the terminals and the coils.
All capacitors with a nominal voltage upwards of 280 V are filled with oil or resin after the coils have been inserted and then hermetically sealed. This protects the coils from environmental influences and reduces partial discharge, which contributes to a long service life and stable capacitance. The effects of partial discharge only play a minor role for capacitors with a nominal voltage of under 280 V so that these devices do not need to be filled.
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