An intermediate circuit (DC-link) capacitor is used in the intermediate circuit of converters of different kinds where it couples different electrical grids to one DC voltage level (Figure 1).
Due to its high capacitance and its ability to supply power very quickly, the DC voltage intermediate circuit is stabilised, and a constant DC voltage value can be realised even if high current peaks are generated by the system.
To comply with this field of application, DC-link capacitors must be designed for high DC voltages which occur permanently and which may be superimposed with high-frequency ripple voltages. Rated voltages of 500 V to 1500 V d.c. are typical for intermediate circuit capacitors. Based on their industrial use it is, besides a long lifetime and a robust and safe terminating configuration, the temperature range of -55°C to +105°C which is of decisive importance.
In general, aluminium electrolytic capacitors are used in power electronics due to their very high power density. However, in an increasing number of applications it is film capacitors with polypropylene film that are selected as they show some fundamental advantages over electrolytic capacitors:
* Three times higher dielectric voltage strength.
* Very low dissipation factor (ESR).
* Very high insulation resistance .
* Temperature resistance up to -55°C .
* Considerably higher reliability through outstanding self-healing properties.
* Long life expectancy.
* Non-polarised construction.
* High vibration and shock resistance.
* Excellent mechanical stability.
WIMA DC-Link capacitors
WIMA DC-Link capacitors are constructed of low-loss, metallised polypropylene films. They are available in several product ranges both in prismatic and cylindrical shape versions.
The rectangular box-type WIMA DC-Link MKP 4 range is available in capacitances of 2 mF up to 150 mF and at rated voltages of 600 V up to 1300 V d.c. It is available in two-pin or four-pin versions.
The WIMA DC-Link MKP 5 range is designed with a cylindrical plastic case available in capacitances of 16 mF to 260 mF and voltages of 500 V, 700 V, 900 V, 1100 V and 1300 V d.c. and exhibits tinned wire terminations for PCB mounting.
WIMA DC-Link MKP 6 capacitors have a cylindrical aluminium housing and are available in capacitances of 165 mF to 1560 mF and in voltage ranges of 600 V, 700 V, 900 V, 1100 V, 1300 V and 1500 V d.c. They are designed with M6 screw connections and M12 earth bolts for bus bar mounting.
Due to their internal construction and their non-polarised termination design, WIMA DC-Link HC capacitors can be connected in three different wiring options. So for example an individual capacitor can be wired as 2 x 2250 mF/400 V d.c., 4500 mF/400 V d.c. or also as 1125 mF/800 V d.c. Depending on the dimensions and wiring options, values between 85 mF/1600 V d.c. and 4500 mF/400 V d.c. are available.
DC-Link HC capacitors can be selected both in moulded versions and with solvent-resistant, flame-retardant plastic casing with or without screw fixing. Customised solutions can be supplied on request.
Reliability and lifetime
Plastic film capacitors offer two decisive advantages when compared to electrolytic capacitors: self-healing and dry construction.
Compared to electrolytic capacitors, metallised plastic film capacitors self-heal in the event of an electrical breakdown of the dielectric. A breakdown always occurs at the weakest point of the dielectric and only takes nanoseconds. Temperatures of several thousand Kelvin happen at one spot, which causes the metal layer to evaporate and transmute the dielectric into a highly compressed plasma.
In the spreading plasma, discharge is continuing via the metal electrodes. A metal-free zone (insulating halo) is formed around the breakdown channel. A proper self-healing process depends on the metallisation thickness, the chemical composition of the dielectric and the voltage level applied whereby apart from the chemical composition – the manufacturing parameters have to provide the base for an optimum self-healing process.
Contrary to electrolytic capacitors, WIMA DC-Link capacitors have a dry construction. This means the absence of additives which for other types of capacitors are necessary in the form of impregnants or electrolytes. Hence, the phenomenon of continuing desiccation over a certain time which is generally known for electrolytic capacitors does not occur with WIMA plastic film capacitors.
In general the mode of vacuum deposition of the metal electrodes provides corresponding self-securing measures to further improve the self-healing properties of the dielectric and thus to considerably increase the energy content of those capacitors. An additionally improved contact area between electrode and schoopage enables the application of highest pulse currents and voltage gradients. These measures have a positive effect on the life expectancy and reliability of WIMA DC-Link capacitors.
Application examples
Railway technology
In an electric traction engine, e.g. a locomotive, DC-link capacitors are used to feed energy from the traction power AC voltage grid into the intermediate circuit via an H-bridge where the AC grid voltage is converted into a DC voltage (intermediate circuit voltage).
This energy can, during traction operation, again be converted into an AC voltage with variable frequency (typically 0 to 150 Hz) by means of a pulse inverter and again be placed at the disposal of the drive motor. Since the pulse inverter also acts as H-bridge, the energy flow can also be effected vice versa, eg during braking operation.
Wind power units
DC-link capacitors are used in the DC voltage intermediate circuit of wind power units, eg for voltage stabilisation. The DC current intermediate circuit capacitor of a wind turbine requires a capacitance of about 3300 mF to 4700 mF and a high rated voltage of 600 V to 1000 V.
Due to the self-healing effect after an electrical breakdown of the dielectric, their dry construction and their low sensitivity against high temperature variations, film capacitors used in wind turbines offer considerably higher reliability and a significantly longer lifetime than electrolytic capacitors.
Solar plants
In solar inverters, DC-link capacitors are set in parallel to the source (either the solar generator directly or the intermediate batteries) prior to the buck inverter module. The capacitor is subjected to a high-frequency ripple voltage being superimposed on the primary DC voltage. There is only one capacitor needed in a simple two-phase solar inverter.
Additional applications
Modern circuits and control devices of electric motors in today’s drive engineering technology necessitate intermediate circuits in all kinds of applications, for example in industrial and drive converters, frequency converters for pumps and ventilation, lifting and locomotion applications, and also for servo drives in machine tools and industrial robotics.
To answer the question of which kind of capacitor is best for an application in a given circuit position, it is necessary to obtain thorough knowledge of the switching mechanism of the inverter and the parasitic shares of the circuit. The synthesis of a constant sinusoidal voltage for connection to the public or local mains requires high switching rates in different inverter valve combinations so that the output current can follow the sinusoidal current desired.
The ripple depends on the DC voltage, the inductivity of the circuit and the switching duration. The switching frequency of a modern inverter based on IGBT technology is typically between 1 kHz and 20 kHz. The ripple current of a two-phase or three-phase track adds up and may cause severe damage to the generator and any other element switched on (eg batteries).
The intermediate circuit capacitor is thus needed to absorb the switching ripples.
That is why the DC-link capacitor is the most important passive component in inverter circuits, as it is the component decisive for the total lifetime of the device.
Conclusion
In modern drive engineering the intermediate circuit capacitor manufactured on the basis of metallised low-loss polypropylene film scores with its robustness, its insensitivity to high temperatures and its temperature adaptability. Above all, in cases where a high load transfer by an increasing intermediate circuit voltage occurs, reliable operation for long lifetime is enabled even without susceptible cascading of capacitances.
Its tolerance for high ripple currents and the option of a low-inductive construction (values of approximately 10 nH at a capacitance of 1000 mF are possible) enable a low-resonance frequency response which is advantageous for the entire circuit.
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