Capacitors are one of the three classic passive components in electrical engineering and fulfil many different tasks. A capacitor has the ability to store electrical energy in the form of an electric field. Basically, a capacitor consists of two electrodes and an insulating material called a dielectric. The electrodes are usually in contact with conductive surfaces. The dielectric can be air, paper, plastic or any other material.
1. What is a capacitor
The simplest capacitor is a plate capacitor, which consists of two conducting electrodes with the same surface area A and a defined distance d between them via the dielectric. The dielectric has a material-dependent dielectric conductivity, known as the permittivity ε. This conductivity consists of the electric field constant ε0 and the relative permittivity εr:
Considering the plate capacitor shown in Figure 1, the following relationship applies to a capacitance:
Capacitance is expressed in Farad [F].
Figure 1: Structure of a plate capacitor
The main functions of capacitors in inverters include:
DC link stability: The DC link in an inverter is an energy storage, usually fed by the Maximum Power Point Tracker (MPPT) circuit, that is designed to maintain a defined voltage level. The inverter circuit draws electrical energy from this DC link to feed back into the grid.
Filters: Several filters are used in an inverter. For example, a sine wave filter that processes the pulse width modulated square wave signal from the power transistors into a clean sine wave, or an EMI filter that filters out harmonics from the mains.
2. Types of power capacitors in inverters
The two most common types of capacitors used in power electronics applications are [1]:
Aluminium electrolytic capacitors with liquid electrolyte
Plastic film capacitors (often metallised polypropylene or polyethylene film capacitors)
An electrolytic capacitor is a polarised component, i.e. its electrodes are divided into a positive anode and a negative cathode, whereas film capacitors are not polarised. The cathode of an electrolytic capacitor is directly connected to the conductive electrolyte and the dielectric consists of an aluminium oxide layer on the anode. Typically, an electrolytic capacitor is rolled up, giving it its typical cylindrical shape.
In a film capacitor, the plastic used is the dielectric. The metal surfaces form the two electrodes. These are connected to the plastic metallisation via a contact layer with a laser-formed margin. This creates multiple capacitor layers to increase the capacitance density.
Figure 2: (a) Electrolytic capacitor: 1: Anode foil, 2: Anode oxide, 3: Electrolyte, 4: Paper spacer, 5: Cathode foil. (b) Film capacitor: 1: Terminals, 2: Contact layer, 3: Plastic film (Dielectric), 4: Metallization, 5: Laser-formed margin, 6: Housing
3. Life expectancy of power capacitors
In general, the expected life t_0 is based on nominal operation. This life is influenced by a number of factors.
3.1. Life expectancy of electrolyic capacitors
In the case of an electrolytic capacitor, three factors are taken into account when calculating the resulting life tres,Al [2]:
Temperature factor kT,Al: Arrhenius' 10 Kelvin rule describes the most significant factor. Lowering the operating temperature by 10 K doubles the basic life, raising it by 10 K halves it.
Ripple current factor kI,Al: The ripple current has a direct effect on the self-heating of the component and therefore on its life. This factor is often estimated by the manufacturer and published in application notes.
Voltage factor kU,Al: If an electrolytic capacitor is operated below its rated voltage, the self-healing effects of this type of component cannot operate adequately. The thickness of the dielectric of an electrolytic capacitor, i.e. its anode oxide layer, is directly dependent on the potential difference across the electrodes, that is the operating voltage. The process of building up the oxide is called formation [3]. If the voltage is too low, the dielectric runs the risk of reacting with the aqueous electrolyte and being degraded, which reduces the dielectric strength and can also lead to hydrogen gas formation or other chemical reactions that contribute to accelerated ageing. This factor is also estimated by the manufacturer.
Please refer to the literature supplied for a detailed discussion of each of these factors. This gives the total life of an electrolytic capacitor:
3.2. Life expectancy of film capacitors
Three factors are taken into account when calculating the resulting life of a film capacitor t_(res,Fo) [4]:
Temperature factor kT,Fo: The film capacitor is also subject to an exponential acceleration of ageing with temperature. The activation energy or reaction rate is a temperature dependent variable.
Voltage factor kU,Fo: An electric field in metallised film capacitors creates a self-healing effect in the component: if a breakdown occurs in the polymer due to a high potential difference, the current through the defect increases sharply at the electrode near the breakdown. At this point, the current density is high enough to vaporise the metal layer on the plastic, 'healing' the defect. At the same time, the capacitance is slightly reduced as this area is no longer available for the electric field. If a much higher rated voltage is applied to the capacitor for a prolonged period of time, uncontrolled self-healing processes will occur, affecting the capacitance to such an extent that the function of the component is lost.
Humidity factor kH,Fo: Humidity can penetrate through the plastic and have various effects within the capacitor itself:
Electrode demetallization
Electrode corrosion
The presence of humidity in the dielectric film increases the dissipation factor and reduces the insulation resistance, resulting in increased leakage current and heat generation.
Please refer to the literature supplied for a detailed discussion of each of these factors. This results in the total life of a film capacitor:
4. Power capacitor failure mechanisms
In general, the causes of capacitor failure can come from two sources:
Errors in the manufacturing process (e.g. material selection or batch errors)
Unsuitable operating conditions
As the various mechanisms and types of failure follow a complicated and complex logic, Figure 3 is intended to help. Here, the two types of capacitor under consideration are categorised according to the cause, the failure mechanism at work and the resulting failure pattern.
At the end of the causal chain, the individual causes have three different consequences:
Loss of function: The component loses its function in its specific application because, for example, the capacitance or series resistance changes to such an extent that the intended functionality is no longer achieved.
Explosion: Thermal runaway causes the internal material of the capacitor to expand, resulting in a sudden expansion of that same material.
Fire: For example, leakage of conductive materials (e.g. the electrolyte) can cause external short circuits around the capacitor circuit, ultimately leading to abnormal current flow and fire.
Figure 3: Fault patterns with their upstream failure mechanisms and causes for electrolytic and film capacitors [4] [5]
5. Conclusion
The greatest accelerator of degradation for capacitors used in photovoltaic power electronics systems is temperature. Another important factor, especially for film capacitors, is humidity.
In addition, it is important to ensure that the inverter operates at nominal voltage at all times to avoid unnecessary voltage fluctuations across the DC link capacitors.
During a general overhaul by Eternus Technology GmbH, the power capacitors, which are subject to increased weakening, are replaced as a preventive measure. We use components that have extended load limits and are therefore more reliable.
[1] R. S. Farswan, B.G. Fernandes (2015): Analysis of different PWM schemes for 3-level boost converter to reduce current stress in DC link capacitors of single phase NPC inverter, 2015 IEEE 42nd Photovoltaic Specialist Conference (PVSC), pp. 1-6. DOI:10.1109/PVSC.2015.7356395.
[2] A. Albertsen (2009): Elko-Lebensdauerabschätzung, Fachartikel von Jianghai Europe GmbH.
[3] T. R.Münninghoff (2012):Mechanismenderanodischen Auflösung von Metallen und Legierugen bei extrem hohen Stromdichten, Dissertation an der Mathematisch Naturwissenschaftlichen Fakultät der Heinrich-Heine Universität Düsseldorf.
[4] R. Gallay (2014): Metallized Film Capacitor Lifetime Evaluation and Failure Mode Analysis, 2014 CAS – CERN Accelerator School: Power Converters in Genf, Schweiz. DOI: 10.5170/CERN-2015-003.45
[5] A. Albertsen (2010): Zuverlässigkeit von Elektrolytkondensatoren, elektroniknet.de: www.elektroniknet.de/e-mechanik-passive/passive/zuverlaessigkeit-von-elektrolytkondensatoren.29188.html