Selection of energy buffer capacitors in solar inverters

Preface

Solar inverters act as an interface between solar panels and the main grid. As shown in Figure 1, two power conversion processes occur inside the solar inverter. A DC/DC converter controls the operating point of the solar panel to achieve maximum power output. The DC/AC converter delivers this power to the main grid and implements various control rules established by the main grid operator. The energy buffer absorbs the difference in power flow between the two converters. Successfully implementing this process depends heavily on the capacitors you choose for your energy buffer. This article explains how to choose between film capacitors and aluminum capacitors, and the factors you need to consider when choosing one or the other.

Initial considerations

By installing energy buffers, you can separate the issue of extracting maximum power from solar panels from how to inject that energy into the main transmission and distribution system. Moreover, doing so will reduce the complexity of the design.

The size of the energy buffer is determined by how much energy needs to be stored, which depends on the power level of the solar inverter and the length of time between storing and releasing energy. For reference, if the time difference is less than 1 second, it is okay to use film capacitors or aluminum capacitors. If it takes longer than 1 second, you should consider using an electrochemical double layer capacitor or battery pack. But both technologies require each battery cell to have a voltage between 1V and 4V. The higher voltage level required to connect to the main grid requires several batteries to be connected in a series, so additional electronic devices are required to maintain balance. Since there are some special requirements for driving the battery or the electrochemical double layer capacitor unit, this function is usually implemented by a separate power electronics module, a detailed introduction to the module is beyond the scope of this article.

Film capacitors and aluminum capacitors have some usage limitations that affect the service life and reliability of the solar inverter. Therefore, you need to list in detail the changes under long-term working conditions. The important parameters are the device ambient temperature, operating voltage, ripple current and duration. Table 1 gives a very simple example of a working situation.

Capacitor basics

Film capacitors and aluminum capacitors are two types of capacitors that evolved from parallel substrate capacitors.

The film capacitors used as energy buffers in solar inverters consist of two layers of metalized polypropylene rolled together. The thickness of the polypropylene determines the voltage level, which can reach thousands of volts. The metallized portions of the polypropylene are contacted by metal particles sprayed onto the rolled film. The connector leads are soldered to this metallization.

Aluminum capacitors are composed of two layers of aluminum foil, sandwiched between one or two layers of paper, and filled with conductive liquid, that is, electrolyte. The joints are welded to each aluminum layer. The first layer of aluminum has holes to increase the surface area, and is covered with a thick oxide layer. The second layer of aluminum is only used to contact the electrolyte. The voltage level is limited by the thickness of the oxide layer and the composition of the electrolyte. In practical applications, it is generally around 500V.

Performance characteristics of film capacitors and aluminum capacitors

Film capacitors are almost ideal capacitors. The capacitance of this capacitor does not change significantly with temperature and generates almost no heat during charging/discharging (ripple current). Due to the structure of the capacitor, which is a short circuit to the current loop, the inductance is very low, allowing it to be used over a wide frequency range, often up to several megahertz.

Aluminum capacitors have more problems. The fine pores and the moderately conductive electrolyte allow the capacitance of this capacitor to change with temperature and frequency. Ohmic losses in the aluminum and paper/electrolyte combination structures, as well as frequency-dependent losses in the imperfect oxide layer, cause the capacitor to heat up when charging/discharging, limiting the capacitor's ripple current. processing power. The last point is particularly important. Since the electrolyte will react chemically with other materials in the capacitor, the electrical characteristics will change after a period of time, causing the failure rate of the capacitor to increase after it reaches the end of its service life. Since the speed of chemical reactions decreases as the temperature of the capacitor decreases, the life of the capacitor needs to be calculated based on the working conditions of the solar inverter.

If something goes wrong in an aluminum capacitor, the results are even harder to predict. Damage to the joint due to dielectric breakdown can result in short circuit, open circuit, or something in between, such as increased leakage current. If an aluminum capacitor overheats and is connected to a power source, its temperature will rise above the boiling point of the electrolyte, which can reach approximately 200 °C. The internal pressure generated causes the pressure relief device to open, the electrolyte to flow out, and the winding layer to dry out.

Design points of film capacitors and aluminum capacitors

When it comes to choosing between two technologies, performance is not all a factor. Component size is also important, and price is also a factor. Something to always keep in mind is how do film capacitors and aluminum capacitors achieve the desired results?

The space efficiency of aluminum capacitors is definitely higher than that of film capacitors. A 470 µF/450 V aluminum capacitor is only 15% the size of a 470 µF/450 V film capacitor.

On the other hand, aluminum capacitors have a limited life and greater losses. For a solar inverter that requires 20 years of operation or high power levels, film capacitors are a better choice due to their lower losses and unlimited life.

In terms of component cost alone, aluminum capacitors have an absolute advantage. The same is 470 µF/450 V. The cost of film capacitors is 5 times or more than the corresponding aluminum capacitors. However, aluminum capacitors generally require additional protection circuitry. In contrast, film capacitors require few peripheral components to protect against failure. Especially for solar inverters at high power levels, capacitors that can handle heating issues are a better choice as this can help significantly reduce costs, for example by not requiring the use of water to cool components.

  in conclusion

Choosing the energy buffer technology in a solar inverter is not a simple matter. You need to consider many factors, which may involve some details of the component's behavior. Therefore, it is wise to find an experienced supplier and leverage the supplier's design expertise early in the design process to ensure that the technology you choose will ultimately meet all your needs.