AC filter capacitors, such as the MKP1847 series, offer an extended capacitance range, various connection configurations, and, for enhanced safety, employ so-called segmented film technology compliant with UL810 standards.
As power density increases, considerations for overload and fault behavior become a focus. Damage can take the form of a short circuit, open circuit, or something in between (higher leakage current); if overheating occurs, the electrolyte can leak due to pressure drop and the windings drying out.
Despite load variations, the high voltage remains stable.
The increasing integration of renewable energy sources such as wind and solar power presents new challenges to the power grid. Stabilizing capacitors in high-voltage power grids operate in a completely different field than ordinary capacitors, with different design requirements and dimensions. They are used to maintain standard requirements, according to which the power supply voltage deviation for end users must not exceed 230 VAC, ±10%.
Providing additional capacitive reactive power can stabilize voltage; voltage can be increased or decreased by slightly shifting the phase into the capacitive or inductive region. For this purpose, parallel reactors or capacitor banks can be connected as needed. Overhead lines generate induction under high loads. If the phase shifts due to capacitive reactive power, the voltage will drop and then rise again.
Besides voltage stability, voltage quality is also a crucial consideration for grid operators. Harmonics—always superimposed voltages that are multiples of the fundamental frequency—are added to the fundamental frequency during operation. The third harmonic (150 Hz) is typically most pronounced in heavily loaded grids and therefore must be significantly reduced. Corresponding filtering devices are typically located in the power range between 200 MVA and 300 MVA.
Reactive power reduction is provided by damped mechanically switched capacitor banks (MSCDN). If a large load exists in the grid and voltage must be supported, this is achieved by connecting capacitors to each phase. Capacitors C1 and C2, in coordination with the high-voltage reactor L, allow a 50 Hz current component to flow unimpeded through C2. However, frequencies close to the center frequency pass through the resistor and are converted into heat. The interference frequency is significantly reduced.
Single capacitor design
Capacitors are formed from wound elements. These devices can operate optimally at voltages up to approximately 2 kV, therefore multiple elements must be connected in series to achieve the required withstand voltage of 250 kV to 300 kV. To allow for the modular transport and installation of these large capacities, specialized manufacturers can now assemble the wound elements in a stainless steel housing and weld them to provide sealed joints. This type of device is called a medium-voltage capacitor.
The high voltage connected to the first capacitor (C1) is distributed across a series of 30 to 40 capacitors, resulting in a voltage of approximately 7.5 kV for each capacitor. The weight of the capacitors is limited to a maximum of 100 kg, and fewer than 10 parallel capacitors are allowed to be connected in each series. The capacitance of a single C1 capacitor is 35 µF to 40 µF. These capacitors consist of several winding elements internally connected to form a series winding group. In the second capacitor (C2), the 30 kV to 40 kV connection is distributed across approximately five capacitor series. This results in a voltage of 7 kV and a capacitance of approximately 45 µF for each capacitor.
The implementation of the technology resulted in a very large factory. In addition to the dielectric, which consists of several layers of polypropylene foil, the electrodes of a capacitor winding element are made of aluminum foil. If all the foils needed for such a project were lined up, it would create an 8,000,000-meter-long strip. That's more than half the length of the world's axis, and the area covered by the foil could cover 350 standard FIFA football fields.
In terms of weight, this will require over 10 tons of aluminum and approximately 25 tons of polypropylene. To package this large active surface in a compact form, aluminum and polypropylene foils are first wound into circles and then flattened. These flat windings are stacked, connected, insulated, and then assembled and sealed in a rectangular housing. The total weight of the capacitor alone, including the housing and connectors, exceeds 50 tons. To handle 250 kV to 300 kV relative ground high voltage as specified, frames were developed for the capacitors, with each horizontal assembly using a single capacitor mounted on the frame. These frames are electrically isolated using insulators and assembled at the customer's site into a tower 7 to 10 meters high. Depending on the plant's power requirements, a total of 30 to 45 frames are needed.
These numerous examples demonstrate the diversity of capacitor applications in power electronics and electrical engineering. They can also serve as a complement to other applications, such as hybrid and electric vehicles, and the control of meters and high-power drives.
