What are self-healing shunt capacitors? The working principle and its application

What are self-healing shunt capacitors?

Self-healing capacitors are characterized by their self-healing properties. When the dielectric is broken down, the short-circuit current will melt and evaporate the metal film around the breakdown site, thereby restoring non-conductivity, so it has a high operating guarantee. The time required for self-healing after dielectric breakdown is several microseconds. When the weak point of the dielectric in the capacitor breaks down to form a path, an arc is formed in a very short time, causing the local temperature and pressure to rise sharply, and the metal layer evaporates violently. The more the radius expands, the arc is broken, and a circular area with the breakdown point as the center loses the metal coating on the surface of the medium, and the self-healing process is completed. When the damaged self-healing capacitor is opened, it can be found that the self-healing effect is obvious at the edge of the dielectric in the capacitor.

BSMJ type self-healing low-voltage shunt capacitor capacitance tolerance is -5% to +10%, and the three-phase phase-to-phase balance is 《1.08.

Scope of application

BSMJ self-healing low-voltage shunt capacitors are capacitor units and capacitor banks used to improve the power factor of AC power systems with a nominal voltage below 1kV and a frequency of 15Hz to 60Hz.

The working principle of self-healing shunt capacitor

Self-healing low-voltage shunt capacitors are relatively complex.

There are many types of capacitors, such as aluminum electrolytic capacitors, solid capacitors, safety capacitors, and cbb capacitors.

The main working principle of the self-healing low-voltage shunt capacitor is to adjust the frequency, change the power, and make the electrical device play a good role through the capacitance absorption. Therefore, in general, the function of self-healing low-voltage parallel capacitors is to adjust the frequency and change the power through capacitive absorption to make the appliance very stable.

The components used in dry self-healing high-voltage shunt capacitors are self-healing capacitor components. Ordinary aluminum foil capacitor components are made by sandwiching a non-conductive medium (film-paper composite or full film) between two layers of aluminum foil electrodes, and then rolling, pressing, welding, vacuum drying and liquid dipping.Due to the electrical weakness of the medium, in order to avoid the short circuit of the two electrodes caused by the breakdown of the medium, the medium should be more than two layers, so that the electrical weaknesses of each layer of the medium are staggered. The self-healing capacitor is different, its medium is a single-layer PP film, and a thin layer (less than 1/100 μm) of metal is evaporated on the surface as a conductive electrode.When a voltage is applied, the electrical weak point of the PP film is broken down, and the breakdown current will pass through the breakdown point. Due to the sharp increase of the current density of the conductive metallized coating and the high heat generated by the metallized layer, the metal conductor around the breakdown point evaporates and dissipates quickly, forming a blank area of the metal coating, and the breakdown point automatically restores insulation.The dielectric film produces a small hole with a diameter of several micrometers, and the area of the metallized coating that disappears during the self-healing process is about a few millimeters in diameter.

According to the requirements of self-healing performance, the thinner the metallized plate coating of the capacitor, the better.The thinner the coating, the lower the energy generated during self-healing, the smaller the temperature rise, the smaller the damage to the capacitor, and the better the self-healing property.However, according to the relationship between the dielectric strength and the coating resistance (see the attached picture), the thinner the coating, the greater the contact resistance and the greater the breakdown field strength. The parts are overheated and damaged, causing the capacitor to fail.Therefore, solving the above contradiction is the focus of dry-type self-healing capacitor manufacturing. Now, there are several ways to solve this problem.For example, Jinzhou capacitor factory adopts a different coating process - trapezoidal coating edge thickening technology, which makes the metallized coating gradually thinner in a large area, and the electrode lead-out edge is thickened, which not only improves the breakdown field strength, but also reduces the capacitance loss, while increasing the surge current capability of the component.

Self-healing shunt capacitor function

Self-healing low-voltage shunt capacitors (MKPS.BSMJ/BCMJ/BZMJ) are suitable for parallel connection with the load in AC power systems with a power frequency of 50HZ or 60HZ rated voltage of 1000V and below, to increase the power factor of the system, reduce line loss and improve voltage quality. Give full play to the efficiency of power generation and power supply equipment; local compensation for inductive equipment such as motor transformers and power frequency electric furnaces; electric traction locomotives and UPS systems and power system harmonic filtering and other aspects.