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What are the main characteristics of capacitors

MV switchgear

⑴ The main characteristics of the capacitor
①Nominal capacitance
The capacitance marked on the capacitor. However, there is a deviation between the actual capacitance of the capacitor and the nominal capacitance, and the accuracy level has a corresponding relationship with the allowable error. Generally, capacitors are usually graded Ⅰ, Ⅱ, and Ⅲ, and electrolytic capacitors are grade Ⅳ, Ⅴ, and Ⅵ, depending on the application. The capacitance value of the electrolytic capacitor depends on the impedance presented when working under AC voltage. The capacitance value will change with the change of the operating frequency, temperature, voltage and measurement method.
② Rated voltage
The effective value of the highest DC voltage that can be continuously added to the capacitor at the lowest ambient temperature and rated ambient temperature. If the operating voltage exceeds the withstand voltage of the capacitor, the capacitor will be broken down, causing damage. In practice, as the temperature increases, the withstand voltage value will become lower.
③ Insulation resistance
The DC voltage is added to the capacitor to produce a leakage current. The ratio between the two is called the insulation resistance. When the capacitance is small, it mainly depends on the surface state of the capacitor; when the capacity is greater than 0.1 μF, it mainly depends on the medium. The greater the insulation resistance, the better.
④ Loss
Under the action of the electric field, the energy consumed by heat in a unit time is called loss. The loss is related to the frequency range, dielectric, conductance, resistance of the metal part of the capacitor, etc.
⑤ Frequency characteristics
As the frequency increases, the capacitance of general capacitors shows a law of decline. When the capacitor works below the resonant frequency, it shows capacitive, and when it exceeds its resonant frequency, it shows inductive. At this time, it is not a capacitor but an inductor. Therefore, it is necessary to avoid the capacitor working above the resonance frequency.

⑵ Selection of several commonly used parameters of capacitors
①Temperature coefficient, that is, the range of capacitance value changes with temperature.
② Dissipation factor, because of the leakage resistance, equivalent series resistance and equivalent series inductance of the capacitor, these three indexes are almost always difficult to separate, so many capacitor manufacturers will combine them into one index, called the loss factor, which is mainly used To describe the ineffectiveness of capacitors. The dissipation factor is defined as the ratio of the energy lost per cycle of the capacitor to the stored energy. Also known as loss tangent. 
③ Q value, also known as a quality factor, is the reciprocal of the loss factor. The general capacitance manual will be marked with a Q or dissipation factor. Three-phase isolation transformer price
④ The difference in dielectric constant K and capacitance is mainly the difference in the filling medium. The size of the dielectric constant depends on the volume of the capacitor and the absorption of the medium. The dielectric constant is large, and a large capacity can be integrated into a small volume, but The medium absorption is very serious.

Date:2020-07-09 14:43

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