What Is Power Capacitor? Formula explanation

The Essence and Formula of Power capacitor’s Capacitance

The figure of the power capacitor is 卝, the symbol C.

The core function of a low-voltage AC power capacitor is closely related to its physical structure. The essence of capacitance (C) can be expressed by the following formula:

uThereinto:

ε is the dielectric constant of the dielectric material;

S is the area of the electrode plate;

d is the distance between the two plates.

uThe game between plate area and distance: 

the capacity is proportional to the plate area S and inversely proportional to the plate spacing d. For example, metallized polypropylene films are commonly used as electrodes in the industry, and the effective area is increased by the winding process, while the thickness of the film is controlled in the micron range (e.g., 5-6 μm), thereby significantly increasing the capacitance density. However, too low a thickness can also reduce the lifetime of the ac power capacitor.

uThe key role of dielectric materials: 

Although the dielectric constant of polypropylene films (about 2.2) is lower than that of ceramic materials, they have high dielectric strength (600 V/μm electric field) and low loss characteristics , making it an ideal medium for AC low-voltage power capacitors.Barium titanate ceramic

Here are some of the dielectric materials and dielectric constant

Precautions：

Dielectric constant ≠ The only indicator of capacitive performance, which needs to be comprehensively selected in combination with parameters such as withstand voltage, temperature stability, and loss angle (tanδ).

Polypropylene film or metallized film capacitors are commonly used in power systems because of their high voltage resistance, non-polarity and suitability for AC environments.

Dry text is always hard to understand, but that's okay I've found the Toutube video for you:

https://www.youtube.com/watch?v=f_MZNsEqyQw

This video visually illustrates the principle of an AC power capacitor in the form of an animation. After knowing the principle of capacitor, you will definitely be curious about the process flow and product structure required in the reproduction process of three-phase power capacitor in the industrial production process.

The Core Structure of the Power Capacitor

Capacitor Components:

uElectrode: Metallized polypropylene film (self-healing) or aluminum foil. The thickness of the metallized film is only 0.02-0.03μm, and when the local breakdown occurs, the metal layer around the breakdown point evaporates, realizing the "self-healing" function and avoiding short-circuit failures.

uMedium: Multi-layer polypropylene film is superimposed, and the gap between the plates is filled by vacuum impregnated with mineral oil or resin to enhance insulation and heat dissipation.

Enclosure & Protection Design: 

uHousing material: metal shell (such as galvanized steel plate) is suitable for large-capacity capacitors, with explosion-proof pressure relief function; Flame-retardant plastic housings are used in miniaturization scenarios.

uProtection device: Built-in temperature sensor (such as bimetal sheet) and pressure release valve, when the internal temperature exceeds 85°C or the pressure is abnormal, it will automatically cut off the circuit to prevent explosion.

Insulation & Cooling System:

Impregnating agent: 

Mineral oil or dodecylbenzene fills the plate gaps, increases the PD starting voltage (e.g. from 2 kV to 4 kV) and accelerates heat transfer.

Heat dissipation structure:

The large-capacity capacitor adopts a corrugated steel plate shell or an additional heat sink to control the temperature rise within 20K.

The Key Role in the Power System

Power Factor Compensation（Reactive Compensation）:

Inductive loads, such as electric motors, generate hysteresis reactive power when they are running, and the power capacitors pass through the formula

Provides advanced reactive power. For example, when a factory increased the power factor from 0.7 to 0.95, line losses were reduced by about 50%.

Voltage Stabilization and Harmonic Suppression:

In the voltage fluctuation scenario, the capacitor reduces the voltage fluctuation range from ±10% to ±3% by dynamically compensating for the reactive power.For harmonic environments (such as 5th and 7th harmonics), it is necessary to connect a reactor with 7% reactance rate in series to avoid the resonance between the capacitor and the system impedance.

The economic value of energy efficiency improvements:

Taking a 10kV power distribution system as an example, every 1kvar capacitor invested can reduce power loss by about 150-200kWh per year. A typical industrial user can save 3-8% of their electricity bill by reactive power compensation. Therefore, power capacitors can play a very good role in energy saving.