What is a capacitor? Uses, composition and classification

What is a capacitor?

A capacitor is a system of two conductors placed close to each other and separated by an insulating layer (dielectric). It has an effect on electric charge and discharge.

The quantity that characterizes the capacitor’s ability to charge at a certain voltage is called capacitance. It is determined by the quotient between the charge of the capacitor and the voltage between its two plates (C = Q / U).

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What is the use of a capacitor?

In power systems, capacitor banks are used for the purpose of compensating reactive power to enhance the power factor cosφ. Since then to ensure the performance of the grid and avoid fines in accordance with the electricity industry. Installing capacitor banks will significantly reduce the monthly electricity bill (a few dozen% depending on the unit). It is a key component in reactive power compensation cabinets. It works with other devices to ensure a smart, stable and safe operation compensation system. For example, capacitor controller, switch, protection, harmonic filter coils, measuring equipment, display, …

When should a capacitor be installed?

According to electricity regulations, if your system has cos phi <= 0.85, the amount of Q consumed will be calculated. Therefore, in order not to be penalized, we should install a system of capacitor banks cosφ. There are many compensation methods such as hard or soft compensation, automatic or manual compensation. But most of us usually choose soft compensation and automatic compensation.

Low-voltage capacitor banks are divided into different compensation levels. For example 6 levels, 12 levels. There are low-voltage capacitor bank controllers programmed to make the compensation correct and sufficient. Reaching the cosφ factor as required by the electricity, so businesses can avoid the risk of a fine.

how is a capacitor constructed?

The capacitor is usually a special type of oil-soaked paper capacitor. It consists of two plates of long aluminum foil insulated with layers of paper. The whole is fixed in a sealed container. The two ends of the pole are taken out.

Formula to calculate capacitor capacitance?

To select a capacitor to compensate for a particular load, we need to know the power P and the power factor Cosφ (cos phi) of that load. Suppose we have:

• The capacity of the load is P.
• The power factor of the load is Cosφ1 → φ1 → tgφ1 (before compensation, cosφ1 is small and  tgφ1 is large).
• The power factor after compensation is Cosφ2 → φ2 → tgφ2 (after compensation, cosφ2 is large and tgφ2 is small).
• Then the reactive power to be compensated is calculated according to the following formula:

Qb = P * (tgφ1 – tgφ2)

For example, we have a load capacity of P = 100 (kW).

Power factor before compensation is cosφ1 = 0.75 → tgφ1 = 0.88.

Power factor after compensation is Cosφ2 = 0.95 → tgφ2 = 0.33.

So the reactive power to be compensated is Qb = P * (tgφ1 – tgφ2) = 100 * (0.88 – 0.33) = 55 (kVAr).

How are capacitors classified?

Sort by structure

Dried capacitor

The dry capacitor is a long round vase. The advantage of this type is the compactness. They are lightweight, easy to install, replace, take up little space in the electrical cabinet. The cost of the dry capacitor is usually lower than the oil capacitor. This type is often used for small power compensation systems, relatively good electrical quality. The popular dry capacitors in the Vietnam market have the capacity of compensation of 10, 15, 20, 25, 30kVAr. Some brands have small type 2.5, 5kVAr and large type 40, 50kVAr.

Oil capacitors

The oil Capacitor is rectangular (side square or round). The advantage of this type is of higher durability. Oil capacitors are commonly used for all compensation systems. Especially the high power compensation system, bad power quality, with harmonics (used in combination with harmonics filter coils). This type is popular on the Vietnamese market with power capacities of 10, 15, 20, 25, 30, 40 and 50kVAr.

Sort by voltage

• Low voltage single phase type: There are types of voltage 230V, 250V.
• 3-phase low voltage type: There are types of voltage 230, 380, 400, 415, 440, 525, 660, 690, 720, 1100V.

The most common type of capacitor bank according to this classification is 415V and 440V. Type 415V is often used in relatively stable voltage systems at 380V standard voltage. Type 440V is usually used in higher voltage systems than standard voltages, systems with harmonics need to be fitted with harmonic filter inductors.

Source: Wikipedia