CAPACITORS FOR POWER DISTRIBUTIONS SYSTEM

CAPACITORS FOR POWER DISTRIBUTIONS SYSTEM

How does Capacitors installed in the power distribution system affect the operation?

The basic definition of a capacitor would be that it is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric (insulator). Capacitors are used as parts of electrical systems, for example, consist of metal foils separated by a layer of insulating film.

When there is a potential difference (voltage) across the conductors, a static electric field develops across the dielectric, causing positive charge to collect on one plate and negative charge on the other plate. Energy is stored in the electrostatic field. An ideal capacitor is characterized by a single constant value, capacitance, measured in farads. This is the ratio of the electric charge on each conductor to the potential difference between them.

The capacitance is greatest when there is a narrow separation between large areas of conductor, hence capacitor conductors are often called "plates," referring to an early means of construction. In practice, the dielectric between the plates passes a small amount of leakage current and also has an electric field strength limit, resulting in a breakdown voltage, while the conductors and leads introduce an undesired inductance and resistance.

The installation of capacitors on distribution systems improves efficiency in two ways. First, capacitors reduce line currents by causing the overall power factor to move closer to 100% as viewed from all points between each capacitor installation and the supply point. Second, capacitors improve line voltage, which results in lower losses on lines and transformers on the system. The improvement in power factor attained with capacitors results in greater loss reduction than that achieved with the voltage improvement.

Effect of Capacitors

As discussed earlier, the power factor of most consumer loads falls short of 100% because the loads draw kVAR of reactive power as well as kW of useful power. There are two types of kVAR: lagging kVAR and leading kVAR. Virtually all load kVAR are the lagging type because of the need for magnetizing currents for inductive load components, such as motor windings. If no capacitors are installed on the system, the total kVAR load must be supplied from substations and delivery points and flow through lines and transformers to reach consumers. This kVAR flow increases line and tranformer currents and increases losses according to the I squared R law.

Capacitors, on the other hand, draw leading kVAR, which are equivalent to negative lagging kVAR. In other words, capacitors are kVAR generators that supply kVAR instead of absorbing kVAR. THe result is that a capacitor installation has effect of locally generating kVAR to satisfy the lagging kVAR requirements of load in its general vicinity. Line currents on all facilities supplying power to that vicinity are thereby related, thus reducing system losses.