Using capacitors to regulate the voltage in electrical networks

Capacitors in Power Systems

For the reduction of cost and improved reliability, most of the world’s electric power systems continue to be interconnected. Interconnections take advantage of diversity of loads, availability of sources and fuel price for supplying power to loads at minimal cost.

Compensation in power systems is, therefore, essential to alleviate some of these problems. Series/shunt compensation has been in use for the past many years to achieve this objective.

Load compensation is the management of reactive power to improve power quality i.e. voltage profile and power factor. The reactive power flow is controlled by installing shunt compensating devices (capacitors/reactors) at the load end bringing about proper balanced between generated and consumed reactive power.

On power systems, capacitors do not store their energy very long—just one-half cycle. Each half cycle, a capacitor charges up and then discharges its stored energy back into the system. The net real power transfer is zero. Just when a motor with low power factor needs power from the system, the capacitor is there to provide it.

Then, in the next half cycle, the motor releases its excess energy, and the capacitor is there to absorb it.

Capacitor units are made of series and parallel combinations of capacitor packs or elements put together as shown in Figure 1.

Capacitors are made within a given tolerance. The IEEE standard allows reactive power to range between 100% and 110% when applied at rated sinusoidal voltage and frequency (at 25°C case and internal temperature) (IEEE Std. 18-2002).

In practice, most units are from +0.5% to +4.0%, and a given batch is normally very uniform.

Capacitor losses are typically on the order of 0.07 to 0.15 W/kVAr at nominal frequency. Losses include resistive losses in the foil, dielectric losses, and losses in the internal discharge resistor.

Capacitors must have an internal resistor that discharges a capacitor to 50 V or less within 5 min when the capacitor is charged to the peak of its rated voltage. This resistor is the major component of losses within a capacitor.

Also, capacitors are designed to operate at high dielectric stresses, so they have less margin for degraded insulation. Standards specify an upper limit for application of 40°C or 46°C depending on arrangement.

These limits assume unrestricted ventilation and direct sunlight. At the lower end, IEEE standard 18 specifies that capacitors shall be able to operate continuously in a −40°C ambient.