3.7.2 Ratings. Capacitors are rated in continuous kVar (kilovolt-ampere-reactive), voltage,
and frequency. They are designed to give not less than rated and not more than 135 percent rated
kVar when operated at rated voltage and frequency. Capacitor units are normally available in
voltage ratings of 2,400 V to 34,500 V and kVar ratings from 15 kVar to 300 kVar. Various
manufacturers' medium-voltage units up to 200 kVar are interchangeable. Capacitors are
generally rated at a frequency of 60 Hertz (Hz), however, they are also suitable for operation at
frequencies below 60 Hz. There is no physical limit to the under-frequency operation of the
capacitors. The limit is economic, in that the capacitor kVar output is directly proportional to
frequency and applied voltage. If a capacitor is operated at a frequency lower than rated,
consequently, its kVar rating is reduced. Since capacitors are installed in theory to utilize their
rated capacity, utilization at reduced frequencies is not economical, as the unit's design rating can
never be achieved.
3.7.3 Construction. A capacitor unit consists of two aluminum foil strips or plates with thin
high-grade insulating paper or a synthetic film placed between them. The strips or plates are
compactly wound and connected in groups, each of which is connected to a terminal. There is no
contact between the two metal surfaces. When these two surfaces are connected to a source of
power, energy is stored in the capacitor. The capacitor remains charged at, or above, full line
voltage when disconnected from the source of power until a discharge path is provided between
the terminals. Capacitors have a built-in discharge resistor designed to drain off or reduce this
residual charge. National Electrical Code requires capacitors rated 600 V or more to be
discharged to a residual voltage of 50 V or less in 5 minutes. Since the built-in resistor has the
disadvantage that it cannot be visually inspected for an open circuit, it should not be relied upon
for positive drain-off of the residual charge (see subparagraph 3.7.9). The wound plates and
discharge resistor of a capacitor are enclosed in a welded sheet steel or stainless steel container,
which is hermetically sealed to protect the capacitor from deterioration due to entrance of foreign
material or moisture. The contents are vacuum dried and are usually impregnated with a
dielectric fluid. As of 1 October 1977, dielectric fluids containing polychlorinated biphenyls
(PCBs) can no longer be installed. The connecting leads from the capacitor are brought up
through the bushings to a joint at the top directly under the brazed terminal. The bushings
supplied on capacitors are usually made of porcelain. As of 1 October 1988, existing PCB
capacitors in unrestricted areas must be removed.
3.7.4 Types of Installations. The greatest electrical benefits are derived from capacitors
connected directly at the loads. This would permit maximum loss reduction and released line
capacity. However, economics and physical limitations are usually the governing factors.
Capacitors may be divided into two classes, primary capacitors and secondary capacitors.
Primary capacitors are those rated 2400 V and above and secondary capacitors are those used on
the low-voltage side of distribution transformers or at motor terminals and are normally rated 600
V and below. The three most common types of power capacitor installation are: pole-mounted,
metal-enclosed, and open-rack.