As
the name implies, protection current transformers are used to protect
electrical systems from damage. They reduce the high current flowing in a power
system element to lower, manageable values ideal for relay operation.
Protective CTs from power monitoring equipment providers can also isolate the
relay circuit from the primary, while allowing for the use of standardized
current rating for relays, which can either be 5 or 1 ampere.
Indoor Protective CTs
Indoor CTs are typically
designed for high-current relaying and metering applications. They come in two
types: the wound and the bar-primary type with secondary cores. Wound current
transformers have a primary winding that’s physically connected in series to the
conductor carrying the measured current flowing in a given circuit. Bar-types,
on the other hand, use the main circuit’s actual cable as primary winding. Both
employ the single-phase method.
Outdoor Protective CTs
Outdoor
CTs are generally used for metering and relaying circuits. They employ three-phase
and single-phase method up to 33 kilovolts (kV), and are magnetic in nature.
They usually come in three types: capacitor, oil-filled, and combined current
and voltage transformer.
There
are also outdoor CTs that exceed the 33 kV limit. All employ the single-phase
method and come in two general types: magnetic and capacitor voltage
transformer (CVT).
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