Current transformers can be used to supply information for measuring power
flows and the electrical inputs for the operation of protective relays associated
with the transmission and distribution circuits or for power transformers.
These current transformers have the primary winding connected in series with
the conductor carrying the current to be measured or controlled. The
secondary winding is thus insulated from the high voltage and can then be
connected to low-voltage metering circuits.
Current transformers are also used for street lighting circuits. Street
lighting requires a constant current to prevent flickering lights and a
current transformer is used to provide that constant current. In this case
the current transformer utilizes a moving secondary coil to vary the output
so that a constant current is obtained.
Figure 1. Metering current transformers
Figure 2. Pole type constant
Figure 3. 400 kV current transformer