Page 71 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

TRANSFORMERS

3.28 CHAPTER THREE

FIGURE 3.25 Sketch of a current transformer.

● Rated frequency
● Transformer impedance
● Voltage rating for each tap on the transformer
● Wiring schematic of the transformer
● Transformer type designation
● References to operating instructions

Instrument Transformers
Potential transformers and current transformers are two special-purpose transformers used
for taking measurements.
Potential transformer is used to sample the voltage of a power system. It has high voltage
on the primary side and low voltage on the secondary. Its power rating is low. These
transformers are manufactured with different accuracy classes.
Current transformer is used to sample the current in a line (Fig. 3.25). Its secondary wind-
ing is wrapped around a ferromagnetic ring while the single primary line goes through the
center of the ring. A typical rating of a current transformer includes 600 : 5 and 1000 : 5.
Extremely high voltages will appear across the secondary terminals of a current trans-
former if the terminals are open. The current transformer should always be kept short-
circuited to prevent dangerous high voltages from appearing at the secondary terminals.

REFERENCE

1. S. J. Chapman, Electric Machinery Fundamentals, 2d ed., McGraw-Hill, New York, 1991.

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