Page 147 - Practical Power System and Protective Relays Commissioning

146 Practical Power System and Protective Relays Commissioning

FIGURE 14.4 Puffer type circuit breaker.

FIGURE 14.5 Self-blast circuit breaker (CB). (A) CB in open position. (B) Interruption of
small currents. (C) Interruption of short-circuit currents. (D) CB in closed position.

14.4 INSTRUMENT TRANSFORMERS
These devices transform the current and voltages to standardized levels for
protection and control devices to 1 or 5 A for CT and to 100, 110, or 120 V
for VT.
In modern substation and bay control systems, current and VT can be
replaced by sensors. They offer the same accuracy as conventional instru-
ment transformers. The output signal, converted from analog to digital, is
processed by the digital bay control unit, using fiber optics. This is discussed
further in Chapter 10, Current Transformers and Chapter 11, Voltage
Transformers, of this book.