Page 853 - Basic Electrical Engineering

We have studied earlier that a low-range ammeter can be used to measure a
high value of current by using shunts. Similarly, the measuring range of a
voltmeter can be extended by using multipliers. However, there are some

limitations to the use of shunts and multipliers for measurement of high
current and high voltage, respectively by using low-range ammeters and

voltmeters.
For measurement of high current and high voltage, voltmeters and

ammeters of higher ranges are not used. Instead, for such measurement
current transformers (CT) and potential transformers (PT) are used along with

low-range ammeters and voltmeters. Generally ammeters of range 0–5 A and
voltmeter of range 0–110 V are used.
Fig. 11.29 shows the use of CT and PT in the measurement of current and

voltage of higher values. High-voltage and high-current measurements in on
electrical power generation and a distribution system are done by installing

CTs and PTs near the high-voltage and high-current lines and the output of
CTs and PTs are connected to the ammeters and voltmeters of lower rating at

the control station. In Fig. 11.29 is shown a power generation and
transmission lines. Output of the generator which generates electricity at

11000 V is stepped up at high voltage, say 220 kV by using a step-up
transformer and then connected to a long transmission line as shown. Current
and potential transformers have been shown connected to the system. Output

of the PT is connected to a 0–110 V voltmeter whereas the output of the CT
is connected to a 0–5 A ammeter.