26-02-2013, 10:46 AM
Use of C.T. and P.T. to Extend the Range of Ammeter, Voltmeter and Wattmeter
Extend the Range.docx (Size: 13.61 KB / Downloads: 54)
APPARATUS:
•Current Transformer 1
•Potential Transformer 1
•Ammeter 1
•Voltmeter 1
•Wattmeter 1
•Load
•Connecting wires
THEORY:
Instrument transformers are used for measuring voltage and current in electrical power
systems, and for power system protection and control. Where a voltage or current is too
large to be conveniently used by an instrument, it can be scaled down to a standardized,
low value. Instrument transformers isolate measurement, protection and control circuitry
from the high currents or voltages present on the circuits being measured or controlled.
A current transformer is a transformer designed to provide a current in its secondary coil
proportional to the current flowing in its primary coil. Care must be taken that the
secondary of a current transformer is not disconnected from its load while current is
flowing in the primary, as the transformer secondary will attempt to continue driving
current across the effectively infinite impedance. This will produce a high voltage across
the open secondary (into the range of several kilovolts in some cases), which may cause
arcing. The high voltage produced will compromise operator and equipment safety and
permanently affect the accuracy of the transformer.
In current transformer, the secondary current is in the range of 0 – 5A. Primary and
secondary currents are related by CT Ratio.
CT Ratio = Ip/Is
And the line current is given by:
IL = (Reading of Ammeter) x (CT Ratio)
Voltage transformers (VTs), also referred to as "potential transformers" (PTs), are
designed to have an accurately known transformation ratio in both magnitude and phase,
over a range of measuring circuit impedances. A voltage transformer is intended to
present a negligible load to the supply being measured. The low secondary voltage allows
protective relay equipment and measuring instruments to be operated at a lower voltages.
Care must be taken that the secondary of a voltage transformer is not short circuited, as
the transformer secondary will attempt to drive a large current across the effectively zero
impedance. The high current will compromise operator and equipment safety and
permanently affect the accuracy of the transformer.
In voltage transformer, the secondary voltage is in the range of 0 – 150V. Primary and
secondary voltages are related by PT Ratio.
PROCEDURE:
1. Connect the circuit as shown in circuit diagram.
2. Connect potential coil of wattmeter in parallel to voltmeter and secondary of P.T.
3. Connect current coil of wattmeter in series with ammeter and secondary of C.T.
4. Connect primary of P.T. in parallel to AC source and load while primary of C.T is
connected in series with the load.
5. Switch on the power supply. Set C.T and P.T ratios. Vary the load and note the
corresponding readings of ammeter, voltmeter and wattmeter.
6. Change the C.T and P.T ratios and vary load. Again note the corresponding
readings of ammeter, voltmeter and wattmeter.
9
7. Calculate actual current, voltage and power delivered by AC source.