Electrical Measurements and Instrumentation Full course on slide share

1. Instrument Transformers
Current & Potential Transformer
(CT & PT)
Electrical Measurement and Instrumentation (EE-336)

2. Contents
• Introduction
• Classification
• Uses
• Current Transformer (CT)
• Design
• Construction
• Function
• Safety Precautions and Usage
• Potential Transformer (PT)
• Design
• Construction
• Conclusion

3. Introduction
• Transformers are classified as follow
• Power T/F
• Single Phase
• 3 Phase
• D Type
• Y Type
• DY Type
• Instrument T/F
• Current Transformer
• Potential Transformer
• Used for Isolating the power supply side from measurement and control
side
• Used for supply of exact amount of current required for driving instruments

5. Types of Instrument Transformers
• Current Transformer (CT)
• Used for sampling out current from a high/moderate current carrying
wire/bar
• Potential Transformer (PT)
• Used for sampling out voltage from a high/moderate tension wire/bar

6. Current Transformer
• Like any other transformer, a current transformer
has a primary winding, a magnetic core and a
secondary winding
• The primary winding of a current transformer is
connected in series with the power circuit
• The impedance is negligible compared with that
of the power circuit
• The alternating current flowing in the primary
produces a magnetic field in the core & induces a
current in the secondary winding
• The CT's primary circuit consists of a single 'turn'
of conductor, with a secondary of many tens or
hundreds of turns
• The primary winding may be a permanent part of
the current transformer, with a heavy copperbar
to carry current through the magnetic core

7. Function
• Reduce power system current to
lower value for measurement
• Insulate secondary circuits from
the primary
• Permit the use of standard
current ratings for secondary
equipment

8. Construction
• Bar type Primary
• Wound Primary

9. Equivalent Circuit

10. Safety Measures and Usage
• The secondary of a current transformer is not disconnected from its
load while current is flowing in the primary
• The transformer secondary will attempt to continue driving current
across the effectively infinite impedance.
• This will produce a high voltage across the open secondary
• This high peaks of voltage may not be measured by conventional
voltmeter. But these high peaks of induced voltage may breakdown
the CT insulation, and may case accident to personnel
• It is used for monitoring the operation of power grid and collection
of data regarding current in the circuit/bus

11. Potential Transformer (PT)
• A Potential Transformer theory is just like
theory of general purpose step down
transformer
• Primary of this transformer is connected
across the phases or and ground depending
upon the requirement
• PT has lowers turns winding at its secondary
• In an ideal Potential Transformer when rated
burden connected across the secondary the
ratio of primary and secondary voltages of
transformer is equal to the turns ratio
• But in actual transformer there must be an
error in the voltage ratio as well as in the
phase angle between primary and secondary
voltages

12. Types of PT
• Electromechanical PTs
• Similar to a power transformer
• May not be economical above 132kV
• Capacitor PTs (CPT)
• It has a capacitor divider on the front
end
• Used at very high voltages and
tension lines e.g., 220kV

13. Construction of PT
• The construction of a voltage
transformer takes into account the
following factors
• Output – Seldom more than 200-
300VA. Cooling is rarely a problem
• Insulation – Designed for the system
impulse voltage level.
• Insulation volume is often larger
than the winding volume
• Mechanical Design – Not usually
necessary to withstand short-circuit
currents. Must be small to fit the
space available within switchgear

14. Function
• Provides isolation from high
voltages
• Must operate in the linear region to
prevent accuracy problems
• Must be capable of driving the
burden, specified by relay
manufacturer

15. Causes of Error in PT
• The voltage applied to the
primary of the potential
transformer first drops due to
internal impedance of primary
Cause of Error in PT
• Transformed voltage across
secondary winding will again
drops due to internal impedance
of secondary before appearing
across burden terminals

16. Summary and Conclusion
• Current Transformer
• Measuring current
• Monitoring the operation of the
power grid
• CT’s secondary should not be kept
open
• Potential Transformer
• Measuring Voltage
• Provides isolation from high voltages

17. Thanks