Measurement & Instrumentation
Lecture 4
Course Instructor : Md. Sadiqur Rahman
Lecturer , Dept. of EEE , NUB
Topic: Instrument Transformers
Introduction
• Called ‘Instrument Transformer’ as they are mainly used for
instrumentation of the measurement of high-power system parameters, e.g.
voltage, current, power, energy, power factor and frequency.
• Used to step down the voltage/current to be measured. Actual
measurements are done by other measuring instrument, like voltmeter,
ammeter, wattmeter, energymeter.
• Have wide applications in protection circuits of power systems, e.g.
over-current, under-voltage, earth fault and various other kinds of relays
Types:
1. Current transformer (CT): Used to transform high current of a power
line to a lower value suitable for direct connection to an AC ammeter.
2. Potential Transformer (PT): used to transform high voltage of a power
line to a lower value suitable for direct connection to an AC voltmeter.
Functions of an Instrumentation Transformer
Performs two important functions:
1. Extend the range of AC measuring instrument, similar to the shunt or
the multiplier resistors that extends the range of a DC meter.
- CT and PT provide required range extensions through their
transformation ratio.
2. Serve to isolate the measuring instrument from high-voltage power line.
-It is impractical to bring the several-hundred kV high-voltage lines directly to
an instrument panel because of the safety hazards involved and the insulation
problems of high-voltage lines running closely together in the confined space
of the instrument panel.
Ratios of Instrument Transformer
Actual Transformation Ratio (R):
Turn Ratio (n):
CT
Construction and special design consideration for CT:
1. Its construction is very different from the normal power transformer
2. In most cases, the primary is only one turn or a single conductor connected in
series with the load/line whose current is to be measured. The secondary
winding has a larger number of turns and is connected in series with the meter.
3. Often the primary winding is a single conductor in the form of a heavy bar of
copper or brass running through the core of the transformer. Such a CT is called
a bar-type CT.
4. The secondary is generally designed to deliver a current of 5 A. An
800/5-A bar-type CT would have 160 turns in the secondary coil.
5. Power rating of the windings is low, as they are not meant to supply
any load other than the instruments losses
6. Very low voltages are involved with primary and secondary windings.
So unlike PT, high insulation is not required for CT. So its construction
is very simple
Why the secondary of a CT should not be open while the primary
is energized?
When the secondary is open-circuited, the voltage developed across the open
secondary terminals may be very high, because of the step-up ratio. Since a
CT is not designed for high insulation to sustain a high voltage like this, it
can easily break down the insulation of the CT
The secondary terminal of a CT should therefore always be short-circuited,
or connected to a meter or relay coil.
PT
Construction and special design consideration for PT:
1. Construction is similar to the normal power transformers
2. Extremely accurate ratio step-down transformer. Secondary is
usually rated for 230 V irrespective of the primary voltage rating.
3. Low power winding rating, as they are not meant to supply any load
other than the instrument losses
4. For very high primary voltages, the insulation between the primary and
secondary windings must be able to withstand large potential difference.
Basis for
Current Transformer
Comparison Potential Transformer
Definition Transform the voltage from high value
Transform the current from high value to
to the low value
the low value.
Circuit Symbol
Primary It carries the voltage which is to be
It carries the current which is to be
Winding measured.
measured.
It is connected to the meter or
Secondary Winding It is connected to the current winding of
instrument.
the instrument
Connection Connected in parallel with the
Connected in series with the instrument
instrument
Primary Circuit
Has a small number of turns Has a large number of turns
Secondary Has a small number of turns and can be
Has a large number of turns and cannot
Circuit open circuit.
be open circuit
Range 110v
5A or 1A
Transformation Low
High
Ratio
