INSTRUMENTATION & MEASUREMENT • Instrument Transformer & Special Instrument • Electrical & Electronics Engineering Department By Sanjay Jain RKDF UNIVERSITY , BHOPAL RKDF UNIVERSITY, BHOPAL 1
INSTRUMENTATION
& MEASUREMENT
• Instrument Transformer & Special Instrument
• Electrical & Electronics Engineering Department
By
Sanjay Jain
RKDF UNIVERSITY , BHOPAL
RKDF UNIVERSITY, BHOPAL 1
Learning objective
• Study of instrument transformer
• Study of error’s & characteristics of CT & PT.
• Study of application of instrument transformer
• Study of power factor meter, Frequency meter, synchronscope.
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Why use Instrument Transformers?
• Circuit Isolation
• Reduce voltage and
currents to reasonable
working levels.
• Phasor combinations
for summing and
measuring power
Introduction
• Instrument transformers (ITs) are designed to transform voltage or
current from the high values in the transmission and distribution
systems to the low values that can be utilized by low voltage
metering devices.
• There are three primary applications for which:
1. Instrument transformers are used: metering (for energy billing and
transaction purposes)
2.protection control (for system protection and protective relaying
purposes) &
3.load survey (for economic management of industrial loads)
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• Depending on the requirements for those applications, the ITdesign and construction can be quite different.
• Protection ITs require linearity in a wide range of voltages andcurrents. During a disturbance, such as system fault or overvoltagetransients, the output of the IT is used by a protective relay toinitiate an appropriate action (open or close a breaker, reconfigurethe system, etc.) to mitigate the disturbance and protect the restof the power system.
• Instrument transformers are the most common and economicway to detect a disturbance.
• Typical output levels of instrument transformers are 1-5 amperesand 115-120 volts for CTs and VTs, respectively.
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Definitions• Voltage Transformer (VT)
• An instrument transformer used to reflect a primary voltageinto a secondary voltage through a magnetic medium.Always connected in parallel with primary conductoracross a circuit load.
• Secondary (measuring) voltage is usually 115 or 120 voltsnominally. The secondary voltage level is selected for easeof measurement and safety.
• Control Power Transformer (CPT)
• Designed to provide power for contractors, relays anddevices with high inrush currents, Regulation is not ascritical.
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TYPES OF CONSTRUCTION
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Current & Voltage Transformer Basics
(IEEE Standards)
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CURRENT TRANSFORMERS
TYPES OF C.T. CONSTRUCTION
The most common type of C.T. construction is
the “DOUGHNUT” type. It is constructed of an
iron toroid, which forms the core of the
transformer, and is wound with secondary
turns.Secondary Winding Primary Conductor
Iron Core
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Transformer ratio (TR)
Primary Current(100 amps)
Secondary Current (5 amps)
Primary CurrentSecondary Current
Transformer Ratio = _____________________
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• Transformation Ratio R =
=
=
• Nominal Ratio Kn =
=
• Turns Ratio n =
=
For a CT
For a PT
For a CT
For a PT
For a CT
For a PT
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Definition
• Ratio error =
=
• Phase Angle error(θ)=
• Burden: It is convenient to express load across the
secondary Terminal as the out put in volt-ampere at the
rated secondary voltage secondary burden due to load.
(Secondary current) x Impedance of load on
secondary winding
• Turns compensation: For CT the actual transformation
ratio
The correction by reduction in secondary winding turn is
exact only for a particular value of current and burdenimpedance. The CT in this case may be called Turnscompensated. RKDF UNIVERSITY, BHOPAL 13
CHARACTERISTICS OF CT
• Effect Of Power Factor Of Secondary Burden On
Errors
1) Ratio Error
2) Phase Angle
• Effect Of Change Of Primary Current
• Effect Of Change Of Secondary Burden
• Effect Of Change Of Frequency
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CHARACTERISTICS OF PT• Effect Of Secondary Current or VA:
If we increases the secondary burden the secondary
current is increased and therefore the primary current
increases. Both primary and secondary voltage drop
increases and thus for a given value of Vp the value of
Vs decreases and hence the actual ratio increases as
the burden increases the ratio error increases becoming
more negative with increase in burden this variation of
ratio error is almost linear with change in burden.
With increase in burden the phase angle between Vp
and Vs reversed increases become more negative.
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• Effect Of PT Of Secondary Burden:
The transformation ratio increases as the pf of
secondary burden reduces. The phase angle reduces with
decrease in secondary pf
• Effect Of Frequency:
The increase in frequency reduce the flux and hence
voltage ratio decreases.
The phase angle increases as the frequency
increases.
• Effect OF Primary Voltage:
There is a no wide variation of supply voltage to which
the primary winding the PT is connected therefore the
study of variation of ratio and ph
• As angle error with supply voltage are of low importance.
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PHASOR DIAGRAM OF CT
Normal Condition CT Burden has leading PFRKDF UNIVERSITY, BHOPAL 17
PHASOR DIAGRAM OF PT
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Power Measurement Using CT & PT
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Power factor meter
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Working
• A power factor meter is a type of electrodynamo
meter movement when it is made with two
movable coils set at right angles to each other. The
method of connection of this type of power
factor meter, in a 3f circuit, is shown in Figure
14.
• The two stationary coils, S and S1, are
connected in series in Phase B. Coils M and M1 are
mounted on a common shaft, which is free to move
without restraint or control springs.
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• These coils are connected with their series resistors from Phase B to Phase A and from Phase B to Phase C.
• At a power factor of unity, onepotential coil current leads and one lags the current in Phase B by 30°; thus, the coils arebalanced in the position shown in Figure 14.
• A change in power factor will cause the currentof one potential coil to become more in phase and the other potential coil to be more out ofphase with the current in Phase B, so that the movingelement and pointer take a new position of balance toshow the new power factor.
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Frequency Meter
.
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construction
• It consists of two coil mounted perpendicular to each
other. Each coil is divided in two section . The fig
shows the connection for weston’s frequency meter.
• The branch circuit of the coil A has a resistor Ra
connected in series with it. The coil B has a reactance
coil Lb in series. The moving element is a soft iron
needle which is pivoted in the spindle carrying pointer
and damping vanes. No controlling force is present.
The meter is connected across the supply and the two
coil carry currents which set up magnetic field which
are add right angles to each other. These fields,
whose is equal to the value of current flowing through
the coil, at on the needle which take up a position
depending on the relative magnitude of the tow fields.RKDF UNIVERSITY, BHOPAL 24
• Frequency increase , Reactance increase &
Resistance remain same .
• Field of first coil become stronger than second .
• Frequency decreases , opposite action takes place
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Frequency Counter
• Used to measure unknown frequency
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• Consists of
- Basic circuit : to get train of spike in output.
- Time Base : gives accuracy & reduces
error .
- Start & Stop Gate : decides starting &
closing instant of signal .
- Counter : for counting and display
purpose .
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working• The signal whose frequency is to be measure is converted
in to a train of pulses. the number of pulses appearing in a
definite interval of time which being very fast the frequency
signals may be known.
• The unknown frequency signal is fed to Schmitt trigger the
signal is converted in to a square wave with very fast rise
and fall time, than differential and dipped. The prior of this,
the signal may be amplified.
• The out put pulses of Schmitt trigger are fed to start-stop
gate. The counter start the count of pulses when this gate
opens(starts) and stop the counting when gate close(stops).
• If the interval is known than the frequency of in put signal
can be known. If the counter count N pulses in time intervalt, than f=N/t Hz
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THANKS
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