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INSTRUMENTATION & MEASUREMENT Instrument Transformer & Special Instrument Electrical & Electronics Engineering Department By Sanjay Jain RKDF UNIVERSITY , BHOPAL RKDF UNIVERSITY, BHOPAL 1
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INSTRUMENTATION MEASUREMENT

Jan 25, 2022

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Page 1: INSTRUMENTATION MEASUREMENT

INSTRUMENTATION

& MEASUREMENT

• Instrument Transformer & Special Instrument

• Electrical & Electronics Engineering Department

By

Sanjay Jain

RKDF UNIVERSITY , BHOPAL

RKDF UNIVERSITY, BHOPAL 1

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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

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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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RKDF UNIVERSITY, BHOPAL 7

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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

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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

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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

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• 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

RKDF UNIVERSITY, BHOPAL 29