Instrumentation & Sensor - Mod@4.pdf

Instrumentation and Sensor
Mr. K. Sujita Kumar Achary
Assistant Professor
DEPARTMENT OF ELECTRICAL ENGINEERING
VEER SURENDRA SAI UNIVERSITY OF TECHNOLOGY
BURLA, ODISHA - 768018

K Sujita Kumar Achary
Department Electrical Engineering, VSSUT 2
Instrument Transformers
Instrument transformers are used for measurement of voltage, current, energy and power in ac circuits.
• To extend (multiply) the range of the measuring instrument and
• To isolate the measuring instrument from a high-voltage line.
In power systems, levels of currents and voltages handled are very high, therefore direct measurements
with conventional instruments is not possible without compromising operator safety, and size and cost of
instrument.
Advantages of Instrument Transformers
• The large voltage and current of AC Power system can be measured by using small rating measuring
instrument i.e. 5 A, 110 – 120 V.
• By using the instrument transformers, measuring instruments can be standardized. Which results in
reduction of cost of measuring instruments.
• Instrument transformers provide electrical isolation between high voltage power circuit and
measuring instruments. Which reduces the electrical insulation requirement for measuring instruments
and protective circuits and also assures the safety of operators.
• Several measuring instruments can be connected through a single transformer to power system.
• Due to low voltage and current level in measuring and protective circuit, there is low power
consumption in measuring and protective circuits.

Instrument transformers are used for measurement of voltage, current, energy and power in ac
circuits.
• To extend (multiply) the range of the measuring instrument and
• To isolate the measuring instrument from a high-voltage line.
In power systems, levels of currents and voltages handled are very high, therefore direct
measurements with conventional instruments is not possible without compromising operator
safety, and size and cost of instrument.
Types of Instrument Transformer
Such transformers are of two types :
• Current Transformer (or Series Transformer)
• Potential Transformer (or Parallel Transformer)
➢ Current transformers are used when the magnitude of AC currents exceeds the safe value of
current of measuring instruments.
➢ Potential transformers are used where the voltage of an AC circuit exceeds 750 V as it is not
possible to provide adequate insulation on measuring instruments for voltage more than this.

Measurement of current as CT
The primary winding is so connected that the current to be measured passes through it and the
secondary is connected to the ammeter.
The function of CT is to step down the current.

Measurement of voltage by PT
The primary winding is connected to the voltage side to be measured and secondary to the
voltmeter.
The function of PT is to steps down the voltage to the level of voltmeter.

Instrument Transformer
Advantages of Instrument Transformer
1. The measuring instruments can be placed for away from the high voltage side by connecting
long wires to the instrument transformer. This ensures the safety of instruments as well as the
operator.
2. This instrument transformers can be used to extend the range of measuring instruments like
ammeters and voltmeters.
3. The power loss in instrument transformers is very small as compared to power loss due to the
resistance of shunts and multipliers.
4. By using current transformer with tong tester, the current in a heavy current circuit can be
measured.
Disadvantages of Instrument Transformer
1. The only main draw back is that these instruments can not be used in DC circuits.

THEORY OF CURRENT TRANSFORMERS
Parameters of Current Transformer:
• A current transformer is defined by its current Turn Ratio, Accuracy Class, Accuracy
Limit Factor and Instrument Safety Factor (ISF) etc.
• Accuracy class is determined by phase angle error and ratio error.
• Turn Ratio is defined as the ratio of number of secondary winding turns to number of
primary winding turns. It is often denoted by n and given as
n = Number of secondary winding turns / Number of primary winding turns
• Transformation Ratio is defined as the ratio of primary winding current to secondary
winding current. It is denoted by R and given as
R = primary winding current / Secondary winding current
There are always some losses associated in a CT and hence Turn Ratio and
Transformation Ratio are not equal. This gave rise to ratio error.
Burden is defined as the volt ampere (VA) of connected load across the terminals for
secondary winding of CT.

The equivalent circuit and phasor diagram are same as that of power transformer.
The phasor diagram is drawn assuming that connected burden is of lagging power
factor.
As CT is connected in series with the primary circuit whose current is to be
measured, this means that primary current Ip of CT is not dependent on secondary
burden rather it is determined by the primary circuit current.

Transformation Ratio:
• For finding transformation ratio we need to calculate primary current Ip as per
definition and then divide it by secondary current Is.
• From the above phasor, it can be easily observed that primary current Ip is
phasor sum of nIs and I0. The angle between and is (90-α-δ).
• Therefore primary current Ip can be calculated using vector addition formula.
• Ip = [(nIs)2+(I0)2+2nIsI0Cos(90-α-δ)]1/2
= [(nIs)2+(I0)2+2nIsI0Sin(α+δ)]1/2
• Hence, Transformation Ratio,
R = Ip/Is
R = [(nIs)2+(I0)2+2nIsI0Sin(α+δ)]1/2 / Is
= [(nIs)2+(I0 Sin(α+δ))2+2nIsI0Sin(α+δ)]1/2 / Is
= [{nIs+ I0 Sin(α+δ)}2]1/2 / Is
= [nIs+ I0 Sin(α+δ)] / Is
= n + (I0/ Is)Sin(α+δ)
Thus. Transformation Ratio R = n + (I0/ Is)Sin(α+δ)

Transformation Ratio R = n + (I0/ Is)Sin(α+δ)
Following points should be noted from the above expression:
• Transformation ratio is not equal to CT turn ratio.
• Transformation ratio depends on excitation current and excitation angle.
• For transformation ratio to be equal to turn ratio, α = δ =0. This means
that there should not be any core loss in the current transformer and
burden should be purely resistive.

Phase Angle of Current Transformer:
Phase angle of current transformer is the angle between the primary current
Ip and secondary current Is when reversed. In above phasor diagram, θ is the
phase angle.
In the above phasor, consider right angled triangle obc
tanθ = bc/ob
= I0Sin(90-α-δ) / (oa+ab)
= I0Cos(α+δ) / [nIs+I0Sin(α+δ)]
Since θ is very small, hence tanθ will be approximately equal to θ. Therefore we
can write,
θ = I0Cos(α+δ) / [nIs+I0Sin(α+δ)]
Also, I0 is very very small, therefore I0Sin(α+δ)<<<<nIs, neglecting I0Sin(α+δ)
θ = I0Cos(α+δ) / nIs
= [I0CosαCosδ – I0SinαSinδ] / nIs
From phasor, Im = I0Cosα and Ie = I0Sinα, hence
θ = [ImCosδ – IeSinδ] / nIs Radian
= (180/π) [ImCosδ – IeSinδ] / nIs degree

Ratio Error of Current Transformer:
• Ratio Error is defined as the per unit deviation in transformation ratio from nominal ratio. It is expressed in
percentage.
• Percentage Ratio Error E = (Nominal Ration – Transformation Ratio)x100 / Transformation Ratio
• Since the burden of current transformer is usually resistive, therefore power factor of burden is almost unity.
Hence δ=0,
• R = n + (I0Sinα / Is) but I0Sinα = Ie
• = n + Ie/Is
• Therefore, percentage ratio error can be calculated easily.
Phase Angle Error of Current Transformer:
• Ideally the angle between the primary current and secondary current should be 180 degree. But there is some
deviation from 180 degree. This deviation is called phase angle error. As can be seen from phasor, this departure is by
an angle θ. This means,
• Phase difference between primary current and secondary current = θ
• Since the burden of current transformer is usually resistive, therefore power factor of burden is almost unity. Hence
δ=0. Therefore Phase angle error θ = (180/π) Im / nIs degree

CT secondary should never be kept open. An open CT secondary causes a high voltage across the
CT secondary terminals which may cause damage of equipment / personnel.
Causes of Errors in CT
• Primary winding always needs some magnetising MMF to produce flux and, therefore, the CT draws
the magnetising current IM.
• CT no-load current must have a component Ic that has to supply the core losses, i.e., the eddy current
loss and the hysteresis loss.
• Once the CT core becomes saturated, the flux density in the core no longer remains a linear function
of the magnetising force, this may introduce further errors.
• Primary and secondary flux linkages differ due to unavoidable flux leakages.
Reducing Errors in CT
• Low Flux Density
• High Permeability Core Material
• Modification of Turns Ratio

THEORY OF POTENTIAL TRANSFORMERS
• The phase-angle difference θ between the primary voltage VP and the
reflected secondary voltage nVS is called phase angle of the PT.
• Ideally, without any no-load current and without any voltage drop in
winding impedances, these two phasors must have been in the same
phase, i.e., ideally θ = 0.

• The phase-angle difference θ between the primary voltage VP and the
reflected secondary voltage nVS is called phase angle of the PT.
• Ideally, without any no-load current and without any voltage drop in
winding impedances, these two phasors must have been in the same
phase, i.e., ideally θ = 0.
• From the phasor diagram

• Substituting the above values

• Here, RP = equivalent resistance of the PT referred to primary side
• XP = equivalent reactance of the PT referred to primary side
• Thus, actual voltage transformation ratio:
• Thus, actual voltage transformation ratio can again be written as

ERRORS INTRODUCED BY POTENTIAL TRANSFORMERS
Ratio Error and Phase-Angle Error
• It can be seen from the above section that, like current transformers, potential transformers also
introduce errors in measurement. This error may be in terms of magnitude or phase, in the
measured value of voltage.
• The ratio error (difference between nominal ratio and actual transformation ratio) only is
important when measurements of voltage are to be made; the phase angle error is of importance
only while measurement of power.
• In presence of these errors, the voltage applied to the primary circuit of the PT can not be
obtained accurately by simply multiplying the voltage measured by the voltmeter connected
across the secondary by the turns ratio n of the PT.
• These errors depend upon the resistance and reactance of the transformer winding as well as on
the value of no-load current of the transformer.

ERRORS INTRODUCED BY POTENTIAL TRANSFORMERS
Reducing Errors in PT
1. Reducing the loss component and magnetising components, i.e., the no-load component of the
primary current can be achieved by reducing the length of magnetic path in the core, using good
quality core magnetic materials, designing with appropriate value of flux densities in the core,
and adopting precautionary measures while assembling and interleaving of core laminations.
2. Winding resistance can be reduced by using thick conductors and taking care to reduce the
length of mean turn of the windings.
3. Winding leakage flux and hence leakage reactance can be reduced by keeping the primary and
secondary windings as close as permissible from the point of view of insulation requirements.
4. Sufficiently high flux densities in the core will reduce the core cross-section, thereby reducing
the length of winding wound over the core. This, in turn, will reduce the winding resistance. An
optimisation in the core flux density value to be used needs to done, since too high a flux density
will increase the no-load current, which is also not desirable.
5. The actual PT transformation ratio exceeds the turns ratio by an amount (IC rP+ IM XP)/VS. by
reducing the number of turns in the primary winding or increasing the number of turns in the
secondary winding. This makes it possible to make the actual transformation ratio to be equal to
the nominal ratio, at least for a particular value and type of burden.

POTENTIOMETER
• A potentiometer is an instrument which is used for measurement of potential
difference across a known resistance or between two terminals of a circuit or
network of known characteristics.
• A potentiometer measures voltage by comparing it with a standard cell, it can be
also used to measure the current simply by measuring the voltage drop produced by
the unknown current passing through a known standard resistance.
Some important characteristics of potentiometer are the following:
• A potentiometer measures the unknown voltage by comparing it with a known
voltage source rather than by the actual deflection of the pointer. This ensures a
high degree of accuracy.
• As a potentiometer measures using null or balance condition, hence no power is
required for the measurement.
• Determination of voltage using potentiometer is quite independent of the source
resistance.

POTENTIOMETER
• First, the switch S is put in the ‘operate’ position and
the galvanometer key K kept open, the battery
supplies the working current through the rheostat and
the slide wire. The working current through the slide
wire may be varied by changing the rheostat setting.
• The method of measuring the unknown voltage, E1,
depends upon the finding a position for the sliding
contact such that the galvanometer shows zero
deflection, i.e., indicates null condition, when the
galvanometer key K is closed.
• Thus, determination of the values of unknown
voltage now becomes a matter of evaluating the
voltage drop E2 along the portion a–c of the slide
wire.

POTENTIOMETER
• When the switch S is placed at ‘ calibrate’
position, a standard or reference cell is connected
to the circuit, it is used to standardize the
potentiometer
• The process of adjusting the working current so
as to match the voltage drop across a portion of
sliding wire against a standard reference source is
known as ‘ standardisation’.

CROMPTON’S dc POTENTIOMETER
• It consists of a dial switch which has fifteen (or
more) steps. Each steep has 10 Ω resistance. So
the dial switch has total 150 Ω resistance.
• The working current of this potentiometer is 10
mA and therefore each step of dial switch
corresponds to 0.1 volt. So the range of the dial
switch is 1.5 volt.
• The circular slide wire has 10 Ω resistance. So the
range of that slide wire is 0.1 volt. The slide wire
calibrated with 200 scale divisions and since the
total resistance of slide wire corresponds to a
voltage drop of 0.1 volt, each division of the slide
wire corresponds to 0.1/200= 0.0005 volt.

• At first, the combination of the dial switch and
the slide wire is set to the standard cell voltage.
Let the standard sell voltage be 1.0175 volts, then
the dial resistor is put in 1.0 volt and the slide
wire at 0.0175 volts setting.
• The switch ‘S’ is thrown to the calibrate position
and the galvanometer switch ‘K’ is pressed until
the rheostat is adjusted for zero deflection on the
galvanometer. The 10 kW protective resistance is
kept in the circuit in the initial stages so as to
protect the galvanometer from overload.

• After completion of the standardisation, the
switch ‘S’ is thrown to the operate position
thereby connecting the unknown emf into the
potentiometer circuit. With the protective
resistance in the circuit, the potentiometer is
balanced by means of the main dial and the slide
wire adjustment.
• As soon as the balanced is approached, the
protective resistance is shorted and final
adjustments are made to obtain true balance.
• After the final true balance is obtained, the value
of the unknown emf is read off directly from the
setting of the dial switch and the slide wire.

AC POTENTIOMETER
• An ac potentiometer is same as dc potentiometer by principle. In case of dc
potentiometer, only the magnitude of the unknown emf is compared with the
standard cell emf, but in ac potentiometer, the magnitude as well as phase angle
of the unknown voltage is compared to achieve balance.
1. To avoid error in reading, the slide wire and the resistance coil of an ac
potentiometer should be non-inductive.
2. The reading is affected by stray or external magnetic field, so in the time of
measurement they must be eliminated or measured and corresponding
correction factor should be introduced.
3. The sources of ac supply should be free from harmonics, because in presence
of harmonics the balance may not be achieved.
4. The ac source should be as sinusoidal as possible.
5. The potentiometer circuit should be supplied from the same source as the
voltage or current being measured.

AC POTENTIOMETER
1. Polar Potentiometer
As the name indicates the unknown emf is measured in polar form, i.e., in terms of
its magnitude and relative phase. The magnitude is indicated by one scale and the
phase with respect to some reference axis is indicated by another scale. There is
provision for reading phase angles up to 360°.
The voltage is read in the form V – θ.
Example: Drysdale polar potentiometer
2. Coordinate Potentiometer
Here the unknown emf is measured in Cartesian form. Two components along and
perpendicular to some standard axis are measured and indicated directly by two
different scales known as in phase (V1) and quadrature (V2) scales. Provision is
made in this instrument to read both positive and negative values of voltages so that
all angles up to 360° are covered.
Example: Gall–Tinsley and Campbell–Larsen type potentiometer

AC POTENTIOMETER
Drysdale Polar Potentiometer
The different components of a Drysdale polar potentiometer is shown in Figure

AC POTENTIOMETER
The slide wire S1–S2 is supplied from a phase shifting circuit for ac measurement. The
phase shifting circuit is so arranged that the magnitude of the voltage supplied by it
remains constant while its phase can be varied through 360°. Consequently, slide wire
current can be maintained constant in magnitude but varied in phase.

AC POTENTIOMETER
The phase shifting circuit consists of two stator coils connected in parallel supplied
from the same source; their currents are made to differ by 90° by using very accurate
phase shifting technique. The two windings produce rotating flux which induces a
secondary emf in the rotor winding which is of constant magnitude but the phase of
which can be varied by rotating the rotor in any position. The phase of the rotor emf is
read from the circular dial attached in the potentiometer.

AC POTENTIOMETER
Before the ac measurement, the potentiometer is first calibrated by using dc supply for
slide wire and standard cell for test terminals T1 and T2. The unknown alternating
voltage to be measured is applied across test terminals and the balance is achieved by
varying the slide wire contact and the position of the rotor. The ammeter connected in
the slide wire circuit gives the magnitude of the unknown emf and the circular dial in
the rotor circuit gives the phase angle of it.

ADVANTAGES AND DISADVANTAGES OF AC POTENTIOMETER
Advantages
1. An ac potentiometer is a very versatile instrument. By using shunt and volt–ratio
box, it can measure wide range of voltage, current and resistances.
2. As it is able to measure phase as well as magnitude of two signals, it is used to
measure power, inductance and phase angle of a coil, etc.
3. The principle of ac potentiometer is also incorporated in certain special application
like Arnold circuit for the measurement of CT (Current Transformer) errors.

Disadvantages
1. A small difference in reading of the dynamometer instrument either in dc or ac calibration
brings on error in the alternating current to be set at standard value.
2. The normal value of the mutual inductance M is affected due to the introduction of mutual
inductances of various potentiometer parts and so a slight difference is observed in the
magnitude of the current of quadrature wire with compared to that in the in–phase
potentiometer wire.
3. Inaccuracy in the measured value of frequency will also result in the quadrature
potentiometer wire current to differ from that of in–phase potentiometer wire.
4. The presence of mutual inductances in the various parts of the potentiometer and the inter
capacitance, the potential gradient of the wires is affected.
5. Since the standardisation is done on the basis of rms value and balance is obtained
dependent upon the fundamental frequency only, therefore, the presence of harmonics in the
input signal introduces operating problem and the vibration galvanometer tuned to the
fundamental frequency may not show full null position at all.

Instrumentation & Sensor - Mod@4.pdf

Recommended

Recommended

More Related Content

Similar to Instrumentation & Sensor - Mod@4.pdf

Similar to Instrumentation & Sensor - Mod@4.pdf (20)

Recently uploaded

Recently uploaded (20)

Instrumentation & Sensor - Mod@4.pdf