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High frequency and high impulse
current measurements
Contents
• Hall generators
• Faraday Generators
• Magnetic potentiometers or probes
• Conclusion and Reference
1Prepared By S ARUN M.Tech Measurements of Impulse Current
Hall Generators
• Hall effect is used to measure very high direct current.
• Whenever electric current flows through a metal plate
placed in a magnetic field perpendicular to it, Lorenz
force will deflect the electrons in the metal structure in a
direction perpendicular to the direction of both the
magnetic field and the flow of current.
• The change in displacement generates an e.m.f called
“Hall Voltage”
2Prepared By S ARUN M.Tech Measurements of Impulse Current
• Hall Voltage,
where, B-Magnetic Flux density
I-Current
d-Thickness of the metal plate
R-Hall Coefficient (depends on Material of the plate &
temperature)
R is small for metals and High for semiconductors
• When large d.c. currents are to be measured the current carrying
conductor is passed through an iron cored magnetic circuit
d
BI
RV
d
BI
αV
H
H

3Prepared By S ARUN M.Tech Measurements of Impulse Current
• The magnetic field intensity produced by the
conductor in the air gap at a depth ‘d’ is given
by,
• The Hall element is placed in the air gap and a
small constant d.c. current is passed through
the element.
• The voltage developed across the Hall element
is measured and by using the expression for
Hall voltage the flux density B is calculated
and hence the value of current I is obtained.
d2
1
H


4Prepared By S ARUN M.Tech Measurements of Impulse Current
Faraday Generator or Magneto
Optic Method
• These methods of current measurement use the
rotation of the plane of polarization in materials by the
magnetic field which is proportional to the current
(Faraday effect).
• When a linearly polarized light beam passes through a
transparent crystal in the presence of a magnetic field,
the plane of polarization of the light beam undergoes
rotation. The angle of rotation is given by,
θ = α Bl
where,
α = A constant of the crystal which is a function of the wave
length of the light.
B = Magnetic flux density due to the current to be measured in
this case.
l = Length of the crystal.
5Prepared By S ARUN M.Tech Measurements of Impulse Current
• Fig. shows a schematic diagram of Magneto-optic method.
• Crystal C is placed parallel to the magnetic field produced by
the current to be measured.
• A beam of light from a stabilized light source is made incident
on the crystal C after it is passed through the polarizer P1.
• The light beam undergoes rotation of its plane of polarization.
• After the beam passes through the analyzer P2, the beam is
focused on a photomultiplier, the output of which is fed to a
CRO.
6Prepared By S ARUN M.Tech Measurements of Impulse Current
• The filter F allows only the monochromatic light to pass
through it. Photo luminescent diodes too, the momentary
light emission of which is proportional to the current flowing
through them, can be used for current measurement.
• Advantages:
1. It provides isolation of the measuring set up from the main
current circuit.
2. It is insensitive to overloading.
3. As the signal transmission is through an optical system no
insulation problem is faced. However, this device does not
operate for D.C current.
7Prepared By S ARUN M.Tech Measurements of Impulse Current
Magnetic Potentiometer
(Rogowski Coil)
• If the current to be measured is flowing through a conductor
which is surrounded by a coil as shown in Fig.
The voltage across the coil terminals will be:
M is the mutual inductance between the coil and the conductor,
Usually the coil is wound on a non-magnetic former in the form of a toroid and
has a large number of turns, to have sufficient voltage induced which could be
recorded.
dt
di
Mv(t) 
8Prepared By S ARUN M.Tech Measurements of Impulse Current
• The coil is wound cross-cross to reduce the leakage inductance.
• If N is the number of turns of the coil, A the coil area and lm its mean
length, the mutual inductance is given by
• Usually an integrating circuit RC is employed as shown in Fig to obtain
the output voltage proportional to the current to be measured. The
output voltage is given by
• The frequency response of the Rogowski coil is flat upto 100 MHz but
beyond that it is affected by the stray electric and magnetic fields and
also by the skin effect.
m
0
l
NAμ
M 
i(t)
RC
M
di
RC
M
dt
dt
di
M
RC
1
v(t)dt
RC
1
(t)v
t
0
0  
9Prepared By S ARUN M.Tech Measurements of Impulse Current
• Conclusion
From these conclude how to measure high
frequency impulse current in high voltage
engineering
• Reference
“High voltage engineering ” by M S Naidu and
V Kamaraju, Tata McGraw Hill Education, 5th
edition.
10Prepared By S ARUN M.Tech Measurements of Impulse Current

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Impulse current

  • 1. High frequency and high impulse current measurements Contents • Hall generators • Faraday Generators • Magnetic potentiometers or probes • Conclusion and Reference 1Prepared By S ARUN M.Tech Measurements of Impulse Current
  • 2. Hall Generators • Hall effect is used to measure very high direct current. • Whenever electric current flows through a metal plate placed in a magnetic field perpendicular to it, Lorenz force will deflect the electrons in the metal structure in a direction perpendicular to the direction of both the magnetic field and the flow of current. • The change in displacement generates an e.m.f called “Hall Voltage” 2Prepared By S ARUN M.Tech Measurements of Impulse Current
  • 3. • Hall Voltage, where, B-Magnetic Flux density I-Current d-Thickness of the metal plate R-Hall Coefficient (depends on Material of the plate & temperature) R is small for metals and High for semiconductors • When large d.c. currents are to be measured the current carrying conductor is passed through an iron cored magnetic circuit d BI RV d BI αV H H  3Prepared By S ARUN M.Tech Measurements of Impulse Current
  • 4. • The magnetic field intensity produced by the conductor in the air gap at a depth ‘d’ is given by, • The Hall element is placed in the air gap and a small constant d.c. current is passed through the element. • The voltage developed across the Hall element is measured and by using the expression for Hall voltage the flux density B is calculated and hence the value of current I is obtained. d2 1 H   4Prepared By S ARUN M.Tech Measurements of Impulse Current
  • 5. Faraday Generator or Magneto Optic Method • These methods of current measurement use the rotation of the plane of polarization in materials by the magnetic field which is proportional to the current (Faraday effect). • When a linearly polarized light beam passes through a transparent crystal in the presence of a magnetic field, the plane of polarization of the light beam undergoes rotation. The angle of rotation is given by, θ = α Bl where, α = A constant of the crystal which is a function of the wave length of the light. B = Magnetic flux density due to the current to be measured in this case. l = Length of the crystal. 5Prepared By S ARUN M.Tech Measurements of Impulse Current
  • 6. • Fig. shows a schematic diagram of Magneto-optic method. • Crystal C is placed parallel to the magnetic field produced by the current to be measured. • A beam of light from a stabilized light source is made incident on the crystal C after it is passed through the polarizer P1. • The light beam undergoes rotation of its plane of polarization. • After the beam passes through the analyzer P2, the beam is focused on a photomultiplier, the output of which is fed to a CRO. 6Prepared By S ARUN M.Tech Measurements of Impulse Current
  • 7. • The filter F allows only the monochromatic light to pass through it. Photo luminescent diodes too, the momentary light emission of which is proportional to the current flowing through them, can be used for current measurement. • Advantages: 1. It provides isolation of the measuring set up from the main current circuit. 2. It is insensitive to overloading. 3. As the signal transmission is through an optical system no insulation problem is faced. However, this device does not operate for D.C current. 7Prepared By S ARUN M.Tech Measurements of Impulse Current
  • 8. Magnetic Potentiometer (Rogowski Coil) • If the current to be measured is flowing through a conductor which is surrounded by a coil as shown in Fig. The voltage across the coil terminals will be: M is the mutual inductance between the coil and the conductor, Usually the coil is wound on a non-magnetic former in the form of a toroid and has a large number of turns, to have sufficient voltage induced which could be recorded. dt di Mv(t)  8Prepared By S ARUN M.Tech Measurements of Impulse Current
  • 9. • The coil is wound cross-cross to reduce the leakage inductance. • If N is the number of turns of the coil, A the coil area and lm its mean length, the mutual inductance is given by • Usually an integrating circuit RC is employed as shown in Fig to obtain the output voltage proportional to the current to be measured. The output voltage is given by • The frequency response of the Rogowski coil is flat upto 100 MHz but beyond that it is affected by the stray electric and magnetic fields and also by the skin effect. m 0 l NAμ M  i(t) RC M di RC M dt dt di M RC 1 v(t)dt RC 1 (t)v t 0 0   9Prepared By S ARUN M.Tech Measurements of Impulse Current
  • 10. • Conclusion From these conclude how to measure high frequency impulse current in high voltage engineering • Reference “High voltage engineering ” by M S Naidu and V Kamaraju, Tata McGraw Hill Education, 5th edition. 10Prepared By S ARUN M.Tech Measurements of Impulse Current