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Measurement Of High Ac Voltage
Contents
•Electrostatic voltmeter
•Series impedance voltmeter
•Potential dividers : Resistance or Capacitance type
•Conclusion and References
1Prepared by S ARUN M.Tech Measurements of high AC voltages
Electrostatic Voltmeter
• One of the direct methods of measuring high voltages is by means of electro-static
voltmeters.
• For voltages above 10 kV, generally the attracted disc type of electrostatic
voltmeter is used.
• When two parallel conducting plates (cross section area ‘A’ and spacing ‘s’) are
charged q and have a potential difference V, then the energy stored in the is given
by
Newton
s
V
A
2
1
F
s
A
ds
dC
s
A
Ce,capacitancfielduniformFor
Newton
ds
dC
V
2
1
FForce,
dsFdCVdWCVW
2
2
















2
2
22
ε
εε
2
1
2
1
• It is thus seen that the force of attraction is proportional to the square of the
potential difference applied, so that the meter reads the square value (or can
be marked to read the rms value).
2Prepared by S ARUN M.Tech Measurements of high AC voltages
Schematic as shown
3Prepared by S ARUN M.Tech Measurements of high AC voltages
4Prepared by S ARUN M.Tech Measurements of high AC voltages
• Electrostatic voltmeters of the attracted disc
type may be connected across the high
voltage circuit directly to measure up to about
200 kV, without the use of any potential
divider or other reduction method.
• The right hand electrode forms the high
voltage plate.
• The centre portion of the left hand disc is cut
away and encloses a small disc which is
movable and is geared to the pointer of the
instrument.
5Prepared by S ARUN M.Tech Measurements of high AC voltages
• The range of the instrument can be altered by
setting the right hand disc at pre-marked
distances.
• The force of attraction created by the applied
voltage causes the movable part-to which a
mirror is attached-to assume a position at
which a balance of forces takes place.
• An incident light beam will therefore be
reflected toward a scale calibrated to read the
applied voltage magnitude.
6Prepared by S ARUN M.Tech Measurements of high AC voltages
• Advantages:
i. Low loading effect
ii. Active power losses are negligibly small
iii. Voltage source loading is limited to the reactive power
needed to charge the system capacitance.(i.e., For 1V
Voltmeter- Capacitance is few Pico farad)
iv. Voltages upto 600kV can be measured.
• Disadvantage:
i. For constant distance ‘s’, F α V2, the sensitivity is small. This
can be overcome by varying the gap distance d in
appropriate steps.
7Prepared by S ARUN M.Tech Measurements of high AC voltages
Series Impedance Voltmeter
• For power frequency a.c. measurements the series impedance may be a pure
resistance or a reactance.
• But use of resistances yields the followings,
– Power losses
– Temperature problem
– Residual inductance of the resistance gives rise to an impedance different from its ohmic
resistance.
• High resistance units for high voltages have stray capacitances and hence a unit
resistance will have an equivalent circuit as shown in Fig.
• At any frequency ω of the a.c. voltage, R+jXL is connected in parallel with –jXC.
 
 
 
 
CRj
LjR
Z
CRjLCSince
CRjLC
LjR
Cj
LjR
Cj
LjR
Z
ω1
ω
,ωω,
ωω1
ω
ω
1
ω
ω
1
ω
2
2










8Prepared by S ARUN M.Tech Measurements of high AC voltages
 






























CR
R
L
anglePhasewhere
CR
R
L
jRCRjLjRZ
RC
LCRCRjLjR
Z
CRj
CRj
CRj
LjR
Z
ω
ω
tanφ,,
ω
ω
1ωω
ω1
ωωω
ω1
ω1
ω1
ω
1
2
222
22
9Prepared by S ARUN M.Tech Measurements of high AC voltages
• Extended Series Resistance neglecting
inductance is shown in figures.
• Resistor unit then has to be taken as a
transmission line equivalent, for calculating
the effective resistance.
• Ground or stray capacitance of each element
influences the current flowing in the unit, and
the indication of the meter results in an error.
10Prepared by S ARUN M.Tech Measurements of high AC voltages
• Stray ground capacitance effects can be
removed by shielding the resistor ‘R’ by a
second surrounding spiral RS which shunts the
actual resistor but does not contribute to the
current through the instrument.
• By tuning the resistors Ra the shielding resistor
end potentials may be adjusted with respect
to the actual measuring resistor so that the
resulting compensation currents between the
shield and the measuring resistors provide a
minimum phase angle.
11Prepared by S ARUN M.Tech Measurements of high AC voltages
Series Capacitance Voltmeter
• To avoid the drawbacks pointed out Series impedance voltmeter, a series
capacitor is used instead of a resistor for a.c. high voltage measurements.
• Current through the instrument, Ic=jωCV
• The rms value of the voltage V with harmonics is given by,
22
2
2
1 nrms VVVV  
•where V1,V2 ,... ,Vn represent the rms value
of the fundamental, second... and nth harmonics.
The currents due to these harmonics are
I1=ωCV1 , I2=2ωCV2 , ……In=nωCVn
   22
2
2
1 2ω nrms nVVVCI  
• With a 10% fifth harmonic only, the current is 11.2% higher, and hence
the error is 11.2% in the voltage measurement
• Not recommended when a.c. voltages are not pure sinusoidal waves but
contain considerable harmonics.
• Used for measuring rms values up to 1000 kV.
12
Prepared by S ARUN M.Tech Measurements of high AC voltages
• Conclusion
From these we came to know how to measure
high alternating voltages in power system
engineering.
• References
“High voltage engineering ” by M S Naidu and
V Kamaraju, Tata McGraw Hill Education, 5th
edition.
13Prepared by S ARUN M.Tech Measurements of high AC voltages

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Ac voltage

  • 1. Measurement Of High Ac Voltage Contents •Electrostatic voltmeter •Series impedance voltmeter •Potential dividers : Resistance or Capacitance type •Conclusion and References 1Prepared by S ARUN M.Tech Measurements of high AC voltages
  • 2. Electrostatic Voltmeter • One of the direct methods of measuring high voltages is by means of electro-static voltmeters. • For voltages above 10 kV, generally the attracted disc type of electrostatic voltmeter is used. • When two parallel conducting plates (cross section area ‘A’ and spacing ‘s’) are charged q and have a potential difference V, then the energy stored in the is given by Newton s V A 2 1 F s A ds dC s A Ce,capacitancfielduniformFor Newton ds dC V 2 1 FForce, dsFdCVdWCVW 2 2                 2 2 22 ε εε 2 1 2 1 • It is thus seen that the force of attraction is proportional to the square of the potential difference applied, so that the meter reads the square value (or can be marked to read the rms value). 2Prepared by S ARUN M.Tech Measurements of high AC voltages
  • 3. Schematic as shown 3Prepared by S ARUN M.Tech Measurements of high AC voltages
  • 4. 4Prepared by S ARUN M.Tech Measurements of high AC voltages
  • 5. • Electrostatic voltmeters of the attracted disc type may be connected across the high voltage circuit directly to measure up to about 200 kV, without the use of any potential divider or other reduction method. • The right hand electrode forms the high voltage plate. • The centre portion of the left hand disc is cut away and encloses a small disc which is movable and is geared to the pointer of the instrument. 5Prepared by S ARUN M.Tech Measurements of high AC voltages
  • 6. • The range of the instrument can be altered by setting the right hand disc at pre-marked distances. • The force of attraction created by the applied voltage causes the movable part-to which a mirror is attached-to assume a position at which a balance of forces takes place. • An incident light beam will therefore be reflected toward a scale calibrated to read the applied voltage magnitude. 6Prepared by S ARUN M.Tech Measurements of high AC voltages
  • 7. • Advantages: i. Low loading effect ii. Active power losses are negligibly small iii. Voltage source loading is limited to the reactive power needed to charge the system capacitance.(i.e., For 1V Voltmeter- Capacitance is few Pico farad) iv. Voltages upto 600kV can be measured. • Disadvantage: i. For constant distance ‘s’, F α V2, the sensitivity is small. This can be overcome by varying the gap distance d in appropriate steps. 7Prepared by S ARUN M.Tech Measurements of high AC voltages
  • 8. Series Impedance Voltmeter • For power frequency a.c. measurements the series impedance may be a pure resistance or a reactance. • But use of resistances yields the followings, – Power losses – Temperature problem – Residual inductance of the resistance gives rise to an impedance different from its ohmic resistance. • High resistance units for high voltages have stray capacitances and hence a unit resistance will have an equivalent circuit as shown in Fig. • At any frequency ω of the a.c. voltage, R+jXL is connected in parallel with –jXC.         CRj LjR Z CRjLCSince CRjLC LjR Cj LjR Cj LjR Z ω1 ω ,ωω, ωω1 ω ω 1 ω ω 1 ω 2 2           8Prepared by S ARUN M.Tech Measurements of high AC voltages
  • 10. • Extended Series Resistance neglecting inductance is shown in figures. • Resistor unit then has to be taken as a transmission line equivalent, for calculating the effective resistance. • Ground or stray capacitance of each element influences the current flowing in the unit, and the indication of the meter results in an error. 10Prepared by S ARUN M.Tech Measurements of high AC voltages
  • 11. • Stray ground capacitance effects can be removed by shielding the resistor ‘R’ by a second surrounding spiral RS which shunts the actual resistor but does not contribute to the current through the instrument. • By tuning the resistors Ra the shielding resistor end potentials may be adjusted with respect to the actual measuring resistor so that the resulting compensation currents between the shield and the measuring resistors provide a minimum phase angle. 11Prepared by S ARUN M.Tech Measurements of high AC voltages
  • 12. Series Capacitance Voltmeter • To avoid the drawbacks pointed out Series impedance voltmeter, a series capacitor is used instead of a resistor for a.c. high voltage measurements. • Current through the instrument, Ic=jωCV • The rms value of the voltage V with harmonics is given by, 22 2 2 1 nrms VVVV   •where V1,V2 ,... ,Vn represent the rms value of the fundamental, second... and nth harmonics. The currents due to these harmonics are I1=ωCV1 , I2=2ωCV2 , ……In=nωCVn    22 2 2 1 2ω nrms nVVVCI   • With a 10% fifth harmonic only, the current is 11.2% higher, and hence the error is 11.2% in the voltage measurement • Not recommended when a.c. voltages are not pure sinusoidal waves but contain considerable harmonics. • Used for measuring rms values up to 1000 kV. 12 Prepared by S ARUN M.Tech Measurements of high AC voltages
  • 13. • Conclusion From these we came to know how to measure high alternating voltages in power system engineering. • References “High voltage engineering ” by M S Naidu and V Kamaraju, Tata McGraw Hill Education, 5th edition. 13Prepared by S ARUN M.Tech Measurements of high AC voltages