Transcript Measurement.

UNIT 4 : MEASUREMENT
OF VERY HIGH VOLTAGES
AND CURRENTS
4.0 INTRODUCTION
The following table gives the
different methods ( techniques )
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for measurement of very high
voltages :
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HIGH CURRENT MEASUREMENT TECHNIQUES :
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4.1 MEASUREMENT OF HIGH
DC VOLTAGES
The various methods of measuring
very high currents are explained
through the following figures:
4.1.1 High resistance in series
with micro ammeter :
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RESITANCE IN SERIES WITH AMMETER
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Referring to circuit (a) ,the
voltage v(t) = R i(t)
Referring to circuit (b),
v(t) = v2 (t) ( R1 + R2 ) / R2
= v2 (t) (1 + R1/R2)
V = V2 ( 1 + R1/R2)
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4.1.2 Resistance Potential
Dividers:
RESISTANCE POTENTIAL DIVIDER WITH ELECTROSTATIC
VOLTMETER
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300 kV DIVIDER FOR DC ( Ht.210 cm)
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4.1.3 Generating Voltmeters:
The charge stored in a capacitor C
is given by, q = cv
If capacitance varies with time ,
when connected to voltage source,
the current through the capacitor,
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i = dq/dt = v dc/dt + c dv/dt
For DC voltages dv/dt = 0 and
hence , I = dq/dt = v dc/dt
If capacitance varies between the
limits C0 and (C0 + Cm)
sinusoidally as,C = C0+ Cm sin ωt
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the current ‘i’ is given by,
i = v dc/dt = v cm ω cos ωt
I = Im cos ωt
where Im = VωCm
For a constant angular frequency
‘ω’, the current is proportional to
the applied voltage ‘V’.
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SCHEMATIC DIAGRAM OF GENERATING VOLTMETER
(ROTATING VANE TYPE )
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The advantages of a
generating voltmeters are :
1)No source loading by the meter
2)No direct connection to the HV
electrode
3)Scale is linear and extension
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of range is easy and
(4)A very convenient instrument
for electrostatic device such as
Van-de-graff generator and
particle accelerators
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4.2 MEASUREMENT OF HIGH
AC VOLTAGES
For power frequency AC
measurements series impedance
like pure resistor or reactance can
be used. Since resistances involve
power losses, often capacitor is
preferred. Resistance varies with
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temperature and also have stray
capacitances. Hence series
capacitance is mostly used.
4.2.1 Series capacitance
voltmeter:
This method is recommended
only for pure sinusoidal
voltages. i.e., Ic = jωcv
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SERIES CAPACITANCE WITH MILLIAMMETER FOR AC
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MEASUREMENT
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4.2.2 Capacitance potential
dividers: V1 = V2 ( C1+ C2 +Cm)/ C1
CAPACITANCE POTENTIAL DIVIDER
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STANDARD (COMPRESSED GAS) CAPACITOR FOR 1000 kV RMS
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4.2.3 Capacitance voltage
transformer:
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SCHEMATIC
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Resonance occurs when
ω ( L1+L2) equals 1/ ω (C1+C2)
4.2.4 Electrostatic voltmeters:
In electrostatic fields, the
attractive force between the
electrodes of parallel plate
condensor is given by,
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F= - dWs/ds = d/ds ((1/2 )CV2 )
= ½ V2 dc/ds =1/2 ε0 A ( V/s)2
As the force is proportional to the
square of the voltage , the
measurement can be made for
both AC and DC voltages.
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ABSOLUTE ELECTROSTATIC
VOLTMETER
LIGHT BEAM
ARRANGEMENT
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4.2.5 Series capacitance peak
voltmeter: ( Chubb-Frotscue
method):
In this method a half wave rectifier
is connected in series with a
capacitance and an ammeter as
shown in the figure next . The
rectified current reading,I = Vm ω C
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SERIES CAPACITACE PEAK VOLTMETER
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4.2.6 Peak voltmeters with
potential dividers:
PEAK VOLTMETER WITH CAPACITOR POTENTIAL DIVIDER AND
ELECTROSTATIC VOLTMETER
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Discharge resistor Rd is used to
permit variation of Vm when it is
reduced.
4.2.7 Uniform field gaps:
The arrangement of an uniform
field gap is shown in the next slide.
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ELECTRODES FOR 300 kV (rms)
SPARK GAP
BRUCE PROFILE
(half contour)
UNIFORM FIELD ELECTRODE GAP
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Ragowski presented a design for
uniform field electrodes for spark
over voltages upto 600 kV and is
given by,
V= AS + B √S
where ‘A’ and ‘B’ are constants
and ‘S’ is the gap spacing .
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At a temperature of 250 C and
pressure 760 mm of Hg , taking
air density factor ‘d’ into
account sparkover voltage ‘V’ is
given as,
V= 24.4 dS + 7.50 √ dS
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COMPARISON OF SPARKOVER VOLTAGES USING UNIFORM
FIELD GAPS AND SPHERE GAP METHODS AT TEMP. 200 C
AND PRESSURE 760 mm of Hg.
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4.3 MEASUREMENT OF HIGH
IMPULSE VOLTAGES
4.3.1 Potential Dividers:
Potential Dividers for high voltage
impulse, high frequency AC and
fast rising transient
voltage
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measurements are either resistive
or capacitive or mixed element
type. The low voltage arm of
the divider is usually connected
to a fast recording oscilloscope
or a peak reading instrument
through a delay cable.
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SCHEMATIC DIAGRAM OF POTENTIAL DIVIDER WITH DELAY
CABLE AND OSCILLOSCOPE
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4.3.1.1 Resistance potential
dividers for low impulse
voltages:
The wave form of the output
voltage measured across the low
voltage arm should be a correct
replica of the input wave shape.
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RESISTANCE POTENTIAL DIVIDER WITH SURGE CABLE
AND OSCILLOSCOPIC TERMINATION
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For correct compensation the
impedances of the high voltage
and low voltage arms are
chosen as , R1C1=R2Cm
4.3.1.2 Potential dividers for
high impulse voltages:
Resistance Dividers :
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EQUIVALENT CIRCUIT OF A RESISTANCE POTENTIAL
DIVIDER WITH SHIELD AND GUARD RINGS
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4.3.2 Capacitance voltage
dividers:
CAPACITANCE VOLTAGE DIVIDER FOR VERY HIGH VOLTAGES
AND ITS EQUIVALENT CIRCUIT
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CAPACITOR DIVIDER FOR 6 MV IMPULSE VOLTAGE
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4.3.3 Resistance –Capacitance
Dividers:
RESISTANCE-CAPACITANCE
CAPACITANCE
MIXED DIVIDER
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DIVIDER
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4.4 MEASUREMENT OF HIGH
VOLTAGES USING SPHERE
GAPS
Sphere gaps are used to
measure peak values of all types
of high voltages (DC,AC,Impulse
and Switching surges).
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The accuracy with potential
dividers is very high provided
the divider ratio is estimated
correctly.
Whereas the
measurement with sphere gaps
are fool proof though the
accuracy is less.
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Professor,
VERTICAL
SPHERE
GAP
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HORIZONTAL SPHERE GAP
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The clearance around the spheres
for various diameters are given
below:
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50 % DISRUPTIVE DISCHARGE
APPLICABLE TO IMPULSE
VOLTAGE BREAKDOWN
Unlike DC or AC voltages, the
impulse voltage is applied only for
microseconds duration. Provided
we apply sufficient voltage to
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cause a disruptive discharge , the
breakdown may occur once and
may not occur the next time when
the same level of voltage is
applied. Hence we resort to
statistical methods to obtain the
disruptive discharge voltage .
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50% disruptive discharge
voltage is that voltage which
causes disruptive discharges
for 50 % of the total number of
applications . Higher the
number of applications we get
more accurate values.
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There are two methods to
obtain the 50 % disruptive
discharge voltage namely,
Average method and
Up and down method
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AVERAGE METHOD
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UP AND DOWN METHOD
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Disruptive discharge voltages:
The peak disruptive discharge
voltages(50 % disruptive discharge
for impulse voltages) for AC
voltage, negative polarity of both
impulse and switching surge and
DC voltage of both polarities are
given in the following tables.
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Peak disruptive discharge voltages
(50 % disruptive discharge for
impulse voltages) for positive
polarity of both impulse and
switching surge voltages are given
in the following tables at a temp. of
200 C and pressure 760 mm of Hg.
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4.5 MEASUREMENT OF HIGH
FREQUENCY AND IMPULSE
CURRENTS
The most common method for
high
impulse
current
measurements is a low ohmic pure
resistive shunt . The voltage drop
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across the shunt , v(t)= R i(t)
The measuring circuit is shown in
the next slide. There are two
types of current shunts , namely
(1) Bifilar flat strip shunt and
(2) Tubular shunt . As the voltage
drop across the shunt is
measured through an
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LOW OHMIC SHUNT
EQUIVALENT CIRCUIT OF
SHUNT
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oscilloscope , the wave form
should be a true replica of the
current wave form. Hence special
care is taken during the design of
current shunts that they should
be of pure resistance only without
inductance or capacitance.
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BIFILAR FLAT STRIP RESISTIVE SHUNT
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SCHEMATIC ARRANGEMENT OF A COAXIAL OHMIC SHUNT
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