MEASUREMENT OF HIGH VOLTAGES AND
CURRENT
1. What is the Specialty of high voltage / current
measurement?
1. Safety of
men & materials.
2. Accuracy
3. Induction
of over voltage, due to stray coupling.
4. Proper
location.
5. Linear
extrapolation not valid.
6. Electro
magnetic interference.
2. Different devices used for High DC voltages.
1. Series
resistance micro ammeter.
2. Resistance
potential dividers
3. Generating
of Voltmeters
4. Sphere
gap & Spark gaps.
3. What are the various methods used for measurement
of power frequency AC voltages.
1. Series
impedance ammeter.
2. Potential
dividers, resistance or capacitive Type.
3. Potential
Transformers electromagnetic or C. V. T.
4. Electrostatic
voltmeters.
5. Sphere
gap.
4. What is the method available for measurement &
High frequency AC voltages or Impulse voltages or other rapidly rising
voltages?
1. Potential
dividers, resistance Type or capacitance Type with CRO.
2. Peak
Voltmeter.
3. Sphere
gaps.
5. What are the various methods available for
measurement of High direct currents?
1. Resistive
shunt with mille ammeter
2. Hall
effect generators,
3. Magnetic
links
6. What are the methods available for measurement of
High alternating current?
1. Resistive
shunts with milli ammeters
2. Electro
magnetic current Transformers.
7. What are the various methods available for
measurement of High Impulse currents or High frequency ac or fast rising ac?
1. Resistive
shunts
2. Magnetic
pot cut meter.
3. Magnetic
links
4. Hall
Effect generators.
5. Faraday
generators.
8. What are the limitations of resistance potential
dividers?
1. Power
dissipation
2. Source
loading
3. Temperature
effect & long term stately
4. Sensitivity
to Mechanical strain.
5. Direct
connection to HV terminals.
9. What is the principle of generating voltmeters?
It is a
variable capacitance electrostatic voltage generator, generating current
proportional to the applied voltage. It does not absorb power from the voltage
measuring source. It is driven by external synchronous / constant speed motor.
Q =
charge
C =
capacitance
V =
voltage applied I = current
Q = C.V
i = dq/dt
= d/dt (CV) = c (dv/dt) +v (dc/dt) For a dc voltage V is constant
Then i =
V (dc/dt)
Capacitance
is changed by rotating the dielectric materials with a suitable shape by a
constant speed motor.
10.
What are
the advantages of generating voltmeters?
1. No source
loading
2. No direct
contact with HV terminals.
3. Scale is
linear & extension easy.
4. Very
convenient, to measure voltages for Van de graaff generators.
11.
What are
the limitations of generating voltmeters?
1. They
require calibrations
2. Careful
construction necessary
3. Disturbance
in mounting make calibrations invalid.
12. What is the condition for using a generator volt
meter to measure power frequency High voltage?
The speed
of the synchronous motor = Speed of the supply voltage. Or = ½ the speed of the
supply voltage)
Here
speed means angular velocity
13.
What is
the principle of electrostatic voltmeter?
In an
electrostatic field in a parallel plate configuration consisting of two
electrodes, when one of the electrodes is free to move, it will experience a
force when voltage is applied and the force is proportional to the squire of the
applied voltage.
The force
can be measured & from the force, the voltage applied can be calculated
using the calibration chart.
F = -
δW/δs
= - δ/δs
(1/2 CV2) = - 1/2 V2 (δC/δs)
= - 1/2 V2
(δ/δs) (Aε0/s) = -1/2 V2 ε0A (1/s2)
F =V/s)
/2
Since F
is proportional to square of voltage applied, it can be used for ac & dc
voltage measurement.
14. What is the principle of a ‘Sphere gap’ for measurement
of High voltages?
A uniform
field sphere gap will always have a spark over voltage within known tolerance
under constant atmospheric conditions. Hence it can be used for measurement of
peak value of the voltage. It is independent of the voltage wave form and hence
suitable for all types of wave forms, from dc to Impulse voltages of short rise
times (Rise time <0.5-s) and ac voltages up to 1MHz frequency.
15.
What are
the factors influencing the spark over voltage of a sphere gap?
1. Nearby
earthed object
2. Atmospheric
conditions and humidity
3. irradiation
4. Polarity
and rise time of voltage wave forms.
16. What is the principle of Regowski Coil?
If a coil
is placed surrounding a current carrying conductor, the voltage signal induced
in the coil.
V2(t)
= M d/dt ( I(t) )
When V2(t)
is passed through an Integrating network then
Vm(t)
= M (1/RC)∫dI(t)/dt = M I (t)/RC
When R
& C are those of the Integrating circuits. i.e.
Vm(t)
= K I (t)
Thus by
measuring Vm(t), we can calculate I(t), the current.
17.
What are
the qualities of a CRO used for measurement of impulse voltage measurements?
1. Sealed
tube, hot cathode Type
2. Photo
graphic arrangements for recording wave forms.
3. Input
voltage range from 5mv/cm to 20v/cm.
4. Probes
and attenuators to handle signals up to 600v.
5. Band with
and rise time of the CRO should be adequate.
6. Rise time
5 n.s. & band width as high as 500 MHz. may be necessary.
7. Oscilloscopes
are fitted with good cameras for recoding purposes, with writing speed 9cm/n.s.
8. Normally
provided with internal & external triggering facility.
9. It is
necessary that oscilloscope time base should be started before the voltage
reaches the deflecting plates. Such facility should be available.
18. What are the qualities of instrument lead and
management of Test circuits during Impulse measurement?
The
instrument leads and connections etc should be so made that the induced voltage
due to stray Pick ups, due to electro magnetic influences are avoided.
19.
What are
the advantages of CVT, (capacitance voltage transformer)?
1. Simple
& easy to install
2. Can be
used for relaying also.
3. Can be
used as a coupling capacitor for PLCC & relaying.
4. Frequency
independency (unlike a conventional electromagnetic potential Transformer).
5. Provides
isolation for HV & LV.
21.
What are
the demerits of a CVT?
1. Introduces
Ferro resonance in Power system.
2. Voltage
ratio is susceptible to Temperature variations.
22.
What are
the different types of shunts used for impulse voltage measurement?
1. Bifilar
flat strip
2. Coaxial
shunt
3. Squirrel
cage shunt
23. Explain the function of a delay cable in the HV
measurements using a CRO.
With
rapidly changing signals, the CRO time base should be started before the
original signal reaches the CRO y plates otherwise, the signed may be missed by
the CRO.
Therefore
while measurement, using a CRO, the Triggering of the CRO time base is done
directly (X plate) immediately and the signal proportional to voltage /current
to be measured is sent through a delay cable to the vertical (Y) plates so that
the required delay is obtained.
Delay
cable may be a long inter connecting cable 20 to 50m long.
25.
What are
the criteria required to assess the potential dividers used for High Impulse
voltages.
1. The shape
of the voltage in the test arrangement should be transferred with out any
distortion to the LV side.
2. Simple
determination of the transfer function is to be ensured.
3. They
should be suitable for Multi purposes, namely: power frequency, switching
Impulse, lighting Impulse voltages etc. That is the dividers should have broad
band widths.
Glossary
1. Inverted running: When
input to the prime-mover fails, the alternator runs as a synchronous motor and draws some current from the
supply
2. Buchholz Relay: Gas actuated relay installed in
oil immersed transformers for protection
against all kinds of faults
3.
Over
current relays: Providing protection mainly against phase-to-phase
faults and overloading.
4.
Earth-fault
relays: Providing protection against earth-faults only.
5.
Differential
protection: Currents entering and leaving the bus are totalized, fault occurs, the fault current upsets the balance and
produces a differential current to
operate a
relay
6.
Unbalanced
loading: Different phase currents in the alternator.
7.
Rod Gaps
: A rod gap
may be used to measure the peak value of power frequency and impulse voltages.
8.Electrostatic Voltmeter The
electric field according to Coulomb is the field of forces.
9. Voltage Dividers :If the
amplitudes of the impulse voltage is not high and is in the range of a few kilovolts.
10.
Potential
Dividers: The resistance potential dividers are the first to appear because of
their simplicity of construction,
less space requirements, less weight and easy portability.
Glossary
1.Regulation- The change of average voltage
across the load from the no Load.
2.Transient Voltage- It is an
oscillatory wave or a damped oscillatory wave of Frequency ranging for few hundred hertz to few kilo hertz.
3.Tesla coil-The high frequency resonant
transformer is called as a tesla Coil.
4.Duration of the wave -It is
defined as the total time of the wave during which the current is at least 10% of its peak value.
5.Trigatron
gap- It consists of a high voltage
spherical electrode of suitable Size,
an earthed main electrode of spherical shape and a Trigger electrode
through the main electrode.
6.
Half-Wave Rectifier Circuit- The simplest circuit for generation of high direct voltage is the
half wave rectifier.
7.
Electrostatic
Generator- In electromagnetic generators, current carrying
conductors are moved against the
electromagnetic forces acting upon them.
8.
Compensation-
As is
mentioned earlier, the test transformers are used for testing the insulation of various electrical
equipments.
9. Impulse voltage generator circuits The
introduction to the full impulse voltages as defined in the previous section leads to simple circuits for the
generation of the necessary wave shapes.
10.
Tripping In large
impulse generators, the spark gaps are generally sphere gaps or gaps formed by hemispherical electrodes.
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