Important Short Questions and Answers: Measurement of High Voltage and High Currents

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

V_m(t) = M (1/RC)∫dI(t)/dt = M I (t)/RC

When R & C are those of the Integrating circuits. i.e.

V_m(t) = K I (t)

Thus by measuring V_m(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.