CHAPTER 2 MEASUREMENT OF HIGH VOLTAGE AND CURRENTS 2.1 MEASUREMENT - - PDF document

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• 2.1 MEASUREMENT OF HIGH DIRECT VOLTAGES
• 2.2 MEASUREMENT OF HIGH ALTERNATING VOLTAGES
• 2.3 MEASUREMENT OF IMPULSE VOLTAGE & CURRENT

CHAPTER 2 MEASUREMENT OF HIGH VOLTAGE AND CURRENTS

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Introduction

There are difficulties in measurement of such high voltages. The measurement technique should be able to control high

electric fields .

Sometimes the heat dissipation within the measuring circuits

may also have to be controlled.

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Introduction

Generally the mean values of dc voltage and peak value of ac

and impulse voltages are of practical interest.

There are certain measuring techniques which are applicable

to all the three types of voltages (ac, dc and impulse).

There are others which are specific to a particular type of

voltage.

The measuring methods will be classified according to the

types of voltages to be measured.

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Measurement of high dc voltages as in low voltage

measurements, is generally accomplished by extension of meter range with a large series resistance

The net current in the meter is usually limited to one to ten

microamperes for full-scale deflection.

a resistance potential divider with an electrostatic voltmeter

is sometimes better when high precision

Spark gaps such as sphere gaps are gas discharge devices and

give an accurate measure of the peak voltage

the measurement is affected by the atmospheric conditions

like temperature, humidity, etc

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Con’t…

Type of voltage Method (a) D.C voltages 1) Series resistance microammeter 2) Resistance potential divider 3) Generating voltmeters 4) Sphere and other spark gaps (b) A.C voltages 1) Series impedance ammeters 2) Potential dividers 3) Potential transformers 4) Electrostatic voltmeters 5) Sphere gaps (c) Impulse voltages 1) Potential dividers (cathode ray oscillograph) 2) Peak voltmeters 3) Sphere gaps

High voltage Measurement Techniques

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Con’t…

High current Measurement Techniques

Type of current Method (a) Direct current 1) Resistive shunts with miliammeter 2) Hall effect generators 3) Magnetic links (b) Alternating currents 1) Resistive shunts 2) Electromagnetic current transformers (c) Impulse current 1) Resistive shunts 2) Magnetic potentionmeters / Rogowski coils 3) Magnetic links 4) Hall effect generators

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Measurement Of High Direct Voltages

2.1.1 Series Resistance Microammeter

An impedance can be used in series with a micro (or

milli-) ammeter for the measurement of high voltages

The current through the instrument will be proportional

to the applied voltage

If the impedance is a resistor, the current is in phase with

and faithfully represent the voltage

The current I flowing through R is measured by micro

ammeter, where the source voltage is given by V = I R

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Measurement Of High Direct Voltages

Series Resistance Microammeter

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Measurement Of High Direct Voltages

The voltage drop in the meter is insignificant as the

impedance of the meter is only few ohms.

A protective device like a paper gap or zener diode with a

suitable series resistance as a protection against high voltage.

The ohmic value of the series resistance R is chosen such that

a current of one to ten microamperes is allowed for full-scale deflection.

The resistance is constructed from a large number of wire

wound or thin film resistor in series.

The voltage drop in each resistor element is chosen to avoid

surface flashovers and discharges

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Measurement Of High Direct Voltages

The limitations in the series resistance design are: (i) Power dissipation and sources loading (ii) Temperature effect and long time stability (iii) voltage dependence or resistive elements (iv) Sensitivity to mechanical stresses Series resistance meters are built for 500 kV d.c. with an

accuracy better than 0.2%.

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Measurement Of High Direct Voltages

2.1.2 Potential Dividers

A potential divider consists of two impedances, Z1and Z2,

connected in series, to which the voltage to be measured is applied.

The components that constitute the impedances are referred

to as the high-and low-voltage arms of the divider

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Measurement Of High Direct Voltages

Connection between the low-voltage arm and the measuring

instrument must be made through a shielded coaxial cable to avoid the adverse effects of stray capacitance between that connection and the high-voltage arm

High-voltage dividers generally consists of either resistors or

capacitors, but sometimes a combination of resistors and capacitors, in either series or parallel is used

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Measurement Of High Direct Voltages

Resistance Potential Dividers

A resistance potential divider with an electrostatic or high

impedance voltmeter is shown.

Usually employed for the measurement of direct voltages

• P – Protective device
• ESV – Electrostatic voltmeter

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Measurement Of High Direct Voltages

The high voltage magnitude is given by

Where V2 is the dc voltage across the low voltage arm R2.

With sudden change in voltage, such as switching operations,

flash over of the test objects or source short circuits, damage may occur to the divider elements.

To avoid these transient voltages, voltage controlling

capacitors are connected across the elements.

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Measurement Of High Direct Voltages

Exercise 1 It is required to measure 100 kV peak DC voltage using a suitable voltage divider and a suitable low voltage meter. If a 100 volt DC voltmeter is available, design a suitable voltage divider and calculate the values of the elements needed.

Assume that the current I flowing through the circuit is 1mA.

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Measurement Of High Direct Voltages

Sphere Gap

Sphere gaps technique are reliable only for certain gap

configurations.

Normally, only sphere gaps are used. In certain cases uniform

field gaps and rod gaps are also used, but their accuracy is less.

IEC-Publ. 52-1960, BS 358 shows the tabulated breakdown

voltages for standard conditions and various sphere diameters as a function of the gap spacing.

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Measurement Of High Direct Voltages

The actual breakdown voltage Vd at air density d may be found

from the tabulated value, Vdo by the following formula;

Vd = kVdo (kV)

where Vdo = 24.22 S + 6.08 √S for uniform field at standard conditions of temperature T=200C and pressure p=1013 milibars

• r 760mmHg

S is the gap length in cms.

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Measurement Of High Direct Voltages

Sphere gap can be arranged either a)

Horizontal with both spheres connected to the sources voltage or one sphere grounded

b)

Vertically with lower sphere grounded

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Measurement Of High Direct Voltages

The voltage to be measured is applied between the two

spheres and the distance spacing S between them gives a measure of the spark over voltage.

Standard values of sphere diameter are specified. Standard table lists standardized disruptive voltages from the

results of large number of international experiments.

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Tabulated value of breakdown voltage (BS 358)

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Measurement Of High Direct Voltages

Exercise 2 Determine the breakdown voltage for air gaps of 5 mm and 15 mm lengths under uniform field and standard atmospheric

• conditions. Also, determine the voltage if the atmospheric

pressure is 750 mm Hg and temperature 35°C.