Module 5 - Generation of High Voltage

EE8215 High Voltage Engineering
(Program Elective)
B.E. / VII Semester / EEE
Prem Prakash
Department of Electrical and Electronics Engineering

Birla Institute of Technology, Mesra
Ranchi-835215
Module – V
Generation of High
Voltage and Currents
In the fields of electrical engineering and applied physics, high voltages (dc, ac, and impulse) are
required for several applications.
For example,
 Electron microscopes and X-ray units require high dc voltages of the order of 100 kV or more.
 Electrostatic precipitators,
 particle accelerators in nuclear physics, etc. require high voltages (dc) of several kilovolts and
even megavolts.
 High ac voltages of one million volts or even more are required for testing power apparatus
rated for extra high transmission voltages (400 kV system and above).
 High impulse voltages are required for testing purposes to simulate over voltages that occur in
power systems due to lightning or switching action.
 For electrical engineers, the main concern of high voltages is for the insulation testing of
various components in power systems for different types of voltages, namely, power frequency
ac, high frequency, ac switching or lightning impulses.
 Hence, generation of high voltages in laboratories for testing purposes is essential.

Different forms of high voltages are classified as:
(i) high dc voltages,
(ii) high ac voltages of power frequency,
(iii) high ac voltages of high frequency,
(iv) high transient or impulse voltages of very short duration such as lightning over voltages, and
(v) transient voltages of longer duration such as switching surges.

Normally, in high-voltage testing, the current under conditions of failure is limited to a small value
(less than an ampere in the case of dc or ac voltages and few amperes in the case of impulse or
transient voltages).
But in certain cases, like the testing of surge diverters or the short-circuit testing of switchgear,
high-current testing with several hundreds of amperes is of importance.
Tests on surge diverters require high-surge currents of the order of several kiloamperes. Therefore,
test facilities require high-voltage and high-current generators.
High impulse current generation is also required along with voltage generation for testing
purposes.
GENERATION OF HIGH DIRECT-CURRENT VOLTAGES
Generation of high dc voltages is mainly requited in research work in the areas of pure and applied
physics.
Sometimes, high direct voltages are needed in insulation tests on cables and capacitors.
Impulse generator charging units also require high dc voltages of about 100 to 200 kV. Normally,
for the generation of dc voltages of up to 100 kV, electronic valve rectifiers are used and the output
currents are about 100 mA.
The rectifier valves require special construction for cathode and filaments since a high electrostatic
field of several kV/cm exists between the anode and the cathode in the non-conduction period.
The ac supply to the rectifier tubes may be of power frequency or may be of audio frequency from
an oscillator.
The latter is used when a ripple of very small magnitude is required without the use of costly
filters to smoothen the ripple.
Half- and Full-Wave Rectifier Circuits
Rectifier circuits for producing high dc voltages from ac sources may be

(a) half wave,
(b) full wave, or
(c) voltage doubler-type rectifiers.
The rectifier may be an electron tube or a solid-state device.
Single electron tubes are available for peak inverse voltages up to 250 kV, and
semiconductor or solid-state diodes up to 20 kV.
For higher voltages, several units are to be used in series.
When a number of units are used in series, transient voltage distribution along each unit
becomes non-uniform and special care should be taken to make the distribution uniform.
Half- and Full-Wave Rectifier Circuits
The value of the capacitor C is chosen such that

the time constant CR is at least 10 times that of
the period of the ac supply.
The rectifier valve must have a peak inverse

rating of at least 2V.
To limit the charging current, an additional

resistance R is provided in series with the
secondary of the transformer.
The source transformer requires a centre-tapped secondary with a rating of 2 V.
For applications at high voltages of 50 kV and above:
the rectifier valves used are of special construction

 Apart from the filament, the cathode and the anode, they contain a protective shield or grid around the filament
and the cathode.
 The anode will usually be a circular plate. Since the electrostatic field gradients are quite large, the heater and the
cathode experience large electrostatic forces during the non-conduction periods. To protect the various elements
from these forces, the anode is firmly fixed to the valve cover on one side.
 a steel mesh structure or a protective grid kept at the cathode potential surrounds them so that the mechanical
forces between the anode and the cathode are reflected on the grid structure only.
 In modern high-voltage laboratories and testing installations, semiconductor rectifier stacks are commonly used
for producing dc voltages.
 Semiconductor diodes are not true valves since they have finite but very small conduction in the backward
direction.
 The more commonly preferred diodes for high-voltage rectifiers are silicon diodes with Peak Inverse Voltage
(PIV) of 1 kV to 2 kV.
 However, for laboratory applications, the current requirement is small (a few milliamperes, and less than one
ampere) and as such, a selenium element stack with PIV of up to 500 kV may be employed without the use of any
voltage grading capacitors.
 Both full wave and half-wave rectifiers produce dc voltages less than the ac maximum voltage.
 Also, ripple or the voltage fluctuation will be present, and this has to be kept with in a reasonable limit by means
of filters.
Ripple Voltage with Half-Wave
and Full-Wave Rectifiers
When a full-wave or a half-wave rectifier is used along with the smoothing capacitor C, the voltage on no
load will be the maximum ac voltage.
But when on load, the capacitor gets charged from the supply voltage and discharges into load resistance
R whenever the supply voltage waveform varies from peak value to zero value.
When loaded, a fluctuation in the output dc voltage dV appears, and is called a ripple.
The ripple voltage dV is larger for a half-wave rectifier than that for a full-wave rectifier, since the
discharge period in the case of half-wave rectifier is larger.
The ripple dV depends on (a) the supply voltage frequency f, (b) the time constant CR, and (c) the
reactance of the supply transformer X.
For half-wave rectifiers, the ripple frequency is equal to the supply frequency and for full-wave rectifiers,
it is twice that value.
The ripple voltage is to be kept as low as possible with the proper choice of the filter capacitor and the
transformer reactance for a given load R.
Voltage Doubler Circuits
In the voltage doubler circuit:
 the capacitor C1 is charged through rectifier R1 to a voltage of +Vmax during the negative half
cycle.
 As the voltage of the transformer rises to positive +Vmax during the next half cycle, the potential of
the other terminal of C1 rises to a voltage of +2Vmax.
 the capacitor C2 in turn is charged through R2 to 2Vmax.
 the dc output voltage on load will be less than 2Vmax, depending on the time constant C2RL and the
forward charging time constants.
 The ripple voltage of these circuits will be about 2% for RL/r=10 and X/r=0.25, where X and r are
the reactance and resistance of the input transformer. The rectifiers are rated to a peak inverse
voltage of 2Vmax, and the capacitors C1 and C2 must also have the same rating.
 If the load current is large, the ripple also is more.

Cascaded voltage doubler
 Cascaded voltage doublers are used when larger output voltages are needed without changing the input
transformer voltage level.
 The rectifiers R1 and R2 with transformer T and capacitors C1 and C2 produce an output voltage of 2V.
 This circuit is duplicated and connected in series or cascade to obtain a further voltage doubling to 4 V.
 T is an isolating transformer to give an insulation for 2Vmax since the transformer T is at a potential of
2Vmax above the ground.
 The voltage distribution along the rectifier string R1, R2, R is made uniform by having capacitors C1, C2,
C3, and C4 of equal values.
 The arrangement may be extended to give 6V, 8V, and so on, by repeating further stages with suitable
isolating transformers.
 In all the voltage doubler circuits, if valves are used, the filament transformers have to be suitably
designed and insulated, as all the cathodes will not be at the same potential from ground.
 The arrangement becomes cumbersome if more than 4 V is needed with cascaded steps.
Voltage Multiplier Circuits
• Cascaded voltage multiplier circuits for higher voltages are

cumbersome
• require too many supply and isolating transformers.
• It is possible to generate very high dc voltages from single supply

transformers by extending the voltage doubler circuits.
• This is simple and compact when the load current requirement is less
than one milliampere, such as for cathode ray tubes, etc. Valve-type
pulse generators may be used instead of conventional ac supply
• the circuit becomes compact.

One-phase, two-pulse, Three-phase, six-pulse, voltage multiplier circuit
Electrostatic Machines: Basic Principle
(a) Van de Graaff Generators
(b) Electrostatic Generators
GENERATION OF HIGH ALTERNATING VOLTAGES
Cascade Transformers
Resonant Transformers
Generation of High-Frequency ac High Voltages
GENERATION OF IMPULSE VOLTAGES

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EE8215 High Voltage Engineering

Department of Electrical and Electronics Engineering

 Hence, generation of high voltages in laboratories for testing purposes is essential.

(i) high dc voltages,

(ii) high ac voltages of power frequency,

(iii) high ac voltages of high frequency,

(v) transient voltages of longer duration such as switching surges.

Rectifier circuits for producing high dc voltages from ac sources may be

The rectifier may be an electron tube or a solid-state device.

For higher voltages, several units are to be used in series.

The value of the capacitor C is chosen such that

The rectifier valve must have a peak inverse

To limit the charging current, an additional

the rectifier valves used are of special construction

 the capacitor C2 in turn is charged through R2 to 2Vmax.

 If the load current is large, the ripple also is more.

• Cascaded voltage multiplier circuits for higher voltages are

• require too many supply and isolating transformers.

• It is possible to generate very high dc voltages from single supply

• the circuit becomes compact.

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