GENERATION OF HIGH

VOLTAGE
EEE575
Introduction essentials…
• A fundamental knowledge about generators and circuits which are in use for the
generation of high voltages belongs to the background of work on h.v. technology.
• Generally commercially available h.v. generators are applied in routine testing
laboratories; they are used for testing equipment such as transformers, bushings,
cables, capacitors, switchgear, etc. The tests should confirm the efficiency and
reliability of the products and therefore the h.v. testing equipment is required to
study the insulation behaviour under all conditions which the apparatus is likely
to encounter. The amplitudes and types of the test voltages, which are always
higher than the normal or rated voltages of the apparatus under test, are in
general prescribed by national or international standards or recommendations.
Quite often, routine testing laboratories are also used for the development of
new products.
• 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
High voltage generation
• There are different forms of high voltages;
(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.
 High D.C voltages
• In h.v. technology, direct voltages are mainly used for pure scientific research work and
for testing equipment related to HVDC transmission systems. There is also a main
application in tests on HVAC power cables of long length, as the large capacitance of
those cables would take too large a current if tested with a.c. voltages. Although such
d.c. tests on a.c. cables are more economical and convenient, the validity of this test
suffers from the experimentally obtained stress distribution within the insulating
material, which may considerably be different from the normal working conditions
where the cable is transmitting power at low-frequency alternating voltages. However,
for the testing of polyethylene h.v. cables, in use now for some time, d.c. tests are no
longer used, as such tests may not confirm the quality of the insulation.
• High d.c. voltages are even more extensively used in applied physics (accelerators,
electron microscopy, etc.), electromedical equipment (X-rays), industrial applications
(precipitation and filtering of exhaust gases in thermal power stations and the cement
industry; electrostatic painting and powder coating, etc.), or communications electronics
(TV, broadcasting stations)..
• The d.c. voltages are generally obtained by means of rectifying circuits applied to a.c.
voltages or by electrostatic generation.
High D.C voltages
• 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. Nowadays 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. Commonly used half-wave and full-wave rectifiers are
shown in Fig. 6.1. In the half wave rectifier (Fig. 6.1a ) the capacitor is charged to Vmax, the
maximum ac voltage of the secondary of the high voltage transformer in the conducting half cycle.
In the other half cycle, the capacitor is discharged into the load. The value of the capacitor C is
chosen such that the time constant CRL 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 2Vmax. To limit the charging current, an
additional resistance R is provided in series with the secondary of the transformer (not shown in the
figure)
 High D.C voltages; Full and half wave rectifier

• A full-wave rectifier circuit is shown in Fig. 6.1b . In the positive half-cycle, the rectifier A conducts and charges the
capacitor C, while in the negative half-cycle, the rectifier B conducts and charges the capacitor. 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. On the
other side, where the cathode and filament are located, 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 and full wave rectifiers

• 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 RL whenever the supply voltage waveform varies
from peak value to zero value. These waveforms are shown in
Fig. 6.2. When loaded, a fluctuation in the output dc voltage
δV appears, and is called a ripple. The ripple voltage δV 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 as shown in Fig. 6.2. The ripple δV depends
on (a) the supply voltage frequency f, (b) the time constant
CRL, and (c) the reactance of the supply transformer XL. 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 RL
 Voltage doubler circuits
• Both full-wave and half-wave rectifier circuits produce a dc voltage less than the ac maximum
voltage. When higher dc voltages are needed, a voltage doubler or cascaded rectifier doubler
circuits are used. The schematic diagram of voltage doublers are given in Figs 6.3a and b . In the
voltage doubler circuit shown in Fig. 6.3a , the capacitor C1 is charged through rectifier R1 to a
voltage of +Vmax with polarity as shown in the figure 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. Thus, the capacitor C2 in turn is charged through R2 to
2Vmax. Normally, 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 doublers
• Cascaded voltage doublers are used when
larger output voltages are needed without
changing the input transformer voltage level.
A typical voltage doubler is shown in Fig. 6.3b
output waveforms are shown in Fig. 6.3c and
its input and . The rectifiers R1 and R2 with
transformer T1 and capacitors C1 and C2
produce an output voltage of 2 V in the same
way as described above. 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 2 Vmax since the transformer
T2 is at a potential of 2Vmax above the
ground. The voltage distribution along the
rectifier string R1, R2, R3, and R4 is made
uniform by having capacitors C1, C2, C3, and
C4 of equal values. The arrangement may be
extended to give 6 V, 8 V, and so on, by
repeating further stages with suitable
isolating transformers
Voltage multiplier circuits; Cockcroft-Walton circuit
• Cascaded voltage multiplier circuits for higher voltages
are cumbersome and require too many supply and
isolating transformers. In 1932, Cockroft and Walton
suggested an improvement over the circuit developed by
Greinacher for producing high D.C. voltages. Fig. 2.3.
shows a multistage single phase cascade circuit of the
Cockroft Walton type. No Load Operation: The portion
ABM′MA is exactly indentical to Greinarcher voltage
doubler circuit and the voltage across C becomes 2Vmax
when M attains a voltage 2Vmax. During the next half
cycle when B becomes positive with respect to A,
potential of M falls and, therefore, potential of N also falls
becoming less than potential at M′ hence C2 is charged
through D2. Next half cycle A becomes more positive and
potential of M and N rise thus charging C′2 through D′2.
Finally all the capacitors C′1, C′2, C′3, C1, C2, and C3 are
charged. The voltage across the column of capacitors
consisting of C1, C2, C3, keeps on oscillating as the supply
voltage alternates. This column, therefore, is known as
oscillating column. However, the voltage across the
capacitances C′1, C′2, C′3, remains constant and is known
as smoothening column. The voltages at M′, N′, and O′
are 2 Vmax 4 Vmax and 6 Vmax. Therefore, voltage across Cockcroft-Walton voltage multiplier circuit
all the capacitors is 2 Vmax except for C1 where it is Vmax
only. The total output voltage is 2n Vmax where n is the
number of stages.
 Electrostatic Generator
• In electromagnetic generators, current carrying conductors are
moved against the electromagnetic forces acting upon them. In
contrast to the generator, electrostatic generators convert
mechanical energy into electric energy directly. The electric charges
are moved against the force of electric fields, thereby higher
potential energy is gained at the cost of mechanical energy.
• shows belt driven electrostatic generator developed by Van
deGraaf in 1931. An insu lating belt is run over pulleys. The belt,
the width of which may vary from a few cms to metres is driven at
a speed of about 15 to 30 m/sec, by means of a motor connected
to the lower pulley. The belt near the lower pully is charged
electrostatically by an excitation arrangement. The lower charge
spray unit consists of a number of needles connected to the
controllable d.c. source (10 kV–100 kV) so that the discharge
between the points and the belt is maintained. The charge is
conveyed to the upper end where it is collected from the belt by
discharging points connected to the inside of an insulated metal
electrode through which the belt passes. The entire equipment is
enclosed in an earthed metal tank filled with insulating gases of
good dielectric strength viz. SF6 etc. So that the potential of the
electrode could be raised to relatively higher voltage without
corona discharges or for a certain voltage a smaller size of the
equipment will result. Also, the shape of the h.t., electrode should
be such that the surface gradient of electric field is made uniform
to reduce again corona discharges, even though it is desirable to
avoid corona entirely. An isolated sphere is the most favourable
electrode shape and will maintain a uniform field E with a voltage
of Er where r is the radius of the sphere
HIGH A.C VOLTAGES
• Most of the present day transmission and distribution networks are
operating on a.c. voltages and hence most of the testing equipments
relate to high a.c. voltages. Even though most of the equipments on
the system are 3-phase systems, a single phase transformer operating
at power frequency is the most common from of HVAC testing
equipment. Test transformers normally used for the purpose have low
power rating but high voltage ratings. These transformers are mainly
used for short time tests on high voltage equipments. These
High AC voltages
• Cascaded Transformer
For voltages higher than 400 KV, it is desired to cascade two or more transformers
depending upon the voltage requirements. With this, the weight of the whole unit is
subdivided into single units and, there fore, transport and erection becomes easier. Also,
with this, the transformer cost for a given voltage may be reduced, since cascaded units
need not individually possess the expensive and heavy insulation required in single stage
transformers for high voltages exceeding 345 kV. It is found that the cost of insulation for
such voltages for a single unit becomes proportional to square of operating voltage. Fig. 2.9
shows a basic scheme for cascading three transformers. The primary of the first stage
transformer is connected to a low voltage supply. A voltage is available across the
secondary of this transformer. The tertiary winding (excitation winding) of first stage has
the same number of turns as the primary winding, and feeds the primary of the second
stage transformer. The potential of the tertiary is fixed to the potential V of the secondary
winding as shown in Fig. 2.9. The secondary winding of the second stage transformer is
connected in series with the secondary winding of the first stage transformer, so that a
voltage of 2V is available between the ground and the terminal of secondary of the second
stage transformer. Similarly, the stage-III transformer is connected in series with the second
stage transformer. With this the output voltage between ground and the third stage
transformer, secondary is 3V. it is to be noted that the individual stages except the upper
most must have three-winding transformers. The upper most, however, will be a two
winding transformer.
Cascaded transformer
Reactive Power Compensation
• As is mentioned earlier, the test transformers
are used for testing the insulation of various
electrical equipment. This means the load
connected to these transformers is highly
capacitive. Therefore, if rated voltage is
available at the output terminals of the test
transformer and a test piece (capacitive load)
is connected across its terminals, the voltage
across the load becomes higher than the
rated volt age as the load draws leading
current. Thus, it is necessary to regulate the
input voltage to the test transformer so that
the voltage across the load, which is variable,
depending on the test specimen, remains the
rated voltage. Another possibility is that a
variable inductor should be connected across
the supply as shown in Fig. 2.13 so that the
reactive power supplied by the load is
absorbed by the inductor and thus the
voltage across the test transformer is
maintained within limits.
Resonant Transformer
• The equivalent circuit of a high-voltage testing transformer
consists of the leakage reactance of the windings, the
winding resistances, the magnetizing reactance, and the
shunt capacitance across the output terminal due to the
bushing of the high-voltage terminal and also that of the
test object. This is shown in Fig. 6.12a with its equivalent
circuit in Fig. 6.12b . It may be seen that it is possible to
have series resonance at power frequency ω, if (L1 +L2) =
1/ωC. With this condition, the current in the test object is
very large and is limited only by the resistance of the
circuit. The waveform of the voltage across the test object
will be purely sinusoidal.
• This principle is utilized in testing at very high voltages and
on occasions requiring large current outputs such as cable
testing, dielectric loss measurements, partial discharge
measurements, etc. A transformer with 50 to 100 kV
voltage rating and a relatively large current rating is
connected together with an additional choke, if necessary.
The test condition is set such that (ω(Le+L) = 1/ωC where
Le is the total equivalent leakage inductance of the
transformer including its regulating transformer.
Advantages of the resonant transformer
(a) It gives an output of pure sine wave,
(b) Power requirements are less (5 to l0% of
total kVA required),
(c) No high-power arcing and heavy current
surges occur if the test object fails, as
resonance ceases at the failure of the test
object,
(d) Cascading is also possible for very high
voltages,
(e) Simple and compact test arrangement, and
(f) No repeated flashovers occur in case of
partial failures of the test object and
insulation recovery. It can be shown that
the supply source takes Q number of cycles
at least to charge the test specimen to the
full voltage.
Generation of High-Frequency ac High Voltages
• High-frequency high voltages are required for rectifier dc
power supplies as discussed in Sec. 6.1. Also, for testing
electrical apparatus for switching surges, high frequency high
voltage damped oscillations are needed which need high-
voltage high-frequency transformers. The advantages of these
high-frequency transformers are:
(i) The absence of iron core in transformers and hence saving
in cost and size,
(ii) Pure sine-wave output,
(iii) Slow build-up of voltage over a few cycles and hence no
damage due to switching surges, and
(iv) Uniform distribution of voltage across the winding coils
due to subdivision of coil stack into a number of units.
The commonly used high-frequency resonant transformer
is the Tesla coil, which is a doubly tuned resonant circuit
shown schematically in Fig. 6.13a . The primary voltage
rating is 10 kV and the secondary may be rated to as high
as 500 to 1000 kV. The primary is fed from a dc or ac
supply through the capacitor C1. A spark gap G connected
across the primary is triggered at the desired voltage V1
which induces a high self-excitation in the secondary. The
primary and the secondary windings (L1 and L2) are
wound on an insulated former with no core (air-cored)
and are immersed in oil. The windings are tuned to a
frequency of 10 to 100 kHz by means of the capacitors C1
and C2.
Generation of impulse voltages
• Impulse Voltage
An impulse voltage is a unidirectional voltage
which, without appreciable oscillations, rises
rapidly to a maximum value and falls more or
less rapidly to zero Fig. 3.1. The maximum value
is called the peak value of the impulse and the
impulse voltage is specified by this value. Small
oscillations are tolerated, provided that their
amplitude is less than 5% of the peak value of
the impulse voltage. In case of oscillations in the
wave shape, a mean curve should be
considered. If an impulse voltage develops
without causing flash over or puncture, it is
called a full impulse voltage; if flash over or
puncture occur, thus causing a sudden collapse
of the impulse voltage, it is called a chopped
impulse voltage. A full impulse voltage is
characterized by its peak value and its two time
intervals, the wave front and wave tail time
intervals defined below: The wave front time of
an impulse wave is the time taken by the wave
to reach to its maxi mum value starting from
zero value.
 Generation of impulse voltages
• Impulse Generator circuits
Fig. 3.3 represents an exact equivalent circuit of a single stage impulse generator
along with a typical load. C1 is the capacitance of the generator charged from a
d.c. source to a suitable voltage which causes discharge through the sphere gap.
The capacitance C1 may consist of a single capacitance, in which case the
generator is known as a single stage generator or alternatively if C1 is the total
capacitance of a group of capacitors charged in parallel and then discharged in
series, it is then known as a multistage generator.
L1 is the inductance of the generator and the leads connecting the generator to
the discharge circuit and is usually kept as small as possible. The resistance R1
consists of the inherent series resistance of the capacitances and leads and often
includes additional lumped resistance inserted within the generator for damping
purposes and for output waveform control. L3, R3 are the external elements
which may be connected at the generator terminal for waveform control. R2 and
R4 control the duration of the wave. However, R4 also serves as a potential
divider when a CRO is used for measurement purposes.
C2 and C4 represent the capacitances to earth of the high voltage components
and leads. C4 also includes the capacitance of the test object and of any other
load capacitance required for producing the required wave shape. L4 represents
the inductance of the test object and may also affect the wave shape
appreciably. Usually for practical reasons, one terminal of the impulse generator
is solidly grounded. The polarity of the output voltage can be changed by
changing the polarity of the d.c. charging voltage. For the evaluation of the
various impulse circuit elements, the analysis using the equivalent circuit of Fig.
3.3 is quite rigorous and complex. Two simplified but more practical forms of
impulse generator circuits are shown in Fig. 3.4 (a) and (b).
Trial Questions
• Mention the applications or use of high Direct and alternating voltage in power systems
engineering.
• With the aid of a diagram explain the principle of operation of a cascaded Transformer.
• Outline the applications of a cascaded transformer.
• What are the different forms of High voltages?
• With the aid of diagrams explain the half wave and full wave rectifier circuits for
producing high DC voltages from AC sources.
• Using a well detailed diagram describe the Cockcroft-Walton voltage multiplier circuits
operation when (i) unloaded (ii) loaded. What are the advantages of a resonant
transformer and the high frequency AC transformer
• Define ripple voltage. Show that the ripple voltage in a rectifier circuit depends upon the
load current and the circuit parameters.
• Explain and compare the performance of half wave rectifier and voltage doubler circuits
for generation of high d.c. voltages.
• What are the differences between power transformers, potential transformers and
testing transformers?