Chapter 3 Generation of high voltages and current

Unit No.3:
Generation of High Voltages and Current
• Topic Contents:
a) Generation of high ac voltages:
• Cascading of transformers
• Series and parallel resonance system
• Tesla coil
b) Generation of impulse voltages and current:
• Impulse voltage definition
• Wave front and wave tail time
• Multistage impulse generator
• Modified Marx circuit
• Tripping and control of impulse generators
• Generation of high impulse current.
3/16/2020 generation of high alternating volatges (MM) 1

Generation of high Alternating Voltages
• When test voltage requirements are less than about 300 kV, a single transformer can be used
for test purposes.
• The impedance of the transformer should be generally less than 5% & must be capable of
giving the short circuit current for one minute or more depending on the design.
• Addition to normal windings (Low and High voltage windings), third winding known as
meter winding is provided to measure the output voltage.
• For higher voltage requirements, a single unit construction becomes difficult and
costly due to insulation problems.
• Transportation and arranging of large transformers become difficult.
• These drawbacks are overcome by series connection or cascading of the several identical
units of transformers, where in the high voltage windings of all the units effectively come in
series.

Schematic diagram of Cascade transformer for HV AC
Generation
Schematic diagram

Cascade Transformer
• The first transformer is at the ground potential along with its tank.
• The second transformer is kept on insulators and maintained at a potential of V2, the output voltage
of the first unit above the ground.
• The high voltage winding of the first unit is connected to the tank of the second unit.
• The low voltage winding of this unit is supplied from the excitation winding of the first transformer,
which is in series with the high voltage winding of the first transformer at its high voltage end.
• The rating of the excitation winding is almost identical to that of the primary or the low voltage
winding. The high voltage connection from first transformer winding and excitation winding
terminal are taken through bushing to second transformer.
• In a similar manner, third transformer is kept on insulator above ground at potential of 2V2 and is
supplied likewise from second transformer.

Cascade Transformer -connection
• Supply to the units can be obtained from a
motor-generator set or through an induction
regulator for variation of the output voltage.
• Second scheme for providing excitation to
second and third stages is shown.Isolating
transformers Is1,Is2,Is3 are 1:1 ratio
transformers insulated to their respective tank
potentials and are meant for supplying
excitation for 2nd and 3rd stages at their tank
potentials.
• Power supply to isolating transformer is also
fed from same ac input. This scheme is
expensive and requires more space.3/16/2020 generation of high alternating volatges (MM) 5

Advantages of cascade connection
• Natural cooling is sufficient
• Transformers are light and compact
• Ease of transportation & assembly
• Construction is similar to the isolating transformer & cascaded unit
• Either star or delta connection are possible
Draw backs
• More space requirement and expensive
Cascade Transformer

The resonance principle of a series tuned L-C circuit can be made use of to
obtain a higher voltage with a given transformer.
Resonant Circuit

Basic principle of Resonant circuit
Resonant Circuit

• This process occurs in a resonant transformer, an electrical component which consists of two high Q
coils wound on the same core with capacitors connected across the windings to make two coupled
LC circuits.
• Resonant transformer is one of the best choice for high voltage generation which operates on resonance
phenomenon (XL = Xc).
• In resonance condition, the current through test object is very large and that is limited only by the
resistance of the circuit.
• The waveform of the voltage across the test object will be purely sinusoidal
Applications of Resonant Transformer:
• 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.
Resonant Transformers

Series Resonant Transformers

Series Resonant transformer
• Resonant transformer work on the principle, that load capacitance is variable and for certain
loading, when capacitance is equal to inductance of circuit, resonance may occur.
• The equivalent circuit of HV testing circuit consists of
a) leakage reactance of the winding,
b) winding resistance,
c) magnetizing reactance,
d) shunt capacitance across the output
• It is possible to have a series resonance at power frequency w, if
• During the resonance condition current in the test object is very large and is limited only by
the resistance of the circuit.
• The magnitude of the voltage across the capacitance C of the test object will be

• The factor (Xc/R= 1/wCR)is the Q factor of the circuit and gives the magnitude of the voltage
multiplication across the test object under resonance conditions.
• The input voltage required for excitation is reduced by a factor 1/Q, and the output kVA
required is also reduced by a factor 1/Q.
• The secondary power factor of the circuit is unity.

• A voltage regulator of either the auto-transformer type or the induction regulator type is
connected to the supply mains.
• The secondary winding of the exciter transformer is connected across the H.V reactor, L, and the
capacitive load C.
• The inductance of the reactor L is varied by varying its air gap and operating range is set in the
ratio 10 : 1.
• Capacitance C comprises of the capacitance of the test object, capacitance of the measuring
voltage divider, capacitance of the high voltage bushing etc.
• The Q-factor obtained in these circuits will be typically of the order of 50.
• Q factor gives magnitude of voltage multiplication across test object under resonance condition.
Hence Q factor 50 means voltage at test object is 50 times input voltage under resonance
condition.

Advantages of series resonant circuit
• It gives an output of pure sine wave.
• Power requirements are less (5 to 10% of total kVA required).
• No high-power arcing and heavy current surges occur if the test object fails, as resonance
ceases at the failure of the test object.
• Cascading is also possible for very high voltages.
• simple and compact test arrangement.
• No repeated flashovers occur in case of partial failures of the test object and insulation
recovery.
Disadvantages of series resonant circuit
• Requirements of additional variable chokes capable of withstanding the full test voltage and the
full current rating.

Parallel Resonant Transformer
• In the parallel resonant mode the high
voltage reactor is connected as an auto-
transformer and the circuit is connected as
a parallel resonant circuit.
• The advantage of the parallel resonant
circuit is that more stable output voltage
can be obtained along with a high rate of
rise of test voltage.
• Independent of the degree of tuning and the
Q-factor.
• Single unit resonant test systems are built
for output voltages up to 500 kV, while
cascaded units for outputs up to 3000 kV,
50/60 Hz are available.

Tesla coil
• Tesla coil is an electrical resonant transformer circuit designed by inventor Nikola Tesla in 1891.
• It is Used to generate or produce high voltage, low current & high frequency AC electricity.
• High frequency transformer is required.
• The commonly used high frequency resonant transformer is the Tesla coil.
• Tesla coil is a doubly tuned resonant circuit or high frequency resonant transformer.
• The primary voltage rating is 10 kV and the secondary may be rated to as high as 500 to 1000 kV.
• Output frequency range: 50kHz to 1 MHz.
• Damped oscillations can be obtained by using Tesla Coil.

Tesla coil
Applications:
X-ray generation, experiment in electrical Lighting etc

Construction of Tesla coil
• The primary is fed from an AC supply through the condenser C1.
• A spark gap G connected across the primary is triggered at the desired voltage V, which induces high
self excitation in the secondary.
• Spark gap G act as a switch of the circuit.
• The primary and the secondary windings (L1 and L2) are wound on an insulated transformer 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 condensers C1 and C2.

Circuit for Tesla Coil arrangement

Working of tesla coil
• Provide suitable supply with the help of autotransformer,.
• Spark gap going to operate act as switch, capacitor C1 gets charged (store energy).
• L1C1 act as resonant circuit that supports to transfer maximum energy from primary side to secondary side.
• Flux generated on primary side.
• Now, these flux transferred to secondary side.(almost more than 90%)
• Now by faradays law of electromagnetic induction, we get high output voltage.
• Duty of C2:
a) Filtration ( some amount of ripple content filtered out by C2)
b) Act as parallel resonant (L2C2 combination)
• Damped waveform( V2 ) is obtained which has high magnitude peak, high frequency, high voltage
generated.
Application
• Electric Welding Machine
• Film Industries
• Spark gap ignition (Diesel Engine)
• CRT (cathode ray tube)

• The output voltage V is a function of the parameters LI, L2, C1, C2 and the mutual
inductance M.
• Usually, the winding resistances will be small and contribute only for damping of the
oscillations.
Output Voltage
Tesla coil- Circuit Analysis

Output Voltage
Where
K = coefficient of coupling
between the windings L1 and L2

• The peak amplitude of the secondary voltage V2 can be expressed as
Where ,

• A more simplified analysis for the Tesla coil may be presented by considering that the
energy stored in the primary circuit in the capacitance C1 is transferred to C2 via the
magnetic coupling.
• If W1 is the energy stored in C1 and W2 is the energy transferred to C2 and if the efficiency
of the transformer is η, then

Advantages of Tesla coil
• The absence of iron core in transformers and hence saving in cost and size.
• Iron losses can be minimized.
• Pure sine wave output ( Less wave form distortion).
• Slow build-up of voltage over a few cycles and hence no damage due to switching surges.
• Uniform distribution of voltage across the winding coils due to subdivision of coil stack into a
number of units.
Disadvantages of Tesla coil
• Complex circuit
• High cost (high voltage capacitors)
• Tunning of LC ciruit

Impulse voltage
• Transient over voltages due to lightning and switching surges cause steep build up of
voltage on transmission lines and other electrical apparatus.
• Impulse voltage is a large voltage generated within a very small time period.
• 10% to 90% time period is called as front time or rise time. The wave front time of an
impulse wave is time taken by wave to reach its maximum value starting from zero value.
• From 𝑡2 𝑡𝑜 𝑡3 is called as fall or tail time.
• V= 𝑉0 [𝑒−∝𝑡- 𝑒−𝛽𝑡] --- Impulse wave representation
• Impulse voltage defined as 1.2/50 𝜇𝑠𝑒𝑐 and 1000 kV.

Multistage Impulse Generators Marx circuit
• A single capacitor may be used for voltages up to
200kV. Beyond this voltage, a single, capacitor
circuit is inconvenient for following reasons:
1. Charging unit costly
2. physical size is large
3. high dc charging voltage is required
4. switching of very high voltages with spark gaps
is difficult.
A bank of capacitors are charged in parallel
and then discharged in series. This concept was
originally proposed by Marx. It is known as
multistage impulse generators or Marx circuit.

Figure: Modified Marx Circuit

• A schematic diagram of Marx circuit and its modification are shown in figure.
• Generally gap spacing is kept such that breakdown voltage of gap is more than charging voltage.
• Also value of charging current is around 50mA to 100mA and resistor is selected accordingly.
• Time constant of circuit CRs should be about 10 sec to 1 min which becomes deciding factor for selecting
generator capacitance.
• Depending on time constant of circuit all capacitance are charged to V volts in about a minute and during
discharge all capacitors come in series and discharge in test object.
• The discharge time constant CR1/n (for n stages) will be very small (micro seconds), compared to
charging time constant CRs which will be few seconds.
• Hence no discharge takes place through charging resistors Rs. In figure (a) impulse wave shaping circuit
is connected externally to capacitor unit.
• Whereas in Modified Marx circuit, the resistance R1 & R2 are incorporated inside unit.
• R1 is divided into n equal parts to R1/n and put in series with gap G.
• R2 is also divided into n parts and arranged across each capacitor unit after gap G.

• Two methods are available
(i) Three electrode gap arrangement
(ii) Trigatron gap
Tripping and control of the impulse Generator

Tripping and control of impulse generators
Three Electrode gap method
• ‘Three electrode gap arrangement ‘ is one of the
method for triggering and synchronization of
impulse generator.
• The spacing between 2 spheres is adjusted so that
two series gap are able to withstand charging
voltage of impulse generator.
• Central sphere is called control sphere.
• A high resistance is connected between the
outer sphere and its centre point is
connected to control sphere.
• The voltage between outer sphere is equally
divided between two sphere gap.

• The first stage of the impulse generator is fitted with a three electrode gap, and the central
electrode is maintained at a potential in between that of the top and the bottom electrodes
with the resistors R1 and R1.
• The tripping is initiated by applying a pulse to the thyratron G by closing the switch S.
• C produces an exponentially decaying pulse of positive polarity. The pulse goes and
initiates oscilloscope time base.
• The Thyratron conducts on receiving the pulse from the switch S and produces a negative
pulse through the capacitance C1 at central electrode of three electrode gap.
• Voltage between central electrode and the top electrode of the three electrode gap goes
above its sparking potential and gap contacts.
• Time lag required for thyratron firing and breakdown of three electrode gap ensures that
seep circuit of oscilloscopes begin before start of impulse generator voltage.
• The resistance R2 ensures decoupling of voltage oscillations produced at spark gap entering
oscilloscope through common trip circuit.

Trigatron Gap method
• A device, known as "Trigatron", is used to
control the flash over at the spark gaps in order
to get a desired magnitude of the output
voltage repeatedly.
• Function- used as ‘First gap of impulse
generator’
• "Trigatron", consists essentially of three-
electrodes.
• three electrodes are
1. High voltage electrode is a sphere- indication
of HV
2. Earthed electrode is also a sphere. The
spherical configuration gives homogeneous
field
• 3. Metal rod electrode/ Trigger electrode
be the third electrode

• A small hole is drilled into earth electrode
into which metal rod projects (trigger rod).
• The annular gap between the rod and the
surrounding hemisphere is 1 mm.
• A glass tube is fitted over rod electrode.
• The potential of metal electrode and earth
electrodes are same.
• Both are connected through a high resistance.
• Tripping pulse or control pulse applied
between metal and earth electrodes.
• When the tripping pulse is applied, main field
is distorted.
• Reason for dielectric breakdown.
• Single stage or multi stage impulse generator
maintaining a definite “trigatron” distance.
Construction of “Trigatron spark gap”

Tripping circuit of Trigatron
• The capacitor C1 is charged through high resistance R1
• Switch S is closed
• A pulse is applied to a sweep circuit of the oscillograph through the
capacitor C3
• Same time capacitor C2 is charged
• Triggering pulse is applied through trigger electrode (metal rod
electrode)
• The requisite delay in triggering the generator can be provide by R2 and
C2
• The residual charge on the C2 can discharged through R2
• Now a days laser is used for tripping the spark gap
• The trigatron also has a phase shifting circuit associate with the
synchronization of initiation time with external Alternating voltage.
• Design is to prevent the overcharging of capacitor
• An indicating device shows whether the generator is going
to fire properly or not

Impulse Current Generator
• Lightning discharges involve both high voltage
impulses and high current impulses on
transmission lines. Protective gears like surge
diverters have to discharge lightning currents
without damage.
• So generation of high impulse current
waveforms of high magnitude(=100 kA peak)
find application in test work as well as basic
research on non-linear resistors, electric arc
studies and study related to electric plasmas in
high current discharges.
• The wave shapes used in testing surge diverters
are 4/10 and 8/20 𝜇s, the figures representing
nominal wave front and wave tail times is as
shown below.
• The tolerance allowed on these are 10% only

• The impulse of large magnitude are generated
are generated using bank of capacitors. These
banks are connected in parallel, using suitable
voltage source, the banks are charged to
specified voltage level.
• These are discharged through series RL circuit
as shown in figure (a).
• A bank of capacitors C connected in parallel is
charged from DC source to a voltage upto 200
kV. R represents dynamic resistance of test
object and resistance of circuit and shunt. L is air
cored high current inductor.
• If the capacitor is charged to a voltage V and
discharged when spark gap is triggered, the
current 𝑖 𝑚 will be given by the equation,
V= R𝑖 𝑚 + L
𝑑𝑖 𝑚
𝑑𝑡
+
1
𝐶 0
𝑡
𝑖 𝑚 𝑑𝑡

• The circuit is usually underdamped, so that
𝑅
2
<
𝐿
𝐶
hence 𝑖 𝑚 is given by,
𝑖 𝑚 = V/wL[ 𝑒−𝛼𝑡
]sinwt where 𝛼 =
𝑅
2𝐿
, 𝑤 = (
1
𝐿𝐶
−
𝑅2
4𝐿2)
The time taken for current 𝑖 𝑚 to rise from zero to first peak
value is
𝑡1 = 𝑡𝑓=
1
𝑤
𝑠𝑖𝑛− 𝑤
𝐿𝐶
=
1
𝑤
𝑡𝑎𝑛− 𝑤
𝛼
The duration of one half cycle of damped oscillatory wave
𝑡2 is given as
𝑡2=
𝜋
1
𝐿𝐶
−
𝑅2
4𝐿2
the large value impulse current are generated, using
arrangement as shown in figure.
In this circuit number of capacitors charged in parallel and
discharged in parallel into the circuit.

Parts of Impulse Current generator
a) DC charging unit
e) A triggering unit and spark gap
b) Capacitors
c) Air cored inductor
d) Measurement circuits
Gives variable
voltage to
capacitor bank
Range 05-5 𝜇𝐹
(low inductance)
Designed for
high current
value
Oscilloscope and
shunt resistor
Initiation of current
generator

Chapter 3 Generation of high voltages and current

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