VandeGraff

2. High voltage Applications
 Electrostatic Precipitators
 Particle accelerators
 Insulation testing
 Test power apparatus for extra high voltage
transmission voltages (400 kV and above)

3. Different forms of high voltages
 High dc voltages
 High ac voltages of power frequency
 High ac voltages of high frequency
 High transient or impulse voltage of short duration
(lightning surge)
 Transient voltages of longer duration (Switching
surge)

4. Generation of High alternating voltages
 When test voltage requirement is less than 300kV,
 a single transformer is used for test purpose
 Its impedance should be less than 5%
 Should be capable of giving SC current for one minute or
more depending on the design.
 In addition to the normal low and high voltage
windings, meter winding is provided to measure the
output voltage.
 Difficulty:
 For high voltage requirements, a single unit construction
is costly and difficult; insulation problems also arise.
 Transportation and erection problem

5. Cascade Transformer Connection
Where
V1 – Input Voltage
V2 – Output Voltage
g – Insulation support
aa′ – L.V. Primary winding
bb′ – H.V. Secondary winding
cc′ – Excitation winding
bd – Meter winding (200 to 500 V)

6.  first transformer is at the ground potential along with
its tank.
 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.

7.  The high voltage connection from the first
transformer winding and the excitation winding
terminal are taken through a bushing to the second
transformer.
 In a similar manner, the third transformer is kept on
insulators above the ground at a potential of 2V2 and
is supplied likewise from the second transformer.
 The number of stages in this type of arrangement
are usually two to four
 But very often, three stages are adopted to facilitate
a three-phase operation so that √3V2 can be
obtained between the lines.

8. Cascade Transformer Connection unit with
isolating transformers for excitation
T1,T2,T3 – Cascade
transformer units
Is1,Is2,Is3 – Isolation
transformer units
C1,C2,C3 – Capacitance
voltage dividers for h.v.
measurement after 1st, 2nd
and 3rd stages V1,V2,V3 –
For metering after 1st, 2nd
and 3rd stages

9.  Isolating transformers Is1, Is2 and Is3 are 1 : 1 ratio
transformers insulated to their respective tank
potentials and are meant for supplying the excitation
for the second and the third stages at their tank
potentials. Power supply to the isolating transformers
is also fed from the same a.c. input.
 This scheme is expensive and requires more space.


10. Advantages
 Transportation and assembly is easy.
 Construction is identical for isolating transformers
and the high voltage Cascade Transformer
Connection units.
 Three phase connection in delta or star is possible
for three units.
 Testing transformers of ratings up to 10 MVA in
Cascade Transformer Connection to give high
voltages up to 2.25 MV are available for both indoor
and outdoor applications.

11. Advantage:
 Natural cooling is sufficient
 Transformers light and compact
 Transportation and assembly easy
 Three phase connection in star and delta possible

12. Van de Graff G/r
 The generator is usually enclosed in an earthed
metallic cylindrical vessel and is operated under
pressure or in vacuum. Charge is sprayed onto an
insulating moving belt from corona points at a
potential of 10 to 100 kV above earth and is removed
and collected from the belt connected too–the inside
of an insulated metal electrode through which the
belt moves. The belt is driven by an electric motor at
a speed of 1000 to 2000 metres per minute. The
potential of the high voltage electrode above the
earth at any instant is V= Q/C. where Q is the charge
stored and C is the capacitance of the high voltage
electrode to earth. The potential of the high voltage
electrode rises at a rate

14.  A steady potential will be attained by the high voltage
electrode when the leakage currents and the load
current are equal to the charging current. The shape
of the high voltage electrode is so made with re-
entrant edges as to avoid high surface field
gradients, corona and other local discharges. The
shape of the electrode is nearly spherical.

15.  The charging of the belt is done by the lower spray
points which are sharp needles and connected to a
d.c. source of about 10 to 100 kV, so that the corona
is maintained between the moving belt and the
needles. The charge from the corona points is
collected by the collecting needles from the belt and
is transferred on to the high voltage electrode as the
belt enters into the high voltage electrode.

16.  The belt returns with the charge dropped, and fresh
charge is sprayed onto it as it passes through the
lower corona point. Usually in order to make the
charging more effective and to utilize the return path
of the belt for charging purposes, a self-inducing
arrangement or a second corona point system
excited by a rectifier inside the high voltage terminal
is employed.