4. Introduction
Most of the present day transmission and distribution networks are operating on a.c.
voltages and hence most of the testing equipment relate to high a.c. voltages. Even
though most of the equipment 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 equipment. The currents required for these tests on various equipment are
given below:
Insulators, C.B., bushings, Instrumentn transformers = 0.1– 0.5 A
Power transformers, h.v. capacitors. = 0.5–1 A
Cables = 1 A and above
5. Testing transformers
High voltage power frequency test transformers are required to produce single phase
very high voltage.
Their continuous current ratings are very low, usually = 1A . Even 1A is a very high
current rating. This is because a HV test transformer has to supply only the capacitive
charging current to the capacitance formed by the dielectric of the test object.
Howere the voltage rating requirements are high, the test transformers are required to
be produced with very high insulation level.
This increases the size of the test sets tremendously. Hence, single units of test
transformers are produced maximum upto 700k.
6. ultra-high-voltage test:
AC products : 1000kV transformer and shunt reactor with large power.
Other special tests and researching tests In order to fulfill tests mentioned above, the
testing shop equipped with main facilities as following:
Impulse voltage generator : 4800kV/720kJ
AC testing transformer : 630MVA/1700kV
Capacitive divider : 500pF/1500kV
Transformer power loss measurement system : 200kV/4000A TMS580
The ultra-high-voltage test laboratory can carry out not only present products test but
also larger rated power and higher voltage level products test in the future.
7. Future TBEA India Factory Testing Hall
SOME PICTURE OF TESTING TRANSFORMERS
12. 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.
3v
2v
v
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1
2
3
Primary transformer
secondary transformer
Tertiary transformer
Cascade transformers
14. 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.
shows metal tank
construction of
transformers and the
secondary winding is not
divided. Here the low
voltage terminal of the
secondary winding is
connected to the tank.
Cascade transformers
16. Voltage Control in HV Transformers:
The same voltage is not required for all the tests. For some tests like, breakdown voltage
test, variable voltage supply should be available.
To vary the output voltage of HV transformers or of a cascaded set of transformers, the
input voltage has to be changed.
The common method employed for achieving variable voltage is to connect an induction
regulator or a variac on low voltage side.
17. Cascade transformers
Disadvantages :
1) Expensive and requires more space.
Advantages:
1) Natural cooling is sufficient.
2) The transformers are light and compact.
3) Transportation and assembly is easy.
4) The construction is identical for isolating transformer and the hv cascade unit .
18. Application
Cascaded transformer 2 *800 kv (Total rating 1600 kv, 9600 kva) upto.
Dielectric test under dry and wet condition and pollution test.
Energizing the experimental transmission line.
Oscillatory surge (switching ) upto 2000 kv .
(High voltage cascade transformer installation close to Russian city of istra.)
