FIELD OF INVENTION:
This present invention generally relates to the magnitude of isolation
(insulation level) of large power transformer and specifically to the system
of insulation providing 10KV r.m.s between core and clamping structure,
clamping structure and tank, core and yoke bolt.
BACKGROUND OF THE INVENTION:
The core which provides the magnetic path to channel the flux consists of
thin strips of high grade laminations, which are electrically separated by a
thin coating of insulation material. The core is held together by a clamping
structure through structural steel end frames & yoke bolts. The core &
these clamping components are insulated from each other. Each of these
components including core is earthed through a single point accessible on
the transformer tank. This grounding point is available for testing purpose
& also for checking unintentional grounding of core. Multiple grounding of
core can provide a path for circulating currents induced by the main flux as
well as leakage flux thus resulting in stray losses & localized heating.
Generally this isolation of core, its clamping structure & tank is verified by
conducting insulation resistance/flash test at 2KV r.m.s for one minute.
Many utilities consider that for large transformers, isolation of core, end
frame, tank at 2KV r.m.s is not adequate as with latest process techniques
and material this level is easily achievable under the ideal condition within
the manufacturer's works. It should be appreciated that in a power
transformer core & winding forms a very large mass & therefore relatively
minor shock suffered during transportation may lead to damage or
dislocation of components so that the small clearances necessary to
withstand the test voltage of 2KV r.m.s are lost thereby resulting in failure
or low resistance of isolation value at site. In prior art to achieve 2KV
isolation between core to yoke bolt very minor insulation as shown in the
figure eg as 3 radial insulation on bolt with 12 thick pressboard was
adequate to provide the necessary gap between the two parts to withstand
2KV. 2KV isolation between core and clamping structure was simply
achieved by providing the insulation of very low thickness. The width of
insulation has no role as level of isolation of 2KV is too low. To achieve 2KV
between core and tank a very thin sheet of pressboard of low electrical
strength was enough to achieve the isolation of 2KV.
SUMMARY OF THE INVENTION
The main object of the invention is to develop a power transformer
providing improved insulation between core to end frame (clamping
structure), end frame (clamping structure) to tank, core to tank and on
yoke bolts so that they are capable of withstanding 10 KV r.m.s for one
minute instead of 2KV r.m.s of prior art. Large transformers are prone to
shocks during transportation and handling. The small clearances necessary
to maintain 2KV r.m.s of prior art are
breached on relatively minor shocks during transportation of such large
transformers resulting in deterioration of insulation- value at site which
leads to core isolation problem. The present invention has successfully
addressed this problem. Formerly multiple grounding of core was resorted
to which resulted in stray losses and localized heating due to circulating
current induced by the main as well as leakage flux. In the present
invention as each of the components are earthed through a single point
accessible on the transformer tank, the high level of insulation prevents
possibility of multiple grounding of core thereby making transformers more
reliable.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1. shows Insulation arrangement between Core to Yoke Bolt and
Clamping Structure of prior art and present invention for 2 KV
and 10 KV.
Figure 2. shows Insulation arrangement between Core to Clamping
Structure of prior art and present invention for 2 KV and 10 KV.
Figure 3. shows Insulation arrangement between Clamping Structure to
Tank of prior art and present invention for 2 KV and 10 KV.
DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE
INVENTION:
The invention will now be described in an exemplary embodiment as
depicted in the accompanying drawing. There can however be other
embodiments of the same invention, all of which are deemed covered by
this description.
The yoke bolts (3) are provided to hold laminations of the core along with
the clamping structure. In the new arrangement insulation level of 10 KV is
achieved by use of special epoxy bush and increased insulation on the bolt.
This arrangement resulted in achieving the larger creepage path between
the core and yoke bolt which otherwise by simply increasing the thickness
of insulation was not possible.
To enhance the level of insulation the thickness of insulation between core
and clamp plate is increased combined with suitable width such that
adequate creepage path is achieved to withstand the 10 KV.
The achieve the higher level of isolation of 10 KV the level special fibre
glass sheets having high electric strength are provided between clamping
structure (1) and tank (9).
The core (2) clamping structure (1) and tank (9) of the transformer are
earthed through a single point accessible on the transformer tank (9).
WE CLAIM:
1. A power transformer having isolation level of. 10 KV rms for one
minute between
- core (2) and yoke bolt (3)
- core (2) and clamping structure (1)
- core (2) and tank (9)
- clamping structure (1) and tank (9)
2. A power transformer as claimed in claim 1, wherein an epoxy bush
(4), and increased insulation is provided on the yoke bolt (3).
3. A power transformer . as claimed in claim 1, wherein suitable
thickness and width of insulation material (5) between core (2) and
clamping structure (1) is provided such that adequate creepage path
is achieved.
4. A power transformer as claimed in claim 1, wherein special fibre
glass sheet (8) having high dielectric strength is provided between
clamping structure (1) and tank (9).
5. A power transformer as claimed in claim 1, wherein provision is made
for single point earthing of core (2), clamping structure (1) and tank
(9) so that same is accessible on the transformer tank.

A power transformer having suitable insulation provided between core (2)
to yoke bolt (3) and clamping structure (1), clamping structure (1) and
tank (9) to achieve an insulation level so that it can withstand 10 KV r.m.s
for one minute. Special epoxy bush (4) with reinforcement of insulation on
the yoke bolt (3) is provided to achieve the target insulation between core
(2) and yoke bolt (3) and clamping structure (1). Thickness and width of
insulation between core (2) and clamping structure (1) is increased
substantially to provide adequate creepage path, so that it can withstand
10 KV r.m.s for one minute. Special fibre glass insulation (8) and PBD
insulation (7) of adequate thickness provided between clamping structure
(1) and tank bottom (9) to achieve the desired insulation.