FIELD OF APPLICATION
The present invention generally relates to a method for determining the soundness of
bus- bar joints of oil immersed power / rectifier transformers.
BACKGROUND OF THE INVENTION
In a conventional transformer the current flowing through the LV coils is decided by the
amount of power drawn and its voltage class. In a conventional rectifier and large
power transformer heavy current to the tune of kilo amperes flows in the low voltage
side. To cater to the high current flow thick copper plates are used as bus-bars. For
carrying large current a number of bus-bars are employed. These bus-bars are
normally routed through a pre-decided path and joined piece wise intermittently (since
the complete required length of bus-bar in single piece is impractical to obtain and
handle). The bus-bars are joined using bolts. Also the bus-bars are joined to the
bushing terminations through bolts. The bus-bars are very heavy and it is very difficult
to ascertain the full tightness of the bolts.
If the bus-bar joining bolts are not fully tightened, there will be small gaps between the
bus-bars. There will be minor sparking between the bus-bars due to the gaps causing
burning of the bus-bars. These sparking will eventually erode the surfaces and
cause
bigger uneven gaps leading to more sparking. These sparking will lead to further
burning (and pitting) of the bus-bar surfaces, local burning of oil and insulations
(locally) causing generation of gas and deterioration of oil leading to failure of
transformer.
Also, a loosely bolted joint will increase the resistance of the joints causing additional
resistance resulting additional losses causing high local temperature that can result
deterioration of the oil. Under such circumstances, the oil has to be filtered frequently
to restore its quality, which can otherwise lead to the failure of transformer.
There is therefore, a need to ascertain full tightness of the bus-bar bolted joints
indicating their soundness.
OBJECTS OF THE INVENTION
The main object of the invention is to propose a method of ascertaining full tightness of
the bolted joints of bus-bars in-order to prevent the failure of transformer by
eliminating the sparking in the bolted joints.
Another object of the invention is to provide perfect bolted joints in the complete total
length of the bus-bar so as to eliminate any local hot spot that can otherwise
deteriorate the oil and insulation reducing the life of the transformer.
Yet another object of the invention is to reduce the frequency of the filtration of the oil
of the transformer that costs time, money and outage of the transformer from service.
A further object of the invention is to provide perfect bolted joints in the complete total
length of the bus-bar so as to reduce the resistance of the bolted joint, which will
otherwise cause a significant increase in the resistance and resultant power loss and
lower efficiency of the transformer.
One more object of the invention is to provide perfect bolted joints in the complete total
length of the bus-bar so as to explore the possibility of use of the smaller workable
lengths of the bus-bar to achieve a bigger total length thereby increase the total over
economy.
In a preferred embodiment the present invention provides a method for determining
the soundness of bus-bar joints of oil immersed power / rectifier transformers,
comprising the steps of identifying the oil immersed bolted joint locations in core-coil
assembly of the transformer and placing thermally sensitive coloured stickers on the
joints; placing the core-coil assembly in oil filled tank so that all the bolted joints
preferably remain immersed in oil; shorting the bus-bars containing the LV coil
terminations; connecting HV terminations of the coils to a three-phase AC source of
supply with predetermined impedance voltage so that required / rated current flows
through the HV and LV windings of the transformer for 3 to 4 hours and identifying
locations of hot spot generation from emission of oil bubbles and changing of colour of
the coloured stickers, thereby indicating possible loose bolted joints in the bus-bars.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention can now be explained with the reference to the accompanying drawings
where
Figure 1(a) shows in schematic form the arrangement of an oil immersed power /
rectifier transformer.
Figure 1(b) shows detailed view of a bolted joint in the bus-bars.
Figure 2 shows generation of gaps in the bus-bar joints due to loose bolts.
Figure 3 shows the general layout arrangement for carrying out the method of the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
Figure 1 shows in schematic form the arrangement in an oil immersed power / rectifier
transformer having a tank 1 with a tank cover 2. The completely assembled core-coil
assembly comprising core 3 and coils 4 are kept with coils immersed in the oil of the
transformer tank 1. The HV terminations of the coils 4 are connected to a three-phase
supply source through bushings 5.
The LV coil terminations are connected to a bus-bar arrangement provided with bus-bar
bolted joints 6. The details of the bolted joints 6 of the bus-bar arrangement are
shown in Figure 1(b). There can be similar bolted joints (not shown) on the bushings
6.
Generation of gaps between bus-bars in bus-bar joints are shown in Figure 2(a). The
widening of these gaps due to sparking between the bus-bars is shown in Figure 2(b).
The layout for the method of the present invention is shown schematically in Figure 3.
The tank cover 2 is removed for carrying out the method steps and the complete
assembled cold coil assembly is kept oil immersed in the transformer tank 1 so that all
the bolted joints 6 remain immersed in oil upto level 40. The LV coil termination which
involves the bus-bars with bus-bar joints 6 are shorted as shown, with shortening
represented by reference numeral 30.
For energizing the transformer the HV terminations of the coils are to be connected to a
three-phase AC source of supply with a predetermined impedance voltage. This is
usually 10 to 20 % of the rated voltage.
In a normal transformer tank the tank cover is placed on the four walls of the
transformer tank and the cover is not required to be provided for oil filling. In a bell-
shaped transformer, the tank cover is placed on the bottom tank before oil filling and
testing.
In bell-shaped tanks all the inspection covers and the cutout for bushing except HV are
kept in open condition so as to provide access for maximum visibility of the oil level 40.
For ascertaining full tightness of the bolted joints the bolted joints 6 are identified and
thermally sensitive coloured stickers are placed on joints 6 before energizing the
transformer under test.
The transformer is kept energized so that rated / required current in the LV windings
flow for about 3 to 4 hours.
If any joint is loose, then local hot spots are generated and oil bubbles start coming
from those places. Also, the thermally sensitive coloured stickers start changing colour
depending upon the severity of the heat generated locally. Then the core-coil
assemblies are taken out and the joints are inspected. The stickers are inspected for
any change in colour. Wherever the joint remained loose the colour of the sticker
changes.
The bolts are re-tightened and the process is repeated till (a) no bubble comes out from
the oil (b) the colour of the sticker is not changed. This ensures the full tightening of
the bolted joints. The stickers are removed after final tightening and before further
processing.
WE CLAIM
1. A method for determining the soundness of bus-bar joints of oil immersed
power / rectifier transformers, comprising the steps of:
- identifying the oil immersed bolted joint locations in core-coil assembly of the
transformer and placing thermally sensitive coloured stickers on the joints;
- placing the core-coil assembly in oil filled tank so that all the bolted joints
preferably remain immersed in oil;
- shorting the bus-bars containing the LV coil terminations;
- connecting HV terminations of the coils to a three-phase AC source of supply
with predetermined impedance voltage so that required / rated current flows
through the HV and LV windings of the transformer for 3 to 4 hours; and
- identifying locations of hot spot generation from emission of oil bubbles and
changing of colour of the coloured stickers,
thereby indicating possible loose bolted joints in the bus-bars.
2. The method as claimed in claim 1, wherein in case of a bell-shaped transformer,
the tank cover is placed on the bottom tank and all inspection covers and cutouts
for bushing are kept in open condition to access maximum visibility of the oil.
3. The method as claimed in claim 1, wherein said step of identifying locations of
hot spot generation after passing rated current through the HV and LV windings
for 3 to 4 hours can be carried out by taking out core coil assembly for inspecting
the joints and the coloured stickers, and for tightening any loose bolts in the
joints.
4. The method as claimed in the preceding claims, wherein said step of
determination of hot spots and tighten of any detected loose bolts are repeated
till no air bubbles come out from the oil or till the colour of the stickers change.
5. A method for determining the soundness of bus-bars joints of oil immersed
power / rectifier transformers, substantially as herein described and illustrated in
the figures of the accompanying drawings.

A method for determining the soundness of bus-bar joints of oil immersed
power / rectifier transformers, comprising the steps of:
- identifying the oil immersed bolted joint locations in core-coil assembly of the
transformer and placing thermally sensitive coloured stickers on the joints;
- placing the core-coil assembly in oil filled tank so that all the bolted joints
preferably remain immersed in oil;
- shorting the bus-bars containing the LV coil terminations;
- connecting HV terminations of the coils to a three-phase AC source of supply
with predetermined impedance voltage so that required / rated current flows
through the HV and LV windings of the transformer for 3 to 4 hours; and
- identifying locations of hot spot generation from emission of oil bubbles and
changing of colour of the coloured stickers,
thereby indicating possible loose bolted joints in the bus-bars.