This Invention relates to an improved valve winding of a converter transformer.
BACKGROUND Of THE INVENTION.
In most of the filed converter transformers, it has been found that a shining layer
is formed between copper conductor and first layer of paper insulation in the
valve winding. This compound is conducting in nature and doubts have been
expressed that it may deteriorate the insulation strength of the covered paper
insulation. Such conducting material is likely to lead to partial discharges
between strands of copper-paper Insulation system and eventually an internal
fault.
The formation of conducting layer Is due to chemical reaction between
sulphides/mercaptans present in transformer oil & copper, which produces
copper sulphide & thereby deteriorating the insulation properties of paper
insulation. Such deterioration eventually leads to the Insulation failure.
OBJECTS OF THE INVENTION:
An object of this invention is to propose en improved veto winding of a convenor
transformer.
Another object of this invention is to propose an improved valve winding of a
converter transformer having a protective layer of enamel Insulation.
Further object of this invention is to propose an Improved valve winding of a
convenor transformer having no chemical reaction between the
sulphides/mercaptans present in transformer oil and copper.
Still further object of this Invention is to propose en Improved valve winding of a
converior transformer having proper insulation properties.

SUMMARY OF THE INVENTION
According to this invention, there is provided an improved valve winding of a
converter transformer comprising a protective coating disposed between the
copper conductor and a first layer of paper insulation of the valve winding to
protect the valve winding from insulation failure due to formation of a conductive
layer, the conductive layer being formed by the chemical reaction between the
copper conductor and the transformer oil.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWING
Fig-1 New concept applied on normal PICC
Fig-2 New concept applied on glued bunch PICC
Fig-3 EDAX Analysis of copper conductor from Rihand
Fig-4 EDAX Analysis of Chandrapur copper conductor
DETAILED DESCRIPTION OF THE INVENTION
In the new design, direct contact between winding copper (especially valve
windings) and transformer oil is avoided thereby eliminating the chance of
formation of conducting layer due to chemical reaction between conductor and
transformer oil. In this design, a protective layer of enamel insulation is used
over all the valve winding copper to prevent the direct chemical reaction
between copper & sulphides/mercaptans present in transformer oil.
Usually for valve windings, Paper insulated copper conductor (PICC) (1) or Glued
Bunched PICC (4) are used. Conventionally both these type of conductors
without enamel (3) were being used since last many years for all applications as
per Fig la & 2a. Paper (2) used to be the insulating material for these type of
conductors. However the new invention makes the use of protective layer of
enamel (3) as a mandatory requirement over bare copper as per Figure 1b & 2b
to prevent the direct chemical reaction between copper (1) &
sulphides/mercaptans present in transformer oil.
The thickness of protective layer of enamel used in valve winding depends on
the type of winding. The protective layer of enamel used in valve winding
comprises of Polyvinyl acetal enamel class 120 of IEC 317-18-1990.

Published information regarding detailed analysis carried out on refined
transformer oil samples indicates that there are different types of sulphur
compounds present. Some of them are corrosive or reactive. Further analysis
reveals that five basic groups of sulphur and sulphur compounds are found in
crude oil as indicated below:

R- Paraffin with straight or branched chain hydrocarbon or cyclic hydrocarbon.
As shown in table above, the elemental sulphur and sulphur containing
mercaptans are very reactive, sulphides are also reactive. Reactive sulphur is
mainly in form of organic sulphur compounds like R-SH, where the sulphur is
attached at the end of an organic molecule. When the molecule is more complex,
for instance when the sulphur is surrounded or contained within the molecule
then the sulphur compounds are more stable and less reactive like in R-S.S-R.
Thiophenes are the most stable of all the sulphur compounds.
It has been found that deterioration in insulation properties is due to reaction of
mercaptan sulphur and elemental sulphur with exposed copper surface of
conductor leading to formation of conducting layer of copper sulphides and other
compounds.
Probable Phenomena of Failure
The reactive sulphur, once in contact with copper surface, starts reacting with it,
forming copper sulphide even at operating temperature. The rate of reaction
increases with temperature. The reaction is likely to be faster in hot spot region
of the winding and ducts. It is likely that presence of voltage could accelerate the
formation of copper sulphide. However no experimental evidence has been
published yet.

The copper sulphide thus formed may create craters/protrusions in localized area
of conductor giving rise to onset of partial discharge or might permeate through
the paper, reducing the breakdown strength of the insulation. Such occurrence
could be the reason of several failures observed in the converter transformers.
The reactive sulphur, once in contact with copper surface, starts reacting with it,
forming copper sulphide even at operating temperature. The rate of reaction
increases with temperature. The reaction is likely to be faster in hot spot region
of the winding and ducts. It is likely that presence of voltage could accelerate the
formation of copper sulphide.
The copper sulphide thus formed may create craters/protrusions in localized area
of conductor giving rise to onset of partial discharge or might permeate through
the paper, reducing the breakdown strength of the insulation. Such occurrence
could be the reason of several failures observed in the converter transformers.
Analysis and Field of Invention
There are two major projects related to HVDC in India, one is Rihand Dadri & the
other is Chandrapur-Padghe. One sample (copper conductor & paper sample)
was drawn from Rihand Transformer, which had failed & the other (copper
conductor & paper sample) was drawn from the Chandrapur Transformer, which
was withdrawn form operation due to high Dissolved gases. These samples were
subjected to detailed tests & analysis.
The results are shown in Fig.3 & 4 respectively. The sample of Rihand was found
to contain precipitates of copper sulphides, (Fig.3). Whereas in the sample of C-P
project, oxide deposits were found along with traces of copper sulphide (Fig.4).
Thus the copper conductor of valve windings when coated with thin layer of
enamel insulation totally eliminates the contact of Mercaptan sulphur and
elemental sulphur with base copper of valve windings which in turn also
eliminate the failure of converter transformers. This invention relates to
elimination of contact of oil & base copper surface by use of enamel coating over
copper surface. This excludes the formation of conducting layer of copper
sulphides and other compounds thereby preventing the failure of valve winding.

WE CLAIM
1. A novel process for design of convertor transformer valve winding
comprising a protective coating disposed between the copper conductor
(1) and a first layer of paper insulation (2) and a second layer of Enamel
insulation (3) of the valve winding to protect the valve winding from
insulation failure due to formation of a conductive layer, the conductive
layer being formed by the chemical reaction between the copper
conductor (1) and the transformer oil.
2. An improved valve winding of a converter transformer as claimed in claim
1, wherein said protective layer is a thin coating of enamel (3).
3. An improved valve winding of a converter transformer as claimed in claim
1, wherein the said coating of enamel comprises of Polyvinyl acetal
enamel Class 120 of IEC 317-18-1990.

Novel process for design of converter transformer valve winding comprising a
protective coating disposed between the copper conductor (1) and a first layer of
paper insulation (2) and a second layer of Enamel insulation (3) of the valve
winding to protect the valve winding from insulation failure due to formation of a
conductive layer, the conductive layer being formed by the chemical reaction
between the copper conductor (1) and the transformer oil.