Insulation of metallic surfaces in power transformers

A method for improving the electrical insulation of an electrode in a power transformer included in a converter plant of a high voltage direct current (HVDC) transmission system. The method involves winding around the electrode, layers of tape of woven or non-woven fibrous structure of non-conducting cellulose material, inorganic plastics material, or inorganic insulating material. This means that ions approaching the electrode, which are migrating under the influence of a high d.c. field outside the electrodes, do not sense the porous tape insulation as any noticeable obstacle, while at the same time the insulating layer is sufficiently dense to increase the breakdown value in the case of electrical surges.

TECHNICAL FIELD 
This invention relates to a method for the electrical insulation of a 
metallic surface immersed in an electrically insulating liquid medium in a 
transformer which is subjected to a high voltage direct current, HVDC. The 
metallic surface may be an electrode, or other electrically energized 
metallic body of the transformer, but also metallic surfaces and bodies at 
ground potential. Thus the invention embraces the protection of, inter 
alia, busbars, conductors from widings, bushing or lead conductors leading 
to the terminals of a transformer, electrostatic shields, and so on and 
for convenience these will be referred to herein as "electrodes". 
In a transformer to which this invention relates, the transformer core 
windings and internal connections are immersed in a transformer tank which 
is filled with a liquid insulating medium, normally a so-called 
transformer oil. Via openings in the transformer tank, the winding and 
lead conductors connect the transformer windings to the terminals of the 
transformer. These conductors are normally each surrounded by a bushing 
turret which supports the conductors and the terminals. The bushing 
turrets communicate with and are also filled with the same liquid 
insulating medium as the transformer tank. An electrostatic shield is 
normally provided in the bushing turret at the transition between the 
winding conductor and the lead conductor, to avoid excessive electrical 
field gradients developing at the transition. 
In addition to being insulated by the liquid medium, the electrodes are 
provided with additional insulation in the form of a non-conducting layer 
of cellulose material (e.g. paper or pressboard), organic plastics 
material (e.g. a film or varnish layer), or an inorganic insulating 
material (e.g. an enamel layer). 
Technical Problem to be Solved 
Before describing the state of the art with regard to this additional 
insulation, a short account of the special conditions which apply to the 
insulation methods employed in power transformers in converter plants, and 
the problems which arise in this connection, will first be given. 
In an HVDC plant there is often used at least one converter bridge for each 
pole and station. A plurality of bridges are commonly series-connected, 
one of the poles of a first bridge being connected to ground, and the 
other pole being connected to the next bridge so as to achieve the series 
connection. In this way, the d.c. voltage potential of each bridge, 
relative to ground potential, increases with the number of bridges that 
are series-connected. 
Each bridge in the series connection is supplied with a.c. voltage from an 
individual transformer. With increasing d.c. voltage potential on the 
bridges relative to ground potential, the insulation of the windings of 
the transformers which supply the bridges will be subjected to an 
increasingly higher d.c. voltage potential with a superimposed a.c. 
voltage. The insulation of these transformer windings must therefore be 
dimensioned so that it is capable of withstanding the increasingly higher 
insulation stresses to which it is subjected. 
The increasing d.c. potential leads to special problems which do not exist 
in ordinary transformers. This is due to the fact that the insulating 
media that are used, the liquid medium, the cellulose material, 
etc.--although being excellent insulators--do transmit electric current to 
a certain, minor extent. The charges that transport the current in the 
liquid insulating medium are considered to be ions from impurities present 
in the medium. These impurities are disassociated, that is, decomposed and 
form ions with positive and negative charges, respectively. In the case of 
a continuously applied d.c. voltage, the positively charged ions migrate 
towards a negative pole, and the negatively charged ions migrate towards a 
positive pole. Thus, the different kinds of ions migrate in opposite 
directions in the electrical field. Now, if one kind of ion is not able to 
penetrate an electrode coating or barrier in its path, the ions of this 
ion type accumulate immediately outside this barrier, which results in an 
increase in the electrical field across the barrier. Concurrently with the 
increased electrical field, the ion current through the barrier also 
increases until an equilibrium has been reached when the ion current 
flowing towards the barrier is equal to the ion current flowing through 
the barrier. When this occurs, the coating/barrier is polarized to the 
greatest possible extent, that is, it has the greatest voltage difference 
in relation to the electrode metal that it can have under the prevailing 
circumstances. In that event, a considerable part of the total d.c. 
voltage, to which the transformer is subjected, may appear across the 
coating/barrier. Now, if this coating/barrier does not have sufficient 
insulating properties to withstand this highest voltage difference, an 
electrical breakdown will occur even during the build-up of the voltage 
difference. If such a breakdown does occur, the entire insulating device 
is generally destroyed. 
DISCUSSION OF PRIOR ART 
The simplest way of preventing the build-up of the above-mentioned barrier 
potential would be not to have any barrier at all, that is, to use 
unshielded, uninsulated electrodes. This would function quite 
satisfactorily if the electrodes were subjected only to d.c. voltage. 
Since the region nearest the electrodes also has to withstand an a.c. 
voltage and, in an HVDC converter plant, stresses which are associated 
with surge voltages arising in the a.c. network, having unshielded 
electrodes is in fact not a practical solution, since experience indicates 
that the voltage at which breakdown would occur would then be greatly 
reduced. 
According to the prior art, therefore, the electrodes in question are 
provided with such thick insulating coatings that the coating/barrier is 
able to withstand the maximum voltages that may occur without the risk of 
insulation breakdown. To cope with this, coatings of cellulose material of 
a thickness of several centimeters are often needed. Examples of the prior 
art in this respect are to be found, inter alia, in the book Power 
transmission by direct current by E. Uhlmann, Springer Verlag 1975, (see, 
for example, FIG. 18.4). 
One disadvantage of the above-mentioned insulating layers is that they 
efficiently prevent the removal of heat from heat-generating electrodes, 
such as, for example, busbars. Insulating layers of a varnish type may, in 
the event of careless handling, be subjected to scratches which are very 
undesirable from the insulation point of view, since insulation breakdowns 
are often concentrated in such regions. 
Studies of insulation breakdowns caused by a.c. voltage stress have been 
carried out using high-speed photography and are described, for example, 
by U. Gafvert in "Particle and oil motion close to electrode surfaces" in 
Proc. CEIDP Amtrust Mass., USA, October 1982. The studies have shown that 
immediately prior to a breakdown, the emission of ions from discrete 
locations in the liquid medium is particularly great. The ion emission 
manifests itself in the form of a visible turbulence in the medium 
adjacent to discrete locations of the electrode, and this turbulence can 
be demonstrated photographically. 
SUMMARY OF THE INVENTION 
The present invention aims to overcome the abovementioned problems and the 
partially contradictory demands for insulation. It comprises using an 
electrode insulation of such porosity that ions approaching the 
coating/barrier of the electrode do not sense the presence of the 
insulation as a significant obstacle, while at the same time the 
coating/barrier is sufficiently dense to prevent the initiation of a 
breakdown when an a.c. voltage stress occurs. Tests have shown that a 
coating/barrier of the required properties can be realized by using a few 
layers of a wrapping material (e.g. a fabric or (non-woven) felt), each 
layer having pores of an open area in the range 0.2 to 10 mm.sup.2 and 
with an aggregate pore area which is from 20% to 80% of the total area of 
the wrapping material. Woven or non-woven materials made of cotton, glass 
fibers, wood cellulose fibers (e.g. paper) or plastics fibers are 
particularly suitable. 
Thus, by employing the method according to the invention, it is possible to 
obtain (a) passage of ions through the insulating layer, whereby no 
significant d.c. voltage difference can develop across the layer, (b) 
sufficient insulation strength against the expected a.c. voltage and (c) 
better heat-removing properties than in the case of the thick lining of 
cellulose material previously used. Also, the porous coatings employed in 
the method of the invention are not as sensitive to careless treatment 
which, for example in the case of prior art varnish insulations, may cause 
scratches and the like.

DESCRIPTION OF PREFERRED EMBODIMENT 
An embodiment in which a method according to the invention is used in the 
insulation of the above-mentioned electrostatic shield will now be 
described in greater detail. 
In FIG. 1, 1 designates a three-phase transformer comprising an oil-filled 
transformer tank 2 with a transformer core (not shown) arranged therein 
with a primary winding and secondary windings. From the transformer tank 2 
there extend a plurality of bushing caps 3, each of which supports a 
bushing 4 as shown in FIG. 2. Each cap 3 is completely oil-filled and 
communicates with the transformer tank 2 via an opening 2a in the 
transformer tank 2. 
As shown in FIG. 2, a winding conductor 5 passes into the bushing cap 3, 
the upper end of the conductor 5 being electrically connected to the lower 
end of the bushing 4. The upper end of the bushing 4 is connected to a 
vertically extending lead conductor 7. 
An electrostatic shield in the form of a metallic, annular shielding body 
10 surrounds the point of connection of the conductor 5 to the lower end 
portion of the bushing 4. The shielding body 10 is electrically and 
mechanically connected to the conductor 5 by means of a connection means 
shown at 11 in FIG. 2. The shielding body 10 is shaped as a body of 
revolution, the axis of rotation of which substantially coincides with the 
axis 6 of the bushing 4. As shown in FIGS. 3 and 4, the shielding body 10 
is formed as a hollow ring, although alternatively it may be solid. At 
least a major part of the external surface of the shielding body 10, and 
typically the entire external surface thereof, is provided with an 
electrically insulating coating 12 according to the invention. The coating 
12 consists of at least three, and preferably from eight to thirty, 
layers--arranged one upon the other--of a thin flexible and porous 
material. The material can be a knitted or woven fabric or a non-woven 
felt-like material, such as porous paper. The coating 12 can be made of 
basic materials such as cotton, glass fibers, wood or other cellulose 
fibers or plastics fibers. 
FIG. 5 shows the shielding body 10 during a manufacturing stage according 
to the invention, when spiral winding with a tape 13 of a thin flexible 
woven fabric has just commenced. Preferably, each winding turn overlaps a 
previously laid turn. As will be clear from the comments above, the tape 
13 may have a woven structure, as shown in FIG. 5, or it may have a felt 
structure such as porous paper, provided it has adequate permeability to 
the ion current. 
Instead of forming coating 12 by wrapping with a tape-formed material, the 
coating can be formed using a sheet-formed material which, depending on 
the dimensions of the sheet, can either be wrapped directly around the 
body 10 or can first be cut to suitable dimensions to facilitate such 
wrapping. 
In order to attain the required technical effect, it is important for the 
wrapping material to have adequate porosity. The pores should preferably 
each have an open area of 0.2-10 mm.sup.2 and the aggregate area of the 
pores should preferably constitute from 20 to 80% of the total area of the 
wrapping material. In dependence on the selected pore size in the 
individual tape, however, a sufficient number of layers of tape should be 
wrapped one upon another that the metal surface is no longer visible 
through the pores. 
The average thickness of the insulating coating 12 (i.e. the dimension "t" 
in FIG. 4) is preferably in the range of from 1 to 5 mm. 
As will be clear from the foregoing, the object of a method according to 
the invention is to coat any metallic surface in a power transformer which 
might occasion the build-up of a barrier potential--with an electrically 
insulating coating, consisting of tape of the type and material mentioned, 
around the respective electrodes. 
Many modifications can be made to the details of the construction described 
with reference to the drawings and all such modifications which are 
included within the scope and spirit of the following claims are to be 
considered as forming part of this invention.