CA3013140A1

CA3013140A1 - Energy efficient bushing for a transformer

Info

Publication number: CA3013140A1
Application number: CA3013140A
Authority: CA
Inventors: Sizwe Magiya Dhlamini
Original assignee: Power Hv Inc
Current assignee: Power Hv Inc
Priority date: 2018-08-02
Filing date: 2018-08-02
Publication date: 2020-02-02
Anticipated expiration: 2038-08-02
Also published as: CA3013140C

Abstract

An energy efficient bushing for a transformer is provided, the bushing comprising an elongate enclosure body to accommodate a conductor extending along a longitudinal axis and having first and second terminal ends, the ends extending from opposite sides of the enclosure body; and a mounting flange fitted to the enclosure body to enable the bushing to be mounted to an enclosure of the transformer. The enclosure body comprises first and second electrically insulating layers partially surrounding the conductor, the first layer being substantially provided by a first polymeric material, within which are energy efficient screens, and the second layer being substantially provided by a second polymeric material, the layers being arranged about the conductor in such a manner that the bushing is substantially cavity-free. In an embodiment, the first layer defines an inner core, with the second layer providing an outer cover that at least partially covers the inner core.

Description

TITLE: ENERGY EFFICIENT BUSHING FOR A TRANSFORMER
FIELD
[0001] Various embodiments are described herein that relate to an oil-free bushing, also referred to as a dry bushing, which is energy efficient in character, for a transformer.
More particularly, at least one embodiment described herein relates to a transformer including an oil-free bushing with low resistive losses, low eddy current losses and low leakage current losses.
BACKGROUND

[0002] When used with reference to electrical devices or systems, the term "bushing"
refers to an insulated device that allows an electrical conductor to pass safely through a conducting barrier, which is usually earthed. An example of such a conducting barrier is an enclosure, or wall, of a transformer.

[0003] In a power transformer, bushings serve to connect the windings of the transformer to a supply line external to the transformer, while insulating an incoming or outgoing conductor from the enclosure of the transformer.

[0004] A bushing includes a conductor made of a conductive material, which connects the windings of the transformer to a supply line, and insulation partially surrounding the conductor. Bushings employing various types of insulating materials have been developed, including porcelain, paper, resin and fibreglass, such as that shown in Figure 5, which typically uses oil inside a resin.

[0005] Existing bushings have a number of drawbacks, including:
1. Oil-impregnated porcelain bushings tend to suffer from fractures, fires and/or explosions, potentially leading to injuries or fatalities. Similarly, oil-impregnated paper bushings may catch fire or develop oil leaks, and are also prone to moisture ingress. These bushings are also dependent on the availability of oil.

2. A drawback of resin bushings is that the insulation in such bushings may be relatively brittle and lack adequate resistance to shock, thus increasing the risk of failure.
3. Further, bushings having insulation provided by fibreglass may be prone to delaminate due to high electric stress, moisture ingress, temperature extreme fluctuations and/or as a result of pollution. Delamination is also a concern in resin bonded paper designs.
SUMMARY OF VARIOUS EMBODIMENTS

[0006] In accordance with at least one embodiment of the invention, there is provided a bushing for a transformer, the bushing comprising: an elongate enclosure body to accommodate a conductor extending along a longitudinal axis, the conductor having a first terminal end and a second terminal end, the ends extending from opposite sides of the enclosure body; a mounting flange fitted to the enclosure body to enable the bushing to be mounted to an enclosure of the transformer; the enclosure body comprising two electrically insulating layers partially surrounding the conductor, a first layer of the electrically insulating layers being substantially provided by a first polymeric material, which includes co-axial energy efficient screens, and a second layer of the electrically insulating layers being substantially provided by a second polymeric material, the first and second layers being arranged about the conductor in such a manner that the bushing is substantially cavity-free.

[0007]
In at least one embodiment, the first layer including the energy efficient screens defines an inner core, with the second layer providing an outer cover which at least partially covers the inner core.

[0008]
In at least one embodiment, the energy efficient screens comprise high relative permeability materials including one or more of nanocrystalline grain structure ferromagnetic metal coatings, Permalloy or Mumetal.

[0009]
In at least one embodiment, the energy efficient screens have low magnetic anisotropy and low magnetostriction.

[0010] In at least one embodiment, the energy efficient screens have a low coercivity so that they saturate at low magnetic fields.

[0011] In at least one embodiment, the inner core includes a condenser screen arrangement, in the form of fine layers of metallic screens included or inserted in the inner core to perform two functions including voltage control by capacitance grading and magnetic decoupling.

[0012] In at least one embodiment, the two electrically insulating layers may be attached directly to the conductor, thereby providing a substantially cavity-free bushing.

[0013] In at least one embodiment embodiments, the first layer may be moulded directly onto the conductor.

[0014] In at least one embodiment, the second layer may be moulded directly onto the first layer.

[0015] In at least one embodiment, the first layer may be substantially provided by resin and the second layer may be substantially provided by a hydrophobic material. For example, the hydrophobic material may be a polymer. In such cases, the polymer may be an elastic polymer.

[0016] In at least one embodiment, the first layer is substantially provided by resin and the second layer is substantially provided by silicone rubber. In such embodiments, the second layer may thus be provided by a substantially shock resistant material.

[0017] In at least one embodiment, the coefficient of thermal expansion of the conductor and the first layer may be selected so as to be closely aligned with one another, thereby reducing the possibility or extent of delamination due to mechanical stress caused by a temperature gradient between the conductor and the first layer, in use.

[0018] In at least one embodiment, the second layer may provide a plurality of .. coaxial sheds spaced apart along the length of the bushing.

[0019] In at least one embodiment, the conductor may be provided by a tube. In other embodiments, the conductor may be a solid, rod-like conductor.
Alternative design dimensions and materials may be used to minimise resistive losses.

[0020]
In at least one embodiment, the first terminal end of the conductor may be configured for operative connection to an electrically active component of the transformer and the second terminal end of the conductor may be configured for operative connection to an electrically active external component.
For example, the electrically active component of the transformer may be transformer windings and the electrically active external component may be a supply line.

[0021]
In at least one embodiment, the conductor is magnetically isolated from a transformer tank of the transformer using the energy efficient screens.

[0022]
The conductor may be manufactured from any suitable conductive material, e.g. Aluminium or copper.

[0023]
The bushing is preferably a high voltage bushing, for use in phase-to-phase voltages greater than 100 kV and in current ratings ranging from approximately 1250 A to 2700 A. In at least one embodiment, the bushing is a 132 kV bushing. The bushing may be configured for use as a generation, transmission or distribution transformer.

[0024]
In at least one embodiment, the bushing may include a condition monitoring sensor. The condition monitoring sensor may be configured to monitor one or more predefined condition parameters associated with the bushing and to communicate values of one or more monitored parameters to a receiving module remote from the bushing.

[0025]
In such embodiments, the measured condition parameter may be leakage current in both of the two electrically insulating layers, with the sensor taking the form of a coupling capacitor.

[0026]
In such embodiments, the condition monitoring sensor may be arranged to take measurements externally on a surface of the bushing and is not connected to any inner parts of the bushing.

[0027]
In such embodiments, the condition monitoring sensor may obtain measurements on a single phase instead of for all three phases simultaneously.

[0028]
In an embodiment, the bushing includes a transmitter that is coupled to the condition monitoring sensor and is configured to transmit the measured condition parameter or any other measurements made by the condition monitoring sensor to a remote controller, typically in an online manner.

[0029] In another aspect, at least one embodiment of the invention extends to a transformer, which includes at least one bushing as hereinbefore described.

[0030] Other features and advantages of the present application will become apparent from the following detailed description taken together with the accompanying drawings. It should be understood, however, that the detailed description and the specific examples, while indicating preferred embodiments of the application, are given by way of illustration only, since various changes and modifications within the spirit and scope of the application will become apparent to those skilled in the art from this detailed description.
BRIEF DESCRIPTION OF DRAWINGS

[0031] For a better understanding of the various embodiments described herein, and to show more clearly how these various embodiments may be carried into effect, reference will be made, by way of example, to the accompanying drawings which show at least one example embodiment, and which are now described. The drawings are not intended to limit the scope of the teachings described herein.

[0032] FIGURE 1 shows an equivalent circuit diagram of an electrical bushing.

[0033] FIGURE 2 shows a phasor diagram that is related to the equivalent circuit diagram of an electrical bushing of Figure 1.

[0034] FIGURE 3 is a schematic diagram showing how to determine a bushing's dissipation factor DF and capacitance by summing leakage current for bushings at one side of a transfer.

[0035] FIGURES 4a-4c shows measurements of a main insulation (Cl), a tap insulation (C2) and an insulation of bushings (Cl) when the tap is not included, respectively.

[0036] FIGURE 5 shows a perspective view of a bushing for a transformer, according to at least one embodiment of the invention;

. '

[0037] FIGURE 6 shows a cross-sectional side view of the bushing shown in Figure 5.

[0038] FIGURE 7 shows sensors that may be used for condition monitoring attached on the bushing shown in Figure 5.
DESCRIPTION OF THE EMBODIMENTS

[0039] The following description of various embodiments of the invention is provided as an enabling teaching of the invention. Those skilled in the relevant art will recognise that many changes can be made to the embodiments described herein, while still attaining the beneficial results of at least one embodiment of the present invention. It will also be apparent that some of the desired benefits of the embodiments of the present invention can be attained by selecting some of the features of the embodiments of the present invention without utilising other features. Accordingly, those skilled in the art will recognise that modifications and adaptations to the present invention are possible and can even be desirable in certain circumstances, and are a part of the present invention.
Thus, the following description is provided as illustrative of the principles of the present invention and not a limitation thereof.

[0040] It should also be noted that, as used herein, the wording "and/or" is intended to represent an inclusive-or. That is, "X and/or Y" is intended to mean X or Y
or both, for example. As a further example, "X, Y, and/or Z" is intended to mean X or Y or Z or any combination thereof.

[0041] It should be noted that terms of degree such as "substantially", "about" and "approximately" as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. These terms of degree may also be construed as including a deviation of the modified term, such as by 1%, 2%, 5% or 10%, for example, if this deviation does not negate the meaning of the term it modifies.

[0042] Furthermore, the recitation of numerical ranges by endpoints herein includes all numbers and fractions subsumed within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.90, 4, and 5). It is also to be understood that all numbers and fractions thereof are presumed to be modified by the term "about" which means a variation of up to a certain amount of the number to which reference is being made if the end result is not significantly changed, such as 1%, 2%, 5%, or 10%, for example.

[0043]
In one aspect, at least one embodiment of the present invention aims to provide a transformer bushing that addresses the shortcomings of conventional bushings discussed in the background section, at least to some extent. It is also an aim of the invention to enable the condition of the bushing to be readily and accurately determined. In this regard, it is known that one way of determining the condition of a bushing is to calculate the bushing condition assessment variable of power factor (PF) (or the related dissipation factor (DF)) value, for quantifying the condition of bushing insulation systems.
The PF and DF values are related by equations (1) and (2) below:
DF = PF
¨(PF)2 (1) PF = DF
+ (DF)2 (2)

[0044]
Electrically bushings can be represented by the equivalent circuit diagram shown in Figure 1) and the related phasor diagram shown in Figure 2, which show the components of the total current and the applied voltage across the insulation material of the bushing.

[0045]
The cosine of the power angle (A) is called the power factor. The complement of 0 is called the loss angle and is denoted by 6 in Figure 2. If 0 decreases, more resistive current will flow through the insulation, and thus the power factor will increase.

[0046]
The power factor, PF, is the ratio of the real power in watts, W, dissipated in a material, to the complex power which is a product of the effective sinusoidal voltage, V, and current, I, in volt-amperes (VA). The power factor may be expressed as the cosine of the phase angle (0) (or the sine of the loss angle (6)).

[0047] Equation (3) below thus provides the power factor:

=
'r V = I P
PF = cos 0 = sin g == __ ' =
/ V=/ S VG2 ____________________________________________________________ +0).02 (3) where:
I= total current (mA), where 12 = Ic2+1r2;
lc = capacitive current (mA);
Ir = leakage current (mA);
V = voltage applied across the insulation (V);
S is the complex power = Voltage (V) x Current (I) (Volt-Amperes (VA)) P is the real power, as follows:
P=Vx1 Watts (W) P=Vxlx cosine (A) Watts (W) C = equivalent parallel capacitance (F); and G = equivalent ac conductance.

[0048] The dissipation factor, (DF), is the ratio of the resistive current (Ir) to the capacitive current (lc) which is equal to the tangent of its loss angle (6) or the cotangent of its phase angle (0) (see Figures 1 and 2). The DF is also called loss tangent, tan6, tanD or tan delta, and is calculated using equation (4) below:
X

DF = tan(g) = ¨I = cot(0) = = G = 1 I c R coC coC = R
(4) where:
C = equivalent parallel capacitance (F), with C = c, ;
R = equivalent ac parallel resistance (Ohm);
G = equivalent ac conductance;
Xc = parallel reactance; and w = 27cf (assuming a sinusoidal wave shape).

[0049] The reciprocal of the dissipation factor DF is the quality factor, Q, sometimes called the storage factor. When the dissipation factor DF is less than 0.1, the power factor PF differs from the dissipation factor by less than 0.5 %.

[0050] One way of determining the condition of a bushing is to measure leakage current. The underlying principle is that all insulating dielectric materials have some power losses due to leakage current, which will vary depending on:
1. the type of insulation;
2. the amount of dielectric material;
3. the temperature of the dielectric material;
4. the voltage and frequency applied across the insulation;
5. the frequency of the applied voltage;
6. the humidity during operation;
7. the extent of water immersion of the bushing;
8. the extent of weathering;
9. the age of the bushing in operation;
10. the quality of the manufacturing process and 11. conditioning while in operation, as described in ASTM D150 (2011), Standard Test Methods for AC Loss Characteristics and Permittivity (Dielectric Constant) of Solid Electrical Insulation.

[0051] As losses increase due to any or all the above causes, the power factor PF
will also increase, reflecting deterioration in insulation ability. This deterioration is caused by changes in the dielectric material due to:
1. aging of material;
2. inclusion of contaminants during production;
3. absorption of moisture while in service;
4. breakdown of bubble inclusions under voltage stress; and 5. other factors as explained further below.

[0052]
With reference to Figure 3, the sum of the leakage currents for three bushings at one side of a transformer allows the bushing's dissipation factor DF and capacitance (Cl) to be determined, where the C in the above DF equation (i.e. Equation 4) is the total capacitance corresponding to the sum of Cl and C2 in Figure 3.

[0053]
One way of measuring leakage current is to use a sensor in the form of a coupling capacitor. In addition, the phase angles and the frequency are also measured, using the same sensor. To compensate for the assumptions used in the calculation of PF
and capacitance (Cl), algorithms for filtering and smoothing are implemented.
The assumptions are that the line voltage at the bushing terminals is constant on all three phases, and that the phase angles between the phase voltages are constant.

[0054]
Figures 4a-4c show how Cl and C2 are defined, as well as offline measurement methods, done with 10kV.

[0055]
In terms of other factors that may affect the deterioration of the bushing's insulation abilities, these include the following:
1. Exposure of the insulation to a range of frequencies results in permittivity and loss index, as a result of dielectric polarizations which exist in the material. The two most important are dipole polarization due to polar molecules and interfacial polarization caused by inhomogeneities in the material. It is expected that bushing insulation in a substation may be exposed to the entire electromagnetic spectrum, from direct current frequencies (OHz) to radar frequencies of at least 3x101 Hz.
There are only very few materials, such as polystyrene, polyethylene, and fused silica, whose permittivity and loss index are even approximately constant over this frequency range.
2. The major electrical effect of temperature on an insulating material is to increase the relaxation frequencies of its polarizations. They increase exponentially with temperature at rates such that a tenfold increase in relaxation frequency may be produced by temperature increments ranging from 6 to 50 C. The temperature coefficient of permittivity at the lower frequencies would always be positive except for the fact that the temperature coefficients of permittivity resulting from many atomic and electronic polarizations are negative. The temperature coefficient will then be negative at high frequencies, become zero at some intermediate frequency and positive as the relaxation frequency of the dipole or interfacial polarization is approached.
3. Voltage stress causes dielectric polarizations, except interfacial polarization, which are nearly independent of the existing potential gradient until such a value is reached that ionization occurs in voids in the material or on its surface, or that breakdown occurs. In interfacial polarization, the number of free ions may increase with voltage and change both the magnitude of the polarization and its relaxation frequency. The DC conductance is similarly affected.
4. Humidity has the electrical effect on an insulating material of increasing greatly the magnitude of its interfacial polarization, thus increasing both its permittivity and loss index and also its DC conductance. The effects of humidity are caused by absorption of water into the volume of the material and by the formation of an ionized water film on its surface. The latter forms in a matter of minutes, while the former may require days and sometimes months to attain equilibrium, particularly for thick and relatively impervious materials.
5. Water immersion is the effect of water on an insulating material approximates that of exposure to 100 % relative humidity. Water is absorbed into the volume of the material, usually at a greater rate than occurs under a relative humidity of 100 %.
However, the total amount of water absorbed when equilibrium is finally established is essentially the same under the two conditions. If there are water-soluble substances in the material, they will leach out much faster under water immersion than under 100 % relative humidity without condensation. If the water used for immersion is not pure, its impurities may be carried into the material. When the material is removed from the water for measurement, the water film formed on its surface will be thicker and more conductive than that produced by a 100 %
relative humidity without condensation, and will require some time to attain equilibrium.

6. Ageing means that under certain operating conditions of voltage, temperature and mechanical shocks, an insulating material may deteriorate in electric strength because of the absorption of moisture, physical changes of its surface, chemical changes in its composition, and the effects of ionization both on its surface and on the surfaces of internal voids. In general, both the insulating material's permittivity and dissipation factor will be increased, and these increases will be greater the lower the measuring frequency.
7. Weathering, is a natural phenomenon, which includes the effects of varying temperature and humidity, of falling rain, severe winds, impurities in the atmosphere, and the ultraviolet light and heat of the sun. Under such conditions, the surface of an insulating material may be permanently changed, physically by roughening and cracking, and chemically by the loss of the more soluble components and by the reactions of the salts, acids, and other impurities deposited on the surface of the insulating material.

[0056] In another aspect, at least one embodiment of the present invention thus also aims to provide a transformer bushing that, when viewed holistically, is the best possible bushing when taking into account all of the factors mentioned above.

[0057]
Referring to Figures 5 and 6, a bushing 10 for a transformer is shown, the bushing 10 comprising an elongate enclosure body 12 to accommodate a conductor extending along a longitudinal axis. The conductor 14 has a first terminal end 16 a second terminal end 18, the ends 16, 18 extending from opposite sides of the enclosure body 12.
In some embodiments, the conductor 14 comprises a tube. For example, the conductor 14 may preferably comprise a solid, rod-like conductor. The conductor 14 may be manufactured from any suitable conductive material, e.g. Aluminium or copper.

[0058] A mounting flange 20 is fitted to the enclosure body 12 to enable the bushing 10 to be mounted to an enclosure of the transformer. In addition to a test tap 33, condition monitoring sensors 34, 35 are attached at the flange (as shown in Figure 6).

[0059]
The enclosure body 12 comprises two electrically insulating layers 22, partially surrounding the conductor 14. The first layer 22 of the insulating layers is substantially provided by a first polymeric material and the second layer 24 of the insulating layers being substantially provided by a second polymeric material. The layers 22, 24 are arranged about the conductor 14 in such a manner that the bushing is substantially cavity-free (and substantially devoid of oil and paper).

[0060] The first layer 22 typically defines an inner core 26, with the second layer 24 providing an outer cover 28 which at least partially covers the inner core 26.
The two electrically insulating layers 22, 24 may be attached directly to the conductor 14, thereby providing a substantially cavity-free bushing. In some embodiments, the first layer 22 may be moulded directly onto the conductor 14, with the second layer 24 being moulded directly onto the first layer 22.

[0061] The first layer 22 may be substantially provided by resin and the second layer 24 may be substantially provided by a hydrophobic material. The hydrophobic material may be a polymer. The polymer may be an elastic polymer. In one embodiment, the first layer 22 is substantially provided by resin and the second layer 24 is substantially provided by silicone rubber. The second layer 24 may thus be provided by a substantially shock resistant material.

[0062] The coefficient of thermal expansion of the conductor 14 and the first layer 22 may be selected so as to be closely aligned with one another, thereby to reduce the possibility or extent of delamination due to mechanical stress caused by a temperature gradient between the conductor 14 and the first layer 22, in use. The society for materials engineers and scientists (ASM) lists typical values of linear and volumetric expansion (10-6 mirn.K-1) for various materials at 20 C and 101.325 kPa as follows: Water 69 and 207;
Aluminium 23.1 and 69; Copper 17 and 51; PVC 52 and 156; Polypropylene 150 and 450.

[0063] In an embodiment, the inner core 26 includes a condenser screen arrangement, typically in the form of very fine layers of high relative permeability metallic foil screens 30 included or inserted in the inner core 26. A condenser screen arrangement is generally only required at voltages above 88kV, and although three screens 30 are shown in Figure 6, the exact number, arrangement and layout of the screens 30 may vary depending on the application. The screens 30 produce a capacitive effect which dissipates the electrical energy more evenly throughout the inner core 26 and reduces the electric field stress between the energised conductor 14 and any earthed material. It does this by distributing the electric field optimally in the radial and tangential directions, so as to lengthen the lifespan of the insulation materials. If the capacitances due to the screens 30 are equal, then the voltage is distributed as shown in Figure 3. A lower and uniformly distributed voltage within the dielectric materials reduces the electric stress in the bushing 10. The inner core 26 may be assembled so as to minimise electric stress in the bushing and/or on a surface of the bushing 10. Accordingly, the screens 30 can provide for voltage control by capacitance coupling and they can also provide for magnetic decoupling.

[0064] The energy efficient screens 30 comprise high relative permeability materials 10 including one or more of nanocrystalline grain structure ferromagnetic metal coatings, Permalloy or Mumetal. These materials have low magnetic anisotropy and low magnetostriction. The energy efficient screens 30 also have a low coercivity so that they saturate at low magnetic fields.

[0065] The outer cover 28 of the second layer 24 may include a plurality of coaxial sheds 32 spaced apart along the length of the bushing.

[0066] The first terminal end 16 of the conductor 14 may be configured for operative connection to an electrically active component of the transformer and the second terminal end 18 of the conductor 14 may be configured for operative connection to an electrically active external component. The electrically active component of the transformer may be transformer windings and the electrically active external component may be a supply line.

[0067] The bushing 10 is preferably a high voltage bushing 10, for use in phase-to-phase voltages greater than 100 kV and in current ratings ranging from approximately 1250 A to 2700 A. In one embodiment, the bushing 10 is a 132 kV bushing. The bushing 10 may be configured for use as a generation, transmission or distribution transformer.

[0068] In some embodiments, the bushing 10 may include at least one condition monitoring sensor 34, 35. The condition monitoring sensor 34, 35 may be configured to monitor one or more predefined condition parameters associated with the bushing 10 and to communicate values of one or more monitored parameters to a receiving module remote from the bushing 10. In an embodiment, the measured condition parameter is leakage current in both of the two electrically insulating layers, with the sensor 34, 35 taking the form of a coupling capacitor with detection ranging from 80 pF up to 10 nF.
The sensor 34, 35 is typically placed at the flange 20 by means of a circumferential strapped band attachment or a threaded bolt-in device into a connection point that is similar to a test tap that is present on most high voltage bushings.

[0069]
In an embodiment, the sensor 34, 35 includes a transmitter to transmit the measured condition parameter to a remote controller, typically in an online manner.
Communications of measured data is network neutral or network independent. The sensor can thus use any available network such as a powerline carrier, a fibre telecommunications network or a wireless network. On-line monitoring and alarming systems allow for the uploading measured data to a server for remote analysis. This feature saves customers the costs associated with bringing in an expert and paying its staff to accompany someone at the local site to perform advanced diagnostics.

[0070]
The bushing described herein provides increased safety and a significantly lower risk to consumers. Particular advantages of at least one embodiment of the bushing of the invention including at least one feature from the following non-exhaustive list:
1. The bushing is waterproof and paperless.
2. The design may eliminate or reduce the risk of bushing explosions and reduce the probability of burn out fires on power transformers.
3. The bushing is sustainable and environmentally friendly as it does not utilize or depend on fossil fuels, e.g. oil, which is a depleting natural resource and which fluctuates in cost.
4. The bushing is environmentally friendly and meets the requirements of international specifications, which require transformer bushings to "be of technology that provides safe operation of the transformer, maintenance free or require minimum maintenance, environmentally friendly, and as far practically possible does not add fire risk".
5. In some embodiments, the bushing can be monitored and maintained from a remote location.

6. The remote access component optimizes maintenance of the bushing and reduces risk to employees who are hired to service bushings, as physically attending to a bushing would not be required frequently.
7. The design aids in providing the least possible level of partial discharges and also provides mechanical strength.
8. The design can be customised and is suitable for a wide range of transformer application.
9. Polymeric dry bushings can withstand extreme operating conditions, including temperatures ranging from -400 to 60 C, which significantly reduces maintenance and storage costs.
10. The design can be used in many different applications, e.g. generation, transmission and distribution transformers that require increased levels of reliability and safety.
11. In at least one embodiment, the bushing may use shock resistant resin that is housed in elastic polymer in order to provide cushion against shock.
12. The use of a polymer as a main component significantly prolongs the life of the bushing and reduces the probability of combustion over the lifespan of the bushing.
13. Unlike fibreglass composition bushings which delaminate under high electric stress, water ingress and pollution, the proposed dual polymer bushing is highly reliable.
14. Oil impregnated porcelain designed bushings are susceptible to explosion and fires which can result in injury or fatalities of personnel, which the dual polymer bushing embodiments of the invention addresses.
15. Most resin bushings suffer from brittle fractures as they are not shock resistant, so under seismic loading such bushings fail, whereas vibration simulations based on data sheet specifications of shock resistant resin types that may be used in at least one embodiment of this invention of the 132kV polymeric bushing greatly reduces this risk.

16. This gives the invention an overall operating advantage in performance, as opposed to oil insulated paper, resign impregnated paper or oil cooled resign impregnated paper which has a higher probability of combustion over time.
17. The elimination of fibreglass and porcelain increases reliability and reduces or eliminates the risk of fractures, explosions causing fires, as well as delamination.
18. The bushing can withstand a relatively high thermal load.

[0071] While the applicant's teachings described herein are in conjunction with various embodiments for illustrative purposes, it is not intended that the applicant's teachings be limited to such embodiments. On the contrary, the applicant's teachings described and illustrated herein encompass various alternatives, modifications, and equivalents, without departing from the embodiments described herein, the general scope of which is defined in the appended claims.

Claims

CLAIMS:

1. A bushing for a transformer, the bushing comprising:
an elongate enclosure body to accommodate a conductor extending along a longitudinal axis, the conductor having a first terminal end and a second terminal end, the ends extending from opposite sides of the enclosure body;
a mounting flange fitted to the enclosure body to enable the bushing to be mounted to an enclosure of the transformer;
the enclosure body comprising two electrically insulating layers partially surrounding the conductor, a first layer of the electrically insulating layers being substantially provided by a first polymeric material, which includes co-axial energy efficient screens, and a second layer of the electrically insulating layers being substantially provided by a second polymeric material, the first and second layers being arranged about the conductor in such a manner that the bushing is substantially cavity-free.

2. The bushing of claim 1, wherein the first layer including the energy efficient screens defines an inner core, with the second layer providing an outer cover which at least partially covers the inner core.

3. The bushing of claim 1 or claim 2, wherein the energy efficient screens comprise high relative permeability materials including one or more of nanocrystalline grain structure ferromagnetic metal coatings, Permalloy or Mumetal.

4. The bushing of any one of claims 1 to 3, wherein the energy efficient screens have low magnetic anisotropy and low magnetostriction.

5. The busing of any one of claims 1 to 4, wherein the energy efficient screens have a low coercivity so that they saturate at low magnetic fields.

6. The bushing of claim 2, wherein the inner core includes a condenser screen arrangement, in the form of fine layers of metallic screens included or inserted in the inner core_to perform two functions including voltage control by capacitance grading and magnetic decoupling.

7. The bushing of claim 1, wherein the two electrically insulating layers are attached directly to the conductor, thereby providing a substantially cavity-free bushing.

8. The bushing of claim 7, wherein the first layer is moulded directly onto the conductor and the second layer is moulded directly onto the first layer.

9. The bushing of claim 1, wherein the first layer is substantially provided by epoxy and the second layer is substantially provided by a hydrophobic material.

10. The bushing of claim 9, wherein the hydrophobic material is an elastic polymer.

11. The bushing of claim 1, wherein the first layer is substantially provided by epoxy resin and the second layer is substantially provided by silicone rubber.

12. The bushing of claim 1, wherein the coefficients of thermal expansion of the conductor and the first layer are selected to be closely aligned with one another, thereby reducing the possibility or extent of delamination due to mechanical stress caused by a temperature gradient between the conductor and the first layer, in use.

13. The bushing of claim 1, wherein the second layer includes a plurality of coaxial sheds spaced apart along the length of the bushing.

14. The bushing of claim 1, wherein the conductor comprises a tube.

15. The bushing of claim 1, wherein the conductor comprises a solid, or a rod-like conductor.

16. The bushing of claim 1, wherein the first terminal end of the conductor is connected or connectable to an electrically active component of the transformer and the second terminal end of the conductor is connected or connectable to an electrically active external component.

17. The bushing of claim 16, wherein the conductor is magnetically isolated from a transformer tank of the transformer using the energy efficient screens.

18. The bushing of claim 1, wherein the bushing includes a condition monitoring sensor, the condition monitoring sensor being arranged to monitor one or more predefined condition parameters associated with the bushing and to communicate values of one or more monitored parameters to a receiving module remote from the bushing.

19. The bushing of claim 18, wherein the condition monitoring sensor is arranged to take measurements externally on a surface of the bushing and is not connected to any inner parts of the bushing.

20. The bushing of claim 18 or claim 19, wherein the condition monitoring sensor obtains measurements on a single phase instead of for all three phases simultaneously.

21. The bushing of any one of claims 18 to 20, wherein the bushing includes a transmitter that is coupled to the condition monitoring sensor and is configured to transmit the measured condition parameter or any other measurements made by the condition monitoring sensor to a remote controller, in an online manner.