Zhikang Yuan, Gang Sun, Zhiwen Huang, Jun Hu, Jinliang He
Bushings are the key equipment in a high-voltage dc (HVdc) project. HVdc bushings not only provide the insulating medium between the HV conductor and the ground electrode but also physically fix the HV conductor in the transformer or in the feedthrough assembly. In operation, HVdc bushings always experience high electric field and load current. When a fault occurs, the bushing will inevitably face instantaneous overvoltage and overcurrent conditions. This harsh operating condition poses a serious threat to the long-term performance and reliability of HVdc bushings. However, the construction of extra-HVdc projects, ultra-HVdc projects, and flexible dc transmission projects is in full swing. The market demand for HVdc bushings is growing rapidly. Take China, for example: from 2013 to 2020, about 2,600 transformer bushings for ultra-HVdc converter transformers and 400 wall bushings for valve halls were needed. The annual demand for HVdc bushings is about 375, and the cost is more than 1.7 billion CNY per year. Considering the construction of extra-HVdc and flexible dc projects, the market scale of an HVdc bushing will increase in the future.
The electrode structure of an HV bushing is a typical plug-in structure. The HV conductor rod plugs into the center of the grounded flange, as shown in Figure 1, resulting in the uneven distribution of the electric field. The uneven distribution of the radial electric field may lead to partial discharge or even breakdown of the insulating medium, while the uneven distribution of the axial electric field may lead to flashover on the surface of the bushing. The material used for outdoor insulation can be silicone rubber or ceramic. The space between the capacitive core and air side housing is filled by an insulating medium such as SF6 gas, a special kind of foam, or oil. The capacitive core is the dominant component in HV bushings to prevent the concentration of the electric field. Coaxial capacitances are constructed between the flange and conductor layer by layer to withstand the applied HV, so that the high concentration of electric field on the surface of the conductor and at the edges of the flange can be effectively suppressed. Electric field grading is a method to prevent local concentration of electric field and make the electric field distribution in the equipment or environment more even. Construction of a capacitive core is one of the typical electric field grading methods.
At present, most of the capacitive cores in HV transformer and wall bushings are designed using resin-impregnated paper as well as in oil-impregnated paper technology. Other technologies, including glass fiber–reinforced plastic (GRP) and resin-impregnated synthetic, are only used in some specific conditions, occupying a small share of the market. The cross section of the capacitive core is also shown in Figure 1.
Theoretically, the higher the coaxial capacitance is, the better the electric field grading effect of the capacitive core will be, and the more evenly distributed the electric field in the bushing will be. However, the arrangement of the metal foils, as the electrodes of the coaxial capacitances, in the bushing requires very high technology manufacturing capability. The distance between each layer of metal foil must be accurately controlled. Partial discharge in bushings can be caused by poor manufacturing quality, including the missed, misplaced, and excessive placement of the conductive foils. An improper casting process, especially the shrinkage process during the curing of the resin or residual bubbles between the conductive foils, will also lead to electric failure of the capacitive core. Compared with the HV ac (HVac) bushing, the HVdc bushing will face operating conditions of dc polarity reversal and ac–dc superposition, which put higher requirements on manufacturing technology.
CIGRE Working Group A2.43 has collected more than 300 bushing failure cases from 22 countries and reported the reasons and proportions of bushing failures. The most abundant bushing failure is induced by capacitive core defect, which is almost half (41%) of all the collected failure cases, as shown in Figure 2. However, 37.8% of transformer failures with fire or explosion as a consequence are caused by bushing failure. The CIGRE report illustrates the shortage and the high manufacturing quality requirements of HVdc bushing. To enhance the reliability of HVdc bushings and further strengthen the stability of the power system, a new electric field grading scheme to replace the capacitive structure in the HVdc bushing is required.
Materials with nonlinear conductivity or permittivity, also known as nonlinear materials (NMs), show an electric field dependent on conductivity or polarizability. Compared with the temperature-dominated electric parameters of ordinary materials, the electric parameters of NMs vary with the electric field intensity. Under low electric field intensity, the NM is equivalent to insulating material, maintaining low electrical conductivity or permittivity. The leakage current and dielectric loss are also low. When the intensity of electric field reaches a certain value, the conductivity or permittivity of the NM will increase sharply, and the material will behave as a conductor. This certain value of the electric field is like a threshold value to “switch” the material from the insulating state to the conducting state, so the certain value of the electric field is also known as the switching threshold value. As is known, the conductors bear low electric field intensity and insulations bear high electric field intensity when an HV is applied.
When the area of high electric field concentration is covered by nonlinear composite, the conductivity of the nonlinear composite tends to increase instantly. However, when the conductivity becomes high, the electric field intensity the nonlinear composite bears will be lowered, which means that the conductivity will also be lowered. This negative feedback process will ultimately lead to a balance between the conductivity of the NM and the electric field distribution of the environment. A comparison between ordinary materials and NMs is shown in Figure 3. The negative feedback between the electric parameters of the NM and the electric field distribution makes adaptive electric field grading possible in HV equipment by using NMs.
The more uneven the electric field distribution is, the more significant the electric field grading effect of the NM on high electric fields will be. In HVdc bushings, electric field grading is a critical issue that increases the difficulty and cost of manufacturing greatly. NM shows promise for solving the problem of uneven electric field distribution in the bushing. NMs may even create an innovative path to subvert the traditional technology for HV bushings.
In this article, we first illustrate the theory of electric field adaptive grading in HV equipment by using NMs. Second, the NM, ZnO microvaristor-based composites, adopted in the HVdc bushing is introduced. The source and the adjusting methods for the nonlinear conductivity of the composite are also discussed. Third, the structure and the electric field distribution in the traditional bushing and the bushing with electric field adaptive grading structure are compared to highlight the advantages of the electric field adaptive grading structure. Last, the prototypes of a ±100-kV transformer bushing and a ±200-kV wall bushing are presented.
Two ways exist to achieve electric field grading in HV equipment. One way is to adjust the shape of the electrode. Electric field grading rings at the ends of HV outdoor insulators and stress cones in cable terminals are two typical cases that optimize the local electric field distribution by adjusting the shape of the electrodes. The other way to achieve electric grading is to use electric field grading materials, such as materials with semiconductivity or nonlinear conductivity. Materials with nonlinear conductivity (also known as NMs), that is, conductivity that can change with the electric field, have been used in cable terminals and motor coils to suppress the local concentration of electric field. As a matter of fact, these two electric grading methods have something in common. The adopted NMs in HV equipment can be regarded as a special shape of electrode, which can realize adaptive shape optimization.
In this section, the concept of an equivalent electrode is proposed to build the relationship between electrode shape adjustment and the adoption of NMs. First, we set a specific conductance as the dividing line between the conductive part and the insulating part in the NM according to the electric field condition. The part of the NM where the conductance is higher than the specific conductance value is thought of as the electrode part. The part of the NM where the conductance is lower than the specific conductance value is thought of as the insulating part. Then, the conductivity contour map is drawn based on the conductivity distribution. The conductivity contour line, which corresponds to the specific conductance value, is the outline of the equivalent electrode.
To further illustrate the theory of the electric field grading methods, a needle-plate electrode model is established, as shown in Figure 4. In Figure 4(a), a voltage of 5.0 kV is applied to the needle electrode to reproduce the high electric field situation in HV equipment. When the needle electrode is exposed to the air and the distance from the needle tip to the grounded plate electrode is 10 mm, as shown in Figure 4(c), the electric field concentrates near the needle tip, and the highest electric field intensity is about 4.5 × 106 V/m, which reaches the same magnitude of electric field intensity in HVdc bushings. The conductivity distribution (S/m) is shown in Figure 4(f). The needle and ground electrodes are high conductivity, and the air is the insulation medium with low conductivity.
The material, which is used to grade the electric field, wraps the needle electrode, as the green rectangle shows in Figure 4(b). Figure 4(d) and (g) show the electric field distribution and conductivity distribution, respectively, when the needle tip is wrapped by high conductance material (HCM). Figure 4(g) illustrates that the HCM and the needle are both in the state of high conductivity and, in combination, become the HV electrode. That is why the electric field intensity in HCM is low, as shown in Figure 4(d). The high electric field area moves from the needle tip to the edge of the HCM, and the highest intensity of electric field decreases significantly to about 1.0 × 106 V/m.
When an NM is adopted, the electric field distribution map and conductivity distribution map are shown in Figure 4(e) and (h), respectively. The high electric field areas appear at the needle tip and the edge of the NM. The highest electric field intensity is suppressed effectively. In Figure 4(h), the conductivity of the NM shows gradient distribution near the needle tip. A circle equivalent electrode, as outlined by the black dotted line in Figure 4(h), is formed adaptively due to the electric field-dependent conductivity of the NM. It is worth noting that the radius of the circle equivalent electrode will change accordingly with the applied voltage.
NMs are materials with nonlinear conductivity or permittivity, prepared by filling the polymer matrix with nonlinear inorganic fillers or weak ionic conductive organic fillers. They are also named nonlinear composites. Up to now, few composites have existed with nonlinear permittivity but no nonlinear conductivity. Only several barium titanate (BaTiO4)-filled composites have shown a low nonlinear coefficient of permittivity, which was too low to meet the requirements in electric field grading applications. So NM researchers pay the most attention to the materials with nonlinear conductivity.
In the early days, the main fillers for nonlinear composites were the powder of silicon carbide (SiC), carbon black (CB), zinc oxide (ZnO), aluminum oxide (Al2O3), etc., and the preferred polymer matrixes were polyethylene (PE), epoxy resin, and silicone rubber (SR). When the volume fraction of the fillers exceeds the percolation threshold value, which means that at least one continuous channel of the fillers is formed in the composite, the composite shows nonlinear conductivity. The interface between the filled powders, as shown by the red lines in Figure 5(a), is the source of the nonlinear conductivity of the composite. Figure 5(c) shows the appearance of SiC powders. The nonlinear coefficient of the contact surfaces is low, and the contact between the fillers is not stable.
To meet the application requirements for HVdc bushings for different voltage levels, the nonlinear conductivity of the nonlinear composite must be flexibly adjusted. The switching threshold value is a parameter of great significance that determines the applicable electric field environment for the nonlinear composite. The number of the grain boundary and the contact interface in the conducting channel affects the switching threshold value directly. The lower the number is, the lower the switching threshold value will be. We can tailor the switching threshold value of the composite by controlling the size of ZnO microvaristors and grains, as shown in Figure 5(e). The numbers of both the grain boundary and the contact interface reach their highest when small fillers with small grains are added in the composite. Large fillers with large grains will be chosen if a low switching threshold value of the nonlinear composite is needed.
However, the volume fraction of ZnO microvaristors in the composite influences the conducting channel. A higher volume fraction will lead to a straighter conducting channel, thus decreasing the number of the grain boundary and the contact interface. The schematic views of the methods that can realize the nonlinear conductivity adjustment of ZnO microvaristor-based composites are shown in Figure 5(e). In addition, the polymer matrix, the shape of the fillers, and the addition of a second type of filler will also contribute to the nonlinear conductivity adjustment. The nonlinear conductivity with a wide range of switching threshold values of ZnO microvaristor-based composites can be obtained by adopting the methods above. The nonlinear conductivities of six different formulas of the composites, named from type 1 to 6, are shown in Figure 5(f). In summary, the ZnO microvaristor-based composite is the first choice to be adopted in the HVdc bushing for electric field adaptive grading due to the high nonlinear coefficient, the stable nonlinear conductivity, and the adjustable nonlinear conductivity.
As Figure 1 shows, the capacitive core with multiple concentric metal foils is used for grading the electric field between the center conductor and the flange in the traditional HV bushing. Figure 6(a) shows the structure of the capacitive core in the bushing. Considering that the nonlinear composite can achieve electric field adaptive grading with the electric field-dependent conductivity, an electric field adaptive grading structure for an HVdc bushing is presented in Figure 6(b). The capacitive core, which requires high manufacturing technology, is eliminated in the bushing. Only the grounded metal foil is preserved. There are three layers from the center conductor to the flange. The first layer is made by the nonlinear composite of the high switching threshold, which is used to grade the high electric field at the surface of the conductor. The second layer is a layer of insulation. Polymers of high dielectric strength can be used as the insulation medium in the layer. The third layer is made by the nonlinear composite of the low switching threshold. The grounded metal foil is wrapped by the nonlinear composite. A measuring wire, which is used to obtain the operational state of the bushing by measuring the capacitance and dielectric loss, is attached to the grounded metal foil. The nonlinear composite is used to grade the electric field around the metal foil adaptively. Between the nonlinear composite and the grounded flange, there remains a thin layer of insulation.
Figure 6(c) shows the electric field distribution map at the end of the metal foils in the traditional bushing when the rated dc voltage is applied to the conductor. Electric field concentrates at the end tip of the foil. Figure 6(d) shows the electric field distribution map at the end of the nonlinear composite, which wraps the grounded metal foil. The electric field intensity at the end tip of the nonlinear composite is significantly lower than that of the metal foil in the traditional bushing. Figure 6(e) shows the electric field distribution from the conductor surface to the grounded metal foil.
In the traditional bushing, the highest electric field area is located at the conductor surface. The saddle-shaped electric field distribution shows that the electric field intensity decreases at first and then increases from the conductor surface to the grounded metal foil. In the bushing, the electric field distributes evenly in the nonlinear composite on the conductor surface. A slight increase of electric field intensity appears at the interface between the nonlinear composite and the insulation layer. Then, the electric field intensity decreases gradually in the insulation layer from inside out. It can be summarized that the electric field uniformity of the bushing with electric field adaptive grading structure is better than the traditional one.
To turn the HVdc bushing with electric field adaptive grading structure from concept to reality, the HV research group from Tsinghua University cooperated with the well-known bushing manufacturer, Nanjing Electric Co., Ltd., and produced prototypes of a ±100-kV transformer bushing and a ±200-kV wall bushing. In the condenser body of the bushing, we choose glass fiber reinforced epoxy resin (GRP), which has a high dielectric and mechanical strength, as the material for the insulation layer. To coordinate with the production of insulation layer, the nonlinear composite is prepared by a wet winding technique with polyester fiber cloth. The technique can also prevent the ZnO microvaristors from settling in the resin. The appearance of the ±100-kV transformer bushing is shown in Figure 7(a). To exhibit the electric field adaptive grading structure, part of the condenser body in the oil side has been dissected. The nonlinear composite, which wraps the grounded metal foil, and the insulation layer are exposed.
The ±100-kV transformer bushing has passed the partial discharge test under power frequency, the ±100-kV polarity reverse test, the full wave negative lightning impulse test, and the ac 200-kV withstand test. The ±200-kV wall bushing has passed the partial discharge test under power frequency, the ±250-kV polarity reverse test, the full wave negative lightning impulse test, and the dc 300-kV withstand test. The ±200-kV wall bushing during test is shown in Figure 7(b). It is worth noting that the outdoor insulation of these two bushings is different. The ceramic envelope is selected as the outdoor insulation in the air side of the ±100-kV transformer bushing. SF6 is used to fill in the space between the ceramic envelope and the condenser body. As for the ±200-kV wall bushing, the silicone rubber envelope is stuck on the condenser body directly. The successful application of these two kinds of materials as the bushing envelope indicates that the electric field adaptive grading structure in the bushing is compatible with both techniques of outdoor insulation.
The electric field adaptive grading structure is a new research direction for nonlinear composites. This work is a preliminary attempt to use the structure in HV bushings. From the aspects of industry applications, there remains a long way to go from the prototype to mature production. However, the successful application of nonlinear composites in HVdc bushings has proved the effectiveness and advantages of the electric field adaptive grading structure. Nonlinear composites show great potential for solving the electric field concentration problems for composite insulators and cable terminals, which may bring new insights for electric field grading in HV equipment. In recent years, the development of wide band gap semiconductor materials supports the development of power electronic modules toward the goals of higher voltage, higher current, and lower loss.
This work was supported in part by the National Key R&D Program of China (2018YFE0200100), Natural Science Foundation of China (51921005 and 52207029), State Key Laboratory of Power Systems (SKLD22KZ02), and Shanghai Sailing Program (22YF1450300).
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Zhikang Yuan is with Tsinghua University, 100084 Beijing, China, and Tongji University, 201804 Shanghai, China.
Gang Sun is with Tsinghua University, 100084 Beijing, China.
Zhiwen Huang is with Tsinghua University, 100084 Beijing, China.
Jun Hu is with Tsinghua University, 100084 Beijing, China.
Jinliang He is with Tsinghua University, 100084 Beijing, China.
Digital Object Identifier 10.1109/MPE.2022.3230891