4.8.2 The Performance Characteristics of Composite Insulators
Composite insulators can take wind and rain and have good self-cleaning performance under wind and rain, so need checking for pollution only once every 4–5 years, and requiring less time for the repair and power interruption. Since the core rod has higher extension strength, composite insulators can result in very light overall weight. Their weight is only 10–20% of the weight of porcelain insulator strings of the same voltage class. Their length can be shortened by more or less 10% in the same voltage class, which can greatly reduce the labor of workers in transportation and field operation.
The composite insulator has many advantages, but also disadvantages, e.g., the loss of hydrophobicity, the risk of the core rod becoming brittle and breaking, lightning strike and birds droppings, which can all make composite insulators lose efficiency.
Since the diameter of the sheds of the composite insulator is less, the minimum electric arc distance is less than for the same length of porcelain insulator strings, and the lightning withstand level is also less than for the same length of porcelain insulator strings.
After a lightning strike, the only effect on composite insulators is some white electric erosion; there is no change in their insulating property. But attention must be paid to the erosion of both ends of the fitting.
The internal insulation distance of composite insulators is nearly equal to that of the external insulation, and the structure is in the group of puncture-proof insulators, and therefore does not have the problem of having to detect zero value insulators, and this greatly reduces the workload of operation maintenance.
The material of sheds and sheaths of composite insulators are silicone rubber and the surface is a low energy surface. The creepage distance of composite insulators is bigger and the diameter of the umbrella shed is smaller, and the surface has hydrophobicity and migration of hydrophobicity. Even in a humid and polluted environment, the shed surface of composite insulators will not form a continuous water film; therefore, its antipollution performance is superior to that of a porcelain insulator.
The main component of the composite insulator is a silicone rubber sheath. Silicone rubber is formed by the linkage of high-molecule polymers of polydimethyl siloxane and organic oxygen compounds; the main chain is formed by a silicon oxygen bond. Since the bond energy of the silicon oxygen bond is larger it has good thermostability, and can work at temperatures of −100 to +350°C. Silicon rubber has good ozone resistance performance, and whereas butadiene-propylene propylene butadiene rubber can be readily broken, under room temperature tension on ozone of 150 ppm, however, silicone rubber can last for several months and not be broken.
Composite insulators have been introduced as a good alternative to ceramic and glass insulators. After nearly 30 years of launching the first type of composite insulators and making design improvements and consuming materials, they are used as well-known and suitable products in HV [1,15,16].
In the resin insulators, the oxygen composition with certain plastics is made in the form of a polyidison; that is, together with the composition, large molecules are produced. In order to harden the compound, usually additional materials such as quartz are added, and finally, the resin is made up by heating and casting. These types of insulators are not used in open space due to their lack of ultraviolet radiation in the sun and are used only in interior spaces and inside panels. In Fig. 6.4, samples of resinous insulators have been shown [1,15,16].
Composite insulators consist of at least two insulating materials; one of which is the task of providing electrical properties and the other providing mechanical properties. Composite insulators, as shown in Fig. 6.5, are composed of components such as [1,29]:
Figure 6.5. Different components of composite insulators containing of composite core, polymeric housing, and metallic connectors in different view [9,28].
1.
Core
2.
Housing
3.
End connections
Core: The main principles of composite insulations are based on the use of a composite core, whose function is to withstand the mechanical load brought on by the conductor wire and transfer this tensile force to the tower.
6.4.4.1 Process of Manufacturing Insulator Core [1]
As shown in Fig. 6.6, the core of a composite insulator consists of a composite rod consisting of two main components of the matrix and reinforcing fibers. The matrix is made of epoxy resin and E-glass reinforcing fiber, which is made of glass fiber in parallel and in the same direction throughout the rod. The core composite quadrilateral depends on the design of the insulator and the tensile load that it has to withstand and is made in different diameters. However, its range with the numbers mentioned by different manufacturers can be set between 14 mm and 70 mm. The fibers in the core of a composite insulator are two main tasks, one that acts as the main insulation component, and the other is the task of bearing mechanical load. Composite core construction is done by pultrusion process. In general, pultrusion is a process used to produce continuous composite sections, such as rods, tubes. The main parts of the pultrusion process are schematically shown in Fig. 6.6. In this process, the fiberglass is fed from the fiber feeder to the resin dipping and entered into the mold after passing through the preforms. In the form of heat treatment, the impregnation and curing of the resin takes place and the profile forms the cross-sectional shape of the mold. The advantages of fabricated parts include lightweight, lower maintenance costs, and more corrosion resistance; the most important advantage in strength (rigidity to weight) is relatively high due to the high percentage of fiber and its continuity in the structure of these components. It should be noted that there are other methods for the production of composite cores, including manual warping and filament winding techniques, but since the highest strength and the highest mechanical properties of the pultrusion process are obtained, the method pultrusion is preferred to other methods [1,30].
Figure 6.6. Pultrusion process [1,30].
Housing (coating)[1,3]: The function of this coating is to protect the core to the weathering and moisture-damaging effects, as well as to increase the voltage and creeping current. This coating is usually made up of silicone rubber and other additives such as TiO2 and aluminum three hydrate [1]. The insulating properties of composite insulators are largely related to their coating.
The coatings used in composite insulators include:
a.
Ethylene propylene monomer
b.
Ethylene propylene diamine monomer
c.
Silicone rubber
d.
Ethylene-propylene rubber
Today, the most common rubber coatings used in composite insulators are silicon rubber. The reason for this is the long-term stability of silicone rubber against different weather conditions, hydrophobicity. Waterproofing properties of silicon separate it from other insulating materials. This property means that water cannot to be dispersed on the insulator, but remains in the form of a water droplet concentrated in one part. This is why, in the case of contaminated environments, the leakage current at the level of the silicone rubber insulators is much lower than that of ceramic, glass, and even ethylene propylene insulators. Therefore, in extremely polluted areas, there is no electrical arc on these insulators. In addition, the hydrophobicity of silicone rubber is always consistent with other polymers [1,31,32].
6.4.4.2 Process of Manufacturing Core Housing
In 2013, Vijayalekshmi [33] performed a two-mill roller machine to prepare a silicon-binder mix in the presence of reinforcing particles. Fig. 6.6 shows the device image of the two-way mixer. Mixing takes place in the room for about 15 min, and then the prepared hot pressing mixture is placed at 140°C for 1 h until the baking takes place.
Gafti [1] has performed a research on manufacturing housings of insulators by compression molding of composite containing HTV silicone rubber reinforced by TiO2 and Aluminate hydroxide.
Connectors[33–36]: Connectors are the parts that are located between the tower and the conductive cable.
Due to this variety in different parts of composite insulators and their construction methods, it can be clearly seen that the performance of a composite insulator is heavily related to the correct selection of raw materials and the technology of manufacturing the insulator [9].
The advantages and disadvantages of composite insulators are as follows:
1.
Flexible and unbreakable, and suitable for areas where breakdown of insulators is commonplace by human factors.
2.
Silicone insulators have the ability to dispose of water and are technically and economically very suitable for wet areas.
3.
The use of these insulators in airlines due to no necessity of periodic washings greatly reduces line maintenance costs.
4.
Due to the lightweight silicone-based insulators in comparison to the glass and ceramic insulators, it is easier to transport and install them. On the other hand, due to this feature, the cost of constructing the line in terms of mechanical calculations of the towers and the design of the foundation will be reduced significantly.
5.
The breakage probability of composite insulators during the transportation and installation is negligible.
6.
Composite insulators contain a higher performance in snowy and frost areas due to the shape of the insulator and the small diameter of housings.
Despite the particular merits of composite insulators, these insulators also have disadvantages. The disadvantages of composite insulators can be classified as follows [32,37,38]
1.
The higher price of raw materials as compared to other insulators.
2.
The lack of experienced labor.
Environmental Effect Parameters in Selecting Insulators for transmission and distribution Lines [32,37,38]:
a.
Climate
b.
Different temperature parameters humidity
c.
Radiation of thermal and ultraviolet radiation (UV)
d.
Rainfall
e.
Lightning and Isocraonic levels of the area
f.
Wind
g.
The severity and type of pollution of the area
h.
Types of insulator profiles
i.
Height above sea level
j.
Installation arrangement: Transmission, end, or chain type
5.3.2.4 Pollution flashover characteristics of composite insulators
Silicone rubber composite insulators have good antipollution flashover capability. In the same pollution and wetting condition and of the same structural height, its flashover or withstand voltage is 2–3 times higher than that of porcelain insulators. Composite insulators with shed sheaths of silicon rubber material have good antipollution flashover capability, and the main reasons are the following:
1.
The surface of silicon rubber sheds is a low energy surface with good hydrophobicity. The hydrophobicity of silicon rubber also migrates to the pollution layer’s surface, which makes the pollution layer have hydrophobicity.
2.
As the pollution layer in the silicon rubber surface also has hydrophobicity, and the water absorbed by the pollution layer surface will not form a continuous water film, and only presents in the form of discontinuous small water droplets. Under sustained voltage action, a thin small arc is distributed over the whole insulation surface, which does not form a concentrated and strong arc like electrical porcelain or glass insulators. This characteristic is the deciding factor meaning that composite insulators do not easily form a concentrated discharge path, a partial arc does not develop easily, and there is higher pollution flashover voltage.
3.
Under the same circumstances, the time required for silicon rubber composite insulators to get damp and become saturated is several times that required for porcelain insulators, so it is hard for silicon rubber composite insulators to get wet, and natural pollution flashover is unlikely to occur.
4.
The rod diameter (or equivalent diameter) of composite insulators is small with a big shape coefficient. In the condition that the surface is dirty, its surface resistance is much larger than that of an insulator with a small shape coefficient. Pollution flashover voltage has direct relation with surface resistance, the bigger the surface resistance, the higher the pollution flashover voltage.
5.
The plasticity of silicon rubber material is high, and it is easy to shape, which is helpful in optimizing and selecting insulators’ structure and shape, with little pollution and higher pollution flashover voltage.
15.5.6.2 Reliability of Composite Insulators for ±800-kV Lines
1.
Application of composite insulators in DC lines in China
China takes the world lead in the study and manufacture of composite insulators. So far, more than 3,000,000 composite insulators have been put into operation in China, and some of these have been in operation for up to 17 years. In 1995, China developed DC composite insulators successfully and put them into operation. Currently, up to 11,000 composite insulators are used in the Gezhouba–Nanqiao line, Tianshengqiao–Guangzhou line, Longquan–Zhengping line, Sanxia–Guangdong line, and Guizhou–Guangdong line. Seventy-five per cent of insulator strings in the Gezhouba–Nanqiao line and one-third of insulator strings in the Longquan–Zhengping line have been replaced with composite insulators. In new lines (including 500-kV AC/DC lines), composite insulators made of high-temperature vulcanized (HTV) silicon rubber have been widely used. In particular, in the Guizhou–Guangdong ±500-kV double-circuit line, suspension insulators are all composite insulators.
In February 2004, CEPRI made an investigation into operating conditions of the Gezhouba–Nanqiao line and Longquan–Zhengping line. It was found that among the 55 towers along the lines, 14 were installed with composite insulators (including jumper strings of tension tower), and among the remaining 41 towers installed with porcelain insulator strings, at least two-thirds experienced discharge of varying degrees, while towers installed with composite insulators were free of any discharge.
For example, in No. 900 tower of the Gezhouba–Nanqiao line, composite insulators and porcelain insulators were used, respectively, on pole 1 and pole 2. It was found that in rainy and foggy days, the former was free of discharge while the latter experienced discharge accompanied with dotted sparks. In No. 1649 tower of the Gezhouba–Nanqiao line, porcelain insulators were installed initially and severe discharge occurred in rainy and foggy days, while after the porcelain insulators were replaced with composite insulators, no discharge occurred. In No. 541 tower of the Longquan–Zhengping line, two parallel strings, each consisting of 38 insulators, were used and suffered discharge accompanied with electric welding light on foggy days; in contrast, the adjacent No. 542 tower which was installed with composite insulators suffered no discharge. In No. 1637 tower of the Longquan–Zhengping line, the porcelain insulator strings suffered discharge on foggy days with sparks on tension strings; meanwhile No. 1636 and No. 1638 tangent towers on both sides which were installed with composite insulators were free of discharge. In No. 1614 tension tower of the Longquan–Zhengping line, the tension string was comprised of four parallel strings each consisting of 39 insulators, and the jumper string of pole 1 comprised 44 insulators, yellow sparks were observed on humid days, while the adjacent No. 1615 tower, which was installed with composite insulators was free of discharge. No. 1821, No. 1822, and No. 1823 towers of the Longquan–Zhengping line were subjected to severe pollution from several chemical plants, the former two towers using strings each consisting of 37 XZP-300 porcelain insulators experienced arc discharge on cloudy and rainy days; while the third which was installed with composite insulators was free of discharge.
From 2005 to 2006, CEPRI also conducted investigations on the Jiangling–Echeng line, Tianshengqiao–Guangzhou line, and Gaoming–Zhaoqing line, and confirmed that use of composite insulators can significantly enhance the operational reliability of HVDC lines.
2.
Reliability of composite insulators for UHVDC lines
Most ±800-kV DC lines are erected across untraversed mountains and hills, and composite insulators are convenient for transportation and installation. In addition, use of composite insulators can reduce the length of strings, and height and strength of towers, thereby saving investment in towers. Moreover, composite insulators are maintenance-free, thereby effectively improving the maintenance efficiency and reliability of lines.
CEPRI carried out a study on the reliability of DC composite insulators and the manufacture of DC rod-type suspension composite insulators, and addressed the main technical difficulties in the application of composite insulators to DC lines. A study on the reliability of composite insulators intended for DC lines involved mechanical reliability and electrical reliability. Mechanical reliability mainly involves performance change of heavy composite insulators in the long term; while electrical reliability involves the aging of composite insulators, performance of silicone rubber against ablation due to DC arcing, performance of end fittings against corrosion due to leakage current, and impacts on the core due to ion transfer under DC voltage.
a.
Mechanical reliability. The loads on composite insulator change over long-term operation; however the primary basic load is tensile load. As such, study on composite insulator's long-term mechanical performance mainly focuses on the tensile performance. One of the major characteristics of long-term tensile performance of composite insulators is that, when the loads are above a certain limit, the strength will decline with time; this is known as creep. Currently, the creep slope of composite insulators is generally specified as 8% in international and national standards, which is determined based on tests many years ago, and is not necessarily suitable for current insulators as the crimping technology has been improved largely. As such, to ensure that the strength of insulators after a long period of operation can still meet the load requirements of the line, it is essential to conduct a full study on the creep of strength of insulators to maintain safe operation of insulators for a long time.
In addition, besides static loads from conductors during operation, insulators are subjected to aeolian vibration, subspan vibration, or galloping under the effects of wind, icing, ice shedding, etc. Previous studies have shown that, for internally wedged composite insulators, aeolian vibration has little impact on its mechanical performance given its structural characteristics; however, for a crimped insulator, the impact of vibration on its mechanical performance is unclear, and especially under the effect of galloping with large amplitudes and causing significant load changes, whether the composite insulator can maintain a sufficient mechanical reliability is an issue that must be confirmed.
In order to ensure that the mechanical strength of composite insulators is still no lower than the specified mechanical load (SML) (with a probability of 90%) after 30 years of operation, UHV composite insulators shall be able to withstand 96 h 1.2SML test. In this case, 1.2SML is the load that fittings with a ball head and ball socket can withstand, and thus, can be used to perform tests on real insulators installed with fittings with a ball head and ball socket.
b.
Aging performance. Shed housing provides necessary creepage distance for composite insulators and protects the core from erosion by the atmospheric environment; hence, an excellent aging performance is required. Silicone rubber has a high performance against sunlight radiation, acid, and alkali. Studies show that aging is mainly attributable to arcing, and is known as electrical aging. Therefore, silicone rubber shed housing of composite insulators should have excellent performance against current leakage tracking and electrical erosion. At present, according to relevant international and national standards, performance against current leakage tracking and electrical erosion of composite material samples can be tested with an inclined plane method, and composite insulator products can be tested with a 1000-h salt spray test method, wheel method, and 5000-h multistress test method; however, these test methods all have to be carried out under AC voltage. Due to the different performance of DC and AC arcs, electrical aging tests shall be performed under DC voltage.
c.
Ion transfer. To investigate the impact of ion transfer on mechanical strength of glass fiber and core, first, perform an accelerated aging test on the core that bears certain tensile loads; then perform a tensile failing load test on the core sample with ion transfer; finally, compare the test results with those of core sample without ion transfer. With reference to relevant test standards for cap and pin porcelain and glass insulators, the aging degree of the core of the composite insulator is still expressed as the quantity of electric charges flowing through cumulatively over 50 years.
Through analysis of test results, it is concluded that:
i.
Volume current of core is roughly linear with temperature in a semilog coordinate. An increase of volume current across the test sample resulting from temperature rise is smaller than that of a porcelain insulator, and therefore, the time taken for a composite insulator to complete ion transfer is longer than that for a porcelain insulator.
ii.
Cores made of domestic glass fibers are different from those made of imported glass fiber in components, resulting in a difference in the volume current flowing through, and to be specific, the volume current flowing through the latter is smaller than that flowing through the former.
iii.
Mechanical strength tests of core indicate that, after ion transfer, under long-term DC voltage, ion transfer will not lead to a remarkable decrease in mechanical strength of composite insulators made of low-alkali glass fiber.
iv.
In order to prevent glass fiber containing excessive alkali from being used to manufacture DC composite insulators, a limit shall be imposed on the volume resistivity of core. For example, the standard that volume resistivity of core made of glass fiber shall not be lower than 1010 Ω·m at the temperature of 140°C is specified in technical specifications for composite insulators intended for DC lines. As the test is performed under high temperature and high voltage, it can be deemed that thermal breakdown resistance of the contact face between domestically manufactured core and its housing is also examined.
d.
Measures against electrolytic corrosion. Corrosion of end fittings of the insulator is mainly attributable to current leakage on the surface of a polluted insulator under continuous high-humidity conditions. The corrosion of end fittings on the positive pole side generally starts from the zinc coating at the joint of the sealing layer and fittings, and then extends to the body of fittings. If the corrosion is not severe, only the zinc coating is damaged, causing rusting of fittings, while the mechanical performance of joint connecting fittings and the core will not be affected. However, if the electrolytic corrosion leads to damage to the zinc coating between the sealing layer and fitting or to the sealing layer, the leakage current may flow into fittings directly, causing corrosion of the fittings and thereby threatening the core directly. In addition, water vapor may also flow into fittings, which is likely to cause brittle fracture of the core.
The special investigation of CIGRE shows that, after the composite insulators used in coastal areas of New Zealand’s Inter-island DC Transmission Line had been in operation for 6–7 years, severe corrosion occurred on end fittings. According to Faraday’s law, weight loss of metallic materials due to electrolytic corrosion is dictated by the magnitude of leakage current and operation time of insulators. Data measured during the operation of DC lines in Japan show that the quantity of annually accumulated electric charges on the surface of a porcelain insulator is 200–300 C in inland areas and 500 C in coastal areas. It may be up to 1500–3000 C in heavily polluted areas, for example, the coastal area of New Zealand’s Inter-island DC Transmission Line. According to data observed at the natural dust accumulation test station set up by CEPRI for the DC line from the right bank of the Three Gorges to Shanghai, the quantity of annually accumulated electric charges on the surface of a porcelain insulator due to leakage current is 600 C. And data observed at the offshore natural pollution accumulation test station in Sweden indicate that the leakage current on the surface of silicone rubber insulators is half that of a porcelain insulator during saline pollution storm, and the maximum pulse current is roughly the same as that of a porcelain insulator. In addition, the density of current flowing through the surface of end fittings of composite insulators is one-third to half of that flowing through the steel pin of cap and pin insulators. It is estimated that such a degree of corrosion is high enough to cause damage to the zinc coating of end fittings.
To prevent electrolytic corrosion on end fittings of composite insulators due to leakage current, a zinc collar is installed at the joint between fittings and the core to substitute the fittings to participate in the electrolytic reaction. According to the calculation of weight loss of zinc collar due to electrolytic corrosion, the exposed part of the zinc collar should be 5 g at least. Through design and calculation, it is confirmed that the thickness of the zinc collar made of pure zinc with a purity no less than 99.8% should be 3 mm at least, so as to ensure the installation stability of the zinc collar and to endure possible corrosion depth.
Composite insulators of 210–550 kN are used in UHV AC lines. Table 8.14 summarizes their technical data, and Figure 8.30 gives a picture of composite insulators used in the UHV AC pilot and demonstration project in China.
Table 8.14. Technical Data of 1000-kV Composite Insulators.
No.
Item
Technical Data
1
Rated operating voltage (kV)
1000
2
Rated 1 min withstand tensile load (kN)
210, 300, 420, and 550
3
1 min wet power frequency withstand voltage (rms, kV)
≥990
4
Dry lightning impulse withstand voltage (peak value, kV)
≥3200
5
Wet switching impulse withstand voltage (peak value, kV)
≥1675
6
Visual corona voltage (kV)
≥700
7
Height (mm)
9750/10,530
8
Dry arcing distance (mm)
>9000
9
Creepage distance (mm)
>32,000
10
Markings of fittings for assembly
20, 24, 28, 32
Note: These parameters apply only for areas at altitudes below 1000 m and with a pollution class of B, C, D, or E. For altitudes above 1000 m, corrections should be made according to GB 311.1—2012 Insulation Co-ordination—Part 1: Definitions, Principles and Rules.
Figure 8.30. Composite insulators used in China’s UHV AC pilot and demonstration project.
2.
Design and manufacture
The following summarizes the key technologies involved in design and manufacture of composite insulators intended for use in UHV AC lines to ensure operational safety and reliability of UHV transmission projects:
a.
Eccentricity during injection. A 1000-kV composite insulator is generally 9750 mm or longer, much longer than a 500-kV insulator. This will easily result in eccentricity during injection and, thus, non-uniform thickness of the housing for the core. This problem can be solved by the following means:
i.
Design of an automatic eccentricity location device
ii.
Proper setting of process parameters, including the injection pressure, rate, and volume
iii.
Increase in the design thickness of the housing; the minimum thickness should be no less than 6 mm
iv.
Eccentricity check with the acoustic emission thickness gauge immediately after injection of each half-finished section
b.
Shed mounting. Compared with a 500-kV composite insulator, a 1000-kV AC composite insulator has a longer core, a larger shed diameter, and more sheds.
By improving the manufacturing equipment and processes, insulator manufacturers have successfully developed a semi-automatic shed-mounting device, which can be used for manufacturing 12-m-long composite insulators. In addition, a new high-temperature vulcanization process is used, during which the extruded housing for the core is first vulcanized at high temperatures instantaneously for setting and then vulcanized at high temperatures, with more supports provided in the vulcanization can.
c.
Design optimization of grading devices. With the increase in voltage class, distortion of electric field distribution of the insulator on the conductor side and earthing side becomes more severe, especially for UHV insulators. To solve this problem, the three-dimension simulation calculation technology is used for optimizing the dimensions of the grading device used for composite insulators, such as the structural type, installation location and diameter, pipe diameter of grading ring, radio interference tests, and corona tests, and potential distribution tests are performed, verifying that the grading devices intended for use for various tower types and various types of insulator strings comply with the design requirements of UHV projects.
d.
Technical data of silicon rubber. Compared with EHV or lower-voltage insulators, UHV AC insulators place a higher requirement on silicon rubber. Table 8.15 gives the technical data of silicon rubber. It can be seen from the table that the mechanical data of the UHV insulator are higher than required in DL/T 376—2010 General Technical Requirements of Silicon Rubber Insulation Materials for Composite Insulators.
Table 8.15. Technical Data of Silicon Rubber Insulation Materials.
Item
Unit
Specified Value in DL/T 376—2010
Required Value for UHV Insulator
Tear strength (right-angle method)
kN/m2
≥10
≥12
Ultimate tensile strength
MPa
≥4
≥4.0
Elongation at break
%
≥150
≥200
e.
120% rated mechanical loads 24-h withstand test. This test is added for 1000-kV composite insulators. During the test, 1.2-times rated tensile loads are imposed on regular products. They are considered qualified if not damaged within 24 h. The purpose of this test is to assure a reliable mechanical strength of insulators for a long time, that is, the tensile failing load in the service life of 40 years is no less than the rated load.
f.
Water diffusion test on the rod. The test is intended to verify the compactness and hydroscopicity of the rod made of epoxy and glass fiber and to check whether the interface between the housing and rod is free from defects. During the test, the leakage current for a 1000-kV composite insulator should be no more than 0.1 mA.
For HV circuit breakers, composite insulators are used increasingly in place of ceramic insulators. Indeed, composite insulators are light, resilient, do not explode under impact, have good seismic behaviour, and withstand pollution well. However, the ageing of such insulators is not well known, and their air-tightness is difficult to manage. The design of insulators, as well as choice of contact material, must be considered, because when the arc is formed, metal droplets of contact and nozzle materials will be in contact with decomposed and polluted SF6 (Domejean et al, 1997).
Composite insulators used in HV circuit breakers generally comprise a composite tube (a), metal flanges (b), and elastomeric silicon sheds (c) – see Fig. 5.4. The composite is made of glass fibres and epoxy resin. The sheds are made out of silicone rubber.
5.4. (a–c) Composite insulator structure. See text for explanation.
During their lifetime of about 30 years, insulators must withstand the temperature requirements and heavy atmospheric conditions, and support mechanical stresses. Therefore, the qualification procedures of such insulators are very demanding, and are described in the IEC 61462 International Standard.
4.2.3.2 Physical degradation and failure (e.g. cycling)
The physical degradation of composite insulators occurs via numerous and sometimes competing mechanisms. The utility pole case study thereafter shows the diversity of environmental loads responsible for degradation of high voltage line components.
Case study: Utility poles
Thanks to corrosion resistance and excellent dielectric properties glass fiber reinforced composites are being introduced as poles supporting transmission lines. This application is a large market for composite materials. Indeed, in most developed countries, transmission lines were installed around 50 years ago and need replacement within the next 10 years. The North American utility pole refurbishment market alone is estimated at one billion dollars [21].
Utility poles are constantly exposed to mixed environmental loads such as moisture, animal and insect damage (e.g. repeated impact from woodpeckers), wind, ice, ultra violet radiation or guy and brace forces. Wood, concrete and steel are traditional materials for power lines but replacement by composite materials provides many benefits ranging from weight savings to higher energy absorption upon vehicle impact. Indeed, the composite poles are expected to have lifetimes three times longer than wooden ones. Typical weight savings obtained by using polymer composites are around 50% for wood and 30% for steel poles replacement. Such drastic weight differences translate in significant cost savings for installation and transportation. For example, larger quantities of poles can be carried on a truck (Figure 4.20) or composite pole installations in remote areas may necessitate smaller investments, such as the use of smaller helicopters (Figure 4.21).
Figure 4.20. Composite pole installation.
(Courtesy of Strongwell.)
Figure 4.21. Helicopter installation of composite pole.
(Courtesy of Strongwell.)
Glass-fiber reinforced polymers can be tailored to provide excellent insulation characteristics, thereby enhancing safety during repair and maintenance. Lightning strikes on concrete poles were seen to induce flashovers resulting in surges large enough to damage household appliances [21]. The use of glass-fiber reinforced polymers can help prevent such occurrences.
Carbon-fiber reinforced poles can also exhibit an excellent resistance to fire: unlike 3000 wooden poles that were fully destroyed, a Powertrusion composite pole installed in San Diego for demonstration survived the 2003 California fire which destroyed over 2000 km2 of land [19].
Stricter environmental regulations are further motivating the use of composite materials. Indeed, wood poles are usually treated with preservatives (creosote, copper chromium arsenate or pentachlorophenol) which are detrimental to the environment and in the process of getting banned [20].
Despite those advantages, the introduction of composites as utility poles and cross-arms that started around 40 years ago is still very slow. Grid providers and utility companies constitute a group of very traditional industries and innovations are accepted under the condition of the demonstration of multiple operating references. Despite original difficulties in the early introduction, composite cross-arms are generally better accepted than poles by the transmission community, probably due to the fact that the product can be cost-competitive even on a as sold basis. In the 1960s and 1970s, composite cross-arms degraded strongly with UV radiation and exhibited fiber blooming. Erosion of the matrix would lead to fiber exposure on the surface [21]. This problem was solved by applying more elaborate UV protections such as inhibiting polyurethane-based paints. Today, necessary ultra-violet protection and required esthetically pleasant surface finish can also be achieved in one step by using a polymer film such as a polyester veil.
Poles are traditionally being manufactured using filament winding or pultrusion process. The fiber reinforced profiles are then generally filled with foam to eliminate nesting pest problems. Pultrusion manufacturing allows precise fiber orientation. The use of multiaxial fabrics and continuous strand mat results in excellent axial and bending strength even for large poles. Obtaining the proper axial strength by filament winding however can be challenging due to the natural difficulty to orient fibers axially. Despite this, 21 m Class 1 filament wound poles were successfully manufactured and 26 m poles achieved for lower classes [35].
In addition to the environmental factors detailed in the various chapters of this book, two mechanisms tightly related to the electrical field environments accelerate the materials degradation, namely partial discharge erosion and tracking and surface erosion.
By definition, composite materials possess large intern interface surfaces. These interfaces are preferred locations for defects such as contamination and voids. Voids can, for example, facilitate moisture absorption. Moisture usually decreases resistivity and dielectric strength while increasing the permittivity of the composite [1]. Voids might also favor internal erosion. Indeed, cavities in the composite are generally filled with a gas of lower dielectric strength. Therefore, breakdowns within the cavity, also called partial discharges, can occur. Discharges occurring in air are a special case of partial discharges and are generally referred to as corona emissions.
Partial discharges occur only in cavities and not in the material. However, the electrons traveling in the cavity hit the materials surface and might induce chain scission and irreversible material degradation. This erosion phenomenon increases the size of the cavity, which increases in turn the partial discharge process.
Partial discharge erosion is a major concern for many electrical applications. The size and the number of voids in the composite need to be precisely controlled for medium and high voltage applications. In power cables, no voids that could be seen (i.e. bigger than about one micrometer) would be tolerated [17]. Partial discharge erosion is often a limiting factor for the use of long-fiber composite materials in which the presence of microscopic voids is almost inevitable.
To make matter worse, ozone (O3) is created as a by-product of air cavity discharges. Therefore, an excellent resistance to ozone exposure is required for composite insulators. Indeed, partial discharges are inevitable in the composite and generation and diffusion of ozone will occur (see Chapter 3 for gaseous diffusion). Therefore, experimental methods of Chapter 3 should be used to insure that ozone does not create irreversible damage in the material.
The loss tangent was shown to be a powerful tool in measuring the degree of damage in the composite. Partial discharges can also be used to assess the degree of void content in the material. However, loss factor increase and partial discharge measurements usually do not coincide perfectly. Indeed, loss tangent (tan δ) measurements are influenced by the defects in the material as well as by polarization mechanisms (which do not mean degradation) when partial discharges are affected only by the voids: unlike partial discharges, loss measurements are, for example, strongly influenced by relaxation mechanisms and the viscoelastic nature of the composite. Furthermore, partial discharges measurement methods heavily rely upon statistical analysis and empirical rules, and do not always provide a clear picture of the state of damage in the material. Indeed, under the presence of the electric field, a given void might experience a partial discharge which can be measured. However, repeated discharges in the cavity may lead to carbonization in which a thin carbon layer forms on the surface of the cavity. The conductivity of this thin layer might suffice to equalize the potentials and new measurements show a recession in the partial discharge activity even though the void is still present. The thin carbonized layer does not, however, significantly influence the tan δ measurements due to the small dimensions of the layer. Therefore, a recession in the partial discharge activity (indicating that the bar would actually become better) should be clearly distinguished from the loss relaxation phenomenon discussed in Section 4.2.2.4 (that also showed a bettering of the insulation).
Based on these observations, partial discharge measurements should be interpreted with care and it is commonly agreed within the industrial community that such measurements are more an art than a science.
Discharges can also occur on the surface of the material. At the surface of insulators, an erosion phenomenon similar to the cavity process can take place. This erosion can be accompanied by an accumulation of carbon on the surface creating a conductive area (tracking). Moisture accumulation in this conductive layer results in low surface resistance and losses. The losses produce heat which tends to dry off some moisture from given areas. Discharges can then occur between dry and wet areas leading to further damage and eventually to carbonization. Carbonization on the surface can be such that the insulation cannot withstand the operation voltage anymore. Surface discharges in air can also combine with ambient molecules to form corrosive gases. This is the case of PE where surface discharges result in nitric acid when combined with ambient moisture. The nitric acid will in turn attack the insulation resulting in irreversible damage [1].
The use of nonceramic and composite insulators in place of traditional porcelain and glass insulators for line insulation has become widespread in the last 20 years. Such insulators have several advantages over porcelain and glass insulators, such as lighter weight, easier handling, better resistance to damage from vandals, lower cost (in some countries), and superior contamination performance. Different material families have been used for the exposed part of the insulator (hereafter called the housing). High temperature vulcanized (HTV) silicone rubber, ethylene propylene rubber (EPR), cycloaliphatic epoxy, and EVA are among the materials proven suitable for outdoor use, with the first two varieties dominating for transmission voltages.
A nonceramic and composite insulator's performance under polluted conditions merits careful consideration. For porcelain or glass insulators, flashover resulting in a temporary outage is the end result of a contamination event. This does not usually cause any major permanent damage to the insulator string although burning of the glaze and/or fragmentation of the bells in contact with the power-follow fault current can occur. There is little risk of the insulator failing mechanically. It is also unlikely that the inorganic dielectric is degraded due to surface discharge activity, which could be long-lasting.
For nonceramic insulators, there is actually less risk than with the porcelain or glass insulators from the flashover event itself due to the elastic nature of the material. But it is from the surface discharge activity that the exposed insulation can be subjected to degradation, and this can be a major concern. In addition, the organic nature of the insulating materials can make them vulnerable to degradation from natural elements, such as heat, UV from sunlight, moisture, and chemicals. A permanent reduction of their desirable properties under service conditions can occur with time, referred to as aging. It is also important to note that some degradation modes may actually occur even in clean conditions, such as from exposure to corona activity. In fact, mechanical failure of nonceramic insulators from a mode of failure called brittle fracture has been experienced in relatively benign outdoor conditions. Users should be aware of all these possibilities.
Despite these concerns, it should be said that judicious selection and application of nonceramic insulators has resulted in improved reliability and lower installation costs for both transmission and distribution lines. Progress at all fronts, namely research, development, testing, manufacturing, and usage, has made this possible.
Just from geometrical considerations alone, nonceramic insulators should offer superior performance under contaminated conditions when compared to their porcelain and glass counterparts due to their smaller diameter. Additional improvement in contamination performance can be obtained by using materials that are hydrophobic, hence suppressing leakage current and discharge activity; they can remain in this state for a long time in service. Silicone rubber is one type of material that fits into this category. Within a particular material family, the leakage current suppression capability is dependent on the formulation, but in general, silicone polymers have better leakage current suppression capability than other outdoor insulating materials.
The typical practice is to use a leakage distance similar to porcelain for silicone rubber insulators; and for nonceramic insulators employing materials other than silicone rubber, the leakage distance is about 20 to 30% higher than the distance for porcelain insulators.
2.2.3.1 Examples of UV detection on line equipment
1.
Detection covering the grading ring. Owing to thoughtless design or careless construction, some composite insulators have no grading ring installed, and this leads to a corona, as shown in Fig. 2.28.
Figure 2.28. Short-shipped grading ring leading to corona.
Fig. 2.28 shows a composite insulator without a grading ring; the electrical field is seriously distorted, and the corona is strong in the whole end. Compared with the other end with a grading ring, the number of the corona ultraviolet photons is about five times as many (the number of the ultraviolet photons is 5000/min, and less than 1000/min with a grading ring). The grading ring needs to be installed when performing maintenance at power off.
The rough surface of the grading ring leads to a corona, as shown in Fig. 2.29. In the situation depicted in Fig. 2.29, it is sunny and windless with a temperature of 22°C, and relative humidity of 40%. The observation spot is 45 m away from the insulator, and the number of ultraviolet photons is 1927/min. Meanwhile the same kind of equipment on the side phase and the adjacent tower is observed, and no corona appears. Therefore, design-based causes of issues arising in the field can be excluded, basically. It is sunny, with no relative humidity, e.g., dense fog, condensation, drizzle, sleet, etc., so pollution on the surface of the grading ring can be excluded basically as a cause. Therefore, it can be assumed that the abnormal corona results from the distortion of the electric field caused by the roughness of the grading ring surface.
Figure 2.29. Roughness of the grading ring surface leading to corona.
Compared with similar ultraviolet photons with the general count value of 300–800/min, the ultraviolet photon of this abnormal corona image has a larger count value, which needs to be listed in the plan for maintenance at power off.
2.
Detection of inferior insulators. In the laboratory, the power frequency voltage is imposed respectively on the 110 kV insulator string, including inferior insulators, the cracking part of the composite insulator housing, and the broken part of the composite insulator shed. Their ultraviolet images are observed when they are discharging through the ultraviolet imager, as shown in Figs. 2.30–2.32.
Figure 2.30. Under the power frequency voltage, the discharging ultraviolet image of the insulator ring when it contains inferior insulators.
Figure 2.31. Under the power frequency voltage, the discharging ultraviolet image of the cracking part of the composite insulator housing.
Figure 2.32. Under the power frequency voltage, the discharging ultraviolet image of the broken part of the composite insulator shed.
At this moment, as regards the insulator string and the composite insulators on the actual lines, their discharging ultraviolet images are observed when they have defects revealed through the ultraviolet imager, as shown in Figs. 2.33 and 2.34.
Figure 2.33. Discharging ultraviolet image of the third piece of the insulator, which is the inferior insulator by the conductor side on the transmission line.
Figure 2.34. Ultraviolet image of corona discharge at the end of the composite insulator on the transmission line.
When the transmission lines are measured on the spot, the appropriate cars or helicopters can be used for vehicle detection or airborne detection, according to actual requirements. Figs. 2.35 and 2.36 show the conditions of vehicle UV detection and airborne UV detection in foreign countries.
Figure 2.35. Vehicle UV detection.
Figure 2.36. Airborne UV detection.
3.
Detection with polluted insulators. The ultraviolet imager is used to study the surface discharge process of the polluted composite insulator. Through the test, the ultraviolet imaging figure of the composite insulator and the corresponding leakage current waveform figure are obtained under the condition of different salt densities, as shown in Fig. 2.37. The distance between the ultraviolet detector and the composite insulator is S=3 m. Under the condition of different salt densities, the relation between the leakage current Ileak and the number of ultraviolet photons N is seen in Table 2.6.
Figure 2.37. Ultraviolet imaging figure of the composite insulator and the corresponding leakage current waveform figure (A) initial corona stage (B) stable corona stage (C) glow discharge stage (D) arc discharge stage.
Table 2.6. Under the Condition of Different Salt Densities, the Relation Between the Leakage Current Ileak and the Number of Ultraviolet Photons N
Discharge Stage
Salt Density (ESDD) (mg/cm2)
0.042
0.094
0.14
0.25
Ileak (mA)
N(times/s)
Ileak (mA)
N(times/s)
Ileak (mA)
N(times/s)
Ileak (mA)
N(times/s)
Initial corona stage
0.5–1
50–300
1–2
200–4300
2–3
200–500
3–5
500–1000
Stable corona stage
1–2
500–1000
1.5–3
1000–2000
4–7
500–1000
8–10
1000–2000
From Fig. 2.37 and Table 2.6, we can see that the leakage current and the number of ultraviolet photons increase with the increase of salt density. According to the characteristics of the waveform changes for the leakage current, the corona stages observed through ultraviolet imaging can be divided into four, i.e., the initial corona stage, the stable corona stage, the glow discharge stage, and the arc discharge stage (see Fig. 2.37).
a.
Initial corona stage: ultraviolet photon counting begins; the distribution area of the photons is focused only on the junction between the high-end core and the steel cap of the insulator. The corresponding leakage current waveform is the standard sine wave.
b.
Stable corona stage: the number of the ultraviolet photons increases, and the distribution area expands around the shed at the high-voltage end of the insulator. The corresponding leakage current waveform is an approximate triangular wave.
c.
Glow discharge stage: the distribution area of the ultraviolet photons continues to expand, and a large number of ultraviolet photons are also generated around the shed at the low-voltage end of the insulator. The corresponding leakage current waveform contains a richer harmonic wave.
d.
Arc discharge stage: the number of the ultraviolet photons increases greatly, and the distribution area covers the whole insulator. The corresponding leakage current waveform appears like saw teeth.
4.
Detection of lines and fittings. Corona phenomena are very common in conductors and fittings. Besides coronas caused by broken strands and serious scratches, coronas caused by pollution, burr, a slight deformation, or scratches will not have obvious effects on the equipment itself. Those corona phenomena are usually listed as general problems and observed more commonly. However, if a stronger corona occurs in some part of the conductor, instruments such as a telescope or an infrared instrument shall be used to coobserve it, and to judge whether there are broken strands, loose strands, serious surface damage, or other phenomena (see Fig. 2.38).
Figure 2.38. Abnormal coronas of line fittings.
5.
Detection on the line terminal. When Fig. 2.39 was shot it was sunny and windless with a temperature of 31°C, and a relative humidity of 60%. The discharge occurred at the connection of the 110 kV line terminal and the cable, and it was strong. Whether the cause of the discharge is severe pollution or the cracks formed on the porcelain sleeve surface, such a strong discharge will have a serious impact on the equipment itself; and so the equipment should be listed in the maintenance schedule.
Figure 2.39. Abnormal coronas of line terminal.
6.
Detection on the discharge gap of the optical fiber composite overhead ground wire (OPGW). Fig. 2.40 shows the ultraviolet imaging of a 500 kV line into and out of a transformer substation. The figure shows that when the lines were operating normally, discharge phenomena occurred constantly in the interior of the gap, and a clear sound of discharge could be heard, owing to the small size of the OPGW discharge gap. After analysis, it is considered that the discharge occurred because the OPGW discharge gap was not large enough. Since the discharge is very strong, it should be put into a maintenance schedule, and the discharge gap adjusted in power-off maintenance.
Figure 2.40. Inadequate discharge gap leading to corona.
The check contents of insulators and fittings are as follows:
1.
Whether porcelain insulators have breaks or cracks, and whether the glass insulator is damaged.
2.
Whether the composite insulator cluster parachute has breaks and burned areas, and at the same time whether the fitting and grading ring has deformed, twisted, and corroded.
3.
Whether the insulator has traces of flashover.
4.
Whether the insulator string has any serious deviation.
5.
Whether the fittings corrode, deform, wear, or have cracks, and whether the cotter pin or the spring pin has a defect or has come off.