Composite Insulators for Application on UHV AC & DC Transmission Lines
UTILITY PRACTICE & EXPERIENCE
Composite Insulators for Application on UHV AC & DC Transmission Lines
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INMR® Q3 2010
CompositE insulators
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appliCation
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uHV aC & dC transmission linEs
Introduction The newly constructed 1000 kV AC and ± 800 kV DC transmission lines presently operating across China’s vast expanse pass through areas with varying degrees of pollution. Because of this, the decision to specify composite insulators for these lines has been seen as the most effective way to achieve reduced string lengths and tower heights and in the process lower construction costs as well. It was also expected to offer additional long-term benefits such as decreasing the need for future maintenance work (such as deenergizing lines to manually clean insulators) while at the same time increasing both reliability and availability. However, the electrical and mechanical requirements for composite insulators used at these types of voltage levels are extremely high. This article, contributed by Su Zhiyi and Yin Yu of the Electric Power Research Institute in Beijing as well as Liang Xidong and Wang Jiafu of Tsinghua University, reviews such key issues as how maximum performance of the core rods and shed material has been ensured for composite insulators currently in service on Chinese UHV AC and DC lines. It also discusses how creepage distance has been selected and corona ring design optimized to improve axial electric field distribution.
In recent years composite insulators have begun to be broadly used on overhead transmission lines in China (including at 500 kV AC and DC) to an estimated level of about 2.9 million units annually. For the most part, these insulators have been constructed using glass fiber reinforced epoxy resin core rods and high temperature vulcanized (HTV) silicone rubber sheaths. In addition, the latest production methods have been used for molding the housings so as to ensure high performance at the critical rod-to-shed interface, while attachment of end fittings has been done with modern crimping techniques. Still, because of the voltage and capacity at UHV, the demands in regard to electrical and mechanical performance for such insulators is significantly greater than for most other transmission line applications. More stringent design margins and test requirements must therefore be put into place for composite insulators in order to assure maximum long-term reliability.
Electrical Performance Requirements at UHV 1. Choice of Creepage & Practical Engineering Applications On the new Chinese 1000 kV AC overhead lines, the border phases on tangent towers are I-string while the middle phase is V-string. The V-string configuration is also used for tangent towers on ± 800 kV DC lines and is regarded as allowing for reduced pollution deposition and more efficient natural washing by rain as well as increased ice flashover voltage in areas prone to icing. Two methodologies have been used in China to select the creepage distance of UHV composite insulators: one based on the pollution flashover characteristics of porcelain insulator strings combined with operating experience on existing 500 kV lines; the second based on the pollution flashover characteristics of composite insulators. Figure 1 shows the pollution flashover characteristics of both porcelain and
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CompositE insulators for appliCation on uHV aC & dC transmission linEs
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Q3 2010 INMR®
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Photo courtesy of State Grid Corp.
Typical 1000 kV AC tangent tower.
Typical V-string used at ± 800 kV.
composite insulators used on 1000 kV AC and ± 800 kV DC lines.
for maximum operational reliability, creepage distance and length of the composite insulators used on ± 800 kV DC lines was increased even further.
According to the withstand voltage curves from Fig. 1a, the required creepage distance for 1000 kV AC composite insulators used in medium to heavy pollution areas at altitudes of less than 1000 m should be from 27,970 to 32,000 mm (corresponding to an insulator length of 7.6 ~ 8.7 m). These insulators would have alternating shed geometry and a ratio of creepage to arcing distance of 3.68. Moreover, as a precaution to ensure such creepage, insulator length would be increased by 20%. Because pollution performance is even more critical at DC, V-type composite insulators were selected for China’s first new ± 800 kV DC lines. According to the withstand voltage curves in Fig. 1b, the creepage distance of these insulators had to be at least 28,520 mm (i.e. an insulator length of 9.2 m) in areas of light pollution and below 1000 m altitude. In this case, the geometry would be one big and two small sheds, with a ratio of creepage to arcing distance of 3.1.
2. Voltage Distribution & Choice of Corona Rings Usually, voltage distribution along a UHV DC insulator is highly non-uniform and greatly affected by ionization due to corona. The proper design of grading a. AC 1000 kV porcelain and composite insulators
INMR® Q3 2010
A three-dimensional finite element methodology was used to calculate the electric field on an I-string at 1000 kV AC and a V-string at ± 800 kV DC. The effect of the tower and conductor were also considered in the calculation, yielding a general design scheme as shown in Figure 2. For UHV composite insulator strings, one big and one small ring have been placed on the conductor side and a single ring on the tower side. Such a configuration was found to offer improved electric field distribution along the insulator surface, especially at the end-fittings on the conductor side as well as the interface of the sheath with the core rod. 3. Requirements for Core Rods and Rod-Sheath Interface
b. DC ± 800 kV porcelain and composite insulators
In order to improve electrical performance at the interface of the core rod and sheath, desired test
Fig. 1: Pollution flashover characteristics of porcelain and composite insulators used on UHV lines.
For medium to heavy pollution areas below 1000 m altitude, the creepage distance required would increase to a minimum of 31,310 mm (i.e. an insulator length of 10.1 m). Moreover,
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rings in such applications is therefore extremely important.
CompositE insulators
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appliCation
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PREVIEW
uHV aC & dC transmission linEs
Photo: INMR ©
Different types of corona rings used on ± 800 kV DC lines in China.
values for the rod material and quality of sheath bonding were increased: • Water diffusion test for rods Specimens were cut from normal insulators having silicone rubber sheaths that included not only the rod but also the adhesive interface between it and the sheath. The allowed leakage current, that according to IEC 61109:1992 should be no more than 1 mArms, was then modified to be not more than 0.1 mArms. • Dry AC voltage test for interface and connection to end fittings Insulator specimens were taken directly from the production line of suppliers and their arcing distance tested. These specimens were then required to withstand 80% of the flashover voltage for a period of 30 min, after which the temperature rise of the shed and sheath was immediately measured. The test values
Fig. 2: Schematic representation of UHV composite insulator and grading ring.
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deemed acceptable were required to be only half that allowed as per IEC 61109:1992.
Mechanical Performance Requirements for UHV UHV overhead lines typically employ conductors with large cross-sections to satisfy the demands for large transmission capacity. For example, the 1000 kV AC project in China has used 8 × 500 conductors and 8 × 630 conductors for double-circuit towers. Similarly, the ± 800 kV DC projects have used 6 × 630 and 6 × 720 conductors. Based on this, the specified mechanical load (SML) for composite insulators selected for such lines has been required to be at least 300 kN. 1. Long-Term Mechanical Test & Acceptance Criteria • Mechanical test on crimped composite insulators with high SML At present, according to such international standards for composite insulators as IEC 61109, the mechanical load-time characteristic is examined under the requirement of 8% MAV per decade, i.e. three insulators must withstand 60% MAV for 96 h each. This requirement is based on long-term test results with different types of end fittings twenty to thirty years ago. Currently, under IEC 61109, the
CompositE insulators
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mechanical load-time performance of composite insulators is examined by means of relative standard deviation where σ=8% of short-time failure load (MAV) while the slope of the creep curve is k=8% MAV/logarithmic time scale (minute). This means three insulators should withstand 60% MAV for 96 hours each, i.e. MW = MAV (1–k1gt)(1–1.82σ), where MW is the 96 hours withstand load. For most major manufacturers in China, the relative standard deviations (σ) of crimped composite insulator failure loads distribute mainly in the range of about 2~3% and their creep slope (k) is usually less than 4%. For 300 kN specimens, k usually distributes between 2.49~3.82% while, in the case of 400 kN specimens, k is usually less than 3.58%. Considering the difficulty in controlling dispersion of these parameters in mass production, k=4% and σ=5% have been selected as the maximum allowable values for China’s UHV projects. According to this criterion and at the test load over 96 h, 1.82σ should be 77.2% of MAV (the integer 75% MAV has therefore been chosen to take the place of the 60% limit as per the current IEC standard). • Test requirements of composite insulators with high SML used for UHV overhead lines Figure 3 shows the curve of load in service and mechanical withstand load for composite insulators.
uHV aC & dC transmission linEs
Because of the improved creep performance of composite insulators, it can be ensured that their mechanical strength will remain at no less than their SML even after 30 years in service (at a 90% probability level). The short-time failure strength factor x can be calculated by: M30 = xSML(1-7.20k)(1-1.28 σ) where 7.20 corresponds to log time unit of 30 years and 1.28σ corresponds to the 90% single probability. If k=4% and σ=5%, the result is x=1.50. As such, MAV ≥1.50 SML is the lowest requirement of UHV composite insulators and the 96 hours withstand test load shown as SML can be calculated as 0.772 MAV � 0.772 × 1.55 SML=1.20 SML.
Using 1.20 SML to replace 75%MAV means that insulators from different manufacturers could be examined under the same test standard, because insulators with the same SML but from alternative suppliers may have different failure loads. Meanwhile, a 1.2 SML tensile load is acceptable for ball and socket coupling, so that actual such insulators can be directly examined. • Recommendation of test standard In China’s UHV tender documents, the tensile load of the mechanical loadtime test has therefore been modified from 60% (as per IEC 61109) to 75% of MAV, and to withstand 96 h with no
Fig. 4: Dynamic fatigue property of composite insulators for UHV.
failures. At the same time, to ensure uniform and reliable high quality, measuring the creep performance of composite insulators is added as a sample test. In this regard, three insulators are selected at random from the production line and subjected to a load of 120% of the SML for a duration of 96 hours. Using the SML as the reference value makes it easy to examine insulators from different manufacturers according to an identical test standard. • Dynamic mechanical performance under conductor galloping
Fig. 3: Curve of load in service and mechanical withstand load of composite insulators.
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CompositE insulators
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Apart from the static mechanical loads of conductors, composite insulators for UHV applications must also be able withstand different types of dynamic loads during service. These can vary from vibrations due to wind and sub-span oscillations during normal weather to jumping from ice-shedding during winter. Considering such broad differences in load situations, special attention in the case of UHV projects has been paid to low frequency, high amplitude oscillations, i.e. galloping. According to the typical characteristics of conductor galloping, the overall dynamic load applied to suspension insulators includes a vertical vibration element, a horizontal torsion element and a flexing-type element, although the values of each load are often difficult to estimate. Based on discussions with various research and design institutes in China, the dynamic mechanical test for 300 kN composite insulators is therefore considered basically as a 0 ~150 kN alternate load test, with the torsion and flex loads omitted. For example, dynamic mechanical tests have been carried out with maximum loads of 150 kN, 100 kN and 50 kN. Test results show that for those specimens with good control of crimping, damage usually occurs at the ball end-fittings. Failures of rods were all at the crimp stress concentration points (i.e. the ‘intracavity’ of the end-fitting). After 0 ~ 50 kN testing for 3×106 cycles, the residual failure
uHV aC & dC transmission linEs
strength of unbroken specimens should display no obvious differences from the new specimens. Figure 4 shows the fatigue property curve of composite insulators under different loads. As part of China’s UHV tender documents, fatigue testing for composite insulators is provisionally being carried out as per the following: the test specimens are 300 kN composite insulators while the continuous alternate load applied to them is 0 ~ 100 kN. Total cycle times are required to be greater than 5×105 before any specimens break.
mechanical withstand test for 96 hours and inspection of slivers. During production, mechanical strength must be verified on a minimum percentage of rod specimens. • Use of acoustic emission inspection Each manufacturer judged qualified to bid on supplying insulators for UHV projects in China is required to submit their own proven methodology for using the acoustic emission (AE) system. Moreover, the AE wave for each product manufactured must be recorded so that it is available if requested under a random spot-check.
Crimping Technology to Assure Mechanical Reliability Composite insulators used in China on 500 kV or higher AC and DC overhead lines are made only with crimping technology having a high level of precision. Still, to ensure that every insulator will perform reliably when used for UHV projects, the crimping process of qualified insulator manufacturers has been strictly monitored.
1. The creepage distance and insulation distance required for UHV AC and DC applications can to some extent be extrapolated from the operating experience of 500 kV composite insulators. Nevertheless, it has been decided that the creepage distance in the case of UHV composite insulators should have a larger insulation margin. 2. New structural designs and optimized dimensions of corona rings have been developed for UHV composite insulators based on electric field calculations. The ring on the high voltage side consists of big and small ‘double rings’ (the larger ring to improve electric field distribution on the conductor side and the smaller ring to reduce electric field at the endfitting and to protect the seal). 3. A new design principle in regard to the mechanical performance of composite insulators has been proposed in the case of UHV applications. During type testing, short specimens should be used to verify the 96 h withstand load at 75% MAV; in sample testing, three insulators should be selected at random from production and used to verify the 96 h withstand load at 120% of SML. These methods will best ensure that the mechanical strength of these insulators will be no less than their SML even after 30 years service (at a 90% probability level).
• Control of materials The dimensional tolerance of each core rod used on a composite insulator for a UHV line must be measured and its Young’s modulus sampled. In addition, the dimensions of endfittings are required to be verified and ultrasonic inspection to be conducted on each unit. Moreover, the mechanical strength of a sample from each batch of fittings has to be tested with the standard deviation of failure strength allowed to be no higher than 5%. After polishing, the diameter of each rod must be measured again with the tolerance of the fit between the rod and end-fitting falling only in the range of 0.05 ~ 0.10 mm.
Conclusions
• Change in end-fitting connecting mode
To avoid steady pin deformation in V-string insulators, the end joint mode has now been changed from ball-andsocket to ring-and-ring type for V-strings on all Chinese UHV DC overhead lines. • Control of crimping technology The cross arm and conductor clip are For each batch of rods and end-fittings then connected to the insulator end used on UHV lines in China, a number fitting by a U type ring. of crimping tests are required initially In order to avoid excessive torque in order to optimize all the various process parameters. This also requires during service, the rings at the two ends are staggered 90°. that tests be performed such as crimping under different parameters, short-time mechanical tests,
4. A dynamic mechanical test method was recommended for composite insulators with high SML requirements. According to this methodology, a continuous alternate load of 0 ~ 100 kN is applied to the specimens and total cycle times should be more than 5×105 until they are broken. 5. Several additional specific measures were presented to ensure the long-term mechanical reliability of composite insulators with high SML values. 6. Although these new test methods were presented mainly for composite insulators used on UHV transmission lines, it has been suggested that composite insulators used on extra high voltage (EHV) lines and other important transmission lines in China be required to follow them as well.
CompositE insulators for appliCation on uHV aC & dC transmission linEs
PREVIEW
Q3 2010 INMR®
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