CHAPTER 2 STRUCTURE AND CRITICAL E-FIELDS

Transcription

1 25 CHAPTER 2 STRUCTURE AND CRITICAL E-FIELDS 2.1 INTRODUCTION Polymeric insulators have been commercially produced and utilized for more than four decades now. Their demand is still increasing rapidly due to its superior performance. This has resulted in large-scale research investigations aimed at improving the in-service operation such that it could last for at least thirty to forty years, as in the case of ceramic insulators (Rahisham Abd Rahman 2012). The major area of concern in improving the in-service operation of polymeric insulator is its E-field distribution and control. The E-field distribution and discharge activity in and around the polymeric insulators are interlinked with each other. The location and magnitude of discharges depend on the magnitude and direction of the local E-field. Maximum magnitude of E-field should be kept below the critical values, in order to prevent or reduce the discharge activity. The E-field distribution along polymeric insulators can be calculated either by measurement or by numerical methods. Measurement methods are expensive, time consuming and cannot be measured in regions of interest (i.e., internal to the insulator or close to the rubber weather-shed surface). Hence, numerical technique seems to be a better option. Currently,

2 26 FEM (Finite Element Method) and BEM (Boundary Element Method) are most frequently used for E-field calculation (Andrew Phillips et al 2008). To start with, this chapter provides an overview on the structural components of polymeric transmission line insulators. Next, the significance of E-field along with its critical values and control techniques are presented. Finally, the available dimensions of insulator and rings from the manufacturer s catalogue and literature are collected and presented for all voltage classes starting from 66 kv to 1200 kv. Based on which, the dimensions for carrying out the E-field analysis is chosen. 2.2 STRUCTURE OF POLYMERIC INSULATOR The basic structure of polymeric insulator (Figure 2.1) used in overhead transmission line consists of three major parts: Central core made of Fibre Reinforced Plastic (FRP) Housing made of Silicone Rubber (SiR) End fittings made of metal Fibre reinforced Plastic (FRP) Polymeric housing (Silicone Rubber) End Fitting (L.V) End Fitting (H.V) Triple junction (L.V) Triple junction (H.V) Figure 2.1 Structure of Polymeric Insulator The design of each of the component needs to be optimized to yield satisfactory electrical and mechanical performance over the life time of polymeric insulators.

3 Central core The core of the polymeric insulator has the dual burden of being the major insulating part and the load (tensile, cantilever and compressive) bearing member. The core is made up of fibreglass reinforced polymer (FRP) rod. The fibre is usually alkali-borosilicate glass (boron-free E-glass) constituting 70% to 75% of the total content in FRP while the polymer (polyester, vinyl ester or epoxy) constitute the remaining portion. Although epoxy is considered better of the two, because of lower cost, the core used today is usually polyester. Brittle fracture (Figure 2.2) is the major failure mechanism in core, which occurs due to the breach of the end seal exposing the rod to atmospheric pollutants and moisture. Figure kv insulator failed in-service by brittle fracture (IEEE Electrical Insulation Magazine 2005) The failure occurs due to the corrosion of the E-glass fibre which can be minimized by usage of chemical resistant alkali-aluminosilicate glass fibre (ECR) which can even withstand acid attacks. End seal is also

4 28 considered to be the most important element in minimizing failure due to brittle fracture (Weiguo Que 2002 and Khan et al 2006) Housing The weather sheds and sheath form the housing (protective covering) for the central core. They are shaped and spaced over the central core to provide the required leakage distance from the high voltage end to ground end. The design of housing depends entirely on the environmental conditions and is designed as per IEC In general, the materials used for housing should have excellent ageing resistance under different stresses like Thermal, Electrical, Ambient and Mechanical (TEAM). The possible materials for high voltage applications are listed in Table 2.1. Each material offers different characteristics. Table 2.1 Housing materials (Khan et al 2006) High voltage applications Low voltage applications Silicone Rubber (SiR) Ethylene-propylene diene monomer (EPDM) Ethylene-propylene rubber (EPR) Ethylene-propylene monomer (EPM) High density polyethylene (HDPE) Polytetrafluoroethylene (PTFE) Polyurethane (PUR) Polyolefin elastomers The long-term performance of these materials in polluted environments is still a major area of concern, while SiR is exceptionally good. Silicone rubber is an elastomer composed of silicone-silicon together with carbon hydrogen and oxygen atoms. The properties of silicone rubber come from the structure of the polymer (Structure and Properties of SiR are given in Appendix 1). Heat stability, resistance to UV radiation and weathering are

5 29 derived from the strength of Si-O ( ) bonds. A flexible polymer chain gives low surface energy where the low molecular weight compounds migrate to the surface of pollution layer and form a thin layer. Hence the water tends to beed up rather than form a continuous film (hydrophobicity-figure 2.3). Water beeds Figure 2.3 Water beeds on Silicone Rubber The carbon (organic) side groups attached to the silicon atoms allow cross linking and tailored applications (i.e increased heat resistance, solvent resistance, increased strength, decreased compression set, increased arc track resistance or increased cure rate). Ethylene Propylene Diane Monomer (EPDM) and silicone elastomeric materials containing a minimum of 70% by weight of hydrated alumina are also favored for weather sheds by most of the manufacturers. In addition to this, fillers are also added to enhance the resistance to tracking, erosion as well as to provide improved mechanical performance in tensile strength, abrasion resistance, tear strength, modulus and to reduce flammability. Typical fillers used are aluminium tri-hydrate (ATH), Al 2 O 3 3H 2 O or hydrated alumina and silica (quartz powder). The former is now very extensively used and forms between 40% and 55% of the total SiR and EPDM compounds. Quartz flour is widely used in cycloaliphatic epoxy resins.

6 End Fittings The end fittings are the insulator attached hardware that are connected to the core. They are used for transferring the load from the conductor to the core. The end fittings are usually metal-forged steel, ductile cast iron, malleable iron or aluminium. The shape of the end fittings is also an important factor to limit the production of corona discharges. Corona discharge might cause the polymeric material to degrade leading to the failure of the insulators. (a) Socket (b) Ball Figure 2.4 End fittings The most common type of end fitting used in transmission line tower is shown in Figure 2.4. The ball type is usually used at the H.V end and the socket type at the L.V end. 2.3 REGIONS OF CRITICAL E-FIELDS E-fields play a significant role in degradation of polymeric material and they are recognized as a significant factor in the aging mechanisms of polymeric insulators. The E-field magnitude is high near the high voltage and ground end of the insulator. Typically, the high voltage end is subjected to high E-field magnitude and its location occurs adjacent to the end fittings while in some cases, a short distance from the end fitting.

7 31 There are three critical regions of interest while considering E-field distribution along polymeric insulators. If the E-field value in any of these regions exceeds the critical limit, discharge activity takes place, affecting the long and short term performance of polymeric insulator. Hence, in order to prevent the discharge activity, the maximum magnitude of E-field should be kept below the critical values. These values form the criteria for the optimization procedure. The critical regions and their corresponding critical E-field values in kv/mm (peak) for dry unpolluted polymeric insulator are listed in this section Critical Region 1 - Surface Of Silicone Rubber Silicone rubber being organic in nature is more prone to deterioration due to high E-field on the surface of it and surrounding the end fitting. Under contaminated conditions, leakage current flows on the surface of silicone rubber leading to dry band arcing. Continuous dry band arcing will result in decomposition of silicone rubber, further leading to tracking and erosion (Yoshimura et al 1999). The potential for occurrence and the magnitude of this dry band arcing are influenced by the E-field magnitude. The E-field value on the surface of weather-shed material and surrounding the end fitting seal (measured 0.5 mm above the surface of the sheath from triple junction H.V to L.V) must be kept below 0.64 kv/mm (Andrew Phillips et al 2008) Critical Region 2- Corona and grading rings High magnitude of E-field values on the surface of energized and grounded end fittings and rings (corona and grading) results in corona activity which results in radio interference and audible noise. Corona discharges can produce temporary loss in the hydrophobic property of silicone rubber and the ozone in it can cause aging of the polymeric insulators (Yoshimura et al 1999). E-field values on the surface of metallic end fittings and rings are

8 32 controlled as they pass the radio interference/corona test indicated in ANSI, CEA, IEC standards and IEEE guide. A value of 2.97 kv/mm under dry condition (measured around the rings) is used as a reference for design purpose (Andrew Phillips et al 2008) Critical Region 3-Fibre Reinforced Plastic (FRP) and Silicone Rubber (SiR) If the E-field internal to the FRP (core) and SiR exceeds the critical value, then it may result in damage to the core or silicone rubber, ultimately resulting in either electrical or mechanical breakdown. Thus the E-field value should be kept below 4.25 kv/mm to avoid breakdown (Andrew Phillips et al 2008). Corona ring (L.V) SiR 0.5 mm measuring line Grading ring measuring line Grading ring Corona ring (L.V) measuring line F B SiR A E Corona ring (H.V) Corona ring (H.V) measuring line D Triple junction (L.V) FRP Triple junction (H.V) C Figure 2.5 Critical regions in polymeric insulator All of the three measuring regions are shown in Figure 2.5 and are explained as follows: Line AB-measured 0.5mm above surface of SiR sheath starting from triple junction H.V to triple junction L.V (Critical region 1). Lines around the corona and grading rings (Critical region 2). CD and EF line-inside FRP and SiR (Critical region 3).

9 CONTROL OF E-FIELD DISTRIBUTION In-service performance of polymeric insulators depends on the E-field distribution along polymeric insulators. The E-field distribution is controlled by optimized designs of: End Fitting Corona and grading rings End Fitting The electrical performance of polymeric insulator varies with different types of end fittings (Figure 2.6) End fitting close to the last shed 2. End fitting at a short distance away from the last shed 3. End fitting completely covered by silicone rubber 4. End fitting partly covered by silicone rubber Figure 2.6 Types of rounded end fittings (Rahisham Abd Rahman 2012) Properly designed and manufactured end fittings tend to reduce the peak magnitude of E-field values in close proximity to the end fittings (Triple junction). End fitting of larger size with rounded edges (Figure 2.6 (b) reduces the peak value of E-field near the triple junction area and is also mostly used in service due to less manufacturing cost (amount of silicone rubber and less

10 34 molding time) (Nihal Mohan 2014). This type of end fitting is used for further analysis Design of Corona and Grading Rings Conventionally designed rings (corona and grading) control the maximum E-field magnitude and also move the position away from triple junction. The dimensions and location of ring have a significant influence on the E-field distribution along polymeric insulators. Improper application i.e incorrect location of ring is most common in service operation of polymeric insulators. Figure 2.7 Image of dry corona activity near the triple junction of an in-service 500 kv composite insulator installed without a corona ring (Andrew Phillips 2008) Figure 2.7 shows the image of 500 kv composite insulator installed without a corona ring experiencing a discharge activity near the triple junction area. Hence manufactures provide a permanent attachment method to minimize the installation error. Depending on the tower, set/string configuration and composite insulator design, corona rings are now installed at the line end for 132 kv lines, whereas in the past, corona rings were used only from 220 kv lines and

11 35 a corona ring at both the line end and ground end for 345 kv lines (Figure 2.8). In EHV and UHV (345kV and higher) lines grading rings are necessary in addition to corona rings to accomplish field grading and power arc protection. Grading rings are effective in leveling E-field along the insulator axis and the corona ring in reducing the E-field near the triple junction point (interface of housing, hardware and air). End Fitting (L.V) Corona ring (L.V) Weather Shed (Silicone Rubber) FRP Corona ring (H.V) Grading ring End Fitting (H.V) 66 kv 132 kv to 220 kv 345 kv Above 400 kv Figure 2.8 Structure of polymeric insulators with rings* *Figure is indented to show the number of rings for all voltage classes and it does not carry any significance to actual dimensions 2.5 DETERMINATION OF E-FIELD DISTRIBUTION In order to design and utilize composite insulators effectively, a fundamental understanding of the E-field distribution and its effect on the insulator performance is needed. The available methods for the E-field calculations are shown below in Figure 2.9.

12 36 E-field analysis Experimental techniques Analytical techniques Numerical techniques Electrostatic Probe Spherical dipole Optical sensors Direct Integration Method of Images Finite difference method Finite Element method Boundary Element method Variable separable Conformal transformation Charge Simulation method Figure 2.9 Different techniques of E-field analysis Experimental Techniques Experimental field analysis techniques are either done in laboratory (actual or equivalent setup of the insulators along with tower, conductors are made) or real time conditions (Weiguo Que 2002). However the measurement of E-field distribution has some limitations including: inability to measure in regions of interest (i.e., internal to the insulator or close to the rubber weather-shed surface) distortion of the E-field being measured thus reducing accuracy Expensive and time consuming Analytical Techniques Analytical techniques make use of formulae to calculate the E-field of relatively simpler geometries where the conducting surfaces are cylinder, spheres etc. But high voltage equipments have complex geometries and hence the formulae become extremely complicated and the analysis becomes cumbersome. Numerical methods of E-field analysis, using computer software, are replacing all other methods of E-field analysis.

13 Numerical Computation In numerical field analysis: all the electromagnetic field problems expressed in terms of partial differential equations (Maxwell Equations) are converted to differential or integral equations by using Green s function. Then these equations are converted to algebraic equations which are solved iteratively till the solution converges with a certain predefined tolerance limit. There are two different categories of numerical methods: The domain methods and the boundary methods. The comparisons of these methods are shown below in Table 2.2. Table 2.2 Comparison of domain and boundary methods (Tanushri Doshi 2010) Domain methods Boundary methods Differential Equations are solved. Integral Equations are solved. For a 3-D field analysis the For a 3-D field analysis the volume integral is differential equations are solved converted to surface integral-thereby reducing directly. the order by one. Computational time is more but less Computational time is more. when compared with boundary methods. They are preferably used for They are preferably used for problems with in problems with closed or limited open boundary. boundary. Ex: Insulator, Bushing. Ex: Transformer, Motor. Finite Element Method Charge Simulation Method Finite Difference Method Boundary Element Method The Matrix to be solved in Boundary element method is fully populated and unsymmetric. This means that the storage requirements and computational time will tend to grow according to the square of the problem size. Hence it is limited for solving small-scale models. It is also virtually impossible to generalize this procedure to nonlinear solids. Hence, in this work, E-field calculations on 2-D and 3-D models have been carried out using finite element based package.

14 Finite Element Method The finite element method is one among the numerical analysis methods of solving Maxwell s equation in differential form and has been widely used in electric and magnetic field analyses since the late 1970s. The partial differential equation (PDE) for electrostatic problems is given by Laplace Equation. (2.1) where, ε is the permittivity of the dielectric medium. In FEM, the entire problem space, including the surrounding region is discretized into small number of non-overlapping subdivisions called finite elements which normally have triangular shape for 2-D models and tetrahedron for 3-D models. This process is called meshing. The PDE is then approximated using weighted residual approach to obtain a system of algebraic equations, where the unknowns are the solution values (voltage) at the discretization points. This system of equations is then solved on a computer by iterative or direct techniques. The error is computed by minimizing the energy function I(V), ( ) ε [( ) ( ) ] (2.2) which is equivalent as solving (Marungsri et al 2008). This error is usually termed as energy error in FEM and the minimized energy corresponds to the true solution of the field equation. FEM is very flexible and can be applied to the most complicated geometries. It is capable of giving highly accurate results and the accuracy of results depends on the number of elements considered in the geometry (Weiguo Que 2002 and Tanushri Doshi 2010).

15 DIMENSIONS OF POLYMERIC INSULATOR Polymeric insulators are available in different dimensions for same voltage class based on its pollution, mechanical and electrical operating condition. The major structural parameters (Figure 2.10) of the insulator which define its in-service performance are: 1. Dry arcing distance 2. Sectional Length 3. Diameter of the FRP rod 4. Dimensions of weather sheds a. Outer radius b. Inner radius c. Spacing between sheds 5. Dimensions of rings (Corona and grading) End Fitting (L.V) Dry arcing distance Spacing between sheds HV end Fitting B A Sectional length Inner radius Outer radius Radius of FRP rod Figure 2.10 Structural parameters of polymeric insulator

16 Dry arcing distance(mm) 40 The dimensions of the FRP rod and end fittings depend on the mechanical loading on the insulator, while dry arcing distance and sectional length vary based on its electrical performance. Dimensions of the shed (outer radius, inner radius and shed spacing) are decided based on the pollution performance. A survey of existing dimensions on the structural parameters listed above is presented in this section Dry Arcing Distance Dry arcing distance is the shortest distance or the sum of the shortest distances, along the insulating parts of the insulator between those parts which normally have the operating voltage between them (Figure 2.10). The range of variation of dry arcing distance for all classes of voltage is shown in Figure Voltage rating (kv) Figure 2.11 Dry arcing distance As expected, the dry arcing distance varies almost linearly with voltage class. Manufacturers and utilities tend to increase the dry arcing distance to increase the basic insulation level, depending upon the electrical operating conditions. It is also observed that the range of variation is predominant in higher voltage class.

17 Sectional length (mm) Sectional Length Sectional length is the sum of the dry arcing distance and lengths of the end fittings of live and ground end. Insulators of different sectional length are available for the same dry arcing distance where the size of end fittings is either increased or decreased based on the mechanical loading on the insulator. The variations of sectional length for all the voltage classes are shown in Figure Voltage rating (kv) Figure 2.12 Sectional length The range of variation of dry arcing and sectional length are similar to each other. Although similar, increase in sectional length by increasing the size of the end fittings will reduce the peak magnitude of E-field along the surface of silicone rubber Diameter of the FRP rod The diameter of the FRP rod depends on the mechanical loading i.e number of conductors per bundle (bundled conductor) and the orientation (suspension and tension) of the insulator. Upto twelve conductors per bundle are used for 1200 kv systems. The variation of the diameter is shown in

18 Dimensions (mm) Diameter of the FRP rod (mm) 42 Figure It is observed that the variation is same for voltage class up to 400 kv where a two conductor bundle is common on transmission lines Voltage rating (kv) Figure 2.13 Diameter of FRP rod Dimensions of the weather shed Dimensions and configuration (plane, alternate, and under ribs) of weather sheds are almost constant with voltage class. They vary with respect to pollution performance and to an extent on the mechanical loading on the insulator. Weather sheds are designed as per IEC The major structural dimensions of the weather shed followed by the manufacturers and utilities are shown in Figure Outer diameter Inner diameter Spacing between sheds Weather shed parameters Figure 2.14 Dimensions of weather shed

19 Dimensions of Rings Appropriately designed rings are used to reduce and move the maximum E-field magnitudes away from the triple junction (interface of housing, hardware and air). The exact dimensions and location of the rings have not been discussed in any of the standards and hence inappropriate grading might result in an increase in E-field leading to corona and also increase in the basic insulation level. Ring parameters (H.V end) Grading ring (r 1 -R 1 -h 1 ) Corona tube radius (r 1 )- Corona ring radius (R 1 ) Distance from H.V end (h 1 ) Corona ring (r-r-h) Corona tube radius (r)- Corona ring radius (R)- Distance from H.V end (h) R 2 r 2 Triple junction (L.V) Corona ring (L.V) Ring parameters (L.V end) Corona ring (r 2 -R 2 -h 2 ) Corona tube radius (r 2 )- Corona ring radius (R 2 )- Distance from H.V end (h 2 ) R h 2 R 1 r Triple junction (H.V) r 1 h 1 Grading ring (0, 0) h Corona ring(h.v) Figure 2.15 Structural parameters of the corona and grading rings The major structural parameters of the rings are shown in Figure 2.15 and the variations of them with respect to the voltage classes are shown in Figure 2.16, 2.17 and 2.18.

20 Dimensions of R and R 1 (mm) Dimensions of r and r 1 (mm) Voltage rating (kv) 1200 r r 1 Figure 2.16 Corona ring (r) and grading ring (r 1 ) tube radius at H.V end Voltage rating (kv) R R 1 Figure 2.17 Corona ring (R) and grading ring (R 1 ) radius at H.V end

21 Dimensions of h and h 1 (mm) Voltage rating (kv) h h 1 Figure 2.18 Distance of corona ring (h) and grading ring (h 1 ) from H.V end The dimensions-location are mentioned with respect to the origin (0, 0) shown in Figure The following major conclusions are made from the detailed survey on the existing dimensions-location of rings: The dimensions-location of grading ring (r 1, R 1, h 1 ) increase almost linearly from 400 kv up to 1200 kv. The dimensions-location of corona ring (r, R, h) vary depending upon grading ring. o o o Ring radius (R) is almost constant with voltage class. Distance of corona ring from H.V end (h) is constant up to 400 kv and then increases almost linearly up to 1200 kv. Ring tube radius (r) increases linearly up to 400 kv and reduces after that. From reference literatures and manufacturers catalogues it is found that the dimensions of the LV end corona rings are r 2-25 to 30, R to 125 for the voltage ratings from 400 kv to 1200 kv.

22 46 Based on the data collected from the manufacturer s catalogue like Deccan Enterprises, Goldstone, Balestro, DCI Composite Insulators, BHEL and K-Line Insulators Limited, the dimensions of insulator and rings for further analysis are chosen. Dry arcing distance and sectional length are changed for corresponding increase in voltage class and its dimensions along with the ring s dimensions for all voltage classes are listed in Table 2.3(a). The other structural parameters which remain constant with voltage class are shown in Table 2.3(b). Table 2.3 (a) Dimensions of structural parameters of insulator and rings for all voltage classes Voltage class (kv) Dry arcing distance (mm) Sectional length (mm) Dimensions of Corona ring (mm) at H.V end Dimensions of Grading ring (mm) at H.V end Dimensions of Corona ring (mm) at L.V end r R h r R h r R h Table 2.3 (b) Dimensions of structural parameter of insulator (Common to all voltage classes) Structural parameter Dimensions (mm) Diameter of the FRP rod 20 Weather shed diameter Outer diameter-144 Inner diameter-91 Spacing between sheds 32.5

23 SUMMARY The structure of polymeric insulator along with the critical regions of interest is discussed. The maximum E-field values at all critical regions must be kept below a critical value for the satisfactory operation of polymeric insulators. The critical E-field values are chosen as the optimization criteria for design of polymeric insulators and rings in forthcoming analysis. The E-field distributions have a significant influence on the performance of polymeric insulators, which is improved by optimally designed end fittings and rings (corona and grading). The E-field distributions are determined either by measurement or computational techniques. Measurement methods are expensive and time consuming and hence computational techniques are usually preferred. The dimensions of insulator and rings are collected from literature and manufacturer s catalogue and are presented for voltage classes from 66 kv to 1200 kv. The dimensions of the insulators and rings have a wide range of variation for the same voltage class, depending upon the operating conditions. Based on which dimensions of insulator and rings are chosen for further analyses. In the next chapter, E-field analysis is carried out in 2-D utilizing its axi-symmetry nature and in 3-D considering the effect of hardware fitting, tower, three phase and conductor.