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Power Line Insulator Application


Definition [1]

An electrical insulator is a material whose internal electric charges do not flow freely, and therefore make it nearly impossible to conduct an electric current under the influence of an electric field. This contrasts with other materials, semiconductors and conductors, which conduct electric current more easily. The property that distinguishes an insulator is its resistivity; insulators have higher resistivity than semiconductors or conductors.

A perfect insulator does not exist, because even insulators contain small numbers of mobile charges (charge carriers) which can carry current. In addition, all insulators become electrically conductive when a sufficiently large voltage is applied that the electric field tears electrons away from the atoms. This is known as the breakdown voltage of an insulator. Some materials such as glass, paper and Teflon, which have high resistivity, are very good electrical insulators. A much larger class of materials, even though they may have lower bulk resistivity, are still good enough to prevent significant current from flowing at normally used voltages, and thus are employed as insulation for electrical wiring and cables. Examples include rubber-like polymers and most plastics.

Insulators are used in electrical equipment to support and separate electrical conductors without allowing current through themselves. An insulating material used in bulk to wrap electrical cables or other equipment is called insulation. The term insulator is also used more specifically to refer to insulating supports used to attach electric power distribution or transmission lines to utility poles and transmission towers. They support the weight of the suspended wires without allowing the current to flow through the tower to ground.
 

Overhead power line in Gloucestershire, England.

Figure 1 - Overhead power line in Gloucestershire, England.
Ceramic insulator used on electrified railways

Figure 2 - Ceramic insulator used on electrified railways 

Description

For power applications, the electrostatic analysis module of EMS can be used to analyze 3D models of electrode devices and power-line insulators (Figure 3).

The model of interest in this example is a power-line pole with a three-phase insulation scheme. The central-phase line (Figure 4) operates at a phase-to-ground voltage of 80kV rms (phase-to-phase voltage of 138.56 kVrms). The upper and lower lines operate at 40 kV rms. The field is calculated at the point of the AC waveforms, when the center phase voltage is at its peak. The model is composed of aluminum conductor lines and copper clamps (Figure 5). The clamps connect the conductors to the silicon rubber and fibre glass insulators, which in turn connect to the tower. The electrostatic module determines the electric field and the displacement field due to the aforementioned conditions.
 
These fields can be viewed in a full 3D plots and 2D plots showing the field at certain positions in the model. For instance, the electric field is obtained along a segment (Figure 6) that passes through the air as well as the silicon rubber insulator.


Power line with Three-phase insulation scheme         Close-up view of the middle-phase insulator

Figure 3 - Power line with Three-phase insulation scheme               Figure 4 - Close-up view of the middle-phase insulator


Close-up view of the middle-phase insulator.               Section view of the middle-phase insulator with fibreglass and silicon rubber components

Figure 5-  Close-up view of the middle-phase insulator.                       Figure 6 - Section view of the middle-phase insulator
                                                                                                                                   with fibreglass  and silicon rubber components



3D Model of insulator

Figure 7 - 3D Model of insulator

 

Study

The Electrostatic module of EMS is used to compute the Electric Field, the Displacement Field, the Potential and the Electrostatic Force density. Also, an important parameter is calculated by EMS, which is the Safety Factor. This factor  is a very useful parameter to determine whether the electric field exceeds the breakdown voltage, in each point or region in the model.
Materials
In the Electrostatic analysis of EMS, the  required material property is the relative permittivity and the dielectric strength, shown in Table 1.

 

Components / Bodies Material Relative permittivity Dielectric Strength
Contact 1 Copper 1 None
Ground_ Contact Copper 1 None
Conductor Aluminum 1 None
Contact 2 Copper 1 None
Insulator Silicon Rubber 4 25.00e+006 V/m
Fibre Fibreglass 5.5 60.00e+006 V/m
Hanger Aluminum 1 None
Inner_Air Air 1 3.00e+006 V/m
Outer_Air Air 1 3.00e+006 V/m
 
Table1 -  Table of materials, relative permittivity and dielectric strength
 

Load and Restraints

Loads and restraints are necessary to define the electric and magnetic environment of the model. The results of analysis directly depend on the specified loads and restraints. Loads and restraints are applied to geometric entities as features that are fully associative to geometry and automatically adjusted to geometric changes.
In this study, we have just to assign the fixed voltage.

 
Components / Bodies Fixed voltage
Contact 1 80 kV
Hanger 80 kV
Conductor 80 kV
Ground contact 0
Contact 2 0
 

Meshing

Meshing is a very crucial step in the design analysis. EMS estimates a global element size for the model taking into consideration its volume, surface area, and other geometric details. The size of the generated mesh (number of nodes and elements) depends on the geometry and dimensions of the model, element size, mesh tolerance, and mesh control. In the early stages of design analysis where approximate results may suffice, you can specify a larger element size for a faster solution. For a more accurate solution, a smaller element size may be required.
We can control the mesh size form a body to another depending in its dimensions and its importance in the final result by applying a mesh control.

Name Mesh size Components /Bodies
Mesh control 1 2.00 mm Ground-Contact / Contact 1/ Contact 2 /
Conductor/ Hanger
Mesh control 2 5.00 mm Insulator /
Fibre


 
 

Results

After a successful run, the Electrostatic module produces five result folders and a result table. The folders contain the Electric Field E, the Electric Displacement D, the Potential distribution V, the Force Density F and the Safety Factor mentioned above. The results table contains the electrostatic energy and the total charge in the fixed voltage bodies. Furthermore, all of the results can be visualized in various formats such as fringe, vector, contour, section, line, and clipping plots. The results can easily be exported, and dissected.
After hiding all components except the fiber and the insulator,  the following Electric Field plot (Figure 8) is obtained:

 
Electric Field in the insulator and the fiber (fringe plot)

Figure 8 - Electric Field in the insulator and the fiber (fringe plot)

Evolution of the Electric Field between two points located at the extremity of the fiber
Figure 9 - Evolution of the Electric Field between two points located at the extremity of the fiber


Potential in the whole model

Figure 10 - Potential in the whole model


Safety Factor

Figure 11 - Safety Factor
 

As shown in the figure (Figure 11) above, the maximum value is  0.378: so there is no breakdown voltage in our model.

Conclusion

To examine if there is a breakdown voltage, the Safety Factor folder in EMS Results is used. It gives the  value of the ratio between Electric Field and Dielectric Strength in a specific point. If the Safety Factor reaches 1,  then there is a risk of breakdown voltage in that point. The Electrostatic Module of EMS helps to avoid the breakdown voltage and the corona effect in such applications. 

Reference

[1] https://en.wikipedia.org/wiki/Insulator_(electricity)



Videos

Electrostatic Analysis of a Powerline Insulator

Electrostatic Analysis of a Powerline Insulator