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Saturday, 27 September 2014

Polymer Insulators for reduction in Infrastructure cost

Porcelain Insulators:

Porcelain has enjoyed a virtual monopoly; as an insulating material for electrical equipment such as transformers, switchgears etc and in transmission lines. The advantages of porcelain are its high insulation strength. But it has a low strength to weight ratio and are prone to fragmentation under stress.

Polymer materials, on the other hand, have high insulation value at par with porcelain and acceptable strength both under compression and tension. It has a better water and sleet shedding properties and is therefore more useful in contaminated and polluted environment. Although the cost of both porcelain and polymer material are the same but polymer is more favoured because of its better handling. It has a better strength to weight ratio i.e. it is much lighter in weight than porcelain. 

The weight of suspension type polymer insulator for the 138 kV class is only 8% of the porcelain insulator of the same voltage level. This vital property of polymers permits the use of lighter supporting structures, more compact design, narrow Right of Way (ROW) requirements and thus significant reduction in cost.   
  
In the polymer insulator a fiberglass insulation rod or shaft serves as the internal structure and around which the polymer insulator is attached usually in the shape of petticoats or rainsheds. The fiberglass rod has a high compression and tensile strength. The metal fittings at both the ends are crimped directly to the fiberglass rod. Proper sealing is done to avoid ingress of moisture or contamination into the fiberglass rod. The petticoats, as in porcelain insulators, provide a longer leakage path between conductor and the support, so as to keep the leakage current to a minimum value. A variety of polymer insulators are shown in Figure1 .

Fig.1: Polymer Insulators for different voltage range.

The rate at which the petticoats or rainsheds dry up is crucial and depends on a number of factors such as the contamination level of the area, temperature, humidity and wind velocity following the cessation of the rough weather. In areas with extreme contamination; for example near an industrial area or power plant, insulators with various petticoat sizes are used, so as to obtain a greater distance between outer edges of petticoats, eventually to avoid a flashover.

The polymer suitable for High Voltage (H.V) applications are Ethylene Propylene (EP) and Silicon Rubber (SR). Ethylene Propylene polymer has high resistance to corrosion and better physical properties whereas Silicon Rubber shows better performance under contamination and offers higher resistance to Ultra Violet (UV) sun rays. A combined EP and SR polymer has better hydrophobic (water repellent), electro-mechanical properties and high resistance to industrial pollution. Table 1 shows typical properties of Polymer insulators used in distribution and transmission lines of voltages ranging from 11 kV to 400 kV.

Table 1: Properties of Polymer Insulators

Sr. No
Nominal System Voltage kV
Sectional Length mm
Mechanical Strength kN
Creepage Distance mm
Impulse Withstand Voltage kVp
Type of Metal Fitting
1
11
260
5
320
80
Pin
2
33
375
10
980
210
Pin
3
132 Suspension type
1344
70-90
4495
650
B&S
4
132 Tension type
1536
120
5016
650
B&S
5
220 Suspension type
2119
70-90
7595
1050
B&S
6
220 Tension type
2256
120
7975
1050
B&S
7
400 Suspension type
3335
120
13020
1550
B&S
8
400 Tension type
3910
160
14500
1550
B&S

These insulating materials are also used in bushings of transformers, reactors, switchgears, capacitors, instrument transformers, lightning arresters etc.