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.