In this topic, you study Electrical Insulators – Definition, Theory, Diagram & Types.
Electrical Insulators stop the leakage current to the earth from supports. They are mounted on line conductors in such a way that they provide proper insulation and necessary clearance.
Electrical Insulator Failures
The main causes of an insulator failure are abnormal electrical and mechanical stress, cracks, flash over and ageing. Flow of leakage current through a portion of the insulator may heat the localised portion and may cause its failure. However, the most common cause of insulator failure is due to flash-over, which creates an arc through the air surrounding the insulator. Switching and lightning surge are also responsible for insulator failure and subsequent short circuit of the conductor. In fact, arcing horns are used to pass away the impulse voltage in the transmission line.
- Post Insulators
Post insulators are used for for supporting bare busbars, isolator switches etc. and are extensively used in busbar panels, switchgear panel boards in sub-stations. These insulators are similar to a pin type insulator but has a metallic base with a metal cap so that a number can be cascaded. These insulators are mostly solid core insulators made of resin. In these insulators porcelain is also used.
- Bushings
Bushings are used in transformers, Switchgears. Isolators etc. to take out live conductors through earthed tanks or metallic parts. In bushing design, the central conductor is in the form of brass or copper or stranded copper conductors passing through brass tube or Aluminum tube supporting the bushing core.
Materials used for Electrical insulators
Porcelain Insulator
In case of overhead lines of power systems insulators are connected to the cross arm of the supporting structures and the post conductor passes through the clamp of the insulator. These insulators are mainly used of porcelain and free from defects and of special design for particular use. These insulators should be vitrified, because the presence of pores of air in the porcelain will lower down its dielectric strength. Moreover, any sealed air impurity will also lower the dielectric strength of porcelain. The dielectric strength of porcelain should be 15 kV to 17 kV for every one tenth inch thickness. Porcelain is mechanically strong, less affected by temperature and has minimum leakage problem.
Glass Insulator
Besides porcelain, toughened glass in also sometimes used for insulators because it has higher dielectric strength (around 35 kV for one tenth inch thickness) which makes it possible to make use of single piece construction whatever be the operating voltage. Glass being transparent it is very easy to detect any flow like air trapped inside, cracks etc.
Electrical Insulator Testing
All the manufacturers of insulators have to carry out tests electrical and mechanical. The tests include the following: (1) Mechanical tests, (2) Electrical insulation test, (3) Corona and radio interference test, (4) Environmental test, including temperature cycle test.
Below a brief discussion is made on these tests:
- Mechanical Tests Include
(a) Tensile strength. (6) Compressing test, (c) Bending test, (d) Mechanical vibration test, (e) torsional test.
- Electrical Test
Power frequency withstand test – During the power frequency voltage withstand test Voltage set (for say 11 kV it is 28 kV, 33 k it is 70 kV 132, kV is applied for a period it is 270 kV, for 220 kV it is 460 kV) of one minute. There should be no flash-over or puncture during the test. However, the application of test voltage at site is about 75% of the test voltage at works.
- Environmental Tests
Electrical insulators are subjected to alternate temperature cycles, sudden temperature changes, pollution and other environmental stresses. For EHV insulators following test are