Explaining flashover tests for overhead transmission lines

1. Flashover Tests
Testing Of Overhead Line Insulators :
Proper operation of a transmission or distribution line is highly dependent upon the proper
working of insulators. A good insulator should have a good mechanical strength to withstand
the mechanical load and stresses. It should have a high dielectric strength to withstand
operating and flashover voltages. Also, an insulator must be free from pores or voids, which
may damage it. Therefore, to ensure desired performance of insulators, each insulator has to
undergo various tests. The most important one of them is the flashover test.
Flashover Tests Of Insulators :
Porcelain insulators are designed so that spark over occurs at a lower voltage than puncture,
thus safeguarding the insulator, in service against destruction in the case of line disturbances.
Flash-over tests are very important in this case .
The flash-over is due to a breakdown of air at the insulator surface, and is independent of the
material of the insulator. As the flash-over under wet conditions and dry conditions differ , tests
such as the one minute dry flash-over test and the one minute wet flash-over test are
performance.
Three types of flashover tests are conducted before the insulator is said to have passed the
flashover test.
1. Power frequency dry flashover test
2. Power frequency wet flashover test
3. Impulse frequency flashover test

2. 1) Power Frequency Dry Flashover Test
The insulator to be tested is mounted in the same manner in which it is to be used. Then, a
variable voltage source of power frequency is connected between the electrodes of the
insulator. The voltage is gradually increased up to the specified voltage. This specified voltage is
less than the minimum flashover voltage. The voltage at which surrounding air of the insulator
breaks down and become conductive is known as flashover voltage. The insulator must be
capable of withstanding the specified voltage for one minute without flashover.
2) Power Frequency Wet Flashover Test (Rain Test)
In this test also, the insulator to be tested is mounted in the same manner in which it is to be
used. Similar to the above test, a variable voltage source of power frequency is connected
between the electrodes. Additionally, in this test, the insulator is sprayed with water at an
angle of 45° in such a manner that its precipitation should not be more than 5.08 mm/min. The
voltage is then gradually increased up to the specified voltage. The voltage is maintained at the
specified value for 30 seconds or one minute and the insulator is observed for puncture or
breakdown. If the voltage is maintained for one minute, this test is also called as one-minute
rain test.
3) Impulse Frequency Flashover Test
This test is to ensure that the insulator is capable of sustaining high voltage surges caused by
lightning. The insulator under test is mounted in the same manner as in above tests. An impulse
voltage generator which generates a very high voltage at a frequency of several hundred
kilohertz is connected to the insulator. This voltage is applied to the insulator and spark-over
voltage is noted. The ratio of impulse spark-over voltage to spark-over voltage at power
frequency is called as the impulse ratio. This ratio should be approximately 1.4 for pin type
insulators and 1.3 for suspension type insulators.

3. In the case of the testing of insulating materials , it is not the voltage which produces spark-over
breakdown which is important , but rather the voltage for puncture of a given thickness ( ie.
dielectric strength ) . The measurements made on insulating materials are usually , therefore ,
those of dielectric strength and of dielectric loss and power factor , the latter been intimately
connected with the dielectric strength of the material.
It is found that the dielectric strength of a given material depends , apart from chemical and
physical properties of the material itself, upon many factors including,
(a) thickness of the sample tested
(b) shape of the sample
(c) previous electrical and thermal treatment of the sample
(d) shape , size , material and arrangement of the electrodes
(e) nature of the contact which the electrodes make with the sample
(f) waveform and frequency of the applied voltage (if alternating )
(g) rate of application of the testing voltage and the time during which it is maintained at a
constant value .
(h) temperature and humidity when the test is carried out
(i) moisture content of the sample.