TRANSMISSION AND DISTRIBUTION UNIT-4 INSULATORS TESTING OF INSULATORS

TESTING OF INSULATORS: The insulator should have good mechanical and dielectric strengths to withstand the load and operating or flashover voltages, respectively. However, it should be free from pores or voids, which may damage the insulators. For this, the following three tests are performed. Flash over tests Performance tests Routine tests

FLASHOVER TESTS: Three types of flashover tests are to be conducted before the insulator can be said to have passed the flashover test. Power frequency dry flashover test Power frequency wet flashover test Impulse frequency flashover test POWER FREQUENCY DRY FLASHOVER TEST: In this test, voltage is applied between the electrodes of the insulator mounted in the manner in which it is to be used. Gradually the applied voltage is increased until the Surrounding air breaks down. This voltage is known as flashover voltage, and must be Greater than that of the minimum specified voltage. The insulator must be capable of withstanding the minimum Specified voltage for one minute.

POWER FREQUENCY WET FLASHOVER TEST: In this test, again the insulator is mounted in the same manner as that of dry flashover test and the voltage is applied gradually, in addition to that, the insulator is sprayed with water at an angle 45˚ in such a manner that its precipitation should not be more than 5.08 mm/min. The insulator must be capable of withstanding minimum voltage for 30 sec. IMPULSE FREQUENCY FLASHOVER TEST: In this test, a generator develops a very high voltage at a frequency of several hundred kilohertz. 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 impulse ratio.

PERFORMANCE TEST: Before giving the satisfactory performance of the insulators, the following tests are to be conducted. Puncture voltage test Mechanical strength test Electro-mechanical test Porosity test PUNCTURE VOLTAGE TEST: The purpose of this test is to determine the puncture voltage. In this test, the insulator is suspended in insulating oil. The voltage is applied and increased gradually until the puncture takes place. The voltage at which the puncture starts is called This voltage for suspension type insulator is 30% higher than that of the dry flashover voltage.

MECHANICAL STRENGTH TEST: This test is conducted to determine the ultimate mechanical strength of pin type insulator. The insulator is mounted on a steel pin and 250% of working load is applied for one minute. ELECTRO-MECHANICAL TEST: This is conducted only for suspension type insulators. In this test, a tensile stress, which is equal to 250% of working tensile strength is applied for one minute. After this, the insulator is tested for 75% of dry spark-over voltage. POROSITY TEST: This test is conducted to determine the degree of porosity. In this, a freshly fired insulator sample is taken and broken into pieces and immersed in a 1% alcohol solution of fuchsine dye under pressure 150 kg/cm2. After one hour, the pieces are removed from the testing pot and are observed for the penetration of the dye. This gives the degree of porosity indication.

ROUTINE TEST: Before the finally recommended insulator is used in the field, it is required to satisfy the following routine test. High voltage test Proof load test Corrosion test HIGH VOLTAGE TEST: In this test, the pin insulators are inverted and are placed in water up to the neck. The spindle hole is also filled with water, and high voltage is applied for 5 min. After the completion of the test, the insulator should remain undamaged.

PROOF LOAD TEST: In this test, all types of testing insulators are assembled and a tensile load of 20% in excess of the working load is applied for one minute. After completion of this test, no damage should occur to the insulator.

CORROSION TEST: In this case, the insulator with its fittings is suspended in a copper sulphate (CuSO4) solution at 15.2˚C for one minute. Then, the insulator is removed, wiped, cleaned and put again in CuSO4 solution. This procedure is repeated four times, which results in zero metal deposits over the insulator.