The leading cause of electrical failures is a breakdown in the insulating materials. This means testing on insulators is an extremely high priority for knowing how your assets are performing.
Partial electrical discharges are the key indicator of insulator deterioration. There are almost always some sparks—partial discharges—that weaken the material. That’s why constant monitoring and testing are so important.
But what types of high-voltage tests can you perform on insulators?
Tests on insulators are roughly divided into three groups: type tests, sample tests, and routine tests. Let’s take a look at each of these in more detail.
1. Withstand test
Withstand tests are designed to simulate practical conditions—this means mounting the insulator exactly as it is intended to be. Then, a 1/50 s wave is applied at a specific voltage. The voltage used is determined by taking the voltage that the insulator will experience in practical conditions, then adjusting for humidity, air density, and other factors.
During a withstand test, you are specifically looking for two things.
a) A flashover—this indicates deterioration in the insulation material
b) A puncture—if you find one of these, the insulator is permanently (and in most cases, irreparably) damaged
Withstand tests are repeated for a total of five times for each polarity.
2. Flash-over test
Similar to the withstand test, a 1/50 s wave is applied. The voltage, however, is different. It builds up to a level of 50% of the flash-over voltage, for both polarities. Also similar with the withstand test, any puncture indicates that the insulator is damaged beyond repair.
3. Dry tests
There are two types of dry tests. First, there’s the dry one-minute test. Similar to the flash-over test, the insulator, completely clean and dry, should be mounted correctly and have its standard voltage (that will be used) pass through it. The voltage should be brought up gradually and must be kept at a standard for one minute. As with the previous tests mentioned, a puncture or a flash-over during the test is a sign of deteriorating material or irreparable damage.
Second, there is a dry flash-over test. Once the dry one-minute test has concluded, the voltage is gradually increased until flash-over occurs. This process is then repeated for ten iterations. If performed correctly, there should be no damage to the insulator.
4. Wet tests
It is critical to test insulators in a range of conditions. Therefore, just as we had dry tests, the insulators must be examined under wet conditions. There are two basic types of wet test. First, there is a one-minute rain test. This involves spraying the insulator throughout the test with artificial rain. The ‘rain’ must be within 10 degrees (centigrade) of the ambient temperature and sprayed on a 45-degree angle. The rate of rain is 3mm/minute, with resistivity 100 ohm-m plus or minus 10%. The prescribed voltage then needs to be maintained for one minute.
Once this test has been performed, just as with the one-minute dry test, the voltage is increased until flash-over occurs. This is called the “wet flash-over test”, and similar with the dry version, it is vital that there is no damage to the insulator.
5. Visible discharge test
This is a relatively simple test, where the room is darkened, and the specified voltage then runs through the insulator. It is required that there are no signs of corona after five minutes.
There are 5 standard sample tests performed on insulators. They are called sample tests because each test is done on only a small sample portion of the insulator, not the entirety of it.
The temperature cycle test is very simple—the sample is heated, cooled, and reheated (cycled through a total of 5 times). Each transfer must not exceed 30 seconds.
The mechanical loading test simply means loading the insulator until it fails. When the insulator fails, the load is recorded. The load that causes failure must exceed 2000 lbf.
The electro-mechanical test is similar to the mechanical test, with the addition of an electrical load. The voltage used should be 75% of the dry flash-over voltage, and there should be no damage.
The overvoltage test is designed to test for punctures. The insulator is immersed, usually in oil, to prevent any flash-overs. While immersed, a predetermined overvoltage is applied. It’s important that the insulator does not puncture at this point. Then, the voltage is brought up until failure (puncture) occurs.
The porosity test is used to check absorption of liquids (broken pieces of porcelain are immersed in a dye for 24 hours).
All insulators should undergo routine tests, which start with very low voltages that increase until flash-over occurs. Once flash-overs are occurring every few seconds, the voltage is held steady for a total of 5 minutes. This is a very standard test designed to check that the insulator doesn’t sustain any damage.
In conclusion, there are a lot of tests that can and need to be done on insulators to ensure that they perform well under a broad range of conditions that mimic the loads the insulators will experience in their lifespan. It is extremely important to test regularly, so you know exactly how your insulators are performing.