High Voltage Testing is one of the most important safety measures in the entire electrical field. That’s because “overvoltages” occur, and we need to be sure that any equipment can withstand surges in power, without breaking down or failing.
Without testing the equipment well beyond the specifications, we can never really be sure that it is safe.
Typically, high voltage testing falls into two fairly broad categories.
First, there’s testing on insulating materials. When testing on insulating materials, which are also sometimes called ‘dielectric’, there are three key things tested for:
- Measurement of permittivity
- Power factor (or total dielectric loss)
- Dielectric strength of the material
The second type of high voltage testing is done on completed equipment. The tests for this category are designed to measure the following things:
- Measurement of capacitance
- Power factor (or total dielectric loss)
- Ultimate breakdown voltage
- Flash-over voltage
Pretty much all equipment that will have an electrical current pass through it not only needs to be safe but also needs to withstand the overvoltages without sustaining any damage. For this reason, the voltage used during testing is often about twice the standard voltage.
What are the most frequent high voltage tests?
By far, the most popular high voltage testing done involves a “sustained low-frequency test”, usually at a power frequency of 50Hz. This test is performed on insulating materials (dielectrics) on a broad range of apparatus.
This test, despite being at low frequency, is performed at the highest ultimate stress possible—that means very large voltages are applied, sometimes up to 2000kV. These tests are also usually performed after the manufacturing process but before installation.
It’s also very important, during these tests, that access to the area is properly restricted and personnel can’t put themselves in harm’s way. In all tests, we take precautions to make sure the testing area is completely off limits, especially in high voltage tests.
When the sustained low-frequency tests cannot be used, sometimes, a high voltage direct current test takes its place. With these tests, a direct current is applied for anywhere between fifteen minutes and ninety minutes. There are various technical differences between direct current testing and alternating current, so it’s important to make sure the tests are measuring performance according to the current type, as well as everything else.
It’s very important to make sure all high-voltage tests are performed at the manufacturing stage. This provides enough testing to aid in the design of an appropriate insulator that will be able to handle the conditions it will face.
Together with the two types of low-frequency tests (A.C. and D.C.), high-frequency tests are also used. These are particularly useful when high frequencies are expected (or even just possible) in the lines. Radio transmitting stations often carry out these kinds of tests, since their lines will carry high-frequency loads.
Switching operations or external causes can also cause high-frequency spikes in porcelain insulators, where the breakdown of “flashover” can occur. For this reason, these are also tested at high frequencies to ensure safety.
Another case where high-frequency testing is vitally important is with power line suspension insulators. Faults, or switching operations in the line, can cause breakdown or flashover. And in addition, flashover can be caused by resonant effects as a result of sudden interruptions. These resonant effects can produce high-frequency voltage waves, which can cause flashover of the insulators.
It’s may not be obvious, but insulating materials perform very differently in different frequency ranges. That makes it very important to examine the type of use and current the lines or equipment will be subjected to so that appropriate tests can be carried out.
For impulse tests, we need to check surges in transmission lines. Such surges may be the result of lightning strikes, which power lines need to be able to withstand. A lightning strike usually lasts only about 100µs, and the entire lightning stroke could be as much as a few seconds long.
To simulate lightning strikes, the standard wave is the IEC Standard impulse wave of 1.2/50µs is used.
For surge tests, the direct voltage needs to go from very low (preferably zero), to extremely high—hence the surge—in a very short time. Then, decay over a much longer time. This simulates real conditions and allows for materials to be thoroughly tested.
In conclusion—why high-voltage testing is so important
Ultimately, high-voltage testing is vitally important, both to ensure equipment is safe for service engineers, and also, that the materials will not degrade or break down when placed under high-voltage loads. There is wide variation in the levels of voltage that equipment will need to withstand, especially surges. For that reason, a broad range of tests is available to simulate real conditions.