A bushing is an insulated device usually made from porcelain (although other materials are becoming more popular). Bushings are used extensively in the electrical industry to allow an electrical conductor to pass safely through a grounded conducting barrier—basically through transformers or circuit breakers.
As such, it’s highly important to test your bushings regularly to ensure they are still able to perform according to specified requirements.
What types of bushings are there?
To be honest, there are quite a lot. Here are the main ones:
First, there’s the Composite Bushing. This is a bushing in which the insulation is made from two or more layers of different insulating materials.
Next, there’s the Compound-filled Bushing. As the name suggests, this bushing fills in the space between the major insulation and the inside surface of a protective weather casing (in most cases—porcelain).
There’s also the Condenser Bushing. These are made by arranging cylindrical conducting layers within the insulating material. The key defining factor in condenser bushing is their ability to control the distribution of the electric field.
Dry or Unfilled Type Bushings are for lower voltages (under 25kV) and are basically just a porcelain tube with no filler in the space between the shell and the conductor.
Oil-filled Bushings are very similar to compound-filled bushings. The main difference being that the ‘filling’ material, in this case, is an insulating oil.
Oil-immersed Bushings, as you might have guessed, involved completely immersing the major insulations in a bath of insulating oil.
And, finally, Solid (ceramic) Bushings are made by using a ceramic or similar material as the major insulation.
How often should bushings be checked and tested?
As a general rule of thumb, you don’t want your bushings to go more than five years without a thorough inspection. Most businesses check, test, and assess every three years. It’s also vital that inspections for all bushings over 115kV include power-factor tests.
Obviously, even lower-voltage bushings should be tested if there are any indications of deterioration. If there are signs that the materials or bushings are degrading, you should increase the test frequency to 6 months, rather than waiting for 3-5 years.
It goes without saying that anything that looks dangerous should be removed and replaced.
Specifically, when checking the bushings, you should check for:
1. Tight terminal caps and connectors—make sure these are tight and have good contact
2. Proper grounding for the bushings and correct gasketing so moisture can’t get in
3. Crumbling, chipped, or cracked cement
4. Any deterioration, looseness, or leaking of the gaskets
5. Any cracks or breaks in the metal pads such as clamping rings and washers
6. Any cracks or leaks in the solder seals
On top of that, you should also make more detailed, annual, visual inspections with binoculars on the oil level (if the type of bushing uses oil) and the porcelain. Make sure that the porcelain is free of chips, cracks, or contamination, which can seriously compromise its insulating properties.
What kind of maintenance checks are required/recommended?
There are 4 basic tests that need to be performed on bushings to ensure they can perform without causing issues owing to failure.
1. Power-factor Test
This is the most important and thorough test that is performed on bushings. The reason it is so useful is that it is the most effective field test you can use to detect any bushing contamination or deterioration. It also allows you to detect such issues very early.
In addition, the test provides an accurate measure of the AC test current, which is directly proportional to the bushing capacitance, which is obviously useful to know.
Depending on what type of bushing you are testing, there are a range of options for how to test: the grounded specimen test, the hot-guard test, the ungrounded-specimen test, and the hot-collar test. Each of these methods has advantages and disadvantages over the other methods, and you should select which one based on the particular circumstances you are faced with.
2. The Radio Influence Test (RIV) test.
This test is designed to detect any corona in resin-bonded, solid-core non-condenser bushings. If your bushing is liquid-filled (such as an oil-filled bushing), you would normally expect a very low RIV value. A high RIV value may indicate that the filling liquid is at an insufficient level, especially if you can’t reduce the RIV value simply by cleaning the porcelain.
3. The DC Insulation Resistance Test
This is a much less used test because it usually does not detect any issues until they are at an advanced stage. By the time a DC insulation resistance test indicates there may be a problem, it will require immediate attention.
4. Hot-wire test for moisture
Any moisture can seriously compromise your bushing’s ability to insulate and, therefore, any potential leakages need to be tested for. You can do this by inserting a red-hot rod into the compound and listening for a crackling or hissing sound. If there is no moisture, the compound will simply melt and you have nothing to worry about.