See the Invisible
It felt like a fine spring morning in eastern Oregon as my colleague and I rolled up the power plant driveway, although the date on my Blackberry indicated mid-February. After some friendly greetings and hand-shaking with maintenance personnel, we signed in at the control room and viewed the not too lengthy or boring safety video. Then it was time to don our hard hats and head toward the substation – all half-million volts of it.
It was dry and sunny as our work boots crunched against the coarse substation gravel rocks. The massive bushings and insulators did not appear to have much, if any, condensation from the night before as we gazed across an acre of GSU transformers, SF6 dead-tank breakers and other equipment. Ideal conditions for a corona image survey. After powering up our new corona camera and making a few minor adjustments, we began doing what no other person had ever done before in this substation – we were viewing an invisible phenomenon known as corona. What we and plant personnel saw during the next few hours that morning would better our understanding of high voltage engineering in a manner that no text book possibly could. We saw corona from insulators that required cleaning, sharp edges from hardware connections, poor bus work corona suppression, and dangerous insulator corona caused by cooling tower residue deposits. As the prevailing wind puffed another cloud from the cooling tower onto the westernmost insulators, we could hear the corona crackling intensify as the camera revealed near flashover conditions. “When it sounds like this, I tell my guys to get out of the substation,” our customer stated. Not a bad idea, I thought. Next, we followed our customer to a corner of the substation were we examined a flash-damaged insulator laying in the gravel, then observed the additional weather sheds added recently to the in-service insulators nearest the cooling tower. For the most part, the increased surface creep age of the modified insulators were doing their job by eliminating most of the corona. However, as can be seen in Figure 1 one insulator was clearly showing dangerous corona that consistently occurred from a defect located half way down the insulator. Additionally, we found severe corona activity occurring from the center phase GSU transformer bushing as shown in Figure 2.
We left the power plant feeling that the customer greatly appreciated the value of our new technology and would use the knowledge gained that day to reduce future flashover likelihood. One of the greatest difficulties when working with electricity is that it cannot be seen. The technician must generally determine its presence based upon meter deflection or simple visual or audible warning signals. Perhaps even more difficult to appreciate is the concept of electrical fields surrounding conductors or insulators in air. A visual representation of the electrical field is shown in Figure 3.
which illustrates the voltage gradient surrounding an insulator. Medium- and high-voltage equipment is carefully designed so that electrical fields are contained. If the electrical field is compromised by foreign materials, poor design, or other factors, corona will occur. Corona is responsible for the generation of ultraviolet radiation, ozone, acids, heat, mechanical-erosion through ion bombardment, electric power loss, and electromagnetic interference of radio communications. Corona by-products destroy insulation and lead to catastrophic failure while remaining invisible to the eye. Corona can occur due to: