By Patricia Irwin, P.E., Contributing Editor
On that fall day in 2001, it was hard to tell which was hotter, the transformer, which was engulfed in flames, or the temper of the petrochemical plant’s manager. A bushing had failed on the main transformer (138/13.8-kV, 40 MVA) serving the facility. This caused the fire that destroyed the unit, but more importantly (at least to the plant manager) it interrupted service. Although some timely switching restored power, the damage was done. It now falls on the shoulders of lawyers to straighten out who owes what.
Bushing failures, like the one described above, are not uncommon. According to Doble Engineering client questionnaires, the leading cause (35 percent) of transformer failures (above 100 MVA) is bushing problems. While most utilities periodically test bushings, there is a solid argument that periodic testing is not enough.
Some companies are moving to continuous monitoring, as opposed to periodic testing, to warn of impending equipment failures. Pictured here is a bushing outfitted with a continuous monitor.
Some companies are now relying on continuous monitoring, as opposed to periodic testing, to warn of impending failure–especially on suspect and aging bushings on large power transformers and high-voltage current transformers, as well as bushings on transformers where a failure would have serious consequences.
For those still relying only on periodic testing, there are some compelling reasons to consider continuous monitoring. While it obviously costs money to purchase and install an online monitoring system, it’s important to note that periodic bushing testing is not without its costs as well. Regardless of the frequency, testing takes time, ties up workers and requires an outage. And, periodic testing is performed at lower voltages and under temperatures conditions that differ very much from the operating ones.
But most importantly, periodic testing misses problems that can arise between tests.
That last point is one that Claude Kane, engineering manager for Eaton Cutler-Hammer, stresses as particularly important. Eaton Cutler-Hammer currently has two systems on the market–the InsulGard and InsulGard G2–which provide continuous monitoring of insulation deterioration in medium- and high-voltage equipment.
“A lot of bushing problems develop in a fairly short time–over a period of weeks to months,” said Kane. “So, testing every two or three years is obviously going to fail to detect quickly progressing problems.”
Anatomy of a Failure
If you lose a bushing, it is not necessarily true that the transformer will be damaged beyond repair. But, following a bushing failure, there is typically some amount of damage. How much depends on how the bushing failed.
A typical bushing failure occurs because the dielectric degrades. If no corrective action is taken, there will eventually be internal arcing, which often leads to a violent failure.
If the bushing fails at the air end, substation crews have to contend with flying porcelain, which is a real danger to anyone in the substation. Further, those flying pieces of porcelain can damage nearby equipment–most notably, other porcelain bushings.
If the bushing fails at the oil end, the oil in the main tank may be contaminated, the transformer coils may be damaged and/or the tank might rupture. And, there is always the concern about fire. Oil, oxygen and arcing are not a good mix. With luck, the fire will be limited to the oil contained in the bushing. In a worst-case scenario, oil from the main tank will also ignite–leading to a major fire and the complete loss of the transformer.
How it Works
Bushing tests are implemented to help avoid such potentially catastrophic equipment failures.
The physics behind a bushing test is simple. To evenly distribute the electrical stress in a high-voltage bushing, manufacturers include conductive layers (aluminum foils or semi-conductive coatings) within the oil-impregnated paper insulation of the bushing. The outer conductive layer is connected to the test tap. One way to monitor bushing health is to monitor the change in capacitance between the test tap and ground (Csub2) and within the bushing itself (Csub1). Figure 1 shows an equivalent circuit for a bushing.
Changes in a bushing’s capacitance and power factor can be determined easily, and if they begin to change, it is evident that something is changing within the bushing. (Adjustments for changes in bushing temperature must be made.)
An online testing system runs the three capacitors’ currents through balancing circuits and into a summing device. This sum should be zero. If not, the magnitude of the current will reflect the severity of the problem, and the current’s phase angle will indicate which bushing is bad.
The capacitance and power factor also can be used to produce a value called Gamma. Once the nominal value of Gamma is determined, it can be used as the null point on the monitoring unit, and any changes in Gamma will trigger alerts or alarms. Figure 2 is a block diagram of how Gamma is reached.
A bushing’s power factor and capacitance are both temperature dependent, so Gamma will vary with the temperature as well. The main source of heat transfer to the bushing comes from the oil in the transformer. Therefore, the top oil temperature must also be measured and factored in.
Continuous monitoring equipment can typically be tied into supervisory control and data acquisition (SCADA) systems and can send system nominal, alert and alarm signals. The signal itself cannot tell the engineers exactly what is wrong, but as with most monitoring systems, the goal is to give the bushing a way to say, “Hey, I have a problem out here. Send someone with some expertise to look at me.”
Bushing testing can detect many types of problems, including manufacturer defects and a breakdown of the capacitors’ dielectric. For oil-filled bushings, testing can also detect sludging and moisture in the oil.
Some continuous monitoring systems also allow the utility to perform periodic (or continuous) partial discharge tests that can detect other types of problems in the bushings or the transformer windings.
Keeping an Eye on Old Bushings
Manitoba Hydro, of Winnipeg, Manitoba, Canada, is one utility company that is using an online bushing monitoring system called InsulGuard G2 from Eaton Cutler-Hammer. Manitoba engineers field-tested the system before installing it on more transformers.
“We first monitored some bushings that we knew were deteriorated, so we got interesting results and did see definite trends in the Gamma reading,” said Bill McDermid, manager, insulation engineering at Manitoba Hydro. “They did not indicate progressive deterioration, but more of a cyclical change that was related to temperature.” McDermid said that this change was greater than one would expect from a “healthy” bushing.
Following the trial, Manitoba installed the system on several mid-aged bushings on critical transformers. Unlike the first experience, engineers have not seen much variation in Gamma on these units over the few years since installation–indicating that everything is OK.
Choosing which bushings to monitor did require some consideration.
“We had bushings that had a poor track record and were old, so we just replaced them with no thought of online monitoring,” McDermid said. “It just wouldn’t have made sense. But, we had some intermediate-age bushings that didn’t have a bad track record, but were aged. The consequences of their failure would be very great. In these cases, a continuous monitoring device seems to fit the situation very well.”
Installation and Payback
Systems vary, but can typically be installed in about a day (assuming the preliminary work is done in advance). At Manitoba it took several days because they decided to run the leads in metal conduit. The work was completed during a scheduled outage, so crews were not under any time constraints.
The price tag varies depending on manufacturer and what extras are chosen, but buyers can expect to pay $6,000 and up per transformer (monitoring three bushings). The payback period varies depending on how frequently the bushings were inspected before monitor installation. Obviously, detecting and avoiding a failure will greatly shorten the payback period.
Some utilities are specifying that continuous monitoring systems be installed at the factory prior to transformer purchases. Others are using the monitors to delay or avoid costly bushing replacements on aging devices. The monitors are also gaining popularity with industrial facility managers whose factories are dependent on a few critical transformers.
Patricia Irwin is a professional engineer, who began her career as a substation engineer with an East Coast utility. She is an experienced technical writer and has written articles for a variety of publications, including the Washington Post. She can be reached at EditorEW@aol.com.