Today, the transmission system is required to carry higher loads without interruptions while government regulations require periodic maintenance deemed necessary to prevent unexpected outages. There was a time when U.S. utilities would schedule major maintenance on their transmission systems around the July 4th holiday because many industrial customers would shut down to let employees take a week of vacation.
In late July 2012, there were two massive power outages in India. One of them started around 2 a.m. when the load was supposed to be low but was not. This and other experiences reinforce the reality that it seems to be getting harder and harder to find the best time for outages to do much-needed maintenance.
Ask transmission engineers what is the weakest component in a transmission line, and the answers range quite a bit. Some will suggest it is the conductors, which are spliced together with a compression sleeve using a multi-ton press, others will nominate suspension insulators, which have more than a 2:1 factor of safety for mechanical loads.
It is really line switches that are the weakest link on the grid. Line switches are installed at many tap lines to serve new load. These switches have multiple moving parts with full-load electrical contacts that simply slide together. Yet, they are expected to carry heavy loads, function during severe storms and carry whatever fault conditions may happen.
What is the minimum requirement for switch maintenance? It is so easy to reference IEEE Standard C37.30.1-2011, Section D4, which addresses switch maintenance. However, a transmission line maintenance engineer with 20 years of experience opening more than 1,000 energized switches knows it is not as simple as reading a standards document. But, first, why is an engineer doing switching? Years ago, a young, naïve engineer would get to maintenance sites early in the morning to switch lines for maintenance crews so they could start work on time. Then, when the job was done, the engineer would switch the lines back in the evening. Vocal complaints about the condition of the switches led to this present job and all of that switching experience.
Testing and Installation
The conclusion of that experience is at least 75% of switch problems are caused by improper installations of the new switch. Whose fault is that? It is shared equally by the utility and the switch manufacturer. The utility requires all switches pass the requirements of the IEEE/American National Standards Institute (ANSI) standards. Therefore, switch manufacturers use an indoor lab to test a switch designed to be operated outdoors and perform all tests under what one might consider ideal conditions.
The fault duty test is illustrative of the folly of this practice. The switch is bolted down on a table in a lab and is subjected to, and passes, the fault duty testing. Then, out in the real world, a utility installs the same switch on a 125-ft (38-m)-tall transmission structure, which is relatively flexible, and expects the switch to remain closed under fault conditions. Another example of testing versus reality is the requirement that a switch successfully perform 1,000 mechanical operations. Not only is this test done in the safety of a lab, but the operations are performed on a de-energized switch.
The switch manufacturer assumes the utilities know how to properly install and adjust a switch so it remains tightly closed during a fault. The utility assumes the manufacturer's switch will handle fault duty by meeting the minimum requirements of the installation instructions. A line switch must either latch or use over toggle on the interphase linkage to remain closed during fault conditions. However, the installation crew thinks the lock on the operating handle is the only one needed to hold the switch closed. Depending on the height of the transmission structure, the locked operating handle might be 50 ft (15 m) from the switch, which leaves a lot of room for movement of the operating linkage.
The Real Problem
Here is the root of the problem: Most line switches are installed with part-time switch installation crews. Most transmission line crews are very well trained in framing, setting structures and pulling wire, but little, if any, training goes into installing switches. Even if crews are trained, they might install one or two line switches on a job and then go six months or a year before installing the next one. Then, if the crew personnel have changed by that time, all bets are off.
Having just any crew install a line switch is like asking a flight attendant to fly the plane because the attendant has seen it done so many times. Remember, as stated earlier, many switch manufacturers assume the utility knows how to install the switch. The installation instructions could sometimes be better. Another part of the problem, utilities seem to always purchase switches from a low-cost supplier. Seldom are maintenance and repair costs considered when purchasing a new switch.
Imagine a utility doing maintenance on an improperly installed switch and expecting the switch to work. Not only that, the more the utility finds wrong with the switch, the more maintenance it does on the switch. Einstein said insanity is doing the same thing over and over again and expecting different results.
Insanity also might be putting a remote-controlled motor operator on a poorly adjusted switch and expecting it to operate properly. Then, since the motor is remotely operated, implement a maintenance procedure to operate the switch annually, because IEEE standards suggest this. Following this procedure does allow the discovery of dangerous switches. It also might increase customer outages if the switches fail.
Imagine a switch is properly installed and works great but the system changes. A line once lightly loaded now could have several new customers running huge motors, causing a large lagging power factor. The switch can be opened with little or no arcing. But, if the switch is going to tie two sources together, one side has no power factor and the other has a tremendous lagging power factor, then it is best to have the video camera ready. The display will be quite exciting. In the name of line switch maintenance, this operation will probably lessen the switch's life significantly because of excessive arcing caused by tying the two unbalanced lines together. By the way, the IEEE standard does recommend de-energizing the switch if possible when doing maintenance.
What to Do?
Do not purchase switches strictly based on the initial cost. Instead, evaluate the initial cost plus the expected maintenance and repair costs based on a utility's experience with each manufacturer. Try to find highly trained switch crews or train crews and, most importantly, keep them trained. Take advantage of the training many switch manufacturers provide. Operate energized switches only when necessary or jumper them out to reduce unnecessary arcing. Use infrared and visual inspections on switches that are difficult to obtain an outage for maintenance. Lastly, use a smartphone to take videos when operating energized switches to share any excitement. Most importantly, be safe.
David L. McMullen ([email protected]) is the lead product engineer for transmission switch with the Southern Co. He has more than 38 years experience in transmission line design, operations and maintenance. His job includes product support for transmission line switches and automation. It includes setting up engineering standards and drawing, approving suppliers, making maintenance recommendations and investigating switch failures.
Editor's note: This article was originally presented at the Southeast Electric Exchange meeting in June 2012.