Public transportaion is more vulnerable than you might think
Power quality and the problems that arise when the quality of the supply doesn’t match the requirements of the equipment are most often thought of in terms of the resulting loss of productivity in commercial applications such as data centers and automated processes. Users are well aware of the economic impact of an interruption to the process, especially in the financial markets where the losses are measured in millions of dollars or more each time.
This type of problem has other far-reaching effects—such as the potential loss of public safety and security and the general inconvenience that can result from power quality phenomena. Each industry and user has its own concerns. For example, keeping the radar systems critical to maintaining the aircraft safety running is being addressed by organizations such as the Federal Aviation Administration and London Air Traffic Control Center. Besides having multiple redundant power systems, new state-of-the-art power quality monitoring systems are being increasingly deployed to make sure these systems are performing as expected.
Sags and drops
For years, an area of the New Jersey shore would experience sags and even sustained interruptions and outages during the vacation months of the summer. The problem wasn’t overly complex —the area was a barrier island that had an inadequate source of supply to match the growing load demanded by users. If one of the feeders was out or the weather was too hot, causing increased energy demand, Ohm’s and Kirchoff’s laws would result in interruptions to the supply, mostly temporary. Eventually, it led to a several-day blackout that garnered an in-depth analysis of the problem with a study on the distribution generation of power and the installation of new transmission lines.
Many forms of public transportation are vulnerable to power quality phenomena, as they also rely on electricity of a certain quality for proper operation. This includes an airport monorail , subways under cities, traffic-light controls and even elevators in multistory buildings. One of the largest elevator companies indicated the majority of service calls for elevators are the result of power quality problems, especially now that most elevator control systems involve adjustable speed drives.
The saga continues. Recently, power quality experts did an investigation at the apartment of an investment fund manager in a major East Coast city. He was complaining about blinking/flickering lights. As is typical with many power quality investigations, a monitor was connected to an outlet in the room where he saw the lights blinking. The characteristics of the electricity supply and the loads usually change throughout a week, so the minimum monitoring period to make a proper assessment is normally one week.
As it turned out, there were about 100 events captured over the time period. To him, it was enough evidence to confront the electric utility and get them to eliminate the problem.
However, that data alone isn’t enough to determine the source of the disturbance. In addition, only a few of the events captured were the source of his complaint. The next location to monitor to assess the problem would be at the point where the building electrical supply comes into his apartment, to determine if the origin is there or back upstream near the supply. Then, the monitor is moved to the service entrance, which is referred to as the point-of-common-coupling, where the electric utility service meets the building’s electrical system. Each time, the data must be examined to determine whether you are getting closer to the source of the problem or further away. It is like a fire investigation, where you start at the outside and work your way along the trail until you come to the point of origin.
Oust the source
Once the origin of the power quality disturbances is found, it doesn’t mean that the problem will go away. In the United States, there aren’t any overall power quality standards to which suppliers of electricity must comply. Most public utility commissions use indices such as SAIFI and CAITI to measure the performance of an electric utility. These only count interruptions that last longer than five minutes to generate the report card. Even in parts of the world where there are standards for power quality phenomena such as sags, harmonics and flicker, they often use the 95 percent rule—95 percent of the time, the various PQ parameters that are defined in the standard must be within the specified limits.
One could look at that as a 5 percent acceptable inconvenience factor. To the public, does that mean that out of 20 trips up and down in an elevator, you would expect to get stuck one time, or that the lights would be flickering for an hour per day? The good news is that the quality of supply in most places is already far better than that. The bad news is that when it isn’t acceptable, it can be a challenging task to have someone else improve it, rather than employing mitigation equipment within the facility. It often comes down to a final analysis of the cost of the fix versus the cost of the problem. EC