Every Thursday around lunchtime our patrol would line up on the marching field. Over the period of an hour we would attempt our right flanks and left columns. I never seemed to be in step with the rest of the patrol. Maybe it was because I was shorter than the other guys. Maybe it was because I daydreamed too much. Maybe it was because I kept forgetting my left from my right. In any case, I'm sure my college ROTC (Reserve Officers Training Corp) experiences were as embarrassing for my marching instructors as they were for me.
Control room operators must contend with an electrical system that does not respond to marching orders any better than I did. Electricity seems to have a mind of its own. Yes, there are laws of physics and sophisticated modeling schemes that tell us where current will flow. But grid systems are becoming so complex that single events have a ripple effect on the entire system, making it nearly impossible to take an action without encountering an unanticipated consequence somewhere down the line.
In an effort to get electrons to line up and march straight, American Electric Power (AEP), Columbus, Ohio, U.S., partnered with Siemens Westinghouse and the Electric Power Research Institute (EPRI) to demonstrate the first use of a Unified Power Flow Controller (UPFC) near Inez, Kentucky, U.S.
Ben Mehraban, principal engineer with AEP, invited me to attend the UPFC dedication ceremony this past June. Based on industry hype, I expected to be overwhelmed with the complexity of the technology. Instead, I realized the concepts were fairly easy to grasp. Here's a simplified description of how the technology works:
-Create a dc bus voltage (in this case 24 kV) to serve as the basic building material from which to construct desired waveforms. -Use two sets of gate turn off (GTO) thyristor inverters to convert the 24-kV dc bus voltage into desired ac waveforms. -Smooth the voltage of the resultant waveforms through an intermediate transformer. -Tie each resultant waveform to the line via an appropriate series or shunt transformer.
The Siemens Westinghouse UPFC actually contains two sets of GTOs. One set is tied to the line through a shunt transformer and the other set is connected to the line through a series transformer. This series-parallel connection enables the UPFC to control voltage, line impedance and phase angle resulting in flexible control of both active and reactive power flows. The UPFC is connected to AEP's 138-kV line that runs between the Inez and Big Sandy substations.
This location was selected to address imbalances that occur under contingency conditions such as the loss of a 765-kV line. Early operating experiences have been positive, but AEP will provide an update after operators have gained additional operating experience.
Now that the first UPFC is on-line, utilities will turn their attention toward applications rather than focusing on the technology itself. Not all utilities will need the UPFC, but that doesn't mean they shouldn't understand the impact of the technology behind it. Other power electronic devices will likely find their way into most utilities: devices that provide stability control, power factor correction and voltage support. Other power electronic devices help mitigate power quality concerns by reducing the impact of switching surges, flicker and voltage dips. AEP recently installed one such device, the distribution static compensator (DSTATCOM), on a 12.4 / 7.2 kV circuit for VAR control to mitigate flicker on the line caused by varying motor loads encountered at three independent rock crushing facilities. This device enabled AEP to clean up the distribution circuit and to keep other customers on the line from being adversely affected while enabling the rock crushing companies to add capacity.
To keep in step with a rapidly changing industry, utilities will increasingly rely on advanced power electronic devices to enhance the operation of their power delivery systems and to address individual customer power quality concerns. Those utilities that decide to go low cost and ignore value-enhancing technologies might find themselves out of step with their customers and might ultimately discover they are no longer in the march.