Like other California electric utilities, San Francisco-based Pacific Gas & Electric (PG&E) has been scrambling to meet the state’s renewable portfolio standard (RPS), which requires...
Ultracapacitors and batteries work together to solve power quality problems.
Power quality was not the important issue it is today when electrical appliances consisted of incandescent bulbs, induction motors and other devices of similar design. None of these items was overly sensitive to voltage sag, voltage swell, momentary interruptions or transients. Maybe a bulb would flicker or a motor would momentarily operate at reduced power; however, with the advent of microcomputers and sophisticated control circuitry, power quality has become important. Recent advances in storage devices such as improved batteries and ultracapacitors have made intermediate storage of electrical energy more practical and cost effective—which in turn helps utilities to deliver high-quality power at the local level.
Unlike most other products, electrical energy isn’t easily stored, so when an unexpected spike in demand occurs, it often leads to a reduction in power quality and manifests in voltage sag, harmonic distortion and variations in frequency. In severe cases, excessive load can cause a low frequency, forcing a generating facility to drop off line. This can put excessive load on other generating facilities, in some cases causing a domino effect and widespread power outage. The lack of efficient storage methods requires utilities to keep a vigilant eye on demand and be prepared to bring reserve facilities into action at a moment’s notice in order to prevent power interruptions.
New storage technologies are helping to make that job easier.
Demand changes come in two flavors: long- and short-term. The long-term changes are those that occur at a gradual rate, and are illustrated by the summertime increase in demand that accompanies a period of sultry summer weather resulting in the widespread use of air conditioners. This type of demand increase can be handled by the utility, provided it has sufficient capacity or can buy reserves from a neighboring utility via the grid network. More ubiquitous are short-term increases in demand that result from a sudden change in load that might occur when large machinery is brought online in a manufacturing facility. Indeed, the majority of fluctuations occur for a period of less than two seconds, and such momentary changes in demand represent the biggest headaches for utilities, as they lead to reduced power quality.
Power quality also is affected by aging delivery infrastructure. Transmission lines in many areas are overloaded, leading to higher line and power losses due to reactance created by an ever-increasing inductive load. With the proliferation of electronic devices and electric motors, inductive load is increasing, which in turn requires more and more reactive power to reduce the real power that’s delivered by utilities. This not only reduces profits to operators, but exacerbates the problems associated with the efficient delivery of quality power to the end user.
It would be advantageous for independent system operators (ISO) to have a fully integrated network so that power could be shifted from locations that are separated by great distances. However the