Research Of Unified Power Quality Conditioner

Recently, because of the frangibility of power system, noneffective management of power industry, and the increasing use of nonlinear loads, all kinds of power quality problems arise, among which the harmonic problem is the most familiar power quality problem. Owing to harmonics, power quality deteriorates, and harmonics does harm to both power system and customers. People make great efforts to mitigate the influence of harmonics. LC passive power filters have the merits of simple structure, low price, and so on, and they are still extensively used today, but they also have a lot of disadvantages. Active power filter, as a new kind of power filter, with many advantages, which counteracts the harmonics in the grid, through injecting harmonics out of phase with the harmonics in the grid, are used more and more extensively.Parallel active power filter, the most important active power filter, which is suitable for compensating current harmonics, is used widely. Series active power filter is suitable for compensating voltage harmonics, and it can also be used to isolate current harmonics. Unified power quality conditioner is the integration of parallel active power filter and series active power filter. Unified power quality conditioner integrates the advantages of parallel active power filter and series power filter, and it can compensate current harmonics and voltage harmonics. Theoretically, unified power quality conditioner can solve most power quality problems.As for active power filters, harmonics detection and circuit control are the two important parts. The paper proposes a new harmonics detection method, in which detection of current and voltage harmonics under 5 cases is considered. Control is the core of active power filters. Fuzzy-PID control is used to control series active power filter, where fuzzy theory is used to adjust PID parameters dynamically. Hysteresis control, which is frequently used in current tracking technology, is used to control parallel active power filter. These two control methods are also used in series active power and parallel active power filter, respectively, in unified power quality conditioner. Results from the simulation show good filtering characteristics of active power filters.