ELECTRICAL DISTRIBUTION
RETHINKING POWER FACTOR CORRECTION
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David Gale
Low switching loss, high temperature semiconductors combined with pulse width modulation (PWM) and control algorithms based on Park-Clark transformations, providing a simple orthogonal vector presentation of three-phase vectors as the basis for control, are the backbone of active filters.
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he same technology is now being applied to displacement power factor correction giving rise to a smaller physical package, extended longevity, minimal maintenance requirements and superior power factor control. It is surprising, given the limitations of capacitor-based power factor correction, that there has not, to date, been a wider adoption of solid-state technology given that its use in HV and MV VAR compensation is well established. Self-commutated VAR compensators, in particular three-level compensated systems, are increasingly used in MV networks. For HV networks, the switching levels are increased to four and beyond. In flexible AC transmission system (FACTS) controllers, the technology has been well established. The Sinexel SVG three-level compensator, using PWM inverter control technology, is designed for LV power factor correction and offers some advantages over capacitor-based systems, chiefly: infinitely fine control of compensating reactive current; per phase compensation; leading as well as lagging compensation; rapid dynamic response, eg, regenerative drives (eg, overhauling
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elevators, winders, etc); line voltage independent compensating reactive current. Capacitive power factor correction for LV reticulation grew out of its use in MV and HV transmission and distribution where it was often a more economic and simpler alternative to synchronous condensers for the control of voltage. The application to LV is obvious, but what is less obvious are the engineering considerations to provide reliability and safety, speed of response and overcoming the limitations on correction dynamic range as well as fineness of control. As will be seen, providing for all these factors is, practically speaking, an impossibility. The use of PWM inverter control technology (see Figure 1) to provide lagging or leading correction of load current from -90° to +90° provides a unified method which can only be approximately simulated by a switched capacitor–inductor paralleled system. Although this combination is seen in transmission line compensation, it is not applied in LV reticulation where therefore only lagging power factor can be controlled. It might be argued that
JANUARY/FEBRUARY 2016 - ECD SOLUTIONS 41