Unified Power Quality Conditioner is a combination of a series and a shunt FACTS. Its target is to improve power quality compensating voltage flicker, unbalance, negative-sequence current and harmonics. Fig. 6 shows the scheme of connection of UPQC. UPQC (Khadkikar et al, 2004) comprises two voltage source inverters connected back to back and sharing a dc link. The shunt inverter helps in compensating load harmonic current and maintains dc voltage at constant level. The second inverter is connected in series by using a series transformer and helps in maintaining the load voltage sinusoidal and compensate voltage dips and swells.
Control system of UPQC is formed by the positive sequence detector, the series inverter control and the shunt inverter control.
Fig. 6. Scheme of Unified Power-Quality Conditioner.
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