This paper presents the study, analysis and practical implementation of a versatile unified power quality conditioner (UPQC), which can be connected in both three-phase three-wire or three-phase four-wire distribution systems for performing the series-parallel power-line conditioning. Thus, even when only a three-phase three-wire power system is available at a plant site, the UPQC is able to carry out power-line compensation for installed loads that require a neutral conductor to operate. Different from the control strategies used in the most of UPQC applications in which the controlled quantities are non-sinusoidal, this UPQC employs a dual compensation strategy, such that the controlled quantities are always sinusoidal. Thereby, the series converter is controlled to act as a sinusoidal current source, whereas the parallel converter operates as a sinusoidal voltage source. Thus, because the controlled quantities are sinusoidal, it is possible to reduce the complexity of the algorithms used to calculate the compensation references. Therefore, since the voltage and current controllers are implemented into the synchronous reference frame, their control references are continuous, decreasing the steady-state errors when traditional proportional-integral controllers are employed. Static and dynamic performances, as well as the effectiveness of the dual UPQC are evaluated by means of experimental results.
1. Active filter
2. Dual control strategy
3. Power conditioning
4. Three-phase distribution systems
Fig. 1. 3P4W distribution system based on UPQC topology connected to 3P3W power system.
EXPECTED SIMULATION RESULTS:
Fig. 2. Experimental results for the loads presented in Table III: (a) UPQC currents for unbalanced three-phase -phase load (1) (20 A/div, 5 ms/div): Load currents (𝑖𝐿𝑎, 𝑖𝐿𝑏, 𝑖𝐿𝑐) and 𝑖𝐿𝑛, Compensated source currents (𝑖𝑠𝑎, 𝑖𝑠𝑏, 𝑖𝑠𝑐), and Currents of the parallel converter (𝑖𝑐𝑎, 𝑖𝑐𝑏, 𝑖𝑐𝑐) and 𝑖𝑐𝑛; (b) Currents and voltages of phase “a” of the UPQC for the unbalanced three-phase load (2) (20 A/div, 100V/div, 5 ms/div): Load currents (𝑖𝐿𝑎, 𝑖𝐿𝑏, 𝑖𝐿𝑐); Currents of phase “a”: load 𝑖𝐿𝑎, parallel converter 𝑖𝑐𝑎 and source 𝑖𝑠𝑎; voltages and currents of phase “a”: load current 𝑖𝐿𝑎 , source current 𝑖𝑠𝑎, utility voltage 𝑣𝑠𝑎 and load voltage 𝑣𝐿𝑎, (c) UPQC currents for three-phase load (1) (2.5 ms/div): Load currents (𝑖𝐿𝑎, 𝑖𝐿𝑏, 𝑖𝐿𝑐) (5 A/div), Source compensated currents (𝑖𝑠𝑎, 𝑖𝑠𝑏, 𝑖𝑠𝑐) (10 A/div), Parallel converter currents (𝑖𝑐𝑎, 𝑖𝑐𝑏, 𝑖𝑐𝑐) (10 A/div).
Fig. 3. Voltages of the UPQC under utility harmonics and unbalances for the unbalanced three-phase load (1): (a) Utility voltages (𝑣𝑠𝑎, 𝑣𝑠𝑏, 𝑣𝑠𝑐) (50 V/div, 2,5ms/div), Load voltages (𝑣𝐿𝑎, 𝑣𝐿𝑏, 𝑣𝑠𝐿) (50 V/div, 2,5ms/div) and series compensating voltages (𝑣𝐶𝑎, 𝑣𝐶𝑏 and 𝑣𝐶𝑐) (50 V/div, 2,5ms/div); (b) (a) Utility voltages (𝑣𝑠𝑎, 𝑣𝑠𝑏, 𝑣𝑠𝑐) (50 V/div, 2,5ms/div), Load voltages (𝑣𝐿𝑎, 𝑣𝐿𝑏, 𝑣𝑠𝐿) (50 V/div, 2,5ms/div) and series compensating voltages (𝑣𝐶𝑎, 𝑣𝐶𝑏 and 𝑣𝐶𝑐) (50 V/div, 2,5ms/div)
Fig. 4. Voltages and current of the UPQC for the unbalanced three-phase load 1: (a) DC-bus voltage (𝑉𝐷𝐶) (100 V/div, 500ms/div) and load currents (𝑖𝐿𝑎, 𝑖𝐿𝑏, 𝑖𝐿𝑐) (20 A/div, 500ms/div); (b) DC-bus voltage (𝑉𝐷𝐶) (100 V/div, 500ms/div) and source currents (𝑖𝑠𝑎, 𝑖𝑠𝑏, 𝑖𝑠𝑐) (20 A/div, 500ms/div); (c) DC-bus voltage (𝑉𝐷𝐶) (100 V/div, 5ms/div) and details of the source currents (𝑖𝑠𝑎, 𝑖𝑠��, 𝑖𝑠𝑐) after the first load transient (20 A/div, 5ms/div).
Fig. 5. UPQC under voltage sag disturbance (phase ‘a’): utility voltage (𝑣𝑠𝑎), load voltage (𝑣𝐿𝑎) and series compensating voltage (𝑣𝐶𝑎) (200 V/div, 25ms/div).
This paper presents a practical and versatile application based on UPQC, which can be used in three-phase three-wire (3P3W), as well as three-phase four-wire (3P4W) distribution systems. It was demonstrated that the UPQC installed at a 3P3W system plant site was able to perform universal active filtering even when the installed loads required a neutral conductor for connecting one or more single-phase loads (3P4W). The series-parallel active filtering allowed balanced and sinusoidal input currents, as well as balanced, sinusoidal and regulated output voltages. By using a dual control compensating strategy, the controlled voltage and current quantities are always sinusoidal. Therefore, it is possible to reduce the complexity of the algorithms used to calculate the compensation references. Furthermore, since voltage and current SRF-based controllers are employed, the control references become continuous, reducing the steady-state errors when conventional PI controllers are used. Based on digital signal processing and by means of extensive experimental tests, static and dynamic performances, as well as the effectiveness of the dual UPQC were evaluated, validating the theoretical development.
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