With increased penetration of renewable power and the nonlinear loads in the distributed generation (DG) systems, increased power quality concerns are exhibited in the active distribution networks, especially the challenges associated with the current and voltage harmonics in the system. Various conventional harmonic compensation techniques are developed for voltage-controlled DG inverters in past, majority involve either multiple proportional-integral (PI) or proportional-resonant (PR) controllers in eliminating grid current harmonics. The current controlled inverters, on the other hand, are not preferred in industrial applications, accounting to their wide variations in the switching frequency. A novel and adaptive harmonic voltage control is developed here, for voltage-controlled DG inverters, which neither uses any PI regulators nor imposes stability issues associated with nonideal implementation of infinite gains of PR controllers. Interestingly, the developed control logic can be used for DG inverters, both in grid-connected and off-grid operational modes. Furthermore, this strategy allows a network operator to use this as an additional supplement that can be enabled/disabled as per the network requirement. The control logic exploits the property of an optimal-switching-vector controller, i.e., accurate output voltage tracking. Simulations results demonstrate the effectiveness of the controller, to suppress grid current harmonics and load voltage harmonics in grid-interfaced (GI) and off-grid modes, respectively, ultimately satisfying the mandatory IEEE standard-1547. Experimental results verify the viability of the controller for practical applications.
- Distributed generation (DG)
- Optimal switching- vector (OSV) control and voltage source inverter (VSI)
- Power quality
Fig. 1. System configuration.
EXPECTED SIMULATION RESULTS:
Fig. 2. Salient internal signals of the harmonic voltage controller upon enabling the harmonic control switch (a) vpabc5, vpabc7, vpabc11, and vpabc13, (b) |vp|abc5, |vp|abc7, |vp|abc11, and |vp|abc13, (c) λpabc5, λpabc7, λpabc11, and λpabc13, and (d) vc abc5, vc abc7, vc abc11, and vc abc13.
Fig. 3. Reference voltages generated by the harmonic voltage controller.
Fig. 4. Salient internal signals of the OSV controller upon enabling the harmonic control switch (a) voαβ, ioαβ, and vcαβ with SS1, and voαβ of future sampling instant with SS1, (b) “eα” of future sampling instant with SS1, MGPC with SS1, SS2, and SS3, and (c) MGPC with SS4, SS5, SS6, and SS0.
Fig. 5. Optimal minimization function and the corresponding switching sequences generated by the OSV controller.
Fig. 6. Performance of a single DG-VSI system in GI mode (a) without any harmonic voltage control and (b) with the presented control.
A harmonic voltage control strategy using optimal switching vector controller has been explored for a three-phase grid connected and off-grid DG system. A minimization criterion is used in an OSV controller to achieve accurate output voltage tracking performance and flexibly control the DG output harmonic voltage. In this way, the harmonic currents entering the grid are precisely regulated in the grid-connected mode of operation. In stand-alone mode of operation, the power quality is improved by elimination of PCC voltage harmonics caused by nonlinear load in the system. The controller eliminates the usage of multiple PR controllers, PI regulators, cascaded feedback loops, or phase locked loops in the system. The simulation and Fig. 17. System performance with OSV-based harmonic control (a) vgab with iga and iLa, (b) salient internal signals of the OSV controller reference voltages corresponding to gating pulses, (c) VDC, iga, and harmonic currents absorbed by the DG, and (d) performance without any harmonic control—iLa, vgab, iga, and ioa. Fig. 18. (a) THD of vgab. (b) THD of iLa. (c) THD of iga. experimental performances are evaluated to confirm the viability of the algorithm. The employed modern DG systems increased renewables and are subject to rapidly increasing nonlinear loads, and the presented control strategy is a possible solution for voltage-controlled DG inverters. As this controller is possible to be appended in existing DG inverter controls, it can be easily enabled or disabled flexibly, as per the system operator need.
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