Parallel operation of inverter modules is the solution to increase the reliability, efficiency, and redundancy of inverters in microgrids. Load sharing among inverters in distributed generators (DGs) is a key issue. This study investigates the feasibility of power-sharing among parallel DGs using a dual control strategy in islanded mode of a microgrid. PQ control and droop control techniques are established to control the microgrid operation. P-f and Q-E droop control is used to attain real and reactive power sharing. The frequency variation caused by load change is an issue in droop control strategy whereas the tracking error of inverter power in PQ control is also a challenge. To address these issues, two DGs are interfaced with two parallel inverters in an islanded AC microgrid. PQ control is investigated for controlling the output real and reactive power of the DGs by assigning their references. The inverter under enhanced droop control implements power reallocation to restore the frequency among the distributed generators with predefined droop characteristics. A dual control strategy is proposed for the AC microgrid under islanded operation without communication link. Simulation studies are carried out using MATLAB/SIMULINK and the results show the validity and effective power-sharing performance of the system while maintaining a stable operation when the microgrid is in islanding mode.
- Inverter parallel operation control strategy
- Droop control strategy
- Frequency restore
- Power sharing
Fig. 1 Enhanced droop control diagram
EXPECTED SIMULATION RESULTS:
Fig. 2 Output voltage and current waveforms at PCC under the proposed dual –control
Fig. 3 Tracking output power of PQ control inverter
Fig. 4 Active power dynamics response at PCC
Fig. 5 Reactive power dynamics response at PCC
Fig. 6 Frequency variation with and without FRS
This letter proposes a compact nine-level T-type packed U-cell inverter. Compared with other nine-level inverters, the proposed topology has fewer power semiconductor devices and only needs two isolated dc sources. Furthermore, the proposed PWM scheme only uses one carrier, which can reduce the design and control complexity. Since the T-type leg will generate the high frequency switching waveform, it can be replaced by SiC MOSFETs for significantly reducing switching losses. Experimental results verified the performance of the proposed multilevel topology.
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