**7. Conclusions**

It has been expected that the mass application of RE-based distributed generation (DG) microgrids or smart grids, and various static and dynamic nonlinear loads is the future trend of development in power systems. To ensure an acceptable PQ level, the development of advanced PQ compensation schemes using new technologies in terms of power-switching devices and state-of-the-art control algorithms is a significant task. In this aspect, this paper has demonstrated, for the first time, a shunt-type GaN HEMTs-based three-phase APF controlled by DSP systems and type II controllers to achieve simultaneous compensation for current harmonics, load imbalance, and reactive currents. Based on the results obtained from simulation and the hardware tests, the proposed 2-kVA GaN-based three-phase shunt APF prototype with digitally integrated control scheme is able to achieve satisfactory compensation results in improving system-wide load current quality of a complex load network consisting of distorted, non-linear and unbalanced loads. In this study, the TPH3207 power switching devices and Si8271 driving integrated circuits (ICs) are successfully adopted. It has been found that GaN HEMTs provide superior performance to conventional Si-based power switches in terms of switching frequency, temperature feature and system e fficiency. To further evaluate the system performance of the constructed GaN-based circuit prototype, in terms of e fficiency, hotspot distribution and power losses in components, a thermographic analysis has been carried out. Results and discussions for improving the proposed implementation scheme have been presented. With the proposed APF operating at 50 kHz switching frequency, the THD and UR of the three-phase grid currents can be greatly reduced. The best THD improvement recorded is from 20.23% to 4.15% and UR is from 14.83% to 2.13% and the highest system e fficiency of 97.2% has been achieved. For future research works, better circuit components and GaN HEMT driving methods based on a bootstrap design can be used for achieving higher switching frequency and better system performance.

**Author Contributions:** The corresponding author, C.-T.M. conducted the research work, proposed the design methods, designed the simulation and hardware test scenarios, verified the technical contents and polished the final manuscript. Z.-H.G., a postgraduate student in the department of electrical engineering, national united university, performed the paper search, managed figures and checked the related data. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by MOST, Taiwan, with gran<sup>t</sup> numbers, MOST 108-2221-E-239-007, MOST 107-2221-E-239-036 and MOST 106-2221-E-239-026 and the APC was funded by MOST 108-2221-E-239-007.

**Acknowledgments:** The author would like to thank the Ministry of Science and Technology (MOST) of Taiwan for financially support the energy-related research regarding key technologies and the design of advanced power and energy systems.

**Conflicts of Interest:** The authors declare no conflict of interest.
