With the continuously increasing penetration of networked microgrids (MGs) on the local utility grid (UG), MGs face the challenge to avoid increasing system fault currents during low-voltage ride-through (LVRT). To solve this challenge, an active fault current limitation (AFCL) method is proposed with three parts: 1) a novel phase angle adjustment (PAA) strategy is conducted to relieve the impact of MGs output fault current on system fault current; 2) the current injection (CI) strategy for LVRT is formulated to fit the function of PAA;
3) a novel converter current generation (CCG) strategy is developed to achieve a better voltage support ability by considering network impedance characteristics. The proposed AFCL method is applied to the back-to-back converter, as a connection interface between MGs and UG. Extensive tests and pertinent results have verified the improvements of proposed AFCL method with better LVRT performance, while the networked MGs output fault current does not increase the amplitude of system fault current.
- Networked microgrids
- Back-to-back converter
- Low-voltage ride-through
- Fault current limitation
Fig. 1. Structure of networked MGs and the corresponding fault current flow.
EXPECTED SIMULATION RESULTS:
Fig. 2. The PCC voltage of MG#1 and MG#2 with existing FCL method in -.
Fig. 3. The PCC voltage of MG#1 and MG#2 with proposed AFCL method.
Fig. 4. The MG#1 and MG#2 fault current with existing FCL method in -.
Fig. 5. The MG#1 and MG#2 fault current with proposed AFCL method.
Fig. 6. The MG#1 and MG#2 power injected by existing method in -.
Fig. 7. The MG#1 and MG#2 power injected by proposed AFCL method.
Fig. 8. DC voltage of BTB converter with proposed/existing FCL in -.
Fig. 9. The UG fault current with proposed/existing FCL in -.
Fig. 10. The system fault current with existing FCL method in -.
Fig. 11. The system fault current with proposed AFCL method.
Under the UG fault condition, in view of the high-level system fault current during the LVRT of networked MGs, an AFCL method is proposed to avoid monotonically increasing system fault currents during the LVRT of networked MGs. In this method, in order to improve the voltage control ability of LVRT, the CCG strategy is proposed by embedding the network impedance characteristics. Then, in order to achieve a better fault current limitation by relieving the impact of MGs fault current, the PAA strategy is proposed with considering voltage’s phase angle difference from UG and MGs to fault branch.
Meanwhile, the CI strategy is conducted to fit the feature of PAA. Numerous simulation results have validated the improvements of the proposed AFCL method with a successful LVRT, meanwhile, the networked MGs fault current does not increase the system fault current amplitude. Considering the fields with a high proportion of sensitive load, the BTB converter is widely used for the PCC connection point of DGs and MGs to provide high power quality. To reduce the fault current level, the AFCL method can be applied to the BTB converter, and can be also used to the other inverter products, such as wind and photovoltaic inverter, AC/DC microgrids, and HVDC transmission system.
 Q. Zhou, M. Shahidehpour, et al, “ Distributed Control and Communication Strategies in Networked Microgrids,” IEEE Communications Surveys & Tutorials, vol. 22, no. 4, pp. 2586-2633, Fourth quarter 2020.
 X. Zhao, J. M. Guerrero, et al, “Low-Voltage Ride-Through Operation of Power Converters in Grid-Interactive Microgrids by Using Negative-Sequence Droop Control,” IEEE Trans. Power Electron., vol. 32, no. 4, pp. 3128–3142, April 2017.
 I. Sadeghkhani, M. E. H. Golshan, A. Mehrizi-Sani, J. M. Guerrero, “Low-voltage ride-through of a droop-based three-phase four-wire grid-connected microgrid,” IET Gener. Transm. Distrib., vol. 12, no. 8, pp. 1906–1914, 2018.
 Y. He, M. Wang and Z. Xu, “Coordinative Low-Voltage-Ride-Through Control for the Wind-Photovoltaic Hybrid Generation System,” IEEE Journal of Emerging & Selected Topics in Power Electronics, vol. 8, no. 2, pp. 1503–1514, Jun. 2020.
 Y. Yang, F. Blaabjerg, and Z. Zou, “Benchmarking of grid fault modes in single-phase grid-connected photovoltaic systems,” IEEE Trans. Ind. Appl., vol. 49, no. 5, pp. 2167–2176, Sep./Oct. 2013.