Conventional fundamental frequency zero-sequence voltage (FFZSV) injection-based fault-tolerant operation methods cause power reversion under submodule (SM) failure conditions with low-power factor, which leads to overcharge risk in the cascaded H-bridge converter-based battery energy storage system (CHB-BESS). To solve this issue, a novel fault-tolerant operation approach has been proposed for the CHB-BESS. First, the power reversion mechanism and overcharge risk are analyzed with the conventional FFZSV injection-based methods. On this basis, the operation conditions of the CHB-BESS are divided into three stages according to the safe FFZSV injection region, the FFZSV is modified, and the negative sequence current is injected when necessary. As a result, the power flow of battery packs among the three phases can maintain the same direction, and the power reversion and overcharge risk can be avoided. Experimental results under various conditions verify that the proposed fault-tolerant operation approach of the CHB-BESS can achieve uninterrupted operation and avoid overcharge risk.

Grid-forming converter faults in islanded hybrid AC/DC microgrid clusters could lead to voltage and frequency in-stability risk in the subgrid, thus threatening system operating safety. To solve this issue, this paper has proposed an uninter-ruptable hierarchical distributed coordinated control scheme of is-landed hybrid AC/DC microgrid clusters. Firstly, when grid-form-ing converter faults occur in a certain AC microgrid (ACMG), the interlinking converter connected with the faulty subgrid switches its control mode into grid-forming, providing stable voltage and frequency support for the faulty ACMG. Then, a two-layer coor-dinated control scheme is presented, where the first layer is de-signed for the remaining interlinking converters, and the second layer is designed for converters in normal subgrids and grid-fol-lowing converters in abnormal ACMG together with its interlink-ing converter. By the control mode switch of the IC and the two-layer coordinated control scheme, accurate power sharing can quickly fill the power deficit in the faulty subgrid, and the voltage and frequency of the microgrid clusters can remain stable. Verifi-cation results indicate that with the proposed scheme, the islanded AC/DC microgrid clusters can operate uninterruptedly under dif-ferent kinds of grid-forming converter faults.

To fully explore the frequency support ability of the cascaded multilevel converters (CMCs), this article has proposed an adaptive virtual inertia emulation and control scheme. First, the basic inertia emulation mechanism is introduced, which is achieved by absorbing and releasing the capacitor energy in the CMC. Second, considering the influence of reactive power generation of the CMC, the available variation range of the dc components of submodule (SM) capacitor voltages is redefined and recalculated. As a result, the maximum power exchange in SM capacitors in the CMC, which is used for frequency support, can be precisely characterized. Afterwards, the highest/lowest frequency value is estimated under different source/load power disturbance, and the adaptive virtual inertia emulation and control method is designed according to the dynamic frequency extremum estimation and the maximum variation range of SM capacitor voltages. Finally, simulation and experimental results verify that by the proposed method, the CMC can enhance its inertia support ability and reduce frequency deviation of the system. Under 0.02 p.u. load step condition, the frequency deviation is reduced by about 32.8%.