Three-phase three-leg inverter is one of the most popular topologies for inverters. When short-circuit fault occurs, the inverter is generally switched to the current-controlled mode from voltage-controlled mode to limit the fault currents. In this situation, the inverter becomes equivalent to a symmetric, three-phase, positive sequence current source. However, the voltage limiting may occur under asymmetrical fault. In this paper, a closed-loop model based on time-domain is originally developed as a means to investigate the fault current characteristics under asymmetrical fault. Based on the proposed model, a more intuitive insight into fault currents is presented as follows.1) The positive sequence fundamental component of three-phase currents can still track the symmetric current references. 2) The magnitude of fault phase currents (ignoring the harmonics) will vary between 1 and √3 times of magnitude of current limiting references. Based on the conclusion, although the magnitudes of fault phase currents can be reduced by adopting smaller current limiting references, the actual magnitudes of fault phase currents will still vary in a range depending on the load impendence of healthy phase. Therefore, the reliable action of the protection devices is greatly affected and the selective protection cannot be realized. Furthermore, in order to achieve completely controllable fault currents under asymmetrical fault, two alternative current limiting strategies are presented with reverse current limiting references, and with current limiting references based on the introduced phase angle regulation. The theoretical results are validated by simulations and experimental results.

Voltage-controlled three-phase three-leg (TPTL) inverters are widely applied in various occasions. The inverters are generally switched to the current-controlled mode to limit the fault currents when short-circuit faults happen. Under symmetrical fault, the inverter can be controlled as a symmetrical, three phase current source. However, the voltage limiting could happen under asymmetrical fault. As a result, the fault phase currents will be distorted and cannot be completely controlled. The switching devices could be damaged while the setting design and reliable action of the protection devices will be also greatly affected. Considering the voltage limiting is mainly caused by large load impedance of healthy phase under asymmetrical fault, the voltage limiting is avoided by reducing the load impedance by means of the virtual impedance in parallel with the output filter capacitor in this paper. As a result, the current limiting loop is not broken and the fault phase currents can be effectively controlled. In addition, the implementation of virtual impedance in control system and the influence of virtual impedance on the stability of system are also analyzed in detailed. The theoretical results are validated by experiments.

In two parallel three-level T-type converters (3LT²Cs) with the sharing DC-bus and AC-bus directly, the neutral-point (NP) potential balancing and the zero-sequence circulating currents (ZSCCs) have to be considered to ensure normal operation of parallel system. In the conventional control scheme, the distribution factor of small vectors with SVPWM strategies (or the injected zero sequence voltage with SPWM strategies) has to carry out two tasks simultaneously: NP potential balancing and ZSCCs suppression. However, the common challenge is how to coordinately control ZSCCs and NP potential due to only one controllable degree of freedom for two different targets. In this paper, the conventional scheme is investigated comprehensively. Theoretical analysis shows that the control authority over ZSCCs and NP potential is limited. Furthermore, a coordinated control strategy is proposed as follows: the distribution factor of one converter is in charge of the common NP potential balancing with the auxiliary NP bus, while the distribution factor of the other converter is in charge of ZSCCs regardless of NP potential. Based on the previous strategy, the distribution factor of every individual converter only needs to control one target, achieving the independent control of NP potential and ZSCCs. In addition, the performance of the proposed strategy is also evaluated. Theoretical analysis shows that NP potential balancing and ZSCCs suppression can be achieved simultaneously in two parallel 3LT²Cs. Finally, the effectiveness of the proposed strategy is verified by experimental results.

When a three-phase stand-alone inverter feeds several loads simultaneously, to ensure the high reliability and continuity of power supply for the large load network, the inverter needs to operate continuously, even during faulty conditions. Moreover, the inverter should provide large enough current by switching from voltage control mode (VCM) to current control mode (CCM) to trigger the circuit breaker connecting the short-circuit fault branch to recover the power supply. Therefore, when a load short-circuit fault occurs, the inverter will switch from VCM to CCM. In CCM, the short-circuit currents can be controlled by the current limiting strategy. Once the fault is cleared, to quickly supply the remaining loads, the output voltages should be restored to the rated voltage as soon as possible. However, the fast recovery process from the load short-circuit fault may cause significant voltage overshoot and oscillation. The voltage overshoot may lead to load damage, and the voltage oscillation may extend fault recovery time. To effectively solve above problems, the voltage overshoot introduced by large fault current and post-fault load impedance after fault clearance is firstly analyzed in this paper, then the suppression strategy based on the parallel virtual impedance is proposed; Secondly, the voltage oscillation introduced by the initial phase difference between the voltage references and the output voltages when the inverter switches from CCM to VCM is analyzed, and then the effective suppression strategy which can achieve seamless mode switching after fault clearance is proposed. Experimental results have been carried to verify the effectiveness of the proposed strategies.

The three-level neutral-point-clamped (NPC) inverter has attracted significant interest for high power/voltage applications, due to its significant advantages. A drawback of NPC inverter is that it is prone to voltage drifts and low-frequency voltage ripples of the neutral-point. In this paper, a novel control strategy of neutral-point balancing under SVPWM-based switching strategy is proposed which can be generally applied for all NPC inverters. Different from the previous control strategies, the proposed strategy utilizes a proportional resonant (PR) controller, which not only can eliminate voltage drifts, but also can minimize low-frequency voltage ripples at the same time. The effectiveness of the approach is documented by experiments obtained from a grid-tie three-phase NPC inverter.

The standalone solar‐storage AC supply system is an efficient form of utilizing renewable energy. The solar‐storage supply system normally includes two types of converters, grid‐forming (GFM) and grid‐following (GFL) converters. Nevertheless, the incompatibility of the dynamic performance between the GFM and GFL converters due to different control schemes may introduce the transient instability risk to the solar‐storage supply system. To address this issue, it is extremely necessary to perform transient stability analysis and stability‐enhanced control for such a system. In this paper, a sixth‐order model of the dc‐link timescale for the solar‐storage supply system is developed, whose accuracy and applicability are verified by comparing with the detailed electromagnetic model. Based on the derived model, the analysis demonstrates that the solar‐storage supply system faces the potential risk of transient instability under large disturbances of sudden load variation, due to the mismatch of the transient response speeds between virtual excitation control of the GFM converter and dc voltage control of the GFL converter. The transient stability boundary of the system is quantitatively investigated, by depicting the basin of attraction of the post‐disturbance equilibrium point. Furthermore, from the perspective of improving the coordination of the transient behaviours between the GFM converter and the GFL converter, the parameters related to virtual excitation control and dc voltage control are designed to enhance the transient stability of the system. Time‐domain simulations and real‐time simulations are conducted to validate the theoretical analysis. DC–AC power convertors, microgrids, power system control, power system transient stability, renewable energy power conversion