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This article proposes an improved hybrid parallel compensator (IHPC) for enhancing the power transfer capability of a photovoltaic (PV) grid-connected inverter. A thyristor switched capacitor (TSC) module is used in series with a conventional inductive-coupling voltage source inverter to reduce the dc-link voltage. Under a low dc-link voltage, the voltage across the insulated gate bipolar transistor (IGBT) will also be low, which will reduce the voltage stress of the IGBT, thereby improving the reliability and reducing the loss of the voltage resource inverter. First, the mechanism of reactive compensation for improving the PV inverter power transmission capability is analyzed. Then, the circuit configuration and operating principle of the IHPC are introduced. After that, an optimal control strategy for the IHPC is proposed to address the resonance problem caused by the TSC connected in series with the L filter and to improve the compensation performance of the IHPC. Finally, the effectiveness of the proposed IHPC and the control method is validated by the simulation and experiments. The results show that the IHPC with optimal control strategy can achieve continuous reactive power compensation under a low dc-link voltage, thereby enhance PV power transfer capability. [ABSTRACT FROM AUTHOR]

A three-phase four-wire inverter is highly suitable for low-voltage distributed grids owing to its zero sequence control; however, essential aspects of grid-following inverters during faults, such as the power oscillation control flexibility and power injection capability, are rarely discussed in detail. Thus, by introducing zero sequence instantaneous power into a four-wire system, this article proposes a flexible control strategy that enhances the power injection capability of the inverter in the photovoltaic generation system during asymmetrical grid faults. First, a generalized reference current generation with flexible control of positive, negative, zero sequence current (FPNZSC), and power oscillations is presented. Second, a maximum allowable power injection (MAPI) algorithm based on the actual value constraints of power oscillations and peak current is presented according to the requirement. The proposed MAPI-FPNZSC strategy further extends the existing fault ride through methods in three-wire inverter systems, which is also suitable for single line-to-ground and double line-to-ground faults and can be easily realized in the natural or stationary frame. The effectiveness of the proposed strategy is verified by comparing the provided three typical objectives with the traditional method, and tested in a real-time hardware-in-the loop experimental platform. [ABSTRACT FROM AUTHOR]

• By the characteristic analysis of VSG, the causes and influencing factors of TPAI and FOC are revealed, and the inherent relationship between TPAI and FOC is found. • Based on the phase portrait theory, the stability analysis of different influencing factors is carried out. It is found that the proposed method can significantly enhance the TPAS of VSG under serious grid faults. • According to the verification results, the proposed method can simultaneously realize the TPAS and FCS of VSG during the fault, and the virtual inductance based on QSA can effectively suppress IIC. In addition, this method has the advantages of simple operation, no need to freeze the reactive power loop, and strong adaptability to engineering application problems such as considering the change of fault impedance and the difficulty of grid fault voltage measurement. When a serious grid fault occurs, a virtual synchronous generator (VSG) tends to lose transient power angle stability (TPAS) and cause the fault current to exceed the limit. Most existing research usually neglects the inherent relationship between transient power angle instability (TPAI) and fault over-current (FOC), which leads to the two problems being solved separately rather than simultaneously. In this paper, the transient power angle and fault current characteristics of VSG are first studied, and the causes and influencing factors of TPAI and FOC are explored. Then, the TPAS analysis is carried out based on the phase portrait theory. A TPAS control method for VSG considering fault current suppression (FCS) is proposed. This method is fulfilled by the combined regulation of active power reference and reactive voltage regulation coefficient, in which the reference and coefficient can be adjusted adaptively according to different fault degrees and application scenarios. In addition, a quasi-static approximate (QSA)virtual inductance is introduced to limit the instantaneous inrush current (IIC). The proposed method achieves both the TPAS and FCS at the same time during the fault. Finally, simulations and experiments verify the correctness of the theoretical analysis and the proposed method. [ABSTRACT FROM AUTHOR]

In the single-phase photovoltaic (PV) grid-connected distributed generation (DG) system located at the end of the feeder, it is necessary to provide quickly active and reactive power to the local loads. In this paper, an improved reactive current detection and power control method are proposed to realize active power control and reactive power compensation. To detect quickly the loads reactive current, a fast reactive current detection method using the derivative and ip\!\!-\!\!iq algorithm is presented, and it can overcome the long delay in a conventional phase-shift method. In the inner current loop, the quasi-proportional-resonant (QPR) control method with grid voltage feed-forward is presented to control accurately the grid current with zero steady error, and to reduce the current distortion due to the frequency offset and distortion of the grid voltage. The inner loop stability is analyzed, and the proper parameters are selected. In the outer loop, proportional–integrator (PI) controller is applied to stabilize the dc-link voltage, and power feed-forward is introduced to speed up system response. Simulation and experimental results verified the validity of the proposed control method. [ABSTRACT FROM PUBLISHER]

• MF-ASD can realize reactive power compensation and arc suppression. • The variation law of the DC-link voltage of the MF-ASD under different conditions is analyzed. • The proposed alternate arc suppression method does not need additional DC power supply devices. • The MF-ASD can also realize the DC-link voltage stability when the active component of the fault current is large. To fully compensate reactive and active components of the single line-to-ground (SLG) fault current, additional DC power supply devices are usually used for arc suppression devices. Therefore, the alternate arc suppression method is proposed for the non-neutral multi-function arc suppression device (MF-ASD) in this paper, which can achieve energy self-balance in the MF-ASD during the SLG fault without additional DC power supply devices. Firstly, the DC-link charge–discharge characteristic of the MF-ASD is analyzed under different SLG fault conditions. Then, the alternate arc suppression principle and energy flow law between two non-faulty phases of the MF-ASD are described in detail. Further, the relationship between the alternate period, switching time, and the DC-link voltage are deduced as well. Then, the control parameters are reasonably designed for the DC-link voltage stability of two non-faulty phases of the MF-ASD. Finally, the correctness and feasibility of the proposed method are verified by simulation and experiment results. [ABSTRACT FROM AUTHOR]