Phase-locked loop (PLL) technique is a critical control module applied in photovoltaic (PV) grid-connected control system to synchronize with grid voltage. The PLL error directly affects the response characteristic of PV grid-connected inverter under faults, leading to a mass of harmonic components in the output current of the PV inverter. Especially in the scenario of short-circuit fault with deep voltage sag, the PLL detection error is too significant to be ignored, posing challenges to transformer differential protection. This paper proposes an improved scheme that utilizes a digital computer algorithm to record and store pre-fault voltage, aiming to address the complexity and implementation challenges for phase-lock error in practical engineering applications. By utilizing the memory voltage, the PLL detection error is eliminated, mitigating harmonic distortion in the PV output current and ensuring the reliability of the transformer differential protection. However, introducing memory voltage may increase the short-circuit current of the PV output. Therefore, it is recommended to reduce the amplitude of the short-circuit current by multiplying the inverter port voltage reference value by an appropriate limiting coefficient k and outputting it to the physical system. Finally, the effectiveness of the proposed scheme is verified through MATLAB/Simulink simulation.

Turn to turn short circuit is a common fault of the Magnetic Valve Controllable Reactor(MVCR), but the current research on the characteristics of turn to turn faults is not clear. What’s more, the corresponding protection also has certain limitations. In this paper, a novel type of MVCR that has been put into operation in engineering is studied. Firstly, the topology structure of the novel MVCR is introduced, and the electrical characteristics of its steady-state operation are analyzed. Based on this, the fault characteristics of the excitation winding and working winding of the novel MVCR when turn to turn faults occur are studied. The study shows that when turn to turn faults occur, the amplitude of DC and AC component in the thyristor current both increase significantly, resulting in large current distortion. Due to the fault characteristics, this paper introduces the current distortion coefficient to quantitatively analyze the different characteristics of the two components before and after the faults. Furthermore, a protection scheme against turn to turn fault based on the thyristor current distortion is proposed. Finally, the feasibility and effectiveness of the proposed turn to turn protection scheme are verified by Matlab/Simulink simulation experiments.