Xiao Liu

Under load disturbance, the DC-side voltage of the three-phase pulse-width modulated (PWM) rectifier is prone to fluctuation. To improve the robustness of the DC-side voltage, a single-loop voltage direct control strategy of the three-phase PWM rectifier is proposed based on the active disturbance rejection technology, which has no current loop. In addition, by building the transfer function model, the performance of the three-phase PWM rectifier under the linear disturbance and nonlinear disturbance are analyzed and compared. Compared with the voltage-current double-loop progressive control architecture, the proposed voltage single-loop control strategy can directly regulate the DC-side voltage with using the current information, which can enhance the dynamic performance of the three-phase PWM rectifier. Meanwhile, due to the application of active disturbance rejection control technology, the steady-state performance of the PWM rectifier is also significantly improved. Finally, experimental results verify the feasibility and effectiveness of the proposed control strategy.

Under weak grid, the three-phase electric drive system is prone to cause sub/super-synchronous oscillation (SSO) due to the impedance interaction, which seriously threatens power system stability. Firstly, the SSO mechanism and characteristics of the electric drive system under weak grid are analyzed based on impedance method. Owing to the negative resistance characteristics of the motor system, the system stability declines with the de crease of grid strength, which can cause SSO. Then, by analyzing and establishing the specific relationship between the dc-side oscillation and the ac-side oscillation, a coupling oscillation self-suppression control strategy of the rectifier is proposed. By applying the dc-side voltage fluctuation, the proposed control can directly adjust the ac-side voltage without the need of voltage and current loop control, which can improve the response speed and enhance the system's oscillation suppression capability. Compared with the existing oscillation suppression strategies, the proposed control strategy can more effectively damp SSO and improve the steady state performance of the system. Finally, the proposed control and the theoretical analysis are verified by simulation and experimental results.

Due to the diversity and uncertainty of loads, the DC voltage of the PWM rectifier is prone to fluctuation. To address this issue, a DC voltage robust control of the PWM rectifier is proposed based on real-time load impedance observation. Firstly, based on the DC-side voltage and AC-side current, a Luenberger observer is proposed, which can real-time estimate the load im-pedance. Subsequently, by applying the observed load impedance, a sliding mode control based on feedback linearization is de-signed, which can quickly adjust the control algorithm and then improve the DC-side voltage robust performance of the PWM rectifier under load fluctuations. Moreover, the parameter design method of the proposed sliding mode controller is presented and the system stability is analyzed based on Lyapunov theory. Final-ly, the proposed control strategy is verified by simulation and experiment.

Under series-compensated grid, the load virtual synchronous machine (LVSM) without voltage and current control loop is prone to subsynchronous resonance (SSR) due to the impedance interaction. First, the sequence impedance model of LVSM is established, and the SSR mechanism and characteristics of the LVSM under series-compensated grid are revealed based on the impedance method. Then, by analyzing and establishing the mathematical relationship between the dc-side voltage fluctuation and the ac-side reactive power fluctuation, a SSR self-damp control strategy is proposed, which can automatically suppress the SSR of LVSM under series-compensated grid. Moreover, the mechanism, characteristics and design method of the SSR self-damp control strategy is introduced in detail. Finally, the proposed control and theoretical analysis are validated by simulation and experimental results.

Under load disturbance, the voltage quality of the pulse width modulation (PWM) rectifier-based dc power supply system will deteriorate, and the system may even become unstable. First, under constant power load, the oscillation mechanism and characteristics of the PWM rectifier-based dc power supply system is analyzed based on impedance method. To address this issue, a harmonic self-absorption control strategy of dc active power filter (DC-APF) is proposed, which calculates the distorted current by using the distorted voltage. Due to the voltage source control mode and small equivalent internal impedance, the proposed DC-APF can self-absorb the dc current harmonics and enhance the system stability. Compared with the traditional DC-APF, the proposed DC-APF can suppress the dc voltage oscillation and improve the dc power quality without using any harmonic extraction filters. Finally, the proposed control strategy and theoretical analysis are validated by simulation and experimental results.

Generally, the output power of photovoltaic (PV) is variable and uncertain, which can easily lead to dc-bus voltage fluctuation of the PV grid-connected inverter. To address this issue, a dc-bus voltage robust control strategy of PV dc modules grid-connected system is proposed based on the input power observation, which can improve the dc-bus voltage robustness and reduce grid current harmonics. First, based on the power conservation, a nonlinear disturbance observer of the PV grid-connected inverter is proposed to obtain the dc-side input power quickly and accurately. Subsequently, a feedback linearization-based sliding mode control of PV grid-connected inverter is designed based on the input power observation, which can improve the stability of dc-bus voltage. Meanwhile, the design method of control parameters is introduced, and the system stability is analyzed based on Lyapunov theory. Finally, the effectiveness of the proposed control strategy is verified by simulation and experiment.

Under weak grid, the photovoltaic (PV) power station is easy to occur subsynchronous oscillations (SSO) owing to the impedance interaction. First, with and without considering the static synchronous compensator (STATCOM), the SSO mechanism and characteristics of the PV power station under weak grid are analyzed and compared based on impedance method. The stability of the PV power station declines as the grid stiffness decrease, which can cause SSO. To address this problem, a voltage self-stability control of STATCOM is proposed by compensating for the distorted current based on the distorted voltage in d-axis. Under the proposed control, the parallel impedance of improved STATCOM and grid is attenuated significantly, which equivalently enhances the grid stiffness from the point-of-view of impedance. Without using any harmonic extraction filters, the proposed control of STATCOM can effectively improve the stability of PV power station, which is simpler. Finally, the proposed control and theoretical analysis are validated by simulation and experimental results.

With the application of single-phase converter, the single-phase phase-locked loop (PLL) has received widespread attention. In this article, a single-phase self-feedback PLL (SF-PLL) is proposed, which constructs an orthogonal voltage signal by utilizing its own outputs. In addition, a low-pass filter is applied to filter out the harmonics of voltage v_q, which can improve the steady-state performance of proposed SF-PLL. The mechanism, characteristics and parameter design method of the proposed single-phase SF-PLL are introduced in detail. Compared to the traditional second-order generalized integrator PLL (SOGI-PLL), the proposed single-phase SF-PLL is simpler and has superior dynamic and steady-state performance under grid disturbance. Finally, the effectiveness of the proposed singlephase SF-PLL is validated by simulation and experiment.