Aimed at the issue of DC offset injection to the grid when the topology of a quasi-Z-source grid-connected inverter without isolation transformer is adopted, a DC offset suppression strategy based on an observer under weighted average current control is proposed in this paper. First, the topology of the quasi-Z-source inverter and the causes of DC offset are analyzed. Second, the order of an LCL filter is reduced from the third-order to first-order by weighted average current control, thus simplifying the system model and eliminating the resonance problem. Then, a disturbance observer is utilized to observe the DC offset of inverter output voltage in realtime, the DC offset disturbance is dynamically compensated by feedback, and its stability is analyzed. Finally, the proposed strategy was validated by experiments. Experimental results show that the proposed strategy can effectively reduce the DC offset of grid-connected current, meet the grid-connected DC offset requirement of less than 0.5%, and ensure good steady-state and dynamic performances of the inverter.

To address the problem that phase-controlled and fully-controlled rectifiers cannot combine high power level, high hydrogen production efficiency and high reliability in the field of electrolytic hydrogen production at present, a hybrid rectifier topology which consists of a main power rectifier and an auxiliary converter in parallel and its control strategy are proposed. The main power rectifier is a thyristor rectifier, and the auxiliary converter consists of a pulse width modulated voltage source converter and a phase-shifted full-bridge converter in cascade. Through an analysis of the mathematical model of the auxiliary converter, the compensation of ripple current and the absorption of harmonic current are taken as control objectives, and a current control method of repetitive control with proportional resonance control is given to realize an efficient operation of the hydrogen production unit while optimizing the input current quality. In addition, three fault-tolerant operation modes of the hybrid rectifier topology are proposed to improve its reliability. An electrolytic hydrogen production platform was built and semi-physical simulations were performed to verify the hybrid rectifier topology and its control strategy, and results proved the topology's correctness and effectiveness.

Aimed at the problems of active power fluctuation and current asymmetry of a Vienna rectifier caused by asymmetric grid, active power fluctuation suppression control (APFSC) and symmetric positive-sequence current control (SPSCC) are introduced. Due to the current limitation of power devices, the use of APFSC will reduce the maximum active power of the Vienna rectifier. In comparison, the use of SPSCC can expand its maximum active power at the cost of significant active power fluctuation. With the comprehensive consideration of disadvantages of the above two methods, an improved power control (IPC) strategy is proposed. When the processing power of the Vienna rectifier is small, keeping the active power without fluctuation is the primary control target. In this case, the effects of IPC and APFSC are the same. The active power fluctuation under IPC is effectively suppressed, while that under SPSCC is large. When the processing power of the Vienna rectifier is large, meeting the power demand is the primary control target, but the active power fluctuation should be reduced as much as possible. Under this circumstance, IPC can meet the active power demand, but APFSC cannot meet the power demand. On the premise of meeting the power demand, the fluctuation of active power under IPC is smaller than that under SPSCC. Finally, experimental results verified the advantage of the proposed control method.

The contradictions faced in practical engineering and design have proved the necessity and urgency of research on electrified railway dual-network interconnection converters. The major needs and engineering challenges in practical applications of dual-network interconnection converters are analyzed. According to the characteristics of the railway electric power supply and distribution network, a topological structure of an electrified railway dual-network interconnection converter with current limiting is proposed, its working principle and composition are analyzed in detail, and its variable impedance control strategy in voltage control mode and current limiting switching control mode is put forward. Moreover, the feasibility of the variable impedance control strategy is proved by a simulation model. In particular, an engineering prototype was used to prove that the voltage control mode of the converter has good practicability, and it can solve the difficulty in actual survey design, high cost, power quality degradation and other issues. Finally, the development potential of the interconnected converter in the future is prospected.

During the dynamic process of a system, the current control of a converter realizes a balance between current and reference by adjusting the output voltage when the grid-side current changes, which is represented as voltage response under the current excitation. Since the AC voltage of the converter is generated by controlling the amplitude/frequency and the amplitude/frequency of voltage is required to be maintained during the system operation, the current control should be described as the internal voltage amplitude/frequency response under active/reactive current excitation when a power imbalance occurs. At the same time, from the perspective of a grid-connected converter's influence on the grid, the active/reactive current response under the terminal voltage amplitude/frequency excitation is another perspective for understanding the characteristics of the grid-connected converter. Therefore, for the current control of the converter, the active/reactive current-internal voltage characteristics used in the dynamic analysis of large systems and the terminal voltage-active/reactive current characteristics from the perspective of the influence of equipment (e.g., single machine) on the grid are presented. On this basis, it is determined that the two kinds of characteristics are essentially equivalent by explaining the redundant relationship between current and terminal voltage under the current control. Finally, the equivalence is verified by simulations, and the influence of phase-locked control parameters on current control characteristics is preliminarily explored.

A cascaded H-bridge (CHB) multilevel inverter is composed of many unit modules, and the probability of failure is relatively high. The fault-tolerant operation can effectively improve the reliability and performance of a system. By injecting zero-sequence voltage to enhance the output capability after fault, the fault-tolerant control method has advantages of not adding any additional cost and simple implementation. As a result, it has attracted wide attention. The zero-sequence voltage value and fundamental component value injected by different implementation methods are different, and an excessive zero-sequence voltage will have a great impact on the motor bearing and insulation in the motor drive. Aimed at this problem, a zero-sequence voltage (containing third harmonic component) injection method is discussed to reduce the fundamental component of injected zero-sequence voltage, so as to decrease the influence of injected zero-sequence voltage on the motor operation. The effectiveness of the proposed fault-tolerant control strategy was verified by simulations and a 10 kW 7-level converter prototype.

The connection of distributed generations to DC grid can improve the consumption level of new energy. In view of the fact that the doubly-fed wind turbine is connected to medium-voltage DC grid through a diode uncontrolled rectifier, a wide-frequency operation control method is proposed, which can limit the rotor-side feed power in the whole wind speed range and further greatly reduce the cost of a rotor-side converter. When the wind speed and rotational speed are high, the stator frequency is properly increased, and the rotor-side power is limited to a small constant power with a certain negative slip. When the wind speed is low, the rotor-side power is operated at a constant stator frequency lower than the power frequency, so as to make the slip rate change positively or negatively in a small range and limit the absolute value of rotor-side power accordingly. A simulation example is taken, in which the absolute value of rotor-side power of a 1.5 MW doubly-fed wind turbine is limited within 140 kW in the full wind speed range, and results verify the effectiveness of the proposed wide-frequency operation control method in reducing the capacity of the rotor-side converter. Finally, the feasibility of the proposed wide-frequency operation control technology was verified through a laboratory-scale experimental platform of 5.5 kW doubly-fed unit connected to DC grid with uncontrolled rectifier.

In a flexible DC transmission system for offshore wind power generation, surplus power will be generated in the system when the land end is greatly unloaded or the AC system fails, resulting in a rapid increase in the DC transmission voltage. A DC energy-consuming device can quickly dissipate the surplus power, which is a key technical solution to ensuring smooth fault ride-through and reliable operation of the system. However, the modulation voltage of the energy-consuming valve will become negative when the surplus power is large. To solve this problem, a hybrid DC energy-consuming device based on half-bridge and full-bridge sub-modules and its modulation strategy are proposed in this paper. By using the negative-voltage output capability of the full-bridge sub-module, the matching range of the surplus power can be expanded, and the recovery speed of DC voltage can be speeded up. The effectiveness of the proposed energy-consuming device and its modulation strategy is verified through theoretical analysis. Besides, a simulation model of the offshore wind power flexible DC transmission system was built on a PSCAD/EMTDC electromagnetic transient simulation platform, and the feasibility of the proposed DC energy-consuming device and corresponding modulation strategy was verified under conditions of different values of surplus power.

To improve the low voltage ride-through capability of a permanent magnet synchronous generator (PMSG) grid-connected wind power system, a coordinated control method based on series dynamic braking resistors (SDBR) and a supercapacitor energy storage system (SCESS) is proposed. When the grid voltage drops, the SDBR connected in series across the grid will consume part of the unbalanced energy while raising the voltage at the grid-connected point, and the remaining unbalanced energy will be absorbed by the SCESS connected in parallel to the DC chain. Simulation results based on MATLAB/SIMULINK show that the SDBR-SCESS coordinated control strategy can effectively suppress the peak DC chain voltage and raise the voltage at the grid-connected point under fault, reduce the peak grid-connected current at the moment of fault removal, effectively shorten the recovery time of DC chain voltage and grid-connected current after fault removal, and improve the low voltage ride-through capability of the PMSG. Compared with the control based on SDBR or SCESS alone, the SDBR-SCESS coordinated control has a superior effect.

Phase-locked loop (PLL) is one of the important structures to control photovoltaic inverters to realize grid-connected synchronization. Aimed at the problem that αβ-PLL will have large errors in the detection frequency and phase when unbalanced faults and harmonic interference occur in power grid, a novel prefiltering PLL (PFPLL) based on cross decoupling complex coefficient filter (CDCCF) and second-order generalized integrator (SOGI) is proposed in this paper. The SOGI with the capability of eliminating specific-order harmonics is used as the prefiltering link of PFPLL, and the CDCCF with a feedback loop is used as the signal decoupling link of PFPLL. The proposed PFPLL structure can eliminate voltage imbalance and harmonic components, thus realizing the frequency self-adaptation. A simulation model and an experimental platform were built, and results verified the filtering capability of PFPLL.