Leming Zhou

Fast Reactive Power Sharing, Circulating Current and Resonance Suppression for Parallel Inverters Using Resistive-Capacitive Output Impedance

In this paper, an inverter using resistive-capacitive output impedance (RC-type inverter) is proposed not only to provide fast reactive power sharing to support microgrid voltage, and but also to reduce circulating currents and damp high-frequency resonances among inverters. Introducing the RC virtual impedance loop, the inverter provides fast transient response. Based on the RC-type inverter modeling, the comparative frequency-domain analysis of equivalent output impedances is discussed, and the impact of the virtual complex impedance over the circulating currents and high-frequency resonances among parallel inverters is quantitatively analyzed. The control parameters are systematically selected, and effect of virtual complex impedance on the inverter output voltage is depicted. The RC-type inverter can reduce circulating currents and damp resonances due to different equivalent output impedances of inverter, and line impedances. Simulation and experimental results verify the effectiveness of the proposed method.

A Virtual Inertia Control Strategy for DC Microgrids Analogized With Virtual Synchronous Machines

In a dc microgrid (DC-MG), the dc bus voltage is vulnerable to power fluctuation derived from the intermittent distributed energy or local loads variation. In this paper, a virtual inertia control strategy for DC-MG through bidirectional grid-connected converters (BGCs) analogized with virtual synchronous machine (VSM) is proposed to enhance the inertia of the DC-MG, and to restrain the dc bus voltage fluctuation. The small-signal model of the BGC system is established, and the small-signal transfer function between the dc bus voltage and the dc output current of the BGC is deduced. The dynamic characteristic of the dc bus voltage with power fluctuation in the DC-MG is analyzed in detail. As a result, the dc output current of the BGC is equivalent to a disturbance, which affects the dynamic response of the dc bus voltage. For this reason, a dc output current feedforward disturbance suppressing method for the BGC is introduced to smooth the dynamic response of the dc bus voltage. By analyzing the control system stability, the appropriate virtual inertia control parameters are selected. Finally, simulations and experiments verified the validity of the proposed control strategy.

Multilevel Power Conditioner and its Model Predictive Control for Railway Traction System

In the rapid development process of a high-speed electrified railway, power quality problems in traction power grid have become increasingly deteriorative. In order to ensure a three-phase balanced traction power grid, a modular multilevel converter based railway traction power conditioner (RTPC) is presented. The RTPC consists of four H-Bridge clusters and filter inductors, and it can be directly connected to traction feeders in a co-phase traction system without insulation transformers. According to the equivalent circuit analysis of RTPC system, it can be considered as four single-phase inversion systems. Based on the equivalent control model of each single-phase cluster, the relationship between the multilevel output voltage and current slopes in a control period is analyzed, and an improved model predictive control is proposed. Moreover, a linear combination of two different output levels is proposed to improve tracking performance of cluster current control, and enhance the waveform quality of ac current. Finally, both simulation and experiment results are presented to verify the effectiveness of the structure and its control method.

Second Ripple Current Suppression by Two Bandpass Filters and Current Sharing Method for Energy Storage Converters in DC Microgrid

With the increase in ac loads injected into the dc microgrid (MG) through inverters, the second ripple current (SRC) in the front-end energy storage converter (ESC) and the circulating current among the ESCs in dc MG become more and more serious. In this paper, the SRC suppression method by introducing two bandpass filters (BPFs) into the output voltage and inductance current feedback of the ESC is proposed. Compared with the traditional dual-loop control method, the proposed method effectively reduces the SRC and improves the dynamic performance in case of a lower cutoff frequency in the outer voltage loop. Simultaneously, an adaptive droop control method by introducing the fine-tuning virtual resistances is adopted to reduce the output voltage deviation of parallel ESCs and improve the output current sharing among the ESCs. Considering the allowed range of deviation between the output voltage and the rated voltage for each ESC, the impacts of the line power loss and circulating current power loss caused by the introduced virtual resistances are analyzed in detail. While the sum of the line power loss and circulating current power loss reaches the minimum value, the appropriate control parameters are obtained. Simulation and experimental results verify the validity of the proposed method.

Seamless transfer control strategy for three-phase inverter in microgrid

In this paper, a seamless transfer control strategy for three-phase inverter in microgrid is proposed to reduce the impact of grid-injection current during the grid-tied transient period, and to restrain the dc side voltage fluctuation during the islanding transient period. To improve the transient response, a soft-start virtual impedance and single loop current feedback control is proposed to transfer between the islanding mode and the grid-tied mode seamlessly. During the grid-tied transfer transient period, weighted average and phase advance control of the reference output voltage is presented to introduce into power droop control loop, and soft-start virtual impedance and single loop current feedback control is also introduced in order to reduce the impact of the inverter output current and the grid-injection current. Otherwise, while the inverter cuts off the grid, the single loop current feedback control is only introduced to accelerate the discharge of the grid current, and to restrain the dc side voltage fluctuation due to the imbalance of instantaneous current. Simulation and experiments verify the effectiveness of the proposed method.

A Double Update PWM Method to Improve Robustness for the Deadbeat Current Controller in Three-Phase Grid-Connected System

In the grid-connected inverter based on the deadbeat current control, the filter inductance variation and single update PWM affect the distortion of the grid current, stability, and dynamic of the system. For this, a double update PWM method for the deadbeat current controller in three-phase grid-connected system is proposed, which not only effectively decreases the grid current distortion and control delay, but also improves the system stability and dynamic response speed due to reducing the characteristic root equation order of the closed-loop transfer function. The influence of the filter inductance deviation coefficient on the system performance is analyzed. As a conclusion, the corresponding filter inductance deviation coefficient in the system critical stability increases with increase in the parasitic resistance of the filter inductance and line equivalent resistance and decreases with increase in the sampling frequency. Considering the system stability and dynamic response, the optimal range of the control parameters is acquired. Simulation and experimental results verify the effectiveness of the proposed method.

Virtual Positive-Damping Reshaped Impedance Stability Control Method for the Offshore MVDC System

For the offshore medium-voltage direct-current (MVdc) system, the dc-side medium voltage can easily cause high-frequency oscillation and even instability owing to the complex impedance interactions. The virtual-resistance stability control method aiming at rectifier station is first introduced from low-voltage dc micro-grid application for mitigating its dc-side oscillation without affecting the load performance of the inverter station. Viewed from the dc input terminal, the small-signal dc impedance modeling of the overall system is established by considering the influences of dc cable, ac grid inductance, and input-parallel output-series structure of rectifier station. Then, the oscillation mechanism is analyzed by the impedance-based Nyquist stability criterion. It is found that only the virtual resistance deteriorates the stability of the MVdc system under the low switching-frequency condition, because the high-frequency oscillation peak may easily exceed the narrow control bandwidth of the rectifier station and fall into the negative-damping region, resulting in a poor robustness against the dc cable variation. To address this issue, the virtual positive-damping reshaped impedance stability control method is further proposed to maintain a larger positive damper in the actual oscillation frequency range regardless of the variation of dc cable length. Thus, the dc-side oscillation of the offshore MVdc system is effectively mitigated at the low switching frequency. Finally, simulation and experimental results validate the proposed control method.

A novel two degrees of freedom grid current regulation for single-phase LCL-type photovoltaic grid-connected inverter

For single-phase LCL-type photovoltaic (PV) grid-connected system installed at the end of the grid, a novel two degrees of freedom grid current regulation (2DOFGCR) is proposed, which includes the synchronous reference frame quasi-proportional-integral (SRFQPI) controller and grid-current-feedback active damping (GCFAD) controller. The SRFQPI controller compensate reactive power quickly and regulate the instantaneous output grid current with zero steady-state errors regardless of the fluctuation range of fundamental frequency. The GCFAD controller not only damps LCL-resonance, but also improves both transient and steady-state performances. Finally, the optimized design of 2DOFGCR is proposed with the careful examinations of the system dynamic performance, stability margin and amplitude error. Simulation and experimental results verified the validity of the proposed method.

A novel hybrid single/three-phase microgrid structure and its energy coordination control method

Due to the intermittency of output power of the distributed generation(DG) and variability of load, the traditional single/three-phase hybrid microgrid operating in islanded mode is subjected to voltage fluctuations and three-phase power unbalances. A novel hybrid single/three-phase microgrid structure and its energy coordinating control method are proposed in this paper. The proposed structure consists of power sharing unit (PSU), energy storage unit (ESU), single and three-phase DG units. The energy coordinating control method based on droop control of PSU is implemented to achieve fast support for active and reactive power when the power of DG units or load mutate so as to reduce voltage fluctuation. The PSU has ability to coordinate power exchanges among three phases so as to alleviate three-phase power unbalances. Besides, the ESU can stabilize the DC-link voltage of the PSU, and achieve the buffering of the power fluctuation in the hybrid microgrid. Experimental results indicate the rationality and effectiveness of the proposed method.