Yandong Chen

An Improved Reactive Current Detection and Power Control Method for Single-Phase Photovoltaic Grid-Connected DG System

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.

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.

Dual-Loop Power Control for Single-Phase Grid-Connected Converters with LCL Filter

Grid-connected converters have widely adopted LCL filters to acquire high harmonic suppression. However, the LCL filter increases the system order so that the design of the system stability would be complicated. Recently, sole-loop control strategies have been used for grid-connected converters with L or LC filters. But if the sole-loop control is directly transplanted to gridconnected converters with LCL filters, the systems may be unstable. This paper presents a novel dual-loop power control strategy composed of a power outer loop and a current inner loop. The outer loop regulates the grid-connected power. The inner loop improves the system stability margin and suppresses the resonant peak caused by the LCL filter. To obtain the control variables, a single-phase current detection is proposed based on PQ theory. The system transfer function is derived in detail and the influence of control gains on the system stability is analyzed with the root locus. Simulation and experimental results demonstrate the feasibility of the proposed control.

Hierarchical Direct Power Control of Modular Multilevel Converter for Tundish Heating

In this paper, the full bridge-based modular multilevel converter (MMC) is discussed for the application of tundish induction heating power supply (TIHPS). TIHPS features three-phase to single-phase ac/ac conversion and variable output frequency, and has intense demand for the power transmission stability and output current control precision. MMC is suitable for this application and especially the bulky and costly transformer and ac filter in the conventional topology can be dispensable. Therefore, it is interesting to investigate the applicable control method for MMC-TIHPS. According to the instantaneous power theory, a hierarchical direct power control based on the improved deadbeat current control is presented to assure the energy balance inside and outside MMC-TIHPS and reduce the sensitiveness of the current controller to parameter uncertainties. The proposed control strategy dispenses with the phase-locked loop, and does not exist the coupling among control variables of internal and external currents. Then, parameters design of energy controller is studied for stability analysis. Finally, MMC-TIHPS and proposed control method are verified by experimental results of a down-scaled physical prototype.

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.

Analysis and Comparison of Modular Railway Power Conditioner for High-Speed Railway Traction System

With the rapid development of modern electrified railway, negative-sequence current minimization is one of the most important considerations in the high-speed railway traction system. In the past, many multiple or multilevel topologies with high compensation capacity have been introduced for railway power conditioner (RPC). This paper presents a simplified quantitative comparison of five previous modular RPC topologies for negative sequence compensation in V/V and SCOTT traction systems, aiming for an optimal selection of the compensators. Performance criteria such as transformer requirement, voltage stress and current stress of a power switch, numbers of the power switches and capacitor are derived by analytical methods. Moreover, the numerical comparison of operating controllers is completed for modular RPCs. In addition, power losses of five modular RPCs are obtained by theoretical analysis, IPOSIM calculation as well as PSIM simulation. These calculations are validated via simulations results in PSIM. The main conclusion is that presented modular RPCs can be divided into general purpose RPC and special purpose RPC in terms of the behavior and efficiency. It is helpful to choose the appropriate topology for specific applications.

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.

Resonance features of multi-paralleled grid-connected inverters and its damping method

The resonance problem in multi-paralleled grid-connected inverters in microgrid is more complex than single one. The Nortons equivalent circuit associated with grid-connected inverter with LCL filter is figured out in this paper. And in terms of different reasons of causing resonance, resonance problems associated with multi-paralleled grid-connected inverters are categorized into three types: individual resonance, parallel resonance, series resonance. In order to attenuate resonance peak, an active damping compensation term associated with filter capacitor voltage feedback is embedded into current control forward path. The proposed active damping approach is based on the concept of virtual damping resistor which acts as an additional resistor paralleled with filter capacitor. The virtual resistance is deliberately designed to guarantee the requirement of effective resonance damping. Comparative study is conducted to verify the feasibility and effectiveness of resonance damping.

Rapid reactive power control method for parallel inverters using resistive-capacitive output impedance

For parallel multi-inverters, an inverter using resistive-capacitive output impedance (RC-inverter) is proposed. The equivalent output impedances of RC-inverter are designed as resistive-capacitance by introducing virtual resistive-capacitive impedance into the feedback of output current. RC-inverters can not only provide rapid reactive power for low-voltage microgrid to maintain the system voltage stability, but also restrain high frequency resonance between output impedance of inverters and the grid impedance. Based on the equivalent modeling of RC-inverter, a multi-loop power sharing control method for parallel inverters is presented, which mainly includes the outer power droop control, the virtual impedance, and the output voltage control. The simulation and experimental results verify the validity of the proposed control 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.