Baocheng Wang

Dynamic Phasors-Based Modeling and Stability Analysis of Droop-Controlled Inverters for Microgrid Applications

System modeling and stability analysis is one of the most important issues of inverter-dominated microgrids. It is useful to determine the system stability and optimize the control parameters. The complete small signal models for the inverter-dominated microgrids have been developed, which are very accurate and could be found in literature. However, the modeling procedure will become very complex when the number of inverters in microgrid is large. One possible solution is to use the reduced-order small signal models for the inverter-dominated microgrids. Unfortunately, the reduced-order small signal models fail to predict the system instabilities. In order to solve the problem, a new modeling approach for inverter-dominated microgrids by using dynamic phasors is presented in this paper. Our findings indicate that the proposed dynamic phasor model is able to predict accurately the stability margins of the system, while the conventional reduced-order small signal model fails. In addition, the virtual ω-E frame power control method, which deals with the power coupling caused by the line impedance X/R characteristic, has also been chosen as an application example of the proposed modeling technique.

Asymmetrical Grid Fault Ride-Through Strategy of Three-Phase Grid-Connected Inverter Considering Network Impedance Impact in Low-Voltage Grid

This letter presents a new control strategy of three-phase grid-connected inverter for the positive sequence voltage recovery and negative sequence voltage reduction under asymmetrical grid faults. Unlike the conventional control strategy based on an assumption that the network impedance is mainly inductive, the proposed control strategy is more flexible and effective by considering the network impedance impact, which is of great importance for the high penetration of grid-connected renewable energy systems into low-voltage grids. The experimental tests are carried out to validate the effectiveness of the proposed solution for the flexible voltage support in a low-voltage grid, where the network impedance is mainly resistive.

A Multifunctional and Wireless Droop Control for Distributed Energy Storage Units in Islanded AC Microgrid Applications

A multifunctional and wireless droop control method for distributed energy storage units (DESUs) in ac microgrids is presented in this paper. This paper achieves the state-of-charge (SoC) balancing by employing the SoC-based P-f droop control method locally for the purpose to prolong the service life of DESUs and effectively utilizing the available capability of the DESUs. Besides, inspired by the conventional P-f droop control, a f Qdt - V droop control is proposed in this paper to eliminate the reactive power sharing errors. The f Qdt - V droop control is realized by reducing the voltage proportional to the integration of the reactive power instead of the reactive power itself. Meanwhile, the voltage compensation term which is proportional to the integration of the accurately shared active power is added to this method in order to restore the voltage to the acceptable range. The control strategy is straightforward to implement and does not require communication links as well as the gain scheduling. This paper also presents the analysis of the corresponding small-signal stability of the proposed droop control method. Simulation and experimental results are provided to validate the feasibility and effectiveness of the proposed approach.

Dual-Bridge LLC Resonant Converter With Fixed-Frequency PWM Control for Wide Input Applications

This paper proposes a dual-bridge (DB) LLC resonant converter for wide input applications. The topology is an integration of a half-bridge (HB) LLC circuit and a full-bridge (FB) LLC circuit. The fixed-frequency pulsewidth-modulated (PWM) control is employed and a range of twice the minimum input voltage can be covered. Compared with the traditional pulse frequency modulation (PFM) controlled HB/FB LLC resonant converter, the voltage gain range is independent of the quality factor, and the magnetizing inductor has little influence on the voltage gain, which can simplify the parameter selection process and benefit the design of magnetic components as well. Over the full load range, zero-voltage switching (ZVS) and zero-current switching (ZCS) can be achieved for primary switches and secondary rectifier diodes, respectively. Detailed analysis on the modulation schedule and operating principle of the proposed converter is presented along with the converter performance. Finally, all theoretical analysis and characteristics are verified by experimental results from a 120-V to 240-V input 24 V/20 A output converter prototype.

A Modified Dual Active Bridge Converter With Hybrid Phase-Shift Control for Wide Input Voltage Range

By inserting a small inductor between the transformer center tap and the midpoint of two split output capacitors in the dual active bridge (DAB) topology, this paper proposes a modified DAB converter for wide-input applications. A hybrid phase-shift (HPS) control scheme is proposed to allow all power switches to achieve practical ZVS over the full operating range; thereby, significantly minimizing the switching losses and alleviating electromagnetic interference. Moreover, the proposed control scheme does not significantly increase the conduction losses in comparison with the extended phase-shift (EPS) control. Therefore, the modified DAB can operate efficiently. The topology derivation and description are first presented. Then, the EPS and triple phase-shift (TPS) modulations are applied, and the corresponding operating principles and characteristics, including the soft-switching, power transfer, and root-mean-square current, are investigated in detail. To achieve practical ZVS operation of all switches over full operating range, while minimizing the conduction losses, the EPS and TPS are combined, and an HPS control scheme is proposed. Finally, experimental results from a 1.4-kW converter prototype with 200-400-V input and 400-V output are presented to verify the feasibility and advantages of the converter and control.

Alternative Source-Port-Tolerant Series-Connected Double-Input DC–DC Converter

The two input sources of a series-connected double-input dc/dc converter (SCDIC) can power the load simultaneously or individually. But the two sources must supply the load simultaneously when the output voltage is greater than either of the two input voltages. This is a big issue, especially in double-input alternative power systems in which two input ports are, respectively, interfaced with a renewable energy source and a storage element due to the randomness and intermittency of renewable energy sources. This paper proposes a topology scheme using a combination of a charging switch and an SCDIC. In the case of one port powering or one input being short-circuit, the proposed topology forms a bootstrap circuit with the help of the charging switch and SCDIC switches to charge the powered off port and maintain the expected output voltage, which enhances the reliability of double-input conversion, making it robust against one input powering off and fault tolerance against one input short-circuit. The operation principle and the corresponding power management strategy are presented in detail. In addition, charge-discharge feature of the bootstrap mode is analyzed and the charging loss is calculated. Experimental results validate the feasibility of the proposed scheme.

A Double-Resistive Active Power Filter System to Attenuate Harmonic Voltages of a Radial Power Distribution Feeder

Harmonic propagation between power-factor correction capacitors and system inductors seriously deteriorates power quality in a radial power distribution feeder. Installation of a resistive active power filter (RAPF) at the end bus only suppresses harmonic propagation, not attenuates harmonic voltages. This paper proposes a double-resistive active power filter (D-RAPF) system consisting of a terminal-RAPF and an attenuation-RAPF for each individual harmonic. The terminal-RAPF operating as the characteristic impedance of the feeder is installed at the end bus to suppress harmonic propagation at all harmonic frequencies. The attenuation-RAPF, whose control gain can be set according to the requirements of the harmonic voltage distortion limit, is installed at a specific position for each individual harmonic to attenuate the corresponding harmonic voltages. The D-RAPF system not only suppresses harmonic propagation and attenuates harmonic voltages more effectively, but also has the same rated power capacity as the RAPF. However, both the harmonic damping performance and the rated power capacity must be considered to choose the D-RAPF system or the RAPF when a harmonic current source exists at a position between the attenuation-RAPF and the terminal-RAPF. Simulation and experiment results verify the theoretical analysis and demonstrate the effectiveness of the D-RAPF system.

A Single DC Source Cascaded Seven-Level Inverter Integrating Switched-Capacitor Techniques

In this paper, a novel cascaded seven-level inverter topology with a single input source integrating switched-capacitor techniques is presented. Compared with the traditional cascade multilevel inverter, the proposed topology replaces all the separate dc sources with capacitors, leaving only one H-bridge cell with a real dc voltage source and only adds two charging switches. The capacitor charging circuit contains only power switches, so that the capacitor charging time is independent of the load. The capacitor voltage can be controlled at a desired level without complex voltage control algorithm and only use the most common carrier phase-shifted sinusoidal pulse width modulation strategy. The operation principle and the charging-discharging characteristic analysis are discussed in detail. A 1-kW experimental prototype is built and tested to verify the feasibility and effectiveness of the proposed topology.

Frequency response analysis of a Rogowski coil transducer for railgun pulse current measurement

Some of intriguing advantages of Rogowski coils, such as high bandwidth, capability of measuring large currents, non-saturation and potential-free, make it possible to detect and diagnose large pulsed current from an electromagnetic launch system. In this paper, high frequency behavior for a Rogowski coil is analyzed. Considering the model of the coil frequency response, a design method for the transducer is presented. The damping ratio, based on termination resistors, can divide the whole frequency band of the coil into three sub-bands, i.e. derivation band, proportion band and resonance band. Therefore, the corresponding outside integrator circuit is built based upon the characteristics in each sub-band. A novel compound integral circuit is, then, described, which consists of self-integral, passive RC integral and active RC integral. This compound integrator improves the transducer measurements upper bandwidth limit around the coil natural frequency. Finally, a coil of 100 turns is constructed using compound integrator of 20Hz~2.0MHz and sensitivity of 1mV/A. Simulation and experimental results verify that the transducer can operate within both the measurement bandwidth of power grid current and the railgun discharging current from the pulse forming unit.

Center-tapped transformer based bidirectional dc-dc converter with wide input voltage range

In this paper, a center-tapped transformer based bidirectional dc-dc converter with full ZVS operating range is proposed for wide input voltage range applications. The soft switching analysis in different modes is carried out and a simple and effective control strategy, allowing extending the practical ZVS to the whole operating range, is proposed. By dynamically adjusting the phase-shift angle between the two output half-bridges, all power switches can operate with ZVS over the entire load range. At heavy loads, no output-bridge phase-shift angle is required and the two output bridges operate in parallel to minimize the conduction loss. Experimental results are given to verify the effectiveness and advantages of the proposed converter.