Control of three-phase power converters in the synchronous reference frame (SRF) is now a mature and well-developed research topic. However, for single-phase converters, it is not as well established as three-phase applications. This paper deals with the design of an SRF multiloop control strategy for single-phase inverter-based islanded distributed generation systems. The proposed controller uses an SRF proportional-integral controller to regulate the instantaneous output voltage, a capacitor current shaping loop in the stationary reference frame to provide active damping and improve both transient and steady-state performances, a voltage decoupling feedforward to improve the system robustness, and a multiresonant harmonic compensator to prevent low-order load current harmonics to distort the inverter output voltage. Since the voltage loop works in the SRF, it is not straightforward to fine tune the control parameters and evaluate the stability of the whole closed-loop system. To overcome this problem, the stationary reference frame equivalent of the voltage loop is derived. Then, a step-by-step systematic design procedure based on a frequency response approach is presented. Finally, the theoretical achievements are supported by experimental results.

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Analysis, Design, and Experimental Verification of a Synchronous Reference Frame Voltage Control for Single-Phase Inverters

An active method for double-frequency power ripple decoupling in single-phase inverters is presented in this paper, exhibiting the main advantage of not using additional power semiconductors besides the H-bridge. The proposed method requires only two capacitors placed between the midpoint and one end of each inverter leg. An original control solution of the inverter ensures the power ripple transfer toward the two decoupling capacitors without affecting the inverter output voltage. The simple design makes the proposed solution easy to adapt for single-phase inverters in H-bridge configuration. This paper focuses on the autonomous operation mode of the inverter, detailing its operating principle and the control analysis. The system performances, including the impact of the decoupling circuit on the inverter efficiency, are assessed by means of experimental results.

Power Decoupling Method for Single-Phase H-Bridge Inverters With No Additional Power Electronics

In this paper, an improved control method for multiple bidirectional power converters is proposed to reduce the circulating current and power-sharing deviation among converters when the hybrid ac/dc microgrid operated in island mode, which can enhance the security of parallel converters. First, a unified detection method for circulating current and power-sharing deviation is described. Then, the improved control method for bidirectional converters in hybrid microgrid operated in island mode is presented to reduce circulating current and power-sharing deviation, which includes the droop controller used to achieve automatic power sharing and the improved virtual impedance controller used to further reduce circulating current and power-sharing deviation. At last, simulation and experiment results verified that the proposed control method can simultaneously achieve circulating current reduction and automatic power sharing, and does not decrease the output power capability of converter. The proposed control method is suitable for the configuration that has a specific bus between bidirectional power converters, and the ac bus connected to ac loads and the host grid.

An Improved Control Method for Multiple Bidirectional Power Converters in Hybrid AC/DC Microgrid

A modified hierarchical control structure for distributed generation (DG) in microgrid is applied in accordance to the recommendations by the IEEE Std. 1676. In the inverter control layer, a generalized control algorithm is developed to enable the DGs to operate in all three modes (i.e., grid-forming, grid-feeding, and grid-supporting modes) with a single control structure, facilitating the seamless transition between the operating modes. This feature is obtained by designing the multiloop controller in the inverter control layer using inverse plant modeling techniques so that the dynamics of the inverter and the $LC$ filter are fully compensated, virtually transforming the controlled DG in the unity gain from the application layer perspective. Thus, effects of disturbances associated with the mode transitions are thus fully eliminated. The effectiveness of the proposed control algorithm is validated by numerical simulations and hardware-in-the-loop experiments. The results show that the DGs can fulfill the tasks defined in the system control layer with fast dynamics, desired accuracy, and good transient behavior.

Design of a Generalized Control Algorithm for Parallel Inverters for Smooth Microgrid Transition Operation

Non-singular terminal sliding-mode control of DC–DC buck converters

The output characteristics of a single-phase inverter with voltage and current dual closed-loop feedback control are analyzed, and the equivalent circuit model of a parallel single-phase inverter system is introduced. By taking both resistance and inductance components of the equivalent output impedance into consideration, a current decoupling control strategy of the parallel inverter system is then proposed. Furthermore, by constructing a three-phase balanced current according to the output current of the single-phase inverter, an active and reactive current decomposition method is presented to decompose the output current of the single-phase inverter into active and reactive currents according to the instantaneous reactive power theory. The block diagram of the active and reactive current decomposition method is given, and current decomposition simulation results are shown to verify the analysis results. The prototype is developed, and the control of two parallel-connected 2-kVA inverters is realized. The experimental results show its feasibility and effectiveness.

A Current Decoupling Parallel Control Strategy of Single-Phase Inverter With Voltage and Current Dual Closed-Loop Feedback

The operation principles of peak current control, V2 control, and enhanced- V2 control of buck converter in continuous conduction mode are reviewed and compared. By the use of weight factors of inductor current and output voltage, they are unified as constant-frequency peak-ripple-based (CF-PR) analog control. It is revealed that conventional peak current control and V2 control are two boundaries of the unified CF-PR control. The duality items between CF-PR control and constant-frequency valley-ripple-based (CF-VR) control are proposed. As the duality of CF-PR analog control, CF-VR analog control is investigated. Unified CF-PR and CF-VR digital control laws with different modulations and their stabilities are investigated in terms of the duality. Experimental results verify the theory analysis.

Constant-Frequency Peak-Ripple-Based Control of Buck Converter in CCM: Review, Unification, and Duality

Equivalent series resistance (ESR) of output capacitor has a significant effect on the control performance of constant on-time (COT) controlled switching dc-dc converters. In this paper, a discrete-time model of COT-controlled buck converter, with variable sampling frequency, is established. Based upon which, the dynamical effects of the ESR of output capacitor on COT-controlled buck converter are revealed and analyzed. The time-domain numerical simulations of the exact switched state equations, the bifurcation diagrams, and maximal Lyapunov exponents of the discrete-time model are obtained. These results verified by experimental circuit indicate that the ESR of output capacitor is a critical factor for COT-controlled buck converter, which can eliminate the unique pulse bursting phenomenon and shift operation mode from discontinuous conduction mode to continuous conduction mode, as well as control stability.

Dynamical Effects of Equivalent Series Resistance of Output Capacitor in Constant On-Time Controlled Buck Converter

In this paper, the effect of equivalent series resistance (ESR) of an output capacitor on the performance of a constant-on-time (COT)-controlled buck converter is studied, and a pulse bursting phenomenon is revealed. It indicates that the ESR of the output capacitor is one of the key factors causing pulse bursting phenomenon in COT-controlled buck converters, and the critical ESR is derived via time-domain analysis and stability analysis in the s-domain. It is pointed out that, when the ESR of the output capacitor is smaller than the critical ESR, pulse bursting phenomenon occurs; otherwise, it disappears. Simulation and experimental results are provided to verify the theoretical analysis results.

Analysis of Pulse Bursting Phenomenon in Constant-On-Time-Controlled Buck Converter

A novel control technique, called bifrequency pulse-train (BF-PT) control technique, for switching dc-dc converters is proposed. In contrast to the conventional pulsewidth-modulation-based control technique of switching dc-dc converters, which realizes the output voltage regulation by adjusting the duty ratio of the control pulse cycle by cycle, the BF-PT control technique realizes the output voltage regulation of switching dc-dc converters based on the presence and absence of preset high- and low-frequency control pulses. In BF-PT control, the output voltage of switching dc-dc converter is sampled at the beginning of each switching cycle and compared with a reference voltage to determine whether a high- or low-frequency control pulse should be selected as the control pulse. The BF-PT control technique is simple and cost effective and enjoys fast transient response. In addition, two types of control pulses with different switching frequencies spread the emission spectrum over discrete frequencies, resulting in lower electromagnetic interference (EMI) noise and easier EMI filter design. Furthermore, more low-frequency control pulses at lighter load improve the light-load power conversion efficiency. A buck converter operating in discontinuous conduction mode is taken as an example to illustrate the application and benefits of BF-PT control technique. Simulation and experimental results are presented to verify the analytical results.

Jinping Wang

Papers

A Current Decoupling Parallel Control Strategy of Single-Phase Inverter With Voltage and Current Dual Closed-Loop Feedback

Constant-Frequency Peak-Ripple-Based Control of Buck Converter in CCM: Review, Unification, and Duality

Dynamical Effects of Equivalent Series Resistance of Output Capacitor in Constant On-Time Controlled Buck Converter

Analysis of Pulse Bursting Phenomenon in Constant-On-Time-Controlled Buck Converter

Bifrequency Pulse-Train Control Technique for Switching DC–DC Converters Operating in DCM