Active damping is a common way to stabilize the current control of LCL-filtered converters. In this paper, the stable region of -180° phase crossing is first identified within a predefined range of grid impedance and LCL parameter variations. Once the phase of the current control loop is in the identified region, a stabilization control can be attained. Subsequently, digital filters can be adopted to achieve active damping by reshaping the open-loop phase. Various digital filters are selected and benchmarked in this paper. It is confirmed that the all-pass filter has a unity gain and adjustable lagging phase before the Nyquist frequency, thereby being a promising solution to the phase reshaping. Therefore, the all-pass filter is employed to move the phase of the open-loop control (i.e., -180° phase crossing) into the targeted region for active damping. Notably, the current controller and the all-pass filter-based active damping can be separately designed, indicating the easy implementation of the active damping. Experimental tests demonstrate that the proposed method can ensure the system stability over a wide range of parameter variations (e.g., grid impedance changes and LCL-filter parameter drifts) while maintaining fast dynamics with the grid-side current control.

Differential inverters provide a costeffective solution to the second order ripple power issue in single phase systems. Most existing differential inverter based power decoupling methods are for linear loads, which may not work well for nonlinear loads. When supplying nonlinear loads, differential inverters may suffer from harmonics at the AC terminal, which may propagate to the DC side and deteriorate the performance of power decoupling. In this paper, the harmonic mitigation is realized by reshaping capacitor voltages, and it is applied to Buck, Boost and Buck-Boost type differential inverters with detailed harmonics compensation capacity analysis. Then, a feedback linearization based DC current feedback control scheme is proposed to realize the harmonic mitigation, hence both the DC side ripple power and the nonlinearity found in differential inverters can be decoupled simultaneously. The proposed control scheme is developed based on a generalized Half-bridge model and can be applied to Buck, Boost or Buck-Boost type differential inverters with minor revisions. Experimental results are presented to validate the performance of the proposed control scheme and theoretical analysis.

High-power pulsed load (HPPL) such as airborne laser weapon and radar poses a severe challenge to aircraft electrical power system. Since peak power cannot be satisfied only by generator, hybrid energy storage system (HESS) should be well sized to match their energetic performances versus instantaneous power requirement and improve reliability of electrical power system. To deal with these challenges, a sizing optimization for HESS with considering energy management strategy (EMS) based on virtual-droop control in aircraft is proposed in this paper. By analyzing the spectrum characteristics of HPPL profile, splitting frequency between different types of energy storage devices is specially analyzed and designed to give full play to HESS. HESS weight, loss and parameters in EMS are taken into account in this multi-objective optimization based on Non-dominated Sorting Genetic Algorithms II (NSGA-II). Series of Pareto fronts can be obtained through the optimization process, and the most suitable solutions of different HESS combinations are selected by a decision-making means which combines subjective and objective weighting methods. Finally, optimization results of four different combinations are compared. Simulation results are obtained in MATLAB/Simulink environment to prove the rationality and applicability of the optimal sizing tool. The research has resulted in a solution of HESS and EMS optimization for HPPL used in aircraft.

The single-phase differential mode buck inverter is recently introduced with a differential mode for power transfer and a common mode for actively decoupling the second-order power oscillation. However, it is limited to islanded applications with an LC filter. This paper addresses the stability and control of a grid-connected differential mode buck inverter with an LCL filter. A generalized small-signal model of the inverter is built first with the averaged switching model. It is shown that the LCL filter resonance merely occurs in the differential mode, while an LC filter resonance exists in the common mode, provided that the filter parameters of the two bridges are kept the same. A unified active damping control approach is then proposed for stabilizing the inverter and improving the transient performance under a wide range of grid impedance. Lastly, experimental tests are carried out and the results confirm the effectiveness of the theoretical analysis and proposed control approach.

DC microgrids have attracted more and more attention with its universality, high efficiency and potential application market. However, DC fault protection is still a challenge to the development of DC microgrids. In this paper, a novel bidirectional DC solid-state circuit breaker based on coupled inductor is introduced. It can realize bidirectional energy flow and bidirectional isolation and interruption of short-circuit faults with fewer components. The working principle of the topology and the working process under different conditions are described, and some properties of the novel circuit breaker are explored, including minimum fault current ramp rate and load step analysis, which provides a reference for the selection of component parameters. Finally, the feasibility and performance of the novel bidirectional circuit breaker is evaluated by simulations and experiments.

LCL filters are widely used in Pulse Width Modulation (PWM) inverters. However, it also introduces a pair of unstable resonant poles that may challenge the controller stability. The passive damping is a convenient possibility to tackle the resonance problem at the cost of system overall efficiency. In contrast, the active damping does not require any dissipation elements, and thus has become of increasing interest. As a result, a vast of active damping solutions have been reported, among which multi-loop control systems and additional sensors are necessary, leading to increased cost and complexity. In this paper, a notch filter based active damping without the requirement of additional sensors is proposed, where the inverter current is employed as the feedback variable. Firstly, a design method of the notch filter for active damping is presented. The entire system stability has then been investigated, which has revealed that negative variations of the resonant frequency can seriously affect the system stability. In order to make the controller more robust against grid impedance variations, the notch filter frequency is thus designed smaller than the LCL filter resonant frequency, which is done in the z-domain. Simulations and experiments are carried out to verify the proposed active damping method. Both results have confirmed that the notch filter based active damping can ensure the entire system stability in the case of resonances with a good system performance.