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In this paper, the control strategy for a flexible microgrid is presented. The microgrid presented here consists of several line-interactive uninterruptible power supply (UPS) systems connected in parallel. The control technique is based on the droop method to avoid critical communications among UPS units. Thus, a flexible microgrid is obtained to operate in either grid-connected or islanded mode. A small-signal analysis is presented in order to analyze the system stability, which gives rules to design the main control parameters. Simulation and experimental results are presented, showing the feasibility of the proposed controller. Index Terms--Distributed generation (DG), droop method, microgrids, uninterruptible power systems.

When the control parameters and operating conditions remain unchanged, it is usually believed that the admittance characteristic of grid-connected inverter is time-invariant. However, considering the effect of frequency-coupling, an abnormal phenomenon is observed in the admittance measurement of the grid-connected inverter: the phase of the off-diagonal elements Y12 and Y21 in the admittance matrix is always changed with time, which presents a time-varying admittance characteristic. To explore this phenomenon, the theoretical analyses based on the traditional frequency-coupling admittance measurement method are made. Because the initial phase of the signal cannot be obtained from Fast Fourier Transform (FFT) in experiments and the disturbance frequency and coupling frequency are different, the phase difference between the two frequency components changes with time. However, the influence is not considered in the traditional frequency-coupling admittance measurement method, which makes the measurement results exhibit the confusing time-varying characteristic. The phenomenon is defined as pseudotime-varying admittance characteristic in this letter. To accurately measure the admittance characteristic of the grid-connected inverter, a frequency-coupling admittance measurement method based on phase correction is proposed, which corrects the phase of disturbance frequency components and coupling frequency components. Finally, the effectiveness of the proposed measurement method is verified by experiments. [ABSTRACT FROM AUTHOR]

Single-stage power factor corrector (PFC) ac/dc converter with galvanic isolation is preferable in some industrial applications due to its characteristic of cost-effective and high-power density. However, the transformer used for isolation in such topologies can be easily saturated. In this paper, the generation principle of flux density bias for the transformer involved in the single-stage (SS)-type full-bridge (FB) power factor corrector (PFC) converter is analyzed in detail. The maximum flux density bias during every line period is obtained by recursion. Conventional suppressing strategies of flux density bias are analyzed, and a digital suppressing approach is proposed. A 5 kW SS-FB PFC prototype is built and tested, and the proposed control strategy is implemented by means of a digital signal processor (DSP) board based on the TMS320LF2407A. The feasibility of the proposed approach is well verified through the experimental results. Index Terms--Digital signal processor (DSP), flux density bias, full bridge (FB), power factor correction, single-stage (SS).

This paper deals with an important aspect in the operation of single-phase grid-connected photovoltaic inverters, i.e., injecting low harmonic current into the electrical grid. Specifically, we propose a linear current control scheme with a resonant harmonic compensator connected in series with a tracking regulator (the standard harmonic compensator location is in parallel with the tracking regulator). The series connection provides an efficient attenuation of the grid voltage background distortion, an accurate synchronization with the grid voltage, and a low computational time in relation to the standard control scheme. Experimental results from a digital signal processor-based laboratory prototype validate the features of the proposed control scheme. Index Terms--Current harmonic content, grid-connected inverter, linear current regulator, photovoltaic (PV) power system.

This paper presents a design methodology for voltage hysteretic regulators powering digital integrated circuits with low voltage, high current, and high slew rate current transients. The design approach optimizes the transient response during large consumption changes by imposing constant closed-loop output impedance. This paper also suggests a novel compensator network for the adaptive voltage positioning feedback loop, which leads to a robust transient response performance against load disturbances. The application of the design methodology to the proposed hysteretic controller provides the suitable control parameter values for optimal transient response. Simulation and experimental results validate the theoretical predictions for the proposed controller, particularly the constant output impedance operation and the robust transient response performance. Index Terms--Computer power supplies, design methodology, hysteretic control, voltage regulators.

In this paper, a novel wireless load-sharing controller for islanding parallel inverters in an ac-distributed system is proposed. This paper explores the resistive output impedance of the parallel-connected inverters in an island microgrid. The control loops are devised and analyzed, taking into account the special nature of a low-voltage microgrid, in which the line impedance is mainly resistive and the distance between the inverters makes the control intercommunication between them difficult. In contrast with the conventional droop-control method, the proposed controller uses resistive output impedance, and as a result, a different control law is obtained. The controller is implemented by using a digital signal processor board, which only uses local measurements of the unit, thus increasing the modularity, reliability, and flexibility of the distributed system. Experimental results are provided from two 6-kVA inverters connected in parallel, showing the features of the proposed wireless control. Index Terms--Distributed generation (DG), droop method, inverters, microgrids.

In this paper, a method for the parallel operation of inverters in an ac-distributed system is proposed. This paper explores the control of active and reactive power flow through the analysis of the output impedance of the inverters and its impact on the power sharing. As a result, adaptive virtual output impedance is proposed in order to achieve a proper reactive power sharing, regardless of the line-impedance unbalances. A soft-start operation is also included, avoiding the initial current peak, which results in a seamless hot-swap operation. Active power sharing is achieved by adjusting the frequency in load transient situations only, owing to which the proposed method obtains a constant steady-state frequency and amplitude. As opposed to the conventional droop method, the transient response can be modified by acting on the main control parameters. Linear and nonlinear loads can be properly shared due to the addition of a current harmonic loop in the control strategy. Experimental results are presented from a two-6-kVA parallel-connected inverter system, showing the feasibility of the proposed approach. Index Terms--Distributed generation (DG), droop control method, microgrids, nonlinear loads.

This paper deals with the design of the output impedance of uninterruptible power system (UPS) inverters with parallel-connection capability. In order to avoid the need for any communication among modules, the power-sharing control loops are based on the P/Q droop method. Since in these systems the power-sharing accuracy is highly sensitive to the inverters output impedance, novel control loops to achieve both stable output impedance and proper power balance are proposed. In this sense, a novel wireless controller is designed by using three nested loops: 1) the inner loop is performed by using feedback linearization control techniques, providing a good quality output voltage waveform; 2) the intermediate loop enforces the output impedance of the inverter, achieving good harmonic power sharing while maintaining low output voltage total harmonic distortion; and 3) the outer loop calculates the output active and reactive powers and adjusts the output impedance value and the output voltage frequency during the load transients, obtaining excellent power sharing without deviations in either the frequency or the amplitude of the output voltage. Simulation and experimental results are reported from a parallel-connected UPS system sharing linear and nonlinear loads. Index Terms--DC-AC power conversion, pulsewidth-modulated (PWM) inverters, uninterruptible power systems (UPSs).

The purpose of this paper is to explore the problem of designing proper sliding-mode controllers to regulate the output voltage of the dc-to-dc quantum series-parallel resonant converter. A control-oriented dynamic model, which appropriately describes the large-signal behavior of the power circuit by average state variables, is first developed. Using input-output feedback linearization, a control design methodology is then presented, which leads to a family of sliding surfaces that make the output voltage behave following a particular large-signal linear dynamics. Among these surfaces, the final configuration is selected taking into account control circuit simplicity as the basic premise. Besides exhibiting the absence of output-voltage errors in steady state, the control solution leads to robust operation with respect to parameter variations and external disturbances. Simulations and experimental results are reported to validate the expected features of the proposed control solution. Index Terms--Feedback linearization, quantum modulation, resonant converter, sliding-mode control.