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

Wireless-Control Strategy for Parallel Operation of Distributed-Generation Inverters

The virtual impedance concept is increasingly used for the control of power electronic systems. Generally, the virtual impedance loop can either be embedded as an additional degree of freedom for active stabilization and disturbance rejection, or be employed as a command reference generator for the converters to provide ancillary services. This paper presents an overview of the virtual-impedance-based control strategies for voltage-source and current-source converters. The control output impedance shaping attained by the virtual impedances is generalized first using the impedance-based models. Different virtual impedances and their implementation issues are then discussed. A number of practical examples are demonstrated to illustrate the feasibility of virtual impedances. Emerging applications and future trends of virtual impedances in power electronic systems conclude this paper.

Virtual-Impedance-Based Control for Voltage-Source and Current-Source Converters

Remote laboratories have been introduced during the last few decades into engineering education processes as well as integrated within e-learning frameworks offered to engineering and science students. Remote laboratories are also being used to support life-long learning and student's autonomous learning activities. In this paper, after a brief overview of state-of-the-art technologies in the development of remote laboratories and presentation of recent and interesting examples of remote laboratories in several areas related with industrial electronics education, some current trends and challenges are also identified and discussed.

Current Trends in Remote Laboratories

Inductive power transfer (IPT) and capacitive power transfer (CPT) are the two most pervasive methods of wireless power transfer (WPT). IPT is the most common and is applicable to many power levels and gap distances. Conversely, CPT is only applicable for power transfer applications with inherently small gap distances due to constraints on the developed voltage. Despite limitations on gap distance, CPT has been shown to be viable in kilowatt power level applications. This paper provides a critical comparison of IPT and CPT for small gap applications, wherein the theoretical and empirical limitations of each approach are established. A survey of empirical WPT data across diverse applications in the last decade using IPT and CPT technology graphically compares the two approaches in power level, gap distance, operational frequency, and efficiency, among other aspects. The coupler volumetric power density constrained to small gap sizes is analytically established through theoretical physical limitations of IPT and CPT. Finally, guidelines for selecting IPT or CPT in small gap systems are presented.

A Survey of Wireless Power Transfer and a Critical Comparison of Inductive and Capacitive Coupling for Small Gap Applications

PEM fuel cell system control: A review

In the future, a major role of fuel cells in combined heat and power generation systems is envisaged. It is well known that a fuel cell's efficiency is highly dependent on the operating conditions, such as temperature, humidity, and air flow. In the aim to assure optimal operating conditions as well as to minimize the losses, parameter identification, diagnostics, and control are going to play an important role. The paper deals with parameter identification and diagnostics of a low-power proton-exchange membrane fuel cell considering a system identification approach. This multi-input multi-output electrochemical system can be tested or monitored by identifying its parameters, which has real importance in the manufacturing, exploitation, and maintenance phase. Therefore, parameter identification represents the core of any diagnostics or online monitoring procedure. Furthermore, online parameter estimation might be used for control purposes to maintain the fuel cell in an optimal operating point with minimal losses. The proposed parameter identification method is simple, cost-effective, and can be extended easily for fuel-cell stacks too.

Fuel-cell parameter estimation and diagnostics

The concept of the variable active-passive reactance (VAPAR) has been proposed previously. VAPAR consists of an inverter and a few passive elements with a control system, which behaves like a two-terminal reactive element, and the produced active reactance works as a passive reactive component, including in transient state. The main purpose of this paper is to analyze the characteristics of negative inductance and to show how to produce negative inductance by VAPAR. First, the principle and realization of VAPAR and complete two-terminal VAPAR (CTT-VAPAR) are explained. Second, the characteristics of negative inductance are analyzed from the point of view of frequency characteristics of impedance and transient response, using capacitance as a reference. Finally, through simulations and experiments, it is verified that VAPAR can generate negative inductance, and it is demonstrated that the actual inductance is cancelled.

Realization of negative inductance using variable active-passive reactance (VAPAR)

This paper offers two Maximum Power Point Tracking (MPPT) systems for Photovoltaic (PV) applications. The first MPPT method is based on a fixed frequency Model Predictive Control (MPC). The second MPPT technique is based on the Predictive Hysteresis Control (PHC). An experimental demonstration shows that the proposed techniques are fast, accurate and robust in tracking the maximum power under different environmental conditions. A DC/DC converter with a high voltage gain is obligatory to track PV applications at the maximum power and to boost a low voltage to a higher voltage level. For this purpose, a high gain Switched Inductor Quadratic Boost Converter (SIQBC) for PV applications is presented in this paper. The proposed converter has a higher gain than the other transformerless topologies in the literature. It is shown that at a high gain the proposed SIQBC has moderate efficiency.

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Novel Predictive Maximum Power Point Tracking Techniques for Photovoltaic Applications

Variable-structure piecewise-linear nonlinear dynamic feedback systems emerge frequently in power electronics. This paper is concerned with the stability analysis of these systems. Although it applies the usual well-known and widely used approach, namely, the eigenvalues of the Jacobian matrix of the Poincare/spl acute/ map function belonging to a fixed point of the system to ascertain the stability, this paper offers two contributions for simplification as well that utilize the periodicity of the structure or configuration sequence and apply an alternative simpler and faster method for the determination of the Jacobian matrix. The new method works with differences of state variables rather than derivatives of the Poincare/spl acute/ map function (PMF) and offers geometric interpretations for each step. The determination of the derivates of PMF is not needed. A key element is the introduction of the so-called auxiliary state vector for preserving the switching instant belonging to the periodic steady-state unchanged even after the small deviations of the system orbit around the fixed point. In addition, the application of the method is illustrated on a resonant dc-dc buck converter.

Stability analysis of nonlinear power electronic systems utilizing periodicity and introducing auxiliary state vector

There are three method for wireless power transfer from the viewpoint of transfer field. The first is inductive power transfer (IPT) using magnetic field which is currently the most popular way to realize wireless power transfer. The second is wireless power transfer using radio wave. The third is capacitive power transfer (CPT) using electric field. There are only few studies about CPT because of small power density compared to another method due to small coupling capacitance. CPT is however investigated recently to utilize the advantages in CPT, such as simple structure of couplings, lightweight, low cost, position flexibility and high frequency applicability. In order to overcome this problem, a lot of CPT system were proposed using LC resonance. The authors have proposed a CPT system using one pulse switching active capacitor (OPSAC). The proposed system improves power transfer efficiency without LC resonance so that it is robust against parameter change. In this paper, the basic principle and characteristics of the proposed circuit is described at first. Next, transfer power of CPT system is analyzed with comparison between CPT with LC resonance and CPT with OPSAC.

Hirohito Funato

Papers

Fuel-cell parameter estimation and diagnostics

Realization of negative inductance using variable active-passive reactance (VAPAR)

Novel Predictive Maximum Power Point Tracking Techniques for Photovoltaic Applications

Stability analysis of nonlinear power electronic systems utilizing periodicity and introducing auxiliary state vector

Analysis of transfer power of capacitive power transfer system