Motivated by the recent and growing interest in smart grid technology, we study the operation of DC/AC inverters in an inductive microgrid. We show that a network of loads and DC/AC inverters ...equipped with power-frequency droop controllers can be cast as a Kuramoto model of phase-coupled oscillators. This novel description, together with results from the theory of coupled oscillators, allows us to characterize the behavior of the network of inverters and loads. Specifically, we provide a necessary and sufficient condition for the existence of a synchronized solution that is unique and locally exponentially stable. We present a selection of controller gains leading to a desirable sharing of power among the inverters, and specify the set of loads which can be serviced without violating given actuation constraints. Moreover, we propose a distributed integral controller based on averaging algorithms, which dynamically regulates the system frequency in the presence of a time-varying load. Remarkably, this distributed-averaging integral controller has the additional property that it preserves the power sharing properties of the primary droop controller. Our results hold for any acyclic network topology, and hold without assumptions on identical line admittances or voltage magnitudes.
A mode-adaptive decentralized control strategy is proposed for the power management of a dc microgrid with multiple renewable distributed generators and energy storage systems. In the presented ...solution, the dc bus voltage signal is used not only to enable power sharing among different sources, but also to designate microgrid operation modes and facilitate seamless mode transitions. With this mode-adaptive operation mechanism, a greater control freedom can be achieved than the conventional dc voltage droop control scheme. More importantly, this approach features fully self-disciplined regulation of distributed converters without an extra control center or communication link. Therefore, both reliability and flexibility can be enhanced. Meanwhile, a novel mode definition criterion is also provided to highlight the special characteristics of the dc microgrid which is different from an ac one. Three typical operation conditions are summarized according to which type of sources are dominating the power balance. Finally, the effectiveness of the proposed technique is verified experimentally based on a composite dc microgrid test system.
The injected grid current regulator and active damping of the LCL filter are essential to the control of LCL-type grid-connected inverters. Generally speaking, the current regulator guarantees the ...quality of the injected grid current, and the active damping suppresses the resonance peak caused by the LCL filter and makes it easier to stabilize the whole system. Based on the proportional-integral (PI) and proportional-resonant (PR) compensator together with capacitor-current-feedback active-damping which are widely used for their effectiveness and simple implementations, this paper proposes a simple step-by-step controller design method for the LCL-type grid-connected inverter. By carefully dealing with the interaction between the current regulator and active damping, the complete satisfactory regions of the controller parameters for meeting the system specifications are obtained, and from which the controller parameters can be easily picked out. Based on these satisfactory regions, it is more convenient and explicit to optimize the system performance. Besides, the insight of tuning the controller parameters from these satisfactory regions is also discussed. Simulation and experimental results verify the proposed step-by-step design method.
This paper proposes an improved model predictive direct power control (MPDPC) for a pulse width modulation (PWM) rectifier by using a duty cycle control. The conventional MPDPC achieves good ...steady-state performance and quick dynamic response by selecting the best voltage vector, which minimizes the errors between the reference power and the real power. However, due to the limited number of voltage vectors in a two-level converter, the sampling frequency has to be high to achieve satisfactory performance. This paper introduces the concept of a duty cycle control in the MPDPC by allocating a fraction of control period for a nonzero voltage vector and the rest time for a zero vector. The nonzero vector is selected by evaluating the effects of each nonzero vector and its duration is obtained based on the principle of power errors minimization. Simulation and experimental results prove that, compared to the conventional MPDPC, the proposed MPDPC with duty cycle achieves further steady-state performance improvement without affecting the dynamic response at a small cost of control complexity increase.
Passive damping is the most adopted method to guarantee the stability of LCL-filter-based grid converters. The method is simple and, if the switching and sampling frequencies are sufficiently high, ...the damping losses are negligible. This letter proposes the tuning of different passive damping methods and an analytical estimation of the damping losses allowing the choice of the minimum resistor value resulting in a stable current control and not compromising the LCL-filter effectiveness. Stability, including variations in the grid inductance, is studied through root locus analysis in the z -plane. The analysis is validated both with simulation and with experiments.
Inductive power transfer (IPT) is an emerging technology that may create new possibilities for wireless power charging and transfer applications. However, the rather complex control method and low ...efficiency remain the key obstructing factors for general deployment. In a regularly compensated IPT circuit, high efficiency and controllability of the voltage transfer function are always conflicting requirements under varying load conditions. In this paper, the relationships among compensation parameters, circuit efficiency, voltage transfer function, and conduction angle of the input current relative to the input voltage are studied. A design and optimization method is proposed to achieve a better overall efficiency as well as good output voltage controllability. An IPT system design procedure is illustrated with design curves to achieve a desirable voltage transfer ratio, optimizing between efficiency enhancement and current rating of the switches. The analysis is supported with experimental results.
In order to address the load sharing problem in islanding microgrids, this paper proposes an enhanced distributed generation (DG) unit virtual impedance control approach. The proposed method can ...realize accurate regulation of DG unit equivalent impedance at both fundamental and selected harmonic frequencies. In contrast to conventional virtual impedance control methods, where only a line current feed-forward term is added to the DG voltage reference, the proposed virtual impedance at fundamental and harmonic frequencies is regulated using DG line current and point of common coupling (PCC) voltage feed-forward terms, respectively. With this modification, the impacts of mismatched physical feeder impedances are compensated. Thus, better reactive and harmonic power sharing can be realized. Additionally, this paper also demonstrates that PCC harmonic voltages can be mitigated by reducing the magnitude of DG unit equivalent harmonic impedance. Finally, in order to alleviate the computing load at DG unit local controller, this paper further exploits the band-pass capability of conventionally resonant controllers. With the implementation of proposed resonant controller, accurate power sharing and PCC harmonic voltage compensation are achieved without using any fundamental and harmonic components extractions. Experimental results from a scaled single-phase microgrid prototype are provided to validate the feasibility of the proposed virtual impedance control approach.
This paper describes a modified phase-shifted carrier-based pulsewidth-modulation (PSC-PWM) scheme for modular multilevel converters (MMC). In order to reduce the average device switching frequency, ...a reduced switching-frequency (RSF) voltage balancing algorithm is developed. This paper also proposes a circulating current suppressing controller (CCSC) to minimize the inner circulating current in an MMC. Based on the double line-frequency, negative-sequence rotational frame, the three-phase alternative circulating currents are decomposed into two dc components and are minimized by a pair of proportional integral controllers. Simulation results based on a detailed PSCAD/EMTDC model prove the effectiveness of the modified PSC-PWM method and the RSF voltage-balancing algorithm. The proposed CCSC not only eliminates the inner circulating current but also improves the quality of the converter ac output voltage. A simple loss evaluation demonstrates that the RSF voltage-balancing algorithm and the CCSC reduce the converter power losses.
Nowadays, going beyond the 2D conventionally laminated structures is essential for improving the functionality of an electrical machine during the design process. Additive manufacturing (AM) offers ...unmatched 3D freedom for processing metal-based materials optimized for weight and cost effectiveness. Using this technology, new families of electrical machines can be manufactured which are difficult to be built using conventional methods. This paper is mainly targeting printing and testing of ferromagnetic material. Using different AM techniques, different parts are built using the silicon steel (Fe-3 wt%Si) powder. The magnetic properties of these parts are measured and compared. Accordingly, a proper AM technology is identified as a promising method to manufacture magnetic materials. Additionally, quasi-static measurements are performed at low frequencies to compare the hysteresis losses of the 3D printed samples with those in conventional Fe-Si laminations. Results shows that good magnetic properties can be obtained from the 3D printed samples. Finally, the potential of 3D multi-material printing is highlighted for electrical machines.
A method is proposed in this article to place sensors in an electrical machine in order to be able to reconstruct the magnetic field distribution. This method is based on the empirical interpolation ...method combined with the Maxvol technique. The results applied on a surface-mounted permanent magnet machine at no load show that the field distribution can be accurately reconstructed even when the sensor location is imposed in the airgap of the rotating machine.
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