The enabling of ac microgrids in distribution networks allows delivering distributed power and providing grid support services during regular operation of the grid, as well as powering isolated islands in case of faults and contingencies, thus increasing the performance and reliability of the electrical system. The high penetration of distributed generators, linked to the grid through highly controllable power processors based on power electronics, together with the incorporation of electrical energy storage systems, communication technologies, and controllable loads, opens new horizons to the effective expansion of microgrid applications integrated into electrical power systems. This paper carries out an overview about microgrid structures and control techniques at different hierarchical levels. At the power converter level, a detailed analysis of the main operation modes and control structures for power converters belonging to microgrids is carried out, focusing mainly on grid-forming, grid-feeding, and grid-supporting configurations. This analysis is extended as well toward the hierarchical control scheme of microgrids, which, based on the primary, secondary, and tertiary control layer division, is devoted to minimize the operation cost, coordinating support services, meanwhile maximizing the reliability and the controllability of microgrids. Finally, the main grid services that microgrids can offer to the main network, as well as the future trends in the development of their operation and control for the next future, are presented and discussed.

Control of Power Converters in AC Microgrids

This paper presents a new multiresonant frequency-adaptive synchronization method for grid-connected power converters that allows estimating not only the positive- and negative-sequence components of the power signal at the fundamental frequency but also other sequence components at other harmonic frequencies. The proposed system is called MSOGI-FLL since it is based on both a harmonic decoupling network consisting of multiple second-order generalized integrators (MSOGIs) and a frequency-locked loop (FLL), which makes the system frequency adaptive. In this paper, the MSOGI-FLL is analyzed for single- and three-phase applications, deducing some key expressions regarding its stability and tuning. Moreover, the performance of the MSOGI-FLL is evaluated by both simulations and experiments to show its capability for detecting different harmonic components in a highly polluted grid scenario.

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Multiresonant Frequency-Locked Loop for Grid Synchronization of Power Converters Under Distorted Grid Conditions

A synchronverter is an inverter that mimics synchronous generators, which offers a mechanism for power systems to control grid-connected renewable energy and facilitates smart grid integration. Similar to other grid-connected inverters, it needs a dedicated synchronization unit, e.g., a phase-locked loop (PLL), to provide the phase, frequency, and amplitude of the grid voltage as references. In this paper, a radical step is taken to improve the synchronverter as a self-synchronized synchronverter by removing the dedicated synchronization unit. It can automatically synchronize itself with the grid before connection and track the grid frequency after connection. This considerably improves the performance, reduces the complexity, and computational burden of the controller. All the functions of the original synchronverter, such as frequency and voltage regulation, real power, and reactive power control, are maintained. Both simulation and experimental results are presented to validate the control strategy. Experimental results have shown that the proposed control strategy can improve the performance of frequency tracking by more than 65%, the performance of real power control by 83%, and the performance of reactive power control by about 70%.

https://www.sheffield.ac.uk/polopoly_fs/1.323811!/file/IEEEPE_SelfSynch.pdf

Self-Synchronized Synchronverters: Inverters Without a Dedicated Synchronization Unit

We have collected a comprehensive set of multi-unit data on dissociated cortical cultures. Previous studies of the development of the electrical activity of dissociated cultures of cortical neurons each focused on limited aspects of its dynamics, and were often based on small numbers of observed cultures. We followed 58 cultures of different densities – 3000 to 50,000 neurons on areas of 30 to 75 mm2 – growing on multi-electrode arrays (MEAs) during the first five weeks of their development. Plating density had a profound effect on development. While the aggregate spike detection rate scaled linearly with density, as expected from the number of cells in proximity to electrodes, dense cultures started to exhibit bursting behavior earlier in development than sparser cultures. Analysis of responses to electrical stimulation suggests that axonal outgrowth likewise occurred faster in dense cultures. After two weeks, the network activity was dominated by population bursts in most cultures. In contrast to previous reports, development continued with changing burst patterns throughout the observation period. Burst patterns were extremely varied, with inter-burst intervals between 1 and 300 s, different amounts of temporal clustering of bursts, and different firing rate profiles during bursts. During certain stages of development bursts were organized into tight clusters with highly conserved internal structure. Dissociated cultures of cortical cells exhibited a much richer repertoire of activity patterns than previously reported. Except for the very sparsest cultures, all cultures exhibited globally synchronized bursts, but bursting patterns changed over the course of development, and varied considerably between preparations. This emphasizes the importance of using multiple preparations – not just multiple cultures from one preparation – in any study involving neuronal cultures. These results are based on 963 half-hour-long recordings. To encourage further investigation of the rich range of behaviors exhibited by cortical cells in vitro, we are making the data available to other researchers, together with Matlab code to facilitate access.

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An extremely rich repertoire of bursting patterns during the development of cortical cultures

Grid synchronization algorithms are of great importance in the control of grid-connected power converters, as fast and accurate detection of the grid voltage parameters is crucial in order to implement stable control strategies under generic grid conditions. This paper presents a new grid synchronization method for three-phase three-wire networks, namely dual second-order generalized integrator (SOGI) frequency-locked loop. The method is based on two adaptive filters, implemented by using a SOGI on the stationary αβ reference frame, and it is able to perform an excellent estimation of the instantaneous symmetrical components of the grid voltage under unbalanced and distorted grid conditions. This paper analyzes the performance of the proposed synchronization method including different design issues. Moreover, the behavior of the method for synchronizing with highly unbalanced grid is proven by means of simulation and experimental results, demonstrating its excellent performance.

A Stationary Reference Frame Grid Synchronization System for Three-Phase Grid-Connected Power Converters Under Adverse Grid Conditions

Phase, frequency, and amplitude of single-phase voltages are the most important and basic information required for single-phase grid-connected applications. This paper proposes a method for instantly and robustly estimating the phase, frequency, and amplitude of frequency-varying single-phase signals for such applications, which is a phase-locked loop (PLL) method based on a structure. The proposed method has the following attractive features: 1) the estimation system results in a nonlinear system, but it can be stabilized; 2) all subsystems constructing the system can be easily designed; 3) "two-phase signal generator"; can autotune a single system parameter in response to varying frequency of the injected signal; 4) high-order "PLL-Controllers" allowing fast tracking can be stably used; and 5) even in hostile envelopments, favorable instant estimates can be obtained. This paper proposes the estimation method and verifies its usefulness by extensive numerical experiments.

A Robust Single-Phase PLL System With Stable and Fast Tracking

This paper proposes a new sensorless vector control method for salient-pole permanent-magnet synchronous motors. With regard to phase estimation, the sensorless vector control method is featured by a new high-frequency voltage injection method (i.e., carrier modulation method), which is distinguished from the conventional ones by a unique ellipse shape of spatially rotating high-frequency voltage, and by a new phase-locked-loop method as a demodulation method whose input is a high-frequency current component multiplied signal. The new vector control method established by two innovative technologies for modulation/demodulation can have the following high-performance and attractive characteristics. 1) It can allow 250% rated torque at standstill. 2) It can operate from zero to the rated speed under the rated motoring or regenerating load. 3) It accepts instant injection of the rated load even for zero-speed control. 4) Phase estimation is robust against inverter dead time, and proper phase estimate can be obtained even under circumstances where stator current crosses the zero at high frequency. 5) Computational load for estimating rotor phase is very small. Usefulness of the proposed new vector control method is verified through extensive experiments.

A New Speed-Varying Ellipse Voltage Injection Method for Sensorless Drive of Permanent-Magnet Synchronous Motors With Pole Saliency—New PLL Method Using High-Frequency Current Component Multiplied Signal

This paper proposes a new sensorless vector control method for a salient-pole permanent-magnet synchronous motor (SP-PMSM). The proposed method is directly based on a new principle called "mirror-phase characteristics" and has the following high-performance and attractive features: 1) it can allow 250% rated torque at standstill, under which strong flux saturation usually occurs; 2) it can allow ultralow-speed control (the observed minimum speed ratio to the rated speed under the rated motoring or regenerating load is 1/1800); 3) in spite of the ultralow-speed performance, it can operate up to the rated speed; 4) it accepts instant injection of the rated load even for zero-speed control; 5) it accommodates a load with huge moment of inertia.; 6) it is insensitive to all motor parameters; 7) it is also insensitive to saturation phenomena of stator flux; and 8) it is so simple that it can be implemented with no additional hardware. The high performance and usefulness of the new "mirror-phase vector control" method are verified through experiments.

New "mirror-phase vector control" for sensorless drive of permanent-magnet synchronous motor with pole saliency

Pmsmセンサレス駆動のためのモデルマッチング形位相推定法のパラメータ誤差起因・位相推定誤差に関する統一的解析と軌道指向形ベクトル制御法 —回転子磁束推定・誘起電圧推定の場合—

This paper proposes a new n-th order two-input/output filter in a module for two-phase signal processing. The filter can make a distinction between positive and negative frequencies of two-phase signals and can allow one to process the signals based on frequency-polarity. The filter can also change dynamically its filtering characteristics by simply injecting a shift-signal to the filter. Filtering effects equivalent to ones by parallel arranged filters with vector rotators can be obtained, but in a much simpler manner. A total of unified analyses of attractive general properties of the proposed filter is also given for easy designs and realization. The effectiveness of the analyses and usefulness of the filter are examined and confirmed through experiments. The proposed two-input/output filter in module has potential usefulness to a variety of polyphase signal filtering applications including sensorless vector controls of three-phase AC motors that require to process positive or/and negative sequences based on frequency and polarity.

Shinji Shinnaka

Papers

A Robust Single-Phase PLL System With Stable and Fast Tracking

A New Speed-Varying Ellipse Voltage Injection Method for Sensorless Drive of Permanent-Magnet Synchronous Motors With Pole Saliency—New PLL Method Using High-Frequency Current Component Multiplied Signal

New "mirror-phase vector control" for sensorless drive of permanent-magnet synchronous motor with pole saliency

Pmsmセンサレス駆動のためのモデルマッチング形位相推定法のパラメータ誤差起因・位相推定誤差に関する統一的解析と軌道指向形ベクトル制御法 —回転子磁束推定・誘起電圧推定の場合—

A new characteristics-variable two-input/output filter in D-module-designs, realizations, and equivalence