Mikihiko Matsui

A new maximum photovoltaic power tracking control scheme based on power equilibrium at DC link

This paper proposes a new simple MPPT (maximum power point tracking) control scheme for A PV (photovoltaic) array based on the principle of power equilibrium at the DC link. In contrast to the typical conventional MPPT scheme based on the so-called mountain climbing method, the proposed scheme needs no detection or calculation of the power and no decision making logic or look up tables, except for several numbers of operational amplifiers. Its operation principle is explained quantitatively. In order to confirm the availability of the scheme, a simulation study using general purpose circuit simulation software PSIM and an experiment have been carried out. Although the stability study and system design are left for future study, the validity of the proposed new MPPT method has been clarified through the simulation and experimental results.

Power sensor-less MPPT control scheme utilizing power balance at DC link-system design to ensure stability and response

The authors previously proposed a new MPPT (maximum power point tracking) control scheme for PV (photovoltaic) power systems connected in parallel with the AC system line. It is characterised by utilizing power balance at the DC link under steady state conditions, hence eliminating the array output power detection and achieving power sensorless operation. However, the system response and stability greatly depend on its main circuit and control system parameters. This paper shows a system design to ensure stability and response based on system transfer functions. Response of the experimental system has been proved fast enough for practical use. Full-daytime testing data show the successful tracking results of the system.

Analysis and Design of a Digital Phase-Locked Loop for Single-Phase Grid-Connected Power Conversion Systems

A new digital phase-locked-loop (DPLL) method is proposed for single-phase grid-connected power conversion systems in this paper. The analysis of a phase detector using trigonometric function transformation is presented in detail. A sliding-window-based data sampling method is utilized to obtain the orthogonal signal related to the original input signal according to the periodic characteristic of the steady-state sine fundamental component, and the fictitious active and reactive power components are constructed. On this basis, the DPLL method is developed. It consists of an even harmonics elimination network, a grid voltage feedforward network, and a fictitious instantaneous power feedback phase correction network. The fundamental signal of the grid voltage is extracted by an even harmonics elimination network and a digital Butterworth filter and a unit value controller to generate a unity sine reference signal for the feedforward network, whereas only the signs of the fictitious instantaneous active and reactive powers are used to revise the phase shift of the unity sine reference signal by means of a real-time variable step-size control in the feedback phase correction network. The method is validated to have good dynamic and static performances through MATLAB simulations, simulator investigations, and experiments with a grid-connected photovoltaic system under different voltage conditions.

Fuzzy-Logic-Based

Since natural distribution power generation systems such as wind energy contain electric power fluctuation, flywheel energy storage (FWES) equipment for power compensation is used. Fuzzy-logic-based V/f control of the induction motor is proposed for the speed sensorless power-leveling system in this paper. Only two sensors, including one DC voltage sensor and one DC current sensor, are utilized. The flywheel is composed of two modes, namely, the speed picking-up control mode and the power control mode. According to the DC-link average current of an inverter, the rotating speed of the flywheel in the speed picking-up control mode is detected by regulating the output frequency and the output voltage based on fuzzy logic control. The power control mode consists of the outer voltage loop and the inner current loop. Although the outer loop is also realized by fuzzy logic control, the inner loop is the key object in this paper, i.e., the proportional and integral gains of a proportional-integral regulator are optimized by means of fuzzy logic reasoning for the purpose of the reliability and rapid response of leveling power. The experiments are carried out with the FWES equipment of 40-kJ energy, and the results verify that the proposed method is reliable, and better dynamic and static performance is demonstrated.

V/f

Since islanding of photovoltaic (PV) generation can cause safety problems, it should be prevented. This paper proposes a correlation-based islanding-detection method (IDM) using current-magnitude disturbance for a PV system. As an islanding-detection index, this paper uses the correlation factor between current-magnitude disturbance and the corresponding inverter output voltage. The proposed IDM uses the characteristics in which the up/down current disturbance has a strong correlation with the PV output voltage when islanding occurs. The proposed method has fast islanding capability and high power quality. For verification of the proposed method, the simulated and experimental results of a 3-kW PV inverter by IEEE Std. 929-2000 and IEEE Std. 1547.1-2005 are provided. The proposed correlation technique can be used in other active antiislanding methods based on the correlation between the current and voltage after islanding occurs.

Control of an Induction Motor for a DC Grid Power-Leveling System Using Flywheel Energy Storage Equipment

In centralized or string photovoltaic (PV) systems, PV modules must be connected in series in order to generate a sufficiently high voltage to avoid further amplification and to efficiently drive further converters. This always requires dozens of PV modules, however some of them maybe suffer from partial shadow caused by trees, clouds or other things. In this case, power generated from each PV module becomes unbalanced so that total output powers greatly decrease. Furthermore, hot-spot effect caused by partial shadow is likely to damage the PV cells and affect the security of PV system. In order to solve these problems, a novel topology of PV system composed of many series-connected PV modules with corresponding energy feedback circuits is proposed in this paper, a feedback circuit is independently utilized to feed the output energy of PV system to the corresponding circuit including shadowed PV module. The simulation and experimental results verify that the proposed topology can make each PV module operate on the maximum power point individually regardless of partial shadow. The energy feedback circuits do not operate without partial shadow, they have no power loss under this condition, therefore the circuit efficiency is improved.

A Correlation-Based Islanding-Detection Method Using Current-Magnitude Disturbance for PV System

The effect of the magnetizing inductance of the high frequency transformer in two-transistor forward converter is investigated in the paper. Relationship of the magnetizing inductance, duty cycle, switching frequency and capacitance between drain and source of main switches is found to satisfy magnetic resetting requirement of the transformer. Finally, simulation and experiment results are given to verb the analysis.

A novel topology for solving the partial shading problem in photovoltaic power generation system

This paper proposes a new switching pattern generating algorithm based on SVM (space vector modulation) technique for a high-frequency link soft-switching converter. Because of its totally soft-switched operation, isolated output stage and bidirectional power flow, the proposed converter system is suited for interfacing a new energy system, such as a PV (photovoltaic) system having a battery, with AC system line. The restriction of output vector transition in applying SVM technique is discussed first. Then, a suitable pattern which satisfies soft-switching operation is proposed. The validity of the proposed scheme has been proved with simulation study using PSIM and experimental results with DSP based prototype system.

Study on magnetizing inductance of high frequency transformer in the two-transistor forward converter

A novel digital phase-locked-loop (DPLL) method is proposed for single-phase grid-connected power generation systems in this paper. It consists of a grid voltage feed-forward network and a fictious instantaneous power feedback correction network. The fundamental signal of the grid voltage is extracted by a digital Butterworth filter to generate a unity sine reference signal for the feed-forward network, while only the signs of fictious instantaneous active and reactive powers are used to revise the phase shift of the unity sine reference signal by means of a real-time variable step-size control in the feedback phase correction network. The method is validated to have good dynamic and static performances through MATLAB simulations and experiments with a grid-connected photovoltaic (PV) power generation system under different conditions.

New space vector modulation scheme based high-frequency link soft-switching converter for AC system line interface

The parallel resonant link high frequency power distribution system can be an attractive system as a supplementary local distribution line linked with an existing AC commercial system line in a building or a plant, if some new energy sources and energy storage systems are combined and connected with the link. This paper deals with some basic issues of a unit converter for such systems from both microscopic (stable switching operation) and macroscopic (energy flow) points of view. A PDM (pulse density modulation) gate control circuit for stable operation and self start-up from standstill is tested. The effect of PDM sampling period on steady state waveforms is compared for three cases. The bad effect of parasitic capacitance of the switching arm is shown, and its countermeasure is given. Based on the probabilistic switching function technique, a DC equivalent circuit model suited for simulating macroscopic energy flow is proposed.