Masahide Hojo

DC voltage sensorless single-phase PFC converter

We propose a simple DC voltage sensorless single phase PFC converter by detecting an AC line voltage waveform. Both DC voltage and AC current sensors used in the conventional PFC converter are not required to construct the control system. The conventional converter circuit with a boost chopper circuit in the DC side from a rectifier circuit is used as the main PFC converter circuit. In the control system, the circuit parameters such as a series inductance L and equivalent load resistance value R/sub d/ are used to generate the sinusoidal current waveform. The DC voltage is directly controlled by the command input signal k/sub d/(=E/sub d//E/sub a/) for the boost chopper circuit. The DC voltage regulation is small because of the feed forward control for the AC line voltage E/sub a/ and no dependence of the circuit parameters. The sinusoidal current waveform in phase with the AC line voltage can be obtained. The feasibility of the proposed control system is verified by some simulation and experimental results.

A method of three-phase balancing in microgrid by photovoltaic generation systems

Recently, a small-scale power system has been able to perform autonomous or islanding operation with some distributed generators. But in such a microgrid, it is afraid that serious voltage imbalance may be caused by unbalanced loads during the islanding operation. Fortunately, grid interactive inverters of the distributed generators often have some amount of surplus capacity. This paper deals with a three-phase balancing method utilizing the surplus capacity. By the proposed method, the inverters output negative sequence currents to suppress the voltage imbalance within their surplus capacities. Operating characteristics and its effectiveness are confirmed by some simulation results.

Fault current limiting and power system stabilization by static synchronous series compensator

In this paper, a control scheme of the static synchronous series compensator (SSSC) for fault current limiting and power system stabilization has been proposed. The apparatus injects a voltage of variable magnitude in series with a transmission line. This injected voltage is in quadrature with the line current. Under normal condition, it is controlled in order to compensate for reactance of the transmission line. When a disturbance occurs, injected voltage from the phase shifter is applied to damping control of generator swing. As a result, the proposed controller is expected to improve steady-state stability. This apparatus is also expected to be a promising fault current limiter by using the leakage reactance of a series transformer. The effectiveness of the proposed controller is evaluated from the viewpoints of current limiting and steady-state stability improvement based on numerical analyses.

Observation of frequency oscillation in western Japan 60 Hz power system based on multiple synchronized phasor measurements

Western Japan 60 Hz power system is a longitudinal power system which consists of interconnected six major electric power companies. Though the power system has some interesting oscillation characteristics, there were few researches based on observation as a whole system. Authors have constructed an online global monitoring system of the power system dynamics, which is based on multiple synchronized phasor measurement technique using time stamp of the Global Positioning System (GPS) and Internet. This paper presents some results of observing system frequency oscillation and discussed the difference among the measured point. System dynamics due to the load frequency control (LFC) can be extracted from observed frequency oscillations by time-frequency analysis based on the Wavelet transform. Because variations of phase difference between the interconnected two areas are closely related with tie line power flow variations, relations between the frequency oscillations and the LFC are also investigated.

Power System Dynamic Performance Measured by Phasor Measurement Unit

In recent years, there has been an increasing interest in monitoring and analyzing dynamic performances of power systems, such as observation of power system oscillations and evaluation of machine parameters. However, in Japan, there was little observation as a whole power system. In this paper the authors present a global monitoring system of power system dynamics by using the synchronized phasor measurement of demand side. Authors have placed phasor measurement units (PMU) connected to 100 V outlets in some universities in Japan and been researching into oscillation characteristics in power systems. Phasor measurements are synchronized based on the global positioning system (GPS) and measurement data are transmitted via Internet. Some oscillative features and variation of phasor in a day, a week or a year are analyzed using the acquired data. The purpose of this paper is to show a power system dynamic performance by the use of measurement data at an unexpected disturbance and monitors a global and a local oscillation mode.

Monitoring power system dynamics based on phasor measurements from demand side outlets developed in Japan Western 60 Hz System

Japan Western 60 Hz System consists of six major electric power companies. Due to its longitudinal structure there are some significant oscillation modes represented by long term oscillation among whole system. So far, some oscillative characteristics have been measured in own area or in the interconnected tie lines. However, there was little observation as a whole power system. In this paper the authors present a global monitoring system of power system dynamics by using the synchronized phasor measurement of demand side outlets, which are located in some universities in the 60 Hz power system. Phasor measurements are synchronized based on the global positioning system (GPS) and measured data are transmitted via Internet. Some oscillative features and variation of phasor in a day, a week or a year are analyzed using the acquired data.

Unified Power Quality Conditioner for dynamic voltage restoration and fault current limitation

In recent years, power system has employed many applications of power electronics technologies for power quality improvement. Unified Power Quality Conditioner (UPQC), which consists of double converters connected by a DC capacitor with each other, has great potential because of its high controllability. Various attractive applications for power quality improvement are considered but the active power handled by each converter should be exchanged simultaneously through a DC link. Therefore, when one converter exchanges active power with the power system, both the other converter controller and a capacitance of the DC link must be carefully designed. This paper proposes a utilization of the left-shunt UPQC for both dynamic voltage restoration and fault current limitation in order to improve power quality. The DC link capacitance was designed to be suitable for the applications. Some simulation studies are introduced to prove the effectiveness of the UPQC.

AC line voltage harmonics compensator with excessive current control

We investigate an instantaneous common terminal voltage-controlled harmonics compensator constructed by a shunt active filter with an appropriate series inductance including the line impedance. This compensator can reduce or compensate both the ac line voltage distortion derived from the downstream utility source voltage harmonics and the upstream current harmonics by nonlinear loads at the same time. The control system can be easily constructed without directly detecting the common terminal voltage to be compensated. Therefore, the main circuit configuration and the control system are simple. The harmonics compensation level and the compensation current can be easily adjusted by changing the feedback gain for the sensing inductance voltage drop. In this paper, we describe the basic principle of the control method, the modified control method, the circuit construction by the pulsewidth-modulation-controlled shunt active filter and the control system of the compensator. Then, we show some operating waveforms for the cases of the downstream voltage distortion and the upstream harmonics current from the nonlinear loads by simulation analysis and experiments to verify the feasibility.

Software-based adjusting P-I gain controller for speed-tracking servo system using traveling-wave type ultrasonic motor

This paper describes the review of traveling-wave type ultrasonic motor (T-USM) and a newly-proposed optimum speed tracking controller of T-USM which is produced on the basis of computer software-based variable-gain adjustment strategy. Its improved speed characteristics are concerned with speed ripple factor, speed regulation and speed tracking properties are feasibly demonstrated in experiment as compared with a previously-developed speed controller of the T-USM actuated servo system. It is proved that the T-USM speed control scheme developed herein is practically acceptable for speed servo direct drive system.<<ETX>>

Effectiveness of constant power factor control of mega-solar system for voltage regulation with long distribution line

When a large-scale photovoltaic system, such as a mega-solar system, is connected to a distribution system, voltage variations on the distribution line may be caused by the output fluctuation of the photovoltaic system. This paper reports the result examined by numerical analysis and an experiment using a real system about mitigating the voltage variation by applying the reactive power control based on constant power factor control of the PCS in a large-scale photovoltaic system. It is investigated that the loss change on a distribution line influences the results of the reactive power control, especially when the large-scale photovoltaic system is interconnected with a long distance from a substation. This paper reveals that it is possible to mitigate a voltage variation sufficiently with the distance of about 5km from a substation to the interconnection point, and this method can be applied in many cases under the condition.