Masaaki Ohshima

A novel utility‐interactive electrical energy storage system making use of electrical double‐layer capacitors and an error tracking mode PWM converter

A novel electrical energy storage system interacting with a commercial ac network was designed and experimentally evaluated. It consists of an ac/dc converter modulated by a constantly sampled PWM scheme called Error Tracking Mode and electrical double-layer capacitors paralleled by monitor circuits. The ac ratings are 100 V, 500 W, and 1.0 power factor. The storage capacity is designed to be 500 Wh. Since the voltage of the capacitor bank decreases to one-fourth with discharge, a reversible step-down chopper is inserted between the bank and the ac/dc converter. ac/dc converter efficiencies of 95.6 and 95.8% were obtained in rectifier operation and inverter operation, respectively. The average switching frequencies were set between 10.4 and 11.2 kHz for all operating conditions. Chopper efficiencies of 94.6% and 93.1% were measured in the charge and discharge modes, respectively. The storage efficiency of the capacitor bank was 89.8%. This novel utility-interactive energy storage system has demonstrated good and rapid performance.

A single-phase PCS with a novel constantly sampled current-regulated PWM scheme

The error-tracking mode (ETM) is a novel constantly sampled pulsewidth modulation (PWM) scheme for a voltage-source power conversion system (PCS) interconnecting with a utility network to regulate its AC current waveform. It guarantees, theoretically, the AC actual current to be settled within an arbitrary target allowable error. Generated current harmonics can be restricted by a relatively small filter. Both the minimum on- and off-duration times of switching commands are assured by the PWM scheme itself. A four-quadrant 1-kVA PCS in which gate commands are produced by the novel PWM scheme has been successfully experimented upon for the first time, and it has justified the ETM-PWM theory. This paper describes the design procedure and experimental results of the trial setup.

The P,Q controllable domain of a single phase PWM converter to preserve sinusoidal AC current waveform

The P,Q controllable domain for any single phase utility-interactive voltage-source pulse width modulation (PWM) converter to preserve the AC current waveform sinusoidally, is derived mathematically to form an ellipse determined by its mains circuit parameters, independently of the PWM types. The elliptical domain was examined by and verified from both digital simulation and experiment.

Coordinate control of distributed generations with power converters in a micro grid

For extensive utilization of distributed generation with the objectives of more efficient use of energy and introduction of renewable energy sources, the construction of a micro grid at demand end is inevitable. The micro grid should operate independently and have a good neighbor relation with the power grid, which supplies base power to it. A coordinate control scheme of distributed generations is studied. It has three layers of structure. The integrated control system is quite similar to that of power system other than its scale. Because of the limited number of generations and customers, the variation of power unbalance between supply and demand is fast and large. To stabilize the system and maintain power supply quality, the micro grid is subdivided into local distribution network groups. The local group control system regulates generations inside it in coordination with other groups and transmits operational information in it to the integrated control system. Each controllable generation has its own control system and regulates its output with the command from the local group control system. The distributed generation on the market, including the battery energy storage, is generally connected to the network through the power converter system. Because it can not autonomously follow the power variation, a novel control scheme should be incorporated into the control system of power converter system and the local level control. Their performances in operation of a micro grid are verified with simulations using EMTDC/PSCAD. The coordination between local group control systems for the load following control is under test in a model grid for experiment. A demonstration system is planned to construct on this control scheme

Supply power quality of a micro grid based on distributed generations in relation with power converter control

In order to maintain both the stable system operation and supply power quality in a micro grid, a model system is studied on the coordinate control operation between a power converter based distributed generation and a conventional rotating machine. The performances of the micro grid are verified by simulation studies

Development of uninterruptible secondary battery system

This paper presents the system configuration and control schemes of a USBS (uninterruptible secondary battery system), which acts as a battery energy storage system when no fault occurs in a commercial power network and functions as a UPS in case of network faults. If a fault such as a voltage interruption occurs on the network, the USBS disconnects critical loads from it instantly, and supplies power to them as a UPS. It takes less than 2 ms to restart supply power to the loads in case of an emergency, so no damage occurs. Simulations and experimental tests are carried out to confirm the feasibility of the proposed system.

Immunity test results of a quick power failure detector of uninterruptible secondary battery system

An uninterruptible secondary battery system (USBS) consists of secondary batteries and a multi-functional power converter having plural operational modes including local load leveling and uninterruptible power supply (UPS). For the USBS, a quick power failure detector is essential for the smooth operational mode changes. However, commercial AC line voltage harmonic distortion or voltage unbalance may cause misoperation of the quick power failure detector. In this paper we present immunity test results of the USBS. The test results proved that the proposed detector has sufficient immunity to harmonic distortion and unbalance of the commercial AC line voltage.

Development of a novel power apparatus to make up for voltage dips and interruptions based on quick reversible operation of a PWM converter

Along with wide and deep penetration of electronics application into our modern society, voltage dips and momentary interruptions, mainly due to lightning and snow damage on overhead transmission lines, have become a disturbance to normal operation of loads. A UPS, which is mostly employed to protect sensitive loads from disturbances, has such limitations that it dissipates no less energy in running operation because currents flow through two converters in series, it is poor in feeding electric motors which occupy the main loads in industrial plants and that its costs are high to be commonly applied. UPS can be regarded not as omnipotent against voltage dips and short interruptions. We have developed a novel power apparatus termed QBS (quick backup system) to supplement the above drawbacks in UPS, employing one reversible AC/DC converter and one high-speed switch. Normally QBS supplies power directly from a commercial network to critical loads while the reversible converter interconnects with it to charge the battery. When the critical bus voltage deviates from the tolerance range QBS disconnects the high-speed switch and continues to feed the critical loads in UPS mode. The whole power transition is completed within 2 ms. QBS of several thousands kVA has already been put to commercial use in telecommunication centers, electronics manufacturers and others.