New concept of DIGITAL GRID composed of Digital Grid Routers (DGR) is proposed to admit a large amount of renewable energy by segmenting the huge synchronous grid into smaller stand-alone grids, called “cell-grids (CELL)” and reconnecting them via asynchronous connection with DGRs. In order to accept high penetration of distributed fluctuating renewable energy generation, current grid of large synchronous system shall be segmented to avoid cascading blackout and other local issue. And also current grid of planned management shall be shifted to market base management. Digital Grid is based on main grid with segmented cell grids with asynchronous connection will fit with high penetration of renewable energy generation in future.

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Digital Grid Router (DGR) enables asynchronous interchange of electricity between grids. DGR consists of multiple inverters connected to a common DC bus on a back-to-back basis. DGR communicates with a central controller which manages the interchange. In this article, we present structure and operation of DGR, and report evaluation result of prototype DGR.

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Achieving digital grid requires controllers that can control electrical power and the information tagged to the electricity. The controller is called “Digital Grid Router (DGR)”. DGR combines analog circuit and digital controller in a system, and in its prototyping, it needs to respond to various specification changes promptly. In this chapter, the attempt in prototyping the DGR and the platform used for that attempt (system design software NI LabVIEW and NI Single-Board hardware) will be discussed.

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We propose a platform which centrally controls distributed Digital Grid routers to enable arbitrary electricity interchange between multiple grids. The platform adds additional information to electric power such as source and CO₂ emission to allow users to choose any combination of their power sources on the basis of the information. We also present an experimental software to identify and visualize power distribution and sources in the grids.

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The key features of the Digital Grid are as follows. Large synchronous grids are divided into smaller segments called “cells”, and they are connected asynchronously. Each cell has balanced supply and demand and has suitable energy storage so that the cell can be operated in the stand-alone mode. Cells are connected by “Digital Grid Router” (DGR) and these routers communicate with each other and share power among the cells. We developed computer software that enables to visualize electricity interchanging and fluctuation of energy generation by renewable energy sources.

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To realize reduction in environment load, electric vehicles (EV) have been attracting great attentions and expectations since they can run at good fuel efficiency without exhaust gases. It is expected that the EVs will become more and more popular. Generally, EVs would be used for driving during daytime and be charged during nighttime considering our lifestyle and time-of-use program. If most of EV owner adopt the use pattern, feeder voltage profiles could be greatly affected. It would cause severe voltage drop.
In this paper, battery charger for EV consists of self-commutated inverter, which can control not only active power but also reactive power in principle. We propose a basic feeder voltage regulation algorithm, “PQ control”, using EVs by means of adjustment of EV charging schedule and reactive power injection. In addition, we propose “prediction control” which is based on “PQ control” and is considering prediction of receiving voltage. And advanced regulation algorithms, “communication control”, utilizing communication among EVs and the distribution system operator are proposed. The purpose of this paper is to confirm the effectiveness of the three proposed algorithms by simulations.

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This paper presents a new control method of wind turbines group by use of a dynamic control including Pump-Up Operation. Under a condition of oscillating wind below the rated speed of the wind turbine, Maximum Power Point Tracking (MPPT) operation is a key to improve its efficiency. Even though, owing to the huge inertia of the wind turbine, the operating point of the wind turbine yields to the unavoidable delay to the Maximum Power Point. By use of the dynamic control, wind turbines respond to the wind speed change more rapidly. The wind turbines hence reach the Maximum Power Point rapidly and capture the energy from the wind more efficiently. On the other hand, because of the dynamic operation, the output from a wind turbine fluctuates more severe than that of the normal operation. In the proposed solution, by taking advantage of the kinematic energy stored in other wind turbines which belong to the same windfarm, not only the efficiency of output power, but also the severity of the fluctuation is improved. In the result of some simulation, group dynamic operation enables wind famrs to improve its efficiency and smooth the fluctuation of the output from the windfarm.

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This paper deals with estimation of total output of solar photovoltaic generators (PV's) widely distributed in future power systems. In the proposed framework of estimation, the estimated signal in each frequency band is a linear combination of observed signals. The observation gain for each observed signal in each frequency band is designed through following two steps. First, an estimation error function is defined using assumed root mean square of output of PV's, predicted distribution of future PV's in the area, and correlation coefficients, which are identified in advance based on actual synchronized measurements of PV's. Second, observation gain in each frequency band is designed so as to minimize the estimation error function. The appropriateness of the proposed framework of estimation is demonstrated in small scale and large scale examples.

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The capacity of distributed generators (DGs) connected to grid by inverters is growing. The inverters are generally controlled by PLL (Phase Locked Loop) in order to synchronize with power system frequency. It cannot have synchronous power. Power systems will become unstable, if the capacity of inverter type DGs is increased. The concept that inverters are controlled to behave like a synchronous generator is studied in this paper, which is called “Virtual Synchronous Generator (VSG)”. VSG has synchronizing power. Power system becomes stable by VSG. VSG is useful for parallel operation of distributed generators. VSG can synchronize with other VSGs, synchronous generators or grid. In this paper the method of synchronous running for a VSG is studied and experimental result of the VSG which is running parallel with synchronous generator and other VSG is shown.

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As part of an ongoing collaboration between Tohoku Electric Power Company, Vaisala, Sankosha, and the University of Arizona, propagation corrections have been implemented in Tohoku's two overlapping LLS networks: a 6-sensor LS700x research LLS and the operational 9-sensor IMPACT (141T) LLS. In addition, new arrival-time onset-corrections have been implemented in the six LS700x sensors. Sensor arrival-time consistency was improved by (roughly) a factor of two in the older IMPACT LLS (as a result of propagation corrections), and more than a factor of three in the new LS700x LLS (propagation and onset corrections). Estimates of “relative” location accuracy (LA) improvements, based on an analysis of location differences for return strokes that shared the same ground strike point, indicate that the IMPACT LLS median LA improved from ∼400m to ∼270m using propagation corrections, and that the LA for the LS network is ∼100m using both propagation and onset corrections. Analysis of return stroke locations produced by the LS network that were associated with four transmission line faults showed locations that were between 60m and 390m from the verified fault location. The actual line-attachment locations cannot be determined, so these errors are likely an upper-bound on the actual location error.

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Insulation discharge behavior of small-air-gap for the molded insulation system has been investigated in view of the Paschen curve. Two electrode systems Metal-Gap-Metal (MGM) and Metal-Gap-Insulator (MGI) were employed as the test samples. It was found that the V_s (breakdown voltage of the air gap) in the Paschen left region was determined in terms of glow-like discharge with slow micro-second order of discharge-current probably due to the ionic current for both MGM and MGI electrode systems. In the minimum and right regions of the Paschen curve for MGM, the partial discharge showed the streamer-like discharge current with relatively short rise-time and short discharge-duration. For the MGI-electrode system, however, the discharge type was found to lie in all glow-like discharge with small discharge current. The glow-like discharge, Townsend and streamer discharges were also recognized by the high speed CCD camera photographs.

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High-voltage supplied customers install static capacitors to improve their power factor (PF) at their interconnection points. However, most of them are excessive and this makes system PF in distribution network extremely-leading. This is caused by two economical grounds. One is because utilities assume excess reactive power for their load to get full discount for PF improvement. The other is expensiveness of static capacitor (SC) controllers.
The authors propose new SC control logic that involves simple electric current measurement without voltage or reactive power, which enables economical controller productions. The basic functions are mounted on verification test devices and evaluated in fields for several months. This paper presents the logic and verification test result.

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The hollow porcelain insulator is widely used in the gas-insulated power apparatus and oil-filled power apparatus. Previously, it had been thought that the aged deterioration does not occur in the hollow porcelain insulator. However, the gas leaks and oil leaks because of alkali silica reaction in the porcelain of cementing part have been reported in recent years.
We experimentally verified the generation process of the alkali silica reaction by using the hollow porcelain insulator removed from the substation, and examined the inhibition method of alkali silica reaction that used the lithium nitrite. As the result, it was clarified that the alkali silica reaction occurs in the hollow porcelain insulator and the lithium nitrite is effective for inhibition of alkali silica reaction.

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