Transformers with on-load tap changer and step voltage regulators are now mainly used for regulating the distribution network voltage. Therefore, several problems exist in the present system, i.e. the voltage control is in discrete steps and the response time is not enough satisfactory. In the meantime, power factor (p. f.) compensation relies on capacitors installed by electricity customers, which give another problems of unexpected over voltage during light load and increase of power losses. To solve those problems, the authors have proposed a new voltage and p. f. control system for distribution networks which adopts sophisticated power electronics technology. This paper describes the study for the operating theory of a inverter controlled regulator and a thyristor controlled reactor, which bear important roles in this system, and the result of simulation through which the usefulness of these equipment has been ascertained.

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Grounding wires and enclosed ZnO elements have been incorporated generally in 6.6kV distribution systems in TEPCO for the reduction of lightning overvoltages. And the reliability to lightning surges is tolerably good in present times. However, the facility of grounding wires is not inexpensive but also its maintenance hard due to corrosion and disconnection in some areas. Typical model simulating TEPCO field adopting enclosed ZnO elements has been developed and the evaluation of relative failure risk conducted systematically according to the conditions with or without grounding wires against lightning overvoltages. Here two kinds of failures discussed in the paper are the flashover of insulation and the overduty of ZnO elements, and two kinds of induced and direct lightning overvoltages are studied in flashover. The greatest problem in no grounding wire is the increase of ZnO elements' duty but it was proved to be clear that a short partial grounding wire around ZnO elements or the selection of more heavy ZnO elements gives the solution.
The main features of this paper is to clarify the relative failures risk systematically according to realistic field conditions, the risk of small stroke currents having long duration to ZnO elements' duty and the countermeasures against ZnO elements' overduty.

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Recently, high quality and high reliability of power supply is required. Responding to these needs, we have developed an advanced switching system for distribution networks.
This system realizes that only fault section of a distribution line is cut off without breaking FCB of distribution substation and interruption of other sound sections.
This paper describes the outline of this system and results of testing in a simulated circuit, focusing the process of finding a fault section of distribution line.

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We have developed a high-speed fault current limiting equipment for 6kV distribution system. The main components of this equipment are a high-speed vacuum switch (VCB) and gate turn-off thyristors (GTO) connected in parallel and a current limiting impedance of low linear resistance in parallel with the switch. The usual load current is carried by the VCB. In case of a power system fault, the fault current is commutated to and interrupted by the GTO by opening the VCB. As result tests, it has been confirmed that the equipment can interrupt an estimated short-circuit current of 13 kArms by limiting to less than 3.5 kAp. Interrupting time was less than 2ms. This response time is about 100 times as fast as that of conventional mechanical circuit breaker.
The current limiter, which has fast interruption, can protect distribution systems within a few ms from severe short circuit fault occurrence. The primary operational benefit of this equipment would be the reduction of instantaneous voltage drop at the bus or at sound lines and of current stress on power system equipment during a power failure.

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This paper proposes an algorithm to obtain an approximate optimal solution for the load balancing of large scale radial distribution system. Since the problem is formulated as a combinatorial optimization problem, it is difficult to solve a large-scale combinatorial optimization problem accurately within the reasonable computational time. Therefore, in order to find an approximate optimal solution quickly, the authors develop a solution method based on the network flow model. Firstly, the optimal load balance between feeders is found through the incremental algorithm with allowing the loop configuration. Secondly, the loop is eliminated through heuristic way to find the radial load balanced distribution system and finally, the branch exchange algorithm is used for finding the better-balanced radial status (approaching to the optimal solution) because the optimal load balance has been deteriorated when the loops are eliminated. Numerical calculations are carried out, and their results are compared with those calculated through branch and bound method and GA to demonstrate the validity of the proposed algorithm.

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The problem of the distribution system expansion planning in urban area is how to minimize new facility installation cost under the constraints of (1) line and transformer capacity, (2) voltage drop at each load point, (3) demand/supply balance and (4) radial configuration of the system. Many approaches to solve the distribution system planning problem have been proposed^[1]-[8]. These approaches assume that new installation candidates are known beforehand so as to simplify the problem. However, for the expansion planning in urban area, the task to determine the candidates of the distribution lines to be expanded includes the task to find the optimal candidate which satisfies the specified constraints. Therefore, it is useless to solve the optimization problem again once the candidates are selected. From the above background the authors have been proposing a new distribution systems expansion planning method which can determine the optimal expansion facilities by searching all the candidates which satisfies the expansion constraints such as current capacity, voltage drop as well as geographical installation restrictions.
In this paper, the authors discuss the determination method of the geographical allocation of the newly installed feeders. Since it is difficult to find the space to install new facilities in urban area, a part of the specified existing feeder may be used as a newly installed feeder. In such a case, so as to eliminate the over load feeders and to accomplish the load balance of feeders, load sections of over loaded feeders are transferred to the specified feeder by multi-stage load transfer, and the part of the feeder is cut off to form a new feeder. A new method to accomplish the multi-stage load transfer for this purpose is proposed in this paper. The proposed method can be used for the tool of the AM/FM-GIS, because it can deal with the geographical information. Through numerical examples, the validity of the proposed method is examined.

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Direct lightning strokes are considered to be a main cause of the damage to surge arresters on power distribution lines up to the present. Recently lightning performance of distribution lines has been observed using still cameras and lightning-caused distribution outages on hill-top areas on the coast of the Sea of Japan has been investigated. These research has shown a possibility that lightning backflow current flowing from customer's facility into distribution lines cause the damage to surge arresters on the distribution lines.
We have investigated the lightning backflow current flowing from customer's facility into distribution lines from the point of causing damage to surge arresters. The main results are as follows:
(1) The ratio of electric charge of the backflow current flowing into distribution lines is more than 60% of that of the lightning stroke current.
(2) In the case that grounding resistance of customer's facility is not low, the failure rates of a surge arrester caused by the backflow current due to winter lightning is about more than 90% of that caused by direct lightning strokes.

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In recent years, the transmission line arresters have been installed to protect an overhead power transmission line from a back flashover caused by lightning. It has been also expected that lightning surges in the substations incoming via transmission lines can be suppressed by them. However, the suppression effect by those with series gaps has not been investigated sufficiently. Author has measured lightning surges at two 77kV substations from 1990 to 1993. As the transmission line arresters have been mounted on the towers in this field since 1992, the lightning surges due to the sparkover of the series gap of the transmission line arresters can be observed at the substations. In this paper, the lightning surge waveforms due to the sparkover of them have been analyzed in detail at first. Next, the accurate EMTP simulation method considering the induced lightning surge voltages has been proposed. Finally, the suppression effect by them has been investigated by the simulation corresponding to each mounting method.

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A major problem in the current LFC system lies in that its effectiveness cannot be checked. This is because it is very difficult to grasp the amount of the control residue of each area. For each area, the sum of the control residue, self regulation and the net power fluctuations through its tie-lines must be zero. Since fluctuations through the tie-lines are measurable, control residue or self regulation complement to each other. That is, the knowledge of either one makes the other known. For the central and western Japan 60 Hz system which consists of four interconnected systems, we obtain ten independent demand-supply balance equations with fourteen unknown variables. If it is assumed that control residues of different systems have no correlation to each other, six variables are eliminated, thus the balance equations become redundant.
This paper proposes an analysis method to resolve this contradiction, which is similar to recursive analysis. In other words, measured data are manipulated to extract as much information as possible and to enhance the assumption of the orthogonality among control residues of constituent systems. Furthermore, in order to cope with the case where the orthogonality assumption does not hold, a countermeasure has been incorporated to minimize the effect of violation of the assumption. At the same time, the method has a capability of checking the degree of correctness of the assumption to adopt or reject obtained analysis results and can be regarded as a very practical method.

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Recently, expansion in the scale of power systems and development of the localized power sources are leading to an increase in fault current of 500kV systems. In future, it is quite likely that the fault current at the interconnection of such power systems may exceed the rated short-time current of existing electric power facilities. As one of the solutions to this problem, a thyristor controlled series resonant type fault current limiter (FCL) is proposed to restrain the fault current. This paper deals with the FCL system configuration, the placement method of the FCL in power system, the outline of the FCL's specification and the operation method of the protective relay in the multi-machine system. Finally, the effectiveness of the FCL is evaluated from the viewpoints of limiting the fault current by simulation analysis. The FCL is shown to be a useful protection device for large, high capacity power systems.

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