This paper proposes new decision method of additional meter location for state estimation of power system, which is desirable from the viewpoint of (N-1) observability. (N-1) observability means “Even if one meter is failed to observe, still the network can be observable with remaining meters”. This criterion denotes the flexibility of the network topology of power system with respect to the trouble of meter.
“(N-1) observable area” is also defined as the index of a reliable measurement set. The additional meter location can be decided with maximum (N-1) observable area by the proposed method.
Small and middle systems are used to illustrate the principles and the effectiveness of the proposed approach.

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Once a fault occurs in the power system, the fault section must be estimated from inadequate information on protective relays and circuit breakers. There have been several studies to develop an automatic estimation method: knowledge engineering, neural network, etc. However, it was very difficult to make a method which is powerful enough and easy to maintain.
In this paper, the discrimination analysis theory is applied to the fault section estimation. Using the theory, it is expected that the inference speed can be improved. It is also expected that the knowledge about the fault section estimation can be collected.
First, using the small model system, fault section estimation method with the discrimination analysis theory is described and compared to the method with neural network. Next, it is illustrated that the method can be applied to the large system.

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An advanced fuzzy logic control scheme has been proposed for micro-computer based power system stabilizer (PSS) to enhance the stability of power systems. In the proposed control scheme, additional integral information of generator speed is taken into account to enlarge the stability region together with propotional and differential information of generator speed. Namely, the advanced control rules are based on the PID information of generator speed. Various simulations have been performed subject to several types of large and small disturbances using a one-machine and an infinite-bus model system. In addition, comparison studies have been made between a conventional analog-type PSS and the fuzzy logic PSS. The results show that the stable region of the study system is highly enlarged up to the physically stabilizable boundary by the proposed fuzzy logic control scheme.

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This paper presents a method for obtaining preventive control strategies based on non-linear optimization for power systems which will incur voltage instability when load demand increases. The purpose of this algorithm is to determine an appropriate and effective control action taking into account the operating constraints, in order to widen the margin between the present operating point and a voltage collapse point, hence improving the system state. The method proposed here is based on a complex-valued load-flow technique using the Newton-Raphson method which has been already developed by the authors. Consequently, the preventive control algorithm also can evaluate the voltage instability for the case that the present operating point becomes closer to the critical point and system state becomes unfeasible at increased loading point. And in optimization, two-types of objective function are introduced so that voltage stabilization and dissolution of constraint violation can be attained simultaneously. In addition, this paper discusses the extension of the proposed preventive control to the successive control method which carries out the control action for avoiding voltage collapse while the load demand is increasing. Numerical examples for various model systems demonstrate the effectiveness of the proposed method.

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This paper describes a unified automatic generation control (AGC) scheme coordinating EDC (Economical Load Dispatching Control) and LFC (Load Frequency Control). So far, in absorption of the load fluctuations occurring on power systems, both of EDC and LFC systems have been considered separately each other. By coordinating both control systems, undesirable system behaviours are avoided. The responses of frequency deviation, turbine power fluctuation and tie-line power flow deviation are smoothed using the proposed AGC scheme.
The background theory is based on servo-type optimal control theory. The structure of the proposed control method is a decentralized type. So, the implementation in multi-area power systems is easy.
The effectiveness of the proposed unified AGC is illustrate_??_ by an example, and the simulation results on three-area power systems show that the method is encouraging. In this paper, some of the problems associated with the application of this method to the multi-area AGC are discussed.

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This paper reports a method of forecasting the load curve for the following day. Coefficients in the multiple regression analysis used for forecasting the load are estimated by processing data of the previous year. When only a simple multiple regression analysis method is used, an hourly set of coefficients is provided exclusively by data of the corresponding time. To get a better approximation of the hourly coefficients with insufficient data, the proposed method estimates a daily set of the coefficients assuming a daily cyclic change in them. We call our proposed method a cyclic coefficients model.
This assumption does not mean the hourly acute change of the coefficients. The influence of daylight is neutralized in the load data before the estimation of the daily set of coefficients, because of the sharp change in the coefficients of the amount of light at sunset.
Practical data of the Hokkaido area are simulated by both the basic application of the multiple regression analysis and the proposed model. The accuracy of the proposed method is shown by the comparison of results.

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This paper deals with voltage control in restoration of EHV power system. Restoration of power system is performed by energizing transmission lines in sequence from initial power sources. These operations have already been progammed into the expert system developed by the authors. However, energizing no-load transmission lines causes some bus voltages to rize 20% more than rated voltages. Transformers can tolerate 20% voltage rize only for 1 minute. Here, we supplement the expert system with a function of checking bus voltages and reactive powers of initial sources, and automatically switching reactors on. Conditions for switching reactors and selecting appropriate reactors, are written by rules, hence it is easy to modify them. Target of voltage control is to keep bus voltages within ±5%, and to keep reactive powers of generators within 30% of generator capacity. Applying this method to three basic restoration patterns of a model power system, shows that small adjustment of switching conditions is enough for two patterns, and some modification of restoration procedure is necessary for the other pattern to obtain good voltage profiles through the restoration process.

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Large power system frequency/voltage excursion due to severe faults in trunk transmision line or major generating unit may cause trip-off of other facilities and result in large-scale power system blackout. To prevent such blackout, development of an accurate and fast long-term power system dynamic simulation tool is required. In the development stage, selection of a numerical integration method available for the step size adjustment and development of a scheme for the adjustment are principal subjects.
The trapezoidal and the 2nd Gear methods were selected as possible candidates for the step size adjustment from the viewpoint of numerical stability, then the trapezoidal method are finally selected from the viewpoint of accuracy and applicability to power system dynamic simulations.
A new scheme for the automatic step size adjustment is proposed for the trapezoidal method to achieve fast simulation of the long-term power system dynamics. The validity of the proposed scheme is verified through simulation studies on a single-machine-to-infinite bus system.

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A transient stability simulation algorithm that can handle the flexibility of system components is studied. The algorithm is explained comparing to a conventional method. Since no iteration process is used, there is no mathematical instability occurred at critical conditions. Precision of integration algorithms are compared to choose an appropriate integration method.
Studies on a control system for phase-shifter's angle and on fast reclosing of separated system are also conducted for a model system with a generator and an infinite bus connected by two transmission lines.

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This paper presents a new method to express saturated magnetizing inductances of synchronous generators. In standard power system simulations, the saturation of main flux in synchronous generators is expressed in terms of variable magnetizing inductance as a function of magnetizing currents. This method, however, requires repeated calculations of many coefficients in machine equations at every time step of the simulation, because mathematical equations of the saturated synchronous machines contain many coefficients which consist of the magnetizing inductances. In this paper, auxiliary current to express the saturation is introduced and new model expressed in terms of the current is proposed. The new model contains only constant coefficients which are calculated from unsaturated values of the magnetizing inductances, and thus it improves the efficiency of the simulations.

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Advanced voltage and var control equipment requires the maximum voltage deviation of 0.1% despite of highly distorted waveforms and the frequency variation. Digital signal processing can satisfy this requirement using fast sampling and very fast 32-bit floating point operations by a DSP (Digital Signal Processor).
This paper describes the design philosophy of the voltage measuring method using an adaptive filter with high-precision measurement. In addition, total voltage measuring deviation characteristics are described in terms of adaptive digital filter characteristics, peak value operating principle, influence of gain deviations of element filters and influence of sampling frequency for adaptive filter.

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Supply and demand in power system planning and operation is required to be balanced. An operational reserve to provide against faults or accidental demands are also required. Therefore load forecasting is one of the most important fields and various load forecasting methods have been applied.
In this paper the grey system theory, which treats uncertain information, is applied to the long term load forecasting from three aspects: the point prediction, the interval prediction, the topological forecasting. In the point prediction, the annual total demand is predicted. In the interval prediction, the annual peak demand is predicted. And in the topological forecasting, the date where a yearly maximum peak demand would occur is predicted. The grey dynamic model (abbreviated to GM model) is adopted as the predicted model. The GM model is a differential equation model which is different from the most forecasting models. The GM model is quite powerful when combined with the preliminary transformation called the accumulated generating operation (AGO).
This paper proposes a new method for the long term load forecasting problems involving uncertainty. The predicted results have been found very satisfactory. The grey system theory is a new tool very efficient for load forecasting.

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At the request of air-operation-less subsations, air-operated circuit breakers are going to be replaced to oli-operated or spring-operated circuit breakers recently. To apply a spring-actuater of lower operating energy, it is necessary to reduce the operating energy of interrupters.
The authors investigated a hybrid circuit breaker of series connection of thermal puffer gas interrupter and vacuum interrupter. The withstand-voltage ability of thermal puffer gas interruter, which requires small operating energy, is high, but high-di/dt or high-dV/dt interrupting ability is not so high. On the other hand, high-di/dt or high-dV/dt interrupting ability of vacuum circuit breaker is high, but its unit voltage is difficult to increase. So the authors invented to connect them is series. Its operating energy will be quite lower than that of conventional puffer type gas circuit breaker.
One of the subjects for developping the hybrid circuit breaker is to reduce the distribution of transient recovery voltage of vacuum interrupter during high current interruption. Interrupting characteristics were examined experimentally. As a result, interrupting ability of thermal puffer gas interrupter was improved by changing gas flow from single-flow to double-flow. The principle of voltage sharing of the hybrid circuit breaker were confirmed. This means that the hybrid circuit breaker of series connection of thermal puffer gas interrupter and vacuum interrupter will be realized as a quite low operating enegy pufferless circuit breaker.

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