From power system on-line operation and security control view point, state estimation, a methodology used to obtain reliable estimate of power system state, has become one of the important issues. Among some state estimation methods, the fast-decoupled state estimator is commonly used as a prevailed method and has been implemented by many utilities. However, it has been recognized that its convergence characteristics may become deteriorated when it encounters bad system conditions. Therefore, in this paper, first of all, we present the fast-decoupled state estimator which is used in present power systems. Next, we propose a method to improve the convergence characteristics of fast-decoupled state estimator (in rectangular coordinates) using the optimal multiplier μ, followed by a reliable technique to detect bad data. The proposed method has been tested on several types of load flow test systems and successful results have been obtained.

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A high efficiency cogeneration system (CGS) utilizing high temperature exhaust gas from a gas engine is proposed. In the proposed CGS, saturated steam produced in the gas engine is superheated with a super heater utilizing regenerative burner and used to drive a steam turbine generator. The heat energy is supplied by extracting steam from the steam turbine and turbine outlet low-temperature steam. Both of the energy saving characteristics of the proposed CGS and a CGS constructed by using the original gas engine (GE-CGS) were investigated and compared, by taking a case where energy for office buildings was supplied by the conventional energy systems. It was shown that the proposed CGS has energy saving rate of 24.5%, higher than 1.83 times, compared with that of the original GE-CGS.

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In a distribution system, in order to enhance the reliability of power supply, the distribution feeder is divided into several sections by installing sectionalizing switches, and then each of sectionalized sections is connected to different feeder. For example, one feeder is divided into three sections by two sectionalizing switches, and then each of divided sections is connected to other feeder through sectionalizing switch. Since a distribution system with many feeders has many sectionalizing switches, the system configuration is determined by states (opened or closed) of sectionalizing switches. Usually, power utility tries to obtain distribution loss-minimum configuration among large numbers of configuration candidates. However, it is very difficult to determine the loss-minimum configuration that the mathematical optimality is guaranteed, because it is well known that determination of distribution system's configuration is to decide whether each sectionalizing switch is opened or closed by solving a combinatorial optimization problem.
In this paper, the authors propose a determination method of loss minimum configuration which the mathematical optimality is guaranteed for a three sectionalized and three connected distribution feeder network. A problem to determine the loss minimum configuration is formulated as a combinatorial optimization problems with four operational constraints ((1) feeder capacity, (2) voltage limit, (3) radial structure and (4) three sectionalization). In the proposed method, after picking up all partial configurations satisfied with radial structure constraint by using enumeration method, optimal combination of partial configurations is determined under the other operational constraints by using conventional optimization method. Numerical simulations are carried out for a distribution network model with 140 sectionalizing switches in order to examine the validity of the proposed algorithm in comparison with one of conventional meta-heuristics (Tabu search).

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Participation of distributed generators (DG), such as wind turbines, co-generation system etc., is natural trend from ecological point of view and will increase more and more. The outputs of these DGs mainly depend on weather condition but don't correspond to the changes of electrical load demand necessarily. On the other hand, due to the deregulation of electric power market, the power flow in power system will uncertainly vary with several power transactions. Thus, complex power flow by DGs or transactions will cause the voltage deviation. It will be difficult to sustain the voltage quality by using the conventional voltage/reactive power control in near future.
In this paper, in order to avoid such a voltage deviation and to decrease the frequency of transformer tap actions, the coordinated voltage control scheme of transformer taps on account of hierarchical structure in power system is proposed. In the proposed scheme, integral of voltage deviation at each layer bus is applied to decide the timing of each transformer tap action. It is confirmed by some numerical simulations that the proposed scheme is able to respond to every conditions on voltage deviation.

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To apply lithium-ion secondary batteries to electric power storage apparatus in distributed power generation systems, electric vehicles, etc., it is necessary to take into consideration their transient cell voltage response to the change of the charge/discharge current. The authors, therefore, develop a simulation method that can accurately calculate this transient cell voltage response of lithium-ion secondary battery even when the cell temperature and state of charge vary as the time proceeds in the time-dependent calculation. The transient cell voltage responses of a commercially available small-sized lithium-ion secondary battery to the square-wave charge/discharge and discharge currents are numerically simulated, and the calculation results are compared and examined with the experimental results. It is made clear that the calculation results coincide well with the experimental results not only for the charge/discharge current with short period under constant temperature and state of charge condition, but also for the discharge current with long period under variable temperature and state of charge condition, indicating the validity of the developed simulation method.

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To improve GIS insulation specifications, it is important to recognize the insulation characteristics under oscillatory overvoltage waveforms occurring in the field. The actual lightning surge waveform (called non-standard lightning impulse waveform) in actual substations is different from the standard lightning impulse waveform (1.2/50μs). The actual lightning surge waveform generally rises steeply, however, the decay of the overvoltage is large, and the insulation requirements are not as severe as those of the standard lightning impulse waveform. Since SF₆ gas was identified as a greenhouse gas at COP3 in 1997, alternative insulation gases to SF₆ have been investigated. The CO₂ gas insulation is one of the candidates, which is a natural gas and has lower global warming potential (GWP). V-t characteristics with CO₂ showed a steep decline than SF₆ and the effects of oscillation on the insulation characteristics with CO₂ are not definite under the condition of frequency up to 4.0MHz.
This paper describes the insulation characteristics of CO₂ gas for single-frequency oscillatory waveforms with various frequencies from 5.3 to 20.0MHz and damping ratios. From experimental investigation, similarly to SF₆, it might be possible to reduce the test voltage of CO₂ gas insulation system by evaluating actual lightning surge waveform in terms of the equivalent standard lightning impulse waveform.

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SF₆ gas is made a problem as greenhouse gas, and a study on a substitute gas for SF₆ is widely done. This paper describes a short line fault (SLF) interruption characteristic of CF₃I gas, which has low environmental effects and a high dielectric strength, and CF₃I-CO₂ gas mixture, and it is considered to apply CF₃I to the gas insulated switchgear. A SLF interruption characteristic is evaluated by measuring a di/dt-dv/dt characteristic. Pure CF₃I gas has about 0.9 times of SF₆ gas for a SLF interruption characteristic. Also, a SLF interruption characteristic of CF₃I-CO₂ gas mixture, which rate of CF₃I is 20%, is equivalent of pure CF₃I gas. Thus, CF₃I gas can lower the liquefaction temperature by using its mixture with carbon dioxide.

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Synchronously controlled switching to suppress transient overvoltage and overcurrent resulting from when the circuit breakers on medium voltage systems are closed is described. Firstly, by simulation it is found that if the closing time is synchronously controlled so that the contacts of the circuit breaker close completely at the instant when the voltage across contacts of the breaker at each of the three individual phases are zero, the resulting overvoltage and overcurrent is significantly suppressed when compared to conventional three phase simultaneous closing. Next, an algorithm for determining the closing timing based on a forecasted voltage zero waveform, obtained from voltage sampling data, is presented. Finally, a synchronous closing experiment of voltage 22kV utilizing a controller to implement the algorithm and a VCB with an electromagnetic operation mechanism is presented. The VCB was successfully closed at the zero point within a tolerance range of 200 microseconds.

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Lightning surge analysis is very important from the viewpoint of insulation design of transmission lines and substations. Lightning surge analysis has many parameters, which include lightning surge characteristics of transmission towers, back flashover phenomena at an arcing horn, characteristics of footing resistance, effects of corona wave deformation, characteristics of electromagnetic fields caused by lightning, and other parameters. This paper describes experimental and analytical studies on lightning surge characteristics of a buried bare wire. The measurement of the lightning surge characteristics of the buried bare wire is carried out under various experimental conditions. The experimental parameters controlled in these experiments include earth resistance, length of the buried bare wire and waveform of the injected current. The measured results are compared with analytical results based on the theoretical study by Sunde. A comparison of the measured results with the analytical results shows good agreement.

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