Recently，DSPs which include high speed AD　converters and high resolution PWMs　have been produced，and digitally controlled switching converters are being researched. In these converters，digitalization has been the method used to digitize analog operational amplifier circuits in order to obtain the desired output characteristics. This requires many parameters to obtain excellent output characteristics. However，it is difficult to tune the control parameters to their most suitable values. This paper proposes an experimental design as a decision method of the control parameters. When a controller has many control parameters this method can be used in conjunction with a combination of the most suitable parameter values. Alternatively，with a smaller number of experiments we can look for the most suitable parameters affecting output characteristics and their tuning directions. Finally，it is possible to quickly tune the parameters by following the direction and only using the parameters affecting output characteristics. We were able to significantly improve results by applying the digitized controller to a modified analog PID operational amplifier circuit.

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In order to prevent the troubles of electrical facilities, it is very important to adequately keep the power quality in distribution system. The imbalances of voltage and current are one of power quality, and many troubles caused by the imbalance of voltage are reported. In order to improve the imbalance of voltage, it has been recommended by guideline that the imbalance rate of facilities is kept below 30% by high voltage (HV) consumers. However, the actual condition of the imbalance of load in HV consumers has not been clarified. In the paper, it was clarified by measurement that the imbalance rate of load is over 30% in many HV consumers. On the other hand, it has been recently reported that the static capacitor (SC) in HV consumer influences the power quality, such as voltage and harmonics. From this background, the influence of SC to the imbalance of voltage and current was investigated. Two methods for improving power factor by using SC are discussed by simulation by using the equivalent model of distribution system.

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