This paper describes measurements, causes, and countermeasures of voltage reduction, spread of voltage unbalance, and voltage flicker, which are technical issues of voltage regulation in distribution system in recent years. The voltage measured in the distribution feeder with large reverse power flow from photovoltaic systems is explained by relationship between voltage rise by active power and voltage reduction by reactive power. The spread of voltage unbalance, which is also related to the large reverse power flow, is caused by unbalanced mutual impedance of three-phase distribution wires arranged horizontally or vertically. Frequency feedback method with step injection of reactive power, which is one of islanding detections installed into PCS, is attributed to voltage flicker in the distribution system.
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