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Regulating Distribution System Voltages: Fundamental Concepts

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Regulating Distribution System Voltages: Fundamental Concepts

A power distribution system distributes power from a substation to the point of power utilization. Voltage drops occur naturally on distribution systems. It is designed to keep the voltages near the nominal values, and usually within a few percent limit. Besides load balancing, utilities commonly use both voltage regulators and shunt capacitors to maintain voltages within the design limits. This paper will address what factors to be considered when deciding whether regulators, capacitors, or load balancing or a combination is most appropriate to provide the needed voltage support.

A distribution system typically is not balanced due to a high concentration of single-phase loads. The load is also very dynamic and varies over time. New customers are added, while other customers may relocate, abandon, or change load at their facilities. All of these factors contribute to difficulty in controlling the distribution voltage within certain limits. The circuit model for a distribution system can be thought of as a series of impedances, with loads connected in between. The type of network often used to model a distribution circuit is a "ladder" network, and the repetitive nature of the calculations is most appropriate for computer analysis.

Performing accurate voltage calculations on a distribution system is a difficult task. An average feeder could easily serve 1,000 or more customers, but modeling these many nodes for one feeder is impractical. Therefore, the individual customer loads must be grouped, and modeled as a reasonable number of nodes to obtain acceptable accuracy.

When an inductive current flows through an impedance, a voltage drop occurs. A typical model of the circuit conditions is given in Fig. l(a), and the corresponding phasor diagram for one phase is shown in Fig. l(b). The difference (,1V) in voltage magnitude between V1 and V2 can be approximated for lagging power factor load conditions as ( by ignoring the out-of phase voltage difference )

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Fundamental Concepts of Regulating Distribution System Voltages