In this paper, transmission line performance with voltage sensitive loads is studied. Three types of load, namely constant power, constant current and constant impedance loads are considered, individually as well as in a mixed combination, and it is shown that the transmission line loss is highest with the constant power load and lowest with the constant impedance load. The line loss for mixed load is higher than that for constant power load when the constant power component is more pronounced in the mixed load.

Transmission lines constitute the arteries of an electric power system. The availability of a well-developed, high capacity system of transmission lines makes it technically and economically feasible to move large blocks of power over long distances.

Usually, transmission line performance is studied by considering constant power loads. The variation of power and reactive power taken by a load with various voltages is of importance when considering the manner in which such loads are represented in transmission line studies.

Most textbooks include transmission line
performance studies assuming constant power loads at the receiving end. The objective of the present paper is to incorporate the effects of the load characteristics into long-line theory to determine transmission line performance.

Although classical power flow load models which are functions of voltage have been used in production grade programs for load flow studies for years,  not much work has been done to study the performance of a single transmission line with voltage-sensitive loads.

Since a transmission line is an important component of the power system, the effects of voltage sensitive loads on its performance could form a part of the syllabus while presenting the transmission line theory in undergraduate classes. The work presented in this paper is based on the authors’ previous work on line losses and shunt compensation of EHV compensated transmission systems.

For constant power factor load, the active and reactive power demands, depending on the type of load, may remain constant with the voltage, change linearly with the voltage, or change as a function of the voltage squared. Constant impedance loads such as water heaters, electric ranges, series inductors and shunt capacitors are represented by RLC circuits and the active and reactive powers consumed  by such loads vary as the square of the voltage. The active power consumed by a lighting load containing incandescent lamps varies with voltage approximately as V.

The active power consumed by a lighting load consisting of fluorescent  lamps depends only slightly on voltage. Lighting load consumes no reactive  power. The active power consumed by a synchronous motor is approximately constant with change in voltage. For induction motors, the PV and QV characteristics are determined from the equivalent circuit, assuming shaft load to remain constant.

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