EFFECTS OF VOLTAGE SENSITIVE LOADS ON THE VOLTAGE STABILITY OF COMPENSATED EHV-TRANSMISSION LINES

ABSTRACT

In this paper, effects of voltage sensitive loads, namely, constant power, constant current and constant impedance type loads on the voltage collapse phenomenon of EHV transmission lines with different types of compensation schemes are studied and discussed. Exponential modelling is used for the three types of loads and exact long line equations are used in the analysis. It is seen that the power transfer capability of a long line for any type of compensation scheme is the highest for a constant impedance load, and the lowest for a constant power load. The power transfer capability for a constant current load has an intermediate value compared to those for constant impedance and constant power loads respectively. Further, the transformer tap control has an adverse effect on the voltage stability for constant current and constant impedance type loads. Improved power factor improves the voltage stability condition for all types of compensation schemes. In all compensation schemes, an increased degree of series compensation makes the system more voltage stable. The effect of increasing the degree of shunt compensation is also similar, but marginally small. The increase of line loss-factor increases the power transfer capacity for all types of loads. Scheme - C with series and shunt compensation at both ends of the line and Scheme - SH with shunt compensation at load-end only with constant power loads are worse than the uncompensated line, from the voltage stability point of view. However, Schemes C and SH are better than the uncompensated line for the constant current and constant impedance type of loads. The results reported in this paper consider the effects of voltage sensitive loads on the voltage stability of different series and shunt compensated schemes.