where V is the bus voltage magnitude.
As the reactive power demand at the bus varies, the susceptance is varied subject to the limit. However, the reactive power as shown above in (4) is a function of the square of the bus voltage. So, the reactive power generation decreases, if the bus voltage decreases. The equivalent susceptance in the SVC control system is used to determine the firing angle. The phase locked loop (PLL) in the synchronizing system is used in the secondary voltage side and produces pulse using pulse generator for thyristor switching.
5. Simulation results and discussion
In this work, simulations are performed through Matlab/Simulink software. Simulations have been carried out considering both balanced (3LG: three-phase-to-ground) and unbalanced (1LG: single-phase-to-ground) permanent faults. F1, F2 and F3 are considered as the fault locations in the system model of Fig. 1. The simulation time and time step are considered as 20 s and 50 μs, respectively.
5.1. Effect of coordinated operation of conventional reclosing of circuit breakers and SVC
For conventional reclosing of the circuit breakers, it is considered that the fault occurs at 0.1 s at any of points of F1, F2 and F3. Circuit breakers are opened at 0.2 s and reclosed at 1.0 s. As the fault considered is permanent in type, the circuit breakers are reopened at 1.1 s. It is assumed that the circuit breakers clear the line when the current through it crosses the zero level. The SVCs connected at the terminal of the generators come into operation as soon as the fault occurs and the circuit breakers open.
For the evaluation of transient stability, in this work we have used the stability index Wc[10], [12] and [13] which is given by
equation(5)