3. Optimal reclosing technique
Conventional auto-reclosing of circuit breakers can affect the stability, as it is dependent on the generator state of reclosing instances. So, to enhance the transient stability, circuit breakers should be closed at an optimal reclosing time (ORCT), when the system disturbance has no effect after reclosing operation.
In literature, there are few optimal reclosing techniques available for circuit breakers. But they lack in online application and some of them require various calculations considering different system operating conditions [2], [3] and [15]. Moreover, some of them are only suitable for single machine systems. The optimal reclosing technique used in this work has online application advantage and can be implemented easily for multi-machine power system. In this method, the time when the total kinetic energy oscillation of the generators without reclosing operation becomes the minimum is determined as ORCT. The reason behind this assumption is as follows.
In order to properly establish the effect of total kinetic energy based ORCT of the circuit breakers on improving transient stability, it is worth to mention that the rotor speed of each generator in the system is considered for determining total kinetic energy. Rotor speed response indicates the state of the generators in case of any major disturbance in the system. So, if we are going to consider the time when the total kinetic energy oscillation of generators without reclosing operation becomes the maximum as ORCT then the generators will be accelerated with some more added energy and will become out of step. Therefore, the first swing stability will not be possible and the system will be more unstable. Furthermore, if we are going to reclose the circuit breakers at the time when the total kinetic energy oscillation without reclosing operation becomes the minimum, then the generators in the system will be rotating with lower energy and there will be no added accelerated energy. So, they will not go out of step and will easily come back to previous state.
The optimal reclosing should happen when the deionization time Tr< reclosing time, where Tr = (10.5 + kV/34.5) cycles and KV indicates the line-to-line rms voltage of the system. The deionization time equation is based on statistical data and power system engineer's years of experience. Based on the 20 kV line voltage and 50 Hz frequency (1 cycle = 0.02 s) of the power system model in Fig. 1, the value of Tr is 0.223 s.
The total kinetic energy, WTotal, can be calculated easily by knowing the rotor speed of each generator and can be expressed as
equation(1)