An investigation of transient and dynamic stability of multimachine interconnected power system during line-to-line faults

In view of the increasing complexity of present-day power systems, their design and operation require a more detailed analysis of possible performance modes. Therefore, digital simulation technique has become a very important analytical tool in planning the power system and its stability studies. Although the balanced, three-phase to ground fault has been the dominant type of fault used in power system stability studies, in reality the unbalanced faults contribute the maximum percentage of faults occurred in power systems.; In this research, a computer program has been developed to simulate a multi-machine interconnected power system. This program is capable of predicting the performance of the multi-machine system after the occurrence of a line-to-line fault. This computer program has many advantages in that it permits study of the transient behavior of both the synchronous and induction machines during the unbalanced line-to-line faults and is not limited to a single machine or pairs of machines, but can handle a number of machines and is limited only by the storage capacity of the digital computer used in its implementation.; A sample interconnected power system has been selected and simulated using this program where the electrical and mechanical changes in the system during a fault period of 0.3 seconds have been presented. An illustration has been made if a balanced three-phase-to-ground fault would have been applied to the same system. Finally, the effect of the negative-sequence currents on the performance of the system during the fault has been discussed. The contributions of the research are summarized as: (1) The mathematical modeling of synchronous generators and induction motors during line to line fault have been fully represented. (2) The contribution of each machine in the multi-machine power system during the fault period (300 m sec) has been evaluated. (3) The prediction of the transient behavior of every machine in the system and the theoretical calculation of fault current of the faulted bus to a fair degree of accuracy during line-to-line fault have been done, which would be very helpful in determining circuit breaker rating and switchgear design. (4) The results have shown that unbalanced line-to-line fault can have a significant effect on motor transient stability. It creates a large amount of unbalance in both the motor currents and the negative sequence torque. (5) The results confirm that the three-phase balanced fault is the most severe by causing the drop in motor speed to be the largest, but also show that the motor transient behavior due to line-to-line fault is significantly different from that caused by a balanced fault.