Discrete time models of a continuous power system stabilizer

The rise of electricity demand in a power system requires the use of discrete-time devices. They are widely spread and play an essential task in the operation and control of power systems. Several kinds of digital controlled devices have been put into practical use in power systems for the last decade, such as power system stabilizer PSS, automatic voltage regulator AVR, and proportional-integral plus derivative PID controllers. In this paper, an online digital PSS is designed for single machine infinite-bus systems. In this study the power system (plant) is represented by nonlinear model while the PSS (controller) is represented by a three-pole three-zero, lead-lag type PSS. Tustin´s discretization method is used to discretize the transfer function of the PSS. The effect of sampling intervals on the proposed digital PSS parameters is examined through simulations. They show that this approximation yields a good digital PSS for sufficiently small intervals. It was also found that, to counter larger disturbances, smaller sampling intervals are desired (about 2 ms-8 ms). For larger sampling intervals, fine-tuning of digital PSS parameters is required. The digital PSS is applied to one machine infinite-bus system, tested for different load conditions and disturbances, and found to be satisfactory.