Stability Analysis of Thyristor Current Controllers

The operational stability areas of proportional-integral (PI) controllers implemented in analogue, analogue with discrete sampling, and digital form have been theoretically investigated. Whereas in the continuous-data control system, stability may be-maintained by compensating any increase of proportional gain Kp with integral gain, Ki, the discretized system tends to become unstable if either Kp or Ki is increased. A new stability area is shown to exist for a discrete-sampled system with puire integral control and large gain (which is unstable when implemented in analogue form). This result is confirmed experimentally.