The PID Optimized Control Provides Power Stabilization for Slow Speed Machines


Over the years, Basler Electric has tuned DECS Excitation Systems using the Pole Zero cancellation solution for optimizing generator voltage response. This approach toward power stabilizer tuning became very popular because the method is fast, accurate, simple, and machine time constant data is not as critical to achieve the desired optimum voltage response of the synchronous machine. The Pole Zero cancellation solution is based upon having a high Proportional Gain (KP) and an Integral Gain (KI) approximately four times lower than the Proportional Gain (KP). For rotating exciter systems, a Derivative gain (KD) term is introduced to dampen voltage overshoot in the system during a disturbance.

The gain values used for the Pole Zero Cancellation solution are typically in the range 80 for the KP Proportion gain or higher and a KI integral gain of 20. This general guideline works well for high speed machines (900 rpm and greater). However, for low speed units, such as hydro generators, where the machine time constants can be quite large for both the rotating exciter and generator main field, the controller gains need to be tuned more aggressively to achieve the best generator and voltage regulator performance.

DECS-400 Excitation System
Interior of a custom single-channel DECS-400 excitation system with a single rectifier bridge
 DECS-400 Excitation System
Close-up view of an IDP-801 Interactive Display Panel and two DECS-400 controllers in a custom system

In this Application Note, we discuss the final tuning gains that were implemented at the generating plant for a large dam in the Western United States after installation of new Basler excitation systems.

This dam has 16 low speed generators, rated for 140 MVA, 133 MW, 0.975 PF with a machine voltage of 16.5 kVac. Each generator spins at 180 rpm, having a 2.6 second exciter time constant and a 5.4 second main field time constant. The brush-type rotating exciter has commutators and brushes. Because the rotating exciter gain was lower than expected, much more aggressive gains were required for the DECS-400 controller to achieve optimum performance.

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