The function of a power system stabilizer (PSS) is to extend the small–signal rotor-angle stability limits of the power system by modulating the synchronous machine’s excitation to damp the rotor speed oscillations. To provide positive damping, the stabilizer must produce a component of electrical torque in the rotor in phase with the speed deviations.

The attached project shows an example in which a rotor-speed based PSS is tuned using a single-machine infinite-bus system with the objective to achieve a minimum damping ratio of 15 % for the system with PSS.

Each study case corresponds to one step of the following tuning procedure:

1. Step 1 - Base case without PSS: This case is used to analyze the stability of the system without PSS. A modal analysis is run to identify the dominant oscillation mode of the system, which has a frequency of 1.23 Hz and a damping ratio of 3.35 %. An RMS simulation (ComSim) is run to verify the time domain response of the system to a step change in the reference signal of the automatic voltage regulator (AVR).
2. Step 2 - Phase compensation requirements: The PSS phase compensation requirements are obtained from the Bode plot of the generator-exciter-power system (GEP) open-loop transfer function calculated with the machine’s inertia set to an infinite value. The Bode plot is obtained from the execution of the Calculation of Frequency Response command (ComFreqresp) in which the input signal is the PSS input signal (upss) of the AVR, and the output signal is the synchronous machine’s electrical torque (xme). From the GEP transfer function, the PSS should ideally compensate 88.72° at 1.23 Hz.
3. Step 3 - Phase shaping: The PSS model is added to the synchronous machine’s frame. The time constants of the PSS phase compensation blocks are chosen to match the ideal phase compensation requirement calculated in the previous step. The ComFreqresp command is run to obtain the Bode plots of the PSS transfer function GPSS and the compensated transfer function GEPxGPSS. The numerator and denominator time constants of the lead-lag blocks of GPSS are set to 0.256 s and 0.044 s, respectively (i.e. each block adding 45° at 1.5 Hz). It is verified that the phase of GEPxGPSS is 2.8° at 1.23 Hz and lies within +/-30° in the typical range of electromechanical frequencies from 0.2 Hz to 2.0 Hz.
4. Step 4 - PSS gain selection: The modal analysis is iteratively run for increasing values of PSS gain (K) until the damping ratio of the dominant mode is at least 15%, the selected value being 24 pu. The time domain response of the system to a step change in the AVR reference signal is simulated and compared with the response without PSS (Step 1) in the same plots.

The following scripts provided in the "Scripts” folder of the project library can be used to aid in the tuning process:

• DPL command “Lead_Lag_Time_Constants”: calculates the time constants of two first-order lead-lag blocks for given center frequencies and maximum angles.
• Python command “wrapTo180”: wraps the results of the phase Bode plot to ±180°. It must be executed while the plot page with the Bode plot results is being displayed.

A similar procedure can be applied to tune power damper controllers (POD) installed in inverter-based resources, FACTS devices or HVDC converters. In those cases, the proper POD input and output signals must be selected and the ComFreqresp configured accordingly.