Excitation systems with high gain and fast response times greatly aid transient stability (synchronizing torque), but can also reduce small signal stability (damping torque). Power system stabilizer (PSS) control provides a positive contribution by damping generator rotor angle swings, which are in a broad range of frequencies in the power system. These range from low frequency intertie modes (typically 0.1 - 1.0 Hz), to local modes (typically 1 - 2Hz), to intra-plant modes (about 2 -3 Hz). The low frequency modes, commonly called intertie or interarea modes, are caused by coherent groups of generators swinging against other groups in the interconnected system. These modes are present in all interconnected systems and the damping is a function of tie line strength and unit loading factors. Weak ties due to line outages and heavy system loads can lead to poorly damped intertie modes. PSS control can generally provide significant improvements in intertie mode damping, by applying stabilizers to most units that participate in power swing modes.
PSS performance is often evaluated from the damping of the “local mode,” the generator swinging against the rest of the power system. This mode is usually at frequencies between 1 and 3 Hertz. Stronger system ties and lighter loading tend to give higher local mode frequencies, and weaker ties and heavier loading tend to give lower local mode frequencies. PSS performance must be designed to provide acceptable performance over a wide range of system conditions, which may result from different operating conditions (such as lines out-of-service and varying load levels).
GE Energy Consulting performs PSS Tuning and Testing studies to help customers achieve the best practical performance. The use of small-signal frequency-domain techniques has proven to be very effective in these studies, and GE Energy Consulting has been developing and improving simulation tools for over 30 years. In addition to the damping of the low frequency modes, which are of primary importance, it is well known that the PSS can also introduce undesirable effects at the characteristic modes of the turbine-generator mechanical torsional system. GE's experience proves that such interaction must be strictly limited for GE turbine-generator designs. There are effective means to reduce torsional signal levels built into GE PSS designs; the settings for these filters are determined from Energy Consulting screening studies.
The PSS provides modulation of field voltage that damps out power and speed oscillations through normal AVR control. The tuning study determines the optimum PSS settings, based on the particular generator, AVR settings, and system characteristics. Special purpose detailed models are used for this analysis. Our studies determine the key adjustment of PSS phase compensation. To complement this offering, we also perform PSS/Torsional Interaction screening for steam turbines with low modal frequencies. These studies are performed to determine if a torsional filter is required.
Testing of the AVR/PSS Controls
Testing of the PSS is normally performed during plant commissioning. The test condition for PSS is at or near base load output of the plant. The testing of modern excitation systems is facilitated by the use of internal data recording and test signals.
The basic types of tests usually performed are given in the following list.
1. Step test in AVR reference (base load – without PSS).
2. Gain margin test to determine the PSS gain to be used.
3. Step test in AVR reference (base load – with PSS).
Optional additional testing for new design units or per customer requirements are:
1. AVR uncompensated transfer function.
2. PSS transfer function