CHARACTERISTICS OF GENERATOR EXCITER POWER (GEP) SYSTEMS
The characteristics of GEP are established by extensive system investigations. All plant operating modes must be examined to identify the conditions of marginal stability. In general, the periods of low system demand (at night) are the most critical. The generator operates at a lead- ing power factor during these times. In pumped-storage plants (where the turbine is used to pump the water upstream during the night) this is done because the price of electricity is very low at night. The same water is allowed down through the turbine during the day because the price of electricity is higher, so the situation is more critical. This is because of the large rotor angle (angle between the rotor flux and stator flux—this angle normally increases with the load when the generator operates) in comparison with the remaining machines on the system during the night. The generator is operating as a motor in this situation.
A number of simulations are done on the unit (including AVR and PSS). The PSS set- tings are adjusted for optimum performance of excitation under all critical operating con- ditions. These settings are then used during plant commissioning. This is done to reduce on-site testing, which can be expensive.
Excitation System Analysis
The generator excitation system has the primary responsibility for power system dynamic and transient stability. Dynamic stability refers to the performance of the system following small load changes. This can result in sustained oscillations around 0.5 Hz when large power is transferred over long distances. These oscillations must be rapidly attenuated. Otherwise, the transmission system will be severely limited. Transient stability refers to the ability of a generator or group of generators to maintain synchronous operation following system faults.
Following a fault, a boost of synchronous torque is required to maintain the generator in synchronism. (Note: The synchronous torque is the torque used to maintain the generator in synchronism. It is created by active power sent to the grid. This is done by increasing the field current.) In this situation, the AVR bucks (resists, opposes) and/or boosts the field current to develop the additional synchronizing torque. Therefore, the AVR must be properly tuned to play an essential role in maintaining stable system operation under all operating conditions.