THE POWER SYSTEM STABILIZER
When a generator is synchronized with the grid, it is magnetically coupled to hundreds of other generators. This coupling is not rigid like a mechanical coupling. It is a flexible coupling similar to a connection with elastic bands. During normal operation, the generator oscillates slightly with respect to the grid. These oscillations are similar to vibrations of a mass attached to a rigid surface by a spring. These electromechanical oscillations normally have a frequency of 0.2 to 2.0 Hz. This frequency depends on the load and location of the generator with respect to other large generators. Each machine can have different modes of oscillation. The frequency of these oscillations can be 0.3 Hz or 1 to 2 Hz normally. Therefore, the electric power produced by the generator is not matching the mechanical power produced by the turbine at every instant. However, the average mechanical power produced matches the electric power generated by the unit.
In some cases, groups of generators at one end of a transmission line oscillate with respect to those at the other end. For example, in a four-unit generating plant, the four generators tend to be coherent. They tend to oscillate as a group. An oscillation of 10 to 50 MW (above and below the 600-MW rating) is expected. These oscillations are called power system oscillations. They depend on the load. They must be prevented. Otherwise, they can severely limit the MW transfer across the transmission system.
Following a system fault, an accelerating torque will be applied to the generator as a result of changes in the electrical transmission system. The generator must produce a breaking torque in this situation to counter the accelerating torque. The damper winding will pro- duce a countertorque (breaking or damping).
Note that the damper winding are bars normally made of copper or brass. They are inserted in the pole face slots and connected at the ends. They form closed circuits as in squirrel-cage winding. During normal operation, the generator is operating at synchronous speed. The damper winding also moves at the same speed. Thus, it is inactive. During a transient, the generator speed changes. The damper winding is now moving at a different speed from the synchronous speed. The currents induced in the damper winding generate an opposing torque to the relative motion. This action helps return the rotor to its normal speed.
The losses such as windage and bearing friction are speed-dependent. They also pro- duce a countertorque that will help to reduce the overspeed. (Windage losses increase with the cube of the speed. The journal bearing losses increase with the square of the speed. The axial thrust bearing loss increases with the speed.)
The power system stabilizer (PSS) is added when there is insufficient countertorque (damping). All units over 10 MW must be reviewed for need of PSS. Most of them require a PSS. The PSS measures the shaft speed and real power generated. It determines the difference between the mechanical power and the electric power. It produces a signal based on this difference that changes the speed of the machine (it produces a component of generator torque in phase with the speed changes). The objective of the PSS is to keep the power leaving the machine constant. It changes the excitation current at the same frequency as the electromechanical oscillations. This action changes the generator voltage with respect to the voltage in the grid. Power will flow from the grid to the unit to provide the countertorque required when the speed of the shaft exceeds the synchronous speed. It is important to men- tion that an improperly tuned PSS can lead to disastrous consequences. This is due to the voltage variations that it creates during operation. If the voltage variations are incorrect, excessive torque changes can occur, leading to significant damage.
In summary, these are the salient features about the PSS:
1. The PSS acts as a shock absorber to dampen the power swings.
2. The AVR cannot handle power swings because it is monitoring the voltage only.
3. A fault on the line can excite a unit severely if the damping is poor.
4. The PSS monitors the change in power and change in speed.
5. During steady-state operation, the PSS will not interfere.
6. When a fault occurs, the voltage drops. The field current must be increased to push as much active power out to increase the synchronizing torque. However, this increase in synchronizing torque (active power out) lasts for a few seconds only, for these reasons:
a. Active power flow between the generator and the load is proportional to
where ex is the angle between the phasors of Vgenerator and Vload and the reactance includes the step-up transformer only. If excitation is increased, Vgenerator increases, but ex changes within a few seconds so that the active power flow remains unchanged.
b. The active power flow is determined by the turbine.
It is also important to mention that the reactive power flow is proportional to
Any change in Vgenerator will result in significant change in the reactive power sent to the grid. Therefore, when the excitation changes, Vgenerator will change, resulting in significant transfer of reactive power.