The maximum power limit Pmax at δ = 90° for the cylindrical rotor
is called the steady-state stability limit. Any swing in δ beyond 90° may cause the rotor to lose synchronism and its power generation capability. Therefore, it is desirable to keep δ below 90° under all conditions, including any transient that may be encountered during normal and abnormal operations.
Big load change on ship generator
For example, if the generator load were suddenly changed from P1 to P2 in one step, the rotor power angle would increase from δ1 at old load P1 to δ2 at new load P2. This takes some time due to the mechanical inertia of the rotor. No matter how short or long it takes, the rotor inertia and the electromagnetic restraining torque will set the rotor in mass-spring-damper
type oscillations, swinging the rotor power angle beyond its new steady state value of δ2
If δ exceeds 90° any time during this swing, the machine stability and the power generation capability may be lost.
Load limits on ship generators
For this reason, the machine can be loaded only to the extent that, even under the worst-case load step, planned or accidental or during all possible faults, the power angle swing remains below 90° with sufficient margin. This limit on loading the machine is called the transient or dynamic stability limit, which is generally the power output for which δ is about 25° to 35° under normal steady-state operation.
For damping the transient oscillations of the rotor following a step load change, each pole face is provided with copper bars running along the length of the machine. All bars on each pole face are shorted at both ends, forming a partial squirrel cage of copper conductors on each pole surface.
Power oscillations on ship generator system
When the rotor oscillates around the synchronous speed, these bars see a relative slip with respect to the stator flux that runs exactly at the synchronous speed. This slip induces currents in the bars, as in the squirrel cage induction motor. The resulting I2R power loss in the bars
depletes the oscillation energy, cycle by cycle, until the oscillations are completely damped out and the induced currents subside.
Adjusting synchronous generator
Thus, there is a small induction motor superimposed on the synchronous generator.
It contributes damping only when the rotor oscillates around the constant synchronous speed. It is also used to start the machine as an induction motor to bring the generator near full speed before applying the dc excitation to the rotor and making it a synchronous machine then onward.
Ship ac motors and impact on 3-phase generator
Since many ac motors with direct online start are used on ships, such as
winches, the transient behavior of the generator needs close consideration. It may be necessary to select the shipboard generator with low reactance to improve the around the synchronous speed to improve transient stability limit and to minimize the voltage dips, but that may increase shortcircuit
Step load design for protection of ship generator
Many shipboard loading events may constitute step load on the generator, such as turning on the bow and stern thruster, cargo and ballast pumps, cranes, main circulating pump on steam ships, high-power weapons on combat ships, and tripping a large load circuit breaker accidentally or for fault protection purposes.
To maintain dynamic stability under such transients, sudden loading on the generator should be limited to no more than 25% to 30% of the rated power in one step, but the exact limit can be determined by the equal area criteria.