Page 44 - Practical Power System and Protective Relays Commissioning

40 Practical Power System and Protective Relays Commissioning

FIGURE 4.9 Brushless excitation system cross-section.

exciter provides DC current to the generator field winding and gives the reg-
ulator control of the exciter output based upon the generator output voltage.

4.4.2 Synchronous Generator Ratings
Short-circuit Ratio (SCR) is defined as follows:
Field current required for rated open circuit voltage
Short circuit ratio 5
Field current required for rated stator Current on SC
This ratio is inversely proportional to the direct axis synchronous reac-
tance X d , its typical values lie between 0.5 and 0.6 for turbogenerators and
1.0 and 1.5 for hydro generators. A higher SCR will give a lower synchro-
nous impedance. High short circuit currents and a lower SCR will inversely
affect the voltage regulation and stability.

4.4.3 Steady-State Stability and Transient Stability of
Synchronous Generators

During a system disturbance, such as a sudden increase in load or sudden
switching power swings, the synchronous generator has oscillations of torque
angle about the mean position. If a generator has a loss of synchronism due
to this disturbance, this called the loss of stability state. If the system can
respond to a small, gradual change in power at a given point in the system,
this called steady-state stability. The maximum power that can be transferred
at a given point in the system with a gradual change in power without loss
of synchronism is called the steady state-stability limit. If the system can
respond to a big, fast change in power at a given point in the system, this
called transient stability. The maximum power that can be transferred at a
given point in the system with sudden large change in power without loss of
synchronism is called transient-stability limit.