592 ROTATING MACHINES
The line-to-line excitation voltage for machineAis√
3√
4. 62 + 1272 = 8 .26 kV.
E ̄fB=V ̄t+I ̄BZ ̄B=( 6. 6 /√
3 )+( 131. 1 −j 124 )( 0. 4 +j 12 )× 10 −^3
= 5. 35 +j 1 .52 kV per phase
Power angleδB=tan−^1 ( 1. 52 / 5. 35 )= 15 .9°
The line-to-line excitation voltage for machineBis√
3√
5. 352 + 1. 522 = 9 .6kV.
The corresponding phasor diagram is sketched in Figure E13.3.3(b).IA IBZA ZBIL+−EfA+−EfB+−Vt Load(a)Vt(b)δBφBδA
φAIAIAZAIBZBIBEfAEfB Figure E13.3.3Steady-State Stability
The property of a power system that ensures that it will remain in equilibrium under both normal
and abnormal conditions is known aspower-system stability. Steady-state stabilityis concerned
with slow and gradual changes, whereastransient stabilityis concerned with severe disturbances,
such as sudden changes in load or fault conditions. The largest possible flow of power through
a particular point, without loss of stability, is known as thesteady-state stability limitwhen
the power is increased gradually, and as thetransient-stability limitwhen a sudden disturbance
occurs.
For a generator connected to a system that is very large compared to its own size, the system
in Figure 13.3.5 can be used. The power-angle equation (neglecting resistances) for the system
under consideration becomes