Analysis Detail
For this analysis, the power system network to be studied is designed and simulated using the software, CYME 5.02 Power System Simulation Software.
The system chosen for the study is that of a two machines system, involving two synchronous generators connected to a substation grid through circuit transmission lines. In order to gain much further comparison of the results, it is considered also the two different types of synchronous generators’ rotor types, salient-pole rotor and non-salient pole (round) type rotor.
Transient stability studies are associated to the effects of faults on the synchronism of the generators. These disturbances may include faults on loss of load, loss of generation or loss of system components such as transformers or transmission lines. When fault is applied, the power generated from the nearby machine is reduced, producing speed differences during the time interval of the fault occurrence.

Fig 1. Two machines system

When the generator electrical load is suddenly increased, the electrical power exceeds the mechanical power input. This power deficiency is supplied by the kinetic energy stored in the rotating system. The reduction in kinetic energy resulted turbine speed to drop and as well as the frequency of the generator. This change of speed can be adjusted by a governor, which is an attachment to a machine for automatic control or limitation speed; it controls the energy through the prime mover to control its power. Governor changes the input valve to change mechanical power output, thus bringing the speed to a new steady state.

Another aspect to be considered in this analysis chapter is the application of the excitation system to the system. The exciter acts as the "backbone" of the generator control system. It is the power source that supplies the dc magnetizing current to the field windings of a synchronous generator thereby inducing ac voltage and current in the generator armature. The amount of excitation required to maintain the output voltage constant is a function of the generator load. As the generator load increases, the amount of excitation increases.

Simulation Results

1. The power system network is simulated without any adjustments to the synchronous machines for comparison purposes further in this chapter. The generators are in set up as classical model type of generators. For this analysis, fault will be applied to the system at different cycle variations. In this study case, the fault occurrence is at bus number 4 of the power system network.

a) Fault occurrence at 0.05 seconds (3 cycles).

b) Fault occurrence at 0.10 seconds (6 cycles).

c) Fault occurrence at 0.15 seconds (9 cycles).

d) Fault occurrence at 0.20 seconds (12 cycles).

e) Fault occurrence at 0.25 seconds (15 cycles).

f) Fault occurrence at 0.30 seconds (18 cycles).

2. The synchronous generators are in set up to type 3 model which is the salient pole rotor with transient and sub-transient effects model. The simulation procedure is repeated for the same cycle variations.

a) Fault occurrence at 0.05 seconds (3 cycles).

b) Fault occurrence at 0.10 seconds (6 cycles).

c) Fault occurrence at 0.15 seconds (9 cycles).

d) Fault occurrence at 0.20 seconds (12 cycles).

e) Fault occurrence at 0.25 seconds (15 cycles).

f) Fault occurrence at 0.30 seconds (18 cycles).

3. The synchronous generators are in set up to type 5 model which is the round rotor with transient and sub-transient effects model. The simulation procedure is repeated for the same cycle variations. a) Fault occurrence at 0.05 seconds (3 cycles).

b) Fault occurrence at 0.10 seconds (6 cycles).

c) Fault occurrence at 0.15 seconds (9 cycles).

d) Fault occurrence at 0.20 seconds (12 cycles).

e) Fault occurrence at 0.25 seconds (15 cycles).

f) Fault occurrence at 0.30 seconds (18 cycles).