Equation (4.8a) is plotted in Figure 4.8 to illustrate the dependence of the generated active power on the load angle. Note that a synchronous generator (SG) connected to an infinite bus is capable of generating a maximum active power PBmax . at 8 = 90°. Any additional applied mechanical torque will increase the load angle beyond 90 °, with a consequential decrease in electrical power. Physically, the peripheral magnetic forces linking the two fields together is insufficient to maintain the locking effect. The power balance between mechanical and electrical powers has now been lost, the excess mechanical power accelerates the rotor beyond synchronous speed and the synchronous generator has lost its ability to act as a stable power converter. In this unstable regime the SG is described as having lost synchronism i.e. its rotor generates an internal voltage VA of a higher frequency than the infinite bus voltage VB. Operation under this condition, known as pole slipping, results in large overcurrents, is highly undesirable and protection equipment will be brought into action to disconnect the synchronous machine from the mains.
During system contingencies violent transient changes may take place that cause swings in the load angles of synchronous generators. To ensure safe stability margins so that pole slipping is prevented, the steady state load angles of SGs are kept well below 30° . Power system stability is a major topic in its own right involving the simultaneous solution of the differential equations characterizing all the network components. This is beyond the scope of this book, but it is worth noting that connection of large RE sources on to the grid will alter the system dynamics. Therefore studies may be required to assess the new system stability margins.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.