Stability of a Synchronous Machine

Synchronous machines are widely used in the electrical power system for various applications. The stability of a synchronous machine refers to its ability to maintain a steady and synchronized operation when subjected to disturbances or variations in operating conditions.

The Impact of Excitation on Stability

The excitation of a synchronous machine refers to the magnetic field produced by the field winding in the rotor. The excitation level is typically controlled by the excitation system, which regulates the direct current (DC) supplied to the field winding.

Influence of Excitation on Stability

The stability of a synchronous machine is affected by its excitation level. Here, we will discuss the influence of excitation on stability.

1. Field Current and Synchronous Reactance: The field current determines the strength of the magnetic field, which affects the synchronous reactance of the machine. The synchronous reactance determines the machine's ability to withstand transient disturbances and maintain synchronization with the power system. Therefore, an increase in excitation, which leads to a higher field current, increases the synchronous reactance and enhances the stability of the machine.

2. Short Circuit Ratio: The short circuit ratio (SCR) is the ratio of the synchronous reactance to the subtransient reactance of the machine. A higher SCR indicates a higher stability margin. Increasing the excitation level increases the synchronous reactance, which leads to a higher SCR and improved stability.

3. Power Angle: The power angle is the angle between the stator voltage and the rotor magnetic field. It determines the active power flow in the machine. Higher excitation levels result in a smaller power angle, reducing the stress on the machine during disturbances and improving stability.

4. Field Saturation: Saturation refers to the point where the increase in the excitation current does not produce a proportional increase in the magnetic field. When a synchronous machine operates in the unsaturated region, increasing the excitation level improves stability. However, in the saturated region, further increasing excitation may have a negligible impact on stability.

Conclusion

Based on the above points, it can be concluded that the stability of a synchronous machine increases with an increase in its excitation. The higher excitation level leads to an increased synchronous reactance, improved short circuit ratio, reduced power angle, and enhanced stability margin. However, it is essential to note that there is a limit to the excitation level beyond which stability may not be significantly affected due to field saturation.