Short Answer:

Over-excitation in a synchronous machine occurs when the rotor’s field excitation is increased beyond the normal operating level, which leads to a leading power factor and the production of reactive power. This helps improve voltage stability and power factor correction but can cause increased losses and heating if excessive. On the other hand, under-excitation occurs when the excitation is reduced, leading to a lagging power factor and a shortage of reactive power. This can cause voltage instability and affect the machine’s ability to support the power system.

Both conditions impact the machine’s efficiency, stability, and the overall performance of the power system.

Detailed Explanation:

Over-Excitation and Its Effects on Synchronous Machines

Over-excitation refers to the condition where the rotor’s field excitation current exceeds the normal level required for synchronous operation. The excitation current controls the strength of the rotor’s magnetic field, and increasing it beyond the rated value can have significant effects on the synchronous machine’s performance. When over-excitation occurs, the machine produces excessive reactive power, which can be used for power factor correction in the grid, but this must be managed carefully to avoid damaging the machine.

1. Leading Power Factor

• In an over-excited synchronous machine, the rotor’s magnetic field becomes stronger than necessary, leading to a leading power factor. A leading power factor means the current leads the voltage, which is the opposite of what is typical in most loads that draw lagging power. This can help improve the power factor of the overall system, especially in systems with large inductive loads like motors and transformers.
• A leading power factor can be beneficial for power factor correction because it supplies reactive power (VARs) to the grid, improving voltage stability. This is especially useful in power systems where the load is inductive, and there is a demand for reactive power.

2. Increased Reactive Power Generation

• Over-excitation increases the amount of reactive power that the synchronous machine generates. This is useful in some situations, such as when reactive power is needed for maintaining voltage levels in the grid, but excessive generation of reactive power can lead to voltage instability and even cause damage to the machine if the system’s voltage exceeds safe limits.

3. Potential Risks of Over-Excitation

• While over-excitation helps in power factor correction and voltage regulation, excessive excitation can lead to increased losses in the machine. This results in higher core losses and additional heat generation, which may cause the machine to overheat and possibly suffer from insulation failure or winding damage if not properly managed.
• Magnetic Saturation: Over-excitation can also push the machine’s magnetic circuit into saturation, causing non-linearities that could affect the machine’s performance and reduce efficiency.

Under-Excitation and Its Effects on Synchronous Machines

Under-excitation, in contrast, occurs when the rotor’s excitation current is reduced below the normal required level. In this condition, the magnetic field produced by the rotor is weaker than necessary, which has different effects compared to over-excitation.

1. Lagging Power Factor

• Under-excitation results in a lagging power factor, where the current lags behind the voltage. A lagging power factor is typical for loads that consume reactive power (like motors and inductive loads). The machine, in this case, becomes a reactive power consumer, drawing power from the grid rather than supplying it.
• The machine’s ability to provide reactive power is significantly reduced, and in some cases, it may require external compensation, such as capacitors, to ensure the system’s voltage remains stable.

2. Reactive Power Deficiency

• The primary impact of under-excitation is a shortage of reactive power. Reactive power is necessary to maintain voltage stability in power systems, especially in systems with large inductive loads. Under-excitation reduces the synchronous machine’s ability to support the system with reactive power, leading to voltage instability, especially during periods of high demand.

3. Reduced Voltage Support

• Under-excitation can also result in lower voltage support within the machine and the overall power system. As the machine operates with less excitation, its capability to maintain a stable voltage at the terminals diminishes. In extreme cases, if the excitation is too low, the machine may fail to synchronize with the power grid and could lose synchronization, leading to a loss of stability.

4. Lower Efficiency

• Under-excitation reduces the motor’s efficiency because the rotor cannot produce sufficient torque, which means the motor has to work harder to maintain the same output. It can also increase core losses, as the rotor field is insufficient for the required power output.

4. Impact on System Stability and Performance

Both over-excitation and under-excitation affect the system stability and the performance of synchronous machines. Over-excitation leads to increased reactive power generation, improving voltage regulation but potentially causing overheating and excessive losses. On the other hand, under-excitation leads to insufficient reactive power supply, resulting in poor voltage support and the risk of instability in the power system.

Conclusion

The operation of a synchronous machine is greatly affected by over-excitation and under-excitation. While over-excitation can improve power factor correction and voltage stability, it can lead to excessive reactive power generation and higher losses if not controlled. On the other hand, under-excitation causes a lagging power factor, reducing the machine’s ability to supply reactive power, potentially leading to voltage instability and system performance issues. Proper excitation control is crucial for maintaining the balance between reactive power production and consumption, ensuring stable and efficient operation of synchronous machines in various power systems.