What are the main causes and effects of voltage and frequency fluctuations in a synchronous generator?

1 Voltage fluctuations

Voltage fluctuations are variations in the magnitude of the terminal voltage of a synchronous generator. They can be caused by changes in the load demand, faults, switching operations, or external disturbances. Voltage fluctuations can affect the power quality, the efficiency, and the lifespan of the generator and the connected equipment. They can also cause overvoltage or undervoltage conditions, which can trigger protection devices or damage the insulation.

To regulate the terminal voltage of a synchronous generator, an automatic voltage regulator (AVR) is used. The AVR senses the terminal voltage and adjusts the field current of the generator to maintain a constant voltage. The AVR can also compensate for the voltage drop due to the armature reaction and the synchronous reactance of the generator.

2 Frequency fluctuations

Frequency fluctuations are variations in the rotational speed of a synchronous generator. They can be caused by changes in the mechanical power input, the electrical power output, or the grid frequency. Frequency fluctuations can affect the synchronism, the stability, and the efficiency of the generator and the power system. They can also cause overfrequency or underfrequency conditions, which can lead to loss of load, loss of generation, or system collapse.

To regulate the frequency of a synchronous generator, a governor is used. The governor senses the speed deviation and adjusts the mechanical power input to maintain a constant speed. The governor can also coordinate with other generators in the system to share the load and frequency regulation.

3 Voltage and frequency control

Voltage and frequency control are interrelated in a synchronous generator. A change in one variable can affect the other, and vice versa. For example, an increase in the load demand can cause a drop in both the terminal voltage and the speed of the generator. To restore the voltage and frequency to their nominal values, both the AVR and the governor have to act simultaneously. However, this can create a feedback loop that can destabilize the system.

To avoid this problem, a coordination scheme is needed between the AVR and the governor. One common method is the droop control, which introduces a negative feedback between the voltage and frequency deviations. The droop control reduces the sensitivity of the AVR and the governor to each other's actions, and allows a smooth and stable operation of the generator and the system.

4 Transient stability

Transient stability is the ability of a synchronous generator to maintain synchronism with the grid after a large disturbance, such as a fault, a load rejection, or a generation loss. During a transient event, the generator experiences a sudden change in its electrical power output and mechanical power input, which creates a torque imbalance and a speed deviation. The generator can either recover from the disturbance and return to its steady state, or lose synchronism and become unstable.

To analyze the transient stability of a synchronous generator, a swing equation is used. The swing equation relates the speed deviation to the accelerating torque, which is the difference between the mechanical and electrical torques. The swing equation can be solved using numerical methods or graphical methods, such as the equal area criterion. The equal area criterion determines the stability of the generator by comparing the areas under the power-angle curve before and after the disturbance.

5 Small-signal stability

Small-signal stability is the ability of a synchronous generator to maintain synchronism with the grid under small and continuous disturbances, such as load variations, noise, or control actions. These disturbances can cause small oscillations in the voltage, frequency, and angle of the generator, which can grow over time and affect the system stability and performance.

To analyze the small-signal stability of a synchronous generator, a linearized model is used. The linearized model represents the generator and its controls as a set of differential equations that describe the dynamic behavior of the state variables. The linearized model can be solved using eigenvalue analysis or modal analysis. These methods determine the stability of the generator by examining the eigenvalues or the modes of the system matrix.

6 Here’s what else to consider

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