Which of the following statements is CORRECT? 

Explanation:

Correct Option Analysis:

The correct option is:

Option 1: A grid-connected induction generator always supplies lagging reactive power to the bus.

Induction generators are widely used in various applications, especially in renewable energy systems like wind turbines. Their operation and interaction with the power system are crucial for maintaining stability and efficiency. Let’s break down why Option 1 is the correct answer.

Working Principle of Induction Generators:

Induction generators work on the principle of electromagnetic induction. When the rotor of an induction generator spins faster than the synchronous speed of the connected grid, it begins to generate electrical power. Unlike synchronous generators, induction generators do not have a self-excitation mechanism, meaning they rely on the grid to provide the necessary reactive power for excitation.

Reactive Power Characteristics:

Induction generators always draw reactive power from the grid for magnetizing purposes. This reactive power is necessary for establishing the magnetic field within the machine. Since they consume reactive power, the power factor is lagging, which means the current lags the voltage. This is a fundamental characteristic of induction machines, whether they are operating as motors or generators.

Why Option 1 is Correct:

When a grid-connected induction generator is in operation, it always supplies active power to the grid while consuming reactive power. This consumption of reactive power results in a lagging power factor, which is why the statement “A grid-connected induction generator always supplies lagging reactive power to the bus” is correct.

Applications:

• Induction generators are commonly used in wind energy systems and small hydroelectric plants.
• They are suitable for applications where the grid can supply the necessary reactive power for excitation.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: An overexcited synchronous motor draws current at a lagging power factor.

This option is incorrect. An overexcited synchronous motor operates at a leading power factor because it generates reactive power rather than consuming it. This characteristic is utilized in power systems to improve the overall power factor and voltage regulation.

Option 3: An under excited synchronous generator connected to an infinite bus works at a leading power factor.

This option is incorrect. An under excited synchronous generator operates at a lagging power factor because it consumes reactive power from the grid. The leading power factor occurs when the generator is overexcited, meaning it produces reactive power.

Option 4: The load angle of a synchronous machine is the angle between the excitation voltage and the load current.

This option is incorrect. The load angle, also known as the torque angle or power angle, is the angle between the rotor’s magnetic field and the stator’s magnetic field in a synchronous machine. It is not the angle between the excitation voltage and the load current.

Conclusion:

The correct answer, Option 1, highlights the fundamental characteristic of grid-connected induction generators, which is their consumption of reactive power resulting in a lagging power factor. Understanding this principle is vital for designing and operating power systems that incorporate induction generators. Additionally, the analysis of other options sheds light on the operational characteristics of synchronous machines, helping to clarify common misconceptions.