What type of synchronous machine produces reactive power and acts as a shunt capacitor?

Explanation:

Synchronous Machine Producing Reactive Power:

Definition: A synchronous machine is an alternating current (AC) machine whose speed under steady state conditions is proportional to the frequency of the current in its armature. Synchronous machines can operate as generators or motors and can be used to control the power factor in the system. When a synchronous machine is over-excited, it acts like a capacitor, generating reactive power to the system.

Working Principle: The working principle of a synchronous machine is based on the interaction of the magnetic field of the stator and the magnetic field of the rotor. The stator carries a three-phase winding and is connected to the AC supply. The rotor, which carries a DC field winding, is supplied with direct current through slip rings or a brushless exciter. When the machine operates under normal conditions, the rotor field locks in with the rotating magnetic field of the stator, allowing synchronous operation.

Correct Option Analysis:

The correct option is:

Option 4: Over excited

An over-excited synchronous machine produces reactive power and acts as a shunt capacitor. When the field current of the synchronous machine is increased beyond the normal excitation level, the machine is said to be over-excited. In this condition, the machine supplies reactive power (VARs) to the power system, similar to the operation of a capacitor.

In an over-excited state, the synchronous machine has a leading power factor. The machine generates reactive power, which helps in improving the voltage profile and power factor of the system. This is particularly useful in power systems where voltage regulation and stability are critical.

Advantages of Over-Excited Synchronous Machines:

• Improves voltage regulation by supplying reactive power to the system.
• Enhances power factor correction, reducing the need for additional capacitors.
• Provides better stability and control over the power system.
• Flexible operation as the machine can be adjusted to supply or absorb reactive power as needed.

Applications: Over-excited synchronous machines are commonly used in power systems for voltage regulation and power factor correction. They are also employed in industries where large inductive loads are present and require reactive power compensation.

Additional Information

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

Option 1: Under excited

An under-excited synchronous machine absorbs reactive power and acts like an inductor. When the field current is below the normal excitation level, the machine operates with a lagging power factor and absorbs reactive power from the system. This is the opposite of what is required to produce reactive power and act as a shunt capacitor.

Option 2: Critically excited

A critically excited synchronous machine neither produces nor absorbs significant amounts of reactive power. It operates at unity power factor where the reactive power is zero. This condition is not suitable for producing reactive power or acting as a shunt capacitor.

Option 3: Half excited

The term "half excited" is not commonly used in the context of synchronous machines. It might imply a machine operating at half of its normal excitation, which would typically result in under-excitation. As discussed, under-excited machines absorb reactive power rather than producing it.

Conclusion:

Understanding the excitation conditions of synchronous machines is essential for determining their role in reactive power management. An over-excited synchronous machine is the correct option as it produces reactive power and acts as a shunt capacitor. This capability is crucial for improving voltage regulation and power factor correction in power systems. The other options, such as under-excited and critically excited, do not produce reactive power and therefore do not fit the requirement described in the statement.