Phase Modifiers in AC Transmission Line

In AC transmission lines, phase modifiers are devices or equipment used to modify the phase angle of the transmitted electrical power. The main purpose of phase modifiers is to control and stabilize the flow of power in the transmission system.

Synchronous Machines
Synchronous machines are one of the phase modifiers used in AC transmission lines. These machines operate at synchronous speed and are capable of generating or absorbing reactive power. They have the ability to control the power factor of the transmission line by adjusting the excitation of the machine. By changing the excitation, the phase angle of the power can be modified, thus acting as a phase modifier. This helps in maintaining a stable and efficient transmission system.

Induction Machines
Induction machines, including both induction motors and generators, are not typically used as phase modifiers in AC transmission lines. Although they can generate or consume reactive power, their ability to control the phase angle is limited. Induction machines rely on the supply frequency and do not have the same level of control over the phase angle as synchronous machines.

DC Machines
DC machines, which include DC generators and motors, are not suitable as phase modifiers in AC transmission lines. DC machines operate on a different principle than AC machines and do not have the capability to control the phase angle of the power. They are primarily used in DC transmission systems, where the power flow is unidirectional.

Transformers
Transformers are not phase modifiers in AC transmission lines. They are used to change the voltage level of the power but do not modify the phase angle. Transformers operate based on the principle of electromagnetic induction and are essential components in power transmission and distribution systems.

In conclusion, the correct answer is option 'A' - Synchronous machines. Synchronous machines have the capability to modify the phase angle of the transmitted power, making them suitable as phase modifiers in AC transmission lines.