Synchronous phase modifier
• Constant voltage transmission
For constant voltage transmission , especially designed synchronous motor , called the synchronous phase modifiers, are installed at receiving end , which maintain the voltage drop across the line constant with the change in load,power factor of the system is changed by the synchronous motors and thus voltage drop along the line remains constant. The advantage and disadvantage is given below .
Merits of constant voltage transmission
1. Possibility of carrying increased power for a given conductor size in case of long distance heavy power transmission
2. Improvement of power factor at times of moderate and heavy loads.
3. Possibility of better protection for the line due to possible use of higher terminal reactances.
4. Availability of steady voltage at all loads at the line terminals.
5. Improvement in system stability due to inertia effect of synchronous phase modifier and reduction in effect of sudden changes in load.
Demerits of constant voltage transmission
1. Increase of short –circuit current of the system and, therefore, increase in the circuit breaker ratings.
2. Increase risk of interruption of supply due to falling of synchronous motors, called the synchronous phase modifiers, out of synchronicity.
3. Lower reserve of lines in case of line trouble.
Synchronous phase modifier
It is well known that a synchronous motor can be made to take either a lagging or leading current from the line by alternating its excitation. Idle-running synchronous motor were first employed in connecting in connection with power plants to correct for low power factor of the load, and thus reduced the current and power losses in the feeders and generators. The synchronous machines used for power factor improvement are usually referred to as synchronous condensers as they are always required to take a leading current. However and idle-running synchronous motor can be used for voltage regulating purposes by connecting it in parallel with the load at the receiving end of a line , as shown in diagram since the machines used for voltage regulation as a synchronous modifiers . They are usually of salient pole design with 6 or 8 poles with rating up to 60MVA, 11KV and connected to high voltage system through transformers.
Synchronous phase modifier differ from the ordinary synchronous motor is as much as they are built for the highest economical speeds, and provided with smaller shafts and bearings and special attention is paid for securing a high overall efficiency . Standard machines for this purpose are designed to give their full-load output at leading power factor, and can carry about 50% of their rated capacity at lagging power factor. machines can , however, be designed to operate at full rating on both leading and lagging power factor s but they are larger in size , have poor efficiency and are more expensive than standard machines .