What is a synchronous capacitor

Synchronous capacitor

Synchronous capacitor

Chapter 13 - AC Motors

Synchronous motors load the power line with a leading power factor. This is often useful in offsetting the more common lag power factor caused by induction motors and other inductive loads. Large industrial synchronous motors originally came into widespread use because of their ability to correct the lagging power factor of induction motors.

This leading power factor can be exaggerated by removing the mechanical load and overexcitation of the synchronous motor. Such a device is known as a synchronizing capacitor. In addition, the conduction power factor can be adjusted by varying the field excitation. This makes it possible to raise an arbitrarily lagging power factor to almost one by connecting the lagging load in parallel with a synchronous motor. A synchronous capacitor is operated in a limit state between a motor and a generator without mechanical load in order to fulfill this function. It can either compensate for a leading or a lagging power factor by consuming or supplying reactive power. This improves the line voltage regulation.

Since a synchronous capacitor does not provide any torque, the output shaft can be dispensed with and the unit can easily be enclosed in a gas-tight envelope. The synchronous condenser can then be filled with hydrogen to aid cooling and reduce air loss. Since the density of hydrogen is 7% of that of air, the air loss for a unit filled with hydrogen is 7% of that found in air. In addition, the thermal conductivity of hydrogen is ten times that of air. Thus, heat removal is ten times more efficient. As a result, a hydrogen-filled synchronizing capacitor can be driven harder than an air-cooled unit, or it can be physically smaller for a given capacity. There is no risk of explosion as long as the hydrogen concentration is kept above 70%, typically above 91%.

The efficiency of long power transmission lines can be increased by placing synchronous capacitors along the line to compensate for lagging currents caused by line inductance. More real power can be transferred over a fixed-size line if the power factor is brought closer to unity using synchronous capacitors that absorb reactive power.

The ability of synchronous capacitors to temporarily absorb or generate reactive power stabilizes the power grid against short circuits and other temporary fault conditions. Temporary voltage drops and voltage drops lasting milliseconds are stabilized. This supplements longer response times for the fast-acting voltage regulation and the excitation of generating plants. The synchronous capacitor supports voltage regulation by drawing the pilot current when the mains voltage drops, which increases the generator excitation and thereby restores the mains voltage. (Image below) A capacitor bank does not have this ability.

The synchronous capacitor improves the line voltage regulation.