A synchronous condenser is an over-excited synchronous motor that is running on no load condition.
A synchronous motor takes a leading current when run over-excited and, as a result behaves as a capacitor. When such a motor is connected in parallel with the power supply, it takes a leading current which partly neutralizes the lagging reactive component of the load. As a result, it can improve the power factor.
The figure below displays a 3-phase load and a 3-phase synchronous motor. Here the synchronous motor is used as synchronous condenser in order to achieve power factor improvement. The three phase AC load takes current IL at lagging power factor ϕL.
Now, the synchronous motor working as a condenser takes current Im that leads voltage by an angle ϕM.
The phasor sum of Im and IL is I and it lags behind voltage by an angle ϕ.
It is evident that ϕ is smaller compared to ϕL and cos ϕ is greater than cos ϕL.
The power factor is improved from cosϕL to cosϕ.
That’s an overview of how synchronous condensers are employed for power factor improvement.
Synchronous condensers are generally employed at power supply substations in order to improve the power factor.