Synchronous condenser is an synchronous machine running under no load with controlled excitation, for the purpose of supplying leading and lagging reactive powers,(drawing leading and lagging currents)

If it is under exited, it acts as a reactive power drawing unit (inductor)

If it is over exited, it acts as a reactive power supplying unit (capacitor)

In is Used as Dynamic compensator for Reactive power compensation as it supplies both leading and lagging VARs ,This is generally connected at mid point buses for Transmission line compensation ,there by we can improve steady state stability ,i.e. power transfer capability of the line.

Generally gases below atmospheric pressure is present inside the casing of the synchronous condenser ,casing is to be done as it is running under no load speed will be dangerous ,also as it is filled with hydrogen like gases ,when it mixes with fire cause explosive, so casing is mandatory