In electrical engineering, a synchronous condenser (sometimes called a syncon, synchronous capacitor or synchronous compensator) is a DC-excited synchronous motor, whose shaft is not connected to anything but spins freely. Its purpose is not to convert electric power to mechanical power or vice versa, but to adjust conditions on the electric power transmission grid. Its field is controlled by a voltage regulator to either generate or absorb reactive power as needed to adjust the grid's voltage, or to improve power factor. The condenser’s installation and operation are identical to large electric motors and generators (some generators are actually designed to be able to operate as synchronous condensers with the prime mover disconnected).
Increasing the device's field excitation results in its furnishing reactive power (measured in units of var) to the system. Its principal advantage is the ease with which the amount of correction can be adjusted.
Synchronous condensers are an alternative to capacitor banks for power-factor correction in power grids. One advantage is that the amount of reactive power from a synchronous condenser can be continuously adjusted. Reactive power from a capacitor bank decreases when grid voltage decreases while the reactive power from a synchronous condenser inherently increases as voltage decreases. However, synchronous machines have higher energy losses than static capacitor banks. Most synchronous condensers connected to electrical grids are rated between 20 MVAR (megavar) and 200 MVAR and many are hydrogen cooled. There is no explosion hazard as long as the hydrogen concentration is maintained above 70%, typically above 91%. A syncon can be 8 metres long and 5 meters tall, weighing 170 tonnes.
Synchronous condensers also help stabilize grids. The kinetic energy stored in the rotor of the machine and its inductance can help stabilize a power system during rapid fluctuations of loads such as those created by short circuits or electric arc furnaces. For this reason, large installations of synchronous condensers are sometimes used in association with high-voltage direct current converter stations to supply reactive power to the alternating current grid.
Theory
A rotating coil in a magnetic field tends to produce a sine-wave voltage. When connected to a circuit some current will flow depending on how the voltage on the system is different from this open-circuit voltage. Note that mechanical torque (produced by a motor, required by a generator) corresponds only to the real power. Reactive power does not result in any torque.
As the mechanical load on a synchronous motor increases, the stator current increases regardless of the field excitation. For both under- and over-excited motors, the power factor (p.f.) tends to approach unity with increase in mechanical load. This change in power factor is larger than the change in with increase in load.
The phase of armature current varies with field excitation. The current has larger values for lower and higher values of excitation. In between, the current has minimum value corresponding to a particular excitation (see graph on right). The variations of with excitation are known as curves because of their shape.
For the same mechanical load, the armature current varies with field excitation over a wide range and so causes the power factor also to vary accordingly. When over-excited, the motor runs with leading power factor (and supplies vars to the grid) and when under-excited with lagging power factor (and absorbs vars from the grid). In between, the power factor is unity. The minimum armature current corresponds to the point of unity power factor (voltage and current in phase).