Synchronous condenser

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A synchronous condenser is fundamentally an AC synchronous motor that is not attached to any driven equipment. Its field is controlled by a voltage regulator to either generate or absorb reactive power as needed by the system. It operates at full leading power factor and puts VARs onto the network as required to support a system’s voltage or to maintain the system power factor at a specified level. The condenser’s installation and operation are identical to large electric motors.

Increasing the device's field excitation results in its furnishing magnetizing power (kilovars) to the system. Its principal advantage the ease with which the amount of correction can be adjusted.

[edit] Theory

As load on the motor increases, the armature (stator) current I_a increases regardless of excitation.
For under and over excited motor, the power factor (p.f.) tends to approach unity with increase in load.
Both with under and over excitation, change in p.f. is greater than in I_a with increase in load.
With normal excitation, when load is increased, change in I_a is greater than in p.f. which tends to become increasingly lagging.

The magnitude of armature current varies with excitation. The current has large value both for low and high values of excitation (though it is lagging for low excitation and leading for higher excitation). In between, it has minimum value corresponding to a certain excitation. The variations of I with excitation are known as V curves because of their shape.
For the same input, the armature current varies over a wide range and so causes the power factor also to vary accordingly. When over-excited, motor runs with leading power factor and with lagging power factor when under-excited. In between, the power factor is unity. The curve for power factor looks like inverted V curve. Also, the minimum armature current corresponds to unity power factor.
As per the first point, an over-excited motor can be run with leading power factor. This property renders it extremely useful for phase advancing (and so power factor correction) purposes in the case of industrial loads driven by induction motors and lighting and heating rods supplied through transformers.
Both transformers and induction motors draw lagging currents from the line. Especially on light loads, the power drawn by them has a large reactive component and the power factor has a very low value. This reactive component, though essential for operating the electrical machinery, entails appreciable losses in many ways. By using synchronous motors in conjunction with induction motors and transformers, the lagging reactive power required by the latter is supplied locally by the leading reactive component taken by the former, thereby relieving the line and generators of much of the reactive component. Hence, they now supply only the active component of the load current.
Most synchronous condensers are rated between 20 MVAR and 200 MVAR and many are hydrogen cooled.

A synchronous condenser provides step-less automatic power factor correction with the ability to produce up to 150% additional MVARs. The system produces no switching transients and is not affected by system electrical harmonics (some harmonics can even be absorbed by synchronous condensers). They will not produce excessive voltage levels and are not susceptible to electrical resonances. Because of the rotating inertia of the condenser, it can provide limited voltage support during short power outages.

The use of rotating synchronous condensers, was common through the 1950s, however they are now making a comeback as an alternative (or a supplement) to capacitors for power factor correction because of problems that have been experienced with harmonics causing capacitor overheating and catastrophic failures. Synchronous condensers are also very good for supporting voltage in situations such as starting large motors,or where power must travel long distances from where it is generated to where it is used, as is the case with power wheeling (distribution of electric power from one geographical location to another within an electric power distribution system.)