Synchronous condenser

In electrical engineering, a synchronous condenser (sometimes synchronous compensator) is a synchronous motor that is not attached to any driven equipment. [B. M. Weedy, Electric Power Systems Second Edition, John Wiley and Sons, London, 1972, ISBN 0471924458 page 149] Its field is controlled by a voltage regulator to either generate or absorb reactive power as needed 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 is the ease with which the amount of correction can be adjusted. The energy stored in the rotor of the machine can also help stabilize a power system during short circuits or rapidly fluctuating loads such as electric arc furnaces. Large installations of synchronous condensors are sometimes used in association with high-voltage direct current converter stations to supply reactive power.

Unlike a capacitor bank, the value of reactive power can be continuously adjusted. However, the synchronous condensor does have higher losses than a static capacitor bank. [B. M. Weedy, Electric Power Systems Second Edition, John Wiley and Sons, London, 1972, ISBN 0471924458 page 149] Most synchronous condensers connected to electrical grids are rated between 20 MVAR and 200 MVAR and many are hydrogen cooled.

Theory

As the load on a synchronous motor increases, the armature (stator) current I_a increases regardless of excitation. For under and over excited motors, the power factor (p.f.) tends to approach 1 with increase in load. The change in power factor is greater than the change in I_a with increase in load.

The magnitude of armature current varies with excitation. The current has large value both for low and high values of 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 output load, the armature current varies 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 with lagging power factor when under-excited. In between, the power factor is unity. The minimum armature current corresponds to unity power factor.

Application

An over-excited synchronous motor has a leading power factor. This makes it useful for power factor correction of industrial loads. Both transformers and induction motors draw lagging currents from the line. On light loads, the power drawn by induction motors has a large reactive component and the power factor has a very low value. The current flowing to supply reactive power creates losses in the power system. In an industrial plant, synchronous motors can be used to supply some of the reactive power required by induction motors. This improves the plant power factor and reduces supply current.

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. They remain 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. The reactive power produced by a capacitor bank is in direct proportion to the square of its terminal voltage, where a synchronous condensor's reactive power declines less rapidly, and can be adjusted to compensate for falling terminal voltage. This reactive power improves voltage regulation 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).

Synchronous Condensers may also be referred to as "Dynamic Power Factor Correction" systems. These machines can prove very effective when advanced controls are utilized. A PLC based controller with pf controller and regulator will allow the system to be set to meet a given power factor or can be set to produce a specified amount of KVAR. In both cases, the output of the machine obviously cannot exceed the capability of the machine.

References

External links

* [http://www.pserc.org/ecow/get/generalinf/presentati/presentati/heydt_synchronous_mach_sep03.pdf A Short Course on Synchronous Machines and Synchronous Condensers]
* [http://www.idealelectricco.com/downloads/synchronous_condensers.pdf Specifying and Using Synchronous Condensers for Power Factor Correction and Voltage Support]
* [http://www.pscpower.com/Whitepapers/What%20is%20Power%20Factor.pdf Whitepaper on Power Factor and PF Correction as it relates to Power Quality]