Synchronous condenser
Encyclopedia
Electrical engineering
Electrical engineering is a field of engineering that generally deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after commercialization of the electric telegraph and electrical...
, a synchronous condenser (sometimes syncronous capacitor or synchronous compensator) is a
device identical to a synchronous motor
Synchronous motor
A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets at the same rate as the power supply frequency and resulting rotating magnetic field which drives it....
, whose shaft is not connected to anything but spins freely. Its purpose is not to convert electric power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
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
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
, or to improve power factor
Power factor
The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load over the apparent power in the circuit, and is a dimensionless number between 0 and 1 . Real power is the capacity of the circuit for performing work in a particular time...
. The condenser’s installation and operation are identical to large electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
s.
Increasing the device's field excitation results in its furnishing magnetizing power (kvars) to the system. Its principal advantage is the ease with which the amount of correction can be adjusted. The energy stored
Inertia
Inertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. It is proportional to an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to...
in the rotor of the machine can also help stabilize a power system during short circuit
Short circuit
A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....
s or rapidly fluctuating loads such as electric arc furnace
Electric arc furnace
An electric arc furnace is a furnace that heats charged material by means of an electric arc.Arc furnaces range in size from small units of approximately one ton capacity up to about 400 ton units used for secondary steelmaking...
s. Large installations of synchronous condensers are sometimes used in association with high-voltage direct current
High-voltage direct current
A high-voltage, direct current electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses...
converter stations to supply reactive power.
Unlike a capacitor bank, the value of reactive power from a synchronous condenser can be continuously adjusted. In addition, reactive power from a capacitor bank decreases with voltage decrease, while a synchronous condenser can increase current as voltage decreases. However, it does have higher losses than a static capacitor bank. Most synchronous condensers connected to electrical grids are rated between 20 Mvar and 200 Mvar and many are hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
cooled.
Theory
As the load on a synchronous motor increases, the stator current Ia increases regardless of excitation. For under and over excited motors, the power factorPower factor
The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load over the apparent power in the circuit, and is a dimensionless number between 0 and 1 . Real power is the capacity of the circuit for performing work in a particular time...
(p.f.) tends to approach 1 with increase in load. The change in power factor is greater than the change in Ia with increase in load.
The magnitude of armature current varies with excitation. The current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
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 curve
Curve
In mathematics, a curve is, generally speaking, an object similar to a line but which is not required to be straight...
s 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 harmonic
Harmonic
A harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency, i.e. if the fundamental frequency is f, the harmonics have frequencies 2f, 3f, 4f, . . . etc. The harmonics have the property that they are all periodic at the fundamental...
s (some harmonics can even be absorbed by synchronous condensers). They will not produce excessive voltage levels and are not susceptible to electrical resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
s. Because of the rotating inertia
Inertia
Inertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. It is proportional to an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to...
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 capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
s 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 condenser'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
Wheeling (electric power transmission)
In electric power transmission, wheeling is a term used to describe either of the following:* the act of transporting electric power over transmission lines, or;...
, the transmission of electric power from one geographic region through another within a set of interconnected electric power systems.
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
Programmable logic controller
A programmable logic controller or programmable controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many industries and machines...
based controller with PF controller and regulator
Regulator (automatic control)
In automatic control, a regulator is a device which has the function of maintaining a designated characteristic. It performs the activity of managing or maintaining a range of values in a machine. The measurable property of a device is managed closely by specified conditions or an advance set...
will allow the system to be set to meet a given power factor or can be set to produce a specified amount of reactive power.
On an electric power system, synchronous condensors can be used to control the voltage on long transmission lines, especially for lines with relatively high ratio of inductive reactance to resistance.