![Templestowe Synchronous Condenser 1](https://upload.wikimedia.org/wikipedia/commons/5/5f/Templestowe_Synchronous_Condenser_1.jpg)
In
electrical engineering
Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics, and electromagnetism. It emerged as an identifiable occupation in the l ...
, a synchronous condenser (sometimes called a syncon, synchronous capacitor or synchronous compensator) is a DC-excited
synchronous motor
A synchronous electric motor is an AC electric motor in which, at steady state,
the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integral number of AC cycles. Sync ...
, whose shaft is not connected to anything but spins freely.
[B. M. Weedy, Electric Power Systems Second Edition, John Wiley and Sons, London, 1972, page 149] Its purpose is not to convert
electric power
Electric power is the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt, one joule per second. Standard prefixes apply to watts as with other SI units: thousands, millions and billions o ...
to mechanical power or vice versa, but to adjust conditions on the
electric power transmission grid
Electric power transmission is the bulk movement of electrical energy from a power generation, generating site, such as a power plant, to an electrical substation. The interconnected lines that facilitate this movement form a ''transmission net ...
. Its field is controlled by a voltage regulator to either generate or absorb
reactive power as needed to adjust the grid's
voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
, or to improve
power factor
In electrical engineering, the power factor of an AC power system is defined as the ratio of the '' real power'' absorbed by the load to the ''apparent power'' flowing in the circuit. Real power is the average of the instantaneous product of v ...
. The condenser’s installation and operation are identical to large
electric motor
An electric motor is an Electric machine, electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a Electromagneti ...
s and generators (some generators are actually designed to be able to operate as synchronous condensers with the
prime mover
Prime mover may refer to:
Philosophy
*Unmoved mover, a concept in Aristotle's writings
Engineering
* Prime mover (engine), motor, a machine that converts various other forms of energy (chemical, electrical, fluid pressure/flow, etc) into energy ...
disconnected).
Increasing the device's field excitation results in its furnishing reactive power (measured in units of
var
Var or VAR may refer to:
Places
* Var (department), a department of France
* Var (river), France
* Vār, Iran, village in West Azerbaijan Province, Iran
* Var, Iran (disambiguation), other places in Iran
* Vár, a village in Obreja commune, Ca ...
) to the system. Its principal advantage is the ease with which the amount of correction can be adjusted. The
kinetic energy stored in the rotor of the machine can help stabilize a power system during rapid fluctuations of loads such as those created by
short circuit
A short circuit (sometimes abbreviated to short or s/c) is an electrical circuit that allows a current to travel along an unintended path with no or very low electrical impedance. This results in an excessive current flowing through the circuit ...
s or
electric arc furnace
An electric arc furnace (EAF) is a furnace that heats material by means of an electric arc.
Industrial arc furnaces range in size from small units of approximately one-tonne capacity (used in foundries for producing cast iron products) up to ab ...
s. Large installations of synchronous condensers are sometimes used in association with
high-voltage direct current
A high-voltage direct current (HVDC) electric power transmission system (also called a power superhighway or an electrical superhighway) uses direct current (DC) for electric power transmission, in contrast with the more common alternating curre ...
converter stations to supply reactive power to the alternating current grid.
Synchronous condensers are an alternative to
capacitor bank
A static VAR compensator (SVC) is a set of electrical devices for providing fast-acting reactive power on high-voltage electricity transmission networks. SVCs are part of the flexible AC transmission system device family, regulating voltage, pow ...
s for
power-factor correction
In electrical engineering, the power factor of an AC power system is defined as the ratio of the '' real power'' absorbed by the load to the ''apparent power'' flowing in the circuit. Real power is the average of the instantaneous product of v ...
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 a synchronous condenser can increase reactive current 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.
Theory
![V curve synchronous motor](https://upload.wikimedia.org/wikipedia/commons/0/0a/V_curve_synchronous_motor.svg)
A rotating coil in a
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
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
In electrical engineering, the power factor of an AC power system is defined as the ratio of the '' real power'' absorbed by the load to the ''apparent power'' flowing in the circuit. Real power is the average of the instantaneous product of v ...
(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
Phase or phases may refer to:
Science
*State of matter, or phase, one of the distinct forms in which matter can exist
*Phase (matter), a region of space throughout which all physical properties are essentially uniform
* Phase space, a mathematic ...
of armature current varies with field excitation. The
current
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
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
curve
In mathematics, a curve (also called a curved line in older texts) is an object similar to a line (geometry), line, but that does not have to be Linearity, straight.
Intuitively, a curve may be thought of as the trace left by a moving point (ge ...
s 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).
As in a synchronous motor, the stator of the machine is connected to a three-phase supply of voltage
(assumed to be constant), and this creates a rotating magnetic field within the machine. Likewise, the rotor is excited with a DC current
to act as an electromagnet. In normal operation the rotor magnet follows the stator field at synchronous speed. The rotating electromagnet induces a three-phase voltage
in the stator windings as if the machine were a synchronous generator. If the machine is considered to be ideal, with no mechanical, magnetic, or electrical losses, its equivalent circuit will be an AC generator in series with the winding inductance
of the stator. The magnitude of
depends on the excitation current
and the speed of rotation, and as the latter is fixed,
depends only on
. If
is critically adjusted to a value
,
will be equal and opposite to
, and the current in the stator
will be zero. This corresponds to the minimum in the curve shown above. If, however,
is increased above
,
will exceed
, and the difference is accounted for by a voltage
appearing across the stator inductance
:
where
is the stator reactance. Now the stator current
is no longer zero. Since the machine is ideal,
,
and
will all be in phase, and
will be entirely reactive (i.e. in phase quadrature). Viewed from the supply side of the machine's terminals, a negative reactive current will flow out of the terminals, and the machine will therefore appear as a capacitor, the magnitude of whose reactance will fall as
increases above
. If
is adjusted to be less than
,
will exceed
, and a positive reactive current will flow into the machine. The machine will then appear as an inductor whose reactance falls as
is reduced further. These conditions correspond to the two rising arms of the V-curves (above). In a practical machine with losses, the equivalent circuit will contain a resistor in parallel with the terminals to represent mechanical and magnetic losses, and another resistor in series with the generator and L, representing copper losses in the stator. Thus in a practical machine
will contain a small in-phase component, and will not fall to zero.
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 (magnetising) currents from the line. On light loads, the power drawn by
induction motors
An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. An induction mot ...
has a large reactive component and the power factor has a low value. The added current flowing to supply reactive power creates additional 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 the reactive current required from the grid.
A synchronous condenser provides stepless automatic power-factor correction with the ability to produce up to 150% additional vars. The system produces no switching transients and is not affected by system electrical
harmonics
A harmonic is a wave with a frequency that is a positive integer multiple of the '' fundamental frequency'', the frequency of the original periodic signal, such as a sinusoidal wave. The original signal is also called the ''1st harmonic'', ...
(some harmonics can even be absorbed by synchronous condensers). They will not produce excessive voltage levels and are not susceptible to electrical
resonances
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillat ...
. Because of the rotating
inertia
Inertia is the idea that an object will continue its current motion until some force causes its speed or direction to change. The term is properly understood as shorthand for "the principle of inertia" as described by Newton in his first law ...
of the synchronous condenser, it can provide limited voltage support during very short power drops.
Rotating synchronous condensers were introduced in 1930s and were common in 1950s, but due to high costs were eventually displaced in new installations by the
static var compensators (SVCs). They remain an alternative (or a supplement) to
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of ...
s for power-factor correction because of problems that have been experienced with harmonics causing capacitor overheating and catastrophic failures. Synchronous condensers are also useful for supporting voltage levels. The reactive power produced by a
capacitor bank
A static VAR compensator (SVC) is a set of electrical devices for providing fast-acting reactive power on high-voltage electricity transmission networks. SVCs are part of the flexible AC transmission system device family, regulating voltage, pow ...
is in direct proportion to the square of its terminal voltage, and if the system voltage decreases, the capacitors produce less reactive power, when it is most needed, while if the system voltage increases the capacitors produce more reactive power, which exacerbates the problem. In contrast, with a constant field, a synchronous condenser naturally supplies more reactive power to a low voltage and absorbs more reactive power from a high voltage, plus the field can be controlled. This reactive power improves voltage regulation in situations such as when 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'', the transmission of electric power from one geographic region to another within a set of interconnected electric power systems.
When compared to an SVC, the synchronous condenser has a few advantages:
* the rotational inertia allows it to
ride-through
In electrical power engineering, fault ride through (FRT), sometimes under-voltage ride through (UVRT), or low voltage ride through (LVRT), is the capability of electric generators to stay connected in short periods of lower electric network volt ...
the short circuit condition;
* reactive power delivery does not depend on the line voltage;
* it is relatively insensitive to overloads and typically can operate for half an hour at 110-120% of capacity and can briefly deliver up to 200% of rated reactive power.
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 reactive power.
On electric power systems, synchronous condensers can be used to control the voltage on long transmission lines, especially for lines with a relatively high ratio of
inductive reactance
In electrical circuits, reactance is the opposition presented to alternating current by inductance or capacitance. Greater reactance gives smaller current for the same applied voltage. Reactance is similar to resistance in this respect, but does ...
to resistance.
[Donald Fink, Wayne Beaty (ed) ''Standard Handbook for Electrical Engineers Eleventh Edition'', Mc Graw Hill, 1978, ,page 14-33]
Gallery
See also
*
Static synchronous compensator A static synchronous compensator (STATCOM), originally known as a static synchronous condenser (STATCON), is a regulating device shunt-connected to alternating current electricity transmission network. It is based on a power electronics voltage-sou ...
*
Static VAR compensator
*
Unified power flow controller
A unified power flow controller (UPFC) is an electrical device for providing fast-acting reactive power compensation on high-voltage electricity transmission networks. It uses a pair of three-phase controllable bridges to produce current that is ...
References
Sources
*
:Electric power transmission
External links
A Short Course on Synchronous Machines and Synchronous Condensers
{{Authority control
Electric motors
Electromagnetic components
Energy conversion
Synchronous machines