In electrical engineering, a synchronous condenser (sometimes synchronous capacitor or synchronous compensator) is a device identical to a synchronous motor, whose shaft is not connected to anything but spins freely.

1. In electrical engineering, a synchronous condenser (sometimes synchronous
capacitor or synchronous compensator) is a device identical to a 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. 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 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 condensers 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 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 cooled.
As the load on a synchronous motor increases, the stator current Ia 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 Ia 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.

2. V- curves for a synchronous machine. A synchronous condenser operates at
nearly zero real power. As the machine passes from under-excited to over-
excited, its stator current passes through a minimum
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

3. 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, 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 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 an electric power system, synchronous condensers can be used to control the
voltage on long transmission lines, especially for lines with relatively high ratio of
inductive reactance to resistance.
Conclusion for effect for field change with constant load on power factor
• For motor with increased (decreased) excitation power factor becomes
leading (lagging).
• For generator with increased (decreased) excitation power factor becomes
lagging (leading).
• Unloaded overexcited synchronous motors are sometimes used to improve
power factor. They are known as synchronous condensers.

4. A Series SC is an unloaded or lightly loaded synchronous machine that will deliver
the required reactive power dynamically. The SC is connected to the power line
and is intentionally run in an overexcited condition. The level of excitation is
dependent on the amount of power factor correction desired and the amount of
power factor sensed by the condenser controls. The condenser will adjust the
excitation level automatically to maintain the power factor at the correct setting.
Once the Series SC is installed, it continuously monitors the power factor and
produces the right amount of VAR needed to correct any power factor without
switching transients and is not troubled by harmonic currents produced by solid
state motor drives.
The Series SC also helps overall power quality by reducing voltage transients and
by reducing the problems associated with harmonic distortion found in many
manufacturing process. The Series SC is available from 100 kVAR to 10,000
kVAR modules, from 480 volts up to 15 KV. Applying a synchronous condenser for
power factor correction provides many advantages with no risks. Correcting power
factor with a condenser is much smoother and will not adversely affect a system
loaded with current harmonics. The condenser is a low impedance source and
appears inductive to loads. In addition the Series SC is easy to maintain (a simple
annual lubrication) and adapts easily to a plant’s changing loads.(Dynamic
Correction).
The SC has many user friendly features such as the automatic restart after power
loss. The SC also has a state-of-the-art integrated touch screen display and
control interface. The controls are designed to be friendly and easily understood so
that the user can control the SC from this panel. The touch screen will display
voltage, current, power factor, and power information as a minimum.

5. Reactive Power compensation devices
Synchronous condensers
Every synchronous machine (motor or generator) has the reactive power
capabilities the same as synchronous generators. Synchronous machines that are
designed exclusively to provide reactive support are called synchronous
condensers. Synchronous condensers have all of the response speed and
controllability advantages of generators without the need to construct the rest of
the power plant (e.g., fuel-handling equipment and boilers). Because they are
rotating machines with moving parts and auxiliary systems, they require
significantly more maintenance than static compensators. They also consume real
power equal to about 3% of the machine’s reactive-power rating. Synchronous
condensers are used in transmission systems: at the receiving end of long
transmissions, in important substations and in conjunction with HVDC converter
stations.
Small synchronous condensers have also been used in high-power industrial
networks to increase the short circuit power. The reactive power output is
continuously controllable. The response time with closed loop voltage control is
from a few seconds and up, depending on different factors. In recent years the
synchronous condensers have been practically ruled out by the thyristor controlled
static VAR compensators, because those are much more cheaper and have
regulating characteristics similar to synchronous condensers.
Static VAR compensators
An SVC combines conventional capacitors and inductors with fast switching
capability. Switching takes place in the sub cycle timeframe (i.e., in less than 1/50
of a second), providing a continuous range of control. The range can be designed
to span from absorbing to generating reactive power. Advantages include fast,
precise regulation of voltage and unrestricted, largely transient-free, capacitor
bank switching. Voltage is regulated according to a slope (droop) characteristic.
Static VAR compensator could be made up from:
TCR (thyristor controlled reactor);
TSC (thyristor switched capacitor);
TSR (thyristor switched reactor);
FC (fixed capacitor);
Harmonic filter.
Because SVCs use capacitors they suffer from the same degradation in reactive
capability as voltage drops. They also do not have the short-term overload
capability of generators and synchronous condensers. SVC applications usually
require harmonic filters to reduce the amount of harmonics injected into the power
system by the thyristor switching. SVCs provide direct control of voltage (C.W.
Taylor, 1994); this is very valuable when there is little generation in the load area.
The remaining capacitive capability of an SVC is a good indication of proximity to
voltage instability. SVCs provide rapid control of temporary overvoltages.
But on the other hand SVCs have limited overload capability, because SVC is a
capacitor bank at its boost limit. The critical or collapse voltage becomes the SVC
regulated voltage and instability usually occurs once an SVC reaches its boost
limit. SVCs are expensive; shunt capacitor banks should first be used to allow
unity power factor operation of nearby generators.

6. Static synchronous compensator (STATCOM)
The STATCOM is a solid-state shunt device that generates or absorbs reactive
power and is one member of a family of devices known as flexible AC transmission
system (FACTS) devices. The STATCOM is similar to the SVC in response speed,
control capabilities, and the use of power electronics. Rather than using
conventional capacitors and inductors combined with thyristors, the STATCOM
uses self-commutated power electronics to synthesize the reactive power output.
Consequently, output capability is generally symmetric, providing as much
capability for production as absorption. The solid-state nature of the STATCOM
means that, similar to the SVC, the controls can be designed to provide very fast
and effective voltage control (B. Kirby, 1997). While not having the short-term
overload capability of generators and synchronous condensers, STATCOM
capacity does not suffer as seriously as SVCs and capacitors do from degraded
voltage. STATCOMs are current limited so their MVAR capability responds linearly
to voltage as opposed to the voltage-squared relationship of SVCs and capacitors.
This attribute greatly increases the usefulness of STATCOMs in preventing voltage
collapse.
Series capacitors and reactors
Series capacitors compensation is usually applied for long transmission lines and
transient stability improvement. Series compensation reduces net transmission line
inductive reactance. The reactive generation I2XC compensates for the reactive
consumption I2X of the transmission line. Series capacitor reactive generation
increases with the current squared, thus generating reactive power when it is most
needed. This is a self-regulating nature of series capacitors. At light loads series
capacitors have little effect.
Shunt capacitors
The primary purposes of transmission system shunt compensation near load areas
are voltage control and load stabilization. Mechanically switched shunt capacitor
banks are installed at major substations in load areas for producing reactive power
and keeping voltage within required limits. For voltage stability shunt capacitor
banks are very useful in allowing nearby generators to operate near unity power
factor. This maximizes fast acting reactive reserve. Compared to SVCs,
mechanically switched capacitor banks have the advantage of much lower cost.
Switching speeds can be quite fast. Current limiting reactors are used to minimize
switching transients.
There are several disadvantages to mechanically switched capacitors. For voltage
emergencies the shortcoming of shunt capacitor banks is that the reactive power
output drops with the voltage squared. For transient voltage instability the
switching may not be fast enough to prevent induction motor stalling. Precise and
rapid control of voltage is not possible. Like inductors, capacitor banks are discrete
devices, but they are often configured with several steps to provide a limited
amount of variable control. If voltage collapse results in a system, the stable parts
of the system may experience damaging over voltages immediately following
separation.
Shunt reactors
Shunt reactors are mainly used to keep the voltage down, by absorbing the
reactive power, in the case of light load and load rejection, and to compensate the
capacitive load of the line.

7. Other
Other equipment can be involved in the provision of reactive power and energy,
such as:
 Unified Power Flow Controllers (UPFC) and other advanced FACTS (flexible ac
transmission system) devices;
 Tap staggering of transformers connected in parallel;
 Disconnection of transmission lines;
 Load shedding;

8.  Investments in reactive compensation
This section will try to answer the question: How much does 1 MVar cost to install
and to produce? The table below from (B. Kirby, 1997) gives some numbers.
These are, of course, approximate and may vary according to equipment producer
and type.
Capital and operating costs of reactive power compensation equipment
Equipment type Capital costs ($/kVar) Operating costs
Generator Difficult to separate High
Synchronous condenser 30-35 High
Capacitor, reactor 08.-10 Very low
Static VAR compensator 45-50 Moderate
STATCOM 50-55 Moderate
Conclusions
In this work we tried to define the value of reactive power using four different
valuation methods: Voltage Sensitivity, PV curve, Equivalent Reactive
Compensation and Back-up generation. A simple test system model was build and
tested with Power World software. Preliminary results showed different nature of
reactive power from the sources. For the same test system model we have tried to
find out the advantages and disadvantages of having zero reactive power flow in
the system transformers. So the results showed that for every extra MVar towards
the zero reactive power flow in the system transformers we need to install 2.5
MVar of reactive power compensation devices. Having zero flow system losses
and transfer capacity changes very little.
In the introduction part we defined some questions we wanted to answer in this
work. So the answers and the final conclusions are like this:
 Which reactive power source is the most important to the system? What are
the criteria?
We tried to answer this question with the help of all four proposed methods
of reactive power valuation. They give different results, but at the same time
showed different nature of reactive power. Voltage Sensitivity and PV curve
methods show how generators react with their reactive power output to the
changing load in the system. ERC and Back-up generation methods help to
realize the importance of the reactive power reserve of the sources.
Generator closest to the relatively high load is the most important to the
system.
 How much MVar from other sources would be needed to replace Q from the
selected source?
Back-up generation method is the answer to this. It describes the
importance of the source to system reactive power reserve and shows how
much of additional reactive power we have to supply to cover the Q

9. shortage from one or other reactive power source to keep bus voltages (at
PV buses) at the predetermined values.
 What does 1 MVar from one or other source do to system losses?
Active and reactive power losses sensitivities to additional MVar injected
into the
system can be obtained using Voltage Sensitivity method. Marginal losses
produced by the source depend on the network configuration and location
of the source in the system.
 What does this 1 MVar mean to transfer capacity?
With PV curve method we were able to see what influence each generator
in our test system had on the transfer limit. During high load condition the
transfer limit was obtained when one of the sources reached the limits of
reactive power output. So the source closest to the loads and with relatively
low reactive power output limits is crucial to the system.
 How much does this MVar cost to install and to produce?
Reactive power sources and sinks were analyzed in the section 2.3 and in
table 2 approximate numbers about the installation and production costs of
reactive power equipment were given. The most expensive, but the best
sources of reactive power support are machinery equipment – generators,
synchronous condensers.
For further investigation in this area, the results should be checked on the
real electric power system to find out if the proposed methods are suitable.

10. 9. Generator reactive power capability
9.1 A generator‘s output capabilities depend on the thermal limits of
various parts of the generator and on system stability limits. Thermal limits
are physical limits of materials such as copper, iron and insulation, if the
generator overheats, insulation begins to degrade and over time this could
result in equipment damage. Increasingly real power output of a generator
heats up the armature. Increasing reactive power output heats up the field
windings and the armature.
9.2 Power Generators shall be able to supply or absorb reactive power
according to the reactive power capability curve that is defined in
connection agreement and shall perform yearly inspection to ensure that
generators can perform according to the reactive power capability curve.
9.3 To supply reactive power, the generator must increase the magnetic
field to raise the voltage it is supplying to the power system; this means
increasing the current in the field windings, which is limited by the thermal
properties of the metal and insulation. The field current is supplied by the
generator exciter, which is a DC power supply connected to the generator.
The field current can be quickly adjusted by automatic control or with a dial
to change the reactive power supplied or consumed by the generator.
9.4 At any given field setting, the generator has a specific terminal
voltage it is attempting to hold. If the system voltage declines, the generator
will inject reactive power into the power system, tending to raise system
voltage. If the system voltage rises, the reactive output of the generator will
drop and ultimately reactive power will flow into the generator, tending to
lower system voltage.
9.5 The voltage regulator will accentuate this behavior by driving the
field current in the appropriate direction to obtain the desired system
voltage. Because most of the reactive limits are thermal limits associated
with large pieces of equipment, significant short-term extra reactive-power
capability usually exists.
9.6 Power-system stabilizers also control generator field current and
reactive power output in response to oscillations on the power system. This
function is a part of the network-stability ancillary service.
9.7 Stability limits are determined by the ability of the power system to
accept delivery of power from the connected generator under a defined set
of system conditions including recognized contingencies. All generators
connected to a power system operate at the same electrical frequency; if a

11. generator loses synchronism with the rest of the system, it will trip offline to
protect itself.
9.8 Capacitors supply reactive power and have leading power factors,
while inductors consume reactive power and have lagging power factors.
The convention for generators is the reverse. When the generator is
supplying reactive power, it has a lagging power factor and its mode of
operation is referred to as overexcited. When a generator consumes reactive
power, it has a leading power factor region and is under-excited.
9.9 The capability-set limits are thermal limits for different parts of the
generator, if the generator output approaches these limits, an alarm will
notify the generator operator of the problem; if the operator does not bring
the generator back to a safe operating point, the generator‘s protection
scheme (relays, circuit breakers, fuses) will operate, resulting in
disconnection of the generator from the network; finally, if the protection
equipment fails and the operator does not act in time, the generator will
overheat, potentially causing equipment damage. Because generators are
expensive, generator operators generally will not operate the generator in a
way that risks damaging the equipment and losing revenue during repair.
9.10 The ability of a generator to provide reactive support depends on its
real power production which is represented in the form of generator
capability curve or D - curve. Figure 9.1 shows the combined limits on real
and reactive production for a typical generator. Like most electric
equipment, generators are limited by their current-carrying capability. Near
rated voltage, this capability becomes an MVA limit for the armature of the
generator rather than a MW limitation, shown as the armature heating limit
in the Figure.
Fig 9.1 GENERATOR CAPABILITY CURVE or D CURVE

12. 9.11 At the edges of the D-curve, the opportunity cost of extending
generator real or reactive power supply amounts to the millions of rupees
that would be needed to replace damaged generator equipment and lost
revenue during repair. The characteristics of the generator step-up
transformer that connects the generator to the electric transmission system,
as well as operational policies of the transmission system, may impose
further limits on generator output.
9.12 Generator capability may be extended by the coolant used in the
generator. A more efficient coolant allows the generator to dissipate more
heat, thereby extending thermal limits. Most large generators are cooled
with hydrogen; increasing the hydrogen pressure cools the generator
equipment more effectively, increasing the generator‘s capability.
9.13 SYNCHRONOUS CONDENSERS:
9.13.1. Synchronous condenser is another reactive power device,
traditionally in use since 1920s. A synchronous Condenser is a synchronous
machine running without a prime mover or a mechanical load. Like
generators, they can be over-exited or under-exited by varying their field
current in order to generate or absorb reactive power, synchronous
condensers can continuously regulate reactive power to ensure steady
transmission voltage, under varying load conditions.
9.13.2. They are especially suited for emergency voltage control under loss of
load, generation or transmission, because of their fast short-time response.
Synchronous condensers provide necessary reactive power even exceeding
their rating for short duration, to arrest voltage collapse and to improve
system stability. It draws a small amount of active power (about 3%) from
the power system to supply losses.
9.13.3. Synchronous machines that are designed exclusively to provide
reactive support are called synchronous condensers. Synchronous
condensers have all of the response speed and controllability advantages of
generators without the need to construct the rest of the power plant (e.g.,
fuel-handling equipment and boilers). Because they are rotating machines
with moving parts and auxiliary systems, they may require significantly
more maintenance than static alternatives. They also consume real power
equal to about 3% of the machine‘s reactive-power rating. That is, a 50-
MVAR synchronous condenser requires about 1.5 MW of real power.
9.14 Synonymous terms are synchronous compensator and synchronous
phase modifier. The synchronous compensator is the traditional means for
Continuous control of reactive power. Synchronous compensators are used

13. in transmission systems: at the receiving end of long transmissions, in
important substations and in conjunction with HVDC inverter stations.
Small synchronous compensators have also been installed in high-power
industrial networks of steel mills; few of these are in use today.
Synchronous compensators in use range in size from a few MVA up to
hundreds of MVA.
9.15 Some hydro/gas generators can operate as synchronous condensers.
Such gas based units are often equipped with clutches which can be used to
disconnect the turbine from the generator when active power is not required
from them. In case of hydro, water supply is blocked and units run with
loads of only air friction
9.16 A synchronous Compensator has several advantages over static
compensators. Synchronous compensators contribute to system short circuit
capacity. Their reactive power production is not affected by the system
voltage. During power swings (electro mechanical oscillations) there is an
exchange of kinetic energy between a synchronous condenser and the
power system.
9.17 During such power swings, a synchronous condenser can supply a
large amount of reactive power, perhaps twice its continuous rating. Unlike
other forms of shunt compensation, it has an internal voltage source and is
better to cope with the low voltage conditions. Because of their high
purchase and operating costs, they have been largely superseded by static
var compensators.
9.17 In recent years the synchronous compensator has been practically
ruled out by the SVC, in the case of new installations, due to benefits in cost
performance and reliability of the latter. One exception is HVDC inverter
stations, in cases where the short-circuit capacity has to be increased. The
synchronous compensators can do this, but not the SVC.
9.18 Comparison between Synchronous Condenser and shunt capacitor is
explained in the below table:-
Sl.No Synchronous condenser Shunt capacitor
1. Synchronous condenser can
supply kVAR equal to its rating
and can absorb up to 100% of its
KVA rating
Shunt capacitor should be associated
with a reactor to give that performance
2. This has fine control with AVR This operates in steps
3. The output is not limited by the
system voltage condition. This
gives out its full capacity even
when system voltage decreases
The capacitor output is proportional to
V2 of the system. Hence its
performance decreases under low
voltage conditions

14. 4. For short periods the
synchronous condenser can
supply KVAR in excess of its
rating at nominal voltage
The capacitor cannot supply more than
its capacity at nominal voltage. Its
output is proportional to V2.
5. The full load losses are above
3% of its capacity
The capacitor losses are about 0.2%
6. These cannot be economically
deployed at several locations in
distribution
The capacitor banks can be deployed at
several locations economically in
distribution
7. The synchronous condenser
ratings cannot be modular
The capacitors are modular. They can
be deployed as and when system
requirements change
8. A failure in the synchronous
condenser can remove the entire
unit ability to produce KVAR.
However failures are rare in
synchronous condensers
compared to capacitors
A failure of a single fused unit in a
bank of capacitors affects only that unit
and does not affect the entire bank
9. They add to the short circuit
current of a system and
therefore increase the size of
(11kV etc.) breakers in the
neighbor-hood.
The capacitors do not increase the
short circuit capacity of the system, as
their output is proportional to V2
10. This is a rotating device. Hence
the O&M problems are more
These are static and simple devices.
Hence O&M problems are negligible
9.19 As per planning philosophy and general guidelines in the Manual on
Transmission planning criteria issued by CEA (MOP, India), Thermal /
Nuclear Generating Units shall normally not run at leading power factor.
However for the purpose of charging unit may be allowed to operate at
leading power factor as per the respective capability curve.

15. 9.20 List of synchronous condenser at all India level is given :