Reactive power compensation is often most effective
way to improve both power transfer capability and voltage stability. The
control of voltage levels is accomplished by controlling the production,
absorption and flow of reactive power. The generating units provide the basic
means of voltage control, because the automatic voltage regulators control
field excitation to maintain scheduled voltage level at the terminals of the
generators. To control voltage throughout the system we have to use addition
devices to compensate reactive power. Reactive compensation can be divided into
series and shunt compensation. It can be also divided into active and passive
compensation. But mostly consideration will be focused on shunt capacitor
banks, static var compensator (SVC) and Static Synchronous Compensators
(STATCOM), which are the part of group of active compensators called Flexible
AC Transmission Systems (FACTS). The devices used for these purposes may be
classified as follows:
way to improve both power transfer capability and voltage stability. The
control of voltage levels is accomplished by controlling the production,
absorption and flow of reactive power. The generating units provide the basic
means of voltage control, because the automatic voltage regulators control
field excitation to maintain scheduled voltage level at the terminals of the
generators. To control voltage throughout the system we have to use addition
devices to compensate reactive power. Reactive compensation can be divided into
series and shunt compensation. It can be also divided into active and passive
compensation. But mostly consideration will be focused on shunt capacitor
banks, static var compensator (SVC) and Static Synchronous Compensators
(STATCOM), which are the part of group of active compensators called Flexible
AC Transmission Systems (FACTS). The devices used for these purposes may be
classified as follows:
·
Shunt capacitors
Shunt capacitors
·
Series capacitors
Series capacitors
·
Shunt reactors
Shunt reactors
·
Synchronous condensers
Synchronous condensers
·
SVC
SVC
·
STATCOM
STATCOM
Shunt Capacitors
Shunt capacitors and reactors and series capacitors
provide passive compensation. They are either permanently connected to the
transmission and distribution system or switched. They contribute to voltage
control by modifying the network characteristics. Synchronous condensers, SVC
and STATCOM provide active compensation (Larson, 2010). The voltages of the
buses to which they are connected together with the generating units, they
establish voltages at specific points in the system. Voltages at other
locations in the system are determined by active and reactive power flows
through various elements, including the passive compensating devices (Miller,
2007).
provide passive compensation. They are either permanently connected to the
transmission and distribution system or switched. They contribute to voltage
control by modifying the network characteristics. Synchronous condensers, SVC
and STATCOM provide active compensation (Larson, 2010). The voltages of the
buses to which they are connected together with the generating units, they
establish voltages at specific points in the system. Voltages at other
locations in the system are determined by active and reactive power flows
through various elements, including the passive compensating devices (Miller,
2007).
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.
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.
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 capacitors banks are always connected to the
bus rather than to the line. They are connected either directly to the high
voltage bus or to the tertiary winding of the main transformer. Shunt capacitor
banks are breaker-switched either automatically by a voltage relays or manually
(Taylor, 2014). The primary purpose of transmission system shunt compensation
near load areas is voltage control and load stabilization. In other words,
shunt capacitors are used to compensate for I2X losses in
transmission system and to ensure satisfactory voltage levels during heavy load
conditions. Shunt capacitors are used in power system for power factor
correction. The objective of power factor correction is to provide reactive
power close to point where it is being consumed, rather than supply it from
remote sources (Trehan, 2011).
bus rather than to the line. They are connected either directly to the high
voltage bus or to the tertiary winding of the main transformer. Shunt capacitor
banks are breaker-switched either automatically by a voltage relays or manually
(Taylor, 2014). The primary purpose of transmission system shunt compensation
near load areas is voltage control and load stabilization. In other words,
shunt capacitors are used to compensate for I2X losses in
transmission system and to ensure satisfactory voltage levels during heavy load
conditions. Shunt capacitors are used in power system for power factor
correction. The objective of power factor correction is to provide reactive
power close to point where it is being consumed, rather than supply it from
remote sources (Trehan, 2011).
Switched shunt capacitors are also used for feeder
voltage control. They are installed at appropriate location along the length of
the feeder to ensure that voltages at all points remain the allowable minimum
or maximum limits as the loads vary. For voltage stability, shunt capacitor
banks are very useful on allowing nearby generators to operate near unity power
factor. This maximizes fast acting reactive reserve. The biggest disadvantage
of shunt capacitors is that the reactive power output drops with the voltage
squared. Thus, during the severe voltage decays these devices are not efficient
enough when compared to static var compensators, mechanically switched
capacitor banks have the advantage of much lower cost. Switching speeds can be
quite fast. Following a transmission line outage, capacitor bank energization
should be delayed to allow time for line reclosing. However, capacitor
switching should be before significant amounts of load are restored by
transformer tap changers or distribution voltage regulators.
voltage control. They are installed at appropriate location along the length of
the feeder to ensure that voltages at all points remain the allowable minimum
or maximum limits as the loads vary. For voltage stability, shunt capacitor
banks are very useful on allowing nearby generators to operate near unity power
factor. This maximizes fast acting reactive reserve. The biggest disadvantage
of shunt capacitors is that the reactive power output drops with the voltage
squared. Thus, during the severe voltage decays these devices are not efficient
enough when compared to static var compensators, mechanically switched
capacitor banks have the advantage of much lower cost. Switching speeds can be
quite fast. Following a transmission line outage, capacitor bank energization
should be delayed to allow time for line reclosing. However, capacitor
switching should be before significant amounts of load are restored by
transformer tap changers or distribution voltage regulators.
Despite of many advantages of mechanically switched
capacitors, there is couple of disadvantages as well. Firstly, for transient
voltage instability, the switching may not be fast enough to prevent induction
motor stalling. If voltage collapse results in system breakdown, the stable
parts of the system may experience damaging over voltages immediately following
separation. Over voltages would be aggravated by energizing of shunt capacitors
during the period of voltage decay (Devitt, 2011).
capacitors, there is couple of disadvantages as well. Firstly, for transient
voltage instability, the switching may not be fast enough to prevent induction
motor stalling. If voltage collapse results in system breakdown, the stable
parts of the system may experience damaging over voltages immediately following
separation. Over voltages would be aggravated by energizing of shunt capacitors
during the period of voltage decay (Devitt, 2011).
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. Other equipment
can be involved in the provision of reactive power and energy, such as:
down, by absorbing the reactive power, in the case of light load and load
rejection, and to compensate the capacitive load of the line. 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;
Unified Power Flow Controllers
(UPFC) and other advanced FACTS (flexible ac transmission system) devices;
·
Tap staggering of transformers
connected in parallel;
Tap staggering of transformers
connected in parallel;
·
Disconnection of transmission
lines;
Disconnection of transmission
lines;
·
Load shedding
Load shedding
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 high-voltage, direct current (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 cheaper and have regulating characteristics similar to
synchronous condensers.
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 high-voltage, direct current (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 cheaper and have regulating characteristics similar to
synchronous condensers.
Static VAR compensators (SVC)
An SVC combines conventional capacitors and
inductors with fast switching capability. Switching takes place in the sub
cycle time frame (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:
inductors with fast switching capability. Switching takes place in the sub
cycle time frame (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);
TCR (thyristor controlled
reactor);
·
TSC (thyristor switched
capacitor) ;
TSC (thyristor switched
capacitor) ;
·
TSR (thyristor switched reactor);
TSR (thyristor switched reactor);
·
FC (fixed capacitor).
FC (fixed capacitor).
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 (Taylor, 2014); 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 over voltages. 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.
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 (Taylor, 2014); 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 over voltages. 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.
Static synchronous compensator
(STATCOM)
(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 (Kirby
& Hirst, 2007). 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.
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 (Kirby
& Hirst, 2007). 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 I2X compensates for the reactive consumption I2X
of the C 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.
for long transmission lines and transient stability improvement. Series
compensation reduces net transmission line inductive reactance. The reactive
generation I2X compensates for the reactive consumption I2X
of the C 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.
