Voltage and reactive power control involves proper
coordination among the voltage and reactive power control equipment in the
distribution system to obtain an optimum voltage profile and optimum reactive
power flows in the system according to the objective function and operating
constraints. According to Erche and Petersson (2009), many distribution network
operators (DNOs) operate on-load tap-changer (OLTC) and shunt capacitors
locally by using conventional controllers, e.g., voltage controller for the
OLTC and either voltage, reactive power or time controllers for the capacitors;
to perform basic voltage and reactive power control functions, e.g., to
maintain the voltages in the distribution system within the acceptable range
and to minimize power losses.
coordination among the voltage and reactive power control equipment in the
distribution system to obtain an optimum voltage profile and optimum reactive
power flows in the system according to the objective function and operating
constraints. According to Erche and Petersson (2009), many distribution network
operators (DNOs) operate on-load tap-changer (OLTC) and shunt capacitors
locally by using conventional controllers, e.g., voltage controller for the
OLTC and either voltage, reactive power or time controllers for the capacitors;
to perform basic voltage and reactive power control functions, e.g., to
maintain the voltages in the distribution system within the acceptable range
and to minimize power losses.
Different voltage and reactive power control
methods have been proposed. Properly locating and sizing shunt capacitors will
decrease power losses. As an improvement to the capacitor planning based on the
load size, methods to include customer load profiles and characteristics in the
capacitor (Gabby, 2010). Proper capacitor planning will also improve the
voltage profile in the distribution system. The capacitor locating and sizing
is studied and executed in the planning stage of the distribution system. In
order to enhance the distribution system further, the capacitor should also be
switched properly in the operation stage of the distribution system (Raj, 2013),
using different types of available capacitor control. Most recently, many
researchers have addressed the problem of voltage and reactive power control in
distribution systems by focusing on automated distribution systems. At the
moment, the voltage and reactive power control based on automated distribution
systems can be divided into two categories: offline setting control and real
time control (Clair, 2013).
methods have been proposed. Properly locating and sizing shunt capacitors will
decrease power losses. As an improvement to the capacitor planning based on the
load size, methods to include customer load profiles and characteristics in the
capacitor (Gabby, 2010). Proper capacitor planning will also improve the
voltage profile in the distribution system. The capacitor locating and sizing
is studied and executed in the planning stage of the distribution system. In
order to enhance the distribution system further, the capacitor should also be
switched properly in the operation stage of the distribution system (Raj, 2013),
using different types of available capacitor control. Most recently, many
researchers have addressed the problem of voltage and reactive power control in
distribution systems by focusing on automated distribution systems. At the
moment, the voltage and reactive power control based on automated distribution
systems can be divided into two categories: offline setting control and real
time control (Clair, 2013).
The offline setting control, for instance, aims to
find a dispatch schedule for the capacitor switching and the OLTC movement based
on a one day ahead load forecast. Meanwhile, the real time control (Taylor,
2014), for instance, aims to control the capacitor and OLTC based on real time measurements
and experiences
find a dispatch schedule for the capacitor switching and the OLTC movement based
on a one day ahead load forecast. Meanwhile, the real time control (Taylor,
2014), for instance, aims to control the capacitor and OLTC based on real time measurements
and experiences
The application of dispatch schedule based load
forecasting is motivated by the fact that although there is a random
fluctuation in the load variation, the major component of the load variations
is related to weather conditions. Furthermore, there is a deterministic load
pattern during the day due to social activities (Kundur, 2014). Therefore, the
load profile is quite predictable. It can be forecasted one-day ahead with an
average error less than 2% (Lach, 2007). Different objective functions and
operating constraints have been proposed in voltage and reactive power control
with automated distribution systems.
forecasting is motivated by the fact that although there is a random
fluctuation in the load variation, the major component of the load variations
is related to weather conditions. Furthermore, there is a deterministic load
pattern during the day due to social activities (Kundur, 2014). Therefore, the
load profile is quite predictable. It can be forecasted one-day ahead with an
average error less than 2% (Lach, 2007). Different objective functions and
operating constraints have been proposed in voltage and reactive power control
with automated distribution systems.
Nevertheless, all researchers (Erche &
Petersson, 2009) still consider loss minimization and keeping the voltage
within the acceptable range as the main objective and constraint in the voltage
and reactive power control. Another objective that is commonly proposed is
flattering the voltage profile. Commonly added operating constraints include
the maximum number of OLTC operations and capacitor switchings (Gil, San, Rios
& Martin, 2000), and the minimum distribution system (Hao &
Papalexopoulos, 2007). Other references, such consider minimization of OLTC
operations and capacitor switching as the objective function. The automated
control with offline setting proposed by Kibry and Hirst (2007) fully replaces
the local control operation of the conventional OLTC and capacitor operations
with a remotely controlled operation. The main obstacle application of this
method is its dependency on communication links and remote control to all
capacitors. However, many DNOs do not have communication links downstream to
the feeder capacitor locations
Petersson, 2009) still consider loss minimization and keeping the voltage
within the acceptable range as the main objective and constraint in the voltage
and reactive power control. Another objective that is commonly proposed is
flattering the voltage profile. Commonly added operating constraints include
the maximum number of OLTC operations and capacitor switchings (Gil, San, Rios
& Martin, 2000), and the minimum distribution system (Hao &
Papalexopoulos, 2007). Other references, such consider minimization of OLTC
operations and capacitor switching as the objective function. The automated
control with offline setting proposed by Kibry and Hirst (2007) fully replaces
the local control operation of the conventional OLTC and capacitor operations
with a remotely controlled operation. The main obstacle application of this
method is its dependency on communication links and remote control to all
capacitors. However, many DNOs do not have communication links downstream to
the feeder capacitor locations
Distribution system voltage
control
control
Distribution system voltage control is sometimes
referred to as volt/var control (VVC), which Gyugyi and Schauder (2008) specify
as follows: The objective is to minimize the peak power and energy losses while
keeping the voltage within specified limits for a variety of nominal load
patterns. This objective is formulated as an optimization problem that is
solved offline, based on nominal load patterns. The optimization variables are
the locations, sizes and control dead bands of capacitors and tap changer
voltage regulators. Tap changers are normally automatically controlled by a
relay controller that measures and regulates the secondary side voltage of the
transformer. The control of transformers operating in parallel in the same
substation must be coordinated to minimize circulating reactive power flows
(Anderson & Fouad, 2006).
referred to as volt/var control (VVC), which Gyugyi and Schauder (2008) specify
as follows: The objective is to minimize the peak power and energy losses while
keeping the voltage within specified limits for a variety of nominal load
patterns. This objective is formulated as an optimization problem that is
solved offline, based on nominal load patterns. The optimization variables are
the locations, sizes and control dead bands of capacitors and tap changer
voltage regulators. Tap changers are normally automatically controlled by a
relay controller that measures and regulates the secondary side voltage of the
transformer. The control of transformers operating in parallel in the same
substation must be coordinated to minimize circulating reactive power flows
(Anderson & Fouad, 2006).
Transmission system voltage
control
control
In practical operation of transmission systems, the
voltage needs to be continuously monitored and controlled to compensate for the
daily changes in load, generation and network structure. In fact, the control
of voltage is a major issue in power system operation. Kundur (2014) identifies
the main objectives of voltage control as: Voltage at the terminals of all
equipment in the system should be kept within acceptable limits, to avoid
malfunction of and damage to the equipment. Keeping voltages close to the values
for which stabilizing controls are designed, to enhance system stability and allow
maximal utilization of the transmission system. Minimize reactive power flows,
to reduce active as well as reactive power losses whereas distribution systems
as a rule are operated in radial configuration. Consequently, more
sophisticated control schemes than those used in distribution systems are
necessary. The control of voltage is often divided into the normal, preventive
and emergency state control. A brief overview of the strategies used in the
different operating states follows:
voltage needs to be continuously monitored and controlled to compensate for the
daily changes in load, generation and network structure. In fact, the control
of voltage is a major issue in power system operation. Kundur (2014) identifies
the main objectives of voltage control as: Voltage at the terminals of all
equipment in the system should be kept within acceptable limits, to avoid
malfunction of and damage to the equipment. Keeping voltages close to the values
for which stabilizing controls are designed, to enhance system stability and allow
maximal utilization of the transmission system. Minimize reactive power flows,
to reduce active as well as reactive power losses whereas distribution systems
as a rule are operated in radial configuration. Consequently, more
sophisticated control schemes than those used in distribution systems are
necessary. The control of voltage is often divided into the normal, preventive
and emergency state control. A brief overview of the strategies used in the
different operating states follows:
(a)
Primary
Control
Primary
Control
Primary voltage controllers are used in all power
systems to keep the terminal voltages of the generators close to reference
values given by the operator or generated by a secondary controller. An
automatic voltage regulator (AVR) acts on the exciter of a synchronous machine,
which supplies the field voltage and consequently the current in the field
winding of the machine and can thereby regulate its terminal voltage. The response
time of the primary controller is short, typically fractions of a second for
generators with modern excitation systems. Furthermore, many generators use a
so-called power system stabilizer (PSS) to modulate the terminal voltage of the
machine based on a local frequency measurement to contribute to damping of
electromechanical oscillations. Although the power system stabilizer in most
cases is integrated in the AVR, it only introduces fast oscillations around the
mean value given by the AVR as long as the generator remains synchronized with
the rest of the network.
systems to keep the terminal voltages of the generators close to reference
values given by the operator or generated by a secondary controller. An
automatic voltage regulator (AVR) acts on the exciter of a synchronous machine,
which supplies the field voltage and consequently the current in the field
winding of the machine and can thereby regulate its terminal voltage. The response
time of the primary controller is short, typically fractions of a second for
generators with modern excitation systems. Furthermore, many generators use a
so-called power system stabilizer (PSS) to modulate the terminal voltage of the
machine based on a local frequency measurement to contribute to damping of
electromechanical oscillations. Although the power system stabilizer in most
cases is integrated in the AVR, it only introduces fast oscillations around the
mean value given by the AVR as long as the generator remains synchronized with
the rest of the network.
(b) Secondary Control
Secondary voltage control acts on a time-scale of
seconds to a minute and within regions of the network. The aim of secondary
voltage control is to keep an appropriate voltage profile in a region of the
system and to minimize circulating reactive power flows and maximize reactive
reserves. Normally, the network is divided in a number of geographic regions.
One or a few so-called pilot nodes, which are assumed to be representative of
the voltage situation in the region, are selected for voltage regulation by the
secondary controller. The main actuators are the set point voltages of the
primary controllers of the generators within a region, although the French
implementation also uses static compensation devices such as capacitor banks
and reactors. The set point values are calculated by an optimization procedure
using a linearized static network model to make each generator in the region
contribute to the control of the pilot node voltage(s).
seconds to a minute and within regions of the network. The aim of secondary
voltage control is to keep an appropriate voltage profile in a region of the
system and to minimize circulating reactive power flows and maximize reactive
reserves. Normally, the network is divided in a number of geographic regions.
One or a few so-called pilot nodes, which are assumed to be representative of
the voltage situation in the region, are selected for voltage regulation by the
secondary controller. The main actuators are the set point voltages of the
primary controllers of the generators within a region, although the French
implementation also uses static compensation devices such as capacitor banks
and reactors. The set point values are calculated by an optimization procedure
using a linearized static network model to make each generator in the region
contribute to the control of the pilot node voltage(s).
(c)
Tertiary Control
Tertiary Control
Tertiary voltage control acts system-wide on a time
scale of about ten to thirty minutes. The traditional method of tertiary
control is so-called reactive power optimal power flow (OPF) (Gyugyi, 2005).
The desired operating conditions are specified in the form of a cost function,
which is minimized using nonlinear optimization techniques. Usually, the main
goal is to minimize losses and to keep voltages close to rated values. A
secondary goal may be to maximize and distribute reactive reserves. The main
control variables are voltage set points for the generators, or pilot nodes if
secondary control is used, and switching orders to compensation devices such as
shunt capacitors and reactors. The power flow equations (Hingorani &
Gyugyi, 2009) are specified as equality constraints in the optimization whereas
operational limits such as transfer limits, limits on reactive reserves and
voltages are specified as inequality constraints.
scale of about ten to thirty minutes. The traditional method of tertiary
control is so-called reactive power optimal power flow (OPF) (Gyugyi, 2005).
The desired operating conditions are specified in the form of a cost function,
which is minimized using nonlinear optimization techniques. Usually, the main
goal is to minimize losses and to keep voltages close to rated values. A
secondary goal may be to maximize and distribute reactive reserves. The main
control variables are voltage set points for the generators, or pilot nodes if
secondary control is used, and switching orders to compensation devices such as
shunt capacitors and reactors. The power flow equations (Hingorani &
Gyugyi, 2009) are specified as equality constraints in the optimization whereas
operational limits such as transfer limits, limits on reactive reserves and
voltages are specified as inequality constraints.