08-07-2014, 02:27 PM
POWER FLOW CONTROL BY UPFC
[attachment=65749]
ABSTRACT
: Continuous and fast improvement of power electronics technology has made FACTS (Flexible AC Transmission System) a promising concept for power
system development in the coming decade. By means of appropriate FACTS
technology, power flow along the transmission network can be more flexibly
controlled, as the name implies. Among a variety of FACTS controllers, UPFC is
chosen as the focus of investigation for it embraces all the basic attributes of
transmission power flow control. Computation of power flow for UPFC embedded
power systems is fundamental need for power system analysis and planning purposes.
In this project a method is proposed to calculate the load flow of power
system in which Unified Power Flow Controllers (UPFCs) are embedded. First the
load flow equations of power system including the UPFCs are derived and the
algorithm is developed based on the Newton Raphson Load Flow (NRLF) technique.
The method inherits the basic properties of the NRLF approach
INTRODUCTION
Continuous and fast development of power system has made FACTS an
effective tool for its development. Among various FACTS controllers Unified Power
Flow Control [UPFC] is chosen as it embraces all basic attributes of the transmission.
The mathematical model of the UPFC is developed and employed for the load
flow control studies. By using UPFC the power flow control becomes more flexible
than ever. But it has a drawback which requires pre-specified condition such as the
power flow in the transmission line where it is being embedded. As no one has prior
knowledge about this the pre-specified power flow and voltage are arbitrary.
This projects aims to present a systematic and efficient method for performing
load flow calculation of a generalized power system with multi-machines and multiUPFC’s.
Since Newton-Raphson Load Flow [NRLF] method together with the
techniques of sparsity and optimal ordering has been proved to be more effective. The
load flow equations are similar to that of the NRLF and algorithm for load flow
studies of UPFC’s is developed based on the traditional NRLF method. The approach
keeps the conventional NRLF method intact, during iteration process, it receives
almost four power mismatches and few elements of Jacobian Matrix for each UPFC.
As for each UPFC, it only refers to a few elements of the Jacobian Matrix and hence
additional computation burden incurred is very little. The developed algorithm is
tested on IEEE 14-bus system indicates the effectiveness and reliability of this
algorithm.
INTRODUCTION TO FACTS
We need transmission interconnections because, apart from delivery, the
purpose of the transmission network is to pool plants and load centers in order to
minimize the total power generation capacity and fuel cost. Transmission
interconnections enable taking advantage of diversity of loads, availability of sources,
and fuel price in order to supply electricity to the loads at minimum cost with a
required reliability. In general, if a power delivery system was made up of radial lines
from individual local generators without being part of a grid system, many more
generation resources would be needed to serve the load with the same reliability, and
the cost of electricity would be much higher. With that perspective, transmission is
often an alternative to a new generation resource. Less transmission capability means
that more generation resources would be required regardless of whether the system is
made up of large or small power plants. In fact small distributed generation becomes
more economically viable if there is a backbone of a transmission grid. One cannot be
really sure about what the optimum balance is between generation and transmission
unless the system planners use advanced methods of analysis which integrate
transmission planning into an integrated value – based transmission/generation
planning scenario. The cost of transmission lines and losses, as well as difficulties
encountered in building new transmission lines, would often limit the available
transmission capacity. It seems that there are many cases where economic energy or
reserve sharing is constrained by transmission capacity, and the situation is not getting
any better. In a deregulated electric service environment, an effective electric grid is
vital to the competitive environment of reliable electric service.
SERIES CONTROLLERS
The series controller could be a variable impedance, such as capacitor, reactor,
etc., or a power electronics based variable source of main frequency, sub synchronous
and harmonic frequencies to serve the desired need. In principle, all series controllers
inject voltage in series with the line. Even a variable impedance multiplied by the
current flow through it, represents an injected series voltage in the line. As long as
the voltage is in phase quadrature with the line current, the series controller only
supplies (or) consumes variable reactive power
SHUNT CONTROLLERS
As in the case of series controllers, the shunt controllers may be variable
impedance, Variable source, or a combination of these. In principle, all shunt
controllers inject current into the system at the point of connection. Even a variable
shunt impedance connected to the line voltage causes a variable current flow and
hence represents injection of current into the line. As long as the injected current is in
phase quadrature with the line voltage, the shunt controller only supplies or consumes
variable reactive power. The basic type of shunt controllers are shown in figure3.2.
COMBINED SERIES-SHUNT CONTROLLERS
This could be a combination of separate shunt and series controllers, which are
controlled in a coordinated manner, or a Unified Power Flow Controller with series
and shunt elements. In principle, combined shunt and series controllers inject current
into the system with the shunt part of the controller and voltage in series in the line
with the series part of the controller. However, when the shunt and series controllers
are unified, there can be a real power exchange between the series and shunt
controllers via the power link
BASIC OPERATING PRINCIPLES
From the conceptual view point, the UPFC is a generalized synchronous
voltage source (SVS), represented at the fundamental (power system) frequency by
voltage phasor Vpq with controllable magnitude Vpq (0< Vpq < Vpqmax) and angle (0
<< 2), in series with the transmission line, as illustrated for the usual elementary
two-machine system (or for two independent systems with a transmission link
intertie) in Figure 4.1. In this functionally unrestricted operation, which clearly
includes voltage and angle regulation, the SVS generally exchanges both reactive and
real power with the transmission system. Since, as established previously, a SVS is
able to generate only the reactive power exchanged, the real power must be supplied
to it, or absorbed from it, by a suitable power supply or sink. In the UPFC
arrangement the real power exchanged is provided by one of the end buses (e.g., the
sending-end bus), as indicated in Figure 4.1.
In the presently used practical implementation, the UPFC consists of two
voltage-source converters. These back-to-back converters, labeled “Converter 1” and
Converter 2” in the figure 4.1, are operated from a common dc link provided by a dc
storage capacitor. As indicated before, this arrangement functions as an ideal ac-to-ac
power converter in which the real power can freely flow in either direction between
the ac terminals of the two converters, and each converter can independently generate
(or absorb) reactive power at its own ac output terminal.
Converter 2 provides the main function of the UPFC by injecting voltage Vpq
with controllable magnitude Vpq and phase angle in series with the line via an
insertion transformer. This injected voltage acts essentially as a synchronous ac
voltage source. The transmission line current flows through this voltage source
resulting in reactive and real power exchange between it and the ac system. The
reactive power exchanged at the ac terminal (i.e., at the terminal of the series insertion
transformer) is generated internally by the converter. The real power exchanged at
the ac terminal is converted into dc power which appears at the dc link as a positive or
negative real power demand.
The basic function of Converter 1 is to supply or absorb the real power
demanded by Converter 2 at the common dc link to support the real power exchange
POWER FLOW CONTROL
Actual flow on the line could be higher depending on
other prevailing system conditions. The UPFC, therefore, may be required to slightly
reduce the line loadings. Line reactive power flow and its direction will be monitored
to help maintain the dynamic reactive power margin of the shunt inverter.
The series power flow control becomes important during contingency
conditions. The control objective is to increase the line loading. This control is to be
activated as soon as any one of the line loadings exceeds 90% of their respective
emergency thermal ratings. The UPFC has to increase line loading until the critical
line loadings are reduced below the defined levels or the UPFC reaches its rating
limit.
[attachment=65749]
ABSTRACT
: Continuous and fast improvement of power electronics technology has made FACTS (Flexible AC Transmission System) a promising concept for power
system development in the coming decade. By means of appropriate FACTS
technology, power flow along the transmission network can be more flexibly
controlled, as the name implies. Among a variety of FACTS controllers, UPFC is
chosen as the focus of investigation for it embraces all the basic attributes of
transmission power flow control. Computation of power flow for UPFC embedded
power systems is fundamental need for power system analysis and planning purposes.
In this project a method is proposed to calculate the load flow of power
system in which Unified Power Flow Controllers (UPFCs) are embedded. First the
load flow equations of power system including the UPFCs are derived and the
algorithm is developed based on the Newton Raphson Load Flow (NRLF) technique.
The method inherits the basic properties of the NRLF approach
INTRODUCTION
Continuous and fast development of power system has made FACTS an
effective tool for its development. Among various FACTS controllers Unified Power
Flow Control [UPFC] is chosen as it embraces all basic attributes of the transmission.
The mathematical model of the UPFC is developed and employed for the load
flow control studies. By using UPFC the power flow control becomes more flexible
than ever. But it has a drawback which requires pre-specified condition such as the
power flow in the transmission line where it is being embedded. As no one has prior
knowledge about this the pre-specified power flow and voltage are arbitrary.
This projects aims to present a systematic and efficient method for performing
load flow calculation of a generalized power system with multi-machines and multiUPFC’s.
Since Newton-Raphson Load Flow [NRLF] method together with the
techniques of sparsity and optimal ordering has been proved to be more effective. The
load flow equations are similar to that of the NRLF and algorithm for load flow
studies of UPFC’s is developed based on the traditional NRLF method. The approach
keeps the conventional NRLF method intact, during iteration process, it receives
almost four power mismatches and few elements of Jacobian Matrix for each UPFC.
As for each UPFC, it only refers to a few elements of the Jacobian Matrix and hence
additional computation burden incurred is very little. The developed algorithm is
tested on IEEE 14-bus system indicates the effectiveness and reliability of this
algorithm.
INTRODUCTION TO FACTS
We need transmission interconnections because, apart from delivery, the
purpose of the transmission network is to pool plants and load centers in order to
minimize the total power generation capacity and fuel cost. Transmission
interconnections enable taking advantage of diversity of loads, availability of sources,
and fuel price in order to supply electricity to the loads at minimum cost with a
required reliability. In general, if a power delivery system was made up of radial lines
from individual local generators without being part of a grid system, many more
generation resources would be needed to serve the load with the same reliability, and
the cost of electricity would be much higher. With that perspective, transmission is
often an alternative to a new generation resource. Less transmission capability means
that more generation resources would be required regardless of whether the system is
made up of large or small power plants. In fact small distributed generation becomes
more economically viable if there is a backbone of a transmission grid. One cannot be
really sure about what the optimum balance is between generation and transmission
unless the system planners use advanced methods of analysis which integrate
transmission planning into an integrated value – based transmission/generation
planning scenario. The cost of transmission lines and losses, as well as difficulties
encountered in building new transmission lines, would often limit the available
transmission capacity. It seems that there are many cases where economic energy or
reserve sharing is constrained by transmission capacity, and the situation is not getting
any better. In a deregulated electric service environment, an effective electric grid is
vital to the competitive environment of reliable electric service.
SERIES CONTROLLERS
The series controller could be a variable impedance, such as capacitor, reactor,
etc., or a power electronics based variable source of main frequency, sub synchronous
and harmonic frequencies to serve the desired need. In principle, all series controllers
inject voltage in series with the line. Even a variable impedance multiplied by the
current flow through it, represents an injected series voltage in the line. As long as
the voltage is in phase quadrature with the line current, the series controller only
supplies (or) consumes variable reactive power
SHUNT CONTROLLERS
As in the case of series controllers, the shunt controllers may be variable
impedance, Variable source, or a combination of these. In principle, all shunt
controllers inject current into the system at the point of connection. Even a variable
shunt impedance connected to the line voltage causes a variable current flow and
hence represents injection of current into the line. As long as the injected current is in
phase quadrature with the line voltage, the shunt controller only supplies or consumes
variable reactive power. The basic type of shunt controllers are shown in figure3.2.
COMBINED SERIES-SHUNT CONTROLLERS
This could be a combination of separate shunt and series controllers, which are
controlled in a coordinated manner, or a Unified Power Flow Controller with series
and shunt elements. In principle, combined shunt and series controllers inject current
into the system with the shunt part of the controller and voltage in series in the line
with the series part of the controller. However, when the shunt and series controllers
are unified, there can be a real power exchange between the series and shunt
controllers via the power link
BASIC OPERATING PRINCIPLES
From the conceptual view point, the UPFC is a generalized synchronous
voltage source (SVS), represented at the fundamental (power system) frequency by
voltage phasor Vpq with controllable magnitude Vpq (0< Vpq < Vpqmax) and angle (0
<< 2), in series with the transmission line, as illustrated for the usual elementary
two-machine system (or for two independent systems with a transmission link
intertie) in Figure 4.1. In this functionally unrestricted operation, which clearly
includes voltage and angle regulation, the SVS generally exchanges both reactive and
real power with the transmission system. Since, as established previously, a SVS is
able to generate only the reactive power exchanged, the real power must be supplied
to it, or absorbed from it, by a suitable power supply or sink. In the UPFC
arrangement the real power exchanged is provided by one of the end buses (e.g., the
sending-end bus), as indicated in Figure 4.1.
In the presently used practical implementation, the UPFC consists of two
voltage-source converters. These back-to-back converters, labeled “Converter 1” and
Converter 2” in the figure 4.1, are operated from a common dc link provided by a dc
storage capacitor. As indicated before, this arrangement functions as an ideal ac-to-ac
power converter in which the real power can freely flow in either direction between
the ac terminals of the two converters, and each converter can independently generate
(or absorb) reactive power at its own ac output terminal.
Converter 2 provides the main function of the UPFC by injecting voltage Vpq
with controllable magnitude Vpq and phase angle in series with the line via an
insertion transformer. This injected voltage acts essentially as a synchronous ac
voltage source. The transmission line current flows through this voltage source
resulting in reactive and real power exchange between it and the ac system. The
reactive power exchanged at the ac terminal (i.e., at the terminal of the series insertion
transformer) is generated internally by the converter. The real power exchanged at
the ac terminal is converted into dc power which appears at the dc link as a positive or
negative real power demand.
The basic function of Converter 1 is to supply or absorb the real power
demanded by Converter 2 at the common dc link to support the real power exchange
POWER FLOW CONTROL
Actual flow on the line could be higher depending on
other prevailing system conditions. The UPFC, therefore, may be required to slightly
reduce the line loadings. Line reactive power flow and its direction will be monitored
to help maintain the dynamic reactive power margin of the shunt inverter.
The series power flow control becomes important during contingency
conditions. The control objective is to increase the line loading. This control is to be
activated as soon as any one of the line loadings exceeds 90% of their respective
emergency thermal ratings. The UPFC has to increase line loading until the critical
line loadings are reduced below the defined levels or the UPFC reaches its rating
limit.