In a deregulated electricity market, the generation, transmission and distribution sectors are operated by different companies. This may give rise to congestion in transmission lines due to lack of co ordination between different sectors. It may also occur in a vertically integrated market but it can easily be taken care of since all the three sectors are controlled by one entity. Congestion can disrupt the demand and supply market. It can directly affect the profits of the market players. Without congestion market price will be uniform but with congestion the prices in some areas will increase and in other areas will decrease. Congestion changes the pre dispatch plan of generating units in order to meet the energy requirements with cheaper generating units reducing their output whereas expensive units increasing their outputs in the congested areas. This makes the energy market less efficient.
By congestion we mean that the transmission network is unable to contain all of the desired transactions due to violation of the system limits. These could be thermal, voltage, small signal stability limits etc. These limits are violated due to the disruptions in power flow in transmission lines. Therefore it becomes imperative to take measures that would prevent or get rid of congestion in transmission lines.
Let's have a look at a congestion example. Suppose that there is a region where the electricity supply becomes very cheap. This would result in an increasing demand as more and more people would want to go for that electricity putting a huge amount of load on the system. In order to meet the increasing demand, there would be heavy flow of power over the transmission lines. This would incur losses as well as the stability limits would be threatened resulting in congestion.
There are two ways to reduce congestion. One is the technical way, by using the FACTS devices and the transformer tap adjustments whereas the other one is related to the market e.g auctioning, re-dispatching etc.
1.2 Objectives
The objectives of this thesis project are as follows:
To Study the FACTS devices
To Identify the different problems due to congestion
To demonstrate the use of FACTS devices in managing congestion in pool market
To smoothen the Locational Marginal Prices (LMP) and maximize the social welfare
Economic consideration in placement of FACTS
1.3 Scope of Thesis
The thesis will start with the literature review on optimal power flow, Power flow constraints, FACTS devices, power flow control, how FACTS are used to increase the transmission capacity in order to deal with the congestion management in transmission systems. Electricity market model will be generated and by using the FACTS devices the LMP will be smoothened and the social welfare will be maximized.. In the end based on the results, recommendations and conclusions would be given accordingly.
Literature Review
2.1 Optimal Power Flow
In 1960 some extensions were made in the conventional economic load dispatch problem called optimal power flow (OPF). OPF simulation is used to find the optimal solution to a problem. A system can be modeled by an objective function.
This objective function could be minimizing the production cost, minimizing the transmission losses etc. It depends on what type of market the objective function represents. The OPF is used to control different variables in such a way that the objective function can be minimized resulting in the increase in overall efficiency of the system.
2.2 Power System Constraints
With the growth of electric utilities all over the world, the transmission systems are being pushed closer to their stability limits whilst more and more quality of power is being demanded. The transmission of power over an area or a region may be limited due to certain power constraints such as :
Steady state power transfer limit
Voltage stability limit
Dynamic stability limit
Transient stability limit
Oscillations damping limit
Thermal limit
These are called the bottlenecks as they limit the amount of power flowing through transmission lines.
2.3 Congestion Management
Congestion management is a major problem faced by the independent system operators (ISO) in the deregulated electricity environment as it has a huge negative impact on market prices and the market trade resulting in disruptions and monetary penalties under some condition. FACT devices like TCSC, TCPAR (more on this in the next chapter) can help to reduce congestion, smoothen locational marginal price and maximize the social welfare. [1] The distinct congestion management systems widely being employed are
Nodal pricing method
Zonal pricing method
These are the congestion management models that are based on optimal power flow algorithm [2].
In [3], two approaches dealing with the management costs are studied. The first approach is the nodal pricing which forms the framework for the pool model. The second approach is based on cost allocation procedures proposed for the bilateral model. Pool and the bilateral model are compared. The bilateral model would easily have been the obvious choice for customers if it hadn't been for the special characteristics of electricity. These characteristics produce two problems. The first problem relates to the presence of transmission constraints whereas the second one relates to transmission losses.
The pool adjusts these special characteristics of electricity in trading process. The locational aspects of pool model are based on theory of nodal spot pricing. In [4],zonal congestion management has been presented. Importance of real and reactive power dispatches in alleviating the congestion management has been highlighted. Congestion management in pool model is formulated as a Non Linear Programming (NLP) and it is solved by using the fitness distance ratio particle swarm intelligence.
In [5], coordination between the pool and the bilateral trading in congestion dispatch in hybrid market model is investigated. A mathematical model of coordinated congestion dispatch is created which relates the rate of pool purchasing cost and the curtailment rate of contracts to the priority of different trades in the hybrid electricity market.
2.4 FACTS
The conventional ways of enhancing power system control were to use Series capacitor to control the impedance, switched shunt capacitor and reactor to control the voltage, phase shifting transformer to control the angle etc. Now in the modern world FACTS controllers are being used.
The concept of FACTS was first introduced in 1990. FACTS stand for Flexible AC Transmission Systems. FACTS is defined by the IEEE as
"a power electronic based system and other static equipment that provide control of one or more AC transmission system parameters to enhance controllability and increase power transfer capability[2] .
The FACTS devices recently became popular due to their efficient power controllability in power transmission lines. There are a number of FACT devices that are currently being used each having their pros and cons. The power is generated and distributed to the consumers over transmission networks. The power flow is related to the impedance of these transmission lines. Greater the impedance lesser will be the power flow. When there is a surge in demand, the power lines with low impedance will get overloaded before the high impedance ones limiting the power that can be transmitted through the high impedance lines. There are two ways through which the rising demand can be met. They are :-
Build more transmission lines in order to increase the transmission capacity
Use the FACTS devices in the existing transmission network in order to increase their transmission capacity by controlling the power flow.
The former one is not feasible since it requires a huge investment and has some environmental concerns associated with it. The latter one is what we would be looking at.
The basic applications of FACT devices are:
They increase the existing transmission line power capability
Control the power flow
Reduction in reactive power benefiting generating companies because lesser the reactive power more will be the real power. The customers are charged for the real power they consume.
To increase the stability
Voltage control
Increase the security of the system.
In [6], congestion management methods are considered. The first is the re dispatch of the generation schedule formed by the market. Congestion management cost is evaluated using the generation schedule. The installation and the operation of flexible alternating current transmission systems (FACTS) devices in the transmission lines for the purpose of countering congestion is also considered. A cost benefit analysis is proposed which justifies the economic feasibility of FACTS devices. After carrying out tests on IEEE 14 bus system, this paper concludes that when FACTS devices are employed in transmission lines at optimal positions they were able to reduce the amount of power to be re dispatched as well as the cost of congestion.
In [7], congestion caused by the increasing number of bilateral contracts is alleviated by using two methods. Cost free and non cost free means. The former deals with the phase shifters, FACTS devices. The system with and without FACTS devices is compared. The optimal power flow (OPF) result shows that the individual power transactions suffer less curtailment with FACTS. The use of FACTS device called thyristor controlled series compensator (TCSC) is discussed. The implementation of FACTS devices in bilateral dispatch framework to maintain system stability and security is proposed.
The development of simple and efficient models for optimal location of FACTS devices to tackle congestion management by the optimal control of their parameters is proposed in [8]. In a congested environment, the sensitivity factors can effectively be used to determine the optimal location of FACTS devices, TCSC and TCPAR. However this paper does not deal with the optimal location of other FACTS devices such as SVC, STATCOM.
In [9], steady state models of FACTS devices are formulated based on PIM and incorporated into optimal power flow (OPF). By doing this, FACTS tend to get flexible due to greater degree of freedom for the OPF solution space. The problem is solved using sequential quadratic programming (SQP) in MatLab. The results clearly show that the FACTS devices greatly enhance the voltage stability as well as the power transfer capability.
2.5 Controllability of Power Flow
This section will shed light on the parameters that affect the power flow in transmission lines. This is essential in understanding the working of FACTS for congestion management.
To keep things simple consider a two bus system in figure 2.1
Figure 2.1 Two Bus System
The transfer of power between the two points is given by
(2-1)
Where is is Power received
is Voltage sent
is voltage received
is characteristic impedance of the line
eristic impedance of the line system in figure 2.1
power flow in transmission lines. This is essential in understand is transmission angle
We have assumed that all the line losses are neglected.
For a short line this equation can be simplified to
(2-2)
Since
Equation 2-2 simply states that the power flowing through a transmission line depends on three things, voltage, angle and impedance. The most important conclusion that can be drawn from this equation is that higher the impedance of a power line lesser will be the power flow through it. Power flow is always through those lines that have got low impedance.
Chapter 4
Electricity Market
This chapter talks about the deregulated electricity market. How this market structure came into existence, what advantages and disadvantages does this current structure has is discussed in the deregulated electricity market section. It then sheds light on the different market models that are adopted by various countries. The three market models that exist and discussed in this chapter are Pool model, Bilateral Model and Hybrid Model. An objective function for the pool market is formulated. Here the objective is the social welfare maximization.
Methodology
The first step is to review the different kinds of FACT devices. Get familiar with their operation. The next step involves the usage of these FACT devices to alleviate congestion in power lines. After that a market model would be generated that would help us to study the effects of FACTS devices on the market parameters such as LMP, social welfare etc. A software will be used to carry out this task. The results would be shown with the help of graphs
Categories of FACTS Devices
FACTS devices can be classified into the following categories.
Series Compensation
In series compensation, the FACTS devices are connected in series in transmission lines to inject voltage. It works as a controllable voltage source. These series FACTS devices can be a variable impedance e.g a capacitor. Series inductance occurs over long transmission lines causing a voltage drop. To compensate that voltage drop, the series capacitors are coupled.
Shunt Compensation
In shunt compensation, FACTS devices are connected in parallel to the transmission lines. These devices inject current at the point of connection in the transmission lines. They act as a controllable current source.
Shunt compensation if of two types:-
Shunt capacitive compensation
This method is implemented when there is a need to improve the power factor. Whenever there is an inductive load connected to the power system, it's going to result in a lagging current causing the power factor to lag. This is not desirable since the low power factor relates to high amount of current drawn by the load resulting in huge energy losses. A shunt capacitor is connected that draws current leading the voltage source which helps improve the power factor.
Shunt inductive compensation
This method is used when there is a very low load connected at the receiving end. This causes a very low current to flow through the transmission lines. Shunt capacitance in the transmission line causes voltage amplification. This phenomenon is known as Ferranti effect. The voltage at the receiving end becomes greater than the voltage at the sending end. To compensate, shunt inductors are connected across the transmission lines.
Combined Series-Series Compensation
Two FACTS devices are connected separately in series in a multi line transmission system. Each of the devices provides independent series reactive compensation for each of the lines.
Combined Series-Shunt Compensation
In this configuration, a combination of a series and shunt FACTS devices are used. The shunt device injects current into the system whereas the series device injects voltage. In order words this kind of configuration allows to have an independent control over real and real and reactive power flowing through the transmission lines.
Examples of Series Compensation
Static Synchronous Series Compensator (SSSC)
Thyristor Controlled Series Capacitor (TCSC)
Thyristor Controlled Series Reactor (TCSR)
Thyristor Switched Series Capacitor (TSSC)
Thyristor Switched Series Reactor (TSSR)
Examples of Shunt Compensation
Static Synchronous Compensator (STATCOM)
Static Var Compensator (SVC)
Example of Combined Series-Series Compensation
Interline Power Flow Controller (IPFC)
Example of Combined Series-Shunt Compensation
Unified Power Flow Controller (UPFC)
Unified Power flow controller (UPFC) is the most powerful FACTS controller yet. It has the ability to control both the reactive and active power flow in transmission lines whereas the other FACTS controllers named above could either control real or active power flow. It is capable of controlling all the parameters such as voltage, impedance and angle that are responsible for the power flow. The most widely used FACTS devices are the ones that involve the thyristor technology.
We will study only two FACTS devices in detail since those are the ones that will be implemented for congestion management. Those two FACTS devices are:-
Static Synchronous Compensator(STATCOM)
Static Synchronous Series Compensator (SSSC)
Static Synchronous Compensator (STATCOM)
The static synchronous compensator (STATCOM) was previously known as static synchronous condenser (STATCON). It is based on a solid state synchronous voltage source that is just like an ideal synchronous machine without a rotating mass. These devices generate a set of sinusoidal voltages at fundamental frequencies with rapidly controllable amplitude and phase angles. Fig 4.1 shows the generalized structure of a voltage source converter. The input of a STATCOM is a DC voltage which it converts into an AC output voltage at a fundamental frequency in order to compensate for the reactive and active power needed by the system. Qref and Pref are the reference signals that control the amplitude of the output voltage V and the phase angle β, respectively.
Fig 4.1 Generalized synchronous voltage source
So by changing the Qref, in other words by varying the voltage the reactive power exchange between the inverter and the AC system can be controlled. If the amplitude of the output voltage is greater than that of the AC system voltage, the inverter produces reactive power for the AC system. If the amplitude of the output voltage is varied such that it becomes less than the AC system voltage, the inverter absorbs the reactive power and if both the AC system voltage and the output voltage is the same, there is no reactive power exchange.
By varying phase angles between the inverter output and AC system voltages, the real power exchange between the inverter and the AC system can be controlled. If the inverter output voltage leads the AC system voltage, real power is supplied to the AC system by the inverter. If the inverter output voltage lags the AC system voltage, the real power is absorbed from the AC system by the inverter.
Figure 4.2 shows the basic structure of STATCOM and it's V-I characteristics are shown in Figure 4.3.
By observing figure 3.3 it's clear that the controller can provide both reactive and inductive compensation and is able to control the output current between the specified maximum capacitive and inductive range independent of the AC system voltage. It can provide full capacitive output current at any practical voltage. It's more effective than SVC in providing transmission voltage support as well as improving the stability of the system since SVC can supply thinning output current with decreasing system voltage as judged by the designed maximum equivalent capacitive admittance. The comparison of STATCOM with SVC can expected to give a fifty percent reduction in the physical size of the installation. Another advantage of STATCOM over SVC is that it is able to support higher loads than the SVC can for the comparable MVAr rating.
The STATCOM may have an increased transient rating in both the inductive and capacitive operating regions, which can further enhance its dynamic performance. The conventional SVC were equipped with transient var absorption capability only. There was no way that they could transiently increase the var generation since the maximum capacitive current it can draw is austerely determined by the value of its maximum capacitance and the magnitude of the system voltage. The transient rating of the STATCOM relies on the characteristics of the power semiconductors used and the maximum junction temperature at which each of these devices can be operated. Moreover, STATCOM does not significantly change the existing system impedance, which is another advantage over the static var compensators (SVCs).
To cut the discussion short, STATCOM has better characteristics over SVC; the maximum reactive power output will not be affected by the voltage magnitude. Therefore, it exhibits constant current characteristics when the voltage falls below the limit. The steady state power exchange between the controller and the AC system is mostly reactive, as active power is only consumed to supply for the internal losses.
Fig 4.2 Basic structure of STATCOM
Figure 4.3 V-I characteristics
STATCOM can be controlled in two ways.
Phase control
In this technique the phase shift β is varied in order to control the STATCOM's output voltage magnitude.
Pulse Width Modulation (PWM)
This technique allows the independent control of the magnitude of the output voltage as well as the phase shift that represents angle of the output voltage. The control of DC voltage is separate from that of AC output voltage.
Synchronous Series Compensator (SSSC)
A synchronous voltage source that has a DC to AC inverter with gate turn off thyristor can be used for series compensation of transmission lines. SSSC is pretty similar to STATCOM as it involves DC capacitor fed voltage source input which generates a three phase voltage at fundamental frequency. This in turn is injected into the transmission line with the help of a transformer connected in series with the system.. What SSSC does is that it directly controls the current and indirectly controls the power flow by controlling the reactive power exchange between the SSSC and the AC system. It has got an advantage over another FACTS controller called TCSC, which is that SSSC doesn't suffer from the resonance problem that can be encountered in TCSC since it does not affect the impedance of the transmission lines that much.
Fig 4.4 Basic structure of SSSC
Figure 4.4 shows the basic structure of SSSC. It shows a voltage source that produces an appropriate at the fundamental AC system frequency in series with the line to cancel the voltage drop, Vc to some extent. The output of synchronous voltage source is locked with the lagging relationship to the current and it is then injected in series with the transmission line. If injected voltage magnitude equals that of the line current, a series compensation equivalent to that provided by a series capacitor at fundamental frequency is acquired.
Mathematically it can be represented as
where Vc is the compensating voltage phasor
Iac represents the line phasor
X is line impedence
k is called degree of series compensation
The main thing is to control the rms magnitude Iac of the AC current and the rms magnitude Vc of the controller's AC voltage phasor in other words to control the reactive power injected or absorbed by the controller. Assuming the reactance of coupling transfer is negligible, the AC output voltage can be directly controlled through the inverter voltage Vinv which can be controlled by charging or discharging the capacitor. In steady state if the controller is delivering reactive power, the phase shift β of the inverter voltage with respect to the AC current is -90 degrees. On the other hand if the controller is absorbing reactive power then the phase shift of the inverter voltage with respect to the AC current becomes 90 degrees. Hence we can change the DC voltage Vdc and the corresponding inverter output voltage Vin just by varying the phase shift parameter that would charge or discharge the capacitor.
Fig 4.5 Operation of SSSC
Figure shows the operation of SSSC. By assuming that the injected voltage in series with the line can be achieved if the DC energy storage has infinite capacity, the voltage phase angle can be chosen separately of the line current over 360 degrees with the magnitude that is between 0 and Vc max. This means that the synchronous voltage source must be able to generate and absorb both the real and reactive power. The controller is responsible for the production of the reactive power whereas the real power comes from the DC energy storage device. This storage device also generates a little active power that is just enough to overcome the controller losses.
Time Frame
Week
Objectives
1
2
3
4
5
6
7
8
9
10
11
12
13
Project proposal
FACTS implementation
Market model Simulation
FACTS in market model
Results
Analysis
Conclusion
Recommendations
