Implementation of Unified Power Flow Controller (UPFC) for Improvement of Voltage Stability in a Congested Electric Network
Chapter One
Aim and Objective
Aim
To implement unified power flow controller (UPFC) for improvement of voltage stability in a congested electric network
Objective
To develop mathematical models for transmission systems and UPFC, which can to be blended together, coded, and used extensively.
To illustrate the controllable features of UPFC in active and reactive power flows in a transmission line.
To maintain the nodal voltage magnitudes in a power system in the limit for system security
CHAPTER TWO
LITERATURE REVIEW
The concept of voltage stability and reactive power compensation has been thoroughly examined in various books and other publications in the last few decades. In effect, numerous methods have been identified by different authors and researchers for reactive power compensation ranging from the conventional to the fast acting thyristor based compensating devices. This chapter looks at these methods in a way of making comparison between them.
The Concept of FACTS
Power electronics is an ubiquitous technology that has affected every aspect of electrical power networks, not just HVDC transmission but also alternating current (AC) transmission, distribution, and utilization.
In its most general expression, the FACTS concept is based on the substantial incorporation of power electronic devices and methods into the high-voltage side of the network, to make it electronically controllable.
Many of the ideas upon which the foundation of FACTS rests evolved over a period of many decades. Nevertheless, FACTS, an integrated philosophy, is a novel concept that was brought to fruition during the 1980s at the Electric Power Research Institute (EPRI), the utility arm of North American utilities. FACTS looks at ways of capitalising on the many breakthroughs taking place in the area of high-voltage and high current power electronics, aiming at increasing the control of power flows in the high voltage side of the network during both steady-state and transient conditions. The new reality of making the power network electronically controllable has started to alter the way power plant equipment is designed and built as well as the thinking and procedures that go into the planning and operation of transmission and distribution networks. These developments may also affect the way energy transactions are conducted, as high-speed control of the path of the energy flow is now feasible. Owing to the many economical and technical benefits it promised, FACTS received the distinctive support of electrical equipment manufacturers, utilities, and research organisations around the world.
Several kinds of FACTS controllers have been commissioned in various parts of the world. The most popular are: load tap changers, phase-angle regulators, static VAR compensators, thyristor-controlled series compensators, interphase power controllers, static compensators, and unified power flow controllers.
Voltage Stability
Voltage stability is the ability of a power system to maintain steady acceptable voltages at all buses in the system under normal operating conditions and after being subjected to a disturbance. A system enters a state of voltage instability when a disturbance, increase in load demand, or change in system condition causes a progressive and uncontrollable drop in voltage. The main factor causing instability is the inability of the power system to meet the demand for reactive power. The heart of the problem is usually the voltage drop that occurs when active power and reactive power flow through the inductive reactance associated with the transmission network [3]. These events may trigger loss of synchronism between generators, thereby pushing the system further towards instability.
Voltage Collapse
This phenomenon remains a major issue for power system networks. In the words of Carson Taylor, as quoted in [3], “A power system at a given operating state and subject to a given disturbance undergoes voltage collapse if post-disturbance equilibrium voltages are below acceptable limits.” Voltage collapse may be total (blackout) or partial. Lack of adequate reactive power resources in a power system is a major contributing factor to the process of voltage collapse. As loads in a power system increase, voltages across the network tend to decrease and reactive power losses increase as a result of high current the reactive loss is proportional to square of the current. This increased reactive power demand would be supplied by voltage regulating devices such as generators, static VAR compensators, or capacitors, if possible.
However, due to physical limitations, such devices cannot supply unlimited amounts of reactive power. Often, sustained load growth will result in some source of reactive power, or perhaps a number of such sources, reaching their physical limits. Once a reactive power source has reached its maximum limit, it can no longer regulate voltage. Therefore, sustained load growth results in an accelerated voltage decay, and hence an even greater reactive power requirement. This may force other voltage regulating devices to their limits, with a subsequent further acceleration in the rate of decline of voltages. This leads to loss of voltage in a significant part of the system and as described a voltage collapse phenomenon. The Institution of Electrical and Electronic Engineers, USA, (IEEE) defines voltage collapse and voltage instability: “Voltage collapse is the process by which voltage instability leads to loss of voltage in a significant part of the system.” A system enters a state of voltage instability when a disturbance, increase in load, or system change causes voltage to drop quickly or drift downward, and operators and automatic system control fail to halt the decay. The voltage decay may take just a few seconds but can also be prolonged of the order of 10-20 minutes [3]. If the decay continues unabated, steady state angular instability or voltage collapse will occur.
CHAPTER THREE
METHODOLOGY
The remit of this chapter is to study the methodology adapted in modeling and simulating the power system network and the UPFC models used in controlling the voltage the bus voltages in the system. It also gives a detailed mathematical models of solutions employed in the simulations.
One-line diagram of the power system used as the model for this project work is
shown in figure 3.1.
LOCATION OF UPFC
To improve the voltage stability level of the system, UPFC has to be placed at the proper locations. To determine the best location, a load flow study was performed to determine the weak buses in the system. There are many load flow study methods but Newton Raphson method has proved most successful owing to its strong convergence characteristics. Hence, it was used in this project for determining the steady state voltages at all the buses in the system.
Newton Raphson Load Flow Method [7] [4]
This approach uses iteration to solve the following set of nonlinear algebraic equations.
(3. 1)
where F represents the set of ‘n’ nonlinear equations, and X is the vector of ‘n’ unknown state variables.
The essence of the method consists of determining the vector of state variables X by performing a Taylor series expansion of F(X) about an initial estimate X (0):
higher-order terms (3. 2)
where J(X (0)) is a matrix of first-order partial derivatives of F(X) with respect to X,
termed the Jacobian, evaluated at X=X (0).
This expansion lends itself to a suitable formulation for calculating the vector of state variables X by assuming that X (1) is the value computed by the algorithm at iteration 1 and that this value is sufficiently close to the initial estimate X (0). Based on this premise, all high-order derivative terms in Equation (3.2) may be neglected. Hence
CHAPTER FOUR
SIMULATION AND RESULTS ANALYSIS
The project simulation as noted earlier was performed in MATLAB. The MATLAB scripts has is detailed in appendix A. This chapter presents the results obtained from the simulations and analyzed in line with the stated objective of the project.
Case 1: Newton-Raphson load flow carried out without the UPFC, considered as the normal condition. This is result is depicted in tables 4.1 and 4.2.
CHAPTER FIVE
CONCLUSION AND RECOMMENDATIONS
Elements of AC systems produce and consume two kinds of power: real power (measured in watts) and reactive power (measured in volt-amperes reactive, or var). Real power accomplishes useful work (e.g., running motors and lighting lamps). Reactive power supports the voltages that must be controlled for system reliability.
Voltage profile is improved by controlling the production, absorption and flow of reactive power throughout the network. Reactive power flows are minimized so as to reduce system losses.
Power flow solution is a solution of the network under steady state conditions subjected to certain constraints under which the system operates. The power flow solution gives the nodal voltages and phase angle as well power flows, given a set of power injections at all buses and specified voltages. Voltage regulation is achieved by controlling the production, absorption and flow of reactive power throughout the network. Sources and sinks of reactive power, such as shunt capacitors, shunt reactors, rotating synchronous condensers, UPFC’s and other FACTS devices are used for this purpose. The UPFC’s are versatile devices that controls almost all the controllable parameters in a power system using power electronic switching components.
This Project focused on the use of UPFC’s to increase active power flows in transmission lines as well as reactive power compensation. Newton- Raphson load flow method has been used for solving the power-flow equation. To study the effect of UPFC on power system, a modified power flow model of the UPLC was performed. UPLC was placed at between buses 3 and 4 and the modified load flow program was used to access the effect of UPFC on the system. The simulation was done in MATLAB. Load flow study of a five bus system was considered and the results showed that the voltage profile of the system improved and it was illustrated using plots. It was also evident that the voltage magnitude of that particular bus at which UPFC is placed was maintained at 1 p.u.
From the results obtained, it can be concluded that the UPFC has the most impart on voltage compensation as well power flows to the bus nearest to it while its effect depreciates as its distance from the bus increases.
Recommendations and Future Work
The completion of one research project opens the avenues for work in many other related areas. The following areas are identified for future work:
- The load flow study can be done on larger interconnected power system like IEEE 14 & IEEE 30 and IEEE 118 bus and even larger.
- IPFC, STATCOM and other FACTS controller can also be incorporated along with the UPFC and their effect on the system can be studied
- Optimal location of UPFC can be found out using Genetic Algorithm and fuzzy logic
- Economic Assessment of FACTS devices against other methods can be studied
References
- Sindekar, A & Parate, J. (2012). Reactive Power Control and Transmission loss reduction with realization of SVC and TCSC. (Electronic version). International Journal of Engineering Science and Technology. 4 (7), 3592 – 3600
- Enrique, A etal. (2004). FACTS Modelling and Simulation in Power Networks. India: Thomson Press Ltd, New Delhi
- Akwukwaegbu, I. & Okwe, G. (2013). Concepts of Reactive Power Control and Voltage Stability Methods in Power System Network. (Electronic version). Journal of Computer Engineering. 11 (2), 15 – 25
- . Kundur, P. (2010). Power System Stability and Control. India: Tata McGraw-Hill Companies
- Sankara, S.S (2010). Effects Of Flexible Ac Transmission System (FACTS) On The Performance Of Distance Protection Relays. (Electronic version)
- Gabriela, G. (2005). Flexible Alternating Current Transmission Systems. (Electronic version).
- Satyendra, S etal. (2014). Load Flow Analysis on STATCOM Incorporated Interconnected Power System Networks Using Newton Raphson Method. Journal of Electrical and Electronics Engineering. 9, (4). 61 – 68
