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voltage stability by compensating reactive power

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voltage stability by compensating reactive power by using shunt capacitor banks.

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voltage stability by compensating reactive power

  1. 1. 1 A REPORT ON STABILIZATION OF VOLTAGE BY COMPENSATING REACTIVE POWER BY M.SAI MANOBHIRAM 2012A3PS224H DURGA RAO GUNDU 2012A3PS255H MOHITH DEVATHI 2012A3PS166H UNDER THE SUPERVISION OF Dr. ALIVELU MANGA PARIMI ASSISTANT PROFESSOR, EEE DEPARTMENT BIRLA INSTITUTE OF TECHNOLOGY AND SCIENCE, PILANI HYDERABAD CAMPUS
  2. 2. 2 ACKNOWLEDGEMENT We would like to thank the Department of Electrical & Electronics, BITS Pilani Hyderabad Campus and in particular Dr. Alivelu Manga Parimi, Asst. Professor for giving us the opportunity to do this project and also for her guidance over the course of completing this project which helped us immensely. We would also like to thank Mr. Ramachandra and Gayatri mam for the support and encouragement during the project. ABSTRACT Voltage stability problems usually occur in heavily loaded systems. While the disturbance leading to voltage collapse may be initiated by a different kinds of contingencies, the underlining problem is an inherent weakness in the power system. This project will illustrate the basic issues related to voltage instability by considering the characteristics of transmission system and afterwards examining how we can improve voltage stability by using reactive power compensation devices. Main consideration in this project will be focused on delivering the reactive power directly to buses in a distributing system, by installing sources of reactive power. The reason is that transmission lines can be operated with varying load and nearly constant voltage at both ends if adequate sources of reactive power are available at both ends. Before these considerations, there will be the description of the voltage stability phenomena and ways of improving it, because only with the description and researches this project will be understandable and complete.
  3. 3. 3 Table of contents 1. ABSTRACT 03 2. BACKGROUND 04 3. OBJECTIVES 04 4. BASIC CONCEPTS AND REFERENCES 05 5. REACTIVE POWER &VOLTAGE CONTROL 05 6. WHY ONLY REACTIVE POWER? 06 7. ELEMENTS OF THE SYSTEM 07 8. WAYS OF IMPROVING 07 9. SHUNT CAPACITORS 08 10. POWER FACTOR CORRECTION 08 11. ADVANTAGES AND DISADVANTRAGES 09 12. RESULTS 09 13. CALCULATION OF SHUNT CAPACITANCE 10 14. WAVEFORMS 12 15. SVC 13 16. TCR 13 17. SIMULATION AND RESULTS 14 18. ADVANTAGES AND DISADVANTAGES 16 19. CONCLUSIONS 16 20. REFERANCES 16
  4. 4. 4 Background of the project: Voltage stability problems usually occur in heavily loaded systems. While the disturbance leading to voltage collapse may be initiated by a different kinds of contingencies, the underlining problem is an inherent weakness in the power system. Loss of power system stability may cause a total blackout of the system. It is the interconnection of power systems, for reasons of economy and of improved availability of supply across the broader areas that makes widespread disruptions possible. Current civilization is susceptible to case of power system blackout, the consequences of systems failure are social and economic as well. Even short disturbance can be harmful for industrial companies, because restarting of process might take several hours. In recent years, voltage instability has been responsible for several major network collapses. - New York Power Pool disturbances of September 22, 1970 - Florida system disturbance of December 28,1982 - French system disturbance of December 19, 1978 - Northern Belgium system disturbance of August 4, 1982 - Swedish system disturbance of December 27, 1983 - Japanese system disturbance of July 23, 1987 This project will illustrate the basic issues related to voltage instability by considering the characteristics of transmission system and afterwards examining how we can improve voltage stability by using reactive power compensation devices. Objectives of the project: Explaining basic principles of equipment used for improving voltage stability. Main consideration in this project will be focused on delivering the reactive power directly to bus (in this project we will consider one bus) in a distributing system, by installing sources of reactive power. The reason is that transmission lines can be operated with varying load and nearly constant voltage at both ends if adequate sources of reactive power are available at both ends. Before these considerations, there will be the description of the voltage stability phenomena and ways of improving it.
  5. 5. 5 Basic concepts and definitions: What is Voltage Stability? Power system stability may be defined as that property of a power system that enables it to remain in a state of operating equilibrium under normal operating conditions and to regain an acceptable state of equilibrium after being subjected to a disturbance. Traditionally, the stability problem has been the rotor angle stability, maintaining the synchronous operation between two or more interconnected synchronous machines. Instability may also occur without loss of synchronism, in which case the concern is the control and stability of voltage. A criterion for voltage stability is that, at a given operating condition for every bus in the system, the bus voltage magnitude increases as the reactive power injection in the same bus is increased. A system is voltage unstable if, for at least one bus, the bus voltage magnitude decreases as the reactive power injection in the same bus is increased. In other words, power system is voltage stable if voltages after disturbances are close to voltages at normal operating conditions. A power systems becomes unstable when voltages uncontrollably decrease due to outage of equipment, increment in load, decrement in production or in voltage control. Even though the voltage stability is generally the local problem, the consequences of voltage instability may have a widespread impact. The result of this impact is voltage collapse, which results from a sequence of contingencies rather than from one particular disturbance. It leads to really low profiles of voltage in a major part of power system. The main factors causing voltage instability are: • High reactive power consumption at heavy loads • Generating stations are too far from load centres. • Source voltages are too low. • Poor coordination between various control and protective systems • The inability of the power system to meet demands for reactive power in the heavily stressed system to keep voltage in the desired range. Reactive Power and Voltage Control: For efficient and reliable operation of power system, the control of voltage and reactive power should satisfy the following objectives • Voltages at all terminals of all equipment in the system are within acceptable limits • System stability is enhanced to maximize utilization of the transmission system • The reactive power flow is minimized so as to reduce RI2 and XI2 losses. This ensures that the transmission system operates mainly for active power.
  6. 6. 6 Why only reactive power?? i.e., Voltage drop depends on Q. if reactive power flows over the transmission line, there will be a voltage drop. Elements of the system, which are producing and absorbing reactive power Loads- A typical load bus supplied by a power system is composed of a large number of devices. The composition changes depending on the day, season and weather conditions. The composite characteristics are normally such that a load bus absorbs reactive power. Both active and reactive powers of the composite loads vary due to voltage magnitudes. Loads at low-lagging power factors cause excessive voltage drops in the transmission network. Industrial consumers are charged for reactive power and this convinces them to improve the load power factor. Underground cables- They are always loaded below their natural loads, and hence generate reactive power under all operating conditions. Overhead lines- Depending on the load current, either absorb or supply reactive power. At loads below the natural load, the lines produce net reactive power, on the contrary, at loads above natural load lines absorb reactive power.
  7. 7. 7 Synchronous generators- Synchronous generators can generate or absorb reactive power depends on the excitation. When overexcited they supply reactive power, and when under excited they absorb reactive power. Compensating devices- They installed in power system to either supply or absorb reactive power. Ways of improving voltage stability and control: 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. The devices used for these purposes may be classified as follows: • Shunt capacitors • Series capacitors • Shunt reactors • Synchronous condensers • SVC (Static Var Compensator) • STATCOM (Static synchronous Compensator) 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. The reactive power absorbed or supplied by them is automatically adjusted so as to maintain 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. NOTE: But in this project our main concentration is on Shunt compensation (by capacitive banks) and SVC (by using thyristors and PWM blocks).
  8. 8. 8 SHUNT CAPACITORS: 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. 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 XI2 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. 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 POWER FACTOR CORRECTION The initial reactive power we have is VA uncorrected. By using shunt compensation we make power factor to unity. (i.e. phi2 is nearly equal to 0)
  9. 9. 9 Advantages  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. Disadvantages  The biggest disadvantage of shunt capacitors is that the reactive power output drops with the voltage squared.  During the severe voltage decays these devices are not efficient enough.  If voltage collapse results in system breakdown, the stable parts of the system may experience damaging over voltages immediately following separation. Results (Matlab implementation): Transmission system without any compensation
  10. 10. 10 System Data Electrical Component Parameter Absolute measurement Voltage Source Type Single phase Frequency 50Hz Voltage 132KV(RMS) Resistance 2.645 Ω Inductance 70.2mH Line conductor resistance 5.2Ω Inductance 138mH Capacitance 1.934µF Load Active Power(P) 75MW Reactive Power 20MVAR Power Factor 0.96623 Nominal voltage 132KV(RMS) Nominal frequency 50Hz Uncompensation Results Sending end current 506.34A(RMS) Load Current 526.9.76A(RMS) Sending end voltage 132KV(RMS) Load voltage 118.276KV(RMS) Calculation of calculation of capacitance: Compensation Capacitance can be calculated by using the formula Here Q can be calculated by Q = (reactive power by compensating device(Qc)) + (reactive power drop across transmission line) + ( a correction factor of 8-9% of the load rated reactive power)
  11. 11. 11 E is the compensated voltage By taking above results into consideration we get Qc =19323575.1VAR; Reactive power drop across Transmission line = 11665070.64VAR Q = 32998645.74VAR By applying above formulas for calculation of capacitance we get C=6.06µF Simulation Circuit (transmission system after compensation): Compensation results: Sending end current 506.34A(RMS) Load Current 587.74A(RMS) Sending end voltage 132KV(RMS) Load voltage 131.932KV(RMS)
  12. 12. 12 Waveforms of voltage and current before and after compensation respectively Before compensation After compensation
  13. 13. 13 Static Var Compensator (SVC): Static var compensators are shunt-connected static generators or/and absorbers whose outputs are varied so as to control specific parameters of an electric power system. SVCs overcome the limitation of mechanically switched shunt capacitors or reactors. Advantages include fast, precise regulation of voltage and unrestricted, transient free capacitor switching. The basic elements of SVCs are capacitor banks or reactors in series with a bidirectional thyristors. Basic types of SVCs: • Saturated reactor (SR) • Thyristor-controlled reactor (TCR) • Thyristor-switched capacitor (TSC) • Thyristor-switched reactor (TSR) • Thyristor control transformer (TCT) • Self- or line-commutated converter (SCC/LCC) NOTE: In this project we considered SVC type as Thyristor-controlled reactor (TCR). The Thyristor-Controlled reactor (TCR): An elementary single-phase thyristor-controlled reactor TCR consists of fixed reactor of inductance L, and a bidirectional thyristor valve. Thyristor-controlled reactor The thyristor conducts on alternate half cycles of the supply frequency depending on the firing angle α. The magnitude of the current in the reactor can be varied continuously by this method of delay angle control form maximum (α=0) to zero at (α=90). The adjustment of current in the reactor can take place only once in each half cycle, in the zero to 90o interval. The ability of the Thyristor-controlled Reactor (TCR) to limit current is a vital part of controlling power flow. The current is controlled by the firing angle (_), which at 0º the switch is permanently closed, then slowly limits current as _ increases to 180° where current is then zero. Limiting the current ultimately limits the reactive current which results in how much reactive power can be added to or subtracted from the system.With the SVC,when α =0 the current passes fully through the inductor bypassing the capacitor. Yet as α increase to
  14. 14. 14 180°, current will be forced to slowly pass through the capacitor, and thus raise the lagging power factor of the system. Therefore the firing angle α may be directly related to how much power factor correction we need for the system. Simulation and results Circuit basically look like System data Electrical Component Parameter Absolute measurement Voltage Source Type Single phase Frequency 50Hz Voltage 132KV(RMS) Resistance 2.645 Ω Inductance 70.2mH Line conductor resistance 5.2Ω Inductance 138mH Capacitance 1.934µF Load Active Power(P) 75MW Reactive Power 20MVAR Power Factor 0.96623 Nominal voltage 132KV(RMS) Nominal frequency 50Hz PID controller Proportional constant 5 Integrational constant 0.01 PWM generator Carrier frequency 300Hz
  15. 15. 15 Calculation of Inductance Voltage current Power Power factor 118.276KV 526.976A 75MW 0.96623 Power factor angle = 14.931o S=VI*= (118.276)526.976L (14.931) = 62328.6134=44443.16396+i (43699.67073) Imaginary part is XL= 43699.67073 By equating it to XL =2πf*L We get L=0.2H The capacitance that we got in the shunt compensation result can be used here After substituting the above results and compensating using SVC compensator we got the simulation result as Sending end current 506.34A(RMS) Load Current 588.02A(RMS) Sending end voltage 132KV(RMS) Load voltage 131.996KV(RMS) Current and Voltage waveforms:
  16. 16. 16 Advantages • The main advantage of SVCs over simple capacitor bank compensation schemes is their near-instantaneous response to changes in the system voltage • Dynamic compensation can be achieved by SVC Disadvantages:  However, static VAR compensators are more expensive than Capacitor banks Conclusions: In this project we have briefly studied the construction and working of various reactive power compensating devices (ex capacitor banks and SVC) in order to stabilize the voltage and then the same were implemented in matlab and then simulated. The achievement of ours in this project is theoretical values and simulated results are closely matched. To conclude, since there is huge increase in load demand and it is very hard to build new transmission lines and generation plants, the existing power network has to be utilized more efficient and hence voltage stability is becoming very important issue. In this project we tried to introduce methods which can improve utilization of exciting network and voltage stability. References  “Power system stability and control” Prabha Kundur ISBN 0-07-035958-X  “Power system voltage stability” Carson W. Taylor ISBN 10-0071137084  “Power system analysis” Hadi Saadat ISBN 0-07-116758-7  “Shunt versus Series compensation in the improvement of Power system performance by Irinjila Kranti Kiran ,Jaya Laxmi.A” INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGUL Volume 2, No 1, 2011  “A Small Scale Static VAR Compensator for Laboratory Experiment” 2nd IEEE International Conference on Power and Energy (PECon 08), December 1-3, 2008, Johor Baharu, Malaysia.  “A New Approach to Reactive Power Calculation of the Static VAr Compensator” Alexander Osnach Department of Optimization of Electricity Supply Systems, Institute of Electrodynamics NAS, Kiev, Ukraine.  “Controlled sources of reactive power used for improving voltage stability” Institute of Energy Technology Electrical Power Systems and High Voltage Engineering Aalborg University Aalborg

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