SeminarOn STATIC SYNCHRONOUS COMPENSATOR (STATCOM) SUBMITTED IN PARTIAL FULFILLMENT OF REQUIREMENT OF THE DEGREE OF B.Tech – BACHELOR OF TECHNOLOGY IN ELECTRICAL ENGINEERING Supervised by Dr. G.K. Joshi Head Submitted by Dimpal Soni Enroll. No. : - 12/22526 Roll No. :B.Tech (Electrical Eng.) DEPARTMENT OF ELECTRICAL ENGINEERING M.B.M. ENGINEERING COLLAGE JAI NARAYAN VYAS UNIVERSITY JODHPUR 2014 Page | 1 ACKNOWLEDGEMENT By the blessings of “Lord Shiva”, the academic outcome in the form of a seminar work on “STATIC SYNCHRONOUS COMPENSATOR (STATCOM)” could take the shape of reality. It is privilege for me to express my sincere gratitude towards my esteemed guide without the support of whom, this would have been very difficult for me to bring out this seminar in this form. I m grateful to Dr. G.K. Joshi (Head of Department) for all the valuable guidance, constant moral encouragement extended at his end. I like to remember the motivation initiated by My Father Shree Brijesh Soni and My Mother Smt. Meena Soni whose love and latent blessings are the basis for me to bring out this seminar. I am thankful to my friend Deepak Soni for his constant encouragement and all those who helped me directly or indirectly in my endeavor. This acknowledgement is intended to be a thanks giving gesture to all those people involved directly or indirectly with my work. Wednesday, November 13, 2013 Dimpal Soni Enroll. No. : - 12/22526 Roll No.: B.Tech (Electrical Eng.) Department of Electrical Engineering J.N.V. University Jodhpur Page | 2 CERTIFICATE This is to certify that Miss Dimpal Soni, B.tech. Scholar in Electrical engineering bearing Roll no. 111103365 & Enroll. No. 12/22526 has carried out her seminar on “STATIC SYNCHRONOUS COMPENSATOR (STATCOM)” under my supervision. The work presented in this seminar has not been submitted elsewhere for award of any other degree or diploma. Dr. G. K. Joshi Supervisor Head Deptt. Of Electrical Eng. J.N.V. University, Jodhpur Counter Signed by Prof. Manoj Kumar Bhaskar Deptt. Of Electrical Engineering J.N.V. University, Jodhpur Page | 3 LIST OF TABLES Table No. Particulars Page No. 1 Partial derivatives of STATCOM model 33 Page | 4 LIST OF FIGURES Fig. 1.3 SVC building blocks and voltage / current Characteristic 6 1.4 SVC Outlook 6 1.5 SVC using a TCR and FC 7 1.1 Reactive power generation by a STATCOM 10 2.6 Comparison of the loss characteristics of TSC–TCR.9 V-I characteristic of a STATCOM 18 2.4 Voltage controlled block diagram of STATCOM 12 2.1 Operational Limits of Transmission lines for different voltage levels 2 1. Particulars Page No.7 Two machine system with STATCOM 15 2. No.3 Current controlled block diagram of STATCOM 11 2.10 STATCOM structure and voltage / current characteristic 19 Page | 5 .2 STATCOM operating in inductive or capacitive modes 11 2.8 Transmitted power versus transmission angle characteristic of a STATCOM 17 2.5 Static Synchronous Compensator 14 2. TCR–FC compensators and Synchronous condenser 7 1.7 SVC of combined TSC and TCR type 8 2.2 Overview of Major Facts Devices 3 1.6 Waveform for Operation of Statcom 15 2. 12 STATCOM Equivalent Circuit 21 2.4 Principle configuration of an UPFC 26 3.1 TCSC Circuit and Characteristics 23 3.3 Operational diagram of a DFC 25 3.13 Substation with a STATCOM 21 3.2.2 Principal configuration of DFC 24 3.1 Thevenin Equivalent Circuit Diagram of STATCOM: (a) STATCOM Schematic Diagram. (b) STATCOM Equivalent Circuit 30 Page | 6 .5 UPFC functional scheme 27 4.11 6 Pulses STATCOM 20 2. 4 BASIC CONFIGURATION AND PRINCIPLE OF OPERATION 13 2.3.3 Voltage Controlled STATCOM 10 10 11 12 2.3.1 Shunt Devices 1.2 FACTS DEVICES 1.6 SVC USING A TCR AND TSC 8 Chapter 2 STATIC SYNCHRONOUS COMPENSATOR (STATCOM) 9-21 2.6 STATCOM V-I CHARACTERISTIC 18 2.2 Current Controlled STATCOM 2.1 SVC of the FC/TCR type 6 7 1.5 SVC USING A TCR AND AN FC 1.1 INTRODUCTION 1 1.2 STRUCTURE OF STATCOM 9 2.4.4 CONFIGURATION OF FACTS DEVICES 1.5.2 SVC 4 4 4 1.3 MAJOR FACTS DEVICES 3 1.3.1 Two Modes of Operation 2.Chapter 1 INTRODUCTION 1-8 1.5 CHARACTERISTICS OF STATCOM 15 2.4.3 CONTROL OF STATCOM 2.1 Facts for Transmission System 1 2 1.2.1 INTRODUCTION 9 2.7 FUNCTIONAL REQUIREMENTS OF STATCOM 18 Chapter 3 OTHER SERIES AND SHUNT DEVICES 22-28 Page | 7 . 1 OPERATING PRINCIPLE OF UPFC 26 26 Chapter 4 STATIC SYNCHRONOUS COMPENSATOR POWER FLOW MODEL 29-33 4.1 STATCOM POWER FLOW MODEL 4.2 TCSC 3.4 UNIFIED POWER FLOW CONTROLLER 3. CONCLUSION AND FUTURE WORK 34-35 5.3.3 CONCLUSION 34 35 References……………………………………………………………………36 Page | 8 .1 SERIES DEVICES 22 3.1 (TSC / TSR) 23 24 3.2 SCOPE FOR FUTURE RESEARCH 5.2.4.1 APPLICATIONS OF STATCOM 34 5.1 Advantages 22 23 3.3.3 NEWTON-RAPHSON-ALGORITHM 29 31 31 Chapter 5 APPLICATIONS.3 DYNAMIC POWER FLOW CONTROLLER 3.2 LINEARISED POWER EQUATION 4. Several FACTS-devices have been introduced for various applications worldwide.2 FACTS DEVICES:Flexible AC Transmission Systems. got in the recent years a well known term for higher controllability in power systems by means of power electronic devices.1 INTRODUCTION:Flexible AC transmission system (FACTS) controllers are power electronics based controllers. A number of new types of devices are in the stage of being introduced in practice. 1. FACTS-devices provide a better adaptation to varying operational conditions and improve the usage of existing installations. Of all the VSC the most widely used is the STATCOM. There are mainly two models of STATCOM which have well tested in power systems. which is a shunt type controller. the most advanced type is the controller that employs Voltage Sourced Converter (VSC) as synchronous sources. The PIM models the STATCOM as shunt voltage source behind an equivalent reactance or impedance.CHAPTER: 1 INTRODUCTION 1. the Static Series Compensator (SSSC). Computation and control of power flow for power systems embedded with STATCOM appear to be fundamental for power system analysis and planning purposes. The CIM STATCOM has a current source connected in shunt the bus for voltage magnitude control. for instance like upgrades or additions of substations and power lines. This steady state power injection model of STATCOM has proved reliable when incorporated in power systems and is well documented. Representative of the VSC type FACTS controllers are the Static Synchronous Compensator (STATCOM). The basic applications of FACTS-devices are: • Power flow control. Page | 9 . bus voltage magnitude and power flow along the transmission lines can be more flexibly controlled. • Increase of transmission capability. The use of this STATCOM in power system simulators has therefore increased over the last one decade and is therefore adopted implementation in this work with the voltage expressed in rectangular coordinate. There are the Current Injection Model (CIM) and the Power Injection Model (PIM). Among the FACTS controllers. With the applications of FACTS technology. which is a series type controller and the Unified Power Flow Controller (UPFC). It can provide bus voltage magnitude control. which is also referred to as voltage source model (VSM). called FACTS. In most of the applications the controllability is used to avoid cost intensive or landscape requiring extensions of power systems. Power flow studies incorporating STATCOM requires accurate model in solution algorithms. a combined series-shunt type controller. • Flicker mitigation.• Voltage control.1 shows the basic idea of FACTS for transmission systems. It can be seen that with growing line length. • Power conditioning. The power electronic allows very short reaction times down to far below one second. Voltage and stability limits shall be shifted with the means of the several different FACTS devices. voltage or impedance controllers.1 FACTS FOR TRANSMISSION SYSTEM:Figure 1. the opportunity for FACTS devices gets more and more important. Page | 10 . The usage of lines for active power transmission should be ideally up to the thermal limits. • Stability improvement.2. • Interconnection of renewable and distributed generation and storages. • Power quality improvement. series compensation or phase shift control. • Reactive power compensation. The influence of FACTS-devices is achieved through switched or controlled shunt compensation. The devices work electrically as fast current. 1. For the FACTS side the taxonomy in terms of 'dynamic' and 'static' needs some explanation.3 MAJOR FACTS DEVICES:The development of FACTS-devices has started with the growing capabilities of power electronic components. Figure 1. Therefore most of the FACTS-devices can equally be static and dynamic. Page | 11 .2 shows a number of basic devices separated into the conventional ones and the FACTS-devices. The overall starting points are network elements influencing the reactive power or the impedance of a part of the power system. The term 'dynamic' is used to express the fast controllability of FACTS-devices provided by the power electronics.1. The term 'static' means that the devices have no moving parts like mechanical switches to perform the dynamic controllability. This is one of the main differentiation factors from the conventional devices. Devices for high power levels have been made available in converters for high and even highest voltage levels. 4. Voltage Source Converters provide a free controllable voltage in magnitude and phase due to a pulse width modulation of the IGBTs or IGCTs. distribution and industrial networks are: Page | 12 .1 Shunt Devices: The most used FACTS-device is the SVC or the version with Voltage Source Converter called STATCOM. The FACTS-devices contain these elements as well but use additional power electronic valves or converters to switch the elements in smaller steps or with switching patterns within a cycle of the alternating current. Therefore special designs of the converters are required to compensate this. These shunt devices are operating as reactive power compensators. inductance or capacitance together with transformers.2 contains the conventional devices build out of fixed or mechanically switch able components like resistance. The disadvantage is that with an increasing switching frequency. The main applications in transmission. High modulation frequencies allow to get low harmonics in the output signal and even to compensate disturbances coming from the network.4 CONFIGURATION OF FACTS DEVICES: 1. The right column of FACTS-devices contains more advanced technology of voltage source converters based today mainly on Insulated Gate Bipolar Transistors (IGBT) or Insulated Gate Commutated Thyristors (IGCT). These valves or converters are well known since several years. They have low losses because of their low switching frequency of once a cycle in the converters or the usage of the Thyristors to simply bridge impedances in the valves. 1. The left column of FACTS-devices uses Thyristor valves or converters. the losses are increasing as well.The left column in Figure 1. • Compensation of Thyristor converters e. To reduce temporary over voltages b. Industry as well as commercial and domestic groups of users require power quality. in conventional HVDC lines. SVCs are also used 1.• Reduction of unwanted reactive power flows and therefore reduced network losses. in transmission systems a.2 SVC: Electrical loads both generate and absorb reactive power. • Compensation of consumers and improvement of power quality especially with huge demand fluctuations like industrial machines. at the extreme a voltage collapse. nor are interruptions of industrial processes due to insufficient power quality. Almost half of the SVC and more than half of the STATCOMs are used for industrial applications. To damp power oscillations in interconnected power systems Page | 13 . To damp sub synchronous resonances c. A rapidly operating Static Var Compensator (SVC) can continuously provide the reactive power required to control dynamic voltage oscillations under various system conditions and thereby improve the power system transmission and distribution stability. To damp power oscillations c. metal melting plants. The result can be unacceptable voltage amplitude variations or even a voltage depression.4. To increase active power transfer capacity and transient stability margin b. Applications of the SVC systems in transmission systems: a. Railway or underground systems with huge load variations require SVCs or STATCOMs.g. • Keeping of contractual power exchanges with balanced reactive power. To achieve effective voltage control In addition. railway or underground train systems. Flickering lamps are no longer accepted. 1. • Improvement of static or transient stability. Since the transmitted load varies considerably from one hour to another. the reactive power balance in a grid varies as well. The coordinated control of a combination of these branches varies the reactive power as shown in Figure. In arc furnaces a. in traction systems a. i. In HVDC systems a. The first commercial SVC was installed in 1972 for an electric arc furnace. Fig 1. To provide reactive power to ac–dc converters 4. To balance loads b. Air core reactors and high voltage AC capacitors are the reactive power elements used together with the Thyristor valves. To reduce voltage variations and associated light flicker Installing an SVC at one or more suitable points in the network can increase transfer capability and reduce losses while maintaining a smooth voltage profile under different network conditions. Page | 14 . The most important is the Thyristor valve. SVC installations consist of a number of building blocks. stack assemblies of series connected anti-parallel Thyristors to provide controllability. To improve voltage regulation 3.3 SVC building blocks and voltage / current characteristic In principle the SVC consists of Thyristor Switched Capacitors (TSC) and Thyristor Switched or Controlled Reactors (TSR / TCR).e.2. On transmission level the first SVC was used in 1979. Since then it is widely used and the most accepted FACTS-device. The step up connection of this equipment to the transmission voltage is achieved through a power transformer. In addition an SVC can mitigate active power oscillations through voltage amplitude modulation. To improve power factor c. 1.5. By changing the firing angle of the thyristor controlling the reactor from 90° to 180°.1 SVC of the FC/TCR type: Page | 15 . two or more FC (fixed capacitor) banks are connected to a TCR (thyristor controlled reactor) through a step-down transformer.5 SVC USING A TCR AND AN FC: In this arrangement. The rating of the reactor is chosen larger than the rating of the capacitor by an amount to provide the maximum lagging vars that have to be absorbed from the system. the reactive power can be varied over the entire range from maximum lagging vars to leading vars that can be absorbed from the system by this compensator.Fig.4 SVC Outlook 1.1.5 SVC using a TCR and FC 1. Fig. In view of the smaller rating of each capacitor bank. thus reducing the harmonics generated by the reactor. Triplex harmonics are canceled by arranging the TCR and the secondary windings of the step-down transformer in delta connection. Page | 16 . or separate thyristor-switched overload reactors must be employed. Further losses are high due to the circulating current between the reactor and capacitor banks. In those situations where harmonics have to be reduced further. Fig. In applications requiring overload capability. 1.6 Comparison of the loss characteristics of TSC–TCR. a small amount of FCs tuned as filters may be connected in parallel with the TCR. seventh. The capacitor banks with the help of series reactors are tuned to filter fifth. the rating of the reactor bank will be 1/n times the maximum output of the SVC. These harmonics are filtered in the following manner. and other higher-order harmonics as a highpass filter.1. TCR–FC compensators and synchronous condenser These SVCs do not have a short-time overload capability because the reactors are usually of theair-core type. TCR must be designed for shorttime overloading.6 SVC USING A TCR AND TSC:This compensator overcomes two major shortcomings of the earlier compensators by reducing losses under operating conditions and better performance under large system disturbances.The main disadvantage of this configuration is the significant harmonics that will be generated because of the partial conduction of the large reactor under normal sinusoidal steady-state operating condition when the SVC is absorbing zero MVAr. In Page | 17 . its capacitive or inductive output currents can be controlled independently from its terminal AC bus voltage.Fig. In the TSC–TCR scheme. due to the flexibility of rapid switching of capacitor banks without appreciable disturbance to the power system. The capital cost of this SVC is higher than that of the earlier one due to the increased number of capacitor switches and increased control complexity. and hence the transients in the system can also be avoided. CHAPTER: 2 STATIC SYNCHRONOUS COMPENSATOR (STATCOM) 2. STATCOM provides much faster response as compared to the SVC.7 SVC of combined TSC and TCR type When large disturbances occur in a power system due to load rejection. Because of the fast-switching characteristic of power converters. oscillations can be avoided. The LC circuit of the SVC in the FC compensator.1 INTRODUCTION:The STATCOM is a solid-state-based power converter version of the SVC. Operating as a shunt-connected SVC. there is a possibility for large voltage transients because of oscillatory interaction between system and the SVC capacitor bank or the parallel. 1. there is a tendency for STATCOM to inject capacitive power to support the dipped voltages. the concept of voltage source converters and the corresponding control techniques are illustrated. Moreover. the leakage inductances of the step-up power transformers can function as coupling reactors. weight. and cost reduction • Equality of lagging and leading output • Precise and continuous reactive power control with fast response • Possible active harmonic filter capability This chapter describes the structure. For example. the capacitor voltage does not change instantaneously. basic operating principle and characteristics of STATCOM. In a very-highvoltage system. In addition. therefore. Page | 18 . The main purpose of the coupling inductors is to filter out the current harmonic components that are generated mainly by the pulsating output voltage of the power converters. Therefore. a step-up coupling transformer. 2. in the event of a rapid change in system voltage. if the system voltage drops for any reason.addition. STATCOM effectively reacts for the desired responses.2 STRUCTURE OF STATCOM:Basically. STATCOM is very effective during the power system disturbances. much research confirms several advantages of STATCOM. and a controller. STATCOM is comprised of three main parts (as seen from Figure below): a voltage source converter (VSC). STATCOM is capable of high dynamic performance and its compensation does not depend on the common coupling voltage. These advantages compared to other shunt compensators include: • Size. 2. when the system voltage is higher than the converter voltage. it regards the STATCOM as a capacitive reactance and the STATCOM is considered to be operating in a capacitive mode. and a Thevenin reactance.3. Figure 3. the STATCOM regards the system as a capacitive reactance and the STATCOM is considered to be operating in an inductive mode Page | 19 . inductive mode and the capacitive mode.1 Two Modes of Operation There are two modes of operation for a STATCOM. connected to a system with a voltage source. 2. Hence. The STATCOM regards an inductive reactance connected at its terminal when the converter voltage is higher than the transmission line voltage.1 Reactive power generation by a STATCOM 2. Similarly.Fig. XTIEX_THVTH. the system regards an inductive reactance connected at its terminal.4 shows a simplified diagram of the STATCOM with a converter voltage source __1E and a tie reactance.3 CONTROL OF STATCOM The controller of a STATCOM operates the converter in a particular way that the phase angle between the converter voltage and the transmission line voltage is dynamically adjusted and synchronized so that the STATCOM generates or absorbs desired VAR at the point of coupling connection. from the system’s point of view. Hence. Fig.2 Current Controlled STATCOM Fig. 2. leads (THVE−1) by 90º. the reactive current component of the STATCOM. This dual mode capability enables the STATCOM to provide inductive compensation as well as capacitive compensation to a system. 2.. Basically the control system for a STATCOM consists of a current control and a voltage control. 2. it is in capacitive mode. when a typical inductive load is about 20% of the full load.4. looking at the phasor diagrams on the right of Figure 3. when1I. Inductive compensation of the STATCOM makes it unique.3. This inductive compensation is to provide inductive reactance when overcompensation due to capacitors banks occurs.2 STATCOM operating in inductive or capacitive modes In other words. it is in inductive mode and when it lags by 90º. and the capacitor banks along the transmission line provide with excessive capacitive reactance due to the lower load. This happens during the night.3 Current controlled block diagram of STATCOM Page | 20 . il. v1. vl. of the converter current is defined to be either positive if the STATCOM is emulating an inductive reactance or negative if it is emulating a capacitive reactance. An instantaneous threephase set of measured converter currents. The DC capacitor voltage. of the converter voltage with respect to the transmission line voltage. vDC. respectively. is compared with the desired reference value. I1q* and the error is passed through an error amplifier which produces a relative angle. 2. V1*. The phase angle. is dynamically adjusted in relation with the converter voltage. The droop factor. will regulate the line voltage. V1q. for the inner current control loop.3. θ. of the converter voltage is calculated by adding the relative angle. α. vla . α.3 Voltage Controlled STATCOM In regulating the line voltage. which is phase-locked to the phase a of the line voltage. The control scheme described above shows the implementation of the inner current control loop which regulates the reactive current flow through the STATCOM regardless of the line voltage. of the converter voltage and the phase – lock-loop angle. at BUS 1 is decomposed into its real or direct component. I1q*. (adjusted by the droop factor. θ. An instantaneous three-phase set of line voltages. and reactive or quadrature component. I1q. 2. is decomposed into its real or direct component.4 Voltage controlled block diagram of STATCOM Figure shows a voltage control block diagram of the STATCOM. The outer voltage control loop would automatically determine the reference reactive current for the inner current control loop which. I1q*. Kdroop. Kdroop) and the error is passed through an error amplifier which produces the reference current. V1d. an outer voltage control loop must be implemented. in turn. Fig. at BUS 1 is used to calculate the reference angle.Figure above shows the reactive current control block diagram of the STATCOM. is defined as the allowable voltage error at the rated reactive current flow through the STATCOM. I1d. and reactive or quadrature component. The quadrature component is compared with the desired reference value. An instantaneous three-phase set of measured line voltages. Page | 21 . θ1. The reference quadrature component. The modeled STATCOM using VSC topology is being used in the test system to supply reactive power to increase the transmittable power and to make it more compatible with the prevailing load demand. In this type of VSC. IGBTs or IGCTs) to synthesize a voltage V2 from a DC voltage source. Two VSC technologies can be used for the VSC. a diode has to be placed in series with each of the switches. When system voltage is low. When system voltage is high. which draws from or injects current into the system at the point of connection. variable source or a combination of these. the shunt connected FACTS device should be able to minimize the line over voltage under light load condition and maintain voltage levels under heavy load condition. As long as the injected current is in phase quadrature with the line voltage. This type of inverter uses sinusoidal Pulse-Width Modulation (SPWM) technique to synthesize a sinusoidal waveform from a DC voltage source with a typical chopping frequency of a few kilohertz. So. But the VSC topology is preferred because CSC topology is more complex than VSC in both power and control circuits. Harmonic voltages are cancelled by connecting filters at the AC side of the VSC. Thus. The DC link energy storage element in CSC topology is inductor where as that in VSC topology is a capacitor. Typically four three-level inverters are used to build a 48-step voltage waveform. Therefore Vdc has to be varied for controlling the reactive power. Variable shunt impedance connected to the line voltage causes a variable current flow and hence represents injection of current into the line. the fundamental component of output voltage is proportional to the voltage Vdc. In another type VSC is constructed with GTO-based square-wave inverters and special interconnection transformers. the efficiency of a CSC is expected to be lower than that of a VSC. One of them. the STATCOM generates reactive power (STATCOM capacitive). This almost doubles the conduction losses compared with the case of VSC. the shunt controller is therefore a good way to control the voltage at and around the point of connection through injection of reactive current (leading or lagging) alone or a combination of active and reactive current for a more effective voltage control and damping of voltage dynamics. The shunt controller is like a current source. The shunt controller may be variable impedance. This type of VSC uses a DC link voltage Vdc. Thus. it absorbs reactive power (STATCOM inductive). Thus modulation index has to be varied for controlling the reactive power injection to the transmission line. shunt connected FACTS device can be realized by either a VSC or a CSC. The VSC uses forced-commutated power electronic devices (GTOs. Output voltage is varied by changing the modulation index of the SPWM modulator. In CSC such as GTO (Gate Turn Off Thyristor) is used. VSC is constructed with IGBT/GTO-based SPWM inverters. Any other phase relationship will involve handling of real power as well. the shunt controller only supplies or consumes reactive power. Page | 22 . The variation of reactive power is performed by means of a VSC connected on the secondary side of a coupling transformer.2.4 BASIC CONFIGURATION AND PRINCIPLE OF OPERATION Basically. Special interconnection transformers are used to neutralize harmonics contained in the square waves generated by individual inverters. In steady state the voltage V has to be phase shifted slightly behind E in order to compensate for transformer and VSC losses and to keep the capacitor charged. On the reverse.5 Static Synchronous Compensator E is the line voltage of transmission line.The real power (P) and reactive power (Q) are given by: Fig. V is the generated voltage of VSC. Page | 23 . 2. the voltage V generated by the VSC is in phase with E (δ=0). Since we are using here a VSC based on SPWM inverters hence modulation index is varied for controlling the reactive power injection to the transmission line. If V is lower than E. so that only reactive power is flowing (P=0). if V is higher than E. Q is flowing from E to V (STATCOM is absorbing reactive power). X is the equivalent reactance of interconnection transformer and filters and δ is the phase angle of E with respect to V. A capacitor is connected on the DC side of the VSC acts as a DC voltage source. In steady state operation. Q is flowing from V to E (STATCOM is generating reactive power). Fig.7 Two machine system with STATCOM Page | 24 . 2. Fig. Using the variables defined in Figure below and applying Kirchoffs laws the following equations can be written.5 CHARACTERISTICS OF STATCOM The derivation of the formula for the transmitted active power employs considerable calculations. 2.6 Waveform for Operation of Statcom 2. The fact that Iq is shifted by 90◦ with regard to UR can be used to express Iq as Applying the sine law to the diagram in Figure below the following two equations result From which the formula for sin α is derived as Page | 25 . a formula for the current I1 is obtained as Where UR is the STATCOM terminal voltage if the STATCOM is out of operation. when Iq = 0.e. i.By equaling right-hand terms of the above formulas. Since this thesis only reflects on the voltage control and power increase. the requirements of the STATCOM would be further elaborated. Page | 26 . Fig. 2.The formula for the transmitted active power can be given as To dispose of the term UR the cosine law is applied to the diagram in Figure above Therefore. it is thus important to utilize these principles in accommodating shunt compensation to any system.8 Transmitted power versus transmission angle characteristic of a STATCOM With these concepts of STATCOM. Fig.2. operating in capacitive mode only.7 FUNCTIONAL REQUIREMENTS OF STATCOM:The main functional requirements of the STATCOM in this thesis are to provide shunt compensation. • Transient stability during disturbances in a system or a change of load.15 pu. 2. As can be seen.6 STATCOM V-I CHARACTERISTIC:A V-I characteristic of a STATCOM is depicted in Fig. The characteristic of a STATCOM reveals strength of this technology: that it is capable of yielding the full output of capacitive generation almost independently of the system voltage (constantcurrent output at lower voltages).9 V-I characteristic of a STATCOM 2.9 . the STATCOM can supply both the capacitive and the inductive compensation and is able to independently control its output current over the rated maximum capacitive or inductive range irrespective of the amount of ac-system voltage. the STATCOM can provide full capacitive-reactive power at any system voltage even as low as 0. • Voltage stability control in a power system.2. This compensation of voltage has to be in synchronism with the AC system regardless of disturbances or change of load. as to compensate the loss voltage along transmission. That is. This capability is particularly useful for situations in which the STATCOM is needed to support the system voltage during and after faults where voltage collapse would otherwise be a limiting factor. Page | 27 . in terms of the following. Fig. a lower investment cost and lower operating and maintenance costs. such as better dynamics. A STATCOM is build with Thyristors with turn-off capability like GTO or today IGCT or with more and more IGBTs. The performance for power quality and balanced network operation can be improved much more with the combination of active and reactive power. the STATCOM keeps its full capability. The next step in STATCOM development is the combination with energy storages on the DC-side. The STATCOM has a characteristic similar to the synchronous condenser. The static line between the current limitations has a certain steepness determining the control characteristic for the voltage. 2. In the distributed energy sector the usage of Voltage Source Converters for grid interconnection is common practice today. The advantage of a STATCOM is that the reactive power provision is independent from the actual voltage on the connection point. This can be seen in the diagram for the maximum currents being independent of the voltage in comparison to the SVC. that even during most severe contingencies. but as an electronic device it has no inertia and is superior to the synchronous condenser in several ways.• Direct voltage support to maintain sufficient line voltage for facilitating increased reactive power flow under heavy loads and for preventing voltage instability • Reactive power injection by STATCOM into the system The design phase and implementation phase (as presented in the next chapter) would refer to the theoretical background of STATCOM in providing the requirements In 1999 the first SVC with Voltage Source Converter called STATCOM (STATic COMpensator) went into operation. This means.10 STATCOM structure and voltage / current characteristic Page | 28 . fundamental frequency. then leading or capacitive VARS are produced. steady state basis. This is achieved by firing the GTO/diode switches in a manner that maintains the phase difference between the ac bus voltage ES and the STATCOM generated voltage VS. electronic equivalent of a synchronous condenser.11 6 Pulses STATCOM The three phases STATCOM makes use of the fact that on a three phase. Page | 29 . Es. The reactive power in each phase is supplied by circulating the instantaneous real power between the phases.STATCOMs are based on Voltage Sourced Converter (VSC) topology and utilize either GateTurn-off Thyristors (GTO) or Isolated Gate Bipolar Transistors (IGBT) devices. Vs. If the STATCOM voltage. and the instantaneous power entering a purely reactive device must be zero. Fig 2. A practical STATCOM requires some amount of energy storage to accommodate harmonic power and ac system unbalances. when the instantaneous real power is non-zero. The maximum energy storage required for the STATCOM is much less than for a TCR/TSC type of SVC compensator of comparable rating. If Vs is smaller then Es then lagging or inductive VARS are produced. (which is proportional to the dc bus voltage Vc) is larger than bus voltage. Ideally it is possible to construct a device based on circulating instantaneous power which has no energy storage device (ie no dc capacitor). The STATCOM is a very fast acting. This approach allows for simpler transformer topologies at the expense of higher switching losses. This staircase voltage can be controlled to eliminate harmonics. thus eliminating harmonics even further. Another possible approach for harmonic cancellation is a multi-level configuration which allows for more than one switching element per level and therefore more than one switching in each bridge arm. but will generally utilize more complex transformer topologies. which turn on and off the GTO or IGBT switch more than once per cycle. The ac voltage derived has a staircase effect. The 6 Pulse STATCOM using fundamental switching will of course produce the 6 Nth harmonics.13 Substation with a STATCOM Page | 30 .Fig. There are a variety of methods to decrease the harmonics. Pulse Width Modulated (PWM) techniques. can be used. a complete elimination of 5th and 7th harmonic current using series connection of star/star and star/delta transformers and a quasi 12 pulse method with a single star-star transformer. dependent on the number of levels. Fundamental switching of the GTO/diode once per cycle can be used. These methods include the basic 12 pulse configuration with parallel star / delta transformer connections. and two secondary windings. Fig 2. using control of firing angle to produce a 30 degree phase shift between the two 6 pulse bridges. 2.12 STATCOM Equivalent Circuit Several different control techniques can be used for the firing control of the STATCOM. This approach will minimize switching losses. As an alternative. This method can be extended to produce a 24 pulse and a 48 pulse STATCOM. a phenomenon that involves an interaction between large thermal generating units and series compensated transmission systems. The TCSC also can regulate steady-state power flow within its rating limits. Secondly it can overcome the problem of Sub Synchronous Resonance (SSR).2 TCSC:Thyristor Controlled Series Capacitors (TCSC) address specific dynamical problems in transmission systems. • Limitation of short circuit currents in networks or substations. including the Thyristor valve that is used to control the behavior of the main capacitor bank. damping of oscillations. • Avoidance of loop flows resp. and allows for rapid readjustment of line power flow in response to various contingencies. The main applications are: • Reduction of series voltage decline in magnitude and angle over a power line. Page | 31 . The firing angle and the thermal limits of the Thyristors determine the boundaries of the operational diagram. From a principal technology point of view. The TCSC's high speed switching capability provides a mechanism for controlling line power flow. which permits increased loading of existing transmission lines. • Improvement of system damping resp. Likewise the control and protection is located on ground potential together with other auxiliary systems. Figure shows the principle setup of a TCSC and its operational diagram. power flow adjustments.CHAPTER: 3 OTHER SERIES AND SHUNT DEVICES 3. • Reduction of voltage fluctuations within defined limits during changing power transmissions. 3. All the power equipment is located on an isolated steel platform. Firstly it increases damping when large electrical systems are interconnected. the TCSC resembles the conventional series capacitor.1 SERIES DEVICES:Series devices have been further developed from fixed or mechanically switched compensations to the Thyristor Controlled Series Compensation (TCSC) or even Voltage Source Converter based devices. Damping of electromechanical (0.1 TCSC Circuit and Characteristics 3.2.2. The Dynamic Flow Controller consists of the following components: • A standard phase shifting transformer with tap-changer (PST) • Series-connected Thyristor Switched Capacitors and Reactors Page | 32 .3 DYNAMIC POWER FLOW CONTROLLER:A new device in the area of power flow control is the Dynamic Power Flow Controller (DFC). 3.1 Advantages Continuous control of desired compensation level Direct smooth control of power flow within the network Improved capacitor bank protection Local mitigation of sub synchronous resonance (SSR). This permits higher levels of compensation in networks where interactions with turbine-generator torsional vibrations or with other control or measuring systems are of concern. 3.5-2 Hz) power oscillations which often arise between areas in a large interconnected power network. These oscillations are due to the dynamics of inter area power transfer and often exhibit poor damping when the aggregate power tranfer over a corridor is high relative to the transmission strength. A functional single line diagram of the Dynamic Flow Controller is shown in Figure 3.Fig. The DFC is a hybrid device between a Phase Shifting Transformer (PST) and switched series compensation. influence on reactive power balance and effectiveness at high/low loading the two parts of the series voltage has Page | 33 .1 (TSC / TSR) • A mechanically switched shunt capacitor (MSC). However. (This is optional depending on the system reactive power requirements) Fig. Normally the reactance of reactors and the capacitors are selected based on a binary basis to result in a desired stepped reactance variation. Assuming that the power flow has a load factor close to one. • The relieve of overload and work in stressed situations is handled by the TSC / TSR. in terms of speed of control.2 Principal configuration of DFC Based on the system requirements. • The total reactive power consumption of the device can be optimized by the operation of the MSC. a DFC might consist of a number of series TSC or TSR. The overall control objective in steady state would be to control the distribution of power flow between the branch with the DFC and the parallel path. the principle of phase-angle control used in TCSC can be applied for a continuous control as well. • The switching of the PST tap-changer should be minimized particularly for the currents higher than normal loading. In order to visualize the steady state operating range of the DFC. in general.3. The controllable reactance will inject a voltage in quadrature with the throughput current. The operation of a DFC is based on the following rules: • TSC / TSR are switched when a fast response is required. If a higher power flow resolution is needed. This control is accomplished by control of the injected series voltage. tap changer and the switched capacities and reactors.3. a reactance equivalent to the half of the smallest one can be added. we assume an inductance in parallel representing parallel transmission paths. the two parts of the series voltage will be close to collinear. The switching of series reactors occurs at zero current to avoid any harmonics. The mechanically switched shunt capacitor (MSC) will provide voltage support in case of overload and other conditions. The PST (assuming a quadrature booster) will inject a voltage in quadrature with the node voltage.3. However. where the x-axis corresponds to the throughput current and the y-axis corresponds to the injected series voltage. At zero current. assuming maximum tap and inductance. The maximum series voltage in the first quadrant is obtained when all inductive steps are switched in and the tap is at its maximum. Starting at rated current (2 kA) the short circuit reactance by itself provides an injected voltage (approximately 20 kV in this case). giving an almost constant maximum voltage in the second quadrant.3 .3. whereas operation in the second and fourth quadrants corresponds to increasing the power flow through the DFC.g. moving into the second quadrant. the capacitive step is approximately as large as the short circuit reactance of the PST. to changing loading of the system) the series voltage will decrease. If more inductance is switched in and/or the tap is increased.quite different characteristics. The slope of the line passing through the origin (at which the tap is at zero and TSC / TSR are bypassed) depends on the short circuit reactance of the PST. Consequently. the series voltage increases and the current through the DFC decreases (and the flow on parallel branches increases). it will not matter whether the TSC / TSR steps are in or out. Now. The operating point moves along lines parallel to the arrows in the figure. the series voltage at zero current corresponds to rated PST series voltage. Next. Page | 34 . if the throughput current decreases (due e. the operating range will be limited by the line corresponding to maximum tap and the capacitive step being switched in (and the inductive steps by-passed). The steady state control range for loadings up to rated current is illustrated in Figure 3. Operational diagram of a DFC Operation in the first and third quadrants corresponds to reduction of power through the DFC. In this case. they will not contribute to the series voltage. The slope of these arrows depends on the size of the parallel reactance. Fig3. an UPFC is getting quite expensive.4. which limits the practical applications where the voltage and power flow control is required simultaneously. which are connected via two voltage source converters with a common DC-capacitor.21.3. as shown in Figure 1. The series converter needs to be protected with a Thyristor bridge. 3.4 UNIFIED POWER FLOW CONTROLLER:The UPFC is a combination of a static compensator and static series compensation. It acts as a shunt compensating and a phase shifting device simultaneously. and connected to the power system through coupling transformers. while the other one is connected in series through a series transformer.3.4. Fig3. One VSI is connected to in shunt to the transmission system via a shunt transformer. provides the full controllability for voltage and power flow.5 Page | 35 . This setup. The DC-circuit allows the active power exchange between shunt and series transformer to control the phase shift of the series voltage. A basic UPFC functional scheme is shown in fig.1 OPERATING PRINCIPLE OF UPFC The basic components of the UPFC are two voltage source inverters (VSIs) sharing a common dc storage capacitor. Principle configuration of an UPFC The UPFC consists of a shunt and a series transformer. Due to the high efforts for the Voltage Source Converters and the protection. ish into the transmission line. of controllable magnitude and phase angle in series with the line to control active and reactive power flows on the transmission line. The UPFC has many possible operating modes. the net real power absorbed from the line by the UPFC is equal only to the losses of the inverters and their transformers. and the active power is transmitted to the dc terminals. the shunt inverter is operating as a STATCOM that generates or absorbs reactive power to regulate the voltage magnitude at the connection point. The reactive power is electronically provided by the series inverter. is also required.Fig. The shunt inverter control translates the var reference into a corresponding shunt current request and adjusts gating of the inverter to establish the desired current. this inverter will exchange active and reactive power with the line.3. the shunt inverter is operating in such a way to inject a controllable current. The remaining capacity of the shunt inverter can be used to exchange reactive power with the line so to provide a voltage regulation at the connection point. UPFC functional scheme The series inverter is controlled to inject a symmetrical three phase voltage system (Vse). Vdc. So in that case. Instead. Page | 36 .5. The two VSI’s can work independently of each other by separating the dc side. and hence the power low on the transmission line. So. For this mode of control a feedback signal representing the dc bus voltage. The shunt inverter can be controlled in two different modes: VAR Control Mode: The reference input is an inductive or capacitive VAR request. the series inverter is operating as SSSC that generates or absorbs reactive power to regulate the current flow. The shunt inverter is operated in such a way as to demand this dc terminal power (positive or negative) from the line keeping the voltage across the storage capacitor Vdc constant. So. In particular. Automatic Voltage Control Mode: The shunt inverter reactive current is automatically regulated to maintain the transmission line voltage at the point of connection to a reference value. Page | 37 . The series inverter controls the magnitude and angle of the voltage injected in series with the line to influence the power flow on the line. For this mode of control. The actual value of the injected voltage can be obtained in several ways. voltage feedback signals are obtained from the sending end bus feeding the shunt coupling transformer. Automatic Power Flow Control Mode: The reference inputs are values of P and Q to maintain on the transmission line despite system changes. Direct Voltage Injection Mode: The reference inputs are directly the magnitude and phase angle of the series voltage. Line Impedance Emulation mode: The reference input is an impedance value to insert in series with the line impedance. Phase Angle Shifter Emulation mode: The reference input is phase displacement between the sending end voltage and the receiving end voltage. The current expression in (2) is transformed into a power expression by the VSC and power injected into bus k as shown in equations (4) and (5) respectively. (4) (5) Page | 38 . This STATCOM model is known as Power Injection Model (PIM) or Voltage Source Model (VSM). If a VSC is shunt-connected to a system via a coupling transformer as shown in Fig. Vk represents bus k voltage and Vstc represents the voltage source inverter. 4.CHAPTER: 4 STATIC SYNCHRONOUS COMPENSATOR POWER FLOW MODEL 4.1. Also.1 (1) is expressed in Norton equivalent form (2) (2) where In these expressions. Z SC and Y SC are the transformer’s impedance and short-circuit admittance respectively. IN is the Norton’s current while Istc is the inverter’s current. The STATCOM voltage injection V STC bound constraints is as follows: (3) Where VSTC min and VSTC max are the STATCOM’s minimum and maximum voltages. controllable magnitude and phase angle.1 STATCOM POWER FLOW MODEL The STATCOM is a FACTS controller based on voltage sourced converter (VSC). A VSC generate a synchronous voltage of fundamental frequency. the resulting STATCOM can inject or absorb reactive power to or from the bus to which it is connected and thus regulate the bus voltage magnitude. Steady state modeling of STATCOM within the Newton-Raphson method in rectangular co-ordinates is carried out as follows: The Thevenin equivalent circuit representing the fundamental frequency operation of the switched-mode voltage sourced converter and its transformer is shown in Figure 4. e k and f k are the real and imaginary parts of the bus voltage respectively.1 Thevenin Equivalent Circuit Diagram of STATCOM: (a) STATCOM Schematic Diagram. Where V STC and δ STC are the STATCOM voltage magnitude and angle respectively. The active and reactive powers for the STATCOM and node k respectively are: (6) (7) Page | 39 . (b) STATCOM Equivalent Circuit Using the rectangular coordinate representation. e STC and f STC are the real and imaginary parts of the STATCOM voltage respectively.Fig. 4. e. We assume a suitable solution for all the buses except the slack bus. If that is the case.0+j0. Page | 40 . 2.0 for p=1. Check if a bus is a slack bus.2. We then set a convergence criterion = ε i.…. skip to step 10. Set the iteration count K=0. 4. p≠s.And (8) (9) 4.1 ahead. The solution of the combined system of non-linear equations is carried out by iteration using the full NewtonRaphson method. 3. Set the bus count p=1. if the largest of absolute of the residues exceeds ε. Vp=1. or else its terminated. The set of linearised power flow equations for the complete system is The Jacobian elements in equation (10) are given in table 4. the process is repeated.n. 5. Vs=a+j0. We assume a flat voltage profile i.3 NEWTON-RAPHSON-ALGORITHM 1. The Jacobian used in conventional power flow is suitably extended to take account of the new elements contributed by the STATCOM. The inclusion of one STATCOM model augments the number of equations by two.2 LINEARISED POWER EQUATION A single-phase power network with n-buses is described by 2×(n-1) non-linear equations. 4.e.0. 14. Page | 41 . Calculate the new bus voltage of all voltages. 9. Evaluate 8. fix the reactive power generation to the corresponding limit and treat the bus as a load bus for that iteration and go to the next step.6. Evaluate the Jacobian matrix elements. 12. compare Qkp with the limits. i. go back to step 5. 11. Evaluate 10. If the largest of the absolute value of the residue is less than ε. If it exceeds the limits. Calculate the real and reactive powers Pp and Qp respectively. 17. Check if the bus p is a generator bus. go to step 17. 7.e. Or else. If that is the case. Evaluate bus and line powers and output the results. using the equations derived for the same earlier. 13. Determine the largest value among the absolute value of residue. p = p+1 and finally check if all the buses have been taken into consideration. If lower limit is violated. If the limit is not violated evaluate the voltage residue. Evaluate cosδ and sinδ 16. set Q sp=Qp min. Calculate the voltage increments 15. Increment the bus count by 1. Advance iteration count K=K+1 and go back to step 4. TABLE 1 Page | 42 . it absorbs reactive power. The response time of a STATCOM is shorter than that of an SVC. Improvement of steady-state power transfer capacity. The STATCOM also provides better reactive power support at low AC voltages than an SVC. Reactive compensations of AC-DC converters and HVDC links.2 SCOPE FOR FUTURE RESEARCH Although this research has covered most of the interesting issues and challenges of the advanced STATCOM and several aspects of the integration of ESS into STATCOM. when the amplitude of the voltage source is lower than the AC voltage. The reactive power at the terminals of the STATCOM depends on the amplitude of the voltage source. since the reactive power from a STATCOM decreases linearly with the AC voltage (as the current can be maintained at the rated value even down to low AC voltage).1 APPLICATIONS OF STATCOM Usually a STATCOM is installed to support electricity networks that have a poor power factor and often poor voltage regulation. the most common use is for voltage stability. on the other hand. a fault protection scheme to enhance the ride-though capability in various faults scenarios remains as an important challenge In the investigation of the interface topology. Effective voltage regulation and control. the STATCOM generates reactive current. Damping of subsynchronous oscillations. The voltage source is created from a DC capacitor and therefore a STATCOM has very little active power capability. with the voltage source behind a reactor. Balanced loading of individual phases. if the terminal voltage of the VSC is higher than the AC voltage at the point of connection. Damping of power system oscillations. For example. Improvement of transient stability margin. mainly due to the fast switching times provided by the IGBTs of the voltage source converter. There are however. the ES was assumed to be charged to a voltage level that is not higher than the DC-side voltage of the VSC. CONCLUSION AND FUTURE WORK 5. A STATCOM is a voltage source converter (VSC)-based device. its active power capability can be increased if a suitable energy storage device is connected across the DC capacitor. However.CHAPTER: 5 APPLICATIONS. there are certain aspects that might be interesting for future investigations which are given below: Due to the excessive number of semiconductor devices and passive components. It might be valuable to Page | 43 . 5. STATCOM has following applications in controlling power system dynamics. Reduction of rapid voltage fluctuations (flicker control). other uses. Reduction of temporary over-voltages. the midpoint sitting controls a larger reactive power because each side of the STATCOM device addresses only half the line impedance and not the full line impedance as in the case of the transmission line receiving end sitting and sending end sitting. However. A power flow model of the STATCOM is attempted and it is seen that the modified load flow equations help the system in better performance. Hence our objective to maintain voltage stability has been successfully achieved with the incorporation of Static Synchronous Compensator (STATCOM). The location of the shunt FACTS device depends on the application for which it is installed. etc. The bus system shows improved plots and the thus we can conclude that the addition of a STATCOM controls the output of a bus in a robust manner.investigate the possibility of charging ES to a higher extent and the related issues such as protection issues. damp the power system oscillations ore effectively and stabilize the system faster if the STATCOM-SMES controller is located at the midpoint. The midpoint sitting of STATCOM also facilitates the independent control of reactive power at both the ends of the transmission line. The Newton raphson method has been presented to solve the power flow problem in the power system with static synchronous compensator (STATCOM). The simulation study shows that a STATCOM with real power capability can improve the real power and enhance load stability margin. the reactive components used in the STATCOM are much smaller than those in the SVC. the STATCOM basically circulates power with the connected network. When connected at the midpoint the real power is improved and the load ability margin. A comparison between the STATCOM and the SVC is made and based on several aspects it is concluded that a STATCOM is more preferred when compared to SVC and other compensation devices. such as bus voltage regulation and improving HVDC link performance. midpoint of the lines is the best location for shunt connected multi pulse STATCOM device. Various concepts regarding the FACTS technology and the important features of some of the FACTS devices have been presented. from simulation results it is observed that for increasing the power transfer capability of long transmission lines (tie lines connecting two major grids). Shunt compensation FACTS devices are installed at the end points of transmission lines (buses) when used for applications. Therefore. Instead of directly deriving reactive power from the energy storage components. The study of the basic principles of the STATCOM is carried out as well as the basics of reactive power compensation using a STATCOM. For a given voltage limit. Page | 44 . Research on the CMC based topology with ESS can be implemented for real and reactive power compensation in wind farms with FSIGs or Double Fed Induction Generator 5. the shunt controller STATCOM have shown feasibility in terms of cost effectiveness in a wide range of problem-solving abilities from transmission to distribution levels.3 CONCLUSION Among FACTS controllers. 20. Xiaorong..S. H. no. S. M. Edris A. New York. “An Open Source Power System Analysis Toolbox.19. and Edris. Paserba. “Understanding FACTS”. IEEE Transactions on Power Systems 19(4). Yam C. Chichester. Ambriz-Perez. M. 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