FACTS Device a Remedy for Power Quality and Power System Stability Problem: A Review

Transcription

1 FACTS Device a Remedy for Power Quality and Power System Stability Problem: A Review Vinit T. Kullarkar, B. Ajay Krishna, Rahul Lekurwale Assistant Professor, Department of Electrical Engineering, KITS College of Engineering, Ramtek, Maharashtra, India vinit.kullarkar@gmail.com, aj7tab@gmail.com, rahullekurwale18@gmail.com Abstract: Modern power systems are continuously being expanded and upgraded to cater the need of ever growing power demand. In the last few decades, power demand has increased abruptly but the expansion of power generation and transmission has been severely restricted because of limited resources and environmental restrictions. So, because of increasing power demand there are lots of power quality problems in the modern power system. This paper explains the problems that are due to poor Power Quality in electrical systems and shows their possible financial consequences and improvement of power quality by using Flexible AC Transmission System (FACTS) device. Power Quality is characterized by parameters that express harmonic distortion, reactive power and load unbalance. In earlier days filters were used to mitigate the problems like voltage sag, voltage swell and many problems related with power system. Nowadays Flexible AC transmission systems (FACTS) devices are used to control the power flow, to overcome the power system stability problems, to improve the power quality and many more problems associated with the power system are used. Finally, all the FACTS devices are explained with their basic circuit. Keywords: Power Quality, FACTS, STATCOM, SSSC, SVC. 134 I. INTRODUCTION Power quality is an issue that is becoming increasingly important to electricity consumers at all levels of usage. The sources of problems that can disturb the power quality are: power electronic devices, arcing devices, load switching, large motor starting, embedded generation, sensitive equipment, storm and environment related damage, network equipment and design. If the Power Quality of the network is good, then any loads connected to it will run satisfactory and efficiently. Installation running costs and carbon footprint will be minimal. If the Power Quality of the network is bad, then loads connected to it will fail or will have a reduced lifetime, and the efficiency of the electrical installation will reduce. Installation running costs and carbon footprint will be high and/or operation may not be possible at all. In this paper the technology of FACTS is discussed which are useful to solve the power quality issues. The term power quality is rather general concept. Broadly, it may be defined as provision of voltages and system design so that user of electric power can utilized electric energy from the distribution system successfully, without interference on interruption. This paper critically discusses about the power quality problems, issues and related standards, assessment of power quality issues and methods for its correction with giving a thorough knowledge of harmonics, power quality indices, parameters effecting electric power etc. II. IMPACT OF LOW POWER QUALITY Power Quality is gaining increasing attention in the electric power industry. The consumer of electrical energy requires electric power with a certain quality, but loads can have a negative impact on the electrical system and are thus also subject to an assessment in terms of quality. Power quality is therefore intrinsically linked to the interaction between the electrical system and loads and must take into account both the voltage quality and power quality. A. Possible consequences of low power quality that affect business costs are: Power failures (Release switches, fuses blowing). Breakdowns or malfunctions of machines. Overheating of machines (transformers, motors, etc.) leading to reduced useful life. Damage to sensitive equipment (computers, production line control systems, etc.). Electronic communication interference. Increased distribution system losses. The need to oversize systems to cope with additional electric stress, resulting in higher installation and operational costs. Luminosity flickering Interruption of production due to these impacts of low power quality entails high costs due to production loss and the associated waste. For the Industrial sector, the estimated costs due to poor power quality represent 4% of turnover (Source: Studio Leonardo Energy. The impact of production interruptions is greatest in companies with continuous production. B. The main causes of poor power quality in low voltage are: Excessive reactive power, because it charges useless power to the system. Harmonic pollution, which causes additional stress on the networks and systems, causing them to operate less efficiently. Voltage variations, because equipment operates less efficiently. The solutions vary for each cause. Excessive reactive power is regulated by a power factor correction system,

2 which not only avoids any penalties due to excessive reactive energy, but reduces the unnecessary electrical current that flows into the lines and power components, yielding substantial benefits, such as reducing voltage drops along the lines and leakages due to the Joule effect. Harmonic pollution is caused by large amounts of nonlinear consumption (from inverters, soft starters, rectifiers, power electronics, non-filament lighting, presses, etc.). Such devices deform the electrical current causing disturbances and problems to the system. Harmonic pollution is solved by active filters that are capable of eliminating the current harmonics in the system by measuring and injecting the same current, but in the opposite phase. Hence there is a need to employ such system that could provide pure power without any disturbance at consumer terminal, therefore FACTS devices are discussed in this paper to improve the power quality and power system stability at consumer terminal. III. OVERVIEW OF FACTS DEVICES The concept of FACTS (Flexible Alternating Current Transmission System) refers to a family of power electronics based devices able to enhance AC system controllability and stability and to increase the power transfer capability. The design of thee different schemes and configurations of FACTS devices is based on the combination of traditional power system components (such as transformers, reactors, reactors, switches and capacitors) with power electronics elements (such as various types of transistors and thyristors). Over the last few years, the current rating of thyristors has evolved into higher nominal values making power electronics capable of high power applications of tens, hundreds and thousands of MW. FACTS devices, because of their speed and flexibility are able to provide transmission system with several advantages such as: transmission capacity enhancement, power flow control, transient stability improvement, power oscillation damping, voltage stability and control, power quality improvement etc. Depending on the type and rating of the selected device and on the specific voltage level and local network conditions, a transmission capacity enhancement up to 40-50% may be achieved by installing a FACTS element. In comparison to traditional mechanically-driven devices, FACTS controllers give more reliability and require low maintenance. Costs, complexity and reliability issues represent nowadays the main barriers to the integration of these promising technologies from the TSOs perspective. Further FACTS penetration will depend on the technology provider s ability to overcome these barriers, for more standardization, interoperability and economics of scale. The FACTS technology has a collection of controllers, that can be used individually or co-ordinate with other controls installed in the network, thus permitting to profit better of the network s characteristics of control. Classification of FACTS Devices: There are different classifications for the FACTS devices: 1. Depending on technological features, the FACTS devices can divided into two generations: First generation: used thyristors with ignition controlled by gate (SCR). Second generation: semiconductors with ignition and extinction controlled by gate (GTO s, IGBT s IGCT s etc). The main difference between first and second generation devices is the capacity to generate reactive power and to interchange active power. The various first and second generation FACTS device are shown in the table I and II below. 2. Depending on the type of connection to the network FACTS devices can differentiate four categories Series Controllers Shunt Controllers Combined Series-Series Controllers Combined Series-Shunt Controllers Table I. First Generation Facts Devices FACTS Devices Static Var SVC (TCR, TCS, TRS) Controlled Series Compensations (TCSC,TSSC) Controlled Reactor Series (TCSR, TSSC) Controlled Phase Shifting Transformer (TCPST,TCPR) Controlled Voltage Regulator (TCVR) Controlled Functions Voltage control and stability, compensation of VAR s. muffling of oscillation Current control, muffling of oscillations transitory, dynamics and of voltage stability, limitation of fault current. Current control, muffling of dynamics and of voltage stability, limitation of fault current. Control of active power, muffling of oscillations, transitory, dynamics and of voltage stability. Control of reactive power, voltage control, muffling of dynamics and voltage stability. Limits of transitory and dynamic Voltage. 135

3 Voltage Limited (TCVL) Table II. Second Generation Facts Devices FACTS Devices Synchronous Static (STATCOM ) Synchronous Static (STATCOM with storage) Static Synchronous Series (SSSC) Unified Power Flow Controller (UPFC) Interline Power Flow Controller (IPFC) Back to Back (BtB) Functions Voltage control, compensation of VAR s, muffling of oscillations, Stability of voltage. Voltage control and stability, compensation of VAR s, muffling of oscillations, transitory, dynamics and of tension stability. Current control, muffling of dynamics and of voltage stability, limitation of fault current. Control of active and reactive power, voltage control, compensation of VAR s, muffling of oscillations, transitory, dynamics and of voltage stability, limitation of fault current. Control of reactive power, voltage. Control, muffling of dynamics and of voltage stability. 1. Series Controller: Series controllers as shown in Fig.1 below are being connected in series with the line as they are meant for injecting voltage in series with the line. These devices could be variable impedances like capacitor, reactor or power electronics based variable source of main frequency, sub synchronous or harmonic frequency, or can be a combination of these, to meet the requirements. If the injected voltage is in phase quadrature with the line current, then only supply or consumption of variable reactive power is possible. In order to handle real power also, any other phase relationship has to be involved. These type of controllers include: SSSC Static synchronous series compensator TCSC controlled series capacitor TCSR controlled series reactor TSSC switched series capacitor TSSR switched series reactor 2. Shunt Controllers: Shunt controllers will be connected in shunt with the line so as to inject current into the system at the point of connection. They can also be variable impedance, variable source, or a combination of these. If the injected line current is in quadrature with the line voltage, variable reactive power supply or consumption could be achieved. But any other phase relationship could involve real power handling as well. The shunt FACTS controller is as shown in Fig.2 below. Fig. 2. Shunt FACTS Controller This category includes STATCOM (Static synchronous compensator) and SVC (Static VAR compensator). The common Static VAR compensators are: TCR controlled reactor TSR switched reactor TSC switched capacitor 3. Combined Series-Series Controllers: This category comprises of separate series controllers controlled in a coordinated manner in the case of a multiline transmission system. It can also be a unified controller in which the series controllers perform the reactive power compensation in each line independently whereas they facilitates real power exchange between the lines via the common DC link because, in unified seriesseries controllers like Interline Power Flow Controller (IPFC), the DC terminals of the controller converters are all connected together. 136 Fig. 1. Series FACTS Controllers Fig. 3. Combined Series-Series Controllers

4 4. Combined Series-Shunt Controllers: It is a combination of separate series and shunt controllers, being operated in a coordinated manner. Hence, they are capable of injecting current into the line using the shunt part and injecting series voltage with the series part of the respective controller. exchange capacitive or inductive current so as to maintain or control specific parameters of the electrical power system (typically bus voltage). SVC is based on thyristors without gate turn-off capability. The operating principal and characteristics of thyristors realize SVC variable reactive impedance. SVC includes two main components and their combination: controlled and -switched Reactor (TCR and TSR); and -switched capacitor (TSC). Fig. 4. Combined Series Shunt Controller If they are unified, there can be real power exchange between the shunt and series controllers via the common DC power link, as in the case of Unified Power Flow Controllers (UPFC) Factors to be considered for FACTS devices Installation: There are three factors to be considered before installing FACTS devices: 1. The type of FACTS device 2. The amount of power to be transferred 3. Optimized location of device Out of these three factors, the last one is of great importance, because the desired effect and the proper features of the system depend of the location of FACTS. Steps for the identification of FACTS Projects: 1. The first step should always be to conduct a detailed network study to investigate the critical conditions of a grid or grids connections. These conditions could include: risks of voltage problems or even voltage collapse, undesired power flows, as well as the potential for power swings or sub synchronous resonances; 2. For a stable grid, the optimized utilization of the transmission lines e.g. increasing the energy transfer capability could be investigated; 3. If there is a potential for improving the transmission system, either through enhanced stability or energy transfer capability, the appropriate FACTS device and its required rating can be determined; 4. Based on this technical information, an economical study can be performed to compare costs of FACTS devices or conventional solutions with the achievable benefits. IV. FACTS DEVICES USED IN POWER SYSTEMS A. Static VAR (SVC): Static Var is a shunt-connected static Var generator or absorber whose output is adjusted to 137 Fig. 5. Static Var TCR and TSR are both composed of a shunt-connected reactor controlled by two parallel, reverse-connected thyristors. TCR is controlled with proper firing angle input to operate in a continuous manner, while TSR is controlled without firing angle control which results in a step change in reactance.tsc shares similar composition and same operational mode as TSR, but the reactor is replaced by a capacitor. The reactance can only be either fully connected or fully disconnected zero due to the characteristic of capacitor. With different combinations of TCR/TSR, TSC and fixed capacitors, a SVC can meet various requirements to absorb/supply reactive power from/to the transmission line. B. STATCOM or Static Synchronous (SSC): It is a shunt device, which uses force-commutated power electronics (i.e. GTO, IGBT) to control power flow and improve transient stability on electrical power networks. It is also a member of the so-called Flexible AC Transmission System (FACTS) devices. The STATCOM basically performs the same function as the static var compensators but with some advantages. The term Static Synchronous is derived from its capabilities and operating principle, which are similar to those of rotating synchronous compensators (i.e. generators), but with relatively faster operation. STATCOMs are typically applied in long distance transmission systems, power substations and heavy industries where voltage stability is the primary concern.

5 In addition, static synchronous compensators are installed in select points in the power system to perform the following: Voltage support and control Voltage fluctuation and flicker mitigation Unsymmetrical load balancing Power factor correction Active harmonics cancellation Improve transient stability of the power system The SSC comprises a voltage source inverter which is connected to power system through a transformer. The SSC can draw either capacitive or inductive current. The voltage source is created from a Capacitor and therefore a SSC has very little active power capability. However, its active power capability can be increased if a suitable energy storage device is connected across the DC capacitor. Static Synchronous Series (SSSC) is a modern power quality FACTS device that employs a voltage source converter connected in series to a transmission line through a transformer. The SSSC operates like a controllable series capacitor and series inductor. The primary difference is that its injected voltage is not related to the line intensity and can be managed independently. This feature allows the SSSC to work satisfactorily with high loads as well as with lower loads. The Static Synchronous Series has three basic components: a. Voltage Source Converter (VSC): main component. b. Transformer: couples the SSSC to the transmission line. c. Energy Source: provides voltage across the DC capacitor and compensate for device losses. Fig. 6. GTO Based Static Synchronous C. Controlled Series Capacitor (TCSC): The TCSC consists of a series capacitor bank, shunted by a Controlled Reactor to provide a smoothly variable series capacitive reactance. Fig. 3 shows a TCSC in series with a transmission line. It injects a series voltage proportional to the line current but in quadrature with it. Inserting a TCSC modifies the equivalent reactance of the line, and the active power flow can be varied. The TCSC is a one-port circuit in series with a transmission line; it uses natural commutation; its switching frequency is low; it contains insignificant energy storage elements; and it has no dc port. Fig. 8. Static Synchronous Series E. Interphase Power Controller (IPC): The IPC is a series controller of active and reactive power. It consists of inductive and capacitive branches subjected to separately phase-shifted voltages. The active and reactive power can be set independently by adjusting the phase shifters and/or branch impedances, using mechanical or electronic switches. The IPC can regulate both the direction and the amount of active power transmitted through a transmission line. The IPC is a two-port circuit (in series with a transmission line and in parallel with a bus bar); it uses natural commutation; its switching frequency is low; it has insignificant energy storage; and it has no dc port. Fig. 7. Controlled Series Capacitor D. Static Synchronous Series (SSSC): 138 Fig. 9. Interphase Power Controller F. Unified Power Flow Controller (UPFC): The UPFC is a combination of an STATCOM and an SSSC, sharing a common dc link. The UPFC can

6 control both the active and reactive power flow in the line. It can also provide independently controllable shunt reactive compensation. In other words, the UPFC can provide simultaneous control of all The basic transmission line parameters. The UPFC is a two-port circuit (in series with a transmission line and parallel with a bus bar), it uses forced commutation; its switching frequency is high; it has capacitive energy storage; and it employs a dc port. The UPFC allows a secondary but important function such as stability control to suppress power system oscillations improving the transient stability of power system. It combines together the features of the Static Synchronous (STATCOM) and the Static Synchronous Series (SSSC). In practice, these two devices are two Voltage Source Inverters (VSI s) connected respectively in shunt with the transmission line through a shunt transformer and in series with the transmission line through a series transformer, connected to each other by a common dc link including a storage capacitor. The fast control of SVC Light will improve the power quality and particularly reduce the flicker levels generated. Advantages of using FACTS devices in the power system network: The benefits of utilizing FACTS devices in electrical transmission systems can be summarized as follows: Better utilization of existing transmission system assets. Increased transmission system reliability and availability Increased dynamic and transient grid stability and reduction of loop flows Increased quality of supply for sensitive industries Environmental benefits Better utilization of existing transmission system asset. Provide dynamic reactive power support and voltage control. Reduce the need for construction of new transmission lines, capacitors, reactors regulatory concerns. Improve system stability. Control real and reactive power flow. Mitigate potential Sub-Synchronous Resonance problems. V. CONCLUSION Fig. 10. Unified Power Flow Controller An example of installed SVC: An ABB SVC Light rated at 13.2 kv, 0-64 Mvar (capacitive) has been installed at the Gerdau plant in Charlotte, N.C., USA operating an electric arc furnace (EAF) with continuous charging for scrap-based steel production. The EAF, rated at 30/33 MVA, as well as a ladle furnace (LF) rated at 18 MVA are taking their power from a 100 kv supply grid. Flexible Alternating-Current Transmission Systems (FACTS) is a recent technological development in electrical power systems. It builds on the great many advances achieved in high-current, high-power semiconductor device technology, digital control and signals gained with the commissioning and operation of high-voltage direct-current (HVDC) links and Static VAR compensator (SVC) systems, over many decades, may have provided the driving force for searching deeper into the use of emerging power electronic equipment and techniques. Due to the, every time higher requirements of the liability and quality of the electricity the implantation of devices capable of guaranteeing these requirements will keep increasing. Hence, FACTS devices are improving the operation of an electric power system and also improve the power quality. VI. REFERENCES [1] Arindam Ghosh and Gerard Ledwich, Power Quality Enhancement Using Custom Power Devices, Springer, [2] Compensation And Harmonic Filtering, Middle East Technical University, Fig.11. An ABB SVC Light Rated at 13.2 kv, 0-64 Mvar (Capacitive) has been Installed at the Gerdau Plant in Charlotte, N.C., USA 139 [3] N. G. Hingorani, "Introducing custom power, vol.32,pp.41-48,june

7 [4] Mohod. S. W and Aware. M. V, Power quality issues &it s mitigation technique in wind energy conversion, in Proc. of IEEE Int.Conf. Quality Power & Harmonic, Wollongong. [5] Stones and A. Collinson,"Power quality, "Power Eng.Journal,vol.15,pp.58 64,April [6] A. K. Jindal, A. Ghosh, and A. Joshi, Interline unified power quality conditioner, IEEE Trans. Power Del, vol. pp , Jan [7] Tan, Y.L., Analysis of line compensation by shunt connected FACT controller STATCOM, IEEE Transaction on Power Engineering Review,vol.19,pp57-58,AUG [8] M. Karthikeyan, Dr. P. Ajay D-Vimalraj, Optimal Location of Shunt Connected Facts Device for Power Flow Control IECTECT, pp [9] Elahe NADERI, Determination of Performance of The Distribution Static Compansator In Distribution Network, 22nd International Conference On Electricity Distribution, pp 1147, June