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Impact of Distributed Generation on Smart Grid Transient Stability

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Impact of Distributed Generation on Smart Grid Transient Stability
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  Smart Grid and Renewable Energy , 2011, 2, 99-109 doi:10.4236/sgre.2011.22012 Published Online May 2011 (http://www.SciRP.org/journal/sgre)Copyright © 2011 SciRes.   SGRE 99 Impact of Distributed Generation on Smart Grid Transient Stability Nur Asyik Hidayatullah, Zahir J. Paracha, Akhtar Kalam School of Engineering and Science, Victoria University, Melbourne, Australia.Email: nurasyik.hidayatullah@live.vu.edu.au, {zahir.paracha, akhtar.kalam}@vu.edu.auReceived February 7 th , 2011; revised March 13 th , 2011; accepted March 17 th , 2011. ABSTRACT  In the 21  st century Smart Grid and Renewable Energy technologies are an important issue with regards to global cli-mate change problem and energy security. The evolution of current conventional or centralized generation in form of distributed generation and Smart Power Grid  ( SPG ) has great opportunity and potentially can eradicate several issuesassociated with energy efficiency , energy security and the drawback of aging power system infrastructures. In order tomeet the rising electrical power demand and increasing service quality as well as reducing pollution , the existing power  grid infrastructure should be developed into Smart Grid  ( SG ) that is flexible for interconnectivity with the distributed  generation. However  , integrating distributed generation to power system causes several technical issues especially sys-tem stability . To make the power grid become “  smarter  ”, particularly in terms of stability , Flexible AC TransmissionSystem (  FACTS  ) device especially Static VAR Compensator  ( SVC  ) is used. This paper explores Smart Grid technologiesand distributed generation systems. Furthermore, it discusses the impact of distributed generation on Smart Grid, par-ticularly its system stability after installing distributed generation in the Smart Grid. This was done by examining the system stability during interconnection and faults on the system and validated with Dig-SILENT Power Factory Soft-ware V  13.2.   Keywords : Smart Grid  , Future Grid  , Distributed Generation , Flexible AC Transmission System (  FACTS  ), Static VARSystem ( SVS  )   1. Introduction  The current electric power industry is undergoing majorchanges from being centralized generation into decen-tralized generation. The advances in technology have cre-ated rapid growth in the utilization of distributed genera-tions which leads to energy market becoming more at-tractive and competitive. Moreover, due to electricityderegulation, environmental issues as well as governmentincentives, this technology has created interest in devel-opment further amongst industrial countries throughoutthe globe. In addition, these issues also results in theSmart Grid platform to end the traditional electric powerindustry which traditionally was vertically integrated andcongested resulting in a higher energy costs.Nowadays, most of aging and large remote power sys-tem stations with central dispatch suffers from disturbancesdue to lack of intelligent interoperability units. The sys-tem also becomes vulnerable when utility abnormalitiesare present, for example on protection or failures to con-trol coordination and human operation errors. Therefore,there is a need to transform this model into Smart Gridthat can enhance power quality and fully integrate withadvanced grid elements such as intelligent sensing anddigital metering.Smart Grid is recognized as a new platform for futurepower industry. The rapid rise on this issue is also lead-ing to the fast growth of distributed generation technol-ogy markets such as in fuel cells (FC), photovoltaic (PV),wind turbine (WT) and energy storage (ES). This trendwill have profound impact on future electricity technol-ogy which allows Information and Communication Tech-nologies (ICT) and advanced power electronic devices tobe installed and embedded throughout the network. Thisis the challenge where current bulk generation and dis-tributed generation will co-exist with higher power reli-ability and quality in the form of Smart Grid. To empha-size these, this paper will provide a fundamental under-standing of distributed generation issues and frameworkof Smart Grid. Lastly, it finishes off by providing ananalysis impact of DG on Smart Grid transient stability.  Impact of Distributed Generation on Smart Grid Transient Stability100 2. Overview of Distributed Generation,Smart Grid and SVC Technologies 2.1. Distributed Generation Distributed Generation (DG) technology incorporates windturbines, micro turbines, photovoltaic systems, fuel cells,energy storage and synchronous generator applications tosupply active power to distributed systems connected closeto the consumers load. This concept is becoming a majorplayer for Green House Gases (GHG) mitigation andpower system reliability. Therefore, many developed coun-tries such as Australia, are encouraging DG utilization forlocal power source and to avoid concentration of newpower system transmission or distribution construction. The increasing number of DG connection to the gridfor the last decade proves that DG intrinsically offersvarious technical, economical and environmental advan-tages for customers, utilities and power operators. How-ever, the presence of DG in the current existing powersystem has an impact on its operation and configuration.Moreover, since most of current aging power system isconsidered as passive system and with the insertion of DG, the network becoming active system where both of them can act as power sources.Various studies in the past examined the impact of DGin power system and identified some vital aspects con-cerning their operation and connection. In reference [1]the analysis on change of power flow direction in corpo-ration of DG integration is verified. In reference [2] theinfluence of DG on system reliability and stability duringpeak load is investigated whilst reference [3] focuses onsystem protection as the short-circuit fault level on thenetwork tends to increase due to DG connection. Refer-ence [4] investigates the voltage variation and protectionby comparing the real system behavior with softwaresimulation. The authors conclude that the level of trans-former tap changer is increased with an increase on thepenetration level of DG. Reference [5] has also reviewsthe impact of DG on distribution system (DS). At presentDG provides much benefit in order to improve voltagequality, loss reduction and reliability. Reference [6] alsoshows that during interconnection to the distributionnetwork, DG unit can be operated under island mode. Ithighlights the interconnected DG performance and showsthis is greater during islanding condition, especially fromsecurity and quality of supply point of view. In addition,the study in [7] demonstrates the positive impact of DGon DS where during faults period, the rotor angle devia-tion and voltage drop are found to decrease. It means thatthe transient stability of the system improves with anincrease in the penetration level of the DG.However, overall these research scenario investigatesthe technical impact of DG on DS where almost all of them are coupled to the medium and low voltage levels.Moreover, none of the research scenario deals with theissues from Smart Grid perspectives. With the presenceof Smart Grid concepts and rapid growth of DG andSmart Grid technologies, it is critical to evaluate the sys-tem performance precisely. Thus, the framework andconcepts of Smart Grid can be applied appropriately andthis avoid degradation of power quality and reliability. 2.2. Smart Grid [8] Nowadays, the intelligent automation on electrical dis-tribution network has been driving the development of future power system like Smart Grid, which might leadto the network model being changed and certainly thiswill change the electric network operations. Smart Gridis a relatively new concept for future power system thatintegrates electricity and communication on power sys-tem network, which supplies digital information on realtime network operation for the operator and ultimatelythe consumer. The main characteristics of Smart Grid are self-healing,empowering the customers, improving power quality andability to accommodate various distributed generation. Inaddition, advanced control methods, digital sensing andmetering, advanced grid devices such as FACTS (Flexi-ble AC Transmission Systems) and SCADA (Supervi-sion Control and Data Acquisition) system are few of themajor technologies involved in the implementation of Smart Grid. FACTS is alternating current transmissionsystem device used to enhance control ability and increasepower transfer capability based on power electronic andother static controllers, meanwhile the SCADA systemrefers to the control and communication data system usedfor operation, monitoring and controlling power systemgrids. However, the standard of smart grid, how all ele-ments are connected with each other and how the com-munication or energy flow works still remains the majorconcerns for enabling Smart Grid implementation. The National Institute of Standard and Technology(NIST) reveals that there are seven important Smart Gridareas that need to be addressed to make the grid “Smar-ter” including; bulk generation, transmission, distribution,customer, operation, market domain and service provider[9]. These domains are integrated based on the concep-tual model of Smart Grid as shown in Figure 1 . As canbe seen from the figure, it is obvious that Smart Grid isan integrated system which involves complex co-ordina-tion strategy for implementation. In addition, Smart Gridtechnology broadens power knowledge and involves in-terdisciplinary research area such as: communication,automation, sensor and control. Furthermore, Copyright © 2011 SciRes.   SGRE  Impact of Distributed Generation on Smart Grid Transient Stability101   Figure 1. Smart Grid topology network [9].   Smart Grid also requires advanced technology to makethe grid as an ideal “Smart Grid”. Nevertheless, due tothe complexities of the system, it is unclear and evenconfusing to define the networks being “smart” if a fewof its key characteristics are neglected. Instead, it is pref-erable to consider the term “Smart Grid” as the chance toenhance the power system performance and improveoperational capabilities [8,10]. 2.3. Static VAR Compensator (SVC) FACTS controller history initially began when Hingoraniproposed the scheme of power compensation in electricalpower system using power electronic applications [11].Afterward, numerous researches were conducted on theapplication of FACTS using self commutated semicon-ductor and thyristor in transmission system. Originally,FACTS devices were developed for transmission systemswhich consisted of electrical conductors that have resis-tance, inductance and capacitance (R-L-C). The capaci-tive and inductive reactance in the transmission line gen-erates and absorbs reactive power. The reactive powerthat flow along the line often causes further loss in theresistance of the conductor. For this reason FACTS de-vice is required to enhance the power transfer capabilityand stability in the transmission system. However, for thelast decade this concept has been extended for improvingpower quality on distribution systems operation as well[12]. The application shown in [12] is to control and main-tain the system stability where in the distribution levelmost of the loads are non-linear or dynamic.Flexible AC Transmission System is one of the ad-vanced technologies used for power system compensa-tion. The devices including Static VAR Compensator(SVC), Static Synchronous Series Compensator (SSSC),Static Synchronous Compensator (STATCOM), Thyris-tor-Unified Power Flow Controller (UPFC), ThyristorCon-trolled Series Compensator (TCSC), Thyristor-Swi-tched Series Capacitor (TSSC) and Thyristor ControlledReactor (TCR) etc. These devices have many configura-tions, commonly it can be classified into series-connectedcontrollers, shunt-connected controllers and a combinationof them. The detail description of series and shunt-con-nected controller can be found in [11].Static VAR Compensator (SVC) is “a shunt-connectedstatic VAR generator or absorber whose output is ad- justed to exchange capacitive or inductive current so asto maintain or control specific parameters of the electri-cal power system (typically bus voltage) [11]”. SVC isbased on thyristor-controlled and switched shunt com-ponent without gate turn-off capability. It is a variableimpedance device using back to back connected thyristorvalves to control the current flow through reactor. SVCas a control device offers fast response time and muchfaster than traditional mechanically switched reactors orcapacitors. The configuration of SVC as shown in Figure2 consists of two main components and their combina-tion:     Thyristor-controlled and Thyristor-switched Reactor(TCR and TSR)     Thyristor-switched Capacitor (TSC) TCR and TSR constitute of a shunt-connected reactorcontroller using pair parallel back to back-connected thy-ristor. Using phase angle control, TCR generates an equi-lent and constant variable inductive reactive power fromzero to maximum. Whereas, TSR is controlled withoutphase angle control which results in a step change in re-tance and provides fixed inductive admittance. TSC has similar operational characteristic and compo-sition as TSR. It consists of a back to back thyristor pairin series with capacitors. The TSC is not continuously con-rolled because of transient phenomena at switch-on, how-ever instead is switched on and off independently. There-fore, TSC cannot inject a reactive current with variableamplitude into the system. The transient phenomenon in TSC does not generate harmonics but if they appear, itisnot a serious problem [13]. 3. Equal Area Criterion for TransientStability Analysis It is well known that large or small-scale integration of distributed generation may have significant impact on po-wer system stability with respect to the rotor angle, vol-tage and frequency stability. Reactive power compensa-tion and voltage control is fundamental to make the gridbecome smarter. Without this control, the presence of distributed generation may potentially cause system col-lapse. Therefore, a dynamic shunt reactive power com-pensator such as SVC is required to mitigate these issuessince it can be used to enhance transient stability marginin power system. The potential effectiveness of SVC ontransient stability enhancement can be examined through Copyright ©2011 SciRes.   SGRE  Impact of Distributed Generation on Smart Grid Transient StabilityCopyright © 2011 SciRes.   SGRE 102 tems are subjected to the same fault for the same periodof time. Before the fault, each system transmits power P M  and is operating at angle δ 1 and δ c1 . During the fault, thetransmitted power decreases significantly while the me-chanical input power remains constant (P M ). As conse-quence, the generator accelerates from δ 1 to δ 2 and δ c1 to δ c2 at which the fault clears. After fault clearing, thetransmitted power exceeds the mechanical input powerand the generator starts to decelerate. Nevertheless, theirangle increases due to the kinetic energy stored in therotors. At δ 3 and δ c3 the maximum rotor angle arereached when the decelerating energies that representedby areas ‘A 2 ’and ‘A c2 ’ become equal to the acceleratingenergies defined by areas ‘A 1 ’ and ‘A c1 ’. The maximumof transient stability limits is achieved at δ 3 and δ ccrit . Theareas of A margin and A c margin represent the transient stabil-ity margin of the system. Figure 2. SVC configuration [11]. the following discussion.As depicted in Figure 3 , assume that a single powergeneration machine with interconnecting lossless lineshave a reactance (  X) . Denoting the terminal voltages of generator machine and the infinite bus by 1 V      and 2 , power angle0 V        and the transmitted power as  P  ,then without the SVC installed in the line, the value of the power transfer can be expressed by,From above description, it is evident that SVC is ableto enhance transient stability limit. In addition, it is ob-vious that the transient stability is determined by power(  P  ) against power angle ( δ ) .   2 sin2 V  P  X      (1) 4. Experimental Setup  To understand the advantages of SVC, assume that anSVC is installed at the mid-point of the interconnectingline. In that case, the reactance of the SVC between themachine and infinite bus is  X/  2 ohms. Subsequently, thevalue of transmitted power can be calculated asIn order to make the power system become “Smarter”,this research considers small and simple system configu-ration with easier control and design as shown in Figure5 . The one-line system network is derived from [6]which presents successful islanding operation of distrib-uted generation into the Smart Grid environment. How-ever, this research only focuses on islanding examinationand does not consider analyzing the system stability. Therefore, it is necessary to developed further underSmart Grid concept by simplifying the system in respectof control where Flexible AC Transmission System(FACTS) devices particularly Static VAR Compensator(SVC) is used. The typical network intends to evaluatethe impact of distributed generation on Smart Grid’ssteady state voltage profile and system stability or dy-namic behavior, which is essential to establish an appro-priate applied model.   22 sin2sin22 VV  XX  2          (2)From Equation (2) it can be seen that the value of transmitted power is doubled i.e. , from V  2  /X  it goes to   2( V  2  /X  ). Accordingly, the transient stability level is alsoincreased. This fact can also be validated by the transientstability equal area criterion with SVC and without SVC,as shown in Figure 4 .Consider that Figure 4(a) is system without SVC and Figure 4(b) is system with SVC. Assume that both sys-Consider Smart Grid network consisting of a 132 kV;50 Hz sub transmission system with maximum of shortcircuit levels 5000 MVA and 4000 MVA respectively. The grid feeds a 33 kV distribution system through a 132kV/33 kV,  g  Y   transformers with rated power equal to T  =90 MVA, Short Circuit Voltage13.18%. Inthe system, two type of distributed generation technolo-gies are considered as dispersed sources; named DG Iand DG II. Each of them has rated power of 28.1 MVAand are separately coupled into the 11 kV bus through33/11 kV transformers. The grid load in Smart Grid zoneis 1 MW while the maximum load of the DGs is 23 MW. S   )( cc V    Figure 3.   Simplified power systems (a) without SVC and (b)  The SVC, which letter on this is called Static VAR with SVC [14].    Impact of Distributed Generation on Smart Grid Transient Stability103   (a) (b) Figure 4. Equal areas illustrating transient stability limit without SVC (a) and with SVC (b) [15].   Figure 5. Smart Grid topology network.   System (SVS), is installed in the grid to make the systembecome “smarter” particularly to maintain the systemstability during the integration of distributed generation. Figure 6 shows the model of SVS controller frame usedin this simulation [16]. It is based on IEEE model withthe objective for voltage and power reactive control inthe system. The SVS is installed at bus 2 at the mid-pointof the system. The basic data parameters are given in  Table 1 . Figure 6 shows that the measurement element(ElmSsc*) obtains active and reactive power, voltage,current and other signals from power system. The Copyright © 2011 SciRes.   SGRE
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