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- 1. ISSN: 2277 – 9043 International Journal of Advanced Research in Computer Science and Electronics Engineering Volume 1, Issue 2, April 2012 A study on Angle Stability Solution Using coordinated Facts Devices Dr. K.T. Chaturvedi, Assistant Professor Dept. of Electrical Engineering UIT RGPV Bhopal Kripashankar Pandeya UIT RGPV Bhopal this model an iterative linear stability analysis isAbstract— This paper presents a Angle Stability Solution performed. This approach analyzes all possibleUsing coordinated Facts Devices located in different combinations of loads and the sustainable electricityareas of a power system. Analysis of initial conditions and generations. Eventually, this method reveals the mosta contingency analysis to determine the voltage stability vulner- able operating points of the system andmargin and voltage variations at different critical nodes consequently can be used as an indicator of smallare held. The response is carried out by the coordination disturbance angle stability.of multiple-type FACTS devices due to which the reactivepower is compensated, improving the voltage stabilitymargin at the critical nodes. II. PROBLEM FORMULATION Index Terms— FACTS Devices, Transmission Postfault rotor angle oscillations lead to power swings. BothLoadability,Angle stability, Voltage Stability, Continuation unstable and stable swings can induce distance relay tripping.Power Flow, Tangent Vector Technique, Real Power Flow For unstable swings, a new computational procedure toPerformance Index Sensitivity Factor locate all the electrical centers is developed. It simplifies the work associated with visual screening of all the R-X plots. For stable swings, a generic three tier hierarchy of stability I. INTRODUCTION related norms defined by branch norm, fault norm and system Currently, electric power systems are experiencing struc- norm is proposed. Ranking by branch norm leads to ranking tural changes due to the growing incorporation of renewable of power swings. Ranking by fault norm leads to ranking of sources of energy. Another determinant factor in modifying faults or contingencies. Magnitude and rate of change of the configuration of electric power systems is the system norm can be used to detect an out-of-step condition. liberalization of electricity market and restructuring the Results on a 10-machine system and a utility system with trade of energy. Also, in near future, the number of detailed models are also presented. Voltage stability is interconnections in electric power systems will increase. concerned with the ability of a power system to maintain Thus, the future electric power systems will operate closer acceptable voltage at all nodes in the system under normal to their limits. Considering the aforesaid reasons, together conditions or after being subjected to a disturbance [1]. In with the major switching actions concerning the connection order to detect the system conditions and to predict voltage of renewable energy sources, the stability of future power instability, it is necessary to conduct a voltage stability study systems is undertaking a more highlighted role. Being large at all nodes. To prevent or correct voltage instability, solution scale and complex, are features of modern power methods based on the results of the power system study must systems. Also, because of deregulation, the configu- ration be applied; these methods allow to improve the voltage of interconnected networks is routinely in a state of change. stability margin and to avoid voltage collapse. Therefore, an indicator of stability, which covers the vast spectrum of states, is of interest. Such a methodology Study of Voltage Stability should reconcile the stochastic behavior of the The methods for studying voltage stability are used to find renewable energy sources and the deterministic approach the operation state, the voltage stability margins and limits of stability and to study the system variation and element responses. The analysis. voltage stability study can be conducted using analytical or A methodology to indicate the small disturbance angle monitoring methods. stability in future power systems is to make use of an 1) Analytical Methods: These methods allow a detailed study iterative- stochastic approach. By applying such an of the variables, parameters and elements behavior of the approach, the prob- abilistic nature of sustainable energy power system, in order to find design solutions and operation sources is modeled and subsequently, for each sample of criteria that allow the system to work far from the instability point. Each of these methods uses a mathematical technique, which is implemented in a computational tool with a great number of nodes, lines and loads. These methods arebased on All Rights Reserved © 2012 IJARCSEE 60
- 2. ISSN: 2277 – 9043 International Journal of Advanced Research in Computer Science and Electronics Engineering Volume 1, Issue 2, April 2012conventional power flows, progressive power flows anddynamic analyses. Conventional power flow techniques are 2) Control of Elements: these techniques are used to avoidbased on mathematical calculations made for each load voltage instabilities and collapses in the power system bycondition of the power system and represent voltage variation controlling elements that change the operation conditions asat all nodes due to the change of the active and reactive power protection relays [14], current limiters [15] and TAPof the load. These techniques allow the calculation of the changers.system operation state and the voltage stability limits andmargins, in normal operation conditions and after 3) System Changes: these techniques are based on thecontingencies. Some of these techniques are: sensitivity entrance of elements or the shedding of loads to avoidanalysis [4], reduction of the Jacobian matrix (matrix voltage instability in the system. They are used to increase thesingularity [5] and modal analysis [6]), network equivalent, power transmission capacity and to alleviate the powervectorial difference [7] and energy-based techniques. system overloads. These methods are divided in undervoltageProgressive flow techniques are static analysis methods load shedding [16], and system configuration changes [17].based on consecutive power flow results used to find thevoltage stability margins and limits of the nodes with higher 4) Mixed: this is a technique based on the combination of thenumerical accuracy [8]; they use a prediction tangent vector above mentioned techniques to create a voltage stabilityto estimate a solution to another load value, which is then prevention and correction scheme using different elementscorrected [9]. These methods have been widely used because [18].of their precision in the voltage stability limitcalculation.Dynamic analysis techniques are based on thesolution ofalgebraic equations in time-domain [1] and theyare used for transient and small signal stability events IV. VOLTAGE STABILITY PROBLEManalysis. These techniques allow to create different scenarios Each node of a power system initially operates with a voltagethat include contingencies or normal operation states in order stability margin as shown in Fig. 1(a). Once a contingencyto determine the variables behavior and response of power occurs, voltage decreases to a stable point as shown in Fig.system elements in the occurrence of an event [10]. 1(b) or it becomes unstable as shown in Fig. 1(c). Also when the node operates near the voltage stability limit, Fig. 1(b), a 2) Monitoring Methods: these methods are based on data voltage collapse can occur after other events or operations measurements of the power system variables such as that decrease the reactive transfer capacity to the critical voltages, current, active power, reactive power and vector nodes; therefore it is necessary a prompt response to increase angles, to find the operation state, voltage stability limit and the voltage stability margins as shown in Fig. 1(d). margin as well as the critical nodes of the system. They can be used as a tool for the on-line and off-line voltage stability detection and prediction III. SOLUTION OF VOLTAGE STABILITYThe methods used for the solution of the voltage stability arebased on the prevention and correction of the problem.Prevention methods are aimed at maintaining voltagestability at all nodes, avoiding them to get close to the limit;correction methods are aimed at restoring the unstableconditions to return voltage stability to the nodes. Thesemethods are divided in: reactive compensation, control ofelements and power system changes. Control actions can beautomatic or manual and must be used according to the timeof response to improve in a short, middle or long-termstability.1) Reactive Compensation: these techniques are based on thecompensation of reactive power to the system, from powergeneration sources [11], transmission lines [12], transformers Fig. 1. PV curves and Voltage stability margins[13] and load nodes. The line compensation can be carriedout with a constant value of reactive power, called static Line compensation using FACTS devices compared withcompensation, using switched elements to increase the other techniques, allows a fast response after disturbances,voltages at the nodes; also, a variable compensation can be allows reactive power injection, does not change the systemcarried out, controlled by power electronic devices as configuration or sheds loads, does not interfere with theFACTS; this is called dynamic compensation and is used to availability of generators and can change the direction of therespond during transitory and small signal variations that power flow from one area to another. These techniques canoccur in the power system due to disturbances. be used to respond to the events occurred after a contingency, 61 All Rights Reserved © 2012 IJARCSEE
- 3. ISSN: 2277 – 9043 International Journal of Advanced Research in Computer Science and Electronics Engineering Volume 1, Issue 2, April 2012with a static or dynamic increasing of the voltage at thenodes. Some methods seek to solve voltage instability by the This methodology is based on a fast injection and theoptimal location of FACTS devices close or in the critical direction of power through the transmission path of lowernodes [19]; other methods are based on the coordination of losses to increase the reactive power supply to the criticalFACTS devices, as in 1998, when a method to coordinate nodes of a power system. This method avoids load shedding,thyristor controlled series and shunt compensators (TCSC increases the time for other slower reactive control elementsand SVC) was presented in order to improve angle and to operate, can be applied to global power systems and doesvoltage stability, using a disturbance response method based not need to relocate FACTS devices so the costs are noton the Disturbances Auto-Rejection Control (DARC) theory increased. The coordination of FACTS devices to improve[20]. In 2003, a secondary voltage control method was voltage stability is made in several stages: FACTS selection,proposed to eliminate voltage violations at the nodes after a design and implementation.contingency, using the coordination of SVC and STATCOMdevices to provide reactive power; the secondary voltage A. FACTS Selectioncontrol is implemented by a learning fuzzy logic controller The FACTS controllers used in power systems are classified[21]. In 2005, the development of a control system and in: shunt controllers (SVC, STATCOM, BESS, SMESS,control strategies capable of governing multiple flexible AC SSG, TCBR, TCR, TSC, TSR, SVG, SVS), series controllerstransmission system (FACTS) devices in coordination with (SSSC, TCSC, TSSC, TCSR, TSSR, TCPST), combinedload shedding was proposed to remove overloads caused by shunt and series connected controllers (UPFC, TCPST, IPC)lines outages in transmission networks, based on linearized and other types (TCVL, TCVR) [23]. These devices allowexpressions in steady state [21]. In 2005, a method for controlling power flow, increasing transmission linecoordination of FACTS devices as SVC, TCSC and TCPST capacity, improving stability and reducing reactive losses.was presented, based on the optimal power flow to avoid The devices that should be used for voltage stability solutioncongestion, to give greater security and to minimize the are those that allow reactive power injection and poweractive losses in transmission lines [22]. FACTS devices transfer with lower losses.location and coordination techniques mentioned above havebeen based on the voltage stability improvement in an areanear the devices, improving voltages in steady state, B. Designpreventing violations of the voltage limits, coordinating few The design stage is based on the determination of thequantity and types of FACTS devices, they do not allow to coordinated control strategies of the FACTS devices. Thehandle reactive power in the lines and aim at relocating the steps to find the control strategies are shown in Fig. 3.devices increasing the costs. Table I shows the comparisonamong the techniques used for the solution of voltagestability problems after a contingency. The rating of theindexes was done with numbers that indicate the low (1),middle (2) and high (3) levels of the objective functions ofthe proposed method. V. PROPOSALThe proposed methodology consists on compensating thereactive power in an interconnected power system of severalareas as shown in Fig. 2, by a coordinated control strategy ofa wide quantity of different FACTS devices located inseveral parts of an interconnected power system in order toincrease the voltage stability margin during critical voltagevariations created by a contingency. 1) Initial Conditions: the initial conditions of a system are calculated to determine the operation state and the voltage stability margin of each node. 2) Contingency Analysis: a contingency analysis to determine nodes with voltage instability or near the limit is carried out. The selection of the All Rights Reserved © 2012 IJARCSEE 62
- 4. ISSN: 2277 – 9043 International Journal of Advanced Research in Computer Science and Electronics Engineering Volume 1, Issue 2, April 2012 determine the type of stability and the magnitude of the disturbance. 2) Determination of Instability Type and Magnitude: one ofTABLE I the most important parts of the control strategy is theCOMPARISON OF THE TECHNIQUE FOR THE SOLUTION OF determination of the type of instability and the magnitude of aVOLTAGE STABILITY disturbance occurred in the system, because it is necessary to establish the time for the operation of the devices and the reactive power quantity to be transferred to the critical nodes. 3) Determination of the Coordinated Control: after determining the type and magnitude of the event, themost critical contingencies and the nodes for the coordinated actions of the available devices are determined,measurement of the entrance variables of the control strategy transmitting through the transmission path of lower losses toare carried out. reduce reactive losses and to allow a higher reactive supply to the nodes.3) Voltage Analysis: based on the selection of the mostcritical contingencies and the nodes for the variables 4) Sending Signals to Controllers: the operation signals formeasurement, a dynamic study of the voltages after the FACTS controllers are sent according to the obtained controlcontingency to determine the system response is carried out. decisions; they control the operation of capacitors and inductors of each FACTS device, allowing the reactive4) Coordination of FACTS: the control strategy of FACTS injection and power flow direction to the critical nodes.devices is developed to improve voltage stability at criticalnodes of the system. These devices should allow the reactive 5) Devices Operation: the devices operate based on thepower supply to the weak nodes and the directing of the signals sent and inject reactive power or allow the pass of thereactive power flow. power flow to a specific node based on the transmission path of lower losses.5) Operation Verification: after determining the control REFERENCESstrategy, an appropriate response for the selectedcontingencies should be verified and voltages must be [1] P. Kundur, ―Power System Stability and Control,‖ Ed.adjusted in order to comply with the system constraints. New York: McGraw-Hill, 1994, pp. 17-39, 959-1020. [2] C.W. Taylor, ―Power System Voltage Stability,‖ Ed. New York: McGraw-Hill, 1994. C. Implementation [3] J. Bucciero, and M. Terbrueggen ―Interconnected PowerThe implementation stage is based on the operation form of System Dynamics Tutorial: Dynamics of Interconnectedthe coordinated control strategy of FACTS devices. The Power Systems Tutorial,‖ Electric Power Reasearch Institute,operation form of the control strategy is shown in Fig. 4. EPRI, 3rd ed, Jan. 1998, pp. 6.1 – 6.55. [4] T.O. Berntsen, N. Flatabo, J.A. Foosnaes, and A. Johannesen, ―Sensitivity signals in detection of network condition and planning of control actions in a power system,‖ CIGRE-Ib AC Symposium on Control Applications for Power System Security, 1983, Paper 208-03. [5] N. Flatabo, R. Ognedal, and T. Carlsen, ―Voltage stability condition in a power transmission system calculated by sensitivity methods,‖ IEEE Transaction on Power Systems, Vol. 5, No. 4, Nov. 1990, pp. 1286 – 1293. [6] N.T. Hawkins, G. Shackshaft, and M.J. Short, ―Online algorithms for the avoidance of voltage collapse: reactive power management and voltage collapse margin assessment,‖ Third International Conference on Power System Monitoring and Control, London, UK, 1991, pp. 134-139. [7] T.A.M. Sharaf, and G.J. Berg, ―Probabilistic voltageFig. 4. Operation form of the voltage stability control stability indexes,‖ IEEE Proceedings, Generation,strategy. Transmission and Distribution, Vol. 138, No. 6, 1991, pp. 499 – 504.1) Measurement of System Signals: the measurements must [8] V.A. Venikov, et al., ―Estimation of Electrical Powerbe taken constantly at nodes identified as critical in the System Steady- State Stability in Load flow Calculations,‖contingency analysis; the obtained data is used as an input to IEEE Transaction on PAS Vol. PAS-94, No. 3, May. 1975, pp. 1034 – 1041. 63 All Rights Reserved © 2012 IJARCSEE
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