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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 is Abstract— This paper presents a Angle Stability Solution performed. This approach analyzes all possible Using coordinated Facts Devices located in different combinations of loads and the sustainable electricity areas of a power system. Analysis of initial conditions and generations. Eventually, this method reveals the most a contingency analysis to determine the voltage stability vulner- able operating points of the system and margin and voltage variations at different critical nodes consequently can be used as an indicator of small are held. The response is carried out by the coordination disturbance angle stability. of multiple-type FACTS devices due to which the reactive power is compensated, improving the voltage stability margin at the critical nodes. II. PROBLEM FORMULATION Index Terms— FACTS Devices, Transmission Postfault rotor angle oscillations lead to power swings. Both Loadability,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 to Performance 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 ISSN: 2277 – 9043 International Journal of Advanced Research in Computer Science and Electronics Engineering Volume 1, Issue 2, April 2012 conventional power flows, progressive power flows and dynamic analyses. Conventional power flow techniques are 2) Control of Elements: these techniques are used to avoid based on mathematical calculations made for each load voltage instabilities and collapses in the power system by condition of the power system and represent voltage variation controlling elements that change the operation conditions as at all nodes due to the change of the active and reactive power protection relays [14], current limiters [15] and TAP of the load. These techniques allow the calculation of the changers. system operation state and the voltage stability limits and margins, in normal operation conditions and after 3) System Changes: these techniques are based on the contingencies. Some of these techniques are: sensitivity entrance of elements or the shedding of loads to avoid analysis [4], reduction of the Jacobian matrix (matrix voltage instability in the system. They are used to increase the singularity [5] and modal analysis [6]), network equivalent, power transmission capacity and to alleviate the power vectorial difference [7] and energy-based techniques. system overloads. These methods are divided in undervoltage Progressive flow techniques are static analysis methods load shedding [16], and system configuration changes [17]. based on consecutive power flow results used to find the voltage stability margins and limits of the nodes with higher 4) Mixed: this is a technique based on the combination of the numerical accuracy [8]; they use a prediction tangent vector above mentioned techniques to create a voltage stability to estimate a solution to another load value, which is then prevention and correction scheme using different elements corrected [9]. These methods have been widely used because [18]. of their precision in the voltage stability limit calculation.Dynamic analysis techniques are based on the solution ofalgebraic equations in time-domain [1] and they are used for transient and small signal stability events IV. VOLTAGE STABILITY PROBLEM analysis. These techniques allow to create different scenarios Each node of a power system initially operates with a voltage that include contingencies or normal operation states in order stability margin as shown in Fig. 1(a). Once a contingency to 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 STABILITY The methods used for the solution of the voltage stability are based on the prevention and correction of the problem. Prevention methods are aimed at maintaining voltage stability at all nodes, avoiding them to get close to the limit; correction methods are aimed at restoring the unstable conditions to return voltage stability to the nodes. These methods are divided in: reactive compensation, control of elements and power system changes. Control actions can be automatic or manual and must be used according to the time of response to improve in a short, middle or long-term stability. 1) Reactive Compensation: these techniques are based on the compensation of reactive power to the system, from power generation sources [11], transmission lines [12], transformers Fig. 1. PV curves and Voltage stability margins [13] and load nodes. The line compensation can be carried out with a constant value of reactive power, called static Line compensation using FACTS devices compared with compensation, 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 system carried out, controlled by power electronic devices as configuration or sheds loads, does not interfere with the FACTS; this is called dynamic compensation and is used to availability of generators and can change the direction of the respond during transitory and small signal variations that power flow from one area to another. These techniques can occur in the power system due to disturbances. be used to respond to the events occurred after a contingency, 61 All Rights Reserved © 2012 IJARCSEE ISSN: 2277 – 9043 International Journal of Advanced Research in Computer Science and Electronics Engineering Volume 1, Issue 2, April 2012 with a static or dynamic increasing of the voltage at the nodes. Some methods seek to solve voltage instability by the This methodology is based on a fast injection and the optimal location of FACTS devices close or in the critical direction of power through the transmission path of lower nodes [19]; other methods are based on the coordination of losses to increase the reactive power supply to the critical FACTS 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 elements and SVC) was presented in order to improve angle and to operate, can be applied to global power systems and does voltage stability, using a disturbance response method based not need to relocate FACTS devices so the costs are not on 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 STATCOM devices to provide reactive power; the secondary voltage A. FACTS Selection control 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 controllers transmission system (FACTS) devices in coordination with (SSSC, TCSC, TSSC, TCSR, TSSR, TCPST), combined load 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 allow expressions in steady state [21]. In 2005, a method for controlling power flow, increasing transmission line coordination 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 solution congestion, to give greater security and to minimize the are those that allow reactive power injection and power active losses in transmission lines [22]. FACTS devices transfer with lower losses. location and coordination techniques mentioned above have been based on the voltage stability improvement in an area near the devices, improving voltages in steady state, B. Design preventing violations of the voltage limits, coordinating few The design stage is based on the determination of the quantity and types of FACTS devices, they do not allow to coordinated control strategies of the FACTS devices. The handle 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 comparison among the techniques used for the solution of voltage stability problems after a contingency. The rating of the indexes was done with numbers that indicate the low (1), middle (2) and high (3) levels of the objective functions of the proposed method. V. PROPOSAL The proposed methodology consists on compensating the reactive power in an interconnected power system of several areas as shown in Fig. 2, by a coordinated control strategy of a wide quantity of different FACTS devices located in several parts of an interconnected power system in order to increase the voltage stability margin during critical voltage variations 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 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 of TABLE I the most important parts of the control strategy is the COMPARISON OF THE TECHNIQUE FOR THE SOLUTION OF determination of the type of instability and the magnitude of a VOLTAGE 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, the most 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 to are carried out. reduce reactive losses and to allow a higher reactive supply to the nodes. 3) Voltage Analysis: based on the selection of the most critical contingencies and the nodes for the variables 4) Sending Signals to Controllers: the operation signals for measurement, a dynamic study of the voltages after the FACTS controllers are sent according to the obtained control contingency to determine the system response is carried out. decisions; they control the operation of capacitors and inductors of each FACTS device, allowing the reactive 4) 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 critical nodes of the system. These devices should allow the reactive 5) Devices Operation: the devices operate based on the power supply to the weak nodes and the directing of the signals sent and inject reactive power or allow the pass of the reactive power flow. power flow to a specific node based on the transmission path of lower losses. 5) Operation Verification: after determining the control REFERENCES strategy, an appropriate response for the selected contingencies 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 Power The implementation stage is based on the operation form of System Dynamics Tutorial: Dynamics of Interconnected the coordinated control strategy of FACTS devices. 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