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A Novel Improved Performance of Direct Power Control of Unified Power-Flow Controller Fed Induction Drive System K.VAJRALABABU M-tech Student Scholar Department of Electrical & Electronics Engineering, Prasad .v Potluri Siddhartha institute of technology, Vijayawada-520007 M.DEVIKA RANI Assistant Professor Department of Electrical & Electronics Engineering, Prasad .v Potluri Siddhartha institute of technology, Vijayawada-520007 Abstract— Very Large electric drives and utility applications require advanced power electronics converter to meet the high power demands. As a result, power converter structure has been introduced as an alternative in high power and medium voltage situations. an electrical power system is a large interconnected network that requires a careful design to maintain the system with continuous power flow operation without any limitations. Flexible Alternating Current Transmission System (FACTS) is an application of a power electronics device to control the power flow and to improve the system stability of a power system. This controller is used to maximize the power flow while maintaining stability of the transmission grid. The direct power control is developed from the instantaneous power theory. The direct power control is a important control technique for a unified power flow controller (UPFC). This controller can be used with a voltage source converter. A compared to other power flow controllers, this UPFC provides better results under balanced and unbalanced conditions. Direct power control is a valuable control technique for a UPFC, and the presented controller can be used with any topology of voltage-source converters. The dynamic analysis of proposed method is evaluated by using Matlab/Simulink platform and results are conferred. Energy resources). Unified power-flow controllers (UPFC) enable the operation of power transmission networks near their maximum ratings, by enforcing power flow through well-defined lines [2]–[4]. These days, UPFCs are one of the most versatile and powerful flexible ac transmission systems (FACTS) devices. Index Terms— Direct Power Control, Flexible Ac Transmission Control (FACTS), Multilevel Converter, Sliding Mode Control, Unified Power-Flow Controller (UPFC), Induction Motor Drive. I. INTRODUCTION Electric power flow through an alternating current transmission line is a function of the line impedance, the magnitudes of the sending-end and receiving-end voltages, and the phase angle between these voltages. Essentially, the power flow is dependent on the voltage across the line impedance. Electricity market deregulation, together with growing economic, environmental, and social concerns, has increased the difficulty to burn fossil fuels, and to obtain new licenses to build transmission lines (rights-of-way) and high power facilities. This situation started the growth of decentralized electricity generation (using renewable The existence of a dc capacitor bank originates additional losses, decreases the converter lifetime, and increases its weight, cost, and volume. In the last few decades, an increasing interest in new converter types, capable of performing the same functions but with reduced storage needs, has arisen [10]. In the last few decades, an increasing interest in new converter types, are performing the same functions but with reduced storage needs, has arisen. Matrix converter is highly applicable for ac to ac conversion, allowing bidirectional power flow, assurance of near sinusoidal input and output currents, voltages with variable amplitude, and adjustable power factor [8]. These minimum energy storage ac/ac converters have the capability to allow independent reactive control on the UPFC shunt and series converter sides, while guaranteeing that the active power exchanged on the UPFC series connection is always supplied/absorbed by the shunt connection. Unified Power Flow Controller (UPFC) proposed by Gyugyi. The combination of static synchronous compensator (STATCOM) and a static series compensator (SSSC) which are coupled via a common dc link, to allow bidirectional flow of real power between the series output terminals of the SSSC and the shunt output terminals of the STATCOM are controlled to provide concurrent real and reactive series line compensation without an external electric energy source. It is the most versatile FACTS controller for the regulation of voltage and power flow in a transmission line. Fig.1. Schematic diagram of UPFC. UPFC is the most versatile FACTS controller for the regulation of voltage and power flow in a transmission line. The general configuration of UPFC is shown in above fig.1. It consists of two voltage source converters (VSC) one shunt connected and the other series connected. The DC capacitors of the two converters are connected in parallel. If the switches 1 and 2 are open, the two converters work as STATCOM and SSSC controlling the reactive current and reactive voltage injected in shunt and series respectively in the line [1]. The closing of the switches 1 and 2 enable the two converters to exchange real (active) power flows between the two converters. The active power can be either absorbed or supplied by the series connected converter [1]-[5]. It is important to note that whereas there is a closed direct path for the real power negotiated by the action of series voltage injection through Converters 1 and 2 back to the line, the corresponding reactive power exchanged is supplied or absorbed locally by Converter 2 and therefore does not have to be transmitted by the line[6]. Thus, Converter 1 can be operated at a unity power factor or be controlled to have a reactive power exchange with the line independent of the reactive power exchanged by Converter 2. Obviously, there can be no reactive power flow through the UPFC de link. II. UPFC SERIES CONVERTER MODEL The series and shunt converter of a UPFC are HV power electronics. To minimize the voltage stress on all components while increasing the system voltage level, multilevel neutral point clamped inverters are a promising topology. The DPC control method described in this paper is divided in two parts—a general external part and an internal topology-specific part. The design principles for both are explained in detail. The external part is universal the internal part can easily be adapted to different topologies of voltage-source converters. In this paper, a three-level neutral point clamped converter is used. Other converter topologies use the converter independent part without further theoretical development. The converter topology dependent part can be deduced analogously to the given example. Fig. 2. Schematic of the equivalent circuit of the UPFC system. During model construction and controller design, power sources VS, VR are assumed to be infinite bus. We assume series transformer inductance and resistance negligible compared to transmission-line impedance. Under these assumptions, we can simplify the grid as experienced by the UPFC to Fig. 2. Sending and receiving end power sources VS, VR, are connected by transmission line r L. The total current drawn from the sending end iT consists of the current flowing through the line iS and the current exchanged with the shunt converter iP. Shunt transformer inductance and resistance are represented by LP and rP. The series inductance and resistance are commonly accepted as a model for overhead transmission lines of lengths up to 80 km. The power to be controlled is the sending end power, formed by the current iS and the sending end voltage VS. This is the most realistic implementation for control purposes. The UPFC shunt converter model is similar and is not described in this paper; its functions and control are well described in literature [1], [2] and the performance of the shunt converter is only of secondary influence on the control system described in this paper, as demonstrated in previous work. Effects of dc bus dynamics are negligible in the control bandwidth of the power flow. For all simulations and experiments in this paper, the shunt converter is only used to satisfy active power flow requirements of the dc bus. Using the model of Fig. 2, differential equations that describe the current in three phases can be formulated. Voltages Vabc=VSabc+VCabc-VRabc are used for notation simplicity. The differential equations for the UPFC model are given as L (1) Applying the Clarke and Park transformation results in differential equations in dq space. Voltages V d=Vsd+VcdVRd and Vq=VSq+VCq-VRq are introduced for notation simplicity. It is assumed that the pulsation of the grid is known and varies without discontinuities. Applying the Laplace transformation and with substitution between the two dq space transfer functions, (2) is obtained, where currents isd(s),isq(s) are given in function of voltages Vd(s) and Vq(s). (2) The active and reactive power of the power line is determined only by the current over the line and the sending end voltage. Without losing generality of the solution, we synchronize the Park transformation on VSa, resulting in Vsq=0. Assuming relative voltage stability, Vsd(s) =Vsd, VRdq(s) =VRdq .Active and reactive power at the sending end are calculated as Fig.3. Schematic of the three-level neutral point clamped converter. (6) (3) Substituting (2) into (3), we receive the transfer functions, linking PS(s), QS(s) to VS, VR, and VC(s). Both active and reactive power consists of an uncontrollable constant part, which is determined by power source voltages V S, VR, and line impedance L, r and a controllable dynamic part, determined by converter VC(s) voltage, as made explicit in It is interesting to take a further look at the components of the dynamic part of the active and reactive power , , especially at the response to steps in series converter injected voltage ,VCd/s, VCq/s. Using the initial value theorem on (6), we receive (4) Splitting in a constant uncontrollable and a dynamic controllable part results in (5) and (6). For notation simplicity, VCd(s), VCq(s), are replaced by VCd(s), VCq. (7) It is clear that only VCd (t) affects the derivative d instantaneously, and only VCq (t) affects the derivative d instantaneously. III.THREE-LEVEL NEUTRAL POINT CLAMPED CONVERTER (5) A schematic of a three-level neutral point clamped converter is given in Fig. 3. This topology and its mathematical model have been diligently described in. Each leg k of the converter consists of four switching components Sk1, Sk2, Sk3, Sk4, and two diodes Dk1and Dk2. The diodes Dk1, Dk2, clamp the voltages of the connections between Sk1, Sk2, and Sk3, Sk4, respectively, to the neutral point, between capacitors C1,C2.There are three possible switching combinations for each leg k, thus three voltages umk. The three levels for voltages umk produce five different converter phase-output voltages Uk .The upper and lower leg currents Ik, or their respective sum i, can be described in function of the output line currents ik .The system state variables are the line currents i1, i2, i3, and the capacitor voltages UC1,UC2. This system has the dc-bus current i0 and the equivalent load source voltages Ueqk as inputs. Under the assumption that the converter output voltages Uk are connected to an req, Leq system with a sinusoidal voltage source ueq with isolated neutral, as in Fig. 3, we can write the equations for the three-phase currents i1,i2 ,i3 as in (8) The capacitor voltages UC1, UC2 are influenced by the sum of the upper and lower leg currents i, and the input current , as in (9) From the restrictions on the states of the switching devices in each leg of the converter, we can define the ternary variable , representing the switching state of the entire leg, as (13) With this variable , and the derived variables and , straightforward equations can be found for the description of the other variables in the system. Combining the equations of the system dynamics (8) and (9), the complete system equation is (14), where , , are aiding functions describing the precise dynamics in function of the switching state. It is important to realize that this system equation is not constant, nor continuous. (14) (10) To simplify notation, combinations of this variable and are introduced , , (11) (12) Fig. 4.Vector arrangement in five levels in , for three-level threephase converter. (a) Five levels in , . (b) Five levels in , . If we assume the voltage balance of the capacitors C1, C2, the 27 possible combinations of leg switching state variables , , lead to 27 sets of phase voltages U1,U2,U3 and 27 voltage vectors after Clark transformation to -space. The 27 voltage vectors can be divided in 24 active vectors and 3 null vectors. The 24 active vectors form 18 unique vectors; 12 vectors form 6 redundant pairs. TABLE I OUTPUT VOLTAGE VECTORS Vector C . d( - 1 2 1 1 1 1 1 0 0 3 1 1 -1 0 4 1 0 -1 5 1 0 0 6 1 0 1 7 1 -1 1 8 1 -1 0 9 1 -1 -1 10 0 -1 -1 11 0 -1 0 12 0 -1 1 13 0 0 1 14 15 0 0 0 0 0 -1 16 0 1 -1 17 0 1 0 18 0 1 1 19 -1 1 1 20 -1 1 0 21 -1 1 -1 22 -1 0 -1 23 -1 0 0 24 -1 0 1 25 -1 -1 1 26 -1 -1 0 27 -1 -1 -1 )/dt . 0 sign of the derivative of the voltage unbalance UC1-UC2, the sign of the instantaneous active power P will be used. Since UDC will always be positive, the sign of P depends only on the sign of . Assuming perfect voltage balance, the instantaneous outgoing power P of the converter is given by the internal product of the switching state variables and outgoing line currents scaled by the capacitor voltage by (16) 0 IV. MATLAB/SIMULINK RESULTS 0 Here simulation is carried out in several different cases, in that 1). Proposed Three level NPC Based DPFC, 2). Proposed Three level NPC Based DPFC with Induction Machine Drive. 0 0 Case 1: Proposed three level NPC Based DPFC 0 0 0 0 0 The 3 null vectors also form only 1 unique vector. This results in 19 different voltage vectors. To simplify the vector selection, the 27 vectors are grouped into 5 levels in the , and dimension, based on their component in this dimension. The levels and vector grouping are represented in Fig. 4. Each combination of levels , corresponds to one unique voltage vector. Assuming that the capacitors C1 and C2 have equal capacity and using the relation of the three line currents i1+i2+i3=0, the dynamics of the voltage balance UC1-UC2 can be derived from (14), leading to (15) In Table I, the effect of the output voltage vectors on the capacitor voltage balance is listed. Comparing these values with those of for the values of the redundant vectors, given in bold, they depend on the same currents and except for the sign, are equal. To know the Fig.5 Matlab/Simulink Model of Proposed Three level NPC Based DPFC Fig.5 shows the Matlab/Simulink Model of Proposed Three level NPC Based DPFC using Matlab/Simulink platform. (a) (b) Fig.6. UPFC series converter controlling power flow under balanced conditions, 2.5-s view during stepwise changes of active and reactive power flow reference (a) and (b) Active & Reactive power, Three Phase Currents. Fig.8. UPFC series converter controlling power flow, comparison between controllers Simulation under unbalanced conditions, 70% single-phase voltage sag. Case 2: Proposed Three level NPC Based DPFC with Induction Machine Drive (a) Fig.9 Matlab/Simulink Model of Proposed Three level NPC Based DPFC with Induction Machine Drive (b) Fig. 7. UPFC series converter controlling the power flow under balanced conditions, 250-ms view during stepwise change of active and reactive power flow reference of active power & reactive power, currents and unit voltage values. Fig.10 Stator Currents, Speed, Electromagnetic Torque Fig.10 shows the Stator Currents, Speed, and Electromagnetic Torque of Proposed Three level NPC Based DPFC with Induction Machine Drive. [6] L.Liu, P.Zhu,Y.Kang, and J.Chen, “Power-flow control performance analysis of a unified power-flow controller in a novel control scheme,” IEEE Trans. Power Del., vol. 22, no. 3, pp. 1613–1619, Jul. 2007. [7] S. Ray and G. Venayagamoorthy, “Wide-area signal-based optimal neuro controller for a upfc,” IEEE Trans. Power Del., vol. 23, no. 3, pp. 1597–1605, Jul. 2008. [8] H. Fujita, Y. Watanabe, and H. Akagi, “Control and analysis of a unified power flow controller,”IEEE Trans. Power Electron., vol. 14, no. 6, pp. 1021–1027, Nov. 1999. [9] J. Guo, M. Crow, and J. Sarangapani, “An improved upfc control for oscillation damping, ”IEEE Trans. Power Syst., vol. 24, no. 1, pp. 288– 296, Feb. 2009. [10] M. Zarghami, M. Crow, J. Sarangapani, Y. Liu, and S. Atcitty, “A novel approach to interarea oscillation damping by unified power flow controllers utilizing ultra capacitors, ”IEEE Trans. Power Syst., vol. 25, no. 1, pp. 404–412, Feb. 2010. Fig.11 3-Level Output Voltage Fig.11 shows the 3-Level Output Voltage of Proposed Three level NPC Based DPFC with Induction Machine Drive. V.CONCLUSION However, the extensive use of power electronics based equipment with pulse width modulated variable speed drives are increasingly applied in many new industrial applications that require superior performance. The DPC technique was applied to a UPFC to control the power flow on a transmission line. The technique has been described in detail and applied to a three-level NPC converter. The main benefits of the control technique are fast dynamic control behavior with no cross coupling or overshoot, with a simple controller, independent of nodal voltage changes. The realization was demonstrated by simulation and experimental results on a scaled model of a transmission line. The controller was compared to two other controllers under balanced and unbalanced conditions, and demonstrated better performance, with shorter settling times, no overshoot, and indifference to voltage Unbalance. By the use of UPQC attain constant regularized power and attain the load condition with respect to good stability factor. REFERENCES [1] L. Gyugyi, “Unified power-flow control concept for flexible ac transmission systems,”Proc. Inst. Elect. Eng., Gen., Transm. Distrib. vol. 139, no. 4, pp. 323–331, Jul. 1992. [2] L. Gyugyi, C. Schauder, S. Williams, T. Rietman, D. Torgerson, and A. Edris, “The unified power flow controller: A new approach to power transmission control,” IEEE Trans. Power Del., vol. 10, no. 2, pp. 1085–1097, Apr. 1995. [3] X. Lombard and P. The rond, “Control of unified power flow controller: Comparison of methods on the basis of a detailed numerical model,” IEEE Trans. Power Syst., vol. 12, no. 2, pp. 824–830, May 1997. [4] H. Wang, M. Jazaeri, and Y. Cao, “Operating modes and control interaction analysis of unified power flow controllers,”Proc. Inst. Elect. Eng., Gen., Transm. Distrib. vol. 152, no. 2, pp. 264–270, Mar. 2005. [5] H. Fujita, H. Akagi, and Y. Watanabe, “Dynamic control and performance of a unified power flow controller for stabilizing an ac transmission system,”IEEE Trans. Power Electron., vol. 21, no. 4, pp. 1013–1020, Jul. 2006.