International Journal of Engineering Technology, Management and Applied Sciences www.ijetmas.com May 2015, Volume 3, Issue 5, ISSN 2349-4476 MAT Lab Implementation of Sen Transformer as a FACTS Device P Yogananda Reddy Assistant Professor VR Siddhartha Engineering College, Vijayawada, A.P.,India R Giridhar Balakrishna Assistant Professor VR Siddhartha Engineering College, Vijayawada, A.P.,India ABSTRACT In conventional AC transmission system, the ability to transfer AC power is limited by several factors like thermal limits, voltage limit, short circuit current limit etc. These limits define the maximum electric power which can be efficiently transmitted through the transmission line without causing any damage to the electrical equipments and the transmission lines. This is normally achieved by bringing changes in the power system layout. Flexible ac transmission systems (FACTS) have been developed for better control of electric power flow through the efficient utilization of existing transmission lines. , In its most exotic form we are using Unified Power Flow Controller (UPFC), static synchronous compensator techniques. But these are employed with power electronic components which are complex in nature. A method is proposed by using the traditional technology of transformer and tap changer. This novel technique includes simplicity of control and by avoiding unnecessary complexity of power electronics. A MAT LAB model is designed for the proposed method of Sen Transformer. increased voltage variation, and ―loop flow‖ of power. The construction of new transmission lines is becoming increasingly difficult because of various reasons, such as unfavorable regulatory, environmental, and public policies and the escalating cost. The power industry is in constant search for the most economic way to transfer bulk power along a desired path. The significant increase in the utilization of the existing transmission systems should be limited by the thermal and not the stability limit. Electric power flow through an ac transmission line is a function of the line impedance (R, XL), the magnitude of the sending-end voltage VS , and the receiving –end voltage Vr, and the phase angle between these voltages as shown in Figure 1. The expressions for power flow at the receiving-end of the line are shown, considering the line is represented in its simplest form with a reactance XL. 1. INTRODUCTION The demand for electrical energy around the world is continuously increasing. The locations for electric generation are based on energy availability and environmental acceptability. The transmission lines are becoming overloaded and experiencing reduced stability, 316 P Yogananda Reddy, R Giridhar Balakrishna An uncompensated active and reactive power flow in a transmission line is typically not optional. If the reactive power flow in the line is reduced, the freed up capacity of the line can be effectively utilized to carry an increased amount of active power. As a consequence, the International Journal of Engineering Technology, Management and Applied Sciences www.ijetmas.com May 2015, Volume 3, Issue 5, ISSN 2349-4476 generator is no longer required to supply the reactive power. The efficiencies of the generator and its coupling transformer also increase. Therefore, the independent control of active and reactive power flow in a transmission line delivers the most revenue from an ac transmission system. Each of the power flow control parameters (voltage, angle, and reactance) can be changed with the use of the following existing solutions: Shunt inductor/capacitor for voltage regulations. Phase-shifting transformer for phase angle regulation. Series inductor/capacitor for series reactance regulation. By changing any one of the parameters using a power flow controller, both the active and reactive power flow in a transmission line can be affected. Consider that the point of compensation in the transmission line is at its sending-end. Assuming that there are no charges in the transmission line‘s impedance and the voltage at the receiving-end, a power flow controller can control the flow of active and reactive power ( P and Q) to be a particular pair of values by modifying the transmission line‘s sending –end voltage to be of one particular magnitude and at a particular angle. A series-connected compensating voltage can modify the transmission line voltage. For a desired amount of active and reactive power flow in the line, the compensating voltage has to be of one particular magnitude and at a particular angle with respect to the line voltage. The compensating voltage is also at any angle with the prevailing line current, and therefore, emulates in series with the transmission line a capacitor that increases the power flow of the line or an inductor that decreases the power flow of the line and a positive resistor that absorbs active power from the line or a negative resistor that delivers active power to the line. Therefore, the desired compensating 317 P Yogananda Reddy, R Giridhar Balakrishna voltage is actually an impedance emulator. Through the use of either the voltage-sourced converter (VSC)-based unified power flow controller (UPFC) or the traditional technology of transformer and tap changer-based ―Sen‖ Transformer (ST), a variable series-impedance is emulated. A 160 MVA-rated UPFC was commissioned in 1998. This UPFC demonstrated for the first time that active and reactive power flow in a transmission line could independently be regulated while maintaining a fixed line voltage at the point of compensation. The VSC-based technology has the capability of providing a fast dynamic response, but this capability is not required in most utility applications where the need is to regulate the line voltage and the power flow in the line(s) in a ―slow‖ manner. Although the UPFC is the most versatile power flow controller that has been ever built, its high installation and operating costs must be reduced before it can be successful commercially in utility applications. The ST is a promising, lowcost power flow controller that provides voltage regulation at a point in a transmission line. Additionally, the ST provides the same independent active and reactive power flow control as the UPFC, albeit at a reduced dynamic rate. The ST uses reliable, costeffective, and proven transformer and tap changer-based technology. Hence, the ST is adequate and economically attractive to meet today‘s utility‘s need for independent control of active and reactive power flow in a transmission line. The objective in this document is to compare the merits and demerits of the traditional technology of transformer and tap changer with the emerging technology of VSC and reveal the need for the new ST as a cost-effective power flow controller. Within the scope of this paper, an ST and a UPFC are studied with both power flow controllers connected to a simple two-bus network. International Journal of Engineering Technology, Management and Applied Sciences www.ijetmas.com May 2015, Volume 3, Issue 5, ISSN 2349-4476 2. TRANSFORMER AND TAP CHANGING TECHNOLOGY The main use of the traditional transformer and tap changer-based technology is as follows. 2.1. Voltage Regulation A voltage-regulating transformer (VRT) connects a voltage that is either in-phase or outof-phase with the phase-to-neutral voltage of the transmission line and in series with the transmission line as shown in Figure2. The result is that the voltage at any point in a transmission line is regulated. The bipolar compensating voltage in any phase is induced, through autotransformer action, in two windings placed on the same phase of the transformer core. In this configuration as shown in Figure 2, a VRT is a single-core transformer. The exciter unit consists of a three-phase (A, B and C). Yconnected primary winding and is connected in shunt with the line. The three-phase primary winding is excited from the three-phase line voltage (VsA,VsB and VsC). A three-phase bipolar compensating voltage (Vs’sA,Vs’sB and Vs’sC) that is either in-phase or out-of-phase with the corresponding phase-to-neutral voltage is generated from the induced secondary voltages. The voltage-regulating unit consists 318 P Yogananda Reddy, R Giridhar Balakrishna of a total of six secondary windings (two windings in each phase). With the use of taps, the magnitude of the compensating voltage Vs‘s is varied. The line is regulated at a voltage Vs‘ from the uncompensated voltage Vs. The corresponding phasor diagram is shown in Figure. The controller, as shown in Figure, is fed with two input signals-one is the exciting voltage Vs and the other is the reference voltage Vs ‗*. The tap control unit, in the controller, monitors the magnitude of the exciting voltage Vs and the reference voltage Vs ‗*, and turns on the appropriate tap, in the voltage-regulating unit, in order to regulate the line voltage at Vs * ‗ .Figure shows the schematic diagram of a thyristor-controlled tap changer. A transformer winding is tapped at various places. Each of the tapped points is connected to one side of a back-to-back thyristor (triac) switch. The other side of each thyristor switch is connected together at point A. Depending on which thyristor is on, the voltage between points A and B can be varied between zero and the fullwinding voltage with desired steps in between. In the mechanical version of this arrangement, a load tap changer (LTC) connects with one of various taps to give a variable number of turns between the connected tap and one end of the winding. 2.2 Phase Angle Regulation A phase angle regulator (PAR), also known as a phase-shifting transformer, connects a voltage that is in quadrature with the phase-to-neutral voltage of the transmission line in series with the transmission line as shown in Figure. The series-connected compensating voltage introduces a phase shift [Figure] whose magnitude (for small change) in radians varies with the magnitude of the compensating voltage in per unit where the phase-to-neutral voltage of the transmission line is the base voltage. In this configuration as shown in Figure, a PAR is a single-core transformer. The exciter unit consists of a three-phase (A, B and C), Y- International Journal of Engineering Technology, Management and Applied Sciences www.ijetmas.com May 2015, Volume 3, Issue 5, ISSN 2349-4476 connected primary winding and is connected in shunt with the line. The three-phase primary winding is excited from the three-phase line voltage (VsA, VsB and VsC). A three-phase bipolar compensating voltage (Vs’sA,Vs’sB and Vs’sC) that is in quadrature with the corresponding phase-to-neutral voltage is generated from the phase-to-phase of the induced secondary voltages. The phase angleregulating unit consists of a total of 12 secondary windings (four windings in each phase). With the use of taps, the magnitude of the compensating voltage Vs‘s is varied. The three-phase compensating voltage is electrically isolated and connected in series with the transmission line. The line is regulated at a voltage Vs‘ from the uncompensated voltage Vs. The corresponding phasor diagram is shown in Figure. together, it requires the use of a single-core transformer with a three-phase, Y-connected primary winding in the exciter unit and a total of 18 windings (six for voltage regulation and 12 for phase angle regulation) and nine LTCs (three for voltage regulation and six for phase angle regulation) in the voltage and phase angle-regulating unit. It would be advantageous to use a scheme that is based on a single-core, three-phase transformer and tap changers in order to generate the required compensating voltage Vs‘s that modifies the effective sendingend voltage Vs‘. The new ST requires the use of only nine secondary windings as compared to 18 windings that are needed when the compensating voltage is segregated into its direct and quadrature components and controlled separately using a VRT and a PAR, respectively. 3. The magnitude and the angle of the effective sending-end voltage Vs‘ can be regulated with the use of a VRT and a PAR, respectively. In order to implement both of these functions 319 P Yogananda Reddy, R Giridhar Balakrishna SEN TRANSFORMER International Journal of Engineering Technology, Management and Applied Sciences www.ijetmas.com May 2015, Volume 3, Issue 5, ISSN 2349-4476 Figure.3 shows an ST, which is a single-core, three-phase transformer with a Y-connected primary winding and nine secondary windings. The ST provides two functions Voltage regulation; Impedance regulation for independent control of bidirectional active and reactive power flow As shown in Figure 3, the voltage Vs at any point in the electrical system is applied to a shunt-connected single-core, three-phase transformer‘s primary windings. A total of nine secondary windings (a1, c2, and b3 on the core of A-phase, b1, a2, and c3 on the core of Bphase, and c1, b2, and a3 on the core of Cphase) constitute the voltage and impedanceregulating unit. By choosing the number of turns of each of the three windings, and therefore, the magnitudes of the components of the three 120degree phase-shifted induced voltages, the compensating voltage Vs’s in any phase is derived from the phasor sum of the voltages induced in a three-phase winding set (a1, a2, and a3 for injection in A-phase, b1, b2, and b3 for injection in B-phase, and c1, c2, and c3 for injection in C-phase). The compensating voltage is of line frequency and is connected in series with the line through autotransformer action. When an ST is used as a voltage regulator [Fig.3(b)], the in-phase component of the compensating voltage for any phase is induced in a winding that is placed on the corresponding phase of the transformer core. The out-of-phase component of the compensating voltage for that phase is derived from the phasor sum of the voltages induced in two equal-turn windings that are placed on the remaining two phases of the transformer core. For example, the in-phase component of the compensating voltage for the A-phase is induced in a winding that is placed on the core with the exciting primary winding of the A-phase. The out-of-phase component of the compensating voltage for the A-phase is derived from the phasor sum of the voltages 320 P Yogananda Reddy, R Giridhar Balakrishna induced in two equal-turn windings that are placed on the core with the exciting primary windings of the B-phase and the C-phase, respectively. The effect is such that the transmission line voltage at a point is regulated. When an ST is used as an impedance regulator [Fig.3(c)], the series-connected compensating voltage Vs’s modifies the effective sending-end voltage Vs’ in order to selectively control the active and the reactive power flow of the line. The compensating voltage is at any angle with the prevailing line current. The active or direct component of the compensating voltage provides the series resistance emulation, whereas the reactive or quadrature component provides the series reactance emulation. Both functions of voltage regulation and impedance regulation can be implemented in just one unit by proper programming of the tap control unit. Notably, each of a1, b1, and c1 is tapped at the same number of turns; each of a2, b2, and c2 is tapped at the same number of turns; each of a3, b3, and c3 is tapped at the same number of turns. However, the number of turns in the a1-b1-c1 set, a2-b2-c2 set, and a3b3-c3 set can be different from each other. The series-connected compensating voltage is derived from the line voltage through transformer action with the shunt-connected primary windings. Therefore, the exchanged active and reactive power with the line must flow through the primary windings to the line. A series-connected compensating voltage that is X% of the line voltage provides a shunt current that is the same X% of the line current. The shunt current through the exciter unit has both active and reactive components. The International Journal of Engineering Technology, Management and Applied Sciences www.ijetmas.com May 2015, Volume 3, Issue 5, ISSN 2349-4476 loading effects of these two currents on the power system network are independent of each other as shown in Fig.4. Therefore, if it is desirable to compensate the combined loading effects of the active and the reactive current through the exciter unit into the power system network, a separate shunt-connected reactance compensator, such as a static var compensator (SVC) may be considered. There are instances when only any one of the three secondary windings in each phase is required to carry the rated current at rated voltage. Therefore, each of the nine secondary windings must be designed to carry the rated current at rated voltage. This require the magnetic rating of the ST to be 2p.u. Note that the efficiency of an ST is in the range of 99.7% when mechanical LTCs are used and in the range of 99% when thyristor-controlled LTCs are used. The ST, with 360degree of voltageinjection capability, uses only three primary windings and at the most six secondary windings at any given operating point. The remaining three secondary windings stay inactive. Therefore, it is possible to achieve a 360degree operating range just by using six secondary windings instead of nine secondary windings with hardware configuration in every 120degree of operation. In this case, the magnetic rating of the ST is only 1.5 p.u., instead of 2 p.u. Note that by using six secondary windings, one of the six operating regions (0 to 120°, 120 to 240°, 240 to 360°, 60 to 60°, 60 to 180°, and 180 to 300°) can be selected. 321 P Yogananda Reddy, R Giridhar Balakrishna A comment on the ST‘s operation during line faults is as follows. Generally, to limit a phaseto-neutral fault current, additional inductive impedance in the path of the fault is needed. If, for example, the additional emulated impedance is equal to the equivalent line impedance, the steady-state fault current is reduced to half of the natural fault current. The steady-state voltage across the emulated impedance is half of the phase-to-neutral voltage. In the worst case, the emulated impedance must withstand twice the voltsecond that is needed to support half of the steady-state phase-to-neutral voltage in order to avoid saturation. The relatively insignificant (typically 0.05 to 0.15 p.u.) voltage rating of the series core of an ST would saturate early in the fault cycle. The saturated windings do not provide significant current-limiting effect in the ST. Therefore, the ST rides through a fault. Figure.4.3. shows a basic ST model. The voltage and impedance-regulating unit injects a voltage Vs’s whose active and reactive components with load convention Vd and Vq, respectively, in series with the transmission line. This, in turn, changes the voltage VX across the transmission line, and hence, the current and the power flow through the transmission line change. The compensating voltage Vs’s is at any angle with the prevailing line current I. The component Vd of the compensating voltage that is either in-phase or out-of-phase with the line current emulates a positive or a negative resistor in series with the transmission line. The remaining component Vq that is in quadrature with the line current emulates either an inductor a capacitor in series with the transmission line. The compensating voltage Vs’s delivers and absorbs both active and reactive power (Peach and Qeach), which are defined as Peach = - Vs’s.I=VdI=VsId Qeach= -Vs’s*I=VqI=VsIq Note that when the series-connected compensating unit emulates a capacitor, the International Journal of Engineering Technology, Management and Applied Sciences www.ijetmas.com May 2015, Volume 3, Issue 5, ISSN 2349-4476 shunt-connected exciter unit emulates an inductor and vice versa. Also, when the seriesconnected compensating unit emulates a ―+R‖, the shunt-connected exciter unit emulates a ―R‖ and vice versa. 4. ALGORITHM The series compensating voltage Vs‘s in any phase is derived from the contributions of the compensating windings of the ST from three different phases. If the phase angle of the series compensating voltage is exactly at 0degree, 120degree or 240degree, it can be constructed from only one of the three phases a, c or b, respectively. For any other angle, the series compensating voltage is constructed from two adjacent voltages. Consider an ST, as shown in Fig.5, which has four tap position in each of the nine compensating secondary windings. Each tap position provides a voltage of 0.1 p.u. and therefore, a maximum of 0.4 p.u. is obtained from each phase. The possible combinations of voltage tap-setting positions are shown by the dotted grid in Fig.4.2. Let Vs‘s be the required compensating voltage, at an angle with reference to the corresponding phase angle. Then, one of the four combinations enclosed by the dashed circle must be selected. In addition, the selected combination must be the nearest to the voltage vector, Vs‘s. In Fig.6.1, the circle is shown in an enlarged view, where the four combinations marked as 1, 2, 3 and 4 are vector distances T1, T2, T3 and T4, respectively, apart from the desired series voltage Vs‘s. These four vector distances indicate the error introduced due to the selection of any particular combination. Of the four distances, the one with the least magnitude will introduce the least error and the corresponding tap-setting combination would be selected to construct Vs‘s. Let Vs‘sx and Vs‘sy donate the two rectangular components of Vs‘s in the Cartesian coordinate system. Similarly, let Vkx and Vky be the components of Vk (k=1, 2, 3, 4), formed by 322 P Yogananda Reddy, R Giridhar Balakrishna the four possible tap-setting combinations. Then, the error with respect to Vs‘s is defined as The tap-setting combination corresponding to Vk with the smallest is selected as the best tap setting to implement Vs‘s. Thus, the algorithm to determine the best tap setting for Vs‘s consists of the following steps. Step 1) Get the input (magnitude Vs‘s, and leading phase angle, β) about required series voltage injection in phase a. Step 2) Based on β, identify the zone into which the series voltage phasor falls: Zone 1 (0 °< β ≤ 120°), Zone2: (120°< β ≤240°) and Zone3: (240°< β≤ 360°). Next, identify the contributing (one or two) phase(s) and set the contribution of the zero other zero phase(s). When β is exactly equal to 0°, 60°, 120°, 180°, 240° and 300° and the magnitude is exactly midpoint in between two consecutive grid positions, select the higher position. International Journal of Engineering Technology, Management and Applied Sciences www.ijetmas.com May 2015, Volume 3, Issue 5, ISSN 2349-4476 Step 3) Based on the magnitude of Vs‘s, identify the four nearest tap-setting positions [dot positions on the grid in Fig.6.1]. Step 4) Calculate the normalized vector distances between voltages produced by these four tap positions and the compensating voltage Vs‘s. This will give the magnitude of errors ϵk (k= 1, 2, 3, 4) that would be introduced due to the selection of corresponding tap settings. Step 5) Compare the errors and identify the tapsetting combination that yields the minimum error. Step 6) Implement the tap setting in corresponding phase(s) in the ST through the use of load tap changers. Step 7) Implement similar tap settings for voltages to be added in series with phases b and c. 5. MODELLING OF “SEN” TRANSFORMER A digital computer simulation model of the ST has been developed using MAT Lab. The model consists of two subsystems: the electrical subsystem and the tap-selection algorithm subsystem. Figure illustrates the interface between the two subsystems. 5.1. ELECTRICAL SYSTEM Figure 7 show the electrical system and the ST. The electrical system is comprised of two ac systems connected by a three phase transmission line. The ST is connected at the sending end of the transmission line. Table1 gives the parameters for both the ST and the network. 5.1.1. Electrical Network Model: The ac sources at both sending and receiving ends are modeled as infinite sources with the same magnitude but at a phase difference of 20° (the receiving end voltage lags the sending end voltage). The transmission line is modeled as lumped series impedance. 5.1.2. ST Model: The ST is a specially designed transformer with multiple windings having multiple tap positions in the secondary. 323 P Yogananda Reddy, R Giridhar Balakrishna The model for such a transformer is not available in MATLAB. Therefore, nine singlephase transformers, each having on-load tap changing capability have been used to model the ST. By using single-phase transformers, inter-phase mutual flux linkage and thus mutual inductance has not been considered, which may cause some discrepancies in the results. These nine single-phase transformers are modeled with a small resistance and leakage reactance as shown in Figure. Output voltages of three transformers (contributing from phase a, b and c) are added in series and then fed to one phase of the transmission line. The nine outputs (aa, ab, ac, ba, bb, bc, ca, cb and cc) from the tapselection algorithm supply the value of tap setting to all nine transformer Tap terminals. Should these outputs undergo any changes; the transformers readjust their tap positions and produce the required compensating voltages. 5.1.3. Tap Changer Model: In a practical transformer, tap changing is performed through a tap selector; where a resistor or inductor is used in parallel with the tap positions to limit the current through a shorting winding segment between two consecutive taps. In Figure, an example of tap-changing operation has been shown along with the equivalent MATLAB model for each position of the tap selector. Although, practical transformers on load tap changers (OLTC), such as the ones form Reinhausen use taps with voltage difference in the range of 0.02 p.u. to 0.067 p.u., in this model a voltage difference of 0.1 p.u. between taps has been assumed for the clarity of International Journal of Engineering Technology, Management and Applied Sciences www.ijetmas.com May 2015, Volume 3, Issue 5, ISSN 2349-4476 simulation. The time required to move the tap selector between adjacent tap positions is 2 s. In order to move the tap selector from its initial position, terminal E (0.2 p.u.) to its final position, terminal C (0.1 p.u.), the following four steps of approximately 0.5 s each are performed. Step 1: In position 1, the tap selector is connected to terminal E; therefore, in the model the circuit breaker (CB) connecting terminal E to line is closed and the rest of the CBs are open. Now, the selector is moved to a position where it connects both terminal D and E; however, the current flows through terminal E alone. To model this situation, the CB connecting terminal D is closed. Step 2: The tap selector moves further down and is connected to terminal D. The line current now flows through the resistor, and depending on the value of the resistance, a slight dip in the voltage may occur. In the model, CB connecting terminal E is opened. Step 3: In this step, the tap selector is moved to a position, where it connects both terminals C and D. This is the situation where a circulating current will flow within the loop formed by the terminals C, D and the resistor. The higher the value of the resistor, the lower will be the circulating current, however, too high a value of resistance would a voltage sag. In the model, the CB connecting terminal C is now closed. Step 4: In the final step, the tap selector is moved to allow contact with terminal C 324 P Yogananda Reddy, R Giridhar Balakrishna alone, which is the required final position. In the model, the CB connecting terminal D is opened. 5.1.4. Measurements: Measurement blocks are used to measure electrical signs and hence, to calculate powers at the receiving end using the following equations: Pr= (vaia+ vbib+ vcic) Qr= √3 (vaic – vcia) Where va, vb, and vc voltages of phase a, b and c, respectively, and ia, ib and ic are the currents in the respective phases. 5.2. TAP-SELECTION ALGORITHM: The tap-selection algorithm has been implemented as a FORTRAN program linked to the electrical system using an interfacing block created in MATLAB. Although MATLAB has the capability of interfacing other scripts such as MATLAB, C or C++, FORTRAN has been chosen for its simplicity and the speed of implementation. The inputs to the interfacing block are the magnitude (Vs‘s) and the phase angle (β) of the compensating voltage and the outputs are the tap positions for the nine compensating windings of the ST. Any change in the demand of series voltage is passed to the FORTRAN program which implements the necessary tap positions which are sent to the electrical system and implemented through the on-load tap changer of the transformer. A Time block has been used to synchronize the instant of inputs (Vs‘s and β) with the simulation time. Two table blocks have been introduced to serve the purpose of a lookup table. The inputs which vary with respect to time, can be predefined through these blocks for both Vs‘s and β. Based on these inputs, the tap-selection program produces the value of tap ratio through the outputs aa, ab, ac, ba, bb, bc, ca, cb and cc. The first letter of the output variable (for example, aa) denotes the contributing phase and the second letter indicates the phase in which the voltage is added in series. These outputs are International Journal of Engineering Technology, Management and Applied Sciences www.ijetmas.com May 2015, Volume 3, Issue 5, ISSN 2349-4476 then passed to the compensating transformers for the readjustment of tap setting. 6. RESULTS All the off-line and real-time simulations are performed using a time step of 100. In these simulations, it is assumed that each secondary winding is capable of injecting up to 0.4 p.u. series voltage in the line (9 tap positions for each secondary winding), although in Fig.4.2. only 5 tap positions are shown. Before injecting any series voltage, the system was simulated in an uncompensated mode with the network parameters listed Steady State Results: In this case the simulation of the ST is carried out for a compensated series voltage of 0.1 p.u 0.4 p.u. The angle β is varied at a discrete step of 1° in the range of 0° to 360° for a tap resolution of 5%. The variation of both Pr and Qr are shown in Fig.10 respectively, for four magnitudes of injected voltage as well as the uncompensated mode (zero voltage injection). It is evident that both of the active and reactive power follows an almost first-order-hold sinusoidal pattern by varying the phase angle. Nevertheless looking at the active power profile, it can be seen that the difference between the peak magnitude of the compensated and uncompensated active power in the first half cycle is smaller than that in the second half cycle. In other words the ST has a higher capacity when regulating active power below the natural power rather than above it. The reason for this asymmetry in the behavior of the ST can be described as follows. As we reached the peak magnitude of active power, a large amount of in phase currents flow through 325 P Yogananda Reddy, R Giridhar Balakrishna the transmission line and ST impedances. Also, the ST is modeled as a set of 12 coupled windings with nonzero coupling coefficients between the secondary windings. This means that if more than one secondary winding has a non-zero tap value, then there would be a secondary effect voltage drop or voltage boost in the ST injected voltage due to the currents flowing through the other secondary windings. Table II shows the tap positions of the ST secondary windings for four extreme values of Pr and Qr, As can be seen, at the peak magnitude of active power ( Prmax) secondary windings of S1 and S2 are in series with the line and both of them are in their full tap positions ( maximum resistance and inductance ). The effect of interaction of these two sets of secondary winding on the ST injected voltage at high levels of active power can be explained by the Phasor diagram shown in fig 11, which is drawn based on the following assumptions. Resistances of secondary windings are neglected. All the inter phase coupling coefficients are assumed to be zero, in practice inter phase couplings are very small compared to same-phase couplings. Quadrature component of line current is neglected in the peak value of active power, in the simulation results it was about 7% of the inphase component. For the case of reactive power, the situation is reversed. Flow of high quadrature current in the line will increase the ST injected voltage rather than decreasing it. This can be seen from fig 9.1, where the distance between the negative peak value of Qr and Qn is larger than the International Journal of Engineering Technology, Management and Applied Sciences www.ijetmas.com May 2015, Volume 3, Issue 5, ISSN 2349-4476 distance between positive peak value of Qr and Qn. It is to be noted that if separate units (zero mutual couplings) are used for different sets of secondary windings, the mutual effect of secondary‘s on each other will be eliminated and the above asymmetry in active and reactive powers will disappear. The relationship between Pr and Qr for Vs1s of 0.1 p.u, 0.2 p.u, 0.3 p.u and 0.4 p.u is shown fig . It was found that the ST produces a nearly hexagon profile compared to the circular profile of UPFC(2). 7. CONCLUSION The ST is a unique device for controlling voltage. Phase angle and power flow in a transmission network. Despite its relatively slower response in comparison to the UPFC. The advantages of the ST lie in the simplicity of its control. the absence of unnecessary complexity of power electronics. and the overall lower cost and higher efficiency of the power-flow controller. Up to now a detailed transient model of the ST was not available. Which prompted users to employ approximate models of simplified transformers available in EMTP—type programs. This precluded transparency in the simulation and control design stages. This paper has developed an accurate ST transient model from 326 P Yogananda Reddy, R Giridhar Balakrishna re-alistic components thus providing greater insight into the ST functionality. The two salient features of this model are the effect of mutual coupling in the ST secondary windings. and the nonlinear excitation characteristic heretofore ignored in the previous works. The inclusion of these effects made the ST model developed more realistic. This also opens the way for incorporating the model into commercial EMTP—type programs. Real-time implementation of the entire system model and the controller was carried out on a state-of-the art simulator. The real-time simulation is fully interactive as though the user is communicating with the real system. Realtime oscilloscope results are given to demonstrate the performance of the model and the controller. The execution time of the simulation is quite low: on a 100µs time-step the ST model execution is only 75 µs. These numbers show that the developed ST model is computationally efficient and that the control algorithm is suitable for online applications. REFERENCES K. K. Sen and M. L. Sen, ―Introducing the family of "sen" transformers : A set of power flow controlling transformers,‖ IEEE Trans. Power Del.,vol. 18, no. 1, pp. 149–157, Jan. 2003. M. O. Faruque and V. Dinavahi, ―A tap-changing algorithm for the implementation of ―sen‖ transformer,‖ IEEE Trans. Power Del., vol.22, no. 3, pp. 1750–1757, Jul. 2007. Babak aschari, M. Omar faruque and Venkata Dinavahi‖, Detailed real time transient model of the ―sen‖ transformer‖, IEEE Trans. Power Del. Vol.23,no.3,July 2008. K. K. Sen and M. L. Sen, ―Comparison of the sen transformer with the unified power flow controller,‖ IEEE Trans. Power Del., vol. 18, no. 4,pp. 1523–1533, Oct. 2003. Understanding FACTS by NARAIN G. HINGORANI, LASZLO GYUGYI.