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1 A novel control strategy for Mitigation of Inrush currents in Load Transformers using Series Voltage source Converter Pulijala Pandu Ranga Rao *1, VenuGopal Reddy Bodha *2 #1 PG student, Power Electronics Department, SVEC, Suryapet, Nalgonda,Telangana, India. #2 Assistant professor, Power Electronics Department, SVEC, Suryapet, Nalgonda,Telangana, India Abstract Voltage sag problems are the most frequently occurs in three-phase loads. When voltage sags are occurred in distribution system due to the nonlinear loads, transformers which are connected to critical loads are affected by inrush currents. Voltage sag compensator of series voltage source inverter type with transformer coupling is proposed in this paper for voltage sag compensation in power system. Flux linkage problems are occurs at time of voltage recovery from external device. Inrush currents are the severe problems, these causes failure of distribution equipments when entered into the system. At the time rectification of problem interruption in supply is occurs. In this paper an efficient inrush mitigation technique for series voltage sag compensator with feed-back controller is described. The performance of the proposed voltage compensator is verified by using MATLAB/simulink software. Performance characteristics are described in simulation results of this paper. Index terms Inrush current mitigation, voltage sag compensator, voltage sag, flux linkage problem, PI controller, and power-quality enhancement. I I. INTRODUCTION nrush current, input surge current or switch-on surge refers to the maximum, instantaneous input current drawn by an electrical device when first turned on. The transformers which are connected to critical loads, at the time of starting transformer get subjects high currents. This current in transformer maybe caused by energizing an unloaded transformer, occurrence of an external fault, voltage recovery after clearing an external fault and out-of-phase synchronizing of connected generator. It is very well known that a transformer will experience magnetizing inrush current during energization. For the explanation of the mechanism causing inrush current in a transformer s primary winding when connected to an AC voltage source, consider equation, where λ and v are the instantaneous flux in a transformer core and voltage drop across the primary winding, respectively. FACTS controlling technique is having wide range of applications power quality improvement of power systems [2]-[4]. In this paper a series voltage sag compensator is proposed for voltage sag compensation, and is connected in series to transmission system with transformer coupling, having operation regulates output voltage by injecting voltage in synchronous to input supply. For the controlling of output levels of VSC, a feed-back controller is proposed in this paper. PI controller is used controlling purpose; the new integrated control can successfully reduce inrush current of load transformers and improve the disturbance rejection capability and the robustness of the sag compensator system. The schematic diagram of a series voltage sag compensator integrated to transmission system is shown in fig. (1). V = From above equation, the change in flux linkage in core of the transformer is directly proportional to voltage drop in primary winding. For the rectification of inrush currents in load transformers voltage source converter is proposed [1]. Fig.1.schematic diagram of series voltage sag compensator IJCSIET-ISSUE4-VOLUME2-SERIES4 Page 1

2 This paper is structured as follows: In section II, system description. The proposed closed-loop control strategies series voltage sag compensator in section III. The simulation test results are presented in section IV. And finally section V gives the conclusion of the paper. The synchronous reference transformation is applied for the above equations II. SYSTEM DESCRIPTION The voltage sag compensator consists of a threephase voltage-source inverter (VSI) and a coupling transformer for serial connection. The voltage source inverter is fed from dc source as an external supply. The operation of voltage sag compensator as: when voltage sags occur, the voltage sag compensator injects the required compensation voltage through the coupling transformer to protect critical loads from being interrupted [5] [6]. As shown in fig. 1, the voltage sag compensator consists of three-phase voltage-source inverter and coupling transformer for connection of VSI to transmission system. Dynamics of voltage sag compensator: The equivalent circuit diagram of voltage sag compensator is shown in fig. (2). Fig.2. equivalent circuit of voltage sag compensator The dynamics of equivalent of voltage sag compensator can be written as follows: The circuit dynamics in d-q reference frame can be Here ϕ = And is the angular frequency of the grid. Here is the three-phase inverter output voltage is the compensation voltage is the three-phase inverter current is three-phase capacitor voltage is three-phase load current In the proposed system, the turns ratio of the coupling transformer is set on unitary (N1:N2 = 1:1) to provide the maximum compensation voltage of the 50% of nominal value. Load transformer and coupling transformer inrush mitigations: One of the advantages of the series-connected sag compensator is that it can restore the load voltage with a converter rated at fractions of the load. A similarly rated coupling transformer is thus adopted for the series connection. The coupling transformer is having higher potential of magnetic saturation due to its smaller cores [7]. The general equation that gives the amplitude of inrush current as a function of time can be written as Here maximum applied voltage - Total impedance under inrush, including system - Energization angle For the purpose of designing a protective system for transformer, the peak value of inrush current is an important factor. In these cases, a simplified equation can be used to calculate the peak value of the first cycle of the inrush current. This equation is as follow IJCSIET-ISSUE4-VOLUME2-SERIES4 Page 2

3 Here - maximum applied voltage R Total dc resistance of the transformer, are normal rated, remnant and saturation flux density of the transformer The value of inrush current is dependent to the parameters of transformer and operating condition. III. CLOSED LOOP CONTROL STRATEGY The inrush current mitigation technique is having several control methods are feedback, feed forward and decoupling control. In the proposed paper the state feedback controller is applied [8], shown in fig. 3. It consists of (1) voltage sag detection; (2) feedback control and decoupling terms; (3) feed forward control; (4) inrush mitigation and pulse width modulator. A. Sag detection For the rectification power-quality problem, first need to detect the magnitude of the positive sequence component of the line voltages to identify voltage sag [9]. The positive sequence components are components are eliminated by disturbance filter. Voltage sag detection block then calculates magnitude of positive sequence voltages. The voltage command for compensator can be calculated by difference between pre-fault voltages and a faulted voltage is applied to feed forward controller and feedback controller. B. Feedback control The feedback control consists of capacitor voltage outer loop and inductor current inner loop. This controller is to improve capability of disturbance rejection. The outer voltage loop is based on proportional-integral (PI) controller. An inner current loop is based on proportional regulator. Control circuit diagram of series voltage sag compensator is shown in fig. (3), and block diagram representation of voltage and current loop control is shown in fig. (4). Fig.3. Control circuit of series voltage sag compensator IJCSIET-ISSUE4-VOLUME2-SERIES4 Page 3

4 Fig.4.block diagram of voltage and current loop control C. Feed forward control Feed forward control for the compensator is to get fast dynamics response for effective protection against voltage sag conditions. The voltage drop across the filter inductor is considered to ensure sufficient voltage is produced across the filter capacitor. The feed forward commands, the feedback commands, the command cross-coupling terms, and the decoupling terms are all summed up to establish the complete command voltages for the compensation inverter, then the inrush mitigation and the pulse width modulator will produce the gating pulses of the inverter switch poles. D. Inrush mitigation and pulse width modulator The inverter starts injecting the compensation voltage, when coupling transformer is affected by inrush current due non-linear loads on system end [10]. In the proposed system, an inrush mitigation procedure is adopted by suppressing the voltage commands if the projected flux linkage of the transformer exceeds a certain pre-determined limit. A space-vector pulse width modulator is then applied. Flux linkage of transformer during voltage compensation is written as: IV. SIMULATION TEST RESULTS The verification of proposed series voltage sag compensator is done in MATLAB/simulink software. A simulation model designed for transmission line system with series voltage sag compensator. Control circuit is designed for generating switching pulses for IGBT s in voltage source converter. Closed loop control technique is used for getting of better accuracy in output voltage. Simulation model design for voltage sag compensator integrated with power system is shown in fig. (5). IJCSIET-ISSUE4-VOLUME2-SERIES4 Page 4

5 Fig.5. simulation model of VSC integrated to transmission system Fig.6. simulation model for voltage and current closed loop control The voltage and current loop controller design is based on generation of dq reference frame variables, these variables are taken as input parameters to PI controller. The parameters of PI controller and inrush mitigation block are subjected to closed loop controller circuit. The voltage and current closed loop controller circuit is shown in fig. (6). Inrush mitigation circuit and PI controller circuit is shown in fig. (7). Fig.7. Simulation model of inrush mitigation technique and PL controller. In Simulation results, the input three phase source voltage ( ) and load voltage ( ) waveforms are shown in fig.8. And flux linkage of load transformer three phase current and load current ( ) wave forms are shown in fig.9. IJCSIET-ISSUE4-VOLUME2-SERIES4 Page 5

6 Total harmonic distortion (THD) of load voltage is observed as 1.61% and fundamental component is is shown in fig.11. Fig.8.simulation results of source voltage ( voltage ( ) ) and load Fig.9.simulation results of flux linkage current and load current ( ) Simulation results for flux linkage current of dq axis wave form are shown in fig.10. Fig.10. simulation results for flux linkage current of dq axis In the proposed paper, the voltage sag compensator is designed for power-quality enhancement due to that harmonic distortion is done. Disturbance rejection capability can be understand by studying total harmonic distortion (THD) value. Fig.11. Total Harmonic Distortion (THD) of load voltage V. CONCLUSION In this paper an inrush current mitigation technique is proposed for voltage sag compensator for power-quality enhancement of power system with critical non-linear loads. The control system is proposed with closed loop control both voltage and current loop control. Flux linkage problem when voltage ssag compensation in transformer coupling is eliminated. Inrush current mitigation technique is proposed for VSC, transformer coupling is used for series connection of inverter into transmission system. Feedback control technique is proposed. Dynamic analysis of voltage sag compensator is explained, and flux linkage deviation in load transformer windings is eliminated. The sag detection method is proposed in this paper for the accuracy in sag compensation in the transmission system. The combined voltage and current loop control of voltage sag compensator is utilized for integration with power system. Simulation test results are described for verification of power-quality compensation in outputs. The proposed system is simulated and the results demonstrate improvement in operating characteristics when compared with conventional approaches. The main features of proposed compensator is having easy of control and cost effectiveness. REFERENCES [1] M. S. J. Asghar, Elimination of inrush current of transformers and distributionlines, in Proc. IEEE Power Electron, Drives Energy Syst. Ind. Growth, 1996, vol. 2, pp IJCSIET-ISSUE4-VOLUME2-SERIES4 Page 6

7 [2] G. Zenginobuz, I. Cadirci, M. Erims, and C. Barlak, Performance optimization of induction motors during voltage-controlled soft starting, IEEE Trans. Energy Convers., vol. 19, no. 2, pp , Jun [3] J. Nevelsteen and H. Aragon, Starting of large motorsmethods and economics, IEEE Trans. Ind. Appl., vol. 25, no. 6, pp , Nov./Dec [4] H. Yamada, E. Hiraki, and T. Tanaka, A novel method of suppressing the inrush current of transformers using a series-connected voltage-source PWM converter, in Proc. IEEE Power Electron. Drives Syst.PEDS 2005 Int. Conf., 2006, vol. 1, pp [5] S. Martinez, M. Castro, R. Antoranz, and F. Aldana, Off-line uninterruptible power supply with zero transfer time using integrated magnetics, IEEE Trans. Ind. Electron., vol. 36, no. 3, pp , Aug [6] IEEE Recommended Practice for the Design of Reliable Industrial and Commercial Power Systems, IEEE Standard , [7] P. T. Cheng, W. T. Chen, Y. H. Chen, C. L. Ni, and J. Lin, A transformer inrush mitigation method for series Voltage sag compensators, IEEE Trans. Power Electron., vol. 22, no. 5, pp , Sep [8] J. G. Nielsen, M. Newman, H. Nielsen, and F. Blaabjerg, Control and testing of a dynamic voltage restorer (DVR) at medium voltage level, IEEE Trans. Power Electron., vol. 19, no. 3, pp , May [9] P.T. Cheng; C. -C. Huang; C. -C. Pan; S. Bhattacharya, "Design and implementation of a series voltage sag compensator under practical utility conditions", IEEE Trans. Ind. Applicat., Vol. 39, pp , May-June [10] P.T. Cheng, W.T. Chen, Y.H. Chen, C.H. Wang, "A Transformer Inrush Mitigation Method for Series Voltage Sag Compensators," Industry Applications Conference, Vol. 2, pp , 2-6 Oct Pulijala Pandu Ranga Rao received the B.Tech degree in electrical engineering from JNTU Hyderabad university in 2012 and currently he pursuing M.Tech in power electronics from SV Engineering College (suryapet,nalgonda). His interests include power electronics, distribution level control and renewable energy systems. Venugopal Reddy Bodha received the B.tech degree in electrical engineering from jntu hyderabad in 2007, M.tech in power electronics from jntu hyderabad in 2012, and currently he is working as an assistant professor in eee dept at sv engineering college, suryapet His research interests include, active power filters, power converters distributed power systems and power factor-correction circuits. IJCSIET-ISSUE4-VOLUME2-SERIES4 Page 7