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Regenerative loading system 2
 

Regenerative loading system 2

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    Regenerative loading system 2 Regenerative loading system 2 Document Transcript

    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 4, July-August (2013), © IAEME 213 REGENERATIVE LOADING SYSTEM Navonita Sharma1 , Abanishwar Chakrabarti2 , Prabir Ranjan Kasari3 , Bikram Das4 1 Electrical engineering M.Tech National Institute of Technology, Agartala 2 Department of Electrical Engineering, National Institute of Technology, Agartala 3 Electrical Engineering, National Institute of Technology, Agartala 4 Department of Electrical Engineering, National Institute of Technology, Agartala ABSTRACT In this paper, the concept of a” REGENRATIVE LOAD”, is proposed. In this proposal a converter system is designed to make a regenerative load set, that can emulate various active and reactive power load and their various combinations, and also capable of regenerating the consumed power by the load back to the grid. The vector control approach is used for independent control of active and reactive power. The proposed scheme is simulated in MATLAB simulink and gives satisfactory result. I. INTRODUCTION Testing of machine is limited by the size of load and associated cost of energy. Since it is not feasible or economical to test machine on full load, different methodologies are developed to evaluate the performance characteristics of machine. These test methods such as no load test, short circuit test, back to back test, Z.P.F test etc gives approximate result. The converter proposed in this paper is capable of loading a machine up to the ratings of the converter. The power consumed by the converter is feedback to the grid as a result the net power consumed and hence energy cost is very low. Such a loading arrangement may find wide spread application in laboratories where there is diversified requirement of load. INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET) ISSN 0976 – 6545(Print) ISSN 0976 – 6553(Online) Volume 4, Issue 4, July-August (2013), pp. 213-224 © IAEME: www.iaeme.com/ijeet.asp Journal Impact Factor (2013): 5.5028 (Calculated by GISI) www.jifactor.com IJEET © I A E M E
    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 4, July-August (2013), © IAEME 214 Fig1: Basic Block Diagram of Project model. The variable regenerative load proposed here consists of a rectifier and an inverter. The rectifier acts as the load to the machine and the power consumed by the load is controlled by controlling Iq (Quadrature axis current) signal. The inverter acts as the regenerating part, and regenerates the consumed power back to the source/grid. The concept of this loading arrangement is derived from the concept of U.P.F.C (Unified Power Flow Controller) which feed reactive power to the grid. UPFC is primarily used for independent control of real and reactive power in transmission lines for a flexible, reliable and economic operation and loading of power system [1]. The independent control of active and reactive power is done using the vector control approach in dq reference frame [2]. To convert a 3 phase quantity to synchronously rotating d-q reference frame, and vice versa, Park’s transform and Inverse Park’s transform is used [3]. For converting the 3 phase parameters in dq frame a unit vector (sinθ, cosθ) is generated. The unit vector rotates at the synchronous speed along with the dq reference frame. Various methods are there to generate unit vector, the one which is used in the paper is using LPF (Low pass filter) [2] and PLL (Phase Locked Loop) [4]. Both current and voltage are converted into dq reference frame and d-axis and q-axis current controls the reactive and active power respectively. The paper is articulated as follows: Section (II) deals with the theory of the proposed idea. Section (III) gives the introduction to synchronously rotating frame. Section (IV) deals with unit vector generation. Design of controller is discussed in Section (V). Section (VI) deals with converter schematic and feed forward terms. Section (VII) shows current and voltage controller design procedure. Simulation results and analysis is given in Section (VIII). Conclusion and Index is given in Section (IX) and Section (X) respectively. II. THEORY Power flow between any two electrical sources is governed by the equations [5]: s i ni j i j V V P X δ = (1) 2 c o si ji ij V VV Q X X δ = − (2) ( )i jV Vδ = ∠ − (3)
    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 4, July-August (2013), © IAEME 215 From these equations it is clear that, for power flow between two sources, there must be a voltage difference or a phase angle difference between them. It is well known that the active power flows from the leading voltage to the lagging voltage and the reactive power flows from the higher voltage to the lower voltage. Therefore, both active power and reactive power can be controlled by controlling the phase and magnitude of the converter voltage fundamental component with respect to the grid voltage. But phase angle and voltage is present in both equation of active and reactive power, thus there exist a coupling between them, thus to control active and reactive power independently, the above equation cannot be followed since varying voltage or phase angle will vary both active and reactive power. In order to control active and reactive power independently, vector control approach has been taken into account. Vector control is a popular method for speed control of three-phase induction motors, by converting the 3 phase parameters to dq reference frame. The basic idea of vector control scheme is to control the flux producing component (direct axis current) and the torque-producing component (quadrature axis current) in a decoupled manner. Keeping analogy with the above convention, in this paper, the current component is divided into its direct axis and quadrature axis component. The control of quadrature axis current will control the active power and control of direct axis component will control the reactive power. The equations of active and reactive power using direct and quadrature axis components are follow [2]: 2 3 s q s qP v i= (4) 2 3 s q s dQ v i= (5) III. SYNCHRONOUS ROTATING FRAME (D-Q AXIS) The DQ transformation is a transformation of coordinates from the three-phase stationary coordinate system to the dq rotating coordinate system. Let a three phase voltage is represented by these following equations: 2 c o sa sV V tω= (6) 2 cos( 120 )o b sV V tω= − (7) 2 cos( 240 )o c sV V tω= − (8) The transformation from three phases to dq is made in two steps [3]: 1). A transformation from the three-phase stationary coordinate system to the two-phase, so-called ab/αβ, stationary coordinate system: 3 3 2 c o s 2 2 a sV V V tα ω= = (9) 3 3 ( ) 2 sin 2 2 b c sV V V V tβ ω= − = (10) 2) A transformation from the ab/αβ stationary coordinate system to the dq rotating coordinate system: ( c o s sin )dV V Vα βθ θ= + (11) ( sin cos )qV V Vα βθ θ= − + (12) , ( )d w h e r e V V αθ = ∠ − (13)
    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 4, July-August (2013), © IAEME 216 Fig2 [2]: Stationary and synchronously revolving frames of reference. The inverse transformation equations for dq to ab/αβ to abc transformation is given by [3]: ( cos sin )d qV V Vα θ θ= − (14) ( sin cos )d qV V Vβ θ θ= + (15) aV V α= (16) 1 ( 3 ) 2 bV V Vβ α= − (17) 1 ( 3 ) 2 cV V Vβ α= − + (18) IV. UNIT VECTOR GENERATION In order to transform any vector from stationary reference frame into d–q reference frame, the quantities sin (θ) and cos(θ), which are the components of a revolving unit vector are required. These quantities should have the same frequency as that of the system voltage. A. Using LPF Filter The voltage vector Vs is at an angle ωt with respect to the a-axis. A low-pass filter, whose corner frequency equals the mains frequency, delays vsa and vsb by 45◦ , two such filters in cascade delay vsa and vsb by 90◦ . Such filtering and normalization yields cosθ and sinθ, where θ is the angle between a-axis and d-axis required for the transformation. Fig3: Block diagram for unit vector generation using LPF
    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 4, July-August (2013), © IAEME 217 Equations to generate unit vector [2]: 3 3 2 cos( ) 2 cos 4 2 4 s sx V t V π ω θ= − = (19) 3 3 2 sin( ) 2 sin 4 2 4 s sy V t V π ω θ= − = (20) 2 2 2 2 cos sin x x y NORMALISATION y x y θ θ   =  +     =  +  (21) Simulation results using LPF to obtain cosθ and sinθ: Fig4: Generation of cosθ and sinθ using LPF. B. Using PLL PLL techniques cause one signal to track another one. It keeps an output signal synchronized with a reference input signal in frequency and phase. In three phase grid connected system, PLL can be implemented using the d-q transformation and with a proper design of loop filter. Fig.5 shows the block diagram of three phase PLL, where Vabc is the sensed grid voltage which is transformed in to DC components using coordinate transformation abc-dq and the PLL gets locked by setting Vd* to zero. The loop filter PI is a low pass filter. It is used to suppress high frequency component and provide DC controlled signal to voltage controlled oscillator (VCO) which acts as an integrator. The output of the PI controller is the grid frequency that is integrated to obtain phase angle of the converter θ. When the difference between grid phase angle and converter phase angle is reduced to zero PLL becomes active which results in synchronously rotating voltages Vd = 0 and Vq gives magnitude of grid voltage. Fig5: General structure of three phase d-q PLL
    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 4, July-August (2013), © IAEME 218 Simulation results using LPF to obtain cosθ and sinθ: Fig6: Generation of cosθ and sinθ, using PLL. ANALYSIS From Fig4 and Fig6, it is clear that generation of unit vector using PLL gives a much better response compare to LPF filter, as LPF filter response contains transients from 0 to 0.01 sec simulation time. In this paper, we have used PLL to generate cosθ and sinθ. V. DESIGN OF CONTROLLER The overall block diagram of a vector controlled converter is shown in figure7. The grid voltages and the line currents are transformed into d–q reference frame, and are used as feedback variables for the controller as shown in the figure7. The controllers are designed in d–q reference frame. Fig7: Block diagram of a vector controlled front-end converter. As shown in figure 8, the controller has an outer voltage loop to control Vdc. The voltage controller sets the reference to the inner q-axis current controller. The q-axis current loop controls the flow of real power P. There is an independent loop for the control of isd, which controls reactive power as per equations (4) and (5).
    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 4, July-August (2013), © IAEME 219 Fig8: Voltage and current controllers in a vector controlled front-end converter. VI. CONVERTER SCHEMATIC AND FEEDFORWARD TERMS [2] Fig9: Schematic of converter. The voltage equations of the converter in the d–q reference frame are given by (22) and (23), where Rs and Ls are the resistance and inductance, respectively, of the line inductor. 0sd s sd s s sq id di R i L L i V dt ω+ − + = (22) 0sq s sq s s sd iq di R i L L i V dt ω+ + + = (23) A cross coupling exists between the d-axis and the q-axis quantities as seen from the above equations To ensure decoupled control of isd and isq , feed forward terms vdff and vqff respectively, are added to the outputs of the d-axis controller and the q-axis controller as shown in figure8, given by the equations: * '' '' s sq d id dff id L i v v v v G ω = − + = − + (24) * '' '' sq s sd q iq qff iq v L i v v v v G G ω = − + = − + − (25) 2 dc c V G V = (26)
    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 4, July-August (2013), © IAEME 220 VII. CURRENT AND VOLTAGE CONTROLLER DESIGN The converter is modeled using its gain G and the delay time Td. The delay Td is equal to half the time period of the carrier signal. The Bode Plot of the linear transfer function of the model is plotted and the gain of the PI controller is calculated depending upon the desired gain and phase margin [6]. The gain margin, phase margin and gains of PI controller of the controlled system are: Gain Margin: infinite db Phase Margin: 72.2o Proportional Gain: 10 Integral Gain: 2 Fig10: Design of inner loop current controller Similarly with the current controller parameters, the outer loop voltage controller is designed, using the liner transfer function bode plot of the following block diagram: Fig11: Design of outer loop voltage controller 2 3 sq dc sq sq dc v I i K i V = = (27) The Gains of the outer loop PI controller for voltage control are: Proportional Gain: 100 Integral Gain: 1.3
    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 4, July-August (2013), © IAEME 221 VIII. SIMULATION RESULT AND ANALYSIS A 2 KVA, 400V machine is loaded using the proposed converter (Simulated in MATLAB simulink). The system parameters are shown in the table: Table I: Specification of system parameter Sl. No. Parameters Value 1. Source Voltage 400V 2. Inductor 0.2 H 3. Dc Link Capacitor 50 uF 4. Converter Gain (G) 326 5. Carrier Frequency Of Sine PWM Generator (Fsw) 5 KHz 6. Delay Time Td 100us OBSERVATIONS A. For a given reference value of Id = 2A and Iq = 6A, we have obtain the following graph of Id and Iq, which shows that, the reference and output value is almost equal, and both Id and Iq can be controlled independently. Fig12: Control of Id and Iq.
    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 4, July-August (2013), © IAEME 222 B. The power flow equation in d-q reference frame I given by equations (4) and (5), thus, with Vq = 450V, Id = 2A and Iq = 6A, calculated active power P = 1800 watt, and reactive power Q = 600 watt. The following graph shows that the calculated and measured values are almost equal: Fig13: Active and reactive power flowing from machine to load, controlled using vector control. C. The inverter regenerates the power consumed by the load and feedback it to the grid. Keeping id=0, (i.e. the reactive power consumed by the load is fully delivered by the inverter without loading the grid), and controlling iq with Vdc=600V, we observe that some of the active power is consumed, by the power electronic devices but the inverter can feedback almost equal active power to the grid, and the reactive power is zero, as per given reference. The negative value of the power shows that, it is being feedback. Fig14: Active and reactive power being feedback from load grid.
    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 4, July-August (2013), © IAEME 223 OBSERVATIONS TABLE Table II: Control of Id and Iq from machine to load side REFERENCE OBSERVED Id (A) Iq (A) Id (A) Iq (A) 2 6 2 6 Table III: Power flow from machine to load side Calculated Power (W) Measured Power (W) Deviation (%) P Q P Q P Q 1800 600 1740 605 3.33 0.8 Table IV: Power flow from load to grid side Desired Power To Be Feedback (W) Actual Feedback Power (W) Deviation (%) P Q P Q P Q -1800 0 -1710 0 5.0 0 IX. CONCLUSION A variable regenerative load, using a power electronic converter is designed, and controlled using vector control approach. The design of controllers is based on the specification of the system parameters given in table I. PI controllers gain has been calculated using BODE PLOT method. The vector control method is used for independent control of active and reactive power from machine to load and load to source. The controller is designed to give high accuracy that can be seen from the output. The machine is load using various reference values of Id and Iq and the converter give satisfactory output under all conditions employed in the simulation. X. INDEX Vi = voltage of ith source. Vj = voltage of jth source. Pij = Active power Qij = Reactive power X = reactance vsq = quadrature axis voltage component. isq = quadrature axis current component. isd = direct axis current component. Rs = Resistance of Front-End converter. Ls = Inductance of Front-End converter. G= Gain of converter. Vs = source voltage Td = converter time delay C0= dc link capacitor fsw= carrier frequency of sine PWM generator. Vc = the peak of the triangular carrier used in PWM. vdff = d- axis feed forward term. vqff = q-axis feed forward term.
    • International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 4, July-August (2013), © IAEME 224 XI. REFERENCES [1]. Higorani, N.G., Gyugyi,L., Understanding FACTS Devices, IEEE Press 2000. [2]. J S Siva Prasad, Tushar Bhavsar, Rajesh Ghosh and G Narayanan “Vector control of three- phase AC/DC front-end converter”. Sadhana Vol.33, Part 5, October, 2008. [3]. Park, Robert (1929). "Two Reaction Theory of Synchronous Machines". Trans. of the AIEE 48: 716–730. [4]. Sangita R Nandurkar and Mini Rajeev, “Design and Simulation of three phase Inverter for grid connected Photovoltaic systems”. Proceedings of Third Biennial National Conference, NCNTE- 2012, Feb 24-25. [5]. Kundur .P “Power System Stability and control”, McGraw Hill, Inc, 1994. [6]. Norman S. N., “Control System Engineering”. John Wiley & Sons, Inc, 2011. [7]. Ivan Jadric “Modeling and Control of a Synchronous generator With Electronic Load”. Thesis submitted to the Faculty of Virginia Polytechnic Institute and State University, 1998. [8]. JiahuGuo, Luhua Zhang, Fujing Deng, “Decoupled Control of the Active and Reactive Power in Three-phase PWM Converter Based on Inverse System Theory”. International Conference on Automation and Logistics August 18 - 21, 2007, Jinan, China. [9]. SIMULINK, “Model-based and system-based design using Simulink”, Mathworks Inc, Natick, MA, 2000. [10]. Ogata, K., “Modern Control Engineering”, Englewood Cliffs, NJ: Prentice Hall, 2001. [11]. Mohan N. “Power Electronics”, John Wiley and Sons, 1989. [12]. B.Kiran Kumar, Y.V.Sivareddy and M.Vijayakumar, “Comparative Analysis of Sine Triangle and Space Vector PWM for Cascaded Multilevel Inverters”, International Journal of Electrical Engineering & Technology (IJEET), Volume 4, Issue 2, 2013, pp. 155 - 164, ISSN Print: 0976-6545, ISSN Online: 0976-6553. [13]. Manoj Kumar, Dr. F. Ansari, Dr. A. K. Jha, “Analysis and Design of Grid Connected Photovoltaic System”, International Journal of Electrical Engineering & Technology (IJEET), Volume 3, Issue 2, 2012, pp. 69 - 75, ISSN Print : 0976-6545, ISSN Online: 0976-6553. [14]. Bikram Das, Prabir Rn. Kasari, Abanishwar Chakraborti, Prasul Jain, Sanjay Raghuwanshi and Pawan Kr. Navin, “Battery Energy Storage and Power Electronics Based Voltage and Frequency Controller for WECS Connected to Grid”, International Journal of Electrical Engineering & Technology (IJEET), Volume 4, Issue 2, 2013, pp. 283 - 292, ISSN Print: 0976-6545, ISSN Online: 0976-6553.