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ACEEE Int. J. on Electrical and Power Engineering, Vol. 02, No. 03, Nov 2011Modeling, Analysis and Simulation of VFT for P...
ACEEE Int. J. on Electrical and Power Engineering, Vol. 02, No. 03, Nov 2011frequencies on both grids. Regardless of power...
ACEEE Int. J. on Electrical and Power Engineering, Vol. 02, No. 03, Nov 2011Since the machine behaves like a transformer, ...
ACEEE Int. J. on Electrical and Power Engineering, Vol. 02, No. 03, Nov 2011and power system#2 is simulated. The simulated...
ACEEE Int. J. on Electrical and Power Engineering, Vol. 02, No. 03, Nov 2011                          REFERENCES          ...
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Modeling, Analysis and Simulation of VFT for Power Flow Control through Asynchronous Power Systems

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Variable Frequency Transformer (VFT) is a
controllable bi-directional transmission device that can
transfer power between asynchronous networks. It avoids both
HVDC link and FACTS based power transmission control
system. Basically, it is a rotatory transformer whose torque is
adjusted in order to control the power flow. In this paper, a
simulated model of VFT is used as a controllable bidirectional
power transmission device that can control power flow through
the connected asynchronous power systems. A simulation
model of VFT and its control system models are developed
with MATLAB and a series of studies on power flow through
asynchronous power systems are carried out with the model.
The response characteristics of power flow under various
torque conditions are discussed. The voltage, current, torque
and power flow plots are also obtained.

Published in: Technology, Business

Modeling, Analysis and Simulation of VFT for Power Flow Control through Asynchronous Power Systems

  1. 1. ACEEE Int. J. on Electrical and Power Engineering, Vol. 02, No. 03, Nov 2011Modeling, Analysis and Simulation of VFT for PowerFlow Control through Asynchronous Power Systems Farhad Ilahi Bakhsh1, Shirazul Islam2, and Mohammad Khursheed3 1 Department of Electrical Engineering, Aligarh Muslim University, Aligarh, India. Email: farhad.engg@gmail.com 2 Department of Electrical Engineering, Teerthankar Mahavir University, Moradabad. Email: shiraz.zhcet@yahoo.com 3 Department of Electrical Engineering, Integral University, Lucknow, India. E-mail: khursheed20@gmail.comAbstract—Variable Frequency Transformer (VFT) is a II. CONCEPT AND COMPONENTS OF VFTcontrollable bi-directional transmission device that cantransfer power between asynchronous networks. It avoids both A variable frequency transformer (VFT) is a controllable,HVDC link and FACTS based power transmission control bidirectional transmission device that can transfer powersystem. Basically, it is a rotatory transformer whose torque is between asynchronous networks [4]. The construction ofadjusted in order to control the power flow. In this paper, a VFT is similar to conventional asynchronous machines, wheresimulated model of VFT is used as a controllable bidirectional the two separate electrical networks are connected to thepower transmission device that can control power flow through stator winding and the rotor winding, respectively. One powerthe connected asynchronous power systems. A simulation system is connected with the rotor side of the VFT and themodel of VFT and its control system models are developed another power system is connected with the stator side ofwith MATLAB and a series of studies on power flow throughasynchronous power systems are carried out with the model. the VFT. The electrical power is exchanged between the twoThe response characteristics of power flow under various networks by magnetic coupling through the air gap of thetorque conditions are discussed. The voltage, current, torque VFT and both are electrically isolated [4-6].and power flow plots are also obtained. The VFT consists of following core components: a rotary transformer for power exchange, a drive motor to control theIndex Terms— Variable frequency transformer (VFT), movement or speed of the rotor and to control the transfer ofMATLAB, Asynchronous power systems, Power flow. power. A drive motor is used to apply torque to the rotor of the rotary transformer and adjust the position of the rotor I. INTRODUCTION relative to the stator, thereby controlling the magnitude and The world’s electric power supply systems are widely direction of the power transmission through the VFT [5]. Theinterconnected. This is done for economic reasons, to reduce world’s first VFT, was manufactured by GE, installed andthe cost of electricity and to improve reliability of power commissioned in Hydro-Quebec’s Langlois substation, wheresupply [1]. There are two ways of transmission it is used to exchange power up to 100 MW between theinterconnection. One is ac interconnection, just connected asynchronous power grids of Quebec (Canada) and New Yorkthe two synchronous networks with ac transmission lines. It (USA) [6].is simple and economic but increases the complexity of power A stable power exchange between the two asynchronoussystem operation and decreases the stabilities of the power systems is possible by controlling the torque applied to thesystem under some serious faults. Another is Back-to-Back rotor, which is controlled externally by the drive motor. WhenHVDC interconnection. The Back-to-Back HVDC is the power systems are in synchronism, the rotor of VFTasynchronous interconnection, which is implemented via remains in the position in which the stator and rotor voltageHVDC for most cases at present. It is easy for bulk power are in phase with the associated systems. In order to transfertransfer and also flexible for system operation. But the design power from one system to other, the rotor of the VFT is rotated.of HVDC system is quite complicated and expensive. The If torque applied is in one direction, then power transmissionHVDC link requires a very costly converter plant at sending takes place from the stator winding to the rotor winding. Ifend and an inverter plant at receiving end. Alternatively torque is applied in the opposite direction, then powerrecently, a new technology known as variable frequency transmission takes place from the rotor winding to the statortransformer (VFT) has been developed for transmission winding. The power transmission is proportional to theinterconnections [2-17]. By adding different devices with it, magnitude and direction of the torque applied. When thepower transmission or power flow can be controlled within two power systems are no longer in synchronism, the rotorand between power system networks in a desired way [3]. of the VFT will rotate continuously and the rotational speed will be proportional to the difference in frequency between the two power systems (grids). During this operation the power flow is maintained. The VFT is designed to continuously regulate power transmission even with drifting© 2011 ACEEE 4DOI: 01.IJEPE.02.03. 23
  2. 2. ACEEE Int. J. on Electrical and Power Engineering, Vol. 02, No. 03, Nov 2011frequencies on both grids. Regardless of power transmission, where,the rotor inherently orients itself to follow the phase angle Vs = Voltage magnitude on stator terminal,difference imposed by the two asynchronous systems [2]. Vr = Voltage magnitude on rotor terminal and Xsr = Total reactance between stator and rotor terminals. III. VFT MODEL AND ANALYSISA. VFT Model In the model, the VFT is a doubly-fed wound rotorinduction machine (WRIM), the three phase windings areprovided on both stator side and rotor side. The two powersystems (#1 and #2) are connected through the VFT as shownin Fig. 1. The power system#1 is connected to the stator sideof the VFT, energized by voltage, VS with phase angle, èS. Thepower system#2 is connected to the rotor side of the VFT,energized by voltage, Vr with phase angle, èr. A drive motor ismechanically coupled to the rotor of WRIM. A drive motor Fig. 2 Power flow from power system #1 to power system #2 usingand control system are used to apply torque, TD to the rotor VFTof the WRIM which adjusts the position of the rotor relative Also θnet = θs - (θr+ θrs) (3)to the stator, thereby controlling the direction and magnitude where,of the power transmission through the VFT. θs = Phase-angle of ac voltage on stator, with respect to a reference phasor, r = Phase-angle of ac voltage on rotor, with respect to a reference phasor and rs = Phase-angle of the machine rotor with respect to stator. Thus, the power flow through the VFT is given by: PVFT = ((Vs Vr/ Xsr) sin(θs - (θr + θrs)) (4) The phasor diagram showing reference phasor, Vs, Vr, s, r, rs and net is shown in Fig. 3. Fig. 1 The VFT model representation It is better to represent the VFT model by an equivalentVFT power transmission or power flow model, as shown inFig. 2. The power flow direction shows the power transmissionfrom power system#1 to power system#2 through VFT. Infact, the direction of power flow could be from powersystem#1 to power system#2 or vice-versa depending onthe operating conditions. If torque is applied in one directionthen power flow takes place from power system #1 to power Fig. 3 The phasors of VFTsystem#2. If torque is applied in opposite direction then power For stable operation, the angle ¸net must have an absoluteflow reverses as shown in Fig. 4. Here, in the power flow value significantly less than 90Ú. The power transmission orprocess, only real power flow is being discussed. power flow will be limited to a fraction of the maximumB. VFT Analysis theoretical level given in (2). Here, the power transmission equations are analyzed based on assumption that VFT is an i) Power Flow from Power system#1 to Power system#2 ideal and lossless machine, with negligible leakage reactanceThe power flow through the variable frequency transformer and magnetizing current. The power balance equation requires(VFT) can be approximated as follow: that the electrical power flowing out of the rotor winding PVFT = PMAX Sin θnet (1) must flow into the combined electrical path on the statorwhere, winding and the mechanical path to the drive system, i.e.. PVFT = Power flow through VFT from stator to rotor, PMAX = Maximum theoretical power flow possible through Pr = Ps + PD (5)the VFT in either direction which occurs when the net angle where,¸ net is near 90Ú. The PMAX is given by: Ps = electrical power to the stator windings, PMAX = Vs Vr / Xsr (2) Pr = electrical power out of the rotor windings and PD = mechanical power from the torque-control drive system.© 2011 ACEEE 5DOI: 01.IJEPE.02.03. 23
  3. 3. ACEEE Int. J. on Electrical and Power Engineering, Vol. 02, No. 03, Nov 2011Since the machine behaves like a transformer, mmf provided where,by the ampere-turns of stator must balance the rotor mmf: Ps = electrical power out of the stator windings, Ns*Is = Nr*Ir (6) Pr = electrical power to the rotor windings andwhere, PD = mechanical power from the torque-control drive Ns = number of turns on stator winding, system. Nr = number of turns on rotor winding, Since the machine behaves like a transformer, the ampere- Is = current to the stator winding and turns must balance between stator and rotor: Ir = current out of the rotor winding. Ns*Is = Nr*Ir (14)Both the stator and rotor windings link the same magnetic Both the stator and rotor windings link the same magneticflux but their frequency differs such that the voltage will also flux but their frequency differs such that the voltage will alsodiffers by the same ratio, therefore differs by the same ratio, therefore Vs = Ns* fs*ψa, (7) Vs = Ns* fs*ψa, (15) Vr = Nr* fr *ψa, (8) Vr = Nr* fr *ψa, (16) and Vr/ Nr = Vs/ Ns * fr / fs (9) and Vr/ Nr = Vs/ Ns * fr/ fs (17)where, where, fs = frequency of voltage on stator winding (Hz), fs = frequency of voltage on stator winding (Hz), fr = frequency of voltage on rotor winding (Hz), and fr = frequency of voltage on rotor winding (Hz), and ψa = air-gap flux. ψa = air-gap flux. The nature of the machine is such that in steady state, the The nature of the machine is such that in steady state, therotor speed is proportional to the difference in the frequency rotor speed is proportional to the difference in the frequency(electrical) on the stator and rotor windings, (electrical) on the stator and rotor windings, frm = fs - fr , (10) frm = fs - fr , (18) and ωrm = f rm *120/ NP (11) and ωrm = f rm *120/ NP (19)where, Combining the above relationships gives the power frm = rotor mechanical speed in electrical frequency (Hz), exchanged with the drive system as NP = number of poles in the machine, and PD = Ps - Pr = Vs*Is - Vr*Ir ωrm = rotor mechanical speed in rpm. = Vs*Is - (Nr*Vs/Ns*fr/fs)*(Ns*Is/Nr)Combining the above relationships gives the power = Vs*Is*(1 - fr/fs)exchanged with the drive system as or, PD = Ps*(1 - fr/fs) (20) PD = Pr - Ps = Vr*Ir - Vs*Is It shows that the electrical power flowing out of the stator = Vr*Ir - (Ns*Vr/Nr*fs/fr)*(Nr*Ir/Ns) winding being only proportional to mechanical power of the = Vr*Ir*(1 - fs/fr) drive system, rotor frequency and stator frequency. Hence, if or, PD = Pr*(1 - fs/fr) (12) the rotor frequency and stator frequency are kept constant,It shows that the electrical power flowing out of the rotor then the electrical power flowing out of the stator windingwinding being proportional to mechanical power of the drive being only proportional to mechanical power of the drivesystem, stator frequency and rotor frequency. Hence, if the system.stator frequency and rotor frequency are kept constant, thenthe electrical power flowing out of the rotor winding being IV. DIGITAL SIMULATION OF VFTonly proportional to mechanical power of the drive system.ii) Power Flow from Power system#2 to Power system#1 A. MATLAB Simulation Model The power balance equation requires that the electrical For MATLAB, here VFT is represented as a wound rotorpower flowing out of the stator winding must flow into the induction machine (WRIM). The WRIM is doubly-fed and iscombined electrical path on the rotor winding and the simulated with the asynchronous machine SI units inmechanical path to the drive system, i.e. MATLAB Simulink [15]. The power system#1 and power Ps = PD + Pr (13) system#2 are simulated with three phase voltage sources as shown in Fig. 5. The three phase voltage source 1 is connected to the stator side of WRIM and the three phase voltage source 2 is connected to rotor side of WRIM. The drive motor is simulated with constant block which gives constant torque. The torque is applied to WRIM as mechanical torque Tm. To simulate various power flow functions, other blocks are also used. The power system#1 is kept at 400V (L- L) and 60Hz whereas power system#2 is kept at 300V (L-L) and 50 Hz. Then this simulated model, as shown in Fig. 5, isFig. 4 Power flow from power system #2 to power system #1 using used to solve electric power system using VFT. Under different VFT torque conditions, the power flow through power system#1© 2011 ACEEE 6DOI: 01.IJEPE.02.03. 23
  4. 4. ACEEE Int. J. on Electrical and Power Engineering, Vol. 02, No. 03, Nov 2011and power system#2 is simulated. The simulated results areshown in Table I, Table II and Fig. 6. Fig. 5 MATLAB simulation model of VFTB. MATLAB Simulation Results represents the power flow towards the power system#1. The power flow through power system#1 and power system#2i) Power Flow from Power system#1 to Power system#2 with the applied torque achieved is shown in Fig. 6. It is evident from the simulated results that under differentexternal torque condition, the power flow through the powersystem#1 and power system#2 is not zero. The magnitudeand frequency of voltage are kept same for all operatingconditions and the power flow through power sytem#1 andpower system#2 under different torque condition are shownin Table I. TABLE I: MATLAB SIMULATION RESULTS FOR VFTIt is clear from table I that under zero torque condition the Fig. 6 The power flow with the applied torquepower flow through the VFT is zero even though there ispower flow through power system#1 and power system#2 CONCLUSIONSi.e. VFT is taking power from both the power systems. The From the simulated results it is evident that both thenegative sign represents the power flow towards the power magnitude and direction of the power flow through thesystem#2. connected power systems are controllable by the externalii)Power Flow from Power system#1 to Power system#2 torque applied to the rotor. Moreover, power flow is directly When the applied torque is in opposite direction then proportional to the applied torque. Hence VFT technologypower flow direction reverses as shown in Table II. provides an option for achieving real power flow control TABLE II: MATLAB SIMULATION RESULTS FOR VFT through asynchronous power systems. The model developed is successfully used to demonstrate the power flow control through asynchronous power systems. The direction and the magnitude of power flow control are achieved. Thus, the VFT concept discussed and its advantages are verified by simulation results.It is clear from table II as the applied torque direction reversesthe power flow direction also reverses. The negative sign© 2011 ACEEE 7DOI: 01.IJEPE.02.03. 23
  5. 5. ACEEE Int. J. on Electrical and Power Engineering, Vol. 02, No. 03, Nov 2011 REFERENCES [8] D. McNabb, D. Nadeau, A. Nantel, E. Pratico, E. Larsen, G. Sybille, V. Do, and D. Paré, “Transient and Dynamic Modeling of[1] N. G. Hingorani, and L. Gyugyi, “Understanding FACTS: the New Langlois VFT Asynchronous Tie and Validation withConcepts and Technology of Flexible AC Transmission Systems,” Commissioning Tests,” Int. Conf. On Power System TransientsIEEE Press/Standard Publishers Distributors, Delhi, 2001. IPST05-075, Montreal, Canada, June 2005.[2] A. Merkhouf, P. Doyon, and S. Upadhyay, “Variable [9] N. Miller, K. Clark, R. Piwko, and E. Larsen, “VariableFrequency Transformer—Concept and Electromagnetic Design Frequency Transformer: Applications for Secure Inter-regionalEvaluation,” IEEE Transactions on Energy Conversion, vol. 23, Power Exchange,” PowerGen Middle East, Abu Dhabi, Januaryno. 4, pp. 989-996, December 2008. 2006.[3] J. J. Marczewski, “VFT Applications between Grid Control [10] M. Spurlock, R. O’Keefe, D. Kidd, E. Larsen, J. Roedel, R.Areas,” IEEE PES General Meeting, Tampa, FL, June 2007, pp. 1- Bodo, and P. Marken, “AEP’s Selection of GE Energy’s Variable4. Frequency Transformer (VFT) for Their Grid Interconnection[4] E. Larsen, R. Piwko, D. McLaren, D. McNabb, M. Granger, Project between the United States and Mexico,” North AmericanM. Dusseault, L-P. Rollin, and J. Primeau, “Variable Frequency T&D Conference, Montreal, Canada, 2006.Transformer - A New Alternative for Asynchronous Power [11] P. Truman, and N. Stranges, “A Direct Current Torque MotorTransfer,” Canada Power, Toronto, Ontario, Canada, September for Application on a Variable Frequency Transformer,” IEEE PES28-30, 2004. General Meeting, Tampa, FL, June 2007.[5] P. Doyon, D. McLaren, M. White, Y .Li, P. Truman, E. Larsen, [12] B. Bagen, D. Jacobson, G. Lane, and H. Turanli, “EvaluationC. Wegner, E. Pratico, and R. Piwko, “Development of a 100 MW of the Performance of Back-to-Back HVDC Converter and VariableVariable Frequency Transformer,” Canada Power, Toronto, Ontario, Frequency Transformer for Power Flow Control in a WeakCanada, September 28-30, 2004. Interconnection,” IEEE PES General Meeting, Tampa, FL, June[6] M. Dusseault, J. M. Gagnon, D. Galibois, M. Granger, D. 2007.McNabb, D. Nadeau, J. Primeau, S. Fiset, E. Larsen, G. Drobniak, [13] P. Marken, J. Roedel, D. Nadeau, D. Wallace, and H. Mongeau,I. McIntyre, E. Pratico, and C. Wegner, “First VFT Application “VFT Maintenance and Operating Performance,” IEEE PES Generaland Commissioning,” Canada Power, Toronto, Ontario, CANADA, Meeting, Pittsburgh, PA, July 2008.September 28-30, 2004. [14] P. Marken, J. Marczewski, R. D’Aquila, P. Hassink, J. Roedel,[7] R. Piwko, and E. Larsen, “Variable Frequency Transformer – R. Bodo, “VFT – A Smart Transmission Technology That IsFACTS Technology for Asynchronous Power Transfer,” IEEE PES Compatible With the Existing and Future Grid,” IEEE PES PowerTransmission and Distribution Conference and Exposition, Dallas, Systems Conference and Exposition, Seattle, WA, March 2009.TX, 2005. [15] F. I. Bakhsh, M. Irshad, and M. S. J. Asghar, “Modeling and Simulation of Variable Frequency Transformer for Power Transfer in-between Power System Networks,” Indian International Conference on Power Electronics (IICPE), Delhi, India, 28-30 Jan., 2011.© 2011 ACEEE 8DOI: 01.IJEPE.02.03. 23

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