Impact of DFIG base Win Energy Conversion System on Fault Stuies an Power Swings Likin Simon Electrical Engineering Department Inian Institute of Technology, Maras Email: ee133@ee.iitm.ac.in K Shanti Swarup Electrical Engineering Department Inian Institute of Technology, Maras Email: swarup@ee.iitm.ac.in Abstract The integration of renewable energy sources into power systems changes the transient an fault current characteristics of the conventional gri. The variation in the system response uring gri isturbance like suen loa changes or fault woul cause the failure of traitional protection an control. Since the operating conition of DFIG epens on the characteristics of the gri an its control, the short circuit current in the system becomes more complex. This paper analyzes the short circuit characteristics of DFIG an evelops an analytical expression for the three phase fault current. Depening on the severity of the fault in terms of voltage rop, the fault current responses are evelope with an without crow bar resistance. The power swings that originates in the system when the fault is cleare are also analyze in the paper. The transient simulation stuies are performe in PSCAD/EMTDC by integrating DFIG to WSCC system. Case stuies are performe to analyze the impact of location of fault to the power swing an fault current contribution. Inex Terms Doubly fe inuction generator (DFIG), fault, power oscillation, reactive power, rotor angle oscillation, short circuit. I. INTRODUCTION The rise in electrical power eman an environmental concerns are riving the power gri towars greener an renewable energy integration. Among the renewable energy integration, win plants are growing fast in recent years [1]. With The DFIG base Win Energy Conversion System (WECS) is the most common in win power plants ue to its variable spee operation [2]. With the increase penetration of DFIG, the fault stuies becomes more complicate an power oscillations ue to any isturbance in the system nee to be analyze in etail. The capability of Low Voltage Rie Through (LVRT) [3] an four quarant operation which makes possible to operate at any esire power factor, makes DFIG base WECS ifferent from other renewable sources. The intermittent nature of win energy can also be mitigate upto an exten by proviing the battery storage in the DC link of DFIG [4]. DFIG base WECS consists of three parts as shown in Fig. 1: 1) Mechanical win riven turbine which can operate at variable win spees, 2) Doubly Fe Inuction Machine whose rotor terminals are externally available an 3) back to back voltage source converter (VSC) which ensures the biirectional power flows uring variable spees. The stator terminal of the machine is normally irectly connecte to the gri an the rotor terminals 978-1-4799-5141-3/14/$31. c 216 IEEE Fig. 1. Configuration of DFIG are connecte to the back to back converters. This converter is hitche back to the gri through a transformer to reuce the voltage rating of the converter. The back to back VSC converters are normally low rate (one thir rating of the machine). The integration of win energy makes the short circuit stuies more intrinsic. The 3 generator 9 bus, Western System Coorinating Council (WSCC) system is selecte for etaile moeling for transient stability stuies. The excitation control, governor control are moele for analyzing the rotor angle oscillation of the synchronous generators. Suen loa isturbance, in the extreme case of faults, will isturbs the rotor torque an correspons to rotor angle oscillation. These isturbance in the rotor angle will reflect as power oscillation in the entire system an in worst case, it can leas the system to collapse. The integration of low inertia renewable sources will make the system more weak [5]. The transient stability improvement with DFIG system is iscusse in [5]. The change in ynamics an operational characteristics of the conventional gri ue to the large penetration of DFIG is aresse. A ecouple FRT technique to enhance the system stability is iscusse in [6]. The optimal crowbar resistance is foun to improve the power transfer capability of the system. The etaile stuy on impact of the penetration of win integration shoul be one before implementing the control schemes. This paper iscusses the short circuit analysis an power angle oscillation with high penetration of win integration with power system.
II. TRANSIENT MODELING OF DFIG The basic circuit configuration of DFIG is as shown in Fig. 1. The stator terminal of woun rotor inuction machine is connecte irectly to the gri an rotor terminals through a step up transformer to reuce the voltage rating of the converters. During ifferent moe of operations like sub synchronous an super synchronous, the rotor circuit nees to supply power in both the irections. The back to back converters of IGBT switches with anti parallel ioes are employe as shown in Fig. 1 which allows the biirectional power flow. The DFIG is moele in synchronous reference frame (-q) to get the inepenent control of active an reactive powers [7]. The electromagnetic torque evelope in the machine can be expresse in terms of irect an quarature axis currents an flux as follows, T em = 3 2 P L m L s (ψ qs i r ψ s i qr ) (1) Where, P is the no of poles, L m an L s are mutual an self inuctances, ψ s an ψ qs are the axis an q axis stator flux, i r an i qr are the axis an q axis rotor currents. The synchronous rotating frame is selecte such that axis is aligne with the stator flux which results, ψ s = ψ an ψ qs = (2) Then the torque equation reuces to Similarly, Where, T em = 3 2 P L m L s ( ψ s i qr ) (3) ims = (1+σ s )i s +i r (4) ims : Magnetizing current σ s : Stator Leakage factor. σ s = Ls L m i s an i sq : axis an q axis stator currents Therefore the reactive power require to provie the magnetizing current can be fe either from stator sie or rotor sie. In this paper, all the magnetizing current is fe from rotor sie to maintain unity power factor at the DFIG terminals. The voltage balance equations at the gri sie converter terminals can be written as, v a i a v b = R i b +L i b + v bg (5) t v c i c i c v cg i a v ag Where, v a,v b,v c an v ag,v bg,v cg are the converter terminal an gri voltages respectively. The three phase active an reactive power can be expresse in q frame as, p = v i s +v q i qs (6) q = v q i s v i qs (7) For gri sie converter, axis is aligne with the stator voltage, v = V an v q = (8) Fig. 2. frame Vs I s R s σ sl σ rl L + jω rψ r Equivalent circuit of DFIG for transient stuies in stator reference The active an reactive power expression can be reuce to p = v i s an q = v i qs (9) Therefore, i s is proportional to active power flow an i qs is proportional to reactive power flow through the gri sie converter. The Rotor Sie Converter (RSC) is employe to control the electromagnetic torque evelope in the machine by regulating i qr as given in eqn 1 an thus to control the rotor spee. The part of magnetization current require by the machine can be supplie from the RSC by regulating i r as shown in eqn 4. The crowbar resistance is employe to limit the RSC current uring gri faults [8]. The crow bar is switche on once the stator voltage is ippe an provie a bypass for the rotor current thus preventing any amage to RSC ue to over current. The etaile analysis of fault current is iscusse in III. The main objective of Gri Sie Converter (GSC) to maintain the DC link voltage constant irrespective of power flow uring super synchronous an sub synchronous operation of DFIG. The GSC can also control the amount of reactive power injecte to the gri by regulating i qs as shown in eqn 9. Even if the RSC is isconnecte uring gri faults an crowbar bypasses the excess current from rotor, the GSC can maintain the DC link voltage constant [9]. This helps DFIG to stay connecte to the gri an resume its operation soon after the removal of fault. III. SHORT CIRCUIT CHARACTERISTICS OF DFIG The equivalent circuit of DFIG in stator reference frame is shown in Fig. 2. The stator an rotor equations can be written as V s = R s i s +(1+σ s )L t (i s)+l t i re jǫ (1) V r = R r i r +σ r L t i r +L t (i se jǫ ) The flux function equations corresponing to voltage balance eqn 1 can be expresse as. V s = R s i s + ψ s t V r = R r i r + ψ r t jω rψ r ψ s = σ s L i s +L o i r ψ r = σ r L i r +L o i s R r I r V r (11) (12)
Where, V s an V r : Stator an rotor voltage i s an i r : Stator an rotor currents R s an R r : Stator an rotor resistances L : Magnetizing inuctance σ s an σ r : Stator an rotor leakage factor ω ms : Synchronous spee ψ s an ψ r : Stator an rotor flux ǫ : Angle between stator an rotor axis ω r : Electrical angular velocity The rotor currents an the stator voltages are within the limits uring the normal operation of DFIG. Assume a fault occurs at a time of t at the gri which causes the stator voltage to rop to V s. It affects the flux linkages at stator an rotor. The analytical expression for the fault current without crowbar an with crowbar resistance are evelope in III-A an III-B respectively. A. Without crowbar resistance in RSC For the normal gri operations, the space vector of stator voltage is rotating at synchronous spee with constant amplitue. Assume a three phase fault occurs at the gri, the space vector voltage ecreases to a value of V s. Therefore, V s (t t ) = V s e jωs V s (t t ) = V se jωs The stator flux can be written as, (13) t ψ s + R s σ s L ψ s = V s R s σ s i r (14) As the current injecte from RSC to rotor is in slip frequency, the rotor flux will have the same spee as that of stator flux, ie, the rotor injecte current, slip frequency + rotor spee = stator flux frequency. There fore, the stator flux eqn 14 before an after fault can be written as, σ s L ψ s (t t ) = (V s I r R s )e jωst R s +jω s σ s L σ s L ψ s (t t ) = (V s I r R s )e jωst + (15) R s +jω s σ s L where, ψ e Rs σsl t ψ = σ sl (V s V s) R s +jω s σ s L e jωst Since the stator resistance is negligible compare to stator reactance, the eqn 15 can be reuce to, ψ s (t t ) = V s e ωst jω s ψ s (t t ) = V s e jωst + V s V s e jωst e Rs σsl t jω s jω s The stator current in terms of stator flux can written from eqn 12 as, i s = ψ s L i r (17) σ s L (16) Substituting the value of stator flux from eqn 16 in eqn 17, i s = V s V s e jωst e t/τs jω s σ s L {{ Transient DC Term V s + e jωst L i r jω s σ s L σ s L {{ Steay State Term (18) where, τ s : Stator time constant = σsl R s The DC transient term makes sure that the flux linkages are not change ue to abrupt ip in the stator voltage. The DC transient magnitue epens on the amount of voltage reuce ue to the fault an the stator time constant. The steay state term epens on the stator voltage magnitue an the amount of excitation current require by the machine. B. With Crowbar resistance The crowbar resistances are usually employe to limit the rotor current to safe margin uring the gri faults. Due to the limitation of RSC current, the rotor spee/electromagnetic torque cannot be controlle uring low voltage at the terminals. Since the DFIG terminals experiences a low voltage at its terminals, ue to the gri fault, it cannot eliver the electrical power to the gri, which will increase the rotor spee to a angerous value. The crowbar can be turne on uring the gri faults to issipate the energy from the mechanical turbine to a limit. Different types of crowbar connections are iscusse in [1], [11]. The analytical expression for the fault current provie from the stator is evelope in this section. Once the crowbar is turne on, the RSC is isconnecte from the rotor wining. Therefore, the rotor inuce current will be only ue to the stator flux. The stator flux equation in terms of stator voltage an stator parameters can written as, t ψ s + R s σ s L ψ s = V (19) As iscusse in III-A, the stator flux before an after fault can expresse as, ψ s (t t ) = V s e jωst jω s (2) ψ s (t t ) = V s e jωst + V s V s e jωst jω s jω s Therefore, eqn 2 an eqn 12 can use to fin the analytical expression for fault current contribution from DFIG. i s = V s V s e jωst e t/τs jω s σ s L {{ Transient DC Term + V s e jωst jω s σ s L {{ Steay State Term (21) By analyzing the fault currents of DFIG with an without crowbar resistances, the DC transient term is present ue to the constant flux linkage theorem. The transient term epens on the low voltage cause at the DFIG terminals ue to the gri fault. More the voltage ip in the terminal, more the severity of the fault current. But the transient term may ie out epening on the value of time constant of the stator. When the crowbar is not connecte in the rotor circuit, the term corresponing to the rotor current provie from the RSC will be present as shown in eqn 18. Since the RSC
(a) Stator Current in Amp RMS Satator Current in Amp 2 1-1 -2 18 16 14 12 1 8 3.7 3.8 3.9 4 4.1 4.2 4.3 4.4 6 (a) (c) 1.8.6 5-5 5 From 2-7 From 3-9 From 7-5 From 9-6 -5 Fig. 5. DFIG Response for Case 1 : Three phase fault applie at. (a) a phase fault current supplie from DFIG Stator RMS Fault Current () 38 375 Gen at Gen at The fault current contribution from DFIG is shown in Fig. 7. The terminal voltage of DFIG is ecrease to.6 p.u. causing the DC component in the stator. Once the fault is cleare, the terminal voltage is taking some time to reach back to 1p.u. The fault current is contribute from DFIG at this time. The DC component transient term ecay epens on the stator time constant of DFIG. Once the terminal voltage reaches at its nominal value of 1 p.u aroun 4.6 secon, the DFIG resumes its normal operation. B. Case 2 : Fault at Generator Bus A three phase sli fault is applie at bus 2 (generator bus) at a simulation time of 4 secon. The fault is cleare in 15 millisec resuming the prefault network. The bus voltage magnitue an phase angles are shown in Fig. 6 (a) an. Due to the avancement of generator rotor uring fault, the voltage phase angles are isturbe. This in turn causes the power oscillations in the entire system. The power flows through selecte lines are as shown in Fig. 6 (c). The generator rotor ynamics are shown in Fig. 6 (). The DFIG ynamics are shown in Fig. 7. The DFIG terminal voltage falls to.65 p.u. uring the fault an it is resume to its nominal value aroun 6.5 secons. Since the fault at bus 2 is nearer to DFIG terminals compare to Case 1, it is taking more time to reach the steay state after clearing the fault. The fault current from a phase reaches to a value of 17 A which is more than Case 1. Since the fault is at the Generator 2 terminals, it experiences more fluctuations compare to other generators. Therefore generator 2 an generator 3 becomes incoherent an they swing ifferently. The maximum swing between two machines is aroun 21 between generator 2 an generator 3. The TSI for the system for the fault at bus 2 is foun to be 88.9 which shows that the system is stable for the conition. C. Case 3 : Fault at the transmission line The three phase soli fault occurs at the mipoint of transmission line connecte between bus 5 an bus 7 at a simulation time of 4 secon. The fault is cleare by opening 37 Fig. 6. System Response for Case 2 : Three phase fault applie at. (a) Bus Voltages in p.u. Bus voltage Phase angles in egrees (c) Power Flows in selecte lines in MW () Generator rotor angle spee variations (a) Stator Current in Amp RMS Stator Current in AMP 3 2 1-1 -2-3 3.7 3.8 3.9 4 4.1 4.2 4.3 4.4 18 16 14 12 1 8 6 Fig. 7. DFIG Response for Case 2 : Three phase fault applie at. (a) a phase fault current supplie from DFIG Stator RMS Fault Current the circuit breakers at both the en of the faulte line after 15 milli secon. Therefore, the system configuration is change after clearing the fault. The system response to the line fault is shown in Fig. 8. The bus voltage magnitue an phase angles are shown in Fig. 8 (a) an. Since the system configuration is change after clearing the fault, the voltage phase angles are settle at a ifferent value from the prefault conition. The power flows in the line are also change an settles to a new operating points as shown in Fig. 8 (c). The power flowing through 7-8 an 4-5 are mostly affecte ue to the removal of the faulte line. The DFIG ynamics uring the fault are shown in Fig. 9. Since the fault is nearer to the DFIG compare to other cases, the fault current is maximum for Case 3. The DFIG terminal voltage settles own to the nominal value aroun 5.2 secons an the current reaches its steay state at the time. The stator
(a) 1.1 1.9.8.7 (c) 4 2 3 2 From 2-7 1 From 3-9 From 7-5 From 9-6 -1 378 377.5 () 377 376.5 Gen at Gen at Fig. 8. System Response for Case 3 : Three phase fault applie at line between 5 an 7. (a) Bus Voltages in p.u. Bus voltage Phase angles in egrees (c) Power Flows in selecte lines in MW () Generator rotor angle spee variations (a) Stator Current in Amp RMS Stator Current in Amp 4 2-2 -4 3.7 3.8 3.9 4 4.1 4.2 4.3 4.4 25 2 15 1 5 Fig. 9. DFIG Response for Case 3 : Three phase fault applie at the mipoint of the line 5-7. (a) a phase fault current supplie from DFIG Stator RMS Fault Current time constant epens the rate of ecay of the DC transient term of the fault current supplie from DFIG. The rms fault current supplie from on of the phases is aroun 258 A which is much larger than the other two cases. The TSI of the system for line fault at 5-7 is foun to be aroun 76. This shows the system is stable but not to the exten of other two cases. Once the fault is nearer to the DFIG terminals, which ecreases the terminal voltage to a low value, causes a very high fault current to flow from DFIG. VI. CONCLUSION The transient performance of DFIG connecte to a large system is iscusse in the paper. An analytical expression for the fault current contribute from DFIG with an without crowbar resistance is evelope. The fault current from DFIG consists of two terms viz, DC transient term which epens on the stator time constant (higher the value of resistance, higher the ecay rate) an steay state term which epens on the amount of voltage ippe at the DFIG terminal. The power oscillations in the system ue to the rotor ynamics after clearing the fault is stuie by applying fault at ifferent buses. It is shown from the simulation result that nearer the location of fault to the DFIG system, more the fault current contribution. 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