DRECT POWER CONTROL OF A DFG-BASED WND TURBNE UNDER UNBALANCED GRD VOLTAGE WTHOUT ROTOR POSTON SENSOR AL ZANLO, SAYYED ASGHAR GHOLAMAN * AND MOHAMMAD VERJ KAZEM Faculty of Electrical and Computer Engineering, Babol Nohirvani Univerity of Technology, Babol, ran. * Correponding author: gholamian@nit.ac.ir (Received: 4 th Apr. 016; Accepted: 0 th Jun. 016; Publihed online: 30 th May 017) ABSTRACT: n thi paper, the behavior of a doubly fed induction generator (DFG) i propoed under an unbalanced grid voltage and without uing a rotor poition enor. There are two main method that have been ued for the detection of rotor poition: ue of a haft enor and ue of a enorle algorithm. n thi paper the haft enor i eliminated and a poition enorle algorithm i ued for etimating the rotor poition. Senorle operation i more deirable than uing haft enor, becaue haft enor have everal diadvantage related to cot, cabling, robutne, and maintenance. Alo, during network imbalance, three electable control target are identified for the rotor ide converter (RSC), i.e., obtaining inuoidal and ymmetrical tator current, mitigation of active and reactive power ripple and the cancellation of electromagnetic torque ocillation. The effectivene of the propoed control trategy i confirmed by the imulation reult from a -MW DFG ytem. t i concluded that the enorle algorithm i able to produce reult with imilar accuracy to haft enor ue and the enorle algorithm can be ued in practical application. ABSTRAK: Dalam kerta ini, tingkah laku penjana induki dua uapan (DFG) dicadangkan di bawah voltan grid tidak eimbang dan tanpa menggunakan enor kedudukan rotor. Dua kaedah utama telah digunakan untuk mengean kedudukan rotor: penggunaan enor haf dan penggunaan algoritma tanpa enor. Dalam kerta ini enor haf dihapukan dan kedudukan algoritma tanpa enor digunakan untuk menganggarkan kedudukan rotor. Operai tanpa enor jauh lebih baik dari menggunakan enor haf, kerana enor haf mempunyai beberapa kelemahan yang berkaitan dengan ko, pemaangan kabel, keteguhan, dan penyelenggaraan. Juga, dalam ketidakeimbangan rangkaian, tiga aaran kawalan yang boleh dipilih telah dikenal pati untuk converter rotor ii (RSC), iaitu, mendapatkan aru tator inuoidal dan imetri, pengurangan kuaa riak aktif dan reaktif dan pembatalan ayunan tork elektromagnet. Keberkeanan trategi kawalan yang dicadangkan diahkan oleh keputuan imulai daripada item - MW DFG. Adalah dirumukan bahawa algoritma tanpa enor mampu menghailkan keputuan dengan tepat ama eperti menggunakan enor yaf dan algoritma tanpa enor boleh digunakan dalam aplikai praktikal. KEYWORDS: double fed induction generator (DFG); direct power control (DPC); unbalanced voltage; rotor poition etimation 1. NTRODUCTON Wind energy ha become an important ource for electricity generation in many countrie. t i expected that wind energy will provide about 10% of the world electrical energy in 00. Nowaday, many wind farm are baed on DFG technology with 57
converter that have generator rating of 0%-30%. n the global wind market, DFG ha become one of the mot popular configuration. DFG ha everal advantage including variable peed, four-quadrant active and reactive power operation capabilitie, and decoupled active and reactive power control. t alo allow the ue of a partially rated converter which reduce ytem cot. A chematic diagram of the DFG-baed wind energy converion ytem i hown in Fig. 1 [1]. Fig. 1: Schematic diagram of DFG-baed wind energy ytem. The wind power ytem hown in Fig. 1 conit of a DFG, where the tator winding i directly connected to the network and the rotor winding i connected to the network through a four-quadrant power converter compried of two back-to-back pule width modulation (PWM) voltage ource inverter (VS). Uually, the controller of the rotor ide converter regulate the electromagnetic torque and upplie part of the reactive power to maintain the magnetization of the machine. On the other hand, the controller of the upply ide converter regulate the DC link voltage. Traditionally, the technique to control the rotor-ide converter (RSC) and the grid-ide converter (GSC) of DFG-baed wind turbine include vector control (VC) and direct torque control (DTC) or direct power control. The direct power control (DPC) for DFG ha been the focu of ome tudie [1]- [7] and proven to have everal advantage over the conventional vector control trategy, uch a imple implementation, fat dynamic repone, and robutne to DFG parameter variation. n [1] a DPC trategy baed on an optimal witching table that ue the etimated tator flux information wa propoed. However, like a conventional DTC, DPC ha a witching frequency that varie ignificantly with active and reactive power variation, the power controller hyterei bandwidth, and the machine operating peed (rotor peed). A a reult, thi method complicate the AC filter deign becaue of it variable witching frequency. Conequently, the DPC with pace vector modulation (SVM) [] and predictive control [3, 4] ha been propoed to achieve a contant witching frequency. Sliding mode control (SMC) trategy i an effective and high frequency witching control for nonlinear ytem with uncertaintie. t feature imple implementation, diturbance rejection, trong robutne and fat repone. Thu, a SMC baed DPC drive for DFG wa propoed in [5]. Paper [6] preent the comparion of three different trategie for the control of a DFG in wind energy converion ytem (WECS). Thee trategie are: vector control, direct torque control, and direct power control. n [7], witching vector for the rotor ide converter were elected from the optimal witching table uing the etimated tator flux 58
poition and the error of the active and reactive power. n thi reference, the increaed number of voltage vector with application of the Dicrete Space Vector Modulation (DSVM) will be preented. Then a new witching table in uper-ynchronou and ubynchronou frame will be propoed. A imple poition-enorle method for rotor ide field oriented control of a wound rotor induction machine i decribed in [8]. The algorithm i baed on axi tranformation. Compared to the other method, it i more direct and the dependence on machine parameter i alo largely reduced. For mot of the tudie reported, a ymmetrical tator voltage upply wa aumed even during network diturbance. However, both tranmiion and ditribution network can have mall teady tate and large tranient voltage unbalance. f DFG control ytem do not take into account the voltage unbalance, the tator current can become highly unbalanced even with mall voltage unbalance. The unbalanced current create unequal heating on the tator winding. Subequently, the pulation with double grid frequency will be generated in the tator output active and reactive power, which are harmful to the tability of the connected power grid. Furthermore, the electromagnetic torque tart to ocillate at the double grid frequency, which will caue durative tre concuion to the rotor haft and poible damage to the mechanical ytem of the wind turbine. t ha been found that wind farm connected to ditribution network periodically experience voltage unbalance of greater than %, and thi ha caued a large number of trip. More recently, everal author have analyzed the generator behavior in thi ituation [9-14]. Baically, the available unbalance control method were baed on the ymmetrical component theory, which tate that an unbalanced ytem i a combination of the poitive and negative network. n [9], a new algorithm for generating the power reference for DPC wa preented. t wa hown that the ocillation term of electromagnetic torque can be eliminated without any equence extraction. n [10], an improved vector control for DFG under unbalanced grid voltage wa propoed; two notch filter have been ued to extract the poitive and negative equence of grid voltage. So according to a pecified target like balanced tator current, thee component were applied to generate the reference rotor current. n [11], an improved DPC trategy uing two reonant controller and a P controller i propoed for maintaining contant tator power, to eliminate torque pulation and to maintain inuoidal tator current. The author in [1] introduced the unbalanced control trategy with the Vector Oriented Control (VOC) technique, in which the detrimental influence on the DFG ytem caued by negative component of the grid voltage wa alo analyzed. Alo, the ytem i deigned to operate for different target. The ocillation in power and torque are coniderably reduced. n [13] and [14], a model for predictive direct power control with power compenation and three elective control target wa preented, -obtaining inuoidal and ymmetrical grid current, removing active and reactive power ripple and mitigation torque ocillation- in order to enhance the control flexibility and performance of the DFG when the network i unbalanced. Thi paper decribe an algorithm for enorle control of a DFG-baed wind turbine ytem under unbalanced grid voltage. Thi paper i organized a follow: ection decribe the DFG modeling; the repone of DFG under unbalanced condition i propoed in ection 3 wherea the power compenation trategie are illutrated in ection 4; the rotor poition etimation trategy i preented in ection 5; ection 6 dicue the imulation reult on a MW DFG ytem to demontrate the effect and performance of the propoed control algorithm; and finally, the concluion are preented in ection 7. 59
. DFG MODELNG The equivalent circuit of a DFG expreed in the tator tationary frame i hown in Fig.. The mathematical equation for a DFG can be expreed a (1) V d R dt d V R r r r r j dt r r L L m r r L m L r r 3 * 3 Te p m = p 3 * 3 Re = 3 * 3 m = P V V V Q V V V 3. RESPONSE OF DFG UNDER UNBALANCED GRD VOLTAGE Under unbalanced grid condition, the ytem variable contain double frequency component additionally. With the conventional decoupled power control and with no additional control meaure in place, the control i ineffective ince the tator and grid current exhibit ignificant low frequency harmonic. A a conequence, there are evere ocillation in torque, active and reactive power, and the DC link voltage. According to the ymmetrical component theory, and auming no zero equence component, if the grid upply i unbalanced, any three phae quantity, e.g., voltage, current, or flux, can be eparated into the poitive and negative equence component a in (9) V - ( - ) ( - V V V V j V V ) - ( - ) ( - j ) - ( - ) ( - j ) Subtituting (8) and (9) into (6) and (7), we get (1) () (3) (4) (5) (6) (7) (8) (9) (10) Where 3 P ( P1 P P3 P4 ) 3 Q ( Q1 Q Q3 Q4 ) * * 3 3 P1 Re V. V V 3 3 P Re V. V V (11) (1) (13) (14) 60
* * V * V V * * * P 3 3 3 Re. V V V 3 3 P4 Re V. V V 3 3 Q1 m. 3 3 Q m V. V V Q 3 3 3 m. V V V 3 3 Q4 m V. V V (15) (16) (17) (18) (19) (0) Fig. : Equivalent circuit of a DFG in the tator tationary frame. The pulation term P1, Q1 and P, Q are contant at teady tate, becaue they are compoed by the ame equence product. However, term P3, Q3 and P4, Q4 ocillate at twice the grid frequency pulation, ince they are compoed of poitive and negative equence product. Alo with ubtituting the poitive and negative equence component of the tator current and flux into the expreion (5), we obtain 3 T m * * * * e p n addition, tator flux can be expreed uing the tator voltage from (1) a j R V Subtituting the flux from expreion () in torque equation (1), we get (1) () T 3 Re * * * * ( e ) p V V V V R According to the active power expreion (11), the torque equation (3) become 3 p Te P 1 P P3 P4 T 0 (3) (4) 61
Where T0 R ( ) t hould be noted, in thi tudy, that the DPC technique i adopted for both RSC and GSC, a enunciated in [1] and [15]. 4. RESPONSE OF DFG UNDER UNBALANCED GRD VOLTAGE n an unbalanced condition, reference power are generated baed on the expreion of intantaneou power that include the teady tate, inuoidal, and coine component of active and reactive power. Reference power for RSC can be generated baed on different individual control target uch a minimizing the double frequency ocillation of inuoidal or coine term of tator active or reactive power, electromagnetic torque, and negative equence winding current. n other word, in thi ection the following three objective will be introduced and will be ued Target 1: Eliminating the double frequency pulation of tator active and reactive power to make the intantaneou tator active and reactive power contant. Target : Maintaining a contant electromagnetic torque in order to reduce the mechanical tre on the turbine ytem. Target 3: Balancing the tator current to enure balanced heating in the three phae tator winding. 4.1 Active and Reactive Power Ocillation Cancellation Strategy (target 1) Thi control target i to allow the exitence of negative equence current component but eliminate the tator output active and reactive power ripple. n order to obtain contant active and reactive power, both of the reference mut be kept contant, thu the active and reactive power ripple mut be zero, i.e. [9] P3P4 0 Q3Q4 0 A a reult, the required power for compenation become Pref Pcont P1 P Qref Qcont Q1Q According to (4) and (6), even if the tator active and reactive power pulation can be removed by the unbalanced DPC, the torque pulation will till exit. 4. Torque Ocillation Cancellation Strategy (target ) Thi objective i to mitigate the torque ocillation when the network i unbalanced. According to (4), the torque ocillation can be eliminated by impoing the requirement [9] P3P4 0 Which reult in an electromagnetic torque, uch that: (5) (6) (7) (8) (9) (30) 6
3 p Te P 1 P T 0 Under thi ituation, the required power and the reference tator active power are related by the following expreion: Pref Pcont P3 Pcont P4 Since the condition for torque ocillation cancellation in (30) i not related to reactive power, no compenation i needed for the reactive power; in other word, Qref=0. 4.3 Sinuoidal and Balanced Stator Current Exchanged with Grid (target 3) f P and Q are added to the original power reference to obtain the new power reference, P1 and Q1 can be forced to zero; that i, the negative equence current i eliminated. n other word, and have to be zero if P1 and Q1 are to be maintained a zero. Therefore, the compenating power pulation become [13] Pref Pcont P4 Qref Qcont Q4 n the above equation, Pcont and Qcont are equal to the original power reference under the balanced grid voltage condition. 5. ROTOR POSTON ESTMATON STRATEGY To increae the reliability of the grid ynchronization proce, it i important to implement the enorle rotor poition control. There are two main method that have been ued for the detection of the rotor poition: 1) derived from a haft enor and, ) derived from a enorle algorithm. Senorle operation i more deirable than uing a haft enor, becaue the haft enor ha everal diadvantage related to the cot, cabling between the enor and the controller, robutne, and maintenance. The rotor current of DFG i available for meaurement, which i not poible in cage rotor induction machine. Thi provide more flexibility in deigning enorle control cheme for DFG. Seen from the tator coordinate ytem, (r) make an angle. Figure 3 how the ame rotor current make an angle r with the rotor axi. The problem, therefore, i to compute and r, o that ( m ) et r can be determined. Rotor current magnitude and unit template of rotor current aligned to rotor axi are calculated a [8] (31) (3) (33) (34) = r r r co in r = r r r = r r (35) (36) (37) 63
The rotor current unit template co,in aligned to the tator coordinate ytem ha to be calculated. Uing previou unit template ample two phae rotor current r, r are converted into tator coordinate a r ' [ k ] r [ k ]* co m [ k 1] r [ k ]* in m [ k 1] et r ' [ k ] r [ k ]* co m [ k 1] r [ k ]* in m [ k 1] et Where k i the preent ample, (k-1) i the previou ample, the upercript ' indicate the intermediate variable. Where co m[ k 1] and et in m[ k 1] are the unit vector et of rotor poition etimated in the previou ample. n term of the tator and rotor current, the tator flux magnetizing current (m) in the tator coordinate ytem can be expreed a ' [ ] 1 [ ] ' m k k r [ k ] ' [ ] 1 [ ] ' m k k r [ k ] et et (38) (39) (40) (41) ' ' ' m m m Where L L L L L r m r i the tator leakage factor. (4) The tator flux magnetizing current vector (m) make an angle of 90 with the tator axi. The expreion of α, β component can be written a m min m m co Where in co V V V V V V (43) (44) (45) (46) Fig. 3: Phaor diagram for rotor poition etimation trategy [8]. 64
Uing above m and meaured value of tator current (), the rotor current can be computed in the tationary frame a r m 1 r m 1 (47) (48) r r r The unit vector of rotor current in tationary coordinate ytem are given by in co r r r r (49) (50) (51) Equation (36-37) and (50-51) repreent the unit vector in the two reference frame, the former rotating at ynchronou peed and, the latter at lip frequency. The unit vector pertaining to the rotor poition ( m ) et r can now be eaily computed m r r r co co co co +in in et m r r r in in in co co in et The unit vector of lip angle are ued in below equation for finding the tator flux vector in rotor reference frame. Thi i equation a: j j r m et m et = co in Where the tator flux linkage are calculated a: = V - R dt = V - R dt Figure 4 how the chematic diagram of the improved control cheme for enorle DPC of grid connected DFG when the network voltage i unbalanced. There are no report related to the ue of enorle algorithm for DFG control when the network voltage i unbalanced. Alo, Fig. 4 how the propoed compenation method for power, torque, and tator current ocillation, a well a the rotor poition etimation trategy under unbalanced grid voltage condition. t can be een from Fig. 4, that the compenating power are calculated according to Eqn. (8), (9) or (3) or (33), (34), a hown in the dotted line block. A notch filter turned at double grid frequency ha been adopted to extract poitive and negative equence tator current and voltage. The filter may involve ome amplitude and phae error that may woren the control effect. However, ince the notch filter i jut ued to calculate the compenating power and it lie out of the power control loop, the inherent defect caued by equential decompoition, uch a low dynamic repone and low bandwidth of the control loop, could be avoided. (5) (53) (54) (55) (56) 65
Fig. 4: The propoed chematic diagram of the enorle DPC for DFG under unbalanced condition. 6. SMULATON RESULTS Simulation of the propoed control trategy for a DFG baed wind power generation ytem were carried out uing MATLAB/Simulink and Fig. 5 how the cheme of the implemented ytem. The DFG i rated at MW with it parameter given in Table 1. The nominal converter DC-link voltage wa et at 100 V. During imulation, the ampling frequency i 0 KHZ and the bandwidth of the active and reactive power hyterei controller are et at 4% of the rated generator power of MW. Table 1: Parameter of the DFG imulated Parameter Value Rated Power MW Stator Voltage 690 V Stator/rotor turn ratio 0.3 R 0.0108 pu Rr 0.011 pu (refer to the tator) Lm 3.36 pu L σ 0.10 pu L σr 0.11 pu (refer to the tator) Lumped inertia contant 0.5 Number of pole pair 66
The performance of the ytem will be analyzed under a teady-tate operation condition. The grid voltage unbalance programmed for thi experiment i hown in Fig. 6 and it value are (Va=555 V <0 0, Vb= 484. V <-113 0, Vc=557.5 V <-31 0 ). The DFG wa aumed to be in peed control, meaning that the rotor peed i et externally, a the large inertia of the wind turbine reult in a low change of rotor peed. The tator active and reactive power reference are et to 0.166 and 0.33 p.u. at a contant peed of 0.73 p.u. where the ynchronou peed wa defined a 1 unit. n the econd half of the experiment, the behavior of the machine ide magnitude and the grid ide magnitude are preented when the ytem i commanded with the ocillation cancellation trategie, a propoed in ection 4. Before thi, in the firt half of the experiment, the cancellation trategie are diabled. Fig. 5: Scheme of the imulated ytem. A hown in the previou ection, Target 3 give good attenuation of both torque and active power ocillation. For Target 1, while the active power ocillation i greatly reduced, the torque pulation i relatively large. Similar obervation can alo be noticed for Target. The election of the control target i highly dependent on the deign of the turbine ytem and the operation of the network. n Fig. 7, Target 1 (Active and reactive power ocillation cancellation trategy) wa elected a the control objective. n Fig. 8, Target (Electromagnetic torque ocillation cancellation trategy) wa elected a the control objective. The econd half, of Fig. 8(a) how the power tracking behavior in order to cancel the electromagnetic torque ocillation. Figure 8(b) how the current exchanged through the tator under thee operation condition. n the imulation reult, a trong deterioration of the current can be oberved before 0.15. Thi fact i due to the ocillating behavior of the electromagnetic torque during thi firt half of the experiment. n addition, when the torque ocillation cancellation trategy i operating, the tator current are unbalanced but inuoidal. n Fig. 9, Target 3 of the power compenation cheme i implemented. A hown in Fig. 9, there are ocillating component in both the active and reactive power becaue of the two ocillating term, P4 and Q4, added to the original contant active and reactive power. However, thee eliminate the negative equence current. A a reult, the tator current become quite inuoidal and ymmetric, thu power quality i improved ignificantly. 67
Simulation of the Stator flux poition in the rotor reference frame, for cae with and without the rotor poition enor, with contant rotor peed were carried out and the reult are hown in Fig. 10 (a) and (b) for rotor peed of 1. and 0.8 p.u. repectively. Alo, Fig. 10(c) how the reult of the propoed method operating with variable peed from 1. p.u. to 0.8 p.u. t can be een that the tator flux poition in the rotor reference frame can be correctly etimated and there i very little deterioration in ytem performance with the enorle cheme. n the rotor poition enorle control invetigation, the rotor encoder wa till intalled on the haft, but it poition output wa not ued in the ytem control. ntead, it wa ued for comparion with the etimated tator flux poition value. Fig. 6: Unbalanced grid voltage. Q P Q P Fig. 7: Simulation reult comparion with [B] and without [A] rotor poition enor, with power compenation for target 1. (a) Stator active, and reactive power (p.u). (b) tator current (p.u). (c) electromagnetic torque (p.u). 68
Q P Q P Fig. 8: Simulation reult comparion with [B] and without [A] the rotor poition enor, with power compenation for target. (a) Stator active, and reactive power (p.u). (b) tator current (p.u). (c) electromagnetic torque (p.u). Q P Q P Fig. 9: Simulation reult comparion with [B] and without [A] rotor poition enor, with power compenation for target 3. (a) Stator active, and reactive power (p.u). (b) tator current (p.u). (c) electromagnetic torque (p.u). 69
Fig. 10: Stator flux poition in the rotor reference frame for with and without rotor poition enor under contant and variable rotor peed. (a) Contant peed 1. (p.u). (b) Contant peed 0.8 (p.u). (c) Variable peed from 1. to 0.8 (p.u). 7. CONCLUSON Thi paper ha preented a new control trategy for DFG-baed wind energy generation ytem operating under unbalanced network condition. Thi method ued a imple poition-enorle algorithm to eliminate the rotor poition enor. The imulation reult how the accuracy of the propoed rotor poition etimation trategy in all relevant ituation. Alo, tator active and reactive power and generator electromagnetic torque have been fully defined under an unbalanced voltage upply, which indicate that ignificant pulation at twice the upply frequency could exit. Method for providing enhanced ytem control and operation for DFG-baed wind turbine during network unbalance i.e., power, torque, or tator current ocillation minimization, are identified. Thee three electable control target are ued for compenation. REFERENCES [1] Lie Xu, Phillip Cartwright. (006) Direct Active and Reactive Power Control of DFG for Wind Energy Generation. EEE Tran. Energy Conv., 1(3):750-758. [] Dawei Zhi, Lie Xu. (007) Direct Power Control of DFG With Contant Switching Frequency and mproved Tranient Performance. EEE Tran. Energy Conv., (1):110-118. [3] Dawei Zhi, Lie Xu, Barry W. William. (010) Model-Baed Predictive Direct Power Control of Doubly Fed nduction Generator. EEE Tran. Power Electron., 5():341-351. 70
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