Novel Control Strategy for Multi-Level Active Power Filter without Phase-Locked-Loop

Transcription

1 Energy and Power Engineering, 2010, 2, doi: epe Published Online November 2010 ( Novel Control Strategy for Multi-Level Ative Power Filter without Phase-Loked-Loop Guojun Tan, Xuanqin Wu, Hao Li, Meng Liu Shool Information and Eletrial Engineering, China University Mining and Tehnology, Xuzhou, China Reeived June 13, 2010; revised August 9, 2010; aepted September 21, 2010 Abstrat Ative power filter (APF using novel virtual line-flux-linkage oriented ontrol strategy an not only realizes no phase-loked-loop (PLL ontrol, but also ahieves a good inhibitory effet to interfere. However, there are some problems in the onventional method, suh as the error amplitude, the shift phase angle and the non-determinay initial oriented angle. In this paper, two one-order low-pass filters are adopted instead the pure integrator in the virtual line-flux-linkage observer, whih an steady the phase and amplitude. Furthermore, an original sheme harmonis detetion under the rotating oordinate is advaned based on the simplified spae vetor pulse width modulation (SPWM strategy. Meanwhile, by using the new SPWM algorithm, the voltage spae vetor diagram the three-level inverter an be simplified and applied into that two-level inverter, and this makes the ontrol for Neutral Point potential easier. Keywords: Ative Power Filter, Harmonis Detetion, irtual Line-Flux-Linkage Observer, Ative Power Filter Control without Phase-Loked-Loop, Spae etor Pulse Width Modulation 1. Introdution The use nonlinear loads suh as power eletroni devies leads to serious harmonis pollution and lager voltage flutuation. Moreover, lager unbalane voltage and urrent distortions in the power system are harmful to the eletrial equipment and power systems. Shunt ative power filter (SAPF an well ompensate the harmoni whose frequeny and amplitude both hanges, and it is also reognized as an effetive way to manage the grid harmoni, to reative power pollution and to improve the quality the power. Ative power filter (APF is a new power eletronis devie for dynami harmonis restrition and reative ompensation without influened by system indutane [1,2], besides it has suh funtions: adaptive ability for the parametri variation system and load, automati traing and ompensation for the varying harmonis [3,4]. There are some remarkable advantages in nature three-level APF, suh as lower distortion output waveform, lower endure voltage and less swith loss, high effiieny and low eletromagneti interferene (EMI, so it helps to raise the installed apaity and improves the harmonis ompensation effet as well as system reliability. On the one hand, the traditional harmoni urrent detetion whih is based on instantaneous reative power adopts phase-lok-loop (PLL to aquire the voltage vetor angle [5-7], when the grid voltage flutuation is more serious, PLL will be in unloked ondition beause the larger frequeny fsets whih an not aurately trak the phase position. To solve the problem, this paper uses the virtual line-flux-linkage orientation to observe the vetor angle, whih onverts the observation vetor angle to the flux. By using the voltage integral on the AC side the ative filter to estimate the grid flux, PLL an be omitted; meanwhile, the power grid interferene an be inhibited well. On the other hand, the traditional harmoni urrent detetion requires that the integrated vetor ombined by the sine and osine funtion should be synhronous and phase oinidene to the integrated vetor the three-phase positive sequene fundamental voltage, otherwise, the detetion auray the fundamental positive sequene reative omponent will be affeted by phase differene [8]. Therefore, the harmoni detetion priniple based on the rotating oordinates is proposed in this paper. The appliation virtual flux in ative filter system also inludes ontrolling generation ompensation urrent, so it has a very good ontrol to harmoni urrent detetion and ompensation urrent ge-

2 G. J. TAN ET AL. 263 neration, and the voltage spae vetor modulation strategy SPWM (Spae etor Pulse Width Modulation an be easily applied to ative filter ontrol. In this paper, the simplified three-level SPWM with neutral point potential adaptive ontrol is applied to ontrol multi-level ative filter, based on this, a novel multilevel voltage ative filter phase reonstrution algorithm is proposed. 2. Mathematial Model for Three-Level APF and the Rotating Coordinate Based Harmonis Detetion Diagram Figure 1 is the main iruit NPC three-level APF, while vetor diagram the virtual line-flux-linkage oriented system is shown in Figure 2. When the virtual line-flux-linkage is at d axis, then mathematial model three-level APF is in dq referene frame [9,10]: where: Z X AX Be (1 Z diag L L C C 0 (2 s s d d 0 LS S1 d S2d 0 LS 0 S1 q S2 0 q A S1 d S1 q S2d S2q T d q d d (3 X i i (4 B diag (5 ed 0 e q E m In (2 to (6, L s : AC side indutane APF; Cd : apaitane DC side; Em : peak voltage power system; id, iq: the D-axis and Q-axis AC urrent APF; d 1, d2: the upper and down DC voltage relevant to neutral point; S1 d, S1 q, S2d, S2q : the D-axis and Q-axis omponents swithing funtion. Aording the virtual line-flux-linkage oriented strategy showed in Figure 2, the referene ompensation urrent an be obtained: i i dh qh i i Ld Lq (6 (7 i i i i i i Ld Ld Ld Lq Lq Lq where: ild, i Lq: The D-axis and Q-axis urrent nonlinear load; i, i : The D-axis and Q-axis fundamental urrent Ld Lq nonlinear load through low pass filter from i, i ; Ld Lq (8 i, i : The D-axis and Q-axis omponent harmonis Ld Lq urrent nonlinear load. 3. The diagram Novel irtual Line-Flux-Linkage Observer The essene virtual line-flux-linkage oriented method is to gain aurate spae angle ( estimation the oriented vetor, as shown in Figure 2, the angle an be obtained: sin os Then the estimation is onverted to the evaluation and omponents the virtual line-fluxlinkage. di e L v dt di e L v dt (9 (10 In (10, v and v are the, voltage quantity at AC side three-phase APF, respetively. Integrated both sides (10, vdtli vdtli (11, :, value virtual line-flux-linkage. It an be seen from (11 that the flux linkage an be evaluated by the voltage integral at AC side APF, while it brings a problem the integral initial value whih ause an error the flux linkage, furthermore, the method pure integrator doesn t retrain the DC omponent the input signal, even a little DC omponent an make the integrator saturation. In this manner, the flux linkage in axes an be a irular trajetory with DC fset orresponding to the entre the irle, meanwhile, it ause the inauray virtual line-fluxlinkage oriented angle and impat both the authentiity urrent feedbak and the veraity voltage spae

3 264 G. J. TAN ET AL. Figure 1. Topology a diode NPC three-level shunt APF. use the first order inertia fil ter ( 1 s f instead the q I i Lq E i LI e i q i d e i Figure 2. etor diagram the virtual line-flux-linkage oriented system. vetor supply, and even aggravate the performane APF seriously. As a result, it brings large urrent shok in the proess starting, and even an t start. Therefore, in order to ahieve an aurate flux estimate, it s neessary to make some measures to eliminate influene the integral initial value. In the traditional virtual flux estimation, in order to solve initial value problems pure integrator, it usually i Ld d 1 pure integrator ( in flux estimation. It an be seen s from the formula the first order filter that the first or- der inertia is similar to the pure integrator when the frequeny the sinusoidal input is far greater than f. However, it is also required that the first order inertia should have a ertain deay to the diret flow, that means f should be a ertain value, and not too small, otherwise the deay will be very slow.so the first order low pass filter has ontraditions between approximating pure points and deaying the DC omponent, that means f must maintain a ertain value to deay, while it also should be far less than to approximate the integral. Know from the above analysis. Low-pass filter is adopted by onventional virtual line-flux-linkage estimate to substitute for the purely integrator to least the DC drift, howev er, it would lead to errors amplitude and phase angle. As desribed by the N N equation ( Ws, the novel virtual line- s s flux-linkage observer and the omparison three observers are showed in Figure 3 and Figure 4 respetively. A ording to Figure 4, it is quite easy to found that the proposed algorithm respond faster than that onventional way. It an be seen from the bode figure

4 G. J. TAN ET AL. 265 Magnitude (db Phase (deg v i i v N N s s N N s s Figure 3. The novel virtual line-flux-linkage observer Figure 4. The omparison the three observers Bode Diagram Frequeny (radse System: sys Frequeny (radse: 314 Magnitude (db: System: sys Frequeny (radse: 314 Phase (deg: -90 Figure 5. Bode diagram the novel virtual line-flux-linkage observer. that it an replae the pure integrator by making the amplitude deay 49.9db and phase shift 90 at the same time while 314 Rad s. 4. The Simplified SPWM Algorithm and Novel oltage Estimation for Three-Level APF In this paper, the simplified three-level SPWM algo- is adapted, as shown in Figure 6. The voltage rithm spae vetor diagram the three-level inverter an be regarded as a hexagon omposed by six small two-level spae vetors, and all the hexes are entered by vertexes inner one. Hene, two-level SPWM algorithm an be applied to alulate the duration-time and the swith sequene voltage vetor [11]. Beause inherent problem in the topology the diode-lamping three-level onverter, the various swith states have different impats on the neutral-point potential. The middle vetors an ause neutral-point voltage deviation beause asymmetri parameter in pratie. Small vetors will ause the flutuation neutral-point potential. There is a striking ontrast effet between the two different middle vetors orresponding to different swith status vetor [11]. There exist the regions that are overlapped by adjaent small hexagons as shown in Figure 6. So if the referene voltage vetor stays at those regions, S an have any values that are possible. s1_ ref is the orreted referene voltage vetor when the index S has the value 1, and s2_ ref is the orreted referene voltage vetor when the index S has the value 2. If the two-level plane S = 1 is seleted, the swithing sequene deided by referene voltage vetor is given as follows: (0 1 1 (0 0 1 (1 0 1 ( N 2N 8 1P 1N, 2N are respetively voltage spae vetors the orresponding swithing sequene (0-1-1 and (0 0-1, and they are both negative short vetors; 1 P (1 0 0 is a positive short vetor voltage spae vetor. T, T 1N, T 2N, T8 1P are dwelling times the orresponding voltage vetors. The voltage vetor 1N and 1P have the same output voltage 1, and dwelling times the 1N, 1P are equal. If the above swithing sequene is adop ted, the dwelling time negative short vetors is longer than the positive ones; neutral-point potential will deline. If the two-level plane S = 2 is seleted, the swithing sequene is given as follows: (001 (101 (100 (110, the dwelling time negative short vetors is shorter than the positive ones, neutral-point potential will rise. So the neutral-point potential an be ontrolled by hanging the orresponding the value index S. As an be seen from (11, on the basis the proposed algorithm, the voltage at the terminals three-level retifier is estimated, whih is shown in Figure 7. When the referene voltage is in the first setor, esti-

5 266 G. J. TAN ET AL. Figure 6. The simplified three-level SPWM algorithm. three-phase for two-level algorithm, respetively. 5. The Main Ciruit Design and Experimental Results Analysis Figure 7. The novel three-level voltage estimation algorithm. To verify the viability proposed non PLL ontrol sheme for multi-level APF and evaluate performane this method, the test platform is established and operated, and its ontrol sketh is showed in Figure 9. As shown in Figure 10, the struture full-digital ontroller is omposed DSP (TMS320F2812 TI and FPGA. The system an implement DC bus voltage ontrol, har- mated three-phase voltages are given by: uaal ud ud ( taon tbon ton ubal ud os(60 ud ( tbon taon ton ual ud os(60 ud ( ton taon tbon (15 where, u aal, u bal, u al are the estimated voltage three-level retifier. u d is DC bus voltage threelevel retifier. t aon, t bon, t on are the duration-time Figure 8. Experiments waveform the novel three-level voltage estimation algorithm.

6 G. J. TAN ET AL. 267 L a Non Linear Load L b L ia i b u AlphaCal sa sb s ila ilb u BetaCal sin os u u IPARK sin os PARK u d u q u d u d i Lq i Ld HPF i d i dh i q i q i qh Figure 9. Novel diagram the virtual-flux oriented vetor ontrol diagram APF. Figure 10. Struture full-digital ontroller. monis detetion, losed-loop urrent ontrol, fault diagnosis and et The Priniples the Main Ciruit Parameter Calulation 1 The design inlet indutane on AC side The design AC filter indutor has two priniples: for one thing, it is the power ative power filter to trak and ontrol ompensation urrent, and make sure it an still produe a orresponding ompensation urrent while the load urrent has larger urrent rate hange; for another, it is to satisfy the requirements traking the size ompensation urrent ripple. It an be known from [12] that: 4U d Lmax (16 9 (10 20 f 2.3I The maximum indutane value an be aquired by the above equation, while the minimum indutor value is determined by the size allowable ripple urrent, the size the ripple urrent should be limited within the presribed range while hoosing the indutor value. 2UdTs Lmin (17 9 imax Combined with (16 (17, It an be obtained: 2Ud Ts 4Ud L (18 9imax 9 (10 20 f 2.3I a where, U d is the DC bus voltage; T s is swithing frequeny; imax is the maximum allowable value that ompensation urrent deviate from the referene urrent; f is the urrent frequeny the fundamental wave; I a is the effetive referene urrent. 2 The design the DC bus apaitor Owing to both the energy pulse and swithing losses aused by harmoni and reative urrent in the APF ompensation urrent and the energy pulse aused by the filter indutor energy storage on AC side, the apaitane is diffiult to maintain a onstant. The larger the apaitor is, the smaller suh flutuations would be, but the a

7 s G. J. TAN ET AL. apaitane value is not unlimited, so the design priniples DC bus apaitor is to work as a minimum apaitane while APF an be under normal operative ondition. It an be known from [12] that: ST Cmin (19 2 (1 Ud U U d max d (20 where, S is the ompensating apaity APF, T is the ontrol yle DC bus voltage, U d is the DC voltage, is the voltage wave pae, U d max is the maximum allowable value the voltage wave. 3 The ontrollable range DC bus voltage DC bus voltage seletion should be onsidered as follows: it is not only neessary to ahieve a ertain urrent funtion, but also should not lead to too fiere urrent hanges. The DC voltage threshold three-phase APF should be greater than the peak value AC power grid line voltage, that is Ud 3Em, otherwise, the urrent an not be traked. Take grid flutuations and linear on- and other fators into onsideration, the DC trol range voltage an be Ud 3Em 2. Where, E m is the peak value the phase voltage on AC side. Aording to the above design priniples, the main iruit parameters the APF test platform are as follow: a Power line voltage: Hz, transformer: , and system impedane is negleted; b Three-phase unontrolled retifier bridge is adopted as the nonlinear load, its R = 8 indutane L = 1.5 mh; Inoming indutane APF: L = 1.7 mh, DC bus voltage: U = 320, apaitane: C = 2300 uf. d 5.2. The Experimental Results Analysis The grid-side transformer is used to buk-boost in the high-power appliations system, the effet aused by the leakage reatane transformer an not be ignored if the non-linear load is a large apaity or in high proportion harmoni. As an be seen in Figure 11, the voltage distortion aused by transformer leakage reatane makes the phase-lok-loop in unloked ondition thus it an not aurately trak the phase position whih aused the whole system shok. Thus, in the event that the orientation angle is not exatly, APF an not ompensate the harmoni the nonlinear loads, on the ontrary, it will ause system shok. The virtual flux observer proposed in this paper uses two first-order low-pass filters instead pure integrator. As the integral part has harateristis a low-pass filter, the amplitude n times harmoni attenuate by n times fundamental when it omes through pure integrator, whih has some effet on filtering high harmoni, that is to say, the virtual flux oriented has a good inhibition on the grid voltage distortion. It an be seen from Figure 12, while using ative filter ontrol based on novel virtual fl ux, the virtual line-flux-linkage swith smoothly before and after net-side power ompensation, the virtual flux angle an be able to aurately trak the phase position without the effet grid voltage distor- The traditional virtual line-flux-linkage uses low-pass tion. filter instead pure integrator whih auses a ertain error amplitude and phase angle, the new virtual line-flux-linkage observer with two first-order low-pass filter instead pure integral an be made no amplitude error and no phase shift. As an be seen in Figure 13, the orientation angle and amplitude errors the virtual flux vetor lead to higher urrent vetor burr, and the novel algorithm responses faster than traditional methods whih makes the urrent vetor stabilize rapidly. A (10 urrent and s (60 Rad voltage PLL (4.5 ide s Grid Angle PLL t ( 10ms Figure 11. Control the ative filter based on phase-lokphase., web (0.2 Rad linkage (1.25, flux linkage line flux (100 rtual ne k lin vi li DC virtual oltage Components Angle ud t ( 20ms e a i a Figure 12. ontrol the ative filter based on novel virtual flux.

8 G. J. TAN ET AL. 269 (a easy to found that the urre nt is nearly lose to sinusoidal and the third, fifth, seventh, ninth, eleventh and thirteenth harmonis are filtered f mostly. But there exist high harmonis by seleting the swithing frequeny 2 khz, and it is easy to be solved by the method a high-pass filter (HPF in parallel with the power system. Figure 16 is the system response in the ondition abrupt hange 50% load more. The system has an exellent dynami performane that it an be stable after 2-3 periods. Aording to Figure 16(a, the APF based on virtual flux oriented vetor ontrol system uses a voltage outer loop and the DC bus voltage oinide with the given voltage when the system omes to be stable. The voltage amplitude flutuation is minimal whih means that there s energy exhange just between the DC side ative filter and load while the system arrive at a steady state ondition. As an be seen in Figure 16(b, the DC side voltage and neutral point potential flutua- tion is minimal, and it shows that the strategy used in this system whih is based on DC side voltage and apaitane detetion and neutral point potential adaptive Fundamental Current(20A and Harmoni Current(20A i La i Ld t ( 10ms (b Figure 13. The vetorgraph power urrent in axes, (a Using low-pass filter instead pure integrator; (b Novel virtual flux observer. Figure 14 shows the harmoni and fundamental wave deteted by the load urrent rotational oordinates detetion method virtual line-flux-linkage orientation based. The method realizes harmoni urrent detetion without PLL, and ompare with the traditional i p i q detetion method PLL based, it is not restrained by frequeny shift and it an omit the PLL iruit avoiding the effet that out ontrol PLL brings to system the ative filter. Moreover, not only the fundamental omponent the deteting urrent beomes more flexible but also the detetion auray harmoni urrent is higher. Figure 15 is the final ompensated line urrent. It is Figure 14. The harmonis urrent and the fundamental wave. A Soure Current (20 i sa FFT Analysis t( 10ms Figure 15. The ompensated soure urrent and FFT analysis.

9 270 G. J. TAN ET AL. d an 200 k( DC lin oltages and 0 nk(10 DC li oltages (10A Currents Soure int (50 po neutral oltages u d t( 50ms (a i sa u d _ up u d _ down u neutral_point t ( 50ms (b Figure 16. The dynami response the system, (a The power system urrent and DC bus voltage response under abrupt ondition; (b DC bus voltage and neutral point potential response under abrupt ondition. ompensation an effetively ontrol the neutral-point potential both in stati and dynami operating onditions. These results proved that the novel ontrol algorithm with virtual line-flux-linkage-oriented based ontrol system possess strong robustness, and also demonstrated that the simplified three-level SPWM strategy with neutral point potential self-adaptation is valid. 6. Conlusions The experimental results show that the sheme adopting the proposed virtual line-flux-linkage-oriented observer an realize the non-pll ontrol for APF, and displayed that the load urrent detetion under rotating o- only detets harmonis urrent rapidly but also makes ordinate oriented by virtual line-flux-linkage an not the ompensating urrent traked referene fast, and indiated that APF system with the method disussed flux oriented and rotating oordinate based harmonis detetion has the harateristis dereasing line total harmoni distortion (THD signifiantly and superior dynami property, thus it is obvious that leading the simplified three-level SPWM strategy with neutral point potential self-adaptation in ontrol an improve the system s overall performane. 7. Referenes [1] M. EI-Habouk, M. K Darwish and P. Mehta, Ative Power Fliter. A Review, IEE Proeedings Eletron Power Appliation, ol. 147, No. 7, January 2000, pp [2] M. Izhar and C. M. Hadzer, Performane for Passive Filter Inreduing Harmonis in the Distribution System, Power and Energy Conferene, Selangor, November 2004, pp [3] M. Jasinski, D. Swierzynski and M. P. Kazmierkowski, Novel Sensorless Diret Power and Torque Control Spae etor Modulated ada Converter, IEEE International Symposium, ol. 2, May 2004, pp [4] Y. Ye, M. Kazerani,. H. Quintana, A Novel Modeling and Control Method for Three-Phase PWM Converters, IEEE 32th Annual, PESC, anouver, ol. 1, June 2001, pp [5] C. K. Lee, S. Y. Ron Hui and H. S.-H. Chung. A 31- Level Caseade Inverter for Power Appliations, IEEE Transations on Industrial Eletris, ol. 49, No. 3, June 2002, pp [6] L. S. Czarneki, On Some Misinterpretation the In- A New Method to Elimate stantaneous Reative Power p-q Theory, IEEE Transations on PE, ol. 19, No. 3, May 2004, pp [7] M. Malinowski, M. P. Kazmerkowski, S. Hansen, FBlaabjerg and C. D. Marques, irtual-flux-based Diret Power Control Three-phase PWM Retifiers, IEEE Transations on Industry Appliations, ol. 37, No. 4, August 2001, pp [8] H. Sasaki and T. Mahida A New Method to Elimate AC Harmoni Currents by Magneti Flux Compensation Considerations on Basi Design, IEEE Transations on Power Appliation System, ol. 90, No. 5, 1971, pp [9] R. D. Zhao and Y. K. He, Simulation Study on the irtual Line Flux Oriented etor Control the PWM Retifier, Proeedings the CSU 2EPSA, ol. 17, No. 5, Otober 2005, pp [ 10] H. Sasaki and T. Mahida AC Harmoni Currents by Magneti Flux Compensation Considerations on Basi Design, IEEE Transations on Power Appliation System, ol. PAS-90, No. 5, Septem- ber 1971, pp [11] J. H. Seo, C. H. Choi and D. S. Hyun, A New Simplified Spae-etor PWM Method for Three-Level Inverter, IEEE Transations on Power Eletronis, ol. 16, No. 4, July 2001, pp [12] E. Teresa and J. A. Pomilio, Shunt Ative Power Filter Synthesizing Resistive Load, IEEE transations on Power Eletron, ol. 17, No. 2, Marh 2002, pp