Internatonal Conference on Advances n Energy and Envronmental Scence (ICAEES 015) Study on Shunt Actve Power Flter wth Improved Control Method Yaheng Ren1,a*, Xaozh Gao,b, Runduo Wang3,c 1 Insttute of Appled Mathematcs, Hebe Academy of Scences, 46 Youy Street, Sh Jazhuang, Hebe Provnce, 050000, PRC School of Electrcal Engneerng, Hebe Unversty of Scence and Technology 6 Yuxang Street, Sh Jazhuang, Hebe Provnce, 050018, PRC 3 Automoble and motorcycle ndustry Development Bureau of Qnghe County Aoje Technology Buldng, Qnghe County, Hebe Provnce 054800, PRC a yahengren@16.com,bgaoxaozh@hebust.edu.cn,cqmjbgs@163.com Keywords SAPF, mproved control method, Ip-Iq current detecton method, repettve control, PI control, harmonc compensaton Abstract.In order to mprove power qualty and ensure stable operaton of power system, shunt actve power flter (SAPF) s wdely used o flter out harmoncs. Ths paper proposed an mproved control method for SAPF. The whole mproved control strategy could be dvded nto two parts: IpIq current detecton method to obtan compensaton current reference value, and repettve control combned wth PI control to generate PWM pulse to control SAPF to delver compensaton current. The MATLAB smulaton results show that SAPF wth the proposed control method has good harmonc compensaton performance. Introducton The wde use of power electronc devces n modern power system has ntroduced a large number of harmoncs, whch has caused serous damage to power supply lne, power consumpton equpments and communcaton equpments and so on. In order to mprove power qualty and ensure stable operaton of power system, many nds of measures could be adopted to flter out the harmoncs, and flter s the frst choce. Flter could be dvded nto passve flter and actve flter. As passve flters may cause resonance, and ts flterng effect depends on the system mpedance characterstcs and s easly nfluenced by the external condtons, scholars pay more attenton on actve flter [1]. Wth advantages of harmonc and reactve power smultaneous compensaton, flexble swtchng and easy control, shunt actve power flter (SAPF) s the most wdely used actve flter devce [][3]. By a certan control method, SAPF detects the harmonc components and then the compensaton current s generated n opposte to the harmonc components to acheve the flterng purpose. Therefore, SAPF harmonc compensaton performance s manly determned by the accuracy of the harmonc detecton and the accuracy of the current tracng control, whch means an effectve control method. However, many scholars dd researches on SAPF wth dfferent control methods [4]-[9]. Based on the analyss about these references, we can see that there are two common nds of methods for harmonc detecton: p_q method and Ip-Iq method. The actve current obtaned by p-q method s always dstorted when the grd voltage contans dstorted and asymmetrc components, and the Ip-Iq method s not affected. Therefore, ths paper adopted the latter method to detect harmoncs. And also we can see that compensaton current control method has been another focus of domestc and foregn scholars. Commonly used technologes are hysteress current control method and trangular wave modulaton control method. And also some artfcal ntellgence methods are n research. For hysteress current control method, the current tracng accuracy has been lowered down because of the fxed hysteress wdth. For trangular wave modulaton control method, the tracng accuracy has been weaen due to harmoncs wth the same frequency as the trangular carrer. 015. The authors - Publshed by Atlants Press 336
Therefore, ths paper proposed repettve control and PI control together to generate PWM pulse to control SAPF to delver more accurate compensaton current. SAPF worng prncple The schematc dagram of SAPF s shown n Fg.1. The worng prncple of SAPF s as follows: the current wth harmoncs s detected frst, and then the compensaton current reference value s obtaned from reference current calculaton crcut and as the nput to current tracng control crcut, and control sgnal outputtng from current tracng control crcut s to drve the SAPF crcut to delver the desred compensaton current offsettng the harmoncs and reactve current eventually. Fg.1. The schematc dagram of SAPF The mproved control method The whole mproved control strategy could be dvded nto two parts: Ip-Iq current detecton method to obtan compensaton current reference value, and repettve control and PI control together to generate PWM pulse to control SAPF to get compensaton current. A. Ip-Iq current detecton method Fg.. Current detecton method The current detecton method s shown as Fg. [10]. Frst, the current detected from non-lnear load sde s transformed from statonary a-b-c coordnate system to rotatng d-q-0 coordnate system by equaton (1) and the current dc values are obtaned. q = C p Where α β = CC 3 a b c 1 C 3 = 3 0 sn ω t C = cos ω t (1) 1 1 3 3 cos ω t sn ω t And snwt and coswt are extracted from the phase-loced loop (PLL) segment. 337
Then p and q are fltered by LPF (Low Pass Flter) respectvely to get fundamental current p and q... The fundamental value of current n a-b-c coordnate system can be expressed as Equaton (), af bf = C cf Where 3 αf βf = C 3 C 1 q Fnally the compensaton current ( ah p, bh C 3 1 0 = 1 3 1 3 and ch ) can be calculated as ah a af bh = b bf ch c cf (3) Current tracng control--- repettve control combned wth PI control together. Based on nternal model prncple, repettve control has the advantages of mprovng the system accuracy and realzng zero steady-state error tracng, whch maes t a good current control method. Repettve control s to correct the orgnal perodc sgnal by controllng the devaton between perodc sgnal and feedbac sgnal so as to elmnate the harmoncs and then mprove the tracng accuracy of the system. The repettve controller s made up of two parts, the nternal module and the compensator, as shown n Fg.3 [10][11]. () Fg 3. The repettve controller The nternal module prncple s to mplant the dynamc model of external sgnal to controller to form a hgh precson feedbac control system. The value of Q s generally chosen to be a constant less than 1 or a functon of a low pass property. In ths paper we set Q as 0.95 based on emprcal value. The compensator s to offer a control sequence accordng to the error sgnal of last cycle and then offset the error qucly and effectvely n the next cycle. Compensator can be expressed as equaton (4) Gc ( z) = K r z S( z) (4) Where K s the gan value for the ampltude compensaton of repettve control, and K r [ 0,1], z s Lead compensator to elmnate phase lag factor, and S(z) s low pass flter because the gan of repettve control s small n hgh frequency secton and S(s) can be expressed as equaton (5) ω n S( s) = (5) s + ξω + ω n n Wth sutable ξ and ω n, S(z) can be obtaned by S(s) dscretzaton. However, the repettve controller s a functon of the next cycle and has to be combned wth better dynamc performance control method n practcal applcaton to output sgnal wth steady state performance and quc dynamc response. In ths paper, we choose PI control to assst repettve control. The current tracng control scheme s shown as Fg.4. 338
Fg.4 The current tracng control scheme---repettve control + PI control Accordng to the practcal experence, the formula of proportonal-ntegral part s * L * ωc p = u * = ω c dc * p (6) Where L s the total equvalent nductance seeng from the actve power flter nverter sde to system sde, ωc s the trangular carrer frequency; udc s the voltage of SAPF nverter DC sde. The worng process s: the dfference between reference compensaton current and SAPF output compensaton current s as the nput sgnal for repettve controller and PI controller, and then the output sgnals from the two controllers are added together and then modulated by trangular wave to generate PWM sgnals to control converter and then to mae SAPF output compensaton current real-tme tracng reference current calculated by current detecton controllng module. Therefore, SAPF realzes real-tme and accurate harmonc compensaton. Smulaton Results The whole system (Fg.1) s verfed n Matlab/Smuln smulaton. It s composed of AC power (0V, 50Hz), nonlnear load (We choose three-phase brdge rectfer as harmonc source because ths nd of equpment s wdely used n power system and causes serous harmonc polluton. ) and SAPF wth mproved control method. Fg.5 s the current waveform of system wthout SAPF. It can be seen that current contans many harmonc components. If we use FFT to analyze ths waveform, we can see that the THD reaches to 14.6% as shown n Fg.6. Fg.5 the current waveform and ts THD of system wthout SAPF From Fg.6, we can see that the current harmoncs have been elmnated by addng SAPF to the system. And the THD of current drops to.15% as shown n Fg.6. 339
Fg.6 the current waveform and ts THD of system wthsapf To testfy the accuracy of the mproved control method, power qualty mprovement results of SAPF wth dfferent control methods are shown n Table 1. From Table.1 t can be seen that the lowest THD of current s SAPF wth the proposed control method, whch means that the proposed method s the most effectve among these control methods. Table1. Power qualty mprovement results of SAPF wth dfferent control methods Control methods Ip-Iq method hysteress current control method Ip-Iq method trangular wave modulaton control method the proposed control method p_q method repettve control + PI control THD 3.86% 4.1%.15%.95% Conclusons Ths paper proposed repettve control combned wth PI control as current tracng control method for SAPF to elmnate harmonc nfluence n control and mprove the current tracng accuracy. Smulaton results confrm the effectveness of the mproved control method. Compared wth othe control methods, SAPF wth the proposed control method shows a better performance n harmonc compensaton. References [1]Das J C. Passve flters Potentaltes and lmtatons. IEEE Transacton on Industry Applcatons, vol. 40(1), pp. 3-41, 004. []Aag H, Ogasawara S, Hyosung K. The theory of nstantaneous power n three-phase four-wre systems: a comprehensve approach [C]. Proceedngs of Industry Applcatons Conference. New Yor: IEEE,1999: 431-439. [3]Bhm Sngh, Kamal Al-Haddad, Ambrsh Chandra. "A revew of actve flters for power qualty mprovement". IEEE Trans, on I.E, vol. 46(5), pp. 1-1,1999 [4] Ucar M, Ozdemr E, Kale M. An analyss of three-phase four-wre actve power flter for harmonc elmnaton reactve power compensaton and load balancng under non-deal mans 340
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