On-Line Capacitance Estimation of DC-Link Electrolytic Capacitor by Input Current Injection for ac/dc PWM Converters. I.

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1 On-Lne Capactance Estmaton of DC-Lnk Capactor by Input Current Injecton for PWM Conerters On-Lne Capactance Estmaton of DC-Lnk Electrolytc Capactor by Input Current Injecton for ac/ PWM Conerters Dong-Choon Lee, Kang-Ju Lee, Jul-K Seok, Jong-Woo Cho Yeungnam Un., 4-, Daedong, Kyungsan, Korea phone: / fax: lee@yu.ac.kr / Kyungpook Nat. Un., 370, Sankyuk-dong, Buk-gu, Daegu, Korea jwcho@ee.knu.ac.kr Abstract : In ths paper, a noel on-lne capactance estmaton of lnk capactor usng nput current njecton s proposed for the ac/ PWM conerters. At no load, a controlled nput current at a lower frequency than the lne frequency s njected nto the nput sde, whch causes the oltage rpples at the output sde. Wth the ac oltage and current rpple components on the sde extracted by the dgtal flters, the capactance can be calculated smply through a oltage-current relatonshp. Ths method can be mplemented only by software wthout any addtonal hardware. It s shown that the estmaton error s less than.5% from the expermental result. Keywords: PWM conerters, lnk, electrolytc capactor, current njecton, dgtal flters. I. Introducton In recent, the three-phase ac//ac PWM conerter s ncreasngly used for ndustral applcatons such as mll dres, eleators, wnd power generaton system, UPS, and so on. The ac//ac PWM conerters usually hae a lnk wth large electrolytc capactors as an energy buffer. The lfetme of the electrolytc capactor s usually shorter than that of the other components of the power conerter. The capactance of the electrolytc capactor s decreased accordng to agng []. It s known that ts lfe s ended when the capactance s reduced by more than 5% from the ntal alue []. Thus, t s necessary to decde the approprate exchangng tme of the capactor for safe operaton. In spte of the mportance of the large electrolytc capactor n the power conerters, the research results of them are ery few. Ref. [] proposed a fault dagnoss of the capactor by estmatng the ESR(equalent seres resstance) at specfc temperature. Howeer, t requres an addtonal hardware such as temperature sensors. Some references dealt wth a lfe preton of the capactor [3] [5], whch algorthms are ery complcated and they are dffcult to estmate the lfetme precsely snce the capactor characterstcs are senste to the operatng frequency and temperature. Furthermore, these methods requre the capactor to be operated contnuously wthout the pause. The capactor used for the most power conerters s nstalled nsde the system, so that t should be detached from the system to measure the capactance by the nstrument. It s ery troublesome to measure the capactance n the power conerter of the wnd power generaton system whch s located at the hgh tower or off-shore. To oercome these dffcultes, ths paper proposes a noel one-lne capactance estmaton method n the PWM conerter systems. To dagnose the degree of deteroraton, the capactance of the capactor s estmated peroally by usng nput current njecton. At ntal state before load applcaton, a certan leel of ac nput current at a lower frequency than the power lne frequency s njected nto the ac/ PWM conerter. Then all the output current of the conerter flows through the capactor snce the load s dsconnected at ntal state. The njected nput current causes the rpple oltages on the output sde. Snce the lnk oltage s usually measured for the oltage regulaton, the rpple oltage component can be extracted easly from the measured oltage by band-pass flters. On the other hand, EPE Toulouse ISBN : P.

2 On-Lne Capactance Estmaton of DC-Lnk Capactor by Input Current Injecton for PWM Conerters the -sde current s not usually measured unlke ac-sde currents used for current control and oer-current protecton. Instead, here, t can be estmated from the ac nput current and the gatng tme of the swtchng deces. Wth the alue of these rpple current and oltage, I and V, respectely, the capactance s calculated by C = I V, where s an angular elocty of the ac component. Compared wth the capactance alue measured by the RLC meter, t s confrmed that the estmaton error s less than.5%. The expermental result erfes the effecteness of the proposed algorthm.. Current njecton II. Estmaton of Capactance of DC Lnk Capactor Frst, consder the operaton of the ac//ac PWM conerter at no load, whch means the nerter sde s dsconnected wth the lnk. If the output oltage s well controlled at the reference alue, the q-axs current reference s zero and the oltage s kept constant except the swtchng frequency-related rpple component. Here, f ac current at a low frequency, for example, of 30Hz s njected nto the PWM conerter, the pulsed current of whch fundamental frequency s 30Hz flows through the capactor. Ths ac current ncurs the rpple oltage component at 30Hz. Snce the capactance of the capactor s calculated n steady state of snusod as I C = () V n n s the angular elocty of the rpple component, t s possble to calculate the capactance n where () wth I and V. con n e as e bs e cs L as bs cs a b c a b c I.M. Low-Pass Flter Band-Pass Flter Band-Pass Flter Band-Stop Flter Cal. of )/ T, cal = ( Tga as + Tgb bs + Tgc cs Cal. of V, bsf samp - V I Voltage Controller Capactance Calcuaton I C = V n de qs Current qeo qe Controller SVPWM ds qe,n Current njecton = 5 sn(π 30 ) = 0 qe, n t Fg. Block dagram of estmatng capactance for three-phase PWM conerters EPE Toulouse ISBN : P.

3 On-Lne Capactance Estmaton of DC-Lnk Capactor by Input Current Injecton for PWM Conerters The lower the frequency of the njected current at the same magntude, the larger the rpple oltage. Thus the low frequency current component s desrable to obtan a sgnfcant component of the rpple oltage snce the allowable rpple current leel of the capactor s not so hgh[6]. Fg. shows an oerall block dagram for estmatng the capactance of the PWM conerter whch noles the output oltage and ac nput current control. The ac current s transformed nto the d- and q-axs currents n a synchronous reference frame. The d-axs current s controlled to be zero at unty power factor of the source sde and the q-axs current s regulated to control the oltage. At no load, the q-axs current becomes zero. When a certan leel of ac current reference at 30[Hz] s added to the q-axs current reference, ac rpple current at 30[Hz] flows through the capactor, whch causes ac rpple oltage at the same frequency. Wth these rpple current and oltage, the capactance can be found by ().. DC lnk rpple current To calculate the capactance n (), t s needed to measure the capactor current. Whle the ac nput current s measured for current control and oer-current protecton, the lnk current s scarcely done. Instead, the lnk current can be reconstructed from the ac nput currents and swtchng functon as [7], [8] = S + S + S () a as b bs c cs Snce the nstantaneous current of the capactor n () s of the pulsed-waeform, t s not easy to obtan the alue. After elmnatng the hgh frequency component by the low-pass flter, only the rpple component at the frequency of the njected current can be obtaned by usng the bans-pass flter. Howeer, ths process s complcated. as 0 bs cs UPPER ON SEQUENCE UPPER OFF SEQUENCE + A T ga T gb + B T gc + C as cs cs as, cal T samp T samp Fg. Instantaneous and aeraged -lnk currents accordng to gatng pulses and phase currents EPE Toulouse ISBN : P.3

4 On-Lne Capactance Estmaton of DC-Lnk Capactor by Input Current Injecton for PWM Conerters Instead, usng the gatng tme and the phase currents, the mean alue of the lnk current each samplng perod can be found as = ( T + T + T ) / T (3) where T ga ga as gb bs gc cs samp, T gb, andt gc are gatng tmes of each phase and T samp s the samplng perod. Fg. shows the relatonshp between the phase current and the lnk current accordng to the swtchng state. The waeform of shows a natural flterng effect. If the dead tme of the swtchng deces s compensated well, ts effect n (3) can be neglected[9]. 3. Dgtal flterng To extract the ac rpple component requred from the measured oltage and the estmated lnk current n (3), a second-order band-pass flter s used[0], []. The transfer functon of t s as K ( Q ) s H ( s) = (4) s + ( Q ) s + where K s a gan, Q s a qualty factor, f s a cut-off frequency, and = πf. The flter output s not so senste to the qualty factor snce there s no other components near 30[Hz]. Fg. 3 shows frequency response of the band-pass flter n case that K =, Q = 4, and f = 30[ Hz]. On the other hand, the rpple oltage component should be rejected snce the aerage alue of the lnk oltage should be fed back for the oltage control. For ths, so, a second-order band-stop flter s used, of whch transfer functon s gen by K ( s + ) H ( s) = (5) s + ( / Q ) s + where K s a gan, Q s a qualty factor, f s a cut-off frequency, and = πf. Fg. 4 shows the frequency response of the band-stop flter n case that K =, Q =, and f = 30[ Hz]. H ( j ) H ( j) K Q K K Q Fg. 3 Gan cure of the band pass flter Fg. 4 Gan cure of the band stop flter EPE Toulouse ISBN : P.4

5 On-Lne Capactance Estmaton of DC-Lnk Capactor by Input Current Injecton for PWM Conerters The oltage fltered by the band-pass flter stll contans the hgh frequency components due to the swtchng acton of the deces. To elmnate these components, a second-order low-pass flter s used, of whch transfer functon s gen by K H ( s) = (6) s + ( Q ) s + where K =, Q =, f = 00[ Hz], and = πf are chosen for experment. 4. Calculaton of the alue The alue of the ac rpple oltage and current fltered by the band-pass flter s calculated by V = N N k= ( k) (7) and I = N ( k) N k = where N s the number of the samplng pont for one cycle of the ac rpple component. Substtutng for (7) and (8) nto (), the capactance can be calculated. III. Expermental Results To erfy the effecteness of the proposed scheme, the experment has been carred out at laboratory. Fg. 5 shows the expermental set-up. The experment condton s lsted n Table I. The nomnal capactances used for test are compared wth that of the measured ones. Fg. 6 shows the oltage controller output qe0 at no load and the njected q-axs current reference qe,n, and shows the modfed reference qe whch s obtaned by addng these two references. Snce the d-axs current s controlled to be zero at unty power factor, the q-axs current s kept almost zero at no load. So, qe s equal to qe,n. (8) Fg. 5 Expermental set-up Table I System parameters Input oltage 3-phase 0[V] Input boost nductance 3.5[mH] Lne resstance 0.5[ Ω ] Conerter capacty 3[kVA] Swtchng frequency 3.5[kHz] Injecton current 5[A]pk, 30[Hz] -lnk oltage 350[V] Nomnal capactance Measured ones Capa- C = 3300[ µ F] C = 3077[ µ F] ctor C = 350[ µ F] C = [ µ F] C = 500[ µ ] C = 474[ µ ] 3 F 3 F EPE Toulouse ISBN : P.5

6 On-Lne Capactance Estmaton of DC-Lnk Capactor by Input Current Injecton for PWM Conerters Fg. 7 shows the q-axs current control performance. Control oer ac component s well done, too. Fg. 8 shows the lnk currents, where s the current measured drectly by the current probe and s the waeform reconstructed from (3), and s the band pass-fltered one from the waeform of. Fg. 9 shows the harmonc spectrum of lnk currents correspondng to Fg. 8. It s seen that the other frequency components are completely elmnated after passng the. Fg. 0 shows the output oltages, where s the measured one and s the band stop-fltered waeform for oltage feedback control, s the band pass-fltered oltage from for the alue. Fg. shows the harmonc spectrum of the rpple oltage correspondng to Fg. 0. The performance of the flters s ery satsfactory. Fg. shows the araton of the capactance alue, lnk rpple oltage, and current n case that C3 s open abruptly whle C and C3 n Table I are beng operated n parallel connecton. Een though the capactance alue s changed abruptly, t s obsered that t s well estmated. Table II shows the comparson of the estmated and measured data for the dfferent capactance of the capactor. The estmaton error s less than.5% n all cases. qe, n (a ) 0[ A] qeo qe qe (b ) 0[ A].5[A]/d 0[ A] qe.5[a]/d T/d = 0[ms] T/d = 0[ms] Fg. 6 Voltage controller output and njected current Fg.7 Q-axs current and ts reference, cal, cal, bpf T/d = 0[ms].5[A]/d, bpf 30[ Hz] 50[Hz/d] Fg. 8 Dc-lnk currents measured calculated band-pass fltered Fg.9 Harmoncs spectrum of -lnk currents measured calculated band-pass fltered EPE Toulouse ISBN : P.6

7 On-Lne Capactance Estmaton of DC-Lnk Capactor by Input Current Injecton for PWM Conerters 350[V] 0[V] 350[V], bsf 0[V],, bsf 0[V] bpf 5[V]/d T/d = 0[ms] Fg. 0 Dc-lnk oltages Measured band-stop fltered band-pass fltered, bpf 0[V] 50[Hz/d] Fg. Harmonc spectrum of -lnk oltages measured band-stop fltered band-pass fltered ) C = 598 [ µ F ], bpf, bpf T/d = 00[ms] ) C = 3 [ µ F ].5[V]/d.5[A]/d Fg. Capactance estmaton at abrupt araton of C Table II Measured and calculated capactances of capactors Capactance Voltage and current (30[Hz]) of -lnk capactor of oltage of current Calculated capactance Calculaton Error 3077[ µ F ] 4.386[V].58[A] 3[ µ F] +.46% 596[ µ F] 5.73[V].58[A] 598[ µ F ] +0.08% [ µ F] 6.436[V].575[A] 3[ µ F] -0.43% IV. Conclusons In ths paper, a noel on-lne capactance estmaton method for the three-phase ac//ac PWM conerter has been proposed and mplemented. Ths scheme requres no extra hardware, whch s executed n software only by njectng ac nput current and sgnal processng wth dgtal flters. It s obsered expermentally that the capactance estmaton error s less than.5%. The proposed algorthm s ery effecte for dagnoss of the electrolytc capactor s deteroraton for the ac//ac PWM conerter system. References []. P. Venet, F. Persse, M. H. El-Hussen, and G. Rojat. "Realzaton of a Smart Electrolytc Capactor Crcut," IEEE Industry Applcaton Magazne, pp. 6-0, 00, Jan/Feb. []. Htach AIC Inc., Estmated lfe of Alumnum Electrolytc Capactors, Tokyo, Japan, 999, August. [3]. D.-C. Lee and H.-J. Km. "Lfe Estmaton of Electrolytc Capactors for Inerters," Tran. of KIEE, Vol. 50B, No.7, pp , 00, Jul. EPE Toulouse ISBN : P.7

8 On-Lne Capactance Estmaton of DC-Lnk Capactor by Input Current Injecton for PWM Conerters [4]. M. L. Gasper. "A Method for Pretng the Expected Lfe of Bus Capactor," IEEE IAS Conf. Rec., pp , 997, October. [5]. A. Lahyan, P. Venet, G, Grellet, and Verge. "Falure Preton of Electrolytc Capactors durng Operaton of a Swtch mode Power Supply," IEEE Trans. on Power Electon., Vol. 3, pp , 998, No. [6]. F. G. Hayatee. "Heat Dsspaton and Rpple Current Ratng n Electrc Capactors," Electrocomponent Sc. Tech., Vol., pp09-4, 975. [7]. D.-C. Lee, G.-M. Lee, and D.-S. Lm. "A noel Control Scheme of Three-Phase PWM Rectfers Elmnatng ac-sde Sensors," of KIPE, Vol. 5, No. 6, pp , 000, Dec. [8]. W.-C. Lee and D.-S Hyun. Sngle Sensor Current Control of a Three-Phase Voltage-Source PWM Conerter Usng Prete State Obserer, Tran. of KIEE, Vol. 4, No.3, pp. 4956, 999, June. [9]. J.-W. Cho and S.-K. Sul. Inerter Output Voltage Synthess Usng Noel Dead Tme Compensaton, IEEE Trans. on Power Electron., Vol., No., 996, March. [0]. Rulph Chassang, Dgtal Sgnal Processng, John WILEY & SONS, INC, 999. []. J. H. Mcclellan, R. W. Schafer, and M. A. Yoder. DSP Frst, Prentce Hall, 999. EPE Toulouse ISBN : P.8