An Isolated Three-Port Bidirectional DC-DC Converter for Photovoltaic Systems with Energy Storage

Transcription

1 Univesity of Nebaska - Lincoln DigitalCommons@Univesity of Nebaska - Lincoln Faculty Publications fom the Deatment of Electical and Comute Engineeing Electical & Comute Engineeing, Deatment of 15 An Isolated Thee-Pot Bidiectional DC-DC Convete fo Photovoltaic Systems with Enegy Stoage Jianwu Zeng Univesity of Nebaska-Lincoln, jzeng@huskes.unl.edu Wei Qiao Univesity of Nebaska-Lincoln, wqiao@eng.unl.edu Liyan Qu Univesity of Nebaska-Lincoln, lqu@unl.edu Follow this and additional woks at: htt://digitalcommons.unl.edu/electicalengineeingfacub Pat of the Comute Engineeing Commons, and the Electical and Comute Engineeing Commons Zeng, Jianwu; Qiao, Wei; and Qu, Liyan, "An Isolated Thee-Pot Bidiectional DC-DC Convete fo Photovoltaic Systems with Enegy Stoage" (15). Faculty Publications fom the Deatment of Electical and Comute Engineeing. 38. htt://digitalcommons.unl.edu/electicalengineeingfacub/38 This Aticle is bought to you fo fee and oen access by the Electical & Comute Engineeing, Deatment of at DigitalCommons@Univesity of Nebaska - Lincoln. It has been acceted fo inclusion in Faculty Publications fom the Deatment of Electical and Comute Engineeing by an authoized administato of DigitalCommons@Univesity of Nebaska - Lincoln.

2 14-IACC-19.R1 1 An Isolated Thee-Pot Bidiectional DC-DC Convete fo Photovoltaic Systems with Enegy Stoage Jianwu Zeng, Student Membe, IEEE, Wei Qiao, Senio Membe, IEEE, and Liyan Qu, Membe, IEEE Abstact This ae ooses a new isolated, thee-ot, bidiectional, DC-DC convete fo simultaneous owe management of multile enegy souces. The oosed convete has the advantage of using the least numbe of switches and soft switching fo the main switch, which is ealized by using a LCL-esonant cicuit. The convete is caable of intefacing souces of diffeent voltage-cuent chaacteistics with a load and/o a DC micogid. The oosed convete is constucted fo simultaneous owe management of a hotovoltaic (PV) anel, a echageable battey, and a load. Simulation and exeimental esults show that the oosed convete is caable of maximum owe oint tacking contol fo the PV anel when thee is sola adiation and contolling the chage and dischage of the battey when thee is sulus enegy and owe deficiency with esect to the load, esectively. Index Tems Battey, bidiectional DC-DC convete, isolated convete, multiot convete, hotovoltaic (PV), soft switching, zeo-cuent switching (ZCS). I. INTRODUCTION To integate multile DC enegy souces of diffeent tyes to a owe gid, multile indeendent DC-DC convetes ae commonly used to ste u the time-vaiant, low-level souce voltages to a constant high-level voltage that is equied by a gid-tie invete. Comaing to that solution, a multiot DC-DC convete is efeable owing to the advantages of using fewe comonents, lowe cost, highe owe density, and highe efficiency [1], []. The multiot convete toologies can be classified into two categoies: nonisolated and isolated toologies [3]. Nonisolated multiot convetes ae usually used in the alications whee a low voltage egulation atio is equied [4], [5]. In contast, in the alications equiing a high voltage egulation atio, isolated convetes which contains a tansfome is efeed [6]-[8]. Manuscit eceived Ail, 14; evised Novembe 17, 14; acceted Januay 17, 15. This wok was suoted by the U.S. Fedeal Highway Administation unde Ageement DTFH61-1-H-3. Any oinions, findings, and conclusions o ecommendations exessed in this ublication ae those of the authos and do not necessaily eflect the view of the Fedeal Highway Administation. The authos ae with the Powe and Enegy Systems Laboatoy, Deatment of Electical and Comute Engineeing, Univesity of Nebaska-Lincoln, Lincoln, NE USA ( jzeng@huskes.unl.edu; wqiao@eng.unl.edu; lqu@unl.edu) The cuently used isolated multiot toologies include the isolated full-bidge convete [6], which uses fou contollable owe switches fo each souce, the isolated half-bidge convete [9], which uses two switches fo each souce, and the isolated single-switch convete [1], which only uses one switch fo each souce. In some actical alications, enegy stoage, such as batteies, is commonly used to handle the intemittence of sola and wind enegy souces. This equies that at least one ot of the multiot convete is bidiectional. The afoementioned toologies ae all unidiectional and cannot satisfy such alications [11]. Seveal bidiectional toologies, such as full-bidge [1], [13] and half-bidge [14], [15] toologies, have been oosed. These two toologies utilize many switches with comlicated dive and contol cicuits. Recently, a thee-ot toology was oosed by adding one middle banch to the taditional half-bidge convete [16], [17]. It uses less contollable owe switches than the half-bidge toology and can achieve zeo-voltage switching (ZVS) fo all main switches. Howeve, the voltage of the imay souce should be maintained at a high value to chage the battey and the battey is both chaged and dischaged within a switching eiod. Such a high-fequency chage/dischage has a negative effect on the battey lifetime. This ae ooses a new isolated, thee-ot, bidiectional, DC-DC convete. It contains an inducto-caacito-inducto (LCL)-esonant cicuit to achieve zeo-cuent switching (ZCS) fo the main switch. Comaed with the convete in [17] using five contollable switches, the oosed convete only use thee switches; moeove, when using the same enewable enegy souce to chage a battey, the nominal voltage of the battey connected to the oosed convete can be highe than that connected to the convete in [17]. The oosed convete is alied fo simultaneous owe management of a hotovoltaic (PV) system with a battey in this ae. The PV system and the battey ae connected to the unidiectional ot and the bidiectional ot of the convete, esectively. A maximum owe oint tacking (MPPT) algoithm is designed fo the PV anel to geneate the maximum owe when sola adiation is available. A chage and dischage contolle is designed to contol the battey to eithe absob the sulus owe geneated by the PV anel o suly the deficient owe equied by the load. Simulation and exeimental esults ae ovided to validate the oosed convete.

3 14-IACC-19.R1 Fig. 1. Poosed isolated, thee-ot, bidiectional, DC-DC convete fo a PV and battey system. II. TOPOLOGY AND OPERATING PRINCIPLE OF THE PROPOSED CONVERTER A. Toology of the Poosed Convete The cicuit diagam of the oosed convete is shown in Fig. 1, which consists of a low-voltage-side (LVS) cicuit and a high-voltage-side (HVS) cicuit connected by a high-fequency tansfome. The LVS consists of two ots, an enegy stoage caacito C s, the imay winding of the tansfome, and a LCL-esonant cicuit consisting of two inductos L and L and a caacito C, whee L includes the added inductance L 1 and the leakage inductance of the tansfome L'. The HVS consists of the seconday winding of the tansfome and a full-bidge ectifie imlemented with the diodes D s1 ~D s4. The tansfome s tun atio is defined as: n = N / N s, whee N and N s eesent the numbes of tuns of the imay and seconday windings, esectively. Among the switches, S 1 is called the main switch because it not only contols the owe geneated by the souce connected to Pot 1 (P 1 ), but also changes the diection of the cuent flowing though the tansfome. In this ae, the two ots on the LVS ae connected to a PV anel and a battey. To simlify the analysis, the oosed convete is analyzed by two seaate convetes: one is a single-switch LCL-esonant convete [18], and the othe is the battey-elated buck and boost convete consisting of L, S, and S 3. B. Single-Switch LCL-Resonant Convete fo PV Panel In a switching eiod, the voltages acoss C 1 and C s can be taken as constant values. Paticulaly, in the steady state, V Cs = V 1, whee V 1 is the outut voltage of the PV anel. The convete has seven oeating modes deending on the states of the switch S 1 and the esonant cicuit. Fig. shows the equivalent esonant cicuit in diffeent modes. The diffeential equations of the esonant cicuit in Mode k (k = 1,, 7) ae oeating mode, esectively. Then v can be solved fom (1) and has the following fom. [ ( t t )] + B sin ( t t ) [ ] V v ( t) = A cos k ω k + whee ω () ω = 1/ L C (3) is the esonant fequency in Mode k; V (k) is the aticula solution of equation (1) in Mode k, and A (k) and B (k) ae coefficients, which can be exessed as: A = v( t ) V (4) B k I1 i ( tk ) i( tk ) = ω C whee v(t k ), I 1, i (t k ), and i(t k ) eesent the voltage acoss C and the cuents of L 1 (i 1 can be viewed as a constant value I 1 because of a lage L 1 ), L, and L at time t k, esectively. Equations (4) and (5) indicate that only ω (k) and V (k) ae equied to detemine the aametes of (). The steady-state wavefoms and equivalent cicuits of the seven oeating modes of the convete ae shown in Figs. 3 and 4, esectively. To facilitate the exlanation of the convete oeation, define V T = n V dc the equivalent outut voltage of the convete efeed to the imay side of the tansfome. Mode 1: t [t 1, t ] (see Fig. 3). Pio to Mode 1, S 1 is off; the cuents though L and L ae zeo and a ositive value of I 1, esectively, i.e., i(t 1 ) =, i (t 1 ) = I 1. When S 1 is on, as shown in Fig. 4, L and L esonate with C, the cuent of the inducto L inceases and the voltage of the caacito C deceases. Due to the existence of L, the cuent though the switch S 1 inceases slowly so that the switch is tuned on unde a low di/dt condition. The esonant fequency and the aticula solution in this mode can be exessed as: (1) ω = 1/ L // L C (6) ( ) (5) di v = L dt (1) dv i = + 1 C i dt whee v eesents the voltage of the caacito C ; L (k) (k) and i eesents the equivalent esonant inductance and the cuent though the equivalent esonant inducto in the k th (k = 1,, 7) Fig.. Equivalent esonant cicuit.

4 14-IACC-19.R1 3 Fig. 3. Steady-state wavefoms of the oosed convete. L V + (1) = 1 L + L ( V V ) whee // eesents that L and L ae connected in aallel. In this mode, the cuent i m though the imay magnetizing inductance L m inceases, the cuent i T though the seconday T (7) side of the tansfome is ositive, which indicates the conduction of the D s1 and D s3. At the end of Mode 1, i (t ) = i m (t ), i achieves its maximum value I max, i T (t ) =, v(t ) =, and v changes its olaity fom ositive to negative. Mode : t [t, t 3 ], duing which S 1 is on, i(t) >, and i (t) = i m (t), D s1 D s4 ae evese biased such that i T =. As shown in Fig. 4, L m, L, and L esonate with C. Since L m >> L, L m >> L, then () ω = 1/ ( L + L ) // L C 1/ L C (8) [ m ] () L L V = V V (9) 1 L + L + Lm Lm At the end of Mode, v (t 3 ) = V T, v(t 3 ) = V 1 V T, the diodes D s and D s4 begin to conduct. Mode 3: t [t 3, t 4 ], duing which S 1 is on, i(t) >, v (t) = V T, i T <. As shown in Fig. 4(c), L and L esonate with C ; the enegy stoed in L is eleased to chage the caacito C ; v is clamed to V T ; and i T is negative, which indicates the conduction of D s and D s4. Comaed to Mode 1, the only diffeence in the equivalent cicuit in this mode is the sign of v. Thus, ω (3) = ω (1), and ( 3) L V = ( V VT ) (1) 1 L + L This mode teminates at time t 4 when the cuent of L deceases to zeo, i.e., i(t 4 ) =. Mode 4: t [t 4, t 5 ], duing which S 1 is on, i(t) <, v (t) = V T, i T <, and D s and D s4 conduct. As shown in Fig. 4(d), a negative cuent flows though the intenal diode of the switch S 1 ; the gate signal can be emoved to tun off the switch, e.g., at time t 5, unde the ZCS condition. The cicuit equations ae the same as those in Mode 3. Thus, ω (4) = ω (1), V (4) = V (3). At the end of Mode 4, i(t 5 ) = and the voltage acoss the switch S 1 is the (c) (d) (e) (f) (g) Fig. 4. Equivalent cicuits fo diffeent oeating modes. Mode 1: S 1 is on, i >, v = V T ; Mode : S 1 is on, i > and i = i m ; (c) Mode 3: S 1 is on, i >, and v = V T ; (d) Mode 4: S 1 is being tuned off, i < and v = V T ; (e) Mode 5: S 1 is off, i =, v = V T ; (f) Mode 6: S 1 is off, i =, i = i m ; (g) Mode 7: S 1 is off, i =, v = V T.

5 14-IACC-19.R1 4 same as that acoss the caacito C, i.e., v ds1 (t 5 ) = v. Mode 5: t [t 5, t 6 ], duing which S 1 is off, i(t) =, v = V T, and i T <. As shown in Fig. 4(e), L and the switch S 1 can be neglected in the cicuit. The inducto L esonates with C, and the diection of i changes fom negative to ositive. The following can be obtained. (5) ω = 1/ L (11) C ( 5) V = V 1 V T (1) At the end of Mode 5, i (t 6 ) = i m (t 6 ), i T (t 6 ) =, and v changes its olaity fom negative to ositive. Mode 6: t [t 6, t 7 ], duing which S 1 is off, i(t) =, i (t) = i m (t), D s1 D s4 ae evese biased such that i T =. As shown in Fig. 4(f), L m and L esonate with C, and C is chaged. The following can be obtained. ω = 1/ ( L + L ) C 1/ L C (13) 6 m (6) V = V (14) 1 At time t 7, v(t 7 ) = V 1 + V T and v (t 7 ) = V T. Mode 7: t [t 7, t 8 ], duing which S 1 is off, i(t) =, v (t) = V T, D s1 and D s3 conduct. As shown in Fig. 4(g), L esonates with C, the cicuit equations ae the same as those in Mode 5 excet the sign of v, then ω (7) = ω (5), and ( 7) V = V 1 + V T (15) Once S 1 is tuned on at time t 8, Mode 7 switches to Mode 1. Thee ae five inductances L 1, L, L, L 1, and L m in the oosed convete that need to be oely designed. L m is designed based on the following citical inductance L mc [18]. VT T Lmc = (16) 4 I m, k whee T is the switching eiod of the switch S 1 ; I m,k is the eak cuent though the magnetizing inducto. In this ae, the oot mean squae (RMS) value of the magnetizing cuent is designed to be % of the RMS value of i. Then L m is designed to be lage than L mc. Once the tansfome is designed, the leakage inductance L' of the tansfome can be measued. Given the load esistance R L and the tansfome s tun atio n, the quality facto Q of this LCL-esonant convete can be calculated [19]: 8 n RL Q = (17) π Z whee Z is the chaacteistic imedance of the esonant cicuit defined below. ' L //( L 1 + L ) Z = (18) C Given the desied value of Q and the value of R L, the value of Z can be calculated fom (16). In this ae, Q is selected in the otimal ange of [1.5, 5]. Secifically, the value of Q is 3.7 when the nominal load is alied. Then, given the esonant fequency, C can be calculated fom (6) and (18). Consideing the necessay condition L > L to achieve ZCS [], L = L 1 is selected such that the cuents though the switch S 1 and the tansfome ae close duing the esonant stage. Then L and L 1 can be calculated fom (18) with the measued value of L'. The values of L 1 and L ae designed accoding to thei m desied cuent iles [1]. In this ae, it is exected that the cuent iles ae within 5% of thei nominal cuents. C. Buck and Boost Convete fo Battey The buck and boost convete consists of the inducto L, switches S and S 3, and caacito C s. When the geneated sola owe is lage than the owe equied by the load, S 3 is inactive and S is switched on to fom the buck convete. Then, the sulus enegy geneated fom the PV anel is stoed in the battey. In contast, when the geneated sola owe is less than the owe equied by the load, S is switched off and S 3 is switched on to fom the boost convete. The battey is dischaged to C s to ovide the deficient enegy equied by the load. III. POWER MANAGEMENT OF THE PROPOSED CONTROLLER Two contolles ae needed to manage the owe in the LVS. Thei objectives ae to egulate the outut DC-link voltage to a constant value and manage the owe fo the two souces, esectively. Accoding to the availability of the sola owe, thee ae thee woking scenaios of the convete, as illustated in Fig. 5. A. Thee Woking Scenaios Scenaio 1 ( 1 out ): the available sola owe is moe than the load demand. As shown in Fig. 5, the PV convete woks in the MPPT mode; the battey is chaged so that the DC-link voltage is contolled at a constant value. Scenaio ( < 1 < out ): thee is sola adiation but the sola owe is not sufficient to suly the load. As shown in Fig. 5, the PV anel is contolled in the MPPT mode by the MPPT algoithm descibed late. On the othe hand, the deficient owe is sulied by the battey, which is dischaged by the boost convete, so that the DC-link voltage can be maintained at a constant value. Scenaio 3 ( 1 = ): thee is no sola owe available and, thus, the battey is dischaged to suly the load, as shown in Fig. 5(c). The active switches ae S 1 and S 3. Poe contolles ae designed to manage the owe of the system in diffeent scenaios. Fig. 6 shows the oveall system with contolles, which include a MPPT contolle fo the PV anel and chage and dischage contolles fo the battey. B. MPPT Contolle fo PV Panel The oosed convete is alied fo MPPT contol of a PV anel using the etubation and obsevation (P&O) MPPT algoithm [1] to maximize the PV anel s outut efficiency. Fig. 7 shows the flowchat of the MPPT algoithm. A atio c is defined to secify the elative owe change (RPC) of the PV anel between two consecutive samling stes. P1( k) P1( k 1) c = (19) P1 ( k 1) whee P 1 (k) and P 1 (k 1) eesent the measued outut owe of the PV anel in the k th and (k 1) th stes, esectively.

6 14-IACC-19.R1 5 Fig. 6. Oveall block diagam of the system with contolles. (c) Fig. 5. Thee woking scenaios of the convete (The aows show the diections of enegy flow). Scenaio 1 ( 1 > out ): PV woks in MPPT mode and the battey woks in chage mode to absob the sulus sola enegy; Scenaio ( 1 < out ): PV woks in MPPT mode and battey woks in dischage mode to ovide the deficient enegy; (c) Scenaio 3 ( 1 = ): thee is no sola enegy available and battey is dischaged to suly load. It can be seen that fo the same owe vaiation value, c is ootional to 1/P 1 (k 1). In this ae, the switching eiod (T) will not be changed if the RPC is lowe than a edefined value (e.g., 1-4 ). As shown in Fig. 7, the P&O MPPT algoithm is ealized by the fequency modulation method [], whee the conduction time of S 1, t on, is fixed so that S 1 can achieve soft switching. C. Chage and Dischage Contolles fo Battey Figs. 8 and show the equivalent cicuit of the battey and the convete when the battey woks in the chage and dischage mode, esectively. To simlify the analysis, the battey is modeled as a caacito C b connected in seies with its intenal esistance b. Since C b is sufficiently lage, the teminal voltage of the battey, v bat, can be calculated as V boc i bat b, whee V boc is the oen-cicuit voltage of the battey. Then the tansfe function between the battey cuent i bat and the duty cycle d of the switch S in the chage mode can be deived: G ( s) = c ( V V ) s + 1 bat i ( ) C bat s b = d ( s) 1 / b 1 + D L C () s + + s + L b C whee V 1 and V bat eesent the aveage voltages of the PV anel and the battey, esectively; and b eesent the aasitic esistance of the inducto L and the intenal esistance Fig. 7. The flowchat of the MPPT algoithm. Fig. 8. Equivalent cicuit of the battey with the buck convete in chage mode and the boost convete in dischage mode. of the battey, esectively; D is the steady-state value of the duty cycle of the switch S.

7 14-IACC-19.R1 6 Similaly, the tansfe function between i bat and the duty cycle d 3 of the switch S 3 in the dischage mode is: 1 V1 s i ( s) + C bat G ( s) b (1) d = = d 3( s) 1 / b + 1 s + + s + L b C L C To contol the cuent of the battey, a ootional-integal (PI) contolle, K + K i /s, is used in the chage/dischage mode seaately, as shown in Fig. 6. Each battey cuent PI contolle takes the cuent eo as the inut to geneate the duty cycle fo S o S 3 in the chage o dischage mode, esectively. When the efeence cuent I * bat is zeo o negative, the chage PI contolle is selected such that d and d 3 =. Othewise, when the efeence cuent I * bat is ositive, the dischage PI contolle is selected such that d 3 > Magnitude (db) Befoe comensation Afte comensation and d =. The bode lots of i bat (s)/d (s) and i bat (s)/d 3 (s) without the PI comensations (i.e., the oen-loo tansfe functions) ae shown in Fig. 9. The lots imly that the two oen-loo systems have low gains and db/decade sloes in the low-fequency egion. Theefoe, the design objective the PI comensation is to incease the low-fequency gains and make them coss the db line with a db/decade sloe, while maintaining a sufficiently lage hase magin (> 45º) and a high cossove fequency. By setting the cossove fequency in the ange of one to seveal hunded Hz with a hase magin of 7º, the chage and dischage PI contolles can be deived. The bode lots of i bat (s)/d (s) and i bat (s)/d 3 (s) with the PI comensations (i.e., the closed-loo tansfe functions) ae shown in Fig. 9 as well. Afte the comensations, the low-fequency gains have been inceased and the low-fequency sloes ae changed to be db/decade. The cossove fequencies coesonding to the chage and the dischage contolles ae 3 Hz and 4 Hz, esectively. Once the cuent contolles ae designed, the oute-loo DC-link voltage contolle which has a lowe cutoff fequency than the cuent loo is then designed. The PI aametes of the cuent and voltage contolles used in this ae ae listed in Table I. Table I: Contolle Paametes Phase (deg) Magnitude (db) Phase (deg) Fequency (Hz) Befoe comensation Afte comensation Fequency (Hz) Fig. 9. Bode lots. i bat (s)/d (s), and i bat (s)/d 3 (s). Cuent contolle (chage).5 5 Cuent contolle (dischage) Voltage contolle 1 K IV. SIMULATION RESULTS Simulations ae caied in MATLAB/Simulink to validate the oosed convete and the contolles. The aametes of the convete ae as follows: tansfome tun atio n = 5:14, L = 3.3μH, L = 3.5 μh, and C =. μf. A SunWize SW-S55P PV anel is used, whose oen-cicuit voltage V oc and the shot-cicuit cuent I sc ae V and 3.15 A, esectively. The nominal voltage and intenal esistance, b, of the battey ae 7.5 V and.16 Ω, esectively. The on-time of the switch S 1, t on, is 3 µs and the switching fequency vaies in a ange of 1 khz to 17 khz. The esistive load R L = 1 Ω. The desied DC-link voltage, V * dc, and nominal owe of the load ae 5 V and 5 W, esectively. To test the dynamic chaacteistic of the contolles, the sola adiation is ste changed to examine the esonses of the DC-link voltage and outut owe of the PV anel, as shown in Fig. 1. Fig. 1 shows that the initial sola adiation is zeo and thee is no owe geneated by the PV anel, as shown in Fig. 1(c). This indicates that the convete woks in Scenaio 1 and all of the owe is sulied by dischaging the battey. Fig. 1 shows that the DC-link voltage quickly eaches its efeence value of 5 V. Scenaio 1 does not teminate until the sola adiation changes fom zeo to 4 W/m at the 1 st second. Afte that, the maximum owe geneated by the PV anel is W, which is K i

8 14-IACC-19.R1 7 Sola adiation (W/m ) DC Link voltage (V) Time (sec) Time (sec) PV Powe (W) DC Link Voltage (V) Ideal MPPs PV Powe 8: 1: 1: : 4: Time of the day : 1: 1: : 4: Time of the day Fig. 11. Simulation esults using the NREL data. The geneated PV owe and the DC-link voltage PV Powe (W) 1 Ideal MPPs PV Powe Time (sec) (c) Fig. 1. Ste esonses. The ofile of sola adiation, DC-link voltage esonse, and (c) PV owe esonse. less than the load demand of 5 W. Thus, the battey still woks in the dischage mode to ovide the deficient owe equied by the load and the vaiation of the DC-link voltage is negligible duing the tansition. Fom 1 second to 1.5 second, the convete woks in Scenaio. The PV anel geneates the maximum owe as indicated in Fig. 1(c). At 1.5 second, the sola adiation is changed fom 4 W/m to 6 W/m, which coesonds to 3-W maximum owe. Then the battey stos dischaging and stats to absob the sulus owe geneated by the PV anel. It takes some time to change the diection of the battey cuent, which not only esults in an aoximately -V oveshoot in the DC-link voltage, but also leads to the PV owe geneated less than the ideal maximum owe duing the tansient eiod, as shown in Fig. 1(c). Afte.3 seconds, both the DC-link voltage and PV owe each the desied value and the ideal maximum owe oint (MPP), esectively. To testify the effectiveness of the convete fo MPPT contol of the PV anel, eal-wold sola adiation data ovided by the National Renewable Enegy Laboatoy (NREL) [3] was used. The data was collected fom the South Table Mountain site in Golden, Coloado on Feb. 7, 13. The duation of the dataset is 6 minutes with a esolution of one data oint e minute. Fig. 11 shows the simulation esults of the PV owe and the DC-link voltage. As shown in Fig. 11, the enegy extacted fom the PV anel closely follows the ideal MPP by using the oosed convete and MPPT contol algoithm. The DC-link voltage is well contolled at its desied value of 5 V. The maximum voltage eo is aoximately 1.%, as shown in Fig. 1, which occus aound 1:1 am when the MPP inceases to 5 W. At that time, the battey switches fom the dischage mode to the chage mode. The steady-state voltage eo is always less than.6%. V. EXPERIMENTAL RESULTS The system simulated is constucted in hadwae to futhe validate the oosed convete and contol algoithm via exeimental studies. Fig. 1 shows the ototye of the whole system. It consists of the oosed DC-DC convete, which is connected to a SunWize SW-S55P PV anel, a battey, and an ezds F81 contol boad. As shown in Fig. 1, the battey consists of fou Samsung ICR1865-8A echageable lithium-ion cells, whee two cells ae connected in seies to

9 14-IACC-19.R1 8 PWM 1 v v ds1 i 35.8 V Fig. 1. The exeimental system setu. Time: ( us/div) v: ( V/div) v ds1 : ( V/div) i: (5 A/div) fom a ack and two acks ae connected in aallel. The nominal voltage, standad chaging cuent, and caacity of the battey ae 7.5 V,.8A, and 56 mah, esectively. The contol algoithm is imlemented in a TMS3F81 DSP located on the contol boad. The aametes of the system ae the same as those used in the simulation. Othe aametes of the ototye convete ae listed in Table II. Table II: Comonent Secifications of the Pototye Convete. L 1, L 3 μh C 1, C, C, C s 1 μf L m 75 μh C. μf (1VDC) L 3.3 μh Ds 1 ~Ds 4 RHRP154 L μh S 1 IPB17N3 L'. μh S, S 3 FDP363 A. Steady-State Wavefoms Fig. 13 shows the steady-state wavefoms of the esonant comonents, the switch S 1, and the tansfome when Pot 1 is connected to a 16 V voltage souce. As shown in Fig. 13, when the switch S 1 is tuned on, the voltage acoss C, v, deceases and the cuent though L and S 1, i, inceases. The sinusoidal wavefoms of v and i indicate the esonance between L and C. The cuent though the switch S 1 inceases slowly such that it is tuned on unde a low di/dt (1 A/μs) condition. When the cuent dos to zeo, the switch S 1 is fully tuned off unde the ZCS condition. Then the voltage acoss the switch S 1, v ds1, is the same as that acoss the esonant caacito C. The eak value of v ds1 is 35.8 V, which is aoximately twice the inut voltage. Fig. 13 shows the voltage (v ) and cuent (i and i T ) wavefoms of the tansfome. The diection of v is the same as i T egadless the state of the switch. This indicates that the owe is deliveed to the seconday side of the tansfome in the entie switching eiod. B. Voltage Regulation Ratio Owing to the use of the LCL-esonant cicuit, the fequency modulation method is used such that S 1 can achieve soft switching. The voltage egulation atio, which is defined as V T /V 1, is a function of the switching fequency when the fequency modulation method is used. Fig. 14 shows that the Fig. 13. The steady-state wavefoms of the convete. The voltage and cuent wavefoms of the esonant comonents and the voltage and cuent wavefoms of the tansfome. voltage egulation atio inceases almost linealy with the nomalized switching fequency f s /f, whee f s and f ae the switching fequency and the esonant fequency, esectively. C. Thee Scenaios Fig. 15 shows the wavefoms when the convete woks in Scenaio 1. As shown in Fig. 15, the owe geneated by the PV anel, 1, in the steady state is W, which is highe than the load owe of 5 W. The negative battey cuent (i bat =.87 A) indicates that the battey woks in the chage mode. The DC-link voltage (v dc = 5.18 V) is close to the efeence value of 5 V, which demonstates that the chage contolle is effective to maintains a constant DC-link voltage. Fig. 15 shows the oen-cicuit tansient esonses of the voltage v 1, cuent i 1, and owe 1 of the PV anel when it is connected to Pot 1. In the exeiment, C 1 is initially fully dischaged such that the initial value of v 1 is zeo; the thee switches S 1, S, and S 3 ae off; and the contolles ae deactivated. As shown in Fig. 15, v 1 inceases fom zeo to its maximum value and i 1 deceases fom a ositive value to zeo. As a esult, 1 fist

10 14-IACC-19.R1 9 Voltage egulation atio (V T /V 1 ) Nomalized fequency (f S /f ) Fig. 14. Voltage egulation atio as a function of the nomalized fequency. Fig. 16. The wavefoms when the convete woks in Scenaio. The steady-state wavefoms; the P-V chaacteistic cuve of the PV anel. Fig. 15. The wavefoms when the convete woks in Scenaio 1. The steady-state wavefoms and the P-V chaacteistic cuve of the PV anel. inceases fom zeo to its MPP (37.96 W) and then deceases to zeo. Based on the esults, the owe-voltage (P-V) chaacteistic cuve of the PV anel can be geneated, as shown on the ight hand side of Fig. 15. It is assumed that the oeating oint of the PV anel does not change within thee minutes since it was a clea day (Dec., 13 in Lincoln, NE). As shown in Fig. 15, the maximum owe of the PV anel is Fig. 17. The wavefoms when the convete woks in Scenaio W, which is slightly highe than the measued mean value of W in Fig. 15. Such a small deviation is caused by the P&O MPPT algoithm in which the duty cycle of the main switch S 1 vaies slightly aound the otimal duty cycle fom time to time. Fig. 16 shows the wavefoms when the oosed convete

11 14-IACC-19.R1 1 woks in Scenaio. As shown in Fig. 16, the owe 1 geneated by the PV anel is W, which is less than that equied by the load. The ositive battey cuent (i bat = 1.4 A) indicates that the battey woks in the dischage mode to suly the deficient owe to the load. The DC-link voltage (v dc = 5.4 V) is close to the efeence value of 5 V, which demonstates the DC-link voltage is well contolled by the dischage contolle. Similaly, the measued PV owe is also close to its maximum owe 19.5 W shown in Fig. 16. Theefoe, the oosed convete is caable of MPPT contol fo the PV anel as long as the sola enegy is available. Fig. 17 shows the steady-state wavefoms of the convete when it woks in Scenaio 3. In this scenaio, no sola enegy is available. In the exeiment, the PV anel was disconnected fom Pot 1. As shown in Fig. 17, the duty cycle of the switch S 1 is fixed aound.4 and S 3 is active to fom the boost convete to dischage the battey. The battey dischages with a cuent of 3.83 A, inceasing fom 1.4 A in Scenaio. Theefoe, the battey ovides moe enegy in Scenaio 3 than it does in Scenaio. Meanwhile, the DC-link voltage is also well contolled at the desied value. D. Comaison with the Had-Switched Convete A novelty o advantage of the oosed convete is that the main switch S 1 is tuned off unde the ZCS condition by using the LCL-esonant cicuit imlemented in Pot 1. To testify the benefit of using the LCL-esonant cicuit, the voltage stess of S 1 of the oosed convete is comaed to that of the coesonding had-switched convete, which is obtained by emoving the LCL-esonant cicuit L, C, and L 1 in the dash-line block of the oosed convete in Fig. 1. Fig. 18 shows the voltage and cuent wavefoms of the switch S 1 of the had-switched convete when V 1 = 16 V. As shown in Fig. 18, the gate signal is emoved when the cuent of the main switch S 1 is 4 A; theefoe, the main switch is not tuned off unde the ZCS condition and the eak voltage acoss the main switch is 65.9 V. When tuning on the main switch, the value of di/dt in the had-switched convete is 118 A/μs. In comaison, in the oosed convete, the main switch is tuned off unde the ZCS condition and the voltage stess and di/dt ae educed to 35.8 V and 1 A/ μs, esectively, as shown in Fig. 13. The efficiencies of the two convetes ae measued when the load is sulied by a voltage souce connected to Pot 1. Fig. 19 comaes the measued efficiencies of the oosed soft-switched convete and the had-switched convete. As shown in Fig. 19, the efficiencies of both convetes fist incease with the load owe, each the eak values when the load owe is aound 6 W, and then decease with the load owe when it is highe than 7 W. The oosed soft-switched convete always has a highe efficiency than the had-switched convete and has achieved the eak efficiency of 94.5%. VI. CONCLUSIONS This ae has oosed a new isolated, thee-ot, bidiectional, DC-DC convete which uses the minimum numbe of switches. The oosed convete has been used fo Fig. 18. The voltage and cuent wavefoms of the switch S 1 in had-switched convete. Efficiency (%) Had-switched Soft-switched Load owe (W) Fig. 19. Comaison of efficiency vs. load owe of the oosed soft-switched convete and the had-switched convete. simultaneous owe management of multile enegy souces, i.e., a PV anel and a battey, in this ae. Simulation esults have shown that the convete is not only caable of MPPT fo the PV anel when thee is sola adiation, but also can contol the chage/dischage of the battey to maintain the DC-link voltage at a constant value. Moeove, the voltage stess and the value of di/dt of the main switch have been educed comaed with the coesonding had-switched convete. The oosed convete is alicable to othe tyes of enewable enegy souces, such as wind tubine geneatos. REFERENCES [1] C. Onwuchekwa and A. Kwasinski, A modified-time-shaing switching technique fo multile-inut DC-DC convetes, IEEE Tans. Powe Electon., vol. 7, no. 11, , Nov. 1. [] A. Khaligh, J. Cao, and Y. Lee, A multile-inut DC-DC convete toology, IEEE Tans. Powe Electon., vol. 4, no. 4, , Ma. 9. [3] J. Lee, B. Min, D. Yoo, R. Kim, and J. Yoo, A new toology fo PV DC/DC convete with high efficiency unde wide load ange, in Poc. Euoean Conf. Powe Electon. Al., Set. 7, [4] C. Lohmeie, J. Zeng, W. Qiao, L. Qu, and J. Hudgins, A cuent-sensoless MPPT quasi-double-boost convete fo PV systems, in Poc. IEEE Enegy Conves. Cong. Exosit., Set. 11,

12 14-IACC-19.R1 11 [5] K. Sayed, M. Abdel-Salam, A. Ahmed, and M. Ahmed, New high voltage gain dual-boost DC-DC convete fo hotovoltaic owe system, Elect. Powe Comon. Syst., vol. 4, no. 7, , A. 1. [6] Y. Chen, Y. Liu, and F. Wu, Multi-inut DC/DC convete based on the multi winding tansfome fo enewable enegy alications, IEEE Tans. Ind. Al., vol. 38, no. 4, , Aug.. [7] Y. Jang and M. Jovanovic, Isolated boost convete, IEEE Tans. Powe Electon., vol., no. 4, , Jul. 7. [8] E. Yang, Y. Jiang, G. Hua, and F. Lee, Isolated boost cicuit fo owe facto coection, in Poc. IEEE Al. Powe Electon. Conf. Exosit., Ma. 1993, [9] Y. Lembeye, V. Bang, G. Lefeve, and J. Feieux, Novel half-bidge inductive DC-DC isolated convetes fo fuel cell alications, IEEE Tans. Enegy Conves., vol. 4, no. 1,. 3-1, Ma. 9. [1] J. Zeng, W. Qiao, L. Qu, and Y. Jiao, An isolated multiot dc-dc convete fo simultaneous owe management of multile diffeent enewable enegy souces, IEEE J. Emeging Sel. Toics Powe Electon., vol., no. 1,. 7-78, Ma. 14. [11] H. Tao, A. Kotsooulos, J. Duate, and M. Hendix, Family of multiot bidiectional DC-DC convetes, IEE Poc. Elect. Powe Al., vol. 153, no. 3, , May 6. [1] C. Zhao, S. Round, and J. Kola, An isolated thee-ot bidiectional DC-DC convete with decouled owe flow management, IEEE Tans. Powe Electon., vol. 3, no 5, , Set. 8. [13] J. Duate, M. Hendix, and M. Simoes, Thee-ot bidiectional convete fo hybid fuel cell systems, IEEE Tans. Powe Electon., vol., no., , Ma. 7. [14] G. Su and F. Peng, A low cost, tile-voltage bus DC-DC convete fo automotive alications, in Poc. IEEE Al. Powe Electon. Conf. Exosit., Ma. 5, [15] D. Liu and H. Li, A ZVS bi-diectional DC-DC convete fo multile enegy stoage elements, IEEE Tans. Powe Electon., vol. 1, no. 5, , Set. 6. [16] H. Al-Atash, F. Tian, and I. Bataseh, Ti-modal half-bidge convete toology fo thee-ot inteface, IEEE Tans. Powe Electon., vol., no. 1, , Jan. 7. [17] Z. Qian, O. Abdel-Rahman, and I. Bataseh, An Integated fou-ot DC/DC convete fo enewable enegy alication, IEEE Tans. Powe Electon., vol. 5, no. 7, , Jul. 1. [18] B. Lu, W. Liu, Y. Liang, F.C. Lee, and J. Van Wyk, Otimal design methodology fo LLC esonant convete, in Poc. IEEE Al. Powe Electon. Conf. Exo., Ma. 6, [19] J. Zeng, W. Qiao, and L. Qu, A single-switch LCL-esonant isolated DC-DC convete, in Poc. IEEE Enegy Conves. Cong. Exosit., Set. 13, [] J. Zeng, W. Qiao, and L. Qu, LCL-esonant single-switch isolated DC-DC convete, IET Powe Electon., to be ublished. [1] C. Hua, J. Lin, and C. Shen, Imlementation of a DSP-contolled hotovoltaic system with eak owe tacking, IEEE Tans. Ind. Electon., vol. 45, no. 1, , Feb [] B. Yok, W. Yu, and J. Lai, Hybid-fequency modulation fo PWM-integated esonant convetes, IEEE Tans. Powe Electon., vol. 8, no., , Feb. 13. [3] htt:// [4] J. Zeng, W. Qiao, and L. Qu, A single-switch isolated dc-dc convete fo hotovoltaic systems, in Poc. IEEE Enegy Conves. Cong. Exosit., Raleigh, NC, Set. 1, uninteutible owe sulies. His eseach inteests include owe electonics, enewable enegy, micogids, and comutational intelligence fo electic owe and enegy systems. Wei Qiao (S 5 M 8 SM 1) eceived the B.Eng. and M.Eng. degees in electical engineeing fom Zhejiang Univesity, Hangzhou, China, in 1997 and, esectively, the M.S. degee in high efomance comutation fo engineeed systems fom Singaoe-MIT Alliance (SMA), Singaoe, in 3, and the Ph.D. degee in electical engineeing fom Geogia Institute of Technology, Atlanta, GA, USA, in 8. Since August 8, he has been with the Univesity of Nebaska Lincoln (UNL), USA, whee he is cuently an Associate Pofesso in the Deatment of Electical and Comute Engineeing. His eseach inteests include enewable enegy systems, smat gids, micogids, condition monitoing and fault diagnosis, enegy stoage systems, owe electonics, electic machines and dives, and comutational intelligence. He is the autho o coautho of 3 book chates and moe than 14 aes in efeeed jounals and intenational confeence oceedings and has 5 intenational/u.s. atents ending. D. Qiao is an Associated Edito of the IEEE Tansactions on Enegy Convesion, the IET Powe Electonics, and the IEEE Jounal of Emeging and Selected Toics in Powe Electonics, and the Coesonding Guest Edito of a secial section on Condition Monitoing, Diagnosis, Pognosis, and Health Monitoing fo Wind Enegy Convesion Systems of the IEEE Tansactions on Industial Electonics. He was an Associate Edito of the IEEE Tansactions on Industy Alications in He was the eciient of a 1 U.S. National Science Foundation CAREER Awad, the 1 IEEE Industy Alications Society (IAS) Andew W. Smith Outstanding Young Membe Awad, the 1 UNL College of Engineeing Faculty Reseach & Ceative Activity Awad, the 11 UNL Haold and Esthe Edgeton Junio Faculty Awad, and the 11 UNL College of Engineeing Edgeton Innovation Awad. He has eceived fou best ae awads fom IEEE IAS, PES, and PELS. Liyan Qu (S 5 M 8) eceived the B.Eng. (with the highest distinction) and M.Eng. degees in electical engineeing fom Zhejiang Univesity, Hangzhou, China, in 1999 and, esectively, and the Ph.D. degee in electical engineeing fom the Univesity of Illinois at Ubana Chamaign, USA in 7. Fom 7 to 9, she was an Alication Enginee with Ansoft Cooation. Since Januay 1, she has been with the Univesity of Nebaska Lincoln (UNL), whee she is cuently an Assistant Pofesso in the Deatment of Electical and Comute Engineeing. He eseach inteests include enegy efficiency, enewable enegy, numeical analysis and comute aided design of electic machiney and owe electonic devices, dynamics and contol of electic machiney, emanent-magnet machines, and magnetic mateials. Jianwu Zeng (S 1) eceived the B.Eng. degee in electical engineeing fom Xi an Univesity of Technology, Xi an, China, in 4, and the M.S. degee in contol science and engineeing fom Zhejiang Univesity, Hangzhou, China, in 6. Cuently, he is woking towad the Ph.D. degee in electical engineeing with the Univesity of Nebaska-Lincoln, USA. In 6, he joined Eaton Electical (Shenzhen) Comany, Ltd., Shenzhen, China, whee he was an Electonic Enginee involved in eseach and develoment on soft-switching and bidiectional DC-DC convetes fo