DESIGN AND IMPLEMENTATION OF FULL BRIDGE MODULAR DC-DC CONVERTER FOR SOLAR CONVERSION SYSTEMS

Transcription

1 VOL. 13, NO. 4, FEBRUARY 18 ISSN ARPN Journal o Engineering an Applie Sciences 6-18 Asian Research Publishing Network (ARPN). All rights reserve. DESIGN AND IMPLEMENTATION OF FULL BRIDGE MODULAR DC-DC CONVERTER FOR SOLAR CONVERSION SYSTEMS Sharon P, K. Parkavi Kathirvelu an R. Balasubramanian Department o Electrical Engineering, Shanmugha Arts, Science, Technology an Research Acaemy University, Inia sharon.paul17@gmail.com ABSTRACT Solar energy is anticipate to become the worl s largest source o electricity an hence an eective esign o Solar Energy Conversion System (SECS) that converts solar energy to electricity is manatory. In huge rating power plants, converter with high voltage hanling capability is require. In this work three level Input Series Output Series (ISOS) moular ull brige DC-DC converter has been propose or Solar Energy Conversion System to meet high input voltage an high output voltage requirements. The Input Series Output Series (ISOS) connecte moular DC-DC converter system enables the use o switches with lower voltage ratings or applications that eman high input an output voltage. As the input o the moular converter is varying with respect to irraiation variation, three loop control (TLC) is propose in this work. The main objective o this work is obtain equal sharing o input an output voltages among the converters an to control input current an the output voltage to a esire constant value or a wie range o irraiance an loa variations. Inex terms: moular DC-DC converters, input series output series, photovoltaic array, zero voltage switching, phase shite ull brige, voltage sharing. 1. INTRODUCTION Photovoltaic (PV) systems are rapily gaining worlwie popularity ue to its easy availability, nonpolluting nature an renewable characteristics. It is also a ast growing inustry with an estimate worlwie annual prouction o about 79 GW in 16 [1] Solar energy conversion system is classiie into a) Gri connecte operating moe b) Islane operating moe. In gri connecte solar power systems, the solar panels are connecte to the utility gri, such that when the surplus power prouce by the panels is e back to the gri. In Islane moe or Stan alone moe, the solar panels are not connecte to the gri, instea they are use to charge a bank o batteries which are then use by the electrical loas. In PV systems, DC-DC converters play a major role to boost the DC voltage an to extract the maximum energy harvesting. A gri connecte PV system usually has the ollowing components, PV arrays, DC-DC converters, Inverters an Isolation transormers, as shown in Figure-1. Because o the growing prominence o solar power, power gris must hanle more photovoltaic (PV) input than beore. The challenge occurs in integrating the PV systems with the utility networks, as it requires DC-DC converters with high voltage conversion ratios to hanle the high PV input voltage an also to provie isolation between the PV panels an the inverter stage [] Figure-1. Typical PV power plant layout. In general Buck converter, Boost converter, Buck-boost converter, CUK an SEPIC converters are use to boost the voltage [3]. Though, their voltage conversion ratio is less, Full Brige DC-DC converters have been use. Conventional converters are not suitable because o the high voltage prouce by the string o PV panels an it becomes necessary that the switching losses across the DC-DC converter moules shoul be reuce. This can be one by using a moular architecture or the converters. In a moular converter system as shown in Figure- the voltage rom the array o solar panels is equally ivie among the DC-DC Boost converter moules by using an input iviing capacitor (C in, n ) which equally ivies the high input voltage across the moules. C in, n supply the corresponing converter moule an the voltage stresses across the switches in each converter moule is reuce consierably. Thereore, Low voltage an low power converter blocks calle sub moules are connecte in series or parallel in the input or in the output 143

2 VOL. 13, NO. 4, FEBRUARY 18 ISSN ARPN Journal o Engineering an Applie Sciences 6-18 Asian Research Publishing Network (ARPN). All rights reserve. sie to achieve esire voltage an current speciication epening upon the application requirement. The avantages o moular converters inclue: a) Reunancy gives reliability- in which the moule at ault is by-passe an the other converter moules carry out the operation with little to no change in eiciency. b) Stanarization leas to shorter prouction cycles- as only one converter nees to be esigne which is cost eective an less time consuming c) Higher switching requencies o the converters lea to reuce ilter size. Even though the input series output series (ISOS) moular DC-DC converter has many avantages, obtaining equal sharing o input an output voltage is iicult. In orer to obtain equal sharing o input an output voltage many control schemes are use. Although it is mentione in [4] that common uty ratio scheme use in ISOS system will result in a runaway conition in which the entire input voltage appears across only a particular converter even when small mismatches in turns ratio occurs, [5] proposes that or some circuit topologies like ISOS systems with two ull brige converters, common uty ratio scheme can be implemente. Though Duty cycle exchange control scheme, as in [6] provie the require results, the control strategy is eective with only ISOS systems with two converter moules an cannot be extene to any more converters. Control proceures in reerences [4]-[6] use a centralize controller with a master moule which compromises the reliability an the reunancy property inherent to the moular converter shoul the master moule ail to operate. been propose in this work to obtain an equilibrium conition the moules. Because o the high input voltage an to reuce the size o the passive components, the power evices are operate at high requencies. This results in increase switching losses which lea to eterioration o converter eiciency. Thereore in the propose work, to reuce switching losses, each moule which consists o a phase shite ull brige DC-DC converter is operate with Zero Voltage Switching (ZVS). The transormer leakage inuctance (L lk ) an parasitic capacitances o the MOSFETs can be use avantageously here to achieve ZVS; hence aitional snubber circuits are not require. This urther reuces the size o the overall system leaing to a converter structure that is compact an stanarize In this paper, Section I an Section II gives an overview an avantages o Input Series Output Series moular coniguration. Section III bries the nee or an input voltage controller an in section IV three loop control scheme or ynamic changes in input voltages is eine. Small signal analysis is represente in Section V. Section VI an Section VII epicts simulation an experimental results respectively.. INPUT SERIES OUTPUT SERIES CONFIGURATION Input series connection is preerre in applications where the input voltage is very high. Apart rom moularity input series connection has several avantages such as: a) As the input voltage is equally ivie among the moules the voltage stresses on the power evices is less, thereore low voltage MOSFETs can be use. b) Use o ilters with reuce ratings ue to interleaving o switches. Figure- shows moular converters implemente with three ISOS connecte converters an Table-1 shows the converter parameters o Solar panel interace ISOS connecte moular DC-DC converter Table-1. Converter parameters. Figure-. Solar panel interace ISOS connecte moular DC-DC converter. Thereore, a three loop control proceure or active sharing o Input Voltage (IVS) an Output Voltage (OVS) which improves the ynamic perormance by iniviual input voltage an output current sensing or ISOS connecte moular converters is implemente in [7].The propose work nees a controller that has to accommoate wie variations on both the input sie an on the output sie; hence a three loop control scheme has Terms V in, i I in, i V o, i I o, i I c, i I c, i I cin, i I co, i Parameters Input voltage o i number o converter Input current o i number o converter Output voltage o i number o converter Output current o i number o converter Current through i number o Input capacitive ilter Current through i number o output capacitive ilter Input current o i number o converter system Output current o i number o the converter system 144

3 VOL. 13, NO. 4, FEBRUARY 18 ISSN ARPN Journal o Engineering an Applie Sciences 6-18 Asian Research Publishing Network (ARPN). All rights reserve. The suix i represents the converter number. Input an output ilter capacitor currents are not inclue. Then, by power conservation, we have V in1 I in1 η 1 = V o1 I o1 V in I in η = V o I o V in n I in n η n = V o n I o n (1) Where η 1, η... η n are the eiciencies o the iniviual converter moules [8]. For ISOS ull brige DC-DC, the input an output currents to each converter moule is equal. Thus rom (1) we get, V in1 η 1 :V in η : : V inn η n = V o1 : V o : : V o n () or ISOS systems, rom (), i the input voltages o the moule are controlle to be share equally among the converters, the moule output voltages will be nearly equal. 3. NEED FOR AN INPUT VOLTAGE CONTROLLER For converters with ientical parameters, the controller applies the same uty ratio to all the converters. Even when converters o the same parameters are taken, some mismatches can occur in terms large transormer leakage inuctance (L lk ) an turns ratio (n) which are iicult to maintain to a constant value. Hence, without an input voltage controller the uty ratio to each converter will be ierent. The entire input voltage appears across the converter with the higher uty ratio resulting in a run way conition [4], an there is no input voltage sharing. The propose work has a solar panel array as the source whose voltage varies wiely accoring to the irraiance conition an it becomes necessary to have a proper orm o control on the input sie such that the same uty ratio is applie to all the converter moules. 4. THREE-LOOP CONTROL (TLC) SCHEME FOR ISOS CONFIGURATION Figure-3 shows the overall control proceure or three converters. The scheme consists o current moe input voltage sharing (IVS) control with three control loops to ensure input an output voltage sharing. Current reerence to the inner current loop is provie by the output voltage loop that is common to all the three converter moules. G Vo is the compensator or the outer voltage loop an G Io is the compensator or iniviual inner current loops. In this control scheme two voltage control loops an one current control loop is use to control ISOS DC-DC converter. The outer voltage loop changes the reerence to the inner current (I re ). I re to the input voltage loop is taken as the average o the input voltage o each converter moule. Figure-3. TLC with input voltage reerence or three moular converters. G Vo an G Io are the output voltage loop gain an inner current loop gain respectively. A proportional controller K is use to regulate the input voltage control loops in accorance with conitions such as input voltage change ue to changes in irraiation conition an is chosen accoring to the mismatches between input an output voltage regulation loops. The ierences in the moule eiciencies epen on the mismatches in the switches an parasitic resistances in iniviual moules [9]. Practically, as the topologies o the moules are ientical, mismatches ue to the above reasons are not so large; the ierences among the moules are negligible. Thereore a common uty ratio is given to all the converter moules. Figure-4 shows the control block iagram o moule 1, moule an moule 3. G vo is the common output voltage compensator, while G Io1, G Io, G Io3 are iniviual current compensators or inner current loops. G 1i, G i, G 3i are the transer unctions o the three moules respectively. 145

4 VOL. 13, NO. 4, FEBRUARY 18 ISSN ARPN Journal o Engineering an Applie Sciences 6-18 Asian Research Publishing Network (ARPN). All rights reserve. incorporate into the average small signal analysis o the conventional boost PWM converter by replacing the uty cycle, by the total change o e which is given by e (5) i v The contribution o i an v is given as two separate controlle sources, i an v occurs ue to perturbation in i L an v in an is not controllable by the control circuit. The transer unction o the output ilter is obtaine as H 1 ( s 1 LC sl/ R 1) (6) Input impeance o the output ilter is Figure-4. Control scheme block or moule 1, moule, an moule SMALL SIGNAL ANALYSIS When Small signal analysis is perorme by state space averaging as in [1], it woul be teious as it requires solving thir-orer system consisting o six systems o equations corresponing to six moes o operation o a phase shite PWM converter. In this work, small signal transer unctions has been erive with the eects introuce ue to phase shiting operation an the uty cycle moulation introuce ue to change in ilter inuctor current an input voltage. The small-signal transer unctions o this converter, thereore, will epen on the perturbations o the ilter inuctor current L lk, s, I L, V in an uty cycle o the primary voltage [11]. Smallsignal circuit moel o the propose PS-PWM converter is given in Figure-5 uty cycle moulation o the ilter inuctor current is given as R I L i (3) nv in Where R 4n L lk N n N s is the turns ratio p s Duty cycle moulation o the input voltage is given as Z R (7) 1 src Output impeance o the output ilter is Z n Where sl (8) s LC sl/ R 1 The control-to-output transer unction is G Or G s nv in v (9) s s( 1/ RC R / L) nv in v (1) Where, Ɛ is the amping o the secon orer enominator given by 1 R L R R C L / R I V L in v (4) nvin This eect can be taken as an aitional ee orwar o input voltage. The above results can be is Control-to-ilter inuctor current transer unction 146

5 VOL. 13, NO. 4, FEBRUARY 18 ISSN ARPN Journal o Engineering an Applie Sciences 6-18 Asian Research Publishing Network (ARPN). All rights reserve. G i in (11) Z nv R The speciication o the system is given in the Table-. Table-. System speciication o ISOS connecte moular DC-DC converters. Components Rate Input voltage, V in Output Voltage, V out Rate Loa, R Switching requency, F s Values 6 V 1 V 4 Ω 5KHz Transormer turns ratio, n Filter inuctance, L 1µH Filter capacitance, C 1µF Transormer leakage inuctance, L lk 6µH Input iviing capacitor, C 1 = C = C 3 4nF For output voltage regulation loop (OVR), compensator G vo is implemente by proportional integral (PI) type controller, an is expresse as, G ( s) k k s (1) vo p i / The compensator or the inner current loop is also a PI type which is expresse as, G Io (s) = 9.6 x /s (13) Thus we can conclue that stability can be achieve or ISOS connecte DC-DC moular converters with three loop control scheme i PI parameters or the voltage compensators an current compensators are selecte properly. Figure-5. Small-signal circuit moel o the propose PS-PWM converter. 4. SIMULATION RESULTS The propose scheme is simulate or three moular converter systems with ientical parameters. Figure-7 an Figure-8 gives iniviual input an output voltages o each moule an also the total input an output voltage or an irraiance o 1W/m uner STC. The input an the output voltages are equally share. While Figure-9 an Figure-1 gives iniviual input an output voltages o each moule an also the total input an output voltage or a lower irraiance o 7 W/m. Figure-11 gives Total input current rom the panel an the total output current or an irraiance o 7 W/ m. It can be seen that in spite o the wie variation in irraiance, the input an the output voltages are maintaine constant resulting in power balance. Eiciency o the converter is as high as 91%. Figure-6. Primary transormer voltage (1V/iv), Inuctor current (5A/iv), Drain source voltage (5V/iv), Gate pulses (5v/iv). 147

6 VOL. 13, NO. 4, FEBRUARY 18 ISSN ARPN Journal o Engineering an Applie Sciences 6-18 Asian Research Publishing Network (ARPN). All rights reserve. Figure-7. Total output voltage (V/iv), iniviual output voltages o each moule or an irraiance o 1 W/ m. Figure-1. Total input voltage (V/iv), iniviual input voltages o each moule or an irraiance o 7 W/ m.. Figure-8. Total input voltage (V/iv), iniviual input voltages o each moule or an irraiance o 1 W/ m. Figure-11. Total input current rom the panel (V/iv), total output current or an irraiance o 7 W/ m. Table-3. Eiciency table o converters uner varying irraiance conition. Loa Ω PV cell voltage V in Output voltage V Converter eiciency η (%) Figure-9. Total output voltage (V/iv), iniviual output voltages o each moule or an irraiance o 7 W/ m. Table-4. Eiciency table o converters uner varying irraiance conition. Irraiance (W/m ) PV cell voltage V in Output voltage V Converter eiciency η (%)

7 VOL. 13, NO. 4, FEBRUARY 18 ISSN ARPN Journal o Engineering an Applie Sciences 6-18 Asian Research Publishing Network (ARPN). All rights reserve. Eiciency (%) Irraiance (W/m ) % loa 4% loa 6% loa 8% loa Figure-1. Eiciency curve or various values o irraiance an loa conitions. 7. EXPERIMENTAL RESULTS To experimentally veriy the propose connection s operation, a prototype is esigne an implemente accoring to Table VI an interace with a PV panel with speciications as given in Table-5. The laboratory prototype is implemente with two converter moules with an input voltage (V in ) o 8V which is booste by a high requency transormer with a switching requency (F s ) o 5 KHz to an output voltage (V out ) o 16V across a 6W loa. Photograph o the implemente prototype is given in Figure-13. Figure-15. Gate pulses or the Mosets M an M3. Figure-16. Total output voltage (5V/iv) or an Irraiance o 1W/m or the two converter system supplie by the PV panel with speciications as given in Table-5. Table-5. PV panel speciications. Figure-13. Photograph o the implemente laboratory prototype. Parameter Speciications P mp (Wp) 5 I mp (A).59 V mp (V) 8.5 I sc (A).66 V oc (V) 1.7 Gate pulses are generate or all the switches in the moules with the controller implemente using PIC16877a. Figure-14 to Figure-15 shows the phase shite gate pulses to the switches M1 to M8. Figure-16 shows the total output voltage o the two series connecte moular converters. Figure-14. Gate pulses or the Mosets M1 an M4. 149

8 VOL. 13, NO. 4, FEBRUARY 18 ISSN ARPN Journal o Engineering an Applie Sciences 6-18 Asian Research Publishing Network (ARPN). All rights reserve. Table-6. Main component list. Mosets (M1 to M8) Dioes Capacitors Drivers Phase shite PWM controller IRF84 IN47 µf, 63V FAN739N PIC16877a CONCLUSIONS An Input Series Output Series (ISOS) connecte converter topology is explore an analyze or PV interace moular DC-DC converter system. The ollowing has been achieve in this work, a) High converter eiciency 9% at an irraiation o 1 W/m STC an 81% at an irraiation o 7 W/m b) A lat eiciency curve i.e., a constant eiciency has been obtaine as shown in Figure-1 uner ull loa an uner lower loas or ierent irraiation values. c) Input voltage an output voltage sharing an hence power balance an equilibrium conition ensure among the converter moules. ) Low voltage rate switches are use because o series connection in the input an in the output e) PV panel current i.e., Input current an hence the input voltage has been maintaine to a require constant value or a wie range o irraiation values ) PV panel voltage has been booste by the series connecte moular converter system to the require loa voltage an maintaine at the optimum constant value or a wie range o loa conitions. REFERENCES [1] Om V. Bapatl an Vishram N. Bapat. 16. An Overview o Solar Energy Policy o Inia an Few Prominent Nations in the Worl. IEEE International Conerence on Power Electronics, Intelligent control an Energy systems, 4-6 July 16. [] Ruben Otero-De-Leon, Ne Mohan. 15. Full- Brige Moular Multilevel Converter with High Frequency Link or Photovoltaic Applications. IEEE 4 th International Symposium on Inustrial Electronics. or PV applications. IEEE Applie Power Electronics Conerence an Exposition (APEC), 5-9 Feb. [4] Raja Ayyanar, Ramesh Giri an Ne Mohan. 4. Active Input-Voltage an Loa-Current Sharing in Input-Series an Output-Parallel Connecte Moular DC-DC Converters Using Dynamic Input-Voltage Reerence Scheme. IEEE transactions on power electronics. 19(6). [5] Ramesh Giri, Raja Ayyanar, N. Mohan. 6. Common Duty Ratio Control o Input Series Connecte Moular DC-DC Converters with Active Input Voltage an Loa Current Sharing. IEEE Trans. On In. Electronics. 4(4): [6] Deshang Sha, Kai Deng an Xiao Zhong Liao. 1. Duty Cycle Exchanging Control or Input-Series- Output-Series Connecte Two PS-FB DC-DC Converters. IEEE Transactions on Power Electronics. 7(3). [7] Ramesh Giri, Raja Ayyanar an Enrique Leezma. 4. Input-Series an Output-Series Connecte Moular DC-DC Converters with Active Input Voltage an Output Voltage Sharing. Applie Power Electronics Conerence an Exposition. 3, -6 Feb. [8] Wu Chen, Xinbo Ruan, Hong Yan, an Chi K. Tse. 9. DC/DC Conversion Systems Consisting o Multiple Converter Moules: Stability, Control, an Experimental Veriications. IEEE transactions on power electronics. 4(6). [9] Chen, Wu, Kai Zhuang an Xinbo Ruan. 9. An Input-Series- an Output-Parallel-Connecte Inverter System or High-Input-Voltage Applications. IEEE Transactions on Power Electronics. 4(9). [1] P. Wang, C. Liu, L. Guo. 13. Moelling an Simulation o Full-Brige Series Resonant Converter Base on Generalize State Space Averaging. Applie Mechanics an Materials : [11] Vlatko Vlatkovic C, Juan A. Sabate, Raymon B. Riley, Fre C. Lee an Bo H. Cho Small- Signal Analysis o the Phase-Shite PWM Converter. IEEE Transactions on Power Electronics. 7(1). [3] Omar Abel Rahim, Mohame Orabi, Ema Abelkaim. 1. Switche inuctor boost converter 15