Photovoltaic Power Conversion Systems

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1 Photovoltaic Power Conversion Systems Presentation at IEEE Virginia Mountain Section Blacksburg, Virginia January 31, 2013 Jih-Sheng (Jason) Lai, Ph.D. James S. Tucker Professor Virginia Polytechnic Institute and State University Future Energy Electronics Center 106 Plantation Road Blacksburg, VA

2 Outline 1. PV Market Outlook and Cost Targets 2. PV Cell Characteristics and Maximum Power Point Tracking 3. Energy Production Comparison for Different PV System Architectures 4. Commercial PV Power Conversion Circuits and Efficiency Profiles 5. Future Trends 2

3 Outline Part 1 1. PV Market Outlook and Cost Targets 2. PV Cell Characteristics and Maximum Power Point Tracking 3. Energy Production Comparison for Different PV System Architectures 4. Commercial PV Power Conversion Circuits and Efficiency Profiles 5. Future Trends 3

PV Market Outlook 2013 2012 PV market continues growing especially in Asia; China grew 470% in 2011; US

4 PV Market Outlook PV market continues growing especially in Asia; China grew 470% in 2011; US looks to 3GW in 2012 PV industry generates $93B global revenues with 27.4GW installation in 2011 (~$3/W) (GW) 4

5 Desertec 2050 Within 6 hours deserts receive more energy from the sun than humankind consumes within a year. Dr. Gerhard Knies MENA desert power can supply around two-thirds of the region s rising energy demand and 17 percent of EU consumption. In 2009, DESERTEC Foundation was founded with financial plan to 2050 Invest in renewable energy and interconnected grids in EU-MENA Tunisia started 2-GW CSP in 2011 and scheduled to deliver power in

6 PV System Cost Targets US SunShot Initiative (Feb. 2012) 75% solar system cost reduction through Reducing solar technology costs Reducing grid integration costs Accelerating solar deployment By 2020, cost targets are: $1/W for utility-scale PV systems $1.25/W for commercial rooftop PV $1.50/W for residential rooftop PV Inverter cost targets: $0.10/W for utility-scale systems $0.11/W for commercial systems $0.12/W for residential systems 6

7 Power Electronics Cost Reduction Solving fundamental power electronics problems at the component level. Reducing the cost of advanced components (SiC, GaN). Addressing reliability failures due to thermal cycling of materials. Developing technologies that allow high penetrations of solar technologies onto grid (VAR control, storage). Developing PV system technologies that reduce overall Balance of System costs (high-voltage systems). Developing technologies that harvest more energy from sun (MPPT and micro-inverters). Integrating micro-inverters into modules, reducing installation effort and achieving further cost reductions through mass production. 7

8 Outline Part 2 1. PV Market Outlook and Cost Targets 2. PV Cell Characteristics and Maximum Power Point Tracking 3. Energy Production Comparison for Different PV System Architectures 4. Commercial PV Power Conversion Circuits and Efficiency Profiles 5. Future Trends 8

9 State-of-the-Art Solar PV Technologies Cell Type *Best Reported Efficiency Comments Silicon Mono crystalline Poly crystalline Thin Films Amorphous Si Cadmium Telluride (CdTe) Cadmium Indium gallium diselenide (CIGS) Nano Silicon 25.0% 27.6% 12.5% 16.7% 20.0% 16.7% Conventional technologies Cost became competitive with Asian manufacturing Used in everything, from solar watches to solar shingles to megawatt Ink jet printing cost down will challenge Si Concentrating PV (CPV) 41.6% Need solar concentrators Too expensive in near term Organic and dye sensitized 11.1% Powering portable electronics * Efficiency data referred to J. Milliken, Solar Energy Technologies Program Overview, Apr. 2010, NREL 9

10 PV Cell V-I Characteristics Representation I D + V I D I D -V curve V I D + I sc + V D D V I I D I sc I D -V curve V I-V curve I I sc I-V curve V Derivation of I-V Curve I = I sc I D Flip current direction V-I Curve 10

11 Solar Cell V-I Characteristics as a Function of Temperature 70 I sc change +0.04%/ K Output Current (ma) Hot Maximum power point (MPP) Cold 10 0 V oc change 0.34%/ K Output voltage (V) 11

12 Typical Solar Cell V-I Characteristics Output Current (ma) Illumination level (125 mw/cm 2 ) 100 mw/cm 2 Maximum power point Output voltage (V) 12

13 Solar Spectrum: Atmosphere Influence Air mass definition: AMx = A x /A 1 = 1/cos ( ) h AM s s 1 h 2 AM1 means = 0 AM1.5 means =

14 Example PV Panel Specifications Kyocera KD180GX-LP 48 cells Dimension: 1341mm x 990mm x 36mm (52.8in x 39.0in x 1.4in) Weight: 16.5 kg (36.4 lbs) 14

15 Current (A) Manufacturer s Datasheet Kyocera KD180GX-LP At 1000W/m 2 25 C 50 C 75 C Voltage (V) (a) Temperature effect Current (A) W/m 2 800W/m 2 600W/m 2 400W/m 2 200W/m Voltage (V) (b) Irradiance effect At 25 C 15

16 Four-Quadrant Photovoltaic Cell Characteristics I D + I sc + V D D V I PV cell may operate in different quadrants: Q1, Q2, Q4. Q1 is normal operating zone. Cell is generating power. Q2 has reverse cell voltage, may appear in seriesconnected cells. Cell is dissipating power. Q4 has reverse cell current, may appear in parallel-connected cells. Cell is dissipating power. + I Q2 Q1 + + Q4 V 16

Series String PV Cell Under Shaded Condition V I o o V a V b V c

b V o V c DC-DC Converter or DC-AC Inverter V BD I I o 2

or failure when V c > V BD V BD V diode I o I V 2 non-shaded

17 Series String PV Cell Under Shaded Condition V I o o V a V b V c V I o + V o a V a V b V c DC-DC Converter or DC-AC Inverter V + b V o V c DC-DC Converter or DC-AC Inverter V BD I I o 2 non-shaded cells 1 shaded cell V c V a +V b Potential hot spot or failure when V c > V BD V BD V diode I o I V 2 non-shaded cells 1 shaded cell V a +V b Adding anti-paralleled diode avoids failure V 17

18 Bypass Diodes in a Typical PV Panel Junction box + A PV panel typically consists of 48 to 72 cells with 3 bypass diodes. A bypass diode covers 16 to 24 cells. For 24 cells case, if each cell voltage is 0.5V, and diode voltage drop is 0.6V, then the worst case shaded cell reverse voltage = = 12.6 V (< breakdown voltage) 18

19 Bypass Diode Power Dissipation Junction box + I o Assume I o = 8A. Diode dissipations: Si diode: 0.7V 8A = 5.6W Schottky diode: 0.4V 8A = 3.2W Active diode (synchronous rectification): (8A) 2 (5m ) = 0.32W 19

20 Lightning Impact i lt M i D v D Case a: Bypass diode stressed in forward direction High forward current (~ka) may destroy diode i km L D i lt k: frame reduction factor ( ) M: mutual inductance ( nh) L: loop inductance of each string (1-3 µh) i lt M i D v D Case b: Bypass diode stressed in reverse direction High induced voltage may exceed diode breakdown voltage VBD and result in overvoltage failure v D M di dt lt 20

21 Lightning Catastrophic Failure Case + V PV Inverter Normally, PVs are protected with metal oxide varistor (MOV), i.e., R GND = 0, V PV = V MOV, but when the ground wire is broken, lightning surge can conduct through parasitic capacitors, i.e., R GND =, V PV = kv surge, k<1. Photos show a catastrophic failure when ground wires were stolen in a MW PV farm installation. All cells were shattered, and diodes were broken. 21

22 Maximum Power Point of a PV Cell Maximum Power Point (MPP) Typically, I sc 1.05 to 1.15 I MPP, V MPP 0.8V oc Defining fill factor (FF) PMPP V FF Typical fill factors are: VocI sc V C-Si: 0.75 ~ 0.85 A-Si: 0.5 ~ 0.7 MPP oc I I MPP sc 22

23 Multiple Peaks Due to Shading Effect with 3-Bypass Diode Configuration Shading condition: Modules a & b: full-sun condition Module c: shaded condition 12-cells in series a b c I V Power (W) Current (A) full-sun condition partial shaded condition full-sun condition partial shaded condition Voltage (V) 23

24 Case with Mismatched Panels and Different Irradiation Levels Among Them Two different types of PV panels in series with different irradiation levels among them can result in multiple MPPs 1200 MPP Voltage--Power 1000 Power(W) V 252V 329V Voltage(V) 24

25 Maximum Power Point Tracking (MPPT) MPP control is required to harness as much energy as possible. Poor MPPT method is equivalent to having additional loss. Important MPPT design considerations: Tracking speed Control loop stability Oscillation around MPP including double line frequency oscillation Global MPP versus local MPP Which stage performs MPPT, DC-DC stage or DC-AC stage? Control of VPV or IPV? Step size? Digital versus analog Almost 20 distinct published methods, ranging from ripple correlation control (a fast method that uses converter ripple to find the MPP) to fuzzy logic controls. 25

26 Fractional Open-Circuit Voltage For most PV cell types, there is a nearly linear relationship between V MPP and V OC, V MPP xv oc x depends on PV material, typically 0.74 to 0.8 V oc Measure V OC at infrequent intervals, then use the known fraction as the basis for control Only an approximation operation practically never exactly at the MPP Simple, low cost, fast, and robust Poor accuracy, lost power during V oc measurement Current (A) Voltage (V) Power (W) 26

27 Hill-Climbing/Perturb & Observe Methods Alter the operating point, by changing a duty ratio slightly. Check whether the power rises or falls. Keep changing to get higher power. Perturbation Change in Power Next Perturbation Positive Positive Positive Positive Negative Negative Negative Positive Negative Negative Negative Positive Key Features Clear and effective. Convergence depends on perturbation step size and converter settling times. Goes to a local maximum power point. 27

28 Outline Part 3 1. PV Market Outlook and Cost Targets 2. PV Cell Characteristics and Maximum Power Point Tracking 3. Energy Production Comparison for Different PV System Architectures 4. Commercial PV Power Conversion Circuits and Efficiency Profiles 5. Future Trends 28

29 Different PCS Architectures Centralized DC/AC with distributed series strings ~MW Distributed series strings + DC/AC ~kw Centralized DC/AC with distributed series strings + DC/DC Centralized DC/AC with Series DC modules Distributed AC modules ~100 s W DC/DC 100 s W DC/AC ~kw Centralized DC/AC with Paralleled DC modules 29

30 Panel Level Power Electronics Becomes More Popular Choices DC-AC DC-AC (a) String inverter 3 12kW (e.g. SunnyBoy) (c) Series DC DC power optimizer (e.g. SolarMagic TM ) DC-AC DC-AC (b) Microinverter W (e.g. Enphase) (d) Paralleled DC DC power optimizer (e.g. VT IntelliSOLAR TM ) 30

Power Output Comparison with and without Power

PV 300V DC-AC DC-AC I PV I PV DC-DC 4A 5s

(b) With SolarMagic TM Using SolarMagic TM as

Without SolarMagic TM, the PV inverter output

b) With SolarMagic TM, the PV inverter output

31 Power Output Comparison with and without Power Optimizer Under Shaded Condition V PV 300V V PV 300V DC-AC DC-AC I PV I PV DC-DC 4A 5s shaded (a) Without SolarMagic TM 4A 5s shaded (b) With SolarMagic TM Using SolarMagic TM as power optimizer for series connected panels a) Without SolarMagic TM, the PV inverter output power drops from 1300W to 60W (95% reduction) b) With SolarMagic TM, the PV inverter output power drops from 1450W to 1200W (17% reduction) 31

VT Solar House PCS Configured with SunnyBoy, SolarMagic TM, and Enphase Inverters AC Grid Configuration SunnyBoy input peak voltage is 520V For each SolarMagic or Enphase, input

32 VT Solar House PCS Configured with SunnyBoy, SolarMagic TM, and Enphase Inverters AC Grid Configuration SunnyBoy input peak voltage is 520V For each SolarMagic or Enphase, input peak voltage is 40V Each PCS branch consists of 26 PV panels with 1.95kW peak power feeding into a mini micro grid A Mini Micro Grid (AC Nano Grid) 32

VT IntelliSOLAR PCS Configuration VT FEEC building Front view of PV

Grid Configuration VT IBR converter (>97% eff.

) 400V DC Bus I dc A DC Micro Grid Other DC sources 2kW other DC sources

) 5kW inverter 240V AC 3kW PV source Paralleled power optimizers for

integrated boost resonant (IBR) converter and softswitching inverter No

33 VT IntelliSOLAR PCS Configuration VT FEEC building Front view of PV panels DC link cable Weather tight connections Back view of PV panels DC Grid Configuration VT IBR converter (>97% eff.) VT IBR converter (>97% eff.) VT IBR converter (>97% eff.) 400V DC Bus I dc A DC Micro Grid Other DC sources 2kW other DC sources VT soft sw. inverter (>99% eff.) 5kW inverter 240V AC 3kW PV source Paralleled power optimizers for individual panels High overall system efficiency (>96%) with VT integrated boost resonant (IBR) converter and softswitching inverter No aluminum electrolytic capacitors; no cooling fans DC micro grid architecture with arc detection and protection at local PV panels Potentially low cost with more integration i ac 33

34 W/m 2 P o (W) P o (W) PCS Power Outputs and Irradiance Level Microinverter Un-shaded Case March 17, kWh Late 10 start Centralized inverter kWh 0 Irradiance March 17, Hour PCS outputs well associate with the irradiance level Without much shading, centralized inverter may produce more energy than microinverter does even with late start in the morning 34

35 Comparison of Energy Production under Un- Shaded Condition for a Two-Month Period Energy production (kwh) Enphase: 482 kwh Sunnyboy: 481 kwh 4/1/2011 5/31/2011 SolarMagic: 469 kwh day With the same 95.5% CEC efficiency, string inverter produces almost the same amount of energy as microinverter does over a two month period. Adding SolarMagic TM power optimizer does not improve the energy output because the solar house is in a wide open area. 35

1600 1200 800 400 0 1600 1200 800 400 0 1000

Irradiance Level Manually Shaded Case June 21,

2% reduction) 0 2 4 6 8 10 12 14 16 18 20 22

5% reduction) 0 2 4 6 8 10 12 14 16 18 20 22

36 PCS Power Outputs and Irradiance Level Manually Shaded Case June 21, 2011 Microinverter Centralized inverter kwh / 11 kwh (19.2% reduction) Irradiance 9.73kWh / 11 kwh (11.5% reduction) Hour Significant power/energy output reduction on centralized inverter case. 3 out of 26 panels were covered 36

37 Comparison of Energy Production under Partial Shaded Condition for a Two-Month Period Energy production (kwh) Enphase: 514 kwh SolarMagic: 470 kwh Sunnyboy: 449 kwh 6/1/2011 7/31/ day Cumulated energy production over a two month period further verifies the partial shading impact to the centralized inverter 37

38 Outline Part 4 1. PV Market Outlook and Cost Targets 2. PV Cell Characteristics and Maximum Power Point Tracking 3. Energy Production Comparison for Different PV System Architectures 4. Commercial PV Power Conversion Circuits and Efficiency Profiles 5. Future Trends 38

39 Inverter Efficiency Standards California Energy Commission (CEC) All inverters must meet the requirements in Emerging Renewables Program, Final Guidebook, Eighth Edition, Section C Inverters. There are no set minimum requirements, but the conversion efficiency must be tested and reported to CEC as defined here. IEC 61683:1999, First Edition, , Photovoltaic systems Power conditioners Procedures for measuring efficiency. This standard describes guidelines for measuring the efficiency of power conditioners used in standalone and utility interactive photovoltaic systems, where the output of the power conditioner is a stable ac voltage of constant frequency or a stable dc voltage. The efficiency is calculated from a direct measurement of input ad output power in the factory. An isolation transformer is included where it is applicable. China: GB/T Photovoltaic systems Power conditioners Procedure for measuring efficiency. Based on IEC 61683:

40 CEC and IEC Weighted Efficiency Measurement CEC IEC 5% % % % % % %

41 41 CEC Efficiency Evaluation for a 200-W PV Inverter Sample #5 Sample #4 Sample #3 Sample #2 Sample #1 Specified Eff. Input voltage Output power Eff. Input voltage Output power Eff. Input voltage Output power Eff. Input voltage Output power Eff. Input voltage Output power Input voltage Output power % % % % % Vmax 10% % % % % % Vnom 10% % % % % % Vmin 10% (%) (Vdc) (W) (%) (Vdc) (W) (%) (Vdc) (W) (%) (Vdc) (W) (%) (Vdc) (W) (Vdc) (% of rated)

42 Efficiency (%) CEC Efficiency Evaluation Report for a 200-W Microinverter 98 V 97 max = 50 V V nom = 40.5 V 94 V min = 31 V % 20% 40% 60% 80% 100% Percent output power CEC Efficiency V min : 95.4% V nom : 95.7% V max : 95.7% Average: 95.6% Minimum 5 samples are needed for efficiency evaluation. Efficiencies need to be measured at different input voltages and different load conditions. The final reported CEC efficiency is the average of CEC efficiencies at three voltages. 42

43 A Grid-Tie Solar Power Conversion System I PV V PV V dc V ac + V PV HF PWM DC/AC HF Xformer AC/DC HF SPWM DC/AC PV source DC-DC converter with isolation (Low to High voltage DC) DC-AC inverter with AC filtering (Low frequency) Utility grid 43

44 Isolated Single- v.s. Two-Stage Configurations V ac + V in HF PWM DC/AC High Freq. Xformer AC/DC Rectifier LF unfolding DC/AC (a) Single PWM stage type, no high voltage energy storage V ac + V in HF PWM DC/AC High Freq. Xformer AC/DC Rectifier V dc HF SPWM DC/AC (b) Two PWM stage type, with high voltage DC bus and energy storage Single PWM stage is generally more efficient, but requires large storage capacitor at the input to stabilize MPPT Two PWM stages mean more costly components and higher switching loss, but the system allows high voltage DC bus to absorb 120Hz ripple and thus eliminating electrolytic capacitor 44

45 S 1 Switching with Hybrid Line and PWM Frequencies S 1 S 3 S 4 S 3 S 2 S 2 S 4 v o Features: Line frequency switching for bottom or top switches (IGBT as the switching device) PWM switching for top switching (MOSFET as the switching device) Ultra fast reverse recovery diodes can be used for freewheeling Potential high efficiency Less shoot-through concerns Ground loop leakage current is an issue with unipolar PWM 45

46 Ground Loop Leakage Current Issues With thin-film PV getting more popular, the parasitic capacitance between PV and ground also draws more attention due to the increase of the ground loop leakage current. Different inverter modulation methods may produce the same output load current, but they may see different output common mode voltage against the neutral and ground, and thus producing different ground loop leakage current. The ground loop leakage current may be alleviated by providing circuit isolation, modulation methods, or different circuit topologies. 46

leakage current IGBT s S 1 and S 2 serve as low frequency selection network S 6 is for over-voltage protection PV

47 Non-isolated H5 TM Inverter Avoid Leakage Current with Unipolar Switching (SMA 8000TL) Basic Operation: S 3 and S 5 operates in SPWM on negative cycle S 4 and S 5 operates in SPWM on positive cycle Complementary PWM at the input No leakage current IGBT s S 1 and S 2 serve as low frequency selection network S 6 is for over-voltage protection PV voltage range AC output Power Nominal voltage Max. efficiency CEC efficiency V 208 V 8 kw 345 V 98.3 % 98.0 % 47

48 Full-Bridge Inverter with Low-Frequency Transformer Isolation (SMA 5000US) PV array 30 s Transformer with leakage inductance Inverter uses a full-bridge IGBT module Transformer contains leakage inductance to serve as differential mode inductance Reasonable efficiency Very bulky and heavy (150 lbs for a 5- kw inverter) PV voltage range AC output Power Nominal voltage Max. efficiency CEC efficiency V 240 V 5 kw 310 V 96.4% 95.5% 48

49 A Microinverter with Two-stage Power Conversion Push-pull DC-DC Full-bridge DC-AC v ac V PV Exeltech Features: Two power stages, two PWM stages Push-pull stage converts the input voltage to around 230V (26 khz) Inverter uses fast reverse recovery diodes and IGBTs (30 khz) Low THD Relatively low efficiency PV voltage range AC output voltage Power Nominal voltage Max. efficiency CEC efficiency V 120 V 212 W 50 V 95.3 % 94.5 % 49

50 Active-Snubber Flyback Microinverter D 2 Q 1 Q 3 S x2 S 2 D 1 Q 2 Q 4 Q5 PV voltage range V Power 190 W Nominal voltage 32.5 V S x1 S 1 Enphase Max. efficiency 95.4 % Basic Operation: CEC efficiency 95 % Interleaved flyback converters serve as single-stage power conversion. S x1 and S x2 are auxiliary switches for active snubber. Q 1, Q 2, Q 3, and Q 4 thyristors serve as polarity selection switches. Q 5 helps commutate thyristors under low dc bus voltage condition. Key Design Features: Single-stage power conversion, good overall efficiency Good waveform fidelity, low THD Burst mode operation at load below 30% of rated power Concern on the life span of electrolytic capacitors 50

Main switches (S 1 ~ S 4 ) operate in switching frequencies Auxiliary switches (S 5 and S 6 ) with fast reverse recovery diodes

51 Highly Efficient and Reliable Concept (Heric) Inverter S 1 S 3 S 1,S 4 I S1 a R o I o L o b I S2 S 5 S 2,S 3 V dc I S3 S 6 I S4 S 6 V o S 2 S 4 S 5 Fraunhofer Society Unipolar operation to reduce current ripple. Main switches (S 1 ~ S 4 ) operate in switching frequencies Auxiliary switches (S 5 and S 6 ) with fast reverse recovery diodes operate in low-frequency to serve as the freewheeling path to avoid reverse recovery problem of slow body diode of the main switch, which are CoolMOS with very low conduction drop. 51

52 Outline Future Trend 1. PV Market Outlook and Cost Targets 2. PV Cell Characteristics and Maximum Power Point Tracking 3. Energy Production Comparison for Different PV System Architectures 4. Commercial PV Power Conversion Circuits and Efficiency Profiles 5. Future Trends 52

53 Future Trend in Cost Reduction Cost Target with 10 /W as cost target, pressure is high to further reduce the cost of power electronics More Power Electronics Integration with 50/60-Hz low-frequency component, cost and size reduction on inverter side is difficult, but DC-DC will have chance for more integration More Use of Wide Bandgap Devices By pushing frequency to MHz range, GaN and SiC devices will be adopted for significant size and cost reduction Plug-and-Play Ease of installation is a way to reduce labor cost. We may see more integration with PV panels and power electronics 53

54 Future Trend in Efficiency Figures Historical figures: Enphase inverter was 95.5% in Gen-1 and 96% in Gen-2 SMA inverter was 95.5% with isolated version and 98% with non-isolated version Efficiency will continue moving up. Future development needs to target: Micro-inverter 97% String/centralized inverter 99% Key factors driving up efficiency Wide bandgap and super-junction devices More efficient converter/inverter circuit with reduced circulating current for conduction loss reduction With soft switching for switching loss elimination 54

55 55

Comparison and Design of High Efficiency Microinverters for Photovoltaic Applications

Comparison and Design of High Efficiency Microinverters for Photovoltaic Applications Jason C. Dominic Thesis submitted to the faculty of the Virginia Polytechnic Institute and State University in partial