July 23 2018 SolarPhotovoltaic DC Systems Basics and

July 23, 2018 Solar/Photovoltaic DC Systems: Basics and Safety ESW 2018 -05 Peter Mc. Nutt, NREL Bill Sekulic, NREL Gary Dreifuerst, LLNL (retired) 2018 ELECTRICAL SAFETY WORKSHOP 1

Introduction • PV systems are common and growing -- 42. 4 GW of installed capacity in the United States – 15 GW added in 2016 [1] • Our talk deals only with the DC side of solar/PV systems • PV DC system basics for electrical safety 2

Background - Why? • At the 2017 IEEE ESW a presentation was given on “Hazards In the Installation and Maintenance of Solar Panels” (ESW 2017 -3) by White and Doherty [2] • Generated a lot of good, interesting questions about solar/PV systems, e. g. : Can a moonlit PV array kill someone? 2) Do all PV strings have the same number of modules? 3) Why would workers be exposed to hazardous DC voltages in a PV system? 1) 3

Outline Abbreviated interactive discussion of our March 2018 IEEE ESW talk touching on some of the following: • • Basic PV array operating characteristics Workers’ exposure to DC system hazards Arc flash hazard PV module testing and standards NEC and PV DC system safety for emergency responders PV DC accidents and injuries Resources 2018 ELECTRICAL SAFETY WORKSHOP 4

PV Terminology • • • Cells Modules Panels Strings Subarrays Arrays. NEC 690. 2 [3] Source: NREL 5

PV System Arrays Fixed tilt Single-axis tracker Dual-axis tracker Source: NREL 6

Outline… • Basic PV array operating characteristics • Workers’ exposure to DC system hazards • Arc flash hazard • PV module testing and standards • NEC and PV DC system safety • PV DC system safety for emergency responders • PV DC accidents and injuries • Resources. Source: NREL 2018 ELECTRICAL SAFETY WORKSHOP 7

Typical PV Module Operating Curve Pmp STC (Standard Test Conditions): 1000 W/m^2 25°C Tpv AM 1. 5 (lab conditions) Source: NREL

Typical PV Module Operating Curves A SUNNIER array produces higher current A COOLER array produces higher voltage Source: NREL

Outline… • Basic PV array operating characteristics • Workers’ exposure to DC system hazards • Arc flash hazard • PV module testing and standards • NEC and PV DC system safety • PV DC system safety for emergency responders • PV DC accidents and injuries • Resources. Source: NREL 2018 ELECTRICAL SAFETY WORKSHOP 10

Commissioning, Maintenance, and Periodic Testing System interactions may include: Installation/Commissioning: • Insulation resistance (megger) • I-V curve traces • Verify torque of terminals and connections Performance/Periodic Testing: • I-V curve traces • Verify torque of terminals and connections. Source: NREL 11

Current (per unit) Maintenance and Periodic Testing Performance/Periodic Testing: • I-V curve traces Source: NREL 12

Maintenance and Periodic Testing Performance/Periodic Testing: • IR thermography Source: NREL 13

PV DC System Safe Operating Procedures and PPE • Disconnects and fused switches must be rated for switching DC voltage under load (load-break) • PPE sized for voltage levels, mainly for shock and thermal burns • Arc flash calculations need to be performed to identify the AF PPE that is needed for workers. (This will typically consist of protection for the face and eyes, upper and lower body of sufficient energy rating to protect the worker for the calculated incident energy levels. ) • Solar panels are current limited, so string-based arc flash energy is limited. Recombiner backflow is primary cause for arc flash hazards at the combiners. 14

Outline… • Basic PV array operating characteristics • Workers’ exposure to DC system hazards • Arc flash hazard • PV module testing and standards • NEC and PV DC system safety • PV DC system safety for emergency responders • PV DC accidents and injuries • Resources. 2018 ELECTRICAL SAFETY WORKSHOP 15

DC Arc Flash Calculator – DOE EFCOG • The Excel spreadsheet (formulas locked) [14] can be found at: • http: //efcog. org/wp-content/uploads/2016/11/DC-Arc Flash-Calculator-EFCOGESW-DC-SC. xlsx • The calculator has four sheets, with the fourth sheet providing equations and references: • The calculator includes the NFPA 70 E [15] Annex D “Doan” approach (linear), the “Enrique” PV method (non-linear - NL), and the Stokes and Oppenlander (SO) equations for DC arc current, sometimes referred to as the “Ammerman” method, which give a realistic arc gap voltage. • Spreadsheet Calculator - instructional pages can be found at [16]: • http: //efcog. org/wp-content/uploads/2016/11/Best-Practice-194 -DC-Arc-Hazard. Spreadsheet. pdf 16

Importance of the “Enrique” Method for PV Arrays • Eduardo Enrique, “DC Arc Flash Calculations for Solar Farms, ” 2013 1 st IEEE Conference on Technologies for Sustainability [17] • For the standard example in his paper, the IEpv/IEmax was 3. 24; when the temperature compensation factor (raises the PV output voltage for cold temperatures) was included, the factor was 3. 94. • Further publications from Doan [25] and Klement [26] show arc flash calculation approaches which take the non-linear nature of a PV system into consideration. • The EFCOG DC Working Group is investigating the accuracy of each of these methods. 17

NREL PV DC Arc Energy Testing • Bill is building a PV DC arc generator test stand • arc probes • calorimeter • Will measure arc energy • Will report on findings – 2019 IEEE ESW 18

PV DC AF Model Verification Testing • PV DC Arc Energy Calculations IV Curve – Arc Data (5/8” Gap) Current • Max Power Point 10. 0000 • Short Circuit Current • Calculated vs. Measured • Preliminary Results • Arcing Energies for Strings • Arcing Energies for Arrays 9. 0000 8. 0000 Current (amperes DC) • Arc Resistance Arc Data 7. 0000 6. 0000 5. 0000 4. 0000 3. 0000 2. 0000 1. 0000 0 50 100 150 Voltage (volts dc) 200 250 19

Outline… • Basic PV array operating characteristics • Workers’ exposure to DC system hazards • Arc flash hazard • PV module testing and standards • NEC and PV DC system safety • PV DC system safety for emergency responders • PV DC accidents and injuries • Resources. 2018 ELECTRICAL SAFETY WORKSHOP 20

Solar Panel Standards and Qualifications • UL 1703 [6] and IEC 61215 [7] Qualification Certifications o Protocols are almost identical - IEC is the international standard - UL is the U. S. standard. • Solar modules receives certification only if they pass all testing. • Module testing is similar to NRTL listing process for consumer goods and electrical equipment.

Bulk conductivity Origins of Standard PV Module Accelerated Tests PVB EVA Define associated accelerated tests PVB Modules corroded Design modules to pass tests Identify failures Deploy modules Wohlgemuth, IEEE 43 Solar Cells, 28 (1990) Jet Propulsion Laboratory executed a series of “block buys” in 1975 -1986. [8]

Power Output for PV Module Over 20 Years 98 96 Power 94 92 90 88 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Power Output (% of Full Power) 100 Power Warranty (0. 5% loss per year for 20 Years) Year Manufacturers’ Warranty Standard Power Warranty is < 0. 5% loss per year Source: NREL

Power Output for Real PV Modules over 20 years Date Power (DC Watts) 1994 54 2014 47. 5 Change -12% Data from NREL’s SERF rooftop mounted crystalline silicon system, installed 1994. 12% loss over 20 years, or 0. 6% loss per year.

Modules are Tough • Mechanically resistant to impacts from wind (and even leaping researchers) • Thanks to mechanical-load testing • DO NOT do this!!! May lead to electrical problems. NREL Now – May 2017 Source: NREL 25

Modules Survive Hail • Hail storm in Golden, CO, May 8, 2017 – golf-ball size hail • Only 6 modules on our site damaged (there are more than 3, 500 modules on the NREL campus) • Thanks to hail-impact testing. NREL Now – May 2017 DOE EERE – May 2017 26

Outline… • Basic PV array operating characteristics • Workers’ exposure to DC system hazards • Arc flash hazard • PV module testing and standards • NEC and PV DC system safety • PV DC system safety for emergency responders • PV DC accidents and injuries • Resources. 2018 ELECTRICAL SAFETY WORKSHOP 27

Example of Poor Installation Practice Improper Torque Proper Torque Improperly torqued terminal, leads to heating and insulation discoloration. NEC 110. 14(D) Notice insulation discoloration. Terminal torque is critical to prevent fires Source: NREL

Ungrounded PV Array – Shock Potential Fault 1 Ungrounded Conductor Fault 2 Ungrounded Conductor Equipment Ground Conductor • During normal system operation a person can only be shocked (or fault created) by contacting two ungrounded leads, not one lead and earth ground. • Recently, the inverter test standard, UL 1741, has been updated to include requirements for ungrounded inverters. [11] • Manufacturers and installers now more willing to bring ungrounded inverters into U. S.

Rapid Shutdown for PV Systems • NEC 690. 12 Rapid Shutdown of PV Systems on Buildings – introduced in 2014 NEC. • Products starting to become available: inverters, module-level electronics. • Goal: < 80 V inside array boundary [12 & 13]. • Create safer systems for maintenance crews and firefighters. Source: B. Brooks 30

Rapid Shutdown Labels [12] Rapid shutdown of array and conductors exiting the array -both reduced to less than 80 V NEC 690. 56 Rapid shutdown only the conductors exiting the array Source: B. Brooks 31

Outline… • Basic PV array operating characteristics • Workers’ exposure to DC system hazards • Arc flash hazard • PV module testing and standards • NEC and PV DC system safety • PV DC system safety for emergency responders • PV DC accidents and injuries • Resources. Source: UL 2018 ELECTRICAL SAFETY WORKSHOP 32

Alternative Light Sources • Moonlight • Artificial light sources • Light from fire. Source: Randy Hunsberger Source: UL 33

Full Moon Poses No Shock Hazard • Full-moon night January 12, 2017 • Array voltage essentially dropped to 0 V • Other studies showed similar results. UL [21], Sandia [24] Solar Irradiance PV Voltage 01 -17 -2017 - Sunset: 6: 30 PM Moonlit night at NREL Golden, Colorado 34

PV Firefighter Safety Recommendations • “CESA Solar Photovoltaic Fire Safety Training” report CESA [19] • Report summarizes safe firefighting techniques; UL findings UL [21] : • Water sprayed at 1000 -V copper plate • At least 20 ft away for smooth bore • At least 10° angle for adjustable nozzle • UL 401 Standard, 30° min cone angle • “Portable Spray Hose Nozzles for Fire-Protection Service” Source: UL • Zero leakage current from 1000 V array. Possible to greatly minimize shock hazard 35

PPE for Firefighters • Boots and gloves can provide some electrical insulation and protect against electric shock, up to 1000 Vdc. • Must be dry and intact • FF PPE must not be relied on to protect against electrical shock. ICLEI [22] 36

Outline… • Basic PV array operating characteristics • Workers’ exposure to DC system hazards • Arc flash hazard • PV module testing and standards • NEC and PV DC system safety • PV DC system safety for emergency responders • PV DC accidents and injuries • Resources. 2018 ELECTRICAL SAFETY WORKSHOP 37

PV DC System Electrical Accidents • 137, 000+ solar/PV U. S. installers (2016) Solar Foundation [23] • We were unable to find a summary of electrical injuries and deaths due explicitly due to PV DC systems. • Data up to 2016 indicates there were no U. S. firefighter deaths due to PV systems. CESA [19] • In Germany, no firefighter has been injured by PV power while fighting a fire as of 2017. • With more than 1. 4 million PV plants in Germany, over 20 years, 120 systems caught fire, 75 resulting in severe damage , and the entire building burned to the ground in 10 cases. • Acceptance tests by a third party are recommended. Fraunhofer [18] 38

Hazards Unique to PV DC Systems • Arc-flash calculations need to be updated for any change in the array. • Some meters (DMMs) may default to AC voltage when first powered on. • Non-contact (proximity) voltage sensors only detect AC, not DC. • Partial shading may damage some module types. • Don’t walk/jump on modules. • Cold water may cause thermal shock – don’t wash when array is hot. • Broken glass or torn back sheet can compromise voltage rating of module (tools/rocks, weed whackers, hardhat hitting back of module, etc. ). • Check modules after wind or hail storms. Periodically inspect arrays. • Consult PV professional if any damage is noticed. 39

Resources To Learn More Training • • UL / DHS “PV and Firefighter Safety” on line training: https: //lms. ulknowledgeservices. com/catalog/disp lay. resource. aspx? resourceid=352901. IAFF Firefighters / IREC Solar PV Safety for Firefighters online training: http: //client. prod. iaff. org/#page=Solar. PVHome. Reports • M. Piantedosi, T Granato, Solar Photovoltaic (PV) Fire Safety Training, CESA, 2012. [19] • R. Backstrom, D. Dini, Firefighter Safety and Photovoltaic Installations Research Project, UL, Nov 2011. [21] • R. Backstrom, D. Grubb, Firefighter Safety and Photovoltaic (PV) Systems, ICLEI, DOE Sun. Shot, 2012. [22] • O. Lavrova, et al, Updated Evaluation of Shock Hazards to Firefighters Working in Proximity of PV Systems, Sandia, 2017. [24] 40

Conclusions • PV systems are a simple, reliable, and growing source of power. • The systems must be engineered, installed, operated, and maintained properly. • Safe PV array operation is possible with a good understanding of PV DC arrays basics. • Safe Operating Procedures must be in place and followed. • Good training resources and reports are available for emergency responders on how to work safely around the systems in emergency situations. • Avoid contact with PV arrays especially those damaged by fire or water. 41

July 23, 2018 Solar/Photovoltaic DC Systems: Basics and Safety ESW 2018 -05 Peter Mc. Nutt , NREL peter. mcnutt@nrel. gov Bill Sekulic, NREL bill. sekulic@nrel. gov Gary Dreifuerst, LLNL (retired) gdreifuerst@gmail. com 2018 ELECTRICAL SAFETY WORKSHOP 42

Acknowledgements • • Heath Garrison - NREL Lloyd Gordon - LANL Randy Hunsberger – contractor, NREL Sarah Kurtz - NREL John Wohlgemuth – NREL retired Ingrid Repins - NREL Tim Silverman - NREL • • • Robert Backstrom - UL Matt Piantedosi – Cadmus Group Bill Brooks – Brooks Engineering John Wiles – SWTDI Geraly Amador - NREL This work was supported by the U. S. Department of Energy under Contract No. DE-AC 3608 GO 28308 with Alliance for Sustainable Energy, LLC, the Manager and Operator of the National Renewable Energy Laboratory. Funding provided by U. S. Department of Energy Office of Energy Efficiency and Renewable Energy Solar Energy Technologies Office. 43

References 1 P. Mc. Nutt, B. Sekulic, G. Dreifuerst, PV DC Systems: Basics & Safety, IEEE ESW, Fort Worth, TX, March 2018. [1] http: //www. seia. org/research-resources/us-solar-market-insight [2] J. White and M. Doherty, “Hazards in the Installation and Maintenance of Solar Panels (ESW 2017 -3), . [3] NFPA 70, 1996 National Electrical Code, Quincy, MA: NFPA. [4] https: //www 3. fronius. com/froniusdownload/tool. html [5] Solar ABCs Solar Design Temps: http: //www. solardesigntemps. com/northamerica [6] UL 1703 Flat-Plate Photovoltaic Modules and Panels, UL, 2002. [7] IEC 61215 -1: 2016, Terrestrial photovoltaic (PV) modules - Design qualification and type approval, IEC, 2016. [8] R. G. Ross and M. I. Smokler, Flat –Plate Solar Array Project Final Report , V 6: Engineering Sciences and Reliability, JPL, October 1986. [9] J. Wohlgemuth, IEC 61215: What it is and isn’t. 2012 PV Module Reliability Workshop, February 2012. [10] B. Brooks, Solar PV Safety for the Fire Service, Solar ABCs, 2015. [11] B. Brooks, Ground-Fault Protection Blind Spot: a Safety Concern for Larger PV Systems in the U. S. , Solar ABCs, January 2012. [12] B. Brooks, Rapid Shutdown Requirements for NEC 2017, AEESolar Dealer Conference, CMP 4, 2017. [13] B. Brooks, personal email, August 9, 2017. [14] http: //efcog. org/wp-content/uploads/2016/11/DC-Arc-Flash-Calculator-EFCOG-ESW-DC-SC. xlsx 44

References 2 [15] NFPA 70 E 2015 Standard for Electrical Safety in the Workplace, Quincy, MA: NFPA. [16] http: //efcog. org/wp-content/uploads/2016/11/Best-Practice-194 -DC-Arc-Hazard-Spreadsheet. pdf. [17] E. Enrique, “DC Arc Flash Calculations for Solar Farms”, 2013 1 st IEEE Conference on Technologies for Sustainability, 2013. [18] H. Wirth, “Recent Facts about Photovoltaics in Germany, ” Fraunhofer ISE, Germany, January 9, 2017. [19] M. Piantedosi, T Granato, Solar Photovoltaic (PV) Fire Safety Training, CESA, 2012. [20] J. Herrman, Decoding the Science of Light, Popular Mechanics, September 20, 2011. [21] R. Backstrom, D. Dini, Firefighter Safety and Photovoltaic Installations Research Project, UL, Nov 2011. [22] R. Backstrom and D. Grubb, Firefighter Safety and Photovoltaic (PV) Systems, ICLEI, 2012. [23] “Solar Jobs Map 2016, ” The Solar Foundation, http: //Solar. States. org , 2016. [24] O. Lavrova, et al, Updated Evaluation of Shock Hazards to Firefighters Working in Proximity of PV Systems. , Sandia National Laboratories, 2017. [25] D. R. Doan and R. M. Derer, “Arc Flash Calculations for a 1. 3 MW Photovoltaic System, ” 2014 IEEE Electrical Safety Workshop, Paper No. ESW 2014 -03. [26] K. Klement, ”DC Arc Flash Studies for Solar Photovoltaic Systems: Challenges and Recommendations”, IEEE Transactions on Industry and Applications, Vol 51, No. 5, pp 4239 -4244, September/October 2015 [27] M. Barrett, IREC, personal email, February 7, 2018. 45

Other References B. Brooks, “Rapid Shutdown for PV Systems, ” Solar. Pro, January/February 2015. http: //solarprofessional. com/articles/designinstallation/rapid-shutdown-for-pv-systems? v=disable_pagination#. WVUz. E 2 cw 2 BQ. B. Hibbard, NREL PV Reliability Workshop (2011). D. Jordan, B. Sekulic, B. Marion, S. Kurtz, Performance and Ageing of a 20 -year old Silicon PV system, IEEE Journal of Photovoltaics Vol. 5 (3) May 2015 pp. 744 -751. R. Mayfield, “Common Residential PV System Code Violations”, Solar. Pro, Dec/Jan 2010. S. Kurtz, J. Wohlgemuth, M. Kempe, N. Bosco, P. Hacke, D. Jordan, D. Miller, T. Silverman, N. Phillips, T. Earnest, R. Romero, Photovoltaic Module Qualification Plus Testing, NREL/TP-5200 -60950, December 2013. Sun. Wize web site – Shading Effects on PV Modules - https: //www. sunwize. com/module-shading NASA JPL Block Buy Module Development and Test Experience - https: //www 2. jpl. nasa. gov/adv_tech/photovol/Pub_blockbuys. htm T. Zgonena, et al. , Hazard Analysis of Fire Fighter Interactions with Photovoltaic Arrays, UL and Sandia, February 2018. N. Goodell, et al. , PV Commissioning Tips and Best Practices, Solar Pro, March/April 2018. 46

DC Power Profiles Based on Time of Day Fixed tilt Array Single-Axis & Dual-Axis Trackers Arrays • Highest output +/- 3 Hours around Solar Noon • Higher output throughout day Source: NREL 47

PV Arrays § N Modules Connected in Series § K Strings Connected In Parallel. § Each String is Fused. § Total DC Output is Fused. § Fuses Protect Strings from Back Fed Fault Currents. § Module Frames are Grounded to Structure and Earth (complying with NEC 250) Source: B. Sekulic

NEC Table 690. 7(A): Voltage Correction Factors for Cold Modules • The colder a PV array, the higher the voltage • Such an important concept that NEC Table 690. 7 was created (for crystalline and multicrystalline silicon modules, when values not provided by a module manufacturer) • A module’s nameplate Voc is specified at STC (25°C) • At freezing 0° C, the correction factor is 1. 10 o A nominal 550 V string at 25°C could produce 605 V at 0°C Inverter-sizing calculators account for this when sizing array strings.

PV Module and Array Connectors • Typical connector used for termination of PV modules • Ensure no current is flowing in DC lines • NEC 690. 33 (since 2008) o Connectors are touch safe o Tool required to open connectors. Source: NREL 50

Example of Poor Installation Practice Source: NREL Wiring must be supported and protected – NEC 690. 31

PV Firefighter Safety • Arvada, CO FPD Station with PV & solar hot water heat systems on roof • PV systems can pose a risk to firefighters; however, no deaths attributed to PV electrocution. Fraunhofer [18], CESA [19] PV SHWH Source: NREL 52

Mesa Verde PV Array: Case Study • 67 -k. Wp, 482 Vdc for NPS • Fire-protection specialist involved with commissioning • What steps should be taken? • Don’t climb on or touch arrays • Open all disconnects • Spray water around & under • 30 foot defensible perimeter • May have to allow fire to selfextinguish Source: NREL 53

Alternative Light Sources – Artificial Lights • Illuminated 1000 V array with common firefighting lights after dark • Depending on the test configuration o Voc ranged between 340 V and 836 V o Isc ranged between 1. 5 m. A & 212 m. A. Source: UL Hazardous voltages and currents may exist. UL [21] 54

Alternative Light Sources – Fire Light • Depending on the test configuration: o Voc ranged between 81% and 89% o Isc ranged between 52 m. A and 62 m. A. Hazardous voltages and currents may exist – if the array is damaged UL [21] Source: UL 55

Outline… • Basic PV array operating characteristics • PV module testing and standards • NEC and PV DC system safety • Workers’ exposure to DC system hazards • Arc flash hazard • PV DC system safety for emergency responders • PV DC accidents & injuries • Resources. 2018 ELECTRICAL SAFETY WORKSHOP 56

PV Shock Potential Solar systems come in two varieties in residential markets. 1. Grounded system a. Negative or positive wire connected to ground 2. Ungrounded system a. Solar module voltage floats from ground What does this mean?

Grounded PV Array – Shock Potential Ungrounded Conductor Fault 1 Grounded Conductor • During normal operation a person could be shocked (or a fault created) by contacting a single ungrounded conductor and earth ground. • Sophisticated monitoring is required to detect and protect against ground faults.

PV Systems and Electrical Fires • Fire can break out usually due to poor-quality components or installation. 3” conduit in a 383 -k. W array - B. Brooks & Solar ABCs [10] • It’s important to adhere to the entire NEC as well as manufacturers’ instructions. • Expansion fittings are required where necessary - NEC 300. 7(B). 59

IEC 61215 Current Testing Matrix (2016) Testing Matrix – Crystalline Silicon [7] 1 Module • Control 1 Module • Temperature Coefficient • Cell Temperature • Performance • Outdoor Exposure • Thermal Diode • Hot Spot Endurance Start of Test – All 8 Modules • Preconditioning • Visual Inspection • Maximum Power Determination • Insulation Testing • Wet Leakage Testing 2 Modules • Thermal Cycle 50 cycles (-40 ˚C to +85 ˚C) • Humidity Freeze 10 cycles (-40 ˚C to +85 ˚C) 85% RH • Robustness of Termination 2 Modules • Damp Heat 1000 hours (+85 ˚C/ 85 RH) • Wet Leakage Current • Mechanical Load • Hail Impact 2 Modules • Thermal Cycle 200 cycles (-40 ˚C to +85 ˚C) Source: NREL

PV DC System Safety and NEC • NEC safeguard persons and property from electrical hazards – shock & fire • PV systems must adhere to entire NEC in addition to Article 690 • 690 represents many years of experience from many PV experts • Arrays will produce power 20+ years • Systems should be professionally designed and installed. Source: NREL 61

PV System Inverters Micro inverters String inverters Central inverter Source: NREL 62

Sizing a PV String (number of modules in series) Typical Sizing Tool [4] (most string inverter manufacturers offer a sizing tool) Inverter Model Inverter 10 kw (240 VAC) String sizing dependent on: • Inverter parameters • Module parameters • Local weather. Module MFG 300 Watt Module In Denver, record cold was -25°C [5], string limited to 11 modules (Voc ~ 592 V). In Phoenix, record cold was +1°C, string limited to 12 modules (Voc ~ 596 V).