Photovoltaic System Overcurrent Protection

1 Photovoltaic System Overcurrent ProtectionIntroductionSolar Photovoltaic (PV) systems have, overthe last fifty years, evolved into a mature, sustainable and adaptive technology is improving as solar cellsincrease in efficiency and modules attainbetter aesthetic a result, solar power is gaining moreacceptance and is becoming an increasingly cost-effective and clean alternative to conventional energy installations and demand for PV systems increases so does the need for effective electrical systems, as with all electrical power systems.

2 Must have appropriateovercurrent Protection for equipment and Bussmann (the world leader inovercurrent Protection products) has developed a revolutionary new fuse link forprotecting Photovoltaic development was implementedthrough coordinated research and testingwith leading Solar Panel/Solar System Protection System from Cooper BussmannPhotovoltaic System Overcurrent ProtectionlAnumber of PV panels in series is termed a stringlAnumber of strings in parallel is termed an array2 2009 Cooper BussmannSolar Power generation systems are made ofPhotovoltaic cells and Power inverters.

3 The Photovoltaic cells utilise the power of sunlight toconverters photons to clean DC (Direct Current) Electricity generated by the Solar Cells is thenfed into a Power Inverter (PV inverter) that convertsand regulates the DC source into usable AC AC power can then be used locally for specificremote equipment, residential homes or fed directlyback into the power grid and used as environmentally clean content of sunlight: sunlight has an energycontent of 1kW (1000 watts) per square typical Solar Panel achieves between 10% and15% efficiency Solar Power Systems workThe voltage output of a Solar Panel/Array is defined bythe number of individual cells in series.

4 An individualpanel (see Fig. 1) is made up of a series string of Photovoltaic there is a push for utilizing higher voltages(trending to 1000 Vdc and above).lA number of PV panels in series is termed a stringlA number of strings in parallel is called an arrayThe vast majority of large Solar Farms in NorthAmerica are 600 Vdc, but following the lead fromEurope to increase voltages up to 1000 Vdc to achievemore Power Protection System from Cooper BussmannV+3 2009 Cooper BussmannFigure 1 Variations of Solar Panel OutputOvercurrent Protection of PV SystemsThe National Electrical Code defines the maximum circuit current as 125% ofthe short-circuit current of the PV module (Isc).

5 The conductors and the Overcurrent protective device are then sized at 125% of the maximum circuitcurrent or x Isc. Additionally, International standards such as BS EN7671 Sec 712 for Solar Photovoltaic (PV) Power Supply systems specify that conductors current carrying ability must be equal to or greater than xIscSTC* at any location. The Iscis published by the PV module manufacturerson datasheets. The Iscis typically only 110-115% of the maximum power current (Ipm) of the PV means that unlike typical grid connected AC systems, the available shortcircuit current is limited and the Overcurrent protective devices will need tooperate effectively on low levels of fault current.

6 For this reason CooperBussmann has conducted extensive research and development of fuses thatare specifically designed and tested to safely protect PV systems with highDC voltages and low fault currents. lDCM - 600 VdclPV - 1000 Vdc*IscSTC: The Electrical data applies under Standard Test Conditions (STC): Radiation 1000 W/m2with a spectrum of AM and at cell temperature of Fuse - 1000 VdcThe most widely used Solar Panels for systems greater than20kW are the 4 , 5 and 6 Poly-crystalline silicon Silicon type panel can achieve up to approx maximum power current per panel.

7 Again there is no specificpreference as economics also play a role in the selection ofSolar Cell word of caution is do not assume all 4 , 5 and 6 Solarpanel designs are equal between different manufacturers. The maximum power output current of the panels can vary asmuch as 35% between manufacturers of equal solar cell dimension always select proper conductors/fusesbased on the specific Isc* characteristics of the manufacturersspecification.*Isc: Short circuit current 4 2009 Cooper BussmannDCM - 600 VdcPhotovoltaic System Overcurrent ProtectionDepending on the desired capacity of the PV System , theremay be several PV strings connected in parallel to achievehigher currents and subsequently more power.

8 PV systems that have three or more strings connected in parallel need to have each string protected (systems thathave less than three strings will not generate enough faultcurrent to damage the conductors/equipment and therefore do not present a safety hazard as long as theconductor was sized properly based on local code requirements). Where three or more strings are connected in parallel a fuseon each string will protect the conductors from damage andhelp minimise any safety hazards. It will also isolate the faulted string so that the rest of the PV System can continueto generate of ConductorslIsolate damaged PV modulesSelecting Fuse Links for PV String Protection5 2009 Cooper BussmannDefine Solar Panel SpeclIpmlIsclMax System VoltageDefine number of parallelsolar panels/stringsN = parallel panels/stringDefine conductor/fuse size per x Isc= I conductor/fuse rating withstand @ 80oC(Panel Back plane temperature influence,ambient temp influence)Ipm:current at maximum powerIsc.

9 Short-circuit currentDoes array max Iscexceed continuouscurrent rating of conductor selected?Max Array Isc= (N-1) *IscPV String Fuse In= x IscSelect equal to or next higher std fuse rating PV String Fuse Vn=> Max System VoltageApply PV fuse to + and - StringsNo PV StringFusing requiredPV Fuse Selection Flow-ChartggggggYe sNoNoFuse Rating for PV ApplicationsOnce it has been determined that maximum short-circuit current ([N-1]*Isc) exceeds continuous current rating of conductor, follow the recommendations of selecting the proper PV string 1.

10 Solar Panel String FusingIsc= System Voltage = 1000 Vdc (max value of series panels)Conductor Size Formula = * Isc= * = Size = 14 AWG or @ 80oCN = 4 (4parallel Solar Panel Strings)Array Max Isc= (N-1) * Isc= (4-1) * = Max Iscis greater than conductor withstand, therefore string fuses are requiredIn= x Isc(individual panel only) = min fuse ratingSelect next higher std rating of 10A: PV-10A10 FFuse selected will protect selected conductorMin wire size: 14 AWG or @ 80oCModule DescriptionCell TypePolycrystalline SiliconCell Size125mm2(5")No of Cells and Connection72 in SeriesMaximum System Voltage1,000 VdcElectrical DataMaximum Power Voltage (Vpm) Circuit Voltage (Voc) Power Current (Ipm) Circuit Current (Isc) Solar Panel Specification6 2009 Cooper BussmannPhotovoltaic System Overcurrent Protection7 2009 Cooper BussmannDCM Fuses Technical Data1/10 - 30A/600 VdcStandards/Approvalsll UL Listed STD 248-14 (File E19180, Guide JDYX)