1

Designing and Installing Code-Compliant PV Systems

Presented by:

Bill Brooks, PEfor the

Nevada SolarGenerations Programfrom materials developed by

Endecon Engineeringand the

Florida Solar Energy Center

Course Acknowledgements

• This course is the present status of the ongoing development of materials from a variety of sources including:– Florida Solar Energy Center– Southwest Technology Development Institute– Sandia National Laboratories– National Renewable Energy Laboratories– PVUSA– Endecon Engineering– Arco/Siemens Solar

2

Course Disclaimers and Notices

• Opinions expressed in the presentations are those of the presenter and are not necessarily those of the SolarGenerations Program.

• No product endorsement is intended or implied.• Neither the SolarGenerations Program nor Bill Brooks are in any way

responsible for the decisions made by installers as a result of taking this training course.

• None of the materials in this course manual may be reproduced without the express written consent of Bill Brooks.

Course Objectives• This course is intended to focus the installer on the important aspects of

system design and installation so as to achieve predictable results in the field.

• The course will provide the attendee with a basic background in– photovoltaic (PV) fundamentals– National Electrical Code requirements and interpretation– key design issues related to PV systems– generally accepted installation practices based on many years

of field experience– available PV-specific products

3

Course Objectives (NOT)

• This course is NOT intended to:– Make participants PV system designers or PV experts– Guarantee that participants will automatically have good

experiences with products chosen as a result of taking this course

– Endorse or reject any product– Make decisions for participants on what products to use

Tales from the Crypt: The california buydown program

4

Field Performance Issues

• Design Issues—Lack of Experience

• Installation Issues—Need Checkout procedures

• Equipment Issues—Need better equipment setup documentation

• Performance Estimation Issues—Lack of consistency and understanding of how to calculate performance—need meters!

Conclusions from California

• Early system performance is not encouraging.• Training is essential to quality design and installation.• Follow-up is necessary especially for new contractors.• Metering is required to provide feedback to the system

owner.

5

Photovoltaic System Fundamentals

Some Benefits of Solar Electricity

Energy independenceEnvironmentally friendly“Fuel” is already delivered free everywhereMinimal maintenanceMaximum reliabilityReduce vulnerability to power lossSystems are easily expanded

6

Difference between PV and Thermal

• Photovoltaic (photo = light; voltaic = produces voltage) or PV systems convert light directly into electricity using semi-conductor technology. (@ 10% efficiency)

• Thermal systems (hot water, pool heaters) produce heat from the sun’s radiation(@ +40 % efficiency)

• Large difference in value of energy types.

What Are Solar Cells?• Thin wafers of silicon

– Similar to computer chips– much bigger – much cheaper!

• Silicon is abundant (sand)

– Non-toxic, safe• Light carries energy into cell• Cells convert sunlight energy into electric current- they do not

store energy• Sunlight is the “fuel”

7

How Solar Cells Change Sunlight Into Electricity• Light knocks loose electrons from

silicon atoms• Freed electrons have extra

energy, or “voltage”• Internal electric field pushes

electrons to top of the cell• Electric current flows on to other

cells or to the load• Cells never “run out” of electrons

Definitions: PV Cell

• Cell: The basic photovoltaic device that is the building block for PV modules.

8

Connect Cells To Make Modules• One silicon solar cell produces 0.5 volt• 36 cells connected together have enough voltage to

charge 12 volt batteries and run pumps and motors• 72-cell modules are the new standard for grid-

connected systems having a nominal voltage of 24-Volts and operating at about 30 Volts.

• Module is the basic building block of systems• Can connect modules together to get any power

configuration

Definitions: PV Module

• Module: A group of PV cells connected in series and/or parallel and encapsulated in an environmentally protective laminate.

BP Solar MSX6060 watt polycrystalline

Shell SP7575 watt single crystal

9

Definitions: Encapsulation

• Encapsulation: The method in which PV cells are protected from the environment, typically laminated between a glass superstrate and EVA substrate.

• Newer light weight flexible laminates use a polymer superstrate and a thin aluminum or stainless steel substrate.

Definitions: Junction Box• Module Junction Box: An enclosed terminal block on

the back of PV modules which allows the module to be connected in the electrical system. Newer modules often replace the junction box and use only plug-and-receptacle connectors

10

Definitions: PV Panel• Panel: A group of modules that is the basic building

block of a PV array.

Definitions: PV Array• Array: A group of panels that comprises the

complete direct current PV generating unit.

11

Definitions: Balance of System (BOS)

• BOS: The balance of the equipment necessary to integrate the PV arraywith the site load (building). This includes the array circuit wiring, fusing, disconnects, and power processing equipment (inverter).

Handling requirements of PV modules

• Most PV modules have a tempered glass cover, an anodized aluminum frame, and a Tedlar back cover.

• Amorphous silicon modules are made of annealed glass and can be either framed or unframed. (unframed annealed glass is very susceptible to breakage).

• All PV modules must be handled carefully.

12

Review Basics of Energy and Power

• Voltage– Like water pressure– Measured in volts

• Current– Like flow rate (like gallons per minute)– Measured in amperes or amps

Review of Power

• Power– Rate of flow X pressure of the flow– Measured in watts– Power = Current X Voltage– More pressure or more flow means more power– Is a RATE of doing work, not an amount of work

13

Review of Energy• Energy

– The total AMOUNT of work done by the pressurized flow over a period of time

– Power X time gives an amount of work generated (by the modules) or consumed (by the loads)

– Is an AMOUNT, not a rate– Measured in watt-hours (or kilowatt-hours, kWh)– Energy = Power X Time = Current X Voltage X Time

Review of Energy and Power

• So Energy is the AMOUNT you have to work with

• And Power is the RATE that you work with it

14

Fundamentals of Solar Energy

Objectives

• Understand the relationships between irradiance, and insolation.

• Explain the effects of the earth’s motion on the solar energy received at a given location.

• Describe the use of insolation data in determining photovoltaic systems performance.

15

Annual Sun Position

Sun Paths for Nevada Latitudes

North

PointO

East

West

Zenith

Ju ne 22 12 noo n

March 21 and September 23: 12 noo n

Ju ne 22 12 noo n

10 am

8 am2 pm

4 pm

4 pm

4 pm

8 am

8 am2 pm

2 pm

10 am

10 am

South

December 21: 12 Noon

16

Sun Path Calculator

Solar Irradiance

• Radiant power per unit area• Units:, Watts/m2, or kW/m2 or mW/cm2

• Peak value: 1000 Watts/m2, or 1 kW/m2 (100 mW/cm2)• Nominal value: 800 Watts/m2 , or 0.8 kW/m2 (roughly

half of PV energy is delivered below and half above this value)

17

Solar Constant• The irradiance on a surface normal to

the sun’s rays immediately outside the earth’s atmosphere.

• Solar constant = 1.36 kW/m2

Solar Irradiation

• Often called Insolation• Radiant energy per unit area• Irradiance integrated over time• Units: kWh/(m2-day)

18

Solar Insolation(Peak Sun Hours)

Time of Day

Sola

r Ir

radi

ance

(W/m

2 )

1000 W/m2 peak sun hours

Reno Annual Irradiation

0

1

2

3

4

5

6

7

8

9

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Sola

r Ene

rgy

(kW

h/m

^2/d

ay)

Horiz Lat-15 Lat Lat+15 Vert

19

Las Vegas Annual Irradiation

0

1

2

3

4

5

6

7

8

9

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Sola

r Ene

rgy

(kW

h/m

^2/d

ay)

Horiz Lat-15 Lat Lat+15 Vert

Magnetic Declination

20

PV Performance Parameters– Open-circuit voltage (Voc)– Short-circuit current (Isc)– Maximum power voltage (Vmp)– Maximum power current (Imp)– Maximum power (Pmp)– Fill factor (ff = Pmp /(Voc*Isc))

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.2 0.4 0.6 0.8 1 1.2 1.4Voltage

Curr

ent,

Pow

er

I P

FF = 0.5580

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.2 0.4 0.6 0.8 1 1.2 1.4Voltage

Cur

rent

, Pow

er

I P

FF = 0.6920

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.2 0.4 0.6 0.8 1 1.2 1.4Voltage

Cur

rent

, Pow

er

I P

FF = 0.815

Cur

rent

(A)

Voltage (V)Voc

Isc

Vmp

ImpPmpx

Typical PV Module Label

21

Current varies with irradianceSiemens Solar Module SP75

Performance at Different Irradiances

0

1

2

3

4

5

6

0 5 10 15 20 25

Voltage (volts)

Cur

rent

(am

ps) 1000 W/m2, 25 oC

800 W/m2, 25 oC600 W/m2, 25 oC400 W/m2, 25 oC200 W/m2, 25 oC

Voltage varies with temperatureSiemens Solar Module SP75

Performance at Different Cell Temperatures

0

1

2

3

4

5

6

0 5 10 15 20 25

Voltage (volts)

Cur

rent

(am

ps)

1000 W/m2, 0 oC1000 W/m2, 25 oC1000 W/m2, 45 oC1000 W/m2, 60 oC

22

Performance Calculations

The Need For Simple Energy Estimates• System sizing often based on module STC• Insolation variability and several important

loss factors often ignored• Tendency toward overpredicting energy

performance (by 20-50%)• Field engineering estimates within 10-15%

are possible without a computer

23

The Need For Simple Energy Estimates

• Dealers and installers need a simple field estimating tool that does not rely on a computer.

• Customers need immediate feedback when looking at array mounting options.

This Method Defined• Starting point is module rating (STC or PTC or

any defined condition)• A series of performance adjustment multipliers

are factored in as appropriate to arrive at long-term energy estimate

• Individual factors that exceed 4% were separately identified. Remaining factors were lumped into more inclusive categories

24

PV System Performance• Energy Production from PV system is a function

of several factors including:– Temperature (efficiency)– System configuration (battery,

non-battery)– Solar Resource – Orientation– Soiling– Shading

Temperature (efficiency):• Module performance only has meaning when

the rating conditions are specified:

– Standard Test Conditions (STC)– Nominal Operating Cell Temperature (NOCT) – PVUSA Test Conditions (PTC)

25

Standard Test Conditions (STC)

• Irradiance: 1,000 W/m2

• Cell temperature: 25o C• Air mass: 1.5• ASTM standard spectrum

Nominal Operating Cell Temperature (NOCT)• Irradiance: 800 W/m2

• Ambient Temp: 20o C• PV Array: open circuit• Wind Speed: 1.0 m/s (calm)

• Sensitivity analysis showed that annual module temperatures were close to NOCT for most locales, and only small errors were incurred for extreme climates (Miami, FL, and Intl.Falls, MN).

26

PVUSA Test Conditions (PTC)• Irradiance: 1000 W/m2

• Ambient Temp: 20oC (not cell temp.—typical cell temp. is 50oC under these conditions)

• PV Array: Max Power• Wind Speed: 1.0 m/s (calm)

System Rating--PVUSA Test Conditions (PTC)• The average annual efficiency is well represented

by the module temperatures rendered by PTC, NOCT, and INOCT

• STC is not useful for energy or power ratings since it poorly represents efficiency of the PV module.

• Shell SQ-150 is 132.5 Watts dc at PTC conditions. WARNING: THIS IS NOT A SYSTEM RATING—this is based strictly on manufacturers information

27

Solar Resource:Data - How to find

• Resource information from the National Renewable Energy Laboratory online or order hardcopy: http://rredc.nrel.gov/solar/pubs/redbook/

• To determine average solar radiation for your system, take the average number for collectors facing south at latitude tilt from the solar insolation table. (orientation dealt with later)(e.g. 6.5 kWh/m2/day for Vegas and 5.8 kWh/m2/day for Reno)

Array Orientation-Optimum Angle

• Application Angle Best Fixed Array

• Maximum Energy Normally Latitude Production (but local climatic (net metering) variations can change

this--California=30) • Winter Peak Load Latitude plus 15

(example: off grid)• Summer Peak Load Latitude minus 15

(time-of-use net metering) (Nevada resource is same asLatitude tilt)

28

Proper Module Rating

• The Shell SQ-150-PC is rated at 132.5 Watts dc at normal operating conditions.

• Must also account for overrating by manufacturer.– Manufacturers typically have either a +/-10%

or +/-5% tolerance. Always assume the low end of the range.

• 0.88 x 0.95 = 0.84 Module Correction Factor• 150-Watt module delivers 126-Watts dc

System Configuration – battery or non-battery• Wiring and max power tracking losses account

for roughly 7% losses high voltage PV systems. (factor of 0.93)

• Battery-based systems also have similar wiring losses, but they also have charge controller losses and require a small trickle charge to maintain float voltage for a total of 15% losses (factor of 0.85)

29

Inverter Efficiency• Annual energy-weighted inverter efficiency ranges from

85% to 90% (use 85% for older battery-based systems and 90% for batteryless systems)– Covers the fact that inverter has an efficiency curve and

is not always operating at peak efficiency)

TOTAL SYSTEM LOSSES:(module x system config. x inverter)

• Non-battery systems -- (0.84 x 0.93 x 0.90 = 0.70)• Typ. battery systems -- (0.84 x 0.85 x 0.85 = 0.61)

Orientation Factor

For Annual Energy Productionin California (Nevada similar-not exact)

ROOF PITCHFACING Flat 4:12 7:12 12:12 21:12 Vert.South 0.89 0.97 1.00 0.97 0.89 0.58SSE,SSW 0.89 0.97 0.99 0.96 0.88 0.59SE, SW 0.89 0.95 0.96 0.93 0.85 0.60ESE,WSW 0.89 0.92 0.91 0.87 0.79 0.57E, W 0.89 0.88 0.84 0.78 0.70 0.52

30

Soiling:• Three basic categories for

Rainy/Dry Season Areas– Washed as often as necessary - 1.0– Washed once in July – 0.96– Never Washed - 0.93

• Factors affecting number– Rainy/Dry Seasons– Dirt roads– Near agricultural activity– Close to road surface of busy street – In airport flight path

Monthly Soiling Loss Profile

-25

-20

-15

-10

-5

0

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Soili

ng L

oss,

%

Wet (-4%) Normal (-7%) Dry (-11%) Weighted (-7%)

31

Shading Factor• Shading has a significant impact on PV system

operation.• Electrical layout of PV modules can reduce or

enhance the impact of shading.• Goal is to be free of shade from 9:00 a.m. to

3:00 p.m. every day of the year (10-4 in summer)

• Solar Pathfinder™, or similar device, is critical to get ballpark value on shading.

Don’t do this!

32

Is This Method Good Enough?• So far, method has been compared with a variety

of real systems with good agreement.• Solar resource data uncertainty

(+/-9%) is as large or larger than the uncertainty associated with this method suggesting that this simple method is adequate for most design purposes.

Photovoltaic Array Electrical Design

33

Objectives• Describe basic PV design principles• Describe series and parallel wiring strategies for PV

modules and arrays.• Explain the function and location of protection diodes

in a PV electrical system.• Identify the appropriate ratings and locations for

overcurrent protection and disconnect devices in PV systems.

Initial Design Philosophy

• A choice must be made between lower voltage and higher voltage options

• These two classes of inverters have some definite advantages and disadvantages that must be reviewed in the context of the specific application.

34

System Comparison• Lower Voltage benefits

– Somewhat lower risk of shock or fire.

– Can be more tolerant of shading because array is in smaller segments

– Allows installation on smaller roof sections

– Allows for battery-backup systems

• High Voltage benefits– Less wiring (more

modules per series string-fewer circuits smaller conduit)

– Lower voltage drops (smaller wire—possibly more efficient inverters)

Photovoltaic Array Wiring• First, build series strings to obtain desired array output

voltage.• Next, Connect series strings in parallel to obtain desired

output power.– Example--2,400 Watt, 48-volt system using 24-volt, 150-

Watt(STC) modules:• 2 modules in series to reach proper voltage. (2 x 150W

= 300-watts per string)• 2,400W/300W = 8 series strings (in parallel)• Total modules 2 x 8 = 16, 150-Watt modules

35

Building a PV Array

Voltage (V)

Cur

rent

(A)

# of Modules in Series for High Voltage Inverters

• First, determine overall voltage limits of inverter.• Next, determine voltage limits of max power tracking

for inverter.• Next, determine maximum number of modules

allowed in series.• Lastly, check to make sure inverter can max power

track voltage at hottest array temperature

36

Example• SMA Sunny Boy SB2500U Inverter:

– Max DC voltage—600V– Tracking voltage range 250V – 550V DC

• Max number of modules in series– Voc = 43V, max voltage per module = 43V x 1.25 (1.17 for Vegas)

= 53.75 V; 600V/53.75V = 11 modules• Module voltage at hottest condition

– Vmp = 27V (from graphs in literature-typical value); 250V/27V = 9 modules

• Maximum range of “24V” modules--9 to 11; recommendation from manufacturer—10 or 11

DC Over-voltage

• Power electronics limitations• Causes

– Poor array design– Poor choice of inverter

Inverter VdcRange

Trouble

37

DC Under-voltage• Design issue for selecting inverter• Voltage limits may be a problem with

amorphous silicon modules• Causes

– Poor choice of inverter– Degradation of array– Excessive temperature or

unexpectedly large Voc temperature sensitivity

• Consequence: Lost energy

Max Power

Protection Diodes

• Diodes are semiconductor devices that allow current to flow in only one direction.

• Analogous to check valves in plumbing systems.• The two uses of diodes in PV system electrical

design are:– Blocking diodes– Bypass diodes

38

Protection Diode Selection

• Silicon diodes have 0.7 volt drop.• Germanium and Shottky diodes have less than 0.3

volt drop.• Rated for current and voltage.• May require heat-sinking.

Blocking Diodes• Placed in series with a module or "string" of

modules to prevent reverse current flow.– As a result of faults being fed by paralleled PV

source circuits.– As a result of reverse power being fed from grid

or battery-tied malfunctioning inverter.• Conduct current during normal operation.• Typically only used for amorphous silicon systems

operating above 100 Volts dc.• Becoming less common

39

Blocking Diode Sizing

• Size for 1.5 times the rated array source circuit Isc

• Size for 1.5 times the rated array source circuit Voc.

Blocking Diode

module or series stringmodule or series string

PVPV

blocking blocking diodediode

40

Bypass Diodes

• Also called "shunt" diodes, used to pass current around, rather than through a group of cells or modules.

• Permit the power produced by other parts of the array to pass around groups of cells that develop an open-circuit or high resistance condition.

• Installed in all UL-listed crystalline modules

Bypass Diode Selection

• One diode for 9-24 series-connected cells recommended. Most 36-cell crystalline modules have 2 bypass diodes. Many 72-cell modules have 3 bypass diodes

• Size for 1.5 times the module Isc rating.• Size for greater than 1.5 times the Voc of cells

bypassed.

41

Bypass Diode Location

PVPV

series connectedseries connectedcells or modulecells or module

bypassbypassdiodediode

ASE Module Shading TestsShading Along Module Horizontal Dimension (Width)

0

1

2

3

4

5

6

7

0 10 20 30 40 50 60

Voltage (V)

Cur

rent

(A) Wid th Shad ed (2 rows o f cells )

Wid th Shad ed (1 row o f cells )Wid th Shad ed (3 /4 ro w o f cells )Wid th Shad ed (1 /2 ro w o f cells )Wid th Shad ed (1 /4 ro w o f cells )No Shad ing

42

ASE Module Shading TestsShading Along Module Vertical Dimension (Length)

0

1

2

3

4

5

6

7

0 10 20 30 40 50 60

Voltage (V)

Cur

rent

(A)

Leng th Shad ed (4 ro ws o f cells )Leng th Shad ed (3 ro ws o f cells )Leng th Shad ed (2 ro ws o f cells )Leng th Shad ed (1 -1 /2 rows o f cells )Leng th Shad ed (1 ro w o f cells )Leng th Shad ed (3 /4 ro w o f cells )Leng th Shad ed (1 /2 ro w o f cells )Leng th Shad ed (1 /4 ro w o f cells )No Shad ing (full o utp ut)

Overcurrent Protection and Disconnects

43

Overcurrent Protection

• Used to protect conductors from creating a fire hazard under circuit overload conditions.

• Options include fuses and circuit breakers. • Use devices with appropriate dc ratings. [NEC 690.9(D)]

Overcurrent Protection• Size string fuses or circuit breakers according to UL

max series fuse listing-- should be at least 1.56 times maximum current (Isc) (to protect PV module). [NEC 690.8]

• Size fuse holders for maximum voltage (Voc) (depends on Temp.). [NEC 690.7]

44

Module Interconnect Methods

Individualcircuits tocombiner

Unacceptable Acceptable

daisy chain

PV Combiner Boxes

45

PV Circuit Conductors• Minimum size according to series fuse rating.• May be larger size due to voltage drop issues.

Inverter Circuit Conductors

• Inverter may draw sizable dc currents if connected to a battery bank.

• Conductors must be rated to handle 125% of the maximum full load inverter dc current.

46

Disconnect Requirements• Required to isolate major system components, e.g., the

PV array, inverter, battery, and load.• Switches or circuit breakers may be used. Use

appropriate dc rated components.• Maximum of six (6) disconnect devices are allowed,

must be marked and labeled.

PV System Electrical Design—Review

• Explained the function and location of protection diodes in a PV electrical system.

• Discussed appropriate ratings and locations for overcurrent protection and disconnect devices in PV systems.

47

Photovoltaic ArrayMechanical Design

Objectives

• Assess system structural loads.• Compare alternative roof mounting

techniques.• Produce a mechanical design that is safe and

appropriate for the site and application.

48

Array Mounting System Design• Minimize installation costs.• Maximize array performance.• Provide accessibility for maintenance.• Meet local code requirements.• Make aesthetically pleasing.

Minimizing Array Installation Costs

• Use pre-engineered designs that incorporate plug connectors to connect modules together.

• Pre-assemble modules into panels.• Pre-wire modules in each panel. (Check modules

visually and Voc & Isc before hauling onto the roof)

• Minimize the number of attachment points and roof penetrations.

49

Maximize Array Performance

• Find location that has a minimum of shade while being closest to a southern orientation (must use Solar Pathfinder)

• NO INTERROW SHADING• ALL modules in a series string MUST be in the

SAME orientation• Keep array as cool as possible.

Inter-row shading—Big Problem

50

Example of series string with multiple orientations-WRONG!!

Array Cooling

• Improves efficiency• Increases reliability• Extends life of solar array

51

Installed Nominal Operating Cell Temperature• Rack mount INOCT = NOCT - 3 ° C• Direct mount INOCT = NOCT + 18 ° C• Standoff/Integral INOCT = NOCT + X

W (inches) X(°C) typical* INOCT @Tamb of 45°C 1 11 56°C 81 °C3 2 47°C 72 °C6 -1 44°C 69 °C

Rack mount 42°C 67 °CDirect mount 63 °C 88 °C

where W is standoff, entrance or exit height or width, whichever is minimum. Add 4°C if channeled. (Reference SAND85-0330)

* Typical NOCT is around 45°C for Tamb of 20°C

Array Orientation• Orientation is specified by relative angle from south

and tilt angles.• Array orientation may be fixed, adjustable, or sun

tracking.• Selection of array orientation should be based on

location, application, type of array, type of system and cost. (generally roof-mounted systems governed by roof orientations)

52

Array Mounting Configurations

• Ground-mounted (rack, shade structure, pole, or tracking)• Roof-mounted (rack, integral, or standoff)

Standoff-Mounted Arrays

• Above and parallel to roof slope• Allows for clearing of debris • Promotes array cooling by allowing air to circulate beneath the

array• Reduces heat gain into buildings by separation from roof• Provides underside access on roof top for diagnostics and

maintenance• Can be used effectively for new construction or retrofit on

existing homes• Recommended

53

Standoff-Mounted Arrays

Rack-Mounted Arrays

54

The California/Nevada Patio Cover

55

Integrally-Mounted Arrays

• Often referred to as Building-Integrated PV or BIPV• Replaces conventional roofing materials• Labor intensive• Dimensional tolerances important• Increases operating temperature• Amorphous modules may perform okay in these

applications

Roof-Integrated PV Products

56

Where are the PV modules?

Photo courtesy of USSC

Photo courtesy of USSC

Solar Electric Metal Roofing

57

“Peel and Stick” solution from UniSolar

Roof Slates

58

GE Energy Gecko

Photos courtesy of GE Energy

Primary Structural Concerns for the Building Official

• Mounting structurally sound• Roof properly weather proofed

59

Structural Loads on PV Arrays

• Dead loads 3 - 8 psf• Wind loads 25 - 55 psf

(corresponds to 85 mph to 120 mph winds)• Snow loads 15 psf (Sierra’s can

require 10-20 times typical snow loads)

Mounting Structurally Sound• Two main types of loading to consider

– dead load (typically 3-5 lbs./ft2 for standard standoff install and 2-12 lbs./ft2 for integrated products)

– wind load (25 lbs./ft2 maximum in California (higher in select mountain regions)

• Structure must be capable of supporting dead load and module attachment method must be capable of withstanding the wind load trying to lift the PV array from the roof (or relevant structure).

60

Mounting Structurally Sound (cont.)

• Most modern truss roofs are capable of handling the extra 3-5 lbs./ft2 dead load provided that the roof is not masonry. (similar to additional layer of comp shingles

• Masonry roofs may require a structural analysis to add weight. An alternative is to remove the existing product and replace it with comp shingles in the area of PV array.

• Attachment method must be capable of keeping the PV array on the structure.

Attachment Method:Lag Screw Mounting

• Penetration depth: (7 to 10) x (screw diameters)• Pilot hole diameter : (½ to ¾) x (shank diameter)

Example: 5/16” lag screwDrill 3/16” (i.e., 3/5 x 5/16”) pilot hole 2.8” (i.e., 9 x 5/16”)into support member.

61

Fastener Withdraw Loads-Wood (conservative)

Allowable Withdraw (lb./in)Screw Size Redwood Spruce Doug Fir#8 64 72 87#10 74 100 119#14 (1/4") 94 113 152#18 (5/16") 114 154 194#20 (3/8") 124 168 207

Sample Structural Calculations

• 2kW system• 2 x 4 truss roof on 24” centers with composite shingle

roof (acceptable for 5 psf added weight)• 18 modules @ 12 ft2 per module = 216 ft2.• 24 mounting brackets• 3” fastener depth in Doug Fir• 25 lb/ft2 uplift force

62

Sample Calculations

• 216 ft2 x 25 lb/ft2 = 5,400 lb (total uplift)

• 5,400 lb /24 brackets = 225 lb per mount

• 194 lb/in x 3 in depth = 582 lb (more than twice as strong as necessary)

PV System Mechanical Design: Review

• Structural loads and code compliance• Ease of installation and access• Array output and thermal performance

63

PV Inverter Fundamentals

Inverter Basics• Convert battery or PV array DC power to AC power

for use with conventional utility-powered appliances.• Inverters can be motor-generator (not discussed

further here) or (more commonly) electronic types.• Vary in utility interaction, power ratings, efficiency,

and performance.

64

Overview• Why: Need ac power from dc source• How: Power electronics, supervisory control• Where: In the shade, if possible• Who: Nobody, except you when it is broken!• And...

Overview (continued)• What:

– PCU: Power Conditioning Unit– Inverter: Power electronics and controls

PV Array PCU

Utility/ Standalone

Grid

Inverter

DC Disconnects

AC Disconnects

Transformers

Batteries

65

Common AC Waveformssquare wavesquare wave

sine wavesine wavequasiquasi--sine wavesine wave

TimeTime

Am

plitu

deA

mpl

itude

One CycleOne Cycle

Standalone Sources

• Inverter creates voltage waveform• Load current demands may create phase shifted or

non-sinusoidal current waveform• Examples

– Motor current lags voltage– Computers demand current at peaks of voltage

waveform

66

Grid-Connected Sources

• Grid creates voltage waveform– Inverter unlikely to distort voltage unless there is a

poor connection• Inverter behaves like a negative load, feeding current

back into grid• Nearby loads respond to grid voltage, not inverter

current

Power Quality• Distortion

– IEEE limits utility-interconnected inverters to 5% current distortion

– Good Utility Grid Voltage < 3% voltage distortion• Radio Frequency Interference

– FCC classifies equipment for allowable radiation of high frequencies from electronic devices. Inverter should be tested to meet FCC regulations. (Class B-Part 15)

67

Effects of Harmonic Distortion

• Some electronic equipment may not work– Sensitive to extra zero crossings

(clocks may run fast)• Not useful power

– Many devices cannot utilize harmonic power

– Excessive heating in motors and transformers

Inverter Classifications• Stand-Alone Inverters: Operate from batteries,

independent of the electric utility. Can provide control/protection functions for hybrids.

• Utility-Interactive or Grid-Connected Inverters: Operate only in conjunction with the electric utility, synchronizing theoutput phase, frequency and voltage with the utility. Directly connected to the PV array.

• Utility-Interactive with Backup Power Mode: Can operate in conjunction with utility but provide backup power if utility fails.

68

Inverter Characteristics

• Voltage ratings• Power and surge ratings• Efficiency

Voltage Ratings• Limited choice (wide input range preferred)• Low voltages imply higher currents and higher losses• Battery systems

– e.g. 48 Vdc nominal• Grid-tied systems

– e.g. 350 Vdc nominal

69

Power Rating• Derived from device current/temp. limits• Surge

– Can handle higher power output for short periods as specified by manufacturer (backup/standalone power systems)

• Power Factor– Derate backup power inverter for low power factor

loads (Inverter output capability based on Volt-Amps not Watts)

Inverter DC Input Voltage• For battery-based systems, nominal battery voltage

dictates the input voltage ratings for the inverter.• Higher output power levels require higher DC input

voltage specification to reduce input currents.

70

Inverter Sizing (UPS loads)• Size inverter to supply expected continuous load

demand.• Sum all AC loads which may operate from inverter at

same time.• Surge ratings must be capable of starting motor loads

such as pumps, compressors and other machinery.• Anticipate growth of load in inverter size selection.

Power Conversion Efficiency• Efficiency = Pac/Pdc• High Efficiency…(peak>94%)

– Avoids inverter overheating– Most power per module $

• Lower efficiency (peak>91%) may allow wider input voltage range or better maximum PV power tracking

71

More About Efficiency• Some inverters work best

at low load– E.g. Xantrex SW5548

• Others work best at mid-range power– E.g. SMA SB2500U

• ...or full-range power– E.g. Xantrex PV20208

Power

Effic

ienc

y

Normal Operation

• How PV Array I-V curves change over time• Automatic sequencing• Manual adjustments• Maximum Power Tracking

72

PV Array I-V Curves Variation• Envision curve shape sliding

up/down with light level (irradiance)

• Isc changes quickly w/ irrad.• Voc changes little above low light

levels• Shape (thus Voc) changes a little

with temperature• Shape also changes with

degradation, shading, or faults in array

I

V

Batteryless Sequencing• Morning Wakeup (Turn On)

– Identify available power (typ. Voc>Vstart)– Identify grid power availability (V and F within

limits)– Safely sync inverter with grid or generator

• Operation– Maximum power tracking

• Shutdown– Saves energy (should disconnect transformer)

73

Manual Adjustments• Battery Float Voltage Setting (temperature

compensated)• Grid settings (only with utility approval for

large units)– O/U Voltage & Frequency limits

• Manual Peak-Power Adjustment (testing-rarely available on small units)

Maximum Power Tracking• Typically performed by

perturbing operating point– Vary voltage, current, or

effective resistance (PWM rate); the other two will change in response

– Timing (200ms-2 minute period, depending on design)

– MPT tracking errors are one common reason for poor system performance

VIP

74

MPT with Batteries

• MPT requires varying the PV voltage

• Batteries require voltage levels for charging and discharging

• MPT dc/dc converter separates voltages

PV

MPT

Batteries

Inverter

Backed-up Loads Utility

Grid Problems• Inverter should shut down when grid power goes out

of frequency or voltage limits– Avoid dumping power into a short– Avoid Islanding (Inverter powering loads on

disconnected local portion of grid)• Safety hazard for working on lines

• Inverter should restart after grid is ok!(5 minute wait according to IEEE 929)

75

Other Grid Problems• Bad connections = Bad Power Quality

– Current distortion encounters high resistance, yields voltage distortion

– Voltage distortion affects nearby loads• Inductive Loading on Feeder

– High Currents → Voltage Drop → Raised Voltage at Substation

– Load/PF Correction Capacitor Switching → Wide Voltage Variations

Sample Grid-Tie Only Inverters• Review of characteristics of some residential grid-

tied inverters– Fronius IG 2000, IG 3000– PV Powered PVP1100, PVP1800, PVP 2800– SMA Sunny Boy 2500, 1800, 1100, 700– Sharp SunVista JH-3500U– Xantrex/Trace Engineering Sun-Tie XR

76

Fronius IG 2000, IG 3000• 1.8 and 2.5 kW ac rating• 150-450V dc input• 240Vac • 94% eff. at 75% load• Utility-interactive only• IGBT-type inverter• Forced air cooling• Reverse polarity protected• Built-in approved meter

PV Powered• 1.1, 1.8, and 2.8kW ac rating• 1100W—90-180Vdc input• 1800W—120-360Vdc input• 2800W—200-450V dc input• 208 or 240Vac for 2800• 120Vac for 1100 and 1800• 93-94% eff. at 75% load (higher claimed)• Utility-interactive only• IGBT-type inverter• Reverse polarity protected• Passive air cooling

77

SMA Sunny Boy• 0.7, 1.1, 1.8, and 2.5kW ac rating• 700 W—100-250Vdc input• 1100W—129-400Vdc input• 1800W—139-400Vdc input• 2500W—250-550V dc input• 240Vac for 1100 and 2500• 120Vac for 700 and 1800• 93-94% eff. at 75% load• Utility-interactive only• IGBT-type inverter• Reverse polarity protected• Passive air cooling• Meter optional

Sharp SunVista JH-3500U • 3.5 kW ac maximum output

(3kW rating) • 160-380 Volt input voltage• Allows up to 3 different input

voltages on one system • 240Vac single-phase output• 92% eff. @ 75% load• Utility-interactive only• Forced air cooling• Meter for house included

78

Xantrex/Trace Engr. Sun-Tie XR• 1.5, 2.5kW models • 42-85 Volt input voltage• 240Vac single-phase output• 89-90% eff. @ 75% load• Utility-interactive only• FET-type inverter• Forced air cooling• Built-in meter• Built-in combiner and disconnects

Sample Grid-Tie Only Inverters

• Review of characteristics of some commercial grid-tied inverters– Ballard Power Systems EPC-PV-480-75KW– SMA Sunny Central 125– SatCon Power Systems AE-100-60-PV-A – Xantrex/Trace Technologies PV-xx208

79

Ballard EPC-PV-480-75KW

• 75 kW ac rating• 95% efficiency @ 75% Load

(w/ xfmr)• 330-600V dc input• 208V or 480V 3φ ac output• Utility-interactive• IGBT-based• Forced liquid cooling

SMA Sunny Central 125

• 125 kW ac rating• 94% efficiency @ 75% Load

(w/ xfmr)• 275-600V dc input• 480V 3φ ac output• Utility-interactive• IGBT-based• Forced air cooling

80

• 100 kW ac rating• 95% efficiency @ 75% Load

(w/ xfmr)• 330-600V dc input• 480V 3φ ac output• Utility-interactive• IGBT-based• Forced air cooling• Fused disconnect for 6 PV

subarrays

SatCon Powergate AE-100-60-PV

Xantrex PV-XX208• 5, 10, 15, 20, 30, 45, 100, and

225 kW ac ratings• 96% efficiency @ 75% Load

(xfmr not counted)• 300-600V dc input• 208V 3φ ac output• Utility-interactive• IGBT-based• Forced air cooling

81

Sample Grid-Tie Inverters w/ backup capabilities

• Review of characteristics of some grid-tied inverters with backup capabilities

– Beacon Power M-5– Outback Power FX2548, VFX3648– Xantrex/Trace Engineering SW

Beacon Power M-5• 5 kVA ac rating, 7 kVA surge

capability• 42-62 V dc input• 90% eff.@ 75% load• 120Vac single-phase• FET-type inverter• Integral charge controller

– Max power tracking• Forced air cooling

82

Outback Power FX2548, VFX3648• 2.5 and 3.6 kVA, 6 kVA surge

capability• 40-66 Vdc input• 90% eff.@ 75% load• 120Vac single-phase• FET-type inverter• Forced air cooling on VFX,

passive cooling on FX

Trace 5548 Power Module

• 5.5kVA ac rating, 10 kVA surge capability

• 44-60V dc input• 89% eff.@ 75% load• 120(240)Vac single-phase• FET-type inverter• Batteries and controls all

in the same cabinet• Forced air cooling

83

Inverter Review • Match array to inverter limitations

– Be aware of voltage limits of inverter and how they interact with the array.

• Think about shading, temperature, and degradation when selecting inverter

• Be cautious selecting inverter for a-Si since Vmp is much less than Voc.

• Still the weak link in reliability– Power electronics failures– Software malfunctions

List of key web resources• www.solargenerations.com• www.californiasolarcenter.org• www.consumerenergycenter.org/erprebate/index.html• www.consumerenergycenter.org/renewable/estimator/• System suppliers:

– www.kyocerasolar.com/products/mygen.htm– www.rweschottsolar.com– solar.sharpusa.com– www.bpsolar.com– www.shellsolar.com– www.solardepot.com

84

Utility Interconnection Issues

Utility Interconnection Issues

• Personnel safety• Equipment protection• Service reliability• Power quality

85

Utility Interconnection Requirements

• IEEE standard 929• UL standard 1741• Utility practice and requirements• Public utility commission statutes

P929Recommended Practice forUtility Interface ofPhotovoltaic (PV) Systems

Prepared by the Utility Working Group ofStandards Coordinating Committee 21, on Photovoltaics

Copyright © 1998 by the Institute of Electrical and Electronic Engineers, Inc.345 East 47th StreetNew York, NY 10017, USAAll Rights Reserved

This is an IEEE Standards Project, subject to change. Permission is hereby granted forIEEE Standards committee participants to reproduce this document for purposes ofIEEE standardization activities, including balloting and coordination. If this documentis to be submitted to ISO or IEC, notification shall be given to the IEEE CopyrightsAdministrator. Permission is also granted for member bodies and technical committeesof ISO and IEC to reproduce this document for purposes of developing a nationalposition. Other entities seeking permission to reproduce portions of this document forthese or other uses must contact the IEEE Standards Department for the appropriatelicense. Use of information contained in the unapproved draft is at your own risk.

IEEE Standards DepartmentCopyrights and Permissions445 Hoes Lane, P.O. Box 1331Piscataway, NJ 08855-1331, USA

IEEE 929-2000• Passed by IEEE

Standards Board in January, 2000.

• Represents an excellent primer on PV inverter interconnection issues.

86

UL 1741 Standard for Inverters, Converters and Controllers for Use in Independent Power Systems

• First released in May of 1999• Was revised to match IEEE 929-

2000• Will be revised to match IEEE 1547.1

when it becomes approved.

Underwriters Laboratories Inc.®

Subjects 1741 333 Pfingsten RoadNorthbrook, IL 60062March 3, 1999

TO: Industry Advisory Group of Underwriters Laboratories Inc. for Power Conditioning Units for Use in Residential Photovoltaic Power Systems,Electrical Council of Underwriters Laboratories Inc.,Subscribers to UL's Listing Services for Photovoltaic Charge Controllers (QIBP),Subscribers to UL's Listing Services for Photovoltaic Power Systems Accessories (QIIO),Subscribers to UL's Listing Services for Power Conditioning Units for Use in Residential Photovoltaic Power Systems (QIKH), andSubscribers to UL's Listing Services for AC Modules (QHYZ)

SUBJECT: Request for Comments on the Proposed First Edition of the Standard for StaticInverters and Charge Controllers, UL 1741; PROPOSED EFFECTIVE DATE

Attached as Appendix A for your review and comment are proposed requirements for UL 1741.Questions regarding interpretation of requirements should be directed to the responsible UL Staff.Please see Appendix B of this bulletin regarding designated responsibility for the subject productcategories.

Please note that proposed requirements are of a tentative and early nature and are for reviewand comment only. Current requirements are to be used to judge a product until theserequirements are published in final form.PROPOSED EFFECTIVE DATE

The proposed requirements will necessitate a review and possible retest of currently Listedproducts. Therefore, UL proposes that the new requirements become effective 18 months afterpublication. This is intended to provide manufacturers with sufficient time to submit modifiedproducts for investigation and to implement the necessary changes in production. Please notethat this also includes the time that will be needed by UL to conduct a review of the modifiedproduct.

RATIONALE

The first edition of the Standard for Static Inverters and Charge Controllers for Use inPhotovoltaic Power Systems, UL 1741, was proposed in the subject bulletin dated August 1,1997. The proposed Standard has been significantly revised since then. The revisions are aresult of the comments UL received from industry members regarding the proposed draft. Inaddition, the revisions were made to align the proposed Standard with the Ninth Draft ofRecommended Practice for Utility Interface of Photovoltaic (PV) Systems, P929, and other ULStandards. This bulletin proposes the revised first edition of the Standard for Static Inverters andCharge Controllers for Use in Photovoltaic Power Systems, UL 1741.

U L®

What is UL 1741 and how does it relate to IEEE 929?

• First official version published in May of 1999. “Final” version released November of 2000.

• 1741 incorporates the testing required by IEEE 929 (frequency and voltage limits, power quality, non-islanding inverter testing)

• 1741 testing includes design (type) testing and production testing.• Line-tie inverters should have the words

“Utility-Interactive” printed directly on the listing label—this makes identification of the listing much more straightforward (several inverter manufacturers currently using this designation).

87

What remains to be done?• Additional testing standards need to be developed for PV

inverter performance and reliability (goto www.endecon.com to read latest version of document)

• These things won’t happen overnight, but we’re getting there.

Terms and Conditions for Interconnection

• May involve the following:– Metering options– Size restrictions on metering options– Carryover credit on monthly billings– Net Meter or differing buy and sell rates– Outdoor disconnect requirements– Insurance requirements– Interconnection costs

88

Net Metered Systems in California(Similar to some other states)

• Requires a contract with the serving utility company.• Inverter must be acceptable to utility for

interconnection.• Utility also may have a list of acceptable manual

disconnects to choose from.• Starting January 1, 1999, systems are to be billed on

annual basis.• Starting January 1, 2001, systems may elect time-of-

use net metering (up to 30% increased financial benefit)

Utility Interconnection Planning

• Contact the utility well in advance.• Become familiar with the terms and conditions for

interconnecting.• Always be courteous when working with utility or

inspection personnel -- They have a lot of control over the destiny of the project!

89

Battery Systems

Battery Functions in PV Systems

• To store energy produced by the PV array and supply it to electrical loads as needed.

• To power electrical loads at stable voltages, suppressing transients.

• To supply surge currents to electrical loads or appliances.

90

Battery Types and Classifications

Motive Power or Traction Batteries

• Designed for deep discharge cycle service, typically used in electric vehicles and equipment.

• Fewer number and thicker plates than SLI (starter, lighting, ignition) batteries.

• Lead-antimony grids provide good deep cycle performance.

91

Lead-Antimony Batteries

• Advantages include:– improved mechanical strength– excellent deep discharge and high discharge rate

performance• Disadvantages include:

– high self-discharge rate– need for frequent water additions.

Valve Regulated Lead-Acid (VRLA) Batteries

• Electrolyte is immobilized, sometimes called captive or starved electrolyte batteries.

• Electrolyte can not be replenished, intolerant of overcharge

• Minimal maintenance, spill proof

92

VRLA Batteries

Gelled VRLA Batteries • Lead-calcium grids• Electrolyte is 'gelled' by the addition of a silica based gel.

Silica gel will not liquefy at 40o C. Only ultrasonic vibration or extreme discharge can liquefy gel.

• Constant-voltage, current-regulated, temperature compensated charging is recommended.

• Recommended for remote off-grid systems that need maintenance-free batteries.

93

Absorbed Glass Mat (AGM) VRLA Batteries

• Electrolyte is absorbed in glass mat separators.• Intolerant to overcharge and high operating

temperatures.• Similar charging recommendations as for gelled

VRLA batteries.• Not as tolerant to deep discharge cycling as gel

batteries, but better at float than gels.• Recommended for Grid-Connected systems

Battery Charge Control

94

Functions of Battery Charge Regulators and System Controls

• Prevent Battery Overcharge• Provide Load Control Functions• Provide Status Information• Interface and Control Backup Energy Sources • Divert PV Energy to an Auxiliary Load• Serve as a Wiring Center

Backup Charge Controller Algorithms• On-Off• Interrupting, Constant-voltage• Pulse Width Modulation (PWM)• Charge control setup for backup systems is critical

– Must be set above inverter operating voltage to prevent control before PV power reaches battery

– Typically set one volt higher than inverter (inverter—53.6Vdc; controller 54.6Vdc)

95

Battery at Low State of Charge

Battery at High State of Charge

Until the battery is fully charged,the current pulses to the battery are wide.

Once the battery becomes fullycharged, the current pulses to the battery become narrower.

Pulse-Width-Modulated (PWM) Controller Design

Battery Gassing is Key to Regulator Set Point Selection

• Flooded batteries require some level of gassing.• Gelled and AGM batteries are not tolerant to

excessive gassing.• Float voltage for all lead acid batteries is

approximately 53.2 Volts to 54.0 Volts (temperature compensated) for nominal 48 Volt dc systems (2.21 -2.25 V/cell)

96

Voltage Regulation Set Point - Temperature Compensation

• Adjusts end of charge voltage based on battery temperature.

• Increase regulation voltage when battery is cold, improving state of charge.

• Decreases regulation voltage when battery is warm, decreasing electrolyte loss.

• Coefficient approx -5 mV/oC/cell for lead-acid batteries.

Lead-Acid Battery Charging Characteristics

2.02.12.22.32.42.52.62.72.82.93.0

0 20 40 60 80 100

Battery State of Charge (%)

Cel

l Vol

tage

(vol

ts) Lead-Antimony Grids

Charge Rate

C/20C/5

C/2.5

Gassing Voltage at 27 oC

Gassing Voltage at 0

Gassing Voltage at 50 oC

97

Battery Selection Criteria

Battery Selection Criteria

PerformanceLifetimeSize and dimensionsMaintenanceCostsAvailability

98

Battery System Issues

• PV system design and autonomy requirements

• Ambient and environmental conditions

Autonomy

• Autonomy refers to the time a fully charged battery can supply energy to the system loads when there is no energy input from the PV array.

• Determine average load and choose capacity that will run that load for the required time.

• Example: 1 kW load for 8 hours = 8 kWh8000 Wh/48 Volts = 167 Amp-hoursChoose an AGM battery like Concorde PVX-12210180 amp-hours at 8-hour rate (210 A-h at 20 hour rate)

99

Ambient and Environmental Conditions

• Batteries prefer ambient conditions of 77oF.• Battery temperature tends to run slightly warmer than

the average daily ambient temperature due to thermal mass of the battery.

• Battery capacity is reduced to half at 32oF battery temperature (very cold ambient conditions).

• Battery life is reduce by half for every 18oF of continuous operation above 77oF.

Battery System Design Considerations

• Installation, maintenance, and structural requirements• Overcurrent protection and disconnects• Battery enclosure

– Corrosion resistant– Non-flammable– Vented

• Cost and warranty• List battery enclosures now exist• PV industry needs listed battery systems

100

Inverters Require High DC Input Currents

• Example: Inverter rated to supply a resistive load of 5500 Volt-Amps AC at 120 volts at 80% efficiency. Battery voltage is 42 volts DC.– DC input power level:

• 5,500/0.80=6,875 watts DC– DC input current at 42 volts:

• 6,875/42 = 164 amps DC.(also should add RMS AC current of 35 amps– 164 + 35 = 199

– Battery cable rating• 200 amps x 1.25 = 249 amps (4/0)

Battery Overcurrent and Disconnect Requirements

• Proper dc rated overcurrent and disconnect devices are supplied from several manufacturers

• If additional devices are necessary, they must have adequate ampere interrupt rating (AIR).

101

Battery Fundamentals:Review

• Defined types and classifications of batteries used in PV backup power systems.

• Discussed characteristics of charge controllers used in PV backup systems.

• Presented design and selection criteria for batteries in PV backup systems.

PV Array Installation Issues

102

Lag Screw Mounting

Lag Screws must be screwed into at least 1” of wood (usually the rafter or truss)

103

Point Attachment Through Tile or Shake Roof

Weather Proofing of Roof

• Attachments must be properly sealed to preclude leakage.

• Urethane caulks such as Sikaflex 1a are both temperature and UV resistant. Silicones must be UV-rated. Roofing tars are less durable and can leak over time.

• Post and flashing method provides excellent weather proofing

104

Post and flashing method

Foot Installation

105

Materials Selection

• Good materials practices include:– Avoid contact dissimilar metals – Use only high quality fasteners– Use only sunlight resistant materials

• Structural members– corrosion resistant aluminum, 6061 or 6063– hot dip galvanized steel per ASTM A123– Stainless Steel

Fasteners• stainless steel greatly corrosion

issues• galvanized should only be used if

5/16” diameter or larger due to wood auguring.– Not recommended in coastal

installations

106

Material Selection—Review

• Weather sealants– Urethane sealants recommended that are UV and

temperature resistant• Avoid

– contact of dissimilar metals– contact of aluminum with concrete– low quality fasteners

More inter-row shading-ain’t that purdy

107

…and this is so much prettier…

Vent pipe shading

108

mud shading

Module Shading Effects-50% loss of power

109

Module Shading Effects

-50% loss of power

Module Shading Effects

-100% loss of power

110

Shading Effects on Module Output

ASE Module Shading TestsShading Along Module Horizontal Dimension (Width)

0

1

2

3

4

5

6

7

0 10 20 30 40 50 60

Voltage (V)

Cur

rent

(A) Wid th Shad ed (2 rows o f cells )

Wid th Shad ed (1 row o f cells )Wid th Shad ed (3 /4 ro w o f cells )Wid th Shad ed (1 /2 ro w o f cells )Wid th Shad ed (1 /4 ro w o f cells )No Shad ing

Balance of System (BOS) Installation Issues

111

Objectives

• Understand accepted engineering practices associated with the design and installation of photovoltaic and electrical power systems.

• Identify common problem areas in the electrical design of PV systems.

• Discuss differences between PV systems and conventional ac electrical systems.

Objectives (cont.)

• State the purpose of the National Electrical Code and discuss Articles applicable to PV electrical systems.

• Evaluate the types and sizes for dc and ac conductors based on ampacity and voltage drop requirements.

112

Electrical System Design: Requirements

• A PV system designer or installer should have working knowledge of electrical codes and understand the basic design principles and hazardsassociated with electrical and photovoltaic power systems.

Electrical System Design: Considerations

• Safe, reliable and easily maintained electrical systems

• Compliance with electrical codes and standards• Use wiring strategies, types, sizes and

terminations compatible with DC systems• Properly apply Overcurrent protection and

disconnect devices• Observe proper Grounding and surge protection

techniques

113

Differences Between PV and Conventional Electrical Systems

• PV systems have dc circuits that require special design and equipment.

• PV systems can have multiple energy sources, and special disconnects are required to isolate components.

• Energy flows in PV systems may be bi-directional.

Differences Between PV and Conventional Electrical Systems

• PV systems may require an interface with the ac utility-grid and special considerations must be adopted. (involve utility personnel early)

• As the maximum current in a PV array is short-circuit limited, a fault may not generate currents high enough to clear fuses. This is normal for PV systems but abnormal for many other electrical systems.

114

PV System Electrical Design:Common Problem Areas

• Insufficient conductor ampacity and insulation• Excessive voltage drop • Unsafe wiring methods• Lack of or improper placement of overcurrent

protection and disconnect devices • Use of unlisted, or improper application of listed

equipment (e.g. ac in dc use)• Lack of or improper equipment or system grounding• Unsafe installation and use of batteries

NEC Article 690 overview

115

The National Electrical Code (NEC)

• A guide on safe and reliable practices for electrical system design and installation.

• Purpose is to safeguard persons and property from electrical hazards.

• Not intended as an instruction manual for untrained persons.

• Handbook is available with additional explanation.

Scope of the NEC• Covers nearly all electrical power systems,

including grid-connected and stand-alone PV systems operating at any voltage.

• Covers PV systems for outdoor lighting, RVs, and other remote applications.

• Exceptions are automobiles, railway cars, boats, self-contained electronic devices, and utility-owned properties.

116

PV Systems and the NEC

• Article 690 addresses safety standards for the installation of PV systems.

• Many other articles of the NEC may also apply to most PV installations.

NEC Sections Applicable to PV Systems• Article 110: Requirements for Electrical Installations• Chapter 2: Wiring and Protection

– Most of the chapter--especially– Article 250: Grounding

• Chapter 3: Wiring Methods and Materials– Most of the chapter—especially– Article 300: Wiring Methods– Article 310: Conductors for General Wiring

• Article 480: Storage Batteries• Article 690: Solar Photovoltaic Systems

117

NEC Article 690:Solar Photovoltaic Systems

• I. General (definitions, installation)• II. Circuit Requirements (sizing, protection)• III. Disconnect Means (switches, breakers)• IV. Wiring methods (connectors)• V. Grounding (array, equipment)• VI. Markings (ratings, polarity, identification)• VII. Connection to Other Sources• VIII. Storage batteries• IX. Systems over 600 Volts

Electrical Equipment Listing

• AHJs generally require listing for components and electrical hardware.

• Some components available for PV systems may not have applicable or any listing.

• Recognized testing laboratories include:• Underwriters Laboratory (UL)• ETL Semko (Formerly Edison Testing

Laboratories)

118

PV Issues in the NEC

AC Point of Connection• 690.64 (b) Exception: allows the installation of

additional overcurrent devices up to 120% of busbar rating.– 100-Amp Panel allows up to 20 Amp breaker.– Main breaker can be derated. (e.g. 125-amp busbar

can take a 100-Amp and a 50-amp breaker)– Connection can also be make on the line side of

the main breaker (with double lugs or bolt on lugs)

119

Sub-panel fed by inverter

• must first carefully characterize all loads to be connected to sub-panel.

• Article 702--Optional Standby Systems– Allows sizing based on supply of all equipment

intended to be operated at one time (702.5).

Safety AlertMultiwire Branch Circuits and PV Systems

• Inverters—100 watt - 6 kW @ 120 volts• Load Centers—120/240 v @ 100 - 200 amps• Multiwire Branch Circuits—common• Neutral Overload Possible

BatteryBank

PV Array

120 Volt 60 Hz

Inverter

HN

G

Load Center

Multiwire BranchCircuit

Neutral

120 V BranchCircuits

120 VAC

120 VAC

120

multi-wire branch circuits--consider options

– install 240-Volt transformer on the output of the inverter to supply these circuits.

– rerun two separate runs to replace multi-wire branch circuits to be powered by inverter.

– stay away from those circuits. (laundry and furnace are on dedicated circuits in newer homes)

Electrical Code Compliance: Issues

• Many PV systems may not comply with the NEC because:– Designers/installers have little experience with dc

electrical systems.– Listed equipment is not widely available for PV

components and dc hardware.– Suggestions that PV systems are easily

designed/installed by untrained personnel.

121

Electrical Inspection Guidelines

• Many local inspectors have little experience with PV systems.

• Contact the local inspector in advance to discuss system requirements.

• Electrical inspectors normally expect equipment used in PV systems to be listed.

Wiring Methods

122

Wire Termination Methods• Use as few connections as possible.• Make sure each connection is the highest quality

(think about 20-year connections).• Bad terminations result in high-resistance ‘hot spots’

and power losses.• Coat connections in outdoor locations with anti-

oxidation grease to minimize corrosion.

Crimped Terminals• Useful for securing a conductor to a ring or spade

type terminals.• Prone to developing high resistance over time.• Should be crimped with heavy-duty electricians tool.• Soldering recommended on 12-Volt systems• Use high-grade UL listed terminals.

123

Binding Post Terminals• Often called box or pressure terminals.• Accept a bare wire and secure it by pressure from a

screw. • Eliminates the one extra connection required with

crimped terminals. • May need to be retightened periodically to ensure a

good connection.• One of the most common array problems is loosening

of these connections

Twist-On Wire Splices• Often called wire nuts.• Frequently oxidize and develop high-resistance

‘hot spots’.• Allowed on DC systems if installed according to

manufacturers directions and applied in the correct location (e.g. wet, dry).

• Available in waterproof, non-corrosive version

124

Plug and Receptacle Type Connectors

• Advantages include:– rapid connections with no tools—

labor savings– polarized and non-

interchangeable– protect live parts– easy for troubleshooting and

maintenance

Plug and Receptacle Type Connectors

• Disadvantages include:– higher material cost– possible corrosion and increasing resistance over time if not installed

according to design. – Most connectors do not have interrupt rating.– Installers often get lazy by not supporting wire and not fully engaging the

connectors

125

Junction Boxes• Connection between

conductors in an outdoor location generally must be done within a rainproof junction box, NEMA 3R or 4 (unless with approved connector)

• Junction boxes are commonly used on PV modules and in combining array source circuits.

Don’t do this!

Conductors and Wiring Methods

• Conductors are the wiring and cable used in electrical systems.

• Available in a variety of types, sizes and ratings.• Copper conductors are recommended over

aluminum.• Single vs. multi-stranded options.

126

Conductor Insulation Types• Protects wire from coming into contact with personnel

or equipment.• Insulation types specifications include:

– temperature rating– sunlight, oil or water resistance– location (dry, wet)

• Insulation type dictates the ampacity rating at operating temperatures.

Types of Conductors in PV SystemsApplication Wire Type

• Array Wiring USE-2, TC(holds XHHW-2 or THWN-2),(sun resistant-check temp

rating)THWN-2 or XHHW-2 (conduit)

• BOS Wiring THWN or XHHW (conduit), orTHWN-2 or XHHW-2 (conduit)

• Battery Cables THW Flexible (hundreds of strands)

127

Conductor Locations

• Location affects ampacity and temperature derating for conductors.

• Single conductor cables outside of conduit only allowed in the PV array.

• Interior exposed cable runs are only permissible with sheathed type cables (must be clearly delineated from AC wiring with tagging or labeling)

Conductor Color Codes

• The insulation on grounded conductors must be white, or if larger than No.6 AWG it may be marked white as long as the insulation color is not green (almost always black).

• Module frame and equipment grounding conductors must be bare wire or have green colored insulation.

• In negative grounded PV systems, the positive conductormay be any color except for green or white, and the negative grounded conductor must be white.

128

Conductor Sizing• Conductor sizing is based on ampacity and voltage

drop considerations. • Sizes are specified by American Wire Gage (AWG),

the higher the number or gage, the smaller the conductor.

Ampacity of Conductors• Ampacities are determined by:

– wire type (copper or aluminum)– wire gage (AWG)– insulation rating (wet rating for outdoor)– highest insulation temperature– location (free air, conduit or buried)

• Ampacity decreases with increasing temperature:– I2 = I1 x factor

129

Temperature Corrected Ampacity: Example

• A No. 10 AWG USE-2 cable has a 90oC rating and ampacity of 40 amps in ambient temperatures of 26-30o C.

• When the same conductor is used for module interconnect wiring for rooftop applications it may experience temperatures of 71-80o C. In this situation its temperature corrected ampacity is reduced to 16 amps.

Temperature Corrected Ampacity: Example

• A No. 10 AWG USE cable has a 75oC rating and ampacity of 35 amps in ambient temperatures of 26-30o C.

• When the same conductor is used for module interconnect wiring on an open rack it may experience temperatures of 61-70o C. In this case its temperature corrected ampacity is reduced to 11.5 amps.

130

Conductor Sizes and Ratings

Resistances for 7Resistances for 7--strand copper conductors at 75strand copper conductors at 75ooC.C.Ampacities for USE at 61Ampacities for USE at 61--7070ooC in conduit.C in conduit.

Conductor Size (AWG)

Ampacity (amps)

Resistance (ohms/kft)

4 28 0.32 6 21 0.51 8 17 0.81

10 11 1.29 12 8 2.05

Resistance of Conductors

• Resistances are determined by:– wire type (copper or aluminum)– wire gage (AWG)– ambient temperature– length of wire

131

Voltage Drop Considerations• Voltage drop is a major factor in low-voltage systems.• Voltage drop increases with increasing current and

decreasing conductor size.• May affect the operation of battery charge controller

set point functions or inverter sensing circuits.

Voltage Drop Considerations

• Factors contributing to voltage drop include terminations, fuses and disconnects.

• According to Ohm’s law, voltage drop is calculated by:Voltage Drop = Current x Resistance/Length x Length

132

Voltage Drop: Example

• Determine the Inverter to service entrance voltage drop with 500 feet (250-foot one-way distance) of #10 AWG cable round-trip with a nominal 10 amp current:

Voltage Drop = 1.29 ohms/kft x 0.5 kft x 10 amps= 6.45 volts (5.4% drop @ 120 Volts)

Grounding and Surge Protection

133

Objectives• Explain acceptable methods for equipment and

conductor grounding in PV electrical systems.• Discuss various surge and lightning protection

strategies for electric circuits.

Electrical System Grounding• The NEC defines grounding as a connection to the

earth, where electrical charge can dissipate safely. • Grounding of electrical systems offers personnel

safety and minimizes the effects of lightning and surges on equipment.

134

Electrical Grounding Types

• System Ground: Connecting the circuit to ground (i.e. the negative of a dc array, the neutral of a split single-phase system, or the neutral of a bi-polar dc system)

• Equipment Ground: Connecting all non-current carrying metal parts to ground (metal enclosure, module frame, etc…)

Grounding and Grounded Conductors: Definitions

• A grounded conductor normally carries current in the system and is grounded. Normally, the negative leg of a two conductor dc PV system is grounded.

• A grounding conductor does not normally carry current, and is used to connect all the exposed metal portions of equipment or the grounded conductor to the grounding system.

135

Grounding Requirements in PV Systems• For two-wire PV systems operating above 50

volts, one dc conductor must be grounded. • In a three-wire system, the neutral or center tap

of the dc subsystem shall be grounded.• Some 3-phase inverters do not allow the ac

neutral to be grounded at the inverter. Grounding is accomplished through the dc array circuit and the isolation transformer.

• Ungrounded systems are allowed in 2005 NEC

Grounding Requirements in PV Systems• Non-current-carrying metal components must be

grounded.• The equipment grounding conductor for the DC-side

of the system is based on Table 250.122 in the 2002 NEC as long as the ground fault protection is used. (otherwise must be same size as current-carrying conductors)

• Disconnect switches must not open-circuit the grounded conductor.

136

Grounding Electrodes• The grounding electrode in most PV systems should be a

corrosion-resistant rod, a minimum of 5/8 inch in diameter and driven a minimum of 8 feet into the earth.

• A secondary grounding electrode may be connected to the PV module and array frames and must be connected to the primary grounding electrode with an appropriately sized grounding electrode conductor according to 2002 NEC Table 250.66. (at least 6 ft away from primary electrode)

Grounding Electrodes (Cont.)• There should be only be one point in the system where

the grounding conductor is attached to the grounded conductor in the PV system. This mitigates the possibility of circulating ground currents under normal operating conditions.– exception—grounding of separate structures

• The dc system grounding electrode shall be in common with or be bonded to the ac system grounding electrode.

137

Grounding Electrodes (Cont.)Grounding of Separate Structures

• For arrays that are mounted a long distance from the service entrance building (more than 20 feet), grounding of separate structures allows for a grounding electrode to be installed at the array with no separate grounding electrode conductor.

• Grounding electrode conductor is essentially the grounded conductor and is connected to the grounding electrode at each end and run in non-metallic conduit.

Traditional Equipment GroundingR1

R4

R2ac supply

R4

R3 R3

R1

R2

motor

motor

138

Grounding and Safety in PV Systems

R2

R5 R1

R3

R6R4

module framecell circuit

R4

R2

R5R6 R3

R1

Ground-Fault Protection

• NEC Article 690-5 requires ground-fault detection and interruption (GFDI) for PV arrays mounted on the roofs of dwellings.

• This requirement is for fire protection in dwellings constructed of wood, not for personnel protection.

139

Ground-Fault Protection Equipment

• Most inverters have these capabilities built-in.• A GFP device automatically:

– senses excess ground-fault currents– interrupts the currents– open the circuit between the array and load

PV Array Current Leakage

• A current path that develops between the cells in a module and the module frame.

• Valuable current is lost and a safety hazard exists.• Grounding reduces the safety hazard.

140

Measuring Leakage Currents

Ammeter

A

leakage path

module frame

Lightning Protection

• Direct strike interception• Equipment grounding• Surge arrestors• Prayer

141

Surge Arrestors• Metal Oxide Varistors (MOVs), Silicone Oxide

Varistors (SOVs), and Zinc Oxide Non-Linear Resistors (ZNRs) are three types of surge arrestors.

• Transient Voltage Suppressors (TVSs) include the above surge arrestors and also include very fast acting devices used close to sensitive electronic equipment that are typically not replaceable.

• Provide protection from current and voltage surges.

Surge Arrestors - SOVs

142

Surge Arrestors- MOVs

• Whole House Surge Arrestors

• Some fit under meter collar

• Some fit inside Main Service Panel or in conduit knockout hole

Surge Arrestors – Both sides of inverter

143

Surge Arrestor Ratings• High impedance devices under normal conditions, short-circuit to ground

under surge conditions.• Rated for breakdown or sparkover voltage (>Voc), current and energy

dissipation capability.

Surge Arrestor I-V Characteristics

0

20

40

60

80

100

120

30 40 50 60 70 80 90 100

110

120

130

140

150

160

170

180

190

Volts

Am

pere

s

Operating Max Open Circuit Breakdown Clamping

Surge Arrestor Locations

• Locate on all ungrounded legs of array strings, and on electronic equipment.

• Locate where catastrophic failure will not damage other wiring or components.

144

Marking and Labeling

The NEC requires a variety of Marking and Labeling

• Any fuse or circuit breaker that can be energized in either direction must be labeled as such.

(NEC 690.17)• UL listing covers markings on PV modules.

(NEC 690.51)• System Ratings: operating current, voltage, max

voltage, and short-circuit current. (NEC 690.53)

145

Marking and Labeling (cont.)

• Interactive Point of Connection (NEC 690.54)

• Provide any additional information that you think would be helpful to the homeowner, inspector, or fire officials.

• If system contains uninterruptible capabilities, provide a diagram at the external house disconnect to notify fire fighters. (NEC 702.8)

Final Inspection and System Acceptance

146

Checklist for PV System Installation• PV Array• Conductors• Overcurrent Protection• Charge Controllers• Disconnects• Batteries• Inverters• Grounding• Safety Signs

SAFETY SIGNS• Make sure all required "warning" and "caution" signs

are installed in the proper locations as required in the NEC.

147

System Final Inspection• Have all system information and

documentation available to address any potential questions from the inspector.

• Point out system labeling.• If issues arise that need to be addressed, deal

with them as quickly as possible.• Always be courteous -- never condescending.

(e.g. I know PV-you don’t)

System Acceptance• Plan to do acceptance on as sunny a day as possible.• Check Voc and Isc for each source circuit• Compare current from each source circuit to a single

module at same orientation to see if there is reasonable agreement. (Note any variations not due to clouds.)

• Compare currents under load (inverter on)

148

System Documentation and Walkthrough

• Provide customer with copy of all drawings and product literature.

• Thoroughly cover any operating procedures that the customer must observe.

• Provide information for what to do in the case of any emergency or system malfunction.