Commissioning and Troubleshooting PV Arrays

with the

Solmetric PV Analyzer

Paul Hernday

Senior Applications Engineer

paul@solmetric.com

cell 707-217-3094

November 14, 2013

• Review of I-V Curves

• Introduction to the Solmetric PV Analyzer

• Setup & Measurement

• Live demo of PVA software

• Irradiance and module temperature measurement

• Data analysis and reporting

• Measurement examples

• Bypass diodes

• Effects of shading, soiling, snow

• Troubleshooting using the selective shading method

Topics

Review of I-V Curves

I-V and P-V Curves Expect this shape for un-shaded, healthy modules & strings

Cu

rre

nt

Voltage

Isc

Voc

I-V curve

Vmp

Imp

Po

we

r

P-V curve

Pmax

*P-V curve is calculated from the measured I-V curve

Max power point

I-V Curve Deviations Each represents a reduction of generating power

Any reduction of the knee of the curve

means reduced output power.

Cu

rre

nt

(A)

Voltage (V)

Isc

Voc

Increased

slope

Reduced

slope

Mismatch losses

(incl. shading)

Normal I-V curve

Reduced

current

Reduced

voltage

Conventional measurements do not reveal many of these effects

How I-V Curve Tracing Works (in concept)

Current

(I) Voltage

(V)

Cu

rre

nt

Voltage

(I)

(V)

(I, V)

Adjust the

load

resistor

1

3 Plot the

point

Read I & V

2

• A curve tracer instrument

does all of this automatically

• The load may be resistive,

capacitive, or electronic

Solmetric PV Analyzer

• 1000V, 20A

• Measured vs. predicted (3 dots)

• PC-based (control, display, storage)

• Wireless interface for convenience and workplace safety

• Automated data analysis & reporting

• Database of >10,000 modules, with automatic updates

Solmetric 1000V PV Analyzer Kit PVA-1000S I-V Curve Tracer & SolSensor Wireless Reference Sensor

SolSensor™

Wireless PV Reference Sensor

• Clamps to module frame in plane of array

• Silicon photodiode irradiance sensor with

temperature compensation and angle of

incidence corrections

• Two thermocouples

• Tilt sensor

• Sensors triggered at same time as I-V sweep to

optimize accuracy in unstable sky conditions

• >300ft wireless range

• Auxiliary input for external irradiance sensors

(software under development)

• Uses same wireless USB adapter as the PVA

• Rechargeable battery

• Optional tripod mounting kit

Optional SolSensor Tripod Kit Under development

Leveling unit, tilt and pan unit, mounting clip

• Allows locating SolSensor for

best line-of-sight transmission

across large PV arrays.

• Also useful in situations where

array is not accessible.

• Thermocouple lead can be

extended to reach array, or

temperature can be determined

from the measured I-V curve.

Wireless Sensor Kit Irradiance & temperature sensors

Irradiance

transmitter

Receiver (USB)

Temperature

transmitter

K-type

thermocouple

Omega Part #

5SRTC-GG-K-

30-72

.

Built-in PV models

PV Module Irradiance

Module temperature Tilt

Azimuth Latitude

Longitude Date & time

3 dots predict curve shape

How It Works

Irradiance

Temperature

Tilt

I-V

Data

Controller

&

Wireless

‘I’ sense

‘V’ sense Capacitor (1 of 3)

PV Test

Leads

NEMA 4X FG Enclosure

• The PV Analyzer uses a capacitor as the load.

• Current and voltage change smoothly, at controlled rate, across the voltage range,

ideal for accurately testing high efficiency modules.

Bleed

resistor Switch

How It Works

PV Analyzer Benefits

• One measurement per string (& just one hookup at combiner box)

• Allows testing array performance before inverter is online

• Instant performance check via built-in PV models

• Automated data analysis and reporting

• Faster troubleshooting

Greater productivity

Greater insight

• I-V curve is the most complete performance measurement possible,

capturing Isc, Voc, Imp, Vmp, Pmax, plus the entire I-V curve

• Independent Pmax measurement for each string

• Helps users “think like a PV array” and develop troubleshooting skills

Setup & Measurement – Large Systems

Setup and Measurement Example: Measuring strings at a combiner box

Hardware setup (do once at each combiner box):

1. Deploy or move the sensors (to stay in wireless range)

2. Open the DC disconnect of the combiner box

3. Lift the string fuses

4. Clip the PV Analyzer test leads to the buss bars

1. Insert a string fuse

2. Press “Measure”

3. View and save results

4. Lift the fuse

Electrical measurement (repeat for each string):

10-15 seconds, typically

Example Measurement Setup

Courtesy of Chevron Energy Solutions © 2011

Measurement Process

Courtesy of Portland Habilitation Center

and Dynalectric Oregon

1. Open the DC disconnect

for the sub-array that you

want to test.

Measurement Process

Courtesy of Portland Habilitation Center

and Dynalectric Oregon

2. Locate the

combiner box

Measurement Process

Courtesy of Portland Habilitation Center

and Dynalectric Oregon

3. With a clamp-meter,

verify that the load has

been disconnected.

4. Then lift all of the

fuses. Combiner box

Measurement Process

Courtesy of Portland Habilitation Center

and Dynalectric Oregon

5. Clip the PV Analyzer

to the buss bars.

6. Push down a fuse and

make an I-V curve

measurement. Lift

fuse again.

7. View and save results.

8. Repeat for the other

strings.

Combiner box

Setup & Measurement – Residential Systems

SolSensor™ Setup

• Clamp SolSensor to a module frame

• Tape thermocouple tip to backside

• Press the ‘ON’ button (glows red)

Accessing PV Source Circuits for performance measurements

DC Combiner

Box

2

Inverter

J-Box

2 5

DC Disco

DC Disco

AC

AC Inverter

DC Disco

DC Disco

4

3

4

Small Residential System

Larger Residential System

• Access = Isolation + Connection

• Choose the safest, most convenient point to isolate and connect to strings

• Sometimes you’ll need two access points for one string

1

1

5

3

Warning: Shut down inverter and open DC disconnect before accessing PV source circuits.

Accessing PV Circuits at an integrated DC disconnect

1. Open the AC and DC disconnects

2. Lift the string fuses

3. Connect the PV Analyzer test leads to

the grounded conductor terminal strip

and to the ungrounded conductor fuse

clip (supply side), observing correct

polarity.

• Use the Solmetric Test Lead kit. Alligator clip the positive lead to the fuse clip.

• Connect the negative lead to the negative terminal block via a pigtail or using a probe in place of the

alligator clip (for example the Fluke Fused Test Probe, 10A max).

Unbroken Conductors

• In systems with non-integrated DC

disconnects, the ‘grounded’ conductors

often pass ‘unbroken’ through a DC

disconnect switch.

• You can isolate the PV source circuits

by opening the switch and clipping

your test lead to a supply side terminal.

• Connect to the grounded conductors

by attaching a pigtail at the inverter

terminal block.

Live Demo of PV Analyzer Software

Irradiance & Temperature Measurement

Low Irradiance

Problem:

When you measure array

performance at very low

irradiance, the data is a

poor basis for estimating

performance at high

irradiance, where

performance matters most.

At low irradiance the I-V

curve changes shape, and

this causes greater error in

translating the data to STC.

Solution:

Negotiate for new date.

If that’s not possible, test

for function, and re-test

later for performance.

Unstable Irradiance

Problem: Unstable irradiance

introduces ‘scatter’ in the

Performance Factor values,

especially if there is a time

delay between I-V and

irradiance measurements. .

Solution: Use SolSensor, which is

triggered simultaneously

with the I-V measurement.

Trigger tests at moments

when irradiance is high

Effect of Time Delay Between I-V and irradiance measurements

• If the irradiance changes

significantly between the I-V curve

and irradiance measurements, it

introduces irradiance error in the

performance prediction against

which the I-V curve is evaluated.

• The longer the time delay and the

steeper the irradiance ramp, the

greater the irradiance error.

• The PVA software sends

simultaneous trigger signals to the

I-V curve tracer and SolSensor,

assuring that irradiance samples

are accurate.

• Manual measurement and entry of

irradiance usually causes long

delays.

10s 10s 10s 10s 10s

980

960

940

920

900

10-second intervals

Irra

dia

nce

(W/m

2)

30

• Like irradiance, module temperature is used

by the PV model to predict expected

performance.

• Choose a location that is typical of the

average temperature of the array.

• Avoid the edges of the array, which are

cooler than average.

• Avoid areas where heat is trapped or

concentrated (for example, the upper edge

of a flush-mounted rooftop array).

• For single module tests (shown here), mount

the thermocouple 2/3 of the way between the

corner and center of a module.

• Use high-temperature tape (eg 1-3/4 inch

Kapton dots**). Roll the tape firmly across

the thermocouple to force out wrinkles and

assure firm contact with backside. ** MOCAP MCD-PE 1.75 poly dot

~$80/roll of 1000 dots

customerservice@mocap.com

Mounting the backside thermocouple

I-V Data Analysis Tool

Displays Generated by the I-V Data Analysis Tool

1950

2000

2050

2100

7

6

5

4

3

2

1

0

Fre

qu

en

cy

Pmax (Watts)

7

6

5

4

3

2

1

0

Cu

rren

t (A

mp

s)

0 100 200 300 400 500

Voltage (Volts)

7

6

5

4

3

2

1

0

Cu

rren

t (A

mp

s)

0 100 200 300 400 500

Voltage (Volts)

I-V curve overlay graph

(one per combiner) Table

Histogram

(one for each parameter)

Statistics

Limits

Parameter

Values String ID’s

Example Report

Measurement Examples

0

1

2

3

4

5

6

7

8

0 50 100 150 200 250 300 350 400

Voltage - V

Cu

rren

t -

A

String 4B14

String 4B15

High Series Resistance

Neighboring

strings Faulty module

Example of a High Resistance Failure Module cord separates from buss ribbon in J-box

Probably failure mode:

Heat cycling bond degradation resistive heating

Backside view Backside view, closeup

Frontside view

Example of a Hot Spot Failure

String of Field-aged, Early TF Modules Degraded fill factor, lower output power

Array-as-sensor mode for viewing relative changes in curve shape

Dropped Cell String Conducting or shorted bypass diode

Bypass Diodes

Bypass Diodes

• PV modules designed for grid-tie systems have

extra components – semiconductor “bypass

diodes” – designed to protect shaded, badly

soiled, or cracked cells from electrical and

thermal damage. Bypass diodes also allow non-

shaded modules to keep producing, by shunting

current around groups of shaded cells.

• In most module designs, the bypass diodes are

mounted in the junction box on the module

backside. In many cases, the junction box can

be opened to test and replace the bypass

diodes.

• Each bypass diode protects a different group of

cells within the module. For example, in a 72-cell

crystalline silicon module there may be three

bypass diodes, each protecting a group of 24

cells, usually laid out as two adjacent columns

as viewed in portrait mode.

Bypass Diodes

• This sketch shows current flow in a typical 72-

cell PV module with 3 bypass diodes (shown

at top).

• If none of the cells is seriously shaded or

otherwise impaired in its ability to generate

current, the current flows as shown by the

green path. The bypass diodes do not

conduct current.

• In the next slide, we shade a cell and the

bypass diode protecting that cell group turns

on, routing current around the partially

shaded group.

Bypass Diodes

• In this sketch, a cell at lower right has been

shaded. Assuming this module is loaded, the

bypass diode protecting that group of cells

turns on, routing current around that group,

protecting the shaded cell.

• Another benefit of bypass diodes is that by

eliminating the shade-induced restriction to

current flow, they preserve the production of

the non-shaded modules and cell groups.

Shading

I-V Curve of a Partially Shaded String

• Multiple ‘knees’ multiple power peaks

• Peaks evolve as conditions change

• Inverter tries to find and track the highest peak

Cu

rre

nt

Voltage

Isc

Voc

Po

we

r

Bypass diode

turning on

Depth of step is

proportional to shading

factor on most shaded cell

Partially shaded residential array Measure the single string mounted along lower edge of roof

I-V Curve of the partially shaded string Single string mounted along lower edge of roof

Approximately 40% reduction in string’s output power

Shade 2 cells in the same cell-string Single module with 72 cells and 3 bypass diodes

Shading one

cell string

drops 1/3 of

PV module

voltage and

power

Shade 2 cells in adjacent cell-strings Single module with 72 cells and 3 bypass diodes

The same

amount of

shade,

oriented

differently,

drops 2/3 of

PV module

voltage and

power.

Tapered Shading

Tapered shading From adjacent row, parapet wall, railing, etc

• This effect produces an I-V

curve deviation similar to that

of shunt loss

• The tapered sliver of shade

causes a slight current

mismatch across cell groups

and modules

• In tilt-up system, the impact of

this shade is felt only early

and late in the day, at low sun

angles

• In general, inter-row shading

losses are greater if rows are

‘crowded’ to increase peak

capacity

Cu

rre

nt

Voltage

Isc

Voc

rows not

parallel

Effect of

tapered

shade

Shade ‘taper’ across a cell-string Single module with 72 cells and 3 bypass diodes

Non-Uniform Soiling

Random Non-uniform Soiling (seagull example)

• Effect is similar to random

partial shading

• Shows up as steps or

notches in the I-V curve

Lower Edge Soiling (“dirt dam”) Common in arrays with tilt less than 10 degrees

Dirty

Clean

50% of the

power loss 50% of the

power loss

Snow

Snow on Array (light cover)

Snow on Array (heavier cover)

Troubleshooting Using Selective Shading

Max power point

I-V Curve Deviations Each represents a reduction of generating power

Any reduction of the knee of the curve

means reduced output power.

Cu

rre

nt

(A)

Voltage (V)

Isc

Voc

Increased

slope

Reduced

slope

Mismatch losses

(incl. shading)

Normal I-V curve

Reduced

current

Reduced

voltage

Conventional measurements do not reveal many of these effects

Selective Shading Technique

Photo courtesy of Harmony Farm Supply

and Dave Bell (shown)

Key to the technique: Blocking the light turns on the bypass diode, shorting out the cell group.

String with one

bad module

Cu

rre

nt

Voltage

Isc

Voc

This is the string

with no shade,

showing a step.

Shading any good

module produces

this red curve, still

showing the step

Troubleshooting using selective shading to identify a bad module

The method can also be used to identify

a bad cell string in a single module

Shading the bad

module produces this

blue curve, with no

step.

• Under development

• Input invited (examples, failure

modes, I-V curves and

photos)

• Will be made available as a

wall poster

Commissioning and Troubleshooting PV Arrays

with the

Solmetric PV Analyzer

Paul Hernday

Senior Applications Engineer

paul@solmetric.com

cell 707-217-3094

November 14, 2013