PV Array Commissioning and Troubleshooting

with the

Solmetric PV Analyzer

Paul Hernday

Senior Applications Engineer

paul@solmetric.com

cell 707-217-3094

April 11, 2013

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

• 600V, 20A (1000V version coming)

• Measured vs predicted (5 dots)

• PC-based (control, display, storage)

• Wireless interface for convenience and workplace safety

Solmetric PVA-600 PV Analyzer

Irradiance sensor (in plane of array)

Temperature sensor (module backside)

Built-in PV models

Module make & model Tilt

Azimuth Irradiance

Module temperature Latitude

Longitude Date & time

5 dots predict curve shape

All wireless

How it works

PV Module or string

How the Solmetric PV Analyzer Works

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

Solmetric PV Analyzer Users I-V curve tracing is embedded in the PV community

EPC organizations

System Integrators

Consulting Engineers

Training Organizations Technical colleges

IBEW

Training Centers

O&M Companies

Electrical contractors

I

V

Module Manufacturers

Inverter Manufacturers

PV Analyzer Benefits

•One measurement per string (& 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 us think like a PV array”

Setup & Measurement - Commercial Systems

Setup and Measurement Example: Measuring strings at a combiner box

Hardware setup (do once at each combiner box):

1. Move the sensors (if necessary to stay in wireless range)

2. Open the DC disconnect of the combiner box

3. Lift the string fuses

4. Clip 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

Wireless Sensor Kit Irradiance & temperature sensors

Irradiance

transmitter

Receiver (USB)

Temperature

transmitter

K-type

thermocouple

Omega Part #

5SRTC-GG-K-

30-72

.

SolSensor™ Wireless PV Reference Sensor

• Integrated construction

• Silicon irradiance sensor with

temperature compensation and

software

• Compatible with external irradiance

sensors via auxiliary port

• Two thermocouples

• Tilt sensor

• Clamps to module frame in plane of

array

• 100m wireless range

• Tripod mounting option

• Rechargeable

Measurement Process 860kW 7-inverter system

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 Combiner box wiring

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.

Measurement Process Insert a single fuse to test the corresponding string

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.

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 = Isolating the source circuit + Connecting to it

• Choose the safest, most convenient point

• 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 armored, high-current test leads. Attach the supplied alligator clips or high-current

probes like the Fluke model FTP.

• Usually can’t alligator clip to a terminal block because of the insulating plastic cover. Install a short,

insulated, temporary wire pigtail in the terminal block, to clip to.

Unbroken Conductor Runs

• The most common example is a

negative-grounded system with the

negative conductors passing unbroken

through a DC disconnect switch.

• Grounded conductors passing through

the enclosure ‘unbroken’

• Can isolate the PV source circuits by

opening the switch, and connect to

them at the top (supply side) of the

switch.

• Connect to the grounded conductors

by attaching a pigtail to their terminal

block at the inverter.

Live Demo of PV Analyzer Software

Irradiance Issues

Low Irradiance

Problem: Max power performance at

low irradiance is not a good

predictor of performance at

high irradiance.

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 error in

irradiance and temperature

measurements, making

data analysis less

accurate.

Solution:

Trigger tests at moments

when irradiance is high

Use real-time, wireless

sensors rather than manual

sensors & data entry.

Effect of Time Delay Between I-V and irradiance measurements

PV performance measurements can be less accurate

under conditions of unstable irradiance, even if the

irradiance is nominally high. Here is why.

There is typically some small time delay between the

I-V curve and irradiance measurements. As shown in

this graph, if the irradiance changes during this time

delay, the measured irradiance value will not be

representative of the condition under which the I-V

curve was measured. In other words, under unstable

irradiance conditions, a time delay between the two

measurements translates into irradiance

measurement error.

The longer the time delay, the more vulnerable the

performance measurement is to this source of

irradiance error.

This error is minimized when using real-time, wireless

irradiance sensing with very short time intervals

between irradiance measurements.

At the other extreme, the error is maximized when

you measure and enter irradiance values manually,

because of the time these steps require.

10s 10s 10s 10s 10s

980

960

940

920

900

10-second intervals

Irra

dia

nce

(W

/m2)

30

Data Analysis

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)

String I-V ‘combo’ graph

(one per combiner)

String table

(one row per string)

Distribution graph

(for each parameter)

Tour of the Data Analysis Tool

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

J-box failure Example of a series resistance problem

Probably failure mode:

Heat cycling bond degradation resistive heating

Backside view Backside view, closeup

Frontside view

Hot Spots

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 from shorted bypass diode

• Shorted bypass diode, or

• Mismatch causing diode to turn on

when current starts flowing

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.

PV Array Commissioning and Troubleshooting

with the

Solmetric PV Analyzer

Paul Hernday

Senior Applications Engineer

paul@solmetric.com

cell 707-217-3094

April 11, 2013