2014 pv reliability, operations & maintenance workshop: commissioning pv systems with an o&m...

Commissioning PV Systems

with an O&M Focus

Paul Hernday

Senior Applications Engineer

[email protected]

cell 707-217-3094

May 7, 2014

Sandia/EPRI 2014 PV Systems Symposium

mailto:[email protected]

Topics

• Guide to Performance and Safety Measurements

• Guidelines for interpreting I-V curves

• Techniques for visualizing I-V data from large arrays

• Complementary use of I-V curve tracing and IR imaging

• Enriching the value of zone monitoring with manual I-V curve testing

Reduced O&M burden & improved

project ROI

Reduced risk, cost of capital, & LCOE for future projects

Cost of capital ~ risk ~ 1/knowledge Roger Hill, Sandia

Earlier exposure & resolution of issues

Improved commissioning measurement

practices

More detailed assessment of performance

Improved design & construction practices

Guide to Performance & Safety Measurements for Commercial PV System Commissioning

Context / Opportunity for commissioning measurements

• Rigorous commissioning ( + resolution of issues) results in higher early

and long term production and reduced maintenance burden.

• Larger systems have tended to receive more rigorous commissioning.

• In commercial systems, there is wide variation in which measurements are

performed and how the data is evaluated.

• Formal commissioning standards such as IEC-62446 are very relevant to

the commercial space but are not firmly embedded in US practice.

• What appears to be lacking is awareness of:

• The business case for effective commissioning

• The reasons for including particular measurements

• Guidance on performing the measurements, including environmental

conditions, so that the measurements are a useful reference for

ongoing maintenance work.

• How to interpret the data and what to report

• Introduction

• System Performance

• Array Performance

• Insulation Resistance

• IR Measurement

• Solar Access

Chapters

Goal

Reduce the Levelized Cost of Energy (LCOE) of commercial scale

PV generation by advancing the use of best measurement practices

in the commissioning of new systems.

In Scope Out of Scope

Commercial systems Residential & utility

Commissioning, re-, retro- Detailed troubleshooting

Performance and safety measurements

Testing of monitoring, comm’s & other systems

Measurement practice Specific electronic tools

Weather conditions Required conditions

Example test limits Required test limits

What to report Reporting format



SECONDARY COMMISSIONING – ENERGY TEST – ENERGY PERFORMANCE INDEX (EPI) Method 1 – Use commissioning test measurements of actual hourly weather for input to PV System Model 1 to

calculate hourly and monthly Expected Energy output kWh. Compare Measured to Expected using EPI.

Commissioning Data

Excerpt from draft of

system performance chapter


Method 2 – Use Regression model developed from measured data from

as-built system to Expected Energy output during first month (Month 1)

of system operation during commissioning tests.

Commissioning Data

Excerpt from draft of

system performance chapter

Topics






Example I-V Curve Measurement Setup Curve tracer connected to combiner buss bars

Courtesy Chevron Energy Solutions © 2011

Fastest

• One measurement per string at the combiner box

• No need to return to clamp-meter the strings

Most complete

I-V curve plus Isc, Voc, Imp, Vmp, Pmax, Performance Factor and Fill Factor

Most specific

Results are independent of inverter performance and issues with shading or soiling elsewhere in the array.

Best troubleshooting clues

• I-V curve shape suggests possible causes

• Selective shading method reduces troubleshooting time

Safest

Combiner is isolated from the inverter and other sub-arrays, reducing arc flash hazard.

Benefits of I-V Curve Tracing For PV source performance testing

• Classification system for

how I-V curves deviate

from expectations.

• Descriptive, not physical.

• When coupled with likely

causes, it becomes a

powerful troubleshooting

aid.

Deviations from Normal I-V Curve Shape

Measuring Actual String Performance

• Actual hardware performance is often

obscured by issues related to weather,

obstructions, or measurement technique.

• Peeling away the external effects is easy

once a few simple concepts are mastered.

• The benefits should include:

Measurement Issues • Irradiance sensor not in POA

• Thermocouple not attached

• Thermocouple location

• Resistive losses

Actual string

performance

Weather Issues • Low irradiance

• Variable irradiance

• Wind

Obstruction

Issues • Shade

• Soiling

Hmm…

Deeper understanding of

commissioning data

Less costly troubleshooting

Reliable basis for module

warranty claims.

Understanding of module

degradation rates

Using Selective Shading to troubleshooting underperforming strings

Courtesy Dave Bell &

Harmony Farm Supply

• Finds the bad module with wiring intact,

based on repeated string I-V curve

measurements with modules electrically

removed one at a time by bypass diodes.

• Blocking light to 2+ rows of cells forces the

bypass diodes ON.

• If one module in a string of N modules is

bad, (N-1) I-V curves will show its effects

and one I-V curve will be normal. The one

normal curve = shading the bad module:

Topics






Fill Factor The underutilized performance metric

Isc

Voc Voltage

Voltage Ratio (Vmp/Voc)

Imp

Vmp

Max Power Point

Current Ratio (Imp/Isc)

Curr

ent

Imp x Vmp

Isc x Voc Fill Factor = =

Green area

Blue area

• FF (0….1) is a measure

of the squareness of the

I-V curve

• Higher FF means more

output for a given Isc and

Voc:

• Relatively independent

of irradiance under high

light conditions

• Excellent screen for

comparing curve shapes

• I-ratio and V-ratio provide

valuable hints about

series or shunt

resistance

Useful Data Analysis Tools

1950

2000

2050

2100

7

6

5

4

3

2

1

0

Fre

qu

en

cy

Pmax (Watts)Pmax (W)

Count

Fill Factor Histogram Example: Broad bell curve with single outlier

Fill Factor and Voc Histograms From I-V curves of strings with PID issues

Voc (Volts)

Inverter 1 Inverter 1

Fill F

acto

r

Date

Performance Trends Example: Trends for a particular string

Perf

orm

an

ce

Facto

r

0 0

100 1.0

2014 2015 2016 2017

Array Navigator

Performance Trends Example: Population trends

Sta

nd

ard

Devia

tion

%

Date

Perf

orm

an

ce

Facto

r %

0 0

100 5.0

2014 2015 2016 2017

Array Navigator

Topics






Infrared Imaging

Image 383

22 C 45 C

One

shaded

cell

Global Solar & Alternative Energies

Solmetric

Solmetric

• A PV system is a thermal

system too, but we need eyes

to see these processes.

• In addition to finding areas of

localized power dissipation,

IR imaging finds sections of

the array that are not

producing (far left, top and

bottom).

• Natural companion tool to I-V

curve tracing (see quote).

“We find that our I-V curve tracers and our infrared cameras are

very often used together to diagnose array performance issues.

For example, if we see an I-V curve with an irregular shape we

immediately view the string with the IR camera to look for

anomalies. Likewise, if we see hot spots on an array or in the

wiring, we use the I-V curve tracer to record and document the

actual performance”.

Jeff Gilbert, Dir. O&M Services, Vigilant Energy Management

Aerial Thermography

Portland Habilitation Center, PV by Dynalectric Image courtesy of Oregon Infrared http://www.oregoninfrared.com/

Open strings

Module issues

Topics






Granularity & blind spots

Graphics from:

Determining Optimal PV System

Monitoring Granularity

SolarPro, Issue 6.5, Aug/Sep '13

Casey Miller & Todd Miklos

• Zone monitoring provides useful

granularity with lower up-front and

maintenance costs and lower

operational complexity than string

level monitoring.

• How can manual array measurements

supplement zone monitoring to make

it even more economically attractive?

‘Curve Into the Zone’ Complementing zone monitoring with I-V curve tracing

• Supplementing zone monitoring

with periodic manual string I-V

curve tracing and IR imaging

boosts sensitivity to performance

issues.

• Curve tracing can be performed

programmatically and in response

to zone alarms.

• All of the I-V curve data adds to

the detailed array performance

database.

• It is likely that the zone alarm

levels can be further optimized

based on I-V curve tracing data.

Commissioning PV Systems

with an O&M Focus

Paul Hernday

Senior Applications Engineer

[email protected]

cell 707-217-3094

May 7, 2014

Sandia/EPRI 2014 PV Systems Symposium

mailto:[email protected]

2014 pv reliability, operations & maintenance workshop: commissioning pv systems with an o&m...

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