C00/ 4094-7 SAFETY PROCEDURES FOR THE 25-kW SOLAR ... · ABSTRACT Since the 25-kW solar photovoltaic agricultural field test system at the University of Nebraska is a unique electrical

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C00 / 4094-7

SAFETY PROCEDURES FOR THE 25-kW SOLAR PHOTOVOLTAIC

ARRAY AT MEAD, NEBRASKA

S. E. Forman

E. E. Landsman

Massachusetts Institute of Technology

Lincoln Laboratory

Lexington, Massachusetts 02173

7 April 1978

Prepared For

THE U.S. DEPARTMENT OF ENERGY

UNDER CONTRACT NO. EY-76-C-02-4094

This report was prepared as an account of work sponsored by the United States Government. Neither the Uni ted States nor the United States Department of Energy, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights.

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SAFETY PROCEDURES FOR THE 25-kW SOLAR PHOTOVOLTAIC

.ARRAY AT MEAD, NEBRASKA

S. E. Forman

E • E. Landsman

C00-4094-7

7 April 1978

Massachusetts Institute of Technology Lincoln Laboratory

Lexington, Massachusetts 02173

ABSTRACT

Since the 25-kW solar photovoltaic agricultural field test system

at the University of Nebraska is a unique electrical power system, special

safety rules and regulations are needed to govern its operation. Field

inspection and maintenance operations require the handling of electrically

active elements during daylight hours. This report serves to enumerate the

methods and techniques necessary to perform these operations in a safe manner

and to make field personnel more safety conscious as well.

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ABSTRACT

LIST OF FIGURES

BACKGROUND AND GLOSSARY OF TERMS

GENERAL INSTRUCTIONS

SAFETY PROCEDURES

CONTENTS

I. Physical Inspection and Cleaning of Array Front Surface

II. Electrical Work on Back Surface of Array Frames

III. Locating an Open Module in a Pair or Quad

IV. Removing a Defective Panel EXCEPT for Completely Open-Circuited Panels

V. Removing a Completely Open-Circuited Panel

VI. Locating a Ground-to-Frame

VII. Locating an Open Panel in a String

VIII. Alternative Method to Find an Open Panel or Bad Interconnects

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LIST OF FIGURES Page

Fig. 1. Aerial view of Mead, Nebraska, photovoltaic array. viii

Fig_. 2. Close-up view of photovoltaic array showing two 2 rows of frames and photovoltaic modules.

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Fig. 1. Aerial view of Mead, Nebraska, photovoltaic array .

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BACKGROUND AND GLOSSARY OF TERMS

As part of the DOE's demonstration program for terrestrial photovoltaic

modules, MIT/Lincoln Laboratory has designed and built a 25-kW experimental

test facility at the University of Nebraska Field Station in Mead, Nebraska.

At the time of its incept~on in July 1977, it was the largest active

photovoltaic power source in the world. An aerial view of the site is

shown in Figure 1.

The lifetime goal for the photovoltaic modules which make up the array

is 20 years. In order to assess whether this goal is being met, it is necessary

to periodically inspect the modules in the field, to remove modules from the

array for more detailed analysis in the laboratory, to make measurements which

locate electrically defective modules, to wash and clean the modules, and

to perform various other general maintenance and testing operations on the

modules. Since the modules represent active electrical elements and many

of the aforementioned operations must be performed in daylight hours, it

is necessary to develop a set of safety guidelines for the personnel involved

in performing the operations.

The Nebraska test site is a "first of its kind" type of facility and,

therefore, many of the safety rules and regulations associated with its

operation are unique. For example, the solar cells in the photovoltaic modules

are encapsulated in clear silicone rubber as opposed to being covered with

glass or hard plastic. In some instances, components of the modules were

inadequately covered by the silicone rubber and eventually became exposed due

to weathering. This type of hazard can be found only through inspection and

touching, and special rules must be developed so that there is no danger of

electrical shock to the inspector. In addition, this system uses batteries

for energy storage. Special guidelines for the handling of batteries in an

enclosed trailer are also needed. The basic intent of this report is to list

all the possibly hazardous operations which need to be performed on the array

subsystem and to ensure that each operation has a set of safety guidelines so

that each may be carried out with no danger.

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Fig . 2. · ff s a nd photovoltaic modul, Clos e -up of photovoltaic array showing two rows o r ame

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Prior to enumerating specific safety guidelines, a glossary is provided

to establish the terminology needed to describe various elements of a

photovoltaic system.

1. Module

2. Pair

3. Quad

As-manufactured assembly of encapsulated photovoltaic cells arranged on a substrate.

Two modules wired in parallel.

Four modules wired in parallel. (The word "panel" may be used interchangeably with pair or quad.)

4. String A number of panels electrically connected in series. (Pairs and quads are never mixed.)

5. Frame Metal structure to which one or more strings are attached. (The frame and its strings are often referred to as an array or array frame.)

6. Row Group of frames aligned to form a row. (This is often called an array group or a subfield.)

7. Field One or more rows of frames

8. Field Control Panel Electrical panel that controls the operational behavior of the field and various subsets of the field, i.e., frames and strings.

9. Frame Junction Box

10. Array Test Box

Electrical junction box attached to the rear side of each frame. Each string on the frame is wired directly to this box.

An electrical test box which can be used to isolate up to three strings from ground and also make various electrical measurements.

As shown in Figure 2, the field consists of two rows, each having fourteen

array frames. The front row contains modules from Solarex Corporation, and

the back row contains modules from Sensor Technology, Inc. Two Solarex modules

wired in parallel constitute a pair; ten pairs in series make up a string.

Four Sensor Tech modules wired in parallel are called a quad; nine quads

in a series make up a string.

Two-and-one-half Solarex strings are located on each array frame in the

front row. Three Sensor Tech strings are located on each of the fourteen array

frames in the back row.

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GENERAL INSTRUCTIONS

The purpose of this report is to enumerate a set of safety procedures

which must be followed in performing various operations at the Nebraska test

facility involving handling of active electrical components and wiring.

If at any time during the execution of the safety regulations described herein

any confusion or doubt arises as to the meaning or intent of the instructions

to be followed, a supervisory person should be contacted immediately.

Prior to working on any section or sections of the field, such areas

should be placed in the open-circuit mode at the field control panel and so

designated by signs that state: People at Work - Do Not Touch. The toggle

switches for specific areas to be worked on should be locked in the open

position - using the special switch panel locking cover which will be

provided at the site.

The following test equipment should be assembled prior to beginning work

on the photovoltaic installation:

1. One or more array test boxes that include separate meters to measure:

Leakage-current to ground String open-circuit voltage String short-circuit current

2. Multimeter for voltage, current, and resistance measurements.

3. Six, 2-foot-long, insulated wires with alligator clips at each end to ground panels or wires.

4. Log book containing ranges of values for particular measurements to be made.

As an overall precaution, no wiring should be considered safe until

checked personally with an appropriate measuring instrument. All entries

in the log book should be accompanied with the date, the time of day, and the

signature of the person making the entry.

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SAFETY PROCEDURES

I. Physical Inspection and Cleaning of Array Front Surface

A. Place those sections of the field to be worked on in the open-circuit

mode via the field control panel in the trailer.

CAUTION: A high voltage exists within module strings.

Avoid contact with modules.

B. Ground the array test box to the metal support frame being worked on

by attaching the clip lead to the frame.

c. Verify that test box is set at open-circuit for all strings.

D. Pull all connectors from junction box of frame being worked on.

NOTE: There should be no arcing when disconnecting connectors.

If arcs occur, notify supervisor before proceeding.

Cap open junction box connectors.

E. Attach the connectors from each string on the frame to the test box

via mating connectors that are in place on test box. Match string

numbers when possible (i.e., unless they are not noted on connectors

at the frame junction box).

F. Test all strings for leakage-current to ground.

NOTE: If meter dial registers more than 10 microamps, STOP, do not

proceed with inspection.

Notify supervisor.

Return all ground test switches to the OPEN position.

G. Measure and record in the log book the open-circuit voltage, short

circuit current, and ground currents for all strings being worked on.

(Log book contains range of acceptable values.)

NOTE: Notify supervisor if discrepancies occur.

H. Place all strings in shorted condition in the test box.

I. Proceed with inspection and/or cleaning.

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J. After completing inspection and/or cleaning, return strings to open

circuit mode and remove string connectors from test box.

K. Remove frame junction box caps and reconnect string connectors

to proper points on junction box.

NOTE: No arcing should occur.

L. Verify that strings are working properly by noting the open-circuit

voltage and short-circuit current of the frame worked on via field

control panel in the trailers.

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II. Electrical Work on Back Surface of Array

A. Preparation

1. Place those sections of the field to be worked on in the open

circuit mode via the field control panel in the trailer.



2. Ground the array test box to the metal support frame being worked

on by attaching the clip lead to the frame.

3. Verify that test box is set at open-circuit for all strings.

4. Pull all connectors from junction box of frame being worked on.




5. Attach the connectors from each string on the frame to the

test box via mating connectors that are in place on test box. Match

string numbers when possible (i.e., unless they are not noted on

connectors at the frame junction box).

6. Test all strings for leakage-current to ground.



Notify supervisor.

Return all ground switches to the OPEN position.

7. Measure and record in the log book the open-circuit voltage,

short-circuit current, and ground currents for all strings being

worked on. (Log book contains range of acceptable values.)


8. Place all strings in shorted condition in the test box.

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B. Connect a ground as close as electrically possible to each wire

connected to the work site, but not at the work site.

OBJECTIVE: Guarantee that all wires are grounded even if disconnected

during the work.

NOTE: A. Grounding connections may, under some conditions, draw

an arc. Thus, keep eyes and skin away from the point of

electrical contact.

B. When frame grounding, always attach lead to frame first,

then to the point being grounded.

C. Measure voltage directly from the work sites to the frame. Proceed

with planned operation unless dangerous voltages are noted (over 50 volts).

D. Upon completing planned operation, remove ground carefully as an arc

may be drawn.

NOTE: Remove lead from grounded point first, then remove lead from frame

ground.

E. Open-circuit all strings.

F. Remove string connectors from test box.

G. Reconnect string connectors to proper points on frame junction box.

NOTE: There should be no arcing.

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III. Locating an Open Module in a Pair or Quad

A. Preparation





2. Ground the array test box to the metal support frame being worked

on by attaching the clip lead to the frame.





Cap junction box connectors.


test box via mating connectors that are in place on test box.

Match string numbers when possible (i.e., unless they are not noted

on connectors at the frame junctio~ box).

6. Test all strings for leakage current to ground.



Notify supervisor.


B. With strings open circuited and test box ground switches in open

condition, measure short-circuit current of panels in question with a

multi-meter switched to a current scale greater than the short-circuit

current of a string.

NOTE: Accidental shorts of measuring leads to the support frame should

produce no arcing.

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C. A short-circuit current that is an integer fraction less than one,

related to the number of modules in parallel, indicates an open module in

a panel.

EXAMPLE: I is 3/4 normal in a 4-module set or 1/2 normal in a SC

2-module set. See table of expected values in the log book.

D. Upon identifying the defective panel, cast a shadow over each module

in the panel separately to locate the one that produces no change in

measured short-circuit current and note its serial number.

NOTE: Modules wired backwards can produce strange results in the above

steps. For pairs, the short-circuit current will be zero if one module

is wired backwards. For quads the short-circuit current will be reduced

from the normal value and shadowing each module in the quad will produce

meter response.

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IV. Removing a Defective Panel EXCEPT for Completely Open-Circuited Panels

A. Identify panel to be removed per Section III.

B. Preparation

1. Place those sections of the field to be worked on in the

open-circuit mode via the field control panel in the trailer.



2. Ground the array test box to the metal support frame

being worked on by attaching the clip lead to the frame.

3. Verify the test box is set at open-circuit for all strings.

4. Pull all connectors from junction box of frame being

worked on.





test box via mating connectors that are placed on test box. Match






Notify supervisor.


7. Measure and record in the log book the open-circuit voltage,




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C. Ground both sides of panel to be removed to array frame.

NOTE: 1. If defective panel is at the end of a series string

of panels, it is necessary to ground at the adjacent panel and

at the other end of the string.

OBJECTIVE: Remove defective panel while leaving all ground wires

in place so that no remaining wires can become hot electrically.

2. When grounding a panel to the frame, always attach lead wire

to the frame first, then to the point on the panel being grounded.

3. Grounding connections may draw an arc; thus keep eyes, skin, and

clothing away from the point of electrical contact.

D. Measure voltage directly from the work sites to the array frame.

Proceed if voltages do not exceed 50 volts.

E. Remove panel's electrical mating plugs. Remove defective panel.

F. Install new panel and mate electrically.

G. Remove ground leads.

NOTE: Remove lead from panel first and then from frame.

CAUTION: Arcing as noted in step C-3 may occur.

H. Test all strings for leakage-current to ground.



Notify supervisor.


I. Measure and record in the log book the open-circuit voltage,



NOTE: Notify supervisor if discrepancies occur. Also record serial

numbers of removed panel and newly installed panel.

J. Remove string connectors from test box.

K. Reconnect string connectors to proper points on frame junction box.


L. Verify proper operation of strings by noting open-circuit voltage

and short-circuit current via field control panel in the trailer. 12

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V. Removing Completely Open-Circuited Panel

(Cause may be a bad diode, which rarely occurs.)

A. Identify panel to be removed per Section III.

B. Preparation

1. Place those.sections of the field to be worked on in the

open-circuit mode via the field control panel in the trailer.



2. Ground the array test box to the metal support frame being

worked on by attaching the clip lead to the frame.







test box via mating connectors that are in place on test box. Match






Notify supervisor.

Return all ground test switches to the OPEN position.

7. Measure and record in log book open-circuit voltage, short

circuit current, and ground currents for all strings being worked

on. (Log book contains range of acceptable values.)



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C. Open-circuit the string to be worked on.

D. Ground both sides of panel to be removed to array frame.

NOTE: If defective panel is at the end of a series string of panels,

it is necessary to ground at the adjacent panel and at the

other end of the string.

OBJECTIVE: Remove defective panel while leaving all ground

wires in place so that no remaining wires can become hot

electrically.

NOTE: 1. When grounding a panel to the frame, always attach

lead wire to the frame first, then to the point on

the panel being grounded.

2. Grounding connections may draw an arc; thus

keep eyes, skin, and clothing away from the point of

electrical contact.

E. Short-circuit string being worked on.

NOTE: All strings connected to test box should now be shorted.

F. Unmate defective panel electrically.

G. Remove defective panel.

H. Install new panel and mate electrically.

I. Remove ground leads.

NOTE: Remove leads from panel first and then from frame.

CAUTION: Arcing as noted in Step D-2 may occur.

J. Read and record in log book short-circuit current and open-circuit

voltage per test box meter. New module is functioning satisfactorily if

reading is within range of values in log book.


K. Test all strings for leakage-current-to-ground.

L. Return all ground test switches to OPEN position.

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M. Return strings to open-circuit mode and remove string·

connectors from test box.

' N. Remove frame junction box caps and reconnect string

connectors to proper points on junction box.


O. Verify that strings are working properly by noting the

open-circuit voltage and short-circuit current of the frame

worked on via field control panel in the trailer •

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VI. Locating a Ground-to-Frame

A. Preparation





2. Ground the array test box to the metal support frame being

worked on by attaching the clip lead to the frame.


4. Pull all connectors from junction box of frame being worked

on.




5. Attach the connectors for each string on the frame to the

test box via mating connectors that are placed on test box. Match



6. Test all strings for leakage-current to ground to verify that

frame ground exists. The ground will cause values of leakage-current

greater than 10 microamps.

NOTE: Return all ground switches to the OPEN position.

B. Measure string voltage (V) between positive and negative test points s

on the test box using a voltmeter.

C. Using a voltmeter, measure voltages from positive-to-ground and

negative-to-ground on test box (V+ and V ).

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D. V+ plus V_ should equal Vs. The ratio (V+/V_) should indicate how

far from either end of the string the ground is.

EXAMPLE: If V = 230 and V = -89 and there are nine panels in the s string, the ground is in the middle of the fourth panel from the

negative side. To zero in, check voltage on both sides of unit 4

to ground. If voltages are of opposite polarity to ground (i.e.,

voltage changes sign) defective panel has been located. Ratio of

either V+ or V to Vs will indicate the approximate position of the

ground in the panel.

E. Remove defective panel per Section IV.

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VII. Locating an Open Panel in a String

A. Short-circuit suspected string at field control panel. (Minus

side of string is assumed permanently grounded.)

NOTE: String is not open if a current is indicated at field control

panel.

B. Measure voltages within string to ground.

NOTE: When voltage changes sign the the open is passed.

CAUTION: Connect one voltmeter lead to ground first and then connect

the other voltmeter lead to the point to be measured.

DANGER: Be careful not to accidentally make bodily contact with any

terminals.

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VIII. Alternative Method to Find Open Panel or Bad Interconnects

A. Follow Section III, but measure panel short-circuit current between

negative side of panel to negative side of adjacent panel. No short

circuit current will be present if an open exists.

NOTE: The same test can be made between positive sides •

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Documents

C00/ 4094-7 SAFETY PROCEDURES FOR THE 25-kW SOLAR ... · ABSTRACT Since the 25-kW solar photovoltaic agricultural field test system at the University of Nebraska is a unique electrical