Download Choosing 600 or 1000 VDC in Photovoltaic

A P P L I C AT I O N N O T E
Choosing 600 or 1000 VDC
in Photovoltaic Projects
Introduction
The photovoltaic (PV) industry continues to benefit from the lower costs of solar modules and advances in
inverter technology. One of the more significant trends in recent years is the migration from 600 Volt DC
CONTENTS
•Introduction
Page 1
• Code Considerations
systems to 1000 Volt DC systems. For several years, 1000 V systems have dominated the utility “behind
the fence” market to reduce installation costs and improve performance. To achieve these same advantages,
some commercial systems are now being installed with 1000 V strings. With proper planning and practices,
1000 V systems can be just as safe as 600 V systems. This application note highlights the key considerations
when choosing between 600 V and 1000 V PV systems.
Page 1–2
• Electrical Design
Considerations
Page 2
•Conclusion
Page 3
Code Considerations
The National Electric Code (NEC) covers photovoltaic systems, but does not specify a maximum voltage
except for residential applications in Article 690.7 (C).
Article 690.7 (C) – Limits systems to 600 V on one- and two-family dwellings; multitenant dwellings over two units and commercial sites with systems over 600 V shall
comply with article 690 Part IX.
Article 690 Part IX (section 690.80) – Requires that PV systems over 600 V comply
with article 490.
Article 490 – While not written explicitly for PV systems, requires use of UL-listed
components when installing systems over 600 V.
The NEC only prohibits PV systems over 600 V in one- and two-family dwellings. A dwelling for three
or more families or any commercial site could, therefore, be a candidate for a 1000 V system. The way
the local authority having jurisdiction (AHJ) interprets the code is what determines which residential and
nonresidential applications are permitted to have systems over 600 V. Article 490 is vague and requires
the most interpretation by the AHJ. Ground-mount systems are generally the most allowable for 1000 V
installations, but in 2013, roof-mounted and carport systems using 1000 V began being allowed by many
AHJs. With systems over 600 V, there is a need to prohibit access by unqualified personnel, and there are
also higher rating requirements for the DC equipment. These additional requirements for enclosures are
covered in the following articles of the NEC.
A P P L I C AT I O N N O T E
Section 101.31 – Electrical equipment operating
above 600 V must be housed within a metal
enclosure that is marked with signs prohibiting
access by unqualified persons. This requirement
covers junction boxes, combiner boxes,
disconnect switches, and all other enclosures
that house systems operating above 600 V.
Any of this equipment that is not installed in a
secured equipment room or fenced-in area must
be protected with a lock. Electrical equipment
operating above 600 V that is located outdoors
must have at least 10 feet of clearance behind
a locked fence.
Electrical Design Considerations
At a mid-northern latitude, a 1000 V string using 72-cell modules
would have 20 modules in series. A 600 V string using the same
modules would only have 12 modules in series. 1000 V systems will,
therefore, require fewer combiner boxes to provide the same power
as a 600 V system. This means fewer strings, fuses, disconnects, and
combiner boxes. This balance of system (BoS) savings can yield a
reduction of up to 40% in the cost of the DC BoS.
In the sample system below, the 40% difference when going to
1000 V is illustrated in the following table. The strings have 40%
more modules in series and those strings carry 40% more power
when wired for 1000 V. These systems have 40% less parallel
Section 300.3 – Conductors over 600 V
connections and they require 40% less combiner boxes.
cannot occupy the same equipment wiring
enclosure, conduit, or raceway as conductors
rated for less than 600 V, including Ethernet
or other network cabling.
Table 1. Balance of System Savings Comparison
Sample 1 MW DC
System
There are other considerations for 1000 V systems, so it is important
to reference the version of the NEC that applies to the installation.
Owing to the increased use of 1000 V systems, the 2014 version of
the NEC is expected to bring greater clarity to high-voltage system
requirements.
Many utilities use systems based on standards published by the
Institute of Electrical and Electronics Engineers (IEEE), but these
600 V
12
20
+8 modules
(40% more)
Watts Per String
with 300 Watt
Modules
3,600
6,000
+2,400 W
(40% more)
Parallel Connections
278
167
-111 Strings
(40% less)
24 String Combiner
Boxes Needed
12
7
-5 String
Combiner
(40% less)
600 V. This is another reason it has been more common for utilities
to use 1000 V systems. Because these systems were typically secured
“behind the fence,” they do generally comply with the requirements
outlined here.
100.00%
AC BOS
MV transformer
60.00%
Inverters
40.00%
DC BOS
20.00%
Racking
Module
0.00%
600 V
1000 V
*BOS costs include labor, shipping, and taxes
Total Installed Cost Comparison – 1 MW
Page 2
Difference
Going to
1000 V
Modules
Per String
standards do not require a UL listing on products operating above
80.00%
1000 V
AE 500NX-1kV inverter
A P P L I C AT I O N N O T E
Additional savings are possible based on the size of the conductors.
1000 V-rated wire can cost more than that rated for 600 V, but more
energy is carried at 1000 V. Increasing the voltage of a conductor
requires more insulation, but the cost of insulation is much less than
the cost of copper or aluminum carrying the current. Conductive
losses are 25% less when voltage is doubled for the same amount of
power, and 1000 V systems are also less susceptible to losses at the
Conclusion
1000 V PV systems are standard in Europe because they are
supported by the European International Electrotechnical
Commission (IEC). Many manufacturers are now getting dual ratings
on their products to comply with both IEC and UL standards for
1000 V, which can also be used, of course, in 600 V applications.
inverter caused by voltage drop.
For these reasons, the availability of products rated for operation
In a sampling of PV system reports with the same parameters
has dropped to be more competitive with those rated for 600 V.
at 1000 V has continued to grow, and the cost of these products
for the same-size system, Ohmic wiring losses were about 1.5% less
on the 1000 V simulation. The table below compares systems using
The trend is clear: More designers are choosing 1000 V systems
two 500 KW inverters in 600 V and 1000 V versions.
owing to the cost savings and other benefits that result from
Table 2. System and Revenue Loss Comparison
The applicable codes and standards are also changing to better
having fewer circuits, smaller wire, less labor, and lower losses.
accommodate this growing demand for 1000 V systems. To begin
1 MW AC
600 V
1000 V
Difference
Going to
1000 V
Ohmic Wiring Loss
55.46 MWh/yr
21.22 MWh/yr
-34.24 MWh/yr
at 1000 V. And expect to discover that 1000 V systems are, when
Loss In System
Efficiency
2.6%
1%
1.6%
properly designed and installed, just as safe as 600 V systems, which
Energy Revenue Lost
@$0.15/KWh
$8,319/yr
$3,183/yr
-$5,136
Energy Revenue Lost
over 20 Yrs
$166,380
$63,660
-$102,720
taking advantage of this trend, check with your AHJ to learn which
applications are now (or will be when installed) eligible for operation
will likely cause you to begin choosing 1000 V for every project
allowed by your AHJ.
Advanced Energy has a variety of central and string inverters
®
that are rated for both 600 V and 1000 V use. Please contact
your regional salesperson to obtain a quote and one of our field
application engineers will assist with the design. For new customers,
AE has a “Contact Us” form on our website (http://solarenergy.
advanced-energy.com/en/Contact_Sales_ Representative.html) and
a dedicated phone line available at 877.312.3832.
AE 3TL 1000 V connection box
AE 3TL 600 V connection box
AE 500NX-1kV installation in Arizona
Page 3
A P P L I C AT I O N N O T E
For more information on AE inverters, visit www.advanced-energy.com.
Specifications are subject to change without notice.
Advanced Energy Industries, Inc.
1625 Sharp Point Drive • Fort Collins, CO 80525 U.S.A.
www.advanced-energy.com
800.446.9167 • [email protected] • [email protected]
Advanced Energy is a U.S. trademark of Advanced Energy Industries, Inc.
®
© 2014 Advanced Energy Industries, Inc.
All rights reserved. Printed in U.S.A.
ENG-600Vor1000V-260-02 0M 2/14