choosing 600 or 1000 vdc in photovoltaic projects

A P P L I C AT I O N N O T E

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 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.

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; multi-tenant 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.

Choosing 600 or 1000 VDC in Photovoltaic Projects

CONTENTS

• Introduction

• Code ConsiderationsPage 1–2

• Electrical Design ConsiderationsPage 2

• ConclusionPage 3


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. Section 300.3 – Conductors over 600 V cannot occupy the same equipment wiring enclosure, conduit, or raceway as conductors rated for less than 600 V, including Ethernet or other network cabling.

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 standards do not require a UL listing on products operating above 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.

0.00%

20.00%

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600 V 1000 V

AC BOS

MV transformer

Inverters

DC BOS

Racking

Module

*BOS costs include labor, shipping, and taxes

Total Installed Cost Comparison – 1 MW

Electrical Design ConsiderationsAt 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 connections and they require 40% less combiner boxes.

Table 1. Balance of System Savings Comparison

Sample 1 MW DC System

600 V 1000 V Difference Going to 1000 V

Modules Per String

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)

AE 500NX-1kV inverter


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 inverter caused by voltage drop.

In a sampling of PV system reports with the same parameters for the same-size system, Ohmic wiring losses were about 1.5% less on the 1000 V simulation. The table below compares systems using two 500 KW inverters in 600 V and 1000 V versions.

Table 2. System and Revenue Loss Comparison

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

Loss In System Efficiency

2.6% 1% 1.6%

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

AE 3TL 1000 V connection box AE 3TL 600 V connection box

Conclusion1000 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. For these reasons, the availability of products rated for operation at 1000 V has continued to grow, and the cost of these products has dropped to be more competitive with those rated for 600 V.

The trend is clear: More designers are choosing 1000 V systems owing to the cost savings and other benefits that result from having fewer circuits, smaller wire, less labor, and lower losses. The applicable codes and standards are also changing to better accommodate this growing demand for 1000 V systems. To begin taking advantage of this trend, check with your AHJ to learn which applications are now (or will be when installed) eligible for operation at 1000 V. And expect to discover that 1000 V systems are, when properly designed and installed, just as safe as 600 V systems, which 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 500NX-1kV installation in Arizona


Advanced Energy® is a U.S. trademark of Advanced Energy Industries, Inc.

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.com800.446.9167 • [email protected] • [email protected]

© 2014 Advanced Energy Industries, Inc.

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choosing 600 or 1000 vdc in photovoltaic projects

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