High Voltage Photovoltaics
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The standard maximum dc voltage for PV modules and solar inverters installed in the US is 600 volts. The National Electrical Code drives these voltage classifications by definition: in NEC 490.2, high voltage is defined as “more than 600 volts, nominal.” As such, circuits in the US operating at more than 600 V to ground require special high voltage equipment and are subject to additional safety guidelines defined in the Code.
In Europe, however, the standard maximum voltage for PV equipment is 1,000 Vdc. As a result, some solar products available for use in the US carry a maximum rating of 1,000 Vdc. While NEC 690.7(C) specifically excludes the use of circuits over 600 V in one- and two-family dwellings, other installations may allow for the use of high voltage circuits. For example, some “behind the fence” systems owned and operated by utilities and installed on utility-owned property are configured for 1,000 Vdc maximum. What are the benefits of higher PV utilization voltages?
What challenges do higher utilization voltages present to PV system designers and installers? What code or certification issues would need to be addressed to lift the voltage ceiling to 1,000 Vdc here in the US, and would that benefit the US PV industry?
Challenges and Benefits of 1,000 Vdc
Bill Brooks / Brooks Engineering / Vacaville, CA / brooksolar.com
Section IX of NEC Article 690, “Systems over 600 Volts,” was added in the 1999 revision in preparation for high voltage systems. Integrators in Europe have been installing systems that operate at up to 1,000 Vdc for several years now. However, key differences between PV systems installed in Europe and the US do exist, the most notable being that in Europe system conductors are not grounded. Ungrounded system conductors tend to float evenly on either side of the ground reference, with the voltage to ground averaging approximately 500 Vdc. In addition, not having a hard system ground allows ground fault detection circuits to be much more sensitive, providing protection in the 10s or 100s of milliamps rather than the 5,000 milliamps setting commonly used with most large inverters in the US.
Moving to 1,000 Vdc in the US presents many challenges, but the NEC and the positive design benefits clearly encourage a move in this direction. Pushing US systems to 1,000 V, however, brings up issues in system design and equipment that do need to be addressed.
First, system design must be reviewed. The ungrounded European method seems like an excellent idea above 600 Vdc; in general, we need to migrate toward ungrounded, grid-tied PV systems in the US. In fact, to be consistent with NEC 250.162, most PV systems operating at over 300 Vdc should probably be ungrounded. Currently, ungrounded systems must meet the wiring requirements set forth in NEC 690.35, “Ungrounded PV Power Systems.”
Second, some equipment issues must be addressed. In 1,000 Vdc systems, all conductors, disconnects, fuses and inverters on the dc side must be rated for at least 1,000 Vdc. This greatly narrows product offerings. For example, in the US market there are few fuses listed for 600 Vdc, and it is much more difficult to source fuses that are listed for 1,000 Vdc. European fuses are generally not listed to US standards, and, quite frankly, are probably inferior to the fuses used in the US.
The equipment issues are complicated not only by fuse availability but also by the ratings of other equipment. PV Wire, also called PV Cable, is required for ungrounded systems and is currently available in the US with a 600 Vdc rating only. It should be straightforward to order 1,000 Vdc cable, but it takes a large order to get a company to manufacture it. Disconnects are another challenge, requiring much more expensive medium voltage switchgear or getting European products listed for the US market. The latter is not necessarily difficult, but it is time consuming and expensive. Lastly, inverters must be rated for 1,000 Vdc. This means the US testing laboratories may need to upgrade some of their test equipment to evaluate higher voltage inverters. These are all significant hurdles.
So what is the upside to 1,000 Vdc? The positives are plentiful. First, there is a huge benefit in wiring the array. Just think about being able to increase string length from 12 modules to 20 modules for a typical 5-inch cell, 175 W module. This increases the power per string from 2.1 kW to 3.5 kW. More power per string means more power per labor hour, since the wiring costs per string are heavily weighted by terminations in combiner boxes and running homerun conductors. Now consider a larger 6-inch cell, 224 W module. In California, 24 of these modules could be wired in series without exceeding 1,000 Vdc, resulting in a whopping 5.4 kW per string. Now we are talking about some real installation efficiency gains.