A few months ago, I was driving home from the Los Angeles area and suddenly found myself surrounded by thousands of wind turbines lining both sides of Interstate 10. Even though I have made this trip many times, I am still in awe at the scale of it all. Looking through my windshield (but still watching the road) at the miles of wind turbines and substations that make up the San Gorgonio Pass Wind Farm, I began to wonder how much electricity is actually being produced by these giant machines. I later learned it is a 615-megawatt (MW) facility.
Recently, I was at another large-scale renewable-energy facility. This time, it was a photovoltaic (PV) plant known as Agua Caliente in Yuma, Ariz. This site recently observed a major milestone as its capacity hit the 100-MW mark. When completed, the total capacity will reach 290 MW, making it the largest PV facility on the planet. Once again, I was in awe as I looked out over a sea of PV modules.
As we continue to hear the battle cry for American energy independence and renewable energy, it is exciting to see how many people are on a crusade to meet the challenge. I finally realized this is where my awe came from. It wasn’t so much the physical scale of the systems as it was the sheer number of people involved to make it all happen. Yes, we still know how to rise to the occasion.
Wind and PV—the numbers
The statistics speak for themselves. According to the American Wind Energy Association (AWEA), by the end of 2011, there were 46,919 MW of utility-scale wind power generation capacity. For 2011, 6,810 MW of new capacity was installed, and as of the end of 2011, there was an additional 8,320 MW under construction.
According to the National Renewable Energy Laboratory (NREL), the United States has an onshore wind resource potential of 10,400 gigawatts (GW) and an additional 4,150 GW available offshore.
In regard to PV, though small, individual module arrays are common, grid-connected PV systems are beginning to see unprecedented growth. Some estimates place the capacity of projects under development or construction well into double-digit gigawatts. The Agua Caliente project may be on track to become the world’s largest PV power plant; however, that record won’t stand long. There are two much larger projects already under construction in California, topping out at 550 MW each, and it is just the tip of the PV iceberg.
What do these numbers mean? The obvious answer is that wind and PV are playing an ever-increasing role in our energy supply. The less obvious answer is that electrical safety for these systems has become more important than ever.
Green = caution?
The ANSI Z535 standard is used to define signal words and colors. Although energy sources, such as wind and PV, are commonly referred to as “green energy,” electrical hazards such as shock and arc flash happen to be color-blind, and sometimes they can be well hidden. In fact, the same cautions that apply to conventional electrical sources also apply to green ones.
Electricity (and electrical hazards) from the wind
The concept of harnessing energy from the wind has been around for centuries. Although it may not be new, today’s wind turbines have taken it to a whole new level. Many land-based wind turbine generators (WTG) have capacities now reaching several megawatts each, and we keep pushing the envelope.
With wind turbines sprawling out over a large area, sometimes covering several miles, a wind farm requires a large distribution system to connect them all together and to the utility grid. The “collector system,” as it is called, is made up of underground and sometimes overhead medium-voltage feeders frequently operating at 34.5 kilovolts (kV).
The output voltage of a WTG typically ranges from 480 to 690 volts (V), and connecting it to the collector system requires a step-up transformer. The power from the individual WTGs is combined and connected to a more traditional substation where it is ultimately interconnected with the electric utility grid.
Just like a more traditional electrical system, the shock and arc flash hazards must be considered part of electrical safety practices. However, the arc flash hazard may not be straightforward. If an arc flash occurs, the resulting incident energy will not only be dependent on the short-circuit current from the individual WTG but from all of the other WTGs that are online as well.
An additional and often more menacing problem is that the utility system will also contribute to the arc flash, and utility short-circuit currents can be quite high. Depending on the operating speed of protective devices, the result could be a very dangerous and potentially deadly event.
Consequently, the arc flash hazard has caught the attention of the Occupational Safety and Health Administration (OSHA). In 2009, OSHA reported the investigation of 32 arc flash accidents at wind farms. As one example, in 2010, OSHA investigated an arc flash at a wind farm where a worker was severely injured.
Fast forward to 2012, OSHA is now targeting the wind-energy industry with a National Emphasis Program (NEP). OSHA announced that the operation and maintenance of wind turbines fall under the same regulations that govern electrical safety of more traditional power plants.
Electricity (and electrical hazards) from the sun
There may be some similarities between utility-scale PV installations and conventional power generation, but there are also some major differences. One key difference is with the electricity production itself. Large arrays of PV modules are lined up, basking in the sunshine. No sound, no moving parts and no indication at all that anything is happening. This absence of visual or audible indication has created an unintended hazard. If the PV modules are just quietly sitting there, how can there possibly be a hazard?
Without any audible or visual triggers, a person can be lulled into a false sense of security, and it has become a safety concern in the PV community. Working within the PV culture requires personnel to be trained to understand that they should not touch anything during daylight hours.
The biggest defining trait of a PV system is the energy source—the sun. Since you can’t exactly flip a switch and turn it off, PV modules are always energized during the day. This seems to present a problem when trying to establish an electrically safe work condition, to comply with NFPA 70E, the Standard for Electrical Safety in the Workplace. This condition requires, among many steps, de-energizing and verifying the absence of voltage.
I discussed large-scale PV systems with Rob Rynar of Tempe, Ariz.-based First Solar, which is building the Agua Caliente plant and the two 550-MW plants in California. I asked how First Solar can establish an electrically safe work condition?
“It’s easy,” he said. “We have a scheduled outage once every 24 hours. It’s called nighttime. Any live work is simply deferred until nondaylight hours.”
Even after the sun goes down, the workers treat the system as if it were energized until they verify the absence of voltage. In addition, arc-rated protective clothing and personal protective equipment (PPE) is always used, whether it is day or night.
Typical voltages on utility-scale PV systems are increasing all the time, and many systems operate upward of 1,000V. There is pressure to further increase the voltage to more than 1,500V. This means workers on PV systems are not immune to the electric shock hazard. Fortunately, the 2012 Edition of NFPA 70E introduced Table 130.4(C)(b) for approach boundaries for direct current (DC) voltage systems, and it can be used to help address this hazard.
Although the arc flash hazard technically exists on the DC side of a PV system, it is not nearly as big of a hazard as on the alternating current (AC) side. PV modules are inherently current--limiting, meaning they do not individually produce much short-circuit current. The larger arc flash hazards exist on the inverter/utility AC side of the system. Short-circuit current flowing from the utility interconnection can result in a significant amount of incident energy, requiring special attention to the proper selection of arc-rated protective clothing and PPE.
Utility owned? Which standard?
There is often a debate about which codes and standards apply when it comes to electrical safety of utility-scale wind and PV systems. Of course, OSHA is at the center of it all, and the National Electrical Safety Code (NESC) is normally used for the substation, distribution and transmission portions. But what about NFPA 70E? Section 90.2 excludes certain installations, including those under the exclusive control of an electric utility system with certain conditions, such as the installation being located on property owned or leased by the electric utility for the purpose of generation. There are other conditions that reference locations, easements and recognition by public service commissions and similar regulatory agencies having jurisdiction. This language often spurs a debate over whether NFPA 70E applies.
Regardless of whether a system is technically exempt from NFPA 70E, it’s good practice for wind and PV installers to base their electrical safety practices on this important standard. Having to tell someone injured by an electrical hazard, “Sorry, this system was exempt from NFPA 70E,” is not in anyone’s best interest.
Keep it green and safe
As thousands of people continue to rise to the occasion and move us toward the goal of energy independence, let’s ensure they also rise to the occasion of making electrical safety part of the same goal.
PHILLIPS, founder of www.brainfiller.com and www.ArcFlashForum.com, is an internationally known educator on electrical power systems and author of “Complete Guide to Arc Flash Hazard Calculation Studies.” His experience includes industrial, commercial and utility systems, and he is a member of the IEEE 1584 Arc Flash Working Group. Reach him at firstname.lastname@example.org.