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Abstract
2012 was a historic year for the U.S. solar industry. There were 3,313 megawatts (MW) of photovoltaic (PV) capacity installed throughout the year, which represents 76% growth over 2011’s record deployment totals. The fourth quarter of 2012 was the largest quarter on record as 1,300 MW came online, driven in part by unprecedented installation levels in the residential and utility markets. SEIA and GTM Research forecast that the market will continue to grow at a steady clip with over 4,200 MW of PV and 940 MW of concentrating solar power (CSP) expected to come online in 2013.1 PV installation (Courtesy of SID AllEarth installation)
With the growth of PV residential installations and increased incentives, many utilities across the nation are investigating in the impact of both residential and industrial installations upon their existing electrical infrastructure. This case study examines one such project at North Attleboro Electric Department in Massachusetts.
Initial Conditions
North Attleboro Electric Department (NAED) is a public power utility headquartered in North Attleboro Mass. In August of 2012, the board of electric commissioners reviewed results of a customer survey. The results included 38% (as compared to 31% in 2010) of those surveyed believed that it was important to their business that NAED research alternative energy sources and include them as a generation options. As a pro-active public service, NAED commissioned a local study on several residential properties to assess and monitor their performance and electrical power generation from solar photovoltaic panels installed on residential homes. One of the major test criteria was whether the excess power generated on each PV system met with the ANSI C84.1.2006 operating tolerances and standards for residential voltage delivery. Each of the systems tested generated between 5-10kW; excess power not consumed by the customer was delivered back to NAED and the customer received a credit.
ANSI C84.1.2006 defines a voltage range of a 3 wire nominal low voltage system as a minimum of 228 VRMS and a maximum of 252 VRMS. With this as a reference standard NAED measured end of line residential PV customers at their service panel nominal of 240 VRMS, using a PMI Boomerang form 2S voltage monitor. Once powered up, this meter records an average every 1 second and transmits 1800 of these averages as a 30 minute highly compressed data package using its built-in cellular network service. This data was transmitted directly to a web hosting server known as Canvass. Figure 1 shows a recent week of activity where, NAED set up a custom scale to easily view if any voltage tolerances exceeded the ANSI.C84.1.2006 limits.
Figure 1. Voltage tolerances exceeded
One such condition was detected and can be clearly seen on the graph shown in Figure 1.
With this important information captured with a specific date, time, location and voltage minimum, NAED were able to conduct further investigations on their delivery system, or determine if a major loading factor had caused a significant voltage drop.
Residential PV Power Trending
The State of Massachusetts was ranked 6th in the Nation for Solar Generation in 2012 (see Figure 2) and is actively involved in many pilot studies to offer residential customers incentives to be more self-sufficient and less reliant on their local utility to provide their electrical power needs.
Figure 2. 2012 YIR ranking of solar generation
NAED is currently reviewing data from an existing PV residential study to determine to overall effect of these individual solar installations. A concern is the quality of the power generated by the solar PV systems and if they comply with expected and accepted practices.
It is important to understand that like any other residential load, Solar PV Panels on the roof of a residential home must meet standards for the industry and not unduly affect other residents or the overall utility voltage delivery system, because unlike typical domestic electrical loads, PV Panels generate electrical power through a AC/DC inverter and power not consumed by the residential home is delivered through a transfer switch back to the local utility provider.
Solar PV Power Profile
Arguably the most important factor when reviewing Solar PV systems is efficiency and solar production of kW. The clear advantage of a an independent power generating PV installation is its ability to provide additional Watts sufficient enough to provide power for the residential consumers daily demand, and below we see a weekly trending of kW from such an installation. Most of the NAEC testing was conducted on systems that generated between 5-10kW each. We can clearly see in Figure 3 a typical profile of PV power generation during off-peak times of the day when customer domestic loads such as pumps, motors, cooling or heating demands were at their lowest.
Figure 3. Canvass Daily Profile of kW from PV solar residential property
One of the issues facing NAED was the raw nature of electrical power generation and delivery. The instant that power is delivered, it’s consumed, so storing power for future use is still a major challenge in this growing renewable industry. Having a DC battery device to store excess power is a consideration; but, from an environmental standpoint, adding DC Batteries to off-grid storage, would appear to be counterproductive to the initial benefits of a renewable source of power.
What is interesting with this profile is to compare the daily weather patterns also during this recording to determine if real power production varies during daily weather patterns, as shown in Figure 4.
Figure 4. April 2013 weather pattern (www.weather.com)
On Saturday April 20th there does appear to be a reduction of power generated as reflected by the reduction of excess delivered back to the NAED network, and certainly the lack of sun during a cloudy overcast day, would appear to confirm that.
Conclusion
There is no doubt that using that solar PV generation for residential homes has come a long way over the last 10 years. For NAED and the State of Massachusetts, the cost of power for residents and a growing demand for more power is a key factor. As a progressive utility provider, NAED is setting the standards for future partnerships with its residential and commercial customers.
PMI is proud to partner with NAED to support important projects on renewable energy by providing leading edge equipment. Having 24 x 7 access to detailed power studies is providing North Attleboro Electric Department with invaluable data.
John Carroll
BSEE. M.InstEEE Sales Engineer
TheEngineers@powermonitors.com
https://www.powermonitors.com
(800) 296-4120
1. All data from Solar Energy Industries Association (SEIA)/GTM research “U.S. Solar Market Insight 2012 Year-In-Review” unless otherwise noted.