When I switched careers from defense R&D to renewable energy in 1994 it was a thoughtful decision. When I started my own renewable energy business it was a calling. And when I started having children it became a responsibility. I had to take a step back and really analyze whether this could work as a career choice. There were two questions, technical feasibility and economics.
By time I was active in the industry, robust, reliable, safe solar modules were being manufactured, they were expensive but it was clear that the key technical problems had been solved. By the mid to late 1990’s modules had Underwriters Laboratories Listing (UL1703) and 20 year warranties were becoming standard. The UL listing provided an assurance of safety that made the PV modules acceptable in many applications including installation on residential construction.
The final technical frontier was the development of safe, reliable inverters. Inverters are the devices that convert the power from the modules, Direct Current (DC), like out of a car battery, into Alternating Current (AC), that is compatible with what is used in our homes and businesses. Much of the basic feasibility was demonstrated in the 1980’s by American Power Conversion (APC) with their SunSine inverters that were deployed in a pilot test in Gardner Massachusetts. At that time power semiconductors were an emerging technology that held the promise of inexpensively converting DC to AC. A group of scientists and engineers at MIT Lincoln Laboratories were learning about the potential of switch mode power conversion using power semiconductors. These devices could be used to convert from AC to DC very efficiently and fairly inexpensively. Realizing the potential of this technology they set out on their own and launched American Power Conversion (APC). They quickly commercialized the SunSine inverters which were installed in operational utility grid connected solar projects. However utilities were expressing considerable concern over the safety and viability of these inverters on “their” grid. The concerns centered around two issues.
What would happen if a line were disconnected from the utility and a PV inverter kept running. Would a lineman be exposed to electrocution. For example, if someone crashed a car into a power pole and the line was broken. The linemen repairing it would shut off the switches and isolate that line. They would then work on the line assuming that it was not energized. But a solar inverter could continue to operate quietly and without their knowledge exposing them to lethal voltage. This run on is called islanding. The circuits and algorithms to prevent it are referred to as anti-islanding technology.
The second concern was the quality of the power produced by the inverters. Utility grid power alternates from a positive voltage to a negative voltage in a repeating fashion. If you graph the voltage over time it is a very smooth curve called a sinewave. If the sinewave is distorted, it can degrade devices that are using or transmitting that power. Of particular concern to the power companies was their distribution transformers. Any diminished of the lifetime or reliability of these systems was of great concern to them.
As it turns out, switch mode power conversion can be utilized to make very “clean” power. That is, power whose sinewave is very smooth. Further, it is relatively straight forward to build anti-islanding circuitry that will shut off the inverter in most islanding scenarios. The APC inverter incorporated this type of technology and it worked well in practice. However, “practice” at that time was placing solar arrays in a very few demonstration projects. The utilities remained concerned about how multiple inverters, close together, might interact. Massachusetts Electric Company (Mass Electric) was a progressive utility and they wanted to put this to the test.
Mass Electric found a neighborhood and offered each resident their own solar array if they would agree to be part of a test program and allowed their systems to be monitored. 40 solar arrays were deployed and all concerns were allayed. The systems worked well. The power quality was good and there have been no published incidents of unsafe operation. The program had the added benefit of demonstrating the longevity claims of the industry with systems still operational after 20 years. However, while remarkably reliable, these inverters did require maintenance and they were somewhat costly on an inflation adjusted basis.
Further, at the time these inverters were introduced there was no UL standard to assure inverter safety so an electrical inspector had no way to know whether an inverter was safe. The National Electric Code had little to cover the DC wiring from the solar array to the inverters so electricians has no guidance on safely wiring the inverters. Finally there were no interconnection guidelines to for inverter designers to utilize in the design of their inverters. For example, just how “clean” did the inverter output waveform need to be. How far off the expected frequency was permissible before the inverter decided that there was a problem and automatically shut off. During the 1990’s all of these issues were resolved. The National Electric Code introduced Article 690 that provided electricians with the guidance they needed for installing solar arrays and inverters. Underwriters Laboratories (UL) developed UL 1741. If an inverter gained the UL 1741 listing, electrical inspectors could rest assured that it would not start a fire in a home or building. The IEEE in conjunction with Sandia National Laboratories and a variety of industry and utility stakeholders, developed IEEE 929 a set of recommended practices that addressed issues such as anti-islanding to assure proper operation of a solar energy system interconnected to the utility grid. IEEE 929 was implicitly given the weight of a standard by its inclusion into UL 1741. It has since been superseded by IEEE 1547 which is an interconnection standard – http://grouper.ieee.org/groups/scc21/dr_shared/ These codes and standards as well as maturing switchmode power conversion moved inverter technology to the mainstream, assuring safe products for interconnecting to the utility grid.
Fast forward to today and power electronics and microprocessor technology have evolved and 100’s of thousands of inverters have been deployed giving the industry an immense experience base. Inverters today are highly safe, efficient and operate reliably for years. The industry is rapidly approaching inverters that can be counted on to operate for decades like the solar modules that they are connected to. See the post on SiC
Copyright 2011, Clayton Handleman, all rights reserved