Alternating current modules at the farm in Concord

Worlds first ac PV module
by E C Kern Jr, Applied Power Corporation, USA

The first PV system in the world made from integrated alternating current (ac) modules provides 5.5 kW of electricity to power a home and an electric car at a farm in Concord, Massachusetts. Produced by Ascension Technology1, these modules are designed specifically for interconnecting with standard electrical systems used in commercial and residential buildings.

Background

Twenty years of government-supported research into roof-mounted PV systems in the USA converged in 1984 with the construction of the Gardner Model Photovoltaic Community project in Gardner, Massachusetts. This was the first project of its kind in the world. Special large-area PV modules were installed on the roofs of 30 homes, connected with 2 kW SunSine¨ inverters in the basements. These inverters, among the first produced for interconnecting consumer PV systems to the electricity grid, provided power with very low harmonic distortion and unity power factor – the most desirable and safest type of electricity for utility system protection. Since then, the project has served as a national test bed for investigating distributed electric power technologies.

The next steps

The next development in rooftop PV systems in the USA came as a result of environmental rather than energy trends. In the early 1990s, the US Environmental Protection Agency began a project to investigate reducing atmospheric emissions of toxins and carbon dioxide from electric power plants. The project, carried out over the course of five years in conjunction with local electric utilities and the Electric Power Research Institute, aimed to verify what emissions could be offset by PV systems installed on buildings. By the end of the project, more than 30 rooftop PV systems, with a combined capacity of more than 400 kW, had been installed in 20 US states.

The Gardener Model Photovoltaic Community Project

One company, Ascension Technology, quickly identified the ac module as the ideal solution for distributed, grid-connected PV installations because such modules eliminate the need for customised electrical engineering on each project. These modules are standardised and pre-approved for standard electrical connections. As well as increasing ease of installation and operational reliability, such standardisation allows for best control of the maximum power-point voltage of individual modules and the best inverter-protective features for interfacing with utility power equipment. In order to achieve this, however, the company had to identify how to include the power electronics inside the modules themselves.

Further developments

Commercial development of the ac module began in 1995 with the award of a contract under the US Department of Energy’s PV Manufacturing Technology (PVMaT) initiative. Twenty electric power companies co-funded the development and testing of 100 beta-production units. Through PVMaT, the SunSine¨300 was the first ac module to gain listing from Underwriters Laboratories for product safety. To accomplish this, SunSine¨300 inverters had to survive the same environmental testing as the modules themselves, including humidity, freeze cycles, high temperatures, etc.

In December 1997, the first ac modules were ready for field testing by utility sponsors. By the end of 1998, all 100 units were installed; they are currently in operation in more than 20 states. During the past year, the results have been very encouraging and lessons have been learned that have led to improvements. For example, a few months after installation of the first modules by New York Power Authority, one of the 20 inverters began to detach from the back of the module. Engineers determined that insufficient cure time had been allowed for the adhesive and the bond did not hold. As a result, a series of standards for manufacturing and assembling components were established.

The Pentagon Building PV system

A second problem that surfaced was even more instructive. In this case, output did not match expectations, yet the module seemed to function correctly most of the time. At first, it was thought that the module was defective, but the cause could not be identified. Finally, it was observed that the module was tripping off-line when there were disturbances on the utility power line. During normal operation, electronic sensors located inside the module will trip a switch and take the module off-line if the current produced is not up to specification. However, in this case, it was found that the sensors were taking readings from the utility power on the grid side of the connection in addition to those of the output from the module. The module was then tripping off-line when the power grid experienced fluctuations and lower quality of electricity, even if the module was functioning smoothly. The protective functions of the module were, therefore, changed to tolerate a lower power quality on the grid side of the interconnection, but not on the side of the module.

In 1998, an additional 200 SunSine®300 modules were produced. These have found markets in a variety of applications, ranging from single-panel units on homes to a 60-panel installation on the Pentagon. By the end of the year, the entire inventory was installed and operating.

The future

The first 300 ac modules produced by Ascension Technology have proved reliable and safe, and the company is now hard at work on the next generation, which is expected to be introduced in March 2000. Using past experience, the aim is to trim manufacturing costs and reduce the module’s size and weight.

For more information contact the CADDET US National Team at Golden, Colorado.

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