Connecting the power grid to an ocean of distributed electrical sources poses some special challenges to utilities, which have, until now, been able to tightly control their power generation resources. Now, every large corporation is reevaluating its energy strategy, and many state and local governments are actively involved in creating renewable energy policy. For example, the Oregon legislature has mandated that 25% of the state’s energy needs be met with renewable power by 2025. What are the challenges and how will we meet them?
Communication and control
One of the main areas for development is in enhanced communications and control. Whereas even the largest utilities have been accustomed to managing a relatively small number of power generating sites, the number of grid-tied PV power sources is growing exponentially. Along with the proliferation of resources comes the challenge of communicating with them and coordinating their operation. For example, utilities would like to be able to use power from PV sources to shore up the grid when instabilities occur, while maintaining the ability to shut off power from solar plants when necessary to avoid damage either to the solar system or the grid. The isolation of a PV source is called “islanding,” and the goal is to control when islanding should occur.
This is one of the problems being researched and solved by a team of solar technology vendors led by PV Powered, Inc. under a grant by the US Department of Energy as part of the Solar Energy Grid Integration System program (SEGIS). The solution that this group is pursuing employs synchrophasor technology. Synchrophasors are devices that take measurements at different locations in a power system using the same absolute time base in order to provide an easy method of correlating values from different locations that take different amounts of time to arrive at a common collection point. This enables the inverter to differentiate between a true unintentional island case and a case where grid support from the PV plant is required.
The use of synchrophasor measurements has other positive implications for utilities that can use these measuring techniques to improve the quality of power on the grid by identifying weaknesses and helping to optimize grid loading and solve problems before outages occur.
The SEGIS team also includes Portland General Electric (PGE), a progressive utility that is already well along the way to meeting Oregon’s green initiative, and the new technical developments are being tried out on PGE’s “Oregon Solar Highway,” a 100kW system that contains about 8,000 square feet of solar panels extending about the length of two football fields (Fig. 1).
To facilitate communications with PGE’s command and control systems, the solar inverter on the site has been connected to the utility’s GenOnSys distributed generation demand response control system. PGE has stated that its goal with GenOnSys is to make solar power more dispatchable by treating all inverters, whether owned by PGE or its customers, as a sort of “virtual power plant.”
Efficient energy harvesting and reliability
Another challenge impacting the nation’s solar inverter fleet is efficiency of energy harvest, which is much more than simply the electrical efficiency of the solar inverter. The efficiency of the solar power plant as a whole depends not only on electrical conversion efficiency, but also on efficacy of the maximum power point tracking algorithm in the inverter. The maximum power point tracking (MPPT) algorithm adjusts PV system voltage continually as environmental conditions change to extract maximum energy from the solar array. The SEGIS development program is also focused on increasing the energy harvest of solar systems by improving the MPPT algorithms that inverters use.
Figure 1. The 100kW array set up by the Oregon Dept. of Transportation as the country’s first solar highway project is being used to test developments coming from the SEGIS team.
Another challenge that utilities face is maintaining a reliable supply of energy in a power generation environment that is heavily comprised of power sources owned by others. In this regard, avoiding system downtime has implications far beyond the impact of an outage on the owner of the solar plant. Historically, the weak link in solar power systems has been the inverter. Due in large part to the harsh environments in which they operate, solar inverters, which typically represent some 6%-8% of the system cost, have caused about 80% of system downtime.
Improving the reliability of solar inverters has required a total rethinking of how inverters are designed. As a result of a three-year development program in partnership with Boeing completed this past December, we have been able to deliver a grid-tied utility-grade solar inverter that is designed for operating lifetimes in excess of 20 years. Completing this project involved developing new design rules and incorporating new materials selected to deliver maximum reliability and uptime. Sophisticated techniques brought from other mission critical industries including the semiconductor and aerospace fields have been employed throughout the design process. Achieving a 20+ year design life has entailed an intentional design approach that is employed end to end across the product lifecycle.
With proper attention to system element design, the modular design of solar power systems can actually contribute positively to the reliability of the grid as a whole. For example, large-scale PV power plants employ multiple arrays, inverters, and other system components arranged modularly such that single point failures cannot by themselves or through a propagating fault cause immediate shutdown of an entire field.
The most likely management scenario for distributed power generation is a corresponding distributed hierarchy. Under this hierarchy, smart inverters would be monitored and controlled centrally by the utility, either directly or via another system, such as a plant controller. In this manner, multiple sources of distributed generation will be aggregated to form a network of “virtual power plants.”
We are actively participating with standards organizations to ensure interoperability and adoption of the advancements being made under the SEGIS program and elsewhere to continue to make smart inverters the focal point for highly productive and reliable grid-tied solar power systems..
Mesa Scharf received his Bachelor of Science in Electrical Engineering from Oregon State University and is Director of Development at PV Powered, Inc., PO Box 7348 Bend, OR 97708; ph.: (541) 312-3832; email [email protected].