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Green Power Control

Preparing the grid for large-scale renewables.

Fortnightly Magazine - June 2011

As many wind generation developers have done in the past, solar generation developers are now seeking to connect large-scale renewable projects to utility transmission and sub-transmission systems. Renewable portfolio standards have created a market for the construction of merchant wind and solar generation and many state renewable programs require that these plants be interconnected to distribution and sub-transmission systems to receive renewable energy credits from the state. Solar inverters are now commercially available in 500 kW and 1,000 kW sizes, and photovoltaic panel prices have fallen in recent years—which makes bulk power grid connectivity, and the associated higher interconnection costs for these solar arrays, more feasible.

Wind turbines that produce up to 3 MW per turbine are now commonplace in the United States, with more than 40,000 MW installed in the country at the end of 2010, according to the American Wind Energy Association (AWEA). Additionally, there are many planned off-shore projects that are likely to use turbines at or greater than 5 MW per turbine. The transmission interconnection queues across the country have proposed future wind and solar projects ranging from 10 MW to more than 500 MW at a site with interconnection voltage generally aligned with the size of the project—the larger the project, the higher the interconnection voltage.

Analysis of potential reliability impacts of connecting large-scale wind farms and solar arrays to the transmission grid is relatively new to long-range transmission planners. The definition of large-scale, for the purposes of this article, are renewable projects exceeding 10 MW in total capacity and connected to the transmission system at voltages above 30 kv. In the past, smaller wind farms were built with Department of Energy funding to test the viability of successfully integrating wind generation into the grid. Likewise, solar arrays were previously installed behind the meter, posing little to no reliability concern. However, these smaller facilities still need to be properly metered and accounted for in order to reveal the true state of load growth on distribution circuits. Not doing so would pose a risk in skewing load forecasts, which could then lead to transmission providers not being adequately prepared to execute system upgrades to both transmission and distribution networks in a timely manner.

Transmission service providers need to contemplate the following concern: with larger solar arrays and wind farms being proposed to connect to transmission and sub-transmission systems, are utility companies sufficiently prepared to handle the challenge of integrating these large intermittent resources?

Power companies have to go beyond simply building an interconnection station for the renewable resources to inject their green energy onto the grid. It will require a function of refining, and for some companies, even redefining the generation interconnection requirements for new renewable projects. The days of selecting a generic voltage schedule and assigning the standard operational power factor limits for these new renewable resources are over. These schedules won’t sufficiently address future

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