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Solar Screen Test

Making room on the local grid for small-scale PV.

Fortnightly Magazine - May 2012

the challenge, the theory goes, that’s all it should take to allay any risk of islanding or reverse flows, and thus justify fast tracking approval for DG interconnections.

And even on a mild sunny day, when the weather is perfect for solar energy, SEIA sees very little probability that all interconnected solar generators could outpace load by reaching maximum energy output at the same time.

“In practice,” SEIA claims, “diversity between PV systems, including tilt, orientation and temperature effects, will generally result in nonsimultaneous maximum output.” (Answer of SEIA to Comments and Protests, p. 10, Dkt. RM12-10, filed April 11, 2012.)

In fact, SEIA appears to have tailored its proposal specifically to match the findings and recommendations contained in a joint study recently conducted by EPRI, the U.S. DOE, Sandia National Labs and the National Renewable Energy Laboratory and released in February, just as SEIA filed its petition at FERC. That study, Updating Interconnection Screens for PV System Integration , recommends reliance on minimum daytime circuit loads as one of three logical short-term approaches to streamline interconnection procedures for distribution-level PV.

The study analyzes data taken from 500 residential and commercial feeders in an unnamed U.S. city located in the Southwest, showing that for most feeder lines, the minimum daytime load occurring between 10 a.m. and 2 p.m. is greater than 30 percent of the daily peak—FERC’s proxy for the minimum load. That suggests that SEIA’s proposed rule would allow for more solar DG PV than under FERC’s current policy. (See Figure 1.)

Granted, the study acknowledges that “it may be difficult to establish minimum daytime load unless reliable historical data is available.” However, it adds that “most utilities now have access to feeder minimum load and feeder peak load data via SCADA systems.

“If SCADA data is unavailable, minimum daytime load can be estimated based on standard load profiles for various customer classes that many utilities maintain and update on an annual basis.”

Nevertheless, as the study also warns, historic minimum loads offer no guarantee of future minimum load levels, suggesting a need for better continuing communications between DG owners and utility grid controllers. (NREL Technical Paper, TP-5500-54063, February 2012).

As of mid-April, reaction to SEIA’s petition was predictable, with solar developers and environmental advocates voicing support, and the power industry in opposition. Groups such as Edison Electric Institute, American Public Power Association, and the National Rural Electric Cooperative Association, warned of reliability threats to distribution networks. EEI in particular suggests that minimum customer load was confidential and market-sensitive; that any mandated release of such data would grant a “subsidy” to the solar industry, to the detriment of retail ratepayers, who would be asked to fund the cost of modernizing local grids and reconfiguring software to allow for trustworthy measurements of minimum loads on individual feeder circuits.

The two public utility commissions that commented on the proposal—California and New Jersey, which rank first and second, respectively, in solar PV installed capacity—each lent support to reform, recommending that even if FERC didn’t favor all the terms of SEIA proposal, the commission should