Unlike base load power plants such as nuclear and some coal plants which operate near full capacity for days at a time, solar photovoltaic (PV) and wind farms are variable resources whose output is dependent on the minute-by-minute change in weather conditions. For solar PV arrays, clouds and atmospheric interference are the sources of variability. While for wind power installation, gusts and weather patterns are the main culprits. This difference in operating characteristic for variable resources requires a novel approach to determining the impact of transmission capacity on the size of the plant.

But first a side note on plant sizing. Again, unlike large scale steam power plants, PV and wind farms are more easily scalable; i.e., the installed capacity may be modified during the planning, construction and even during operations phases of the project. The developer may decide, for reasons that may include the potential impact of transmission congestion, that the original size of the project was too large and reduce the project scale by removing some PV arrays or wind turbines. Or, alternatively, the developer may decide to increase the capacity during construction or even after the farm has been in operation, by adding additional arrays, turbines and feeder circuits. Conceivably, some turbines or solar arrays may be dismantled and re-installed at another site for the right economic reasons.

This Blog addresses two factors relating to transmission capacity that generally influence project size: interconnection capacity and curtailment expectation.

Curtailment Issues

We have heard wind farm operators say that in general their installations will be subject to some amount of curtailment during the course of a year for various reasons including reliability calls of the grid operator or constraints on output due to outages inside the farm facilities. Let’s say for discussion purposes that the amount of this curtailment is 0.5% of the total available energy from a PV or wind farm facility in a year. By this experience, developers plan for projects that can tolerate the economic impact of this level of curtailment. If the actual curtailment turns out to be 5% instead of the expected 0.5%, the economic viability of the project may become moot.

Figure 1: Wind Farm Curtailment by Plant Size

Using a technique described in an earlier Blog (See A Methodology for Estimating Potential Curtailment of Wind Farms, September 2010), the developer can prepare a set of curves that relate the percent expected energy curtailed (EEC) to the size of the plant. Such a curve is shown in Figure 1 for a wind farm. Using 1% as the target EEC, the ideal plant size is about 115 MW in 2011 and about 120 MW in 2014. If planned system upgrades are in service by 2014, the supported plant size increases to about 160 MW; and if a coal plant nearby is retired at the same time, just over 200 MW is available on the transmission system.

For a sample solar PV project at the same interconnection point, the relationship between EEC and plant size is shown in Figure 2.

Figure 2: Solar Farm Curtailment by Plant Size

In this case, the ideal PV plant size for a 1% EEC is about 100 MW in 2011 and 2014. If system upgrades are made in 2014, the plant size can be increased to about 125 MW, and with retirement of a nearby coal plant, to 150 MW.

Transmission Interconnection

The other aspect of transmission that may constrain PV and wind plant sizes is interconnection capacity.

The capacity that a plant may install to interconnect to an existing transmission system is determined through the interconnection process of the grid’s operator. Interconnection procedures vary from grid to grid but is governed overall by the Open Access rules of the Federal Energy Regulatory Commission (FERC). In general, a power plant may interconnect within the reliability capability of the grid and consistent with the standards for electrical installations used by the transmission facility owner.

As an illustrative example, for a transmission line that is rated 150 MW (long-term emergency rating or post-contingency rating), the maximum plant size that can tap the line is 150 MW. Other factors may further reduce the interconnection capacity such as limits on adjacent transmission circuits, deliverability constraints related to pockets of generation and other reliability-related limitations that may come about from, say, voltage and system stability.

Unfortunately, most interconnection processes require re-study when the plant developer/owner desires to change the plant capacity after the initial interconnection study has been completed. Some independent system operators (ISOs) have allowed conditional interconnection options that may give owners of variable resources the option to apply capacity changes.

Curtailment and Interconnection Relationship

The plant size determined by curtailment indices and by the interconnection process share the same underlying reliability criteria. However, the values determined by the two processes are not the same. One of the key reasons for the difference in values is that the interconnection process looks at selected operating states, not more than 10 such states, while the curtailment assessment looks at perhaps all hours of an operating year and can take into account probabilities of weather conditions, plant availabilities and line outages. In the case of wind farms, the grid may be extremely constrained during summer peak load conditions but the wind farm may have low curtailment risk at such times since the projected output is zero or very small in summer during the daytime peak hour (See A Closer Look at Wind Curtailment, September 2010).

In grids that apply the Minimum Interconnection Standard (for example, New York ISO and ISO New England), the interconnection capacity can be much higher than that indicated by curtailment analysis. Developers of variable resources in these regions include curtailment analysis as part of project risk assessment and to determine optimum plant sizes.

In grids that apply deliverability testing as part of the interconnection process, the interconnection capacity (without upgrades) may be smaller than what curtailment risk tests indicate. The developer in this case may take a conditional or energy-only interconnection for the portion of the plant that is greater than the interconnection limit up to the level of curtailment they are willing to tolerate.

Conclusion

In accounting for transmission capacity, variable resources such as wind and solar PV farms must address both interconnection capacity determined by the grid operator’s interconnection process as well as curtailment risk. In the planning stage, the developer of such resources may account for differences in the transmission capacity indicated by the two methods in determining the plant size. It is also important to consider the methodology that the grid operator applies when determining interconnection capacity as the methods may vary by grid. Even after the planning stage and well into energy production, the renewable resources are amenable to adjustments in the plant capacity as the curtailment risk may change due to system upgrades and/or generation changes. Conditional or energy-only interconnections give power plant owners the flexibility to take advantage of such changes to increase the capacity of installed facilities.