Cloud cover and inconsistent winds are natural assumptions when it comes to integrating renewable energy. Their variable and non-dispatchable nature requires utilities to take a number of precautions when integrating some types of renewable energy. Utilities take precautions because they are required to ensure the safety, reliability and power quality of the grid, and thankfully, they take this responsibility seriously. Many utilities have recently gained considerable experience integrating renewables and have started to allow higher penetrations of solar on the distribution grid than previously allowed. Distribution-level interconnection can save a lot of money for developers, compared to transmission interconnections, and substantially lower the cost of the developing new renewable generating capacity.
For example, we have begun hearing about utilities charting new territory for increasing grid penetration. One of these projects, a 10-MW solar facility, is located in Atlantic City Electric’s (ACE) territory in Cumberland, New Jersey and developed by Lincoln Renewable Energy (LRE). While facilities of this size are usually connected to higher voltage, subtransmission lines, this facility is in fact connected to a dedicated 12kV distribution line. This New Jersey Oak Solar project consists of 53,200 solar modules constructed on a 100-acre site and connects to ACE’s distribution system.
From an engineering perspective, this means the facility is operating at the upper end of the feeder’s capacity. When lines get overloaded, the excess heat can expand the conductive material of the lines, making them sag, which could damage the lines or cause a public safety concern. Additionally, if there are tree limbs or other obstructions when they sag, it could cause the lines to short out and cause a fault condition. As a result, utilities must ensure that the total power load traveling along a utility line generally remains within a certain range of its peak capacity. Typical utility practice is to allow the aggregate load on distribution lines to reach 80% of rated capacity, leaving 20% as reserve capacity for emergency switching and operating practices. The Oak Solar facility alone could theoretically use about 80% of that line’s thermal capacity, while still ensuring the 20% safety margin.
Maximizing the penetration of solar on the distribution grid in this way also helps to maximize the locational benefits that distributed renewable energy provides to the grid and ratepayers. Locating generation closer to load, for example, avoids much of the line-loss associated with long-distance transmission. It might also allow utilities to make smaller capacity additions that more closely follow incremental load requirements, which could, in turn, help them to defer or avoid capital expenditures. There are other benefits as well, including the obvious environmental benefits related to displacing fossil-based resources.
This New Jersey project, and others like it, offers an encouraging example of a developer and utility working together to maximize the solar potential of a site, and maximize the capabilities of the distribution grid. Because New Jersey has a much higher solar penetration than do most other states, utilities are likely much more comfortable with larger systems. In fact, Chris McKissack, Director of Transmission at LRE, said about the project, “ACE and LRE worked very well together, with constant communication being a big key to the Oak Solar Project's success. The ACE team was very efficient, designing and building out the entire interconnection facilities in less than 6 months. The ACE team was also quite flexible, working with LRE to provide temporary backfeed for inverter testing while, at the same time, maintaining system reliability for their other customers.”
Hopefully this is a good omen for where other up-and-coming states will be heading, once they gain more experience. Additionally, with smarter grid sensors on the market, utilities are better able to use more of their existing distribution grid capacity. These aspects allow utilities to make more efficient use of the existing grid without imposing unnecessary costs, which ultimately has a positive impact on everyone.
This blog was originally published in IREC's March 2013 Connecting to the Grid Newsletter.
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