PV Generation and Its Effect on Utilities
Inside this Article
The PV industry depends on a stable electric power system. Fortunately, PV systems can facilitate an electric grid that is more reliable and intelligent than today's.
Given that PV installers and designers focus on optimizing system efficiency, ensuring electrical safety, evaluating structural adequacy and so forth—while constantly looking for opportunities to improve the value proposition to clients without compromising reliability—it is no surprise that they tend to treat the impact distributed power generation has on the electric power system as an afterthought.
From the point of view of PV project developers, engineers and contractors, 100-page interconnection agreements, long review periods and much-feared impact studies—which can delay or even derail a proposed PV system—may seem like unnecessary obstacles. However, if industry professionals ignore the challenges that increasing amounts of PV penetration present for grid operators, it is at their own peril. Grid stability is essential to the reliable operation of the systems they design and deploy, and the long-term success of the PV industry depends on its ability to provide value to grid operators rather than create new problems.
In this article, we discuss the basic architecture of the electric power system in the continental US. We present some of the challenges utility engineers will likely face in the future and explain why the growth of distributed renewable energy systems could exacerbate some of these. Finally, we explain why we expect PV systems to defy conventional wisdom and become an essential part of the solution for grid operators by providing benefits that ultimately lead to a more reliable and intelligent grid.
The basic architecture of the existing electric power grid is essentially the same today as it was 100 years ago, with centralized generation sources pushing power out to a wide variety of loads. While the first electric power grid used low-voltage direct current and served only a few city blocks, today’s electric power system is many orders of magnitude larger and is standardized around alternating current.
As the electric grid expanded from its origins in downtown metropolitan areas, utility service areas increased. Eventually, adjacent utilities were able to interconnect previously autonomous grid systems and provide more-reliable service by sharing excess generation capacity. Today, the electric power system is highly interconnected, consisting of three independently synchronized grids: the Eastern Interconnection, the Western Interconnection and the Electric Reliability Council of Texas.
As summarized in part one of an MIT study, “The Future of the Electric Grid” (see Resources), “The electric power system is composed of four interacting physical elements: energy generation, high-voltage transmission, lower-voltage distribution and energy consumption.” The electric power system originates with large centralized power plants, located far away from most electricity consumers. These generation sources feed step-up transformers that output transmission-level high voltages, ranging from 34.5 kV to 800 kV, which facilitate long-distance transmission paths to utility substations located in the vicinity of the electric load. Distribution transformers located at these substations step transmission-level voltages down to the medium-voltage levels used in distribution lines, typically in the 12 kV to 25 kV range.
At this point, the structural design of the electric power system generally changes from a mesh of redundant bidirectional transmission paths to radial circuits fed from distribution substations, as shown in Figure 1. These lower-voltage lines are much easier for utilities to manage. They allow for the use of relatively inexpensive customer-sited transformers that step the voltage down further to low-voltage levels (under 600 V) for commercial and residential consumers. Under normal operation, power flows in one direction, from the point of generation through transformers, switching devices and miles of conductors before finally reaching the load.
In addition to describing these physical elements, the authors of the MIT report also identify “operational, regulatory and governance structures” that are essential to the management of the electric grid. They explain, “Two less tangible elements are also important: the operational systems that protect and control the physical elements, and the regulatory and governance structures that shape the system’s evolution.”
While the interconnected nature of the electrical power system greatly improves system reliability, it can also allow a fault in one area to cascade through other areas. For example, the August 2003 blackout that affected the northeast portion of the Eastern Interconnection began with the unexpected shutdown of a power plant in Ohio. Losing this single plant had a cascading effect, and neighboring plants tripped off-line as they failed to meet the new load requirements.