Access to high voltage transmission lines is integral for utility-scale solar projects. Transmission is part of the high voltage system that connects generation (like utility-scale solar power plants or coal-fired power plants) with load centers, subtransmission, and distribution. Essentially, high voltage transmission lines take the electricity from where it’s produced to where it needs to go to be used. In addition to connecting generators to load centers, transmission also connects one network to another, connecting neighboring utilities.

North American Power Grid Interconnections

The bulk power systems in North America consist of 4 distinct power grids, called interconnections: the Eastern Interconnection, the Western Interconnection, Québec Interconnection, and the Electricity Reliability Council of Texas (ERCOT) Interconnection. Interconnections are zones where utilities are electrically tied together. Each interconnection operates independent of each other.

Photo credit: United States Electricity Industry Primer, Office of Electricity Delivery and Energy Reliability, U.S. Dept, of Energy, July 2015

North American Electric Reliability Corporation 

The North American Electric Reliability Corporation (NERC) is designated as the government’s electrical reliability organization by the Federal Energy Regulatory Commission (FERC). This gives them power to oversee and regulate the electrical market and the bulk power system according to reliability standards. NERC activities include developing and enforcing Reliability Standards, annually assessing seasonal and long-term reliability, monitoring the bulk power system through system awareness, and educating and certifying industry personnel. 

NERC does not have jurisdiction over distribution. Distribution is under state public service commissions or local utility regulatory agencies. In the past, distribution systems were viewed as relatively passive load, which is no longer the case. With the integration of more distributed energy resources (DER) on the distribution systems, NERC is concerned with the impact on reliability of the bulk power system. NERC is also concerned with the risk of cybersecurity attacks, particularly on aggregated amounts of DER and the subsequent impact on the bulk power system.

Related to these concerns and increasing utility-scale solar development, the Solar Energy Industries Association (SEIA) advocates for a coordinated and cooperative planning effort to develop interstate transmission lines. Because current transmission planning occurs on a state-by-state basis, all too often transmission lines that provide regional and system-wide renewable energy and reliability benefits are deprioritized. 

Transmission Voltages and Limitations

There are no set voltage levels across transmission lines. Because our utility grid was built over time, there is a progression of maximum nominal voltages of transmission systems. As there was more research and development at higher voltages, more power could be transmitted. 

Today common AC voltages are 69, 115, 230, 345, and 500 kV. The higher the voltage, the more power that can be moved at longer distances. 

Photo credit: ISO – New England Public

There are limiting factors to the amount of power transfer on transmission lines. Some factors include equipment thermal constraints, voltage drop, system security, and system stability constraints. 

Ready to dive deeper into the utility side of utility-scale solar? Enroll in Tim Taylor’s Connecting Utility-Scale Solar PV to Distribution Systems course to learn much more about utility distribution systems, utility-scale solar PV plants, and the many considerations in the interconnection of the two.