Annual electric generation in 2020 was approximately 4,200 billion kilowatt-hours (kWh) and has been relatively flat over the past decade. Renewable sources reached 800 billion kWh in 2020, which is about the same amount as the total electrical generation in the United States in 1960.
While this number is lower than some other similar industrial countries, it is a formidable amount of generation that operates with a different paradigm than the traditional prime-mover-powered generator sources. That generation technology usually involves a source of power, such as water or steam, that spins a turbine that turns a generator. These power generators have a large amount of mechanical inertia, as there is a lot of mass spinning at a constant and reasonably stable speed, even as daily loads vary. In much of the U.S. grid, the variation in the frequency of voltage is between 59.95 Hz and 60.05 Hz, which is less than 0.1%.
PQ near the distribution network
Many renewable-power sources are referred to as distributed-energy resources (DER) and are often connected to the grid at the distribution level. About a decade ago, I raised a question to the experts in the IEEE PQ Subcommittee meeting: Will the increase in DERs result in an increase or decrease in the power quality in the immediate area of the electrical distribution network? The answers were “yes,” “no” and “it depends.”
A similar question was raised recently, with a narrower focus: Will the frequency stability during fault conditions improve as more DERs that comply with IEEE 1547-2018, Standard for Interconnection and Interoperability of Distributed Energy Resources with Associated Electric Power Systems Interfaces, are installed? Prior to the 2018 revision, DERs would drop offline during fault conditions rather than stay on as long as possible to help provide power at the line frequency.
A study was conducted in the Hawaiian Islands to examine this scenario in a more controlled situation with a higher proportion of renewable power and fewer interconnections to generators with inertia. Hawaii leads the nation in percentage of photovoltaic (PV) power, which creates a significant concern for the utilities over what happens during a transmission fault or loss of one of the prime-mover generators. If many DERs trip off during under-frequency, this would exasperate the change in frequency and jeopardize grid stability.
The National Renewable Energy Laboratories and Hawaiian electric companies began a project in 2016 to determine if a fast frequency response from DERs in milliseconds would improve grid stability during such events, or if the interaction between the DERs, conventional generators and motor-generator sets used for backup power would create an “electron battle.”
DERs typically employ inverter-based voltage generation technologies, which takes electrical energy from one form, such as DC voltage produced by PV, and converts it to AC voltage by electronic power components and control systems that recreate a sine wave at 60 Hz, or the desired frequency as used in adjustable-speed drives.
During normal operation, the utility grid power provides the frequency reference to which all generators on the grid need to synchronize. Grid operators adjust this frequency in response to load changes. We tend to ignore frequency variations as a power quality problem because the frequency in the United States is so stable during normal operations. This isn’t the case in some parts of the world, even with a facility motor-generator set used for backup power. The frequency will “droop” during a sudden load increase and will continue for 5–10 seconds as the control system tries to recover and correct.
Coordinating the response of DERs that comply with IEEE 1547-2018 in a large-scale DER deployment as well as verifying the response and whether it helped or hindered the grid recovery requires extensive monitoring. Massive amounts of data need to be verified, analyzed and compared against models and recommended changes in the control/response operation in a dynamic environment. This stems from the fact that loads and power generation from PV change, since the sun isn’t a constant, even in Hawaii. Most of the installed base of DERs can respond to an overfrequency event but cannot provide the power needed during an under-frequency event. Few of the DERs are connected to the utility communication system because protocols are still evolving.
Things don’t change just because Federal Energy Regulatory Commission Order 842, issued in February 2018, “require[s] that all new generating facilities install, maintain and operate a functioning governor or equivalent controls as a precondition of interconnection” in recognition that primary frequency response is “an essential service in ensuring the reliability and resilience of the North American Bulk Power-System.” California and Hawaii now require such control for all new DERs installed in their systems, but the protocols and standards for testing are evolving and the certified test labs are inadequate.
From the information available to answer the frequency question, the answer so far appears to be “yes,” “no” and ”it depends.” Sounds familiar.