When trying to determine the effects of renewable energy sources on the quality of the electrical supply, one must first carefully examine the statistics and what goes into them.
Lately, the term “renewable” has been focused on wind and solar power, while some statistics also include other sources that have been around for years: hydropower, tidal power and biofuels. These traditional, geographically dispersed prime movers used in the generation of electricity are typically connected into the electrical grid at the substations that feed transmission lines, as are other large generators such as nuclear, gas- and coal-fired plants. The electrical power is then transmitted downstream to the distribution substations, and then on eventually to the consumers of electrical power.
Distributed energy resources
The growth of wind- and solar-powered generation has allowed for more dispersed sources connected at the distribution level or at the point of consumption, such as rooftop photovoltaic systems. These distributed energy resources (DER) present a different set of potential power quality issues than the traditional sources because it’s difficult to parse their individual effect from the pool of generated power as a whole. In the United States, wind power has now exceeded hydropower, and solar power is about one-third of wind. Combined, renewable sources provide about 10% of the electrical energy consumed in the nation—a far smaller share than countries such as Germany and Great Britain that have a similar percentage of industrial and commercial usage per capita, but steadily growing.
The start of significant amounts of wind and solar generators being dispersed throughout the electrical grid over 20 years prompted the Federal Energy Regulatory Commission to push IEEE to rapidly develop an interconnect standard at the distribution level to allow integration of additional smaller generation into the grid. This resulted in IEEE 1547-2003, IEEE Standard for Interconnecting Distributed Energy Resources with Electric Power Systems. This standard was expanded in the last 20 years to include a series of documents that more completely define the specifications and performance requirements for DERs to connect to the grid, including power quality. Subsequent standards include IEEE P2800, Interconnection and Interoperability of Inverter-Based Resources Interconnecting with Associated Transmission Electric Power Systems, which also includes performance requirements related to power quality.
Do we have an answer to the question, “Do DERs improve, or are they a detriment to, the quality of the electrical supply?” The answer a decade ago from a panel of experts was “yes, they improve it,” though it still seems that “it depends.” Wind and solar power are not based on a grid frequency technology, since wind speed that turns the turbines is not a constant and the photovoltaic cells in solar panels produce DC voltage, not AC. This means they need to employ inverter-based technologies to convert the generated electrical energy into grid-frequency (typically 50 or 60 hertz) voltage and current waveforms.
Like other inverter-based electrical equipment that uses rectified inputs with switching-mode power supplies, they can be the source of significant harmonic and interharmonic currents, which become harmonic voltages that can affect other equipment. When the power source is generating harmonics, even more consumers can be affected than from nonlinear loads within a facility.
The earlier performance requirements were for DERs to drop offline when a voltage sag of a specified magnitude occurred. Most sags are the result of excessive current levels reducing the voltage levels, which creates less energy available for the loads, so losing more generation at this time can result in a deeper sag. New requirements are for DERs to stay online for a specified time to help ride through typical-duration events. Eventually, they are to cut off so as to not backfeed into utility lines that are thought to be de- energized and unprotected by electric utility system protection equipment.
A July 2016 ELECTRICAL CONTRACTOR article, “The Curious Case of Voltage Sags,” looked into whether rooftop solar panel systems could help minimize voltage sags’ effects caused by large HVAC startup conditions. The data showed no improvement in the depth or duration of the voltage sags that occurred every 22 minutes during the peak usage time. Having an 8-hour window of energy production (if it wasn’t cloudy) didn’t help meet peak load requirements, which lasted well after dark. Hence, DERs cannot be relied on as part of an uninterruptible power system unless they are used to charge batteries, rather than selling back to the utility or providing partial power during favorable conditions.
There are stories circulating among the utilities’ PQ engineering community about unusual things happening that have been traced back to unanticipated interactions with large-scale DER systems. Hopefully, as we learn more and the standards evolve to address such issues, we may be able to unambiguously answer the question, and it will be for the better of the quality of the electrical supply.