The conventional utility power grid is vulnerable to a number of threats that can cause failure over wide areas. Most people understand that power failure can be caused by frequent, low-impact events such as ice storms and hurricanes. Less well understood are high-impact, low-likelihood events on the power grid, such as geomagnetic disturbance (GMD) caused by solar storms, or deliberate cyber or electromagnetic pulse (EMP) attack.
In fact, the Congressional EMP Commission and others have warned multiple times of the probability of widespread, long-duration power outages in the wake of a large GMD or EMP attack. Some of the failure mechanismsentified by the Congressional EMP Commission include permanent damage to extra high-voltage (EHV) power transformers and damage to electronic controls in multiple networks, including electric power distribution, fuel pipelines, transportation, communications, and other critical infrastructures. The extent and duration of these failures would be without precedent.
One question sometimes asked by people contemplating long-duration failure of the power grid is, “Are alternative energy systems such as home solar photovoltaic (PV) power systems or wind farms viable sources of electricity should a widespread grid outage occur?”
The bad news, and shortcoming of many small alternative energy systems, is that most alternative energy systems would not be able to deliver power after a power grid outage. The good news, though, is that, with planning and knowledge, some of these systems can be designed ahead of time, or retrofitted, to operate in “off-grid” or “island” mode, such that they can deliver power after a grid failure.
“Island Mode” & Power Grid Most emergency generators and microgrids – networks of two or more generating sources, such as photovoltaic, wind, and combustion engines – are capable of operating either independent of, or in conjunction with, the conventional utility power grid. When the power fails, these sources serve as “islands” of electric power – hence, the terms “island mode” and “off grid.” Two pieces of equipment are essential to island operation: (a) a prime mover (solar panels, wind turbine, combustion engine); and (b) a transfer switch to isolate the “island” from the utility grid. Even large microgrids such as university campuses, airports, and other large facilities are equipped with the means to connect to, and disconnect from, the utility grid.
What differentiates “islanding” capability from other generating sources – alternative or otherwise – is that a generating “island” is designed to operate independently from the conventional power grid. Unless the owner of a particular generating asset such as wind farm or home PV system clearly specified that the system must operate in “island mode,” the asset is unlikely to function after a grid outage. In other words, just because solar panels are installed on a roof does not mean that the power they generate is available when the power grid fails. Before discussing why rooftop PV systems stop running when the grid fails, it is necessary to understand how and why the power grid functions.
Known as the most complex machine ever built, the U.S. power grid is a vast network of generating plants, EHV transformers, high-voltage transmission lines, substations, a low-voltage distribution system and computer controls throughout. Assuming that one power plant in a network of thousands goes offline, all remaining plants in the system will make up the difference such that no customers lose power. If power were unavailable from multiple plants simultaneously, demand for electricity would greatly exceed supply. The result of this imbalance is a collapse of alternating current frequency and voltage.
To prevent this collapse of frequency and voltage from propagating across a wide region, including to customers whose local power plants and transmission resources are not damaged, automatic protection systems in the power grid isolate areas where demand exceeds capacity. Normally these systems function quickly and seamlessly, but sometimes they fail in spectacular fashion. When a tree limb fell on a transmission line in Ohio in August 2003, for example, power was lost across large portions of the mid-Atlantic region and into Canada. Failure to isolate intact power grid assets from the failed regions causes an almost immediate cascading failure.
Alternative Energy Systems & Power Grid Failures Unless specified as an “off-grid” or “island” system, most home solar PV systems – including rooftop solar power systems – are synchronized to the alternating current frequency of the power grid. This enables seamless import of utility power to the home at night and export of power from the solar panels during times of peak production. When failure of the power grid in the PV-equipped home’s neighborhood occurs, protective systems in the solar power inverter shut down the home system for two reasons:
- To prevent damage to the system inverter from massive overload when one home system attempts to power an entire neighborhood; and
- To eliminate the risk that power produced by the PV system will energize downed utility lines and expose repair crews to dangerous voltage – these power systems are subject to the Underwriters Laboratories standard (UL 1741) and to the National Electrical Code (NFPA 70)
The cheapest and simplest way to perform these tasks is to just shut off the home inverter unless grid power is present. Unfortunately, this solution leaves the homeowner literally in the dark, unable to access the power generation resource on his roof during a utility power failure
A similar mechanism is designed into wind farms, including even the largest farms with hundreds of megawatt-rated or larger turbines. When the power grid fails, each turbine is designed to feather its blades such that the turbine stops turning. Even though output of the wind farm might be large by household standards, the suddenly massive load applied to the wind farm by the wide area grid is just as overwhelming to the wind farm as the neighborhood was to the homeowner’s small PV system. The turbines shut down to protect from this overload, and to insure that transmission lines are de-energized and thus not a threat to repair crews.
Specifying a Residential Microgrid Although technical specifications are beyond the scope of this article, the following guidelines provide important considerations when specifying a new, or retrofitting an existing, alternative energy source, such that electric power will be available in the event of a utility outage:
The system must be specified to operate completely independently of the power grid. One test to determine whether an existing power system is capable of off-grid operation is to shut off the main power feed to the home or facility in question. If the alternative source cannot deliver power when it ought to – for example, during midday for a solar PV system – the system is not independent of the power grid.
There is a manual or automatic transfer switch installed at the utility entrance point that enables the alternative energy source to be disconnected, either automatically or manually, from the utility grid. The transfer switch protects the home system from the relatively infinite load presented by the grid, and prevents the home system from energizing neighborhood power lines. Transfer switches add cost and complexity to alternative energy system, but are essential for safe off-grid operation.
- The alternative energy system includes either on-site energy storage assets, such as a battery, or a fuel-consuming generator, such as a gas or diesel engine or a fuel cell. Without either of these elements, power will not be available from the alternative energy system during times when the alternative energy source is nonproductive – for example, darkness for a PV system and lack of wind for a windmill.
It is not difficult to specify a new alternative energy system, or convert an existing one to operate “off grid.” It does, however, demand sufficient knowledge about distributed energy and alternative energy systems and a contractor experienced in building “off-grid” energy systems. Some suppliers are even able to harden off-grid energy systems to the effects of EMP attacks and geomagnetic storms. During a utility power grid outage, people who properly specified and built their own “off-grid” system will stand a good chance of enjoying the many benefits of electric power, and will be in a position to help their less-prepared neighbors in a time of need.
William Kaewert is the founder of two power protection companies and has over 30 years of experience applying technology-based solutions that assure continuity of electrical power to critical applications. He is currently president and chief technology officer of Colorado-based Stored Energy Systems LLC (SENS), an industry-leading supplier of nonstop DC power systems essential to electric power generation and other critical infrastructures. The company also produces commercial off-the-shelf (COTS)-based power converters used in military systems hardened for electromagnetic pulse (EMP), including ground power for the Minuteman III ballistic missile system and the Terminal High Altitude Area Defense (THAAD) missile interceptor. He received his AB in history from Dartmouth College and MBA from Boston University. He serves on the board of directors of the Electrical Generation Systems Association and on the management team of the FBI InfraGard Electromagnetic Pulse Special Interest Group.