Grid-connected systems
Grid-connected systems are integrated electrical networks that link multiple power generation sources to consumers, enhancing the reliability and quality of electricity supply. In contrast to early single-generator setups, which often led to power outages and inconsistent service, modern grid systems allow for a mix of energy sources, including traditional fossil fuels and renewable options like solar and wind. These interconnected grids help normalize voltage and frequency across larger service areas, providing a stable power supply even when individual generation sources fail.
The systems also facilitate the integration of smaller-scale power generators, enabling them to contribute excess energy back into the grid, which can be particularly beneficial for intermittent sources like solar power. To connect safely and efficiently, these systems must include sophisticated switching devices that ensure synchronization of voltage and frequency, allowing for seamless transitions between local generation and grid power. Backup power solutions, such as emergency generators, are critical in many installations, particularly in essential services like hospitals, where power loss can have severe consequences. By leveraging diverse power sources and advanced technology, grid-connected systems are crucial in meeting the growing energy demands while accommodating the shift towards more sustainable energy practices.
On this Page
Subject Terms
Grid-connected systems
Summary: One of the original problems with electric generation for power is the quality and reliability of the electricity produced by the source of the power. Most industrialized countries have switched from single-generation sources to electric grids.
One of the original problems with electric generation for power is the quality and reliability of the electricity produced by the source of the power. In the early days of electric service, small generators were connected to a single load without any connection to other generation sources. Any malfunction of the single generator resulted in a power outage for the service, so this type of system resulted in the provision of low-quality and unreliable power. Variations in voltage, current, and if applicable frequency can and will cause damage to the equipment being powered by the generation source.
By the twenty-first century, most industrialized countries had changed from single-generation sources to electric grids to reduce the severity of the problems mentioned above. In many cases, nations even share electical grids. Most grids are for alternating current (AC) generation, and the loads and generation sources are all connected to a common electric transmission system. If the area served is large, the amount of energy the transmission line holds is a significant amount of power in potential energy form, and the voltage, currents, and frequency will be normalized. If excess power is put into this grid by the generation sources, normally called spinning reserve, the loss of individual generation sources does not interrupt service to the grid and the served load is unaffected by individual generation source problems. Interconnected grid systems can be thought of as spiderwebs, where the power can move several ways to get to any connected point on the web.
Electric grids developed over time and are maintained by large commercial and municipal utilities using well-established generation sources, such as coal-fired, natural-gas-fired, hydroelectric, and nuclear power. As energy prices increase and fossil fuel sources get smaller, smaller generation sources, such as solar and wind power, are being used to power consumer loads. If these smaller generators are connected to the large electric grids, the grids can supply power if the load source is damaged or out of service, and any excess power produced by the small source can be put into the grid.
Alternative Sources
Competition for grid-provided power is readily available. Microturbines, solar power systems, wind power systems, and small-scale hydroelectric power all can be used to provide electricity for a specific single location. Almost all electric providers are required to allow customers to connect a small single source to the electric grid, provided they use the utility’s suggested installation scheme. For electrical safety in connecting to a grid, the generation source must be able to be synchronized in voltage and frequency to the grid. All cabling must be sized to carry full amperage plus some safety factor, depending on codes in the area. There must be an interrupt device, and codes may require fuses, circuit breakers, and ground fault interrupters. For direct current (DC) generation sources, a DC-to-AC power inverter must be used, and if 24-hour service is required, battery banks must be employed for grid backup.
It is advantageous to use grid-connected systems with small-scale solar power, because solar unavailability is very high. Solar systems can provide power only when the sun is shining at sufficient intensity to generate the power; night and cloud cover can temporarily cut off solar energy generation. If the solar system is grid-connected, however, power will be available off the grid when the sun cannot provide the needed power.
Natural-gas-fired or oil-fired microturbine generators might be a good economic choice for customers who have a low-cost source of natural gas or oil. If these microturbines are grid-connected with an automatic switchover mechanism, the customer can run the microturbine when the microturbine generator can generate power less expensively than the grid source and switch to grid power when it is more economical. The same principle applies for microhydroelectric-generating stations. Very small streams on land sufficiently steep to allow water to fall or move rapidly downhill can be used to generate power when water is flowing. If the water-based microturbines are connected to a grid, the customer can use water when available but at times of drought can use grid-connected power.
Switching and Backups
The key to a grid-connected power system is the switching device, which allows the customer to change from the local source to grid power. These controls must be sophisticated enough that sources can the synchronized in frequency and voltage before the switch occurs to avoid any interruption in power. These devices normally sense incoming voltage to a high degree of accuracy and can switch to the alternative system automatically if voltage drops to a preset value, assuming the alternative source is available and at the correct voltage. Being able to detect power drops before the power source becomes unavailable and switch to the backup source is normally referred to as a hot transfer. A momentary loss of power before transfer, by contrast, is often referred to as a dead bus transfer or cold transfer of power.
Grid-connected systems are sometimes the normal source of power, with backup emergency generators that start if the grid becomes unavailable. This type of system is common in hospitals and other critical installations where a loss of power would result in serious property damage, injury, or loss of life. Most of these systems have an on-site fuel storage tank that holds gasoline or diesel fuel used to provide fuel to a piston-driven engine-based generator. In the United States, nuclear power facilities are required to have backup emergency power of this type for critical services. Emergency diesel-powered generators supply energy to critical systems when grid power is lost and are a normal part of the installation at all commercial nuclear plants in the United States. The backup diesels have enough fuel to power the nuclear plant’s emergency systems for several days without any fuel deliveries, until the plant becomes subcritical and is in shutdown.
Bibliography
"About Capstone." Capstone Green Energy Corporation, 2024, www.capstonegreenenergy.com/about. Accessed 1 Aug. 2024.
"Grid-Connected Renewable Energy Systems." US Department of Energy, www.energy.gov/energysaver/grid-connected-renewable-energy-systems. Accessed 1 Aug. 2024.
“How to Size a Grid Connected Solar System.” National Renewable Energy Laboratory, August 2002, publicservice.vermont.gov/energy/ee‗files/solar/sizing-solar-electric-systems.pdf. Accessed 1 Aug. 2024.
"Interconnecting Countries for Energy." International Electrotechnical Commission, 28 Feb. 2023, www.iec.ch/blog/interconnecting-countries-energy. Accessed 1 Aug. 2024.