by Principal Engineer , Dr M Khan, MAESP, MAIENG
Introduction
Due to electrical energy supplies being diversified, mainly due to the introduction of renewable energy technology used to generate electricity; adequate energy distribution systems need to be established to transmit electricity (Sun et al., 2015).
Due to the increasing electrical energy demand, a greater need for improved infrastructure is needed to transmit energy to consumers, however, due to environmental and land-use regulations, adequate transmission of electrical energy is not possible; resulting in bottlenecks (Asare et al., 1994).
A solution to these issues was the Flexible AC Transmission System (FACTS), which was discussed in this report. The history and current status of transmission systems were addressed to highlight issues faced with current transmission systems and the need for an improved transmission system.
The impact of renewable energy technology on energy generation was discussed; which lead to the need for an alteration in conventional energy distribution systems. The possibility of integrating the flex-grid distribution system in South Africa was also discussed in this paper.. History of and Current Status of Transmission Systems History of and Current Status of Transmission Systems
History of and Current Status of Transmission Systems
In the 19th century, DC electrical energy was supplied via short transmission lines. As the demand for electrical energy grew and the distance between power stations and consumers grew, AC was used to transmit current over longer distances with less loss of power. Voltages ranging from 400 kilovolts to 1000 kilovolts (pending in China) are frequently used in the 21st century (Kirschner et al., 2005).
With an increase in the global efforts to reduce the carbon footprint of the world, the use of renewable energy technology has grown. Because conventional energy distribution infrastructure is inadequately equipped to deal with the integration of renewable energy technology, flex-grid configurations were developed. With an increase in electrical energy demand, worldwide, energy producers had to develop a means to transmit electricity effectively (Rogers & Overbye, 2009).
There is only a certain amount of electrical energy able to be transferred through the grid, via transmission lines. The factors, which limit the amount electrical energy distributed through transmission lines, include (Paserba, n.d.):
- The voltage stability limit
- Thermal limit of transmission lines
- Dynamic voltage stability
Currently, due to the increasing population and the increasing life expectancy, a greater toll is taken by infrastructure and resources, resulting in a greater impact on the environment (Retzmann, 2007).
Flexible AC Transmission Systems
Due to the infrequent nature of renewable energy, a constant energy load would not be able to be produced by renewable energy sources, due to external forces (e.g. fluctuating weather patterns). An embedded system, supplying energy to a grid, would therefore be complex to control due to numerous variables. Flexible AC Transmission systems (FACTS) would therefore be a viable option for efficient energy transmission.
FACTS distribution methods aim to develop a configuration which would be able to vary distribution of electrical energy using real-time data. With real-time data technology, the distribution system would be able to adequately deal with varying load demands and varying load resource availability, with respect to renewable energy technology (Rogers & Overbye, 2009).
Benefits and Disadvantages of Flexible AC Transmission Systems
Flexible AC transmission systems would resolve issues around the distribution of greater amounts of electrical energy, without the need of more transmission lines, which saves costs and benefits the environment (certain areas are restricted from the construction of transmission lines). These systems would be able to integrate and distribute the load generated from renewable energy technologies (Okeke & Zaher, 2013). Flex-grid transmission holds more benefits, such as (Sun et al., 2015):
- Greater energy efficiency
- Improvement in power quality
- Faster emergency response times
- Decrease in power failures
There are, however, disadvantages to the flex-grid system, such as the high capital costs. Because the configuration makes use of renewable energy technology, the use storage devices would become a key component in the configuration (Sun et al., 2015). Because of varying loads, consumers use different amounts of energy at different times of the day and the use of renewable energy sources, such as solar PV systems, which generate electrical energy during the day when sunlight is abundant; energy needs to be stored and used when energy demands are great (IEA, 2011).
FACTS and High Voltage Direct Current (HVDC)
Due to the enhancement of thyristor component technologies, HVDC and FACTS systems are able to transmit power over distances of 1 000 km to 2 000 km with overhead transmission lines (Retzmann, 2007).
According to (Retzmann, 2007), energy transmission over distances up to 700 km are feasible using AC transmission, however, HVDC transmission becomes more economically feasible for distances greater than 700 km with power greater than 1 GW. Current DC transmission systems in use deliver 600-800 MW over distances greater than 300 km; with projects where power is transmitted over distances over 1 000 km. Due to these advancements in HVDC systems, it is deemed a reliable system for transmission.
According to (Beck et al., 2006), development of hybrid transmission systems for large systems would be feasible. These hybrid systems consist of HVDC and FACTS subsystems hold various advantages such as lower maintenance costs, increased reliability and stability in the transmission system. With hybrid transmission systems HVDC would be used to serve larger power systems over long distances, while systems with shorter distances and less power demand are served by AC power. The benefit of the hybrid system allows the HVDC links to strengthen AC connections to avoid system instability. A schematic of a HVDC – FACTS hybrid system can be seen in Figure 1
Current Flexible AC Transmission Systems
Due to a blackout in the United States of America, which occurred in 2003, Siemens and Neptune developed an HVDC transmission link, between Sayreville, New Jersey and Long Island, New York, with a transmission distance of 105 km, and a power rating between 600 MW and 660 MW. The installation supplied 700 000 households with electricity during a 2007 blackout in New York, with power being supplied from New Jersey (Retzmann, 2007).
Germany implemented an HVDC and FACTS system for an application which transmitted electrical energy of the magnitude of 600 MW from Germany to Sweden (Retzmann, 2007).
Flexible AC Transmission Systems for South Africa
The IRP 2010-2030, aims to meet South Africa’s energy demand by spreading the load across various energy generation systems, such as coal fired power stations, nuclear power stations, renewable energy and gas fired power stations (Department of Energy, 2011). The IRP aims to decrease the dependency of coal use, by delegating larger portions of South Africa’s load to cleaner sources of energy and diversify energy production.
Because of the diversification of energy production systems, adequate distribution methods need to be in place. The flex-grid configuration may be a viable option for South Africa as there is a considerable influence of renewable energy technology in South Africa’s aim to expand the energy market.
Costs would be greatly negated in terms of storage mechanisms. Because of South Africa’s landscape, pumped storage devices which are efficient and have a low localised cost of energy when compared to other storage devices (Eskom, 2010).
Because South Africa transmits power to neighbouring countries, such as Zimbabwe, Namibia and Mozambique, to name a few, which range in distances in excess of 1 000 km; FACTS may be a financially feasible system of energy transportation (Beck et al., 2006).
Conclusion
With a worldwide attempt to harness energy from various sources for the production of electrical energy, namely renewable energy technology, nuclear power and reducing the reliance on fossil fuels; electrical energy distribution methods need to be improved to deliver electricity efficiently.
The flex-grid distribution configuration holds many advantages such as power bottleneck reduction (Kirschner et al., 2005), integration of various energy sources, decrease in power failures and improves response times to emergencies (Sun et al., 2015).
Flex-grid distribution may be viable for South Africa as one of the aims of the Integrated Resource Plan (IRP) 2010-2030, was to diversify energy sources. A flex-grid distribution configuration would be able to integrate these intermittent sources of electricity.
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