The need to supply steady, continuous energy worldwide for decades to come to its citizens is in the forefront of national and international strategic planning for all nations, especially now, as alternative energy sources come online. Energy distributed through the power grids securely and efficiently ensures there is enough power available to supply all activities within a nation – even move power across borders. There are key considerations in moving power from grid to grid, location to location:
Compatible infrastructure must be available
Power losses over long hauls (1000 km – 2000 km) must be minimized,
Asynchronous networks will have to be seamlessly, efficiently connected so that power may move back and forth, uninterrupted.
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"Voltage Source Converters for HVDC".
With the current focus being squarely on petroleum and natural gas, the need to supply and distribute electrical power more efficiently (and uniformly) has been sometimes overlooked. Aging infrastructure, where it exists, can’t be expanded due to land requirements and losses: existing solutions, such as using High Voltage Direct Current assemblages to increase capacity and connect asynchronous networks, are nearing their limits.
Even more critical, as significant amounts of power generated with alternative, green technology come online, the demand for technology to connect these sources into the grid and match existing power V/I characteristics increases dependence on VSC control to manage HVDC transmission. Revisiting components in the HVDC assemblage to improve power output, or apply the technology to short haul transmission and lower losses represent one way to increase power supply to meet variable demand.
It is proposed to explore both current and future applications of Voltage Source Converter (VSC) technology in HVDC for future power transmission design. Industry consensus is that the VCS configurations to boost maximum power and range over which HCDC can move electricity in the grid have not yet been maximized. The technology, as used presently, is mature, in other words, but there is ample room for improvement. Components and configurations of the components are two areas were additional work can be explored; these include looking at current applications of HVDC with VSCs, examining how stackable VSCs drive higher voltage, lower losses, and longer haul transmission of power by HVDC, and new or proposed applications of HVDC with emergent/green power sources.
At present, the most common application of HVDC is in the connection or linking of power grids, including the transmission of power from one area to the other. HVDC has been the preferred technology, because it can move as much as 10 gW at 800 kV across runs of up to 1000km (Siemens A.G., 2015; Bahrman, 2003) with losses of no more than 3.5%. Power transmission via AC technology, over the same distance, experiences losses of up to 6.7% at 735 kV, with a maximum capacity of 3 gW (Siemens A.G., 2015; Alston, 2011). The basic technology is mature and well-understood, having been in use over 100 years. New applications which are just coming on line or close to coming on-line include transmission of high-power electricity over longer distances in between HVDC/VSC control stations, electrical transmission to off-shore facilities, connection of wind-farms, nuclear plants, and solar generation plants into existing infrastructure to move power into usage areas far away from the generation site. Because of the distances, higher demand, and changes in high-load areas, newer, lower-costs solutions to increase transmission capacity are being sought. The VSC are one suitable area where additional work can be done to increase capacity, minimize power loss, and increase transmission distance.
A preferred configuration for control and stabilization of voltage and current flow in HVDC, Voltage Source Converters, or VSCs, are the preferred method to control electrical flow during HVDC transmission: it can switch current on and off at will, and in some configurations, notably uploading of solar- or wind generated electricity into a conventional electricity generator grid, or during transfer of power between grids in a shared network arrangement, VSCs can – and do – permit two-directional electricity flow: a form of ‘‘power-trading’ (Siemens, 2015; Alstrom, 2011; Navpreet, et. al., 2012).
The VSC is the heart of an HVDC-VSC system: it converts DC power to AC, and vice-versa. Its key feature is that it does so without significant power losses to resistance or current ‘leakage’. It avoids common losses of power that can occur in AC transmission, especially the need to transmit power in 3-phase configuration, then step down for application, very well. How well, how fast, and at what maximum voltages power can be pushed in an HVDC-VSC system, depends upon how the VSCs in the system are configured; at one time, these parameters also depended on VSC size at one time, but VSCs are getting smaller and smaller, daily. Siemens, for example, now touts its ability to place an HVDC-VSC control system anywhere, in smaller areas (more than 50% less than that required by a comparable AC transmission control system; 2015).
A preferred configuration is to use VSCs in a stacked array (VSCS connected in parallel) to convert large amounts of power, and speed up on-off switching for traffic control within the grid, at the HVDC-VSC linkages. While power outputs and through-puts are high, stacked VSCs seem to have reached their upper limit, thus other configurations are being explored and tested, to produce what industry is terming UHVDC, or ultra-high voltage direct current or move larger amounts of power off-shore to support the boom in off-shore, sideways drilling. These alternative configurations will be contrasted in detail.
Recent developments, tested via simulation and in the field, have shown that addition of IGBTs (Insulated Gate Bipolar Transistors) to an HVDC-VSC system produces VSCs with even high voltage maximums than exist today. Simulations coupled with field testing have been conducted by several industrial and academic groups; one, in India, suggests that the properties of new, higher-power VSC “… include independent control of active and reactive power, operation against isolated [AC networks] with no generation of their own, very limited need of filters and no need of transformers for the conversion process.” (Navpreet, et. al., 2012). They are working from the premise that by increasing the level of stacking and adding more valves to the VSC, one may vary AC voltage without increasing DC voltage, described as flexible AC transmission , by exploiting harmonics in the system (Navpreet, et. al., 2012), but testing against VSC-HVDC, and other systems to compare outputs. This study uses physical models, as well as mathematical analysis.
Another approach to simulation is taken by Mohamed, et. al.; they have performed MatLab simulations of a faulted system in order to explore active and reactive transmission schema experiencing fault conditions, in order to survey, systematically, how VSCs perform to stabilize current flow in the transmission grid. This tested the pulse-width modulation features as well as other switching features that facilitate connection to weak unstable AC networks, mitigate reactive events, or integrate unconventional power generation sources into the electrical grid. The two studies will be contrasted and compared, along with a third (Asplund, et. al.), which discusses state of the art for the mature technologies, with limited data.
In summary, a study of VSC-HVDC systems, current technology, and emerging improvements intended to increase transmission distances, lower power losses, and fine-tune control over output voltages or reactive events, will be explored. As alternate power generation sources come online, opportunities to increase power in the grid at points when and where it is needed most, furnish a large area in which to explore modifications to VSCs in power transmission. At the same time, that alternative energy sources are developing further and further away from the conventional electrical grid, it is critical that technology to move the new power rapidly and effectively into the standard grid, be tested and developed. The area exhibiting the most promise is in developing new configurations and procedures to apply VSC.
