Power Conversion with Photoconductive Switches
Much of the nation’s power generation exists far from urban areas where electricity demand is highest. However, the current transmission system is built on an alternating current (AC) power network that is not ideal for long-distance power transmission. Medium and High Voltage Direct Current (MVDC/HVDC) systems offer advantages for high efficiency, long distance transmission. They are attractive for transmitting electricity from offshore wind farms, since undersea AC transmission cables require a charging current to feed the cable, which adds to the cost of transmission and limits the practical length of cables that can be used. For above ground applications, very long distance power transmission (hundreds of miles) with DC can offer lower costs than AC, but the current cost of power conversion is too high for wide deployment of DC even at long distances. Realizing the advantages of marrying expanded MVDC/HVDC power transmission with the existing AC power grid will require DC to AC power conversion that is both highly efficient and cost effective. Achieving high power conversion efficiency in high power electronics systems requires high voltage capable, low-loss power semiconductor switches, but modern silicon-based technology has a number of limitations including high switching and conduction losses, low switching frequency, and poor high-temperature performance. Gallium Nitride (GaN) and its alloys (AlGaN) are wide-bandgap (WBG) semiconductors that can replace silicon at higher voltages and switching frequencies and are capable of low-loss, higher efficiency operation.
Project Innovation + Advantages:
Sandia National Laboratories will develop a new type of switch, a 100kV optically controlled switch (often called photoconductive semiconductor switch or PCSS), based on the WBG semiconductors GaN and AlGaN. The capabilities of the PCSS will be demonstrated in high-voltage circuits for medium and high voltage direct current (MVDC/HVDC) power conversion for grid applications. Photoconductivity is the measure of a material's response to the energy inherent in light radiation. The electrical conductivity of a photoconductive material increases when it absorbs light. The team will first measure the photoconductive properties of GaN and AlGaN in order to assess if they operate similarly to gallium arsenide, a conventional semiconductor material used for PCSS, demonstrating sub-bandgap optical triggering and low-field, high-gain avalanche providing many times as many carriers by the electric field as created by the optical trigger. These two effects provide a tremendous reduction in the optical trigger energy required to activate the switch. The team will then design and fabricate GaN and AlGaN-based photoconductive semiconductor switches. The team predicts that WBG PCSS will outperform their predecessors with higher switch efficiency, the ability to switch at higher voltages, and will turn-off and recover faster, allowing for a higher frequency of switching. Ultimately, this will enable high-voltage switch assemblies (50-500kV) that can be triggered from a single, small driver (e.g. semiconductor laser). These modules will be substantially smaller (~10x) and simpler than existing modules used in grid-connected power electronics, allowing the realization of inexpensive and efficient switch modules that can be used in DC to AC power conversion systems on the grid at distribution and transmission scales.