For combating the climate crisis, generating electricity from renewable resources, such as wind and solar, is essential. These resources are economical primarily in large scales and are often located far away from the metropolitan areas, hence need transmission lines. Protective relays are installed on these transmission lines to detect faults caused either by lighting or other short circuits, and disconnect faulted lines. The existing protection equipment is designed for electricity that is generated in power plants using conventional sources such as coal and natural gas. However, this protection infrastructure will not work with the existing control methods used in renewables-based generation. With many states opting to move towards hundred-percent generation by renewables, all existing protective relays will need to be replaced. The objective of the proposed research is to allow the existing protection equipment, such as relays, to continue to be used even as the electricity generation transitions from conventional sources to renewables, by controlling the renewable-based generation such that it emulates the conventional generation in the event of a fault. This will avoid incurring the huge cost of the system-wide upgrade of the protection infrastructure. The proposed approach is based on first determining the type of fault that has occurred and then to take appropriate action such that the existing relays perform as they do in the case of conventional sources. This research will be a very important step in keeping the cost of renewables low in order to enhance their penetration into the utility grid. It will be transformative in advancing knowledge in the critical field of protecting the transmission infrastructure and becoming a norm used by hardware equipment manufacturers. This research is at the intersection of power electronics, power systems, and controls, and will be very educational for the next generation of power engineers.

The objective of this proposal is to find a solution to the real and immediate need to enhance the integration of utility-scale renewables. Existing inverters of utility-scale renewable resources, such as PVs and wind, are controlled to generate balanced three-phase currents even during unbalanced faults. However, to operate, the commonly-used protection relays depend upon unbalanced currents that are generated, such as by the synchronous generators of conventional power plants during unbalanced faults. With the increasing penetration of such inverter-based resources, toward the goal of hundred-percent renewables, the existing protection schemes will not work. The proposed research is on injecting appropriate unbalanced current during faults so that the use of the existing transmission-protection infrastructure continues, without having to incur the huge cost of the system-wide upgrade of the protection infrastructure. The proposed approach is based on first identifying the type of fault that has occurred and then to inject the appropriate currents to make the relays operate, knowing only the inverter terminal voltages and the transformer-winding configuration. The proposed control scheme is based on emulating the time-tested behavior of conventional generators during faults, thus requiring no modifications in the existing protection infrastructure, i.e., the inverters operate based solely based on local measurements. This research will thoroughly investigate and develop the control scheme by software emulation and hardware verification. The research will also examine the impact of this control on the inverter components. The aim of the technology transfer is that the control scheme becomes a norm that can be mandated by transmission-system owners and independent power producers, who in turn, would require it of inverter manufacturers.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

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University of Minnesota Twin Cities
United States
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