# Research Projects

## Fault Detection and Identification in DC Microgrids

 DC microgrids are gaining popularity due to their lower need of power conversion stages and greater efficiency. However, the constant dc bus voltage present enables the possibility of series dc arc faults. These types of faults occur in series with the circuit, making them difficult to detect (as opposed to line to ground faults). In addition, the noise generated by a series arc fault can travel to adjacent line sections, which can (mis) trigger detectors using frequency domain signatures. In this project, we are investigating both continuous and discrete time parameter and state estimation techniques for localization of series dc arc. In particular, the network's line parameters are estimated online, detecting when a change (e.g. a series arc fault) occurs. Probabilistic techniques, such as Quickest Change Detection (QCD), have also been developed to detect when a change in the probability distribution of the network injection currents occurs. Such change will be characterized with series faults at different locations and can be quickly detected online. Several insights can also be gained on the observability of the network for fault detection at every line using graph theoretic techniques.

# Control of Power Electronics/Systems

## Time Scale Separation for Controller Design

 Power systems are nowadays composed of power electronics based sources (e.g. PV, wind, etc.) and loads, with dynamics ranging from the order of nano seconds (power converters) to minutes and hours (e.g. economic dispatch, unit commitment). With regards to microgrids, these time scales also enable the design of hierarchical control, generally referred to as primary (inner-outer loop control of power electronics), secondary (current sharing, voltage averaging), and tertiary (optimization, forecasting). In this project, we are developing control techniques to exploit these time scales to create more robust and efficient controllers. For example, a Nonlinear Model Predictive Control (NMPS) of Interior Permanent Magnet Synchronous Generator/active rectifier was developed by simplifying the outer loop using time scale separation. The proposed technique is able to regulate the dc bus voltage while at the same time operating the generator at an optimal (efficient) setting. In addition, field weakening can be achieved through the constraints of the NMPC problem.

## Voltage Balancers for the MEA

 As the power rating of dc microgrids used in aircraft and ships continues to increase, a higher voltage can be helpful in reducing the size and weight of cables and components. However, this higher voltage can create problems, specially at high altitudes. Voltage balancers can be useful for providing bi-polar voltages, e.g. $$\pm 270\;\text{V}_\text{dc}$$, reducing the available potential between any line and ground (by half). In addition, it enables the possibility of supplying both line to line ($$\pm 270\;\text{V}$$) and line to ground ($$270\;\text{V}$$)loads. In this project, we compare both statically and dynamically different types of voltage balancers. In addition, we have developed control schemes for buck, cuk, and interleaved buck balancers. The proposed controllers are then tested using control Hardware in the Loop (HIL) and complete hardware testbed.

## Stability Analysis of DC Microgrids

 For this work, nonlinear control techniques are utilized to compute safety regions for the microgrid. Utilizing optimization techniques, the controller gains can be optimized to increase the robustness and safety region of these systems. Lastly, hierarchical control methods are currently being investigated for the optimal operation of microgrids.