Popularly used impedance-based methods need voltage and current waveform as well as line impedance per unit length to estimate distance to fault location. For a non-homogenous system with different line configuration, these methods assume that the system is homogenous and use the line impedance of the most frequently occurring line configuration. Load present in the system before fault is an important parameter which affects fault location accuracy. Impedance-based methods like Takagi and positive-sequence method assume that the load is lumped beyond the fault point which may not be true for a typical distribution system. As a result, accuracy of the impedance-based methods in estimating distance to fault is affected. Another short-coming of impedance-based methods are that they are unable to identify the branch in which the fault may be located. To minimize these errors, this thesis proposes a short-circuit fault current profile approach to complement impedance-based algorithms. In the short-circuit fault current profile approach, circuit model of the distribution feeder is used to place faults at every bus and the corresponding short-circuit fault current is plotted against reactance or distance to fault. When a fault occurs in the distribution feeder, fault current recorded by relay is extrapolated on the current profile to get location estimates. Since the circuit model is directly used in building the current profile, this approach takes into account load and non-uniform line impedance. Using the estimates from short-circuit fault current profile approach and impedance-based methods, the path on which the fault is located is identified. Next to improve fault location estimates, a median value of the estimates is computed. The median is a more robust estimate since it is not affected by outliers. The strategy developed above is tested using modified IEEE 34 Node Test Feeder and validated against field data provided by utilities. For the IEEE 34 Node Test Feeder, it is observed that the median estimate computed from impedance-based methods and the short-circuit fault current profile approach is very close to the actual fault location. Error in estimation is within 0.58 miles. It was also observed that if a 0.6 mile radius is built around the median estimate, the fault will lie within that range. Now the IEEE 34 Node Test Feeder represents a typical distribution feeder and has also been modeled to represent the worst case scenario, i.e. load current is around 51% of the fault current for the farthest bus. Hence the 0.6 mile radius around the median estimate will hold true for most distribution feeders and will be used when computing the fault range for field case events. For the field events, it was seen that the actual faults indeed lie within the 0.6 mile radius built around the median estimate and the path of the fault location has also been accurately estimated. For certain events, voltage waveform was not useful for analysis. In such situations, short-circuit fault current profile alone could be used to estimate fault location. Error in estimation is within 0.1 miles, provided the circuit model closely represents the distribution feeder. / text

Current trends in electric power consumption indicate an increasing use of dc in end-user equipment, such as computers and other electronic appliances used in households and offices. With a dc power system, ac/dc conversion within these loads can be avoided, and losses reduced. AC/DC conversion is instead centralized, and by using efficient, fully controllable power-electronic interfaces, high power quality for both ac and dc systems during steady state and ac grid disturbances can be obtained. Connection of back-up energy storage and small-size generation is also easier to realize in a dc power system. To facilitate practical application, it is important that the shift from ac to dc can be implemented with minimal changes. Results from measurements carried out on common household appliances show that most loads are able to operate with dc supply without any modifications. Furthermore, simple, and yet sufficiently accurate, load models have been derived using the measurement results. The models have been used for further analysis of the dc system, both in steady state and during transients. AC microgrids have gained research interest during the last years. A microgrid is a part of power systems which can operate both connected to the ac grid, and autonomously in island mode when the loads are supplied from locally distributed resources. A low-voltage dc microgrid can be used to supply sensitive electronic loads, since it combines the advantages of using a dc supply for electronic loads, and using local generation to supply sensitive loads. An example of a commercial power system which can benefit from using a dc microgrid is data center. The lower losses due to fewer power conversion steps results in less heat which need to be cooled, and therefore the operation costs are lowered. To ensure reliable operation of a low-voltage dc microgrid, well-designed control and protection systems are needed. An adaptive controller is required to coordinate the different resources based on the load-generation balance in the microgrid, and status of the ac grid. The performance of the developed controller has been studied and evaluated through simulations. The results show that it is possible to extend use of the data center dc microgrid to also support a limited amount of ac loads close to the data center, for example an office building. A protection-system design for low-voltage dc microgrids has been proposed, and different protection devices and grounding methods have been presented. Moreover, different fault types and their impact on the system have been analyzed. The type of protection that can be used depends on the sensitivity of the components in the microgrid. Detection methods for different components have been suggested in order to achieve a fast and accurate fault clearing. An experimental small-scale dc power system has been used to supply different loads, both during normal and fault conditions. A three-phase two-level voltage source converter in series with a Buck converter was used to interconnect the ac and the dc power systems. Together the converters have large controllability, high power quality performance, and allow bi-directional power flow. This topology can preferably be used together with energy storage. The tests confirm the feasibility of using a dc power system to supply sensitive electronic loads. / QC 20100908