In this paper, the heuristic rules are proposed to derive the rephasing strategy of laterals and distribution transformers to improve the three phase unbalance of distribution systems. The distribution feeder network has been obtained by retrieving the attribute data of distribution components from the database of outage management system (OMS) in Taipower. The topology process and node reduction have also been executed to identify the network configuration and to prepare the input data for load flow analysis. With the monthly energy consumption of customers served by each transformer, which has been retrieved from the Customer Information System (CIS), the hourly loading of each distribution transformer can be derived. By performing the three phase load flow analysis, the three phase currents and neutral current of each primary trunk line section and each lateral can be calculated. The heuristic rule is employed to determine the phase adjustment strategy laterals and distribution transformers for rephasing to achieve three phase balancing. In order to demonstrate the effectiveness of the proposed methodology for three phase balancing, two practical distribution feeders in Taipower Fengshan District are selected for simulation. After rephasing the distribution transformers and laterals proposed by this paper, the three phase currents and netural of the test feeders have been collected. By companing to the neutral current before rephasing, it is found that the neutral current of test feeders have been reduced significantly and there phase balancing has been obtained by executing the proposed strategy derived using the heuristic rule.

Unbalanced operation of distribution feeders not only affects equipment utilization, voltage level and system protection, but it also increases extra energy losses. This leads to a deterioration of service quality, reliability and operation efficiency of a distribution system. This dissertation analyzes the problems of unbalanced three-phase distribution feeders, and offers potential solutions. Due to the voluminous data involved in a distribution system, analyzing the system by retrieving system data from paper maps is tedious and difficult. Thus, this dissertation uses data from the already constructed Outage Management System (OMS) of Taiwan Power Company (Taipower) to support distribution feeder three-phase unbalance analysis. The distribution feeder network was obtained by retrieving the connectivity table and attribute data of distribution components from the database of OMS. The topology process and node reduction were executed to identify the network configuration and to prepare the input data for load flow analysis. The hourly loading of each distribution transformer was derived using data of monthly energy consumption of customers served by each transformer, as retrieved from the Customer Information System (CIS), and the typical daily load patterns of customer classes. By performing three-phase load flow analysis, phase currents and neutral current of each primary trunk line section and each lateral could be calculated. Finally, an expert system is proposed to establish the rephasing strategy of laterals and distribution transformers to improve the imbalance of the three phases of the unbalanced distribution feeders. The heuristic rules adopted by distribution engineers are incorporated in the knowledge base of the expert system in the problem-solving process. The neutral current reduction algorithm is developed to support the inference engine to derive the rephasing strategy to reduce the neutral current of distribution feeder. In doing so, customer service interruption due to unexpected tripping of low energy over current relay (LCO) can be prevented, and furthermore the customer service interruption costs and labor costs to implement the rephasing strategy can be justified by the reduced power loss. To demonstrate the effectiveness of this proposed methodology in improving the three-phase unbalance of the distribution feeders, two actual distribution feeders in the Taipower Fengshan District were selected for computer simulation. After Taipower engineers implement the proposed rephasing strategies, the data of phase currents and neutral current of test feeders were collected from the SCADA system of Distribution Dispatch Control Center (DDCC). By comparing to the neutral current of test feeders before rephasing, it is concluded that the proposed rephasing strategy is effective in achieving three-phase balance of the distribution feeders after executing the rephasing of laterals and distribution transformers.

Taipower Company has already completed the installation of the Outage Management System (OMS) in all service districts. The attributes of all distribution equipments and customers have been included in the computerized mapping system. However, the phasing attributes of distribution transformers are not very accurate in the database because they are very difficult to be identified for the distribution systems. The phase information of transformers in the OMS database is often inconsistent with the actual service phase, which deteriorates the performance of distribution system analysis, planning, and operation of Taipower distribution systems. The objective of this thesis is to develop an innovative Phasor Measurement Unit (PMU) to support the phasing identification of distribution transformers in a very effective way. The proposed PMU is used to measure the low voltage signal at the secondary side of transformers to prevent the survey personnel from safety problem. With the measured phases information of distribution transformers stored in the embedded system, the attributes of transformer phases in OMS can be updated to improve the accuracy of database. For the underground distribution systems, it is very difficult to receive the 1PPS signal of GPS system for timing synchronous to support the proposed transformer phasing measurement because all transformers are located at the building basement. This thesis proposes a timing synchronous module by using the Temperature Compensated Crystal Oscillator, TCXO to maintain the timing accuracy with high precision. Before executing the phasing measurement, this module is calibrated using the GPS 1PPS signal with fuzzy control calibration algorithm. It is found that the proposed PMU module can maintain the timing synchronous with 6˚, during two hours time period which will support the transformer phasing measurement by providing the reference timing synchronous even without the GPS 1PPS signal.

The contents of this thesis are the introduction to mesh grids, the method of dispatch control of these grids and the description of technology used in mesh grids distribution nodes in the city of Brno. This thesis also describes low voltage switchgears used in these types of grids. The development of dispatch control in ECD company is also mentioned. The development describes the grid dispatch control methods prior to implementing the OMS system and the changes which followed after the implementation. The process improvements of the new system resulting from this thesis can be found in this thesis as well. The improvements are incorporated in the OMS system and are used for more efficient dispatch control of the low voltage mesh grids. There is a more detailed analysis of the mesh grid Brno – Bohunice in the practical part of this thesis. The practical part contains analyses of mesh grids stabilized conditions measurements and analyses of mesh grids fault conditions measurements. The analyses assess the power load in transformers, minimal and maximal phase current and maximal power load at the time they were observed. The analysis of the measured data is accompanied by the assessment of differences in current phase measurements and differences in voltage phase and combined measurements. The result, based on the analysis and collection of the data, is a proposition of adding switchgears for the support of the dispatch control of the mesh grid Brno – Bohunice. In the summary of the thesis there is a comparison of the result of the factual and theoretical analysis. An experimental model of the mesh grid Brno – Bohunice was created in PS CAD software as a part of the theoretical analysis.