Power Electronics Assisted Voltage Regulators for Modern Distribution Systems
Step voltage regulator (SVR) has been utilized in power distribution systems for decades. The induced arc from the conventional SVR tap change and the voltage instability from the renewable energy penetration impose constraints on the conventional SVRs’ lifetime. With more distributed power generation and renewable energy penetration, voltage fluctuation and power generation variation can be observed more frequently in the modern power distribution network. More tap change operations are required for SVR to regulate the line voltage. However, the tap changing mechanism of the conventional SVR always generates an electric arc when tap changes, which imposes constraints on conventional SVRs, such as lifetime and maintenance period. Meanwhile, the voltage regulation accuracy cannot be guaranteed since the SVR regulates the voltage step by step. The power electronics transformer solution was proposed but requires the power converter capacity proportional to the voltage regulation range, which significantly increases the system cost.Motivated by the issues mentioned above, several PE-assisted arcless tap change topologies are proposed to reduce the contact erosion rate of tap changers in SVR. The system efficiency is the same with the conventional SVR in normal operation, while the converter power rating is only 0.3% of the total system power, which also reduces the system cost compared with the full power electronics solutions. Based on the proposed arcless tap change mechanism, a hybrid voltage regulator is proposed. Stepless load voltage regulation is achieved while the tap changer mechanism remains in the system, which helps to promote the upgrade to the existing power distribution systems. A scaled-down prototype of the arcless SVR is developed to verify the proposed arcless tap changing method. The hardware test results verified the proposed arcless step voltage regulator can eliminate arcing during the tap change and reduce the contact erosion rate by over 10,000 times the conventional arcing SVR. Other advantages of the novel method over the conventional SVR, such as advanced load voltage regulation and volt/var control, are also verified. The proposed hybrid voltage regulator was simulated and experimentally validated. The experimental results demonstrate arcless tap change operation, stepless voltage regulation, and load voltage continuity during the tap change. For PE-based hybrid voltage regulators, many functions, such as fast voltage regulation, flicker compensation, and var control, can be accomplished, which cannot be achieved from the conventional SVR. This research also proposed a new topology of the hybrid voltage regulation transformer (VRT). The feasibility and capability of var control are investigated for different load power factors and input voltage percentage when the voltage regulation does not exceed the power converter capacity. The simulation results illustrate the feasibility of implementing var control while the load voltage is being regulated. This dissertation also proposed a new hybrid transformer based on interline power converters for voltage regulation. The maximum power delivered by converters is reduced in half compared with the conventional series compensation configuration for the same voltage regulation range. Therefore, the proposed hybrid transformer exhibits a higher overall efficiency covering a wide range of voltage regulation. Comparison between the conventional series voltage compensation method and the proposed interline power converter-based method is presented based on the operation principle, the converter power, and the overall system efficiency.