| The power supply reliability, which is the core index to reflect the performance of power system, characterizes the keeping power ability. The duration interruption (more than3min) is focused on in the traditional reliability evaluation code, which could meet the requirement of normal load that is lightly affected with poor power reliability. But with the development of digital economy, sensitive equipments to the interruption have been widely used and the load loss caused by interruption especially by the temporary interruption is enormous. So improving the reliability and particularly solving the temporary interruption has become the attention focus by the power enterprises and even the whole society.Distribution network lies in the end of power system, facing to the load directly, and70%of interruption is caused by fault in the distribution system, so the distribution is the key link to improve the supply reliability. Generally, distribution network is designed in closed loop and operated in open loop. Even though the advanced technical measure is taken, such as distribution automation system, the temporary interruption caused by the load transfer couldn’t be avoided. Closed loop distribution network with single source could solve the fault in the feeder based on the differential protection, but it is helpless against the source fault. The closed loop distribution system with dual sources could cope with fault in the feeder and sources, in which there might be much circle power or even reverse power. But the traditional power flow control methods have some shortcomings, such as adjusting frequency and continuity, which couldn’t ensure the controlling effect. With the development of power electronic and advanced control theory, UPFC is applied successfully in the transmission system, which could improve the system stability and transmission ability. If UPFC is used in the distribution system, the key issue is not technology but economy. With the development of power semiconductor technology, the unit cost has been reduced gradually. Moreover, the needed capacity of UPFC is limited because the power flow in the distribution system is relatively less than that in the transmission system. More importantly, interruption could bring about huge losses on sensitive load, and the electric customers pay more attention to the interruption loss than to the electricity consumption. Thus many positive factors have created conditions for the UPFC to be applied in the distribution.From the internal demand of digital economy to improve the power supply reliability, the fault self-healing supply system and it’s power flow control method in the low and medium voltage distribution network is studied in this thesis.The fault seamless self-healing system of medium voltage closed-loop distribution with dual sources is presented to solve the temporary interruption, and the power flow is adjusted by UPFC to ensure the stable operation of the closed-loop system. The optimal control method of power flow is the focus to ensure the application economy of UPFC. A power flow control method of the low-voltage bus is presented based on UPFC to decrease the reverse power in the spot network with multi low-voltage sources parallel supplying. The main contents are as following.(1) Research on the fault seamless self-healing system of the medium voltage closed-loop distribution network with dual sources. Based on the analysis of the reason of temporary interruption and the limitation of the traditional measurement to improve the supply reliability, the fault seamless self-healing system of the medium voltage closed-loop distribution network with dual sources is presented, compared with the traditional method in technology and economy. The constitution and the key technology is given, including the power flow control method and the fast self-healing algorithm. And the control of power flow is the foundation to ensure the stable operation of the system.(2) Research on the optimal control strategy of the power flow in the medium voltage distribution system with closed-loop operation based on UPFC. The characteristic of power flow in the closed-loop distribution network is deduced and the difference between the distribution and transmission system is analyzed. Against the traditional power flow control measurement, the control method based on UPFC is presented, as the foundation of closed loop operation. In order to break the neck bottle of UPFC’s application economy, with the precondition of adjusted demand on power flow of the system, the optimal power flow control strategy based on UPFC is studied. And three optimal strategies are presented, including the minimum compensated voltage, the minimum active power, the minimum capacity of UPFC, thus ensuring the economy from various angles. And the simulation results prove the validity of the presented methods.(3) Research on the optimal power flow control method based on UPFC. The optimal power flow model considering the loss and nodal voltage in the closed loop distribution network is deduced, which is available for the active distribution system. And the simulation results prove that the loss and nodal voltage could be further improved. In order to combine the economy of UPFC application with the optimal power flow, the membership function of the loss, nodal voltage and UPFC’s cost is designed individually. And the comprehensive objective function is founded by weighted average, transforming the multi-objective problem into a single-objective one. And the application economy of UPFC with above strategies is compared.(4) Research on the power flow control method of the spot network with multi low-voltage sources parallel operation system based on UPFC. The constitution of spot network and it’s advantages in improving supply reliability are introduced. The output power expression of any source is deduced, describing the relationship between the output of the source with source voltage, system equivalent impedance and load at the low voltage bus. The reason causing the unbalanced output power, even reverse power is analyzed and the power flow control method at the low voltage bus based on UPFC is presented to balance the output power among the sources. In case the system parameter is unknown, the parameter identification based on disturbing signal on line and power control method is presented. In order to ensure the exited source to operate in parallel in time, the control method of the parallel spot voltage is introduced based on UPFC. And the simple match principle between the power flow control and protective relay is presented, ensuring the normal operation of multi sources in parallel and the exited source going back to work in time.(5) The development of testing system based on UPFC. Considering the laboratory condition, the physical experimental platform is designed and relative experiment about stable power flow control and voltage control as operating to parallel in the spot network is carried out. The results show that this method could balance the output power, and adjust the voltage at the parallel operation spot flexibly. Besides, a significant amount of power could be adjusted by a little of UPFC’s capacity with high accuracy.In a word, the fault seamless self healing system of the closed loop distribution network with dual sources could avoid the temporary interruption effectively, and the power flow control method considering the economy of UPFC could balance the power flow distribution, establishing the foundation for the normal closed loop distribution network and spot network to operate stably. The research conclusion of this thesis offers a new solution to improve the supply reliability and achieve the fault seamless self-healing target for smart grid, especially for smart distribution grid. |
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