Distributed Feeder Automation Technology For Smart Distribution Network

With economic development and social progress, the requirement for high power quality is increasing. For the high-tech digital equipment, even a short interruption lasting in a few seconds will cause serious economic losses and malign social influence. It normally takes more than one minute for the traditional local feeder automation (FA) and centralized FA to restore service. Therefore, it is urgent to adopt advanced feeder automation technology (FA) to further improve the power supply quality and reduce outage costs The distributed FA, which employs peer to peer communication network to exchange fault and control information among feeder terminal units, can restore the service within 1 second. Distributed FA has become a hot spot in the research of FA.In addition, as more and more distributed energy resources access to the distribution network, the distribution network becomes to active distribution network. It proposed further problems to the fault location of distribution lines for the two-way flow of fault current. Therefore, it has important theoretical significance and engineering application value to research distributed FA technology for smart distribution network and achieve fault location for the active distribution network.With the analysis of local FA, centralized FA, and distributed FA, the realization mode of distributed FA which can automatically recognize the position of tie switch is proposed. In addition, with research on impact of distributed energy resources on short circuit current, fault location based on fault current amplitude comparison or phase comparison is proposed. The main research contents and innovations are as follows:(1) The system composition, control principle and the implementation mode of the distributed FA are proposed. The distributed FA is composed of smart terminal units, peer to peer communication system and the master station. Smart terminal units exchange information and independently find fault section to realize distributed fault location, isolation and service restoration. The peer to peer communication system carries data exchanging among smart terminal units and between smart terminal units and master station. The master station justly receives the final result of fault handling. The distributed FA designs algorithm for fault location, isolation and service restoration orienting to the type of switches which are classified into the power switch, main line switch and branch line switch. Depending on the control method, the distributed FA has two implementation modes-cooperative mode or agent-based mode. The cooperative mode employs multiple smart terminal units co-determination to realize distributed fault location, isolation and service restoration. The agent-based mode employs one smart terminal unit as the agent to collect fault information of all switches to realize fault location, isolation and service restoration.(2) The position of tie switch automatically recognized by the STU by means of "relay inquiry" is proposed. The local topological information is configured on the STU, and the STU can recognize the feeder topology by "relay inquiry". According to state information (whether"opening") and the position of the switch (whether contacting power switch on both sides), the STU recognizes the position of the tie switch. The identification method which could adapt to the feeder topology changes is reliable. Besides, it is economic and easy to realize because it requires no installation of PT.(3) The peer to peer communication technology for distributed FA is introduced. Based on analysis of the requirements of distributed FA to communication system, two kinds of optical fiber communication network-industrial ethernet and EPON are present. In order to improve the management efficiency of the communication network, the STUs among associated feeders are set in a local area network by the virtual LAN technology. Three modes (GOOSE transmission mechanism in digital substation, TCP protocol supporting priority and UDP protocol supporting re-transmission mechanism)to transmit real-time controlling data rapidly is introduced, and the mode which adopts the UDP protocol to transmit real-time data meets the requirements of distributed FA applications.(4) Analyze the fault current characteristics of the active distribution network, and research the penetration capacity of DER considering the constraint of voltage variation. Introduce the characteristics of fault current provided by the synchronous generator, induction generator, doubly-fed generator and inverter, and analyze the influence of the DER on the fault current at the fault point, upstream line of the fault point and downstream line of the fault point. Considering the constraint of voltage variation, derive the relationship of penetration capacity of DER and voltage variation and the system short circuit capacity at access point of DER when DER takes non-participation in voltage regulation.(5) Propose the fault location method for active distribution network based on fault current amplitude comparison or fault current phase comparison. Research the relationship of fault current amplitude and phase between both sides of the healthy section and fault section.According to the relationship of amplitude between the maximum fault current provided by DER and fault current provided by the grid when the penetration capacity of DER considers the constraint of voltage variation and DER takes non-participation in voltage regulation, it is proposed to locate the fault by the comparison of fault current amplitude between both sides of a feeder section, and the implementation scheme in cooperative mode distributed FA is given. While the permeability of the DER is further increased, it is proposed to locate the fault by the comparison of fault current phase between both sides of a feeder section, and the time instant when the fault signal appears is used to synchronize the phase angle measurement. The fault location method for active network is simple and easy to realize in engineering.(6) The results of static simulation and field trial operation prove the accuracy of the distributed FA. Construct the testing system to verify the function of the distributed FA. With building the "hand in hand "cable line and setting STUs access EPON system, the distributed FA action logic is confirmed and its real-time performance is analyzed by setting fault at different points. Finally, the engineering applicability of the distributed FA system is verified in Xiamen, Kunming and some other places.The proposed distributed FA technology speeds up fault processing in the distribution network and the restoration time is shortened to within 1 second; and the proposed fault location method for the active distribution network based on amplitude and phase comparison is simple in principle and easy to apply in engineering. The research results have great significance to promote the development of FA technology, to further improve the reliability of power supply of the smart distribution network and to solve the fault location for the active network.