The Comprehensive Application Of Signal Injection Methods In The Protection And Control Of Active Distribution Network

In active distribution network, the advantages of distribution generation (DG) can be applied very well and DGs can play a positive effect on distribution network, reduce the overall investment of power system and satisfy the increasing requirement of electric power, so the active distribution network is the development trend of the future distribution network. But in active distribution network, some problems will arise due to the interconnection of DGs, islanding operation is one of them. While islanding operation occurs, the supplied voltage and frequency can't be guaranteed, the fault arc extinction and reclosing will be influenced due to that the feeder in islanding continues to be alive, and the equipments of power system and maintenance workers may be damaged. Therefore, islanding protection should be applied in active distribution network, which can disconnect DGs from distribution network while islanding operation occurs. In active distribution network, automatic tuning of arc suppression coil (ASC), namely, the current caused by ASC can completely compensate the current caused by distributed capacitance while a single-phase grounding fault occurs by tracking the capacitive current and adjusting ASC in real-time, is an important technique measure to ensure distribution network operating reliably. Fault line selection, namely, fault line should be identified quickly and the fault should be removed manually after the necessary loads are transferred while a single-phase grounding fault occurs, can avoid the system insulation being damaged by the over voltage caused by the fault and more serious phase-to-phase fault occurring, so it is an important technique measure to reduce the loss caused by the fault and improve power supply reliability.At present, a lot of islanding protection methods have already been proposed, in which, carrier islanding protection has good application prospect due to that it is easy to implement, has little effect on output power quality of DGs and can apply to active distribution network that many DGs are interconnected into. But the protection has poor reliability and may cause mal-operation and fail to trip, so it still has some way from application. Therefore, a new islanding protection method based on signal injection which has smaller non-detection zone and higher practicability is important for the development of active distribution network. All of the methods of distributed capacitance detection in traditional distribution network are suitable for active distribution network. Among which, the methods based on signal injection are widely used, but they have either poor measurement accuracy or complicated manipulation, and their applications are ineffective, so they need to be gone into further research. All of the methods of fault line selection in traditional distribution network can apply to active distribution network except those which based on negative-sequence components. Among which, the methods based on signal injection also have wide application, but the practical effects show that the accuracies are low. Therefore, a new method which has higher accuracy needs to be developed. Moreover, it can be known that signals need to be injected from the substation bus while islanding protection, distributed capacitance detection and fault line selection based on signal injection are adopted, so it makes the integrative design of the three devices become possible. Thus, it can not only decrease hardware equipments and reduce cost, but also avoid the injected signals interfering with each other and optimize each performance, so the selection of the shared signal type and frequency, signal injection mode, signal detection method and comprehensive application also should be studied.In this paper, methods based on signal injection oriented islanding protection, distributed capacitance detection and fault line selection are studied to improve the reliability of islanding protection and fault line selection, also the measurement accuracy of distributed capacitance. And the comprehensive application of signal injection methods is studied to simplify the structure of system and reduce the cost. The details of the research work are as follows:(1) Bring forward a new integrated design idea synthesizing three kinds of technology including islanding protection, automatic tuning of ASC, and fault line selection in active distribution network, which not only can simply the hardware architecture and wiring of equipments, but also improve the cooperation of different technical and functions to obtain optimum performance.(2) Propose carrier based islanding protection method, which adds carrier blocking in(on) the basis of over/under voltage protection and over/under frequency protection to improve sensitivity and reliability and reduce the non-detection zone, and stops injecting carrier signal in bus islanding to avoid the protection failing to trip. This method has characteristics of high sensitivity, simple principle, easy realizing, and small influence on the power quality, and also it is suitable for DGs in different grid-connected way. (3) Put forward a new method of distributed capacitance detection to lay the foundation for automatic tuning of ASC by injecting a constant current signal from the secondary side of voltage transformer of ASC and detecting the voltage of neutral point to calculate the distributed capacitance. The method needn't adjust ASC manually or regulate the signal source. It has features of being easy to operate, high calculation accuracy, fast response and no negative impact on the system. While calculating distributed capacitance using the above method, we can also estimate the insulation resistance of the system to obtain damping rate for the on-line monitor of the system insulation.(4) Propose a new method of fault line selection based on square wave in resonance grounding system. The method can identify the fault line by injecting a constant current square wave into system through the fault phase of TV, detecting zero sequence current of each branch and analyzing the relationship between fundamental current and the 3rd harmonic current. The method is almost free from the influence of distributed capacitance, and has high sensitivity and accuracy for the faults with high impedance grounded. And this method can judge the fault line only using the zero current of the fault line itself.(5) Proposed the frequency selection criteria of the injected square wave is the premise and key of the fault line selection method; considering the requirements of distribution system, signal injection equipment is designed to make the proposed fault line selection method put into practical application, which can not only put out constant current fundamental signal and the 3rd harmonic signal, but also have little influence on the system; signal processing method is developed to lay the foundation for selecting fault line accurately, which could detect the injected signal with sensitive precision and eliminate various harmonic interference effectively.(6) Considering the three proposed methods including carrier based islanding protection, distributed capacitance detection based on constant current signal, and fault line selection based on square wave, a comprehensive application scheme of signal injection methods is developed, which is implemented by selecting the shared signal injection mode, signal type and signal frequency satisfy the needs of the three functions. Thus, it can avoid the injected signals interfering with each other and the working performances are influenced, also it can reduce the cost and decrease the influence on the measurement and detection of power system.(7) To satisfy the design requirements that islanding protection, distributed capacitance detection and fault line selection are integrated in one device, the feasibilities of realizing islanding protection and fault line selection with signal injecting from the voltage transformer of ASC and with the same square wave were proved. The signal source employed to islanding protection, the signal source and detection system employed to automatic tuning of ASC, and the signal source and detection system employed to fault line selection are all integrated in one device which can implement all functions, besides, a detection and control system are installed in the grid-connected point of DG. Finally, the functions and the flow charts of each components of the device are introduced.The three methods proposed in this paper, including carrier based islanding protection, distributed capacitance detection based on constant current signal, and fault line selection based on square wave, can solve the problems existed in each technique. The development of the comprehensive application of signal injection methods can reduce the cost and obtain better performance. The researches of this paper are of great importance in pushing on the developments of the protection and monitor technology of active distribution network and improving the supplied reliability of power system.