1 Research background
The protection control problem after distributed generation access to the power distribution system has always been one of the important challenges facing the power grid. The access of distributed power sources changes the power flow distribution of traditional distribution networks, and the selectivity and sensitivity of traditional protections decline. At the same time, the grid-connected distributed power supply is connected to the grid through power electronic devices. Different control strategies determine different fault characteristics. It is not ideal to protect the ultra-high voltage system protection in the distribution system of distributed power supply. Program.
Therefore, a new principle and scheme of transient polarity comparison protection based on fault transient component is proposed. Because the fault transient component reflects the fault transient characteristics of the grid itself, it has nothing to do with the type and capacity of the power supply on both sides of the system. Overcoming the problem of short-circuit current difference caused by the difference between distributed generation access capacity and system capacity; at the same time, the fault transient high-frequency component has a duration of milliseconds. With the development of microprocessors and sensors, Provide high-speed protection to adapt to the fast-moving requirements of distribution network protection with distributed power supply, which is beneficial to ensure the coordination relationship between distribution network protection and distributed power supply low-voltage ride-through capability.
2 Transient polarity comparison protection principle
Transient polarity comparison protection uses wavelet transform to extract a certain frequency band information of fault transient high-frequency signal as the basis for fault judgment. By comparing the polarity of transient high-frequency signal, the fault location is quickly and accurately determined.
The protection criterion of transient polarity comparison protection is precisely the polarity of the high-frequency component of the transient current signal. The concept of cross-correlation function is used to describe the similarity degree of the two transient signals. When the transient high-frequency signal flowing through the two ends of the line meets the high positive correlation condition, it can be judged as the fault in the line area according to this, and vice versa, when the condition is highly negatively correlated, the fault is an out-of-area fault.
Transient polarity comparison protection covers all fault types. More importantly, the distribution network integrated protection method based on transient information is not affected by factors such as distribution network architecture, distributed power supply type, capacity, and access location. Moreover, the protection method is adaptive. The data sampling rate, the discrete wavelet transform extraction frequency band and the fault determination time are all related to each other. According to the sampling frequency and data processing capability that can be achieved by different hardware platforms, the wavelet transform can extract transient high-frequency signals of different frequency bands according to specific needs. As the frequency band increases, the signal attenuation speed increases, so the polarity comparison is based. The protection principle is greatly improved.
3 Integrated protection of distributed power multi-point access distribution system
With the access of new energy grid-connected power generation systems in the distribution network, the distribution network must be integrated into the medium-voltage distribution network from the micro-grid and large-scale distributed power sources in a centralized manner to maximize the grid consumption. The ability to regenerate energy creates a more distributed and more interactive active distribution network. At present, the performance of distributed sensing devices has been rapidly developed and improved in the development and construction of intelligent substations, which provides the necessary technical basis for the engineering application of integrated protection of power distribution systems. In addition, the data-time-based and optical fiber-to-synchronous technology between intelligent electronic devices based on global positioning system is also well applied in engineering practice. Based on the proposed principle of transient polarity comparison protection for distributed generation multi-point access distribution system, an active distribution network protection system combining regional centralized control and local protection control system can be constructed. An integrated protection unit (IR) is set up at the busbar to protect the local single electrical equipment bus and line based on local information and fault information of adjacent protection units. At the same time, the distribution network integrated protection unit integrates multiple points of information according to different situations to complete the protection control function based on multi-point information, thereby completing the functions of rapid fault location and backup protection, and realizes protection and automation of the distribution network.
When a fault occurs, the high-frequency component of the fault will propagate from the fault point to the entire network. The polarity of the transient current signal detected by each current transformer will have a certain regularity, that is, a set of current transformers that point to the fault point. The transient currents are in the same polarity direction, and the transient currents detected by a group of current transformers that are facing away from the fault point are in the same direction. Each integrated protection unit can detect the relative position of the fault relative to the busbar. By comprehensively comparing the information of each integrated protection unit in the entire distribution network, the location of the fault can be identified. When it is determined that the bus is faulty, the protection unit issues a trip command to all circuit breakers connected to the bus. When the fault is determined as a line fault, the circuit breaker connected to the fault line on the bus side is tripped by the trip command, thereby isolating the fault area. The integrated network protection unit utilizes the integrated protection unit polarity determination information to implement the backup protection function.
4 Conclusion
The transient polarity comparison protection principle proposed in this paper is not affected by the distributed power supply access capacity and short-circuit current difference, which improves the fastness of protection. The integrated protection scheme formed on this basis realizes the rapid fault location and isolation of the distributed power distribution system, which helps to improve the acceptance capability of the active distribution network for renewable energy in the future and ensure the reliability of the system power supply.
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