Introduction to multi-terminal flexible DC system fault protection scheme

The DC system has outstanding advantages in reducing line loss, increasing transmission capacity, and large-scale centralized access of new energy power generation. With the rapid development of power electronics technology in recent years, DC system has become a hot research and application field in the field of power systems at home and abroad. . Among them, the multi-terminal flexible DC grid has become a major trend in the development of power systems in the future due to its huge advantages in power supply reliability. DC fault protection of flexible DC grid is one of the key technologies for its development. The main technical difficulty lies in the reliable identification and location of fault lines. Combined with the current state of the art, this paper analyzes in detail the technical feasibility of using DC circuit breakers and using self-clearing converters to achieve fault isolation. Combined with the pre-acceleration protection idea of ​​"handshake method", a multi-terminal flexible DC system fault protection scheme based on DC circuit breaker and non-DC circuit breaker is designed to realize fault identification, isolation and rapid recovery of remaining network.

Problem and meaning

DC fault protection is a key technical issue for flexible DC grids. Due to the immature DC fault isolation technology, flexible DC engineering currently uses AC circuit breakers to achieve fault isolation. In view of the actual project, the related literature puts forward the idea of ​​"handshake method" to realize the reliable identification and location of faulty lines. However, the performance of this method in terms of motion speed, system recovery capability, power supply reliability, etc. is far from meeting the requirements of flexible DC grid.

With the gradual maturity of DC circuit breaker technology and converter self-cleaning technology, the DC fault isolation scheme of flexible DC system is gradually clear. The paper aims at the two most promising fault isolation schemes of "half-bridge MMC+ DC circuit breaker" and "self-clearing converter + DC switch". Based on the "pre-acceleration" idea, the corresponding DC fault protection scheme is designed. Taking the "half-bridge MMC + DC circuit breaker" isolation scheme as an example, the paper proposes a protection action scheme as shown in Figure 1: 1) using a directional component to select a pre-trip DC circuit breaker; 2) once the fault detection criterion action, immediately jump off The DC circuit breaker is preselected to achieve reliable removal of the faulty line, and it is ensured that at least one end of the sound line is still connected to the converter station and continues to operate. 3) The line is charged by the live signal, and if the line is still charged, the corresponding trip is broken. The devices overlap, otherwise they do not coincide, thus achieving rapid recovery of the remaining network.

The protection scheme designed in this paper can realize the reliable identification and location of faults without communication, and the protection fault speed and system power supply reliability are greatly improved compared with the traditional DC fault protection scheme based on AC breaker.

Figure 1 is based on the "half-bridge MMC + DC circuit breaker" front acceleration protection scheme

Important content

The paper analyzes the technical feasibility of two fault isolation technologies: “Half Bridge MMC+ DC Circuit Breaker” and “Self-Clear Inverter + DC Switch”. On this basis, combined with the "pre-acceleration" protection idea, a DC grid protection scheme based on single-ended information is designed.

in conclusion

This paper proposes a flexible DC grid protection scheme based on the idea of ​​“pre-acceleration”. The scheme only needs the local signal, and the directional component and the coincidence judgment can realize the reliable positioning isolation of the faulty line and the rapid recovery of the remaining network. Compared with the traditional DC breaker-based DC fault protection scheme, the protection action speed and system power supply reliability are significantly improved, which can be used as a backup protection for flexible DC grid lines (especially in the case of communication failure).

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