The voltage-source converter (VSC) based high-voltage direct-current (HVDC) grid is an effective means for large-scale integration of renewable energies. This paper proposes a pilot protection scheme as the backup protection for transmission lines in VSC-HVDC grid. The traveling wave (TW) propagating processes and waveform characteristics at both ends of the transmission line are analyzed under different fault conditions. Based on the different characteristics between internal and external fault conditions, the protection is formulated utilizing similarity measure of TW waveforms. A four-terminal VSC-HVDC grid is modelled on PSCAD/EMTDC platform to test the performance of the proposed protection. Various simulation results demonstrate that the proposed protection is able to accurately discriminate internal faults from external faults and identify the faulty pole. The protection performance analysis shows that the protection is insensitive to fault resistance, protection sampling rate and the inductance value of the reactors at the end of lines. The protection algorithm is simple with small calculation amount, and does not require data synchronism. Furthermore, the proposed protection has superiority in fault resistance withstand ability compared with other backup protections.

The Modular Multilevel Converter-High Voltage Direct Current (MMC-HVDC) system is recognized worldwide as a highly efficient strategy for transporting renewable energy across regions. As most of the MMC-HVDC system electronics are weak against overcurrent, protections of the MMC-HVDC system are the major focus of research. Because of the insufficiencies of the conventioned fault diagnosis method of MMC-HVDC system, such as hand-designed fault thresholds and complex data pre-processing, this paper proposes a new method for fault detection and location based on Bidirectional Gated Recurrent Unit (Bi-GRU). The proposed method has obvious advantages of feature extraction on the bi-directional structure, and it simplifies the pre-processing of fault data. The simplified pre-processing avoids the loss of valid information in the data and helps to extract detailed fault characteristics, thus improving the accuracy of the method. Extensive simulation experiments show that the proposed method meets the speed requirement of MMC-HVDC protections (2 ms) and the accuracy rate reaches 99.9994%. In addition, the method is not affected by noise and has a high potential for practical applications.

The fast DC line protection technology is one of the main challenges to high‐voltage direct current (HVDC) grids. Based on the Zhangbei HVDC grid project, this paper proposes a non‐unit overhead transmission line protection scheme for bipolar voltage‐source converter‐based HVDC grids equipped with DC circuit breakers. The voltage gradient is used to detect faults rapidly and start the protection algorithm, and the voltage derivative criterion is adopted to discriminate the internal faults from the external ones. Two kinds of faulty pole identification methods are proposed based on the propagation characteristics between the line‐ and zero‐mode voltages, ensuring the selectivity of the protection. The Zhangbei HVDC grid model is built in power systems computer‐aided design/electromagnetic transients including DC and the performance of the proposed protection scheme is verified by simulation results of various fault conditions.

Non-destructive inspection methods that use broadband reflecting signals to identify microscopic flaws and aging characteristics in power cables are often employed, however they suffer from limited detection distances and a single detection target. This study proposes a scheme for harnessing power line communication (PLC) devices to accomplish the detection and identification of multiple cable aging. A model of the propagation of a high-frequency carrier signal through a cable is first combined with mathematical models of various types of aging, such as water-tree aging, electric-tree aging, and thermal aging, to simulate the network channel state in the process of different insulation degradation of the cable. The channel frequency domain response (CFR) at the receiver can then reflect the incidence of cable aging, and the proposed approach, in conjunction with machine learning algorithms, can recognize different types and severity of aging under interference. Computer simulations and field experiments were conducted to validate the viability of the suggested approach, and the results shown that 98% correctness could be attained in on-line monitoring despite interference considerations. In theory, the range is doubled when compared to the reflected signal approach with the same bandwidth and power.

The modular multilevel converter (MMC) high-voltage direct current (HVDC) transmission technology is essential for overcoming the challenges of large-scale renewable energy integration. Line protection is critical for ensuring system safety. However, existing protection methods for MMC-HVDC transmission lines face difficulties in withstanding both high resistance and noise interference, frequently leading to failures in detecting internal high-resistance faults or triggering false operations due to noise. This paper first derives the theoretical expression of the line-mode voltage through analytical methods. By analyzing the second derivative of the line-mode voltage under different fault conditions, this paper constructs a criterion based on the ratio of the integrals of the positive and negative components of the second derivative of the line-mode voltage. This criterion enables effective fault discrimination by utilizing the characteristic differences in the second-derivative waveform. The proposed criterion allows for precise fault identification, requiring only a 0.5 ms time window to detect faults. Additionally, this criterion is highly resistant to transition resistance, remaining unaffected by resistances up to 500 Ω. Moreover, an entropy-based auxiliary criterion is introduced to prevent false operations caused by noise interference. Simulation results using PSCAD/EMTDC demonstrate that the proposed protection scheme can swiftly and reliably detect faults, with a detection time of 0.5 ms and robust performance against both high transition resistance and noise interference.

The hybrid high‐voltage direct current (HVDC) transmission system comprising a line‐commutated converter (LCC) on the rectifier side and a voltage source converter (VSC) on the inverter side combines the advantages of both LCC and VSC. It has broad prospects in many application fields. However, at the VSC‐based inverter side of the hybrid system, there is no dc filters, and the inductance of the smoothing reactor is much smaller than that of the LCC station. Therefore, it is hard to distinguish line internal and external faults at the VSC side for the conventional protections based on the attenuation characteristic of the line boundary. In order to solve this problem, this study proposes a non‐unit protection for hybrid HVDC transmission lines. The protection judges the fault section based on the differences of the backward travelling waveforms under internal and external fault conditions. Simulation results show that the proposed protection is able to distinguish internal faults from external ones, identify the faulty pole exactly, and has enough sensitivity under large resistance faults.