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Double-circuit transmission lines are widely used in UHV transmission, which can improve the reliability of the lines. The pilot zero-sequence directional protection has a high sensitivity to asymmetric ground faults of the entire power grid. During the period when complex faults occur on one circuit of double-circuit transmission lines on the same tower, due to the existence of multiple fault sources, the pilot zero-sequence directional protection may refuse to operate. Based on the principle of pilot zero-sequence directional protection, taking the single-phase grounding-disconnection complex fault of typical double-circuit lines as the research object, this paper qualitatively puts forward the influencing factors on the misoperation of pilot zero-sequence directional protection. Finally, through RTDS modeling and simulation, the influence of sending & receiving power flow and transition resistance on pilot zero-sequence directional protection when complex faults occur at different fault points is verified.

As the main protection of AC system, the current differential protection performs well in rapidity and selectivity. However, for DC system, current differential protection can only be used as backup protection; in the meantime to avoid the influence of external fault and unbalanced current and ensure the selectivity of current differential protection, such protection often needs quite a delay. Starting from the characteristics of DC transmission lines, this paper mainly theoretically analyzes the characteristics of distributed capacitance and frequency variation of impedance parameters of DC transmission lines, and studies the influence of line characteristics on current differential protection of DC system. In addition, the DC transmission line solution model is compared and analyzed, and the factors causing poor performance of current differential protection are analyzed from the perspective of the solution model. Finally, the research of current differential protection is prospected.

In order to solve the problems of insufficient dynamic reactive power compensation and small short-circuit capacity of ultra-high voltage direct current (UHVDC) transmission system, it is a practical and effective scheme to install a certain number of synchronous condensers (SCs) at the receiving end of UHVDC. However, the connected SCs will provide a lot of sub-transient and transient currents in the case of AC system fault, which may affect the operation of AC protection. This paper takes distance and current differential protections of high-voltage transmission network as examples to analyze the influence of the large-scale SC on relay protection. Firstly, the system modeling is carried out. Secondly, the influences of SC's output current on the measuring resistance and differential current are analyzed. Finally, in PSCAD, the electromagnetic transient simulation model of the AC power grid including the SC and its excitation control system is established, and the theory analysis results is verified by simulation. The research shows that the branch coefficient of the adjacent lines will be increased with the connection of the SC, which will increase the measurement impedance, reduce the sensitivity of the distance protection and even cause the protection's refusing action. The connection of the SC will increase the differential current slightly, and the differential protection can still operate reliably.

Diacritics are very important in diacritical languages, because the meaning of sentences can be changed in accordance to diacritics. Writing without diacritics makes the sentences ambiguous; however, there are several reasons make people do not write words with diacritics, such as fast typing, convenience, or texting on unsupported diacritics devices. As a result, these texts are very difficult to process on further natural language processing (NLP) tasks like machine translation, sentiment analysis, or question answering system. Therefore, diacritics restoration is critical for further usage or processing in NLP related tasks. In this study, we propose a method which combines convolutional neural network (CNN) and bidirectional gated recurrent unit (Bi-GRU) to restore diacritics. In addition, we use residual block to resolve vanishing gradient problem of recurrent neural networks. We applied the model for restoring diacritics of Vietnamese language that has the highest ratio of diacritics in words. This approach has character accuracy at 98.63% and word accuracy at 94.77%.

Sentiment analysis has an important role in social media monitoring as it extracts public opinions, emotions, and feelings about certain products or services. There are several publications in building a system to identify opinions from text using rule-based approach, lexicon-based approach, or machine learning. In this paper, we propose and compare several deep learning models to solve sentiment analysis problem of the Internet Movie Database (IMDb) review sentiment dataset. The feature extractor consists of a convolutional layer, followed by a max pooling layer and a batch normalization layer. To solve the vanishing gradient problem, we use a residual connection to concatenate the input values with the extracted features before feeding the output into a recurrent layer. Our best model has an accuracy of 90.02%.

As a backup protection for HVDC lines, legacy current differential protection needs long time delay to avoid the influence of distributed capacitor current under external faults. A new protection principle is proposed based on the fact that under internal faults, the backward traveling wave at the side away from the fault point and the forward traveling wave at the side near the fault point will not satisfy the transmission function of the DC line, whereas the backward traveling wave at the side away from the fault point and the forward traveling wave at the side near the fault point satisfy the transmission function under external faults. The proposed principle is verified in PSCAD/EMTDC, and simulation results show that, it can identify the fault reliably and rapidly, while having better tolerance to high fault resistance.

When the hierarchical connection mode is adopted, the high-end and low-end inverters of HVDC system may occur commutation failure at the same time, which will interrupt the transmission power of the system instantaneously, weaken advantages of hierarchical connection system, and affect the stability of system. This paper proposed an improved extinction angle control strategy based on DC current variation. The dynamic fluctuation characteristics of DC current during fault and recovery process is fully utilized. Based on the DC current variation, improved extinction angle controller quickly increases the extinction angle, reduces the commutation failure and facilitates the recovery of system. This control method does not need to detect receiving end AC faults to mitigate commutation failure, so there is no detection delay. Moreover, no additional investment such as reactive power compensation device is required.

Medium voltage direct current (MVDC) distribution grids, that is proposed to improve power quality, enable high penetration of renewable sources, decrease operating losses, has drawn more and more attraction. However, dc faults ride-through remains to be a technical bottleneck constraining the extensive industry application of MVDC distribution grids, and the method based on DC circuit breaker (DCCB) is one of the most promising solution to deal with this problem. With features of no operating losses, low cost and free maintenance, the hybrid DCCB with current commutation drive circuit (CCDC) could be widely used in MVDC distribution system to interrupt DC fault current. During the current interruption process of DCCB with CCDC, circuit switching occurs twice, so the transient voltages are inevitable. In this paper, a simulation model of DCCB with CCDC is established by PSCAD, and the transient voltages are calculated. According to the simulation results, the transient voltages should be suppressed. Then, based on the mechanisms of the transient voltages, mitigation ways are proposed. In the end, the simulation results show that the transient voltages have been effectively suppressed and the reliability of DCCB with CCDC has been improved.

This paper presents a novel protection principle for turn to turn fault of valve side winding of UPFC series transformer. First, with a domestic 500 kV UPFC project as the basis, the difference of connection mode of line side winding of series transformer is presented. Next, problems of traditional current differential protection are analyzed. Traditional current differential protection may have less sensitivity or even refuse to operate in certain fault cases. Then, double K zero-sequence current differential protection is proposed to overcome the problem in traditional protection scheme. Finally, RTDS Simulation tests verify the problems of traditional protection and the validity and superiority of double K zero-sequence current differential protection. The proposed protection principle has higher reliability and sensitivity and has a good applicability in practical engineering project.

DC line protection technology is one of the key technical issues to ensure the reliability of flexible DC grids supply. In the practice of flexible DC engineering, DC reactors are installed at both ends of the DC line to reduce the fault hazard and improve the survivability from the fault crossing of the converter station and the grid fault. The line boundary feature provides a breakthrough for the study of the line protection of the flexible DC grid. In this paper, the DC reactors installed at both ends of the line in the DC grid are used as the line boundaries. The transient characteristics of the internal faults and external faults are analyzed. A pilot line protection principle based on the amplitude ratio of the voltage wavelet transform detail coefficient on both sides of the DC reactor and the transient current fault characteristics which could quickly distinguish the internal faults and external faults was proposed. A four-terminal MMC type flexible DC grid simulation model was built on the PSCAD/EMTDC simulation platform, and the simulation was verified. A large number of simulation examples show that the protection scheme is fast, adapt to transition resistance, and is not affected by the inverter blocking. It can be used as the main protection for flexible DC transmission lines.

In this paper, analytical and simulation results are carried out about the application of distance protection with the presence of the unified power flow controller (UPFC) in the AC system. Combined with the UPFC protection, the impact of UPFC on distance protection is analyzed under different operation conditions. The influence to the distance protection is bigger when the series unit remains in the system after fault occurs. The simulation results show the impact of UPFC on protection for different operation conditions and different fault locations.