The traditional auto-reclosing (AR) method based on fixed time delay cannot avoid blind reclosing on permanent faults. Moreover, with the increasing penetration of inverter interfaced distribution generations (IIDG) in the active distribution networks (ADNs), the method encounters significant challenges in properly setting the delay. The main reason is that it cannot actively identify the differences between transient and permanent faults. To realize the actively fault nature identification (FNI), an adaptive reclosing method for ADNs based on multi-harmonic current injection by IIDG was proposed in the paper. Firstly, the differences in response voltage characteristics between transient and permanent faults after the injection of the multi-harmonic currents are analysed. Subsequently, an additional control for IIDGs is designed to enable IIDGs to actively inject multi-harmonic current. Then, a FNI criterion based on the characteristics of response voltages, which result from the injected multi-harmonic currents, is established. On the basis above an adaptive reclosing method that accelerates reclosing for transient faults while reliably locking out for permanent faults. Through additional control of existing IIDGs in the network after a fault, the proposed method uses the injected multi-harmonic currents to achieve an adaptive reclosing correctly, eliminating the need for extra signal injection equipment. Furthermore, the proposed FNI criterion relies on local voltage information from PT, thus requiring no communication. Simulation and hardware-in-loop experimental results validate that under different fault scenarios, the proposed method can substantially reduce the power outage time during transient faults, while avoiding the risk of blind reclosing, which improving the reliability of the ADNs.

Thyristor controlled phase shifting transformer (TCPST) is a new type of power flow control equipment. Due to its series and shunt coupling topology and special connection to the power grid, it is necessary to consider the handling strategy in case of power grid faults. Based on the structure and principle of TCPST, the influence of grid fault on TCPST is analyzed in this paper. It is clarified the distribution of fault current and characteristics of overcurrent and overexcitation within TCPST. On this basis, the control strategy of TCSPT for deal with grid fault impact is proposed, and the fault state identification criterion and control sequences for the TCPST is constructed. By identifying the faults within and outside the zone of TCPST and the recovery state of power grid faults with the electrical quantities, the out of service and auto-restart strategy for TCPST are realized. The temporarily withdraw method for TCPST is used to protect equipment safety in the event of severe power grid faults. Finally, some simulations are conducted for the proposed fault ride-through strategy of TCPST. It is shown that the reliable ride-through of TCPST during power grid faults can be achieved by using the strategy, which has a good application for feasibility and effectiveness of TCPST. It is benefit to take advantage of the potential of power flow regulation function of TCPST.

Flexible interconnection and dynamic power flow regulation in distribution networks are achieved by soft open point (SOP), optimizing the configuration and scheduling of distribution resources. To improve the load voltage during asymmetric ground faults, it is proposed that positive and negative sequence currents are output by the converter during faults. This provides voltage support and enhances fault ride-through capability. However, actual operating conditions make open-loop optimization schemes susceptible. Poor performance is shown by traditional negative sequence voltage-based closed-loop support. In this paper, the conventional negative sequence voltage outer loop is improved by controlling the amplitude of negative sequence current to suppress the negative sequence voltage. The phase of the negative sequence current is adjusted to optimize the suppression effect. To enhance the inverter's capacity utilization and limit active power fluctuations, a comprehensive scheme is proposed for limiting the positive and negative sequence current amplitudes. A short-circuit fault simulation is constructed using PSCAD simulation software, based on the actual parameters of the distribution network. The simulation results show that the proposed control strategy significantly enhances voltage performance during fault conditions, improving the low-voltage ride-through capability.

Non-isolated flexible interconnected distribution networks as the research object are explored, focusing on the adaptability issue of typical grounding fault detection methods when single-phase grounding faults occur in AC systems due to zero-sequence transfer characteristics. Firstly, The zero-sequence equivalent topology network for non-isolated soft open point (SOP) is established, with transmission equations for zero-sequence components proposed to quantitatively analyzed their magnitude when transmitted to the non-fault side. The suppression effects of zero-sequence components are evaluated in various scenarios. Then, an adaptability analysis of typical protections is conducted for interconnected systems with low-current, low-resistance connections and hybrid low-current/low-resistance grounding methods. Results show that in non-isolated flexible interconnected systems, zero-sequence components transmission to the healthy side may cause protection maloperation or false alarms. Finally, a typical non-isolated flexible interconnected distribution network model is established in PSCAD to verify the adaptability analysis of protections in different interconnected systems, and the theoretical analysis is confirmed to be correct. The findings provide a theoretical basis for protection configuration in non-isolated flexible interconnected distribution networks.

The adaptive reclosing of the distribution network does not have the ability to identify the nature of faults, and may blindly reclose with permanent faults, causing secondary impact on the system and electrical equipment. Therefore, an adaptive reclosing method for distribution networks based on IIDG active detection is proposed. By actively injecting multi frequency detection signals through IIDG, the feeder side voltage phase characteristic differences under transient/permanent faults were analyzed, and the identification of fault nature was achieved. If it is a permanent fault, the recloser blocked, otherwise reclose it immediately. The proposed method does not require communication or additional equipment, making it easy to implement in engineering. The simulation results have verified the effectiveness of the proposed method.

This paper proposes an improved noniterative fault location method on untransposed transmission lines without utilizing line parameters. Two-end three phase voltage and current phasor measurements before and during the fault are typically required. First, the parameter-free fault location problem is formulated through multi-section transmission line models, and the necessary condition of noniterative solutions is carefully investigated to determine the maximum possible section number of the multi-section line model. Afterwards, with the determined section number, the analytical solution of the fault location is derived with full utilization of the inherent characteristics of the line parameter matrices. Instead of solving all the parameters, two key variables inside the parameter matrices are extracted and the fault location is obtained by analytically solving a polynomial equation. The method provides a closed-form analytical solution, and therefore avoids convergence issues of iterative algorithms. In addition, line asymmetry of untransposed transmission lines are fully considered to minimize fault location errors. Extensive numerical experiments show that the proposed method has improved fault location accuracy compared to the existing method, with different fault types, locations and impedances.

According to the practical requirements of intelligent operation and maintenance of relay protection, an IEC61850 dynamic model expansion method for on-line monitoring and intelligent diagnosis of relay protection is proposed, which expands the carrying capacity of DA (Data Attribute). The container class DO (Data Object) division method is used to form the dynamic data extension logic node modeling, so as to realize the modeling of determined type values and multi type document data. JSON lightweight text format is introduced for dynamic data transmission to realize the modeling and submission of equipment monitoring, early warning, patrol inspection and other data in the station. This method is piloted in practical projects and applied to the advanced application of relay protection intelligent operation and maintenance of control cloud master station. It has a demonstration effect on comprehensively mastering the health status of secondary equipment and realizing equipment defect early warning.

In order to improve the transferability of transmission line fault identification models, this paper divides transmission lines into source lines and target lines based on transfer learning theory, and proposes a method for identifying transmission line fault types based on deep-transfer learning. First, time series data during transmission line faults is generated by combining different fault conditions, and by preprocessing the data, obtained input data samples for the Convolutional Neural Network (CNN). Second, the initial convolutional neural network is pre-trained using the source data to obtain a pre-trained model of the source line fault type identification. Next, the maximum mean difference method is used to perform a similarity test on the source line and the target line to screen out the source pre-trained model to be migrated. Finally, the target data is used to fine-tune the transfer training to obtain the final target fault identification model. The simulation results show that using the target data of 5% of the source data to fine-tune the transfer training of the pre-trained model, the accuracy of the target model for the target line fault identification is more than 99%.

The fifth generation mobile communication (5G communication) technology provides new ideas for current differential protections of distribution networks by virtue of its advantages in bandwidth, transmission capacity, transmission rate and delay. This paper firstly introduces the characteristics of 5G communication technology and tests the 5G communication delay characteristics in a laboratory environment. The test results show that the 5G communication delay can meet the requirements of current differential protections in distribution networks. On this basis, an adaptive current differential protection is proposed, which includes the use of a distribution management system to realize the automatic topology identification of the distribution network and the automatic establishment of communication links. Finally, simulations on the RTDS platform are implemented to verify the effectiveness of the proposed protection scheme. Moreover, the protection scheme can still play a protective role when a single terminal of the distribution network fails or its communication fails, which greatly improves the reliability of the current differential protection of the distribution networks based on 5G communication.

Considering the frequency-dependent characteristics of line parameters, a novel fault location method for high voltage direct current (HVDC) transmission lines is proposed in this paper. It is of great significance to accurately establish transmission line model. Firstly, the distributed and frequency-dependent characteristics of transmission line parameter are analyzed. Then, according to the idea of digital filter, a more practical formula for calculating the voltage and current of transmission line considering the frequency-dependent line parameters is proposed. Using the new formula, a fault location algorithm can be constructed based on the principle that the voltage distribution calculated from two ends of the line is equal at the fault point. Theoretical analysis and simulation results show that more accurate fault-location results can be obtained by the proposed algorithm.

The lack of analytical mechanism of continuous commutation failure (CCF) process hinders the development of CCF early warning algorithm and further obstructs the coordination strategy of relay protection between AC and DC size in hybrid grid. However, summarizing the protection threshold of CCF in the form of electrical quantity analytically under different fault types is a nontrivial pattern recognition problem. To overcome the difficulty of directly analysing the electromagnetic process of CCF, a statistical learning modelling approach is adapted to propose a CCF warning system, in which XGBoost algorithm was used as the backbone to regress on the relationship between AC/DC electrical quantities and the possibility of the occurrence of CCF. The design of the feature vector in XGBoost-based classifier considers not only the analytical expression of key electrical quantity during single commutation failure but also the important state variable of control system. Additionally, a special data windowing method was proposed to enrich of information encoded by the feature vector while ensuring the speed requirement posed by the protection system. Validation on multi-fault scenes simulation shows that the proposed early warning system can trigger warning signal correctly with sufficiently small measured data required, proving the practicability in engineering practice.

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.

研究了定功率控制（PQ控制）策略下的逆变型分布式电源的多电源故障等值方法,通过分析单个逆变型电源、多个等容量逆变型电源、多个不等容量逆变型电源以及多个逆变型电源异地接入的输出特性,进而得到多个逆变型电源的故障等值方法,该方法简化了含多个逆变型电源的复杂网络故障分析时的计算复杂度。同时,提出了逆变型电源的故障等值分区方法,并利用迭代法对区域内的多个逆变型电源进行迭代计算得到等值模型。最后利用PSCAD搭建了含多个逆变型电源的配电网络仿真模型。在配电网络不同线路发生故障时,对配电网进行区域划分,并利用文中提出的等值方法计算等值后的逆变型电源输出特性,通过将计算得到的结果与仿真得到的结果相比较,验证了故障等值方法的正确性和有效性。

为了实现基于磨削参数预测工件的摩擦性能,并减少测量真实表面形貌和开展摩擦磨损试验等环节,在考虑砂轮磨粒切削刃与工件运动干涉的条件下,结合单点金刚石修整和参数设定建立了磨削后的工件表面形貌模型,并利用混合润滑雷诺方程分析不同磨削形貌下的压力分布及摩擦系数.同时,以机床导轨磨削平面的润滑特性为例验证了所建模型的有效性.结果表明,当磨削表面上、下纹理方向夹角均为0°时,摩擦副的摩擦系数最大.

With the promotion of distributed energy and direct current (DC) loads, the DC microgrid is able to provide a higher power quality and improve the grid efficiency. Various technical issues in DC microgrids are still to be addressed, particularly a proper protection scheme for fault detection and isolation in DC microgrids utilizing voltage source converters (VSCs). In this paper, the pole-to-pole DC fault transient behavior of the VSC-based microgrids is firstly analyzed with four successive stages, and then the exact requirements for protections are presented. Furthermore, a novel single-ended protection scheme based on local transient signals is proposed, which needs no data transmission or synchronization between two ends, ensuring the speed. A four-terminal DC microgrid model was built in PSCAD/EMTDC. Numerous simulations have demonstrated the validity of the proposed scheme.

Current differential protection is normally designed as the primary protection for the high-voltage transmission lines. However, for the transmission line that is connected with the large-scale inverter-interfaced generators (IIGs), the performance of the conventional current differential protection is significantly affected by the difference between the sequence components of the fault currents on both sides of this line. This may result in the mal-operation of protection relays. This study proposes a sequence-component-based current differential protection from a suitable fault model of IIGs under positive-sequence control strategy. A differential coefficient is introduced to overcome the low sensitivity and poor reliability of the conventional differential protection. The proposed scheme discriminates the internal faults accurately from the external faults and the normal operating conditions. Additionally, the robustness analysis shows that this scheme is immune to different fault resistances, fault locations, and system parameters. Simulation results in power systems computer-aided design/electro-magnetic transient design and control validate the effectiveness of the proposed sequence-component-based protection scheme.

Direct-current (DC) line faults are a major issue in ultra-high-voltage direct-current systems due to the complex and wicked environment. Thus, reliably detecting and isolating the DC line faults are not a trivial engineering task. This study proposes a DC line protection scheme based on resonance frequency. The scheme mainly consists of a fault discrimination criterion and a faulty pole selection criterion. The fault discrimination criterion is based on the phase of the resonance frequency impedance, and it is deployed to differentiate the faults on the protected DC line from the external faults. The faulty pole selection criterion is based on the ratio of the resonance frequency currents, and it is employed to select the faulty pole. The sensitivity analysis demonstrates that the performance of the proposed protection is promising. The simulations carried out in PSCAD/EMTDC under various conditions verify the feasibility and validity of the proposed protection scheme.

In order to avoid problems caused by data synchronization of distributed bus bar protection and current transformer (CT) transient saturation, a novel fast bus bar protection was proposed based on fault superimposed current polarity comparison. The bus protection takes the pre-fault voltage and pre-fault current as the new reference for polarity comparison, and it utilizes the relationship between the pre-fault current and superimposed current to identify the fault direction. For internal faults to bas bur, the polarities of the branches connected to the faulted bus bar are identical, for external faults, the polarities are not exactly same. The time window length of fast bus bar protection is very short, and the relay is also immune to the CT transient saturation. The theoretical analysis and PSCAD/EMTDC simulation tests show that the bus protection operates reliably and rapidly, and that its performance is immune to the influences of various fault types, fault path resistance and fault inception angle.

Transformer can be used as a cross-voltage and cross-interval device in substation,so the transformer protection is able to install on-site approaching to primary devices.It is ideal to adopt circuit breaker distributed scheme,particularly after canceling human-machine interaction interface.Thus,how to simplify distributed transformer protection operation and maintenance in circuit breaker arrangement is what we need to solve.Taking 220 kV step-down transformer as an example,the paper proposes transformer protective device can get setting parameters by IEC61850 station layer communication,producing protection fixed setting automatically according to sensitivity constraint condition,improving the intelligent level of protection device and reducing the impact of human factors on the reliability of protection.