Fast and reliable single-ended protection is crucial for multi-terminal DC(MTDC) line protection. Traveling-wave-based protection, characterized by rapid operation, immunity to the influence of renewable energy control strategies, and strong tolerance to transition resistance, has become one of the mainstream approaches for single-ended protection schemes in MTDC grids. In this paper, a concept “line length difference degree” is proposed based on polarity variation of reflected and refracted fault-induced traveling waves, which is used to classify the protected lines in MTDC grids. Corresponding protection schemes are designed according to different “line length difference degree”. For grids with a line length difference degree greater than 30%, a protection method using traveling wave time differences is developed. For grids with a line length difference degree less than 30%, a dual-criterion protection method combining the port voltage change rate and traveling wave time differences is developed. Finally, a four-terminal DC grid model is established in PSCAD/EMTDC, the feasibility and superior performance of proposed protection scheme are verified through simulations.

The article deduces analytic expression of voltage travelling wave for hybrid cascaded DC system and puts forward protection based on amplitude coefficients of line-mode fault-component voltage (LFCV). By analyzing time-domain expressions of fault voltage travelling waves, it is derived that under internal faults, for the head of line, the ratio of two amplitude coefficients of the LFCV is 1:2, while for the end of line, the ratio of two amplitude coefficients of the LFCV is 1:0. Then, it is further derived that under internal faults, for either side of line, the sum of two amplitude coefficients of the LFCV is far less than 0. However, under external faults, for the near faulty terminal of the line, the ratio of two amplitude coefficients of the LFCV is 1:-1; thus, the sum of two amplitude coefficients is 0. Therefore, the amplitude coefficients of LFCV can be utilized to distinguish the internal fault from the external fault. Simulation verifies that the method can reliably discriminate the internal and external faults even with high fault resistances and has high robust, ss confronting noise interference.

DC fault in diode-rectifier (DR) based MMC-HVDC system causes significant offshore AC voltage drops, threatening the stability of grid-forming (GFM) wind turbines (WTs). This paper first analyzes the mechanism of offshore AC voltage drop induced by DC faults, and reveals the reasons for overcurrent and loss of active power control. Secondly, a DC fault ride-through (FRT) strategy is proposed to improve the FRT capability, which comprises power reference modification and additional feedforward control. The proposed strategy effectively limits voltage and frequency deviations within 6%, restores active power controllability within 50 ms, and limits overcurrent to below 1.2 p.u. during DC faults. Thirdly, an additional pitch angle control is proposed to eliminate the excessive power and enhance the response speed of both pitch angle and rotor speed. Finally, leveraging the passivity-based control method, an improved inner current control loop is proposed to enhance the dynamic and static performance of the GFM WTs during DC faults. Various case studies performed on PSCAD/EMTDC and RTDS validate that the proposed strategy significantly improves the stability and FRT capability of GFM WTs compared to existing strategies.

Abstract The large-scale integration of renewable energy has led to complex topologies in 110 kV distribution networks, posing significant challenges for accurate fault section location. Meanwhile, high-resistance grounding faults complicate fault type identification. To address these problems, this paper first proposes a novel fault diagnosis method based on mapping time series measurements to RGB images. Specifically, three-phase currents and voltages are measured at the terminals of 110 kV lines and mapped to three channels of RGB images. The fault diagnosis problem, which includes fault type identification and fault section location, is reformulated as two image classification tasks. Then, a lightweight ResNet (LW-ResNet) is proposed to solve the fault type identification problem by refining the residual blocks in traditional ResNet. Finally, a squeeze-and-excitation (SE) attention module is combined with LW-ResNet (LW-SE-ResNet) to improve the accuracy of fault section location, especially for faults near T-junctions. Simulation results demonstrate that under simulation conditions with synchronization and measurement errors, LW-ResNet achieves 97.75% accuracy in fault type identification, and LW-SE-ResNet achieves 95.69% accuracy in fault section location. Compared with traditional methods, the proposed fault diagnosis method offers superior accuracy and robustness. The real-time capability is verified, as the proposed methods achieve high fault diagnosis accuracy using data from the initial 0.25 cycle after a fault occurs, and the inference latencies are less than 5 ms.

A new line protection for HVDC systems using the ratio of currents of a DC filter and smoothing reactor is presented. Characteristic analysis on a DC filter shows that it behaves like an inductor at a high frequency band whose inductance is far less than the sum of smoothing reactor inductance plus converter equivalent inductance. Based on this characteristic, fault analysis shows that during high-frequency DC line faults (on either side), the DC filter current amplitude significantly exceeds that of the smoothing reactor, with a current ratio much greater than 1. Whereas for external faults, near faulty end, DC filter current amplitude is less than the one of a smoothing reactor and current ratio at this end is smaller than 1, while the current ratio at the distance faulty end is still far greater than 1. The effectiveness of the protection is verified by a large number of simulation experiments. It does not need synchronization of signals at both ends. In addition, it has high sensitivity for high-resistance faults, and is insensitive to the length of DC transmission line and power flow change.

When a single-phase to ground fault occurs on the 110 kV transmission line, the neutral point voltage (NPV) offset of the gap-grounded transformer will be generated. After the line protection near the main power supply trips, if inverter-interfaced distributed generators (IIDGs) are connected to the distribution network, neutral point overvoltage (NPOV) issues may occur. Therefore, several studies about transformer NPV and gap protection are discussed in this paper. First of all, NPV values are theoretically analyzed based on an inverter-interfaced distributed photovoltaic and energy storage (IIDPES) connected distribution network. Then, the matching degree between the rated capacity of IIDPES and local load is introduced considering the impact of local load on NPV, which aims to investigate the critical proportion that leads to NPOV issues. Moreover, since energy storage (ES) can balance the output power of IIDPES and load power, this paper also researches the influence of ES proportion on NPV. Finally, suggestions for installing the gap protection of 110 kV transformer are proposed considering different ES proportions and control strategies. Simulation results show that the PQ-controlled IIDG significantly increases the value of NPV and master-slave-controlled IIDGs are demanded to consider the impact of specific ES proportions on the NPV.

Offshore wind farms (OWF) connected to diode rectifier (DR) and modular multilevel converter (MMC)-based HVDC confront challenges of surplus power induced by onshore AC faults. This paper proposes an internal energy distribution control (IEDC) strategy, which utilizes the rotor kinetic energy (KE) of wind turbines (WT) and the capacitor energy of MMC submodules to achieve fault ride-through (FRT) and postfault recovery (PFR). Firstly, the mechanism of OWF is analyzed, and an onshore AC fault detection method based on local measure-ments is proposed. Then, a two-stage FRT control strategy is proposed. Three preset power reduction and energy absorption curves are designed to utilize the internal energy to actively absorb excess power, and flexibly distribute surplus power to KE and MMC energy. An additional pitch angle control (APAC) is devised, which can reduce captured wind power and eliminate surplus power when the internal energy reaches its maximum value. Thirdly, a two-stage PFR control strategy is proposed. The preset power and energy recovery curves are designed to achieve fast active power recovery and release of stored excess internal energy after fault clearance. Case studies are performed on 2-terminal and 4-terminal test systems to validate the performance and effectiveness of the proposed strategy.

When a single-phase to ground fault (SPGF) occurs near the main power source in an active distribution network, distance protection section II (DPS-II) located at the distributed generator (DG) side operates with a delay. In gap-grounded transformers, this delay can lead to gap breakdown due to neutral-point overvoltage, which adversely affects the operation of DPS-II on the DG side. To address this issue, this paper proposes an improved DPS designed for active distribution networks with gap-grounded transformers. First, the factors influencing the additional impedance are analyzed after gap breakdown. To mitigate the effects of the additional impedance on DPS performance, an improved DPS based on real short-circuit impedances is introduced for active distribution networks. This scheme utilizes the negative-sequence current distribution factor on the DG side to accurately calculate the additional impedance angle, ensuring reliable protection. Simulation results demonstrate that the proposed scheme effectively operates under forward faults across various DG capacities, fault locations, local loads, and fault transition resistances. In addition, it avoids tripping under reverse faults, thereby confirming its reliability and superiority.

A new protection for hybrid cascaded DC transmission line using phase difference of voltage travelling waves is put forward. According to fault travelling waves propagation process, the equivalent fault analysis circuits for internal fault and external fault are obtained. Then it is found that after internal fault, for either terminal of the line, at high frequency, the phase difference between the voltages at two sides of current-limiting reactor (PDVR) is approximately 0°. Whereas after external fault, the PDVR near the faulty terminal of line is approximately 90°, and at the distance faulty side, PDVR equals about 0°. Therefore, PDVR can be used to identify the faults. Simulations verify that the protection can differentiate the internal and external faults accurately, and reliably respond to high-resistance faults. It only needs to transmit the logic value of fault discrimination to two terminals, having low requirements for communicating channels. Besides, the identification criterion threshold is derived based on theory, providing the theoretical basis for the setting of on-site protection.

Hybrid DC circuit breaker (HCB) is an effective solution to realize fault isolation when DC side fault occurs in DC grid. However, the main breaker (MB) in the conventional two-port HCB requires a large number of insulated gate bipolar transistors (IGBTs) in series, which raises the cost. With the expansion of the DC grid, a single DC bus in the multi-terminal DC grid is usually connected to numerous DC lines. In order to ensure absolute selectivity of protection, the two-port HCB needs to be installed on each DC line, which is extremely costly. Additionally, a crucial assurance for the fault isolation of the HCB is the successful opening of the ultra-fast disconnector (UFD). Nevertheless, the majority of the existing HCBs ignore remedial actions in the case of UFD failure. In response to the above problems, this article proposes a multi-port HCB (MPCB) with a shared MB. The proposed MPCB integrates n HCBS on the DC bus to become an n-port HCB sharing two MBs. The two MBs are able to achieve fault isolation on either line or the DC bus, guaranteeing fault current interruption performance while significantly reducing economic costs. Moreover, to avoid the problem of fault isolation failure caused by UFD failure, the proposed MPCB is equipped with UFD failure protection function by adding a UFD failure auxiliary switch.

The reliability of protection is the precondition for the security of hybrid multi-terminal high-voltage direct-current (HVDC) transmission system. For AC system at the receiving terminal, due to the infeed of DC system, the conventional AC line protection may malfunction. In this context, a new protection utilizing zero-sequence voltage ratio is proposed in this article. Based on the fault analysis of zero-sequence voltage distribution, the comparative voltage is defined, and find that under an internal fault, for two sides of line, the ratios of zero-sequence voltages to their corresponding comparative voltages are less than 1. Under an external fault, for the distance faulty end, the voltage ratio is less than 1, however, near faulty side, the voltage ratio is larger than 1. Thereby, this difference is utilized to construct the fault identification criterion. Simulations show that the proposed protection can correctly identify internal and external faults. Meanwhile, it is immune to DC commutation failure, and has a good tolerance to high-resistance faults.

The petal-shaped distribution network under closed-loop operation has more complex fault characteristics,which makes it difficult for traditional protection methods to satisfy the requirements of selectivity and speed. Aiming at this problem,the short circuit fault characteristics of the petal-shaped distribution network are analyzed. The variation trend of both upstream and downstream fault currents with the changing of fault location is revealed. And the problems encountered in the application of definite-time and standard inverse-time overcurrent protection to petal-shaped distribution network are clarified. Then,combined with the fault current characteristics of the petal-shaped distribution network,a parameter adaptive correction based inverse-time overcurrent protection（PACITO） method is proposed. The PACITO only uses the local current information to adaptively correct the inverse-time characteristic,which can significantly improve the speed of protection while ensuring selectivity. A 10 kV petal-shaped distribution network model is established in PSCAD/EMTDC simulation software to verify the PACITO method. The simulative results prove that the proposed protection can improve protection selectivity and speed while also overcoming the drawbacks of traditional petal-shaped distribution network protection methods.

For the hybrid multi-infeed HVDC system in which the receiving-end grid is a strong AC grid including LCC-HVDC subsystems and multiple VSC-HVDC subsystems, it has higher voltage support capability. However, for weak AC grid, the voltage support capability of the multi-VSC-HVDC subsystems to the LCC-HVDC subsystem (voltage support capability-mVSCs-LCC) can resist the risk of commutation failure. Based on this consideration, this paper proposes an evaluation index called Dynamic Voltage Support Strength Factor (DVSF) for the hybrid multi-infeed system, and uses this index to qualitatively judge the voltage support capability-mVSCs-LCC in weak AC grid. In addition, the proposed evaluation index can also indirectly judge the ability of the LCC-HVDC subsystem to suppress commutation failure. Firstly, the mathematical model of the power flow of the LCC and VSC networks in the steady-state is analyzed, and the concept of DVSF applied to hybrid multi-infeed system is proposed. Furthermore, the DVSF index is also used to qualitatively judge the voltage support capability-mVSCs-LCC. Secondly, the influence of multiple VSC-HVDC subsystems with different operation strategies on the DVSF is analyzed with reference to the concept of DVSF. Finally, the indicators proposed in this paper are compared with other evaluation indicators through MATLAB simulation software to verify its effectiveness. More importantly, the effects of multi- VSC-HVDC subsystems using different coordinated control strategies on the voltage support capability of the receiving-end LCC-HVDC subsystem are also verified.

Faulty phase selection (FPS) methods are used to solve the problem of single phase to ground fault (SPGF) in the distribution network. When a false FPS (FFPS) issue occurs, the short-circuit current in the faulty line suddenly increases. It is crucial to diminish damages caused by the FFPS issue and rapidly reselect the faulty phase. Therefore, this article proposes a fault-tolerant method based on an active-intervention-type arc suppression device with a soft switch mode (AASD-SSM) and a fast FPS method based on wavelet packet transform (WPT). To strengthen the fault-tolerant performance of FPS, the AASD-SSM is first developed with an intermediate resistance and a soft switch. Then, the value of intermediate resistance is determined by the short-circuit current in AASD-SSM. To realize the accuracy and rapidity of FPS, a modified WPT (MWPT) method based on frequency band energy is presented. In different fault scenarios, the number of wavelet decomposition layer is updated by MWPT, and the SPGF issues in the faulty lines can be monitored in real time. Simulation results demonstrate that the AASD-SSM improves the fault-tolerant performance of FPS. The phase selection accuracy of MWPT is better than that of WPT in all fault scenarios, and the computational complexity of MWPT is reduced for more than 10%.

This article proposes an innovative DC line protection utilizing current correlation coefficient for hybrid cascaded high-voltage direct-current (HVDC) system. First, the equivalent discharge circuit after DC line fault is inferred, and then fault characteristic analysis on the superimposed network reveals that for the transmission lines connecting line commutated converter (LCC) and modular multilevel converters (MMCs), high-frequency currents of faulty line and non-fault line are negatively correlated, their correlation coefficient is theoretically −1. Whereas, high-frequency currents of two non-fault lines show a strong positive correlation, their correlation coefficient equals theoretically 1. Then, cosine correlation coefficient is adopted to describe current correlations and construct the identification criterion. Simulation demonstrates that protection could differentiate the faulty line accurately. Additionally, it has high sensitivity for high-resistance fault. And it is simple and has low computational complexity, and is insensitive to the line boundary element.

In a high voltage direct current (HVDC) grid, hybrid DC circuit breakers (HCBs) are one of the effective solutions to interrupt fault currents. The conventional two-port HCB features high economic costs since the expensive insulated gate bipolar transistors (IGBTs) are needed. Each line in the multiterminal HVDC grid must be equipped with the HCB, which is extremely costly to implement. In addition, the likelihood of temporary faults is considerably raised by the fact that the HVDC grid transfers power across overhead lines (OHLs). By reclosing operations after temporary faults, the HVDC grid can resume normal operation. However, it is vital to discern between temporary faults and permanent faults before reclosing operation to prevent secondary shocks to the HVDC grid caused by blindly reclosing on permanent faults. For the problems of high cost and blind reclosing of HCBs, this article proposes a multiport HCB (MHCB) with high economy and adaptive reclosing capability. All ports of the MHCB share a single main breaker (MB) for fault isolation. The MHCB utilizes the capacitor voltage to lower the operating voltage of the metal oxide varistor (MOV), which reduces the number of IGBTs connected together at both ends of the MOV. With the above design, the expense of the MHCB is significantly decreased. Additionally, the MHCB achieves adaptive reclosing by using the difference of current amplitudes in the capacitor branch under permanent and temporary faults.

The hybrid cascaded LCC/MMC high voltage direct current (HC-HVDC) system is a prospective topology that can avoid the commutation failure problem of LCC-HVDC while ensuring bulk power transmission capability. However, the complex topology results in different DC fault characteristics at the receiving end, which brings challenges to fault detection and current limiting technology. As a result, it is of great theoretical importance to investigate the fault characteristics of the HC-HVDC system and propose the corresponding fault current limiting strategy. Firstly, this paper simplifies the equivalent discharging circuit of the hybrid cascaded LCC/MMC inverter into a first-order RL circuit by assuming the firing-advance angle is constant after fault, and proposes the fault current expressions by Laplace transform. Secondly, by changing the insertion sequence of submodules, an active fault current limiting strategy based on the submodule sorting algorithm (SMS-FCL) is proposed, which can accomplish fault current suppression, reactive power and DC voltage support, and energy loss reduction simultaneously. Moreover, the proposed SMS-FCL can cooperate with other FCLs to further suppress the fault current, and the cooperation prerequisites are analyzed. Finally, an LCC-LCC/MMC HVDC system is built in PSCAD/EMTDC to validate the accuracy of the proposed fault current expressions and the effectiveness of the current limiting strategy.

An accurate faulty phase detection (FPD) method in the case of a single-phase grounding fault (SGF) is the foundation for the reliable operation of an active-intervention-type arc-extinguishing device (AAD). Due to asymmetric system parameters, the applicability of traditional FPD methods exists several difficulties under high -resistance and arc grounding faults. In this paper, an adaptive FPD method is proposed based on wavelet packet reconstruction energy (WPRE) of three-phase voltage signals in resonant grounded distribution systems. Firstly, the fault characteristics of three-phase voltages are analyzed by constructing equivalent circuits with simplified parameters under single-phase low-resistance, high-resistance underdamped, and high-resistance overdamped grounding faults. Secondly, the threshold value of integral |Delta u0|dt is utilized to identify low-resistance faults. Mean-while, the high-resistance faults are distinguished from switching events by calculating the wavelet packet reconstruction energy entropy (WPREE) of Delta u0. Thirdly, the WPRE of three-phase voltage signals is compared to detect the faulty phase. Besides, the time windows of the algorithm are adaptively adjusted under different faults. Furthermore, simulation results show that the proposed FPD method can be implemented speedily and sensitively in asymmetric systems and various fault conditions considering noise interference.

多端柔性直流电网(multi terminal DC,MTDC)要求直流线路发生故障后在几毫秒内隔离故障线路,如何实现故障线路快速可靠识别是MTDC直流线路保护的难点之一.该文利用区内外故障时保护安装处暂态电流与本侧模块化多电平换流器(modular multilevel converter,MMC)故障电流的比值差异,提出了一种基于暂态电流比值的直流线路保护方案.该方法通过测量线路双端电气量来判断故障位置,可以保护线路全长.最后,在PSCAD/EMTDC仿真平台搭建了四端柔性直流电网模型,通过仿真验证所提保护方案的可行性与优越性.

在基于虚拟同步发电机(VSG)控制的光伏及混合储能系统中,不同类型的储能之间存在协调配合问题,其荷电状态(SOC)也与VSG的控制策略密切相关.针对该问题,提出了一种基于VSG的光伏及混合储能系统的协调控制策略.在逆变器直流侧引入混合储能系统,并基于VSG控制原理对其进行功率分配.根据储能SOC与VSG虚拟惯性之间的定量关系,设计了一种改进的虚拟惯性自适应控制策略,并给出相关参数的选取原则,在改善系统输出频率和功率动态响应的同时,对储能SOC进行控制.基于MATLAB/Simulink进行仿真,结果表明所提控制策略可以有效改善系统电压和频率的稳定性,实现混合储能之间功率的合理分配,提高储能的充放电性能并延长其寿命.