Battery energy storage stations (BESSs) are increasingly integrated into modern grids as a crucial solution for stabilizing the volatility of renewable energy resources. However, effectively protecting the grid-tied BESS with uncertain discharge/charge status during grid faults remains an urgent issue in practical applications, yet has received limited attention in existing research. This paper demonstrates that even the superimposed positive-sequence current (SPSC)-based current differential protection (CDP), traditionally known for its high sensitivity to faults and currently employed as the primary protection for BESS direct-tie lines, may also encounter insufficient sensitivity problems or potential malfunctions in the presence of BESS. To assure the protection of BESS during low-voltage grid faults, this paper proposes an enhanced SPSC-based CDP scheme capable of accommodating both normal and excessive discharge/charge modes. Without compromising the resistant reliability in external fault scenarios, the proposed scheme utilizes pre-fault and post-fault positive-sequence current magnitude variations and the SPSC magnitude ratio to improve sensitivity in detecting internal faults. Moreover, by solely relying on current magnitude measurements, the proposed scheme eliminates the need for investments in high-bandwidth communication channels or relay hardware upgrades, providing cost-effective insights for implementing or updating protection configuration in remote BESS installations with underdeveloped infrastructure. Comprehensive test results, incorporating different BESS discharge/charge statuses, fault properties, and synchronization error scenarios, validate the superior sensitivity performance of the proposed scheme.

Torque ripple is a key source of vibration in permanent magnet motors, whose formation mechanism requires in-depth investigation. Current research often directly targets torque ripple or vibration for optimization, overlooking their fundamental origins and limiting mitigation effectiveness. This paper analyzes the torque ripple mechanism using a two-dimensional spatio-temporal air-gap magnetic field model. Focusing on an 8-pole 48-slot permanent magnet motor, the main electromagnetic force waves contributing to torque ripple and their critical flux-density harmonic combinations are identified. Simulations and experiments confirm that these dominant force waves significantly affect motor vibration. The findings provide a basis for reducing torque ripple and vibration noise via targeted air-gap magnetic field design.

Addressing the controversy over mixing TPY and TPZ-class current transformers (CTs) in large generator protection, this paper conducts a quantitative simulation study. First, the electromagnetic characteristics of air-gapped cores are analyzed to establish transient models for both CT classes. Specifically, an improved inverse Jiles-Atherton (J-A) model accounting for the core air gap is employed for the TPY-class CT, with its nonlinear magnetization parameters identified via a hybrid intelligent algorithm. Conversely, the TPZ-class CT is modeled using an explicit analytical expression obtained through direct data fitting. Next, a joint simulation platform incorporating a typical biased differential protection logic is established. Results show that under severe out-of-zone faults (e.g., a sudden three-phase short circuit with a 300 ms primary time constant and full DC offset), mixing TPY and TPZ-class CTs generates transient unbalanced currents exceeding 5 A in the differential circuit. This directly threatens protection reliability. Therefore, the GB/Z 20840.100-2023 standard's claim that “For CTs within the same protection zone, the use of different accuracy classes-with TPZ class and non-gapped core representing the extreme cases-generally does not result in practical defects” has strict limitations for large generators. This study provides a solid theoretical basis for CT selection and risk assessment of mixed configurations in generator protection systems.

The wind farm collector relaying system (WFCRS) plays a crucial role in ensuring the power supply reliability of wind turbine generators (WTGs) during internal collector line (CL) faults. However, the existing WFCRS is designed with a conventional two-section overcurrent protection (OCP) arrangement, leaving it vulnerable to both vertical and horizontal reliability issue that are critical but have not been comprehensively addressed in existing research. These reliability challenges are further compounded by the evolving grid code requirements for WTGs. This paper proposes a three-step comprehensive scheme, encompassing inverse-time fitting optimization and WTG tripping-off risk classification assessment, to enhance the reliability performance of the WFCRS. On the one hand, the proposed scheme is capable of adaptively adjusting its tripping logic by considering the operational status of WTGs on both faulty and non-faulty CLs. Additionally, the proposed scheme does not necessitate high-bandwidth communication channels and additional control improvement for WTGs, providing valuable insights for existing WFCRS installations often located in remote areas with limited infrastructure. Extensive simulation results under various fault conditions confirm the identified reliability issue of the WFCRS and demonstrate the anticipated performance of the proposed scheme.

The current differential protection (CDP) functions as the primary protection for the lines emanating from wind plants. This paper reveals that the limited current contribution capability of grid-following (GFL) controlled doubly-fed induction generators (DFIG) is just one of the challenges that CDP may encounter; due to the isolation from the external grid during balanced faults, the fault current output of GFL-DFIG could conflict with the realistic power balance constraints when complying with modern grid code (GC) requirements. Such contradictions may manifest as a frequency deviation pattern detected by the differential relay, with the resulting errors in magnitude and phase angle extraction potentially compromising sensitivity or causing CDP failure. To enhance the performance of CDP in the presence of weak-infeed lines connecting GFL-DFIG-based wind plants, this paper devises a new CDP scheme based on the ρ plane. The novel differential criterion utilizes the current magnitude ratio to optimize the restraint boundary with concave characteristics and employs the composite dynamic time warping (DTW) distance metric to indirectly calculate the current phase difference. In comparison to traditional CDP methods, the proposed approach does not rely on synchronized data and maintains high adaptability to the unique fault current signature of GFL-DFIG. Detailed simulation results under various fault scenarios, including instances of CT saturation and high-frequency harmonic interference, validate the effectiveness of the proposed CDP scheme.

The protection of rotor winding in variable speed pumped hydro (VSPH) machines is currently a research hotspot in the energy storage field. In this paper, based on the rotor winding internal fault characteristics of the VSPH machines, the fast Fourier transform (FFT) algorithm is used to calculate the amplitudes of FFT spectral lines that correspond to the main harmonic components in the stator winding. Subsequently, a rotor protection scheme is introduced based on the distinctive features of FFT spectral lines to distinguish between internal and external faults. Simulation results using a multi-loop model of an AC excitation dynamic electrical machine demonstrate that the proposed protection scheme can quickly and sensitively respond to the rotor winding internal fault.

Battery energy storage stations (BESSs) hold promising market potential within microgrids, serving as a complementary solution to mitigate fluctuations in renewable distributed generations and providing backup power during microgrid outages or emergencies. However, the distinct fault signatures of BESSs, compared to conventional synchronous generator (SG)-based sources, can result in misoperation of the widely-used superimposed current magnitude (SCM)-based faulty phase selector (FPS), which has received limited attention in previous research. Moreover, the variable operation modes of the microgrid further deteriorate the working condition of the SCM-based FPS. As a consequence, the potential misidentification of faulty phase(s) in the presence of BESS violates the selective phase tripping requirements in existing microgrids. To achieve precise selection of faulty phase(s), this paper proposes a novel FPS based on the generalized magnitude ratio (GMR). Firstly, the proposed FPS applies a compensation method to SCM and extends it to the superimposed voltage magnitude (SVM), mitigating the adverse impacts caused by the unbalanced sequence impedance of BESS. Subsequently, the GMR, which integrates both the compensated SCM and SVM information, is constructed to enhance the adaptability of phase selection to the uncertain infeed level at the BESS side. Finally, the faulty type and specific faulty phase(s) are determined based on the sorted results of the GMR values for each phase. Comprehensive simulation results affirm the expected performance of the proposed FPS under various microgrid modes, as well as different BESS discharge/charge statuses.

DL/T 684—2012 Guide of Calculating Settings of Relay Protection for Large Generator Transformer （hereinafter referred to as the “2012 Guide”） has played a significant role in improving the correct operation rate of relay protection devices for generator transformers and ensuring the safe operation of electrical equipment in power plants. However， with the increase in capacity of individual generator units in China and the emergence of new faults， the “2012 Guide” has proved inadequate， requiring revision to better guide engineering practice. Therefore， this paper analyzes the problems in the “2012 Guide” and puts forward specific revision suggestions to address problems such as misoperation of generator differential protection under sympathetic inrush current， misoperation of false power-on protection under poor synchronous conditions， and tripping of gap zero-sequence current protection of main transformer before reclosing in the case of transient single-phase ground faults on the line. The suggestions can provide reference for the revision of the “2012 Guide”.

In the presence of inverter-interfaced renewable energy sources (IIRESs), the conventional superimposed current magnitude (SCM)-based faulty phase selector (FPS) is prone to insufficient sensitivity and even malfunction issues. To remedy the defects of the SCM-based FPS in such a problematic situation, this paper proposes a novel FPS. In the proposed FPS, a compensation method is first applied to both the SCM and the superimposed voltage magnitude (SVM) to mitigate the adverse impact caused by the unbalanced sequence impedance of the IIRES. Subsequently, the generalized magnitude ratio (GMR) that integrates both the compensated SCM and SVM information is constructed to enhance the adaptability of phase selection to the uncertain infeed level at the IIRES side. Finally, the faulty type and specific faulty phase(s) are identified according to the sorted results of the GMR. Simulation results demonstrate the expected performance of the proposed FPS.

The vibroacoustic level has become a key factor for evaluating permanent magnet motor performance in recent years. However, the influence of tangential force is usually ignored, which will lead to errors in vibration noise calculation and limit the idea of reducing motor vibration noise. In this article, a comprehensive investigation of the tangential - and radial forces and their effects on the motor vibration is conducted. First, the analytical method of the electromagnetic force is presented. Next, the finite element method is applied to analyze the electromagnetic laws of radial force and tangential force. Then, the Theorem of Translation of A Force (TTF) is introduced to describe the influence of tangential force on radial vibration. After that, the influencing factors of stator teeth and load on the radial- and tangential-force are discussed, especially the phase relationship. Finally, the electromagnetic vibration law caused by radial force and tangential force is simulated and verified by experiments. The results can provide guidance for design of low vibroacoustic motor considering radial force and tangential force.

This paper proposes a multi-agent system (MAS)-assisted relaying scheme to identify the fault section in a distribution network (DN) that incorporates doubly-fed induction generators (DFIG)-based wind distributed generations (DGs). Firstly, to mitigate the adverse effect of the uncertain fault behavior of DFIGs on conventional directional elements, the fault direction identifiers (FDIs) are assigned at the I-type and II-type buses in the DN using the high-frequency component principle. Furthermore, a MAS-assisted relaying structure is devised to collect and integrate these identifiers and then accomplish the fault section location. On the one hand, since the setting of FDIs only requires local current information and the logical signals are solely exchanged within each predetermined associated region, the proposed fault section location scheme can be implemented in an immature DN with a limited number of voltage transformers or under weak synchronization conditions. On the other hand, the autonomy and cooperation of intelligent Agents also enable the proposed scheme to determine the fault section in a decentralized decision-making (DDM) mode, eliminating the dependence on strong central control and reducing redundant data transmission in a multi-bus distribution system. Simulation results under representative faulty scenarios validate the expected performance of the protection scheme.

Doubly-fed motors are extensively employed as high-capacity variable speed pumped hydro (VSPH) machines. The Fengning Pumped Storage Power Station in Hebei province constructed the first two VSPH machines in China. However, the rotor protection for the Fengning VSPH unit has not been completed to date, and traditional protection methods exhibit inherent deficiencies. To design an appropriate rotor protection scheme for the VSPH machines, it is vital to ascertain the internal fault characteristics through an accurate simulation model. Nonetheless, the commonly used dq0 model fails to account for all harmonics. To address this limitation, this paper proposes a multi-loop model to analyze the internal fault characteristics of VSPH machine windings. Experimental validation conducted on a 12kW VSPH prototype demonstrates the model's ability to accurately predict voltage and current waveforms under different internal fault conditions. Subsequently, the internal fault characteristics of the investigated VSPH prototype are determined by performing harmonic analysis on the stator and rotor currents. The observation and analysis results indicate that the utilization of fractional order currents in stator branches can effectively safeguard the rotor windings of VSPH machines.

The Jiles-Atherton (J-A) theory is widely adopted in modeling electromagnetic transformers. Its performance is determined by five parameters, so parameter identification becomes a key issue. This paper quantitatively compares and analyzes the performance of three intelligence algorithms on identifying J-A model's parameters. Three intelligence algorithms are as follows: particle swarm optimization (PSO), artificial fish swarm algorithm (AFSA) and simulated annealing algorithm (SAA). The evaluation indexes are accuracy, speed and stability. The main conclusion is drawn that SAA performs better.

Multi-phase ring brushless exciter is widely used in million-kilowatt nuclear power plants at present. However, due to its lack of perfect relay protection, multiple faults on site have brought serious losses. The difficulty in implementing this protection lies in the fact that the exciter is a rotating-armature-type motor, and the only measurable quantity is the stator excitation current.Therefore, a single electrical characteristic is required to protect and distinguish various faults. Firstly, based on the characteristics of rotor armature current and spatial harmonic magnetic field before and after various faults, the fault characteristics and mechanism of exciter stator and rotor windings and rotating rectifier are deeply studied. According to the mutual induction relationship between stator and rotor currents in the asymmetric winding structure, the transient process and steady-state spectrum distribution characteristics and evolution laws of various faults are obtained. Secondly, taking the 11-phase ring brushless exciter as an example, the experiments and simulations with different faults are carried out on the prototype, and the experimental and simulation results verify the correctness of the theoretical analysis. Finally, a new protection principle based on stator excitation current is proposed. The prototype experiment and the field test results show that this protection method can not only realize reliable and sensitive protection from faults, but also distinguish the faults selectively.

The rotating asynchronous machine （RAM） system in the power stations of China General Nuclear Power Group （CGN）includes three types of generators —i.e., Shangdian generator, Zhongchuan generator and Remeng generator. The Shangdian generator is under self-shunt excitation, and the other two types are under phase compound excitation. When setting the overcurrent protection of RAM generators with different excitation modes, the power stations don’t consider the influence of excitation difference on value setting and delay time setting of overcurrent protection, and there may be dangers of overcurrent protection refusal and protection mismatch with other abnormal condition protections （malfunction）. In this paper, the PSACD models of RAM generators with different excitation modes are established. The influence of excitation difference on the short circuit current decay process of RAM generator is illustrated by theoretical analysis and simulation, and the correctness of simulation analysis is verified by prototype experiment. Based on the theoretical analysis and simulation results, the setting value and delay time of RAM generator overcurrent protection under different excitation modes are set reasonably, which has been applied in the power stations of CGN.

In traditional analysis, several studies have discussed the vibration behavior of electric machines from the point of view of radial force. With the increasingly strict vibroacoustic index, the tangential force effects on the vibration are analyzed and investigated, and it shows the contribution of tangential force on electromagnetic vibration cannot be ignored. However, these studies are based mainly on finite element multi-physics models, and experiments are not conducted. Furthermore, the mechanism of tangential force is not clearly explained. In this paper, a special motor stator structure is proposed to explore the effect of tangential force on vibration. First, the analytical calculation of the tangential force and the tangential effect on motor vibration are described. Next, the Theorem of Translation of A Force is introduced to convert the effective tangential tooth force to a radial force couple acting on the stator yoke. Then, a novel motor stator is proposed to investigate the vibration characteristics caused only by tangential force, and the simulation is analyzed. Finally, a vibration test of the prototype is conducted and the result shows the tangential force can induce the radial vibration with corresponding order.

The torque amplification (TA) in turbine generators connected to a series-compensated network could cause excessive fatigue loss-of-life (FLoL) or even a sudden shaft failure, and thus poses a major threat to the shaft safety. This paper develops an instantaneous transient torque protection (ITTP) scheme to ensure shaft safety. It adopts the shaft speed to estimate transient torque in real-time and then judges whether to trip the generator by comparing the estimated transient torque with the pre-set tripping thresholds. As the key parameters of the proposed protection scheme, the tripping thresholds are delicately tuned based on the most severe circumstance, ensuring the inflicted FLoL less than the tolerable value and avoiding mal-operation under all possible circumstances. A prototype ITTP is developed and its performance is thoroughly verified through real-time digital simulator (RTDS)-based hardware-in-the-loop (HIL) tests on a practical series-compensated power system. The results under a variety of operating conditions and disturbances have well demonstrated the effectiveness of the developed ITTP in estimating transient torque and protecting the generator from damage caused by impulsive transient torque.

针对发电机定子单相接地故障导致定子铁芯烧损的问题,提出了基于电弧模型和定子铁芯温度场模型的仿真分析方法.首先,根据单相接地等值电路构建电弧模型,分析电路参数对电弧电压和电弧功率的影响.其次,建立定子铁芯温度场模型,利用有限元法计算电弧功率、故障时间等参数不同时铁芯的烧熔体积.仿真结果表明,电弧功率与接地故障电流大小、铁芯烧熔体积与电弧功率呈近似线性关系.最后,分析定子铁芯燃弧烧损试验数据,提出电弧功率模型,修正温度场模型,明确铁芯烧熔体积与接地故障电流大小的关系,并通过仿真与试验对比验证仿真分析方法的有效性.结合电弧烧损铁芯的仿真和试验以及电机制造厂家的铁芯局部修复水平,可以确定大型发电机的接地故障电流允许值最大为25 A.

多相无刷励磁机已广泛应用于大容量核电机组,但现阶段对常见故障缺乏可靠的检测及保护措施.该文设计了一种用于无刷励磁机故障检测的磁极探测线圈,并研究了不同磁极下探测线圈的组合联结方式.针对多相环形绕组无刷励磁机的励磁绕组匝间短路、二极管开路、电枢绕组断线和内部短路等常见故障,分析了正常运行和各种故障工况下多相电流及其产生的合成电枢反应磁场的特点,推导出各种故障在磁极探测线圈端口引起的电压谐波特征,并通过样机实验进行了验证.理论分析与实验结果表明,应用该文设计的组合磁极探测线圈,可根据探测线圈端口电压的频率特征,检测并区分各种故障,为提高多相无刷励磁机的安全可靠性提供了新思路.

某电厂主变压器(三相变压器组)拟对C相进行单独更换,由于新C相与旧A/B相的阻抗及对地电容值存在差异,需要对主变压器新C相与旧A/B相联合运行下的电气特征量变化进行理论推导和仿真分析,校核与过电流保护相关的三相电流不平衡度,以及与定子接地保护相关的位移电压的变化等,以确定相关继电保护配置及定值整定的应对措施,为后续主变压器C相更换的实施提供决策依据.上述做法将为后续电站类似技改工作提供借鉴.