Transformer internal arcing faults relieve significant fault energy, causing the formation and rapid expansion of arc-induced bubble. This leads to a sudden surge in oil pressure. When the pressure exceeds the ultimate structural capacity of the tank, structural failure and even fire may occur. Recently, transformer tank ruptures and explosions caused by internal arcing faults have occurred repeatedly, causing substantial economic losses and social impacts. Rooted in the immense energy released by the arc, a method for enhancing transformer safety involving a superconducting fault current limiter (SFCL) is developed. This approach leverages the rapid transition of superconducting materials to a high-impedance state under fault conditions, effectively limiting fault energy and mitigating arc-induced structural failure. An adaptive finite element method-smoothed particle hydrodynamics (FEM-SPH) coupling approach is proposed to simulate the transient transformer rupture behavior and quantitatively assess the structural failure proportion. The results show that the application of an SFCL to a 40 MVA/110 kV transformer significantly reduces the number of structural failures under various fault conditions. Under a 30% interturn short-circuit fault, the structural failure proportion is reduced by 90.1%, preventing transformer tank rupture.

As critical protection devices for achieving low voltage ride-through (LVRT) in a doubly-fed induction generator (DFIG), DC choppers are widely used because of their low costs and high reliability. The conventional DC choppers are limited to DC-link voltage regulation and cannot provide rotor current control. This inherent limitation has spurred growing research interest in multifunctional DC choppers (MDCCs). However, the absence of a systematic approach for deriving the topologies of MDCCs has significantly constrained their practical applications, resulting in only a limited number of proposed configurations. To address this challenge, in this study, a systematic topology derivation methodology for MDCCs is proposed. The hierarchical three-stage topology derivation method sequentially generates unweighted, weighted, and directed-weighted graphs. Moreover, the specific characteristics of an MDCC are modeled via formally established topology filtering criteria, which ensures the completeness of the generation process. The proposed code implemented method significantly improves the efficiency of MDCC design while maintaining high configuration accuracy. A hardware-in-the-loop experimental validation conducted on a real-time digital simulator (RTDS) confirms that the optimized topology (with only two insulated-gate bipolar transistors (IGBTs), two diodes and one resistor) achieves satisfactory performance during LVRT transients.

Transformer interturn short-circuit (ITSC) faults that occur during energization are highly challenging to identify for existing protection, particularly when slight faults occur, because the fault component is often masked by inrush current. Accordingly, using an equivalent magnetic circuit, the variations in the magnetomotive force and main magnetic flux during the ITSC faults are analyzed, revealing that the nonsaturated region contains unique fault information insensitive to the inrush current. On this basis, a magnetics-guided multitask convolutional neural network (CNN) that embeds this knowledge into a data-driven framework is proposed for ITSC detection during energization. Specifically, the auxiliary task eliminates the inrush current as noise and guides the primary task to utilize the informative nonsaturated region of magnetic parameters to classify operating states. A validation through experiments conducted on a 30 kVA/2 kV transformer shows that the proposed method achieves sensitive detection of slight ITSC faults during energization and offers a data-driven solution with greater generalization capability for enhancing the safety of power transformers.

In scenarios where synchronous generators (SGs) and grid-following renewable energy sources (GFLR) are co-located, existing research, which mainly focuses on the first-swing stability of SGs, often overlooks ongoing dynamic interactions between GFLRs and SGs throughout the entire rotor swing period. To address this gap, this study first reveals that the angle oscillations of SG can cause periodic grid voltage fluctuations, potentially triggering low-voltage ride-through (LVRT) control switching of GFLR repeatedly. Then, the periodic energy changes of SGs under “circular” and “rectangular” LVRT limits are analyzed. The results indicate that circular limits are detrimental to SG's first-swing stability, while rectangular limits and their slow recovery strategies can lead to SG's multi-swing instability. Conservative stability criteria are also proposed for these phenomena. Furthermore, an additional controller based on feedback linearization is introduced to enhance the entire period transient stability of SG by adjusting the post-fault GFLR output current. Finally, the efficacy of the analysis is validated through electromagnetic transient simulations and controller hardware-in-the-loop (CHIL) tests.

With the increase in voltage levels of power systems, converter transformers are more susceptible to internal high-energy arcing faults because of their compact structure, causing pressure to increase inside the tank and ultimately resulting in deflagration accidents. Conventional pressure relief devices (PRDs) suffer limitations in terms of response speed and pressure relief capacity, making them inadequate for severe arcing faults within converter transformers. In this context, a novel approach based on resistive-type superconducting fault current limiters (R-SFCLs) is proposed to suppress overpressure. Specifically, a coupled model is established, in which the magnetic field, electric circuit, arc nonlinear characteristics, R-SFCLs insertion, and fluid-structure interactions are integrated. Afterward, a corresponding numerical method is developed and implemented. Simulations performed involving an actual 500 kV converter transformer reveal that the deployment of R-SFCLs effectively limits the arc power and the released energy. Moreover, combined with PRDs operation, the proposed method significantly mitigates overpressure inside the tank. Compared with the scenario without R-SFCLs, the peak pressure is reduced by 21.1% under a severe interturn fault shorting 30% of the grid side winding, and this effect increases with increasing fault severity.

As the most prevalent internal transformer faults, interturn short-circuit (ITSC) faults can lead to destructive accidents if not cleared in time. To analyze and prevent ITSC faults, it is necessary to develop an accurate and efficient numerical model. In this paper, a 3-D hybrid modeling method combining lumped and distributed magnetic circuits is proposed, which enables accurate and efficient ITSC fault analysis. Lumped and distributed magnetic circuits are used to represent the main and leakage fluxes, respectively, as the main flux is nearly uniform and the leakage flux is strongly distorted during ITSC faults. The hybrid magnetic circuit is constructed by meshing the 3-D structure of the transformer using four types of elements. To solve the coupled electric-magnetic circuit system, a direct coupling solution algorithm is developed and implemented in Python, achieving ITSC fault calculation within seconds. The proposed method is validated through experiments conducted on a realistic 30 kVA/2 kV three-phase transformer. The experimental and calculated results are in close agreement, demonstrating that the proposed method can accurately reproduce the electrical transient characteristics of ITSC faults with errors less than 1%, thus providing an accurate and efficient numerical approach for the analysis and prevention of transformer ITSC faults.

To facilitate the integration and long-distance delivery of renewable energy, configurations such as wind-thermal-bundled power transmission via line commutated converter-based high-voltage direct current (LCC-HVDC) systems have been widely adopted. However, the inverter commutation failure (CF) is a primary cause of sending-end transient overvoltage in LCC-HVDC systems, thus causing cascading tripping of wind farms. In this study, transient overvoltage is determined to be related to the reduction in both the transverse and longitudinal voltage drop components following power interruption. To address this issue, a resistive superconducting fault current limiter (RSFCL)-based reduction method is proposed. By deploying an RSFCL at the sending end, the equivalent system impedance is rapidly regulated, which increases the transverse voltage drop component while reducing the longitudinal voltage drop component during post-fault conditions, thereby effectively alleviating the CF-induced overvoltage phenomenon. A series of PSCAD/EMTDC simulations under various fault conditions rigorously validate the proposed approach.

The adoption of centralized current-limiting reactor (CCLC) configurations in MMC-HVDC grids significantly weakens transmission line boundary effects, thereby challenging the performance of conventional boundary-dependent protection principles. This paper proposes a protection scheme for CCLC-MMC-HVDC ring grids. Analytical derivation reveals that the drop feature of the forward traveling wave (FTW) exhibits a monotonic correspondence with the fault location. Based on this characteristic, a single-ended interval criterion is formulated to discriminate internal faults from external ones. Complementing the interval criterion, a directional element based on the relative amplitude difference between the line-mode voltage (LMV) and the FTW is introduced to ensure robustness across both normal and open-ring conditions. Extensive simulations validate that the integrated scheme provides accurate fault discrimination across the entire line length, maintaining reliable performance under high-impedance fault conditions up to 1000 Ω and in noisy environments with a signal-to-noise ratio as low as 10 dB. The method offers a viable and cost-effective protection solution for CCLC-MMC-HVDC ring grids.

Under stiff grid conditions, grid-forming (GFM) converters are prone to subsynchronous oscillations (SSOs). Phase compensators are widely used as effective means to address SSO issues. However, owing to the lack of precise design regarding the location and configuration, the performance of phase compensators within multi-loop GFM control remains underutilized. To this end, the performance of the phase compensator is quantitatively evaluated by the negative damping area in the impedance model. With a parametric analysis of this area, the SSO-critical control loops and governing parameters are pinpointed, thereby identifying four optimal embedding locations for compensators. To effectively demonstrate the robustness of SSO damping under varying grid conditions, the required configuration properties of the compensator for each location are proposed, and a feasible and general compensator form is provided. Furthermore, the proposed design proves effective in a model incorporating frequency coupling. The hardware-in-loop (HIL) experimental results validate the theoretical insights as well as the effectiveness and robustness of the presented damping strategy. The systematic design framework can be adapted to guide the phase compensator in diverse multi-loop GFM controllers for effective SSO damping.

Interturn short-circuit (ITSC) faults in transformers can result in catastrophic failures if they are not identified and isolated in time, thus, relay protection devices with properly calibrated operating thresholds are deployed. However, the lack of fast and accurate calculation methods for generating extensive fault cases under diverse conditions impedes relay protection validation and transformer commissioning in engineering. To this end, an efficient calculation method for transformer ITSC faults based on a physics-informed neural network (PINN) surrogate model is proposed. With ground truth finite element simulation datasets, the PINN surrogate model is constructed for predicting winding inductance matrices under diverse fault conditions while ensuring compliance with physical laws. Utilizing the output of the PINN and the generalized inductance-based circuit model, ITSC fault calculation can be performed rapidly. Experimental validation of the proposed method is performed on a 30 kVA/2000 V three-phase transformer. The experimental results demonstrate that the combination of the PINN with the inductance-based circuit model enables fault representation with errors less than 4.3%. Compared with existing calculation methods, this approach achieves both computational speed and accuracy, significantly reducing computational costs for transformer fault analysis and relay protection device calibration.

Doubly fed induction generators (DFIGs) are confronted with fault ride through (FRT) issues during grid faults whose fault path resistance often comprises a transition resistance and a time-varying resistance of AC arcs. In existing FRT analyses for DFIGs, the arc resistance is often simplified to a fixed value, whose nonlinear characteristics have been neglected. This paper quantitatively analyzes the arc-induced distortions in the stator voltage, stator flux and rotor electromotive force. Under a severe arc fault, the impact of the additional arc component on the FRT process is found to be nonnegligible. In view of this unique signature, an improved arc component-based FRT control strategy with proposed flux observation and component separation method is developed for enhancing the FRT abilities from multiple perspectives, including overcurrent suppression, reactive power supply, and power and electromagnetic torque oscillation elimination. The correctness of the analysis and the advancement of the approach are demonstrated on real-time digital simulator (RTDS). The findings provide new insights when developing FRT methods based on the separated components and the approach can be universally applicable to FRT scenarios of DFIGs with notably distorted stator voltages.

Flexible DC grid is of great significance for effective consumption of large-scale renewable energy. However, conventional traveling wave protections for DC transmission lines encounter challenges such as limited tolerance to fault resistance and susceptibility to lightning interference, leading to potential misoperations and reduced the reliability. To tackle these issues, this paper begins by deriving time-domain analytical expressions for the initial forward traveling wave (FTW) under conditions of internal faults, external faults, and lightning interference. These expressions reveal the characteristic differences of the initial FTW under each case. Subsequently, combined with the mathematical method of waveform mirroring and quadratic polynomial fitting, the non-unit protection method based on the mirror waveform opening index of the initial forward traveling wave is proposed. The method takes advantage of the differences in FTW characteristics under different conditions, theoretically unaffected by fault resistance. Extensive simulation results demonstrate that the proposed protection method operates fast and accurately identifies different DC line faults and lightning interference within 1 ms, and it is able to withstand fault resistances up to 1000 ohm. Furthermore, the proposed protection method is highly robust to noise conditions and is applicable to various grid topologies.

An on-load tap changer (OLTC) is a key subassembly inside transformers. Pressure relief devices (PRDs) are widely installed on the top cover of OLTC compartments to mitigate internal overpressure. At present, the limitations and performance of a PRD under internal arcing faults have not been quantitatively determined, which is hampered in part by a lack of theoretical and experimental evidence. In this paper, a numerical method considering the interaction of internal oil pressure behaviors and transient PRD operation is developed and validated by arcing fault experiments conducted inside a full-scale OLTC compartment. Using this method, the arc current levels and time durations that the OLTC compartment can withstand are identified. To explore the improvement limit of the PRD performance, an optimal solution for the structural parameters is obtained via the surrogate-assisted approach, achieving an increase of 27.3% in the oil discharge capacity. The venting diameter is found to have the most significant impact on the evacuation volume, whereas the spring stiffness has the minimal impact. Moreover, the optimized PRD extends the permissible arcing duration by 28.6% under an arc current of 50 kA, but this effect decreases significantly as the arc current level increases.

Considerable efforts have been made to address the resonance issue of the Direct-drive Permanent Magnet Synchronous Generator (D-PMSG) wind farm integrated power systems. However, the D-PMSG controller structure and parameters are concealed because of commercial secrecy, thus the target system exhibits grey-box characteristics. The existing resonance damping methods are either unavailable for grey-box systems or economically infeasible, which makes resonance damping of grey-box systems extremely challenging. To address this issue, this paper proposes an Additional Resonance Damping Control (ARDC) specifically for the grey-box D-PMSG system. This strategy is achieved by incorporating an additional control loop outside the D-PMSG controller. Firstly, the external impedance characteristics are obtained by the frequency sweeping technique offline and then the key parameter of the additional control loop is determined by the Bode-diagram-based method under the worst stability scenario. Once the resonance occurs, the external impedance of the black-box D-PMSG is reshaped online to increase the magnitude stability margin of the system, thus providing effective resonance damping. The ARDC's effectiveness is finally verified in the simulation and controller-hardware-in-the-loop experiment under various operating conditions.

Generally, crowbar circuits and DC choppers are employed to prevent rotor-side converter overcurrent and DClink overvoltage, respectively, during the fault ride-through (FRT) process in doubly fed induction generators (DFIGs). However, once the crowbar is activated, the DFIG cannot fulfill the grid code requirements. This letter introduces a multifunctional DC chopper that can operate in current-limiting and voltage-limiting modes without requiring a crowbar, thereby avoiding RSC blocking. The proposed design is simple and facilitates the implementation of nonlinear control. Hardware-in-the-loop tests performed on a real-time digital power system simulator show that the proposed scheme suppresses both the DC-link voltage and rotor current under severe faults and provides better transient performance. This outcome not only enhances the FRT capability of DFIGs but also advances FRT research into renewable energy systems with grid-connected inverters.

In traditional views, the build-up of accelerating energy during faults can cause the well-known first-swing angle instability in synchronous generators (SGs). Interestingly, this letter presents a new insight that the accumulation of decelerating energy due to the low voltage ride-through (LVRT) and recovery control of grid-following inverter-based resources (GFL-IBRs), might also result in transient angle instability in SGs. The transient energy accumulated during angle-decreasing swing transforms into the acceleration energy of the subsequent swing, hence such phenomena often manifest as multi-swing instability. Both theoretical analysis and simulation support these findings.

Vacuum-type on-load tap changers (OLTCs) are widely adopted in ultra-high voltage converter transformers to perform frequent voltage regulation without interrupting the load current. The diverter switch topology is the key basis of switching procedures and mechanical design for OLTCs. Due to the lack of an effective topology derivation method, the acquisition of diverter switch topologies with symmetrical features remains a long-standing challenge. In this paper, the diverter switch topology is abstracted as a weighted graph with designated external nodes based on graph theory. Meanwhile, a computer-aided topology derivation method is developed to obtain diverter switch topologies with given components. Specifically, according to proposed electrical criteria and transition function criteria, all possible topologies are enumerated to screen out feasible diverter switch topologies, among which structurally symmetric topologies are further identified. Graph generation, screening and identification are implemented with MATLAB code. The proposed method for automatic topology derivation can also be applicable to the structure design of other intricate power equipment.

Additional active power control (AAPC) of wind turbines (WTs) is essential to improve the transient frequency stability of low-inertia power systems. Most of the existing research has focused on imitating the frequency response of the synchronous generator (SG), known as virtual inertia control (VIC), but are such control laws optimal for the power systems? Inspired by this question, this paper proposes an optimal AAPC of WTs to maximize the frequency nadir post a major power deficit. By decoupling the WT response and the frequency dynamics, the optimal frequency trajectory is solved based on the trajectory model, and its universality is strictly proven. Then the optimal AAPC of WTs is constructed reversely based on the average system frequency (ASF) model with the optimal frequency trajectory as the desired control results. The proposed method can significantly improve the system frequency nadir. Meanwhile, the event insensitivity makes it can be deployed based on the on-line rolling update under a hypothetic disturbance, avoiding the heavy post-event computational burden. Finally, simulation results in a two-machine power system and the IEEE 39 bus power system verify the effectiveness of the optimal AAPC of WTs.

When an oil-immersed transformer experiences an external short-circuit fault, the current flowing through the windings increases abruptly. The large electromagnetic forces generated by this fault current can induce violent vibrations in the windings and subsequent oil surges. Once the Buchholz relay (BR) detects an oil flow velocity surpassing its threshold, it erroneously cuts off the transformer. Such BR malfunctions have resulted in successive widespread power outages and economic losses in recent years. Given that significant fault currents passing through windings are the root cause of intense oil surges, a novel approach for suppressing transformer oil surges under external short-circuit faults by superconducting fault current limiters (SFCLs) is proposed. Specifically, a sophisticated electromagnetic-mechanical-fluid coupling model, which allows for the quantitative calculation of the mechanical response of transformer windings and the transient oil surge characteristics, is developed to estimate the efficiency of the suggested approach. The results indicate that applying an SFCL to a transformer rated at 40 MVA can reduce the velocity of oil flow through the BR by approximately 60%, effectively preventing unplanned transformer shutdown.

Damages of transformer windings caused by short-circuit (SC) faults have long been an issue of concern. In this paper, a numerical method on the basis of electromagnetic-mechanical (E-M) coupling analysis is presented. Considering the nonlinear stress-strain relations of copper conductors and insulation spacers, winding mechanical behaviors are simulated under multiple SC faults by implementing the finite element method (FEM) in ANSYS. Furthermore, an actual full-scale 110 kV transformer is subjected to actual SC faults. Due to initial deformations in the winding, significant distortion occurred in the B-phase winding after five SC fault tests. Numerically simulated results and on-site experimental evidences indicate that once the winding undergoes slight deformation, the impact of SC currents will create a significant imbalance in stress distribution, and the increased stress in the fault areas exacerbates the deformation of the winding under multiple SC faults. These intensified effects could ultimately lead to damage or collapse of the winding during multiple SC fault events.