Unexpected open-circuit (OC) faults are a significant problem in cascaded H-bridge (CHB) converters and can cause secondary failure of other components. Therefore, it is important to localize the OC fault quickly. This paper proposed an efficient localization method for the OC fault in the CHB converter. Firstly, the operation mechanisms are analyzed under the healthy and faulty state of a CHB converter, and the grid current and the DC capacitor voltage are selected as parametric to locate OC faults. Then, the simulation model is built to validate the feasibility of fault localization using these two parameters. Finally, an OC fault localization strategy based on the capacitor voltage and the grid current is proposed, which is used to localize the OC fault in only two steps.
Nowadays, a doubly-fed induction generator (DFIG) performs an important role in wind power generation. Stator inter-turn short circuit faults are one of the major faults in DFIGs and can adversely affect the operation of DFIGs. Therefore, it is crucial to detect the inter-turn short circuit fault for the proper operation of the DFIG. In this paper, a voltage and current method, without additional sensors and complex operations, is proposed to diagnose the short circuit fault. Firstly, a doubly-fed wind power generation system model is established by using MATLAB/Simulink software. The voltage and current of phase A stator windings under the normal state and inter-turn short circuit faults are obtained by this model. Then, Lissajous figures under different states of phase A stator windings are synthesized from the above voltage and current. Finally, the feasibility of the proposed voltage and current method for detecting the short circuit between turns of the stator winding is proved by comparing the Lissajous under normal and fault conditions, and the variation characteristics of the Lissajous figure are deduced. The simulation results show that using the voltage and current method to detect DFIG stator inter-turn short circuit faults is feasible.
Fault localization and severity evaluation of transformer winding is crucial for its mechanical condition assessment. Ladder network synthesis of winding entity based on its frequency response analysis data is an elegant solution to this problem. This article proposes a generic method for high-frequency (HF) ladder network synthesis of winding in different mechanical conditions for its fault diagnosis. The simplified HF network used in the synthesis process is first built. Afterward, the synthesis algorithms with high precision and efficiency and the mathematical model for normal HF ladder network are elaborated. Further, the synthesis method for defective network is proposed to locate the winding fault positions and evaluate their severities. Finally, the network synthesis method is applied on a distribution transformer, with the fact that the obtained network components comply with all the constraints and the diagnostic results also match well with the actual mechanical conditions of winding, which verifies its feasibility and accuracy.
In this paper, we investigate the joint optimization of power and subcarrier allocation for maximizing the energy efficiency (EE) in downlink OFDMA systems. The problem of EE maximization is formulated as a stochastic optimization to determine the optimal power and subcarrier allocation, in which many practical factors including channel estimation errors, delay requirements, and time-varying channel are considered. We first propose a dynamic power and subcarrier allocation (DPSA) algorithm to solve the formulated problem. Then, we derive the two bounds of EE and delay and reveal the tradeoff between them. The theoretical analysis and simulation results demonstrate the variations of EE and delay with channel estimation errors.
Abstract Metal tip defect is a typical insulation defect in gas insulated switchgear (GIS). Simulation research on the defect under multi‐physical field coupling was conducted in order to explore the realistic electric field distribution. The results of the simulation were confirmed by a partial discharge experiment. On the one hand, the influence of defect location and structural parameters on electric field distribution was investigated in the established defect model. On the other hand, the partial discharge experimental platform was built to obtain the initial voltage of partial discharge under this defect. The results show that the presence of the defects can increase the electric field distortion by several‐fold or more in the surrounding electric field. The closer the defect on the outside of the conductor is to the basin‐type insulator, the lower the electric field distortion is around the conductor. However, the opposite is true on the inside of the shell. The change in the top radius of the defect has the greatest impact on the electric field distribution. The experimental results verify the validity of the simulation model. This study augments the research on insulation defects in GIS, offering a valuable reference for the manufacturing and installation of GIS.
To study sweep frequency impedance (SFI) features of short-circuit (SC) faults easily, this paper proposes a broadband electric circuit model of a transformer winding and solves its three key problems. The first problem is the calculation of lumped-circuit parameters considering frequency-dependent complex anisotropic permeabilities (FDCAPs), which are caused by the physical characteristics, such as skin, proximity, and geometrical effects and anisotropic properties, of the transformer core and winding materials. The other issue is the establishment of the electric circuit model based on the SFI measurement connection mode, the transformer winding parameters, and a double-ladder network (DLN). Another issue is the construction of the state-space model of the electric circuit toward different SFI values to obtain all network branch voltages and currents. The accuracy of the proposed model is assessed by comparing its SFI signatures with those of the simulation model, without considering FDCAPs under healthy winding, and the corresponding physical transformer model during healthy winding and SC faults. It is observed that the SFI results of the proposed model are closer to the experimental measurements, and the model can be effectively used to study the SFI features of SC faults. Moreover, the impacts of different types of SC faults on the SFI data are concluded in this paper.
Ladder network synthesis is very significant for winding fault diagnosis within transformer. To synthesize the network reversely based on the measurable frequency response analysis (FRA) data is the only applicable way. To date the iron core and non-tested winding, which result in the frequency-dependent network components and multiple ladders in some frequency ranges, are usually not considered in the reverse process and thus the synthesis precision is largely limited. This paper proposes to synthesize multiple ladder networks in different frequency regions to handle this problem. Firstly, the sensitivities and frequency-dependent features of network components are analyzed, based on which the topology and components of the networks in diverse frequency ranges are determined afterwards. Further, the network synthesis strategies in different frequency ranges are elaborately designed. Finally, the synthesis method is applied on a distribution transformer successfully, with the fact that the extracted network parameters meet all the constraints and its FRA curve matches well with the measured one. An intact method of multiple-network synthesis for transformer winding in wide frequency range, considering the iron core and non-tested winding, is offered in this paper.
At present, the most widely used method of winding fault detection is frequency response analysis (FRA). To surmount the defects of slow convergence and low classification accuracy caused by inappropriate parameter selection of support vector machines (SVM), this paper proposes a transformer winding fault classification method based on bald eagle search (BES) algorithm to optimize the kernel parameter g and the penalty coefficient C in the SVM model, denoted as BES-SVM. The analysis of transformer winding faults shows that the BES- SVM model can diagnose faults more accurately. Compared with the traditional SVM and PSO-SVM methods, this method has higher diagnostic accuracy.
To investigate the impact of temperature on the internal electric field distribution and oil streamer discharge development under AC-DC composite voltage, the dielectric parameters of insulating oil and insulating pressboard at different temperatures are measured experimentally. The nonlinearity of dielectric parameters with temperature is compared and analyzed in terms of its effect on electric field distribution. Additionally, based on the bipolar carrier transport model and drift-diffusion model, the influence of temperature on the discharge development process is simulated and studied. The results indicate that considering the non-linear temperature dependence of dielectric parameters, the electric field strength in oil is positively correlated with the ratio of dielectric constants of insulating pressboard to that of insulating oil, and negatively correlated with the ratio of conductivity. There is an opposite law for insulating pressboard. Increasing temperature exponentially enhances charge mobility by altering conductivity which exacerbates discharge development process. The attraction of insulating pressboard to streamer discharge is positively correlated with the difference in dielectric constants between oil and pressboard，which indirectly affects the time and morphology of the discharge.
The frequency response method is an effective means to detect the deformation of power transformer windings. The study of power transformer frequency response characteristics by means of finite element simulation and winding equivalent circuit calculation is an effective tool. In this paper, by means of finite element simulation, the equivalent electric group is obtained in three cases: considering only skin effect, considering turn-to-turn proximity effect, and considering disc-todisc proximity effect. By comparing the obtained equivalent resistance and frequency response curves, the influence of the proximity effect on the calculation of the equivalent resistance of the winding in the finite element calculation is discussed in depth, and its effect on the accuracy of the frequency response is described.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)