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An increase in moisture content within GIS （gas insulated switchgear） can compromise its insulation integrity. To monitor insulation gas humidity inside GIS， a humidity sensor using dual-cone microfibers is developed. Firstly， utilizing beam propagation method （BPM）， the optical field of the dual-cone microfibers is simulated to investigate their field distribution and mode interference theory. This analysis reveals the correlation between SF6 and moisture content. Subsequently， the fused biconical taper （FBT） is used to fabricate the dual-cone microfiber humidity sensor and conduct humidity sensing experiments. Based on the experimental data， it is determined that the humidity sensor using dual-cone microfibers exhibits a detection sensitivity of -51.75 pm/%RH， enabling continuous online monitoring of humidity levels in electrical equipment.

The realization of digital twinning technology for transformers largely depends on the rapid calculation of various physical fields within the equipment. Therefore， the response surface methodology based on enhanced radial basis function （RBF） is employed to construct a response surface model of the POD （proper orthogonal decomposition） modal coefficients for winding conditions and then quickly calculate the transformer winding temperature field based on reduced order mode. Upon comparison of these results with those obtained from full-order calculations， it is observed that the maximum average relative error of the reduced-order model is 1.91% across 100 test cases. Remarkably， the calculation of these 100 test cases is completed in a mere 31.26 seconds， signifying an efficiency level approximately 61，266.79 times superior to that of the 12-core parallel full-order calculation. The computational findings affirm the high computational accuracy and efficiency of the proposed POD reduced-order algorithm founded on response surface methodology.

When partial discharge occurs within transformers， the pulse current signal propagates along the winding. Detecting this pulse current signal at both ends of the winding enables the identification of partial discharge within the transformer. To this end， a wideband equivalent circuit model for the transformer is established. In high-voltage windings where partial discharges are likely to occur， partial discharge pulses are applied between different wound disks and the ground to simulate the discharge to the ground at different locations of the high-voltage winding. Pulse current signals are then collected at the end screen of the high-voltage casing and at the neutral point to study the propagation rules of the partial discharge pulse current signal at different locations within the transformer windings. Through calculations， it is observed that the energy ratios of pulse current signals in two distinct frequency bands are linearly correlated with the distance ratio of pulse current propagation， and the maximum error observed in this context is 9.80 wound disks. This method facilitates localization of partial discharge within the transformer， and the results of field experiment subsequently validate the effectiveness of this localization technique.

The temperature rise at winding hotspots is a critical metric for assessing the operational condition and remaining lifespan of oil-immersed transformers. In the context of a 110 kV oil-immersed self-cooling transformer， a comprehensive two-dimensional closed-loop full-scale thermal-fluid simulation model incorporating the equivalent geometric structures of radiators is constructed. The model predicts temperatures with an error of less than 5 ℃ when compared to temperature rise test results， demonstrating a high level of accuracy. Simulation results bring to light the presence of an oil stagnant zone within the horizontal oil duct of the windings. To optimize oil circulation in this area， the impact of horizontal oil duct width and the number of baffle plates on oil flow rate and winding temperature rise is analyzed. The study findings suggest that widening the horizontal oil duct reduces both the average winding temperature and hotspot temperature. Additionally， the installation of baffle plates significantly improves oil flow rate in the horizontal oil duct， consequently reducing the average winding temperature rise. Under the specified conditions， the installation of five baffle plates emerges as the optimal solution for achieving comprehensive heat dissipation performance.

The analysis of transient voltage and current characteristics of DPFC （distributed power flow controller） under lightning strikes is an essential consideration in its design and operational management. Firstly， the structure and operational principles of DPFC are introduced， and simulation models are respectively constructed for two scenarios： a centralized arrangement of DPFC sub-modules in substations and a decentralized arrangement along transmission lines. Subsequently， the electromagnetic transient characteristics of DPFC are simulated and analyzed in the case of back strikes on the conductors and circling strikes on the transmission line， and the overvoltage and overcurrent levels at critical positions of DPFC during lightning strikes are thereby obtained. Finally， the differences in overvoltage and overcurrent between the centralized and decentralized arrangement of DPFC sub-modules are compared and analyzed in the case of back strikes and circling strikes. It is concluded that when lightning strikes the lightning conductor， the overvoltage to earth and the overvoltage at the submodule port of the centrally arranged DPFC are larger than the results of the decentralized arrangement； the overcurrents of the submodule of the centrally arranged DPFC and the arrester are smaller than the results of the decentralized arrangement.

Distributed power flow controller （DPFC） can improve power flow distribution in the power grid and solve the problem of power flow section excess. Based on the topology structure and operation principle of DPFC and with the backing of the world’s first 220 kV DPFC demonstration project， a distributed power flow control simulation model is established. The cases of distributed power flow controllers centrally installed in substations and dispersedly installed in transmission lines are considered， respectively. The overvoltage and overcurrent levels of DPFC key nodes in the case of short-circuit faults on the transmission lines are simulated. Moreover， it is analyzed how fault location influences the DPFC's overvoltage and overcurrent. The calculation results in this paper can provide a reference for DPFC insulation coordination in engineering practice.

Exploiting the load capacity of operating transformers is an important measure to ensure the safe operation of the power grid under emergency load conditions. Based on the typical methods for top oil temperature measurement and hot-spot temperature measurement， this paper proposes an optimal measurement of R×K2 and gr for special transformers to exert the original load capacity of the transformer still further. Meanwhile， to improve the cooling capacity of transformers， a series of solutions including installing extra fans， replacing low-speed fans， and frequency conversion of power supply for fans. Thus， the load capacity of the operating transformers is improved. The paper analyzes the transformers’ safety risks during the high-load operating condition， and proposes a new safety management method based on the online monitoring of oil levels and combustible gas， providing technical support for ensuring the high-load operation safety of transformers.

Two cases of abnormal acetylene content rise in the oil chromatogram of ultra-high voltage reactors are introduced in this paper. To monitor the internal condition of the reactor in real time and judge the type of defect， a partial discharge critical monitoring device is used for long-term monitoring. Critical monitoring results in case 1 show that there is a partial discharge caused by floating potential and weak insulation defects inside the reactor that lead to the rapid content rise of acetylene. Critical monitoring results in case 2 show that the intermittent partial discharge signal inside the reactor is an external interference signal near the north side of the ultra-high voltage reactors， and there is no obvious internal discharge signal. As to the iron core clamp ground current test， the internal defect type is the high temperature and overheating gas produced by the multi-point grounding of the magnetic circuit， and the chromatogram growth has nothing to do with the partial discharge. It is discovered by disassembly that the equipotential copper tape of the ground screen connected to the clamp on reactor body 1 in case 1 has broken discharge traces and the insulation paperboard is damaged. In case 2， the iron yoke of reactor body 1 and the magnetic shield of the upper body are overlapped， and the contact surface is obviously ablated with no obvious discharge trace inside， which excludes the possibility of internal discharge. The disassembly results are consistent with the analysis conclusions of the partial discharge critical monitoring system， verifying the effectiveness of the partial discharge critical monitoring method in power equipment defect analysis. The method can evade misjudgment due to external signal interference.

C4F7N gas has the most potential to replace SF6 because of its excellent insulation and environmental protection characteristics. The paper expounds the insulation and arc extinguishing performance of C4F7N-CO2 gas mixture， summarizes the influence of different electric field types， air pressure， mixing ratio and oxygen content on the insulation performance of C4F7N-CO2 gas mixture， and analyzes the arc burning characteristics of C4F7N-CO2 gas mixture on different arc extinguishing structure load switches （compressor type， deionization grid type and magnetic blow type）. The research results provide a reference for the application of C4F7N-CO2 gas mixture as insulation and arc extinguishing medium in gas insulation ring main units. Finally， on the basis of the existing research， the research directions of C4F7N-CO2 gas mixture applied in ring main units in the future are put forward： The first is to further optimize the existing switch structure and gas mixture ratio， and the second is to focus on the biosafety of decomposed gas after arc extinguishing of C4F7 Ngas mixture.