Recently, quality problems were found successively in more than ten current transformers (CTs), which were produced in a same batch, in Guangming switch station of Inner Mongolia province. Both the capacitance and dielectric loss did not meet the requirement of criteria. And the contents of dissolved gas in oil in those CTs exceeded the warming value. In order to explore the formative reasons of defects, the characteristics of defects in this batch of the transformers were sorted firstly. A typical CT which was used to be installed at an A-phase outgoing of the No.159 interval was selected for disassembly analysis. However, no abnormality was found in all parts of the transformer during the disassembling process. In view of this, two defective models were prepared by using the LB6-IIOW type of transformers separately. One of them was improper dried whereas the other was simply oil-immersed without using vacuum. The development process of defects in current transformer was studied. The simulation test showed that under the external applied voltage and current stresses, the capacitance and dielectric loss of models increased rapidly. And the dissolved gases were produced very fast in oil. H 2 is the dominating dissolved gas followed by CH 4 . A small amount of C 2 H 2 could be also found in oil. It was believed that the unsatisfactory vacuum immersion oil is the main cause for the failure of this batch of CTs. Based on this research, corresponding technical suggestions were put forward.

When the wind farm line fails, the unreasonable neutral grounding mode leads to the simultaneous disconnection of multiple wind turbines, resulting in increasingly serious power loss. Therefore, it is necessary to study the selection of neutral grounding mode. Firstly, taking typical wind farm as the research object, the electromagnetic transient lumped parameter calculation model is established. In addition, the neutral grounding modes are ungrounded, grounded through arc suppression coil and grounded through resistance. Under the condition of different transition resistance, the overvoltage and overcurrent levels of three grounding modes are calculated when single-phase short-circuit grounding fault occurs. Besides, the advantages and disadvantages of different grounding modes are compared and analyzed. Finally, the suggestions for the selection of grounding modes on the 35 kV side of the transformer in the wind farm are given: The 35kV system of transformer in wind farm gathering station is not suitable for neutral ungrounded mode; The compensation function of arc suppression coil greatly reduces the energy of arc reburning, and the arc can extinguish itself; However, due to the large capacitive current of the typical wind farm, the fault overvoltage phenomenon is obvious. It is more suitable to adopt the neutral point grounding through resistance.

More than 60% of oil-immersed current transformers are the vertical type in the Inner Mongolia Power Grid, which affects the operation stability of power system directly. This paper presents a diagnosing case of partial discharge defects of a 110kV vertical oil-immersed current transformer (VOCT). It is found that the Dissolved Gas Analysis (DGA) and dielectric loss of the VOCT exceeded warming thresholds. To explain the abnormal experimental results, byproducts in oil were examined and dielectric loss were re-measured under different voltage levels. In addition, the results of dielectric loss and grounding capacitance measurements indicated that the insulation of cable-paper tapes was good and the discharge was not attributed to moisture. Scanning Electron Microscope (SEM) and Energy Dispersive Spectrometer (EDS) showed that the copper belts and cable tapes are neither oxidized nor aged. Based on the diagnosis and analysis, the causes of defects were cleared, and technical suggestions for instrument transformer were proposed.