The effect of temperature and field strength on the resistivity of transformer oil
Safety operation of total ultra-HVDC transmission system could be directly affected by the operation reliability of converter transformer which is one of the most important equipments in the system. The transformer oil is the main liquid dielectric medium in the converter transformer. Thus, it is significant to study the dielectric properties of the transformer oil. In this paper, the dc resistivity of transformer oil under different conditions has been systematically investigated, and then the temperature and field strength were the test parameter variables. The variation range of temperature was 25~80°C, and under each temperature point the field strength changed ranging from 0.5V/mm to 10kV/mm. Meanwhile, 15 min is selected as the duration of testing resistivity. In this paper, the authors obtained the systematical data points and analysis curves of resistivity under different temperature and field strength, and obtained a empirical equation which describe the relationship between resistivity, temperature and field strength. It enhanced the understanding of the resistivity for transformer oil and provided the theoretical and test basis to expand the engineering application of this transformer oil.
Resistivity of pressboard immersed in mineral insulation oil
The pressboard is an important solid insulation dielectric in AC-DC converter transformers. Thus, it is very significant to understand the dielectric properties of the oil-impregnated pressboard. In this paper, the volume resistivities of oil-impregnated pressboard under different conditions have been systematically investigated. The pressboard was immersed in two kinds of mineral insulation oil, respectively. The measuring device of resistivity was full of oil and isolated from outside. All samples were measured at temperature range of 25-80°C under the field strength range of 1-30kV/mm. The influences of the temperature and the field strength on the resistivity of oil-impregnated pressboard have been investigated. The results of the volume resistivity of oil-impregnated pressboard showed that the resistivity of oil-impregnated pressboard decreased with the increase of temperature, and the activation eneregy increased with the increase of field strength. Meanwhile, with the increase of field strength, the resistivity changed slightly at first and then decreased significantly.
Actuating Principle of Online Automatic Balancer with Counter Weight Driven by Magnetic Force
Kinematic precept of mass block in automatic balancer is proposed following the principle of online automatic dynamic balancing.By combination of electromagnet and permanent magnet,the magnetic circuit of the balancer with ring coil and rare earth permanent magnet is designed.Actuating process of the balancer is discussed and then a step drive voltage waveform is defined.For the high-speed grinding motorized spindle 170MD12Y16,the mechanical structure and dimensions of the balancer are schemed out.The electromagnetic simulations for the designed balancer are conducted and the better coil ampere-turns and reluctance adjusting ring thickness are then determined.The validity of simulation results is verified by a balancer driving experiment and the desired voltage value and its duration time are also determined.the designed balancer has a stable operation process without out-of-step and fluctuation around balanced position when the ampere-turns are considered as 1 kA,the reluctance adjusting ring thickness 0.2 mm,the driving voltage 12.4 V and the voltage duration time 120 ms.