E-field distribution analysis on three types of converter double valve in 800kV valve hall
Converter valve possesses the function of AC-DC voltage conversion and power transmission. It is an important device in the valve hall of UHVDC converter station. Since the corona discharge has a serious impact on the safe and stable operation of converter valve, it is necessary to analyze its surface electric field (E-field) distribution. In this paper, the structure characteristics of three typical double valve designs have been analyzed, which were respectively used in three high voltage (400-800kV) valve halls of one ±800kV UHVDC transmission project in China. Based on the project drawings, the equivalent model of each type of double valve with shielding structure was built with three-dimensional (3D) finite-element method (FEM) software ANSYS. Considering the interaction of double valves in actual valve hall, three double valve FEM models of same type in one hall, which formed the upper 6-pulse bridge in the 12-pulse converter unit, were built for the analysis of E-field distribution. Due to the hybrid AC-DC operating voltage, the transient E-field analysis with discrete loading voltage waveforms was adopted in these models. After calculation, the E-field distribution characteristic of different double valve types were summarized and compared under the normal operating condition. The results showed that the surface E-field strength on the shielding structure of all the double valves was lower than the critical value of corona inception and could meet the project requirements. This research could provide valuable references for shielding structure design and optimization of converter valve in the UHVDC transmission project.
Electrical field calculation and optimization of suspension insulator with cone conformation in valve hall
As a crucial part of Ultra High Voltage Direct Current (UHVDC) transmission line, the security of the valve hall is stressed much importance. Therefore, it is vital to figure the insulator and shielding apparatus properly and further to obtain the applicable grading ring design, or corona discharge is possibly to take place, thus to risk the safety of the whole substation. In this paper, the model of a new-style suspension insulator in front of the valve tower with triple ones in cone conformation was established using finite-element method (FEM) software ANSYS. The surface field distribution was calculated and was compared to the critical field. In addition, several types of optimization schemes for grading ring were put forward to reduce the maximum field. The results indicated that the triple grading ring design could help reduce the maximum field.
The effect of collimation on image quality and radiation safety in digital radiography of small animals
Collimating the primary x-ray beam to the specific area of anatomic interest has traditionally been strongly recommended to produce high quality radiographic images, even though some small animal patients can fit their entire torso on a single radiographic casette or detector. However, modern digital radiography may allow for whole-body projections for these patients at similar quality to specific collimated projections. Lateral and ventrodorsal projections of 31 small dogs were obtained to evaluate the effect of collimation on the quality of the resulting digital radiograph. Each projection was performed using both a collimated and whole-body technique. Scatter radiation was detected during each exposure as well to determine the effect of collimation on radiation safety for veterinary personnel. The resulting images were cropped to portray only the thorax, and were evaluated by five radiologists blinded to the procedure used for each image. Visual grading characteristics analysis was performed. An overall trend towards higher image quality was found in the collimated approach compared to the whole-body, but in most cases this change was not significant. Overall image quality for the ventrodorsal images was found to be significantly better in the collimated approach compared to the whole-body. Evaluation of image histograms suggests that the application of digital processing and the look-up table differs between them, and this may account for a large portion of any quality difference. Over twice as much scatter radiation was detected during the whole-body images as during the collimated images. Overall image quality was rated as adequate to high for almost all images with both approaches. Adequate collimation remains recommended to provide the highest quality images with the lowest exposure to veterinary personnel.