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.
Electric field evaluation and optimization of shielding electrodes for high voltage apparatus in ±1100 kV indoor DC yard
The indoor DC yard of the ±1100 kV UHVDC transmission project is preferred to the outdoor solution, and the rational design of shielding electrodes for the ±1100 kV apparatus becomes the key to system security. In this paper, the design requirements of indoor DC yards were described and the preliminary designs of the indoor shielding electrodes for the key ±1100 kV DC apparatus were proposed. The whole 3-D FEM model of one ±1100 kV indoor DC yard was established. An empirical formula of the critical E-field strength for the evaluation of relatively large shielding electrodes was proposed based on the corona tests of typical ball and ring electrodes. The electric field distribution of the whole indoor DC yard was calculated and the surface electric field strength of all the shielding electrodes for the key ±1100 kV apparatus was evaluated. The results show that after optimization, the maximum surface field strength on the shielding electrode of ±1100 kV DC filter capacitor decreased from 1507 V/mm to 1335 V/mm, lower than the critical value for evaluation, then the corona free design of all the ±1100 kV indoor shielding electrode could be predicted. The method of electric field evaluation and optimization could provide important references for the development of shielding electrodes in the ±1100 kV or other UHV indoor DC yards.
Long term response stability of a well-type ionization chamber used in calibration of high dose rate brachytherapy sources
Well-type ionization chamber is often used to measure strength of brachytherapy sources. This study aims to check long term response stability of High Dose Rate (HDR)-1000 Plus well-type ionization chamber in terms of reference air kerma rate (RAKR) of a reference 137Cs brachytherapy source and recommend an optimum frequency of recalibration. An HDR-1000 Plus well-type ionization chamber, a reference 137Cs brachytherapy source (CDCSJ5), and a MAX-4000 electrometer were used in this study. The HDR-1000 Plus well-type chamber was calibrated in terms of reference air kerma rate by the Standards Laboratory of the International Atomic Energy Agency (IAEA), Vienna. The response of the chamber was verified at regular intervals over a period of eight years using the reference 137Cs source. All required correction factors were applied in the calculation of the RAKR of the 137Cs source. This study reveals that the response of the HDR-1000 Plus well-type chamber was well within ±0.5% for about three years after calibration/recalibration. However, it shows deviations larger than ±0.5% after three years of calibration/recalibration and the maximum variation in response of the chamber during an eight year period was 1.71%. The optimum frequency of recalibration of a high dose rate well-type chamber should be three years.