Characteristics of interface charges on oil-pressboard insulation and their influence on flashover under polarity reversal voltage
Converter transformer is one of the important apparatus of high voltage direct current (HVDC) transmission system. Because of the complex structures and operating conditions, the winding of valve side of converter transformer will withstand DC voltage, combined AC-DC voltage and even polarity reversal voltage, and the insulating properties of which under polarity reversal voltage has drawn great attention. Under the special condition, the interface charge will be accumulated on the oil-pressboard interface, which can directly affect the electric field distribution of the converter transformer dielectric and even decrease the flashover voltage. Therefore, it is necessary to study the characteristics of interface charges and their influence on flashover voltage. The oil-pressboard insulation test platform is designed in this paper, including the interface charge of oil-pressboard insulation measurement platform and polarity reversal test platform. The minimum spatial resolution of interface charge measurement platform is about 1mm2, and its measuring sensitivity can reach 0.129pC/mm2. This paper adopts a needle-plate test model for measurement. The reversal time of the polarity reversal test apparatus is on the level of millisecond that conforms to the actual operation of the converter transformer. The experimental results under polarity reversal voltage indicated that the interfacial charge could greatly reduce the flashover voltage of polarity reversal, and the surface potential produced by charges can be up to 40% of polarization voltage. The changes of oil-pressboard interface charge density before and after the polarity reversal have been measured. It is found that the electric field produced by interface charge can strengthen the maximum synthesis field intensity of the needle electrode after polarity reversal, thus greatly decreases the flashover voltage.
The influence of polarity reversal time on interface charges in oil-impregnated pressboard insulation under non-uniform electric field
The polarity reversal test is one of the routine tests for the evaluation of electric insulation in converter transformers. The polarity reversal time (PRT) has significant effect on the accumulation of interface charges in oil-impregnated pressboard insulation. IEC standard recommends that polarity reversal should be performed within 2 min so as to match with the dispersion rate of interface charges in uniform filed, but not necessarily in the non-uniform field. In order to investigate the influence of PRT on the dissipation rate of interface charges in non-uniform electric field, the density of interface charges was measured by an electrostatic capacitance probe in the needle-plate non-uniform field at the PRT of 2 ms, 4 ms and 2 min respectively. The results showed that the polarity of interface charges did not change with the applied voltage jump at the PRT of 2 ms and 4 ms, while the polarity of interface charges almost synchronously changed with the polarity reversal of applied voltage at the PRT of 2 min. For the PRT of 2 ms and 4 ms, the electric field nearby the needle electrode would be enhanced with applied electric field of the same direction, whilst the total electric field of insulation system would be weakened at the PRT of 2 min. It was inferred that polarity of interface charge might have not enough time to change in polarity reversal test within the PRT of 4 ms under non-uniform electric field, which was more close to the severe condition of covert transformer. The paper suggest that the value of PRT be reconsidered when the polarity reversal test is performed in non-uniform electric field.
Mentally disordered offenders and the law: Research update on the insanity defense, 2004-2019.
The insanity defense is among the most controversial legal constructs that has attracted the attention of scholars, practitioners and policy makers. Here, we conducted a systematic review of the literature spanning 2004 to 2019 that produced 58 studies of insanity defense research. Findings are organized according to: (1) assessments and assessment-related issues, (2) juror decision-making in defense trials, (3) characteristics of insanity acquittees, (4) release recommendations for insanity acquittees, (5) revocation of conditional release status of insanity acquittees, and (6) additional areas of insanity defense research. Implications of the research for the insanity defense and cognate legal issues are proffered.