Abstract
Multiple valve unit (MVU), which converts AC to DC and DC to AC, is one of the key elements of high voltage DC (HVDC) transmission. Therefore, the insulation design of MVU against overvoltage should be considered for the stable and reliable operation of HVDC transmission system. Especially, the air clearance of MVU should be calculated based the switching impulse, since it is fatal to MVU in terms of electrical insulation. However, the previous studies were limited to wave front, and the air clearance of the switching impulse is specified only for an ultra-high voltage (UHV) above 750 kV. As a result, it is difficult to calculate the air clearance of MVU which must endure for a switching impulse under 750 kV. In addition, when the switching impulse introduced while the MVU is in normal operation, it is superimposed to DC and creates the most severe situation, but the studies on such subjects are also insufficient. Therefore, as a fundamental step to calculate the air clearance of MVU, the dielectric characteristics of switching impulse and DC superimposed switching impulse in air have been investigated. The experiments on switching impulse showed that the critical flashover voltage was varied according to the curvature of electrode in the gap distance, up to eight times of the electrode radius. However, beyond that gap distance, the critical flashover voltage became similar, regardless of the radius of electrodes. In case of the superimposed experiment, it was performed according to DC pre-stress level and the polarities of switching impulse. The results were most severe when the positive switching impulse was superimposed on the positive DC, and the peak voltage at which flashover occurs was independent of DC pre-stress.
Highlights
The high voltage DC (HVDC) transmission system consists of three basic parts: (1) converter station to convert AC to DC, (2) transmission line, (3) second converter station to convert back to AC
The multiple valve unit (MVU) is loaded high operating voltage, and many elements of MVU, such as thyristors or insulated gate bipolar transistors (IGBTs), are connected in series and these are operated in a valve hall in a HVDC converter station [1]
Research on the precise air clearance calculation that does not cause the flashover between MVU and the ground, the wall, and the ceiling of HVDC converter valve hall should be preceded
Summary
The high voltage DC (HVDC) transmission system consists of three basic parts: (1) converter station to convert AC to DC, (2) transmission line, (3) second converter station to convert back to AC. Paris et al had reported the dielectric characteristics of air for diverse electrode configuration, such as rod–plane, rod–rod, conductor–rod, etc The experiments include both switching impulse and lightning impulse for both polarities, positive and negative. M. Rizk had reported the dielectric characteristics of the switching impulse in air for double toroid electrodes and sphere–plane electrode configuration up to gap distance 4.55 m, but impulse wave front was 5 to 500 μs [10]. It is necessary to study the dielectric characteristics of the DC superimposed switching impulse in air For these reasons, in this work, the critical flashover voltage of air of standard switching impulse according to gap distances from tens of millimeters to thousands of millimeters and the curvature of electrodes have been measured as a fundamental study for the precise calculation of the air clearance of MVU. The dielectric characteristics of DC superimposed standard switching impulse in air had been investigated according to DC pre-stress level and polarities of standard switching impulse
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