Lightening Impulse Breakdown of Vacuum Gaps in Series—Part II: Bridging Resistor

Abstract

In part I of this series article, it was found that a negative charge process after a partial breakdown (PB) could reduce the breakdown voltage of vacuum gaps in series. This part proposes the use of a bridging resistor to mitigate this negative charge process and improve the breakdown voltage of the entire arrangement. Two commercial vacuum interrupters (VIs) of the same model were connected in series, with each VI having a grading capacitor connected in parallel. A bridging resistor was then bridged between two floating potential middle points: at the middle point between the VIs and at the middle point between the grading capacitors. Experiments were performed using various gap distance arrangements and resistance values. A noncontact measurement method using an electric field sensor was proposed to measure the floating potentials of the two VIs. Experimental results showed that the negative charge process could be mitigated using the bridging resistor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{g}$ </tex-math></inline-formula> by blocking the discharge current from the grading capacitor into the broken-down VI. The voltage distribution between the two VIs became uneven because of the bridging resistor, but the breakdown voltage of the series-connected gaps increased significantly, regardless of the resistance value. The increase rate was influenced by the gap distance arrangement. With a symmetrical gap distance arrangement for the two VIs, a maximum increase rate of 37.4% was reached when <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{g} =40\,\,\text{k}\Omega $ </tex-math></inline-formula> . With asymmetrical arrangements, when smaller gaps shared higher voltages, the increase rate reached a maximum of 52.8% with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{g} =70\,\,\text{k}\Omega $ </tex-math></inline-formula> , but it only reached a maximum of 38.9% when larger gaps shared higher voltages with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{g} =10\,\,\text{k}\Omega $ </tex-math></inline-formula> .