AIR LOAD BREAK SWITCH DESIGN PARAMETERS

Abstract

Current interruption is vital in the power system, as this makes it possible to control the use of different loads, change the grid configuration, and minimize damage when faults occur. This thesis presents a study of the different switch design and test circuit parameters involved in medium voltage air load break switching and how they affect the thermal interrupting capability. Mediumvoltage load break switches are common in the distribution grid, and are a cheaper option than installing circuit breakers. Medium voltage load current ratings are typically in the range of 6 – 36 kV and 400 A up to around 1 kA (50 Hz). Air is considered an environmentally benign alternative as an interrupting medium compared to SF6 for these ratings, and is also thought to be cost-competitive compared to vacuum. However, no compact air load break switch for 24 kV is currently available for commercial use. Thus, it is necessary to have a good understanding of the design parameters involved, and how they affect the interrupting capability of the switch. This thesis addresses medium-voltage load current interruption in air. It is an empirical study based on an extensive test program in a medium voltage test lab, and the main results and contents of this thesis are presented in five papers. The two first are switch design parameter studies. Using a test switch that is simple and axisymmetric, yet in many aspects similar to commercial puffer devices, one test switch parameter has been changed at a time to find the air flow over-pressure needed for successful interruption. The test circuit settings are also varied to find how the interrupting capability changes with load currents in the range 400 – 880 A, and with a transient recovery voltage corresponding to IEC’s 24 kV ”mainly active load” test duty. Only the thermal phase of current interruption has been considered, i.e. the first tens of microseconds after current zero. The over-pressures needed to interrupt the load currents were typically from 0.2 to 0.4 bar. The third paper presents a logistic regression analysis of all the conducted interruption tests, with the goal of describing the interruption performance as a function of the main test switch design parameters and transient recovery voltage stresses. More than 3 000 interruption tests are used as input data for this analysis, which produce a mathematical expression that summarizes all the empirical results. The nozzle-to-contact diameter ratio has been found to be an important design factor. Low ratios require a substantially lower air over-pressure than high nozzle-to-contact diameter ratios in order to interrupt successfully. The choice of contact diameter important as well, where larger contact diameters require lower over-pressures, but higher mass flow rates. The nozzle length does not influence the interrupting capability very much, but the chance of successful interruption is greater when the pin contact has moved out of the nozzle at current zero. The interruption becomes more difficult with increasing current and the rate of the voltage build-up across the contacts after interruption. The other two papers are based on the current interruption experiments mentioned above, but concern details of the arc behavior and arc voltage under different currents, test design variations and air flow conditions. For typical medium-voltage and load current ratings, the arc greatly affects the air flow during current interruption. The flow through the tulip contact and nozzle is clogged during the high current part of the half-cycle, even for moderate currents and relatively large contact dimensions. The typical over-pressures needed for successful interruption correspond to air velocities that are well below supersonic level. The arc voltage is a function of several parameters, and rises with increasing air over-pressure, decreasing current, and a larger contact gap. There is also a clear visible difference in the arc appearance when it is either subjected to forced cooling or not. The typical arc voltage is a few hundred volts.