Disconnectors are low-cost switching devices in high voltage electrical power supply systems that basically have an insulation function only. Nevertheless, they have a very limited capability to interrupt current (below one Ampere), e.g. from unloaded busbars or short overhead lines. The present study is a search for possibilities to increase the current interruption capability with auxiliary devices interacting with the switching arc. In this project the state of the art of disconnector switching is investigated and an inventory is presented of models of the free burning arc in air. A series of experiments were arranged at different laboratories. The switching arc and the interruption process are studied in detail through electrical and optical measurements during the switching process for a disconnector with (without) auxiliary devices under high voltage (300 kV) conditions. Three options for auxiliary devices were investigated: (i) arc cooling by forced air flow; (ii) fast interrupting by high-velocity opening contacts; (iii) reduction of arc energy by added resistive elements. Finally, a qualitative description is provided on the physical nature of the arc and how the evaluated methods affect the arc characteristics. All results are obtained by analysis of highresolution measurement of arc current (including all relevant transients), voltages across the disconnector and high-speed video observation. It was found that, depending on the current to be interrupted, the interruption process is governed by dielectric and/or thermal processes. In the dielectric regime, the interrupted current is low (roughly below 1A) and the switching arc is characterized by a high rate of repetition of interruptions and restrikes that only cease after a sufficient gap spacing has been reached. The restrikes interact severely with the circuitry in which the disconnector is embedded, exciting transients in current and voltage with frequencies up to the megahertz range. High overvoltages can be generated. Their magnitudes can be limited by a proper choice of the capacitance at supply side of the disconnector. The arc-circuit interaction has been studied and relevant processes have been modelled and verified by experiments in full-power test-circuits. In the thermal regime, the switching arc behaves less vehemently, interrupting and re-igniting basically occur at every power frequency current zero. Because of the presence of sufficient thermal energy in the switching gap along the arc path, the voltage to re-ignite the arc is limited, and the arc-circuit interaction is less pronounced. Though not producing very severe overvoltages, the arc duration is longer and the current may not be interrupted at every current zero crossing. The ultimate thermal regime is reached when the arc continues to exist after power frequency current zero without any appreciable voltage to re-ignite. This situation must be avoided because arcing goes on until a higher level breaker interrupts the current. Before this, the arc can reach far away from its roots and can greatly reduce insulation clearance. The main factors influencing the interruption performance are the level of current to be interrupted, the system voltage, the ratio of capacitances at both sides of the disconnector and the gap length. These factors influence the energy supplied to the arc upon re-strike. This energy extends the arcing time by lowering the breakdown voltage. It has been observed that the arc in its thermal mode always re-ignites in its former trajectory. Key to the interruption process is the reduction of breakdown voltage in this path, created by hot gases remaining from the former arc. The existing breakdown models are reviewed in order to understand the influence of high temperature air on the breakdown process. Based on the observed arc behaviour, various methods have been researched to increase the interruption capability. The most successful methods are those that remove the residual (partially) ionized air from the arc path. Experiments were carried out to demonstrate the effectiveness of air flow directed into the arc’s foot point. A substantial gain in interruption capability is demonstrated, but at the cost of generating re-ignition transients at a very rapid succession. Specifically, the experiments showed that 7.5 A could be interrupted successfully at 90 kVrms voltage with a shorter arcing duration (a factor of 0.5 was observed) than without air flow. With application of air flow, the frequency of re-ignitions occurring, and the breakdown voltage are much higher than without air flow. Another method, the assistance of an auxiliary switch able to produce a very fast opening, was also successful. Herein, the arc is forced mechanically into ambient cool air, thus avoiding accumulation of thermal energy in the arc path. Specifically, it can interrupt currents up to 7 A at 100 kVrms safely and 9 A at 90 kVrms in the experiments with arcing time only a few tens of milliseconds instead of a few seconds. The arc exhibits a "stiff" (linear) character instead of the "erratic" (randomly moving) arc mode with a disconnector alone. This method reduces the number of re-strikes. The possible influence of energy absorbing elements (resistors) is investigated through circuit modelling, supported by some laboratory experiments. Other methods, such as the application of series auxiliary interrupting elements (vacuum, SF6 interrupter and ablation assisted approaches) have been evaluated. From the practical point of view, the auxiliary fast-opening interrupter is recommended due to its economic, simple and effective merits. Other approaches have certain disadvantages. The method with air flow needs a complex construction in order to introduce the compressed air flow into the disconnector, and the hazard for nearby equipments from the overvoltages caused by the interruption is greater. The method of inserted resistor requires very expensive arrangement. Regarding the application of auxiliary interrupters, vacuum interrupters have to be applied in considerable numbers in series and SF6 interrupters have good performance but at very high cost. An ablation assisted approach seems less promising because the level of the interrupted current is too low to be effective.
|Qualification||Doctor of Philosophy|
|Award date||14 Mar 2012|
|Place of Publication||Eindhoven|
|Publication status||Published - 2012|