|Jochen's High Voltage Page|
I used this experimental setup during a lecture for high school students. It demonstrates how a pair of horn-shaped electrodes can be used to protect power lines from overvoltage.
Experimental setup for demonstrating the horn-shaped lightning arrestor. The equipment on the left table simulates a thunderstorm - a flyback feeds a 100kV cascade, which charges a simple plate capacitor. The "hot" plate of this capacitor (the hot-water bottle) acts as the cloud. The right table carries the power source ("potential transformer" inverted for 20kV at high current), the horn-shaped electrodes and the simulated power line (horizontal rod supported by insulators).Click on image for a larger version with labels.
At the beginning of the experiment, the "power line" is energized to 20kVp. (The switching itself may cause an overvoltage and trigger the lighting arrestor!). Then the flyback is activated, and the "thunderstorm" begins to form. When the "cloud" is charged to 100kV, it sparks to the energized "power line". For a short moment, the voltage across the horn-shaped electrodes is up to 120kV instead of just 20kVp. As a result, an arc forms at the point of smallest seperation. The "cloud" is quickly discharged, but the power line source continues to feed the arc (see Jacobs ladder for images). While this arc is burning, the power line voltage is much lower than usual, as it is nearly short-circuited. But the arc rises, finally breaks up and the "power line" returns to its normal state of operation, until the next lightning stroke.
It is clear that any spark gap can be used for overvoltage protection: it is adjusted so that break-through does not occur at normal operating voltage. If a higher voltage spike, e.g. as a result of lightning, arrives, the gap ignites and the resulting arc clamps the voltage to a relatively low level.
The problem is how to get the gap non-conducting again ("to quench the gap"). If the available power, and/or the normal operating voltage, is too low to sustain an arc, this happens automatically. With power lines however, this is not usually the case. Even 220V (or 110V) lines have enough power and voltageb to form an arc. What happens then is usually that some fuse blows and interrupts the current. (This leads to the next problem, namely how to interrupt high currents without another arc forming...). Besides of the practical problems of designing fuses for high voltage power lines, you may want to avoid the necessity to manually exchange a fuse every time an overvoltage occurs somewhere on a large power grid.
The classical solution to this problem, overvoltage protection with automatic return to normal, is the horn-shaped electrode pair. The advantage of this construction over, say, a pair of spherical electrodes (which would be just as good for the overvoltage part), is that the arc is not stable. Thermal buoyancy (and magnetic forces) drive it upwards, and force it to become longer and longer until the line power and voltage cannot sustain it any more, and that's it. Of course, if the electrodes are not properly designed for the line power, it may happen that even at the top end of the electrodes the arc is sustained - but this can be avoided by making the the electrodes and their seperation at the top large enough.
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