Overshoot at Lines with High Resistive or Capacitive Termination and Fast Rising Clamped or Crowbarred Input Voltage

Abstract:
An overshooting voltage can occur at the end or termination of a low-voltage cable, even though an overvoltage protection device is operating at the cable beginning. The effect is caused by amplification of travelling waves that occur during the propagation of rapidly increasing impulse voltages fed in at the cable beginning. For this purpose, a fundamental linetheoretical investigation with an analytical description of the travelling wave and oscillation processes is carried out. Due to reflections at the open-circuited termination of the line, there is in maximum doubling for a ramp-step or triangular or trapezoidal with one half-wave fed-in voltage, and a maximum quadrupling for a trapezoidal fed-in voltage with two half-waves of opposite polarity, whereby there is significant dependency on the ratio of travel time (time of flight ∼ line length) to front time of the injected pulse. A capacitance as termination at the line end can cause both voltage reduction or voltage increase depending on the ratio of time constant (capacitance times characteristic line impedance) to front time. If travelling wave reflections and the oscillation between termination capacitance and line inductance interfere unfavorably, the voltage at line end can overshoot by up to 5 times the voltage amplitude in case of a two half-wave trapezoidal fed-in voltage.