Module 5 : Generation of High Test Voltages
Lecture 29 : Triggering methods of Impulse Generator

In a multistage impulse generator; it is desired that triggering or flashover takes place first at the stage No. 1 and then in sequence at the higher number stages. Hence, the gap distance setting requirement is such that the gap distance set at stage one is smallest and the gap distances in higher stages are in increasing order by a small difference.

There are three ways of triggering an impulse generator;

  1. Fix the gap distance between the spheres and increase the stage applied dc voltage till the flashover occurs.
  2. Set the gap distance between the spheres large enough, apply a desired voltage across them and then reduce the gap distance till flashover takes place.
  3. Fix both, the desired stage voltage and corresponding gap distance within prescribed limits (Fig. 29.3). Then apply the trigger pulse to the trigatron on the first stage.

The third method is widely practiced since this is the only method to get impulses of exact desired magnitude repeatedly, Fig. 29.2.

 

A device, known as "Trigatron", is used to control the flash over at the spark gaps in order to get a desired magnitude of the output voltage repeatedly.
  • "Trigatron", consists essentially of three-electrodes.
  • The main electrode - indicated as HV and the earthed electrode are equal size spherical electrodes in the first stage of the generator.
  • A small hole, drilled in the earthed electrode, holds a metal rod through it inside with the help of a bushing. The annular gap formed between the rod and the surrounding sphere is typically about 1 mm.
  • The metal rod, or the trigger electrode forms third electrode, is essentially at the same potential as the sphere since it is connected to it over a high resistance R.  The control or trigger pulse is applied between these two electrodes on the rod.
  • For this special arrangement, a glass tube is fitted over the rod and it is surrounded by a metal foil connected to the main electrode at the ground potential.
  • The function of this glass tube is to produce corona or surface discharges around the rod. The surface discharge or corona causes photoionization in the pilot gap on applying an impulse voltage to the rod. Due to photoionization enough initiatory electrons are made available in the annular gap which breaksdown without appreciable  time delay  consistantly at a given voltage across the gap 'd'.

FIG.  29.2  The Trigatron spark gap.

 
  • Thus a surface discharge is caused by the triggering pulse.
  • If a voltage U is applied across the main gap, which is not too low but always lower than the peak voltage at which self-firing-i.e. the breakdown in the absence of any trigger pulse-occurs, this main gap will breakdown at a voltage even appreciably lower than the self-firing voltage, if a tripping pulse is applied.
  • The Trigatron requires a pulse of some kilovolts, typically ≤ 10 kV, and the tripping pulse should have a steep front with steepness 0.5 kV/nsec to keep the jitter of the breakdown as small as possible.
  • The essential operating characteristic of a Trigatron refers to the voltage operating limits at which a steady operation or triggering is possible. Such characteristics are sketched in Fig. 29.23 where the operating voltage U, the voltage limits across the main gap, are shown for increasing gap distance d.

FIG. 29.3 Schematic of functional characterstics of a Trigatron
 
1