Tripping and Control of Impulse Generators

Tripping and Control of Impulse Generators

Q. With a neat sketch explain trigatron spark gap used in impulse generators.

Figure A

Tripping :

• The gaps are stanged such that the parking of one pap results in automatic sparking of other gaps as overvoltage is impressed on the other. In order to have consistency in sparking irradiation in the form of U-V lamp is provided at the bottom of all the gaps.

Control of tripping :

• The control over tripping enables us to trip the generate at a predetermined time. One way of doing this is by mounting the spark taps on a movable frame and by reducing the gap distance by moving the electrodes closer.
• This method is however difficult and does not assure consistent and controlled tripping.
• The other method is by use of three electrode pap as shown in Figure C. In this, the switch S is closed which initiates the sweep circuit of the oscillograph.
• The same impulse is applied to the grid of the thyratron tube. The inherent time delay of the thyratron ensures that the sweep circuit begins to operate before the start of the high voltage impulse.
• A further delay can be introduced if required by means of a capacitance – resistance circuit R₁ C₁
•  The tripping impulse is applied through the capacitor C₂. During the charging period of the generator the anode of the thyratron tube is held at a positive potential of about 20 kV.
• The grid is held at negative potential with the help of B battery so that it does not conduct during charging period.
• As the switch S is closed, the trigger pulse is applied to the grid of the thyratron tube which conducts and negative impulse of 20 kV is applied to the central sphere which triggers the impulse generator.

Figure B

• The three electrode gap requires larger space and elaborate construction.
• The trigatron in comparison requires much smaller gap.

It consists of :

1. High voltage spherical electrode of suitable size
2. An earthed main electrode of spherical shape
3. A trigger electrode through the main electrode

• When a tripping pulse is applied to the rod, the field is distorted in the main gap and the latter breaks down at a voltage appreciably lower than that required to cause its breakdown in the absence of the tripping pulse.

Figure C

• Figure C shows a typical tripping circuit for a trigatron. The capacitor C₁ is charged through a high resistance R. As the remotely controlled switch S is closed, a pulse is applied to the sweep circuit of the oscillograph through the capacitor C₃. 
• At the same time the capacitor C₂ is charged up and a triggering pulse is applied to the trigger electrode of the trigatron.
• The requisite delay in triggering the generator can be provided by suitably adjusting the values of R₂  and C₂. The residual charge on C can be discharged through a high resistance R_3.

Figure D

• These days lasers are also used for tripping the spark.
• The trigatron also has a phase shifting circuit associated with it so as to synchronize the initiation time with an external alternating voltage.
• The trigatron is designed so as to prevent the overcharging of the impulse capacitor in case of an accidental failure of triggering.

Figure E

Figure F

• The students can get an idea of the different methods of generation of high voltages at a glance with the help of tece chart shown in Figure G.

Figure G