WEEK 8

WEEK 8

Activity : 
Research on applications of Marx Generator in various industry.

Objective : 

• To wider knowledge on applications of Marx Generator.
• to get the idea of future development of Marx Generator in industries.

Content:

    

1           BOX SWITCHING OPERATION

Box switching is one of the application of Marx generator and it is also called as a boxcar switching of a pockels cell. Four Marx generators are used in this application, two electrodes of the Pockels cells being connected to a positive pulse generator and another one is a negative pulse Marx generator. Two generators are having opposite polarity, in that one of the electrode is first fired to charge the Pockels cell into one charging polarity. These techniques are  used to charge the two generators, but not trigger them to the pulses of each generator, Because they needed only partly charged and leak through the Marx resistors which needs to be compensated by a small bias current through the generator. At the trailing edge of the boxcar, the two other generators are fired to “reverse” the cell.

Marx generators are used to provide high voltage pulses for the testing of insulation of an electrical apparatus such as large power transformers or Insulators used for supporting power transmission lines. The voltages applied may exceed two million volts for high-voltage apparatus of the Marx generators.

2           OPERATION OF LOW JITTER FOR PULSE SYNCHRONIZATION

The low temporal jitter of the impulse Marx generator may be required for multi source applications or timing applications. Marx generator is a variable candidate for phased array systems, low jitter must be reduced to a small fraction of  pulse width of 200 ns to the 1ns impulse signal. The main function of this low jitter to be achieved for multiple pulse addition circuits such as Gatling systems and bi static radar systems applications.

Based on the closure of the trigger switch, the reflected pulse of -1/2 the spark gap-charge voltage and a length is twice of the charged transmission line propagates  toward the spark gap, as shown in Figure. Arriving at the end of the trigger pin, the pulse doubles in magnitude, resulting in a potential of minus one-half the charge voltage. This results in a highly distorted field between the electrodes due to the presence of the sharp pin at the negative potential.

The low jitter performance is measured using the three Marx generator system is described. Each Marx generator is connected to a common trigger circuit, This is a krytron-based circuit designed to hold off 15KW. These Marx generator is normally connected to a single output transmission line. We connect temporally because we have to separate the individual pulses, The trigger lines are  connected to each Marx generator to the trigger circuit that are unique in the  same length.

   

3           GENERATION of UWB SIGNALS

The impulse Marx generator was tested for its ability to directly generate microwave energy in the form of an UWB signal. The Marx generator directly drives a rudimentary TEM horn antenna signal. Generally it measures 100meters from the source, For example an EMCO 3106 antenna was used for radiate measurements. Additional measurements were made with a crystal detector.

                                                 Generation of UWB Signals.

4           SINGLE PULSE SYSTEM WITH BWO

In this application we are using a narrow band Marx  generator, used to  drive the cathode of a Russian made BWO. It was measured in Texas tech university. The max generator directly groove the cathode and was temporarily aligned with the magnetic field pulse. An Uncalibrated,   b-dot probe was used to monitor the generated signal as well as the Uncalibrated integrated B-dot probe is used to monitor the generated signal, then as well as a florescent witness plate.

Now the BWO  was designed to deliver a 35 Hz  signal within a 3-4ns. The system is delivered to appear a 20ns window of microwave energy that was approximated to be 30 MW in peak power. This power delivers at TM0 mode of the BWO.  The BWO delivered a TM01 mode.

The three Marx generator is orthogonally connected to the common transition line, And for the purpose of demonstrating the generator have a common trigger system. A capacitive voltage probe is placed at the trigger input of Marx generator, and a current viewing resistor is placed at the output of a common transmission line system. So each of the generator will be designed to deliver a 140 KW pulse with sub Nano sec  rise time.

The output of the current viewing resistor demonstrates the system’s ability to generate three district high voltage pulse, each having an amplitude excess of 125kv range.

The Gatling Marx generator system was connected to the TEM horn used to earlier  in this Marx generated system. Now we have to take precautions were taken to ensure the antenna did not break down during the first pulse, so later pulse would be radiated.

5       LIGHTNING IMPULSE TESTING ON POWER TRANSFORMER

Lighting is a common phenomenon in transmission lines because of their tall height. This lightning stroke on the line conductor causes impulse voltage. The terminal equipment of transmission line such as power transformer then experiences this lightning impulse voltages. Again during all kind of online switching operation in the system, there will be switching impulses occur in the network. The magnitude of the switching impulses may be about 3.5 times the system voltage. Insulation is one of the most important constituents of a transformer. Any weakness in the insulation may cause failure of transformer. To ensure the effectiveness of the insulation system of a transformer, it must confirms the dielectric test. But the power frequency withstand test alone cannot be adequate to demonstrate the dielectric strength of a transformer. That is why impulse test of transformer performed on it. Both lightning impulse test and switching impulse test are included in this category of testing.

The lightning impulse is a pure natural phenomenon. So it is very difficult to predict the actual wave shape of an lightning disturbance. From the data compiled about natural lightning, it may be concluded that the system disturbance due to natural lightning stroke, can be represented by three basic wave shapes.

i) Full wave                                    

 ii) Chopped wave

 iii) Front of wave

Although the actual lightning impulse disturbance may not have exactly these three shapes but by defining these waves one can establish a minimum impulse dielectric strength of a transformer. If lighting disturbance travels some distance along the transmission line before it reaches the transformer, its wave shape may approach to full wave. If during traveling, if flash-over occurs at any insulator of the transmission line, after the peak of the wave has been reached, the wave may become in form of chopped wave. If the lightning stroke directly hits the transformer terminals, the impulse voltage rises rapidly until it is relieved by a flash over. At the instant of flash - over the voltage suddenly collapses and may form the front of wave shape.
The effect of these wave forms on the transformer insulation may be different from each other. We are not going here in detail discussion of what type of impulse voltage wave forms causes what type of failure in transformer. But whatever may be the shape of lightning disturbance voltage wave, all of them can cause insulation failure in transformer. So lighting impulse test of transformer is one of the most important type test of transformer.

6       AIRCRAFT LIGHTNING PROTECTION

Aircraft certification and flight safety authorities around the world require that aircraft structures and systems that are critical or essential to the safe flight of an aircraft be protected from significant lightning-induced damage or upset. These requirements are fulfilled through a certification plan that details the lightning protection engineering and shows  through verification testing so that the engineering meets all applicable standards and regulations.

Lightning can affect avionics systems in two ways: directly, causing physical damage due to heat or shock, and indirectly, as a result of earth voltage rises occurring in the aftermath of a strike. Proper testing to ensure lightning strike protection must take into account both factors.

• Direct effects —  Direct testing equipment consists of high voltage Marx-type impulse generators that are capable of producing up to 1.5 million volts, and high-current generators specially outfitted to produce over 250,000 amperes of current.
• Indirect effects —Earth voltage rises can cause equipment failure in devices up to a kilometer away from the impact site. For this reason, it is essential to test sensitive electrical and electronic components against the indirect effects of a lightning strike. 

CONCLUSION
 For this week,I have studied about the applications of Marx Generator in various industries. This is important for me to get the idea of added value to my project. I am also looking forward to enhance this project for future development.