Impulse current

In comparison to ordinary dielectrics, the permittivities of the so-called ferroelectric materials are about 103 times larger. The ferroelectric material can be transformed into a new type of material called piezoelectric material by heating the ferroelectric above its Curie temperature and then cooling it in a powerful electric field. A piezoelectric crystal changes its polarization once subjected to a mechanical strain. As a result, it can deform mechanically under an electric field or produce electric impulses as a result of mechanical impulses. Currently, piezoelectric materials are widely used as force or pressure transducers with fast response times and very sensitive output. Permittivities of common dielectric and ferroelectric materials are given in Table 1.9. 

Inexpensive silicon diodes or photocells are often used in lower priced instruments. These consist of a chip of pure silicon doped with a specific element in which a photon striking it causes an electric impulse (current), which is amplified for readout. Diodes are prepared to be sensitive to specific colors of light. 

Impulse current A current that rises rapidly to a peak then decays to zero without oscillating. 

The measurement and registration of electrical impulse currents or voltages require that the electrical circuits employed do not alter the temporal or amplitudinal shape of the impulse. As long as the duration of the impulse exceeds about 0.1 (is the measurement circuit can be considered as consisting of discrete elements (resistors, inductances, capacitors). For the measurement of shorter impulses, the circuit has to be treated as consisting of distributed elements. 

A voltage waveform with 1.2/50 ps (wave front Tf= 1.2 ps, wave tail = 50 ps) and a current waveform with 8/20 ps are used as lightning impulses to test the withstand voltage of home appliances or electronic equipment in general. The impulse voltage is mainly used for high-impedance equipment, and its maximum value is taken as 6 kV. On the other hand, the impulse current is mainly for low-impedance equipment, and its peak value is taken as 3 kA [18,19]. 

An IG is the impulse current source used in the experiment. It is represented by a charged capacitance and a resistance [25,26]. is the mutual grounding impedance between various grounding electrodes, and a value of 2-10 Q is used [24,30]. 

Analytical Formula of Electrode Voltage 1. Impulse current application... 

An electrode voltage is derived when the following impulse current is applied ... 

The second term of Equation 7.26 is produced by inductance L of the circuit and corresponds to Equation 7.24 when an impulse current is applied. The maximum voltage of Vi(t) appears theoretically at t = 0, as is clear from Equations 7.24, 7.25 and 7.26. hi the circuit of Figure 7.26a with a distributed-parameter line, a voltage given by is... 

Figure 7.30 illustrates an experimental circuit with electrode length x = 8 m, radius r = 1 mm, and buried depth h = 0.22 m. The soil resistivity was measured to be 140 Qm. An impulse current with the amplitude Iq = 1 A and the wavefront duration Tf = 20 ns is applied. To make an FDTD simulation easy, the same conductor as the electrode is used as a lead wire with length x. Figure 7.31 shows a comparison of the simulation results for x = 0.22 m with a measured result. It should be clear that the FDTD simulation gives a satisfactory accuracy. 

Xuetong Z, Jian3rin L, Fluan L, Shengtao L. The impulse current degradation of ZnO varistor ceramics. Proceedings of 2011 International Conference on Electrical Insulating Materials (ISEIM),2011 no 43-46. DOI 10.1109/ISEIM.2011.6826272... 

The equation system of eq.(6) can be used to find the input signal (for example a crack) corresponding to a measured output and a known impulse response of a system as well. This way gives a possibility to solve different inverse problems of the non-destructive eddy-current testing. Further developments will be shown the solving of eq.(6) by special numerical operations, like Gauss-Seidel-Method [4]. 

Methods from the theory of LTI-systems are practicable for eddy-current material testing problems. The special role of the impulse response as a characteristic function of the system sensor-material is presented in the theory and for several examples. 

So, a comparison of different types of magnetic field sensors is possible by using the impulse response function. High amplitude and small width of this bell-formed function represent a high local resolution and a high signal-to-noise-characteristic of a sensor system. On the other hand the impulse response can be used for calculation of an unknown output. In a next step it will be shown a solution of an inverse eddy-current testing problem. 

Due to its importance the impulse-pulse response function could be named. .contrast function". A similar function called Green s function is well known from the linear boundary value problems. The signal theory, applied for LLI-systems, gives a strong possibility for the comparison of different magnet field sensor systems and for solutions of inverse 2D- and 3D-eddy-current problems. 

The EU contains capacitor battery (CB), current source (CS), accumulator (Ac), controlled by CS, impulse former (IF),power supply (PS), two comparators (Cpl,Cp2),... 

The operator sets TU on the area to be controlled and puts the button B Under the command of SC EDC impulse of proper value and duration is formed in source controllable current (SCC). 

As a result by using the energy accumulated in CB, current impulse of the value of 0.2...0.6 A and the duration of 10...200 ms is created, and this impulse goes through the area being controlled. 

Dispersion Model An impulse input to a stream flowing through a vessel may spread axially because of a combination of molecular diffusion and eddy currents that together are called dispersion. Mathematically, the process can be represented by Fick s equation with a dispersion coefficient replacing the diffusion coefficient. The dispersion coefficient is associated with a linear dimension L and a linear velocity in the Peclet number, Pe = uL/D. In plug flow, = 0 and Pe oq and in a CSTR, oa and Pe = 0. 

An impulse is an intentionally applied voltage or current in a laboratory. It is in the form of an aperiodic and unidirectional waveform (Figure 17.2). It rises rapidly without appreciable oscillations to a maximum value and then falls, usually less rapidly, to zero, with small, if any, loops of opposite polarity (Figure 17.4). The parameters... 

Figure 17.3 Defining a voltage or a current impulse waveform...

For instance, when lightning of, say, a nominal discharge current of 10 kA strikes a 400 kV (r.m.s.) overhead line, having a surge impedance of 350 Q, then two parallel waves will be produced each of amplitude 10 x 350/2 or 1750 kV which may be more than the impulse withstand level of the system and cause a flashover between the conductors and the ground, besides damaging the line insulators and the terminal equipment (Table 13.2). It is therefore imperative that the system is protected against such eventualities. 

Figure 18.6 Arrester voltage and current oscillograms for 10 kA, 8/20 /iS current impulse lest...

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