what's the difference between wave impedence and surge impedence of the conductor?

[jyj982001]

what's the difference of wave impedence and surge impedence of the conductor?
when in lightning protection calculation,there will be surge impedence of the conductor involved. But could anyone tell me what's the difference between wave impedence and surge impedence of the conductor?

 

[Skogsgurra]

Considering that the frequency components involved in a surge (Dirac, if extreme) are the same as in most HF applications, I have always considered wave and surge impedance to be one and the same thing. But there may also be a difference based on the sometimes much higher energies that can be found in some surges and that may cause saturation and other nonlinear phenomena.

But then, the impedance isn't linear any more and should not be treated as one. Someone with more specialized knowledge may have a better answer. What is the application?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

A suggestion, Gunnar;
Wave impedance; The impedance at the normal operating frequency, eg; 50 Hz or 60 Hz.
Surge impedance; The impedance at the eqivalent frequency of a fast rising waveform, such as the leading edge of a lightning strike.
Comments??

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

Dictionary of Engineering definition:
Surge Impedance
The impedance of a circuit, which when connected to the output terminals of a uniform transmission line, makes the line appear infinitely long. Under these conditions, the transmission line has no standing waves, and the ratio of voltage to current at any given frequency is the same at any point of the line. Its symbol is Z0. Also called characteristic impedance. Zo=V/I=SQRT(L/C)
ITS Institute of Telecommunication Sciences definition:
wave impedance: At a point in an electromagnetic wave, the ratio of the electric field strength to the magnetic field strength. Note 1: If the electric field strength is expressed in volts per meter and the magnetic field strength is expressed in ampere-turns per meter, the wave impedance will have the units of ohms. The wave impedance, Z , of an electromagnetic wave is given by
Z=SQRT(miu/eps)
where miu is the magnetic permeability and eps is the electric permittivity. For free space, these values are 4 × 10-7 H/m (henries per meter) and (1/36 ) F/m (farads per meter), from which 120*pi , i.e., 377, ohms is obtained. In dielectric materials, the waveimpedance is 377/n, where n is the refractive index. Note 2: Although the ratio is called the wave impedance, it is also the impedance of the free space or the material medium.

 

[Skogsgurra]

No, Bill. Wave impedance is the transmission line impedance. Typical values are fifty to a few hundred ohms and irrespective of "cable" length. If you measure the 50 or 60 Hz or DC resistance, you will typically see milliohms. The definition is sqrt(L/C) and that should not be any different for an impulse, a surge or an HF sinewave.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Thanks Gunnar.
Got it now.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FPelec]

The difference is mainly in the propagation modes involved in the considered phenomena:
Characteristic (or surge) impedance of power line refers to power frequency (50 or 60 Hz) direct-sequence phenomena, so that the L and C values that are used for evaluating the surge impedance are the positive sequence ones.
Wave impedance generally refers to electromagnetic wave propagation rather than circuit analysis. However when this term is used in transmission line impedance, it usually refers to the ratio between voltage and current involved in a fast (lightning or switching) transient. As such, its value can significantly differs from the surge impedance one, because the involved propagation mode is different.
As an example, for a 400 kV transmission line the surge (i.e. characteristic) impedance is about 265 Ohm, whereas the wave impedance (for a conductor of the same line) for lightning overvoltage study is about 350 - 400 Ohm, depending on its height above ground.


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