What is impedance?

Impedance is the total opposition to current flow. It includes both resistance and reactance (capacitive and inductive). Impedance varies with frequency, while plain resistance does not.

The symbol for impedance is usually Z and it may be represented by writing its magnitude and phase in the form

The term impedance was coined by Oliver Heaviside in July 1886.

Impedance is defined as the frequency domain ratio of the voltage to the current. In other words, it is voltage–current ratio for a single complex exponential at a particular frequency ?. In general, impedance will be a complex number, but this complex number has the same units as resistance, for which the SI unit is the ohm.

The reciprocal of impedance is admittance (i.e., admittance is the current-to-voltage ratio, and it conventionally carries mho or Siemens units).

Impedance is written as Z=R+jX

Where R = Resistance(Real part) & X= Reactance(img. Part)

What is reactance?

Reactance is a measure of the combined effects of capacitance and inductance on an alternating current.  The amount of such opposition varies with the frequency of the current.  The reactance of a capacitor decreases with an increase in frequency; the opposite occurs with an inductance.

Here

Reactance is of two types:

1.      Capacitive reactance:

Capacitors themselves offer a very real opposition to current flow.at a given voltage and frequency, the number of electrons which go back and forth from plate to plate is limited by the storage ability-that is, the capacitance-of the capacitor. As the capacitance is increased, a greater number of electrons change plates every cycle, and (since current is a measure of the number of electrons passing a given point in a given time) the current is increased. Increasing the frequency will also decrease the opposition offered by a capacitor. This occurs because the number of electrons which the capacitor is capable of handling at a given voltage will change plates more often. As a result, more electrons will pass a given point in a given time (greater current flow). The opposition which a capacitor offers to ac is therefore inversely proportional to frequency and to capacitance. This opposition is called CAPACITIVE REACTANCE.

2.      Inductive reactance:

When the current flowing through an inductor continuously reverses itself, as in the case of an ac source, the inertia effect of the cemf is greater than with dc. The greater the amount of inductance (L), the greater the opposition from this inertia effect. Also, the faster the reversal of current, the greater this inertial opposition. This opposing force which an inductor presents to the FLOW of alternating current cannot be called resistance, since it is not the result of friction within a conductor. The name given to it is INDUCTIVE REACTANCE because it is the “reaction” of the inductor to the changing value of alternating current. Inductive reactance is measured in ohms and its symbol is X_L.

What is surge?

A short-duration (microsecond to millisecond) increase in power line voltage.

What is surge impedance?

The ratio of voltage to current on that line for a high speed wave propagating down the line. The surge impedance of a line is a constant which depends on the line geometry and conductor characteristics. On power transmission lines, these waves are typically generated by lightning strokes, circuit breaker switching, etc.

Surge impedance also known as Characteristics impedance

What is surge impedance loading (SIL)?

Of a transmission line, the characteristic impedance with resistance set to zero (resistance is assumed small compared to reactance). The power that flows in a lossless transmission line terminated in a resistive load equal to the linear surge impedance is denoted as the surge impedance loading of the line.

What is surge response voltage?

The voltage that appears at the output terminals of surge protection equipment and is seen by loads connected to that device both during and after a surge condition.