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Surge impedance, also referred to as characteristic impedance, is a term utilized in the analysis of the electrical transmission lines.

It is defined as the voltage/current ratio of a moving wave on a line. Surge impedance is especially relevant in the presence of high-frequency signals (or) transient phenomena like as lightning strikes or switching surges.

The formula for surge impedance (Z0)

(Z0) = âˆš(L/C)

Where,

L – Inductance/Length &

C â€“ Capacitance/Length

of transmission line.

Thus, from the given formula, we clearly observe two key conclusions.

• The surge impedance of a transmission line is determined by its inductance and capacitance.
• The surge impedance is independent of transmission line’s length.

In this post, we will explore the concept of surge impedance loading.

• Pure inductors have infinite surge impedance (Z0=âˆž) since their capacitance is zero.
• A pure capacitor has no surge impedance (Z0=0) since its inductance is zero.
• Transformers typically have a surge impedance of around (Z0=5000Î©).

Surge impedance loading (SIL) can be described as the amount of power delivered when the load connected to transmission line equals the surge impedance.

Where,

Vs = source voltage (or) operating voltage, and

Z0 = surge impedance.

In other terms, SIL is the megawatt power delivered by a load with an impedance equal to the transmission line’s surge impedance (Z0).

So we can express it as,

SIL = 3 Vph IphcosÏ†

The load is resistive, therefore cosÏ†=1

IPh = Vph/R = Vph/Z0 = VL/âˆš3 Z0

SIL = 3Ã—VL/âˆš3Ã—VL//âˆš3Z0 = VL2/Z0

SIL = VR2/Z0= VS2/Z0=VS2âˆšC/L

Where

VS and VR represent line voltage

C & L represent phase value

Surge impedance loading (SIL) is proportional to the square of source (or) line voltage.

SIL âˆž VS2

SIL is proportional to square root of capacitance.

SIL âˆž âˆšC

SIL is inversely proportional to square root of inductance.

SIL âˆž 1/âˆšC

The different methods to reduce L include:

• By utilizing parallel lines,
• Utilizing series capacitance,
• Using bundled conductor.

It is square root of the ratio of the series impedance to the shunt admittance. It indicates that characteristic is the transmission line’s impedance at any given point.

Zc = âˆšZ/Y

Where

Zc – Characteristic Impedance

Z – Series Impedance