Surge Impedance Loading or SIL

Surge Impedance Loading is a key parameter for analyzing power systems because it predicts the maximum capacity of transmission lines.

Before exploring SIL, it’s important to understand Surge Impedance (Z_s), which can be explained in two ways: a basic and a more detailed approach.
Method 1
Long transmission lines (> 250 km) inherently possess distributed inductance and capacitance. When activated, the capacitance feeds reactive power into the line, and the inductance absorbs it.

Now if we take the balance of the two reactive powers we arrive at the following equation

Capacitive VAR = Inductive VAR

Where,
V = Phase voltage
I = Line Current
X_c = Capacitive reactance per phase
X_L = Inductive reactance per phase
Upon simplifying

Where,
f = Frequency of the system
L = Inductance per unit length of the line
l = Length of the line
Hence we get,

This quantity having the dimensions of resistance is the Surge Impedance. It can be considered as a purely resistive load which when connected at the receiving end of the line, the reactive power generated by capacitive reactance will be completely absorbed by inductive reactance of the line.
It is nothing but the Characteristic Impedance (Z_c) of a lossless line.

Method 2
A detailed analysis of long transmission line provides an equation that models the voltage and current at any point, x, from the receiving end.

Where,
V_x and I_x = Voltage and Current at point x
V_R and I_R = Voltage and Current at receiving end
Z_c = Characteristic Impedance
δ = Propagation Constant

Z = Series impedance per unit length per phase
Y = Shunt admittance per unit length per phase
Putting the value of δ in above equation of voltage we get

Where,

We observe that the instantaneous voltage consists of two terms each of which is a function of time and distance. Thus they represent two travelling waves. The first one is the positive exponential part representing a wave travelling towards receiving end and is hence called the incident wave. While the other part with negative exponential represents the reflected wave. At any point along the line, the voltage is the sum of both the waves. The same is true for current waves also.
Now, if suppose the load impedance (Z_L) is chosen such that Z_L = Z_c, and we know

Thus

and hence the reflected wave vanishes. Such a line is termed as infinite line. It appears to the source that the line has no end because it receives no reflected wave.
Hence, such an impedance which renders the line as infinite line is known as surge impedance.It has a value of about 400 ohms and phase angle varying from 0 to –15 degree for overhead lines and around 40 ohms for underground cables.

The term surge impedance is however used in connection with surges on the transmission line which may be due to lightning or switching, where the line losses can be neglected such that

Now that we have understood Surge Impedance, we can easily define Surge Impedance Loading.
SIL is defined as the power delivered by a line to a purely resistive load equal in value to the surge impedance of that line. Hence we can write

The unit of SIL is Watt or MW.
When the line is terminated by surge impedance the receiving end voltage is equal to the sending end voltage and this case is called flat voltage profile. The following figure shows the voltage profile for different loading cases.

It should also be noted that surge impedance and hence SIL is independent of the length of the line. The value of surge impedance will be the same at all the points on the line and hence the voltage.
In case of a Compensated Line, the value of surge impedance will be modified accordingly as

Where, K_se = % of series capacitive compensation by C_se

K_Csh = % of Shunt capacitive compensation by C_sh

Kl_sh = % of shunt inductive compensation by L_sh

The equation for SIL will now use the modified Z_s.