How to Calculate Surge Impedance Loading

Surge Impedance Loading or SIL for short, has a quite confusing definition.

We can define the Surge Impedance Loading (SIL) as:

  • A quantity in a transmission line to determine the injected reactive power (VAR) by its capacitance to the system and the consumed VAR by its inductance. SIL is measured in real power (Watt).
  • Amount of real power in the transmission line when generated or absorbed VAR doesn’t exist.
  • The load power when the reactive power in the transmission line is zero.
  • The condition where there are no losses in the transmission line or capacitance and inductance is negligible.

There is no way to understand what surge impedance loading exactly is only from the list of explanations above. To fully understand it, we should learn about what a surge impedance is first.

What is a Surge Impedance

In the field, there will be voltage drops in the transmission line when the power is transmitted from one to another spot. This voltage drop is caused by the inductance in the transmission line.

The capacitance in the transmission line can compensate for the voltage drop caused by the inductance.

Since the capacitance always exists in the transmission line, it is an important parameter along with its generated reactive voltage.

Observe the illustration below.

surge impedance loading 1

From the illustration above, both inductances and capacitances are the irreplaceable things in a transmission line.

When the capacitive reactive volt-amperes is generated by the shunt capacitance, the reactive volt-amperes in the series inductance will be absorbed by the transmission line.

The surge impedance load is the load that exists when both inductive and reactive volt-amperes have equal value and opposite of each other.

Surge impedance load is also known as natural load because there is no dissipated power in the transmission line.

So what is the surge impedance loading?

Surge impedance loading is when the voltage and current have the same phase at all points in the line. Surge impedance loading is also when the surge impedance generated in the line terminates the power delivered by it.

Below is the illustration of shunt capacitance when the start of the line is closed while the end of the line is open.

surge impedance loading 2

The reactive power (VAR) generated by the shunt capacitance is


When both start and end of the line is closed, the series inductance generated by the line will consume the electrical energy

surge impedance loading 3

The absorbed reactive power (VAR) is


Since there is a surge impedance load, the reactive power is terminated and the load is purely resistive.

The surge impedance load is calculated from

Surge Impedance Loading Summary

We can conclude that a surge impedance loading or natural load is an ideal load where both voltage and current are in phase since the reactive power is terminated (absorbed).

Surge impedance load has dependence on the voltage transmitted in the line. The value of surge impedance load is always less than the maximum loading capacity of the transmission line.

Leave a Comment