Capacitance and reactance are the main parameters of the transmission line. It is distributed uniformly along the line. These parameters are also called distributed parameters. When the voltage drops occur in transmission line due to inductance, it is compensated by the capacitance of the transmission line.
The transmission line generates capacitive reactive volt-amperes in its shunt capacitance and absorbing reactive volt-amperes in its series inductance.The load at which the inductive and capacitive reactive volt-amperes are equal and opposite, such load is called surge impedance load.
It is also called natural load of the transmission line because power is not dissipated in transmission. In surge impedance loading, the voltage and current are in the same phase at all the point of the line. When the surge impedance of the line has terminated the power delivered by it is called surge impedance loading.
Shunt capacitance charges the transmission line when the circuit breaker at the sending end of the line is close. As shown below
Capacitive volt-amperes (VAr) generated in the line
Surge impedance loading is also defined as the power load in which the total reactive power of the lines becomes zero. The reactive power generated by the shunt capacitance is consumed by the series inductance of the line.
If Po is its natural load of the lines, (SIL)1∅ of the line per phase
If the load is less than the SIL, reactive volt-amperes are generated, and the voltage at the receiving end is greater than the sending end voltage. On the other hand, if the SIL is greater than the load, the voltage at receiving end is smaller because the line absorbs reactive power.
If the shunt conductance and resistance are neglected and SIL is equal to the load than the voltage at both the ends will be equal.
Surge impedance load is the ideal load because the current and voltage are uniform along the line. The wave of current and voltage is also in phase because the reactive power consumed are equal to the reactive power generated by the transmission line.