Line as source and sink of reactive power, effects

A transmission line has both series inductance and shunt capacitance, so it both absorbs and generates reactive power. Which one dominates depends on the load.

Why a line acts as a source or a sink

• Shunt capacitance generates reactive power: $Q_C = \omega CV^2$ per phase (per unit length). It depends on voltage only, so it is almost constant (voltage stays near rated).
• Series inductance absorbs reactive power: $Q_L = \omega LI^2$. It depends on the square of load current.

Comparing the two:

At SIL, $V/I = \sqrt{L/C} = Z_c$, so the two balance exactly.

Effects on the power system

As a source (light load):

• Receiving-end voltage rises above sending voltage (Ferranti effect); overvoltages stress insulation.
• Generators may have to absorb Mvar (under-excited operation), which reduces their stability margin.
• Shunt reactors are needed at line ends.

As a sink (heavy load):

• Voltage falls along the line and at the receiving end; voltage regulation becomes poor.
• Extra reactive current increases losses and reduces the capacity for active power.
• Generators must supply more Mvar; risk of voltage instability or collapse.
• Shunt capacitors, series capacitors, synchronous condensers or SVCs are needed.

So the voltage profile of a line changes with load, and reactive compensation must be adjustable (switched reactors and capacitors, SVC/STATCOM) to keep voltage within limits throughout the day.