Reactance

Reactance is the component of impedance that opposes alternating current flow due to energy storage in inductance or capacitance, measured in ohms. Unlike resistance, reactance does not dissipate energy as heat but causes current and voltage to be 90 degrees out of phase, directly affecting reactive power flow in the system.

Key Aspects of Reactance:

• Inductive Reactance (X_L): Calculated as X_L = 2πfL, it increases linearly with frequency and dominates in transmission lines, transformers, and generator windings. Inductive elements absorb reactive power and cause voltage to lead current.
• Capacitive Reactance (X_C): Calculated as X_C = 1/(2πfC), it decreases with increasing frequency and appears in capacitor banks, cable insulation, and long transmission lines. Capacitive elements supply reactive power and cause current to lead voltage.
• Reactive Cancellation: Since inductive and capacitive reactance are 180° apart, they can partially or fully cancel each other, a principle exploited in series compensation of long transmission lines to reduce effective line reactance and increase power transfer capability.
• Transmission Line Behavior: Overhead lines typically have characteristic impedance of 300–400 Ω dominated by inductive reactance, while underground cables have significantly lower impedance due to higher shunt capacitance.
• X/R Ratio: The ratio of reactance to resistance is an important parameter: transmission lines and large transformers are highly reactive (high X/R), whereas distribution feeders have more significant resistance relative to reactance. The X/R ratio affects fault current asymmetry and power factor.