Harmonics in Power Systems

Harmonics are sinusoidal voltages or currents having frequencies that are integer multiples of the fundamental power frequency. They degrade power quality in 3-phase motor drives and active rectifiers.

Core Concept

Non linear loads like diode rectifiers draw non sinusoidal current from the grid. This current can be decomposed into fundamental and harmonic components. The third harmonic is 150 Hz in a 50 Hz system.

Switching power supplies often use active power factor correction to reduce harmonic injection. Firing angle control in SCR circuits generates significant low order harmonics. A firing angle of 60 degrees produces severe distortion.

Harmonics cause excessive heating in transformers and neutral wire overloading. Commutation overlap in rectifiers slightly reduces high order harmonic amplitudes.

Key Formulas

Total Harmonic Distortion THD evaluates the ratio of RMS harmonic content to the fundamental. THD equals square root of sum of squares of harmonic RMS voltages divided by fundamental RMS voltage.

Given: V1 = 230 V, V3 = 46 V, V5 = 23 V
Formula: THD = sqrt(V3^2 + V5^2) / V1
Steps: THD = sqrt(2116 + 529) / 230 = 51.42 / 230
Final Answer: 22.3 percent

Exam Tip: Triplen harmonics which are multiples of 3 add up in the neutral wire of a 3 phase star system. This causes dangerous neutral currents.

Performance Parameters

• IEEE 519 limit: current THD under 5 percent
• Fundamental frequency: 50 Hz
• Third harmonic: 150 Hz
• Fifth harmonic: 250 Hz
• Harmonic factor: measures individual distortion

Quick Revision

• Non linear loads generate harmonics
• THD quantifies distortion
• Triplens stack in neutral wires
• Delta windings trap triplen currents
• Passive filters attenuate specific frequencies
• Exam trap: Forgetting that THD is a ratio to fundamental not total RMS