Understanding THDi vs. TDD in Power Quality Analysis

THDi (Total Harmonic Distortion) and TDD (Total Demand Distortion) are critical parameters used to evaluate harmonic distortion in electrical systems. While both metrics assess the presence of harmonics, they differ in their definitions, calculations, and significance.

THDi (Total Harmonic Distortion)

Definition: THDi measures the harmonic distortion in a signal, representing the ratio of the sum of harmonic component powers to the fundamental power.

Application: In power systems, THDi quantifies the level of voltage or current harmonic distortion, providing insights into system performance.

Calculation Formula: THDi is typically expressed as a percentage and calculated using the formula: THDi = (Σ RMS value of harmonic voltages / RMS value of fundamental voltage) × 100%.

Significance: Lower THDi values indicate reduced peak currents, less heating, lower electromagnetic radiation, and fewer iron losses in motors, contributing to improved system efficiency and reliability.

TDD (Total Demand Distortion)

Definition: TDD calculates harmonic current distortion at full load (demand) levels in an electrical system, providing a comprehensive assessment of distortion impact.

Relation: At full load, TDD equals THD (Total Harmonic Distortion) and provides a better reflection of harmonic distortion effects on the system.

Importance: TDD accounts for harmonic distortion effects at varying load conditions, ensuring a more accurate representation of system performance. Even with high THDi values, the impact on the system is smaller at lower loads.

Calculation Formula: TDD is typically calculated as TDD (I) = (Harmonic current distortion / Full load (demand) current) × 100%.

By understanding the differences between THDi and TDD and their implications, power quality analysts can effectively assess and mitigate harmonic distortion issues, ensuring optimal system operation and reliability.