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In power systems, maintaining the quality of electrical power is crucial for the efficient operation of electrical equipment and the overall reliability of the grid. Two important metrics used to assess power quality are Total Harmonic Distortion (THD) and Total Demand Distortion (TDD). While both metrics are significant, it is important to note that from the grid's perspective, TDD is more important. This is because TDD provides a measure of harmonic distortion relative to the system's demand, offering a more accurate representation of the potential impact on the grid. Moreover, utility companies are particularly concerned with the harmonic distortion at the Point of Common Coupling (PCC), where multiple facilities connect to the grid. This document provides a comprehensive explanation of THD and TDD, including their definitions, formulas, usage, and purposes based on the IEEE 519-2022 standard. An example will also be provided to illustrate the concepts, and we will focus exclusively on current distortion and not voltage distortion.
Definitions and Formulas
Definition:
THD is a measure of the harmonic distortion present in a current waveform. It quantifies the extent to which the harmonics of a signal deviate from the fundamental frequency.
Formula:
Where:
Usage and Purpose
THD is used to assess the quality of current delivered to electrical equipment. High THD levels can cause overheating, malfunction, or reduced lifespan of equipment. However, high THD is not necessarily bad because it is relative to the load current. If the load current is low, even if the THD is high, the absolute distorted current is not high and won't be a concern for the grid. IEEE 519-2022 recommends THD limits to ensure that the current quality remains within acceptable standards for various voltage levels.
Definition:
TDD is a measure of the harmonic distortion of a system's current relative to its maximum demand load current. It provides a normalized measure of distortion that takes into account the system's loading conditions.
Formula:
Where:
Usage and Purpose
TDD is crucial for grid operators as it provides a measure of the harmonic distortion relative to the system's demand. Unlike THD, which can be misleading during low load conditions, TDD reflects the true impact of harmonics on the power system. High TDD levels can indicate potential issues with the system's capacity to handle harmonic currents, which can lead to overheating of transformers, cables, and other components. In the IEEE 519 standard, TDD is emphasized because it reflects the exact amount of harmonic current, providing a clearer understanding of the impact on the grid.
The IEEE 519-2022 standard provides recommended practices and requirements for controlling harmonics in electrical power systems. According to the table 2 of this standard:
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