As the number of non-linear loads in power networks constantly rises, there is increasing "noise on the grid," also referred to as grid distortion effects. This is akin to environmental pollution, but within our power networks. Ideally, generators produce purely sinusoidal current at the output terminals. This sinusoidal current form is considered the ideal alternating current form, and any deviation from it is designated mains interference.
Increasing non-linear loads are extracting non-sinusoidal current from the grid. When subjected to the Fast Fourier Transform (FFT), this "noisy" current form results in a broad spectrum of harmonic frequencies, commonly known as harmonics.
Harmonics pose a substantial risk of damage to electrical networks and can be harmful, even dangerous, to connected loads -- much like polluted water's detrimental effects on the human body. The presence of harmonics results in overloads, diminished equipment service lives and, in some instances, premature failure of electrical and electronic loads.
Harmonic loads are the primary cause of unseen power quality problems and can result in significant maintenance and investment costs associated with the replacement of defective devices. High levels of grid distortion effects leading to poor power quality can consequently cause issues in production processes, sometimes even leading to production downtime.
Harmonics are currents or voltages whose frequency is multiple times the 50/60 Hz mains frequency. These harmonic currents don't contribute to the effective power but cause a thermal load on the network. They flow alongside "active" sinusoidal oscillations, causing electrical losses within the electrical installation. These losses can lead to thermal overloads, and losses in the load can cause overheating, reducing the service life of the equipment.
Assessment of harmonic loads typically takes place at the connection or transition point to the public mains supply network of the energy supplier. This point is known as a Point of Common Coupling (PCC). In certain scenarios, it may be necessary to determine and analyze the harmonic load through individual operating equipment or equipment groups to identify internal power quality issues and potentially uncover their root causes.
The following parameters are used to assess harmonic loads:
Total Harmonic Distortion (THD) serves as a measure to quantify the proportion of distortion due to non-linear distortion of an electrical signal. Specifically, it calculates the ratio of the effective value of all harmonics to the effective value of the mains frequency. The THD value is used across low-, medium- and high-voltage systems. Conventionally, THDi refers to the distortion of current, while THDu is used for the distortion of voltage.
Total Demand Distortion (TDD) is a term related to harmonics. TDD is a figure that corresponds to THDi, but here, the total harmonic distortion is related to the fundamental oscillation portion of the nominal current value. Therefore, TDD illustrates the relationship between the current harmonics (analogous to the THDi) and the effective current value under full load conditions that arises within a set interval. Standard intervals are typically 15 or 30 minutes.
TDD gives the relationship between the current harmonics (THDi) and the effective current value with a full load.