System Harmonic Pollution and Total Harmonic Distortion
Mar 23, 2026| "Your power distribution system suffers from severe harmonic pollution. The harmonic distortion rate is as high as over 30% and requires treatment!""The harmonics are excessive and pose a serious threat to electrical equipment. Look, the harmonic distortion rate on the meter is 45%!""The harmonic distortion rate is only 6%. Is it really that serious?"
In recent years, as the issue of harmonics in power systems has been widely discussed, every electrical manager has gained sufficient understanding of harmonics. However, how to judge the degree of harmonic pollution in a system seems to be both clear and ambiguous.
It is clear because the national standard Power Quality - Harmonics in Public Supply Networks provides clear definitions and judgment criteria for harmonic pollution levels, and power quality meters can measure specific harmonic parameters.
It is ambiguous because the commonly used harmonic distortion rate-a percentage that can be easily and directly measured by power quality meters-cannot always definitively reflect harmonic pollution.
Total Harmonic Distortion (THD) is the most frequently used indicator to evaluate the degree of harmonic pollution in power systems. It is adopted in technical standards and electrical instruments as the key metric for power harmonic pollution. THD is intuitive and easy to understand: simply put, it is the percentage of harmonic content relative to the fundamental content, similar to the alcohol content in alcoholic beverages.
THD is a general term. In practical applications, the power quality of a power system is evaluated from two perspectives: voltage and current, represented by Total Voltage Harmonic Distortion (THDu) and Total Current Harmonic Distortion (THDi) respectively. Their definitions are refined to the percentage of harmonic voltage/current content relative to the fundamental voltage/current content.
Let us first look at THDu.Normally, the voltage of a power system is fixed, with different voltage levels for various applications, such as low-voltage 400 V, medium- and high-voltage 10 kV, 35 kV, etc. For a grid with a given voltage level, the value of THDu can scientifically and clearly reflect its harmonic pollution level. This is because the denominator in the THDu formula-the fundamental voltage U₁-is constant. Thus, the larger the harmonic voltage UH, the higher the THDu. National standards use THDu as the intuitive indicator for limiting harmonic voltage.
| Nominal Grid Voltage (kV) | Total Voltage Harmonic Distortion (%) | Individual Harmonic Voltage Content (%) | |
| Odd-order Harmonics | Even-order Harmonics | ||
| 0.38 | 5.0 | 4.0 | 2.0 |
| 6 | 4.0 | 3.2 | 1.6 |
| 10 | |||
| 35 | 3.0 | 2.4 | 1.2 |
| 66 | |||
| 110 | 2.0 | 1.6 | 0.8 |
Power Quality - Harmonics in Public Supply Networks Limits for Harmonic Voltage
Next, let us examine Total Current Harmonic Distortion (THDi).Based on its definition and formula, THDi appears similar to THDu. But can THDi measure the harmonic pollution of a system as scientifically and clearly as THDu?
Take an example:A 37 kW / 380 V motor equipped with a frequency converter.
Under normal load: fundamental current = 50 A, harmonic current = 15 A, so THDi = 30%.
Under light load: fundamental current = 10 A, harmonic current = 5 A, so THDi = 50%.
From this example, we can see that THDi often cannot fully and accurately reflect the actual harmonic current condition.This is because the denominator in the THDi calculation-the fundamental current-is not fixed. Therefore, the calculated total current harmonic distortion is only meaningful under a specific load rate, similar to the analogy of alcohol concentration.
A more important indicator for evaluating the harm of harmonic current to the system is the actual harmonic current magnitude, which reflects how much a harmonic source contributes to system pollution. Reasonable limits can also be set for the harmonic current injected into the grid according to the scale of the power system. National standards use the allowable harmonic current values to restrict the harmonic current injected by users at the point of common coupling.
| Nominal Voltage (kV) | Short-Circuit Capacity (MVA) | Harmonic Order and Allowable Harmonic Current (A) | |||||||||||||||||
| 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | ||
| 0.38 | 10 | 78 | 62 | 39 | 62 | 26 | 44 | 19 | 21 | 16 | 28 | 13 | 24 | 11 | 12 | 9.7 | 18 | 8.6 | 16 |
| 6 | 100 | 43 | 34 | 21 | 34 | 14 | 24 | 11 | 11 | 8.5 | 16 | 7.1 | 13 | 6.1 | 6.8 | 5.3 | 10 | 4.7 | 9.0 |
| 10 | 100 | 26 | 20 | 13 | 20 | 8.5 | 15 | 6.4 | 6.8 | 5.1 | 9.3 | 4.3 | 7.9 | 3.7 | 4.1 | 3.2 | 6.0 | 2.8 | 5.4 |
| 35 | 250 | 15 | 12 | 7.7 | 12 | 5.1 | 8.8 | 3.8 | 4.1 | 3.1 | 5.6 | 2.6 | 4.7 | 2.2 | 2.5 | 1.9 | 3.6 | 1.7 | 3.2 |
| 66 | 500 | 16 | 13 | 8.1 | 13 | 5.4 | 9.3 | 4.1 | 4.3 | 3.3 | 5.9 | 2.7 | 5.0 | 2.3 | 2.6 | 2.0 | 3.8 | 1.8 | 3.4 |
| 110 | 750 | 12 | 9.6 | 6.0 | 9.6 | 4.0 | 6.8 | 3.0 | 3.2 | 2.4 | 4.3 | 2.0 | 3.7 | 1.7 | 1.9 | 1.5 | 2.8 | 1.3 | 2.5 |
Power Quality - Harmonics in Public Supply Networks Limits for Harmonic Current
After reading the above content, you should have a clearer understanding of the concept and application of Total Harmonic Distortion (THD).THD is widely used in harmonic measurement and analysis, standard formulation, filter effect evaluation, and other scenarios. Correctly understanding the characteristics and limitations of THDu and THDi in voltage and current harmonic analysis helps us scientifically judge harmonic pollution levels, reasonably select filtering methods, and implement accurate effect evaluation.