How do grid conditions affect harmonics pollution?
The most important factor in determining the harmonic pollution of a supply grid is the system impedance.
The system impedance is mostly dependent on the transformer size in relation to the total power consumption of installed loads. The bigger the transformer is in relation to non-sinusoidal power consumption, the smaller the pollution.
The power grid is an interconnected system of power supplies and power consumers all connected via transformers. All loads drawing a non-sinusoidal current contribute to the pollution of the power grid – not just at the low voltage supply but also at higher voltage levels.
When measuring at a power socket, some degree of pollution will thus always be present. This is referred to as harmonic pre-distortion. As not all consumers draw three-phase current, the load on each phase is dissimilar. This leads to unequal voltage values on each phase, causing phase imbalance.
Different harmonic solutions have diff erent immunity against pre-distortion and imbalance and so this has to be evaluated when determining the most suitable harmonic mitigation solution.
What application aspects must be considered?
Harmonic distortion increases with the amount of power consumed by the non-linear load and so both the number of drives installed, and their individual power sizes and load profi les, must be considered.
The distortion of a drive is defined by the total harmonic current distortion (THDi) which is the relationship between the sum of harmonic components and the fundamental frequency.
The loading of each drive is important because the THDi increases at partial load, thus over-sizing drives increases the harmonic pollution on the grid.
Additionally, environmental and physical constraints must be taken into account because the diff erent solutions all have characteristics making them more or less suited to specifi c conditions.
What needs to be considered is, for example, wall space, cooling air (polluted/contaminated), vibration, ambient temperature, altitude, humidity, etc.
Are compliance with standards consistent globally?
To ensure a certain grid quality, most power distribution companies demand that consumers comply with standards and recommendations.
Different standards apply in different geographical areas and industries but all of them have one basic goal – to limit the grid voltage distortion.
Standards depend on grid conditions and so it is impossible to guarantee standards compliance without knowing the grid specifications.
Standards themselves do not compel a specific mitigation solution and so an understanding of standards and recommendations is important to avoid unnecessary cost for mitigation equipment.
What areas of cost must be considered when applying harmonic mitigation?
Finally, the initial costs and running expenses have to be evaluated to ensure that the most cost-effective solution is found.
The initial cost of the different harmonic mitigation solutions in comparison to the drive varies with the power range. The mitigation solution that is most cost efficient for one power range is not necessary best cost fit over the full power range.
The running costs are determined by the efficiency of the solutions across the load profi le and their lifetime maintenance/service costs.
Compared to active solutions, passive solutions often do not require regular maintenance. On the other hand, active solutions tend to keep the true power-factor close to unity over the entire load range, resulting in better energy utilization at partial load.
Also, future development plans for the plant or system need to be taken into account because although one solution will be optimal for a static system, another will be more flexible if the system needs to be extended.