Harmonic study: When two do the same, it is not the same

What factors make the difference

It is a realistic assumption that not every piece of information is available and some assumptions need to be made. Let’s look at some examples:

• Let’s start with the VFD: What spectrum is used? Idealized one (e.g. showing only characteristic harmonics) or realistic spectrum considering some residual harmonics of a 6-pulse bridge even when using a multi-pulse rectifier?
• Grid transformers are mostly defined with rated power, voltage ratio, vector group and short-circuit impedance. The no-load and load losses are often not indicated. However, the network model requires such information to calculate e.g. the winding resistance and thus the damping. A conservative approach considers rather lower transformer losses. Too low losses lead to unrealistically low damping in the system.
• Power cables contribute with their capacitance to a parallel resonance. The specific capacitance (e.g. μF/km) is provided in the technical datasheet. However, note that the value is subject to tolerance that can easily be ±10% or more.
• Frequency dependent impedance and skin effect: It is well-known that the resistance increases at higher frequencies due to skin effect. Thus, there is much more damping that helps to limit the distortion, particularly in case of a resonance. Not every network software allows the modelling of skin effect and even if it does, it is up to the user to select the factor. The results are highly sensitive to the factor representing skin effect.
• Information about other consumers (loads):  Other loads connected to the same grid, typically inside the plant, help reducing the grid harmonic distortion as they represent parallel paths for the harmonic currents. Ignoring or neglecting those loads lead to pessimistic results.
• Full load does not mean rated transformer power: A typical worst case considers the drive (VFD) running at full load. However, full load does not mean full power on the input transformer. First of all, there are design margins: electric motor is some 10…15% oversized compared to the driven load. The VFD may have additional margin on top of the VFD rating. And input transformer has some margin to consider the undervoltage.
• Positive, negative and zero sequence impedance: The VFD generates harmonics that can be positive, negative or zero sequence. To get correct results, the user shall enter corresponding positive, negative and zero sequence impedance. Often, only positive sequence impedance is known.
• Overall philosophy from optimistic, over realistic and conservative to ultra-conservative approach. Some conservatism can be justified and even recommended. However, combining multiple “worst case” factors will result in totally unrealistic scenario.