Limitation of the spread of high frequency harmonics through series filters

Origin of HF-harmonics

Often power electronic based equipment emits both LF harmonics and HF harmonics. Passive power-electronic converters (based on diodes) and grid-commutated converters (based on thyristors) mainly emit LF harmonics. The origin of the HF harmonics current is mainly from active power electronic converters like the ones used for Active Power Factor Correction (APFC).

Origin of the HF harmonics current is presented in Figure 1.

When the LF harmonics are reduced by active PFC technology often, as a backlash, the levels of HF harmonics are increased. The HF harmonics can be seen as an unintended consequence of the reduction of the LF harmonics.

The “synchronized” behavior is present in the current as a “zero crossing distortion” or “crossover distortion” close to the current zero crossing.

The “random” behavior is a “nondamped oscillation” present in between the zero-crossing oscillations (i.e., with current maximum or minimum). This oscillation is associated with the power-electronic switching used in the active PFC circuits.

Propagation of HF harmonics

The lack of synchronization between different HF harmonic current sources, together with the difference in impedance at higher frequencies between the grid and neighboring end-user equipment, make a clear difference in the propagation of HF harmonics compared to LF harmonics. This in turn impacts the way in which they should be assessed and treated.

It has been shown by Rönnberg in ‘Limits for voltage distortion in the frequency range 2 to 9 kHz’ that the HF-harmonic current meanly flows between the different electronic equipment instead of upstream to the transformer. The explanation for this is that the lowest impedance path seen by the HF- harmonic current is into the equipment instead of into the grid.

As a consequence, the current component flowing into the grid is not as an indication of the HF- harmonic current “disturbances level” downstream of the measurement point. The other way around: high levels of HF harmonics emitted by individual devices are not a concern for the grid in the same way as emission of LF harmonics.

The measurements also confirm a preliminary conclusion that the “zero crossing distortion” increases almost linearly with the number of devices, since its occurrence is synchronized with the voltage zero crossing, which is the same for all devices.

Once multiple devices of different character are located close to each other, they could together result in completely different emission patterns that each one of them individually.