The site includes many generators of harmonic pollution, mainly the 5th harmonic. The loads generate harmonic current which create voltage pollution. Figure 8 shows a typical condition – maximum 3.3% THD V, maximum 20% THD I, 6.5V and 140A at 5th harmony. Normally, measurements are performed in accordance to IEC 61000-4-30. This standard requires an averaging of all parameters over a 200 ms period (10 cycles at 50Hz or 12 cycles at 60Hz). However, this averaging may hide shorter phenomena. In some cases, short events don’t have the required energy to cause damage. However, it depends on the weakness of the most sensitive device. Figure 10 shows the different results of the voltage and current THD when calculating every cycle (red) and according to IEC 61000‑4-30 (green). Rather than maximum current THD of 19%, the true value is 53%. Similarly, the voltage THD is 12% and not 4%.
Why is it Important?
Due do superposition, the network is represented at the 5th harmonic differently than in the fundamental (Figure 10). The load functions as current source (IH). Normally, the network equivalent impedance (Zs = ZSOURCE = utility network plus transformer) is the lowest, and the harmonic current flows to the utility
network which can withstand pollution. However, in some cases the network impedance is increased (e.g., connection or disconnection of capacitors) and the harmonic current flows to the lowest impedance, which is a sensitive load. In addition, due to the increased impedance, the current source cannot supply the same amount of current, and during this resonance the current is smaller.
Normally, harmonics are averaged because the importance is the total energy in the harmonics. In this case, the peak power of this resonance is 3kVA (Figure 11). Obviously, the 200 ms average is less, but it was still enough to cause damage. Moreover, sensitive loads probably fail before the 200 ms period is over.