If we consider an individual non-linear load such as a
6-pulse rectifier, the input harmonic current distortion is largely a function
of effective source impedance. Generally speaking, the higher the total input
impedance, the lower the harmonic current distortion. However, there is a point of diminishing
returns. Typical effective impedance values range from about 0.5%
(corresponding to about 100% THD-i) to about 5% (corresponding to about 35%
impedance). If a system had a total effective impedance of 10%, current
distortion would be about 26% THD-I. But, if the impedance was due to AC
reactance, then the corresponding ac voltage drop (about 10%) would be too high
for this to be a practical solution. While nameplate impedance (%) is based on
full rated current, effective impedance is based on actual circuit
(fundamental) current. If a 500KVA, 5%
impedance transformer supplies power to a 300KVA motor drive, then although
nameplate impedance states 5%, the effective impedance will be about 3% (5%
times 300/500) and harmonic current distortion at full load will be about 45%
THD-i. Current distortion, as a
percentage of fundamental current, will increase as load is reduced (lower
effective impedance), although the rms value of harmonic current will decrease.
.
