Noncharacteristic line current harmonics in diode rectifier bridges produced by network asymmetries

In the case of an asymmetric network, i.e. unbalanced three-phase voltages or/and different line reactances, noncharacteristic harmonics appear in the line current of diode rectifiers. To investigate the basic mechanism of the generation of these noncharacteristics, a discrete small-signal modeling is proposed consisting of difference equations of the DC current and firing angle determinations. Thereby, the influence of network symmetries is described analytically taking into account the commutation process and DC current ripple effects. With this, basic dependencies can be investigated and an effective evaluation of the noncharacteristics is obtained. The results drawn as loci in the complex plane lead to a compact and user friendly representation. Thereby, arbitrary network asymmetries can be considered easily by corresponding phasor additions, especially for the estimation of the maximum values. Comparing digital simulation studies confirm the validity of small-signal modeling in the practical range of network asymmetries