ETEP zy Generation of Uncharacteristic Harmonics z in Electrical Plants zyxw with ACDC Converters Nl. Loggini, G. C. Montanari, A. Cavallini Abstract zyxwvutsrq In this paper an investigation is carried out on the correlation between dissymmetry imbalances and the gen- eration of uncharacteristic harmonics in low-voltage electrical plants supplying AC/DC six-pulse converters. After a general introduction, some typical cases of dissymmetries and imbalances are considered, relevant to supply voltage, line impedances, thyristor injection angles and filter capacitors, and the corresponding un- characteristic harmonic currents derived by means zyxwv of the simulation program EMTP. Moreover, evaluation of harmonics is made for various values of plant and load parameters in order to derive the behaviour of ampli- tude and phase of uncharacteristic harmonics in a wide range of plant conditions. The results provide clear relationship between kind and degree of dissymmetry and presence of uncharacteristic harmonics with useful indications for plant design. 1 Introduction The design of compensation systems for electrical plants supplying distorting loads, is generally made under the implied assumption that only characteristic harmonic currents and voltages, with significant ampli- tude values, are present in the plant. On thecontnry, how- ever, it is a common experience to measure not negligible values of uncharacteristic harmonics in electrical plants or networks, which can sometimes be comparable to the amplitude of the lowest-ordercharacteristic harmonics. The purpose of the paper is to examine the situation pertinent to low-voltage electrical plants supplying six- pulse zyxwvutsr AC/DC converters with DC loads (e. g. motors), where the lowest characteristic harmonic is the 5th one A$ Y Converter ETlll. I A Fig. 1. Equivalent circuit of the examined electrical plant zyxwvu (E electro-motive force (emt)) in order to investigate the causes of the often noticed presence of uncharacteristic even and odd harmonic cur- rents and voltages, e. g. Znd, 3rd. and 4th ones. It has already been shown zyxw [ I -41 that imbalances and dissymmetries of the electrical plants, like dissymmetry of supply voltage, unbalanced load distribution, shift- ings of bridge-thyristor injection angles, can generate uncharacteristic harmonics. Moreover, it is well-known that the 3rd harmonic is a consequence of transformer saturation. However, some topics still need to be more extensively dealt with. In particular, it seems interesting to clarify what is the action of imbalances and dissym- metries on converters, such as to compel generation of uncharacteristic harmonic currents. Furthermore, also the influence of plant and load parameters on the values of amplitude and phase of uncharacteristic harmonics should be clarified. Finally, the behaviour of the com- pensation systems constituted by AC filters or capaci- tors which operate in the presence of plant dissymme- tries, and then of uncharacteristic harmonics (particular- ly when the same compensation systems are unbalanced loads), is an important matter for the purpose of non-ac- tive power compensation. To these aims, the reference electrical plant of Fig. 1 has been considered, and computer simulation (by means of the EMTP program [5]) or analytical solutions (in some particular cases) have been used to evaluate amplitudes and phases of the uncharacteristic harmon- ics for different conditions of the electrical plant and the previously mentioned dissymmetry imbalances (ex- cluding transformer saturation, elsewhere widely inves- tigated). Therefore, the values of short-circuit ratio r,, AC protection and DC smoothing inductances (Lp and LK, respectively), converter delay angle a, DC power Pw, DC load. electro-motive force (emf) E, are varied in each case of dissymmetry. From this approach it follows that also the kind of converter control can condition uncharacteristic har- monic current generation [3.4]. ETEP Vol. 4, No. 3, May/June 1994 I87