INFLUENCE OF UNITS FORCED CAPACITIVE COMMUTATION ON A HARMONIC COMPOSITION OF A VOLTAGE EXCITATION ASYMMETRICAL BRIDGE EXCITERS OF SYNCHRONIC GENERATORS

Abstract

Synchronous generators (SG) autonomous generating units (AGU) most often operate in modes close to the nominal. However, when consumers of total power are connected to the AGU and the output voltage of the generator is stabilized, relay or parametric excitation forcing is applied by affecting the control angles thyristors of the exciter. The voltage boost factor lies within 3 ... 5 p.u. depending on the SG power, which leads to significant control angles thyristors of the exciter in the nominal mode and causes the consumption of significant reactive power by the exciter and a decrease in its energy indicators.To increase the energy performance of bridge exciters, especially asymmetric ones, capacitors are used in the power circuits of the excitation transformer, or two zero diodes are used that are connected in series and connected to the exciter output, and their common point is connected to zero of the supply transformer. A significant improvement in energy performance can be achieved by using forced capacitive commutation units. However, the improvement of energy indicators by the inclusion of zero diodes and capacitors leads to a significant change in the harmonic composition of the excitation voltage. In the paper, the harmonic composition of the excitation voltage of synchronous generators with asymmetric bridge exciters using the units of forced capacitive commutation is studied.The results of the spectral analysis made indicate that the excitation voltage during stabilization of the nominal voltage of the generator contains a significant third harmonic, which is 66 percent of the amplitude of the first harmonic. A significant value of the third harmonic is caused the large angles of control of the thyristors of the exciter in the nominal mode of operation of the synchronous generator. Because of the significant third harmonic, the secondary winding of the rectifier transformer must be connected in a triangle. The presence of even harmonics is caused by asymmetry voltage respect to the abscissa axis.