Real-time monitoring and calculation of the derating of single-phase transformers under nonsinusoidal operation.

摘要

The extensive use of power electronic devices in the last two decades have degraded the quality of the power system by introducing voltage and current harmonics as well as DC excitations. Such phenomena cause additional losses in transformers, resulting in elevated temperatures of transformers above their rated temperatures. This added heat degrades the insulating material of the windings, decreasing the rated lifetime of transformers. For this reason, transformers feeding nonlinear loads must be derated; that is. by limiting either their output apparent or real power such that rated temperatures are not exceeded. It is of advantage to measure the derating of transformers which are already in service, for given nonlinear loads and to calculate the derating of large transformers that cannot readily be tested in laboratories. It is one of the objectives of this thesis to validate the computed derating values of transformers with corroborating measurements.; A 25kVA 7200V/240V single-phase pole transformer is analyzed using two dimensional field analysis based on the diffusion equation, employing either rectangular or polycentric grid structures. Such a field analysis allows us to visualize the complex vector potential and flux density distributions inside the unsaturated transformer operating under short-circuit conditions. One can calculate the eddy currents within conducting materials, such as copper and aluminum windings, from complex vector potential values. Short-circuit tests applied to low and high voltage windings allow us to calculate the eddy currents inside each winding and consequently their eddy-current losses. The frequency dependent AC winding resistance RAC as well as the per-unit eddy-current loss coefficient PEC-R are computed. These values are used to determine the derating of transformers via either the K-factor as proposed by Underwriters Laboratory, Inc., or via the harmonic loss factor FHL, as favored by IEEE and IEC.; A new digital data-acquisition method for the separate on-line measurement of the iron-core and copper losses of transformers under any (non)linear load condition is introduced. Accuracy requirements of the instruments (voltage, current sensors, volt and current meters) employed are addressed. The maximum errors of the measured copper and iron core losses mainly depend on the accuracy of sensors used and are less than 6%.; The derating of the 25kVA 7200V/240V transformer has been determined utilizing the on-line monitoring method, where the transformer is loaded with three diode-bridge and/or thyristor-bridge rectifiers to generate varying total harmonic voltage and current distortions such that at the maximum permissible apparent power S the total losses are identical with the rated total losses. For total harmonic distortion of current (THD _i) values ranging from 0 up to 90%, the apparent power S derating and the real power P capability of the above-mentioned transformer are measured. The effects of high-order harmonics as compared with those of low-order harmonics on the derating of the transformer for given (same) THD_i values are explained.; The analysis and validation of this thesis points to the fact that the transformer should not be derated with respect to apparent output power S but with respect to real output power P. This is so because the derating of the latter is much more severe for a given THD_i.