Selecting the Power Transformers Rated for 10(6)/0.4 kv Substations Taking into Account the Influence of Nonlinear Load
The factors influencing the choice of power transformers rated for substations with a voltage of 10(6)/0,4 kV are considered. A review of methods that allow for estimating the permissible load level of power transformers with a non-sinusoidal nature of the load current is given. The requirements of normative and technical documents regarding compliance with operating modes of power transformers are presented. Recommendations on the choice of the calculated load factor КL of power transformers depending on the design of the transformer, operational characteristics, the adopted backup mode and the share of responsible consumers of electric energy are formulated. An algorithm for selecting the power transformers rated STR.N is proposed, taking into account the influence of non-sinusoidal load currents, which assumes the use of the coefficient of reduction of the permissible load of the KPL. The dependences of the coefficient KPL on the share of the nonlinear consumers total power NLL with different harmonic composition of the load current are obtained. It is established that for the considered harmonic compositions of the load current, when the NLL values change from 0 to 100%, the values of the KPL coefficient change in the range from 1.0 to 0.712. The results of selecting the power transformers rated according to the proposed algorithm are analyzed for different backup modes (different values of the calculated load factor KL). The power transformers rated was selected for three typical objects characterized by different harmonic composition of the load current, with a change in the share of the total power of non-linear consumers NLL from 0 to 100 %. It is established that in order to ensure the standard service life of power transformers in these cases, their rated power STR.N should be increased by 25...60 %. It is advisable to select the power transformers rated taking into account the influence of non-linear load using information about the harmonic composition of the load current. For this purpose, the stage of calculating the electrical loads should involve the definition of the full design power of electric devices that have nonlinear current-voltage characteristic and of the harmonic part of the load current. The proposed algorithm for selecting the power transformers rated can be used as part of the method for calculating electrical loads and selecting the power electrical equipment when designing transformer substations for industrial facilities, infrastructure facilities, residential and public buildings.