Study On Operation Characteristic Of Substation With Cable Outlets

The development of the center city district and the rapid growth of electricity load bring the medium and high voltage urban grids the opportunity to replace overhead lines with cables, so there must be a new substation style with cable outlets. Because the traditional protection and running modes worked with out regard to the larger and larger proportion of cable outlets, then there may be some potential harm to the equipments.The models of substations with cable outlets and overhead line outlets are built respectively to analyze the crest of over-voltage and over-current which take place when a single-phase ground fault has occurred and the three-phase reclosing operation took action. A scale down experiment was made to prove the correctness of the simulation models. Because the equivalent capacitance per kilometer of cable line is large, the trapped electric is dangerous to the equipments and the human. It's necessary to analyze the relationship between the trapped electric and the over-voltage. And the feasibility of reclosing operation was studied. The relationship between crest of the over-currents in cable and overhead line outlets was analyzed too.Analysis on the models of substations with cable outlets and overhead line outlets shows that over voltages due to reclosing are affected by some factors, such as the magnitude of the trapped charge on the line, the reclosing moment and the number of the outlets. Generally over voltages taking place in cable grid are higher than the over voltages in overhead line grid, however if there are many other outlets in the grid, the over voltage will be depressed much and will be even lower if these outlets are carrying lodes. So the over voltages will not endanger the grid. On the other hand, the over-currents which cable could endure depend on the time for which fault lasts, and the over-currents that transformer in the grid could endure depends on the transformer's mechanical characteristic. Over currents in cable grid are much higher than in overhead line grid, so the mechanical characteristic of transformer should be checked carefully.