Optimal reactive power control strategies for rural substation

Reasonable reactive power sources compensation of rural substations has been becoming a hot issue since Chinese rural electric network alteration. The principal reactive power compensation mode of rural substations is still using fixed compensation capacitor to control voltage and reactive power at present in China. This compensation mode has some problems such as capacity adjustment requires manual intervention under power outage, the phenomenon of over and under compensation may always happen, the rate of putting into operation of reactive power compensation is relatively low, and so on. At the same time, there is no sampling function at the primary side of the main transformer because of the special devices in rural substations. In order to realize the objectives that the power factor is not less than 0.95 at primary side and not less than 0.9 at secondary side at the highest load, in this paper, some optimal reactive power control strategies for rural substation were proposed. In accordance with the reactive power flow conditions of the rural distribution network, the pros and cons of two control strategies were analyzed. One of the strategies was sampling at the primary side of the main transformer, the other was sampling at the secondary side and switching control by power factor of secondary side. After comparison of such analysis, an optimal control strategy was proposed. The data were sampled in the substation secondary side, then the sampled data were evaluated in equivalence to the primary side, and then the power factor assessment criteria of primary side were used to control capacitor switching. The compensation capacity should be calculated after electric motor compensation, transformer compensation and distributed compensation on distribution line. The sampled values at secondary side and active loss and reactive loss of the main transformer were used to calculate compensation capacity to meet the power factor objectives of primary side. Through the example calculation and analysis by applying actual substation data a result were obtained. The result met appraisal standards and the power factor of main transformer primary side was above 0.95 at the highest load. If the power factor of main transformer secondary side was above 0.98, there was no need to compensate for substation. If the power factor of main transformer secondary side was under 0.97, after the compensation by using the proposed optimal compensation capacity and the primary side power factor control method, the power factor of the main transformer secondary side was not less than 0.98 and the primary side reaches 0.95. These results show that the proposed optimal control strategy and compensation capacity calculation method are feasible, and the research has practical significance of making full use of reactive power supply in rural distribution network.