A practical method for calculation of over-excited region in the synchronous generator capability curves

The capability curves are used for loading the synchronous generators as a useful and essential tool. Moreover, one of the important applications of the capability curves is setting the relays. The main purpose of the proper loading and accurate setting of the relays is stable operating of the synchronous generators in desired margins. Therefore, the accurate calculation of the curves is significant. Although in the papers some methods and formula for calculating and drawing the capability curves have been presented, the obtained results do not coincide with the original capability curves provided by the manufacturers. It could be potentially because of disregarding the real conditions of synchronous generators such as saturation, temperature, mechanical considerations, altitude and etc. Also, there is no complete source about the capability curves which cover all parts of the curve. In this paper, in addition to briefly assess all parts of the capability curve, the latter is precisely calculated by taking into account all parameters and operational conditions of the generator. The results in the armature current limit and the under-excitation limit are in agreement with the original capability curves provided by the manufacturers. In spite of this coincidence, the over-excitation part of the graph drawn on the basis of the above-mentioned considerations and calculations does not follow the manufacturer´s curves, although the conditions and limitations of the generator are considered. Accordingly, in this paper, a new simple and applicable procedure is proposed for calculating the over-excitation part of the curve, based on authors´ experience in designing the synchronous generators. The results of the calculations are in good agreement with over-excitation limit of the manufacturers´ curves. Simplicity, fast calculation time, precision and error minimization main features of this method.