Method and Software Tool for Assessment of Seasonal Step-Up Transformer Optimal Tap Settings

In most European countries the generators’ step-up transformers are demanded to be equipped with tap changers, explicitly or through tanϕ.-voltage requirement. Releasing the maximum available reactive power range of generator or at different voltage levels at the point of common coupling is the prime purpose of required transformers variable turns ratio by keeping the generator voltage close to rated value. It is suggested in the literature that turns ratio should be calculated as a result of power system load flows studies. However, those outputs are not unique and very often they are not conclusive since commercially available software don’t offer maximum reactive reserve at generating unit terminals as an optimization factor. The care should be taken that transfer ratios of all step-up transformers connected to same point of common coupling (PCC) should be the same in order to avoid bounding of generator reactive powers due to terminal voltage limits. With change of tap position the step-up transformer reactance, turns ratio, and generator coupling to network changes as well as the generator terminal voltage and reactive power flow through step-up transformer. Taking all these factors into account a method for assessment of seasonal optimal settings of generator step-up transformer taps is suggested. The main objective of the proposed methodology for optimal tap setting of steep-up transformer is to determine the optimal tap positions in order to maximize the reactive capability of generating units. In Serbian power network the generator step-up transformers generally are equipped with no--load taps that are infrequently changed so their optimum positions need to be determined for meeting power system discrete states over long periods, or seasons, covering the annual light-load, peak-load, and emergency conditions. Those states are determined by a software tool which uses the network voltage data for past years (big data analysis)) to assess the optimal position for next year. The software output is the suggested tap positions and the date of change for the next year. By optimal setting of the step-up transformer tap changer, the available reactive capability of the synchronous generator under scope is fully utilized for expecting voltage levels at PCC within season so that the generator terminal voltage is no more limiting factor for available reactive power during production/absorption of reactive power.