A new approach to predict the additional costs of reactive power system services delivered by large hydrogenerators is proposed in this letter. It is based on the application of the accumulated average efficiency (AAE), which has recently been proposed. An optimal operational path within the capability diagram with minimal losses is derived. This path can be used to calculate additional losses from operational regimes deviating from the optimal one for each active power level. Finally, the additional losses are accumulated in a similar manner as the AAE to estimate the extra cost of the operational regime, with ideal operation as the reference. In addition, the accuracy of a data clustering approach is explored to speed up the computation of the AAE and the accumulation of additional costs.

The energy transition is aimed to take advantage of the operational flexibility of hydropower to extend the in- tegration of intermittent renewable sources. Consequently, the hydrogenerators will have to operate in regimes far away from their designed best-point operation. In order to accurately assess the impact, this paper presents a useful approach to determine the overall operating efficiency of synchronous generators under intermittent operation. An accumulated average efficiency (AAE) model is proposed and compared against an alternative approach; the weighted average efficiency (WAE) model. It is found that the WAE approach produces unrealistic low efficiencies when the generator operates in synchronous condenser mode (SCM) for long periods. In general, the AAE supersedes the WAE for all the different load distributions that were investigated. This was further illustrated by a worked example and by constructing more complex load distributions. A load distribution dominated by SCM yields a difference as high as 33.18%, while an even distribution deviates 1.43 % in their respective efficiencies. Finally, a yearly on-site measurement of our studied 103MVA generator’s concentrated load distribution revealed a discrepancy of 0.67 %, which is a significant deviation considering what the operating regime would mean in terms of economic implications.