Abstract
The energy transition is aimed to take advantage of the operational flexibility of hydropower to extend the integration 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 103 MVA 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.
Highlights
O VER the last decade, the load cycling intervals of large hydropower plants have changed fundamentally from rated conditions to a diverse set of operating points
As a result of a significant integration of intermittent renewable energy sources (RES) to the grid, the classical generators have responded more often to dispatch the fluctuations in both voltage and power [1]–[3]
Most of the generators today are not designed for these kinds of operational regimes as they are conventionally specified for a constant duty mode, and a nominal operating point [8]
Summary
O VER the last decade, the load cycling intervals of large hydropower plants have changed fundamentally from rated conditions to a diverse set of operating points. Future operating regimes imply frequent step changes in active and reactive power production, and several start-stop cycles during the day and week [4]–[7], depending on the RES’s weather pattern It implies running the generators in synchronous condenser mode (SCM), providing intermittent power reserves, buffers, and rotating inertia. The WAE has one primary weakness; it weights the efficiency for one loading point at a time and not the accumulation of the loading points This means that if the generator is operated purely as a synchronous condenser ( i.e., P = 0 pu) for a long time, one can get a large dataset of samples of zero efficiencies in the summation of average efficiencies.
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