Because of the energy transition, the share of volatile electricity production by wind and photovoltaics is increasing strongly. Figure 1 shows the feed-in contribution of wind and solar energy into the control area of "50Hz Transmission GmbH" for 2015. The short-term feed-in power varies between 0 and about 3.4 times the average feed-in power. This requires fast and flexible control energy to balance this volatility. Hydroelectric power plants (especially pumped-storage power plants) are used for this purpose. However, most power plants are old and were not designed for such variable operation.
This can lead to critical operating conditions in the plants. For example, the water control system can surge during unfavorable operations with large cyclic load changes and cause the surge tank to run dry or overflow. While such large cyclic load changes are unlikely, they cannot be ruled out since the power plants are normally controlled remotely by the dispatcher. Therefore, in order to avoid unnecessary operational restrictions, there is an attempt to apply new control strategies to manage these critical operating conditions when they occur.
As an example, a control concept is shown here to prevent the surge tank of a plant from running dry. In this plant, the surge tank can run empty if, on the one hand, the headwater level is very low and there is then a cyclic load demand.
In this concept, an attempt is made to create as few restrictions on operation as possible. For this purpose, the surge tank protection is only activated when the headwater level is below the critical value and when the level in the surge tank falls below a critical limit value.
The corresponding control algorithm is shown in Fig. 3. If both criteria (critical headwater level and critical surge tank level) are fulfilled, the permissible power change is limited. This ensures that even in this critical case an overflow of the surge tank can be avoided. If the level in the surge tank falls below the critical HUK value, an emergency shutdown of the system is automatically initiated to prevent a further drop in the surge tank level.
Results of the investigation
Figure 4 shows the behavior of the system with and without WS protection for a cyclic load demand between 0 and 75%. After 670 s, the surge tank oscillates below the switch-on point of the protection. Without protection, the surge tank would run empty.
With protection, the power demand Psoll-neu is changed. This prevents the surge tank from running empty. As a result, the power output of the system is temporarily smaller and does not correspond to the power demanded by the dispatcher. However, after another approx. 200 s the system has stabilized again and the power control takes place again without restriction.
With such protective measures, even older plants that were not designed for extremely flexible operation can be used completely flexibly without operational restrictions and thus provide a larger, necessary contribution to control energy for the energy transition.