BS EN 61362-2012 pdf download.Guide to specification of hydraulic turbine governing systems.
4.2.2 Speed control
The purpose of the speed control basically is to maintain constant frequency. In the various modes of operation this means that:
— in the isolated network mode with only one unit (small network), the actual speed and therefore the frequency corresponds to the command signal setting; in the isolated network mode with more than one unit (medium network). the speed control contributes to the frequency control through the permanent droop avoiding oscillation between the units;
— in the operation on the grid, where the speed is determined by the network frequency, the speed control contributes to the network frequency control through the permanent droop and the dynamic characteristics of the controlled system;
— in the no load mode (before synchronization and after separation from the network), the actual speed corresponds to the command signal or the existing network frequency with some small deviation.
4.2.3 Power output control
The power output control with a separate power controller is applied with the unit connected to the grid, its purpose is to control the power output of the unit according to a power command signal irrespective of head variations Any frequency variations influence the power level additionally via the permanent droop.
It is noted that in the cases where head variations can be ignored, a closed loop power output control, i.e., a power output controller, may not be necessary. In such a case, the calculation of the appropriate opening via a linearization may suffice (see 4.3.2). In this case also, any frequency variations influence the power level additionally via the permanent speed droop.
4.2.4 Opening control
The opening control serves to position the opening of the servomotor according to an opening command signal. either as a follow-up control in master control operations (for example speed control) or as a specific operating mode in grid control. In that last case, the usual configuration includes the permanent speed droop, which creates the relationship between the frequency of the grid and the opening of the servomotor. around the opening set-point from the unit control system; it is an alternative solution for power control with frequency influence, which is the preferred solution using modern digital controllers.
4.2.5 Water level control
For run-of-the-river hydropower plants, it is often required to control the water level of the upper part of the river, in order to keep it relatively constant or inside a specified range around a fixed value. The corresponding water level control is usually operated by an external controller of the power plant control system. But in some cases. it can also be managed by the turbine governing system itself, especially if there is no need of grid frequency control.
In the first case, by using an external water level control, the water level controller operates as a secondary controller. For that, it compares a water level measurement with a level set-point or level limit values, and by a specific algorithm, modifies the command signal of the main controller of the turbine governing system (speed or opening or power controller), in order to control the water level. Attention should be paid, in case of participation of the unit to the frequency control of the grid, that time constant of the water level control has to be enough long, in order to allow the action of the primary frequency control, with a duration as required by the TSO (transmission system operator) according to the corresponding grid code.
In the second case, the internal water level controller also compares the level measurement with the level set-point or level limit values, and modifies the command signal to the servopositioners, or to the opening limiter of the speed controller. If there are several units in the power plant, a level-opening aroop see 5.3.2) has to be implemented in each governor, in order to fix the operating point of each unit.
4.2.6 Flow control
For run-of-the-river hydropower plants, especially in case of several cascade power plants along a river (see 5.16.4). it can be required to control the flow across the different turbines in operation in the concerned power plant. With that objective, a flow control system can be implemented in the power plant control system or inside the turbine governing system.
In both cases, the usual configuration is as following: the flow controller compares a flow measurement” with a flow set-point, and by a specific flow control algorithm, modifies the command signal to the servo-positioners or to the main controller of the turbine governing system (speed or opening or power controller). The flow measurement Is generally an indirect measurement, i.e. calculated using characteristic curves from the runner blade opening (In case of Kaplan or bulb turbines) or from the guide vane opening, if necessary with a dependency on the measured head.BS EN 61362-2012 pdf download.