The Pelton turbine is a hydraulic action turbine, designed to operate in high and low falls. It consists of a rotor with blades in the shape of shells or buckets and has one or more injectors. The jets of water coming from the injectors when colliding with the rotor blades generate the mechanical torque that is applied to the shaft of the turbine. In order to control the flow of water in the injectors, there is a needle type valve, or simply needle, which alters the area of the section through which the water passes through the injector and reaches the turbine. The position of the needle is controlled by the speed governor.
Due to the slow closing of the needles, Pelton turbines are equipped with deflectors, whose purpose is to divert the water jet that leaves the injectors so that the water does not reach the turbine. The deflectors can act very quickly on the water jet, reducing the mechanical torque. This action allows avoiding turbine overspeed in situations of sudden load reduction, such as during the load rejection and loading taking.
The deflector position control is also carried out by the speed governor. Typically, two control options are used for the deflector: actuation of the ON / OFF type, with the deflector fully open or totally closed; and proportional action, in which the deflector can be placed in intermediate positions. In the latter case the position of the deflector is determined from the position of the needle, so that the deflector remains close without diverting the water.
In the occurrences of system disconnection and load shedding, the mechanical torque needs to adapt to the new load. As, in general, the closing time of the needles is long, it is necessary to use the deflectors to quickly reduce the mechanical torque, in order to reduce the machine rotation. As soon as the deflectors close, completely deflecting the water jet, the speed drops abruptly. Thus it is necessary to re-open the deflectors so that the machine does not decelerate below a predetermined limit. In this situation there is a control problem very different from that for which the traditional speed control loops were projected and tuned to.
These characteristics of Pelton lead this type of turbine to have dynamic responses very different from the others, such as the Kaplan or Francis which are reaction turbines.
ANALYSIS OF THE PROBLEM
The analysis of the problem is presented in a real case of insufficient performance of the traditional control by the deflector in the Speed / Frequency control after the load shedding. The position of the deflector, by the traditional control, is given by the output of the conjugated curve with the reference of needle position.
FIGURE 2 presents the real case of a rejection of 90% of the load with the traditional control, where it is possible to observe the oscillatory behavior of the deflector, in addition to the sinking of the frequency. The turbine in question has the following characteristics:
- Needle closing time = 62 seconds
- Inertia constant (2H) = 3,82 seconds
After the load rejection, the needles are closing at their maximum closing rate to reach a position according to the no load position. The closing speed of the needles is mechanically limited. It is during this movement of the needle that the deflector remains closed until the needle reaches a position value close to the no load position.
This traditional strategy causes greater excursions of the speed, especially a considerable drop after the first impact of the over speed, as the deflectors must remain closed until the needles reach the no-load condition.
The insufficient performance of the deflector in the speed control can also occur in the case of a grid disconnection, when after the opening of the connecting switch, the plant continues generating power for an isolated system with much smaller load taking than the generation before the occurrence. If the synchronous machine, or the equivalent system has a low constant of inertia, the speed will then drop very quickly and may cause bumps and locks by the action of the sub function (ANSI 81 <).
REIVAX CONTROL SOLUTION
The aim of the REIVAX strategy is to improve the performance of the speed regulator in the face of speed droops that cause a speed excursion, as it is the case of load rejections and isolations from the power plant . The solution is to place the deflector independently of the conjugation curve of the needle, as shown in FIGURE 3.
The switching of the deflector position reference is made from the disconnected state detection. The speed governor, upon detecting that the unit is disconnected, automatically disables the conjugation curve, so the position reference of the deflector is now determined by the output of a PID loop which controls frequency / speed. This strategy is performed by monitoring the variables of active power and frequency.
With this new philosophy of control the speed recovery time is much faster than the traditional control methodology. From the commutation, the deflector will be positioned by the output of a speed PID. In the period where the speed governor is in the alternative mode with deflector control enabled, the needle is commanded by a reference that matches the value of the new operating point.
REIVAX solution is demonstrated as the real case above, where the problem of speed governor occurred. The load rejection on this case was 50% of nominal and with the adoption of the independent positioning strategy of the deflector, there were no more oscillations of the frequency and of the deflector, nor has the sub frequency occurred.
The positioning strategy of the deflector in the Pelton turbines, available in the RVX Power speed governors , is vital for the performance of the unit in events such as disconnecting from grid and load rejection.
Cases where the closing time of the needles is high and the inertial constant of the Turbine / Generator set is small, the primary frequency regulation may have insufficient performance when the deflector is positioned only through the conjugation curve.
REIVAX solution in the control, independent of the deflector, presents a performance superior to the traditional one that uses only the conjugation curve for the control of the deflector, which features a contribution to the control of Pelton turbines and to the primary regulation as a whole, once it shortens the time of speed restoration and the reduction of the frequency is avoided.
The strategy of independent control of positioning of the Pelton turbine deflector is available in the RVX Power speed governors.