A new method for regulating Pelton turbine based on ejection of discharged flow

. Hydropower plants (HPPs) contribute a significant share of Russia's electricity generation. With the prospects of developing this sector and harnessing the hydroenergy potential, it is necessary to consider methods for modernizing hydroelectric installations. This article examines a new method for regulating active (Pelton) turbines based on modifying the nozzle flow rate through the ejection of spent liquid from the runner using a jet pump. The relevance of this solution lies in the potential to increase the operational duration of high-head pumped-storage hydropower plants (PSHPPs) and improve reliability by eliminating or reducing the intensity of operation of the regulating needle in the guide apparatus. Various structural configurations for incorporating the jet pump into the hydrosystem of HPPs/PSHPPs with Pelton hydro turbines are discussed. Parallel and series configurations offer specific advantages and disadvantages. A methodology for determining the expression to calculate the parameters of the injecting device, depending on the turbine's characteristics, is presented based on a mathematical model of the jet pump with a central nozzle.


Introduction
It is well known that in conditions of high head and low flow rate, the use of Pelton turbines is advisable.Leading hydro turbine manufacturers confirm this trend through numerous implemented projects [1][2].Increasing the unit power of HPPs/PSHPPs leads to improved operational efficiency.The main method of regulating Pelton turbines involves the use of a guide apparatus with a regulating needle, which, under high head conditions, has certain drawbacks that require an analysis of the basic design and operation of the turbine.

Research Problem
A Pelton turbine belongs to the class of impulse turbines.It consists of a runner wheel 1, a guide apparatus 2, and a deflector 3. The guide apparatus includes a nozzle 4 and a regulation system 5, which consists of a needle inserted into the nozzle and a servomotor controlling it (Figure 1) [3].There are Pelton turbines with horizontal and vertical shaft arrangements.Horizontal installations are suitable for turbines with no more than two nozzles, while vertical installations can have up to six nozzles.One significant advantage of the vertical arrangement is that the nozzles can be symmetrically positioned around the wheel [4].The runner wheel of a Pelton turbine comprises a shaft, a guide bearing, and a set of buckets.The inlet edge of the buckets divides the blade in the middle into two equal parts.In the nozzle apparatus, the high-pressure flow is transformed into a high-velocity jet, which impinges on the buckets, transferring its kinetic energy to the runner wheel and converting it into mechanical rotational energy.The efficiency of this conversion, among other factors, depends on the number of jets.On Figure 2, the basic design of the guide apparatus is shown [4].A guide apparatus with a regulating needle is installed on each nozzle apparatus and serves to control the flow rate.Since the needle is surrounded by a water flow, the hydraulic force acts against any changes in the needle's position.This hydraulic force is generated due to the pressure distribution on the needle's surface.Depending on the position of the needle, this force can have a closing or opening direction.
With large displacements of the needle, the hydraulic force is directed to push it deeper into the nozzle, resulting in an opening tendency.To compensate for the influence of the hydraulic force, a spring is introduced.However, due to the decrease in the flow pressure acting on the needle, there is a possibility of sudden closure of the nozzle under the action of the spring, leading to water hammer.To avoid water hammer, a deflector is employed, which also significantly reduces the rotational acceleration frequency of the rotor.
Despite the simplicity of the flow regulation method using a needle, it requires additional devices to ensure its functionality.Furthermore, this method is based on the throttling principle.For high-head hydro units, this leads to increased fluid velocities, resulting in losses and accelerated equipment wear.
In this regard, a new method for regulating the flow of Pelton hydro turbines is proposed, which is based on modifying the nozzle flow by ejecting the fluid discharged after the turbine wheel using a jet pump.The active flow is provided by the station's head, while the passive flow is extracted from the turbine outlet.The combined flow is then directed to the nozzle.Consequently, under conditions of a constant flow rate through the penstock (based on river discharge calculations), it becomes possible to increase the flow rate through the nozzle, thereby altering the power output.Wherein, the regulation of the jet pump operation can be achieved by manipulating the low-pressure passive flow with a gate valve, which can reduce equipment wear.Furthermore, it should be noted that in certain cases, unlike the use of regulating needles to adjust the flow, it is possible to install only one ejector before the branching of the conduits in the guide apparatus of the Pelton hydro turbine.This can contribute to equipment cost reduction, taking into account the synchronization system.This can contribute to equipment cost reduction, taking into account the synchronization system.The proposed regulation method offers advantages for PSHPPs, including the potential to increase the overall emptying time of the upper reservoir due to the extraction of the exhausted working fluid.This fact theoretically could lead to a reduction in the required volume of the upper surge tank, thereby reducing capital investments during the initial stages of PSHPP construction.

The Proposed Method of Flow Regulation
The implementation of the flow regulation method for Pelton turbines, based on modifying the nozzle flow by ejecting the discharged fluid from the turbine wheel, can be realized through various schemes for integrating the jet pump into the hydraulic system of the hydro unit: the series and parallel connection schemes of the jet pump (Figure 3).In the case of series installation of the jet pump for ejecting the discharged flow, the entire flow passing through the nozzle is used as the working fluid.Regulation can be achieved by adjusting the gate valve on the passive flow line.In the fully closed position, the flow will correspond to the standard operating scheme of the Pelton turbine, considering the additional resistance of the jet pump.Since the pressure in the passive flow line is low, this regulation method offers operational advantages compared to needle valve regulation, which involves throttling the high-pressure flow.
When a jet pump is installed in parallel, only a part of the total flow through the nozzle can be used as a working flow.In this case, it is possible to achieve higher ejection efficiency values, however, to ensure the operability of the circuit, it is necessary to tune the system in terms of the formation of a pressure drop between the lines of the active and mixed flow of the jet pump, which can be achieved by installing a valve in parallel.
For this case, we can write: ( Here: -flow bypassing and through the jet pump respectively [m 3 /s]; upstream flow [m 3 /s].
It is known that the power of the turbine can be determined as follows: By analyzing the flow expressions, it can be concluded that the inclusion of a jet pump in the system leads to an increase in flow rate and should contribute to an increase in power.However, the jet pump acts as a resistance and generates additional flow by converting part of the head, thus reducing the specific energy and potentially resulting in a corresponding decrease in power.To assess the actual impact of the jet pump on turbine power, it is necessary to develop a mathematical model.However, an important step in this process is the coordination of the ejector's parameters with the parameters of the hydraulic system.
For the implementation of the selection methodology, we will consider a basic model of a jet pump with a central circular nozzle and a cylindrical mixing chamber, for which the relationships between dimensions and energy parameters are well-known.
The calculation scheme of the jet pump is shown in (Figure 4).method is new, and the obtained tool allows for the evaluation of the ejector's efficiency for different hydro units.

Conclusions
The article reviewed the design of the Pelton turbine with its characteristic method of flow regulation using a needle.Despite its simplicity and effectiveness, this method has its drawbacks, leading to the proposal of an alternative flow regulation approach using the ejection of fluid discharged from the turbine wheel using a jet pump.
For the proposed method, a mathematical model was developed to select the parameters of the jet pump for the hydraulic system of the Pelton turbine.

Fig. 4 .
Fig. 4. Schematic diagram of the flow path of a jet pump.