Transition processes during the start-up of the pumping unit of happ

. The transition processes of the centrifugal pump in hydro accumulation power plant (HAPP) are studied in the article. The reliability of the pumping system is increased by the correct selection of the parameters of the transition process when regulating the operation of the pumping units. This prevents vibration damage and erosion. Based on the results obtained by calculating the hydraulic shock and the pressure loss in the system together, damage and failures in the pumping equipment and the network can be avoided. The results and graphs allow you to choose the right electric motor for the pump unit, taking into account the required pressure and water supply. Technical and economic efficiency is achieved by choosing a rated electric motor for the pump unit. Constructing graphic descriptions of the pump, determined the magnitude of the hydraulic effect in the grapho-analytical method.


Introduction
Hydro accumulation power plant (HAPP) are built in large power systems to replenish energy during times of high energy demand.The energy storage feature of HAPP is based on the use of electricity that is idle in the power system for some time.The main function of the HAPP is to pump water from the lower pool to the upper pool, using it to generate electricity when needed when electricity is tight.Efficient use of water and energy resources requires proper use of HAPP pumps.
However, until now, the modes of soft start of HAPP hydraulic units in the pumping mode remain insufficiently studied and imperfect [1].At the same time, in practice, the current domestic and foreign HAPPs use a variety of methods for starting hydraulic units in a pumping mode, but all known methods have their own advantages and disadvantages.In particular, with the most technically simple method of direct asynchronous start (without excitation), huge starting currents arise, leading to premature wear of the hydraulic unit and deterioration in the quality of the voltage of the power system [2]- [4].
It is known that most thermal power plants (nuclear power plants abroad) are currently used for the production of electricity.In our republic, 85% of all energy is produced with the help of thermal power plants.One of the specific features of the operating mode of thermal E3S Web of Conferences 420, 07023 (2023) https://doi.org/10.1051/e3sconf/202342007023EBWFF 2023 power plants is that their nominal (installed) power differs significantly from the minimum power required at night.For this reason, it is necessary to reduce the power of the thermal power plant by 25-50% at night, and to stop some units completely.This has many negative aspects, for example, changing the power of the units, frequent stopping and starting will cause the equipment to malfunction and fail before the specified time.For example, according to the sources given in [5]- [8] a 20% change in the power of the TPS leads to a reduction in the service time of the equipment by 15-20%.Therefore, using HAPPs as a consumer when energy consumption is low at night and as a producer during hours of high energy consumption during the day is very effective.HAPPs are more affordable, efficient and promising than other devices performing this task (gas turbine, steam gas turbine).In some cases, HAPP can participate not only in daily energy distribution, but also in weekly energy accumulation.In this case, water energy is collected on two weekends (days with low energy consumption), and on the remaining five days it is used to generate electricity.A transition occurs when starting and stopping pump units.During the transition, the pump unit is stressed and has a negative effect on the operation of the pump unit.When the pump unit is started, the load on the electric motor is 5-7 times higher than the nominal one.By regulating the mode of operation of pumping units at pumping stations, it is possible to ensure the uninterrupted, normal operation of the system and prevent various negative consequences.Centrifugal pumps are widely used in engineering practice.The pump parameters fluctuate greatly during power outages over time, and such vibrations lead to high pressure pulsations in the flow and vibrations in the pump system.After a power outage, the transient process mainly experiences four modes, namely pump mode, brake mode, turbine mode, and escape mode [4], [9]- [12].

Materials and methods
Due to the fact that the non-return valve is not installed in the pressure pipes of the pumping stations, the pipe is empty when the pump unit is started.When the pump starts, the water movement in the water transmitter starts immediately, and the shock wave created by the sudden increase in pressure spreads along the pipe.Therefore, during start-up, the torque on the electric motor shaft is greater than the torque of the pumps.This torque difference is ensured by increasing the rotation frequency of the rotor of the pump unit.As the number of revolutions of the impeller increases, the pressure generated by the pump increases.Due to the partial opening of the valve after the pump, water consumption Q n is transmitted through the pump.The difference between the pressure created by the pump and the pressure created in the pipe represents the pressure loss in the valve, the reduction of the pressure in the network depends on the consumption of water driven by the pump, and it is calculated as follows: Here H n -pressure created by the pump, m; H Q -pressure in the pipe, m; Q n -consumption of water delivered by the pump, m 3 /sek; S -the characteristic coefficient of the pipe depends on the geometric dimensions of the pipe, the material of the pipe, the coefficient of local resistance and the coefficient of hydraulic friction, and the following formula is used: E3S Web of Conferences 420, 07023 (2023) https://doi.org/10.1051/e3sconf/202342007023EBWFF 2023 Here λhydraulic friction is the coefficient of resistance, which depends on the order of fluid movement and the thickness of the pipe wall; ξthe coefficient of local hydraulic resistance, which depends on the speed of the liquid in the pipe, the change in direction and the equipment.
When starting a centrifugal pump, the valve in the delivery pipe must be closed.The transfer pipe must be filled with water for back pressure during start-up.When the pump pushes water into the pipe, the check valve slot opens and water starts to flow and pressure builds up in the delivery pipe.When the liquid passes through the check valve, the water in the pipe begins to compress, as a result of which the pressure increases and the pipe expands, a pressure wave spreads along the length of the pipe.In this case, a hydraulic blow begins to form in the pipe.The appearance of pressure pulsation in the pumping station causes failure of hydromechanical devices and elements of pumping stations.To eliminate this situation, it is necessary to perform the following sequence when starting the pump unit:  synchronization of rotation frequency;  increase the pressure in the pipe by synchronizing the rotation during the filling of the pipe;  actuation of shut-off devices after pressure build-up;  achieve accounting order at output.It is not possible to do all this at the same time, therefore, it is necessary to determine an effective regime.If a check valve is installed in the delivery pipe, the pipe will be full of water when the pump starts.Therefore, at the time t j when the pump starts, the water flow through it will be Q nj = 0 [13].When the check valve opens, if there is not enough water after the valve, there will be an unstable movement in the water transmitter, which will cause vibration in the pipe.When the valve is open, the calculation of the pump start-up is performed in the following order.In order to simplify the calculation work, the length of the calculation piece Δl is taken uniformly to simplify the calculation work.If the wave propagation speed a in the water transmitter is constant along the entire length, the time that determines the pressure change will be the same for all sections: The current consumption of water driven by one pump is determined as follows: Here v0 -velocity of the water in the water pipe at the time being viewed., m/sec; ωcross-sectional area of the pipe, m 2 ; nthe number of pumps working in parallel; n TR -the number of water transmitters working in parallel.When the pumps are running in parallel, the pump that is started is under pressure.When starting and stopping the pump, the movement of water in the water transmitter is unstable, and the speed of such unstable water can be determined from the following formula [14]: Here v o -water velocity in the water transmitter at the initial time (v o = 0 m/sec), gfree fall acceleration, m/sec 2 ; avelocity of wave propagation in pressure change, m/sec; φ − ψthe sum of the wavelengths of pressure changes, respectively, in the direction of water movement and in the direction opposite to water movement.
Effort in unstable motion is defined as follows: Here H 0 -initial pressure height, H 0 = H i .Due to the short length of the pump's suction pipe, the hydraulic shock at the suction is not considered in most cases [14].Hydraulic hammer is mainly for water transmitters.When calculating the hydraulic stroke at the start and stop of the pump, it is calculated from the Zhukovsky formula [15]: Here Kmodulus of elasticity of water, Pa; Emodulus of elasticity of the pipe, Pa; δthe thickness of the pipe wall, m; dthe diameter of the pipe, m.Pressure loss is not taken into account in the pressure increase in the Zhukovsky formula.Here, hydraulic shock is reduced due to pressure loss, which is considered as a special case.The analysis of the phenomenon of hydraulic shock takes an important place in the systems where centrifugal pumps are used.If automated centrifugal pumps are used in a small water supply system, the valve on the drive water transfer must be open when the pump is started.In this case, due to the small inertia in the moving liquid, the pump consumes less power, so it quickly acquires a sufficient frequency of rotation, high pressure is created in the pipe until the liquid moves.In a short time, the pump drives the fluid as if the valve were closed.In this case, the liquid itself is compressed, the pipe through which the liquid is being transferred fills up, and the pipe wall expands under pressure.In such conditions, i.e. during pump startup, liquid consumption is minimal and the generated working pressure is maximal.To take into account the above, it is necessary to analyze the working characteristics of the pump and the pipeline together.This can be obtained directly from the Q-H characteristic of the centrifugal pump.The maximum pressure value at the start of the pump is determined from the working point of intersection of the pump characteristics and pipe characteristics.The equation of motion of the liquid is as follows: pressure during pump start-up, ;   -coefficient of movement quantity (  = 1 can be accepted in calculations),   -movement cross-sectional surface of the water transfer,  2 .The curve (1) in Figure 1 is the  = () characteristic of the pump and is taken from the pump passport or pump catalog.Curve ( 2) is the characteristic of the pipe for steady motion and is defined by the following relationship: Here characteristic coefficient of the pipe.During the normal operation of the pump, the working point A is determined, where the pump characteristic and the pipe characteristic intersect.Formula ( 11) is used to determine the maximum pressure during pump start-up, and based on this formula, a graph of the pressure in the pipeline is drawn.For this purpose, by giving successive values H, the consumption of the liquid driving the pump is determined.Based on the obtained values, the 4th straight line on the graph is constructed and point C is found.Since the graph uses the exact formula (11), the pressure loss in the water transmitter is found in exact values [16].
Due to the formation of a hydraulic shock during the pump start-up, the Zhukovsky formula is used to calculate the pressure: Since  in this formula has a variable value, line 3 in the figure is constructed, and based on this, the operating point  is found.
If a non-return valve is installed in the water transfer pipe, there is no significant danger to the water transfer pipe when the pump is stopped.Mainly, the increase in pressure leads to the occurrence of danger in the water transmission pipe when the continuity of the flow is broken.
If the continuity of the flow is not broken, the maximum pressure increase is determined by the following formula [14]: Here -time, ; usually  0 phase duration: 1 -the distance from the pumping station to the point in question, ; natural logarithm base (e=27183); is the wave attenuation coefficient, which in practice is determined from the following formula: ℎ  -loss of pressure, ; the length of the pipe, ; 3 Results and discussions As a result of the experiment, formula (13) was fully studied [14].Therefore, the calculated effort in high-power pumps is slightly different in practice, analytical calculations for small pumps give an accurate value, but in practice, large-power pumps are used for water transfer.If the pump is not equipped with a non-return valve, the calculation becomes more complicated, because if the non-return valve is installed, there is a reverse flow when the pump is stopped, and the pump impeller rotates in reverse, that is, the pump works as a turbine.
Reverse rotation of the pump impeller causes the electric motor to fail.If the moment of inertia in the rotor of the pump unit is too large, the check valve will not be installed.Because a large hydraulic shock causes damage to the system.It is possible to determine the value of pressure increase and decrease in the pump using the approximation method when estimating the hydraulic shock [16]- [18].Based on such calculations, a graph is drawn (Fig. 2).On the ordinate of the graph, the pressure reduction coefficient is   and the increase coefficient is   , which is obtained in % compared to the normal operation of the pump.The value of  1  is placed on the abscissa axis, where: Here water consumption of the pump, Maximum pressure increase: Here H p -working pressure, m; H o -static pressure, m.

Conclusions
The correct analysis of the hydraulic shock in the pump system allows for convenient and effective organization of the work order and increases the reliability of the system.
Correct selection of transition process parameters in regulating the operation of pumping units in HAPP increases the reliability of the entire system except for the pump, prevents vibrations, vibration damage and erosion.
The correct analysis of the hydraulic shock generated in the pumping system increases the reliability and efficiency of the system, and the parameters of the pump are selected accordingly.According to the information studied and obtained above, it is necessary to increase the downtime by preventing the stress during the start and stop of the pump by reducing the hydraulic shock, which requires the use of additional devices.This increases the cost of the network, but prevents the pump and network failure.
Based on the results and graphs, the correct electric motor for the pump unit can be selected depending on the required pressure and water supply.Technical and economic efficiency is achieved by choosing the right engine.
When performing the above calculations, based on the results obtained by calculating the hydraulic shock and the pressure loss in the system together, damage and malfunctions in the pumping device and the network will be avoided.

Fig. 1 .
Fig. 1.Combined characteristics of a centrifugal pump and a pipeline.1-pump characteristic, 2-pipe characteristic of fluid in steady motion, 3, 4-pipe characteristic of fluid in unstable motion.

Fig. 2 .
Fig. 2. The graph for determining the values of increase and decrease in the maximum pressure of the pumps.
Based on  1  and 2 values,   and   values were determined.The decrease and increase in pressure are determined as follows: