Experimental study on micro-pump turbine test bed and pump performance under working conditions

In order to explore the actual performance of a micro-pump turbine under pump conditions, this paper builds a micro-pumped storage test platform based on micro-mixed-flow pump turbine, and then conducts energy characteristics tests in pump conditions and hump region on this basis. The final determination of the hump area range of 10.6 m3/h∼18.2 m3/h; When the flow rate is 52 m3/h, the pump turbine efficiency reaches the maximum 81.91%, and the head is 16.683 m. In the test, it is found that when the unit runs to the hump area, the noise and vibration of the flow parts are generated, and the output power and efficiency curves show nonlinear growth of abnormal speed. In this paper, by setting up a test bench and conducting real machine tests, the pump operation of the micro-pump turbine is explored, and the hump phenomenon found in the test is further tested, which provides data support and improvement ideas for the next application and optimization design of the micro-pumped storage power station.


Introduction
In order to meet the large-scale application of renewable energy such as solar power generation and improve the power supply security and comprehensive utilization efficiency of conventional power systems and new energy systems, energy storage technology has developed vigorously [1].According to the form of energy storage, electric energy storage can be divided into mechanical energy storage, electric energy storage, chemical energy storage and heat storage.Mechanical energy storage includes pumped storage, compressed air energy storage and flywheel energy storage [2].Among them, pumped storage technology has gradually matured after 100 years of development, featuring large capacity, high efficiency, rapid response to grid demand, long energy storage cycle, etc., and has been applied on a large scale all over the world [3].In the short term, pumped storage can still occupy the large-scale and large-capacity energy storage market with its low cost and perfect operation mechanism [4,5].Although the traditional extraction and storage system has all the above advantages, it still has certain limitations due to its harsh site selection, long application approval cycle and high construction cost [6,7].Therefore, small and micro pumped storage system came into being [8], small and micro pumped storage system generally refers to the installed capacity of 20 to 300 megawatts, water head of 90 to 365m systems [9].The system is extremely flexible, allowing the power system to remain stable under special circumstances, and can also be tailored to the special needs of users [10].Looking at the development of small and micro pumped storage at home and abroad, it only stays in the theoretical research stage, without practical application cases, so it is very necessary to carry out research and analysis of small and micro pumped storage.Therefore, a micro-pumped storage test platform is built in this paper, and the pump performance of the micro-mixed-flow pump turbine is analyzed and studied.

Overview of miniature pumped storage test bench
The micro-pumped storage test bed built in this paper innovatively uses the new supercharged pumped storage technology, gets rid of the strict geographical conditions, has the characteristics of flexible layout, short construction period, no pollution to the environment and low operation and maintenance costs, and has a maximum water head of 100 m, breaking through the key technologies such as a new wide-load efficient pump turbine and a new type of accumulator.Figure 1 shows the final result.He main features are: the bearing unit is the micro-mixed-flow pump turbine studied in this paper; The energy storage tank can be regarded as the upper storage tank is connected with a screw compressor, which can be pressurized in the tank, so as to realize the free adjustment and control of the water head under the working condition of the turbine, and the electric pressure regulator is added to control the output pressure of the stable screw compressor.The storage tank is also connected with a water ring vacuum pump, which can reduce the pressure in the tank to realize the study of cavitation performance under the pump condition.Water circulation is realized through pipeline optimization; The inlet and outlet of the pump turbine are also equipped with a special pressure device, which can realize the accurate measurement of the inlet and outlet pressure.
Based on the graphic programming software Labview, the corresponding measurement and control system is developed to realize the real-time display of the internal pressure and temperature of the accumulator.Real-time display of pressure data at inlet and outlet pressure ring of pump turbine; Electromagnetic flowmeter flow data display; Pump turbine speed and torque data display; The water level data in the tank is displayed, and all the read data is saved to the specified path in real time at the frequency of 1 s/.The final transformation result can achieve the following functions: (1) Carry out energy characteristic test of pump and turbine working condition; (2) Carry out cavitation characteristic test of pump working condition; (3) Carry out zero flow test under pump condition; (4) Carry out full characteristic test of turbine working condition; (5) The water storage capacity of the storage tank is about 6 tons, and when the water storage tank is insufficient for the test, the water in the storage tank can be pressed back to the storage tank by the pump to achieve water replenishment.(1) Pressure transmitter The PTX5072-TC-A1-CA-H0-PA type pressure transmitter measures gauge pressure ranging from 0 to 1.6 MPa, and can output pressure data as current signal with measurement accuracy of 0.2 FS.It is mainly arranged at the inlet and outlet pressure rings of the pump turbine, which is convenient to read the real-time pressure data of the pump inlet and outlet, which is shown in Figure 3.
(2) Liquid level sensor The BLD1B1A2A5AL2000+TS model magnetic flip plate with reed tube remote liquid level sensor is located at the bottom of the water tank, can output the pressure data of the tank bottom current signal, the accuracy level is ± 0.5 FS, can feedback the liquid level change in the water tank in real time, in addition, the sensor has a magnetic flip plate type liquid level gauge outside, can observe the water level change in the water tank in real time, which is shown in Figure 4.
(3) Torque speed sensor The ZH07-100 speed and torque sensor is connected between the pump turbine runner and the motor, which can convert the pump turbine speed into pulse signal and torque into frequency signal, and realize the output of speed and torque signal into current signal, the accuracy level is ±0.5 FS, and the pump turbine speed and torque can be fed back in real time, which is shown in Figure 5.
(4) Frequency converter The test bench uses the inverter as a high-performance vector inverter, which can provide an output frequency of 0~200 Hz to adjust the speed of the pump.The flowmeter has a full scale of 200 m 3 /h, the applicable material is water, and the applicable medium temperature is minus 10 degrees Celsius to 80 degrees Celsius, which is shown in Figure 6.

Test bench parameter.
The basic geometric parameters of the pump-turbine studied in this paper are shown in Table 2.The micro-mixed-flow pump-turbine is made of acrylic (polymethylmethacrylate) with extremely high transparency and strength, except that the blades are made of stainless steel.The volute as a whole is made of two acrylic plates bonded together, and the shaft seal is also made of acrylic material.In order to facilitate the observation of the inlet and outlet flow, the inlet and outlet of the pump turbine are provided with acrylic short tubes, as shown in Figure 7.
Where: H1 -total inlet head, m; P1 -water pump inlet pressure, Pa; P2 -pump outlet pressure, Pa; U -flow rate, m/s; U1 -water pump inlet flow rate, m/s; U2 -pump outlet flow rate, m/s; g -acceleration of gravity, 9.81 m/s 2 ; Z1 -the height of the center of the runner to the datum, m; Z2 -height from exit point to datum, m; ρ -density of water at current temperature, kg/m 3 ; Q -volume flow, m 3 /s; M -torque, Nm; n -rotational speed, r/min; D -impeller diameter, m.

Energy characteristic test of pump working condition
This test was carried out according to the standards of <<GB/T 3216-2016>> and <<GB/T 15613-2008>> .The main test methods and results are as follows.
(1) Test purpose Determine pump efficiency, head, hump zone range and input power.
(2) Test procedure The specific test process is as follows: Step 1 Ensure that the water in the storage tank is 0.75 m, open the corresponding valve of the pump working pipe to the full open state, run the pump turbine, and adjust the frequency of the frequency converter to the design speed of the pump turbine (3000 rpm); Step 2 Adjust the opening of the inlet valve of the water pump pipeline so that the reading of the electromagnetic flowmeter is slightly higher than the initial target value of the test, and the parameters will be read after the water level in the tank drops to 0.70 m and the unit runs stably for 20 s.
Step 3 Stop the test immediately after the completion of the experiment in the first group of working conditions.In the working condition of running water pump, water is added to the energy storage tank until the water level reaches 0.75 m; Step 4 Continuously adjust the opening of the pump outlet valve, repeat steps 2 and 3, change the inlet flow of the pump, read the external characteristic parameters under the corresponding test conditions, until the system flow reaches the maximum test target value ( 65 m 3 /h); Step 5 After all conditions are tested, the generator is powered off, the test bench is shut down, and the test water inside the turbine impeller of the pump is emptied by the blowdown valve at the lower part of the DN80 pressure taking ring. (

3) Test results
According to the test results, its characteristic curves are drawn as follows: According to the experimental results, the analysis is as follows: ① head-flow curve.According to Figure 8 and Figure 9, when the pump turbine is running under the pump condition, its overall head gradually decreases with the increase of flow rate.It can be mainly divided into four operating areas.When the unit runs in the small flow area of 7 m 3 /h~ 12 m 3 /h, its head decreases significantly with the decrease of flow rate, and it shows a downward trend of first fast and then slow.As the flow rate increases (12 m 3 /h~ 18 m 3 /h), the pump head increases significantly, forming a fluctuation area similar to the "hump" shape on the curve.Among them, the lowest point of the hump region corresponds to a flow of 12 m 3 /h and a corresponding lift of 21.871 m.The highest point in the hump region corresponds to a flow of 18m 3 /h and a corresponding lift of 22.467 m.When operating in this area, the unit flow rate will change rapidly, resulting in extremely unstable operation, and different operating flows correspond to the same head.During the test, the draft pipe was found to vibrate violently, and a "buzzing" noise could be heard.In the transition zone (18 m 3 /h~ 50.5 m 3 /h), the head increases linearly as the flow decreases.When running in the large flow area (50.5 m 3 /h7 0 m 3 /h), the head decreases linearly and rapidly with the decrease of flow, and the decrease speed is greater than that in the transition area.
As can be seen from Figure 10, when a pump-turbine operates as a pump, its output power generally increases first and then decreases with the increase of ground flow in the unit, and the curve is similar to an inverted parabola.In the process of approaching the optimal working condition (7m 3 /h~ 52 m 3 /h), the output power of the unit continues to increase with the increase of flow rate, and the output power reaches the maximum value of 2360 W when approaching 52 m 3 /h.Then, when the unit runs to the high flow condition (52 m 3 /h~ 70 m 3 /h), the output power value of the unit begins to decline with the decrease of the flow rate.
As can be seen from Figure 11, when the pump turbine operates as a pump, its efficiency generally increases first and then decreases as the flow rate increases.Its curve is similar to the output powerflow curve, and it can be seen from the efficiency-flow curve that there is a relatively wide efficiency zone.The efficiency reaches the maximum when approaching 52 m 3 /h.Through several tests in a small range near the flow point, it is found that when the flow rate is 52 m 3 /h, the efficiency of the pump turbine reaches the maximum of 81.91 %, the output power is 2360 W, and the input power is 2880 W.Then, when the unit runs to the high flow condition (52 m 3 /h~ 70 m 3 /h), the efficiency value of the unit decreases linearly and rapidly with the decrease of flow rate.

Hump area test under pump condition (1) Test purpose
It can be seen from the head-flow characteristic curve of the pump condition that when the micromixed-flow pump turbine runs at the low flow point of the pump condition, the unit head fluctuates continuously in an unstable operation state, and one head corresponds to multiple flows on its characteristic curve.Therefore, the characteristic curve at the low flow condition also presents a shape similar to "hump".However, due to the rough selection of working conditions in the energy characteristic test, the range of the hump region can only be roughly determined, and when the unit is running in the hump region, the flow rate will change rapidly, making the test operation more difficult.Therefore, the main purpose of the pump hump region test is to measure the characteristic parameters of the pump hump region through detailed measurement, and accurately draw the characteristic curves of the pump in the hump region.
(2) Test procedure Step 1 Ensure that the water level in the energy storage tank is 0.8 m, open the pump working pipeline valve, run the pump turbine, and adjust the generator speed to the design speed (3000 rpm); Step 2 Adjust the opening of the inlet valve of the pump pipeline to make the reading of the electromagnetic flowmeter slightly higher than the initial target value of the test, and read the first set of parameters (flow rate, water head, torque, speed, etc.) after the water level in the tank drops to 0.75 m and the unit runs stably for 15 s; Step 3 During the test, the amount of water in the storage tank is considered and the maximum running time is calculated.When the water level in the tank reaches 0.7 m, the test is stopped, and water is added to the storage tank until the water level reaches 0.8 m in the working condition of running the water pump.
Step 4 Continuously adjust the opening of the inlet valve of the pump, repeat steps 3 and 4, and read the corresponding value until the system flow reaches the maximum target value of the test (22 m 3 /h); Step 5 After all conditions are tested, the generator is powered off, the test bench is shut down, and the test water inside the turbine impeller of the pump is emptied by the blowdown valve at the lower part of the DN80 pressure taking ring.
(3) Test results According to the test results, the head-flow curve, head-output power curve and head-efficiency curve are drawn respectively:   energy characteristic test (12 m 3 /h~ 18 m 3 /h), the flow and lift range of the working point in the hump region are further accurate.Among them, the measured hump region ranges from 10.6 m 3 /h to 18.2 m 3 /h, and the measured lowest hump region corresponding flow is 10.6 m 3 /h, and the lift is 21.815 m.The highest point in the hump region corresponds to a flow of 18.2 m 3 /h, and the head is 22.531 m.
During this period, it was found that the draft pipe, the connection between the draft pipe and the runner inlet and the volute outlet produced relatively obvious vibration, and emitted a noise similar to that of an electric drill.② Output power-flow curve.
As shown in Figure 13, when the pump-turbine is running in the hump region (10.6 m 3 /h~ 18.2 m 3 /h), its output power generally shows a trend of gradually increasing with the increase of flow rate.However, since the pump turbine is in an unstable state when running in the hump region, the flow rate and the head increase synchronously, it can be seen from the output power-flow characteristic curve that when running in the hump region, the output power will produce nonlinear growth at an abnormal speed, and the characteristic curve will fluctuate.
As shown in Figure 14, when the pump-turbine is running in the hump region (10.6 m 3 /h~ 18.2 m 3 /h), its efficiency generally shows a trend of gradually increasing with the increase of flow rate.However, due to the unstable running state of the pump turbine in the hump region, the abnormal phenomenon of synchronous increase of flow and head appears, and its efficiency also produces an abnormal increase similar to the output power, and the fluctuation is more drastic.

Analysis of pump working condition test results
The internal flow field of pump turbine is complicated when it is running under pump condition, which is difficult to be observed by experiment.By drawing all kinds of external characteristic curves, the performance of the pump can be reflected in detail, concrete and direct, so as to master the operation of the pump turbine under this condition.In addition, there is an unstable operation area in the lift-flow characteristic curve obtained from the preliminary energy test, which is shown as a "hump-like" fluctuation on the curve.In view of this phenomenon, more precise and accurate tests are carried out in this paper, and more accurate results and variation trends of the "hump" range and external characteristic parameters of the unit operating in this area are obtained.
According to the test results, the micro-mixed-flow pump turbine tested in this test has an unstable operation area of "hump" when it runs in the area of low flow and high lift under the pump working condition, resulting in an operation situation in which the head that does not conform to the operation law of the pump increases synchronously with the flow rate.An unstable operating state occurs in which one flow corresponds to multiple heads.In addition to the abnormal change of head, the output power and efficiency also increase abnormally with a certain fluctuation, which reflects the instability of the external characteristic parameters.At the same time, it is found that the draft pipe, the connection between the draft pipe and the runner inlet and the volute outlet produce relatively obvious noise and vibration during operation in this area, which needs to be further analyzed and studied by numerical simulation.
When the pump turbine is running as the pump, the overall trend is that with the increase of flow, the head gradually decreases, the output power and the efficiency rise first and then fall.Finally, the range of the hump region was determined (10.6 m 3 /h~18.2m 3 /h), in which the lowest point of the hump region corresponds to a flow of 10.6 m 3 /h, corresponding to a lift of 21.815 m, and the highest point corresponds to a flow of 18.2 m 3 /h, corresponding to a lift of 22.531 m.After repeated measurements in a small range, it is determined that when the flow rate is 52 m 3 /h, the pump turbine efficiency reaches the maximum value of 81.91 %.At this time, the pump turbine head is 16.683 m, the output power is 2360 W, and the input power is 2880 W. At this time, the noise at the draft pipe and volute gradually becomes smaller, and the whole unit is stable and no abnormal sound is generated.

Conclusions
In this paper, a micro-pumped storage test platform based on micro-mixed-flow pump turbine is built, and then the pump working condition energy characteristic test and hump region energy characteristic test are carried out on this basis, which provides data support and improvement ideas for the next application and optimization design of micro-pumped storage power station.
The test results show that when the pump turbine is running in the pump condition, the overall trend is that with the increase of flow, the head decreases, the output power and efficiency rise first and then decrease.Finally, the hump region range (10.6 m 3 /h~ 18.2 m 3 /h) was determined.The lowest point of the hump region corresponds to a flow of 10.6 m 3 /h and a corresponding lift of 21.815 m.The highest point in the hump region corresponds to a flow of 18.2 m 3 /h and a corresponding lift of 22.469 m.It is also found that when the unit runs to the hump area, the draft pipe, the connection between the draft pipe and the runner inlet and the volute outlet produce relatively obvious noise and vibration, the output power and efficiency increase nonlinear with abnormal speed, and the characteristic curve fluctuates.When the flow rate is 52 m 3 /h, the optimal working condition is reached, and the efficiency of the pump turbine reaches the maximum value of 81.91 %.At this time, the head of the pump turbine is 16.683 m, the output power is 2360 W, and the input power is 2880 W. At this time, the noise at the draft pipe and volute gradually becomes smaller, and the whole unit is stable, and no abnormal sound is produced.

Figure 2 .
Figure 2. Data acquisition system.The main equipment used in this test is frequency converter, pressure transmitter, water level sensor, torque speed sensor, electromagnetic flowmeter, etc.The following part of the important equipment is introduced.(1)Pressure transmitter The PTX5072-TC-A1-CA-H0-PA type pressure transmitter measures gauge pressure ranging from 0 to 1.6 MPa, and can output pressure data as current signal with measurement accuracy of 0.2 FS.It is mainly arranged at the inlet and outlet pressure rings of the pump turbine, which is convenient to read the real-time pressure data of the pump inlet and outlet, which is shown in Figure3.(2)Liquid level sensor The BLD1B1A2A5AL2000+TS model magnetic flip plate with reed tube remote liquid level sensor is located at the bottom of the water tank, can output the pressure data of the tank bottom current signal, the accuracy level is ± 0.5 FS, can feedback the liquid level change in the water tank in real time, in addition, the sensor has a magnetic flip plate type liquid level gauge outside, can observe the water level change in the water tank in real time, which is shown in Figure4.(3)Torque speed sensor The ZH07-100 speed and torque sensor is connected between the pump turbine runner and the motor, which can convert the pump turbine speed into pulse signal and torque into frequency signal, and realize the output of speed and torque signal into current signal, the accuracy level is ±0.5 FS, and the pump turbine speed and torque can be fed back in real time, which is shown in Figure5.(4)Frequency converter The test bench uses the inverter as a high-performance vector inverter, which can provide an output frequency of 0~200 Hz to adjust the speed of the pump.The flowmeter has a full scale of 200 m 3 /h, the applicable material is water, and the applicable medium temperature is minus 10 degrees Celsius to 80 degrees Celsius, which is shown in Figure6.

Figure 4 .
Figure 4. Speed and torque sensor of the ZH07-100 model.

Figure 14 .
Figure 14.Hump area flow-efficiency curve.① head-flow curve.According to Figure 12, through accurate measurement, the lift-flow curve of the pump turbine shows a more obvious "hump" shape.Compared with the hump region tested in the initial pump

Table 1 .
2.1.1Testbenchparameter.Table1shows the parameters of the test bench.Parameters of the test bench.Test bench parameter.The test bench is an open test bench, which consists of two parts: water circulation system and data collection system.The water circulation system includes energy storage tank, water ring vacuum pump, pump turbine and motor, electric and manual regulating valve, carbon steel water pipe, water storage tank and other equipment components.The data collection system includes pressure gauge, vacuum meter, flow meter, torque meter, DC voltage regulator power supply, pressure sensor, acquisition board and computer equipment components.The program of the test bench test system is compiled based on the graphical programming software Labview, which mainly includes three main parts: acquisition chassis, display and acquisition card.The schematic diagram is shown in Figure2.

Table 2 .
Basic parameters of micro-pump turbine.Therefore, the uncertainty of measurement parameters in pump turbine tests is mainly caused by random errors, as shown in Table3.
2.2.1 Error analysis.Test error is the difference between the actual measured value and the real value, which is divided into systematic error, random error and gross error.Errors are unavoidable in the test process, and gross error and systematic error can be eliminated by guaranteeing the test method, test system and test conditions.

Table 3 .
(7)ameters of the test bench.Test data processing methods.Data such as flow rate, total pressure at inlet and outlet, torque, speed, etc. are recorded in the test measurement.After processing, the following parameters can be obtained, as shown in formulas (1) to(7).