Pump set switching control method under variable same-frequency control mode

Abstract

The invention discloses a pump set switching control method under a variable same-frequency control mode, which relates to the technical field of pump set switching control and comprises a first switching strategy based on total flow required by a refueling branch and a second switching strategy based on lift output rate of a pump set. The pump set switching control method under the variable same-frequency control mode has the advantages that a switching strategy is designed on the basis of the total flow of a refueling branch, firstly, the required flow condition is obtained in time, and switching action does not need to be carried out after the pressure of a looped network exceeds a threshold value; secondly, a plurality of pumps can be switched at one time, and the time of the switching process is shortened; thirdly, the pump set is prevented from working in a low-flow or excess-flow area, namely the pump set is ensured to work in an efficient working area; the switching strategy based on the pump set lift output rate has the advantage that the problem that the pump set working condition is in a non-efficient working area in the pressure maintaining and adjusting process due to the fact that the inlet pressure of the pump set is changed due to the liquid level change of the oil storage tank can be solved.

Description

Technical Field

[0001] The invention relates to the technical field of pump group switching control, and in particular to a pump group switching control method under a multi-variable same-frequency control mode. Background Art

[0002] The aviation linear refueling system consists of multiple parallel centrifugal pumps (usually 4 in a group), a refueling ring network, several refueling units and related pipe networks, such as Figure 1 As shown. In order for the refueling unit to work properly, the refueling ring network needs to maintain a certain pressure (generally 0.35-0.8MPa), depending on different refueling conditions. As the required flow changes, the pressure loss in the pipe network will also change accordingly. Therefore, it is necessary to adjust the number of pumps put into use and their speed to match different flow requirements, which is called pressure maintenance control.

[0003] At present, the most commonly used pressure-maintaining control scheme is "multiple industrial frequencies and one variable frequency" control, that is, one pump in the pump group is variable frequency operated, and the other pumps are fixed frequency operated, and the output flow is adjusted by the operating frequency of the variable frequency pump. Its characteristics are high control accuracy and simple process, but there are also problems such as long control parameter debugging cycle, unbalanced operating load of the variable frequency pump and the industrial frequency pump, and easy wear of the variable frequency pump. In order to solve these problems, a newer control method "multi-pump synchronous variable frequency" control has emerged, that is, the output flow is controlled by synchronous variable frequency of the pump group, which is characterized by high operating efficiency, convenient debugging, and relatively balanced operating load of the pump group.

[0004] Regardless of the control method, it involves pump switching control, that is, increasing (switching) or reducing (cutting) the number of pumps put into operation as needed. The current practice is to set the upper (lower) limit threshold of the ring network pressure, and to cut (switching) the pumps in accordance with the lower (upper) limit of the frequency. Specifically, when the operating frequency of the pump group reaches the upper limit and the ring network pressure reaches the lower limit threshold, it means that the number of pumps put into operation is insufficient, and one operating pump is added; conversely, when the operating frequency of the pump group reaches the lower limit and the ring network pressure reaches the upper limit threshold, it means that the number of pumps put into operation is too large, and one operating pump is reduced.

[0005] Using the above switching control strategy, two major problems were found in engineering applications: (1) Pump switching is carried out one by one. When the refueling demand flow changes greatly, the adjustment time is too long, resulting in the inability to maintain the ring network pressure within the specified range in a short period of time; (2) Due to the change in the liquid level of the oil storage tank, the inlet pressure of the pump group changes. During the pressure maintenance adjustment process, the pump group operating condition may be in a non-efficient working area.

[0006] Therefore, it is urgent to improve this shortcoming. The present invention studies and improves the existing technology and its shortcomings, and provides a pump group switching control method under a multi-variable same-frequency control mode. Summary of the invention

[0007] The object of the present invention is to provide a pump group switching control method under a multi-variable same-frequency control mode to solve the problems raised in the above background technology.

[0008] To achieve the above object, the present invention provides the following technical solutions: a pump group switching control method under a multi-variable same-frequency control mode, wherein the first pump group switching control method uses the total flow required by the refueling branch as a switching strategy, and comprises the following steps:

[0009] S1. Drawing of HQ characteristic curve, high-efficiency operation area and working trajectory of pump group;

[0010] S2. Calculation of pump change flow rate at different pump group inlet pressures;

[0011] S3. Design of pump switching logic.

[0012] Furthermore, in step S1, it is assumed that the details of the efficient operation area of ​​a certain type of pump group and the trajectory curve at different inlet pressure working points are as follows:

[0013] The characteristic curves of double pumps, triple pumps and quadruple pumps are obtained by the horizontal addition method (measured data can also be used for fitting) through the single pump operation characteristic curve, and the high-efficiency operation areas are divided, and the more efficient combination is selected when there is overlap between the areas;

[0014] The four working point trajectory curves from top to bottom (orange, purple, green and blue respectively) correspond to the pump group inlet pressures generated by water columns at heights of 0, 5, 10 and 15 meters respectively. As the pump group inlet pressure increases, the actual working point curve will shift downward accordingly.

[0015] Furthermore, in step S1, according to the assumption, the efficient operation area of ​​a certain type of pump group and the working point trajectory curve at different inlet pressures are drawn, and in the figure, the intersection of the working point trajectory curve and the efficient operation area can be divided into two cases: one is when the pump group inlet pressure is low, the intersection is on the power frequency characteristic curve of the pump group; the other is when the pump group inlet pressure is high, there will be a situation where the intersection is on the frequency reduction trajectory curve of a certain operating point (the three vertical purple thick solid lines in the figure); in summary, the intersection (pump change) flow value Qc (m 3 / h) with pump inlet pressure H PumpI (meters of water column) changes.

[0016] Furthermore, in step S2, based on the drawn efficient operating area of ​​a certain type of pump group and the trajectory curve of the working point at different inlet pressures, a mathematical model of the pump change flow rate changing with the inlet pressure of the pump group is established, and the trajectory curve and the power frequency characteristic curve near the intersection are approximated as straight lines to obtain a calculation model.

[0017] Furthermore, in step S3, in order to avoid mis-tripping of the pump due to flow fluctuation, the following logic design may be adopted:

[0018] 1) Set a ΔQ value so that the pump reduction flow rate is slightly smaller than the pump addition flow rate Qc, which is Qc-ΔQ;

[0019] 2) Set the delay time Td, that is, the pump change operation will not be started until the pump change condition is met for more than Td seconds;

[0020] The parameters of the switching pump logic include the current number of working pumps, the current total flow (which can be obtained by summing the flow of each branch or the outlet flow of each pump), and the pump group inlet pressure (used to calculate the switching flow); the current number of working pumps is m, the total branch flow is Q, and the switching flow between different numbers of pumps is calculated as Qc ij , where i, j∈{1,2,3,4} and |ij|=1.

[0021] Furthermore, the second pump group switching control method is a switching strategy based on the pump group head output rate, which is used in the case of lack of required total flow data as an alternative to the first method, and includes the following steps:

[0022] Definition: head output rate η;

[0023] The head output rate η is defined as the ratio of the current head of the pump to the maximum head of the pump at the current frequency (head when the flow rate is 0), that is:

[0024]

[0025] In the above formula, H is the working head of the current pump, H f is the maximum head of the pump at the current working frequency, H0 is the maximum head of the pump at the working frequency, ρ is the oil density, g is the acceleration of gravity, k is the ratio of the current frequency speed to the working frequency speed, p out 、p in are the outlet and inlet pressures of the pump respectively.

[0026] According to the similarity law, the head of the centrifugal pump after frequency conversion and the head before frequency conversion have the following relationship as shown in Formula 2:

[0027] H'(Q')=k 2 H(Q)--Formula 2;

[0028] Then the head output rate of the operating point on the same frequency reduction trajectory is:

[0029]

[0030] The above formula shows that the operating points on the same frequency reduction trajectory have the same head output rate. Since the high-efficiency working areas of different pump numbers are bounded by the frequency reduction trajectory, it is possible to determine whether the current pump is in the high-efficiency working area based on the head output rate of the operating point.

[0031] S1. Drawing of HQ characteristic curve, high-efficiency operation area and working trajectory of pump group;

[0032] S2. Calculation of pump change flow rate at different pump group inlet pressures;

[0033] S3. Design of pump switching logic.

[0034] Furthermore, in step S1, it is assumed that the details of the efficient operation area of ​​a certain type of pump group and the head output rate corresponding to different operating points are as follows:

[0035] The characteristic curves of double pumps, triple pumps and quadruple pumps are obtained by the horizontal addition method (measured data can also be used for fitting) through the single pump operation characteristic curve, and the high-efficiency operation areas are divided, and the more efficient combination is selected when there is overlap between the areas;

[0036] As the pump unit inlet pressure increases, the actual operating point curve will shift downward accordingly.

[0037] Furthermore, in step S1, a head output rate situation diagram corresponding to the frequency reduction trajectory line at different operating points is drawn according to the assumption, and in the figure, the head output rate corresponding to the left boundary of the efficient operation area (approximately corresponding to the 70% most efficient flow point) is 0.969, and the head output rates corresponding to the intersection of the pump group operating trajectory line and the efficient area boundary (power frequency characteristic curve) during pressure maintenance are 0.896, 0.908, and 0.922, respectively, corresponding to the lower limit of the head output rate when the three combinations of single pump, double pump, and three pumps are efficiently operated, that is, the pump head output rate η c+ The head output rates of the frequency reduction trajectory lines (black dotted lines) intersecting the intersection points are 0.987, 0.971, and 0.966, respectively, corresponding to the head output rates of the pump reduction when the two-pump, three-pump, and four-pump combinations are η c- .

[0038] Furthermore, in step S2, according to the head output rate situation diagram corresponding to the frequency reduction trajectory line at different operating points, a mathematical model of the pump change flow rate changing with the pump group inlet pressure is established, and the trajectory curve near the intersection and the power frequency characteristic curve are approximated as a straight line, or curve fitting can be performed to establish the head output rate η when changing the pump c With pump inlet pressure H PumpI (meters of water column) changes.

[0039] Furthermore, in step S3, the parameters of the pump switching logic include the current number of working pumps, the inlet and outlet pressures of the pump group, and the current operating speed (which can be calculated by the current frequency). The current number of working pumps is m, the inlet and outlet pressure difference of the pump group is Δp, and the current transformation ratio is k. The current head output rate η is calculated, and the inlet pressure of the pump group is converted into the water column height, so as to calculate the pump head output rate η between different numbers of pumps. ij , where i, j∈{1,2,3,4} and |ij|=1.

[0040] The present invention provides a method for controlling the switching of pump groups under a multi-variable same-frequency control mode, which has the following beneficial effects:

[0041] 1. The advantages of designing the switching strategy based on the total flow of the refueling branch are: first, the required flow situation can be known immediately, and there is no need to wait for the ring network pressure to exceed the threshold before switching; second, multiple pumps can be switched at one time, shortening the switching process time; third, the pump group is prevented from working in the low flow or excess flow area, that is, ensuring that the pump group works in the high-efficiency working area; these three advantages can avoid the occurrence of problems (1) and (2) in the background technology.

[0042] 2. Advantages of the switching strategy based on the head output rate of the pump group: Since the high-efficiency working areas of different numbers of pumps are bounded by the frequency reduction trajectory line, it is possible to determine whether the current pump is in the high-efficiency working area based on the head output rate of the operating point. Using this as the basis for switching control can very conveniently solve the problem (2) raised by the background technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 It is a schematic diagram of an aviation linear refueling system of a pump group switching control method under a multi-variable same-frequency control mode of the present invention;

[0044] Figure 2 It is a surface diagram of the operating efficiency of different pump number combinations of a certain type of pump group in a pump group switching control method under a multi-variable same-frequency control mode of the present invention;

[0045] Figure 3 It is a graph showing the efficient operation area of ​​a certain type of pump group and the trajectory of the working point at different inlet pressures in a pump group switching control method under a multi-variable same-frequency control mode of the present invention;

[0046] Figure 4 A pump change flow calculation table for different inlet pressures of a pump group switching control method under a multi-variable same-frequency control mode of the present invention;

[0047] Figure 5 A pump group switching control logic diagram based on the total flow rate of a pump group switching control method under a multi-variable same-frequency control mode of the present invention;

[0048] Figure 6It is a schematic diagram of the head output rate corresponding to the frequency reduction trajectory lines at different working points of a pump group switching control method under a multi-variable same-frequency control mode of the present invention;

[0049] Figure 7 A calculation table of pump change head output rate at different inlet pressures of a pump group switching control method under a multi-variable same-frequency control mode of the present invention;

[0050] Figure 8 The present invention discloses a pump group switching control logic diagram based on the head output rate in a pump group switching control method under a multi-variable same-frequency control mode. DETAILED DESCRIPTION

[0051] The following embodiments of the present invention are described in further detail in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0052] Embodiment 1

[0053] like Figure 1-Figure 5 As shown, a pump group switching control method under a multi-variable same-frequency control mode, the first pump group switching control method uses the total flow required by the refueling branch as the switching strategy, including the following steps:

[0054] S1. Drawing of HQ characteristic curve, high-efficiency operation area and working trajectory of pump group:

[0055] In this embodiment, it is assumed that the efficient operation area of ​​a certain type of pump group and the trajectory curve at different inlet pressure working points are as follows: Figure 3 As shown, the four shaded areas in the figure are the high-efficiency operating areas when single pump, dual pump, triple pump and quad pump are in operation (obtained by horizontal addition). When there is overlap between the areas, a more efficient combination is displayed (the corresponding efficiency is higher when the number of pumps is small). The four working point trajectory curves from top to bottom (orange, purple, green and blue, respectively) correspond to the pump group inlet pressures generated by water columns at heights of 0, 5, 10 and 15 meters, respectively. It can be seen that as the pump group inlet pressure increases, the actual working point curve will shift downward accordingly. (In fact, the HQ characteristic curve of the pump group rises, which is equivalent to the working point trajectory curve moving downward. In addition, the trajectory curve should not exceed the HQ characteristic curve of the pump group. It is retained for the convenience of drawing. This is hereby explained.)

[0056] The intersection of the working point trajectory curve and the high-efficiency operation area in the figure can be divided into two situations. One is that when the pump group inlet pressure is low, the intersection point is on the pump group's power frequency characteristic curve; the other is that when the pump group inlet pressure is high, the intersection point will be on the frequency reduction trajectory curve of a certain working point (the three vertical purple thick solid lines in the figure). We can establish the intersection (pump change) flow value Qc (m 3 / h) with pump inlet pressure H PumpIMathematical model of the change of (meters of water column);

[0057] S2. Calculation of pump change flow rate at different pump group inlet pressures:

[0058] according to Figure 3 A mathematical model of the pump change flow rate changing with the pump group inlet pressure is established. For convenience, the trajectory curve near the intersection and the power frequency characteristic curve are approximated as straight lines, and the model is established as follows: Figure 4 As shown;

[0059] S3. Design of pump switching logic:

[0060] according to Figure 3 , it is easy to calculate the flow value of different pump switching at a specific inlet pressure. For example, assuming the pump inlet pressure is 10 meters of water column (ignoring the inlet pipeline pressure loss, the corresponding tank liquid level should be 10 / ρ 油 ), Qc of switching between 1 pump and 2 pumps can be based on Figure 4 Calculated by formula 1#, it is 225.5, that is, when the total flow exceeds this value, two pumps are switched to work, and when it is less than this value, one pump is switched to work.

[0061] Of course, in order to avoid mis-cutting the pump due to flow fluctuation, one can set a ΔQ value so that the pump reduction flow is slightly smaller than the pump addition flow Qc, which is Qc-ΔQ; the other is to set a delay time Td, that is, the pump change operation is started only when the pump change condition is reached and maintained for more than Td seconds. In addition, this parameter can be slightly smaller than the calculated value in the application to avoid pressure drop caused by untimely pump addition.

[0062] The parameters required to implement this strategy are the current number of working pumps, the current total flow (which can be obtained by summing the flow of each refueling branch or the outlet flow of each pump), and the pump group inlet pressure (used to calculate the switching flow). Let the current number of working pumps be m, the total branch flow be Q, and the calculated switching flow between different numbers of pumps is Qc ij , where i, j∈{1,2,3,4} and |ij|=1, such as Qc 23 Indicates the flow value when switching from 2 pumps to 3 pumps, Qc 32 Can be used with the same Qc 23 , or a smaller value can be obtained, then the pump group switching control logic is as follows Figure 5 shown.

[0063] Embodiment 2

[0064] like Figure 6-Figure 8 As shown in the figure, a pump group switching control method under a multi-variable same-frequency control mode is used. The second pump group switching control method is a switching strategy based on the pump group head output rate. It is used in the absence of required total flow data as an alternative to the first method, and includes the following steps:

[0065] Definition: head output rate η;

[0066] The head output rate η is defined as the ratio of the current head of the pump to the maximum head of the pump at the current frequency (head when the flow rate is 0), that is:

[0067]

[0068] In the above formula, H is the working head of the current pump. f is the maximum head of the pump at the current working frequency, H0 is the maximum head of the pump at the working frequency, ρ is the oil density, g is the acceleration of gravity, k is the ratio of the current frequency speed to the working frequency speed, p out 、p in are the outlet and inlet pressures of the pump respectively.

[0069] According to the similarity law, the head of the centrifugal pump after frequency conversion and the head before frequency conversion have the following relationship as shown in Formula 2:

[0070] H'(Q')=k 2 H(Q)--Formula 2;

[0071] Then the head output rate of the operating point on the same frequency reduction trajectory is:

[0072]

[0073] The above formula shows that the operating points on the same frequency reduction trajectory have the same head output rate. Since the high-efficiency working areas of different pump numbers are bounded by the frequency reduction trajectory, it is possible to determine whether the current pump is in the high-efficiency working area based on the head output rate of the operating point.

[0074] S1. Drawing of HQ characteristic curve, high-efficiency operation area and working trajectory of pump group:

[0075] In this embodiment, it is assumed that the efficient operation area of ​​a certain type of pump group and the head output rate corresponding to different operating points are as follows: Figure 6 As shown. The four shaded areas in the figure are the high-efficiency operating areas when single pump, dual pump, triple pump and quad pump are running (obtained by horizontal addition). When there is overlap between the areas, a more efficient combination is shown (the corresponding efficiency is higher when the number of pumps is small). The working point trajectory curve is shown by the purple solid line (horizontally). As the inlet pressure of the pump group increases, the actual working point curve will shift downward accordingly. (In fact, the HQ characteristic curve of the pump group rises, which is equivalent to the working point trajectory curve shifting downward. This is specially explained)

[0076] The head output rate corresponding to the left boundary of the high-efficiency operation area (approximately corresponding to the 70% most efficient flow point) is 0.969. The head output rates corresponding to the intersection of the pump group operating trajectory line and the high-efficiency area boundary (power frequency characteristic curve) during pressure maintenance are 0.896, 0.908, and 0.922, respectively, which correspond to the lower limit of the head output rate when the three combinations of single pump, double pump, and three pumps are running efficiently, that is, the pump head output rate η c+ The actual value can be slightly larger. The head output rates of the frequency reduction trajectory lines (black dotted lines) intersecting the intersection point are 0.987, 0.971, and 0.966, respectively, corresponding to the head output rate of the pump reduction when the double pump, three pumps, and four pumps are combined. c- , the actual value can be slightly larger;

[0077] S2. Calculation of pump change flow rate at different pump group inlet pressures:

[0078] according to Figure 6 A mathematical model can be established for the pump change flow rate as the pump group inlet pressure changes. For convenience, the trajectory curve near the intersection and the power frequency characteristic curve are approximated as a straight line, and the head output rate η when the pump is replaced can be established. c With pump inlet pressure H PumpI (meters of water column) changes, such as Figure 7 As shown:

[0079] S3. Design of pump switching logic:

[0080] The parameters required to implement this strategy are the current number of working pumps, the inlet and outlet pressures of the pump group, and the current operating speed (which can be calculated from the current frequency); the current number of working pumps is m, the inlet and outlet pressure difference of the pump group is Δp, and the current transformation ratio is k. Based on this, the current head output rate η is calculated, and the inlet pressure of the pump group is converted into the water column height, so as to calculate the pump head output rate η between different numbers of pumps ij , where i, j∈{1,2,3,4} and ij|=1, such as η 23 Indicates the head output rate when switching from 2 pumps to 3 pumps, corresponding to Figure 7 η in c+ , η 32 Indicates the head output rate when switching from 3 pumps to 2 pumps, corresponding to Figure 7 η in c- , the control logic of switching pump is as follows Figure 8 shown.

[0081] The embodiments of the present invention are given for the purpose of illustration and description, and are not intended to be exhaustive or to limit the invention to the disclosed forms. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiments are selected and described in order to better illustrate the principles and practical applications of the present invention and to enable those of ordinary skill in the art to understand the present invention and thereby design various embodiments with various modifications suitable for specific uses.

Claims

1. A method for controlling the switching of pump groups under a multi-variable same-frequency control mode, characterized in that: The first pump group switching control method is a switching strategy based on the total flow required by the refueling branch, including the following steps: S1. Drawing of HQ characteristic curve, high-efficiency operation area and working trajectory of pump group; S2. Calculation of pump change flow rate at different pump group inlet pressures; S3. Design of pump switching logic.

2. The method for controlling the switching of pump groups under the multi-variable same-frequency control mode according to claim 1 is characterized in that: In step S1, it is assumed that the details of the efficient operation area of ​​a certain type of pump group and the trajectory curve at different inlet pressure working points are as follows: The characteristic curves of double pumps, triple pumps and quadruple pumps are obtained by horizontal addition through the single pump operation characteristic curve, and the high-efficiency operation areas are divided, and the more efficient combination is selected when there is overlap between the areas; The four working point trajectory curves correspond to the pump group inlet pressures from top to bottom, which are the pressures generated by water columns at heights of 0, 5, 10, and 15 meters respectively. As the pump group inlet pressure increases, the actual working point curve will shift downward accordingly.

3. The method for controlling the switching of pump groups under the multi-variable same-frequency control mode according to claim 2 is characterized in that: In step S1, according to the assumption, the high-efficiency operation area of ​​a certain type of pump group and the working point trajectory curve at different inlet pressures are drawn, and in the figure, the intersection of the working point trajectory curve and the high-efficiency operation area can be divided into two cases: one is that when the pump group inlet pressure is low, the intersection point is on the power frequency characteristic curve of the pump group; the other is that when the pump group inlet pressure is high, there will be a situation where the intersection point is on the frequency reduction trajectory curve of a certain working point; in summary, the intersection flow value Qc can be established with the pump inlet pressure H in two cases PumpI Mathematical model of change.

4. The method for controlling the switching of pump groups under the multi-variable same-frequency control mode according to claim 1 is characterized in that: In step S2, based on the drawn efficient operating area of ​​a certain type of pump group and the trajectory curve of the working point at different inlet pressures, a mathematical model of the pump change flow rate changing with the inlet pressure of the pump group is established, and the trajectory curve and the power frequency characteristic curve near the intersection are approximated as straight lines, or a fitting curve is used for calculation to obtain a calculation model.

5. The method for controlling the switching of pump groups under the multi-variable same-frequency control mode according to claim 1 is characterized in that: In step S3, in order to avoid mis-tripping of the pump due to flow fluctuation, the following logic design may be adopted: 1) Set a ΔQ value so that the pump reduction flow rate is slightly smaller than the pump addition flow rate Qc, which is Qc-ΔQ; 2) Set the delay time Td, that is, the pump change operation will not be started until the pump change condition is met for more than Td seconds; The parameters of the switching pump logic include the current number of working pumps, the current total flow rate, and the pump group inlet pressure; the current number of working pumps is m, the total branch flow rate is Q, and the switching flow rate between different numbers of pumps is calculated as Qc ij , where i, j∈{1,2,3,4} and |ij|=1.

6. A method for controlling the switching of pump groups under a multi-variable same-frequency control mode, characterized in that: The second pump switching control method is a switching strategy based on the pump head output rate, including the following steps: S1. Drawing of HQ characteristic curve, high-efficiency operation area and working trajectory of pump group; S2. Calculation of pump change flow rate at different pump group inlet pressures; S3. Design of pump switching logic.

7. The method for controlling the switching of pump groups under the multi-variable same-frequency control mode according to claim 6 is characterized in that: In step S1, it is assumed that the details of the efficient operation area of ​​a certain type of pump group and the head output rate corresponding to different operating points are as follows: The characteristic curves of double pumps, triple pumps and quadruple pumps are obtained by horizontal addition through the single pump operation characteristic curve, and the high-efficiency operation areas are divided, and the more efficient combination is selected when there is overlap between the areas; As the pump unit inlet pressure increases, the actual operating point curve will shift downward accordingly.

8. The method for controlling the switching of pump groups under the multi-variable same-frequency control mode according to claim 7 is characterized in that: In the step S1, a head output rate situation diagram corresponding to the frequency reduction trajectory line at different operating points is drawn according to the assumption. In the figure, the head output rate corresponding to the left boundary of the efficient operation area is 0.969, and the head output rates corresponding to the intersection of the pump group operating trajectory line and the efficient area boundary during pressure maintenance are 0.896, 0.908, and 0.922, respectively, corresponding to the lower limit of the head output rate when the three combinations of single pump, double pump, and three pumps are efficiently operated, that is, the pump head output rate η c+ The head output rates of the frequency reduction trajectory lines intersecting the intersection point are 0.987, 0.971, and 0.966, respectively, corresponding to the head output rate of the pump reduction when the double pump, three pumps, and four pumps are combined. c- .

9. The method for controlling the switching of pump groups under the multi-variable same-frequency control mode according to claim 6 is characterized in that: In step S2, according to the head output rate situation diagram corresponding to the frequency reduction trajectory line at different working conditions, a mathematical model of the pump change flow rate changing with the pump group inlet pressure is established, and the trajectory curve near the intersection and the power frequency characteristic curve are approximated as a straight line to establish the head output rate η when changing the pump c With pump inlet pressure H PumpI Mathematical model of change.

10. The method for controlling the switching of pump groups under the multi-variable same-frequency control mode according to claim 6 is characterized in that: In step S3, the parameters of the pump switching logic include the current number of working pumps, the inlet and outlet pressures of the pump group, and the current operating speed. The current number of working pumps is m, the inlet and outlet pressure difference of the pump group is Δp, and the current transformation ratio is k. The current head output rate η is calculated, and the inlet pressure of the pump group is converted into the water column height, so as to calculate the pump head output rate η between different numbers of pumps. ij , where i, j∈{1,2,3,4} and ij|=1.

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