Method for operating a multi-pump system with at least two pumps which can be operated hydraulically in parallel, and multi-pump system

Abstract

The invention relates to a method for operating a multi-pump system with at least two pumps which can be operated hydraulically in parallel and each comprise a hydraulic centrifugal pump, an electric drive motor for driving the centrifugal pump, and an inverter, wherein the pumps deliver into a common pressure line and are operated at a setpoint rotational speed, and wherein at least one switch-off curve is stored in a controller, and the controller is configured to switch off at least one of the at least two pumps which are operated in parallel in a manner which is dependent on the current operating point position of the multi-pump system and the switch-off curve, characterized in that an inactive pump is switched on if one of the currently active pumps does not reach its setpoint rotational speed.

Description

[0001] Description

[0002] Method for operating a multi-pump system with at least two hydraulically parallel operated pumps and multi-pump system

[0003] The invention relates to a method for operating a multi-pump system with at least two hydraulically operated pumps in parallel, each comprising a hydraulic centrifugal pump, an electric drive motor for driving the centrifugal pump and a converter, wherein the pumps deliver into a common pressure line and are operated at a target speed, and wherein at least one switch-off curve is stored in a controller and the controller is configured to switch off at least one of the at least two pumps operated in parallel depending on the current operating point position of the multi-pump system and the switch-off curve.

[0004] Multi-pump systems are used primarily in systems with highly fluctuating consumption and high total pressures, such as waterworks. Multi-pump systems comprise two or more pumps connected hydraulically in parallel, which pump into a common pressure line. The number of pumps operating simultaneously is selected based on demand, with the systems usually capable of independently switching their pumps on and off as needed via a control system.

[0005] An additional pump is usually switched on when the demand cannot be met by the currently active pumps. However, if the current demand drops to such an extent that the demand could be met by fewer than the currently active pumps, the unused pumps should be switched off.

[0006] So-called switching curves have previously been defined as one possible switching criterion; these are generally derived from the characteristic pump curve. The pump curve of an individual pump characterizes the relationship between its flow rate and its achievable head and consequently limits its maximum possible operating range, which lies below the characteristic curve in the Q / H diagram. If more than one pump is running actively, the flow rate values ​​of the individual pump curves are, in the simplest case, added together to obtain the curve or operating range of the n active pumps. Figure 1 shows the resulting operating ranges of a multi-pump system in parallel operation with a different number of active pumps. Curve 3 defines the operating range of an individual active pump and therefore corresponds to its characteristic pump curve.In the following, a pump is defined as a system consisting of a hydraulic centrifugal pump, an electric drive motor, and a converter. Curve 4 describes the operating range for two pumps operating in parallel, curve 5 the operating range for three active pumps, and curve 6 for four active pumps.

[0007] In the simplest case, the pumps in the multi-pump system are identical, and an identical pump characteristic curve is assumed for all pumps, which is permanently stored in the system's control system. The curves for the operating range for n active pumps are therefore calculated by multiplying the flow values ​​of the pump characteristic curve by the number n of active pumps. Curve 3 of the operating range of a pump can then also be used as the switch-off curve for operation with two simultaneously active pumps. If the current system operating point, i.e. the associated flow rate / head value pair, lies within curve 3 of the operating range of a pump, one of the active pumps is switched off. In the case of three pumps running simultaneously, curve 4 of the operating range for two pumps is used as the switch-off curve.If the corresponding flow / head value pair of the current system operating point during operation with n active pumps is instead on or outside the curve defining the operating range of the n active pumps, an additional pump must be activated. In the case of two running pumps, an additional pump would be activated if the current flow / head value pair is on curve 4 or instead outside the operating range defined by curve 4. The operating point may be outside the defined operating range because the pump characteristic curves / shutdown curves used usually do not correspond to the actual curves.

[0008] The pump characteristic curve of a pump and thus curves 3-6 of the operating range are always only known imprecisely in reality. Although the pump characteristic curve can be determined by the manufacturer in the laboratory, there are differences between the individual pumps produced due to production variations. In addition, external influences such as temperatures can change the operating range of the motor and thus the operating range of the pump consisting of the hydraulic pump, motor, and converter. Therefore, it is not the curve of the operating range measured in the laboratory that is stored in the control system as a curve, but rather a significantly narrower curve. This additional safety corridor ensures that even in the worst-case scenario, the pumps do not switch off too early and that an adequate supply is always guaranteed.This leads to more pumps operating in the transition area than would actually be necessary to meet current demand, but this comes at the expense of increased energy consumption.

[0009] We are therefore looking for a more advanced solution to achieve optimized switching on and off of the pumps, which should ultimately result in reduced energy consumption.

[0010] This object is achieved by a method according to the features of claim 1. Advantageous embodiments of the method are the subject of the dependent claims. According to the invention, it is proposed that the switching on is not carried out as a function of a stored switching curve, but instead is made dependent on whether one of the currently active pumps does not reach its predetermined target speed. In the following, a pump is understood to be a system consisting of a hydraulic centrifugal pump, an electric motor that drives the centrifugal pump, and a converter that feeds the electric motor. The pumps do not each have to be a single product in which the components centrifugal pump, drive motor, and converter are permanently installed. It can also be provided that the three components are designed individually as separate parts and interact to implement a pump function with variable speed adjustment, e.g.by assembling them into a pump or connecting them together for operation at the place of use.

[0011] If one of the active pumps does not reach its target speed, this is an indication that the pump is operating at its current limit and can no longer provide the speed required to maintain the supply. This situation is automatically detected by the system control system and at least one additional pump is switched on. Unlike what is proposed in the prior art, the invention therefore only switches on an additional pump at a later time, namely only when the current consumption cannot be ensured by the number of active pumps. By avoiding the premature switching on of an additional pump, the actually achievable operating range of the active pumps can be better utilized, so that overall energy savings can be achieved and the efficiency of the system increases.Only when one of the pumps no longer reaches its target speed can it be assumed that the current demand cannot be met by the number of active pumps, so that the activation of at least one additional pump is absolutely necessary.

[0012] The stored shutdown curve is preferably a Q / H curve, i.e. a curve that characterizes the possible operating range of n active pumps. Ideally, the stored shutdown curve is first determined taking into account the pump characteristic curve in the Q / H diagram stored in the control system. If more than one pump is actively running, the flow values ​​of the individual pump characteristic curves are added together to obtain the curve or the operating range of the n active pumps. This means that only the vector with the flow values ​​or flow support points of the curve is added together; the delivery head remains the same. In the simplest case, the pumps in the multi-pump system are identical, and an identical pump characteristic curve is assumed for all pumps, which is permanently stored in the system control system.The curves for the operating range for n active pumps are therefore calculated by multiplying the flow values ​​of the pump characteristic curve by the number n of active pumps. The shutdown curve for n active pumps corresponds to the operating range of n-1 active pumps.

[0013] According to an advantageous embodiment of the invention, however, the aforementioned shutdown curve, stored or determined based on the stored pump characteristic curve, is not used directly as the shutdown criterion; instead, the invention preferably provides for an optimization of this shutdown curve. In particular, the current operating point at the time an additional pump is switched on, i.e., the operating point at which at least one of the active pumps does not reach its target speed, is stored in the controller and subsequently used to optimize the shutdown curve applicable to the currently active pumps.

[0014] As already demonstrated with the state of the art, the shutdown curves stored in the control system do not correspond to the actual operating range of a pump, but are instead narrowed by a corresponding safety corridor. By determining the switch-on point according to the invention, the corresponding shutdown curve can be brought closer to the actual operating range and thus optimized, allowing earlier shutdown of a pump in the event that demand can be met with fewer pumps. This results in overall energy savings.

[0015] For example, the optimized shutdown curve can be calculated based on the stored operating point at the time of activation and the shutdown curve for the number of active pumps, either stored or determined on the basis of the pump characteristic curve stored in the controller. Such a calculation preferably includes a transformation of the shutdown curve into a linear representation. If, for example, the operating range of the pump(s) specified in the Q / H diagram is used as the shutdown curve, it is clear that there is no linear relationship between the respective parameters, which complicates optimization because the distance between the determined shutdown point and the corresponding curve point does not allow any conclusions to be drawn about the remaining curve points.However, by transforming it into a linear representation, it is possible to shift the curve as a whole in parallel, so that an optimized shutdown curve can be determined for all conceivable operating points.

[0016] For example, the linear curve representation is shifted parallel to such an extent that it passes through the operating point of the switch-on point.

[0017] The aforementioned transformation of the shutdown curve can be performed using affinity laws, particularly in combination with one or more additional pump-specific curves. Such characteristic pump curves can, for example, include the relationship between flow rate and mechanical power or other parameters.

[0018] It is conceivable, for example, that the stored shutdown curve is a Q / H representation and that this curve is converted into a speed / torque diagram by transformation. The relationship between torque and electrical winding current is linear. This means that the torque limit can be shifted parallel to the current limit (which is constant) so that the curve can be shifted in the speed / torque representation. The saved operating point represents a value pair between flow rate and head. The corresponding value pair in the speed / torque range is either already available or is first determined by transformation. The distance between the speed / torque value pair and the corresponding curve point, i.e. the difference between the torque value of the saved operating point and the corresponding curve value, determines the extent of the parallel shift of the entire curve.In other words, the determined difference value is added to all torque values ​​of the transformed curve.

[0019] After subsequent back-transformation to the original representation, in particular the Q / H representation, the optimized shutdown curve is obtained, which is then used as the shutdown criterion for the parallel operation of several active pumps. If the current operating point lies within the operating range defined by the optimized shutdown curve, at least one of the active pumps is shut down. This measure allows for the timely shutdown of an active pump in the event that the current demand can also be met by less active pumps. Overall, this results in significant energy savings.

[0020] Alternatively, a shutdown curve can already be stored in the speed / torque display in the control system. In this case, the necessary transformation of the curve display is eliminated, and the already stored curve is shifted until it passes through the stored operating point of the switch-on time. The corresponding speed / torque values ​​of the stored operating point are either already available in the motor control system or are converted or estimated based on other parameters. Monitoring whether a pump can be shut down is then preferably carried out directly in the speed / torque display.

[0021] In addition to the method according to the invention, the present invention also relates to a multi-pump system with at least two pumps that can be connected hydraulically in parallel. The pumps each comprise a hydraulic centrifugal pump, an electric drive motor for driving the centrifugal pump, and a converter assigned to the pump, wherein the pumps deliver into a common pressure line. A control system of the system is configured to carry out the method according to the invention. The multi-pump system therefore offers the same advantages and properties as already demonstrated above with reference to the method according to the invention. For this reason, a repeated description is omitted. The pumps do not need to be a separate product in which the components centrifugal pump, drive motor, and converter are permanently installed.It can also be provided that the three components are designed individually as separate components and interact to implement a pump function with variable speed adjustment, for example, by being assembled into a pump at the point of use or connected to each other for operation. The control system can be designed as a central control system for the pumps. It is also conceivable for individual control systems of the pumps to interact with each other to implement the method according to the invention.

[0022] Furthermore, the invention also relates to a control system for a multi-pump system consisting of at least two pumps that can be operated hydraulically in parallel. The pumps of the system each comprise a hydraulic centrifugal pump, an electric drive motor for driving the centrifugal pump, and a converter associated with the pump, with the pumps delivering into a common pressure line. The control system is configured to carry out the method according to the invention.

[0023] Further advantages and features of the invention will be explained in more detail below using an exemplary embodiment illustrated in the figures. They show:

[0024] Figure 1 : different operating ranges or shutdown curves for the operation of 1 - 4 pumps of the multi-pump system,

[0025] Figure 2: a Q / H representation of the operating range of one active pump and the optimized shutdown curve for operation with two active pumps,

[0026] Figure 3: a schematic diagram of the method according to the invention for optimizing the switch-off curve.

[0027] The invention is described below using an exemplary embodiment of a multi-pump system. The system consists of n pumps arranged hydraulically in parallel. A pump is understood to be a system consisting of a hydraulic centrifugal pump, an electric drive unit, and a converter. The pumps do not necessarily have to be a single product in which the converter, motor, and centrifugal pump components are permanently installed. The three components can also be separate modules that are installed at the point of use and interact with each other.

[0028] The system's n pumps are to be switched on or off by the control system as needed, so that only as many pumps run in parallel as are necessary to meet the current demand. When the pumps are switched on, they are operated at a specific target speed, whereby the target speed is usually set to be identical for all active pumps, at least initially. The target speed can be measured via a sensor or indirectly derived from electrical or other measured values.

[0029] In the described embodiment, the n pumps of the multi-pump system are of identical design. For at least one of the pumps, the characteristic pump curve (Q / H diagram) was determined by the manufacturer and stored in the control system. As already described in the introductory section with reference to Figure 1, curves 3-6 can be determined to limit the operating range depending on the number of active pumps. These curves are also referred to below as non-optimized shutdown curves. Unlike in the prior art, however, these curves are not used as shutdown curves without modification, but are instead optimized as part of a procedure.

[0030] Firstly, the control system is configured in such a way that the decision as to whether another pump should be switched on during ongoing system operation is not made taking curves 3-6 into account, but instead depends on whether the active pumps actually reach their specified target speeds. Using this information on the switch-on point, the actual possible operating range of the pumps can then be determined in order to optimize the respective switch-off curves 3-6. For a detailed description, refer to Figure 2. For the sake of simplicity, the process is described using two pumps. However, it can be applied to any number of pumps. The figure shows the stored switch-off curve 3 (operating range), which corresponds to the pump characteristic curve of a single pump. At operating point 7, the control system determined that the pumps were not reaching their target speed and the second pump was switched on.This actually occurred operating point 7 is saved and is used to calculate an optimized switch-off curve 8, which runs noticeably shifted to the right through the switch-on point 7.

[0031] The process therefore consists of two steps: Switching on if the target speed is not reached. This operating point with the value pair flow rate / discharge head is saved (step 1). Step 2 then involves calculating the optimal switch-off curve. To do this, it is first important to understand why the operating range is limited. This limitation is ultimately due to a limitation of the motor's torque-generating current (hereinafter referred to as the current limit). This current limit is a motor-specific parameter and cannot be changed. The torque-generating current is proportional to the torque, so it is essentially a torque limit.

[0032] As a result of step 1, the controller now has the following information available to calculate the optimal shutdown curve: the non-optimized Q / H curve 3, which is stored in the controller (Figure 2), and the actual cut-in point 7 (Figure 2). Since the stored shutdown curve 3 was deliberately narrowed, the actual cut-in point, as shown in Figure 2, will lie outside the stored curve 3.

[0033] Figure 3a shows, as an example, the interpolation points 3a of the stored Q / H curve 3 as well as the Q / H value pair for the activation point 7. The control system then transforms this representation into a speed / torque representation (Figure 3b) with the resulting interpolation points 9 of such a curve representation. The conversion is carried out using the affinity laws and stored pump-specific curves. The activation point 7 is also transformed into the speed / torque representation and now corresponds to a speed / torque value pair 7a. Alternatively, the speed / torque pair 7a can already be present in the control system, since the speed is determined in the control system anyway and is present at the respective activation point.

[0034] In practice, it may be the case that the system does not have separate sensors for measuring the flow rate or the discharge head and these values ​​are instead calculated using an operating point estimate.

[0035] Since the limitation of the operating range depends significantly on the current limit (and thus on the torque limit), it can be assumed that the one detected connection point 7 is representative of all other points along the shutdown curve 3. The differential torque AM shown in Figure 3b corresponds to the difference between the torque detected at the connection point 7 (transformed connection point 7a) and the torque value at the corresponding speed in the support points 9 of the speed / torque curve. Consequently, the entire curve 9 can be shifted by AM

[0036] Finally, the speed / torque interpolation points 9 are transformed back to the flow / conveyance representation, resulting in the new cut-off curve 8 with interpolation points 8 (Figure 3 c). Due to the parallel shift by the value AM, the optimized cut-off curve now passes through the cut-in point 7.

[0037] In principle, it is conceivable that instead of Q / H curves, only speed / torque curves could be used for shutdown. In this case, a transformation or inverse transformation of the curves could be omitted.

[0038] As already mentioned above, the head and flow rate in one embodiment of the invention are estimated by the control system using operating point estimation. This will be explained below as a precautionary measure.

[0039] In order for the pump control system to adjust the speed as needed, it requires knowledge of the current operating point (flow rate Q and head / - / ). For cost reasons, a flow sensor and, in some cases, a pressure sensor are usually not present. Instead, the pump control system estimates the current operating point based on the motor torque and speed, which are either provided directly by the motor control system or can be derived from the variables provided by the motor control system.

[0040] From the torque, the mechanical power can be determined according to (Eq. 1 ):

[0041] P = 2* pi *M *n (Eq. 1 )

[0042] Both data provide a mathematical model of the motor, which runs redundantly to the pump on the processor. The operating point is estimated based on affinity laws, characteristic curves stored in the control system (the relationship between mechanical power and flow rate, as well as head and flow rate at rated speed), and a correction for mechanical power losses. The affinity laws are well known in the literature and state that power, flow rate, and head behave as follows when the speed changes:

[0043] The input variables for the operating point estimation include, for example, the values ​​for speed and mechanical power (torque) provided by the motor control. After subtracting the mechanical losses from the motor power, the affinity laws can be applied to obtain the estimated values ​​for the head and flow rate. The calculation can be performed in reverse order, for example, to transform the stored shutdown curve into a linear speed / torque representation.

Claims

Patent claims Method for operating a multi-pump system with at least two hydraulically parallel operated pumps and multi-pump system 1. Method for operating a multi-pump system with at least two pumps which can be operated hydraulically in parallel and which each comprise a hydraulic centrifugal pump, an electric drive motor for driving the centrifugal pump and a converter, wherein the pumps deliver into a common pressure line and are operated at a target speed, and wherein at least one switch-off curve is stored in a controller and the controller is configured to switch off at least one of the at least two pumps operated in parallel as a function of the current operating point of the multi-pump system and the switch-off curve, characterized in that an inactive pump is switched on if one of the currently active pumps does not reach its target speed.

2. Method according to claim 1, characterized in that the shutdown curve is a Q / H characteristic curve which defines the upper limit of the possible operating range for n pumps.

3. Method according to claim 2, characterized in that the shutdown curve for n active pumps corresponds to the curve of the operating range of n-1 active pumps.

4. Method according to one of the preceding claims, characterized in that an optimized shutdown curve (8) is used as the shutdown criterion for at least one pump.

5. Method according to claim 4, characterized in that the current operating point at the time of switching on another pump is stored and used for the optimization of the stored switch-off curve.

6. Method according to claim 5, characterized in that the optimized switch-off curve (8) is calculated on the basis of the stored operating point (7) at the time of switching on and the stored switch-off curve (3-6).

7. Method according to claim 6, characterized in that the calculation comprises a transformation of the shutdown curve into a transformed curve representation in order to carry out a parallel shift of the transformed shutdown curve as a function of the stored operating point.

8. Method according to claim 7, characterized in that the transformation is carried out using the affinity laws, preferably in conjunction with one or more stored pump-specific curves.

9. Method according to one of the preceding claims 7 or 8, characterized in that the transformed representation of the switch-off curve is a speed / torque representation.

10. Method according to one of the preceding claims 7 to 8, characterized in that the transformed switch-off curve is shifted in parallel so that the shifted switch-off curve passes through the stored operating point. 11 . Method according to one of the preceding claims 7 to 10, characterized in that the optimized switch-off curve (8) is obtained by back-transforming the parallel-shifted linear curve representation into the original representation.

12. Method according to claim 6, characterized in that the switch-off curve is stored in a speed / torque representation in the control system and, in order to optimize the switch-off curve, the latter is shifted in parallel until the shifted switch-off curve passes through the stored operating point.

13. Multi-pump system with at least two pumps which can be operated hydraulically in parallel, each consisting of a hydraulic centrifugal pump, an electric drive motor for driving the centrifugal pump and a converter, wherein the pumps deliver into a common pressure line, and with a control system configured to carry out the method according to one of the preceding claims.

14. Control system for controlling a multi-pump system with at least two hydraulically parallel operated pumps, each consisting of a hydraulic Centrifugal pump, an electric drive motor for driving the centrifugal pump and a converter, wherein the pumps deliver into a common pressure line, characterized in that the controller is configured to carry out the method according to one of claims 1 to 12.

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