Motor driving system for water chilling unit and control method

Abstract

The invention relates to an air conditioning technology and a motor control technology, in particular to a motor driving system for a water chilling unit, the water chilling unit comprising the motor driving system, a control method for the water chilling unit and a computer readable storage medium on which a computer program for implementing the method is stored. The motor driving system for the water chilling unit according to one aspect of the application comprises: a variable frequency driving unit configured to drive a first motor; the power frequency driving unit is configured to drive the second motor; and the control unit is configured to adapt to the load requirement of the water chilling unit by controlling the output power of the variable frequency driving unit and the power frequency driving unit.

Description

Technical Field

[0001] The present application relates to air conditioning technology and motor control technology, and in particular to a motor drive system for a chiller, a chiller including the motor drive system, a control method for the chiller, and a computer-readable storage medium storing a computer program for implementing the above method. Background Art

[0002] Screw chillers are water chillers that use various forms of screw compressors as the main unit. They are increasingly used in the field of refrigeration and air conditioning. A typical screw chiller includes a screw compressor, condenser, evaporator, expansion mechanism, oil separator, automatic control components and instruments, etc.

[0003] In order to save energy and increase efficiency, the motors used in compressors often use variable frequency drives as drive components. However, variable frequency drives are expensive, which leads to a longer payback period, especially when the screw chiller contains multiple compressors. Summary of the invention

[0004] According to one aspect of the present application, a motor drive system for a chiller is provided, the chiller comprising a first compressor, a first motor for the first compressor, a second compressor and a second motor for the second compressor, the motor drive system comprising:

[0005] a variable frequency drive unit configured to drive the first motor;

[0006] an industrial frequency driving unit configured to drive the second motor; and

[0007] A control unit is configured to adapt to the load demand of the chiller by controlling the output power of the variable frequency drive unit and the industrial frequency drive unit.

[0008] Optionally, in the above motor drive system, the rated power of the first compressor is greater than or equal to the rated power of the second compressor.

[0009] Optionally, in the above motor drive system, the number of the first compressor is 1, and the number of the second compressor is 1 or more.

[0010] Optionally, in the above motor drive system, the control unit controls the output power of the variable frequency drive unit and the industrial frequency drive unit in the following manner:

[0011] When the load demand is less than or equal to a first critical value, adapting the load demand by operating only the variable frequency drive unit, wherein the variable frequency drive unit operates with a variable output power;

[0012] When the load demand is greater than a second critical value and less than a third critical value, adapting the load demand by operating the variable frequency drive unit and the industrial frequency drive unit, wherein the variable frequency drive unit operates at a first output power and the industrial frequency drive unit operates at a variable output power, and the second critical value is greater than or equal to the first critical value;

[0013] When the load demand is greater than or equal to the third critical value, the load demand is adapted by operating the variable frequency drive unit and the industrial frequency drive unit, wherein the variable frequency drive unit operates with a variable output power and the industrial frequency drive unit operates with a second output power.

[0014] Further optionally, in the above motor drive system, the first output power is less than the output power of the variable frequency drive unit when the load demand is equal to the first critical value, and the second output power is equal to the maximum output power of the industrial frequency drive unit.

[0015] Or further optionally, in the above motor drive system, the first compressor and the second compressor are compressors of the same type, the chiller is a screw chiller, and the first critical value is 50% of the rated load demand of the screw chiller.

[0016] Or further optionally, in the above motor drive system, the first compressor and the second compressor are compressors of different types, the chiller is a screw chiller, and the first critical value is 40% of the rated load demand of the screw chiller.

[0017] According to another aspect of the present application, there is provided a chiller, comprising:

[0018] The first compressor;

[0019] a first motor for said first compressor;

[0020] Second compressor;

[0021] a second motor for the second compressor;

[0022] Motor drive system, including:

[0023] a variable frequency drive unit configured to drive the first motor;

[0024] an industrial frequency driving unit configured to drive the second motor; and

[0025] A control unit is configured to adapt to the load demand of the chiller by controlling the output power of the variable frequency drive unit and the industrial frequency drive unit.

[0026] According to another aspect of the present application, a control method for a chiller is provided, the chiller comprising a first compressor, a first motor for the first compressor, a second compressor, and a second motor for the second compressor, the method comprising:

[0027] A variable frequency drive unit and an industrial frequency drive unit are used to drive the first motor and the second motor respectively, wherein the load demand of the chiller is adapted by controlling the output power of the variable frequency drive unit and the industrial frequency drive unit.

[0028] According to another aspect of the present application, a computer-readable storage medium is provided, on which a computer program suitable for being executed on a processor of a terminal device is stored, and the execution of the computer program causes the steps of the method described above to be performed. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The above and / or other aspects and advantages of the present application will become clearer and easier to understand through the following description of various aspects in conjunction with the accompanying drawings, in which the same or similar units are represented by the same reference numerals. The accompanying drawings include:

[0030] Figure 1 It is a schematic block diagram of a screw chiller according to some embodiments of the present application.

[0031] Figure 2 The comparative results of the operating performance of the screw chiller when using the cross mode, balanced mode and priority mode are shown.

[0032] Figure 3 A schematic block diagram of a typical processing device.

[0033] Figure 4 The present invention is a flowchart of a control method for a screw chiller according to some other embodiments of the present application. DETAILED DESCRIPTION

[0034] The present application is described more fully below with reference to the accompanying drawings in which illustrative embodiments of the present application are illustrated. However, the present application may be implemented in different forms and should not be interpreted as being limited to the embodiments given herein. The above embodiments are given to make the disclosure herein comprehensive and complete, so as to more fully convey the scope of protection of the present application to those skilled in the art.

[0035] In this specification, terms such as "comprise" and "include" indicate that in addition to the units and steps directly and explicitly stated in the specification and claims, the technical solution of the present application does not exclude the situation where there are other units and steps that are not directly or explicitly stated.

[0036] Unless otherwise specified, terms such as "first" and "second" do not indicate the order of the units in terms of time, space, size, etc., but are merely used to distinguish the units.

[0037] Figure 1 Schematic diagram of a screw chiller according to some embodiments of the present application. Figure 1 In addition to the units shown, the screw chiller also contains other units that are essential to implement the refrigeration function. However, for the sake of simplicity, Figure 1 The following description will only relate to units directly related to the technical problem solved by the present application. In addition, the screw chiller described here is only exemplary, and the embodiments described in this specification can also be extended to other types of chillers.

[0038] Figure 1 The screw chiller 10 shown includes a first compressor 110A, a second compressor 110B, a first motor 120A, a second motor 120B and a motor drive system 130. Exemplarily, the first compressor 110A and the second compressor 110B are respectively connected to different circuits of the screw chiller, and the first motor 120A and the second motor 120B serve as power sources for the first compressor 110A and the second compressor 110B, respectively. In addition, the motor drive system 130 is configured to drive the first motor 120A and the second motor 120B so that the first motor 120A and the second motor 120B operate at the required output power.

[0039] Continue to see Figure 1 The motor drive system 130 includes a variable frequency drive unit 131, an industrial frequency drive unit 132 and a controller 133. The variable frequency drive unit 131 can be implemented, for example, by a variable frequency drive, which is configured to drive the first motor 120A in a variable frequency manner under the control of the control unit 133. The industrial frequency drive unit 132 can be implemented, for example, by an industrial frequency (fixed frequency) drive, which is configured to drive the second motor 120B at a fixed frequency under the control of the control unit 133. The control unit 133 is configured to control the output power of the variable frequency drive unit 131 and the industrial frequency drive unit 132, so that the first compressor 110A is driven by the first motor 120A to operate at the required power, and the second compressor 110B is driven by the second motor 120B to operate at the required power, thereby adapting the load demand of the screw chiller 10. By equipping different compressors with variable frequency drive units and industrial frequency drive units, the number of variable frequency drives can be reduced, which is beneficial to reducing equipment costs.

[0040] For illustrative purposes only, Figure 1In the illustrated embodiment, it is assumed that the number of the first compressor 110A and the number of the second compressor 110B are both 1. However, those skilled in the art will recognize after reading the specification of this application that the content disclosed in this application is also applicable to the case where the compressors have other numbers. For example, the number of the first compressor is 1, and the number of the second compressor is 2 or more; or the number of the first compressor is 2 or more, and the number of the second compressor is 1 or more.

[0041] exist Figure 1 In the illustrated embodiment, the first compressor 110A and the second compressor 110B may be compressors of the same type or compressors of different types. In addition, optionally, when the compressors have different rated powers, the compressor with a larger rated power may be designated as the first compressor and the compressor with a smaller rated power may be designated as the second compressor, that is, the compressor with a larger rated power may be equipped with a variable frequency drive unit and the compressor with a smaller rated power may be equipped with an industrial frequency drive unit.

[0042] The control logic of the control unit 133 over the variable frequency drive unit 131 and the industrial frequency drive unit 132 is further described below.

[0043] In order to save energy and increase efficiency, Figure 1 It is a natural idea to give priority to the operation of the variable frequency drive unit in the screw chiller shown. For example, when the load demand of the screw chiller increases, priority is given to increasing the output power of the variable frequency drive unit 131 to adapt to the increase in load demand. Only when the output power of the variable frequency drive unit 131 reaches the maximum value and still cannot adapt to the load demand, the operation of the industrial frequency drive unit 132 is considered; on the other hand, when the load demand of the screw chiller decreases, priority is given to reducing the output power of the industrial frequency drive unit 132. Only when the output power of the industrial frequency drive unit 132 reaches the minimum value, the output power of the variable frequency drive unit 131 is considered to be reduced. This control logic of giving priority to the operation of the variable frequency drive unit is referred to as the "priority mode" below.

[0044] In the control logic called "balanced mode" to be described below, the activation of the variable frequency drive unit and the industrial frequency drive unit is more balanced compared to the priority mode. Specifically, when the load demand of the screw chiller starts to rise from zero, priority is still given to increasing the output power of the variable frequency drive unit 131 to adapt to the load demand, but unlike the priority mode, after the output power of the variable frequency drive unit 131 reaches a higher value (for example, 70% of the maximum value), the subsequent load demand increment is adapted by alternately increasing the output power of the industrial frequency drive unit 132 and the variable frequency drive unit 131; on the other hand, when the load demand of the screw chiller decreases from a high point, the output power of the industrial frequency drive unit 132 and the variable frequency drive unit 131 is alternately reduced to adapt to the reduction in load demand.

[0045] After in-depth research, the inventors of the present application found that when the following control logic (hereinafter referred to as "cross mode") is adopted, the energy-saving and efficiency-enhancing effects can be significantly improved compared with the above-mentioned priority mode and balance mode.

[0046] Table 1 shows the output power P of the variable frequency drive unit 131 in the crossover mode. VFD and the output power P of the power frequency drive unit 132 FD An example of changes with the increase of load demand TOL_CAP. In Table 1, the output power P VFD and P FD It is expressed as a percentage relative to the respective maximum output power, and the load demand TOL_CAP is expressed as a percentage relative to the maximum load demand.

[0047] Table 1

[0048]

[0049]

[0050] Table 2 shows the output power P of the variable frequency drive unit 131 in the crossover mode. VFD and the output power P of the power frequency drive unit 132 FD As an example of the change with the decrease of load demand TOL_CAP, in Table 2, the output power P VFD and P FD It is expressed as a percentage relative to the respective maximum output power, and the load demand TOL_CAP is expressed as a percentage relative to the maximum load demand.

[0051] Table 2

[0052]

[0053]

[0054] In the examples shown in Tables 1 and 2, the load demand TOL_CAP of the screw chiller is divided into the following multiple intervals:

[0055] Interval I: [0, 40%]

[0056] Interval II: (40%, 50%)

[0057] Interval III: [50%, 75%)

[0058] Interval IV: [75%, 100%]

[0059] Interval I

[0060] When the load demand TOL_CAP is within the interval I, the load demand is adapted by operating only the variable frequency drive unit 131. For example, the variable frequency drive unit 131 operates with a variable output power to adapt to the change of the load demand within the interval I. In this example, the upper limit of the interval I, 40%, is equivalent to a critical point or a critical value (hereinafter referred to as the first critical value T1). When the critical value is not exceeded, only the variable frequency drive unit 131 is in operation, and once it is exceeded, the power frequency drive unit 132 will cooperate with the variable frequency drive unit 131 to match the required load demand.

[0061] Interval II

[0062] After the load demand TOL_CAP enters interval II, the load demand is adapted by simultaneously operating the variable frequency drive unit 131 and the power frequency drive unit 132. In particular, when the load demand increases from 40% to 50%, the load demand is adapted by gradually reducing the output power of the variable frequency drive unit 131 and gradually increasing the output power of the power frequency drive unit 132; on the other hand, when the load demand decreases from 50% to 40%, the reduced load demand is adapted by simultaneously reducing the output power of the variable frequency drive unit 131 and the power frequency drive unit 132.

[0063] Table 3 shows the output power P of the variable frequency drive unit 131 when the load demand TOL_CAP is within interval II. VFD and the output power P of the power frequency drive unit 132 FD An example of changes as the load demand TOL_CAP increases.

[0064] Table 3

[0065] TOL_CAP <![CDATA[P VFD ]]> <![CDATA[P FD ]]> 40 80 0 47.5 80 15 50 80 20 50 75 25 50 70 30 50 65 35 50 60 40 50 55 45 50 50 50

[0066] In the example shown in Table 3, when the load demand TOL_CAP reaches 50%, the output power of the variable frequency drive unit 131 is rapidly reduced from 80% to 50%. At the same time, the output power of the industrial frequency drive unit 132 is increased from 20% to 50% to compensate for the reduction in the output power of the variable frequency drive unit 131, so that the overall output power of the first and second compressors is adapted to the load demand.

[0067] Table 4 shows the output power P of the variable frequency drive unit 131 when the load demand TOL_CAP is within interval II. VFD and the output power P of the power frequency drive unit 132 FD An example of changes with a decrease in load demand TOL_CAP.

[0068] Table 4

[0069] TOL_CAP <![CDATA[P VFD ]]> <![CDATA[P FD ]]> 50 50 50 45 45 45 40 40 40 40 80 0

[0070] In the example shown in Table 4, when the load demand TOL_CAP reaches 40% (the aforementioned first critical value T1), the output power of the variable frequency drive unit 131 rapidly increases from 40% to 80%. At the same time, the output power of the industrial frequency drive unit 132 rapidly decreases from 40% to 0, so the overall output power of the first and second compressors is still adapted to the load demand.

[0071] Interval III

[0072] When the load demand TOL_CAP is within interval III, the load demand is adapted by operating the variable frequency drive unit 131 and the industrial frequency drive unit 132 simultaneously. In particular, the load demand changes within interval III are adapted by operating the variable frequency drive unit 131 at the first output power P1 and operating the industrial frequency drive unit 132 at a variable output power. In this example, the lower limit of interval III, 50%, is equivalent to a critical point or critical value (hereinafter referred to as the second critical value T2), and once the critical value is exceeded, the variable frequency drive unit 131 will operate at a fixed output power P1. Optionally, the output power P1 can be set to be less than the output power of the variable frequency drive unit 131 when the load demand is equal to the first critical value T1 (for example, 80% of the maximum output power of the variable frequency drive unit 131 as shown in Table 1), for example, the output power P1 can be set to 50% of the maximum output power of the variable frequency drive unit 131 as shown in Table 1).

[0073] Interval IV

[0074] When the load demand TOL_CAP is within the interval IV, the load demand changes within the interval IV are adapted by operating the variable frequency drive unit 131 with a variable output power and operating the industrial frequency drive unit 132 with a second output power P2. In this example, the lower limit of the interval IV, 75%, corresponds to a critical point or a critical value (hereinafter referred to as the third critical value T3). Once the critical value is exceeded, the variable frequency drive unit 131 will operate with a variable output power and the industrial frequency drive unit 132 will operate with a fixed output power. Optionally, the output power P2 can be set to the maximum output power of the industrial frequency drive unit 132.

[0075] Figure 2 The comparison results of the operating performance of the screw chiller in the cross mode, the balanced mode and the priority mode are shown, wherein the cross mode is based on the exemplary methods shown in Tables 1 and 2 and includes the first critical value T1 being 40%, 50% and 60%. Figure 2 In the figure, the vertical axis is the integrated part load performance coefficient (IPLV) used to describe the operating performance of the screw chiller.

[0076] Depend on Figure 2 It can be seen that the IPLV in the cross mode is higher than that in the balanced mode and the priority mode, which shows that using the cross mode as the control logic of the variable frequency drive unit and the industrial frequency drive unit can significantly improve the effect of energy saving and efficiency improvement.

[0077] It should be pointed out that the interval division method of the load demand shown in Table 1 and Table 2 is only exemplary. Optionally, the upper limit or first critical value T1 of interval I can be determined based on the type of the first compressor and the second compressor. For example, when the first and second compressors are of different types, the first critical value T1 can be set to about 40% of the rated load demand of the screw chiller, and when the first and second compressors are of the same type, the first critical value T1 can be set to about 50% of the rated load demand of the screw chiller. In addition, the number of intervals shown in Table 1 and Table 2 is also non-limiting. For example, in some other examples, the load demand TOL_CAP of the screw chiller is divided into the following multiple intervals:

[0078] Interval I': [0, 50%]

[0079] Interval II': (50%, 75%)

[0080] Interval III': [75%, 100%]

[0081] In the above division method, the first critical value T1 and the second critical value T2 can be combined into a critical value T. For example, when the critical value T is not exceeded, only the variable frequency drive unit 131 is in operation, and once the critical value T is exceeded, the variable frequency drive unit 131 will operate with a fixed output power P1' and the industrial frequency drive unit 132 will operate with a variable output power. Optionally, the output power P1' can be set to an output power (e.g., 50%) that is smaller than the output power (e.g., 80%) of the variable frequency drive unit 131 at the upper limit of interval I.

[0082] Figure 3 A schematic block diagram of a typical processing device. Figure 3 The processing device shown can be used to implement Figure 1 The control unit in the motor drive system shown.

[0083] like Figure 3 As shown, the processing device 30 includes a communication unit 310 , a memory 320 (eg, a non-volatile memory such as a flash memory, a ROM, a hard drive, a magnetic disk, an optical disk, etc.), a processor 330 and a computer program 340 .

[0084] The communication unit 310 serves as a communication interface and is configured to establish a communication connection between the processing device and external devices (such as the variable frequency drive unit 131, the industrial frequency drive unit 132 and sensors, etc.) or a network (such as the Internet).

[0085] The memory 320 stores a computer program 340 executable by the processor 330. In addition, the memory 320 may store data generated when the processor 330 executes the computer program and data or commands received from the outside via the communication unit 310.

[0086] The processor 330 is configured to execute a computer program 340 stored on the memory 320 and access data on the memory 320 .

[0087] Figure 4 Flow chart of a control method for a screw chiller according to some other embodiments of the present application. Figure 3 That is, Figure 3 The computer program 340 in the embodiment may include computer instructions for implementing each step of the following method, so that when the computer program 340 is executed on the processor 330, the corresponding method can be implemented.

[0088] Figure 4 The method shown includes the following steps:

[0089] Step 401 : the processing device 30 obtains or determines the current load demand of the screw chiller 10 .

[0090] Step 402 : The processing device 30 executes the control logic based on the aforementioned cross mode based on the current load demand obtained or determined in step 401 , thereby adapting the load demand of the screw chiller 10 by controlling the output power of the variable frequency drive unit 131 and the industrial frequency drive unit 132 .

[0091] The control logic based on the cross mode has been described in detail above and will not be repeated here.

[0092] Step 403: The processing device 30 determines whether it is necessary to continue to execute the control logic. If so, it returns to step 401; otherwise, it exits. Figure 4 The process shown.

[0093] According to another aspect of the present application, a computer-readable storage medium is provided, on which a computer program is stored, and when the program is executed by a processor, the above-mentioned Figure 4 One or more steps included in the method.

[0094] The computer-readable storage medium referred to in this application includes various types of computer storage media, which can be any available medium that a general or special-purpose computer can access. For example, a computer-readable storage medium can include RAM, ROM, EPROM, E2PROM, register, hard disk, removable disk, CD-ROM or other optical disk storage, disk storage or other magnetic storage device, or can be used to carry or store a desired program code unit with an instruction or data structure form and can be accessed by a general or special-purpose computer, or a general or special-purpose processor. Any other temporary or non-temporary medium. The above combination should also be included in the protection scope of the computer-readable storage medium. An exemplary storage medium is coupled to a processor so that the processor can read and write information from / to the storage medium. In an alternative, a storage medium can be integrated into a processor. The processor and the storage medium can reside in an ASIC. The ASIC can reside in a user terminal. In an alternative, the processor and the storage medium can reside in a user terminal as discrete components.

[0095] Those skilled in the art will appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described herein may be implemented as electronic hardware, computer software, or a combination of both.

[0096] In order to show the interchangeability between hardware and software, various schematic components, blocks, modules, circuits and steps are generally described above according to their functionality. Such functionality is implemented in hardware form or software form depending on the specific application and the design restrictions imposed on the overall system. Those skilled in the art can implement the described functionality in a variable manner for specific specific applications, but such implementation decisions should not be understood as causing deviations from the scope of the present application.

[0097] Although only some of the specific embodiments of the present application are described, it should be understood by those skilled in the art that the present application can be implemented in many other forms without departing from its subject matter and scope. Therefore, the examples and embodiments shown are considered to be illustrative rather than restrictive, and the present application may include various modifications and substitutions without departing from the spirit and scope of the present application as defined in the appended claims.

[0098] The embodiments and examples set forth herein are provided to best illustrate embodiments according to the present technology and its specific applications, and thereby enable those skilled in the art to implement and use the present application. However, those skilled in the art will appreciate that the above description and examples are provided only for ease of illustration and example. The description set forth is not intended to cover all aspects of the present application or to limit the present application to the precise form disclosed.

Claims

1. A motor drive system for a water chiller, the water chiller comprising a first compressor, a first motor for the first compressor, a second compressor and a second motor for the second compressor, the motor drive system comprising: a variable frequency drive unit configured to drive the first motor; an industrial frequency driving unit configured to drive the second motor; as well as A control unit is configured to adapt to the load demand of the chiller by controlling the output power of the variable frequency drive unit and the industrial frequency drive unit.

2. The motor drive system according to claim 1, wherein: The rated power of the first compressor is greater than or equal to the rated power of the second compressor.

3. The motor drive system according to claim 1, wherein: The number of the first compressor is one, and the number of the second compressor is one or more.

4. The motor drive system according to claim 1, wherein: The control unit controls the output power of the variable frequency drive unit and the industrial frequency drive unit in the following manner: When the load demand is less than or equal to a first critical value, adapting the load demand by operating only the variable frequency drive unit, wherein the variable frequency drive unit operates with a variable output power; When the load demand is greater than a second critical value and less than a third critical value, adapting the load demand by operating the variable frequency drive unit and the industrial frequency drive unit, wherein the variable frequency drive unit operates at a first output power and the industrial frequency drive unit operates at a variable output power, and the second critical value is greater than or equal to the first critical value; When the load demand is greater than or equal to the third critical value, the load demand is adapted by operating the variable frequency drive unit and the industrial frequency drive unit, wherein the variable frequency drive unit operates with a variable output power and the industrial frequency drive unit operates with a second output power.

5. The motor drive system according to claim 4, wherein: The first output power is less than the output power of the variable frequency drive unit when the load demand is equal to the first critical value, and the second output power is equal to the maximum output power of the industrial frequency drive unit.

6. The motor drive system according to claim 4, wherein: The first compressor and the second compressor are compressors of the same type, the chiller is a screw chiller, and the first critical value is 50% of the rated load requirement of the screw chiller.

7. The motor drive system according to claim 4, wherein: The first compressor and the second compressor are compressors of different types, the chiller is a screw chiller, and the first critical value is 40% of the rated load requirement of the screw chiller.

8. A chiller comprising: The first compressor; a first motor for said first compressor; Second compressor; a second motor for the second compressor; as well as A motor drive system as claimed in any one of claims 1 to 7.

9. A control method for a water chiller, the water chiller comprising a first compressor, a first motor for the first compressor, a second compressor and a second motor for the second compressor, the method comprising: A variable frequency drive unit and an industrial frequency drive unit are used to drive the first motor and the second motor respectively, wherein the load demand of the chiller is adapted by controlling the output power of the variable frequency drive unit and the industrial frequency drive unit.

10. The method of claim 9, wherein: The rated power of the first compressor is greater than or equal to the rated power of the second compressor.

11. The method of claim 9, wherein: The number of the first compressor is one, and the number of the second compressor is one or more.

12. The method of claim 9, wherein: The output power of the variable frequency drive unit and the industrial frequency drive unit is controlled in the following manner: When the load demand is less than or equal to a first critical value, adapting the load demand by operating only the variable frequency drive unit, wherein the variable frequency drive unit operates with a variable output power; When the load demand is greater than a second critical value and less than a third critical value, adapting the load demand by operating the variable frequency drive unit and the industrial frequency drive unit, wherein the variable frequency drive unit operates at a first output power and the industrial frequency drive unit operates at a variable output power, and the second critical value is greater than or equal to the first critical value; When the load demand is greater than or equal to the third critical value, the load demand is adapted by operating the variable frequency drive unit and the industrial frequency drive unit, wherein the variable frequency drive unit operates with a variable output power and the industrial frequency drive unit operates with a second output power.

13. The method of claim 12, wherein: The first output power is less than the output power of the variable frequency drive unit when the load demand is equal to the first critical value, and the second output power is equal to the maximum output power of the industrial frequency drive unit.

14. The method of claim 12, wherein: The first compressor and the second compressor are compressors of the same type, the chiller is a screw chiller, and the first critical value is 50% of the rated load requirement of the screw chiller.

15. The method of claim 12, wherein: The first compressor and the second compressor are compressors of different types, the chiller is a screw chiller, and the first critical value is 40% of the rated load requirement of the screw chiller.

16. A computer-readable storage medium having instructions stored therein, characterized in that: When the instructions are executed by a processor, the processor is caused to perform the method according to any one of claims 9 to 15.

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