EP4160102A1

EP4160102A1 - Oil return control method for multi-split system

Info

Publication number: EP4160102A1
Application number: EP21800140.2A
Authority: EP
Inventors: Baitian ZHUO; Bin Shi; Shaojiang CHENG; Ruigang Zhang; Jun Wang
Original assignee: Qingdao Haier Air Conditioning Electric Co Ltd; Haier Smart Home Co Ltd
Current assignee: Qingdao Haier Air Conditioning Electric Co Ltd; Haier Smart Home Co Ltd
Priority date: 2020-05-25
Filing date: 2021-02-08
Publication date: 2023-04-05
Anticipated expiration: 2041-02-08
Also published as: CN113719963B; WO2021223481A1; EP4160102A4; EP4160102B1; CN113719963A

Abstract

An oil return control method for a multi-split system. The multi-split system comprises a plurality of indoor units. At least one indoor unit among the plurality of indoor units is not powered on. The oil return control method comprises: determining the oil storage amount q of all the indoor units; determining the system oil return duration T of the multi-split system; determining, according to the oil storage amount q and the system oil return duration T, the current opening degree k of an expansion valve of one indoor unit which is not powered on; comparing the magnitude relation between the current opening degree k and the maximum opening degree threshold k<sub>smax</sub> and the minimum opening degree threshold k<sub>smin</sub>; and according to the comparison results, selectively adjusting the opening degree of the expansion valve of the indoor unit which is not powered on, and maintaining the expansion valve to operate at the adjusted opening degree in the system oil return duration T. The oil return control method can accurately control the opening degree and the oil return time of the expansion valve of each indoor unit which is not powered on, and ensure that an indoor unit which is normally powered on has the sufficient refrigerant while providing a compressor with the sufficient lubricating oil, thereby considering both the oil return efficiency and the heat exchange performance of the system.

Description

FIELD OF THE INVENTION

The present disclosure belongs to the technical field of air conditioning, and specifically provides an oil return control method for a multi-connection system.

BACKGROUND OF THE INVENTION

A multi-connection system usually consists of a plurality of indoor units and at least one outdoor unit. During the actual operation, some indoor units are in a shutdown or standby state. If an expansion valve of the shutdown or standby indoor unit is closed, a lubricating oil in connecting pipes and heat exchanger of the indoor unit cannot be circulated into the system. As the time elapses, there will be more and more lubricating oil in the indoor unit, causing the compressor to suffer severe wear due to lack of the lubricating oil or causing damage to the compressor due to overheating. In order to ensure a normal operation of the multi-connection system, oil return control is required so that the lubricating oil can be normally circulated into the system.
In traditional oil return control methods, the oil is usually returned according to a fixed cycle. During the oil return process, the expansion valve is opened by a fixed opening degree, and the system terminates the oil return after an oil return operation time is reached.
However, since the actual working condition of each indoor unit is different, after the oil return of the system is completed, the lubricating oil stored in some indoor units may have returned to the system long ago, but their expansion valves are still in the open state, so that a refrigerant can continue to flow in these indoor units, which will not only generate noise, but also lead to problems such as insufficient refrigerant and poor heat exchange performance of the system in normally turned-on indoor units; even worse, when the opening degree of the expansion valve is too large, there will be a liquid hammer phenomenon in the compressor. However, there may still be a large amount of lubricating oil left in some other shutdown or standby indoor units that has not yet returned to the system, and there is still a problem of insufficient lubricating oil in the compressor.
As can be seen from the above, the traditional oil return control methods have not fundamentally solved the problem of insufficient lubricating oil in the compressor, and may also cause the problem of poor overall heat exchange performance of the system due to the insufficient amount of refrigerant in the normally turned-on indoor units. That is, the traditional oil return control methods cannot give considerations to both the oil return effect and the heat exchange performance of the system.
Accordingly, there is an urgent need in the art for a new oil return control method to solve the above technical problem existing in the traditional oil return control methods.

SUMMARY OF THE INVENTION

In order to solve the above technical problem existing in the traditional oil return control methods, the present disclosure provides an oil return control method for a multi-connection system.
In the oil return control method for the multi-connection system, the multi-connection system includes a plurality of indoor units, at least one of which is not turned on, and the oil return control method includes: determining an oil storage quantity q of the indoor units; determining a system oil return time T of the multi-connection system; determining a current opening degree k of an expansion valve of one indoor unit that is not turned on according to the oil storage quantity q and the system oil return time T; comparing the current opening degree k with a maximum opening degree threshold k_smax and a minimum opening degree threshold k_smin; selectively adjusting the opening degree of the expansion valve according to a comparison result, and keeping the expansion valve operating at the adjusted opening degree for the system oil return time T.
In a preferred technical solution of the above oil return control method, the step of "determining a current opening degree k of an expansion valve of the one indoor unit that is not turned on according to the oil storage quantity q and the system oil return time T" specifically includes calculating the current opening degree k using the following formula: $k = q (k_{smax} \times T_{smax}) / (q_{smax} \times T);$
where q_smax represents a maximum oil storage quantity threshold of the indoor unit, k_smax represents the maximum opening degree threshold of the expansion valve, and T_smax represents a maximum system oil return time threshold of the multi-connection system.
In a preferred technical solution of the above oil return control method, the step of "selectively adjusting the opening degree of the expansion valve according to a comparison result, and keeping the expansion valve operating at the adjusted opening degree for the system oil return time T" specifically includes: determining an indoor unit oil return time t of the one indoor unit that is not turned on according to the following formula if k<k_smin: $t = (k \times T) / k_{smin};$
adjusting the opening degree of the expansion valve to k_smin, and keeping the expansion valve operating at the adjusted opening degree for the time t; adjusting the opening degree of the expansion valve to zero, keeping the expansion valve operating at the adjusted opening degree for a time T-t, and then returning to the step of determining q.
In a preferred technical solution of the above oil return control method, the step of "selectively adjusting the opening degree of the expansion valve according to a comparison result, and keeping the expansion valve operating at the adjusted opening degree for the system oil return time T" specifically includes: keeping the expansion valve operating at the current opening degree k for the system oil return time T if k_smin≤ k≤k_smax, and then returning to the step of determining q.
In a preferred technical solution of the above oil return control method, the step of "selectively adjusting the opening degree of the expansion valve according to a comparison result, and keeping the expansion valve operating at the adjusted opening degree for the system oil return time T" specifically includes: adjusting the opening degree of the expansion valve to k_smax if k>k_smax, keeping the expansion valve operating at the adjusted opening degree for the system oil return time T, and then returning to the step of determining q.
In a preferred technical solution of the above oil return control method, before returning to the step of determining q, the expansion valve is kept operating at the adjusted opening degree for an oil return interval time Ti.
In a preferred technical solution of the above oil return control method, the step of "keeping the expansion valve operating at the adjusted opening degree for an oil return interval time T_i" specifically includes: determining a total oil storage quantity Q of the multi-connection system according to formula (5): $Q = \sum_{i = 1}^{n} q_{i};$
where n represents the number of the indoor units, and qi represents the oil storage quantity of the i^th indoor unit; and
judging whether the total oil storage quantity is larger than a maximum total oil storage quantity threshold Q_smax; if yes, it means that the expansion valve has operated at the adjusted opening degree for the oil return interval time Ti.
In a preferred technical solution of the above oil return control method, the step of "keeping the expansion valve operating at the adjusted opening degree for an oil return interval time T_i" specifically includes: acquiring a shutdown time t_sp of the indoor unit; and judging whether the shutdown time t_sp is larger than a maximum shutdown time threshold t_spmax; if yes, it means that the expansion valve has operated at the adjusted opening degree for the oil return interval time Ti.
In a preferred technical solution of the above oil return control method, the oil storage quantity q of the indoor unit is determined according to the following formula in the oil return control method: $q = P \times t_{sp} \times (P_{d} - P_{s}) \times R_{o};$
where P represents a capacity horsepower of the indoor unit, t_sp represents the shutdown time of the indoor unit, P_d represents a system high pressure of the multi-connection system, P_s represents a system low pressure of the multi-connection system, and R_o represents an oil storage coefficient of the indoor unit.
In a preferred technical solution of the above oil return control method, the step of "determining a system oil return time T of the multi-connection system" includes: acquiring a maximum oil storage quantity q_max of the indoor unit; judging whether the maximum oil storage quantity q_max is larger than the maximum oil storage quantity threshold q_smax; if yes, determining the system oil return time T as the maximum oil return time threshold T_smax; otherwise, determining the system oil return time T according to the following formula: $T = q_{\max} / q_{smax} \times T_{smax} .$
The oil return control method for the multi-connection system provided by the present disclosure includes: determining an oil storage quantity q of the indoor units; determining a system oil return time T of the multi-connection system; determining a current opening degree k of an expansion valve of one indoor unit that is not turned on according to the oil storage quantity q and the system oil return time T; comparing the current opening degree k with a maximum opening degree threshold k_smax and a minimum opening degree threshold k_smin; selectively adjusting the opening degree of the expansion valve according to a comparison result, and keeping the expansion valve operating at the adjusted opening degree for the system oil return time T.
The oil return control method of the present disclosure can precisely control the opening degree of the expansion valve and the oil return time of each indoor unit that is not turned on, so that at the same time of providing sufficient lubricating oil to the compressor, it can also be ensured that there is sufficient amount of refrigerant in the normally turned-on indoor units, thus giving considerations to both the oil return effect and the heat exchange performance of the system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the typical schematic structural view of a multi-connection system;
FIG. 2 is a flowchart illustrating main steps of an oil return control method for a multi-connection system of the present disclosure;
FIG. 3 is a flowchart illustrating detailed steps of the oil return control method for the multi-connection system of the present disclosure;
FIG. 4 is a flowchart illustrating detailed steps of step S200 in FIG. 3;
FIG. 5 is a flowchart illustrating detailed steps of an embodiment of step S600 in FIG. 3; and
FIG. 6 is a flowchart illustrating detailed steps of another embodiment of step S600 in FIG. 3.

DETAILED DESCRIPTION OF THE EMBODIMENT(S) OF THE INVENTION

Preferred embodiments of the present disclosure will be described below with reference to the accompanying drawings. It should be understood by those skilled in the art that these embodiments are only used to explain the technical principle of the present disclosure, and are not intended to limit the scope of protection of the present disclosure.
In the description of the present application, a "controller" may include hardware, software, or a combination thereof. A module may include hardware circuits, various suitable sensors, communication ports and memories, and may also include software parts, such as program codes, or a combination of software and hardware.
In order to facilitate understanding, in the following, a typical structure of a multi-connection system will be first briefly introduced with reference to FIG. 1. It should be noted that the dotted line "- - - " in FIG. 1 represents liquid pipes, and the solid line "-" in FIG. 1 represents gas pipes.
Referring to FIG. 1, a multi-connection system includes an outdoor unit and n indoor units. Each of the indoor units is connected to the outdoor unit through a gas pipe and a liquid pipe to form a refrigerant circulation system, and each of the indoor units further includes an expansion valve VE. The expansion valve VE is arranged on the liquid pipe between the outdoor unit and the corresponding indoor unit, and is configured to adjust the amount of refrigerant flowing through the indoor unit. When the refrigerant circulates between the outdoor unit and individual indoor units of the multi-connection system, the lubricating oil also circulates with the refrigerant in the system so as to lubricate and cool inner components of a compressor.
When some of the indoor units of the multi-connection system are turned on and working, some other indoor units are in a shutdown or standby state. Therefore, in order to improve readability, the indoor units in the shutdown or standby state are collectively referred to as indoor units that are not turned on herein. In order to prevent the problem of insufficient lubricating oil in the compressor caused by the accumulation of lubricating oil in the indoor units that are not turned on, generally, an opening degree of the expansion valves VE of the indoor units that are not turned on is adjusted to a fixed value within a preset period of time, so that the lubricating oil accumulated in the indoor units that are not turned on flows back into the compressor with the refrigerant, which is called an oil return control process of the multi-connection system.
However, due to the difference in the actual working conditions of various indoor units that are not turned on, after the oil return control process of the system is completed, a large amount of lubricating oil is still accumulated in some indoor units, causing the compressor to still have the problem of insufficient lubricating oil, and the lubricating oil in some other indoor units has completely flowed back into the compressor before the oil return control process is completed, but the expansion valves VE thereof still maintain a fixed opening degree. When the opening degree of the expansion valves VE is too large, a large amount of refrigerant in the system continues to flow in these indoor units, which will not only cause the liquid hammer phenomenon in the compressor, but also will cause the problems of insufficient amount of the refrigerant in the normally turned-on indoor units and poor heat exchange performance of the system.
For this reason, the present disclosure provides an oil return control method for a multi-connection system, which can precisely control the opening degree of the expansion valve of each indoor unit that is not turned on and the oil return time thereof, so that at the same time of providing sufficient lubricating oil to the compressor, it can also be ensured that there is sufficient amount of refrigerant in the normally turned-on indoor units, thus giving considerations to both the oil return effect and the heat exchange performance of the system.
In order to facilitate understanding the oil return control method, a main step flow of the oil return control method for the multi-connection system of the present disclosure will be described hereinafter with reference to FIG. 2.
Referring to FIG. 2, the main step flow of the oil return control method of the present disclosure includes:

Step S100: determining an oil storage quantity q of all the indoor units;
Step S200: determining a system oil return time T of the multi-connection system;
Step S300: determining a current opening degree k of an expansion valve of one indoor unit that is not turned on according to the oil storage quantity q and the system oil return time T;
Step S400: comparing the current opening degree k with a maximum opening degree threshold k_smax and a minimum opening degree threshold k_smin; and
Step S500: selectively adjusting the opening degree of the expansion valve according to a comparison result, and keeping the expansion valve operating at the adjusted opening degree for the system oil return time T.

It should be noted that the expansion valve of the turned-on indoor unit is always in an open state before the oil is returned. In order to ensure the heat exchange effect of the multi-connection system, the opening degrees of the expansion valves of all the turned-on indoor units are normally adjusted according to a control logic before the oil return.
Further, in order to better understand the oil return control method of the present disclosure, a control flow of the detailed steps of the oil return control method of the present disclosure will described in detail below with reference to FIG. 3.
Referring to FIG. 3, a detailed step flow of the oil return control method of the present disclosure includes the following steps.
Step S100: determining an oil storage quantity q of all the indoor units.
Specifically, in step S 100, the oil storage quantity q of the indoor units is determined according to formula (1). $q = P \times t_{sp} \times (P_{d} - P_{s}) \times R_{o}$
where P represents a capacity horsepower of the indoor unit, t_sp represents the shutdown time of the indoor unit, P_d represents a system high pressure of the multi-connection system, P_s represents a system low pressure of the multi-connection system, and R_o represents an oil storage coefficient of the indoor unit.
It should be noted that P represents the capacity horsepower of the indoor unit. This value is a performance parameter of the indoor unit, which has been clearly set at the factory and can be acquired directly when needed, without additional calculation. t_sp represents the shutdown time of the indoor unit. The shutdown time t_sp of each indoor unit is stored in a controller of the multi-connection system, and can be read directly when needed. P_d and P_s represent the system high pressure and system low pressure of the multi-connection system respectively. These two specific values can be collected by two pressure sensors installed on the outdoor unit, and then transmitted to the controller in a wired or wireless manner for storage. Also, they can be read directly when needed.

R_o represents the oil storage coefficient of the indoor unit, and this value depends on an ambient temperature T_ai of the indoor unit, a saturation temperature T_pd corresponding to the system high pressure and a saturation temperature T_ps corresponding to the system low pressure under different working conditions of the system. Table 1 shows the oil storage coefficient R_o obtained under different working conditions of the indoor unit through experimental verification, and an appropriate value can be chosen from Table 1 according to the actual working conditions of the indoor unit during calculation.

Table 1

cooling		heating
T_ai - T_ps (°C)	R_o	T_ai - T_pd (°C)	R_o
>=30	0.5	>=30	0.7
[25,30)	0.7	[25,30)	0.9
[20,25)	1.0	[20,25)	1.1
[15,20)	1.2	[15,20)	1.5
[10,15)	1.4	[10,15)	1.7
<=10	1.8	<=10	2.0

Step S200: determining a system oil return time T of the multi-connection system.
Referring to the detailed step flow chart of step S200 in FIG. 4, step S200 specifically includes:
Step S201: acquiring the maximum oil storage quantity q_max of the indoor unit.
The controller of the multi-connection system calculates and stores the oil storage quantity q of each indoor unit according to the aforementioned formula (1). The oil storage quantity q of the indoor unit varies according to the different working condition of the indoor unit. In step S201, the controller traverses all the oil storage quantities q of the indoor unit that have been stored, finds out the maximum oil storage quantity q_max therefrom and reads it directly.
Step S202: judging whether the maximum oil storage quantity q_max is larger than the maximum oil storage quantity threshold q_smax; if yes, the process proceeds to step S203; otherwise, the process proceeds to step S204.
Step S203: determining the system oil return time T as the maximum oil return time threshold T_smax.
Step S204: determining the system oil return time T according to formula (2): $T = q_{\max} / q_{smax} \times T_{smax}$
It should be noted that the values of the maximum oil storage quantity threshold q_smax and the maximum oil return time threshold T_smax depend on various performance parameters of the indoor unit, and they can be set by those skilled in the art according to the specific performance parameters of the indoor unit.
After the oil storage quantity q of the indoor unit and the system oil return time T are determined, with continued reference to FIG. 2, the oil return control method proceeds to step S300.
Step S300: determining the current opening degree k of the expansion valve of the indoor unit that is not turned on using the formula (3) according to the oil storage quantity q and the system oil return time T: $k = q (k_{smax} \times T_{smax}) / (q_{smax} \times T)$
where q_smax represents the maximum oil storage quantity threshold of the indoor unit, k_smax represents the maximum opening degree threshold of the expansion valve, and T_smax represents the maximum system oil return time threshold of the multi-connection system. It should be noted that the values of q_smax, k_smax and a minimum opening degree threshold k_smin that will appear in subsequent steps depend on various performance parameters of the indoor unit, and they can be set by those skilled in the art according to the specific performance parameters of the indoor unit. The value of T_smax depends on the actual working parameters of the multi-connection system, and it can be set by those skilled in the art according to the actual situation.
Step S400: comparing the current opening degree k with the maximum opening degree threshold k_smax and a minimum opening degree threshold k_smin.
Step S500: selectively adjusting the opening degree of the expansion valve according to a comparison result, and keeping the expansion valve operating at the adjusted opening degree for the system oil return time T.
In detail, step S500 specifically includes:
proceeding to step S501 if k<k_smin, and determining the indoor unit oil return time t of the indoor unit according to formula (4): $t = (k \times T) / k_{smin}$
Step S502: adjusting the opening degree of the expansion valve to k_smin, and keeping the expansion valve operating at the adjusted opening degree for the indoor unit oil return time t.
Step S503: adjusting the opening degree of the expansion valve to zero, and keeping the expansion valve operating at the adjusted opening degree for a time T-t; then the process returns to the step of determining q.
If k_smin ≤ k ≤ k_smax, the process proceeds to step S504 of keeping the expansion valve operating at the current opening degree k for the system oil return time T, and then the process returns to the step of determining q.
If k>k_smax, then the process proceeds to step S505 of adjusting the opening degree of the expansion valve to k_smax and keeping the expansion valve operating at the adjusted opening degree for the system oil return time T, and then the process returns to the step of determining q.
Further, referring to FIG. 3, in order to enable the oil return control method to run periodically, before returning to the step of determining q, the oil return control method further includes:
Step S600: keeping the expansion valve operating at the adjusted opening degree for an oil return interval time Ti.
In detail, the present disclosure will provide two methods of judging whether the expansion valve has operated at the adjusted opening degree for the oil return interval time T_i, and the two methods will be described in detail below with reference to FIGS. 5 and 6. It should be noted that in order to improve readability, the following two methods are distinguished by different step labels. Specifically, steps of the first method are denoted by a label S600, and steps of the second method are denoted by a label S600'.
Referring to FIG. 5, step S600 of the first method specifically includes:
Step S601: determining a total oil storage quantity Q of the multi-connection system according to formula (5): $Q = \sum_{i = 1}^{n} q_{i}$
where n represents the number of the indoor units, and qi represents the oil storage quantity of the i^th indoor unit.
Step S602: judging whether the total oil storage quantity Q is larger than a maximum total oil storage quantity threshold Q_smax; if yes, the process proceeds to step S603; otherwise, the process returns to step S601.
Step S603: meaning that the expansion valve has operated at the adjusted opening degree for the oil return interval time Ti.
Referring to FIG. 6, step S600' of the second method specifically includes:

Step S601': acquiring a shutdown time t_sp of the indoor unit;
Step S602': judging whether the shutdown time t_sp is larger than a maximum shutdown time threshold t_spmax; if yes, the process proceeds to step S603'; otherwise, the process returns to step S601'.
Step S603': meaning that the expansion valve has operated at the adjusted opening degree for the oil return interval time Ti.

It can be understood that the method for judging whether the expansion valve has operated at the adjusted opening degree for the oil return interval time T_i is not limited to the above two methods, and those skilled in the art can also use other conventional means to judge.
In addition, the present disclosure also provides a multi-connection air conditioning system including a controller, and the controller is configured to be capable of executing the above refrigerant flow control method. It should be noted that the basic functional components constituting the multi-connection air conditioning system and the working principle are basically the same as those in the prior art, and can be realized by those skilled in the art completely based on the prior art, so a repeated description thereof will not be given herein.
Hitherto, the technical solutions of the present disclosure have been described in connection with the preferred embodiments shown in the accompanying drawings, but it is easily understood by those skilled in the art that the scope of protection of the present disclosure is obviously not limited to these specific embodiments. Without departing from the principles of the present disclosure, those skilled in the art can make equivalent changes or replacements to relevant technical features, and all the technical solutions after these changes or replacements will fall within the scope of protection of the present disclosure.

Claims

An oil return control method for a multi-connection system, the multi-connection system comprising a plurality of indoor units, at least one of which is not turned on, wherein the oil return control method comprises:
determining an oil storage quantity q of all the indoor units;

determining a system oil return time T of the multi-connection system;

determining a current opening degree k of an expansion valve of one indoor unit that is not turned on according to the oil storage quantity q and the system oil return time T;

comparing the current opening degree k with a maximum opening degree threshold k_smax and a minimum opening degree threshold k_smin; and

selectively adjusting the opening degree of the expansion valve of the one indoor unit that is not turned on according to a comparison result, and keeping the expansion valve operating at the adjusted opening degree for the system oil return time T.
The oil return control method according to claim 1, wherein the step of "determining a current opening degree k of an expansion valve of the one indoor unit that is not turned on according to the oil storage quantity q and the system oil return time T" specifically comprises calculating the current opening degree k using the following formula: $k = q (k_{smax} \times T_{smax}) / (q_{smax} \times T);$
where q_smax represents a maximum oil storage quantity threshold of the indoor unit, k_smax represents the maximum opening degree threshold of the expansion valve, and T_smax represents a maximum system oil return time threshold of the multi-connection system.
The oil return control method according to claim 2, wherein the step of "selectively adjusting the opening degree of the expansion valve of the one indoor unit that is not turned on according to a comparison result, and keeping the expansion valve operating at the adjusted opening degree for the system oil return time T" specifically comprises:
determining an indoor unit oil return time t of the one indoor unit that is not turned on according to the following formula if k<k_smin: $t = (k \times T) / k_{smin};$

adjusting the opening degree of the expansion valve of the one indoor unit that is not turned on to k_smin, and keeping the expansion valve operating at the adjusted opening degree for the time t; and

adjusting the opening degree of the expansion valve to zero, keeping the expansion valve operating at the adjusted opening degree for a time T-t, and then returning to the step of determining q.
The oil return control method according to claim 2, wherein the step of "selectively adjusting the opening degree of the expansion valve of the one indoor unit that is not turned on according to a comparison result, and keeping the expansion valve operating at the adjusted opening degree for the system oil return time T" specifically comprises:
keeping the expansion valve operating at the current opening degree k for the system oil return time T if k_smin≤k≤k_smax, and then returning to the step of determining q.
The oil return control method according to claim 2, wherein the step of "selectively adjusting the opening degree of the expansion valve of the one indoor unit that is not turned on according to a comparison result, and keeping the expansion valve operating at the adjusted opening degree for the system oil return time T" specifically comprises:
adjusting the opening degree of the expansion valve to k_smax if k>k_smax, keeping the expansion valve operating at the adjusted opening degree for the system oil return time T, and then returning to the step of determining q.
The oil return control method according to any one of claims 3 to 5, wherein before returning to the step of determining q, the expansion valve is kept operating at the adjusted opening degree for an oil return interval time Ti.
The oil return control method according to claim 6, wherein the oil return interval time T_i is determined through the following steps:
determining a total oil storage quantity Q of the multi-connection system according to the following formula: $Q = \sum_{i = 1}^{n} q_{i};$

judging whether the total oil storage quantity Q is larger than a maximum total oil storage quantity threshold Q_smax;

if yes, it means that the expansion valve has operated at the adjusted opening degree for the oil return interval time T_i;

where n represents the number of the indoor units, and qi represents the oil storage quantity of the i^th indoor unit.
The oil return control method according to claim 6, wherein the oil return interval time T_i is determined through the following steps:
acquiring a shutdown time t_sp of the one indoor unit that is not turned on; and

judging whether the shutdown time t_sp is larger than or equal to a maximum shutdown time threshold t_spmax;

if yes, it means that the expansion valve has operated at the adjusted opening degree for the oil return interval time Ti.
The oil return control method according to any one of claims 1 to 8, wherein the oil storage quantity q of the indoor unit is determined according to the following formula in the oil return control method: $q = P \times t_{sp} \times (P_{d} - P_{s}) \times R_{o};$
where P represents a capacity horsepower of the indoor unit, t_sp represents the shutdown time of the indoor unit, P_d represents a system high pressure of the multi-connection system, P_s represents a system low pressure of the multi-connection system, and R_o represents an oil storage coefficient of the indoor unit.
The oil return control method according to claim 9, wherein the step of "determining a system oil return time T of the multi-connection system" comprises:
acquiring a maximum oil storage quantity q_max of the indoor unit;

judging whether the maximum oil storage quantity q_max is larger than the maximum oil storage quantity threshold q_smax;

if yes, determining the system oil return time T as the maximum oil return time threshold T_smax; otherwise, determining the system oil return time T according to the following formula: $T = q_{\max} / q_{smax} \times T_{smax} .$