Low-temperature freezing oil return control method for ultralow-temperature variable-frequency heat pump water heater

Abstract

The invention relates to the technical field of air conditioners, in particular to an ultralow-temperature variable-frequency heat pump water heater low-temperature freezing oil return control method which comprises the steps that a plurality of temperature sensors are used for obtaining temperature parameters, and the temperature parameters comprise the condensation temperature, the exhaust temperature, the water tank upper temperature, the water tank lower temperature and the environment temperature; the obtained temperature parameters are analyzed, a temperature change curve of the temperature parameters when the heat pump works is determined, and a temperature change coefficient is obtained; obtaining a working frequency curve of the compressor corresponding to the temperature change curve, and calculating the working frequency curve according to a corresponding time point to obtain a frequency stability coefficient of the working frequency curve; according to the obtained temperature change coefficient and frequency stability coefficient, a control strategy when the heat pump works is selected, and the specific situation of implementation of the control strategy is verified; the temperature control precision, the response speed and the energy efficiency of the system are improved, and the service life of equipment is prolonged.

Description

Technical Field

[0001] The invention relates to the technical field of air conditioning, and in particular to a low-temperature refrigeration oil return control method for an ultra-low-temperature variable-frequency heat pump water heater. Background Art

[0002] At present, low-temperature refrigeration units are used in low-temperature cold storage. The storage temperature is generally -20℃ or lower, and the evaporation temperature reaches below -30℃. As the evaporation temperature decreases, the viscosity of the oil increases, and the reliability of oil return is the key concern of the low-temperature unit. In order to avoid the difficulty of oil return on the evaporation side of the system, an oil separator is generally added to the high-pressure side during the system design process to ensure that the oil discharged from the compressor can return to the compressor smoothly, effectively preventing the compressor oil from being discharged in large quantities to the low-pressure side of the system. Although this method can ensure the reliability of oil return of the unit under normal working conditions. However, when the outdoor temperature is very low, it is easy for the refrigerant to migrate into the compressor to dilute the refrigeration oil, which will cause the compressor to be in a poor lubrication state for a long time, resulting in a shortened compressor life or even damage.

[0003] For example, Chinese patent publication number CN115930497A discloses a refrigeration unit control method, control system, air conditioner and control device, which relates to the field of air conditioner technology and solves the technical problem that the direct oil separator in the prior art relies on pressure difference to return oil. When the unit is running at high pressure difference, the lubricating oil in the oil separator quickly returns to the compressor, which is easy to cause oil return gas. The control method includes the following contents: detecting the oil temperature T oil return in the oil return pipe; judging the oil temperature T oil return in the oil return pipe; if the oil temperature T oil return in the oil return pipe is judged to be high temperature, then the oil return solenoid valve is controlled to be closed; if the oil temperature T oil return in the oil return pipe is judged to be low temperature, then the oil return solenoid valve is controlled to be opened.

[0004] However, when the existing technology focuses on the oil pipe temperature and ambient temperature, it is easy to ignore the conditions of other equipment, such as changes in the internal conditions of the compressor. At this time, it is necessary to determine the refrigerant in the compressor when the heat pump water heater stops running to prevent the refrigerant from mixing with the refrigeration oil, so as to improve the overall stability of the heat pump system and adjust the corresponding working strategy to complete the overall control. Summary of the invention

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is: a low-temperature refrigeration oil return control method for an ultra-low temperature variable frequency heat pump water heater, comprising: using multiple temperature sensors to obtain temperature parameters, the temperature parameters including condensing temperature, exhaust temperature, upper water tank temperature, lower water tank temperature and ambient temperature.

[0006] The acquired temperature parameters are analyzed to determine the temperature variation curve of the temperature parameters when the heat pump is working, and the temperature variation coefficient is obtained.

[0007] The operating frequency curve of the compressor corresponding to the temperature change curve is obtained, and the frequency stability coefficient of the operating frequency curve is calculated according to the corresponding time point.

[0008] According to the obtained temperature variation coefficient and frequency stability coefficient, the control strategy for the heat pump operation is selected and the specific implementation of the control strategy is verified.

[0009] According to the specific circumstances of the implementation of the control strategy, adjust the implemented control strategy and determine the adjustment plan of the control strategy.

[0010] The beneficial effects of the present invention are as follows: 1. The present invention can more accurately predict and control the behavior of the refrigerant by using multiple temperature sensors to obtain temperature parameters at key locations and analyzing the temperature change curve in combination with these parameters.

[0011] 2. By calculating the temperature variation coefficient and the frequency stability coefficient, the present invention can more scientifically select and adjust the control strategy, so that the system can maintain optimal temperature control under different operating conditions; the condensing temperature control strategy, the exhaust superheat control strategy and the ambient temperature control strategy can be adaptively adjusted according to the actual operating conditions to improve the temperature control accuracy of the system.

[0012] 3. Through real-time monitoring and rapid response, the system can reach the set temperature in a short time, thereby improving the comfort and satisfaction of users.

[0013] 4. The present invention selects the most appropriate control strategy by comprehensively considering the temperature variation coefficient, frequency stability coefficient, system response time and energy efficiency ratio, thereby improving the overall energy efficiency of the system; under different ambient temperatures, by adjusting the duration of the condensing temperature control strategy and the exhaust superheat control strategy, it ensures that the system can operate efficiently under various conditions and reduce energy consumption. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0015] Figure 1 It is a flow chart of a low-temperature refrigeration oil return control method for an ultra-low-temperature variable-frequency heat pump water heater.

[0016] Figure 2 It is a flow chart of step S2 of a low-temperature refrigeration oil return control method for an ultra-low-temperature variable-frequency heat pump water heater.

[0017] Figure 3 It is a logic diagram of the control strategy of the low-temperature refrigeration oil return control method of the ultra-low-temperature variable-frequency heat pump water heater. DETAILED DESCRIPTION

[0018] The embodiments of the present invention are described in detail below. The embodiments described below are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention. If no specific techniques or conditions are specified in the embodiments, the techniques or conditions described in the literature in the art or the product specifications are used.

[0019] See also Figure 1 The invention discloses a low-temperature refrigeration oil return control method for an ultra-low-temperature variable-frequency heat pump water heater, comprising: S1, using a plurality of temperature sensors to obtain temperature parameters, the temperature parameters including condensing temperature, exhaust temperature, upper water tank temperature, lower water tank temperature and ambient temperature.

[0020] S2, analyzing the acquired temperature parameters, determining the temperature variation curve of the temperature parameters when the heat pump is working, and obtaining the temperature variation coefficient.

[0021] S3, obtaining the operating frequency curve of the compressor corresponding to the temperature change curve, and calculating the frequency stability coefficient of the operating frequency curve according to the corresponding time point.

[0022] S4, according to the obtained temperature variation coefficient and frequency stability coefficient, select the control strategy when the heat pump is working, and verify the specific situation of the implementation of the control strategy.

[0023] S5, according to the specific situation of the implementation of the control strategy, adjust the implemented control strategy and determine the adjustment plan of the control strategy.

[0024] In this invention, the main purpose is to verify how the heat pump corresponding system should work in a low temperature environment to prevent the oil return. At this time, it is mainly necessary to collect multiple temperature parameters, such as selecting the ambient temperature to determine the target condensing temperature required at this time. The target condensing temperature refers to the temperature that the condenser (i.e., the heat exchanger in the water heater) is to reach. This temperature is set according to the design of the system and the current operating conditions, with the purpose of ensuring that the heat pump can effectively transfer heat from the refrigerant to the water in the water tank.

[0025] The condensing temperature and exhaust temperature are used to determine the exhaust superheat to limit the heat emitted by the corresponding data during exhaust; the upper temperature of the water tank is also used to prevent local high temperature conditions, to determine the corresponding heating conditions of the heat pump during operation, and to prevent corresponding abnormalities.

[0026] That is to say, the temperature change curve obtained at this time actually identifies each temperature parameter separately, with the goal of identifying whether there is an abnormal temperature value, and calculating the corresponding exhaust superheat and target condensation temperature to verify the temperature change coefficient at this time; the exhaust superheat is the difference between the exhaust temperature and the condensation temperature, and the target condensation temperature is a value selected by identifying the ambient temperature and the lower temperature of the water tank.

[0027] For the obtained temperature change curve, it is necessary to obtain the temperature change curves corresponding to the condensing temperature, exhaust temperature, upper water tank temperature, lower water tank temperature and ambient temperature contained in the temperature parameters, and verify whether this part of the obtained temperature change curve will change accordingly, for example, use the average temperature change amplitude and the average temperature change rate to quantify the temperature change coefficient corresponding to the temperature change curve at this time.

[0028] like Figure 2 As shown, the method for obtaining the temperature variation coefficient is as follows: S21, obtaining the average temperature variation amplitude and the average temperature variation rate of the corresponding data in the temperature variation curve to obtain the temperature variation coefficient.

[0029] The average temperature change amplitude is expressed as the average value corresponding to the difference between the maximum and minimum values ​​of the measured temperature parameter. During verification, multiple sets of data will be collected cyclically, and the changes in the maximum and minimum values ​​of the collected temperature parameters will be quantified in each case according to the corresponding working conditions of the equipment at this time. At the same time, the average temperature change rate represents the ratio of the change rate between the average temperature of the sampled data and the standard value after multiple samplings, to verify which temperature has a large change at this time.

[0030] The temperature variation coefficient is shown in the following manner.

[0031] TVC=TCA×ω1+TCR×ω2; wherein TVC represents the temperature variation coefficient, TCA represents the average temperature variation amplitude, TCR represents the average temperature variation rate, ω1 represents the weight of the average temperature variation amplitude, and ω2 represents the weight of the average temperature variation rate.

[0032] At this time, the corresponding temperature variation coefficient can be obtained. Since there are several values ​​that need to be combined among the five temperature values ​​identified at this time, for example, when controlling the heat pump system, it is necessary to obtain the exhaust superheat by taking the difference between the exhaust temperature and the condensing temperature, and select the adjustment value of the lower temperature of the water tank according to the ambient temperature to obtain the target condensing temperature, and implement the overall control strategy according to the target condensing temperature; and the corresponding upper temperature of the water tank can reflect the working condition of the water tank under certain circumstances; then the temperature variation coefficient obtained at this time needs to be divided into the temperature variation coefficients corresponding to the ambient temperature, condensing temperature, exhaust temperature, upper water tank temperature and lower water tank temperature respectively on the basis of the current acquisition.

[0033] S22, according to the obtained temperature variation coefficient, sequentially calculating a first correlation between the ambient temperature and the lower temperature of the water tank, a second correlation between the exhaust temperature and the condensation temperature, and a third correlation between the upper temperature of the water tank and other temperature parameters.

[0034] At this time, TVC1, TVC2, TVC3, TVC4, and TVC5 will respectively represent the temperature variation coefficients corresponding to the condensing temperature, exhaust temperature, upper water tank temperature, lower water tank temperature, and ambient temperature, and the values ​​corresponding to the first correlation, the second correlation, and the third correlation will be calculated one by one.

[0035] For example, the first correlation cor1 is expressed as: Among them, TVC4 represents the temperature variation coefficient of the lower temperature of the water tank, TVC5 represents the temperature variation coefficient of the ambient temperature, TVC′4 represents the standard value of the temperature variation coefficient of the lower temperature of the water tank, and TVC′5 represents the standard value of the temperature variation coefficient of the ambient temperature.

[0036] Then the calculation method for the second correlation is the same as that for the first correlation. At this time, the temperature variation coefficients of the exhaust temperature and the condensing temperature are used as input to calculate the value corresponding to the second correlation. If there are multiple corresponding values ​​when calculating the first correlation and the second correlation, the average value is taken as the corresponding value of the first correlation and the second correlation.

[0037] For the third correlation, it is necessary to calculate the value between the upper temperature of the water tank and other temperature parameters. At this time, the third correlation needs to calculate the correlation values ​​of the upper temperature of the water tank and the condensing temperature, exhaust temperature, lower temperature of the water tank and ambient temperature in sequence according to the calculation method of the first correlation, and the weighted average of the correlation values ​​is used as the third correlation.

[0038] S23, taking the weighted average of the first correlation, the second correlation, and the third correlation to obtain a comprehensive temperature coefficient. When the comprehensive temperature coefficient, the first correlation, the second correlation, and the third correlation are all greater than the preset standard value, the temperature change coefficients corresponding to the comprehensive temperature coefficient, the first correlation, the second correlation, and the third correlation are output.

[0039] After completing the calculation of the first correlation, the second correlation, and the third correlation, determine whether the value of the correlation at this time is within the normal range. Since when the heat pump system is controlled, the measured temperatures have a high correlation as expected, the multiple correlation values ​​calculated at this time should be greater than the set preset standard values, thereby reflecting the stability of the corresponding temperature at this time.

[0040] The frequency stability coefficient is mainly calculated by calculating the average absolute error of the working frequency at this time as the frequency stability coefficient set at this time; for example, the frequency stability coefficient is expressed as shown below.

[0041] Wherein, MAD is the frequency stability factor, f i Expressed as the operating frequency at the i-th time point, Represents the average value of the operating frequency, N represents the number of time points, i=1,2,,,N.

[0042] According to the output temperature variation coefficient and frequency stability coefficient, the control coefficient corresponding to the temperature variation coefficient and the frequency stability coefficient is calculated, and the control strategy is selected according to the control coefficient.

[0043] The control coefficient is expressed as a weighted average of the temperature variation coefficient and the frequency stability coefficient. After the control coefficient is obtained, the control strategy with the highest control coefficient similarity is used as the output control strategy.

[0044] When selecting a control strategy based on the temperature variation coefficient and the frequency stability coefficient, the temperature variation coefficient, the frequency stability coefficient, and the standard values ​​of the temperature variation coefficient and the frequency stability coefficient in the database are used as inputs, and the Pearson correlation coefficient is used to calculate the value of the Pearson correlation coefficient of the control coefficient at this time. According to the value of the Pearson correlation coefficient of the control coefficient, data with correlation coefficient values ​​greater than 0.7 are selected, and this part of the data is sorted according to the value of the preset control coefficient in the control strategy, and the control strategy with the smallest difference between the preset control coefficient and the current control coefficient is used as the control strategy output at this time; the control strategy obtained at this time is regarded as the control strategy with the highest similarity, and the preset control coefficient used at this time will set the value after normalizing the temperature variation coefficient and the frequency stability coefficient. The preset control coefficient is set to 0.6, and this preset control coefficient is compared with the normalized values ​​of the temperature variation coefficient and the frequency stability coefficient to determine whether there is a strategy with high similarity at this time.

[0045] At this time, the implementation method of the adjustment plan is mainly to verify the situation after the control strategy is implemented at this time, and verify the performance indicators at this time, determine the implementation effect of the control strategy, and based on the obtained implementation effect, determine the adjustment plan corresponding to the implementation effect.

[0046] The performance indicators are expressed as temperature variation coefficient, frequency stability coefficient, system response time and energy efficiency ratio to obtain the performance indicators of the control strategy. The performance indicators here are the indicator values ​​of the implementation effect of the control strategy.

[0047] Then the implementation method of the adjustment plan at this time is: obtain the temperature change coefficient, frequency stability coefficient, system response time and energy efficiency ratio after the implementation of the control strategy.

[0048] The performance index is calculated based on the temperature variation coefficient, frequency stability coefficient, system response time and energy efficiency ratio.

[0049] Based on the obtained performance indicators, the adjustment plan of the control strategy is determined.

[0050] At this time, the system response time is expressed as the time to reach the set temperature after the heat pump is started. This time also represents the time to reach the target condensing temperature when the control strategy is implemented.

[0051] The energy efficiency ratio represents the ratio of the output heat to the consumed electrical energy, and is used to quantify whether there is an abnormal increase in energy consumption when the control strategy is selected for implementation.

[0052] The performance index is expressed as: PI = α1 × λ TVC +α2×λ MAD +α3×λ SRT +α4×λ COP ; Among them, PI represents the performance index, λ TVC represents the temperature variation coefficient after the control strategy is implemented, λ MAD represents the frequency stability coefficient after the control strategy is implemented, λ SRT represents the system response time, λ COP represents the energy efficiency ratio, α1 represents the weight of the temperature variation coefficient after the implementation of the control strategy, α2 represents the weight of the frequency stability coefficient after the implementation of the control strategy, α3 represents the weight of the system response time, and α4 represents the weight of the energy efficiency ratio.

[0053] According to the obtained value of the performance index, an adjustment scheme corresponding to the performance index value is determined from preset adjustment schemes to complete the control of the current heat pump system.

[0054] like Figure 3 As shown, in one embodiment of the present invention, the control strategy includes a condensing temperature control strategy, an exhaust superheat control strategy, and an ambient temperature control strategy to achieve control of the heat pump.

[0055] At this time, a temperature sensor is set in the middle of the condenser to obtain the condensing temperature T1, a temperature sensor is set at the compressor exhaust port to obtain the exhaust temperature T2, a temperature sensor is set on the upper part of the water tank to obtain the upper temperature T3 of the water tank, a temperature sensor is set at the lower part of the water tank to obtain the lower temperature T4 of the water tank, and a temperature sensor is set in the external environment to obtain the ambient temperature T5.

[0056] The condensing temperature control strategy is shown below.

[0057] First, according to the ambient temperature T5 and the temperature of the lower part of the water tank T4, the target condensing temperature T6 is obtained; the first working time when the operating frequency of the compressor reaches the preset operating frequency and the second working time when the compressor works at the preset operating frequency are judged; when the first working time and the second working time both meet the preset time values, whether the condensing temperature T1 at this time is consistent with the target condensing temperature T6 is judged; if the condensing temperature T1 at this time is consistent with the target condensing temperature T6, the current operating frequency of the compressor is recorded; if they are inconsistent, the operating frequency of the compressor is adjusted until the corresponding values ​​of the condensing temperature T1 at this time and the target condensing temperature T6 are consistent.

[0058] The method of obtaining the target condensation temperature T6 based on the ambient temperature T5 and the lower temperature of the water tank T4 is as follows.

[0059] When the ambient temperature T5<-15°C, the target condensation temperature T6=T4+5; when the ambient temperature T5 is between (-15°C, -7°C), the target condensation temperature T6=T4+6; when the ambient temperature T5>-7°C, the target condensation temperature T6=T4+7.

[0060] For example, the first working time set at this time is 60s. At this time, the working frequency of the compressor is increased from 0hz to 30Hz at a speed of 0.5Hz / s, and it runs at the frequency point of 30hz for 30 seconds. This is the second working time of 30s. After 30 seconds, the values ​​corresponding to the condensing temperature T1 and the target condensing temperature T6 are identified to determine whether the condensing temperature T1 and the target condensing temperature T6 are consistent. If they are consistent, the current working frequency of the compressor is recorded. If they are inconsistent, the working frequency of the compressor is adjusted until the corresponding values ​​of the condensing temperature T1 and the target condensing temperature T6 are consistent; the frequency of adjusting the compressor working frequency is once every 10 seconds; for every 0.5℃ difference between the condensing temperature T1 and the target condensing temperature T6, the compressor working frequency is adjusted by 1hz, that is, when the difference between the condensing temperature T1 and the target condensing temperature T6 is 0.5℃, the compressor frequency is lowered by 1hz, and when the difference is -0.5℃, the compressor frequency is increased by 1hz, so as to achieve the purpose of stability; so that the condensing temperature and the target condensing temperature can be consistent at the corresponding working frequency of the compressor.

[0061] The exhaust superheat control strategy is shown below.

[0062] The exhaust gas superheat TD is obtained, where the exhaust gas superheat TD is the difference between the exhaust gas temperature T2 and the condensation temperature T1.

[0063] The exhaust superheat TD is controlled at 7-12℃. If the exhaust superheat TD is less than 7℃, the electronic expansion valve opening will be increased by 1-5 steps for every 1℃ decrease. If the exhaust superheat TD is greater than 12℃, the electronic expansion valve opening will be reduced by 1-5 steps for every 1℃ increase. Adjust every 10 seconds.

[0064] The ambient temperature control strategy is shown below.

[0065] When the compressor is shut down for 5 hours; when the ambient temperature T5 is below -15°C, the duration of the condensing temperature control strategy and the exhaust superheat control strategy is 10 minutes; when the ambient temperature T5 is between -15 and -7°C, the duration of the condensing temperature control strategy and the exhaust superheat control strategy is 8 minutes; when the ambient temperature T5 is greater than -7°C, the duration of the condensing temperature control strategy and the exhaust superheat control strategy is 6 minutes; the purpose of the ambient temperature control strategy used at this time is to adjust the condensing temperature and exhaust when the compressor is started after a long period of shutdown, so as to prevent oil return at this time.

[0066] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations on the present invention. A person skilled in the art may make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention and they are still covered by the protection scope of the present invention.

Claims

1. A method for controlling low-temperature refrigeration oil return of an ultra-low-temperature variable-frequency heat pump water heater, characterized in that: include: Using multiple temperature sensors to obtain temperature parameters, the temperature parameters include condensing temperature, exhaust temperature, upper water tank temperature, lower water tank temperature and ambient temperature; Analyze the acquired temperature parameters, determine the temperature variation curve of the temperature parameters when the heat pump is working, and obtain the temperature variation coefficient; Obtain the operating frequency curve of the compressor corresponding to the temperature change curve, and calculate the frequency stability coefficient of the operating frequency curve according to the corresponding time point; According to the obtained temperature variation coefficient and frequency stability coefficient, select the control strategy when the heat pump is working, and verify the specific implementation of the control strategy; According to the specific circumstances of the implementation of the control strategy, adjust the implemented control strategy and determine the adjustment plan of the control strategy.

2. The low-temperature refrigeration oil return control method of the ultra-low-temperature variable-frequency heat pump water heater according to claim 1 is characterized in that: The obtained temperature variation curve includes the condensing temperature, the exhaust temperature, the upper temperature of the water tank, the lower temperature of the water tank and the ambient temperature.

3. The low-temperature refrigeration oil return control method of the ultra-low-temperature variable-frequency heat pump water heater according to claim 1 is characterized in that: The temperature variation coefficient is obtained as follows: Obtain the average temperature change amplitude and average temperature change rate of the corresponding data in the temperature change curve to obtain the temperature change coefficient; According to the obtained temperature variation coefficient, the first correlation between the ambient temperature and the lower temperature of the water tank, the second correlation between the exhaust temperature and the condensation temperature, and the third correlation between the upper temperature of the water tank and other temperature parameters are calculated in sequence; The weighted average of the first correlation, the second correlation, and the third correlation is taken to obtain a comprehensive temperature coefficient. When the comprehensive temperature coefficient, the first correlation, the second correlation, and the third correlation are all greater than the preset standard value, the temperature change coefficients corresponding to the comprehensive temperature coefficient, the first correlation, the second correlation, and the third correlation are output.

4. The low-temperature refrigeration oil return control method of the ultra-low-temperature variable-frequency heat pump water heater according to claim 3 is characterized in that: The temperature variation coefficient is shown in the following way: TVC=TCA×ω1+TCR×ω2; Among them, TVC represents the temperature variation coefficient, TCA represents the average temperature variation amplitude, TCR represents the average temperature change rate, ω1 represents the weight of the average temperature variation amplitude, and ω2 represents the weight of the average temperature change rate.

5. The low-temperature refrigeration oil return control method of the ultra-low-temperature variable-frequency heat pump water heater according to claim 1 is characterized in that: The frequency stability factor is expressed as follows: Wherein, MAD is the frequency stability factor, f i Expressed as the operating frequency at the i-th time point, Represents the average value of the operating frequency, N represents the number of time points, i=1,2,,,N.

6. The low-temperature refrigeration oil return control method of the ultra-low-temperature variable-frequency heat pump water heater according to claim 1 is characterized in that: According to the output temperature variation coefficient and frequency stability coefficient, the control coefficient corresponding to the temperature variation coefficient and the frequency stability coefficient is calculated, and the control strategy is selected according to the control coefficient; The control coefficient is expressed as the weighted average of the temperature variation coefficient and the frequency stability coefficient, and the control strategy with the highest control coefficient similarity is used as the output control strategy.

7. The low-temperature refrigeration oil return control method of the ultra-low-temperature variable-frequency heat pump water heater according to claim 1 is characterized in that: The adjustment scheme is implemented by: obtaining the temperature variation coefficient, frequency stability coefficient, system response time and energy efficiency ratio after the control strategy is implemented; Calculate the performance index based on the temperature variation coefficient, frequency stability coefficient, system response time and energy efficiency ratio; Based on the obtained performance indicators, the adjustment plan of the control strategy is determined.

8. The low-temperature refrigeration oil return control method of the ultra-low-temperature variable-frequency heat pump water heater according to claim 7, characterized in that: The performance index is expressed as: PI=α1×λ TVC +α2×λ MAD +α3×λ SRT +α4×λ COP ; Among them, PI represents the performance index, λ TVC represents the temperature variation coefficient after the control strategy is implemented, λ MAD represents the frequency stability coefficient after the control strategy is implemented, λ SRT represents the system response time, λ COP represents the energy efficiency ratio, α1 represents the weight of the temperature variation coefficient after the implementation of the control strategy, α2 represents the weight of the frequency stability coefficient after the implementation of the control strategy, α3 represents the weight of the system response time, and α4 represents the weight of the energy efficiency ratio.

9. The low-temperature refrigeration oil return control method of the ultra-low-temperature variable-frequency heat pump water heater according to claim 1, characterized in that: The control strategies include condensing temperature control strategy, exhaust superheat control strategy and ambient temperature control strategy.

10. The low-temperature refrigeration oil return control method of the ultra-low-temperature variable-frequency heat pump water heater according to claim 9, characterized in that: The condensing temperature control strategy is shown below: According to the ambient temperature and the temperature of the lower part of the water tank, the target condensation temperature is obtained; Determine the first working time when the working frequency of the compressor reaches the preset working frequency and the second working time when the compressor works at the preset working frequency, and when both the first working time and the second working time meet the preset time values, determine whether the condensing temperature at this time is consistent with the target condensing temperature, and if the condensing temperature at this time is consistent with the target condensing temperature, record the current working frequency of the compressor, and if they are inconsistent, adjust the working frequency of the compressor until the condensing temperature at this time is consistent with the corresponding value of the target condensing temperature; The exhaust superheat control strategy is as follows: Obtain exhaust superheat, which is the difference between exhaust temperature and condensation temperature; The exhaust superheat TD is controlled at 7-12℃. If the exhaust superheat is less than 7℃, the electronic expansion valve opening is increased by 1-5 steps for every 1℃ decrease. If the exhaust superheat is greater than 12℃, the electronic expansion valve opening is reduced by 1-5 steps for every 1℃ increase. Adjust once every 10 seconds. The ambient temperature control strategy is as follows: When the compressor shutdown duration is 5 hours; when the ambient temperature is below -15℃, the duration of the condensing temperature control strategy and the exhaust superheat control strategy is 10 minutes; when the ambient temperature is between -15 and -7℃, the duration of the condensing temperature control strategy and the exhaust superheat control strategy is 8 minutes; when the ambient temperature is greater than -7℃, the duration of the condensing temperature control strategy and the exhaust superheat control strategy is 6 minutes.

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