Power frequency and variable frequency undisturbed mutual switching control method

Abstract

The invention discloses a power frequency and variable frequency undisturbed mutual switching control method. The method comprises the steps that a console initiates a frequency switching command; if the current working condition meets the switching condition, executing a frequency switching command; wherein the switching process of frequency-power switching comprises the following steps of: controlling the frequency converter to gradually rise to the power frequency; finely adjusting the operation frequency of the frequency converter to the power grid frequency according to the phase locking angle of the power grid; adjusting the operation phase of the frequency converter to enable the difference angle between the operation phase and the power grid phase to be within a preset range; after the motor is connected to the power frequency system, the frequency converter is stopped in a delayed mode, and electric connection among the frequency converter, the motor and the power grid is sequentially disconnected; the switching process of the power frequency switching comprises the following steps: recovering the electric connection between the frequency converter and the power grid, starting up the frequency converter, and delaying the recovery of the electric connection between the frequency converter and the motor; and after the frequency converter automatically captures and outputs the phase locking angle of the power grid and stably outputs the voltage the same as that of the power grid, the electric connection between the motor and the power frequency system is disconnected.

Description

Technical Field

[0001] The present invention relates to the technical field of motor frequency conversion control, and particularly to a method for seamless switching between power frequency and frequency conversion control. Background Art

[0002] With the development of human society and the progress of science and technology, in the fields of industrial production and photovoltaic, the variable frequency speed regulation technology is known for its high efficiency and energy saving, and has been widely used in motor drive occasions. In particular, inverters are commonly used for on-site transformation of motor systems such as fans and pumps. In some production occasions with high requirements for load power supply, such as VOC waste gas treatment in the photovoltaic industry, the motor should avoid inrush current during startup. If the machine stops after being put into on-site production, it will cause great losses. Therefore, in this working condition, the seamless switching between power frequency and frequency conversion of the fan variable frequency technology is very suitable.

[0003] Under general conditions, the mechanical part of the fan body and the failure rate of the main fan motor are not high. Through comprehensive analysis of past fan failure data, the inverter control part is the key to affecting the stable operation of the system. The inverter belongs to a part of the power electronic devices and is deeply affected by the characteristics of the power electronic devices. It is inevitable that the failure rate is high in actual operation.

[0004] In the current application of motor technology, some working condition control methods are relatively backward, and manual experience is required for operation, resulting in unstable production and poor economic and technical indicators. Therefore, a set of mature automatic control system needs to be applied to the system to monitor the system in real time and control the system stability in real time, while reducing the blindness based on experience in the past. On the other hand, the failure of the inverter itself or the tripping of the fan induced by external factors will affect the safe and stable operation of the system, and even cause safety accidents. At the same time, after the fan trips, it will trigger the fast load shedding logic, and the wind smoke and combustion systems will be affected, and the air volume and air pressure will fluctuate greatly, thus affecting the operation efficiency of the entire system. Therefore, how to improve the operation stability of the inverter is an urgent problem to be solved at present. Summary of the Invention

[0005] In view of this, an embodiment of the present invention provides a method for seamless switching between power frequency and frequency conversion control to solve the problem of insufficient stability of the power frequency and frequency conversion switching control of the motor in the prior art.

[0006] An embodiment of the present invention provides a method for seamless switching between power frequency and frequency conversion control, including:

[0007] The console issues a frequency switching command;

[0008] If the current working condition meets the switching conditions, the frequency switching command is executed;

[0009] Among them, when the frequency switching command is frequency-industrial frequency switching, the switching process includes:

[0010] Controlling the frequency converter to gradually rise to the industrial frequency;

[0011] Fine-tuning the operating frequency of the frequency converter to the grid frequency according to the phase-locked angle of the power grid;

[0012] Adjusting the operating phase of the frequency converter so that the phase difference angle from the grid phase is within the preset range;

[0013] After the motor is connected to the industrial frequency system, making the frequency converter delay shutdown and disconnecting the electrical connections between the frequency converter and the motor and the power grid in sequence;

[0014] When the frequency switching command is industrial frequency switching, the switching process includes:

[0015] Restoring the electrical connection between the frequency converter and the power grid, turning on the frequency converter, and delaying the restoration of the electrical connection between the frequency converter and the motor;

[0016] After the frequency converter automatically captures and outputs the phase-locked angle of the power grid and stably outputs the same voltage as the power grid, disconnecting the electrical connection between the motor and the industrial frequency system.

[0017] Optionally, the input end of the frequency converter is electrically connected to the power grid through a first switch; the output end of the frequency converter is electrically connected to the input end of the motor through a second switch; the input end of the motor is electrically connected to the power grid through a third switch; wherein, the first switch, the second switch, and the third switch are controlled by the console to switch the on / off state.

[0018] Optionally, when the frequency switching command is frequency-industrial frequency switching, the switching conditions include:

[0019] The third switch is in the off state;

[0020] The frequency converter is operating in variable frequency;

[0021] The frequency converter has no faults;

[0022] The industrial frequency switching command is prohibited;

[0023] The shutdown mode of the frequency converter is set to free shutdown;

[0024] The isolation contactor of the frequency converter is switched to the console control mode.

[0025] Optionally, when the frequency switching command is industrial frequency switching, the switching conditions include:

[0026] The first switch and the second switch are in the off state;

[0027] The frequency converter is in the stop state;

[0028] The frequency converter has no faults;

[0029] The industrial frequency switching command is prohibited;

[0030] The shutdown mode of the frequency converter is set to free shutdown;

[0031] The isolation contactor of the frequency converter is switched to the console control mode.

[0032] Optionally, during the switching process of industrial frequency switching, adjust the phase difference angle between the operating phase of the frequency converter and the grid phase within 2°.

[0033] Optionally, when the frequency switching command is industrial frequency switching, the switching process further includes:

[0034] After the frequency converter locks the phase successfully, close the third switch after a delay of 2 to 10 seconds;

[0035] After closing the third switch, stop the frequency converter after a delay of 200 ms, and then open the first switch and the second switch after a delay of 200 ms.

[0036] Optionally, when the frequency switching command is industrial frequency switching, the switching process further includes:

[0037] After closing the first switch, start the frequency converter after a delay of 15 to 25 seconds;

[0038] After the frequency converter accelerates and increases the frequency to the industrial frequency, the input and output lines of the frequency converter start to lock the phase after a delay of 5 to 6 minutes;

[0039] After the phase locking is successful, control the second switch to close after a delay of 10 seconds;

[0040] After the second switch is closed, open the third switch after a delay of 200 ms.

[0041] Optionally, it further includes:

[0042] Conduct real-time fault detection on the frequency converter;

[0043] When the console receives the fault signal of the frequency converter, open the first switch and the second switch, and at the same time close the regulating baffle at the inlet of the fan; the console determines the delayed closing time of the third switch according to the load and the system air pressure, and enters the independent operation state of the industrial frequency.

[0044] Optionally, it further includes:

[0045] Collect the air pressure information in the system air duct through the air pressure sensor;

[0046] The console adjusts the opening degree of the regulating baffle at the inlet of the fan in real time according to the air pressure information.

[0047] The beneficial effects of the present invention:

[0048] 1. The embodiment of the present invention provides a method for seamless mutual switching control between power frequency and variable frequency, which reduces the number of standby inverters, avoids repeated shutdowns in case of failures, does not affect the load operation, and reduces economic losses.

[0049] 2. When performing the frequency conversion / power frequency switching of the inverter, the impact current on the grid side is minimized, avoiding the tripping of the incoming switch of the electrical cabinet and the switch on the transformer side, not affecting the power grid, motor and other electrical equipment, significantly improving the energy-saving effect and operation stability of the motor, reducing the unplanned shutdown of equipment, and having very important significance in terms of cost reduction and efficiency improvement.

[0050] 3. The method for seamless mutual switching control between power frequency and variable frequency provided by the embodiment of the present invention enables the motor speed to be as stable as possible before and after the frequency conversion / power frequency switching of the inverter, without causing great disturbance to the production process.

[0051] 4. It avoids the risk that the full voltage during power frequency startup may exceed the designed pressure bearing of the air duct, tearing the air duct and expansion joint.

[0052] 5. It avoids the large fluctuations in air volume, air pressure and negative pressure from affecting the stability of system smoke exhaust and combustion, and damaging the material circulation in the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] The features and advantages of the present invention will be more clearly understood by referring to the accompanying drawings. The drawings are schematic and should not be construed as any limitation to the present invention. In the drawings:

[0054] Figure 1 The schematic diagram of a seamless mutual switching control system between power frequency and variable frequency applied to a fan motor in the embodiment of the present invention is shown;

[0055] Figure 2 The circuit diagram of a fan motor operating at variable frequency in the embodiment of the present invention is shown;

[0056] Figure 3 The frequency-power switching flow chart in a method for seamless mutual switching control between power frequency and variable frequency in the embodiment of the present invention is shown;

[0057] Figure 4 The circuit diagram of a fan motor operating at power frequency in the embodiment of the present invention is shown;

[0058] Figure 5 The power frequency switching flow chart in a method for seamless mutual switching control between power frequency and variable frequency in the embodiment of the present invention is shown;

[0059] Figure 6 The schematic diagram of a phase-locked algorithm in the embodiment of the present invention is shown. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0060] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Apparently, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0061] An embodiment of the present invention provides a method for seamless switching control between power frequency and variable frequency, including:

[0062] Step S1, the console issues a frequency switching command.

[0063] In this embodiment, the console is a PLC controller.

[0064] Step S2, if the current working condition meets the switching conditions, execute the frequency switching command.

[0065] In this embodiment, when the frequency switching command is for power frequency to variable frequency switching, the switching conditions include: the third switch is in the off state, the frequency converter is in variable frequency operation, the frequency converter has no faults, the power frequency switching command is prohibited, the stop mode of the frequency converter is set to free stop, and the frequency converter isolation contactor is switched to the console control mode.

[0066] When the frequency switching command is for variable frequency to power frequency switching, the switching conditions include: the first switch and the second switch are in the off state, the frequency converter is in the stopped state, the frequency converter has no faults, the variable frequency to power frequency switching command is prohibited, the stop mode of the frequency converter is set to free stop, and the frequency converter isolation contactor is switched to the console control mode.

[0067] Hardware devices such as Figure 1 As shown, according to the actual working condition environment and functional requirements on site, industrial computers / DCSs and PLCs are mainly used for core control. The function of the upper computer / DCS system is to monitor the system and is strictly responsible for receiving production plan commands and dispatching commands, complete the production operation monitoring and production dispatching of the entire system, and at the same time statistically analyze historical curves and real-time data, leave corresponding tabular materials for later inspection, and form data communication with the PLC through optical fibers and DTUs to transmit control commands.

[0068] Taking the Siemens PLC1200 series as an example, the PLC is used to complete the monitoring of production working conditions and the execution of algorithms and logic control. In the automatic control mode, according to the set process parameters, the feed rate and air pressure of the system are automatically adjusted and monitored through algorithms. In the semi-automatic mode, the motor speed can be adjusted by professionals through the buttons on the electrical cabinet panel and the upper computer / DCS operation interface according to the production needs of the workshop, so that the feed rate and air pressure meet the process requirements of the system.

[0069] In the waste gas treatment industry, the on-site environmental conditions are generally relatively harsh and there is a large amount of dust. Therefore, the frequency converter is installed in a frequency conversion cabinet where the ventilation and airtightness meet the protection level requirements to prevent conductive dust from falling into the frequency conversion cabinet and burning it, causing losses to the entire production.

[0070] As Figure 2 shown, the frequency converter uses the ABB510 series frequency converter. The input end of the frequency converter is electrically connected to the power grid through the first switch KM1; the output end of the frequency converter is electrically connected to the input end of the motor through the second switch KM2; the input end of the motor is electrically connected to the power grid through the third switch KM3; among them, the first switch, the second switch and the third switch are controlled by the console to switch the on / off state.

[0071] After the PLC controls KM3 to automatically disconnect, then close KM1 and KM2 in sequence, and at the same time the PLC inputs a start command to make the frequency converter perform frequency modulation and voltage regulation operation. The system is currently in frequency conversion operation. At this time, the frequency switching command for frequency conversion to industrial frequency switching can be executed, and the switching process is as Figure 3 shown:

[0072] Control the frequency converter to gradually rise to the industrial frequency.

[0073] Fine-tune the operating frequency of the frequency converter to the power grid frequency according to the phase-locked angle of the power grid;

[0074] Adjust the operating phase of the frequency converter so that the phase difference angle from the power grid phase is within the preset range;

[0075] After the motor is connected to the industrial frequency system, make the frequency converter delay shutdown, and disconnect the electrical connections between the frequency converter and the motor and the power grid in sequence.

[0076] The frequency converter first maintains the existing frequency, gradually rises to the industrial frequency of 50Hz, and monitors the power grid voltage in real time through the frequency converter. The frequency converter automatically fine-tunes the operating frequency to the power grid frequency according to the phase-locked angle of the power grid. The frequency converter automatically adjusts the phase so that the operating phase difference from the power grid phase is within 2°. The frequency converter maintains the current working state and feeds back the angle adjustment completion signal to the PLC. The PLC commands to close KM3 after a 10-second delay. The frequency converter delays for 200ms to shutdown, and then delays for 200ms to disconnect KM2 and KM1 in sequence. At this time, the seamless switching process from frequency conversion to industrial frequency is completed.

[0077] If the switching conditions are not met, the frequency converter has a serious fault and the running signal disappears, then the PLC automatically switches from frequency conversion to industrial frequency once.

[0078] If the frequency converter does not have a serious fault and the running signal is normal, then this switching is exited.

[0079] If the frequency converter fails to lock the phase successfully, then this switching is exited.

[0080] As Figure 4 shown, the system is currently operating at power frequency. At this time, a frequency switching command for power frequency switching can be executed, and the switching process is as Figure 5 shown:

[0081] When the frequency switching command is for power frequency switching, the switching process includes:

[0082] Restore the electrical connection between the frequency converter and the power grid, turn on the frequency converter, and delay restoring the electrical connection between the frequency converter and the motor;

[0083] After the frequency converter automatically captures and outputs the phase-locked angle to the power grid and stably outputs the same voltage as the power grid, disconnect the electrical connection between the motor and the power frequency system.

[0084] In this embodiment, the PLC first automatically closes KM1, closes KM2 after a period of time, turns on the frequency converter, the frequency converter automatically captures the phase-locked angle and outputs a stable and same voltage as the power grid, and then the PLC controls KM3 to disconnect, completing the seamless switching from power frequency to frequency conversion.

[0085] In a specific embodiment, after closing the first switch, the frequency converter is started after a delay of 20 seconds. After the frequency converter accelerates to the power frequency, the incoming and outgoing lines of the frequency converter start to lock the phase after a delay of 6 minutes. After successful phase locking, after a delay of 10 seconds, the second switch is controlled to close. After the second switch closes, the third switch is disconnected after a delay of 200 ms.

[0086] As an optional implementation manner, it further includes: performing real-time fault detection on the frequency converter; when the console receives a fault signal from the frequency converter, disconnect the first switch and the second switch, and at the same time close the regulating baffle at the inlet of the fan; the console determines the delay closing time of the third switch according to the load and the system air pressure, and enters the independent operation state of power frequency.

[0087] As an optional implementation manner, it further includes: collecting the air pressure information in the system air duct through an air pressure sensor; the console adjusts the opening degree of the regulating baffle at the inlet of the fan in real time according to the air pressure information.

[0088] The following explains the relevant principles of motor frequency conversion:

[0089] At present, constant-speed operation and speed regulation operation are two main operation modes of the motor. Constant-speed operation generally refers to the motor directly operating in the power frequency mode, and the adjustment of the flow rate requires constantly controlling the opening of the valve or changing the number of operating units of the system unit; speed regulation often uses a frequency converter for speed regulation. Theoretically speaking, frequency conversion speed regulation is a speed regulation method with the most significant energy-saving effect.

[0090] Variable frequency starting and speed regulation are the main ways for motors to operate efficiently. When starting with a frequency converter, the current is less than the rated current, the load requirement for the power grid is relatively small, and the impact generated by the mechanical load and the motor itself is also very small. In the photovoltaic waste gas treatment industry, most motors are three-phase asynchronous motors, and the speed regulation principle is as follows:

[0091] nr = (1 - s)·60f / P

[0092] In the above formula, nr represents the mechanical speed of the motor (r / min); f represents the power supply frequency (Hz); s represents the slip ratio; p represents the number of pole pairs. It can be seen from the above formula that when the slip ratio s and the number of pole pairs p remain unchanged, the size is proportional to the power supply frequency. Therefore, by smoothly adjusting the power supply frequency, stepless speed regulation of the motor can be achieved.

[0093] The expression for the air-gap magnetic flux of an asynchronous motor is:

[0094] φm = E1 / (4.44f*k s N s )

[0095] In the formula, E1 is the effective value of the electromotive force per phase of the magnetic flux in the stator. When the resistance voltage drop and leakage reactance voltage drop of the stator winding are ignored, it can be approximately equal to the stator phase voltage; k s and N s are the stator winding coefficient and the number of turns respectively.

[0096] In order to keep the magnetic flux constant, the constant voltage ratio (V / F) control method is often used to regulate the speed of the motor, that is, when evenly adjusting the frequency, the voltage amplitude output by the regulated power supply is taken in proportion.

[0097] The core of the seamless switching technology is that the frequency converter adjusts the amplitude, frequency and phase of the output voltage, and makes the adjustment to reach the switching requirements of the power frequency power grid before switching, so as to achieve the effect of no impact on the current during the switching process.

[0098] Accurately locking the phase of the power grid is the prerequisite for seamless switching. The three-phase AC system currently used can use a phase-locking algorithm such as Figure 6 . In the figure, U A , U B , U C represent the three-phase voltages, and their expressions can be represented as:

[0099] U A =√2 U S *cos(ωt)

[0100] U B =√2 U S *cos(ωt - 120°)

[0101] U C= √2 U S *cos(ωt + 120°)

[0102] In the above formula, U S represents the effective value of the grid voltage; ω represents the phase angular frequency. The collected grid voltage is transformed through coordinates and synchronized to the dq coordinate system. The PI controller is used to control the voltage component on the q-axis to 0. When the rotation speed in the dq coordinates is consistent with the frequency and phase of the grid voltage, it indicates that the switching between power frequency and variable frequency can be performed under the current state.

[0103] When driving the motor to run under load, tracking the grid phase, the control system needs to control the speed of phase tracking. The frequency converter control system obtains the output voltage angle of the frequency converter and the angle of the grid voltage through phase-locking. The two angles are adjusted through PI operation to obtain the step size that the frequency converter needs to output. This step size plus 50HZ is used as the output frequency of the frequency converter. If the grid voltage phase is ahead of the phase of the frequency converter, the frequency converter will drive the motor to do work; when the grid voltage phase lags behind the frequency converter, the frequency converter drives the motor to track the phase and absorbs power from the motor until the phase of the frequency converter is consistent with the phase of the grid.

[0104] During the switching process, if a serious fault occurs in the frequency converter and the switching from variable frequency to power frequency cannot be carried out smoothly, it will cause the motor to stop running.

[0105] To solve this problem, add a fault detection point for the frequency converter in the DCS system. When a frequency converter fault signal appears, directly send commands to disconnect the KM1 and KM2 contactors, and at the same time close the fan baffle to avoid overloading the air duct pressure and tearing the air duct during power frequency operation. Select an appropriate delay time to close KM3 for independent operation at power frequency, and maintain the system stability by controlling the baffle opening through the PLC.

[0106] When the equipment operates under overload, it may damage the equipment; when the full pressure exceeds the designed bearing pressure of the air duct, it will tear the air duct and expansion joint; large fluctuations in air volume, air pressure, and negative pressure will affect the stability of boiler combustion and damage the material circulation of the system. To solve this problem, it is necessary to control the opening of the regulating baffle at the fan inlet. First, the baffle must be able to respond in time by controlling the angle through the PLC after the switching is completed; second, confirm the linear relationship between the fan baffle and the air volume of the variable frequency.

[0107] The full pressure of the fan is proportional to the square of the rotational speed, and the rotational speed of the fan is proportional to the flow rate. In actual projects, the working frequency of the fan is determined by the on-site working conditions. When a fault occurs in the frequency converter, the operating frequency cannot be detected in time. Using the variable frequency instruction at the moment of switching to control the fan inlet baffle can well achieve the equality of the fan frequencies before and after automatic switching.

[0108] The wind pressure sensor with a 4-20mA DC current and a working voltage of 24V DC is used to detect the wind pressure in the system duct in real time, and then feedback it to the PLC. The PLC outputs a signal in real time according to the process requirements to control the opening of the fan baffle, ensuring the stability of the internal wind pressure of the system.

[0109] When the frequency converter fails, the system automatically switches to power frequency operation and automatically adjusts the opening of the fan baffle. At this time, the frequency converter is overhauled. In order to ensure that the system can handle emergencies when they occur, a disconnect switch QF1 is set at the front end of the frequency converter, and a manual emergency stop knob is set in the control circuit under both power frequency and variable frequency control states to ensure that the system operation can be manually disconnected.

[0110] Both excessive slip and the phase of the stator electromotive force will cause impact current. Since the coasting-down process of the fan is relatively long, generally more than one minute, past experience shows that the motor drops about 50 revolutions within 3S and will not cause impact current due to excessive slip. If the switching time is greater than the total electromagnetic transition time of 2s, the induced electromotive force can be ignored after 2s and will not cause a greater impact, avoiding the impact caused by the stator induced electromotive force being in phase with the power supply voltage.

[0111] In summary, the fan system applying the power frequency and variable frequency non-disturbing mutual switching control method provided by this embodiment realizes the following functions:

[0112] 1. Monitor the system working conditions and actuators in real time, and feedback the parameters of the motor operation conditions, working conditions, and the states of contactors and intermediate relays to the upper computer system for display in the form of dynamic graphics and curves.

[0113] 2. The motor driving the fan can achieve soft start and soft braking.

[0114] 3. Control the feeding amount and differential pressure. According to the process requirements, use the PID algorithm to adjust the speed of the motor to control the system differential pressure and achieve constant pressure control of the system pressure.

[0115] 4. Bidirectional non-disturbing switching between variable frequency and power frequency.

[0116] An embodiment of the present invention provides a method for seamless mutual switching control between power frequency and variable frequency, which reduces the number of standby inverters, avoids repeated shutdowns in case of failures, does not affect the load operation, and reduces economic losses. When performing the frequency conversion / power frequency switching of the inverter, the impact current on the grid side is minimized, the tripping of the incoming switch of the electrical cabinet and the switch on the transformer side is avoided, and the power grid, motor and other electrical equipment are not affected. The energy-saving effect and operation stability of the motor are significantly improved, and the unplanned shutdown of equipment is reduced, which is of great significance in cost reduction and efficiency improvement. The method for seamless mutual switching control between power frequency and variable frequency provided by the embodiment of the present invention enables the motor speed to be as stable as possible before and after the frequency conversion / power frequency switching of the inverter, and does not cause great disturbance to the production process. The risk that the full voltage during power frequency startup may exceed the designed pressure bearing of the air duct and tear the air duct and expansion joint is avoided. The large fluctuations in air volume, air pressure and negative pressure are avoided from affecting the stability of system smoke exhaust and combustion and destroying the material circulation in the system.

[0117] Although the embodiments of the present invention are described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the present invention, and such modifications and variations fall within the scope defined by the appended claims.

Claims

1. A power frequency and variable frequency seamless switching control method, characterized in that Including: The console issues a frequency switching command; If the current working condition meets the switching condition, execute the frequency switching command; Wherein, when the frequency switching command is frequency-to-power switching, the switching process includes: Control the frequency converter to gradually rise to the power frequency; Fine-tune the operating frequency of the frequency converter to the power grid frequency according to the phase-locked angle of the power grid; Adjust the operating phase of the frequency converter so that the phase difference angle from the power grid phase is within the preset range; After the motor is connected to the power frequency system, make the frequency converter delay shutdown, and disconnect the electrical connections between the frequency converter and the motor and the power grid in sequence; When the frequency switching command is power-to-frequency switching, the switching process includes: Restore the electrical connection between the frequency converter and the power grid, turn on the frequency converter, and delay to restore the electrical connection between the frequency converter and the motor; After the frequency converter automatically captures and outputs the phase-locked angle of the power grid and stably outputs the same voltage as the power grid, disconnect the electrical connection between the motor and the power frequency system.

2. The power frequency and variable frequency seamless switching control method according to claim 1, wherein The input end of the frequency converter is electrically connected to the power grid through a first switch; the output end of the frequency converter is electrically connected to the input end of the motor through a second switch; the input end of the motor is electrically connected to the power grid through a third switch; wherein, the first switch, the second switch and the third switch are controlled by the console to switch the on / off state.

3. The power frequency and variable frequency seamless switching control method according to claim 2, characterized in that, When the frequency switching command is frequency-to-power switching, the switching conditions include: The third switch is in the off state; The frequency converter is in variable frequency operation; The frequency converter has no faults; The power-to-frequency switching command is prohibited; The shutdown mode of the frequency converter is set to free shutdown; The isolation contactor of the frequency converter is switched to the console control mode.

4. The power frequency and frequency conversion non-interference mutual switching control method according to claim 2, characterized in that, When the frequency switching command is power-to-frequency switching, the switching conditions include: The first switch and the second switch are in the off state; The frequency converter is in the stop state; The frequency converter has no faults; The frequency-to-power switching command is prohibited; The shutdown mode of the frequency converter is set to free shutdown; The isolation contactor of the frequency converter is switched to the console control mode.

5. The power frequency and frequency conversion non-interference mutual switching control method according to claim 1, characterized in that, During the switching process of power-to-frequency switching, adjust the phase difference angle between the operating phase of the frequency converter and the power grid phase to be within 2°.

6. The power frequency and frequency conversion non-interference mutual switching control method according to claim 2, wherein, When the frequency switching command is frequency-to-power switching, the switching process further includes: After the frequency converter is successfully phase-locked, delay 2 to 10 seconds to close the third switch; After closing the third switch, delay 200ms to stop the frequency converter, and then delay 200ms to disconnect the first switch and the second switch.

7. The power frequency and variable frequency seamless switching control method according to claim 2, wherein When the frequency switching command is power-to-frequency switching, the switching process further includes: After closing the first switch, delay 15 to 25 seconds to start the frequency converter; After the frequency converter accelerates and rises to the power frequency, delay 5 to 6 minutes for the input and output lines of the frequency converter to start phase-locking; After successful phase-locking, delay 10 seconds to control the second switch to close; After the second switch is closed, delay 200ms to disconnect the third switch.

8. The power frequency and variable frequency seamless switching control method according to claim 2, wherein Also including: Conduct real-time fault detection on the frequency converter; When the console receives the fault signal of the frequency converter, it disconnects the first switch and the second switch, and at the same time closes the regulating baffle at the inlet of the fan; the console determines the delayed closing time of the third switch according to the load and the system air pressure, and enters the independent operation state of the power frequency.

9. The power frequency and variable frequency seamless switching control method according to claim 8, wherein It further includes: Collecting the air pressure information in the system air duct through the air pressure sensor; The console adjusts the opening degree of the regulating baffle at the inlet of the fan in real time according to the air pressure information.

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