Frequency converter and method for controlling the same

Abstract

The application discloses a frequency converter and a control method thereof. The control method comprises the following steps: controlling the frequency converter to output a signal with a first voltage and a first frequency, so that a brushless exciter outputs a first electromagnetic torque, and then starting the operation of an electrically excited synchronous motor and a load; after a preset time, controlling the frequency converter to output a signal with a second voltage and a second frequency, so that the brushless exciter outputs a second electromagnetic torque, and then maintaining the operation of the electrically excited synchronous motor and the load; wherein the first electromagnetic torque is greater than the second electromagnetic torque. The application controls the frequency converter to output a signal with a first voltage and a first frequency, so that the brushless exciter outputs a larger electromagnetic torque motor, overcomes the starting static friction torque of the compressor and the main motor, and makes the motor rotate; then controls the frequency converter to output a signal with a second voltage and a second frequency, so that the brushless exciter outputs a smaller electromagnetic torque motor, and maintains the rotation of the compressor and the main motor; without additional devices, the system cost is saved.

Description

Technical Field

[0001] This application relates to the field of frequency converter technology, and in particular to a frequency converter and its control method. Background Technology

[0002] In the fields of oil and gas transportation and storage, electric drive units typically use brushless excitation synchronous motors to drive the compressor to rotate and output power.

[0003] When the compressor is stopped, the motor shaft, being relatively long, can bend if left stationary at one angle for an extended period. To prevent this, a turning gear is used to rotate the compressor and motor shafts at low speed for a certain time after the unit has been stopped for a period of time. This allows the motor to be positioned at different angles when stationary. Additionally, a turning gear is also needed to rotate the motor at low speed during unit maintenance.

[0004] Generally, the device for driving the compressor and the electrically excited synchronous motor to rotate requires additional equipment, which increases the system cost. Alternatively, the rotating function can be achieved directly using a frequency converter that drives the electrically excited synchronous motor, but this method requires high-voltage electricity, poses safety hazards, and involves complex preparation procedures. Summary of the Invention

[0005] This application provides a control method for a frequency converter, wherein the frequency converter is used to drive a brushless exciter, the brushless exciter is used to supply excitation power to an electrically excited synchronous motor, and the electrically excited synchronous motor can drive a load; the method includes:

[0006] The frequency converter outputs a signal with a first voltage and a first frequency so that the brushless exciter outputs a first electromagnetic torque, thereby starting the operation of the electrically excited synchronous motor and the load;

[0007] After a preset time, the inverter is controlled to output a second voltage and a second frequency signal, so that the brushless exciter outputs a second electromagnetic torque, thereby enabling the electrically excited synchronous motor and the load to continue working.

[0008] Wherein, the first electromagnetic torque is greater than the second electromagnetic torque.

[0009] This application also provides a frequency converter, including a memory, a processor, and a control program for the frequency converter stored in the memory and executable on the processor. When the control program for the frequency converter is executed by the processor, it implements the steps of the control method for the frequency converter.

[0010] The frequency converter and its control method provided in this application first control the frequency converter to output a first voltage and a first frequency signal so that the brushless exciter outputs a large electromagnetic torque motor to overcome the starting static friction torque of the compressor and the main motor and make the motor rotate; then control the frequency converter to output a second voltage and a second frequency signal so that the brushless exciter outputs a smaller electromagnetic torque motor to maintain the rotation of the compressor and the main motor; no additional turning device is required, saving system costs. Attached Figure Description

[0011] Figure 1 A schematic diagram of the excitation system provided in the embodiments of this application;

[0012] Figure 2 This is a schematic diagram of the equivalent circuit of the brushless exciter provided in the embodiments of this application;

[0013] Figure 3 This is a schematic diagram of the torque curve of the compressor provided in an embodiment of this application;

[0014] Figure 4 This is a schematic diagram of the control method for a frequency converter provided in an embodiment of this application;

[0015] Figure 5 A block diagram of a frequency converter provided in an embodiment of this application;

[0016] Figure 6 This is a schematic diagram of the excitation system workflow provided in an embodiment of this application;

[0017] Figure 7 The diagram shows the output of the excitation inverter, the torque of the brushless exciter, and the motor speed provided in the embodiments of this application.

[0018] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0019] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer and more understandable, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of this application and are not intended to limit the scope of this application.

[0020] In the description of this application, it should be understood that the terms "center," "upper," "lower," "front," "rear," "left," and "right," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are used only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this application. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0021] The variables and their definitions involved in the embodiments of this application are as follows:

[0022] T em Electromagnetic torque of a brushless exciter

[0023] ω r Angular frequency of the brushless exciter

[0024] ω s Stator angular frequency of a brushless exciter

[0025] ω sli p: Slip frequency of the brushless exciter

[0026] n p Number of pole pairs of a brushless exciter

[0027] P1: Rotor-side power loss of the brushless exciter

[0028] P2: Power consumption of the brushless exciter inverter

[0029] P3: Mechanical power output of the brushless exciter

[0030] L m The excitation inductance of a brushless exciter

[0031] L s Stator leakage inductance of brushless exciter

[0032] L r Rotor leakage inductance of a brushless exciter

[0033] R r Rotor resistance of a brushless exciter

[0034] R s Stator resistance of a brushless exciter

[0035] V s Stator voltage of brushless exciter

[0036] V r Rotor voltage of brushless exciter

[0037] is Stator current of a brushless exciter

[0038] i r Rotor current of brushless exciter

[0039] i m Excitation current of brushless exciter

[0040] Slip: The slip ratio of a brushless exciter.

[0041] T SF static friction torque of the compressor

[0042] like Figure 1 As shown, the output terminal of the electrically excited synchronous motor inverter T1 is connected to the stator winding S1 of the electrically excited synchronous motor. The power of the electrically excited synchronous motor inverter T1 is generally in the tens of MW level, and the power supply uses a medium voltage power supply. The output terminal of the excitation inverter T2 is connected to the stator winding Q1 of the brushless exciter. Its power is generally in the tens of kW level, and the power supply uses a low voltage power supply, such as 380V three-phase AC power.

[0043] The rotor winding Q2 and stator winding Q1 of the brushless exciter can be compared to a rotatable transformer. The AC voltage applied to the stator winding Q1 will also generate an AC voltage in the rotor winding Q2. After uncontrolled rectification by the rotating rectifier Z1, the rotor winding Q2 of the brushless exciter supplies excitation power to the excitation winding S2 of the electrically excited synchronous motor, enabling the synchronous motor to establish a rotor magnetic field.

[0044] The excitation winding S2, the rotating rectifier Z1, and the rotor winding Q2 constitute rotating components and are all mounted on the rotor shaft of the motor. These components rotate as the motor rotor rotates. The motor rotor and the compressor rotor are coaxially connected, thereby enabling the motor to drive the compressor to rotate and output power.

[0045] When an electrically excited synchronous motor drives a compressor, the motor speed reaches several thousand revolutions per minute, and the power reaches tens of MW. However, during the turning cycle, the motor only needs to rotate at a low speed, and the power requirement of the motor is also very small when the compressor rotates at a low speed.

[0046] For a brushless exciter, its equivalent circuit model can be treated as that of an asynchronous motor. For example... Figure 2 As shown, the circuit parameters include the stator resistance R. s Rotor resistance R r Magnetizing inductance L m Stator leakage inductance L s Rotor leakage inductance L r Parameters such as stator leakage inductance L. s Rotor leakage inductance L r Much smaller than the magnetizing inductance L m .

[0047] The main difference between the equivalent circuit model of a brushless exciter and that of an asynchronous motor lies in the fact that, due to the presence of the excitation resistance in an electrically excited synchronous motor, the equivalent rotor resistance R of a brushless exciter... r The rotor resistance R is very high, typically at a 50Hz power frequency. r It is even larger than the excitation reactance. This makes it difficult for the brushless exciter to output torque.

[0048] Electromagnetic torque T of the brushless exciter em The formula can be approximated as:

[0049]

[0050] Among them, the slip frequency ω of the brushless exciter slip =ω s -ω r ω s ω is the stator angular frequency of the brushless exciter. r n is the angular frequency of the brushless exciter. p P1 is the number of pole pairs of the brushless exciter, P2 is the rotor-side power loss of the brushless exciter, P3 is the power consumed by the frequency converter of the brushless exciter, and P4 is the mechanical power output of the brushless exciter.

[0051] During normal operation of an electrically excited synchronous motor driving a compressor at output power, the brushless exciter is supplied with a positive-sequence voltage. In this case, the brushless exciter absorbs mechanical power from the motor shaft of the electrically excited synchronous motor to excite it. When using a brushless exciter to drive both the synchronous motor and the compressor in a rotating position, a reverse-sequence voltage needs to be supplied to the brushless exciter to make the motor rotate in the forward direction.

[0052] When the motor is stationary or rotating at low speed, it can be seen from equation (1) that the rotor-side power loss P1 can be increased and the stator angular frequency ω can be decreased. slip This is to improve the electromagnetic torque output of the brushless exciter.

[0053] When the brushless exciter is stationary or at low speed, its slip ratio (Slip) is close to 1, due to the stator resistance R of the brushless exciter. s Stator leakage inductance L s Rotor leakage inductance L r Much smaller than the rotor resistance R r The stator resistance R can be ignored. s Stator leakage inductance L s Rotor leakage inductance L r The voltage drop across the stator is approximated by the stator voltage V of the brushless exciter. s and rotor voltage V r They are equal. The rotor-side power loss P1 can be calculated as follows:

[0054]

[0055] Further obtain the electromagnetic torque T of the brushless exciter em The formula is:

[0056]

[0057] Therefore, increasing the stator voltage V of the brushless exciter... s Reduce stator angular frequency ω s This can increase the electromagnetic torque T of the brushless exciter. em As can be seen from equation (3), increasing the stator voltage V s To improve electromagnetic torque T em The effect is better than reducing the stator angular frequency ω s More effective. It should be noted that due to the excitation inductance L of the brushless exciter... m The existence of stator angular frequency ω s Reducing the current i of the brushless exciter will significantly increase its excitation current. m .

[0058] From the perspective of reducing the excitation current, while increasing the same electromagnetic torque T... em The following should be done by increasing the stator voltage V. s Achieved, but the stator voltage V is increased. s This will increase the excitation losses of the synchronous motor. From the perspective of reducing losses, increasing the electromagnetic torque T by the same amount... em This should be achieved by reducing the stator angular frequency ω. s In practical applications, the voltage and frequency output by the excitation inverter T2 during turning gear operation should be selected by comprehensively considering the excitation power supply power, voltage output capability, and the heating conditions of the exciter and synchronous motor.

[0059] refer to Figure 3 The given torque curve of the compressor shows that when the compressor is stationary, it needs to overcome the static friction torque T. SF This is necessary to make the motor rotate. Once the motor rotates, the resistance torque decreases rapidly, and the brushless exciter only needs to maintain a small electromagnetic torque T. em This will maintain the motor's rotation.

[0060] Based on the above analysis, such as Figure 4 As shown, one embodiment of this application provides a control method for a frequency converter. The frequency converter is used to drive a brushless exciter, such as the aforementioned excitation frequency converter T2. The brushless exciter is used to supply excitation power to an electrically excited synchronous motor. For example, the rotor winding Q2 of the aforementioned brushless exciter is rectified uncontrolled by a rotating rectifier Z1 and then supplies excitation power to the excitation winding S2 of the electrically excited synchronous motor, thereby enabling the synchronous motor to establish a rotor magnetic field. The electrically excited synchronous motor can drive a load, including but not limited to the aforementioned compressor. The method includes:

[0061] Step S11: Control the inverter to output a signal of a first voltage and a first frequency so that the brushless exciter outputs a first electromagnetic torque, thereby starting the operation of the electrically excited synchronous motor and the load;

[0062] Step S12: After a preset time, control the inverter to output a second voltage and a second frequency signal so that the brushless exciter outputs a second electromagnetic torque, thereby enabling the electrically excited synchronous motor and the load to continue working.

[0063] Wherein, the first electromagnetic torque is greater than the second electromagnetic torque.

[0064] In one example, the signal controlling the inverter to output a first voltage and a first frequency includes:

[0065] When the frequency converter enters the turning mode and the turning command is valid, the frequency converter is controlled to output a signal with a first voltage and a first frequency.

[0066] Specifically, when the electrically excited synchronous motor is in a stopped state and the turning mode switch is active, the frequency converter is controlled to enter the turning mode. Otherwise, the frequency converter does not enter the turning mode.

[0067] When the frequency converter enters the turning gear mode, the electrically excited synchronous motor frequency converter T1 is prohibited from running, and its high-voltage switch is prohibited from being closed.

[0068] In one example, the first voltage is greater than or equal to the second voltage, and / or the first frequency is greater than or equal to the second frequency. Preferably, the absolute value of the first frequency is greater than or equal to the absolute value of the second frequency.

[0069] In one example, the first frequency or the second frequency is in the opposite phase sequence to the frequency at which the inverter is operating normally, thereby ensuring that the motor does not reverse.

[0070] In one example, the second frequency is between 25Hz and 60Hz, preferably between 25Hz and 50Hz, more preferably between 25Hz and 45Hz, and even more preferably between 25Hz and 40Hz; the first frequency is between 50Hz and 75Hz, preferably between 55Hz and 75Hz, and even more preferably between 60Hz and 75Hz.

[0071] In one example, the first voltage and the second voltage are negative sequence voltages, causing the motor to rotate in the forward direction.

[0072] In one example, the first voltage is between 200V and 400V, preferably between 200V and 350V, and more preferably between 200V and 300V; the second voltage is between 80V and 220V, preferably between 200V and 350V, and more preferably between 200V and 300V.

[0073] In one example, the preset time is between 1s and 20s, preferably between 5s and 20s, more preferably between 10s and 20s, and even more preferably between 15s and 20s.

[0074] like Figure 5 As shown, another embodiment of this application provides a frequency converter, including a memory 21, a processor 22, and a control program for the frequency converter stored in the memory 21 and executable on the processor 22. When the control program is executed by the processor 22, it implements the steps of the control method for the frequency converter. The control method for the frequency converter can be found in the foregoing description.

[0075] The following combination Figure 1 , Figures 6-7 The working process of the excitation system is explained as follows:

[0076] The excitation frequency converter T2 first supplies a voltage V1 and a frequency f1 to output a large torque, causing the motor to start from a standstill; after a period of time, T... th Then, another V2 with frequency f2 is supplied to output a smaller torque to keep the motor rotating.

[0077] S1: When the motor is stopped and the turning mode is active, the excitation frequency converter T2 enters the turning mode.

[0078] In this state, the electric synchronous motor inverter T1 is prohibited from operating, and the high-voltage switch of the electric synchronous motor inverter T1 is prohibited from being closed. When the motor is in normal operation or the turning mode is invalid, the excitation inverter T2 does not enter the turning mode, that is, it is in the non-turning mode.

[0079] S2: After entering the turning gear mode, if the turning gear command is valid, the excitation frequency converter T2 will close.

[0080] like Figure 5 As shown, the excitation inverter T2 first outputs a signal with voltage V1 and frequency f1, causing the brushless exciter to output a larger torque T. em1 This allows the motor and compressor to start from a standstill; for a period of time T th Then, the excitation inverter T2 outputs another voltage V2 and frequency f2, causing the brushless exciter to output a smaller torque T. em2 This is to keep the motor and compressor rotating.

[0081] S3: After receiving the shutdown command, the excitation frequency converter T2 stops running.

[0082] The preferred embodiments of this application have been described above with reference to the accompanying drawings, but this does not limit the scope of the claims. Any modifications, equivalent substitutions, and improvements made by those skilled in the art without departing from the scope and spirit of this application shall be within the scope of the claims.

Claims

1. A control method for a frequency converter, wherein the frequency converter is used to drive a brushless exciter, the brushless exciter is used to supply excitation power to an electrically excited synchronous motor, the output terminal of the frequency converter is connected to the stator winding of the brushless exciter, and the electrically excited synchronous motor can drive a load, the load including a compressor; characterized in that, The method includes: When the electrically excited synchronous motor and the load are in a stationary state, the frequency converter is controlled to output a signal of a first voltage and a first frequency so that the brushless exciter outputs a first electromagnetic torque, thereby starting the operation of the electrically excited synchronous motor and the load. After a preset time, the inverter is controlled to output a second voltage and a second frequency signal, so that the brushless exciter outputs a second electromagnetic torque, thereby enabling the electrically excited synchronous motor and the load to continue operating; wherein, the first electromagnetic torque is greater than the second electromagnetic torque; The signals controlling the inverter to output the first voltage and the first frequency include: When the frequency converter enters the turning gear mode and the turning gear command is valid, the frequency converter is controlled to output a signal of a first voltage and a first frequency. When the electrically excited synchronous motor is in a stopped state and the turning mode switch is active, the frequency converter is controlled to enter the turning mode.

2. The control method according to claim 1, characterized in that, The first voltage is greater than or equal to the second voltage, and / or the first frequency is greater than or equal to the second frequency.

3. The control method according to claim 1, characterized in that, The first frequency or the second frequency has the opposite phase sequence to the frequency during normal operation of the inverter.

4. The control method according to claim 1, characterized in that, The second frequency is between 25Hz and 60Hz, and the first frequency is between 50Hz and 75Hz.

5. The control method according to claim 1, characterized in that, The first voltage and the second voltage are negative sequence voltages.

6. The control method according to claim 1, characterized in that, The first voltage is between 200V and 400V, and the second voltage is between 80V and 220V.

7. The control method according to claim 1, characterized in that, The preset time is between 1s and 20s.

8. A frequency converter, characterized in that, The device includes a memory, a processor, and a control program for a frequency converter stored in the memory and executable on the processor. When executed by the processor, the control program for the frequency converter implements the steps of the control method for the frequency converter as described in any one of claims 1 to 7.

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