Frequency converter synchronous switching system

Abstract

The utility model discloses a frequency converter synchronous switching system. The switching system comprises a user switch QF1, a power frequency switch QF2, a frequency converter input switch QF3, a frequency converter output switch QF4, a frequency converter VFD, a current sensor and a motor M, a lower port of the power frequency switch QF2 is provided with a current sensor which is used for collecting current in a switching system loop after the power frequency switch QF2 is switched on; and the frequency converter VFD is used for receiving the current and sending breaking signals of the frequency converter input switch QF3 and the frequency converter output switch QF4. According to the utility model, an electric reactor used for suppressing the impact current of the frequency converter in synchronous switching is canceled, and the closing state of the switch can be obtained in an extremely short time by detecting the current signal of the lower port of the power frequency switch, so that the cost of the system is reduced, and the grid-connected reliability can be improved.

Description

Technical Field

[0001] The utility model relates to the technical field of frequency converter synchronous switching, and particularly relates to a frequency converter synchronous switching system. Background Art

[0002] In many occasions with large-power motor loads, due to the limitation of the grid capacity, if the motor is directly switched on, it will cause the grid voltage to drop or even the switch to trip due to protection. Therefore, generally, a soft start method needs to be adopted to start the motor. Using a voltage-source frequency converter to drive the motor for soft start is a current mainstream solution, and its main circuit is as Figure 1 shown. Its principle is as follows: The frequency converter VFD drives the motor load M from zero speed to 50Hz, and then synchronizes with the grid voltage. When the voltage amplitude and phase output by the frequency converter VFD are exactly the same as those of the grid, the industrial frequency switch QF2 is closed. After the frequency converter VFD operates in parallel with the grid, the frequency converter VFD immediately exits, and the motor M is powered by the industrial frequency power supply to complete the synchronous switching. During the entire switching process, the stator of the motor M is not de-energized, so there is no current impact on the grid at all. However, this solution has two problems: First, there is a process of parallel operation between the frequency converter VFD and the grid. The voltage output by the frequency converter VFD is a PWM waveform, and there is always a deviation from the pure sine voltage of the grid. Therefore, a reactor L needs to be connected in series at the output end of the frequency converter VFD to reduce the impact current formed by the deviation between the voltage output by the frequency converter VFD and the grid voltage. However, adding the reactor L not only increases the cost of the system but also increases the volume of the equipment. Second, after the frequency converter VFD completes synchronization, it sends a closing signal for the industrial frequency switch QF2. The industrial frequency switch QF2 closes, and then the switch position is fed back to the frequency converter VFD. There will be a certain delay (generally dozens of ms) in the entire signal transmission path. During this delay time, between the frequency converter VFD and the grid, there are two voltage sources in parallel. If not controlled, it is very likely to cause overcurrent output of the frequency converter VFD or overvoltage shutdown due to grid charging, resulting in switching failure.

[0003] In view of this, this application is specifically proposed. Content of the Utility Model

[0004] The technical problem to be solved by the utility model is that in the existing frequency converter synchronous switching system, adding the reactor L not only increases the cost of the system but also increases the volume of the equipment; it is very likely to cause overcurrent output of the frequency converter VFD or overvoltage shutdown due to grid charging, resulting in switching failure. The purpose of the utility model is to provide a frequency converter synchronous switching system, which cancels the reactor for suppressing the impact current of the frequency converter during synchronous switching, and can obtain the closing state of the switch in a very short time by detecting the current signal at the lower port of the industrial frequency switch, which not only reduces the cost of the system but also improves the reliability of grid connection.

[0005] The utility model is realized through the following technical solutions:

[0006] A frequency converter synchronization switching system, which includes a user switch QF1, a power frequency switch QF2, a frequency converter input switch QF3, a frequency converter output switch QF4, a frequency converter VFD, a current sensor, and a motor M;

[0007] One end of the user switch QF1 is connected to the power grid, and the other end of the user switch QF1 is connected to the frequency converter input switch QF3. The other end of the user switch QF1 is also connected to the power frequency switch QF2; the frequency converter input switch QF3 is connected to the frequency converter VFD, and the frequency converter VFD is connected to the motor M through the frequency converter output switch QF4; the power frequency switch QF2 is connected to the motor M;

[0008] A current sensor is provided at the lower port of the power frequency switch QF2, which is used to collect the current in the switching system loop after the frequency converter VFD drives the motor M to run in parallel with the power grid and the power frequency switch QF2 is closed;

[0009] The frequency converter VFD is used to receive the current and send out the breaking signals of the frequency converter input switch QF3 and the frequency converter output switch QF4.

[0010] Preferably, the current sensor includes a first current sensor CT1 and a second current sensor CT2;

[0011] The first current sensor CT1 is arranged on the A phase of the three-phase transmission line at the lower port of the power frequency switch QF2;

[0012] The second current sensor CT2 is arranged on the C phase of the three-phase transmission line at the lower port of the power frequency switch QF2.

[0013] Preferably, the frequency converter VFD includes a frequency converter main controller MCB and a synchronization switching controller SC. The frequency converter main controller MCB is connected to the synchronization switching controller SC, and the synchronization switching controller SC is connected to the power frequency switch QF2;

[0014] The frequency converter main controller MCB is also connected to the first current sensor CT1 and the second current sensor CT2.

[0015] Preferably, the model of the frequency converter main controller MCB is MB210Z.

[0016] Preferably, the model of the synchronization switching controller SC is SMART200.

[0017] Preferably, a phase-locked loop is provided in the frequency converter main controller MCB;

[0018] A phase-locked loop is used to make the voltage phase output by a variable-frequency drive (VFD) switch to be consistent with the grid voltage phase based on the phase deviation between the grid and the voltage phase of its own output, so as to achieve phase locking.

[0019] Preferably, the switching system further includes a first grid voltage sensor PT1 and a second grid voltage sensor PT2;

[0020] The first grid voltage sensor PT1 is disposed between phase A and phase B of the three-phase transmission line between the user switch QF1 and the power-frequency switch QF2;

[0021] The second grid voltage sensor PT2 is disposed between phase B and phase C of the three-phase transmission line between the user switch QF1 and the power-frequency switch QF2.

[0022] Preferably, the switching system further includes a first VFD output voltage sensor PT3 and a second VFD output voltage sensor PT4;

[0023] The first VFD output voltage sensor PT3 is disposed between phase A and phase B of the three-phase transmission line between the VFD main controller MCB and the VFD output switch QF4;

[0024] The second VFD output voltage sensor PT4 is disposed between phase B and phase C of the three-phase transmission line between the VFD main controller MCB and the VFD output switch QF4.

[0025] Preferably, the synchronization switching controller SC is disposed inside the VFD.

[0026] Preferably, the synchronization switching controller SC is disposed in a cabinet outside the VFD for multiple sets of switching systems to be used simultaneously.

[0027] Compared with the prior art, the present utility model has the following advantages and beneficial effects:

[0028] 1. For a VFD synchronization switching system of the present utility model, first, the grid connection reactor required for traditional VFD synchronization and grid connection is cancelled, which can reduce the system cost and the volume of equipment; second, a current sensor is disposed at the lower port of the power-frequency switch QF2 to quickly detect the closing state of the QF2 switch and immediately stop the VFD output, preventing the circulating current generated by the parallel operation of the VFD and the grid from causing the VFD to shut down.

[0029] 2. A VFD synchronization switching system of the present utility model can be used in high-voltage or low-voltage VFD starting systems. For the synchronization scheme without an output reactor, after synchronization switching, a fault of overcurrent output by the VFD can be used to trigger shutdown. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The accompanying drawings described herein are used to provide a further understanding of the embodiments of the present utility model, form a part of this application, and do not limit the embodiments of the present utility model. In the drawings:

[0031] Figure 1 It is a schematic structural diagram of a frequency converter synchronous switching system in the prior art;

[0032] Figure 2 It is a schematic structural diagram of a frequency converter synchronous switching system of the present utility model;

[0033] Figure 3 It is a timing diagram of each signal during the switching process of the present utility model. Detailed implementation manners

[0034] In the following, the term "comprising" or "may comprise" that can be used in various embodiments of the present utility model indicates the presence of the functions, operations, or elements of the present utility model, and does not limit the addition of one or more functions, operations, or elements. In addition, as used in various embodiments of the present utility model, the terms "comprising", "having", and their cognates are only intended to indicate specific features, numbers, steps, operations, elements, components, or combinations of the foregoing items, and should not be construed as first excluding the existence or addition of one or more other features, numbers, steps, operations, elements, components, or combinations of the foregoing items.

[0035] In various embodiments of the present utility model, the expression "or" or "at least one of A or / and B" includes any combination or all combinations of the words listed simultaneously. For example, the expression "A or B" or "at least one of A or / and B" may include A, may include B, or may include both A and B.

[0036] Expressions (such as "first", "second", etc.) used in various embodiments of the present utility model may modify various constituent elements in various embodiments, but do not limit the corresponding constituent elements. For example, the above expressions do not limit the order and / or importance of the elements. The above expressions are only used for the purpose of distinguishing one element from other elements. For example, the first user device and the second user device indicate different user devices, although both are user devices. For example, without departing from the scope of various embodiments of the present utility model, the first element may be referred to as the second element, and similarly, the second element may also be referred to as the first element.

[0037] It should be noted that: If it is described that a component is "connected" to another component, the first component can be directly connected to the second component, and a third component can be "connected" between the first component and the second component. Conversely, when a component is "directly connected" to another component, it can be understood that there is no third component between the first component and the second component.

[0038] The terms used in the various embodiments of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the various embodiments of the present invention. As used herein, the singular form is intended to also include the plural form unless the context clearly indicates otherwise. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the various embodiments of the present invention belong. The terms (such as those defined in a general use dictionary) will be interpreted as having the same meaning as the contextual meaning in the relevant technical field and will not be interpreted as having an idealized meaning or an overly formal meaning unless clearly defined in the various embodiments of the present invention.

[0039] To make the purpose, technical solutions and advantages of the present invention more clear and understandable, the present invention will be further described in detail below in conjunction with embodiments and drawings. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and do not constitute a limitation to the present invention.

[0040] The present invention designs a frequency converter synchronous switching system, which cancels the reactor for suppressing the impact current of the frequency converter during synchronous switching, and can obtain the closing state of the switch in an extremely short time by detecting the current signal at the lower port of the power frequency switch, which not only reduces the cost of the system but also improves the reliability of grid connection.

[0041] The main circuit of a frequency converter synchronous switching system is as Figure 2 shown, where QF1 is the user switch, QF2 is the power frequency switch, QF3 is the frequency converter input switch, and QF4 is the frequency converter output switch. This system cancels Figure 1The reactor L at the output end of the medium - frequency inverter is directly connected to the motor through QF4. Two current sensors for current detection are installed on the A - phase and C - phase of the lower port of QF2 in this system, and the output signals of the sensors are connected to the main controller MCB of the frequency converter VFD. The system is configured with a synchronization switching controller SC for synchronization switching control. The synchronization switching controller SC can be built into the frequency converter VFD or set separately outside the frequency converter VFD. Specifically, when the synchronization switching controller SC is set separately outside the frequency converter VFD, the synchronization switching controller SC is set in a cabinet outside the frequency converter VFD and is used for multiple sets of switching systems to be used simultaneously.

[0042] Embodiment

[0043] As Figure 2 As shown, a frequency - converter synchronization switching system of the present utility model includes a user switch QF1, a power - frequency switch QF2, a frequency - converter input switch QF3, a frequency - converter output switch QF4, a frequency converter VFD (including a main controller MCB and a synchronization switching controller SC), current sensors (including a first current sensor CT1 and a second current sensor CT2), a motor M, a first grid - voltage sensor PT1, a second grid - voltage sensor PT2, a first frequency - converter output - voltage sensor PT3, and a second frequency - converter output - voltage sensor PT4. Compared with the traditional synchronization switching system, the reactor L at the output end of the frequency converter is cancelled, and CT1 and CT2 at the lower port of the power - frequency switch QF2 are added.

[0044] One end of the user switch QF1 is connected to the power grid, and the other end of the user switch QF1 is connected to the frequency - converter input switch QF3. The other end of the user switch QF1 is also connected to the power - frequency switch QF2; the frequency - converter input switch QF3 is connected to the frequency converter VFD, and the frequency converter VFD is connected to the motor M through the frequency - converter output switch QF4; the power - frequency switch QF2 is connected to the motor M;

[0045] The lower port of the power - frequency switch QF2 is provided with a first current sensor CT1 and a second current sensor CT2, which are used to collect the current in the switching - system loop after the frequency converter VFD drives the motor M to complete parallel operation with the power grid and the power - frequency switch QF2 is closed. Among them, the first current sensor CT1 is arranged on the A - phase of the three - phase transmission line at the lower port of the power - frequency switch QF2; the second current sensor CT2 is arranged on the C - phase of the three - phase transmission line at the lower port of the power - frequency switch QF2;

[0046] A variable frequency drive (VFD) is used to receive the currents in the switching system loop collected by CT1 and CT2, and send out the tripping signals of the input switch QF3 and the output switch QF4 of the VFD. The VFD includes a main controller MCB of the VFD and a synchronization switching controller SC. The main controller MCB of the VFD is connected to the synchronization switching controller SC, and the synchronization switching controller SC is connected to the power frequency switch QF2. The main controller MCB of the VFD is also connected to a first current sensor CT1 and a second current sensor CT2.

[0047] Specifically, a first grid voltage sensor PT1 is disposed between phase A and phase B of the three-phase transmission line between the user switch QF1 and the power frequency switch QF2; a second grid voltage sensor PT2 is disposed between phase B and phase C of the three-phase transmission line between the user switch QF1 and the power frequency switch QF2.

[0048] Specifically, a first VFD output voltage sensor PT3 is disposed between phase A and phase B of the three-phase transmission line between the main controller MCB of the VFD and the output switch QF4 of the VFD; a second VFD output voltage sensor PT4 is disposed between phase B and phase C of the three-phase transmission line between the main controller MCB of the VFD and the output switch QF4 of the VFD.

[0049] Specifically, a phase-locked loop is provided in the main controller MCB of the VFD. The phase-locked loop is used to make the voltage phase output by the VFD switch to be consistent with the grid voltage phase based on the phase deviation between the grid and the voltage phase output by itself, so as to achieve phase locking.

[0050] In specific implementation, the model of the main controller MCB of the VFD is MB210Z, and the model of the synchronization switching controller SC is SMART200. The pin CN2 of the main controller MCB of the VFD is connected to the pin DO6 of the synchronization switching controller SC, and both the first current sensor CT1 and the second current sensor CT2 are connected to the pin CN5 of the main controller MCB of the VFD.

[0051] The system switching process is as follows: First, the synchronization switching controller SC sends a start command to the frequency converter VFD. After the frequency converter VFD starts successfully, it drives the motor M to run automatically from zero speed to 49 Hz. When the operating frequency of the frequency converter VFD reaches 49 Hz, it locks the phase of the grid voltage. The main controller MCB of the frequency converter sends a synchronization permission command to the synchronization switching controller SC. After receiving the synchronization permission command, the synchronization switching controller SC detects that all switches meet the switching conditions and then issues a synchronization command to the main controller MCB of the frequency converter. After receiving the switching command, the main controller MCB of the frequency converter locks the phase of the grid voltage through the first grid voltage sensor PT1 and the second grid voltage sensor PT2, and obtains the frequency and amplitude of the grid voltage. Then, it increases the output voltage frequency of the frequency converter from 49 Hz to the frequency corresponding to 0.2 Hz higher than the grid frequency (for example, if the grid frequency is 50 Hz, the output frequency of the frequency converter will be increased to 50.2 Hz), and at the same time increases the amplitude of the output voltage to be the same as the grid voltage. Since there is a deviation between the grid voltage frequency and the output frequency of the frequency converter VFD, the phase of the output voltage of the frequency converter VFD and the grid voltage phase will change continuously. The main controller MCB of the frequency converter obtains the deviation between the two in real time. When the phase deviation is less than 3°, the phase of the output voltage of the frequency converter VFD is switched to be the same as the grid voltage phase, completing the locking with the grid voltage.

[0052] After the voltage locking is successful, when the amplitude deviation between the output voltage of the frequency converter VFD and the grid voltage detected by the first frequency converter output voltage sensor PT3 and the second frequency converter output voltage sensor PT4 is less than 50 V, the main controller MCB of the frequency converter sends a command to the synchronization switching controller SC to allow the industrial frequency switch QF2 to close. After detecting that all switch states are normal, the synchronization switching controller SC sends a closing command for the industrial frequency switch QF2. After receiving the closing command, the industrial frequency switch QF2 closes after about 60 - 100 ms. Once QF2 closes, there will immediately be current flowing through its lower port. The main control MCB of the frequency converter detects that there is current at the lower port of QF2 through the first current sensor CT1 and the second current sensor CT2. After a 3 - ms delay confirmation, it immediately blocks the output pulse and stops running, and at the same time sends out disconnection signals for the input switch QF3 and the output switch QF4 of the frequency converter. The input switch QF3 and the output switch QF4 of the frequency converter disconnect after about 60 - 100 ms, and the switching process is completed. The timing of each signal during the switching process is as Figure 3 shown.

[0053] During the above switching process, although there is no reactor at the output end of the frequency converter VFD, the time of parallel operation with the power grid is extremely short (less than 5 ms). Therefore, the voltage and phase deviation are not large, and it will not cause the frequency converter to report overcurrent or overvoltage faults. During the entire synchronization process of the motor M, the stator side has experienced three processes: power supply by the frequency converter, simultaneous power supply by the frequency converter and the power grid, and power supply by the power grid, without any power loss state. Therefore, there is no current impact at all, nor will it cause an impact on the power grid, realizing a seamless start and switching.

[0054] The specific embodiments described above further elaborate on the purpose, technical solutions, and beneficial effects of the present invention. It should be understood that the above description is only the specific embodiments of the present invention and is not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A frequency converter synchronous switching system, characterized in that: The switching system includes a user switch QF1, a power frequency switch QF2, a frequency converter input switch QF3, a frequency converter output switch QF4, a frequency converter VFD, a current sensor and a motor M; One end of the user switch QF1 is connected to the power grid, the other end of the user switch QF1 is connected to the inverter input switch QF3, and the other end of the user switch QF1 is also connected to the power frequency switch QF2; the inverter input switch QF3 is connected to the inverter VFD, and the inverter VFD is connected to the motor M through the inverter output switch QF4; the power frequency switch QF2 is connected to the motor M; The lower port of the power frequency switch QF2 is provided with a current sensor, which is used to collect the current in the switching system loop after the power frequency switch QF2 is closed; The frequency converter VFD is used to receive the current and send out disconnection signals of the frequency converter input switch QF3 and the frequency converter output switch QF4.

2. A frequency converter synchronous switching system according to claim 1, characterized in that: The current sensor includes a first current sensor CT1 and a second current sensor CT2; The first current sensor CT1 is arranged on the three-phase transmission line phase A at the lower port of the power frequency switch QF2; The second current sensor CT2 is disposed on the three-phase transmission line phase C of the lower port of the power frequency switch QF2.

3. A frequency converter synchronous switching system according to claim 2, characterized in that: The frequency converter VFD comprises a frequency converter main controller MCB and a synchronous switching controller SC, wherein the frequency converter main controller MCB is connected to the synchronous switching controller SC, and the synchronous switching controller SC is connected to the power frequency switch QF2; The inverter main controller MCB is also connected to a first current sensor CT1 and a second current sensor CT2.

4. A frequency converter synchronous switching system according to claim 3, characterized in that: The model of the inverter main controller MCB is MB210Z.

5. The inverter synchronous switching system according to claim 3, characterized in that: The model of the synchronous switching controller SC is SMART200.

6. The inverter synchronous switching system according to claim 3, characterized in that: The frequency converter main controller MCB is provided with a phase-locked loop; The phase-locked loop is used to switch the voltage phase output by the frequency converter VFD to be consistent with the grid voltage phase based on the grid phase and the voltage phase deviation of its own output, thereby achieving phase locking.

7. The inverter synchronous switching system according to claim 1, characterized in that: The switching system also includes a first grid voltage sensor PT1 and a second grid voltage sensor PT2; The first grid voltage sensor PT1 is arranged between phase A and phase B of the three-phase transmission line between the user switch QF1 and the power frequency switch QF2; The second grid voltage sensor PT2 is disposed between phases B and C of the three-phase transmission line between the user switch QF1 and the power frequency switch QF2.

8. The inverter synchronous switching system according to claim 3, characterized in that: The switching system also includes a first inverter output voltage sensor PT3 and a second inverter output voltage sensor PT4; The first inverter output voltage sensor PT3 is arranged between phase A and phase B of the three-phase transmission line between the inverter main controller MCB and the inverter output switch QF4; The second inverter output voltage sensor PT4 is arranged between phase B and phase C of the three-phase transmission line between the inverter main controller MCB and the inverter output switch QF4.

9. The inverter synchronous switching system according to claim 3, characterized in that: The synchronous switching controller SC is arranged inside the frequency converter VFD.

10. The inverter synchronous switching system according to claim 3, characterized in that: The synchronous switching controller SC is arranged in a cabinet outside the frequency converter VFD and is used for multiple switching systems to be used simultaneously.

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