Optimization Method and Device for Phase Selection and Closing Control Strategy of Converter Station

Abstract

The present application provides an optimization method and device for the control strategy of selective closing of a converter station, including: controlling the residual magnetism of each phase core to be close to zero; after receiving a closing start signal, sending a first closing signal to control the circuit breaker at the first closing phase to close; calculating the distortion rate of the transient process of the feedback voltage of the first closing phase; within the first maximum waiting delay time limit, when the distortion rate of the transient process of the feedback voltage of the first closing phase is less than a first threshold, sending a second closing signal; in response to the second closing signal, controlling the circuit breaker at the second closing phase to close; calculating the distortion rate of the transient process of the feedback voltages of the first closing phase and the second closing phase; within the second maximum waiting delay time limit, when the distortion rates of the transient processes of the feedback voltages of the first closing phase and the second closing phase are less than a second threshold, sending a third closing signal; in response to the third closing signal, controlling the circuit breaker at the last closing phase to close.

Description

Technical Field

[0001] This application relates to the technical field of suppressing magnetizing inrush current of power transformers. Specifically, it relates to an optimization method and device for the phase selection closing control strategy of a converter station. Background Technique

[0002] The converter transformer is one of the core equipment of UHV DC transmission. Due to the hysteresis and non-linear saturation characteristics of the iron core, at the moment of transformer closing and charging, the iron core may be saturated under the action of the sudden DC component of the magnetic flux linkage, resulting in a sharp decrease in the magnetizing reactance and a surge in the magnetizing current. A large magnetizing inrush current will reduce the service life of the transformer and cause misoperation of the relay protection system; a large number of harmonic components (mainly low-order harmonics) in the inrush current will cause serious distortion of the commutation voltage, which may lead to commutation failure of the converter, and even continuous commutation failure. The waveform distortion caused by the impact of the magnetizing inrush current will also bring risks such as DC blocking, power reduction, and power oscillation.

[0003] In view of the fact that the converter transformer capacity of the UHV DC system is larger, the peak value of the magnetizing inrush current is higher, and the harm to the AC and DC systems is more serious, technical means should be adopted to suppress the magnetizing inrush current.

[0004] At the present stage, the common phase selection closing control strategy for converter transformer groups is that the first closing phase closes at the peak of the system voltage of this phase, and the other two phases close simultaneously with a 1 / 4 cycle lag behind the first closing phase.

[0005] However, when the mechanical closing time of the circuit breaker has a large dispersion, or the first closing phase deviates from the predetermined target phase due to external factors, it often causes waveform distortion of the grid-side voltage of the transformer in the already closed phase, and the distortion duration is not fixed.

[0006] At this time, if the other two phases of the circuit breaker are closed simultaneously during the voltage distortion period, it will cause serious distortion of the magnetic flux in the transformer iron core, thus increasing the rate of change of the magnetic flux in the iron cores of the other two phases, and ultimately deteriorating the suppression effect of the magnetizing inrush current of each phase of the transformer, posing a threat to the safe and stable operation of the converter station, as Figure 1 shown.

[0007] Patent CN108767826A, an invention of a method for suppressing the magnetizing inrush current of an un-demagnetized converter transformer after a DC resistance test, delays the closing of the first closing phase M, and closes the remaining two phases simultaneously after a delay of 2-3 cycles. There are still deficiencies: 1. Although the distortion of the first-phase voltage will be improved to a certain extent after adding a time of 2-3 cycles and closing simultaneously, when the first closing phase deviates greatly from the target point or the voltage distortion is very serious, the closing effect of the remaining two phases still cannot be guaranteed; 2. When the remaining two phases are closed simultaneously, the difference in the dispersion of the closing time will cause the phase with a faster closing speed of the two phases to complete the closing first, and the un-closed phase will complete the closing in a short period, which will exacerbate the target control deviation and also exacerbate the impact of the closing transient process.

[0008] Therefore, it is necessary to propose an optimization method and device for the in-phase closing control strategy of the converter station, which can effectively improve the suppression effect of the magnetizing inrush current during the no-load charging operation of the converter transformer bank.

[0009] The above information disclosed in the background art section is only used to enhance the understanding of the background of the present application, and thus it may include information that does not constitute the prior art known to those of ordinary skill in the art. Summary of the Invention

[0010] In view of the deficiencies in the prior art, the present application provides an optimization method and device for the in-phase closing control strategy of the converter station. The issuing time of the in-phase closing command for the later closing phase is dynamically delayed according to the feedback voltage distortion rate of the earlier closing phase, preventing the magnetizing inrush current impact caused by reclosing during the transient process of voltage distortion. Compared with the conventional in-phase closing strategy, it can effectively improve the suppression effect of the magnetizing inrush current during the no-load charging operation of the converter transformer bank.

[0011] According to the first aspect of the present application, an optimization method for the in-phase closing control strategy of the converter station is proposed, including:

[0012] Controlling the residual magnetism of each phase core to be close to zero and waiting for the closing start signal;

[0013] After receiving the closing start signal, sending a first closing signal to control the breaker at the first closing phase to close;

[0014] Calculating the distortion rate of the transient process of the feedback voltage of the first closing phase;

[0015] Within the first maximum waiting delay time limit, when the distortion rate of the transient process of the feedback voltage of the first closing phase is less than the first threshold, sending a second closing signal;

[0016] In response to the second closing signal, controlling the breaker at the second closing phase to close;

[0017] Calculating the distortion rates of the transient processes of the feedback voltages of the first closing phase and the second closing phase;

[0018] Within the second maximum waiting delay time limit, when the distortion rates of the transient processes of the feedback voltages of the first closing phase and the second closing phase are less than the second threshold, sending a third closing signal;

[0019] In response to the third closing signal, controlling the breaker at the last closing phase to close.

[0020] According to some embodiments, the controlling the residual magnetism of each phase core to be close to zero includes performing a degaussing operation on the converter transformer bank or controlling the residual magnetism of each phase core to be close to zero through phase-selective opening.

[0021] According to some embodiments, the closing target angle for closing the circuit breaker at the first closing phase is at 90° of the first closing phase voltage of the bus voltage transformer on the system side.

[0022] According to some embodiments, the optimization method further includes:

[0023] Within the first time limit, when the transient process distortion rate of the feedback voltage of the first closing phase is greater than or equal to the first threshold, after the end of the first time limit, the second closing signal is sent; or

[0024] After the end of the first time limit, trip the circuit breaker of the first closing phase and wait for the first closing signal.

[0025] According to some embodiments, the optimization method further includes:

[0026] Within the second time limit, when the transient process distortion rates of the feedback voltages of the first closing phase and the second closing phase are greater than or equal to the second threshold, after the end of the second time limit, the third closing signal is sent; or

[0027] After the end of the second time limit, trip the circuit breakers of the first closing phase and the second closing phase and wait for the closing start signal.

[0028] According to some embodiments, the closing target angle for closing the circuit breaker at the second closing phase is at 180° of the first closing phase voltage of the bus voltage transformer on the system side.

[0029] According to some embodiments, the closing target angle for closing the circuit breaker at the last closing phase is at any angle of the first closing phase voltage of the bus voltage transformer on the system side.

[0030] According to some embodiments, it further includes:

[0031] Before the circuit breaker is energized, perform a mechanical characteristic dispersion test on the three-phase mechanism of the circuit breaker, and select the first closing phase, the second closing phase, and the last closing phase.

[0032] According to some embodiments, the phase with the largest closing time dispersion is taken as the last closing phase, the phase with the smallest closing time dispersion is taken as the first closing phase, and the remaining phase is taken as the second closing phase.

[0033] According to some embodiments, the transient process distortion rate of the feedback voltage is calculated by the total voltage harmonic distortion rate and the hth harmonic voltage content rate, where h is one or more odd or even harmonics, or a combination of odd and even harmonic components, which have a greater influence weight on the voltage waveform distortion.

[0034] According to some embodiments, the sum of the first time limit and the second time limit is less than or equal to the operating time of the breaker's single-phase operation protection, or less than the maximum sustainable single-phase operation duration acceptable to the system.

[0035] According to the second aspect of the present application, a phase selection control device for a converter station's phase selection closing control strategy is proposed, which is used to execute the optimization method of the converter station's phase selection closing control strategy as described in any item of the first aspect. The phase selection control device is characterized in that it includes a sampling and calculation unit, an action unit, and a signal transmission unit, where:

[0036] The sampling and calculation unit is used to collect the feedback voltage of each phase breaker of the converter station and calculate the transient distortion rate of the feedback voltage;

[0037] The signal transmission unit is used to transmit the transient process distortion rate of the feedback voltage and receive the closing command and opening command of each phase breaker;

[0038] The action unit is used to close or disconnect the corresponding phase of the breaker according to the closing command and the opening command.

[0039] The present application provides an optimization method and device for a converter station's phase selection closing control strategy. By adopting a phase-by-phase dynamic delay closing strategy, compared with the conventional phase selection closing strategy, it has one or more of the following advantages:

[0040] 1. The issuing time of the closing command for each phase of the breaker is dynamically adjusted according to the voltage distortion rate of the already closed phase, avoiding the magnetic flux distortion of the iron core of the later closed phase caused by insufficient delay time, thus ensuring the suppression effect of the exciting inrush current of the later closed phase.

[0041] 2. The second closing phase and the last closing phase complete closing at different times, and the better one can be selected from the mechanical dispersion of the two-phase breakers to improve the control accuracy of the second closing phase. At the same time, it avoids exacerbating the closing transient process due to dispersion when the two phases close simultaneously.

[0042] It should be understood that the above general description and the following detailed description are only exemplary and do not limit the present application. Description of the Drawings

[0043] By referring to the accompanying drawings and describing its exemplary embodiments in detail, the above and other objectives, features, and advantages of the present application will become more obvious. The following described drawings are only some embodiments of the present application and do not limit the present application.

[0044] Figure 1 A waveform diagram showing that the conventional phase selection closing strategy in the prior art causes a large exciting inrush current;

[0045] Figure 2Shows a wiring schematic diagram of an exemplary single - pole twelve - pulse converter transformer and a phase - selection control device;

[0046] Figure 3 Shows a flowchart of a phase - selection closing control strategy of an exemplary embodiment;

[0047] Figure 4 Shows a schematic diagram of the core magnetic - flux change during phase - selection closing of an exemplary embodiment;

[0048] Figure 5 Shows another embodiment of the flowchart of the exemplary phase - selection closing control strategy;

[0049] Figure 6 Shows a schematic diagram of a phase - selection control device for a phase - selection closing control strategy of an exemplary embodiment. Detailed implementation manners

[0050] Example embodiments will now be described more fully with reference to the accompanying drawings. However, the example embodiments can be implemented in various forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this application will be thorough and complete, and will fully convey the concept of the example embodiments to those skilled in the art. Like reference numerals in the figures denote like or similar parts, and thus their repeated description will be omitted.

[0051] The described features, structures, or characteristics can be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided to give a thorough understanding of the embodiments of the present disclosure. However, those skilled in the art will realize that the technical solutions of the present disclosure can be practiced without one or more of these specific details, or can be implemented in other ways, components, materials, devices, etc. In these cases, well - known structures, methods, devices, implementations, materials, or operations will not be shown or described in detail.

[0052] The flowcharts shown in the drawings are merely illustrative and not necessarily include all the contents and operations / steps, nor are they necessarily executed in the order described. For example, some operations / steps can be decomposed, while some operations / steps can be combined or partially combined, so the actual execution order may change according to the actual situation.

[0053] In the description, claims, and the above-mentioned drawings of this application, terms such as "first" and "second" are used to distinguish different objects, rather than to describe a specific order. In addition, the terms "comprising" and "having" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units is not limited to the listed steps or units, but optionally further includes steps or units not listed, or optionally further includes other steps or units inherent to these processes, methods, products, or devices.

[0054] Those skilled in the art can understand that the drawings are only schematic diagrams of exemplary embodiments, and the modules or processes in the drawings are not necessarily essential for implementing this application, so they cannot be used to limit the protection scope of this application.

[0055] Figure 1 A waveform diagram showing that the conventional phase-selection closing strategy in the prior art causes a large inrush current is shown.

[0056] As Figure 1 shown, the closing of phase A causes distortion of the feedback voltage waveform. During the voltage distortion period, the other two phases, phase B and phase C, of the circuit breaker are closed simultaneously, resulting in severe distortion of the transformer core magnetic flux, increasing the rate of change of the magnetic flux in the cores of the remaining two phases, deteriorating the suppression effect of the inrush current in each phase of the transformer, and posing a threat to the safe and stable operation of the converter station.

[0057] Figure 2 A wiring schematic diagram of an exemplary single-pole twelve-pulse converter transformer and a phase-selection control device is shown.

[0058] As Figure 2 shown, the single-pole twelve-pulse converter transformer bank includes 3 single transformers with a YNyn0 wiring method and 3 single transformers with a YNd11 wiring method. The three-phase cores of each phase of the converter transformer are independent, and the two groups of transformers operate in parallel. Their high-voltage sides are connected to the AC system bus through phase-separated circuit breakers, and the low-voltage sides are connected to the converter valves.

[0059] Figure 3 A flowchart of the phase-selection closing control strategy of an exemplary embodiment is shown.

[0060] Before the circuit breaker is energized, a mechanical dispersion test is performed on the closing times of the three-phase mechanisms of the circuit breaker. The phase with the largest closing-time dispersion is used as the last closing phase, the phase with the smallest closing-time dispersion is used as the first closing phase, and the remaining phase is used as the second closing phase. Assume that the test results are: the closing-time dispersion of phase A is ±0.5 ms, the closing-time dispersion of phase B is ±1 ms, and the closing-time dispersion of phase C is ±1.5 ms. Then, phase A is selected as the first closing phase, phase B is selected as the second closing phase, and phase C is selected as the last closing phase to implement phase-selection control.

[0061] According to some embodiments, the present application takes two transformers operating in parallel with the wiring modes of YNyn0 and YNd11 in a converter station as an example, but the present application is not limited thereto, and the present application is applicable to devices such as transformers composed of equivalent variant combinations of Yy and Yd wiring modes.

[0062] S1. Control the residual magnetism of each phase core to be close to zero and wait for the closing start signal.

[0063] According to the exemplary embodiments, before the circuit breaker closes, first perform a demagnetization operation on the single-pole twelve-pulse converter transformer bank, or control the residual magnetism of each phase core of the transformer to be close to zero through phase-selective opening and closing, and wait for the closing start signal.

[0064] S2. Control the circuit breaker at the first closing phase to close at the peak value of the system first closing phase voltage.

[0065] According to some embodiments, through the background control, the system issues a remote control closing instruction. After receiving the closing signal, the phase-selective control device completes the closing of the first closing phase, and the closing target angle is selected at 90° of the first closing phase voltage of the system-side bus voltage transformer, that is, at the peak value of the phase voltage, or at other preferred closing target angles.

[0066] According to the exemplary embodiments, in the present application, phase A is the first closing phase. Through the background control system, a remote control closing instruction is issued. After receiving the closing signal, the phase-selective control device completes the phase-selective closing of phase A, and the closing target angle is selected at 90° of the phase A voltage of the system-side bus voltage transformer, or at other preferred closing target angles.

[0067] According to the exemplary embodiments, the closing point of phase A happens to be at the zero crossing point of the pre-induced magnetic flux of the phase core. Therefore, on the premise of relatively small residual magnetism, magnetic flux saturation will not occur, and it directly enters the steady-state operation, and the inrush current of phase A is better suppressed.

[0068] According to the exemplary embodiments, after phase A completes closing, due to the electrical connection of the valve-side delta winding of the transformer with the wiring mode of YNd11, an induced voltage will be generated in the valve-side windings of phases B and C, with a magnitude of half of UA and a reverse direction. The three-phase magnetic fluxes of the YNd11 transformer remain balanced, the magnetic flux lags the voltage by 90°, and the magnetic fluxes of phases B and C are also half of the magnetic flux of phase A and in the reverse direction, as Figure 4 shown.

[0069] According to the exemplary embodiments, since the two single-phase converter transformers operating in parallel for each phase are switched by the same circuit breaker, their grid-side windings are directly connected and the neutral points are grounded. Therefore, the voltage drops of the grid-side windings of the two transformers are the same, and thus the magnetic fluxes of the cores of the two groups of converter transformers also remain consistent. After phase A closes, since the initial magnetic flux of the YNyn0 transformer is zero, the magnetic fluxes of phases B and C are also half of the magnetic flux of phase A and in the reverse direction, as Figure 4 shown.

[0070] S3. Determine the distortion rate of the feedback voltage of the first switched-on phase.

[0071] According to some embodiments, the phase selection control device collects and monitors the feedback voltage of the first switched-on phase that has been switched on, and calculates in real time. Taking the establishment of the A-phase feedback voltage as the starting timing point, it is checked in real time whether it simultaneously satisfies that the total harmonic distortion rate of the phase voltage is less than the first set value, and the harmonic voltage content rates of the 2nd, 3rd, 4th, and 5th harmonics are all less than the second set value. If such a situation that meets the above conditions occurs within the first time limit T1, the closing output of the second switched-on phase B is immediately opened; if there is no situation that meets the above conditions within the first time limit T1, after reaching the time T1, the closing output of the second switched-on phase B is immediately opened.

[0072] According to some embodiments, the transient process distortion rate criterion of the feedback voltage includes a combination of one or more of the following criteria: the total harmonic distortion rate of the voltage is less than the first set value, the harmonic voltage content rate of the hth harmonic is less than the second set value. In practical engineering applications, h can be selected as one or more odd or even harmonics with a relatively large influence weight on the voltage waveform distortion, or a combination of odd and even harmonic components. The first set value and the second set value are set according to empirical values.

[0073] According to some embodiments, the first set value is 10%, and the second set value is 5%. In actual implementation, it can also be modified and set according to engineering requirements.

[0074] According to some embodiments, considering the non-full-phase protection operation time of a typical circuit breaker of 2 - 2.5 s, the maximum waiting delay T1 = 500 ms is set. In actual implementation, it can also be modified and set according to engineering requirements.

[0075] S4. Control the circuit breaker at the second switched-on phase.

[0076] According to the exemplary embodiments, after the A phase is switched on, the B-phase circuit breaker is controlled to be switched on. The closing target angle is selected at 180° of the A-phase voltage of the system-side bus voltage transformer, or at other preferred closing target angles. At this time, the bias magnetic generated by the B-phase magnetic flux is zero, so it directly enters the steady-state operation, and the inrush current of the B-phase excitation is better suppressed.

[0077] S5. Determine the distortion rates of the feedback voltages of the first switched-on phase and the second switched-on phase.

[0078] According to the exemplary embodiment, after the closing of the second closing phase, i.e., phase B, the feedback voltages of the closed phases A and B are collected and monitored by the phase selection control device, and the transient distortion rate of the feedback voltages is evaluated. Taking the establishment of the feedback voltage of phase B as the starting timing point, before the arrival of time T2, if the total harmonic distortion rate is less than the first set value, and the harmonic voltage content rates of the 2nd, 3rd, 4th, and 5th harmonics are all less than the second set value, the closing output of the last unclosed phase, i.e., phase C, is immediately enabled; or when the distortion rate criterion cannot be satisfied all the time, at the moment when T2 arrives, the closing output of the last unclosed phase, i.e., phase C, is immediately enabled.

[0079] According to some embodiments, the sum of the maximum waiting delay T1 and the maximum waiting delay T2 should not exceed the non-full-phase protection operation time of the circuit breaker, or be less than the maximum non-full-phase continuous duration acceptable to the system, with a margin. The maximum waiting delay T2 = 500 ms, and in actual implementation, it can also be modified according to engineering requirements.

[0080] S6, control the circuit breaker at the last unclosed phase.

[0081] According to the exemplary embodiment, the closing output of the last unclosed phase, i.e., phase C, is enabled, and the closing of the circuit breaker of phase C is controlled. The closing angle of the last unclosed phase, i.e., phase C, is selected at any angle or other preferred closing target angles to complete the phase selection closing control of the three-phase circuit breaker.

[0082] According to the exemplary embodiment, the issuing time of the closing command for each phase of the circuit breaker is dynamically adjusted according to the voltage distortion rate of the closed phases, so as to avoid the distortion of the iron core magnetic flux of the later-closing phase caused by insufficient delay time, thereby ensuring the suppression effect of the inrush current of the later-closing phase.

[0083] According to the exemplary embodiment, the second closing phase and the last unclosed phase are closed separately, and the optimal one can be selected from the mechanical dispersions of the two-phase circuit breakers to improve the control accuracy of the second closing phase, and at the same time avoid aggravating the closing transient process due to the dispersion when the two phases are closed simultaneously.

[0084] Figure 5 Another embodiment of the flowchart showing the exemplary phase selection closing control strategy is shown.

[0085] Before the circuit breaker is energized, a mechanical dispersion test is carried out on the closing time of the three-phase mechanism of the circuit breaker. The phase with the largest closing time dispersion is taken as the last unclosed phase, the phase with the smallest closing time dispersion is taken as the first closing phase, and the remaining phase is taken as the second closing phase. Suppose the test results are: the closing time dispersion of phase A is ±0.5 ms, the closing time dispersion of phase B is ±1 ms, and the closing time dispersion of phase C is ±1.5 ms. Then phase A is selected as the first closing phase, phase B is selected as the second closing phase, and phase C is selected as the last unclosed phase to implement phase selection control.

[0086] According to some embodiments, the present application takes two groups of transformers operating in parallel with connection modes of YNyn0 and YNd11 in a converter station as an example, but the present application is not limited thereto, and the present application can be applied to devices such as transformers composed of equivalent variant combinations of Yy and Yd connection modes.

[0087] S1. Control the residual magnetism of each phase core to be close to zero and wait for the closing start signal.

[0088] According to the exemplary embodiments, before the circuit breaker closes, first perform a demagnetization operation on the single-pole twelve-pulse converter transformer bank, or control the residual magnetism of each phase core of the transformer to be close to zero through phase-selective opening.

[0089] S2. Control the circuit breaker at the first closing phase to close at the peak value of the system's first closing phase voltage.

[0090] According to some embodiments, through background control, the system issues a remote control closing command. After the phase-selective control device receives the closing signal, it completes the closing of the first closing phase, and the selected closing target angle is selected at 90° of the first closing phase voltage of the system-side bus voltage transformer, that is, at the peak value of the phase voltage, or at other preferred closing target angles.

[0091] According to the exemplary embodiments, in the present application, phase A is the first closing phase. Through the background control system, a remote control closing command is issued. After the phase-selective control device receives the closing signal, it completes the phase-selective closing of phase A, and the selected closing target angle is selected at 90° of the phase A voltage of the system-side bus voltage transformer, or at other preferred closing target angles.

[0092] According to the exemplary embodiments, the closing point of phase A happens to be at the zero-crossing point of the pre-induced magnetic flux of the core of this phase. Therefore, on the premise of relatively small residual magnetism, magnetic flux saturation will not occur, and it directly enters the steady-state operation, and the inrush current of phase A is better suppressed.

[0093] According to the exemplary embodiments, after phase A completes the closing, due to the electrical connection of the valve-side delta winding of the transformer with the connection mode of YNd11, an induced voltage will be generated in the valve-side windings of phases B and C, with a magnitude of half of UA and the direction opposite. The three-phase magnetic fluxes of the YNd11 transformer remain balanced, and the magnetic flux lags the voltage by 90°. The magnetic fluxes of phases B and C are also half of the magnetic flux of phase A and the direction is opposite, as Figure 4 shown.

[0094] According to the exemplary embodiments, since the two single-phase converter transformers operating in parallel for each phase are switched by the same circuit breaker, their grid-side windings are directly connected and the neutral points are grounded. Therefore, the voltage drops of the grid-side windings of the two transformers are the same, and thus the core magnetic fluxes of the two groups of converter transformers also remain consistent. After phase A closes, since the initial magnetic flux of the YNyn0 transformer is zero, the magnetic fluxes of phases B and C are also half of the magnetic flux of phase A and the direction is opposite, as Figure 4 shown.

[0095] S3. Determine the distortion rate of the feedback voltage of the first switched-on phase.

[0096] According to some embodiments, the selected-phase switching control device collects and monitors the feedback voltage of the first switched-on phase that has been switched on, and calculates in real time. Taking the establishment of the A-phase feedback voltage as the starting timing point, it is checked in real time whether it simultaneously meets the conditions that the total harmonic distortion rate of the phase voltage is less than the first set value, and the harmonic voltage content rates of the 2nd, 3rd, 4th, and 5th harmonics are all less than the second set value. If such a situation that meets the above conditions occurs within the first time limit T1, the switching output of the second switched-on phase B is immediately opened; if there is no situation that meets the above conditions within the first time limit T1, then after reaching the time T1, the already switched-on phase A is tripped, and it returns to step S1 to wait for a new switching start signal.

[0097] According to some embodiments, the transient process distortion rate criterion of the feedback voltage includes a combination of one or more of the following criteria: the total harmonic distortion rate of the voltage is less than the first set value, the harmonic voltage content rate of the hth harmonic is less than the second set value. In practical engineering applications, h can be selected as one or more odd or even harmonics with a relatively large influence weight on the voltage waveform distortion, or a combination of odd and even harmonic components. The first set value and the second set value are set according to empirical values.

[0098] According to some embodiments, the first set value is 10%, and the second set value is 5%. In actual implementation, it can also be modified and set according to engineering requirements.

[0099] According to some embodiments, considering the non-full-phase protection operation time of a typical circuit breaker is 2 - 2.5 s, the maximum waiting delay T1 = 500 ms is set. In actual implementation, it can also be modified and set according to engineering requirements.

[0100] S4. Control the circuit breaker at the second switched-on phase.

[0101] According to the exemplary embodiments, after the A-phase is switched on, control the B-phase circuit breaker to switch on. The selected closing target angle is at 180° of the A-phase voltage of the system-side bus voltage transformer, or at other preferred closing target angles. At this time, the bias magnetic generated by the B-phase magnetic flux is zero, so it directly enters the steady-state operation, and the inrush current of the B-phase excitation is better suppressed.

[0102] S5. Determine the distortion rates of the feedback voltages of the first switched-on phase and the second switched-on phase.

[0103] According to the exemplary embodiment, after the closing of the second closing phase, i.e., phase B, the feedback voltages of the already closed phases A and B are collected and monitored by the phase selection control device, and the transient distortion rate of the feedback voltages is evaluated. Taking the establishment of the feedback voltage of phase B as the starting timing point, before the arrival of time T2, if the total harmonic distortion rate is less than the first set value and the harmonic voltage content rates of the 2nd, 3rd, 4th, and 5th harmonics are all less than the second set value, the closing output of the last unclosed phase, i.e., phase C, is immediately opened; in the case where the distortion rate criterion cannot be satisfied all the time, at the moment when T2 arrives, the already closed phases A and B are tripped, and step S1 is executed again, waiting for a new closing start signal.

[0104] According to some embodiments, the sum of the maximum waiting delay T1 and the maximum waiting delay T2 should not exceed the non - all - phase protection operation time of the circuit breaker, or be less than the maximum non - all - phase continuous duration acceptable to the system, with a margin. The maximum waiting delay T2 = 500 ms, and in actual implementation, it can also be modified according to engineering requirements.

[0105] S6, control the circuit breaker at the last unclosed phase.

[0106] According to the exemplary embodiment, the closing output of the last unclosed phase, i.e., phase C, is opened, and the closing of the circuit breaker of phase C is controlled. The closing angle of the last unclosed phase C is selected at any angle or at other preferred closing target angles, and the selective closing control of the three - phase circuit breaker is completed.

[0107] According to the exemplary embodiment, the issuing time of the closing command for each phase of the circuit breaker is dynamically adjusted according to the voltage distortion rate of the already closed phase, so as to avoid the distortion of the iron - core magnetic flux of the later - closing phase caused by insufficient delay time, thereby ensuring the suppression effect of the inrush current of the later - closing phase.

[0108] According to the exemplary embodiment, the second - closing phase and the last - unclosed phase complete closing at different times, and the better one can be selected from the mechanical dispersions of the two - phase circuit breakers to improve the control accuracy of the second - closing phase. At the same time, the transient closing process is not aggravated due to the dispersity when the two phases close simultaneously.

[0109] Figure 6 The schematic diagram of a phase selection control device for a selective closing control strategy showing an exemplary embodiment is shown.

[0110] As Figure 6 shown, a phase selection control device for a selective closing control strategy includes a sampling and calculation unit 601, an action unit 603, and a signal transmission unit 605.

[0111] According to the exemplary embodiment, the sampling and calculation unit 601 collects the feedback voltages of the three phases A, B, and C, and calculates the transient process distortion rate of the feedback voltages. The transient process distortion rate includes the total harmonic distortion rate and the harmonic voltage content rate.

[0112] According to an exemplary embodiment, the signal transmission unit 605 is configured to receive the closing instruction and opening instruction for each phase of the circuit breaker.

[0113] According to an exemplary embodiment, the action unit 603 is configured to close or open the corresponding phase of the circuit breaker according to the closing instruction and opening instruction.

[0114] It should be clearly understood that this application describes how to form and use specific examples, but this application is not limited to any details of these examples. On the contrary, based on the teachings of the content disclosed in this application, these principles can be applied to many other embodiments.

[0115] In addition, it should be noted that the above-mentioned drawings are only schematic illustrations of the processes included in the method according to the exemplary embodiments of this application, rather than for limiting purposes. It is easy to understand that the processes shown in the above-mentioned drawings do not indicate or limit the chronological order of these processes. Additionally, it is also easy to understand that these processes can be executed, for example, synchronously or asynchronously in multiple modules.

[0116] The exemplary embodiments of this application have been specifically shown and described above. It should be understood that this application is not limited to the detailed structures, settings, or implementation methods described herein; on the contrary, this application is intended to cover various modifications and equivalent settings included within the spirit and scope of the appended claims.

Claims

1. An optimization method for the selected-phase closing control strategy of a converter station, characterized in that, it includes: Before the circuit breaker is energized, conduct a mechanical characteristic dispersion test on the three-phase mechanism of the circuit breaker, and select the first closing phase, the second closing phase, and the last closing phase. Among them, the phase with the largest dispersion of closing time is used as the last closing phase, the phase with the smallest dispersion of closing time is used as the first closing phase, and the remaining one phase is used as the second closing phase; Control the residual magnetism of each phase core to be close to zero and wait for the closing start signal; After receiving the closing start signal, send the first closing signal to control the circuit breaker at the first closing phase to close; Calculate the distortion rate of the transient process of the feedback voltage of the first closing phase; Within the first maximum waiting delay time limit, when the distortion rate of the transient process of the feedback voltage of the first closing phase is less than the first threshold, send the second closing signal; In response to the second closing signal, control the circuit breaker at the second closing phase to close; Calculate the distortion rates of the transient processes of the feedback voltages of the first closing phase and the second closing phase; Within the second maximum waiting delay time limit, when the distortion rates of the transient processes of the feedback voltages of the first closing phase and the second closing phase are less than the second threshold, send the third closing signal; In response to the third closing signal, control the circuit breaker at the last closing phase to close.

2. The optimization method according to claim 1, characterized in that, The control of the residual magnetism of each phase core to be close to zero includes performing a demagnetization operation on the converter transformer bank, or controlling the residual magnetism of each phase core to be close to zero through selected-phase opening.

3. The optimization method according to claim 1, characterized in that, The closing target angle of the circuit breaker at the first closing phase is at 90° of the voltage of the first closing phase of the bus voltage transformer on the system side.

4. The optimization method according to claim 1, characterized in that, The optimization method further includes: Within the first time limit, when the distortion rate of the transient process of the feedback voltage of the first closing phase is greater than or equal to the first threshold, after the first time limit ends, send the second closing signal; or after the first time limit ends, trip the circuit breaker of the first closing phase and wait for the closing start signal.

5. The optimization method according to claim 1, characterized in that, The optimization method further includes: Within the second time limit, when the distortion rates of the transient processes of the feedback voltages of the first closing phase and the second closing phase are greater than or equal to the second threshold, after the second time limit ends, send the third closing signal; or After the second time limit ends, trip the circuit breakers of the first closing phase and the second closing phase and wait for the closing start signal.

6. The optimization method according to claim 1, characterized in that, The closing target angle of the circuit breaker at the second closing phase is at 180° of the voltage of the first closing phase of the bus voltage transformer on the system side.

7. The optimization method according to claim 1, characterized in that, The closing target angle of the circuit breaker at the last closing phase is at any angle of the voltage of the first closing phase of the bus voltage transformer on the system side.

8. The optimization method according to claim 1, characterized in that, Calculate the transient distortion rate of the feedback voltage through the total harmonic distortion rate of voltage and the harmonic voltage content rate of the h-th harmonic, where h is one or more odd or even harmonics with a relatively large influence weight on the voltage waveform distortion, or a combination of odd and even harmonic components.

9. The optimization method according to claim 1, characterized in that the sum of the first time limit and the second time limit is less than the non-full-phase protection operation time of the circuit breaker, or less than the maximum non-full-phase duration acceptable to the system.

10. A device for the phase selection closing control strategy of a converter station, which is used to execute the optimization method of the phase selection closing control strategy of the converter station according to any one of claims 1-9, characterized in that the phase selection control device includes a sampling and calculation unit, an action unit and a signal transmission unit, wherein: the sampling and calculation unit is used to collect the feedback voltage of each phase circuit breaker of the converter station and calculate the transient distortion rate of the feedback voltage; the signal transmission unit is used to transmit the transient process distortion rate of the feedback voltage and receive the closing instruction and opening instruction of each phase of the circuit breaker; the action unit is used to close or open the corresponding phase of the circuit breaker according to the closing instruction and the opening instruction.

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