Hybrid cascade DC system subsequent commutation failure prediction method considering MMC fault inrush current

Abstract

The invention relates to a hybrid cascade direct current system subsequent commutation failure prediction method considering MMC fault inrush current. The method comprises the following steps: analyzing an MMC converter transformer fault inrush current generation mechanism in a hybrid cascade direct current system; analyzing the rule of harmonic distortion of the bus voltage of the commutation converter of the power grid caused by the fault inrush current of the MMC converter transformer; analyzing the influence of bus voltage harmonic distortion on the commutation process; and according to the fault inrush current amplitude, the bus voltage harmonic distortion rate and the commutation failure judgment condition, predicting the subsequent commutation failure risk of the inverter side power grid commutation converter of the hybrid cascade direct current system. According to the method, the electrical coupling relation between the inverter side LCC and the MMC and the relevance between the fault inrush current and the subsequent commutation failure are brought into a prediction system, a linkage induction mechanism that the fault inrush current causes bus voltage harmonic distortion, the turn-off angle of the LCC is reduced, and the subsequent commutation failure is triggered is completely revealed, the induction factors of the subsequent commutation failure are accurately positioned, and the reliability of the subsequent commutation failure is improved. And the prediction accuracy is improved.

Description

Technical Field

[0001] This application relates to the field of power system relay protection technology, specifically to a method for predicting subsequent commutation failure in a hybrid cascaded DC system that considers MMC fault inrush current. Background Technology

[0002] Commutation failure, a typical fault in DC transmission systems, is characterized by a sudden drop in DC voltage and a sharp rise in DC current, which can lead to power imbalance between the sending and receiving AC systems. Under severe AC faults, the first commutation failure in a DC transmission system is difficult to suppress, but its impact on the power grid is limited. Subsequent commutation failures, however, seriously threaten the safe and stable operation of the power grid, and may even cause DC blocking, resulting in power transmission interruption and posing a significant threat to the safe and stable operation of the system.

[0003] Hybrid cascaded DC systems combine the advantages of conventional DC and flexible DC, and have become a new way to achieve long-distance, high-capacity power transmission. However, because their inverter side uses a cascaded structure of a line commutated converter (LCC) and a modular multilevel converter (MMC), and there is a complex electrical coupling between the LCC and MMC, a fault inrush current caused by a ground fault on the MMC converter transformer valve side may cause subsequent commutation failure of the LCC, jeopardizing the safe and stable operation of the system.

[0004] However, current research only covers the inrush current mechanism induced by ground faults on the converter transformer valve side of hybrid cascaded DC transmission systems and its impact on the differential protection of the converter transformer, as well as subsequent commutation failures in line-commutated converter based high-voltage direct-current (LCC-HVDC) transmission systems. There is no research on subsequent commutation failures in hybrid cascaded DC transmission systems caused by inrush currents in the inverter-side MMC converter transformer.

[0005] Therefore, in order to address the above problems, there is an urgent need for an LCC subsequent commutation failure analysis method that can take into account the complex coupling relationship between different converters on the inverter side and the impact of faulty inrush current of the MMC converter transformer in hybrid cascaded DC transmission systems, so as to accurately predict the risk of LCC subsequent commutation failure in hybrid cascaded DC systems. Summary of the Invention

[0006] To overcome the shortcomings of the prior art, this application provides a method for predicting subsequent commutation failure in a hybrid cascaded DC system that considers MMC inrush current, specifically employing the following technical solution:

[0007] A method for predicting subsequent commutation failure in a hybrid cascaded DC system considering MMC inrush current, wherein the rectifier side of the hybrid cascaded DC system uses a grid-commutated converter, the high-side valve group on the inverter side of the hybrid cascaded DC system uses a grid-commutated converter, and the low-side valve group uses multiple modular multilevel converters connected in parallel and then connected in series with the high-side valve group on the DC side, the method comprising: Based on the topology of the hybrid cascaded DC system, the control strategies for the converters corresponding to the rectifier side and the inverter side are obtained; Based on the converter control strategy, the generation mechanism of fault inrush current of MMC converter transformer in the hybrid cascaded DC system is analyzed, and the correlation between the fault inrush current amplitude and the saturation characteristics of the MMC converter transformer is clarified. Based on the electrical coupling between the high-end valve group and the low-end valve group on the inverter side, the law of harmonic distortion of the bus voltage of the grid commutator on the inverter side caused by the fault inrush current of the MMC converter transformer is analyzed, and the correlation between the amplitude of the fault inrush current of the MMC converter transformer and the harmonic distortion rate of the bus voltage of the grid commutator is established. The influence of the harmonic distortion of the bus voltage of the grid commutator on the commutation process of the grid commutator is analyzed, and the judgment conditions for commutation failure of the hybrid cascaded DC system are determined. Based on the fault inrush current amplitude of the MMC converter transformer under different saturation characteristics, the harmonic distortion rate of the grid commutator bus voltage, and the commutation failure judgment conditions, the risk of subsequent commutation failure of the grid commutator on the inverter side of the hybrid cascaded DC system is predicted.

[0008] Optionally: The fault inrush current generation mechanism of the MMC converter transformer in the hybrid cascaded DC system includes: after a single-phase ground fault occurs on the valve side of the MMC converter transformer, the bridge arm current increases sharply, causing the IGBT to be blocked due to overcurrent. The fault phase current contains a large DC component and flows into the valve side of the MMC converter transformer, causing the core flux of the MMC converter transformer to accumulate continuously, causing the core operating point to enter the saturation region, thereby generating fault inrush current.

[0009] Optional: When obtaining the control strategies of the converters corresponding to the rectifier side and the inverter side, the UMEC model should also be used to simulate the different saturation characteristics of the MMC converter by simulating different current-voltage characteristic curves.

[0010] Optionally: When analyzing the fault inrush current generation mechanism of the MMC converter transformer in the hybrid cascaded DC system according to the converter control strategy, the higher the saturation degree of the MMC converter transformer, the greater the amplitude of the fault inrush current.

[0011] Optionally: The fault inrush current contains 2nd to 7th harmonic components, with the 2nd harmonic component having the highest amplitude; the amplitude of the fault inrush current is positively correlated with the harmonic distortion rate of the grid commutator bus voltage. The larger the amplitude of the fault inrush current, the higher the harmonic distortion rate of the grid commutator bus voltage and the more severe the voltage waveform distortion.

[0012] Optionally: The impact of the harmonic distortion of the bus voltage of the grid-commutated converter on the commutation process of the grid-commutated converter includes: The harmonic distortion of the bus voltage of the grid commutation converter will cause the zero-crossing point of the commutation voltage waveform of the grid commutation converter to shift forward and the voltage amplitude to decrease. This will increase the commutation overlap angle and decrease the turn-off angle of the grid commutation converter. The time area provided by the commutation voltage cannot meet the commutation process requirements of the grid commutation converter, and the risk of commutation failure will increase.

[0013] Furthermore: When analyzing the law of harmonic distortion of the bus voltage of the grid commutator on the inverter side caused by the fault inrush current of the MMC converter transformer, the process from the occurrence of the fault to the stabilization of various electrical quantities is divided into four stages according to the size of the turn-off angle: the first commutation failure stage, the recovery stage after the first commutation failure, the subsequent commutation failure stage, and the recovery stage after the subsequent commutation failure. The influence of fault inrush current on harmonic distortion of bus voltage is analyzed by combining the electrical quantity characteristics of each stage: the fault inrush current has the most significant impact on the harmonic distortion of the bus voltage of the commutator converter in the subsequent commutation failure stage, and the harmonic distortion rate of the bus voltage reaches its peak in this stage. In the first commutation failure stage and the recovery stage, the harmonic distortion is dominated by the fault voltage drop. In the recovery stage after the subsequent commutation failure, the amplitude of the fault inrush current decreases, and the harmonic distortion rate of the bus voltage gradually decreases.

[0014] Furthermore, the criteria for determining commutation failure in the hybrid cascaded DC system include: Determine whether the actual turn-off angle during the commutation process of the grid-commutated converter is less than 7°: when the actual turn-off angle is less than 7°, it is determined that the grid-commutated converter on the inverter side has a risk of commutation failure; when the actual turn-off angle is greater than or equal to 7°, it is determined that the grid-commutated converter on the inverter side does not have a risk of commutation failure.

[0015] Furthermore, when predicting the risk of subsequent commutation failure of the grid-commutated converter on the inverter side of the hybrid cascaded DC system, the change in system electrical quantities of the MMC converter transformer under ideal unsaturated conditions should be used as a reference. If no subsequent commutation failure occurs in the grid-commutated converter on the inverter side of the hybrid cascaded DC system under ideal unsaturated conditions, but the grid-commutated converter meets the commutation failure judgment conditions under saturated conditions, then it is determined that the grid-commutated converter on the inverter side of the hybrid cascaded DC system has the risk of subsequent commutation failure, and the fault inrush current is the inducing factor for subsequent commutation failure.

[0016] Furthermore: the control strategy of the converter on the rectifier side adopts the constant current control strategy of the grid-commutated converter; the control strategy of the converter of the high-end valve group on the inverter side adopts the constant voltage control strategy or constant turn-off angle control strategy of the grid-commutated converter; and the control strategy of the converter of the low-end valve group adopts a combination of the constant DC voltage control strategy and the constant active power control strategy of the modular multilevel converter.

[0017] The technical solution of this application achieves the following beneficial effects: This application's method for predicting subsequent commutation failures in hybrid cascaded DC systems, considering inrush currents in the MMC converter transformer, breaks through the limitations of traditional commutation failure analysis that attributes subsequent commutation failures in the LCC solely to voltage dips in the converter bus. For the first time, it incorporates the electrical coupling between the inverter-side LCC and MMC, and the correlation between inrush currents in the MMC converter transformer and subsequent commutation failures into the prediction system. It fully reveals the cascading mechanism by which inrush currents induce harmonic distortion of the bus voltage, leading to a decrease in the LCC turn-off angle and triggering subsequent commutation failures, filling a theoretical gap in the prediction of this type of fault in hybrid cascaded DC systems. This method, by introducing saturation characteristic analysis of the MMC converter transformer, establishing a quantitative correlation between the amplitude of the inrush current and the harmonic distortion rate of the LCC converter bus voltage, and using the ideal unsaturated state of the MMC converter transformer as a benchmark, can accurately pinpoint inrush currents as the core inducing factor for subsequent commutation failures, effectively distinguishing between the initial commutation failure caused by voltage dips and subsequent commutation failures caused by inrush currents. This method avoids omissions and misjudgments during the prediction process. Furthermore, by combining the development of commutation failure with a phased risk analysis and using a turn-off angle threshold, it quantifies and assesses the risk of commutation failure in stages. This clearly presents the timing and development trend of subsequent commutation failures, making the prediction results more relevant for engineering applications. In addition, the method clarifies the impact of the saturation characteristics of the MMC converter transformer on the risk of subsequent commutation failures. This provides a theoretical basis for the selection of MMC converter transformers and the optimization of protection configurations in hybrid cascaded DC systems. It also addresses the induction path from fault inrush current to commutation failure, providing a basis for the design of subsequent fault inrush current suppression and bus voltage harmonic control strategies. This allows for accurate prediction of subsequent commutation failure risks in advance, providing technical support for maintenance personnel to prevent and respond to emergencies. It effectively avoids serious consequences such as DC blocking and power transmission interruptions caused by subsequent commutation failures, significantly improving the safety, stability, and reliability of the hybrid cascaded DC system and its supporting AC / DC grid. Attached Figure Description

[0018] Figure 1 This is a flowchart of a method for predicting subsequent commutation failures in a hybrid cascaded DC system that considers MMC fault inrush current in an embodiment of this application.

[0019] Figure 2 This is a topology diagram of the hybrid cascaded DC system in the embodiments of this application.

[0020] Figure 3 This is a flowchart illustrating the control strategy of the converter in the hybrid cascaded DC system according to an embodiment of this application. Figure 3 (a) shows the constant current control strategy diagram of the LCC on the rectifier side; Figure 3 Figure (b) shows the constant voltage control strategy or constant turn-off angle control strategy of the LCC on the inverter side; Figure 3 (c) shows the DC voltage control strategy diagram for the MMC1 on the inverter side; Figure 3The diagram in (d) shows the constant active power control strategy of the inverter MMC2 / 3.

[0021] Figure 4 The converter transformers in the embodiments of this application are different Schematic diagram of characteristic curves.

[0022] Figure 5 This is a schematic diagram of the fault current loop of the MMC1 bridge arm after locking in an embodiment of this application.

[0023] Figure 6 This is a schematic diagram of the inrush current failure of the MMC1 converter transformer in the embodiments of this application.

[0024] Figure 7 This is a schematic diagram of commutation from thyristor VT3 to VT5 in an embodiment of this application.

[0025] Figure 8 This is a schematic diagram of the commutation voltage area in an embodiment of this application. Figure 8 (a) is a schematic diagram of the phase AC voltage waveform; Figure 8 (b) is a schematic diagram of a normal commutation process; Figure 8 (c) is a schematic diagram of the commutation process considering harmonics.

[0026] Figure 9 This is a waveform diagram illustrating the commutation failure development in an embodiment of this application.

[0027] Figure 10 This is a diagram showing the amplitude variation of the 2nd to 7th harmonics of the faulty inrush flow in the embodiments of this application.

[0028] Figure 11 Different in the embodiments of this application Plot showing the change in LCC turn-off angle under the characteristic curve.

[0029] Figure 12 This is a waveform diagram showing the commutation failure development under an ideal commutator transformer in the embodiments of this application.

[0030] Figure 13 Examples of embodiments in this application The graph shows the changes in fault inrush current and LCC turn-off angle under characteristic curve 1. Figure 13 (a) is Graph showing the inrush flow variation due to commutation transformer failure under characteristic curve 1; Figure 13 (b) is Characteristic curve 1 shows the change in the LCC turn-off angle of the inverter.

[0031] Figure 14 Examples of embodiments in this application The graph shows the changes in fault inrush current and LCC turn-off angle under characteristic curve 2. Figure 14 (a) is Characteristic curve 2 shows the change in inrush flow due to commutation transformer failure; Figure 14 (b) is Characteristic curve 2 shows the change in the LCC turn-off angle of the inverter.

[0032] Figure 15 Examples of embodiments in this application The graph shows the changes in fault inrush current and LCC turn-off angle under characteristic curve 3. Figure 15 (a) is Characteristic curve 3 shows the change in inrush flow due to commutation transformer failure. Figure 15 (b) is Characteristic curve 3 shows the change in the LCC turn-off angle of the inverter.

[0033] Figure 16 This is a structural diagram of a subsequent commutation failure prediction device for a hybrid cascaded DC system considering MMC fault inrush current in an embodiment of this application.

[0034] Figure 17 This is a structural diagram of an electronic device according to an embodiment of this application. Detailed Implementation

[0035] The present application will now be further described with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present application and should not be construed as limiting the scope of protection of the present application. It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of the present application.

[0036] Specifically, such as Figure 1As shown in Embodiment 1, this invention discloses a method for predicting subsequent commutation failures in a hybrid cascaded high-voltage direct current (HC-HVDC) system considering MMC inrush current. The rectifier side of the HC-HVDC system employs conventional DC technology, such as a line commutated converter (LCC), a traditional high-voltage DC converter technology based on semi-controlled thyristors. It relies on the AC grid voltage for current commutation, consuming significant reactive power and posing a risk of commutation failure. Therefore, it requires large reactive power compensation and filtering devices in engineering applications and is commonly used for high-capacity, long-distance power transmission. The inverter side of the HC-HVDC system combines conventional DC and flexible DC technologies. Specifically, its high-end valve group employs conventional DC technology, such as a line commutated converter. The low-end valve group uses multiple modular multilevel converters connected in parallel, which are then connected in series with the grid-commutated converter of the high-end valve group on the DC side. The modular multilevel converter (MMC) is an advanced voltage source converter used in high-voltage direct current transmission. Its core is the synthesis of high-quality multilevel AC waveforms through the series connection of numerous standard sub-modules, offering advantages such as low harmonics, strong scalability, and flexible control. This structure combines the large capacity and low loss of the LCC with the flexible control, strong grid support capability, and no risk of commutation failure of the MMC.

[0037] The hybrid cascaded DC system described in this embodiment is equipped with a converter transformer, which is the core equipment of the UHVDC transmission system and is located between the AC grid and the converter valve. It undertakes the core functions of voltage transformation, electrical isolation, and limiting short-circuit current. This converter transformer typically consists of one Y / Y connected transformer and one Y / D connected transformer operating in parallel to form a 12-pulse converter transformer group. The two converter transformers in a group are always switched on and off simultaneously. Therefore, in addition to configuring separate differential protection (small differential protection) for each of the two transformers in the group as the main protection, a large differential protection is also configured for this group of converter transformers. Currently, both the large and small differential protection of the converter transformer in engineering practice adopt differential protection based on the second harmonic braking principle. In this hybrid cascaded DC system, the converter bridge directly connected to the converter transformer valve side is a nonlinear element, and the converter valve has unidirectional conduction. This causes the fault current to be biased to one side of the time axis after a fault occurs on the converter transformer valve side. This current flows through the converter transformer, causing the current on the converter transformer valve side to be biased to one side of the time axis, resulting in a DC component. This changes the operating point of the converter transformer core and generates a certain amount of inrush current.

[0038] Specifically, this embodiment predicts subsequent commutation failures in a hybrid cascaded DC system, taking into account the inrush current caused by the failure of the MMC converter transformer. The specific method includes: First, control strategies for the converters on the rectifier and inverter sides are obtained based on the topology of the hybrid cascaded DC system.

[0039] like Figure 2 As shown, this is the topology diagram of the hybrid cascaded DC system in this embodiment, and the control strategies for each converter on the rectifier side and inverter side are as follows: Figure 3 As shown. The control strategy for the converter on the rectifier side adopts the constant current control strategy for grid-commutated converters, such as... Figure 3 As shown in (a), the control strategy for the inverter side high-end valve group converter adopts either the grid-commutated converter constant voltage control strategy or the constant turn-off angle control strategy, as shown in Figure (a). Figure 3 As shown in (b). The low-end valve group is equipped with three modular multilevel converters, namely MMC1, MMC2, and MMC3. Among them, the control strategy of MMC1 adopts the modular multilevel converter constant DC voltage control strategy, such as... Figure 3 As shown in (c). The control strategy of MMC2 / 3 adopts a modular multilevel converter constant active power control strategy, such as... Figure 3 As shown in (d).

[0040] Furthermore, in this embodiment, when obtaining the control strategies for the converters corresponding to the rectifier and inverter sides, the UMEC model can also be used. By setting different volt-ampere characteristic curves, different saturation characteristics of the MMC converter transformer can be simulated, such as... Figure 4 As shown, under the same operating conditions, the converter transformer is more prone to saturation under characteristic curve 1, followed by characteristic curve 2 and characteristic curve 3.

[0041] Subsequently, based on the converter control strategy, the generation mechanism of fault inrush current in the MMC converter transformer in the hybrid cascaded DC system was analyzed, and the correlation between the fault inrush current amplitude and the saturation characteristics of the MMC converter transformer was clarified.

[0042] Specifically, in combination Figure 2 As shown, this embodiment analyzes the inrush current generation mechanism of the MMC converter transformer in a hybrid cascaded DC system when a single-phase ground fault occurs at point K1 on the valve side of the MMC1 converter transformer. The mechanism includes: after a fault occurs on the valve side of the MMC1 converter transformer, the bridge arm current rapidly increases. Approximately 3-5 ms later, the IGBT is blocked due to overcurrent. The topology diagram of the upper and lower bridge arm submodules of MMC1 after blocking is shown below. Figure 4 As shown in the dashed box, it consists of two diodes and a capacitor connected in series. Furthermore, after IGBT overcurrent lockout, the schematic diagram of the fault current loop for the upper and lower arms of the MMC is as follows. Figure 5 As shown, after the upper bridge arm capacitor is charged to its peak value, there is no current loop in the upper bridge arm; the fault phase current in the lower bridge arm slowly decays to zero due to the presence of bridge arm inductance and resistance; the diodes in the non-fault phases conduct when their cathode voltage is negative, and the current with a large DC component flows into the MMC converter transformer valve side. At this time, the expression for the converter transformer core flux linkage is as follows: ; in: ; ; ; ; In the above formula This indicates the flux linkage in the commutator core. This represents the change in flux linkage of the commutator core with time t. For the flux linkage amplitude of the converter transformer core, To compensate for the commutator excitation branch voltage, I n Rated current, and These are the magnetizing resistor and the magnetizing inductor, respectively. L For bridge arm inductance, U This is the effective value of the valve side line voltage. and The phase of the line voltage. The equivalent phase angle of the power supply potential at the time of the fault. k This represents the shunting coefficient of the AC component flowing into the converter transformer in the excitation branch. Represents the equivalent AC voltage amplitude. Represents the initial phase angle. This represents the corrected voltage phase angle.

[0043] According to the above formula, a fault on the valve side of the MMC1 converter transformer will cause the continuous accumulation of magnetic flux in the core, leading to the core operating point entering the saturation region and thus generating a fault inrush current. At this time, the fault inrush current of the MMC1 converter transformer is as follows: Figure 6 As shown. And combined with Figure 4 and Figure 6 The higher the saturation level of the MMC converter, the greater the amplitude of the fault inrush current.

[0044] Next, based on the electrical coupling between the high-end valve group and the low-end valve group on the inverter side, the law of harmonic distortion of the grid commutator bus voltage on the inverter side caused by the fault inrush current of the MMC converter transformer is analyzed, and the correlation between the amplitude of the fault inrush current of the MMC converter transformer and the harmonic distortion rate of the grid commutator bus voltage is established.

[0045] In this embodiment, Figure 7 Taking the commutation from thyristor VT3 to thyristor VT5 as an example, the diagram shows... It is direct current. and These represent the current flowing through converter valves VT3 and VT5, respectively.L c To replace the equivalent inductance of the current transformer, L d It is a DC-side smoothing inductor.

[0046] Combination Figure 7 The commutation process, according to Kirchhoff's laws, can be obtained as follows: ; ; Combining the above formula, we can obtain: ; Set the commutation start time as ,and , The thyristor firing angle is; the commutation end time is... ,and , This represents the thyristor turn-off angle; by integrating both sides of the above equation simultaneously, combined with the commutation process... , We can obtain: ; Because the commutation process is short and a large smoothing inductor is installed on the DC side, this embodiment sets the DC current at the end of the commutation to be the same as the current at the beginning of the commutation. At this point, we can obtain: ; According to the above formula, the commutation voltage-time area required for the commutation process is... DC current at the start of commutation Decide.

[0047] In this embodiment, when the hybrid cascaded DC system is operating normally, the AC line voltage of the LCC converter bus contains only the fundamental component, which satisfies the following equation: ; in E 1 represents the fundamental line voltage of the AC system.

[0048] The time area provided by the commutation voltage As shown below: ; If considered n The influence of subharmonics, generally n If ≥2, then the time area provided by the commutation voltage is... Represented as: ; in E n forn Second harmonic voltage amplitude; for n Second harmonic phase angle.

[0049] In this embodiment, to ensure successful commutation, it is necessary to ensure the commutation voltage-time area required for the commutation process. Not greater than the time area provided by the commutation voltage And the minimum shut-off angle is That is, satisfying the following formula: ; Combination Figure 8 The analysis of the above process reveals that, due to harmonic distortion, voltage waveform zero-crossing point shifts forward and voltage amplitude decreases, thereby affecting the commutation overlap angle. Increase, shut-off angle Reduced, or even severe <7°, which leads to commutation failure.

[0050] Furthermore, in analyzing the law of harmonic distortion of the bus voltage of the grid commutator on the inverter side caused by the fault inrush current of the MMC converter transformer, this embodiment divides the process from the occurrence of the fault to the stabilization of various electrical quantities into four stages according to the size of the turn-off angle: the first commutation failure stage, the recovery stage after the first commutation failure, the subsequent commutation failure stage, and the recovery stage after the subsequent commutation failure.

[0051] like Figure 9 As shown, stage 1, i.e. t 11 ~ t 22 This is the initial commutation failure stage. t At time 0, a single-phase ground fault occurred on the valve side of the MMC1 converter transformer, causing a voltage drop on the LCC converter bus. t 11 The first commutation failure occurs at a certain moment.

[0052] Phase 2, namely t 22 ~t 33 This is the recovery phase I after the initial commutation failure. Following the initial commutation failure, due to the system's self-regulation... t 22 The system enters the recovery phase, and the converter valve resumes normal commutation.

[0053] Phase 3, namely t 33 ~t 44 This is the subsequent commutation failure stage. When subsequent commutation failure occurs, the DC current value is 1. puAround the same time, the voltage of the LCC converter bus on the inverter side is close to that during normal operation, therefore and The voltage waveform was close to that during normal operation, but there was obvious distortion in the converter bus voltage waveform, and the system subsequently failed to commutate.

[0054] Phase 4, namely t 44 This is followed by recovery phase II after a subsequent commutation failure. After the subsequent commutation failure is completed, the system enters the recovery phase again.

[0055] Furthermore, this embodiment analyzes the impact of fault inrush current on bus voltage harmonic distortion by combining the electrical quantity characteristics of each stage: The transformer inrush current contains a large number of harmonics, with the second harmonic being the most abundant. Fault inrush current and excitation inrush current are essentially the same, both being "excitation currents" caused by transformer saturation. Based on this embodiment and the expression for the converter transformer core flux linkage, the expression for fault inrush current is as follows: ; in For saturated magnetic flux; This represents the slope of the saturation segment of the excitation curve.

[0056] Combination Figure 10 As shown, the inrush current in this embodiment also contains a large number of harmonics, namely the amplitudes of the 2nd to 7th harmonics. Figure 10 It can be seen that the second harmonic amplitude is relatively high. Combined with... Figure 2 As can be seen, there is an electrical connection between the inverter-side LCC and MMC1 in this embodiment. Therefore, the fault inrush current of the MMC1 converter transformer will cause the harmonic content of the inverter-side LCC converter bus voltage to rise, resulting in waveform distortion, which in turn will have an adverse effect on the commutation process.

[0057] Furthermore, in this embodiment, considering only the 2nd to 7th harmonics, the harmonic distortion rate of the LCC converter bus voltage can be calculated using the following formula: ; In the above formula U 1 represents the amplitude of the fundamental voltage at the LCC converter bus. U n For n Secondary harmonic voltage amplitude.

[0058] Because the converter transformer is more prone to saturation, the larger its fault inrush current amplitude, the greater its impact on the LCC converter bus voltage.

[0059] In this embodiment, the MMC1 converter uses different... Under the characteristic curve, the inverter-side LCC turn-off angle is as follows: Figure 11 As shown. The MMC1 converter transformer uses... Under characteristic curves 1, 2, and 3, subsequent commutation failures occurred in the LCC on the inverter side.

[0060] Furthermore, in this embodiment, when the MMC1 converter transformer is in an ideal state, i.e., saturation will not occur, the time period from the occurrence of a fault to the stabilization of electrical quantities is divided into two stages based on the magnitude of the LCC turn-off angle on the inverter side: the first commutation failure stage (…). ) and the recovery phase after the first commutation failure ( Afterwards, the electrical quantities of the inverter-side LCC are as follows: Figure 12 As shown. t 33 for Figure 9 The figure shows the moment when subsequent commutation failure occurs in the inverter-side LCC under MMC1 converter transformer saturation conditions. At this time, under ideal converter transformer conditions, the converter bus voltage of the inverter-side LCC is close to that under normal operation, and the waveform has no obvious distortion. The DC current value is around 1. pu Left and right, the system does not experience subsequent commutation failures.

[0061] Each of the embodiments in this example Characteristic curves and, under ideal conditions, the amplitude of the fault-induced inrush current of the MMC1 converter transformer and the harmonic distortion rate of the LCC converter bus voltage at the moment of subsequent commutation failure. As shown in Table 1:

[0062] As shown in Table 1, the more easily the MMC1 converter transformer saturates, the larger its fault inrush current amplitude. This, in turn, affects the harmonic distortion rate of the LCC converter bus voltage through the coupling relationship between the MMC converter and the LCC inverter. The larger the voltage, the more severe the voltage distortion of the LCC converter bus, and the more prone the hybrid cascaded DC transmission system is to subsequent commutation failures. Therefore, in this embodiment, the fault inrush current has the most significant impact on the harmonic distortion of the grid commutator converter bus voltage during the subsequent commutation failure stage, and the bus voltage harmonic distortion rate reaches its peak during this stage. In the initial commutation failure stage and the recovery stage, the harmonic distortion impact is dominated by the fault voltage drop. In the recovery stage after subsequent commutation failures, the amplitude of the fault inrush current decreases, and the bus voltage harmonic distortion rate gradually decreases.

[0063] Subsequently, the influence of harmonic distortion of the bus voltage of the grid commutator on the commutation process of the grid commutator was analyzed, and the criteria for determining commutation failure of the hybrid cascaded DC system were determined.

[0064] Specifically, in this embodiment, harmonic distortion of the bus voltage of the grid-commutated converter causes the zero-crossing point of the commutation voltage waveform to shift forward and the voltage amplitude to decrease, resulting in an increase in the commutation overlap angle and a decrease in the turn-off angle of the grid-commutated converter. In this embodiment, to ensure successful commutation, it is necessary to ensure the required commutation voltage time area during the commutation process. Not greater than the time area provided by the commutation voltage And the minimum shut-off angle is That is, the turn-off angle must be greater than or equal to 7°. If the time area provided by the commutation voltage cannot meet the commutation process requirements of the grid commutation converter, the risk of commutation failure will increase.

[0065] Therefore, in this embodiment, the commutation failure determination condition of the hybrid cascaded DC system can be set as follows: determine whether the actual turn-off angle of the grid commutation converter during the commutation process is less than 7°: when the actual turn-off angle is less than 7°, it is determined that the grid commutation converter on the inverter side has the risk of commutation failure; when the actual turn-off angle is greater than or equal to 7°, it is determined that the grid commutation converter on the inverter side does not have the risk of commutation failure.

[0066] Furthermore, in this embodiment, since the fault inrush current contains 2nd to 7th harmonic components, the amplitude of the 2nd harmonic component is the highest; and the amplitude of the fault inrush current is positively correlated with the harmonic distortion rate of the grid commutator bus voltage, the larger the amplitude of the fault inrush current, the higher the harmonic distortion rate of the grid commutator bus voltage, and the more severe the voltage waveform distortion.

[0067] Finally, based on the fault inrush current amplitude of the MMC converter under different saturation characteristics, the harmonic distortion rate of the grid commutator bus voltage, and the commutation failure judgment conditions, the risk of subsequent commutation failure of the grid commutator on the inverter side of the hybrid cascaded DC system is predicted.

[0068] In this embodiment, when predicting the risk of subsequent commutation failure of the grid-commutated converter on the inverter side of the hybrid cascaded DC system, the change of system electrical quantities of the MMC converter transformer under ideal unsaturated state is used as a reference. If no subsequent commutation failure occurs in the grid-commutated converter on the inverter side of the hybrid cascaded DC system under ideal unsaturated state, and the grid-commutated converter meets the commutation failure judgment condition under saturated state, then it is determined that the grid-commutated converter on the inverter side of the hybrid cascaded DC system has the risk of subsequent commutation failure, and the fault inrush current is the inducing factor for subsequent commutation failure.

[0069] Combination Figures 13-15 As shown, in this embodiment, the converter transformer is in different... Different saturation characteristics under different curve parameters result in different fault inrush amplitudes. The effects of different current types on subsequent commutation failures vary. Generally speaking, the more easily the converter transformer saturates, the larger its fault inrush current amplitude, and the more likely it is to experience subsequent commutation failures.

[0070] Furthermore, such as Figure 16 As shown, this embodiment also discloses a subsequent commutation failure prediction device for a hybrid cascaded DC system considering MMC inrush current. The rectifier side of the hybrid cascaded DC system uses a grid-commutated converter, the high-side valve group on the inverter side uses a grid-commutated converter, and the low-side valve group uses multiple modular multilevel converters connected in parallel and then connected in series with the high-side valve group on the DC side. The prediction device includes: The parameter acquisition module is used to obtain the control strategies of the converters corresponding to the rectifier side and the inverter side according to the topology of the hybrid cascaded DC system. The first correlation analysis module is used to analyze the fault inrush current generation mechanism of MMC converter transformer in hybrid cascaded DC system according to the converter control strategy, and to clarify the correlation between the fault inrush current amplitude and the saturation characteristics of MMC converter transformer; The second correlation analysis module is used to analyze the law of harmonic distortion of the bus voltage of the grid commutator on the inverter side caused by the fault inrush current of the MMC converter transformer based on the electrical coupling relationship between the high-end valve group and the low-end valve group on the inverter side, and to establish the correlation between the amplitude of the fault inrush current of the MMC converter transformer and the harmonic distortion rate of the bus voltage of the grid commutator. The judgment condition determination module is used to analyze the impact of harmonic distortion of the bus voltage of the grid commutator on the commutation process of the grid commutator and determine the judgment conditions for commutation failure of the hybrid cascaded DC system. The predictive analysis module is used to predict the risk of subsequent commutation failure of the grid commutator on the inverter side of the hybrid cascaded DC system based on the fault inrush current amplitude of the MMC converter transformer under different saturation characteristics, the harmonic distortion rate of the grid commutator bus voltage, and the judgment conditions for commutation failure.

[0071] The apparatus provided in this application embodiment can achieve... Figure 1 To avoid repetition, the various processes implemented in the method embodiments will not be described again here.

[0072] like Figure 17 As shown in the illustration, this application also provides an electronic device, including a processor and a memory. The processor and memory are communicatively connected to a bus interface. A program or instruction stored in the memory and executable on the processor is provided. When the program or instruction is executed by the processor, it implements the following: Figure 1 The various processes of the method embodiments shown are all capable of achieving the same technical effect, and will not be described again here to avoid repetition.

[0073] This application embodiment also provides a readable storage medium storing a program or instructions that, when executed by a processor, implement the above-described functionality. Figure 1 The various processes described in the embodiments of the method described herein can achieve the same technical effect, and will not be repeated here to avoid repetition.

[0074] This application also provides a computer program product, including computer instructions, which, when executed by a processor, implement the above-described... Figure 1 The various processes described in the embodiments of the method described herein can achieve the same technical effect, and will not be repeated here to avoid repetition.

[0075] It should be understood that the phrase "one embodiment" or "an embodiment" throughout the specification means that a specific feature, structure, or characteristic related to the embodiment is included in at least one embodiment of this application. Therefore, "in one embodiment" or "in an embodiment" appearing throughout the specification does not necessarily refer to the same embodiment. Furthermore, these specific features, structures, or characteristics can be combined in any suitable manner in one or more embodiments. It should be understood that in the various embodiments of this application, the sequence numbers of the above-described processes do not imply a sequential order of execution; the execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application. The sequence numbers of the above-described embodiments are merely descriptive and do not represent the superiority or inferiority of the embodiments.

[0076] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element.

[0077] In the several embodiments provided in this application, it should be understood that the disclosed devices and methods can be implemented in other ways. The device embodiments described above are merely illustrative. For example, the division of units is only a logical functional division, and in actual implementation, there may be other division methods, such as: multiple units or components may be combined, or integrated into another device, or some features may be ignored or not executed. In addition, the coupling, direct coupling, or communication connection between the various components shown or discussed may be through some interfaces, and the indirect coupling or communication connection between devices or units may be electrical, mechanical, or other forms.

[0078] The units described above as separate components may or may not be physically separate. The components shown as units may or may not be physical units. They may be located in one place or distributed across multiple network units. Some or all of the units may be selected to achieve the purpose of this embodiment according to actual needs.

[0079] In addition, each functional unit in the various embodiments of this application can be integrated into one processing unit, or each unit can be a separate unit, or two or more units can be integrated into one unit; the integrated unit can be implemented in hardware or in the form of hardware plus software functional units.

[0080] Those skilled in the art will understand that all or part of the steps of the above method embodiments can be implemented by hardware related to program instructions. The aforementioned program can be stored in a computer-readable storage medium. When the program is executed, it performs the steps of the above method embodiments. The aforementioned storage medium includes various media that can store program code, such as mobile storage devices, read-only memory (ROM), magnetic disks, or optical disks.

[0081] Alternatively, if the integrated units described above are implemented as software functional modules and sold or used as independent products, they can also be stored in a computer-readable storage medium. Based on this understanding, the technical solutions of the embodiments of this application, or the parts that contribute to the prior art, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a device (which may be a terminal or platform, etc.) to execute all or part of the methods described in the various embodiments of this application. The aforementioned storage medium includes various media capable of storing program code, such as mobile storage devices, ROMs, magnetic disks, or optical disks.

[0082] The above description is only a preferred embodiment of this application. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of this application, and these improvements and modifications should also be considered within the scope of protection of this application.

Claims

1. A method for predicting subsequent commutation failure in a hybrid cascaded DC system considering MMC inrush current, wherein the rectifier side of the hybrid cascaded DC system employs a grid-commutated converter, the high-side valve group on the inverter side of the hybrid cascaded DC system employs a grid-commutated converter, and the low-side valve group employs multiple modular multilevel converters connected in parallel and then connected in series with the high-side valve group on the DC side, characterized in that, The method includes: Based on the topology of the hybrid cascaded DC system, the control strategies for the converters corresponding to the rectifier side and the inverter side are obtained; Based on the converter control strategy, the generation mechanism of fault inrush current of MMC converter transformer in the hybrid cascaded DC system is analyzed, and the correlation between the fault inrush current amplitude and the saturation characteristics of the MMC converter transformer is clarified. Based on the electrical coupling between the high-end valve group and the low-end valve group on the inverter side, the law of harmonic distortion of the bus voltage of the grid commutator on the inverter side caused by the fault inrush current of the MMC converter transformer is analyzed, and the correlation between the amplitude of the fault inrush current of the MMC converter transformer and the harmonic distortion rate of the bus voltage of the grid commutator is established. The influence of the harmonic distortion of the bus voltage of the grid commutator on the commutation process of the grid commutator is analyzed, and the judgment conditions for commutation failure of the hybrid cascaded DC system are determined. Based on the fault inrush current amplitude of the MMC converter transformer under different saturation characteristics, the harmonic distortion rate of the grid commutator bus voltage, and the commutation failure judgment conditions, the risk of subsequent commutation failure of the grid commutator on the inverter side of the hybrid cascaded DC system is predicted.

2. The method for predicting subsequent commutation failure in a hybrid cascaded DC system considering MMC inrush current as described in claim 1, characterized in that, The fault inrush current generation mechanism of the MMC converter transformer in the hybrid cascaded DC system includes: after a single-phase ground fault occurs on the valve side of the MMC converter transformer, the bridge arm current increases sharply, causing the IGBT to be blocked due to overcurrent. The fault phase current contains a large DC component and flows into the valve side of the MMC converter transformer, which leads to the continuous accumulation of magnetic flux in the core of the MMC converter transformer, causing the core operating point to enter the saturation region, thereby generating fault inrush current.

3. The method for predicting subsequent commutation failure in a hybrid cascaded DC system considering MMC inrush current as described in claim 1, characterized in that, When obtaining the control strategies for the converters corresponding to the rectifier and inverter sides, the UMEC model is also required. Different volt-ampere characteristic curves are set to simulate the different saturation characteristics of the MMC converter transformer.

4. The method for predicting subsequent commutation failure in a hybrid cascaded DC system considering MMC inrush current as described in claim 1, characterized in that, When analyzing the fault inrush current generation mechanism of the MMC converter transformer in the hybrid cascaded DC system based on the converter control strategy, the higher the saturation degree of the MMC converter transformer, the greater the amplitude of the fault inrush current.

5. The method for predicting subsequent commutation failure in a hybrid cascaded DC system considering MMC inrush current as described in claim 1, characterized in that, The fault inrush current contains 2nd to 7th harmonic components, with the 2nd harmonic component having the highest amplitude. The amplitude of the fault inrush current is positively correlated with the harmonic distortion rate of the grid commutator bus voltage. The larger the amplitude of the fault inrush current, the higher the harmonic distortion rate of the grid commutator bus voltage and the more severe the voltage waveform distortion.

6. The method for predicting subsequent commutation failure in a hybrid cascaded DC system considering MMC inrush current as described in claim 1, characterized in that, The impact of the harmonic distortion of the bus voltage of the grid-commutated converter on the commutation process of the grid-commutated converter includes: The harmonic distortion of the bus voltage of the grid commutation converter will cause the zero-crossing point of the commutation voltage waveform of the grid commutation converter to shift forward and the voltage amplitude to decrease. This will increase the commutation overlap angle and decrease the turn-off angle of the grid commutation converter. The time area provided by the commutation voltage cannot meet the commutation process requirements of the grid commutation converter, and the risk of commutation failure will increase.

7. The method for predicting subsequent commutation failure in a hybrid cascaded DC system considering MMC inrush current as described in claim 1, characterized in that, When analyzing the law of harmonic distortion of the bus voltage of the grid commutator on the inverter side caused by the fault inrush current of the MMC converter transformer, the process from the occurrence of the fault to the stabilization of various electrical quantities is divided into four stages according to the size of the turn-off angle: the first commutation failure stage, the recovery stage after the first commutation failure, the subsequent commutation failure stage, and the recovery stage after the subsequent commutation failure. The influence of fault inrush current on harmonic distortion of bus voltage is analyzed by combining the electrical quantity characteristics of each stage: the fault inrush current has the most significant impact on the harmonic distortion of the bus voltage of the commutator converter in the subsequent commutation failure stage, and the harmonic distortion rate of the bus voltage reaches its peak in this stage. In the first commutation failure stage and the recovery stage, the harmonic distortion is dominated by the fault voltage drop. In the recovery stage after the subsequent commutation failure, the amplitude of the fault inrush current decreases, and the harmonic distortion rate of the bus voltage gradually decreases.

8. The method for predicting subsequent commutation failure in a hybrid cascaded DC system considering MMC inrush current as described in claim 1, characterized in that, The criteria for determining commutation failure in the hybrid cascaded DC system include: Determine whether the actual turn-off angle during the commutation process of the grid-commutated converter is less than 7°: when the actual turn-off angle is less than 7°, it is determined that the grid-commutated converter on the inverter side has a risk of commutation failure; when the actual turn-off angle is greater than or equal to 7°, it is determined that the grid-commutated converter on the inverter side does not have a risk of commutation failure.

9. The method for predicting subsequent commutation failure in a hybrid cascaded DC system considering MMC inrush current as described in claim 1, characterized in that, When predicting the risk of subsequent commutation failure of the grid-commutated converter on the inverter side of the hybrid cascaded DC system, the change of system electrical quantities of the MMC converter transformer under ideal unsaturated conditions should be used as a reference. If no subsequent commutation failure occurs in the grid-commutated converter on the inverter side of the hybrid cascaded DC system under ideal unsaturated conditions, and the grid-commutated converter meets the commutation failure judgment conditions under saturated conditions, then it is determined that the grid-commutated converter on the inverter side of the hybrid cascaded DC system has the risk of subsequent commutation failure, and the fault inrush current is the inducing factor for subsequent commutation failure.

10. The method for predicting subsequent commutation failure in a hybrid cascaded DC system considering MMC inrush current as described in claim 1, characterized in that, The control strategy for the converter on the rectifier side adopts the constant current control strategy of the grid-commutated converter; the control strategy for the converter of the high-end valve group on the inverter side adopts the constant voltage control strategy or constant turn-off angle control strategy of the grid-commutated converter; and the control strategy for the converter of the low-end valve group adopts a combination of the constant DC voltage control strategy and the constant active power control strategy of the modular multilevel converter.

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