Hybrid series converter valve topological structure based on CCM and control method thereof

Abstract

The invention discloses a CCM-based hybrid series converter valve topological structure and a control method, and belongs to a high-voltage direct-current power transmission technology. According to the converter valve topological structure, a controllable capacitor converter valve topological structure based on hybrid series connection of a half-control device and a full-control device is adopted, and the topology flexibly switches a capacitor by controlling on and off of the full-control device so as to increase the commutation voltage time area and accelerate the commutation process, and the capability of resisting commutation failure is improved. Meanwhile, in order to improve the adaptive suppression capability of the topological face to the fault depth of the alternating current system, the invention provides a control method for dynamically adjusting the turn-off time sequence of a device based on real-time alternating current voltage measurement, and the commutation failure risk is effectively reduced.

Description

Technical Field

[0001] The present invention belongs to a high-voltage direct current (HVDC) power distribution system control technology, and in particular relates to a CCM-based hybrid series converter valve topology structure and a control method thereof. Background Art

[0002] Line commutated converter-based high voltage direct current (LCC-HVDC) transmission technology, with its advantages of long distance, large capacity, and low losses, has become a core means of transmitting renewable energy power. With the advancement of the "dual carbon" initiative, the high penetration of renewable energy has led to reduced system strength and weakened voltage support, significantly exacerbating the risk of commutation failure in LCC-HVDC. Commutation failures not only cause DC power interruptions but also trigger transient overvoltages and power angle instability. In weak grid coupling scenarios, they can easily evolve into continuous commutation failures, seriously threatening the safe and stable operation of new power systems. Traditional converter valves rely on the AC system for commutation voltage support and, due to device physical characteristics, cannot actively intervene in the shutdown process. Therefore, research is urgently needed on the design of hybrid series converter valve topologies with active commutation failure mitigation capabilities. By incorporating fully controlled devices and establishing a dynamic commutation compensation mechanism, these technologies can retain the advantages of traditional HVDC transmission while overcoming the inherent limitations of conventional LCC. It has great engineering value and technical significance for improving the independence of the receiving-end converter station in fault conditions, enhancing the ability of the HVDC system to resist commutation faults, and improving the transient performance of the overall system.

[0003] To address commutation failure in LCC-HVDC systems, existing research focuses on three key areas: AC voltage regulation, commutation failure prediction (CF prediction), and converter valve topology design. AC voltage regulation improves the AC system's voltage support capability by deploying dynamic reactive power compensation devices, but large-scale equipment deployment is costly and has limited adaptability to grid fault scenarios. Commutation failure prediction relies on real-time monitoring of electrical parameters and provides fault warnings through algorithms or transient feature extraction, but its scope of defense is limited by the accuracy and response speed of the prediction model. Converter valve topology design, by improving hardware structure and optimizing the commutation process, offers advantages such as strong proactive intervention capabilities and controllable economics, making it a current research hotspot.

[0004] Currently, research in converter valve topology design focuses on two main areas: thyristor-based converter valve topology design and fully controlled device-based converter valve topology design. Among thyristor-based converter valve topology designs, existing technologies include capacitor-commutated converter valves embedded in anti-parallel thyristor bidirectional full-bridge modules. These valve-side parallel capacitors achieve commutation voltage compensation, significantly improving shutdown margin. Innovative valve-side converter transformer structures have led to the design of novel topologies with DC current suppression capabilities. Another example is a modular thyristor-based cascade converter valve. This utilizes cascaded thyristor commutation modules and a modular design with capacitors and lightning arresters in parallel. In the event of a fault, capacitors actively compensate for commutation voltage, while the lightning arresters dissipate energy and dynamically adjust impedance, completely eliminating commutation failures. However, the capacitor-injection method of this thyristor-based full-bridge converter valve is limited by the characteristics of semi-controlled devices and its modular design, resulting in inherent drawbacks such as reliance on trigger reliability, complex capacitor pre-charge maintenance, and delayed dynamic response.

[0005] Furthermore, in the context of grid-commutated valve topologies based on fully controlled devices, existing technologies include integrating fully controlled devices with thyristors to form enhanced grid-commutated valves. When the AC voltage drops, the fully controlled devices are turned off to rapidly switch submodule capacitors to compensate for commutation voltage and prevent commutation failures. This includes integrating resistor-capacitor submodules into the valve. When a fault occurs, switching capacitors provides voltage support while damping resistors limit the fault current, preventing commutation failure.

[0006] The aforementioned topologies all rely on bridge structures to mitigate commutation failures. However, this structure results in larger equipment, higher construction costs, limited topology flexibility, and increased control complexity. A structure combining half-controlled and fully-controlled devices in series can mitigate commutation failures, but relying solely on the snubber circuits of fully-controlled devices to mitigate commutation failures presents a limited protection range and a high risk of device damage. Therefore, topologies combining half-controlled and fully-controlled devices in series require further optimization. Summary of the Invention

[0007] Purpose of the invention: The present invention provides a hybrid series converter valve topology based on CCM, analyzes the mechanism by which this topology resists commutation failure under fault conditions, and provides a control method for dynamically adjusting the shutdown timing of devices based on real-time AC voltage measurement.

[0008] Technical solution: A CCM-based hybrid series converter valve topology, comprising a three-phase six-bridge-arm circuit, wherein each bridge arm is connected in series with a controllable capacitor module and a semi-controlled device, wherein the controllable capacitor module serves as a fully controlled device;

[0009] The controllable capacitance module is composed of a turn-off device, a diode, a damping resistor and a high-voltage capacitor connected in series and in parallel, wherein the damping resistor and the high-voltage capacitor are connected in series and in parallel with the turn-off device, the damping resistor is also connected in parallel with the diode, and the turn-off device is also connected in parallel with the diode;

[0010] The controllable capacitor module is turned off by IGBT. The topology structure dynamically adjusts its operating state when the LCC-HVDC system fails. There are three commutation operating modes according to different commutation states:

[0011] Mode 1: The half-controlled device and the turn-off device are turned on at the same time, and the high-voltage capacitor is bypassed;

[0012] Mode 2: The turn-off device is actively turned off. The current flows through the damping resistor and the high-voltage capacitor. The current flowing through the damping resistor generates a voltage to start the diode. The damping resistor is bypassed and the current gradually decreases.

[0013] Mode 3: The turn-off device and the high-voltage thyristor are turned on at the same time, the current gradually increases, and the energy stored in the high-voltage capacitor is released through the damping resistor and the turn-off device, accelerating the current flow.

[0014] Furthermore, the semi-controlled device is an LCC commutation valve composed of high-voltage thyristors connected in series.

[0015] During the commutation process under fault conditions in the LCC-HVDC system, the arm to be shut down switches from mode 1 to mode 2, while the arm to be turned on is in mode 3. The commutation process consists of the following two stages:

[0016] The first stage: When the current of the pre-shutdown bridge arm does not reach the preset shutdown current i ref When , the turn-off device in the controllable capacitor module is not actively turned off, that is, the bridge arm works in mode 1, and the current decay executes the current control achieved by the LCC converter valve;

[0017] The second stage: the current of the bridge arm to be turned off decays to the preset turn-off current i ref , the turn-off device is actively turned off, and the high-voltage capacitor begins to assist in commutation, that is, the bridge arm works in mode 2.

[0018] Furthermore, the capacitance of the high-voltage capacitor is 40-60 μF. The voltage of the fully-controlled device is the same as that of the high-voltage capacitor, the maximum withstand voltage of the fully-controlled device is 30 kV, and the on-state current is greater than or equal to 150% of the DC current.

[0019] Furthermore, based on the three models of the controllable capacitor module, the corresponding turn-off devices controlled by IGBT exist:

[0020] Mode 1: The voltage of the turn-off device is 0 and the current is the two-phase short-circuit current;

[0021] Mode 2: The voltage of the turn-off device is the voltage of the high-voltage capacitor, and the current is the difference between the two-phase short-circuit current and the high-voltage capacitor current;

[0022] Mode 3: The voltage of the turn-off device is 0, and the current is the sum of the two-phase short-circuit current and the high-voltage capacitor current.

[0023] The present invention also provides a control method for dynamically adjusting the shutdown timing of devices based on real-time AC voltage measurement. The method includes determining the time when a controllable capacitor module in a hybrid series converter valve topology is actively shut down, using instantaneous voltage measurement to quickly detect the commutation bus voltage amplitude in real time, and determining changes in the system state by detecting the bus amplitude; when the bus amplitude is less than a preset threshold value, the shutdown signal of the turnable device in the topology is adjusted accordingly, thereby achieving the topology adaptively selecting the optimal shutdown time according to system changes;

[0024] The busbar amplitude is calculated as follows:

[0025]

[0026] Where u a 、u b 、u c are the instantaneous values ​​of the three-phase voltages; U a 、U b 、U c are the effective values ​​of the three-phase voltages, ω is the system frequency;

[0027] The optimal shutdown time refers to the real-time current change rate meeting the following conditions:

[0028]

[0029] Where i ref Indicates the preset shutdown current, i off is the basic value, p and q are unknown constants and the optimal value is determined by system simulation. q 、U p Represents the set voltage threshold and divides the voltage into three intervals to ensure optimal system control.

[0030] Beneficial Effects: This invention addresses the commutation failure problem of traditional HVDC inverters by designing a controllable capacitor commutation valve topology based on a hybrid series connection of semi-controlled and fully controlled devices. This topology's ability to resist commutation failure is theoretically analyzed, resulting in the conclusion that during the commutation process, the topology flexibly switches capacitors by controlling the on / off switching of fully controlled devices to increase the commutation voltage-time area and accelerate the commutation process, thereby improving its ability to resist commutation failure. Furthermore, the invention also offers the following significant benefits:

[0031] (1) By controlling the switches of the fully controlled devices to achieve dynamic switching of capacitors, the commutation voltage time area during the fault period is effectively increased, thereby significantly reducing the probability of commutation failure.

[0032] (2) The dynamic control strategy proposed in the present invention can adaptively adjust the shutdown timing of the fully controlled components according to the real-time status of the system, so that the converter valve can flexibly respond to faults of different severity, thereby improving the reliability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 The hybrid series converter valve topology structure based on CCM described in the present invention;

[0034] Figure 2 is the working mode of the hybrid series converter valve;

[0035] Figure 3 It is a CCM-based hybrid series converter commutation equivalent circuit;

[0036] Figure 4 It is based on the working principle of CCM hybrid series converter;

[0037] Figure 5 It is a logic diagram of the control method provided by the present invention;

[0038] Figure 6 is the current decay curve under different capacitance values;

[0039] Figure 7 It is the voltage change curve under different capacitance values;

[0040] Figure 8 This is the LCC bridge arm current diagram in Case 3;

[0041] Figure 9 is a comparison diagram of the system state in steady state, where Figure 9(a) corresponds to DC current, Figure 9(b) corresponds to DC voltage, and Figure 9(c) corresponds to AC voltage;

[0042] Figure 10 is a comparison diagram of the system status under transient conditions (0.7H), where Figure 10(a) corresponds to the AC voltage analysis, Figure 10(b) corresponds to the DC voltage analysis, Figure 10(c) corresponds to the DC current analysis, and Figure 10(d) corresponds to the turn-off angle analysis.

[0043] Figure 11 is a comparison diagram of the system status under transient conditions (0.3H), where Figure 11(a) corresponds to the AC voltage analysis, Figure 11(b) corresponds to the DC voltage analysis, Figure 11(c) corresponds to the DC current analysis, and Figure 11(d) corresponds to the turn-off angle analysis.

[0044] Figure 12 is the capacitor voltage waveform;

[0045] FIG13 is a diagram of electrical stress of a switching device, wherein FIG13( a ) corresponds to the current effect, and FIG13( b ) corresponds to the voltage effect. DETAILED DESCRIPTION

[0046] In response to the commutation failure problem of traditional high-voltage direct current transmission inverters, the present invention provides a hybrid series converter valve topology based on CCM. The converter valve is also a controllable capacitor converter valve topology based on a hybrid series connection of semi-controlled devices and fully controlled devices. The theoretical basis for the topology to resist commutation failure is established, and it is concluded that during the commutation process, the topology flexibly switches capacitors by controlling the on-off of fully controlled devices to increase the commutation voltage-time area and accelerate the commutation process, thereby improving the ability to resist commutation failure. By establishing a mathematical model of the commutation process, the influence of capacitor capacitance on commutation voltage-time-area compensation is revealed. Combined with the electrical stress characteristics of the switching devices, the design principles of capacitor capacitance and device parameters are proposed, and then the parameter combination that takes into account the improvement of commutation capability and the safety margin of the equipment is determined. At the same time, in order to improve the topology's adaptive suppression capability in the face of AC system fault depth, a control strategy for dynamically adjusting the shutdown timing of switching devices based on real-time monitoring technology of AC bus voltage is proposed, which effectively reduces the risk of commutation failure. Finally, the simulation model constructed on the PSCAD / EMTDC platform verifies that the proposed topology and control strategy can effectively suppress commutation failure under different fault severities, and the selected device parameters meet the operating requirements and are within the design allowable range.

[0047] (1) Hybrid series converter valve topology

[0048] The hybrid series type converter valve provided by the present invention is combined with Figure 1 As shown, each bridge arm of the converter is composed of the original LCC bridge arm and the controllable capacitor module (CCM) in series. Among them, the high-voltage thyristors are connected in series to form the LCC converter valves VT1-VT6, which constitute the semi-controlled devices described below; the controllable capacitor module CCM is composed of a turn-off device, a diode, a damping resistor R and a high-voltage capacitor C in series and parallel. For the turn-off device, the present invention adopts an IGBT module for controlled shutdown, and the controllable capacitor module CCM is used as a fully controlled device. In the event of a fault, the opening and closing of the turn-off device is controlled to realize the flexible switching of the capacitor during the commutation process. The capacitors put in provide the system with auxiliary commutation voltage, accelerate the commutation process, and ensure the reliable operation of the system under fault conditions.

[0049] The controllable capacitor module CCM dynamically adjusts its operating state when the system fails. It has three corresponding real-time working modes according to different commutation states, such as Figure 2 shown. Figure 2Middle: The blue solid line represents the current conduction; the blue dotted line represents the current gradually increasing from zero; the red solid line represents the current not flowing; the red dotted line represents the current gradually decreasing to zero. Figure 2 It can be seen that:

[0050] Mode 1: The half-controlled device and the fully-controlled device are turned on at the same time. In this case, the high-voltage capacitor C is bypassed.

[0051] Mode 2: The IGBT module is actively turned off, and the current flows through the damping resistor R and the high-voltage capacitor C. The current flowing through the damping resistor R generates a voltage that turns on the diode, the damping resistor R is bypassed, and the current gradually decreases.

[0052] Mode 3: The IGBT module and thyristor are turned on simultaneously, and the current gradually increases. The energy stored in the high-voltage capacitor C is released through the damping resistor R and the IGBT, accelerating the current flow.

[0053] In the above mode, the controllable capacitor module CCM is operated by a switchable control device, and the switchable control device is controlled by the IGBT module. That is, the controllable capacitor module CCM in the specification has the same function as the fully controlled device, and the semi-controlled device is the existing LCC converter valve structure.

[0054] (2) Analysis of the mechanism of hybrid series converter to resist commutation failure

[0055] Commutation failure occurs when a converter valve fails to completely close when subjected to reverse voltage, causing the valve that was intended to conduct to switch to the valve that was intended to close when the valve voltage changes from negative to positive. In LCC-HVDC systems, during a fault, the commutation voltage-time area decreases as the AC voltage drops. According to the commutation voltage-time-area criterion, a core criterion for assessing commutation failure risk in HVDC systems, commutation failure occurs when the commutation voltage-time-area falls below the critical commutation voltage-time-area required for system commutation.

[0056] On the LCC-HVDC inverter side, the traditional converter is replaced by a CCM-based hybrid series converter. The commutation process under fault conditions is as follows: the bridge arm to be shut down switches from mode 1 to mode 2, and the bridge arm to be turned on is in mode 3. The equivalent circuit of the CCM-based hybrid series valve commutation process is as follows: Figure 3 shown.

[0057] According to the converter mode change, the commutation process can be divided into two stages.

[0058] The first stage: When the current of the pre-shutdown bridge arm does not reach the preset shutdown current i ref When the IGBT is not actively turned off, the bridge arm operates in mode 1, and the current decay method is similar to the existing LCC converter valve control method. According to Kirchhoff's voltage law, the commutation process has the following relationship:

[0059]

[0060] in:

[0061] I d =i open (t)+i close (t) (2)

[0062] Where i open (t) is the current flowing through the bridge arm to be opened; i close (t) is the current flowing through the bridge arm to be turned off; U is the effective value of the commutation voltage on the inverter side of the system; ω is the system frequency; L is the system commutation reactance; I d is the DC current; α is the trigger angle;

[0063] Substituting formula (2) into formula (1) and integrating within (0, t1) yields:

[0064]

[0065] Where, I d (α) is the initial DC current of commutation; I d (t1) is the DC current at time t1; t1 is the IGBT active turn-off time;

[0066] The second stage: At t1, the current of the bridge arm to be turned off decays to i ref , the IGBT is actively turned off, and the capacitor begins to assist in commutation, that is, the bridge arm works in mode 2. At this time, the commutation process has the following relationship:

[0067]

[0068] Where u c (t) is the capacitor voltage in CCM.

[0069] For equation (4), integrating at time (t1, μ / ω) yields:

[0070]

[0071] Where μ is the commutation angle; I d (α+μ) is the DC current at the end of commutation;

[0072] By adding equation (3) and equation (5), we can get:

[0073] L(I d (α+μ)+I d (α))=A+A c (6)

[0074] in:

[0075]

[0076] Analyzing formula (6), we know that the commutation voltage time area during the commutation process is the sum of the commutation voltage time area provided by the system and the commutation voltage time area provided by the capacitor. Formula (8) shows that the commutation voltage time area provided by the capacitor is affected by the IGBT active turn-off time and the capacitor voltage.

[0077] Secondly, by arranging the phase shift of equation (6), we can get:

[0078]

[0079] Where, X = ωL; I dz =I d (α)+I d (α+μ);

[0080] According to formula (9), the LCC system has the following relationship:

[0081]

[0082] Comparison can be obtained:

[0083]

[0084] Analyzing formula (10), we can know that the commutation voltage time area A provided by the capacitor is c Negatively correlated with μ, μ increases with A c The increase of θ decreases. Analyzing formula (11), we can know that under the same trigger angle, the hybrid series converter based on CCM requires a shorter commutation time μ than the traditional DC transmission converter. The working principle of the hybrid series converter based on CCM is as follows: Figure 4 As shown in the figure, by controlling the IGBT to switch on and off flexibly, the capacitor is switched on and off in a timely manner, providing the system with additional commutation voltage time area, accelerating the commutation process, and enhancing the system inverter side's ability to resist commutation failure. line (t) is the commutation voltage provided by the system, t1 is the IGBT active shutdown moment, t2 and t3 are the current zero-crossing moment in the topology structure of the present invention and the current zero-crossing moment of the LCC system, respectively.

[0085] (3) Control strategy for dynamically adjusting device shutdown timing by real-time AC voltage measurement

[0086] According to the above analysis, the IGBT active shutdown moment determines whether the commutation is successful, and the reasonable design of its shutdown current reference value i refHowever, the commutation process is inherently dynamic, influenced by multiple factors, including AC system impedance, fault severity, and DC current magnitude. Therefore, the ideal shutdown time should vary with system operating conditions. This invention combines the pre-shutdown current with a fault identification algorithm to detect fault severity and dynamically adjust the pre-shutdown current threshold, thereby achieving adaptive shutdown control under different operating conditions.

[0087] Currently, zero-sequence voltage or dq decomposition detection methods vary significantly under different fault conditions and cannot meet the requirements for detecting all faults. The sin-cosine component algorithm has a time lag in detecting bus voltage amplitude under asymmetric operating conditions, resulting in insufficient dynamic tracking accuracy of the AC voltage amplitude at the converter station. This paper utilizes instantaneous voltage to rapidly detect the commutation bus voltage amplitude, effectively improving the measurement response speed under grid disturbances and providing sufficient time for subsequent control adjustments.

[0088] The instantaneous value of the AC voltage of the inverter side AC system can be expressed as:

[0089]

[0090] Where u a 、u b 、u c are the instantaneous values ​​of the three-phase voltages; U a 、U b 、U c are the effective values ​​of the three-phase voltages respectively; integrating equation (12) yields:

[0091]

[0092] According to U 2 sin 2 (ωt)+U 2 cos 2 (ωt)=U 2 , adding the squares of equations (12) and (13) together, we can get the amplitude of the AC voltage in the AC system as:

[0093]

[0094] Based on the instantaneous voltage, the above formula can realize real-time and rapid detection of the voltage amplitude of the AC system. Based on the real-time voltage detection, the change of voltage means the change of system state. At this time, it is necessary to adjust the pre-shutdown current threshold accordingly to improve the stability and efficiency of the system. After confirming that the current change rate and the voltage rise rate have strong coupling characteristics, and the AC voltage amplitude determines the current change rate, based on this, the instantaneous line voltage amplitude U is defined. L Proportional adjustment of pre-shutdown current i ref :

[0095]

[0096] Where i off is the base value; U LN is the line voltage rating; k is the constant coefficient to be determined.

[0097] After simulation verification, it was found that the above formula did not put the system in a relatively optimal state. Therefore, the relationship between the pre-shutdown current and the line voltage amplitude was further classified into the following:

[0098]

[0099] A piecewise function is used to divide the system into three state intervals to represent the relationship between the current threshold and the voltage change. Among them, p and q are unknown constants whose optimal values ​​are determined through system simulation. q 、U p It represents the voltage threshold and divides the voltage into three intervals to ensure optimal system control.

[0100] As shown in formula (16), the pre-shutdown current is inversely proportional to the bus voltage drop amplitude. The dynamic pre-shutdown current control strategy can adaptively select the optimal shutdown point by real-time monitoring of the AC voltage amplitude: lowering the pre-shutdown current threshold under high current change rate conditions, thereby reducing the voltage stress and switching loss borne by the device; when a low current change rate is detected, the shutdown action is triggered in advance, and the system safety margin is guaranteed by actively adjusting the shutdown time. While minimizing the overvoltage risk, it ensures that the commutation process has sufficient blocking recovery time. The specific process of the control strategy is as follows: Figure 5 shown.

[0101] (4) Capacitor and switch device design

[0102] From the above part (2), we can know that the capacitor voltage can change A c , which directly affects the success rate of the commutation process and system performance. The capacitor voltage can be expressed as:

[0103]

[0104] Where i c (t) is the capacitor current;

[0105] According to Equation (17), the capacitor voltage is negatively correlated with the capacitance value. Secondly, during the commutation process, the fully controlled device with active shutdown capability operates before the thyristor and shuts down early. During the shutdown process, the turn-off transistor will have a large voltage rise rate du / dt. The capacitor connected in parallel with it can suppress the overvoltage of the fully controlled device, and the capacitance value is positively correlated with the overvoltage suppression capability. Therefore, considering both voltage regulation and the ability to resist commutation failure, the rational selection of capacitors is a key design step in achieving efficient and reliable commutation.

[0106] According to the analysis in Section 2, the capacitor participation in the shutdown process can be roughly divided into two stages.

[0107] The first stage is when the IGBT is actively turned off, and the current flowing through the IGBT decreases from zero. At this point, current flows into the capacitor to charge it, and the capacitor voltage begins to increase. During this time (t1, t2) (t2 is the time when the current flowing through the IGBT is zero):

[0108] i c (t) = i close (t)-i IGBT (t) (18)

[0109] The second stage is when the IGBT is completely turned off, and the current flows through the path formed by the thyristor and capacitor until the bridge arm current decays to zero. During this time (t2, μ / ω):

[0110] i c (t) = i close (t) (19)

[0111] Substitute equations (18) and (19) into equation (17) respectively, integrate them within the corresponding time and add them together to obtain:

[0112]

[0113] The IGBT turn-off time is in the microsecond range and the device characteristics are less affected by external factors. Its internal current decreases linearly with the rate of change of m. Through simulation experiments, it is known that i close (t) and u c (t) presents a linear downward and linear upward trend in the integral interval, and the area enclosed by the horizontal line is approximately a triangle. close (t1)=i ref ;i close (μ / ω)=0;u c (t1) = 0; Substituting into the above formula, we can get:

[0114]

[0115] Substituting formula (21) into formula (8) yields:

[0116]

[0117] From formula (22), it can be seen that the additional commutation voltage time area is related to the IGBT turn-off moment, pre-turn-off current and capacitor value. From formula (22) and formula (21), it can be seen that when only the capacitor value is changed, a small capacitor can achieve a larger voltage value in the same time, and accordingly can provide more commutation voltage time area, but too small a capacitor will cause overvoltage problems. On the contrary, in the same time, although a large capacitor can maintain a smaller voltage value, the commutation area provided will be relatively small. Therefore, there is an optimal solution for the capacitor value. Without changing the fault type and the fault occurrence and processing time. (AC bus single-phase grounding; fault grounding inductance is 0.6H; fault time is 1.5s). Based on the CCM hybrid series converter topology solution, the changes in valve current and capacitor terminal voltage under different capacitance values ​​are analyzed.

[0118] Figure 6 The width of graph A represents the time it takes for the current to decay to zero when the capacitance is 20μF compared to 40μF; the width of graph B represents the time it takes for the current to decay to zero when the capacitance is 40μF compared to 60μF; the width of graph C represents the time it takes for the current to decay to zero when the capacitance is 60μF compared to 80μF. Figure 7 It can be seen that the capacitance value is negatively correlated with the bridge arm current decay rate. When the capacitance value increases, the bridge arm current decay rate decreases. When the capacitance value is equal to 80μF, the current decay rate is too slow, causing the system to reverse phase after the current passes through zero, and the current changes from zero to positive, resulting in phase change failure.

[0119] Depend on Figure 7 As can be seen in the figure, as the capacitance value gradually increases, the voltage at the capacitor terminals decreases rapidly. However, when the capacitance value reaches 80μF, the system fails to commutate, and the capacitor voltage collapses. As the capacitance value gradually decreases, the voltage at the capacitor terminals also rises rapidly; however, when the capacitance changes between 40 and 60μF, the voltage at the terminals changes little.

[0120] Based on the above analysis, taking into account the commutation resistance and device cost, the selected capacitance value is 40μF.

[0121] Furthermore, the CCM module is divided into three modes, so the electrical quantities of the switchable tube are analyzed according to the three modes. The analysis results are shown in Table 1. sc is the two-phase short-circuit current)

[0122] Table 1. Electrical stress parameters of the shut-off tube

[0123]

[0124] Table 1 shows that the voltage of the fully controlled device is the same as the capacitor voltage. Based on system reliability and safety margin, a maximum withstand voltage of 30 kV is selected for the fully controlled device. Furthermore, the on-state current must be greater than or equal to 150% of the DC current. Therefore, the Infineon FZ3600R17HP4_B2 IGBT meets this design requirement.

[0125] Example: To verify the effectiveness of the controllable capacitor module converter and control strategy based on the integration of half-control and full-control to suppress commutation failure, the present invention built the following three cases in PSCAD / EMTDC and conducted simulation comparison experiments from two aspects: system operation characteristics and commutation failure prevention capabilities.

[0126] Case 1: CIGRE standard test model.

[0127] Case 2: Coupled shutdown model of existing half-controlled and fully-controlled devices in series;

[0128] Case 3: The inverter station in the CIGRE model uses a CCM-based hybrid series converter. Table 2 shows the system model parameters for Case 3.

[0129] Table 2. System parameters for Case 3

[0130]

[0131] The system operation characteristics analysis is carried out for the above case: Under steady-state operation, the DC voltage, DC current, AC voltage and bridge arm current waveforms on the inverter side are as follows: Figure 8 , as shown in Figure 9. Compared to a traditional LCC converter, the waveforms are essentially identical. Under transient operation, and with the aforementioned design parameters, to compare and analyze the system characteristics of Cases 1, 2, and 3, a single-phase inductor ground fault was set at the inverter-side AC bus. The fault initiation time was 1.5 seconds, the fault clearing time was 1.55 seconds, and the fault inductances were 0.7 ohms (minor fault) and 0.3 ohms (severe fault), respectively.

[0132] Figure 10 shows a comparison of the inverter-side system operating characteristics for three cases under minor faults.

[0133] Figure 10 shows that under a minor fault condition, the AC bus voltage and DC voltage on the inverter side of Case 1 begin to drop sharply at 1.5s, reaching their lowest point at 1.535s, at which point the bus voltage is 0.82 pu and the DC voltage drops to 0. After the fault occurs, the DC current begins to rise, reaching a maximum of 2.4 pu at 1.52s. During the fault, the shutdown angle γ returns to zero, and the system experiences commutation failure. In Case 2, the AC bus voltage and DC voltage on the inverter side begin to drop at 1.5s, reaching their lowest point at 1.56s, at which point the AC bus voltage is 0.9 pu and the DC voltage is 0.865 pu. After the fault occurs, the DC current begins to rise, reaching a maximum of 1.2 pu at 1.51s, then slowly decreases, reaching a minimum of 0.9 pu at 1.56s. The turn-off angle γ was 11° during the fault, and the system commutated successfully. In Case 3, the AC bus voltage and DC voltage on the inverter side began to slowly decrease at 1.5 seconds, reaching their lowest point at 1.6 seconds, at which point the AC bus voltage was 0.94 pu and the DC voltage was 0.9 pu. During the fault, the DC current fluctuated within a narrow range around 1 pu, and the turn-off angle γ remained at 14.5°, allowing the system to commutate successfully. A comparison of the three cases shows that in Case 1, the DC current experienced a surge of 2.4 times its rated value and a significant voltage drop after the fault, resulting in slow system recovery after the fault was cleared. In Case 2, the voltage dropped by approximately 10% during the fault, but the recovery time was essentially the same as in Case 3. In Case 3, the AC bus voltage remained above 0.94 pu and the DC voltage remained around 0.9 pu during the fault. The turn-off angle barely changed, and system characteristics remained unchanged. After the fault was cleared, the system returned to normal within 0.12 seconds. Therefore, Case 3 maintained system stability during the fault while improving the system's dynamic recovery speed.

[0134] The inverter side system operating characteristics of three cases under severe faults are as follows: Figure 12 shown.

[0135] Figure 11 shows that under severe fault conditions, in Case 1, the AC and DC voltages on the inverter side begin to drop sharply at 1.5s, with the DC voltage instantly returning to zero. The AC voltage reaches a minimum of 0.83 pu at 1.525s. Simultaneously, the DC current surges, reaching a maximum of approximately 2.5 pu at 1.51s. During the fault, the turn-off angle γ is 0°, and the system experiences commutation failure. In Case 2, the voltage curve also begins to drop rapidly at 1.5s. After the fault ends, the AC voltage also reaches a minimum of 0.83 pu, but the DC voltage reaches a minimum of 0.62 pu. At the onset of the fault, the DC current briefly rises to approximately 1.2 pu before rapidly decreasing. At 1.6s, it reaches a minimum of 0.77 pu and then gradually rises to the rated value. The turn-off angle γ remains at 6.7° during the fault, and the system experiences commutation failure. In Case 3, after the fault occurs, the AC and DC voltages begin to drop, reaching a minimum of 0.85 pu at 1.56s, and the corresponding DC voltage reaches a minimum of 0.71 pu. Secondly, the DC current rose to 1.2 pu in 1.51 seconds before beginning to decline, reaching a minimum of 0.85 pu at 1.57 seconds. The shutdown angle remained stable above 10° during the fault, and the system successfully commutated. Furthermore, comparing the recovery characteristics, in Case 1, the voltage rose rapidly after the fault ended, resuming normal operation approximately 0.27 seconds later. In Case 2, the voltage and current remained near their lowest points for 0.04 seconds after the fault ended. In Case 3, recovery was rapid, returning to normal within approximately 0.1 seconds, representing a system stabilization speed improvement of approximately 60% compared to Case 1.

[0136] Therefore, the hybrid series converter valve and control strategy based on CCM can effectively improve the system's ability to resist commutation failure and enable the system to quickly return to stable operation after the fault is cleared, significantly shortening the duration of voltage fluctuations and improving the dynamic performance of the system.

[0137] Capacitor voltage and electrical stress analysis of switching devices: To verify the correctness of the capacitor and switching device parameter design, the electrical characteristics during the fault period are analyzed through simulation under severe fault conditions. Case 3 of severe fault condition: Capacitor voltage and electrical stress of switching devices are as follows: Figure 12 and as shown in Figure 13.

[0138] Observe the changes in the CCM capacitor voltage and the voltage and current of the switching device in Case 3 after the fault occurs. Figure 12 This shows that the maximum capacitor voltage stress in the controllable capacitor module CCM is 25kV. Figure 13 shows that the maximum switching device stress in Case 3 is 25kV, and the maximum current stress is 2.325kA, which meets the design requirements and ensures that the switching device can operate within the normal operating range.

Claims

1. A hybrid series converter valve topology based on CCM, characterized in that: The hybrid series converter valve topology structure includes a three-phase six-bridge arm circuit, wherein each bridge arm is provided with a controllable capacitor module and a semi-controlled device in series, and the controllable capacitor module serves as a fully controlled device; The controllable capacitance module is composed of a turn-off device, a diode, a damping resistor and a high-voltage capacitor connected in series and in parallel, wherein the damping resistor and the high-voltage capacitor are connected in series and in parallel with the turn-off device, the damping resistor is also connected in parallel with the diode, and the turn-off device is also connected in parallel with the diode; The controllable capacitor module is turned off by IGBT. The topology structure dynamically adjusts its operating state when the LCC-HVDC system fails. There are three commutation operating modes according to different commutation states: Mode 1: The half-controlled device and the turn-off device are turned on at the same time, and the high-voltage capacitor is bypassed; Mode 2: The turn-off device is actively turned off. The current flows through the damping resistor and the high-voltage capacitor. The current flowing through the damping resistor generates a voltage to start the diode. The damping resistor is bypassed and the current gradually decreases. Mode 3: The turn-off device and the high-voltage thyristor are turned on at the same time, the current gradually increases, and the energy stored in the high-voltage capacitor is released through the damping resistor and the turn-off device, accelerating the current flow.

2. The hybrid series converter valve topology structure according to claim 1, characterized in that: The semi-controlled device is an LCC commutation valve composed of high-voltage thyristors connected in series.

3. The hybrid series converter valve topology structure according to claim 1, characterized in that: During the commutation process under fault conditions in the LCC-HVDC system, the arm to be shut down switches from mode 1 to mode 2, while the arm to be turned on is in mode 3. The commutation process consists of the following two stages: The first stage: When the current of the pre-shutdown bridge arm does not reach the preset shutdown current i ref When , the turn-off device in the controllable capacitor module is not actively turned off, that is, the bridge arm works in mode 1, and the current decay executes the current control achieved by the LCC converter valve; The second stage: the current of the bridge arm to be turned off decays to the preset turn-off current i ref , the turn-off device is actively turned off, and the high-voltage capacitor begins to assist in commutation, that is, the bridge arm works in mode 2.

4. The hybrid series converter valve topology structure according to claim 1, characterized in that: The capacitance value of the high-voltage capacitor is 40-60 μF.

5. The hybrid series converter valve topology structure according to claim 4, characterized in that: The voltage of the fully controlled device is the same as that of the high-voltage capacitor, and the maximum withstand voltage of the fully controlled device is 30kV; and the on-state current is greater than or equal to 150% of the DC current.

6. The hybrid series converter valve topology structure according to claim 1, 4 or 5, characterized in that: Based on the three models of controllable capacitor modules, there are corresponding devices that can be turned off by controlling IGBTs: Mode 1: The voltage of the turn-off device is 0 and the current is the two-phase short-circuit current; Mode 2: The voltage of the turn-off device is the voltage of the high-voltage capacitor, and the current is the difference between the two-phase short-circuit current and the high-voltage capacitor current; Mode 3: The voltage of the turn-off device is 0, and the current is the sum of the two-phase short-circuit current and the high-voltage capacitor current.

7. A control method for dynamically adjusting device shutdown timing based on real-time AC voltage measurement, characterized in that: The method includes determining the time to actively shut down a controllable capacitor module in a hybrid series converter valve topology, using instantaneous voltage measurement to quickly detect the commutation bus voltage amplitude in real time, and determining changes in system status by detecting the bus amplitude; when the bus amplitude is less than a preset threshold value, adjusting the shutdown signal of the turnable device in the topology accordingly, thereby achieving the topology adaptively selecting the optimal shutdown time according to system changes; The busbar amplitude is calculated as follows: Where u a 、u b 、u c are the instantaneous values ​​of the three-phase voltages; U a 、U b 、U c are the effective values ​​of the three-phase voltages, ω is the system frequency; The optimal shutdown time refers to the real-time current change rate meeting the following conditions: Where i ref Indicates the preset shutdown current, i off is the basic value, p and q are unknown constants and the optimal value is determined by system simulation. q 、U p Represents the set voltage threshold and divides the voltage into three intervals to ensure optimal system control.

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