Hybrid cascade system commutation failure suppression method caused by sending end fault

Abstract

The invention discloses a hybrid cascade system commutation failure suppression method caused by sending end faults, and belongs to the technical field of high-voltage direct-current power transmission systems. The invention aims to provide the method for suppressing the commutation failure of the hybrid cascade system caused by the sending-end fault, which comprises the following steps of: monitoring the deviation between a direct current instruction value and an actual value in a system fault recovery period in real time, and dynamically adjusting a rectifier-side trigger angle instruction according to the deviation so as to control the direct current recovery rate and amplitude. According to the method, the dynamic recovery characteristic of the sending-end bus voltage and the constant direct-current voltage control effect of the inversion-side MMC are combined, and key influence factors of commutation failure are systematically analyzed; a commutation failure suppression strategy based on a rectification side additional trigger angle is provided. According to the method, the influence of the phase dynamic change of the alternating current bus voltage at the rectification side and the constant direct current voltage effect of the MMC at the inversion side on the commutation failure in the fault recovery process is fully considered, and reference is provided for safe and stable operation of the hybrid cascaded high-voltage direct current power transmission system.

Description

Technical Field

[0001] This invention belongs to the technical field of high voltage direct current transmission systems. Background Technology

[0002] Commutation failure, a typical fault in high-voltage direct current (HVDC) transmission systems, seriously threatens the safe and stable operation of the power grid. Hybrid cascaded high-voltage direct current (HC-HVDC) systems combine the advantages of grid-connected phase-commutation converters and modular multilevel converters (MMCs), offering advantages such as large transmission capacity, lower construction costs, and high operational reliability, and have become an important development direction in the HVDC field. However, due to the presence of traditional LCC converters, the inverter side of the system still faces the risk of commutation failure.

[0003] Regarding commutation failure caused by sending-end faults, existing research has made some progress in mechanism analysis and suppression strategies. However, most studies still focus on inverter-side characteristics and fail to fully consider the impact of dynamic phase changes in the rectifier-side AC bus voltage on commutation failure during fault recovery. Furthermore, since the inverter side of an HC-HVDC system includes both LCC and MMC converter structures, there are more complex coupling relationships between various electrical quantities and control loops, making previous commutation failure theories based on LCC-HVDC systems difficult to apply directly. Summary of the Invention

[0004] The purpose of this invention is to suppress commutation failure in a hybrid cascaded system caused by a fault in the rectifier side by dynamically adjusting the rectifier side firing angle command based on the deviation between the DC current command value and the actual value during real-time monitoring of the system fault recovery period, thereby controlling the DC current recovery rate and amplitude.

[0005] The steps of this invention are: S1, The inverter side model of the hybrid cascaded system is: (1) (2) (3) (4) (5) (6) (7) In the formula: I dP d These represent the inverter-side DC current and active power, respectively; U di U is the DC voltage on the inverter side. dLCC U dMMC These are the high and low voltage DC voltages on the inverter side, respectively; U LCC φ is the effective value of the LCC converter bus voltage; φ is the power factor angle; P LCC P MMC1 / 2 They are LCC and MMC respectively. 1 / 2 Power output; Q c The reactive power generated by the reactive power compensation device; α, μ, and γ are the LCC trigger delay angle, commutation overlap angle, and turn-off angle, respectively; X T is the leakage reactance of the LCC converter transformer; k is the turns ratio of the converter transformer between the LCC and the AC system; S2. Analysis of commutation failure in a hybrid cascaded HVDC transmission system caused by sending-end fault: S21. Dynamic response characteristics of AC / DC systems during faults: Phase I is the fault phase: the AC bus voltage at the sending end, U aci Sudden drop, DC current I dc This decrease leads to a drop in the output β of the inverter-side CC control. CC Gradually increase; simultaneously, CEC control starts, Δγ CEC Increase the output β of the inverter-side CEA control. CEA It increases accordingly; Phase II is the fault recovery phase: After the fault at the sending end is cleared, the AC bus voltage U aci Recovery begins; the inverter-side system experiences DC current I under the influence of VDCOL. dc As it gradually rises, the rectifier side switches back to CC control; Phase III is the commutation failure phase: DC current I dc The continued recovery of β CC <β CEA The inverter side switches back to CEA control; S22. Phase shift of the sending-end bus voltage causes triggering deviation. Take U s As a reference phasor, the transverse component δU and longitudinal component ΔU of this voltage drop are expressed as follows: (8) (9) Combining equations (8) and (9), we can derive U. r with U s Phase difference δ between them: (10) Ignoring the minor factors in equation (10), we simplify to obtain U. r Phase offset φ: (11) rectifier firing angle α r Obtained from the following formula: (12) In the formula: U dr α is the DC voltage on the rectifier side. r This refers to the rectifier firing angle; S23, The Influence of MMC Constant Voltage on DC Current Recovery The VDCOL output expression is as follows: (13) In the formula: , U d This is the actual operating voltage on the DC side; U dh and U dl These are the upper and lower limits of the VDCOL control start-up voltage, respectively; I dh and I dl These are the upper and lower limits of the DC current, respectively; U dN and I dN These are the rated values ​​of DC voltage and DC current, respectively. The actual operating voltage on the DC side is rewritten as U d =ULCC d+UMMC d, therefore, the change in the current command value output by VDCOL during the fault recovery process is: (14) In the formula, ULCC d and UMMC d are the DC voltages of LCC and MMC, respectively; S3. Methods to suppress commutation failure in hybrid cascaded HVDC transmission systems caused by sending-end faults: S31, Additional trigger angle control module (15) S32. Fault Detection Module: The judgment logic is as follows: Fault detection uses the CFPREV fault detection module, which detects the phase voltage u of the AC bus at the sending end through the sine and cosine component method. a u b and u c The amplitude of the voltage is measured and the magnitude of its zero-sequence voltage component is calculated to determine whether a single-phase short-circuit fault has occurred; at the same time, the abc-αβ coordinate transformation is used to detect three-phase short-circuit faults, thereby avoiding controller malfunctions.

[0006] This invention fully considers the dynamic phase change of the AC bus voltage on the rectifier side and the effect of the constant DC voltage of the MMC on the inverter side on commutation failure during the fault recovery process. It explores the commutation failure mechanism and suppression strategy of the hybrid cascaded system caused by the sending end fault, and hopes to provide reference for the safe and stable operation of the hybrid cascaded high voltage DC transmission system. Attached Figure Description

[0007] Figure 1 This is a diagram of the hybrid cascaded system topology and control strategy; Figure 2 This is an analysis diagram of the impact of phase shift on rectifier-side triggering deviation; Figure 3 This is a diagram of the improved control strategy on the rectifier side; Figure 4 This is a diagram showing the changes in electrical and control quantities of the system under a 0.4H three-phase ground fault; where: (a) is the phase of the sending-end bus voltage; (b) is the control command on the rectifier side; (c) is the DC voltage of MMC1; (d) is the control command on the inverter side; (e) is the DC current; and (f) is the inverter side... Measured value; Figure 5 This is a diagram showing the changes in electrical and control quantities of the system under a 0.07H single-phase ground fault; where: (a) is the phase of the sending-end bus voltage; (b) is the rectifier-side control command; (c) is the DC voltage of MMC1; (d) is the inverter-side control command; (e) is the DC current; and (f) is the inverter-side... Measured value; Figure 6 This is a diagram showing the suppression effect under a 0.4H three-phase ground fault; where: (a) is the DC current; (b) is the inverter side. Measured values; (c) is the additional firing angle on the rectifier side; (d) is the control command on the rectifier side; Figure 7 This is a diagram showing the suppression effect under a 0.07H single-phase ground fault; where: (a) is the DC current; (b) is the inverter side. Measured value; (c) is the additional firing angle on the rectifier side; (d) is the control command on the rectifier side. Detailed Implementation

[0008] This invention addresses the commutation failure problem in hybrid cascaded systems caused by sender-end faults. First, it identifies the dominant factors contributing to commutation failure. Based on this, it systematically analyzes the key influencing factors of commutation failure by combining the dynamic recovery characteristics of the sender-end bus voltage with the constant DC voltage control function of the inverter-side MMC. Finally, it proposes a commutation failure suppression strategy based on an additional firing angle on the rectifier side. By monitoring the deviation between the commanded and actual DC current values ​​during system fault recovery in real time, the rectifier-side firing angle command is dynamically adjusted to control the recovery rate and amplitude of the DC current, thus suppressing commutation failure while ensuring stable DC current recovery.

[0009] The purpose of this invention is to analyze the commutation failure mechanism of a hybrid cascaded HVDC transmission system caused by a sending-end fault and to propose a corresponding suppression strategy. This strategy dynamically adjusts the rectifier-side firing angle command based on the real-time monitoring of the deviation between the commanded and actual DC current values ​​during system fault recovery, thereby controlling the recovery rate and amplitude of the DC current. This suppresses commutation failure while ensuring stable DC current recovery.

[0010] The problem described in this invention is solved by the following technical solution: (1) The response process of AC / DC system during the fault was analyzed in stages based on the dynamic response characteristics of each electrical quantity and control quantity. It was pointed out that the excessively fast recovery and excessive amplitude of DC current were the main reasons for the failure of commutation at the receiving end. (2) The effects of the rectifier side firing angle deviation and the inverter side MMC constant DC voltage on DC current recovery during fault recovery were studied; (3) A control strategy based on the additional firing angle of the rectifier side is proposed. The firing angle command of the rectifier side LCC is dynamically adjusted to suppress the excessive recovery of DC current, thereby suppressing commutation failure.

[0011] The hybrid cascaded high-voltage direct current transmission system described in this invention has the following topology and basic control strategy: Figure 1 As shown: The hybrid cascaded high-voltage direct current transmission system adopts a dual 6-pulse LCC series structure on the rectifier side, and the inverter side consists of a single 6-pulse LCC and two sets of parallel MMCs in series, where the LCC is the high-voltage side and the MMC set is the low-voltage side.

[0012] The mathematical model for the inverter side of the hybrid cascade system is as follows: (1) (2) (3) (4) (5) (6) (7) In the formula: I d P d These represent the inverter-side DC current and active power, respectively; U di U is the DC voltage on the inverter side. dLCC U dMMC These are the high and low voltage DC voltages on the inverter side, respectively; U LCC φ is the effective value of the LCC converter bus voltage; φ is the power factor angle; P LCC P MMC1 / 2 They are LCC and MMC respectively. 1 / 2 Power output; Q c The reactive power generated by the reactive power compensation device; α, μ, and γ are the LCC trigger delay angle, commutation overlap angle, and turn-off angle, respectively; X T is the leakage reactance of the LCC converter transformer; k is the turns ratio of the converter transformer between the LCC and the AC system.

[0013] In the hybrid cascaded system, the rectifier-side LCC employs constant current (CC) control and minimum firing angle control; the inverter-side LCC uses constant extinction angle (CEA) control in steady state, and constant current control is activated when a system fault occurs. To ensure smooth switching between the two modes, the inverter side is also equipped with a current error controller (CEC).

[0014] Vector current control is used in the inverter-side MMC groups. MMC1 and MMC2 employ constant DC voltage control and constant active power control, respectively, to maintain stable DC voltage on the inverter side and achieve reasonable power distribution. Meanwhile, considering the voltage stability of the receiving-end AC grid, both MMCs employ constant reactive power control.

[0015] The main analysis process of the commutation failure mechanism of the hybrid cascaded HVDC transmission system caused by sending-end faults proposed in this invention is as follows: (1) Dynamic response characteristics of AC / DC systems during faults: When a fault occurs at the sending end of a hybrid cascaded system, electrical quantities such as voltage and current will undergo significant transient changes. Based on the dynamic response characteristics of electrical and control quantities, the response process of the AC / DC system during the fault occurrence and recovery period can be divided into the following three stages.

[0016] Phase I is the fault phase: the AC bus voltage at the sending end, U aci Sudden drop, DC current I dcThis decrease leads to a drop in the output β of the inverter-side CC control. CC Gradually increasing. Simultaneously, CEC control is activated, Δγ CEC Increase the output β of the inverter-side CEA control. CEA This increases accordingly. To maintain the stability of the DC current, the rectifier-side CC control gradually decreases the LCC firing angle, until it reaches its minimum value α. min At this time, the rectifier side switches to minimum firing angle control; after one cycle of minimum turn-off angle acquisition, the inverter side turn-off angle γ increases sharply, β CEA Decrease, when less than β CC When the inverter side switches to CC control, the system reaches fault steady state.

[0017] Phase II is the fault recovery phase: After the fault at the sending end is cleared, the AC bus voltage U aci Recovery begins; the inverter-side system experiences DC current I under the influence of VDCOL. dc As it gradually rises, the rectifier side switches back to CC control. With I... dc The recovery, β CC It continues to decrease, but is still greater than β. CEA The inverter-side control did not switch, so both sides of the system operated in CC control mode. During this stage, a deviation gradually appeared between the commanded LCC firing angle value and the actual firing angle value on the rectifier side, affecting the recovery of DC current. The DC current recovery rate was relatively fast, and towards the end of the fault recovery period, the DC current had recovered to near the current command value, and the CEC output gradually decreased to 0.

[0018] Phase III is the commutation failure phase: DC current I dc The continued recovery of β CC <β CEA The inverter side switches back to CEA control. At this time, I dc The recovery amplitude is large, and the CEC output remains at 0. Simultaneously, because the constant turn-off angle control uses the minimum turn-off angle and the turn-off angle setpoint from the previous cycle as inputs, there is a certain lag in the control output response. At the instant the inverter side switches back to CEA control, β... CEA The level is too low, making it difficult to meet the shut-off margin requirements of the converter valve, ultimately causing the shut-off angle to drop to the critical value γ. min Subsequently, the pre-shutdown valve is reactivated, and the inverter experiences commutation failure.

[0019] (2) The phase shift of the sending-end bus voltage caused triggering deviation The sending end of a hybrid cascaded system mainly includes an AC power grid, an LCC rectifier, a converter transformer, and reactive power compensation devices. After the AC fault at the sending end is cleared, U... r Recovery begins, and active power gradually increases. Subject to equivalent impedance Z s The impact, Ur with U s A voltage drop will occur between them. (U) s Using the reference phasor, the transverse component δU and longitudinal component ΔU of this voltage drop can be expressed as follows: (8) (9).

[0020] Combining equations (8) and (9), U can be further derived. r with U s Phase difference δ between them: (10).

[0021] Furthermore, in high-voltage direct current transmission systems, the reactance is typically much greater than the resistance (X >> R). Therefore, δU is mainly affected by active power, and the change in δ is mainly related to δU. Thus, the change in active power ΔP at the sending-end grid directly determines the bus voltage U. r The phase shift amplitude. Furthermore, U s As an ideal voltage source, the phase remains constant, and the phase difference δ actually represents the sending-end bus voltage U. r The phase shift, ignoring the minor factors in equation (10), simplifies to U. r Phase offset φ: (11).

[0022] As can be seen from equation (11), as the active power recovers, φ always < 0, meaning that the sending-end bus voltage always lags behind the voltage source voltage. The change in φ reflects the bus voltage U to a certain extent. r The degree of phase shift. After the fault is cleared, ΔP gradually increases, U r As the absolute value of the phase offset φ continuously increases, the phase of the AC bus voltage at the sending end gradually shifts backward relative to the ideal voltage source. This characteristic change will inevitably affect the accurate acquisition of the bus voltage phase by the rectifier-side phase-locked loop, thereby causing triggering deviation of the rectifier-side LCC.

[0023] To reduce non-characteristic harmonics and lower filter costs, LCC converters now generally employ equal-interval triggering control. Taking an LCC connected to a Yy-type converter transformer as an example, the phase-locked loop tracks the positive-sequence voltage of the rectifier-side AC bus and outputs a ramp signal θ synchronized with the A-phase voltage. PLL After delaying this signal by 30°, the line voltage U is obtained. acThe phase of the pulse is used as the input signal for the LCC valve 1 trigger pulser; under symmetrical operation conditions, the commutation voltage phases of each valve are equally spaced at 60° intervals, so by delaying the input signal of valve 1 by 60° sequentially, the ramp signals required for the other valves can be obtained. Finally, when the trigger command α... ord When the signal is equal to the ramp signal, the trigger pulse generator outputs a pulse signal of equal width (usually 120°) to trigger the corresponding converter valve.

[0024] like Figure 2 As shown, the phase of the sending-end bus voltage shifts backward due to the active power recovery. Because the phase-locked loop (PLL) itself lacks sufficient dynamic performance, it cannot quickly and accurately track the actual phase change of the bus voltage, resulting in a shift in the output phase θ. PLL The phase θ of the phase A AC bus voltage is continuously ahead of the actual phase. act At this point, the pulse generator uses a leading ramp signal θ. PLL With trigger command α ord When the two are equal, an advanced trigger pulse will be output, causing the actual trigger angle of the rectifier-side LCC to be less than the trigger angle command value. The converter valve is equivalent to being triggered prematurely, with a trigger advance of Δα. According to equation (12), the trigger angle α is... r The reduction in voltage will lead to an increase in the DC voltage output on the rectifier side, accelerating the recovery rate of the DC current. At the same time, during the fault recovery phase, the AC bus voltage at the sending end tends to rise sharply. Under the combined effect of triggering deviation and voltage rise, the difference between the DC voltage at the sending and receiving ends may increase significantly, causing the DC current to rise rapidly, thereby increasing the risk of commutation failure at the receiving end.

[0025] (12) In the formula: U dr α is the DC voltage on the rectifier side. r This is the rectifier firing angle.

[0026] (3) The effect of constant voltage of MMC on DC current recovery When a fault occurs at the sending end, the DC voltage drops rapidly. When the DC voltage drops to the VDCOL start-up threshold, the current controller automatically lowers the current reference value. After the fault is cleared, as the DC voltage gradually recovers, the current reference value is gradually raised again. The VDCOL output expression is as follows: (13) In the formula: , U d This is the actual operating voltage on the DC side; U dh and U dl These are the upper and lower limits of the VDCOL control start-up voltage, respectively; Idh and I dl These are the upper and lower limits of the DC current, respectively; U dN and I dN These are the rated values ​​for DC voltage and DC current, respectively.

[0027] Due to the presence of the MMC on the inverter side, the actual operating voltage on the DC side of the hybrid cascaded system can be rewritten as follows: U d =ULCC d+UMMC d, therefore, the change in the current command value output by VDCOL during the fault recovery process is: (14) In the formula, ULCC d and UMMC d are the DC voltages of LCC and MMC, respectively.

[0028] During the recovery process, the DC voltage output by the inverter-side MMC1 remains essentially constant, effectively providing a constant voltage source for the high-voltage LCC. This constant voltage effect reduces the fluctuation amplitude of the system's DC voltage, which may delay or weaken the triggering condition of the LCC-VDCOL. If the dynamic response of the MMC is fast enough, the LCC may not even need to enter current-limiting mode. Under the same severity of fault at the sending end, compared to the traditional LCC-HVDC system, the hybrid cascaded system maintains a higher DC voltage during fault and recovery, resulting in Uhc d > Ulcc d. Consequently, the VDCOL current command value Ihc d > Ilcc d, increasing the recovery rate and magnitude of the system's DC current.

[0029] Based on the above analysis of the AC / DC system response process and commutation failure influencing factors during the fault and recovery period of a hybrid cascaded system, the following conclusions can be drawn: 1) During the fault at the sending end and at the moment of fault clearing, the DC current is always at a low level. Regardless of the control mode, the inverter side will maintain a high lead trigger angle command, and the inverter will maintain a high turn-off margin, so commutation failure will not occur.

[0030] 2) During the fault and fault recovery phases, the AC bus voltage on the inverter side fluctuates little and remains near the rated value. Therefore, the magnitude of the turn-off angle mainly depends on the change in DC current. Excessive DC current recovery rate and recovery amplitude are more likely to cause commutation failure on the inverter side.

[0031] 3) During the fault recovery process, due to the constant DC voltage control of the MMC, the DC voltage of the hybrid cascaded system remains at a high level, resulting in a large VDCOL current command output. At the same time, the phase shift of the AC bus voltage at the sending end leads to a decrease in the actual firing angle on the rectifier side. Under the combined effect of these two factors, the system struggles to suppress the excessively rapid recovery rate of the DC current. Towards the end of the recovery phase, the DC current amplitude becomes too large, causing commutation failure on the inverter side.

[0032] The commutation failure suppression strategy for hybrid cascaded HVDC transmission systems caused by sending-end faults proposed in this invention: The commutation failure suppression strategy mainly includes an additional firing angle control module and a fault detection module. An additional firing angle control is added to the original constant current control. During system recovery, the excessively rapid recovery of the DC current is suppressed by increasing the firing angle output on the rectifier side in real time. The specific control block diagram is shown below. Figure 3 As shown.

[0033] (1) Additional trigger angle control module.

[0034] After the fault at the sending end is cleared, the DC current recovers rapidly under the control of the rectifier-side firing angle deviation and VDCOL. When the actual value of the DC current is detected to be greater than the command value, a firing angle increment Δα is output based on the deviation value ΔI between them as the input signal and added to the rectifier-side CC control. Δα increases with ΔI at a certain slope k. When ΔI increases to a certain value ΔI... H At that time, Δα remains at its maximum value Δα max .

[0035] Δα max The value of is determined by the DC voltage on the rectifier side. To ensure no commutation failure occurs during system fault recovery while avoiding excessive active power loss, the DC voltage on the rectifier side after the firing angle is added should not drop too much. This paper limits the maximum voltage drop to 20% of the rated operating voltage. As shown in the following formula: (15) The rated firing angle α on the rectifier side N Substituting 30° into the above equation, we get: Δα max =16°.

[0036] In Δα max Once determined, the slope k is determined by the maximum current deviation value ΔI. H The decision is as follows: if the value of k is too small, it is difficult to suppress the excessively fast recovery rate of the DC current; if the value of k is too large, it is not conducive to the recovery of the system's active power. Taking all factors into consideration, this paper takes the maximum current deviation value as 10% of the current command value, i.e., ΔI. H =0.1pu.

[0037] (2) Fault detection module.

[0038] As a starting criterion for improving the control strategy, the fault detection module is mainly used to identify the occurrence and clearing of AC faults at the sending end. Its judgment logic is as follows: Fault occurrence detection adopts a conventional CFPREV fault detection module, which detects the phase voltage u of the AC bus at the sending end through the sine and cosine component method. a u b and u c The amplitude of the voltage is measured and its zero-sequence voltage component is calculated to determine whether a single-phase short-circuit fault has occurred. Simultaneously, an abc-αβ coordinate transformation is used to detect three-phase short-circuit faults, thus preventing controller malfunctions. The latter determines whether the fault has been cleared by detecting whether the AC bus voltage change rate exceeds a threshold k*. When both detection conditions are met simultaneously, the module outputs a start signal (on), engaging the additional trigger angle control module; otherwise, it outputs a stop signal (off), and the rectifier side reverts to the original DC current control mode.

[0039] A simulation model of the hybrid cascaded system was built in PSCAD / EMTDC to verify the correctness of the above commutation failure mechanism and the effectiveness of the proposed suppression strategy.

[0040] To verify the correctness of the commutation failure mechanism, a three-phase ground fault with a ground inductance of 0.4H and a single-phase ground fault with a ground inductance of 0.07H were set to occur on the AC bus at the sending end of the system at 3.0s. The faults lasted for 0.05s. The simulation results are as follows: Figure 4 , Figure 5 As shown.

[0041] Depend on Figure 4 As can be seen in (a) of the diagram, under a three-phase ground fault at the sending end, from the time the fault is cleared in 3.05 seconds until the commutation failure occurs, the phase of the converter bus voltage at the sending end continuously shifts backward as active power recovers. During this period, the rectifier-side phase-locked loop cannot track the converter bus voltage phase in a timely and accurate manner, resulting in a deviation in the triggering of the rectifier-side LCC, such as... Figure 4 As shown in (b) above, the actual firing angle α act Always less than the trigger angle command value α ord At the same time, by Figure 4 As can be seen in (c), during the fault and fault recovery period, the DC voltage output of MMC1, which controls the DC voltage on the inverter side, remains near its steady-state value, compensating for part of the DC voltage drop in the system, thus causing the current command value I output by VDCOL control to remain near its steady-state value. vdcol Maintaining a consistently high level, the LCC doesn't even need to enter current-limiting mode, accelerating the DC current recovery rate. Figure 4 In (d) and (e), before the inverter-side LCC switches from CC control to CEA control, I dThe inverter output has recovered to above the rated value of 1.0 pu, and the CEC control output has dropped to 0. After the inverter side switches back to CEA control, the inverter side control command value β is now... CEA If the value is too small, it is difficult to meet the shut-off margin requirements of the converter valve, and the shut-off angle drops significantly, leading to commutation failure. The simulation results are consistent with the theoretical analysis above.

[0042] Depend on Figure 5 As shown in (a) and (b), similar to the three-phase fault condition, under a single-phase ground fault at the sending end, the phase of the converter bus voltage at the sending end also continuously shifts backward during the fault recovery period, resulting in a triggering deviation of the rectifier-side LCC. It is worth noting that when a single-phase asymmetrical fault occurs, negative sequence and harmonic components are introduced into the grid voltage. These components interfere with the dynamic tracking capability of the phase-locked loop, causing the actual triggering angle α of the rectifier-side LCC to decrease during the fault period. act The rectifier exhibits periodic fluctuations and cannot maintain minimum firing angle control on the rectifier side. However, this does not affect the firing angle change trend after the 3.05s fault is cleared. After the fault is cleared, the actual firing angle α of the LCC... act Gradually increases, and remains less than the trigger angle command value α. ord This aligns with the theoretical analysis above. Similarly, as... Figure 5 As shown in (c), the DC voltage output by the inverter-side MMC remains stable, the VDCOL current command value is relatively large, and I d Recovery too fast. Figure 5 In (d) and (e), when the inverter-side LCC switches to CEA control, I d It has been restored to above the rated value of 1.0 pu, and the CEA control command value β CEA If the angle is too small, it will be difficult to meet the turn-off margin requirements of the converter valve, the turn-off angle will drop significantly, and commutation failure will occur on the inverter side.

[0043] To verify the effectiveness of the proposed suppression strategy, the same ground fault as in the above mechanism analysis was set for the hybrid cascaded systems of the following two control schemes. The fault operation characteristics of the hybrid cascaded systems before and after implementing the suppression strategy were compared and analyzed. The simulation results are as follows: Figure 6 , Figure 7 As shown.

[0044] Control Scheme 1: Original Control Strategy for Hybrid Cascaded System Control scheme 2: Implement the suppression strategy of this invention.

[0045] Depend on Figure 6 As can be seen in (a) and (b), under a three-phase ground fault at the sending end, after the fault is cleared at the sending end in 3.05s, I d Recovery begins, with the rectifier side controlled by α. minWhen control switches back to CC control, the original control system experiences a surge in DC current at the end of the fault recovery phase, leading to commutation failure on the inverter side. After the suppression strategy is implemented, the system detects that the DC current has recovered to a certain value, and the additional trigger angle module on the rectifier side activates, such as... Figure 6 As shown in (c) and (d), the rectifier-side LCC output has an additional firing angle increment, which is superimposed on the original current control output. Compared to control scheme 1, the rectifier-side firing angle command value in control scheme 2 is larger. This compensates for the firing angle deviation caused by the phase-locked loop and, to some extent, suppresses the tendency of the rectifier-side DC voltage to surge, thereby slowing down the recovery rate of the system DC current. At the end of the recovery period, I... d When the current rises to near the rated value, the magnitude of the current is within the acceptable range for normal commutation of the inverter-side system, and commutation failure will not occur.

[0046] Figure 7 As can be seen from (a) and (b) in the diagram, under a single-phase ground fault at the sending end, the DC current of the original control system surges at the end of the fault recovery period, leading to commutation failure on the inverter side. When a suppression strategy is implemented, as shown in Figures (a) and (b), the DC current surges. Figure 7 As shown in (c) and (d), the additional firing angle control outputs an additional firing angle increment in real time based on the degree of DC current recovery, which is superimposed on the original current control output. Compared with control scheme 1, the rectifier-side firing angle command value in control scheme 2 is larger, compensating for the firing angle deviation caused by the phase-locked loop, and also suppressing the tendency of sudden increase in rectifier-side DC voltage to a certain extent, resulting in a smooth recovery of DC current. At the end of the recovery period, I... d The system stabilized near the rated value and did not experience commutation failure.

[0047] This invention fully considers the dynamic phase change of the rectifier-side AC bus voltage and the impact of the inverter-side MMC constant DC voltage on commutation failure during fault recovery. It analyzes the commutation failure mechanism of a hybrid cascaded HVDC transmission system caused by a sending-end fault and proposes a corresponding suppression strategy. This strategy dynamically adjusts the rectifier-side firing angle command based on the real-time monitoring of the deviation between the commanded and actual DC current values ​​during system fault recovery, thereby controlling the recovery rate and amplitude of the DC current. This suppresses commutation failure while ensuring stable DC current recovery.

[0048] This invention relates to the mechanism and suppression strategy of commutation failure in hybrid cascaded systems caused by sending-end faults. It mainly includes: (1) the response process of AC / DC system during the fault period was analyzed in stages according to the dynamic response characteristics of each electrical quantity and control quantity, and it was pointed out that the excessively fast recovery and excessive magnitude of DC current are the main reasons for the failure of receiving-end commutation; (2) the influence of the rectifier side firing angle deviation and the inverter side MMC constant DC voltage on the recovery of DC current during the fault recovery period was studied; (3) a control strategy based on the additional firing angle of the rectifier side was proposed. By monitoring the deviation between the DC current command value and the actual value during the fault recovery period of the system in real time, the firing angle command of the rectifier side was dynamically adjusted accordingly, thereby controlling the recovery rate and magnitude of DC current. While suppressing commutation failure, it can also ensure the smooth recovery of DC current.

Claims

1. A method for suppressing commutation failure in a hybrid cascaded system caused by a sending-end fault, characterized in that: The steps are as follows: S1, The inverter side model of the hybrid cascaded system is: （1） （2） （3） （4） （5） （6） （7） In the formula: I d P d These represent the inverter-side DC current and active power, respectively; U di U is the DC voltage on the inverter side. dLCC U dMMC These are the high and low voltage DC voltages on the inverter side, respectively; U LCC φ is the effective value of the LCC converter bus voltage; φ is the power factor angle; P LCC P MMC1 / 2 They are LCC and MMC respectively. 1 / 2 Power output; Q c The reactive power generated by the reactive power compensation device; α, μ, and γ are the LCC trigger delay angle, commutation overlap angle, and turn-off angle, respectively; X T is the leakage reactance of the LCC converter transformer; k is the turns ratio of the converter transformer between the LCC and the AC system; S2. Analysis of commutation failure in a hybrid cascaded HVDC transmission system caused by sending-end fault: S21. Dynamic response characteristics of AC / DC systems during faults: Phase I is the fault phase: the AC bus voltage at the sending end, U aci Sudden drop, DC current I dc This decrease leads to a drop in the output β of the inverter-side CC control. CC Gradually increase; simultaneously, CEC control starts, Δγ CEC Increase the output β of the CEA control on the inverter side. CEA It increases accordingly; Phase II is the fault recovery phase: After the fault at the sending end is cleared, the AC bus voltage U aci Recovery begins; the inverter-side system experiences DC current I under the influence of VDCOL. dc As it gradually rises, the rectifier side switches back to CC control; Phase III is the commutation failure phase: DC current I dc The continued recovery of β CC <β CEA The inverter side switches back to CEA control; S22. Phase shift of the sending-end bus voltage causes triggering deviation. Take U s As a reference phasor, the transverse component δU and longitudinal component ΔU of this voltage drop are expressed as follows: （8） （9） Combining equations (8) and (9), we can derive U. r with U s Phase difference δ between them: （10） Ignoring the minor factors in equation (10), we simplify to obtain U. r Phase offset φ: （11） rectifier firing angle α r Obtained from the following formula: （12） In the formula: U dr α is the DC voltage on the rectifier side. r This refers to the rectifier firing angle; S23, The Influence of MMC Constant Voltage on DC Current Recovery The VDCOL output expression is as follows: （13） In the formula: , ;U d This is the actual operating voltage on the DC side; U dh and U dl These are the upper and lower limits of the VDCOL control start-up voltage, respectively; I dh and I dl These are the upper and lower limits of the DC current, respectively; U dN and I dN These are the rated values ​​of DC voltage and DC current, respectively. The actual operating voltage on the DC side is rewritten as U d =ULCC d+UMMC d, therefore, the change in the current command value output by VDCOL during the fault recovery process is: （14） In the formula, ULCC d and UMMC d are the DC voltages of LCC and MMC, respectively; S3. Methods to suppress commutation failure in hybrid cascaded HVDC transmission systems caused by sending-end faults: S31, Additional trigger angle control module （15） S32. Fault Detection Module: The judgment logic is as follows: Fault detection uses the CFPREV fault detection module, which detects the phase voltage u of the AC bus at the sending end through the sine and cosine component method. a u b and u c The amplitude of the voltage is measured and the magnitude of its zero-sequence voltage component is calculated to determine whether a single-phase short-circuit fault has occurred; at the same time, the abc-αβ coordinate transformation is used to detect three-phase short-circuit faults, thereby avoiding controller malfunctions.

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