Method for suppressing LCC-HVDC commutation failure based on VSC-HVDC reactive power coordination control

Abstract

The invention relates to a method for suppressing LCC-HVDC commutation failure based on VSC-HVDC reactive power coordination control, and belongs to the technical field of high-voltage direct-current power transmission. According to the method, the running state of a sending end alternating current system is monitored in real time, and when it is detected that a fault enters a recovery stage, a reactive power coordination control mode of the VSC-HVDC system is triggered; the VSC-HVDC calculates the maximum output reactive power according to the current active power and the capacity of the VSC-HVDC, and absorbs reactive power from a sending end alternating current system at the maximum value, so that the rise of alternating voltage is inhibited, and the commutation failure risk of the inverter side of the LCC-HVDC is reduced; according to the method, the rapid reactive power regulation capability of the VSC-HVDC is fully utilized, the problems of voltage abnormity and commutation failure caused by reactive power imbalance in the sending end fault recovery process are effectively solved, and the transient stability of the hybrid DC power transmission system is improved.

Description

Technical Field

[0001] This invention relates to the field of high-voltage direct current transmission technology, specifically a reactive power coordination control method for suppressing commutation failure caused by AC faults at the sending end in an LCC-HVDC (high-voltage direct current transmission based on grid commutator) system in the power grid. Background Technology

[0002] Currently, in LCC-HVDC systems, when an AC fault occurs at the sending end, the electrical coupling and control coordination between the rectifier and inverter sides lead to frequent switching of the inverter-side control mode during fault recovery. In particular, the frequent triggering of current deviation control further reduces the inverter-side firing angle, potentially causing commutation failure. While some studies have proposed using reactive power clamping control or coordinated control strategies at the receiving end to suppress commutation failure, these methods primarily target receiving-end faults, with limited research on commutation failures caused by sending-end faults. Furthermore, they do not fully utilize the rapid reactive power regulation capabilities of VSC-HVDC (Voltage Source Converter-based High Voltage Direct Current Transmission) systems. In addition, existing control strategies are prone to abnormal AC voltage increases due to reactive power imbalance during sending-end fault recovery, further exacerbating the risk of commutation failure.

[0003] In summary, there is an urgent need for a method to effectively suppress commutation failures in LCC-HVDC caused by AC faults at the sending end, in order to improve the stability and reliability of hybrid DC transmission systems. Summary of the Invention

[0004] To overcome the problem of commutation failure caused by frequent switching of inverter-side control modes and reactive power imbalance during AC fault recovery in existing LCC-HVDC systems, this invention proposes a reactive power coordination control method based on VSC-HVDC. By utilizing the maximum reactive power regulation capability of VSC, the rise in AC voltage at the sending end is suppressed, thereby effectively reducing the probability of commutation failure.

[0005] This invention provides a method for suppressing commutation failure in LCC-HVDC based on VSC-HVDC reactive power coordination control, comprising the following steps: A method for suppressing commutation failure in LCC-HVDC based on VSC-HVDC reactive power coordination control includes the following steps: Step 1: Monitor the operating status of the sending-end AC system in real time to determine if a fault has occurred; Step 2: When a fault is detected in the AC system at the sending end and the recovery phase is entered, if the voltage of the converter bus at the sending end is higher than 0.8 pu and the voltage rise rate of the converter bus at the sending end is greater than 2 p.u. / s, the reactive power coordination control mode of the nearby VSC-HVDC system is triggered. If any condition is not met, the coordination control mode is exited. Step 3: In the reactive power coordinated control mode, the VSC-HVDC system determines the active power P it is currently operating under. VSC and its own capacity S VSC Calculate its maximum output reactive power Q. max_VSC ,in ; Step 4: Control the VSC-HVDC system to output its maximum reactive power Q. max_VSC Reactive power is absorbed from the sending-end AC system.

[0006] Furthermore, the faults in the sending-end AC system described in step one include three-phase grounding faults.

[0007] Furthermore, the recovery phase described in step two is the transient process during which the system voltage begins to recover after the fault in the sending-end AC system is cleared.

[0008] Furthermore, the reactive power absorbed from the sending-end AC system in step four serves to suppress the rise of the sending-end AC voltage, maintain the stability of the sending-end AC voltage, and avoid frequent switching of the control mode and arc extinction angle loss on the LCC-HVDC inverter side due to overvoltage, thereby reducing the risk of commutation failure on the LCC-HVDC inverter side.

[0009] Beneficial effects: This invention utilizes the rapid reactive power regulation capability of the VSC-HVDC system. When a fault occurs at the sending end and is cleared, it actively provides maximum reactive power support during the recovery phase based on judgment conditions. This effectively suppresses abnormal increases in AC voltage at the sending end, reduces frequent switching of control modes on the inverter side, and lowers the probability of commutation failure. The method has a simple structure, rapid response, and is suitable for hybrid LCC-VSC transmission systems, improving the transient stability and operational reliability of the system. Attached Figure Description

[0010] Figure 1 This is a topology diagram of the hybrid LCC-VSC delivery system of the present invention;

[0011] Figure 2 The simulation results are shown below, illustrating the control method of this invention.

[0012] Figure 3 The figure shows the simulation results using the traditional control method. Detailed Implementation

[0013] A method for suppressing commutation failure in LCC-HVDC based on VSC-HVDC reactive power coordination control includes the following steps: Step 1: Monitor the operating status of the sending-end AC system in real time to determine if a fault has occurred; Step 2: When a fault is detected in the AC system at the sending end and the recovery phase is entered, if the voltage of the converter bus at the sending end is higher than 0.8 pu and the voltage rise rate of the converter bus at the sending end is greater than 2 p.u. / s, the reactive power coordination control mode of the nearby VSC-HVDC system is triggered. If any condition is not met, the coordination control mode is exited. Step 3: In the reactive power coordinated control mode, the VSC-HVDC system determines the active power P it is currently operating under. VSC and its own capacity S VSC Calculate its maximum output reactive power Q. max_VSC ,in ; Step 4: Control the VSC-HVDC system to output its maximum reactive power Q. max_VSC Reactive power is absorbed from the sending-end AC system.

[0014] It should be noted that the faults in the sending-end AC system mentioned in step one include three-phase grounding faults. Considering that three-phase grounding faults are the most serious of all fault types in the sending-end AC system and have the greatest impact on the system, if the method provided by this invention can effectively solve three-phase grounding faults, then solving other types of faults will not be difficult.

[0015] It should be noted that the recovery phase described in step two is the transient process during which the system voltage begins to recover after the fault in the sending-end AC system is cleared.

[0016] It should be understood that the fault diagnosis mentioned in step one, the recovery phase mentioned in step two, and the detection of the judgment conditions, including the start and stop of the reactive power coordinated control mode of the VSC-HVDC system, all require the data acquisition and control command transmission from the sensors connected to the monitoring system of the distribution network. This is a common technical means in this field.

[0017] It should be noted that the reactive power absorbed from the sending-end AC system in step four is intended to suppress the rise of the sending-end AC voltage, maintain the stability of the sending-end AC voltage, and avoid frequent switching of the control mode and arc extinction angle loss on the LCC-HVDC inverter side due to overvoltage, thereby reducing the risk of commutation failure on the LCC-HVDC inverter side. Example

[0018] In a typical embodiment of the present invention, PSCAD / EMTDC is constructed as follows: Figure 1The simulation model of the hybrid LCC-VSC sending-end system is shown. System parameters are as follows: short-circuit ratio (SCR) at both the sending and receiving ends is 2.5, AC voltage is 345kV, LCC-HVDC rated power is 1000MW, and VSC-HVDC rated power is 600MW. Real-time monitoring of the sending-end AC system voltage and current is performed. At 1 second, a three-phase ground fault is detected at the common coupling point (PCC) of the sending-end system. The fault is cleared at 1.1 seconds. After the fault is cleared, the system voltage enters the recovery phase. At this time, the voltage of the sending-end converter bus is detected to be 1.2 pu, higher than 0.8 pu, and the voltage rise rate of the sending-end converter bus is 5.4 pu / s, greater than 2 p.u. / s. Therefore, the reactive power coordination control mode of the VSC-HVDC system is immediately triggered. The VSC-HVDC system adjusts the reactive power based on the current active power P. VSC =400MW and capacity S VSC =600MVA, calculate the maximum reactive power. After rounding, the value is 447.2 Mvar. The VSC-HVDC system then absorbs 447.2 Mvar of reactive power from the sending-end AC system to suppress voltage rise and maintain voltage stability, thereby preventing commutation failure on the LCC-HVDC inverter side. For detailed simulation results, please refer to [link to simulation results]. Figure 2 As shown: like Figure 2 As shown in Figure a, when a three-phase ground fault occurs at the common coupling point PCC of the sending-end system within 1 second, the PCC voltage drops to approximately 0.66 pu, and then rises to approximately 1.2 pu after the fault is cleared in 1.1 seconds. Figure 2 Figure b shows that in the reactive power coordination control mode of the hybrid LCC-VSC sending-end system in the VSC-HVDC system, the arc extinction angle is always greater than the minimum arc extinction angle, and no commutation failure occurs. Figure 2 The 'c' in the figure shows that in the reactive power coordinated control mode of the VSC-HVDC system, only one control switching process occurred in the sending-end system, and the arc extinction angle remained under control. Figure 2 The 'd' in the figure indicates that the AC voltage at the sending end recovers to a stable state within a short time without overvoltage.

[0019] Comparative Example 1 Build in PSCAD / EMTDC as follows Figure 1The simulation model of the hybrid LCC-VSC sending-end system shown has the following system parameters: short-circuit ratio (SCR) of 2.5 at both the sending and receiving ends, AC voltage of 345kV, rated power of 1000MW for the LCC-HVDC, and rated power of 600MW for the VSC-HVDC. A three-phase ground fault occurs in the PCC of the sending-end system within 1 second, and the fault is cleared in 1.1 seconds. A traditional control mode is adopted, with the VSC-HVDC not participating in reactive power regulation throughout the process to suppress commutation failure of the LCC-HVDC. For detailed simulation results, please refer to [link to simulation results]. Figure 3 As shown: like Figure 3 As shown in Figure a, when a three-phase ground fault occurs in the PCC of the sending-end system within 1 second, the PCC voltage drops to approximately 0.66 pu, then rises to approximately 1.2 pu after the fault is cleared in 1.1 seconds, and drops to 0 at 1.15 seconds. Figure 3 Figure b shows that the arc extinction angle of the inverter-side LCC decreases to approximately 0 degrees at about 1.15s, which means that commutation failure has occurred in the inverter-side LCC. Accordingly, the DC voltage of the LCC drops to a very low level (even to 0). Figure 3 The 'c' in the figure shows that after the fault was cleared, multiple control switching occurred on the inverter side, causing the arc extinction angle to become uncontrollable, which in turn led to commutation failure. Figure 3 The 'd' in the figure indicates that during the fault recovery process, excessive reactive power compensation occurred on the rectifier side, resulting in an increase in the AC voltage at the PCC point, exceeding 1.1 pu, which increased the risk of commutation failure.

[0020] By comparing Example 1 and Comparative Example 1, it can be concluded that the present invention can indeed actively provide maximum reactive power support during the recovery phase based on the judgment conditions after a three-phase ground fault occurs at the sending end and the fault is cleared. This effectively suppresses the abnormal rise of AC voltage at the sending end, reduces the frequent switching of control modes on the inverter side, and lowers the probability of commutation failure. In addition, the present invention has a rapid response and is applicable to hybrid LCC-VSC transmission systems, which can improve the transient stability and operational reliability of the system.

Claims

1. A method for suppressing commutation failure in LCC-HVDC based on VSC-HVDC reactive power coordinated control, characterized in that, Includes the following steps: Step 1: Monitor the operating status of the sending-end AC system in real time to determine if a fault has occurred; Step 2: When a fault is detected in the AC system at the sending end and the recovery phase is entered, if the voltage of the converter bus at the sending end is higher than 0.8 pu and the voltage rise rate of the converter bus at the sending end is greater than 2 p.u. / s, the reactive power coordination control mode of the nearby VSC-HVDC system is triggered. If any condition is not met, the coordination control mode is exited. Step 3: In the reactive power coordinated control mode, the VSC-HVDC system determines the active power P it is currently operating under. VSC and its own capacity S VSC Calculate its maximum output reactive power Q. max_VSC ,in ; Step 4: Control the VSC-HVDC system to output its maximum reactive power Q. max_VSC Reactive power is absorbed from the sending-end AC system.

2. The method for suppressing LCC-HVDC commutation failure based on VSC-HVDC reactive power coordination control as described in claim 1, characterized in that: The faults in the sending-end AC system mentioned in step one include three-phase grounding faults.

3. The method for suppressing LCC-HVDC commutation failure based on VSC-HVDC reactive power coordination control as described in claim 1, characterized in that: The recovery phase described in step two is the transient process during which the system voltage begins to recover after the fault in the sending-end AC system is cleared.

4. The method for suppressing LCC-HVDC commutation failure based on VSC-HVDC reactive power coordination control as described in claim 1, characterized in that: The reactive power absorbed from the sending-end AC system in step four serves to suppress the rise of the sending-end AC voltage, maintain the stability of the sending-end AC voltage, and avoid frequent switching of the control mode and arc extinction angle loss on the LCC-HVDC inverter side due to overvoltage, thereby reducing the risk of commutation failure on the LCC-HVDC inverter side.

Images