A method for suppressing transient overvoltage in AC system at the sending end

Abstract

In the face of high-risk events that may occur in AC / DC hybrid systems under large-scale new energy grid connection, the present invention discloses a method for suppressing transient overvoltages in the sending-end AC system. This evaluation index comprehensively considers the system short-circuit ratio SCR and reactive compensation device parameters to quantitatively evaluate the stability of the system under sending-end faults. When it is applied to short-circuit faults on the AC side of large-scale AC / DC systems, it can quantify the impact of the strength of the AC system and the reactive compensation capacity on the system fault response, reflect the dynamic changes of transient voltage during short-circuit faults on the AC side, and can more accurately and quickly evaluate the fault risk of the sending-end AC system. The application of the indicators of the present invention provides theoretical guidance for the planning and operation of high-voltage DC transmission systems and the switching of reactive compensation devices.

Description

Technical field:

[0001] The invention belongs to the field of ultra-high voltage direct current transmission, and relates to a method for suppressing transient overvoltage of a sending-end alternating current system, which is suitable for quantitative evaluation of transient voltage rise after short-circuit fault and DC blocking fault on the AC side of a large-scale AC / DC system. Background technology:

[0002] With the development of UHV DC transmission, my country's "strong DC and weak AC" transmission structure is prone to transient faults in the AC and DC systems. Transient faults in the AC system mainly include short-circuit faults in the AC system on the rectifier and inverter sides, AC single-phase reclosing faults, AC filter faults, etc. In particular, when a short-circuit fault occurs in the AC system at the sending end, the DC system is prone to phase-change failure during the system recovery phase after the fault is removed, which will cause transient overvoltage in the AC system at the sending end; when the fault is more serious, multiple DC locking faults will cause interruption of DC power transmission, and driven by the surplus of reactive power compensation, it will cause transient overvoltage in the sending end system, resulting in large-scale chain disconnection of wind turbines, and ultimately causing the entire system to collapse.

[0003] In order to reflect the transient process of AC / DC system under fault conditions, the establishment of transient performance indicators of AC / DC power system can quickly evaluate the AC system's ability to accept DC power, preliminarily determine the safety and stability level of AC / DC power system and possible safety and stability problems. Therefore, quantitative evaluation of transient performance indicators of AC / DC system and proposing improvement measures are of great guiding significance for the planning, design and dispatching operation of AC / DC power system. Summary of the invention:

[0004] The present invention derives a transient overvoltage calculation method based on reactive compensation surplus and AC system short-circuit capacity, and proposes an improved transient voltage index of the sending-end AC system to quantitatively evaluate stability, so that it can be applied to short-circuit faults on the AC side of large-scale AC / DC systems. It is not only limited to the transient voltage boost problem in DC blocking faults, but also considers the dynamic changes of transient voltage during the short-circuit fault of the sending-end AC system. It can quickly and accurately evaluate the fault risk of the sending-end AC system and provide theoretical guidance for the capacity configuration of the reactive compensation device.

[0005] The technical solution of the present invention is as follows:

[0006] A method for suppressing transient overvoltage in a sending-end AC system, comprising:

[0007] Obtain the short circuit ratio S when a short circuit fault occurs in the AC system CR ;

[0008] Determine the voltage drop depth after obtaining system faults;

[0009] Calculate the index reference value R under this fault level based on the voltage drop depth r0 , R r1 ;

[0010] Get the capacitance impedance X at the commutation bus c and the AC filter impedance X at the commutation bus f , based on the AC system short-circuit ratio S CR , the capacitance impedance at the commutation bus is X c and the AC filter impedance X at the commutation bus f Three measurement quantities are used to extract the transient voltage rise evaluation index R of the actual system r ;

[0011] According to the indicator R r The value of the transient overvoltage level of the system after the fault is evaluated, and whether the capacity configuration of the reactive power compensation device needs to be re-planned;

[0012] Switch on and off the reactive power compensation device as needed until the indicator R r0 <R r <R r1 .

[0013] The method for suppressing transient overvoltage of a sending-end AC system specifically comprises:

[0014] After an AC system failure, the reactive power compensation capacity of the AC filter and capacitor will increase with the rise of transient voltage, and the remaining reactive power compensation power Q of the capacitor will increase. cr1 and the residual reactive power compensation power Q of the AC filter cr2 , respectively expressed as:

[0015]

[0016] Where U'2 is the bus voltage value after the fault, so the residual reactive power of the system after the fault is Q cr for:

[0017]

[0018] Combined with the AC system short circuit ratio S CR , then the extracted index R r for:

[0019] The method for suppressing transient overvoltage of the AC system at the sending end is as follows:

[0020] By comparing the transient voltage rise evaluation index R r And transient voltage rise evaluation index reference value R r0 , R r1 To determine whether the capacity of the reactive power compensation device needs to be reconfigured;

[0021] The commutation bus voltage can be expressed as:

[0022]

[0023] Among them, P dN Rated power transmission for DC system, U 2N is the rated voltage of the commutation bus, and U2' is the voltage value of the commutation bus after the fault;

[0024] According to the high voltage ride-through standard for new energy units, when the side grid voltage suddenly rises to 130% of the rated voltage, the wind turbine unit should maintain continuous operation for 0.5s without disconnecting from the grid, and the photovoltaic power station should continue to operate for at least 0.5s; therefore, the specified indicator R r0 The corresponding transient overvoltage rises to 1.3pu, R r1 Corresponding to the nominal voltage 1.0pu. R r ≤R r0 When a short circuit fault occurs at the commutation busbar of the sending-end AC system, the transient overvoltage of the AC system exceeds 1.3pu, and the new energy unit is in danger of being disconnected after a fault. The system needs to reconfigure the capacity of the reactive power compensation device; the method of switching the reactive power compensation device is adopted to the index R r0 <R r <R r1 . R r0 <R r <R r1 When the transient voltage of the AC system is calculated to be within the range of 1.0pu to 1.3pu, after a short-circuit fault occurs at the commutation busbar of the AC system at the sending end, there is no danger of the new energy unit being disconnected after the fault, and there is no need to switch on the reactive power compensation device; R r ≥R r1 When the calculated transient voltage of the AC system is less than 1.0pu, reactive power compensation equipment needs to be put into use.

[0025] The utilization index R r The method for evaluating transient overvoltage in the sending-end AC system is:

[0026] Step (1) According to the high voltage ride-through standard of new energy units, when the voltage of the high-voltage side grid rises suddenly to 130% of the rated voltage, the wind turbine unit should maintain continuous operation for 0.5s without disconnecting from the grid, and the photovoltaic power station should continue to operate for at least 0.5s; when a short-circuit fault occurs in the sending-end AC system, the voltage drop depth after the fault is obtained, and the system parameter reference value of the AC system short-circuit ratio S when the transient overvoltage of the sending-end AC system is 1.3pu under this fault degree is taken CR , static capacitance impedance X at the commutation bus c0 and the AC filter impedance X at the commutation bus f0 , then R r0 for:

[0027]

[0028] Take the system parameter reference value of the AC system short-circuit ratio S when the transient overvoltage of the AC system at the sending end is 1.0pu under the same fault degree CR , static capacitance impedance X at the commutation bus c1 and the AC filter impedance X at the commutation bus f1 , then R r1 for:

[0029]

[0030] Step (2) Obtain the static capacitance impedance X at the commutation bus c and the AC filter impedance X at the commutation bus f , based on S CR , X c , X f Three measurement quantities, calculate the transient voltage rise evaluation index R r ;

[0031] Step (3) If the indicator R r <R r0 , after a short-circuit fault occurs at the commutation busbar of the sending-end AC system, the transient overvoltage of the AC system exceeds 1.3pu, and the new energy unit is in danger of being disconnected after a fault. The AC system strength is low and the reactive power surplus is large. The system needs to reconfigure the capacity of the reactive power compensation device. The method of cutting out the reactive power compensation device is adopted to the index R r0 <R r <R r1 Return to step (2) to obtain the AC system short-circuit ratio S of the new system. CR , static capacitance impedance X at the commutation bus c and the AC filter impedance X at the commutation bus f , and calculate the index R r , re-evaluate the transient overvoltage; if R r0 <R r <R r1 When the transient voltage of the AC system is calculated, it does not exceed 1.3pu, indicating that the transient overvoltage of the AC system will not exceed 1.3pu after a short-circuit fault occurs in the AC system at the sending end. There is no danger of the new energy unit being disconnected after a fault. The AC system has a high strength and a small reactive power surplus. The risk of system instability is low, and there is no need to reconfigure the capacity of the reactive power compensation device. r ≥R r1 When the calculated transient voltage of the AC system is less than 1.0pu, it indicates that the strength of the AC system is high and the reactive power compensation margin is insufficient, and reactive power compensation equipment needs to be invested.

[0032] The above-mentioned method is applied in the fault risk assessment of the sending-end AC system.

[0033] In addition, the present invention also discloses a computer-readable storage medium storing instructions, which can implement any of the above-mentioned methods when executed.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] The present invention derives the transient overvoltage calculation method based on reactive power compensation surplus and AC system short-circuit capacity, and proposes an improved transient voltage index of the sending-end AC system to quantitatively evaluate stability. The index includes the AC system short-circuit ratio S CR And reactive power compensation device parameters X c , X f , to evaluate the degree of transient voltage rise after the fault; when an AC system short circuit fault occurs in the system, as the short circuit ratio decreases and the reactive power compensation margin increases, the index R r The transient overvoltage rise degree after the fault increases; compared with the existing transient overvoltage assessment indicators, the indicator of the present invention can more accurately quantify and assess the fault risk of the sending-end AC system, and can assess the degree of commutation failure at the receiving end caused by the sending-end fault, providing theoretical guidance for the planning and operation of the UHV DC transmission system and the capacity configuration of the reactive compensation device. Description of the drawings:

[0036] Figure 1 It is a control flow chart of the present invention in the embodiment;

[0037] Figure 2 Equivalent circuit diagram of high voltage direct current transmission sending end system;

[0038] Figure 3 This is the transient overvoltage change diagram when a three-phase short-circuit fault occurs on the rectifier side when the short-circuit ratio is small;

[0039] Figure 4 This is the transient overvoltage change diagram of a three-phase short circuit fault on the rectifier side after reconfiguring the reactive compensation capacity;

[0040] Figure 5 This is the transient overvoltage change diagram when a three-phase short circuit fault occurs on the rectifier side when the short circuit ratio is large;

[0041] Figure 6 This is the transient overvoltage change diagram when a three-phase short circuit fault occurs on the rectifier side after reconfiguring the reactive compensation capacity. Specific implementation method:

[0042] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is described in detail below in conjunction with the accompanying drawings and implementation examples. It should be understood that the specific implementation examples described herein are only used to explain the present invention and are not used to limit the invention.

[0043] Application Example 1:

[0044] In this embodiment, the equivalent circuit diagram of the high-voltage direct current transmission sending end system is as follows: Figure 2 As shown in the figure, the fault is set as a three-phase short circuit fault at the commutation bus of the sending-end AC system, and the voltage drops to 30%. The fault occurs at 2s and lasts for 0.2s.

[0045] (1) Obtaining the AC system short-circuit ratio S CR Take the system parameters when the AC system transient overvoltage is 1.3pu under the above fault level as the reference value. At this time, the AC filter impedance X f0 =-237.01Ω, static capacitance impedance at the commutation bus X c0 =-192.92Ω, let the transient voltage rise index standard at this time be the reference value, and make it normalized to R r0 =1; obtain the system parameters when the transient overvoltage of the AC system is 1.0pu under the same fault. At this time, the AC filter impedance X f1 =-237.01Ω, static capacitance impedance at the commutation bus X c1 =-397.89Ω, index R r1 =1.397;

[0046] (2) Obtain the static capacitance impedance X at the commutation bus c =-144.69Ω and AC filter impedance X at the commutation bus f = -237.01Ω, based on S CR , X c , X f Three measurement quantities, calculate the transient voltage rise evaluation index R r =0.845;

[0047] (3) If the transient voltage rise is higher than 1.3 pu, the indicator R r Less than 1, the transient overvoltage waveform after the fault is shown in Figure 3, indicating that the system strength is low and the reactive power surplus of the converter station is large, and the new energy unit should be cut off immediately. At this time, the static capacitor is cut out to the index R r0 <R r <R r1 Return to step (2) to obtain the AC system short-circuit ratio S of the new system. CR =2.5, static capacitance impedance at the commutation bus X c=-244.85Ω and AC filter impedance X at the commutation bus f =-237.01Ω, and calculate the index R r =1.132. The transient overvoltage curve of the new system after the fault is as follows Figure 4 shown.

[0048] The transient voltage change of the three-phase short-circuit fault of this embodiment is shown in Table 1. Comparison of the table data and the transient overvoltage waveform shows that the transient voltage rise level is significantly improved and the risk of system instability is reduced.

[0049] Table 1 Transient voltage changes of three-phase short-circuit fault based on reactive power compensation capacity

[0050] SCR <![CDATA[Reactive power reserve Q cr / Mva]]> <![CDATA[Indicator R r > Transient voltage / pu 2.5 2980.51 0.845 1.455 2.5 1341.924 1.132 1.162

[0051] Application Example 2:

[0052] In this embodiment, the fault is set to be a three-phase short circuit fault at the commutation bus of the sending-end AC system, and the voltage drops to 30%. The fault occurs at 2s and lasts for 0.2s.

[0053] (1) Obtaining the AC system short-circuit ratio S CR =2.96. The system parameters when the AC system transient overvoltage is 1.3pu are taken as the reference values. At this time, the AC filter impedance X f0 =-237.01Ω, static capacitance impedance at the commutation bus X c0 =-151.58Ω, let the transient voltage rise index standard at this time be the reference value, and make it normalized to R r0 =1; obtain the system parameters when the transient overvoltage of the AC system is 1.0pu under the same fault. At this time, the AC filter impedance X f1 =-237.01Ω, static capacitance impedance at the commutation bus X c1 =-424.413Ω, index R r1 =1.645;

[0054] (2) Obtain the static capacitance impedance X at the commutation bus c =-106.10Ω and AC filter impedance X at the commutation bus f =-237.01Ω, calculate the transient voltage rise evaluation index R r =0.793.

[0055] (3) If the transient voltage rise is higher than 1.3 pu, the indicator R r Less than 1, the transient overvoltage waveform after the fault is shown in Figure 5, indicating that the system strength is low and the reactive power surplus of the converter station is large, and the new energy unit should be cut off immediately. At this time, the static capacitor is cut out to the index R r0 <Rr <R r1 Return to step (2) to obtain the AC system short-circuit ratio S of the new system. CR =2.96, static capacitance impedance at the commutation bus X c =-212.207Ω and AC filter impedance X at the commutation bus f =-237.01Ω, at this time the indicator R r =1.211. The transient overvoltage curve of the new system after the fault is as follows Figure 6 shown.

[0056] The transient voltage change of the three-phase short-circuit fault of this embodiment is shown in Table 2. Comparison of the table data and the transient overvoltage waveform shows that the transient voltage rise level is significantly improved and the risk of system instability is reduced.

[0057] Table 2 Transient voltage changes of three-phase short-circuit faults based on AC system strength

[0058] SCR <![CDATA[Reactive power remaining Q cr / Mva]]> <![CDATA[Indicator R r > Transient voltage / pu 2.96 3357.195 0.845 1.502 2.96 1640.89 1.211 1.205

[0059] In some examples of the present invention, there is also a computer-readable storage medium storing instructions. When the instructions are executed, the above-mentioned method for suppressing transient overvoltage indicators of the AC system at the sending end can be implemented. More specifically, the instructions can be a computer-readable language. The above-mentioned computer can be a general-purpose computer device or a special-purpose computer device. In a specific implementation, the computer can be a desktop computer, a portable computer, a network server, a PDA (Personal Digital Assistant), a mobile phone, a tablet computer, a wireless terminal device, a communication device or an embedded device. The storage medium can be any available medium that can be accessed by the computer or a data storage device such as a server or a data center that includes one or more available media. For example, the storage medium is, for example, but not limited to, a magnetic medium (for example, a floppy disk, a hard disk, a tape), an optical medium (for example, a digital versatile disc (DVD)), or a semiconductor medium (for example, a solid-state drive (SSD)).

Claims

1. A method for suppressing transient overvoltage in a sending-end AC system, characterized in that: The following steps are involved: Obtain the short circuit ratio S when a short circuit fault occurs in the AC system CR ; Determine the voltage drop depth after obtaining system faults; Calculate the index reference value R corresponding to the fault degree based on the voltage drop depth r0 , R r1 ; Get the capacitance impedance X at the commutation bus c and the AC filter impedance X at the commutation bus f , based on the AC system short-circuit ratio S CR , the capacitance impedance at the commutation bus is X c and the AC filter impedance X at the commutation bus f Three measurement quantities are used to extract the transient voltage rise evaluation index R of the actual system. r ; According to the indicator R r The value of the transient overvoltage level of the system after the fault is evaluated, and whether the capacity configuration of the reactive power compensation device needs to be re-planned; Switch on and off the reactive power compensation device as needed until the indicator R r0 <R r <R r1 ; The transient voltage rise evaluation index R of the actual system is extracted. r Extraction methods include: After an AC system failure, the reactive power compensation capacity of the AC filter and capacitor will increase with the rise of transient voltage, and the remaining reactive power compensation power Q of the capacitor will increase. cr1 and the residual reactive power compensation power Q of the AC filter cr2 , respectively expressed as: Where U'2 is the bus voltage value after the fault, so the residual reactive power of the system after the fault is Q cr for: Combined with the AC system short circuit ratio S CR , then the extracted index R r for: According to the indicator R r The method for evaluating the transient overvoltage level of the system after a fault is: By comparing the transient voltage rise evaluation index R r And transient voltage rise evaluation index reference value R r0 , R r1 To determine whether the capacity of the reactive power compensation device needs to be reconfigured; The calculation method of commutation bus voltage is: Among them, P dN Rated power transmission for DC system, U 2N is the rated voltage of the commutation bus, U'2 is the voltage value of the commutation bus after the fault; R r ≤R r0 When a short circuit fault occurs at the commutation busbar of the sending-end AC system, the transient overvoltage of the AC system exceeds 1.3pu, and the new energy unit is in danger of being disconnected after a fault. The system needs to reconfigure the capacity of the reactive power compensation device; the method of switching the reactive power compensation device is adopted to the index R r0 <R r <R r1 , R r0 <R r <R r1 When the transient voltage of the AC system is calculated to be within the range of 1.0pu to 1.3pu, after a short-circuit fault occurs at the commutation busbar of the AC system at the sending end, there is no danger of the new energy unit being disconnected after the fault, and there is no need to switch on the reactive power compensation device; R r ≥R r1 When the calculated transient voltage of the AC system is less than 1.0pu, reactive power compensation equipment needs to be put into use.

2. A method for suppressing transient overvoltage in a sending-end AC system according to claim 1, characterized in that: The transient voltage rise evaluation index reference value R r0 , R r1 The calculation method is: When a short-circuit fault occurs in the AC system at the sending end, the voltage drop depth after the fault is obtained, and the system parameter reference value when the transient overvoltage of the AC system at the sending end is 1.3pu under this fault degree is taken. The AC system short-circuit ratio S CR , static capacitance impedance X at the commutation bus c0 and the AC filter impedance X at the commutation bus f0 , then R r0 for:

3. Application of the method described in any one of claims 1 to 2 in fault risk assessment of a sending-end AC system.

4. A computer-readable storage medium storing instructions, which, when executed, can implement the method according to any one of claims 1 to 2.

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