A method and system for transient current protection of a flexible direct current transmission system

Abstract

The application discloses a kind of flexible direct current transmission system transient current protection methods, comprising: when detecting that ground fault occurs between bridge arm reactor and converter valve, obtain fault related parameters;Wherein, the fault related parameters include fault current;According to the fault related parameters, calculate the voltage maximum transient value before and after locking;According to the voltage maximum transient value, the current rise rate of unblocking fault phase is calculated;When the fault current and the current rise rate meet the preset condition, di / dt protection is triggered;The application can guarantee the safety of system and equipment under extreme fault condition through the collaborative design of di / dt protection and valve control overcurrent protection, so that the flexible direct current transmission system can operate safely and stably.

Description

Technical Field

[0001] This invention relates to the field of flexible DC transmission technology, and in particular to a method and system for transient current protection of flexible DC transmission systems. Background Technology

[0002] Currently, flexible DC transmission has entered a stage of rapid development, with a large number of flexible DC transmission projects emerging around the world. As the demand for long-distance power transmission increases, it is gradually moving towards higher voltage levels and larger capacities. The transmission capacity of the converter valve is a crucial factor restricting the development of flexible DC transmission technology. With the increase in transmission capacity, the utilization of converter valve components is more thorough, and the overcurrent problems they face during transient processes become more severe, requiring more comprehensive and in-depth research.

[0003] Traditional overcurrent protection strategies for flexible DC transmission mainly rely on valve-controlled overcurrent protection. When the arm current exceeds its set value, the converter valve of that arm is triggered to lock out after a certain time delay. However, in cases where the fault current develops too rapidly, the converter valve may not lock out in time, causing damage to the device. Therefore, existing strategies also include di / dt protection to address this issue. However, when a fault occurs between the arm reactor and the converter valve, or when two-phase or three-phase ground faults occur simultaneously, the fault current development rate may exhibit phased characteristics, increasing the difficulty of protection judgment. How to analyze the system fault characteristics and achieve system and equipment safety under various extreme fault conditions through the coordinated operation of valve-controlled overcurrent protection and di / dt protection is the key to the development of overcurrent protection technology for flexible DC transmission and a core issue that needs to be addressed. Summary of the Invention

[0004] This invention provides a transient current protection method for flexible DC transmission systems. By coordinating valve-controlled overcurrent protection and di / dt protection, the method can ensure the safety of the system and equipment under extreme fault conditions, enabling the flexible DC transmission system to operate safely and stably.

[0005] In a first aspect, embodiments of the present invention provide a transient current protection method for a flexible DC transmission system, comprising: when a ground fault is detected between a bridge arm reactor and a converter valve, acquiring fault-related parameters; wherein, the fault-related parameters include fault current;

[0006] Based on the fault-related parameters, calculate the maximum voltage transient before and after blocking;

[0007] Calculate the rate of rise of the current in the unblocked fault phase based on the maximum transient voltage value.

[0008] When the fault current and the current rise rate meet preset conditions, di / dt protection is triggered.

[0009] Furthermore, the fault-related parameters also include the average voltage of the submodule, and the calculation process for the average voltage of the submodule includes:

[0010] Obtain the number of full-bridge submodules, the number of half-bridge submodules, and the maximum voltage across the converter valve;

[0011] The average voltage of the sub-modules is calculated based on the number of full-bridge sub-modules, the number of half-bridge sub-modules, and the maximum voltage.

[0012] Furthermore, calculating the maximum voltage transient value before and after blocking based on the fault-related parameters includes:

[0013] Calculate the minimum valve-side voltage before locking and the maximum valve-side voltage after locking;

[0014] The absolute value of the difference between the minimum and maximum valve-side voltage is calculated to obtain the maximum transient voltage value.

[0015] Furthermore, the step of calculating the current rise rate of the unblocked fault phase based on the maximum transient voltage value includes: obtaining the equivalent reactance of the transformer;

[0016] Based on the equivalent reactance of the transformer and the maximum transient voltage value, the current rise rate of the unblocked fault phase is calculated.

[0017] Furthermore, the step of triggering di / dt protection when the fault current and the current rise rate meet preset conditions includes:

[0018] When the fault current is greater than or equal to the preset protection action setting and the current rise rate is greater than or equal to the preset current rise rate setting, the di / dt protection is triggered.

[0019] Furthermore, the fault-related parameters also include the submodule's safe and stable operating current value, the valve-controlled overcurrent protection setting value, and the valve-controlled overcurrent protection action delay. Therefore, the process for determining the current rise rate setting value includes:

[0020] The current rise rate setting is determined based on the safe and stable operating current value of the submodule, the valve-controlled overcurrent protection setting, and the valve-controlled overcurrent protection action delay.

[0021] Secondly, embodiments of the present invention provide a transient current protection system for a flexible DC transmission system, comprising: a data acquisition module, used to acquire fault-related parameters when a ground fault is detected between a bridge arm reactor and a converter valve; wherein the fault-related parameters include fault current;

[0022] The voltage calculation module is used to calculate the maximum transient voltage value before and after blocking based on the fault-related parameters.

[0023] The current calculation module is used to calculate the current rise rate of the unblocked fault phase based on the maximum transient voltage value.

[0024] The protection strategy module is used to trigger di / dt protection when the fault current and the current rise rate meet preset conditions.

[0025] Furthermore, the fault-related parameters also include the average voltage of the submodule, and the calculation process for the average voltage of the submodule includes:

[0026] Obtain the number of full-bridge submodules, the number of half-bridge submodules, and the maximum voltage across the converter valve;

[0027] The average voltage of the sub-modules is calculated based on the number of full-bridge sub-modules, the number of half-bridge sub-modules, and the maximum voltage.

[0028] Furthermore, the voltage calculation module is also used for:

[0029] Calculate the minimum valve-side voltage before locking and the maximum valve-side voltage after locking;

[0030] The absolute value of the difference between the minimum and maximum valve-side voltage is calculated to obtain the maximum transient voltage value.

[0031] Furthermore, the current calculation module is also used for:

[0032] Obtain the equivalent reactance of the transformer;

[0033] Based on the equivalent reactance of the transformer and the maximum transient voltage value, the current rise rate of the unblocked fault phase is calculated.

[0034] Compared with existing technologies, the transient current protection method for flexible DC transmission systems provided by this invention has the following advantages: When a ground fault is detected between the bridge arm reactor and the converter valve, fault-related parameters are obtained; wherein, the fault-related parameters include the fault current; based on the fault-related parameters, the maximum transient voltage value before and after blocking is calculated; based on the maximum transient voltage value, the current rise rate of the unblocked fault phase is calculated; when the fault current and the current rise rate meet preset conditions, di / dt protection is triggered; this enables valve-controlled overcurrent protection and di / dt protection to coordinate and cooperate, ensuring the safety of the system and equipment under extreme fault conditions, and enabling the flexible DC transmission system to operate safely and stably. Attached Figure Description

[0035] To more clearly illustrate the technical features of the embodiments of the present invention, the drawings used in the embodiments of the present invention will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 This is a schematic flowchart of a transient current protection method for a flexible DC transmission system provided in an embodiment of the present invention;

[0037] Figure 2 This is a schematic diagram of the di / dt limit value of a transient current protection method for a flexible DC transmission system provided in an embodiment of the present invention;

[0038] Figure 3 This is a schematic diagram of the first waveform of a transient current protection method for a flexible DC transmission system provided in an embodiment of the present invention;

[0039] Figure 4 This is a schematic diagram of the second waveform of a transient current protection method for a flexible DC transmission system provided in an embodiment of the present invention;

[0040] Figure 5 This is a schematic diagram of the third waveform of a transient current protection method for a flexible DC transmission system provided in an embodiment of the present invention;

[0041] Figure 6 This is a fourth waveform diagram of a transient current protection method for a flexible DC transmission system provided in an embodiment of the present invention;

[0042] Figure 7 This is a schematic diagram of the structure of a transient current protection system for a flexible DC transmission system provided in an embodiment of the present invention. Detailed Implementation

[0043] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0044] In a first aspect, embodiments of the present invention provide a transient current protection method for a flexible DC transmission system, see [link to relevant documentation]. Figure 1 This is a flowchart illustrating an embodiment of a transient current protection method for a flexible DC transmission system provided by the present invention.

[0045] like Figure 1 As shown, the method includes the following steps:

[0046] S1: When a ground fault is detected between the bridge arm reactor and the converter valve, fault-related parameters are acquired; wherein, the fault-related parameters include the fault current;

[0047] S2: Calculate the maximum transient voltage values ​​before and after blocking based on the fault-related parameters;

[0048] S3: Calculate the rate of rise of the current in the unblocked fault phase based on the maximum transient voltage value;

[0049] S4: When the fault current and the current rise rate meet the preset conditions, the di / dt protection is triggered.

[0050] In specific implementation, this invention only considers the cases of two-phase ground faults and three-phase ground faults occurring on the valve side of the bridge arm reactor. When a two-phase ground fault occurs on the valve side of the bridge reactor, the transient current development before blocking includes two stages. The first stage is when neither of the two faulty phases is blocked, AC and DC currents are fed in, and the bridge arm fault current continues to rise. The second stage is when one of the faulty phases is blocked, causing a transient change in the voltage of the unblocked faulty phase, and the AC current fed into the unblocked faulty phase rises rapidly, accelerating the growth rate of the fault current. When a three-phase ground fault occurs on the valve side of the bridge reactor, the transient current development before blocking includes three stages. The first stage is when none of the three faulty phases are blocked, AC and DC currents are fed in, and the bridge arm fault current continues to rise. The second stage is when one of the faulty phases is blocked, causing a transient change in the voltage of the other two unblocked faulty phases, and the AC current fed into the unblocked faulty phase rises rapidly, accelerating the growth rate of the fault current. The third stage is when another faulty phase is blocked on the basis of the second stage, further accelerating the growth rate of the fault current.

[0051] In summary, when a ground fault is detected between the bridge arm reactor and the converter valve, this invention acquires fault-related parameters, including the fault current. Based on these parameters, the maximum transient voltage values ​​before and after blocking are calculated. Based on the maximum transient voltage values, the current rise rate of the unblocked fault phase is calculated. When the fault current and the current rise rate meet preset conditions, di / dt protection is triggered. This invention comprehensively considers the phased fault characteristics of two-phase and three-phase ground faults on the bridge reactor valve side, achieving a coordinated design of protection strategy settings and improving the safety and reliability of the flexible DC transmission system.

[0052] In one optional implementation, the fault-related parameters further include the average submodule voltage, the calculation process of which includes:

[0053] Obtain the number of full-bridge submodules, the number of half-bridge submodules, and the maximum voltage across the converter valve;

[0054] The average voltage of the sub-modules is calculated based on the number of full-bridge sub-modules, the number of half-bridge sub-modules, and the maximum voltage.

[0055] Specifically, based on the number of full-bridge submodules, the number of half-bridge submodules, and the average voltage of the submodules, the maximum voltage transient before and after blocking is estimated. First, the voltage on the high-side valve of the converter valve during normal steady-state operation is as follows:

[0056] U dc (1-1 / 2 / n)+U v cos(ωt+ψ);

[0057] Among them, U dc U is the DC-side electrode-to-ground voltage under steady-state conditions, where n is the number of cascaded valve groups on the electrode. For high- and low-end valve groups connected in series, n = 2; for a single valve group, n = 1. v ω = 2πf, where f is the fundamental frequency of 50Hz and ψ is the phase angle of the valve-side voltage.

[0058] Based on this voltage value, the range of voltage variation on the valve side is [U]. dc (1-1 / 2 / n)-U v U dc (1-1 / 2 / n)+U v Furthermore, the voltage dU across the bridge arm converter valve... bridge The voltage difference between the DC pole voltage and the valve-side voltage, when the pole voltage is the rated value U. dc Its maximum value occurs when the valve-side voltage is at its minimum, which is dU. bridge =U dc / 2 / n+U v Furthermore, the average value of the submodule capacitor voltage dU sm It should be the maximum voltage across the bridge arm converter valve of a single submodule, i.e.

[0059] dU sm =dU bridge / (x+y);

[0060] Where x represents the number of full-bridge submodules and y represents the number of half-bridge submodules.

[0061] It should be noted that after a ground fault occurs on the bridge-side valve, the valve-side voltage drops instantaneously, with the drop amplitude before and after the fault being U. dc At this time, the range of the valve-side voltage variation is [-dU]. bridge [0] Since the protection action is relatively fast, it is assumed that the average voltage of the submodule remains unchanged during the period from the occurrence of the fault to the global lockout of the converter valve.

[0062] In one optional implementation, calculating the maximum voltage transient values ​​before and after blocking based on the fault-related parameters includes:

[0063] Calculate the minimum valve-side voltage before locking and the maximum valve-side voltage after locking;

[0064] The absolute value of the difference between the minimum and maximum valve-side voltage is calculated to obtain the maximum transient voltage value.

[0065] Specifically, the protection action process includes two stages: temporary locking of a single bridge arm and permanent locking of the converter valve. At the instant a single bridge arm is locked, the valve-side voltage changes from negative to positive, and the voltage across the engaged half-bridge submodule changes from dU... sm When the voltage becomes 0, the voltage across the full-bridge submodule becomes the submodule capacitor voltage. Therefore, the maximum value of the valve-side voltage after locking is as follows:

[0066] xdU sm ;

[0067] When a two-phase ground fault occurs on the bridge arm of the valve, the two faulty bridge arms are locked sequentially. The valve-side voltage difference between the unlocked faulty phase and the locked faulty phase, i.e., the maximum transient voltage value dU, is measured immediately after the first bridge arm is locked. max_f The calculation formula is as follows: dU max_f =dU bridge +xdU sm .

[0068] In one optional implementation, calculating the current rise rate of the unblocked fault phase based on the maximum voltage transient value includes:

[0069] Obtain the equivalent reactance of the transformer;

[0070] Based on the equivalent reactance of the transformer and the maximum transient voltage value, the current rise rate of the unblocked fault phase is calculated.

[0071] Specifically, the rate of current rise is related to voltage changes and the system's equivalent impedance. To obtain the transformer's equivalent reactance, for a Y / Δ transformer, the formula for calculating the rate of current rise of the unblocked fault phase is as follows:

[0072] di / dt=dU max_f / L T / 2;

[0073] Where di / dt is the rate of rise of current, dU max_f L is the maximum value of the voltage transient. T This is the equivalent reactance of the transformer.

[0074] In one optional implementation, triggering di / dt protection when the fault current and the current rise rate meet a preset condition includes:

[0075] When the fault current is greater than or equal to the preset protection action setting and the current rise rate is greater than or equal to the preset current rise rate setting, the di / dt protection is triggered.

[0076] Specifically, the transient current protection method of the present invention combines valve-controlled overcurrent protection with di / dt protection. The di / dt protection setting includes the protection action setting and the current rise rate setting di / dt. set Two parts, of which the protection action setting should satisfy the following formula: I set_didt arm_lim -di / dt max *T delay_didt ;

[0077] Among them, I set_didt To set the protection action value, I arm_lim For the safe and stable operation current of the submodule, di / dt max T is the maximum rate of rise of the current. delay_didt Delay the di / dt protection action.

[0078] di / dt protection requires I>I to simultaneously satisfy I>I set_didt and di / dt>di / dt set That is, the protection can only be triggered when the fault current is greater than or equal to the protection action setting and the current rise rate is greater than or equal to the current rise rate setting. When the two conditions cannot be met simultaneously, the coordination with valve-controlled overcurrent protection should be considered. For I set_didt <I<I set_oc And di / dt <di / dt set In this case, the prerequisite for effective di / dt protection is that the di / dt of the next stage of the fault satisfies the following equation:

[0079] di / dt<I set_oc / T delay_didt ;

[0080] Among them, I set_oc T is the setpoint for valve-controlled overcurrent protection. delay_didt Delay the di / dt protection action.

[0081] Understandably, I set_didt set_oc This means that the protection setting for the rate of change of current is lower than that for valve-controlled overcurrent protection. The rate of change of current protection can detect potential faults earlier, di / dt. <di / dt set ​​This indicates that the detected current rise rate is less than the current rise rate setpoint, meaning that the di / dt protection has not yet been triggered. set_oc / T delay_didt This means that if the fault current continues to increase at this rate during the delay time, it will reach the overcurrent protection threshold. In order to ensure that the di / dt protection is effective during the delay time and will not trigger the overcurrent protection due to the current continuing to increase rapidly during this period, the current change rate in the next stage must be lower than this limit value. In this way, even if the di / dt protection has a delay, it can act before the current increases to the overcurrent protection threshold, thereby effectively preventing more serious faults from occurring.

[0082] For I set_didt And di / dt > di / dt set In this case, the current rise rate has exceeded the current rise rate setting, but the fault current has not reached the protection action setting of di / dt protection. As the fault develops, the fault current will gradually increase until both conditions are met simultaneously, thereby triggering di / dt protection.

[0083] In an optional implementation, the fault-related parameters further include the submodule's safe and stable operating current value, the valve-controlled overcurrent protection setting value, and the valve-controlled overcurrent protection action delay. The process for determining the current rise rate setting value then includes:

[0084] The current rise rate setting is determined based on the safe and stable operating current value of the submodule, the valve-controlled overcurrent protection setting, and the valve-controlled overcurrent protection action delay.

[0085] Specifically, the rate of current rise during valve-controlled overcurrent protection operation should satisfy the following formula:

[0086] di / dt<(I arm_lim -I set_oc ) / T delay_oc ;

[0087] Where di / dt is the rate of rise of the current, I arm_lim For the safe and stable operation current of the submodule, I set_oc T is the setpoint for valve-controlled overcurrent protection. delay_oc Delay for valve-controlled overcurrent protection operation.

[0088] Understandably, the above formula represents the maximum rate at which the fault current may rise during the time period before the valve-controlled overcurrent protection trips, and the time delay T before the valve-controlled overcurrent protection trips. delay_oc Inside, if the fault current flows from I set_oc Rise to I arm_lim Therefore, it is necessary to consider triggering di / dt protection.

[0089] ​Therefore, when setting the current rise rate setting for di / dt protection, the following formula should be satisfied:

[0090] di / dt max ≤di / dt set ≤(I arm_lim -I set_oc ) / T delay_oc ;

[0091] Wherein, di / dt set As a constant for the rate of rise of the current, di / dt max This represents the maximum rate of increase of the current.

[0092] Setting a reasonable current rise rate setting can ensure that the protection mechanism can act in a timely manner when a fault occurs, while avoiding malfunctions under normal operating conditions, thus ensuring the safety and reliability of the system.

[0093] In summary, the transient current protection method for flexible DC transmission systems proposed in this application further considers the phased fault characteristics of two-phase grounding and three-phase grounding on the bridge reactor valve side, based on the traditional fault current development characteristics. It expands the fault types that overcurrent protection can cover, realizes the collaborative design of protection strategy settings, effectively avoids the formation of dead zones of the two types of protection, and avoids the problem of unclear protection action types in the overlapping protection area. Through the collaborative design of di / dt protection and valve-controlled overcurrent protection, the safety and reliability of flexible DC transmission systems are improved.

[0094] As an example, the following is an embodiment of the transient current protection method for flexible DC transmission systems using the present application; see [link to relevant documentation]. Figures 2 to 6 , Figure 2 This is a schematic diagram of the di / dt limit value for a transient current protection method for a flexible DC transmission system provided in an embodiment of the present invention. Figure 3 This is a schematic diagram of the first waveform, i.e., the waveform of a two-phase ground fault on the bridge reactance valve side before adopting the di / dt protection strategy, where U v dblk_a1, dblk_b1, and dblk_c1 represent the current differences between phases A, B, and C in the bridge circuit, respectively, and can indicate asymmetry or grounding faults in the circuit. dblk_a2, dblk_b2, and dblk_c2 represent the voltage differences between phases A, B, and C in the bridge circuit, respectively, and can indicate asymmetry or grounding faults in the circuit. UC1 represents the voltage at a certain point in the bridge circuit. Figure 4The diagram shows the second waveform, representing the three-phase ground fault waveform on the bridge reactor valve side before the di / dt protection strategy is implemented. Here, didt1, didt2, didt3, and didt4 represent the measured values ​​of the rate of change of current (di / dt), corresponding to four different current measurement points. ipa2, ipb2, and ipc2 represent the measured values ​​of the three-phase current, corresponding to the currents of phases A, B, and C, respectively. ina2, inb2, and inc2 represent the measured values ​​of the three-phase voltage, corresponding to the voltages of phases A, B, and C, respectively. DIDT_TRIP represents the trigger threshold for di / dt protection. When the rate of change of current exceeds this threshold, the protection device will activate to prevent further deterioration of the fault. Figure 5 This is a schematic diagram of the third waveform, which is the waveform of a two-phase ground fault on the bridge reactance valve side after adopting the di / dt protection strategy. Figure 6 The fourth waveform diagram shows the three-phase ground fault waveform on the bridge reactor valve side after adopting the di / dt protection strategy. Here, didt_pa, didt_pb, and didt_pc represent the rate of change of the current in phase A, phase B, and phase C of the bridge arm, respectively, showing the changes of the current in phase A, phase B, and phase C during the fault. didt_na, didt_nb, and didt_nc represent the rate of change of the neutral current, corresponding to phase A, phase B, and phase C, respectively.

[0095] In this embodiment, the flexible DC voltage level is ±800kV, the transmission capacity is 8000MW, the system adopts a symmetrical bipolar structure, each pole is composed of high and low end valve groups connected in series, the valve-controlled overcurrent protection setting is set to 8.5kA, the valve-controlled overcurrent protection action delay is 100μs, the bridge arm reactor is 25mH, each bridge arm contains 124 full-bridge sub-modules and 76 half-bridge sub-modules, the sub-module capacitance is 24mF, the maximum safe operating current of the IGBT device is 10kA, and the equivalent leakage reactance of the converter transformer is 12.27mH. The bridge arm reactor is used to suppress current fluctuations. When the fault point is not located between the bridge arm reactor and the valve (i.e., external fault), the bridge arm reactor can effectively limit the rate of rise of the fault current flowing through the converter valve. For external faults, the maximum voltage that the bridge arm can withstand is 400kV, and the corresponding fault current rise rate of the bridge arm reactor is 8A / µs. Based on the valve-controlled overcurrent protection setting, the safe operating current of the device, and the valve-controlled overcurrent protection action delay, the valve-controlled overcurrent protection can promptly block faults with a fault current rise rate within 15A / µs. The protection and the bridge arm reactor are properly coordinated and there is no dead zone. Therefore, the subsequent analysis only considers the development trend of the fault current under two-phase and three-phase ground faults on the bridge reactor valve side.

[0096] At time t0, a phase AB ground fault and a three-phase ground fault are triggered on the reactor valve side of the upper arm of the converter valve. According to the fault current development process, the first arm is temporarily blocked at time t1, the second arm is temporarily blocked at time t2, and all arms are blocked at time t3. The fault development process is as follows:

[0097] For the time interval t0 to t1, both the valve-side voltage and the DC-side voltage drop, and the maximum negative voltage of a single bridge arm is -400kV. Therefore, the valve-side voltage of the faulty phase should be between -400kV and 0kV immediately after the high-end converter valve failure (before the first lockout). The simulated value before the lockout is -100kV.

[0098] For times t1 to t2, similar to time t0, for the locked fault phase, the valve-side voltage after locking should be between 0kV and 248kV, with a simulated value of 230kV; for the unlocked fault phase, the valve-side voltage after the fault is between -400kV and 0kV. Therefore, the maximum voltage difference between the two fault phases after the first locking is 648kV, with a simulated value of 491kV.

[0099] Based on the voltage difference mentioned above, the maximum rate of increase of the valve-side current is 26.4 A / µs, while the actual value is 20 A / µs, which exceeds the effective range of the valve-controlled overcurrent protection. Since the voltage on the valve side of the unblocked faulty phase did not drop significantly after being blocked, its impact on the rate of increase of the faulty bridge arm current is not considered for the time being.

[0100] For times t2 to t3, when the fault is a two-phase ground fault on the bridge reactive valve side, all faulty bridge arms are already locked, and the rate of increase of the bridge arm current decreases significantly. When the fault is a three-phase ground fault on the bridge reactive valve side, one faulty bridge arm remains unlocked, and both locked phases form a fault path with this phase, causing the rate of increase of the bridge arm current to increase again. If the bridge arm current before time t2 is greater than 7kA but less than 8.5kA, and di / dt is less than the trigger value, the di / dt protection will not trigger. Since the upper limit of the effective range of the di / dt protection is limited by the protection action delay, the effective range of the di / dt protection after time t2 will shrink. In extreme cases, if the current is infinitely close to 8.5kA before time t2, the effective range of the di / dt protection after time t2 will shrink to the trigger value of 15A / µs, which may cause the di / dt protection to fail in some cases. To avoid the above situation, the di / dt protection should trigger during the time period t1 to t2. set Setting it to 8A / us will meet the requirements.

[0101] Since the current rise rate at times t1 to t2 in this embodiment exceeds the valve-controlled overcurrent protection range, di / dt protection needs to be added. To avoid dead zone, in the di / dt protection, I set_didt and di / dt setBoth overlap with the valve-controlled overcurrent protection settings. To avoid false tripping, the fault current rise rate and di / dt corresponding to the bridge arm reactor are... set The regions do not overlap, therefore I set_didt Set to 7kA, di / dt set Set to 8A / us.

[0102] The di / dt protection works in coordination with the overcurrent protection. For faults such as two-phase grounding on the bridge reactive valve side, the fault current rise rate before blocking includes two stages. This protection is effective for faults with a bridge arm current rise rate between 8A / us and 30A / us. It can be seen that the addition of di / dt protection effectively suppresses the rise of fault current for two-phase and three-phase grounding faults on the bridge reactive valve side, and effectively controls the converter valve current before blocking to within 10kA.

[0103] Secondly, embodiments of the present invention provide a transient current protection system for a flexible DC transmission system, see [link to relevant documentation]. Figure 7 The diagram shown is a structural schematic of a transient current protection system for a flexible DC transmission system provided in an embodiment of the present invention.

[0104] like Figure 7 As shown, the system includes:

[0105] The data acquisition module 21 is used to acquire fault-related parameters when a ground fault is detected between the bridge arm reactor and the converter valve; wherein, the fault-related parameters include the fault current;

[0106] Voltage calculation module 22 is used to calculate the maximum transient voltage value before and after blocking based on the fault-related parameters; current calculation module 23 is used to calculate the current rise rate of the unblocked fault phase based on the maximum transient voltage value.

[0107] The protection strategy module 24 is used to trigger di / dt protection when the fault current and the current rise rate meet preset conditions.

[0108] In one optional implementation, the fault-related parameters further include the average submodule voltage, the calculation process of which includes:

[0109] Obtain the number of full-bridge submodules, the number of half-bridge submodules, and the maximum voltage across the converter valve;

[0110] The average voltage of the sub-modules is calculated based on the number of full-bridge sub-modules, the number of half-bridge sub-modules, and the maximum voltage.

[0111] In an optional implementation, the voltage calculation module 22 is further configured to:

[0112] Calculate the minimum valve-side voltage before locking and the maximum valve-side voltage after locking;

[0113] The absolute value of the difference between the minimum and maximum valve-side voltage is calculated to obtain the maximum transient voltage value.

[0114] In an optional implementation, the current calculation module 23 is further configured to:

[0115] Obtain the equivalent reactance of the transformer;

[0116] Based on the equivalent reactance of the transformer and the maximum transient voltage value, the current rise rate of the unblocked fault phase is calculated.

[0117] In an optional implementation, the protection strategy module 24 is further configured to:

[0118] When the fault current is greater than or equal to the preset protection action setting and the current rise rate is greater than or equal to the preset current rise rate setting, the di / dt protection is triggered.

[0119] In an optional implementation, the fault-related parameters further include the submodule's safe and stable operating current value, the valve-controlled overcurrent protection setting value, and the valve-controlled overcurrent protection action delay. Then, the protection strategy module 24 is further used for:

[0120] The current rise rate setting is determined based on the safe and stable operating current value of the submodule, the valve-controlled overcurrent protection setting, and the valve-controlled overcurrent protection action delay.

[0121] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. It should be noted that, for those skilled in the art, several equivalent obvious modifications and / or equivalent substitutions can be made without departing from the technical principles of the present invention, and these obvious modifications and / or equivalent substitutions should also be considered within the scope of protection of the present invention.

Claims

1. A transient current protection method for a flexible DC transmission system, characterized in that, include: When a ground fault is detected between the bridge arm reactor and the converter valve, fault-related parameters are acquired; wherein, the fault-related parameters include the fault current; Based on the fault-related parameters, calculate the maximum voltage transient before and after blocking; Calculate the rate of rise of the current in the unblocked fault phase based on the maximum transient voltage value. When the fault current and the current rise rate meet the preset conditions, di / dt protection is triggered; The fault-related parameters also include the average voltage of the submodule, and the calculation process for the average voltage of the submodule includes: Obtain the number of full-bridge submodules, the number of half-bridge submodules, and the maximum voltage across the converter valve; The average voltage of the sub-modules is calculated based on the number of full-bridge sub-modules, the number of half-bridge sub-modules, and the maximum voltage. The step of calculating the maximum voltage transient before and after blocking based on the fault-related parameters includes: Calculate the minimum valve-side voltage before locking and the maximum valve-side voltage after locking; The absolute value of the difference between the minimum and maximum valve-side voltage is calculated to obtain the maximum transient voltage value.

2. The transient current protection method for flexible DC transmission systems as described in claim 1, characterized in that, The step of calculating the current rise rate of the unblocked fault phase based on the maximum voltage transient value includes: Obtain the equivalent reactance of the transformer; Based on the equivalent reactance of the transformer and the maximum transient voltage value, the current rise rate of the unblocked fault phase is calculated.

3. The transient current protection method for flexible DC transmission systems as described in claim 1, characterized in that, The step of triggering di / dt protection when the fault current and the current rise rate meet preset conditions includes: When the fault current is greater than or equal to the preset protection action setting and the current rise rate is greater than or equal to the preset current rise rate setting, the di / dt protection is triggered.

4. The transient current protection method for flexible DC transmission systems as described in claim 3, characterized in that, The fault-related parameters also include the submodule's safe and stable operating current value, the valve-controlled overcurrent protection setting value, and the valve-controlled overcurrent protection action delay. Therefore, the process for determining the current rise rate setting value includes: The current rise rate setting is determined based on the safe and stable operating current value of the submodule, the valve-controlled overcurrent protection setting, and the valve-controlled overcurrent protection action delay.

5. A transient current protection system for a flexible DC transmission system, characterized in that, include: The data acquisition module is used to acquire fault-related parameters when a ground fault is detected between the bridge arm reactor and the converter valve; wherein, the fault-related parameters include the fault current; The voltage calculation module is used to calculate the maximum transient voltage value before and after blocking based on the fault-related parameters. The current calculation module is used to calculate the current rise rate of the unblocked fault phase based on the maximum transient voltage value. The protection strategy module is used to trigger di / dt protection when the fault current and the current rise rate meet preset conditions; The fault-related parameters also include the average voltage of the submodule, and the calculation process for the average voltage of the submodule includes: Obtain the number of full-bridge submodules, the number of half-bridge submodules, and the maximum voltage across the converter valve; The average voltage of the sub-modules is calculated based on the number of full-bridge sub-modules, the number of half-bridge sub-modules, and the maximum voltage. The voltage calculation module is also used for: Calculate the minimum valve-side voltage before locking and the maximum valve-side voltage after locking; The absolute value of the difference between the minimum and maximum valve-side voltage is calculated to obtain the maximum transient voltage value.

6. The transient current protection system for flexible DC transmission systems as described in claim 5, characterized in that, The current calculation module is also used for: Obtain the equivalent reactance of the transformer; Based on the equivalent reactance of the transformer and the maximum transient voltage value, the current rise rate of the unblocked fault phase is calculated.

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