Transient current protection method and system for flexible direct current power transmission system

Abstract

The invention discloses a transient current protection method for a flexible direct-current power transmission system, and the method comprises the steps: obtaining fault related parameters when a ground fault is detected between a bridge arm reactor and a converter valve; wherein the fault related parameters comprise fault current; according to the fault related parameters, calculating the maximum transient value of the voltage before and after locking; according to the voltage maximum transient value, calculating a current rise rate of an unlocked fault phase; when the fault current and the current rise rate meet preset conditions, di / dt protection is triggered; through collaborative design of di / dt protection and valve control over-current protection, system and equipment safety under the extreme fault working condition can be guaranteed, and the flexible direct-current power transmission system can operate safely and stably.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible DC power transmission, and particularly to a transient current protection method and system for a flexible DC power transmission system. Background Technique

[0002] At present, flexible DC power transmission has entered a stage of rapid development. A large number of flexible DC power transmission projects have emerged one by one on the world map, and gradually, with the increasing demand for long-distance power transmission, it is moving towards higher voltage levels and larger capacities. The transmission capacity of the converter valve is an important factor restricting the development of flexible DC power transmission technology. As the transmission capacity increases, the devices of the converter valve are utilized more fully, and the overcurrent problem it faces during the transient process is also more severe, requiring more comprehensive and in-depth research.

[0003] Traditional flexible DC overcurrent protection strategies mainly rely on valve control overcurrent protection. When the arm current exceeds its set value, after a certain time delay, the converter valve of that arm is triggered to lock. However, for the situation where the fault current develops too fast, the converter valve may not be locked in time, resulting in device damage. Therefore, the existing strategies also include di / dt protection to address this problem. However, when a fault occurs between the arm reactance and the converter valve or when a two-phase or three-phase ground fault occurs simultaneously, the development speed of the fault current may show stage characteristics, increasing the difficulty of protection discrimination. How to analyze the system fault characteristics and achieve the safety of the system and equipment under various extreme fault conditions through the coordinated cooperation of valve control overcurrent protection and di / dt protection is the key and core problem to be solved in the development of flexible DC power transmission overcurrent protection technology. Summary of the Invention

[0004] An embodiment of the present invention provides a transient current protection method for a flexible DC power transmission system. Through the coordinated cooperation of valve control overcurrent protection and di / dt protection, it can ensure the safety of the system and equipment under extreme fault conditions and enable the flexible DC power transmission system to operate safely and stably.

[0005] In a first aspect, an embodiment of the present invention provides a transient current protection method for a flexible DC power transmission system, including: when a ground fault occurs between the arm reactor and the converter valve is detected, obtaining fault-related parameters; wherein, the fault-related parameters include fault current; Calculating the maximum transient values of the voltage before and after locking according to the fault-related parameters; Calculating the current rising rate of the un-locked fault phase according to the maximum transient value of the voltage; When the fault current and the current rising rate meet a preset condition, triggering di / dt protection.

[0006] Furthermore, the fault-related parameters further include the average value of the sub-module voltage, and the calculation process of the average value of the sub-module voltage includes: Obtain the number of full-bridge sub-modules, the number of half-bridge sub-modules of the commutation valve, and the maximum voltage at both ends of the commutation valve; Calculate the average sub-module voltage based on the number of full-bridge sub-modules, the number of half-bridge sub-modules, and the maximum voltage.

[0007] Further, the calculating the maximum voltage transient values before and after blocking according to the fault-related parameters includes: Calculate the minimum valve-side voltage before blocking and the maximum valve-side voltage after blocking; Calculate the absolute value of the difference between the minimum valve-side voltage and the maximum valve-side voltage to obtain the maximum voltage transient value.

[0008] Further, the calculating the current rising rate of the unblocked fault phase according to the maximum voltage transient value includes: obtaining the equivalent reactance of the transformer; Calculate the current rising rate of the unblocked fault phase based on the equivalent reactance of the transformer and the maximum voltage transient value.

[0009] Further, the triggering the di / dt protection when the fault current and the current rising rate meet the preset conditions includes: When the fault current is greater than or equal to the preset protection operation setting value and the current rising rate is greater than or equal to the preset current rising rate setting value, trigger the di / dt protection.

[0010] Further, the fault-related parameters further include the safe and stable operating current value of the sub-module, the valve control overcurrent protection setting value, and the valve control overcurrent protection operation delay. Then, the determination process of the current rising rate setting value includes: Determine the current rising rate setting value according to the safe and stable operating current value of the sub-module, the valve control overcurrent protection setting value, and the valve control overcurrent protection operation delay.

[0011] In a second aspect, an embodiment of the present invention provides a transient current protection system for a flexible DC transmission system, including: a data acquisition module, configured to obtain fault-related parameters when a ground fault occurs between the arm reactor and the commutation valve; wherein, the fault-related parameters include the fault current; A voltage calculation module, configured to calculate the maximum voltage transient values before and after blocking according to the fault-related parameters; A current calculation module, configured to calculate the current rising rate of the unblocked fault phase according to the maximum voltage transient value; A protection strategy module, configured to trigger the di / dt protection when the fault current and the current rising rate meet the preset conditions.

[0012] Further, the fault-related parameters further include the average sub-module voltage, and the calculation process of the average sub-module voltage includes: Obtain the number of full-bridge sub-modules, the number of half-bridge sub-modules of the converter valve, and the maximum voltage at both ends of the converter valve; Calculate the average sub-module voltage according to the number of full-bridge sub-modules, the number of half-bridge sub-modules, and the maximum voltage.

[0013] Further, the voltage calculation module is further configured to: Calculate the minimum valve-side voltage before blocking and the maximum valve-side voltage after blocking; Calculate the absolute value of the difference between the minimum valve-side voltage and the maximum valve-side voltage to obtain the maximum voltage transient value.

[0014] Further, the current calculation module is further configured to: Obtain the equivalent reactance of the transformer; Based on the equivalent reactance of the transformer and the maximum voltage transient value, calculate the current rising rate of the unblocked fault phase.

[0015] Compared with the prior art, the beneficial effect of a transient current protection method for a flexible DC transmission system provided by an embodiment of the present invention is that: when a ground fault occurs between the arm reactor and the converter valve is detected, fault-related parameters are obtained; wherein, the fault-related parameters include fault current; according to the fault-related parameters, calculate the maximum voltage transient values before and after blocking; according to the maximum voltage transient values, calculate the current rising rate of the unblocked fault phase; when the fault current and the current rising rate meet a preset condition, trigger di / dt protection; it can make the valve-controlled overcurrent protection and di / dt protection coordinate and cooperate to ensure the safety of the system and equipment under extreme fault conditions, and make the flexible DC transmission system operate safely and stably. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical features of the embodiments of the present invention, the drawings required to be used in the embodiments of the present invention will be briefly introduced below. Obviously, the following described drawings 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 efforts.

[0017] Figure 1 is a schematic flowchart of a transient current protection method for a flexible DC transmission system provided by an embodiment of the present invention; Figure 2 is a schematic diagram of the di / dt limit value of a transient current protection method for a flexible DC transmission system provided by an embodiment of the present invention; Figure 3It is the first waveform schematic diagram of a transient current protection method for a flexible DC transmission system provided by an embodiment of the present invention; Figure 4 It is the second waveform schematic diagram of a transient current protection method for a flexible DC transmission system provided by an embodiment of the present invention; Figure 5 It is the third waveform schematic diagram of a transient current protection method for a flexible DC transmission system provided by an embodiment of the present invention; Figure 6 It is the fourth waveform schematic diagram of a transient current protection method for a flexible DC transmission system provided by an embodiment of the present invention; Figure 7 It is the structural schematic diagram of a transient current protection system for a flexible DC transmission system provided by an embodiment of the present invention. Specific embodiments

[0018] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0019] In the first aspect, an embodiment of the present invention provides a transient current protection method for a flexible DC transmission system. Refer to Figure 1 , which is the flowchart of an embodiment of a transient current protection method for a flexible DC transmission system provided by the present invention.

[0020] As Figure 1 shown, the method includes the following steps: S1: When a ground fault occurs between the arm reactor and the converter valve, obtain the fault-related parameters; wherein, the fault-related parameters include the fault current; S2: Calculate the maximum voltage transient values before and after blocking according to the fault-related parameters; S3: Calculate the current rising rate of the unblocked fault phase according to the maximum voltage transient value; S4: When the fault current and the current rising rate meet the preset conditions, trigger the di / dt protection.

[0021] In specific implementation, the present invention only considers the cases of two-phase grounding fault and three-phase grounding fault occurring on the valve side of the arm reactor. When a two-phase grounding fault occurs on the valve side of the bridge reactor, the development of the transient current before blocking includes two stages. In the first stage, when neither of the two faulty phases is blocked, the AC-side current and the DC-side current are fed in, and the arm fault current continues to rise. In the second stage, when one of the faulty phases is blocked, it causes the voltage of the unblocked faulty phase to transiently change, and the AC-side current fed into the unblocked faulty phase rises rapidly, and the growth rate of the fault current accelerates. When a three-phase grounding fault occurs on the valve side of the bridge reactor, the development of the transient current before blocking includes three stages. In the first stage, when none of the three faulty phases is blocked, the AC-side current and the DC-side current are fed in, and the arm fault current continues to rise. In the second stage, when one of the faulty phases is blocked, it causes the voltage of the other two unblocked faulty phases to transiently change, and the AC-side current fed into the unblocked faulty phase rises rapidly, and the growth rate of the fault current accelerates. In the third stage, based on the second stage, another faulty phase is blocked, and the growth rate of the fault current further accelerates.

[0022] In summary, when the present invention detects a grounding fault between the arm reactor and the converter valve, it obtains the fault-related parameters; wherein, the fault-related parameters include the fault current; according to the fault-related parameters, it calculates the maximum transient voltage values before and after blocking; according to the maximum transient voltage values, it calculates the current rising rate of the unblocked faulty phase; when the fault current and the current rising rate meet the preset conditions, it triggers the di / dt protection; comprehensively considering the staged fault characteristics of two-phase grounding and three-phase grounding on the valve side of the bridge reactor, it realizes the collaborative design of the protection strategy setting values, and improves the safety and reliability of the flexible DC transmission system.

[0023] In an optional implementation manner, the fault-related parameters further include the average sub-module voltage, and the calculation process of the average sub-module voltage includes: Obtain the number of full-bridge sub-modules, the number of half-bridge sub-modules of the converter valve, and the maximum voltage at both ends of the converter valve; Calculate the average sub-module voltage according to the number of full-bridge sub-modules, the number of half-bridge sub-modules, and the maximum voltage.

[0024] Specifically, to estimate the maximum transient voltage values before and after blocking according to the number of full-bridge sub-modules, the number of half-bridge sub-modules, and the average sub-module voltage, first, the voltage at the high-end valve side of the converter valve during normal steady-state operation is as follows: U dc (1 - 1 / 2 / n) + U v cos(ωt + ψ); wherein, U dc is the DC-side pole-to-ground voltage under steady state, n is the number of cascaded valve groups in the pole. For the case of series connection of high- and low-end valve groups, n = 2, and for a single valve group, n = 1, Uv is the peak value of the AC component, ω = 2πf, f is the fundamental frequency of 50 Hz, and ψ is the phase angle of the valve-side voltage.

[0025] According to this voltage value, the variation range of the valve-side voltage is [U dc (1 - 1 / 2 / n) - U v , U dc (1 - 1 / 2 / n) + U v . Furthermore, the voltage dU bridge across the arm commutation valve is the voltage difference between the DC pole line voltage and the valve-side voltage. When the pole line voltage is the rated value U dc , its maximum value appears when the valve-side voltage is the minimum, that is, dU bridge = U dc / 2 / n + U v . Further, the average value dU sm of the sub-module capacitor voltage should be the maximum value of the voltage across the arm commutation valve averaged to a single sub-module, that is dU sm = dU bridge / (x + y); where x is the number of full-bridge sub-modules and y is the number of half-bridge sub-modules.

[0026] It should be noted that after the occurrence of the valve-side grounding fault of the bridge reactor, the valve-side voltage drops instantaneously, and the drop amplitude before and after the fault is U dc . At this time, the variation range of the valve-side voltage is [-dU bridge , 0]. Since the protection action is fast, it is considered that the average value of the sub-module voltage remains unchanged during the process from the occurrence of the fault to the global locking of the commutation valve.

[0027] In an alternative embodiment, calculating the maximum voltage transient values before and after locking according to the fault-related parameters includes: Calculating the minimum valve-side voltage before locking and the maximum valve-side voltage after locking; Calculating the absolute value of the difference between the minimum valve-side voltage and the maximum valve-side voltage to obtain the maximum voltage transient value.

[0028] Specifically, the protection action process includes two stages: temporary locking of a single arm and permanent locking of the commutation valve. At the moment of locking a single arm, the valve-side voltage changes from negative to positive, and the voltage across the half-bridge sub-modules put into operation changes from dU sm to 0, and the voltage across the full-bridge sub-modules becomes the sub-module capacitor voltage. Therefore, the maximum valve-side voltage after locking is as follows: xdU sm ; When a two-phase grounding fault occurs on the valve side of the bridge anti-valve, the two faulty bridge arms are blocked successively. At the moment when the first bridge arm is blocked, the voltage difference between the non-blocked faulty phase and the blocked faulty phase on the valve side, that is, the maximum voltage transient value dU max_f The calculation formula is as follows: dU max_f = dU bridge + xdU sm .

[0029] In an alternative embodiment, calculating the current rising rate of the non-blocked faulty phase according to the maximum voltage transient value includes: Obtaining the equivalent reactance of the transformer; Based on the equivalent reactance of the transformer and the maximum voltage transient value, calculating the current rising rate of the non-blocked faulty phase.

[0030] Specifically, the current rising rate is related to the voltage change and the system equivalent impedance. Obtaining the equivalent reactance of the transformer, for a Y / Δ transformer, the calculation formula for the current rising rate of the non-blocked faulty phase is referred to as follows: di / dt = dU max_f / L T / 2; Where, di / dt is the current rising rate, dU max_f is the maximum value of the voltage transient, L T is the equivalent reactance of the transformer.

[0031] In an alternative embodiment, when the fault current and the current rising rate meet the preset conditions, triggering the di / dt protection includes: When the fault current is greater than or equal to the preset protection action setting value and the current rising rate is greater than or equal to the preset current rising rate setting value, triggering the di / dt protection.

[0032] Specifically, the transient current protection method of the present invention combines the valve-controlled overcurrent protection with the di / dt protection. The di / dt protection setting value includes two parts: the protection action setting value and the current rising rate setting value di / dt set Among them, the protection action setting value should satisfy the following formula: I set_didt < I arm_lim - di / dt max * T delay_didt ; Where, I set_didt is the protection action setting value, I arm_lim is the current for the sub-module to operate safely and stably, di / dt max is the maximum value of the current rising rate, T delay_didt is the di / dt protection action delay.

[0033] The di / dt protection needs to satisfy both I > Iset_didt and di / dt > di / dt set , that is, the protection action can be triggered only when the fault current is greater than or equal to the protection action setting value and the current rising rate is greater than or equal to the current rising rate setting value. When the two conditions cannot be satisfied simultaneously, the cooperation relationship with the valve-controlled overcurrent protection should be considered. For I set_didt < I < I set_oc and di / dt < di / dt set In the case of, the prerequisite for the effectiveness of the di / dt protection is that the di / dt in the next stage of the fault satisfies the following formula: di / dt < I set_oc / T delay_didt ; wherein, I set_oc is the valve-controlled overcurrent protection setting value, and T delay_didt is the di / dt protection action delay.

[0034] It can be understood that I set_didt < I set_oc means that the protection action setting value of the current change rate is lower than the valve-controlled overcurrent protection setting value, and the current change rate protection can detect potential faults earlier. di / dt < di / dt set indicates that the currently detected current rising rate is less than the current rising rate setting value, that is, the current situation has not triggered the di / dt protection. I set_oc / T delay_didt means that within the delay time, if the fault current continues to increase at such a rate, it will reach the threshold of the overcurrent protection. To ensure the effectiveness of the di / dt protection within the delay time and prevent the overcurrent protection from being triggered due to the continuous rapid increase of the current during this period, the current change rate in the next stage must be lower than this limit value, so that 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.

[0035] For the case of I < I set_didt and di / dt > di / dt set , the current rising rate has exceeded the current rising rate setting value, but the fault current has not reached the protection action setting value of the di / dt protection. As the fault develops, the fault current will gradually increase until both conditions are satisfied simultaneously, thereby triggering the di / dt protection.

[0036] In an alternative embodiment, the fault-related parameters further include the sub-module safe and stable operating current value, the valve-controlled overcurrent protection setting value, and the valve-controlled overcurrent protection action delay. Then, the determination process of the current rising rate setting value includes: Determine the current rising rate setting value according to the sub-module safe and stable operating current value, the valve-controlled overcurrent protection setting value, and the valve-controlled overcurrent protection action delay.

[0037] Specifically, the current rise rate within the valve-controlled overcurrent protection operation should satisfy the following formula: di / dt < (I arm_lim - I set_oc ) / T delay_oc ; Wherein, di / dt is the current rise rate, I arm_lim is the current for the safe and stable operation of the sub-module, I set_oc is the valve-controlled overcurrent protection setting value, and T delay_oc is the valve-controlled overcurrent protection operation delay.

[0038] It can be understood that the above formula represents the maximum rate at which the fault current may rise during the period before the valve-controlled overcurrent protection operation. During the delay T delay_oc before the valve-controlled overcurrent protection operation, if the fault current rises from I set_oc to I arm_lim , then it is necessary to consider triggering the di / dt protection.

[0039] Therefore, when setting the current rise rate setting value for the di / dt protection, the following formula should be satisfied: di / dt max ≤ di / dt set ≤ (I arm_lim - I set_oc ) / T delay_oc ; Wherein, di / dt set is the current rise rate setting value, and di / dt max is the maximum value of the current rise rate.

[0040] Setting a reasonable current rise rate setting value can ensure that the protection mechanism can act in a timely manner when a fault occurs, while avoiding misoperation under normal operating conditions, and ensuring the safety and reliability of the system.

[0041] In summary, the transient current protection method for the flexible DC transmission system of the present application further comprehensively considers the staged fault characteristics of two-phase grounding and three-phase grounding on the valve side of the bridge reactor based on the traditional fault current development characteristics, expands the fault types that can be covered by the overcurrent protection, realizes the collaborative design of the protection strategy setting values, effectively avoids the formation of the action dead zones of the two protections, and the problem of unclear protection action types in the protection overlapping area. Through the collaborative design of the di / dt protection and the valve-controlled overcurrent protection, the safety and reliability of the flexible DC transmission system are improved.

[0042] Exemplarily, the following is an embodiment adopting the transient current protection method for the flexible DC transmission system of the present application. Refer to Figures 2 to 6 , Figure 2Schematic diagram of the di / dt limit for a transient current protection method of a flexible DC transmission system provided by an embodiment of the present invention Figure 3 It is the first waveform schematic diagram, that is, the waveform diagram of a two-phase grounding fault on the valve side of the bridge reactor before adopting the di / dt protection strategy, where U v is the valve side voltage, representing the voltage measured on the valve side of the bridge circuit. dblk_a1, dblk_b1, and dblk_c1 respectively represent the current differences of phase A, phase B, and phase C in the bridge circuit, which can indicate asymmetry or grounding faults in the circuit. dblk_a2, dblk_b2, and dblk_c2 respectively represent the voltage differences of phase A, phase B, and phase C in the bridge circuit, which can indicate asymmetry or grounding faults in the circuit. UC1 represents the voltage at a certain point in the bridge circuit. Figure 4 It is the second waveform schematic diagram, that is, the waveform diagram of a three-phase grounding fault on the valve side of the bridge reactor before adopting the di / dt protection strategy, where didt1, didt2, didt3, and didt4 represent the measured values of the current change rate (di / dt), corresponding to four different current measurement points respectively. ipa2, ipb2, and ipc2 represent the measured values of the three-phase currents, corresponding to the currents of phase A, phase B, and phase C respectively. ina2, inb2, and inc2 represent the measured values of the three-phase voltages, corresponding to the voltages of phase A, phase B, and phase C respectively. DIDT_TRIP represents the trigger threshold of the di / dt protection. When the current change rate exceeds this threshold, the protection device will act to prevent the fault from deteriorating further. Figure 5 It is the third waveform schematic diagram, that is, the waveform diagram of a two-phase grounding fault on the valve side of the bridge reactor after adopting the di / dt protection strategy Figure 6 It is the fourth waveform schematic diagram, that is, the waveform diagram of a three-phase grounding fault on the valve side of the bridge reactor after adopting the di / dt protection strategy, where didt_pa, didt_pb, and didt_pc respectively represent the change rates of the currents of phase A, phase B, and phase C of the bridge arm, showing the change conditions of the currents of phase A, phase B, and phase C during the fault. didt_na, didt_nb, and didt_nc represent the change rates of the neutral line currents, corresponding to phase A, phase B, and phase C respectively.

[0043] In this embodiment, the flexible DC voltage level is ±800 kV, the transmission capacity is 8000 MW, the system adopts a symmetrical bipolar structure, each pole is composed of a series connection of high and low end valve groups, the setting value of the valve-controlled overcurrent protection is set to 8.5 kA, the action delay of the valve-controlled overcurrent protection is 100 μs, the arm reactor is 25 mH, each arm contains 124 full-bridge sub-modules and 76 half-bridge sub-modules, the sub-module capacitance is 24 mF, the maximum safe operating current of the IGBT device is 10 kA, and the equivalent leakage reactance of the converter transformer is 12.27 mH. The arm reactor is used to suppress current fluctuations. When the fault point is not located between the arm reactor and the valve (i.e., an external fault in the valve hall), the arm reactor can effectively limit the rising rate of the fault current flowing through the converter valve. For an external fault in the valve hall, the maximum voltage borne by the arm is 400 kV, and the corresponding rising rate of the fault current of the arm reactor is 8 A / μs. According to the setting value of the valve-controlled overcurrent protection, the safe operating current of the device, and the action delay of the valve-controlled overcurrent protection, the valve-controlled overcurrent protection can lock out the fault in time when the rising rate of the fault current is within 15 A / μs. The protection cooperates properly with the arm reactance and there is no dead zone for action. Therefore, the subsequent analysis only considers the development trend of the fault current under the two-phase and three-phase grounding faults on the valve side of the arm reactor.

[0044] At time t0, a ground fault of phase AB and three-phase ground fault on the valve side of the arm reactor of the upper bridge arm of the converter valve is triggered. According to the development process of the fault current, the first arm is triggered to be temporarily blocked at time t1, the second arm is triggered to be temporarily blocked at time t2, and all arms are triggered to be blocked at time t3. The fault development process is as follows: For the time period from t0 to t1, both the valve side voltage and the DC side voltage drop. The maximum negative voltage of a single arm is -400 kV. Therefore, the valve side voltage of the fault phase immediately after the high-end converter valve fails (before the first blocking) should be between -400 kV and 0 kV, and the simulation value at the moment before blocking is -100 kV.

[0045] For the time period from t1 to t2, in the same way as at time t0, for the blocked fault phase, the valve side voltage after blocking should be between 0 kV and 248 kV, and the simulation value is 230 kV; for the unblocked fault phase, the valve side voltage after the fault is between -400 kV and 0 kV. Therefore, the maximum voltage difference between the two fault phases after the first blocking is 648 kV, and the simulation value is 491 kV.

[0046] According to the above voltage difference, the maximum rising rate of the valve side current is 26.4 A / μs, and the actual value is 20 A / μs, exceeding the effective range of the valve-controlled overcurrent protection. Since the valve side voltage of the unblocked fault phase does not drop significantly after blocking, its influence on the rising rate of the fault arm current is not considered temporarily.

[0047] For the time period from t2 to t3, when the fault is a two-phase grounding fault on the valve side of the bridge reactor, both fault arms are blocked at this time, and the rising rate of the arm current decreases significantly. When the fault is a three-phase grounding fault on the valve side of the bridge reactor, there is still one fault arm that is not blocked at this time, and fault paths are formed between the two blocked phases and this phase. The rising rate of the arm current increases again. When the arm current before t2 is greater than 7 kA and less than 8.5 kA, and di / dt is less than the trigger value, the di / dt protection is not triggered. Since the upper limit of the effective range of the di / dt protection is restricted by the protection action delay, the effective range of the di / dt protection after t2 will be reduced. In extreme cases, when it is infinitely close to 8.5 kA before t2, the effective range of the di / dt protection after t2 is reduced to the trigger value to 15 A / us, which may cause the di / dt protection to fail in some cases. To avoid the above situation, the di / dt protection should be triggered during the time period from t1 to t2, and di / dt set Set to 8 A / us can meet the requirements.

[0048] Since the current rising rate during the time period from t1 to t2 in this embodiment exceeds the range of the valve-controlled overcurrent protection, it is necessary to add di / dt protection. To avoid dead zones, in the di / dt protection, I set_didt and di / dt set both overlap with the settings of the valve-controlled overcurrent protection. To avoid misoperation, the rising rate of the fault current corresponding to the arm reactor does not overlap with the di / dt set region. Therefore, I set_didt is set to 7 kA, and di / dt set is set to 8 A / us.

[0049] The di / dt protection and the overcurrent protection cooperate. For faults with two stages in the rising rate of the fault current before blocking, such as two-phase grounding on the valve side of the bridge reactor, this protection is effective for faults where the rising rate of the arm current is between 8 A / us and 30 A / us. It can be seen that adding the di / dt protection effectively suppresses the rising of the fault current in two-phase and three-phase grounding faults on the valve side of the bridge reactor, and effectively controls the current of the converter valve before blocking within 10 kA.

[0050] In a second aspect, an embodiment of the present invention provides a transient current protection system for a flexible DC transmission system. Refer to Figure 7 shown, which is a schematic structural diagram of a transient current protection system for a flexible DC transmission system provided by an embodiment of the present invention.

[0051] As Figure 7 shown, the system includes: A data acquisition module 21, configured to acquire fault-related parameters when a grounding fault occurs between the arm reactor and the converter valve; wherein, the fault-related parameters include fault current; A voltage calculation module 22, configured to calculate the maximum voltage transient values before and after blocking according to the fault-related parameters; a current calculation module 23, configured to calculate the current rising rate of the unblocked fault phase according to the maximum voltage transient value; A protection strategy module 24, configured to trigger di / dt protection when the fault current and the current rising rate meet preset conditions.

[0052] In an alternative embodiment, the fault-related parameters further include the average sub-module voltage, and the calculation process of the average sub-module voltage includes: Obtaining the number of full-bridge sub-modules, the number of half-bridge sub-modules of the converter valve, and the maximum voltage at both ends of the converter valve; Calculating the average sub-module voltage according to the number of full-bridge sub-modules, the number of half-bridge sub-modules, and the maximum voltage.

[0053] In an alternative embodiment, the voltage calculation module 22 is further configured to: Calculate the minimum valve-side voltage before blocking and the maximum valve-side voltage after blocking; Calculating the absolute value of the difference between the minimum valve-side voltage and the maximum valve-side voltage to obtain the maximum voltage transient value.

[0054] In an alternative embodiment, the current calculation module 23 is further configured to: Obtain the equivalent reactance of the transformer; Calculating the current rising rate of the unblocked fault phase based on the equivalent reactance of the transformer and the maximum voltage transient value.

[0055] In an alternative embodiment, the protection strategy module 24 is further configured to: Trigger di / dt protection when the fault current is greater than or equal to a preset protection action setting value and the current rising rate is greater than or equal to a preset current rising rate setting value.

[0056] In an alternative embodiment, the fault-related parameters further include the sub-module safe and stable operating current value, the valve control overcurrent protection setting value, and the valve control overcurrent protection action delay, then, the protection strategy module 24 is further configured to: Determine the current rising rate setting value according to the sub-module safe and stable operating current value, the valve control overcurrent protection setting value, and the valve control overcurrent protection action delay.

[0057] The above description is only a preferred embodiment of the present invention, but the protection scope of the present invention is not limited thereto. It should be pointed out that for those skilled in the art, several equivalent obvious variations and / or equivalent substitutions can be made without departing from the technical principles of the present invention. These obvious variations and / or equivalent substitutions should also be regarded as the protection scope of the present invention.

Claims

1. A transient current protection method for a flexible direct current 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 a fault current; Calculating the maximum transient voltage value before and after blocking according to the fault-related parameters; Calculating the current rise rate of the unblocked fault phase according to the maximum transient value of the voltage; When the fault current and the current rising rate meet preset conditions, the di / dt protection is triggered.

2. The transient current protection method for a flexible direct current transmission system according to claim 1, characterized in that: The fault-related parameters also include a submodule voltage average value, and the calculation process of the submodule voltage average value includes: Obtaining the number of full-bridge submodules, the number of half-bridge submodules and the maximum voltage across the converter valve of the converter valve; The submodule voltage average value is calculated according to the number of the full-bridge submodules, the number of the half-bridge submodules and the maximum voltage.

3. The transient current protection method for a flexible direct current transmission system according to claim 1, characterized in that: The step of calculating the maximum transient voltage value before and after blocking according to 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 value of the valve side voltage and the maximum value of the valve side voltage is calculated to obtain the maximum transient value of the voltage.

4. The transient current protection method for a flexible direct current transmission system according to claim 1, characterized in that: The step of calculating the current rise rate of the unblocked fault phase according to the maximum transient value of the voltage includes: Get the equivalent reactance of the transformer; Based on the transformer equivalent reactance and the maximum voltage transient value, the current rise rate of the unblocked fault phase is calculated.

5. The transient current protection method for a flexible direct current transmission system according to claim 1, characterized in that: When the fault current and the current rising rate meet preset conditions, triggering di / dt protection includes: When the fault current is greater than or equal to a preset protection action setting and the current rise rate is greater than or equal to a preset current rise rate setting, the di / dt protection is triggered.

6. The transient current protection method for a flexible direct current transmission system according to claim 5, characterized in that: The fault-related parameters also include the submodule safe and stable operation current value, the valve-controlled overcurrent protection constant value and the valve-controlled overcurrent protection action delay. Then, the determination process of the current rise rate constant value includes: The current rise rate constant is determined according to the safe and stable operating current value of the submodule, the valve-controlled overcurrent protection constant and the valve-controlled overcurrent protection action delay.

7. A transient current protection system for a flexible direct current transmission system, characterized in that: include: A data acquisition module, used for acquiring 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; A voltage calculation module, used to calculate the maximum transient voltage value before and after blocking according to the fault-related parameters; A current calculation module, used for calculating the current rise rate of the unblocked fault phase according to the maximum transient value of the voltage; A protection strategy module is used to trigger di / dt protection when the fault current and the current rise rate meet preset conditions.

8. The transient current protection system for a flexible direct current transmission system according to claim 7, characterized in that: The fault-related parameters also include a submodule voltage average value, and the calculation process of the submodule voltage average value includes: Obtaining the number of full-bridge submodules, the number of half-bridge submodules and the maximum voltage across the converter valve of the converter valve; The submodule voltage average value is calculated according to the number of the full-bridge submodules, the number of the half-bridge submodules and the maximum voltage.

9. The transient current protection system of the flexible direct current transmission system according to claim 7, characterized in that: 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 value of the valve side voltage and the maximum value of the valve side voltage is calculated to obtain the maximum transient value of the voltage.

10. The transient current protection system for a flexible direct current transmission system according to claim 7, characterized in that: The current calculation module is also used for: Get the equivalent reactance of the transformer; Based on the transformer equivalent reactance and the maximum voltage transient value, the current rise rate of the unblocked fault phase is calculated.

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