Misoperation locking risk assessment method for high-voltage direct-current power transmission system and related equipment

Abstract

The invention discloses a misoperation locking risk assessment method for a high-voltage direct-current power transmission system and related equipment, and relates to the technical field of high-voltage direct-current power transmission, and the key points of the technical scheme are as follows: constructing a first circuit equivalent model in which a variable resistor is connected in parallel at a resistance-capacitance voltage division position of a direct-current voltage divider; constructing a second circuit equivalent model in which a voltage dependent resistor is connected in series at a discharge gap of the direct-current voltage divider; voltage disturbance signals are applied to the two ends of the piezoresistor of the second circuit equivalent model, and a resistance change curve of the piezoresistor caused by voltage disturbance is simulated; taking the resistance value change curve as an input parameter of a variable resistor of a first circuit equivalent model, and simulating a voltage value when the voltage of a low-voltage arm of the direct-current voltage divider drops to the lowest point when the variable resistor changes according to the resistance value change curve; and comparing the voltage value with a starting voltage value of low-voltage current-limiting protection, and determining a risk analysis result of misoperation locking of the high-voltage direct-current power transmission system based on a comparison result.

Description

Technical Field

[0001] This invention relates to the field of high voltage direct current (HVDC) transmission technology, and more specifically, to a method and related equipment for assessing the risk of accidental blocking in a HVDC transmission system. Background Technology

[0002] The basic principle of high-voltage direct current (HVDC) transmission is as follows: at the power transmission end of the DC transmission system, a converter is used to rectify the three-phase alternating current into direct current. The electrical energy is transmitted through the HVDC transmission line, and then at the power receiving end of the system, a converter is used to invert the direct current into three-phase alternating current. The electrical energy is then sent into the AC system at the power receiving end.

[0003] The existing literature, "Research on the Dynamic Characteristics of DC Voltage Dividers in UHVDC Transmission Projects and the Mechanism of False Lockout Caused by Them," analyzes the basic process of UHVDC false lockout caused by the transient characteristics of DC voltage dividers and explains the response characteristics of each link in the control system under abnormal rectifier-side voltage command measurement conditions. However, it considers that the low-voltage arm of the voltage divider does not have a parallel varistor, meaning that all voltage loss is considered as complete voltage loss. In actual projects, after configuring varistors, the degree of voltage loss varies under different voltage disturbance conditions, and the risk of false lockout in the UHVDC transmission system also varies. The existing technical solution cannot analyze this. Summary of the Invention

[0004] The purpose of this invention is to provide a method and related equipment for assessing the risk of accidental blocking in a high-voltage direct current (HVDC) transmission system. This solves the problem that existing technologies cannot analyze the risk of accidental blocking in a HVDC transmission system after the low-voltage arm of the DC voltage divider loses voltage due to lightning disturbance.

[0005] The above-mentioned technical objective of the present invention is achieved through the following technical solution:

[0006] In a first aspect, the present invention provides a method for assessing the risk of accidental blockage in a high-voltage direct current transmission system, the method comprising:

[0007] A first circuit equivalent model is constructed with a variable resistor connected in parallel at the RC voltage divider of the DC voltage divider, and a second circuit equivalent model is constructed with a varistor connected in series at the discharge gap of the DC voltage divider.

[0008] A voltage perturbation signal is applied across the varistor in the second circuit equivalent model to simulate the resistance change curve of the varistor caused by the voltage perturbation.

[0009] Using the resistance change curve as the input parameter of the variable resistor in the first circuit equivalent model, the voltage value of the low-voltage arm of the DC voltage divider when the voltage drops to the lowest point is simulated when the variable resistor changes according to the resistance change curve.

[0010] The voltage value is compared with the starting voltage value of the low-voltage current limiting protection, and the risk analysis results of the false blocking of the high-voltage direct current transmission system are determined based on the comparison results.

[0011] In one implementation, the equivalent model of the first circuit includes a first capacitor, a second capacitor, a first resistor, a second resistor, and a variable resistor;

[0012] The first capacitor and the second capacitor are connected in series to form a capacitor branch, the first resistor and the second resistor are connected in series to form a resistor branch, the capacitor branch and the resistor branch are connected to form an H-bridge, and the variable resistor is connected in parallel with the second resistor;

[0013] The input to the first capacitor equivalent model is the DC rated voltage of the high-voltage direct current transmission system.

[0014] In one implementation, the equivalent model of the second circuit includes a first resistor, a second resistor, a first capacitor, a second capacitor, a third capacitor, a discharge gap, and a varistor.

[0015] The first capacitor and the second capacitor are connected in series to form a capacitor branch, and the first resistor and the second resistor are connected in series to form a resistor branch. The capacitor branch and the resistor branch are connected in parallel.

[0016] The discharge gap is connected in series with the varistor to form a branch and is connected in parallel across the two ends of the second resistor, and the third capacitor is connected in parallel across the two ends of the second capacitor;

[0017] The input to the second capacitor equivalent model is a voltage disturbance signal.

[0018] In one implementation, the resistance of the resistance change curve first decreases and then increases.

[0019] In one implementation scheme, the voltage value is compared with the starting voltage value of the low-voltage current limiting protection. Based on the comparison result, the risk analysis results of the false blocking of the high-voltage direct current transmission system are determined, including:

[0020] If the voltage value is greater than the starting voltage value, the electrical quantities of the high-voltage direct current transmission system are not disturbed, and there is no risk of blockage in the high-voltage direct current transmission system.

[0021] If the voltage value is less than or equal to the starting voltage value, the electrical quantities of the high-voltage direct current transmission system are not disturbed, and the high-voltage direct current transmission system is at risk of being blocked.

[0022] In a second aspect, the present invention provides a risk assessment system for accidental blocking in a high-voltage direct current transmission system, the system comprising:

[0023] The model building module is used to build a first circuit equivalent model with a variable resistor connected in parallel at the DC voltage divider, and a second circuit equivalent model with a varistor connected in series at the discharge gap of the DC voltage divider.

[0024] The resistance simulation module is used to apply a voltage disturbance signal across the varistor in the equivalent circuit model of the second circuit to simulate the resistance change curve of the varistor caused by the voltage disturbance.

[0025] The voltage simulation module is used to take the resistance change curve as the input parameter of the variable resistor in the first circuit equivalent model, and simulate the voltage value of the low voltage arm of the DC voltage divider when the voltage drops to the lowest point as the variable resistor changes according to the resistance change curve.

[0026] The risk analysis module is used to compare the voltage value with the starting voltage value of the low-voltage current limiting protection, and to determine the risk analysis result of the false blocking of the high-voltage direct current transmission system based on the comparison result.

[0027] In one implementation, the equivalent model of the first circuit includes a first capacitor, a second capacitor, a first resistor, a second resistor, and a variable resistor;

[0028] The first capacitor and the second capacitor are connected in series to form a capacitor branch, the first resistor and the second resistor are connected in series to form a resistor branch, the capacitor branch and the resistor branch are connected to form an H-bridge, and the variable resistor is connected in parallel with the second resistor;

[0029] The input to the first capacitor equivalent model is the DC rated voltage of the high-voltage direct current transmission system.

[0030] In one implementation, the equivalent model of the second circuit includes a first resistor, a second resistor, a first capacitor, a second capacitor, a third capacitor, a discharge gap, and a varistor.

[0031] The first capacitor and the second capacitor are connected in series to form a capacitor branch, and the first resistor and the second resistor are connected in series to form a resistor branch. The capacitor branch and the resistor branch are connected in parallel.

[0032] The discharge gap is connected in series with the varistor to form a branch and is connected in parallel across the two ends of the second resistor, and the third capacitor is connected in parallel across the two ends of the second capacitor;

[0033] The input to the second capacitor equivalent model is a voltage disturbance signal.

[0034] In a third aspect, the present invention provides an electronic device, including a memory and a processor;

[0035] A memory for storing computer programs, the computer programs including program instructions;

[0036] A processor is configured to execute the program instructions to cause the electronic device to perform the steps of a method for assessing the risk of accidental blocking of a high-voltage direct current transmission system as provided in the first aspect of the present invention.

[0037] In a fourth aspect, the present invention provides a computer-readable storage medium comprising a computer program that, when executed by one or more processors, implements a method for assessing the risk of accidental blocking in a high-voltage direct current transmission system as provided in the first aspect of the present invention.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] This invention analyzes whether a high-voltage direct current (HVDC) transmission system will experience accidental blocking due to loss of voltage in the low-voltage arm of a DC voltage divider under any lightning strike-induced ground potential disturbance condition. The analytical method provided by this invention can be used to predict operational risks of ultra-high-voltage direct current (UHVDC) converter stations or guide the selection of parameters for varistor and RC voltage divider circuits. Attached Figure Description

[0040] The accompanying drawings, which are included to provide a further understanding of embodiments of the invention and form part of this application, do not constitute a limitation thereof. In the drawings:

[0041] Figure 1 This is a schematic diagram of a DC voltage divider.

[0042] Figure 2 The equivalent circuit diagram of a DC voltage divider;

[0043] Figure 3 This is a diagram showing the basic control characteristics of a high-voltage direct current transmission system.

[0044] Figure 4 This is a diagram showing the characteristics of low-voltage current limiting control.

[0045] Figure 5 This is a schematic diagram of the low-voltage arm of a DC voltage divider losing voltage.

[0046] Figure 6 This is a graph showing the change in the system operating point after the low-voltage arm of the DC voltage divider loses voltage (the low-voltage arm voltage is higher than UD_High).

[0047] Figure 7 This is a graph showing the change in the system operating point after the low-voltage arm of the DC voltage divider loses voltage (the low-voltage arm voltage is slightly lower than UD_High).

[0048] Figure 8 This is a graph showing the change in the system operating point after the low-voltage arm of the DC voltage divider loses voltage (the low-voltage arm voltage is significantly lower than UD_High).

[0049] Figure 9(a) is a schematic diagram of the DC voltage response of the system under different degrees of voltage loss in the low-voltage arm;

[0050] Figure 9(b) is a schematic diagram of the system firing angle response under different degrees of pressure loss in the low-pressure arm;

[0051] Figure 10(a) is a schematic diagram of the DC voltage, DC current and firing angle waveforms of the low-voltage arm of the I-pole DC voltage divider;

[0052] Figure 10(b) is a schematic diagram of the DC voltage, DC current and firing angle waveforms of the low-voltage arm of the II DC voltage divider;

[0053] Figure 11 A flowchart illustrating a method for assessing the risk of accidental blocking in a high-voltage direct current transmission system, provided in an embodiment of the present invention;

[0054] Figure 12 This is a schematic diagram of a risk assessment system for accidental blocking of a high-voltage direct current transmission system provided in an embodiment of the present invention. Detailed Implementation

[0055] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention.

[0056] It should be noted that the terms "comprising" or "may include" used in the various embodiments of this application indicate the presence of the claimed function, operation, or element, and do not limit the addition of one or more functions, operations, or elements. Furthermore, as used in the various embodiments of this application, the terms "comprising," "having," and their cognates are intended only to indicate a specific feature, number, step, operation, element, component, or combination of the foregoing, and should not be construed as primarily excluding the presence of one or more other features, numbers, steps, operations, elements, components, or combinations of the foregoing, or adding one or more combinations of the foregoing.

[0057] It should be understood that terms such as "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0058] DC voltage dividers are key devices in ultra-high voltage direct current (UHVDC) transmission systems used for real-time monitoring of line voltage; their measurement results directly affect the reliability of the control and protection system. Their structure is as follows: Figure 1 As shown, the DC voltage divider utilizes the principle of resistor-capacitor voltage division to achieve two-stage transformation. The first stage reduces the 800kV primary voltage to 70V and introduces it into the balancing module. In the subsequent voltage divider module, the 70V input voltage is further reduced to 5V and then input to the control and protection system after passing through an isolation amplifier.

[0059] Based on the actual operating parameters of the converter station, the equivalent circuit of the DC voltage divider is given as follows: Figure 2 As shown, by Figure 2 It can be seen that the DC voltage divider uses the resistor-capacitor voltage divider principle. Resistor R1 and capacitor C1 form the high-voltage arm, and resistor R2 and capacitor C2 form the low-voltage arm, working together to measure the DC voltage. Simultaneously, the resistors and capacitors of the high and low voltage arms satisfy the following matching principle: The function of the discharge gap is to quickly conduct and discharge surge energy after a lightning strike, thereby limiting the voltage amplitude and preventing equipment damage due to overvoltage. A varistor is connected in series in the protective gap circuit to ensure a certain voltage exists in the circuit after the discharge gap breaks down. The analysis of the response characteristics of the low-voltage arm of the DC voltage divider in a high-voltage direct current transmission system after a short circuit is as follows: assuming no varistor, for... Figure 2 Circuit analysis was performed. Under normal operating conditions, if a lightning strike or other disturbance causes the discharge gap to break down and short-circuit, the air gap will then recover.

[0060] Considering that U1 maintains an amplitude of V throughout the entire process D Given a DC voltage and ignoring C3, the transient response of U2 during this process satisfies: From the above equation, it can be seen that at the instant of recovery from a short circuit in the low-voltage arm of the DC voltage divider, due to the charging process of the low-voltage arm capacitor, the voltage of U2 remains at 0, gradually changing to a steady-state value through the first-order dynamic response, and its time constant satisfies: .

[0061] Current margin control is a common control method for maintaining the stable operation of DC transmission systems, such as... Figure 3As shown. The rectifier-side characteristic consists of two straight lines: constant current and minimum firing angle; the inverter-side characteristic consists of two characteristics: constant DC current and constant turn-off angle or constant DC voltage. To avoid simultaneous operation of the current regulators at both ends, which could cause instability, the setpoint of the inverter-side current regulator is generally 0.1 pu smaller than that of the DC side; this is the current margin. During normal operation, the inverter operates with constant current on the rectifier side and constant turn-off angle or constant DC voltage on the inverter side. When the AC voltage on the rectifier side decreases or the AC voltage on the inverter side increases significantly, causing the rectifier to enter the minimum firing angle limit, the DC current will be less than the steady-state value of 0.1 pu, and the inverter will automatically switch to constant DC current control. This combination of rectifier and inverter control characteristics constitutes the current margin control characteristic, ensuring that the DC current transmitted by the DC transmission system does not fluctuate significantly due to changes in the voltage of the connected AC system, thus guaranteeing stable DC power transmission.

[0062] For low-voltage current limiting control in ultra-high voltage direct current (UHVDC) transmission systems, the control targets on the rectifier and inverter sides are DC current and voltage, respectively. However, considering that if the aforementioned steady-state control strategy is still adopted after a system fault, the voltage drop will lead to a significant reduction in the arc-extinguishing angle γ on the inverter side, posing a serious risk of commutation failure to the converter. Simultaneously, the sudden increase in DC current will severely impact system equipment. Therefore, low-voltage current limiting (VDCOL) stages are incorporated into the control strategy design for both the rectifier and inverter sides, with their external characteristic curves as shown in the figure. Figure 4 As shown. In the VDCOL circuit, the input variable is the DC line midpoint voltage U. dc_middle The output variable is the constant current control command value I. dcref The minimum value between this value and the upper-level current command value serves as the input current reference value for the subsequent PI circuit. According to the curve characteristics shown in the figure, when a system fault causes an abnormal drop in DC voltage, VDCOL will automatically reduce the DC line current command value based on the degree of voltage drop, reducing the reactive power absorbed by the converter. This is beneficial for the subsequent recovery of AC voltage and ensures uninterrupted and stable power transmission during DC voltage anomalies. After the fault disappears, the current command value increases gradually under the control of the VDCOL circuit, improving the transient characteristics of AC and DC voltages during fault recovery.

[0063] The low-voltage arm of a DC voltage divider experiences two types of voltage loss: complete voltage loss (without a varistor) and partial voltage loss (with a varistor). For complete voltage loss, referring to the transient response and time constant expressions above, the transient process is measured in hundreds of milliseconds. It takes 68.5 milliseconds for the voltage to recover to 0.35 pu (higher than the low-voltage protection setting), 256 milliseconds to recover to 0.8 pu (higher than the VDCOL upper limit setting), and 477 milliseconds to end the transient process. For partial voltage loss, i.e., with a varistor, the dynamic process after the discharge gap breaks down is more complex. First, under the voltage condition of discharge gap breakdown, the resistance of the varistor will rapidly drop to the ohm level or even lower, but not to zero. Therefore, the low-voltage arm voltage cannot be equated to an instantaneous drop to zero; rather, it needs to undergo an extremely rapid transient discharge process. Secondly, during the process of capacitor discharge and rapid reduction of low-voltage arm voltage, the resistance of the varistor also changes dynamically. After the voltage drops, the resistance will increase rapidly, which will cause the time constant of the discharge circuit to increase sharply. This will cause the rate of decrease of low-voltage arm voltage to slow down rapidly or even start reverse charging until the discharge gap is restored, and then the capacitor charging process will restart with the current state as the initial state.

[0064] After a lightning strike causes a momentary change in the ground grid potential, the high potential difference leads to a short-term breakdown of the discharge gap. The dynamic process of the low-voltage arm of the DC voltage divider losing voltage is as follows: Figure 5 As shown, although the discharge gap breaks down, it is caused by a change in ground potential. The discharge gap-varistor path is not the main energy discharge path (even without breakdown, the ground potential can drop naturally, and the energy is discharged through the ground). Therefore, the voltage loss process of the low-voltage arm of the DC voltage divider can be approximated as the low-voltage arm simply having a variable resistor Rc connected in parallel, with its resistance decreasing and then increasing. The resistance value is only related to the absolute value of the voltage across its terminals. At the instant of discharge gap breakdown, the varistor experiences overvoltage, and its resistance drops to the ohm level or even lower. At this time, R2||Rc≈Rc, and the time constant RcC2 is on the order of microseconds. Although this time constant is very small, it is still not negligible in the lightning disturbance analysis scenario. As the varistor resistance recovers, the discharge time constant R2||Rc·C2 of C2 increases rapidly, and the voltage drop of the low-voltage arm slows down or even turns from decreasing to increasing. Until the discharge gap recovers, C2 continues to charge slowly with a time constant R2C2, and the voltage recovers.

[0065] During the instantaneous loss of voltage on the low-voltage arm of the DC voltage divider and the subsequent charging recovery process, the primary electrical parameters of the DC system show no abnormalities. However, during this process, deviations in secondary measurements may cause abnormal actions in the control system, or even trigger a short-term loss of control in the DC system, leading to protective lockout. The following is a detailed analysis based on the DC system operating characteristic curves. After the low-voltage arm of the DC voltage divider loses voltage, the actual operating point of the system does not change, but the operating point observed by the rectifier-side secondary system drops instantaneously. Depending on the magnitude of the drop, three types of consequences may occur: first, the electrical quantities on the DC primary side of the system are not disturbed; second, the system is slightly disturbed but the overall process is under control; and third, the system is significantly disturbed and experiences a loss of control.

[0066] Assuming the DC primary side electrical quantities of the system remain undisturbed, and that under the action of the varistor, the discharge gap breaks down but the low-voltage arm does not experience severe voltage loss, and the voltage remains higher than the VDCOL start-up voltage UD_High, the DC primary side electrical quantities will remain undisturbed. The changes in the system operating point are as follows: Figure 6 As shown, under normal system operation, the rectifier-side control remains stable at a constant current. After the low-voltage arm of the DC voltage divider momentarily loses voltage, the system operating point observed by the rectifier-side secondary system drops from the red point to the yellow point in the diagram. At this time, the measured current remains unchanged, VDCOL is not activated, and the current command remains unchanged; therefore, no additional control adjustments are made on the rectifier side. Furthermore, there are no abnormalities in the electrical quantities or secondary system on the inverter side; therefore, no additional control adjustments are made on the inverter side. As the low-voltage arm charges, the measured voltage of the rectifier-side secondary system gradually recovers to the correct value, as shown by the gray arrow in the diagram. Throughout this process, the actual electrical quantities of the DC system are not subjected to any disturbance.

[0067] Assuming the system experiences a slight disturbance but the overall process remains under control, and that the low-voltage arm experiences significant voltage loss after the discharge gap breaks down, with the voltage slightly below the VDCOL start-up voltage UD_High, the DC primary side electrical quantities will be slightly disturbed, but the overall process will remain under control. The changes in the system operating point are as follows: Figure 7As shown, the system operating point observed by the rectifier-side secondary system drops from the red point to the yellow point. Although the measured current remains unchanged, VDCOL starts, and the control system increases the rectifier-side firing angle (reducing the DC voltage) in an attempt to reduce the DC current, shifting the observed operating point, which has deviated from the operating characteristic curve, towards one side of the curve (leftward). Consequently, the actual operating point deviates from the steady-state operating point and also moves to the lower left. After detecting the actual operating point shifting to the lower left (reduced voltage and current), the inverter side, due to the small deviation, can use constant turn-off angle control to maintain the operating point near the operating characteristic curve, and may also switch to constant current control mode. Subsequently, as the low-voltage arm charges, the measured voltage of the rectifier-side secondary system gradually rises and recovers, and the system operating point smoothly returns to the steady-state operating point. Throughout this process, the rectifier-side control makes a misjudgment due to measurement issues, but this can be mitigated by appropriate adjustments to the inverter-side control during the low-voltage arm charging process, ensuring a smooth system recovery. The operating point is controlled throughout and moves only within the green area shown in the figure.

[0068] Assuming the system is subjected to significant disturbances and experiences a runaway process, and that the low-voltage arm suffers severe voltage loss after the discharge gap breaks down, with the voltage significantly lower than the VDCOL start-up voltage UD_High, the DC primary side electrical quantities will be significantly disturbed and experience a runaway process. The changes in the system operating point are as follows: Figure 8 As shown, the system operating point observed by the rectifier-side secondary system drops sharply from the red point to the yellow point, VDCOL is activated, and the control system continuously increases the rectifier-side firing angle (reducing the DC voltage) in an attempt to quickly reduce the DC current. This causes the actual operating point to deviate from the steady-state operating point, continuously moving to the lower left. After the inverter side switches from constant turn-off angle control to constant current control, the rectifier side continues to reduce the current. Once the current drops below the inverter-side current command value, the control trend on the inverter side changes to increasing the DC current by reducing the DC voltage. Subsequently, both the rectifier-side and inverter-side controls exhibit a voltage-reducing trend, resulting in mutual amplification and non-convergence of control effects. The system deviates further and further from the steady-state operating point, and the DC voltage becomes uncontrolled and drops rapidly. In summary, the magnitude of the instantaneous voltage loss on the low-voltage arm of the DC voltage divider determines the subsequent control system response. The more severe the voltage loss, the more difficult it is for the system to regulate and recover, and it may even trigger DC voltage runaway, leading to a series of consequences such as low-voltage protection activation.

[0069] Based on the above, the response characteristics of the system under different degrees of voltage loss of the low-voltage arm caused by the breakdown of the discharge gap of the low-voltage arm of the DC voltage divider were simulated and analyzed. Considering that the low-voltage arm voltage is 70V under normal operating conditions, the overall residual voltage Urem of the low-voltage arm after voltage loss was set to decrease from 65V to 0V in 5V increments. The measured values ​​of the rectifier-side DC voltage and the firing angle response curves under 14 operating conditions are shown in Figures 9(a) and 9(b). The residual voltage Urem... remThe five operating conditions with voltages greater than or equal to 45V belong to case one analyzed above (electrical quantities are not disturbed). After the low-voltage arm of the DC voltage divider loses voltage, its voltage drops from the corresponding U... rem It began to recover smoothly, with no adjustment process for the trigger angle during this period. rem The two operating conditions, 40V and 35V, fall under scenario two analyzed above (slight disturbance but overall process under control). The firing angle undergoes an adjustment process, but control convergence is achieved. After a brief deviation, the firing angle returns to its normal operating value, and subsequently, the DC voltage recovers smoothly. Residual voltage U rem The seven operating conditions with voltages less than or equal to 30V fall under scenario three analyzed above (significant disturbance and uncontrolled process). After the voltage loss occurs, the measured DC voltage undergoes a brief recovery process, after which the system loses its steady-state operating point, and the voltage begins to drop rapidly. Simultaneously, the more severe the low-voltage arm voltage loss, the shorter the normal voltage recovery process (i.e., the earlier the uncontrolled inflection point arrives), and the faster and deeper the voltage drop. In general, the more severe the low-voltage arm voltage loss caused by the breakdown of the discharge gap in the DC voltage divider, the more significant the disturbance to the system, and the more difficult the voltage recovery. Simulation results show that, under single voltage loss conditions, considering the Jinsu DC low-voltage protection activation value of 0.35pu, the low-voltage protection can only be guaranteed not to operate if the remaining voltage after the low-voltage arm voltage loss at the Jinping station still exceeds 30V. Based on the above analysis, considering only a single lightning strike disturbance, without the installation of a varistor, the remaining voltage of the low-voltage arm after voltage loss is 0V. Under the combined effects of low-voltage arm voltage recovery and control failure, the voltage remains negative 80ms after voltage loss, and only recovers to about 0V after 150ms (refer to U in Figure 9(a)). rem The voltage drop (=0V curve) was far below the low-voltage protection trip value of 0.35 pu, indicating that the low-voltage protection would activate. After installing a varistor, the remaining voltage of the low-voltage arm corresponding to the first lightning strike in this incident was 50V and 52.5V (pole I and pole II), and the low-voltage protection did not activate in either case. The low-voltage protection for pole I ultimately activated because it suffered three lightning strikes in a short period, each with a significant voltage drop, making recovery more difficult. The voltage drop was compounded by the additional voltage drop before the low-voltage arm charging was complete, causing the voltage to gradually decrease and eventually become uncontrollable. Pole II, however, experienced a less severe second voltage drop and a higher degree of voltage recovery before the third voltage drop. Therefore, the third voltage drop did not cause a runaway DC voltage in the system, and ultimately, the low-voltage protection did not activate.

[0070] In summary, based on the analysis described above:

[0071] 1) The loss of voltage in the low-voltage arm is essentially caused by the rapid recovery of the capacitor C2 after the resistance of the varistor drops sharply. During this process, the time constant of the RC discharge circuit changes by orders of magnitude.

[0072] 2) The rapid discharge process of the capacitor is synchronized with the recovery of the resistance value of the varistor. The final voltage drop depends on the inflection point of C2 "from discharge to charge". It is not only affected by the parameters of the varistor, but also by the complex influence of the peak value, shape and duration of the lightning disturbance waveform, and has a certain degree of randomness.

[0073] 3) When the low-voltage arm loses voltage, the voltage division ratio of the DC voltage divider changes. The measured DC voltage value is related not only to the voltage loss condition of the low-voltage arm but also to the actual DC voltage. In this incident, the control action caused the actual DC voltage to continuously decrease, and the low-voltage arm lost voltage multiple times, resulting in a "step-like" drop in the measured voltage.

[0074] 4) Without the addition of a varistor, the complete loss of voltage in the low-voltage arm of the DC voltage divider is equivalent to the variable resistor Rc directly switching between the "high resistance - short circuit" state.

[0075] Based on the analysis above regarding the undervoltage mechanism of the low-voltage arm of the DC voltage divider and the dynamic response characteristics of the DC control system after undervoltage, in actual scenarios, when multiple lightning strikes occur, the grounding grid voltage in the station rises multiple times, causing the discharge gap of the low-voltage arm of the DC voltage divider (pole I and pole II) to break down multiple times, resulting in the instantaneous undervoltage of the low-voltage arm of the DC voltage divider.

[0076] Figures 10(a) and 10(b) show the DC voltage, DC current, and firing angle waveforms of the PCP control hosts for poles I and II during the period when the measured DC voltage of pole 1 dropped from 800kV to 0kV in this event. It can be seen that during this period, the discharge gap breakdown caused the low-voltage arm to lose voltage three times, with the voltage decreasing progressively. Ultimately, after the third voltage loss, the measured voltage of pole I fell below the DC line undervoltage protection threshold, and the protection activated after an 80ms delay, issuing a restart signal. After the third voltage loss, the measured voltage of pole II remained above the DC line undervoltage protection threshold, and the protection did not activate. In summary, compared to the bipolar blocking event caused by the low-voltage arm loss of the DC voltage divider in a converter station without a varistor, this event resulted in the low-voltage arm experiencing three significant voltage losses before the DC system restarted, indicating a greater number of disturbances. However, because a varistor is connected in series in the discharge gap circuit, there is still a certain residual voltage in the circuit after the discharge gap breaks down, and the value of the residual voltage has a certain degree of randomness. The low-voltage protection for pole I is triggered and restarted only after the third voltage loss, while the low-voltage protection for pole II does not activate. Therefore, although adding a varistor can effectively prevent low-voltage protection activation caused by a single lightning strike leading to low-voltage arm voltage loss, if multiple lightning strikes occur in a short period, and each voltage loss is significant, the voltage loss can be compounded before the low-voltage arm charging is complete, easily causing the voltage to drop step by step and eventually become uncontrollable, thus triggering false lockout.

[0077] The above analysis describes the basic process of UHVDC erroneous blocking caused by the transient characteristics of the DC voltage divider, and explains the response characteristics of each link in the control system under abnormal rectifier-side voltage command measurement. However, the low-voltage arm of the voltage divider considered in this analysis does not have a parallel varistor, meaning that all voltage loss is considered as complete voltage loss. In engineering practice, after configuring varistors, the degree of voltage loss varies under different voltage disturbance conditions, and the risk of erroneous blocking of the UHVDC transmission system also varies. Existing technical solutions cannot analyze this.

[0078] Therefore, to address the problem of analyzing the risk of false blocking, this invention provides a method for assessing the risk of false blocking in high-voltage direct current (HVDC) transmission systems. This method analyzes whether an HVDC transmission system will experience false blocking due to loss of voltage in the low-voltage arm of the DC voltage divider under any given lightning-induced ground potential disturbance condition. The analysis method provided by this invention can be used to predict the operational risks of ultra-high-voltage direct current (UHVDC) converter stations or to guide the selection of parameters for varistor and RC voltage divider circuits.

[0079] The control method for accidental blocking of a high-voltage direct current transmission system provided in this invention will be described in detail below with reference to specific implementation methods. Figure 11 This is a flowchart illustrating the risk assessment method for false blocking in a high-voltage direct current transmission system provided in an embodiment of the present invention, as shown below. Figure 11 As shown, the method includes:

[0080] S1101, constructing a first circuit equivalent model with a variable resistor connected in parallel at the RC voltage divider of the DC voltage divider, and a second circuit equivalent model with a varistor connected in series at the discharge gap of the DC voltage divider.

[0081] Specifically, such as Figure 5 As shown, the first circuit equivalent model includes a first capacitor, a second capacitor, a first resistor, a second resistor, and a variable resistor; the first capacitor and the second capacitor are connected in series to form a capacitor branch, the first resistor and the second resistor are connected in series to form a resistor branch, the capacitor branch and the resistor branch are connected to form an H-bridge, and the variable resistor and the second resistor are connected in parallel; the input of the first capacitor equivalent model is the DC rated voltage of the high voltage direct current transmission system.

[0082] like Figure 2 As shown, the equivalent model of the second circuit includes a first resistor, a second resistor, a first capacitor, a second capacitor, a third capacitor, a discharge gap, and a varistor; the first capacitor and the second capacitor are connected in series to form a capacitor branch, the first resistor and the second resistor are connected in series to form a resistor branch, and the capacitor branch and the resistor branch are connected in parallel; the discharge gap and the varistor are connected in series to form a branch and are connected in parallel across the second resistor, and the third capacitor is connected in parallel across the second capacitor; wherein, the input of the equivalent model of the second capacitor is a voltage disturbance signal.

[0083] It is understood that the first and second circuit equivalent models constructed in this embodiment can be built in electromagnetic transient simulation software. The values ​​of capacitance and resistance in the two circuit equivalent models can be set according to the actual component parameters in the engineering. The parameters of the discharge gap and varistor are set according to the actual component parameters in the engineering. The parameters of the discharge gap mainly include the breakdown voltage value, and the parameters of the varistor mainly include the nominal voltage, residual voltage, current flow rate, temperature coefficient, etc.

[0084] S1102, apply a voltage disturbance signal across the varistor in the equivalent circuit model of the second circuit to simulate the resistance change curve of the varistor caused by the voltage disturbance.

[0085] In this embodiment, the voltage disturbance signal is the voltage experienced by the low-voltage arm of the DC voltage divider under the ground potential change caused by lightning strikes. It generally increases rapidly and then recovers rapidly. The discharge gap conducts under overvoltage. The characteristic of the varistor is that its resistance value is related to the voltage; after the voltage increases to a certain level, the resistance value decreases rapidly. Therefore, under the action of the voltage disturbance signal, the resistance change curve shows a rapid decrease followed by a rapid recovery.

[0086] S1103, using the resistance change curve as the input parameter of the variable resistor in the first circuit equivalent model, simulates the voltage value of the low-voltage arm of the DC voltage divider when the voltage drops to the lowest point as the variable resistor changes according to the resistance change curve.

[0087] In this embodiment, a DC voltage source is connected to the high-voltage end of the DC voltage divider model, and after all capacitors have been charged, the variable resistor R is adjusted. c Its resistance value changes according to the resistance change curve. c A rapid decrease in voltage causes a rapid overall decrease in the equivalent resistance of the low-voltage arm, resulting in a rapid discharge of the low-voltage arm capacitor and a momentary drop in voltage; R c Rapid recovery leads to a rapid overall recovery of the equivalent resistance, charging of the low-voltage arm capacitor (but the charging speed is slower than the discharging speed due to the increased loop resistance), and a gradual recovery of the voltage. Record the lowest voltage drop point U during this process. LOW That is, the voltage value when it drops to its lowest point.

[0088] S1104 compares the voltage value with the starting voltage value of the low-voltage current limiting protection, and determines the risk analysis result of the false blocking of the high-voltage DC transmission system based on the comparison result.

[0089] Specifically, the risk analysis results for accidental blocking of the high-voltage direct current transmission system are determined based on the comparison results, including:

[0090] If the voltage value is greater than the starting voltage value, the electrical quantities of the high-voltage direct current transmission system are not disturbed, and there is no risk of blockage in the high-voltage direct current transmission system.

[0091] If the voltage value is less than or equal to the starting voltage value, the electrical quantities of the high-voltage direct current transmission system are not disturbed, and the high-voltage direct current transmission system is at risk of being blocked.

[0092] Specifically, compared to U LOW and the start-up voltage U of low-voltage current limiting control VDCOL th_h If U LOW >U th_h The output conclusion is "The electrical quantities of the real DC system are not disturbed, and there is no risk of system error-induced blockage"; if U LOW ≤U th_h The output conclusion is: "Measurement deviation leads to incorrect control adjustment, the electrical quantities of the real DC system are disturbed, and the system faces the risk of false lockout."

[0093] Figure 12 A schematic diagram of a risk assessment system for accidental blocking in a high-voltage direct current transmission system provided in this embodiment of the invention is shown below. Figure 12 As shown, the system includes:

[0094] The model building module 1201 is used to build a first circuit equivalent model with a variable resistor connected in parallel at the DC voltage divider, and a second circuit equivalent model with a varistor connected in series at the discharge gap of the DC voltage divider.

[0095] The resistance simulation module 1202 is used to apply a voltage disturbance signal across the varistor in the equivalent circuit model of the second circuit to simulate the resistance change curve of the varistor caused by the voltage disturbance.

[0096] The voltage simulation module 1203 is used to take the resistance change curve as the input parameter of the variable resistor in the first circuit equivalent model, and simulate the voltage value of the low voltage arm of the DC voltage divider when the voltage drops to the lowest point when the variable resistor changes according to the resistance change curve.

[0097] The risk analysis module 1204 is used to compare the voltage value with the starting voltage value of the low-voltage current limiting protection, and to determine the risk analysis result of the false blocking of the high-voltage direct current transmission system based on the comparison result.

[0098] The present invention provides a risk assessment system for false blocking in high-voltage direct current (HVDC) transmission systems, which analyzes whether a false blocking will occur due to the loss of voltage in the low-voltage arm of a DC voltage divider under any ground potential disturbance caused by a lightning strike. The analysis method provided by this invention can be used to predict the operational risks of ultra-high-voltage direct current (UHVDC) converter stations, or to guide the selection of parameters for varistor and RC voltage divider circuits.

[0099] In some embodiments, the equivalent model of the first circuit includes a first capacitor, a second capacitor, a first resistor, a second resistor, and a variable resistor;

[0100] The first capacitor and the second capacitor are connected in series to form a capacitor branch, the first resistor and the second resistor are connected in series to form a resistor branch, the capacitor branch and the resistor branch are connected to form an H-bridge, and the variable resistor is connected in parallel with the second resistor;

[0101] The input to the first capacitor equivalent model is the DC rated voltage of the high-voltage direct current transmission system.

[0102] In some embodiments, the equivalent model of the second circuit includes a first resistor, a second resistor, a first capacitor, a second capacitor, a third capacitor, a discharge gap, and a varistor;

[0103] The first capacitor and the second capacitor are connected in series to form a capacitor branch, and the first resistor and the second resistor are connected in series to form a resistor branch. The capacitor branch and the resistor branch are connected in parallel.

[0104] The discharge gap is connected in series with the varistor to form a branch and is connected in parallel across the two ends of the second resistor, and the third capacitor is connected in parallel across the two ends of the second capacitor;

[0105] The input to the second capacitor equivalent model is a voltage disturbance signal.

[0106] This invention also provides an electronic device. The electronic device includes a processor, a memory, a communication interface, and at least one communication bus for connecting the processor, the memory, and the communication interface. The memory includes, but is not limited to, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (PROM), or portable read-only memory (CD-ROM), and is used for related instructions and data.

[0107] The communication interface is used to receive and send data. The processor can be one or more CPUs; if the processor is a single CPU, it can be a single-core CPU or a multi-core CPU. The processor in the electronic device reads one or more programs stored in memory and performs the following operations: constructing a first circuit equivalent model with a variable resistor connected in parallel at the RC voltage divider of the DC voltage divider, and a second circuit equivalent model with a varistor connected in series at the discharge gap of the DC voltage divider; applying a voltage perturbation signal across the varistor in the second circuit equivalent model to simulate the resistance change curve of the varistor caused by the voltage perturbation; using the resistance change curve as the input parameter of the variable resistor in the first circuit equivalent model to simulate the voltage value when the voltage of the low-voltage arm of the DC voltage divider drops to its lowest point as the variable resistor changes according to the resistance change curve; comparing the voltage value with the starting voltage value of the low-voltage current limiting protection, and determining the risk analysis result of the false blocking of the high-voltage DC transmission system based on the comparison result.

[0108] It should be noted that the specific implementation of each operation can be described above. Figure 11 The corresponding description of the method embodiments shown indicates that electronic devices can be used to execute a method for assessing the risk of accidental blockage in a high-voltage direct current transmission system according to the above-described method embodiments of this application, which will not be described in detail here.

[0109] This invention also provides a computer-readable storage medium, which is a memory device in a computer device for storing programs and data. It is understood that the computer-readable storage medium here can include both the built-in storage medium in the computer device and extended storage media supported by the computer device. The computer-readable storage medium provides storage space that stores the operating system of the terminal. Furthermore, the storage space also stores one or more instructions suitable for loading and execution by a processor. These instructions can be one or more computer programs (including program code). It should be noted that the computer-readable storage medium here can be high-speed RAM or non-volatile memory, such as at least one disk storage device. The processor can load and execute one or more instructions stored in the computer-readable storage medium to implement the corresponding steps of the above-described method for assessing the risk of accidental blocking in a high-voltage direct current transmission system. Those skilled in the art should understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0110] This invention also provides a computer program product containing program instructions. The computer program product can be software or program products containing program instructions, capable of running on a computing device or stored on any usable medium. When the computer program product runs on at least one electronic device, it causes the at least one electronic device to perform a method for assessing the risk of accidental blocking in a high-voltage direct current transmission system.

[0111] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for risk assessment of a false trip in a high voltage direct current power transmission system, characterized by, The method comprises: a first circuit equivalent model is constructed in parallel with a variable resistor at a blocking capacitor voltage divider of a DC voltage divider, and a second circuit equivalent model is constructed in series with a voltage-dependent resistor at a discharge gap of the DC voltage divider; a voltage disturbance signal is applied across the voltage-dependent resistor of the second circuit equivalent model to simulate a resistance change curve of the voltage-dependent resistor caused by the voltage disturbance; the resistance change curve is taken as an input parameter of the variable resistor of the first circuit equivalent model, and a voltage value when a voltage of a low-voltage arm of the DC voltage divider drops to a minimum point is simulated according to the resistance change curve of the variable resistor; the voltage value is compared with a starting voltage value of low-voltage current-limiting protection, and a risk analysis result of false locking of the high-voltage DC power transmission system is determined based on a comparison result.

2. The method of claim 1, wherein, The first circuit equivalent model comprises a first capacitor, a second capacitor, a first resistor, a second resistor and a variable resistor; the first capacitor and the second capacitor are connected in series to form a capacitor branch, the first resistor and the second resistor are connected in series to form a resistor branch, the capacitor branch and the resistor branch are connected to form an H-bridge, and the variable resistor is connected in parallel with the second resistor; wherein the input of the first capacitor equivalent model is a DC rated voltage of the high-voltage DC power transmission system.

3. The method of claim 1, wherein, The second circuit equivalent model comprises a first resistor, a second resistor, a first capacitor, a second capacitor, a third capacitor, a discharge gap and a voltage-dependent resistor; the first capacitor and the second capacitor are connected in series to form a capacitor branch, the first resistor and the second resistor are connected in series to form a resistor branch, and the capacitor branch and the resistor branch are connected in parallel; the discharge gap and the voltage-dependent resistor are connected in series to form a branch connected across the second resistor, and the third capacitor is connected across the second capacitor; wherein the input of the second capacitor equivalent model is a voltage disturbance signal.

4. The method of claim 3, wherein, The resistance value of the resistance change curve first decreases and then increases.

5. The method of claim 1, wherein, The voltage value is compared with a starting voltage value of low-voltage current-limiting protection, and a risk analysis result of false locking of the high-voltage DC power transmission system is determined based on a comparison result, which comprises: if the voltage value is greater than the starting voltage value, the electrical quantity of the high-voltage DC power transmission system is not disturbed, and the high-voltage DC power transmission system does not have a locking risk; if the voltage value is less than or equal to the starting voltage value, the electrical quantity of the high-voltage DC power transmission system is not disturbed, and the high-voltage DC power transmission system has a locking risk.

6. A high voltage direct current power transmission system misblock risk assessment system, characterised in that, The system comprises: a model construction module configured to construct a first circuit equivalent model in parallel with a variable resistor at a blocking capacitor voltage divider of a DC voltage divider, and to construct a second circuit equivalent model in series with a voltage-dependent resistor at a discharge gap of the DC voltage divider; a resistance simulation module configured to apply a voltage disturbance signal across the voltage-dependent resistor of the second circuit equivalent model to simulate a resistance change curve of the voltage-dependent resistor caused by the voltage disturbance; a voltage simulation module configured to take the resistance change curve as an input parameter of the variable resistor of the first circuit equivalent model, and to simulate a voltage value when a voltage of a low-voltage arm of the DC voltage divider drops to a minimum point according to the resistance change curve of the variable resistor; a risk analysis module configured to compare the voltage value with a starting voltage value of low-voltage current-limiting protection, and to determine a risk analysis result of false locking of the high-voltage DC power transmission system based on a comparison result.

7. The system of claim 6, wherein, The first circuit equivalent model comprises a first capacitor, a second capacitor, a first resistor, a second resistor and a variable resistor; The first capacitor and the second capacitor are connected in series to form a capacitor branch, the first resistor and the second resistor are connected in series to form a resistor branch, and the capacitor branch and the resistor branch are connected to form an H-bridge, and the variable resistor is connected in parallel with the second resistor. The input of the first capacitor equivalent model is a DC rated voltage of the high-voltage direct current power transmission system.

8. The system of claim 6, wherein, The second circuit equivalent model comprises a first resistor, a second resistor, a first capacitor, a second capacitor, a third capacitor, a discharge gap and a voltage-dependent resistor; The first capacitor and the second capacitor are connected in series to form a capacitor branch, the first resistor and the second resistor are connected in series to form a resistor branch, and the capacitor branch and the resistor branch are connected in parallel; The discharge gap and the voltage-dependent resistor are connected in series to form a branch connected in parallel across the second resistor, and the third capacitor is connected in parallel across the second capacitor; The input of the second capacitor equivalent model is a voltage disturbance signal.

9. An electronic device, comprising: comprise a memory and a processor; The memory is configured to store a computer program, and the computer program comprises program instructions; The processor is configured to execute the program instructions to enable the electronic device to perform the steps of the high-voltage direct current power transmission system mis-blocking risk assessment method according to any one of claims 1 to 5.

10. A computer-readable storage medium, characterized in that, The computer readable storage medium comprises a computer program, and the computer program, when executed by one or more processors, implements the high-voltage direct current power transmission system mis-blocking risk assessment method according to any one of claims 1 to 5.

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