A method, system and electronic device for detecting commutation failure in a direct current power transmission system

Abstract

The application provides a DC power transmission system commutation failure detection method, system and electronic equipment, and belongs to the field of DC power transmission. The DC power transmission system commutation failure detection method comprises the following steps: obtaining the current oscillation frequency of the sending end of the DC power transmission system; obtaining the sending end electrical quantity, the receiving end electrical quantity and the receiving end control quantity of the DC power transmission system within a set time period based on the current oscillation frequency of the sending end; and determining whether the DC power transmission system has a commutation failure according to the sending end electrical quantity, the receiving end electrical quantity and the receiving end control quantity of the DC power transmission system within the set time period. The influence of the sending end oscillation frequency on the receiving end commutation failure is considered, and the accuracy of the DC power transmission system commutation failure detection is improved.

Description

A method, system, and electronic equipment for detecting commutation failure in a DC transmission system. Technical Field

[0001] This invention relates to the field of DC power transmission, and in particular to a method, system, and electronic device for detecting commutation failure in a DC power transmission system based on sending-end oscillation. Background Technology

[0002] High-voltage direct current (HVDC) transmission has been widely used in long-distance power transmission and asynchronous grid interconnection due to its advantages such as long transmission distance, large transmission capacity, and low transmission loss. Commutation failure (CF) is one of the most common faults in DC transmission systems. Commutation failure leads to a surge in DC current and a sharp drop in DC voltage, and in severe cases, even DC blocking, seriously impacting the safe and stable operation of the power grid. It is generally believed that commutation failure is caused by faults in the receiving-end AC system, and the mechanism by which these faults cause commutation failure is relatively well understood. However, with the construction of large-scale new energy bases focusing on deserts, Gobi, and arid regions, the sending-end systems of these new energy systems lack conventional generating units for support, making them highly susceptible to fault disturbances. Due to the electrical and control coupling between the sending and receiving ends of the DC transmission line, disturbances to the sending-end (rectifier side) system may cause changes in the electrical response and control switching at the receiving end, thus leading to commutation failure at the receiving end.

[0003] Currently, research on the mechanism of receiving-end commutation failure caused by sending-end disturbances mainly focuses on large disturbances such as sending-end faults. The main research methods fall into three categories: 1. Classifying sending-end faults into severe and minor faults, and combining simulation phenomena, the quasi-steady-state formula of DC transmission systems, and inverter-side control actions to analyze the changes in electrical quantities such as system voltage and current, and the control quantities of the control system during the recovery process of different faults, thus deriving the mechanism of commutation failure; 2. Analyzing in detail the changes in the control systems of both the sending and receiving ends during receiving-end commutation failure under different sending-end faults, and combining this with the reactive power consumption of the receiving-end converter to analyze the mechanism of receiving-end commutation failure; 3. From the perspective of different voltage drop degrees under large receiving-end disturbances, analyzing the control methods adopted by the rectifier and inverter when the voltage drop degree of the rectifier-side converter bus is different, and deriving the expression for the turn-off angle when the voltage drop degree is different. This further analyzes the mechanism of receiving-end commutation failure.

[0004] Existing research mainly analyzes the impact characteristics of large disturbances in the sending-end AC system on the electrical quantities and control system at the receiving end. It has provided a preliminary understanding of the mechanism by which a fault in the sending-end system leads to commutation failure at the receiving end. However, it does not consider the possibility of small disturbances in the sending end causing commutation failure at the receiving end, resulting in low accuracy in detecting commutation failure at the receiving end. Summary of the Invention

[0005] The purpose of this invention is to provide a method, system, and electronic device for detecting commutation failure in a DC transmission system. Based on the oscillation detection at the sending end, it can improve the detection accuracy of commutation failure in a DC transmission system.

[0006] To achieve the above objectives, the present invention provides the following solution:

[0007] A method for detecting commutation failure in a DC transmission system, the DC transmission system comprising a sending-end AC power supply, a rectifier, an inverter, and a receiving-end AC power supply connected sequentially via DC transmission lines, the method comprising:

[0008] Obtain the current oscillation frequency at the sending end of the DC transmission system;

[0009] Based on the current oscillation frequency of the sending end, the electrical quantities of the sending end, the electrical quantities of the receiving end, and the control quantities of the receiving end of the DC transmission system within a set time period are obtained.

[0010] Based on the sending-end electrical quantities, receiving-end electrical quantities, and receiving-end control quantities of the DC transmission system within a set time period, it is determined whether the DC transmission system has experienced a commutation failure.

[0011] Optionally, the sending-end electrical quantity includes the sending-end DC voltage; the receiving-end electrical quantity includes the receiving-end DC current and the receiving-end DC voltage; and the receiving-end control quantity includes the inverter's lead-fire angle under constant current control and constant turn-off angle control.

[0012] Alternatively, the DC voltage at the sending end can be determined using the following formula:

[0013]

[0014] Among them, U dr E is the DC voltage at the sending end. r α is the commutation line voltage of the rectifier. r γ is the firing angle of the rectifier. r The sending end is the shut-off angle.

[0015] Alternatively, the receiving-end DC current can be determined using the following formula:

[0016]

[0017] Among them, I d U1 is the DC current at the receiving end, and E is the AC power supply at the sending end. r x1 is the commutation line voltage of the rectifier, and x2 is the AC line reactance at the sending end.

[0018] Alternatively, the receiving-end DC voltage can be determined using the following formula:

[0019]

[0020] Among them, U di U is the DC voltage at the receiving end. dr The DC voltage at the sending end. α r X is the firing angle of the rectifier. cr R is the commutation reactance of the rectifier, x1 is the AC line reactance at the sending end, and R is the commutation reactance of the rectifier. d U1 is the DC circuit resistor, and U2 is the AC power supply at the sending end.

[0021] Optionally, the lead firing angle of the inverter under constant current control can be determined using the following formula:

[0022] β cc =K pcc (I dr-order -I di -0.1)+K icc ∫(I dr-order -I di -0.1)dt;

[0023] Where, β cc The leading firing angle of the inverter under constant current control, K pcc K is the proportional gain of the PI controller in constant current control. icc I is the integral gain of the PI controller in constant current control. dr-order I is the DC current command value transmitted from the inverter to the rectifier. di This is the measured value of the inverter's DC current.

[0024] Optionally, the lead firing angle of the inverter under constant turn-off angle control can be determined using the following formula:

[0025] β cea =K pcea (γ ref -γ+Δγ rec )+K icea ∫(γ ref -γ+Δγ rec )dt;

[0026] Where, β cea The inverter's leading firing angle under constant turn-off angle control, K pcea K is the proportional gain of the PI controller in constant turn-off angle control. icea γ is the integral gain of the PI controller in constant turn-off angle control. ref Δγ is the reference value for the turn-off angle under constant turn-off angle control, γ is the measured value of the inverter turn-off angle, and Δγ is the reference value for the turn-off angle. recThis is the output value of the inverter under current deviation control.

[0027] Optionally, determining whether a commutation failure has occurred in the DC transmission system based on the sending-end electrical quantities, receiving-end electrical quantities, and receiving-end control quantities of the DC transmission system within a set time period specifically includes:

[0028] For any given moment within a set time period, determine whether the change in the current electrical quantity at the sending end of the DC transmission system compared to the previous electrical quantity at the sending end is greater than a first set threshold, whether the change in the current electrical quantity at the receiving end compared to the previous electrical quantity at the receiving end is greater than a second set threshold, and whether the change in the current control quantity at the receiving end compared to the previous control quantity at the receiving end is greater than a third set threshold.

[0029] If so, the DC transmission system has experienced a commutation failure; otherwise, the DC transmission system has not experienced a commutation failure.

[0030] To achieve the above objectives, the present invention also provides the following solution:

[0031] A commutation failure detection system for a DC transmission system, comprising:

[0032] The oscillation frequency detection unit is used to obtain the current oscillation frequency at the sending end of the DC transmission system;

[0033] An electrical control quantity determination unit, connected to the oscillation frequency detection unit, is used to obtain the electrical quantities at the sending end, the electrical quantities at the receiving end, and the control quantities at the receiving end of the DC transmission system within a set time period based on the current oscillation frequency at the sending end.

[0034] The commutation failure determination unit is connected to the electrical control quantity determination unit and is used to determine whether the DC transmission system has experienced a commutation failure based on the sending-end electrical quantity, receiving-end electrical quantity, and receiving-end control quantity of the DC transmission system within a set time period.

[0035] To achieve the above objectives, the present invention also provides the following solution:

[0036] An electronic device includes a memory and a processor, the memory storing a computer program, and the processor running the computer program to cause the electronic device to perform the above-described method for detecting commutation failure in a DC transmission system.

[0037] According to specific embodiments provided by the present invention, the following technical effects are disclosed: First, the current oscillation frequency of the sending end of the DC transmission system is obtained; then, based on the current oscillation frequency of the sending end, the electrical quantities of the sending end, the electrical quantities of the receiving end, and the control quantities of the receiving end of the DC transmission system within a set time period are obtained; finally, based on the electrical quantities of the sending end, the electrical quantities of the receiving end, and the control quantities of the receiving end of the DC transmission system within the set time period, it is determined whether a commutation failure has occurred in the DC transmission system. By considering the influence of the sending end oscillation frequency on the receiving end commutation failure, the accuracy of commutation failure detection in the DC transmission system is improved. Attached Figure Description

[0038] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0039] Figure 1 is a flowchart of the DC transmission system commutation failure detection method of the present invention;

[0040] Figure 2 shows the waveform diagram of commutation failure development;

[0041] Figure 3 shows the distribution of participation factors at a frequency of 1.2155 Hz;

[0042] Figure 4 is a schematic diagram of the CIGRE HVDC standard test model;

[0043] Figure 5 shows the waveform of the DC voltage at the sending end as a function of the firing angle;

[0044] Figure 6 is a schematic diagram of the evolution mechanism of the first commutation failure;

[0045] Figure 7 is a schematic diagram of the evolution mechanism of subsequent commutation failure;

[0046] Figure 8 is a schematic diagram of the module of the DC transmission system commutation failure detection system of the present invention.

[0047] Symbol explanation:

[0048] Oscillation frequency detection unit-1, electrical control quantity determination unit-2, commutation failure judgment unit-3. Detailed Implementation

[0049] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0050] The purpose of this invention is to provide a method, system, and electronic device for detecting commutation failure in a DC transmission system. Based on the phenomenon that a small disturbance at the sending end causes commutation failure at the receiving end, the invention detects whether a commutation failure has occurred in the DC transmission system, thereby improving detection accuracy.

[0051] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0052] This invention proposes a quasi-steady-state model for HVDC. Based on the commutation failure at the receiving end caused by small disturbances at the sending end, it proposes a method for analyzing the commutation failure at the receiving end. This method is based on small-signal analysis of the DC system, studies the natural frequency and dominant dynamics of the DC system, and combines the quasi-steady-state model of the DC system to analyze the process of the transmission of the sending end oscillation to the inverter side in stages and the interaction law between the oscillation and the inverter side control. The method is then verified by simulation using PSCAD.

[0053] Example 1

[0054] As shown in Figure 1, the DC transmission system commutation failure detection method provided in this embodiment includes:

[0055] S1: Obtain the current oscillation frequency at the sending end of the DC transmission system.

[0056] S2: Based on the current oscillation frequency of the sending end, obtain the sending end electrical quantity, receiving end electrical quantity, and receiving end control quantity of the DC transmission system within a set time period.

[0057] In this embodiment, the sending-end electrical quantity includes the sending-end DC voltage. The receiving-end electrical quantity includes the receiving-end DC current and the receiving-end DC voltage. The receiving-end control quantity includes the inverter's lead-fire angle under constant current control and constant turn-off angle control.

[0058] The DC voltage at the sending end is determined using the following formula:

[0059]

[0060] Among them, U dr E is the DC voltage at the sending end. r α is the commutation line voltage of the rectifier. r γ is the firing angle of the rectifier. r The sending end is the shut-off angle.

[0061] The receiving-end DC current is determined using the following formula:

[0062]

[0063] Among them, Id U1 is the DC current at the receiving end, and E is the AC power supply at the sending end. r x1 is the commutation line voltage of the rectifier, and x2 is the AC line reactance at the sending end.

[0064] The receiving-end DC voltage is determined using the following formula:

[0065]

[0066] Among them, U di U is the DC voltage at the receiving end. dr The DC voltage at the sending end. α r X is the firing angle of the rectifier. cr R is the commutation reactance of the rectifier, x1 is the AC line reactance at the sending end, and R is the commutation reactance of the rectifier. d U1 is the DC circuit resistor, and U2 is the AC power supply at the sending end.

[0067] HVDC systems typically have CC (constant current) control and constant minimum firing angle control on the rectifier side, and CC control, CEA (constant extinction angle) control, VDCOL (voltage dependent current order limiter) link, and CEC (current error controller) control on the inverter side.

[0068] The lead firing angle of the inverter under constant current control is determined using the following formula:

[0069] β cc =K pcc (I dr-order -I di -0.1)+K icc ∫(I dr-order -I di -0.1)dt;

[0070] Where, β cc The leading firing angle of the inverter under constant current control, K pcc K is the proportional gain of the PI controller in constant current control. icc I is the integral gain of the PI controller in constant current control. dr-order I is the DC current command value transmitted from the inverter to the rectifier. di This is the measured value of the inverter's DC current.

[0071] The lead firing angle of the inverter under constant turn-off angle control is determined using the following formula:

[0072] β cea =K pcea (γ ref -γ+Δγ rec )+K icea ∫(γ ref -γ+Δγ rec )dt;

[0073] Where, β cea The inverter's leading firing angle under constant turn-off angle control, K pcea K is the proportional gain of the PI controller in constant turn-off angle control. icea γ is the integral gain of the PI controller in constant turn-off angle control. ref Δγ is the reference value for the turn-off angle under constant turn-off angle control, γ is the measured value of the inverter turn-off angle, and Δγ is the reference value for the turn-off angle. rec Δγ is the output value of the inverter under current deviation control. rec =k cec (I dr-order -I di ), k cec The maximum limit and slope of CEC control.

[0074] S3: Determine whether the DC transmission system has experienced a commutation failure based on the sending-end electrical quantity, receiving-end electrical quantity, and receiving-end control quantity of the DC transmission system within a set time period.

[0075] Specifically, for any given moment within a set time period, it is determined whether the change in the current sending-end electrical quantity of the DC transmission system compared to the previous sending-end electrical quantity is greater than a first set threshold, whether the change in the current receiving-end electrical quantity compared to the previous receiving-end electrical quantity is greater than a second set threshold, and whether the change in the current receiving-end control quantity compared to the previous receiving-end control quantity is greater than a third set threshold. If so, the DC transmission system has experienced a commutation failure; otherwise, the DC transmission system has not experienced a commutation failure.

[0076] To better understand the solution of the present invention, further explanation is provided below with reference to specific embodiments.

[0077] This invention first performs small-signal analysis on the DC transmission system to obtain the oscillation frequency of the DC transmission system, studies the interaction between the sending-end oscillation and the inverter-side control, and analyzes the influence characteristics of the control strategy and parameters of the DC transmission system on commutation failure. Then, based on the quasi-steady-state model of HVDC, the relationship between DC current and sending-end DC voltage with respect to the sending-end commutation voltage is derived, and the expression for the receiving-end DC voltage with respect to the sending-end DC voltage is also derived. Furthermore, the dynamic behavior of the receiving-end electrical quantities and control quantities coupled with the sending-end oscillation is analyzed. Finally, the mechanism of commutation failure caused by the resonance between the sending-end oscillation and the control system leading to abnormal control quantities and control switching is determined.

[0078] In this embodiment, the phenomenon of receiving-end commutation failure caused by sending-end oscillation is demonstrated using the CIGRE HVDC standard test model. Three-phase voltage sources of different frequencies and low amplitudes are added to the sending-end AC bus to simulate oscillation disturbances at the sending end. The results show that when an oscillation source of approximately 1.2Hz is injected, the electrical and control quantities of the DC transmission system change significantly, accompanied by receiving-end commutation failure. The simulation results are shown in Figure 2. From the simulation waveforms in the figure, it can be seen that after injecting a 1.2Hz oscillation into the system, the changes in the system's electrical and control quantities can be roughly divided into four stages: Stage 1: The DC transmission system remains in a stable state; Stage 2: After approximately 0.2s, the DC transmission system begins to oscillate, and the electrical and control quantities begin to show slight changes; Stage 3: The amplitude of the changes in electrical and control quantities increases significantly, specifically manifested as an increase in DC current, and the DC transmission system experiences its first commutation failure; Stage 4: The DC transmission system continues to oscillate, repeating the process of Stage 3, and subsequent commutation failures occur during the next switch between CEA control and CC control.

[0079] The above phenomena indicate that when oscillation occurs at the sending end, the coupling of electrical quantities at the sending and receiving ends will affect the control quantities at the receiving end, thereby causing large-scale oscillations in the electrical and control quantities of the DC transmission system, ultimately leading to commutation failure at the receiving end.

[0080] The following is a detailed description of the overall process for commutation failure analysis in this invention:

[0081] (1) Perform small-signal analysis on the DC transmission system to obtain the inherent oscillation frequency of the DC transmission system, study the interaction law between the sending end oscillation and the inverter side control, and analyze the influence characteristics of the control strategy and parameters of the DC transmission system on commutation failure.

[0082] An HVDC small-signal model is established for analysis, and the state variables of the DC transmission system are selected:

[0083]

[0084] Where x is the state variable of the DC transmission system, I dr I is the DC line current on the rectifier side. dr ′ represents the DC current after filtering by the first-order measurement stage. This refers to the output of the integral element in the PI controller during constant current control at the sending end. The output of the integrator in the rectifier-side phase-locked loop PI converter. θ is the output of the integrator in the inverter-side phase-locked loop PI converter. PLLr For the output phase of the rectifier-side phase-locked loop, θ PLLi For the inverter-side phase-locked loop output phase, I di θ is the DC line current on the inverter side, γ is the inverter side turn-off angle, μ is the output of the integral element in the PI controller during the received-end fixed turn-off angle control, and θ is the DC line current on the inverter side. π2 To account for the voltage phase angle change during the voltage dynamic process, V C This is the voltage of the capacitor at the midpoint of the DC line.

[0085] The eigenvalues, oscillation frequency, and state variables related to the maximum participation factor of the HVDC system were calculated, and the results are shown in Table 1. The HVDC system eigenvalues ​​refer to the matrix eigenvalues ​​obtained from small-signal analysis of the system, and are related to the oscillation frequency. If the real part of the eigenvalue is negative, oscillations may occur.

[0086] Table 1 Modal Results of LCC-HVDC Model

[0087]

[0088] Modal analysis reveals five inherent oscillation modes in the HVDC system. Among them, the 1.2155Hz inherent oscillation mode is closest to the sending-end oscillation frequency that causes receiving-end commutation failure, and is prone to resonance with the sending-end oscillation. The participation factor distribution of this oscillation mode is shown in Figure 3, with the largest participation factor variable being the output of the sending-end constant current control. Other variables with significant impact include the output μ controlled by the end-point turn-off angle and the phase-locked loop output θ. PLLi This indicates that the oscillation mode is significantly affected by the DC control system.

[0089] (2) Based on the quasi-steady-state model of HVDC, the relationship between DC current and DC voltage at the sending end with respect to the commutation voltage at the sending end is derived. At the same time, the expression of DC voltage at the receiving end with respect to DC voltage at the sending end is derived. Then, the dynamic behavior of electrical quantities and control quantities at the receiving end coupled with the oscillation at the sending end is analyzed, and the commutation failure mechanism is analyzed.

[0090] The CIGRE HVDC standard test model, as shown in Figure 4, consists of a sending-end AC power supply, a receiving-end AC power supply, a rectifier, an inverter, and a DC transmission line. In the figure, r and i represent the rectifier side and the inverter side, respectively; U1 and U2 are the sending-end and receiving-end AC power supplies, respectively; E and δ are the commutation line voltage and phase angle, respectively; P1 and P2 are the active power on the rectifier side and the inverter side, respectively; x1 and x2 are the AC line reactances on the sending-end and receiving-end, respectively; R is the resistance at both ends of the DC line; L is the reactance at both ends of the DC line; and C is the terminal capacitance of the DC line. For ease of analytical analysis, the effects of the distributed capacitance and inductance of the DC line are ignored. The equations of the HVDC quasi-steady-state model are as follows:

[0091]

[0092] Among them, E r E is the commutation line voltage of the rectifier. i I is the commutation line voltage of the inverter. r I is the AC current at the sending end. i I is the alternating current at the receiving end. d For direct current, U dr U is the DC voltage at the sending end. di α is the DC voltage at the receiving end. r α is the firing angle of the sending end. i X is the firing angle of the receiving end. cr X is the commutation reactance of the rectifier. ci R is the commutation reactance of the inverter. d =R+R is the resistance of the DC circuit.

[0093] Based on the above-mentioned HVDC quasi-steady-state model and the control modes of the rectifier and inverter sides, the mechanism of commutation failure at the receiving end caused by oscillation at the sending end is analyzed.

[0094] When oscillation first occurs, resonance will occur when the sending-end system exhibits an oscillation close to the natural frequency dominated by the DC control system, causing fluctuations in control quantities such as the firing angle, and electrical quantities such as voltage and current. These quantities in the system will not respond immediately, but will only begin to show significant waveform fluctuations after a period of time due to the cumulative effect of the PI integral controller. Therefore, when oscillation first occurs, the system maintains normal operation, all electrical quantities remain stable, the rectifier side is in CC control mode, and the inverter side CEA control leads the firing angle output value β. cea The output value β of the control lead angle is greater than CC. cc The firing angle is adjusted by the CEA closed-loop control.

[0095] After the oscillation continues for a period of time, as the HVDC system resonance continues, the system's electrical and control quantities begin to oscillate significantly, and commutation failures at the receiving end continue to occur. This can be divided into the following two stages:

[0096] 1) Initial commutation failure

[0097] System resonance causes the firing angle of the constant current control output on the rectifier side to oscillate within a certain range. This is because there exists a minimum firing angle control α on the rectifier side. min =5°, meaning the minimum firing angle is 5°; considering that the minimum turn-off angle is 7°, the commutation angle is generally 20°, and the safety margin angle is 10°, the maximum firing angle can be calculated to be 180° - 7° - 20° - 10° = 143°. Therefore, during system resonance, the firing angle varies within the range of α ∈ (5°, 143°).

[0098] The direct current can also be determined using a formula: The formula for the DC voltage at the sending end is:

[0099]

[0100] In the formula, the commutation line voltage E of the rectifier is not considered. r If we consider the variation of the firing angle as a rated value, then when the firing angle gradually increases within the variation range, the DC voltage U at the sending end will... dr With the trigger angle α r The changing waveform is shown in Figure 5. It can be seen that U dr With α r It decreases as it increases.

[0101] The expressions for DC current, sending-end DC voltage, and receiving-end DC voltage in quasi-steady-state mode can be obtained by solving the quasi-steady-state equations of the HVDC system:

[0102]

[0103] U dr =k1E r -b1;

[0104]

[0105] From the above equation, it can be seen that the DC voltage at the receiving end, U... di With the DC voltage U at the sending end dr The changing trend is the same, but the DC voltage U at the receiving end is... di The waveform slope is greater than the DC voltage U at the sending end. dr The DC voltage at the receiving end is larger, therefore the DC voltage U diThe rate of decrease is faster. Meanwhile, from the expression for the DC current at the receiving end... It can be seen that if the resistance of a DC line is small, a certain DC voltage difference can generate a large DC current pulse.

[0106] In summary, after the electrical and control quantities begin to oscillate violently, the converter bus voltage drops, the DC voltage at both ends decreases as the firing angle increases, the DC current increases as the DC voltage decreases, and the inverter-side turn-off angle decreases as the firing angle increases, thus causing the first commutation failure. Therefore, the direct cause of the first commutation failure at the receiving end under sending-end oscillation is the increased pulse current after the oscillation. The specific evolution mechanism of the entire process is shown in Figure 6.

[0107] 2) Subsequent commutation failure

[0108] After the initial commutation failure, the receiving-end AC bus voltage rises and recovers to near the command value, the DC current begins to decrease, and the turn-off angle increases. From the formulas for constant current control and constant turn-off angle control on the inverter side, it can be seen that the inverter's lead firing angle β under constant current control... cc Increase the inverter's lead firing angle β under constant turn-off angle control. cea As the voltage decreases, the inverter side switches from CEA control to CC control. Simultaneously, the rectifier side is also under CC control, and the DC current oscillates between the CC command values ​​on both sides, i.e.:

[0109] I dr-order -0.1 < I d dr-order ;

[0110] Where 0.1 is the threshold between the current command values ​​at both ends.

[0111] From the above formula, we can see the output command value Δγ for current deviation control. rec >0, inverter-side CEC triggered. When the DC current rises for the second time, causing the turn-off angle to decrease, the inverter's lead firing angle β under constant current control... cc Reduce the inverter's lead firing angle β under constant turn-off angle control. cea As the inverter increases, the CC control switches to CEA control. During the switching process, the turn-off angle is uncontrolled, leading to commutation failure. Therefore, the direct cause of subsequent commutation failures is the switching of inverter-side control.

[0112] Furthermore, during the switch from CC control to CEA control on the inverter side, the lead firing angle β on the inverter side... i Increase to raise the shut-off angle γ m This leads to the trigger angle α i The power factor decreases, while the expression for the inverter-side power factor is:

[0113] ​

[0114] Where, φ i The inverter-side power factor angle.

[0115] As can be seen from the above equation, the inverter-side power factor will change due to the turn-off angle γ. m The increase and firing angle α i The decrease in the voltage leads to an increase in reactive power consumed by the converter, which in turn causes the AC bus voltage at the receiving end to drop, further increasing the risk of commutation failure at the receiving end.

[0116] Therefore, based on the above analysis, the specific evolution mechanism of the subsequent commutation failure process is shown in Figure 7. In the figure, Q... C E is the reactive power consumed by the inverter-side converter. i This refers to the converter bus voltage on the inverter side.

[0117] To better understand the solution of this invention, four examples are provided below for further explanation.

[0118] Case 1: System Frequency Scan

[0119] Simulation verification was performed on the PSCAD / EMTDC platform using the CIGRE HVDC standard test model. First, a frequency scan was performed on the model to verify the existence of the natural frequency of 1.2Hz. Then, an oscillator was set at the AC bus on the rectifier side of the system, and the oscillator frequency was adjusted. Changes in electrical and control quantities were observed, with the lowest turn-off angle γ... min =7° to determine if commutation failure has occurred at the receiving end. The SCRs on the rectifier side and AC side of this system are 2.5 and 2 respectively, and the reactive power compensation on both sides is balanced.

[0120] Case 2: The oscillation frequency at the sending end is 1.2 Hz.

[0121] The oscillation source frequency was set to 1.2Hz and the amplitude to 3kV. The changes in the electrical and control quantities of the system were obtained. After 0.2s of oscillation at 1.2Hz, the quantities began to change, with the rectifier side firing angle gradually decreasing to the minimum firing angle control. After 0.4s, the oscillation became more intense, the firing angle α began to rise, and the DC voltage U at the sending end increased. dr and the DC voltage at the receiving end U di It begins to decrease. Therefore, the DC current I... d The pulse value rises, reaching its peak at t = 2.44s, leading to a decrease in the turn-off angle and initial commutation failure. Simultaneously, the inverter-side CEA command decreases, while the CC command increases, switching from CEA control to CC control at 2.5s. Afterwards, with the I... d Decrease, γ increases, β cc Decrease, β ceaWhen the control frequency increases, a second commutation failure occurs when CC control switches to CEA control at t = 2.6s. Without control, commutation failures will continue to occur at control switching times.

[0122] Case 3: The oscillation frequency at the sending end is 7.6 Hz and 66 Hz.

[0123] The frequency of the oscillation source was set to 7.6Hz or 66Hz, and the amplitude was 3kV, respectively, to obtain the changes in the electrical and control quantities of the system.

[0124] As shown in Table 1, 7.6Hz is the system's inherent oscillation mode dominated by the mid-terminal capacitor voltage of the DC line, while 66Hz is the system's inherent oscillation mode dominated by the DC line current on the rectifier side. Although both 7.6Hz and 66Hz are inherent oscillation modes of the system, since their dominant dynamics are independent of the DC control system, they will not cause drastic changes in the system control variables or commutation failure at the receiving end.

[0125] Case 4: The oscillation frequency at the sending end is 0.6 Hz and 0.8 Hz.

[0126] The frequencies of the oscillation source were set to 0.6Hz and 0.8Hz respectively, with an amplitude of 3kV. The changes in the electrical and control quantities of the system were obtained, and the changes were the same as at 1.2Hz.

[0127] This invention is the first to study the phenomenon of receiving-end commutation failure caused by sending-end oscillation. By using small-signal analysis methods, combined with changes in electrical quantities and control switching processes in the DC transmission system, the mechanism of receiving-end commutation failure caused by sending-end oscillation is analyzed. Finally, the phenomenon and mechanism of receiving-end commutation failure caused by sending-end oscillation are verified in the CIGRE DC transmission standard test system using PSCAD / EMTDC full electromagnetic transient simulation.

[0128] Example 2

[0129] In order to implement the method corresponding to Embodiment 1 above and achieve the corresponding functions and technical effects, a DC transmission system commutation failure detection system is provided below.

[0130] As shown in Figure 8, the DC transmission system commutation failure detection system provided in this embodiment includes: an oscillation frequency detection unit 1, an electrical control quantity determination unit 2, and a commutation failure determination unit 3.

[0131] Among them, the oscillation frequency detection unit 1 is used to obtain the current oscillation frequency of the sending end of the DC transmission system.

[0132] The electrical control quantity determination unit 2 is connected to the oscillation frequency detection unit 1. The electrical control quantity determination unit 2 is used to obtain the electrical quantity at the sending end, the electrical quantity at the receiving end, and the control quantity at the receiving end of the DC transmission system within a set time period based on the current oscillation frequency at the sending end.

[0133] The commutation failure determination unit 3 is connected to the electrical control quantity determination unit 2. The commutation failure determination unit 3 is used to determine whether the DC transmission system has experienced a commutation failure based on the sending-end electrical quantity, receiving-end electrical quantity, and receiving-end control quantity of the DC transmission system within a set time period.

[0134] Compared with the prior art, the DC transmission system commutation failure detection system provided in this embodiment has the same beneficial effects as the DC transmission system commutation failure detection method provided in Embodiment 1, and will not be repeated here.

[0135] Example 3

[0136] This embodiment provides an electronic device, including a memory and a processor. The memory is used to store computer programs, and the processor runs the computer programs to enable the electronic device to execute the DC transmission system commutation failure detection method of Embodiment 1.

[0137] Alternatively, the aforementioned electronic device may be a server.

[0138] In addition, this embodiment of the invention also provides a computer-readable storage medium storing a computer program that, when executed by a processor, implements the DC transmission system commutation failure detection method of Embodiment 1.

[0139] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0140] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for detecting commutation failure in a DC transmission system, the DC transmission system comprising a sending-end AC power supply, a rectifier, an inverter, and a receiving-end AC power supply connected sequentially via DC transmission lines, characterized in that, The method for detecting commutation failure in a DC transmission system includes: obtaining the current oscillation frequency at the sending end of the DC transmission system; based on the current oscillation frequency at the sending end, obtaining the electrical quantities at the sending end, the electrical quantities at the receiving end, and the control quantities at the receiving end of the DC transmission system within a set time period; the electrical quantities at the sending end include the DC voltage at the sending end; the electrical quantities at the receiving end include the DC current at the receiving end and the DC voltage at the receiving end; the control quantities at the receiving end include the lead-fire angle of the inverter under constant current control and constant turn-off angle control; and using the formula... Determine the DC voltage at the sending end; where U dr E is the DC voltage at the sending end. r This is the commutation line voltage of the rectifier. The firing angle of the rectifier. The cut-off angle at the sending end; using the formula Determine the receiving-end DC current; where I d U1 is the DC current at the receiving end, U1 is the AC power supply at the sending end, and x1 is the AC line reactance at the sending end; the formula is used. Determine the DC voltage at the receiving end; where U di The receiving end DC voltage, ， ， ， X cr R is the commutation reactance of the rectifier. d For DC circuit resistance; use the formula Determine the lead firing angle of the inverter under constant current control; where, The leading firing angle of the inverter under constant current control, K pcc K is the proportional gain of the PI controller in constant current control. icc I is the integral gain of the PI controller in constant current control. dr-order I is the DC current command value transmitted from the inverter to the rectifier. di This is the measured DC current value of the inverter; the formula is used. Determine the lead firing angle of the inverter under constant turn-off angle control; whereby... The inverter's leading firing angle under constant turn-off angle control, K pcea K is the proportional gain of the PI controller in constant turn-off angle control. icea The integral gain of the PI controller in constant turn-off angle control. This is a reference value for the shut-off angle under fixed shut-off angle control. This is the measured value of the inverter's turn-off angle. The output value of the inverter under current deviation control; based on the sending-end electrical quantity, receiving-end electrical quantity, and receiving-end control quantity of the DC transmission system within a set time period, it is determined whether the DC transmission system has experienced commutation failure.

2. The method for detecting commutation failure in a DC transmission system according to claim 1, characterized in that, The step of determining whether the DC transmission system has experienced commutation failure based on the sending-end electrical quantity, receiving-end electrical quantity, and receiving-end control quantity of the DC transmission system within a set time period specifically includes: for any moment within the set time period, determining whether the change in the sending-end electrical quantity of the DC transmission system at the current moment is greater than the change in the sending-end electrical quantity at the previous moment is greater than a first set threshold, whether the change in the receiving-end electrical quantity at the current moment is greater than the change in the receiving-end electrical quantity at the previous moment is greater than a second set threshold, and whether the change in the receiving-end control quantity at the current moment is greater than the change in the receiving-end control quantity at the previous moment is greater than a third set threshold; if so, the DC transmission system has experienced commutation failure; otherwise, the DC transmission system has not experienced commutation failure.

3. A commutation failure detection system for a DC transmission system, characterized in that, The DC transmission system commutation failure detection system includes: an oscillation frequency detection unit for acquiring the current oscillation frequency at the sending end of the DC transmission system; and an electrical control quantity determination unit connected to the oscillation frequency detection unit for acquiring, based on the current oscillation frequency at the sending end, the electrical quantities at the sending end, the electrical quantities at the receiving end, and the control quantities at the receiving end within a set time period. The electrical quantities at the sending end include the DC voltage at the sending end; the electrical quantities at the receiving end include the DC current at the receiving end and the DC voltage at the receiving end; and the control quantities at the receiving end include the lead-fire angle of the inverter under constant current control and constant turn-off angle control. The system uses a formula... Determine the DC voltage at the sending end; where U dr E is the DC voltage at the sending end. r This is the commutation line voltage of the rectifier. The firing angle of the rectifier. The cut-off angle at the sending end; using the formula Determine the receiving-end DC current; where I d U1 is the DC current at the receiving end, U1 is the AC power supply at the sending end, and x1 is the AC line reactance at the sending end; the formula is used. Determine the DC voltage at the receiving end; where U di The receiving end DC voltage, ， ， ， X cr R is the commutation reactance of the rectifier. d For DC circuit resistance; use the formula Determine the lead firing angle of the inverter under constant current control; where, The leading firing angle of the inverter under constant current control, K pcc K is the proportional gain of the PI controller in constant current control. icc I is the integral gain of the PI controller in constant current control. dr-order I is the DC current command value transmitted from the inverter to the rectifier. di This is the measured DC current value of the inverter; the formula is used. Determine the lead firing angle of the inverter under constant turn-off angle control; whereby... The inverter's leading firing angle under constant turn-off angle control, K pcea K is the proportional gain of the PI controller in constant turn-off angle control. icea The integral gain of the PI controller in constant turn-off angle control. This is a reference value for the shut-off angle under fixed shut-off angle control. This is the measured value of the inverter's turn-off angle. The output value of the inverter under current deviation control; the commutation failure determination unit, connected to the electrical control quantity determination unit, is used to determine whether the DC transmission system has experienced a commutation failure based on the sending-end electrical quantity, receiving-end electrical quantity, and receiving-end control quantity of the DC transmission system within a set time period.

4. An electronic device, characterized in that, The device includes a memory and a processor, wherein the memory stores a computer program and the processor runs the computer program to cause the electronic device to perform the DC transmission system commutation failure detection method according to any one of claims 1 to 2.

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