An improved VDCOL curve-based AC / DC system sending end transient overvoltage suppression method

Abstract

The application belongs to the field of power system transient stability, and provides a method for suppressing transient overvoltage of an AC-DC system sending end based on an improved VDCOL curve. The method comprises the following steps: 1. building an LCC-HVDC system with a new energy sending end, collecting the power consumption of each bus and normalizing it; calculating the transient overvoltage characteristic quantity of each node by substituting the formula; combining the overvoltage characteristics, voltage change rate, reactive power surplus and new energy penetration coefficient to establish a comprehensive risk index; when the index exceeds the threshold, adaptively adjusting the VDCOL multi-parameter, updating the control curve; relying on the new curve to suppress the overvoltage of the sending end, and matching the hysteresis switching and change rate constraint to ensure stable control. According to the short-circuit ratio, the application deduces the expression of the transient overvoltage of the LCC-HVDC system sending end bus, constructs the transient overvoltage characteristic quantity, and combines the bus dynamic voltage information, reactive power state information and new energy output information to form a comprehensive risk discrimination index, realizes online setting of the VDCOL control parameter, and improves the transient voltage stability of the new energy high proportion AC-DC system.

Description

Technical Field

[0001] This invention belongs to the field of power system transient stability and proposes a method for suppressing transient overvoltage at the sending end of AC / DC systems based on an improved VDCOL curve. Background Technology

[0002] To address the uneven distribution of energy resources and significant transmission losses, my country employs a hybrid AC / DC transmission method for power transmission. In recent years, inter-regional transmission technology for new energy sources has further developed. High-voltage DC transmission systems can achieve large-capacity, long-distance power transmission, and hybrid AC / DC transmission has become an important pathway for new energy transmission. However, considering the connection of wind turbine generators to the sending-end grid, AC faults in the receiving-end system may lead to transient overvoltages at the wind turbine grid connection point and the sending-end bus, affecting the stable operation of the AC / DC system.

[0003] Commutation failure leads to a bypass on the inverter side, causing a short circuit on the DC side and a rapid increase in DC current. This results in a rapid increase in reactive power consumption by the rectifier, drawing a large amount of reactive power from the sending-end AC system, thus causing a drop in the sending-end AC grid voltage. Under the action of the low-voltage dependent current order limiter (VDCOL), the rectifier-side firing angle rapidly increases, and the DC current rapidly decreases, even dropping to zero. At this time, the reactive power consumed by the rectifier decreases rapidly, while the AC filter of the rectifier station continues to operate, resulting in a reactive power surplus at the rectifier station, which feeds a large amount of reactive power into the sending-end AC system, thereby generating a transient overvoltage in the sending-end AC grid.

[0004] Existing VDCOL control typically employs fixed inflection points or fixed curve shapes. Under conditions of significant fluctuations in renewable energy output, substantial dynamic changes in reactive power support, and complex voltage recovery processes, fixed control parameters have limited adaptability to transient overvoltage risks at the sending end, making it difficult to simultaneously achieve rapid suppression, smooth recovery, and control stability. Therefore, proposing a suppression method that can characterize transient overvoltage risks at the sending end and enable online tuning of VDCOL control parameters is of great significance for improving the transient voltage stability of AC / DC systems with a high proportion of renewable energy. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve, which can accurately identify the risk of transient overvoltage at the sending end and adjust the VDCOL control parameters in real time according to the system status to suppress transient overvoltage and improve the system's operational stability.

[0006] A method for suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve includes the following:

[0007] 1. Clarify the generation mechanism of transient overvoltage.

[0008] Commutation failure leads to a bypass on the inverter side, causing a short circuit on the DC side. This results in a rapid increase in DC current, leading to a rapid increase in reactive power consumption by the rectifier. A large amount of reactive power is absorbed from the sending-end AC system, causing a drop in the sending-end AC grid voltage. Under the influence of VDCOL, the rectifier-side firing angle increases rapidly, and the DC current decreases rapidly, even dropping to zero. At this time, the reactive power consumption of the rectifier decreases rapidly, but the AC filter of the rectifier station continues to operate, resulting in a reactive power surplus in the sending-end system. This causes the sending-end AC bus voltage to rise, forming a transient overvoltage.

[0009] 2. Construct the expression for transient overvoltage and calculate the characteristic quantities of transient overvoltage.

[0010] With the same network topology in a power system, different parameters for each component result in different system short-circuit ratios. They are also different:

[0011]

[0012] In the formula, The rated reactive power of the DC system; The short-circuit capacity of the commutating bus can be expressed as:

[0013]

[0014] In the formula, The equivalent reactance of the sending-end AC system; The rated voltage of the sending-end AC system;

[0015] Based on the equivalent AC / DC short-circuit capacity and the stable power transmission to the AC system, considering the remaining reactive power capacity of the converter station and transient voltage rise compensation, the transient voltage at the sending-end bus after the fault is derived:

[0016]

[0017] In the formula, This refers to the transient voltage at the sending-end bus after the fault. The rated voltage of the sending-end AC system; The reactive power consumed by the converter station; This represents the system short-circuit ratio.

[0018] Busbar reactive power compensation devices are divided into capacitors and AC filter Therefore, the total reactive power compensation capacity on the sending bus is Represented as:

[0019]

[0020] In the formula, The reactive power compensation capacity provided for the capacitor; The reactive power compensation capacity provided for the AC filter; The equivalent reactance of the capacitor; Let be the equivalent reactance of the AC filter. For ease of calculation, let the transient overvoltage characteristic value be... for:

[0021]

[0022] Then there exists an expression:

[0023]

[0024] The above equation is a quartic equation. The transient voltage at the sending-end bus after the fault is solved using the Ferriera root formula method, as shown in the following equation:

[0025]

[0026]

[0027] In the formula, This refers to the intermediate variable introduced when using Ferrari's root formula to solve a quartic equation. The square root of the intermediate variable is taken as the root of the square root of the intermediate variable. In the above formula, all radical signs are taken such that... Non-negative real roots with practical physical meaning. The transient overvoltage characteristic quantity. It is approximately proportional to the voltage value after a bus fault. The larger the voltage, the more pronounced the tendency for transient overvoltage to occur on the sending-end bus.

[0028] 3. Construct comprehensive risk assessment indicators.

[0029] To accurately characterize the transient overvoltage risk of AC / DC systems containing renewable energy units during fault recovery, this invention selects transient overvoltage characteristic quantities, bus voltage change rate, sending-end reactive power surplus coefficient, and renewable energy penetration coefficient to construct a comprehensive risk discrimination index G:

[0030]

[0031] In the formula, This represents the normalized value of the transient overvoltage characteristic quantity. This is the normalized value of the bus voltage change rate. This is the normalized value of the reactive power surplus coefficient at the sending end. This represents the normalized value of the new energy penetration coefficient, where... , , , These are the weighting coefficients corresponding to the normalized values ​​of transient overvoltage characteristic quantities, bus voltage change rate, sending-end reactive power surplus coefficient, and new energy penetration coefficient, respectively. .

[0032] Residual reactive power coefficient at the sending end satisfy:

[0033]

[0034] New energy penetration coefficient satisfy:

[0035]

[0036] In the formula, As the baseline reactive power capacity, For the total output of new energy units, This is the total output of the sending system.

[0037] The comprehensive risk discrimination index G is used to evaluate the transient overvoltage risk level of each node. When G is large, it indicates that the transient overvoltage risk of the system during the fault recovery phase is increased, and the VDCOL control suppression capability needs to be enhanced.

[0038] 4. Design the adaptive VDCOL control curve.

[0039] During the boost process, the rectifier side is typically in a constant current control state under VDCOL. Based on the current DC voltage, a DC current limit value is obtained, which is then compared with a current reference value to output a current command and adjust the rectifier side firing angle. Fixed three-stage VDCOL control is usually achieved by setting... , and Current limiting has limited adaptability to curve shape. This invention utilizes the tanh function combined with a comprehensive risk discrimination index G to achieve online tuning of VDCOL parameters. Let the basic parameters be as follows: , , , , The parameters after online correction are as follows:

[0040]

[0041]

[0042]

[0043]

[0044]

[0045] In the formula, This is the baseline value for the low-voltage inflection point of the VDCOL circuit. This is the baseline value for the high-voltage inflection point of the VDCOL circuit; This is the base value for the minimum current limit of the VDCOL circuit; and These are the basic shape parameters of the VDCOL control curve; , , , , The parameters for low-voltage inflection point, high-voltage inflection point, minimum current limit, and curve shape after online correction; It is a positive adjustment coefficient; This is the minimum current limit. When the comprehensive risk assessment index G increases, the intervention threshold of the VDCOL curve is advanced, the minimum current limit is appropriately reduced, and the slope of the transition section increases, thereby improving the ability to suppress transient overvoltages.

[0046] The adaptive VDCOL link function curve as follows:

[0047]

[0048] In the formula, This refers to the transient voltage at the sending-end bus. and These are the normalized transition function and the current-limiting correction function, respectively, constructed based on the transient voltage at the sending end, used to determine the smooth transition shape of the adaptive VDCOL curve between the low-voltage inflection point and the high-voltage inflection point. Their analytical expressions are as follows:

[0049]

[0050]

[0051] Through the , , , and The online correction enables adaptive adjustment of the shape of the control curve.

[0052] 5. Set hysteresis switching and rate of change constraints.

[0053] To avoid frequent switching of control modes, this invention sets up hysteresis switching logic: when the comprehensive risk judgment index G is greater than or equal to the entry threshold... And the duration is greater than the preset time. When the comprehensive risk discrimination index G is less than or equal to the exit threshold, it enters the enhanced suppression mode; when the comprehensive risk discrimination index G is less than or equal to the exit threshold, it enters the enhanced suppression mode. And the duration is greater than the preset time. At that time, exit the enhanced suppression mode, where .

[0054] To reduce control fluctuations caused by drastic parameter changes, a rate-of-change constraint is applied to the VDCOL control parameters or DC current command:

[0055]

[0056]

[0057] In the formula, , , These are the preset maximum rates of change for the rectifier-side DC current command, the VDCOL low-voltage inflection point voltage, and the VDCOL high-voltage inflection point voltage, respectively.

[0058] Through the above technical solution, this invention can generate a comprehensive risk assessment index based on the system's electrical state, and realize online tuning of VDCOL control parameters based on the comprehensive risk assessment results. This method takes into account rapid suppression of transient overvoltage, smooth recovery of DC current, and stability of the control process, and is suitable for suppressing transient overvoltage at the sending end of AC / DC systems with a high proportion of new energy sources. Attached Figure Description

[0059] The present invention will be further described below with reference to the accompanying drawings and specific embodiments:

[0060] Figure 1 This is a schematic diagram of the structure of an LCC-HVDC AC / DC system containing new energy generating units;

[0061] Figure 2 VDCOL control block diagram;

[0062] Figure 3 The curve showing the relationship between the comprehensive risk assessment index G and the transient voltage of the sending-end bus;

[0063] Figure 4 A comparison of traditional VDCOL curves and adaptive VDCOL curves under different risk levels;

[0064] Figure 5 A comparison curve of transient voltages at the sending-end bus;

[0065] Figure 6 A comparison curve of reactive power at the sending end;

[0066] Figure 7 This is a comparison curve of DC current recovery. Detailed Implementation

[0067] This invention discloses a method for suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve, combined with... Figures 1 to 6 This includes the following steps:

[0068] 1. Construct an LCC-HVDC system with new energy units at the sending end. This embodiment takes... Figure 1 The system architecture diagram will be used as an example for explanation.

[0069] 2. Construct the expression for transient overvoltage.

[0070] Under the same network topology of the power system, the system short-circuit ratio The expression is as follows:

[0071]

[0072] In the formula, The rated reactive power of the DC system; The short-circuit capacity of the commutating bus can be expressed as:

[0073]

[0074] In the formula, The equivalent reactance of the transmitting AC system; To transmit the rated voltage of the AC system;

[0075] Assuming line impedance is ignored, the voltage increment at the sending end bus can be seen from the power flow calculation formula as follows:

[0076]

[0077] When the LCC-HVDC system fails, the total reactive power compensation on the sending bus is: The voltage at the sending end bus is After a fault occurs, the voltage on the sending-end bus rises, and the reactive power compensation capacity of the sending-end bus increases after the fault. It also increases accordingly:

[0078]

[0079] When a fault occurs, the voltage on the sending-end bus rises. Transformed into:

[0080]

[0081] Based on the equivalent AC / DC short-circuit capacity and the stable power transmission to the AC system, considering the remaining reactive power capacity of the converter station and transient voltage rise compensation, the transient voltage at the sending-end bus after the fault is derived:

[0082]

[0083] 3. Calculate the characteristic quantities of transient overvoltage.

[0084] As can be seen from the above, the reactive power compensation capacity of the sending-end bus after the fault... As voltage changes; bus reactive power compensation devices are divided into capacitors. and AC filter Therefore, the total reactive power compensation capacity on the transmitting bus is Represented as:

[0085]

[0086] For ease of calculation, let the transient overvoltage characteristic quantity be... for:

[0087]

[0088] Then there exists an expression:

[0089]

[0090] The above equation is a quartic equation. The transient voltage at the sending-end bus after the fault is solved using the Ferriera root formula method, as shown in the following equation:

[0091]

[0092]

[0093] Transient overvoltage characteristic quantity The larger the voltage, the higher the risk of transient overvoltage on the sending-end bus.

[0094] 4. Construct comprehensive risk assessment indicators.

[0095] Selecting transient overvoltage characteristic quantities A comprehensive risk assessment index G is constructed based on the bus voltage change rate coefficient, the reactive power surplus coefficient at the sending end, and the renewable energy penetration coefficient.

[0096]

[0097] in, It reflects the degree of drastic change during the bus voltage recovery process; It reflects the degree of residual between the reactive power compensation of the sending-end system and the reactive power consumption of the converter station; This reflects the proportion of renewable energy output in the total output of the sending-end system. The larger the comprehensive risk assessment index G, the higher the risk of transient overvoltage during the system's fault recovery phase.

[0098] 5. Design the adaptive VDCOL control curve.

[0099] 1) tanh function:

[0100]

[0101] The hyperbolic tangent function is used to design the control curve of the VDCOL process, where x is the independent variable of the hyperbolic tangent function. and These represent exponential functions with base e.

[0102] 2) VDCOL parameter online tuning rules.

[0103] Let the basic parameters be respectively , , , , When the system detects an increase in the comprehensive risk assessment index G, it will be adjusted online according to the following rules:

[0104]

[0105]

[0106]

[0107]

[0108]

[0109] 3) Adaptive VDCOL link function curve as follows:

[0110]

[0111] in and The analytical expression is as follows:

[0112]

[0113]

[0114] In the formula, This refers to the transient voltage at the sending-end bus. , These are the minimum and maximum values ​​of the transient voltage after online correction; It is direct current; This is the minimum DC current after online correction; through the... and Online adjustments can alter the curve shape, enabling it to better suppress transient overvoltages under different risk conditions.

[0115] 6. Model calculation process.

[0116] 1) First, construct an LCC-HVDC system that includes new energy generating units at the sending end.

[0117] 2) Obtain the power consumption of the bus, the rate of change of the bus voltage, the reactive power compensation at the sending end and the output of new energy sources at each node of the LCC-HVDC system through WAMS, and perform normalization processing.

[0118] 3) Substitute the obtained electrical quantities into the transient overvoltage expression to calculate the transient overvoltage characteristic quantities of each node; calculate the comprehensive risk discrimination index G based on the transient overvoltage characteristic quantities, bus voltage change rate, sending-end reactive power surplus coefficient and new energy penetration coefficient.

[0119] 4) When the comprehensive risk judgment index is higher than the identification threshold, the low voltage inflection point, high voltage inflection point, minimum current limit and curve shape parameters of the VDCOL link are adjusted online, and the adaptive VDCOL control curve is called to suppress transient overvoltage.

[0120] 5) Introduce hysteresis switching and rate of change constraints during the control process to avoid frequent switching of control modes and drastic fluctuations in parameters.

[0121] In-depth analysis Figures 3 to 6 The experimental results show that:

[0122] 1) By Figures 3 to 4 It can be seen that there are significant differences between traditional VDCOL curves and adaptive VDCOL curves in terms of control characteristics and transient response. Figure 3 This indicates that the adaptive VDCOL curve can adjust the low-pressure inflection point, high-pressure inflection point, and transition slope online according to changes in the comprehensive risk discrimination index. When the comprehensive risk is high, the control curve can intervene in advance and enhance the current limiting regulation capability. When the comprehensive risk is low, the control curve remains relatively flat to balance transient suppression and steady-state recovery. Figure 4 The results show that when there is a risk of transient overvoltage at the sending-end bus, the peak value of the transient voltage at the bus is significantly reduced and the voltage recovery process is smoother after adopting adaptive VDCOL control, indicating that this method can effectively improve the transient voltage stability of the sending-end system.

[0123] 2) By Figures 5 to 7It is evident that the dynamic changes in reactive power and DC current at the sending end both alter with the control method. During a fault, the voltage at the sending-end bus drops, causing rapid fluctuations in reactive power. In the recovery phase after fault clearance, if the sending-end system faces a high risk of transient overvoltage, adaptive VDCOL control can online adjust control parameters based on the comprehensive risk assessment results, thereby reducing the reactive power surge amplitude at the converter bus and improving the smoothness of reactive power recovery. Simultaneously, the DC current recovers faster and oscillates less under adaptive VDCOL control, indicating that the online tuning control strategy can suppress transient overvoltage while simultaneously considering DC current recovery performance, thus improving the overall stability of the AC / DC system during fault recovery.

[0124] The calculation conditions, illustrations, etc., in the embodiments of this invention are only used to further illustrate the invention and are not exhaustive, nor do they constitute a limitation on the scope of protection of the claims. Equivalent substitutions or modifications made by those skilled in the art based on the teachings gained from the embodiments of this invention, without departing from the spirit and substance of the invention, should all fall within the protection scope of this invention.

Claims

1. A method for suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve, characterized in that, Includes the following steps: 1) Construct an LCC-HVDC system with new energy units at the sending end; obtain the power consumption of each node bus in the LCC-HVDC system and perform normalization processing; 2) Substitute the obtained electrical quantities into the transient overvoltage expression to calculate the transient overvoltage characteristic quantities of each node; 3) Based on the obtained transient overvoltage characteristics, bus voltage change rate, sending-end reactive power surplus coefficient and new energy penetration coefficient, construct a comprehensive risk discrimination index, and evaluate the transient overvoltage risk level of each node based on the comprehensive risk discrimination index. 4) When the comprehensive risk judgment index is higher than the identification threshold, the low-voltage inflection point, high-voltage inflection point, minimum current limit and curve shape parameters of the VDCOL link are corrected online to form an adaptive VDCOL control curve. 5) The adaptive VDCOL control curve is used to adjust the rectifier side current command, and the transient overvoltage at the sending end is suppressed by hysteresis switching and rate of change constraint.

2. The method for suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve according to claim 1, characterized in that, The transient overvoltage expression includes: Under the same network topology of the power system, the system short-circuit ratio The expression is as follows: ； In the formula, The rated reactive power of the DC system; The short-circuit capacity of the commutating bus can be expressed as: ； In the formula, The equivalent reactance of the sending-end AC system; The rated voltage of the sending-end AC system; The voltage increment at the sending end bus is: ； In the formula, This represents the voltage increment at the sending end bus. The reactive power compensation capacity of the power supply bus after a fault; When the LCC-HVDC system fails, the total reactive power compensation on the sending bus is: The voltage at the sending end bus is After a fault occurs, the voltage on the sending-end bus rises, and the reactive power compensation capacity of the sending-end bus increases after the fault. It also increases accordingly: ； When a fault occurs, the voltage on the sending-end bus rises. Transformed into: ； Based on the equivalent AC / DC short-circuit capacity and the stable power transmission to the AC system, considering the remaining reactive power capacity of the converter station and transient voltage rise compensation, the transient voltage at the sending-end bus after the fault is derived: ； In the formula, This refers to the transient voltage at the sending-end bus after the fault. The reactive power consumed by the converter station; This represents the system short-circuit ratio.

3. The method for suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve according to claim 2, characterized in that, The calculation of the transient overvoltage characteristic quantities includes: Reactive power compensation capacity of the sending-end bus after a fault As voltage changes; bus reactive power compensation devices are divided into capacitors. and AC filter Therefore, the total reactive power compensation on the sending bus is Represented as: ； In the formula, The reactive power compensation capacity provided for the capacitor; The reactive power compensation capacity provided for the AC filter; The equivalent reactance of the capacitor; Let the transient overvoltage characteristic be the equivalent reactance of the AC filter. For ease of calculation, let it be... for: ； Then there exists an expression: ； The above equation is a quartic equation. The transient voltage at the sending-end bus after the fault is solved using the Ferrari root formula method, as shown in the following equation: ； ； In the formula, This refers to the intermediate variable introduced when using Ferrari's root formula to solve a quartic equation. The square root of the intermediate variable is taken as the root of the square root of the intermediate variable. In the above formula, all radical signs are taken such that... Non-negative real roots with actual physical meaning; the transient overvoltage characteristic quantity It is approximately proportional to the voltage value after a bus fault. The larger the voltage, the more pronounced the tendency for transient overvoltage to occur on the sending-end bus.

4. The method for suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve according to claim 3, characterized in that, The comprehensive risk assessment index G is represented as follows: ； In the formula, This represents the normalized value of the transient overvoltage characteristic quantity. This is the normalized value of the bus voltage change rate. This is the normalized value of the reactive power surplus coefficient at the sending end. This represents the normalized value of the new energy penetration coefficient, where... , , , These are the weighting coefficients corresponding to the normalized values ​​of transient overvoltage characteristic quantities, bus voltage change rate, sending-end reactive power surplus coefficient, and new energy penetration coefficient, respectively. .

5. The method for suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve according to claim 4, characterized in that, Residual reactive power coefficient at the sending end satisfy: ； New energy penetration coefficient satisfy: ； In the formula, As the baseline reactive power capacity, For the total output of new energy units, This is the total output of the sending system.

6. A method for suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve, as described in claim 4 or 5, characterized in that, The online correction rules for VDCOL control parameters include: ； ； ； ； ； In the formula, This is the baseline value for the low-voltage inflection point of the VDCOL circuit. This is the baseline value for the high-voltage inflection point of the VDCOL circuit; This is the base value for the minimum current limit of the VDCOL circuit; and These are the basic shape parameters of the VDCOL control curve; , , , , The parameters for low-voltage inflection point, high-voltage inflection point, minimum current limit, and curve shape after online correction; It is a positive adjustment coefficient; This is the minimum current lower limit.

7. A method for suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve, as described in claim 6, is characterized in that... The adaptive VDCOL control curves include: 1) tanh function: ； In the formula, x is the independent variable of the hyperbolic tangent function; and Let represent exponential functions with base e; 2) Adaptive VDCOL cyclic function curve : ； In the formula, This refers to the transient voltage at the sending-end bus. and These are the normalized transition function and the current-limiting correction function, respectively, constructed based on the transient voltage at the sending end, used to determine the smooth transition shape of the adaptive VDCOL curve between the low-voltage inflection point and the high-voltage inflection point; their analytical expressions are as follows: ； 。 8. The method for suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve according to claim 1, characterized in that, The method sets up hysteresis switching logic, making To enhance the threshold for suppressing modes, To enhance the exit threshold of the suppression mode, and These are the preset durations required to enter and exit Enhanced Suppression Mode, respectively. ; When the comprehensive risk assessment index G is greater than or equal to the threshold And the duration is greater than the preset time. At that time, it enters enhanced inhibition mode; When the comprehensive risk assessment index G is less than or equal to the exit threshold And the duration is greater than the preset time. At that time, exit the enhanced suppression mode.

9. A method for suppressing transient overvoltage at the sending end of an AC / DC system based on an improved VDCOL curve, as described in claim 1, is characterized in that... Apply a rate-of-change constraint to the VDCOL control parameters or DC current command, satisfying: ； ； In the formula, The preset maximum rate of change of the rectifier-side DC current command. The preset maximum rate of change of the low-voltage inflection point voltage of VDCOL, The preset maximum rate of change for the VDCOL high-voltage inflection point voltage.

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