A method and device for suppressing DC overvoltage of a doubly-fed wind power flexible DC transmission system

Abstract

The application discloses a double-fed wind power flexible DC transmission system DC overvoltage suppression method and device, and the method comprises the following steps: collecting the DC line voltage of the flexible DC system, calculating the surplus power by using the charging effect of the converter submodule; determining the dynamic voltage reduction coefficient of the wind farm side flexible DC converter station WFMMC according to the surplus power amount; determining the value range of the dynamic voltage reduction coefficient to ensure that the double-fed wind farm enters the low voltage ride-through interval; after the double-fed wind farm enters the low voltage ride-through interval, determining the reduction amount of the power output according to the surplus power amount; and distributing the reduction amount to each wind turbine connected to the AC side of the WFMMC according to the power reduction distribution coefficient; detecting the DC voltage of the wind turbine converter, and if the DC voltage exceeds the action value, the DC voltage of the converter is suppressed. The application effectively suppresses the DC system overvoltage problem from the DC system converter and the double-fed wind turbine generator set side.

Description

Technical Field

[0001] This invention relates to a method and apparatus for suppressing DC overvoltage in a doubly-fed wind power flexible DC transmission system, belonging to the field of DC power transmission technology. Background Technology

[0002] As the proportion of new energy sources connected to the grid continues to increase, wind power, as a major form of new energy power generation, is seeing its installed capacity and the total number of wind turbines connected to the grid continuously rise. Consequently, the impact of wind turbine grid connection on the power grid is also gradually increasing. my country has abundant wind resources, but due to the long distances between wind farms and grid connection points, and the large transmission capacity, large-scale wind power grid connection via flexible DC transmission systems has become a mainstream trend, leveraging the advantages of flexible DC transmission distance and providing stable voltage and frequency support to passive networks.

[0003] When the wind turbine generator set is a doubly fed wind turbine, the flexible DC transmission system, the doubly fed wind turbine rotor converter, and the grid-side converter form a multi-converter system. The fault ride-through of the converter itself and the coordinated control between the converters become important factors to ensure the stable operation of the entire transmission system.

[0004] When a flexible DC transmission system experiences an AC fault at the receiving end, the control strategies between the flexible DC system and the converters at each stage of the doubly-fed induction generator (DFIG) wind turbine will affect the transmission of active power across the entire system. When the transmitted power is mismatched, problems such as power surplus and power deficit will arise, manifesting as overvoltage and undervoltage phenomena in the flexible DC system. In severe cases, this can lead to unipolar or bipolar blocking of the flexible DC system, interrupting the entire power transmission channel. Therefore, research on fault ride-through and coordinated control of wind power flexible DC transmission systems is of great significance in mitigating this phenomenon. Summary of the Invention

[0005] This invention provides a method and apparatus for suppressing DC overvoltage in a doubly-fed wind power flexible DC transmission system, which effectively suppresses DC system overvoltage problems from both the DC system converter and the doubly-fed wind turbine generator side.

[0006] The technical solution of this invention is: a method for suppressing DC overvoltage in a doubly-fed wind power flexible DC transmission system, comprising the following steps:

[0007] Step 1: Collect the DC line voltage of the flexible DC system and calculate the surplus power using the charging effect of the converter submodule;

[0008] Step 2: Determine the dynamic voltage reduction coefficient of the WFMMC at the wind farm-side flexible DC converter station based on the surplus power.

[0009] Step 3: Determine the range of values ​​for the dynamic voltage reduction factor to ensure that the doubly-fed wind farm enters the low-voltage ride-through range;

[0010] Step 4: After the doubly fed wind farm enters the low-voltage transmission zone, determine the reduction in power output based on the surplus power; and distribute the reduction to each wind turbine connected to the AC side of the WFMMC according to the power reduction distribution coefficient.

[0011] Step 5: Detect the DC voltage of the wind turbine converter and suppress the DC voltage of the converter.

[0012] Step 1 includes:

[0013] When a fault occurs in the AC power grid at the receiving end, the overvoltage of the DC line in the flexible DC system is calculated. The surplus power of the DC line causing the overvoltage in the flexible DC system is defined as ΔP. ​​The surplus power is calculated using the charging effect of the equivalent capacitance of the converter. ΔP is calculated using the following formula:

[0014]

[0015] In the formula, C represents the sum of the equivalent capacitances of the converter submodules; U dc For the DC line voltage of the flexible DC system; U dcmax U is the upper limit threshold for overvoltage in DC lines. dc -U dcmax This represents the overvoltage value.

[0016] Step 2 includes:

[0017] When an overvoltage occurs in a doubly-fed wind power flexible DC transmission system, the power flowing into the WFMMC should be reduced to decrease the surplus power. The reduced power P' WFMMC The calculation formula is:

[0018] P' WFMMC =P WFMMC -ΔP

[0019] In the formula, P WFMMC P' represents the power flowing into the WFMMC (Flexible Direct Current Converter Station) on the wind farm side of the wind farm. WFMMC This indicates the power output of the wind farm-side flexible DC converter station after WFMMC reduction;

[0020] The power balance equation is obtained based on the power balance relationship before and after the power reduction on the WFMMC side:

[0021]

[0022] In the formula: U' acWFMMC To reduce the AC side voltage of WFMMC after power inflow; U acWFMMC To reduce the AC side voltage of WFMMC before the power flows in; I is the effective value of the AC side current flowing into WFMMC; The power factor of the WFMMC AC side;

[0023] Based on the power balance equation, the expression for the dynamic buck coefficient in the WFMMC buck control strategy is obtained:

[0024] U' acWFMMC =U acWFMMC -K(U dc -U dcmax )

[0025] In the formula: K represents the dynamic voltage drop coefficient of the V / f control on the AC side of WFMMC during the fault period.

[0026] Step 3 includes: determining the dynamic voltage reduction coefficient range to ensure that the AC side voltage of the WFMMC is reduced to the low voltage ride-through range of the doubly fed wind farm during the fault, and dynamically reducing the AC side voltage of the WFMMC according to the DC overvoltage value.

[0027]

[0028] In the formula: U * acWFMMC The reference value for the AC side voltage of WFMMC before the fault; the dynamic voltage drop factor K can be divided into three stages according to the degree of DC overvoltage. When the overvoltage degree is less than the upper limit of DC voltage U... dc max When the overvoltage level exceeds the upper limit of DC voltage U, the step-down control is not activated; dc max However, when the voltage reduction is insufficient to allow the doubly fed wind farm to enter the low voltage ride-through range, the voltage reduction factor is set to reduce the voltage on the AC side of the WFMMC to the voltage of the low voltage ride-through operating range of the wind farm. When the overvoltage is more severe, the voltage reduction amplitude on the AC side of the WFMMC is larger, and the voltage reduction factor will be set to a dynamic value according to the degree of overvoltage.

[0029] Step 4 includes:

[0030] Total power reduction ΔP of WFMMC AC side wind turbine wind The active power surplus ΔP of the flexible DC system is greater than or equal to the following:

[0031] ΔP≤ΔP wind

[0032] When the number of wind turbines connected to the WFMMC side of the flexible DC system is n, and each wind turbine is in operation; the active power command value of each wind turbine is dynamically changed to P. *' :

[0033] P *' =P * -n i ΔP wind

[0034] In the formula: P* P represents the active power command value of the wind turbine before power reduction. *' This represents the active power command value of the wind turbine after power reduction; n i Let be the power reduction distribution coefficient for the i-th wind turbine in the wind farm.

[0035] The power reduction distribution coefficient for the i-th wind turbine in the wind farm is determined by the following method: if the operating conditions of each wind turbine are the same, it is considered to be distributed proportionally, then n i =1 / n; if the operating conditions of each fan are different, the power of the i-th fan before power reduction shall be allocated proportionally to the total fan capacity.

[0036] Step 5 includes:

[0037] The power consumed by the resistor after the Chopper circuit is connected is:

[0038]

[0039] In the formula: P Chopper This represents the power dissipated across the resistor after the Chopper circuit is connected, in V. dc R represents the DC voltage of the wind turbine converter. Chopper This indicates the resistance value of the Chopper circuit;

[0040] After a doubly-fed induction generator (DFIG) wind farm enters the low-voltage ride-through range, if the DC voltage of the wind turbine converter rises, it will cause the Chopper circuit to activate. At this time, the DC overvoltage of the wind turbine converter should be suppressed. The activation of the Chopper circuit must meet the following conditions:

[0041] V * dc ≤V dc

[0042] In the formula: V * dc This indicates the action value of the Chopper circuit.

[0043] According to another aspect of the present invention, a DC overvoltage suppression device for a doubly-fed wind power flexible DC transmission system is provided, comprising:

[0044] The calculation module is used to collect the DC line voltage of the flexible DC system and calculate the surplus power using the charging effect of the converter submodule.

[0045] The first determining module is used to determine the dynamic voltage reduction coefficient of the WFMMC of the flexible DC converter station on the wind farm side based on the surplus power.

[0046] The second determining module is used to determine the range of values ​​for the dynamic voltage reduction factor to ensure that the doubly fed wind farm enters the low voltage ride-through range.

[0047] The third determining module is used to determine the reduction in power output based on the surplus power after the doubly fed wind farm enters the low-voltage transmission range; and to distribute the reduction to each wind turbine connected to the AC side of the WFMMC according to the power reduction distribution coefficient.

[0048] The suppression module is used to detect the DC voltage of the wind turbine converter and suppress the DC voltage of the converter.

[0049] The beneficial effects of this invention are:

[0050] 1. This invention analyzes the mechanism of overvoltage during faults, and proposes an optimization scheme for fault ride-through of the converter in a flexible DC system by utilizing the low-voltage ride-through characteristics of the doubly-fed wind turbine. Furthermore, it proposes a collaborative control optimization scheme for the doubly-fed wind turbine by combining the low-voltage ride-through characteristics of the doubly-fed wind turbine, thereby further solving the system power imbalance problem and effectively suppressing the DC system overvoltage problem from both the DC system converter and the doubly-fed wind turbine side.

[0051] 2. This invention can effectively suppress overvoltage phenomena of different degrees caused by different types of faults at the receiving end of the system, thereby achieving the goal of wind turbines not disconnecting from the grid and maximizing the power transmission from the grid-side GSMMC to the AC grid system, and thus improving the fault ride-through capability of the entire wind power flexible DC transmission system. Attached Figure Description

[0052] Figure 1 This is a flowchart of the present invention;

[0053] Figure 2 Wiring diagram for a doubly-fed wind power flexible DC transmission system;

[0054] Figure 3 This is the wiring diagram for DFIG grid connection and the working principle diagram for RSC.

[0055] Figure 4 This is a block diagram of the converter control for a flexible DC system.

[0056] Figure 5 This is a block diagram of the coordinated control strategy for each level of converter proposed in this invention;

[0057] Figure 6 The diagrams show the effect without additional control strategies. Figures a, b, and c show the DC bus voltage of the flexible DC transmission system, the 35kV AC voltage at the wind farm outlet, and the active power generated by the wind farm, respectively, without additional control strategies.

[0058] Figure 7 The effects of the additional control strategy are shown in Figures a, b, and c, which represent the DC bus voltage of the flexible DC transmission system, the 35kV AC voltage at the wind farm outlet, and the active power generated by the wind farm, respectively, under the additional control strategy. Detailed Implementation

[0059] The invention will be further described below with reference to the accompanying drawings and embodiments, but the scope of the invention is not limited to the description.

[0060] Example 1: As Figure 1-7 As shown, a method for suppressing DC overvoltage in a doubly-fed induction generator (DFIG) flexible DC transmission system includes the following steps: Step 1: Collect the DC line voltage of the flexible DC system and calculate the surplus power using the charging effect of the converter submodule; Step 2: Determine the dynamic voltage reduction coefficient of the WFMMC (Flexible DC-DC converter station) on the wind farm side based on the surplus power; Step 3: Determine the range of values ​​for the dynamic voltage reduction coefficient to ensure that the DFIG wind farm enters the low-voltage ride-through range; Step 4: After the DFIG wind farm enters the low-voltage ride-through range, determine the reduction in power output based on the surplus power and distribute the reduction to each wind turbine connected to the AC side of the WFMMC according to the power reduction distribution coefficient; Step 5: Detect the DC voltage of the wind turbine converter and suppress the DC voltage of the converter.

[0061] Further, step 1 includes:

[0062] When a fault occurs in the AC power grid at the receiving end, excess power accumulates on the converters on both sides of the flexible DC system, causing a rapid rise in the DC capacitor voltage. First, the overvoltage of the DC lines in the flexible DC system is calculated. The excess power of the DC lines causing the overvoltage is defined as ΔP. ​​The excess power is calculated using the charging effect of the converter's equivalent capacitor. ΔP is obtained using the following formula:

[0063]

[0064] In the formula, C represents the sum of the equivalent capacitances of the converter submodules; U dc For the DC line voltage of the flexible DC system; U dcmax The upper limit threshold for overvoltage in a DC line is generally set to 1.1 pu to 1.2 pu of the steady-state operating voltage of the DC line; U dc -U dcmax This represents the overvoltage value.

[0065] Further, step 2 includes:

[0066] When an overvoltage occurs in a doubly-fed wind power flexible DC transmission system, the power flowing into the WFMMC should be reduced to decrease the surplus power. Ignoring converter power losses, the reduction should be equal to the surplus power ΔP, thereby ensuring that the DC voltage does not exceed U. dcmax This aims to maximize the power supplied by the receiving-end converter station's GSMMC to the AC system; the reduced power P' WFMMC The calculation formula is:

[0067] P'WFMMC =P WFMMC -ΔP (2)

[0068] In the formula, P' WFMMC To reduce the inflow power, P WFMMC To reduce the inflow power before;

[0069] The power balance equation is obtained based on the power balance relationship before and after the power reduction on the WFMMC side:

[0070]

[0071] In the formula: U' acWFMMC To reduce the AC side voltage of WFMMC after power inflow; U acWFMMC To reduce the AC side voltage of WFMMC before the power flows in; I is the effective value of the AC side current flowing into WFMMC; The power factor of the WFMMC AC side;

[0072] Based on the power balance equation, the expression for the dynamic buck coefficient in the WFMMC buck control strategy is obtained:

[0073] U' acWFMMC =U acWFMMC -K(U dc -U dcmax (4)

[0074] In the formula: K represents the dynamic voltage drop coefficient of the V / f control on the AC side of WFMMC during the fault period.

[0075] Further, step 3 includes: determining the dynamic voltage reduction coefficient range to ensure that the AC side voltage of the WFMMC is reduced to the low voltage ride-through range of the doubly fed wind farm during a fault, and dynamically reducing the AC side voltage of the WFMMC according to the DC overvoltage value;

[0076]

[0077] In the formula: U * acWFMMC The reference value for the AC side voltage of WFMMC before the fault; the dynamic voltage drop factor K can be divided into three stages according to the degree of DC overvoltage. When the overvoltage degree is less than the upper limit of DC voltage U... dcmax When the overvoltage level exceeds the upper limit of DC voltage U, the step-down control is not activated; dcmax However, when the voltage reduction is insufficient to allow the doubly fed wind farm to enter the low voltage ride-through range, the voltage reduction factor is set to reduce the voltage on the AC side of the WFMMC to the voltage of the low voltage ride-through operating range of the wind farm. When the overvoltage is more severe, the voltage reduction amplitude on the AC side of the WFMMC is larger, and the voltage reduction factor will be set to a dynamic value according to the degree of overvoltage.

[0078] Further, step 4 includes:

[0079] The wind turbine converter and the flexible DC transmission system converter form a multi-stage converter coordinated control system. The wind turbine converter responds to the step-down response of the WFMMC converter, dynamically adjusting the active power output; the total power reduction ΔP of the wind turbine on the AC side of the WFMMC is... wind The active power surplus ΔP of the flexible DC system is greater than or equal to the following, and the preferred option is generally the total power reduction ΔP. wind This equals the active power surplus ΔP of the flexible DC system, thereby minimizing power waste in wind farms.

[0080] ΔP≤ΔP wind (6)

[0081] When the number of wind turbines connected to the WFMMC side of the flexible DC system is n, and each wind turbine is in operation; the active power command value of each wind turbine is dynamically changed to P. *' :

[0082] P *' =P * -n i ΔP wind (7)

[0083] In the formula: P * P represents the active power command value of the wind turbine before power reduction. *' This represents the active power command value of the wind turbine after power reduction; n i Let be the power reduction distribution coefficient for the i-th wind turbine in the wind farm.

[0084] Furthermore, the power reduction distribution coefficient for the i-th wind turbine in the wind farm is determined by the following method: if the operating conditions of each wind turbine are the same, it is considered to be distributed proportionally, then n i =1 / n; If the operating conditions of each fan are different, the power distribution is proportional to the ratio of the power output of the i-th fan before power reduction to the total fan capacity. That is, the larger the ratio of the power output of the i-th fan before power reduction to the total fan capacity, the greater the ratio of n. i The larger the value (e.g., if there are 2 wind turbines, the power of the first wind turbine before power reduction is 5, and the power of the second wind turbine before power reduction is 3; then the power reduction allocation coefficient n1 of the first wind turbine is 5 / 8, and the power reduction allocation coefficient n2 of the second wind turbine is 3 / 8).

[0085] Further, step 5 includes:

[0086] The power consumed by the resistor after the Chopper circuit is connected is:

[0087]

[0088] In the formula: P Chopper This represents the power dissipated across the resistor after the Chopper circuit is connected, in V. dc R represents the DC voltage of the wind turbine converter. Chopper This indicates the resistance value of the Chopper circuit;

[0089] After a doubly-fed induction generator (DFIG) wind farm enters the low-voltage ride-through range, if the DC voltage of the wind turbine converter rises, it will cause the Chopper circuit to activate. At this time, the DC overvoltage of the wind turbine converter should be suppressed. The activation of the Chopper circuit must meet the following conditions:

[0090] V * dc ≤V dc (9)

[0091] In the formula: V * dc This indicates the activation value of the Chopper circuit, which is generally set to 1.05pu to 1.1pu of the DC voltage of the wind turbine converter during steady-state operation.

[0092] As described above, this invention analyzes the control strategy for a doubly-fed induction generator (DFIG) flexible DC transmission system during normal operation. Based on the normal operation control strategy, it analyzes the generation mechanism of overvoltage and power surplus under fault conditions; derives a method for calculating surplus power based on the generation mechanism; calculates the dynamic voltage reduction coefficient of the wind farm-side converter (WFMMC) based on the power surplus, and limits the voltage reduction coefficient to the low-voltage ride-through range of the wind turbine; determines the reduction range of the active power reference value during the low-voltage ride-through of the wind turbine based on the upper limit of the DC voltage of the flexible DC system; and optimizes the DC unloading circuit (Chopper) setting value in the low-voltage ride-through measures of the wind turbine by comprehensively utilizing the low-voltage ride-through characteristics of the DFIG wind turbine. This solves the system power imbalance problem and further ensures that the system overvoltage is suppressed within the required range.

[0093] Furthermore, based on experimental data, the present invention provides the following optional specific implementation process:

[0094] S1. When the AC power grid fails and the power transmission channel is blocked, the resulting surplus power causes the DC voltage to rise. The DC line voltage of the flexible DC system is collected, and the surplus power is calculated using the charging effect of the converter submodule.

[0095] S2, based on the degree of DC voltage exceeding the limit U dc -U dcmax The dynamic step-down factor of WFMMC is calculated based on the surplus power.

[0096] S3. Based on the dynamic voltage reduction coefficient calculated in S2, dynamically change the voltage reference value U in the V / f control of WFMMC.* ' acWFMMC And limit the voltage reference value to a range that allows the wind turbine to enter low voltage ride-through;

[0097] S4. Calculate the reduction in power output after the doubly fed wind farm enters the low-voltage transmission zone; and distribute the reduction to each wind turbine connected to the AC side of the WFMMC according to the power reduction distribution coefficient.

[0098] S5. Optimize the setting value of the DC Chopper protection circuit of the wind turbine converter to ensure that the DC voltage of the converter is stable after the doubly fed wind turbine enters the low voltage ride-through range, and to ensure that the DC voltage of the flexible DC system is suppressed within a reasonable range.

[0099] Taking a 900MW doubly-fed induction generator (DFIG) wind farm transmitted via a flexible DC transmission system as an example, assuming a three-phase short-circuit fault occurs after 7 seconds of stable system operation, the specific parameters of the DFIG induction generator are as follows:

[0100] Table 1 Parameters of Doubly Fed Induction Generator

[0101] parameter Stator equivalent resistance R s ]]> Rotor-side equivalent resistance R r ]]> Frequency f Rotor leakage inductance L σr ]]> Numerical value (pu) 0.0054 0.0060 1 0.11 parameter Stator leakage inductance L σs ]]> Excitation inductance L m ]]> synchronous rotational speed ω1 <![CDATA[Rotational speed ω1 of the rotor]]> Numerical value (pu) 0.10 4.5 1 1.05

[0102] Table 2 Parameters of Flexible DC System Converter and DC Line

[0103]

[0104] 1. According to the wiring diagram of the doubly-fed wind turbine generator set via the flexible DC transmission system, as follows: Figure 2 As shown, the doubly-fed wind power flexible DC transmission system includes a doubly-fed wind farm and a flexible DC system; the flexible DC system includes a converter transformer, a wind farm-side flexible DC converter station WFMMC, a receiving-end converter station GSMMC, and a DC line. The specific parameters of the doubly-fed wind turbine are shown in Table 1, and the specific parameters of the flexible DC system are shown in Table 2. At time 0s, a three-phase short-circuit fault occurs at point f. At this time, the power transmission channel of GSMMC is blocked, and excess power accumulates on the converters on both sides of the flexible DC system, causing the DC capacitor voltage to rise rapidly. The surplus power can be calculated using the charging effect of the equivalent capacitor of the converter. ΔP can be calculated using the above formula (1); in formula (1), the capacitor value is the sum of the equivalent capacitors of the sub-modules, U dcmax Set to 1.2 PU, take U dc The detection signal is the DC bus voltage of the flexible DC system, and the surplus power ΔP is calculated using equation (1).

[0105] 2. Calculate the dynamic voltage drop factor of the WFMMC. Reduce the power P flowing into the WFMMC. WFMMC To reduce the surplus power and ignore converter power losses, the reduction should be the surplus power ΔP, thus ensuring that the DC voltage does not exceed U. dcmaxThis prevents DC line blocking and maximizes the power output of the GSMMC to the AC system. The reduced power P' WFMMC The above formula (2) can be used to calculate the power balance equation. Based on the power balance relationship before and after the power reduction on the WFMMC side, the power balance equation can be obtained, namely formula (3). Eliminate the following equations according to formula (3). The expression for the dynamic pressure reduction coefficient in the WFMMC pressure reduction control strategy can be obtained, namely equation (4); the pressure reduction relationship in equation (4) is superimposed on the V reference value in the V / f control of the WFMMC side, thus realizing the dynamic pressure reduction control of WFMMC.

[0106] 3. Determine the range of the voltage drop factor K. The dynamic voltage drop relationship on the WFMMC side should also ensure that the AC voltage of the WFMMC is reduced to the low voltage ride-through range of the wind turbine during a fault, and the AC voltage of the WFMMC is dynamically reduced according to the degree of DC overvoltage. At this time, the voltage drop factor K can be limited to the following range according to the above formula (5).

[0107] 4. Calculate the power reduction of the wind farm. When the dynamic voltage reduction measure of WFMMC is after the doubly fed wind turbine enters the low voltage ride-through range, the wind turbine converter will respond to the voltage reduction response of the WFMMC converter and dynamically adjust the active power output. At this time, the total power reduction of the wind turbine should be greater than or equal to the active power surplus of the flexible DC system, that is, satisfy the above equation (6); when the number of wind turbines connected to the WFMMC side of the flexible DC system is n, and each wind turbine is put into operation, the active power command value of each wind turbine is dynamically changed to P *' That is, the above formula (7); if the operating conditions of each fan are the same, it is considered as an equal distribution, that is, n i = 1 / n.

[0108] 5. Optimize low-voltage ride-through measures for wind farms. The DC voltage variation between the converters of doubly-fed induction generator (DFIG) wind turbines is limited by the DC bus capacitance, and the resistance of the Chopper resistor generally remains constant during operation. Therefore, the Chopper can be considered an approximately constant load. The power consumed on the resistor after the Chopper circuit is activated is expressed by equation (8). After the wind turbine enters the low-voltage ride-through range, the DC voltage of the converter rises, causing the Chopper circuit to be activated. At this time, the DC overvoltage of the wind turbine converter should be suppressed, and the activation of the Chopper circuit satisfies equation (9). When the DC voltage of the DFIG wind turbine converter is greater than V... * dc At that time, the Chopper will consume excess power on the doubly-fed induction generator (DFIG) converter, further ensuring the stability of the converter's DC line voltage and the derating power during the DFIG's low-voltage ride-through process, meeting the overvoltage suppression requirements of the flexible DC system; V * dcTake 1.1 pu.

[0109] Figure 6 (ac) The horizontal axis of the three graphs represents the system operating time, and the vertical axes represent: DC bus voltage of the flexible DC system, 35kV bus voltage after the main transformer at the wind farm outlet, and total active power transmitted from the wind farm to the WFMMC. Figure 2 Because WFMMC uses V / f control in the system, it can be controlled by Figure 6 (b) It can be seen that the 35kV bus voltage after the main transformer at the wind farm outlet remained almost unchanged during the fault period. The active power of the wind turbine is as follows: Figure 6 (c) The change in the wind turbine outlet voltage is insufficient to cause the wind turbine to enter a low-voltage ride-through process; therefore, the active power injected into the WFMMC by the wind farm remains almost unchanged. Ignoring transformer and converter losses, the injected power on the wind farm side remains unchanged at around 900MW. Figure 6 (a) It can be seen that the surplus power accumulates in the converters on both sides of the flexible DC transmission system, resulting in overvoltage in the DC line. The maximum overvoltage is around 820KV.

[0110] exist Figure 2 Additional control strategies are added to the system. Figure 7 (ac) The horizontal axis of all three graphs represents system operating time, and the vertical axes represent: DC bus voltage of the flexible DC system, 35kV bus voltage after the main transformer at the wind farm outlet, and total active power transmitted from the wind farm to the WFMMC, respectively. Figure 7 As shown in (a), when a three-phase ground fault occurs at point f, the fault occurrence time is 7.0s, the fault duration is 0.3s, and all system parameters remain unchanged. Dynamic voltage reduction control is added to the WFMMC side of the system during the fault period. On the wind farm side, dynamic active power reduction command value control is added to the wind turbine converter during low-voltage ride-through, and Chopper circuit protection is added to the wind turbine converter. It can be seen that the DC line overvoltage situation is significantly improved during the fault period, with the maximum overvoltage level around 720kV, and the overvoltage phenomenon is significantly suppressed. Figure 7 (b) The change in AC voltage after the main transformer at the wind farm outlet after the WFMMC adds a dynamic voltage reduction control strategy during the fault period. It can be seen that the degree of AC voltage reduction varies with the degree of DC bus overvoltage, but the overall voltage reduction is below 0.9 pu, ensuring that the wind turbine enters the low voltage ride-through process and thus reduces the generated power. Figure 7 (c) As the wind turbines enter the low-voltage ride-through range, the wind farm's output power dynamically decreases based on the degree of DC bus overvoltage, thereby suppressing DC bus overvoltage. At this point, U in the optimization strategy... dcmaxWith a setting of 1.1 pu, the maximum DC line overvoltage was suppressed to around 720 kV, showing a significant suppression effect.

[0111] Example 2: A DC overvoltage suppression device for a doubly-fed wind power flexible DC transmission system, comprising:

[0112] The calculation module is used to collect the DC line voltage of the flexible DC system and calculate the surplus power using the charging effect of the converter submodule.

[0113] The first determining module is used to determine the dynamic voltage reduction coefficient of the WFMMC of the flexible DC converter station on the wind farm side based on the surplus power.

[0114] The second determining module is used to determine the range of values ​​for the dynamic voltage reduction factor to ensure that the doubly fed wind farm enters the low voltage ride-through range.

[0115] The third determining module is used to determine the reduction in power output based on the surplus power after the doubly fed wind farm enters the low-voltage transmission range; and to distribute the reduction to each wind turbine connected to the AC side of the WFMMC according to the power reduction distribution coefficient.

[0116] The suppression module is used to detect the DC voltage of the wind turbine converter and suppress the DC voltage of the converter.

[0117] The sequence numbers of the above embodiments of the present invention are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0118] In the above embodiments of the present invention, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0119] The specific embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A method for suppressing DC overvoltage in a doubly-fed wind power flexible DC transmission system, characterized in that: Includes the following steps: Step 1: Collect the DC line voltage of the flexible DC system and calculate the surplus power using the charging effect of the converter submodule; Step 2: Determine the dynamic voltage reduction coefficient of the WFMMC at the wind farm-side flexible DC converter station based on the surplus power. Step 3: Determine the range of values ​​for the dynamic voltage reduction factor to ensure that the doubly-fed wind farm enters the low-voltage ride-through range; Step 4: After the doubly fed wind farm enters the low voltage ride-through range, determine the reduction in power output based on the surplus power; and distribute the reduction to each wind turbine connected to the AC side of the WFMMC according to the power reduction allocation factor. Step 5: Detect the DC voltage of the wind turbine converter and suppress the DC voltage of the converter; Step 1 includes: When a fault occurs in the receiving-end AC power grid, the overvoltage of the DC line in the flexible DC system is calculated, and the surplus power of the DC line causing the overvoltage in the flexible DC system is defined as... The surplus power is calculated using the charging effect of the converter's equivalent capacitance. It is calculated using the following formula: In the formula, C is the sum of the equivalent capacitances of the converter submodules; This refers to the DC line voltage of a flexible DC system. This refers to the upper limit threshold for overvoltage in DC lines. Represents the overvoltage value; Step 2 includes: When an overvoltage occurs in a doubly-fed wind power flexible DC transmission system, the power flowing into the WFMMC should be reduced to decrease the surplus power. The calculation formula is: In the formula, This indicates the power flowing into the WFMMC (Flexible Direct Current Converter Station) on the wind farm side. This indicates the power output of the wind farm-side flexible DC converter station after WFMMC reduction; The power balance equation is obtained based on the power balance relationship before and after the power reduction on the WFMMC side: In the formula: To reduce the AC side voltage of WFMMC after power inflow; To reduce the AC side voltage of WFMMC before power flows in; This represents the effective value of the AC-side current flowing into the WFMMC. The power factor on the AC side of WFMMC; Based on the power balance equation, the expression for the dynamic buck coefficient in the WFMMC buck control strategy is obtained: In the formula: K represents the dynamic voltage drop coefficient of the V / f control on the AC side of WFMMC during the fault period; Step 3 includes: determining the dynamic voltage reduction coefficient range to ensure that the AC side voltage of the WFMMC is reduced to the low voltage ride-through range of the doubly fed wind farm during the fault, and dynamically reducing the AC side voltage of the WFMMC according to the DC overvoltage value. In the formula: The reference value for the AC side voltage of WFMMC before the fault; the dynamic voltage drop factor K can be divided into three stages according to the degree of DC overvoltage. When the overvoltage degree is less than the upper limit of DC voltage... When the overvoltage level exceeds the upper limit of the DC voltage, the step-down control is not activated; when the overvoltage level exceeds the upper limit of the DC voltage... However, when the voltage reduction is insufficient to allow the doubly fed wind farm to enter the low voltage ride-through range, the voltage reduction factor is set to reduce the voltage on the AC side of the WFMMC to the voltage of the low voltage ride-through operating range of the wind farm; when the overvoltage is more severe, the voltage reduction amplitude on the AC side of the WFMMC is larger, and the voltage reduction factor will be set to a dynamic value according to the degree of overvoltage. Step 4 includes: Total power reduction of WFMMC AC side wind turbine Active power surplus greater than or equal to that of flexible DC systems The details are as follows: When the number of wind turbines connected to the WFMMC side of the flexible DC system is n, and each wind turbine is in operation; the active power command value of each wind turbine is dynamically changed to : In the formula: This indicates the active power command value of the fan before power reduction. This indicates the active power command value of the wind turbine after power reduction; Let be the power reduction distribution factor for the i-th wind turbine in the wind farm; The power reduction distribution coefficient of the i-th wind turbine in the wind farm can be determined in the following ways: If all wind turbines are operating in the same condition, then the distribution is considered proportional. =1 / n; If the operating conditions of each fan are different, the power of the i-th fan before power reduction shall be allocated proportionally to the total fan capacity.

2. The DC overvoltage suppression method for a doubly-fed wind power flexible DC transmission system according to claim 1, characterized in that: Step 5 includes: The power consumed by the resistor after the Chopper circuit is connected is: In the formula: This indicates the power consumed by the resistor after the Chopper circuit is connected. This indicates the DC voltage of the wind turbine converter. This indicates the resistance value of the Chopper circuit; After a doubly-fed induction generator (DFIG) wind farm enters the low-voltage ride-through range, if the DC voltage of the wind turbine converter rises, it will cause the Chopper circuit to activate. At this time, the DC overvoltage of the wind turbine converter should be suppressed. The activation of the Chopper circuit must meet the following conditions: In the formula: This indicates the action value of the Chopper circuit.

3. A DC overvoltage suppression device for a doubly-fed wind power flexible DC transmission system implementing the method of claim 1, characterized in that: include: The calculation module is used to collect the DC line voltage of the flexible DC system and calculate the surplus power using the charging effect of the converter submodule. The first determining module is used to determine the dynamic voltage reduction coefficient of the WFMMC of the flexible DC converter station on the wind farm side based on the surplus power. The second determining module is used to determine the range of values ​​for the dynamic voltage reduction factor to ensure that the doubly fed wind farm enters the low voltage ride-through range. The third determining module is used to determine the reduction in power output based on the surplus power after the doubly fed wind farm enters the low voltage ride-through range; and to distribute the reduction to each wind turbine connected to the AC side of the WFMMC according to the power reduction allocation coefficient. The suppression module is used to detect the DC voltage of the wind turbine converter and suppress the DC voltage of the converter.

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