Flexible DC surplus power consumption control method and system

Abstract

The invention relates to a flexible DC surplus power consumption control method and system, and belongs to the field of flexible DC power transmission. The method comprises the following steps: when a sending-end converter judges that surplus power occurs in a flexible direct current system, judging a fault working condition causing the surplus power; determining an alternating-current voltage limit value according to the judged fault working condition, and taking the alternating-current voltage limit value as an alternating-current voltage reference value of the transmitting-end converter; respectively calculating current reference values of an inner ring current controller under the d axis and the q axis according to an AC voltage reference value of a sending end converter and AC voltage components under the d axis and the q axis, calculating a sending end AC voltage through the inner ring current controller, and inputting an energy consumption device of a sending end fan generator set according to the obtained sending end AC voltage. And surplus power of the wind turbine generator is consumed. According to the invention, the power transmission is reduced by controlling the AC voltage, the probability of fault occurrence and the cost of surplus power consumption are reduced, and the reliability of system operation is improved.

Description

Technical Field

[0001] The invention relates to a flexible direct current surplus power consumption control method and system, belonging to the field of flexible direct current power transmission. Background Art

[0002] In order to solve the global energy crisis, wind power generation has developed rapidly around the world, and my country has proposed a strategic adjustment of the energy structure of "carbon peak and carbon neutrality". In offshore wind power systems, when there is surplus power in the flexible DC system, the DC voltage will continue to increase until the system trips, which will affect the safe and stable operation of the transmission system.

[0003] The Chinese patent application publication document with publication number CN117498460A discloses an offshore wind power DC transmission system and control method. When facing surplus power in the flexible DC system, on the one hand, the system reduces the overall volume, weight and cost of the system by canceling the DC energy consumption device centrally configured on the DC side of the onshore converter station; on the other hand, when the method detects an AC fault in the receiving AC power grid, it determines and triggers the call of the wind turbine energy consumption device in the wind power converter by calculating the estimated value of surplus power, thereby achieving the purpose of maintaining the balance of surplus power. However, although this method also absorbs surplus power by calling the wind turbine energy consumption through the flexible DC control system, it relies on high-speed and large-scale optical fiber to communicate with the wind turbine control system in real time when estimating surplus power, which increases the probability of failure and the cost of stable operation of the system. Summary of the invention

[0004] The purpose of the present invention is to provide a flexible DC surplus power consumption control method and system to solve the problem that the surplus power consumption in the prior art needs to rely on the strong communication capability of the wind turbine control system, which leads to a high probability of failure and high operating cost.

[0005] To achieve the above object, the solution of the present invention includes:

[0006] A flexible DC surplus power consumption control method of the present invention comprises:

[0007] When the sending-end converter determines that the flexible DC system has surplus power, the fault condition that causes the surplus power is determined; the AC voltage limit value is determined according to the determined fault condition, and the AC voltage limit value is used as the AC voltage reference value of the sending-end converter, and the sending-end converter adopts V / F control; the current reference values ​​of the d-axis and q-axis of the inner-loop current controller are calculated according to the AC voltage reference value of the sending-end converter and the AC voltage component under the d-axis and the AC voltage component under the q-axis, respectively, the sending-end AC voltage is calculated through the inner-loop current controller, and the energy consumption device of the sending-end generator set is input according to the obtained sending-end AC voltage to consume the surplus power of the generator set.

[0008] Furthermore, the fault conditions include receiving-end AC instantaneous fault conditions and DC line instantaneous fault conditions; if it is a receiving-end AC instantaneous fault, the AC voltage limit value is the AC voltage amplitude during the receiving-end AC fault; if it is a DC line instantaneous fault, the AC voltage limit value is 0.

[0009] Furthermore, the steps for the sending-end converter to determine whether the flexible DC system has surplus power are as follows: the sending-end converter collects the voltages of each bridge arm submodule of the sending-end converter in real time; calculates the real-time submodule voltage average value; when the submodule voltage average value is greater than the submodule voltage threshold, it is determined that the flexible DC system has surplus power.

[0010] Furthermore, the submodule voltage threshold is 1.2 pu.

[0011] Furthermore, when the fault condition disappears, the AC voltage reference value of the sending-end converter is increased to the sending-end AC voltage setting value, and the generator set cancels the input of its energy-consuming device.

[0012] Furthermore, the AC voltage reference value of the sending-end converter is increased to the sending-end AC voltage setting value at a set rate.

[0013] Furthermore, the sending-end converter adopts a full-power submodule, which includes a full-bridge submodule and a half-bridge submodule, and the ratio of the full-bridge submodule to the half-bridge submodule is greater than 6:4.

[0014] Furthermore, the inner loop current controller adopts a PI controller.

[0015] The present invention also provides a flexible DC surplus power consumption control system, comprising a processor, wherein the processor is used to execute a computer program to implement the steps of any one of the above flexible DC surplus power consumption control methods.

[0016] The beneficial effects of the present invention are as follows: as an improved invention, when surplus power is detected in the system, it is determined that the operating condition causing the surplus power is a receiving-end AC instantaneous fault or a DC line instantaneous fault, and the sending-end converter determines different AC voltage limit values ​​according to different fault conditions, and uses the AC voltage limit value as the AC voltage reference value of the sending-end converter V / F control, so as to achieve the reduction of power transmission by controlling the AC voltage; then, according to the AC voltage reference value and the AC voltage components under the d and q axes, the current reference values ​​of the inner loop current controller d and q axes are calculated respectively, and the sending-end AC voltage is calculated by the inner loop current controller, and finally, according to the obtained sending-end AC voltage, the energy consumption device of the sending-end wind turbine generator set is input to consume the surplus power of the wind turbine set. Therefore, the flexible DC control system of the present invention does not need to rely on the powerful interactive function of the flexible DC control system and the wind turbine control system when performing surplus power consumption control, and reduces power transmission by controlling the AC voltage, thereby reducing the probability of fault occurrence and the cost of absorbing surplus power, and improving the reliability of system operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a topological diagram of the grid connection of new energy via a flexible DC system;

[0018] Figure 2 It is the topological structure of the wind turbine system at the sending end;

[0019] Figure 3 This is the block diagram of the V / F control adopted by the sending-end converter. DETAILED DESCRIPTION

[0020] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is described in detail in a clear and complete manner in conjunction with the accompanying drawings and embodiments.

[0021] The idea of ​​the present invention is that when surplus power is detected in the flexible DC system, the corresponding AC voltage limit value is determined according to the fault condition causing the surplus power, and it is used as the AC voltage reference value of the V / F control, and then the current reference value i of the d-axis and q-axis of the inner loop current controller is calculated. dref and i qref , and then the sending-end AC voltage is determined through the inner-loop current controller. The wind turbine controller enters low voltage ride-through control according to the determined sending-end AC voltage, and automatically puts the wind turbine energy consumption device into use to consume the surplus power of the wind turbine.

[0022] Method Example:

[0023] The present invention is applicable to all power plants with internal energy consumption devices. This embodiment only takes a wind farm as an example. Figure 1The schematic diagram of the grid-connected topology of the new energy through the flexible DC system shown in the figure mainly consists of wind turbines, sending-end converters, DC lines (cables, overhead lines), receiving-end converters and receiving-end power grids. Among them, the new energy wind turbines are connected to the AC busbar, the AC side of the sending-end converter is connected to the AC busbar, the DC side of the sending-end converter is connected to the DC side of the receiving-end converter through a DC line (cable, overhead line), and the AC side of the receiving-end converter is connected to the receiving-end power grid through a transformer. Both the sending-end converter and the receiving-end converter use full-power submodules, which include full-bridge submodules and half-bridge submodules. The ratio of full-bridge submodules to half-bridge submodules is generally greater than 6:4, which meets the transient fault requirements of the overhead line of the flexible DC system.

[0024] The voltage of each bridge arm submodule of the converter is collected in real time through the sending-end converter, and the real-time submodule voltage average is calculated. If the submodule average voltage is greater than a certain threshold (such as 1.2pu), it is determined that the flexible DC system has surplus power. At this time, the energy consumption device of the wind turbine at the sending end absorbs the surplus power of the wind turbine. The topological structure of the wind turbine system at the sending end is as follows: Figure 2 As shown in Figure 1, this topology mainly consists of a wind turbine, a machine-side converter, a grid-side converter and an energy-consuming device. The blades and the wind turbine are connected to the AC side of the machine-side transformer, and an energy-consuming device is connected in parallel between the DC side of the machine-side converter and the DC side of the grid-side converter to absorb the surplus power of the wind turbine at the sending end and maintain the stable operation of the entire transmission system. The AC side of the grid-side converter is connected to the AC system at the sending end through a step-up transformer.

[0025] In addition, the sending-end converter also provides a stable grid-connected AC voltage for the sending-end wind turbine. The converter adopts V / F control with a fixed frequency of 50Hz. Its control block diagram is as follows: Figure 3 As shown. set Indicates the AC voltage setting value at the sending end, U lim Indicates the AC voltage limit value of the sending-end converter, U ref AC voltage reference value, U d It represents the d-axis component of the AC voltage at the sending end after dq transformation, U q It is represented by the q-axis component of the AC voltage at the sending end after dq transformation; the current reference value i of the d-axis and q-axis of the inner loop current controller is calculated by the following formula dref and i qref , the calculation formula is as follows:

[0026]

[0027] In the formula, kp represents the proportional coefficient of the voltage controller link, ki represents the integral coefficient of the voltage controller link, dimensionless, and U ref Select different AC voltage reference values ​​according to different situations. When the system has no surplus power, Uref U is the AC voltage setting value at the sending end set ; When the AC failure occurs at the receiving end of the system, U ref is the AC voltage limit value U of the sending-end converter lim ; When the DC line fails, U ref is 0.

[0028] After obtaining the current reference values ​​i of the d-axis and q-axis dref and i qref After, i dref and i qref Respectively with i d and i q Then, the PI controller is used to obtain V d and V q , V d and V q The AC voltage V at the sending end is obtained through the dq→αβ and αβ→abc converters. a 、V b and V c .

[0029] After the sending-end converter determines that the flexible DC system has surplus power based on the average voltage of the submodule being greater than a certain threshold, it then determines the fault condition that causes the surplus power, and then determines different AC voltage limit values ​​based on different fault conditions and puts the wind turbine energy consumption device into operation. Specifically,

[0030] (1) When a transient AC fault occurs at the receiving end:

[0031] If an AC instantaneous fault occurs at the receiving end, the controller of the receiving end converter will detect the AC fault signal and send the AC voltage amplitude during the AC fault signal to the controller of the sending end converter through inter-station communication.

[0032] When the average voltage of the converter submodule is detected to be beyond the limit, if the controller of the sending-end converter receives a signal generated by the receiving-end AC fault, the AC voltage amplitude during the receiving-end AC fault is used as the AC voltage limit value U of the sending-end converter. lim , reduce the AC voltage at the sending end (usually the AC voltage setting value at the sending end is U set The rated value is 1.2pu, and the AC voltage amplitude during the fault is lower than 0.85pu). The wind turbine controller automatically enters low voltage ride-through control according to the reduced AC voltage at the sending end, and automatically puts the wind turbine energy consumption device into use to consume the surplus power of the wind turbine.

[0033] When the controller of the sending-end converter receives the signal that the AC fault at the receiving end has disappeared, the AC voltage at the sending end will increase to the AC voltage setting value U at a certain rate. set; As the AC voltage at the sending end recovers, the wind turbine controller automatically exits the low voltage ride-through control, and the surplus power consumption under the AC fault condition at the receiving end ends.

[0034] (2) When a DC line instantaneous fault occurs:

[0035] When the average voltage of the converter submodule is detected to be over the limit, if the controller of the sending-end converter receives a DC line fault signal, the AC voltage limit value of the sending-end converter is switched to 0, and the sending-end AC voltage is reduced to 0. The wind turbine controller automatically enters low voltage ride-through control according to the reduced sending-end AC voltage, and automatically puts the wind turbine energy consumption device into use to consume the surplus power of the wind turbine.

[0036] When the controller of the sending-end converter receives the DC line fault disappearance signal, the AC voltage at the sending end will increase to the AC voltage setting value U at a certain rate. set ; As the AC voltage at the sending end recovers, the wind turbine controller automatically exits the low voltage ride-through control, and the surplus power consumption under the DC line fault condition ends.

[0037] System Example:

[0038] A flexible DC surplus power consumption control system of the present invention includes a processor, and the processor is used to execute computer program instructions to implement a flexible DC surplus power consumption control method introduced in the method embodiment. The method has been introduced clearly enough in the method embodiment and will not be repeated here.

Claims

1. A flexible DC surplus power consumption control method, characterized in that: The method includes: When the sending-end converter determines that the flexible DC system has surplus power, the fault condition that causes the surplus power is determined; the AC voltage limit value is determined according to the determined fault condition, and the AC voltage limit value is used as the AC voltage reference value of the sending-end converter, and the sending-end converter adopts V / F control; the current reference values ​​of the d-axis and q-axis of the inner-loop current controller are calculated according to the AC voltage reference value of the sending-end converter and the AC voltage component under the d-axis and the AC voltage component under the q-axis, respectively, the sending-end AC voltage is calculated through the inner-loop current controller, and the energy consumption device of the sending-end generator set is input according to the obtained sending-end AC voltage to consume the surplus power of the generator set.

2. The flexible DC surplus power consumption control method according to claim 1 is characterized in that: The fault conditions include receiving-end AC instantaneous fault conditions and DC line instantaneous fault conditions; if it is a receiving-end AC instantaneous fault, the AC voltage limit value is the AC voltage amplitude during the receiving-end AC fault; if it is a DC line instantaneous fault, the AC voltage limit value is 0.

3. The flexible DC surplus power consumption control method according to claim 1 is characterized in that: The steps for the sending-end converter to determine whether the flexible DC system has surplus power are as follows: the sending-end converter collects the voltages of each bridge arm submodule of the sending-end converter in real time; calculates the real-time submodule voltage average value; when the submodule voltage average value is greater than the submodule voltage threshold, it is determined that the flexible DC system has surplus power.

4. The flexible DC surplus power consumption control method according to claim 3 is characterized in that: The submodule voltage threshold is 1.2 pu.

5. The flexible DC surplus power consumption control method according to claim 1 is characterized in that: When the fault condition disappears, the AC voltage reference value of the sending-end converter is increased to the sending-end AC voltage setting value, and the generator set cancels the input of its energy-consuming devices.

6. The flexible DC surplus power consumption control method according to claim 5 is characterized in that: The AC voltage reference value of the sending-end converter is increased to the sending-end AC voltage setting value at a set rate.

7. The flexible DC surplus power consumption control method according to claim 1, characterized in that: The sending-end converter adopts a full-power submodule, which includes a full-bridge submodule and a half-bridge submodule, and the ratio of the full-bridge submodule to the half-bridge submodule is greater than 6:

4.

8. The flexible DC surplus power consumption control method according to claim 1, characterized in that: The inner loop current controller adopts a PI controller.

9. A flexible DC surplus power consumption control system, comprising a processor, characterized in that: The processor is used to execute a computer program to implement the steps of a flexible DC surplus power consumption control method as described in any one of claims 1 to 8.

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