DRU-MMC new energy sending end alternating current system and fault ride-through method thereof

Abstract

The invention relates to a DRU-MMC new energy sending end alternating current system and a fault ride-through method thereof, and belongs to the technical field of new energy direct current sending. According to the method, an alternating-current energy consumption device is arranged on the alternating-current side of a DRU sending-end converter valve, when a sending-end alternating-current system breaks down, the fault depth of the alternating-current system is detected and divided into a single-phase earth fault, a two-phase earth fault and a three-phase earth fault, and if the fault depth is the single-phase earth fault, the alternating-current energy consumption device is triggered to absorb redundant active power emitted by a power generation field, so that the fault depth of the sending-end alternating-current system is determined. Shutdown of the whole system caused by over-voltage and over-current faults of an SVG converter valve in the auxiliary converter is avoided; if the fault is a two-phase grounding fault or a three-phase grounding fault, whether an alternating current energy consumption device is triggered or not is selected according to whether the average voltage value of the SVG sub-modules is larger than a set threshold value or not or whether the energy capable of being absorbed by the SVG is larger than the energy needing to be absorbed by the system or not. The problem of fault ride-through failure of the sending-end alternating-current system caused by saturation failure of an SVG control loop when the sending-end alternating-current system has a grounding fault in the prior art is solved.

Description

Technical Field

[0001] The invention relates to a DRU-MMC new energy sending end AC system and a fault ride-through method thereof, belonging to the technical field of new energy direct current transmission. Background Art

[0002] An existing hybrid offshore wind DC transmission system is Figure 1 As shown, the new energy delivery system includes a DRU sending-end converter valve, an auxiliary converter SVG, a DC line and an MMC receiving-end converter valve. Each wind turbine is connected to the AC bus. The AC side of the DRU sending-end converter valve is connected to the AC bus through a transformer, and the DC side is connected to the DC side of the receiving-end MMC through a DC transmission line. The AC side of the receiving-end MMC is used to access the power grid. The auxiliary converter is connected to the AC bus through a transformer. The auxiliary converter is provided with an SVG for reactive power compensation. For the DRU-MMC new energy delivery system, when a ground fault occurs in the sending-end AC system, since the fault level detected by the wind turbine is relatively light compared to the actual fault level, the wind farm will still send out some excess active power. At this time, this part of the active power will continue to charge the SVG submodule in the auxiliary converter, resulting in saturation failure of the SVG control loop, so that the SVG in the auxiliary converter will have overvoltage and overcurrent conditions, and ultimately lead to the failure of the sending-end AC system to cross the fault. Summary of the invention

[0003] The purpose of the present invention is to provide a DRU-MMC new energy sending-end AC system and a fault ride-through method thereof, so as to solve the problem in the prior art that when a ground fault occurs in the sending-end AC system, the SVG control loop saturates and fails, resulting in failure of the sending-end AC system to ride through the fault.

[0004] To achieve the above object, the solution of the present invention includes:

[0005] A DRU-MMC new energy sending-end AC system fault ride-through method of the present invention comprises the following steps:

[0006] When the sending-end AC system encounters a fault, the fault depth of the AC system is detected and the reference value of the SVG inner loop current in the auxiliary converter is set to 0; the fault depth includes single-phase grounding fault, two-phase grounding fault and three-phase grounding fault. If it is a single-phase grounding fault, the AC energy consumption device is triggered; if it is a two-phase grounding fault / three-phase grounding fault, it is determined whether the AC energy consumption device needs to be triggered; the AC energy consumption device is used to be set on the AC side of the DRU sending-end converter valve.

[0007] Furthermore, if it is a two-phase grounding fault / three-phase grounding fault, the AC energy consumption device needs to be triggered when: the average voltage of the SVG sub-mode is greater than the set threshold or the energy that the SVG can absorb is less than the energy that the system needs to absorb.

[0008] Furthermore, the AC energy consumption device includes three energy consumption branches, and the three energy consumption branches are connected end to end in a triangle connection or a star connection to be connected to the AC side of the DRU sending-end converter valve.

[0009] Furthermore, the energy dissipation branch includes a controllable switch device and at least one energy dissipation resistor which are arranged in series.

[0010] Furthermore, there are three energy-consuming resistors, and two of the energy-consuming resistors are respectively connected in parallel with two controllable switch devices.

[0011] Furthermore, if it is a single-phase grounding fault, the controllable switch device connected in series with the resistor on the AC energy-consuming bridge arm and the two controllable switch devices connected in parallel on the two energy-consuming resistors are triggered; if it is a two-phase grounding fault or a three-phase grounding fault and the AC energy-consuming device needs to be triggered, the controllable switch device connected in series with the resistor on the AC energy-consuming bridge arm is triggered.

[0012] Furthermore, when a ground fault occurs, the energy E that the system needs to absorb is fault Calculated by the following formula:

[0013]

[0014] Among them, u a 、u b 、u c is the phase voltage value of the grid connection point after the fault drops; i a 、i b 、i c is the three-phase rated phase current value when the rated power is transmitted; t is the time for the active power to drop from the rated value to 0 under the control of SVG after the fault occurs.

[0015] Furthermore, the energy E that SVG can absorb SVG Calculated by the following formula:

[0016]

[0017] Among them, U max is the maximum voltage of the SVG submodule; U0 is the rated voltage of the SVG submodule; N is the rated number of SVG submodules in a single bridge arm; C is the capacitance of the SVG submodule.

[0018] Furthermore, the resistance R of a single energy-consuming resistor of the energy-consuming branch is determined according to the effective value of the line voltage and the active capacity of the system. The specific formula is as follows:

[0019]

[0020] Among them, U is the effective value of the line voltage, and P is the active capacity of the system.

[0021] The present invention also provides a DRU-MMC new energy sending-end AC system, including an auxiliary converter for being arranged at the sending end and a DRU sending-end converter valve, the DC side of the DRU sending-end converter valve is used to be connected to the receiving end through a DC transmission line, and the AC side of the DRU sending-end converter valve is used to be connected to the AC line at the sending end. The system adopts any of the above DRU-MMC new energy sending-end AC system fault crossing methods.

[0022] The beneficial effects of the present invention are as follows: as an improved invention, the present invention sets an AC energy dissipation device on the AC side of the DRU sending-end converter valve. When the sending-end AC system fails, the fault depth of the AC system is detected, and the fault depth is divided into a single-phase grounding fault, a two-phase grounding fault, and a three-phase grounding fault. If it is a single-phase grounding fault, the AC energy dissipation device is triggered to absorb the excess active power generated by the power plant to avoid saturation failure of the SVG control loop in the auxiliary converter; if it is a two-phase grounding fault / three-phase grounding fault, whether to trigger the AC energy dissipation device is determined based on whether the average voltage of the SVG submodule is greater than the set threshold or whether the energy that the SVG can absorb is greater than the energy that the system needs to absorb. Therefore, the present invention solves the problem in the prior art that when a grounding fault occurs in the sending-end AC system, the SVG control loop saturation failure leads to the failure of the sending-end AC system to cross the fault. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a hybrid offshore wind DC transmission system in which the sending-end DRU and the auxiliary converter are connected in parallel in the prior art;

[0024] Figure 2 is the DRU-MMC delivery system topology of this implementation mode;

[0025] Figure 3 It is the topology of the AC energy consumption device of the DRU-MMC delivery system of this implementation mode;

[0026] Figure 4 It is a single energy consumption branch of the AC energy consumption device of the DRU-MMC sending system of this implementation mode;

[0027] Figure 5 This is the AC fault ride-through strategy at the sending end of the DRU-MMC sending system of this implementation mode. DETAILED DESCRIPTION

[0028] 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.

[0029] The idea of ​​the present invention is that when the sending-end AC system encounters a fault, the depth of the sending-end fault is first detected, the reference value of the inner loop current of the auxiliary converter SVG is set to 0, and then the energy-consuming device is put into operation according to different fault depths to absorb the excess power generated by the wind farm, thereby avoiding overvoltage and overcurrent in the SVG. Finally, after the fault is resolved, the energy-consuming device is stopped from being put into operation, and the SVG resumes normal control.

[0030] Method Example:

[0031] like Figure 2 The DRU-MMC new energy transmission system topology shown is mainly composed of wind turbines, AC busbars (also called AC lines), transformers, DRU sending-end converter valves, auxiliary converters SVG, AC energy consumption devices, DC transmission lines, MMC receiving-end converter valves and receiving-end AC power grids. Each new energy wind turbine is connected to the AC busbar, the AC side of the DRU sending-end converter valve is connected to the AC busbar through a transformer, the auxiliary converter is connected to the AC busbar through another transformer, and the DC side of the DRU sending-end converter valve is connected to the MMC receiving-end converter valve through a DC transmission line, and then connected to the receiving-end AC power grid through a transformer. As other implementation methods, the wind farm can also be other power plants, such as a hydropower plant.

[0032] When a ground fault occurs in the AC system, in order to avoid the problem that the SVG control loop in the auxiliary converter is saturated and fails, resulting in a fault ride-through failure, an AC energy consuming device is provided on the AC busbar at the sending end to absorb the excess active power generated by the wind farm. Specifically, the topology of the AC energy consuming device is as follows: Figure 3 As shown, the AC energy consumption device has three energy consumption branches, which are connected end to end in a triangle connection. As another implementation method, the three energy consumption branches can also be connected in a star connection. When the wind turbine encounters a grounding fault and generates more active power to charge the SVG submodule, the AC energy consumption device can be used to absorb the excess power generated by the wind turbine to avoid overvoltage and overcurrent in the SVG. The topology diagram of the energy consumption branch is shown in Figure 4 As shown, it is composed of an anti-parallel thyristor T1 and three resistors in series, wherein two resistors are respectively connected in parallel with anti-parallel thyristors T2 and T3. The parameters of resistor R are as follows:

[0033]

[0034] U is the effective value of line voltage, P is the active capacity of the system;

[0035] When the energy consumption device is not put into operation, the system line voltage is mainly borne by the anti-parallel thyristor T1. After the energy consumption device is put into operation, the triggering or non-triggering of T2 and T3 can be selected according to the fault depth. As other implementations, the anti-parallel thyristor here can be replaced by other controllable switch devices, such as MOS tubes; the number of resistors on each energy consumption branch is not fixed, and can be set according to the actual energy consumption demand. For example, the number of resistors on the energy consumption branch may not be three, and can be set to two resistors or more resistors to absorb the excess power generated by the power plant. The number of corresponding controllable switch devices is also adjusted according to the number of resistors.

[0036] The sending-end AC fault ride-through strategy of this implementation is as follows: Figure 5 As shown, when the sending-end AC system encounters a fault, the depth of the sending-end fault is first detected. The fault depth can be divided into single-phase grounding fault, two-phase grounding fault and three-phase grounding fault, and then different fault crossing strategies are selected according to the fault depth.

[0037] Specifically, when the detected fault is a single-phase ground fault, firstly, the reference value of the inner loop current of the auxiliary converter SVG is set to 0, that is, I dref Set to 0 and I qref Set to 0; then trigger the anti-parallel thyristor T1 on the bridge arm and the two anti-parallel thyristors T2 and T3 on the bridge arm connected in parallel with the two resistors according to the actual energy consumption. According to the above formula, the greater the actual energy consumption, the fewer resistors should be put into use. The anti-parallel thyristors T2 and T3 here are used to bypass the resistors connected in parallel with them. By controlling the number of resistors put into use on the energy consumption branch, the AC energy consumption device absorbs the excess power generated by the wind farm to avoid overvoltage and overcurrent in SVG; finally, after the fault ends, the energy consumption device is terminated and SVG resumes normal control. The triggering of the energy consumption branch has nothing to do with which phase has a grounding fault. For example, if phase A is single-phase grounded, all three T1s in the energy consumption branch need to be triggered.

[0038] When the detected fault is a two-phase ground fault / three-phase ground fault, firstly, the reference value of the inner loop current of the auxiliary converter SVG is set to 0, that is, I dref Set to 0 and I qrefSet to 0; then determine whether it is necessary to put the energy-consuming device into operation. In this embodiment, the situation where the AC energy-consuming device needs to be triggered is: the average voltage of the SVG sub-module is greater than the set threshold or the energy that the SVG can absorb is less than the energy that the system needs to absorb. Specifically, if the average voltage of the SVG sub-module is greater than 1.2pu, that is, when the system generates surplus power, execute strategy 2 to trigger the anti-parallel thyristor T1 on the AC energy-consuming bridge arm to make the energy-consuming device absorb the energy of the wind turbine; otherwise, execute strategy 1 and do not trigger the energy-consuming device; or, if the energy that the SVG can absorb is greater than the energy that the system needs to absorb, execute strategy 1 and do not trigger the energy-consuming device. For example, if there is a three-phase grounding fault and the voltage drops to 0, the energy that needs to be absorbed is 0, so there is no need to trigger the energy-consuming device; if the energy that the SVG can absorb is less than the energy that the system needs to absorb, execute strategy 2 to trigger the anti-parallel thyristor T1 on the AC energy-consuming bridge arm to absorb the excess power generated by the wind turbine to avoid overvoltage and overcurrent in the SVG. Finally, after the fault ends, the energy-consuming device is terminated and the SVG resumes normal control.

[0039] For example, when a three-phase ground fault occurs, the energy E that the system needs to absorb is fault for:

[0040]

[0041] Wherein, ua, ub, uc are the phase voltage values ​​of the grid connection point after the fault drop, and the three-phase voltage is 0 when the fault occurs; ia, ib, ic are the rated phase current values ​​of the three phases when the rated power is transmitted; t is the time for the active power to drop from the rated value to 0 under the control of SVG after the fault occurs.

[0042] Energy E that SVG can absorb SVG for:

[0043]

[0044] Where U max is the maximum voltage of the SVG submodule, which is 2.4 kV here; U0 is the rated voltage of the SVG submodule, which is 2 kV here; N is the rated number of SVG submodules in a single bridge arm, which is 58 here; C is the submodule capacitance value of the SVG valve, which is 20 mF here.

[0045] Since the energy that the SVG can absorb, 3 MJ, is greater than the energy that the system needs to absorb, 0 J, strategy 1 is executed and the energy consumption device is not triggered.

[0046] System Example:

[0047] A DRU-MMC new energy sending-end AC system of the present invention includes an auxiliary converter for being arranged at the sending end and a DRU sending-end converter valve, the DC side of the DRU sending-end converter valve is used to be connected to the receiving end through a DC transmission line, and the AC side of the DRU sending-end converter valve is used to be connected to the AC line at the sending end. The system implements a DRU-MMC new energy sending-end AC system fault ride-through method introduced in a method embodiment. The method has been introduced clearly enough in the method embodiment and will not be repeated here.

Claims

1. A fault ride-through method for a DRU-MMC new energy sending-end AC system, characterized in that: The steps include: When the sending-end AC system encounters a fault, the fault depth of the AC system is detected and the reference value of the SVG inner loop current in the auxiliary converter is set to 0; the fault depth includes single-phase grounding fault, two-phase grounding fault and three-phase grounding fault. If it is a single-phase grounding fault, the AC energy consumption device is triggered; if it is a two-phase grounding fault / three-phase grounding fault, it is determined whether the AC energy consumption device needs to be triggered based on the average voltage of the SVG submodule or the energy that the SVG can absorb; the AC energy consumption device is used to be set on the AC side of the DRU sending-end converter valve.

2. The DRU-MMC new energy sending end AC system fault ride-through method according to claim 1 is characterized in that: If it is a two-phase grounding fault / three-phase grounding fault, the AC energy consumption device needs to be triggered when: the average voltage of the SVG sub-mode is greater than the set threshold or the energy that the SVG can absorb is less than the energy that the system needs to absorb.

3. The DRU-MMC new energy sending end AC system fault ride-through method according to claim 1 is characterized in that: The AC energy consumption device comprises three energy consumption branches, and the three energy consumption branches are connected end to end in a triangle connection or a star connection to be connected to the AC side of the DRU sending end converter valve.

4. The DRU-MMC new energy sending-end AC system fault ride-through method according to claim 3 is characterized in that: The energy dissipation branch includes a controllable switch device and at least one energy dissipation resistor arranged in series.

5. The DRU-MMC new energy sending end AC system fault ride-through method according to claim 4 is characterized in that: There are three energy-consuming resistors, and two of them are connected in parallel with two controllable switch devices respectively.

6. The fault ride-through method of the DRU-MMC new energy sending-end AC system according to claims 4 and 5, characterized in that: If it is a single-phase grounding fault, the controllable switch device connected in series with the resistor on the AC energy-consuming bridge arm and the two controllable switch devices connected in parallel on the two energy-consuming resistors will be triggered; if it is a two-phase grounding fault or a three-phase grounding fault and the AC energy-consuming device needs to be triggered, the controllable switch device connected in series with the resistor on the AC energy-consuming bridge arm will be triggered.

7. The DRU-MMC new energy sending end AC system fault ride-through method according to claim 2, characterized in that: When a ground fault occurs, the energy E that the system needs to absorb fault Calculated by the following formula: Among them, u a 、u b 、u c is the phase voltage value of the grid connection point after the fault drops; i a 、i b 、i c is the three-phase rated phase current value when the rated power is transmitted; t is the time for the active power to drop from the rated value to 0 under the control of SVG after the fault occurs.

8. The DRU-MMC new energy sending-end AC system fault ride-through method according to claim 2 is characterized in that: The energy E that the SVG can absorb SVG Calculated by the following formula: Among them, U max is the maximum voltage of the SVG submodule; U0 is the rated voltage of the SVG submodule; N is the rated number of SVG submodules in a single bridge arm; C is the capacitance of the SVG submodule.

9. The DRU-MMC new energy sending end AC system fault ride-through method according to claim 4, characterized in that: The resistance R of a single energy-consuming resistor of the energy-consuming branch is determined according to the effective value of the line voltage and the active capacity of the system. The specific formula is as follows: Among them, U is the effective value of the line voltage, and P is the active capacity of the system.

10. A DRU-MMC new energy sending-end AC system, comprising an auxiliary converter for setting at the sending end and a DRU sending-end converter valve, the DC side of the DRU sending-end converter valve is used to connect to the receiving end through a DC transmission line, and the AC side of the DRU sending-end converter valve is used to connect to the AC line at the sending end, characterized in that: The system adopts the DRU-MMC new energy sending end AC system fault ride-through method as described in any one of claims 1-9.

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