Flexible direct current system and black start control method thereof

Abstract

This invention belongs to the field of DC energy storage and flexible DC transmission technology, specifically relating to a flexible DC system and its black-start control method. The flexible DC system includes a DC energy storage valve and a flexible DC converter valve. The AC side of the flexible DC converter valve is connected to the AC power grid, and the DC side of the DC energy storage valve is connected to the flexible DC converter valve. The DC energy storage valve includes multiple sub-modules connected in series. In the event of a power outage in the AC power grid connected to the flexible DC converter valve, the battery pack of the DC energy storage valve provides DC voltage support to the DC line side of the flexible DC system. Then, the converter valve of the flexible DC system is activated to provide AC voltage support to the connected AC power grid, thereby restoring the AC power grid. This improves system stability, ensuring power supply to critical loads and continuous stable operation of equipment. Under certain conditions, it can also provide starting power for the restoration of the main power grid, shortening the overall system recovery time and reducing losses caused by power outages.

Description

Technical Field

[0001] This invention belongs to the field of DC energy storage and flexible DC transmission technology, specifically relating to a flexible DC system and its black-start control method. Background Technology

[0002] Energy storage technology is a key technology supporting the construction of future new power systems. It can effectively solve the problems of intermittency and fluctuation of new energy power, and realize frequency regulation and peak shaving of large power grids, friendly grid connection of wind and solar power, support the operation of microgrids and user demand-side response, which has far-reaching significance.

[0003] AC energy storage power stations suffer from several problems, including low conversion efficiency, complex module topology and numerous components, and an inability to address power surplus and power surges in flexible DC systems. Therefore, they are not an economically effective solution for integrating new energy sources into flexible DC systems. For flexible DC systems, fault ride-through at the receiving end of the AC system is typically achieved only by configuring large-capacity energy-consuming devices. However, there are no effective solutions for the fluctuations and randomness of new energy power, as well as the power surges caused by new energy field faults. Furthermore, the large-capacity energy-consuming devices in AC energy storage power stations are unsuitable for flexible DC systems containing DC energy storage, resulting in system instability and poor applicability.

[0004] Flexible DC systems incorporating DC energy storage are a crucial technological means to address the aforementioned issues. However, there are currently no research findings on flexible DC systems with DC energy storage, both domestically and internationally, and related DC-side energy storage designs are in a technological vacuum. Black-start control, which involves charging the DC energy storage valve and flexible DC converter valve via battery packs to complete the startup process after an AC power grid outage due to a fault, and further driving the restoration of AC power supply, has significant practical implications. Summary of the Invention

[0005] The purpose of this invention is to provide a flexible DC system and its black-start control method to solve the problem that the configuration of large-capacity energy-consuming devices in existing AC energy storage power stations is not suitable for flexible DC systems containing DC energy storage, leading to system instability.

[0006] To address the aforementioned technical problems, this invention provides a black-start control method for a flexible DC system, comprising the following steps:

[0007] 1) In the event of a power outage in the AC grid connected to the flexible DC converter valve, connect the DC energy storage valve and the flexible DC converter valve.

[0008] 2) The battery pack in the DC energy storage valve charges the capacitors in the flexible DC converter valve and the DC energy storage valve.

[0009] 3) When the capacitor in the DC energy storage valve is fully charged, unlock all sub-modules included in the DC energy storage valve so that the DC line voltage between the DC energy storage valve and the flexible DC converter valve is stabilized at the rated DC voltage.

[0010] 4) When the flexible DC converter valve is fully charged, unlock the flexible DC converter valve. When the AC side voltage of the flexible DC converter valve rises to the rated AC voltage, connect the flexible DC converter valve to the AC power grid to complete the black start.

[0011] Its beneficial effects are as follows: In the event of a power outage in the AC grid connected to the flexible DC converter valve, the present invention provides DC voltage support to the DC line side of the flexible DC system through the battery pack of the DC energy storage valve, and then starts the converter valve of the flexible DC system to provide AC voltage support to the connected AC grid, thereby completing the restoration of the AC grid. The black start of the flexible DC system of the present invention is based on DC energy storage, which improves the system stability. It can not only ensure the power supply of critical loads and the continuous and stable operation of equipment, but also provide starting power for the restoration of the large power grid under certain conditions, shortening the restoration time of the entire system and reducing the losses caused by power outages.

[0012] Furthermore, in step 1), when connecting the DC energy storage valve and the flexible DC converter valve, the capacitor in the DC energy storage valve is first uncontrolled charged through the pre-charge branch, and the flexible DC converter valve is also uncontrolled charged through the DC line; when the current in the pre-charge branch is less than the current setting threshold, the pre-charge branch is bypassed to controllably charge the capacitor in the DC energy storage valve.

[0013] Its beneficial effects are: uncontrolled charging of the capacitors in the DC energy storage valve and the flexible DC converter valve, and controlled charging of the capacitors in the DC energy storage valve by bypassing the pre-charge branch when the pre-charge branch current is less than the current setting threshold, so as to enable the capacitors in the DC energy storage valve and the flexible DC converter valve to be charged quickly and promote the recovery of the AC power grid.

[0014] Furthermore, in step 1), when connecting the DC energy storage valve and the flexible DC converter valve, the grounding switch connected to the DC energy storage valve must be in the open position.

[0015] Its beneficial effect is that the grounding switch connected to the DC energy storage valve is in the open position, thus preventing the system from being grounded.

[0016] Furthermore, in step 3), when the DC energy storage valve is fully charged, the sub-modules included in the DC energy storage valve are unlocked using a constant DC voltage control mode.

[0017] Its beneficial effects are: the constant DC voltage control mode unlocks all sub-modules included in the DC energy storage valve, stabilizing the voltage during system operation and further improving system stability.

[0018] Further, in step 4), when the flexible DC converter valve is fully charged, the flexible DC converter valve is unlocked using a constant AC frequency and AC voltage mode.

[0019] Its beneficial effects are: by using a constant AC frequency and AC voltage mode to unlock the flexible DC converter valve, the flexible DC converter valve can be unlocked quickly and the system can be kept stable.

[0020] To address the aforementioned technical problems, this invention also provides a flexible DC system. The flexible DC system includes a DC energy storage valve and a flexible DC converter valve. The AC side of the flexible DC converter valve is connected to the AC power grid, and the DC side of the DC energy storage valve is connected to the flexible DC converter valve. The DC energy storage valve includes multiple sub-modules connected in series. The flexible DC system is used for black start using the following method:

[0021] 1) In the event of a power outage in the AC grid connected to the flexible DC converter valve, connect the DC energy storage valve and the flexible DC converter valve.

[0022] 2) The battery pack in the DC energy storage valve charges the capacitors in the flexible DC converter valve and the DC energy storage valve.

[0023] 3) When the capacitor in the DC energy storage valve is fully charged, unlock all sub-modules included in the DC energy storage valve so that the DC line voltage between the DC energy storage valve and the flexible DC converter valve is stabilized at the rated DC voltage.

[0024] 4) When the flexible DC converter valve is fully charged, unlock the flexible DC converter valve. When the AC side voltage of the flexible DC converter valve rises to the rated AC voltage, connect the flexible DC converter valve to the AC power grid to complete the black start.

[0025] Its beneficial effects are as follows: In the event of a power outage in the AC grid connected to the flexible DC converter valve, the present invention provides DC voltage support to the DC line side of the flexible DC system through the battery pack of the DC energy storage valve, and then starts the converter valve of the flexible DC system to provide AC voltage support to the connected AC grid, thereby completing the restoration of the AC grid. The black start of the flexible DC system of the present invention is based on DC energy storage, which improves the system stability. It can not only ensure the power supply of critical loads and the continuous and stable operation of equipment, but also provide starting power for the restoration of the large power grid under certain conditions, shortening the restoration time of the entire system and reducing the losses caused by power outages.

[0026] Furthermore, each DC energy storage valve submodule includes a pre-charge branch, which is connected in parallel with the main switch connected in series on the main branch between the capacitor and the battery pack. The pre-charge branch is equipped with a pre-charge switch and a pre-charge resistor. Uncontrolled charging is first performed on the capacitor in the DC energy storage valve through the pre-charge branch, and uncontrolled charging is also performed on the capacitor in the flexible DC converter valve. If the current in the pre-charge branch is less than the current setting threshold, the pre-charge branch is bypassed, and the capacitor in the DC energy storage valve enters controlled charging.

[0027] The capacitors in the DC energy storage valve and the flexible DC converter valve are charged uncontrolled, and when the pre-charge branch current is less than the current setting threshold, the pre-charge branch is bypassed to charge the capacitors in the DC energy storage valve in a controlled manner, so as to enable the capacitors in the DC energy storage valve and the flexible DC converter valve to charge quickly and promote the recovery of the AC grid.

[0028] Furthermore, in step 1), when connecting the DC energy storage valve and the flexible DC converter valve, the grounding switch connected to the DC energy storage valve must be in the open position.

[0029] Its beneficial effect is that the grounding switch connected to the DC energy storage valve is in the open position, thus preventing the system from being grounded.

[0030] Furthermore, in step 3), when the capacitor in the DC energy storage valve is fully charged, the sub-modules included in the DC energy storage valve are unlocked using a constant DC voltage control mode.

[0031] Its beneficial effects are: the constant DC voltage control mode unlocks all sub-modules included in the DC energy storage valve, stabilizing the voltage during system operation and further improving system stability.

[0032] Further, in step 4), when the flexible DC converter valve is fully charged, the flexible DC converter valve is unlocked using a constant AC frequency and AC voltage mode.

[0033] Its beneficial effects are: by using a constant AC frequency and AC voltage mode to unlock the flexible DC converter valve, the flexible DC converter valve can be unlocked quickly and the system can be kept stable. Attached Figure Description

[0034] Figure 1 This is a diagram of the DC energy storage topology of the present invention;

[0035] Figure 2 This is a topological diagram of the DC energy storage valve submodule of the present invention;

[0036] Figure 3 This is a block diagram of the flexible DC system structure based on DC energy storage according to the present invention. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0038] Example of a flexible DC system:

[0039] The flexible DC system in this embodiment is as follows: Figure 3 As shown, it includes a DC energy storage valve and a flexible DC converter valve. The AC side of the flexible DC converter valve is connected to the AC power grid via AC side control switch Q0, and the other end is connected to the DC energy storage valve via a DC line. The specific connection relationship between the flexible DC converter valve and the DC energy storage valve is as follows: Figure 1As shown, the DC energy storage valve is connected to the flexible DC converter valve via disconnect switches Q11 and Q12, and the system also includes grounding disconnect switches Q21 / Q22 / Q23 / Q24. The DC energy storage valve comprises multiple DC energy storage valve sub-modules (which can be simply referred to as sub-modules) arranged in series. The circuit diagram of each DC energy storage valve sub-module is shown below. Figure 2 As shown. In this embodiment, the DC energy storage valve submodule is a half-bridge submodule. The capacitor in the DC energy storage valve submodule is connected in parallel with the battery pack, and a main branch is connected in series between the capacitor and the battery pack. A main switch Q1 is connected in series on the main branch, and a pre-charge branch is connected in parallel across the main switch. A pre-charge switch Q2 and a pre-charge resistor are connected in series in the pre-charge branch.

[0040] Meanwhile, the entire system is equipped with multiple control systems. The battery pack is equipped with a battery management system (BMS), each DC energy storage valve submodule is equipped with a submodule control system (SMC), and the flexible DC converter valve is equipped with a flexible DC converter valve control system.

[0041] The flexible DC system in this embodiment uses a black-start control method for flexible DC systems according to the present invention for black-start, and the specific process is as follows:

[0042] 1) Close the disconnect switches Q11 and Q12 between the flexible DC converter valve and the DC energy storage valve, such as Figure 1 As shown, the grounding switches Q21 / Q22 / Q23 / Q24 are in the open position, allowing the flexible DC converter valve and the DC energy storage valve to be interconnected and not grounded, thus enabling the flexible DC converter valve and the DC energy storage valve to be charged.

[0043] 2) The battery management system (BMS) of the DC energy storage valve commands the closure of the pre-charge switch Q2 between the DC energy storage valve submodule capacitor and the battery pack, such as... Figure 2 As shown, the pre-charge branch of the battery pack is used to perform uncontrolled charging of the capacitor of the DC energy storage valve submodule, and the DC uncontrolled charging of the flexible DC converter valve is performed through the DC line.

[0044] 3) When the current flowing through the pre-charge resistor of the DC energy storage valve is lower than I set When this happens, the main switch Q1 of the battery pack will automatically close, such as... Figure 2 As shown, the bypass pre-charge resistor is disconnected, the pre-charge switch Q2 is opened, and the DC energy storage valve enters the controllable charging state.

[0045] 4) After the DC energy storage valve is fully charged, the DC energy storage valve submodule control system (SMC) adopts a constant DC voltage control mode, systematically issuing unlock commands to the DC energy storage valve submodule to gradually establish the DC line voltage, eventually stabilizing the DC line voltage at U. dcset Rated voltage value, flexible DC converter valve enters DC controllable charging.

[0046] 5) After the flexible DC converter valve is fully charged, the flexible DC converter valve control system uses a constant AC frequency of 50Hz and an AC voltage of 0kV to unlock the flexible DC converter valve.

[0047] 6) The flexible DC converter valve control system operates according to the set rate U acrate Increase the AC side voltage to directly reach the rated AC voltage U required by the AC power grid. acset .

[0048] 7) Close the switch Q0 between the flexible DC converter valve and the AC power grid, such as... Figure 3 As shown, black start control is completed after the AC power grid is energized.

[0049] Example of a black-start control method for flexible DC systems:

[0050] This embodiment is based on, as follows Figure 1 , 3 The flexible DC system shown is subjected to black-start control. The specific structure and composition of the flexible DC system have been described in detail in the system embodiment of the flexible DC system, and will not be repeated here. The steps of the flexible DC black-start control method in this embodiment are as follows:

[0051] 1) Close the switch between the flexible DC converter valve and the DC energy storage valve to connect the flexible DC converter valve and the DC energy storage valve to each other without grounding, allowing the flexible DC converter valve and the DC energy storage valve to be charged.

[0052] 2) The battery management system of the DC energy storage valve commands the closure of the pre-charge switch between the DC energy storage valve submodule capacitor and the battery pack, uses the battery pack pre-charge circuit to perform uncontrolled charging of the DC energy storage valve submodule capacitor, and performs uncontrolled DC charging of the flexible DC converter valve through the DC line.

[0053] 3) When the current flowing through the pre-charge resistor of the DC energy storage valve is lower than the threshold, the main switch of the battery pack is automatically closed, the pre-charge resistor is bypassed, the pre-charge switch is opened, and the DC energy storage valve enters controllable charging.

[0054] 4) After the DC energy storage valve is fully charged, the DC energy storage valve submodule control system adopts a constant DC voltage control mode, and issues unlocking commands to the DC energy storage valve submodule in an orderly manner to gradually establish the DC line voltage. Finally, the DC line voltage stabilizes at the rated voltage value, and the flexible DC converter valve enters DC controllable charging.

[0055] 5) After the flexible DC converter valve is fully charged, the flexible DC converter valve control system unlocks the flexible DC converter valve using a constant AC frequency and AC voltage mode.

[0056] 6) The flexible DC converter valve control system increases the AC side voltage at a certain rate to reach the rated AC voltage required by the AC power grid.

[0057] 7) Close the switch between the flexible DC converter valve and the AC power grid. After the AC power grid is energized, the black start control is completed.

[0058] In summary, this invention provides a flexible DC system and a black-start control method. In the event of a power outage in the AC grid connected to the flexible DC system based on DC energy storage, the DC energy storage provides DC voltage support to the DC line side of the flexible DC system. Then, the converter valve of the flexible DC system is activated to provide AC voltage support to the connected AC grid, thereby restoring the AC grid. Black start not only ensures power supply to critical loads and the continuous and stable operation of equipment, but also, under certain conditions, provides starting power for the restoration of the main power grid, shortening the overall system recovery time and reducing losses caused by power outages.

[0059] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product embodied on several computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0060] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit its scope of protection. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that after reading the present invention, they can still make various changes, modifications or equivalent substitutions to the specific implementation of the invention. However, all such changes, modifications or equivalent substitutions are within the scope of protection of the pending claims of the invention.

Claims

1. A black start control method of a flexible direct current system, characterized by, Includes the following steps: 1) In the event of a power outage in the AC grid connected to the flexible DC converter valve, connect the DC energy storage valve and the flexible DC converter valve. 2) The battery pack in the DC energy storage valve charges the capacitors in the flexible DC converter valve and the DC energy storage valve. 3) When the capacitor in the DC energy storage valve is fully charged, unlock all sub-modules included in the DC energy storage valve so that the DC line voltage between the DC energy storage valve and the flexible DC converter valve is stabilized at the rated DC voltage. 4) When the flexible DC converter valve is fully charged, unlock the flexible DC converter valve. When the AC side voltage of the flexible DC converter valve rises to the rated AC voltage, connect the flexible DC converter valve to the AC power grid to complete the black start.

2. The black-start control method for a flexible DC system according to claim 1, characterized in that, In step 1), when the DC energy storage valve and the flexible DC converter valve are connected, the battery pack in the DC energy storage valve first performs uncontrolled charging of the capacitor in the DC energy storage valve through the pre-charge branch, and then performs uncontrolled charging of the flexible DC converter valve through the DC line; when the pre-charge branch current is less than the current setting threshold, the pre-charge branch is bypassed to perform controlled charging of the capacitor in the DC energy storage valve.

3. The black start control method of a flexible DC system according to claim 1, characterized by, In step 1), when connecting the DC energy storage valve and the flexible DC converter valve, the grounding switch connected to the DC energy storage valve must be in the open position.

4. The black start control method of a flexible DC system according to claim 1, characterized by, In step 3), when the capacitor in the DC energy storage valve is fully charged, the sub-modules included in the DC energy storage valve are unlocked using a constant DC voltage control mode.

5. The black start control method of a flexible DC system according to claim 1, characterized by, Step 4) When the flexible DC converter valve is fully charged, unlock the flexible DC converter valve using a constant AC frequency and AC voltage mode.

6. A flexible DC system characterized by, The flexible DC system includes a DC energy storage valve and a flexible DC converter valve. The AC side of the flexible DC converter valve is connected to the AC power grid, and the DC side of the DC energy storage valve is connected to the flexible DC converter valve. The DC energy storage valve includes multiple sub-modules connected in series. The flexible DC system is used for black start using the following method: 1) In the event of a power outage in the AC grid connected to the flexible DC converter valve, connect the DC energy storage valve and the flexible DC converter valve. 2) The battery pack in the DC energy storage valve charges the capacitors in the flexible DC converter valve and the DC energy storage valve. 3) When the capacitor in the DC energy storage valve is fully charged, unlock all sub-modules included in the DC energy storage valve so that the DC line voltage between the DC energy storage valve and the flexible DC converter valve is stabilized at the rated DC voltage. 4) When the flexible DC converter valve is fully charged, unlock the flexible DC converter valve. When the AC side voltage of the flexible DC converter valve rises to the rated AC voltage, connect the flexible DC converter valve to the AC power grid to complete the black start.

7. The flexible DC system of claim 6, wherein, Each DC energy storage valve submodule includes a pre-charge branch, which is connected in parallel with the main switch that is connected in series on the main branch between the capacitor and the battery pack. The pre-charge branch is equipped with a pre-charge switch and a pre-charge resistor. The capacitor in the DC energy storage valve is first uncontrolled charged through the pre-charge branch, and the flexible DC converter valve is also uncontrolled charged through the DC line. When the current in the pre-charge branch is less than the current setting threshold, the pre-charge branch is bypassed to controllably charge the capacitor in the DC energy storage valve.

8. The flexible DC system of claim 6, wherein, In step 1), when connecting the DC energy storage valve and the flexible DC converter valve, the grounding switch connected to the DC energy storage valve must be in the open position.

9. The flexible DC system of claim 6, wherein, In step 3), when the capacitor in the DC energy storage valve is fully charged, the sub-modules included in the DC energy storage valve are unlocked using a constant DC voltage control mode.

10. The flexible DC system of claim 6, wherein, Step 4) When the flexible DC converter valve is fully charged, unlock the flexible DC converter valve using a constant AC frequency and AC voltage mode.

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