A control method for reducing operation of a cascade hybrid direct current voltage

Abstract

The application discloses a control method for reducing cascade type hybrid DC voltage operation, comprising the following steps: step 1, unlocking LCC inverter station and MMC inverter station in the cascade type hybrid DC; step 2, increasing voltage instruction value of the LCC inverter station and three MMC inverter stations to the rated instruction value; step 3, unlocking LCC rectifier station in the cascade type hybrid DC; step 4, setting reactive power of three inverter side low end MMC inverter stations as 0; step 5, increasing current instruction value of the LCC rectifier station to the rated instruction value; step 6, controlling voltage instruction value of the inverter side high end LCC inverter station to decrease to 80%; step 7, reducing cascade type hybrid DC voltage to 90%; and step 8, realizing normal operation of the cascade type hybrid DC after reducing to 90%. The application can directly use low end MMC to absorb reactive power for balance, does not need to operate the AC side equipment, can realize millisecond level reactive control, and ensures normal operation of the cascade type hybrid DC under the condition of reducing to 90%.

Description

Technical Field

[0001] The present invention belongs to the technical field of hybrid direct current (DC) transmission, and in particular relates to a control method for reducing the voltage operation of a cascaded hybrid DC transmission. Background Art

[0002] Hybrid DC transmission has become an important development direction of DC transmission technology in recent years because it combines the advantages of conventional DC (LCC-HVDC) and flexible DC (VSC-HVDC). Figure 1 The rectifier station for the receiving-end cascaded hybrid DC transmission system shown in the figure consists of two sets of 12-pulse LCCs connected in series, and the inverter station consists of a 12-pulse phase-commutated converter (LCC) and a voltage source converter (VSC) connected in parallel in series. The low-end VSC is expanded to multiple VSCs connected in parallel, with the connection points located in different regional grids. While increasing the transmission power of the hybrid DC system, the multi-connection structure also facilitates phased construction of the project. Figure 1 The MMC in this system stands for modular multilevel converter, a type of VSC. The receiving-end converter station consists of multiple VSC converter stations connected in parallel and then cascaded with an LCC converter station, effectively forming a multi-terminal system and enabling power distribution across multiple VSC inverter stations.

[0003] Due to the increasing impact of DC transmission on AC systems, DC voltage reduction is necessary when AC systems are weak or when DC systems are participating in grid restoration. When the DC system voltage is reduced, both its reactive power consumption and its own active power undergo significant changes. However, the voltage reduction operation of cascaded DC transmission differs from that of conventional DC transmission. Therefore, conventional DC voltage reduction methods are not suitable for cascaded DC transmission, and corresponding strategies are needed to address this issue. Summary of the Invention

[0004] To address the issue of hybrid DC operating at abnormal voltages, the present invention discloses a control method for reducing the voltage operation of a cascaded hybrid DC. After the LCC reduces the DC voltage, the low-end MMC can be used to directly absorb reactive power for balancing, without requiring AC-side equipment to operate. This not only achieves millisecond-level reactive power control, but also ensures the normal operation of the cascaded hybrid DC at a 90% voltage reduction.

[0005] The specific technical solutions of the present invention are as follows:

[0006] A control method for reducing cascade hybrid DC voltage operation, the specific steps are as follows:

[0007] Step 1: Unlock the LCC inverter station and three MMC inverter stations in the cascade hybrid DC system;

[0008] Step 2: Increase the voltage command values ​​of the LCC inverter station and the three MMC inverter stations to the rated command values;

[0009] Step 3: Unlock the LCC rectifier station in the cascade hybrid DC;

[0010] Step 4: Set the reactive power of the three inverter-side low-end MMC inverter stations to 0;

[0011] Step 5: Increase the current command value of the LCC rectifier station to the rated command value, and the cascade hybrid DC-AC system operates at the rated state;

[0012] Step 6: Control the voltage command value of the inverter-side high-end LCC inverter station to be reduced to 80% of the rated command value;

[0013] Step 7: After the high-end LCC inverter station on the inverter side reduces the voltage command value to 80% of the rated command value, the three low-end MMC inverter stations on the inverter side maintain the rated voltage command value unchanged. Then, the overall DC voltage of the cascaded hybrid DC system is actually reduced to 90% of the rated command value.

[0014] Step 8: After the high-end LCC inverter station on the inverter side reduces the voltage command value to 80% of the rated command value, the reactive power compensation equipment still compensates according to the reactive power consumption under the rated state, and there is a surplus compared with the actual reactive power consumption. Therefore, the three low-end MMC inverter stations on the inverter side are controlled to absorb the surplus reactive power on average to ensure the reactive power balance on the AC side and achieve normal operation of the cascaded hybrid DC after the voltage is reduced to 90%.

[0015] Preferably, in step 5, when the cascaded hybrid DC / AC system operates in a rated state, the cascaded hybrid DC active power is 1 pu, and the reactive power consumption of the inverter-side high-end LCC inverter station is 40% of the active power, i.e., 0.4 pu.

[0016] Preferably, in step 6, when the voltage command value of the inverter-side high-end LCC inverter station is reduced to 80% of the rated command value, the cascaded hybrid DC active power is reduced to 0.8 pu, and the reactive power consumption of the inverter-side high-end LCC inverter station is 0.32 pu.

[0017] Preferably, in step 8, the reactive power compensation device still compensates according to the reactive power consumption of 0.4pu under the rated state, and 0.08pu of reactive power remains compared with the actual reactive power consumption. At this time, the reactive power control instructions for controlling the three low-end MMC inverter stations on the inverter side are respectively -0.0267pu, and a negative value indicates absorption.

[0018] Beneficial effects: The present invention discloses a control method for reducing the voltage operation of a cascade hybrid DC system, which has the following advantages over the prior art:

[0019] When the LCC reduces the DC voltage, the low-end MMC can be directly used to absorb reactive power for balancing. This allows the cascaded hybrid DC to operate normally under low voltage conditions without operating the AC-side equipment. Compared with the conventional method of performing reactive balancing by switching filters on the AC-side equipment in seconds, the present invention uses the reactive control instructions of the MMC to directly achieve matching reactive power absorption, achieving millisecond-level reactive control, thereby ensuring the normal operation of the DC voltage after the system DC voltage is reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a topological diagram of a hybrid cascaded DC / AC system in a specific embodiment of the present invention;

[0021] Figure 2 This is a diagram showing the change in DC voltage of the high-end LCC of the inverter station in the present invention;

[0022] Figure 3 This is a diagram showing the DC voltage variation of the low-end MMC of the inverter station in the present invention;

[0023] Figure 4 This is a graph showing the overall DC voltage variation of the cascade hybrid DC in the present invention;

[0024] Figure 5 This is a diagram showing the transmission power changes of each cascade hybrid DC converter station in the present invention. DETAILED DESCRIPTION

[0025] The following makes some improvements and modifications to the present invention in conjunction with the accompanying drawings, and these improvements and modifications should also be considered as the scope of protection of the present invention.

[0026] Example

[0027] like Figure 1 The figure shows the topology of the hybrid cascaded DC / AC system of this embodiment. In the figure, the receiving-end cascaded hybrid DC transmission rectifier station is composed of two groups of 12-pulse LCCs connected in series, and the inverter station is composed of a group of 12-pulse phase-commutated converters (LCCs) and voltage source converters (VSCs) connected in parallel in series. The low-end VSC is expanded to multiple VSCs connected in parallel and deployed in different regional power grids. Figure 1 MMC stands for modular multilevel converter, which is a type of VSC.

[0028] For the above hybrid cascaded DC / AC system, the specific steps of the control method for reducing the cascaded hybrid DC voltage operation are as follows:

[0029] Step 1: Unlock the LCC inverter station and three MMC inverter stations in the cascade hybrid DC system;

[0030] Step 2: Increase the voltage command values ​​of the LCC inverter station and the three MMC inverter stations to the rated command values;

[0031] Step 3: Unlock the LCC rectifier station in the cascade hybrid DC;

[0032] Step 4: Set the reactive power of the three low-end MMC inverter stations on the inverter side to 0, in preparation for the reactive power command coordination of the low-end MMC inverter stations on the inverter side;

[0033] Step 5: Increase the current command value of the LCC rectifier station to the rated command value. The cascaded hybrid DC / AC system operates at the rated state, with an active power of 1 pu. The reactive power consumption of the high-end LCC inverter station on the inverter side is 40% of the active power, that is, 0.4 pu.

[0034] Step 6: Control the voltage command value of the high-end LCC inverter station on the inverter side to 80% of the rated command value. At this time, the active power consumption of the cascaded hybrid DC system is reduced to 0.8 pu and the reactive power consumption is 0.32 pu.

[0035] Step 7: After the high-end LCC inverter station on the inverter side reduces the voltage command value to 80% of the rated command value, the three low-end MMC inverter stations on the inverter side maintain the rated voltage command value unchanged. Then, the overall DC voltage of the cascaded hybrid DC system is actually reduced to 90% of the rated command value.

[0036] Step 8: After the high-end LCC inverter station on the inverter side reduces the voltage command value to 80% of the rated command value, the reactive power compensation equipment still compensates according to the reactive power consumption of 0.4pu under the rated state. Compared with the actual reactive power consumption, there is a remaining reactive power of 0.08pu. At this time, the reactive power control command of the three low-end MMC inverter stations on the inverter side is controlled to -0.0267pu. The negative value indicates absorption, thereby ensuring the reactive power balance on the AC side and achieving normal operation of the cascaded hybrid DC after the voltage is reduced to 90%.

[0037] by Figure 1 The system is used as an example for verification. The verification scheme is as follows:

[0038] After the system starts normally, the high-side LCC DC voltage on the inverter side is reduced to 80% in the first 2.5 seconds, and the group filter is put into operation. The reduction range of the LCC DC voltage is as follows: Figure 2 As shown, the DC voltage reduction of MMC (VSC) is as follows Figure 3 As shown, the overall DC voltage reduction is as follows Figure 4 As shown in the figure, it can be seen that the DC voltage of MMC (VSC) remains unchanged, and the DC voltage of LCC is reduced to 80%, so the overall DC voltage is reduced to 90%. The active power changes of each converter station in the system are as follows: Figure 5 As shown, it can be seen that the system can run smoothly.

[0039] The above description is only an illustration of the present invention and is a preferred embodiment of the present invention. It should be noted that those skilled in the art may make several improvements and modifications without departing from the present invention, and such improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A control method for reducing the voltage operation of a cascaded hybrid DC system, characterized in that: The specific steps are as follows: Step 1: Unlock the LCC inverter station and three MMC inverter stations in the cascade hybrid DC system; Step 2: Increase the voltage command values ​​of the LCC inverter station and the three MMC inverter stations to the rated command values; Step 3: Unlock the LCC rectifier station in the cascade hybrid DC; Step 4: Set the reactive power of the three inverter-side low-end MMC inverter stations to 0; Step 5: Increase the current command value of the LCC rectifier station to the rated command value, and the cascade hybrid DC-AC system operates at the rated state; Step 6: Control the voltage command value of the inverter-side high-end LCC inverter station to be reduced to 80% of the rated command value; Step 7: After the high-end LCC inverter station on the inverter side reduces the voltage command value to 80% of the rated command value, the three low-end MMC inverter stations on the inverter side maintain the rated voltage command value unchanged. Then, the overall DC voltage of the cascaded hybrid DC system is actually reduced to 90% of the rated command value. Step 8: After the high-end LCC inverter station on the inverter side reduces the voltage command value to 80% of the rated command value, the reactive power compensation equipment still compensates according to the reactive power consumption under the rated state, and there is a surplus compared with the actual reactive power consumption. Therefore, the three low-end MMC inverter stations on the inverter side are controlled to absorb the surplus reactive power on average to ensure the reactive power balance on the AC side and achieve normal operation of the cascaded hybrid DC after the voltage is reduced to 90%.

2. The control method for reducing the cascade hybrid DC voltage operation according to claim 1, characterized in that: In step 5, when the cascaded hybrid DC / AC system operates at rated power, the active power of the cascaded hybrid DC is 1 pu, and the reactive power consumption of the high-end LCC inverter station on the inverter side is 40% of the active power, that is, 0.4 pu.

3. The control method for reducing the cascade hybrid DC voltage operation according to claim 2, characterized in that: In step 6, when the voltage command value of the inverter-side high-end LCC inverter station is reduced to 80% of the rated command value, the cascaded hybrid DC active power is reduced to 0.8 pu, and the reactive power consumption of the inverter-side high-end LCC inverter station is 0.32 pu.

4. The control method for reducing the cascade hybrid DC voltage operation according to claim 2, characterized in that: In step 8, the reactive power compensation device still compensates according to the reactive power consumption of 0.4pu under the rated state, and 0.08pu of reactive power remains compared with the actual reactive power consumption. At this time, the reactive power control instructions for controlling the three low-end MMC inverter stations on the inverter side are respectively -0.0267pu, and a negative value indicates absorption.

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