Dc interconnected dc group system based on mmc and control method thereof

Abstract

The present specification belongs to the field of power system transmission and distribution technology, and provides a DC interconnected type DC group system based on MMC and a control method thereof. The system comprises at least two DC group subsystems, each of which is configured with a hydropower unit cluster, an AC collection network, a sending-end MMC converter station, a receiving-end MMC converter station and a receiving-end AC power grid. The hydropower unit cluster is used to generate a first AC electric signal, the sending-end MMC converter station is used to convert the first AC electric signal into a DC electric signal based on a modular multi-level conversion mode, and the receiving-end MMC converter station is used to convert the DC electric signal into a second AC electric signal and transmit the second AC electric signal to the receiving-end AC power grid. The DC ports of the sending-end MMC converter stations in each DC group subsystem are connected through DC interconnected lines to form a sending-end DC side interconnection channel for DC power mutual aid between the DC group subsystems. The scheme improves the system operation flexibility, power supply reliability and engineering practicability.

Description

Technical Field

[0001] The embodiments in this specification relate to the field of power system transmission and distribution technology, and in particular to a DC interconnected DC group system based on MMC and its control method. Background Technology

[0002] With the continuous advancement of clean energy development and utilization, giant cascade hydropower bases have become a crucial support for the construction of new power systems and the low-carbon transformation of energy. Cascade hydropower bases are generally located far from load centers, and the generated electricity is mainly transmitted over long distances via DC transmission. Currently, the industry mostly adopts an architecture where multiple independent DC transmission lines operate separately to transmit hydropower. These DC transmission systems are independent of each other, lacking power interconnection and coordinated control links. Under this operating mode, system scheduling flexibility is insufficient, the transmission lines cannot support each other, overall power supply reliability is limited, and it is difficult to achieve optimal allocation and full utilization of cascade hydropower resources.

[0003] The power grid infrastructure in areas where giant cascade hydropower projects are located is relatively weak, with insufficient support from the sending-end AC system. In some scenarios, there is even a lack of effective AC system support. Traditional ultra-high-voltage direct current (UHVDC) transmission often uses grid-commutated converters. These devices are highly dependent on the AC grid for commutation voltage and are prone to instability under weak AC grid conditions at the sending end, making them unsuitable for the actual grid conditions of hydropower bases. In contrast, modular multilevel converters (MMCs) offer advantages such as strong adaptability to weak grids, no risk of commutation failure, and independent decoupling control of active and reactive power, making them more suitable for hydropower transmission scenarios.

[0004] However, the existing independent DC channel architecture not only lacks the ability to cross-channel power mutual assistance, but also fails to give full play to the flexible regulation advantages of MMC, making it difficult to meet multiple needs such as safe and stable transmission of large-scale hydropower, flexible power allocation and grid condition adaptation. Summary of the Invention

[0005] In view of this, the embodiments of this specification provide a DC interconnected DC group system based on MMC and its control method to solve the technical defects existing in the prior art.

[0006] According to a first aspect of the embodiments of this specification, a DC interconnected DC group system based on MMC is provided, comprising: at least two DC group subsystems, each of the DC group subsystems being configured with a hydropower unit cluster, an AC collection network, a sending-end MMC converter station, a receiving-end MMC converter station, and a receiving-end AC power grid; The hydropower unit cluster is connected to the sending-end MMC converter station through the AC collection network. The sending-end MMC converter station is connected to the receiving-end MMC converter station through a DC transmission line. The receiving-end MMC converter station is connected to the receiving-end AC power grid. The hydropower unit cluster is used to generate a first AC signal. The sending-end MMC converter station is used to convert the first AC signal into a DC signal based on a modular multilevel converter and transmit it to the receiving-end MMC converter station. The receiving-end MMC converter station is used to convert the DC signal into a second AC signal and transmit it to the receiving-end AC power grid. In this system, the DC ports of the sending-end MMC converter stations within each DC group subsystem are connected via DC interconnection lines to form a sending-end DC-side interconnection channel for mutual DC power exchange between the various DC group subsystems.

[0007] In one possible implementation, the DC group subsystem is configured as two or three; When configured as three DC group subsystems, the DC ports of the sending-end MMC converter stations in the first, second, and third DC group subsystems are connected in series via DC interconnection lines.

[0008] In one possible implementation, the sending-end MMC converter station includes: a sending-end MMC body and a first connecting transformer, and the receiving-end MMC converter station includes: a receiving-end MMC body and a second connecting transformer. The first connecting transformer is connected in series between the AC collection network and the sending-end MMC body, and the second connecting transformer is connected in series between the receiving-end MMC body and the receiving-end AC power grid. Both the sending-end MMC body and the receiving-end MMC body contain multiple sub-converters.

[0009] In one possible implementation, the AC aggregation network includes: internal AC aggregation lines, external AC aggregation lines, and an AC aggregation bus. Each hydropower unit in the hydropower unit cluster is connected through the internal AC collection line, and the hydropower unit cluster is connected to the AC collection bus through the external AC collection line. The AC collection bus is connected to the sending-end MMC converter station.

[0010] According to a second aspect of the embodiments of this specification, a control method for a DC interconnected DC group system based on MMC is provided. This method is based on any of the systems described above, and includes: The control unit cluster generates the first AC signal, which is then collected by the AC collection network and transmitted to the sending end MMC converter station. The sending-end MMC converter station is controlled by a modular multilevel converter method, which converts the first AC signal into a DC signal and transmits it to the receiving-end MMC converter station. The DC ports of the sending-end MMC converter stations in each DC group subsystem complete the mutual allocation of DC power through DC interconnection lines. The control unit at the receiving end MMC converter station inverts the DC signal into a second AC signal and transmits it to the receiving end AC power grid.

[0011] In one possible implementation, the control-end MMC converter station converts the first AC signal into a DC signal, including: Obtain the sending-end arm current and sending-end AC grid strength of the sending-end MMC converter station; Based on the AC grid strength at the sending end, a target control mode is determined, and a sending end current reference value and a sending end reference phase angle are generated according to the target control mode. The sending end current reference value and the sending end reference phase angle are input into a pre-built current controller to obtain the sending end AC voltage component; The current of the sending end bridge arm is input into a pre-built circulating current controller to obtain the sending end compensation amount; The modulation ratio of the sending end is obtained by performing an algebraic sum operation on the AC voltage component of the sending end and the compensation amount of the sending end. Based on the modulation ratio of the transmitting end, a modulation signal of the transmitting end is generated, and the modulation signal of the transmitting end is converted into a trigger pulse signal of the transmitting end; The sending end trigger pulse signal drives the sending end MMC converter station to convert the first AC signal into a DC signal.

[0012] In one possible implementation, determining the target control mode based on the sending-end AC grid strength, and generating the sending-end current reference value and the sending-end reference phase angle according to the target control mode, includes: If the AC grid strength at the sending end is above a preset threshold, the target control mode is determined to be the grid-following control mode. In the grid-following control mode, the active power, reactive power, power reference value, and common bus voltage of the sending end MMC converter station are obtained. Based on the active power, reactive power, and power reference value of the sending end MMC converter station, the sending end current reference value is generated. Based on the common bus voltage of the sending end, the sending end reference phase angle is generated. If the AC grid strength at the sending end is lower than a preset threshold, the target control mode is determined to be the grid construction control mode. In the grid construction control mode, the AC voltage component, AC voltage reference value, active power, and active power reference value of the sending end MMC converter station are acquired. Based on the AC voltage component and AC voltage reference value of the sending end MMC converter station, the sending end current reference value is generated. The active power and active power reference value of the sending end MMC converter station are phase synchronized to generate the sending end reference phase angle.

[0013] In one possible implementation, during the process of controlling the receiving-end MMC converter station to convert the DC signal into a second AC signal, one of the receiving-end MMC converter stations of the DC group subsystem adopts a constant DC voltage control strategy, while the receiving-end MMC converter stations of the other DC group subsystems adopt a constant power control strategy.

[0014] In one possible implementation, the constant DC voltage control strategy includes: Obtain the DC voltage component, DC voltage reference value, reactive power, reactive power reference value, receiving-end bridge arm current, and receiving-end common bus voltage of the receiving-end MMC converter station; The DC voltage component, DC voltage reference value, reactive power, and reactive power reference value of the receiving-end MMC converter station are input into a pre-built DC voltage and reactive power controller to generate the receiving-end current reference value, and the receiving-end reference phase angle is generated based on the common bus voltage of the receiving-end side. The receiving end current reference value and the receiving end reference phase angle are input into a pre-built current controller to obtain the receiving end AC voltage component; The current of the receiving end arm is input into a pre-built circulating current controller to obtain the receiving end compensation amount; The receiving-end AC voltage component and the receiving-end compensation amount are summed algebraically to obtain the receiving-end modulation ratio. Based on the receiving-end modulation ratio, a receiving-end trigger pulse signal is generated, and the receiving-end MMC converter station is driven by the receiving-end trigger pulse signal to convert the DC signal into a second AC signal.

[0015] In one possible implementation, the constant power control strategy includes: Obtain the active power, reactive power, power reference value, receiving-end arm current, and receiving-end common bus voltage of the receiving-end MMC converter station; The active power, reactive power, and power reference value of the receiving-end MMC converter station are input into a pre-built active and reactive power controller to generate the receiving-end current reference value, and the receiving-end reference phase angle is generated based on the receiving-end side common bus voltage of the receiving-end MMC converter station. The receiving end current reference value and the receiving end reference phase angle are input into a pre-built current controller to obtain the receiving end AC voltage component; The current of the receiving end arm is input into a pre-built circulating current controller to obtain the receiving end compensation amount; The receiving-end AC voltage component and the receiving-end compensation amount are summed algebraically to obtain the receiving-end modulation ratio. Based on the receiving-end modulation ratio, a receiving-end trigger pulse signal is generated, and the receiving-end MMC converter station is driven by the receiving-end trigger pulse signal to convert the DC signal into a second AC signal.

[0016] The above-described scheme provided in this manual establishes a DC-side interconnection channel at the sending end by setting up at least two DC group subsystems and connecting the DC ports of each sending-end MMC converter station using DC interconnection lines. This enables flexible mutual support of DC power among the subsystems, effectively optimizes the allocation of cascade hydropower resources, and enhances the hydropower absorption capacity. Relying on a modular multi-level converter scheme to complete AC / DC power conversion, it possesses stronger adaptability to weak power grids compared to traditional converter equipment, ensuring stable system operation under fluctuating grid strength conditions at the sending end. Furthermore, the entire architecture enables integrated operation of hydropower collection, power conversion, long-distance transmission, and cross-system power support, significantly improving the overall system's operational flexibility, power supply reliability, and engineering practicality. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structural architecture of a DC interconnected DC group system based on MMC, provided in one embodiment of this specification; Figure 2 This is a schematic diagram of an example structure of a DC interconnected DC group system based on MMC; Figure 3 This is a schematic diagram of another example structure of a DC interconnected DC group system based on MMC; Figure 4 This is a flowchart illustrating a control method for a DC interconnected DC group system based on MMC, provided in one embodiment of this specification. Figure 5 This is a schematic diagram illustrating the implementation principle of the network switching control strategy; Figure 6 This is a schematic diagram illustrating the implementation principle of a constant DC voltage control strategy. Figure 7 This is a schematic diagram illustrating the implementation principle of the constant power control strategy.

[0018] Figure label: 10. DC subsystem; 101. Hydropower unit cluster; 1011. Hydropower unit; 102. AC collection network; 103. Sending-end MMC converter station; 1031. First connecting transformer; 1032. Sending-end MMC body; 104. Receiving-end MMC converter station; 1041. Receiving-end MMC body; 1042. Second connecting transformer; 105. Receiving-end AC grid; 106. DC interconnection line; 107. DC transmission line; 21. First DC subsystem; 22. Second DC subsystem; 23. Third DC subsystem. Detailed Implementation

[0019] Many specific details are set forth in the following description to provide a full understanding of this specification. However, this specification can be implemented in many other ways than those described herein, and those skilled in the art can make similar extensions without departing from the spirit of this specification. Therefore, this specification is not limited to the specific implementations disclosed below.

[0020] The terminology used in one or more embodiments of this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the one or more embodiments of this specification. The singular forms “a” and “the” as used in one or more embodiments of this specification and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used in one or more embodiments of this specification refers to and includes any or all possible combinations of one or more associated listed items.

[0021] It should be understood that although the terms first, second, etc., may be used to describe various information in one or more embodiments of this specification, such information should not be limited to these terms. These terms are only used to distinguish information of the same type from one another. For example, first may also be referred to as second without departing from the scope of one or more embodiments of this specification, and similarly, second may also be referred to as first. Depending on the context, the word "if" as used herein may be interpreted as "when," "when," or "in response to a determination."

[0022] This specification provides a DC interconnected DC group system based on MMC, and also relates to the control method of the DC interconnected DC group system based on MMC, which will be described in detail in the following embodiments.

[0023] See Figure 1 , Figure 1The diagram illustrates a structural diagram of a DC interconnected DC group system based on MMC according to an embodiment of this specification. The system specifically includes: at least two DC group subsystems 10, each DC group subsystem 10 being used for the collection and transmission of mega-scale hydropower. Each DC group subsystem 10 is equipped with a hydropower unit cluster 101, an AC collection network 102, a sending-end MMC converter station 103, a receiving-end MMC converter station 104, and a receiving-end AC power grid 105.

[0024] The hydropower unit cluster 101 is connected to the sending-end MMC converter station 103 through the AC collection network 102. The sending-end MMC converter station 103 is connected to the receiving-end MMC converter station 104 through the DC transmission line. The receiving-end MMC converter station 104 is connected to the receiving-end AC power grid 105.

[0025] The hydropower unit cluster 101 is used to generate a first AC signal. The first AC signal is collected by the AC collection network 102 and then transmitted to the AC port of the sending-end MMC converter station 103. The sending-end MMC converter station 103 is used to convert the first AC signal into a DC signal based on the modular multilevel converter method and transmit it to the DC port of the receiving-end MMC converter station 104 through its DC port. The receiving-end MMC converter station 104 is used to invert the DC signal into a second AC signal and transmit it to the receiving-end AC power grid 105 through its AC port.

[0026] In this system, the DC ports of the sending-end MMC converter station 103 in each DC group subsystem are connected through the DC interconnection line 106 to form a sending-end DC side interconnection channel for DC power mutual assistance between each DC group subsystem.

[0027] The system provided in this embodiment consists of at least two DC group subsystems connected by DC interconnection lines. The AC power generated by the hydropower unit cluster 101 is transmitted to the sending-end MMC converter station 103 via the AC collection network 102. After the AC-DC conversion is completed, the AC power is transmitted to the receiving-end MMC converter station 104 via the DC transmission line. The receiving-end MMC converter station 104 then inverts the AC power to supply the receiving-end AC power grid 105. On the other hand, the DC ports of the sending-end MMC converter stations 103 of each subsystem are connected by the DC interconnection line 106, thereby establishing a sending-end DC interconnection channel and realizing the mutual allocation of DC power between different subsystems.

[0028] The above system setup breaks the traditional operating mode of independent DC channels that are not interconnected. It relies on the MMC converter station to complete the efficient conversion of hydropower and electricity, and at the same time relies on DC interconnection lines to achieve power mutual assistance, which greatly improves the hydropower absorption capacity of the cascade, the system operation flexibility and the overall power supply reliability.

[0029] In one embodiment, the DC group subsystem can be configured as two or three. Figure 2The diagram illustrates a scenario with two DC group subsystems, in which the DC ports of the sending-end MMC converter stations of the first DC group subsystem 21 and the second DC group subsystem 22 are connected via DC interconnection line 106.

[0030] Figure 3 The diagram illustrates the configuration of three DC group subsystems. When configured as three DC group subsystems, the DC ports of the sending-end MMC converter stations of the first DC group subsystem 21, the second DC group subsystem 22, and the third DC group subsystem 23 are connected in series via DC interconnection line 106.

[0031] In this embodiment, the number of DC group subsystems is limited to two or three. When three DC group subsystems are configured, the DC ports of the sending-end MMC converter stations of the three DC group subsystems are connected in series by DC interconnection lines to form a chain-type DC interconnection topology. The DC network of multiple DC group subsystems can carry out power transmission and mutual assistance in a coordinated manner.

[0032] In practical applications, the number of DC group subsystems can be flexibly selected according to the actual situation such as the scale of the hydropower base and the power transmission layout. The series interconnection topology is simple, has strong engineering implementation, and is suitable for different capacity cascade hydropower collection and transmission scenarios, and can take into account both network diversity and operational stability.

[0033] It should be noted that the number of DC interconnection lines in the entire system can be set according to the number of DC group subsystems, and the overall configuration follows the principle of adjacent series connection and one-to-one layout. The specific setting rules, corresponding topologies, and explanations are as follows: When the system includes a first DC group subsystem and a second DC group subsystem, only one DC interconnection line is set up. This single line directly connects the DC ports of the sending-end MMC converter stations of the two subsystems, establishing a DC channel for interconnection between the two stations and realizing DC power mutual assistance between them.

[0034] When the system includes the first, second, and third DC group subsystems, a total of two DC interconnection lines are set up, adopting a chain topology in series. The first DC interconnection line connects the DC ports of the sending-end MMC converter stations of the first and second DC group subsystems, and the second DC interconnection line connects the DC ports of the sending-end MMC converter stations of the second and third DC group subsystems. The three DC group subsystems form a through DC interconnection network through the two DC interconnection lines, completing the global power distribution.

[0035] Specifically, combined Figure 2 and Figure 3As shown, the hydropower unit cluster 101 comprises multiple hydropower units 1011. Each hydropower unit 1011 can be selected and configured according to the different natural operating conditions of the giant cascade hydropower base with varying water heads and flow rates. The capacity and rated parameters of a single unit can be kept consistent or differentiated. The entire cluster covers the entire output range of the cascade hydropower, meeting the power generation needs under different water inflow conditions. In addition, the hydropower units 1011 can be connected in parallel, with the AC output terminals of each hydropower unit 1011 connected in parallel to the internal AC collection line. This ensures that when a single hydropower unit 1011 is under maintenance or fails, the remaining hydropower units can operate normally, improving the power supply reliability of the hydropower unit cluster.

[0036] In one embodiment, combined with Figure 2 and Figure 3 As shown, the AC aggregation network 102 specifically includes: internal AC aggregation lines, external AC aggregation lines, and AC aggregation bus.

[0037] Each hydropower unit in the hydropower unit cluster is connected through an internal AC collection line. The hydropower unit cluster is connected to the AC collection bus through an external AC collection line, and the AC collection bus is connected to the sending-end MMC converter station.

[0038] In this embodiment, the AC collection network 102 can be divided into three layers. The power units on the generator side are collected through the internal AC collection line. The collected power is then transmitted to the AC collection bus through the external AC collection line, and then the AC collection bus transmits the power to the sending end MMC converter station.

[0039] Through a hierarchical aggregation architecture, the electrical energy of multiple hydropower units can be integrated in an orderly manner. The aggregation lines and busbars have clear division of labor, resulting in high energy aggregation efficiency and a well-organized line layout, which can adapt to the operating conditions of multiple hydropower units working together.

[0040] In one embodiment, combined with Figure 1 , Figure 2 and Figure 3 As shown, the sending-end MMC converter station 103 specifically includes: the sending-end MMC body 1032 and the first connecting transformer 1031, and the receiving-end MMC converter station 104 specifically includes: the receiving-end MMC body 1041 and the second connecting transformer 1042.

[0041] The first connecting transformer 1031 is connected in series between the AC collection network 102 and the sending-end MMC body 1032. The second connecting transformer 1042 is connected in series between the receiving-end MMC body 1041 and the receiving-end AC power grid 105. The sending-end MMC body 1032 is connected to the receiving-end MMC body 1041 through the DC transmission line 107. Both the sending-end MMC body 1032 and the receiving-end MMC body 1041 contain multiple sub-converters.

[0042] In this embodiment, the two connecting transformers are respectively arranged between the AC network and the MMC body, which can realize voltage adaptation and electrical isolation. The MMC body, which is composed of multi-segment converters, serves as the core unit and can complete AC and DC power conversion.

[0043] The above scheme can effectively isolate faults and match voltage levels through two-part connection transformers, protecting the MMC main equipment; the modular structure of the MMC main body facilitates operation, maintenance and capacity expansion, while ensuring the quality of power conversion, further improving the operational safety and adaptability of the MMC converter station.

[0044] In practical applications, each DC group subsystem can adopt true bipolar wiring. The DC neutral bus of the sending-end MMC converter station and the receiving-end MMC converter station of each DC group subsystem are connected to the grounding electrode through the grounding electrode line, thereby ensuring the operational reliability of the DC group subsystem. In addition, the receiving-end AC grid of each DC group subsystem can belong to different provincial or regional grids, or partly belong to the same grid zone, to adapt to the needs of inter-regional interconnection.

[0045] The system provided in this embodiment, relying on the DC interconnection architecture and the flexible control characteristics of MMC, significantly improves the collaborative operation capability and power supply reliability of the DC group system, and can provide a reliable technical solution for the efficient absorption and stable transmission of giant cascade hydropower.

[0046] Corresponding to the above system embodiments, this specification also provides embodiments of a control method for a DC interconnected DC group system based on MMC. Figure 4 A flowchart illustrating a control method for a DC interconnected DC group system based on MMC, according to one embodiment of this specification, is shown. Figure 4 As shown, the method specifically includes: Step 310: Control the hydropower unit cluster to generate the first AC signal, and transmit it to the sending end MMC converter station after being collected by the AC collection network.

[0047] Step 320: The sending-end MMC converter station is controlled to convert the first AC signal into a DC signal based on the modular multilevel converter method and transmit it to the receiving-end MMC converter station. The DC ports of the sending-end MMC converter stations in each DC group subsystem complete the mutual allocation of DC power through DC interconnection lines.

[0048] Step 330: Control the receiving-end MMC converter station to convert the DC signal into a second AC signal and transmit it to the receiving-end AC power grid.

[0049] This embodiment relies on the aforementioned system to carry out overall control. First, the hydropower units are controlled to generate electricity in clusters. The generated electricity is transmitted to the sending-end MMC converter station via an AC collection network. Then, the sending-end MMC converter station is controlled to complete the AC-DC conversion. At the same time, the power mutual assistance between subsystems is completed through the DC interconnection line. Finally, the receiving-end MMC converter station is controlled to invert the DC power into AC power and transmit it to the receiving-end AC power grid, thus completing the full-link power transmission and regulation.

[0050] The above-mentioned method sets up an integrated control logic for the entire process, which links power conversion, power mutual assistance, grid connection and power transmission, ensuring the orderly and coordinated operation of the entire DC group system and realizing the large-scale and stable transmission of giant cascade hydropower.

[0051] In one embodiment, the sending-end MMC converter stations of each DC group subsystem adopt a grid-based adaptive switching control strategy to control the sending-end MMC converter stations to convert the first AC signal into a DC signal, specifically including: First, obtain the sending-end arm current and the sending-end AC grid strength of the sending-end MMC converter station.

[0052] Then, based on the AC grid strength at the sending end, the target control mode is determined, and the sending end current reference value and sending end reference phase angle are generated according to the target control mode. Specifically, this process mainly includes: If the AC grid strength at the sending end is above a preset threshold, the target control mode is determined to be grid-following control mode. In grid-following control mode, the active power, reactive power, power reference value, and common bus voltage of the sending end MMC converter station are obtained. Based on the active power, reactive power, and power reference value of the sending end MMC converter station, the sending end current reference value is generated. Based on the common bus voltage of the sending end, the sending end reference phase angle is generated.

[0053] If the AC grid strength at the sending end is lower than a preset threshold, the target control mode is determined to be the grid construction control mode. In the grid construction control mode, the AC voltage component, AC voltage reference value, active power, and active power reference value of the sending end MMC converter station are acquired. Based on the AC voltage component and AC voltage reference value of the sending end MMC converter station, the sending end current reference value is generated. The active power and active power reference value of the sending end MMC converter station are phase synchronized to generate the sending end reference phase angle.

[0054] This embodiment uses a preset grid strength threshold as the criterion. When the grid strength meets the standard, the grid-following control mode is activated, which generates current reference values ​​and reference phase angles based on active and reactive power and bus voltage to adapt to strong grid conditions. When the grid strength is insufficient, the control mode is switched to grid-building mode, which completes parameter calculation and phase synchronization based on voltage components and active power to adapt to weak grid conditions.

[0055] The above scheme enables adaptive switching between grid-following and grid-connecting modes, solves the problem that traditional converters cannot adapt to weak AC power grids, significantly improves the system's adaptability to power grids of different strengths at the sending end, and thus avoids the problem of operational instability under weak grid conditions.

[0056] Next, the sending end current reference value and the sending end reference phase angle are input into a pre-built current controller to obtain the sending end AC voltage component.

[0057] At the same time, the current of the sending end bridge arm is input into a pre-built circulating current controller to obtain the sending end compensation amount.

[0058] Subsequently, the AC voltage component at the sending end and the compensation amount at the sending end are summed algebraically to obtain the modulation ratio at the sending end.

[0059] Furthermore, based on the modulation ratio of the sending end, a modulation signal of the sending end is generated, and the modulation signal of the sending end is converted into a trigger pulse signal of the sending end.

[0060] Finally, the sending-end MMC converter station is driven by the sending-end trigger pulse signal to convert the first AC signal into a DC signal.

[0061] This embodiment collects the arm current of the sending-end MMC converter station and the grid strength in real time, and generates a current reference value and a reference phase angle by combining the grid operating conditions. The AC voltage component is output by the current controller. The arm current is used to generate a compensation quantity through the circulating current controller. The modulation ratio is obtained by calculating the two types of output quantities, and then the modulation signal and trigger pulse are generated in sequence to drive the sending-end MMC converter station to complete the AC-DC conversion.

[0062] Combination Figure 5 As shown, the network-based handover control strategy specifically includes network-based control, network-based control, handover control, circulating current control, modulation strategy, and triggering logic control.

[0063] The grid-following control system includes active and reactive power control, current control, and grid-following phase angle generation, adapting to strong AC grid conditions. The active and reactive power control system uses the active power P from the sending-end MMC converter station and its reference value P0. ref Reactive power Q and its reference value Q ref Generate d-axis current reference value I dref and q-axis current reference value I qref. As a reference value for the sending-end current; in the current control loop, I dref and I qref The current controller generates the three-phase AC voltage (A, B, and C), which is the AC voltage component at the sending end. The phase angle generation stage refers to the generation of the sending-end reference phase angle θ, which acts on the current control stage, through the common bus voltage Us on the sending end side.

[0064] The grid control system includes AC voltage control, current control, and phase angle generation, adapting to weak AC grid conditions. The AC voltage control stage is used to generate the phase angle based on the AC voltage d. q0 d-axis voltage U in coordinate system sd and its reference value U sdref q-axis voltage U sq and its reference value U sqref Generate d-axis current reference value I dref and q-axis current reference value I qref As a reference value for the sending-end current; in the current control loop, I dref and I qref The current controller generates three-phase AC voltage (A, B, and C), i.e., the AC voltage component U at the sending end. x The phase angle generation stage refers to the active power P and its reference value P generated by the MMC. ref The phase synchronization process generates a reference phase angle θ at the sending end, which is then applied to the current control circuit.

[0065] The switching control loop is used to switch between the grid-following control loop and the grid-building control loop. It is achieved by switching the output ports of the current control loop in the grid-following control loop and the grid-building control loop, thereby generating AC voltages for the three phases abc.

[0066] The circulating current control link refers to the upper and lower arm currents I of the MMC converter station at the sending end. px and I nx The input is sent to the circulating current controller, which is used to suppress the second-harmonic circulating current within the MMC. The outputs of the current control and circulating current control components in the grid-connected or network-forming control circuit are algebraically summed to generate the modulation ratio M of the AC voltage on the d-axis and q-axis. d and M q The modulation ratio at the sending end is used to generate the modulation signal at the sending end through the modulation strategy stage, and then the trigger pulse of the switching device of each submodule of the MMC is generated through the trigger logic stage, which serves as the trigger pulse signal at the sending end.

[0067] The above solution integrates current control, circulating current suppression, and pulse generation throughout the entire process. It can suppress the second-harmonic circulating current inside the MMC in real time, ensure the safe operation of the converter, and enable the sending-end MMC converter station to smoothly complete the power conversion. It can adapt to complex operating conditions.

[0068] In one embodiment, during the process of controlling the receiving-end MMC converter station to convert a DC signal into a second AC signal, the receiving-end MMC converter station of one DC group subsystem adopts a constant DC voltage control strategy, while the receiving-end MMC converter stations of the other DC group subsystems adopt a constant power control strategy.

[0069] In this embodiment, among multiple receiving-end MMC converter stations, one converter station is selected to implement a constant DC voltage control strategy to provide voltage support for the entire DC network. The remaining receiving-end MMC converter stations adopt a constant power control strategy to allocate and transmit DC power on demand. This division of labor control mode can stabilize the voltage level of the entire DC system while achieving flexible power allocation. It balances the voltage stability of the DC network with the flexibility of power dispatch, ensuring the safe operation of the system under multi-subsystem coordinated power transmission conditions.

[0070] In one embodiment, the constant DC voltage control strategy specifically includes: First, obtain the DC voltage component, DC voltage reference value, reactive power, reactive power reference value, receiving-end bridge arm current, and receiving-end common bus voltage of the receiving-end MMC converter station.

[0071] Then, the DC voltage component, DC voltage reference value, reactive power, and reactive power reference value of the receiving-end MMC converter station are input into the pre-built DC voltage and reactive power controller to generate the receiving-end current reference value, and the receiving-end reference phase angle is generated based on the common bus voltage on the receiving-end side.

[0072] Subsequently, the receiving end current reference value and the receiving end reference phase angle are input into a pre-built current controller to obtain the receiving end AC voltage component.

[0073] At the same time, the current of the receiving end bridge arm is input into a pre-built circulating current controller to obtain the receiving end compensation amount.

[0074] Furthermore, the receiving-end AC voltage component and the receiving-end compensation are algebraically summed to obtain the receiving-end modulation ratio.

[0075] Finally, based on the receiving-end modulation ratio, a receiving-end trigger pulse signal is generated, and the receiving-end MMC converter station is driven by the receiving-end trigger pulse signal to convert the DC signal into a second AC signal.

[0076] Combination Figure 6 As shown, the constant DC voltage control strategy includes DC voltage and reactive power control, current control, circulating current control, modulation strategy, and trigger logic control. The DC voltage and reactive power control is used to control the DC voltage U at the receiving-end MMC converter station. dc and its reference value U dcref Reactive power Q and its reference value Q ref Generate d-axis current reference value I dref and q-axis current reference value I qref As a reference value for the receiving-end current; in the current control loop, I dref and I qrefThe current controller generates three-phase AC voltages (abc) as the receiving-end AC voltage components. Simultaneously, the receiving-end reference phase angle θ, acting on the current control loop, is generated from the receiving-end common bus voltage Us. In the circulating current control loop, the upper and lower arm currents I of the receiving-end MMC converter station... px and I nx The input is sent to the circulating current controller, which suppresses the second-harmonic circulating current within the MMC. The outputs of the current control and circulating current control circuits are algebraically summed to generate the modulation ratio M of the AC voltage on the d-axis and q-axis. d and M q As the receiving-end modulation ratio, the receiving-end modulation ratio generates a modulation signal through the modulation strategy stage, and then forms the trigger pulses of the switching devices of each sub-module of MMC through the trigger logic stage, which serve as the receiving-end trigger pulse signal.

[0077] This embodiment, through a constant DC voltage control strategy, can accurately manage DC voltage and reactive power, simultaneously suppress circulating current within the converter, reliably undertake the DC voltage stabilization function of the entire network, and thus provide a stable voltage reference for the entire DC interconnection network.

[0078] In one embodiment, the constant power control strategy specifically includes: First, obtain the active power, reactive power, power reference value, receiving-end arm current, and receiving-end common bus voltage of the receiving-end MMC converter station.

[0079] Then, the active power, reactive power, and power reference value of the receiving-end MMC converter station are input into the pre-built active and reactive power controller to generate the receiving-end current reference value, and the receiving-end reference phase angle is generated based on the receiving-end common bus voltage of the receiving-end MMC converter station.

[0080] Next, the receiving end current reference value and the receiving end reference phase angle are input into a pre-built current controller to obtain the receiving end AC voltage component.

[0081] At the same time, the current of the receiving end bridge arm is input into a pre-built circulating current controller to obtain the receiving end compensation amount.

[0082] Furthermore, the receiving-end AC voltage component and the receiving-end compensation are algebraically summed to obtain the receiving-end modulation ratio.

[0083] Finally, based on the receiving-end modulation ratio, a receiving-end trigger pulse signal is generated, and the receiving-end MMC converter station is driven by the receiving-end trigger pulse signal to convert the DC signal into a second AC signal.

[0084] Combination Figure 7As shown, the constant power control strategy specifically includes active and reactive power control loops, current control loops, circulating current control loops, modulation strategies, and triggering logic control loops. The active and reactive power control loops are used to determine the active power P at the receiving-end MMC converter station and its reference value P. ref Reactive power Q and its reference value Q ref Generate d-axis current reference value I dref and q-axis current reference value I qref As the reference value for the receiving-end current, the remaining current control loop, circulating current control loop, modulation strategy, and trigger logic control loop are related to... Figure 6 The constant DC voltage control strategies shown are basically the same, so I won't go into too much detail here.

[0085] In this embodiment, under a constant power control strategy, active and reactive power, bridge arm current, and bus voltage are collected at the receiving end. A current reference value is generated by the active and reactive power controllers, and a reference phase angle is determined by combining this with the bus voltage. After current control, circulating current suppression, and modulation calculations, a trigger pulse is generated to drive the converter to complete the inverter power transmission according to the set power. This scheme allows for precise control of the active and reactive power output at the receiving end, and, in conjunction with the constant DC voltage control unit, enables flexible power scheduling, meeting the power access requirements of different receiving-end grids and improving the precision of system power allocation.

[0086] The above is an illustrative scheme of a control method for a DC interconnected DC group system based on MMC according to this embodiment. It should be noted that the technical solution of this control method for a DC interconnected DC group system based on MMC belongs to the same concept as the technical solution of the aforementioned DC interconnected DC group system based on MMC. Details not described in detail in the technical solution of the control method for a DC interconnected DC group system based on MMC can be found in the description of the aforementioned technical solution of the DC interconnected DC group system based on MMC.

[0087] It should be noted that, for the sake of simplicity, the foregoing method embodiments are all described as a series of actions. However, those skilled in the art should understand that the embodiments in this specification are not limited to the described order of actions, because according to the embodiments in this specification, some steps can be performed in other orders or simultaneously. Furthermore, those skilled in the art should also understand that the embodiments described in this specification are all preferred embodiments, and the actions and modules involved are not necessarily essential to the embodiments in this specification.

[0088] In the above embodiments, 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.

[0089] The preferred embodiments disclosed above are merely illustrative of this specification. The optional embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the embodiments described herein. These embodiments are selected and specifically described in this specification to better explain the principles and practical applications of the embodiments, thereby enabling those skilled in the art to better understand and utilize this specification. This specification is limited only by the claims and their full scope and equivalents.

Claims

1. A DC interconnected DC group system based on MMC, characterized in that, include: At least two DC group subsystems, each of which is equipped with a cluster of hydropower units, an AC collection network, a sending-end MMC converter station, a receiving-end MMC converter station, and a receiving-end AC power grid; The hydropower unit cluster is connected to the sending-end MMC converter station through the AC collection network. The sending-end MMC converter station is connected to the receiving-end MMC converter station through a DC transmission line. The receiving-end MMC converter station is connected to the receiving-end AC power grid. The hydropower unit cluster is used to generate a first AC signal. The sending-end MMC converter station is used to convert the first AC signal into a DC signal based on a modular multilevel converter and transmit it to the receiving-end MMC converter station. The receiving-end MMC converter station is used to convert the DC signal into a second AC signal and transmit it to the receiving-end AC power grid. In this system, the DC ports of the sending-end MMC converter stations within each DC group subsystem are connected via DC interconnection lines to form a sending-end DC-side interconnection channel for mutual DC power exchange between the various DC group subsystems.

2. The system according to claim 1, characterized in that, The DC group subsystem is configured to be two or three; When configured as three DC group subsystems, the DC ports of the sending-end MMC converter stations in the first, second, and third DC group subsystems are connected in series via DC interconnection lines.

3. The system according to claim 1, characterized in that, The sending-end MMC converter station includes: the sending-end MMC body and the first connecting transformer; the receiving-end MMC converter station includes: the receiving-end MMC body and the second connecting transformer. The first connecting transformer is connected in series between the AC collection network and the sending-end MMC body, and the second connecting transformer is connected in series between the receiving-end MMC body and the receiving-end AC power grid. Both the sending-end MMC body and the receiving-end MMC body contain multiple sub-converters.

4. The system according to claim 1, characterized in that, The AC aggregation network includes: internal AC aggregation lines, external AC aggregation lines, and AC aggregation busbars; Each hydropower unit in the hydropower unit cluster is connected through the internal AC collection line, and the hydropower unit cluster is connected to the AC collection bus through the external AC collection line. The AC collection bus is connected to the sending-end MMC converter station.

5. A control method for a DC interconnected DC group system based on MMC, characterized in that, Based on the system according to any one of claims 1 to 4, the method comprises: The control unit cluster generates the first AC signal, which is then collected by the AC collection network and transmitted to the sending end MMC converter station. The sending-end MMC converter station is controlled by a modular multilevel converter method, which converts the first AC signal into a DC signal and transmits it to the receiving-end MMC converter station. The DC ports of the sending-end MMC converter stations in each DC group subsystem complete the mutual allocation of DC power through DC interconnection lines. The control unit at the receiving end MMC converter station inverts the DC signal into a second AC signal and transmits it to the receiving end AC power grid.

6. The method according to claim 5, characterized in that, The control-end MMC converter station converts the first AC signal into a DC signal, including: Obtain the sending-end arm current and sending-end AC grid strength of the sending-end MMC converter station; Based on the AC grid strength at the sending end, a target control mode is determined, and a sending end current reference value and a sending end reference phase angle are generated according to the target control mode. The sending end current reference value and the sending end reference phase angle are input into a pre-built current controller to obtain the sending end AC voltage component; The current of the sending end bridge arm is input into a pre-built circulating current controller to obtain the sending end compensation amount; The modulation ratio of the sending end is obtained by performing an algebraic sum operation on the AC voltage component of the sending end and the compensation amount of the sending end. Based on the modulation ratio of the transmitting end, a modulation signal of the transmitting end is generated, and the modulation signal of the transmitting end is converted into a trigger pulse signal of the transmitting end; The sending end trigger pulse signal drives the sending end MMC converter station to convert the first AC signal into a DC signal.

7. The method according to claim 6, characterized in that, The step of determining the target control mode based on the sending-end AC grid strength, and generating the sending-end current reference value and the sending-end reference phase angle according to the target control mode, includes: If the AC grid strength at the sending end is above a preset threshold, the target control mode is determined to be the grid-following control mode. In the grid-following control mode, the active power, reactive power, power reference value, and common bus voltage of the sending end MMC converter station are obtained. Based on the active power, reactive power, and power reference value of the sending end MMC converter station, the sending end current reference value is generated. Based on the common bus voltage of the sending end, the sending end reference phase angle is generated. If the AC grid strength at the sending end is lower than a preset threshold, the target control mode is determined to be the grid construction control mode. In the grid construction control mode, the AC voltage component, AC voltage reference value, active power, and active power reference value of the sending end MMC converter station are acquired. Based on the AC voltage component and AC voltage reference value of the sending end MMC converter station, the sending end current reference value is generated. The active power and active power reference value of the sending end MMC converter station are phase synchronized to generate the sending end reference phase angle.

8. The method according to claim 5, characterized in that, During the process of controlling the receiving-end MMC converter station to convert the DC signal into a second AC signal, one of the receiving-end MMC converter stations of the DC group subsystem adopts a constant DC voltage control strategy, while the receiving-end MMC converter stations of the other DC group subsystems adopt a constant power control strategy.

9. The method according to claim 8, characterized in that, The constant DC voltage control strategy includes: Obtain the DC voltage component, DC voltage reference value, reactive power, reactive power reference value, receiving-end arm current, and receiving-end common bus voltage of the receiving-end MMC converter station; The DC voltage component, DC voltage reference value, reactive power, and reactive power reference value of the receiving-end MMC converter station are input into a pre-built DC voltage and reactive power controller to generate the receiving-end current reference value, and the receiving-end reference phase angle is generated based on the common bus voltage of the receiving-end side. The receiving end current reference value and the receiving end reference phase angle are input into a pre-built current controller to obtain the receiving end AC voltage component; The current of the receiving end arm is input into a pre-built circulating current controller to obtain the receiving end compensation amount; The receiving-end AC voltage component and the receiving-end compensation amount are summed algebraically to obtain the receiving-end modulation ratio. Based on the receiving-end modulation ratio, a receiving-end trigger pulse signal is generated, and the receiving-end MMC converter station is driven by the receiving-end trigger pulse signal to convert the DC signal into a second AC signal.

10. The method according to claim 8, characterized in that, The constant power control strategy includes: Obtain the active power, reactive power, power reference value, receiving-end arm current, and receiving-end common bus voltage of the receiving-end MMC converter station; The active power, reactive power, and power reference value of the receiving-end MMC converter station are input into a pre-built active and reactive power controller to generate the receiving-end current reference value, and the receiving-end reference phase angle is generated based on the receiving-end side common bus voltage of the receiving-end MMC converter station. The receiving end current reference value and the receiving end reference phase angle are input into a pre-built current controller to obtain the receiving end AC voltage component; The current of the receiving end arm is input into a pre-built circulating current controller to obtain the receiving end compensation amount; The receiving-end AC voltage component and the receiving-end compensation amount are summed algebraically to obtain the receiving-end modulation ratio. Based on the receiving-end modulation ratio, a receiving-end trigger pulse signal is generated, and the receiving-end MMC converter station is driven by the receiving-end trigger pulse signal to convert the DC signal into a second AC signal.

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