Direct current power flow controller between high-voltage direct current network lines based on three-bridge-arm MMC (Modular Multilevel Converter)

Abstract

The invention relates to a high-voltage direct-current network interline direct-current power flow controller based on a three-bridge-arm MMC, which comprises a 3Arm-MMC circuit and a control module which are connected with each other, the 3Arm-MMC circuit is a multi-port network and comprises a direct-current bus capacitor and a converter which are connected with each other, the converter comprises converter branches, the number of the converter branches is the same as that of ports, and the converter branches are connected with the control module. Each converter branch comprises an upper arm, a middle arm and a lower arm which are connected to the midpoint of the converter branch, each middle arm provides the port outwards, and each arm comprises N full-bridge sub-modules which are in cascade connection, the control module obtains PWM pulse signals of all the full-bridge sub-modules through external power flow control, internal power flow control and module capacitance balance control. Compared with the prior art which is mainly composed of a two-stage topological structure, the distributed control is carried out on the multi-branch three-bridge-arm MMC converter connected among a plurality of power transmission lines in the high-voltage direct-current power transmission network, so that higher output voltage can be provided.

Description

Technical Field

[0001] This invention relates to the field of DC power flow control technology, and in particular to an inter-line DC power flow controller for high voltage DC networks based on a three-arm MMC. Background Technology

[0002] DC power transmission through the power grid encounters technical challenges related to unbalanced power distribution and inefficiency within the system. This can be due to uneven current distribution, inaccurate power control, voltage regulation limitations, and issues related to handling power fluctuations. These problems lead to system instability, low efficiency, and difficulty in maintaining the power levels required for normal operation. In low-voltage DC power systems, common challenges include power losses, voltage drops on transmission lines, and limited transmission capacity. As the system voltage level decreases, higher voltage drops occur, impacting power transmission efficiency. The presence of conductor resistance results in greater resistive losses at lower voltages, reducing overall system efficiency and causing component overheating. Low-voltage systems are also limited in long-distance transmission, further complicating stability management. Therefore, managing voltage fluctuations and ensuring system stability are crucial, especially under conditions of constantly changing loads or power demands. Low-voltage topologies typically employ a single-stage design, with voltage limits generally below 5000V.

[0003] Flexible DC transmission systems based on voltage source converters (VSCs) have attracted significant attention from academia due to their superior characteristics. These characteristics include: small footprint, decoupling of active and reactive power, and seamless integration with passive systems. Furthermore, compared to line-commutated converters (LCCs), VSCs maintain constant DC voltage polarity even when system power flow is reversed. Therefore, they offer a crucial possibility for establishing multiterminal DC (MTDC) transmission systems, characterized by improved system reliability, multifunctional power flow control, efficient utilization of converters and cables, and better economic efficiency.

[0004] Furthermore, due to advancements in voltage source converter (VSC) technology, modular multilevel converters (MMCs) have become the dominant converter topology for medium- and high-voltage applications. Compared to two-level VSCs and other multilevel converter designs, MMCs stand out due to their modular structure, high efficiency, superior power quality, and excellent fault tolerance. In fact, the industry increasingly favors using modular multilevel converters (MMCs) as the fundamental component for developing multi-terminal direct current (MTDC) transmission systems. This concept has been successfully applied in several practical scenarios, such as the Zhoushan five-terminal flexible DC transmission project and the Nan'ao wind farm three-terminal flexible DC transmission project, achieving significant progress in this field.

[0005] The main structure of an MMC-MTDC transmission system is a mesh structure. However, without power flow control and regulation within the system, unavoidable energy losses and even system instability will result. Therefore, the adoption of a DC control power flow controller (DCPFC) is imperative. It plays a crucial role in improving the performance of MMC-MTDC transmission systems and DC grids, driving significant progress in this field.

[0006] Currently, much research has been conducted in academia on DC power flow control (DCCPFC). Traditional methods for power flow control in DC grids include adjusting line impedance or regulating the voltage at the nodes at both ends of the DC grid line, such as... Figure 1 The variable series resistor scheme shown in (1a) is used. Although the series variable resistor scheme can directly adjust the topology and control structure, they can only adjust the equivalent resistance on the feeder in one direction. This limitation hinders their adjustment capability, increases system operating losses, and often requires additional cooling equipment, increasing costs.

[0007] To adjust DC voltage, one can use, such as Figure 1 The DC transformer shown in (1b) or as Figure 1 The series adjustable DC voltage source shown in (1c) enables bidirectional power flow regulation. When a DC transformer is connected to a power grid with similar or different voltage levels, it can change the power flow distribution by adjusting the voltage ratio. Simultaneously, it can also achieve fault isolation and decoupled operation between connected power grids. Power flow control methods using series adjustable DC voltage sources can be divided into two subcategories: independent adjustable DC voltage sources and inter-line power flow controllers. Series independent adjustable DC voltage sources, due to the typically low resistance of DC lines, only require small changes in DC voltage to regulate line current. This allows for flexible power flow control with lower voltage and capacity, resulting in relatively lower system cost and losses. However, it requires an external power supply and high-voltage isolation, increasing the cost and structural complexity of the DCPFC equipment.

[0008] To eliminate the need for external power supplies and high-voltage isolation equipment, some literature has introduced, for example... Figure 1 The IDCPFC shown in (1d) enables power exchange and flow regulation between different lines without energy interaction with an external network. As illustrated in the dual-capacitor IDCPFC described in the paper "An interline DC power-flow controller (IDCPFC) for multiterminal HVDC system" (W. Chen, X. Zhu, and L. Yao, IEEE Trans. Power Del., vol. 30, no. 4, pp. 2027–2036, Aug. 2015), independent capacitors are integrated in each of the two lines as stable voltage sources, avoiding capacitor switching. However, energy exchange between capacitors relies on DC transformers or coupling inductors, increasing the size and construction cost of the device.

[0009] exist Figure 1 (1e) shows a multi-terminal DC-CPFC (MT-DCPFC) that combines the features of both self-balancing and external power supply types. It achieves power balancing through interaction with the main converter, expanding the DC current regulation range without the need for an additional power supply. However, this scheme still relies on an isolation transformer for electrical isolation, which limits its application.

[0010] Overall, previous research and methods still have the following shortcomings:

[0011] -Due to its single-stage design and two-level topology, the traditional IDCPFC topology cannot provide a high output voltage.

[0012] Traditional two-level topologies limit the voltage to below 5000V, restricting their use in higher voltage networks and affecting transient power control.

[0013] These existing methods are sometimes adapted for high-voltage applications by integrating transformers into complex designs. Summary of the Invention

[0014] The purpose of this invention is to overcome the defects of the prior art and provide a DC power flow controller based on a three-arm MMC for high-voltage DC transmission networks, which realizes power distribution control between multiple transmission lines in a high-voltage DC transmission network.

[0015] The objective of this invention can be achieved through the following technical solutions:

[0016] A high-voltage DC network line-to-line DC power flow controller based on a three-arm MMC includes a 3Arm-MMC circuit and a control module connected in series. The 3Arm-MMC circuit is a multi-port network, including a DC bus capacitor and a converter connected in series. The converter includes a converter branch with the same number of converter branches as the number of ports. Each converter branch includes an upper arm, a middle arm, and a lower arm connected to the midpoint of the converter branch. Each middle arm provides the port to the outside. Each arm includes N cascaded full-bridge sub-modules. The control module obtains the PWM pulse signal of each full-bridge sub-module through external power flow control, internal power flow control, and module capacitor balance control.

[0017] Furthermore, each of the arms also includes a series inductor connected to the cascaded full-bridge submodule.

[0018] Furthermore, each of the full-bridge submodules has a zero state, a negative state, and a positive state.

[0019] Furthermore, the external power flow control includes DC link voltage balancing control and current control for balancing the power flow of multiple transmission lines, wherein the DC link voltage balancing control obtains a DC link voltage balancing control signal, the current control obtains a current control signal, and generates first voltage information for controlling the PWM pulse signal for the upper and lower arms based on the combination of the DC link voltage balancing control signal and the current control signal.

[0020] Furthermore, the DC link voltage balance control signal is obtained by PI control of the error signal between the DC bus voltage and the reference voltage.

[0021] Furthermore, the current control is implemented in at least two branches, and the current control signal is obtained based on PI control of the error signal between the current of the selected branch and the reference current.

[0022] Furthermore, the internal power flow control includes feedforward DC current control and inter-arm AC current control. The feedforward DC current control controls the common-mode current, causing the total power of each branch to converge to zero, generating a feedforward DC current control signal. The inter-arm AC current control is used to control the power output of each arm in each branch, gradually reducing it to zero, generating an inter-arm AC current control signal. Based on the combination of the feedforward DC current control signal and the inter-arm AC current control signal, a second voltage information is generated to control the PWM pulse signal for the upper and lower arms. A differential AC voltage for the middle arm is generated based on the reference voltage and the sinusoidal signal. This differential AC voltage is used to control the PWM pulse signal for the middle arm.

[0023] Furthermore, the AC current control between the bridge arms is achieved by introducing zero-sequence and positive-sequence voltages.

[0024] Furthermore, the module capacitor balance control includes differential mode control and common mode control. The voltage changes between the upper arm, lower arm, and middle arm of each branch are controlled to achieve balanced power distribution. The common mode control adjusts the shared common mode voltage of the upper arm, lower arm, and middle arm of each branch to be consistent, generating third voltage information for controlling the PWM pulse signal.

[0025] Furthermore, when acquiring the PWM pulse signal of each of the full-bridge submodules, the common-mode signal and the differential-mode signal are allocated in different ways for the upper arm and the lower arm, while the common-mode signal and the differential-mode signal are allocated in the same way for the middle arm.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] 1. Traditional IDCPFC topologies, due to their single-stage design and two-level topology, cannot provide high output voltages. These topologies limit the voltage to below 5000V, restricting their application in higher voltage networks and affecting transient power control. This invention, through distributed control of multi-branch three-arm MMC converters connected between multiple transmission lines in a HVDC transmission network, can provide high voltage levels to achieve power distribution control between multiple transmission lines within the HVDC network, making it a potential candidate method for high-voltage and high-power applications. This invention's Multi-Terminal Interleaved DC Power Flow Controller (MT-IDCPFC) based on a 3A-MMC topology achieves specific power flow control across multiple terminal lines, ensuring efficient energy transmission in HVDC networks.

[0028] 2. This invention utilizes multiple control loops to achieve power balance and module voltage balance, thereby solving problems caused by continuous and asymmetrical current flow in the line. A significant advantage of this invention is that it eliminates the need for transformers to ensure power balance within the power grid.

[0029] 3. The control method of this invention can effectively manage line current and ensure DC bus voltage balance across the entire network and submodule capacitors. Employing zero-sequence and positive-sequence voltage injection methods and using a 3A-MMC inter-arm AC current controller, it does not induce any AC current in the AC transmission system. This design emphasizes simplicity, robustness, cost-effectiveness, and minimal power loss.

[0030] 4. The present invention is characterized by simplicity, robustness, low cost, high power density and high efficiency. The proposed MT-IDCPFC has a simple structure, a wide operating range, bidirectional power flow control, and convenient port expansion. Attached Figure Description

[0031] Figure 1For the existing DC power flow control scheme, (1a) add a variable series resistor, (1b) add a DC transformer, (1c) add an adjustable DC voltage source, (1d) IDCPFC, and (1e) MT-IDCPFC;

[0032] Figure 2 This is a schematic diagram of the MT-IDCPFC topology proposed by the present invention based on MMC;

[0033] Figure 3 The common-mode and differential-mode operation modes of the MMC system are defined as follows: (3a) is the common-mode operation mode, and (3b) is the differential-mode operation mode.

[0034] Figure 4 This is a system block diagram of the controller of the present invention;

[0035] Figure 5 This is a schematic diagram of the control principle of the control module and the structure of the sub-modules of the present invention, wherein (5a) is the control principle of the control module and (5b) is the sub-module;

[0036] Figure 6 This invention relates to the voltage control of the MT-IDCPFC.

[0037] Figure 7 This invention relates to the current control of the MT-IDCPFC.

[0038] Figure 8 This is a schematic diagram of the signal generator of the present invention that generates an input PWM signal based on the combination of voltage and current output signals;

[0039] Figure 9 A feedforward DC controller for 3-arm-MMC systems;

[0040] Figure 10 An inter-arm AC current controller for a 3-arm-MMC system is provided, wherein (10a) is the controller structure and (10b) is the controller output signal.

[0041] Figure 11 The AC voltage difference of the intermediate arm;

[0042] Figure 12 This is a schematic diagram of the differential mode voltage balance of the submodule, where (12a) is the upper arm, (12b) is the lower arm, and (12c) is the middle arm;

[0043] Figure 13 This is a schematic diagram of voltage distribution and balance of the sub-modules, where (13a) is the upper arm, (13b) is the lower arm, and (13c) is the middle arm;

[0044] Figure 14The diagram shows the power balance of the submodule, where (14a) represents the common-mode power of the upper and lower arms, and (14b) represents the differential-mode power of the upper and lower arms.

[0045] Figure 15 This is a schematic diagram of the combination of common-mode power and differential-mode power, where (15a) is the feedforward DC controller, (15b) is the AC power flow controller, and (15c) is the output signal combination;

[0046] Figure 16 This is a schematic diagram of the control flow of the present invention;

[0047] Figure 17 For the voltage modulation of the MMC module, (17a) is the voltage modulation of the upper arm module, (17b) is the voltage modulation of the lower arm module, and (17c) is the voltage modulation of the middle arm module.

[0048] Figure 18 This is a schematic diagram of the line currents I1, I2, I3 and I4 in the embodiment. Detailed Implementation

[0049] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0050] This embodiment provides a high-voltage direct current (HVDC) network line-to-line DC power flow controller based on a three-arm modular multilevel converter (3A-MMC) and corresponding control strategy. It is a novel multi-terminal line-to-line DC power flow controller, denoted as MT-IDCPFC, capable of providing high voltage levels to achieve power distribution control among multiple transmission lines within a HVDC network. (Reference) Figure 2 As shown, this high-voltage DC network line-to-line DC power flow controller based on a three-arm MMC includes a connected 3Arm-MMC circuit and a control module. The 3Arm-MMC circuit is a multi-port network, including a connected DC bus capacitor and a converter. The converter includes converter branches with the same number of ports as the number of ports. Each converter branch includes an upper arm, a middle arm, and a lower arm connected to the midpoint of the converter branch. Each middle arm provides the port outward. Each arm includes N cascaded full-bridge submodules. The control module acquires the PWM pulse signals of each full-bridge submodule through external power flow control, internal power flow control, and module capacitor balance control. By adding full-bridge branches, this controller can be extended to applications on multiple transmission lines.

[0051] This embodiment considers using an n-port system to implement the MT-IDCPFC method. The MT-IDCPFC system contains n hysteresis bridges, which are connected to a common-mode DC link capacitor C via a multi-line network. x Link. When using 3A-MMC as an IDCPFC, some problems arise due to the continuous and asymmetrical current in the line, leading to voltage balance issues in the MMC submodule. To address these challenges faced by MT-IDCPFC, this embodiment proposes a control method applied to the control module, allowing multiple control loops to simultaneously act as power balancers and module voltage balancers to control the MT-IDCPFC.

[0052] The aforementioned multi-terminal interleaved DC power flow controller (MT-IDCPFC) based on a 3Arm-MMC topology enables multi-power flow control within a multi-line DC grid structure. This circuit is designed for high-current, high-voltage, and high-power applications and does not rely on transformers. Its control method effectively manages line currents, ensuring DC bus voltage balance across the entire network and submodule capacitors. Employing zero-sequence and positive-sequence voltage injection methods and using a 3A-MMC inter-arm AC current controller, it does not induce any AC current in the AC transmission system. This design emphasizes simplicity, robustness, cost-effectiveness, and minimal power loss.

[0053] This embodiment uses a four-port network as an example. Figure 2 The diagram shows an offshore wind farm located at two of the nodes, and a Controlled Source Voltage (CSV) power station located at the other node. Each voltage source converter (VSC), namely VSC1 and VSC2, is connected to one of the offshore wind farms to regulate power flow in the DC grid. VSC3, located onshore, acts as a voltage regulator. These nodes are connected via transmission lines with specific impedances. An MT-IDCPFC is strategically installed between lines 1 and 3 to enable advanced control of power flow in the system.

[0054] Figure 2The 3Arm-MMC circuit, indicated by dashed lines, features a four-port IDPFC. Each phase is connected to the midpoint of its respective converter branch (1, 2, 3, and 4). This four-phase modular multilevel converter comprises eight arms, each consisting of N cascaded full bridge sub-modules (FBSMs). Each bridge includes three arms: an upper arm, a middle arm, and a lower arm. These arms consist of cascaded sub-modules (SMs) and series inductors (Ls). The total number of output voltage levels is N+1. The DC side, referred to as the DC link, has two ports: positive and negative. The upper arm (U) connects to the positive DC port and the midpoint of each converter branch, while the lower arm (L) connects to the negative DC port and the midpoint of each converter branch. The middle arm (M) connects the upper and lower arms.

[0055] Each full-bridge submodule (FBSM) has three states: zero state (switches Q1 and Q2 are off, or Q3 and Q4 are off), negative state (switches Q2 and Q3 are on, and switches Q1 and Q4 are off), and positive state (switches Q1 and Q4 are on, and switches Q2 and Q3 are off). Each mode has a specific function: positive mode generates a positive DC voltage, negative mode generates a reverse current flow, and zero-voltage mode operation prevents DC faults, thus enabling the MMC to function as a STATCOM. The output voltage (Vsm) of the SM varies according to the specific switching states listed in Table 1. Switching between these states allows control of the unit battery output to obtain the desired voltage waveform.

[0056] Table 1 Sub Module Switching Status

[0057]

[0058]

[0059] The capacitors in the submodule are called floating capacitors because they do not require an external power source to operate. Ensuring that the charging and discharging of the capacitors are balanced over a period of time is crucial for maintaining a consistent voltage within the submodule. This balance is essential for the efficient operation of the inverter. Capacitor balancing also plays a vital role in minimizing the circulating current generated during operation.

[0060] To simplify the analysis, this section will focus on each arm with n submodules. To explain the operational concept of MMC, phase 1 will be used as an example. Figure 2 In the middle, V cx Indicates the DC link voltage. I U and I LThese represent the currents flowing through the upper and lower arms, respectively. Finally, the current I is related to the output current on the line load side. During operation, the common-mode current in the system flows directly from the DC link capacitor C. x After passing through the upper and lower arms, it then returns to the DC link capacitor C. x This cyclic flow is characteristic of the 3A-MMC under certain operating conditions. The differential mode current either flows from the DC link capacitor Cx through the upper arm to the line load side, or from the line load side back to the DC link capacitor Cx through the lower arm. x .

[0061] The analysis identified the common mode and the differential mode, such as Figure 3 As shown. In this case, V cx This represents the DC link voltage, while V U and V L These represent the accumulated voltages of the upper and lower arms, respectively. Additionally, I... c I represents the common-mode current. d This represents the differential-mode current. Common-mode power is distributed between the two arms through a capacitor on a DC link, but differential-mode power generates a varying current, which causes an imbalance in power distribution between the upper and lower arms.

[0062] For common-mode signals, it can be determined according to... Figure 3 The governing equations for (3a) are as follows:

[0063]

[0064] For differential mode signals, it can be determined according to... Figure 3 (3b) establishes the following governing equations:

[0065]

[0066]

[0067] The control block diagram proposed in this invention is as follows: Figure 4 As shown, the design aims to achieve the power flow required for lines 1 and 2, while line 4 maintains the voltage on the capacitor by pooling all power flows. The duty cycle (Qi) of each full bridge (FB) arm is given by formula Q. i =V pwmi / V cuiGiven. Phase-Shift Carrier Pulse Width Modulation (PSC-PWM) ensures a balanced voltage distribution in the 3A-MMC to maintain consistent capacitor voltage, which is crucial for voltage consistency between the upper and lower arm modules. PSC-PWM compares a single reference waveform with a phase-shifted carrier to determine the operating state of the N-stage system unit. Each arm of the converter uses a frequency of f s A triangular carrier signal with a phase shift angle of Δθ, ranging from 0 to 2π / n. By comparing the control signal and the phase-shifted carrier signal, the SM will generate a corresponding... Pulse, such as Figure 5 As shown. The PSC-PWM method in 3A-MMC may have f in each arm s A high-frequency current component is introduced at this point, which will be used later for voltage balance control.

[0068] Assuming the line resistance of the entire network is relatively minimum, and the line voltage V i If P1 / V1 and P3 / V3 are constants, then P1 / V1 and P3 / V3 can be obtained from the constant current source I. A and I B Replacement, such as Figure 4 As shown. The proposed control method aims to maintain the power balance of the transmission line and stabilize the DC link voltage of the capacitor. The proposed control method is divided into different control layers.

[0069] 1. External power flow control layer

[0070] The proposed control method integrates an external power flow controller with two main control layers: the main DC link capacitor C. x DC link voltage V cx Regulating and controlling current flow for balancing multiple transmission lines. In the MMC-based MT-IDCPFC system, the DC link capacitor acts as a power source, ensuring system stability and voltage balance while regulating the DC voltage level of the entire converter.

[0071] a) DC link voltage balance control

[0072] DC link capacitor C x It can store energy, enabling the converter to handle fluctuations in DC voltage levels. It can generate or absorb electrical energy under varying demand conditions. DC link capacitor C x It helps balance the voltage across different arms or modules within the MMC. It ensures a uniform voltage level distribution throughout the entire converter module. Its function is to maintain the required DC voltage level for the system, ensuring the converter's stability and normal operation.

[0073] To maintain voltage balance, the DC link voltage Vcx The adjustment is crucial for multiport converters, such as Figure 6 As shown. From the reference voltage V cx-ref Subtract the measured voltage V across the capacitor from the middle. cx Then the error signal is input to the PI controller to obtain the voltage vector V. p Size. V p With the normalized current vector I g = (I1, I2, I3, I4) multiplied together. The normalized current is shown in formula (20).

[0074] The DC link voltage balance control process is constructed as follows:

[0075]

[0076] In the above formula, K p K i S represents the parameters of the PI controller, and v represents the parameters of the PI controller. pi This represents the DC link voltage balance control signal for the i-th line.

[0077] b) Current control

[0078] In an MT-IDCPFC network, there are typically multiple current controllers, each specifically designed to regulate the current of a single transmission line or port. This embodiment can directly control the current of α-2 lines, while the current of other lines depends on the current on the controlled lines.

[0079] In order to control the current I1 of line 1 in the system, such as Figure 7 As shown. Subtract the reference current I from the measured current I1. 1-ref Then, an error signal ΔI is provided to the PI controller. Therefore, the output of the PI controller will be a voltage V. x According to equation (24), the present invention can define V. x Then multiply it with the current vector I1 to obtain the output V. x I1, then output V x Dividing I1 by the current vector I3 yields n1V. x The divisor of I1 / I3 is equal to the current ratio n1. A negative signal is the current ratio -n1V. x Multiples of.

[0080] The current control process for Line 1 is constructed as follows:

[0081]

[0082] To control the current I2 in line 2 of the network, the reference current I is subtracted from the measured current I2. 2-refThen, an error signal ΔI is provided to the PI controller. Therefore, the output of the PI controller is a voltage vector v. y V y Multiplying by the current vector I2 yields the output V. y I2, output V y Dividing I2 by the current vector I3 yields nV. y The divisor of I2 / I3 is equal to the current ratio n2. A negative signal is the current ratio -n2V. y The minus sign indicates that if the currents are in the same direction and the total power is zero, then the voltage V is a multiple of 1 / 2. x V y The direction should be opposite.

[0083] The current control process for Line 2 is constructed as follows:

[0084]

[0085] -n1V x and -n2V y Adding them together gives V z .

[0086]

[0087] Figure 8 Equations (31)-(34) show the combined output of voltage and current control, i.e., the output V for PWM. eci .

[0088] <![CDATA[V ec1 =V p-1 +V x ]]> (31) <![CDATA[V ec2 =V p-2 +V y ]]> (32) <![CDATA[V ec3 =V p-3 +V z ]]> (33) <![CDATA[V ec4 =V p-4 +V0]]> (34)

[0089] 2. Internal power flow control layer based on DC-AC

[0090] The internal DC-AC power flow control layer has two basic controllers: a feedforward DC power flow controller and an inter-arm AC current controller. The feedforward DC controller maintains stable operation within the specified limits of the common-mode current in each branch, ensuring that the total power is zero. Meanwhile, the inter-arm AC current control method focuses on regulating the power within each arm, gradually reducing the power in all arms to zero.

[0091] a) Feedforward DC power flow controller

[0092] Figure 9 This illustrates a combination of feedforward control methods for common-mode DC current control. The feedforward controller manages the common-mode current, ensuring the system remains stable and operates within the required parameter range. The common-mode current is controlled so that the total power on each line approaches zero. This function is particularly important because the total power in the system approaching zero indicates minimal active power usage or transmission. Under this assumption, the feedforward DC controller law can be expressed as:

[0093]

[0094]

[0095] Feedforward DC controllers help determine the common-mode current, using I c Indicated by measuring current I. c And subtract the reference current I LUc-mea This yields an error signal, which is then sent to the PI controller. The PI controller outputs a voltage vector V. pcj .

[0096] b) Inter-arm AC current controller

[0097] The goal of a feedforward DC current controller is to keep the total power of all lines zero, while the power of a single arm is not zero. For example... Figure 10 As shown, the inter-arm AC current controller monitors the power output of each arm on each branch and gradually reduces it to zero. Due to the DC current controller, an imbalance occurs in the power exchange between the two arms, preventing the required output current from being achieved. Introducing AC current through zero-sequence and positive-sequence injection can solve the limitation of the DC current controller. Unlike traditional topologies, the proposed method involves introducing zero-sequence and positive-sequence voltages to effectively eliminate AC voltage in DC transmission lines. Traditional topologies use DC transformers to manage AC current in the grid. This control mechanism ensures that the power of each submodule is systematically reduced to zero. When the total current rises, the zero-sequence voltage prevents the increase of AC components in the DC transmission network. However, the increase of AC components complicates the stability of DC link voltages, posing challenges to the transmission system. Introducing positive-sequence voltage helps alleviate these AC components, but excessive positive-sequence voltage may lead to an increase in AC voltage levels. To address this issue, introducing negative-sequence voltages in each phase can counteract the effects of AC components, maintain stability, and prevent problems caused by excessive AC voltage.

[0098] The inter-arm AC current controller generates AC current, denoted as I. LUac From the reference current I LUc-mea Subtract the measured current I from the middle LUac This yields the error signal. This error signal is then input into a proportional-resonant (PR) controller to generate a voltage vector V. pcrj As the output, the PR controller effectively reduces steady-state error, suppresses harmonic distortion in the output waveform, and ensures stable output voltage. Under this assumption, the inter-arm AC current controller law can be expressed as:

[0099]

[0100] Inter-arm AC current controller output V pcriand feedforward DC current controller output V pci The merged module forms a common signal V ci In the two arms, the differential-mode power is zero because the differential-mode AC voltage is multiplied by the AC current of the upper and lower arms, then subtracted, resulting in a zero output. In the middle arm, the zero-sequence voltage prevents AC components from entering the DC network as current increases. However, excessive AC components can disrupt the DC link voltage V. cx The stability of the AC component is crucial. Positive-sequence voltage can reduce the AC component, but excessive AC component can increase the AC voltage. Therefore, introducing negative-sequence voltage can balance the AC influence, thereby achieving stability. Figure 11 Describes the voltage from the reference voltage V ref Generate differential-mode AC voltage V from a sinusoidal signal Maci The process.

[0101] 3. Module capacitor balance control

[0102] The module balancing control employs two different methods: differential-mode control and common-mode control. Differential-mode control operates on the voltage difference between the upper and lower arms of each branch, aiming to eliminate these differences and achieve balanced power distribution. Ideally, the power in each arm cancels out, resulting in zero net differential power. On the other hand, common-mode control regulates the shared common-mode voltage between the two arms of each branch, ensuring that the average voltage between the two arms remains consistent. This stability helps maintain the DC link capacitor voltage V. cx The balance is achieved. Changing the capacitor voltage requires changing the power so that the controller's output signal can be converted into a power-dependent adjustment.

[0103] V CiU The average voltage V taken from all submodules avgU The average voltage V of a single submodule avgU The difference is then compared to the zero signal, and the resulting error signal is processed by the PI controller to determine the power of the submodule, using P... diU Indicates, such as Figure 12 As shown in (12a).

[0104] The differential voltage balance of the upper arm module can be expressed as:

[0105]

[0106] Controller P diU-j The output divided by the common current I U The voltage difference V between the modules can be obtained. diU-j . Figure 12 (b) provides alternative expressions for the lower and middle arms of the MMC designed to balance the voltage difference between submodules. Total power P diL-j The sum is zero, and the same applies to the upper arm.

[0107] The average balance of the upper arm submodule is as follows: Figure 13 As shown in (13a).

[0108] The average voltage balance of the upper arm submodule can be expressed as:

[0109]

[0110] The equivalent expression for achieving average voltage balance of submodules in the lower arm of the 3A-MMC is as follows: Figure 13 As shown in (b). The controller output P cjM Divide by common-mode current I M To achieve voltage averaging in the middle arm submodules, such as Figure 13 As shown in (c).

[0111] The common power required for balancing is derived from the average power of the upper and lower arms, while the differential power is derived from the power difference between the upper and lower arms, such as... Figure 14 As shown.

[0112] The common-mode and differential-mode power balance of the upper and lower arm submodules can be expressed as:

[0113]

[0114] Common-mode and differential-mode power, combined with a feedforward DC controller and an inter-arm AC current controller, ensure the main DC link capacitor C in the 3A-MMC. x Transmission power, each phase lags as Figure 15 As shown. As described in the external power flow control section, the DC link voltage V cx It is regulated by a separate controller.

[0115] refer to Figure 16 As shown, the entire control process can include: measuring all inputs; implementing DC link voltage balance control based on formula (21); implementing feedforward DCPFC control based on formulas (35)-(38); implementing ACPFC control based on zero-sequence and positive-sequence injection methods, as shown in formulas (39)-(43); implementing DC link voltage balance of sub-modules based on formulas (44), (48)-(53), and implementing power balance based on formulas (25), (29)-(34); and implementing PSC-PWM control using the signals obtained from each control.

[0116] 4. Signal distribution

[0117] There are two signal allocation types: common-mode signals and differential-mode signals. For example... Figure 17 As shown, all common-mode signals are divided by 2N, while the differential-mode signal for each arm is divided by N. The output voltage V of the external power flow controller... eciIt is a differential-mode signal. In a DC-AC based internal power flow controller, the common-mode voltage signal is V. ci The differential voltage signal is V aci For each module, the capacitor balance controller involves the common-mode power signal P. cj Sum and difference mode power signals P crj Within the entire system, there is also a separate differential voltage signal, namely V. dj-n For the intermediate arm, both the common-mode and differential-mode signals are divided by N, such as... Figure 17 As shown in (17c).

[0118] To verify the efficiency and feasibility of the proposed MT-IDCPFC method, such as Figure 4 As shown, a multiport converter for DC current control demonstrates its practicality and effectiveness. This invention was tested for its ability to enhance wind power integration in a two-port DC system with a controlled voltage source (CVS) and an offshore wind farm. The system operates at ±200 kV DC voltage, with WT1 at 200 MW and WT2 at 300 MW, connected to offshore converters. These converters deliver all generated wind energy to the DC system, regulated by the CVS at node N3. The objective is to control the current in line 1; this is a low-capacity line prone to overheating without regulation. The simulation validates the effectiveness of the control method of this invention in managing the current in line 1, ensuring stability under various conditions. Table 2 lists the parameters of the network proposed in this embodiment.

[0119] Table 2 System Parameters

[0120] parameter value Power source 1 150MW Power Source 2 320MW Controlled voltage source 200kV Line distance L-1 200km Line distance L-2 150km Line distance L-3 150km Line distance L-4 150km Line distance L-5 300km Line resistance 0.0095Ω / km DC link capacitor 100000μF Switching frequency 1kHz

[0121] according to Figure 18 The simulation results shown allow us to draw some conclusions about the behavior of the line current and DC link voltage Vcx in the system. At time "t", the line current I1 in line 1 changes from the reference current I1 of -1450A. 1-ref Controlled, while the line current I2 in line 2 is controlled by the reference current I of -850A. 2-ref Control. Two current controllers operate simultaneously, effectively maintaining the required reference current.

[0122] After time "t1", the reference current I 1-ref It changes to -500A while maintaining a constant DC link voltage of 4500V. cx Similarly, after time "t2", the reference current I... 2-ref It changes to -500A while maintaining a constant DC link voltage of 4500V. cx DC link voltage Vcx The reference current remains constant, while the line currents I1 and I2 are effectively adjusted according to the new reference value. Therefore, the line current I4 is related to the changing reference current I. ref It remains unchanged. At time t3, the reference currents of I1 and I2 are adjusted back to -850A. During this period, the DC link voltage V... cx The current reference remained unchanged. However, within this timeframe, the DC link voltage exceeded its original value, causing saturation within this specific time interval. This saturation prevents the voltage from exceeding ±V. dc limit.

[0123] This result demonstrates the balanced current in the line and highlights the effectiveness of the proposed control method in regulating line currents within the network. Therefore, simulation results show that the proposed control strategy successfully and effectively manages and maintains the ideal power flow and current balance in the system.

[0124] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A high-voltage DC network line-to-line DC power flow controller based on a three-arm MMC, characterized in that, The system includes a connected 3Arm-MMC circuit and a control module. The 3Arm-MMC circuit is a multi-port network, including a connected DC bus capacitor and a converter. The converter includes a converter branch with the same number of ports as the number of ports. Each converter branch includes an upper arm, a middle arm, and a lower arm connected to the midpoint of the converter branch. Each middle arm provides the port to the outside. Each arm includes N cascaded full-bridge sub-modules. The control module obtains the PWM pulse signal of each full-bridge sub-module through external power flow control, internal power flow control, and module capacitor balance control.

2. The high-voltage DC network line-to-line DC power flow controller based on a three-arm MMC as described in claim 1, characterized in that, Each arm also includes a series inductor connected to a cascaded full-bridge submodule.

3. The high-voltage DC network line-to-line DC power flow controller based on a three-arm MMC as described in claim 1, characterized in that, Each of the full-bridge submodules has a zero state, a negative state, and a positive state.

4. The high-voltage DC network line-to-line DC power flow controller based on a three-arm MMC according to claim 1, characterized in that, The external power flow control includes DC link voltage balancing control and current control for balancing the power flow of multiple transmission lines. The DC link voltage balancing control obtains a DC link voltage balancing control signal, the current control obtains a current control signal, and generates first voltage information for controlling the PWM pulse signal for the upper and lower arms based on the combination of the DC link voltage balancing control signal and the current control signal.

5. The high-voltage DC network line-to-line DC power flow controller based on a three-arm MMC according to claim 4, characterized in that, The DC link voltage balance control signal is obtained by PI control of the error signal between the DC bus voltage and the reference voltage.

6. The high-voltage DC network line-to-line DC power flow controller based on a three-arm MMC according to claim 4, characterized in that, The current control is implemented in at least two branches, and the current control signal is obtained based on PI control of the error signal between the current of the selected branch and the reference current.

7. The high-voltage DC network line-to-line DC power flow controller based on a three-arm MMC according to claim 1, characterized in that, The internal power flow control includes feedforward DC current control and inter-arm AC current control. The feedforward DC current control controls the common-mode current, causing the total power of each branch to converge to zero, generating a feedforward DC current control signal. The inter-arm AC current control controls the power output of each arm in each branch, gradually reducing it to zero, generating an inter-arm AC current control signal. Based on the combination of the feedforward DC current control signal and the inter-arm AC current control signal, a second voltage information is generated to control the PWM pulse signal for the upper and lower arms. A differential AC voltage for the middle arm is generated based on the reference voltage and the sine signal. This differential AC voltage is used to control the PWM pulse signal for the middle arm.

8. The high-voltage DC network line-to-line DC power flow controller based on a three-arm MMC according to claim 7, characterized in that, The AC current control between the bridge arms is achieved by introducing zero-sequence and positive-sequence voltages.

9. The high-voltage DC network line-to-line DC power flow controller based on a three-arm MMC according to claim 1, characterized in that, The module capacitor balance control includes differential mode control and common mode control. The voltage changes between the upper arm, lower arm, and middle arm of each branch are controlled to achieve balanced power distribution. The common mode control adjusts the shared common mode voltage of the upper arm, lower arm, and middle arm of each branch to be consistent, generating third voltage information for controlling the PWM pulse signal.

10. The high-voltage DC network line-to-line DC power flow controller based on a three-arm MMC according to claim 1, characterized in that, When acquiring the PWM pulse signal of each full-bridge submodule, the common-mode signal and differential-mode signal are allocated differently for the upper and lower arms, while the common-mode signal and differential-mode signal are allocated in the same way for the middle arm.

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