Hybrid dc transmission system and power system

Abstract

The application provides a hybrid DC power transmission system and a power system. The hybrid DC power transmission system comprises a DC bus, a sending end comprising a sending end converter, a DC side of the sending end converter being electrically connected to the DC bus, the sending end converter comprising a first modular multi-level converter, a receiving end comprising a receiving end converter, a DC side of the receiving end converter being electrically connected to the DC bus, an AC side of the receiving end converter being used for accessing an AC power grid of the receiving end, the receiving end converter comprising a hybrid commutation converter, and a controller used for determining whether the hybrid commutation converter meets a turn-off condition in a case where it is detected that an AC fault occurs in the hybrid DC power transmission system, and controlling the hybrid commutation converter to turn off in a case where the hybrid commutation converter meets the turn-off condition. The application solves the problems of poor stability, economy and adaptability of a DC power transmission system in large-scale renewable energy transmission in the prior art.

Description

Technical Field

[0001] This application relates to the field of power electronics technology, and more specifically, to a hybrid DC transmission system and a power system. Background Technology

[0002] In the field of high-voltage direct current (HVDC) transmission, existing technologies mainly rely on two converter technologies: line commutated converters (LCC) based on thyristors and modular multilevel converters (MMC) based on insulated-gate bipolar transistors (IGBTs). Although LCC-HVDC systems have lower costs, lower losses, and are suitable for long-distance, high-capacity transmission, they are prone to commutation failures when connected to weak grids, limiting their application in renewable energy grid integration scenarios. MMC-HVDC systems can adapt to weak grid connections, do not have commutation failure issues, and are easy to construct multi-terminal DC systems, but they have disadvantages such as high costs, high losses, low power density, and large footprint and size.

[0003] Among related technologies, DC transmission systems have poor stability, economy, and adaptability in large-scale renewable energy transmission, and cannot meet the transmission needs of large-scale wind-solar hybrid renewable energy. Summary of the Invention

[0004] The main objective of this application is to provide a hybrid DC transmission system and power system to at least solve the problems of poor stability, economy and adaptability of DC transmission systems in large-scale renewable energy transmission in related technologies.

[0005] To achieve the above objectives, according to one aspect of this application, a hybrid DC transmission system is provided, comprising: a DC bus; a sending end, including a sending-end converter, the DC side of which is electrically connected to the DC bus, the sending-end converter including a first modular multilevel converter; a receiving end, including a receiving-end converter, the DC side of which is electrically connected to the DC bus, the AC side of which is used to connect to the receiving-end AC grid, the receiving-end converter including a hybrid phase-commutation converter; and a controller, communicatively connected to the first modular multilevel converter and the hybrid phase-commutation converter, wherein the controller is configured to, upon detecting an AC fault in the hybrid DC transmission system, determine whether the hybrid phase-commutation converter meets a shutdown condition, and, if the hybrid phase-commutation converter meets the shutdown condition, control the hybrid phase-commutation converter to shut down.

[0006] Optionally, the shutdown condition includes a fixed-delay shutdown condition, which characterizes the state of the hybrid commutator during its commutation cycle. Determining whether the hybrid commutator meets the shutdown condition, and controlling the hybrid commutator to shut down when it meets the shutdown condition, includes: determining whether the hybrid commutator meets the fixed-delay shutdown condition; and controlling the hybrid commutator to shut down after a preset delay when it meets the fixed-delay shutdown condition.

[0007] Optionally, the shutdown condition includes a current change rate detection shutdown condition, which characterizes the situation where the rate of change of the DC current of the hybrid commutator is greater than a preset current change rate threshold. Determining whether the hybrid commutator meets the shutdown condition, and controlling the hybrid commutator to shut down when the hybrid commutator meets the shutdown condition, includes: determining whether the hybrid commutator meets the current change rate detection shutdown condition; and controlling the hybrid commutator to shut down immediately when the hybrid commutator meets the current change rate detection shutdown condition.

[0008] Optionally, the controller is further configured to: when the DC power value on the DC bus is detected to be lower than a predetermined DC power value, adjust the DC current of the hybrid commutator to a preset DC current threshold, wherein the DC current threshold represents the minimum DC current that the hybrid commutator can carry.

[0009] Optionally, the first modular multilevel converter includes multiple bridge arms, each bridge arm including a series-connected bridge arm reactor and multiple sub-modules, each sub-module including a sub-module capacitor. The controller is further configured to: when the first modular multilevel converter is in normal operating mode, acquire the actual DC voltage value of the first modular multilevel converter to obtain a first actual DC voltage value, and acquire the actual DC current value of the first modular multilevel converter to obtain a first actual DC current value; acquire the actual resonant frequency of the hybrid DC transmission system, and determine the capacitance of a virtual capacitor based at least on the capacitance of the sub-module capacitor and the actual resonant frequency; determine the active power value of the first modular multilevel converter based on the first actual DC voltage value, the first actual DC current value, and the capacitance of the virtual capacitor; acquire the actual value of the d-axis component of the AC current of the first modular multilevel converter; determine the target active power of the first modular multilevel converter based on the active power value, the active power reference value, and the actual value of the d-axis component of the AC current, and adjust the active power of the first modular multilevel converter to the target active power.

[0010] Optionally, determining the active power value of the first modular multilevel converter based on the actual value of the first DC voltage, the actual value of the first DC current, and the capacitance of the virtual capacitor includes: performing a differential calculation on the capacitance of the virtual capacitor; multiplying the differentially calculated capacitance of the virtual capacitor by the first DC voltage to obtain a first current value; calculating the difference between the actual value of the first DC current and the first current value to obtain a first difference; and multiplying the first difference by the first DC voltage to obtain the active power value.

[0011] Optionally, the hybrid commutator includes a smoothing reactor, and the controller is further configured to: when the hybrid commutator is in normal operating mode, acquire the actual value of the DC current of the hybrid commutator to obtain a second actual value of the DC current, and acquire the actual value of the DC voltage of the hybrid commutator to obtain a second actual value of the DC voltage; acquire the actual resonant frequency of the hybrid DC transmission system, and determine the inductance value of the virtual inductor based at least on the actual resonant frequency; obtain the virtual inductor output based on the second actual value of the DC current and the inductance value of the virtual inductor; determine the target DC voltage of the hybrid commutator based on the virtual inductor output, the second actual value of the DC voltage, and the DC voltage reference value, and adjust the DC voltage of the hybrid commutator to the target DC voltage.

[0012] Optionally, obtaining the virtual inductor output based on the actual value of the second DC current and the inductance value of the virtual inductor includes: performing a differential calculation on the inductance value of the virtual inductor; multiplying the differentially calculated inductance value of the virtual inductor with the actual value of the second DC current to obtain the virtual inductor output.

[0013] Optionally, determining the target DC voltage of the hybrid commutator based on the virtual inductor output, the actual value of the second DC voltage, and the DC voltage reference value includes: calculating the difference between the actual value of the second DC voltage and the DC voltage reference value to obtain a DC voltage difference; calculating the difference between the DC voltage difference and the virtual inductor output to obtain the firing angle of the hybrid commutator; and determining the target DC voltage based on the firing angle.

[0014] According to another aspect of this application, an electric power system is provided, comprising: any of the hybrid DC transmission systems described above.

[0015] According to the technical solution of this application, the hybrid DC transmission system includes a DC bus, a sending end including a sending-end converter, a receiving end including a receiving-end converter, and a controller. The DC side of the sending-end converter and the DC side of the receiving-end converter are electrically connected to the DC bus. The sending-end converter includes a first modular multilevel converter, and the receiving-end converter includes a hybrid phase-commutation converter. The controller is communicatively connected to the first modular multilevel converter and the hybrid phase-commutation converter. The controller is used to control the hybrid phase-commutation converter to turn off when an AC fault is detected in the hybrid DC transmission system and the hybrid phase-commutation converter meets the turn-off conditions. Compared with the poor stability, economy, and adaptability of DC transmission systems in large-scale renewable energy transmission in related technologies, this application adopts a hybrid topology consisting of a first modular multilevel converter at the sending end and a hybrid phase-commutation converter at the receiving end. The first modular multilevel converter has self-commutation capability and fast dynamic response, which can be adapted to weak grid access scenarios and ensure good overall stability of the transmission system. The hybrid phase-commutation converter can reduce reactive power demand and reduce investment in reactive power compensation equipment, thereby reducing costs and footprint, while also reducing the risk of commutation failure. The combined topology meets the high-performance requirements of the system while also taking into account economic benefits. It performs well in terms of weak grid integration, footprint reduction, and cost-effectiveness, providing a better solution for long-distance transmission of large-scale renewable energy. In addition, the controller monitors the system status and, when an AC fault is detected, takes rapid measures according to the turn-off conditions of the hybrid phase-commutation converter to reduce the impact of fault current on the system and ensure the safe and stable operation of the system. Attached Figure Description

[0016] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0017] Figure 1 A schematic diagram of the topology of a hybrid DC transmission system provided in an embodiment of this application is shown;

[0018] Figure 2 A schematic diagram of the topology of a first modular multilevel converter in a hybrid DC transmission system according to an embodiment of this application is shown.

[0019] Figure 3 A schematic diagram of the topology of a hybrid phase-commutation converter in a hybrid DC transmission system according to an embodiment of this application is shown.

[0020] Figure 4 A schematic diagram of the control strategy flow of a hybrid DC transmission system according to an embodiment of this application is shown;

[0021] Figure 5A schematic diagram of the equivalent circuit structure of a hybrid DC transmission system according to an embodiment of this application is shown.

[0022] The above figures include the following reference numerals:

[0023] 10. DC bus; 11. First modular multilevel converter; 12. Hybrid commutation converter. Detailed Implementation

[0024] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0025] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort should fall within the scope of protection of the present application.

[0026] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate for the embodiments of this application described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0027] As described in the background section, the DC transmission system in the related art suffers from poor stability, economy, and adaptability in large-scale renewable energy transmission. To address these issues, embodiments of this application provide a hybrid DC transmission system and power system.

[0028] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0029] This application provides a hybrid DC transmission system, such as... Figure 1 As shown, it includes:

[0030] DC bus 10;

[0031] The sending end (not shown) includes a sending end converter (not shown), the DC side of which is electrically connected to the DC bus 10, and the sending end converter includes a first modular multilevel converter 11;

[0032] Specifically, the sending end is the sending end rectifier station.

[0033] The receiving end (not shown) includes a receiving end converter (not shown), the DC side of which is electrically connected to the DC bus 10, and the AC side of which is used to connect to the receiving end AC grid. The receiving end converter includes a hybrid phase-commutation converter 12.

[0034] Specifically, the receiving end is a receiving inverter station.

[0035] A controller (not shown) is communicatively connected to the first modular multilevel converter 11 and the hybrid phase-commutation converter 12, respectively. The controller is used to determine whether the hybrid phase-commutation converter 12 meets the shutdown conditions when an AC fault is detected in the hybrid DC transmission system (not shown), and to control the hybrid phase-commutation converter 12 to shut down when the hybrid phase-commutation converter 12 meets the shutdown conditions.

[0036] The hybrid DC transmission system provided in this application includes a DC bus, a sending end including a sending-end converter, a receiving end including a receiving-end converter, and a controller. The DC side of the sending-end converter and the DC side of the receiving-end converter are electrically connected to the DC bus. The sending-end converter includes a first modular multilevel converter, and the receiving-end converter includes a hybrid phase-commutation converter. The controller is communicatively connected to the first modular multilevel converter and the hybrid phase-commutation converter. The controller is used to control the hybrid phase-commutation converter to turn off when an AC fault is detected in the hybrid DC transmission system and the hybrid phase-commutation converter meets the turn-off conditions. Compared with the poor stability, economy, and adaptability of DC transmission systems in large-scale renewable energy transmission in related technologies, this application adopts a hybrid topology consisting of a first modular multilevel converter at the sending end and a hybrid phase-commutation converter at the receiving end. The first modular multilevel converter has self-commutation capability and fast dynamic response, which can be adapted to weak grid access scenarios and ensure good overall stability of the transmission system. The hybrid phase-commutation converter can reduce reactive power demand and reduce investment in reactive power compensation equipment, thereby reducing costs and footprint, while also reducing the risk of commutation failure. The combined topology meets the high-performance requirements of the system while also taking into account economic benefits. It performs well in terms of weak grid integration, footprint reduction, and cost-effectiveness, providing a better solution for long-distance transmission of large-scale renewable energy. In addition, the controller monitors the system status and, when an AC fault is detected, takes rapid measures according to the turn-off conditions of the hybrid phase-commutation converter to reduce the impact of fault current on the system and ensure the safe and stable operation of the system.

[0037] Specifically, Figure 1 In the circuit, there are two first modular multilevel converters 11, and these two first modular multilevel converters 11 are connected in series; there are also two hybrid commutation converters 12, and these two hybrid commutation converters 12 are connected in series.

[0038] Specifically, the hybrid DC transmission system of this application is suitable for transmission projects with weak grid access, where commutation failure needs to be avoided and where there are requirements on cost and land area.

[0039] It should be noted that in the field of high-voltage direct current (HVDC) transmission, the relevant technologies mainly rely on two types of converter technologies: line-commutated converters (LCCs) based on thyristors and modular multilevel converters (MMCs) based on insulated-gate bipolar transistors (IGBTs). While LCC-HVDC systems have lower costs, lower losses, and are suitable for long-distance, high-capacity transmission, they are prone to commutation failures when connected to weak grids, limiting their application in renewable energy grid integration scenarios. MMC-HVDC systems can adapt to weak grid connections, do not suffer from commutation failure issues, and are easy to construct multi-terminal DC systems, but they have disadvantages such as high cost, high losses, low power density, and large footprint and volume. Large-scale renewable energy has randomness and weak support characteristics, and the receiving end also faces the risk of commutation failure and footprint limitations. Existing solutions are insufficient to meet the reliable and economical requirements of ultra-high-voltage direct current (UHVDC) transmission.

[0040] Specifically, the hybrid commutated converter (HCC) based on the reverse blocking integrated gate-commutated thyristor (RB-IGCT) can reduce system reactive power demand, save costs and reduce equipment size through optimization.

[0041] Specifically, such as Figure 2 As shown, the first modular multilevel converter (MMC) consists of a three-phase six-arm bridge, with each arm comprising an arm reactance L0, an arm equivalent resistance R0, and N. h One half-bridge sub-module (HBSM) and N f It is composed of several full-bridge sub-modules (FBSMs) connected in series, and each bridge arm contains the same number of full-bridge and half-bridge sub-modules with corresponding and consistent parameter settings; dc i represents the DC voltage at the DC-side outlet of the MMC. dc U represents the DC current of the line. pj u nj (j=a,b,c) represent the upper and lower arm voltages of each phase of the MMC, respectively, ipj i nj (j=a,b,c) represent the upper and lower arm currents of each phase of the MMC, respectively, u vj (j=a,b,c) represents the phase voltage at the AC output of the MMC, i vj (j=a,b,c) represents the phase current at the AC output of the MMC, u sj (j=a,b,c) represents the equivalent potential of the MMC AC side system, u o’o L represents the potential difference between the AC and DC neutral points of the MMC. sj (j=a,b,c) represents the equivalent electromotive force of the MMC AC side system and the equivalent inductance of the MMC AC outlet, R sj (j=a,b,c) represents the equivalent electromotive force of the MMC AC side system and the equivalent resistance of the MMC AC outlet.

[0042] Specifically, such as Figure 3 As shown, in HCC, VT1~VT6 represent the converter valves of each bridge arm. The upper bridge arm VT4, VT6, and VT2 are three sets of common anode reverse resistance valves, and the lower bridge arm VT1, VT3, and VT5 are three sets of common cathode reverse resistance valves. The bridge arm devices typically use reverse resistance IGCTs or reverse resistance IGBTs, which can block reverse current and withstand reverse voltage. dc i represents the reactance of the smoothing reactor in a hybrid commutator. ga i gb i gc Let u represent the three-phase currents a, b, and c fed into the AC system at the receiving end, respectively. ga u gb u gc L represents the three-phase voltages a, b, and c at the AC feed point of the receiving end. eq i represents the equivalent commutation reactance, i1 represents the current flowing through VT1, i3 represents the current flowing through VT3, and i a This indicates that VT1 and VT3 are connected through phases a and b and the equivalent commutation reactance L. eq The current in the circuit is formed by the dynamic buffer branch RC, which is used to limit the voltage rise rate dv / dt when the IGCT is turned off and to eliminate the steep wave effect of the surge arrester MOV. The static equalizing resistor R d Used to balance the voltage stress of IGCT under high voltage blocking conditions.

[0043] like Figure 1 As shown, in the main loop topology, U R ∠δ R U represents the voltage vector at the point of common coupling (PCC) on the rectifier side. ph ∠δ ph Uw ∠δ w U th ∠δ th U represents the terminal voltage at the output of the photovoltaic power station, wind turbine, and conventional power unit, respectively. dcR U represents the positive voltage on the rectifier side. dcI P represents the positive voltage on the inverter side. ph +jQ ph P w +jQ w and P th +jQ th P1 + jQ1, P2 + jQ2, and P3 + jQ3 represent the output power of each type of power supply, respectively, and U represents the power fed into the rectifier-side AC bus of each type of power supply. I ∠δ I U s ∠δ s P represents the inverter-side PCC voltage and the equivalent power supply output terminal voltage, respectively. I +jQ I P4+jQ4 represent the power injected into the receiving-end system from the inverter-side AC bus and the power fed into the equivalent power source, respectively; R4+jX4 represents the equivalent impedance of the inverter-side AC grid; X R X I P represents the sum of the leakage reactance and valve reactance of the rectifier-side converter transformer T1 and the inverter-side converter transformer T2, respectively. MMC1 +jQ MMC1 P represents the apparent power flowing from the AC side of the sending end into the positive MMC. HCC1 +jQ HCC1 P represents the apparent power flowing from the positive electrode HCC into the receiving end AC side. MMC2 +jQ MMC2 P represents the apparent power flowing from the AC side of the sending end into the negative MMC. HCC2 +jQ HCC2 This represents the apparent power flowing from the negative electrode HCC into the receiving end AC side.

[0044] Specifically, the hybrid DC transmission system of this application integrates the weak grid adaptability of MMC with the low cost and low loss characteristics of HCC to achieve long-distance and efficient transmission of large-scale renewable energy (such as wind power and photovoltaics), while avoiding commutation failure and reducing land area. The rectifier station, composed of MMC, connects the renewable energy system (wind power, photovoltaic arrays) and the sending-end AC system. It integrates the renewable energy output through the AC bus and converts it into DC power output via the MMC. The MMC, based on a combined HBSM and FBSM structure, possesses fast response and reactive power support capabilities. The inverter station, composed of HCC, connects to the receiving-end AC system, inverting DC power into AC power for grid connection. The HCC adopts a dual twelve-pulse converter structure, achieving stable operation through dynamic buffer branches and static voltage equalization resistors. The DC line connects the MMC and HCC, including DC filters and line impedance, used to suppress harmonics and transmit DC power.

[0045] In one alternative embodiment, the shutdown condition includes a fixed-delay shutdown condition, which characterizes the state of the hybrid commutator during its commutation cycle. Determining whether the hybrid commutator meets the shutdown condition, and controlling the hybrid commutator to shut down when it meets the shutdown condition, includes: determining whether the hybrid commutator meets the fixed-delay shutdown condition; and controlling the hybrid commutator to shut down after a preset delay period when it meets the fixed-delay shutdown condition. In this embodiment, by setting a fixed delay turn-off condition, the HCC can precisely control the turn-off time of the IGCT in the HCC during the commutation cycle, avoiding unstable turn-off during the commutation process, thereby reducing DC current and voltage fluctuations and improving the operational stability of the entire hybrid DC system. When a fault occurs in the AC power grid, the fixed delay turn-off condition can ensure that the IGCT turns off at the appropriate time, avoiding additional resonance or current oscillation caused by premature or late turn-off. This is beneficial for the hybrid DC system to recover quickly under fault conditions and improves the system's fault ride-through capability.

[0046] In practical applications, those skilled in the art can set the above-mentioned delay duration based on empirical values, or it can be obtained through multiple experiments. This application does not impose specific restrictions on this.

[0047] In one exemplary embodiment, the shutdown condition includes a current change rate detection shutdown condition, which characterizes the situation where the rate of change of the DC current of the hybrid commutated converter is greater than a preset current change rate threshold. Determining whether the hybrid commutated converter meets the shutdown condition, and controlling the hybrid commutated converter to shut down when it does, includes: determining whether the hybrid commutated converter meets the current change rate detection shutdown condition; and controlling the hybrid commutated converter to immediately shut down when it does. In this embodiment, current change trends can be intelligently identified, reducing maloperation caused by transient changes and ensuring that the hybrid commutated converter is shut down at the appropriate time, thereby effectively suppressing fault current oscillations and improving the dynamic response speed and stability of the system.

[0048] In practical applications, those skilled in the art can set the above-mentioned current change rate threshold based on empirical values, or it can be obtained through multiple experiments. This application does not impose any specific restrictions on this.

[0049] According to some exemplary embodiments of this application, the controller is further configured to: when the DC power value on the DC bus is detected to be lower than a predetermined DC power value, adjust the DC current of the hybrid commutator to a preset DC current threshold, wherein the DC current threshold represents the minimum DC current that the hybrid commutator can carry. In this embodiment, when the DC power value is low, by adjusting the DC current of the hybrid commutator to the preset DC current threshold, it can be ensured that the hybrid commutator has a certain current flowing through it, preventing the converter from completely cutting off the current. This ensures that the hybrid DC transmission system can still maintain a basic operating state at low power levels, avoiding system instability caused by a sudden drop in power, and further improving the stability of the system.

[0050] In practical applications, those skilled in the art can set the above-mentioned DC current threshold based on empirical values, or it can be obtained through multiple experiments. This application does not impose any specific restrictions on this.

[0051] To address the requirements for system stability and fault ride-through, the coordination control strategy block diagram of this application is as follows: Figure 4As shown, during an AC fault, reference values ​​for positive and negative sequence voltages can be derived immediately, avoiding delays caused by sensors and fault detection. This rapid response helps suppress the negative sequence current component during a fault, thereby reducing the peak fault current and DC voltage ripple. The hybrid commutator (HCC) employs a control strategy of "fixed delay + current rate of change (di / dt) detection" to cut off the fault current, ensuring AC fault ride-through capability. Di / dt detection helps prevent commutation failure during the turn-off process, and the fixed delay strategy ensures timely execution of the turn-off command. When the DC power value on the DC bus is low, the HCC switches to minimum current control (i.e., adjusts the DC current of the hybrid commutator to a preset DC current threshold).

[0052] According to some other exemplary embodiments of this application, the first modular multilevel converter includes multiple bridge arms, each bridge arm including multiple sub-modules, each sub-module including a sub-module capacitor. The controller is further configured to: when the first modular multilevel converter is in normal operating mode, acquire the actual value of the DC voltage of the first modular multilevel converter to obtain a first actual DC voltage value, and acquire the actual value of the DC current of the first modular multilevel converter to obtain a first actual DC current value; acquire the actual resonant frequency of the hybrid DC transmission system, and determine the capacitance of a virtual capacitor based at least on the capacitance of the sub-module capacitor and the actual resonant frequency; determine the active power value of the first modular multilevel converter based on the actual value of the first DC voltage, the actual value of the first DC current, and the capacitance of the virtual capacitor; acquire the actual value of the d-axis component of the AC current of the first modular multilevel converter; determine the target active power of the first modular multilevel converter based on the active power value, the active power reference value, and the actual value of the d-axis component of the AC current, and adjust the active power of the first modular multilevel converter to the target active power. In this embodiment, by introducing the calculation of virtual capacitors, the controller can accurately determine the active power value of the first modular multilevel converter. Virtual capacitors simulate the effect of additional capacitors without the need to actually add physical capacitors, which helps to change the impedance characteristics of the system, suppress mid-to-high frequency resonance, and thus improve the stability of the hybrid DC system. Introducing virtual capacitors into the control of the first modular multilevel converter at the sending end can effectively reduce resonance phenomena caused by changes in system parameters or external disturbances, and ensure the smooth operation of power transmission. Compared with actually adding capacitors, virtual capacitor control avoids the problem of increased equipment costs and operating losses caused by adding physical capacitors. Virtual capacitors are implemented only through software control and do not require additional hardware facilities, thereby reducing the overall construction and operating costs and improving economic efficiency.

[0053] Specifically, in normal operation mode, the outer loop control of the first modular multilevel converter adopts constant power control, which includes target constant active power control and constant reactive power control. Target constant active power control is constant active power control with the introduction of virtual capacitors. In normal operation mode, the inner loop control of the first modular multilevel converter adopts current control to achieve new energy output tracking.

[0054] In other embodiments, determining the active power value of the first modular multilevel converter based on the actual value of the first DC voltage, the actual value of the first DC current, and the capacitance of the virtual capacitor includes: performing a differential calculation on the capacitance of the virtual capacitor; multiplying the differentially calculated capacitance of the virtual capacitor by the first DC voltage to obtain a first current value; calculating the difference between the actual value of the first DC current and the first current value to obtain a first difference; and multiplying the first difference by the first DC voltage to obtain the active power value. In this embodiment, the specific calculation process ensures that the obtained active power value is relatively accurate, thereby ensuring that the subsequently obtained target active power is relatively accurate.

[0055] According to some other exemplary embodiments of this application, determining the target active power of the first modular multilevel converter (MMC) based on the aforementioned active power value, active power reference value, and AC current value includes: inputting the aforementioned active power value and the aforementioned active power reference value to a PI controller to obtain a reference value for the d-axis component of the AC current of the first MMC; and determining the target active power based on the aforementioned reference value for the d-axis component of the AC current and the actual value of the d-axis component of the AC current. In this embodiment, the controller adjusts the reference value of the d-axis component of the AC current using a proportional-integral control algorithm based on the deviation between the current active power value and the reference value, thereby controlling the active power output of the MMC. This enables rapid tracking and adjustment of active power, improves the system's response speed to fluctuations in new energy output, and enhances the system's dynamic stability and fault ride-through capability.

[0056] Specifically, such as Figure 5 As shown, the MMC converter station can be equivalently represented as a station with a parallel equivalent capacitor C. eq1 The controlled current source of the HCC converter station can be equivalently represented as a controlled voltage source with a series equivalent impedance d2. The state-space equation of the hybrid DC transmission system can be expressed as: (1), (2), (3), where i s i represents the equivalent input current of the MMC converter station. cr For the equivalent capacitance C eq1 The current, R line and L lineC represents the equivalent resistance and equivalent inductance of the DC bus. eq1 C is the equivalent capacitance of MMC. eq2 E is the equivalent capacitance of the DC bus. dc d2 is the DC voltage of the controlled voltage source, d2 is the commutation voltage drop ratio, and u is the DC voltage of the controlled voltage source. ll k is the effective value of the line voltage of the inverter-side AC system. T Where γ is the transformer turns ratio, γ is the turn-off angle of the HCC, and X is the transformer turns ratio. r The equivalent commutation reactance of the inverter-side converter transformer is given. Solving and simplifying equations (1) to (3) yields: (4) Performing a Laplace transform on equation (4) yields i s with i dc Transfer function between: (5) In addition, we can also obtain from i s To U dcr and U dci Transfer function:

[0057] (6), (7) Similarly, the DC-side impedance expressions for MMC and HCC can be obtained: (8), (9). Analysis shows that in the low-frequency range, the amplitude of all curves in the phase-frequency response curve is close to 0dB; as the frequency increases, the amplitude first rises to a resonant peak and then gradually decreases, indicating that the system's response characteristics are weakening; in the phase-frequency response curve, the phase of all curves in the low-frequency range is close to 0°; as the frequency continues to increase, the phase gradually decreases, eventually approaching -180° in the high-frequency range, which reflects the significant phase lag phenomenon typical of second-order systems; when L... line As C increases, both the resonant peak value and the resonant frequency decrease; with C... eq1 As the value increases, both the resonant peak value and the resonant frequency decrease.

[0058] To gain a more intuitive understanding of the resonant characteristics of the hybrid DC transmission system, s=jω is substituted into equation (5) to obtain the frequency response equation of the hybrid DC transmission system: (10). As can be seen from the analysis, with C eq1 and L line As the value increases, both the resonant amplitude and resonant frequency decrease; furthermore, it is worth noting that, compared with L... line Compared to the effect of increasing C, the resonant frequency of the system changes with C. eq1 The increase in [something] significantly decreased.

[0059] To improve system stability, increasing the DC inductance and capacitance is the most direct method in related technologies. However, in practical engineering applications, this increases construction costs and operating losses, and may even trigger resonance phenomena in other frequency bands. Therefore, this application introduces virtual inductance and virtual capacitance, and the equivalent circuit of the hybrid DC transmission system is as follows: Figure 5 As shown, in its equivalent circuit, L vd For virtual inductance, C vd This is a virtual capacitor.

[0060] After taking into account virtual impedance (i.e., virtual inductance and virtual capacitance), the expressions for the inverter-side DC voltage and the rectifier-side DC current can be expressed as: (11), (12); To differentiate the DC current, a bandpass or lowpass filter must be used to filter out harmonics. By introducing a virtual impedance, i can be derived. s with i dc New transfer function between: (13); by Figure 4 Equations (8) and (9) provide the DC-side impedance expressions for the MMC and HCC after introducing virtual impedance: (14) (15).

[0061] In other embodiments, the controller is further configured to: when the hybrid commutator is in normal operating mode, acquire the actual value of the DC current of the hybrid commutator to obtain a second actual value of the DC current, and acquire the actual value of the DC voltage of the hybrid commutator to obtain a second actual value of the DC voltage; acquire the actual resonant frequency of the hybrid DC transmission system, and determine the inductance value of the virtual inductor based at least on the actual resonant frequency; obtain the virtual inductor output based on the second actual value of the DC current and the inductance value of the virtual inductor; determine the target DC voltage of the hybrid commutator based on the virtual inductor output, the second actual value of the DC voltage, and the DC voltage reference value, and adjust the DC voltage of the hybrid commutator to the target DC voltage. In this embodiment, by monitoring the actual values ​​of DC current and voltage in real time and combining the dynamic output of the virtual inductor, the controller can quickly respond to fluctuations on the DC side, ensuring that the DC voltage of the hybrid commutator is maintained near the predetermined reference value, thereby enhancing the dynamic voltage regulation capability of the system. The introduction of the virtual inductor can simulate the effect of additional inductance, thereby increasing the damping of the system and improving the dynamic response characteristics. In normal operation mode, the virtual inductor can absorb or release transient energy, playing a role similar to that of a physical inductor, but without the need to actually add hardware, reducing the system size and cost, helping to suppress the intermediate frequency resonance phenomenon, reducing DC voltage and current fluctuations caused by resonance, and enhancing the overall stability of the system.

[0062] Specifically, in normal operation mode, the hybrid commutation converter adopts target constant DC voltage control, which maintains DC voltage stability and reduces reactive power consumption by adjusting the firing angle and lead angle. Target constant DC voltage control is constant DC voltage control that introduces virtual inductance. In normal operation mode, the hybrid commutation converter also adopts constant turn-off angle control.

[0063] Specifically, using a phase margin (PM) ≥ 30° for the hybrid DC transmission system as the stability criterion, the capacitance of the virtual capacitor is determined based on at least the capacitance of the aforementioned submodule capacitors and the aforementioned actual resonant frequency, including: according to the formula Determine the capacitance C of the virtual capacitor. vd C eq1 C represents the equivalent capacitance of the first modular multilevel converter. eq1 It is determined based on the capacitance of the submodule capacitor, L line This represents the equivalent inductance of the DC bus. L represents the actual resonant frequency of the hybrid DC transmission system. vd This represents the inductance value of the virtual inductor. This setting avoids an excessively large virtual capacitor, which would lead to an overabundance of equivalent capacitance in the first modular multilevel converter and instead induce high-frequency resonance.

[0064] Based at least on the actual resonant frequency mentioned above, determine the inductance value of the virtual inductor, including: according to the formula Determine the inductance value L of the virtual inductor. vd X req The preset target equivalent reactance (the preset target equivalent reactance needs to meet the phase margin (PM) requirement of the hybrid DC transmission system ≥ 30°, usually requiring X) req >ωL line To increase impedance amplitude), L line This represents the equivalent inductance of the DC bus. This represents the actual resonant frequency of the hybrid DC transmission system.

[0065] In other embodiments, the virtual inductance output is obtained based on the actual value of the second DC current and the inductance value of the virtual inductor, including: performing a differential calculation on the inductance value of the virtual inductor; and multiplying the differentially calculated inductance value of the virtual inductor with the actual value of the second DC current to obtain the virtual inductance output. In this embodiment, by multiplying the differentially calculated inductance value of the virtual inductor with the actual value of the second DC current, the obtained virtual inductance output is ensured to be relatively accurate, thereby ensuring that the subsequently obtained target DC voltage is relatively accurate.

[0066] According to some other exemplary embodiments of this application, determining the target DC voltage of the hybrid commutator based on the virtual inductor output, the actual value of the second DC voltage, and the DC voltage reference value includes: calculating the difference between the actual value of the second DC voltage and the DC voltage reference value to obtain a DC voltage difference; calculating the difference between the DC voltage difference and the virtual inductor output to obtain the firing angle of the hybrid commutator; and determining the target DC voltage based on the firing angle. In this embodiment, the controller calculates the firing angle of the HCC based on the DC voltage difference and the virtual inductor output, and then adjusts the DC voltage to the target DC voltage, ensuring stable system operation, enhancing system dynamic stability, and enabling rapid adjustment of the DC voltage through firing angle control.

[0067] Specifically, the target DC voltage can be determined based on the firing angle by utilizing the constant DC voltage control of HCC in the existing technology.

[0068] like Figure 4 As shown, the coordinated control strategy of the hybrid DC system includes fault detection, power control, voltage and current control, triggering strategy and active shutdown strategy. When the three-phase fault detection module or the asymmetrical fault detection module detects a fault, it will output a signal to the "Max" (maximum value selection) stage. The output of "Max" will further act on the voltage and current control stage to switch the control mode.

[0069] like Figure 4 As shown, the fault detection module includes a three-phase fault detection module and an asymmetrical fault detection module; the three-phase detection module detects faults by inputting the instantaneous value U of the three-phase AC voltage. a U b U c After the abc / αβ coordinate transformation, the voltage amplitude is calculated. , and voltage amplitude threshold U abz Compare sizes, if The output is larger, producing a positive signal. After a "hold time" delay, it outputs a three-phase fault detection signal. The asymmetrical fault detection module inputs the instantaneous value U of the three-phase AC voltage. a U b U c Find the algebraic sum and then take the absolute value, and sum it with the voltage difference threshold U. diff Compare the magnitudes; if the former is larger, output a positive signal. After a "hold time" delay, output an asymmetric fault detection signal.

[0070] like Figure 4 As shown, the power control module includes a positive sequence control module and a negative sequence control module. The positive sequence control module mainly includes target active power control and target reactive power control.

[0071] Figure 4 In the process of target active power control, the following steps are taken: Given the active power reference value P* of the first modular multilevel converter (MMC), it is compared with the actual active power value P (i.e., the aforementioned active power value). After passing through the PI controller, a reference value i for the positive sequence component of the d-axis AC current on the valve side is generated. vdref(1) The actual active power value P is obtained through the virtual capacitor C. vd After differentiation and rectifier-side DC voltage u dcr Multiply by (i.e., the actual value of the first DC voltage mentioned above), and then multiply by the rectifier-side DC current i. dcr The difference between the actual value of the first DC current and the DC voltage u is calculated. dcr The product obtained after multiplication, i vdref(1) (i.e., the reference value of the d-axis component of the AC current mentioned above) and the actual value of the positive sequence d-axis component of the AC current on the valve side i vd(1) After the difference is calculated, it passes through a PI controller and is then added with the reference value u of the positive sequence component of the grid-side AC voltage. sd(1) Subtract the coupling term (the equivalent inductance of the bridge arm ωL and the actual value of the positive sequence component of the valve-side AC current q-axis i). vq(1) The product of these components yields the differential modulus component and the positive sequence component u along the d-axis. diffd(1) The reactive power control is as follows: Given a reactive power reference value Q*, it is compared with the actual active power value Q. The PI controller then generates a reference value i for the positive sequence component of the valve-side AC current q-axis. vqref(1) i vqref(1) Actual value of the positive sequence component of the q-axis AC current on the valve side, i vq(1) After the difference is calculated, it passes through a PI controller and is then added with the reference value u of the positive sequence component of the grid-side AC voltage q-axis. sq(1) Adding the coupling term (the equivalent inductance value ωL of the bridge arm and the actual value i of the positive sequence component of the d-axis AC current on the valve side), vd(1) The product of these components yields the differential modulus component and the positive sequence component u on the q-axis. diffq(1) Finally, the upper and lower bridge arm currents i of each phase pj(2) i nj(2) (j=d, q) The common-mode voltage d-axis positive-sequence components are generated by the circulating current suppression controller CCSC, and then compared with the previously obtained differential-mode components d-axis positive-sequence components u. diffd(1) u diffq(1) and DC voltage reference value u dcref Half of the value is used to obtain the reference value u of the positive sequence components of the valve-side AC voltage d and q axes through algebraic operations. dref(1) u qref(1) .

[0072] Figure 4 In the negative sequence control module, the reference value i of the d-axis negative sequence component of the valve-side AC current is... vdref(2) Actual value of the negative sequence component of the d-axis AC current on the valve side, i vd(2)After the difference is calculated, it passes through a PI controller and is then added with the reference value u of the negative sequence component of the grid-side AC voltage. sd(2) Adding coupling terms (the equivalent inductance of the bridge arm ωL and the actual value of the negative sequence component of the valve-side AC current q-axis i) vq(2) The product of these components yields the differential modulus component d-axis negative sequence component u. diffd(2) Reference value i of the negative sequence component of the q-axis AC current on the valve side vqref(2) The actual value of the negative sequence component of the q-axis AC current on the valve side, i vq(2) After the difference is calculated, it passes through a PI controller and is then added with the reference value u of the negative sequence component of the grid-side AC voltage q-axis. sq(2) Subtract the coupling term (the equivalent inductance of the bridge arm ωL and the actual value of the negative sequence component of the d-axis AC current on the valve side i). vd(2) The product of these components yields the q-axis negative sequence component u of the differential modulus component. diffq(2) Finally, the upper and lower bridge arm currents i of each phase pj(2) i nj(2) (j=d, q) The common-mode voltage d-axis negative sequence components are generated by the circulating current suppression controller CCSC, and then compared with the previously obtained differential-mode components d-axis negative sequence components u. diffd(2) u diffq(2) and DC voltage reference value u dcref Half of the value is used to obtain the reference value u of the negative sequence components of the valve-side AC voltage d and q axes through algebraic operations. dref(2) u qref(2) .

[0073] Figure 4 In this process, the minimum current control is achieved by setting a given DC current reference value I*. dc , with actual current I dc The comparison is performed, and a signal is generated by the PI controller; the target constant voltage control (i.e., target constant DC voltage control) uses a given DC voltage reference value U*. dc , and the actual DC voltage U dc (i.e., the actual value of the second DC voltage mentioned above) is compared, and the additional virtual inductance L is subtracted. vd The output of the control loop (i.e., the output of the aforementioned virtual inductor) is used by the PI controller to generate a firing angle signal, wherein the additional virtual inductance L vd The output of the control circuit is obtained through the virtual inductor L. vd After differentiation and input, the inverter-side DC current i' dc The result is obtained by multiplying the actual value of the DC current mentioned above.

[0074] Figure 4 In the middle, the input three-phase voltage u sa u sb u sc After the abc / dq coordinate transformation, the phase-locked loop angle θ is output by adjusting the PI controller and combining it with the grid angular frequency ω0.PLL This provides a synchronous phase reference for the control system; in the triggering strategy, when "CP signal unlocked" and "U" is triggered... + When both "signal return" and "signal return" conditions are met, an AND gate (&) is used to generate a trigger signal for the gate-commutated thyristor (IGCT) in the integrated HCC, controlling the power device to turn on. "CP signal unlock" refers to the unlocking of the bridge arm control pulse signal. U + Signal return refers to positive pressure signal return; in the active shutdown strategy, the "optical CT signal" is received, and the time-delay shutdown detection and current change rate di / dt detection are used as two input signals of the OR gate (≥1). The output signal of the OR gate is then used as two input signals of the AND gate (&) along with the "blocking condition (i.e., AC fault occurs in the hybrid DC transmission system)" for logical judgment, generating the shutdown signal of the gate commutated thyristor IGCT in HCC, quickly cutting off the fault current, and ensuring the safe and stable operation of the system. The optical CT signal refers to the photocurrent sensor signal.

[0075] According to some further exemplary embodiments of this application, the sending-end converter further includes a second modular multilevel converter, which is connected in parallel with the first modular multilevel converter; or, the sending-end converter further includes a line-commutated converter, which is connected in parallel with the first modular multilevel converter; the receiving-end converter includes a third modular multilevel converter, which is connected in parallel with the hybrid phase-commutated converter. In this embodiment, the parallel converters can share the load, improve the transmission power and capacity of the system. In addition, when one converter fails and stops operating, the system can continue to operate through other parallel converters, reducing the impact of the fault on the overall system operation and improving the fault ride-through and rapid recovery capabilities.

[0076] In other embodiments, the controller is further configured to: acquire historical output power of the renewable energy system and historical impedance data of the sending-end AC grid; train a machine learning model using the historical output power and historical impedance data to obtain a first target model; acquire the current output power of the renewable energy system and the current impedance data of the sending-end AC grid, and input the current output power and the current impedance data of the sending-end AC grid into the first target model to obtain the predicted output power of the renewable energy system and the predicted impedance data of the sending-end AC grid in the near future; calculate the difference between the predicted output power and the current output power to obtain a power difference, calculate the ratio of the power difference to the current output power to obtain a first ratio; and calculate the predicted impedance data. The impedance difference is calculated by taking the difference between the current impedance data and the impedance value. The ratio of this impedance difference to the current impedance data is then calculated to obtain a second ratio. If the first ratio is greater than a first predetermined ratio (ranging from 15% to 25%) or the second ratio is greater than a second predetermined ratio (ranging from 15% to 25%), the number of active submodules in the MMC is increased. If the first ratio is less than a third predetermined ratio (ranging from -15% to -25%) or the second ratio is less than a fourth predetermined ratio (ranging from -15% to -25%), the number of active submodules in the MMC is decreased to mitigate power fluctuations or impedance fluctuations and improve system response speed and stability. The first and second predetermined ratios are greater than 0, while the third and fourth predetermined ratios are less than 0.

[0077] In other embodiments, the controller is further configured to: acquire historical operating data, historical fault types, and historical fault severity, wherein the historical operating data includes historical voltage and current of the receiving-end AC grid and historical operating status parameters of the HCC; train a machine learning model using historical AC fault data, historical fault types, and historical fault severity to obtain a second target model; acquire current operating data and input the current operating data into the second target model to obtain a prediction result, wherein the prediction result characterizes whether an AC fault will occur and the severity of the fault; and, if the prediction result indicates that an AC fault will occur, dynamically adjust the switching state of the IGCT in the HCC according to the fault severity. Dynamically adjusting the switching state of the IGCT in the HCC according to the fault severity includes: when the prediction result indicates a severe fault, controlling all IGCTs in the HCC to be closed; when the prediction result indicates a moderate fault, increasing the number of IGCTs in the HCC that are closed; and when the prediction result indicates a mild fault, controlling the switching state of all IGCTs in the HCC to remain unchanged. The system can achieve more refined and faster fault response.

[0078] In summary, the hybrid topology composed of the sending-end MMC and the receiving-end HCC in this application features a self-commutation capability and a fast dynamic response of the MMC, which is suitable for weak grid access scenarios. This solves the problem of commutation failure that easily occurs in LCC-HVDC in weak grids. The HCC based on RB-IGCT devices reduces reactive power demand and the cost and size of reactive power compensation equipment by optimizing the turn-off angle and IGCT configuration ratio, while avoiding the risk of commutation failure and making up for the shortcomings of high cost and high loss of MMC-HVDC. The combined topology outperforms traditional LCC and MMC topologies in terms of weak grid integration, reduced footprint, and cost-effectiveness, providing a better solution for large-scale long-distance transmission of renewable energy. Virtual impedance control (i.e., virtual capacitor control and virtual inductor control), by introducing virtual inductors and capacitors, reshapes the system impedance characteristics, suppresses mid-frequency resonance, and eliminates the need to actually increase the DC inductance and capacitance (avoiding problems such as reduced dynamic response speed, increased cost, and losses). This reduces resonance peak value, decreases high-frequency phase lag, and significantly improves system stability. In the event of an AC fault, HCC ensures timely shutdown commands through "fixed-delay shutdown" and prevents commutation failure through "di / dt detection shutdown." Simultaneously, it integrates with a fault detection module to achieve stable DC power when DC power is low. The minimum current control switching effectively reduces DC current oscillation and voltage fluctuation amplitude, and significantly shortens the recovery time after a fault, thus improving the system's anti-interference capability. The four-terminal hybrid UHVDC system based on the MMC-HCC topology (two schemes: sending end "MMC+MMC" + receiving end "MMC+HCC" or sending end "MMC+LCC" + receiving end "MMC+HCC") can achieve stable transmission of 10GW-level power in both schemes, verifying the scalability of the MMC-HCC system. The sending end "MMC+LCC" + receiving end "MMC+HCC" scheme further combines the low-cost characteristics of LCC, reducing the cost of large-scale networking while ensuring stability, and solving the problem that a single topology cannot balance large capacity and economy.

[0079] This application also provides a power system, including any of the above-described hybrid DC transmission systems.

[0080] It is obvious to those skilled in the art that the modules or steps of the present invention described above can be implemented using general-purpose computing devices. They can be centralized on a single computing device or distributed across a network of multiple computing devices. They can be implemented using computer-executable program code, and thus can be stored in a storage device for execution by a computing device. In some cases, the steps shown or described can be performed in a different order than those described herein, or they can be fabricated as separate integrated circuit modules, or multiple modules or steps can be fabricated as a single integrated circuit module. Thus, the present invention is not limited to any particular combination of hardware and software.

[0081] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0082] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0083] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0084] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0085] In a typical configuration, a computing device includes one or more processors (CPU), input / output interfaces, network interfaces, and memory.

[0086] Memory may include non-persistent memory in computer-readable media, such as random access memory (RAM) and / or non-volatile memory, such as read-only memory (ROM) or flash RAM. Memory is an example of computer-readable media.

[0087] Computer-readable media includes both permanent and non-permanent, removable and non-removable media that can store information using any method or technology. Information can be computer-readable instructions, data structures, modules of programs, or other data. Examples of computer storage media include, but are not limited to, phase-change memory (PRAM), static random access memory (SRAM), dynamic random access memory (DRAM), other types of random access memory (RAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), flash memory or other memory technologies, CD-ROM, digital versatile optical disc (DVD) or other optical storage, magnetic tape, disk storage or other magnetic storage devices, or any other non-transferable medium that can be used to store information accessible by a computing device. As defined herein, computer-readable media does not include transient computer-readable media, such as modulated data signals and carrier waves.

[0088] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0089] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element.

[0090] As can be seen from the above description, the embodiments of this application achieve the following technical effects:

[0091] In the hybrid DC transmission system of this application, the hybrid DC transmission system includes a DC bus, a sending end including a sending-end converter, a receiving end including a receiving-end converter, and a controller. The DC side of the sending-end converter and the DC side of the receiving-end converter are electrically connected to the DC bus, respectively. The sending-end converter includes a first modular multilevel converter, and the receiving-end converter includes a hybrid phase-commutation converter. The controller is communicatively connected to the first modular multilevel converter and the hybrid phase-commutation converter, respectively. The controller is used to control the hybrid phase-commutation converter to turn off when an AC fault is detected in the hybrid DC transmission system and the hybrid phase-commutation converter meets the turn-off conditions. Compared with the poor stability, economy, and adaptability of DC transmission systems in large-scale renewable energy transmission in related technologies, this application adopts a hybrid topology consisting of a first modular multilevel converter at the sending end and a hybrid phase-commutation converter at the receiving end. The first modular multilevel converter has self-commutation capability and fast dynamic response, which can be adapted to weak grid access scenarios and ensure good overall stability of the transmission system. The hybrid phase-commutation converter can reduce reactive power demand and reduce investment in reactive power compensation equipment, thereby reducing costs and footprint, while also reducing the risk of commutation failure. The combined topology meets the high-performance requirements of the system while also taking into account economic benefits. It performs well in terms of weak grid integration, footprint reduction, and cost-effectiveness, providing a better solution for long-distance transmission of large-scale renewable energy. In addition, the controller monitors the system status and, when an AC fault is detected, takes rapid measures according to the turn-off conditions of the hybrid phase-commutation converter to reduce the impact of fault current on the system and ensure the safe and stable operation of the system.

[0092] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A hybrid DC transmission system, characterized in that, include: DC bus; The sending end includes a sending end converter, the DC side of which is electrically connected to the DC bus, and the sending end converter includes a first modular multilevel converter; The receiving end includes a receiving-end converter, the DC side of which is electrically connected to the DC bus, and the AC side of which is used to connect to the receiving-end AC grid. The receiving-end converter includes a hybrid phase-commutation converter. The controller is communicatively connected to both the first modular multilevel converter and the hybrid commutator, and is used for: In the event of an AC fault detected in the hybrid DC transmission system, it is determined whether the hybrid commutator meets the shutdown conditions, and if the hybrid commutator meets the shutdown conditions, the hybrid commutator is controlled to shut down. When the hybrid commutator is in normal operating mode, the actual value of the DC current of the hybrid commutator is obtained to obtain the second actual value of the DC current, and the actual value of the DC voltage of the hybrid commutator is obtained to obtain the second actual value of the DC voltage. Obtain the actual resonant frequency of the hybrid DC transmission system, and determine the inductance value of the virtual inductor based at least on the actual resonant frequency; obtain the virtual inductor output based on the actual value of the second DC current and the inductance value of the virtual inductor; determine the target DC voltage of the hybrid commutation converter based on the virtual inductor output, the actual value of the second DC voltage, and the DC voltage reference value, and adjust the DC voltage of the hybrid commutation converter to the target DC voltage.

2. The hybrid DC transmission system according to claim 1, characterized in that, The turn-off condition includes a fixed-delay turn-off condition, which characterizes the state of the hybrid commutator during its commutation cycle. Determining whether the hybrid commutator meets the turn-off condition, and controlling the hybrid commutator to turn off when the turn-off condition is met, includes: Determine whether the hybrid commutation converter satisfies the fixed delay turn-off condition; When the hybrid commutator meets the fixed delay turn-off condition, the hybrid commutator is controlled to turn off after a preset delay period.

3. The hybrid DC transmission system according to claim 1, characterized in that, The shutdown condition includes a current change rate detection shutdown condition, which characterizes the situation where the rate of change of the DC current of the hybrid commutator is greater than a preset current change rate threshold. The process involves determining whether the hybrid commutator meets the shutdown condition, and if the hybrid commutator meets the shutdown condition, controlling the hybrid commutator to shut down, including: Determine whether the hybrid commutation converter meets the current change rate detection turn-off condition; When the hybrid commutator meets the current change rate detection turn-off condition, the hybrid commutator is controlled to turn off immediately.

4. The hybrid DC transmission system according to claim 1, characterized in that, The controller is also used for: If the DC power value on the DC bus is detected to be lower than the predetermined DC power value, the DC current of the hybrid commutator is adjusted to a preset DC current threshold, which represents the minimum DC current that the hybrid commutator can carry.

5. The hybrid DC transmission system according to claim 1, characterized in that, The first modular multilevel converter includes multiple bridge arms, each bridge arm including multiple sub-modules, each sub-module including a sub-module capacitor, and the controller is further configured to: When the first modular multilevel converter is in normal operating mode, the actual value of the DC voltage of the first modular multilevel converter is obtained to obtain the first actual value of the DC voltage, and the actual value of the DC current of the first modular multilevel converter is obtained to obtain the first actual value of the DC current. Obtain the actual resonant frequency of the hybrid DC transmission system, and determine the capacitance of the virtual capacitor based at least on the capacitance of the submodule capacitor and the actual resonant frequency; The active power value of the first modular multilevel converter is determined based on the actual value of the first DC voltage, the actual value of the first DC current, and the capacitance of the virtual capacitor. Obtain the actual value of the d-axis component of the AC current of the first modular multilevel converter; Based on the active power value, the active power reference value, and the actual value of the d-axis component of the AC current, the target active power of the first modular multilevel converter is determined, and the active power of the first modular multilevel converter is adjusted to the target active power.

6. The hybrid DC transmission system according to claim 5, characterized in that, The active power value of the first modular multilevel converter is determined based on the actual value of the first DC voltage, the actual value of the first DC current, and the capacitance of the virtual capacitor, including: The capacitance of the virtual capacitor is calculated by differentiation; The capacitance of the virtual capacitor after differential calculation is multiplied by the first DC voltage to obtain the first current value; Calculate the difference between the actual value of the first DC current and the first current value to obtain the first difference; Multiply the first difference by the first DC voltage to obtain the active power value.

7. The hybrid DC transmission system according to claim 1, characterized in that, Based on the actual value of the second DC current and the inductance value of the virtual inductor, the virtual inductor output is obtained, including: The inductance value of the virtual inductor is calculated by differentiation; The inductance value of the virtual inductor after differential calculation is multiplied by the actual value of the second DC current to obtain the output value of the virtual inductor.

8. The hybrid DC transmission system according to claim 1, characterized in that, The target DC voltage of the hybrid commutator is determined based on the virtual inductance output, the actual value of the second DC voltage, and the DC voltage reference value, including: Calculate the difference between the actual value of the second DC voltage and the reference value of the DC voltage to obtain the DC voltage difference; The firing angle of the hybrid commutation converter is obtained by calculating the difference between the DC voltage difference and the output of the virtual inductor. The target DC voltage is determined based on the firing angle.

9. An electric power system, characterized in that, include: The hybrid DC transmission system according to any one of claims 1 to 8.

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