Fault ride-through method and fault ride-through system for direct-current transformer

Abstract

The invention relates to a fault ride-through method of a direct current transformer and a fault ride-through system of the direct current transformer. Working modes of low-voltage bridge arm direct-current voltage component adjustment, low-voltage port direct-current voltage control and high-voltage bridge arm alternating-current voltage control are switched, and direct current of a low-voltage port is adjusted through a first fault ride-through mode, a constant direct-current power mode and a bridge arm energy balance control mode; during the fault ride-through period, the low-voltage port is controlled to output negative voltage, current opposite to fault current is generated, the direct-current transformer quickly blocks the fault current, voltage balance of the bridge arm sub-module is maintained during the fault period, and stable fault ride-through is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of high-voltage direct current (HVDC) transmission, and in particular to a fault ride-through method and a fault ride-through system for a DC transformer. Background Art

[0002] The DC collection and transmission system has significant advantages such as a wide collection range, long transmission distance, and no need for AC power support. It is the preferred solution for large-scale new energy collection and transmission in my country's desertified and deep-sea areas.

[0003] In DC collection and transmission systems, there are multiple different DC voltage levels. When connecting two DC transmission lines with different voltage levels, a DC transformer becomes a necessary interface device. A DC transformer consists of an input bridge arm, an output bridge arm, and a common bridge arm connected at a common point.

[0004] When a single-pole ground fault or a bipolar short-circuit fault occurs on the DC side of a DC transformer, the protection methods proposed in related technologies block the fault current by blocking the arm or all arm submodules where the fault occurs. However, these methods suffer from drawbacks such as uncontrollable DC transformers and a long system restart process after the fault is cleared, which reduces the reliability of the DC transformer. Therefore, there is an urgent need to research non-blocking DC transformer fault ride-through methods to help the DC transformer quickly block the fault current and maintain voltage balance among the arm submodules during the fault, achieving smooth fault ride-through and rapid recovery after the fault is cleared. Summary of the Invention

[0005] In order to solve the problems existing in the prior art, the present invention provides a fault ride-through method for a DC transformer. The DC transformer includes a high-voltage bridge arm, a low-voltage bridge arm, and a common bridge arm connected by a common point. The output end of the low-voltage bridge arm is used as a low-voltage port. The method includes:

[0006] When the DC transformer is in a fault state, the operating mode of the low-voltage bridge arm DC voltage component regulation is switched to the first fault ride-through mode, the operating mode of the low-voltage port DC voltage control is switched to the constant DC power mode, and the operating mode of the high-voltage bridge arm AC voltage control is switched to the bridge arm energy balance control mode;

[0007] Based on the DC voltage and DC power of the low-voltage port and the average voltage of the high-voltage bridge arm submodule, the DC current and DC voltage of the low-voltage port are adjusted by using the first fault ride-through mode, the constant DC power mode, and the bridge arm energy balance control mode to obtain adjusted DC current and DC voltage;

[0008] Based on the regulated DC current, switching the operating mode of regulating the DC voltage component of the low-voltage bridge arm to a second fault ride-through mode;

[0009] Based on the adjusted DC voltage, the DC transformer is controlled to perform fault ride-through using the second fault ride-through mode.

[0010] Optionally, the adjusting the DC current and DC voltage of the low-voltage port based on the DC voltage and DC power of the low-voltage port and the average voltage of the high-voltage bridge arm submodule by using the first fault ride-through mode, the constant DC power mode, and the bridge arm energy balance control mode to obtain the adjusted DC current and DC voltage includes:

[0011] Based on the DC voltage of the low-voltage port and the DC voltage component of the common bridge arm, using the first fault ride-through mode, obtaining a first DC reference voltage of the low-voltage bridge arm;

[0012] Based on the DC power of the low-voltage port, using the constant DC power mode, obtaining a first reference DC current of the low-voltage bridge arm;

[0013] Based on the reference voltage of the high-voltage bridge arm submodule and the average voltage of the high-voltage bridge arm submodule, the reference AC current of the high-voltage bridge arm is obtained by utilizing the bridge arm energy balance control mode;

[0014] Based on the first DC reference voltage, the first reference DC current, and the reference AC current, the DC current and the DC voltage of the low-voltage port are adjusted to obtain adjusted DC current and DC voltage.

[0015] Optionally, obtaining the first DC reference voltage of the low-voltage bridge arm based on the DC voltage of the low-voltage port and the DC voltage component of the common bridge arm by using the first fault ride-through mode includes:

[0016] Multiplying the DC voltage component of the common bridge arm by a first preset multiple to obtain a standby DC voltage component;

[0017] Calculating the difference between the DC voltage of the low-voltage port and the standby DC voltage component to obtain a voltage difference;

[0018] Based on the voltage difference, a first DC reference voltage of the low-voltage bridge arm is obtained by utilizing the first fault ride-through mode.

[0019] Optionally, the adjusting the DC current and DC voltage of the low-voltage port based on the first DC reference voltage, the first reference DC current, and the reference AC current to obtain the adjusted DC current and DC voltage includes:

[0020] generating a first modulation voltage of the low-voltage bridge arm based on the first DC reference voltage and the first reference DC current;

[0021] generating a standby modulation voltage of the high-voltage bridge arm based on the reference alternating current;

[0022] Based on the first modulation voltage and the standby modulation voltage, the low-voltage bridge arm and the high-voltage bridge arm are modulated to adjust the DC current and DC voltage of the low-voltage port to obtain adjusted DC current and DC voltage.

[0023] Optionally, generating the first modulation voltage of the low-voltage bridge arm based on the first DC reference voltage and the first reference DC current includes:

[0024] Based on the first reference DC current and the DC current of the low-voltage port, a first corrected DC voltage of the low-voltage bridge arm is obtained by using a high-order harmonic filtering link;

[0025] Based on the DC current, using virtual impedance, obtaining a second corrected DC voltage of the low-voltage bridge arm;

[0026] superimposing the first DC reference voltage, the first corrected DC voltage, and the second corrected DC voltage to obtain a reference DC voltage of the low-voltage bridge arm;

[0027] Based on the reference DC voltage, a first modulation voltage of the low-voltage bridge arm is generated by utilizing the low-voltage bridge arm modulation voltage.

[0028] Optionally, switching the operation mode of regulating the DC voltage component of the low-voltage bridge arm to the second fault ride-through mode based on the regulated DC current includes:

[0029] When the regulated DC current is less than a preset current threshold, the operation mode of regulating the DC voltage component of the low-voltage bridge arm is switched to a second fault ride-through mode.

[0030] Optionally, controlling the DC transformer to perform fault ride-through using the second fault ride-through mode based on the regulated DC voltage includes:

[0031] Within a preset fault ride-through time, based on the adjusted DC voltage and a DC voltage component of the common bridge arm having a second preset multiple, using the second fault ride-through mode, a second DC reference voltage of the low-voltage bridge arm is obtained;

[0032] generating a second modulation voltage of the low-voltage bridge arm based on the second DC reference voltage and the first average voltage of the low-voltage bridge arm submodule;

[0033] Based on the second modulation voltage, the DC transformer is controlled to perform fault ride-through.

[0034] Optionally, after the operation mode of regulating the low-voltage bridge arm DC voltage component is switched to the first fault ride-through mode, the operation mode of the low-voltage port DC voltage control is switched to the constant DC power mode, and the operation mode of the high-voltage bridge arm AC voltage control is switched to the bridge arm energy balance control mode, the method further includes:

[0035] Based on the actual AC voltage component and the reference AC voltage component inside the DC transformer, an AC modulation voltage is obtained by using proportional resonance control;

[0036] Based on the AC modulation voltage, an actual AC voltage component of the DC transformer is adjusted.

[0037] Optionally, after controlling the DC transformer to perform fault ride-through using the second fault ride-through mode based on the regulated DC voltage, the method further includes:

[0038] After a preset fault ride-through time, the operating mode of the low-voltage bridge arm DC voltage component regulation is switched to a fault recovery mode, and the operating mode of the low-voltage port DC voltage control is switched to a constant DC voltage mode;

[0039] Using a preset function to adjust the DC voltage component of the common bridge arm to obtain an adjusted DC voltage component;

[0040] Based on the DC voltage and the adjusted DC voltage component, a third DC reference voltage of the low-voltage bridge arm is obtained by utilizing a fault recovery mode;

[0041] Based on the third DC reference voltage, the DC transformer is controlled to return to a normal operating state using the constant DC voltage mode.

[0042] Optionally, controlling the DC transformer to return to a normal operating state based on the third DC reference voltage and utilizing the constant DC voltage mode includes:

[0043] Obtaining the actual DC voltage of the low-voltage port;

[0044] Based on the actual DC voltage and the preset DC voltage, using the constant DC voltage mode, obtaining a second reference DC current of the low-voltage bridge arm;

[0045] Based on the second reference DC current, a third corrected DC voltage of the low-voltage bridge arm is obtained by controlling the low-voltage port DC current;

[0046] Obtaining a third modulation voltage of the low-voltage bridge arm based on the second reference DC current, the third corrected DC voltage, and the second average voltage of the low-voltage bridge arm submodule;

[0047] Based on the third modulation voltage, modulating the low-voltage bridge arm to obtain a regulated actual DC voltage;

[0048] Based on the adjusted actual DC voltage, the DC transformer is controlled to return to a normal operating state.

[0049] Optionally, controlling the DC transformer to return to a normal operating state based on the adjusted actual DC voltage includes:

[0050] When the actual DC voltage after adjustment returns to the preset voltage value, it is determined that the DC transformer has returned to a normal operating state;

[0051] When the actual DC voltage after adjustment does not return to the preset voltage value, it is determined that the operating state of the DC transformer is a permanent fault state.

[0052] Optionally, after determining that the DC transformer has returned to a normal operating state, the method further includes:

[0053] The operating mode of the high-voltage bridge arm AC voltage control is switched to a fixed common point DC voltage mode, so that the DC transformer resumes active power transmission.

[0054] Optionally, the DC transformer further includes a grounding device, and the input end of the high-voltage bridge arm serves as a high-voltage port, and the high-voltage port is connected to the grounding device;

[0055] When the DC transformer is in a fault state, and the fault state is a single-pole grounding fault, the method further includes:

[0056] Based on the positive DC voltage and the negative current voltage of the high-voltage port, a corrected DC voltage of the high-voltage bridge arm is obtained by using proportional-integral control;

[0057] generating a target modulation voltage of the high-voltage bridge arm based on the average voltage and the corrected DC voltage;

[0058] Based on the target modulation voltage, the high-voltage bridge arm is modulated so that the grounding device consumes unbalanced energy in the positive and negative high-voltage bridge arms.

[0059] Based on the same inventive concept, a fault ride-through system for a DC transformer is provided. The DC transformer includes a high-voltage bridge arm, a low-voltage bridge arm, and a common bridge arm connected via a common point. The output end of the low-voltage bridge arm serves as a low-voltage port. The system includes:

[0060] A first working mode switching unit is used to switch the working mode of the low-voltage bridge arm DC voltage component regulation to the first fault ride-through mode, the working mode of the low-voltage port DC voltage control to the constant DC power mode, and the working mode of the high-voltage bridge arm AC voltage control to the bridge arm energy balance control mode when the DC transformer is in a fault state;

[0061] a DC parameter determination unit, configured to adjust the DC current and DC voltage of the low-voltage port based on the DC voltage and DC power of the low-voltage port and the average voltage of the high-voltage bridge arm submodule, using the first fault ride-through mode, the constant DC power mode, and the bridge arm energy balance control mode, to obtain adjusted DC current and DC voltage;

[0062] a fault ride-through mode switching unit, configured to switch the operating mode of regulating the DC voltage component of the low-voltage bridge arm to a second fault ride-through mode based on the regulated DC current;

[0063] A fault ride-through unit is configured to control the DC transformer to perform fault ride-through based on the adjusted DC voltage and using the second fault ride-through mode.

[0064] Optionally, the DC parameter determination unit is specifically used to: based on the DC voltage of the low-voltage port and the DC voltage component of the common bridge arm, use the first fault ride-through mode to obtain the first DC reference voltage of the low-voltage bridge arm; based on the DC power of the low-voltage port, use the constant DC power mode to obtain the first reference DC current of the low-voltage bridge arm; based on the reference voltage of the high-voltage bridge arm submodule and the average voltage of the high-voltage bridge arm submodule, use the bridge arm energy balance control mode to obtain the reference AC current of the high-voltage bridge arm; based on the first DC reference voltage, the first reference DC current and the reference AC current, adjust the DC current and DC voltage of the low-voltage port to obtain the adjusted DC current and DC voltage.

[0065] Optionally, the DC parameter determination unit is specifically used to: multiply the DC voltage component of the common bridge arm by a first preset multiple to obtain a standby DC voltage component; calculate the difference between the DC voltage of the low-voltage port and the standby DC voltage component to obtain a voltage difference; based on the voltage difference, use the first fault ride-through mode to obtain a first DC reference voltage of the low-voltage bridge arm.

[0066] Optionally, the DC parameter determination unit is specifically used to: generate a first modulation voltage for the low-voltage bridge arm based on the first DC reference voltage and the first reference DC current; generate a standby modulation voltage for the high-voltage bridge arm based on the reference AC current; modulate the low-voltage bridge arm and the high-voltage bridge arm based on the first modulation voltage and the standby modulation voltage to adjust the DC current and DC voltage of the low-voltage port and obtain adjusted DC current and DC voltage.

[0067] Optionally, the DC parameter determination unit is specifically used to: obtain a first corrected DC voltage of the low-voltage bridge arm based on the first reference DC current and the DC current of the low-voltage port by using a high-order harmonic filtering link; obtain a second corrected DC voltage of the low-voltage bridge arm based on the DC current by using a virtual impedance; superimpose the first DC reference voltage, the first corrected DC voltage and the second corrected DC voltage to obtain a reference DC voltage of the low-voltage bridge arm; and generate a first modulation voltage of the low-voltage bridge arm based on the reference DC voltage by using a low-voltage bridge arm modulation voltage.

[0068] Optionally, the fault ride-through mode switching unit is specifically configured to switch the operating mode of regulating the DC voltage component of the low-voltage bridge arm to a second fault ride-through mode when the regulated DC current is less than a preset current threshold.

[0069] Optionally, the fault ride-through unit is specifically used to: within a preset fault ride-through time, based on the adjusted DC voltage and the DC voltage component of the second preset multiple of the common bridge arm, utilize the second fault ride-through mode to obtain a second DC reference voltage of the low-voltage bridge arm; based on the second DC reference voltage and the first average voltage of the low-voltage bridge arm sub-module, generate a second modulation voltage of the low-voltage bridge arm; based on the second modulation voltage, control the DC transformer to perform fault ride-through.

[0070] Optionally, after the operation mode of regulating the low-voltage bridge arm DC voltage component is switched to the first fault ride-through mode, the operation mode of the low-voltage port DC voltage control is switched to the constant DC power mode, and the operation mode of the high-voltage bridge arm AC voltage control is switched to the bridge arm energy balance control mode, the system further includes:

[0071] an AC modulation voltage unit, configured to obtain an AC modulation voltage by using proportional resonance control based on an actual AC voltage component and a reference AC voltage component inside the DC transformer;

[0072] An AC voltage regulating unit is configured to regulate an actual AC voltage component of the DC transformer based on the AC modulation voltage.

[0073] Optionally, after controlling the DC transformer to perform fault ride-through using the second fault ride-through mode based on the regulated DC voltage, the system further includes:

[0074] A second working mode switching unit is used to switch the working mode of regulating the DC voltage component of the low-voltage bridge arm to a fault recovery mode and the working mode of controlling the DC voltage of the low-voltage port to a constant DC voltage mode after a preset fault ride-through time.

[0075] a DC voltage component determining unit, configured to adjust the DC voltage component of the common bridge arm using a preset function to obtain an adjusted DC voltage component;

[0076] a DC reference voltage determining unit, configured to obtain a third DC reference voltage of the low-voltage bridge arm based on the DC voltage and the adjusted DC voltage component using a fault recovery mode;

[0077] A recovery control unit is used to control the DC transformer to recover to a normal operating state based on the third DC reference voltage and using the constant DC voltage mode.

[0078] Optionally, the recovery control unit is specifically used to: obtain the actual DC voltage of the low-voltage port; based on the actual DC voltage and the preset DC voltage, use the constant DC voltage mode to obtain a second reference DC current of the low-voltage bridge arm; based on the second reference DC current, use the low-voltage port DC current control to obtain a third corrected DC voltage of the low-voltage bridge arm; based on the second reference DC current, the third corrected DC voltage and the second average voltage of the low-voltage bridge arm sub-module, obtain a third modulation voltage of the low-voltage bridge arm; based on the third modulation voltage, modulate the low-voltage bridge arm to obtain an adjusted actual DC voltage; based on the adjusted actual DC voltage, control the DC transformer to restore to normal operating state.

[0079] Optionally, the recovery control unit is specifically used to: determine that the DC transformer has recovered to a normal operating state when the actual DC voltage after adjustment has recovered to a preset voltage value; and determine that the operating state of the DC transformer is a permanent fault state when the actual DC voltage after adjustment has not recovered to the preset voltage value.

[0080] Optionally, the system further comprises:

[0081] The active power recovery unit is used to switch the working mode of the high-voltage bridge arm AC voltage control to the fixed common point DC voltage mode, so that the DC transformer can restore active power transmission.

[0082] Optionally, the DC transformer further includes a grounding device, and the input end of the high-voltage bridge arm serves as a high-voltage port, and the high-voltage port is connected to the grounding device; when the DC transformer is in a fault state, and the fault state is a single-pole grounding fault, the system further includes:

[0083] a corrected DC voltage determining unit, configured to obtain a corrected DC voltage of the high-voltage bridge arm by using proportional-integral control based on the positive DC voltage and the negative current voltage of the high-voltage port;

[0084] a target modulation voltage generating unit, configured to generate a target modulation voltage of the high-voltage bridge arm based on the average voltage and the corrected DC voltage;

[0085] The high-voltage bridge arm modulation unit is used to modulate the high-voltage bridge arm based on the target modulation voltage so that the grounding device consumes unbalanced energy in the positive and negative high-voltage bridge arms.

[0086] Compared with the prior art, the present invention has the following beneficial effects:

[0087] The present invention provides a fault ride-through method and a fault ride-through system for a DC transformer. The fault ride-through method for a DC transformer switches the operating modes of regulating the DC voltage component of a low-voltage bridge arm, controlling the DC voltage of a low-voltage port, and controlling the AC voltage of a high-voltage bridge arm when the operating state of the DC transformer is a fault state. The DC current of the low-voltage port is regulated through a first fault ride-through mode, a constant DC power mode, and a bridge arm energy balance control mode. During the fault ride-through period, the low-voltage port is controlled to output a negative voltage to generate a current in the opposite direction of the fault current, so that the DC transformer quickly blocks the fault current and maintains voltage balance of the bridge arm submodules during the fault period, thereby achieving smooth fault ride-through.

[0088] Figure Description

[0089] Figure 1 A schematic diagram of a DC transformer topology provided by the present invention;

[0090] Figure 2 A flow chart of a fault ride-through method for a DC transformer provided by the present invention;

[0091] Figure 3 A schematic diagram of a low-voltage port DC current control provided by the present invention;

[0092] Figure 4 A schematic diagram of low-voltage port virtual impedance control provided by the present invention;

[0093] Figure 5 A schematic diagram of a low-voltage bridge arm DC voltage component regulation provided by the present invention;

[0094] Figure 6 A schematic diagram of a low-voltage port DC voltage control provided by the present invention;

[0095] Figure 7 A schematic diagram of AC voltage control of a high-voltage bridge arm provided by the present invention;

[0096] Figure 8 A schematic diagram of the high-voltage side grounding of a DC transformer provided by the present invention;

[0097] Figure 9 A schematic diagram of voltage balancing control between electrodes of a high-voltage port provided by the present invention;

[0098] Figure 10 A schematic diagram of a fault ride-through system for a DC transformer provided by the present invention. DETAILED DESCRIPTION

[0099] Example 1:

[0100] The embodiment of the present invention provides a fault ride-through method for a DC transformer. The DC transformer can be used as follows: Figure 1 In the true bipolar topology shown, each pole can adopt a single-phase, two-phase or multi-phase topology. Each phase consists of three bridge arms, namely a low-voltage bridge arm (represented by L), a high-voltage bridge arm (represented by H), and a common bridge arm (represented by W). Each bridge arm contains a certain number of sub-modules SM. The low-voltage bridge arm adopts a full-bridge sub-module configuration, and the high-voltage bridge arm and the common bridge arm adopt a half-bridge sub-module configuration. The DC transformer includes a high-voltage bridge arm H, a low-voltage bridge arm L, and a common bridge arm W connected by a common point, with the output end of the low-voltage bridge arm L serving as a low-voltage port and the input end of the high-voltage bridge arm H serving as a high-voltage port. The common point may include a first common point S1 and a second common point S2. The first common point S1 is respectively connected to the first sub-bridge arm of the high-voltage bridge arm H, the first end of the common bridge arm W, and the first sub-bridge arm of the low-voltage bridge arm L. The second common point S2 is respectively connected to the first sub-bridge arm of the high-voltage bridge arm H, the first end of the common bridge arm W, and the first sub-bridge arm of the low-voltage bridge arm L. The sub-bridge arm can be formed by connecting a plurality of sub-modules SM in series with a resistor, and the common bridge arm W can be formed by connecting a plurality of sub-modules SM in series with two resistors. It can be understood that the above-mentioned DC transformer structure is only an example, and other DC transformer structures that can be applied to the fault ride-through method provided by the present invention are not excluded.

[0101] Based on the above-mentioned DC transformer, the process of the fault ride-through method of the DC transformer provided by the present invention is as follows: Figure 2 As shown, the following steps may be included:

[0102] In step 101, when the DC transformer is in a fault state, the operating mode of the low-voltage bridge arm DC voltage component regulation is switched to the first fault ride-through mode, the operating mode of the low-voltage port DC voltage control is switched to the constant DC power mode, and the operating mode of the high-voltage bridge arm AC voltage control is switched to the bridge arm energy balance control mode.

[0103] In step 102, based on the DC voltage and DC power of the low-voltage port and the average voltage of the high-voltage bridge arm sub-module, the DC current and DC voltage of the low-voltage port are regulated using the first fault ride-through mode, the constant DC power mode and the bridge arm energy balance control mode to obtain regulated DC current and DC voltage.

[0104] In step 103, based on the regulated DC current, the operation mode for regulating the DC voltage component of the low-voltage bridge arm is switched to a second fault ride-through mode.

[0105] In step 104 , based on the adjusted DC voltage, the DC transformer is controlled to perform fault ride-through using a second fault ride-through mode.

[0106] This process shows a general fault ride-through solution in the fault state of the DC transformer. The fault state of the DC transformer in the above step 101 can include a single-pole grounding fault and a double-pole short circuit fault. The fault of the DC transformer generally occurs at the low-voltage port. Therefore, the low-voltage port of the DC transformer can be monitored to identify whether the DC transformer has a fault and whether the fault is a single-pole grounding fault or a double-pole short circuit fault. For example, by collecting the current at both ends of the low-voltage port, when the current at both ends of the low-voltage port exceeds the preset threshold, it is identified that the DC transformer has a double-pole short circuit fault. When the current at any end of the low-voltage port exceeds the preset threshold, it is identified that the DC transformer has a single-pole grounding fault. In some scenarios, when a single-pole grounding fault occurs in the DC transformer, the system connected to the low-voltage port is a symmetrical single-pole system or a symmetrical double-pole system, and the common bridge arm is controlled to operate the fault pole and the non-fault pole.

[0107] The above can be controlled by different working modes of adjusting the DC voltage component of the low-voltage bridge arm and different working modes of controlling the DC voltage of the low-voltage port. Figure 1 The low voltage bridge arm in the DC transformer shown in the figure can be modulated, and the above can be controlled by different working modes of the high voltage bridge arm AC voltage control. Figure 1The high-voltage bridge arm of the DC transformer is modulated, wherein the operating modes for regulating the DC voltage component of the low-voltage bridge arm can include a steady-state operation mode, a first fault ride-through mode, a second fault ride-through mode, and a fault recovery mode. The operating modes for controlling the DC voltage at the low-voltage port can include a constant DC power mode and a constant DC voltage mode. The operating modes for controlling the AC voltage of the high-voltage bridge arm can include a bridge arm energy balance control mode and a constant common point DC voltage component mode.

[0108] The present invention adds low-voltage bridge arm DC voltage component regulation to the low-voltage bridge arm controller. This dynamically adjusts the DC voltage component of the low-voltage bridge arm at the faulty pole, rapidly suppressing the DC transformer's fault current, in response to DC current overcurrent at the low-voltage port during a single-pole ground fault or a bipolar short-circuit fault. Furthermore, when the DC transformer is in a faulty state, the operating mode of the low-voltage port DC power control is switched from a constant DC voltage mode to a constant power mode, where the DC voltage at the low-voltage port is 1.0 pu and the DC power at the low-voltage port is controlled to zero. This ensures that the DC voltage at the low-voltage port is controlled to the rated value when the DC transformer is in normal operation and that the DC power at the low-voltage port is controlled to zero when the DC transformer is in a faulty state.

[0109] In the above step 101, when the DC transformer is in a fault state, the DC voltage component of the low-voltage bridge arm is regulated and switched to the first fault ride-through mode, and the first fault ride-through mode is used to generate a first DC reference voltage of the low-voltage bridge arm. At the same time, the DC voltage control of the low-voltage port is switched to the constant DC power mode, and the constant DC power mode is used to generate a first reference DC current of the low-voltage bridge arm. Through the first DC reference voltage and the first reference DC current of the low-voltage bridge arm, the DC voltage of the low-voltage port is controlled to become a negative voltage and the DC power of the low-voltage port is controlled to become zero, so that the DC voltage of the low-voltage port is lower than the low-voltage side DC line voltage (zero value), thereby generating a current in the opposite direction to the bridge arm power generation current, quickly blocking the fault current, and switching the AC voltage control of the high-voltage bridge arm to the bridge arm energy balance control mode. Through the average voltage of the high-voltage bridge arm sub-module, closed-loop feedback control is performed on the high-voltage bridge arm to maintain the energy balance of the bridge arm during the fault period and suppress the voltage fluctuation of the sub-module.

[0110] It should be noted that when the DC transformer is in a normal state, the operating mode for regulating the DC voltage component of the low-voltage bridge arm can be a steady-state operation mode, the operating mode for the DC voltage control at the low-voltage port can be a constant DC power mode, and the operating mode for the AC voltage control at the high-voltage bridge arm can be a constant common point DC voltage component mode. When the DC transformer is in a faulty state, the operating mode for the DC voltage control at the low-voltage port can be switched to a constant DC power mode, the operating mode for the AC voltage control at the high-voltage bridge arm can be switched to a bridge arm energy balance control mode, and the operating mode for regulating the DC voltage component of the low-voltage bridge arm can be switched sequentially to a first fault ride-through mode, a second fault ride-through mode, and a fault recovery mode, thereby achieving fault ride-through.

[0111] Possible implementations of the above step 102 may include the following steps 1021 to 1024:

[0112] In step 1021, based on the DC voltage of the low-voltage port and the DC voltage component of the common bridge arm, a first DC reference voltage of the low-voltage bridge arm is obtained using a first fault ride-through mode.

[0113] A possible implementation of this step may include: multiplying the DC voltage component of the common bridge arm by a first preset multiple to obtain a standby DC voltage component; subtracting the DC voltage of the low-voltage port from the standby DC voltage component to obtain a voltage difference; and using the first fault ride-through mode based on the voltage difference to obtain a first DC reference voltage of the low-voltage bridge arm. The first preset multiple may be an integer greater than 2, such as 3, 5, or 6.

[0114] In step 1022 , based on the DC power of the low-voltage port, a first reference DC current of the low-voltage bridge arm is obtained using a constant DC power mode.

[0115] In step 1023 , based on the reference voltage of the high-voltage bridge arm submodule and the average voltage of the high-voltage bridge arm submodule, a reference AC current of the high-voltage bridge arm is obtained by utilizing the bridge arm energy balance control mode.

[0116] In step 1024 , the DC current and DC voltage of the low-voltage port are regulated based on the first DC reference voltage, the first reference DC current, and the reference AC current to obtain regulated DC current and DC voltage.

[0117] Possible implementation methods of the above-mentioned step 1024 may be: based on the first DC reference voltage and the first reference DC current, generate a first modulation voltage of the low-voltage bridge arm; based on the reference AC current, generate a standby modulation voltage of the high-voltage bridge arm; based on the first modulation voltage and the standby modulation voltage, modulate the low-voltage bridge arm and the high-voltage bridge arm to adjust the DC current and DC voltage of the low-voltage port, and obtain the adjusted DC current and DC voltage.

[0118] Optionally, generating the first modulation voltage of the low-voltage bridge arm based on the first DC reference voltage and the first reference DC current in the above embodiment may include: based on the first reference DC current and the DC current of the low-voltage port, using a high-order harmonic filtering link to obtain a first corrected DC voltage of the low-voltage bridge arm; based on the DC current, using a virtual impedance to obtain a second corrected DC voltage of the low-voltage bridge arm; superimposing the first DC reference voltage, the first corrected DC voltage and the second corrected DC voltage to obtain a reference DC voltage of the low-voltage bridge arm; based on the reference DC voltage, using the low-voltage bridge arm modulation voltage to generate the first modulation voltage of the low-voltage bridge arm.

[0119] The low-voltage bridge arm modulation voltage generation may be a low-voltage bridge arm modulation voltage generator, or a low-voltage bridge arm modulation voltage generation algorithm, or a low-voltage bridge arm modulation voltage generation strategy.

[0120] It should be noted that by adding a high-order harmonic filtering step to the low-voltage port DC current control, the impact of double-frequency or even higher-frequency oscillation components contained in the low-voltage port DC current is eliminated, improving the DC current control effect of the low-voltage port. Furthermore, by introducing a virtual impedance into the low-voltage port DC voltage control, the virtual impedance voltage drop corrects the DC voltage of the low-voltage bridge arm, reducing the DC voltage of the low-voltage port, thereby reducing the oscillation amplitude between the fault point and the DC transformer, and the current fed into the fault point by the DC transformer.

[0121] For example, Figure 3 As shown, the low voltage port DC current is controlled by the first reference DC current I dc_L_ref The DC current I of the low voltage port dc_L Closed-loop feedback control is performed, and after passing through the high-order harmonic filtering link (which may include LPF (Low-Pass Filter)) and PI (Proportional-Integral Controller), the first corrected DC voltage E of the low-voltage bridge arm is output. dcL_ref_rev1 , the first corrected DC voltage E dcL_ref_rev1 The modulation voltage is then transferred to the low-voltage bridge arm for generation. Since the DC current at the low-voltage port typically contains double-frequency or even higher-frequency oscillation components, directly using the measured DC current for modulation will not yield optimal modulation results. By adding a high-order harmonic filtering step, the effects of these high-order harmonic oscillation components are eliminated, improving the modulation effect.

[0122] like Figure 4As shown in FIG, the inductive part of the virtual impedance in the virtual impedance control of the low-voltage port is realized by a differential link sL, and the high-frequency noise introduced by the differential link is filtered out by a low-pass filter link. Therefore, a HPF (High-Pass Filter) can be directly used here to realize the virtual impedance. The voltage drop generated by the DC current of the low-voltage port passing through the virtual impedance is used as the second corrected DC voltage E of the low-voltage bridge arm. dcL_ref_rev2 , the correction value will be passed to the low voltage bridge arm modulation voltage generation.

[0123] A possible implementation of step 103 may be: when the regulated DC current is less than a preset current threshold, switching the operation mode of regulating the DC voltage component of the low-voltage bridge arm to the second fault ride-through mode.

[0124] The preset current threshold may be a current close to zero, such as 0.01 pu, 0.015 pu, or 0.012 pu.

[0125] It should be noted that the regulated DC current I dc_LP , when the regulated DC current I dc_LP When it drops to a value close to zero (0.01 pu can be selected here), in order to prevent the reverse overcurrent caused by the continuous overvoltage at the low-voltage port from impacting the DC transformer, the DC voltage component of the low-voltage bridge arm is regulated and switched to the second fault ride-through mode, controlling the DC voltage of the low-voltage port to zero, matching the DC line voltage (zero value) on the low-voltage side.

[0126] Possible implementations of step 104 may include the following steps: within a preset fault ride-through duration, utilizing a second fault ride-through mode to obtain a second DC reference voltage for the low-voltage bridge arm based on the adjusted DC voltage and a DC voltage component of the common bridge arm at a second preset multiple; generating a second modulation voltage for the low-voltage bridge arm based on the second DC reference voltage and a first average voltage of the low-voltage bridge arm submodule; and controlling the DC transformer to perform fault ride-through based on the second modulation voltage. The second preset multiple may be less than or equal to 2, for example, 2, 1.5, or 1.6.

[0127] For example, in the second fault ride-through mode, the DC voltage of the low voltage port is The DC voltage component of the common bridge arm The second DC reference voltage of the low-voltage bridge arm is obtained by taking the difference of 2 times and normalizing it to control the DC voltage of the low-voltage port to zero, so as to prevent the reverse current generated by the negative DC voltage from being too large and causing impact on the DC transformer.

[0128] Optionally, after step 101, proportional resonance control may be used to obtain an AC modulation voltage based on the actual AC voltage component inside the DC transformer and a reference AC voltage component; and the actual AC voltage component of the DC transformer may be adjusted based on the AC modulation voltage.

[0129] For example, using proportional resonance control, the actual AC voltage component E inside the DC transformer is ac_order With the reference AC voltage component E ac Perform closed-loop feedback control and output AC modulation voltage E ac_ref , the modulation voltage of the DC transformer is generated by the AC modulation voltage, and the actual AC voltage component of the DC transformer is adjusted. Among them, when the DC transformer is in normal operation, the actual AC voltage component E inside the DC transformer is ac_order The same value as the DC voltage rating of the low voltage port can be selected. When the DC transformer is in a fault state, the actual AC voltage component E inside the DC transformer is ac_order Adjust it to a value close to zero to reduce the regulation time of the negative voltage output of the low-voltage port.

[0130] Optionally, after step 104, the method may further include the following steps S1 to S4:

[0131] In step S1, after the preset fault ride-through time, the operating mode of the low-voltage bridge arm DC voltage component regulation is switched to the fault recovery mode, and the operating mode of the low-voltage port DC voltage control is switched to the constant DC voltage mode.

[0132] The preset fault ride-through time may be 200ms, or may be determined according to actual conditions, for example, greater than 200ms, or less than 200ms.

[0133] In step S2, the DC voltage component of the common bridge arm is adjusted using a preset function to obtain an adjusted DC voltage component.

[0134] The preset function may be a linear function, for example, a univariate linear function with a slope of K, where K may be 2, 3, or 5.

[0135] In step S3, based on the DC voltage and the adjusted DC voltage component, a third DC reference voltage of the low-voltage bridge arm is obtained by utilizing a fault recovery mode.

[0136] In step S4, based on the third DC reference voltage, the DC transformer is controlled to return to a normal operating state using a constant DC voltage mode.

[0137] For example, after the preset fault ride-through time, the DC transformer voltage is rebuilt, the working mode of the low voltage bridge arm DC voltage component adjustment is switched to the fault recovery mode, and the DC voltage component of the common bridge arm is adjusted by using the preset function. From zero to the rated value, thereby controlling the third DC reference voltage to rise from a negative value to 0, and controlling the DC voltage of the low-voltage port to rise from zero to the voltage when the DC transformer is in normal operating state.

[0138] A possible implementation method of the above-mentioned step S4 can be: obtaining the actual DC voltage of the low-voltage port; based on the actual DC voltage and the preset DC voltage, using the fixed DC voltage mode, obtaining the second reference DC current of the low-voltage bridge arm; based on the second reference DC current, using the low-voltage port DC current control, obtaining the third corrected DC voltage of the low-voltage bridge arm; based on the second reference DC current, the third corrected DC voltage and the second average voltage of the low-voltage bridge arm sub-module, obtaining the third modulation voltage of the low-voltage bridge arm; based on the third modulation voltage, modulating the low-voltage bridge arm to obtain the actual DC voltage after adjustment; based on the actual DC voltage after adjustment, controlling the DC transformer to return to normal operating state.

[0139] Optionally, the specific implementation of controlling the DC transformer to return to a normal operating state based on the regulated actual DC voltage may include:

[0140] When the actual DC voltage after adjustment returns to the preset voltage value, it is determined that the DC transformer has returned to normal operation; when the actual DC voltage after adjustment does not return to the preset voltage value, it is determined that the operating state of the DC transformer is a permanent fault state.

[0141] It should be noted that when the actual DC voltage after adjustment returns to the preset voltage value, the DC voltage at the low-voltage port is successfully rebuilt, indicating that the DC transformer has returned to normal operation and the fault of the DC transformer has been cleared; when the actual DC voltage after adjustment does not return to the preset voltage value, it indicates that the DC voltage at the low-voltage port has failed to be rebuilt, and the operating state of the DC transformer is a permanent fault state, then the DC isolation switches on both sides of the DC transformer are disconnected.

[0142] Optionally, after determining that the DC transformer has returned to a normal operating state, the method may further include:

[0143] The operating mode of the high-voltage bridge arm AC voltage control is switched to the fixed common point DC voltage mode to enable the DC transformer to resume active power transmission.

[0144] It should be noted that after the DC voltage of the low-voltage port is successfully rebuilt, the sub-module voltage of each bridge arm is adjusted to restore to the rated value when the DC transformer is in normal operation, and the working mode of the AC voltage control of the high-voltage bridge arm is switched to the fixed common point DC voltage mode, and the AC voltage of the common point is adjusted to the rated value when the DC transformer is in normal operation. The DC transformer resumes active power transmission, and the DC transmission system resumes active power transmission, and the DC transmission system is restored to the normal operation state before the fault.

[0145] When the DC transformer is in a faulty state, the operating mode of the high-voltage bridge arm AC voltage control can be switched from the fixed common point DC voltage mode to the bridge arm energy balance mode, and the common point DC voltage is controlled to 1.0pu. The sub-module voltage of the bridge arm energy balance control mode is controlled to the average value of the high-voltage, common and low-voltage bridge arm sub-module voltages. This can ensure that when the DC transformer is operating normally, the DC voltage at the common point is the rated value. When the DC transformer fails, the voltage of the high-voltage bridge arm sub-module is balanced and maintained near the rated value.

[0146] For example, Figure 5 As shown, the low-voltage bridge arm DC voltage component is adjusted according to the operating state of the DC transformer, and the low-voltage bridge arm DC voltage component E is adaptively adjusted. dcL_ref_Base , E dcL_ref_Base The modulation voltage will be passed to the low voltage bridge arm for generation.

[0147] When the DC transformer is in normal operation, the DC voltage component of the low-voltage bridge arm is regulated using steady-state operation mode I (also known as zero voltage control). The DC voltage component of the common bridge arm After the difference is made and the per-unit processing is performed, the standby DC reference voltage E of the low-voltage bridge arm is obtained. dcL_Base1 , and equal.

[0148] When the DC transformer is in a fault state, the DC voltage component of the low-voltage bridge arm is adjusted in stages. At the fault initiation stage, in order to quickly suppress the positive and negative fault currents of the DC transformer to zero, the fault ride-through mode II (also known as the maximum negative voltage control, i.e. the first fault ride-through mode) is adopted to adjust the DC voltage of the low-voltage port. AND (ie the first preset multiple) N or N max times the DC voltage component of the common bridge arm After the difference is made and the standardization is performed, the first DC reference voltage E of the low-voltage bridge arm is obtained. dcL_Base2 , where N or N maxis an integer greater than 2, and the DC voltage of the low-voltage port is controlled to a negative voltage value to achieve rapid suppression of the fault current to zero. After the fault current is suppressed to zero, the fault ride-through mode III (also known as negative voltage control, i.e. the second fault ride-through mode) is adopted to control the DC voltage of the low-voltage port to a negative voltage value. The DC voltage component of the common bridge arm is twice (i.e., the second preset multiple) After the difference is made and the standardization is performed, the second DC reference voltage E of the low-voltage bridge arm is obtained. dcL_Base3 , control the DC voltage of the low-voltage port to zero, and prevent the reverse current generated by the negative DC voltage from being too large and causing impact on the transformer.

[0149] When the DC transformer is in the fault recovery state, the DC voltage component of the low-voltage bridge arm is regulated using the fault ride-through mode IV (also known as voltage recovery control, i.e., fault recovery mode). The DC voltage of the common bridge arm adjusted by a preset function After the difference is made and the standardization is performed, the third DC reference voltage E of the low voltage bridge arm is obtained. dcL_Base4 .

[0150] Based on the operating status of the DC transformer, the above-mentioned standby DC reference voltage, the first fault ride-through mode, the second fault ride-through mode or the third DC reference voltage is used as the DC reference voltage of the low-voltage bridge arm to adjust the DC current and DC voltage of the low-voltage port.

[0151] like Figure 6 As shown in Figure 1, the low-voltage port DC voltage control includes two control modes: one is the constant DC voltage mode, and the other is the constant DC power mode. When the DC transformer is in normal operation, the low-voltage port DC voltage control adopts the constant low-voltage port DC voltage mode, using PI to control the DC voltage of the low-voltage port. The actual DC voltage E of the low voltage port dcL Perform closed-loop feedback control and output the reference DC current I of the low-voltage bridge arm dc_L_ref When the DC transformer is in a fault state, in order to quickly suppress the fault current, the DC power mode is used to adjust the DC power of the low voltage port. Set to 0 so that the first reference DC current I generated by the low voltage bridge arm dc_L_ref is 0, and the first reference DC current is passed to the low voltage port DC current control, where P dcL is the actual DC power of the low voltage port.

[0152] like Figure 7As shown, the high-voltage bridge arm AC voltage control can include two control modes, one is the fixed common point DC voltage component mode, and the other is the bridge arm energy balance mode (also known as the high-voltage bridge arm energy balance mode). When the DC transformer is in normal operation, the high-voltage bridge arm AC voltage control adopts the fixed common point DC voltage component mode, using PI to reference the common point DC voltage component. The actual value of the DC voltage E at the common point dcS Perform closed-loop feedback control and output the reference AC current I of the high-voltage bridge arm ac_H_ref When the DC transformer is in a fault state, in order to maintain the energy balance of the high-voltage bridge arm, it switches to the energy balance control mode and uses PI to control the reference voltage of the high-voltage bridge arm submodule. The average voltage U of the high-voltage bridge arm submodule c_avg_H Perform closed-loop feedback control and output the high-voltage bridge arm AC current reference value I ac_H_ref The high voltage bridge arm AC current reference value is passed to the high voltage bridge arm AC current control, where the high voltage bridge arm submodule voltage reference value Select the average voltage of the high-voltage, common, and low-voltage bridge arm sub-modules.

[0153] Optionally, the DC transformer further includes a grounding device, and the input end of the high-voltage bridge arm serves as a high-voltage port, which is connected to the grounding device;

[0154] When the DC transformer is in a fault state and the fault state is a single-pole grounding fault, the following may also be included:

[0155] Based on the positive DC voltage and negative current voltage of the high-voltage port, proportional-integral control is used to obtain the corrected DC voltage of the high-voltage bridge arm; based on the average voltage and the corrected DC voltage, the target modulation voltage of the high-voltage bridge arm is generated; based on the target modulation voltage, the high-voltage bridge arm is modulated so that the grounding device consumes the unbalanced energy in the positive and negative high-voltage bridge arms.

[0156] It should be noted that for the single-pole grounding fault on the low-voltage side of the DC transformer, it is necessary to consider the energy imbalance problem of the positive and negative bridge arms caused by the single-pole grounding. Figure 8 The high-voltage side grounding scheme of the DC transformer shown provides a path for unbalanced energy in the bridge arm. The first end of the grounding device is connected to the AC side of the DC transformer and the AC side of the AC-DC converter. The second end of the grounding device is grounded, and the DC side of the AC-DC converter is connected to the high-voltage port.

[0157] By adding inter-pole voltage balancing control at the high-voltage port, the voltage balance between the positive and negative lines can be maintained when a single-pole ground fault occurs in the DC transformer, controlling the voltage balance between the positive and negative sub-modules of the high-voltage bridge arm. Furthermore, by installing a grounding device on the high-voltage side, a path is provided for the unbalanced energy in the positive and negative high-voltage bridge arms when a single-pole ground fault occurs in the DC transformer, accelerating the restoration of energy balance in the bridge arms and, consequently, accelerating the system's return to normal operation after the fault is cleared.

[0158] For example, Figure 9 As shown, the voltage balancing control between the high-voltage ports has an effect on the positive DC voltage E dcH_P and negative electrode current voltage E dcH_N Subtract and get the voltage deviation E dc_H_diff , and use PI to determine the inter-electrode reference voltage deviation of the high-voltage port With voltage deviation E dc_H_diff Perform closed-loop feedback control and output the corrected DC voltage E of the high-voltage bridge arm dcH_ref_rev , the corrected DC voltage of the high voltage bridge arm is transferred to the AC current control of the high voltage bridge arm, where the inter-electrode reference voltage deviation The value can be set to 0. The high-voltage port inter-pole voltage balance control is used to balance the energy between the positive and negative electrodes of the high-voltage bridge arm during a single-pole ground fault ride-through. Proportional-integral control is used to feedback control the positive DC voltage and negative current voltage of the high-voltage port to obtain a corrected DC voltage for the high-voltage bridge arm. This corrected DC voltage generates a target modulation voltage, which is then modulated to provide a path for unbalanced energy in the bridge arm during a single-pole ground fault. This ensures that the voltages of the positive and negative high-voltage bridge arm submodules quickly return to equilibrium and stabilize near the rated value, thereby achieving smooth fault ride-through of the DC transformer.

[0159] The purpose of the present invention is to provide a fault ride-through control method for single-pole grounding and bipolar short circuit on the low-voltage side of a DC transformer. By utilizing the "negative level" output characteristic of the low-voltage bridge arm full-bridge submodule, the DC voltage component reference of the low-voltage bridge arm where the fault pole is located is dynamically adjusted. During the fault ride-through period, the low-voltage port is controlled to output a negative voltage, generating a current opposite to the fault current and quickly suppressing the fault current. By introducing virtual impedance control in the low-voltage port DC voltage control link, that is, by correcting the low-voltage bridge arm DC voltage component reference through the voltage drop generated by the virtual impedance, the low-voltage port DC voltage is further reduced, achieving the purpose of reducing the oscillation amplitude between the fault point and the transformer and the current fed into the fault point by the transformer. By switching the low-voltage bridge arm AC voltage component control mode from constant DC voltage control to constant DC current control and adding a high-frequency filtering link in the inner loop current control link, the fault current is further suppressed. By switching the high-voltage bridge arm AC voltage component control mode from fixed common point DC voltage control to bridge arm energy balance control, the sub-module voltage fluctuation during the fault period is suppressed and the bridge arm energy balance is maintained. At the same time, in order to address the energy asymmetry problem of the positive and negative bridge arms on the high-voltage side caused by a single-pole grounding fault on the low-voltage side, the positive and negative line voltages are kept symmetrical through the high-voltage side inter-pole voltage balance control, and a grounding point is set on the high-voltage side DC grid to provide a path for the unbalanced energy in the bridge arm during a single-pole grounding fault, ensuring that the voltages of the positive and negative high-voltage bridge arm sub-modules quickly return to balance and stabilize near the rated value, thereby achieving smooth fault ride-through of the system.

[0160] The fault ride-through method of the DC transformer described above provides the fault ride-through process for single-pole grounding and bipolar short circuit on the low-voltage side of the DC transformer, from the occurrence of the fault to the restart after the fault. Among them, for the single-pole grounding fault on the low-voltage side, the fault ride-through process for the low-voltage port connected to the symmetrical single-pole system and the symmetrical bipolar system is given respectively. The proposed fault ride-through process can achieve rapid suppression of the fault current, reduce the voltage fluctuation of the sub-module during the fault, and after the fault is cleared, the voltage of the bridge arm sub-module can be quickly restored to near the rated value, while the system voltage reconstruction and power transmission can be smoothly achieved. Among them, when the DC transformer is operating normally, the internal AC voltage component of the DC transformer can be selected to be the same as the rated value of the DC voltage of the low-voltage port. When the DC transformer fails, the power transmission is terminated and adjusted to a value close to zero to meet the energy balance requirements of the sub-module.

[0161] Example 2:

[0162] Figure 10 This is a schematic diagram of a fault ride-through system for a DC transformer provided by the present invention. The DC transformer includes a high-voltage bridge arm, a low-voltage bridge arm, and a common bridge arm connected through a common point. The output end of the low-voltage bridge arm is used as a low-voltage port, such as Figure 10As shown, the system may include: a first working mode switching unit, which is used to switch the working mode of regulating the DC voltage component of the low-voltage bridge arm to a first fault ride-through mode, switch the working mode of the low-voltage port DC voltage control to a constant DC power mode, and switch the working mode of the high-voltage bridge arm AC voltage control to a bridge arm energy balance control mode when the DC transformer is in a fault state; a DC parameter determination unit, which is used to regulate the DC current and DC voltage of the low-voltage port based on the DC voltage and DC power of the low-voltage port and the average voltage of the high-voltage bridge arm sub-module, using the first fault ride-through mode, the constant DC power mode and the bridge arm energy balance control mode to obtain regulated DC current and DC voltage; a fault ride-through mode switching unit, which is used to switch the working mode of regulating the DC voltage component of the low-voltage bridge arm to a second fault ride-through mode based on the regulated DC current; and a fault ride-through unit, which is used to control the DC transformer to perform fault ride-through based on the regulated DC voltage and DC power and the average voltage of the high-voltage bridge arm sub-module.

[0163] Optionally, the DC parameter determination unit is specifically used to: based on the DC voltage of the low-voltage port and the DC voltage component of the common bridge arm, use the first fault ride-through mode to obtain the first DC reference voltage of the low-voltage bridge arm; based on the DC power of the low-voltage port, use the constant DC power mode to obtain the first reference DC current of the low-voltage bridge arm; based on the reference voltage of the high-voltage bridge arm sub-module and the average voltage of the high-voltage bridge arm sub-module, use the bridge arm energy balance control mode to obtain the reference AC current of the high-voltage bridge arm; based on the first DC reference voltage, the first reference DC current and the reference AC current, adjust the DC current and DC voltage of the low-voltage port to obtain the adjusted DC current and DC voltage.

[0164] Optionally, the DC parameter determination unit is specifically used to: multiply the DC voltage component of the common bridge arm by a first preset multiple to obtain a standby DC voltage component; calculate the difference between the DC voltage of the low-voltage port and the standby DC voltage component to obtain a voltage difference; based on the voltage difference, use the first fault ride-through mode to obtain a first DC reference voltage of the low-voltage bridge arm.

[0165] Optionally, the DC parameter determination unit is specifically used to: generate a first modulation voltage for the low-voltage bridge arm based on a first DC reference voltage and a first reference DC current; generate a standby modulation voltage for the high-voltage bridge arm based on a reference AC current; modulate the low-voltage bridge arm and the high-voltage bridge arm based on the first modulation voltage and the standby modulation voltage to adjust the DC current and DC voltage of the low-voltage port and obtain the adjusted DC current and DC voltage.

[0166] Optionally, the DC parameter determination unit is specifically used to: obtain a first corrected DC voltage of the low-voltage bridge arm based on the first reference DC current and the DC current of the low-voltage port by using a high-order harmonic filtering link; obtain a second corrected DC voltage of the low-voltage bridge arm based on the DC current by using a virtual impedance; superimpose the first DC reference voltage, the first corrected DC voltage and the second corrected DC voltage to obtain a reference DC voltage of the low-voltage bridge arm; and generate a first modulation voltage of the low-voltage bridge arm based on the reference DC voltage by using the low-voltage bridge arm modulation voltage.

[0167] Optionally, the fault ride-through mode switching unit is specifically configured to switch the working mode of regulating the DC voltage component of the low-voltage bridge arm to a second fault ride-through mode when the regulated DC current is less than a preset current threshold.

[0168] Optionally, the fault ride-through unit is specifically used to: within a preset fault ride-through time, based on the adjusted DC voltage and the DC voltage component of the second preset multiple of the common bridge arm, use the second fault ride-through mode to obtain a second DC reference voltage of the low-voltage bridge arm; based on the second DC reference voltage and the first average voltage of the low-voltage bridge arm sub-module, generate a second modulation voltage of the low-voltage bridge arm; based on the second modulation voltage, control the DC transformer to perform fault ride-through.

[0169] Optionally, after the working mode of the low-voltage bridge arm DC voltage component regulation is switched to the first fault ride-through mode, the working mode of the low-voltage port DC voltage control is switched to the constant DC power mode, and the working mode of the high-voltage bridge arm AC voltage control is switched to the bridge arm energy balance control mode, the system also includes: an AC modulation voltage unit, which is used to obtain an AC modulation voltage based on the actual AC voltage component and the reference AC voltage component inside the DC transformer by using proportional resonance control; and an AC voltage adjustment unit, which is used to adjust the actual AC voltage component of the DC transformer based on the AC modulation voltage.

[0170] Optionally, after controlling the DC transformer to perform fault ride-through based on the adjusted DC voltage using the second fault ride-through mode, the system also includes: a second working mode switching unit, used to switch the working mode of adjusting the DC voltage component of the low-voltage bridge arm to a fault recovery mode, and the working mode of the low-voltage port DC voltage control to a constant DC voltage mode after a preset fault ride-through time; a DC voltage component determination unit, used to adjust the DC voltage component of the common bridge arm using a preset function to obtain an adjusted DC voltage component; a DC reference voltage determination unit, used to obtain a third DC reference voltage of the low-voltage bridge arm based on the DC voltage and the adjusted DC voltage component using the fault recovery mode; and a recovery control unit, used to control the DC transformer to return to normal operating state based on the third DC reference voltage using the constant DC voltage mode.

[0171] Optionally, the recovery control unit is specifically used to: obtain the actual DC voltage of the low-voltage port; based on the actual DC voltage and the preset DC voltage, use the fixed DC voltage mode to obtain a second reference DC current of the low-voltage bridge arm; based on the second reference DC current, use the low-voltage port DC current control to obtain a third corrected DC voltage of the low-voltage bridge arm; based on the second reference DC current, the third corrected DC voltage and the second average voltage of the low-voltage bridge arm sub-module, obtain a third modulation voltage of the low-voltage bridge arm; based on the third modulation voltage, modulate the low-voltage bridge arm to obtain an adjusted actual DC voltage; based on the adjusted actual DC voltage, control the DC transformer to restore to normal operating state.

[0172] Optionally, the recovery control unit is specifically used to: determine that the DC transformer has returned to normal operating state when the actual DC voltage after adjustment has returned to a preset voltage value; and determine that the operating state of the DC transformer is a permanent fault state when the actual DC voltage after adjustment has not returned to the preset voltage value.

[0173] Optionally, the system further includes: an active power recovery unit for switching the operating mode of the high-voltage bridge arm AC voltage control to a fixed common point DC voltage mode, so that the DC transformer can restore active power transmission. Optionally, the DC transformer also includes a grounding device, with the input end of the high-voltage bridge arm serving as a high-voltage port, which is connected to the grounding device; when the DC transformer is in a fault state, and the fault state is a single-pole grounding fault, the system further includes: a corrected DC voltage determination unit for obtaining a corrected DC voltage of the high-voltage bridge arm using proportional-integral control based on the positive DC voltage and negative current voltage of the high-voltage port; a target modulation voltage generation unit for generating a target modulation voltage of the high-voltage bridge arm based on the average voltage and the corrected DC voltage; and a high-voltage bridge arm modulation unit for modulating the high-voltage bridge arm based on the target modulation voltage, so that the grounding device consumes the unbalanced energy in the positive and negative high-voltage bridge arms.

[0174] The above are merely embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention are included in the scope of the claims of the present invention to be approved.

Claims

1. A fault ride-through method for a DC transformer, characterized in that: The DC transformer includes a high-voltage bridge arm, a low-voltage bridge arm, and a common bridge arm connected through a common point, and the output end of the low-voltage bridge arm is used as a low-voltage port. The method includes: When the DC transformer is in a fault state, the operating mode of the low-voltage bridge arm DC voltage component regulation is switched to the first fault ride-through mode, the operating mode of the low-voltage port DC voltage control is switched to the constant DC power mode, and the operating mode of the high-voltage bridge arm AC voltage control is switched to the bridge arm energy balance control mode; Based on the DC voltage and DC power of the low-voltage port and the average voltage of the high-voltage bridge arm submodule, the DC current and DC voltage of the low-voltage port are adjusted by using the first fault ride-through mode, the constant DC power mode, and the bridge arm energy balance control mode to obtain adjusted DC current and DC voltage; Based on the regulated DC current, switching the operating mode of regulating the DC voltage component of the low-voltage bridge arm to a second fault ride-through mode; Based on the adjusted DC voltage, the DC transformer is controlled to perform fault ride-through using the second fault ride-through mode.

2. The method according to claim 1, characterized in that The method comprises: adjusting the DC current and DC voltage of the low-voltage port based on the DC voltage and DC power of the low-voltage port and the average voltage of the high-voltage bridge arm submodule, and using the first fault ride-through mode, the constant DC power mode, and the bridge arm energy balance control mode to obtain the adjusted DC current and DC voltage, including: Based on the DC voltage of the low-voltage port and the DC voltage component of the common bridge arm, using the first fault ride-through mode, obtaining a first DC reference voltage of the low-voltage bridge arm; Based on the DC power of the low-voltage port, using the constant DC power mode, obtaining a first reference DC current of the low-voltage bridge arm; Based on the reference voltage of the high-voltage bridge arm submodule and the average voltage of the high-voltage bridge arm submodule, the reference AC current of the high-voltage bridge arm is obtained by utilizing the bridge arm energy balance control mode; Based on the first DC reference voltage, the first reference DC current, and the reference AC current, the DC current and the DC voltage of the low-voltage port are adjusted to obtain adjusted DC current and DC voltage.

3. The method according to claim 2, characterized in that The obtaining of a first DC reference voltage of the low-voltage bridge arm based on the DC voltage of the low-voltage port and the DC voltage component of the common bridge arm by using the first fault ride-through mode includes: Multiplying the DC voltage component of the common bridge arm by a first preset multiple to obtain a standby DC voltage component; Calculating the difference between the DC voltage of the low-voltage port and the standby DC voltage component to obtain a voltage difference; Based on the voltage difference, a first DC reference voltage of the low-voltage bridge arm is obtained by utilizing the first fault ride-through mode.

4. The method according to claim 2, characterized in that The step of adjusting the DC current and DC voltage of the low-voltage port based on the first DC reference voltage, the first reference DC current, and the reference AC current to obtain adjusted DC current and DC voltage includes: generating a first modulation voltage of the low-voltage bridge arm based on the first DC reference voltage and the first reference DC current; generating a standby modulation voltage of the high-voltage bridge arm based on the reference alternating current; Based on the first modulation voltage and the standby modulation voltage, the low-voltage bridge arm and the high-voltage bridge arm are modulated to adjust the DC current and DC voltage of the low-voltage port to obtain adjusted DC current and DC voltage.

5. The method according to claim 4, characterized in that Generating the first modulation voltage of the low-voltage bridge arm based on the first DC reference voltage and the first reference DC current includes: Based on the first reference DC current and the DC current of the low-voltage port, a first corrected DC voltage of the low-voltage bridge arm is obtained by using a high-order harmonic filtering link; Based on the DC current, using virtual impedance, obtaining a second corrected DC voltage of the low-voltage bridge arm; superimposing the first DC reference voltage, the first corrected DC voltage, and the second corrected DC voltage to obtain a reference DC voltage of the low-voltage bridge arm; Based on the reference DC voltage, a first modulation voltage of the low-voltage bridge arm is generated by utilizing the low-voltage bridge arm modulation voltage.

6. The method according to claim 1, characterized in that The step of switching the operation mode of regulating the DC voltage component of the low-voltage bridge arm to the second fault ride-through mode based on the regulated DC current includes: When the regulated DC current is less than a preset current threshold, the operation mode of regulating the DC voltage component of the low-voltage bridge arm is switched to a second fault ride-through mode.

7. The method according to claim 1, characterized in that The controlling the DC transformer to perform fault ride-through by utilizing the second fault ride-through mode based on the regulated DC voltage includes: Within a preset fault ride-through time, based on the adjusted DC voltage and a DC voltage component of the common bridge arm having a second preset multiple, using the second fault ride-through mode, a second DC reference voltage of the low-voltage bridge arm is obtained; generating a second modulation voltage of the low-voltage bridge arm based on the second DC reference voltage and the first average voltage of the low-voltage bridge arm submodule; Based on the second modulation voltage, the DC transformer is controlled to perform fault ride-through.

8. The method according to claim 1, characterized in that After the operation mode of regulating the low-voltage bridge arm DC voltage component is switched to the first fault ride-through mode, the operation mode of the low-voltage port DC voltage control is switched to the constant DC power mode, and the operation mode of the high-voltage bridge arm AC voltage control is switched to the bridge arm energy balance control mode, the method further includes: Based on the actual AC voltage component and the reference AC voltage component inside the DC transformer, an AC modulation voltage is obtained by using proportional resonance control; Based on the AC modulation voltage, an actual AC voltage component of the DC transformer is adjusted.

9. The method according to any one of claims 1 to 8, characterized in that After controlling the DC transformer to perform fault ride-through based on the adjusted DC voltage and utilizing the second fault ride-through mode, the method further includes: After a preset fault ride-through time, the operating mode of the low-voltage bridge arm DC voltage component regulation is switched to a fault recovery mode, and the operating mode of the low-voltage port DC voltage control is switched to a constant DC voltage mode; Using a preset function to adjust the DC voltage component of the common bridge arm to obtain an adjusted DC voltage component; Based on the DC voltage and the adjusted DC voltage component, a third DC reference voltage of the low-voltage bridge arm is obtained by utilizing a fault recovery mode; Based on the third DC reference voltage, the DC transformer is controlled to return to a normal operating state using the constant DC voltage mode.

10. The method according to claim 9, characterized in that The controlling the DC transformer to return to a normal operating state based on the third DC reference voltage and utilizing the constant DC voltage mode includes: Obtaining the actual DC voltage of the low-voltage port; Based on the actual DC voltage and the preset DC voltage, using the constant DC voltage mode, obtaining a second reference DC current of the low-voltage bridge arm; Based on the second reference DC current, a third corrected DC voltage of the low-voltage bridge arm is obtained by controlling the low-voltage port DC current; Obtaining a third modulation voltage of the low-voltage bridge arm based on the second reference DC current, the third corrected DC voltage, and the second average voltage of the low-voltage bridge arm submodule; Based on the third modulation voltage, modulating the low-voltage bridge arm to obtain a regulated actual DC voltage; Based on the adjusted actual DC voltage, the DC transformer is controlled to return to a normal operating state.

11. The method according to claim 10, characterized in that The controlling the DC transformer to return to a normal operating state based on the adjusted actual DC voltage includes: When the actual DC voltage after adjustment returns to the preset voltage value, it is determined that the DC transformer has returned to a normal operating state; When the actual DC voltage after adjustment does not return to the preset voltage value, it is determined that the operating state of the DC transformer is a permanent fault state.

12. The method according to claim 11, characterized in that After determining that the DC transformer has returned to a normal operating state, the method further includes: The operating mode of the high-voltage bridge arm AC voltage control is switched to a fixed common point DC voltage mode, so that the DC transformer resumes active power transmission.

13. The method according to claim 1, wherein The DC transformer further includes a grounding device, and the input end of the high-voltage bridge arm serves as a high-voltage port, and the high-voltage port is connected to the grounding device; When the DC transformer is in a fault state, and the fault state is a single-pole grounding fault, the method further includes: Based on the positive DC voltage and the negative current voltage of the high-voltage port, a corrected DC voltage of the high-voltage bridge arm is obtained by using proportional-integral control; generating a target modulation voltage of the high-voltage bridge arm based on the average voltage and the corrected DC voltage; Based on the target modulation voltage, the high-voltage bridge arm is modulated so that the grounding device consumes unbalanced energy in the positive and negative high-voltage bridge arms.

14. A fault ride-through system for a DC transformer, characterized in that: The DC transformer includes a high-voltage bridge arm, a low-voltage bridge arm, and a common bridge arm connected through a common point, and the output end of the low-voltage bridge arm serves as a low-voltage port. The system includes: An operating mode switching unit is configured to switch the operating mode of the low-voltage bridge arm DC voltage component regulation to the first fault ride-through mode, the operating mode of the low-voltage port DC voltage control to the constant DC power mode, and the operating mode of the high-voltage bridge arm AC voltage control to the bridge arm energy balance control mode when the DC transformer is in a fault state; a DC parameter determination unit, configured to adjust the DC current and DC voltage of the low-voltage port based on the DC voltage and DC power of the low-voltage port and the average voltage of the high-voltage bridge arm submodule, using the first fault ride-through mode, the constant DC power mode, and the bridge arm energy balance control mode, to obtain adjusted DC current and DC voltage; a fault ride-through mode switching unit, configured to switch the operating mode of regulating the DC voltage component of the low-voltage bridge arm to a second fault ride-through mode based on the regulated DC current; A fault ride-through unit is configured to control the DC transformer to perform fault ride-through based on the adjusted DC voltage and using the second fault ride-through mode.

15. The system according to claim 14, wherein: The DC parameter determination unit is specifically configured to: Based on the DC voltage of the low-voltage port and the DC voltage component of the common bridge arm, using the first fault ride-through mode, obtaining a first DC reference voltage of the low-voltage bridge arm; Based on the DC power of the low-voltage port, using the constant DC power mode, obtaining a first reference DC current of the low-voltage bridge arm; Based on the reference voltage of the high-voltage bridge arm submodule and the average voltage of the high-voltage bridge arm submodule, the reference AC current of the high-voltage bridge arm is obtained by utilizing the bridge arm energy balance control mode; Based on the first DC reference voltage, the first reference DC current, and the reference AC current, the DC current and the DC voltage of the low-voltage port are adjusted to obtain adjusted DC current and DC voltage.

16. The system according to claim 15, wherein: The DC parameter determination unit is specifically used to: multiply the DC voltage component of the common bridge arm by a first preset multiple to obtain a standby DC voltage component; calculate the difference between the DC voltage of the low-voltage port and the standby DC voltage component to obtain a voltage difference; based on the voltage difference, use the first fault ride-through mode to obtain a first DC reference voltage of the low-voltage bridge arm.

17. The system according to claim 15, wherein: The DC parameter determination unit is specifically configured to: generate a first modulation voltage for the low-voltage bridge arm based on the first DC reference voltage and the first reference DC current; and generate a standby modulation voltage for the high-voltage bridge arm based on the reference AC current; Based on the first modulation voltage and the standby modulation voltage, the low-voltage bridge arm and the high-voltage bridge arm are modulated to adjust the DC current and DC voltage of the low-voltage port to obtain adjusted DC current and DC voltage.

18. The system according to claim 17, wherein: The DC parameter determination unit is specifically configured to: obtain a first corrected DC voltage of the low-voltage bridge arm based on the first reference DC current and the DC current of the low-voltage port using a high-order harmonic filtering link; obtain a second corrected DC voltage of the low-voltage bridge arm based on the DC current using a virtual impedance; and obtain a reference DC voltage of the low-voltage bridge arm by superimposing the first DC reference voltage, the first corrected DC voltage, and the second corrected DC voltage; Based on the reference DC voltage, a first modulation voltage of the low-voltage bridge arm is generated by utilizing the low-voltage bridge arm modulation voltage.

19. The system according to claim 14, wherein: The fault ride-through mode switching unit is specifically configured to switch the operating mode of regulating the DC voltage component of the low-voltage bridge arm to a second fault ride-through mode when the regulated DC current is less than a preset current threshold.

20. The system according to claim 14, wherein: The fault ride-through unit is specifically configured to: obtain a second DC reference voltage of the low-voltage bridge arm by using the second fault ride-through mode within a preset fault ride-through time based on the adjusted DC voltage and a DC voltage component of the common bridge arm having a second preset multiple; generating a second modulation voltage of the low-voltage bridge arm based on the second DC reference voltage and the first average voltage of the low-voltage bridge arm submodule; Based on the second modulation voltage, the DC transformer is controlled to perform fault ride-through.

21. The system according to claim 14, wherein: After the operation mode of regulating the DC voltage component of the low-voltage bridge arm is switched to the first fault ride-through mode, the operation mode of the low-voltage port DC voltage control is switched to the constant DC power mode, and the operation mode of the high-voltage bridge arm AC voltage control is switched to the bridge arm energy balance control mode, the system further includes: an AC modulation voltage unit, configured to obtain an AC modulation voltage by using proportional resonance control based on an actual AC voltage component and a reference AC voltage component inside the DC transformer; An AC voltage regulating unit is configured to regulate an actual AC voltage component of the DC transformer based on the AC modulation voltage.

22. The system according to any one of claims 14 to 21, characterized in that: After controlling the DC transformer to perform fault ride-through based on the regulated DC voltage and utilizing the second fault ride-through mode, the system further includes: A second working mode switching unit is used to switch the working mode of regulating the DC voltage component of the low-voltage bridge arm to a fault recovery mode and the working mode of controlling the DC voltage of the low-voltage port to a constant DC voltage mode after a preset fault ride-through time. a DC voltage component determining unit, which adjusts the DC voltage component of the common bridge arm using a preset function to obtain an adjusted DC voltage component; a DC reference voltage determining unit, configured to obtain a third DC reference voltage of the low-voltage bridge arm based on the DC voltage and the adjusted DC voltage component using a fault recovery mode; A recovery control unit is used to control the DC transformer to recover to a normal operating state based on the third DC reference voltage and using the constant DC voltage mode.

23. The system according to claim 22, wherein: The recovery control unit is specifically configured to: obtain an actual DC voltage of the low-voltage port; obtain a second reference DC current of the low-voltage bridge arm based on the actual DC voltage and a preset DC voltage using the constant DC voltage mode; and obtain a third corrected DC voltage of the low-voltage bridge arm based on the second reference DC current using the low-voltage port DC current control; Based on the second reference DC current, the third corrected DC voltage and the second average voltage of the low-voltage bridge arm submodule, a third modulation voltage of the low-voltage bridge arm is obtained; based on the third modulation voltage, the low-voltage bridge arm is modulated to obtain an adjusted actual DC voltage; based on the adjusted actual DC voltage, the DC transformer is controlled to return to a normal operating state.

24. The system according to claim 23, wherein: The recovery control unit is specifically used to: determine that the DC transformer has resumed normal operation when the actual DC voltage after adjustment has recovered to a preset voltage value; and determine that the operating state of the DC transformer is a permanent fault state when the actual DC voltage after adjustment has not recovered to the preset voltage value.

25. The system according to claim 24, wherein: After determining that the DC transformer has returned to a normal operating state, the system further includes: The active power recovery unit is used to switch the working mode of the high-voltage bridge arm AC voltage control to the fixed common point DC voltage mode, so that the DC transformer can restore active power transmission.

26. The system according to claim 14, wherein: The DC transformer further includes a grounding device, and the input end of the high-voltage bridge arm serves as a high-voltage port, and the high-voltage port is connected to the grounding device; When the DC transformer is in a fault state, and the fault state is a single-pole grounding fault, the system further includes: A corrected DC voltage determining unit, configured to obtain a corrected DC voltage of the high-voltage bridge arm using proportional-integral control based on the positive DC voltage and the negative current voltage of the high-voltage port; a target modulation voltage generating unit, configured to generate a target modulation voltage of the high-voltage bridge arm based on the average voltage and the corrected DC voltage; The high-voltage bridge arm modulation unit is used to modulate the high-voltage bridge arm based on the target modulation voltage so that the grounding device consumes unbalanced energy in the positive and negative high-voltage bridge arms.

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