Fault Current Limiting Control Method and Control System for T-Type Modular Multilevel DC Transformer
By calculating the port and arm current signals of the transformer and using the output voltage of the energy balance control module to compensate for the virtual impedance voltage, the problem of rapid increase in fault current during T-type modular multilevel DC transformer failure was solved, achieving a balance between rapid current limiting and steady-state performance.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-03-31
- Publication Date
- 2026-06-30
AI Technical Summary
In existing technologies, the fault current of T-type modular multilevel DC transformers rises rapidly during faults, threatening the safe operation of equipment and the stability of the system. There is a lack of effective virtual impedance control strategies for current limiting.
By acquiring the port and arm current signals of the transformer, the virtual absorption differential mode and transmission differential mode impedance voltages of the port, as well as the virtual common mode impedance voltage of the arm, are calculated. These impedance voltages are then used to compensate the output voltage of the energy balance control module, thereby achieving rapid limitation of fault transient current.
Without affecting steady-state performance, it can quickly limit fault transient current, improve the transient stability and safety of the system, and achieve rapid fault ride-through.
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Figure CN122316077A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the technical field of protection for high-power power electronic equipment, and more specifically, relates to a fault current limiting control method and control system for a T-type modular multilevel DC transformer. Background Technology
[0002] Compared to traditional non-isolated DC / DC MMC topologies, the T-type topology modular multilevel DC transformer (T-MMDCT) effectively reduces the required bridge arm inductance by introducing full-bridge submodules into the low-voltage bridge arm, achieving a better current stress distribution with a smaller number of submodules. However, this structure also brings new challenges: due to the low damping of the DC system and the low internal loop impedance of the T-MMDCT, when a short-circuit fault occurs at the DC port or in one of the T-MMDCT's internal bridge arms, the fault current can rise to several times the rated value within milliseconds, seriously threatening the safe operation of the converter equipment and the system stability. A fast and effective fault current limiting control strategy is a key technical challenge that needs to be solved to achieve high-reliability operation of the T-MMDCT.
[0003] Virtual impedance control, a method that introduces equivalent impedance at the converter output without adding extra hardware, has become a popular strategy in various power electronic converters for limiting overcurrent, maintaining voltage source characteristics during fault transients, and improving system transient stability, demonstrating good current limiting effect and fault ride-through capability. However, for T-MMDCTs, there is currently a lack of effective technical solutions for fault current limiting using virtual impedance control strategies.
[0004] Therefore, how to quickly limit the fault transient current of a T-MMDCT without affecting its steady-state performance is a technical solution that urgently needs to be solved. Summary of the Invention
[0005] In view of the above-mentioned defects or improvement needs of the existing technology, the present invention provides a fault current limiting control method and control system for a T-type modular multilevel DC transformer. Its purpose is to quickly limit the fault transient current when the T-MMDCT fails without affecting the steady-state performance.
[0006] To achieve the above objectives, the present invention is proposed.
[0007] According to a first aspect of the present invention, a fault current limiting control method for a T-type modular multilevel DC transformer is provided, comprising: Acquire the transformer port current signal; the port current signal includes the high-voltage port current i H.dcLow-voltage port current i L.dc and the neutral port current i of the multiplexed bridge arm M.dc ; Calculate the port virtual absorption differential mode impedance voltage for port faults based on the port current signal. A compensation voltage is obtained, wherein the compensation voltage includes the virtual absorption differential-mode impedance voltage at the port; wherein... In the formula, , , These represent the virtual absorption differential mode impedance voltages at the ports corresponding to the x-phase high-voltage bridge arm, x-phase low-voltage bridge arm, and x-phase multiplexed bridge arm, respectively, where x represents the phase and B... dm,in The absorption differential modulus coefficient, R dm,in For the preset absorption differential mode controlled impedance, i port,dm,in This indicates that the port absorbs the differential-mode current component. U H and U L These are the high-voltage port voltage and the low-voltage port voltage, respectively. The compensation voltage is used to compensate the bridge arm output voltage calculated by the energy balance control module to obtain a target value of the bridge arm output voltage that can suppress fault current. The target value of the bridge arm output voltage is used as the basis for the adjustment of the bridge arm sub-module so that the actual output voltage of the bridge arm follows its target value. The energy balance control module is a control module for controlling the energy balance of the transformer to make it operate stably.
[0008] According to a second aspect of the present invention, a fault current limiting control system for a T-type modular multilevel DC transformer is provided, comprising: a fault current limiting module; The fault current limiting module has a current signal acquisition unit, a compensation voltage calculation unit, and a compensation unit. The current signal acquisition unit is used to acquire the current signal of the transformer. The compensation voltage calculation unit is used to calculate the compensation voltage. The compensation unit is used to compensate the bridge arm output voltage calculated by the energy balance control module using the compensation voltage to obtain a target value of the bridge arm output voltage that can suppress the fault current. The current signal acquisition unit, the compensation voltage calculation unit, and the compensation unit work together to achieve the above fault current limiting control method.
[0009] Overall, compared with the prior art, the technical solutions conceived by this invention have the following beneficial effects.
[0010] 1. The fault current limiting control method for a T-type modular multilevel DC transformer proposed in this invention calculates the port virtual absorption differential mode impedance voltage for port faults based on the port current signal, obtains a compensation voltage, and uses the compensation voltage to compensate the bridge arm output voltage calculated by the energy balance control module to obtain the target value of the bridge arm output voltage. The port virtual absorption differential mode impedance voltage varies with the port absorption differential mode current component i. port,dm,in Adaptively changes to the changes; when the transformer is in steady-state operation, the port absorbs the differential-mode current component i. port,dm,in Since the voltage of the virtual absorption differential mode impedance is approximately zero, the virtual absorption differential mode impedance will not affect the steady-state performance of the transformer. Furthermore, it can adaptively adjust to quickly suppress the transient current of the port fault during port faults, thus enabling targeted suppression of transient faults.
[0011] 2. Furthermore, in some embodiments, a virtual transmission differential mode impedance voltage for port faults is calculated based on the port current signal. The virtual absorption differential mode impedance voltage and the virtual transmission differential mode impedance voltage are used together as a compensation voltage. Compared with using only the virtual absorption differential mode impedance voltage as a compensation voltage, the limitation effect on transient current of port faults is better.
[0012] 3. Furthermore, in some embodiments, the virtual common-mode impedance voltage of the bridge arm for bridge arm faults is calculated based on the bridge arm current signal. The compensation voltage also includes the virtual common-mode impedance voltage of the bridge arm, which can limit both the fault transient current of the port fault and the fault transient current of the bridge arm fault, making the fault protection mechanism more comprehensive. Attached Figure Description
[0013] Figure 1 This is a schematic diagram of the basic structure of T-MMDCT in one embodiment; Figure 2 This is a schematic diagram of the energy balance control module of a T-MMDCT in one embodiment; Figure 3 This is a flowchart of the fault current limiting control method of T-MMDCT in one embodiment of the present invention; Figure 4 This is a structural block diagram of the fault current limiting control system of T-MMDCT in one embodiment of the present invention; Figure 5 This is a comparison of the suppression effects of different control strategies on port current under a port-level high-voltage port grounding fault in one embodiment. Figure 6 This is a comparison of the suppression effects of different control strategies on the bridge arm current of the fault phase under a port-level high-voltage port grounding fault in one embodiment. Figure 7This is a comparison diagram of the current suppression effects of different control strategies on the faulty phase bridge arm during a single-phase ground fault in one embodiment. Figure 8 This is a comparison diagram of the current suppression effects of different control strategies on the faulty phase bridge arm during a phase-to-phase short-circuit fault in one embodiment. Detailed Implementation
[0014] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.
[0015] To facilitate understanding of this invention, the basic structure of T-MMDCT will be introduced first.
[0016] like Figure 1 The diagram shows a basic structural schematic of a T-MMDCT in one embodiment. The T-MMDCT is a power electronic converter topology suitable for interconnecting medium- and high-voltage DC power grids, particularly for achieving efficient power conversion between DC buses of different voltage levels. It has a high-voltage arm, a low-voltage arm, and a multiplexed arm, which together form a T-shaped topology. Each arm consists of a series of sub-modules (SMs) connected in series, most commonly half-bridge sub-modules (HBSMs) or full-bridge sub-modules (FBSMs). The sub-modules are basic power modules; by controlling the sub-modules, the output voltage of the corresponding arm can be controlled.
[0017] like Figure 2 The diagram shows a schematic of the energy balance control module of a T-MMDCT in one embodiment. This energy balance control module is a conventional controller currently used for controlling T-MMDCTs, as described in the article "Trapezoidal Wave Operation of T-type DC / DC Converter for High-Power Applications." The energy balance control module is used to regulate the T-MMDCT, achieving stable operation through multi-layer energy balance control. This multi-layer energy balance includes: port energy balance, phase energy balance, inter-arm energy balance, and sub-module energy balance within each arm. The basic regulation process involves controlling the average capacitor voltage U of the transformer sub-module. C_avg Average capacitor voltage U of phase module Cx_avg (x = a, b, c), average capacitor voltage U of bridge arm submodules CYx_avg(Y = HV, LV, MV) Closed-loop control is used to achieve this. At the transformer port, low-voltage side constant voltage control is used, and the output current of the high-voltage port is adjusted to achieve the transformer's energy balance. Inside the transformer, phase energy balance is achieved by adjusting the DC current of the high-voltage arm, and bridge arm energy balance is achieved by adjusting the AC current of the high-voltage arm and the low-voltage arm. Finally, the control target is achieved through inner-loop bridge arm current control. The energy balance control of the submodule is similar to that of conventional MMC, and voltage equalization control is achieved through appropriate switching strategies.
[0018] For the T-MMDCT described above, this invention proposes an effective fault current limiting control strategy. Based on this fault current limiting control strategy, the fault transient current of the T-MMDCT can be quickly limited when a fault occurs without affecting the steady-state performance.
[0019] Example 1.
[0020] This embodiment limits the fault transient current during port faults based on the port's virtual absorption differential impedance.
[0021] like Figure 3 The diagram shown is a flowchart of the fault current limiting control method for a T-MMDCT according to an embodiment of the present invention. The following is in conjunction with... Figure 2 The fault current limiting control method is described in detail.
[0022] S1. Obtain the transformer port current signal; the port current signal includes the high-voltage port current i. H.dc Low-voltage port current i L.dc and the neutral port current i of the multiplexed bridge arm M.dc .
[0023] Specifically, the T-MMDCT has three ports: a high-voltage port, a low-voltage port, and a neutral port. The high-voltage port connects to the high-voltage side DC power, and the low-voltage port connects to the low-voltage side DC power. A high-voltage bridge arm connects to the high-voltage port and the common node, a low-voltage bridge arm connects to the low-voltage port and the common node, and a multiplexed bridge arm connects to the common node and the ground wire. The end of the multiplexed bridge arm connected to the ground wire is the neutral port. Each type of bridge arm corresponds to multiple phases: high-voltage bridge arms with multiple phases, low-voltage bridge arms with multiple phases, and multiplexed bridge arms with multiple phases. High-voltage bridge arms, low-voltage bridge arms, and multiplexed bridge arms of the same phase are connected in a T-shaped topology. All high-voltage bridge arms of different phases share the high-voltage port, all low-voltage bridge arms of different phases share the low-voltage port, and all multiplexed bridge arms of different phases share the neutral port.
[0024] During the operation of the T-MMDCT, the high-voltage port current i of the T-MMDCT can be acquired in real time. H.dc Low-voltage port current iL.dc and the neutral port current i of the multiplexed bridge arm M.dc This allows for timely detection of faults and automatic current limiting control.
[0025] S2. Calculate the port virtual absorption differential mode impedance voltage for port faults based on the port current signal to obtain the compensation voltage.
[0026] First, calculate the differential-mode current component i absorbed at the port. port,dm,in The calculation formula is as follows: ; In the formula, U H and U L These are the high-voltage port voltage and the low-voltage port voltage, respectively. This is the differential mode current regulation coefficient.
[0027] The above ports absorb the differential mode current component i port,dm,in This mainly characterizes the magnitude of the current corresponding to the differential mode power absorbed between the ports of a T-MMDCT. Port absorbed power refers to the power flowing into the converter from the connected DC bus or ground wire, while port absorbed differential mode power refers to the differential mode of the power signal absorbed between different ports. During steady-state operation, the transformer satisfies the energy balance mechanism, and the port absorbed differential mode current component is approximately zero. After a port-level fault occurs, the port absorbed differential mode component increases significantly. On the one hand, this can be represented by the port absorbed differential mode current component i... port,dm,in The current amplitude exceeding its corresponding preset amplitude is used as the basis for port fault identification, enabling rapid identification of port faults. On the other hand, it also relies on the differential mode current component i absorbed by the port. port,dm,in The virtual absorption differential impedance voltage at the port is calculated and used as a compensation for the output voltage of the bridge arm, thereby limiting the fault transient current during port faults.
[0028] Among them, based on the port absorption differential mode current component i port,dm,in The formula for calculating the virtual absorption differential-mode impedance voltage at the port is as follows: ; ; In the formula, , , These represent the virtual absorption differential mode impedance voltages at the ports corresponding to the x-phase high-voltage bridge arm, x-phase low-voltage bridge arm, and x-phase multiplexed bridge arm, respectively. dm,in R is the absorption differential mode coefficient. dm,in The preset differential mode absorption impedance R is the differential mode absorption impedance. dm,in The value of can be determined in the early testing phase, and the value selected should have a better limiting effect on the transient current of port faults. In practice, this involves introducing virtual differential-mode absorption impedances at the high-voltage port, low-voltage port, and neutral port. The voltage across these virtual differential-mode absorption impedances can vary with the differential-mode current component absorbed at the port. port,dm,in Adaptively changes to the changes; when the transformer is in steady-state operation, the port absorbs the differential-mode current component i. port,dm,in Since the voltage of the virtual absorption differential mode impedance is approximately zero, the virtual absorption differential mode impedance will not affect the steady-state performance of the transformer. Furthermore, it can adaptively adjust to quickly suppress the transient current of the port fault during port faults, thus enabling targeted suppression of transient faults.
[0029] S3. The bridge arm output voltage calculated by the energy balance control module is compensated using the compensation voltage to obtain the target value of the bridge arm output voltage that can suppress the fault current.
[0030] The target value of the bridge arm output voltage serves as the basis for regulating the bridge arm submodule so that the actual output voltage of the bridge arm follows its target value. The energy balance control module is a control module that controls the energy balance of the transformer to ensure its stable operation.
[0031] Specifically, as mentioned earlier, T-MMDCTs are currently typically controlled using an energy balance control module. This module regulates the T-MMDCT and achieves stable operation through multi-layer energy balance control, including: port energy balance, phase energy balance, inter-arm energy balance, and sub-module energy balance within each arm. While this energy balance control module can achieve stable transformer operation through multi-layer energy balance control, it cannot promptly respond to surges in fault transient currents.
[0032] Therefore, this invention further compensates the bridge arm output voltage calculated by the energy balance control module. The compensation voltage includes the port virtual absorption differential mode impedance voltage calculated above. , , After compensating the bridge arm output voltage calculated by the energy balance control module using the port virtual absorption differential mode impedance voltage, the target value of the bridge arm output voltage can be expressed as: ; In the formula, u Hx_set u Lx_set u Mx_set The bridge arm output voltages calculated by the energy balance control module for the x-phase high-voltage bridge arm, x-phase low-voltage bridge arm, and x-phase multiplexed bridge arm, respectively, u Hx_set,vi u Lx_set,vi u Mx_set,viThe target output voltage values of the bridge arms corresponding to the x-phase high-voltage bridge arm, x-phase low-voltage bridge arm, and x-phase multiplexed bridge arm are obtained after compensation using the port virtual absorption differential mode impedance voltage.
[0033] After obtaining the target output voltage value of each bridge arm, the control parameters of the bridge arm submodule can be calculated so that the actual output voltage of the bridge arm follows its target value.
[0034] Example 2.
[0035] Based on Example 1, the port virtual transmission differential impedance voltage for port faults can also be calculated based on the port current signal. When the compensation voltage is used to compensate the bridge arm output voltage calculated by the energy balance control module, the compensation voltage also includes the port virtual transmission differential impedance voltage.
[0036] First, calculate the differential-mode current component i transmitted at the port. port,dm,thru The calculation formula is as follows: ; In the formula, U H and U L These are the high-voltage port voltage and the low-voltage port voltage, respectively. This is the differential mode current regulation coefficient.
[0037] The above ports transmit the differential mode current component i port,dm,thru This mainly characterizes the magnitude of the current corresponding to the differential-mode power transmitted between the ports of a T-MMDCT. Port-transmitted power refers to the power transmitted through that port, while port-transmitted differential-mode power refers to the differential mode of the power signal transmitted between different ports. During steady-state operation, the transformer satisfies the energy balance mechanism, and the trend of the port-transmitted differential-mode current component is consistent with the magnitude of the transformer's transmitted power, remaining in a stable state. After a port-level fault occurs, the port-transmitted differential-mode current component fluctuates. On the one hand, this can be represented by the port-transmitted differential-mode current component i... port,dm,thru The occurrence of fluctuations and their amplitude exceeding a corresponding preset amplitude are used as criteria for port fault identification, enabling rapid identification of port faults. Furthermore, the differential-mode current component i transmitted at that port is also considered. port,dm,thru The virtual transmission differential impedance voltage at the port is calculated and used as a compensation for the bridge arm output voltage, thereby limiting the fault transient current during port faults.
[0038] Among them, the differential mode current component i is transmitted based on the port. port,dm,thru The formula for calculating the virtual transmitted differential-mode impedance voltage at the port is: ; ; In the formula, , , These represent the virtual transmission differential-mode impedance voltages at the ports corresponding to the x-phase high-voltage bridge arm, x-phase low-voltage bridge arm, and x-phase multiplexed bridge arm, respectively. dm,in To transmit the differential mode coefficient, R dm,thru The preset differential mode control impedance is R. dm,thru The value of can be determined in the early testing phase, and the value selected should have a better limiting effect on the transient current of port faults. In practice, virtual differential-mode impedances are introduced at the high-voltage, low-voltage, and neutral ports to maintain the port current distribution consistent with that before the fault. The voltage across these virtual differential-mode impedances can vary with the differential-mode current component absorbed by the port. port,dm,in The voltage at the port changes adaptively. When the transformer is in steady state, the trend of the differential mode current component at the port changes in the same way as the power transmitted by the transformer. Therefore, the virtual differential mode impedance at the port will not affect the steady-state performance of the transformer. Moreover, it can adaptively adjust to quickly suppress the transient current of the port fault when the port is faulty, and can achieve targeted suppression of transient faults.
[0039] Using both the virtual absorption differential-mode impedance voltage and the virtual transmission differential-mode impedance voltage at the ports as compensation voltages, the target value of the bridge arm output voltage after compensation by the energy balance control module can be expressed as: .
[0040] The virtual absorption differential impedance voltage and the virtual transmission differential impedance voltage at the port are used together as compensation voltages. Compared with using only the virtual absorption differential impedance voltage at the port as compensation voltage, the limitation effect on transient current during port faults is better.
[0041] Example 3.
[0042] The above embodiments 1 and 2 can limit the transient current under port faults. When a bridge arm fault occurs, the transient current of the bridge arm fault will also surge. Therefore, based on embodiment 1 or embodiment 2, this embodiment also adds a solution for bridge arm faults.
[0043] Specifically, the fault current limiting control method further includes: acquiring the transformer arm current signal, the arm current signal including the high-voltage arm current i of each phase. Hx Low-voltage bridge arm current i of each phase Lx and the current i of each phase multiplexed bridge arm Mx That is, during the operation of the T-MMDCT, the high-voltage bridge arm current i of each phase of the T-MMDCT can be acquired in real time. Hx Low-voltage bridge arm current i of each phase Lx and the current i of each phase multiplexed bridge arm MxTaking three phases (a, b, and c) as an example, the phase a arm current of the T-MMDCT is collected in real time, including the phase a high-voltage arm current i. Ha Phase a low-voltage bridge arm current i La and the a-phase multiplexed bridge arm current i Ma It also collects the b-phase bridge arm current of the T-MMDCT, including the b-phase high-voltage bridge arm current i. Hb Phase b low-voltage bridge arm current i Lb and the b-phase multiplexed bridge arm current i Mb It also collected the c-phase bridge arm current of the T-MMDCT, including the c-phase high-voltage bridge arm current i. Hc c-phase low-voltage bridge arm current i Lc and the c-phase multiplexed bridge arm current i Mc .
[0044] Based on the collected bridge arm current signal, the bridge arm common-mode current i is first calculated. x,cm The calculation formula is as follows: ; In the formula, i Hx i Lx i Mx These are the bridge arm currents for the high-voltage bridge arm, the low-voltage bridge arm, and the reused bridge arm, respectively.
[0045] The common-mode current i of the above bridge arms x,cm When the T-MMDCT is in steady-state operation, the common-mode current i in the bridge arm x,cm The common-mode current i of the bridge arm is zero when a bridge arm fault occurs (single-phase grounding or phase-to-phase short circuit). x,cm Since it is not zero, on the one hand, it can be represented by the common-mode current i of the bridge arm. x,cm The current amplitude exceeding its corresponding preset amplitude is used as the basis for bridge arm fault identification, enabling rapid identification of bridge arm faults. On the other hand, it is also based on the bridge arm common-mode current i x,cm The virtual common-mode impedance voltage of the bridge arm is calculated and used as a compensation for the output voltage of the bridge arm, thereby limiting the fault transient current when the bridge arm fails.
[0046] Among them, based on the common-mode current i of the bridge arm x,cm The formula for calculating the virtual common-mode impedance voltage of the bridge arm is as follows: ; In the formula, , , These are the virtual common-mode impedance voltages of the bridge arms corresponding to the x-phase high-voltage bridge arm, the x-phase low-voltage bridge arm, and the x-phase multiplexed bridge arm, respectively. imThe preset common-mode impedance of the bridge arms is equivalent to introducing virtual common-mode impedances into the high-voltage bridge arms, low-voltage bridge arms, and multiplexed bridge arms. The virtual common-mode impedance voltage of these bridge arms can vary with the common-mode current i of the bridge arms. x,cm The common-mode current i in the bridge arm adapts to changes in the transformer's steady-state operation. x,cm Since the voltage is zero, the virtual common-mode impedance voltage of the bridge arm is also zero. Therefore, the virtual common-mode impedance of the bridge arm will not affect the steady-state performance of the transformer, and can adaptively adjust to quickly suppress the transient current of the bridge arm fault during a bridge arm fault, thus achieving targeted suppression of transient faults.
[0047] Using the virtual differential-mode impedance voltage of the port and the virtual common-mode impedance voltage of the bridge arm as compensation voltages, the target value of the bridge arm output voltage after compensation by the energy balance control module can be expressed as: ; By using the port virtual absorption differential-mode impedance voltage, the port virtual transmission differential-mode impedance voltage, and the bridge arm virtual common-mode impedance voltage together as compensation voltages, the target value of the bridge arm output voltage after compensation for the bridge arm output voltage calculated by the energy balance control module can be expressed as: ; The above methods can limit the fault transient current of both port faults and bridge arm faults, making the fault protection mechanism more comprehensive.
[0048] The fault current limiting mentioned in the above embodiments can occur instantly. Although it can limit the transient fault current to a certain extent, for safety reasons, the fault needs to be cleared as soon as possible. Therefore, based on the above embodiments, transformer faults can also be identified, and when a transformer fault is identified, fault clearing measures can be triggered to clear the fault and ensure device safety. The basis for identifying transformer faults is the basis mentioned in the above embodiments: Basis 1: Port absorption of differential mode current component i port,dm,in If the current amplitude exceeds its corresponding preset amplitude, it indicates that a port fault has occurred. Based on 2: Port transmission of differential mode current component i port,dm,thru If fluctuations occur and the amplitude of the fluctuations exceeds the corresponding preset amplitude, it indicates that a port failure has occurred. Basis 3: Bridge arm common mode current i x,cm If the current amplitude exceeds its corresponding preset amplitude, it indicates that a bridge arm fault has occurred.
[0049] In the above embodiments, the energy balance control module controls the energy balance of the transformer, specifically by ensuring that the transformer meets the following requirements: ; In the formula, ΔP port P represents the port power deviation. Hdc and P Ldc Let ΔP represent the power at the high-voltage port and the power at the low-voltage port, respectively. x P represents the power deviation of phase x. HV,arm P LV,arm and P MP,arm These represent the input power ΔP of the high-voltage arm, low-voltage arm, and multiplexed arm, respectively. Y,arm This represents the power deviation of the Y-arm, where Y = HV, LV, MP. Y = HV corresponds to the high-voltage arm, Y = LV to the low-voltage arm, and Y = MP to the multiplexed arm. P Y,arm,ac and P Y,arm,dc These represent the AC and DC input power of the Y-arm, ΔP and ΔP, respectively. N,SM P represents the power deviation of the Nth submodule. N,SM,FON and P N,SM,RON These represent the input power when the Nth submodule is connected in the forward and reverse directions, respectively.
[0050] Example 4 This invention also relates to a fault current limiting control system for a T-type modular multilevel DC transformer, such as... Figure 4 The diagram shows a structural block diagram of a fault current limiting control system for a T-MMDCT according to an embodiment of the present invention. It includes a fault current limiting module, which comprises a current signal acquisition unit, a compensation voltage calculation unit, and a compensation unit. The current signal acquisition unit acquires the transformer's current signal, the compensation voltage calculation unit calculates the compensation voltage, and the compensation unit uses the compensation voltage to compensate the bridge arm output voltage calculated by the energy balance control module to obtain the target value of the bridge arm output voltage. The current signal acquisition unit, the compensation voltage calculation unit, and the compensation unit work together to achieve the fault current limiting control method described above. Furthermore, the fault current limiting control system also includes an energy balance control module; the energy balance control module controls the transformer's energy balance to ensure stable operation.
[0051] Specifically, the above current signal acquisition unit can acquire port current signals and bridge arm current signals. The above compensation voltage calculation unit can calculate the port virtual absorption differential mode impedance voltage and the port virtual transmission differential mode impedance voltage based on the port current signal. It can also calculate the bridge arm virtual common mode impedance voltage based on the bridge arm current signal. The above compensation unit can superimpose the compensation voltage on the bridge arm output voltage calculated by the energy balance control module to obtain the target value of the bridge arm output voltage. The compensation voltage includes the port virtual absorption differential mode impedance voltage, and may also include the port virtual transmission differential mode impedance voltage and / or the bridge arm virtual common mode impedance voltage.
[0052] Furthermore, the control system may also include a fault clearing unit; the fault clearing unit is used to identify transformer faults and, when a transformer fault is identified, to trigger fault clearing measures to clear the fault.
[0053] To verify the control effect of the proposed current limiting strategy, a corresponding simulation model was built on a simulation software platform for analysis.
[0054] For port-level faults, the port current and the faulty phase bridge arm current under a high-voltage port grounding fault are respectively as follows: Figure 5 and Figure 6 As shown. Figure 5 The paper compares the suppression effects of conventional control strategies (without employing the fault current limiting control strategy mentioned in this invention), virtual impedance control strategies based on port transmission of differential mode components, and virtual impedance control strategies based on port absorption of differential mode components on port current. Figure 6 This study compares the suppression effects of conventional control strategies, virtual impedance control strategies based on port-transmitted differential-mode components, and virtual impedance control strategies based on port-absorbed differential-mode components on fault phase arm current. It can be seen that both strategies have some suppression effect, with the latter showing better performance. The maximum fault current is reduced from 5kA to 4kA, the fault transient growth component is reduced by approximately 50%, and the port and arm currents stabilize faster after fault recovery. This results in more stable control and effectively prevents premature blockage of the T-MMDCT due to overcurrent, thus achieving fault ride-through.
[0055] For bridge arm level faults Figure 7 and Figure 8 The bridge arm currents of the faulted phases under single-phase ground fault and phase-to-phase short-circuit faults are shown respectively. Figure 7 The study compared the current suppression effects of different control strategies on the faulty phase arm during a single-phase ground fault. Under conventional control, the common-mode current was significantly higher, with the maximum current in the faulty phase arm exceeding 10kA and severe waveform distortion. Under virtual common-mode impedance control of the bridge arm, the common-mode current was effectively suppressed, with the maximum fault current in the bridge arm not exceeding 8kA, a reduction of 20% compared to conventional control. The transient increase component of the fault was reduced by nearly 30%, and the waveform was more stable. It was able to quickly recover stability after the fault was restored, achieving rapid and stable fault ride-through. Figure 8 The study compares the current suppression effects of different control strategies on the faulty phase arm during phase-to-phase short-circuit faults. For phase-to-phase short-circuit faults, the virtual common-mode impedance control of the arm can also quickly limit the rising trend of the current in the early stage of the fault.
[0056] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as the combination of these technical features does not contradict each other, it should be considered within the scope of this specification. It should be noted that the terms "in one embodiment," "for example," and "again" are intended to illustrate the present invention and are not intended to limit the present invention.
[0057] The above embodiments merely illustrate several implementation methods of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention.
Claims
1. A fault current limiting control method for a T-type modular multilevel DC transformer, characterized in that, include: obtaining a port current signal of the transformer; the port current signal comprises a high-voltage port current i H.dc , a low-voltage port current i L.dc , and a multiplexing bridge arm neutral line port current i M.dc ; Calculate the port virtual absorption differential mode impedance voltage for port faults based on the port current signal. A compensation voltage is obtained, wherein the compensation voltage includes the virtual absorption differential-mode impedance voltage at the port; wherein... In the formula, , , These represent the virtual absorption differential mode impedance voltages at the ports corresponding to the x-phase high-voltage bridge arm, x-phase low-voltage bridge arm, and x-phase multiplexed bridge arm, respectively, where x represents the phase and B... dm,in The absorption differential modulus coefficient, R dm,in For the preset absorption differential mode controlled impedance, i port,dm,in This indicates that the port absorbs the differential-mode current component. U H and U L These are the high-voltage port voltage and the low-voltage port voltage, respectively. The compensation voltage is used to compensate the bridge arm output voltage calculated by the energy balance control module to obtain a target value of the bridge arm output voltage that can suppress fault current. The target value of the bridge arm output voltage is used as the basis for the adjustment of the bridge arm sub-module so that the actual output voltage of the bridge arm follows its target value. The energy balance control module is a control module for controlling the energy balance of the transformer to make it operate stably.
2. The fault current limiting control method as described in claim 1, characterized in that, The method further includes: Calculate the port virtual transmitted differential mode impedance voltage for port faults based on the port current signal. The compensation voltage also includes the port virtual transmission differential-mode impedance voltage; wherein, In the formula, , , These represent the virtual transmission differential-mode impedance voltages at the ports corresponding to the x-phase high-voltage bridge arm, x-phase low-voltage bridge arm, and x-phase multiplexed bridge arm, respectively. B dm,thru To transmit differential mode coefficients, R dm,thru For the preset differential mode control impedance, i port,dm,thru This indicates that the port transmits the differential-mode current component. .
3. The fault current limiting control method as described in claim 1, characterized in that, The method further includes: acquiring a bridge arm current signal of the transformer, the bridge arm current signal comprising a high-voltage bridge arm current i Hx of each phase Lx a low-voltage bridge arm current i Mx of each phase Mx and a multiplex bridge arm current i Mx of each phase Calculate the virtual common-mode impedance voltage of the bridge arm facing a bridge arm fault based on the bridge arm current signal. The compensation voltage further includes the bridge arm virtual common-mode impedance voltage; wherein, , , , These are the virtual common-mode impedance voltages of the bridge arms corresponding to the x-phase high-voltage bridge arm, the x-phase low-voltage bridge arm, and the x-phase multiplexed bridge arm, respectively. im i is the preset common-mode impedance of the bridge arm. x,cm Let i be the common-mode current of the bridge arm in phase x. x,cm =i Lx -i Hx -i Mx .
4. The fault current limiting control method as described in any one of claims 1 to 3, characterized in that, x = a, b, c, where the transformer has phase a, phase b, and phase c.
5. The fault current limiting control method according to any one of claims 1 to 3, characterized in that, The method also includes identifying transformer faults and, upon identification of a transformer fault, triggering fault clearing measures to clear the fault.
6. The fault current limiting control method as described in claim 5, characterized in that, The criteria for identifying transformer faults include any of the following: According to 1: port absorbs differential mode current component i port,dm,in The current amplitude exceeds its corresponding preset amplitude, indicating that a port fault has occurred. Based on 2: Port transmission of differential mode current component i port,dm,thru If fluctuations occur and the amplitude exceeds the corresponding preset amplitude, it indicates a port failure. ; According to 3: Bridge arm common-mode current i x,cm The current amplitude of the bridge arm exceeds its corresponding preset amplitude, indicating that the bridge arm fault occurs; x,cm =i Lx -i Hx -i Mx , i Hx is the high-voltage bridge arm current of the x phase, i Lx is the low-voltage bridge arm current of the x phase, i Mx is the multiplexing bridge arm current of the x phase.
7. The fault current limiting control method as described in claim 1, characterized in that, The energy balance control module controls the energy balance of the transformer, specifically ensuring that the transformer meets the following requirements: ； In the formula, ΔP port P represents the port power deviation. Hdc and P Ldc Let ΔP represent the power at the high-voltage port and the power at the low-voltage port, respectively. x P represents the power deviation of phase x. HV,arm P LV,arm and P MP,arm These represent the input power ΔP of the high-voltage arm, low-voltage arm, and multiplexed arm, respectively. Y,arm This represents the power deviation of the Y-arm, where Y = HV, LV, MP. Y = HV corresponds to the high-voltage arm, Y = LV to the low-voltage arm, and Y = MP to the multiplexed arm. P Y,arm,ac and P Y,arm,dc These represent the AC and DC input power of the Y-arm, ΔP and ΔP, respectively. N,SM P represents the power deviation of the Nth submodule. N,SM,FON and P N,SM,RON These represent the input power when the Nth submodule is connected in the forward and reverse directions, respectively.
8. A fault current limiting control system for a T-type modular multilevel DC transformer, characterized in that, include: Fault current limiting module; The fault current limiting module has a current signal acquisition unit, a compensation voltage calculation unit, and a compensation unit. The current signal acquisition unit is used to acquire the current signal of the transformer. The compensation voltage calculation unit is used to calculate the compensation voltage. The compensation unit is used to compensate the bridge arm output voltage calculated by the energy balance control module using the compensation voltage to obtain a target value of the bridge arm output voltage that can suppress the fault current. The current signal acquisition unit, the compensation voltage calculation unit, and the compensation unit work together to realize the fault current limiting control method as described in any one of claims 1 to 4.
9. The fault current limiting control system as described in claim 8, characterized in that, Also includes: Energy balance control module; The energy balance control module is used to control the energy balance of the transformer to ensure its stable operation.
10. The fault current limiting control system as described in claim 8 or 9, characterized in that, It also includes a fault clearing unit; The fault clearing unit is used to identify transformer faults and, when a transformer fault is detected, to trigger fault clearing measures to clear the fault.