A non-latching fault ride-through method for active bridge dc converters

Abstract

The application discloses a non-locking fault ride-through method for an active bridge type direct current converter and relates to the technical field of direct current converter fault ride-through, and comprises the following steps: keeping the active bridge switch running and performing fault current limiting modulation when a fault occurs on the direct current side; determining a release phase and a residual electromagnetic state after the fault is released; determining a normal cycle track and a corresponding normal bridge state according to a recovery port voltage and normal modulation parameters; determining an accessible state meeting a current safety constraint with the current bridge state and the residual electromagnetic state as a starting point, and screening a target access state; performing corresponding bridge state transition and accessing normal modulation along the target phase. The application can reduce secondary current stress caused by the mismatch between the internal state and the normal modulation state after the fault is released, and improve the continuity and operation safety of the fault recovery process.

Description

Technical Field

[0001] This invention relates to the field of DC-DC converter fault ride-through technology, and more specifically, to a non-blocking fault ride-through method for active bridge DC-DC converters. Background Technology

[0002] Active bridge DC-DC converters are widely used in DC power distribution, energy storage interfaces, and new energy conversion systems due to their bidirectional power transmission, electrical isolation, and high power density. When a fault occurs on the DC side, to avoid directly blocking the active bridge and causing power transmission interruption, existing technologies typically employ non-blocking fault ride-through control. This limits the fault current by adjusting the phase shift relationship, duty cycle, or switching state of the active bridge. Once the external electrical quantities such as voltage and current at the fault-side port are detected to have returned to normal range, the fault current-limiting state is exited, and normal modulation is resumed.

[0003] However, the restoration of external electrical quantities on the fault side does not necessarily mean that the internal state of the converter has returned to a state matching normal modulation. Since the fault clearance time may fall on any switching phase, the equivalent series inductance current, transformer flux linkage, and actual bridge state formed during the fault current limiting period may deviate from the periodic state corresponding to normal modulation at that phase. If existing recovery methods directly switch back to normal modulation based on the external fault clearance result, it can easily cause a discontinuity between the internal electromagnetic state at the moment of fault clearance and the subsequent bridge state switching process. This can lead to additional current changes in the early stages of recovery, and even secondary current stress, affecting the safe operation of power devices and the converter.

[0004] The aforementioned disclosed technical solutions have at least the following technical problems: When existing active bridge DC-DC converters use a non-blocking method for fault ride-through, they typically exit the fault control state based on the recovery of external electrical quantities such as voltage and current on the fault side. However, the fault clearance time may be at any switching phase, and the leakage inductance current, transformer flux linkage, and bridge state formed during the fault current limiting period are not consistent with the periodic state corresponding to normal modulation under that phase. When directly restoring normal modulation, it is difficult to ensure that the internal electromagnetic state and bridge state can be continuously connected, which can easily generate secondary current stress in the first or first few switching cycles after the fault clearance. Summary of the Invention

[0005] To overcome the aforementioned deficiencies of the prior art, embodiments of the present invention provide a non-blocking fault ride-through method for active bridge DC converters. This method determines a safe and reachable target access state by combining the release phase, residual electromagnetic state, and normal periodic trajectory after fault clearance, and restores normal modulation along the corresponding phase. This solves the problem of secondary current stress caused by the mismatch between the internal residual state and the normal modulation state after fault clearance.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A non-blocking fault ride-through method for an active bridge DC-DC converter includes the following steps: During a DC-side fault, the active bridge switch is kept running and fault current-limiting modulation is executed; after fault clearance is detected, the clearance phase and corresponding residual electromagnetic state are determined; based on the recovered port voltage and normal modulation parameters, the normal periodic trajectory from the clearance phase and the normal bridge state corresponding to each phase are determined; starting from the current bridge state and the residual electromagnetic state, state evolution is performed based on the executable bridge states that can be actually transitioned from the current bridge state to determine the reachable state that satisfies the current safety constraint, and the target access state is determined from the reachable states matching the normal inductor current, normal flux linkage state, and normal bridge state of the phase corresponding to the normal periodic trajectory; a bridge state transition to the target access state is executed, and after reaching the target access state, operation continues according to the normal modulation gate command corresponding to the target access phase.

[0007] In a preferred embodiment, maintaining the operation of the active bridge switch and performing fault current limiting modulation includes: detecting a DC-side fault, determining a fault operating state while maintaining the operation of the active bridge switch; determining fault current limiting modulation parameters based on the fault operating state, and forming a controlled bridge state sequence based on the fault current limiting modulation parameters; controlling the active bridge to switch according to the controlled bridge state sequence to limit the fault current, and updating the switch phase and internal electromagnetic state as the bridge state switches, forming a continuously evolving electromagnetic state during the fault.

[0008] In a preferred embodiment, determining the release phase includes: acquiring the operating status of the fault side and determining the fault release time based on the operating status of the fault side; determining the switching cycle to which the fault belongs and the corresponding cycle phase reference based on the fault release time; determining the release phase based on the position of the fault release time relative to the cycle phase reference, and determining the current bridge state based on the release phase.

[0009] In a preferred embodiment, determining the release phase based on the position of the fault release time relative to the periodic phase reference includes: acquiring the fault release time and the period start time of the current switching cycle to which it belongs, and determining the time position of the fault release time relative to the period start time; determining the release phase corresponding to the fault release time based on the time position and the length of the current switching cycle; determining the bridge state phase boundary within the current switching cycle based on the fault current limiting modulation parameters, and determining the current bridge state at the time of fault release based on the correspondence between the release phase and the bridge state phase boundary.

[0010] In a preferred embodiment, the residual electromagnetic state is specifically obtained by: determining the actual current of the equivalent series inductor at the time of fault clearance based on the bridge-side current corresponding to the release phase; determining the cumulative volt-second effect of the high-frequency transformer winding based on the bridge state sequence during fault current limiting modulation; determining the actual flux linkage state at the time of fault clearance based on the initial flux linkage state and the cumulative volt-second effect at the time of fault occurrence; and determining the actual current of the equivalent series inductor and the actual flux linkage state as the residual electromagnetic state corresponding to the release phase.

[0011] In a preferred embodiment, determining the normal periodic trajectory and the normal bridge state corresponding to each phase from the release phase based on the recovery port voltage and normal modulation parameters includes: determining the normal modulation parameters based on the recovery port voltage and the target operating state, and determining the normal initial electromagnetic state and the initial normal bridge state; determining the reference switching relationship based on the normal modulation parameters, and dividing multiple reference switching intervals; starting from the normal initial electromagnetic state and the initial normal bridge state, advancing the state according to each reference switching interval, successively determining the boundary electromagnetic state and the next normal bridge state corresponding to each reference switching phase, until a switching cycle is completed; forming the normal periodic trajectory based on the cycle advancement result, and performing cycle rearrangement starting from the release phase to obtain the normal periodic trajectory and the normal bridge state corresponding to each phase from the release phase.

[0012] In a preferred embodiment, dividing multiple reference switching intervals includes: determining the gate command change position of each active bridge according to normal modulation parameters to obtain a reference switching phase within a switching cycle; determining the corresponding bridge arm switching relationship according to the gate command change before and after each reference switching phase; dividing the reference switching intervals according to the order of the reference switching phases, and associating the bridge arm switching relationship with the corresponding reference switching interval.

[0013] In a preferred embodiment, the step of successively determining the boundary electromagnetic state and the next normal bridge state corresponding to each reference switching phase until a switching cycle is completed includes: determining the corresponding bridge output voltage based on the current normal bridge state, and determining the boundary electromagnetic state corresponding to the next reference switching phase based on the bridge output voltage and its duration; determining the commutation path based on the boundary electromagnetic state and the corresponding gate command change, and determining the next normal bridge state based on the commutation path; using the next normal bridge state and the corresponding electromagnetic state as the starting state for subsequent state advancement, and repeating the above process until a switching cycle is completed.

[0014] In a preferred embodiment, the step of determining reachable states that satisfy current safety constraints by performing state evolution based on executable bridge states that can be actually transitioned from the current bridge state includes: determining executable bridge states that can be directly transitioned to based on the current bridge state and the residual electromagnetic state; predicting the corresponding electromagnetic state changes based on each executable bridge state and selecting the next state node based on the current safety constraints; continuing state evolution from each next state node to form state nodes that satisfy current safety constraints and corresponding bridge state transition paths; and determining the state nodes and corresponding bridge state transition paths as reachable states.

[0015] In a preferred embodiment, determining the target access state from the reachable states matching the normal inductor current, normal flux linkage state, and normal bridge state corresponding to the phase of the normal cycle trajectory includes: determining the electromagnetic state deviation between the reachable state and the state corresponding to the normal cycle trajectory based on the switching phase corresponding to each reachable state, and screening reachable states that meet the access allowable range; determining candidate access states based on the matching relationship between the state-matched reachable states and the corresponding normal bridge states; screening candidate access states that meet the normal commutation conditions based on the candidate access states and their subsequent normal bridge state changes; determining the access sequence based on the recovery process corresponding to the remaining candidate access states, and determining the target access state.

[0016] The technical effects and advantages of the non-blocking fault ride-through method for active bridge DC converters of the present invention are as follows: This invention determines the release phase and residual electromagnetic state after fault clearance, and establishes a normal periodic trajectory and normal bridge state corresponding to the release phase. Starting from the actual bridge state and residual electromagnetic state, it searches for reachable states that meet current safety constraints, and determines the target access state that can be continuously connected with the normal operation state. Then, it restores normal modulation along the corresponding phase. This makes the fault recovery process no longer dependent solely on the recovery results of external electrical quantities such as port voltage and current, but also takes into account the internal electromagnetic state, bridge state, and actual commutation feasibility left after fault current limiting. This reduces the additional current changes and secondary current stress caused by state mismatch after fault clearance, and improves the recovery continuity and operational safety after non-blocking fault crossing. Attached Figure Description

[0017] Figure 1 This is a flowchart illustrating a non-blocking fault ride-through method for an active bridge DC-DC converter according to the present invention. Figure 2 This is a schematic diagram showing the changes in bridge-side current and transformer flux linkage during the non-blocking fault ride-through process of the present invention. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0019] Example 1, Figure 1 This invention provides a non-blocking fault ride-through method for active bridge DC-DC converters, comprising the following steps: S1, in the event of a DC-side fault, keeps the active bridge switch running and performs fault current limiting modulation; In this embodiment, maintaining the operation of the active bridge switch and performing fault current limiting modulation includes: Detect DC-side faults and keep each active bridge in switching operation. The fault current limiting modulation parameters are determined based on the port voltage and bridge-side current after the fault, and each active bridge is controlled to form a corresponding bridge state sequence according to the fault current limiting modulation parameters. The fault current limiting modulation parameters refer to the modulation parameters used to control the switching state and timing of the active bridge during a DC-side fault. The port voltage change state caused by the fault is determined based on the port voltage after the fault, and the current stress is determined in combination with the bridge-side current. The phase shift, duty cycle, or combination thereof of the active bridge is adjusted accordingly to keep the bridge-side current within the allowable range during the fault, while maintaining the active bridge continuously switching according to the determined bridge state sequence. The bridge-side current is limited during a fault according to the bridge state sequence, and the switch phase and internal electromagnetic state are updated as the bridge state switches, so that the internal electromagnetic state during a fault evolves continuously according to the bridge state sequence.

[0020] The purpose of this step is to avoid destabilizing the converter's internal state by blocking the active bridge after a DC-side fault occurs. Instead, it continuously executes fault current-limiting modulation to limit the fault current while maintaining continuous switching of the bridge state. This allows the internal electromagnetic states, such as inductor current and transformer flux linkage, to evolve continuously according to a known bridge state sequence during the fault. Therefore, when the fault is released at any switching phase, the corresponding residual electromagnetic state can be determined based on the fault current-limiting modulation process and the bridge state at the time of release. This provides the initial conditions for subsequently determining the safe reachable normal cycle trajectory position of the residual electromagnetic state and identifying the target access state.

[0021] S2, after the fault is detected to be cleared, determine the clearing phase and the corresponding residual electromagnetic state; In this embodiment, the release phase refers to the phase position of the fault release moment in the current switching cycle, which is used to characterize the bridge state operation stage of the fault current limiting modulation when the fault is released.

[0022] The determination of the release phase includes: Obtain the fault-side port voltage and bridge-side current. When the fault-side port voltage recovers to the fault clearance judgment range and the bridge-side current meets the fault clearance condition, determine the corresponding time as the fault clearance time. Obtain the current switching cycle including the fault clearance time, and use the start time of the switching cycle as the phase reference; Based on the time interval between the fault clearance time and the cycle start time, and the length of the switching cycle, the clearance phase of the fault clearance time in the current switching cycle is determined, and the current bridge state corresponding to the clearance phase is determined.

[0023] The fault clearance determination range is used to determine whether the voltage at the fault-side port has recovered from the fault drop state to the voltage state that allows for entering the fault exit determination. It is determined based on the port's rated voltage and allowable fluctuation range under normal operating conditions.

[0024] Specifically, the average port voltage during the stable operation phase before the fault occurs is used as the normal voltage reference. The allowable deviation range is determined based on the port voltage fluctuation amplitude during normal operation. The allowable deviation range is then corrected by combining the voltage recovery characteristics after fault clearance in fault clearance tests or historical operating data to obtain the fault clearance judgment range.

[0025] The fault clearance conditions are used to eliminate misjudgments caused by incomplete fault clearance or transient oscillations at the moment of fault clearance. These conditions include the bridge-side current falling back from the fault current-limited state to the normal operating range, and the port voltage continuously remaining within the fault clearance judgment range for a preset duration.

[0026] The allowable range of normal operation of the bridge-side current is determined based on the allowable current of the active bridge power device, the range of normal load current variation, and the upper limit of the fault current limiting current; the preset duration is determined based on the typical transient oscillation duration after the fault is cleared, making it greater than the duration of instantaneous recovery fluctuations caused by switching action or parasitic parameters, and less than the allowable delay time that affects the subsequent fault recovery control response.

[0027] When the voltage at the fault-side port is continuously within the fault clearance determination range and the bridge-side current simultaneously meets the normal operating allowable range, the DC-side fault is determined to be cleared, and the first time that the above conditions are met is determined as the fault clearance time.

[0028] The step of determining the release phase of the fault release time in the current switching cycle based on the time interval between the fault release time and the cycle start time and the switching cycle length includes: Obtain the fault clearing time and the start time of the current switching cycle containing the fault clearing time, and determine the time interval between the two: The normalized position of the fault clearance time in the current switching cycle is determined based on the ratio of the time interval to the length of the current switching cycle, and the fault clearance phase is obtained. Based on the fault current limiting modulation parameters, determine the phase boundaries corresponding to each bridge state within the current switching cycle. Compare the fault clearing phase with each phase boundary to determine the bridge state action interval at the fault clearing time. Then, determine the bridge state corresponding to this action interval as the current bridge state at the fault clearing time.

[0029] If the fault clearing time happens to be at the switching boundary between two adjacent bridge states, the current bridge state used for fault clearing is determined based on the actual gate drive signal flipping result; or, the bridge state after the switching is completed is taken as the current bridge state, and the phase corresponding to the switching boundary is taken as the clearing phase.

[0030] The specific calculation formula for the fault clearance phase is as follows:

[0031] In the formula, The fault is cleared in the phase; For time intervals; This represents the current switching cycle length.

[0032] Furthermore, the residual electromagnetic state refers to the electromagnetic state inside the active bridge DC converter that has not yet returned to the normal cycle state when the fault current limiting modulation continues to operate until the fault is cleared, including at least the equivalent series inductor current and the transformer flux linkage state.

[0033] The specific methods for obtaining the residual electromagnetic state include: Obtain the bridge-side current corresponding to the fault-clearing phase, and determine the actual current of the equivalent series inductor when the fault is cleared based on the current transfer relationship of the active bridge DC converter. Based on the bridge output voltage and its operating range corresponding to each bridge state during the fault current limiting modulation, determine the volt-second action of the high-frequency transformer winding in each operating range. Using the normal flux linkage state of the corresponding switch phase at the time of the fault as the initial flux linkage state, the volt-second action during the fault current limiting modulation period is accumulated to determine the actual flux linkage state corresponding to the fault clearance phase. The actual current and actual flux linkage state of the equivalent series inductor are correlated with the phase of fault release to form the residual electromagnetic state when the fault is cleared.

[0034] The specific formula for calculating the volt-second action is as follows:

[0035] In the formula, It is the volt-second action; The voltage of the transformer winding corresponding to the j-th bridge state's operating interval; This refers to the duration of action.

[0036] The actual flux linkage state is specifically calculated using the following formula:

[0037] In the formula, This represents the actual flux linkage state when the fault is cleared. This represents the flux linkage state at the time of the fault.

[0038] like Figure 2 As shown, the horizontal axis represents the operating time, and is divided into the normal operation stage, the fault current limiting modulation stage, and the normal modulation recovery stage after the fault is cleared, according to the time of DC side fault occurrence and fault clearance. Figure 2 The bridge-side current variation curves illustrate how the bridge-side current changes with operating conditions. The solid line represents the bridge-side current when using the non-blocking fault-crossing method of this invention, the dashed line represents the preset current limit, and the dashed line shows the bridge-side current variation under the comparison strategy. After a DC-side fault occurs, the active bridge maintains switch operation and adjusts the bridge state and its timing through fault current limiting modulation, ensuring the bridge-side current remains in a controlled evolution state during the fault. Upon fault clearance, the current bridge state is determined based on the corresponding clearance phase, and subsequent recovery control continues.

[0039] Figure 2 The transformer flux linkage change curves in the figure illustrate the changes in transformer flux linkage corresponding to the above process. The solid line represents the actual flux linkage formed during fault current limiting modulation and recovery, the dashed line represents the flux linkage trajectory corresponding to normal modulation under the same switching phase, and the dotted line represents the flux linkage zero line. During the fault, because the active bridge is not locked, the transformer windings are continuously subjected to the volt-second action corresponding to each bridge state; therefore, the flux linkage evolves continuously with the bridge state. The flux linkage state formed when the fault is cleared, together with the equivalent series inductance current, constitutes the residual electromagnetic state.

[0040] After the fault is cleared, starting from the current bridge state and residual electromagnetic state corresponding to the cleared phase, state evolution is performed based on the normal periodic trajectory and executable bridge states. Among the reachable states that satisfy current safety constraints, a target access state matching the normal inductor current, normal flux linkage state, and normal bridge state under the same switching phase is sought. Figure 2 The fault clearing time in the code is used to indicate the starting position of the search and recovery process for that state, rather than requiring a direct switch to normal modulation at the moment the fault is cleared.

[0041] When the actual electromagnetic state reaches the target access state along the determined bridge state transition path, and the current access deviation, magnetic link access deviation, and actual bridge state meet the corresponding access conditions, the phase from the target access phase continues to run according to the normal cycle trajectory and executes the subsequent gate instruction sequence corresponding to normal modulation, thereby realizing continuous access from the fault current limiting modulation state to the normal cycle operation state.

[0042] S3, based on the recovered port voltage and normal modulation parameters, determine the normal periodic trajectory from the release phase and the normal bridge state corresponding to each phase; In this embodiment, the normal periodic trajectory refers to the electromagnetic state trajectory formed by the equivalent series inductor current and transformer flux linkage changing periodically with the switching phase when the active bridge DC converter operates according to normal modulation parameters under the port voltage conditions after the fault is cleared. It is used to characterize the target electromagnetic state corresponding to each switching phase under normal modulation.

[0043] The normal bridge state refers to the actual conduction state of the active bridge determined by the gate command of normal modulation and the actual commutation process near the corresponding switching phase, including the normal conduction state and the transition bridge state formed during the dead-zone commutation process.

[0044] The step of determining the normal periodic trajectory and the normal bridge state corresponding to each phase from the release phase based on the recovered port voltage and normal modulation parameters includes: The input and output voltages after the fault is cleared are obtained, and the corresponding normal modulation parameters are determined based on the input and output voltages and the target operating state. The reference switching phase of each active bridge and the corresponding gate command change within a switching cycle are determined based on the normal modulation parameters, and multiple reference switching intervals are divided based on the reference switching phase. The normal initial electromagnetic state and initial normal bridge state corresponding to the periodic reference phase are determined based on the recovered port voltage, normal modulation parameters, and target operating state. Further, the normal initial electromagnetic state is determined based on the normal steady-state operating solution corresponding to the recovered port voltage, normal modulation parameters, and target operating state. For an active bridge DC-DC converter using periodic symmetrical modulation, the normal initial inductor current and normal initial flux linkage state corresponding to the periodic reference phase are determined based on the periodic symmetrical relationship between the inductor current and the transformer flux linkage. The initial normal bridge state is determined based on the gate command combination corresponding to the periodic reference phase and the direction of the normal inductor current in that phase.

[0045] Starting from the normal initial electromagnetic state and the initial normal bridge state, the state is advanced in the order of each reference switching interval, and the boundary electromagnetic state and the next normal bridge state corresponding to each reference switching phase are determined one by one until a switching cycle is completed. When the periodic closure condition is met, the equivalent series inductor current, transformer flux linkage and normal bridge state corresponding to each switch phase in a switching cycle are correlated to form a normal periodic trajectory. Starting with the fault-clearing phase, the electromagnetic state, normal bridge state, and corresponding switching phase in the normal periodic trajectory are periodically rearranged according to the normal modulation direction to obtain the normal periodic trajectory from the fault-clearing phase and the normal bridge state corresponding to each phase.

[0046] Further, the step of determining the reference switching phase of each active bridge and the corresponding gate command change within a switching cycle based on normal modulation parameters, and dividing multiple reference switching intervals based on the reference switching phase, includes: The gate command change position of each active bridge relative to the periodic reference phase is determined according to the normal modulation parameters, and the position where each gate command changes is determined as the corresponding reference switching phase. Obtain the gate command combination before and after each reference switching phase, and determine the bridge arm that is switched at the corresponding reference switching phase and its switching direction based on the gate command that changes between the two. The reference switching phases are arranged in ascending order of switching phase, and multiple bridge arm switches located in the same switching phase are merged into the same reference switching phase. Using the periodic reference phase, the sequentially arranged reference switching phases, and the periodic reference phase of the next cycle as interval boundaries, multiple reference switching intervals are formed continuously within a switching cycle, and the gate command combination corresponding to the start and end boundaries of each interval is associated with the reference switching interval.

[0047] Furthermore, the process of advancing the state according to the sequence of each reference switching interval, starting from the normal initial electromagnetic state and the initial normal bridge state, and successively determining the boundary electromagnetic state and the next normal bridge state corresponding to each reference switching phase, until a switching cycle is completed, includes: The normal initial electromagnetic state and the initial normal bridge state are used as the starting state of the first reference switching interval. The primary bridge output voltage and the secondary bridge output voltage are determined according to the current normal bridge state. Based on the voltage difference between the primary bridge output voltage and the secondary bridge output voltage acting on the equivalent series inductor and the actual duration of the current normal bridge state, the boundary inductor current before reaching the next reference switching phase and starting commutation is determined, and the boundary flux state is synchronously determined based on the transformer winding voltage corresponding to the current normal bridge state. Based on the direction of the boundary inductor current and the gate command change corresponding to the next reference switching phase, the bridge arm where commutation occurs and the current freewheeling path after the device is turned off are determined, and the transition bridge state during the dead time is determined based on the current freewheeling path. The boundary inductor current and boundary flux state are updated based on the bridge output voltage and dead zone time formed by the transition bridge state. After the bridge terminal potential corresponding to the target conducting device is established, the actual bridge output state is determined as the next normal bridge state. The next normal bridge state and the corresponding inductor current and flux linkage state at the time of its establishment are used as the starting state of the next reference switching interval. The above process is repeated in the order of the reference switching intervals until a switching cycle is completed.

[0048] Specifically, let the k-th normal bridge state be in Establish, in the next reference phase switching Once commutation begins, the actual hold time is:

[0049] Converting the secondary side to the primary side, the equivalent series inductor voltage is:

[0050] Therefore, the boundary inductance current before commutation begins is:

[0051] In the formula, This refers to the actual holding time; This is the equivalent series inductor voltage; and These are the primary and secondary bridge output voltages corresponding to the bridge state, respectively. The voltage conversion factor for the transformer is determined based on the ratio of the number of turns on the primary side to the number of turns on the secondary side of the transformer. This is the boundary inductance current before commutation begins; The inductor current is the actual inductor current when the k-th normal bridge state is established. It is the equivalent series inductance.

[0052] S4, starting from the current bridge state and the residual electromagnetic state, determine the reachable state that meets the current safety constraints based on the executable bridge state, and determine the target access state from the reachable states located on the normal periodic trajectory and whose bridge state is consistent with the corresponding normal bridge state. In this embodiment, the reachable state refers to a state that can be actually reached after at least one executable bridge state transition, starting from the current bridge state and residual electromagnetic state corresponding to the fault-clearing phase, and in which the equivalent series inductor current does not exceed the current safety limit during the bridge state action process; each reachable state is associated with at least the corresponding switching phase, bridge state, predicted inductor current, predicted flux linkage state, and bridge state transition path from the fault-clearing state to that state.

[0053] The process of determining reachable states that satisfy current safety constraints based on the executable bridge state, starting from the current bridge state and the residual electromagnetic state, includes: Obtain the current bridge state and residual electromagnetic state corresponding to the fault clearance phase, and determine the executable bridge state that can be directly transitioned from the current bridge state based on the complementary conduction relationship of the active bridge, dead time requirements, current equivalent series inductor current direction, and remaining time until the next reference switching phase. For each executable bridge state, based on the actual bridge output voltage after its establishment and the duration of its operation, the equivalent series inductor current and transformer flux change during the operation of the corresponding bridge state are predicted, and the peak current during the operation is determined. Remove bridge state transitions where the peak current exceeds the current safety limit during the operation period. Form the next state node with the predicted electromagnetic state, corresponding bridge state and switching phase at the end of the remaining bridge state transitions, and record the bridge state transition from the current state node to the next state node. Starting from the bridge state and predicted electromagnetic state corresponding to each next state node, the state evolution continues in the order of reference switching phases from the fault clearance phase until the preset state evolution range is reached. The state node in which the peak current of each bridge state's active range does not exceed the current safety limit during the state evolution process, along with its corresponding switch phase, bridge state, and bridge state transition path, is determined as a reachable state.

[0054] Furthermore, determining the executable bridge state that can be directly transitioned from the current bridge state includes: The actual conduction state of each bridge arm is determined based on the current bridge state, and in accordance with the requirement that the upper and lower switching devices of the same bridge arm are complementary in conduction, a candidate bridge state is formed to maintain the current bridge state or to enable at least one commutation bridge arm to perform a complementary switching once. Eliminate candidate bridge states that cause simultaneous conduction of upper and lower switching devices in the same bridge arm, and set dead time for bridge arms that undergo complementary switching; The current freewheeling path of each commutator arm during the dead zone is determined based on the current direction of the current in the current of the current in the current equivalent series inductor, and the actual transition bridge state and bridge output voltage formed during the dead zone are determined based on the freewheeling path. Based on the transition bridge state, dead time, and gate command of the target conducting device, determine the actual establishment time of the bridge output voltage corresponding to the candidate bridge state; Determine the effective duration from the actual establishment time to the next reference switching phase, and determine the candidate bridge state into which the bridge output voltage can be actually established and the effective duration is not less than the minimum effective duration as the executable bridge state into which the current bridge state can directly transition.

[0055] The candidate bridge state must at least satisfy:

[0056]

[0057] In the formula, In candidate bridge state; The minimum effective duration is determined based on the minimum effective pulse width of the power device, the gate drive response time, and the controller time resolution. The time corresponding to the next reference phase switch; This represents the actual time when the candidate bridge state is established.

[0058] The process begins with the bridge state and predicted electromagnetic state corresponding to each next state node as a new starting point, and continues the state evolution according to the reference switching phase sequence starting from the fault-clearing phase, until a preset state evolution range is reached, including: Based on the switching phase corresponding to the current state node, determine the next reference switching phase along the normal modulation direction from the reference switching phases since the fault clearing phase; Using the bridge state, predicted inductor current, and predicted flux linkage state corresponding to the current state node as the new starting state, and in accordance with the method for determining the executable bridge state, determine the subsequent executable bridge state that can actually be established before the next reference switching phase. For each subsequent executable bridge state, predict the changes in inductor current and transformer flux linkage from the current state node to the next reference switching phase, and determine the current peak value of each state evolution branch; Eliminate the state evolution branch where the current peak exceeds the current safety limit, and determine the predicted electromagnetic state, bridge state and switching phase formed by the remaining branches when the next reference switching phase or the corresponding bridge state is actually established as the update state node. Starting from each updated state node, the executable bridge state continues to expand along the subsequent reference switching phase until it covers the preset state evolution range. The preset state evolution range is initially set to one normal switching cycle from the fault clearing phase; when there is no reachable state that meets the access conditions within this range, it expands backward in units of one normal switching cycle, and the cumulative state evolution time does not exceed the preset maximum recovery time.

[0059] When the bridge output voltage remains constant or approximately constant within the current actual bridge state operating range, the equivalent series inductor current is:

[0060] The transformer flux linkage during the current actual bridge operation is:

[0061] For the current actual bridge state operating range, its equivalent series inductor current peak value is determined as follows:

[0062] In the formula, This is the equivalent series inductor current when the current actual bridge state begins to function; This is the equivalent series inductor current when the current actual bridge state ends; This refers to the actual moment when the bridge is established, reflecting its current state. The termination time is the time when the current actual bridge state begins to switch or the time when the current prediction interval ends. The voltage across the equivalent series inductor within the current actual bridge state operating range; The equivalent series inductance referred to the same side includes the transformer leakage inductance and the external series inductance set in the power transmission circuit; and These are the transformer flux linkages at the start and end of the current actual bridge state, respectively. This refers to the voltage actually applied to the corresponding winding used to establish the transformer's main magnetic flux under the current actual bridge condition; when considering the voltage drop due to winding resistance, The induced voltage is obtained by subtracting the voltage drop across the winding resistance from the winding terminal voltage; ignoring the influence of winding resistance. Use the actual winding terminal voltage within this operating range; This represents the predicted peak current within the current actual bridge condition's operating range.

[0063] Further, determining the target access state from reachable states located on a normal periodic trajectory and whose bridge state is consistent with the corresponding normal bridge state includes: Based on the switching phase corresponding to each reachable state, obtain the normal inductor current, normal flux linkage state and normal bridge state corresponding to the same switching phase from the normal periodic trajectory. Determine the inductor current deviation and magnetic flux deviation between the reachable state and the normal electromagnetic state corresponding to the same switch phase, and determine the reachable state in which both the inductor current deviation and magnetic flux deviation are within the corresponding access allowable range as the state matching reachable state. From the state matching reachable states, select states whose bridge state is consistent with the normal bridge state corresponding to the same switch phase to form candidate access states; Based on the inductor current direction corresponding to each candidate access state and the gate command change corresponding to the first reference switching phase after the switching phase of normal modulation, it is determined whether the candidate access state can form a follow-through path consistent with the normal cycle trajectory when it continues to run according to normal modulation, and the candidate access state that cannot form the follow-through path is eliminated. The access order is determined by ranking the recovery time required to reach the candidate access state from the fault clearing state in ascending order. If the recovery times are the same, the access order is determined by ranking the number of bridge state transitions in ascending order. The candidate access state ranked first is determined as the target access state.

[0064] S5, execute the bridge state transition to the target access state, and upon arrival, access normal modulation along the normal bridge state sequence of the corresponding phase.

[0065] In this embodiment, the step of performing a bridge state transition to the target access state and accessing normal modulation along the normal bridge state sequence of the corresponding phase upon arrival includes: Obtain the target access state and its corresponding bridge state transition path, execute each bridge state sequentially according to the bridge state transition path, and complete the switching between adjacent bridge states according to the dead time, state establishment time and action time corresponding to each bridge state. At each bridge state transition node, the actual equivalent series inductor current is obtained, and the transformer flux is updated according to the actual bridge state, the corresponding bridge output voltage and the actual action time, so as to form the actual electromagnetic state corresponding to each bridge state transition node. The actual electromagnetic state and actual bridge state corresponding to each bridge state transition node are verified against the predicted electromagnetic state and predicted bridge state of the corresponding node in the bridge state transition path. If the current safety constraints and the path continuation conditions are met, the next bridge state transition is executed. If not, the current actual bridge state and actual electromagnetic state are used as the new starting point to re-determine the executable bridge state, target access state, and corresponding bridge state transition path. When the execution reaches the target access phase corresponding to the target access state, the normal inductor current, normal flux linkage state and normal bridge state corresponding to the target access phase are obtained from the normal cycle trajectory, and the current access deviation between the current actual inductor current and the normal inductor current, and the flux linkage access deviation between the current actual flux linkage state and the normal flux linkage state are determined respectively. When the current access deviation and magnetic link access deviation are both within the corresponding access allowable range, and the current actual bridge state is consistent with the normal bridge state, the phase reference of the current switching cycle and the current normal bridge state are maintained, and the normal cycle trajectory continues to run from the target access phase. When the next normal switching phase is reached, the subsequent gate instruction sequence corresponding to the normal modulation is executed. When at least one of the current access deviation, magnetic link access deviation, or current actual bridge state fails to meet the corresponding access conditions, non-blocking operation is maintained, and the executable bridge state, target access state, and corresponding bridge state transition path are re-determined with the current actual bridge state and actual electromagnetic state as the new starting point.

[0066] Table 1

[0067] Table 1 presents a comparison of the peak current, flux deviation, and number of cycles required to recover to steady state in the conventional direct recovery method and the method of this invention under different fault clearance phases. It can be seen that the peak recovery current and flux deviation of the conventional direct recovery method fluctuate significantly with the change of fault clearance phase. In contrast, the method of this invention can control the peak recovery current and flux deviation at a low level in all clearance phases, while simultaneously shortening the number of switching cycles required to recover to a stable operating state. This indicates that determining the connection state based on the clearance phase, residual electromagnetic state, and normal cycle trajectory is beneficial for reducing state mismatch and secondary current stress in the early stages of recovery.

[0068] The above formulas are all dimensionless calculations. The formulas are derived from software simulations based on a large amount of collected data to obtain the most recent real-world results. The preset parameters in the formulas are set by those skilled in the art according to the actual situation.

[0069] The above embodiments can be implemented, in whole or in part, by software, hardware, firmware, or any other combination thereof. When implemented using software, the above embodiments can be implemented, in whole or in part, in the form of a computer program product.

[0070] Those skilled in the art will recognize that the modules and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.

[0071] In addition, the functional modules in the various embodiments of this application can be integrated into one processing module, or each module can exist physically separately, or two or more modules can be integrated into one module.

[0072] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

[0073] In conclusion, the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A non-blocking fault ride-through method for an active bridge DC-DC converter, characterized in that, Includes the following steps: In the event of a DC-side fault, the active bridge switch remains operational and fault current limiting modulation is performed; After the fault is detected as cleared, the clearing phase and the corresponding residual electromagnetic state are determined. Based on the recovered port voltage and normal modulation parameters, determine the normal periodic trajectory from the release phase and the normal bridge state corresponding to each phase; Starting from the current bridge state and the residual electromagnetic state, the state evolution is carried out according to the executable bridge state that can be actually transitioned from the current bridge state, the reachable state that satisfies the current safety constraint is determined, and the target access state is determined from the reachable states that match the normal inductor current, normal flux linkage state and normal bridge state corresponding to the phase of the normal periodic trajectory. Execute the bridge state transition to the target access state, and after reaching the target access state, continue operation according to the normal modulation gate instruction corresponding to the target access phase.

2. The non-blocking fault ride-through method for an active bridge DC-DC converter according to claim 1, characterized in that, The method of maintaining the active bridge switch operation and performing fault current limiting modulation includes: Detect DC-side faults and determine the fault operating status while maintaining the operation of the active bridge switch; The fault current limiting modulation parameters are determined based on the fault operating state, and a controlled bridge state sequence is formed based on the fault current limiting modulation parameters; The active bridge is switched according to the controlled bridge state sequence to limit the fault current, and the switching phase and internal electromagnetic state are updated as the bridge state switches, forming a continuously evolving electromagnetic state during the fault.

3. The non-blocking fault ride-through method for an active bridge DC-DC converter according to claim 2, characterized in that, The determination of the release phase includes: Obtain the operating status of the faulty side and determine the fault clearance time based on the operating status of the faulty side; Determine the switching cycle and corresponding cycle phase reference based on the fault clearance time. The release phase is determined based on the position of the fault release time relative to the periodic phase reference, and the current bridge state is determined based on the release phase.

4. The non-blocking fault ride-through method for an active bridge DC-DC converter according to claim 3, characterized in that, The step of determining the release phase based on the position of the fault release time relative to the periodic phase reference includes: Obtain the fault clearance time and the start time of the current switching cycle to which it belongs, and determine the time position of the fault clearance time relative to the start time of the cycle. Determine the release phase corresponding to the fault release time based on the time location and the current switching cycle length; The bridge state phase boundary within the current switching cycle is determined based on the fault current limiting modulation parameters, and the current bridge state at the time of fault clearance is determined based on the correspondence between the release phase and the bridge state phase boundary.

5. The non-blocking fault ride-through method for an active bridge DC-DC converter according to claim 4, characterized in that, The specific methods for obtaining the residual electromagnetic state include: The actual current of the equivalent series inductor when the fault is cleared is determined by the bridge-side current corresponding to the phase of the fault clearing. The cumulative volt-second action of the high-frequency transformer winding is determined based on the bridge state sequence during fault current limiting modulation. The actual flux linkage state at the time of fault clearance is determined based on the initial flux linkage state at the time of fault occurrence and the cumulative volt-second action. The actual current and actual flux linkage state of the equivalent series inductor are determined as the residual electromagnetic state corresponding to the phase release.

6. The non-blocking fault ride-through method for an active bridge DC-DC converter according to claim 5, characterized in that, The step of determining the normal periodic trajectory and the normal bridge state corresponding to each phase from the release phase based on the recovered port voltage and normal modulation parameters includes: Determine the normal modulation parameters based on the restored port voltage and the target operating state, and determine the normal initial electromagnetic state and the initial normal bridge state; The reference switching relationship is determined based on the normal modulation parameters, and multiple reference switching intervals are divided. Starting from the normal initial electromagnetic state and the initial normal bridge state, the state is advanced according to each reference switching interval. The boundary electromagnetic state and the next normal bridge state corresponding to each reference switching phase are determined one by one until a switching cycle is completed. Based on the results of the periodic advancement, a normal periodic trajectory is formed, and the periodic rearrangement is performed starting from the release phase to obtain the normal periodic trajectory from the release phase and the normal bridge state corresponding to each phase.

7. The non-blocking fault ride-through method for an active bridge DC-DC converter according to claim 6, characterized in that, The division of multiple reference switching intervals includes: The gate command change position of each active bridge is determined based on the normal modulation parameters, and the reference switching phase within one switching cycle is obtained. The corresponding bridge arm switching relationship is determined based on the changes in gate commands before and after each reference switching phase; The reference switching intervals are divided according to the order of the reference switching phases, and the bridge arm switching relationships are associated with the corresponding reference switching intervals.

8. The non-blocking fault ride-through method for an active bridge DC-DC converter according to claim 7, characterized in that, The process of successively determining the boundary electromagnetic state and the next normal bridge state corresponding to each reference switching phase until a switching cycle is completed includes: The corresponding bridge output voltage is determined based on the current normal bridge state, and the boundary electromagnetic state corresponding to the next reference switching phase is determined based on the bridge output voltage and its duration. The commutation path is determined based on the boundary electromagnetic state and the corresponding gate command change, and the next normal bridge state is determined based on the commutation path. The next normal bridge state and its corresponding electromagnetic state are used as the starting state for subsequent state advancement. The above process is repeated until a switching cycle is completed.

9. The non-blocking fault ride-through method for an active bridge DC-DC converter according to claim 8, characterized in that, The process of determining reachable states that satisfy current safety constraints based on executable bridge states that can be actually transitioned from the current bridge state includes: The executable bridge state that can be directly transitioned to is determined based on the current bridge state and the residual electromagnetic state. Based on the prediction of the electromagnetic state changes corresponding to each executable bridge state, and the selection of the next state node according to the current safety constraints; Starting from each next state node, continue the state evolution to form state nodes that satisfy current safety constraints and corresponding bridge state transition paths; The state nodes and their corresponding bridge state transition paths are determined as reachable states.

10. The non-blocking fault ride-through method for an active bridge DC-DC converter according to claim 9, characterized in that, Determining the target access state from the reachable states matching the normal inductor current, normal flux linkage state, and normal bridge state corresponding to the phase of the normal periodic trajectory includes: Based on the switching phase corresponding to each reachable state, determine the electromagnetic state deviation between the reachable state and the state corresponding to the normal cycle trajectory, and filter the reachable states that meet the access allowable range. Based on the matching relationship between the reachable state and the corresponding normal bridge state, the candidate access state is determined; Based on the relationship between the candidate access status and its subsequent normal bridge status changes, candidate access statuses that meet the normal switching conditions are selected. The access order is determined based on the recovery process corresponding to the remaining candidate access states, and the target access state is determined.

Images