Method for controlling bus voltage in medium voltage ac-dc system and system therefor

Abstract

The application provides a method and system for controlling bus voltage in a medium-voltage AC / DC system. The method collects AC grid voltage and calculates positive sequence voltage drop depth M according to positive sequence component. M is compared with a preset threshold value. When M is greater than or equal to the threshold value, the DC bus voltage is controlled by a module cascade converter. When M is less than the threshold value, the main control ability is released and a coordinated control mode is triggered. In the coordinated control mode, the control right of the DC bus voltage is equally distributed among at least two power conversion units, including an energy storage bidirectional DC / DC conversion unit and a low-voltage DC / AC conversion unit. When the energy storage bidirectional DC / DC conversion unit has charging and discharging conditions, it is preferentially controlled. When it does not have the conditions, the low-voltage DC / AC conversion unit takes over. PWM driving signals are generated based on the corrected current reference command to adjust the DC bus voltage. When M returns to above the threshold value, the main control of the module cascade converter is restored. The application improves the stability of the bus voltage under fault.

Description

Technical Field

[0001] This invention relates to the field of power system technology, and more specifically, to a method and system for controlling bus voltage in medium-voltage AC / DC systems. Background Technology

[0002] Medium-voltage AC / DC systems refer to power systems with voltage levels between 1kV and 35kV that include both AC and DC power supply links. These systems achieve bidirectional conversion and coordinated control of AC and DC power through power electronic converters, combining the maturity of traditional AC power distribution with the high efficiency of DC power transmission. With the large-scale integration of distributed renewable energy and the rapid growth of DC loads such as data centers and electric vehicle charging stations, medium-voltage AC / DC systems have demonstrated significant advantages in multi-voltage level power conversion, new energy aggregation and consumption, and have become an important direction for future power grid development.

[0003] The stable operation of medium-voltage AC / DC systems depends on the reliable control of the DC bus voltage. Because the numerous power electronic converters connected to the system often exhibit constant power load characteristics, their negative impedance effect can easily lead to bus voltage oscillations or even instability. Simultaneously, the complex system structure, with multiple ports, voltage levels, and operating modes, presents challenges to voltage control. The fundamental contradiction lies in the imbalance between the high-frequency dynamic response capability of low-inertia power electronic equipment and the robustness requirements under complex fault conditions. Maintaining the stability of the DC bus voltage when fault conditions such as voltage dips occur on the AC side is a critical issue that medium-voltage AC / DC systems urgently need to address.

[0004] Existing methods for controlling the bus voltage in medium-voltage AC / DC systems can be mainly categorized as follows: First, adjusting the DC bus voltage using direct voltage control strategies (such as dual closed-loop control) or droop control strategies of the converter, or employing droop control to simulate synchronous generator characteristics and adjust power output based on DC voltage deviations. Second, utilizing the charging and discharging of energy storage devices such as batteries and supercapacitors to smooth power fluctuations and maintain stable bus voltage. Third, using DC / DC converters to regulate DC voltage levels, suitable for multi-voltage DC systems. Fourth, using DC circuit breakers and current limiters to quickly clear faults or limit short-circuit currents to prevent voltage collapse.

[0005] While the aforementioned existing technologies can achieve basic control of the bus voltage under most operating conditions, they still have shortcomings. During faults such as voltage dips in the AC grid of a medium-voltage AC / DC system, the power electronic converter may be unable to effectively maintain the DC bus voltage due to limited control capabilities. Furthermore, when partial faults occur in the system, such as when the DC bus voltage deviates and the battery state of charge exceeds the charge / discharge threshold, the energy storage system will be unable to charge or discharge. In this case, the control strategy relying on energy storage devices to maintain the bus voltage will fail. Under these conditions, existing control methods cannot guarantee the continuous stability of the DC bus voltage, affecting the overall system reliability. Summary of the Invention

[0006] This invention provides a method and system for controlling bus voltage in a medium-voltage AC / DC system. The method obtains the voltage drop level by performing positive sequence component analysis on the AC grid voltage and adaptively judges the AC grid operating status based on the voltage drop depth. Under different operating conditions, the control rights of the cascaded converter and multiple power conversion units are dynamically switched.

[0007] Under normal AC grid operation, the cascaded converter module handles the main control of the DC bus voltage. When a voltage dip is detected, the main control capability of the cascaded converter module is deactivated, triggering a coordinated control mode. This allows the energy storage bidirectional DC / DC converter unit and the low-voltage DC / AC converter unit to enter a peer-to-peer coordinated control state, thereby achieving distributed support for the DC bus voltage from multiple power conversion units. During this coordinated control process, each power conversion unit independently generates current control commands through a voltage deviation control structure and modifies these commands based on operational constraints to adapt to changes in the energy storage device's charging and discharging capabilities and operational limitations.

[0008] When the energy storage bidirectional DC / DC converter unit is ready to charge and discharge, it takes priority in controlling the DC bus voltage. When it is not ready to charge and discharge, the low-voltage DC / AC converter unit automatically takes over the control function, thus achieving seamless migration of control functions. After the grid voltage returns to the normal range, the system control is switched back to the cascaded converter module, achieving a smooth return to the operating mode.

[0009] Through the above technical solution, the present invention can achieve adaptive allocation and seamless switching of control rights among multiple power conversion units under complex operating conditions such as grid voltage drop and limited energy storage. It effectively avoids the problem of bus voltage instability caused by the failure of a single control unit, improves the dynamic stability and power supply reliability of medium voltage AC / DC system under fault conditions, and reduces the dependence on centralized controller, thereby enhancing the robustness of the system and the flexibility of engineering implementation.

[0010] In a first aspect, the present invention provides a method for controlling bus voltage in a medium-voltage AC / DC system, the method comprising: Collect the AC grid voltage of the medium-voltage AC / DC system, and calculate the positive sequence voltage drop depth M based on its positive sequence component. M is the ratio of the positive sequence voltage amplitude during the voltage drop period to the positive sequence voltage amplitude during normal operation. M is compared with a preset threshold: when M is not less than the preset threshold, the DC bus voltage is controlled by the cascaded converter; otherwise, the main control capability of the cascaded converter is released, and the control of the DC bus voltage is equally distributed between at least two power conversion units. The power conversion unit includes at least an energy storage bidirectional DC / DC conversion unit and a low-voltage DC / AC conversion unit. A deviation control structure is set in the controller of each power conversion unit. The deviation control structure includes an upper deviation PI controller and a lower deviation PI controller connected in series. The upper deviation PI controller is used to generate an integral limit to constrain the integral range of the lower deviation PI controller. The lower deviation PI controller is used to generate a current reference command. The deviation control structure is also used to perform operating state constraint correction on the current reference command. When the energy storage bidirectional DC / DC converter unit is ready for charging and discharging, it shall take priority in controlling the DC bus voltage; otherwise, the low-voltage DC / AC converter unit shall take over the control. The corresponding power conversion unit is controlled based on the modified current reference command to regulate the DC bus voltage; When M recovers to a value not less than the preset threshold, the cascaded converter resumes main control of the DC bus voltage, and the power conversion unit exits control.

[0011] Secondly, the present invention also provides a control system for bus voltage in a medium-voltage AC / DC system, the system comprising: The voltage detection module is used to collect the AC grid voltage of the medium-voltage AC / DC system and calculate the positive sequence voltage drop depth M based on its positive sequence component. M is the ratio of the positive sequence voltage amplitude during the voltage drop period to the positive sequence voltage amplitude during normal operation. The mode determination module is used to compare M with a preset threshold: when M is not less than the preset threshold, the DC bus voltage is controlled by the cascaded converter; otherwise, the main control capability of the cascaded converter is deactivated. The equal distribution module is used to equally distribute the DC bus voltage control between at least two power conversion units after the main control capability of the cascaded converter is deactivated. The power conversion unit includes at least an energy storage bidirectional DC / DC conversion unit and a low-voltage DC / AC conversion unit. The deviation control structure is set in the controller of each power conversion unit, including an upper deviation PI controller and a lower deviation PI controller connected in series. The upper deviation PI controller is used to generate an integral limit to constrain the integral range of the lower deviation PI controller, and the lower deviation PI controller is used to generate a current reference command. The deviation control module is also used to perform operating state constraint correction on the current reference command. The switching control module is used to control the energy storage bidirectional DC / DC converter unit to take priority in controlling the DC bus voltage when the energy storage bidirectional DC / DC converter unit is ready for charging and discharging; otherwise, it controls the low-voltage DC / AC converter unit to take over the control. The power control module is used to control the corresponding power conversion unit based on the modified current reference command to regulate the DC bus voltage. The recovery control module is used to restore the main control of the DC bus voltage by the cascaded converter when M recovers to a value not less than a preset threshold, and to control the power conversion unit to exit control.

[0012] Compared with the prior art, the present invention has the following beneficial effects: First, by adopting an equal-coordinated control strategy, the control of the DC bus voltage is switched between the energy storage bidirectional DC / DC converter unit and the low-voltage DC / AC converter unit. This avoids the reliability defects caused by relying solely on the energy storage bidirectional DC / DC converter unit as a backup in the existing technology. When the energy storage bidirectional DC / DC converter unit does not have the conditions for charging and discharging, the low-voltage DC / AC converter unit can seamlessly take over the bus voltage control, thereby ensuring the stability of the intermediate bus voltage and greatly improving the reliability of the system. Second, by introducing a switching coefficient |(Fsoc+Fidc) / 2| based on the battery state of charge flag Fsoc and the discharge current flag Fidc into the inner current loop of the energy storage bidirectional DC / DC converter unit, the energy storage bidirectional DC / DC converter unit can automatically exit the bus voltage control when it cannot charge or discharge. Without the need for complex central controller logic, the autonomous switching of DC bus voltage control between the energy storage bidirectional DC / DC converter unit and the low-voltage DC / AC converter unit can be achieved, which is easy to implement and highly reliable. Third, by real-time detection and segmented response of the positive sequence voltage drop depth M, combined with the dual-equipment redundancy mechanism and negative sequence current suppression control measures, the symmetry of the injected grid current is ensured, the stability of the DC bus voltage is guaranteed, and the system can maintain stable control of the DC bus voltage under complex fault conditions such as AC grid voltage drop. Fourth, without increasing the complexity of system control, each power conversion unit controller works independently, with a clear control structure that is easy to implement in engineering and suitable for widespread application in existing medium-voltage AC / DC systems.

[0013] It should be noted that existing technologies also employ Active Disturbance Rejection Control (ADRC) schemes that use reduced-order ESO observers to actively compensate for the total system disturbance in real time. However, this approach has limitations such as complex observer design (ESO gain parameters need to be tuned in the frequency domain) and sensitivity to high-frequency noise (susceptible to switching noise interference), which significantly restricts its practical engineering applications. In contrast, the series dual-PI deviation control scheme proposed in this invention does not require complex observer design, has a simple control structure, is easy to implement in engineering, and exhibits better stability in switching noise environments. Attached Figure Description

[0014] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0015] Figure 1 This is a schematic diagram of the bus voltage operation control strategy provided in an embodiment of the present invention; Figure 2 This is a control block diagram of a low-voltage DC / AC converter unit provided in an embodiment of the present invention; Figure 3 This is a control block diagram of the energy storage DC / DC converter unit provided in an embodiment of the present invention. Detailed Implementation

[0016] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Invention Overview As mentioned above, this invention provides a method and system for controlling bus voltage in a medium-voltage AC / DC system. By adopting an equal-coordinated control strategy based on the positive-sequence voltage drop depth M of the AC power grid, it effectively realizes the autonomous and seamless switching of control rights of the DC bus voltage between the energy storage bidirectional DC / DC converter unit and the low-voltage DC / AC converter unit without the need for complex central controller logic, thus ensuring the stability of the bus voltage of the medium-voltage AC / DC system under fault conditions.

[0018] Exemplary methods This invention provides a method for controlling bus voltage in a medium-voltage AC / DC system, which includes the following steps: S101: Collect the voltage of the AC grid of the medium-voltage AC / DC system, and calculate the positive sequence voltage drop depth M based on the positive sequence component of the AC grid voltage. The positive sequence voltage drop depth M is the ratio of the positive sequence voltage amplitude during the AC grid voltage drop to the positive sequence voltage amplitude during normal operation.

[0019] The formula for calculating M is:

[0020] In the formula, U d+ and U q+ These are the d-axis and q-axis components of the positive sequence component of the AC grid voltage, respectively. N This represents the positive sequence voltage amplitude during normal operation. A larger M value indicates that the power grid is closer to normal operating conditions, while a smaller M value indicates a more severe voltage drop.

[0021] S102: Compare the positive sequence voltage drop depth M with a preset threshold, and determine the AC grid operating status based on the comparison result: when M is greater than or equal to the preset threshold, the AC grid is determined to be in normal operation, and the DC bus voltage is controlled by the cascaded converter; when M is less than the preset threshold, the AC grid is determined to be in voltage drop state, the main control capability of the cascaded converter over the DC bus voltage is released, and the coordinated control mode is triggered.

[0022] In this step, the preset threshold is used to distinguish between the normal operation state and voltage dip state of the AC power grid, and its value is determined based on engineering experience of low voltage ride-through in power systems. Specifically, when the value of M is large, it indicates that the AC power grid voltage is close to the rated value and the system has normal power transmission capacity; when the value of M is small, it indicates that the AC power grid voltage has dropped significantly and the system faces the risk of power imbalance.

[0023] When M is greater than or equal to the preset threshold, it indicates that the AC grid voltage drop is within the acceptable range of the system, and the cascaded converter still has complete voltage support capability. At this time, it is determined that the AC grid is in normal operation, and the cascaded converter performs the main control of the DC bus voltage. The cascaded converter maintains the stability of the DC bus voltage through its internal control closed loop, without the need for other power conversion units to intervene.

[0024] When M is less than the preset threshold, it indicates that the AC grid voltage drop has exceeded the system's acceptable range, and the voltage support capability of the cascaded converter is insufficient to maintain DC bus voltage stability. This confirms that the AC grid is in a voltage drop state. At this point, the cascaded converter's primary control capability over the DC bus voltage is released, meaning the cascaded converter ceases its regulation of the DC bus voltage. This prevents continued regulation under insufficient capacity from degrading system performance and triggers a coordinated control mode. In this coordinated control mode, the control of the DC bus voltage is no longer solely borne by a single device but is transferred to other power conversion units with voltage support capabilities.

[0025] Preferably, the preset threshold is 0.85. This value corresponds to the AC grid voltage dropping to 85% of its rated value, at which point the voltage drop is approximately 15%, which is a typical critical point in the low-voltage ride-through requirements of the power system.

[0026] Furthermore, when M is less than the preset threshold, negative sequence current is first eliminated through negative sequence current suppression control measures to ensure the symmetry of the injected grid current; and reactive current injection is automatically adjusted according to the positive sequence voltage drop depth M to help the grid voltage recover. Specifically, a q-axis positive sequence current command value i is generated according to the positive sequence voltage drop depth M. qref and d-axis positive sequence current command value i dref This is to control the cascaded converter of the modules.

[0027] The generation of the aforementioned current command value follows these principles: During a voltage dip in the grid, the cascaded converter should inject an appropriate amount of reactive current into the grid to support the grid voltage, based on the depth of the voltage dip, while simultaneously maintaining continuous active power transmission as much as possible to reduce the power impact on the AC / DC grid. Based on this, this embodiment adopts a segmented control strategy, determining the q-axis positive sequence current command value i based on the interval where the M value is located. qref .

[0028] Wherein, the q-axis positive sequence current command value i qref Values ​​are segmented based on the M value: When M≥0.85, i qref =0, meaning that when the voltage drop does not exceed 15%, no additional reactive current needs to be injected, and the cascaded converter will automatically maintain the original operating state. When 0.2 < M < 0.85, i qref =1.10×(0.85-M) / 0.65, that is, when the voltage drop is between 15% and 80%, i qref The reactive current injection increases linearly with decreasing M, indicating that the deeper the voltage drop, the more reactive current is injected to help the grid voltage recover quickly. Here, 1.10 is the upper limit coefficient for reactive current injection, and 0.65 is the normalization coefficient to ensure i qref The saturation value is reached when M approaches 0.2; When M≤0.2, i qref =1.1, meaning that when the voltage drop exceeds 80%, iqref reaches the saturation value of 1.1. At this time, the cascaded converter outputs the maximum allowable reactive current to fully support the grid voltage.

[0029] After determining the q-axis reactive current command, the d-axis active current command also needs to be calculated. Let the power absorbed by the cascaded converter from the AC grid before the voltage dip be... , where u d0i represents the d-axis voltage value of the cascaded converter module before the voltage drop. d0 This represents the d-axis current value of the cascaded converter module before the voltage dip. During the voltage dip, ignoring the transmission losses of the cascaded converter module, the positive-sequence d-axis current command value must meet the following requirements to maintain the active power transmission level: That is, the deeper the voltage drop (the smaller M), the greater the active current required to maintain the same active power. Combined with the converter current limiting condition, the d-axis positive sequence current command value i... dref satisfy: , Where i d0 This represents the d-axis current value of the cascaded converter module before the voltage drop, with its sign indicating the direction of active power transmission. The min function prioritizes active current injection while ensuring the total current does not exceed the limit (1.1 times the rated current). Specifically, The remaining current capacity after deducting the q-axis reactive current from the total current limiting, i d0 / M is the active current required to maintain the active power transmission level before the voltage drop. This i dref The value is determined based on the active power transmission situation before the voltage drop, aiming to maintain the active power transmission level and direction to the greatest extent possible while ensuring that the converter current does not exceed the limit, thereby reducing the disturbance to the AC / DC power grid.

[0030] Through the coordinated operation of the segmented reactive current injection and active current limiting control, the modular cascaded converter can provide reactive power support to the grid to help restore voltage during AC grid voltage drops, and can also maintain the continuous transmission of active power to the maximum extent, avoiding secondary impact on AC and DC grids due to sudden power drops, thus creating favorable conditions for the smooth switching of DC bus voltage control.

[0031] For example, such as Figure 1 As shown, this control strategy first acquires the positive-sequence d-axis component U of the AC grid voltage. d+ and positive sequence q-axis component U q+ Calculate the positive sequence voltage drop depth M, where

[0032] U N This represents the positive sequence voltage amplitude during normal operation.

[0033] Based on the relationship between M and the threshold 0.85, the operating modes of the cascaded converter are divided into Mode 0 and Mode 1: When M ≥ 0.85, the cascaded converter operates in mode 0, and the AC power grid is in normal operation. At this time, no additional coordination control is required, and the DC bus voltage is regulated by the cascaded converter to maintain stability. When M < 0.85, the cascaded converter operates in mode 1. At this time, the AC grid is in a voltage dip state, and the cascaded converter loses its main control capability over the DC bus voltage, triggering the coordinated control mode. The control of the DC bus voltage is equally distributed between the energy storage bidirectional DC / DC converter unit and the low-voltage DC / AC converter unit (three-phase half-bridge inverter). The two units work together to undertake the task of stabilizing the DC bus voltage, thereby achieving a seamless transfer of control from the cascaded converter to other power conversion units.

[0034] S103: In the coordinated control mode, the control of the DC bus voltage is equally distributed among at least two power conversion units, wherein the power conversion unit includes at least an energy storage bidirectional DC / DC conversion unit and a low-voltage DC / AC conversion unit.

[0035] The aforementioned equal distribution means that the control of the DC bus voltage is no longer exclusively held by a single power conversion unit, but is distributed among multiple power conversion units. Each power conversion unit is in an equal control position and has the ability to independently take over the control of the DC bus voltage, without a fixed master-slave or priority relationship.

[0036] For example, when a fault occurs in the 10kV medium-voltage AC power grid or a low-voltage ride-through occurs, the cascaded converter loses control of the 750V DC bus voltage. The low-voltage DC / AC conversion unit or the energy storage bidirectional DC / DC conversion unit seamlessly takes over the control of the 750V DC bus voltage. In coordinated control mode, the allocation of control rights is not predetermined by a fixed priority, but is flexibly determined based on the real-time operating status of each power conversion unit.

[0037] Specifically, the equal-equal allocation is achieved by setting deviation control structures in the controllers of the energy storage bidirectional DC / DC converter unit and the low-voltage DC / AC converter unit, respectively. These deviation control structures enable each power conversion unit to independently respond to fluctuations in the DC bus voltage and autonomously generate corresponding control actions, thus eliminating the need for centralized mode judgment and command scheduling by a central controller.

[0038] Preferably, the peer-to-peer allocation includes: ensuring that both the energy storage bidirectional DC / DC converter and the low-voltage DC / AC converter receive the same DC bus voltage deviation signal, and independently generate their respective current reference commands. That is, the control input for each power conversion unit is the deviation between the same bus voltage feedback value and the reference value, but each unit independently calculates its own current command based on its own control parameters and operating constraints, without interference or dependence on each other, thus achieving true peer-to-peer control.

[0039] As an optional embodiment, the low-voltage DC / AC conversion unit can adopt a three-phase half-bridge inverter structure and regulate the DC bus voltage through current control in the dq coordinate system. This structure achieves decoupled control of active and reactive current by transforming the AC quantities in the three-phase stationary coordinate system to the synchronous rotating coordinate system, which is beneficial to improving control accuracy and dynamic response speed.

[0040] S104: Deviation control structures are respectively set in the controllers of the energy storage bidirectional DC / DC converter unit and the low-voltage DC / AC converter unit. The deviation control structure includes an upper deviation PI controller and a lower deviation PI controller connected in series. The upper deviation PI controller is used to generate an integral limit value based on the first voltage deviation to constrain the integral range of the lower deviation PI controller and prevent control lag or overshoot caused by integral saturation of the lower deviation PI controller. The lower deviation PI controller is used to generate a current reference command based on the second voltage deviation to realize the conversion of the bus voltage deviation into a current command. The deviation control structure is also used to correct the operating state constraint of the current reference command so that the corrected current command can adapt to the actual operating constraints such as the change of the state of charge of the energy storage bidirectional DC / DC converter unit and the current limit.

[0041] The deviation control structure is the core execution link for achieving stable DC bus voltage control in this invention. Its function is to allow the energy storage bidirectional DC / DC converter unit and the low-voltage DC / AC converter unit to take over the regulation of the DC bus voltage after the cascaded converter module exits main control. This deviation control is a closed-loop control strategy that dynamically adjusts based on the error between the actual system output and the desired target. Specifically, it measures the deviation between the actual DC bus voltage value Udc_fdb and the reference value Udc_ref in real time. This deviation signal is used as the input to the PI controller, and after proportional-integral calculation, a corresponding current control command is generated to drive the power conversion unit to perform charging / discharging or power output adjustment to eliminate the deviation and restore the bus voltage to a stable value near the set point. Compared to open-loop control, deviation control can automatically compensate for the impact of system parameter changes and external disturbances on the bus voltage, exhibiting strong robustness.

[0042] In this embodiment, the deviation control structure of each power conversion unit adopts the same series dual-PI architecture, enabling each unit to maintain the consistency of its control logic while operating independently, thereby achieving peer-to-peer coordinated control. Specifically, the deviation control structure includes a series upper deviation PI controller and a lower deviation PI controller. The series connection means that the output of the upper deviation PI controller serves as the input constraint for the lower deviation PI controller, forming a cascaded signal path.

[0043] Specifically, the setpoint of the upper deviation PI controller is the sum of the bus voltage reference value Udc_ref and the upper deviation threshold dU (i.e., Udc_ref+dU), and the feedback value is the measured value of the DC bus voltage Udc_fdb. The difference between the two is used to obtain the first voltage deviation, which reflects whether the bus voltage exceeds the upper limit of the normal adjustment window. When Udc_fdb is higher than Udc_ref + dU, the first voltage deviation is positive, and the upper deviation PI controller starts generating the integral lower limit value of the lower deviation PI controller. The output of the upper deviation PI controller serves as the integral lower limit value of the lower deviation PI controller, used to limit the integral amount of the lower deviation PI controller from falling below this lower limit value, thereby preventing excessive accumulation of the lower deviation PI controller's integral when the bus voltage is continuously high, which could lead to subsequent improper regulation. The setpoint value of the lower deviation PI controller is the difference between the bus voltage reference value Udc_ref and the lower deviation threshold dU (i.e., Udc_ref - dU), and the feedback value is the measured value of the DC bus voltage Udc_fdb. The difference between the two yields the second voltage deviation, which reflects whether the bus voltage exceeds the lower limit of the normal regulation window. When Udc_fdb is lower than Udc_ref - dU, the second voltage deviation is negative, and the lower deviation PI controller starts generating a corresponding current command to increase the bus voltage. The output of the lower deviation PI controller, after being limited, serves as the current reference command. Through the series architecture of the upper and lower deviation PI, the bus voltage is controlled within the deviation band range of Udc_ref-dU to Udc_ref+dU. When the voltage is within this deviation band, the deviation control structure does not output additional adjustment commands, thereby avoiding frequent operation of the controller. When the voltage exceeds this deviation band, the deviation control structure operates accordingly based on the direction and magnitude of the exceedance, driving the power conversion unit to adjust until the bus voltage returns to the deviation band.

[0044] Furthermore, in the deviation control structure on the low-voltage DC / AC converter side, the integral upper limit of the upper deviation PI controller is limited to the discharge current setpoint id_dis_L of the low-voltage DC / AC converter. That is, when the low-voltage DC / AC converter performs bus voltage regulation, its discharge current must not exceed this upper limit to protect the converter power devices from overcurrent. In the deviation control structure on the energy storage bidirectional DC / DC converter side, the integral upper limit of the upper deviation PI controller is limited to the discharge current setpoint idc_dis of the energy storage bidirectional DC / DC converter. That is, when the energy storage DC / DC converter performs bus voltage regulation, its discharge current must also not exceed this upper limit to protect the energy storage bidirectional DC / DC converter and its power devices.

[0045] Furthermore, the current reference command is modified by operating state constraints to generate a modified current reference command. The purpose of the operating state constraint modification is to dynamically adjust the current command according to the actual operating state of the bidirectional DC / DC converter unit, ensuring that the bidirectional DC / DC converter unit always operates within its safe operating range. Specifically, the operating state constraint modification includes: acquiring the battery state of charge flag Fsoc and the bidirectional DC / DC converter unit discharge current flag Fidc. Fsoc reflects whether the battery's current state of charge is within the allowable charge / discharge range (typically between 20% and 90%), and Fidc reflects whether the current discharge current of the DC / DC converter unit exceeds the safety limit. The current reference command is multiplied by a switching coefficient |(Fsoc+Fidc) / 2| to correct it. This switching coefficient is used to determine whether the energy storage bidirectional DC / DC converter has bus voltage control capability. Specifically, when the energy storage bidirectional DC / DC converter is in normal operating condition, Fsoc = 1 and Fidc = 1, and the switching coefficient is equal to 1, indicating that the energy storage bidirectional DC / DC converter has full control capability, and the current command is output according to its original value. When the energy storage bidirectional DC / DC converter cannot charge or discharge, Fsoc = 0 or Fidc = 0, and the switching coefficient is less than 1, indicating that the control capability of the energy storage bidirectional DC / DC converter is limited, and its current command is correspondingly reduced. After the above correction, the corrected current reference command is generated and output to the inner current loop, which then performs the final current closed-loop tracking control.

[0046] Figure 2 This is a control block diagram of a low-voltage DC / AC converter unit provided in an embodiment of the present invention. Figure 2 As shown, the low-voltage DC / AC converter unit employs a dual-loop control structure with an outer voltage loop and an inner current loop. The outer voltage loop generates a current command based on the DC bus voltage deviation, while the inner current loop tracks this command and generates a PWM drive signal. This dual-loop structure enables the system to combine stable voltage regulation with rapid current response. The outer voltage loop uses a series connection of an upper deviation PI controller and a lower deviation PI controller. The output of the upper deviation PI controller in this series connection serves as the integral lower limit of the lower deviation PI controller, thereby dynamically constraining the integral action of the lower deviation PI controller.

[0047] The setpoint of the upper deviation PI controller is the sum of the bus voltage reference value Udc_ref and the upper deviation threshold dU2, i.e., Udc_ref + dU2; its feedback value is the measured DC bus voltage value Udc_fdb. The difference between the two yields the first voltage deviation. When Udc_fdb is higher than Udc_ref + dU2, the deviation is positive, indicating that the bus voltage has exceeded the upper limit of the normal adjustment window, and the upper deviation PI controller starts to operate. The difference between the setpoint and the feedback value is sent to the upper deviation PI controller. The output of the upper deviation PI controller serves as the lower integral limit of the lower deviation PI controller. At the same time, the upper integral limit of the upper deviation PI controller is limited to the discharge current setpoint id_dis_L of the low-voltage DC / AC converter unit, ensuring that the discharge current of the converter does not exceed its safe operating limit.

[0048] The setpoint of the lower deviation PI controller is the difference between the bus voltage reference value Udc_ref and the lower deviation threshold dU2, i.e., Udc_ref - dU2; its feedback value is also the measured DC bus voltage value Udc_fdb. The difference between the two yields the second voltage deviation. When Udc_fdb is lower than Udc_ref - dU2, the deviation is negative, indicating that the bus voltage has exceeded the lower limit of the normal adjustment window, and the lower deviation PI controller starts to operate. The difference between the setpoint and the feedback value is sent to the lower deviation PI controller, and the output of the lower deviation PI controller, after being limited, serves as the setpoint id_ref_L for the d-axis current inner loop. Through the series architecture of the upper and lower deviation PI, the bus voltage is controlled within the deviation band range of Udc_ref-dU2 to Udc_ref+dU2. When the voltage is within this deviation band, the deviation control structure does not output additional adjustment commands, thereby avoiding frequent controller operation. When the voltage exceeds this deviation band, the deviation control structure operates accordingly based on the direction and magnitude of the exceedance, driving the low-voltage DC / AC conversion unit to adjust until the bus voltage returns to the deviation band.

[0049] In the inner current loop, the difference between the d-axis current setpoint id_ref_L and the d-axis current feedback value id_fdb_L is sent to the d-axis current PI controller to control the active current component. Similarly, the difference between the q-axis current setpoint iq_ref_L and the q-axis current feedback value iq_fdb_L is sent to the q-axis current PI controller. Setting the q-axis current setpoint iq_ref_L to zero means that when the low-voltage DC / AC converter takes over DC bus voltage control, it only needs to output active power to maintain bus voltage stability and does not need to participate in reactive power regulation. Setting the q-axis current to zero avoids additional reactive power losses and improves system operating efficiency. The outputs of the d-axis and q-axis current PI controllers are transformed into three-phase modulation signals via dq / abc coordinate transformation. This converts the DC current in the synchronous rotating coordinate system into a sinusoidal modulation wave in the three-phase stationary coordinate system. The signal is then used in a PWM generation stage to generate drive pulses for the switching transistors. By controlling the on / off state of the switching transistors, the three-phase voltage is modulated, thereby controlling the low-voltage DC / AC converter unit and ensuring that the converter outputs the required active power to support the DC bus voltage. Each PI controller output is equipped with a limiting circuit to prevent integral saturation, ensuring the control system remains stable under large disturbances.

[0050] Figure 3 This is a control block diagram of an energy storage DC / DC converter unit provided in an embodiment of the present invention. Figure 3 As shown, the energy storage DC / DC converter unit adopts a dual-loop control structure with an outer voltage loop and an inner current loop. The outer voltage loop generates a current command based on the DC bus voltage deviation, while the inner current loop tracks this command and generates a PWM drive signal. Unlike the low-voltage DC / AC converter unit, the energy storage DC / DC converter unit incorporates an energy storage status monitoring signal into its inner current loop to enable autonomous switching of control authority as the energy storage operating status changes. The outer voltage loop uses a series connection of an upper deviation PI controller and a lower deviation PI controller, while the inner current loop incorporates the energy storage status monitoring signal as the basis for determining control authority switching.

[0051] The setpoint of the upper deviation PI controller is the sum of the bus voltage reference value Udc_ref and the upper deviation threshold dU1, i.e., Udc_ref + dU1; its feedback value is the measured DC bus voltage value Udc_fdb. The difference between the two yields the first voltage deviation. When Udc_fdb is higher than Udc_ref + dU1, the deviation is positive, indicating that the bus voltage has exceeded the upper limit of the normal adjustment window, and the upper deviation PI controller starts to operate. The difference between the setpoint and the feedback value is sent to the upper deviation PI controller. The output of the upper deviation PI controller serves as the lower integral limit of the lower deviation PI controller, preventing excessive accumulation of the integral when the bus voltage remains high. At the same time, the upper integral limit of the upper deviation PI controller is limited to the discharge current setpoint idc_dis of the energy storage DC / DC converter unit, ensuring that the discharge current of the energy storage system does not exceed its safe operating limit, protecting the battery and power devices.

[0052] The setpoint of the lower deviation PI controller is the difference between the bus voltage reference value Udc_ref and the lower deviation threshold dU1, i.e., Udc_ref - dU1; its feedback value is also the measured DC bus voltage value Udc_fdb. The difference between the two yields the second voltage deviation. When Udc_fdb is lower than Udc_ref - dU1, the deviation is negative, indicating that the bus voltage has exceeded the lower limit of the normal adjustment window, and the lower deviation PI controller starts to operate. The difference between the setpoint and the feedback value is sent to the lower deviation PI controller, and the output of the lower deviation PI controller is limited to output the current setpoint idc_ref. Through the series architecture of the upper and lower deviation PI controllers described above, the energy storage DC / DC converter unit participates in maintaining the DC bus voltage within the deviation band of Udc_ref ± dU1.

[0053] In the inner current loop, the battery state-of-charge (SOC) flag (Fsoc) and the energy storage DC / DC converter discharge current flag (Fidc) are introduced as the basis for control switching. Fsoc indicates whether the battery SOC is within the safe range for charge and discharge (typically between 20% and 90%), while Fidc indicates whether the current discharge current of the energy storage DC / DC converter exceeds the safe limit. Together, these two flags determine whether the energy storage DC / DC converter has bus voltage control capability. The current setpoint idc_ref is multiplied by the switching coefficient |(Fsoc+Fidc) / 2|. The value of the switching coefficient is as follows: when Fsoc=1 and Fidc=1, the switching coefficient is equal to 1, and the current command is output according to the original value; when Fsoc=0 or Fidc=0, the switching coefficient is less than 1, the current command is reduced accordingly, and the difference is calculated with the current feedback value idc_fdb of the energy storage DC / DC converter unit. The difference is sent to the current PI controller. The output of the current PI controller generates the drive pulse of the switching transistor through the PWM generation stage, thereby realizing the control of the energy storage DC / DC converter unit.

[0054] S105: When the energy storage bidirectional DC / DC converter unit has charging and discharging conditions, it takes priority in controlling the DC bus voltage. That is, the energy storage bidirectional DC / DC converter unit acts as the primary actuator for DC bus voltage regulation, maintaining bus voltage stability through its deviation control structure. When the energy storage bidirectional DC / DC converter unit does not have charging and discharging conditions, the low-voltage DC / AC converter unit takes over the DC bus voltage control. That is, the low-voltage DC / AC converter unit replaces the energy storage bidirectional DC / DC converter unit as the actuator for bus voltage regulation.

[0055] Specifically, whether the bidirectional DC / DC converter unit has the conditions for charging and discharging is determined by the battery state-of-charge flag Fsoc and the discharge current flag Fidc introduced in S104. When the bidirectional DC / DC converter unit is in normal working condition and has charging and discharging capabilities, both Fsoc and Fidc are 1, the switching coefficient is equal to 1, the current loop is working normally, and the energy storage DC / DC converter unit maintains the DC bus voltage stability according to the instructions of the voltage outer loop. At this time, the low-voltage DC / AC converter unit is in standby mode and does not participate in the bus voltage regulation. When Fsoc = 0 or Fidc = 0, it indicates that the bidirectional DC / DC converter unit cannot charge or discharge because the battery state of charge exceeds the charging and discharging threshold or the discharge current exceeds the limit. At this time, the bidirectional DC / DC converter unit automatically withdraws from the bus voltage control, and the low-voltage DC / AC converter unit takes over the control task.

[0056] The aforementioned takeover process is automatically achieved through the switching coefficient |(Fsoc+Fidc) / 2| in S104: When the switching coefficient is 1 (energy storage has charging and discharging conditions), the bidirectional DC / DC converter unit of the energy storage assumes the bus voltage control; when the switching coefficient is less than 1 (energy storage does not have charging and discharging conditions), the current command of the bidirectional DC / DC converter unit of the energy storage is reduced, its output current gradually decreases, and the energy storage DC / DC converter unit automatically withdraws from the control of the DC bus voltage. The bus voltage regulation task it undertook is gradually transferred to the low-voltage DC / AC converter unit, and the control right is transferred to the low-voltage DC / AC converter unit, realizing the autonomous and seamless switching of the DC bus voltage control right. The entire switching process does not require intervention from the central controller and is completed autonomously based entirely on the status information of each power conversion unit. During the switching process, the bus voltage remains stable, and the load equipment is not affected.

[0057] S106: Generates a PWM drive signal based on the corrected current reference command to control the corresponding power conversion unit in order to regulate the DC bus voltage.

[0058] The corrected current reference command is the final current command output in S104 after correction by operating state constraints. This command carries the current amplitude information required to maintain the stability of the DC bus voltage. This step converts the current command into an actual switching drive signal to achieve precise control of the on / off state of the switching devices in the power conversion unit, thereby enabling the converter to output the desired current and achieve the purpose of regulating the DC bus voltage.

[0059] Specifically, the corrected current reference command is compared with the current feedback value of the corresponding power conversion unit, where the current feedback value is the detected current value of the current actually output by the power conversion unit. The difference between the two is the current deviation signal, which reflects the gap between the current actual output current and the desired current. The difference is adjusted by the current PI controller to generate the PWM drive signal. The current PI controller performs proportional-integral calculations on the deviation signal to quickly respond to the current deviation and eliminate steady-state errors. The generated PWM drive signal controls the output current of the converter by adjusting the duty cycle of the switching transistor, making it track the corrected current reference command, thereby achieving precise closed-loop regulation of the DC bus voltage.

[0060] S107: When the positive sequence voltage drop depth M recovers to a value greater than or equal to the preset threshold, it indicates that the AC grid voltage has returned to normal, the voltage support capability of the cascaded converter has been restored, and the system has the conditions to switch from the coordinated control mode back to the normal operation mode. The cascaded converter resumes the main control of the DC bus voltage and resumes the closed-loop regulation task of the DC bus voltage. The energy storage bidirectional DC / DC conversion unit and the low-voltage DC / AC conversion unit withdraw from the control of the DC bus voltage and no longer participate in the bus voltage regulation.

[0061] During the recovery process, since the cascaded converter requires a certain response time to reconnect to control, if the bidirectional DC / DC converter and the low-voltage DC / AC converter simultaneously and immediately exit control, the DC bus voltage may experience a drop or overshoot at the moment of switching. Therefore, a gradual exit strategy is adopted. That is, after M recovers to a level not less than a preset threshold, the bidirectional DC / DC converter and the low-voltage DC / AC converter do not immediately cut off the current output. Instead, they gradually return the bus voltage regulation task they are undertaking to the cascaded converter by gradually reducing their respective current reference commands. Preferably, when the positive sequence voltage drop depth M recovers to a level greater than or equal to the preset threshold, the bidirectional DC / DC converter and the low-voltage DC / AC converter gradually exit the current reference command output, that is, they gradually reduce the current reference command according to a set rate of change, so that the output current smoothly decreases to zero, thereby achieving a smooth transition of control. The entire recovery process requires no intervention from the central controller. The cascaded converter, energy storage bidirectional DC / DC converter, and low-voltage DC / AC converter autonomously complete the handover of control based on their respective local information. During the switching process, the DC bus voltage remains stable, and the load equipment is unaffected.

[0062] Exemplary System Accordingly, this embodiment of the invention also provides a control system for bus voltage in a medium-voltage AC / DC system. The system provided in this embodiment includes: The voltage detection module is used to collect the AC grid voltage of the medium-voltage AC / DC system and calculate the positive sequence voltage drop depth M based on its positive sequence component. M is the ratio of the positive sequence voltage amplitude during the voltage drop period to the positive sequence voltage amplitude during normal operation. The mode determination module is used to compare M with a preset threshold: when M is not less than the preset threshold, the DC bus voltage is controlled by the cascaded converter; otherwise, the main control capability of the cascaded converter is deactivated. The equal distribution module is used to equally distribute the DC bus voltage control between at least two power conversion units after the main control capability of the cascaded converter is deactivated. The power conversion unit includes at least an energy storage bidirectional DC / DC conversion unit and a low-voltage DC / AC conversion unit. The deviation control structure is set in the controller of each power conversion unit, including an upper deviation PI controller and a lower deviation PI controller connected in series. The upper deviation PI controller is used to generate an integral limit to constrain the integral range of the lower deviation PI controller, and the lower deviation PI controller is used to generate a current reference command. The deviation control module is also used to perform operating state constraint correction on the current reference command. The switching control module is used to control the energy storage bidirectional DC / DC converter unit to take priority in controlling the DC bus voltage when the energy storage bidirectional DC / DC converter unit is ready for charging and discharging; otherwise, it controls the low-voltage DC / AC converter unit to take over the control. The power control module is used to control the corresponding power conversion unit based on the modified current reference command to regulate the DC bus voltage. The recovery control module is used to restore the main control of the DC bus voltage by the cascaded converter when M recovers to a value not less than a preset threshold, and to control the power conversion unit to exit control.

[0063] It should be noted that although the operation of the bus voltage control method in the medium-voltage AC / DC system of the present invention is described in a specific order in the accompanying drawings, this does not require or imply that these operations must be performed in that specific order, or that all the operations shown must be performed to achieve the desired result. Additionally or alternatively, certain steps may be omitted, multiple steps may be combined into one step, and / or one step may be broken down into multiple steps.

[0064] Furthermore, although several devices, units, or modules for the control system of bus voltage in medium-voltage AC / DC systems have been mentioned in the detailed description above, this division is merely exemplary and not mandatory. In fact, according to embodiments of the present invention, the features and functions of two or more modules described above can be embodied in one module. Conversely, the features and functions of one module described above can be further divided and embodied by multiple modules.

[0065] While the spirit and principles of the invention have been described with reference to several specific embodiments, it should be understood that the invention is not limited to the disclosed specific embodiments, and the division of aspects does not imply that features in these aspects cannot be combined for benefit; such division is merely for ease of description. The invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. A method for controlling bus voltage in a medium-voltage AC / DC system, characterized in that, The method includes: Collect the AC grid voltage of the medium-voltage AC / DC system, and calculate the positive sequence voltage drop depth M based on its positive sequence component. M is the ratio of the positive sequence voltage amplitude during the voltage drop period to the positive sequence voltage amplitude during normal operation. M is compared with a preset threshold: when M is not less than the preset threshold, the DC bus voltage is controlled by the cascaded converter; otherwise, the main control capability of the cascaded converter is released, and the control of the DC bus voltage is equally distributed between at least two power conversion units. The power conversion unit includes at least an energy storage bidirectional DC / DC conversion unit and a low-voltage DC / AC conversion unit. A deviation control structure is set in the controller of each power conversion unit. The deviation control structure includes an upper deviation PI controller and a lower deviation PI controller connected in series. The upper deviation PI controller is used to generate an integral limit to constrain the integral range of the lower deviation PI controller. The lower deviation PI controller is used to generate a current reference command. The deviation control structure is also used to perform operating state constraint correction on the current reference command. When the energy storage bidirectional DC / DC converter unit is ready for charging and discharging, it shall take priority in controlling the DC bus voltage; otherwise, the low-voltage DC / AC converter unit shall take over the control. The corresponding power conversion unit is controlled based on the modified current reference command to regulate the DC bus voltage; When M recovers to a value not less than the preset threshold, the cascaded converter resumes main control of the DC bus voltage, and the power conversion unit exits control.

2. The control method according to claim 1, characterized in that, The preset threshold is 0.

85.

3. The control method according to claim 1, characterized in that, The setpoint of the upper deviation PI controller is the sum of the bus voltage reference value Udc_ref and the upper deviation threshold dU, and the feedback value is the measured value of the DC bus voltage Udc_fdb. The output of the upper deviation PI controller is used as the integral lower limit value of the lower deviation PI controller; the setpoint value of the lower deviation PI controller is the difference between the bus voltage reference value Udc_ref and the lower deviation threshold dU, and the feedback value is the measured value of the DC bus voltage Udc_fdb. The output of the lower deviation PI controller is limited and used as the current reference command.

4. The control method according to claim 1, characterized in that, The equal-equal allocation includes: ensuring that both the energy storage bidirectional DC / DC converter and the low-voltage DC / AC converter receive the same DC bus voltage deviation signal, and that each independently generates its own current reference command.

5. The control method according to claim 1, characterized in that, The operational state constraint correction specifically includes: Obtain the battery state of charge flag Fsoc and the energy storage bidirectional DC / DC converter discharge current flag Fidc; The current reference command is modified by multiplying the switching coefficient |(Fsoc+Fidc) / 2|. When the energy storage bidirectional DC / DC converter unit is ready for charging and discharging, it shall take priority in controlling the DC bus voltage; otherwise, the low-voltage DC / AC converter unit shall take over the control. Specifically, this includes: When the energy storage bidirectional DC / DC converter unit is in normal working condition, Fsoc = 1 and Fidc = 1, and the switching coefficient is equal to 1. When the energy storage bidirectional DC / DC converter unit cannot charge or discharge, Fsoc = 0 or Fidc = 0, and the switching coefficient is less than 1.

6. The control method according to claim 1, characterized in that, The control of the corresponding power conversion unit based on the modified current reference command to adjust the DC bus voltage specifically includes: The corrected current reference command is compared with the current feedback value of the corresponding power conversion unit. The difference is adjusted by the current PI controller to generate the PWM drive signal, which controls the corresponding power conversion unit to adjust the DC bus voltage.

7. The control method according to claim 1, characterized in that, In the deviation control structure on the low-voltage DC / AC converter side, the upper limit of the integral of the upper deviation PI controller is limited to the discharge current setpoint id_dis_L of the low-voltage DC / AC converter. In the deviation control structure on the side of the energy storage bidirectional DC / DC converter unit, the upper integral limit of the upper deviation PI controller is limited to the discharge current setpoint idc_dis of the energy storage bidirectional DC / DC converter unit.

8. The control method according to claim 1, characterized in that, When M recovers to a value not less than a preset threshold, the energy storage bidirectional DC / DC converter and the low-voltage DC / AC converter gradually withdraw from current reference command output.

9. The control method according to claim 1, characterized in that, When M is less than the preset threshold, a q-axis positive sequence current command value i is generated based on M. qref and d-axis positive sequence current command value i dref To control the cascaded converter of the modules; Wherein, the q-axis positive sequence current command value i qref Values ​​are segmented based on the M value: When M≥0.85, i qref =0; When 0.2 < M < 0.85, i qref =1.10×(0.85-M) / 0.65; When M≤0.2, i qref =1.1; The d-axis positive sequence current command value i dref satisfy: , where i d0 This refers to the d-axis current value of the cascaded converter module before the voltage drop.

10. A control system for bus voltage in a medium-voltage AC / DC system, characterized in that, The system includes: The voltage detection module is used to collect the AC grid voltage of the medium-voltage AC / DC system and calculate the positive sequence voltage drop depth M based on its positive sequence component. M is the ratio of the positive sequence voltage amplitude during the voltage drop period to the positive sequence voltage amplitude during normal operation. The mode determination module is used to compare M with a preset threshold: when M is not less than the preset threshold, the DC bus voltage is controlled by the cascaded converter; otherwise, the main control capability of the cascaded converter is deactivated. The equal distribution module is used to equally distribute the DC bus voltage control between at least two power conversion units after the main control capability of the cascaded converter is deactivated. The power conversion unit includes at least an energy storage bidirectional DC / DC conversion unit and a low-voltage DC / AC conversion unit. The deviation control structure is set in the controller of each power conversion unit, including an upper deviation PI controller and a lower deviation PI controller connected in series. The upper deviation PI controller is used to generate an integral limit to constrain the integral range of the lower deviation PI controller. The lower deviation PI controller is used to generate a current reference command. The deviation control module is also used to perform operating state constraint correction on the current reference command. The switching control module is used to control the energy storage bidirectional DC / DC converter unit to take priority in controlling the DC bus voltage when the energy storage bidirectional DC / DC converter unit is ready for charging and discharging; otherwise, it controls the low-voltage DC / AC converter unit to take over the control. The power control module is used to control the corresponding power conversion unit based on the modified current reference command to regulate the DC bus voltage. The recovery control module is used to restore the main control of the DC bus voltage by the cascaded converter when M recovers to a value not less than a preset threshold, and to control the power conversion unit to exit control.

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