DC power smoothing system and method based on MMC and port switchable energy storage

Abstract

The present application relates to the technical field of flexible direct current power transmission, and particularly relates to a direct current power stabilizing system and method based on MMC and port switchable energy storage, which comprises the following steps: a modular multilevel converter, the modular multilevel converter comprises a plurality of phase units, each phase unit comprises an upper bridge arm and a lower bridge arm, each upper bridge arm and each lower bridge arm comprises a bridge arm reactor and N same half-bridge sub-modules; a port switchable energy storage module, comprising two anti-parallel connected third switch tubes and fourth switch tubes, a symmetrical bipolar DC converter and a battery, configured to provide positive or negative pressure output to realize charge and discharge control of the port switchable energy storage module according to the power state of the DC side. Through the present application, the problems of system instability caused by direct current power fluctuation in the new energy grid connection scene and the problem that the traditional MMC cannot quickly stabilize the direct current side power fluctuation are effectively solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of flexible direct current power transmission, in particular to a direct current power smoothing system and method based on MMC and port switchable energy storage. BACKGROUND

[0002] In the modern power system with high proportion of new energy grid-connected, direct current power fluctuation suppression is the core challenge to ensure the stable operation of the power grid. Modular multilevel converter (MMC) has become the key equipment of flexible direct current power transmission due to its strong modular expansion capability and low output harmonic. However, when intermittent power sources such as wind power and photovoltaic power are connected to the MMC direct current bus, the random fluctuation of their power will cause the direct current voltage to change dramatically, leading to system protection misoperation, equipment over-stress and even cascading collapse.

[0003] To solve the above problems, the existing technology mainly adopts two types of schemes, but both have fundamental defects: on the one hand, the MMC sub-module integrated energy storage scheme balances power by connecting battery units in parallel on the capacitor side of the sub-module, but it cannot match the microsecond-level power mutation due to the limited response speed of the sub-module switching period, and it is difficult to support large power fluctuation scenarios due to the limited capacitor volume. On the other hand, the direct current bus parallel energy storage system independently configures energy storage devices on the bus side, but it needs to additionally increase the full-power DC-AC converter, and the traditional one-way Buck-Boost converter only supports single polarity output, so the charge and discharge mode switching relies on the delay mechanical switch, resulting in serious lag of power compensation.

[0004] The information disclosed in this BACKGROUND section is only for the purpose of enhancing the understanding of the general background of the disclosure and should not be taken as admitting or implying that this information is prior art known to those of ordinary skill in the art. SUMMARY

[0005] The present application provides a direct current power smoothing system and method based on MMC and port switchable energy storage, which can effectively solve the problems in the background art.

[0006] In order to achieve the above purpose, the technical scheme adopted by the present application is:

[0007] The direct current power smoothing system based on MMC and port switchable energy storage comprises:

[0008] The modular multilevel converter comprises a plurality of phase units, each phase unit comprises an upper bridge arm and a lower bridge arm, each upper bridge arm and each lower bridge arm comprises a bridge arm reactor and N identical half-bridge sub-modules, and the bridge arm reactor and the N identical half-bridge sub-modules are connected in series; the upper bridge arm and the lower bridge arm of the modular multilevel converter are connected to the positive and negative poles of the direct current bus, respectively;

[0009] The port-switchable energy storage module is connected in series between the upper bridge arm and the lower bridge arm of the modular multilevel converter, respectively, to form a DC side topology of the modular multilevel converter, wherein the port-switchable energy storage module comprises two third and fourth switch tubes connected in anti-parallel, a symmetrical bipolar DC converter connected to a battery, and configured to provide positive or negative voltage output to realize charge and discharge control of the port-switchable energy storage module according to a power state of the DC side.

[0010] Further, each of the half-bridge sub-modules comprises two first and second switch tubes connected in anti-parallel and a capacitor;

[0011] The first and second switch tubes are connected in series and are connected in parallel with the capacitor;

[0012] A common connection end point of the first and second switch tubes serves as a first series port of the half-bridge sub-module, and the capacitor and a common connection end point of the second switch tube serve as a second series port of the half-bridge sub-module.

[0013] Further, in the port-switchable energy storage module, the third and fourth switch tubes are connected in series, and a common connection end point of the third and fourth switch tubes serves as an input end of the port-switchable energy storage module;

[0014] An emitter of the fourth switch tube serves as an output end and a second connection port of the port-switchable energy storage module;

[0015] A collector of the third switch tube serves as a first connection port of the port-switchable energy storage module.

[0016] Further, the symmetrical bipolar DC converter comprises an inductor, fifth, sixth, seventh, eighth and ninth switch tubes, first and second clamping capacitors, first and second capacitors, and first and second output resistors.

[0017] Further, in the symmetrical bipolar DC converter, a positive electrode of the battery is connected to one end of the inductor; a negative electrode of the battery is connected to an emitter of the fifth switch tube, and the emitter of the fifth switch tube is further connected to a ground potential terminal;

[0018] The other end of the inductor is connected to a collector of the fifth switch tube, an emitter of the sixth switch tube, and a collector of the seventh switch tube.

[0019] Further, in the symmetrical bipolar DC converter, a collector of the sixth switch is connected to one end of the first clamping capacitor and an emitter of the eighth switch;

[0020] A collector of the eighth switch is connected to one end of the first capacitor and one end of the first output resistor;

[0021] An emitter of the seventh switch is connected to one end of the second clamping capacitor and a collector of the ninth switch;

[0022] An emitter of the ninth switch is connected to one end of the second capacitor and one end of the second output resistor;

[0023] The other end of the first clamping capacitor and the other end of the second clamping capacitor are commonly connected, and the commonly connected terminal is connected to a ground potential terminal;

[0024] The other end of the first capacitor and the other end of the second capacitor are commonly connected;

[0025] The other end of the first output resistor and the other end of the second output resistor are commonly connected.

[0026] Further, in the symmetrical bipolar DC converter, drive signals of the sixth switch and the eighth switch are configured to be complementary to a drive signal of the fifth switch, ignoring a dead zone;

[0027] Drive signals of the seventh switch and the ninth switch are configured to be the same as the drive signal of the fifth switch.

[0028] Further, in the port-switchable energy storage module, the third switch and the fourth switch are configured to control the switching in or out of the port-switchable energy storage module;

[0029] The first connection port and the second connection port of the port-switchable energy storage module are configured to be connected to port B and port C of the symmetrical bipolar DC converter respectively in a first operation mode, so that the symmetrical bipolar DC converter outputs negative voltage;

[0030] The first connection port and the second connection port of the port-switchable energy storage module are configured to be connected to port A and port B of the symmetrical bipolar DC converter respectively in a second operation mode, so that the symmetrical bipolar DC converter outputs positive voltage;

[0031] The first operation mode is a discharging mode, and the second operation mode is a charging mode.

[0032] The DC power suppression method based on MMC and port switchable energy storage comprises the following steps:

[0033] Real-time monitoring of the power abnormal state of the DC bus of the modular multilevel converter, judging whether the power abnormal state is in a surplus state or a deficiency state;

[0034] According to the power abnormal state, the switch unit of the port switchable energy storage module is turned on, so that the port switchable energy storage module is connected between the upper and lower arms of the modular multilevel converter;

[0035] When the power is in the deficiency state, the symmetric bipolar DC converter of the port switchable energy storage module is controlled to output negative voltage to drive the battery to discharge and supplement the deficiency power;

[0036] When the power is in the surplus state, the symmetric bipolar DC converter is controlled to output positive voltage to drive the battery to charge and absorb the excess power;

[0037] Based on the charge and discharge control of the port switchable energy storage module, the DC bus power fluctuation is suppressed, and the voltage across the modular multilevel converter is restored and kept stable.

[0038] The technical scheme of the present application can achieve the following technical effects:

[0039] The problems of system instability caused by DC power fluctuation in new energy grid connection scene and the problem that traditional MMC cannot quickly suppress DC side power fluctuation are effectively solved.

[0040] The above description is only a summary of the technical scheme of the present application, in order to more clearly understand the technical means of the present application, the specific embodiments of the present application can be implemented according to the content of the description, and in order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the following specific embodiments of the present application are described. BRIEF DESCRIPTION OF DRAWINGS

[0041] In order to more clearly illustrate the technical scheme in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiment or prior art description. Obviously, the drawings in the following description are only some embodiments described in the present application, and for those skilled in the art, other drawings can also be obtained without creative labor on the basis of these drawings.

[0042] Figure 1 The topological schematic diagram of the modular multilevel converter;

[0043] Figure 2 The topological schematic diagram of the half-bridge sub-module;

[0044] Figure 3A topology diagram of a port-switchable energy storage module;

[0045] Fig. 1 is a topology diagram of a port-switchable energy storage module (PPS-ES) connected to a modular multilevel converter (MMC) according to an embodiment of the present application. dc , a DC bus voltage; L0, a bridge reactor; SM, a half-bridge submodule; S1, a first switch tube; S2, a second switch tube; a, a first series port; b, a second series port; C, a capacitor; V+, an input terminal; V-, an output terminal; S3, a third switch tube; S4, a fourth switch tube; Port 1, a first connection port; Port 2, a second connection port; S5, a fifth switch tube; S6, a sixth switch tube; S7, a seventh switch tube; S8, an eighth switch tube; S9, a ninth switch tube; R1, a first output resistor; R2, a second output resistor; C o1 , a first capacitor; C o2 , a second capacitor; C x1 , a first clamping capacitor; C x2 , a second clamping capacitor; L, an inductor. DETAILED DESCRIPTION

[0046] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments of the present application.

[0047] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description of the application herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. As used in this description, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0048] Embodiment one;

[0049] As shown in Figure 1 , the present application provides a DC power smoothing system based on MMC and port-switchable energy storage, which comprises:

[0050] a modular multilevel converter, the modular multilevel converter comprising a plurality of phase units, each phase unit comprising an upper bridge arm and a lower bridge arm, each upper bridge arm and each lower bridge arm comprising a bridge reactor and N identical half-bridge submodules, the bridge reactor and the N identical half-bridge submodules being connected in series; the upper bridge arms and the lower bridge arms of the modular multilevel converter being connected to a positive electrode and a negative electrode of a DC bus respectively;

[0051] The port-switchable energy storage module is connected in series between the upper bridge arm and the lower bridge arm of the modular multilevel converter, respectively, to form a DC side topology of the modular multilevel converter, wherein the port-switchable energy storage module comprises two third and fourth switch tubes connected in anti-parallel, a symmetrical bipolar DC converter, and a battery, the symmetrical bipolar DC converter is connected to the battery and is configured to provide positive or negative pressure output to achieve charge and discharge control of the port-switchable energy storage module according to the power state of the DC side.

[0052] Specifically, the modular multilevel converter adopts a plurality of phase units as basic constituent components, each phase unit is composed of an upper bridge arm and a lower bridge arm, wherein each bridge arm is connected in series with an N number of same half-bridge sub-modules and a bridge reactor, the bridge reactor is mainly used for current buffering and voltage balancing, and the half-bridge sub-module is composed of corresponding power switching elements and capacitors, which can provide flexible multi-level output, the upper bridge arm and the lower bridge arm are connected to the positive and negative poles of the DC bus respectively, thereby forming a structured topology connected to the DC system; the topology of the DC side is further optimized for the application design of the port-switchable energy storage module, each energy storage module is composed of two switch tubes connected in anti-parallel, a symmetrical bipolar DC converter, and an energy storage battery, wherein the symmetrical bipolar DC converter is connected to the energy storage battery and has the ability to output positive and negative pressure, and the specific control logic is that according to the state of the DC side power balance, the port-switchable energy storage module realizes the flexible charge and discharge control of the energy storage module by coordinating the switching state and the converter output, the positive pressure output of the energy storage module is used to reduce the instantaneous power peak load of the system, and the negative pressure output is mainly used to eliminate the influence of the system peak energy, thereby maintaining the stable operation of the DC side power; for example, in a typical DC power fluctuation scenario, when the converter system senses that the voltage fluctuation on the DC side bus exceeds the stable range, the control system will instruct the port-switchable energy storage module to enter the working state, first, the third and fourth switch tubes are alternately turned on, so that the energy storage module is connected to the bridge circuit, and the positive and negative pressure outputs are controlled through the symmetrical bipolar DC converter, so as to quickly suppress the bus voltage fluctuation, at this time, if the voltage transient increases beyond the target value, the converter outputs negative pressure to absorb the excess energy in the form of battery charging; if the voltage transient decreases, the converter outputs positive pressure to release the stored energy of the battery for compensation, in this processing process, the modular structure allows the system to operate in the form of multiple parallel energy storage modules, thereby enhancing the suppression capability in high power state; the design of the port-switchable energy storage module also helps to realize efficient fault isolation, when the energy storage module unit or a certain sub-module fails, the faulty unit is quickly removed through the state adjustment of the third and fourth switch tubes to continue to maintain the operation stability, thereby improving the overall reliability.

[0053] The technical scheme of the application effectively solves the problems of system instability caused by DC power fluctuation in a new energy grid-connected scenario and the inability of traditional MMC to quickly suppress DC side power fluctuation.

[0054] Further, as shown in Figure 2 each half-bridge sub-module includes two first and second switch tubes connected in anti-parallel and a capacitor;

[0055] The first and second switch tubes are connected in series, and the first and second switch tubes are connected in parallel with the capacitor;

[0056] The common connection point of the first and second switch tubes serves as a first series port of the half-bridge sub-module, and the capacitor and the common connection point of the second switch tube serve as a second series port of the half-bridge sub-module.

[0057] As a preferred embodiment of the above, each half-bridge sub-module adopts two first and second switch tubes connected in anti-parallel to form its core structure, wherein the second switch tube and the capacitor jointly realize the charging and discharging function of the module, and this design provides flexible power distribution and control capability, which is a key component of MMC; specifically, the first and second switch tubes are connected in series, and the two switch tubes are connected in parallel to a capacitor, through this arrangement, the active elements in the circuit can provide a charging path for the capacitor, and at the same time, the key voltage regulation function in the modular design is realized, the two switch tubes have independent driving control strategies, and the charging and discharging state of the capacitor can be flexibly controlled according to system requirements, the common connection point of the first and second switch tubes constitutes the first series port of the half-bridge sub-module, and the common connection point of the capacitor and the second switch tube constitutes the second series port, this connection mode ensures that the half-bridge sub-module can provide sufficient power gain and stability in both series and parallel states among sub-modules; the preferred embodiment takes into account the complexity of actual system operation, and optimizes the selection of switch tubes and the design of driving circuit during implementation to further improve the response speed and durability of the module, in addition, the selection of the capacitor has a direct impact on the performance of the entire module, and appropriate parameters need to be designed according to practical application scenarios to ensure the effective play of the power suppression function, for example, in a high-frequency environment, a thin-film capacitor with high voltage resistance and low loss can be selected to further improve system efficiency; for example, in a typical DC bus voltage fluctuation management process, each half-bridge sub-module will control the charging and discharging of the switch tube, when the DC bus voltage is higher than the set threshold, the second switch tube is controlled to charge the capacitor to store excess energy; conversely, when the DC bus voltage is lower than the set threshold, the first switch tube is turned on to allow the capacitor to release energy to raise the bus voltage, so as to maintain the stability of the system.

[0058] Further, in the port-switchable energy storage module, the third switch tube and the fourth switch tube are connected in series, and a common connection end point of the third switch tube and the fourth switch tube serves as an input end of the port-switchable energy storage module;

[0059] An anode of the fourth switch tube serves as an output end of the port-switchable energy storage module and a second connection port;

[0060] A cathode of the third switch tube serves as a first connection port of the port-switchable energy storage module.

[0061] As a preferred embodiment of the above-mentioned embodiment, the port-switchable energy storage module is configured with the third switch tube and the fourth switch tube, the two switch tubes form a tank circuit through series connection, and a common connection end point thereof serves as an input end of the port-switchable energy storage module, which can flexibly adapt to dynamic requirements of a direct-current power system, the cathode of the third switch tube is designed as a first connection port of the energy storage module, and the anode of the fourth switch tube is designed as a second connection port of the energy storage module and an output end of the energy storage module, which ensures accurate and efficient power transmission and conversion of the energy storage module in the operation process of the direct-current system; the working principle of the energy storage module is based on the coordination of the switching states of the third switch tube and the fourth switch tube, when the module works, the energy storage module adjusts the closed and open states of the switch tubes according to the dynamic power requirements of the direct-current bus, so as to realize intelligent allocation of power flow, when the third switch tube is turned on, the module allows external power input to the energy storage device, and realizes the charging process; when the fourth switch tube is turned on, the energy storage device releases the stored energy to the system, and realizes the discharging operation, this bidirectional control logic optimizes the flow direction of energy, and meets the power smoothing requirements of the system; in a preferred specific design, the third switch tube and the fourth switch tube are both high-voltage-resistant and low-loss power switching devices, such as IGBT, so as to adapt to the high power transmission requirements in the system, in addition, the connection port layout of the energy storage module is optimized and designed to ensure the durability of signal transmission and power stabilization, in actual application, the input end of the integrated energy storage module is connected to the bridge arm end of the modular multilevel converter, so as to ensure that the energy storage module can be triggered according to the converter signal, so as to match the dynamic regulation and control strategy of the whole system; for example, in the scenario of fluctuation of the direct-current bus voltage, when the load of the direct-current bus suddenly increases, causing power shortage of the system, the energy storage module is triggered by the fourth switch tube to release the energy storage from the battery to the bridge arm port, and compensate for the power gap; when the bus load decreases and the system power is surplus, the energy storage module is triggered by the third switch tube to realize the charging operation, and stores the excess energy for subsequent use, in this process, the connection port design of the energy storage module ensures the rapid response and dynamic operation stability of the whole system.

[0062] Further, the symmetrical bipolar DC converter comprises: an inductor, a fifth switch tube, a sixth switch tube, a seventh switch tube, an eighth switch tube, and a ninth switch tube, a first clamping capacitor and a second clamping capacitor, a first capacitor and a second capacitor, and a first output resistor and a second output resistor.

[0063] As a preferred embodiment of the above, the symmetrical bipolar DC converter aims to achieve high-efficiency energy conversion and dynamic power suppression capability, and the structure of the converter comprises the following main parts: an inductor for ensuring smooth transition of current and acting as a buffer during operation of the converter; fifth to ninth switch tubes as main power electronic devices, achieving bidirectional regulation of energy flow through appropriate logic control; first and second clamping capacitors for limiting voltage fluctuations and ensuring stable voltage operation of the converter; first and second capacitors for storing electrical energy and participating in voltage regulation to meet the output requirements of the energy storage module; and first and second output resistors as load designs to ensure proper distribution of current and power during system operation. In the preferred embodiment, the working logic of the symmetrical bipolar DC converter is managed by a control system, the core of which is to coordinate the on and off states of the switch tubes to achieve positive and negative voltage conversion of the target energy. The fifth and sixth switch tubes are responsible for the positive current path in the energy storage charging mode, the seventh and eighth switch tubes are responsible for the negative current path in the energy storage discharging mode, and the ninth switch tube acts as a safety protection device to trigger a circuit break or clamping under certain abnormal conditions to protect the system from overcurrent, for example, when the system bus voltage is higher than the normal value, the fifth and sixth switch tubes are turned on to store excess electrical energy in the first and second capacitors; when the system bus voltage is lower than the normal value, the seventh and eighth switch tubes are turned on to release the stored electrical energy through the inductor to the system to supplement the bus power demand. Importantly, the first and second clamping capacitors act as voltage limiting devices during operation. In actual design, high-capacity, low-leakage capacitor elements can be selected to ensure operational stability. At the same time, the resistance values of the first and second output resistors also need to be adjusted according to the actual application scenario to optimize power transfer efficiency and loss.

[0064] Further, in the symmetrical bipolar DC converter, the positive electrode of the battery is connected to one end of the inductor; the negative electrode of the battery is connected to the emitter of the fifth switch tube, and the emitter of the fifth switch tube is further connected to the ground potential terminal;

[0065] The other end of the inductor is connected to the collector of the fifth switch tube, the emitter of the sixth switch tube, and the collector of the seventh switch tube.

[0066] As a preferred embodiment of the above-mentioned embodiment, the positive electrode of the battery is directly connected to one end of the inductor, which ensures that the battery can provide source energy for the converter as the main energy source, and the smooth transition of current in the circuit path is realized through the inductor, so that the entire energy flow and power regulation process remain stable. The negative electrode of the battery is connected to the emitter of the fifth switch tube, which ensures that the current path during the conversion process is clear and enhances the reliability of the system operation. At the same time, the emitter of the fifth switch tube is also connected to the ground potential terminal, which provides a reference potential and ensures the voltage stability of the entire system circuit and the integrity of the power loop. The other end of the inductor is connected to the collector of the fifth switch tube, the emitter of the sixth switch tube, and the collector of the seventh switch tube, which ensures efficient integration of the energy flow path within the converter. It performs multiple functions, including regulating the flow and supporting the switching of the switch tube in different operating modes. Specifically, the fifth and sixth switch tubes are responsible for controlling the charging path, and the seventh switch tube handles the reverse discharge path, enabling flexible conversion of charging and discharging. In this connection layout, the inductor not only assists in the conversion process of the current, but also absorbs high-frequency noise in the current through its high-frequency characteristics, further improving the stability of the system. When the system senses a voltage drop on the DC bus, the sixth switch tube is turned on, and the energy is transferred to the energy storage module through the inductor, and the battery charging is realized in the path through the fifth switch tube. When the bus voltage is too high, the seventh switch tube is turned on, and energy is released from the battery to the load. The inductor assists in current regulation during this process to ensure that the power load has no adverse effect on the steady-state operation of the system.

[0067] Furthermore, in the symmetrical bipolar DC converter, the collector of the sixth switch tube is connected to one end of the first clamping capacitor and the emitter of the eighth switch tube.

[0068] The collector of the eighth switch tube is connected to one end of the first capacitor and one end of the first output resistor.

[0069] The emitter of the seventh switch tube is connected to one end of the second clamping capacitor and the collector of the ninth switch tube.

[0070] The emitter of the ninth switch tube is connected to one end of the second capacitor and one end of the second output resistor.

[0071] The other end of the first clamping capacitor and the other end of the second clamping capacitor are commonly connected, and the common connection terminal is connected to the ground potential terminal.

[0072] The other end of the first capacitor and the other end of the second capacitor are commonly connected.

[0073] The other end of the first output resistor and the other end of the second output resistor are commonly connected.

[0074] As a preferred embodiment of the above embodiment, firstly, in the relationship between the switching tube and the clamping capacitor, the collector of the sixth switching tube is connected to one end of the first clamping capacitor and the emitter of the eighth switching tube, which ensures that when the sixth switching tube is turned on, it can guide the excess current to the first clamping capacitor for voltage clamping to stabilize the circuit voltage and prepare for the subsequent energy release path. The collector of the eighth switching tube is connected to one end of the first capacitor and one end of the first output resistor, further completing the energy transfer from the clamping capacitor to the energy storage capacitor and the load. This layout makes the energy flow from the power supply side to the load side efficient and continuous, avoiding overshoot or loss during energy transfer. Secondly, the emitter of the seventh switching tube is connected to one end of the second clamping capacitor and the collector of the ninth switching tube, which functions to dynamically limit the voltage change in the circuit with the second clamping capacitor, while effectively transferring related energy to the ninth switching tube. In this path, the emitter of the ninth switching tube is further connected to one end of the second capacitor and one end of the second output resistor. The second capacitor serves as an energy storage node to store and stabilize the output of the transferred energy to the load. The second output resistor is used to regulate the current flow to ensure reasonable power distribution during energy release. In addition, to achieve closed-loop control of the overall connection, the other end of the first clamping capacitor and the second clamping capacitor is commonly connected and connected to the ground potential terminal through the common connection terminal, providing a stable reference point for the system circuit and maintaining electrical balance throughout the circuit. Similarly, the other end of the first capacitor and the other end of the second capacitor are commonly connected to form a unified output of the energy storage node. The other end of the first output resistor and the other end of the second output resistor are commonly connected to achieve unified energy output at the load end. This common node design ensures that the energy flow between multiple components is coordinated and consistent, ensuring the integrity of the system operation. For example, in the scenario of DC bus voltage fluctuation, when the bus voltage exceeds the set limit, the sixth switching tube is turned on to transfer power to the first clamping capacitor for voltage stabilization, and then the excess energy is stored in the energy storage capacitor. Conversely, when the bus voltage drops, the second clamping capacitor limits the voltage drop, and the seventh and ninth switching tubes are sequentially turned on to release energy through the second capacitor to transfer compensation power to the load and stabilize system operation.

[0075] Further, in the symmetrical bipolar DC converter, the drive signals of the sixth and eighth switching tubes are configured to be complementary to the drive signal of the fifth switching tube, ignoring the dead zone.

[0076] The drive signals of the seventh and ninth switching tubes are configured to be the same as the drive signal of the fifth switching tube.

[0077] As a preferred embodiment of the above-mentioned embodiment, the driving signals of the sixth switch tube and the eighth switch tube are designed to form a complementary relationship with the driving signal of the fifth switch tube, ignoring the dead zone. This complementary relationship means that when the fifth switch tube is in the on state, the sixth switch tube and the eighth switch tube are off; when the fifth switch tube is off, the sixth switch tube and the eighth switch tube are on. This logical relationship ensures that during the charging and discharging process of the energy storage module, regardless of the path of the current, the on-state loss can be effectively reduced and the risk of short circuit caused by cross conduction can be avoided. By ignoring the dead zone design, the dynamic response speed of the operation is further improved, making the system quickly adapt to the change of the DC bus voltage. At the same time, the driving signals of the seventh switch tube and the ninth switch tube are consistent with the driving signal of the fifth switch tube, that is, the driving logic is synchronized. This synchronous configuration enables the seventh switch tube and the ninth switch tube to accurately participate in the control of forward or reverse energy flow during the operation of the energy storage module, thereby achieving efficient transmission of the entire system power. For example, when the fifth switch tube is on, the seventh switch tube and the ninth switch tube are also in the on state, at which time the power path of the entire system forms a forward working mode, and energy enters the bus or load through the energy storage module. When the fifth switch tube is off, the seventh switch tube and the ninth switch tube are also off, and the energy storage module enters a constant voltage stabilization mode without power flow. In the specific circuit design, the driving signal generation of the sixth switch tube and the eighth switch tube adopts an isolated driving mode to avoid interference or false triggering, and the driving signal of the seventh switch tube and the ninth switch tube adopts a synchronous hot standby mode, which ensures that it is always consistent with the signal of the fifth switch tube under different power input states. When the system senses an increase in the demand for DC bus voltage, the on-state of the fifth switch tube causes the energy storage module to enter a mode of releasing energy to the bus. At this time, according to the driving system logic, the off-state of the sixth switch tube and the eighth switch tube ensures the stability of the energy release path, and the synchronous on-state of the seventh switch tube and the ninth switch tube ensures that the energy storage energy is fully transmitted to the bus. When the voltage demand decreases, the driving signal is adjusted in the opposite direction, the fifth switch tube is off, the sixth switch tube and the eighth switch tube are on, and the system enters a charging mode to store excess energy for subsequent release requirements.

[0078] Further, as shown in FIG. 6, the third switch tube and the fourth switch tube in the port-switchable energy storage module are configured to control the input or removal of the port-switchable energy storage module. Figure 3

[0079] The first connection port and the second connection port of the port-switchable energy storage module are configured to be connected to port B and port C of the symmetrical bipolar DC converter respectively in the first operation mode, so that the symmetrical bipolar DC converter outputs negative voltage.

[0080] ​The first connection port and the second connection port of the port-switchable energy storage module are configured to be connected to port A and port B of the symmetrical bipolar DC converter respectively in the second operation mode, so that the symmetrical bipolar DC converter outputs positive voltage;

[0081] The first operation mode is a discharging mode, and the second operation mode is a charging mode.

[0082] As a preferred embodiment of the above-mentioned embodiment, the third switch tube and the fourth switch tube of the port-switchable energy storage module are configured as control elements, and their operation directly affects the working state of the energy storage module in the system, determines whether the energy storage module is put into operation or cut off, and adjusts the on-off state of the two switch tubes through a dynamic switching signal in the system operation, so as to realize flexible participation of the energy storage module. The configuration of the third switch tube and the fourth switch tube ensures that the energy storage module can be effectively activated or disconnected in the power system, and maintains the efficiency and safety of energy flow. The first connection port and the second connection port of the energy storage module are connected to different ports of the symmetrical bipolar DC converter in different operation modes, forming different voltage outputs. For example, in the first operation mode, i.e. the discharging mode, the first connection port and the second connection port of the energy storage module are configured to be connected to port B and port C of the symmetrical bipolar DC converter respectively, so that the converter outputs negative voltage. This negative voltage output mechanism allows the energy storage module to release stored energy to the system when the DC bus voltage demand decreases, thereby providing supplemental energy to the bus through this path and stabilizing the grid voltage. In the second operation mode, i.e. the charging mode, the first connection port and the second connection port of the energy storage module are configured to be connected to port A and port B of the symmetrical bipolar DC converter respectively, so that the converter outputs positive voltage. In this configuration, the energy storage module acts as an energy storage unit and stores excess energy in the battery or energy storage device through the positive voltage output conversion energy path when the system has excess power, in preparation for subsequent demand changes. In the preferred design, the control system automatically adjusts the operation mode of the energy storage module by real-time monitoring of the DC bus voltage state. Specifically, when the bus voltage is lower than the set threshold, the control logic triggers the first operation mode to provide a rapid response when the load demand peaks. Conversely, when the bus voltage is higher than the set threshold and the energy storage module is not fully charged, the second operation mode is activated to achieve safe storage of additional energy. Through this active design, the energy storage module can efficiently adapt to different power states and ensure the dynamic balance of the system.

[0083] Embodiment two;

[0084] Based on the same inventive concept as the MMC and port-switchable energy storage-based DC power smoothing system in the foregoing embodiments, the present application also provides a MMC and port-switchable energy storage-based DC power smoothing method. The method comprises:

[0085] Real-time monitoring of the power abnormal state of the DC bus of the modular multilevel converter, judging whether the power abnormal state is in surplus state or shortage state;

[0086] According to the power abnormal state, the switch unit of the port-switchable energy storage module is turned on, so that the port-switchable energy storage module is connected between the upper and lower arms of the modular multilevel converter;

[0087] When the power is in the shortage state, the symmetric bipolar DC converter of the port-switchable energy storage module is controlled to output negative voltage, so that the battery is discharged to supplement the shortage power;

[0088] When the power is in the surplus state, the symmetric bipolar DC converter is controlled to output positive voltage, so that the battery is charged to absorb the excess power;

[0089] Based on the charge and discharge control of the port-switchable energy storage module, the DC bus power fluctuation is suppressed, so that the voltage across the modular multilevel converter is restored and kept stable.

[0090] The above method in the application can effectively realize the DC power suppression system based on MMC and port-switchable energy storage, and the technical effects are as described in the above embodiments, which will not be repeated here.

[0091] Although the present application is described in conjunction with specific features and embodiments thereof, it is obvious that various modifications and combinations can be made thereto without departing from the spirit and scope of the application. Accordingly, the specification and drawings are to be regarded simply as illustrative of the present application defined by the appended claims, and it is intended to cover any and all modifications, variations, combinations or equivalents that fall within the scope of the present application. Obviously, those skilled in the art can make various modifications and variations to the present application without departing from the scope of the present application. Thus, if these modifications and variations of the present application belong to the scope of the present application and its equivalents, the present application is intended to include these modifications and variations.

Claims

1. A DC power smoothing system based on MMC and port-switchable energy storage, characterized in that, The system includes: A modular multilevel converter includes multiple phase units, each phase unit including an upper bridge arm and a lower bridge arm, each upper bridge arm and each lower bridge arm including a bridge arm reactor and N identical half-bridge submodules, the bridge arm reactor being connected in series with the N identical half-bridge submodules; the upper bridge arm and the lower bridge arm of the modular multilevel converter are respectively connected to the positive and negative terminals of a DC bus; A port-switchable energy storage module is provided, wherein the port-switchable energy storage module is connected in series between the upper arm and the lower arm of the modular multilevel converter to form the DC-side topology of the modular multilevel converter. The port-switchable energy storage module includes two anti-parallel connected third and fourth switching transistors, a symmetrical bipolar DC-DC converter, and a battery. The symmetrical bipolar DC-DC converter is connected to the battery and configured to provide positive or negative voltage output according to the power state of the DC side to realize the charging and discharging control of the port-switchable energy storage module. The symmetrical bipolar DC-DC converter includes: an inductor, a fifth switch, a sixth switch, a seventh switch, an eighth switch and a ninth switch, a first clamping capacitor and a second clamping capacitor, a first capacitor and a second capacitor, and a first output resistor and a second output resistor. In the symmetrical bipolar DC-DC converter, the positive terminal of the battery is connected to one end of the inductor; the negative terminal of the battery is connected to the emitter of the fifth switch, and the emitter of the fifth switch is further connected to the ground potential terminal. The other end of the inductor is connected to the collector of the fifth switch, the emitter of the sixth switch, and the collector of the seventh switch. In the symmetrical bipolar DC-DC converter, the collector of the sixth switch is connected to one end of the first clamping capacitor and the emitter of the eighth switch. The collector of the eighth switch is connected to one end of the first capacitor and one end of the first output resistor; The emitter of the seventh switch is connected to one end of the second clamping capacitor and the collector of the ninth switch. The emitter of the ninth switch is connected to one end of the second capacitor and one end of the second output resistor; The other end of the first clamping capacitor is connected to the other end of the second clamping capacitor, and the common connection terminal is connected to the ground potential terminal; The other end of the first capacitor is connected to the other end of the second capacitor in a common connection; The other end of the first output resistor is connected to the other end of the second output resistor.

2. The DC power smoothing system based on MMC and port-switchable energy storage according to claim 1, characterized in that, include: Each of the half-bridge submodules includes two anti-parallel connected first and second switching transistors and a capacitor; The first switch and the second switch are connected in series, and the first switch and the second switch are connected in parallel with the capacitor. The common connection terminal of the first switch and the second switch serves as the first series port of the half-bridge submodule, and the common connection terminal of the capacitor and the second switch serves as the second series port of the half-bridge submodule.

3. The DC power smoothing system based on MMC and port-switchable energy storage according to claim 1, characterized in that, include: In the port-switchable energy storage module, the third switch and the fourth switch are connected in series, and the common connection terminal of the third switch and the fourth switch serves as the input terminal of the port-switchable energy storage module. The emitter of the fourth switching transistor serves as the output terminal of the port-switchable energy storage module and the second connection port. The collector of the third switching transistor serves as the first connection port of the port-switchable energy storage module.

4. The DC power smoothing system based on MMC and port-switchable energy storage according to claim 1, characterized in that, include: In the symmetrical bipolar DC-DC converter, the drive signals of the sixth and eighth switches are configured to be complementary to the drive signal of the fifth switch, ignoring the dead zone. The drive signals for the seventh and ninth switches are configured to be the same as the drive signal for the fifth switch.

5. The DC power smoothing system based on MMC and port-switchable energy storage according to claim 1, characterized in that, include: The third and fourth switching transistors in the port-switchable energy storage module are configured to control the activation or deactivation of the port-switchable energy storage module. The first and second connection ports of the switchable energy storage module are configured in the first operating mode to be connected to ports B and C of the symmetrical bipolar DC-DC converter, respectively, so that the symmetrical bipolar DC-DC converter outputs a negative voltage. In the second operating mode, the first connection port and the second connection port of the port-switchable energy storage module are configured to be connected to port A and port B of the symmetrical bipolar DC-DC converter, respectively, so that the symmetrical bipolar DC-DC converter outputs positive voltage. The first operating mode is the discharge mode, and the second operating mode is the charging mode.

6. A DC power mitigation method based on MMC and port-switchable energy storage, characterized in that, The method, applied to the DC power smoothing system based on MMC and port-switchable energy storage as described in claim 1, comprises: Real-time monitoring of abnormal power status of the DC bus of the modular multilevel converter, and determination of whether the abnormal power status is in a surplus state or a deficit state. According to the power anomaly state, the switching unit of the port-switchable energy storage module is turned on, so that the port-switchable energy storage module is connected between the upper and lower bridge arms of the modular multilevel converter. When the power is in the aforementioned deficit state, the symmetrical bipolar DC-DC converter of the switchable energy storage module controls the output of negative voltage to drive the battery to discharge and replenish the deficit power. When the power is in the surplus state, the symmetrical bipolar DC-DC converter is controlled to output positive voltage to drive the battery to charge and absorb the excess power. The charging and discharging control of the switchable energy storage module at the port smooths out DC bus power fluctuations, allowing the voltage across the modular multilevel converter to recover and remain stable.

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