A method and system for suppressing voltage sag in a multi-terminal DC distribution network

Abstract

The present invention relates to a method and system for suppressing voltage sags in a multi-terminal DC distribution network. Based on virtual DC motor control and variable droop inertia control, the present invention proposes an inertia coordination control strategy that takes voltage grading support into consideration to suppress voltage sags in the DC distribution network, and provides a timing coordination method. For the drop and recovery sections of the voltage sag, an adaptive inertia coefficient is designed based on factors such as capacity margin and voltage change rate. The proposed strategy can fully coordinate the adjustable equipment in the DC distribution network to participate in bus voltage regulation, share the power regulation pressure of the voltage control node converter equipment during the transient process of voltage sag, and improve the dynamic response of the system in the voltage recovery section, so that the voltage can recover quickly. It can provide active transient voltage support to the DC bus, reduce the sag amplitude, voltage drop rate and energy loss of the DC voltage sag, and effectively suppress the DC voltage sag.

Description

Technical Field

[0001] The present invention relates to the electrical field, and in particular to a method and system for suppressing voltage sag in a multi-terminal direct current distribution network. Background Art

[0002] DC distribution networks offer advantages such as high power efficiency and strong controllability, making them a crucial form of power distribution for the future Energy Internet. However, power quality issues also exist in DC distribution networks, and these networks exhibit unique characteristics due to factors such as the random nature of distributed generation, the spatial variability of DC loads, and the diversity of disturbances. Among these, DC voltage sags can have a significant impact on sensitive loads, causing severe damage and substantial economic losses, making them a key power quality issue in DC distribution networks. Currently, research on voltage sag assessment has largely focused on AC systems, and experience with large-scale construction and maintenance of DC distribution networks is relatively limited, leading to a lack of research on DC voltage sags. Compared to AC distribution networks, DC distribution networks have weaker inertia. Factors such as high renewable energy output fluctuations and line faults can easily cause sudden voltage fluctuations in the system, leading to power quality issues such as voltage sags and interruptions.

[0003] One approach to suppressing voltage sags is to address this from the user side by adding external voltage sag control equipment, including uninterruptible power supplies (UPS), dynamic voltage restorers (DVRs), and solid-state transfer switches (SSTSs). However, this increases investment costs. Another approach to suppressing voltage sags is to address this from the power supply side by utilizing inertial control strategies, using renewable energy and energy storage devices in the DC distribution network to provide active voltage support to the DC bus. However, this approach's effectiveness in suppressing DC voltage sags requires further exploration and analysis, as does the coordinated strategy for multiple devices providing active inertial support to the DC bus. Summary of the Invention

[0004] Aiming at the voltage sag problem of multi-terminal DC distribution network, the present invention proposes a virtual inertia coordinated control strategy based on virtual DC motor control and variable droop inertia control to actively support the DC bus voltage and suppress DC voltage sag.

[0005] The above technical problems of the present invention are mainly solved by the following technical solutions:

[0006] A method for suppressing voltage sag in a multi-terminal DC distribution network. The voltage of the multi-terminal DC distribution network is a double-terminal ring topology. The network obtains power from the AC main grid through two converter stations. The two converter stations are the master converter station and the slave converter station. The voltage classification threshold U is set. M , and define the real-time medium voltage side bus voltage U dc , and the DC bus voltage rating U dcN , select execution based on the following judgment results:

[0007] When U dc =U dcN , execute normal operation mode: the main converter station adopts constant voltage control strategy to control DC voltage, and the slave converter station adopts constant power control strategy; the photovoltaic work is in maximum power point tracking control mode, outputting maximum power to the DC bus; the energy storage equipment adopts constant power control;

[0008] When U M dc dcN , executing the master voltage regulation mode: the slave converter station does not participate in inertia regulation and adopts constant power control mode, while the DC distribution network maintains a master-slave coordinated control mode. The energy storage converter's control strategy switches from constant power control to virtual DC motor control, providing voltage support to the medium-voltage bus and suppressing voltage sags. During transient voltage fluctuations, the energy storage and master converter station work together to stabilize the bus voltage.

[0009] When U dc M , execute the backup support mode: switch from the converter station to variable droop inertia control to provide active voltage support to the DC bus.

[0010] In the above-mentioned method for suppressing voltage sag in a multi-terminal DC distribution network, when the voltage drops to the minimum value Ud, the converter station switches back to constant power control, and the system switches to the main voltage regulation mode.

[0011] In the above-mentioned method for suppressing voltage sag in a multi-terminal DC distribution network,

[0012] Considering the drop and recovery stages of voltage sag, the virtual DC motor control adopts the adaptive inertia coefficient H.

[0013] During the voltage drop period, the inertia coefficient H is selected to be a large fixed value to avoid the voltage losing transient stability due to frequent coefficient changes during the transient process of voltage drop;

[0014] In the voltage recovery stage, the inertia coefficient H is adjusted according to the bus voltage deviation |U dc -U dcN |Adaptive adjustment: when the deviation is large, a smaller H is adopted to speed up the response and make the voltage recover quickly.

[0015] In the above-mentioned method for suppressing voltage sag in a multi-terminal DC distribution network,

[0016] Considering the drop section and recovery section of voltage sag, the variable droop inertia control adopts the adaptive droop coefficient k U .

[0017] ​​​During the voltage drop phase, the faster the voltage drops, the faster the droop coefficient changes according to the maximum capacity limit of the converter station, providing stronger inertial support to the DC bus.

[0018] In the voltage recovery section, the droop coefficient is set to 0 to speed up the system response and enable the voltage to recover quickly.

[0019] A multi-terminal DC distribution network voltage sag suppression system is characterized in that the multi-terminal DC distribution network voltage is a double-terminal ring topology, and obtains power from the AC main grid through two converter stations, the two converter stations are respectively a master converter station and a slave converter station, wherein a voltage classification threshold U is set M , and define the real-time medium voltage side bus voltage U dc , and the DC bus voltage rating U dcN ,

[0020] The judgment execution module selects execution based on the following judgment results:

[0021] When U dc =U dcN , execute normal operation mode: the main converter station adopts constant voltage control strategy to control DC voltage, and the slave converter station adopts constant power control strategy; the photovoltaic work is in maximum power point tracking control mode, outputting maximum power to the DC bus; the energy storage equipment adopts constant power control;

[0022] When U M dc dcN , executing the master voltage regulation mode: the slave converter station does not participate in inertia regulation and adopts constant power control mode, while the DC distribution network maintains a master-slave coordinated control mode. The energy storage converter's control strategy switches from constant power control to virtual DC motor control, providing voltage support to the medium-voltage bus and suppressing voltage sags. During transient voltage fluctuations, the energy storage and master converter station work together to stabilize the bus voltage.

[0023] When U dc M , execute the backup support mode: switch from the converter station to variable droop inertia control to provide active voltage support to the DC bus.

[0024] In the above-mentioned multi-terminal DC distribution network voltage sag suppression system, when the voltage drops to the minimum value Ud, the converter station switches back to constant power control, and the system switches to the main voltage regulation mode.

[0025] In the above-mentioned multi-terminal DC distribution network voltage sag suppression system,

[0026] Considering the drop and recovery stages of voltage sag, the virtual DC motor control adopts the adaptive inertia coefficient H.

[0027] ​​​During the voltage drop period, the inertia coefficient H is selected to be a large fixed value to avoid the voltage losing transient stability due to frequent coefficient changes during the transient process of voltage drop;

[0028] In the voltage recovery stage, the inertia coefficient H is adjusted according to the bus voltage deviation |U dc -U dcN |Adaptive adjustment: when the deviation is large, a smaller H is adopted to speed up the response and make the voltage recover quickly.

[0029] In the above-mentioned multi-terminal DC distribution network voltage sag suppression system,

[0030] Considering the drop section and recovery section of voltage sag, the variable droop inertia control adopts the adaptive droop coefficient k U .

[0031] During the voltage drop phase, the faster the voltage drops, the faster the droop coefficient changes according to the maximum capacity limit of the converter station, providing stronger inertial support to the DC bus.

[0032] In the voltage recovery section, the droop coefficient is set to 0 to speed up the system response and enable the voltage to recover quickly.

[0033] Compared with the prior art, the present invention has the following advantages:

[0034] 1. The proposed strategy can fully coordinate the adjustable devices in the DC distribution network to participate in bus voltage regulation, and share the power regulation pressure of the commutation equipment at the voltage control node during the transient process of voltage sag;

[0035] 2. The proposed strategy can improve the dynamic response of the system in the voltage recovery section, allowing the voltage to recover quickly.

[0036] 3. The proposed strategy can provide active transient voltage support to the DC bus when the bus power is unbalanced, reduce the DC voltage sag depth, voltage drop rate and energy loss, and effectively suppress the DC voltage sag. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a medium and low voltage multi-terminal DC distribution network topology structure in the example application of the present invention.

[0038] Figure 2 It is a schematic diagram of control mode switching in an example application of the present invention.

[0039] Figure 3 It is a schematic diagram of the timing coordination of the voltage sag suppression strategy in the example application of the present invention.

[0040] Figure 4 It is an equivalent calculation model for DC voltage sag in the example application of the present invention taking into account the timing coordination of the voltage sag suppression strategy.

[0041] Figure 5 1 is the voltage waveform of the DC bus I on the medium voltage side of the DC distribution network in the example application of the present invention.

[0042] Figure 6 It is the power change waveform of the DC distribution network master converter station, slave converter station and energy storage in the example application of the present invention. DETAILED DESCRIPTION

[0043] The specific embodiments of the present invention will be further described below in conjunction with the accompanying drawings.

[0044] Example 1

[0045] This embodiment provides a voltage sag suppression strategy for a multi-terminal DC distribution network, which is divided into three operation control modes: normal operation mode, main voltage regulation mode, and backup support mode.

[0046] By voltage classification threshold U M Divide the control operation mode to meet the following requirements: 1) Normal operation mode: U dc =U dcN ;2) Main voltage regulation mode: U M dc dcN ;3) Backup support mode: U dc M .

[0047] In the normal operating control mode, the DC distribution network adopts a master-slave coordinated control strategy. One converter station is the master station, which uses a constant voltage control strategy to control the DC voltage. The remaining converter stations are slave stations, which use a constant power control strategy. The photovoltaic system operates in the maximum power point tracking control mode, outputting active power to the DC bus at maximum power. The energy storage equipment adopts constant power control.

[0048] In main voltage regulation mode, the energy storage system switches to virtual DC motor control, generating virtual inertia by simulating the DC motor's mechanical rotation equations and electromotive force balance equations. This strategy's control effect is equivalent to connecting a virtual capacitor in parallel to the DC bus, thereby increasing the inertia of the DC distribution network and suppressing voltage sags.

[0049] In the backup support control mode, the converter station switches to the variable droop control strategy, and the voltage droop coefficient k is increased. U This allows the droop curve to swing. Reserve power is quickly released from the converter station within an adjustable range to compensate for the power shortfall on the DC bus. This strategy also has the effect of connecting a virtual capacitor in parallel to the DC bus, providing active voltage transient support and suppressing voltage sags.

[0050] Consider the drop section and recovery section of voltage sag:​​​

[0051] 1. The virtual DC motor control adopts an adaptive inertia coefficient H. In the voltage drop section, the inertia coefficient H is selected to a larger fixed value to avoid the voltage losing transient stability due to frequent coefficient changes during the transient process of voltage drop; in the voltage recovery stage, the inertia coefficient H is adjusted according to the bus voltage deviation |U dc -U dcN |Adaptive adjustment: when the deviation is large, a smaller H is adopted to speed up the response and make the voltage recover quickly.

[0052] 2. Variable droop inertia control uses adaptive droop coefficient k U During the voltage drop phase, when the voltage drops faster, the droop coefficient changes faster based on the maximum capacity limit of the converter station, providing stronger inertial support to the DC bus. During the voltage recovery phase, the droop coefficient is set to 0, accelerating the system's response and allowing the voltage to recover quickly.

[0053] Figure 1 Figure 2 shows the topology of a medium- and low-voltage, multi-terminal DC distribution network used in an example application of the present invention. This structure is a two-terminal ring topology, drawing power from the AC mains via two converter stations, H1 and H2. The system has six medium-voltage busbars, while the low-voltage side includes various DC loads, including data centers, residential loads, industrial loads, photovoltaic power plants, and energy storage devices.

[0054] Figure 2 This is a schematic diagram of control mode switching in an example application of the present invention. dc =U dcN , the system is in normal operation control mode, the multi-terminal DC distribution network adopts master-slave coordinated control strategy, one converter station is the master station, adopts constant voltage control strategy to control DC voltage, the other converter station is the slave station, adopts constant power control strategy; the photovoltaic work is in maximum power point tracking control mode, outputs maximum power to the DC bus; the energy storage device adopts constant power control. During the voltage sag, when the medium voltage side bus voltage U M dc dcN , the system is in the main voltage regulation mode, the slave converter station does not participate in inertia regulation, adopts constant power control mode, and the DC distribution network maintains the master-slave coordinated control mode. The control strategy of the energy storage converter switches from constant power control to virtual DC motor control strategy, providing voltage support to the medium voltage side bus and suppressing voltage sag. During the transient process of voltage change, the energy storage and the main converter station work together to stabilize the bus voltage. When the voltage returns to U dcN , the energy storage switches back to the constant power control strategy and the system resumes normal operation mode. During the voltage sag, when the medium voltage side bus voltage U dc M ​​​, the system is in backup support mode, switching from the converter station to variable droop inertia control, providing active voltage support to the DC bus. When the voltage drops to the minimum value Ud, the converter station switches back to constant power control, and the system switches to main voltage regulation mode.

[0055] Figure 3 This is a schematic diagram of the timing coordination of the voltage sag suppression strategy in an example application of the present invention. The timing coordination process can be described as follows:

[0056] (1) At t = t0, a large power shortage occurs on the bus due to transient disturbances, causing the DC bus voltage to drop rapidly. dc dcN , voltage change rate dU dc As / dt increases, the energy storage switches to virtual DC motor control to provide active voltage support to the DC bus;

[0057] (2) At t = t1, the voltage drops to U M , switching from the converter station to variable droop inertia control, and providing voltage support together with energy storage.

[0058] (3) At t = t2, the DC bus voltage drops to the lowest value and the voltage enters the recovery stage. The constant power control strategy is switched back from the converter station to speed up the voltage recovery rate. To avoid system oscillation caused by simultaneous switching of control, the energy storage converter still maintains the virtual DC motor control at this time.

[0059] (4) At t=t3, the DC voltage recovers to the rated value under the constant voltage control of the main converter station and the energy storage virtual DC motor control. At this time, the energy storage switches back to the constant power control mode.

[0060] The following is a theoretical analysis of the proposed strategy's effect on suppressing voltage sag.

[0061] Figure 4 This is an equivalent calculation model for DC voltage sag in the application of the present invention taking into account the timing coordination of voltage sag suppression strategy. Load is the effective value of constant power load power; R L is the line resistance; i R is the load current; ΔP is the instantaneous active power shortage on the bus. If u converters on the medium voltage DC bus adopt the virtual DC motor control strategy and v converters adopt the variable droop control strategy, and u + v = q, the equivalent capacitance on the DC bus increases to

[0062]

[0063] Assume that at time t0, a power shortage occurs on the DC bus due to a transient disturbance, ignoring the DC voltage across the line resistance R. L1 and R​L2 The voltage loss on The transient characteristics of the bus voltage meet the following requirements:

[0064]

[0065] At t=t1, the u converters switch to the virtual DC motor control strategy, providing active inertial support to the bus and generating an equivalent virtual capacitor C on the DC bus. vir1 , at this time the DC side equivalent capacitance increases to C e ' q =C eq +C vir1 The transient expression of DC bus voltage satisfies

[0066]

[0067] Where: U1 is the DC bus voltage at time t1.

[0068] At t=t2, v converters switch to variable droop inertia control, and a virtual capacitor C is equivalent to the DC bus. vir2 , the busbar equivalent capacitance increases to C e ″ q =C eq +C vir1 +C vir2 , the DC bus voltage satisfies

[0069]

[0070] Where: U2 is the DC bus voltage at time t2 obtained from the above formula

[0071] At this time, the minimum DC voltage drop considering the timing coordination of virtual DC motor control and variable droop inertia control is

[0072]

[0073] According to the above analysis, the virtual inertia control makes the equivalent capacitance C of the DC bus eq Increase, the lowest value of the temporary drop U d Increase, sag amplitude U sag =U dcN -U d The voltage drop is reduced and the voltage sag is effectively suppressed.

[0074] In the example application of the present invention, the virtual DC motor adopts an adaptive inertia coefficient. Under the condition of meeting the voltage range of the main voltage regulation mode, the adaptive adjustment equation of the inertia coefficient H considering the voltage deviation and voltage change rate is:

[0075]

[0076] Where: H max 、H min are the maximum and minimum values of the inertia time constant respectively; k x is the virtual inertia adjustment coefficient.

[0077] During the voltage drop process, H is selected as a larger fixed value because of its short duration to avoid the voltage losing transient stability due to frequent coefficient changes during the transient process of voltage drop; in the voltage recovery stage, according to the characteristics of the hyperbolic tangent function, H is adjusted according to the bus voltage deviation |U dc -U dcN |Adaptive adjustment, when the deviation is large, adopt a smaller H to speed up the response time, and as the voltage deviation decreases, the inertia coefficient gradually increases to prevent multiple sags caused by continuous fluctuations in source / load. Thus, H achieves [H min ,H max ] range of adaptive adjustment.

[0078] In the example application of the present invention, the variable droop control adopts an adaptive droop coefficient. Combined with the capacity threshold, voltage change rate, real-time transmission power and other operating status information of the converter station, under the condition of meeting the voltage range of the backup support mode, the adaptive variable droop coefficient constructed using the hyperbolic tangent function is:

[0079]

[0080] In the example application of the present invention, three indicators, namely, sag depth, voltage drop rate and energy loss, are used to evaluate the suppression effect.

[0081] (1) Depth of temporary drop

[0082] In DC distribution networks, the extent of voltage drop can be measured by the drop amplitude U sag To describe:

[0083] U sag =U dcN -U d

[0084] Where: U d is the minimum value of voltage drop; U dcN is the rated value of the DC bus voltage.

[0085] For any DC voltage sag event, if U sag The larger the U is, the greater the severity of the sag is. sag The smaller it is, the less severe it is.

[0086] (2) Voltage drop rate

[0087] Defining the mean sag for

[0088]

[0089] Where: u(t) is the sampling value of the continuous change of the DC bus voltage, t1 is the time when the node voltage drops to 90% of the rated voltage, and t4 is the time when the node voltage recovers to 90% of the rated voltage.

[0090] Define t2 as the time when the voltage reaches At the moment of DC voltage sag, the voltage drop rate δ fall It is defined as the voltage at the node dropping from 90% of the rated voltage to the average value of the sag during a sag event. The voltage change rate is positive, that is:

[0091]

[0092] (3) Energy loss

[0093] According to the IEEE P1564 standard, the energy loss index E of the DC distribution network is defined as vs The calculation formula is

[0094]

[0095] The energy loss index can intuitively represent the active energy lost by the resistive load during a voltage sag event.

[0096] The technical solution of the present invention will be further specifically described below through embodiments and in conjunction with the accompanying drawings.

[0097] Example 2:

[0098] In order to verify the effectiveness of the voltage sag suppression strategy of the present invention, a Figure 1 The electromagnetic transient simulation model of the DC distribution network is shown in Table 1 and Table 2. The simulation parameters are shown in Table 1 and Table 2. It is set that at t = 0.5s, a 12MW instantaneous load is put on the DC bus V. The power difference generated by the transient disturbance is large, causing the DC bus voltage to temporarily drop to U dc M The system automatically switches between the main voltage regulation mode and the backup support mode during transient voltage changes. Simulation results for adaptive inertia coordinated control are compared with those for no-inertia control, virtual DC motor control for energy storage only, and fixed inertia coefficient control.

[0099] Table 1 Main parameters of the medium and low voltage multi-terminal DC distribution network simulation system

[0100] ​

[0101] Table 2 Source and load parameters of medium and low voltage multi-terminal DC distribution network simulation system

[0102]

[0103] Table 3 Voltage sag severity index data

[0104]

[0105] Figure 5 1 is the voltage waveform of the DC bus I on the medium voltage side of the DC distribution network in the example application of the present invention. Figure 6 It is the power change waveform of the main converter station, slave converter station and energy storage of the DC distribution network in the example application of the present invention. It can be found that when the adaptive inertia coordinated control strategy is adopted, the bus voltage drop amplitude is reduced by about 0.35kV, 0.06kV and 0.02kV respectively compared with the other three. It can be seen from the power change curve that in the early stage of voltage drop, the capacitor at the outlet of the converter station H2 has a short-term charging and discharging process due to the change of the bus voltage, and the output active power fluctuates slightly. Under the action of constant power control, it quickly returns to the rated value. When the voltage drops to 0.95pu, the converter station H2 switches from constant power control to variable droop inertia control, instantly increasing a large amount of active power to provide power and active voltage support for the DC bus; and when the bus voltage recovers to 0.95pu, it switches back to the constant power control mode. The output power P2 of the converter station H2 returns to P after a period of time. 2ref The adaptive inertia coefficient provides greater instantaneous power than the fixed inertia coefficient, increasing active power by up to 3.18 MW and being more effective in suppressing voltage drops. The fixed inertia control strategy only increases active power by 1.37 MW. Furthermore, the proposed inertia coordination strategy can mitigate the power impact on the main converter station H1 in the early stages of transient disturbances, thus reducing its power regulation pressure.

[0106] Table 3 shows the calculation results of the suppression effect evaluation indicators under each control mode. A comparison shows that when the proposed voltage sag suppression strategy is adopted, the voltage drop amplitude, drop rate, and energy loss are improved by 23.5%, 24.2%, and 8.5%, respectively, compared to the case without the coordinated inertia control strategy. Compared to the case with only virtual DC motor inertia control, they are improved by 4.6%, 3.9%, and 1.8%, respectively. This effectively suppresses voltage drops and reduces the severity of voltage sags. Furthermore, the adaptive inertia coordination coefficient performs slightly better than the fixed inertia in suppressing voltage sags, with these three indicators improved by 1.5%, 2.2%, and 1.8%, respectively.

[0107] In summary, when a transient disturbance occurs on the bus, the proposed adaptive inertia coordinated control can coordinate the adjustable equipment in the system during the transient change of voltage, provide active voltage support to the DC bus, and improve the inertia of the DC distribution network, thereby effectively suppressing the DC voltage drop and reducing the severity of the voltage sag on the bus. At the same time, the proposed adaptive inertia coefficient can take into account the different inertia requirements of the voltage drop section and the voltage recovery section, and provide a flexible inertial support effect. In summary, based on the simulation results, the effectiveness of the active voltage sag suppression strategy for a multi-terminal DC distribution network of the present invention is verified.

[0108] Example 3

[0109] This embodiment provides a multi-terminal DC distribution network voltage sag suppression system. The multi-terminal DC distribution network voltage is a double-terminal ring topology. It obtains power from the AC main grid through two converter stations. The two converter stations are the master converter station and the slave converter station. The voltage classification threshold U is set. M , and define the real-time medium voltage side bus voltage U dc , and the DC bus voltage rating U dcN ,

[0110] The judgment execution module selects execution based on the following judgment results:

[0111] When U dc =U dcN , execute normal operation mode: the main converter station adopts constant voltage control strategy to control DC voltage, and the slave converter station adopts constant power control strategy; the photovoltaic work is in maximum power point tracking control mode, outputting maximum power to the DC bus; the energy storage equipment adopts constant power control;

[0112] When U M dc dcN , executing the master voltage regulation mode: the slave converter station does not participate in inertia regulation and adopts constant power control mode, while the DC distribution network maintains a master-slave coordinated control mode. The energy storage converter's control strategy switches from constant power control to virtual DC motor control, providing voltage support to the medium-voltage bus and suppressing voltage sags. During transient voltage fluctuations, the energy storage and master converter station work together to stabilize the bus voltage.

[0113] When U dc M , execute the backup support mode: switch from the converter station to variable droop inertia control to provide active voltage support to the DC bus.

[0114] ​​​The specific embodiments described herein are merely illustrative of the spirit of the present invention. Persons skilled in the art may make various modifications, additions, or substitutions to the described specific embodiments without departing from the spirit of the present invention or exceeding the scope of the appended claims.

Claims

1. A method for suppressing voltage sag in a multi-terminal DC distribution network, characterized in that: The voltage of the multi-terminal DC distribution network is a double-terminal ring topology, which obtains power from the AC main grid through two converter stations. The two converter stations are the master converter station and the slave converter station. The voltage classification threshold U is set. M , and define the real-time medium voltage side bus voltage U dc , and the DC bus voltage rating U dcN , select execution based on the following judgment results: When U dc =U dcN , execute normal operation mode: the main converter station adopts constant voltage control strategy to control DC voltage, and the slave converter station adopts constant power control strategy; the photovoltaic work is in maximum power point tracking control mode, outputting maximum power to the DC bus; the energy storage equipment adopts constant power control; When U M dc dcN , execute the master voltage regulation mode: the slave converter station does not participate in inertia regulation, adopts the constant power control mode, and the DC distribution network maintains the master-slave coordinated control mode; the control strategy of the energy storage converter switches from constant power control to the virtual DC motor control strategy, providing voltage support to the medium voltage side bus and suppressing voltage sag; during the transient process of voltage change, the energy storage and the master converter station work together to stabilize the bus voltage;​​ When U dc M , execute the backup support mode: switch from the converter station to variable droop inertia control to provide active voltage support to the DC bus;​ Considering the drop and recovery stages of voltage sag, the virtual DC motor control adopts the adaptive inertia coefficient H; During the voltage drop period, the inertia coefficient H is selected to be a large fixed value to avoid the voltage losing transient stability due to frequent coefficient changes during the transient process of voltage drop; In the voltage recovery stage, the inertia coefficient H is adjusted according to the bus voltage deviation |U dc -U dcN |Adaptive adjustment: when the deviation is large, a smaller H is adopted to speed up the response and make the voltage recover quickly; Considering the drop section and recovery section of voltage sag, the variable droop inertia control adopts the adaptive droop coefficient k U ; During the voltage drop phase, the faster the voltage drops, the faster the droop coefficient changes according to the maximum capacity limit of the converter station, providing stronger inertial support to the DC bus. In the voltage recovery section, the droop coefficient is set to 0 to speed up the system response and enable the voltage to recover quickly.

2. A method for suppressing voltage sag in a multi-terminal DC distribution network according to claim 1, characterized in that: When the voltage drops to the minimum value Ud, the converter station switches back to constant power control and the system switches to the main voltage regulation mode.

3. A multi-terminal DC distribution network voltage sag suppression system, characterized in that: The voltage of the multi-terminal DC distribution network is a double-terminal ring topology, which obtains power from the AC main grid through two converter stations. The two converter stations are the master converter station and the slave converter station. The voltage classification threshold U is set. M , and define the real-time medium voltage side bus voltage U dc , and the DC bus voltage rating U dcN , The judgment execution module selects execution based on the following judgment results: When U dc =U dcN , execute normal operation mode: the main converter station adopts constant voltage control strategy to control DC voltage, and the slave converter station adopts constant power control strategy; the photovoltaic work is in maximum power point tracking control mode, outputting maximum power to the DC bus; the energy storage equipment adopts constant power control; When U M dc dcN , execute the master voltage regulation mode: the slave converter station does not participate in inertia regulation, adopts the constant power control mode, and the DC distribution network maintains the master-slave coordinated control mode; the control strategy of the energy storage converter switches from constant power control to the virtual DC motor control strategy, providing voltage support to the medium voltage side bus and suppressing voltage sag; during the transient process of voltage change, the energy storage and the master converter station work together to stabilize the bus voltage;​​ When U dc M , execute the backup support mode: switch from the converter station to variable droop inertia control to provide active voltage support to the DC bus;​ Considering the drop and recovery stages of voltage sag, the virtual DC motor control adopts the adaptive inertia coefficient H; During the voltage drop period, the inertia coefficient H is selected to be a large fixed value to avoid the voltage losing transient stability due to frequent coefficient changes during the transient process of voltage drop; In the voltage recovery stage, the inertia coefficient H is adjusted according to the bus voltage deviation |U dc -U dcN |Adaptive adjustment: when the deviation is large, a smaller H is adopted to speed up the response and make the voltage recover quickly; Considering the drop section and recovery section of voltage sag, the variable droop inertia control adopts the adaptive droop coefficient k U ; During the voltage drop phase, the faster the voltage drops, the faster the droop coefficient changes according to the maximum capacity limit of the converter station, providing stronger inertial support to the DC bus. In the voltage recovery section, the droop coefficient is set to 0 to speed up the system response and enable the voltage to recover quickly.

4. A multi-terminal DC distribution network voltage sag suppression system according to claim 3, characterized in that: When the voltage drops to the minimum value Ud, the converter station switches back to constant power control and the system switches to the main voltage regulation mode.

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