A Multi-Point Differential Average Method Applied to Digital Multi-Terminal Ring Power Grids

Abstract

The present invention belongs to the technical field of distribution networks, and specifically discloses a multi-point differential average value method applied to a digital multi-terminal looped power grid. First, the present invention conducts an example design on the main converter equipment selection, main wiring form, and grounding method of a multi-terminal looped DC distribution system based on MMC, and establishes an application case of a multi-terminal looped DC distribution system for a certain urban power grid; then, according to the operation and fault characteristics of the multi-terminal looped DC distribution system, combined with the performance and configuration of primary equipment, a DC voltage slope control strategy considering voltage limits is proposed, and the converter station protection configuration of the application case is completed. The present invention can improve the operation control and optimization ability of the multi-terminal looped power grid system.

Description

Technical Field

[0001] The present invention belongs to the technical field of distribution networks and is used for the control and protection of multi-terminal flexible DC distribution networks. Specifically, it is a multi-point differential average value method applied to digital multi-terminal looped power grids. Background Art

[0002] The increasingly narrow space of modern urban power station corridors and the highly concentrated load have become new technical bottlenecks restricting the development of distribution networks. The multi-terminal flexible DC distribution network based on the Modular Multilevel Converter (MMC) has gradually developed into one of the key technologies to support the wide interconnection of modern power energy Internet of Things.

[0003] The multi-terminal flexible DC hybrid distribution network formed by interconnecting medium-voltage distribution networks using the flexible DC technology of MMC integrates the advantages of large-capacity power supply, low transmission loss, high-quality power quality, and convenient access to distributed power supply devices.

[0004] DC voltage control is currently one of the hotspots of multi-terminal flexible DC distribution networks. The control method plays a crucial role in the stability of system operation and power supply reliability.

[0005] Zhang Xue, Pei Wei, Fan Shixiong, Kong Li, Deng Wei, Huang Renle, in the paper "Coordinated Control Method of AC / DC Hybrid Distribution Network with Multi-Terminal Flexible Interconnection Devices", applied the improved battery load shedding control method considering the safety domain to the load margin condition, and proposed a coordinated joint control scheme for interconnected electrical devices such as distributed power sources and chemical energy storage in normal and fault states of the system. However, the load shedding control loses the adjustable maximum electrical energy and has a high cost.

[0006] Zheng Pengwei, in the paper "Research on Voltage Control of Flexible Medium-Voltage DC Distribution Network", proposed to involve the DC transformer configured in the distributed energy storage station in the voltage margin control. Compared with the traditional master-slave control method with lagging information transmission, a voltage boundary medium- and low-voltage control mode switching strategy considering the action priority was formulated. However, the participation of the DC transformer in voltage control requires an increase in capacity configuration, and the action priority method has a large error.

[0007] Ji Yirun, Yuan Zhichang, Zhao Jianfeng, Li Yan, Xu Shukai, in the paper "A Voltage Control Strategy Suitable for Flexible DC Distribution Network", introduced the slack node as the main station VdcQ outer loop control variable, injecting reactive power support into the AC system and providing the calculation basis for the reference voltage value of the distribution network system. The slave station follows the relative voltage value of the main station to broaden the constraints of the converter station power control and voltage limit, achieving the purpose of reasonable energy distribution. However, there is no unified standard for the selection of the slack node, and the credibility is not high.

[0008] The inherent properties of the T-connected line form, AC-DC load interconnection, and flexible configuration of distributed power sources existing in the multi-terminal flexible DC distribution network jointly construct its highly complex network topology. Therefore, new challenges arise in its AC-DC system, converter protection identification, fault isolation and recovery, etc.

[0009] "Research on Key Control Technologies of Medium-Voltage Flexible DC Distribution Network" published by Ji Yirun shows that the increase in equipment makes the network topology more complex, but it is necessary to re-establish the control and protection strategies for the AC-DC system.

[0010] "DC Distribution Network Protection and Fault Isolation Scheme Based on Active Current Limiting Control" published by Zheng Tao, Wu Qiong, Lv Wenxuan, Li Rui, and Xu Lie first controls the fault DC current at 1.2 times and identifies the zero-crossing property of other branches simultaneously. Then, relying on the coordinated action of high-speed power electronic switches and circuit breakers, the current is controlled to drop to zero to achieve the purpose of isolating the fault. However, there is a delay phenomenon in identifying the zero-crossing of the branch, increasing the error.

[0011] "Research on Protection Scheme of Multi-Terminal Flexible DC Distribution Network Based on Modular Multilevel Converter" published by Shangguan Xin proposes a calculation method based on the zero-sequence circuit by combining the common-mode voltage and current with the active power direction discrimination condition, establishes a time sequence logic between the longitudinal component of the third harmonic and the zero-sequence voltage of the converter valve, and verifies the feasibility in the fault state network for DC single-stage ground faults. However, the third harmonic can only be extracted in specific faults.

[0012] "Research on Voltage Control of Multi-Terminal Flexible Medium-Voltage DC Distribution Network" published by Xiao Feng introduces the concept of Pearson correlation after a single-stage ground fault to distinguish the differences in the sub-transient fault voltages, and forms a differential protection for the reverse direction of the current-limiting voltage to identify internal and external faults. However, the sensitivity of the differential protection to the sub-transient voltage is insufficient. Summary of the Invention

[0013] The purpose of the present invention is to provide a multi-point differential average value method applied to a digital multi-terminal looped power grid. First, the interconnected design of the segmented urban medium-voltage distribution network is carried out to design a multi-terminal flexible DC hybrid distribution network, and an example design of the main converter equipment selection is carried out; secondly, considering the curve characteristics of the DC voltage threshold of the converter combined with the translation of the power reference value, a DC voltage slope control method considering voltage limits is proposed to ensure the stable operation of the voltage; and according to the operation and fault characteristics of the DC distribution network, combined with the performance and configuration of primary equipment, a secondary strategy for the integrated control and protection of the converter is formulated; finally, the model is built; to improve the operation control and optimization ability of the multi-terminal looped power grid system.

[0014] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0015] A multi-point differential average method applied to a digital multi-terminal loop power grid is carried out in the following steps in sequence:

[0016] S1. Design a multi-terminal loop DC power distribution system

[0017] The multi-terminal loop DC power distribution system includes the first to seventh buses, the first to third converter stations, the first to third AC circuit breakers, the first to third DC circuit breakers, the first to third connection transformers, the first to fourth converters, the first to third DC transformers, AC loads, DC loads, a new energy system, and an energy storage system;

[0018] The first connection transformer, the first converter, and the first DC circuit breaker are arranged in the first converter station;

[0019] The second connection transformer, the second converter, and the second DC circuit breaker are arranged in the second converter station;

[0020] The third connection transformer, the third converter, and the third DC circuit breaker are arranged in the third converter station;

[0021] The external 10 kV AC power distribution network is connected to the first bus through the first AC circuit breaker and the first converter station in sequence, the external 10 kV AC power distribution network is connected to the third bus through the second AC circuit breaker and the second converter station in sequence, and the external 10 kV AC power distribution network is connected to the fifth bus through the third AC circuit breaker and the third converter station in sequence;

[0022] The AC load is connected to the second bus through the fourth converter; the DC load is connected to the fourth bus through the third DC transformer; the new energy system is connected to the seventh bus through the first DC transformer, and the energy storage system is connected to the sixth bus through the second DC transformer;

[0023] The multi-terminal loop DC power distribution system adopts a single-pole symmetric wiring method;

[0024] S2. Design of control and protection schemes

[0025] S21. DC voltage slope control of additional voltage thresholds

[0026] On the basis of the droop control method, the voltage upper limit threshold U dcmax and the voltage lower limit threshold U dcmin are respectively set to control the voltage fluctuation range of the first to seventh buses. When the voltage of the first to seventh buses reaches the voltage upper limit threshold U dcmax or the voltage lower limit threshold U dcmin , the power of the first to third converter stations is fed back by the corresponding voltage thresholds and the droop curve is shifted;

[0027] S22. Protection configuration of converter stations

[0028] The first to third converter stations are protected by the valve-side DC-side grounding fault protection control logic, and the protection area includes the areas of the first to third connecting transformers and the first to third converters.

[0029] As a limitation: in step S1, neutral points of the high-voltage sides of the first to third DC transformers are grounded through resistors by using concentrated capacitors, and grounding switches of the first to third DC transformers are put into operation in the island mode.

[0030] As a further limitation: in step S21, for any one of the first to third converter stations, a voltage upper limit threshold U dcmax , a voltage lower limit threshold U dcmin and a power reference value P ref are set, and the following steps are carried out in sequence,

[0031] S211. Measure the DC voltage U dc and power P of this converter station;

[0032] S212. Filter out the high-frequency fluctuation components in the DC voltage U dc and power P by using a filter to obtain U and P f respectively;

[0033] S213. If U dcmin ≤U≤U dcmax is satisfied, then step S214 is executed; otherwise, step S215 is executed;

[0034] S214. Judge whether is established. If it is established, then step S215 is executed; otherwise, step S216 is executed;

[0035] S215. Modify the power reference value to P ref =P f and end step S21;

[0036] S216. Keep the power reference value unchanged, that is, P ref =P ref and end step S21.

[0037] As a further limitation: in step S22, the process of protecting the first to third converter stations by using the valve-side DC-side grounding fault protection control logic includes protection in two cases of fault occurrence and fault elimination;

[0038] For any one of the first to third converter stations, the protection after the fault occurs is carried out in the following steps in sequence,

[0039] S221. The grounding resistor is put into operation;

[0040] S222, DC differential protection;

[0041] S223, inverter pulse blocking;

[0042] S224, DC circuit breaker is open;

[0043] S225, AC circuit breaker is open;

[0044] S226, grounding resistance cut-off;

[0045] S227, fault repair;

[0046] For any of the first to third converter stations, the protection after the fault is removed is carried out in the following order:

[0047] T221, AC circuit breaker closed;

[0048] T222, inverter slow start;

[0049] T223, inverter pulse unlocking;

[0050] T224, if the DC voltage rises to the set value, execute step T225, otherwise, repeatedly determine whether the DC voltage rises to the set value until the DC voltage rises to the set value, and execute step T225;

[0051] T225, DC circuit breaker closing;

[0052] T226, the inverter works according to the set mode.

[0053] As a further limitation: the main protection configuration of the first to third converter stations is DC system grounding overcurrent protection; the backup protection configuration of the first to third converter stations is DC voltage unbalance protection and DC undervoltage protection;

[0054] The DC system includes a line connecting the first to seventh busbars, a first DC transformer and a second DC transformer.

[0055] Due to the adoption of the above technical solution, the present invention has the following technical advances compared with the prior art:

[0056] (1) The present invention comprehensively considers the system-side power supply, distributed energy, energy storage and load characteristics, designs a multi-terminal ring DC distribution system based on MMC, and designs an additional balance control strategy for the converter DC capacitor voltage based on carrier phase shift modulation; and performs example design of the main equipment and wiring method according to the main circuit structure, thereby improving the stability margin of the converter DC voltage and ensuring the safe and reliable operation of the multi-terminal ring AC / DC system;

[0057] (2) The present invention designs a method for multi-point differential average value to follow power disturbances, proposes a DC voltage slope control strategy, realizes the translation of the DC voltage and its operation in the stable operation range by automatically modifying the power reference value of the DC voltage-active power droop curve, sets the upper and lower voltage threshold values, and formulates a protection configuration strategy according to the operation and fault characteristics of the flexible DC distribution system, which can dynamically respond to the power fluctuations and voltage dips of the AC-DC system, and ensure the good interaction and coordinated control between the power supply side and the load side;

[0058] (3) The present invention studies the modeling technology of a multi-terminal looped DC distribution system based on MMC, establishes an offline model of a digital multi-terminal flexible DC distribution system, and can verify the effectiveness of the proposed DC voltage slope control strategy and protection configuration scheme through simulation, greatly improving the operation control and optimization ability of the system;

[0059] (4) In the present invention, a concentrated capacitor is set at the high voltage side of the DC transformer to ground the neutral point, which can ensure voltage balance; the design adopts the method of grounding the neutral point through a resistor, which can reduce the single-pole fault current during island operation; setting the grounding switch of the DC transformer to be put into operation in the island mode can avoid the influence of the grounding scheme on the DC distribution side on the grounding parameters of the AC side of the medium-voltage distribution network.

[0060] The present invention belongs to the technical field of distribution networks and can improve the operation control and optimization ability of a multi-terminal looped power grid system. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] The drawings are used to provide a further understanding of the present invention, and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention, but do not constitute a limitation to the present invention.

[0062] In the drawings:

[0063] Figure 1 is the topological structure diagram of the multi-terminal looped DC distribution system designed for the embodiment of the present invention;

[0064] Figure 2 is the waveform schematic diagram corresponding to the five levels in the carrier phase-shifted modulation in the embodiment of the present invention;

[0065] Figure 3 is the schematic diagram of the reference voltage control principle of the bridge arm module in the embodiment of the present invention;

[0066] Figure 4 is the schematic diagram of the structure principle of the DC transformer designed in the embodiment of the present invention;

[0067] Figure 5 is the schematic diagram of the main wiring form principle of the system in the embodiment of the present invention;

[0068] Figure 6Schematic diagram of the grounding scheme for the DC transformer designed in the embodiments of the present invention;

[0069] Figure 7 Control characteristic curve of the additional voltage threshold in the embodiments of the present invention;

[0070] Figure 8 Flowchart of the power reference value for setting the voltage threshold in the embodiments of the present invention;

[0071] Figure 9 Measuring point diagram of the protection signal for the converter protection area in the embodiments of the present invention;

[0072] Figure 10 a is the protection flowchart after a fault occurs in the embodiments of the present invention;

[0073] Figure 10 b is the protection flowchart after the fault is removed in the embodiments of the present invention;

[0074] Figure 11 Load power curve in the simulation of the embodiments of the present invention;

[0075] Figure 12 Power curve of the first converter station in the simulation of the embodiments of the present invention;

[0076] Figure 13 DC voltage curve at the port of the first converter station in the simulation of the embodiments of the present invention;

[0077] Figure 14 Power curve of the second converter station in the simulation of the embodiments of the present invention;

[0078] Figure 15 DC voltage curve at the port of the second converter station in the simulation of the embodiments of the present invention;

[0079] Figure 16 Power curve of the third converter station in the simulation of the embodiments of the present invention;

[0080] Figure 17 DC voltage curve at the port of the third converter station in the simulation of the embodiments of the present invention;

[0081] Figure 18 DC voltage curves of the positive and negative DC buses of the multi-terminal ring-shaped power grid in the simulation of the embodiments of the present invention;

[0082] Figure 19 DC voltage curve of the first converter in the simulation of the embodiments of the present invention;

[0083] Figure 20 DC power curve of the first converter in the simulation of the embodiments of the present invention;

[0084] Figure 21This is the DC power curve of the second converter in the simulation of the embodiment of the present invention;

[0085] Figure 22 This is the DC power curve of the third converter in the simulation of the embodiment of the present invention. Specific embodiments

[0086] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are only for explaining and illustrating the present invention, and are not used to limit the present invention.

[0087] Embodiment A multi-point differential average value method applied to a digital multi-terminal looped power grid

[0088] In this embodiment, first, the main converter equipment selection, main wiring form and grounding method of the multi-terminal looped DC distribution system based on MMC are designed by example, and an application case of the multi-terminal looped DC distribution system of a certain urban power grid is established. Then, according to the operation and fault characteristics of the multi-terminal looped DC distribution system, combined with the performance and configuration of primary equipment, a DC voltage slope control strategy considering voltage limits is proposed, and the converter station protection configuration of the application case is completed.

[0089] In this embodiment, first, step S1 is entered to design a multi-terminal looped DC distribution system.

[0090] Step S1 includes the following processes:

[0091] The designed topological structure of the multi-terminal looped DC distribution system is as Figure 1As shown in the figure, the system includes the first to seventh buses, the first to third converter stations, the first to third AC circuit breakers ACB1 - ACB3, the first to third DC circuit breakers DCB1 - DCB3, the first to third connection transformers, the first to fourth converters VSC1 - VSC4, the first to third DC transformers DCSST1 - DCSST3, AC loads, DC loads, a new energy system, and an energy storage system; the first connection transformer, the first converter VSC1, and the first DC circuit breaker ACB1 are arranged in the first converter station; the second connection transformer, the second converter VSC2, and the second DC circuit breaker ACB2 are arranged in the second converter station; the third connection transformer, the third converter VSC3, and the third DC circuit breaker ACB3 are arranged in the third converter station; the external 10 kV AC distribution network is connected to the first bus through the first AC circuit breaker ACB1 and the first converter station in sequence, the external 10 kV AC distribution network is connected to the third bus through the second AC circuit breaker ACB2 and the second converter station in sequence, the external 10 kV AC distribution network is connected to the fifth bus through the third AC circuit breaker ACB3 and the third converter station in sequence; the AC loads are connected to the second bus through the fourth converter VSC4; the DC loads are connected to the fourth bus through the third DC transformer DCSST3; the new energy system is connected to the seventh bus through the first DC transformer DCSST1, and the energy storage system is connected to the sixth bus through the second DC transformer DCSST2. Figure 1 In the figure, the dashed box represents the converter station.

[0092] The main parameters of the system circuit are shown in Table 1 below:

[0093] Table 1

[0094]

[0095] For the 10 kV DC distribution network, the voltage fluctuation range is considered to be -10% to +5%.

[0096] This step also includes the design of the voltage source converter, the design of the DC transformer, the main wiring and grounding design.

[0097] Regarding the design of the voltage source converter: As Figure 1 shown in the figure, the first to fourth converters VSC1 - VSC4 select MMC - type converters with a rated capacity of 20 MVA, a rated active power of 20 MW, and a maximum reactive power demand of 20 MVar in the STATCOM operation mode. The rated voltage of the AC system is 10 kV, the current is 1154 A, the rated voltage of the DC system is ±10 kV, the current is 1000 A, and the rated current of the bridge arm is 666 A. To meet the system harmonic requirements, the number of bridge arm connections is set to 25, and 1700 V / 1200 A - class devices are selected considering a certain margin.

[0098] The first to fourth converters VSC1 to VSC4 are modulated using the classical carrier phase-shifted modulation (CPSM) method. As Figure 2 shown, it is a schematic diagram of the waveforms corresponding to five levels in carrier phase-shifted modulation.

[0099] Based on CPSM, an additional control strategy for DC capacitor voltage balance is adopted. The energy adjustment of the arm modules is achieved by controlling their reference voltages to achieve balanced distribution. As Figure 3 shown, it illustrates the principle of the reference voltage control of the arm modules. Figure 3 In it, v ci is the capacitor voltage of the i-th sub-module of the arm. represents the average value taken as the capacitor voltages of N sub-modules. Δv ci takes the difference between v ci and as the additional balance control input. The product of Δv ci and the arm current i br obtains the additional control value through a proportional loop and is added to the arm through limiting to obtain the system reference voltage value In the figure, represents the reference value of the arm voltage.

[0100] Regarding the design of the DC transformer: As Figure 4 shown is the structural schematic diagram of the DC transformer. The dual-active-bridge (DAB) structure includes 2 full bridges and 1 isolation transformer. This structure can fully improve the power transmission density. n DABs need to be connected to the medium-voltage and low-voltage DC buses in series and parallel forms at the high-voltage end and the medium-voltage end respectively. Combining with what is shown in Figure 1 it can be known that n = 3 in this embodiment.

[0101] According to the system operation requirements, Figure 1 in

[0102] the first DC transformer DCSST1 and the second DC transformer DCSST2 are used to connect to the wind-solar-storage DC microgrid with power exchange requirements and have the ability to transmit power in opposite directions under the constant DC voltage and constant power control modes. Figure 5 Regarding the main wiring and grounding design: The multi-terminal ring-shaped DC distribution system adopts a single-pole symmetric wiring method. In this embodiment, the first to third connection transformers adopt the scheme shown in

[0103] This embodiment fully considers the possibility of island operation. The first to third DC transformers DCSST1 to DCSST3 control and adjust the ±10 kV medium-voltage DC bus voltage. To ensure voltage balance, a concentrated capacitor is used to set the neutral point grounding at the high-voltage side of the first to third DC transformers DCSST1 to DCSST3. And to reduce the single-pole fault current during island operation, a grounding method with the neutral point grounded through a resistor is designed, as Figure 6 shown. In addition, to avoid the influence of the DC distribution side grounding scheme on the grounding parameters of the AC side of the medium-voltage distribution network, the first to third DC transformers DCSST1 to DCSST3 are set to operate with the grounding switches turned on in the island mode.

[0104] Next, step S2 is executed to design the control and protection scheme. Step S2 includes two steps: DC voltage slope control with an additional voltage threshold and converter station protection configuration.

[0105] S21. DC voltage slope control with an additional voltage threshold

[0106] The droop control method has been widely applied to flexible DC distribution networks. For example, the droop control method is disclosed in "Research Review on Droop Control Technology of DC Microgrids" published by Zhu Shanshan, Wang Fei, Guo Hui, Wang Qifeng, and Gao Yanxia. On this basis, the voltage upper limit threshold U dcmax and the voltage lower limit threshold U dcmin are respectively set to control the voltage fluctuation range of the first to seventh buses. When the voltage reaches the voltage upper limit threshold U dcmax or the voltage lower limit threshold U dcmin , the power of the first to third converter stations is fed back by the corresponding voltage threshold and the droop curve is shifted. The characteristic curve considering the voltage threshold is as Figure 7 shown. When a certain converter station runs along curve 1 to (P1, U1), the voltage reaches the upper limit threshold U dcmax , and P1 is fed back and set as the power reference value P ref to obtain curve 2 and control the converter station to run to the voltage stable point. Similarly, when the voltage reaches the lower limit value U dcmin , it is fed back and runs to the voltage stable point corresponding to curve 3.

[0107] For any one of the first to third converter stations, as Figure 8 shown, the voltage upper limit threshold U dcmax , the voltage lower limit threshold U dcmin , and the power reference value P ref are set, and the following steps are carried out in sequence,

[0108] S211. Measure the DC voltage U dc and the power P of the converter station;

[0109] S212. Filter the DC voltage U dc and the high-frequency fluctuation components in the power P to obtain U and P respectively f ;

[0110] S213. If U dcmin ≤U≤U dcmax is satisfied, then execute step S214; otherwise, execute step S215;

[0111] S214. Judge Whether it holds. If it holds, then execute step S215; otherwise, execute step S216;

[0112] S215. Modify the power reference value to P ref =P f and end step S21;

[0113] S216. Keep the power reference value unchanged, that is, P ref =P ref and end step S21.

[0114] In this step, a filter is used to filter the high-frequency components of U dc to obtain U as the reference value. When the operating voltage reaches the upper and lower threshold values, a feedback reference value is set to obtain a new operating curve to ensure that the voltage operates in the range of [U dcmax、 U dcmin .

[0115] S22. Converter station protection configuration

[0116] The protection of the first to third converter stations includes the areas covered by the first to third connecting transformers and the first to third converters VSC1 - VSC3; for any converter station, the protection signal measuring points are as Figure 9 shown;

[0117] The valve-side DC side ground fault protection control logic is used to protect the first to third converter stations, and the protection process includes protection in two cases: fault occurrence and fault removal.

[0118] As Figure 10 shown in a, for any of the first to third converter stations, the protection after a fault occurs is carried out in the following order of steps

[0119] S221. Insert the grounding resistance;

[0120] S222. DC differential protection;

[0121] S223. Converter pulse blocking;

[0122] S224. DC circuit breaker opening;

[0123] S225, AC circuit breaker trips;

[0124] S226, ground resistance removed;

[0125] S227, fault repair;

[0126] As Figure 10 shown in b, for any one of the first to third converter stations, after the fault is removed, the protection proceeds in the following steps in sequence,

[0127] T221, AC circuit breaker closes;

[0128] T222, converter soft start;

[0129] T223, converter pulse unlock;

[0130] T224, if the DC voltage rises to the set value, execute step T225, otherwise repeatedly judge whether the DC voltage has risen to the set value until the DC voltage rises to the set value, and then execute step T225;

[0131] T225, DC circuit breaker closes;

[0132] T226, converter operates in the set mode.

[0133] In step S22, the main protection of the first to third converter stations is configured as DC system ground overcurrent protection; the backup protection of the first to third converter stations is configured with DC voltage imbalance protection and DC under-voltage protection.

[0134] Next, this embodiment studies the modeling technology of a multi-terminal looped DC distribution system based on MMC in an urban distribution network, and establishes an offline model of a digital multi-terminal flexible DC system based on PSCAD / EMTDC through a designed application case, and verifies the correctness of the system model and control protection configuration through simulation analysis.

[0135] Based on the application case of the ±10kV urban distribution network multi-terminal looped DC distribution system designed in this embodiment, the simulation system is built in the PSCAD software Figure 1 adopting a single-pole symmetric wiring method, and the effectiveness of the control strategy and protection configuration in this embodiment is verified through simulation.

[0136] First, perform a simulation verification of the DC voltage slope control. The specific process is as follows: The simulation time is set to 3 s, the capacity of the converter stations is 10 MVA each, the capacity of the AC load is 4 MVA, which is connected to the DC bus through the first converter station, the capacity of the new energy system is 4 MW, the capacity of the DC load is 5 MW, the upper and lower voltage limits are 20.5 kV and 19.5 kV respectively, and the slopes of the converters are 0.067, 0.05, and 0.04 respectively. The connection times of the AC load, DC load, energy storage system, and new energy system to the DC system are 0.5 s, 0.6 s, 0.7 s, and 0.8 s respectively; it is set that the output power stabilizes after 1 s, and the DC bus voltage initially operates stably at 20 kV. The DC voltage slope control simulation takes the voltage of the first bus as an example. The threshold values of the upper and lower system voltage limits are set to 20.5 kV and 19.0 kV respectively. The power conversion curves of the AC load and the first to third converter stations are as Figures 11 to 15 shown.

[0137] It can be seen from Figure 11 that when the system load suddenly changes, in order to maintain the system power and voltage stability, each converter station adjusts the power and voltage stability in a timely manner according to the set control strategy. The power curves and DC voltage curves of the first to third converter stations are respectively shown in Figures 12 to 17 ; it can be seen that the output powers of the first to third converter stations gradually recover to the initial steady-state values after the system operates normally. Figure 11 In Figure 1 , L1, L3, L4, and L5 respectively correspond to and represent the lines indicated by L1, L3, L4, and L5 in

[0138] The simulation results show that the DC voltage slope control strategy based on the upper and lower voltage threshold values effectively improves the system voltage, as shown in Figure 18 . As the load output power changes, after reaching the voltage threshold boundary, it promptly feeds back and follows the power reference value to move to the stable operating point, ensuring that the system DC voltage operates within the set range.

[0139] Next, perform a simulation verification of the converter fault. The specific process is as follows: Set a ground fault at the valve top of the first converter VSC1 at 1.2 s. The AC voltage of the AC distribution network system connected to the first converter station and the AC voltage on the valve side are distorted. The protection system locks the No. 1 converter in 3 ms and removes the fault in 5 ms. After the first converter VSC1 receives the locking signal, its DC output quickly drops to 0, the positive voltage at the outlet of the first converter VSC1 drops to 0, and the negative voltage drops to -5 kV, as shown in Figure 19 , Figure 20 . Since the second converter VSC2 is under constant power control, the power remains at 9 MW after oscillation, as shown in Figure 21 . The output of the third converter VSC3 quickly increases to supplement the power gap caused by the locking of the first converter VSC1 and stabilizes at 20 MW at 1.5 s, as shown in Figure 22. Due to the sharp oscillation and increase in the DC output of the second converter VSC2 and the third converter VSC3, the AC voltage waveform on the valve side of the corresponding converter shows a slight decrease, and the AC voltage waveform returns to normal after the output of the corresponding converter stabilizes.

Claims

1. A multi-point differential average method applied to a digital multi-terminal loop power grid, characterized in that, Proceed in the following step sequence: S1. Design a multi-terminal looped DC distribution system The multi-terminal looped DC distribution system includes the first to seventh buses, the first to third converter stations, the first to third AC circuit breakers, the first to third DC circuit breakers, the first to third coupling transformers, the first to fourth converters, the first to third DC transformers, AC loads, DC loads, a new energy system, and an energy storage system; The first coupling transformer, the first converter, and the first DC circuit breaker are arranged in the first converter station; The second coupling transformer, the second converter, and the second DC circuit breaker are arranged in the second converter station; The third coupling transformer, the third converter, and the third DC circuit breaker are arranged in the third converter station; The external 10 kV AC distribution network is connected to the first bus through the first AC circuit breaker and the first converter station in sequence, the external 10 kV AC distribution network is connected to the third bus through the second AC circuit breaker and the second converter station in sequence, and the external 10 kV AC distribution network is connected to the fifth bus through the third AC circuit breaker and the third converter station in sequence; The AC load is connected to the second bus through the fourth converter; the DC load is connected to the fourth bus through the third DC transformer; the new energy system is connected to the seventh bus through the first DC transformer, and the energy storage system is connected to the sixth bus through the second DC transformer; The multi-terminal looped DC distribution system adopts a monopolar symmetric wiring mode; S2. Control and protection scheme design S21. DC voltage slope control with an additional voltage threshold Based on the droop control method, set the upper voltage threshold U dcmax and the lower voltage threshold U dcmin to control the voltage fluctuation range of the first to seventh buses. When the voltage of the first to seventh buses reaches the upper voltage threshold U dcmax or the lower voltage threshold U dcmin , the power of the first to third converter stations is fed back by the corresponding voltage threshold and the droop curve is shifted; S22. Converter station protection configuration The first to third converter stations are protected by using the protection control logic for DC side grounding faults at the valve end, and the protection area includes the areas where the first to third coupling transformers and the first to third converters are located.

2. The multi-point differential average value method applied to a digital multi-terminal loop power grid according to claim 1, characterized in that: In step S1, neutral points are grounded through resistors by using concentrated capacitors at the high voltage sides of the first to third DC transformers respectively, and grounding switches of the first to third DC transformers are put into operation in the island mode.

3. The multi-point differential average value method applied to a digital multi-terminal loop power grid according to claim 2, wherein: In the step S21, for any one of the first to third converter stations, set the upper voltage threshold U dcmax , the lower voltage threshold U dcmin , the power reference value P ref , and proceed in the following step sequence S211. Measure the DC voltage U of the converter station dc and the power P; S212. Filter the high-frequency fluctuation components in the DC voltage U dc and the power P to obtain U and P respectively f ; S213. If U satisfies dcmin ≤U≤U dcmax , then execute step S214; otherwise, execute step S215; S214. Determine whether it holds. If it holds, execute step S215; otherwise, execute step S216; S215. Modify the power reference value to P ref = P f and end step S21; S216, the power reference value remains unchanged, i.e., P ref = P ref and step S21 ends.

4. A multi-point differential average value method applied to a digital multi-terminal loop power grid according to claim 3, characterized in that: In step S22, the process of protecting the first to third converter stations by using the protection control logic for DC side grounding faults at the valve end includes protection in two cases: when a fault occurs and when the fault is eliminated; For any one of the first to third converter stations, the protection after a fault occurs proceeds in the following step sequence: S221. Grounding resistor is put in; S222. DC differential protection; S223. Converter pulse blocking; S224. DC circuit breaker trips; S225. AC circuit breaker trips; S226. Grounding resistor is removed; S227. Fault repair; For any one of the first to third converter stations, the protection after the fault is eliminated proceeds in the following step sequence: T221. AC circuit breaker closes; T222. Converter soft start; T223. Converter pulse unlocking; T224. If the DC voltage rises to the set value, then execute step T225, otherwise repeatedly judge whether the DC voltage rises to the set value until the DC voltage rises to the set value and execute step T225; T225. DC circuit breaker closes; T226. Converter operates in the set mode.

5. A multi-point differential average value method applied to a digital multi-terminal loop power grid according to claim 4, characterized in that: The main protection configurations of the first to third converter stations are DC system grounding overcurrent protection; the backup protection configurations of the first to third converter stations are DC voltage imbalance protection and DC undervoltage protection. The DC system includes lines connecting the first to seventh buses, a first DC transformer, and a second DC transformer.

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