Direct current power distribution network fault recovery method and system based on multi-device coordination and time sequence cooperation

Abstract

The invention discloses a direct-current power distribution network fault recovery method and system based on multi-device coordination and time sequence cooperation, and relates to the field of direct-current power distribution systems, aiming at solving the problems that after a direct-current line breaks down, the fault current rising speed is extremely high, the peak value is relatively large, and a direct-current circuit breaker cannot reliably remove the fault, and the invention discloses the direct-current power distribution network fault recovery method and system based on multi-device coordination and time sequence cooperation and relates to the field of direct-current power distribution systems. Detecting and isolating a fault area through a direct-current circuit breaker, and locking a current converter and a direct-current transformer; reclosing the direct-current circuit breaker to judge the fault type, and realizing the recovery of the direct-current power distribution network through the time sequence cooperation of multiple devices according to different fault types; and in the recovery process, the output voltage and current of the converter are controlled. According to the method, the reliability and recovery efficiency of the DC power distribution network are remarkably improved, and powerful support is provided for safe and stable operation of the DC power distribution network.

Description

Technical Field

[0001] The present invention relates to the field of DC distribution systems, and specifically to a DC distribution network fault recovery method and system based on multi-device coordination and timing cooperation. Background Art

[0002] With the development of new energy technologies and the adjustment of the energy structure, flexible DC distribution networks have been widely used due to their advantages such as enabling smooth access of new energy, independent control of active and reactive power, and fast and flexible power transmission. However, flexible DC distribution networks have the characteristics of "low inertia and weak damping". After a fault occurs in the DC line, the fault current rises extremely fast and has a large peak value, and the DC circuit breaker cannot reliably cut off the fault, resulting in the expansion of the fault range and even threatening the stable operation of the entire DC distribution network. In addition, the power supply restoration after the clearance of the flexible DC distribution network line fault is also of great significance to the stable, reliable operation and construction and development of the entire power grid. Summary of the Invention

[0003] Aiming at the deficiencies in the prior art, the present invention provides a DC distribution network fault recovery method and system based on multi-device coordination and timing cooperation. The fault area is quickly detected and isolated by a DC circuit breaker, and according to the location and nature of the fault, the topology structure and operation mode of the DC distribution network are adjusted. After the fault is isolated, the system is restored through the timing cooperation of multiple devices. During the restoration process, by controlling the output voltage and current of the converter, the power supply of the non-fault area can be quickly and effectively restored after the fault occurs, ensuring the stable operation of the system.

[0004] To achieve the above object, the present invention adopts the following technical solutions:

[0005] A DC distribution network fault recovery method based on multi-device coordination and timing cooperation includes the following steps:

[0006] When a fault occurs in the DC distribution network, the fault area is detected and isolated by a DC circuit breaker, and the converter and DC transformer are blocked;

[0007] The DC circuit breaker is reclosed to determine the fault type, and for different fault types, the DC distribution network is restored through the timing cooperation of multiple devices;

[0008] During the restoration process, the output voltage and current of the converter are controlled.

[0009] To optimize the above technical solution, the specific measures taken further include:

[0010] Further, the DC distribution network is a two-terminal DC distribution network, the fault type is an instantaneous fault or a permanent fault, and the fault location is at the medium-voltage bus or the low-voltage bus.

[0011] Furthermore, the DC circuit breaker detecting and isolating the fault area specifically includes:

[0012] If the following starting criterion is detected to be established, a fault occurs, and the DC circuit breaker disconnects to isolate the fault;

[0013] The starting criterion of the DC circuit breaker is shown as the following formula:

[0014] i dc |>I set

[0015] In the formula, i dc is the current of the DC circuit breaker, and I set is the current setting threshold.

[0016] Furthermore, the current setting threshold is 1.1 times the rated current value on the DC side of the converter.

[0017] Furthermore, the reclosing of the DC circuit breaker to determine the type of fault specifically includes:

[0018] If the DC circuit breaker attempts reclosing after deionization and the reclosing of the DC circuit breaker is successful, it is determined that the fault is a transient fault;

[0019] If the DC circuit breaker attempts reclosing after deionization but fails to successfully establish the DC side voltage, it is determined that the fault is a permanent fault.

[0020] Furthermore, the restoration of the DC distribution network through the timing coordination of multiple devices includes:

[0021] For a transient fault, reclosing the DC circuit breakers on both sides of the faulty line, unlocking the converter and the DC transformer, and restoring the power transmission to the DC load;

[0022] For a permanent fault, the AC circuit breaker disconnects, the tie switch operates, cuts off the electrical connection between the fault point and the remaining subnet, switches the control mode of the slave converter to constant DC voltage control, closes the DC circuit breaker, unlocks the converter and the DC transformer, and transmits power to the DC load.

[0023] Furthermore, the restoration of the DC distribution network through the timing coordination of multiple devices also includes:

[0024] When a permanent fault occurs on the medium-voltage bus, the DC distribution network is switched from double-ended power supply operation to single-ended power supply operation through the tie switch;

[0025] If a permanent fault occurs on the medium-voltage bus under open-loop operation, the post-fault recovery strategy is as follows: Isolate the faulty branch. After the only transmission line withdraws due to a permanent fault, the master converter and the DC transformer are blocked. The AC circuit breaker on the master converter side trips cooperatively to cut off the fault, the tie switch at the low-voltage bus is closed, and the slave converter supplies power to the de-energized load on the master converter side through the tie bus;

[0026] If a permanent fault occurs on the medium-voltage bus under closed-loop operation, disconnect the tie switch at the medium-voltage bus to cut off the electrical connection between the fault point and the remaining subnet. After switching the control mode of the slave converter from constant power control to constant DC voltage control, close the DC circuit breaker and unlock the slave converter. After establishing the medium-voltage DC bus voltage, unlock the DC transformer to transmit power to the DC load.

[0027] The present invention also proposes a DC distribution network fault recovery system based on multi-device coordination and timing cooperation, including:

[0028] A DC circuit breaker for detecting and isolating the fault area and determining the fault type through reclosing;

[0029] Converters, including a master converter and a slave converter, which adjust the control mode according to the fault situation;

[0030] A DC transformer for realizing the conversion between different voltage levels;

[0031] A tie switch for adjusting the operation mode of the DC distribution network and isolating the fault area when a fault occurs;

[0032] A control unit for coordinating the timing actions of each device for different fault types to achieve fault recovery and controlling the output voltage and current of the converter during the recovery process.

[0033] The beneficial effects of the present invention are as follows: The present invention provides a DC distribution network fault recovery method and system based on multi-device coordination and timing cooperation. In the case of a permanent fault occurring at the DC grid bus, when the slave converter under constant power control exits operation while the master converter under constant DC voltage control is in operation, it is necessary to promptly change the control mode to become a new balance node of the remaining healthy subnet, ensuring that the system resumes to a new steady state under the new operation mode. At the same time, in order to minimize the power outage area to the greatest extent, the tie switch needs to cooperate in a timely manner to isolate the fault and assist the remaining subnet to resume and restart. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a schematic diagram of the wiring structure of a two-terminal DC distribution network involved in a specific embodiment of the present invention;

[0035] Figure 2It is a timing diagram of fault handling and coordinated recovery of each device for the instantaneous fault at the open-loop f1 in the specific embodiment of the present invention;

[0036] Figure 3 It is the effective values of the medium and low voltage DC bus voltages and DC current on the MMC1 side of the main station converter during the instantaneous fault at the open-loop f1 in the specific embodiment of the present invention;

[0037] Figure 4 It is a timing diagram of fault handling and coordinated recovery of each device for the permanent fault at the open-loop f1 in the specific embodiment of the present invention;

[0038] Figure 5 It is the effective values of the medium voltage DC current and low voltage DC bus voltage on the MMC1 side of the main station converter during the permanent fault at the open-loop f1 in the specific embodiment of the present invention;

[0039] Figure 6 It is the overall flowchart of the DC distribution network fault recovery method based on multi-device coordination and timing cooperation proposed by the present invention. Specific Embodiments

[0040] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present application.

[0041] Embodiment 1

[0042] The present invention proposes a DC distribution network fault recovery method based on multi-device coordination and timing cooperation. The overall process of this method is as Figure 6 shown, and includes the following steps:

[0043] When a fault occurs in the DC distribution network, the fault area is detected and isolated through a DC circuit breaker, and the converter and DC transformer are blocked;

[0044] The DC circuit breaker is reclosed to determine the fault type. For different fault types, the DC distribution network is restored through the timing cooperation of multiple devices;

[0045] During the recovery process, the output voltage and current of the converter are controlled.

[0046] The DC distribution network is a two-terminal DC distribution network, the fault type is an instantaneous fault or a permanent fault, and the fault location is at the medium voltage bus or the low voltage bus.

[0047] Figure 1It is a schematic diagram of the wiring structure of a two-terminal DC distribution network. Among them, the AC / DC converter adopts an MMC, AC1 and AC2 are AC-side circuit breakers, D1 to D14 are hybrid DC circuit breakers, and BK1 and BK2 are the bus tie switches on the medium-voltage side and low-voltage side respectively. The specific system parameters are shown in Table 1. The operation mode is the open-loop operation of the DC distribution network, that is, all the tie switches between the medium-voltage bus and the low-voltage bus on both sides are disconnected. The main station converter MMC1 and the slave station converter MMC2 both adopt constant DC voltage control, and there is no energy exchange between the two converter stations.

[0048] Table 1 Parameters of the two-terminal DC distribution network

[0049]

[0050] The DC circuit breaker detects and isolates the fault area specifically as follows:

[0051] If the following starting criterion is detected to be established, a fault occurs and the DC circuit breaker disconnects to isolate the fault;

[0052] The starting criterion of the DC circuit breaker is shown in the following formula:

[0053] |i dc | > I set

[0054] In the formula, i dc is the current of the DC circuit breaker, and I set is the current setting threshold. The current setting threshold is 1.1 times the rated current value of the DC side of the converter.

[0055] The reclosing of the DC circuit breaker is carried out to determine the type of the fault specifically as follows:

[0056] If the DC circuit breaker attempts to reclose after deionization and the reclosing of the DC circuit breaker is successful, it is determined that the fault is a transient fault;

[0057] If the DC circuit breaker attempts to reclose after deionization but cannot successfully establish the DC side voltage, it is determined that the fault is a permanent fault.

[0058] The restoration of the DC distribution network is achieved through the time-sequence coordination of multiple devices, including:

[0059] For transient faults, the DC circuit breakers on both sides of the faulted line are reclosed, the converters and DC transformers are unlocked, and the power transmission to the DC load is restored;

[0060] For permanent faults, the AC circuit breaker is disconnected, the tie switch operates, the electrical connection between the fault point and the residual subnet is cut off, the control mode of the slave station converter is switched to constant DC voltage control, the DC circuit breaker is closed, the converters and DC transformers are unlocked, and power is transmitted to the DC load.

[0061] When a permanent fault occurs on the medium-voltage bus, the DC distribution network is switched from double-ended power supply operation to single-ended power supply operation through the tie switch;

[0062] If a permanent fault occurs on the medium-voltage bus under open-loop operation, the post-fault recovery strategy is as follows: isolate the fault branch. After the only transmission line withdraws due to a permanent fault, the master converter and the DC transformer are blocked. The AC circuit breaker on the master converter side trips cooperatively to cut off the fault, close the tie switch at the low-voltage bus, and the slave converter supplies power to the de-energized load on the master converter side through the tie bus;

[0063] If a permanent fault occurs on the medium-voltage bus under closed-loop operation, disconnect the tie switch at the medium-voltage bus to cut off the electrical connection between the fault point and the remaining subnet. After switching the control mode of the slave converter from constant power control to constant DC voltage control, close the DC circuit breaker and unlock the slave converter. After establishing the medium-voltage DC bus voltage, unlock the DC transformer to transmit power to the DC load.

[0064] Taking the occurrence of a fault at the typical fault point f1 on the medium-voltage bus as an example, the specific implementation operation of the DC distribution network fault recovery method based on multi-device coordination and timing cooperation is as follows:

[0065] After an instantaneous fault occurs at the typical fault point f1 on the medium-voltage bus under open-loop operation, the protection starts, and the DC circuit breakers D5 and D7 trip to isolate the fault. The converters MMC1 and the DC transformer DCT1 are blocked due to overcurrent caused by the fault. The recovery strategy is as follows: when the fault is cleared, the two DC circuit breakers D5 and D7 deionize and then start to reclose. The main branch is triggered and conducts successfully. Unlock the converter MMC1 to establish the DC side voltage, and unlock the DC transformer DCT1 to resume the DC side power transmission. During the entire fault process, the operation of the MMC2 side is not affected by the fault on the MMC1 side. In the simulation model, a bipolar short-circuit fault with a duration of 0.1 s and a transition resistance of 0.01 Ω occurs on the medium-voltage bus of the MMC1 side at 2.0 s. The specific timing of each device during the fault recovery process is as Figure 2 shown.

[0066] By Figure 2It can be seen that at the initial stage of the fault (0 - t1), electrical quantities such as the arm current in the converter MMC1 and the DC current at the outlet have not exceeded the threshold for the input of current limiting control, and the overcurrent protection in the distribution network has not detected the fault yet, so the DC breakers D5 and D7 do not operate. During the period from t1 to t2, the current limiting control starts to limit the rising rate and peak value of the fault current. The overcurrent protection in the system starts, and the DC breakers D5 and D7 on both sides of the fault line operate to isolate the fault point. The fault detection process takes about 3 ms. At time t3, the arm current in MMC1 continues to rise to twice the rated value, and a blocking command is issued after a delay of 130 μs. The converter and the DC transformer DCT1 are blocked. From t3 to t4 is the deionization time of 295 ms for the DC breaker. At time t4, that is, at 2.3 s, the DC breaker starts to reclose, and the main branch conducts successfully. At time t5, MMC1 is unlocked. When the DC side bus voltage is successfully established at time t7, the DC transformer DCT1 is unlocked. The time from unlocking to reaching the steady state of the system is about 180 ms. The simulation waveforms of the fault recovery of the medium - voltage and low - voltage DC bus voltages and the DC current on the MMC1 side when a transient fault occurs at f1 are as Figure 3 shown.

[0067] When a permanent fault occurs at the medium - voltage bus in the DC distribution network, the system is switched from double - end power supply operation to single - end power supply operation.

[0068] If a permanent fault occurs at the typical fault point f1 under open - loop operation, the post - fault recovery strategy is as follows: After the deionization of the DC breakers D5 and D7, they try to reclose, but the DC side voltage cannot be successfully established. It is determined that the fault is a permanent fault, and then the fault branch is isolated. After the only transmission line withdraws due to the permanent fault, the master converter MMC1 and the DC transformer DCT1 are blocked. Since the half - bridge MMC cannot completely block the discharge from the AC side to the fault point, the AC breaker on the MMC1 side trips cooperatively to cut off the fault. The connection switch BK2 at the low - voltage bus is closed, and the slave converter MMC2 supplies power to the load without power on the MMC1 side through the connection bus, so that the system can resume power supply to the load without power in the non - fault area as much as possible.

[0069] When the DC distribution network is in closed - loop operation, after a permanent fault occurs, the connection switch BK1 at the medium - voltage bus is disconnected to cut off the electrical connection between the fault point and the remaining subnet. After switching the control mode of the slave converter MMC2 from constant - power control to constant - DC - voltage control, the DC breakers D6 and D8 are closed, and the slave converter MMC2 is unlocked. After the medium - voltage DC bus voltage is established, the DC transformer DCT2 is unlocked to transmit power to the DC load.

[0070] In the simulation model, when a permanent fault of 0.01 Ω occurs at the medium - voltage DC bus f1 on the MMC1 side at 2.0 s, the specific time sequence of each device during the recovery process is as Figure 4 shown.

[0071] As can be seen from Figure 4 , when a permanent fault is determined, the MMC1 has a fault overcurrent and locks at 6 ms, i.e., at time t3. About 40 ms later, i.e., at time t4, the AC side breaker AC1 is opened to ensure reliable electrical isolation, facilitating subsequent safety troubleshooting. About 2 ms later, i.e., at time t5, the connection switch BK2 between the low-voltage DC buses is closed. MMC2 transmits power to the central load and commercial load through the DC transformer DCT2 and the connection switch BK2. Since the receiving-end power of the sending-end converter station MMC2 increases, the medium-voltage DC current on the MMC2 side rises slightly, and the DC bus voltage on the low-voltage load side drops slightly. Figure 5 It is the simulation waveform diagram for the recovery of a permanent fault at open-loop f1.

[0072] Embodiment 2

[0073] The present invention provides a DC distribution network fault recovery system corresponding to the method of Embodiment 1 based on multi-device coordination and timing cooperation, including:

[0074] A DC breaker for detecting and isolating the fault area and determining the fault type through reclosing;

[0075] A converter including a master converter and a slave converter, which adjusts the control mode according to the fault situation;

[0076] A DC transformer for realizing the conversion between different voltage levels;

[0077] A connection switch for adjusting the operation mode of the DC distribution network and isolating the fault area when a fault occurs;

[0078] A control unit for coordinating the timing actions of each device for different fault types to achieve fault recovery, and controlling the output voltage and current of the converter during the recovery process.

[0079] The implementation manners of each module and module functions in the system are exactly the same as the steps of the method in Embodiment 1, so details are not described herein again.

[0080] Those of ordinary skill in the art can realize that the units and algorithm steps of each example described in combination with the embodiments disclosed in this application can be implemented by electronic hardware or a combination of computer software and electronic hardware. Whether these functions are executed in a hardware or software manner depends on the specific application and design constraints of the technical solution. Professional technicians can use different methods to implement the described functions for each specific application, but such implementation should not be considered to exceed the scope of this application.

[0081] The above are only the preferred embodiments of the present invention. The protection scope of the present invention is not limited to the above embodiments. All technical solutions falling within the concept of the present invention belong to the protection scope of the present invention. It should be noted that for those of ordinary skill in the art, several improvements and refinements made without departing from the principle of the present invention should be regarded as within the protection scope of the present invention.

Claims

1. A DC distribution network fault recovery method based on multi-device coordination and timing cooperation, characterized in that It includes the following steps: When a fault occurs in the DC distribution network, the DC circuit breaker detects and isolates the fault area, and locks the converter and the DC transformer; The DC circuit breaker is reclosed to determine the fault type. For different fault types, the restoration of the DC distribution network is achieved through the sequential coordination of multiple devices; During the restoration process, the output voltage and current of the converter are controlled.

2. The method for DC distribution network fault recovery based on multi-device coordination and timing cooperation according to claim 1, characterized in that: The DC distribution network is a two-terminal DC distribution network. The fault types are instantaneous faults or permanent faults, and the fault locations are at the medium-voltage bus or the low-voltage bus.

3. The method for DC distribution network fault recovery based on multi-device coordination and timing cooperation according to claim 1, wherein: The specific process of the DC circuit breaker detecting and isolating the fault area is as follows: If the following starting criterion is detected to be established, a fault occurs, and the DC circuit breaker disconnects to isolate the fault; The starting criterion of the DC circuit breaker is shown by the following formula: |i dc |>I set Where, i dc is the current of the DC circuit breaker, and I set is the current setting threshold value.

4. The method for DC distribution network fault recovery based on multi-device coordination and timing cooperation according to claim 3, characterized in that: The set current threshold is 1.1 times the rated current value on the DC side of the converter.

5. The method for DC distribution network fault recovery based on multi-device coordination and timing cooperation according to claim 2, characterized in that The specific process of reclosing the DC circuit breaker to determine the fault type is as follows: If the DC circuit breaker attempts to reclose after deionization and the reclosing is successful, it is determined that the fault is an instantaneous fault; If the DC circuit breaker attempts to reclose after deionization but fails to successfully establish the DC side voltage, it is determined that the fault is a permanent fault.

6. The method for DC distribution network fault recovery based on multi-device coordination and timing cooperation according to claim 2, characterized in that, The restoration of the DC distribution network achieved through the sequential coordination of multiple devices includes: For an instantaneous fault, the DC circuit breakers on both sides of the fault line are reclosed, the converter and the DC transformer are unlocked, and the power transmission to the DC load is restored; For a permanent fault, the AC circuit breaker disconnects, the tie switch operates, the electrical connection between the fault point and the remaining subnet is cut off, the control mode of the slave converter is switched to constant DC voltage control, the DC circuit breaker is closed, the converter and the DC transformer are unlocked, and power is transmitted to the DC load.

7. The method for DC distribution network fault recovery based on multi-device coordination and timing cooperation according to claim 6, characterized in that, The restoration of the DC distribution network achieved through the sequential coordination of multiple devices also includes: When a permanent fault occurs at the medium-voltage bus, the DC distribution network is switched from double-ended power supply operation to single-ended power supply operation through the tie switch; If a permanent fault occurs at the medium-voltage bus under open-loop operation, the post-fault restoration strategy is: isolate the fault branch. After the only transmission line withdraws due to a permanent fault, the master converter and the DC transformer are locked, the AC circuit breaker on the master converter side trips in coordination to cut off the fault, the tie switch at the low-voltage bus is closed, and the slave converter supplies power to the de-energized load on the master converter side through the tie bus; If a permanent fault occurs at the medium-voltage bus under closed-loop operation, the tie switch at the medium-voltage bus is disconnected, the electrical connection between the fault point and the remaining subnet is cut off. After the control mode of the slave converter is switched from constant power control to constant DC voltage control, the DC circuit breaker is closed, and the slave converter is unlocked. After the medium-voltage DC bus voltage is established, the DC transformer is unlocked, and power is transmitted to the DC load.

8. A DC distribution network fault recovery system based on multi-device coordination and timing cooperation, characterized in that, It includes: A DC circuit breaker, which is used to detect and isolate the fault area and determine the fault type through reclosing; A converter, including a master converter and a slave converter, which adjusts the control mode according to the fault situation; A DC transformer, which is used to realize the conversion between different voltage levels; A tie switch, which is used to adjust the operation mode of the DC distribution network during a fault and isolate the fault area; A control unit, which coordinates the timing actions of each device for different fault types to achieve fault recovery and controls the output voltage and current of the converter during the recovery process.

Images