Short-circuit protection method and system for alternating-current and direct-current hybrid micro-grid

Abstract

The invention belongs to the field of short-circuit protection of micro-grids, and particularly relates to a short-circuit protection method and system of an alternating-current and direct-current hybrid micro-grid. The method comprises the following protection steps: when direct current in a direct current bus reaches a blocking action threshold value, on the basis of controlling a converter between a power supply corresponding to the direct current bus and the direct current bus to work in a voltage stabilization mode, blocking connection between the power supply corresponding to the direct current bus and the direct current bus; when the duration for blocking the connection is greater than a set time threshold value and the direct current in the direct current bus is smaller than a blocking action threshold value, the connection is switched on on the basis of controlling the converter to be maintained in a voltage stabilization mode; the blocking action threshold value is smaller than a current value used for triggering short-circuit protection of a device connected with a direct-current bus in the alternating-current and direct-current hybrid micro-grid and larger than the upper limit of the normal current value range of the device in the alternating-current and direct-current hybrid micro-grid, the fault rate of the alternating-current and direct-current hybrid micro-grid system can be effectively reduced, and then the operation reliability of the system is improved.

Description

Technical Field

[0001] The present invention belongs to the field of short-circuit protection of microgrids, and in particular relates to a short-circuit protection method and system for an AC / DC hybrid microgrid. Background Art

[0002] Microgrids are designed to achieve flexible and efficient application of distributed power sources and solve the problem of grid connection of a large number of distributed power sources in various forms. It is a new type of network structure that can fully promote the large-scale access of distributed power sources and renewable energy, and realize highly reliable supply of various energy forms to loads. Microgrids also have the ability of self-control, protection and management, and can operate independently after disconnecting from the external power grid to ensure the power supply of important loads. More and more distributed energy sources such as photovoltaics and wind power are being invested in microgrids. As an important part of smart grids, microgrids have broad development prospects and application potential. With the continuous advancement of technology and continuous policy support, microgrids will play a more important role in future power systems.

[0003] The AC / DC hybrid microgrid contains both an AC bus and a DC bus, which can meet the needs of both AC and DC loads. Compared with ordinary DC microgrids and AC microgrids, it has higher flexibility and reliability, and is a more preferred microgrid system. However, when a DC short circuit fault occurs in the AC / DC hybrid microgrid system, all devices connected to the DC bus will detect the fault, and then all these devices that detect the fault will be shut down. That is, the DC short circuit fault in the AC / DC hybrid microgrid system will trigger the fault protection of each device connected to the DC bus, increasing the failure rate of the system; and it takes a long time to restart and resume operation of the device that has stopped due to the fault, affecting the reliability of the system operation. Summary of the invention

[0004] The purpose of the present invention is to provide a short-circuit protection method and system for an AC / DC hybrid microgrid, which is used to solve the problem in the prior art that when a DC short-circuit fault occurs in an AC / DC hybrid microgrid system, short-circuit protection of a device connected to the DC bus will be caused, resulting in an increased failure rate of the system and affecting the reliability of system operation.

[0005] In order to achieve the above-mentioned purpose, the present invention provides a short-circuit protection method for an AC / DC hybrid microgrid, the method comprising the following protection steps: when the DC current in the DC bus reaches a blocking action threshold, on the basis of controlling the converter between the power supply corresponding to the DC bus and the DC bus to operate in a voltage stabilization mode, the connection between the power supply corresponding to the DC bus and the DC bus is blocked; when the duration of blocking the connection is greater than a set time threshold and the DC current in the DC bus is less than a blocking action threshold, on the basis of controlling the converter to maintain in the voltage stabilization mode, the connection is then connected; the blocking action threshold is less than the current value used to trigger the short-circuit protection of a device connected to the DC bus in the AC / DC hybrid microgrid, and is greater than the upper limit of the normal current value range of the device in the AC / DC hybrid microgrid.

[0006] Furthermore, the method of blocking the connection between the power supply corresponding to the DC bus and the DC bus includes blocking the connection between the DC output of the converter between the power supply corresponding to the DC bus and the DC bus and the DC bus; the method of connecting the connection between the power supply corresponding to the DC bus and the DC bus includes connecting the DC output of the converter between the power supply corresponding to the DC bus and the DC bus and the DC bus.

[0007] Furthermore, if it is determined that a DC bus short-circuit fault occurs and the duration of the DC bus short-circuit fault satisfies a setting that satisfies a maximum fault time allowed for operation, the connection between the power supply corresponding to the DC bus and the DC bus is blocked.

[0008] Furthermore, the DC current in the DC bus in the protection step is DC current information in the DC bus acquired in real time.

[0009] Furthermore, if the DC current in the DC bus is within the normal current value range of the device in the AC / DC hybrid microgrid, the DC bus is determined to be operating normally, and the connection between the power supply corresponding to the DC bus and the DC bus is connected to restore the normal DC voltage supply of the DC bus.

[0010] Furthermore, the method of controlling the converter between the power supply corresponding to the DC bus and the DC bus to operate in a voltage stabilization mode includes: if the converter is currently operating in a voltage stabilization mode, maintaining the current operating mode of the converter; if the converter is currently operating in a power mode, switching the current operating mode of the converter to a voltage stabilization mode.

[0011] As a pioneering invention, the beneficial effects of the short-circuit protection method of the AC / DC hybrid microgrid include: the present invention determines whether the DC current in the DC bus satisfies the condition of being greater than the blocking action threshold, and performs a short-circuit judgment on the DC bus to achieve the corresponding short-circuit protection action; the blocking action threshold does not exceed the protection current value of the device connected to the DC bus in the AC / DC hybrid microgrid, which is equivalent to timely controlling the connection between the power supply corresponding to the DC bus and the DC bus according to the change of the current, so that once the current increases to a level close to the current value that triggers the short-circuit protection of the device connected to the DC bus (that is, the DC current in the DC bus) When the blocking action threshold is reached), the power supply is disconnected to reduce the current. After the power supply is disconnected for a period of time, if the current is successfully reduced to a lower level compared with the current value of the short-circuit protection of the device connected to the DC bus, the power supply is connected to support the DC bus current (that is, the duration of the blocking connection is greater than the set time threshold and the DC current in the DC bus is reduced to less than the blocking action threshold), thereby providing reliable short-circuit current support for the system while ensuring that the current of the bus does not exceed the device protection value, avoiding that when a DC short-circuit fault occurs in the AC / DC hybrid microgrid system, the non-fault devices connected to the DC bus also shut down themselves due to the detection of the fault. Therefore, the present invention can effectively reduce the failure rate of the AC / DC hybrid microgrid system, thereby improving the reliability of system operation.

[0012] The present invention also provides a short-circuit protection system for an AC / DC hybrid microgrid, comprising a processor, wherein the processor is used to execute a computer program to implement the steps of the short-circuit protection method for the AC / DC hybrid microgrid.

[0013] The short-circuit protection system of the AC / DC hybrid microgrid can achieve the same beneficial effects as the short-circuit protection method of the AC / DC hybrid microgrid described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is an application example diagram of an embodiment of a short-circuit protection method for an AC / DC hybrid microgrid;

[0015] Figure 2 It is a schematic diagram of the principle of the blocking module in the embodiment of the short-circuit protection method of the AC / DC hybrid microgrid;

[0016] Figure 3 A fault processing flow chart when a short circuit fault occurs in an embodiment of a short circuit protection method for an AC / DC hybrid microgrid;

[0017] Figure 4 The diagram is a schematic diagram of the on-off of a DC circuit when a short circuit fault occurs in an embodiment of a short circuit protection method for an AC / DC hybrid microgrid. DETAILED DESCRIPTION

[0018] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below with reference to the accompanying drawings and embodiments.

[0019] Embodiment of short-circuit protection method for AC / DC hybrid microgrid

[0020] The present embodiment provides a short-circuit protection method for an AC / DC hybrid microgrid, comprising the following protection steps: when the DC current in the DC bus reaches a blocking action threshold, the connection between the power supply corresponding to the DC bus and the DC bus is blocked on the basis of controlling the converter between the power supply corresponding to the DC bus and the DC bus to operate in a voltage stabilization mode; when the duration of the blocking connection is greater than a set time threshold and the DC current in the DC bus is less than the blocking action threshold, the connection is then made on the basis of controlling the converter to maintain the voltage stabilization mode; the blocking action threshold is less than a current value for triggering short-circuit protection of a device connected to the DC bus in the AC / DC hybrid microgrid, and is greater than an upper limit of a normal current value range of the device in the AC / DC hybrid microgrid.

[0021] Among them, the method of blocking the connection between the power supply corresponding to the DC bus and the DC bus includes blocking the connection between the DC output of the converter between the power supply corresponding to the DC bus and the DC bus and the DC bus; the method of connecting the connection between the power supply corresponding to the DC bus and the DC bus includes connecting the DC output of the converter between the power supply corresponding to the DC bus and the DC bus and the DC bus.

[0022] Specifically, Figure 1 As shown, the AC / DC hybrid microgrid system to which the short-circuit protection method of the AC / DC hybrid microgrid of the present embodiment is applicable includes a DC bus, a power supply corresponding to the DC bus (specifically, it is realized by taking power from the AC power grid through a converter, i.e., a DCAC device), a converter (in this embodiment, a DCAC device), various devices connected to the DC bus, and a blocking module; the blocking module is used to perform the actions of blocking and connecting the line; the AC / DC hybrid microgrid includes two working modes: working mode 1 (i.e., the DCAC device connected to the AC power grid works in a voltage stabilization mode) and working mode 2 (i.e., the DCAC device connected to the AC power grid works in a power mode).

[0023] In this embodiment, Figure 2As shown, it is a schematic diagram of the blocking module in this embodiment, and the blocking module mainly includes a controllable switch device (such as IGBT, etc.) and a current limiting inductor. The blocking module is connected between the DCAC bus and the DC bus in the AC / DC hybrid microgrid, and is used to realize the connection between the DC output of the DCAC and the DC bus segment. The control of the blocking module is mainly to judge the DC current value (which can be realized by the blocking module itself), and according to the result of the comparison between the current value of the DC bus and the action threshold of the blocking module, a blocking pulse is sent to the controllable switch tube to quickly shut down the controllable switch tube in the blocking module (which can be referred to as quickly shutting down the blocking module or pulse blocking the blocking module) to block the connection between the power supply corresponding to the DC bus and the DC bus, and an unlocking pulse is sent to the controllable switch tube to quickly open the controllable switch tube in the blocking module (which can be referred to as quickly opening the blocking module or pulse unlocking the blocking module) to connect the connection between the power supply corresponding to the DC bus and the DC bus.

[0024] On this basis, in this embodiment, the fault handling process corresponding to the above protection steps is as follows: Figure 3 As shown in the figure, when the AC / DC hybrid microgrid operates in working mode 1, if a short circuit fault occurs in the DC bus, the DC bus current will rise rapidly. The DC current in the DC bus is detected and judged first, and then the fast on and off of the blocking module is controlled according to the change of the current. That is, when the current reaches the set blocking module action threshold I dcH In the case of a blocking pulse, the blocking module quickly shuts down by blocking the pulse; at the same time, the pulse blocking time starts. When the timing time (i.e., the duration of the blocking connection) is greater than the set T pwm_lock (i.e. set time threshold) and the DC current in the DC bus is less than the blocking module action threshold I dcH , the blocking module is controlled to be quickly opened through an unlocking pulse, and at this time, short-circuit current support is provided for the DC bus; and in this embodiment, when the AC / DC hybrid microgrid operates in the working mode 1 (that is, the DC-AC device between the power supply corresponding to the DC bus and the DC bus operates in the voltage regulation mode), the blocking module will perform the blocking action.

[0025] When the AC / DC hybrid microgrid operates in working mode 2, if a short circuit occurs in the DC bus, the DC bus current will rise rapidly. The DC current in the DC bus is detected and judged first, and then the blocking module is controlled to be turned on and off quickly according to the change of the current. That is, when the current reaches the set blocking module action threshold I dcH In this case, the DCAC device needs to quickly switch to the voltage regulation mode to stabilize the DC bus (i.e., control the DCAC device to maintain the voltage regulation mode); at the same time, the blocking module quickly shuts down through the blocking pulse, and starts the pulse blocking time timing (i.e., the duration of the blocking connection) greater than the set Tpwm_lock (i.e. set time threshold) and the DC current in the DC bus is less than the blocking module action threshold I dcH , the blocking module is controlled to be quickly opened through an unlocking pulse, and at this time provides short-circuit current support for the DC bus.

[0026] In addition, if it is determined that a DC bus short-circuit fault occurs and the duration of the DC bus short-circuit fault satisfies a setting greater than the maximum fault time allowed for operation, the connection between the power supply corresponding to the DC bus and the DC bus is blocked. In this embodiment, if the DC current in the DC bus is not within the normal current value range of the device in the AC / DC hybrid microgrid, that is, it is determined that a DC bus short-circuit fault occurs, then the fault timing starts at this time, and when the timing time exceeds the maximum fault time allowed for operation of the blocking module, the blocking module will perform pulse blocking (i.e., blocking the connection between the power supply corresponding to the DC bus and the DC bus), which is equivalent to switching the blocking module to a disconnected state, thereby isolating and protecting the fault point from the DCAC device, that is, as long as the short-circuit fault current continues to exist for a long time, the blocking module is disconnected; thereby, it can be avoided that the short-circuit fault in the power grid is not handled for a long time, so that the blocking module may be turned on and off multiple times according to the above-mentioned protection steps, resulting in the continuous fluctuation of the current value in the power grid and the presence of a large current corresponding to the short-circuit fault in the power grid (i.e., the DC current in the DC bus is not within the allowable current value range of the device in the AC / DC hybrid microgrid) for a long total time, etc., which affects the safety of the power grid system.

[0027] The DC current in the DC bus in the above protection step is the DC current information in the DC bus obtained in real time. Specifically, in this embodiment, the real-time adjustment of the fast on and off of the blocking module is performed according to the real-time change of the current; Figure 4 During the short-circuit fault period, the peak section is the period when the blocking module performs pulse unlocking. During this period, the blocking module provides short-circuit current support for the AC / DC hybrid microgrid while ensuring that the current of the bus does not exceed the protection value of the device; the valley section is the period when the blocking module performs pulse blocking. During this period, power supply to the DC bus is stopped to reduce the DC current in the DC bus, thereby effectively avoiding triggering the short-circuit protection of the device connected to the DC bus.

[0028] If the DC current in the DC bus is within the normal current value range of the device in the AC / DC hybrid microgrid, the DC bus is determined to be operating normally, and the connection between the power supply corresponding to the DC bus and the DC bus is connected to restore the normal DC voltage supply of the DC bus.

[0029] Specifically, in this embodiment, after the DC bus fault point is removed, the blocking module judges the state of the DC circuit by judging the real-time value of the DC current. When the DC current value is within the normal current value range of the device, the blocking module will switch to the long-on state and quickly restore the DC bus voltage supply, thereby restoring the normal operation of the entire AC / DC hybrid microgrid.

[0030] Specifically, the method of controlling the DCAC device between the power supply corresponding to the DC bus and the DC bus to work in the voltage stabilization mode includes: if the DCAC device is currently working in the voltage stabilization mode, then the current working mode of the DCAC device is maintained; if the DCAC device is currently working in the power mode, then the current working mode of the DCAC device is switched to the voltage stabilization mode. In this embodiment, it is specifically manifested as follows: when the AC / DC hybrid microgrid operates in working mode 1, the DCAC device connected to the AC grid operates in the voltage stabilization mode, and this working mode has established a stable DC bus voltage and does not need to be changed; when the AC / DC hybrid microgrid operates in working mode 2, this working mode is when the current reaches the set blocking module action threshold I dcH , the DCAC device needs to quickly switch to the voltage stabilization mode to stabilize the DC bus voltage. The switching operation is performed because in the art, it is generally believed that when the DC bus voltage is stable, that is, when the DC-AC device between the power supply corresponding to the DC bus and the DC bus works in the voltage stabilization mode, the blocking module will perform the blocking action; therefore, when the AC / DC hybrid microgrid works in the working mode 2 (that is, the DC-AC device between the power supply corresponding to the DC bus and the DC bus works in the power mode), it is necessary to first control the DC-AC device to switch to the voltage stabilization mode through software to stabilize the DC bus voltage, so that the blocking module can perform the blocking action normally.

[0031] Embodiment of short circuit protection system for AC / DC hybrid microgrid

[0032] This embodiment provides a short-circuit protection system for an AC / DC hybrid microgrid, the system including a processor storing executable program instructions, the executable program instructions being used to implement the short-circuit protection method for the AC / DC hybrid microgrid in the above-mentioned short-circuit protection method embodiment for the AC / DC hybrid microgrid.

[0033] Since the specific working mode and working principle of the short-circuit protection system of the AC / DC hybrid microgrid of this embodiment have been described in detail in the above-mentioned short-circuit protection method embodiment of the AC / DC hybrid microgrid, they will not be repeated here.

[0034] It should be understood that the above specific embodiments of the present invention are only used to illustrate or explain the principles of the present invention, but do not constitute a limitation to the present invention.

Claims

1. A short-circuit protection method for an AC / DC hybrid microgrid, characterized in that: The invention comprises the following protection steps: when the DC current in the DC bus reaches a blocking action threshold, on the basis of controlling the converter between the power supply corresponding to the DC bus and the DC bus to work in a voltage stabilization mode, the connection between the power supply corresponding to the DC bus and the DC bus is blocked; when the duration of blocking the connection is greater than a set time threshold and the DC current in the DC bus is less than a blocking action threshold, on the basis of controlling the converter to maintain in the voltage stabilization mode, the connection is then connected; the blocking action threshold is less than a current value for triggering short-circuit protection of a device connected to the DC bus in an AC / DC hybrid microgrid, and is greater than an upper limit of a normal current value range of a device in the AC / DC hybrid microgrid.

2. The short-circuit protection method for an AC / DC hybrid microgrid according to claim 1, characterized in that: The method of blocking the connection between the power supply corresponding to the DC bus and the DC bus includes blocking the connection between the DC output of the converter between the power supply corresponding to the DC bus and the DC bus and the DC bus; the method of connecting the connection between the power supply corresponding to the DC bus and the DC bus includes connecting the DC output of the converter between the power supply corresponding to the DC bus and the DC bus and the DC bus.

3. The short-circuit protection method for an AC / DC hybrid microgrid according to claim 1 or 2, characterized in that: If it is determined that a DC bus short-circuit fault occurs and the duration of the DC bus short-circuit fault satisfies a setting that satisfies a maximum fault time allowed for operation, the connection between the power supply corresponding to the DC bus and the DC bus is blocked.

4. The short-circuit protection method for an AC / DC hybrid microgrid according to claim 1 or 2, characterized in that: The DC current in the DC bus in the protection step is the DC current information in the DC bus acquired in real time.

5. The short-circuit protection method for an AC / DC hybrid microgrid according to claim 1 or 2, characterized in that: Also includes: If the DC current in the DC bus is within the normal current value range of the device in the AC / DC hybrid microgrid, the DC bus is determined to be operating normally, and the connection between the power supply corresponding to the DC bus and the DC bus is connected to restore the normal DC voltage supply of the DC bus.

6. The short-circuit protection method for an AC / DC hybrid microgrid according to claim 1 or 2, characterized in that: The method of controlling the converter between the power supply corresponding to the DC bus and the DC bus to operate in a voltage stabilization mode includes: if the converter is currently operating in a voltage stabilization mode, maintaining the current operating mode of the converter; if the converter is currently operating in a power mode, switching the current operating mode of the converter to a voltage stabilization mode.

7. A short-circuit protection system for an AC / DC hybrid microgrid, comprising a processor, characterized in that: The processor is used to execute a computer program to implement the steps of the short-circuit protection method for an AC / DC hybrid microgrid as described in any one of claims 1 to 6.

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