Grid-connected and off-grid switching control method of direct-current power distribution system

Abstract

The invention discloses a grid-connected and off-grid switching control method for a direct-current power distribution system, which comprises the following steps: when an STS control system receives an off-grid instruction or detects that the amplitude or frequency of the voltage of a power grid is abnormal, cutting off an electronic switch, separating a converter from the power grid, setting a synchronous grid-connected signal as a high level, and sending the high level to the converter; the converter operates in an off-grid mode after detecting a high level; when the STS control system receives a grid-connected instruction or detects that the grid voltage returns to normal, the STS control system sends a synchronous grid-connected signal to the converter, the converter adjusts the amplitude and phase of the output voltage of the converter according to the amplitude and phase of the grid voltage, and when the STS control system detects that the amplitude and phase difference of the converter and the grid voltage are within a set error range, the STS control system sends the synchronous grid-connected signal to the converter. And setting a synchronous grid-connected signal as a high level, keeping the high level for 15 ms and then setting the low level, closing a power switch, accessing an alternating-current power grid, and performing grid-connected mode operation after the converter detects the low level. According to the method, seamless switching and uninterruptible operation of a load end within 10ms are realized.

Description

Technical Field

[0001] The present invention relates to the control field of direct current power distribution technology, and in particular to a method for controlling on-grid and off-grid switching of a direct current power distribution system. Background Art

[0002] The DC distribution system has three operation modes: grid-connected, off-grid and on-grid and off-grid. The grid-connected operation mode follows the status of the photovoltaic power generation unit on the DC side to switch between charging and discharging in real time, realizing peak shaving and valley filling on the DC side; the off-grid operation mode is disconnected from the grid, and the entire system forms an AC microgrid to achieve stable output of voltage, current and power, and power autonomy within the microgrid; the grid-connected and off-grid operation modes have active and passive grid-connected and off-grid functions; achieving seamless switching between grid-connected and off-grid is a key technical issue of the DC distribution system. The existing grid-connected and off-grid switching schemes simply realize the function of switching between grid-connected mode and off-grid mode, but cannot avoid the fluctuation of voltage and current of the microgrid or even the discontinuous load current during the switching process; or realize the one-way smooth switching from grid-connected mode to off-grid mode, without considering the two-way smooth switching between grid-connected mode and off-grid mode in the case of planned off-grid, unplanned off-grid, planned grid-connected, unplanned grid-connected, etc., thereby reducing the safety, reliability and energy utilization of the microgrid system. Summary of the invention

[0003] The purpose of the present invention is to overcome the deficiencies of the prior art and to provide a method for controlling on-grid and off-grid switching of a DC power distribution system which has a simple control strategy and can achieve seamless switching.

[0004] To achieve the above object, the technical solution adopted by the present invention is: a method for controlling on-grid and off-grid switching of a DC power distribution system, wherein the DC power distribution system includes a DC subnet, an AC subnet, a converter, an STS part and a control system; wherein: The DC subgrid includes photovoltaic power generation units, energy storage batteries, DC loads, DCDC power converters and DC bus; The AC subnet includes the AC power grid and AC load; The STS module includes mechanical switches, electronic switches, and a control system that controls their actions; The control system includes the STS control system and the grid-connected, off-grid and grid-connected and off-grid switching control systems of the energy storage converter; The on-grid and off-grid switching control method includes a planned off-grid control method and an unplanned off-grid control method, wherein: Planned off-grid control methods include: 1.1) When the STS control system in the DC power distribution system receives the off-grid command from the EMS or the backend, it starts to cut off the electronic switch in the STS module to disconnect the converter from the grid. At the same time, the STS control system sets the synchronous grid-connected signal to a high level and sends it to the converter. After the converter detects the high level, it starts to operate in off-grid mode. 1.2) After the STS control system receives the grid-connection instruction, it sends a synchronous grid-connection signal to the converter. The synchronous grid-connection signal is a square wave signal with the grid frequency as the period. The converter adjusts the amplitude and phase of its output voltage through the phase pre-synchronization function according to the amplitude and phase of the grid voltage. The phase pre-synchronization function is to perform closed-loop control on the amplitude and phase of the grid voltage and the amplitude and phase of the converter output voltage to achieve synchronization of the amplitude and phase of the converter output voltage with the amplitude and phase of the grid. When the STS control system detects that the amplitude and phase difference between the converter output voltage and the grid voltage is within the set error range, the synchronous grid-connection signal is set to a high level, which lasts for 15ms and then is set to a low level. At the same time, the power switch in the STS module is controlled to close, and the AC grid is connected. When the converter detects that the synchronous grid-connection signal is continuously at a high level for 15ms, it considers that the synchronous grid-connection is completed. After detecting the low level, it starts to operate in the grid-connection mode. Unplanned off-grid control methods include: 2.1) When the STS control system in the DC power distribution system detects that the voltage amplitude or frequency of the grid is abnormal, it starts to cut off the electronic switch in the STS module to disconnect the converter from the grid. At the same time, the STS control system sets the synchronous grid-connected signal to a high level and sends it to the converter, and the converter starts to operate in off-grid mode; 2.2) When the STS control system detects that the grid voltage has returned to normal, it starts to send a synchronous grid-connected signal to the converter. The converter adjusts the amplitude and phase of its own output voltage through the phase pre-synchronization function according to the amplitude and phase of the grid voltage. The phase pre-synchronization function is to perform closed-loop control on the amplitude and phase of the grid voltage and the amplitude and phase of the converter output voltage to achieve synchronization of the amplitude and phase of the converter output voltage with the amplitude and phase of the grid. When the STS control system detects that the amplitude and phase difference between the converter output voltage and the grid voltage is within the set error range, the synchronous grid-connected signal is kept at a high level for 15ms and then set to a low level. At the same time, the power switch in the STS module is controlled to be closed, and the AC grid is connected. When the converter detects that the synchronous grid-connected signal is kept at a high level for 15ms, it considers that the synchronous grid-connected is completed. After detecting the low level, it starts to operate in the grid-connected mode.

[0005] As a specific implementation method, the grid-connected operation mode is divided into a voltage stabilization mode and a PQ mode, wherein: The voltage stabilization mode uses PI control to perform closed-loop control on the DC bus voltage, and the parallel current sharing between multiple modules is achieved by DC droop control. The PQ mode uses PI control to perform closed-loop control on active power P and reactive power Q respectively.

[0006] As a specific implementation method, the off-grid mode adopts repeated P control to perform closed-loop control of the output voltage, and adopts PI control to perform active amplitude modulation and reactive frequency modulation strategies to suppress the circulating current problem between the voltage source parallel modules. The off-grid mode voltage loop adopts three-phase AC control, and for the three-phase unbalanced load, it will stably output the three-phase balanced voltage to supply the normal operation of the load end; the current loop adopts three-phase AC P control.

[0007] As a specific implementation mode, in the voltage stabilization mode, the converter performs voltage stabilization control on the DC bus and sets the voltage band of the matching system. If the bus voltage on the DC subgrid side is lower than the lower limit of the voltage band, it is considered that the DC bus in the DC subgrid is insufficient to support the DC load. At this time, the converter starts rectification and voltage stabilization charging until the bus voltage reaches the lower limit of the voltage band, stabilizing the DC bus voltage to provide a stable DC voltage for the DC load; if the bus voltage on the DC subgrid side is higher than the upper limit of the voltage band, it is considered that the DC bus in the DC subgrid is too high. At this time, the converter starts to discharge to the grid inverter, pulls the bus voltage to the upper limit of the voltage band, and provides a stable DC voltage for the DC load; if the bus voltage on the DC subgrid side is within the voltage band, the converter does not charge or discharge, and the DC subgrid provides a stable DC voltage for the DC load.

[0008] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art: The grid-connected operation of the converter in the DC distribution system reduces the peak load and fills the valley load of the DC power consumption, thus improving the utilization rate of electric energy; the seamless switching between planned off-grid, unplanned off-grid, planned grid connection and unplanned grid connection forms an independent microgrid, ensuring the stable output of voltage, current and power, thus improving the reliability and safety of the power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 is a system block diagram of the DC power distribution system described in the present invention; Figure 2 This is a control block diagram for switching between grid and off-grid as described in the present invention; Figure 3 This is a control block diagram of switching from off-grid mode to grid-connected mode in the present invention; Figure 4 This is a logic timing diagram of the on-grid and off-grid switching described in the present invention. DETAILED DESCRIPTION

[0010] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and specific embodiments.

[0011] The present invention provides a method for controlling on-grid and off-grid switching of a DC power distribution system. Figure 1 As shown, the DC power distribution system includes a DC subnet, an AC subnet, a converter, an STS part and a control system; among them, The DC subgrid includes photovoltaic power generation units, energy storage batteries, DC loads, DCDC power converters and DC bus; The AC subnet includes the AC power grid and AC load; The STS module includes mechanical switches, electronic switches, and a control system that controls their actions; The control system includes the STS control system and the energy storage converter grid-connected, off-grid and grid-connected and off-grid switching control systems.

[0012] See also Figure 2-4 As shown, in the present invention, the on-grid and off-grid switching control method includes a planned off-grid control method and an unplanned off-grid control method, wherein: Planned off-grid control methods include: 1.1) When the STS control system in the DC power distribution system receives the off-grid command from the EMS or the backend, it starts to cut off the electronic switch in the STS module to disconnect the converter from the grid. At the same time, the STS control system sets the synchronous grid-connected signal to a high level and sends it to the converter. After the converter detects the high level, it starts to operate in off-grid mode. 1.2) After the STS control system receives the grid-connection instruction, it sends a synchronous grid-connection signal to the converter. The synchronous grid-connection signal is a square wave signal with the grid frequency as the period. The converter adjusts the amplitude and phase of its output voltage through the phase pre-synchronization function according to the amplitude and phase of the grid voltage. The phase pre-synchronization function is to perform closed-loop control on the amplitude and phase of the grid voltage and the amplitude and phase of the converter output voltage to achieve synchronization of the amplitude and phase of the converter output voltage with the amplitude and phase of the grid. When the STS control system detects that the amplitude and phase difference between the converter output voltage and the grid voltage is within the set error range, the synchronous grid-connection signal is set to a high level, which lasts for 15ms and then is set to a low level. At the same time, the power switch in the STS module is controlled to close, and the AC grid is connected. When the converter detects that the synchronous grid-connection signal is continuously at a high level for 15ms, it considers that the synchronous grid-connection is completed. After detecting the low level, it starts to operate in the grid-connection mode. Unplanned off-grid control methods include: 2.1) When the STS control system in the DC power distribution system detects that the voltage amplitude or frequency of the grid is abnormal, it starts to cut off the electronic switch in the STS module to disconnect the converter from the grid. At the same time, the STS control system sets the synchronous grid-connected signal to a high level and sends it to the converter, and the converter starts to operate in off-grid mode; 2.2) When the STS control system detects that the grid voltage has returned to normal, it starts to send a synchronous grid-connected signal to the converter. The converter adjusts the amplitude and phase of its own output voltage through the phase pre-synchronization function according to the amplitude and phase of the grid voltage. The phase pre-synchronization function is to perform closed-loop control on the amplitude and phase of the grid voltage and the amplitude and phase of the converter output voltage to achieve synchronization of the amplitude and phase of the converter output voltage with the amplitude and phase of the grid. When the STS control system detects that the amplitude and phase difference between the converter output voltage and the grid voltage is within the set error range, the synchronous grid-connected signal is kept at a high level for 15ms and then set to a low level. At the same time, the power switch in the STS module is controlled to be closed, and the AC grid is connected. When the converter detects that the synchronous grid-connected signal is kept at a high level for 15ms, it considers that the synchronous grid-connected is completed. After detecting the low level, it starts to operate in the grid-connected mode.

[0013] The above-mentioned on-grid and off-grid switching scheme can achieve seamless switching of the load side within <10ms, thus achieving uninterrupted power supply operation on the load side.

[0014] Here, the grid-connected operation mode is divided into voltage stabilization mode and PQ mode.

[0015] Among them, the voltage stabilization mode is that the converter itself performs voltage stabilization control on the DC bus. The voltage stabilization mode uses PI control to perform closed-loop control on the DC bus voltage, and the parallel current sharing between multiple modules is realized by DC droop control. Specifically, the voltage band of the matching system is set. If the bus voltage on the DC subnet side is lower than the lower limit of the voltage band, it is considered that the DC bus in the DC subnet is insufficient to support the DC load. At this time, the converter starts to rectify and stabilize the charging until the bus voltage reaches the lower limit of the voltage band, stabilizing the DC bus voltage to provide a stable DC voltage for the DC load; if the bus voltage on the DC subnet side is higher than the upper limit of the voltage band, it is considered that the DC bus in the DC subnet is too high. At this time, the converter starts to discharge to the grid inverter, pulls the bus voltage to the upper limit of the voltage band, and provides a stable DC voltage for the DC load; if the bus voltage on the DC subnet side is within the voltage band, the converter does not charge or discharge, and the DC subnet provides a stable DC voltage for the DC load.

[0016] The PQ mode uses PI control to perform closed-loop control of active power P and reactive power Q respectively. Specifically, it is controlled by the EMS system or background to achieve balanced load, limit power consumption, limit power generation, and peak and frequency regulation within the voltage band. In the constant power mode, if power mutual assistance is not required at present, it can be directly switched to the voltage stabilization mode according to specific conditions, or it can be switched from the voltage stabilization mode to the PQ mode.

[0017] The DC power distribution system has a continuous and stable DC voltage, so it can operate completely independent of the power grid. The entire system forms an AC microgrid, achieving stable output of voltage, current and power, and power autonomy within the microgrid.

[0018] In off-grid mode, repeated P control is used to perform closed-loop control of the output voltage, and PI control is used for active amplitude modulation and reactive frequency modulation strategies to suppress the circulating current problem between voltage source parallel modules. In off-grid mode, the voltage loop adopts three-phase AC control, and for three-phase unbalanced loads, it will stably output three-phase balanced voltage for normal operation of the load end; the current loop adopts three-phase AC P control, and the inner loop can continue to work stably when the outer loop is switched, and seamless switching can be achieved for both three-phase balanced and three-phase unbalanced systems.

[0019] In the DC power distribution system using the above-mentioned on-grid and off-grid control method, the switching of each operating mode of the converter can be achieved by switching the outer loop, such as Figure 2 As shown, when the grid-connected voltage stabilization mode is in operation, the switch k=1 is closed, and the outer loop is the DC bus voltage outer loop and the DC droop control loop; when the grid-connected PQ mode is in operation, the switch k=2 is closed, and the outer loop is the active and reactive power control loop; when the grid-connected and off-grid mode is in operation, the switch k=0 is closed, and the outer loop is the AC output voltage loop and the active amplitude modulation and reactive frequency modulation loop.

[0020] The on-grid and off-grid switching control strategy of the present invention takes into account the working condition of unbalanced three-phase voltage. The converter in the DC power distribution system realizes peak shaving and valley filling of DC power consumption, and can achieve seamless switching of planned off-grid, unplanned off-grid, planned on-grid and unplanned on-grid, forming an independent microgrid and ensuring stable output of voltage, current and power.

[0021] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with the technology to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for controlling on-grid and off-grid switching of a DC power distribution system, characterized in that: The DC power distribution system includes DC subnet, AC subnet, converter, STS part and control system; among them, The DC subgrid includes photovoltaic power generation units, energy storage batteries, DC loads, DCDC power converters and DC bus; The AC subnet includes the AC power grid and AC load; The STS module includes mechanical switches, electronic switches, and a control system that controls their actions; The control system includes the STS control system and the grid-connected, off-grid and grid-connected and off-grid switching control systems of the energy storage converter; The on-grid and off-grid switching control method includes a planned off-grid control method and an unplanned off-grid control method, wherein: Planned off-grid control methods include: 1.1) When the STS control system in the DC power distribution system receives the off-grid command from the EMS or the backend, it starts to cut off the electronic switch in the STS module to disconnect the converter from the grid. At the same time, the STS control system sets the synchronous grid-connected signal to a high level and sends it to the converter. After the converter detects the high level, it starts to operate in off-grid mode. 1.2) After the STS control system receives the grid-connection instruction, it sends a synchronous grid-connection signal to the converter. The synchronous grid-connection signal is a square wave signal with the grid frequency as the period. The converter adjusts the amplitude and phase of its output voltage through the phase pre-synchronization function according to the amplitude and phase of the grid voltage. The phase pre-synchronization function is to perform closed-loop control on the amplitude and phase of the grid voltage and the amplitude and phase of the converter output voltage to achieve synchronization of the amplitude and phase of the converter output voltage with the amplitude and phase of the grid. When the STS control system detects that the amplitude and phase difference between the converter output voltage and the grid voltage is within the set error range, the synchronous grid-connection signal is set to a high level, which lasts for 15ms and then is set to a low level. At the same time, the power switch in the STS module is controlled to close, and the AC grid is connected. When the converter detects that the synchronous grid-connection signal is continuously at a high level for 15ms, it considers that the synchronous grid-connection is completed. After detecting the low level, it starts to operate in the grid-connection mode. Unplanned off-grid control methods include: 2.1) When the STS control system in the DC power distribution system detects that the voltage amplitude or frequency of the grid is abnormal, it starts to cut off the electronic switch in the STS module to disconnect the converter from the grid. At the same time, the STS control system sets the synchronous grid-connected signal to a high level and sends it to the converter, and the converter starts to operate in off-grid mode; 2.2) When the STS control system detects that the grid voltage has returned to normal, it starts to send a synchronous grid-connected signal to the converter. The converter adjusts the amplitude and phase of its own output voltage through the phase pre-synchronization function according to the amplitude and phase of the grid voltage. The phase pre-synchronization function is to perform closed-loop control on the amplitude and phase of the grid voltage and the amplitude and phase of the converter output voltage to achieve synchronization of the amplitude and phase of the converter output voltage with the amplitude and phase of the grid. When the STS control system detects that the amplitude and phase difference between the converter output voltage and the grid voltage is within the set error range, the synchronous grid-connected signal is kept at a high level for 15ms and then set to a low level. At the same time, the power switch in the STS module is controlled to be closed, and the AC grid is connected. When the converter detects that the synchronous grid-connected signal is kept at a high level for 15ms, it considers that the synchronous grid-connected is completed. After detecting the low level, it starts to operate in the grid-connected mode.

2. The method for controlling on-grid and off-grid switching of a DC power distribution system according to claim 1, characterized in that: The grid-connected operation mode is divided into voltage stabilization mode and PQ mode, among which: The voltage stabilization mode uses PI control to perform closed-loop control on the DC bus voltage, and the parallel current sharing between multiple modules is achieved by DC droop control. The PQ mode uses PI control to perform closed-loop control on active power P and reactive power Q respectively.

3. The method for controlling on-grid and off-grid switching of a DC power distribution system according to claim 1, characterized in that: In off-grid mode, repeated P control is used to perform closed-loop control of the output voltage, and PI control is used for active amplitude modulation and reactive frequency modulation strategies to suppress the circulating current problem between voltage source parallel modules. In off-grid mode, the voltage loop adopts three-phase AC control, and for three-phase unbalanced loads, it will stably output three-phase balanced voltage to supply normal operation of the load end; the current loop adopts three-phase AC P control.

4. The method for controlling on-grid and off-grid switching of a DC power distribution system according to claim 2, characterized in that: In the voltage regulation mode, the converter controls the voltage regulation of the DC bus and sets the voltage band of the matching system. If the bus voltage on the DC subgrid side is lower than the lower limit of the voltage band, it is considered that the DC bus in the DC subgrid is insufficient to support the DC load. At this time, the converter starts rectification and voltage regulation charging until the bus voltage reaches the lower limit of the voltage band, stabilizing the DC bus voltage to provide a stable DC voltage for the DC load; if the bus voltage on the DC subgrid side is higher than the upper limit of the voltage band, it is considered that the DC bus in the DC subgrid is too high. At this time, the converter starts to discharge to the grid, pull the bus voltage to the upper limit of the voltage band, and provide a stable DC voltage for the DC load; if the bus voltage on the DC subgrid side is within the voltage band, the converter does not charge or discharge, and the DC subgrid provides a stable DC voltage for the DC load.

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