Power distribution network and micro-grid cooperative operation method under hierarchical grouping strategy

Abstract

The invention relates to the technical field of smooth switching between a micro-grid and a power distribution network, and particularly discloses a power distribution network and micro-grid cooperative operation method under a hierarchical grouping strategy, which comprises the following steps of: arranging a group of super capacitors in parallel between a micro-grid and a point of common connection PCC; the output voltage of the super capacitor is consistent with the voltage of the micro-grid in an isolated island operation state or a grid-connected state, and a transformer is arranged at the connecting end of the energy storage equipment and the micro-grid and used for enabling the discharge voltage of the energy storage equipment to be consistent with the output voltage of the micro-grid in the isolated island operation state. According to the technical scheme of the invention, a group of super capacitors are arranged in parallel between the micro-grid and the PCC, and when the micro-grid and the power distribution network are cut off or connected, the super capacitors provide corresponding voltage values to ensure that the micro-grid is cut off and connected with the grid at a stable voltage value, so that transient oscillation during grid-connected and off-grid switching is effectively eliminated, and smooth switching is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of smooth switching between microgrids and distribution networks, and specifically to a method for coordinated operation of distribution networks and microgrids under a hierarchical and grouping strategy. Background Art

[0002] Microgrids have two operating modes: grid-connected and islanded. Under normal circumstances, microgrids are connected to the public distribution network, and their voltage and frequency stability are supported by the public distribution network. When a fault occurs in the distribution network or the microgrid, the common connection point (PCC) between the microgrid and the distribution network will be disconnected immediately, and the microgrid will switch from a grid-connected operation state to an islanded operation state. For most microgrids, the output of distributed power sources generated by renewable resources in the microgrid cannot fully meet the load demand. When the microgrid switches from grid-connected to unplanned islanded operation, power shortages often occur, resulting in the microgrid being unable to maintain voltage and frequency stability. In severe cases, the distributed power sources will be shut down, the load will be cut off, and the entire microgrid will collapse. Studying the control methods and strategies for switching from microgrid grid-connected operation to unplanned island operation and from island operation to grid-connected operation can ensure the smooth transition of the microgrid under different operation modes, which is of great significance to improving the power quality and power supply reliability of the microgrid. When the microgrid is disconnected from the public distribution network, it cannot be guaranteed that the microgrid energy storage power will be replenished to the microgrid in time, resulting in a momentary power loss when the microgrid is disconnected from the public distribution network. The switching is not smooth, which makes it difficult for the microgrid to develop rapidly and needs to be improved. Therefore, we propose a method for the coordinated operation of distribution network and microgrid under a layered and grouped strategy. Summary of the invention

[0003] The purpose of the present invention is to provide a method for coordinated operation of a distribution network and a microgrid under a hierarchical grouping strategy, which solves the problems raised in the background technology.

[0004] To achieve the above-mentioned purpose, the present invention provides the following technical solutions: a method for coordinated operation of a distribution network and a microgrid under a hierarchical grouping strategy, comprising a low-voltage microgrid provided with a variety of new energy equipment, the new energy equipment being connected to an energy storage device, the energy storage device being connected to the microgrid, the low-voltage microgrid and the distribution network being provided with a common connection point PCC, the common connection point PCC between the low-voltage microgrid and the distribution network, the connection end of the energy storage device and the switches of all load branches of the microgrid are all monitored by a microgrid energy management system EMS, the microgrid energy management system EMS determines whether the low-voltage microgrid is in an island operation state or a grid-connected operation state by detecting the open / closed state of the microgrid common connection point PCC, a group of supercapacitors are arranged in parallel between the microgrid and the common connection point PCC, the output voltage of the supercapacitor is consistent with the voltage of the microgrid in an island operation state or a grid-connected state, a transformer is arranged at the connection end of the energy storage device and the microgrid, and is used to make the discharge voltage of the energy storage device consistent with the output voltage of the microgrid in an island operation state, and the specific method is as follows: Switching control from grid-connected operation to unplanned islanding: When it is detected that the common connection point PCC is in a disconnected state, the low-voltage microgrid switches from grid-connected operation to unplanned islanding, and the supercapacitor switches from a floating charge state to a discharge state. The supercapacitor temporarily serves as the main power source for the low-voltage microgrid. The energy storage device is then adjusted to switch from an energy storage state to a discharge state. The transformer transforms the electric energy released by the energy storage device and transmits it to the low-voltage microgrid. The energy storage device serves as the main power source for the low-voltage microgrid, and the supercapacitor switches to a floating charge state. Switching control from unplanned island operation to grid-connected operation: When the common connection point PCC is detected to be in a closed state, the distribution network resumes power distribution, and the microgrid is switched from unplanned island operation to grid-connected operation. Before grid connection, the supercapacitor is pre-synchronized and the output voltage of the low-voltage microgrid and supercapacitor is adjusted to be basically consistent with the public distribution network. The energy storage device is then disconnected from the microgrid, and the energy storage device is switched to the energy storage state. The low-voltage microgrid is then connected to the public distribution network, thereby effectively reducing the impact and oscillation when switching from unplanned island operation to grid-connected operation, ensuring a smooth transition during the switching of operation modes.

[0005] As a preferred implementation scheme of the technical solution of the present application, the microgrid EMS system obtains the PCC point disconnection information through optical fiber communication to determine whether the microgrid is operating in an isolated state. When it is detected that the microgrid is operating in an isolated state, the supercapacitor is immediately switched from a floating charge state to a discharge state, and the energy storage device is switched from a floating charge mode to a V / F controlled discharge mode. In order to minimize the impact of the energy storage device on the microgrid, the reference voltage and reference frequency / phase angle of the V / F control of the energy storage device are the voltage and frequency / phase angle of the public distribution network at the moment the PCC point is disconnected.

[0006] As a preferred implementation of the technical solution of the present application, when switching control from unplanned island operation to grid-connected operation, when it is detected that the public distribution network has resumed power supply, the low-voltage microgrid is adjusted to a suitable state and then reintegrated into the distribution network, that is, the microgrid is switched from the unplanned island operation state to the grid-connected operation, and the switching control strategy is as follows: In order to reduce the impact of grid connection, the voltage output by the low-voltage microgrid and supercapacitor and the phase angle of the low-voltage microgrid are adjusted to be basically consistent with the public distribution network through pre-synchronization control before grid connection; After the grid connection is completed, the energy storage device is immediately switched from the V / F control mode to the floating charge mode, the supercapacitor enters the floating charge state, and the power supply to the load branch is restored.

[0007] As a preferred implementation of the technical solution of the present application, the supercapacitor is an adjustable capacitor, which adjusts the output voltage by changing its capacitance value, and is used to adapt the voltage of the microgrid in an island operation state or a grid-connected state.

[0008] As a preferred implementation of the technical solution of the present application, the voltage level of the low-voltage microgrid is 380V, the new energy equipment is wind power generation system equipment and photovoltaic power generation system equipment, and the direct current generated by the new energy equipment is connected to the energy storage device through an inverter.

[0009] As a preferred implementation scheme of the technical solution of the present application, the energy storage device is a flywheel energy storage system device. Direct current is converted into alternating current through an inverter to drive a flywheel motor to rotate and store energy. When the energy storage device is discharged, the flywheel motor is driven to rotate through the flywheel, converting mechanical energy into electrical energy and transmitting it to the low-voltage microgrid.

[0010] As a preferred implementation of the technical solution of the present application, when the low-voltage microgrid is connected to the grid, the circuit of the new energy equipment adopts PQ control, the energy storage device is in a floating charge state, and the supercapacitor is in a floating charge state.

[0011] As a preferred implementation of the technical solution of the present application, when entering the island operation state, the circuit of the new energy equipment adopts PQ control, the energy storage device serves as the main power supply, and V / F control is adopted to maintain the stability of the voltage and frequency of the microgrid.

[0012] Compared with the prior art, the present invention has the following beneficial effects: The technical solution of the present application is to set up a group of supercapacitors in parallel between the microgrid and the common connection point PCC. When the microgrid is disconnected from or connected to the distribution network, the supercapacitors provide corresponding voltage values ​​to ensure that the microgrid is disconnected and connected to the grid at a stable voltage value, effectively eliminating transient oscillations during switching between on and off the grid and achieving smooth switching. DETAILED DESCRIPTION

[0013] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments.

[0014] Embodiment 1, the present invention provides a technical solution: a method for coordinated operation of a distribution network and a microgrid under a hierarchical grouping strategy, comprising a low-voltage microgrid provided with a variety of new energy equipment, the new energy equipment is connected to an energy storage device, the energy storage device is connected to the microgrid, the low-voltage microgrid and the distribution network are provided with a common connection point PCC, the common connection point PCC of the low-voltage microgrid and the distribution network, the connection end of the energy storage device and the switches of all load branches of the microgrid are all monitored by a microgrid energy management system EMS, the microgrid energy management system EMS determines whether the low-voltage microgrid is in an island operation state or a grid-connected operation state by detecting the open / closed state of the microgrid common connection point PCC, a group of supercapacitors are arranged in parallel between the microgrid and the common connection point PCC, the output voltage of the supercapacitor is consistent with the voltage of the microgrid in an island operation state or a grid-connected state, and a transformer is arranged at the connection end of the energy storage device and the microgrid, which is used to make the discharge voltage of the energy storage device consistent with the output voltage of the microgrid in an island operation state.

[0015] In a specific embodiment of the present invention, the common connection point PCC includes the following main parts: a circuit breaker (in island protection, when a power failure is detected in the grid, the circuit breaker will quickly cut off the connection with the main grid to prevent the potential risks caused by the island operation of the microgrid), a protection device (including a current transformer, a voltage transformer, and a relay. The protection device can realize the integration and coordination of multiple protection functions such as island protection and overcurrent protection), a synchronous grid-connected closing device (when the microgrid needs to be converted from an off-grid operation state to a grid-connected operation state, the synchronous grid-connected closing device will detect the voltage, frequency and other parameters of the microgrid and the main grid to ensure that the two are synchronized before closing the switch, thereby reducing the impact current and voltage fluctuations during the grid connection process and ensuring the stable operation of the system), a measuring and monitoring device (including measuring instruments such as ammeters, voltmeters, and power factor meters for real-time monitoring of electrical parameters and operating status at the PCC), a communication interface and communication equipment (including components such as communication modules, routers, and switches for transmitting information such as measurement data and control instructions, and for realizing communication between the PCC and remote monitoring and management systems such as the microgrid control center and the main grid control center); The steps for remotely controlling the voltage value of the supercapacitor are as follows: Wi-Fi wireless communication technology is used to realize remote adjustment of the adjustable capacitor. A wireless communication module is connected to the adjustable capacitor, and the remote command is received through the module to adjust the capacitance value. According to actual needs, a suitable wireless communication module, such as a Wi-Fi module, is selected. The module is connected to the adjustable capacitor and ensures that the communication between them is normal. The module is connected to the adjustable capacitor and ensures that the communication between them is normal. The parameters of the wireless communication module, such as the IP address, port number, etc., are set to ensure that they can correctly access the network. A control program is written according to actual needs. The program can receive remote commands and adjust the capacitance value of the adjustable capacitor according to the commands. The program can run on a remote terminal or a local controller. The command is sent to the wireless communication module through a remote terminal (such as a computer, a mobile phone, etc.). After receiving the command, the wireless communication module transmits it to the local controller. The local controller adjusts the capacitance value of the adjustable capacitor according to the received command. The monitoring circuit can be used to verify whether the capacitance value has been adjusted correctly. When performing remote adjustment, the security of the system should be ensured to prevent unauthorized access and operation. Switching control from grid-connected operation to unplanned islanding: When the microgrid is normally connected to the distribution network, the supercapacitor can be used as an energy storage device to absorb and store excess electrical energy. When the microgrid is disconnected from the distribution network, the supercapacitor can quickly release the stored electrical energy to compensate for the power shortage in the microgrid and ensure a smooth transition of power. The supercapacitor can quickly respond to power fluctuations in the microgrid and balance power through rapid charging and discharging, thereby avoiding sudden changes in voltage and frequency. The microgrid energy management system EMS monitors the status of the distribution network and the power demand of the microgrid in real time, predicts the power shortage and electrical energy demand when the microgrid is disconnected from the distribution network, and detects that the public connection point PCC is in a disconnected state. The low-voltage microgrid switches from grid-connected operation to an unplanned island, and the supercapacitor switches from a floating charge state to a discharge state. The supercapacitor is temporarily The main power supply of the low-voltage microgrid adjusts the capacitance of the supercapacitor so that the output voltage is consistent with the standard voltage of the microgrid in the island operation mode, and then adjusts the energy storage device to switch from the energy storage state to the discharge state. The transformer transforms the electric energy released by the energy storage device and transmits it to the low-voltage microgrid. The energy storage device serves as the main power supply of the low-voltage microgrid. The supercapacitor is switched to the floating charge state, and the energy storage device is switched from the energy storage mode to the V / F control discharge mode. In order to minimize the impact of the energy storage device on the microgrid, the reference voltage and reference frequency / phase angle of the V / F control of the energy storage device are taken from the voltage and frequency / phase angle of the public distribution network at the moment when the PCC point is disconnected. When the microgrid enters the island operation state, the circuit of the new energy device adopts PQ control, and the energy storage device is used as the main power supply. V / F control is used to maintain the voltage and frequency stability of the microgrid. Switching control from unplanned island operation to grid connection: When the common connection point PCC is detected to be in a closed state, the distribution network resumes power distribution, and the microgrid is switched from unplanned island operation to grid connection operation. Before grid connection, the supercapacitor is pre-synchronized and the voltage output by the low-voltage microgrid and supercapacitor is adjusted to be basically consistent with the public distribution network. Then, the energy storage device is disconnected from the microgrid, and the energy storage device is switched to the energy storage state. Then, the low-voltage microgrid is connected to the public distribution network. In order to reduce the impact of grid connection, the voltage output by the low-voltage microgrid and supercapacitor and the phase angle of the low-voltage microgrid are adjusted to be basically consistent with the public distribution network through pre-synchronization control before grid connection. After the grid connection is completed, the energy storage device is immediately switched from V / F control mode to floating charge mode, the supercapacitor is put into floating charge state, and the power supply to the load branch is restored. When the low-voltage microgrid is connected to the grid, the circuit of the new energy equipment adopts PQ control, the energy storage device is in floating charge state, and the supercapacitor is in floating charge state.

[0016] To sum up: by setting a group of supercapacitors in parallel between the microgrid and the public connection point PCC, when the microgrid is disconnected from or connected to the distribution network, the supercapacitor provides the corresponding voltage value to ensure that the microgrid is disconnected and connected to the grid at a stable voltage value. At the same time, according to the unplanned island operation and grid-connected operation mode, the loop control mode of the energy storage equipment and new energy equipment is adjusted in time to effectively eliminate transient oscillations during switching between on and off the grid and achieve smooth switching.

Claims

1. A method for coordinated operation of a distribution network and a microgrid under a hierarchical grouping strategy, comprising a low-voltage microgrid provided with a variety of new energy equipment, wherein the new energy equipment is connected to an energy storage device, and the energy storage device is connected to the microgrid, and a common connection point PCC is provided between the low-voltage microgrid and the distribution network, and the common connection point PCC between the low-voltage microgrid and the distribution network, the connection end of the energy storage device, and the switches of all load branches of the microgrid are monitored by a microgrid energy management system EMS, and the microgrid energy management system EMS determines whether the low-voltage microgrid is in an island operation state or a grid-connected operation state by detecting the open / closed state of the microgrid common connection point PCC, and is characterized in that: A group of supercapacitors is arranged in parallel between the microgrid and the common connection point PCC, and the output voltage of the supercapacitor is consistent with the voltage of the microgrid in an island operation state or a grid-connected state. A transformer is arranged at the connection end between the energy storage device and the microgrid, which is used to make the discharge voltage of the energy storage device consistent with the output voltage of the microgrid in an island operation state. The specific method is as follows: 1- Switching control from grid-connected operation to unplanned islanding: When it is detected that the common connection point PCC is in a disconnected state, the low-voltage microgrid switches from grid-connected operation to unplanned islanding, and the supercapacitor switches from a floating charge state to a discharge state. The supercapacitor temporarily becomes the main power source of the low-voltage microgrid. Then, the energy storage device is adjusted to switch from an energy storage state to a discharge state. The transformer transforms the electric energy released by the energy storage device and transmits it to the low-voltage microgrid. After the energy storage device becomes the main power source of the low-voltage microgrid, the supercapacitor switches to a floating charge state. 2-Switching control from unplanned island operation to grid-connected operation: When the common connection point PCC is detected to be in a closed state, the distribution network resumes power distribution, and the microgrid is switched from unplanned island operation to grid-connected operation. The supercapacitor is pre-synchronized before grid connection, and the output voltages of the low-voltage microgrid and supercapacitor are adjusted to be basically consistent with the public distribution network. The energy storage device is then disconnected from the microgrid, and the supercapacitor temporarily becomes the main power source of the low-voltage microgrid. The energy storage device is switched to the energy storage state, and then the low-voltage microgrid is connected to the public distribution network, thereby effectively reducing the impact and oscillation when switching from unplanned island operation to grid-connected operation, ensuring a smooth transition during the switching of operation modes.

2. The method for coordinated operation of distribution network and microgrid under the hierarchical grouping strategy according to claim 1 is characterized in that: The microgrid EMS system obtains the PCC point disconnection information through optical fiber communication to determine whether the microgrid is operating in an isolated state. When it is detected that the microgrid is operating in an isolated state, the supercapacitor is immediately switched from a floating charge state to a discharge state, and the energy storage device is switched from a floating charge mode to a V / F control discharge mode. In order to minimize the impact of the energy storage device on the microgrid, the reference voltage and reference frequency / phase angle of the V / F control of the energy storage device are taken as the voltage and frequency / phase angle of the public distribution network at the moment the PCC point is disconnected.

3. The method for coordinated operation of distribution network and microgrid under the hierarchical grouping strategy according to claim 1 is characterized in that: When switching from unplanned island operation to grid-connected operation, when it is detected that the public distribution network has resumed power supply, the low-voltage microgrid is adjusted to a suitable state and then reintegrated into the distribution network, that is, the microgrid is switched from unplanned island operation to grid-connected operation. The switching control strategy is as follows: In order to reduce the impact of grid connection, the voltage output by the low-voltage microgrid and supercapacitor and the phase angle of the low-voltage microgrid are adjusted to be basically consistent with the public distribution network through pre-synchronization control before grid connection; After the grid connection is completed, the energy storage device is immediately switched from the V / F control mode to the floating charge mode, the supercapacitor enters the floating charge state, and the power supply to the load branch is restored.

4. The method for coordinated operation of distribution network and microgrid under the hierarchical grouping strategy according to claim 1 is characterized in that: The supercapacitor is an adjustable capacitor, which can adjust the output voltage by changing its capacitance value, and is used to adapt the voltage of the microgrid in an island operation state or a grid-connected state.

5. The method for coordinated operation of distribution network and microgrid under the hierarchical grouping strategy according to claim 1 is characterized in that: The voltage level of the low-voltage microgrid is 380V, the new energy equipment is a wind power generation system equipment and a photovoltaic power generation system equipment, and the direct current generated by the new energy equipment is connected to the energy storage device through an inverter.

6. The method for coordinated operation of distribution network and microgrid under the hierarchical grouping strategy according to claim 5 is characterized in that: The energy storage device is a flywheel energy storage system device. Direct current is converted into alternating current through an inverter to drive a flywheel motor to rotate and store energy. When the energy storage device is discharged, the flywheel motor is driven to rotate through the flywheel, converting mechanical energy into electrical energy and transmitting it to the low-voltage microgrid.

7. The method for coordinated operation of distribution network and microgrid under the hierarchical grouping strategy according to claim 1 is characterized in that: When the low-voltage microgrid is connected to the grid, the circuit of the new energy equipment adopts PQ control, the energy storage equipment is in a floating charge state, and the supercapacitor is in a floating charge state.

8. The method for coordinated operation of distribution network and microgrid under the hierarchical grouping strategy according to claim 1 is characterized in that: When entering the island operation state, the circuit of the new energy equipment adopts PQ control, the energy storage equipment serves as the main power supply, and V / F control is used to maintain the stability of the microgrid voltage and frequency.