A black start method of optical storage micro-grid based on virtual oscillator control

Abstract

The application discloses a black start method of a light storage micro-grid based on virtual oscillator control, and the method comprises the following steps: step one, constructing a micro-grid based on black start, connecting the micro-grid based on black start to a 220kV large grid bus through a grid-connected circuit breaker, and introducing an improved V / f control method to control the black start of the micro-grid; step two, introducing virtual oscillator control to perform parallel self-synchronization control on inverters of photovoltaic units in the multiple micro-grids based on black start in parallel connection. The application has the beneficial effect of reducing circulating current generation and solving the problem of large start-up impact caused by a traditional grid-following control strategy.

Description

Technical Field

[0001] This invention relates to the field of microgrid technology, and in particular to a black-start method for a photovoltaic-storage microgrid based on virtual oscillator control. Background Technology

[0002] With the continuous development of society, the problems of energy crisis and environmental pollution are becoming increasingly prominent, and the development and application of microgrids have become a hot topic in green energy research. The comprehensive utilization of clean, efficient, and environmentally friendly renewable energy sources is an important way to achieve the "dual-carbon" strategic goal. As renewable energy power plants transition from centralized grid connection to distributed grid connection, after necessary or unnecessary power outages, distributed grids are required to achieve rapid regional restoration and ensure power supply to necessary regional loads in the shortest possible time. This requires effective control and smooth transition of the black start (BS) process. Traditional power system restoration methods mainly rely on traditional power generation, and the black start method is not in line with the development trend of green energy.

[0003] Stable control strategies for bus voltage and frequency during the black start process of a microgrid are crucial for ensuring its effective operation. To address the issue of parallel self-synchronization of multiple different inverters in a microgrid, the control method commonly employed by those skilled in the art relies on grid-connected phase-locked loops (PLLs). However, during black start, the grid strength is low, and strong coupling exists between the PLL and the grid impedance, severely reducing the small-disturbance stability of the grid-connected converter and failing to meet the requirements for microgrid black start. Summary of the Invention

[0004] The purpose of this invention is to solve the above-mentioned problems by designing a black-start method for a photovoltaic-storage microgrid based on virtual oscillator control.

[0005] The technical solution of the present invention to achieve the above objectives is a black-start method for a photovoltaic-storage microgrid based on virtual oscillator control, which includes the following steps:

[0006] Step 1: Construct a microgrid based on black start. Connect the microgrid based on black start to the 220 kV main grid bus through a grid-connected circuit breaker. Introduce an improved V / f control method to control the black start of the microgrid.

[0007] Step two involves introducing a virtual oscillator to perform parallel self-synchronization control on the inverters of the photovoltaic units in the parallel black-start microgrid.

[0008] The microgrid in step one utilizes energy storage units to ensure rapid voltage boost at the grid connection point. The energy storage units serve as the main power source for the microgrid's black start process. Necessary and secondary loads are set up in the microgrid, and other units are connected in parallel to the 35 kV small system bus. The 35 kV small system bus is connected to the 220 kV main grid bus through a grid-connected circuit breaker. The grid-connected circuit breaker is always disconnected during the black start process.

[0009] The microgrid black start process in step one includes:

[0010] 1) When there is a large-scale power outage, the new energy photovoltaic and energy storage power station is disconnected from the main power grid, all loads are cut off, and the microgrid's main controller issues a black start command;

[0011] 2) Connect to important loads, switch the energy storage unit as the main power source for black start, start the main power source for black start, and provide voltage and frequency support for the entire microgrid. After power supply to the microgrid begins, connect the photovoltaic unit as the power support for the operation of the microgrid.

[0012] 3) The bus voltage of the small system is boosted to 35KV and connected to the secondary load. The energy storage unit and photovoltaic unit supply the power required by the load and stabilize the black-start power unit.

[0013] The improved V / f control method in step one is as follows: A voltage-current negative feedback dual closed-loop system is used for stable control of the energy storage unit, and the target value of the black start bus voltage is controlled based on time-varying changes. e dref Achieving a uniform rise in microgrid bus voltage; based on energy storage unit V / f control, clamping the microgrid bus voltage, with output power varying with equivalent load, neglecting microgrid line losses, and ensuring active power balance:

[0014] (1)

[0015] In the formula: P PV and P bat They provide power to the photovoltaic unit and the energy storage unit, respectively. P main and P sec For both critical and secondary loads of the microgrid, U This is the microgrid bus voltage. R The equivalent resistance of the microgrid. X Equivalent inductive reactance of microgrid;

[0016] Maintaining the microgrid bus voltage constant, achieving microgrid power balance based on load changes, ensuring necessary load demand through photovoltaic unit output, and determining secondary load access based on the microgrid's renewable energy generation situation.

[0017] In step two, the parallel self-synchronization control process of the inverter of the photovoltaic unit by the virtual oscillator is as follows: Utilizing the spontaneous synchronizing characteristic of two parallel oscillating circuits, the voltage change continues until the voltage difference between the two ports reaches the reference phase voltage phase difference threshold, thus reducing the circulating current and achieving self-synchronization of the virtual oscillator's output voltage. A negative feedback dual-loop control module is used to improve the power factor and anti-interference capability of the photovoltaic unit. The voltage reference value of the dual-loop control module is given by the virtual oscillator based on the sinusoidal voltage output of the energy storage inverter's output port. The current at the A-phase port of the inverter output is collected and self-synchronized by the virtual oscillator. The first photovoltaic unit outputs a single-phase sine wave containing the frequency and phase of the first photovoltaic unit. After phase shifting, it outputs a three-phase sine reference voltage. After coordinate transformation, the d-axis voltage is taken as the voltage outer loop input. The first photovoltaic unit is based on a phase-locked loop structure and is connected to the grid in a traditional way. When the grid connection signal of the second photovoltaic unit is received, the output of the virtual synchronization control loop, which contains the sine wave containing the frequency and phase of the first photovoltaic unit, is used as the voltage outer loop input. At the same time, the voltage outer loop input is set to 0. The phase difference of the current output by the two photovoltaic unit inverters is compared. When the tracking accuracy requirement is met, a self-synchronization flag is issued.

[0018] The virtual oscillation circuit must simultaneously meet the conditions for stable oscillation of the circuit system and the sufficient condition for output voltage synchronization. The design parameters and the conditions to be met are as follows:

[0019] (6)

[0020] In the formula, ω sw It is the resonant angular frequency; f sw This refers to the inverter switching frequency; g s The conductivity of the controlled source of the virtual oscillator; K t , K u These are the proportional coefficients for the sampling current and the sampling voltage, respectively.

[0021] Beneficial effects

[0022] A black-start method for a photovoltaic-storage microgrid based on virtual oscillator control, using the technical solution of this invention, has the following advantages:

[0023] 1. This method uses the energy storage unit as the main power source for black start of the power plant. Compared with wind power or photovoltaic units, the output of the energy storage unit is relatively stable. At the same time, the photovoltaic unit is selected as a non-black start power source to provide power support for the subsequent operation of the power plant. In addition, this method introduces improved V / f control, which uses the energy storage unit to ensure the rapid voltage boost at the grid connection point, exhibiting voltage source characteristics and having voltage and frequency regulation capabilities. Furthermore, compared with the traditional black start method, this method does not require the participation of an external power source, and the control method is more flexible.

[0024] 2. This method proposes a self-synchronization control process for grid-connected photovoltaic inverters based on virtual oscillators, which reduces the requirements of the system's self-synchronization on the tracking capability of the phase-locked loop, and reduces the impact of parameter deviations between inverters in microgrid photovoltaic units and inverter circulating currents on the microgrid. Attached Figure Description

[0025] Figure 1 This is a flowchart of a black start method for a photovoltaic-storage microgrid based on virtual oscillator control, as described in this invention.

[0026] Figure 2 This is a schematic diagram of the topology and power supply system of the wind and solar energy storage station described in this invention;

[0027] Figure 3 This is a schematic diagram of the structure of the photovoltaic-storage microgrid described in this invention;

[0028] Figure 4 This is a flowchart of the microgrid black start described in this invention;

[0029] Figure 5 This is the V / f control diagram of the energy storage unit described in this invention;

[0030] Figure 6 This is a schematic diagram of the structure of the virtual oscillator described in this invention;

[0031] Figure 7 This is the parallel equivalent circuit diagram of the virtual oscillator described in this invention;

[0032] Figure 8 This is a schematic diagram of the self-synchronization control principle of a photovoltaic inverter based on a virtual oscillator according to the present invention, wherein (a) is a self-synchronization module of a photovoltaic inverter based on a virtual oscillator, and (b) is a self-synchronization control structure of a photovoltaic inverter based on a virtual oscillator. Detailed Implementation

[0033] The present invention will now be described in detail with reference to the accompanying drawings, such as... Figure 1 As shown;

[0034] A certain wind-solar-energy storage power station adopts an AC grid structure including photovoltaic units, wind power units, energy storage units, 35 kV AC busbars, and 220 kV AC busbars. Multiple 35 kV AC busbars are connected to the 220 kV mains power grid transmission lines via transformers and circuit breakers, operating in parallel. Its structure is as follows: Figure 2 As shown.

[0035] The station requires that the necessary loads of the wind, solar and energy storage power station, such as lighting, charging screens and socket boxes, about 250 kW, be kept powered without interruption, and that the secondary loads, such as living lighting and air conditioning, about 250 kW, be powered as much as possible.

[0036] To address both necessary and unnecessary power outages, the main power supply for black-start microgrids must meet the following requirements: 1) Fast start-up speed and sufficient power generation capacity; 2) Self-starting through a self-powered auxiliary system to establish and maintain voltage amplitude and frequency in the microgrid; 3) Ability to operate in voltage control or current control mode and withstand the impact of non-black-start main power supplies and load grid connection.

[0037] In renewable energy power plants, an improved V / f control strategy is introduced. The energy storage unit can ensure rapid voltage boost at the grid connection point, exhibiting voltage source characteristics and possessing voltage and frequency regulation capabilities. Furthermore, compared to traditional black-start methods, no external power supply is required, resulting in more flexible control. Therefore, this method uses the energy storage unit as the main power source for black-start power plants. Compared to wind power, photovoltaic units have relatively stable outputs; therefore, photovoltaic units are selected as the non-black-start power source to provide power support for subsequent operation of the power plant.

[0038] Designing microgrid structures that integrate energy storage and photovoltaic units for black start, such as... Figure 3 As shown. Based on the necessary and secondary loads within the station, each unit is connected in parallel to the 35 kV small system bus; the 35 kV small system bus is connected to the 220 kV main grid bus through a grid-connected circuit breaker, and the grid-connected circuit breaker is always disconnected during black start-up.

[0039] Black start in a microgrid establishes the microgrid system voltage when the output voltage is zero, thereby starting other power sources in the system that do not have black start functionality and gradually expanding the microgrid system's power generation capacity.

[0040] A microgrid's black start and zero-start voltage boosting requires bus voltage boosting and maintenance, grid connection of renewable energy units, and stable load power supply. Control strategies include... Figure 4 As shown.

[0041] The black start operation process of a microgrid is as follows:

[0042] 1) When there is a large-scale power outage, the new energy photovoltaic and energy storage power station is disconnected from the main power grid, all loads are cut off, and the main controller of the photovoltaic and energy storage microgrid issues a black start command;

[0043] 2) Connect to important loads, switch to black-start micro-source, start the black-start main power energy storage unit, undertake the voltage and frequency support of the entire microgrid, and after the power supply to the system begins, connect the photovoltaic unit to provide power support for grid operation;

[0044] 3) The bus voltage is boosted to 35KV and connected to the secondary load. The energy storage unit and the photovoltaic unit can supply the power required by the load, and the frequency variation is controlled within an acceptable range to stabilize the black-start power unit.

[0045] Based on the necessary and secondary load capacities, the SOC of the black-start main power energy storage unit is greater than 80%.

[0046] Considering the problems caused by the untimely voltage drop of the microgrid bus due to capacitive devices in the system, which lead to instantaneous voltage surges, a variable parameter d-axis reference value is designed to increase linearly with time, replacing the fixed d-axis reference value and improving the traditional V / f control method.

[0047] A dual closed-loop system with voltage and current negative feedback is used for stable control of the energy storage unit, such as... Figure 5 As shown.

[0048] After two PI stages, the inverter output voltage and current track the V / f control input reference value; in order to obtain a larger active power output, the q-axis reference value is set to 0; based on time, the black start bus voltage control target value d-axis reference value is changed to achieve a uniform rise in the microgrid bus voltage.

[0049] Based on V / f control of the energy storage unit, the microgrid bus voltage is clamped, and the output power varies with the equivalent load. Ignoring microgrid line losses, the active power balance satisfies:

[0050] (1)

[0051] In the formula: P PV and P bat They provide power to the photovoltaic unit and the energy storage unit, respectively. P main and P sec For the important and secondary loads of the microgrid, U This is the microgrid bus voltage. R The equivalent resistance of the microgrid. X This is the equivalent inductive reactance of a microgrid.

[0052] Maintaining a constant bus voltage in the microgrid, power balance is achieved based on load changes. Photovoltaic unit output ensures necessary load demand, determined according to the microgrid's renewable energy generation capabilities, and the access status of secondary loads is also determined.

[0053] Depend on Figure 2 and Figure 3 It is known that there is no DC grid connection between the various power generation units; instead, they are connected in parallel to the microgrid bus via inverters. When the new energy power source is connected to the microgrid system bus, phase information at the connection point is measured through a phase-locked loop.

[0054] In the initial stage of microgrid black start-up, the voltage information measured by the phase-locked loop (PLL) is lagging; the time difference between the grid connection of different photovoltaic (PV) units leads to circulating currents among multiple PV inverters. These circulating currents cause a large phase difference in the in-phase voltage between inverters connected in parallel with multiple PV units, resulting in an increase in inverter port current and problems such as a sudden surge in PV unit power demand and unstable operation.

[0055] To address the aforementioned issues, this method proposes a self-synchronization strategy for grid-connected photovoltaic inverters based on virtual oscillators. This reduces the requirements for phase-locked loop tracking capability in system self-synchronization and minimizes the impact of parameter deviations between inverters in microgrid photovoltaic units and inverter circulating currents on the microgrid.

[0056] Virtual oscillator structure as follows Figure 6 As shown. Among them, u osc The output voltage of the virtual oscillator. i osc Output current for the virtual oscillator; C osc , L osc and R osc These are virtual resonant capacitor, virtual resonant inductor, and virtual resistance, respectively. i Cosc , i Losc and i Rosc These represent the currents flowing through the capacitor, inductor, and resistor, respectively; the control unit is a current-controlled current source. i s The inverter outputs current to control the current output of the control unit. Decide.

[0057] A virtual oscillator synchronization control strategy is introduced into the control of microgrid black-start photovoltaic inverters.

[0058] The equivalent circuit of the inverter parallel connection based on the virtual oscillator control strategy is as follows: Figure 7 As shown. u osc1 , u osc2 The voltage at the ports of the two parallel circuits. i s1 , i s2The output current is for the two control units. Z osc1 , Z osc2 The equivalent impedance of two parallel circuits i zosc1 , i zosc2 For flow Z osc1 , Z osc2 Current, Z 1. Z 2 represents the equivalent series impedance of the two circuits. Z a The equivalent impedance of the line connecting the two circuits. i circle It is a circulating current.

[0059] The designed self-synchronization control loop and structure of the photovoltaic inverter based on the virtual oscillator control strategy are as follows: Figure 8 As shown, a negative feedback dual-loop control module is proposed to improve the power factor and anti-interference capability of the photovoltaic unit. The voltage reference value of the dual-loop control module is given by a virtual oscillator based on the sinusoidal voltage output of the energy storage inverter output port. The inverter output A-phase port current is collected. i oa After passing through a virtual oscillator self-synchronization stage, a single-phase sine wave containing the frequency and phase of the first photovoltaic unit is output. u osoa After phase shifting, a three-phase sinusoidal reference voltage is output. u oso After coordinate transformation, the voltage along the d-axis is taken as the input quantity of the outer voltage loop. u dref .

[0060] The first photovoltaic unit is based on a phase-locked loop structure and is connected to the grid using a traditional method. When it receives the grid connection signal of the second photovoltaic unit, it uses the output of the virtual synchronization control loop, which contains a sine wave with the frequency and phase of the first photovoltaic unit, as the voltage outer loop input. u dref Simultaneously set the voltage outer loop input u qref If the value is 0, a self-synchronization flag is issued when the phase difference of the current output from the two photovoltaic unit inverters is compared and the tracking accuracy requirement is met.

[0061] The virtual oscillation circuit must simultaneously meet the conditions for stable oscillation start-up and sufficient output voltage synchronization. The design parameters and conditions to be met are as follows:

[0062] (6)

[0063] In the formula, It is the resonant angular frequency; This refers to the inverter switching frequency; g s The conductivity of the controlled source of the virtual oscillator; K t , K u These are the proportional coefficients for the sampling current and the sampling voltage, respectively.

[0064] The electronic devices used in this technical solution are all existing products. The technical solution of this application does not have any special requirements or changes to the structure of the above electronic devices. The above electronic devices are all conventional electronic devices.

[0065] During the implementation of this technical solution, those skilled in the art need to connect all electrical components and their compatible power supplies through wires, and should select a suitable controller according to the actual situation to meet the control requirements. The specific connection and control sequence should refer to the working principle described below, where the electrical components are connected in sequence. The detailed connection methods are well-known in the art. The following mainly introduces the working principle and process, and will not describe the electrical control.

[0066] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, the phrase "comprising an element defined as..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0067] The above technical solutions only embody the preferred technical solutions of the present invention. Any modifications that may be made by those skilled in the art to certain parts thereof embody the principles of the present invention and fall within the protection scope of the present invention.

Claims

1. A black-start method for a photovoltaic-storage microgrid based on virtual oscillator control, characterized in that, The method includes the following steps: Step 1: Construct a microgrid based on black start. Connect the microgrid based on black start to the 220kV main grid bus through a grid-connected circuit breaker. Introduce an improved V / f control method to control the black start of the microgrid. Step 2: Introduce a virtual oscillator to perform parallel self-synchronization control on the inverters of multiple photovoltaic units in a microgrid based on black start. In step two, the virtual oscillator performs parallel self-synchronization control on the inverter of the photovoltaic unit by: utilizing the characteristic that two parallel oscillating circuits spontaneously tend to synchronize, the voltage change continues until the voltage difference between the two ports reaches the reference phase voltage phase difference threshold, thereby reducing the circulating current and realizing the self-synchronization of the output voltage of the virtual oscillator. A negative feedback dual-loop control module is adopted to improve the power factor and anti-interference capability of the photovoltaic unit. The voltage reference value of the dual-loop control module is given by the sinusoidal voltage output of the virtual oscillator based on the output port of the energy storage inverter. The current of the A-phase port of the inverter is collected and output through the self-synchronization link of the virtual oscillator, which outputs a single-phase sinusoidal wave containing the frequency and phase of the first photovoltaic unit. After phase shifting, a three-phase sinusoidal reference voltage is output. After coordinate transformation, the d-axis voltage is taken as the voltage outer loop input. The first photovoltaic unit is connected to the grid in a traditional way based on the phase-locked loop structure. When the grid connection signal of the second photovoltaic unit is received, the output of the virtual synchronization control link, which contains the sinusoidal wave containing the frequency and phase of the first photovoltaic unit, is used as the voltage outer loop input. At the same time, the voltage outer loop input voltage reference value is set to 0. The phase difference of the current output of the two photovoltaic unit inverters is compared. When the tracking accuracy requirement is met, a self-synchronization flag is issued.

2. The black-start method for a photovoltaic-storage microgrid based on virtual oscillator control according to claim 1, characterized in that, The microgrid in step one uses energy storage units to ensure a uniform voltage increase at the grid connection point. The energy storage units serve as the main power source for the microgrid's black start process. Necessary and secondary loads are set up in the microgrid, and other units are connected in parallel to the 35 kV small system bus. The 35 kV small system bus is connected to the 220 kV main grid bus through a grid-connected circuit breaker. The grid-connected circuit breaker is always open during the black start process.

3. The black-start method for a photovoltaic-storage microgrid based on virtual oscillator control according to claim 2, characterized in that, The microgrid black start process in step one includes: 1) When there is a large-scale power outage, the new energy photovoltaic and energy storage power station is disconnected from the main power grid, all loads are cut off, and the microgrid's main controller issues a black start command; 2) Connect the necessary loads, switch the energy storage unit as the main power source for black start, start the main power source for black start, and provide voltage and frequency support for the entire microgrid. After the power supply to the microgrid begins, connect the photovoltaic unit as the power support for the operation of the microgrid. 3) The bus voltage of the small system is boosted to 35KV and connected to the secondary load. The energy storage unit and photovoltaic unit supply the power required by the load and stabilize the black-start power unit.

4. The black-start method for a photovoltaic-storage microgrid based on virtual oscillator control according to claim 1, characterized in that, The improved V / f control method in step one is as follows: A voltage-current negative feedback dual closed-loop system is used for stable control of the energy storage unit. The target value of the black-start bus voltage is changed based on time to achieve a uniform rise in the microgrid bus voltage. Based on the energy storage unit's V / f control, the microgrid bus voltage is clamped, and the output power varies with the equivalent load. Microgrid line losses are ignored, and active power balance is satisfied. （1） In the formula: P PV and P bat The photovoltaic unit and the energy storage unit each contribute power, P main and P sec For the necessary and secondary loads of the microgrid, U is the microgrid bus voltage, R is the microgrid equivalent resistance, and X is the microgrid equivalent inductive reactance. Maintaining the microgrid bus voltage constant, achieving microgrid power balance based on load changes, ensuring the output of photovoltaic units to meet necessary load demands, and determining the output of microgrid energy storage units based on the connection status of secondary loads.

5. The black-start method for a photovoltaic-storage microgrid based on virtual oscillator control according to claim 1, characterized in that, The virtual oscillator must simultaneously meet the stable start-up condition of the circuit system and the sufficient condition of output voltage synchronization. The design parameters and the conditions to be met are as follows: （6） In the formula, ω sw f is the resonant angular frequency; sw g is the inverter switching frequency; s K represents the conductivity of the controlled source of the virtual oscillator. t K u These are the proportionality coefficients for the sampling current and the sampling voltage, respectively, L osc For virtual resonant inductance, C osc R is the virtual resonant capacitor. osc This is a virtual resistor.

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