A method for coordinating control of a fixed-frequency grid-forming power supply and a grid-following power supply for a microgrid

Abstract

A coordinated control method for fixed-frequency grid-connected power sources and grid-connected power sources in microgrids belongs to the field of power system automation. Its key features include the following steps: Step a, determining the I-V droop curve parameters of the fixed-frequency grid-connected power source and the P-V droop curve parameters of the grid-connected power source; Step b, data acquisition and processing in the microgrid; Step c, setting the microgrid voltage constructed by the fixed-frequency grid-connected power source and setting the grid-connected power source's grid-connected output power; Step d, coordinated adjustment of the fixed-frequency grid-connected power source and the grid-connected power source based on load characteristics. In this application's technical solution, the fixed-frequency grid-connected power source uses an I-V droop curve to output AC current with a fixed frequency and phase consistent with the synchronization signal. The grid-connected power source tracks the grid phase through a phase-locked loop and adjusts its active power output according to the P-V droop curve. Both the fixed-frequency grid-connected power source and the grid-connected power source allocate active power according to their respective capacities, and each fixed-frequency grid-connected power source allocates reactive power according to its capacity, jointly maintaining the power balance of the microgrid.

Description

Technical Field

[0001] A method for coordinated control of fixed-frequency grid-connected power sources and grid-connected power sources in microgrids belongs to the field of power system automation. Background Technology

[0002] Microgrids are playing an increasingly prominent role in the core support of new power systems, becoming an important component connecting distributed energy resources with the main grid and promoting the energy system's transition to clean and low-carbon energy. In the core architecture of AC microgrids, the parallel operation of multiple power electronic power sources is the mainstream form for achieving efficient energy integration. Among them, power sources with grid-connected capabilities and grid-connected power sources are the two core types of power sources, and their coordinated operation level directly determines the power supply stability and operational reliability of the microgrid.

[0003] Currently, the main control methods for parallel operation of power electronic power sources in AC microgrids are master-slave control and virtual synchronous machine (VSG) control. Among them, the master-slave control method has simple control logic and high power distribution accuracy, but it has shortcomings such as reliability dependence on the main power supply, poor expansion flexibility, and weak adaptability to grid connection / off-grid switching. On the other hand, the virtual synchronous machine control method supports seamless grid connection / off-grid switching, does not depend on the main power supply, is compatible with multiple types of power sources, and has good scalability, but it also has disadvantages such as complex parameter tuning and susceptibility to system oscillation when there are large fluctuations in source and load.

[0004] With the large-scale development of distributed resources and the continuous advancement of energy storage technology, some technical solutions for the coordinated control of grid-connected and grid-linked power sources have emerged in existing technologies. For example, Chinese invention patent application number 202311058994.8, filed on August 22, 2023, entitled "A Coordinated Control Method and System for Grid-Connected and Grid-Linked Energy Storage Converters," proposes a coordinated control strategy for grid-connected and grid-connected energy storage. Since the grid-connected energy storage converter adjusts its output value by following the frequency and voltage of the microgrid system through a phase-locked loop (PLL), while the grid-connected energy storage converter can be controlled by actively setting its frequency and voltage, a novel coordinated control strategy can be formed. By setting the power setpoint of the grid-connected energy storage to output an ideal frequency and voltage amplitude, the grid-connected energy storage converter no longer follows the frequency and voltage amplitude of the microgrid system, but rather follows the frequency and voltage amplitude output by the grid-connected energy storage. Thus, even without the support of any synchronous generator or high-power power system, the system can maintain a stable output voltage amplitude and frequency. The proposed secondary control framework for coordinated grid-connected and grid-based energy storage can operate in a plug-and-play manner. However, this technical solution suffers from several drawbacks: it heavily relies on network communication between grid-connected and grid-based converters, as well as power and voltage coordination control modules; its scalability is poor; and its operation and maintenance complexity is high.

[0005] Chinese invention patent application number 202411938369.7, filed on December 26, 2024, entitled "A Method and System for Coordinated Control of Isolated Grids and Grid-connected Energy Storage," proposes a technical solution. In this solution, the remaining usable capacity of the grid-connected energy storage is monitored in real time. It determines whether the remaining usable active power capacity of the grid-connected energy storage is less than an active power threshold, and further determines whether the remaining usable active power capacity of the grid-connected energy storage is greater than the active power threshold. If greater, frequency deviation control is applied to the grid-connected energy storage, and frequency supplementary control strategy is applied to the grid-connected energy storage. Similarly, it determines whether the remaining usable reactive power capacity of the grid-connected energy storage is less than a reactive power threshold, and further determines whether the remaining usable reactive power capacity of the grid-connected energy storage is greater than the reactive power threshold. If greater, voltage amplitude deviation control is applied to the grid-connected energy storage, and voltage supplementary control strategy is applied to the grid-connected energy storage. The drawback of this technical solution is that it relies on the frequency and voltage amplitude deviation to achieve active and reactive power control, requires frequent switching between grid-based energy storage and grid-connected energy storage working modes, and has a high complexity of control system, which is quite different from the fixed-frequency control grid-based method.

[0006] Fixed-frequency grid-connected power supplies not only fundamentally solve the power oscillation problem, but also have the advantages of fast voltage regulation, good compatibility of multiple power supply parallel connection, and strong adaptability to grid-connected / off-grid switching. However, their control is separate from that of grid-connected power supplies. Fixed-frequency grid-connected power supplies focus on voltage support, while grid-connected power supplies focus on power tracking. Therefore, designing a method that can achieve coordinated control of fixed-frequency grid-connected power supplies and grid-connected power supplies is an urgent problem to be solved in this field. Summary of the Invention

[0007] The technical problem this invention aims to solve is to overcome the shortcomings of existing technologies and provide a coordinated control method for fixed-frequency grid-connected power sources and grid-connected power sources used in microgrids. The fixed-frequency grid-connected power source uses an IV droop curve to output AC current with a fixed frequency and phase consistent with the synchronization signal to support the microgrid voltage. The grid-connected power source tracks the grid phase through a phase-locked loop and adjusts its active power output according to the PV droop curve. Both the fixed-frequency grid-connected power source and the grid-connected power source allocate active power according to their respective capacities, and the fixed-frequency grid-connected power source allocates reactive power according to its capacity, jointly maintaining the power balance of the microgrid.

[0008] The technical solution adopted by this invention to solve its technical problem is: a method for coordinated control of fixed-frequency grid-connected power sources and grid-connected power sources for microgrids, characterized by the following steps: Step a: Determine the IV droop curve parameters of the fixed-frequency grid-connected power supply and the PV droop curve parameters of the grid-connected power supply. Step b: Data acquisition and processing in the microgrid; Step c: Set the fixed-frequency grid power supply to construct the microgrid voltage, and set the grid-connected power supply and grid-connected output power. Step d involves the coordinated adjustment of the fixed-frequency grid power supply and the grid power supply based on load characteristics.

[0009] Preferably, in step a, the expression for the IV droop curve of the fixed-frequency grid power supply is: in, The target value for output current, For fixed-frequency grid power supply port voltage, The effective value of the power supply port is U. max To meet the upper limit of the grid voltage required by the standard, U min The lower limit of the grid voltage that meets the standard requirements, U max =1.1U N U min =0.9U N U N I is the rated voltage of the power grid. N Where k is the rated current of the power supply, and k1 is the droop coefficient of the IV droop curve. The sag intercept; Fixed-frequency network power supply port voltage phasor Transformed to a rotating coordinate system based on the synchronization signal, it is divided into components U. d and component U q and the component U d and component U q In the calculation of the D-axis and Q-axis components of the droop curve intercept, the output current of the power supply is adjusted according to the instantaneous active power and reactive power expressions in the synchronously rotating dq coordinate system, respectively, to characterize the active power and reactive power conditions in the microgrid, thereby realizing the active and reactive power support of the fixed-frequency grid power supply for the microgrid.

[0010] Preferably, the expression for the droop coefficient k1 is: IV. Intercept of the sag curve The expression is: Among them, U max To meet the upper limit of the grid voltage required by the standard, U min The lower limit of the grid voltage that meets the standard requirements, I N This is the rated current value of the power supply.

[0011] Preferably, in step a, the expression for the PV sag curve is: Among them, P s The target value for active power. The effective value of the power supply port is U. max To meet the upper limit of the grid voltage required by the standard, U min The lower limit of the grid voltage that meets the standard requirements, P N Where k is the rated power of the power supply, k2 is the droop coefficient of the PV droop curve, and P s0 This is the intercept of the PV sag curve.

[0012] Preferably, the expression for the droop coefficient k2 is: PV sag curve intercept P s0 The expression is: Among them, U max To meet the upper limit of the grid voltage required by the standard, U min The lower limit of the grid voltage that meets the standard requirements, P N This is the rated power value of the power supply.

[0013] Preferably, the total number of fixed-frequency grid-connected power sources is at least one, and the total rated power is greater than the total reactive power demand of the microgrid.

[0014] Preferably, in step c, the grid-connected power source monitors the voltage at its respective grid connection point in real time, tracks the grid voltage phase through a phase-locked loop, and outputs an AC current in phase with the grid voltage according to the PV droop curve in step a, participating in the active power regulation of the microgrid.

[0015] Preferably, in step e, if the load in the microgrid is purely active and the voltage phase is consistent with the phase of the synchronization signal, the fixed-frequency grid-connected power supply and the grid-connected power supply allocate the active load according to capacity based on the IV droop curve and the PV droop curve, respectively; if reactive load exists, the current of each fixed-frequency grid-connected power supply will have a Q-axis component, the fixed-frequency grid-connected power supply controls its voltage to generate a Q-axis component, and the fixed-frequency grid-connected power supply allocates the reactive load according to capacity based on the IV droop curve.

[0016] Preferably, in a microgrid, a single fixed-frequency grid-connecting power source is used to establish the grid using a fixed-frequency control method, or multiple fixed-frequency grid-connecting power sources are connected in parallel and operate collaboratively to establish the grid using a fixed-frequency control method.

[0017] Preferably, the power source includes a power electronic power source, which includes an energy storage converter, a wind turbine converter, a photovoltaic inverter, and a hydrogen fuel cell converter.

[0018] Compared with the prior art, the beneficial effects of this invention are: In the method for coordinated control of fixed-frequency grid-connected power sources and grid-connected power sources in microgrids according to this application, the fixed-frequency grid-connected power source uses an IV droop curve to output AC current with a fixed frequency and phase consistent with the synchronization signal to support the microgrid voltage. The grid-connected power source tracks the grid phase through a phase-locked loop and adjusts its active power output according to the PV droop curve. Both the fixed-frequency grid-connected power source and the grid-connected power source allocate active power according to their respective capacities, and each fixed-frequency grid-connected power source allocates reactive power according to its capacity, jointly maintaining the power balance of the microgrid.

[0019] Compared to conventional microgrid power electronic power control methods, the technical solution in this application dynamically allocates output based on load changes for both types of power sources, eliminating the need for additional communication and avoiding the problems of slow power response and low power supply reliability caused by communication.

[0020] The technical solution of this application can meet higher quality energy requirements while maintaining the stability of microgrid frequency. The overall microgrid has a fast dynamic response speed. When any power source is put into operation or withdrawn, other power sources can immediately adjust their output to maintain the power balance of the microgrid. It has "plug and play" characteristics and can quickly respond to complex operating conditions and flexibly adjust the power supply configuration. Attached Figure Description

[0021] Figure 1 This is a flowchart of a coordinated control method for fixed-frequency grid-connected power sources and grid-connected power sources used in microgrids.

[0022] Figure 2 The diagram shows the I / V droop characteristic curves for a coordinated control method between fixed-frequency grid-connected power sources and grid-connected power sources used in microgrids.

[0023] Figure 3 P / V droop characteristic curves for a coordinated control method between fixed-frequency grid-connected power sources and grid-connected power sources used in microgrids. Detailed Implementation

[0024] Figures 1-3 This is the preferred embodiment of the present invention, which is described below in conjunction with the accompanying drawings. Figures 1-3 The present invention will be further described below.

[0025] like Figure 1 As shown, the method for coordinated control of fixed-frequency grid-connected power sources and grid-connected power sources in microgrids includes the following steps: Step 1: Determine the IV droop curve parameters of the fixed-frequency grid-connected power supply and the PV droop curve parameters of the grid-connected power supply. Combination Figure 2 The expression for the IV sag curve in step 1 is: in, The target value for output current, For fixed-frequency network power supply ports, U max To meet the upper limit of the grid voltage required by the standard, U min The lower limit of the grid voltage that meets the standard requirements, U max =1.1U N U min =0.9U N U N I is the rated voltage of the power grid. N Let k1 be the rated current of the power supply, and k1 be the droop coefficient of the IV droop curve. The expression for the droop coefficient k1 is: Among them, U max To meet the upper limit of the grid voltage required by the standard, U min The lower limit of the grid voltage that meets the standard requirements, I N This is the rated current value of the power supply.

[0026] The intercept of the sag curve is obtained from the following formula: In the expression for the IV droop curve, the power port phasor of the fixed-frequency network... Transformed to a rotating coordinate system based on the synchronization signal, it is divided into components U. d and component U q and the component U d and component U q Incorporating the sag curve intercept In the calculation of the D-axis and Q-axis components, the microgrid component U is maintained. d and component U q Within reasonable values, and based on the instantaneous active and reactive power expressions in the synchronously rotating dq coordinate system, the active and reactive power conditions in the microgrid are characterized respectively. The power supply output current is adjusted according to the above formula to achieve active and reactive power support for the microgrid by the fixed-frequency grid power supply.

[0027] Combination Figure 3 The expression for the PV sag curve is: Among them, P s This represents the target value for active power. The effective value of the power supply port is U. max To meet the upper limit of the grid voltage required by the standard, U min The lower limit of the grid voltage that meets the standard requirements, P N Let k be the rated power of the power supply, and k2 be the droop coefficient of the PV droop curve. The expression for the droop coefficient k2 is: P s0 The intercept of the PV sag curve is obtained from the following formula: Since the grid-connected power source tracks the grid voltage phase through a phase-locked loop, the active power support of the microgrid is achieved solely according to the above formula.

[0028] In both P / V droop control and I / V droop control, the upper limit of the grid voltage shall not exceed 110% of the microgrid's rated voltage, and the lower limit of the grid voltage shall not be lower than 90% of the microgrid's rated voltage.

[0029] The aforementioned power sources include, but are not limited to, power electronic power sources, which include power electronic power conversion devices such as energy storage converters, wind turbine converters, photovoltaic inverters, and hydrogen fuel cell converters.

[0030] Step 2, Data acquisition and processing in the microgrid; In microgrid operation, sensors deployed within each power source collect real-time instantaneous three-phase current data. These instantaneous current values ​​are then processed using coordinate transformation. For fixed-frequency grid-connected power sources, the satellite synchronization signal phase is used as the reference phase; for grid-connected power sources, the grid voltage phase is used as the reference phase. This transforms the three-phase current from a static coordinate system to components in a rotating coordinate system, representing the current components controlling active power and reactive power, respectively, providing fundamental data support for subsequent power allocation adjustments.

[0031] Step 3: Set the fixed-frequency grid power supply to construct the microgrid voltage, and set the grid-connected power supply and grid-connected output power. The fixed-frequency grid-connected power supply receives a synchronization signal and, based on the grid connection point voltage, adjusts the output AC current according to the IV droop curve to ensure that the output voltage frequency is constant and the amplitude is within the upper limit value U required by the standard. max and lower limit value U min between.

[0032] The power supply monitors its own voltage relative to the grid connection point in real time, tracks the grid voltage phase through a phase-locked loop, and outputs an AC current in phase with the grid voltage according to the PV droop curve in step 1 to participate in the active power regulation of the microgrid.

[0033] Step 4: Coordinated adjustment of fixed-frequency grid-connected power supply and grid-connected power supply based on load characteristics; Signal synchronization: Multiple grid-connected power supplies use a timing signal as the synchronization signal between them. The timing signal is usually a satellite signal. When there is a problem with the timing signal of the fixed-frequency grid-connected power supply, the fixed-frequency grid-connected power supply needs to rely on its internal high-precision crystal oscillator timekeeping mode to provide a reference for the current output phase of the fixed-frequency grid-connected power supply. Each power supply can still maintain a relatively stable output reference for a short period of time.

[0034] Voltage Construction: The fixed-frequency grid-connected power source collects the voltage at the grid connection point and adjusts its output current based on the IV droop curve, thereby controlling the output voltage to maintain a constant frequency (50Hz) and keep the voltage amplitude within the microgrid standard range, meeting the upper and lower limits required by the microgrid standard. The grid-connected power source tracks the grid voltage phase through a phase-locked loop and participates in the active power regulation of the microgrid.

[0035] Power regulation: Regulation is performed based on load characteristics. If the load in the microgrid is purely active, the voltage phase is consistent with the phase of the synchronization signal. Fixed-frequency grid-connected power sources and grid-connected power sources allocate active loads according to their respective droop curves and capacities. If reactive loads exist, the current of each fixed-frequency grid-connected power source will exhibit a Q-axis component. The fixed-frequency grid-connected power source controls its voltage to generate a Q-axis component, and then allocates reactive loads according to their capacities and droop curves.

[0036] If the microgrid load is a purely active load, the voltage phase is consistent with the phase of the synchronization signal. If there is a reactive load, the reactive load will cause the microgrid voltage phase to deviate from the phase of the synchronization signal. With the synchronization signal as the reference phase, the voltage will have a Q-axis component. Each fixed-frequency grid power source will output a Q-axis current according to the magnitude of the Q-axis component, so as to realize the distribution of reactive power by the fixed-frequency grid power source according to its capacity.

[0037] The droop curves for fixed-frequency grid-connected power sources and grid-connected power sources are set according to their capacity and the allowable voltage variation range of the microgrid. Both fixed-frequency grid-connected and grid-connected power sources must allocate active power according to their capacity, and the reactive power of each fixed-frequency grid-connected power source must be allocated according to its capacity. The total rated power of the fixed-frequency grid-connected power sources must be greater than the total reactive power demand of the microgrid.

[0038] In a microgrid, a single fixed-frequency grid-connected power source can be used to establish the grid using a fixed-frequency control method, or multiple fixed-frequency grid-connected power sources can be connected in parallel and operated collaboratively to establish the grid using a fixed-frequency control method.

[0039] In the technical solution of this application, when the reactive load of some lines in the microgrid is too heavy, reactive power compensation power supply can be configured nearby as appropriate, and effective reactive power support can be achieved in conjunction with the fixed frequency grid power supply.

[0040] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for coordinated control of fixed-frequency grid-connected power sources and grid-connected power sources in microgrids, characterized in that: Includes the following steps: Step a: Determine the IV droop curve parameters of the fixed-frequency grid-connected power supply and the PV droop curve parameters of the grid-connected power supply. Step b: Data acquisition and processing in the microgrid; Step c: Set the fixed-frequency grid power supply to construct the microgrid voltage, and set the grid-connected power supply and grid-connected output power. Step d involves the coordinated adjustment of the fixed-frequency grid power supply and the grid power supply based on load characteristics.

2. The method for coordinated control of fixed-frequency grid-connected power sources and grid-connected power sources for microgrids according to claim 1, characterized in that: In step a, the expression for the IV droop curve of the fixed-frequency grid-connected power supply is: in, The target value for output current, For fixed-frequency grid power supply port voltage, U is the effective value of the mains power supply port voltage. max To meet the upper limit of the grid voltage required by the standard, U min The lower limit of the grid voltage that meets the standard requirements, U max =1.1U N U min =0.9U N U N I is the rated voltage of the power grid. N Where is the rated current of the power supply, and k1 is the droop coefficient of the IV droop curve. The sag intercept; Fixed-frequency network power supply port voltage phasor Transformed to a rotating coordinate system based on the synchronization signal, it is divided into components U. d and component U q and the component U d and component U q Incorporating the sag curve intercept In the calculation of the D-axis and Q-axis components, based on the instantaneous active power and reactive power expressions in the synchronously rotating dq coordinate system, the power supply output current is adjusted to characterize the active power and reactive power conditions in the microgrid, so as to realize the active and reactive power support of the fixed-frequency grid power supply to the microgrid.

3. The method for coordinated control of fixed-frequency grid-connected power sources and grid-connected power sources for microgrids according to claim 2, characterized in that: The expression for the droop coefficient k1 is: IV. Intercept of the sag curve The expression is: Among them, U max To meet the upper limit of the grid voltage required by the standard, U min The lower limit of the grid voltage that meets the standard requirements, I N This is the rated current value of the power supply.

4. The method for coordinated control of fixed-frequency grid-connected power sources and grid-connected power sources for microgrids according to claim 1, characterized in that: In step a, the expression for the PV sag curve is: Among them, P s The target value for active power. U is the effective value of the mains power supply port voltage. max To meet the upper limit of the grid voltage required by the standard, U min The lower limit of the grid voltage that meets the standard requirements, P N Where k is the rated power of the power supply, k2 is the droop coefficient of the PV droop curve, and P s0 This is the intercept of the PV sag curve.

5. The method for coordinated control of fixed-frequency grid-connected power sources and grid-connected power sources for microgrids according to claim 4, characterized in that: The expression for the droop coefficient k2 is: PV sag curve intercept P s0 The expression is: Among them, U max To meet the upper limit of the grid voltage required by the standard, U min The lower limit of the grid voltage that meets the standard requirements, P N This is the rated power value of the power supply.

6. The method for coordinated control of fixed-frequency grid-connected power sources and grid-connected power sources for microgrids according to claim 1, characterized in that: The total number of fixed-frequency grid-connected power sources must be at least one, and the total rated power must be greater than the total reactive power demand of the microgrid.

7. The method for coordinated control of fixed-frequency grid-connected power sources and grid-connected power sources for microgrids according to claim 1, characterized in that: In step c, the grid-connected power sources monitor the voltage at their respective grid connection points in real time, track the grid voltage phase through a phase-locked loop, and output an AC current in phase with the grid voltage according to the PV droop curve in step a, participating in the active power regulation of the microgrid.

8. The method for coordinated control of fixed-frequency grid-connected power sources and grid-connected power sources for microgrids according to claim 1, characterized in that: In step e, if the load in the microgrid is purely active, the voltage phase is consistent with the phase of the synchronization signal. The fixed-frequency grid-connected power supply and the grid-connected power supply allocate active load according to capacity based on the IV droop curve and the PV droop curve, respectively. If reactive load exists, the current of each fixed-frequency grid-connected power supply will have a Q-axis component. The fixed-frequency grid-connected power supply controls its voltage to generate a Q-axis component. The fixed-frequency grid-connected power supply allocates reactive load according to capacity based on the IV droop curve.

9. The method for coordinated control of fixed-frequency grid-connected power sources and grid-connected power sources for microgrids according to claim 1, characterized in that: In a microgrid, a single fixed-frequency grid-connected power source is used to establish the grid using a fixed-frequency control method, or multiple fixed-frequency grid-connected power sources are connected in parallel and operate collaboratively to establish the grid using a fixed-frequency control method.

10. The method for coordinated control of fixed-frequency grid-connected power sources and grid-connected power sources for microgrids according to claim 1, characterized in that: The power source includes a power electronic power source, which includes an energy storage converter, a wind turbine converter, a photovoltaic inverter, and a hydrogen fuel cell converter.

Images