A parallel operation control method and system for grid-connected inverters

Abstract

The present invention relates to the field of power electronics technology, and in particular to a parallel operation control method and system for grid-type inverters, including obtaining the inverter output active power and inverter output reactive power in the current state according to the grid-type inverter output current and capacitor voltage; predicting and generating the optimal output active power at the next sampling moment according to the inverter output active power; obtaining the inertia coefficient adjustment law according to the reference angular frequency of the grid-type inverter using a sliding mode adaptive inertia enhancement mechanism; performing active droop control according to the inertia coefficient adjustment law and the optimal output active power at the next sampling moment to obtain the inverter output angular frequency; obtaining the inverter output voltage according to the inverter output reactive power; and generating a grid-type inverter parallel operation control signal according to the inverter output angular frequency and the inverter output voltage. The present invention improves the stability of the parallel operation of the grid-type inverter by introducing multi-band adaptive virtual impedance and sliding mode inertia enhancement technology.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics, and in particular to a parallel operation control method and system for grid-connected inverters. Background Art

[0002] With the development of renewable energy, distributed power sources, as an important form of renewable energy, play an increasingly important role in the power system with their advantages such as flexible access and high power quality. However, the widespread application of distributed power sources has also led to the transformation of the power system from traditional centralized power generation to distributed power generation mode, which has brought new challenges to the planning, operation and control of the power system.

[0003] As a key device for connecting distributed energy sources with the power grid, the importance of grid-forming inverters is becoming increasingly prominent. Grid-forming inverters can actively adjust the output voltage and frequency, and have functions similar to traditional synchronous generators. Especially under off-grid or weak grid conditions, they can independently maintain the stability of voltage and frequency, providing strong support for island operation and autonomous control of microgrids. However, in practical applications, the parallel operation of grid-forming inverters faces a series of technical difficulties that need to be solved urgently. The circulating current problem is one of the most prominent problems. When multiple inverters are operated in parallel, due to factors such as parameter differences between inverters, inconsistent controllers, and unbalanced line impedance, circulating current is easily generated between inverters, which will not only increase system losses and aggravate inverter heating, but may even cause malfunction of protection devices, seriously threatening the safe and stable operation of the system. In addition, the circulating current will also cause distortion of the inverter output voltage and current, thereby reducing the power quality.

[0004] Secondly, the lack of physical inertia of the inverter leads to poor system frequency stability. Traditional synchronous generators can smoothly respond to load changes and maintain system frequency stability due to their rotational inertia. In contrast, inverters based on power electronic devices lack physical inertia and cannot provide sufficient energy storage to buffer power disturbances. When the system is disturbed by sudden load changes or faults, the frequency changes are more drastic, which may cause system oscillations and affect the stability and power quality of the power grid. In addition, in a multi-inverter parallel system, achieving accurate power sharing and synchronous control between inverters is also challenging. The traditional droop control strategy achieves preliminary power distribution by introducing virtual impedance, but there are problems such as large steady-state error and slow dynamic response, which makes it difficult to meet the power distribution accuracy and system stability requirements under complex load conditions and nonlinear loads.

[0005] In summary, in order to solve the problems of circulating current and poor system frequency stability in the parallel operation of grid-connected inverters, it is urgent to provide an efficient and stable parallel operation control method for grid-connected inverters to improve the reliability and operation efficiency of the power system. Summary of the invention

[0006] The object of the present invention is to provide a grid-type inverter parallel operation control method and system, by introducing multi-band adaptive virtual impedance and sliding mode inertia enhancement technology, to improve the stability of the inverter parallel operation.

[0007] In order to solve the above technical problems, the present invention provides a parallel operation control method and system for grid-connected inverters.

[0008] In a first aspect, the present invention provides a method for controlling parallel operation of grid-connected inverters, the method comprising the following steps:

[0009] According to the collected grid-type inverter output current and capacitor voltage, the inverter output active power and inverter output reactive power in the current state are calculated;

[0010] According to the inverter output active power, the optimal output active power at the next sampling moment is predicted and generated by using the power sharing strategy of the integrated model predictive control;

[0011] According to the predetermined reference angular frequency of the grid-forming inverter, the inertia coefficient adjustment law is calculated by using the sliding mode adaptive inertia enhancement mechanism;

[0012] Performing active power droop control according to the inertia coefficient adjustment law and the optimal output active power at the next sampling moment to obtain the inverter output angular frequency;

[0013] According to the reactive power output by the inverter, the inverter output voltage is calculated using a passive filtering multi-band adaptive virtual impedance model;

[0014] Dual closed-loop control is performed according to the inverter output angular frequency and the inverter output voltage to generate a parallel operation control signal for the grid-forming inverters.

[0015] In a further embodiment, the step of predicting and generating the optimal output active power at the next sampling moment using the power sharing strategy of the integrated model predictive control according to the inverter output active power includes:

[0016] Predicting and generating the inverter output active power state at several future sampling moments according to the inverter output active power and a predefined voltage and current control input sequence;

[0017] The power sharing optimization objective function is constructed with the optimization goal of minimizing the deviation between the inverter output active power state and the inverter reference power and the frequency change at several future sampling moments;

[0018] By solving the minimum value of the power sharing optimization objective function, the optimal output active power at the next sampling moment is obtained.

[0019] In a further embodiment, the step of calculating the inertia coefficient adjustment law using the sliding mode adaptive inertia enhancement mechanism according to the predetermined reference angular frequency of the grid-connected inverter comprises:

[0020] Determine a reference angular frequency of a grid-type inverter, and calculate a target output frequency of the inverter according to the reference angular frequency of the grid-type inverter;

[0021] Defining a sliding mode surface according to a frequency error between a target output frequency of the inverter and a command reference frequency and its integral, and constructing a high-order sliding mode control function according to the sliding mode surface and a preset control gain coefficient;

[0022] The high-order sliding mode control function is used to respond to the current state of the sliding mode surface in real time, and the inertia coefficient of the active power droop control of the virtual synchronous generator is dynamically adjusted according to the response result to obtain the inertia coefficient adjustment law.

[0023] In a further embodiment, the step of performing active droop control according to the inertia coefficient adjustment law and the optimal output active power at the next sampling moment to obtain the inverter output angular frequency includes:

[0024] Calculating an active power angular frequency deviation value according to a difference between the active power output by the inverter and the reference angular frequency of the grid-forming inverter;

[0025] According to the active power angular frequency deviation value, the current voltage value of the inverter DC side and the initial DC voltage, the reference angular frequency is adjusted using a frequency synchronization strategy to obtain a synchronous angular frequency;

[0026] Active power droop control is performed according to the inertia coefficient adjustment law, the optimal output active power at the next sampling moment and the synchronous angular frequency to obtain the inverter output angular frequency.

[0027] In a further embodiment, the step of calculating the inverter output voltage using a multi-band adaptive virtual impedance model of passive filtering according to the inverter output reactive power includes:

[0028] Performing reactive droop control on the reactive power output by the inverter using a preset reactive droop coefficient to obtain a voltage deviation value caused by the reactive droop;

[0029] According to the equivalent output admittance of the inverter, the disturbance observer is used to observe the disturbance of the inverter output current, and the estimated value of the inverter output current disturbance change is obtained;

[0030] According to the virtual resistance and virtual inductance, the low-frequency component of the virtual impedance is obtained;

[0031] According to the impedance gain and cut-off frequency of different high-frequency bands, the high-frequency component of the virtual impedance is calculated;

[0032] An adaptive gain coefficient is introduced to calculate a multi-band adaptive virtual impedance according to an estimated value of the inverter output current disturbance change, a low-frequency component of the virtual impedance, and a high-frequency component of the virtual impedance;

[0033] The inverter output voltage is obtained according to the multi-band adaptive virtual impedance and the voltage deviation value.

[0034] In a further embodiment, the mathematical expression of the multi-band adaptive virtual impedance is:

[0035]

[0036]

[0037]

[0038] In the formula, It is a multi-band adaptive virtual impedance; is a virtual resistor; is the complex frequency variable in Laplace transform; is the virtual inductor; is the index of the high frequency band; is the number of high frequency bands; is the impedance gain of the dth high frequency band; is the cutoff frequency of the dth high frequency band; is the adaptive gain coefficient; is the estimated value of the inverter output current disturbance change; is the transfer function parameter of the filter; Output current for grid-type inverter; is the equivalent output admittance of the inverter; Output filter inductor for inverter; is the equivalent series resistance of the filter inductor; is the capacitor voltage; is the filter time constant.

[0039] In a further embodiment, the step of obtaining the inverter output voltage according to the multi-band adaptive virtual impedance and the voltage deviation value comprises:

[0040] Acquire the inverter output current amplitude, and calculate the voltage drop generated by the inverter output current on the multi-band adaptive virtual impedance according to the multi-band adaptive virtual impedance and the inverter output current amplitude;

[0041] Subtracting the inverter command voltage from the voltage deviation value to obtain a voltage reference value taking reactive power droop into consideration;

[0042] The inverter output voltage is obtained according to the voltage reference value considering reactive power droop and the voltage drop generated by the inverter output current on the multi-band adaptive virtual impedance.

[0043] In a second aspect, the present invention provides a grid-connected inverter parallel operation control system, the system comprising:

[0044] The power calculation module is used to calculate the inverter output active power and the inverter output reactive power in the current state according to the collected grid-forming inverter output current and capacitor voltage;

[0045] A power prediction module, used to predict and generate the optimal output active power at the next sampling moment by using the power sharing strategy of the integrated model predictive control according to the active power output by the inverter;

[0046] A sliding mode control module is used to calculate the inertia coefficient adjustment law using a sliding mode adaptive inertia enhancement mechanism according to a predetermined reference angular frequency of the grid-forming inverter;

[0047] An active power droop control module is used to perform active power droop control according to the inertia coefficient adjustment law and the optimal output active power at the next sampling moment to obtain an inverter output angular frequency;

[0048] A virtual impedance analysis module, used to calculate the inverter output voltage using a passive filtering multi-band adaptive virtual impedance model according to the reactive power output by the inverter;

[0049] The control signal generation module is used to perform double closed-loop control according to the inverter output angular frequency and the inverter output voltage, and generate a grid-type inverter parallel operation control signal.

[0050] In a further embodiment, the power prediction module is specifically used to:

[0051] Predicting and generating the inverter output active power state at several future sampling moments according to the inverter output active power and a predefined voltage and current control input sequence;

[0052] The power sharing optimization objective function is constructed with the optimization goal of minimizing the deviation between the inverter output active power state and the inverter reference power and the frequency change at several future sampling moments;

[0053] By solving the minimum value of the power sharing optimization objective function, the optimal output active power at the next sampling moment is obtained.

[0054] In a further embodiment, the sliding mode control module is specifically used to:

[0055] Determine a reference angular frequency of a grid-type inverter, and calculate a target output frequency of the inverter according to the reference angular frequency of the grid-type inverter;

[0056] Defining a sliding mode surface according to a frequency error between a target output frequency of the inverter and a command reference frequency and its integral, and constructing a high-order sliding mode control function according to the sliding mode surface and a preset control gain coefficient;

[0057] The high-order sliding mode control function is used to respond to the current state of the sliding mode surface in real time, and the inertia coefficient of the active power droop control of the virtual synchronous generator is dynamically adjusted according to the response result to obtain the inertia coefficient adjustment law.

[0058] The present invention provides a parallel operation control method and system for grid-type inverters. The method calculates the inverter output active power and the inverter output reactive power in the current state according to the output current and capacitor voltage of the grid-type inverter; predicts and generates the optimal output active power at the next sampling moment according to the inverter output active power by using the power sharing strategy of the integrated model predictive control; calculates the inertia coefficient adjustment law according to the reference angular frequency of the grid-type inverter by using the sliding mode adaptive inertia enhancement mechanism; performs active droop control according to the inertia coefficient adjustment law and the optimal output active power at the next sampling moment to obtain the inverter output angular frequency; calculates the inverter output voltage according to the inverter output reactive power by using the multi-band adaptive virtual impedance model of passive filtering; performs double closed-loop control according to the inverter output angular frequency and the inverter output voltage to generate a grid-type inverter parallel operation control signal. Compared with the existing technology, this method can realize precise power sharing and synchronous control between parallel inverters by combining technologies such as multi-band adaptive virtual impedance model and sliding mode adaptive inertia enhancement mechanism, improve the accuracy of power distribution of parallel inverters and system stability, and solve the circulation problems and poor system frequency stability in parallel operation of grid-connected inverters, thereby improving the reliability and operation efficiency of the power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 It is a schematic flow chart of a parallel operation control method of grid-connected inverters provided in an embodiment of the present invention;

[0060] Figure 2 Schematic diagram of the topological structure of a grid-connected inverter parallel operation system provided by an embodiment of the present invention;

[0061] Figure 3 It is a block diagram of a grid-connected inverter parallel operation control system provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0062] The following specifically illustrates the implementation mode of the present invention in conjunction with the accompanying drawings. The embodiments are provided for illustrative purposes only and cannot be understood as limiting the present invention. The accompanying drawings are provided for reference and illustration only and do not constitute a limitation on the scope of patent protection of the present invention, because many changes can be made to the present invention without departing from the spirit and scope of the present invention.

[0063] refer to Figure 1 The embodiment of the present invention provides a parallel operation control method for grid-connected inverters, such as Figure 1 As shown, the method comprises the following steps:

[0064] S1. According to the collected grid-forming inverter output current and capacitor voltage, the inverter output active power and inverter output reactive power in the current state are calculated.

[0065] S2. Based on the active power output by the inverter, the optimal output active power at the next sampling moment is predicted and generated using the power sharing strategy of the integrated model predictive control.

[0066] In this embodiment, the step of predicting and generating the optimal output active power at the next sampling moment by using the power sharing strategy of the integrated model predictive control according to the output active power of the inverter includes:

[0067] Predicting and generating the inverter output active power state at several future sampling moments according to the inverter output active power and a predefined voltage and current control input sequence;

[0068] The optimization goal is to minimize the deviation between the inverter output active power state and the inverter reference power at several future sampling moments and the frequency change, and construct a power sharing optimization objective function; the power sharing optimization objective function is specifically:

[0069]

[0070] In the formula, Optimizing objective function for power sharing; is the inverter output active power at the (k+i)th sampling moment in the future; k is the sampling moment; i is the i-th sampling moment in the future; N is the number of predicted future moments; is the inverter reference power; is the weight coefficient, which is used to balance the importance of power error and frequency variation; is the angular frequency deviation at the (k+i)th sampling moment in the future;

[0071] By solving the minimum value of the power sharing optimization objective function, the optimal output active power at the next sampling moment is obtained, wherein the mathematical expression of the optimal output active power at the next sampling moment is:

[0072]

[0073] In the formula, is the optimal output active power at the next sampling moment; k is the sampling moment; is the inverter output active power at the kth sampling moment; is the power change at the kth sampling moment.

[0074] Specifically, Figure 2 As shown, in this embodiment, the inverter output current is obtained by a sensor or a measuring device. and capacitor voltage , where the inverter output current The current provided by the inverter to the grid or load, the capacitor voltage is the capacitor voltage at the inverter output end, which is mainly used to maintain the stability of the output voltage. Figure 2 middle, , are the AC filter inductors of inverter 1 and inverter 2 respectively, , The AC filter capacitors of inverter 1 and inverter 2 are used in this embodiment according to the output current of the inverter. , capacitor voltage And the inverter related parameters calculate the inverter output active power in the current state And the inverter output reactive power , then the inverter outputs active power based on the current state and the assumed voltage and current control input sequence, predicting the inverter output active power state at the next N moments, wherein the voltage and current control input sequence may include a voltage reference value, a current reference value, etc. This embodiment may use a machine learning algorithm to predict the inverter output active power state, and the embodiment of the present invention does not impose specific limitations.

[0075] Next, this embodiment takes minimizing the deviation between the inverter output active power state and the inverter reference power and the sum of squares of frequency changes at several future sampling moments as the optimization goal, and constructs a power sharing optimization objective function. The power sharing optimization objective function F is mainly used to measure the deviation between the predicted state and the expected state at the next N moments. Specifically, the power sharing optimization objective function F consists of two parts: the sum of squares of power errors (measures the accuracy of power sharing) and the negative weighted sum of squares of frequency changes (measures the stability of frequency). By solving the optimization problem, this embodiment can obtain the power changes at several future sampling moments, so that the objective function F is minimized, and the optimal output active power to be achieved at the next sampling moment is calculated based on the power change and the current state. , the above process is repeated at the next sampling moment to predict the optimal output active power. In this embodiment, the power sharing optimization objective function F predicts the future state based on the current state and the assumed control input sequence. By adjusting the control input, the output power of the inverter at the next N moments is made as close to the preset reference power as possible, while maintaining the stability of the frequency change, and achieving a balance between power error and frequency stability. This embodiment ensures accurate power sharing and frequency stability through the power sharing optimization objective function, while taking into account the balance between power error and frequency change.

[0076] S3. According to the predetermined reference angular frequency of the grid-connected inverter, the inertia coefficient adjustment law is calculated using the sliding mode adaptive inertia enhancement mechanism.

[0077] In this embodiment, the step of calculating the inertia coefficient adjustment law by using the sliding mode adaptive inertia enhancement mechanism according to the predetermined reference angular frequency of the grid-connected inverter includes:

[0078] Determine a reference angular frequency of a grid-type inverter, and calculate a target output frequency of the inverter according to the reference angular frequency of the grid-type inverter;

[0079] Defining a sliding mode surface according to a frequency error between a target output frequency of the inverter and a command reference frequency and its integral, and constructing a high-order sliding mode control function according to the sliding mode surface and a preset control gain coefficient;

[0080] The high-order sliding mode control function is used to respond to the current state of the sliding mode surface in real time, and the inertia coefficient of the active power droop control of the virtual synchronous generator is dynamically adjusted according to the response result to obtain the inertia coefficient adjustment law.

[0081] Specifically, this embodiment requires presetting the reference angular frequency of the grid-connected inverter , the reference angular frequency of the grid-type inverter The angular frequency corresponding to the desired inverter output AC frequency, and then the inverter target output frequency can be calculated according to the relationship between frequency and angular frequency by setting the grid-type inverter reference angular frequency , that is, the target state that the system is expected to achieve, the relationship between frequency and angular frequency is Next, in order to achieve adaptive inertia enhancement, this embodiment defines the sliding surface , sliding surface Based on the frequency error between the inverter output frequency and the command reference frequency and the integral of the frequency error, the frequency error is calculated according to the difference between the command reference frequency and the inverter target output frequency. In this embodiment, the sliding mode adaptive inertia enhancement mechanism is a high-order sliding mode control. Therefore, this embodiment adopts a high-order function (high-order sliding mode control function) to update the inertia coefficient J in the virtual synchronous generator control strategy. In this embodiment, the high-order sliding mode control function is specifically:

[0082]

[0083]

[0084]

[0085] Therefore, the inertia coefficient adjustment law is calculated according to the following formula:

[0086]

[0087] In the formula, is the inertia coefficient adjustment law; is the initial inertia coefficient; is a high-order sliding mode control function; , are control gain coefficients; is the sliding surface; is the frequency error; is the sliding mode control coefficient, which is used to adjust the integral weight; t is the time; is the maximum response time, i.e., the time period required for the inverter to respond to changes in grid frequency; is the command reference frequency; is the inverter target output frequency; is a sign function used to avoid system chattering and realize adaptive enhancement of system inertia.

[0088] In this embodiment, the sliding surface is defined and the high-order sliding mode control function is calculated according to the frequency error of the system. To dynamically adjust the inertia coefficient J of the active droop control link in the virtual synchronous generator control strategy, so as to achieve adaptive enhancement of the system inertia. After updating the inertia coefficient J, the control strategy of the virtual synchronous generator will be adjusted accordingly to better adapt to system changes and improve the stability and response speed of the system.

[0089] S4. Perform active power droop control according to the inertia coefficient adjustment law and the optimal output active power at the next sampling moment to obtain the inverter output angular frequency.

[0090] In this embodiment, the step of performing active power droop control according to the inertia coefficient adjustment law and the optimal output active power at the next sampling moment to obtain the inverter output angular frequency includes:

[0091] Calculating an active power angular frequency deviation value according to a difference between the active power output by the inverter and the reference angular frequency of the grid-forming inverter;

[0092] According to the active power angular frequency deviation value, the current voltage value of the inverter DC side and the initial DC voltage, the reference angular frequency is adjusted using a frequency synchronization strategy to obtain a synchronous angular frequency;

[0093] Active power droop control is performed according to the inertia coefficient adjustment law, the optimal output active power at the next sampling moment and the synchronous angular frequency to obtain the inverter output angular frequency.

[0094] Specifically, this embodiment calculates the active power angular frequency deviation value and adjusts the reference angular frequency accordingly, so as to effectively reduce the fluctuation of the active power output by the inverter. At the same time, a frequency synchronization strategy that takes into account the voltage change on the DC side of the inverter is used to calculate the synchronous angular frequency. The frequency synchronization strategy is based on data such as the reference angular frequency, the DC voltage, the initial DC voltage, the DC side capacitance, and the energy-to-frequency conversion coefficient. Through the energy balance between the DC side and the AC side of the inverter, the reference frequency is directly adjusted to ensure the frequency synchronization and power sharing of multiple inverters, thereby calculating the synchronous angular frequency. Specifically, this embodiment calculates the synchronous angular frequency by The square difference between the current DC voltage and the initial DC voltage is used to analyze the change in DC side energy, and the DC side energy change is multiplied by the energy-frequency conversion coefficient to calculate the angular frequency adjustment corresponding to the DC side energy change. The synchronous angular frequency is obtained according to the active power angular frequency deviation value, the difference between the active power angular frequency deviation value and the angular frequency adjustment corresponding to the DC side energy change. The calculated synchronous angular frequency is used as the set value of the inverter output angular frequency for the subsequent active droop calculation link. In this embodiment, the synchronous angular frequency reflects the synchronous operation state of the power grid, and the calculation formula of the synchronous angular frequency is:

[0095]

[0096] In the formula, is the synchronization angular frequency; is the reference angular frequency of the grid-connected inverter; is the active power angular frequency deviation value; is the energy-frequency conversion coefficient, that is, the conversion coefficient from energy to frequency; is the DC side capacitance value; is the current voltage value of the DC side of the inverter; is the initial DC voltage; Indicates the angular frequency adjustment corresponding to the energy change on the DC side.

[0097] In this embodiment, the step of performing active power droop control according to the inertia coefficient adjustment law, the optimal output active power at the next sampling moment and the synchronous angular frequency to obtain the inverter output angular frequency includes:

[0098] Determine a reference active power, and obtain an active power deviation according to a difference between the optimal output active power at the next sampling moment and the reference active power;

[0099] A preset droop proportional coefficient is introduced, and the angular frequency correction value is obtained according to the active power deviation and inertia coefficient adjustment law;

[0100] The angular frequency correction value is added to the synchronous angular frequency to obtain the inverter output angular frequency.

[0101] Specifically, in the active droop control link, the present embodiment simulates the droop characteristics of the synchronous generator, that is, the linear relationship between active power and frequency, and adjusts the output voltage of the inverter according to the frequency deviation of the system, thereby affecting the output of active power. In the active droop control process, the present embodiment obtains the active power deviation by comparing the difference between the optimal output active power and the reference active power, and then multiplies the ratio of the active power deviation to the inertia coefficient adjustment law J by the preset droop proportional coefficient to obtain the angular frequency correction value. The angular frequency correction value reflects the deviation between the current power demand and the actual output of the system. The inertia coefficient adjustment law J is used to adjust the influence of this active power deviation on the output angular frequency. After obtaining the angular frequency correction value, the present embodiment adds the angular frequency correction value to the synchronous angular frequency to obtain the output angular frequency of the inverter. The inverter output angular frequency is the basis for the inverter to control its output voltage frequency. By adjusting the frequency of the output voltage, the inverter can control the phase and magnitude of the grid-connected current to achieve precise control of the active power.

[0102] This embodiment inputs the synchronization angular frequency, the inertia coefficient adjustment law and the optimal output active power at the next sampling moment into the active power droop calculation link, and obtains the output angular frequency of the inverter through the active power droop control algorithm, thereby realizing stable operation and efficient energy conversion of the inverter in the microgrid, and enhancing the system's ability to smoothly respond to load changes. At the same time, this embodiment combines the frequency synchronization strategy with the active power droop control strategy to optimize the power distribution between inverters, reduce the circulating current caused by factors such as parameter differences between inverters, inconsistent controllers and unbalanced line impedance, and improve the reliability and operation efficiency of the power system.

[0103] S5. According to the reactive power output by the inverter, the inverter output voltage is calculated using a multi-band adaptive virtual impedance model of passive filtering.

[0104] In this embodiment, the step of calculating the inverter output voltage by using a multi-band adaptive virtual impedance model of passive filtering according to the reactive power output by the inverter includes:

[0105] Performing reactive droop control on the reactive power output by the inverter using a preset reactive droop coefficient to obtain a voltage deviation value caused by the reactive droop;

[0106] According to the equivalent output admittance of the inverter, the disturbance observer is used to observe the disturbance of the inverter output current, and the estimated value of the inverter output current disturbance change is obtained;

[0107] According to the virtual resistance and virtual inductance, the low-frequency component of the virtual impedance is obtained;

[0108] According to the impedance gain and cut-off frequency of different high-frequency bands, the high-frequency component of the virtual impedance is calculated;

[0109] An adaptive gain coefficient is introduced to calculate a multi-band adaptive virtual impedance according to an estimated value of the inverter output current disturbance change, a low-frequency component of the virtual impedance, and a high-frequency component of the virtual impedance;

[0110] The inverter output voltage is obtained according to the multi-band adaptive virtual impedance and the voltage deviation value.

[0111] Specifically, in this embodiment, the calculated inverter output reactive power and a preset reactive droop coefficient are input into the reactive droop control link, and reactive droop control is performed through the reactive droop control link to obtain a voltage deviation value caused by reactive droop, and the voltage deviation value reflects the influence of reactive power on voltage. Next, this embodiment constructs a multi-band adaptive virtual impedance model of passive filtering. The multi-band adaptive virtual impedance model of passive filtering calculates the low-frequency component of the virtual impedance through virtual resistance and inductance to suppress low-frequency circulating current; at the same time, for different high-frequency bands, this embodiment calculates the high-frequency component of the virtual impedance through the impedance gains and cut-off frequencies of multiple high-frequency bands to achieve high-frequency circulating current suppression. On this basis, this embodiment introduces an adaptive gain coefficient in the multi-band adaptive virtual impedance model, and according to the estimated value of the inverter output current disturbance change estimated by the disturbance observer, the virtual impedance The low-frequency and high-frequency components of the pseudo-impedance are dynamically calculated to obtain a multi-band adaptive virtual impedance, wherein the adaptive gain coefficient can dynamically adjust the virtual impedance parameters according to the real-time observed disturbances of the inverter output current and voltage, thereby further realizing the precise suppression of the circulating current, and then the multi-band adaptive virtual impedance is combined with the voltage deviation value to calculate the output voltage of the inverter. This voltage is the final output voltage of the inverter, which can simultaneously suppress the low-frequency and high-frequency circulating currents and improve the stability of the system. In summary, this embodiment realizes the precise suppression of the circulating current by real-time observation of the disturbances of the inverter output current and voltage, and dynamically adjusts the virtual impedance parameters, thereby improving the stability of multiple inverter power supplies under complex load conditions. In this embodiment, the mathematical expression of the multi-band adaptive virtual impedance is:

[0112]

[0113]

[0114]

[0115] In the formula, It is a multi-band adaptive virtual impedance; is a virtual resistor; is the complex frequency variable in Laplace transform; is the virtual inductor; is the index of the high frequency band; is the number of high frequency bands; is the impedance gain of the dth high frequency band; is the cutoff frequency of the dth high frequency band; is the adaptive gain coefficient; is the estimated value of the inverter output current disturbance change; is the transfer function parameter of the filter; Output current for grid-type inverter; is the equivalent output admittance of the inverter; For the inverter output filter inductor (in Figure 2 In the example, the inverter output filter inductor of inverter 1 is , the inverter output filter inductor of inverter 2 is ); is the equivalent series resistance of the filter inductor; is the capacitor voltage of the grid-type inverter (the output voltage of the grid-type inverter); is the filter time constant.

[0116] In this embodiment, the step of obtaining the inverter output voltage according to the multi-band adaptive virtual impedance and the voltage deviation value includes:

[0117] Acquire the inverter output current amplitude, and calculate the voltage drop generated by the inverter output current on the multi-band adaptive virtual impedance according to the multi-band adaptive virtual impedance and the inverter output current amplitude;

[0118] Subtracting the inverter command voltage from the voltage deviation value to obtain a voltage reference value taking reactive power droop into consideration;

[0119] The inverter output voltage is obtained according to the voltage reference value considering reactive power droop and the voltage drop generated by the inverter output current on the multi-band adaptive virtual impedance; wherein the mathematical expression of the inverter output voltage is:

[0120]

[0121] In the formula, is the inverter output voltage; is the inverter command voltage, which indicates the target voltage that the inverter should reach; is the voltage deviation value caused by reactive power droop, where: The result is the voltage reference value considering reactive power droop; It is a multi-band adaptive virtual impedance; is the magnitude of the inverter output current, where It is the voltage drop generated by the inverter output current on the multi-band adaptive virtual impedance. This voltage drop represents the voltage change caused by the current flowing through the multi-band adaptive virtual impedance.

[0122] This embodiment will consider the voltage reference value of reactive power droop and the voltage drop caused by the inverter output current on the multi-band adaptive virtual impedance to obtain the inverter output voltage. The inverter output voltage is the actual output voltage of the inverter. The inverter output voltage takes into account both the influence of reactive power and the direct influence of current on voltage. This embodiment combines the multi-band adaptive virtual impedance model and voltage drop and other contents to effectively suppress the circulating current in the power grid, ensure that the inverter can accurately control the output, and improve the stability and efficiency of the power grid.

[0123] S6. Perform double closed-loop control according to the inverter output angular frequency and the inverter output voltage to generate a parallel operation control signal for the grid-forming inverters.

[0124] This embodiment obtains the parallel operation control signal of the grid-forming inverter through a dual closed-loop control link, and controls the on and off of the power devices (such as IGBT, MOSFET, etc.) in the inverter according to the parallel operation control signal of the grid-forming inverter, thereby realizing the control of the output voltage and current of the inverter. For example: the output current of the inverter is controlled through the current inner loop to make it track the current reference value, and the output voltage of the inverter is controlled through the voltage outer loop to keep it stable and meet the system requirements.

[0125] This embodiment is based on the virtual synchronous generator control strategy. By introducing a multi-band adaptive virtual impedance model of passive filtering, a sliding mode adaptive inertia enhancement mechanism, a frequency synchronization strategy, and a power sharing strategy of integrated model predictive control, an efficient and stable grid-type inverter parallel operation control method is realized, which provides a strong guarantee for the stable operation and efficient power sharing of the inverter power supply. Specifically, this embodiment can effectively suppress the low-frequency and high-frequency circulating current problems generated when multiple inverter power supplies adopt virtual synchronous generator control technology under complex load conditions by constructing a multi-band adaptive virtual impedance model, thereby significantly improving the stability of the system; secondly, this embodiment introduces a sliding mode adaptive inertia enhancement mechanism, and uses a sliding mode The control principle adaptively adjusts the inertia coefficient. When the frequency deviation is large, the system stability is further enhanced by increasing the inertia, while avoiding the common jitter problem in traditional sliding mode control. At the same time, the frequency synchronization strategy adopted in this embodiment directly adjusts the reference angular frequency of the grid-type inverter by accurately controlling the energy balance between the DC side and the AC side of the inverter, and calculates the synchronization angular frequency to ensure that multiple inverters can still maintain frequency synchronization under complex working conditions and realize effective power sharing. In addition, the power sharing strategy of integrated model predictive control proposed in this embodiment predicts future power change trends and adjusts the frequency reference value in advance accordingly, which not only realizes accurate power sharing, but also ensures continuous optimization of frequency stability.

[0126] An embodiment of the present invention provides a parallel operation control method for grid-type inverters. The method obtains the inverter output active power and the inverter output reactive power in the current state according to the output current and capacitor voltage of the grid-type inverter; predicts and generates the optimal output active power at the next sampling moment according to the inverter output active power by using the power sharing strategy of the integrated model predictive control; obtains the inertia coefficient adjustment law according to the reference angular frequency of the grid-type inverter by using the sliding mode adaptive inertia enhancement mechanism; performs active droop control according to the inertia coefficient adjustment law and the optimal output active power at the next sampling moment to obtain the inverter output angular frequency; obtains the inverter output voltage according to the inverter output reactive power by using a multi-band adaptive virtual impedance model of passive filtering; performs dual closed-loop control according to the inverter output angular frequency and the inverter output voltage to generate a grid-type inverter parallel operation control signal. Compared with the existing technology, this method can realize precise power sharing and synchronous control between parallel inverters by combining technologies such as multi-band adaptive virtual impedance model and sliding mode adaptive inertia enhancement mechanism, improve the accuracy of power distribution of parallel inverters and system stability, solve the circulation problem and poor system frequency stability in the parallel operation of grid-connected inverters, improve the stability of parallel operation of inverters, and thus improve the reliability and operation efficiency of the power system.

[0127] It should be noted that the size of the serial numbers of the above-mentioned processes does not mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0128] In one embodiment, Figure 3 As shown, an embodiment of the present invention provides a grid-connected inverter parallel operation control system, the system comprising:

[0129] The power calculation module 101 is used to calculate the inverter output active power and the inverter output reactive power in the current state according to the collected grid-type inverter output current and capacitor voltage;

[0130] A power prediction module 102, configured to predict and generate the optimal output active power at the next sampling moment by using the power sharing strategy of the integrated model predictive control according to the active power output by the inverter;

[0131] The sliding mode control module 103 is used to calculate the inertia coefficient adjustment law using the sliding mode adaptive inertia enhancement mechanism according to the predetermined reference angular frequency of the grid-forming inverter;

[0132] An active power droop control module 104 is used to perform active power droop control according to the inertia coefficient adjustment law and the optimal output active power at the next sampling moment to obtain an inverter output angular frequency;

[0133] A virtual impedance analysis module 105 is used to calculate the inverter output voltage using a passive filtering multi-band adaptive virtual impedance model according to the reactive power output by the inverter;

[0134] The control signal generating module 106 is used to perform double closed-loop control according to the inverter output angular frequency and the inverter output voltage, and generate a grid-connected inverter parallel operation control signal.

[0135] In this embodiment, the power prediction module 102 is specifically used for:

[0136] Predicting and generating the inverter output active power state at several future sampling moments according to the inverter output active power and a predefined voltage and current control input sequence;

[0137] The power sharing optimization objective function is constructed with the optimization goal of minimizing the deviation between the inverter output active power state and the inverter reference power and the frequency change at several future sampling moments;

[0138] By solving the minimum value of the power sharing optimization objective function, the optimal output active power at the next sampling moment is obtained.

[0139] In this embodiment, the sliding mode control module 103 is specifically used for:

[0140] Determine a reference angular frequency of a grid-type inverter, and calculate a target output frequency of the inverter according to the reference angular frequency of the grid-type inverter;

[0141] Defining a sliding mode surface according to a frequency error between a target output frequency of the inverter and a command reference frequency and its integral, and constructing a high-order sliding mode control function according to the sliding mode surface and a preset control gain coefficient;

[0142] The high-order sliding mode control function is used to respond to the current state of the sliding mode surface in real time, and the inertia coefficient of the active power droop control of the virtual synchronous generator is dynamically adjusted according to the response result to obtain the inertia coefficient adjustment law.

[0143] For the specific definition of a grid-type inverter parallel operation control system, please refer to the above-mentioned definition of a grid-type inverter parallel operation control method, which will not be repeated here. A person of ordinary skill in the art will appreciate that the various modules and steps described in conjunction with the embodiments disclosed in this application can be implemented in hardware, software, or a combination of both. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of this application.

[0144] The embodiment of the present invention provides a parallel operation control system for a grid-type inverter, wherein a power calculation module of the system obtains the inverter output active power and the inverter output reactive power in the current state according to the output current and capacitor voltage of the grid-type inverter; a power prediction module predicts and generates the optimal output active power at the next sampling moment according to the inverter output active power by using the power sharing strategy of the integrated model predictive control; a sliding mode control module obtains the inertia coefficient adjustment law according to the reference angular frequency of the grid-type inverter by using the sliding mode adaptive inertia enhancement mechanism; an active power droop control module performs active power droop control according to the inertia coefficient adjustment law and the optimal output active power at the next sampling moment to obtain the inverter output angular frequency; a virtual impedance analysis module obtains the inverter output voltage according to the inverter output reactive power by using a multi-band adaptive virtual impedance model of passive filtering; a control signal generation module performs dual closed-loop control according to the inverter output angular frequency and the inverter output voltage to generate a grid-type inverter parallel operation control signal. Compared with the existing technology, the system can realize precise power sharing and synchronous control between parallel inverters by combining technologies such as multi-band adaptive virtual impedance model and sliding mode adaptive inertia enhancement mechanism, improve the accuracy of power distribution of parallel inverters and system stability, solve the circulation problems and poor system frequency stability in the parallel operation of grid-connected inverters, improve the stability of parallel operation of inverters, and thus improve the reliability and operation efficiency of the power system.

[0145] The above-mentioned embodiments only express several preferred implementation modes of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in the technical field, several improvements and substitutions can be made without departing from the technical principles of the present invention, and these improvements and substitutions should also be regarded as the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be based on the protection scope of the claims.

Claims

1. A method for controlling parallel operation of grid-connected inverters, characterized in that: The following steps are involved: According to the collected grid-type inverter output current and capacitor voltage, the inverter output active power and inverter output reactive power in the current state are calculated; According to the inverter output active power, the optimal output active power at the next sampling moment is predicted and generated by using the power sharing strategy of the integrated model predictive control; According to the predetermined reference angular frequency of the grid-forming inverter, the inertia coefficient adjustment law is calculated by using the sliding mode adaptive inertia enhancement mechanism; Performing active power droop control according to the inertia coefficient adjustment law and the optimal output active power at the next sampling moment to obtain the inverter output angular frequency; According to the reactive power output by the inverter, the inverter output voltage is calculated by using a multi-band adaptive virtual impedance model of passive filtering; Perform double closed-loop control according to the inverter output angular frequency and inverter output voltage to generate grid-type inverter parallel operation control signals; The step of calculating the inertia coefficient adjustment law by using the sliding mode adaptive inertia enhancement mechanism according to the predetermined reference angular frequency of the grid-forming inverter comprises: Determine a reference angular frequency of a grid-type inverter, and calculate a target output frequency of the inverter according to the reference angular frequency of the grid-type inverter; Defining a sliding mode surface according to a frequency error between a target output frequency of the inverter and a command reference frequency and its integral, and constructing a high-order sliding mode control function according to the sliding mode surface and a preset control gain coefficient; The high-order sliding mode control function is used to respond to the current state of the sliding mode surface in real time, and the inertia coefficient of the active power droop control of the virtual synchronous generator is dynamically adjusted according to the response result to obtain the inertia coefficient adjustment law; The step of calculating the inverter output voltage by using a multi-band adaptive virtual impedance model of passive filtering according to the inverter output reactive power comprises: Performing reactive droop control on the reactive power output by the inverter using a preset reactive droop coefficient to obtain a voltage deviation value caused by the reactive droop; According to the equivalent output admittance of the inverter, the disturbance observer is used to observe the disturbance of the inverter output current, and the estimated value of the inverter output current disturbance change is obtained; According to the virtual resistance and virtual inductance, the low-frequency component of the virtual impedance is obtained; According to the impedance gain and cut-off frequency of different high-frequency bands, the high-frequency component of the virtual impedance is calculated; An adaptive gain coefficient is introduced to calculate a multi-band adaptive virtual impedance according to an estimated value of the inverter output current disturbance change, a low-frequency component of the virtual impedance, and a high-frequency component of the virtual impedance; The inverter output voltage is obtained according to the multi-band adaptive virtual impedance and the voltage deviation value.

2. A grid-connected inverter parallel operation control method according to claim 1, characterized in that: The step of predicting and generating the optimal output active power at the next sampling moment by using the power sharing strategy of the integrated model predictive control according to the inverter output active power comprises: Predicting and generating the inverter output active power state at several future sampling moments according to the inverter output active power and a predefined voltage and current control input sequence; The power sharing optimization objective function is constructed with the optimization goal of minimizing the deviation between the inverter output active power state and the inverter reference power and the frequency change at several future sampling moments; By solving the minimum value of the power sharing optimization objective function, the optimal output active power at the next sampling moment is obtained.

3. A grid-connected inverter parallel operation control method according to claim 1, characterized in that: The step of performing active droop control according to the inertia coefficient adjustment law and the optimal output active power at the next sampling moment to obtain the inverter output angular frequency comprises: Calculating an active power angular frequency deviation value according to a difference between the active power output by the inverter and the reference angular frequency of the grid-forming inverter; According to the active power angular frequency deviation value, the current voltage value of the inverter DC side and the initial DC voltage, the reference angular frequency is adjusted using a frequency synchronization strategy to obtain a synchronous angular frequency; Active power droop control is performed according to the inertia coefficient adjustment law, the optimal output active power at the next sampling moment and the synchronous angular frequency to obtain the inverter output angular frequency.

4. A grid-connected inverter parallel operation control method according to claim 1, characterized in that: The mathematical expression of the multi-band adaptive virtual impedance is: In the formula, It is a multi-band adaptive virtual impedance; is a virtual resistor; is the complex frequency variable in Laplace transform; is the virtual inductor; is the index of the high frequency band; is the number of high frequency bands; is the impedance gain of the dth high frequency band; is the cutoff frequency of the dth high frequency band; is the adaptive gain coefficient; is the estimated value of the inverter output current disturbance change; is the transfer function parameter of the filter; Output current for grid-type inverter; is the equivalent output admittance of the inverter; Output filter inductor for inverter; is the equivalent series resistance of the filter inductor; is the capacitor voltage; is the filter time constant.

5. A grid-connected inverter parallel operation control method according to claim 1, characterized in that: The step of obtaining the inverter output voltage according to the multi-band adaptive virtual impedance and the voltage deviation value comprises: Acquire the inverter output current amplitude, and calculate the voltage drop generated by the inverter output current on the multi-band adaptive virtual impedance according to the multi-band adaptive virtual impedance and the inverter output current amplitude; Subtracting the inverter command voltage from the voltage deviation value to obtain a voltage reference value taking reactive power droop into consideration; The inverter output voltage is obtained according to the voltage reference value considering reactive power droop and the voltage drop generated by the inverter output current on the multi-band adaptive virtual impedance.

6. A grid-connected inverter parallel operation control system, characterized in that: The system comprises: The power calculation module is used to calculate the inverter output active power and the inverter output reactive power in the current state according to the collected grid-forming inverter output current and capacitor voltage; A power prediction module, used to predict and generate the optimal output active power at the next sampling moment by using the power sharing strategy of the integrated model predictive control according to the active power output by the inverter; A sliding mode control module is used to calculate the inertia coefficient adjustment law using a sliding mode adaptive inertia enhancement mechanism according to a predetermined reference angular frequency of the grid-forming inverter; An active power droop control module is used to perform active power droop control according to the inertia coefficient adjustment law and the optimal output active power at the next sampling moment to obtain an inverter output angular frequency; A virtual impedance analysis module, used to calculate the inverter output voltage using a passive filtering multi-band adaptive virtual impedance model according to the reactive power output by the inverter; A control signal generation module is used to perform double closed-loop control according to the inverter output angular frequency and the inverter output voltage, and generate a grid-type inverter parallel operation control signal; The sliding mode control module is specifically used for: Determine a reference angular frequency of a grid-type inverter, and calculate a target output frequency of the inverter according to the reference angular frequency of the grid-type inverter; Defining a sliding mode surface according to a frequency error between a target output frequency of the inverter and a command reference frequency and its integral, and constructing a high-order sliding mode control function according to the sliding mode surface and a preset control gain coefficient; The high-order sliding mode control function is used to respond to the current state of the sliding mode surface in real time, and the inertia coefficient of the active power droop control of the virtual synchronous generator is dynamically adjusted according to the response result to obtain the inertia coefficient adjustment law; The virtual impedance analysis module is specifically used for: Performing reactive droop control on the reactive power output by the inverter using a preset reactive droop coefficient to obtain a voltage deviation value caused by the reactive droop; According to the equivalent output admittance of the inverter, the disturbance observer is used to observe the disturbance of the inverter output current, and the estimated value of the inverter output current disturbance change is obtained; According to the virtual resistance and virtual inductance, the low-frequency component of the virtual impedance is obtained; According to the impedance gain and cut-off frequency of different high-frequency bands, the high-frequency component of the virtual impedance is calculated; An adaptive gain coefficient is introduced to calculate a multi-band adaptive virtual impedance according to an estimated value of the inverter output current disturbance change, a low-frequency component of the virtual impedance, and a high-frequency component of the virtual impedance; The inverter output voltage is obtained according to the multi-band adaptive virtual impedance and the voltage deviation value.

7. A grid-connected inverter parallel operation control system according to claim 6, characterized in that: The power prediction module is specifically used for: Predicting and generating the inverter output active power state at several future sampling moments according to the inverter output active power and a predefined voltage and current control input sequence; The power sharing optimization objective function is constructed with the optimization goal of minimizing the deviation between the inverter output active power state and the inverter reference power and the frequency change at several future sampling moments; By solving the minimum value of the power sharing optimization objective function, the optimal output active power at the next sampling moment is obtained.

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