WO2022068146A1 - Method for controlling micro-grid grid-connected inverter using dynamic droop coefficient - Google Patents

Abstract

A method for controlling a micro-grid grid-connected inverter using a dynamic droop coefficient. The method comprises: establishing a micro-grid grid-connected inverter output active and reactive power mathematical model; simplifying the inverter output active and reactive power mathematical model; simulating a droop characteristic of a synchronous generator, so as to control a micro-grid grid-connected inverter, and establishing a traditional droop control equation; improving a droop coefficient in the traditional droop control equation, introducing a dynamic droop coefficient, and reducing a power distribution deviation by means of automatically adjusting the droop coefficient; replacing the traditional droop coefficient with the dynamic droop coefficient, so as to obtain a dynamic droop control equation; and applying the obtained dynamic droop control equation to power and voltage control links of the micro-grid grid-connected inverter.

Description

A Microgrid Grid-connected Inverter Control Method Using Droop Dynamic Coefficient 【Technical field】

The invention relates to a control method of a micro-grid grid-connected inverter using a droop dynamic coefficient, and uses the droop reactive power dynamic coefficient to improve the reactive power-voltage droop control performance of the micro-grid grid-connected inverter.

【Background technique】

In the control system of the microgrid grid-connected inverter device, PQ droop control and constant voltage and constant frequency (V-f) control are mostly used. The traditional droop control strategy performs independent decoupling control on the output active power-frequency and reactive power-voltage of the inverter device by simulating the droop characteristics of the traditional synchronous generator. However, in the actual operation of the microgrid grid-connected inverter, there are problems such as uneven distribution of line impedance and nonlinear output voltage drop, which will lead to the distribution deviation of the output power of the microgrid grid-connected inverter and the stability of the bus voltage. state bias problem.

[Content of the invention]

The purpose of the present invention is to provide a microgrid grid-connected inverter control method using a droop dynamic coefficient, specifically using the droop active power dynamic coefficient to improve the active-frequency droop control performance of the microgrid grid-connected inverter, by automatically adjusting the droop coefficient To reduce the active power distribution deviation; use the droop reactive power dynamic coefficient to improve the reactive power-voltage droop control performance of the microgrid grid-connected inverter, reduce the reactive power compensation range by automatically adjusting the droop coefficient, and effectively reduce the voltage regulation deviation.

The present invention adopts following technical scheme to realize:

A control method for a microgrid grid-connected inverter using a droop dynamic coefficient, comprising the following steps:

1) Establish a mathematical model of the output active and reactive power of the microgrid grid-connected inverter;

2) Simplify the mathematical model of the inverter output active and reactive power in step 1);

3) Simulate the droop characteristics of the synchronous generator to control the microgrid grid-connected inverter, and establish the traditional droop control equation;

4) Improve the droop coefficients m and n in the traditional droop control equation in step 3), introduce dynamic droop coefficients m _i , _ni , i=1, 2, 3,...n, and reduce power by automatically adjusting the droop coefficient allocation bias;

5) replacing step 4) dynamic droop coefficients m _i , n _i with step 3) traditional droop coefficients m, n, the new dynamic droop control equation obtained;

6) The new dynamic droop control equation obtained in step 5) is applied to the power and voltage control links of the microgrid grid-connected inverter to improve the power distribution accuracy of the microgrid grid-connected inverter control system and reduce the voltage regulation deviation.

A further improvement of the present invention is that step 1) establishes a mathematical model of the output active and reactive power of the microgrid grid-connected inverter:

Among them: _{Pi is the active power output by the inverter; Q i is the reactive power output by the inverter; U i is the} output voltage of the inverter; U ₀ is the voltage across the load impedance; δ _i is the power angle; Z _i = R _i +JX _i is the line equivalent impedance; R _i is the line resistance; X _i is the line inductive reactance.

A further improvement of the present invention is that the specific implementation method of step 2) is: according to the multi-inverter parallel connection low-voltage microgrid, the line impedance is resistive, that is _, Ri ＞X _i , Ri _{≈Z i ,} Xi ≈ 0, simplify the mathematical model of the active and reactive power output of the inverter in step 1):

A further improvement of the present invention is that the specific implementation method of step 3) is: simulate the droop characteristic of the synchronous generator to control the grid-connected inverter of the microgrid, and establish a traditional droop control equation:

Among them: ω is the output voltage angular frequency of the controlled inverter; U is the output voltage amplitude of the controlled inverter; ω ₀ is the reference value of the no-load output voltage angular frequency; U ₀ is the no-load output voltage amplitude reference value; m is the active power droop coefficient; n is the reactive power droop coefficient; P is the active power distributed by the load; Q is the reactive power distributed by the load.

A further improvement of the present invention lies in that the specific implementation method of step 4) is: improving the droop coefficients m and n in the traditional droop control equation in step 3), and introducing dynamic droop coefficients m _i and n _i ,

Among them: ω _max , ω _min , U _max , and U _min are the upper and lower thresholds of the output voltage angular frequency and voltage amplitude; ΔP and ΔQ are the difference between the current output active and reactive power and the target active and reactive power.

A further improvement of the present invention is that the specific implementation method of step 5) is: the dynamic droop coefficients m _i and n _i of step 4) are replaced by step 3) traditional droop coefficients m and n, and the obtained novel dynamic droop control equation:

A further improvement of the present invention is that the specific implementation method of step 6) is: applying the novel active power-frequency dynamic droop control equation obtained in step 5) to the power control link of the grid-connected inverter of the microgrid, and the change rate of the output power of the inverter Significantly reduced, avoiding power distribution errors.

Compared with the prior art, the present invention at least has the following beneficial technical effects:

1. The droop active power dynamic coefficient proposed by the present invention can effectively increase the active power distribution accuracy of the microgrid grid-connected inverter;

2. The droop reactive power dynamic coefficient proposed by the present invention can reduce the reactive power compensation range and effectively eliminate the steady-state voltage deviation of the microgrid grid-connected inverter.

【Description of drawings】

Figure 1 is a structural diagram of a microgrid;

Figure 2 is a Thevenin equivalent circuit diagram of two distributed parallel power points;

(a) and (b) in Figure 3 are schematic diagrams for the comparison of active and reactive droop control;

Figure 4 is an analysis diagram of power droop adjustment;

Figure 5 is an analysis diagram of reactive power droop adjustment;

Figure 6 is a control block diagram of a grid-connected inverter;

Fig. 7 is the simulation waveform of active power distribution using conventional droop control for sudden increase in load power;

Fig. 8 is the simulation waveform of active power distribution using dynamic coefficient droop control for sudden increase in load power;

Fig. 9 is the simulation waveform of the RMS voltage of the busbar voltage using conventional droop control for sudden increase in load power;

Fig. 10 is the simulation waveform of the bus voltage RMS simulation waveform using dynamic coefficient droop control for sudden increase of load power.

【Detailed ways】

In order to make those skilled in the art better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only The embodiments are part of the present invention, not all of the embodiments, and are not intended to limit the scope of the present disclosure. Furthermore, in the following description, descriptions of well-known structures and techniques are omitted to avoid unnecessarily obscuring the concepts disclosed in the present invention. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

Various structural schematic diagrams according to the disclosed embodiments of the present invention are shown in the accompanying drawings. The figures are not to scale, some details have been exaggerated for clarity, and some details may have been omitted. The shapes of various regions and layers shown in the figures and their relative sizes and positional relationships are only exemplary, and in practice, there may be deviations due to manufacturing tolerances or technical limitations, and those skilled in the art should Regions/layers with different shapes, sizes, relative positions can be additionally designed as desired.

In the context of the present disclosure, when a layer/element is referred to as being "on" another layer/element, it can be directly on the other layer/element or intervening layers/elements may be present therebetween. element. In addition, if a layer/element is "on" another layer/element in one orientation, then when the orientation is reversed, the layer/element can be "under" the other layer/element.

It should be noted that the terms "first", "second" and the like in the description and claims of the present invention and the above drawings are used to distinguish similar objects, and are not necessarily used to describe a specific sequence or sequence. It is to be understood that the data so used may be interchanged under appropriate circumstances such that the embodiments of the invention described herein can be practiced in sequences other than those illustrated or described herein. Furthermore, the terms "comprising" and "having" and any variations thereof, are intended to cover non-exclusive inclusion, for example, a process, method, system, product or device comprising a series of steps or units is not necessarily limited to those expressly listed Rather, those steps or units may include other steps or units not expressly listed or inherent to these processes, methods, products or devices.

Below in conjunction with accompanying drawing, the present invention is described in further detail:

As shown in Figure 1, there are i (i=1, 2, 3,...n) distributed power points and inverters in the microgrid structure, and the output interface of each inverter is connected to an LCL filter device for Suppressing higher harmonics, R _{1,i=1, 2, 3,...n} represents the equivalent series resistance of the inductor on the inverter side, the line impedance Z _line =R _line +JX _line , R _line is the line resistance, X _line is the inductive reactance of the line, in the low-voltage microgrid, R _line >> X _line . P _{i=1, 2, 3,...n} , Q _{i=1, 2, 3,...n} indicate that the inverter outputs active and reactive power, and Z _load is the equivalent impedance of the load.

As shown in Figure 2, the Thevenin equivalent circuit diagram of two distributed parallel power points is established, in which: U ₁ ∠δ ₁ and U ₂ ∠δ ₂ are the output voltages of the first and second inverters; U ₀ ∠0 is the load The voltage across the impedance; Z ₁ =R ₁ +JX ₁ , Z ₂ =R ₂ +JX ₂ , are the equivalent impedances of the first and second transmission lines.

The output active and reactive power of a single inverter can be expressed as:

The actual transmission line impedance of the microgrid is resistive (R _i ＞＞X _i , Ri _{≈Z i ,} Xi ≈ 0 ₎ , the above formula can be simplified as:

The inverter parameters of each distributed power point are not the same, the impedance of the transmission line has parameter drift, and there is an error in the acquisition, which will lead to the fact that the power delivered to the load cannot be accurately matched according to the actual capacity.

As shown in Figure 3, the microgrid inverter control system controls the inverter by simulating the droop characteristics of the synchronous generator. The traditional droop control equation is:

In formula (3): ω is the output voltage angular frequency of the controlled inverter; U is the output voltage amplitude of the controlled inverter; ω ₀ is the reference value of the no-load output voltage angular frequency; U ₀ is the no-load output voltage amplitude value reference value; m is the active power droop coefficient; n is the reactive power droop coefficient; P is the active power distributed by the load; Q is the reactive power distributed by the load, and the traditional droop control is a differential adjustment.

The droop characteristics of P-ω and Q-U have a linear relationship, and the droop coefficients m and n are fixed values. If the conventional droop control is sampled, it will cause power distribution errors. In the actual operation of multi-inverter parallel low-voltage microgrids, some electrical equipment is more sensitive to power fluctuations. When the power distribution error is large, it is very easy to cause the equipment to be disconnected from the grid.

The invention proposes a control method of a microgrid grid-connected inverter using a droop dynamic coefficient, which reduces power distribution deviation by automatically adjusting the droop coefficient. The droop dynamic coefficients m _i and _ni can be expressed as:

In formulas (4) and (5): in order to ensure the stability of the droop control, the upper and lower thresholds of the output voltage angular frequency and voltage amplitude are set, namely ω _max , ω _min , U _max , U _min ; ΔP, ΔQ It is the difference between the current output active and reactive power and the target active and reactive power.

As shown in Figure 4, when faced with the same active power regulation, the power fluctuation of the new dynamic droop control is smaller. When the power fluctuation occurs at point A, the droop coefficient m _i gradually increases from the upper limit of the voltage angular frequency threshold to the operating point A , gradually decreases from the operating point A to the lower limit of the voltage angular frequency threshold. Facing the same adjustment amount Δω at point A, the power fluctuation ΔP ₁ of the traditional droop curve is larger than the power fluctuation ΔP ₂ of the new droop curve. In the AC microgrid, the change rate of the output power of the inverter is greatly reduced, which effectively avoids the power distribution error.

As shown in Figure 5, when facing the voltage regulation target of U ₁ →U ₂ , the traditional droop reactive power compensation control, because the droop coefficient is fixed, the reactive power regulation amount is ΔQ ₁ . With the dynamic droop reactive power control proposed in this paper, the dynamic reactive power droop coefficient _ni changes in real time with the difference between the current voltage and the target voltage. Facing the same voltage regulation target, the reactive power regulation amount is ΔQ ₂ (ΔQ ₂ <ΔQ ₁ ), the reactive power compensation range is reduced, the voltage regulation deviation is effectively reduced, and the impact on the AC microgrid control system is less.

As shown in Figure 6, the control process of the grid-connected inverter at a single power point is as follows: by measuring the voltage and the output current of the inverter, the coordinates are transformed to the actual values of the voltage and current in the d-q coordinate system, respectively, and obtained through the power calculation link. The active power P and reactive power Q are controlled by the new dynamic droop control link, and the outer loop control component is obtained through the coordinate transformation link. switch drive signal.

In order to verify the effectiveness of the microgrid grid-connected inverter control method using the droop dynamic coefficient proposed by the present invention. In the Matlab/Simulink microgrid simulation model with two distributed power points, the line parameters are shown in Table 1.

Table 1 Simulation parameter table

The simulated load surge condition is: when the system runs stably for 0.2s, the load power suddenly increases from 22kW to 30kW.

As shown in Figure 7, when the microgrid load suddenly increases from 22kW to 30kW in 0.2s, the output power of the DG ₁ inverter using conventional active droop control changes from 11kW to 16.1kW, and the output power of the DG ₂ inverter changes from 10.2 When the kW changes to 12.5kW, the relative error reaches 10%, and there is a problem that the power distribution deviation is too large.

As shown in Figure 8, the microgrid load suddenly increased from 22kW to 30kW in 0.2s, the output power of the DG ₁ inverter using dynamic coefficient active droop control was changed from 11kW to 14.3kW, and the output power of the DG ₂ inverter was changed from 10.2 kW changes to 15.5kW, the relative error is only 2.6%, which avoids the poor characteristics of the traditional droop control strategy. To a certain extent, the capacity of the microgrid is used to dynamically adjust the power distribution to improve the balance control in a short time. Purpose.

As shown in Figure 9, when the load of the grid increases by 12kW in 0.2s, the traditional reactive power droop control faces a sudden increase in the load, and there is an overshoot during the voltage regulation process of the voltage bus and cannot return to the original rated voltage, and there is always a 42V voltage deviation , the voltage cannot be precisely controlled.

As shown in Figure 10, when the grid load is increased by 12kW in 0.2s, the dynamic coefficient reactive power droop control is adopted, the dynamic regulation performance of the bus voltage is good, the voltage regulation can be maintained within the set small offset range, and the voltage can also be Restoring to the rated voltage eliminates the steady-state voltage deviation and improves the utilization of distributed power points in the microgrid.

The above content is only to illustrate the technical idea of the present invention, and cannot limit the protection scope of the present invention. Any changes made on the basis of the technical solution according to the technical idea proposed by the present invention all fall within the scope of the claims of the present invention. within the scope of protection.

Claims (7)

1. A microgrid grid-connected inverter control method using droop dynamic coefficient is characterized in that, it includes the following steps:

   1) Establish a mathematical model of the output active and reactive power of the microgrid grid-connected inverter;

   2) Simplify the mathematical model of the inverter output active and reactive power in step 1);

   3) Simulate the droop characteristics of the synchronous generator to control the microgrid grid-connected inverter, and establish the traditional droop control equation;

   4) Improve the droop coefficients m and n in the traditional droop control equation in step 3), introduce dynamic droop coefficients m _i , _ni , i=1, 2, 3,...n, and reduce power by automatically adjusting the droop coefficient allocation bias;

   5) replacing step 4) dynamic droop coefficients m _i , n _i with step 3) traditional droop coefficients m, n, the new dynamic droop control equation obtained;

   6) The new dynamic droop control equation obtained in step 5) is applied to the power and voltage control links of the microgrid grid-connected inverter to improve the power distribution accuracy of the microgrid grid-connected inverter control system and reduce the voltage regulation deviation.
2. A microgrid grid-connected inverter control method using a droop dynamic coefficient according to claim 1, wherein step 1) establishes a mathematical model of the output active and reactive power of the microgrid grid-connected inverter:

   

   Among them: _{Pi is the active power output by the inverter; Q i is the reactive power output by the inverter; U i is the} output voltage of the inverter; U ₀ is the voltage across the load impedance; δ _i is the power angle; Z _i = R _i +JX _i is the line equivalent impedance; R _i is the line resistance; X _i is the line inductive reactance.
3. A microgrid grid-connected inverter control method using a droop dynamic coefficient according to claim 2, characterized in that, the specific implementation method of step 2) is: according to the multi-inverter parallel connection in the low-voltage microgrid, the line impedance It is resistive, that is, Ri ＞＞X _i , Ri _{≈Z i ,} Xi ≈0 _, and the mathematical model of the active and reactive power output by the inverter in step 1) is simplified:

   
4. A microgrid grid-connected inverter control method using a droop dynamic coefficient according to claim 3, characterized in that, the specific implementation method of step 3) is: simulating the droop characteristic of the synchronous generator to reverse the microgrid grid-connected inverter. The inverter is controlled, and the traditional droop control equation is established:

   

   Among them: ω is the output voltage angular frequency of the controlled inverter; U is the output voltage amplitude of the controlled inverter; ω ₀ is the reference value of the no-load output voltage angular frequency; U ₀ is the no-load output voltage amplitude reference value; m is the active power droop coefficient; n is the reactive power droop coefficient; P is the active power distributed by the load; Q is the reactive power distributed by the load.
5. A microgrid grid-connected inverter control method using droop dynamic coefficient according to claim 4, characterized in that, the specific implementation method of step 4) is: the droop coefficient m, n is improved by introducing dynamic droop coefficients m _i , _ni ,

   

   Among them: ω _max , ω _min , U _max , and U _min are the upper and lower thresholds of the output voltage angular frequency and voltage amplitude; ΔP and ΔQ are the difference between the current output active and reactive power and the target active and reactive power.
6. A microgrid grid-connected inverter control method using a droop dynamic coefficient according to claim 5, wherein the specific implementation method of step 5) is: replacing the dynamic droop coefficients m _i and n _i of step 4) with Step 3) The traditional droop coefficients m, n, the obtained new dynamic droop control equation:

   
7. A microgrid grid-connected inverter control method using a droop dynamic coefficient according to claim 1, wherein the specific implementation method of step 6) is: the new active power-frequency dynamic droop control method obtained in step 5) is used. The equation is applied to the power control of the microgrid grid-connected inverter, and the change rate of the inverter output power is greatly reduced, avoiding the power distribution error.

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