CN112821453A - Power control method and device of grid-connected inverter and grid-connected inverter - Google Patents

Abstract

The scheme discloses a power control method of a grid-connected inverter, which is implemented in an alpha and beta two-phase static coordinate system and comprises the following steps: calculating the feedback power of the power grid based on the power of the power grid and the expansion power; determining a reference power value of the grid-connected inverter based on the limit of the newly-viewed power; and calculating the output voltage of the grid-connected inverter based on the difference value of the reference power value and the feedback power value. The method can realize flexible control of the grid-connected inverter without phase-locked loop and rotating coordinate transformation, thereby realizing flexible regulation of the output reactive power and active power of the grid-connected inverter under asymmetric voltage drop, and having the characteristics of convenient implementation, simple control, fast dynamic response and the like.

Description

Power control method and device for grid-connected inverter and grid-connected inverter

technical field

The invention relates to the technical field of regional security, in particular to a power control method and device of a grid-connected inverter and a grid-connected inverter.

Background technique

Voltage source inverters (VSIs) are widely used as grid interfaces in renewable energy systems, distributed generation systems, and electrified transportation systems to convert direct current to alternating current. However, due to unbalanced loads, short-circuit faults, etc., the VSI may operate under asymmetrical voltage dips. For the control of VSI under asymmetric voltage sag, direct power control (DPC) has been widely used due to its advantages of no current control loop, fast dynamic response, and simple structure. Although the direct power control based on the look-up table method has a simple structure and fast dynamic response, it has problems such as large steady-state control ripple and unstable switching frequency. In order to solve these problems, on the one hand, model predictive control-based direct power control (MPC-DPC) introduces an objective function, and takes the optimal objective function as the basis for selecting the effective voltage vector per cycle, and then introduces a zero-voltage vector for modulation to obtain A stable switching frequency is achieved, and the steady-state power ripple is reduced, but the objective function of all voltage vectors needs to be calculated every cycle, and the calculation is more complicated. The other is space vector modulation based direct power control (SVM-DPC), in which SVM is introduced to synthesize voltage vectors instead of the cost function and voltage vector selection in MPC-DPC. In [1], a Proportional-Integral-Resonant controller (PIR) controller is developed in an improved DPC with adjustable power compensation. However, power compensation is calculated based on positive and negative sequence voltages and currents. Then, in order to eliminate sequence extraction, a sliding mode control method is used in [2], and a DPC based on extended active and reactive power is proposed. However, the implementation of the sliding mode controller is complex and it is difficult to guarantee convergence to its equilibrium point. References [3, 4] proposed a grid voltage modulation DPC with steady-state performance. But this method is only suitable for voltage-balanced grids. In the event of an asymmetric sag of the grid voltage, the converter current may exceed its maximum value to track the predetermined average power due to the drop in the positive sequence voltage. Therefore, the current/power limiter is critical to limit the current to a set threshold below the asymmetric voltage dip. In [5], the current limiter is designed based on its root mean square (RMS). However, the speed of RMS calculation is a major obstacle to application. In [6], an individual current limiter for each phase is proposed based on the current magnitude. However, differences in current magnitude can introduce high-frequency noise and numerical errors.

SUMMARY OF THE INVENTION

One purpose of this solution is to provide a power control method for a grid-connected inverter, which can realize flexible control of the grid-connected inverter without a phase-locked loop and rotational coordinate transformation, so as to achieve a The flexible adjustment of the output reactive power and active power of the grid-connected inverter has the characteristics of convenient implementation, simple control, and fast dynamic response.

Another object of the present solution is to provide an apparatus and apparatus for performing the above-mentioned other methods.

In order to achieve the above purpose, the scheme is as follows:

A power control method for a grid-connected inverter, which is implemented in an αβ two-phase stationary coordinate system, comprising:

Calculate the feedback power of the grid based on the grid power and extended power;

Determine the reference power value of the grid-connected inverter based on the constraints on the new apparent power;

determining a three-phase output voltage of the grid-connected inverter based on the difference between the reference power value and the feedback power value;

Wherein, the grid power includes grid active power and grid reactive power;

The reference power includes reference active power and reference reactive power of the grid-connected inverter;

The extended power is the value of the product of the grid voltage and grid current delayed by a quarter cycle;

The new apparent power is the minimum value of the grid active power under unity power factor and the grid reactive power under zero power factor.

Preferably, the feedback power of the grid is calculated based on the grid power and the extended power as shown in formula (19):

Among them, P _fb represents the feedback power of the grid active power, Q _fb represents the feedback power of the grid reactive power; P _ex represents the extended power of the grid active power, Q _ex represents the extended power of the grid reactive power; P represents the grid active power , Q represents the reactive power of the grid; λ represents an adjustable parameter, λ∈[0,1].

Preferably, the determining the reference power value of the grid-connected inverter based on the restriction on the new apparent power includes: determining the reference reactive power value of the grid-connected inverter according to formula (29),

Among them, Q _ref is the reference reactive power value of the grid-connected inverter, Q _max is the maximum output reactive power of the grid-connected inverter under the positive sequence voltage, U _g+ is the positive sequence voltage, and k _Q is the proportional coefficient;

Determine the reference active power value of the grid-connected inverter according to formula (31),

in,

P _max is the maximum allowable active power of the grid-connected inverter, P _ref is the reference active power of the grid-connected inverter, and APL is the maximum power value provided by the grid-connected inverter.

Preferably, determining the three-phase output voltage of the grid-connected inverter based on the difference between the reference power value and the feedback power value includes:

Obtaining the active modulation voltage v _P and the reactive modulation voltage v _Q of the grid-connected inverter based on the difference between the reference power value and the feedback power value;

obtain the output modulation voltages _vα and _vβ of the grid-connected inverter in the αβ two-phase stationary coordinate system based on the active power modulation voltage v _P and the reactive power modulation voltage v _Q ;

Pulse width modulation is performed on the output modulation voltages v _α and v _β to obtain the three-phase output voltage of the grid-connected inverter.

In a second aspect, a power control device for a grid-connected inverter is provided, the device comprising:

The feedback power calculation unit calculates the feedback power of the grid based on the grid power and the extended power;

a reference power determination unit, which determines a reference power value of the grid-connected inverter based on the restriction on the new apparent power;

an adjustment unit, which determines the three-phase output voltage of the grid-connected inverter based on the difference between the reference power value and the feedback power value;

Wherein, the grid power includes grid active power and grid reactive power;

The reference power includes reference active power and reference reactive power of the grid-connected inverter;

The extended power is the value of the product of the grid voltage and grid current delayed by a quarter cycle;

The new apparent power is the minimum value of the grid active power under unity power factor and the grid reactive power under zero power factor.

Preferably, the feedback power calculation unit calculates the feedback power of the grid based on the following formula:

Among them, P _fb represents the feedback power of the grid active power, Q _fb represents the feedback power of the grid reactive power; P _ex represents the extended power of the grid active power, Q _ex represents the extended power of the grid reactive power; P represents the grid active power , Q represents the reactive power of the grid; λ represents an adjustable parameter, λ∈[0,1].

Preferably, the determination of the reference power value of the grid-connected inverter by the reference power determination unit includes performing the following operations:

Determine the reference reactive power value of the grid-connected inverter according to equation (29),

Among them, Q _ref is the reference reactive power value of the grid-connected inverter, Q _max is the maximum output reactive power of the grid-connected inverter under the positive sequence voltage, U _g+ is the positive sequence voltage, and k _Q is the proportional coefficient;

Determine the reference active power value of the grid-connected inverter according to formula (31),

in,

P _max is the maximum allowable active power of the grid-connected inverter, P _ref is the reference active power of the grid-connected inverter, and APL is the maximum power value provided by the grid-connected inverter.

Preferably, the adjusting unit determining the three-phase output voltage of the grid-connected inverter includes performing the following operations:

Obtaining the active modulation voltage v _P and the reactive modulation voltage v _Q of the grid-connected inverter based on the difference between the reference power value and the feedback power value;

obtain the output modulation voltages _vα and _vβ of the grid-connected inverter in the αβ two-phase stationary coordinate system based on the active power modulation voltage v _P and the reactive power modulation voltage v _Q ;

Pulse width modulation is performed on the output modulation voltages v _α and v _β to obtain the three-phase output voltage of the grid-connected inverter.

In a third aspect, a grid-connected inverter is provided, including:

memory;

processor;

Wherein, an application program executable by the processor is stored in the memory, so as to cause the processor to execute the power control method for a grid-connected inverter as described in any one of the above.

In a fourth aspect, a computer-readable storage medium is provided, and a computer program is stored on the computer-readable storage medium, and when the computer program is executed by a processor, the power of the grid-connected inverter according to any one of the above-mentioned items is realized. Control Method.

The beneficial effects of this program are as follows:

The method is implemented in a two-phase static coordinate system, and can realize flexible control of the grid-connected inverter without the need for phase-locked loop and rotation coordinate transformation, so as to realize the grid-connected inverter under the condition of asymmetric voltage drop. The flexible adjustment of output reactive power and active power has the characteristics of convenient implementation, simple control and fast dynamic response. At the same time, the method has good dynamic response and frequency adaptability, and can realize the trade-off between negative sequence current and oscillating active/reactive power components according to external requirements, so that the grid-connected inverter has flexible grid-connected characteristics.

Description of drawings

In order to illustrate the implementation of this solution more clearly, the following will briefly introduce the accompanying drawings used in the description of the embodiments. Obviously, the accompanying drawings in the following description are only some embodiments of this solution, which are common in the art. As far as technical personnel are concerned, other drawings can also be obtained based on these drawings without any creative effort.

Fig. 1 is a flow chart of a power control method of a grid-connected inverter;

FIG. 2 is a block diagram of controlling the power of the grid-connected inverter;

3 is a schematic diagram of a grid-connected inverter power control device;

4 is an equivalent circuit diagram of a simulation model of the grid-connected inverter of the embodiment;

FIG. 5 is a simulation experiment result of the power control of the grid-connected inverter under the asymmetric voltage drop of the embodiment.

Detailed ways

The embodiments of this solution will be described in further detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the solution, rather than an exhaustive list of all the embodiments. It should be noted that the embodiments in this solution and the features of the embodiments may be combined with each other under the condition of no conflict.

The terms "first", "second", etc. (if present) in the description and claims and in the aforementioned drawings are used to distinguish between similar objects and are not necessarily used to describe a particular order or sequence. It is to be understood that data so used may be interchanged under appropriate circumstances so that the embodiments 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, not necessarily limited to those expressly listed but may include other steps or units not expressly listed or inherent to these processes, methods, products or devices.

It should be understood that the term "and/or" used in this document is only an association relationship to describe the associated objects, indicating that there may be three kinds of relationships, for example, A and/or B, which may indicate that A exists alone, and A and B exist at the same time. B, there are three cases of B alone. In addition, the character "/" in this document generally indicates that the related objects are an "or" relationship.

Depending on the context, the word "if" as used herein can be interpreted as "at" or "when" or "in response to determining" or "in response to detecting." Similarly, the phrases "if determined" or "if detected (the stated condition or event)" can be interpreted as "when determined" or "in response to determining" or "when detected (the stated condition or event)," depending on the context )" or "in response to detection (a stated condition or event)".

After research, the applicant found that the introduction of adjustable parameters to connect the grid power calculated in the classical way with the extended power defined in this solution, and the introduction of a proportional-integral-resonant (Proportional-Integral-Resonant controller, PIR) controller can realize the connection to the grid. Tracking of inverter active and reactive power and elimination of double frequency power oscillation, so that a trade-off between negative sequence current and double frequency power oscillation can be made according to external requirements. In addition, the applicant has also designed a power limiter with adjustable parameters and a voltage unbalance factor, which can ensure that the grid-connected inverter can inject maximum power into the grid under various asymmetrical voltage sag conditions without causing the phase current to exceed its threshold.

In order to facilitate the research, the physical quantities in the abc three-phase stationary coordinate system are transformed into the physical quantities in the αβ two-phase stationary coordinate system through Clark transformation. On this basis, a power control method for grid-connected inverters is proposed.

As shown in Figure 1, the method includes:

S100, calculate the feedback power of the grid based on the grid power and the extended power;

The grid power includes grid active power P and grid reactive power Q; according to the instantaneous power theory, active power P and reactive power Q are shown in formula (3),

In formula (3), u _gαβ is the vector representation of the grid voltage in the αβ two-phase stationary coordinate system, i _αβ is the vector representation of the grid current in the αβ two-phase stationary coordinate system, u _gαβ = u _gα +ju _gβ , i _αβ = i _α +ji _β , "*" represents the conjugate of complex numbers, j is the imaginary unit; u _gα and u _gβ are the grid voltage of VSI in the αβ two-phase stationary coordinate system,

The grid voltages u _gα and u _gβ of VSI in the αβ two-phase stationary coordinate system are expressed as shown in formula (1-1),

Equation (1-1) is to transform the three-phase voltage in the abc three-phase static coordinate system into the grid voltage expression in the αβ two-phase static coordinate system by Clark transformation, u _ga , u _gb , and u _gc are the VSI grid side voltages. grid three-phase voltage,

As shown in Figure 2, the instantaneous values of the three-phase voltage and current on the VSI grid side are collected by sensors, and _{u ga} , u _gb , u _gc , ia , _ib , and _ic represent the collected three-phase voltages on the VSI grid side, respectively , the instantaneous value of the three-phase current,

The relationship between the three-phase voltage and the instantaneous value of the three-phase current on the VSI grid side is shown in formula (1),

In formula (1), v _a , v _b , and v _c are the three-phase voltages output by the VSI respectively, L is the inductance of the incoming line reactor of each phase, and R is the line resistance of each phase including the reactor resistance;

Through Clark transformation, in the αβ two-phase stationary coordinate system, Equation (1) can be expressed as Equation (2),

In formula (2), i _α and i _β are the output current of the VSI in the αβ two-phase static coordinate system, v _α and v _β are the output modulation voltage of the VSI in the αβ two-phase static coordinate system, and v _α is on the α axis. , v _β is on the β axis;

When an asymmetrical voltage dip occurs, the voltage and current can be decomposed into positive sequence and negative sequence components as shown in equation (5).

In formula (5), u _gα and u _gβ are the grid voltages of the VSI in the αβ two-phase static coordinate system, and i _α and i _β are the output currents of the VSI in the αβ two-phase static coordinate system;

The time domain expressions of the positive and negative sequence components are shown in equations (6) and (7),

In formulas (6) and (7), U _g+ , U _g- and I _g+ , I _g- are the amplitudes of positive and negative sequence voltage and current, respectively, θ _u+ , θ _u- and θ ₊ , θ _- are positive The initial phase angle of negative sequence voltage and current, ω is the angular frequency of grid voltage;

According to equations (3) and (5), the active power and reactive power of the grid are expressed as the sum of the DC component and the oscillation component as shown in equation (8),

In formula (8), P represents the grid active power, P _cl0 represents the DC component of the grid active power, P _cli2 represents the double frequency component of the grid active power caused by the positive sequence voltage and negative sequence current, and P _clu2 represents the negative sequence voltage. and the double frequency component of the grid active power caused by the positive sequence current, Q represents the grid reactive power, Q _cl0 represents the DC component of the grid reactive power, and Q cli2 represents the grid reactive power _doubled by the positive sequence voltage and negative sequence current. Frequency component, Q _clu2 represents the double frequency component of grid reactive power caused by negative sequence voltage and positive sequence current;

In formula (8),

The extended active power and extended reactive power are calculated by multiplying the voltage and current delayed by a quarter cycle. The extended active power and reactive power are shown in equation (12),

表示αβ坐标系下延迟四分之一个周期的电压向量的共轭，“*”表示复数的共轭，即

In formula (12),

Represents the conjugate of the voltage vector delayed by a quarter cycle in the αβ coordinate system, and "*" represents the conjugate of the complex number, namely

u′_gα+、u′_gα-、u′_gβ+和u′_gβ-如式(11)所示In formula (i)

u′ _gα+ , u′ _gα- , u′ _gβ+ and u′ _gβ- are shown in formula (11)

i _αβ represents the grid current vector in the αβ two-phase stationary coordinate system, that is, i _αβ = i _α +ji _β , i _α and i _β are shown in the formula

(5) and equation (7) P _ex represents the extended active power, P _ex0 represents the DC component of the extended active power, P _exi2 represents the extended active power double frequency component caused by the positive sequence voltage and negative sequence current, and P _exu2 represents the The double frequency component of the extended active power caused by the negative sequence voltage and the positive sequence current, Q _ex represents the extended reactive power, Q _ex0 represents the DC component of the extended reactive power, and Q _exi2 represents the extended zero caused by the positive sequence voltage and the negative sequence current. is the double frequency component of the active power, Q _exu2 represents the double frequency component of the extended reactive power caused by the negative sequence voltage and positive sequence current,

In formula (12),

Introduce adjustable parameters to connect the grid power calculated in the classical way with the extended power defined in this scheme, and introduce a proportional-integral-resonant (PIR) controller to realize the control of the active power of the grid-connected inverter. and reactive power tracking and elimination of double frequency power oscillation, so that a trade-off between negative sequence current and double frequency power oscillation can be made according to external requirements.

As shown in formula (4), the rate of change of the active power and reactive power of the grid is calculated,

The positive and negative sequence voltages in the αβ coordinate system are introduced to represent the rate of change of the active power and reactive power of the grid,

As shown in Equation (11), the positive and negative sequence voltages in the αβ coordinate system are expressed and related as follows:

Substitute equations (2), (3), (11), (13), (14) into equation (4) to obtain the rate of change of the grid active power and reactive power as shown in equation (15),

u_gα、u_gβ为VSI在αβ两相静止坐标系中的电网电压；v_P表示并网逆变器的有功调制电压，v_Q表示并网逆变器的无功调制电压；以v′_P、v′_Q分别作为式(15)中

表示的电网有功功率和无功功率的变化率经拉普拉斯变换后的表达式；求出v′_P、v′_Q(或

)后通过式(15)，可以求得αβ坐标系中功率调制电压v_P和v_Q；以G(s)表示功率控制器的传递函数，P_fb表示电网有功功率的反馈功率，Q_fb表示电网无功功率的反馈功率，P_ref表示VSI的参考有功功率，Q_ref表示VSI的参考无功功率，得出式(17)，In formula (15), U _g is the grid voltage amplitude,

u _gα and u _gβ are the grid voltage of VSI in the αβ two-phase static coordinate system; v _P represents the active power modulation voltage of the grid-connected inverter, and v _Q represents the reactive power modulation voltage of the grid-connected inverter _; , v′ _Q respectively as in formula (15)

The expression of the change rate of the grid active power and reactive power represented by the Laplace transform; find v′ _P , v′ _Q (or

) and then through formula (15), the power modulation voltages v _P and v _Q in the αβ coordinate system can be obtained; G(s) represents the transfer function of the power controller, P _fb represents the feedback power of the grid active power, and Q _fb represents Feedback power of grid reactive power, P _ref represents the reference active power of VSI, Q _ref represents the reference reactive power of VSI, and Equation (17) is obtained,

The adjustable parameter λ is introduced to connect the grid power calculated in the classical way with the extended power defined in this scheme, and the feedback power in Eq. (17) expressed by the adjustable parameter λ as shown in Eq. parameter λ∈[0,1],

After the feedback power is obtained based on the grid power and extended power calculation, the relationship between the feedback power and the transfer function of the power controller is further analyzed.

Equation (18) is obtained according to equations (9) and (13), equations (10) and (14),

It can be seen from equation (18) that the oscillating power components of grid power and extended power caused by negative sequence current and positive sequence voltage are equal, while the oscillating power components caused by positive sequence current and negative sequence voltage are opposite;

Since the grid power is expressed as the sum of the DC component and the oscillating component in Equation (8), and the feedback power is expressed by the grid power with adjustable parameters and the extended power in Equation (19), it is concluded that the feedback power can be calculated by the DC component and the double frequency oscillation component; and because it is known that the power controller can not only adjust the DC component, but also eliminate the double frequency oscillation component, so the PIR controller with the cut-off frequency ω _c can be used as the power control. Since the PIR controller can eliminate the steady-state error twice the grid frequency, the oscillation component can be set to 0 by setting a specific negative sequence current. Substitute equation (18) into equation (19) to obtain equation (21) The governing equations shown

Substituting equations (9) and (10) into equation (21), the negative sequence current represented by the adjustable parameter λ as shown in equation (22) and the adjustable parameter λ as shown in equation (23) are obtained. The initial phase angle expression of ,

It can be seen from equations (22) and (23) that as long as a proper negative sequence current is injected without injecting third-order harmonic current, the output current of the VSI can be kept highly close to a sine wave;

Therefore, the feedback power calculated based on the grid power and the extended power in this method shows that the output power of the grid-connected inverter can be adjusted without any power compensation. The adjustment is realized by introducing the adjustable parameter λ, so this method can The priority is determined between the negative sequence current and the active/reactive oscillating components according to the external requirements, and all calculations in the power regulation are carried out in the static coordinate system, without the need for phase-locked loops and PARK transformations.

According to the previous discussion, when a PIR controller with cut-off frequency ω _c is used as the power controller, the power controller transfer function G(s) in equation (17) can be expressed as equation (20);

In formula (20), k _p is the proportional coefficient of the proportional link of the PIR controller, _ki is the integral coefficient of the integral link of the PIR controller, k _r is the integral coefficient of the resonant link of the PIR controller, and ω is the grid angular frequency.

S200, determining a reference power value of the grid-connected inverter based on the restriction on the new apparent power;

The reference power of VSI includes reference active power and reference reactive power of VSI;

When the grid voltage drops, the current may exceed the set threshold, tripping the overcurrent protection, so it is necessary to limit the apparent power under the asymmetrical voltage drop.

This method defines an apparent power with a new calculation method and calls it new apparent power (NAP). The new apparent power in this method is the active power under unity power factor and no power under zero power factor. The minimum value in the work power, that is, the new apparent power expression shown in Equation (24),

According to equations (22) and (23), P _cl0 and Q _cl0 are deduced, and the expression when λ∈[0,1] is obtained is as shown in equation (25),

In formula (25), k _pn is the voltage unbalance factor, k _pn =U _g- /U _g+ ,

Therefore, equation (24) can be expressed as,

When the positive sequence and negative sequence current vectors are in the same direction, the maximum current vector coincides with the A (or B, C) axis, then the maximum peak current I _peak of the A (or B, C) phase is equal to the current vector amplitude at this time The value I _max , as shown in equation (27),

I _peak =I _max =I ₊ +I _- =(1+|1-2λ|k _pn )I ₊ ≤I _th (27),

In formula (27), I _th is the safe current threshold value under per unit value; according to formula (26) and (27), the limit value of new apparent power can be obtained as shown in formula (28),

即

Equation (28) expresses the limit on the new apparent power, the limit value

which is

When the voltage drops, the VSI will be required to provide the function of supporting the power grid. Generally, reactive power will be injected first. At this time, the expression of the reference reactive power Q _ref in equation (17) is shown in equation (29),

下VSI的最大输出无功功率，在恶劣条件下，应将VSI的全部容量分配给无功功率，因此应将VSI提供的最大功率设置为APL，即Q_max＝APL；若无特殊说明，比例系数k_Q取2。再根据式(29)得到VSI的最大允许有功功率P_max，P_max的表达式如式(30)所示，In the formula, Q _max is the positive sequence voltage

The maximum output reactive power of the lower VSI, under severe conditions, the full capacity of the VSI should be allocated to the reactive power, so the maximum power provided by the VSI should be set to APL, that is, Q _max = APL; unless otherwise specified, the ratio The coefficient k _Q is taken as 2. Then the maximum allowable active power P _max of the VSI is obtained according to equation (29), and the expression of P _max is shown in equation (30),

During the asymmetric voltage drop, always compare P _max with the reference active power P _ref of the VSI, if P _max >P _ref , the set VSI reference active power can be injected, otherwise only the maximum active power P _max can be injected, the formula The expression of the reference active power P _ref in (17) is shown in equation (31),

S300, determining the three-phase output voltage of the grid-connected inverter based on the difference between the reference power value and the feedback power value;

As shown in Figure 2, after the feedback power is obtained according to equation (19) and the reference power value of VSI is obtained according to equation (29)/(30), the VSI is obtained according to equation (15) and equation (16) when the αβ two-phase static The output modulation voltages v _α and v _β in the coordinate system are pulse width modulated to output the required three-phase voltages to the grid.

In formula (16), v _α represents the output modulation voltage of the grid-connected inverter on the α axis in the αβ two-phase static coordinate system, and v _β represents the β axis of the grid-connected inverter in the αβ two-phase static coordinate system. The output modulation voltage on ; v _P represents the active power modulation voltage of the grid-connected inverter, v _Q represents the reactive power modulation voltage of the grid-connected inverter;

Equation (16) represents the conversion relationship between v _α , v _β and v _P , v _Q. When v _P and v _Q are converted into v _α and v _β according to formula (16), the two-phase αβ coordinate system is v _α and v _β are transformed into three-phase voltages v _a , v _b , v _c in the three-phase abc coordinate system through the Clark inverse transformation shown in equation (16-1), and then the pulse width modulation (PWM) technology is used to convert v _a , v _b , and v _c are transformed into the three-phase output voltage of VSI. Therefore, obtaining v _α and v _β through formula (16) is equivalent to obtaining a switch that can control the output of the inverter, so as to realize the reverse operation of the grid-connected inverter. Flexible control of inverter power. And the whole control of the method is implemented in the αβ two-phase stationary coordinate system, so it does not depend on the phase-locked loop and the rotation coordinate transformation.

As shown in Figure 3, a power control device 1 of a grid-connected inverter includes:

The feedback power calculation unit 10 calculates the feedback power of the power grid based on the power grid power and the extended power;

a reference power determination unit 20, for determining a reference power value of the grid-connected inverter based on the restriction on the new apparent power;

The adjusting unit 30 determines the three-phase output voltage of the grid-connected inverter based on the difference between the reference power value and the feedback power value;

Wherein, the grid power includes grid active power and grid reactive power;

The reference power includes reference active power and reference reactive power of the grid-connected inverter;

The extended power is the product value of the conjugate of the grid voltage vector delayed by a quarter cycle and the grid current;

The new apparent power is the minimum value of the grid active power under unity power factor and the grid reactive power under zero power factor.

The solution also provides a grid-connected inverter, including a memory and a processor;

Wherein, an application program executable by the processor is stored in the memory, so as to make the processor execute the power control method of the grid-connected inverter of the present solution. The memory may be a computer readable storage medium, which may take the form of a portable compact disc read only memory (CD-ROM) and include program code, and which may run on a device such as a personal computer. However, the program product of the present invention is not limited thereto, and in this document, a readable storage medium may be any tangible medium that contains or stores a program that can be used by or in conjunction with an instruction execution system, apparatus, or device.

The program product may employ any combination of one or more readable media. The readable medium may be a readable signal medium or a readable storage medium. The readable storage medium may be, for example, but not limited to, an electrical, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus or device, or a combination of any of the above. More specific examples (non-exhaustive list) of readable storage media include: electrical connections with one or more wires, portable disks, hard disks, random access memory (RAM), read only memory (ROM), erasable programmable read only memory (EPROM or flash memory), optical fiber, portable compact disk read only memory (CD-ROM), optical storage devices, magnetic storage devices, or any suitable combination of the foregoing.

A computer-readable storage medium may include a data signal propagated in baseband or as part of a carrier wave with readable program code embodied thereon. Such propagated data signals may take a variety of forms, including but not limited to electromagnetic signals, optical signals, or any suitable combination of the foregoing. A readable signal medium can also be any readable medium, other than a readable storage medium, that can transmit, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device.

Program code embodied on a computer-readable storage medium may be transmitted using any suitable medium, including but not limited to wireless, wireline, optical fiber cable, RF, etc., or any suitable combination of the foregoing.

Program code for performing the operations of the present scheme may be written in any combination of one or more programming languages, including object-oriented programming languages - such as JAvA, C++, etc., as well as conventional procedural Programming Language - Such as "C" language or similar programming language. The program code may execute entirely on the user computing device, partly on the user computing device and partly on a remote computing device, or entirely on the remote computing device or server. In the case of a remote computing device, the remote computing device may be connected to the user computing device through any kind of network, including a local area network (LAN) or a wide area network (WAN), or may be connected to an external computing device (eg, using an Internet service provider business via an Internet connection).

The solution will be described below with reference to the simulation model shown in FIG. 4 . Use Matlab/Simulink to build the simulation model of the main circuit as shown in Figure 4. The grid voltage frequency is set to 50Hz, the reference active power of the inverter (VSI) is P _ref =1.0, and the reference reactive power of the VSI is Q _ref =0. The VSI DC side voltage source is 1.5kV. In 0.5s, the A-phase and C-phase voltages drop to 0.5 times the original value, and the adjustable parameter λ increases from 0 to 1 at a constant speed within 1 to 2s; the above situation is shown in Figure 5 The simulation results below.

It can be seen from the figure that at 1s, λ=0, the output three-phase current i _abc of the VSI is unbalanced and contains negative sequence components, the reactive power Q output by the VSI contains a double frequency oscillation component, and the active power P remains constant The value is 0.4 (pu), that is, the active power can be controlled to be constant at this time, that is, λ takes 0 to eliminate the double frequency component of the VSI output active power; at 1.5s, λ=0.5, the active power and reactive power both contain twice the frequency frequency oscillation component, and the output current i _abc is three-phase symmetrical without negative sequence current, that is, the three-phase symmetry of the output current can be controlled at this time, that is, λ takes 0.5 to keep the VSI output current highly close to a sine wave; at 2s, λ =1, the output current i _abc is unbalanced and contains negative sequence components, the active power contains double frequency oscillation components, and the reactive power Q maintains a constant value of 0.33 (pu), that is, the reactive power can be controlled to be constant at this time, that is, λ takes 1 can eliminate the double frequency component of VSI output reactive power.

It can be seen that by properly adjusting the values of k _p , _ki and k _r in equation (20), the DC component of the feedback power can be adjusted, and the oscillation component of the double frequency can also be eliminated; as long as the appropriate k _p , _ki and k _r value, set the appropriate λ value according to external requirements, the VSI output current can be kept highly close to the sine wave, and the double frequency component in the VSI output active or reactive power can also be eliminated.

It can be seen that the power control strategy of the grid-connected inverter proposed in this scheme under unbalanced grid voltage can be adjusted according to the size of λ to achieve a trade-off between negative sequence current and oscillating active/reactive power components according to external requirements. , so that the VSI has flexible grid-connected characteristics.

References in the background art are as follows:

1.H.Nian,Y.Shen,H.Yang,and Y.Quan,“Flexible grid connection techniqueof voltage-source inverter under unbalanced grid conditions based on directpower control,”IEEE Trans.Ind.Appl.,vol.51, no.5, pp.4041-4050, Sep-Oct, 2015.

2.D.Sun,X.Wang,H.Nian,and Z.Q.zhu,“A sliding-mode direct powercontrol strategy for DFIG under both balanced and unbalanced grid conditions using extended active power,”IEEE Trans.Power Electron.,vol.33 , no.2, pp.1313-1322, Feb, 2017.

3. Y.H.Gui, C.Kim, C.C.Chung, J.M.Guerrero, Y.J.Guan, and J.C.Vasquez, “Improved direct power control for grid-connected voltage source converters,” IEEE Trans.Ind.Electron., vol.65, no. 10, pp.8041-8051, Oct, 2018.

4. Y.H.Gui, M.S.Li, J.H.Lu, S.Golestan, J.M.Guerrero, and J.C.Vasquez, "AVoltage modulated DPC approach for three-phase PWM rectifier," IEEETrans.Ind.Electron.,vol.65,no.10, pp.7612-7619, Oct, 2018.

5. H.D. Tafti, A.I. Maswood, G. Konstantinou, J. Pou, and P. Acuna, “Active/reactive power control of photovoltaic grid-tied inverters with peak currentlimitation and zero active power oscillation during unbalanced voltage sags,” IET Power Electron .,vol.11,no.6,pp.1066-1073,Jun,2018.

6. B. Mahamedi, M. Eskandari, J. E. Fletcher, and J. Zhu, “Sequence-based control strategy with current limiting for the fault ride-through of inverter-interfaced distributed generators,” IEEE Trans. Sustain. Energy, vol. 11, no.1, pp.165-174, Jan, 2020.

Obviously, the above-mentioned embodiments of the present invention are only examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Changes or changes in other different forms cannot be exhausted here, and all obvious changes or changes derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims (10)

1. A power control method of a grid-connected inverter is characterized in that the method is implemented in an alpha beta two-phase static coordinate system and comprises the following steps:

calculating the feedback power of the power grid based on the power of the power grid and the expansion power;

determining a reference power value of the grid-connected inverter based on the limit of the newly-viewed power;

determining three-phase output voltage of the grid-connected inverter based on the difference value of the reference power value and the feedback power value;

wherein the power of the power grid comprises active power of the power grid and reactive power of the power grid;

the reference power comprises reference active power and reference reactive power of a grid-connected inverter;

the expanded power is the product value of the grid voltage and the grid current with a delay of one quarter cycle;

and the new apparent power is the minimum value of the active power of the power grid under the unit power factor and the reactive power of the power grid under the zero power factor.

2. The power control method according to claim 1, wherein the calculating of the feedback power of the grid based on the grid power and the developed power is as shown in equation (19):

wherein, P_fbFeedback power, Q, representing the active power of the grid_fbFeedback power representing reactive power of the grid; p_exThe developed power, Q, representing the active power of the grid_exRepresenting the expansion power of the reactive power of the power grid; p represents the active power of the power grid, and Q represents the reactive power of the power grid; λ represents an adjustable parameter, λ ∈ [0,1]]。

3. The power control method of claim 1, wherein the determining the reference power value for the grid-tied inverter based on the limit on the newly-seen power comprises: a reference reactive power value of the grid-connected inverter is determined according to equation (29),

wherein Q is_refReference reactive power value, Q, for grid-connected inverter_maxIs the maximum output reactive power of the grid-connected inverter under the positive sequence voltage,

is a positive sequence voltage, k_QIs a proportionality coefficient;

determining a reference active power value of the grid-connected inverter according to equation (31),

wherein,

P_maxfor maximum allowable active power of the grid-connected inverter, P_refThe APL is the maximum power value provided by the grid-connected inverter for the reference active power of the grid-connected inverter.

4. The power control method of claim 1, wherein determining a three-phase output voltage of a grid-tied inverter based on the difference between the reference power value and the feedback power value comprises:

obtaining the active modulation voltage v of the grid-connected inverter based on the difference value of the reference power value and the feedback power value_PAnd a reactive modulation voltage v_Q；

Based on the active modulation voltage v_PAnd a reactive modulation voltage v_QObtaining output modulation voltage v of grid-connected inverter in alpha and beta two-phase static coordinate system_αAnd v_β；

For output modulation voltage v_αAnd v_βAnd performing pulse width modulation to obtain the three-phase output voltage of the grid-connected inverter.

5. A power control device for a grid-connected inverter, characterized by comprising:

the feedback power calculation unit is used for calculating the feedback power of the power grid based on the power grid power and the expansion power;

the reference power determining unit is used for determining a reference power value of the grid-connected inverter based on the limit of the newly-viewed power;

the adjusting unit is used for determining the three-phase output voltage of the grid-connected inverter based on the difference value of the reference power value and the feedback power value;

wherein the power of the power grid comprises active power of the power grid and reactive power of the power grid;

the reference power comprises reference active power and reference reactive power of a grid-connected inverter;

the expanded power is the product value of the grid voltage and the grid current with a delay of one quarter cycle;

and the new apparent power is the minimum value of the active power of the power grid under the unit power factor and the reactive power of the power grid under the zero power factor.

6. The power control apparatus according to claim 5, wherein the feedback power calculation unit calculates the feedback power of the grid based on the following formula:

wherein, P_fbFeedback power, Q, representing the active power of the grid_fbFeedback power representing reactive power of the grid; p_exThe developed power, Q, representing the active power of the grid_exRepresenting the expansion power of the reactive power of the power grid; p represents the active power of the power grid, and Q represents the reactive power of the power grid; λ represents an adjustable parameter, λ ∈ [0,1]]。

7. The power control apparatus according to claim 5, wherein the reference power determination unit determines the reference power value of the grid-connected inverter includes performing operations of:

a reference reactive power value of the grid-connected inverter is determined according to equation (29),

wherein Q is_refReference reactive power value, Q, for grid-connected inverter_maxIs the maximum output reactive power of the grid-connected inverter under the positive sequence voltage,

is a positive sequence voltage, k_QIs a proportionality coefficient;

determining a reference active power value of the grid-connected inverter according to equation (31),

wherein,

P_maxfor grid-connected invertersMaximum allowable active power, P_refThe APL is the maximum power value provided by the grid-connected inverter for the reference active power of the grid-connected inverter.

8. The power control apparatus of claim 5, wherein the regulating unit determining a three-phase output voltage of a grid-tied inverter comprises:

obtaining the active modulation voltage v of the grid-connected inverter based on the difference value of the reference power value and the feedback power value_PAnd a reactive modulation voltage v_Q；

Based on the active modulation voltage v_PAnd a reactive modulation voltage v_QObtaining output modulation voltage v of grid-connected inverter in alpha and beta two-phase static coordinate system_αAnd v_β；

For output modulation voltage v_αAnd v_βAnd performing pulse width modulation to obtain the three-phase output voltage of the grid-connected inverter.

9. A grid-connected inverter, comprising:

a memory;

a processor;

wherein, the memory stores an application program executable by the processor for causing the processor to execute the power control method of the grid-connected inverter according to any one of claims 1 to 4.

10. A computer-readable storage medium, characterized in that a computer program is stored thereon, which when executed by a processor implements the power control method of the grid-connected inverter according to any one of claims 1 to 4.

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