CN108879775B - Power grid unbalanced photovoltaic inverter coordination control method considering current limit value - Google Patents
Power grid unbalanced photovoltaic inverter coordination control method considering current limit value Download PDFInfo
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Abstract
The invention discloses a grid unbalanced photovoltaic inverter coordination control method considering current limit values, which comprises the steps of firstly analyzing a positive-negative sequence current separation scheme based on a wave trap, then quantizing the output power of an inverter under an unbalanced grid, weighting the quantity influencing photovoltaic active output, reactive output and current harmonic to obtain control quantity k and gamma, and finally optimizing k and gamma by taking the minimum active fluctuation and reactive fluctuation as optimization targets and taking the current harmonic distortion rate and the output current limit value as constraint conditions. When the power grid is unbalanced, the invention ensures that the current quality is qualified and the current is not out of limit, minimizes the fluctuation of active power and reactive power, and improves the stability and reliability of the control system.
Description
Technical Field
The invention relates to the field of electrical information, in particular to a coordination control method for a power grid unbalanced photovoltaic inverter considering a current limit value.
Background
One of the most common faults in an electric power system when the power grid is unbalanced, the factors such as the access of a high-power single-phase load, the unbalanced distribution of the single-phase load in a three-phase system, the randomness of the power consumption of the single-phase load and the like can cause the unbalanced three-phase voltage of the power grid, the sunny and the sunny conditions, and the Guiweihua under the condition of the unbalanced power grid, and the control strategy of the grid-connected inverter is reviewed in [ J ] in the technical report of electricians, 2015,30(14):241 plus 246. When the three-phase voltage of the power grid is unbalanced, negative sequence components can be generated in the voltage of the power grid, and at the moment, because the photovoltaic inverter control strategy based on the balanced power grid lacks control over the negative sequence voltage, a large amount of harmonic waves can be injected into the power grid by the photovoltaic inverter, and secondary pulsation of active power and reactive power can be generated. Therefore, the research on the control strategy of the photovoltaic inverter when the power grid is unbalanced is of great significance.
The main contradiction of the control strategy of the grid unbalanced photovoltaic inverter is the power fluctuation and the current harmonic magnitude. For a three-phase system, there are generally two coordinate systems, namely a two-phase stationary α β coordinate system and two-phase rotating dq coordinate systems. Some researches propose a phase-locked loop-free control strategy based on a static alpha beta coordinate system, firstly, the inverter output under the condition of unbalanced power grid is quantized, the control quantity influencing power fluctuation and current harmonic is obtained through Clark conversion, and the coordination control of the output power/current quality of the photovoltaic inverter, Zhaoxin, Jinxinmin, Zhouyi, and the like is realized by utilizing the weighting idea. Some researches analyze factors influencing active fluctuation, reactive fluctuation and current quality of the inverter under the condition of power grid imbalance based on two rotating dq coordinate systems, obtain two coefficients through weighting, and realize multi-target coordination control of the inverter by utilizing a neural network PI controller, and researches on multi-target coordination control strategies of the grid-connected inverter under the condition of power grid imbalance of sunny and Guiweihua [ J ] in the technical and electrotechnical report, 2015,30(11): 148-. Some studies have also proposed a method of limiting inverter current in the event of an unbalanced fault, which multiplies a ratio of rated current to maximum current to a reference current, and finally the output of the photovoltaic inverter gets a limit Liu W Z, Guo X Q, Sulligoi G, et al.
The research mainly focuses on improving methods for the current limiting problem during the coordinated control and unbalanced fault of the unbalanced power grid inverter, and rarely considers the conditions of the coordinated control and the current limit value at the same time, only considers the condition that the current exceeds the limit during the coordinated control fault to cause photovoltaic grid disconnection, and only considers the condition that the photovoltaic output is reduced when the current exceeds the limit to influence the stability of the system.
Disclosure of Invention
For overcoming the defects or requirements of the existing method, the invention provides a coordination control method of a photovoltaic inverter of an unbalanced power grid considering a current limit value, which solves the problems of current out-of-limit during coordination control and photovoltaic output reduction only considering a current out-of-limit control strategy.
The invention is realized by the following technical scheme:
a grid imbalance photovoltaic inverter coordination control method considering current limits comprises the following steps:
collecting three-phase voltage and three-phase current of a power grid, respectively carrying out coordinate transformation on the three-phase voltage and the three-phase current, and extracting positive and negative sequence components of the voltage and the current after the coordinate transformation by adopting a wave trap;
step two, quantizing the output power of the inverter under the unbalanced power grid, and weighting the quantity influencing photovoltaic active output, reactive output and current harmonic to obtain weight coefficients k and gamma of coordinated control;
thirdly, optimizing the weight coefficients k and gamma by taking the minimum active power fluctuation and reactive power fluctuation as optimization targets and taking the current harmonic distortion rate and the output current limit value as constraint conditions, and constructing a control optimization module;
and step four, obtaining the optimal values of the weight coefficient k and the gamma of the coordinated control through the constructed control optimization module, and performing coordinated control on the unbalanced photovoltaic inverter of the power grid.
Preferably, the step one specifically includes:
collecting three-phase voltage and three-phase current of a power grid and respectively carrying out dq coordinate transformation on the three-phase voltage and the three-phase current; and extracting positive and negative sequence components of the voltage and the current after coordinate transformation by adopting a wave trap to obtain: positive sequence component of three-phase voltage in dq rotation coordinate system
And negative sequence component
Positive sequence component of three-phase current in dq rotation coordinate system
And negative sequence component
Wherein E isd +、Eq +、Ed -、Eq -Positive sequence component amplitude and negative sequence component amplitude of the three-phase voltage on a d axis and a q axis respectively; i isd +、Iq +、Id -、Iq -The positive sequence component amplitude and the negative sequence component amplitude of the three-phase current on the d axis and the q axis are respectively.
Preferably, in the second step, the output power of the inverter under the unbalanced power grid is quantized, and the quantization of the output power of the inverter under the unbalanced power grid is specifically represented as:
under the condition of only considering fundamental wave, the three-phase unbalanced grid voltage of the grid is represented by positive and negative sequence components of orthogonal formula as follows if zero sequence components are ignored:
the expressions of obtaining the instantaneous active power and the reactive power of the inverter power grid side by the two expressions are as follows:
P(t)＝P0+Pc2cos(2ωt)+Ps2sin(2ωt)
Q(t)＝Q0+Qc2cos(2ωt)+Qs2sin(2ωt)
in the formula, P0、Q0The average values of active and reactive respectively; pc2、Qc2The cosine values of the secondary active power and the reactive power are respectively; ps2、Qs2The sine values of the secondary active power and the reactive power are respectively; p0、Q0、Pc2、Qc2、Ps2、Qs2The expression of (a) is as follows:
the output power of the inverter when the power grid is unbalanced contains a second harmonic component and a harmonic component besides a fundamental component; p0、Q0、Pc2、Qc2、Ps2、Qs2Through Id +、Id -、Iq +、Iq -And controlling the positive and negative sequence components of the four currents.
Preferably, in the second step, the reference vector expression of the current obtained by weighting the control target in 3 is as follows:
in the formula, k is more than or equal to 0 and less than or equal to 1, and gamma is more than or equal to 0 and less than or equal to 1; different control objectives are achieved by varying the weighting coefficients k and y.
Preferably, in the third step, the optimization of the weight parameters k and γ specifically adopts an interior point method to establish a control optimization module: active power fluctuation and reactive power fluctuation output by the photovoltaic inverter are taken as an optimization objective function, and the expression is as follows:
min∑μΔP+λΔQ
in the formula, Δ P and Δ Q are fluctuation values of active power and reactive power, respectively, and μ and λ are weights of Δ P and Δ Q, respectively;
the constraint conditions of the objective function include:
a. total harmonic distortion rate: i isTHD≤5％
In the formula ITHDIs the total harmonic distortion rate and is calculated by the formula
In the formula I1Is the fundamental component of the current, InAs each harmonic component; the total harmonic distortion rate cannot exceed 5%;
b. current limit constraint: i isdq≤Idq max
In the formula IdqIs a three-phase instantaneous current IabcInstantaneous current, I, in dq coordinate system obtained by Park transformationdq maxAnd transforming the maximum current limit value of the grid-connected inverter to a lower value of a dq coordinate system.
The invention has the following advantages and beneficial effects:
the method takes the optimal active power fluctuation and reactive power fluctuation as optimization targets, and takes the current harmonic distortion rate and the output current limit value as constraint conditions, so that the photovoltaic inverter can minimize the active power fluctuation and the reactive power fluctuation when the power grid has unbalanced faults and the current quality is qualified and the current does not cross the line; the grid-connected current control method can effectively realize grid-connected current control of the photovoltaic inverter when the power grid is unbalanced, and the dynamic performance of the control is good.
Drawings
The accompanying drawings, which are included to provide a further understanding of the embodiments of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principles of the invention. In the drawings:
fig. 1 is a schematic diagram of positive and negative sequence component extraction according to the present invention.
Fig. 2 is a circuit diagram of a voltage-type three-phase grid-connected inverter according to the present invention.
Fig. 3 is a block diagram of grid imbalance photovoltaic inverter coordination control in consideration of current limits according to the present invention.
Fig. 4 is a photovoltaic grid-connected simulation model adopting a power grid imbalance control strategy according to the present invention.
Fig. 5 is a simulation result of the present invention using fixed k and γ.
Fig. 6 is a flow chart of the optimization of k and γ according to the present invention.
Fig. 7 shows the simulation results of the present invention using optimized k and γ.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail below with reference to examples and accompanying drawings, and the exemplary embodiments and descriptions thereof are only used for explaining the present invention and are not meant to limit the present invention.
Example 1
In this embodiment, a power grid unbalanced photovoltaic inverter coordination control method considering a current limit includes the following steps:
1) scheme for extracting positive and negative sequence current and voltage when power grid is unbalanced
The extraction of positive and negative sequence components of voltage is the premise of realizing the control of the unbalanced inverter of the power grid, and the extraction precision of the positive and negative sequence components directly influences the performance of a control system. At present, there are many methods for extracting positive and negative sequence components, such as a positive and negative sequence extraction technology based on decoupling, a positive and negative sequence extraction technology based on orthogonal components, and a positive and negative sequence extraction technology based on filtering. Considering that the control efficiency is influenced by the large calculation amount of the first two methods, the invention adopts a positive and negative sequence extraction method based on a filter. The filters adopted by the positive and negative sequence extraction technology based on the filters generally comprise a low-pass filter and a wave trap, and the low-pass filter can filter out higher harmonics but can reduce the rapidity of the system; although the wave trap can only filter specific harmonic waves, the dynamic characteristic of the wave trap is superior to that of a low-pass filter, secondary harmonic waves of active power and reactive power are mainly generated when the power grid is unbalanced, other harmonic components are few, and the requirements of a control system on reference current can be met by adopting the wave trap to filter positive and negative sequence components, so that the wave trap is adopted to extract the positive and negative sequence components, as shown in fig. 1, firstly, the three-phase voltage e of the unbalanced power grid isa、eb、ecAfter Park transformation, the voltage e is transformed to the dq coordinate systemd、eq，ed、eqThe main components of the waveform of (a) are fundamental wave and second harmonic; however, the device is not suitable for use in a kitchenThen, the second harmonic is filtered by a wave trap, and the left positive sequence component e isd +、eq +Finally, the positive sequence component is subtracted to obtain the negative sequence component e under the dq coordinate systemd -、eq -。
A trap is a special filter that is characterized by the ability to filter out signals at specific frequencies without contributing to other signals. The transfer function is:
in the formula, ω0The angular frequency of the signal to be filtered; k is 1/Q, Q is a quality factor of the filter. The frequency of the power grid is f-50 Hz, then omega0＝2πf＝100π。
2) Inverter mathematical model during power grid unbalance
As shown in FIG. 2, ea、eb、ecIs the three-phase voltage of the grid, ia、ib、icIs a three-phase current, U, injected into the griddcIs the voltage on the dc side, and C is the capacitance on the dc side. Then, considering only the fundamental wave, the grid potential when the grid is unbalanced in three phases is as follows:
in the formula, Em +、Em -Positive sequence voltage amplitude and negative sequence voltage amplitude respectively; omega is the frequency of the power grid; theta+、θ-The initial phase angle of the positive sequence voltage and the initial phase angle of the negative sequence voltage are respectively. Equation (2) ignores the zero sequence component, which can be represented by orthogonal positive and negative sequence components
In the formula (I), the compound is shown in the specification,
can obtain a voltage equation of
In the formula of UdqsAnd IdqsRespectively, inverter voltage and line current, which can be expressed as
The positive sequence and the negative sequence of the power grid voltage can be decoupled by bringing the formula (3) into the formula (7), and the expression is as follows:
in the formula of Udq +、Udq -Are respectively three-phase voltage Ua、Ub、UcA positive sequence component and a negative sequence component in dq rotation coordinate system.
When the grid is unbalanced, the power on the grid side of the inverter can be expressed as
In the formula (I), the compound is shown in the specification,
respectively positive sequence component and negative sequence component of three-phase current under dq rotation coordinate system, and
the expression formula (12) is taken into the formula (3), and the instantaneous active power and the reactive power of the inverter power grid side at the moment can be obtained as
P(t)＝P0+Pc2cos(2ωt)+Ps2sin(2ωt) (13)
Q(t)＝Q0+Qc2cos(2ωt)+Qs2sin(2ωt) (14)
In the formula, P0、Q0The average values of active and reactive respectively; pc2、Qc2The cosine values of the secondary active power and the reactive power are respectively; ps2、Qs2The sine values of the secondary active power and the reactive power are respectively. P0、Q0、Pc2、Qc2、Ps2、Qs2The expression of (a) is as follows:
as can be seen from equations (13) to (20), when the grid is unbalanced, the output power of the inverter contains a second harmonic component and a harmonic component in addition to the fundamental component. P0、Q0、Pc2、Qc2、Ps2、Qs2Can be passed through Id +、Id -、Iq +、Iq -Four reference current controls, but a single control strategy cannot achieve control over all targets.
3) Reference current calculation
Equations (15) - (20) already calculate the reference current in case of grid imbalance, and by selecting different control targets, the corresponding current vector can be calculated.
(1) Control target 1: the negative sequence current is zero, and the current harmonic wave is suppressed, so that the current waveform of the grid-connected inverter is sinusoidal, namely I-0. Simultaneous (15) to (18) obtaining
The reference vector of the positive sequence current can be calculated by the equation (21)
(2) Control target 2: keeping the output active power constant and suppressing the secondary pulsation of the active power, i.e. Ps2＝P c20. Combined type (15) - (18) can obtain reference vector of current
(3) Control target 3: keeping the output reactive power constant and suppressing secondary pulsations of the reactive power, i.e. Qs2＝Q c20. The united type (15), (18) to (20) can obtain the reference vector of the current
In the formula (I), the compound is shown in the specification,
equations (22) to (24) describe a single control target, and the effects of the remaining two control targets are not considered while achieving the single control target, so that it is necessary to achieve coordinated control of 3 control targets by means of weighting. With reference to expressions (22) to (24), the reference vectors of the currents obtained by weighting the 3 kinds of control targets are collectively expressed as
In the formula, k is more than or equal to 0 and less than or equal to 1, and gamma is more than or equal to 0 and less than or equal to 1. From equation (25), it can be analyzed that three different control objectives can be achieved by changing the weighting coefficients k and γ. When k ═ γ ═ 1, control target 1 is achieved; when k ═ γ ═ 0, control target 2 is achieved; when k is 1 and γ is 0, the control target 3 is achieved. In addition, through the formulas k and gamma, smooth switching between 0 and 1 can be realized, and the coordination control of 3 targets is realized.
4) Coordinated control mathematical model taking into account current limits
Although the formula (25) can be used for realizing the coordinated control of the output power of the photovoltaic inverter, the unbalanced fault is a dynamic process, the current is out of limit at the moment of fault generation, and if the current value is too large, the photovoltaic power station is disconnected, so that the stability of the system is influenced. The method uses a current limit value as a constraint condition, and uses an interior point method for optimization, and a mathematical model of a control model is as follows:
(1) objective function
The active power fluctuation and the reactive power fluctuation output by the photovoltaic inverter are taken as optimization targets, and the mathematical expression is as follows:
min∑μΔP+λΔQ (26)
in the formula, Δ P and Δ Q are fluctuation values of active power and reactive power, respectively, and μ and λ are weights of Δ P and Δ Q, respectively.
(2) Constraint conditions
a. Total harmonic distortion rate: i isTHD≤5％
In the formula ITHDIs the total harmonic distortion rate and is calculated by the formula
In the formula I1Is the fundamental component of the current, InAre the respective harmonic components. In addition, according to the national requirements on the quality of the photovoltaic grid-connected current, the total harmonic distortion rate cannot exceed 5%.
b. Current limit constraint: i isdq≤Idq max
In the formula IdqIs a three-phase instantaneous current IabcInstantaneous current, I, in dq coordinate system obtained by Park transformationdq maxAnd transforming the maximum current limit value of the grid-connected inverter to a lower value of a dq coordinate system.
c. Optimizing variable constraints: k is more than or equal to 0 and less than or equal to 1, and gamma is more than or equal to 0 and less than or equal to 1
In the formula, k and gamma are weights of the coordinated control and are also optimization variables of the current control.
As shown in fig. 3, a coordination control block diagram of the grid unbalanced photovoltaic inverter considering the current limit value is obtained by firstly performing coordinate transformation on three-phase voltage and three-phase current of a grid; then extracting positive and negative sequence components according to the wave trap in the formula (1), and simultaneously using the obtained data to complete functions of power calculation, THD calculation, k and gamma optimization and the like in the formulas (26) and (27) by an optimization solving module; then, calculating the reference current according to the formula (25); and comparing the calculated reference current with the current obtained by actual measurement to obtain a control parameter, and finally carrying out coordinate transformation to obtain a control signal of the inverter to complete control.
Example 2
In this embodiment, based on the grid imbalance photovoltaic inverter coordination control method considering the current limit value described in the above embodiment, a photovoltaic grid-connected model of the grid imbalance control method is built for simulation test, where an "unbalance" module in the model is an inverter coordination control module built and packaged according to fig. 3, and inputs of the module are a grid voltage signal, an inverter current signal, and a reference power signal of a photovoltaic cell.
The positive and negative sequence separation principle of the wave trap in this embodiment is obtained by formula (1), and the wave trap can realize the positive and negative sequence separation by filtering a waveform with a specific frequency, so as to provide required data for the implementation of a subsequent scheme.
As shown in fig. 2, the output power characteristics of the inverter when the grid is unbalanced can be analyzed from the graph. According to the parameters in the graph, the grid potential when the three phases of the grid are unbalanced is shown as the formula (2) under the condition of only considering fundamental waves. Neglecting the zero sequence component in the equation (2), which can be expressed by orthogonal positive and negative sequence components to obtain equation (3), and further obtain voltage equation (7); the formula (3) is taken into the formula (7), the positive sequence and the negative sequence of the electromotive force of the power grid can be decoupled to obtain a formula (10) and a formula (11), and the integrated formula (10) and the formula (11) obtain a formula (12); by taking expression (3) into expression (12), expressions (13) to (20) of instantaneous active power and reactive power on the power grid side of the inverter at this time can be obtained, and as can be seen from expressions (13) to (20), the output power of the inverter when the power grid is unbalanced contains a second harmonic component and a harmonic component in addition to a fundamental component. P0、Q0、Pc2、Qc2、Ps2、Qs2Can be passed through Id +、Id -、Iq +、Iq -Four reference current controls, but a single control strategy cannot achieve control over all targets. There are three main control objectives: controlling the target 1, wherein the negative sequence current is zero, and the current harmonic is suppressed, so that the current waveform of the grid-connected inverter is sinusoidal, namely I-The joint type (15) to (18) can obtain a reference vector formula (22) of the current as 0; controlling the target 2 to keep the output active power constant and to suppress the secondary pulsation of the active power, i.e. Ps2＝Pc2A reference vector equation (23) for obtaining a current in the equations (15) to (18) of the associated type at 0; controlling the target 3 to keep the output reactive power constant and to suppress secondary pulsation of the reactive power, i.e. Qs2＝Qc2When the current is 0, the reference vector (24) of the current is obtained by the joint equation (15) and the equations (18) to (20). With reference to expressions (22) to (24), the reference vector unified expression (25) of the current is obtained by weighting the 3 kinds of control targets. The coordination control of each target can be realized through the transformation of k and gamma from 0 to 1; when k is 1, the control target is 1; when k is 0, the control target 2 is set; when k is 1 and γ is 0, the control target 3 is obtained.
As shown in fig. 3, a block diagram of coordinated control shows the whole process of the whole control strategy. The optimization solution module integrates an optimization control strategy of an interior point method, the objective function is the minimum fluctuation of active power and reactive power as shown in the formula (26), and the constraint condition is the total harmonic distortion rate ITHDInstantaneous inverter current I under dq coordinate system not less than 5%dqNot exceeding a high current limit value Idq maxThe control variables k and γ are between 0 and 1. The wave trap module is used for positive and negative sequence separation. In addition, a module which is constructed according to the formula (25) and used for calculating reference current and obtaining control parameters by comparing the current obtained by actual test with the reference current optimized and calculated through the formula (25) is also arranged, and the control parameters are converted into control signals through coordinates to complete optimization control.
As shown in FIG. 4, in order to adopt the photovoltaic grid-connected model of the improved coordination control, the photovoltaic battery adopts an engineering practical model which passes through the short-circuit current IscOpen circuit voltage UocMaximum power current ImAnd maximum power voltage UmThe output characteristics of the photovoltaic cell can be obtained. The parameters of the model are shown in table 1. The weights μ and λ for both the active and reactive ripple are 0.5, the initial values of k and γ are 0.5, the inverter maximum current limit Iabc max is 10A, and the Idq max after Park conversion is 10A.
TABLE 1
And carrying out simulation test based on the acquired parameter data and the control strategy, wherein the test result is as follows:
as shown in fig. 5, the simulation results for fixed k and γ: assuming that the simulation time is 2s, the a-phase voltage generated at 0.8s is reduced by 20%. From fig. 5(a), it can be seen that the control strategy with fixed k and γ has a large current harmonic (THD: 34.5%) when the grid is unbalanced, which does not meet the requirement of photovoltaic grid connection, and the maximum current also goes out of limit. From the results of fig. 5(b) and 5(c), it can be seen that the active and reactive power fluctuation is large when the grid is unbalanced by using the control strategy with fixed k and γ.
As shown in fig. 7, an improved coordination control strategy is adopted: the same failure as in fig. 5, i.e., a simulation time of 2s, was set, and the a-phase voltage generated was reduced by 20% at 0.8 s. The control strategy finally obtains an optimization result of k being 0.875 and gamma being 0.247. From fig. 7(a), the current is only the current out-of-limit condition at the fault instant, because the control strategy optimizes k and γ in real time (as shown in fig. 6), the current quickly returns to the limit value when the optimal solution is found, and the measured THD is 4.78%, which meets the load photovoltaic grid connection requirement. As can be seen from the results of fig. 7(b) and fig. 7(c), the active power and the reactive power are stabilized within a small range after a short fluctuation when the power grid is unbalanced by using the improved coordination control strategy.
In conclusion, by adopting the coordination control strategy with the optimizing function, the optimal solution which is the minimum power fluctuation and is searched under the condition of ensuring the qualified current quality is the correctness of the model.
The above-mentioned embodiments are intended to illustrate the objects, technical solutions and advantages of the present invention in further detail, and it should be understood that the above-mentioned embodiments are merely exemplary embodiments of the present invention, and are not intended to limit the scope of the present invention, and any modifications, equivalent substitutions, improvements and the like made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (4)
1. A grid imbalance photovoltaic inverter coordination control method considering current limit values is characterized by comprising the following steps:
collecting three-phase voltage and three-phase current of a power grid, respectively carrying out coordinate transformation on the three-phase voltage and the three-phase current, and extracting positive and negative sequence components of the voltage and the current after the coordinate transformation by adopting a wave trap;
step two, quantifying the output power of the inverter under the unbalanced power grid, and weighting the quantity influencing the photovoltaic active output, the reactive output and the current harmonic wave to obtain a weight coefficient for coordinated controlkAndγ；
thirdly, optimizing the weight coefficients k and gamma by taking the minimum active power fluctuation and reactive power fluctuation as optimization targets and taking the current harmonic distortion rate and the output current limit value as constraint conditions, and constructing a control optimization module;
the weight parameters in the third stepkAndγthe optimization is carried out by specifically adopting an interior point method to establish a control optimization module: active power fluctuation and reactive power fluctuation output by the photovoltaic inverter are taken as an optimization objective function, and the expression is as follows:
in the formula (I), the compound is shown in the specification,ΔPandΔQthe fluctuation values of active power and reactive power are respectively,μandλare respectivelyΔPAndΔQthe weight of (c);
the constraint conditions of the objective function include:
a. total harmonic distortion rate:
in the formula (I), the compound is shown in the specification,I THD is the total harmonic distortion rate and is calculated by the formula
In the formula (I), the compound is shown in the specification,I 1is the fundamental component of the current and,I nas each harmonic component; the total harmonic distortion rate cannot exceed 5%;
b. current limit constraint:
in the formula (I), the compound is shown in the specification,I dq is a three-phase instantaneous currentI abcInstantaneous current under dq coordinate system obtained by Park transformation,I dq maxconverting the maximum current limit value of the grid-connected inverter to a lower value of a dq coordinate system;
step four, obtaining the weight coefficient of the coordination control through the constructed control optimization modulekAndγand the optimal value is used for carrying out coordination control on the power grid unbalanced photovoltaic inverter.
2. The method for coordinated control of a grid-unbalanced photovoltaic inverter considering current limits as claimed in claim 1, wherein the first step specifically comprises:
collecting three-phase voltage and three-phase current of a power grid and respectively carrying out dq coordinate transformation on the three-phase voltage and the three-phase current; and extracting positive and negative sequence components of the voltage and the current after coordinate transformation by adopting a wave trap to obtain: positive sequence component of three-phase voltage in dq rotation coordinate system
And negative sequenceMeasurement of
(ii) a Positive sequence component of three-phase current in dq rotation coordinate system
And negative sequence component
；
Wherein the content of the first and second substances,E d +、E q +、E d -、E q -positive sequence component amplitude and negative sequence component amplitude of the three-phase voltage on a d axis and a q axis respectively;I d +、I q +、I d -、I q -the positive sequence component amplitude and the negative sequence component amplitude of the three-phase current on the d axis and the q axis are respectively.
3. The method according to claim 2, wherein in the second step, the inverter output power under the unbalanced grid is quantized, specifically represented as:
under the condition of only considering fundamental wave, the three-phase unbalanced grid voltage of the grid is represented by positive and negative sequence components of orthogonal formula as follows if zero sequence components are ignored:
the expressions of obtaining the instantaneous active power and the reactive power of the inverter power grid side by the two expressions are as follows:
in the formula (I), the compound is shown in the specification,P 0、Q 0the average values of active and reactive respectively;P c2、Q c2the cosine values of the secondary active power and the reactive power are respectively;P s2、Q s2the sine values of the secondary active power and the reactive power are respectively;P 0、Q 0、P c2、Q c2、P s2、Q s2the expression of (a) is as follows:
the output power of the inverter when the power grid is unbalanced contains a second harmonic component and a harmonic component besides a fundamental component;P 0、Q 0、P c2、Q c2、P s2、Q s2by passingI d +、I d -、I q +、I q -And controlling the positive and negative sequence components of the four currents.
4. The method for coordination control of grid unbalanced photovoltaic inverter considering current limit as claimed in claim 3, wherein the reference vector expression of the current obtained by weighting 3 control targets in the second step is:
in the formula (I), the compound is shown in the specification,
；0≤k≤1，0≤γless than or equal to 1; by matching the weight coefficientskAndγto achieve different control objectives.
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