CN112821453A - Power control method and device of grid-connected inverter and grid-connected inverter - Google Patents
Power control method and device of grid-connected inverter and grid-connected inverter Download PDFInfo
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- H—ELECTRICITY
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- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
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- H—ELECTRICITY
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- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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- H02J3/48—Controlling the sharing of the in-phase component
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
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- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
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- H02J3/50—Controlling the sharing of the out-of-phase component
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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
Technical Field
The invention relates to the technical field of regional safety, in particular to a power control method and device of a grid-connected inverter and the grid-connected inverter.
Background
Voltage Source Inverters (VSIs) are widely used as grid interfaces in renewable energy systems, distributed power generation systems and electric transport systems to convert direct current into alternating current. However, due to load imbalance, short circuit faults, etc., the VSI may operate under asymmetric voltage sag. For controlling the VSI under the asymmetric voltage drop, the Direct Power Control (DPC) is widely applied due to the advantages of no current control loop, fast dynamic response, simple structure, and the like. The direct power control based on the table lookup method has the problems of large steady-state control pulsation, unfixed switching frequency and the like although the direct power control based on the table lookup method has a simple structure and quick dynamic response. In order to solve the problems, on one hand, the direct power control (MPC-DPC) based on model predictive control obtains stable switching frequency and reduces steady-state power pulsation by introducing an objective function, taking the optimum objective function as an effective voltage vector selection reference in each period, and then introducing a zero voltage vector for modulation, but the objective functions of all voltage vectors need to be calculated in each period, so that the calculation is complex. The other is direct power control (SVM-DPC) based on space vector modulation, in which SVMs are introduced to synthesize voltage vectors, rather than the cost function and voltage vector selection in MPC-DPC. In document [1], a Proportional-Integral-Resonant (PIR) controller was developed in an improved DPC with adjustable power compensation. However, the power compensation is calculated based on the voltage and current of the positive and negative sequences. Then, in order to eliminate sequence extraction, a sliding mode control method is used in document [2], and a DPC based on expanding active power 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. Document [3, 4] proposes a grid voltage modulation DPC with steady-state performance. However, this method is only suitable for voltage-balanced networks. In the case of an asymmetric drop in the grid voltage, the converter current may exceed its maximum value due to the positive sequence voltage drop to track the predetermined average power. Therefore, the current/power limiter is critical to limit the current to a set threshold under an asymmetric voltage drop. In document [5], a current limiter is designed based on its Root Mean Square (RMS). However, the speed of RMS calculation is a major obstacle to application. In document [6], a separate current limiter for each phase is proposed based on the current magnitude. However, differences in current magnitude may introduce high frequency noise and numerical errors.
Disclosure of Invention
One purpose of the scheme is to provide a power control method of a grid-connected inverter, the method can realize flexible control of the grid-connected inverter without phase-locked loops and rotating coordinate transformation, so that flexible regulation of reactive power and active power output by the grid-connected inverter under asymmetric voltage drop is realized, and the method has the characteristics of convenience and rapidness in implementation, simplicity in control, quick dynamic response and the like.
It is another object of the present solution to provide an apparatus and device for performing the above other methods.
In order to achieve the purpose, the scheme is as follows:
a power control method of a grid-connected inverter, which is implemented in an alpha beta two-phase static coordinate system, 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.
Preferably, the feedback power of the power grid is calculated based on the power grid power and the expansion power as shown in formula (19):
wherein, PfbFeedback power, Q, representing the active power of the gridfbFeedback representing reactive power of a power networkPower; pexThe developed power, Q, representing the active power of the gridexRepresenting 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]]。
Preferably, the determining the reference power value of the grid-connected inverter based on the limit on the newly-viewed power includes: a reference reactive power value of the grid-connected inverter is determined according to equation (29),
wherein Q isrefReference reactive power value, Q, for grid-connected invertermaxFor maximum output reactive power, U, of the grid-connected inverter at positive sequence voltageg+Is a positive sequence voltage, kQIs a proportionality coefficient;
determining a reference active power value of the grid-connected inverter according to equation (31),
wherein,
Pmaxfor maximum allowable active power of the grid-connected inverter, PrefThe APL is the maximum power value provided by the grid-connected inverter for the reference active power of the grid-connected inverter.
Preferably, the 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 of the grid-connected inverter based on the difference value of the reference power value and the feedback power valuePAnd a reactive modulation voltage vQ；
Based on the active modulation voltage vPAnd a reactive modulation voltage vQObtaining the output modulation of the grid-connected inverter in an alpha and beta two-phase static coordinate systemBraking voltage vαAnd vβ；
For output modulation voltage vαAnd vβAnd performing pulse width modulation to obtain the three-phase output voltage of the grid-connected inverter.
In a second aspect, there is provided a power control apparatus for a grid-connected inverter, the apparatus including:
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.
Preferably, the feedback power calculation unit calculates the feedback power of the power grid based on the following formula:
wherein, PfbFeedback power, Q, representing the active power of the gridfbFeedback power representing reactive power of the grid; pexThe developed power, Q, representing the active power of the gridexRepresenting 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]]。
Preferably, the reference power determining unit determines the reference power value of the grid-connected inverter includes performing the following operations:
a reference reactive power value of the grid-connected inverter is determined according to equation (29),
wherein Q isrefReference reactive power value, Q, for grid-connected invertermaxFor maximum output reactive power, U, of the grid-connected inverter at positive sequence voltageg+Is a positive sequence voltage, kQIs a proportionality coefficient;
determining a reference active power value of the grid-connected inverter according to equation (31),
wherein,
Pmaxfor maximum allowable active power of the grid-connected inverter, PrefThe APL is the maximum power value provided by the grid-connected inverter for the reference active power of the grid-connected inverter.
Preferably, the adjusting unit determines the three-phase output voltage of the grid-connected inverter includes performing the following operations:
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 valuePAnd a reactive modulation voltage vQ；
Based on the active modulation voltage vPAnd a reactive modulation voltage vQObtaining output modulation voltage v of grid-connected inverter in alpha and beta two-phase static coordinate systemαAnd vβ；
For output modulation voltage vαAnd vβAnd performing pulse width modulation to obtain the three-phase output voltage of the grid-connected inverter.
In a third aspect, there is provided a grid-connected inverter including:
a memory;
a processor;
the storage stores an application program executable by the processor, and the application program is used for enabling the processor to execute the power control method of the grid-connected inverter.
In a fourth aspect, a computer-readable storage medium is provided, on which a computer program is stored, which, when executed by a processor, implements the power control method of a grid-connected inverter as described in any one of the above.
The scheme has the following beneficial effects:
the method is implemented in a two-phase static coordinate system, can realize flexible control on the grid-connected inverter under the condition of not needing a phase-locked loop and rotating coordinate transformation, thereby realizing flexible regulation on the output reactive power and active power of the grid-connected inverter under the condition of asymmetric voltage drop, and has the characteristics of convenient implementation, simple control, quick dynamic response and the like. Meanwhile, the method has good dynamic response and frequency adaptability, and can realize the balance between negative sequence current and oscillation active/reactive power components according to external requirements, so that the grid-connected inverter has flexible grid-connected characteristics.
Drawings
In order to illustrate the implementation of the solution more clearly, the drawings that are needed in the description of the embodiments will be briefly described below, it being obvious that the drawings in the following description are only some embodiments of the solution, and that other drawings may be derived from these drawings by a person skilled in the art without inventive effort.
FIG. 1 is a flow chart of a method for controlling the power of a grid-connected inverter;
FIG. 2 is a block diagram of controlling the power of a grid-tied inverter;
FIG. 3 is a schematic diagram of a grid-connected inverter power control device;
FIG. 4 is an equivalent circuit diagram of a simulation model of an embodiment grid-connected inverter;
fig. 5 is a simulation experiment result of the power control of the grid-connected inverter under the asymmetric voltage drop according to the embodiment.
Detailed Description
Embodiments of the present solution will be described in further detail below with reference to the accompanying drawings. It is clear that the described embodiments are only a part of the embodiments of the present solution, and not an exhaustive list of all embodiments. It should be noted that, in the present embodiment, features of the embodiment and the embodiment may be combined with each other without conflict.
The terms "first," "second," and the like in the description and in the claims, and in the drawings described above, if any, are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It will be appreciated that the data so used may be interchanged under appropriate circumstances such that the embodiments described herein may be practiced otherwise than as specifically illustrated or described herein. Furthermore, the terms "comprises," "comprising," and "having," and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, system, article, or apparatus that comprises a list of steps or elements is not necessarily limited to those steps or elements expressly listed, but may include other steps or elements not expressly listed or inherent to such process, method, article, or apparatus.
It should be understood that the term "and/or" as used herein is merely one type of association that describes an associated object, meaning that three relationships may exist, e.g., a and/or B may mean: a exists alone, A and B exist simultaneously, and B exists alone. In addition, the character "/" herein generally indicates that the former and latter related objects are in an "or" relationship.
The word "if" as used herein may be interpreted as "at … …" or "when … …" or "in response to a determination" or "in response to a detection", depending on the context. Similarly, the phrases "if determined" or "if detected (a stated condition or event)" may be interpreted as "when determined" or "in response to a determination" or "when detected (a stated condition or event)" or "in response to a detection (a stated condition or event)", depending on the context.
Through research, the applicant finds that the introduction of adjustable parameters links the power of a power grid calculated in a classical mode with the expanded power defined by the scheme, and the introduction of a Proportional-Integral-Resonant (PIR) controller can realize the tracking of the active power and the reactive power of the grid-connected inverter and eliminate double-frequency power oscillation, so that the balance between negative sequence current and double-frequency power oscillation can be realized according to external requirements. In addition, the applicant also designs a power limiter comprising adjustable parameters and a voltage unbalance factor, which can ensure that the grid-connected inverter injects the maximum power into the power grid under various asymmetric voltage drops without causing the phase current to exceed the threshold value of the phase current.
For convenience of research, a power control method for a grid-connected inverter is proposed based on converting physical quantities in an abc three-phase stationary coordinate system into physical quantities in an α β two-phase stationary coordinate system by clark conversion.
As shown in fig. 1, the method includes:
s100, calculating feedback power of the power grid based on the power of the power grid and the expansion power;
the power of the power grid comprises active power P and reactive power Q of the power grid; according to the instantaneous power theory, the active power P and the reactive power Q are shown as the formula (3),
u in formula (3)gαβIs a vector representation of the grid voltage in an alpha beta two-phase stationary coordinate system, iαβIs a vector representation of the grid current in an alpha beta two-phase stationary coordinate system, ugαβ＝ugα+jugβ,iαβ＝iα+jiβ"+" denotes the conjugate of the complex number, j is the unit of the imaginary number; u. ofgα、ugβFor the grid voltage of VSI in the alpha beta two-phase stationary frame,
grid voltage u of VSI in alpha beta two-phase stationary coordinate systemgα、ugβExpressed as shown in formula (1-1),
the expression (1-1) is a power grid voltage expression formula in which three-phase voltages in an abc three-phase stationary coordinate system are converted into an alpha beta two-phase stationary coordinate system by using Clark conversion, and uga、ugb、ugcIs the three-phase voltage of the power grid at the VSI grid side,
as shown in FIG. 2, three-phase voltage and current instantaneous values at VSI network side are acquired by sensors, and u isga、ugb、ugc、ia、ib、icRespectively representing the three-phase voltage and the three-phase current instantaneous values of the VSI network side,
the relation between three-phase voltage and three-phase current instantaneous values at the VSI network side is shown as a formula (1),
in the formula (1), va、vb、vcThree-phase voltage is output for the VSI respectively, L is the inductance of the reactor of each phase of incoming line, and R is the resistance of each phase of circuit including the reactor resistance;
in an alpha beta two-phase stationary coordinate system, formula (1) can be expressed as formula (2) by Clark transformation,
in the formula (2), iα、iβFor the output current of VSI in an alpha beta two-phase stationary frame, vα、vβModulating voltage, v, for VSI output in an alpha beta two-phase stationary frameαOn the alpha axis, vβOn the beta axis;
when asymmetric voltage drop occurs, the voltage and the current can be decomposed into positive sequence components and negative sequence components as shown in formula (5),
in the formula (5), ugα、ugβFor the grid voltage of VSI in an alpha beta two-phase stationary frame, iα、iβThe output current of VSI in an alpha beta two-phase static coordinate system is obtained;
the time domain expressions of the positive and negative sequence components are shown in equations (6) and (7),
in formulae (6) and (7), Ug+、Ug-And Ig+、Ig-Amplitude of positive and negative sequence voltage, current, respectively, thetau+、θu-And theta+、θ-Initial phase angles of positive and negative sequence voltage and current are respectively, and omega is the angular frequency of the voltage of the power grid;
according to the formula (3) and the formula (5), the active power and the reactive power of the power grid are expressed as the sum of the direct current component and the oscillation component as shown in the formula (8),
in the formula (8), P represents the active power of the power grid, Pcl0Representing the direct current component of the active power of the grid, Pcli2Representing the double frequency component of the active power of the network, P, caused by the positive sequence voltage and the negative sequence currentclu2Representing the double frequency component of the active power of the grid caused by the negative sequence voltage and the positive sequence current, Q representing the reactive power of the grid, Qcl0Representing the direct component of the reactive power of the network, Qcli2Representing the double frequency component, Q, of the reactive power of the network caused by the positive-sequence voltage and the negative-sequence currentclu2Indicating the voltage of the negative sequence and the current of the positive sequenceThe induced second harmonic component of the reactive power of the power grid;
in the formula (8), the reaction mixture is,
the expanded active power and the expanded reactive power are calculated by taking the product of the voltage and the current delayed by one quarter of a cycle, the expanded active power and the expanded reactive power are shown as a formula (12),
in the formula (12), the reaction mixture is,
denotes the conjugate of the voltage vector in the α β coordinate system, delayed by a quarter of a cycle, "-" denotes the conjugate of the complex number, i.e. the complex number
In the formula (i)
u′gα+、u′gα-、u′gβ+And u'gβ-As shown in formula (11)
iαβRepresenting the grid current vector in an alpha beta two-phase stationary frame, i.e. iαβ＝iα+jiβ，iα、iβSee formula (I)
(5) And formula (7) PexShow the developed active powerRate, Pex0Representing a direct component, P, extending active powerexi2Representing the developed active power double frequency component, P, caused by positive sequence voltage and negative sequence currentexu2Representing the developed active power double frequency component, Q, caused by negative sequence voltage and positive sequence currentexIndicating extended reactive power, Qex0Representing a direct component, Q, of the developed reactive powerexi2Representing the developed reactive power double frequency component, Q, caused by positive sequence voltage and negative sequence currentexu2Representing the double frequency component of the extended reactive power caused by the negative sequence voltage and the positive sequence current,
in the formula (12), the reaction mixture is,
adjustable parameters are introduced to link the power of the power grid calculated in a classical mode with the expanded power defined by the scheme, and a Proportional-Integral-Resonant (PIR) controller is introduced to realize the tracking of the active power and the reactive power of the grid-connected inverter and eliminate the double-frequency power oscillation, so that the balance between the negative sequence current and the double-frequency power oscillation can be carried out according to external requirements.
Calculating the change rate of the active power and the reactive power of the power grid as shown in the formula (4),
the positive and negative sequence voltages introduced under the alpha beta coordinate system represent the change rate of the active power and the reactive power of the power grid,
as shown in formula (11), the positive and negative sequence voltages in the α β coordinate system are expressed and related as follows,
substituting the formulas (2), (3), (11), (13) and (14) into the formula (4) to obtain the change rate of the active power and the reactive power of the power grid shown in the formula (15),
in formula (15), UgFor the amplitude of the voltage of the power network,
ugα、ugβthe grid voltage of the VSI in an alpha and beta two-phase static coordinate system is obtained; v. ofPRepresenting the active modulation voltage, v, of the grid-connected inverterQRepresenting a reactive modulation voltage of the grid-connected inverter; v'P、v′QRespectively as in formula (15)
The expression of the change rate of the active power and the reactive power of the power grid after Laplace transformation is shown; v 'was obtained'P、v′Q(or
) Then, by the equation (15), the power modulation voltage v in the α β coordinate system can be obtainedPAnd vQ(ii) a The transfer function of the power controller is represented by G(s), PfbFeedback power, Q, representing the active power of the gridfbFeedback power, P, representing reactive power of the gridrefReference active power, Q, representing VSIrefRepresenting the reference reactive power of the VSI, equation (17) is derived,
introducing an adjustable parameter lambda to link the power grid power calculated in a classical mode with the expansion power defined by the scheme, obtaining the feedback power expressed by the adjustable parameter lambda in the formula (17) shown in the formula (19), wherein the adjustable parameter lambda belongs to [0,1],
after the feedback power is obtained based on the power grid power and the expansion power calculation, the relation between the feedback power and the transfer function of the power controller is further analyzed,
obtaining an equation (18) according to equations (9) and (13) and equations (10) and (14),
from the equation (18), the oscillation power components of the power grid power and the expansion power caused by the negative sequence current and the positive sequence voltage are equal, and the oscillation power components caused by the positive sequence current and the negative sequence voltage are opposite;
because the power of the power grid is represented as the sum of the direct current component and the oscillation component in the formula (8), and the feedback power is represented by the power of the power grid with adjustable parameters and the expanded power in the formula (19), the obtained feedback power can consist of the direct current component and the frequency doubling oscillation component; since the known power controller is capable of adjusting both the dc component and the frequency-doubled oscillating component, it is possible to use a power controller with a cut-off frequency ωcThe PIR controller is used as a power controller, because the PIR controller can eliminate steady-state error twice the frequency of a power grid, the oscillating component can be 0 by setting a specific negative sequence current, and the formula (18) is replaced by the formula (19) to obtain a control equation shown in the formula (21)
Substituting the formula (9) and the formula (10) into the formula (21) to obtain a negative sequence current expressed by an adjustable parameter lambda shown in the formula (22) and an initial phase angle expression expressed by the adjustable parameter lambda shown in the formula (23),
as can be seen from equations (22) and (23), the output current of VSI can be kept highly close to a sine wave as long as a proper negative-sequence current is injected without injecting a third-order harmonic current;
therefore, the feedback power obtained by calculation based on the power grid power and the expansion power in the method indicates that the output power of the grid-connected inverter can be adjusted without any power compensation, and the adjustment is realized by introducing an adjustable parameter lambda, so that the method can determine the priority between the negative sequence current and the active/reactive oscillation component according to the external requirement, and all the calculation in the power adjustment is carried out in a static coordinate system without phase-locked loop and PARK conversion.
According to the preceding discussion, when employed with a cut-off frequency ωcWhen the PIR controller of (a) is used as a power controller, the power controller transfer function g(s) in equation (17) can be expressed as shown in equation (20);
in the formula (20), kpIs the proportional coefficient, k, of the proportional element of the PIR controlleriIs the integral coefficient, k, of the integral element of the PIR controllerrThe integral coefficient of a resonance link of the PIR controller is shown, and omega is the angular frequency of the power grid.
S200, determining a reference power value of the grid-connected inverter based on the newly-viewed power limit;
the reference power of the VSI comprises a reference active power and a reference reactive power of the VSI;
in the event of a grid voltage drop, the current may exceed a set threshold, tripping the overcurrent protection, and thus limiting the apparent power in the asymmetric voltage drop is desirable.
The method defines an apparent power with a new calculation method and is called New Apparent Power (NAP), the minimum value of active power under the power of unit power factor and reactive power under the zero power factor in the method, namely a new apparent power expression shown in a formula (24),
according to the formulas (22) and (23) to Pcl0And Qcl0Derivation is carried out to obtain lambda epsilon [0,1]The expression of (c) is shown in formula (25),
in the formula (25), kpnIs the voltage unbalance factor, kpn＝Ug-/Ug+，
Accordingly, the formula (24) can be expressed as,
when the positive-sequence current vector and the negative-sequence current vector are in the same direction, the maximum current vector is coincident with the A (or B, C) axis, and the maximum peak current I of the A (or B, C) phasepeakIs equal to the current vector magnitude I at that timemaxAs shown in the formula (27),
Ipeak＝Imax＝I++I-＝(1+|1-2λ|kpn)I+≤Ith (27)，
in the formula (27), IthIs the safe current threshold per unit value; the limit value of the new apparent power obtained according to equations (26) and (27) is shown in equation (28),
equation (28) represents the limit on the newly observed power, the limit value
Namely, it is
In the event of a voltage drop, the VSI is required to provide the function of supporting the grid, and reactive power is generally injected preferentially, in this case the reference reactive power Q in equation (17)refIs expressed by the formula (29),
in the formula, QmaxIs a positive sequence voltage
The maximum output reactive power of the lower VSI, under severe conditions, should be allocated to the reactive power by the full capacity of the VSI, and therefore the maximum power provided by the VSI should be set to APL, i.e., QmaxAPL; if not otherwise specified, the proportionality coefficient kQAnd taking 2. Then obtaining the maximum allowable active power P of VSI according to the formula (29)max，PmaxIs expressed by the formula (30),
during asymmetric voltage sag, P is alwaysmaxReference active power P with VSIrefMaking a comparison if Pmax>PrefThe set VSI reference active power can be injected, otherwise only the maximum active power P can be injectedmaxIn formula (17)Reference active power PrefIs represented by the formula (31),
s300, 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;
as shown in fig. 2, after obtaining the feedback power according to equation (19) and the reference power value of VSI according to equation (29)/(30), the output modulation voltage v of VSI in the α β two-phase stationary coordinate system is obtained according to equations (15) and (16)αAnd vβAnd outputting the required three-phase voltage to the power grid through pulse width modulation.
In the formula (16), vαRepresenting the output modulation voltage, v, of the grid-connected inverter on the alpha axis in the alpha-beta two-phase stationary coordinate systemβThe output modulation voltage of the grid-connected inverter on a beta axis in an alpha and beta two-phase static coordinate system is represented; v. ofPRepresenting the active modulation voltage, v, of the grid-connected inverterQRepresenting a reactive modulation voltage of the grid-connected inverter;
formula (16) represents vα、vβAnd vP、vQWhen v is expressed according to equation (16)P、vQConversion to vα、vβV in a two-phase α β coordinate systemαAnd vβConverted into three-phase voltage v in a three-phase abc coordinate system by inverse Clark transformation as shown in the formula (16-1)a、vb、vcV is then modulated by Pulse Width Modulation (PWM) techniquea、vb、vcConverted into a three-phase output voltage of VSI, and therefore v is obtained by equation (16)α、vβThe method is equivalent to obtaining a switch capable of controlling the output of the inverter, thereby realizing the flexible control of the power of the grid-connected inverter. And the whole control of the method is implemented in an alpha beta two-phase static coordinate system, so that the method does not depend on a phase-locked loop and rotary coordinate transformation.
As shown in fig. 3, a power control apparatus 1 for a grid-connected inverter includes:
the feedback power calculation unit 10 is used for calculating the feedback power of the power grid based on the power of the power grid and the expansion power;
a reference power determination unit 20 that determines a reference power value of the grid-connected inverter based on a limit on the newly viewed power;
a regulating unit 30 for determining a three-phase output voltage of the grid-connected inverter based on a difference value between 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 conjugate of the grid voltage vector and the grid current with the 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.
The scheme also provides a grid-connected inverter which comprises a memory and a processor;
the storage stores an application program executable by the processor, and the application program is used for enabling the processor to execute the power control method of the grid-connected inverter. The memory may be a computer readable storage medium that may employ a portable compact disc read only memory (CD-ROM) and include program code, and may be run on a device, such as a personal computer. However, the program product of the present invention is not limited in this regard and, in the present document, a readable storage medium may be any tangible medium that can contain, or store a program for use by or in connection 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. A readable storage medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any combination of the foregoing. More specific examples (a non-exhaustive list) of the readable storage medium include: an electrical connection having one or more wires, a portable disk, a hard disk, a Random Access Memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing.
A computer readable storage medium may include a propagated data signal with readable program code embodied therein, for example, in baseband or as part of a carrier wave. Such a propagated data signal may take many forms, including, but not limited to, electro-magnetic, optical, or any suitable combination thereof. A readable signal medium may also be any readable medium that is not a readable storage medium and that can communicate, propagate, or transport a program for use by or in connection with an instruction execution system, apparatus, or device.
Program code embodied on a computer readable storage medium may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fiber cable, RF, etc., or any suitable combination of the foregoing.
Program code for carrying out operations for aspects of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as JAvA, C + + or the like and conventional procedural programming languages, such as the "C" programming language or similar programming languages. The program code may execute entirely on the user computing device, 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 (e.g., through the internet using an internet service provider).
This scheme is described below with reference to the simulation model shown in fig. 4. Matlab/Simulink was used to build a simulation model of the main circuit as shown in FIG. 4. Setting the grid voltage frequency to 50Hz, the reference active power P of the inverter (VSI)ref1.0, VSI reference reactive power Qref0. A VSI direct-current side voltage source is 1.5kV, the voltage of the phase A and the phase C drops to 0.5 time of the original voltage when the voltage of the phase A and the phase C drops within 0.5s, and the adjustable parameter lambda rises from 0 to 1 at a constant speed within 1-2 s; fig. 5 shows the simulation result in the above case.
As seen from the figure, at 1s, λ is 0, and VSI outputs three-phase current iabcThe active power P maintains a constant value of 0.4(p.u.), namely the active power can be controlled to be constant, namely lambda is 0, and the double frequency component of the active power output by the VSI can be eliminated; at 1.5s, λ is 0.5, active power and reactive power both contain frequency doubling oscillation component, and output current iabcThe three phases are symmetrical, and the negative sequence current is not contained, namely the three phases of the output current can be controlled to be symmetrical at the moment, namely the lambda is 0.5, so that the VSI output current can be kept to be close to a sine wave in height; at 2s, λ is 1, and current i is outputabcThe unbalance contains a negative sequence component, the active power contains a double frequency oscillation component, and the reactive power Q maintains a constant value of 0.33(p.u.), namely the reactive power can be controlled to be constant at the moment, namely, the lambda is 1, so that the double frequency component of the VSI output reactive power can be eliminated.
It can be seen that k is adjusted by adjusting k appropriately in the formula (20)p、kiAnd krThe value of (3) can adjust the direct current component of the feedback power and also can eliminate the oscillation component of double frequency; as long as the appropriate k is setp、kiAnd krThe value of lambda is set according to the external requirement, so that the VSI output current can be kept to be close to a sine wave, and the double frequency component in the VSI output active or reactive power can be eliminated.
Therefore, the power control strategy of the grid-connected inverter under the unbalanced grid voltage can realize the balance between the negative sequence current and the oscillation active/reactive power component by adjusting the size of lambda according to the external requirement, so that the VSI has flexible grid-connected characteristics.
The references in the background art are as follows:
1.H.Nian,Y.Shen,H.Yang,and Y.Quan,“Flexible grid connection technique of voltage-source inverter under unbalanced grid conditions based on direct power 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 power control 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,“A Voltage modulated DPC approach for three-phase PWM rectifier,”IEEE Trans.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 current limitation 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.
it should be understood that 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, and it will be obvious to those skilled in the art that other variations or modifications may be made on the basis of the above description, and all embodiments may not be exhaustive, and all obvious variations or modifications may be included within the 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, PfbFeedback power, Q, representing the active power of the gridfbFeedback power representing reactive power of the grid; pexThe developed power, Q, representing the active power of the gridexRepresenting 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 isrefReference reactive power value, Q, for grid-connected invertermaxIs the maximum output reactive power of the grid-connected inverter under the positive sequence voltage,
is a positive sequence voltage, kQIs a proportionality coefficient;
determining a reference active power value of the grid-connected inverter according to equation (31),
wherein,
Pmaxfor maximum allowable active power of the grid-connected inverter, PrefThe 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 valuePAnd a reactive modulation voltage vQ；
Based on the active modulation voltage vPAnd a reactive modulation voltage vQObtaining output modulation voltage v of grid-connected inverter in alpha and beta two-phase static coordinate systemαAnd vβ；
For output modulation voltage vαAnd vβAnd 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, PfbFeedback power, Q, representing the active power of the gridfbFeedback power representing reactive power of the grid; pexThe developed power, Q, representing the active power of the gridexRepresenting 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 isrefReference reactive power value, Q, for grid-connected invertermaxIs the maximum output reactive power of the grid-connected inverter under the positive sequence voltage,
is a positive sequence voltage, kQIs a proportionality coefficient;
determining a reference active power value of the grid-connected inverter according to equation (31),
wherein,
Pmaxfor grid-connected invertersMaximum allowable active power, PrefThe 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 valuePAnd a reactive modulation voltage vQ；
Based on the active modulation voltage vPAnd a reactive modulation voltage vQObtaining output modulation voltage v of grid-connected inverter in alpha and beta two-phase static coordinate systemαAnd vβ；
For output modulation voltage vαAnd vβAnd 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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