CN116885781A - Grid-connected converter on-grid-connected operation mode switching method and system - Google Patents
Grid-connected converter on-grid-connected operation mode switching method and system Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
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Abstract
The invention provides a method and a system for switching a grid-connected converter along with a grid-constructed network operation mode. The method provided by the invention can meet the requirement of fast switching speed along with the switching of the network-network operation modes, can solve the problem of slow switching of the traditional method, and does not need to inject disturbance signals when the impedance of the power grid is identified by adopting an extended Kalman filtering method, so that the power quality is not required to be deteriorated.
Description
Technical Field
The invention belongs to the technical field of switching control, and particularly relates to a method and a system for switching a grid-connected converter along with a grid-networking operation mode.
Background
The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.
Along with the aggravation of environmental pollution and exhaustion of fossil energy, clean renewable energy sources such as solar energy, wind energy and the like are paid attention to, and the proportion of new energy to be connected into a power grid is increased year by year. The grid-connected converter plays a key role in new energy power generation and is widely applied to micro-energy, distributed power generation, energy storage, AC/DC micro-grid systems and the like. Because distributed power generation is located in a remote area, the power transmission line is long, transformers are more, and the power grid shows weak power grid characteristics; along with the increase of the specific gravity of distributed power generation, a plurality of converters are often operated in parallel to meet the load requirement, under the working condition, the equivalent power grid impedance of a single converter is increased, and the weak power grid characteristics of the power grid are obvious. The wind curtailment, the light curtailment and the grid fault frequency caused by the weak electric network have seriously threatened the safe and stable operation of the grid and the new energy power generation system. The new energy power generation with high permeability has intermittent, random and output fluctuation, and the power grid impedance often has a problem of large fluctuation.
The traditional grid-connected converter mainly works in a grid-following mode, namely active power output and reactive power output are regulated based on the phase of a power grid, and the traditional grid-connected converter is suitable for a strong power grid. However, under the condition of high permeability, the stability margin is reduced, harmonic resonance and even instability can be caused, and the stability and high-efficiency operation of the new energy grid-connected power generation system are challenged. Therefore, the research on the grid-connected converter stable control scheme under the condition of high permeability when the grid impedance greatly fluctuates is significant for the reliable operation of the new energy power generation grid-connected system.
Unlike the grid-following mode, the grid-forming mode directly realizes output power adjustment by controlling the output voltage vector phase, and is more stable in a weak current grid and unstable in a strong power grid compared with the grid-following mode.
The new energy power generation has intermittence, randomness and output fluctuation, the power grid strength often fluctuates greatly, if the grid-connected converter works alone in a grid-following or grid-constructing mode, the problems of poor stability and insufficient efficiency exist respectively, and based on the complementarity of the two grid-connected modes, the existing implementation scheme provides new energy power generation under high permeability andaccording to a dual-mode control strategy of the grid converter, non-characteristic harmonic disturbance is periodically injected into a power grid, voltage and current information of PCC points is processed through a recursive discrete Fourier algorithm, power grid impedance and a power grid Short Circuit Ratio (SCR) are calculated, and a grid-connected mode is switched when the short circuit ratio reaches a switching boundary. The traditional method adopts an active impedance estimation method, superimposes harmonic current disturbance with the harmonic frequency h on a given current in a specific period, adopts a discrete Fourier algorithm to extract harmonic response components of PCC points, and u g (h) And i g (h) Calculating the impedance Z of the power grid g The method comprises the steps of carrying out a first treatment on the surface of the The periodic harmonic disturbance can interfere the normal operation of electrical equipment such as a grid-connected converter and the like, and the quality of grid-connected power can be reduced. The passive impedance estimation method utilizes noise or disturbance existing in the public coupling point of the power grid to estimate the line impedance, and the problem of harmonic disturbance existing in the traditional method is avoided.
The traditional method adopts a double-loop cascade PI structure under the following network and the networking modes. In the network-following mode, a power reference value P is given ref And Q ref Combining instantaneous power to calculate and obtain grid-connected current reference valueAnd->Obtaining a current reference value at the converter side through a PI regulator, obtaining an adjustment quantity through the PI regulator, and adding the adjustment quantity to the filter capacitor voltage u Cd And u Cq To obtain a modulated wave divided by half of the DC side voltage +.>And obtaining a switch control signal after PWM, and forming a network in the same mode.
By adopting the double-ring PI cascade mechanism, 4 PI regulators are required for controlling the two modes of the follow-up network and the construction network, the parameter design and the selection are complicated, the nested cascade structure also can cause slow dynamic response, and the requirement of the follow-up network mode switching on the switching speed is difficult to meet. The situation that the intensity of the power grid is reduced when the power grid is switched to the constructed power grid is corresponding to the situation that the power grid intensity is reduced, the constructed power grid mode needs to be achieved quickly to support the power grid, the power grid is prevented from being broken, and the PI control can cause instability of the system due to untimely switching. The switching of the network configuration to the network-associated condition corresponds to the increase in the strength of the network, and it is desirable to output the maximum generated power as soon as possible, and the slower switching speed may deteriorate the economy.
Therefore, how to enable the grid-connected converter to realize rapid and accurate on-grid mode switching under the fluctuation of the power grid intensity is a problem to be solved urgently at present.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides a method and a system for switching a grid-connected converter along with a grid-constructed operation mode, which adopts an extended Kalman filter to identify the impedance of a power grid on line, acquire the strength of the power grid, dynamically switch the along with the grid-constructed mode according to the strength of the power grid, obtain a predicted state variable at the next moment through a prediction model, and obtain a corresponding control signal with the minimum cost function so as to realize rapid mode switching when the strength of the power grid reaches a switching condition.
In order to achieve the above object, a first aspect of the present invention provides a method for switching operation modes of a grid-connected converter along with a grid, including:
predicting and updating the power grid impedance model by using relevant parameters in the power grid impedance model as filtering state variables through an extended Kalman filtering algorithm to obtain a real-time power grid impedance estimated value;
judging whether a power grid accessed by the grid-connected converter meets the switching condition of the running mode of the grid-connected converter, and determining whether the running model of the grid-connected converter reaches the switching condition according to the real-time power grid impedance estimated value;
if the power grid accessed by the grid-connected converter meets the switching condition of the running mode of the grid-connected converter, obtaining a predicted grid-connected current value based on a grid-following prediction model or a grid-constructing prediction model established under different running modes, and completing the switching action based on the predicted grid-connected current value so as to minimize the grid-following cost function or the grid-constructing cost function.
A second aspect of the present invention provides a grid-connected converter operation mode switching system along with a grid, including:
a power grid impedance estimation module: predicting and updating the power grid impedance model by using relevant parameters in the power grid impedance model as filtering state variables through an extended Kalman filtering algorithm to obtain a real-time power grid impedance estimated value;
and a judging module: judging whether a power grid accessed by the grid-connected converter meets the switching condition of the running mode of the grid-connected converter, and determining whether the running model of the grid-connected converter meets the switching condition according to the real-time power grid impedance estimated value;
and a switching module: if the power grid accessed by the grid-connected converter meets the switching condition of the running mode of the grid-connected converter, obtaining a predicted grid-connected current value based on a grid-following prediction model or a grid-constructing prediction model established under different running modes, and completing the switching action based on the predicted grid-connected current value so as to minimize the grid-following cost function or the grid-constructing cost function.
A third aspect of the present invention provides a computer apparatus comprising: the system comprises a processor, a memory and a bus, wherein the memory stores machine-readable instructions executable by the processor, when the computer equipment runs, the processor and the memory are communicated through the bus, and the machine-readable instructions are executed by the processor to execute a grid-connected converter grid-connected-networking running mode switching method.
A fourth aspect of the present invention provides a computer readable storage medium having a computer program stored thereon, which when executed by a processor performs a grid-connected converter with grid-connected operation mode switching method.
The one or more of the above technical solutions have the following beneficial effects:
according to the invention, the real-time power grid impedance is measured and calculated through the extended Kalman filtering method, the intensity of the power grid connected with the grid-connected converter is judged, and the running mode of the grid-connected converter is switched according to the intensity of the power grid connected with the grid-connected converter.
In the invention, the predicted state variable at the next moment is obtained by the network following prediction model or the network constructing prediction model, the switching of the operation mode is completed based on the driving signal of the state variable at the next moment corresponding to the minimization of the network following cost function or the network constructing cost function as the control switching signal, the dynamic response speed of the prediction control is far higher than that of the traditional PI control switching mode, the requirement of the network following-network constructing operation mode switching on the faster switching speed can be met, and the problem of slow switching of the traditional method can be solved.
Additional aspects of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the invention.
FIG. 1 is a schematic diagram of prediction control of a network-following operation mode according to a first embodiment of the present invention;
FIG. 2 is a schematic diagram illustrating prediction control of a network operation mode according to a first embodiment of the present invention;
FIG. 3 is a schematic diagram illustrating a control of switching between network-to-network modes according to a first embodiment of the present invention;
fig. 4 is a diagram of a three-phase grid-connected converter according to the first embodiment of the present invention;
fig. 5 is a voltage vector diagram of a three-phase grid-connected inverter according to a first embodiment of the present invention;
fig. 6 is a flowchart of a predictive control algorithm for a grid-connected converter according to a first embodiment of the present invention;
fig. 7 is a single line diagram of an equivalent grid impedance model in accordance with an embodiment of the present invention.
Detailed Description
It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the invention. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments according to the present invention.
Embodiments of the invention and features of the embodiments may be combined with each other without conflict.
Example 1
The embodiment discloses a method for switching operation modes of a grid-connected converter along with a network, which comprises the following steps:
step 1: predicting and updating the power grid impedance model by using relevant parameters in the power grid impedance model as filtering state variables through an extended Kalman filtering algorithm to obtain a real-time power grid impedance estimated value;
step 2: judging whether a power grid accessed by the grid-connected converter meets the switching condition of the running mode of the grid-connected converter, and determining whether the running model of the grid-connected converter meets the switching condition according to the real-time power grid impedance estimated value;
step 3: if the power grid accessed by the grid-connected converter meets the switching condition of the running mode of the grid-connected converter, obtaining a predicted grid-connected current value based on a grid-following prediction model or a grid-constructing prediction model established under different running modes, and completing the switching action based on the predicted grid-connected current value so as to minimize the grid-following cost function or the grid-constructing cost function.
In step 1 of this embodiment, for the grid-connected converter, a Model Predictive Control (MPC) based on-grid converter control architecture is respectively established, and based on the control architecture, extended Kalman Filter (EKF) is adopted to identify the grid impedance online, obtain the grid strength, and dynamically switch the on-grid mode according to the grid strength.
In step 2 of this embodiment, as shown in fig. 1, the on-grid prediction control structure of the on-grid converter obtains a predicted on-grid current value based on the on-grid prediction model, and finishes the switching action based on the predicted on-grid current value with the minimum on-grid cost function, specifically:
step 2-11: at the current moment PCC pointAnd grid-connected current measurement at PCC point +.>As an input of the on-grid prediction model, obtaining a predicted grid-connected current value of the next moment based on the on-grid prediction model>
Step 2-12: grid-connected current instruction value based on d-axis and q-axisAnd->Real-time phase theta of power grid voltage obtained by combining PLL (phase locked loop) processing GFL Obtaining a grid-connected current reference value +.>
Step 2-13: according to the obtained predicted grid-connected current value at the next momentGrid-connected current reference value->And taking the driving information of the predicted grid-connected current value at the next moment corresponding to the minimum network cost function as a control switching signal.
In step 2-12, the DC bus voltage u is set dc And DC bus reference voltageThe grid-connected current instruction values of the d axis and the q axis are obtained after the processing>And->
Optionally, based on the dc bus voltage u dc And DC bus reference voltageObtained by PI regulator->The current control equation is +.>Wherein K is iP 、K iI Proportional and integral regulation gains for the current loop, respectively, are generally made +.>
The mathematical model in the synchronous rotation dq coordinate system is transformed into a mathematical model in the two-phase stationary αβ coordinate system. X is X dq ＝T(γ)X αβ ，X αβ ＝T(γ) -1 X dq In the transformation matrixVector X dq ＝(x d ,x q ) T ，X αβ ＝(X α ,X β ) T X dq ＝T(γ)X αβ ，X αβ ＝T(γ) -1 X dq Hereinafter, X may be i g 、/>i g [k+1]、u g 、/>u g [k+1]、e。
The following network cost function in the steps 2-13 is:
wherein, the liquid crystal display device comprises a liquid crystal display device,respectively is grid-connected variable current reference value->Alpha, beta component of (a); i.e gα [k+1]、i gβ [k+1]Predicted grid-connected current for the next time>Alpha, beta components of (a).
The embodiment can calculate the state variable at the next moment by using the network following prediction model By a given grid-connected current reference value i * Calculating state variable +_of the next moment corresponding to the minimum cost function by using the network-associated cost function and related limiting conditions>
For simplifying analysis, taking an L-filter-based three-phase grid-connected converter as an example, the topological structure of the three-phase grid-connected converter is shown in fig. 4, and the grid-connected converter is connected with a power grid through the L-filter. According to kirchhoff's voltage law, the current reference direction is shown in fig. 4, and the dynamic current equation is:
wherein i is ga 、i gb 、i gc Outputting current for the grid-connected converter; u (u) gaN 、u gbN 、u gcN The method comprises the steps of outputting voltage for a grid-connected converter; u (u) nN The voltage between the neutral point of the power grid voltage and the negative electrode of the direct current bus; e, e a 、e b 、e c Respectively three-phase grid voltages.
Assuming three-phase network voltage balance, i.e. e a +e b +e c The current dynamic equation of the output current of the three-phase grid-connected converter in the stationary αβ coordinate system is:
wherein i is gα 、i gβ The method comprises the steps that alpha and beta components of the grid-connected converter output current under an alpha and beta coordinate system are obtained; u (u) ga 、u gβ The method comprises the steps that alpha and beta components of grid-connected converter output voltage under an alpha and beta coordinate system are obtained; e, e α 、e β The alpha and beta components of the grid voltage under an alpha and beta coordinate system.
The switching state of the grid-connected converter is defined as follows:
the resultant vector of grid-tied converter switching functions can be expressed as:
S＝S a +αS b +α 2 S c (7)
wherein α=e j2π/3 。
The synthesized vector of the output voltage of the grid-connected converter is as follows:
u i ＝Su dc i＝-0,···,7 (8)
wherein u is dc Is the DC bus voltage.
Considering the combination of all the switching states of the grid-connected converter, there are 8 combinations, where u 0 ＝u 7 The 8 combinations result in 7 different voltage vectors, as shown in fig. 5.
According to the DC bus voltage and the switch state S of the grid-connected converter a 、S b 、S c The output voltage u of the grid-connected converter can be obtained gα 、u gβ ：
Discretizing the mixture to obtain:
wherein T is s Is the sampling period.
The method can obtain:
wherein i is gα [k+1]、i gβ [k+1]The output current alpha and beta components of the three-phase grid-connected converter are predicted at the moment k.
Outputting the current i at the time tk+1 according to the system prediction model gα [k+1]、i gβ [k+1]Can be predicted. To predict t k+1 Current i at time instant gα [k+1]、i gβ [k+1]Current i of current sample gα [k]、i gβ [k]Grid voltage e α [k]、e β [k]The switching state of the grid-connected converter and the voltage of the direct current bus are required to be obtained. When predicting t k+1 Current i at time instant gα [k+1]、i gβ [k+1]Then, a cost function J is constructed to evaluate each voltage vector of the grid-connected converter. Which of these voltage vectors minimizes the cost function, this voltage vector will be applied in the next sampling period.
The grid-following predictive control structure of the grid-connected converter is shown in figure 1. Wherein, the liquid crystal display device comprises a liquid crystal display device,grid-connected current command values respectively of d axis and q axis, theta GFL Real-time phase of the grid voltage acquired for the PLL, u dc For DC bus voltage +.>And S is an output gate driving signal, which is the reference voltage of the direct current bus. Grid voltage and grid-connected current measured values from a measuring device +.>And->The state variable +.>By a given grid-connected current reference value->And calculating the state variable of the next moment corresponding to the minimum cost function by using the cost function and the related limiting conditions. Taking the quadratic sum of the current errors as a cost function, the cost function J is expressed as:
according to the model prediction principle and the characteristics of the grid-connected converter, a flow chart of a model prediction control algorithm is shown in fig. 6.
The minimum cost function can be applied in the next sampling period in the voltage vector, and the voltage vector corresponds to the switching state (0, 1 combination) of the grid-connected converter. There are 8 combinations of switching states of the grid-connected inverter, controlled by the gate drive signal, which can be obtained by Pulse Width Modulation (PWM) in order to achieve the desired switching state.
In this embodiment, as shown in fig. 2, the network configuration prediction control structure obtains a predicted grid-connected current value based on a network configuration prediction model, and completes the switching action based on the predicted grid-connected current with minimum network configuration cost function, specifically:
step 2-21: at the current moment of grid voltageAnd grid-connected current measurement +.>As input of a networking prediction model, obtaining a predicted grid-connected current value of the next moment based on the networking prediction model>
Step 2-22: voltage loop command value based on d-axis and q-axis sagging power outer loop output0, phase θ combined with droop power outer loop output GFM The dq-alpha beta coordinate conversion is carried out to obtain the reference value of the power grid voltage in the phase theta GFM Values under the αβ coordinate system +.>Obtained by PI regulator->
Step 2-23: according to the obtained predicted grid-connected current value at the next momentGrid-connected current reference value->And taking a driving signal of the predicted grid-connected current at the next moment corresponding to the minimum networking cost function as a control switching signal.
In steps 2-21 of this embodiment, the grid voltage is based on the current timeAnd grid-connected current measurement +.>Active power setpoint P ref And reactive power setpoint Q ref And obtaining the voltage loop command value output by the droop power outer loop of the d axis and the q axis and the phase output by the droop power outer loop through droop control calculation.
Sag characteristic equation:
the grid-structured predictive control structure of the grid-connected converter is shown in fig. 2, wherein θ GFM Andthe droop power outer ring is respectively set value P according to active power and reactive power ref And Q ref Output phase and d-axis voltage loop command value, < >>Is the voltage loop instruction value, i under the alpha beta coordinate system * For the current loop command value in the alpha beta coordinate system,/->Is the output gate drive signal. Considering that undisturbed switching control is needed during switching of the random-networking modes, the predictive control cost functions of the two models are the same, so that the predictive control variable of the networking converter is also selected as the grid-connected current i g Is consistent with the following net type. Obtaining grid voltage and grid-connected current measurement values from a measurement device>And->The state variable of the next moment can be calculated by using the prediction modelBy a given grid-connected current reference value i * And calculating the state variable of the next moment corresponding to the minimum cost function by using the cost function and the related limiting conditions. Taking the quadratic sum of the current errors as a cost function, the cost function is expressed as:
in this embodiment, the state variable at the next moment corresponding to the minimum cost function is calculated by using the network cost function and the related constraint condition. The prediction of the state variable at the next time based on the network-based prediction model is the same as the above-described method of predicting the state variable at the next time based on the network-following prediction model.
Different control signals, namely driving signals corresponding to the state quantity at the next moment, lead to different cost functions J, so that the control signal with the minimum cost function J is selected in the next control period, and the steady state can be quickly reached in a plurality of control periods during the mode switching, thereby realizing the quick self-adaptive random-structure switching.
As shown in fig. 3, in the present embodiment, the strength of the power grid may be represented by a short-circuit ratio,i.e.Wherein S is DG Incorporating rated capacity of a power grid for a distributed power generation system, S SC U is short-circuit capacity of electric network g For the rated effective value of the power grid voltage Z g Is the grid impedance modulus at the grid fundamental frequency. According to IEEEStandard1204-1997, when SCR>3, when 2<SCR<3, the grid is called weak grid, and the very weak grid is often SCR<2。
According to the formula of SCR, the key point of identifying the strength of the power grid is to accurately and rapidly acquire the power grid impedance of the system.
Under a strong current network, the grid-connected converter has a grid-following working mode, active power and reactive power are regulated by current oriented based on the voltage of the power grid, the phase of the voltage of the power grid is observed by adopting a phase-locked loop, the maximization of the utilization rate of new energy can be realized, and higher electric energy quality can be ensured. Under a weak current network, the grid-connected converter has a grid-formed working mode, and output power adjustment is realized by controlling the phase of an output voltage vector, so that the grid-connected converter has better stability.
In the embodiment, a real-time power grid impedance estimated value is obtained by using an extended Kalman filtering method. A state space model based on line impedance is as in equation (3).
Wherein the sampling time is T s The resistive and inductive components of the grid impedance are R and L respectively, for simplicity of the model, usei gα 、i gβ Respectively is grid-connected current i g Alpha, beta component, i g ＝(i gα ,i gβ ) T ；e gα 、e gβ Respectively the grid side voltage e g Alpha, beta component, e g ＝(e gα ,e gβ ) T ；u gα 、u gβ Respectively the side voltages u of the current transformer g Alpha, beta component, u g ＝(μ gα ,u gβ ) T . To simplify the model, the following approximations and transformations are made: />
The extended kalman recursion equation is as follows:
state one-step prediction:
one-step prediction covariance matrix:
Pk+1k＝Φk+1kPkkΦ T k+1k+Qk+1(17)
filtering gain matrix:
Kk+1＝Pk+1kH T k+1[Hk+1Pk+1kH T k+1+
Rk+1] -1 (18)
and (5) updating the state:
updating the covariance matrix:
P(k+1|k+1)＝[I-Kk+1Hk+1]Pk+1k(20)
wherein, the liquid crystal display device comprises a liquid crystal display device,for the state prediction at time k versus time k+1, representing:
f is a state function representing [ (1-x 7] x 8] Ts ] x 1-Ts ] x 8] x 3 + Ts ] x 5; (1-x [7] x [8] Ts) x [2] -Ts x [8] x [4] +ts x [8] x [6]; x 3; x 4; x 5; x < 6 >; x 7;
x[8]；]；state prediction at time k represents:
(16) The expression in the specific embodiment is (15). h is a measurement function, here representing [ x 1]; x 2; x 3; x 4; x 5; x 6. Pk+1k is the covariance prediction at time K versus time k+1, P (k|k) is the covariance prediction at time K, and K is the Kalman gain. Φ (k+ 1|k) is a state transition matrix, H (k+1) is a measurement matrix, and is jacobian matrices of f and H, respectively. Q is a process noise covariance matrix, R is a measurement noise covariance matrix, Z is an observation value, and I is an identity matrix.
Extended kalman recursion predicts by continually performing (16) - (20) operationsI.e. < ->The latter two element grid impedance values are also included.
According to the embodiment, the real-time line impedance value can be estimated by establishing the micro-grid line impedance model and combining with the extended Kalman algorithm so as to monitor SCR and judge whether the power grid strength and the converter need to switch the working modes. Compared with the traditional method that harmonic current is superimposed on given current, the harmonic disturbance problem existing in a specific period is caused, the extended Kalman filtering method utilizes noise existing at the connecting point of the grid-connected converter to overcome the requirement on active injection disturbance, and the influence on electrical equipment and grid-connected quality is smaller.
The traditional switching method uses PI control, and the cascade PI structure has slower dynamic response during mode switching, which may cause unstable problems such as oscillation and inrush current. Compared with the traditional method, the predictive control of the embodiment can realize rapid mode switching when the power grid strength reaches the switching condition. When the power grid strength is reduced, the converter is required to be quickly switched from a network following mode to a network constructing mode so as to support the system, the dynamic response of the traditional method is in a millisecond level, and the dynamic response of the predictive control is in a microsecond level, so that the system can be supported more quickly; when the power grid strength is increased, the converter is required to be quickly switched from a networking mode to a following mode, and the predictive control can also output power with higher efficiency. The prediction control-based random-networking mode switching strategy gives consideration to system stability and grid-connected output efficiency, and can effectively improve the stability and grid-connected power quality of a converter in a new energy grid-connected power generation system.
Example two
An object of the present embodiment is to provide a grid-connected converter operation mode switching system along with a grid, including:
a power grid impedance estimation module: predicting and updating the power grid impedance model by using relevant parameters in the power grid impedance model as filtering state variables through an extended Kalman filtering algorithm to obtain a real-time power grid impedance estimated value;
and a judging module: judging whether a power grid accessed by the grid-connected converter meets the switching condition of the running mode of the grid-connected converter, and determining whether the running model of the grid-connected converter meets the switching condition according to the real-time power grid impedance estimated value;
and a switching module: if the power grid accessed by the grid-connected converter meets the switching condition of the running mode of the grid-connected converter, obtaining a predicted grid-connected current value based on a grid-following prediction model or a grid-constructing prediction model established under different running modes, and completing the switching action based on the predicted grid-connected current value so as to minimize the grid-following cost function or the grid-constructing cost function.
Example III
It is an object of the present embodiment to provide a computing device comprising a memory, a processor and a computer program stored on the memory and executable on the processor, which processor implements the steps of the method described above when executing the program.
Example IV
An object of the present embodiment is to provide a computer-readable storage medium.
A computer readable storage medium having stored thereon a computer program which when executed by a processor performs the steps of the above method.
The steps involved in the devices of the second, third and fourth embodiments correspond to those of the first embodiment of the method, and the detailed description of the embodiments can be found in the related description section of the first embodiment. The term "computer-readable storage medium" should be taken to include a single medium or multiple media including one or more sets of instructions; it should also be understood to include any medium capable of storing, encoding or carrying a set of instructions for execution by a processor and that cause the processor to perform any one of the methods of the present invention.
It will be appreciated by those skilled in the art that the modules or steps of the invention described above may be implemented by general-purpose computer means, alternatively they may be implemented by program code executable by computing means, whereby they may be stored in storage means for execution by computing means, or they may be made into individual integrated circuit modules separately, or a plurality of modules or steps in them may be made into a single integrated circuit module. The present invention is not limited to any specific combination of hardware and software.
While the foregoing description of the embodiments of the present invention has been presented in conjunction with the drawings, it should be understood that it is not intended to limit the scope of the invention, but rather, it is intended to cover all modifications or variations within the scope of the invention as defined by the claims of the present invention.
Claims (10)
1. The grid-connected converter operation mode switching method along with the grid-constructed grid is characterized by comprising the following steps of:
predicting and updating the power grid impedance model by using relevant parameters in the power grid impedance model as filtering state variables through an extended Kalman filtering algorithm to obtain a real-time power grid impedance estimated value;
judging whether a power grid accessed by the grid-connected converter meets the switching condition of the running mode of the grid-connected converter, and determining whether the running model of the grid-connected converter reaches the switching condition according to the real-time power grid impedance estimated value;
if the power grid accessed by the grid-connected converter meets the switching condition of the running mode of the grid-connected converter, obtaining a predicted grid-connected current value based on a grid-following prediction model or a grid-constructing prediction model established under different running modes, and completing the switching action based on the predicted grid-connected current value so as to minimize the grid-following cost function or the grid-constructing cost function.
2. The method for switching the grid-connected converter with the grid-constructed operation mode according to claim 1, wherein the short-circuit ratio is determined according to the obtained real-time power grid impedance estimated value, and the strength of the power grid is determined according to the short-circuit ratio; if the grid-connected current transformer is in a strong current network, the grid-connected current transformer operation mode is a grid-following mode, and if the grid-connected current transformer is in a weak current network, the grid-connected current transformer and the grid-connected current transformer operation mode are in a grid-constructing mode.
3. The method for switching a grid-connected converter with a grid-constructed operation mode according to claim 1, wherein the method is characterized in that the method for predicting and updating the grid impedance model by using relevant parameters in the grid impedance model as filtering state variables by using an extended kalman filtering algorithm, and specifically comprises the following steps:
predicting a filtering state variable at the next moment according to the grid-connected variable current value, the grid voltage and the grid-connected converter output voltage at the current moment;
predicting a covariance matrix by using the state transition matrix;
predicting a filtering gain matrix by using the covariance matrix;
and updating by using the filtering gain matrix and the obtained filtering state variable at the next moment.
4. The method for switching the grid-connected current transformer on-grid-connected operation mode according to claim 1, wherein the predicted grid-connected current value is obtained based on the on-grid prediction model, and the switching action is completed based on the predicted grid-connected current value so as to minimize the on-grid cost function, specifically:
taking the measured values of the grid voltage and the grid-connected current at the current moment as the input of a grid-following prediction model, and obtaining a predicted grid-connected current value at the next moment based on the grid-following prediction model;
grid-connected current command values based on d axis and q axis are combined with real-time phase of grid voltage to obtain a grid-connected current reference value through dq-alpha beta coordinate conversion;
and taking driving information of the predicted grid-connected current value at the next moment corresponding to the time when the network cost function is minimum as a control switching signal according to the obtained predicted grid-connected current value at the next moment and the grid-connected current reference value.
5. The method for switching a grid-connected converter with grid-connected operation mode according to claim 4, wherein the direct current bus voltage and the direct current bus reference voltage are subjected to PI regulation to obtain grid-connected current command values of d-axis and q-axis.
6. The method for switching the grid-connected current transformer along with the grid-constructed operation mode according to claim 1, wherein the method is characterized in that a predicted grid-connected current value is obtained based on a grid-constructed prediction model, and the switching action is completed based on the predicted grid voltage with minimum grid-constructed cost function, and specifically comprises the following steps:
taking the measured values of the grid voltage and the grid-connected current at the current moment as the input of a grid-constructed prediction model, and obtaining a predicted grid-connected current value at the next moment based on the grid-constructed prediction model;
the voltage loop instruction value output by the droop power outer loop based on the d axis and q axis is combined with the phase output by the droop power outer loop to obtain a grid voltage reference value through dq-alpha beta coordinate conversion, and the grid-connected current reference value is obtained through a PI regulator;
and taking a driving signal of the predicted grid voltage at the next moment corresponding to the minimum grid construction cost function as a control switching signal according to the obtained predicted grid-connected current value at the next moment and the grid-connected current reference value.
7. The grid-connected converter grid-connected operation mode switching method according to claim 6, wherein the voltage loop command value output by the d-axis and q-axis droop power outer loop and the phase output by the droop power outer loop are obtained through droop control calculation according to the measured values of grid voltage and grid-connected current at the present moment and the set values of active power and reactive power.
8. A grid-connected converter grid-connected operation mode switching system, comprising:
a power grid impedance estimation module: predicting and updating the power grid impedance model by using relevant parameters in the power grid impedance model as filtering state variables through an extended Kalman filtering algorithm to obtain a real-time power grid impedance estimated value;
and a judging module: judging whether a power grid accessed by the grid-connected converter meets the switching condition of the running mode of the grid-connected converter, wherein the switching condition of the running mode of the grid-connected converter is determined according to the real-time power grid impedance estimated value;
and a switching module: if the power grid accessed by the grid-connected converter meets the switching condition of the running mode of the grid-connected converter, obtaining a predicted grid-connected current value or predicted grid voltage based on a grid-following prediction model or a grid-constructing prediction model established under different running modes, and completing the switching action based on the predicted grid-connected current value or predicted grid voltage so as to minimize the grid-following cost function or the grid-constructing cost function.
9. A computer device, comprising: a processor, a memory and a bus, the memory storing machine-readable instructions executable by the processor, the processor in communication with the memory via the bus when the computer device is running, the machine-readable instructions when executed by the processor performing a grid-tie converter grid-tie mode switching method as claimed in any one of claims 1 to 7.
10. A computer readable storage medium, wherein a computer program is stored on the computer readable storage medium, and when the computer program is executed by a processor, the computer program performs a grid-connected converter on-grid operation mode switching method according to any one of claims 1 to 7.
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