CN114268116B - State space modeling method of master-slave alternating-current micro-grid considering communication time delay - Google Patents
State space modeling method of master-slave alternating-current micro-grid considering communication time delay Download PDFInfo
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Abstract
The invention discloses a state space modeling method of a master-slave alternating-current micro-grid taking communication time delay into consideration, which comprises a parallel alternating-current micro-grid structure with master-slave control formed by a plurality of energy storage converters. The invention aims at the technical problem of communication delay when the host transmits a current instruction to the slave through low-bandwidth communication, and carries out modeling adjustment on system control parameters affecting stability according to the communication parameters of actual engineering, thereby improving the stability of the system and further improving the productivity.
Description
Technical Field
The invention relates to the field of electric power, in particular to a stability analysis technology of a state space of a master-slave alternating-current micro-grid with communication time delay in the field of electric power.
Background
In an independent alternating current micro-grid, the energy storage system can provide stable voltage and frequency for loads and other distributed power sources, stability and reliability of the micro-grid are enhanced, and the level of the absorption of renewable energy sources such as wind, light and the like is improved. As a mainstream energy storage form, electrochemical energy storage is mainly composed of a battery body, a battery management system, and a power conversion system. The parallel operation of the energy storage converters can effectively improve the reliability of the system, and flexible configuration of the system capacity and fine management of the battery units are realized. The master-slave control is used as a simple multi-machine parallel coordination control method, can provide voltage with constant amplitude and frequency for a load, and the steady-state power distribution among the converters is less influenced by line impedance. However, the number of the converters of the alternating-current micro-grid in the master-slave control mode is large, and multi-time scale coupling exists among links of the control system, so that factors affecting the stability of the system are complex.
How to effectively and reasonably model the alternating-current micro-grid in the master-slave control mode is the basis of stability analysis work. Based on effective and reasonable modeling, the influence of system parameters and control parameters on the stability of the master-slave alternating-current micro-grid under each time scale of the system is analyzed, and the key for improving the stability of the system is provided.
Around master-slave control, the master-slave structure needs to rely on low-bandwidth communication to realize the issuing of slave-to-current instructions, and the low-bandwidth communication interacts with a control system, so that the stability of the system is influenced. As in document [1 ]]Li Xialin, guo Li, wang Chengshan stability analysis in master-slave control mode of microgrid [ J]Electrotechnical journal, 2014, 29 (02): 24-34, establishing a discrete time state space model of a master-slave structure micro-grid system, and discussing the influence of key control parameters, power supply quantity and load change on system stability; as in document [2 ]]Liu Z.,Liu J.,Hou X.,et al.Output Impedance Modeling and Stability Prediction of Three-Phase Paralleled Inverters With Master-Slave Sharing Scheme Based on Terminal Characteristics of Individual Inverters[J]IEEE Transactions on Power Electronics,2016, 31 (7): 5306-5320. Establishing output impedance models of a master machine and a slave machine under a synchronous rotation coordinate system, and providing a stability judging method based on the Nyquist theorem by measuring the external characteristics of each converter; as in document [3 ]]Mortezaei A.,M.G., Savaghebi M.,et al.Cooperative Control of Multi-Master–Slave Islanded Microgrid With Power Quality Enhancement Based on Conservative Power Theory[J]IEEE Transactions on Smart Grid, 2018,9 (4) 2964-2975. In the proposed solution, the slaves are made to bear the asymmetric and harmonic power in the local load, the master outputs only the three-phase symmetric active power, and the droop control and remote converter cluster are adopted to realize power distribution; as in document [4 ]]Chen J.,Hou S.,Chen J.Seamless mode transfer control for master–slave microgrid[J]IET Power Electronics, 2019,12 (12) 3158-3165 provides a master-slave-droop hybrid control-based off-grid seamless switching strategy, wherein a droop control is adopted by a host during grid-connected operation, and the droop coefficient is slowly regulated to be zero by the host during off-grid operation so as to be switched into constant voltage constant frequency control, so that voltage out-of-limit caused by switching of the host control strategy during unplanned island is avoided; such as document [5 ]]Li D.,Man Ho C.N.,A Delay-Tolerable Master–Slave Current-Sharing Control Scheme for Parallel-Operated Interfacing Inverters with Low-Bandwidth Communication[J]IEEE Transactions on Industry Applications,2020, 56 (2): 1575-1586. Around the ac/dc side multi-machine parallel system of grid-connected operation, the influence of communication delay on system stability when the adoption of master-slave control to realize the sharing of dc load is discussed, and the method of low-pass filtering of current command and feedforward of dc voltage is adopted in slave control to improve the stability margin of the system.
The master-slave micro-grid research literature is less in consideration of the influence of low-bandwidth communication delay on system stability, few researches are in consideration of the influence, but a general modeling method is lacked in analysis, so that comprehensive and accurate stability analysis is difficult to perform.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to solve the problems that: the stability analysis is carried out on the independent alternating-current micro-grid adopting the master-slave structure, and the parameter design of the system is assisted, so that the problem of stability of the system due to communication time delay is solved.
Aiming at the problems, the invention provides a state space model of an independent alternating current micro-grid under a master-slave structure considering communication time delay, a zero-order retainer is adopted to model low-bandwidth communication time delay, second-order Pade approximation is carried out, and the dynamics of master-slave control, converter power and load are fully considered in the model.
A state space modeling method of a master-slave alternating-current micro-grid taking communication time delay into consideration comprises a master-slave control parallel alternating-current micro-grid structure formed by a plurality of energy storage converters, wherein the direct-current side of each energy storage converter is independently connected to a battery unit, the alternating-current side is connected to a public coupling point through an LCL filter, and then the load is supplied with power through an isolation transformer together, and a voltage-current controller realizes coordinated starting, stopping and protection of the plurality of energy storage converters.
In the technical scheme, the method comprises the following steps of: step one: establishing a host state space model under a dq coordinate system, selecting an energy storage converter as a host, and adopting constant voltage and constant frequency control by a central controller to establish and maintain the voltage and frequency of a micro-grid system; step two: establishing a slave state space model under a dq coordinate system, taking the rest energy storage converters as slaves, acquiring the amplitude and the phase of the voltage of the micro-grid by a central controller through a phase-locked loop, acquiring the output current of a host in real time through low-bandwidth communication as a reference, and performing current source type control to realize the power equalization of the master and the slave; step three: establishing a load state space model under a dq coordinate system; step four: and establishing an independent alternating current micro-grid state space model.
In the first step, a state space model of the host under the dq coordinate system is established. The state space model of the LCL filter is:
wherein u is inv_dq U, which is the component of the converter port voltage on the dq axis PCC_dq Is the component of the PCC voltage on the dq axis.
In the first step, the state space model of the voltage-current controller is:
wherein x is u_dq Integrator output, x, representing the outer loop of the host dq axis voltage i_dq The integrator output representing the internal loop of the host dq axis current.
In the first step, the host state space model of sampling and modulation delay is:
wherein T is s Represents the switching period, which is 1/(1.5T) of the first-order hysteresis link s +1)。
In step one, a state variable x is selected master ＝[i L1_d ,i L1_q ,u C_d ,u C_q ,i L2_d ,i L2_q ,x u_d ,x u_q ,x i_d ,x i_q , u inv_d ,u inv_q ] T Input variable u master ＝[u PCC_d ,u PCC_q ,u d_ref ,u q_ref ]The state space model of the host is:
in the second step, a state space model of the slave machine under the dq coordinate system is established, the state space model of the LCL filter is consistent with the state space model of the master machine, a zero-order retainer is adopted for carrying out low-bandwidth communication time delay, and second-order Pade approximation is carried out, wherein the state space model is as follows:
wherein T is LBC Representing a low-bandwidth communication period, converting the transfer type into a state space energy control standard model, wherein the state space energy control standard model is as follows:
wherein x is LBC 、u LBC And y LBC State variables, input variables, and output variables, respectively, representing low bandwidth communication delays.
The input variable is host L 1 Current i L1_dq_m The model is:
the slave current instructs the state space of the low-pass filtering link, and then the model is:
selecting state variables x slave ＝[i L1_d ,i L1_q ,u C_d ,u C_q ,i L2_d ,i L2_q ,x i_d_LBC_1 ,x i_q_LBC_1 ,x i_d_LBC_2 , x i_q_LBC_2 ,i d_ref ,i q_ref ,x i_d ,x i_q ,u inv_d ,u inv_q ] T Input variable u slave ＝[u PCC_d ,u PCC_q ,i L1_d ,i L1_q ]The state space model of the slave is:
in the third step, when the load is a resistive load, the load state space model of the PCC voltage is:
where n represents the number of slaves operating in parallel.
In the fourth step, the state space model under the condition of parallel connection of multiple hosts is as follows:
compared with the prior art, the invention has the following advantages: (1) The master-slave alternating-current micro-grid state space modeling method considering communication time delay is provided, and the communication time delay existing when a host transmits a current instruction to a slave through low-bandwidth communication is considered. The method is beneficial to analyzing the influence of communication time delay on the stability of the micro-grid system, and is convenient for flexibly adjusting the system control parameters according to the communication parameters in actual engineering so as to improve the stability of the system. (2) The model can be flexibly expanded according to the number of the actual energy storage converters, and is convenient for quick modeling when the system structure is changed, so that the productivity is improved.
Drawings
The drawings described herein are for providing a further understanding of the invention and should not be construed as unduly limiting the invention. The terminology in the reference numerals is for the purpose of describing and illustrating the invention only more conveniently and is not to be construed as limiting in any way.
FIG. 1 is a diagram of an AC micro-grid junction in a master-slave configuration.
Fig. 2 is a master-slave control block diagram.
FIG. 3 is a graph of a system feature root profile for a parameter change.
FIG. 4 is a graph of a second system feature root profile when parameters are changed.
Fig. 5 is a system feature root profile three as parameters change.
Fig. 6 is a system feature root profile four as parameters change.
Fig. 7 is a system feature root profile fifth as parameters change.
Fig. 8 is a diagram of an experimental waveform.
Fig. 9 is a second experimental waveform.
Detailed Description
The invention will be further described with reference to the drawings and specific examples to provide a clearer and more visual understanding of the invention.
Examples: as shown in an alternating current micro-grid junction diagram under a master-slave structure of FIG. 1, the invention adopts a master-slave control alternating current micro-grid structure, and is a state space modeling method of a master-slave alternating current micro-grid taking communication time delay into consideration. The direct current side of each energy storage converter is independently connected to the battery unit, the alternating current side is connected to the public coupling point through the LCL filter, and the load is supplied with power through the isolation transformer. Selecting one converter as a host, and adopting constant voltage and constant frequency control to establish and maintain the voltage and frequency of the system; the other converters are used as slaves, the amplitude and the phase of the micro-grid voltage are obtained through a phase-locked loop, and the output current of the host is obtained in real time through low-bandwidth communication to be used as a reference, so that the current source type control is realized, and the purpose of power sharing of the master and the slave is achieved. Although the master-slave control only needs to perform data interaction between the master machine and the slave machine, in the actual micro-grid engineering, in order to realize the coordinated starting, stopping and protecting of a plurality of converters, a voltage-current controller is an indispensable important link.
As shown in the master-slave control block diagram of fig. 2, wherein variables are added with suffixes _m Representing host-related variables, adding suffixes _s Representing slave related variables. The host computer is based on the voltage reference u d_ref And u q_ref Realizing constant voltage and constant frequency control. Wherein G is u_m For the host voltage outer loop controller, proportional-integral control (k pu ，k iu )；G i_m For the host current inner loop controller, PI control (k pi ， k ii ). The slave obtains the d-axis component i of the actual output current by obtaining the host sample through low bandwidth communication L1_d_m And q-axis component i L1_q_m Through the low-pass filtering link G LPF_i_ref Respectively obtaining d-axis current references i d_ref_s And q-axis current reference i q_ref_s And constant current control is realized through the current loop. G i_s For slave current inner loop controllers PI control (k pi ，k ii )；G LPF_i_ref The slave current commands a low pass filtering link. i.e L1_dq_s(_m) Is L 1 The value of the current in the dq coordinate system, i L2_dq_s(_m) Is L 2 The value of the current in the dq coordinate system, e dq_s(_m) For modulating the values referenced in the dq coordinate system, e αβ_s(_m) For modulation reference values in the αβ coordinate system, ω is the system angular frequency.
The method is used for further building an independent alternating current micro-grid state space model formed by parallel operation of three energy storage converters under a master-slave structure so as to verify the accuracy of the provided master-slave alternating current micro-grid state space modeling method. The reference data are shown in table 1 below. Table 1:
when analyzing the influence of each parameter on the system stability, the parameters shown in table 1 are used as reference parameters except for the specific description.
On the basis of the above, the system characteristic root distribution when each parameter is changed is as follows, and as shown in the first system characteristic root distribution diagram when the parameter is changed in FIG. 3, the proportion coefficient k is the outer ring proportion coefficient of the host voltage pu_m The eigenvalues eig.1 and eig.2 gradually move towards the imaginary axis from 0.1 in steps 1 to 8, the damping ratio decreasing.
As shown in a second system characteristic root distribution diagram when the parameters of FIG. 4 are changed, the inner loop proportionality coefficient k is related to the host current pi_m The eigenvalues eig.1 and eig.2 gradually move towards the imaginary axis from 1 in steps 1 to 8, the damping ratio decreasing.
As shown in a third system characteristic root distribution diagram when the parameters of fig. 5 are changed, the current loop proportion coefficient k with the slave current pi_s Increasing from 1 in steps of 1 to 5, the dominant feature roots Eig.3 and Eig.4 damping ratio decreases.
As shown in figure 6, the system characteristic root distribution diagram is four when the parameters are changed, and the current command of the slave machine is low-passFilter time constant T LPF_i_ref The characteristic values eig.5 and eig.6 increase the damped oscillation frequency and decrease the damping ratio in steps of 0.001s from 0.008s to 0.001 s.
As shown in fig. 7, the system characteristic root distribution diagram is five, with the low bandwidth communication period T LBC Changing from 0.001s to 0.01s in steps of 0.001s, the eigenvalue eig.6 moves in a direction approaching the imaginary axis, the damping ratio of the eigenvalues eig.7 and eig.8 decreases, and the damped oscillation frequency increases and then decreases.
On the basis of the above, three energy storage converter parallel operation systems adopting a master-slave structure are built to verify the relevant conclusion of the stability analysis of the multi-energy storage converter parallel operation system with the master-slave structure, and the experimental result is as follows.
As shown in the experimental waveform diagram I of FIG. 8, a parameter T is set in the model LPF_i_ref Is 0.001, t 1 The active power load is charged for 18kW at any time, the oscillation of output current appears between the master machine and the slave machine, and the oscillation period is about 2ms and is close to theoretical analysis.
As shown in the experimental waveform diagram II of FIG. 9, a parameter T is set in the model LPF_i_ref ＝0.01，t 1 And the active power load is constantly input with 18kW, so that the stability of the system is improved.
Also, the model proposed by the above-described embodiment can be flexibly extended according to the number of actual energy storage converters, so as to facilitate rapid modeling when the system structure is changed.
According to the embodiment, the communication delay is modeled when the low-bandwidth communication transmits the current instruction by fully considering the dynamic characteristics of the master-slave control, the power part of the converter and the load and adopting the zero-order retainer, and the control coefficient is flexibly adjusted according to the communication parameters in the actual engineering, so that the stability of the energy storage system is greatly improved, and the electricity consumption requirement of a user is ensured.
In addition to the above embodiments, the technical features of the present invention may be rearranged and modified in steps within the scope of the claims and the description of the present invention to constitute new embodiments, which may be implemented by those skilled in the art without inventive effort, and thus, embodiments of the present invention not described in detail should be considered as embodiments of the present invention within the scope of the protection of the present invention.
Claims (7)
1. A state space modeling method of a master-slave alternating-current micro-grid taking communication time delay into consideration is characterized in that a master-slave control parallel alternating-current micro-grid structure is formed by a plurality of energy storage converters, the direct-current sides of the energy storage converters are independently connected to a battery unit, the alternating-current sides are connected to a public coupling point through LCL filters and supply power to loads through isolating transformers, and a voltage-current controller realizes coordinated starting, stopping and protection of the plurality of energy storage converters, and the method comprises the following steps:
s1: establishing a host state space model under a dq coordinate system, selecting an energy storage converter as a host, and adopting constant voltage and constant frequency control by a central controller to establish and maintain the voltage and frequency of a micro-grid system;
s2: establishing a slave state space model under a dq coordinate system, taking the rest energy storage converters as slaves, acquiring the amplitude and the phase of the voltage of the micro-grid by a central controller through a phase-locked loop, acquiring the output current of a host in real time through low-bandwidth communication as a reference, and performing current source type control to realize the power equalization of the master and the slave;
s3: establishing a load state space model under a dq coordinate system;
s4: establishing an independent alternating current micro-grid state space model;
the state space model of the LCL filter is as follows:
wherein u is inv_dq U, which is the component of the converter port voltage on the dq axis PCC_dq Is the component of the PCC voltage on the dq axis;
the zero-order retainer is adopted to carry out low-bandwidth communication time delay and second-order Pade approximation, and the method is as follows:
wherein T is LBC Representing a low-bandwidth communication period, converting the transfer type into a state space energy control standard model, wherein the state space energy control standard model is as follows:
wherein x is LBC 、u LBC And y LBC State variables, input variables, and output variables, respectively, representing low bandwidth communication delays.
2. The method for modeling a state space of a master-slave ac microgrid according to claim 1, wherein the state space model of the voltage-current controller is as follows:
wherein x is u_dq Integrator output, x, representing the outer loop of the host dq axis voltage i_dq The integrator output representing the internal loop of the host dq axis current.
3. The method for modeling a state space of a master-slave ac microgrid according to claim 2, wherein the host state space model of sampling and modulation delay is:
wherein T is s Represents the switching period, which is 1/(1.5T) of the first-order hysteresis link s +1)。
4. A method according to claim 3A state space modeling method of a master-slave alternating-current micro-grid considering communication time delay is characterized by selecting a state variable x master ＝[i L1_d ,i L1_q ,u C_d ,u C_q ,i L2_d ,i L2_q ,x u_d ,x u_q ,x i_d ,x i_q ,u inv_d ,u inv_q ] T Input variable u master ＝[u PCC_d ,u PCC_q ,u d_ref ,u q_ref ]The state space model of the host is:
5. the method for modeling a state space of a master-slave ac microgrid according to claim 1, wherein the input variable is a host L 1 Current i L1_dq_m The model is:
the slave current instructs the state space of the low-pass filtering link, and then the model is:
selecting state variables x slave ＝[i L1_d ,i L1_q ,u C_d ,u C_q ,i L2_d ,i L2_q ,x i_d_LBC_1 ,x i_q_LBC_1 ,x i_d_LBC_2 ,x i_q_LBC_2 ,i d_ref ,i q_ref ,x i_d ,x i_q ,u inv_d ,u inv_q ] T Input variable u slave ＝[u PCC_d ,u PCC_q ,i L1_d ,i L1_q ]The state space model of the slave is:
6. the method for modeling a state space of a master-slave ac microgrid according to claim 1, wherein when the load is a resistive load, the load state space model of the PCC voltage is:
where n represents the number of slaves operating in parallel.
7. The method for modeling a state space of a master-slave ac microgrid according to claim 1, wherein the state space model under the multi-machine parallel condition is as follows:
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