CN116316523A - Distributed cascade secondary control method for bipolar direct-current micro-grid - Google Patents

Abstract

The invention proposes a distributed cascade secondary control method for a bipolar DC microgrid. Aiming at the defects of complex structure and poor robustness of the parallel scheme, the present invention proposes a simplified cascaded secondary control method in order to further reduce the number of controllers used and reduce the operating cost of the system. The method uses a single current regulator and Local voltage compensation enables simultaneous voltage restoration and current sharing, while still ensuring steady-state voltage and current regulation accuracy in the presence of variable delays in information exchange. Especially in the bipolar DC microgrid with a large number of converters, the method proposed by the present invention has more significant practicability, and can compensate the voltage drop caused by the droop control in the bipolar DC microgrid within a fixed time range and uneven current distribution.

Description

Distributed cascade secondary control method for bipolar DC microgrid

technical field

The invention belongs to the technical field of DC micro-grids, in particular to a distributed cascade secondary control method for bipolar DC micro-grids. Specifically, from the perspective of improving system operation reliability, the present invention proposes a distributed cascading secondary control scheme of a peer-to-peer bipolar DC microgrid.

Background technique

The DC micro-grid can efficiently gather distributed power generation and energy storage by using renewable energy such as solar energy and biomass, and it is a multi-energy complementarity of "heat, electricity, cooling, and gas" and integrated operation of "source, grid, load, and storage". The effective carrier is also a useful supplement to the large power grid, and has the advantages of simple control and high energy efficiency. According to the bus architecture, the DC microgrid can be divided into two types: unipolar DC microgrid and bipolar DC microgrid. The bipolar power supply system can provide more voltage-level interfaces for distributed power sources and loads. Under the premise of the same voltage stress between the busbars, the bipolar power supply system has twice the transmission capacity of the unipolar power supply system, which is suitable for high voltage and high power applications, and the bipolar power supply system has high reliability and safety. Even if one of the buses fails, power to the load can still be supplied using the other two buses.

Overall, the bipolar DC microgrid is a power supply structure that efficiently utilizes renewable energy. However, this power supply structure requires the application of more power electronic converters, making the system larger and more complex. In addition, any asymmetry of power supply, load or positive and negative line parameters will generate unbalanced current in the neutral line, which will increase the line loss and cause the bus voltage to deviate from the rated value. In order to alleviate the uneven voltage between poles caused by unbalanced current, compensate the drop of bus voltage, and coordinate the operation of various micro-sources in the network, it is necessary to introduce a suitable operation control method in the bipolar DC micro-grid to improve the efficiency of the micro-grid. The ability of overall planning and dispatching can improve the quality of power supply for end users.

Although a wealth of experience has been accumulated in the operation control of unipolar DC microgrids, due to the large differences in configuration structures, it is difficult to directly apply the existing unipolar system operation control methods and experience in bipolar systems. Based on the above requirements, the present invention proposes a distributed cascaded secondary control scheme for bipolar DC microgrids. Compared with the existing parallel secondary control scheme, it saves half of the time without losing the rapidity of convergence. The regulator has the advantages of simple design and strong practicability.

Contents of the invention

The purpose of the present invention is to solve the problems of bus voltage drop and limited current sharing accuracy caused by droop control of bipolar DC microgrid, and proposes a distributed cascaded secondary control method for bipolar DC microgrid.

The present invention is achieved through the following technical solutions. The present invention proposes a distributed cascaded secondary control method for bipolar DC microgrids. The method specifically includes: the secondary control layer compensates the droop control error by modifying the voltage reference value or the droop coefficient ; The principle of secondary control compensation is expressed as:

The above principles can be regarded as the droop translation method and the impedance adjustment method respectively in the V-I curve; among them, the distributed secondary control expression based on the droop translation method is:

分别表示二次控制产生的电压、电流修正项；In the formula,

Respectively represent the voltage and current correction items generated by the secondary control;

Based on the voltage drop value, that is, the condition that the virtual voltage drop is known, the secondary control voltage correction item can be directly set:

Substituting the voltage correction term into formula (8) to solve the relationship between the voltage reference value, current correction term and output voltage is:

则式(10)中的电流修正项需满足：Considering that the voltage control target is

The current correction term in formula (10) needs to satisfy:

Reference design current control input is as follows:

The current control input is introduced into the integral controller to obtain the intermediate state variable δ _polei , and its mathematical expression is as follows:

δ _polei = ∫ε _polei dt (13)

和右特征向量1_N；由拉普拉斯矩阵左特征值与特征向量关系可知：For an N-order undirected connected graph G=(ν,ε), its corresponding Laplacian matrix must be a positive semi-definite matrix, and the Laplacian matrix has a unique zero eigenvalue λ ₁ (L)=0, corresponding to the left eigenvector

and the right eigenvector 1 _N ; from the relationship between the left eigenvalue and eigenvector of the Laplace matrix:

Based on formula (14), the expression of the current correction item predictor is designed:

式(15)的矢量表达形式为：definition

The vector expression form of formula (15) is:

_uIpole = _Lδpole (16)

计算电流修正项之和：Multiply the unit row vector to the left of equation (16)

Compute the sum of the current correction terms:

The sum of current correction terms satisfies the requirements in formula (11), and since then, the simplified fixed-time consistent quadratic control method composed of fixed-time consistent controller, integral controller and predictor is completed.

Furthermore, in order to eliminate the control hysteresis existing in the simplified cascaded structure, the original structure is combined with the proportional control of the error signal to form a proportional feedforward-fixed time consistent secondary control system;

The mathematical expression of the distributed proportional feedforward-fixed-time consistent quadratic control method is:

In the formula, k _polei represents the feed-forward coefficient.

Further, the bipolar DC microgrid is controlled by a voltage source type bidirectional Buck-Boost converter, and the main control converters in each area adopt droop control, and then output PWM signals to control the converters through voltage and current double closed-loop output voltage and current.

Further, the basic principle of the droop control is to introduce a virtual voltage feedback value linearly proportional to the current in the voltage outer loop, so that the voltage given value can be adjusted according to the load condition; the mathematical expression of the droop control principle The formula is:

Further, the integral controller is used to eliminate the chattering phenomenon caused by rapid error changes, and at the same time generate the input signal δ _polei of the predictor; the final current correction term is generated by the predictor, and is superimposed with the voltage correction term obtained by local real-time measurement , to achieve current balance control and voltage compensation.

The beneficial effects of the present invention are:

The invention relates to the field of DC micro-grids, and specifically proposes a distributed cascading secondary control scheme for peer-to-peer bipolar DC micro-grids from the perspective of improving system operation reliability. The invention solves the problems of bus voltage drop and limited current sharing accuracy caused by bipolar DC micro-grid droop control. Aiming at the defects of complex structure and poor robustness of the parallel scheme, in order to further reduce the number of controllers used and reduce the operating cost of the system, a simplified cascaded secondary control scheme is proposed, which uses a single current regulator and local voltage Compensation enables simultaneous voltage recovery and current sharing, while maintaining steady-state voltage and current regulation accuracy in the presence of variable delays in information exchange. Especially in the bipolar DC microgrid with a large number of converters, the scheme proposed by the present invention has more significant practicability, and can compensate the voltage drop caused by the droop control in the bipolar DC microgrid within a fixed time range and uneven current distribution.

Description of drawings

Figure 1 is a structural diagram of the bipolar DC microgrid.

Fig. 2 is a block diagram of droop control principle of bipolar DC microgrid.

Figure 3 is a parallel equivalent circuit diagram of the droop control converter.

Fig. 4 is a block diagram of a cascaded secondary control based on a fixed-time consistency protocol.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

Combining with Fig. 1-Fig. 4, the present invention takes the bipolar DC micro-grid with ±100V DC bus as the core as the research object, and the specific structure of the system is shown in Fig. 1 .

Among them, six bidirectional Buck-Boost converters are connected in series according to the modular design structure, the series module I is used as the main unit, and the series modules II and III are used as the slave units. As the hub of the coordinated operation of the master-slave control system, the master unit is always connected to the power grid and operates in a manner similar to a voltage source to maintain the system voltage. The slave unit adjusts its operation mode according to the instructions of the master unit and its own energy characteristics. A bipolar DC microgrid provides power. The independent converters connected in series inside the main unit have their common connection point grounded, thereby leading to output terminals with three levels of +100V, 0V, and -100V. The two series converters in any unit are powered by the same type of renewable energy or energy storage device to ensure the energy balance of the two poles and improve efficiency. At the same time, the two series converters use independent control loops to ensure that the positive bus (P) - Neutral (Z) area, Z-negative bus (N) area can independently adjust the voltage. On the load side, the +200V voltage between the positive pole and the negative pole of the common DC bus is converted into the required level of AC and DC power by DC-AC and DC-DC converters, and supplied to various types of loads. If the load balance can be ensured, it can also be symmetrically connected to a more appropriate +100V area to improve the conversion efficiency of the converter.

The left side of Figure 1 shows the control block diagram of the voltage source bidirectional Buck-Boost converter. The main control converters in each area adopt droop control, and then output PWM signals through voltage and current double closed loops to control the output voltage and current of the converter. .

The control objective of the bipolar DC microgrid system mainly includes two parts: one is to adjust the output voltage of the double-sided converter to the rated value, and the other is to ensure that the load current is evenly distributed among the parallel converters, namely:

V _polei = V ^ref (1)

In the formula, the subscript pole represents the power supply electrode of the bipolar DC microgrid, including two power supply electrodes P and N, the subscript i indicates the converter at the i-th node, V ^ref , V _polei , and I _polei are the converter rated voltage, actual output voltage and output current.

Under the hierarchical control structure, droop control is widely used in the underlying control due to its good scalability, plug-and-play characteristics, and no need to lay communication networks. Figure 2 shows the block diagram of droop control for bipolar DC microgrids , the basic principle of droop control is to introduce a virtual voltage feedback value that is linearly proportional to the current in the voltage outer loop, so that the voltage given value can be adjusted according to the load condition. The mathematical expression of the droop control principle is:

表示从第1或2号变换器输出端到直流母线的线缆电阻，R_pole1、R_pole2表示下垂控制引入的虚拟电阻。根据基尔霍夫电压定律，对两回路列写电压方程求得线缆末端电压表达式为：Considering the complexity of the distributed power supply operation mode, for example, the photovoltaic cell or wind generator interface converter works in MPPT mode or current control mode, and the fuel cell or energy storage unit interface converter works in voltage control mode. There are N units in the system The assumption of parallel voltage source converters is established. Figure 3 selects any two parallel converters and draws an equivalent circuit diagram to analyze the control effect of droop control on output voltage and output current. In order to make the equivalent circuit diagram closer to the real physical topology, add

Indicates the cable resistance from the output terminal of No. 1 or 2 converter to the DC bus, and R _pole1 and R _pole2 indicate the virtual resistance introduced by the droop control. According to Kirchhoff's voltage law, write the voltage equation for the two circuits to obtain the voltage expression at the end of the cable:

In view of the parallel connection structure between the converters, the voltages at the cable ends of No. 1 converter and No. 2 converter are equal, and the output current relationship of the two converters is obtained by using the common DC bus voltage V _bus instead of the cable end voltages V ₁ and V ₂ The expression is:

In order to make up for the problems of bus voltage drop and limited current sharing accuracy caused by droop control, it is necessary to introduce distributed secondary control to reflect the operating performance of the DC microgrid through secondary regulation. The implementation of the secondary control requires the underlying control to retain a certain degree of control freedom. Specifically for the droop control scheme, this control degree of freedom comes from the two quantities of the voltage reference value and the droop coefficient, which means that the secondary control layer can be modified by The voltage reference or droop coefficient compensates for droop control errors. The principle of secondary control compensation can be expressed as:

The above principles can be regarded as the droop translation method and the impedance adjustment method respectively in the V-I curve. Among them, the impedance adjustment method takes the feedback loop parameters as the adjustment object, which will have a great impact on the dynamic characteristics of the system. If the variation range of the droop coefficient is not reasonably limited, the stability of the system operation may be threatened. At present, the droop translation method is a more mainstream secondary control scheme.

The distributed quadratic control expression based on the droop translation method is:

分别表示二次控制产生的电压、电流修正项；In the formula,

Respectively represent the voltage and current correction items generated by the secondary control;

Considering the distributed terminal power supply in the system, the diversity of energy sources determines the power capability of the terminal power supply, and the purpose of setting the droop coefficient in distributed control is to quantify the power capability of different terminal power supplies in advance, so as to coordinate the terminal power supply during system operation. contribute. Obviously, for each converter in the system, the droop coefficient is local information, and the voltage drop value caused by the droop coefficient can also be obtained through real-time measurement. Based on the voltage drop value, that is, the condition that the virtual voltage drop is known, the secondary control voltage correction item can be directly set:

Substituting the voltage correction term into formula (8) to solve the relationship between the voltage reference value, current correction term and output voltage is:

则式(10)中的电流修正项需满足：Considering that the voltage control target is

The current correction term in formula (10) needs to satisfy:

Reference design current control input is as follows:

The current control input is introduced into the integral controller to obtain the intermediate state variable δ _polei , and its mathematical expression is as follows:

δ _polei = ∫ε _polei dt (13)

和右特征向量1_N。由拉普拉斯矩阵左特征值与特征向量关系可知：For an N-order undirected connected graph G=(ν,ε), its corresponding Laplacian matrix must be a positive semi-definite matrix, and the Laplacian matrix has a unique zero eigenvalue λ ₁ (L)=0, corresponding to the left eigenvector

and the right eigenvector 1 _N . According to the relationship between the left eigenvalue and eigenvector of the Laplacian matrix:

Based on formula (14), the expression of the current correction item predictor is designed:

式(15)的矢量表达形式为：definition

The vector expression form of formula (15) is:

计算电流修正项之和：Multiply the unit row vector to the left of equation (16)

Compute the sum of the current correction terms:

Obviously, the sum of the current correction items meets the requirements in formula (11). Since then, a simplified fixed-time consistent quadratic control scheme consisting of a fixed-time consistent controller, an integral controller, and a predictor is formed. Among them, the sign function and the power operation link can significantly improve the response speed of the integral controller and shorten the convergence time of the current error, and the integral controller is used to eliminate the chattering phenomenon caused by the rapid change of the error, and at the same time generate the input signal δ _polei of the predictor . The final current correction term is generated by the predictor and superimposed with the voltage correction term obtained from local real-time measurement to realize current balance control and voltage compensation.

According to the characteristics of the integral control, the output of the integral controller is the cumulative effect of the control input on time. With the help of the fast convergence algorithm, although the integral control can get rid of the asymptotic characteristics and eliminate the residual error within a fixed time, but because the later stage prediction If an exchange of information about intermediate state variables is added in the controller, the current control effect will still lag behind the deviation change, and the influence of the disturbance cannot be overcome in time and effectively. In order to eliminate the control hysteresis existing in the simplified cascaded structure, the original structure can be chosen to be combined with the proportional control on the error signal to form a proportional feedforward-fixed-time consistent secondary control system. In this way, the advantages of integrated proportional control and integral control can further improve the adjustment sensitivity under the premise of ensuring the stability of the system.

The mathematical expression of the distributed proportional feedforward-fixed-time consistent quadratic control scheme is:

In the formula, k _polei represents the feed-forward coefficient.

The final cascaded secondary control block diagram is shown in Figure 4. Converters No. 1, No. 3, and No. 5, and No. 2, No. 4, and No. 6 converters are placed in the positive and negative areas. There is an undirected ring communication network between the parallel converters. Similar to the traditional parallel framework, the internal current balance control of each pole needs to use the information exchange between adjacent nodes to generate an error term, combine the proportional and integral control to coordinate the current distribution relationship, and finally generate a current correction term at the output of the predictor to eliminate the current error . In the absence of a voltage recovery controller, the scheme relies on locally measured virtual voltage drops for voltage compensation. Compared with the traditional parallel control scheme, the novel cascaded control scheme designed by the present invention only needs a fixed-time controller and an integral controller, which reduces the number of controllers by half and greatly simplifies the control system.

The invention relates to the field of DC micro-grids, and specifically proposes a distributed cascading secondary control scheme for peer-to-peer bipolar DC micro-grids from the perspective of improving system operation reliability. The invention solves the problems of bus voltage drop and limited current sharing accuracy caused by bipolar DC micro-grid droop control. Aiming at the defects of complex structure and poor robustness of the parallel scheme, in order to further reduce the number of controllers used and reduce the operating cost of the system, a simplified cascaded secondary control scheme is proposed, which uses a single current regulator and local voltage Compensation enables simultaneous voltage recovery and current sharing, while maintaining steady-state voltage and current regulation accuracy in the presence of variable delays in information exchange. Especially in the bipolar DC microgrid with a large number of converters, the scheme proposed by the present invention has more significant practicability, and can compensate the voltage drop caused by the droop control in the bipolar DC microgrid within a fixed time range and uneven current distribution.

The bipolar DC microgrid distributed cascade secondary control method proposed by the present invention has been introduced in detail above. In this paper, specific examples are used to illustrate the principle and implementation of the present invention. The description of the above embodiments is only used To help understand the method of the present invention and its core idea; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation and scope of application. In summary, this specification The content should not be construed as a limitation of the invention.

Claims (5)

1. The bipolar direct current micro-grid distributed cascade secondary control method is characterized by comprising the following steps of: the method specifically comprises the following steps: the secondary control layer compensates sagging control errors by modifying voltage reference values or sagging coefficients; the secondary control compensation principle is expressed as:

the above principle can be considered as a droop translation method and an impedance adjustment method in the V-I curve, respectively; wherein, the distributed secondary control expression based on the sagging translation method is as follows:

in the method, in the process of the invention,

respectively representing voltage and current correction terms generated by secondary control;

based on the voltage drop value, i.e. the condition that the virtual voltage drop is known, the secondary control voltage correction term can be set directly:

substituting the voltage correction term into the formula (8) to obtain the relation among the voltage reference value, the current correction term and the output voltage, wherein the relation is as follows:

considering that the voltage control target is

The current correction term in equation (10) needs to satisfy:

the reference design current control inputs are as follows:

introducing a current control input into an integral controller to obtain an intermediate state variable delta _polei The mathematical expression is as follows:

δ _polei ＝∫ε _polei dt(13)

for the N-order undirected connected graph G= (v, epsilon), the corresponding Laplacian matrix is necessarily a semi-positive definite matrix, and the Laplacian matrix has a unique zero eigenvalue lambda ₁ (L) =0, respectively corresponding to left feature vectors

And right eigenvector 1 _N The method comprises the steps of carrying out a first treatment on the surface of the The relationship between the left eigenvalue and eigenvector of the Laplace matrix is as follows:

based on equation (14), a current correction term predictor expression is designed:

definition of the definition

δ _pole ＝[δ _pole1 ,δ _pole2 ,…,δ _poleN ] ^T The vector expression of formula (15) is:

to (16) the unit row vector of the left multiplication

Calculating the sum of current correction terms:

the sum of the current correction terms satisfies the requirement in the formula (11), and the simplified fixed time consistency secondary control method consisting of the fixed time consistency controller, the integral controller and the predictor is completed.

2. The method of claim 1, wherein to eliminate control hysteresis associated with a simplified cascade structure, the combination of the original structure with proportional control with respect to the error signal is selected to form a proportional feed forward-fixed time consistency secondary control system;

the mathematical expression of the distributed proportional feedforward-fixed time consistency secondary control method is as follows:

wherein k is _polei Representing the feed forward coefficient.

3. The method of claim 2, wherein the bipolar dc micro-grid is controlled by a voltage source type bi-directional Buck-Boost converter, and the master control converter in each region is controlled by droop, and then the output voltage and current of the converter are controlled by a voltage-current double closed loop output PWM signal.

4. A method according to claim 3, characterized in that the basic principle of droop control is to introduce a virtual voltage feedback quantity in the voltage outer loop in linear proportion to the current, so that the voltage set point can be adjusted according to the load condition; the mathematical expression of the droop control principle is:

5. the method of claim 1, wherein the integral controller is configured to eliminate buffeting caused by rapid changes in error while generating the predictor input signal delta _polei The method comprises the steps of carrying out a first treatment on the surface of the The final current correction term is generated by a predictor and is overlapped with the voltage correction term obtained by local real-time measurement, so that current balance control and voltage compensation are realized.

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