CN108448565B - Power distribution method for direct-current micro-grid composite energy storage system - Google Patents
Power distribution method for direct-current micro-grid composite energy storage system Download PDFInfo
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- CN108448565B CN108448565B CN201810296032.9A CN201810296032A CN108448565B CN 108448565 B CN108448565 B CN 108448565B CN 201810296032 A CN201810296032 A CN 201810296032A CN 108448565 B CN108448565 B CN 108448565B
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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
- H02J1/00—Circuit arrangements for dc mains or dc distribution networks
- H02J1/14—Balancing the load in a network
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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
- H02J7/00—Circuit arrangements for charging or depolarising batteries or for supplying loads from batteries
- H02J7/34—Parallel operation in networks using both storage and other dc sources, e.g. providing buffering
- H02J7/345—Parallel operation in networks using both storage and other dc sources, e.g. providing buffering using capacitors as storage or buffering devices
Abstract
The invention discloses a power distribution method of a direct-current micro-grid composite energy storage system, which comprises the following steps: calculating the resistive droop coefficient of each branch of the storage battery pack; calculating the capacitive droop coefficient of each branch of the super capacitor bank; acquiring output voltage and unbalanced power of the storage battery branch converter, performing fuzzy logic processing on the acquired two parameters, and adjusting a resistive droop coefficient by using an output signal processed by the fuzzy logic processing; acquiring the output power of a branch circuit of the super capacitor, the power change rate and the unbalanced power of the converter, performing fuzzy logic processing on the acquired absolute values of the three parameters, and adjusting a capacitive droop coefficient by using an output signal processed by the fuzzy logic processing; and performing resistance-capacitance droop control on the direct current micro-grid according to the adjusted resistance-droop coefficient and the adjusted capacitance-droop coefficient. The invention realizes the frequency-based distribution of unbalanced power among different energy storages, improves the distribution precision, accelerates the response speed of the super capacitor and has good expandability.
Description
Technical Field
The invention belongs to the technical field of electrical engineering, and particularly relates to a power distribution method of a direct-current micro-grid composite energy storage system.
Background
With the development of power electronic technology and the growth of new energy power generation, a micro-grid integrating distributed power sources, loads and energy storage devices receives wide attention. The microgrid may be classified into an ac microgrid and a dc microgrid in terms of types. Compared with an alternating-current micro-grid, the direct-current micro-grid has no problems in the aspects of frequency stability, reactive power loss and the like, meanwhile, the control is relatively simple, the construction cost is relatively low, and the trend of the development of the future micro-grid can be realized. In a direct-current micro-grid, the output of renewable energy sources such as wind power and photovoltaic has randomness and volatility, so that the power of a system is unbalanced, the stability of direct-current voltage is influenced, and the safe operation of the direct-current micro-grid is threatened. The energy storage has a fast power response characteristic and is always an important technical means for maintaining power balance in a power grid. At present, energy storage devices are mainly classified into two types, one type is energy storage represented by a storage battery, and the energy storage devices have the characteristics of high energy density and low cost, but the power density is not high, the cycle life is limited, and the energy storage devices are not suitable for scenes of frequent charging and discharging. The other type is power type energy storage represented by a super capacitor, which has high power density and long cycle life and can be charged and discharged rapidly for many times. Because the direct-current micro-grid needs to maintain energy balance for a long time and guarantee power balance constantly, the composite energy storage system combining the storage battery and the super capacitor can fully exert the advantages of different energy storage, and plays roles of making best of advantages and avoiding disadvantages and supplementing each other, thereby achieving wide application.
The traditional resistive droop control strategy can only realize the fixed proportion distribution of the unbalanced power of the direct current micro-grid according to the droop coefficient, and is difficult to realize the distribution of the unbalanced power according to the frequency characteristic. Through introducing the branch road that has the capacitive characteristic at traditional flagging control circuit, form resistance-capacitance flagging control back, through the electric capacity energy storage with the unbalanced power distribution of high frequency on the capacitive branch road, the unbalanced power distribution of low frequency is the battery energy storage on the resistive branch road, can realize the unbalanced power rational distribution of direct current microgrid. However, the resistance parameters of the lines in the direct-current microgrid have obvious influence on unbalanced power distribution, unbalanced distribution of unbalanced power among energy storage devices of the same type can be caused, and the operating characteristics of the energy storage devices are influenced.
Disclosure of Invention
Aiming at the defects and improvement requirements of the prior art, the invention provides a power distribution method of a direct-current micro-grid composite energy storage system, which aims to realize the distribution control of a storage battery pack and a super capacitor pack according to power frequency through resistance-capacitance droop control, and realize the current sharing control of the storage battery pack and the super capacitor pack based on a fuzzy logic algorithm, so that high-frequency components in the unbalanced power of a direct-current micro-grid are distributed to capacitive energy storage on a capacitive branch, and low-frequency components are distributed to storage battery energy storage on a resistive branch, thereby fully playing the complementary operation characteristics of different energy storage devices in the composite energy storage system, improving the power distribution precision of the storage battery pack and the super capacitor pack, and accelerating the response speed of a super capacitor.
In order to achieve the purpose, the invention provides a power distribution method of a direct current micro-grid composite energy storage system, which comprises the following steps:
(1) obtaining the maximum voltage deviation delta V allowed by the direct current busmaxAnd the maximum output current of each branch of the storage battery pack, and calculating the resistive droop coefficient of each branch of the storage battery pack; wherein, the maximum output current of the ith branch of the storage battery is IimaxThe resistive droop coefficient of the ith branch of the storage battery pack is dri(ii) a The value of i is 1 to Nr，NrThe total number of branches contained in the storage battery pack;
(2) obtaining the response time of each super capacitor and the droop controller adjustment time tsCalculating the capacitive droop coefficient of each branch of the super capacitor bank; wherein the response time of the super capacitor of the jth branch of the super capacitor bank is tcjThe capacitive droop coefficient of the jth branch of the super capacitor bank is dcj(ii) a j takes a value of 1 to Nc，NcThe total number of branches contained in the super capacitor bank;
(3) for the ith branch of the storage battery pack, the output voltage v of the converter is obtainedbiAnd unbalanced power Δ Pbi(ii) a For output voltage vbiAnd unbalanced power Δ PbiPerforming fuzzy logic processing to obtain output signal rriFor adjusting the resistive droop coefficient dri；
(4) Using the output signal rriFor resistance droop coefficient driAdjusting to obtain the adjusted resistive droop coefficient d'riTo realize the current sharing control among the storage battery packs;
(5) for the j branch of the super capacitor bank, obtaining the output power P of the convertercjPower rate of change of converter
And unbalanced power Δ Pcj(ii) a To the output power PcjAbsolute value of (2), power rate of changeAbsolute value of (d) and unbalanced power Δ PcjThe absolute value of the signal is processed by fuzzy logic to obtain an output signal rcjFor adjusting the capacitive sag factor dcj；
(6) Using the output signal rcjFor the sag coefficient d of the capacitancecjAdjusting to obtain the adjusted capacitive droop coefficient d'cjTo realize the current-sharing control among the super capacitor groups;
(7) according to the resistive droop coefficient d'riAnd capacitive droop coefficient d'cjAnd carrying out resistance-capacitance droop control on the direct current micro-grid so as to realize the distribution control between the storage battery pack and the super capacitor pack according to the power frequency.
Further, in the step (3), the output voltage v is adjustedbiAnd unbalanced power Δ PbiFuzzy logic processing is carried out according to the following principle: unbalanced power Δ PbiThe larger the forward direction or the output voltage vbiThe larger the resistance droop coefficient d is, the more the resistance droop coefficient d isri(ii) a Unbalanced power Δ PbiThe larger the negative direction or the output voltage vbiThe smaller the resistance droop coefficient d isri(ii) a Unbalanced power Δ PbiApproaching zero or output voltage vbiWhen the rated value is reached and the power responses among the storage battery packs are consistent, the resistance droop coefficient d is obtainedriNo adjustment is made; the resistive droop coefficient is increased, the power distribution precision among the storage battery branches can be improved, but the voltage deviation of the direct current bus becomes large, the fuzzy logic processing is realized based on the principle, and the power distribution precision among the storage battery branches can be improved as much as possible while the voltage deviation of the direct current bus is not overlarge.
Further, the output signal r is utilized in the step (4)riFor resistance droop coefficient driAdjusting to obtain the adjusted resistive droop coefficient d'riThe calculation formula is as follows: d'ri＝dri-kr∫rridt; wherein k isrIs the integral gain of the corresponding fuzzy logic output.
Further, the air conditioner is provided with a fan,step (5) for output power PcjAbsolute value of (2), power rate of change
Absolute value of (d) and unbalanced power Δ PcjThe absolute value of (a) is subjected to fuzzy logic processing according to the following principle: rate of change of power
Lower, output power PcjSmaller and unbalanced power Δ PcjThe larger the value, the larger the capacitive droop coefficient dcj(ii) a Rate of change of power
The greater, the output power PcjGreater and unbalanced power Δ PcjThe smaller the value, the lower the capacitive droop coefficient dcj(ii) a Unbalanced power Δ PcjWhen the capacitance droop coefficient d is small and the power response among the super capacitor groups is consistentcjNo adjustment is made; the capacitive droop coefficient is increased, the influence caused by line impedance can be reduced, the power response speed of the capacitor branch circuit is improved, the fuzzy logic processing is realized on the basis of the principle, the influence of line resistance on the power distribution of the capacitor branch circuit can be reduced, and the power distribution precision and the response speed of the super capacitor branch circuit are improved.
Further, the output signal r is utilized in the step (6)cjFor the sag coefficient d of the capacitancecjAdjusting to obtain the adjusted capacitive droop coefficient d'cjThe calculation formula is as follows: d'cj＝dcj/s-kc∫rcjdt; where s is an integral operator, kcIs the integral gain of the corresponding fuzzy logic output.
Further, in the step (7), according to the resistive droop coefficient d'riAnd capacitive droop coefficient d'cjAnd the resistance and capacitance droop control on the direct current micro-grid comprises the droop control on a resistance branch and the droop control on a capacitance branch:
droop control for the resistive branch circuit includes: obtaining the ith strip of the storage battery packIncoming current i of the branchboiAnd the resistance droop coefficient d 'is matched with the metal strip'riMultiplying with the rated reference value V of the bus voltagerefMaking difference to obtain a voltage reference value V'rrefi(ii) a Obtaining the measured voltage v of the ith branch of the storage batteryobiAnd is compared with a voltage reference value V'rrefiMaking a difference to obtain a voltage V'rrefi(ii) a To voltage V'rrefiCarrying out proportional integral operation to obtain an inductive current reference value i of the converterbrefi(ii) a Obtaining the measured value i of the inductive current of the converterL1And then the reference value is compared with the inductance current reference value ibrefiAfter the difference is made, the proportional integral operation is carried out on the result obtained by the difference, and the operation result of the proportional integral operation is compared with the triangular carrier wave to obtain a complementary modulation signal s1And a modulated signal s2For controlling the switching on and off of the converter;
droop control for the capacitive branch includes: obtaining the introduced current i of the jth branch of the super capacitor bankcojAnd is subjected to the reaction with a capacitive droop coefficient d'cjMultiplying with the rated reference value V of the bus voltagerefMaking difference to obtain a voltage reference value V'crefj(ii) a Obtaining the measured voltage v of the jth branch of the super capacitor bankocjAnd is compared with a voltage reference value V'crefjMaking a difference to obtain a voltage V "crefj(ii) a To voltage V "crefjCarrying out proportional integral operation to obtain an inductive current reference value i of the convertercrefj(ii) a Obtaining the measured value i of the inductive current of the converterL2And then the reference value is compared with the inductance current reference value icrefjAfter the difference is made, the proportional integral operation is carried out on the result obtained by the difference, and the operation result of the proportional integral operation is compared with the triangular wave carrier wave to obtain a complementary modulation signal s3And a modulated signal s4For controlling the switching on and off of the converter.
Generally, by the above technical solution conceived by the present invention, the following beneficial effects can be obtained:
(1) according to the power distribution method of the direct-current micro-grid composite energy storage system, the storage battery pack and the super capacitor pack are distributed and controlled according to power frequency through resistance-capacitance droop control, current sharing control between the storage battery packs and current sharing control between the super capacitor packs are realized on the basis of a fuzzy logic algorithm, so that high-frequency components in unbalanced power of a direct-current micro-grid are distributed to capacitive energy storage on a capacitive branch, and low-frequency components are distributed to storage battery energy storage on a resistive branch, so that complementary running characteristics of different energy storage devices in the composite energy storage system are fully exerted, power distribution precision between the storage battery packs and between the super capacitor packs is improved, and response speed of the super capacitor is accelerated;
(2) the power distribution method of the direct-current micro-grid composite energy storage system provided by the invention realizes the current sharing control between the storage battery groups and the current sharing control between the super capacitor groups based on fuzzy logic processing, does not need to carry out detailed modeling on the system, can flexibly adapt to different operating conditions, simultaneously realizes the distributed control on the system by respectively controlling the resistive branch and the capacitive branch, and has good adaptability and expandability while fully exerting the complementary operating characteristics of different energy storage devices in the composite energy storage system.
Drawings
Fig. 1 is a schematic diagram of a low-voltage dc microgrid composed of two sets of storage batteries and two sets of super capacitors according to an embodiment of the present invention;
fig. 2 is an equivalent circuit diagram of the composite energy storage system according to the embodiment of the invention;
fig. 3 is a schematic diagram illustrating a power distribution method of the dc microgrid composite energy storage system according to an embodiment of the present invention;
fig. 4 is a membership function of fuzzy logic processing of a branch of a storage battery pack according to an embodiment of the present invention; (a) is a membership function of the output voltage; (b) is a function of the degree of membership of the unbalanced power; (c) is a membership function of the output signal;
FIG. 5 is a membership function of the branch fuzzy logic processing of the supercapacitor bank provided in the embodiment of the present invention; (a) is a membership function of the output power; (b) is a power change rate membership function; (c) is a function of the degree of membership of the unbalanced power; (d) is a membership function of the output signal;
FIG. 6 is a fuzzy logic reasoning result of the fuzzy logic process provided by the embodiment of the present invention; (a) fuzzy logic reasoning results for the storage battery branch; (b) fuzzy logic reasoning results are obtained for the branch of the super capacitor bank;
FIG. 7 is a schematic diagram of the power response of the resistive and capacitive droop control in an embodiment of the present invention when the system power varies; (a) responding for the storage battery power; (b) is the supercapacitor power response; (c) is a dc bus voltage; (d) power is in shortage for the system;
FIG. 8 is a diagram illustrating the effect of the control method according to the embodiment of the present invention when the system power is changed; (a) responding for the storage battery power; (b) is the supercapacitor power response; (c) power is in shortage for the system; (d) power is in shortage for the system;
FIG. 9 is a diagram illustrating the effect of the control method according to the embodiment of the present invention when the line impedance changes; (a) responding for the storage battery power; (b) is the supercapacitor power response; (c) is a dc bus voltage; (d) power is in shortage for the system;
FIG. 10 is a diagram illustrating the effect of the control method according to the present embodiment when a set of storage batteries and super capacitors are added; (a) responding for the storage battery power; (b) is the supercapacitor power response; (c) is a dc bus voltage; (d) power is in shortage for the system;
FIG. 11 is an effect diagram of the control method according to the present embodiment when photovoltaic output variation in an actual day is simulated; (a) responding for the storage battery power; (b) is the supercapacitor power response; (c) is a dc bus voltage; (d) photovoltaic random output is obtained;
FIG. 12 is a graph showing the change of droop coefficients of the control method of the present embodiment shown in FIGS. 8-11; (a) the change curve of the resistive droop coefficient is compared with the traditional droop control; (b) the change curve of the capacitive droop coefficient is compared with the traditional droop control; (c) the change curve of the resistive droop coefficient when the line impedance is changed; (d) the change curve of the capacitive droop coefficient when the line impedance is changed; (e) adding a group of change curves of the resistive droop coefficient after energy storage; (f) adding a group of change curves of the resistive droop coefficient after energy storage; (g) the change curve of the resistive droop coefficient during actual photovoltaic output is considered; (h) the change curve of the capacitive droop coefficient when the actual photovoltaic output is considered.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
Fig. 1 shows a 400V low-voltage DC microgrid system formed by two storage batteries and two super capacitors, where each energy storage unit is connected to a DC/DC converter and then connected to a DC bus via line impedance. Fig. 2 is an equivalent circuit diagram formed by a storage battery and a super capacitor in the system shown in fig. 1, the equivalent of line impedance is a virtual resistance, which may cause steady-state deviation of a dc bus and imbalance of power distribution among energy storages of the same kind, and in addition, the virtual resistance of a branch of the super capacitor may also affect its response speed, thereby affecting the performance of controlling the capacitance-resistance droop of power distribution according to frequency.
For the above system, the control objective of the power distribution method for the dc microgrid composite energy storage system provided in the embodiment of the present invention is to construct an adaptive droop controller by using a fuzzy logic algorithm on the basis of resistance-capacitance droop control under the premise of considering the line resistance of the low-voltage dc microgrid, so as to eliminate the influence of the line resistance, better realize the reasonable distribution of unbalanced power in the composite energy storage system, and simultaneously realize the good operation of the system under different working conditions, and have good adaptability and expandability.
The power distribution method of the direct current micro-grid composite energy storage system, as shown in fig. 3, includes the following steps:
(1) obtaining the maximum voltage deviation delta V allowed by the direct current busmaxAnd the maximum output current of each branch of the storage battery pack, and calculating the resistive droop coefficient of each branch of the storage battery pack; wherein, the maximum output current of the ith branch of the storage battery is IimaxThe resistive droop coefficient of the ith branch of the storage battery pack is dri(ii) a The value of i is 1 to Nr，NrThe total number of branches contained in the storage battery pack; resistive sag factor driSatisfies the following conditions: dri＝ΔVmax/Iimax；
(2) Obtaining the response time of each super capacitor and the droop controller adjustment time tsCalculating the capacitive droop coefficient of each branch of the super capacitor bank; wherein the response time of the super capacitor of the jth branch of the super capacitor bank is tcjThe capacitive droop coefficient of the jth branch of the super capacitor bank is dcj(ii) a j takes a value of 1 to Nc，NcThe total number of branches contained in the super capacitor bank;
droop controller adjust time tsThe adjustment time is the steady state (95%) of the system, so the droop controller adjusts the time tsSatisfies the following conditions: t is t s3 τ; wherein tau is an equivalent time constant of a system under droop control, and satisfies the following conditions: τ ═ dr/dc；
The energy storage response time should be less than the controller settling time, so the following relationship exists: t is ts≥tcj(ii) a And thus the capacitive droop coefficient dcjSatisfies the following conditions:
wherein d isr、dcThe equivalent resistance and the capacitive droop coefficient of the system respectively meet the following requirements:
(3) for the ith branch of the storage battery pack, the output voltage v of the converter is obtainedbiAnd unbalanced power Δ Pbi(ii) a For output voltage vbiAnd unbalanced power Δ PbiPerforming fuzzy logic processing to obtain output signal rriFor adjusting the resistive droop coefficient dri(ii) a Unbalanced power Δ PbiThe calculation formula of (2) is as follows: delta Pbi＝(Pbi-Pb(i+1))/2+(Pbi-Pb(i-1)) 2; wherein, Pb(i-1)、PbiAnd Pb(i+1)Respectively the i-1 th strip in the storage battery packMeasured values of power of the branch, the ith branch and the (i + 1) th branch;
(4) using the output signal rriFor resistance droop coefficient driAdjusting to obtain the adjusted resistive droop coefficient d'riTo realize the current sharing control among the storage battery packs; the calculation formula for adjusting the resistive droop coefficient is as follows: d'ri＝dri-kr∫rridt; wherein k isrIntegral gain for the corresponding fuzzy logic output;
(5) for the j branch of the super capacitor bank, obtaining the output power P of the convertercjPower rate of change of converterAnd unbalanced power Δ Pcj(ii) a To the output power PcjAbsolute value of (2), power rate of change
Absolute value of (d) and unbalanced power Δ PcjThe absolute value of the signal is processed by fuzzy logic to obtain an output signal rcjFor adjusting the capacitive sag factor dcj(ii) a Unbalanced power Δ PcjThe calculation formula of (2) is as follows: delta Pcj＝(Pcj-Pc(j+1))/2+(Pcj-Pc(j-1)) 2; wherein, Pc(j-1)、PcjAnd Pc(j+1)The measured values of the power of the j-1 th branch, the j-1 th branch and the j +1 th branch in the super capacitor bank are respectively measured;
(6) using the output signal rcjFor the sag coefficient d of the capacitancecjAdjusting to obtain the adjusted capacitive droop coefficient d'cjTo realize the current-sharing control among the super capacitor groups; the calculation formula for adjusting the capacitive droop coefficient is d'cj＝dcj/s-kc∫rcjdt; where s is an integral operator, kcIntegral gain for the corresponding fuzzy logic output;
(7) according to the resistive droop coefficient d'riAnd capacitive droop coefficient d'cjDirect current microgridCarrying out resistance-capacitance droop control to realize the distribution control of the storage battery pack and the super capacitor pack according to the power frequency; the method specifically comprises droop control on a resistive branch circuit and droop control on a capacitive branch circuit;
droop control for the resistive branch circuit includes: obtaining the lead-in current i of the ith branch of the storage battery packboiAnd the resistance droop coefficient d 'is matched with the metal strip'riMultiplying with the rated reference value V of the bus voltagerefMaking difference to obtain a voltage reference value V'rrefi(ii) a Obtaining the measured voltage v of the ith branch of the storage batteryobiAnd is compared with a voltage reference value V'rrefiMaking a difference to obtain a voltage V "rrefi(ii) a To voltage V "rrefiCarrying out proportional integral operation to obtain an inductive current reference value i of the converterbrefi(ii) a Obtaining the measured value i of the inductive current of the converterL1And then the reference value is compared with the inductance current reference value ibrefiAfter the difference is made, the proportional integral operation is carried out on the result obtained by the difference, and the operation result of the proportional integral operation is compared with the triangular carrier wave to obtain a complementary modulation signal s1And a modulated signal s2For controlling the switching on and off of the converter;
droop control for the capacitive branch includes: obtaining the introduced current i of the jth branch of the super capacitor bankcojAnd is subjected to the reaction with a capacitive droop coefficient d'cjMultiplying with the rated reference value V of the bus voltagerefMaking difference to obtain a voltage reference value V'crefj(ii) a Obtaining the measured voltage v of the jth branch of the super capacitor bankocjAnd is compared with a voltage reference value V'crefjMaking a difference to obtain a voltage V "crefj(ii) a To voltage V "crefjCarrying out proportional integral operation to obtain an inductive current reference value i of the convertercrefj(ii) a Obtaining the measured value i of the inductive current of the converterL2And then the reference value is compared with the inductance current reference value icrefjAfter the difference is made, the proportional integral operation is carried out on the result obtained by the difference, and the operation result of the proportional integral operation is compared with the triangular wave carrier wave to obtain a complementary modulation signal s3And a modulated signal s4For controlling the switching on and off of the converter.
In this embodiment, straightSetting the reference value of the current bus voltage as 400V and setting the voltage V of the controllerbiThe range is +/-105%, namely 380V-420V. The maximum power shortage of the system is designed to be 8kw, at least two storage batteries and two super capacitors exist, so that the input and output membership function of the storage battery branch fuzzy logic controller is determined as shown in fig. 4, and the input and output membership function of the super capacitor branch fuzzy logic controller is determined as shown in fig. 5. And meanwhile, calculating according to system parameters to obtain an initial resistive droop coefficient: dr1＝dr20.3, initial capacitive sag factor dc1＝dc2＝1。
For the output voltage v in the step (3)biAnd unbalanced power Δ PbiFuzzy logic processing is carried out according to the following principle: unbalanced power Δ PbiThe larger the forward direction or the output voltage vbiThe larger the resistance droop coefficient d is, the more the resistance droop coefficient d isri(ii) a Unbalanced power Δ PbiThe larger the negative direction or the output voltage vbiThe smaller the resistance droop coefficient d isri(ii) a Unbalanced power Δ PbiApproaching zero or output voltage vbiWhen the rated value is reached and the power distribution among the storage battery groups is consistent, the resistance droop coefficient d is adjustedriNo adjustment is made; the resistive droop coefficient is increased, the power distribution precision among the storage battery branches can be improved, but the voltage deviation of the direct current bus becomes large, the fuzzy logic processing is realized based on the principle, and the power distribution precision among the storage battery branches can be improved as much as possible while the voltage deviation of the direct current bus is not overlarge.
For the output voltage v in the step (3)biAnd unbalanced power Δ PbiFuzzy logic processing is carried out to obtain a coefficient d for adjusting the resistive droopriIs output signal rriThe method comprises the following steps:
(31) design output voltage vbiAs shown in FIG. 4(a), for an output voltage vbiWhich are subject to different sets of fuzzy logic { A (minimum), B (small), C (slightly small), D (medium), E (slightly large), F (large), G (maximum) }, with the following degrees of membership:
(32) design of unbalanced power Δ PbiAs shown in FIG. 4(b), for unbalanced power Δ PbiWhich are attached to different sets of fuzzy logic { NL (negative large), NB (negative medium), NS (negative small), Z (zero), PS (positive small), PB (positive medium), PL (positive large) }, with the following degrees of attachment:
(33) design output signal rriFor the output signal r, as in FIG. 4(c)riWhich are attached to different sets of fuzzy logic { NL (negative large), NB (negative medium), NS (negative small), Z (zero), PS (positive small), PB (positive medium), PL (positive large) }, with the following degrees of attachment:
(34) determining a fuzzy rule of the fuzzy logic processing according to the principle of the fuzzy logic processing, as shown in table 1:
TABLE 1
If the output voltage v isbiIs A, unbalanced power Δ PbiNL, then output signal rriIs PL; if the output voltage v isbiIs A, unbalanced power Δ PbiIs NB, then the signal r is outputriIs PL; if the output voltage v isbiIs A, unbalanced power Δ PbiIs NS, then the signal r is outputriIs PB; and so on.
In step (5), the output power P is adjustedcjAbsolute value of (2), power rate of change
Absolute value of (d) and unbalanced power Δ PcjThe absolute value of (a) is subjected to fuzzy logic processing according to the following principle: rate of change of powerLower, output power PcjSmaller and unbalanced power Δ PcjThe larger the value, the larger the capacitive droop coefficient dcj(ii) a Rate of change of power
The greater, the output power PcjGreater and unbalanced power Δ PcjThe smaller the value, the lower the capacitive droop coefficient dcj(ii) a Unbalanced power Δ PcjWhen the capacitance droop coefficient d is small and the power response among the super capacitor groups is consistentcjNo adjustment is made; the capacitive droop coefficient is increased, the influence caused by line impedance can be reduced, the power response speed of the capacitor branch circuit is improved, the fuzzy logic processing is realized on the basis of the principle, the influence of line resistance on the power distribution of the capacitor branch circuit can be reduced, and the power distribution precision and the response speed of the super capacitor branch circuit are improved.
In step (5), the output power P is adjustedcjAbsolute value of (2), power rate of change
Absolute value of (d) and unbalanced power Δ PcjThe absolute value of the droop coefficient d is processed by fuzzy logic to obtain the droop coefficient d for adjusting the capacitancecjIs output signal rcjThe method comprises the following steps:
(51) design output power PcjAs shown in FIG. 5(a), for an output power PcjWhich are attached to different sets of fuzzy logic { a (minimum), B (small), C (medium), D (large), E (maximum) }, with the degrees of attachment as follows:
(52) design power change rate
As shown in FIG. 5(b), for the rate of change of power
Which are attached to different sets of fuzzy logic { a (minimum), B (small), C (slightly small), D (medium), E (slightly large), F (large), G (maximum) }, with the following degrees of attachment:
(53) design output signal rcjAs shown in fig. 5(c), for the output signal rcjWhich are subject to different sets of fuzzy logic { A (minimum), B (small), C (slightly small), D (medium), E (slightly large), F (large), G (maximum) }, with the following degrees of membership:
(54) design output signal rcjAs shown in fig. 5(d), for the output signal rcjBelonging to different fuzzy logic sets { A (minimum), B (small), C (slightly small), D (moderate), E (slightly large), F (large), G (maximum) },the degree of membership is as follows:
(55) determining the fuzzy rule of the fuzzy logic processing according to the principle of the fuzzy logic processing, as shown in table 2:
TABLE 2
When the unbalanced power is Δ PcjIf it is S, then output power PcjIs A, power change rate
Is A, then the signal r is outputcjIs G; if the output power PcjIs A, power change rate
B, then output signal rcjIs G; and so on;
when the unbalanced power is Δ PcjWhen M is reached, if the output power P iscjIs A, power change rate
Is A, then the signal r is outputcjIs F; if the output power PcjIs A, power change rate
B, then output signal rcjIs F; and so on;
when the unbalanced power is Δ PcjIf it is L, if the power P is outputcjIs A, power change rate
Is A, then the signal r is outputcjIs E; if the output power PcjIs A, power change rate
B, then output signal rcjIs E; and so on.
The fuzzy logic reasoning results of the corresponding storage battery branch and the supercapacitor branch are obtained according to the fuzzy rules shown in tables 1 and 2 and are respectively shown in fig. 6(a) and 6 (b).
A simulation model shown in FIG. 1 is built in PSCAD/EMTDC to verify the performance of the control method of the embodiment.
Fig. 7 is a graph that verifies the characteristics of the rc droop control versus the power ripple divide distribution, ignoring the system line impedance. When t is 2s, the system is applied with 8kw load, and then cut off after 3 s. The resistive droop control of the battery branch circuit responds to low-frequency components of power, and fig. 7(a) is a battery power response curve, which slowly rises along with time and responds to all power in a steady state. The supercapacitor is subject to capacitive droop control in response to high frequency components of power, as shown in figure 7(b), Psc rises rapidly and decays gradually to zero over time. When t is 5s, the system cuts off the load, and the storage battery and the super capacitor still respond to the power frequency division. Simulation results prove that the composite energy storage system controlled by the resistance-capacitance droop can fully exert the technical advantages among different energy storages and realize power frequency division distribution. Because the line impedance is zero, equal power distribution is performed among the same energy storages, and response curves are completely overlapped.
Fig. 8 is a control performance of the control method of the present embodiment when the line resistance is present, which is verified by using the control method provided by the present embodiment. Wherein the line resistance value is taken as:
rr_l1＝0.1Ω；rr_l2＝0.03Ω；rr_l1＝0.03Ω；rr_l1＝0.12Ω
the system suddenly adds the load of P8 kw when t is 2s and 9s, cuts off after 3s, and does not add the control method of the embodiment in the process of the first sudden load. As can be seen from the first 9s of fig. 8 (a-b). At the same time, fig. 8(c) shows that the energy storage output voltage also rises to avoid the steady state voltage deviation exceeding the range caused by the too large droop coefficient.
Fig. 9 is a diagram for verifying the adaptability of the control method of the present embodiment to unknown line impedance under the control method provided by the present embodiment. Wherein the line resistance value is taken as:
rr_l1＝0.03Ω；rr_l2＝0.15Ω；rr_l1＝0.12Ω；rr_l1＝0.04Ω
the system suddenly adds the load of P8 kw when t is 2s and 9s, cuts off after 3s, and does not add the control method of the embodiment in the process of the first sudden load. As can be seen from fig. 9, the control method of the present embodiment still has a good control effect. After the starting control, the current equalizing precision of the storage battery branch is remarkably improved, the response speed of the super capacitor branch is increased, and the high-frequency power response is more balanced. The droop coefficients output by the controllers in the above two sets of simulations are adjusted as shown in fig. 12(a-b) (c-d). When the system generates sudden load, the controller can quickly respond and adjust the droop coefficient, and the response performance of the energy storage device to power is improved.
Fig. 10 is a flowchart for verifying the expansion performance of the control method of the present embodiment when the control method provided by the present embodiment is adopted. And a group of storage batteries and super capacitors are added in the system. The initial droop coefficient for the newly added stored energy is the same as for the other groups. The line resistance value is taken as:
rr_l1＝0.1Ω；rr_l2＝0.03Ω；rr_l1＝0.03Ω；rr_l1＝0.12Ω
the system suddenly adds the load of P8 kw when t is 2s and 9s, cuts off after 3s, and does not add the control method of the embodiment in the process of the first sudden load. It can be seen from fig. 10(a-b) that the existence of line resistance still has a significant effect on the power distribution accuracy and the power response speed when the conventional impedance-capacitive control strategy is adopted. After the control method is added, the power distribution precision of the storage battery and the super capacitor is obviously improved, the response speed of the super capacitor is improved, and the voltage drop is still within the range. Fig. 12(e-f) shows the adjustment of the droop coefficient in this example, which indicates that the droop coefficient of each energy storage can be quickly adjusted according to the power fluctuation after the proposed adaptive droop control is adopted, so as to improve the power distribution characteristics of different energy storages.
Fig. 11 is a flowchart for verifying the adaptability of the control method of the present embodiment to random power fluctuation of a distributed power source such as a fan or a photovoltaic power source in an actual system, under the control method provided by the present embodiment. Typical solar photovoltaic output data under actual conditions are selected for simulation verification, and the obtained simulation result is shown in fig. 11. Photovoltaic output fluctuations are shown in fig. 11 (d). As can be seen from the smoothness of the power response of the storage battery and the super capacitor in fig. 11(a-b), the resistance-capacitance droop control can still achieve a good power frequency division distribution effect, the storage battery mainly responds to the low frequency portion, and the super capacitor mainly responds to the high frequency portion. Meanwhile, under the control method of the embodiment, the power responses of the storage batteries are basically overlapped, the capacitive droop coefficient is rapidly adjusted after the photovoltaic output fluctuation is increased, and the power responses of the super capacitor are also highly overlapped. The variation of the resistance-capacitance droop coefficient is shown in FIG. 12 (g-h). The simulation result shows that the control method of the embodiment still has a good control effect on the power fluctuation in the actual system.
The traditional resistive droop control strategy can only realize the fixed proportion distribution of the unbalanced power of the direct current micro-grid according to the droop coefficient, and is difficult to realize the distribution of the unbalanced power according to the frequency characteristic. Through introducing the branch road that has the electric capacity characteristic at traditional droop control circuit, form resistance-capacitance droop control back, can realize the unbalanced power rational distribution of direct current microgrid. The high-frequency unbalanced power is distributed to the capacitive energy storage on the capacitive branch circuit, and the low-frequency unbalanced power is distributed to the storage battery energy storage on the resistive branch circuit. However, the resistance parameters of the lines in the direct-current microgrid have obvious influence on unbalanced power distribution, unbalanced distribution of unbalanced power among energy storage devices of the same type can be caused, the operating characteristics of the energy storage devices are influenced, and negative influence caused by the unbalanced power distribution needs to be eliminated.
Simulation results show that the control method can eliminate negative effects caused by line resistance, effectively and reasonably distribute unbalanced power in the direct-current micro-grid under various conditions, fully exert complementary operation characteristics of different energy storage devices in the composite energy storage system, and have good adaptability and expandability.
In general, the method overcomes the defects that the control of the resistive droop coefficient is difficult to realize the distribution of unbalanced power according to the frequency characteristic, and the virtual capacitor droop control is greatly influenced by the line impedance parameter, and meanwhile, the built fuzzy logic controller does not need to carry out detailed modeling on the system, can adjust the resistive droop coefficient and the capacitive droop coefficient according to the system state, and realizes the response of the composite energy storage system to the shortage power according to the energy storage unit characteristic.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (6)
1. A power distribution method for a direct current micro-grid composite energy storage system is characterized by comprising the following steps:
(1) obtaining the maximum voltage deviation delta V allowed by the direct current busmaxAnd the maximum output current of each branch of the storage battery pack, and calculating the resistive droop coefficient of each branch of the storage battery pack; wherein, the maximum output current of the ith branch of the storage battery is IimaxThe resistive droop coefficient of the ith branch of the storage battery pack is dri(ii) a The value of i is 1 to Nr，NrThe total number of branches contained in the storage battery pack;
(2) obtaining the response time of each super capacitor of the super capacitor group and the adjusting time t of the droop controllersCalculating the capacitive droop coefficient of each branch of the super capacitor bank; wherein the response time of the super capacitor of the jth branch of the super capacitor bank is tcjThe capacitive droop coefficient of the jth branch of the super capacitor bank is dcj(ii) a j takes a value of 1 to Nc，NcThe total number of branches contained in the super capacitor bank;
(3) for the ith branch of the storage battery pack, the output voltage v of the converter is obtainedbiAnd unbalanced power Δ Pbi(ii) a For the output voltage vbiAnd said unbalance power Δ PbiPerforming fuzzy logic processing to obtain output signal rriFor adjusting said resistive droop coefficient dri；
(4) Using said output signal rriFor the resistive droop coefficient driAdjusting to obtain the adjusted resistive droop coefficient d'riTo realize the current sharing control among the storage battery packs;
(5) for the j branch of the super capacitor bank, obtaining the output power P of the convertercjPower rate of change of converter
And unbalanced power Δ Pcj(ii) a For the output power PcjAbsolute value of, the power change rate
And the unbalance power Δ PcjThe absolute value of the signal is processed by fuzzy logic to obtain an output signal rcjFor adjusting said capacitive droop coefficient dcj；
(6) Using said output signal rcjFor the capacitive droop coefficient dcjAdjusting to obtain the adjusted capacitive droop coefficient d'cjTo realize the current-sharing control among the super capacitor groups;
(7) according to the resistive droop coefficient d'riAnd the capacitive droop coefficient d'cjAnd carrying out resistance-capacitance droop control on the direct current micro-grid so as to realize the distribution control between the storage battery pack and the super capacitor pack according to the power frequency.
2.The method for distributing power to the direct-current microgrid composite energy storage system of claim 1, characterized in that in the step (3), the output voltage v is subjected tobiAnd said unbalance power Δ PbiFuzzy logic processing is carried out according to the following principle: said unbalance power Δ PbiThe larger the forward direction or the output voltage vbiIncreasing the resistive droop coefficient d at larger incrementsri(ii) a Said unbalance power Δ PbiThe greater the negative direction or the output voltage vbiThe smaller the resistive droop coefficient d isri(ii) a Said unbalance power Δ PbiApproaching zero or the output voltage vbiWhen the rated value is reached and the power distribution among the storage batteries is consistent, the resistance droop coefficient d is adjustedriNo adjustment is made.
3. The method for distributing power to a direct-current microgrid composite energy storage system according to claim 1, characterized in that the output signal r is utilized in the step (4)riFor the resistive droop coefficient driAdjusting to obtain the adjusted resistive droop coefficient d'riThe calculation formula is as follows: d'ri＝dri-kr∫rridt; wherein k isrIs the integral gain of the corresponding fuzzy logic output.
4. The method for distributing power to the direct-current microgrid composite energy storage system of claim 1, characterized in that in the step (5), the output power P is subjected tocjAbsolute value of, the power change rate
And the unbalance power Δ PcjThe absolute value of (a) is subjected to fuzzy logic processing according to the following principle: the rate of change of powerThe lower the output power PcjThe smaller and the unbalanced power Δ PcjThe larger the sizeIncreasing the capacitive sag factor dcj(ii) a The rate of change of power
The greater the output power PcjThe greater and the unbalanced power Δ PcjThe smaller the capacitive droop coefficient d iscj(ii) a Said unbalance power Δ PcjSmall enough to make the power response among the super capacitor groups consistent, the capacitance droop coefficient dcjNo adjustment is made.
5. The power distribution method for the direct-current microgrid composite energy storage system of claim 1, characterized in that the output signal r is utilized in the step (6)cjFor the capacitive droop coefficient dcjAdjusting to obtain the adjusted capacitive droop coefficient d'cjThe calculation formula is as follows: d'cj＝dcj/s-kc∫rcjdt; where s is an integral operator, kcIs the integral gain of the corresponding fuzzy logic output.
6. The power distribution method of the direct-current microgrid composite energy storage system of claim 1, wherein in the step (7), the power distribution method is according to the resistive droop coefficient d'riAnd the capacitive droop coefficient d'cjAnd the resistance and capacitance droop control on the direct current micro-grid comprises the droop control on a resistance branch and the droop control on a capacitance branch:
droop control for the resistive branch circuit includes: obtaining the lead-in current i of the ith branch of the storage battery packboiAnd the resistance droop coefficient d 'is matched with the resistance droop coefficient'riMultiplying with the rated reference value V of the bus voltagerefMaking difference to obtain a voltage reference value V'rrefi(ii) a Obtaining the measured voltage v of the ith branch of the storage batteryobiAnd is compared with the voltage reference value V'rrefiMaking a difference to obtain a voltage Vrrefi(ii) a For the voltage V ″)rrefiCarrying out proportional integral operation to obtain an inductive current reference value i of the converterbrefi(ii) a Obtaining the measured value i of the inductive current of the converterL1And comparing it with the reference value i of the inductor currentbrefiAfter the difference is made, the proportional integral operation is carried out on the result obtained by the difference, and the operation result of the proportional integral operation is compared with the triangular carrier wave to obtain a complementary modulation signal s1And a modulated signal s2For controlling the switching on and off of the converter;
droop control for the capacitive branch includes: obtaining the introduced current i of the jth branch of the super capacitor bankcojAnd the capacitive droop coefficient d 'is matched with the capacitive droop coefficient'cjMultiplying with the rated reference value V of the bus voltagerefMaking difference to obtain a voltage reference value V'crefj(ii) a Obtaining the measured voltage v of the jth branch of the super capacitor bankocjAnd is compared with the voltage reference value V'crefjMaking a difference to obtain a voltage Vcrefj(ii) a For the voltage V ″)crefjCarrying out proportional integral operation to obtain an inductive current reference value i of the convertercrefj(ii) a Obtaining the measured value i of the inductive current of the converterL2And comparing it with the reference value i of the inductor currentcrefjAfter the difference is made, the proportional integral operation is carried out on the result obtained by the difference, and the operation result of the proportional integral operation is compared with the triangular wave carrier wave to obtain a complementary modulation signal s3And a modulated signal s4For controlling the switching on and off of the converter.
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