CN110556852B - Distributed energy storage system based on SOC dynamic balance submodule retrieval and control method - Google Patents
Distributed energy storage system based on SOC dynamic balance submodule retrieval and control methodDownload PDFInfo
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- CN110556852B CN110556852BCN201910930669.3ACN201910930669ACN110556852BCN 110556852 BCN110556852 BCN 110556852BCN 201910930669 ACN201910930669 ACN 201910930669ACN 110556852 BCN110556852 BCN 110556852B
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for AC mains or AC distribution networks
- H02J3/28—Arrangements for balancing of the load in a network by storage of energy
- H02J3/32—Arrangements for balancing of the load in a network by storage of energy using batteries with converting means
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for AC mains or AC distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/381—Dispersed generators
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of AC power input into DC power output; Conversion of DC power input into AC power output
- H02M7/42—Conversion of DC power input into AC power output without possibility of reversal
- H02M7/44—Conversion of DC power input into AC power output without possibility of reversal by static converters
- H02M7/48—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/483—Converters with outputs that each can have more than two voltages levels
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of AC power input into DC power output; Conversion of DC power input into AC power output
- H02M7/42—Conversion of DC power input into AC power output without possibility of reversal
- H02M7/44—Conversion of DC power input into AC power output without possibility of reversal by static converters
- H02M7/48—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/53—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/537—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
- H02M7/5387—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration
- H02M7/53871—Conversion of DC power input into AC power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current
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- Charge And Discharge Circuits For Batteries Or The Like (AREA)
- Dc-Dc Converters (AREA)
Abstract
Translated fromChinese
Description
Technical Field
The invention relates to an energy storage system and a control method thereof, in particular to a comprehensive control system for performing SOC dynamic balance control on the energy storage system which realizes distributed energy storage through a modular multilevel converter.
Background
A distributed energy storage system based on a Modular Multilevel Converter (MMC) is a novel energy storage system which can be used for large-scale energy storage, and due to the inherent characteristics of the structure, the energy storage system can greatly increase the energy storage scale and greatly enhance the risk resistance of the energy storage system. And due to the characteristics of multiple modules, the distributed energy storage system has quite flexible controllability, so that the control system almost plays a decisive role in the aspects of performance, functions and the like. In recent years, as large computers and various novel control methods are developed rapidly, distributed energy storage systems are receiving more and more attention. At present, a distributed energy storage system is gradually applied to various fields of peak clipping and valley filling, power supply reliability and electric energy quality improvement, frequency modulation, network tide distribution optimization and the like.
The distributed energy storage system is different from a common centralized energy storage system, and the biggest difference is that a converter used by the distributed energy storage system is a multi-level converter which can be infinitely expanded. The modular multilevel converter has very high control freedom, which also leads to a more complex control method compared with the common converter, and as the number of modules increases, the control difficulty increases. Due to the development of distributed energy storage technology in recent years, various new control methods are developed, and the control theory system is also continuously perfected. Compared with the conventional energy storage system control, the control of the distributed energy storage system not only realizes the control of closed loop and energy flow, but also needs to ensure the balance among all the sub-modules. In order to ensure that the distributed energy storage system can be efficient and stable, the SOC balance of the energy storage sub-modules is particularly important. On the basis, the bridge arm circulation is controlled, so that the system loss is reduced, the distortion rate is reduced, and the high-quality work of the system is ensured. For the modular multilevel converter, a modulation method, a closed-loop control method, a circulation control method and an SOC balance strategy of a sub-module energy storage unit are described in documents respectively, the methods are numerous but are generally only controlled aiming at a single aspect, and a practical method for controlling the whole energy storage system is still lacked.
When the number of the sub-modules of the distributed energy storage system is large, the modulation of the modular multi-level converter by using the nearest level approximation method is reasonable, and the characteristics that the more the number of the modules is, the more the waveform is ideal are provided, and compared with other modulation methods, the modulation method is easier to understand and is convenient to implement. On the basis of the modulation method of the converter, system closed loop, energy storage unit charging and discharging control, submodule balance control and bridge arm circulation control are realized, and all parts are matched with each other to realize system coordination control.
Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to design a distributed large-scale energy storage system based on SOC dynamic equalization submodule composite retrieval and provides a control method thereof to realize large-scale distributed energy storage control. The control system can realize the closed-loop stable operation of the converter, the control of the charging and discharging of the energy storage system, the SOC balance of the energy storage sub-modules and the loop current suppression of the bridge arms, and can enable the control of each part to be coordinated.
The distributed energy storage system consists of an energy storage control system and an energy storage system main circuit, the energy storage units are distributed in each submodule of the MMC, the front end of the submodule is a half-bridge type MMC unit, the rear end of the submodule is connected with the energy storage units through a bidirectional DC/DC converter, a submodule capacitor is connected in parallel between the front end and the rear end, the MMC is provided with three-phase bridge arms, each phase of the bridge arms is provided with 2N submodules, wherein, the sub-modules on the same bridge arm are connected in series through a half-bridge MMC unit, a bridge arm inductor is connected between the upper bridge arm and the lower bridge arm, and the output signal of the MMC sub-module composite retrieval system is transmitted to the grid electrodes of all IGBTs of an MMC bridge arm. The energy storage control system transmits direct-current side bridge arm voltage and alternating-current side three-phase current data collected in an energy storage system main circuit to a voltage and current closed-loop control system, a voltage reference signal obtained by superposing a signal output by the voltage and current closed-loop control system and a correction signal generated by a bridge arm circulating current control system is transmitted to an MMC sub-module composite retrieval system, a signal output by the MMC sub-module composite retrieval system is used as a switching tube trigger pulse to directly control the on-off of an IGBT in a half-bridge type MMC unit in each sub-module of an MMC, and a DC/DC converter modulation signal generation module directly generates a trigger signal to control a bidirectional DC/DC converter in the sub-module according to preset parameters.
The energy storage control system comprises a DC/DC converter modulation signal generation module, an MMC sub-module composite retrieval system, a voltage and current closed-loop control system and a bridge arm circulation control system, wherein the MMC sub-module composite retrieval system comprises a nearest level approximation step wave generator and an SOC dynamic balance autonomous optimization combined controller, the nearest level approximation step wave generator generates step waves according to voltage reference waves and determines the number of sub-modules to be put into the bridge arm at each moment according to the relative height of the current steps of the step waves, and the sub-module retrieval and input selection control system retrieves and selects the sub-modules to be put into the bridge arm at the moment according to the number of the sub-modules to be put into the bridge arm. In order to realize the control of the distributed energy storage system, firstly, the converter in the energy storage system is modulated in a matching way, when the DC/DC converter is controlled to work in a Buck mode, the MMC is in an inversion state, energy flows to the energy storage system from a power grid, and an energy storage battery is charged; when the DC/DC converter is controlled to work in a Boost mode, the MMC is in a rectification state, energy flows to a power grid from the energy storage system, and the energy storage battery discharges. The coordinated modulation of the converters is the fundamental stone of the control of the system, the output signals of which are applied directly to the individual switching tubes in each submodule. Due to the inherent characteristics of the MMC multi-module, a sub-module comprehensive selection control system which is formed by sub-module capacitor voltage sequencing and energy storage unit SOC sequencing is integrated in a modulation system of the MMC multi-module. The upper level of the system receives the step wave which is output by a nearest level approximation step wave generator and approximates to the reference wave, and respectively generates control signals of the input quantity of upper and lower bridge arm sub-modules at each moment; and the lower stage outputs trigger pulses of each switching tube in the MMC. The converter modulation system belongs to the keystone part of the control system and directly acts on each switch tube of the converter; above that is a voltage-current dual-loop control system. The system performs double-loop control by taking three-phase current at the alternating current side and direct-current bus voltage as feedback quantities, so that the bus voltage reaches a stable state and keeps constant. The closed-loop control system provides a stable control environment for a low-level system, and ensures the stability and reliability of the whole system. The circulation control system operates in parallel with the rest of the control system. The closed-loop control is carried out on alternating current frequency doubling components in the bridge arms, and the reference value of the bridge arm circulation is set to be 0 so as to inhibit the bridge arm circulation. The output bridge arm circulation control signal is used as a correction signal and is directly superposed into the three-phase reference voltage of the converter modulation system, so that the functions of optimizing the working efficiency of the energy storage system and reducing energy loss can be achieved. The specific contents of each module in the control system are as follows.
The bidirectional DC/DC converter in the submodule is a parallel-connection laminated bidirectional DC/DC converter, the structure of the bidirectional DC/DC converter consists of four SIC MOSFETs, four anti-parallel diodes and two inductors, the four SIC MOSFETs are connected in parallel in pairs to form two bridge arms, each SIC MOSFET is connected with a diode in parallel in a reverse direction, a line taking the energy storage unit as a starting end is divided into two paths, each path is connected with an inductor and then is respectively connected to the two bridge arms; the circuit structure forms a two-layer overlapped parallel bidirectional DC/DC converter, the current flowing out of the energy storage unit is divided into two paths, the current flowing through each SIC MOSFET is only half of the current flowing through a normal bidirectional DC/DC converter, and the ripple wave flowing through the energy storage battery can be reduced. The bidirectional DC/DC converter modulation signal generation module has two working modes: when the energy storage unit of the sub-module needs to be charged, the modulation signal generation module of the DC/DC converter controls the DC/DC converter to work in a Buck mode; when the energy storage unit of the sub-module needs to discharge, the modulation signal generation module of the DC/DC converter controls the DC/DC converter to work in a Boost mode, the two modes both have preset duty ratios, and the system needs to be manually switched during working.
The voltage and current closed-loop control system adopts a closed-loop control method of a voltage outer loop and a current inner loop, the voltage outer loop controls the voltage of a direct-current side bridge arm to keep constant, and the current inner loop controls the output or absorption power of the energy storage system. This voltage current control system passes through PI regulation, makes the input variable track the given quantity all the time, after given direct current voltage value, can guarantee that the total voltage of system direct current side is invariable, creates stable condition for MMC work.
And an input signal of the MMC sub-module composite retrieval optimized combination control system is a three-phase alternating current reference voltage. A three-phase alternating voltage reference wave obtained by superposing a signal output by the voltage and current closed-loop control system and a correction signal output by the bridge arm circulating current control system is used as an input signal of a nearest level approximation stepped wave generator, the nearest level approximation stepped wave generator takes the relative height of zero level of the stepped wave as 0, if 2N sub-modules are arranged in one bridge arm, the relative height of the maximum amplitude of the stepped wave corresponding to the upper bridge arm is taken as N, the relative heights of other steps are from 0, 1, 2, 3 to N, the number of the submodules to be input at a certain moment is determined by the relative height of the corresponding step wave at the moment and is used as the basis for selecting input after the submodules are sequenced, and the duration of a single step of the step wave is used as a refreshing period of the sequencing data of the submodules and used as a bridge arm containing 2N submodules, and the sequencing refreshing time is the period of the voltage reference wave divided by 2N.
The system adopts secondary composite retrieval, namely primary retrieval based on the capacitance and voltage of the sub-modules and secondary retrieval based on the SOC of the energy storage unit, and selects the sub-modules to be put into the same bridge arm at each moment through secondary retrieval to achieve dynamic SOC balance of the energy storage unit.
The control method of the distributed energy storage system based on SOC dynamic balance submodule retrieval comprises the following steps:
firstly, direct current side bridge arm voltage and alternating current side three-phase current data collected in a main circuit of a distributed energy storage system are transmitted to a voltage and current closed-loop control system, a voltage reference signal obtained by superposing a signal output by the voltage and current closed-loop control system and a correction signal generated by a bridge arm loop current control system is transmitted to a nearest level approximation stepped wave generator in an MMC sub-module composite retrieval system, and the nearest level approximation stepped wave generator generates stepped waves by approximating voltage reference waves;
secondly, after the SOC dynamic balance autonomous optimization combined controller receives the step wave signals generated in the first step, firstly, the working state of the system at the moment is judged, if the energy storage system is in a charging state, firstly, capacitor voltage values of bridge arm sub-modules measured at the moment are arranged in an ascending order, the sorted capacitor voltage values are corresponding to the bridge arm sub-modules within a certain error range through fuzzy comparison, a corresponding number of capacitor voltage values are selected in sequence according to the number of sub-modules to be put in at the moment, the sub-modules are retrieved according to the selected voltage values, all sub-modules retrieved within the certain error range are used as preferred sub-modules, and the number of the preferred sub-modules is larger than or equal to the number of the sub-modules to be put in by the bridge arm at the moment; secondly, performing ascending arrangement on the SOC values of the sub-module energy storage units of the optimal sub-modules obtained at the moment, selecting SOC values in corresponding quantity arrangement according to the number of sub-modules to be put into the sub-modules at the moment in a sequence mode, retrieving the sub-modules according to the selected SOC values within a certain error range, and after secondary retrieval, selecting the sub-modules with corresponding quantity from the sub-modules obtained by the secondary retrieval as the sub-modules to be finally selected to be put into through a quantity control module if the quantity of the sub-modules obtained by the retrieval is still larger than the number of the sub-modules to be put into the sub-modules at the moment, wherein the selection here does not set conditions; when the energy storage system is in a discharging state at a certain time, the sorting mode of the capacitor voltage is changed into descending order;
and thirdly, determining the input condition of the sub-module at the moment in the second step, outputting 2N paths of trigger signals by the phase bridge arm MMC sub-module composite retrieval system, wherein the trigger signals corresponding to the sub-modules determined to be in the input state are high level, the trigger signals corresponding to the sub-modules determined to be not in the input state are low level, each path of trigger signals are respectively transmitted to the trigger ends of two IGBTs of the half-bridge MMC in each corresponding sub-module, the upper pipe receives a forward signal, and the lower pipe receives a reverse signal.
Further, a correction signal obtained by multiplying the control signal output by the bridge arm circulating current control system by the weight is superposed on the control signal output by the voltage and current closed-loop control system, and a three-phase alternating voltage reference wave is obtained after superposition.
Further, the relative height of zero level of the step wave is taken as 0 by the nearest level approaching step wave generator, if 2N submodules are totally arranged in one bridge arm, the relative height of the maximum amplitude of the step wave corresponding to the upper bridge arm is taken as N, the relative heights of the rest steps are from 0, 1, 2 and 3 to N, the number of submodules to be input at a certain moment is determined by the relative height of the step wave corresponding to the certain moment, the submodules are used as the basis for selecting input after the submodules are sorted, the duration of a single step of the step wave is used as the refreshing period of the submodule sorting data, the bridge arm containing 2N submodules is used, and the sorting refreshing time is required to divide the voltage reference wave period by 2N.
And further, a fuzzy interval is set when the sub-modules are searched, the sub-modules which correspond to the capacitor voltage value or the energy storage unit SOC value within the range of the number of the sub-modules to be put into are used as the preferred sub-modules if the numerical difference is judged to be corresponding to the fuzzy interval, and the error range during the first search is larger than the error range during the second search.
Further, the MMC sub-module composite retrieval system mainly works in one of the following two states:
state (1): when the difference value of the capacitance and voltage of each submodule in the MMC bridge arm is large, the number of the optimized submodules selected by one-time retrieval is equal to the number of submodules to be put in at the moment, and the secondary retrieval is invalid, wherein the MMC submodule composite retrieval system mainly plays a role in balancing the capacitance and voltage values of the submodules in the MMC bridge arm under the condition;
state (2): when the difference value of the capacitance and voltage of each submodule in the MMC bridge arm is small, the number of the optimal submodules selected by one-time retrieval is close to the total number of the submodules in the bridge arm, and the secondary retrieval is invalid, wherein the comprehensive selection control system of the submodules mainly acts to adjust the SOC balance of the energy storage units of the submodules in the bridge arm;
when the difference value of the capacitance and voltage of each submodule in the MMC bridge arm is large, the number of the optimized submodules selected by one-time retrieval is equal to the number of submodules to be put in at the moment, and the secondary retrieval is invalid, wherein the MMC submodule composite retrieval system mainly plays a role in balancing the capacitance and voltage values of the submodules in the MMC bridge arm under the condition; when the difference value of the capacitance and voltage of each submodule in the MMC bridge arm is small, the number of the optimal submodules selected by one-time retrieval is close to the total number of the submodules in the bridge arm, and the secondary retrieval is invalid, the comprehensive selection control system of the submodules mainly plays a role in adjusting the SOC balance of the energy storage units of the submodules in the bridge arm under the condition that the secondary retrieval is invalid. The MMC sub-module composite retrieval system has a transition process between two working states, in the state (1), due to the effect of one-time retrieval, the capacitance voltage of each sub-module in a bridge arm gradually tends to be consistent after the system is started, the number of the selected optimal sub-modules in one-time retrieval is gradually larger than the number of the sub-modules which should be put into the system at present, the effect of the system on balancing the capacitance voltage value of the sub-modules in the bridge arm is gradually reduced, the effect of adjusting the SOC balance of the energy storage unit of the sub-modules in the bridge arm is gradually increased, and finally the system is transited to the state (2), and when the system is started and runs for a period of time; due to the self property of the circuit, the sub-module capacitor voltage tends to be consistent fast, the energy storage unit SOC is balanced slowly, and therefore the system is in a transient state when the sub-module capacitor voltage does not reach the consistency, primary retrieval plays a leading role, the system is in a steady state after the sub-module capacitor voltage reaches the consistency, and secondary retrieval plays a leading role. The output signal of the MMC sub-module composite retrieval system is composed of a series of narrow pulses, if 2N sub-modules exist in a bridge arm, the phase bridge arm MMC sub-module composite retrieval system outputs 2N paths of pulse signals, wherein the pulse signals of the N paths of an upper bridge arm and the N paths of a lower bridge arm are input to an enabling end of a switch tube in the bridge arm sub-module and directly serve as trigger pulses of the MMC switch tube. The switching tube of the sub-module selected by the MMC sub-module composite retrieval system is triggered to work in an access state, and the sub-module which is in the access state as well as other sub-modules are connected in series in a bridge arm; the sub-modules that are not selected will be in the cut-out state. The sub-modules comprehensively select the control system to control so that the sub-modules with low capacitance voltage and small energy storage unit SOC value are preferentially connected into the bridge arm when the energy storage system is charged; when the energy storage system discharges, the sub-modules with high capacitance voltage and large energy storage unit SOC value are connected into the bridge arm preferentially.
Each period of the step wave generated by the latest level approaching step wave generator has a certain time length, the amplitude of the step wave in the time length is kept unchanged, and the number of submodules to be input is also unchanged, so that the submodule retrieval input selection control system carries out retrieval in a certain period, and the SOC dynamic balance autonomous optimization combined controller works according to the following steps:
step (1): detecting the rising edge or the falling edge of an input step wave, and adding a delay with the time length of ts after the edge is triggered, wherein the delay time length is less than one fourth of the step time length of a single step wave;
step (2): after delaying, sequencing the detected sub-module capacitor voltage values;
and (3): selecting a corresponding number of capacitor voltage values according to the number of submodules to be put into corresponding to the time step in a sequencing order;
and (4): carrying out primary retrieval on the submodules according to the set error range and the selected capacitor voltage value, and selecting a preferred submodule;
and (5): sorting the energy storage unit SOC of the selected optimal sub-module in the step (4), and selecting SOC values of corresponding quantity according to the number of the sub-modules input at the moment;
and (6): carrying out secondary retrieval on the selected optimal sub-module in the step (4) according to the set error range and the selected SOC value of the energy storage unit;
and (7): and controlling the quantity of the submodules selected by the secondary retrieval, and if the quantity of the selected submodules is still larger than the quantity of the submodules to be put in at the moment, unconditionally selecting the submodules with the quantity to be put in.
And (8): putting the finally selected sub-module after two times of retrieval into a bridge arm;
it should be noted that the sorting in the above steps is in an ascending order when the system is in a charging state, and in a descending order when the system is in a discharging state.
And the control system performs coordinated control on the sub-modules of the upper bridge arm and the lower bridge arm in the same bridge arm, so that the input quantity of the sub-modules in the same bridge arm is unchanged at each moment and is equal to half of the total number of the sub-modules in one bridge arm. The input quantity of the upper and lower bridge arm sub-modules at each moment is consistent with the requirement that the recent level approaches to modulation, and the sub-modules are input at each moment and are selected by the sub-module comprehensive selection control system, so that normal modulation of the converter is met, and meanwhile, sub-module capacitance voltage balance and energy storage unit SOC balance are realized.
The method has the advantages that the system realizes the coordination control of the distributed energy storage system, compared with other control methods, the nearest level approximation modulation method used by the system is well adapted to the characteristic of large quantity of sub-modules of the distributed energy storage system, and meanwhile, the principle is simple and has strong practicability; in addition, the recent level approximation modulation method can be well adapted to the designed MMC sub-module composite retrieval system, step waves generated by recent level approximation modulation are input into the MMC sub-module composite retrieval system as input signals to serve as sub-module input quantity control signals, the sub-module input quantity serves as a basis, and the sub-module is subjected to composite retrieval according to the sub-module capacitance voltage and the sub-module energy storage unit SOC value, so that the sub-module can be input or cut off in real time according to an optimal scheme, the sub-module voltage dynamic balance and the energy storage unit SOC dynamic balance are realized by a simple and feasible method, a complex control algorithm is avoided, and the system is more convenient to understand, realize and maintain.
Drawings
Fig. 1 is a connection diagram of a main circuit and a control circuit of a three-bridge-arm n +1 level MMC distributed modular energy storage system according to an embodiment of the present invention;
fig. 2 is a connection diagram of an internal circuit of an energy storage submodule of a distributed energy storage system and a control circuit thereof according to an embodiment of the present invention;
FIG. 3 is a logic flow diagram of an energy storage control method according to an embodiment of the present invention;
FIG. 4 and FIG. 5 are respectively a reference voltage amplitude U according to an embodiment of thepresent invention ref1 and UrefWhen the voltage is equal to 0.6, the upper bridge arm of the closest level approaching step wave generator approaches the reference voltage modulates the step wave;
FIG. 6 is a diagram of DC side voltage waveforms of the energy storage system in accordance with an embodiment of the present invention;
FIG. 7 is a waveform of an output power of an energy storage system according to an embodiment of the invention;
FIG. 8 is an SOC curve of the energy storage unit of the upper bridge arm No. 1 sub-module of the phase bridge arm in the embodiment of the invention;
FIG. 9 is a diagram of waveforms of AC side voltages when switching the bridge arm modes of phase a according to the embodiment of the present invention;
FIG. 10 is a schematic block diagram of voltage current control according to an embodiment of the present invention;
FIG. 11 is a schematic block diagram of bridge arm loop current control in accordance with an embodiment of the present invention;
fig. 12 is a graph of output voltage waveforms of the energy storage unit when a common bidirectional DC/DC converter is used in the upper arm No. 1 sub-module of the phase arm according to the embodiment of the present invention;
fig. 13 is a graph of waveforms of output voltages of the energy storage unit when the parallel stacked bidirectional DC/DC converter is used in the upper arm No. 1 sub-module of the phase arm according to the embodiment of the present invention.
Detailed Description
The present invention is described in detail below with reference to specific embodiments, which will assist those skilled in the art in further understanding the present invention, and the accompanying drawings, but the present invention is not limited thereto in any way.
The invention relates to a distributed large-scale energy storage system based on SOC dynamic balance submodule composite retrieval and a control method thereof, and as shown in a figure 1, the invention is a connection diagram of a main circuit and a control circuit of a three-bridge-arm N +1 level MMC distributed modular energy storage system, wherein the main circuit is divided into three-phase bridge arms, each phase of the bridge arm is divided into an upper bridge arm and a lower bridge arm, the upper bridge arm and the lower bridge arm are respectively provided with N submodules, the upper bridge arm and the lower bridge arm are connected through bridge arm inductors, and the three-phase bridge arms can be connected with a power grid through; the control system mainly comprises a voltage and current control loop and an MMC submodule composite retrieval optimization combination control system, wherein the voltage and current control loop comprises voltage and current double closed loop control and bridge arm circulating current correction. A three-phase alternating current reference voltage signal generated by the voltage and current control loop is output to a nearest level approximation step wave generator in the MMC sub-module composite retrieval system, and the output step wave is transmitted to the sub-module retrieval input selection control system; the submodule retrieves the control signal outputted by the input selection control system and directly connects to each submodule of the three bridge arms as the trigger signal of the MMC switch tube. Fig. 2 is a connection diagram of an internal circuit and a control circuit of an energy storage submodule of a distributed energy storage system, where the submodule is a half-bridge MMC unit at the front end and is connected with the energy storage unit through a parallel stacked bidirectional DC/DC converter at the rear end. Because the half-bridge MMC unit is composed of two IGBTs, the DC/DC converter is composed of four SIC MOSFETs, four anti-parallel diodes and two inductors, the four SIC MOSFETs are connected in parallel in pairs to form two bridge arms, each SIC MOSFET is connected with one diode in parallel in a reverse direction, a line taking the energy storage unit as a starting end is divided into two paths, each path is connected with one inductor and then is respectively connected to the two bridge arms; the circuit structure forms a two-layer stacked parallel bidirectional DC/DC converter, the current flowing out of the energy storage unit is divided into two paths, the current flowing through each SIC MOSFET is only half of that of a normal bidirectional DC/DC converter, and the ripple waves flowing through the energy storage battery are reduced. A sub-module is provided with six switching tubes in total; between the front and back ends of the sub-module are parallel sub-module capacitors.
Fig. 3 is a logic flow diagram of an energy storage control method according to an embodiment, and the energy storage control system is known from the diagram, in which DC-side arm voltage and ac-side three-phase current data collected in a main circuit of a distributed energy storage system are transmitted to a voltage-current closed-loop control system, a voltage reference signal obtained by superimposing a signal output by the voltage-current closed-loop control system and a correction signal generated by a bridge arm loop current control system is transmitted to an MMC sub-module composite retrieval system, the signal output by the MMC sub-module composite retrieval system is used as a switching tube trigger pulse to directly control on/off of switching tubes of each sub-module of an MMC, and a DC/DC converter modulation signal generation module directly generates a trigger signal according to preset parameters to control a parallel stacked bidirectional DC/DC converter in the sub-module.
Based on the scheme provided in the summary of the invention, a three-phase 9-level (N ═ 8) embodiment is given below for explanation:
MMC modulation method
The nine-level MMC may output nine different levels at most. One phase of the bridge has 16 sub-modules, namely eight sub-modules of an upper bridge arm and eight sub-modules of a lower bridge arm. Eight submodules in one phase bridge arm are in the input state at each moment, and different levels can be output according to the difference of the number of the input submodules of the upper bridge arm and the lower bridge arm. The given value of the capacitor voltage in the MMC sub-module is set to
Because the virtual neutral point at the direct current side can be arranged on the voltage bisection point of the direct current bus voltage when the output is three-phase symmetrical sinusoidal voltage, the potential of the upper bridge arm public bus is Udc/2, the potential of the common bus of the lower bridge arm is-U dc2; the relationship between the input quantity and the output voltage of the submodules of the upper and lower bridge arms is as follows:
TABLE 1 output voltage and submodule input condition relation table
(a) when the reference wave is between-1 and-0.875, the output level is "-4";
(b) when the reference wave is between-0.875 and-0.625, the output level is "-3";
(c) when the reference wave is between-0.625 and-0.375, the output level is "-2";
(d) when the reference wave is between-0.375 and-0.125, the output level is "-1";
(e) when the reference wave is between-0.125 and 0.125, the output level is 0;
(f) when the reference wave is in [ 0.125-0.375 ], the output level is 1;
(g) when the reference wave is in [ 0.375-0.625 ], the output level is '2';
(h) when the reference wave is in the range of 0.625-0.875 ], the output level is 3;
(i) when the reference wave is in [ 0.875-1 ], the output level is "4".
The level number output by the MMC is determined by the number of sub-modules input by an upper bridge arm and a lower bridge arm; on the premise that selective input of the sub-modules is not considered, in a certain phase arm, the serial numbers of the eight sub-modules of the upper arm are respectively ' 1, 2, 3, 4, 5, 6, 7 and 8 ', the serial numbers of the eight sub-modules of the lower arm are respectively ' 9, 10, 11, 12, 13, 14, 15 and 16 ', 1 ' represents input, and ' 0 ' represents cut-off. The following drive signal truth table for the submodule can thus be obtained:
TABLE 2 truth table of driving signals for nearest level approximation modulation submodule
According to the relation, the input and the cut-off of each submodule can be controlled, and therefore the target waveform is output.
FIG. 4 shows the reference wave voltage amplitude UrefThe upper arm when 1 modulates the step wave. Because the amplitude of the reference wave reaches the maximum at the moment, the voltage value of the fitted step wave is 0-8, and eight steps are total. FIG. 5 shows the reference wave voltage amplitude UrefThe upper arm when 0.6 is obtained modulates the step wave. At this time, due to the decrease of the reference wave voltage amplitude, it can be seen that the MMC only outputs five levels of "-2", "-1", "0", "1", "2". And (3) outputting trigger pulses on the basis of step waves, wherein each rising edge and each falling edge of the step waves are the trigger time of the switching tube of the submodule, and the number of the added submodules changes once on each rising edge or each falling edge. When the voltage value of the step wave of the upper arm rises from 2 to 3, the upper arm changes from putting two submodules into three, the lower arm changes from putting six submodules into five, and the output level also changes from "2" to "1". And then, the alternating-current side voltage obtains a sine waveform after passing through a bridge arm filter reactance.
Control strategy of DC/DC converter and principle of controlling charging and discharging by changing working state of DC/DC converter
Because the MMC converter can operate in four quadrants, the charging and discharging control of the energy storage system can be effectively realized by reasonably matching the MMC converter with the DC/DC converter. As shown in fig. 2, four switching tubes of the DC/DC converter are S3, S4, S5 and S6, when the voltage of the MMC DC bus is controlled to be constant, the capacitance voltage of the submodules is also substantially constant because the number of the submodules put into one bridge arm at each moment in the normally operating MMC is constant. At this time, if the energy storage battery system needs to be charged, the DC/DC converter is operated in the Buck mode, the switching tubes S3 and S5 are simultaneously subjected to Buck chopper modulation, and the switching tubes S4 and S6 are always turned off. After the capacitor voltage is reduced through the Buck circuit by adjusting the duty ratio of the PWM wave, the voltage value is higher than the rated voltage output by the energy storage battery pack, the sub-module capacitor discharges to the energy storage battery pack, and the energy storage battery pack is in a charging state. At the moment, the sub-module capacitor starts to discharge from the reference voltage value, and the capacitor voltage is in a descending trend; in order to ensure that the voltage of the direct current bus is constant, the closed-loop control system enables the MMC to work in a rectification state, and the power grid outputs energy to the MMC. If the energy storage battery system is required to discharge to the power grid, the DC/DC converter is enabled to work in a Boost mode, the switching tubes S4 and S6 are subjected to Boost chopper modulation at the same time, and the switching tubes S3 and S5 are always turned off. After the output voltage of the energy storage battery pack is boosted through the Boost circuit by adjusting the duty ratio of the PWM wave, the voltage value of the output voltage is higher than the reference voltage of the sub-module capacitor, the energy storage battery pack discharges to the sub-module capacitor, and the energy storage battery pack is in a discharging state. At the moment, the sub-module capacitor starts to be charged from the reference voltage value, and the capacitor voltage is in a rising trend; in order to ensure that the voltage of the direct current bus is constant, the closed-loop control system enables the MMC to work in an inversion state, and the converter feeds energy back to the power grid. Therefore, after appropriate modulation parameters are set, the charging and discharging of the energy storage battery pack can be controlled only by changing the working state of the bidirectional DC/DC converter, namely, the DC/DC converter works in a Buck mode, a power grid charges an energy storage system, and an MMC performs rectification; and enabling the DC/DC converter to work in a Boost mode, enabling the energy storage system to feed back energy to the power grid, and enabling the MMC to work in an inversion mode.
3. Submodule capacitor voltage balance control method
In the starting process of the distributed energy storage system, because the input time values of the submodules are not equal, the charging of the capacitors of the submodules is different, and the difference value of the capacitor voltage is amplified continuously; the difference value of the capacitor voltage is larger after the voltage of the direct current bus reaches a stable value. In order to balance the capacitance and voltage values of each submodule in the same bridge arm, a certain selectivity is needed when the submodules are put into and cut out, and at the moment, the putting into and the cutting out of the submodules are controlled by using a submodule capacitance and voltage sequencing priority determination method. If the positive direction of the bridge arm current is the direction that the upper bridge arm points to the lower bridge arm, when the bridge arm current is positive, the sub-module capacitor is charged; and when the bridge arm current is negative, the sub-module capacitor discharges. The bridge arm is taken as an example: when the upper bridge arm current is detected to be positive, the voltage values of the upper bridge arm sub-module capacitors measured at the moment are arranged in an ascending order, the arranged voltage values correspond to the sub-module input priorities one by one, and the sub-module input priorities are sequentially reduced according to the arrangement order; the capacitor voltage value arranged in the first place corresponds to the submodule with the lowest capacitor voltage value, which has the highest input priority. Then, the first m voltage values in the capacitor voltage arrangement are extracted according to the number m of the submodules to be put in at the moment, then m submodules with the capacitor voltages closest to the extracted m voltage values are selected through fuzzy comparison, and then the m submodules are put in. When the bridge arm voltage is detected to be negative, the capacitor voltage values of the upper bridge arm sub-modules detected at the moment are arranged in a descending order, the control selection method is the same as the bridge arm current is positive, and the sub-module with the highest capacitor voltage is input with the highest priority. For the lower bridge arm, the control selection method is completely the same as that of the upper bridge arm, and the judgment basis is only changed into the current measurement value of the lower bridge arm. It should be noted that, because the sub-modules are selected by fuzzy comparison of voltage values, depending on the comparison accuracy, there may be cases where more than m sub-modules are selected at some time; at this point, a secondary rank selection system is required to further screen the sub-modules, as will be described in more detail below, ignoring this problem which can lead to failure of the selection system.
Because the reference voltage value of the MMC is changed all the time during closed-loop control, frequent charging and discharging can cause the capacitor voltage of each submodule to fluctuate continuously. If the input of the sub-modules is selected at each moment, the switching of the sub-modules is too frequent, and a large amount of unnecessary switching actions are generated. Therefore, the design provides a submodule capacitor voltage edge delay triggering sequencing strategy for a nearest level approximation method. By means of the characteristic that a reference wave generated by a recent level approximation method is a step wave, the method collects the voltage value of the sub-module capacitor only after the reference wave has a rising edge or a falling edge and a short time delay, and keeps the collected voltage value until the next trigger edge; a short delay is provided to prevent repeated edge glitches. The voltage edge delay triggering sorting strategy only specifies the time for reordering the capacitance voltages of the sub-modules, namely, the capacitance voltages of the sub-modules are sorted once at intervals of a certain time value to update the input priority of each sub-module, and the input quantity of the sub-modules has no influence, so that the MMC output level is not influenced, and the method belongs to a sub-module input priority coding method parallel to the switching of the sub-modules.
4. Submodule energy storage unit SOC balance and secondary retrieval system
The submodule SOC sorting and selecting input method is similar to the submodule capacitor voltage sorting and input priority determination method. After the SOC of the energy storage batteries is estimated, the SOC of the energy storage battery packs in each sub-module is sequenced; carrying out SOC ascending sequencing in a charging mode, and preferentially putting the submodules with lower SOC values to charge the battery packs; in the discharging mode, SOC descending order is carried out, sub-modules with higher SOC values are preferentially put in, and the put battery pack is discharged. The submodule retrieval input selection control system works according to the following steps:
step (1): detecting the rising edge or the falling edge of an input step wave, and adding a delay with the time length of ts after the edge is triggered, wherein the delay time length is less than one fourth of the step time length of a single step wave;
step (2): after delaying, respectively sequencing the detected capacitor voltages of 8 sub-modules of the upper and lower bridge arms in the same bridge arm;
and (3): selecting a corresponding number of capacitor voltage values according to the number of submodules to be put into corresponding to the time step in a sequencing order;
and (4): carrying out primary retrieval on the submodules according to the set error range and the selected capacitor voltage value, and selecting a preferred submodule;
and (5): sorting the energy storage unit SOC of the selected optimal sub-module in the step (4), and selecting SOC values of corresponding quantity according to the number of the sub-modules input at the moment;
and (6): carrying out secondary retrieval on the selected optimal sub-module in the step (4) according to the set error range and the selected SOC value of the energy storage unit;
and (7): and controlling the quantity of the submodules selected by the secondary retrieval, and if the quantity of the selected submodules is still larger than the quantity of the submodules to be put in at the moment, unconditionally selecting the submodules with the quantity to be put in.
And (8): putting the finally selected sub-module after two times of retrieval into a bridge arm;
5. voltage and current closed-loop control system
Assuming three-phase symmetry of the grid voltage, the three-phase ac voltage can be expressed as follows:
ua＝Um cos(ωt)
in the formula of UmAnd ω is the amplitude and phase angle of the phase voltage, respectively. Now let the resistance and inductance on the AC side be RsAnd LsThen the following equation can be obtained:
wherein u is [ u ]a,ub,uc]T，is＝[isa,isb,isc]T，ur＝[ura,urb,urc]T；urAnd outputting voltage for the MMC alternating current side. Now, coordinate conversion is performed on the above equation, and the d axis in the dq rotation coordinate system is made to coincide with the voltage vector, so that the equation can be changed into an equation in the following rotation coordinate system through Park transformation:
when the resistance loss of the alternating current side and the sub-module switching loss of the MMC are neglected, the following power balance equation of the alternating current side and the direct current side can be approximately obtained:
therefore, when the d axis is selected to coincide with the voltage vector, the active power p is only equal to isdAfter that, i will besdReferred to as active current; and the reactive power is equal to isqThis is referred to as reactive current. Thus, the DC side bus voltage udcThe active current control can be carried out, the voltage and current double closed-loop control of a given direct current bus voltage reference value is adopted in the design, and the active current reference value of a current inner ring can be obtained by adopting PI regulated direct current voltage outer ring control as long as the direct current bus reference value is given; after the current inner ring is controlled by an algorithm evolved based on formulas (3.3) and (3.4), the obtained dq axis voltage u of the MMC alternating current siderdAnd urqAnd performing inverse Park conversion to finally obtain the MMC three-phase modulation reference wave which can be directly used for MMC nearest level approximation modulation, and realizing closed loop of the system. The control schematic block diagram of the double closed-loop controller is shown in FIG. 10.
6. Loop control
The MMC bridge arm circulating current is formed by superposing direct current bridge arm circulating current and alternating current bridge arm circulating current; the direct current bridge arm circulation is related to direct current of the MMC sub-module, and external characteristics of an alternating current side are not influenced; the AC bridge arm circulating current is related to the characteristics of an AC side power grid. When the three phases of the alternating-current side power grid are not ideal symmetrical three-phase alternating currents, the alternating-current side currents of the MMC are not balanced any more. At the moment, three-phase power at the alternating current side is superposed into time-varying fluctuation quantity, and power oscillation is generated on the MMC converter. When the circulation control is not performed on the system, a large frequency doubling current and voltage fluctuation occur on the direct current side, and the fluctuation has different degrees of influence on the direct current side according to the size of the fluctuation.
DC current I of energy storage system of current submoduleeWhen the current is small, the main component of the bridge arm circulation of the MMC is alternating current double-frequency current, so that the bridge arm circulation of the MMC is restrainedCan be equivalent to suppressing double frequency current in the circulating current. If the difficulty in directly controlling the alternating current bridge arm loop current is high, the frequency-doubled alternating current can be converted into direct current through Park conversion and then controlled, and a logic block diagram of the controller is shown in fig. 11.
Fig. 6 is a voltage waveform diagram of a dc side of an energy storage system according to an embodiment of the present invention, where a set dc side reference voltage value is 4000V, and if the dc side voltage enters steady state after the system is started up after about 0.6s, the dc side voltage is stabilized at 4000V; the system carries out mode switching in 1.5s, the system is in a charging mode before mode switching, and is in a discharging mode after switching, and voltage waveform on a direct current side has small fluctuation during mode switching, and then the system quickly enters a stable state.
FIG. 7 is a waveform of output power of the energy storage system according to an embodiment of the present invention, where the energy storage system absorbs energy from the power grid and the output power is negative when the system is in the charging mode before the mode switching point; and after the switching point, the system is in a discharging mode, the energy storage system outputs energy to the power grid, and the output power is positive.
Fig. 8 is an SOC curve of the energy storage unit of the sub-module No. 1 of the upper bridge arm of the phase bridge arm in the embodiment of the present invention, where the discharge SOC of the energy storage system before the switching point is reduced; the system charge SOC increases after the switch point.
Fig. 9 shows waveforms of ac side voltages of a phase bridge arm according to an embodiment of the present invention, where the waveforms of the ac side voltages generate disturbance when the system switches the mode, and then the waveforms of the ac side voltages recover to the steady state.
The bidirectional DC/DC converter in the sub-module of the invention is a parallel-stacked bidirectional DC/DC converter as shown in FIG. 2, and the switching tube in the converter adopts SIC MOSFET. FIG. 12 is a diagram illustrating an output voltage waveform of an energy storage unit when a normal bidirectional DC/DC converter is used in a submodule according to an embodiment; fig. 13 shows that the DC/DC converter is replaced by a parallel stacked bidirectional DC/DC converter, so that the output voltage ripple of the energy storage unit is reduced and a higher switching frequency can be satisfied after the parallel stacked bidirectional DC/DC converter is used.
The above is a description of a specific embodiment of the present invention, and it should be noted that the present invention is not limited in any way, but is only illustrated and referred to.
Claims (6)
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| CN111688539B (en)* | 2020-06-04 | 2023-07-28 | 国网天津市电力公司电力科学研究院 | Multi-level energy storage electric vehicle DC charging optimization control system and method |
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| CN112803476B (en)* | 2020-12-31 | 2022-04-15 | 山东大学 | Method and system for controlling voltage of variable direct current bus of modular multi-level energy storage converter |
| CN113258803B (en)* | 2021-04-28 | 2023-03-28 | 上海电机学院 | Capacitor voltage balance control system and method of modular multilevel converter |
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