CN109687490B - Variable adjustment factor SOC droop control method of distributed energy storage system - Google Patents
Variable adjustment factor SOC droop control method of distributed energy storage system Download PDFInfo
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Abstract
The invention provides a variable adjustment factor SOC droop control method of a distributed energy storage system, wherein the distributed energy storage system comprises a plurality of energy storage modules, the distributed energy storage system adopts an exponential type SOC droop control strategy based on a double closed loop of a direct current bus voltage outer loop and a current inner loop, and the method comprises the following steps: a droop control setting step, wherein an SOC droop control strategy adopting non-interconnection communication is set for each energy storage module; and a droop control and adjustment step, namely changing the value of the equalizing speed adjustment factor n to change the SOC droop coefficient curve. The invention ensures that the system power has higher response speed in the initial starting stage and the power response stage and has higher convergence speed in the power convergence stage by changing the value of the regulating factor n in different stages, so that the system simultaneously meets the requirements of power response rapidity and convergence rapidity, and the overall response speed of the system is improved.
Description
Technical Field
The invention relates to the technical field of direct current energy storage, in particular to a variable adjustment factor SOC droop control method of a distributed energy storage system.
Background
In a direct-current microgrid, the fluctuation of bus power caused by unstable output, load fluctuation and the like of a distributed power generation unit can possibly threaten the stability of the microgrid system in serious cases. The reasonable operation of the energy storage system in the direct-current micro-grid can stabilize energy fluctuation, stabilize direct-current bus voltage, maintain system power balance and further improve power supply reliability, so that the energy storage system is an indispensable component in the direct-current micro-grid. However, a single energy storage device cannot meet the requirement of a micro-grid system for large capacity, and therefore a plurality of energy storage modules are connected in parallel in a distributed mode to a direct current bus to form a distributed energy storage system. In order to avoid overcharge or overdischarge of the energy storage unit and achieve reasonable distribution of power among the energy storage modules, an effective power distribution strategy is required to enable the energy storage modules to output or absorb power according to a state of charge (SOC) of the energy storage modules, so that the purposes of power balance of the direct-current microgrid and efficient utilization of the energy storage unit are achieved.
Chinese invention patent 201611149350.X discloses a control method of SOC optimal droop factor applied to a distributed energy storage system, which comprises the steps of 1, constructing the SOC optimal droop factor; step 2, describing the control system as a voltage source type converter according to the topological structure of the distributed energy storage converter; step 3, the droop factor and the output port current idcProduct of and output terminal reference voltage udcComparing to form new droop control reference voltage udc(ii) a Step 4, designing a voltage current regulator to ensure that the phase margin and the amplitude margin of the voltage current regulator are large enough; and 5, adding secondary control to compensate the voltage drop of the bus, so that the whole system has higher dynamic response and smaller steady-state error. The invention can effectively distribute the load according to the residual capacity of the storage battery and the super capacitor by simulating and experimentally inducing the optimal droop factor of the SOC, so that the SOC quickly tends to be consistent and the loads tend to be equal. The SOC and the port voltage information are shared through the interconnection line and used for adaptive adjustment of the droop factor and bus voltage recovery. The SOC optimal droop factor control method applied to the distributed energy storage system aims at interconnection communication of the energy storage modules, and the adjustment factor is a constant, so that the SOC optimal droop factor control method is an ideal model and has large errors in actual use.
The distributed droop control method comprises Wuqingfeng, Sunxiafeng, Wangyounan and the like, a micro-grid distributed energy storage system SOC balance strategy [ J ] based on distributed droop control, a report of electrotechnical science, 2018,33(6):1247 and 1256, and aims at solving the problem of SOC imbalance when a distributed energy storage system DESS in an island alternating current micro-grid adopts traditional P-f droop control, and provides the distributed droop control. According to the scheme, the active power output by the inverter is controlled by adding the SOC balance factor on the basis of the traditional P-f droop control, so that SOC balance in the charging and discharging processes of DESS with different capacities is realized, and the frequency cannot deviate in the SOC balance process. When the SOC is balanced, the SOC balance factor is 1, and the control algorithm is automatically changed into the traditional P-f droop control. Since the droop coefficient is typically small, the frequency can be kept within a specified range of ± 1%. Further, by adjusting the droop coefficient of the SOC, the balancing speed of the SOC can be adjusted. The strategy proposed in the document is also directed to interconnection communication, and the SOC information of each energy storage module is communicated, so that the strategy is also an ideal model and has a large error in actual use.
In view of the fact that the charge states of the energy storage modules in the distributed energy storage system are not necessarily equal, in order to achieve efficient utilization and safe operation of the energy storage system, a power exponent type SOC droop control strategy is adopted, so that the output or absorption power of the energy storage modules can be adjusted in real time according to the charge states of the energy storage modules in the charging and discharging processes, and reasonable distribution of load power among the energy storage modules connected in parallel is achieved. Under the control strategy, if the value of the adjusting factor n is small, the power response speed of the system is high, but the convergence speed is low; if the value of the adjustment factor n is larger, the power convergence speed of the system is high, but the response speed is low. That is, according to the existing control strategy, the rapidity and convergence of the system adjustment cannot be considered at the same time.
Disclosure of Invention
The invention solves the problem that the regulation factor n is a fixed value and cannot give consideration to the rapidity of power response and the rapidity of convergence in the prior art, and improves the overall response speed of the system. In view of the above-mentioned drawbacks of the prior art, the present invention provides the following solutions.
A variable adjustment factor SOC droop control method of a distributed energy storage system comprises a plurality of energy storage modules, wherein a control strategy adopted by the distributed energy storage system is an exponential type SOC droop control strategy based on a direct current bus voltage outer ring and a current inner ring double closed loop, and the method comprises the following steps:
a droop control setting step, wherein an SOC droop control strategy adopting non-interconnection communication is set for each energy storage module;
and a droop control and adjustment step, namely changing the value of the equalizing speed adjustment factor n to change an SOC droop coefficient curve, and improving the overall response speed of the system on the premise of ensuring the convergence of the system, wherein the step of changing the value of the equalizing speed adjustment factor n is as follows:
(1) in the initial stage of starting the energy storage module, the equalizing speed adjusting factor n is the minimum adjusting value nsmin；
(2) In the power response stage of the energy storage module, changing an equalizing speed adjusting factor n, wherein the expression is as follows:
(3) in the power convergence stage of the energy storage module, the equalizing speed adjusting factor is the maximum value nsmax。
Further preferably, the SOC droop control strategy is set as:
wherein subscript i denotes the ith transducer;
is a direct current bus voltage reference value; u. ofdci_refOutputting voltage for the i-th converter droop module as the reference voltage of the double closed loop; piOutputting power for the ith converter; rxiThe SOC droop coefficient of the ith converter is a discharge mode droop coefficient when subscript x is equal to d, and is a charge mode droop coefficient when subscript x is equal to c;
the droop coefficient for the exponential SOC is:
the droop coefficient for the power exponent type SOC is:
wherein R isi0The initial droop coefficient of the ith energy storage module is obtained; n isiAdjusting a factor for the equalizing speed of the ith energy storage module; SOCiThe residual capacity of the ith energy storage module is obtained; m is an exponential coefficient of the power exponential type SOC; e is the base of the exponential function, the value of which is constant.
More preferably, the energy storage modules are any two energy storage modules connected in parallel on the direct current bus.
Further preferably, a and b are any two energy storage modules connected in parallel to the dc bus, and the relationship between the output power and the droop coefficient is as follows:
get Ra0＝Rb0，na＝nbWhen n is obtained, the output power relationship of the two energy storage modules in the exponential SOC discharge mode is:
under the power exponent type SOC discharge mode, the output power relation of two energy storage modules is:
under the exponential SOC charging mode, the output power relationship of the two energy storage modules is as follows:
under the power exponent type SOC charging mode, the output power relation of two energy storage modules is as follows:
in the initial starting stage and the power response stage, under the condition of the same SOC, the smaller n is, the larger droop coefficient is, the faster dynamic response speed is, the input or output power of the energy storage module can quickly reach the power distribution point, and the power ratio of the power distribution point and the SOC value thereof satisfy the formula (4); in the power convergence stage, the functional relation between the energy storage module power and the adjustment factor n is as in formulas (5) - (8), and under the condition that the SOC difference value is the same, the larger the adjustment factor n is, the larger the energy storage module power ratio is, and the faster the convergence speed is.
Further preferably, when the exponential SOC droop control strategy is adopted, the droop control expression of each energy storage module is:
when a power exponent type SOC droop control strategy is adopted, the droop control expression of each energy storage module is as follows:
the initial value of the speed regulating factor n of the equalizing speed is nsminAt intervals of time Δ t, the value is incremented by Δ n until a maximum value n is reachedsmax(ii) a The value of the equalizing speed adjusting factor n is changed at different stages, the SOC droop coefficient curve is changed, the overall response speed of the system is improved on the premise of ensuring the convergence of the system, and the SOCs of the energy storage modules are consistent or tend to be consistent, so that power equalization is realized.
The invention has the technical effects that: the invention ensures that the system power has higher response speed in the initial starting stage and the power response stage by changing the value of the adjusting factor n in different stages, and has higher convergence speed in the power convergence stage, so that the system simultaneously meets the requirements of power response rapidity and convergence rapidity, solves the problem that the adjusting factor n is a constant value and cannot give consideration to the power response rapidity and the convergence rapidity in the prior art, and improves the overall response speed of the system.
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Other features, objects and advantages of the present application will become more apparent upon reading of the following detailed description of non-limiting embodiments thereof, made with reference to the accompanying drawings.
Fig. 1 is a flowchart of a variable adjustment factor SOC droop control method of a distributed energy storage system according to an embodiment of the present invention;
FIG. 2 is a diagram of a distributed energy storage architecture;
FIG. 3 is a block diagram of a variable adjustment factor exponential SOC control structure with dual energy storage modules in communication without interconnection;
FIG. 4 is a simulation diagram of power response when the value of the adjustment factor n is different;
FIG. 5 is a graph of droop coefficient as a function of SOC in discharge mode;
FIG. 6 is a plot of energy storage module power ratio as a function of Δ SOC;
FIG. 7 is a graph of the adjustment factor n at various stages;
FIG. 8 is a waveform diagram of output power of each module under the variable adjustment factor SOC droop control strategy;
fig. 9 is a simulation comparison graph of the variable adjustment factor SOC droop control strategy and the adjustment factor n being 8; and
fig. 10 is a simulation comparison graph of the variable adjustment factor SOC droop control strategy and the adjustment factor n being 1.
Detailed Description
The present application will be described in further detail with reference to the following drawings and examples. It is to be understood that the specific embodiments described herein are merely illustrative of the relevant invention and not restrictive of the invention.
Fig. 1 shows a variable adjustment factor SOC droop control method of a distributed energy storage system according to the present invention, wherein the distributed energy storage system 3 includes a plurality of energy storage modules, the distributed energy storage system always employs a dual closed loop control strategy of a dc bus voltage outer loop and a current inner loop, for the distributed energy storage system, as shown in fig. 2, the SOC of each energy storage module connected in parallel to the dc bus is not necessarily equal, an exponential SOC droop control without interconnection is employed, by changing the value of the adjustment factor n at different stages and changing the SOC droop coefficient curve, on the premise of ensuring the convergence of the system, the response speed of the whole system is increased, the SOC of each module tends to be consistent faster, the energy storage system always employs a dual closed loop control strategy of a dc bus voltage outer loop and a current inner loop, so as to ensure that the energy storage module still provides stable dc voltage support for the system under the SOC limit condition, the reliability of the system is improved.
Referring to fig. 1, the method for controlling variable adjustment factor SOC droop of a distributed energy storage system of the present invention includes:
a droop control setting step S101, setting an SOC droop control strategy adopting non-interconnection communication for each energy storage module;
and a droop control and adjustment step S102, wherein an SOC droop coefficient curve is changed by changing the value of the equalizing speed adjusting factor n, the overall response speed of the system is improved on the premise of ensuring the convergence of the system, and the step of changing the value of the equalizing speed adjusting factor n is as follows:
(1) in the initial stage of starting the energy storage module, the equalizing speed adjusting factor n is the minimum adjusting value nsmin；
(2) In the power response stage of the energy storage module, changing an equalizing speed adjusting factor n, wherein the expression is as follows:
(3) in the power convergence stage of the energy storage module, the equalizing speed adjusting factor is the maximum value nsmax。
In one embodiment, the SOC droop control strategy in the droop control setting step S101 is set as:
wherein subscript i denotes the ith transducer;
is a direct current bus voltage reference value; u. ofdci_refOutputting voltage for the i-th converter droop module as the reference voltage of the double closed loop; piOutputting power for the ith converter; rxiThe SOC droop coefficient of the ith converter is a discharge mode droop coefficient when subscript x is equal to d, and is a charge mode droop coefficient when subscript x is equal to c;
the droop coefficient for the exponential SOC is:
the droop coefficient for the power exponent type SOC is:
wherein R isi0The initial droop coefficient of the ith energy storage module is obtained; n isiAdjusting a factor for the equalizing speed of the ith energy storage module; SOCiThe residual capacity of the ith energy storage module is obtained; m is an exponential coefficient of the power exponential type SOC; e is the base of the exponential function, the value of which is constant.
In one embodiment, the step of changing the value of the equalizing speed adjustment factor n in the droop control step S102 is as follows:
(1) in the initial stage of starting the energy storage module, the adjusting factor n is the minimum adjusting value nsmin(ii) a As can be seen from fig. 4, the smaller n is, the larger the discharge droop coefficient is under the same SOC, the faster the dynamic response speed is, and therefore, the output power of the energy storage module can quickly reach the power distribution point, and the power ratio of the power distribution point and the SOC value thereof satisfy equation 4. When the adjustment factor n takes different values, the power response phase is as shown in fig. 5, where a curve F1 represents the output power waveforms of the two energy storage modules when the adjustment factor n is 3; a curve F2 is the output power waveform of the two energy storage modules when the adjustment factor n is 5; curve F3 is the output power waveform of the two energy storage modules when the adjustment factor n is equal to 7.
(2) In the energy storage module power response stage, the adjusting factor n is changed, and the expression is as follows:
(3) in the power convergence stage of the energy storage module, the adjustment factor is the maximum value nsmax。
Furthermore, the energy storage modules are any two energy storage modules which are connected in parallel on the direct current bus.
Furthermore, let a, b be arbitrary two energy storage modules connected in parallel on the dc bus, and the relationship between the output power and the droop coefficient is:
in general, R is takena0＝Rb0，na＝nbWhen n is obtained, the output power relationship of the two energy storage modules in the exponential SOC discharge mode is:
under the power exponent type SOC discharge mode, the output power relation of two energy storage modules is:
under the exponential SOC charging mode, the output power relationship of the two energy storage modules is as follows:
under the power exponent type SOC charging mode, the output power relation of two energy storage modules is as follows:
therefore, in the initial starting stage and the power response stage, under the condition of the same SOC, the smaller n is, the larger droop coefficient is, and the faster dynamic response speed is, so that the input or output power of the energy storage module can quickly reach the power distribution point, and the power ratio of the power distribution point and the SOC value thereof satisfy the formula 4; in the power convergence stage, the functional relation between the power of the energy storage module and the adjustment factor n is as shown in the formula 5-8, and under the condition that the SOC difference value is the same, the larger the adjustment factor n is, the larger the power ratio of the energy storage module is, and the faster the convergence speed is.
wherein Δ SOC is the difference of SOC, and Δ SOC is the SOCa-SOCb。
Fig. 2 is a structural diagram of a distributed energy storage system, and a first side of a dc bus 4 may be connected to a distributed power generation system 5 or a power grid 6, respectively; the second side of the dc bus 4 may be connected to the distributed energy storage system 3 or to a load 7, respectively. Wind power generation or photovoltaic power generation in the distributed power generation system 5 is connected to the direct current bus 4 via an AC/DC or DC/AC converter. On the other side of the DC bus 4, the energy storage modules 1 and 2 in the distributed energy storage system 3 are connected to the DC bus 4 through a bidirectional DC/DC converter. Preferably. In this embodiment, two energy storage modules connected in parallel to a dc bus are exemplified, and a plurality of energy storage modules may be connected in parallel as necessary.
In this example, an exponential SOC droop control is adopted, and a control structure block diagram of the dual energy storage modules without interconnection communication is shown in fig. 2. The two energy storage modules adopt a double closed-loop control strategy of a direct current bus voltage outer ring and a current inner ring to maintain the voltage stability of the direct current bus. The droop control module inputs direct current bus voltage and module output current and outputs reference voltage of a double closed loop. The droop of each energy storage module is controlled as follows:
the speed regulation factor n has an initial value of nsminAt intervals of time Δ t, the value is incremented by Δ n until a maximum value n is reachedsmax. By changing the value of the adjusting factor n at different stages and changing the droop coefficient curve of the SOC, the overall response speed of the system is improved on the premise of ensuring the convergence of the system, and the SOC of each module tends to be consistent more quickly, so that the power is evenly divided.
Taking the example that the energy storage module works in a discharging mode for verification, the simulation parameters are as follows: DC bus voltage
The load power of the bus is 10kW, and the initial droop coefficient R10＝R200.001, SOC of energy storage module 110.9 SOC of the energy storage module 22＝0.8。
For the control strategy of adopting variable adjustment factor SOC droop, the system starts the initial stage n smin1, the control time interval Δ t is 0.5, the control factor increment Δ n is 1, and the value is increased to a maximum value nsmaxAt 8, the power convergence phase n is maintained at a maximum value and the value of the adjustment factor n is varied as shown in fig. 7.
The simulation diagram of the output power of each module adopting the variable adjustment factor SOC droop control strategy is shown in fig. 8, wherein L1 is the waveform of the output power of the energy storage module 1, and L2 is the waveform of the output power of the energy storage module 2. After the output power of the two energy storage modules is subjected to the droop control adjustment of the improved variable adjustment factor SOC, the output power can be stabilized at 5kW, the SOC tends to be consistent, and the system can be maintained stable.
A simulation pair of the variable adjustment factor SOC droop control strategy and the adjustment factor n-1 exponential type SOC control strategy is shown in fig. 9, where B1 is the variable adjustment factor and the output power of the energy storage module 1; b2 is the output power of the energy storage module 2 with variable regulating factors; c1 is the output power of the energy storage module 1 when the adjustment factor n is 1; c2 is the output power of the energy storage module 2 with the adjustment factor n being 1. The adjusting factors n of the two control strategies are both 1 in the initial stage of system starting, the speed of the power initial response stage is the same, in the convergence stage, the value of the adjusting factor n of the improved exponential type SOC control strategy is large, the power ratio is larger, the convergence speed is higher, and the overall response speed of the system is higher.
A simulation pair of the variable adjustment factor SOC droop control strategy and the adjustment factor n-8 exponential type SOC control strategy is shown in fig. 10, where D1 is the variable adjustment factor and the output power of the energy storage module 1; d2 is the output power of the energy storage module 2 with the variable regulating factor; e1 is the output power of the energy storage module 1 with the adjustment factor n being 8; e2 is the output power of the energy storage module 2 with the adjustment factor n being 8. The adjusting factors n of the two control strategies are both 8 in the convergence stage, the convergence speed is the same, in the initial starting stage and the power response stage, the value of the adjusting factor n of the improved exponential type SOC control strategy is smaller, the power response speed is higher, the speed of reaching a power distribution point is higher, and the overall response speed of the system is higher.
Compared with the prior art, the invention has the following advantages: in the prior art, the adjustment factor n is a fixed value, and power response rapidity and convergence rapidity cannot be considered at the same time. The technology ensures that the system power has a faster response speed in the initial starting stage and the power response stage and has a faster convergence speed in the power convergence stage by changing the value of the adjusting factor n in different stages, so that the system simultaneously meets the requirements of power response rapidity and convergence rapidity, and the overall response speed of the system is improved.
For convenience of description, the above devices are described as being divided into various units by function, and are described separately. Of course, the functionality of the units may be implemented in one or more software and/or hardware when implementing the present application.
From the above description of the embodiments, it is clear to those skilled in the art that the present application can be implemented by software plus necessary general hardware platform. Based on such understanding, the technical solutions of the present application may be substantially implemented or contributed to by the prior art in the form of a software product, which may be stored in a storage medium, such as a ROM/RAM, a magnetic disk, an optical disk, or the like, and includes instructions for causing a computer device, such as a personal computer, a server, or a network device, to execute the embodiments or some parts of the methods of the embodiments of the present application.
Finally, it should be noted that: although the present invention has been described in detail with reference to the above embodiments, it should be understood by those skilled in the art that: modifications and equivalents may be made thereto without departing from the spirit and scope of the invention and it is intended to cover in the claims the invention as defined in the appended claims.
Claims (5)
1. A variable adjustment factor SOC droop control method of a distributed energy storage system is disclosed, the distributed energy storage system comprises a plurality of energy storage modules, and a control strategy adopted by the distributed energy storage system is an exponential type SOC droop control strategy based on a direct current bus voltage outer ring and a current inner ring double closed loop, and the method is characterized by comprising the following steps:
a droop control setting step, wherein an SOC droop control strategy adopting non-interconnection communication is set for each energy storage module;
and a droop control and adjustment step, namely changing the value of the equalizing speed adjustment factor n to change an SOC droop coefficient curve, and improving the overall response speed of the system on the premise of ensuring the convergence of the system, wherein the step of changing the value of the equalizing speed adjustment factor n is as follows:
(1) in the initial stage of starting the energy storage module, the equalizing speed adjusting factor n is the minimum adjusting value nsmin；
(2) In the power response stage of the energy storage module, changing an equalizing speed adjusting factor n, wherein the expression is as follows:
(3) in the power convergence stage of the energy storage module, the equalizing speed adjusting factor is the maximum value nsmax。
2. The method of claim 1, wherein the SOC droop control strategy is configured to:
wherein subscript i denotes the ith transducer;
is a direct current bus voltage reference value; u. ofdci_refOutputting voltage for the i-th converter droop module as the reference voltage of the double closed loop; pi is the output power of the ith converter; rxiThe SOC droop coefficient of the ith converter is a discharge mode droop coefficient when subscript x is equal to d, and is a charge mode droop coefficient when subscript x is equal to c;
the droop coefficient for the exponential SOC is:
the droop coefficient for the power exponent type SOC is:
wherein R isi0The initial droop coefficient of the ith energy storage module is obtained; n isiAdjusting a factor for the equalizing speed of the ith energy storage module; SOCiThe residual capacity of the ith energy storage module is obtained; m is an exponential coefficient of the power exponential type SOC; e is an exponential functionThe base number of (d) is constant.
3. The method for controlling the variable adjustment factor SOC droop of the distributed energy storage system according to claim 2, wherein the energy storage modules are any two energy storage modules connected in parallel on a direct current bus.
4. The method for controlling the droop of the variable adjustment factor SOC of the distributed energy storage system according to claim 3, wherein a and b are any two energy storage modules connected in parallel on a DC bus, and the relationship between the output power and the droop coefficient is as follows:
get Ra0＝Rb0，na＝nbWhen n is obtained, the output power relationship of the two energy storage modules in the exponential SOC discharge mode is:
under the power exponent type SOC discharge mode, the output power relation of two energy storage modules is:
under the exponential SOC charging mode, the output power relationship of the two energy storage modules is as follows:
under the power exponent type SOC charging mode, the output power relation of two energy storage modules is as follows:
in the initial starting stage and the power response stage, under the condition of the same SOC, the smaller n is, the larger droop coefficient is, the faster dynamic response speed is, the input or output power of the energy storage module can quickly reach the power distribution point, and the power ratio of the power distribution point and the SOC value thereof satisfy the formula (4); in the power convergence stage, the functional relation between the energy storage module power and the adjustment factor n is as in formulas (5) - (8), and under the condition that the SOC difference value is the same, the larger the adjustment factor n is, the larger the energy storage module power ratio is, and the faster the convergence speed is.
5. The method for controlling the droop of the variable adjustment factor SOC of the distributed energy storage system according to claim 4, wherein when an exponential SOC droop control strategy is adopted, the droop control expression of each energy storage module is as follows:
when a power exponent type SOC droop control strategy is adopted, the droop control expression of each energy storage module is as follows:
the initial value of the speed regulating factor n of the equalizing speed is nsminAt intervals of time Δ t, the value is incremented by Δ n until a maximum value n is reachedsmax(ii) a The value of the equalizing speed adjusting factor n is changed at different stages, the SOC droop coefficient curve is changed, the overall response speed of the system is improved on the premise of ensuring the convergence of the system, and the SOC of each energy storage module is consistent, so that the power is evenly divided.
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