Method for controlling string-type network-forming energy storage converter

Abstract

The invention discloses a string type network energy storage converter control method, and the method comprises the steps: enabling an energy storage converter to take an SOC mean value as the feedback of an SOC equalization ring, and obtaining an SOC equalization ring output value; superposing the output value of the secondary frequency modulation ring, the output value of the SOC equalization ring and the output value of the primary frequency modulation ring as an active given value, and obtaining a reference value of the frequency through a VSG active frequency control ring; taking the output value of the secondary voltage regulating ring as a reactive given value, obtaining a reference value of a VSG reactive amplitude control ring through the primary voltage regulating ring, and obtaining a reference value of voltage through the amplitude control ring; inputting the obtained reference value of the frequency and the reference value of the voltage into a voltage and current double closed loop to obtain an output signal for following the reference values, and further generating a driving signal for controlling a switching tube of the energy storage converter; according to the invention, the communication load rate can be reduced, when large frequency and voltage fluctuation occurs, the voltage and frequency stability problem can be independently solved, the self-recovery control of secondary regulation is realized, and the frequency and the output voltage of the system are dynamically regulated in real time.

Description

Technical Field

[0001] The present invention relates to the field of converters, and in particular to a control method for a string-type grid-connected energy storage converter. Background Art

[0002] As the installed capacity of photovoltaic power stations gradually increases, power generation and maintainability will become important factors in power station design. String converter technology is mature, flexible in design, easy to maintain and highly adaptable, especially in remote areas. As the new favorite of the times, string energy storage converters have developed rapidly in recent years. At the same time, due to the increasing penetration rate of power electronic power sources such as new energy sources, grid-type converters that can adapt to weak power grids have become the research focus of the corresponding field.

[0003] The virtual synchronous generator (VSG) algorithm can provide damping and inertial support as well as stable voltage and frequency for the grid-type energy storage system. However, the coordinated control of the parallel operation of string energy storage inverters also needs to consider the difference-free control of system power distribution and frequency and the over-charging and discharging caused by the SOC difference between the string energy storage converter units, so as to further optimize the performance of the string grid-type energy storage system and ensure the system life. The current string grid-type energy storage system cannot independently solve the voltage and frequency stability problems, the battery over-charging and discharging caused by the SOC difference, and other problems when large frequency and voltage fluctuations occur.

[0004] The above information disclosed in this Background section is included only for enhancement of understanding of the background of the present disclosure and therefore it may contain information that does not form the prior art that is currently known to a person of ordinary skill in the art. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a control method for a string-type grid-connected energy storage converter in view of the above-mentioned defects of the prior art.

[0006] The technical solution adopted by the present invention to solve the technical problem is: constructing a control method for a string-type grid-type energy storage converter, wherein the string-type grid-type energy storage converter includes a plurality of energy storage converters, and the method includes:

[0007] The energy storage converter uses the SOC mean value as feedback of the SOC balancing loop to obtain an SOC balancing loop output value;

[0008] The output value of the secondary frequency modulation loop, the output value of the SOC balancing loop and the output value of the primary frequency modulation loop are superimposed as the active power given value, and the reference value of the frequency is obtained through the VSG active frequency control loop; the output value of the secondary voltage regulation loop is used as the reactive power given value, and the reference value of the VSG reactive amplitude control loop is obtained through the primary voltage regulation loop, and the reference value of the voltage is obtained through the VSG reactive amplitude control loop;

[0009] The obtained frequency reference value and voltage reference value are input into the voltage-current double closed loop to obtain an output signal that follows the reference value, and a drive signal for controlling the switch tube of the energy storage converter is generated based on the output signal.

[0010] Furthermore, in the control method of the string-type grid-type energy storage converter described in the present invention, the method further includes: before superimposing the secondary frequency modulation loop output value, the SOC balancing loop output value and the primary frequency modulation loop output value, the secondary frequency modulation loop output value is subjected to the following constraint processing: if the SOC value itself is less than or equal to the SOC minimum limit value, the secondary frequency modulation loop output value is forcibly set to 0, otherwise the secondary frequency modulation loop output value remains unchanged;

[0011] Furthermore, in the control method of the string-type grid-connected energy storage converter of the present invention, the method further includes: before superimposing the secondary frequency modulation loop output value, the SOC balancing loop output value and the primary frequency modulation loop output value, performing the following two constraints on the SOC balancing loop output value:

[0012] The first constraint range is -K soc *P n ≤P soc ≤K soc *P n , P soc is the SOC balance loop output value, K soc is the coefficient of SOC balanced adjustable power, P n is the rated power;

[0013] The second constraint range is -P limit ≤P soc ≤P limit , P limit =2P n -P one -P sec .

[0014] Furthermore, in the string-type grid-type energy storage converter control method described in the present invention, multiple energy storage converters are divided into a master and multiple slaves, and the slaves upload their own SOC values ​​through weak communication;

[0015] The method also includes: if the energy storage inverter is a host, the secondary frequency modulation loop output value, the secondary voltage regulation loop output value, and the SOC average are calculated locally, and sent to the slave through weak communication; if the energy storage inverter is a slave, the secondary frequency modulation loop output value, the secondary voltage regulation loop output value, and the SOC average are directly obtained from the host through weak communication.

[0016] Furthermore, in the string-type grid-connected energy storage converter control method described in the present invention, the calculation process of the secondary frequency modulation loop output value includes:

[0017] The host obtains the sampled AC output side frequency, subtracts the output side frequency from the rated frequency to obtain the frequency difference, and calculates the frequency change rate of the output side frequency;

[0018] If the product of the frequency difference and the frequency change rate is greater than or equal to zero, the integral coefficient matches the variable integral coefficient of the first underdamping ratio range, otherwise the integral coefficient matches the fixed integral coefficient of the second underdamping ratio range, wherein the second underdamping ratio range is greater than the first underdamping ratio range;

[0019] The frequency difference and the matched integral coefficient are sent to the first closed-loop feedback controller, and the output of the first closed-loop feedback controller is evenly divided according to the number of the energy storage converters to obtain the secondary frequency modulation loop output value.

[0020] Furthermore, in the string grid-type energy storage converter control method described in the present invention, the calculation process of the secondary voltage regulation loop output value includes:

[0021] The host obtains the AC output side voltage sampled by itself, and obtains the voltage difference by subtracting the output side voltage from the given voltage;

[0022] The voltage difference is sent to a second closed-loop feedback controller, and the output of the second closed-loop feedback controller is evenly divided according to the number of the energy storage converters to obtain the secondary voltage regulation loop output value.

[0023] Furthermore, in the control method of the string-type grid-connected energy storage inverter described in the present invention, the SOC mean value is used as feedback of the SOC balancing loop to obtain the SOC balancing loop output value, which specifically includes: taking its own SOC value as a given and taking the SOC mean value as feedback to perform subtraction to obtain the SOC difference value; and sending the SOC difference value to a third closed-loop feedback controller to obtain the SOC balancing loop output value.

[0024] Furthermore, in the control method of the string-type grid-connected energy storage inverter described in the present invention, the process of obtaining the primary frequency modulation loop output value includes: the energy storage inverter obtains the AC output side frequency sampled by itself, subtracts it from the rated frequency and multiplies it by the frequency conversion coefficient to obtain the primary frequency modulation loop output value.

[0025] Further, in the control method of the string-type grid-type energy storage converter described in the present invention, the method also includes: the energy storage converter obtains the AC output side voltage and current sampled by itself, and calculates the active power and reactive power based on the output side voltage and current; uses the calculated active power as the feedback value of the VSG active frequency control loop; uses the calculated reactive power as the feedback value of the primary voltage regulation loop;

[0026] The method of obtaining the reference value of the VSG reactive amplitude control loop through the primary voltage regulating loop specifically includes: subtracting the reactive reference value of the primary voltage regulating loop from the feedback value and multiplying the difference by the voltage conversion coefficient to obtain the reference value of the VSG reactive amplitude control loop;

[0027] The voltage reference value obtained through the VSG reactive amplitude control loop specifically includes: superimposing the reference value of the VSG reactive amplitude control loop on the original voltage set value to obtain a new voltage set value; subtracting the local AC output side voltage from the new voltage set value and sending the result to the fourth closed-loop feedback controller; and superimposing the output of the fourth closed-loop feedback controller with the original voltage set value to obtain the voltage reference value.

[0028] Furthermore, in the string-type grid-type energy storage converter control method of the present invention, a plurality of energy storage converters are divided into a host and a plurality of slaves, and the host and the slaves transmit power frequency synchronization signals through a pulse signal line, and the method further includes:

[0029] The energy storage converter receives a black start command issued by the energy management system;

[0030] The host collects the online number of the energy storage converters, determines whether the total number of the system is consistent with the online number, and if they are consistent, performs a battery side soft start and broadcasts a battery side soft start instruction;

[0031] The slave receives the battery side soft start command broadcast by the host and performs battery side soft start, and then waits for a delay;

[0032] The host checks that all the energy storage converters complete the soft start on the battery side and then sends out the power frequency synchronization signal, and the slave receives the power frequency synchronization signal to perform phase synchronization, specifically including: the host pulls up the pulse in the pulse signal line based on the power frequency information generated by its own local oscillator at the time when the power frequency passes through zero, so that the rising edge of the pulse is consistent with the power frequency zero point, and the slave captures the pulse in the pulse signal line respectively, adjusts its own phase synchronously, and sends a soft start success flag after the adjustment is completed;

[0033] After the host receives the soft start success flag bits sent by all the energy storage converters, it broadcasts an AC switch instruction allowing the slave to close;

[0034] The energy storage converter closes the AC side switch after receiving the AC switch instruction.

[0035] The control method of the string-type grid-type energy storage converter of the present invention has the following beneficial effects: adding a primary frequency modulation and a primary voltage regulation link to the traditional VSG power outer loop, so that the string-type grid-type converter unit can meet the stable current sharing of the droop characteristic in the isolated grid voltage source mode. Compared with the traditional voltage source parallel connection based on communication current sharing, there is no need to carry current sharing information in the communication, so the communication interaction information of each unit does not need to be too frequent, thereby reducing the burden of the communication load rate. In addition, the communication baud rate can be set relatively low, so the communication line can be pulled long enough to meet the distributed characteristics of the string-type grid-type converter; a grid-type control method of VSG secondary frequency modulation, secondary voltage regulation and SOC balancing control is provided. When large frequency and voltage fluctuations occur, the string-type grid-type energy storage system can independently solve the voltage and frequency stability problems, realize the self-recovery control method of secondary regulation, and realize real-time dynamic adjustment of the system frequency and output voltage, eliminating the threat to the safe operation of the system;

[0036] Furthermore, the constraints of SOC balance control and secondary frequency modulation can avoid the problems of over-charging and discharging caused by SOC differences between string energy storage converter units and effective control without difference with secondary frequency modulation when SOC and secondary frequency modulation act at the same time. That is, the string energy storage converter is subject to the double constraints of its own SOC balance and secondary frequency modulation, which not only solves the problems of battery over-charging and discharging caused by SOC differences, but also effectively solves the priority problem of SOC balance and secondary frequency modulation, thus expanding the functionality of the system.

[0037] Furthermore, in weak communication, the power frequency synchronization is performed using the power frequency synchronization pulse signal line, so that each group of string-type networking units synchronizes the phase between the closed side switches in advance during the black start, and a reasonable state interaction startup logic is added, thereby reducing the transient current at the moment when the switches of each unit are closed, solving the shortcoming that the traditional voltage source based on the droop characteristic can only be started in series in parallel. The network control based on weak communication used in the present invention can perform black start in parallel, thereby speeding up the startup time of the entire black start;

[0038] Furthermore, the contradiction between the recovery time and oscillation in the traditional fixed coefficient frequency modulation is overcome. The variable coefficient control of the secondary frequency modulation link divides the secondary frequency modulation process into two areas. When the product of the frequency difference and the frequency change rate is greater than or equal to zero, the variable integral coefficient of underdamping shortens the system recovery time. When the product of the frequency difference and the frequency change rate is less than zero, the fixed integral coefficient of underdamping shortens the frequency-based oscillation and ensures the stability of the system.

[0039] The communication program framework of weak communication is reasonably designed, and the data involved in the communication is very small, which can greatly reduce the communication load rate and ensure the control performance of the network energy storage system. In addition, due to the small amount of communication, the communication baud rate can be set relatively low, thereby ensuring the stable transmission of data in long-distance communication conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art are briefly introduced below. Obviously, the drawings in the following description are only embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on the provided drawings without creative work:

[0041] Figure 1 It is an architecture diagram of a string-type grid-connected energy storage converter system to which the method of the present invention is applied;

[0042] Figure 2 It is a schematic diagram of a control method for a string-type grid-connected energy storage converter;

[0043] Figure 3 The method of the present invention is applied to a string-type grid-type energy storage converter system to realize a parallel black start process;

[0044] Figure 4 It is a schematic diagram of the control model corresponding to the string-type grid-connected energy storage converter system;

[0045] Figure 5 It is a schematic diagram of the control model of the host;

[0046] Figure 6 This is a schematic diagram of the control model of the slave machine;

[0047] Figure 7 A schematic diagram of weak communication data interaction in a string-type grid-connected energy storage converter system to which the method of the present invention is applied;

[0048] Figure 8 It is a schematic diagram of the secondary frequency modulation coefficient of the energy storage converter of the present invention. DETAILED DESCRIPTION

[0049] In order to facilitate the understanding of the present invention, the present invention will be described more fully below with reference to the relevant drawings. Typical embodiments of the present invention are shown in the drawings. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present invention more thorough and comprehensive. It should be understood that the embodiments of the present invention and the specific features in the embodiments are detailed descriptions of the technical solutions of the present application, rather than limitations on the technical solutions of the present application. In the absence of conflict, the embodiments of the present invention and the technical features in the embodiments can be combined with each other.

[0050] The method of the present invention is applicable to a string-type grid-type energy storage converter system. Therefore, before introducing the method of the present invention, Figure 1 A string-type grid-connected energy storage converter system is described.

[0051] refer to Figure 1 The string-type grid-type energy storage inverter system includes an EMS and multiple energy storage inverters connected to the EMS through communication lines. Multiple refers to two or more. Specifically illustrated in the attached figure are n energy storage inverters, which are respectively denoted as PCS#1, PCS#2, ..., PCS#n. Each energy storage inverter includes a battery and a DCAC module. The side of the DCAC module connected to the battery is the battery side, and the other side is the AC side. The AC sides of all DCAC modules and the loads are connected to the same AC bus. The load can be a generator set, a wind turbine set, a photovoltaic power generation unit, etc.

[0052] It should be noted that in a string-type network system, there is one master and the rest are slaves. When the master is offline, another master will be automatically generated. This master-slave competition control method is now very mature, and the specific algorithm will not be described here. Figure 1 The master and slave machines use the CAN local area network for parallel communication. The control method of the present invention has no strict requirements on the communication speed, so there is no strict restriction on the length of the physical connection of the network cable between the string-type grid-type energy storage converters, and in theory, it will not affect the number of parallel machines. In particular, this communication method that is not restricted by the communication distance and communication rate is called weak communication in this article.

[0053] like Figure 7 As shown, in a specific embodiment of the present invention, the baud rate between the host and the slave is 500kbps. The main contents of the communication are: status flags / instructions, secondary frequency modulation, secondary voltage regulation output value and SOC value, etc. For detailed data interaction, refer to Figure 7In addition to the status information, the data sent by the slave to the host only includes the SOC value (State of Charge, which usually indicates the battery charging state, that is, the ratio of the remaining battery capacity to its maximum capacity, usually expressed as a percentage). In addition to the command information, the data sent by the host to the slave only includes the secondary frequency modulation loop output value P sec , output value of secondary voltage regulating ring Q sec , SOC mean Therefore, this embodiment only uses "host fast broadcast frame (2ms / frame)", "host slow broadcast frame (50ms / frame)" and "slave slow upload frame (50ms / frame)" in communication. The specific data transmitted by each frame is as follows: Figure 7 , I will not go into details here.

[0054] contrast Figure 5 and Figure 6 Compared with the energy storage converter as a slave, the energy storage converter as the host needs to perform a secondary frequency modulation loop output value P sec , output value of secondary voltage regulating ring Q sec , SOC mean Calculation.

[0055] Let's combine Figure 5-7 , indicating the additional computing tasks of the host, combined with Figure 2 , Figure 4 Describe the control process shared by the master and slave.

[0056] refer to Figure 5 , the secondary frequency modulation loop output value P sec The calculation process includes:

[0057] S11: The host obtains the sampled AC output frequency, subtracts the output frequency from the rated frequency to obtain the frequency difference Δf, and calculates the frequency change rate of the output frequency

[0058] It should be noted that the frequency mentioned in this article can refer to the usual definition of frequency, or other parameters that can be obtained by simple conversion, such as angular velocity. In this embodiment, this is the case, because the actual sampling is usually angular velocity, and the frequency can be calculated based on the angular velocity, so the attached Figure 4 The figure shows the angular velocity ω of the sampled output side and the rated angular velocity ω n Based on ω=2πf, the corresponding output frequency can be calculated as The rated frequency is Therefore, the frequency difference It refers to the frequency change rate of the output side frequency. Assuming that the output side angular velocity at the current moment is represented by ω(t), and the output side frequency at the previous moment is represented by ω(t-1), then is the output frequency at the current moment The output frequency at the previous moment The difference between the current moment and the previous moment is Δt, that is,

[0059] S12: If the frequency difference Δf and the frequency change rate The product of is greater than or equal to zero, that is Then the integral coefficient Ki matches the variable integral coefficient Kil in the first underdamping ratio range. The calculation formula of the variable integral coefficient Kil is otherwise, When the integral coefficient Ki matches the fixed integral coefficient Kio of the second underdamping ratio range;

[0060] Among them, ka is the adjustment coefficient, which is used to adjust the influence of the frequency change rate, K ilm is the lower limit of the variable integral coefficient. When it reaches the extreme point, that is, When K il =K ilm .

[0061] The second underdamping ratio range is greater than the first underdamping ratio range. For example, in this embodiment, the first underdamping ratio range is 0.1-0.3, and the second underdamping ratio range is 0.6-0.8.

[0062] like Figure 8 , It means the state before the frequency drops / rises to the extreme point. At this time, Ki adopts the variable integral coefficient Kil to suppress the excessive frequency difference and shorten the frequency adjustment time. It means the state after the frequency drops / rises to the extreme point. At this time, Ki uses a fixed integral coefficient Kio to ensure smooth convergence of the system, avoid frequency oscillation, and ensure system stability.

[0063] S13: Send the frequency difference Δf and the matched integral coefficient Ki to the first closed-loop feedback controller, and divide the output of the first closed-loop feedback controller according to the number of the energy storage converters to obtain the secondary frequency modulation loop output value P sec .

[0064] like Figure 5In the figure, the first closed-loop feedback controller, i.e. the variable coefficient PI shown in the figure, is a well-matched PI controller. The integral coefficient Ki of the PI controller is determined by the above step S12, so it is variable and not fixed. The output of the PI controller is divided by the number n of energy storage converters to obtain the secondary frequency modulation loop output value P sec The host calculates the secondary frequency modulation loop output value P sec After that, it will be sent to the slave through weak communication, that is, each converter will get the secondary frequency modulation loop output value P sec .

[0065] It should be noted that in this step, the angular velocity does not need to be actually converted into frequency, but the difference between the angular velocity and the rated angular velocity and the rate of change of the angular velocity are directly used for variable coefficient matching and as input for PI control.

[0066] refer to Figure 5 , the output value of the secondary voltage regulating loop is Q sec The calculation process includes:

[0067] S21: The host obtains the AC output voltage U sampled by itself out , the output voltage U out With given voltage U ref The voltage difference U ref -U out ;

[0068] S22: The voltage difference U ref -U out The output of the second closed-loop feedback controller is sent to the second closed-loop feedback controller, and the output of the second closed-loop feedback controller is evenly divided according to the number of the energy storage converters to obtain the output value Q of the secondary voltage regulation loop. sec .

[0069] The second closed-loop feedback controller is a matched PI controller whose input is U ref -U out , its output divided by n can get the secondary voltage regulating ring output value Q sec , the host calculates the output value Q of the secondary voltage regulating ring sec Then it is sent to the slave through weak communication, that is, each converter gets the output value Q of the secondary voltage regulation loop sec .

[0070] refer to Figure 7 , the SOC mean The calculation process includes: the host accumulates its own SOC value and the SOC values ​​uploaded by each slave, and then divides them equally according to the number of the energy storage converters to obtain the SOC average value. like Figure 6 , Ssoc，i (t) represents the SOC value uploaded by the i-th energy storage converter in real time. The host calculates the average SOC value After that, it will be sent to the slave through weak communication, that is, each converter will get the SOC average value

[0071] Combine the following Figure 2 , introduces the contents that need to be executed by both the host and the slave in the string-type grid-type energy storage converter control method of the present invention. For both the host and the slave, the following steps need to be executed:

[0072] S100: The energy storage converter obtains the secondary frequency modulation loop output value P sec , output value of secondary voltage regulating ring Q sec , SOC mean Primary frequency modulation loop output value P one ;

[0073] Among them, the output value of the primary frequency modulation loop is P one The acquisition process is shown in the figure. Figure 5-6 As shown, it mainly includes: the energy storage converter obtains the AC output side frequency sampled by itself, subtracts it from the rated frequency and multiplies it by the frequency conversion coefficient K fd Get the output value P of the primary frequency modulation loop one Here, the acquisition of the AC output side frequency can refer to the explanation in part S11. Whether it is the master or the slave, the AC output side frequency can be calculated by sampling the angular velocity ω of its own AC output side. The rated frequency can be obtained by the rated angular velocity ω n Calculated.

[0074] Among them, the energy storage converter obtains the secondary frequency modulation loop output value P sec , output value of secondary voltage regulating ring Q sec , SOC mean The acquisition process mainly includes: if the energy storage converter is the host, then the secondary frequency modulation loop output value P is calculated locally sec , output value of secondary voltage regulating ring Q sec , SOC mean And send it to the slave through weak communication; if the energy storage converter is a slave, the secondary frequency modulation loop output value P is directly obtained from the host through weak communication sec , output value of secondary voltage regulating ring Q sec , SOC mean As mentioned above, the slave uploads its own SOC value through weak communication, and the master calculates and shares P sec , Q sec , To the slave.

[0075] refer to Figure 3 The master and slave both include SOC balancing control module, power calculation module, VSG active frequency control module, VSG reactive voltage control module, voltage and current double closed-loop control module and PWM modulation module. Figure 4-5 The SOC balance control module specifically includes a balance control loop. The VSG active frequency control module of the host includes a primary frequency modulation loop, a secondary frequency modulation loop, and a frequency control loop. The slave can obtain the secondary frequency modulation loop output value P sent by the host through weak communication. sec Therefore, the slave VSG active frequency control module has no secondary frequency modulation loop, only a primary frequency modulation loop and a frequency control loop, but because P sec The VSG reactive voltage control module of the host includes a primary voltage regulation loop and a secondary voltage regulation loop. The slave can obtain the secondary voltage regulation loop output value Q sent by the host through weak communication. sec Therefore, the VSG reactive voltage control module of the slave machine has no secondary voltage regulation loop, only a primary voltage regulation loop, but because Q sec Involved in the control, so it also introduces secondary voltage regulation.

[0076] In this embodiment, the host and the slave obtain the secondary frequency modulation loop output value P sec After that, the following constraint processing is required: If the SOC value S soc Less than or equal to the minimum SOC limit S limit , then the secondary frequency modulation loop output value P sec Forced to 0, that is, S soc ≤S limit , then P sec =0; otherwise the secondary frequency modulation loop output value P sec Remain unchanged, that is, S soc >S limit , then P sec =P sec .

[0077] S200: The SOC average As the feedback of the SOC balancing loop, the SOC balancing loop output value P is obtained. soc ;

[0078] refer to Figure 5-6 This step specifically includes: taking its own SOC value as a given value and taking the SOC mean value as a given value. The SOC difference is fed into the third closed-loop feedback controller to obtain the SOC balance loop output value P soc , the third closed-loop feedback controller is specifically a matched P controller.

[0079] In this embodiment, both the host and the slave output the value P of the SOC balancing loop. soc Make the following two constraints:

[0080] The first constraint range is -K soc *P n ≤P soc ≤K soc *P n , K soc is the coefficient of SOC balanced adjustable power, P n is the rated power;

[0081] The second constraint range is -P limit ≤P soc ≤P limit , P limit =2P n -P one -P sec .

[0082] Among them, P limit Greater than K soc *P n .

[0083] S300: Obtaining the AC output side voltage U sampled by itself out and current I out , and based on the output voltage U out and current I out Calculate the active power P out and reactive power Q out ;

[0084] refer to Figure 4 The dotted box part belongs to the internal structure of the converter, and the AC output voltage U out and current I out The sampling points are actually the nodes where each converter is connected to the load, such as Figure 4 As shown in Uoabc and Ioabc.

[0085] S400: The calculated active power P out As the feedback value of the VSG active frequency control loop, the secondary frequency modulation loop output value P sec , SOC balance loop output value P soc The output value P of the primary frequency modulation loop one Superposition as active given P ref , the reference value of frequency is obtained through the VSG active frequency control loop;

[0086] refer to Figure 5-6 , P ref =P sec +P soc +Pone .

[0087] Among them, the VSG active frequency control loop mainly realizes the following control process: the active given P ref Subtract the feedback value P out The output of the differential controller D is then subtracted and the result is sent to the rotor motion module. The output of the rotor motion module is sent to the differential controller D on the one hand and compared with the rated angular velocity ω on the other hand. n After superposition, the reference value W of the angular velocity is obtained by an integrator. Although step S400 refers to obtaining a reference value of the frequency, referring to the explanation of step S11, the frequency here is not necessarily the usual definition of frequency in the strict sense, but can also be other parameters that can be converted to obtain the frequency, such as Figure 5-6 In the specific embodiment, the final value obtained is actually the reference value W of the angular velocity. Since the angular velocity can be used to obtain the frequency through simple calculation, the reference value W of the angular velocity can be regarded as the reference value P of the frequency. ref , there is no need to convert angular velocity into frequency.

[0088] S500: The calculated reactive power Q out As the feedback value of the primary pressure regulating loop, the output value Q of the secondary pressure regulating loop is sc As reactive power given Q sec , the reference value of the VSG reactive amplitude control loop is obtained through the primary voltage regulation loop, and the voltage reference value U is obtained through the VSG reactive amplitude control loop;

[0089] The reference value of the VSG reactive amplitude control loop is obtained through the primary voltage regulating loop, specifically including: the reactive given value Q of the primary voltage regulating loop sent to sec With feedback value Q out Take the difference and multiply it by the voltage conversion coefficient K ud Obtain the reference value of the VSG reactive amplitude control loop;

[0090] The voltage reference value obtained by the VSG reactive amplitude control loop specifically includes: adding the reference value of the VSG reactive amplitude control loop to the original voltage given value U ref The new voltage setting value is obtained; the local AC output side voltage U out The difference between the voltage and the new voltage setting value is sent to the fourth closed-loop feedback controller, that is, the input of the fourth closed-loop feedback controller is K ud *(Q sec -Q out )+U ref -U out ; The output of the fourth closed-loop feedback controller is compared with the original voltage given value U refAfter superposition, a voltage reference value U is obtained. In this embodiment, the fourth closed-loop feedback controller is specifically a PI controller.

[0091] S600: Input the obtained frequency reference value and voltage reference value U into the voltage-current double closed loop to obtain an output signal that follows the reference value, and generate a drive signal for controlling the switch tube of the energy storage converter based on the output signal.

[0092] Voltage and current double closed loop Figure 4 The voltage and current dual closed-loop control module mentioned in the reference includes a voltage outer loop and a current inner loop. Figure 5-6 , only the calculation process of the reference value of the voltage outer loop is shown. Specifically, after the frequency reference value W is obtained in S400 and the voltage reference value U is obtained in S500, the three-phase voltage reference value of the voltage outer loop can be obtained by three-phase synthesis. Specifically, the reference value W is first integrated to obtain the angle θ. Since there are three phases in total, the angle θ is specifically included. a ,θ b ,θ c Then the three-phase voltage is synthesized by the following calculation formula to obtain the three-phase voltage reference value U a , U b , U c :

[0093]

[0094] return Figure 4 The energy storage converter samples the three-phase inductor current Iabc on the output side of the DCAC module in the local converter as the feedback value of the current inner loop, and samples the three-phase voltage Uabc on the output capacitor C1 on the output side of the DCAC module as the feedback value of the voltage outer loop. The voltage outer loop obtains the three-phase voltage reference value U based on the collected feedback value Uabc and the three-phase voltage synthesis. a , U b , U c PI control is performed to obtain the reference value of the current inner loop. The current inner loop performs PI control based on the reference value output by the voltage outer loop and the sampled feedback value Iabc to obtain a sinusoidal signal without static error as the output signal. Then, the sinusoidal signal is modulated by PWM to obtain the drive signal. This drive signal can be used to control the on and off of the switch tube of the converter itself, and complete the secondary frequency modulation, secondary voltage regulation link and SOC balance control of the converter. At this time, the system active output and reactive output satisfy the following formula, where Pload and Qload are the active and reactive power of the load:

[0095]

[0096] It can be observed that the error of the system output frequency and output voltage can be adjusted through the control of the secondary frequency modulation and secondary voltage regulation links, and the SOC balance control output value Psoc is updated over time, and eventually the SOC of each module will tend to be equal.

[0097] In addition, in view of the unpredictability of power grid failures, the grid-connected energy storage converter needs to be able to quickly rebuild the power supply in the event of a system failure or catastrophic event. Therefore, it is necessary to provide a suitable black start solution and black start capacity to improve the recovery capability of the microgrid. Since the method of this embodiment is based on the grid control of weak communication, it provides a basis for parallel black start. Therefore, on this basis, the embodiment of the present invention further proposes to perform parallel black start control when the system is started. Specifically, refer to Figure 2 The dashed lines on both sides of the figure represent the steps performed by the host and the slave respectively. The black start process is as follows:

[0098] S30: The energy management system (EMS) issues a black start command;

[0099] S31: The energy storage converter receives a black start command issued by the energy management system;

[0100] S32: The slave sends its own online status information to the host, and the host collects the online number of the energy storage converter, determines whether the total number of the system is consistent with the number of online modules (modules are energy storage converters), and if they are consistent, performs battery side soft start and broadcasts battery side soft start instructions;

[0101] S33: The slave receives the battery side soft start instruction broadcast by the host and performs battery side soft start, sends the battery test soft start status to the host, and both the host and the slave perform a delay wait after performing the battery side soft start. The purpose of the waiting delay is to synchronize the start command due to the timing difference received by the string-type grid-type energy storage inverter after the EMS sends the command.

[0102] S34: The host checks whether the total number of the system is consistent with the number of successful soft start of the module battery. If so, it determines that all the energy storage converters have completed the battery side soft start;

[0103] S35: The host sends a power frequency synchronization signal, and the slave receives the power frequency synchronization signal for phase synchronization, which specifically includes: the host pulls up the pulse in the pulse signal line based on the power frequency information generated by its own local oscillator at the time when the power frequency passes through zero, so that the rising edge of the pulse coincides with the power frequency zero point, thereby sending the power frequency synchronization information. The slaves capture the pulses in the pulse signal line respectively, adjust their own phase synchronously, and send a soft start success flag after the adjustment is completed, thereby completing the power frequency synchronization between themselves and the host.

[0104] S36: After the host receives the soft start success flag bits sent by all the energy storage converters, broadcast an AC switch instruction allowing the slave to close;

[0105] S37: The energy storage converter closes the AC side switch after receiving the AC switch instruction.

[0106] Since the phase synchronization in S35 is completed, the phase angles of the string-type grid-type energy storage converters are consistent, so there will be no large impact current when the AC side switch is closed. After the AC side switch is closed, each unit of the string-type grid-type energy storage converter no longer uses the power frequency to synchronize its own phase, but uses the droop characteristics of the primary frequency modulation and primary voltage regulation of the VSG mentioned above to balance the active and reactive currents. At this point, the parallel black start of the entire system is completed.

[0107] It can be seen that the control method of the string-type grid-type energy storage converter of the embodiment of the present invention improves the traditional VSG algorithm, and adds a primary frequency modulation and a primary voltage regulation link to the VSG power outer loop, so that each converter unit meets the stable current sharing based on the droop characteristic in the isolated grid voltage source mode. This embodiment also designs a parallel black start control scheme based on weak communication and a bidirectional constraint control scheme combining secondary frequency modulation and SOC balance. In general, this embodiment has the following beneficial effects:

[0108] (1) Primary frequency modulation and primary voltage regulation are added to the traditional VSG power outer loop, so that the string-type grid-connected converter can meet the stable current sharing of the droop characteristics in the isolated grid voltage source mode. Compared with the traditional voltage source parallel connection based on communication current sharing, this method does not contain current sharing information in the communication, so the communication exchange information of each converter unit does not need to be too frequent, thereby reducing the burden of the communication load rate. In addition, the communication baud rate can be set relatively low, so the communication line can be pulled long enough to meet the distributed characteristics of the string-type grid-connected converter.

[0109] (2) In weak communication, the power frequency synchronization IO pulse signal line is used for power frequency synchronization, so that each group of string networking units synchronizes the phase between the black start closing side switches in advance, and adds a reasonable state interaction startup logic, thereby reducing the transient current of each unit at the moment of closing the switch, solving the disadvantage that the traditional voltage source based on the droop characteristic can only be started in series in parallel (the serial black start scheme is to start with a black start power supply, establish the grid voltage frequency, and then start the remaining power supplies one by one, and then network, and the microgrid is restored in series). The network control based on weak communication used in the present invention can be black started in parallel, thereby speeding up the startup time of the entire black start.

[0110] (3) Based on weak communication, a grid-connected control algorithm is provided for VSG secondary frequency regulation, secondary voltage regulation, and SOC balancing control. When large frequency and voltage fluctuations occur, the string-type grid-connected energy storage system can independently solve the voltage and frequency stability problems and implement a self-recovery control method for secondary regulation, so that the system frequency and output voltage can be dynamically adjusted in real time, eliminating the threat to the safe operation of the system.

[0111] (4) Variable coefficient control method for secondary frequency modulation: In view of the contradiction between recovery time and oscillation in the traditional fixed coefficient secondary frequency modulation, a control algorithm with variable integral coefficient is proposed. The value of divides the secondary frequency modulation process into two regions. The variable integral coefficient Kil of underdamping shortens the system recovery time. When the integral coefficient Ki is shortened with a fixed damping ratio, frequency oscillation is avoided and the stability of the system is ensured.

[0112] (5) The constraints of SOC balance control and secondary frequency modulation can avoid the simultaneous action of SOC and secondary frequency modulation, and effectively control the problems of overcharging and discharging caused by SOC differences between string energy storage converter units and the secondary frequency modulation. That is, the string energy storage converter is subject to the double constraints of its own SOC balance and secondary frequency modulation, which not only solves the problems of battery overcharging and discharging caused by SOC differences, but also effectively solves the priority problem of SOC balance and secondary frequency modulation, expanding the functionality of the system.

[0113] (6) The communication program framework of weak communication is reasonably designed. Only "host fast broadcast frame (2ms / frame)", "host slow broadcast frame (50ms / frame)" and "slave slow upload frame (50ms / frame)" are used in the communication to realize all the above functions. This program framework can greatly reduce the CAN communication load rate and ensure the control performance of the networked energy storage system. In addition, due to the small amount of communication, the communication baud rate can be set relatively low, thereby ensuring the stable transmission of data in long-distance communication.

[0114] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0115] Terms including ordinal numbers such as "first" and "second" used in this specification may be used to describe various components, but these components are not limited by these terms. The purpose of using these terms is only to distinguish one component from other components. For example, without departing from the scope of the present invention, the first component may be named as the second component, and similarly, the second component may also be named as the first component.

[0116] A person of ordinary skill in the art can understand that all or part of the processes in the above-mentioned embodiment methods can be implemented by instructing related hardware through a computer program. The program can be stored in a computer-readable storage medium. When the program is executed, it can include the processes of the embodiments of the above-mentioned methods.

[0117] The embodiments of the present invention are described above in conjunction with the accompanying drawings, but the present invention is not limited to the above-mentioned specific implementation modes, which are merely illustrative rather than restrictive. Under the guidance of the present invention, ordinary technicians in this field can also make many forms without departing from the scope of protection of the present invention and the claims, all of which are within the protection of the present invention.

Claims

1. A control method for a string-type grid-type energy storage converter, wherein the string-type grid-type energy storage converter comprises a plurality of energy storage converters, characterized in that: The method comprises: The energy storage converter uses the SOC mean value as feedback of the SOC balancing loop to obtain an SOC balancing loop output value; The output value of the secondary frequency modulation loop, the output value of the SOC balancing loop and the output value of the primary frequency modulation loop are superimposed as the active power given value, and the reference value of the frequency is obtained through the VSG active frequency control loop; the output value of the secondary voltage regulation loop is used as the reactive power given value, and the reference value of the VSG reactive amplitude control loop is obtained through the primary voltage regulation loop, and the reference value of the voltage is obtained through the VSG reactive amplitude control loop; The obtained frequency reference value and voltage reference value are input into the voltage-current double closed loop to obtain an output signal that follows the reference value, and a drive signal for controlling the switch tube of the energy storage converter is generated based on the output signal.

2. The control method of a string grid-type energy storage converter according to claim 1, characterized in that: The method also includes: before superimposing the secondary frequency modulation loop output value, the SOC balance loop output value and the primary frequency modulation loop output value, the secondary frequency modulation loop output value is subjected to the following constraint processing: if the SOC value itself is less than or equal to the SOC minimum limit value, the secondary frequency modulation loop output value is forced to be set to ⊙, otherwise the secondary frequency modulation loop output value remains unchanged.

3. The control method of a string grid-type energy storage converter according to claim 1, characterized in that: The method further includes: before superimposing the secondary frequency modulation loop output value, the SOC balancing loop output value and the primary frequency modulation loop output value, performing the following two constraints on the SOC balancing loop output value: The first constraint range is -K soc *P n ≤P soc ≤K soc *P n , P soc is the SOC balance loop output value, K soc is the coefficient of SOC balanced adjustable power, P n is the rated power; The second constraint range is -P limit ≤P soc ≤P limit , P limit =2P n -P one -P sec .

4. The control method of a string grid-type energy storage converter according to claim 1, characterized in that: Multiple energy storage converters are divided into a master and multiple slaves, and the slaves upload their own SOC values ​​through weak communication; The method also includes: if the energy storage inverter is a host, the secondary frequency modulation loop output value, the secondary voltage regulation loop output value, and the SOC average are calculated locally, and sent to the slave through weak communication; if the energy storage inverter is a slave, the secondary frequency modulation loop output value, the secondary voltage regulation loop output value, and the SOC average are directly obtained from the host through weak communication.

5. The control method of a string grid-type energy storage converter according to claim 4, characterized in that: The calculation process of the secondary frequency modulation loop output value includes: The host obtains the sampled AC output side frequency, subtracts the output side frequency from the rated frequency to obtain the frequency difference, and calculates the frequency change rate of the output side frequency; If the product of the frequency difference and the frequency change rate is greater than or equal to zero, the integral coefficient matches the variable integral coefficient of the first underdamping ratio range, otherwise the integral coefficient matches the fixed integral coefficient of the second underdamping ratio range, wherein the second underdamping ratio range is greater than the first underdamping ratio range; The frequency difference and the matched integral coefficient are sent to the first closed-loop feedback controller, and the output of the first closed-loop feedback controller is evenly divided according to the number of the energy storage converters to obtain the secondary frequency modulation loop output value.

6. The control method of a string grid-type energy storage converter according to claim 4, characterized in that: The calculation process of the output value of the secondary voltage regulating ring includes: The host obtains the AC output side voltage sampled by itself, and obtains the voltage difference by subtracting the output side voltage from the given voltage; The voltage difference is sent to a second closed-loop feedback controller, and the output of the second closed-loop feedback controller is evenly divided according to the number of the energy storage converters to obtain the secondary voltage regulation loop output value.

7. The control method of a string grid-type energy storage converter according to claim 1, characterized in that: The method of using the SOC mean value as feedback of the SOC balancing loop to obtain the SOC balancing loop output value specifically includes: taking the SOC value of the self as a given value and taking the SOC mean value as feedback to perform subtraction to obtain the SOC difference value; and sending the SOC difference value to a third closed-loop feedback controller to obtain the SOC balancing loop output value.

8. The control method of a string grid-type energy storage converter according to claim 1, characterized in that: The process of obtaining the output value of the primary frequency modulation loop includes: the energy storage converter obtains the AC output side frequency sampled by itself, subtracts it from the rated frequency and multiplies it by the frequency conversion coefficient to obtain the output value of the primary frequency modulation loop.

9. The control method of a string grid-type energy storage converter according to claim 1, characterized in that: The method further includes: the energy storage converter obtains the AC output side voltage and current sampled by itself, and calculates the active power and reactive power based on the output side voltage and current; uses the calculated active power as the feedback value of the VSG active frequency control loop; uses the calculated reactive power as the feedback value of the primary voltage regulation loop; The method of obtaining the reference value of the VSG reactive amplitude control loop through the primary voltage regulating loop specifically includes: subtracting the reactive reference value of the primary voltage regulating loop from the feedback value and multiplying the difference by the voltage conversion coefficient to obtain the reference value of the VSG reactive amplitude control loop; The voltage reference value obtained through the VSG reactive amplitude control loop specifically includes: superimposing the reference value of the VSG reactive amplitude control loop on the original voltage set value to obtain a new voltage set value; subtracting the local AC output side voltage from the new voltage set value and sending the result to the fourth closed-loop feedback controller; and superimposing the output of the fourth closed-loop feedback controller with the original voltage set value to obtain the voltage reference value.

10. The control method of a string grid-type energy storage converter according to claim 1, characterized in that: The multiple energy storage converters are divided into a host and multiple slaves, and the host and the slaves transmit power frequency synchronization signals through a pulse signal line. The method also includes: The energy storage converter receives a black start command issued by the energy management system; The host collects the online number of the energy storage converters, determines whether the total number of the system is consistent with the online number, and if they are consistent, performs a battery side soft start and broadcasts a battery side soft start instruction; The slave receives the battery side soft start command broadcast by the host and performs battery side soft start, and then waits for a delay; The host checks that all the energy storage converters complete the soft start on the battery side and then sends out the power frequency synchronization signal, and the slave receives the power frequency synchronization signal to perform phase synchronization, specifically including: the host pulls up the pulse in the pulse signal line based on the power frequency information generated by its own local oscillator at the time when the power frequency passes through zero, so that the rising edge of the pulse is consistent with the power frequency zero point, and the slave captures the pulse in the pulse signal line respectively, adjusts its own phase synchronously, and sends a soft start success flag after the adjustment is completed; After the host receives the soft start success flag bits sent by all the energy storage converters, it broadcasts an AC switch instruction allowing the slave to close; The energy storage converter closes the AC side switch after receiving the AC switch instruction.

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