A droop control method for multi-inverter asynchronous access

Abstract

The application discloses a droop control method for multi-converter asynchronous access, and relates to the technical field of electric power. Firstly, aiming at the requirement of equalizing the output currents of converters under the condition of no communication, rules that should be followed by parameter settings of the converters are obtained through analysis. Then, based on the rule that the no-load voltage parameters of the converters must be dynamically identical, a specific no-load voltage parameter correction mode is designed, and the improved droop control method is based on the mode. The improved droop control method can not only control the load voltage to be always the rated value, but also equalize the output currents of the converters, and the whole process does not need communication, and the improved droop control method also has certain control redundancy.

Description

Technical Field

[0001] The present invention relates to the field of electric power technology, and in particular to a droop control method for asynchronously accessing multiple converters. Background Art

[0002] A DC microgrid comprises distributed generation systems, energy storage systems, and loads. It can connect to the external AC main grid via a DC-AC converter, but can also operate in an islanded state, disconnected from the main grid, for example, following a main grid outage. To maintain constant voltage in an islanded state, DC microgrids typically employ either master-slave or peer-to-peer control. Under master-slave control, the converter in the energy storage system typically serves as the master converter, employing constant voltage control. This means its terminal voltage (i.e., the island voltage) is controlled to a rated value. The other converters serve as slave converters, with their output power referenced to the island voltage. Under peer-to-peer control, all converters have equal power and typically employ droop control, operating in voltage source mode. Master-slave control requires a communication network to ensure a master converter in the microgrid, while peer-to-peer control does not. Therefore, droop control is more cost-effective and offers higher reliability in an islanded state.

[0003] Under certain conditions, droop control can ensure that each converter outputs current proportional to its rated current (power), achieving balanced output current without requiring communication. This is a major advantage of droop control. However, conventional droop control suffers from voltage static error, meaning that the stabilized island voltage (converter-terminal voltage or load-terminal voltage) deviates from its rated value. Numerous improved droop control methods have been proposed to address this issue. However, if the converters are connected at different times, these improved methods can disrupt the conditions for balanced converter output current. Consequently, while voltage static error is eliminated, balanced converter output current cannot be guaranteed. Summary of the Invention

[0004] In response to the deficiencies in the prior art, the present invention provides a droop control method for asynchronously connected multiple converters. This method is based on the regulation of the no-load voltage u0. By improving the regulation method, it eliminates the static voltage difference at the load end while ensuring the balance of the output current of all converters.

[0005] To achieve the above purpose, the present invention adopts the following technical methods:

[0006] A droop control method for asynchronously connecting multiple converters is characterized by comprising:

[0007] After the converter is started, the no-load voltage u0 is continuously adjusted as follows:

[0008] Step 1: Set the no-load voltage u0 of the converter to its initial value u 00 ;

[0009] Step 2: After the load voltage of the DC microgrid reaches a steady state, determine whether the load voltage is within an allowable deviation range of a rated value;

[0010] If yes, continue to monitor the magnitude of the load voltage, and when the magnitude of the load voltage exceeds the allowable deviation range of the rated value, go back to step 1;

[0011] If not, then correct u0, wait for the load voltage to reach a steady state, then monitor the magnitude of the load voltage, and when the magnitude of the load voltage exceeds the allowable deviation range of the rated value, go back to step 1.

[0012] The droop control method for the asynchronous access of multiple converters as described above further uses the following droop control formula to control the converter terminal voltage

[0013] u j =u0-r j i j (1)

[0014] Among them, i j 、u j They represent the output current and voltage of converter j, r j is the droop coefficient of converter j, which is a positive number, and u0 is the no-load voltage of the converter.

[0015] As described above, the droop control method for the simultaneous access of multiple converters, furthermore, all converters agree on the same u 00 and I jN (r j +R linej ) value, where I jN represents the rated current of converter j, R linej is the transmission line impedance corresponding to converter j; each converter is based on the agreed I jN (r j +R linej ) value, its own I jN and the R measured by itself linej Set its j .

[0016] The droop control method for asynchronously connecting multiple converters as described above further modifies u0 according to the following relationship:

[0017] u 0j =u 00 U N / [u 00 -i j (r j +R linej )] (2)

[0018] Among them, u 0j represents u0, u of converter j 00 Indicates the initial value of u0, U N is the rated voltage at the load end.

[0019] The droop control method for asynchronously connecting multiple converters as described above can further ensure that the output current and power of each converter can be kept balanced regardless of when the converter is connected, and the load voltage can be controlled to its rated value.

[0020] The droop control method for asynchronously connecting multiple converters as described above further eliminates the need for communication between the converters regardless of when the converters are connected.

[0021] Compared with the prior art, the present invention has the following advantages: Based on the regulation of the no-load voltage u0 in the droop control formula, the present invention improves the regulation method to eliminate the static voltage difference at the load end while ensuring the balance of the output current of all converters. That is, it can meet the following three requirements:

[0022] 1) The output current (power) of each converter is balanced, regardless of when it is connected;

[0023] 2) Ability to control the load voltage to its rated value;

[0024] 3) No communication required. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical methods in the embodiments of the present invention, the following is a brief introduction to the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0026] Figure 1 Schematic diagram of a DC microgrid according to an embodiment of the present invention.

[0027] Figure 2 is the droop control curve of the embodiment of the present invention.

[0028] Figure 3 This is a circuit diagram of two converters connected in parallel according to an embodiment of the present invention.

[0029] Figure 4 This is the u0 correction curve of two converters connected at different times in the embodiment of the present invention.

[0030] Figure 5 4 is a flow chart of the converter u0 correction process according to an embodiment of the present invention. DETAILED DESCRIPTION

[0031] The following will be combined with the accompanying drawings in the embodiments of the present invention to clearly and completely describe the technical methods in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0032] Example:

[0033] In order to better demonstrate the technical concept of the present invention and clearly explain the improvements of the present invention, the technical difficulties faced by the present invention are now explained:

[0034] Figure 1 The following is a schematic diagram of a DC microgrid, where each converter is based on droop control, and the droop control characteristics are as follows: Figure 2 As shown, the voltage is controlled by the iu droop characteristic, as shown in formula (1).

[0035] Taking two converters connected successively as an example, according to formula (1), we can get Figure 3 As shown in the circuit diagram, R line1 、R line2 Respectively represent the line impedance corresponding to converter 1 and converter 2, u L is the load voltage, and it can be obtained from the figure

[0036] i1=(u0-u L ) / (r1+R line1 ) (3)

[0037] i2=(u0-u L ) / (r2+R line2 ) (4)

[0038] So, there is

[0039] i1(r1+R line1 )=i2(r2+R line2 ) (5)

[0040] Then, as long as the relationship shown in formula (6) is agreed upon, the output current balance can be ensured, that is, i1 / I 1N =i2 / I 2N .

[0041] I 1N (r1+R line1 )=I 2N (r2+R line2 ) (6)

[0042] Among them, I 1N , I 2Nare the rated currents of converter 1 and converter 2 respectively. line1 、R line2 ), the corresponding method has been given in the existing literature (Jiang Weiming, et al. Research on control strategy of independent DC microgrid with voltage self-recovery characteristics [J]. Power System Technology, 2020, 44(9): 3547-3555.).

[0043] According to the droop control theory, the initial output voltage of the newly connected converter is high, and the output current will gradually increase. Figure 2 If the droop curve shown in the figure is followed, the output voltage will gradually decrease. If the load remains unchanged, the current of the previously operating converter will decrease due to the increased current of the newly connected converter, and according to the droop curve, its output voltage will gradually increase. Therefore, the voltages of the newly connected converter and the already operating converter will gradually converge. However, after stabilization, neither the converter terminal voltage nor the load terminal voltage will necessarily reach the rated voltage, that is, there will be a voltage static error, resulting in poor voltage quality. This is a problem with conventional droop control.

[0044] Most of the existing improved droop control methods are devoted to eliminating the voltage static difference at the converter end, usually by adjusting u0 or r j However, these methods will lead to unbalanced output current. As for the method of adjusting u0, since there is no communication between converters, the newly connected converter cannot know the value of u0 of the previously running converter. It can only set u0 to the initial value. Therefore, u0 in equation (3) and equation (4) is actually different, which causes equation (5) to no longer hold, and thus there is no i1 / I 1N =i2 / I 2N ; For adjustment r j Similarly, the newly connected converter cannot know the r of the converter that has been running before. j Adjust to any value, it cannot ensure that equation (6) is valid, and there is no longer i1 / I 1N =i2 / I 2N .

[0045] In view of the above problems, the present invention proposes an improved droop control method for converters. The method is based on the regulation of the no-load voltage u0 in equation (1). By improving the regulation mode, the static voltage difference at the load end is eliminated while ensuring the balance of the output currents of all converters. That is, the method described in the present invention can meet the following three requirements:

[0046] 1) The output current (power) of each converter is balanced, regardless of when it is connected;

[0047] 2) Ability to control the load voltage to its rated value;

[0048] 3) No communication required.

[0049] According to equations (3) and (4), if u0 of each converter is different, output current balance cannot be ensured without communication. Therefore, each converter agrees to set the same u0. In this way, as long as I jN (r j +R linej ) are the same, that is, formula (6) holds, which can ensure the output current balance.

[0050] In summary, in order to balance the output current, each converter has the same u0 and I jN (r j +R linej ) value, in the absence of communication, the same initial u0 and I are agreed in practice. jN (r j +R linej ) value, the converter can set its own r j .

[0051] It's easy to see that the aforementioned identical u0 can be dynamically identical. That is, u0 can vary across converters, but as long as the changed u0 remains the same, output current balance is maintained. Therefore, when adjusting u0 to achieve zero voltage dropout, ensuring that the changed u0 values ​​are the same across converters ensures balanced output current. Based on this concept, the following describes a droop control method for achieving both output current balance and zero load voltage dropout.

[0052] Below, Converter 1 represents all operating converters, and Converter 2 represents a newly connected converter. After Converter 2 is connected, the output currents of the two converters will be unbalanced, as Converter 1's u0 may have been adjusted. However, Converter 2's connection will cause Converter 1's output current to decrease. If Converter 1 adjusts its u0 to its initial value after detecting the change in its output current, Converter 2 and Converter 1 will enter steady state with the same u0. The currents in steady state are necessarily balanced, but there will be a static voltage difference.

[0053] At this time, the droop control formula of each converter is as follows

[0054] u1=u0-r1i1

[0055] u2=u0-r2i2

[0056] Therefore, equation (5) is established. After that, in order to eliminate the voltage static difference, each converter corrects u0 according to the following equation. In order to facilitate distinction, u 0j represents u0, u of converter j 00 Indicates the initial value of u0

[0057] u 0j =u 00 U N / uL =u 00 U N / [u 00 -i j (r j +R linej )] (7)

[0058] Among them, U N is the rated voltage at the load end. The above formula shows that after correcting u0, we have

[0059] u 01 =u 02

[0060] Therefore, when the system stabilizes again, the output currents of the converters are balanced.

[0061] Since the current is balanced in both steady states before and after the correction of u0, the following formula can be obtained, where i L represents the load current, where the numerator and denominator are the values ​​of the same parameter before and after the correction u0.

[0062] i1 / i1′=i2 / i2′=(i1+i2) / (i1′+i2′)=i L / i L ′=u L / u L ′ (8)

[0063] In addition, according to formula (7), the following formula holds:

[0064] u 00 U N / u L -i j ′(r j +R linej )=u L '

[0065] Combining the above formula with formula (8) and considering the relationship shown in formula (7), we can get

[0066] u L ′=U N

[0067] That is, after correcting u0, the load voltage is controlled to the rated value in the steady state.

[0068] At this point, the above three requirements are met, indicating that the proposed improved droop control method effectively overcomes the output current imbalance and voltage static difference problems existing in the existing droop control method.

[0069] The entire u0 correction process mentioned above can be used Figure 4 express. Figure 4In it, the converter 1 is connected at the moment of 0, and then the voltage u is corrected after the voltage is stabilized at the moment of t1. 01 To make the voltage have no static error; the converter 2 is connected at the moment of t2, and then its output current and the output current of the converter 1 gradually change, and the converter voltage and the load voltage will also change; based on the improved droop control method proposed in the present invention, after the converter 1 detects the change in its output current, at the moment of t 21 u 01 is restored to the initial value, and u0 is corrected again after the voltage is stabilized at the moment of t3 to ensure that the voltage has no static error. Since u0 is equal after the moment of t 21 , the output currents of the converters have been balanced thereafter.

[0070] Regarding Figure 4 , there are two points to note:

[0071] (1) If the converter 2 is also connected before the voltage of the converter 1 is stabilized after the converter 1 is connected, that is, t2 < t1, they will tend to the steady state with the same u0, and the whole process is similar to the process after the moment of t 21 , so the output current can be balanced and the voltage has no static error.

[0072] (2) Each converter should correct u0 synchronously at the moment of t3, but under the condition of no communication, they are not actually synchronous. However, there is a transient process in the current change of the converter. According to formula (7), as long as this correction is generally synchronous, that is, the judgment of the voltage stability by each converter is generally synchronous, u0 is still approximately equal, and the output current is still generally balanced.

[0073] In summary, the key of the droop control method proposed in the present invention is the specific way of correcting u0. Therefore, the flowchart of correcting u0 in the converter is drawn as Figure 5 shown.

[0074] Since there are always slight fluctuations in the actual voltage at steady state, u0 is not adjusted when the static error of the load voltage is small. If the rated current of the newly connected converter is small, it may cause a small change in the load voltage and does not trigger the correction of u0, which in turn leads to unbalanced output current. However, in fact, Figure 5 shows that as long as the change in the converter current causes the load voltage to deviate beyond the limit, the correction of u0 can be triggered. And the events that can cause current changes include not only the connection of new converters, but also the disconnection of converters and the connection / disconnection of loads. Therefore, even if u0 of the converter is not corrected or the correction does not meet the expectation (such as the corrections of each converter are not synchronous as described above) in a certain event, as long as there is another event that causes a large current change, u0 can be corrected again. In other words, this droop control method has control redundancy.

[0075] In the description of this specification, the reference terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification and features of different embodiments or examples without contradiction.

[0076] The above embodiments are intended only to illustrate the technical concepts and features of the present invention. Their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. They are not intended to limit the scope of protection of the present invention. Any equivalent changes or modifications made based on the essence of the present invention are intended to be covered by the scope of protection of the present invention.

Claims

1. A droop control method for a DC microgrid with multiple converters connected asynchronously, characterized in that: After the newly connected converter starts, the no-load voltage u0 is continuously adjusted as follows: Step 1: Set the no-load voltage u0 of all running converters to the initial value u 00 ; Step 2: After the load voltage of the DC microgrid reaches a steady state, determine whether the load voltage is within an allowable deviation range of a rated value; If yes, continue to monitor the magnitude of the load voltage, and when the magnitude of the load voltage exceeds the allowable deviation range of the rated value, go back to step 1; If not, correct u0, wait for the load voltage to reach a steady state, then monitor the magnitude of the load voltage. When the magnitude of the load voltage exceeds the allowable deviation range of the rated value, go back to step 1; Among them, the following droop control formula is used to control the converter terminal voltage u j =u0-r j i j Among them, i j 、u j They represent the output current and voltage of converter j, r j is the droop coefficient of converter j, which is a positive number, and u0 is the no-load voltage of the converter; All transformers use the same u 00 and I jN (r j +R linej ) value, where I jN represents the rated current of converter j, R linej is the transmission line impedance corresponding to converter j; each converter is based on the agreed I jN (r j +R linej ) value, its own I jN and the R measured by itself linej Set its j ; Correct u0 according to the following relationship: u 0j =u 00 U N / [u 00 -i j (r j +R linej )] Among them, u 0j represents u0, u of converter j 00 Indicates the initial value of u0, U N is the rated voltage at the load end.

2. The droop control method for multi-converter asynchronous access to a DC microgrid according to claim 1, characterized in that: Regardless of when the converters are connected, the output current and power of each converter can be kept balanced, and the load voltage can be controlled to its rated value.

3. The droop control method for multi-converter asynchronous access to a DC microgrid according to claim 1, characterized in that: No matter when the converter is connected, there is no need for communication between the converters.

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