Power range acquisition method of multi-level converter based on half-bridge sub-module

Abstract

The invention discloses a half-bridge sub-module-based multi-level converter power range acquisition method, and belongs to the field of modular multi-level converters. The problems of low efficiency and poor accuracy when an application range is determined by an existing modular multi-level converter are solved. According to the invention, MATLAB software is adopted to carry out operation simulation on the multi-level converter under different active power conditions, and output voltage, output current, sub-module voltage peak values and stack energy corresponding to the multi-level converter under the active power and reactive power conditions are obtained; and judging whether the multi-level converter meets a safe operation condition or not in the operation process by utilizing the output voltage, the output current, the sub-module voltage peak value and the stack energy, and obtaining the final active power and reactive power range of the multi-level converter through the limitation of the safe operation condition. The method is suitable for determining the power range of the multi-level converter.

Description

Technical Field

[0001] The invention belongs to the field of modular multi-level converters. Background Art

[0002] Modular multilevel converter (MMC) is mainly used in flexible DC transmission system. It has the characteristics of modular design, multi-level structure and high voltage capacity. It is one of the converter types that can be used for VSC-HVDC power transmission. It uses bidirectional controllable power electronics as converter elements to control the switching state, such as insulated gate bipolar transistor (IGBT). MMC power conversion control theory can also apply step wave approximation. The basic control method of MMC is energy control. Further control methods of MMC can ensure the component life of MMC. There are four constraints in the operation PQ area of ​​MMC, among which the voltage should have a safe energy margin to ensure the safe operation of MMC. MMC is highly modular and has many advantages over VSC-HVDC, including less loss, higher output quality, and greater fault handling capability. However, the specific control details of MMC are complex, and when introducing restrictions, a large amount of calculation is required to infer the operation effect and specific scope of application, which has the problems of low efficiency and poor accuracy. Summary of the invention

[0003] The present invention aims to solve the problems of low efficiency and poor accuracy in determining the applicable range of an existing modular multi-level converter, and provides a method for obtaining the power range of a multi-level converter based on a half-bridge submodule.

[0004] The power range acquisition method of the multi-level converter based on the half-bridge sub-module of the present invention comprises:

[0005] Step 1: According to the model of the converter to be simulated, establish the equivalent circuit of the converter to be simulated; at the same time, set the initial value and step size of the active power and the power angle, and calculate the corresponding reactive power;

[0006] Step 2: Based on the equivalent circuit, MATLAB software is used to calculate the output voltage, output current, submodule voltage peak value and stack energy corresponding to the current active power and reactive power;

[0007] Step 3: judging whether the multi-level converter operates within a safe range by the output voltage, output current, submodule voltage peak value and stack energy; if so, recording the active power and reactive power corresponding to the previous iteration, increasing or decreasing the active power by one step according to the power angle, calculating the corresponding reactive power, and returning to step 2; otherwise, executing step 4;

[0008] Step 4: Increase the power angle by one step, calculate the reactive power at this time and the output voltage, output current, submodule voltage peak and stack energy corresponding to the current active power and reactive power, return to execute step 3 until the power angle is greater than or equal to 360 degrees, and use the active power and reactive power recorded in the previous iteration as the final maximum value of the active power and reactive power range.

[0009] Furthermore, in the present invention, in step three, the specific method of increasing or decreasing the active power by one step according to the size of the power angle is: when the active power angle is between 0-90° and between 270-360°, the active power increases by one step; when the active power angle is between 90-270°, the active power decreases by one step.

[0010] Furthermore, in the present invention, in step 1, the initial values ​​and step sizes of active power and power angle are: the initial value of active power is 0, the step size is 0.05, the initial value of power angle is 0°, and the step size is 0.5°.

[0011] Furthermore, in the present invention, in step 1, the formula for calculating the corresponding reactive power is:

[0012] Q=Ptanδ

[0013] Among them, Q is reactive power, P is active power, and δ is the power angle.

[0014] Furthermore, in the present invention, in step 3, the method for judging whether the multi-level converter operates within a safe range by the output voltage, output current, submodule voltage peak value and stack energy is:

[0015] Determine whether the real-time output voltage is greater than 0, if so, the multi-level converter operates within a safe range, otherwise, the multi-level converter does not operate within a safe range;

[0016] Determine whether the output current is less than the maximum stack current threshold Imax, if so, the multi-level converter operates within a safe range, otherwise, the multi-level converter does not operate within a safe range;

[0017] The maximum value of the real-time output voltage is divided by the number of submodules in the stack to obtain the maximum value of the real-time voltage of the stack submodule, and whether the maximum value of the real-time voltage of the stack submodule is less than the maximum threshold value of the submodule voltage is determined. If so, the multilevel converter operates within a safe range, otherwise, the multilevel converter does not operate within a safe range.

[0018] Calculate the real-time energy of the stack, use the real-time energy of the stack to calculate the real-time voltage allowable value of the sub-module in the stack, and determine whether the real-time output voltage of the sub-module is less than the real-time voltage allowable value. If so, the multi-level converter operates within a safe range, otherwise, the multi-level converter does not operate within a safe range.

[0019] Furthermore, in the present invention, the real-time output voltage is:

[0020]

[0021] Among them, V abc (t) is the total output voltage, V DCo is the DC side output voltage, When x is u, it indicates the AC output voltage of the upper arm, and when x is l, it indicates the AC output voltage of the lower arm. Voltage phase, ω e =2πf e , f e is the frequency of alternating current and t is the time.

[0022] Furthermore, in the present invention, the real-time output current is:

[0023]

[0024] Among them, I abc (t) is the output current, δ is the current phase, I DCo is the DC side output current, When x is u, it represents the AC output current of the upper arm, and when x is l, it represents the AC output current of the lower arm.

[0025] Furthermore, in the present invention, the method for calculating the real-time energy of the stack and using the real-time energy of the stack to calculate the real-time allowable voltage of the submodule in the stack is:

[0026] Calculate the stack energy:

[0027]

[0028] The relationship between stack energy and stack neutron module voltage is:

[0029]

[0030] Among them, E stack represents the stack energy, represents the initial energy of the stack, express The average value, E average represents the stack average energy of energy control, V SM Submodule allowable voltage, N represents the number of submodules, C SMIndicates the submodule capacitance value;

[0031] The formula for stack energy changing over time is:

[0032]

[0033] Among them, V SM (t) represents the submodule allowable voltage at time t, E stack (t) represents the stack energy at time t.

[0034] Furthermore, the present invention also includes a step of using MATLAB software to display the range of active power and reactive power.

[0035] The power range acquisition method of the multi-level converter of the present invention uses MATLAB software to simulate the operation of the multi-level converter under different active power conditions, and obtains the output voltage, output current, submodule voltage peak value and stack energy corresponding to the multi-level converter under different active power and reactive power conditions, and uses the output voltage, output current, submodule voltage peak value and stack energy to judge whether the multi-level converter meets the safe operation conditions during the operation process, and obtains the final range of the active power and reactive power of the multi-level converter through the definition of the safe operation conditions. The efficiency and accuracy of determining the power range of the multi-level converter are effectively improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 is a flow chart of the method of the present invention;

[0037] Figure 2 It is a diagram of the MMC topology described in a specific embodiment;

[0038] Figure 3 It is the MMC equivalent circuit diagram described in the specific embodiment;

[0039] Figure 4 It is an equivalent circuit diagram of the MMC after single-phase conversion described in the specific embodiment. DETAILED DESCRIPTION

[0040] The following will be combined with the accompanying drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work belong to the scope of protection of the present invention. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0041] Specific implementation method 1: refer to Figure 1Specifically describing this embodiment, the method for obtaining a power range of a multi-level converter based on a half-bridge sub-module described in this embodiment includes:

[0042] Step 1: According to the model of the converter to be simulated, establish the equivalent circuit of the converter to be simulated; at the same time, set the initial value and step size of the active power and the power angle, and calculate the corresponding reactive power;

[0043] Step 2: Based on the equivalent circuit, MATLAB software is used to calculate the output voltage, output current, submodule voltage peak value and stack energy corresponding to the current active power and reactive power;

[0044] Step 3: judging whether the multi-level converter operates within a safe range by the output voltage, output current, submodule voltage peak value and stack energy; if so, recording the active power and reactive power corresponding to the previous iteration, increasing or decreasing the active power by one step according to the power angle, calculating the corresponding reactive power, and returning to step 2; otherwise, executing step 4;

[0045] Step 4: Increase the power angle by one step, calculate the reactive power at this time and the output voltage, output current, submodule voltage peak and stack energy corresponding to the current active power and reactive power, return to execute step 3 until the power angle is greater than or equal to 360 degrees, and use the active power and reactive power recorded in the previous iteration as the final maximum value of the active power and reactive power range.

[0046] Furthermore, in the present invention, in step three, the specific method of increasing or decreasing the active power by one step according to the size of the power angle is: when the active power angle is between 0-90° and between 270-360°, the active power increases by one step; when the active power angle is between 90-270°, the active power decreases by one step.

[0047] Furthermore, in the present invention, in step 1, the initial values ​​and step sizes of active power and power angle are: the initial value of active power is 0, the step size is 0.05, the initial value of power angle is 0°, and the step size is 0.5°.

[0048] Further, in this implementation, in step 1, the formula for calculating the corresponding reactive power is:

[0049] Q=Ptanδ

[0050] Among them, Q is reactive power, P is active power, and δ is the power angle.

[0051] Further, in this embodiment, in step 3, the method for judging whether the multi-level converter operates within a safe range by the output voltage, output current, submodule voltage peak value and stack energy is:

[0052] Determine whether the real-time output voltage is greater than 0, if so, the multi-level converter operates within a safe range, otherwise, the multi-level converter does not operate within a safe range;

[0053] Determine whether the output current is less than the maximum stack current threshold Imax, if so, the multi-level converter operates within a safe range, otherwise, the multi-level converter does not operate within a safe range;

[0054] The maximum value of the real-time output voltage is divided by the number of submodules in the stack to obtain the maximum value of the real-time voltage of the stack submodule, and whether the maximum value of the real-time voltage of the stack submodule is less than the maximum threshold value of the submodule voltage is determined. If so, the multilevel converter operates within a safe range, otherwise, the multilevel converter does not operate within a safe range.

[0055] Calculate the real-time energy of the stack, use the real-time energy of the stack to calculate the real-time voltage allowable value of the sub-module in the stack, and determine whether the real-time output voltage of the sub-module is less than the real-time voltage allowable value. If so, the multi-level converter operates within a safe range, otherwise, the multi-level converter does not operate within a safe range.

[0056] If any of the conditions in this implementation is not met, the multi-level converter does not operate within the safe range; the power angle is readjusted until the power angle reaches 360 degrees, and the recorded active power and reactive power are used as the power range of the simulated multi-level converter.

[0057] Furthermore, in this implementation, the real-time output voltage is:

[0058]

[0059] Among them, V abc (t) is the total output voltage, V DCo is the DC side output voltage, When x is u, it indicates the AC output voltage of the upper arm, and when x is l, it indicates the AC output voltage of the lower arm. Voltage phase, ω e =2πf e , f e is the frequency of alternating current and t is the time.

[0060] Furthermore, in this embodiment, the real-time output current is:

[0061]

[0062] Among them, I abc (t) is the output current, δ is the current phase, I DCo is the DC side output current, When x is u, it represents the AC output current of the upper arm, and when x is l, it represents the AC output current of the lower arm.

[0063] Furthermore, in this embodiment, the method of calculating the real-time energy of the stack and using the real-time energy of the stack to calculate the real-time allowable voltage of the submodule in the stack is:

[0064] Calculate the stack energy:

[0065]

[0066] The relationship between stack energy and stack neutron module voltage is:

[0067]

[0068] Among them, E stack represents the stack energy, represents the initial energy of the stack, express The average value, E average represents the stack average energy of energy control, V SM Submodule allowable voltage, N represents the number of submodules, C SM Indicates the submodule capacitance value;

[0069] The formula for stack energy changing over time is:

[0070]

[0071] Among them, V SM (t) represents the submodule allowable voltage at time t, E stack (t) represents the stack energy at time t.

[0072] Furthermore, in this embodiment, the step of using MATLAB software to display the range of active power and reactive power is also included.

[0073] Specific implementation example: Simulate the single-phase MMC topology structure and combine Figures 2 to 4 Provide explanation.

[0074] 1. According to the model and application scenario of the modular multi-level converter MMC, obtain relevant parameters:

[0075] Table 2 Multi-level converter MMC model and application scenario parameter table

[0076] Serial number parameter unit describe 1 VDC V DC side voltage 2 VAC V AC side voltage (effective value) 3 Srated VA Rated apparent power 4 Xarm pu (per unit value) Single arm reactance value 5 Xl pu (per unit value) Single-phase reactance value 6 Ra pu (per unit value) Single arm equivalent resistance 7 P pu (per unit value) Active power setting value 8 Q pu (per unit value) Reactive power setting value 9 Imax(rms) pu (per unit value) Maximum stack current limit (effective value) 10 Csm F Submodule capacitance value 11 No.SM N / A Number of submodules 12 Vsm0 V Submodule initial voltage 13 Vsmp V Submodule voltage limit 14 Eaverage J Energy-controlled stack average energy

[0077] 2. Use MATLAB software to read the data input by the user for calculation. The calculation method and steps are as follows:

[0078] First, based on the single-phase MMC model, its topology and equivalent circuit are established. Under the given active and reactive power conditions, the following calculations are performed in combination with KCL and KVL.

[0079]

[0080] Among them, I AC Indicates the AC side output current, V AC(rms) represents the effective value of the AC side output voltage, j represents the imaginary unit, The conjugate of the current value on the AC side of the lower arm;

[0081] In steady state, it can be approximated as follows:

[0082] P ACo +P DCo =P net =0 (6)

[0083] P ACo Indicates the active power output on the AC side, P DCo Indicates the active power output on the DC side, P net is the total power; and the DC current will not flow into the AC side:

[0084]

[0085] It turns out that:

[0086]

[0087] Table 3 Figure 2 And the corresponding meaning table of parameters in the formula

[0088]

[0089]

[0090] To achieve easier calculation, the equivalent circuit is transformed according to Formula 6, as follows: Figure 3 ;

[0091] After equivalent transformation, the current and voltage values ​​of the upper and lower arms can be decomposed into two parts, as shown in formula 10-13

[0092]

[0093] Among them, x can be replaced by DC or AC, so the following formula can be derived in steady state:

[0094]

[0095] is the alternating current on the AC side, is the equivalent AC voltage flow, is the equivalent DC voltage flow, is the resultant AC voltage after equivalent operation, is the composite amount of the equivalent DC voltage; and in steady state, the DC current will not flow into the AC side, according to Figure 2 , the current calculation formula is as follows:

[0096]

[0097] is the equivalent AC current flow, is the equivalent DC current flow, is the resultant amount of equivalent alternating current, is the resultant DC current after equivalent operation.

[0098] thus, can be calculated, and:

[0099]

[0100] After calculation, in order to obtain the waveform diagram under ideal conditions, the above conclusions need to be associated with time. The relationship between voltage and current associated with time is:

[0101]

[0102] in, is the phase angle of the upper arm voltage, is the phase angle of the upper arm current, is the phase angle of the lower arm voltage, is the phase angle of the lower arm current.

[0103] Compared with single-phase MMC, three-phase MMC needs to add the phase difference between the three phases, as shown in the following formula:

[0104]

[0105] P(t)=V(t)I(t) (25)

[0106] E(t)=∫P(t)dt (26)

[0107] ωe ＝ 2πf e

[0108] f e is the frequency of alternating current, which is set to 50 Hz in this invention. is the voltage phase, and δ is the current phase. and Where x can be u or l, is the upper arm AC output voltage, is the lower arm AC output voltage, is the upper arm AC side current, is the AC current of the lower arm. V DCo is the DC side output voltage, I DCo is the DC side output current.

[0109] Determine whether the MMC has exceeded the acceptable operating range under this active and reactive power condition. The specific limitations of the operating range include four points:

[0110] (1) Output current: The current of each arm cannot exceed the bearing capacity of each component in the MMC. The current limit can ensure safe operation and control the loss caused by circuit impedance. This limit is related to the data 9 entered by the user. Data 9 in Table 2 represents the comparison between the maximum stack current limit and the output current result calculated by formula 24. The output current must always be less than the maximum stack current limit;

[0111] (2) Output voltage: In this embodiment, the MMC uses a half-bridge structure submodule, so the negative output stack voltage cannot generate a negative voltage. The output voltage result of formula 23 cannot be negative at any time.

[0112] (3) Submodule voltage peak: The capacitor in the submodule is set with a maximum voltage value (energy) that it can withstand.

[0113] The maximum value of the output voltage of Formula 23 divided by the number of sub-modules (data 11 in Table 2) needs to be less than the sub-module voltage limit given by the user (data 13 in Table 2) to obtain the result.

[0114] (4) Stack energy:

[0115]

[0116] Its average value is always close to 0, calculated by formula 26, E average Give the user a value

[0117]

[0118] In the formula, E stack is the energy of the stack, N is the number of submodules, C SM is the capacitance of the submodule. According to the above formula, V SM is time-varying and must always be less than V SM peak value.

[0119] Each stack can accept a submodule voltage and must be greater than the voltage required by the stack at any time. The energy of the stack must be E safeSafety threshold, therefore, the acceptable submodule voltage for each stack must comply with the following formula:

[0120]

[0121] Currently the default is E safe is 0, and time t can be introduced into Formula 29.

[0122]

[0123] The result obtained by dividing the output voltage of Formula 23 by the number of sub-modules in Data 11 in Table 2 needs to be less than the result obtained by Formula 30 at any time t.

[0124] Although the present invention is described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the present invention. It should therefore be understood that many modifications may be made to the exemplary embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims. It should be understood that the various dependent claims and features described herein may be combined in a manner different from that described in the original claims. It should also be understood that the features described in conjunction with a single embodiment may be used in other described embodiments.

Claims

1. A method for obtaining a power range of a multi-level converter based on a half-bridge submodule, characterized in that: include: Step 1: Establish an equivalent circuit of the converter to be simulated according to the model of the converter to be simulated; At the same time, set the initial value and step size of active power and power angle, and calculate the corresponding reactive power; Step 2: Based on the equivalent circuit, MATLAB software is used to calculate the output voltage, output current, submodule voltage peak value and stack energy corresponding to the current active power and reactive power; Step 3: judging whether the multi-level converter operates within a safe range by the output voltage, output current, submodule voltage peak value and stack energy; if so, recording the active power and reactive power corresponding to the previous iteration, increasing or decreasing the active power by one step according to the power angle, calculating the corresponding reactive power, and returning to execute step 2; Otherwise, go to step 4; Step 4: Increase the power angle by one step, calculate the reactive power at this time and the output voltage, output current, submodule voltage peak and stack energy corresponding to the current active power and reactive power, return to execute step 3 until the power angle is greater than or equal to 360 degrees, and use the active power and reactive power recorded in the previous iteration as the final maximum value of the active power and reactive power range.

2. The method for obtaining the power range of a multi-level converter based on a half-bridge submodule according to claim 1, characterized in that: In step three, according to the size of the power angle, the active power is increased or decreased by one step. The specific method is: when the active power angle is between 0-90° and between 270-360°, the active power increases by one step; when the active power angle is between 90-270°, the active power decreases by one step.

3. The method for obtaining the power range of a multi-level converter based on a half-bridge submodule according to claim 1, characterized in that: In step 1, the initial values ​​and step sizes of active power and power angle are: the initial value of active power is 0, the step size is 0.05, the initial value of power angle is 0°, and the step size is 0.5°.

4. The method for obtaining the power range of a multi-level converter based on a half-bridge submodule according to claim 1, characterized in that: In step 1, the formula for calculating the corresponding reactive power is: Q=Ptanδ Among them, Q is reactive power, P is active power, and δ is the power angle.

5. The method for obtaining the power range of a multi-level converter based on a half-bridge submodule according to claim 4, characterized in that: In step 3, the method for judging whether the multi-level converter operates within a safe range by the output voltage, output current, submodule voltage peak value and stack energy is: Determine whether the real-time output voltage is greater than 0, if so, the multi-level converter operates within a safe range, otherwise, the multi-level converter does not operate within a safe range; Determine whether the output current is less than the maximum stack current threshold Imax, if so, the multi-level converter operates within a safe range, otherwise, the multi-level converter does not operate within a safe range; The maximum value of the real-time output voltage is divided by the number of submodules in the stack to obtain the maximum value of the real-time voltage of the stack submodule, and whether the maximum value of the real-time voltage of the stack submodule is less than the maximum threshold value of the submodule voltage is determined. If so, the multilevel converter operates within a safe range, otherwise, the multilevel converter does not operate within a safe range. Calculate the real-time energy of the stack, use the real-time energy of the stack to calculate the real-time voltage allowable value of the sub-module in the stack, and determine whether the real-time output voltage of the sub-module is less than the real-time voltage allowable value. If so, the multi-level converter operates within a safe range, otherwise, the multi-level converter does not operate within a safe range.

6. The method for obtaining the power range of a multi-level converter based on a half-bridge submodule according to claim 5, characterized in that: The real-time output voltage is: Among them, V abc (t) is the total output voltage, V DCo is the DC side output voltage, When x is u, it indicates the AC output voltage of the upper arm, and when x is l, it indicates the AC output voltage of the lower arm. Voltage phase, ω e =2πf e , f e is the frequency of alternating current, and t is the time.

7. The method for obtaining the power range of a multi-level converter based on a half-bridge submodule according to claim 6, characterized in that: The real-time output current is: Among them, I abc (t) is the output current, δ is the current phase, I DCo is the DC side output current, When x is u, it represents the AC output current of the upper arm, and when x is l, it represents the AC output current of the lower arm.

8. The method for obtaining the power range of a multi-level converter based on a half-bridge submodule according to claim 5, characterized in that: The method for calculating the real-time energy of the stack and using the real-time energy of the stack to calculate the real-time allowable voltage of the submodule in the stack is: Calculate the stack energy: The relationship between stack energy and stack neutron module voltage is: Among them, E stack represents the stack energy, represents the initial energy of the stack, express The average value, E average represents the average energy of the stack controlled by energy, V SM Submodule allowable voltage, N represents the number of submodules, C SM Indicates the submodule capacitance value; The formula for stack energy changing over time is: Among them, V SM (t) represents the submodule allowable voltage at time t, E stack (t) represents the stack energy at time t.

9. The method for obtaining the power range of a multi-level converter based on a half-bridge submodule according to claim 5, characterized in that: The method also includes the step of using MATLAB software to display the range of active power and reactive power.

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