A method for suppressing voltage ripple of submodule capacitor of modular DC transformer

Abstract

The present invention discloses a method for suppressing the voltage ripple of a submodule capacitor of a modular DC transformer, and belongs to the field of power electronic control technology. First, according to the DC bus voltage and the required DC voltage conversion ratio, the total number of a single bridge arm submodule and the number of bridge arm submodules put into use in the positive half cycle are determined, which are recorded as n and m respectively; the optimal switching state of each AC cycle is obtained in sequence, and an optimal submodule switch sequence is formed; the optimal submodule switch sequence is used as the switch sequence of the first submodule of the upper and lower bridge arms, and the other submodules are the same as the first submodule switch sequence, but delayed by one AC cycle in sequence; the midpoint of the bridge arm will continuously output a square wave AC voltage, and a DC transformer output is realized through a rear-stage AC-DC conversion circuit. Compared with other methods, the submodule capacitor energy fluctuation of the described submodule capacitor voltage ripple suppression method is small, the submodule capacitance required under the same voltage ripple requirement is small, and the system hardware cost is lower.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronic control, and in particular relates to a method for suppressing voltage ripple of a submodule capacitor of a modular direct current transformer. Background Art

[0002] As an energy conversion device, modular DC transformers are increasingly widely used in the field of medium and high voltage DC conversion. This type of DC transformer consists of a main structure of multiple sub-modules, which are stacked to produce bridge arm voltages to achieve controllable DC voltage transformation output.

[0003] The stability of submodule voltage and the capacitor voltage ripple within a certain range are the basis for achieving stable operation of the DC transformer. Larger submodule capacitors can suppress their ripples, but the presence of a large number of submodule capacitors in the equipment will lead to large equipment volume, low power density, and high construction costs. Therefore, it is necessary to suppress the capacitor voltage ripple. Summary of the invention

[0004] In order to reduce the submodule capacitance of a modular DC transformer, the present invention proposes a method for suppressing the voltage ripple of the submodule capacitance of a modular DC transformer, comprising the following steps:

[0005] 1. A method for suppressing voltage ripple of a submodule capacitor of a modular DC transformer, characterized by comprising the following steps:

[0006] 1) According to the high-voltage side DC bus voltage V in , submodule rated voltage V c , the amplitude of the primary voltage of the internal transformer V T , determine the total number n of submodules in a single bridge arm;

[0007] 2) The operation of the modular DC transformer is cyclical. An AC cycle can be divided into two stages, namely the positive half cycle and the negative half cycle. Determine the number of sub-modules put into operation in the upper bridge arm in the positive half cycle and the number of sub-modules put into operation in the lower bridge arm in the negative half cycle. The two numbers are equal and are counted as m.

[0008] 3) Define the switch sequence of a submodule as a column vector containing n elements. The switch sequences of the submodules with the same sequence number in the upper bridge arm and the lower bridge arm are consistent. The switch sequence contains m elements 1 and nm elements 0. The 1st to nth elements correspond to the switch states of the submodule in the 1st to nth AC cycles respectively.

[0009] 4) The switching sequence that minimizes the voltage ripple of the submodule capacitor is implemented according to the following steps 5) to 8);

[0010] 5) Let p k is the number of full-investment cycles in the first k cycles, then for any k = 1, 2, ..., n, pk for

[0011]

[0012] Among them, round(x) is the rounding function;

[0013] 6) For k = 1, 2, ..., n, calculate the optimal p in turn k , thus obtaining the optimal switching state s in the kth cycle k , that is, the corresponding AC cycle is a full-time cycle or a half-time cycle, which can be obtained by p k With p k-1 Get: If p k =p k-1 +1, then the kth period is the full investment period, s k =1; if p k =p k-1 , then the kth cycle is a half-input cycle, s k =0, special, definition, p 0 =0; the above statement can be expressed by the formula

[0014] s k =p k -p k-1 ,p 0 =0 Formula IV

[0015] 7) Combined s 1 ,s 2 ,…,s n , obtain the optimal submodule switching sequence with the smallest ripple, and use it as the switching sequence of the first submodule in the upper and lower bridge arms, which is expressed by the column vector as

[0016] sp 1 =[s 1 s 2 s 3 …s n ] T Formula V

[0017] Among them, sp 1 is the switch sequence of the first submodule;

[0018] 8) The switching sequence of other submodules in the bridge arm is deduced from the switching sequence of submodule 1. The switching sequence of the i-th submodule is

[0019] sp i =T i-1 ×sp 1 Formula VI

[0020] Wherein, the subscript i represents the i-th submodule (i=2,3,…,n), the matrix T is an n-row and n-column matrix, in which only the r-1-th element in the r-th row is 1, and the others are 0, the n-th element in the 1st row is 1, and the others are 0,

[0021] Thus, the switching states of all submodules in n AC cycles are obtained;

[0022] 9) With n AC cycles as one cycle, starting from the (n+1)th AC cycle, each submodule repeats the switching sequence of its first AC cycle.

[0023] As a preferred embodiment of the present invention, in step 1), the total number n of submodules in a single bridge arm is determined by the following formula I:

[0024]

[0025] As a preferred embodiment of the present invention, in step 2), m is determined according to formula II:

[0026]

[0027] As a preferred embodiment of the present invention, in the switching sequence of step 3), element 1 represents that the submodule is always put into the circuit in the corresponding AC cycle, which is recorded as a full-on cycle; for the upper bridge arm submodule, element 0 represents that it is bypassed in the corresponding AC positive half-cycle and is only put into the circuit in the AC negative half-cycle; for the lower bridge arm submodule, element 0 represents that it is bypassed in the corresponding AC negative half-cycle and is only put into the circuit in the AC positive half-cycle, so the AC cycle represented by element 0 is recorded as a half-on cycle.

[0028] As a preferred embodiment of the present invention, in step 8), the switching state of the i-th submodule in the j-th AC cycle is the same as the switching state of the i-1-th submodule in the j-1-th AC cycle, i=2,3,…,n, j=2,3,…,n, and the switching state of the i-th submodule in the 1st AC cycle is the same as the switching state of the i-1-th submodule in the nth AC cycle.

[0029] The present invention has the following beneficial effects:

[0030] The submodule capacitor voltage ripple suppression method generates a switching sequence with the smallest submodule capacitor voltage ripple, and by assigning values ​​to all submodules in turn and repeating the sequence, the capacitor voltage distribution of all submodules is consistent and optimal (i.e., the capacitor voltage ripple is minimized). Under the same capacitor voltage ripple limit, the required submodule capacitance is minimized, which helps to reduce system cost, reduce equipment volume, and improve power density. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1is a schematic diagram of a modular DC transformer;

[0032] Figure 2 A flowchart for realizing the proposed submodule capacitor voltage ripple suppression method;

[0033] Figure 3 It is the capacitor voltage of the submodule with larger ripple when the proposed ripple suppression method is not adopted.

[0034] Figure 4 It is the sub-module capacitor voltage with smaller ripple when the proposed ripple suppression method is adopted. DETAILED DESCRIPTION

[0035] The following takes the general modular DC transformer control as an example, and combines the drawings and embodiments to specifically illustrate the purpose, scheme and advantages of the present invention.

[0036] Figure 1 The schematic diagram of the modular DC transformer circuit is shown in Figure 1. The high voltage side of the transformer consists of two DC clamping capacitors (C u and C b ) and a modular multi-level structure, wherein the modular multi-level structure is divided into an upper bridge arm and a lower bridge arm, both of which have the same structure and are composed of bridge arm sub-modules (upper bridge arm sub-module SM u1 , S.M. u2 , …, SM un , lower bridge arm submodule SM b1 , S.M. b2 , …, SM bn ) and the bridge arm inductance (L u and L b ), the upper bridge arm and the lower bridge arm have the same number of bridge arm sub-modules and the same bridge arm inductance; its low-voltage side is composed of a full-bridge circuit; the high-voltage side and the low-voltage side are connected through a transformer (T).

[0037] Figure 2 The following is a flow chart of the implementation of the method of the present invention, and the implementation steps are as follows:

[0038] 1) According to the high-voltage side DC bus voltage V in , submodule rated voltage V c , the amplitude of the primary voltage of the internal transformer V T , determine the total number n of submodules in a single bridge arm according to Formula I;

[0039]

[0040] 2) The operation of modular DC transformer is cyclical. An AC cycle can be divided into two stages, namely the positive half cycle and the negative half cycle. The number of sub-modules put into operation in the upper bridge arm in the positive half cycle is equal to the number of sub-modules put into operation in the lower bridge arm in the negative half cycle, which is m and is determined by the following formula II;

[0041]

[0042] 3) Define the switch sequence of a submodule as a column vector containing n elements. The switch sequences of submodules with the same sequence number in the upper bridge arm and the lower bridge arm are consistent. For example, the first submodule (SM u1 ) and the switching sequence of the first submodule of the lower bridge arm (SM b1 ) is the same as the switch sequence. The switch sequence contains m 1s and nm 0s. The 1st to nth elements correspond to the switch states of the submodule in the 1st to nth AC cycles respectively: element 1 represents that the submodule is always put into the circuit in the corresponding AC cycle, which is recorded as a full-on cycle; for the upper bridge arm submodule, element 0 represents that it is bypassed in the corresponding AC positive half cycle and is only put into the circuit in the AC negative half cycle; for the lower bridge arm submodule, element 0 represents that it is bypassed in the corresponding AC negative half cycle and is only put into the circuit in the AC positive half cycle, so the AC cycle represented by element 0 is recorded as a half-on cycle;

[0043] 4) The switching sequence that minimizes the voltage ripple of the submodule capacitor is implemented according to the following steps 5) to 8);

[0044] 5) Let p k is the number of full-investment cycles in the first k cycles, then for any k = 1, 2, ..., n, p k for

[0045]

[0046] Among them, round(x) is a rounding function, which means rounding to get the integer closest to x;

[0047] 6) For k = 1, 2, ..., n, calculate the optimal p in turn k , thus obtaining the optimal switching state s in the kth cycle k , that is, the corresponding AC cycle is a full-time cycle or a half-time cycle, which can be obtained by p k With p k-1 Get: If p k =p k-1 +1, then the kth period is the full investment period, s k =1; if p k =p k-1 , then the kth cycle is a half-input cycle, s k =0, special, definition, p 0 =0. The above statement can be expressed by the formula

[0048] s k =p k -p k-1 ,p 0=0 Formula IV

[0049] 7) Combined s 1 ,s 2 ,…,s n , obtain the optimal submodule switching sequence with the smallest ripple, and use it as the switching sequence of the first submodule in the upper and lower bridge arms, which is expressed by the column vector as

[0050] sp 1 =[s 1 s 2 s 3 …s n ] T Formula V

[0051] Among them, sp 1 is the switch sequence of the first submodule. Specifically, if sp 1 If the cth element of is 1, it means that the first submodule in the upper and lower bridge arms is fully put into use in the cth AC cycle; if sp 1 If the cth element of is 0, it means that the first submodule in the upper and lower bridge arms is half-activated in the cth AC cycle;

[0052] 8) The switching sequence of other submodules in the bridge arm can be deduced from the switching sequence of submodule 1. The switching sequence of the i-th submodule is

[0053] sp i =T i-1 ×sp 1 Formula VI

[0054] The subscript i represents the i-th submodule (i=2,3,…,n), and the matrix T is an n-row and n-column matrix, in which only the r-1-th element in the r-th row is 1 (r=2,3,…,n), and the others are 0, the n-th element in the 1st row is 1, and the others are 0, that is,

[0055]

[0056] Then the switch state of the i-th submodule in the j-th AC cycle is the same as the switch state of the i-1-th submodule in the j-1-th AC cycle (i=2,3,…,n, j=2,3,…,n), and the switch state of the i-th submodule in the 1st AC cycle is the same as the switch state of the i-1-th submodule in the n-th AC cycle, so the switch states of all submodules in n AC cycles can be obtained;

[0057] 9) With n AC cycles as one cycle, starting from the (n+1)th AC cycle, each submodule repeats the switching sequence of its first AC cycle.

[0058] An application example of the present invention is given below.

[0059] For the modular DC transformer shown in the figure, there are 7 submodules in each of the upper and lower bridge arms, that is, n = 7. By changing the number of submodules in the upper and lower bridge arms in real time, an AC voltage will be generated at the midpoint of the upper and lower bridge arms.

[0060] According to the DC output voltage requirement, if the number of upper bridge arm submodules put into operation in the internal AC positive half cycle is m=3, the optimal number of fully put-in-operation cycles in the first k AC cycles can be obtained according to formula III in step 5) above:

[0061] p 1 =0,p 2 =1,p 3 =1,p 4 =2,p 5 =2,p 6 =3,p 7 =3 Formula VIII

[0062] According to formula IV in step 6), the switch state of each AC cycle submodule 1 is obtained as follows:

[0063] s 1 =0,s 2 =1,s 3 =0,s 4 =1,s 5 =0,s 6 =1,s 7 =0 Formula IX

[0064] According to formula V in step 7) above, the switching sequence of submodule 1 is:

[0065] sp 1 =[s 1 s 2 s 3 …s n ] T =[0 1 0 1 0 1 0] T Formula X

[0066] According to the formula VI in step 7) above, the switch sequence of other submodules is obtained as follows:

[0067]

[0068] The obtained switch sequence, ie, equation X and equation XI, can be sequentially composed into a switch matrix and stored in the controller. When the modular DC transformer is running, the switch sequence is sequentially cycled to control the switch state of the submodule. Figure 3 and Figure 4The simulation results of the submodule capacitor voltage under the same hardware conditions and the same operating conditions are shown in Figure 1. The horizontal axis is time and the vertical axis is voltage. Figure 3 This is the result of the traditional operation mode without the proposed ripple suppression. The submodule capacitor voltage fluctuation is 121V. Figure 4 The result of using the proposed ripple suppression method is that the submodule capacitor voltage fluctuation is 91 V. The simulation results show that under the same circuit parameters, the modular DC transformer submodule capacitor voltage ripple using this method is smaller, so under the same ripple limit, a smaller submodule capacitor can be used to reduce the construction cost of the system.

[0069] The present invention is not limited to the above specific implementations. Those skilled in the art can adopt various other implementations based on the content disclosed in the present invention, such as replacing the full-bridge topology of the rear-stage ac-dc link with a half-bridge topology, replacing the topology of the bridge arm composed of two independent modules with a topology composed of more than two bridge arms, etc. Therefore, the claims are intended to cover all variations within the true concept and scope of the present invention.

Claims

1. A method for suppressing voltage ripple of submodule capacitors of a modular DC transformer, characterized in that The steps include: 1) According to the high-voltage side DC bus voltage V in , submodule rated voltage V c , the amplitude of the primary voltage of the internal transformer V T , determine the total number n of submodules in a single bridge arm; 2) The operation of the modular DC transformer is cyclical. An AC cycle can be divided into two stages, namely the positive half cycle and the negative half cycle. Determine the number of sub-modules put into operation in the upper bridge arm in the positive half cycle and the number of sub-modules put into operation in the lower bridge arm in the negative half cycle. The two numbers are equal and are counted as m. 3) Define the switch sequence of a submodule as a column vector containing n elements. The switch sequences of the submodules with the same sequence number in the upper bridge arm and the lower bridge arm are consistent. The switch sequence contains m elements 1 and nm elements 0. The 1st to nth elements correspond to the switch states of the submodule in the 1st to nth AC cycles respectively. 4) The switching sequence that minimizes the voltage ripple of the submodule capacitor is implemented according to the following steps 5) to 8); 5) Let p k is the number of full-investment cycles in the first k cycles, then for any k = 1, 2, ..., n, p k for Among them, round(x) is the rounding function; 6) For k = 1, 2, ..., n, calculate the optimal p in turn k , thus obtaining the optimal switching state s in the kth cycle k , that is, the corresponding AC cycle is a full-time cycle or a half-time cycle, which can be obtained by p k With p k-1 Get: If p k =p k-1 +1, then the kth period is the full investment period, s k =1; if p k =p k-1 , then the kth cycle is a half-input cycle, s k =0, in particular, definition, p0 = 0; the above statement can be expressed by the formula s k =p k -p k-1 ,p0=0 Formula IV 7) Combined s1, s2, ..., s n , obtain the optimal submodule switching sequence with the smallest ripple, and use it as the switching sequence of the first submodule in the upper and lower bridge arms, which is expressed by the column vector as sp1 = [s1 s2 s3…s n T Formula V​ Among them, sp1 is the switch sequence of the first submodule; 8) The switching sequence of other submodules in the bridge arm is deduced from the switching sequence of submodule 1. The switching sequence of the i-th submodule is: sp i =T i-1 ×sp1 Formula VI Wherein, the subscript i represents the i-th submodule, i=2,3,…,n, and the matrix T is an n-row and n-column matrix, in which only the r-1-th element in the r-th row is 1 and the others are 0, the n-th element in the 1st row is 1 and the others are 0, Thus, the switching states of all submodules in n AC cycles are obtained; 9) With n AC cycles as one cycle, starting from the (n+1)th AC cycle, each submodule repeats the switching sequence of its first AC cycle.

2. The method for suppressing submodule capacitor voltage ripple of a modular DC transformer according to claim 1, characterized in that: In step 1), the total number of submodules n in a single bridge arm is determined according to formula I:

3. The method for suppressing submodule capacitor voltage ripple of a modular DC transformer according to claim 1, characterized in that: In step 2), m is determined according to formula II:

4. The method for suppressing submodule capacitor voltage ripple of a modular DC transformer according to claim 1, characterized in that: In the switch sequence of step 3), element 1 represents that the submodule is always put into the circuit in the corresponding AC cycle, which is recorded as the full-on cycle; for the upper bridge arm submodule, element 0 represents that it is bypassed in the corresponding AC positive half cycle and is only put into the circuit in the AC negative half cycle; For the lower bridge arm submodule, element 0 represents that it is bypassed in the corresponding AC negative half cycle and is only put into the circuit in the AC positive half cycle, so the AC cycle represented by element 0 is recorded as a half-put cycle.

5. The method for suppressing submodule capacitor voltage ripple of a modular DC transformer according to claim 1, characterized in that: In the corresponding step 8), the switching state of the i-th submodule in the j-th AC cycle is the same as the switching state of the i-1-th submodule in the j-1-th AC cycle, i=2,3,…,n, j=2,3,…,n, the switching state of the i-th submodule in the 1st AC cycle is the same as the switching state of the i-1-th submodule in the nth AC cycle.

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