Asymmetric waveform operation method of direct current converter

Abstract

The invention discloses an asymmetric waveform operation method of a direct current converter. The method comprises the following steps: step 1, designing an asymmetric alternating current voltage waveform; 2, designing an asymmetric alternating current waveform; 3, designing an optimal asymmetric alternating current waveform; and step 4, designing a control method under asymmetric waveform operation. According to the invention, by designing the asymmetric AC voltage waveform in the DC converter, the distribution of the number of modules in the DC converter among different branches is adjusted, and the total number of modules is ensured to be unchanged; by designing the asymmetric alternating current waveform of the direct current converter, the alternating current amplitude on one side of the direct current component of the device is reduced, so that the current peak value of the device is reduced, and the power capacity is improved. On the premise that the circuit structure and the device number of the direct-current converter are not changed, the transmission power capacity of the direct-current converter is remarkably improved, and the direct-current converter topology has great significance in application of an existing direct-current converter topology in a direct-current power grid.

Description

Technical Field

[0001] The invention belongs to the field of power electronics and high-voltage direct current transmission, and relates to an asymmetric waveform operation method of a direct current converter. Background Art

[0002] With the development of DC grid technology, DC converters, as the core hub of DC grids, have received more and more attention. However, large capacity has always been a key technical challenge faced by DC converters. Existing DC converters are limited by the device current peak and cannot achieve large capacity.

[0003] CN105576982B discloses a non-isolated DC transformer, which uses the three phases of a modular multi-level converter to be directly connected in parallel at the output to improve the power capacity of the transmission, but because the AC voltage and AC current need to be generated internally to achieve the balance of capacitor energy, a large-capacity filter is required on the DC side to filter out the internal AC voltage and AC current, and the volume and weight are significant. CN117318474A discloses a DC transformer for DC grid interconnection and a control and protection method thereof, which adopts an active filtering method and uses a sub-module cascade bridge arm instead of a passive filter, but because the bridge arm contains both a DC current component and an AC current component, the AC current and the AC voltage are waveforms with symmetrical positive and negative amplitudes, and the sinusoidal AC voltage and AC current have high amplitudes, resulting in the device current peak in the bridge arm being very large after the DC current component amplitude and the AC current component amplitude are superimposed, and the DC converter is difficult to achieve large capacity.

[0004] Therefore, in order to improve the power capacity of the DC converter in the DC power grid, it is urgent to invent a new operating method to reduce the current peak of the DC converter. Summary of the invention

[0005] The present invention provides an asymmetric waveform operation method for a DC converter. By designing an asymmetric AC voltage waveform in a DC converter, the number of modules inside the DC converter is adjusted to be distributed between different branches to ensure that the total number of modules remains unchanged; by designing an asymmetric AC current waveform of the DC converter, the AC current amplitude on one side of the DC current component of the device is reduced, thereby reducing the device current peak and improving the power capacity. The present invention proposes an optimal constraint condition for AC current design, which ensures that the designed asymmetric operating waveform is the optimal operating waveform in terms of current peak value and current effective value, and the device current peak can be significantly reduced. The present invention proposes a control method for asymmetric operation to achieve the stability of DC voltage, branch current and converter energy. The present invention can significantly improve the transmission power capacity of the DC converter without changing the circuit structure and the number of devices of the DC converter, which is of great significance to the application of the existing DC converter topology in the DC power grid.

[0006] The objective of the present invention is achieved through the following technical solutions:

[0007] A method for operating a DC converter in an asymmetric waveform comprises the following steps:

[0008] Step 1: Design an asymmetric AC voltage waveform:

[0009] For each phase in the DC converter, the sum of the first branch voltage and the intermediate branch voltage is equal to the first DC voltage U 1 , the intermediate branch voltage and the second branch voltage are equal to the second DC voltage U 2 ; The first branch voltage, the middle branch voltage, and the second branch voltage all contain DC voltage components and AC voltage components, the AC voltages in the first branch and the middle branch cancel each other out in reverse phase, and the AC voltages in the middle branch and the second branch cancel each other out in reverse phase; the specific steps are as follows:

[0010] For each phase in the DC converter, the positive amplitude of the AC voltage in the intermediate branch is U p , the duration is T p The negative amplitude of the AC voltage in the middle branch is U n , the duration is T n , then U p T p Equal to U n T n ; In order to satisfy the AC voltage offset relationship, the positive amplitude of the AC voltage in the first branch is U n , the duration is T n , the negative amplitude is U p , the duration is T p , the positive amplitude of the second branch AC voltage is U p , the duration is T p , the negative amplitude is U n , the duration is T n ; Under this design method, the AC voltages in the first branch and the middle branch are offset in reverse phase, and the AC voltages in the middle branch and the second branch are offset in reverse phase, and the AC voltage will not affect the DC side; by designing U p Greater than U n , reduce the number of submodules in the first branch, increase the number of submodules in the middle branch and the second branch, and maintain a balance in the total number of submodules;

[0011] Step 2: Design an asymmetric AC current waveform:

[0012] For each phase of the DC converter, the AC current of the middle branch is the sum of the AC current of the first branch and the AC current of the second DC branch. The AC currents of the first branch of the three phases are staggered by 120 degrees and offset each other. The AC currents of the second branch of the three phases are staggered by 120 degrees and offset each other to avoid AC current appearing on the DC side. The specific steps are as follows:

[0013] For each phase in the DC converter, the positive amplitude of the AC current in the intermediate branch is I p , the duration is T p , the negative amplitude of the AC current in the middle branch is I n , the duration is T n , then I p T p Equal to I n T n ; The positive amplitude of the first branch AC current is I p 1. The negative amplitude of the AC current in the first branch is I n 1. The positive amplitude of the second branch AC current is I p 2. The negative amplitude of the AC current in the second branch is I n 2, then the positive amplitude of the AC current in the first branch is I p 1 and the positive amplitude of the AC current in the second branch I p 2 is equal to the positive amplitude of the AC current in the middle branch I p Similarly, the negative amplitude of the AC current in the first branch is I n 1 and the negative amplitude of the second branch AC current I n The sum of 2 is equal to I n ; When the DC current component of the middle branch is negative, the current peak of the middle branch is reduced by designing Ip to be greater than In; when the DC current component of the middle branch is positive, the current peak of the middle branch is reduced by designing Ip to be less than In;

[0014] Step 3: Design the optimal asymmetric AC current waveform:

[0015] For the asymmetric AC circuit waveform of the DC converter, the optimal asymmetric AC current waveform is designed from the two perspectives of minimum AC current peak value and minimum AC current effective value; the specific steps are as follows:

[0016] In a cycle [0, T], in order to ensure that the peak value of the AC current is minimized, the AC current is kept constant in the period [T / 3, 2T / 3], and the amplitude is I p ; To ensure the minimum effective value of the AC current, the AC current is kept constant in the two time periods [0,T / 3] and [2T / 3,T], with an amplitude of I p / 2=I n ;

[0017] Step 4: Design a control method for asymmetric waveform operation:

[0018] According to the DC voltage reference instruction Uref, the DC voltage is controlled by proportional-integral control to obtain the DC current reference instruction Iref of the first branch or the second branch. The instruction is multiplied with the asymmetric AC current waveform designed in step 2 and step 3 to obtain the internal AC current reference; further according to the DC current reference instruction Iref, the DC current of the first branch or the second branch is controlled by proportional-integral control to obtain the voltage reference instruction Us_ref of the first branch or the second branch. The instruction is multiplied with the asymmetric AC voltage waveform designed in step 1 to obtain the internal AC voltage reference, thereby obtaining the branch voltage, and achieving the stability of the converter voltage and the branch current; according to the sub-module capacitor reference instruction UC_ref of the DC converter, the sub-module capacitor voltage average value UC_avg of each branch in the DC converter is controlled by proportional-integral control to obtain the AC voltage amplitude reference instruction Up_ref, thereby further adjusting the AC voltage amplitude in each phase of the DC converter, adjusting the energy absorbed or released by the sub-module capacitors of each branch, and achieving the energy balance of the DC converter.

[0019] Compared with the prior art, the present invention has the following advantages:

[0020] 1. Compared with the sinusoidal operation mode of the existing DC converter, the present invention proposes an asymmetric operation method to transmit power with constant voltage and current amplitude, which can improve the utilization rate of voltage and current and reduce the current peak.

[0021] 2. Compared with the symmetrical operation mode of the existing DC converter, the asymmetrical operation method proposed in the present invention can reduce the AC current amplitude on one side of the DC current component and reduce the current peak value.

[0022] 3. The optimal asymmetric AC current waveform in the present invention can achieve the lowest effective value and peak value of the AC current under the asymmetric operation method.

[0023] 4. The control method under asymmetric operation proposed in the present invention can achieve energy balance of each branch of the DC converter, stable control of DC voltage and stable control of branch current. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of an asymmetric waveform operation method of a DC converter;

[0025] Figure 2 It is a schematic diagram of three-phase asymmetrical AC voltage waveform;

[0026] Figure 3 It is a three-phase asymmetrical alternating current waveform diagram;

[0027] Figure 4 is the voltage waveform of the first branch of the three-phase circuit;

[0028] Figure 5 is the three-phase first DC current waveform;

[0029] Figure 6 is the three-phase middle branch voltage waveform;

[0030] Figure 7 is the current waveform of the three-phase middle branch;

[0031] Figure 8 is the voltage waveform of the second branch of the three-phase circuit;

[0032] Fig. 9 is the current waveform of the second branch of the three-phase circuit;

[0033] Fig.10 This is the control block diagram of the asymmetric operation method of the DC converter. DETAILED DESCRIPTION

[0034] The technical solution of the present invention is further described below in conjunction with the accompanying drawings, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be included in the protection scope of the present invention.

[0035] The present invention provides an asymmetric waveform operation method of a DC converter, such as Figure 1 As shown, the DC converter is composed of three identical phase circuits, each phase circuit includes a first branch, an intermediate branch and a second branch, wherein the first branch and the intermediate branch are formed by cascading half-bridge submodules, and the second branch is formed by cascading full-bridge submodules. 1 and U 2 is the DC side voltage, I 1 and I 2 is the DC side current, and in this embodiment, U 1 Greater than U 2 , I 1 Less than I 2 .U p is the positive amplitude of the AC voltage in the middle branch, U n is the negative amplitude of the AC voltage in the middle branch, I p is the positive amplitude of the AC current in the middle branch, I n It is the negative amplitude of the AC current in the middle branch.

[0036] The following will explain in detail the trapezoidal wave operation mode of the T-type DC transformer:

[0037] 1. Design asymmetrical AC voltage waveform:

[0038] like Figure 4 , Figure 6 , Figure 8 The voltage waveforms of each branch are shown. For each phase in the DC converter, the sum of the first branch voltage and the middle branch voltage is equal to the first DC voltage U 1 , the intermediate branch voltage and the second branch voltage are equal to the second DC voltage U 2 The first branch voltage, the middle branch voltage, and the second branch voltage all contain DC voltage components and AC voltage components, so the AC voltages in the first branch and the middle branch cancel each other out in anti-phase, and the AC voltages in the middle branch and the second branch cancel each other out in anti-phase.

[0039] Specifically, taking one phase as an example, the positive amplitude of the AC voltage in the middle branch is U p , the duration is T / 3, the negative amplitude of the AC voltage in the middle branch is U n , the duration is 2T / 3, so U p Equal to 2U n In order to satisfy the AC voltage offset relationship, the positive amplitude of the AC voltage in the first branch is U n , duration is 2T / 3, negative amplitude is U p , the duration is T / 3. The positive amplitude of the AC voltage in the second branch is U p , duration is T / 3, negative amplitude is U n , the duration is 2T / 3. Under this design method, the AC voltages in the first branch and the middle branch are offset in reverse phase, and the AC voltages in the middle branch and the second branch are offset in reverse phase, and the AC voltage will not affect the DC side. Design U p =2U n ,Compared to the traditional symmetrical operation mode, the number of sub-modules in the first branch is reduced, the number of sub-modules in the middle branch and the second branch is increased, and the total number of sub-modules is balanced.

[0040] 2. Design asymmetrical AC current waveform:

[0041] like Figure 5 , Figure 7 , Fig. 9 The current waveforms of each branch are shown. For each phase of the DC converter, the AC current of the middle branch is the sum of the AC current of the first branch and the AC current of the second DC branch. The AC currents of the first branch of the three phases are staggered 120 degrees and offset each other to prevent AC current from appearing on the DC side. The second branch of the three phases does not contain AC current, but only DC current components.

[0042] Specifically, taking one phase as an example, the positive amplitude of the AC current in the middle branch is I p , the duration is T / 3, and the negative amplitude of the AC current in the middle branch is I n , the duration is 2T / 3, so I p Equal to 2In The positive amplitude of the AC current in the first branch is I p , the negative amplitude of the AC current in the first branch is I n , the AC current of the second branch is 0, and the sum of the AC current of the first branch and the AC current of the second branch is equal to the AC current of the middle branch. The DC current component of the middle branch is (I 1 -I 2 ) / 3<0, Design I p =2I n , reducing the current peak of the middle branch. Therefore, the asymmetrical AC current waveform reduces the current peak of the DC converter and can increase the power capacity.

[0043] 3. Design the optimal asymmetric AC current waveform:

[0044] For the asymmetric AC circuit waveform of the DC converter, the optimal asymmetric AC current waveform is designed from the two perspectives of minimum AC current peak value and minimum AC current effective value. The AC current waveform of the three-phase DC converter meets the periodic characteristics, and a cycle [0, T] is used as an example.

[0045] First, the maximum current peak of the DC converter appears in the [T / 3, 2T / 3] period. When the current is constant during this period, the AC current peak is the smallest, otherwise the current peak of any other shape will increase.

[0046] Secondly, according to the AC current effective value calculation formula:

[0047]

[0048] To ensure the minimum effective value of the AC current, the AC current is kept constant in the two time periods [0,T / 3] and [2T / 3,T], and the amplitude is I p / 2=I n Among them, I RMS is the effective value of the AC current, T is the AC current period, and i(t) is the AC current.

[0049] Finally, considering the line inductance, the AC current changes with a certain slope, presenting a trapezoidal wave, thus obtaining the final three-phase optimal AC current waveform, as shown in Figure 3 shown.

[0050] The three-phase optimal AC current waveform designed by the present invention takes into account the advantages of minimum AC current stress and minimum AC current effective value. Without changing the circuit structure and the number of devices of the DC converter, the device current peak is reduced compared with the traditional symmetrical operation mode, and the power capacity can be improved.

[0051] 4. Design control method under asymmetric waveform operation:

[0052] like Fig.10 As shown, under the asymmetric waveform operation method, according to the DC voltage reference instruction Uref, the DC voltage is controlled by a proportional integral control method to obtain the DC current reference instruction Iref of the first branch or the second branch, and the instruction is multiplied with the asymmetric AC current waveform designed in step 2 and step 3 to obtain the internal AC current reference. Further, according to the DC current reference instruction Iref, the DC current of the first branch or the second branch is controlled by a proportional integral control method to obtain the voltage reference instruction Us_ref of the first branch or the second branch, and the instruction is multiplied with the asymmetric AC voltage waveform designed in step 1 to obtain the internal AC voltage reference, thereby obtaining the branch voltage, and realizing the stability of the converter voltage and the stability of the branch current. According to the submodule capacitor reference instruction UC_ref of the DC converter, the submodule capacitor voltage average value UC_avg of each branch in the DC converter is controlled by a proportional integral control method to obtain the AC voltage amplitude reference instruction Up_ref, thereby further adjusting the AC voltage amplitude in each phase of the DC converter, adjusting the energy absorption or release of each branch submodule capacitor, and realizing the energy balance of the DC converter.

Claims

1. A method for operating a DC converter with an asymmetric waveform, characterized in that The method comprises the following steps: Step 1: Design an asymmetric AC voltage waveform: For each phase in the DC converter, the sum of the first branch voltage and the middle branch voltage is equal to the first DC voltage U1, and the middle branch voltage and the second branch voltage are equal to the second DC voltage U2; the first branch voltage, the middle branch voltage, and the second branch voltage all contain DC voltage components and AC voltage components, and the AC voltages in the first branch and the middle branch cancel each other out in opposite phases, and the AC voltages in the middle branch and the second branch cancel each other out in opposite phases; Step 2: Design an asymmetric AC current waveform: For each phase of the DC converter, the AC current of the middle branch is the sum of the AC current of the first branch and the AC current of the second DC branch. The AC currents of the first branch of the three phases are staggered by 120 degrees and offset each other, and the AC currents of the second branch of the three phases are staggered by 120 degrees and offset each other to avoid AC current appearing on the DC side; Step 3: Design the optimal asymmetric AC current waveform: Aiming at the asymmetric AC circuit waveform of the DC converter, the optimal asymmetric AC current waveform is designed from the two perspectives of minimum AC current peak value and minimum AC current effective value; Step 4: Design a control method for asymmetric waveform operation: According to the DC voltage reference instruction Uref, the DC voltage is controlled by proportional-integral control to obtain the DC current reference instruction Iref of the first branch or the second branch. The instruction is multiplied with the asymmetric AC current waveform designed in step 2 and step 3 to obtain the internal AC current reference; further according to the DC current reference instruction Iref, the DC current of the first branch or the second branch is controlled by proportional-integral control to obtain the voltage reference instruction Us_ref of the first branch or the second branch. The instruction is multiplied with the asymmetric AC voltage waveform designed in step 1 to obtain the internal AC voltage reference, thereby obtaining the branch voltage, and achieving the stability of the converter voltage and the branch current; according to the sub-module capacitor reference instruction UC_ref of the DC converter, the sub-module capacitor voltage average value UC_avg of each branch in the DC converter is controlled by proportional-integral control to obtain the AC voltage amplitude reference instruction Up_ref, thereby further adjusting the AC voltage amplitude in each phase of the DC converter, adjusting the energy absorbed or released by the sub-module capacitors of each branch, and achieving the energy balance of the DC converter.

2. The asymmetric waveform operation method of the DC converter according to claim 1, characterized in that The specific steps of step one are as follows: For each phase in the DC converter, the positive amplitude of the AC voltage in the intermediate branch is U p , the duration is T p The negative amplitude of the AC voltage in the middle branch is U n , the duration is T n , then U p T p Equal to U n T n ; In order to satisfy the AC voltage offset relationship, the positive amplitude of the AC voltage in the first branch is U n , the duration is T n , the negative amplitude is U p , the duration is T p , the positive amplitude of the second branch AC voltage is U p , the duration is T p , the negative amplitude is U n , the duration is T n ; Under this design method, the AC voltages in the first branch and the middle branch are offset in reverse phase, and the AC voltages in the middle branch and the second branch are offset in reverse phase, and the AC voltage will not affect the DC side; by designing U p Greater than U n , reduce the number of sub-modules in the first branch, increase the number of sub-modules in the middle branch and the second branch, and maintain a balance in the total number of sub-modules.

3. The asymmetric waveform operation method of a DC converter according to claim 1, characterized in that The specific steps of step 2 are as follows: For each phase in the DC converter, the positive amplitude of the AC current in the intermediate branch is I p , the duration is T p , the negative amplitude of the AC current in the middle branch is I n , the duration is T n , then I p T p Equal to I n T n ; The positive amplitude of the first branch AC current is I p 1. The negative amplitude of the AC current in the first branch is I n 1. The positive amplitude of the second branch AC current is I p 2. The negative amplitude of the AC current in the second branch is I n 2, then the positive amplitude of the AC current in the first branch is I p 1 and the positive amplitude of the AC current in the second branch I p 2 is equal to the positive amplitude of the AC current in the middle branch I p Similarly, the negative amplitude of the AC current in the first branch is I n 1 and the negative amplitude of the second branch AC current I n The sum of 2 is equal to I n ; When the DC current component of the middle branch is negative, the current peak of the middle branch is reduced by designing Ip to be greater than In; when the DC current component of the middle branch is positive, the current peak of the middle branch is reduced by designing Ip to be less than In.

4. The asymmetric waveform operation method of a DC converter according to claim 1, characterized in that The specific steps of step three are as follows: In a cycle [0, T], in order to ensure that the peak value of the AC current is minimized, the AC current is kept constant in the period [T / 3, 2T / 3], and the amplitude is I p ; To ensure the minimum effective value of the AC current, the AC current is kept constant in the two time periods [0,T / 3] and [2T / 3,T], with an amplitude of I p / 2=I n .

5. The asymmetric waveform operation method of a DC converter according to claim 1, 2 or 3, characterized in that The first branch and the middle branch are formed by cascading half-bridge sub-modules, and the second branch is formed by cascading full-bridge sub-modules.

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