Midpoint Balancing and Circulating Current Control Method and System for Interleaved Parallel Three-Level Converter

Abstract

The present invention discloses a method and system for neutral point balance and circulating current control of an interleaved parallel three-level converter. By equally distributing the neutral point voltage balance control quantity to the first three-phase converter and the second three-phase converter connected in parallel between the bridge arms of at least one interleaved parallel three-level converter, and equally and reversely distributing the zero-sequence circulating current control output to the first three-phase converter and the second three-phase converter, and on this basis, correcting the amplitude of the injected zero-sequence voltage, the coordinated control of zero-sequence circulating current and neutral point balance can be ensured, which is of great significance for the development of converter parallel connection.

Description

Technical Field

[0001] The present invention relates to the technical field of electronic circuits, and particularly to a method and system for neutral point balance and circulating current control of an interleaved parallel three-level converter. Background Art

[0002] In high-power application scenarios such as energy storage and microgrids, the requirements for the power supply capacity are continuously increasing. The capacity of a single converter is limited by the voltage and current ratings of the switching devices and is difficult to increase. Therefore, parallel connection of converters has become a common method to expand the capacity. The parallel connection of converters can be divided into parallel connection between modules and parallel connection between bridge arms. Although the parallel connection between modules is simple in capacity expansion, it increases the volume and cost of the system. Compared with the parallel connection between modules, the parallel connection between bridge arms can increase the capacity of a single module, thereby reducing the volume and cost of the system under the same capacity.

[0003] Currently, the topological characteristics of the three-level converter with parallel connection between bridge arms are as follows: (1) When the three-level converters share a common AC-DC bus and are connected in parallel, there is a zero-sequence circulating current path inside the converter. If the zero-sequence circulating current is not suppressed, it will cause unbalanced currents in the three-phase parallel bridge arms, distorted current waveforms, different currents flowing through the switching tubes, and reduce the service life of the switching tubes. (2) The three-level converter must maintain neutral point balance, otherwise it will cause distortion of the output current waveform.

[0004] The existing three-level SPWM (Sinusoidal Pulse Width Modulation) control strategy is difficult to balance the neutral point and suppress the circulating current at the same time. Especially when the two are coupled with each other, it further increases the control difficulty.

[0005] Therefore, it is necessary to improve the existing technology.

[0006] The above information is given as background information only to assist in understanding the present disclosure, and does not determine or admit whether any of the above content can be used as the prior art relative to the present disclosure. Summary of the Invention

[0007] The present invention provides a method and system for neutral point balance and circulating current control of an interleaved parallel three-level converter to solve the deficiencies of the existing technology.

[0008] To achieve the above object, the present invention provides the following technical solutions:

[0009] In a first aspect, the present invention provides a method for neutral point balance and circulating current control of an interleaved parallel three-level converter, which is applied to at least one interleaved parallel three-level converter. The interleaved parallel three-level converter includes a first three-phase converter and a second three-phase converter connected in parallel between bridge arms. The method includes:

[0010] Collect the three-phase currents \(i_a\), \(i_b\), \(i_c\) of the first three-phase converter and the upper bus capacitor voltage \(U\) dc1 and the lower bus capacitor voltage \(U\) dc2 ;

[0011] Calculate the zero-sequence voltage \(u\) to be injected when the midpoint voltage deviation is zero z_np ;

[0012] Let the amplitude limit value of the charge flowing into the midpoint within one cycle be \(Q\) limit , with a magnitude of \(C\Delta U\) limit , and calculate the allowable zero-sequence deviation range \(u\) to be injected under this condition z_min and \(u\) z_max ;

[0013] Obtain the zero-sequence voltage \(u\) injected into the first three-phase converter through the zero-sequence circulating current controller z_zs and the zero-sequence voltage \(-u\) of the second three-phase converter with the same magnitude but opposite direction of the circulating current z_zs ;

[0014] Calculate the zero-sequence injection voltage \(U\) of the first three-phase converter z1 and the zero-sequence injection voltage \(U\) of the second three-phase converter z2 ;

[0015] Limit the amplitude of \(U\) z1 and \(U\) z2 ;

[0016] Superimpose the amplitude-limited \(U\) z1 onto the modulation wave output \(U\) a1* , \(U\) b1* and \(U\) c1* of the first three-phase converter, and superimpose the amplitude-limited \(U\) z2 onto the modulation wave output \(U\) a2* , \(U\) b2* and \(U\) c2* of the second three-phase converter.

[0017] Furthermore, in the midpoint balance and circulating current control method of the interleaved parallel three-level converter, the step of calculating the zero-sequence voltage \(u\) to be injected when the midpoint voltage deviation is zero z_np includes:

[0018] Calculate the zero-sequence voltage \(u\) to be injected when the midpoint voltage deviation is zero according to the following formula z_np :

[0019]

[0020] where \(C\) is the capacitance value of the upper and lower capacitor groups, \(f\) s is the switching frequency, \(i\) npav is the average midpoint current, \(i\)npo is the neutral point current after injecting zero-sequence voltage.

[0021] Further, in the neutral point balance and circulating current control method of the interleaved three-level converter, the step of setting the charge limit value flowing into the neutral point within one cycle to be Q limit , with a modulus of CΔU limit , and calculating the allowable zero-sequence deviation range u z_min and u z_max includes:

[0022] Set the charge limit value flowing into the neutral point within one cycle to be Q limit , with a modulus of CΔU limit , and calculate the allowable zero-sequence deviation range u z_min and u z_max according to the following formula:

[0023]

[0024] where u z is the superimposed zero-sequence component, and the amplitude of u z has a maximum value and a minimum value, which are u z_min and u z_max respectively.

[0025] Further, in the neutral point balance and circulating current control method of the interleaved three-level converter, the step of separately calculating the zero-sequence injection voltage U z1 of the first three-phase converter and the zero-sequence injection voltage U z2 of the second three-phase converter includes:

[0026] Calculate the zero-sequence injection voltage U z1 of the first three-phase converter and the zero-sequence injection voltage U z2 of the second three-phase converter respectively according to the following formula:

[0027]

[0028] Further, in the neutral point balance and circulating current control method of the interleaved three-level converter, the step of limiting the amplitude of U z1 and U z2 includes:

[0029] Limit the amplitude of U z1 and U z2 according to the following formula:

[0030] u z_min <u z1 ,u z2 <u z_max .

[0031] In a second aspect, the present invention provides a neutral point balance and circulating current control system for an interleaved three-level converter, which is applied to at least one interleaved three-level converter. The interleaved three-level converter includes a first three-phase converter and a second three-phase converter connected in parallel between arms. The system includes:

[0032] An electrical signal acquisition module for acquiring the three-phase currents ia, ib, ic of the first three-phase converter, the upper bus capacitor voltage U dc1 and the lower bus capacitor voltage U dc2 ;

[0033] A first calculation module for calculating the zero-sequence voltage u z_np to be injected when the neutral point voltage deviation is zero;

[0034] A second calculation module for setting the charge limit value flowing into the neutral point within one period to Q limit , with a modulus of CΔU limit , and calculating the allowable zero-sequence deviation range u z_min and u z_max under this condition;

[0035] A voltage acquisition module for obtaining the zero-sequence voltage u z_zs injected into the first three-phase converter through a zero-sequence circulating current controller and the zero-sequence voltage -u z_zs of the second three-phase converter with the same magnitude but opposite direction of the circulating current;

[0036] A third calculation module for respectively calculating the zero-sequence injection voltage U z1 of the first three-phase converter and the zero-sequence injection voltage U z2 of the second three-phase converter;

[0037] A voltage limiting module for limiting U z1 and U z2 ;

[0038] A voltage superposition module for superposing the limited U z1 onto the modulation wave outputs U a1* , U b1* and U c1* of the first three-phase converter, and superposing the limited U z2 onto the modulation wave outputs U a2* , U b2* and U c2* of the second three-phase converter.

[0039] Further, in the neutral point balance and circulating current control system of the interleaved three-level converter, the first calculation module is specifically used for:

[0040] Calculate the zero-sequence voltage u to be injected when the midpoint voltage deviation is zero according to the following formula z_np :

[0041]

[0042] where C is the capacitance value of the upper and lower capacitor groups, f s is the switching frequency, i npav is the average midpoint current, i npo is the midpoint current after injecting the zero-sequence voltage.

[0043] Furthermore, in the midpoint balance and circulating current control system of the interleaved three-level converter, the second calculation module is specifically used for:

[0044] Let the charge limit value flowing into the midpoint within one period be Q limit , with a modulus of CΔU limit , and calculate the allowable zero-sequence deviation range u to be injected under this condition according to the following formula z_min and u z_max :

[0045]

[0046] where u z is the superimposed zero-sequence component, and the amplitude of u z has a maximum value and a minimum value, which are u z_min and u z_max .

[0047] Furthermore, in the midpoint balance and circulating current control system of the interleaved three-level converter, the third calculation module is specifically used for:

[0048] Calculate the zero-sequence injection voltage U of the first three-phase converter and the zero-sequence injection voltage U of the second three-phase converter respectively according to the following formula z1 and z2 :

[0049]

[0050] Furthermore, in the midpoint balance and circulating current control system of the interleaved three-level converter, the voltage limiting module is specifically used for:

[0051] Limit U z1 and U z2 according to the following formula:

[0052] u z_min < u z1 , u z2 < u z_max .

[0053] Compared with the prior art, the present invention has the following beneficial effects:

[0054] A method and system for neutral point balance and circulating current control of an interleaved parallel three-level converter provided by an embodiment of the present invention can ensure the coordinated control of zero-sequence circulating current and neutral point balance by equally distributing the neutral point voltage balance control quantity to at least the first three-phase converter and the second three-phase converter connected in parallel between the bridge arms of the interleaved parallel three-level converter, and equally and reversely distributing the zero-sequence circulating current control output to the first three-phase converter and the second three-phase converter, and on this basis, correcting the amplitude of the injected zero-sequence voltage, which has important significance for the development of converter parallel connection. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention, and those of ordinary skill in the art can also obtain other drawings based on these drawings without creative efforts.

[0056] Figure 1 is a schematic flow chart of a method for neutral point balance and circulating current control of an interleaved parallel three-level converter provided by Embodiment 1 of the present invention;

[0057] Figure 2 is a topology diagram of an interleaved parallel three-level converter including a first three-phase converter and a second three-phase converter connected in parallel between the bridge arms provided by Embodiment 1 of the present invention;

[0058] Figure 3 is a block diagram of coordinated control of neutral point balance and circulating current provided by Embodiment 1 of the present invention;

[0059] Figure 4 is a schematic diagram of functional modules of a system for neutral point balance and circulating current control of an interleaved parallel three-level converter provided by Embodiment 2 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0060] In order to make the objectives, features, and advantages of the present invention more obvious and understandable, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the embodiments described below are only some embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art without creative efforts based on the embodiments of the present invention belong to the scope of protection of the present invention.

[0061] In the description of the present invention, it should be understood that when a component is considered to be "connected" to another component, it can be directly connected to the other component or there may be an intermediate component. When a component is considered to be "disposed on" another component, it can be directly disposed on the other component or there may be an intermediate component.

[0062] In addition, terms such as "long", "short", "inner", "outer", etc. indicating orientation or positional relationship are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention, rather than indicating or implying that the device or element referred to must have this specific orientation or be constructed and operated in a specific orientation, and thus should not be construed as a limitation of the present invention.

[0063] The technical solution of the present invention will be further described below with reference to the drawings and through specific embodiments.

[0064] Embodiment 1

[0065] In view of the defects existing in the above-mentioned existing parallel converter technology, based on the rich practical experience and professional knowledge in the design and manufacture of such products for many years, and in cooperation with the application of theory, the applicant has actively carried out research and innovation in the hope of creating a technology that can solve the defects in the existing technology and make the parallel converter technology more practical. After continuous research, design, and repeated sample making and improvement, the present invention with practical value has finally been created.

[0066] The present invention is applied to at least one interleaved three-level converter. Taking a single interleaved three-level converter as an example, the interleaved three-level converter includes a first three-phase converter and a second three-phase converter connected in parallel between the arms, and the topology diagram is as Figure 1 shown.

[0067] The neutral point control scheme for the two converters is the zero-sequence injection method. By changing the zero-sequence voltage injected into the converter, the neutral point balance control can be realized; the circulating current suppression adopts a PI controller and is finally realized in the way of zero-sequence injection.

[0068] Since the two converters share the DC bus and the neutral point, the neutral point voltage balance control quantity can be equally distributed to the modulation waves of the two converters, and the amplitude of the control quantity injected into each converter is denoted as u z_np .

[0069] The zero-sequence circulating current control can be adjusted by a PI controller. Assuming that the output value of the PI controller is u z_zs , since the zero-sequence circulating currents of the two converters are equal in magnitude and opposite in direction, the zero-sequence circulating current control output can be equally and reversely distributed to the two modules to realize the zero-sequence circulating current control.

[0070] Without affecting normal modulation, the maximum injection amount modulus of the zero-sequence voltage is |1 - max(u refa , u refb , u refc )| = U zmax .

[0071] It should be particularly noted that since both the midpoint balance and the zero-sequence circulating current are controlled in the form of superposition of zero-sequence components, the zero-sequence components must satisfy:

[0072] |u z_np + u z_zs | < U zmax (1);

[0073] According to the above analysis, the zero-sequence components of the modulation wave outputs of the two converters are superimposed as follows:

[0074]

[0075] Currently, most in the industry use PI controllers to achieve midpoint balance and zero-sequence circulating current control. However, its disadvantages are very obvious. According to Equation (2), in the form of zero-sequence injection, the zero-sequence circulating current and midpoint balance are mutually coupled, and it is difficult to decouple the controller design. The problems are as follows:

[0076] The zero-sequence circulating current control and midpoint balance control affect each other. For example, when the midpoint balance controller injects a zero-sequence quantity u z_np as a positive value, the zero-sequence circulating current controller injects a zero-sequence quantity u z_zs as a negative value, then the controller with the larger modulus plays a dominant role, and the role of the other controller weakens or even fails.

[0077] Since midpoint imbalance will cause serious problems such as DC bias harmonic distortion, here, taking midpoint balance as the basic control point, the zero-sequence circulating current is suppressed considering the conditions for midpoint balance control.

[0078] The implementation idea of midpoint balance and circulating current coordinated control is as follows:

[0079] ① The midpoint balance controller uses the calculation method to obtain u z_np (calculated with the goal of controlling the midpoint offset to zero);

[0080] ② Similarly using the calculation method, assuming that the allowable midpoint deviation range is controlled within (-ΔU limit , ΔU limit ), under this condition, the allowable zero-sequence injection amount range (u z_min , u z_max ) is calculated. This range can be used as the limit value of the modulation wave output quantity u z1 , u z2 , and inject it into the modulation wave u z1, u z2 Within this range, it can ensure that the midpoint balance is effectively controlled, and at the same time, a margin is left for zero-sequence circulating current control, realizing the decoupled control of the circulating current and the midpoint.

[0081] The calculation of the amplitude range of zero-sequence voltage injection to ensure midpoint balance is as follows. Taking one of the converters as an example (in actual implementation, because the converters share the common AC and DC busbars, the zero-sequence injection amount of one converter can be equally distributed to the other, which is easier to realize the decoupled control of the midpoint voltage and the zero-sequence circulating current):

[0082] The average charge flowing into the midpoint within one cycle is:

[0083] ΔQ1 = inpav * Ts = (1 - |va|) * ia * Ts + (1 - |vb|) * ib * Ts + (1 - |vc|) * ic * Ts (3);

[0084] Assume that the superimposed zero-sequence component is u z , then the change in the charge flowing into the midpoint after superimposing the zero-sequence component is:

[0085] ΔQ2 = i pno * Ts * u z = [-sign(v a ) * i a -sign(v b ) * i b -sign(v c ) * i c ) * u z (4);

[0086] In the initial state, the midpoint charge is:

[0087] Q0 = C(U dc1 - U dc2 ) (5);

[0088] Let the amplitude limit of the charge flowing into the midpoint within one cycle be Q limit , and its modulus is CΔU limit , which represents the maximum allowable fluctuation range of the midpoint. Then, the amplitude range of the allowable injected zero-sequence voltage can be obtained:

[0089]

[0090] According to the above formula, it can be seen that the amplitude of u z has a maximum value and a minimum value, which are denoted as (u z_min , u z_max ).

[0091] Then, the final method for midpoint balance control is as follows:

[0092]

[0093]

[0094] u z_min <u z1 ,u z2 <u z_max (9);

[0095] The output of the zero-sequence circulating current controller is corrected (clamped) through formulas (7)-(9), so as to ensure that the circulating current is controlled within the maximum range while achieving neutral point balance, increasing the margin of circulating current control.

[0096] In formula (6), ΔU limit determines the deviation degree of the neutral point. The larger its modulus, the larger the margin left for the zero-sequence circulating current controller, which can be adjusted according to the actual situation.

[0097] Please refer to Figure 2 , this embodiment of the present invention provides a method for neutral point balance and circulating current control of an interleaved parallel three-level converter, and the method includes:

[0098] S101. Collect the three-phase currents ia, ib, ic of the first three-phase converter, the upper bus capacitor voltage U dc1 and the lower bus capacitor voltage U dc2 .

[0099] S102. Calculate the zero-sequence voltage u z_np that needs to be injected when the neutral point voltage deviation is zero.

[0100] In this embodiment, the step S102 can be further refined to include the following steps:

[0101] Calculate the zero-sequence voltage u z_np that needs to be injected when the neutral point voltage deviation is zero according to the following formula:

[0102]

[0103] where C is the capacitance value of the upper and lower groups of capacitors, f s is the switching frequency, i npav is the average neutral point current, and i npo is the neutral point current after injecting the zero-sequence voltage.

[0104] S103. Let the charge limit value flowing into the neutral point within one cycle be Q limit , the modulus is CΔU limit , and calculate the allowable zero-sequence deviation range u z_min and u z_max .

[0105] In this embodiment, step S103 can be further refined into the following steps:

[0106] Let the charge limit value flowing into the midpoint within one cycle be Q limit , with a modulus of CΔU limit , and calculate the allowable zero-sequence deviation range u z_min and u z_max under this operating condition according to the following formula:

[0107]

[0108] where u z is the superimposed zero-sequence component, and the amplitude of u z has a maximum value and a minimum value, which are u z_min and u z_max respectively.

[0109] S104. Obtain the zero-sequence voltage u z_zs injected into the first three-phase converter through the zero-sequence circulating current controller z_zs and the zero-sequence voltage -u

[0110] of the second three-phase converter with the same magnitude but opposite direction of the circulating current. z1 S105. Calculate and obtain the zero-sequence injection voltage U z2 of the first three-phase converter and the zero-sequence injection voltage U

[0111] In this embodiment, step S105 can be further refined into the following steps:

[0112] Calculate and obtain the zero-sequence injection voltage U z1 of the first three-phase converter and the zero-sequence injection voltage U z2 of the second three-phase converter respectively according to the following formula:

[0113]

[0114] S106. Limit the amplitudes of U z1 and U z2 .

[0115] In this embodiment, step S106 can be further refined into the following steps:

[0116] Limit the amplitudes of U z1 and U z2 according to the following formula:

[0117] u z_min <u z1 ,u z2 <u z_max .

[0118] S107. Superimpose the limited U z1 onto the modulation wave output U a1* , U b1* and U c1* of the first three-phase converter, and superimpose the limited U z2 onto the modulation wave output U a2* , U b2* and U c2* of the second three-phase converter.

[0119] It should be noted that the block diagram of the neutral point balance and circulating current coordinated control is as Figure 3 shown. By collecting the three-phase currents ia, ib, ic, according to the current modulation waves va, vb, vc, the average charge value flowing into the neutral point within one cycle can be obtained from Equation (3), and its value is Q1. After superimposing the zero-sequence component u z , compared with before superposition, the change in the average charge within one cycle is ΔQ2, and the initial neutral point charge is Q0. Assuming that the charge flowing into the neutral point within one cycle is CΔUlimit, then:

[0120] Q1 + ΔQ2 + Q0 = C * ΔU limit ;

[0121] In the above formula, ΔU limit represents the neutral point voltage fluctuation range (with positive maximum and negative maximum), and the expression is as shown in Equation (6). In the formula, it is expressed as ±CΔU limit . In special cases, when its value is 0, the neutral point fluctuation is zero.

[0122] Since the two converters share a common DC bus and neutral line, and the output powers are the same (the interleaved parallel topology is actually one converter, but it is described as two converters here for convenience), the neutral point balance control quantity can be equally superimposed on the two controllers, and only one calculation is required.

[0123] It can be assumed that the neutral point fluctuation is ±10V. Then, the first three-phase converter (i.e., Figure 3 the #1 converter in Figure 3 ) and the second three-phase converter (i.e.,

[0124] the #2 converter in z1 and u z2 , it is equivalent to the neutral point balance control (Equation (7), the neutral point offset control is zero) uz_np The zero-sequence circulating current control quantity u is superimposed z_zs , but since zero-sequence injection limiting is finally adopted (see Equation (9)), through limiting, the output value of the zero-sequence circulating current controller is superimposed within the allowable range of midpoint fluctuation. This zero-sequence circulating current control quantity u z_zs will not affect the midpoint control;

[0125] Looking at z1 and z2 together, it can be divided into two cases:

[0126] ① If iz1>0 (assuming that the clockwise direction flowing into the power grid is positive), then the output u of the zero-sequence circulating current controller z_zs is less than 0, which is equivalent to reducing the output modulation voltage of Converter #1 (due to the equal and opposite superposition of zero-sequence components, it is equivalent to increasing the output modulation voltage of Converter #2). The zero-sequence circulating current increment is:

[0127] uz1 - uz2 = 2u z_zs <0, then the zero-sequence voltage decreases, and the zero-sequence circulating current naturally decreases.

[0128] ② If iz1<0 (assuming that the clockwise direction flowing into the power grid is positive), then the output u of the zero-sequence circulating current controller z_zs is greater than 0, which is equivalent to increasing the output modulation voltage of Converter #1 (due to the equal and opposite superposition of zero-sequence components, it is equivalent to reducing the output modulation voltage of Converter #2). The zero-sequence circulating current increment is:

[0129] uz1 - uz2 = 2u z_zs >0, then the zero-sequence voltage increases, and the zero-sequence circulating current naturally increases.

[0130] The zero-sequence circulating current is specifically set to be equal and opposite here, which is also to increase the adjustment margin of the zero-sequence circulating current controller and has symmetry.

[0131] A method for midpoint balance and circulating current control of an interleaved parallel three-level converter provided by an embodiment of the present invention. By equally distributing the midpoint voltage balance control quantity to at least one first three-phase converter and a second three-phase converter connected in parallel between the bridge arms of the interleaved parallel three-level converter, and equally and oppositely distributing the zero-sequence circulating current control output to the first three-phase converter and the second three-phase converter, and on this basis, correcting the amplitude of the injected zero-sequence voltage, it can ensure the coordinated control of zero-sequence circulating current and midpoint balance, which is of great significance for the development of converter parallel connection.

[0132] Embodiment 2

[0133] Please refer to Figure 4, which is a schematic diagram of the functional modules of a neutral point balance and circulating current control system for an interleaved three-level converter provided in the second embodiment of the present invention. It is applied to at least one interleaved three-level converter. The interleaved three-level converter includes a first three-phase converter and a second three-phase converter connected in parallel between the arms. This system is suitable for implementing the method of the neutral point balance and circulating current control system for the interleaved three-level converter provided in the embodiment of the present invention. This system specifically includes the following modules:

[0134] The electrical signal acquisition module 201 is used to acquire the three-phase currents ia, ib, ic of the first three-phase converter, the upper bus capacitor voltage U dc1 and the lower bus capacitor voltage U dc2 ;

[0135] The first calculation module 202 is used to calculate the zero-sequence voltage u z_np to be injected when the neutral point voltage deviation is zero;

[0136] The second calculation module 203 is used to set the limit value of the charge flowing into the neutral point within one cycle to Q limit , with a modulus of CΔU limit , and calculate the allowable zero-sequence deviation range u z_min and u z_max under this condition;

[0137] The voltage acquisition module 204 is used to obtain the zero-sequence voltage u z_zs injected into the first three-phase converter through the zero-sequence circulating current controller and the zero-sequence voltage -u z_zs of the second three-phase converter with the same magnitude but opposite direction of the circulating current;

[0138] The third calculation module 205 is used to calculate the zero-sequence injection voltage U z1 of the first three-phase converter and the zero-sequence injection voltage U z2 of the second three-phase converter respectively;

[0139] The voltage limiting module 206 is used to limit the amplitudes of U z1 and U z2 ;

[0140] The voltage superposition module 207 is used to superimpose the limited U z1 onto the modulation wave outputs U a1* , U b1* and U c1* of the first three-phase converter, and superimpose the limited U z2 onto the modulation wave outputs U a2* , U b2* and U c2* of the second three-phase converter.

[0141] Preferably, the first calculation module 202 is specifically configured to:

[0142] Calculate the zero-sequence voltage u to be injected when the midpoint voltage deviation is zero according to the following formula z_np :

[0143]

[0144] where C is the capacitance value of the upper and lower groups of capacitors, f s is the switching frequency, i npav is the average midpoint current, and i npo is the midpoint current after injecting the zero-sequence voltage.

[0145] Preferably, the second calculation module 203 is specifically configured to:

[0146] Let the charge limit value flowing into the midpoint within one period be Q limit , with a modulus of CΔU limit , and calculate the allowable zero-sequence deviation range u z_min and u z_max to be injected under this condition according to the following formula:

[0147]

[0148] where u z is the superimposed zero-sequence component, and the amplitude of u z has a maximum value and a minimum value, which are u z_min and u z_max respectively.

[0149] Preferably, the third calculation module 205 is specifically configured to:

[0150] Calculate the zero-sequence injection voltage U z1 of the first three-phase converter and the zero-sequence injection voltage U z2 of the second three-phase converter respectively according to the following formula:

[0151]

[0152] Preferably, the voltage limiting module 206 is specifically configured to:

[0153] Limit U z1 and U z2 according to the following formula:

[0154] u z_min <u z1 , u z2 <u z_max .

[0155] A neutral point balance and circulating current control system for an interleaved three-level converter provided by an embodiment of the present invention can ensure the coordinated control of zero-sequence circulating current and neutral point balance, which is of great significance for the development of converter parallel connection, by equally distributing the neutral point voltage balance control quantity to the first three-phase converter and the second three-phase converter connected in parallel between the bridge arms of at least one interleaved three-level converter, and equally and reversely distributing the zero-sequence circulating current control output to the first three-phase converter and the second three-phase converter, and on this basis, correcting the amplitude of the injected zero-sequence voltage.

[0156] In summary, after reading this detailed disclosure, those skilled in the art will understand that the foregoing detailed disclosure may be presented only by way of example and may not be restrictive. Although not explicitly stated here, those skilled in the art can understand that the present application is intended to encompass various reasonable changes, improvements, and modifications to the embodiments. These changes, improvements, and modifications are intended to be proposed by the present application and are within the spirit and scope of the exemplary embodiments of the present application.

[0157] In addition, certain terms in the present application have been used to describe the embodiments of the present application. For example, "one embodiment", "embodiment" and / or "some embodiments" mean that the specific features, structures, or characteristics described in connection with the embodiment may be included in at least one embodiment of the present application. Therefore, it should be emphasized and understood that the two or more references to "embodiment" or "one embodiment" or "alternative embodiment" in various parts of this specification do not necessarily all refer to the same embodiment. In addition, the specific features, structures, or characteristics may be appropriately combined in one or more embodiments of the present application.

[0158] It should be understood that in the foregoing description of the embodiments of the present application, for the purpose of helping to understand a feature and for the purpose of simplifying the present application, the present application combines various features in a single embodiment, drawing, or its description. However, this does not mean that the combination of these features is necessary. When reading the present application, those skilled in the art are completely likely to extract some of these features as separate embodiments for understanding. That is to say, the embodiments in the present application can also be understood as the integration of multiple sub-embodiments. And it is also established when the content of each sub-embodiment is less than all the features of a single foregoing disclosed embodiment.

[0159] Each patent, patent application, publication of patent application, and other materials cited herein, such as articles, books, specifications, publications, documents, items, etc., may be incorporated herein by reference. The entire content for all purposes, except any prosecution file history associated therewith, any identical that may be inconsistent or conflict with this document, or any identical prosecution file history that may have a limiting effect on the broadest scope of the claims. Now or hereafter associated with this document. By way of example, if there is any inconsistency or conflict between the description, definition, and / or use of a term associated with any of the incorporated materials and the terms, descriptions, definitions, and / or of this document, the terms of this document shall govern.

[0160] Finally, it should be understood that the embodiments of the application disclosed herein are illustrative of the principles of the embodiments of this application. Other modified embodiments are also within the scope of this application. Therefore, the embodiments disclosed in this application are merely examples and not limitations. Those skilled in the art can adopt alternative configurations based on the embodiments in this application to implement the application in this application. Therefore, the embodiments of this application are not limited to the embodiments precisely described in the application.

Claims

1. A neutral point balance and circulating current control method for an interleaved three-level converter, which is applied to at least one interleaved three-level converter. The interleaved three-level converter includes a first three-phase converter and a second three-phase converter connected in parallel between the bridge arms, and is characterized in that The method includes: Collect the three-phase currents \(i_a\), \(i_b\), \(i_c\) of the first three-phase converter, the upper bus capacitor voltage \(U\) dc1 and the lower bus capacitor voltage \(U\) dc2 ; Calculate the zero-sequence voltage u to be injected when the midpoint voltage deviation is zero z_np ; Let the amplitude limit of the charge flowing into the midpoint within one period be Q limit , with a modulus of CΔU limit , where ΔU limit represents the maximum allowable fluctuation range at the midpoint, and calculate the allowable zero-sequence deviation range u z_min and u z_max ; Obtain the zero-sequence voltage u injected into the first three-phase converter through the zero-sequence circulating current controller z_zs and the zero-sequence voltage -u of the second three-phase converter with the same magnitude but opposite direction of the circulating current z_zs ; Calculate the zero-sequence injection voltage U of the first three-phase converter respectively z1 and the zero-sequence injection voltage U of the second three-phase converter z2 ; Limit U z1 and U z2 for amplitude limiting; The clipped U z1 is superimposed on the modulation wave output U a1* , U b1* and U c1* of the first three-phase converter, and the clipped U z2 is superimposed on the modulation wave output U a2* , U b2* and U c2* of the second three-phase converter; The steps for calculating the zero-sequence voltage u z_np to be injected when the midpoint voltage deviation is zero include: Calculate the zero-sequence voltage u to be injected when the midpoint voltage deviation is zero according to the following formula z_np : Among them, C is the capacitance value of the upper and lower group capacitors, f s is the switching frequency, i npav is the midpoint average current, i npo is the midpoint current after injecting the zero-sequence voltage.

2. The method for neutral point balance and circulating current control of the interleaved parallel three-level converter according to claim 1, characterized in that The limited amplitude of the charge flowing into the midpoint within the other period is Q limit , with a modulus of CΔU limit , and calculate the zero-sequence deviation range u z_min and u z_max The steps include: Let the amplitude limit of the charge flowing into the midpoint within one period be Q limit , with a modulus of CΔU limit , and calculate the allowable zero-sequence deviation range u injected under this condition according to the following formula z_min and u z_max : where, u z is the superimposed zero-sequence component, and the amplitude of u z has a maximum value and a minimum value, which are u z_min and u z_max .

3. The method for neutral point balance and circulating current control of the interleaved parallel three-level converter according to claim 2, wherein The step of separately calculating the zero-sequence injection voltage U z1 of the first three-phase converter and the zero-sequence injection voltage U z2 of the second three-phase converter includes: The zero-sequence injection voltage U of the first three-phase converter is calculated respectively according to the following formula z1 and the zero-sequence injection voltage U of the second three-phase converter z2 :

4. The method for neutral point balance and circulating current control of the interleaved parallel three-level converter according to claim 3, characterized in that, The step of limiting U z1 and U z2 includes the following steps: Clip U according to the following formula z1 and U z2 as follows: u z_min <u z1 ,u z2 <u z_max 。 5. A neutral point balance and circulating current control system for an interleaved three-level converter, which is applied to at least one interleaved three-level converter. The interleaved three-level converter includes a first three-phase converter and a second three-phase converter connected in parallel between bridge arms, and is characterized in that, The system includes: An electric signal acquisition module, configured to acquire three-phase currents ia, ib, ic of the first three-phase converter, the upper bus capacitor voltage U dc1 and the lower bus capacitor voltage U dc2 ; The first calculation module is used to calculate the zero-sequence voltage u to be injected when the midpoint voltage deviation is zero z_np ; A second calculation module, configured to limit the charge amplitude flowing into the midpoint within one period to Q limit , with a modulus of CΔU limit , where ΔU limit represents the maximum allowable fluctuation range of the midpoint, and calculates the allowable zero-sequence deviation range u z_min and u z_max ; A voltage acquisition module, configured to obtain, through a zero-sequence circulating current controller, a zero-sequence voltage u injected into the first three-phase converter z_zs and a zero-sequence voltage -u of the second three-phase converter that is equal in magnitude to the circulating current but opposite in direction z_zs ; A third calculation module, configured to calculate respectively the zero-sequence injection voltage U z1 of the first three-phase converter and the zero-sequence injection voltage U z2 of the second three-phase converter; Voltage limiting module, used to limit U z1 and U z2 for limiting; A voltage superposition module, which is used to superimpose the limited U z1 onto the modulation wave output U a1* 、U b1* and U c1* of the first three-phase converter, and to superimpose the limited U z2 onto the modulation wave output U a2* 、U b2* and U c2* of the second three-phase converter; The first computing module is specifically configured to: Calculate the zero-sequence voltage u to be injected when the midpoint voltage deviation is zero according to the following formula z_np : Among them, C is the capacitance value of the upper and lower group capacitors, f s is the switching frequency, i npav is the midpoint average current, i npo is the midpoint current after injecting the zero-sequence voltage.

6. The neutral point balance and circulating current control system of the interleaved parallel three-level converter according to claim 5, characterized in that The second computing module is specifically configured to: Let the amplitude limit of the charge flowing into the midpoint within one period be Q limit , with a modulus of CΔU limit , and calculate the allowable zero-sequence deviation range u to be injected under this condition according to the following formula z_min and u z_max : where, u z is the superimposed zero-sequence component, and the amplitude of u z has a maximum value and a minimum value, which are u z_min and u z_max .

7. The neutral point balance and circulating current control system of the interleaved parallel three-level converter according to claim 6, wherein The third computing module is specifically configured to: The zero-sequence injection voltage U of the first three-phase converter is calculated respectively according to the following formula z1 and the zero-sequence injection voltage U of the second three-phase converter z2 :

8. The neutral point balance and circulating current control system of the interleaved parallel three-level converter according to claim 7, wherein The voltage limiting module is specifically configured to: Clip U according to the following formula z1 and U z2 as follows: u z_min <u z1 ,u z2 <u z_max 。

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