Isolated MMDC sub-module energy control method and system

Abstract

The invention discloses an isolated MMDC sub-module energy control method and system. At present, in an application scene of a medium-voltage direct-current power distribution network, the voltage level of a medium-voltage side is lower than that of high-voltage direct current, the number of sub-modules of MMDC is relatively small, and the modulation effect of adopting a nearest level approximation strategy is not ideal. MMDC quasi-two-level phase shift modulation is carried out by introducing a small amount of phase shift between control pulses of MMDC sub-modules, and when the maximum phase shift ratios of the control pulses of the primary and secondary side sub-modules of a transformer are the same, a calculation formula of isolated MMDC transmission active power in a steady state is given; active power control of MMDC transmission in a steady state is carried out by adjusting the phase shift ratio D between the no-load voltage of the primary side and the secondary side of the transformer, and energy control of the bridge-arm-free current sampling sub-module of the isolated MMDC is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of direct current converters, and in particular to an isolated MMDC submodule energy control method and system. Background Art

[0002] The mature sub-module energy control methods of traditional modular multilevel AC-DC converters (MMCs) are all based on sinusoidal wave modulation and require real-time detection of the bridge arm current. They cannot be applied to isolated MMDCs (modular multilevel DC / DC converters) in medium and high frequency quasi-two-level modulation modes.

[0003] For sinusoidal pulse width modulation strategies in MMC, such as nearest level approximation and carrier phase shifting, the nearest level approximation strategy requires a larger number of modules to achieve a better modulation effect. However, in the application scenario of medium-voltage DC distribution network, the voltage level on the medium-voltage side is lower than that on the high-voltage DC. Therefore, the number of sub-modules of MMDC is relatively small, and the effect of using this modulation method is not ideal.

[0004] Therefore, it is necessary to study an isolated MMDC sub-module energy control strategy. Summary of the invention

[0005] The purpose of the present invention is to provide an isolated MMDC submodule energy control method and system, which performs isolated MMDC quasi-two-level modulation based on submodule control pulse phase shifting, and performs active power control of isolated MMDC transmission by adjusting the phase shift ratio between the primary and secondary no-load voltages of the transformer, so as to realize the energy control of the isolated MMDC non-bridge arm current sampling submodule.

[0006] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical scheme.

[0007] In a first aspect, the present invention provides an isolated MMDC submodule energy control method, comprising: The MMDC quasi-two-level phase shift modulation is performed by introducing a small amount of phase shift between the sub-module control pulses. D Perform active power control for MMDC transmission; If the MMDC power direction is from the primary side to the secondary side of the transformer, the submodule capacitor voltage is in a balanced state under the initial conditions, and the submodule capacitor voltage ripple is ignored. When the maximum shift ratio of the submodule control pulse on the primary and secondary sides of the transformer is the same, the active power transmitted by the isolated MMDC in steady state is P for: , In the formula, is the quasi-square wave voltage amplitude of the primary side of the transformer equivalent to the primary side; is the quasi-square wave voltage amplitude of the secondary side of the transformer equivalent to the secondary side; Indicates the transformer ratio; Indicates the total inductance value of the AC side, including the leakage inductance of the transformer's primary and secondary bridge arm coupling inductance and the transformer leakage inductance; T h It is the duration of half switching cycle of the submodule; d N It is the maximum shift ratio of the control pulse of the primary side submodule.

[0008] Furthermore, the upper and lower bridge arms adopt a complementary modulation mode, that is, when a submodule is put into / removed in the upper bridge arm, a submodule is correspondingly removed / put into the lower bridge arm, and the sum of the capacitor voltages of the submodules of each bridge arm is equal to the DC side voltage.

[0009] Furthermore, in order to realize the soft switching operation of the isolated MMDC under the rated voltage condition, the maximum shift ratio of the primary submodule control pulse needs to meet the following constraints: .

[0010] Furthermore, due to , the constraints are simplified as follows: .

[0011] Furthermore, in order to achieve soft switching conditions during the removal of the upper bridge arm submodule and the input of the lower bridge arm submodule, when the upper bridge arm submodule is removed, the upper bridge arm current needs to be negative; when the lower bridge arm submodule is input, the lower bridge arm current needs to be positive.

[0012] Furthermore, in order to achieve soft switching conditions during the process of putting the upper bridge arm submodule into operation and removing the lower bridge arm submodule, the upper bridge arm current needs to be positive when the upper bridge arm submodule is removed; and the lower bridge arm current needs to be negative when the lower bridge arm submodule is put into operation.

[0013] Furthermore, the first half cycle of the isolated MMDC t 0 - t 4 The working modes in the first mode are t 0 - t 1 , second mode t 1 - t 2 , the third mode t 2 - t 3 and the fourth mode t 3 - t 4; exist t 0 Before the moment, the lower bridge arm submodules of the primary side A phase bridge arm and the secondary side A' phase bridge arm of the transformer are all removed, and the upper bridge arm submodules are all put into use; the lower bridge arm submodules of the primary side B phase bridge arm and the secondary side B' phase bridge arm are all put into use, and the upper bridge arm submodules are all removed; the primary and secondary voltages of the transformer are both in a high level state; In the first mode t 0 - t 1 Under this condition, the energy absorbed by the AC side from the primary side of the transformer is: , In the formula, is the transformer primary voltage, is the primary current of the transformer; In the second mode t 1 - t 2 Under this condition, the energy absorbed by the AC side from the primary side of the transformer is: ; In the third mode t 2 - t 3 Under this condition, the energy absorbed by the AC side from the primary side of the transformer is: , In the formula, d Ns The maximum shift ratio of the control pulse of the transformer secondary submodule; In the fourth mode t 3 - t 4 Under this condition, the energy absorbed by the AC side from the primary side of the transformer is: .

[0014] Furthermore, the calculation formula for the active power transmitted by the converter in steady state is: .

[0015] Furthermore, to simplify the analysis, it is assumed that the maximum shift of the control pulse of the original and secondary sub-modules is the same, that is, d Ns = d N , the calculation formula of the active power transmitted by the converter in steady state is simplified to: .

[0016] In a second aspect, the present invention provides an isolated MMDC sub-module energy control system for implementing the isolated MMDC sub-module energy control method.

[0017] The present invention analyzes the soft switching operation characteristics and soft switching operation conditions of the converter according to the working mode decomposition and instantaneous current calculation, obtains the relationship between the capacitor energy and the sub-module phase shift by comparing and analyzing the energy changes of different sub-modules in a switching cycle, and controls the active power of MMDC transmission in steady state by adjusting the phase shift ratio D between the primary and secondary no-load voltages of the transformer, thereby realizing the energy control of the armless current sampling sub-module of the isolated MMDC. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0019] Figure 1 It is a control pulse and steady-state voltage and current waveform diagram of the A-phase submodule of the isolated MMDC in the quasi-two-level modulation mode in a specific embodiment of the present invention; Figure 2 is the upper bridge arm current in the specific implementation mode of the present invention Less than zero and the lower arm current When it is greater than zero, the schematic diagram of the switching process when the typical submodule is removed from the upper bridge arm of phase A (and the submodule is put into the lower bridge arm at the same time); Figure 3 is the upper bridge arm current in the specific implementation mode of the present invention Greater than zero and the lower arm current When it is greater than zero, the schematic diagram of the switching process when the typical submodule is removed from the upper bridge arm of phase A (and the submodule is put into the lower bridge arm at the same time); Figure 4 is the upper bridge arm current in the specific implementation mode of the present invention Greater than zero and the lower arm current When it is less than zero, the schematic diagram of the switching commutation process when the upper bridge arm of phase A is put into the submodule (and the lower bridge arm is cut off the submodule at the same time); Figure 5 is the upper bridge arm current in the specific implementation mode of the present invention Greater than zero and the lower arm current When it is greater than zero, the schematic diagram of the switching process when the upper bridge arm of phase A is put into the submodule (and the lower bridge arm is cut off the submodule at the same time); Figure 6It is a simulation waveform diagram of the voltage on the AC side of the transformer in a specific implementation manner of the present invention; Figure 7 This is a bridge arm submodule capacitor voltage waveform diagram obtained by simulation of the present invention. DETAILED DESCRIPTION

[0020] In order to make the purpose, features and advantages of the present invention more obvious and easy to understand, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the embodiments described below are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0021] This embodiment provides an isolated MMDC submodule energy control method, the steps of which are as follows: The MMDC quasi-two-level phase shift modulation is performed by introducing a small amount of phase shift between the submodule control pulses (in this embodiment, the small amount of phase shift refers to the phase shift added between each submodule pulse, and its value is 10us). D Perform active power control for MMDC transmission; If the MMDC power direction is from the primary side to the secondary side of the transformer, the submodule capacitor voltage is in a balanced state under the initial conditions, and the submodule capacitor voltage ripple is ignored. When the maximum shift ratio of the submodule control pulse on the primary and secondary sides of the transformer is the same, the active power transmitted by the isolated MMDC is P for: , In the formula, is the quasi-square wave voltage amplitude of the primary side of the transformer equivalent to the primary side; is the quasi-square wave voltage amplitude of the secondary side of the transformer equivalent to the primary side; Indicates the transformer ratio; Indicates the total inductance value of the AC side, including the leakage inductance of the transformer's primary and secondary bridge arm coupling inductance and the transformer leakage inductance; T h It is the duration of half switching cycle of the submodule; d N It is the maximum shift ratio of the control pulse of the primary side submodule.

[0022] In order to realize the soft switching operation of the isolated MMDC under the rated voltage condition, the maximum shift ratio of the primary submodule control pulse needs to meet the following constraints: , because , The constraints are simplified as follows: .

[0023] The use of quasi-two-level modulation in isolated MMDC can improve the utilization of DC voltage and maintain a small AC side. dv / dt (voltage change rate), introduce the soft switching advantage of the two-level converter, and thus combine the technical advantages of the modular multi-level converter and the traditional two-level DC-DC converter.

[0024] The principle of the above method is described in detail below.

[0025] To simplify the analysis, the MMDC power direction is considered to be from the primary side to the secondary side of the transformer, and the submodule capacitor voltage is assumed to be in a balanced state under the initial conditions, while the submodule capacitor voltage ripple is ignored.

[0026] 1. Realizing MMDC quasi-two-level phase-shift modulation based on sub-module control pulse phase shift Figure 1 The control pulse and steady-state waveform of the A-phase submodule of the isolated MMDC in the quasi-two-level modulation mode are shown. S 1 ... S k ... S N It is the switching control signal of the upper and lower bridge arm neutron modules. S k For example, when S k =1, the submodule it drives is put into operation; when S k =0, the submodule driven by it is cut off. The present invention realizes quasi-two-level modulation by introducing a small amount of phase shift between submodule control pulses. d 1 - d k - d N It is represented as the pulse shift sequence controlled by the primary submodule, corresponding to the pulse S 1 - S k - S N Compared with the shift d 1 is zero, d Nis the maximum phase shift of the primary submodule control pulse. The upper and lower bridge arms adopt complementary modulation mode, that is, when the upper bridge arm puts in (removes) a submodule, the lower bridge arm correspondingly removes (puts in) a submodule. As can be seen from the figure, due to the introduction of a small amount of phase shift between the submodule control pulses, the rising and falling edges of the AC side voltage are steps of N +1 staircase wave, where N is the number of submodules in each bridge arm.

[0027] exist Figure 1 middle, T s is the switching cycle, which is also the AC voltage cycle; T h is the duration of half a switching cycle; is the transformer primary current; , , They are the upper and lower bridge arm currents and common mode current waveforms in the A phase bridge arm, respectively. Under ideal conditions A phase bridge arm output current Half. According to the working principle of isolated MMDC, the upper and lower bridge arms of each phase are in a complementary switching state, and the sum of the capacitor voltages of each bridge arm submodule is equal to the DC side voltage. In a single-phase isolated MMDC, the upper bridge arm of phase B is put into the same voltage as the lower bridge arm of phase A, and the lower bridge arm of phase B is put into the same voltage as the upper bridge arm of phase A. It can be seen that ( is the transformer primary voltage, is the lower bridge arm voltage, is the upper bridge arm voltage), when all the lower bridge arm submodules of phase A are put into operation and all the upper bridge arm submodules are removed In the high level state, its voltage value is V dc1 equal NV c ; When all the lower bridge arm submodules of phase A are removed In the low level state, its voltage value is - V dc1 ,equal- NV c ,in V c is the average voltage value of the submodule. Therefore, the primary voltage The amplitude is V dc1 Similarly, after being equivalent to the primary side of the transformer, the secondary side voltage of the quasi-square wave is The amplitude is nV dc2 .

[0028] 2. Analysis of the operating mode and transmission power of isolated MMDC in quasi-two-level phase-shift modulation mode exist Figure 1 In the example, based on the principle that the area enclosed by the time axis is equal, the transformer primary voltage The rising and falling stages of the step wave can be equivalent to a slope of 2 V dc1 / d N The first-order curve is shown as the red dotted line in the figure. The waveform can be simplified into a trapezoidal wave, and the secondary voltage can be The waveform is equivalently simplified. The steady-state operation process of the isolated MMDC is decomposed into working modes in combination with the primary and secondary equivalent voltage conversion process. Since the voltage and current waveforms on the AC side are symmetrical in the first and second half working cycles, the following focuses on analyzing the working modes of the isolated MMDC in the first half cycle.

[0029] exist t 0 Before the moment, the lower bridge arm submodules of the primary side A phase bridge arm and the secondary side A' phase bridge arm of the transformer are all removed, and the upper bridge arm submodules are all put into use; the lower bridge arm submodules of the primary side B phase bridge arm and the secondary side B' phase bridge arm are all put into use, and the upper bridge arm submodules are all removed; the primary and secondary voltages of the transformer are both in a high level state. t 0 At this moment, the transformer voltage and current can be expressed as: (1) 1) First mode t 0 - t 1 In the interval ( t 0 - t 1 ) In the primary side, the lower bridge arm submodule of phase A follows the shift phase sequence d 1 - d k - d N The upper bridge arm submodules are removed one by one; the lower bridge arm submodules of phase B are removed according to the shift sequence. d 1 - d k - d N The upper bridge arm submodules are switched on one by one. In this mode, the primary voltage Linear rise, secondary side equivalent voltage Maintain as – nV dc2 , the transformer voltage and current can be expressed as: (2) In the formula, L ac is the total inductance of the AC side, including the leakage inductance of the primary-secondary bridge arm coupling inductance and the transformer leakage inductance. Substituting the voltage expression in equation (2) into the current expression, the instantaneous current of the primary side of the transformer can be further sorted out as follows: (3) Considering t 1 - t 0 = d N T h , combining equation (2) and equation (3), we can get t 1 The transformer voltage and current at this moment are: (4) In formula (2), And in formula (3) Multiply and add in the interval ( t 0 - t 1 ), the energy absorbed by the AC side from the primary MMC in this mode is: (5) 2) Second mode t 1 - t 2 t 1 After the moment, the lower bridge arm submodules of the primary phase A and the secondary phase B' are all put into operation, and the upper bridge arm submodules are all removed; the lower bridge arm submodules of the primary phase B and the secondary phase A' are all removed, and the upper bridge arm submodules are all put into operation. Maintain as – nV dc2 , the primary voltage is in a high level state, equal to V dc1 In this mode, the transformer voltage and current are: (6) The voltage expression in equation (6) and the voltage expression in equation (4) are t 1 Substituting the instantaneous current value into the current expression in formula (6), the instantaneous current of the transformer in this mode is obtained as follows: (7) according to Figure 1 Knowable t 2 -t 1 = DT h , combined with formula (7), we can get t 2 The transformer current at the moment is: (8) In the formula, D It is the shift ratio between the no-load voltage on the primary side of the transformer and the no-load voltage on the secondary side.

[0030] Similar to equation (5), find the transformer primary voltage and current product value in the interval ( t 1 - t 2 ), the energy absorbed by the AC side from the primary MMC in this working mode is: (9) 3) The third mode t 2 - t 3 In this mode, the lower bridge arm submodules of the secondary side A' phase are put into operation one by one, and the upper bridge arm submodules are removed one by one; the lower bridge arm submodules of the B' phase are removed one by one, and the upper bridge arm submodules are put into operation one by one; the primary side voltage Maintain high level, the secondary side equivalent voltage Linear increase; the transformer voltage and current can be expressed as: (10) In the formula, d Ns is the maximum shift ratio between the control pulses of the secondary submodule. t 2 Substituting the instantaneous current expression into the current expression in equation (10), the instantaneous current of the transformer in this mode is obtained as follows: (11) according to Figure 1 Knowable t 3 - t 2 = d Ns T h Combining formula (11), we can get t 3 The transformer primary current at the moment is: (12) Similar to equation (5), find the transformer primary voltage and current product value in the interval ( t2 - t 3 ), the energy absorbed by the AC side from the primary MMC in this working mode is: (13) 4) The fourth mode t 3 - t 4 t 3 After the moment, the lower bridge arm submodules of the primary phase A and the secondary phase A' are all removed, and the upper bridge arm submodules are all put into operation; the lower bridge arm submodules of the primary phase B and the secondary phase B' are all put into operation, and the upper bridge arm modules are all removed. The primary and secondary voltages of the transformer are both maintained at a high level, which are equal to V dc1 and nV dc2 In this mode, the transformer voltage The current can be expressed as: (14) Substituting equation (12) and the original secondary voltage value into equation (14), we can obtain: (15) according to Figure 1 Knowable t 4 - t 3 =(1- D - d Ns ) T h Combining formula (15), we can get t 4 The transformer primary current at the moment is: (16) Similar to equation (5), find the transformer primary voltage and current product value in the interval ( t 3 - t 4 ), the energy absorbed by the AC side from the primary MMC in this working mode is: (17) 5) Fifth Mode t 4 - t 5 from t 4At the beginning of the moment, the lower bridge arm submodules of the primary A phase are removed one by one, and the upper bridge arm submodules are put into operation one by one; the lower bridge arm submodules of the B phase are put into operation one by one, and the lower bridge arm submodules are removed one by one; the primary voltage It decreases linearly from high level to low level. In this mode, the state of the secondary submodule remains unchanged. Maintain nV dc2 Therefore, the transformer voltage and current can be expressed as: (18) Substituting the voltage expression in equation (18) and the voltage expression in equation (16) t 4 Substituting the instantaneous current value into the current expression in formula (18), the instantaneous current of the transformer in this mode is obtained as follows: (19) according to Figure 1 It can be seen that t 5 - t 4 = d N T h Combining formula (19), we can get t 5 The primary current of the transformer at this moment is: (20) According to the symmetry of the transformer current waveform: (twenty one) Substituting formula (16) into formula (21), we can obtain: (twenty two) Combining equations (5), (9), (13) and (17), we can get the active power transmitted by the converter in steady state: (twenty three) To simplify the analysis, the maximum shift phase of the control pulse of the original secondary sub-module is taken to be the same, that is, d Ns = d N , then formula (23) can be simplified as: (twenty four).

[0031] 3. Analysis of soft switching conditions of isolated MMDC in quasi-two-level modulation mode According to the soft switching operation principle of the two-level isolated DC-DC converter, the zero voltage condition is met when the anti-parallel diode of the active switch is turned on. At this time, turning on the active switch can achieve zero voltage switching (ZVS). Therefore, whether the isolated MMDC sub-module switch can achieve zero voltage switching needs to be judged in combination with the current direction at the moment of switch action. d 1 - d k - d N When certain constraints are met, the submodule control pulse phase shift modulation proposed in the present invention can achieve ZVS operation of isolated MMDC. In order to analyze the soft switching boundary conditions, the commutation process of each switch device during the switching state of the submodule is discussed in detail by taking the primary phase A as an example. Based on the modulation strategy and the symmetry of the topological structure, similar conclusions can be obtained by analyzing the soft switching boundary conditions of other phases of the isolated MMDC.

[0032] Combination Figure 1 From the above analysis, we can know that in the interval ( t 0 , t 1 ) The upper bridge arm submodules of phase A are removed one by one, and the lower bridge arm submodules are put into use one by one. Starting from a minimum value less than zero, the lower arm current increases It starts from a maximum value greater than zero and gradually decreases. Analysis Figure 1 The bridge arm current waveform shows that during the switching process of the above submodules, there will be Less than zero and the lower arm current Greater than zero sum Greater than zero and the lower arm current There are two cases where the value is greater than zero.

[0033] Figure 2 Shown is the interval ( t 0 , t 1 ) When the upper arm current Less than zero and the lower arm current When it is greater than zero, the switching mode during the process of removing a typical submodule in the upper bridge arm (and simultaneously putting a submodule in the lower bridge arm). As shown in the figure, before the switch action, the upper bridge arm submodule is put into the lower bridge arm submodule and removed. In order to remove the upper bridge arm and put the lower bridge arm submodule into, the active switch needs to be turned off at the same time. and After the switch is turned off, the current in the upper and lower bridge arms is transferred to and The anti-parallel diode enters the dead zone state. Adding a parallel buffer capacitor can effectively reduce the shutdown process and The tube voltage drop can reduce the turn-off loss and even achieve zero voltage turn-off. As the anti-parallel diode is turned on, the tube voltage drop is zero, which satisfies the zero voltage turn-on condition and turns on after a short dead time delay. and It can realize zero voltage conduction. When the bridge arm current reverses, the current naturally commutates from the anti-parallel diode to the active tube.

[0034] Figure 3 Shown is the interval ( t 0 , t 1 ) When the upper arm current When it is greater than zero, the switching mode during the process of removing the typical submodule in the upper bridge arm (while the submodule is put into the lower bridge arm). Before the switch action, the upper bridge arm submodule is put into the lower bridge arm submodule and removed, which is different from Figure 2 ,because Flow through switch The anti-parallel diode conducts instead of , then turn off the switch and The lower bridge arm submodule can be put into use but the upper bridge arm submodule cannot be removed, that is, the switch The shutdown action will not change the switching state of the upper bridge submodule, and the circuit working state is not affected by the active switch action. During the dead zone, the upper bridge arm submodule is always put into the circuit. The uncontrolled input of the upper bridge arm submodule will cause the AC voltage waveform to be distorted, and a pulse voltage will be added to the bridge arm inductor, introducing ripple to the bridge arm common mode current. After the dead zone delay, it is turned on in a hard switch mode. This will force the bridge arm current to The anti-parallel diode circulates to , the reverse recovery process of the diode during this switching process and The turn-on process will increase switching losses and electromagnetic interference.

[0035] Based on the above analysis, it can be seen that the soft switching conditions for realizing the removal of the upper bridge arm submodule and the input of the lower bridge arm submodule are: when the upper bridge arm submodule is removed, it is necessary to ensure that the bridge arm The current is negative; when the lower bridge arm is put into the submodule, it is necessary to ensure that the bridge arm The current is positive. Figure 1 The middle bridge arm current waveform shows that in the interval ( t 0 , t 1) In order to realize that all upper bridge arm submodule removal and lower bridge arm submodule input processes are soft switching actions, the upper and lower bridge arm currents should meet the following conditions: (25) Combination Figure 1 It can be seen that in the interval ( t 4 , t 5 ) The upper bridge arm submodules of phase A are put into operation one by one, and the lower bridge arm submodules are removed one by one. The upper bridge arm current Starting from a maximum value greater than zero, the lower arm current decreases. Starts from a minimum value less than zero and gradually increases. Figure 4 and Figure 5 is the switching process when the upper bridge arm of phase A is put into operation with the submodule (while the lower bridge arm is cut off from the submodule) in this interval. Figure 4 The upper bridge arm current is greater than zero and the lower bridge arm current is less than zero. Figure 5 The currents in the upper and lower bridge arms are both greater than zero. Figure 2 and Figure 3 The analysis process is to analyze Figure 4 and Figure 5 It can be seen that in order to ensure the soft switching operation during the input of the upper bridge arm submodule of phase A and the removal of the lower bridge arm submodule, in the interval ( t 4 , t 5 ), the conditions that the upper and lower bridge arms should meet are: (26) observe Figure 1 The middle arm current waveform combined with equation (10) shows that, since the AC components of the upper and lower arm currents are complementary and contain the same DC bias, when the upper arm current When negative, the lower bridge arm The current must be positive; when the lower bridge arm current When it is negative, the upper arm The current must be positive; at the same time = Therefore, equations (25) and (26) can be integrated and resolved as follows: (27) In the expression of the instantaneous value of the transformer current calculated above, substituting equation (22) into equation (20) yields t 5 The transformer current at the moment is: (28) In addition, ignoring the converter loss, based on the transfer power expression described in equation (24), the input current Idc1 It can be expressed as: (29) Ignoring the power current ripple, the common mode current component in the upper and lower bridge arms of phase A = I dc1 / 2. Substituting the common-mode current component expression and equation (28) into equation (24), we can obtain t 5 The current of the lower bridge arm of phase A at this moment is: (30) According to the analysis of the transmission power characteristics of the phase-shift control converter, in order to achieve the minimum power-free return flow and the highest conversion efficiency on the AC side, the rated voltage on the DC side of the isolated MMDC needs to be designed to be matched, that is, the ratio of the DC voltage is equal to the transformer ratio, which means that under the rated voltage condition: (31) Based on equation (31), equation (30) is simplified to further solve the inequality We can get: (32) Notice (33) Therefore, we can get d N A stronger simplified constraint is: (34) Equations (32) and (34) give the constraints that the submodule control pulse shift ratio needs to meet in order to achieve soft switching operation of the isolated MMDC under rated voltage conditions.

[0036] In order to verify the effectiveness of the isolated MMDC sub-module energy control method described in the present invention, a simulation model of iMMDCT was built on the Matlab / Simulink platform, and the simulation system parameters are shown in Table 1.

[0037] Table 1 iMMDCT simulation parameters

[0038] Figure 6 is the voltage on the AC side of the transformer 、 By slightly shifting the phase between the trigger pulses, Quasi-two-level modulation is achieved, and its rising and falling edges are changed from steep square waves to smooth step waves. 、 The phase shift between them realizes the power transfer, and the simulation results are consistent with the theory.

[0039] Figure 7 : is the voltage waveform of the bridge arm submodule capacitor. From the figure, it can be seen that the submodule energy control method of the present invention is effective, and the submodule capacitor achieves voltage balance.

[0040] This embodiment also provides an isolated MMDC submodule energy control system for implementing the isolated MMDC submodule energy control method. Those skilled in the art can clearly understand that for the convenience and simplicity of description, the specific working process of the system described above can refer to the corresponding process in the aforementioned method, which will not be repeated here.

[0041] As described above, the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features thereof may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. An isolated MMDC submodule energy control method, characterized in that: include: The MMDC quasi-two-level phase shift modulation is performed by introducing a small amount of phase shift between the sub-module control pulses. D Perform active power control of MMDC transmission in steady state; If the MMDC power direction is from the primary side to the secondary side of the transformer, the submodule capacitor voltage is in a balanced state under the initial conditions, and the submodule capacitor voltage ripple is ignored. When the maximum shift ratio of the submodule control pulse on the primary and secondary sides of the transformer is the same, the active power transmitted by the isolated MMDC in steady state is P for: ， In the formula, is the quasi-square wave voltage amplitude of the primary side of the transformer equivalent to the primary side; is the quasi-square wave voltage amplitude of the secondary side of the transformer equivalent to the secondary side; Indicates the transformer ratio; Indicates the total inductance value of the AC side, including the leakage inductance of the transformer's primary and secondary bridge arm coupling inductance and the transformer leakage inductance; T h It is the duration of half switching cycle of the submodule; d N It is the maximum shift ratio of the control pulse of the primary side submodule.

2. The isolated MMDC submodule energy control method according to claim 1, characterized in that: The upper and lower bridge arms adopt a complementary modulation mode, that is, when a submodule is put into / removed in the upper bridge arm, a submodule is correspondingly removed / put into the lower bridge arm, and the sum of the capacitor voltages of the submodules of each bridge arm is equal to the DC side voltage.

3. The isolated MMDC submodule energy control method according to claim 1, characterized in that: In order to realize the soft switching operation of the isolated MMDC under the rated voltage condition, the maximum shift ratio of the primary submodule control pulse needs to meet the following constraints: 。 4. The isolated MMDC submodule energy control method according to claim 3, characterized in that: The constraints are simplified as follows: .

5. The isolated MMDC submodule energy control method according to claim 2, characterized in that: To achieve soft switching conditions during the removal of the upper bridge arm submodule and the input of the lower bridge arm submodule, the upper bridge arm current needs to be negative when the upper bridge arm submodule is removed; and the lower bridge arm current needs to be positive when the lower bridge arm submodule is input.

6. The isolated MMDC submodule energy control method according to claim 2, characterized in that: In order to achieve soft switching conditions during the process of putting the upper bridge arm submodule into operation and removing the lower bridge arm submodule, the upper bridge arm current needs to be positive when the upper bridge arm submodule is removed; and the lower bridge arm current needs to be negative when the lower bridge arm submodule is put into operation.

7. The isolated MMDC submodule energy control method according to claim 1, characterized in that: The first half cycle of the isolated MMDC t 0- t The working modes in 4 are the first mode. t 0- t 1. Second Mode t 1- t 2. The third mode t 2- t 3 and 4th mode t 3- t 4; exist t Before time 0, the lower bridge arm submodules of the primary side A phase bridge arm and the secondary side A' phase bridge arm of the transformer are all removed, and the upper bridge arm submodules are all put into use; the lower bridge arm submodules of the primary side B phase bridge arm and the secondary side B' phase bridge arm are all put into use, and the upper bridge arm submodules are all removed; the primary and secondary voltages of the transformer are both in a high level state; In the first mode t 0 -t 1, the energy absorbed by the AC side from the primary side of the transformer is: ， In the formula, is the transformer primary voltage, is the primary current of the transformer; In the second mode t 1- t 2, the energy absorbed by the AC side from the primary side of the transformer is: ； In the third mode t 2- t 3, the energy absorbed by the AC side from the primary side of the transformer is: ， In the formula, d Ns The maximum shift ratio of the control pulse of the transformer secondary submodule; In the fourth mode t 3- t 4, the energy absorbed by the AC side from the primary side of the transformer is: 。 8. The isolated MMDC submodule energy control method according to claim 7, characterized in that: The calculation formula for the active power transmitted by the converter in steady state is: 。 9. The isolated MMDC submodule energy control method according to claim 8, characterized in that: To simplify the analysis, the maximum shift of the control pulse of the original and secondary sub-modules is assumed to be the same, that is, d Ns = d N , the calculation formula of the active power transmitted by the converter in steady state is simplified to: 。 10. An isolated MMDC submodule energy control system according to claim 1, characterized in that: Used to implement the isolated MMDC sub-module energy control method described in any one of claims 1-9.

Images