Modular multilevel DC converter voltage regulation method based on module number and phase shift angle adjustment

Abstract

The present invention is a voltage regulation method for a modular multi-level DC converter based on the number of modules and phase shift angle adjustment. By adjusting the voltage modulation ratio of the modular multi-level circuit and the switching signal phase shift angle of the bridge arm submodule, continuous and precise regulation of the output voltage under a wide input voltage range is achieved. The present invention determines the number of switching modules of the modular multi-level circuit through a voltage feedforward link, changes the voltage modulation ratio, and adjusts the output voltage of the modular multi-level circuit. Through the output voltage feedback link, the switching signal phase shift angle of the bridge arm submodule is adjusted, so as to accurately and continuously regulate the output voltage. The present invention does not need to adjust the switching frequency, so that the modular DC converter works at the resonance point, can widen the input and output voltage range, and achieve wide range and high efficiency conversion; the design method of the present invention can reduce the number of submodules required for the modular multi-level circuit, reduce the system construction cost, and make the converter have important application value in the field of medium and high voltage DC conversion.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronic control, and in particular relates to a modular multi-level DC converter voltage regulation method based on module number and phase shift angle regulation. Background Art

[0002] With the rapid development of large-scale new energy grid connection, large-scale data centers and rail transit traction, medium-voltage large-capacity converters have been widely studied and applied in many fields. At present, low-voltage power semiconductor devices are connected in series and parallel or low-voltage modules are connected in series and parallel to meet the application requirements of high voltage levels and large-capacity power supplies. In actual application projects, it is often necessary to convert medium and high voltage electric energy (tens of kilovolts) into low voltage (hundreds of volts) to supply power to the subsequent loads. The traditional solution adopts a topological structure in which a single low-voltage module is connected in series on the input side and in parallel on the output side for power conversion. However, as the input and output voltages increase, the level of the isolation transformer inside each module will also increase synchronously, which will increase the size of the isolation transformer and reduce the conversion efficiency and power density of the converter. There are also centralized transformer topology solutions at present, which reduce the volume and loss of magnetic components such as transformers. Among them, the modular multi-level resonant converter uses multiple modules in series on the input side, which can be flexibly used in different voltage levels in a modular way. The LLC resonant circuit voltage regulation solution has been used in the medium voltage power supply field. Under the medium voltage wide input voltage range, only using frequency modulation and voltage regulation strategy will pose severe challenges to the hardware circuit parameter design. When the transformer excitation inductance is small, the converter will have a large excitation current in medium voltage applications, increasing the converter loss. On the other hand, the LLC resonant circuit usually uses the frequency adjustment of the switching device to adjust the output voltage in the voltage regulation solution. The frequency adjustment range is limited and cannot be applied to occasions where the input voltage range changes widely. At the same time, the highest efficiency point of the resonant converter is near the resonant frequency. By changing the frequency adjustment method, it is impossible to achieve wide range voltage regulation and maximize operating efficiency at the same time. Summary of the invention

[0003] In view of the above problems existing in the prior art, the present invention proposes a modular multi-level DC converter based on module number and phase shift angle adjustment, including a DC input capacitor, a modular multi-level circuit, a resonant capacitor, a medium and high frequency transformer, a rectifier circuit, an output capacitor, a feedforward control loop and a feedback control loop;

[0004] The modular multi-level circuit is divided into two bridge arms, an upper bridge arm and a lower bridge arm, each bridge arm is composed of N identical bridge arm sub-modules and a bridge arm inductor connected in series; one pole of the primary side of the medium and high frequency transformer is connected to the connection point of the two DC input capacitors through a resonant capacitor, and the other pole is connected to the connection point of the upper and lower bridge arms, the secondary side of the transformer is connected to the output rectifier circuit, and the output rectifier circuit is connected to the output capacitor; the feedforward control loop is used to adjust the voltage modulation ratio m of the modular multi-level circuit according to the input voltage; the feedback control loop is used to sample the output voltage, and then adjust the switching signal phase shift angle between the bridge arm sub-modules through a proportional integral closed-loop control method to control the output voltage.

[0005] The present invention also provides a voltage regulation method for a modular multi-level DC converter, which is specifically: a feedforward control loop controls the number of bridge arm sub-modules that are continuously put into use in a switching cycle according to the input voltage, adjusts the voltage modulation ratio m of the modular multi-level circuit, and reduces the range of variation of the AC output voltage; a feedback control loop samples the output voltage, and then adjusts the switching signal phase shift angle between the bridge arm sub-modules that are not continuously put into use through a proportional-integral closed-loop control method, changes the fundamental amplitude of the inverter AC voltage, and controls the output voltage.

[0006] As a preferred solution of the present invention, in one switching cycle, K submodules of the bridge arm are continuously put into operation, and the remaining NK submodules are modulated by quasi-square waves. The voltage modulation ratio m is expressed as:

[0007]

[0008] Where K is the number of bridge arm submodules continuously put into use, N is the total number of bridge arm submodules in a bridge arm, and m is the voltage modulation ratio;

[0009] The method for selecting the K value is: when the DC input voltage V ink When it is minimum, it is recorded as V in0 , K value is 0, voltage modulation ratio m = 1,

[0010] When the DC input voltage V ink When the voltage increases, the number of bridge arm sub-modules K continuously put into use in one cycle increases one by one during adjustment (K is 0, 1, 2, 3, ...), and the voltage modulation ratio is V ink Satisfies the following relationship:

[0011]

[0012] K is an integer, and the number of bridge arm submodules added to the bridge arm increases until V ink Greater than the maximum DC input voltage.

[0013] As a preferred embodiment of the present invention, the fundamental amplitude of the inverter AC output voltage is changed by adjusting the switching signal phase shift angle of the bridge arm sub-module. The larger the phase shift angle, the lower the output gain. Under the output feedback control closed-loop regulation, as the input voltage increases, the switching signal phase shift angle increases to maintain the stability of the output voltage. When the number of bridge arm sub-modules continuously put into operation in a cycle increases one by one, the switching signal phase shift angle is actively adjusted to the minimum to reduce the output voltage overshoot, and then closed-loop regulation is performed; when the input voltage decreases and the number of bridge arm sub-modules continuously put into operation in a cycle decreases one by one, the switching phase shift angle is actively adjusted to the maximum switching phase shift angle to reduce the output voltage overshoot, and then closed-loop control is performed.

[0014] As a preferred embodiment of the present invention, the switching signal phase shift angle is the switching signal phase shift angle of the entire bridge arm submodule that is not continuously put into operation, the switching signal phase shift angle of half of the bridge arm submodule that is not continuously put into operation, or the switching signal phase shift angle of a single bridge arm submodule that is not continuously put into operation.

[0015] As a preferred solution of the present invention, when the feedback control loop adjusts the switch signal phase shift angle of the bridge arm submodule that is not continuously put into operation as a whole through a proportional-integral closed-loop control method, the switch signal phase shift angle α of the bridge arm submodule that is not continuously put into operation is adopted by quasi-square wave modulation, and the feedback control loop adjusts the switch signal phase shift angle α between the bridge arm submodules that are not continuously put into operation as a whole according to the output voltage closed-loop. When the output voltage is greater than the set value, the switch signal phase shift angle α is increased, and when the output voltage is less than the set value, the switch signal phase shift angle α is reduced; the switch signal phase shift angle α is greater than 0 and less than

[0016] As a preferred solution of the present invention, when the feedback control loop adjusts the switching signal phase shift angle of half of the bridge arm sub-modules that are not continuously put into operation through a proportional-integral closed-loop control method, the bridge arm sub-module that is not continuously put into operation uses quasi-square wave modulation, and the switching signal phase shift angle between the second half of the bridge arm sub-module and its previous half of the bridge arm sub-module is β. The feedback control loop adjusts the switching signal phase shift angle β of the second half of the sub-module according to the output voltage closed-loop. When the output voltage is greater than the set value, the switching signal phase shift angle β is increased, and when the output voltage is less than the set value, the switching signal phase shift angle β is reduced; the switching signal phase shift angle β is greater than 0 and less than π.

[0017] As a preferred embodiment of the present invention, when the feedback control loop adjusts the switching signal phase shift angle of a single bridge arm sub-module that is not continuously put into operation through a proportional-integral closed-loop control method, the switching signal phase shift angle between the last sub-module of the bridge arm sub-module that is not continuously put into operation using quasi-square wave modulation and the remaining sub-modules is γ, and the feedback control loop adjusts the switching signal phase shift angle γ of the last sub-module after closed-loop adjustment of the output voltage. When the output voltage is greater than the set value, the switching signal phase shift angle γ is increased, and when the output voltage is less than the set value, the switching signal phase shift angle γ is reduced; the switching signal phase shift angle γ is greater than 0 and less than π.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] (1) Compared with the constant voltage modulation ratio control strategy, the present invention changes the number of sub-modules continuously put into operation within a cycle as the input voltage changes. At the same time, the upper and lower bridge arms N+K sub-modules jointly support the DC input voltage, which can reduce the working voltage of the sub-modules. On the other hand, it can reduce the total number of bridge arm sub-modules, thereby reducing the system construction cost and operating loss.

[0020] (2) Variable module number voltage regulation has the characteristics of strong regulation capability but discontinuous regulation, and variable phase shift angle regulation has the characteristics of continuous regulation but limited regulation capability. The present invention combines the two control degrees of freedom to achieve wide range continuous voltage regulation capability.

[0021] (3) Compared with the traditional frequency modulation and voltage regulation strategies, the several phase-shifting and voltage regulation strategies proposed in the present invention do not rely on the transformer excitation inductance, and can avoid a smaller excitation inductance, thereby reducing the transformer excitation current, reducing the transformer loss and the bridge arm submodule conduction loss, and improving the system operation efficiency, especially the operation efficiency under light load conditions, thereby achieving efficient conversion under wide input and wide load conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a topology diagram of a modular multi-level resonant converter.

[0023] Figure 2 This is a diagram of the output voltage control architecture of a modular multi-level resonant converter.

[0024] Figure 3 A voltage modulation ratio control method for a modular multi-level converter is proposed.

[0025] Figure 4 The figure is an example of a method for adjusting the phase shift angle of a switching signal of a single submodule.

[0026] Figure 5 This is an example of a method for adjusting the phase shift angle of a switching signal of half a submodule.

[0027] Figure 6 This is an example of a method for adjusting the phase shift angle of a switching signal of an overall submodule.

[0028] Figure 7 This is a control embodiment of a submodule phase angle adjustment method. DETAILED DESCRIPTION

[0029] The present invention is further described and illustrated below in conjunction with specific embodiments. The embodiments are merely exemplary of the present disclosure and do not define the scope of limitation. The technical features of each embodiment of the present invention may be combined accordingly without conflicting with each other.

[0030] like Figure 2 As shown in FIG. 1 , the present invention proposes a voltage regulation strategy based on the number of modules and phase shift angle control and applied to a wide input range modular multi-level resonant converter. The control framework of the converter proposed in the present invention is as follows: Figure 2 As shown, including the DC input capacitor C in , modular multi-level circuit, resonant capacitor, medium and high frequency transformer, rectifier circuit, output capacitor, feedforward control loop and feedback control loop; the feedforward control loop and feedback control loop sample the input voltage and output voltage respectively, the feedforward control link calculates the voltage modulation ratio of the modular multi-level circuit according to the input voltage, calculates the number of sub-modules that need to be switched, and adjusts the voltage amplitude output by the modular multi-level circuit to a narrower range. At the same time, the feedback control link adjusts the phase shift angle of the bridge arm sub-module switch signal according to the output voltage closed loop, and changes the circuit gain to accurately control the output voltage.

[0031] like Figure 1 As shown, the single-phase modular multi-level resonant converter includes a DC input capacitor C in , modular multi-level circuit MMC, resonant circuit, medium and high frequency transformer, rectifier circuit, output capacitor C o and output equivalent load R L The modular multi-level circuit is divided into two bridge arms, the upper and lower arms, each of which consists of N identical half-bridge sub-modules SM 1 ~SM N and a bridge arm inductor L p / L n The upper and lower bridge arm inductors can be connected in series by coupling winding and wound on the same magnetic core, which can ensure that the inductance of the upper and lower bridge arms is equal and make the equivalent inductance of the bridge arm inductor on the AC side zero, thus realizing the decoupling design of the bridge arm inductor and the inductance in the resonant circuit. The voltage and current of the upper and lower bridge arms are expressed by u p ,i p ,u n ,i n Indicates that the resonant circuit consists of the resonant capacitor C r , Transformer leakage inductance L k and the magnetizing inductance L m The transformer secondary rectifier circuit adopts a full-bridge structure D 1 ~D 4 To reduce the voltage and current stress of a single switching device.

[0032] In one square wave cycle, K sub-modules of the bridge arm are continuously put into use, and the remaining NK sub-modules are modulated by quasi-square waves. The voltage modulation ratio m is expressed as:

[0033]

[0034] Where K is the number of bridge arm submodules continuously put into use, N is the total number of bridge arm submodules in a bridge arm, and m is the voltage modulation ratio;

[0035] The method for selecting the K value is: when the DC input voltage is the minimum, it is recorded as V in0 , K value is 0, voltage modulation ratio m = 1, when the DC input voltage increases to V in1 When K switches from 0 to 1, the voltage modulation ratio is V in1 Satisfies the following relationship:

[0036]

[0037] When the DC input voltage increases to V ink When the number of bridge arm submodules K continuously put into use in one cycle increases from k-1 to k (k is 1, 2, 3, ...), the voltage modulation ratio is V ink Satisfies the following relationship:

[0038]

[0039] K is an integer, and the DC input voltage increases until V ink Greater than the maximum DC input voltage.

[0040] Figure 3 The figure shows the specific modulation strategy of voltage modulation ratio control. The bridge arm has N sub-modules, of which K sub-modules have a duty cycle of 100% in one switching cycle and are continuously put into operation. The remaining NK sub-modules have a duty cycle of 50% in one switching cycle. There is a phase shift angle θ between adjacent drive signals, so that a multi-level quasi-square wave is generated. Similarly, a set of complementary drive signals are applied to the lower bridge arm. Voltage modulation ratio control can be achieved by changing the K value.

[0041] Figure 4 The figure shows the adjustment method of the switching signal phase shift angle of a single submodule. In one bridge arm, the phase shift angle of the switching signal of a single submodule is adjusted, and the remaining submodules adopt the quasi-square wave modulation strategy according to the traditional method, which can reduce the impact of dv / dt on the transformer and reduce the difficulty of insulation design of medium and high frequency transformers. The phase shift angle of the submodule is obtained through the output closed-loop feedback. The gain of the input voltage is different under different phase shift angles. The phase shift angle of the submodule is 0~π.

[0042] Figure 5 The figure shows the adjustment method of the switching signal phase shift angle of half of the submodules. In one bridge arm, the phase shift angle of the switching signal of half of the submodules is adjusted. The phase shift angle of the submodule is obtained through the output closed-loop feedback. The gain of the input voltage is different under different phase shift angles. The phase shift angle of the submodule is 0~π. The number of submodules involved in the phase shift angle adjustment is The phase shift angle of half of the sub-modules is adjusted to change the bridge arm voltage AC output from a quasi-square wave to a quasi-three-level wave, which can reduce the impact of dv / dt on the transformer.

[0043] Figure 6 The figure shows the adjustment method of the phase shift angle of the switching signal of the whole submodule. In one bridge arm, the phase shift angle of the switching signal of the whole submodule is adjusted. The phase shift angle of the submodule is obtained through the output closed-loop feedback. The gain of the input voltage is different under different phase shift angles. The phase shift angle of the submodule is The number of submodules involved in the phase shift angle adjustment is NK. The overall submodule phase shift angle adjustment changes the bridge arm voltage AC output from a quasi-square wave to a quasi-triangular wave.

[0044] In order to demonstrate the implementation effect of the modular multi-level DC converter voltage regulation method based on the number of modules and phase shift angle adjustment proposed in the present invention, a simulation model was built in the PLEC electrical simulation software for verification. The main parameters of the simulation model are as follows: DC input voltage 9kV~18kV, output voltage is constant 375V, maximum output power is 100kW, the number of bridge arm submodules is 18, the resonant frequency is 12kHz, and the phase shift angle of a single submodule is adjusted. Table 1 shows the parameter values ​​of the implementation case.

[0045] Table 1

[0046] parameter DC input voltage 9kV~18kV DC output voltage 375V Rated output power 100kW Number of single bridge arm submodules N 18 Submodule maximum voltage 782V Bridge arm inductance 500μH Transformer ratio 12:1 Transformer leakage inductance 600μH Transformer magnetizing inductance 150mH Resonant capacitor 293nF Resonant frequency, switching frequency 12kHz

[0047] When the input voltage is the minimum V in0 =9kV, the feedforward link sets the voltage modulation ratio to the maximum of 1, that is, in one switching cycle, all 18 sub-modules of the bridge arm participate in square wave switching, and the number of sub-modules K=0 is always put into operation, and the sub-module phase shift angle is a small value; as the input voltage rises, the feedback link will adjust the sub-module phase shift angle to maintain the stability of the output voltage. When the input voltage increases to When the feedforward link sets the voltage modulation ratio to That is, K = 1, and the submodule phase shift angle is actively adjusted to the minimum to reduce the output voltage overshoot, and then the submodule phase shift angle is autonomously adjusted by the feedback closed loop. Similarly, when K = 2, 3, 4, 5, 6, the voltage switching point set by the feedforward submodule adjustment is calculated according to the formula. Table 2 shows the voltage switching point set by the feedforward submodule adjustment.

[0048] Table 2

[0049]

[0050] Figure 7The output voltage regulation effect diagram of the voltage regulation strategy combining the continuous change of input voltage, voltage modulation ratio and phase shift angle adjustment of a single submodule is shown. Under the 100kW output condition, when the input voltage changes continuously from 13kV to 18kV, K needs to be switched from 3 to 5. At the voltage modulation ratio switching point, the output voltage oscillation does not exceed 15V. After 20ms, the feedback control loop adjusts the phase shift angle of a single submodule to restore the output voltage to the set value of 375V, and the phase shift angle range can be controlled within the range of 0 to 120°. In addition, each increase in the K value can reduce the submodule capacitor voltage and reduce the submodule voltage stress. In other words, under the premise that the input voltage and the maximum voltage of the submodule are constant, the voltage regulation scheme of the present invention can reduce the number of submodules required for the bridge arm. The simulation results verify the excellent effect of the voltage regulation method proposed in the present invention.

[0051] In summary, the present invention does not need to adjust the switching frequency, so that the modular DC converter works at the resonance point, which can widen the input and output voltage range and achieve wide-range high-efficiency conversion; the design method of the present invention can reduce the number of sub-modules required for the modular multi-level circuit and reduce the system construction cost, so that the converter has important application value in the field of medium and high voltage DC conversion.

[0052] The above-mentioned embodiments only express several implementation methods of the present invention, and the description is relatively specific and detailed, but it cannot be understood as limiting the scope of the present invention. For ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A voltage regulation method for a modular multi-level DC converter based on module number and phase shift angle adjustment, characterized in that: The modular multi-level DC converter includes a DC input capacitor, a modular multi-level circuit, a resonant capacitor, a medium-high frequency transformer, a rectifier circuit, an output capacitor, a feedforward control loop and a feedback control loop; The modular multi-level circuit is divided into two bridge arms, an upper arm and a lower arm, each bridge arm is composed of N identical bridge arm sub-modules and a bridge arm inductor connected in series; one pole of the primary side of the medium and high frequency transformer is connected to the connection point of the two DC input capacitors through a resonant capacitor, and the other pole is connected to the connection point of the upper and lower bridge arms, the secondary side of the transformer is connected to the output rectifier circuit, and the output rectifier circuit is connected to the output capacitor; the feedforward control loop is used to adjust the voltage modulation ratio m of the modular multi-level circuit according to the input voltage; the feedback control loop is used to sample the output voltage, and then adjust the switching signal phase shift angle between the bridge arm sub-modules through a proportional integral closed-loop control method to control the output voltage; The voltage regulation method comprises: the feedforward control loop controls the number of bridge arm submodules continuously put into operation in a switching cycle according to the input voltage, adjusts the voltage modulation ratio m of the modular multi-level circuit, and reduces the variation range of the AC output voltage; the feedback control loop samples the output voltage, and then adjusts the switching signal phase shift angle between the bridge arm submodules that are not continuously put into operation through a proportional integral closed-loop control method, changes the fundamental amplitude of the inverter AC voltage, and controls the output voltage; In one switching cycle, K sub-modules in the bridge arm are continuously put into operation, and the remaining NK sub-modules are modulated by quasi-square waves. The voltage modulation ratio m is expressed as: Where K is the number of bridge arm submodules continuously put into use, N is the total number of bridge arm submodules in a bridge arm, and m is the voltage modulation ratio; The method for selecting the K value is: when the DC input voltage V ink When it is minimum, it is recorded as V in0 , K value is 0, voltage modulation ratio m = 1, When the DC input voltage V ink When the bridge arm increases, the number of bridge arm sub-modules K continuously put into use in one cycle increases one by one during adjustment, and the voltage modulation ratio is V ink Satisfies the following relationship: K is an integer, and the number of bridge arm submodules added to the bridge arm increases until V ink Greater than the maximum DC input voltage; By adjusting the switching signal phase shift angle of the bridge arm submodule, the fundamental amplitude of the inverter AC output voltage is changed. The larger the phase shift angle, the lower the output gain. Under the closed-loop regulation of output feedback control, as the input voltage increases, the switching signal phase shift angle increases to maintain the stability of the output voltage. When the number of bridge arm sub-modules continuously put into operation in a cycle increases one by one, the switching signal phase shift angle is actively adjusted to the minimum to reduce the output voltage overshoot, and then closed-loop regulation is performed; when the input voltage decreases and the number of bridge arm sub-modules continuously put into operation in a cycle decreases one by one, the switching phase shift angle is actively adjusted to the maximum switching phase shift angle to reduce the output voltage overshoot, and then closed-loop control is performed.

2. The voltage regulation method of the modular multi-level DC converter according to claim 1, characterized in that: The switching signal phase shift angle is the switching signal phase shift angle of the entire bridge arm submodule that is not continuously put into operation, the switching signal phase shift angle of half of the bridge arm submodule that is not continuously put into operation, or the switching signal phase shift angle of a single bridge arm submodule that is not continuously put into operation.

3. The voltage regulation method of the modular multi-level DC converter according to claim 2, characterized in that: When the feedback control loop adjusts the switch signal phase shift angle of the bridge arm submodule that is not continuously put into operation as a whole through the proportional integral closed-loop control method, the switch signal phase shift angle α of the bridge arm submodule that is not continuously put into operation is modulated by a quasi-square wave, and the feedback control loop adjusts the switch signal phase shift angle α between the bridge arm submodules that are not continuously put into operation as a whole according to the output voltage closed-loop. When the output voltage is greater than the set value, the switch signal phase shift angle α is increased, and when the output voltage is less than the set value, the switch signal phase shift angle α is reduced; the switch signal phase shift angle α is greater than 0 and less than 4. The voltage regulation method of the modular multi-level DC converter according to claim 2, characterized in that: When the feedback control loop adjusts the switching signal phase shift angle of the half of the bridge arm sub-modules that are not continuously put into use through the proportional-integral closed-loop control method, the quasi-square wave is used to modulate the bridge arm sub-module that is not continuously put into use, and the switching signal phase shift angle between the second half of the bridge arm sub-module and the first half of the bridge arm sub-module is β. The feedback control loop adjusts the switching signal phase shift angle β of the second half of the sub-module according to the output voltage closed-loop. When the output voltage is greater than the set value, the switching signal phase shift angle β is increased, and when the output voltage is less than the set value, the switching signal phase shift angle β is reduced; the switching signal phase shift angle β is greater than 0 and less than π.

5. The voltage regulation method of the modular multi-level DC converter according to claim 2, characterized in that: When the feedback control loop adjusts the switching signal phase shift angle of a single bridge arm submodule that is not continuously put into operation through a proportional-integral closed-loop control method, the switching signal phase shift angle of the last submodule of the bridge arm submodule that is not continuously put into operation is γ compared with the remaining submodules using quasi-square wave modulation. The feedback control loop adjusts the switching signal phase shift angle γ of the last submodule after closed-loop adjustment of the output voltage. When the output voltage is greater than the set value, the switching signal phase shift angle γ is increased; when the output voltage is less than the set value, the switching signal phase shift angle γ is reduced; the switching signal phase shift angle γ is greater than 0 and less than π.

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