A pre-charging start-up method for a fault-ride-through modular multi-level converter

Abstract

The present invention discloses a pre-charging start-up method for a fault-travel modular multi-level converter, and belongs to the field of power electronic control. The control method is divided into an uncontrolled charging stage and a controlled charging stage. In the uncontrolled charging stage, all submodules are in a locked state, and a current limiting resistor is connected to the AC side to limit the impulse current during the charging process. When the submodule voltage is stable and reaches a steady-state voltage value, it switches to the controlled charging stage, adjusts the DC modulation index to 0, and makes the average voltage of the submodule follow the submodule average voltage command value to rise to a preset rated value through a negative feedback control loop; after the submodule completes charging, the DC modulation index is adjusted to 1 to complete the pre-charging process. By introducing the DC modulation index, the present invention improves the AC side output capacity of the fault-travel modular multi-level converter in the controlled charging stage, effectively suppresses the current impulse in the controlled charging stage, and makes the entire charging process stable and efficient.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronic control, and in particular relates to a pre-charging starting method for a fault-ride-through modular multi-level converter. Background Art

[0002] Voltage source converter high voltage direct current transmission (VSC-HVDC) based on fully controlled devices has been widely used in island power supply, urban power distribution and large-scale wind power grid connection. In the voltage source converter (VSC) topology, compared with the traditional two-level and three-level converters, the modular multilevel converter (MMC) has become one of the preferred topologies in the field of high voltage direct current transmission (HVDC) due to its advantages such as good output waveform quality, low device switching frequency and high modularity. At present, most of the MMC-HVDC flexible direct current projects are based on the half-bridge submodule (HBSM) topology. Although the half-bridge MMC has a small number of devices and high operating efficiency, it lacks DC fault handling capabilities and is limited in larger-scale and larger-capacity flexible direct current projects. Correspondingly, the fault-through MMC also uses the full-bridge submodule (FBSM) as a component of the bridge arm valve string, and still retains good consistency in electrical and thermal stress; compared with HBSM, FBSM has negative input capability, so that the bridge arm as a whole can output enough negative pressure to absorb the fault current, and has excellent fault-through capability.

[0003] At present, the research on MMC mainly focuses on the control and modulation strategies under normal operation and fault conditions. Considering the startup process of MMC, how to ensure that the voltage of submodule capacitors can be controlled and efficiently reach its rated working level still needs to be studied. The current pre-charging strategy of MMC mainly involves taking power from the DC side. With the rise of distributed energy, the DC side of MMC begins to bear the DC voltage support and power transmission for the startup of wind, solar and energy storage units. The startup method of pre-charging MMC from the grid-connected AC side has begun to receive attention.

[0004] For the AC side pre-charging of the half-bridge MMC, usually the uncontrolled charging and the controlled charging of the DC voltage constitute the traditional "two-stage" charging method. However, the submodule capacitor voltage that can be achieved by the fault-through MMC in the uncontrolled charging stage is less than 50% of the rated working voltage. If the charging method is directly used, a huge impulse current will be generated at the beginning of the controlled charging, which will not only destroy the control process, but also may trigger the overcurrent fault protection, resulting in startup failure. Therefore, it is necessary to improve the traditional "two-stage" charging method and study a new charging method suitable for fault-through MMC. Summary of the invention

[0005] In order to solve the problems in the prior art, the present invention proposes a pre-charging starting method for a fault-ride-through modular multi-level converter.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] The present invention discloses a pre-charging start-up method for a fault-ride-through modular multi-level converter, which is divided into an uncontrolled charging stage and a controlled charging stage. The fault-ride-through modular multi-level converter has a submodule, a resistance circuit breaker and an AC circuit breaker. The method comprises the following steps:

[0008] 1) In the uncontrolled charging stage, disconnect all resistor circuit breakers, lock all submodules and AC circuit breakers for grid connection, and charge all submodules; when the voltage of the submodule is stable and reaches the steady-state voltage value, enter the controlled charging stage;

[0009] 2) In the controlled charging stage, all resistance circuit breakers are further closed, the DC modulation index of the fault-ride-through modular multi-level converter is adjusted to 0, and the AC side output voltage command of each phase unit is repeatedly obtained to obtain the number of sub-modules that need to be put into operation for each phase unit at different times, and then the sub-modules that need to be put into operation at different times are determined and charged; when the voltages of all sub-modules reach the preset rated values, the DC modulation index is adjusted to 1 to complete the pre-charging process.

[0010] Compared with the prior art, the present invention has the following beneficial effects:

[0011] (1) The present invention improves the traditional AC side pre-charging method of the half-bridge MMC. The traditional negative feedback control structure of the MMC is used in the controlled charging stage, and the DC modulation index of the MMC is introduced and adjusted, making full use of the negative input capability of the fault ride-through submodule. At the same time, the adjustment of the DC modulation index avoids the occurrence of serious current impulses. The value obtained by real-time sampling calculation is used as the feedback value. The feedback value can accurately track the command value. The charging speed has the advantage of a wide range design, which fully guarantees the controllability and safety of the charging process.

[0012] (2) Compared with the common group charging and step charging methods, the present invention charges all full-bridge sub-modules in the controlled charging stage, and only needs to slightly adjust the control structure in the normal operation scenario to achieve an efficient and simple charging method. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is the topological structure diagram of the fault-travel MMC;

[0014] Figure 2 This is the MMC charging control loop architecture diagram for the controlled charging stage;

[0015] Figure 3 This is the output capacity indicator diagram of the bridge arm when the DC modulation index is 1;

[0016] Figure 4 This is the output capacity indicator diagram of the bridge arm when the DC modulation index is 0;

[0017] Figure 5 This is the simulation waveform of the entire charging process. DETAILED DESCRIPTION

[0018] 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 limit the scope.

[0019] The present invention proposes a pre-charging startup method for a fault-ride-through MMC. The present invention makes full use of the negative input capability of the full-bridge submodule, introduces and reduces the DC modulation index, reduces the DC bias of the bridge arm output voltage, thereby improving the output capability of the industrial frequency AC component, matching the AC side output capability with the control instruction, avoiding excessive impulse current caused by insufficient AC side output capability, and providing guarantee for the safe operation and efficient startup of the MMC.

[0020] The topological structure of the fault-through MMC mentioned in the present invention is as follows: Figure 1 As shown in FIG. 1 , the fault-through MMC is composed of three phase units, each of which is connected in parallel; each phase unit includes two bridge arms, namely, an upper and a lower complementary bridge arm, each of which consists of The full-bridge submodule FBSM, a bridge arm impedance R0 and a bridge arm inductor L0 are connected in series (the bridge arm impedance can usually be ignored in the analysis); in a phase unit, the two bridge arm inductors of the upper and lower bridge arms are connected, and the connection point is used as the AC side output terminal; the AC side of the fault-through MMC consists of the grid-side voltage source, the AC side inductor L0, and the AC side voltage source. ac , current limiting resistor R ac , resistor circuit breaker K s And AC circuit breaker K ac The AC side output end is connected in series with the current limiting resistor, the resistance circuit breaker is connected in parallel with the current limiting resistor, the AC circuit breaker is connected in series with the current limiting resistor, and the AC circuit breaker, the AC side inductor and the grid side voltage source are connected in sequence.

[0021] The present invention proposes a pre-charging start-up method for a fault-ride-through MMC, which is divided into an uncontrolled charging stage and a controlled charging stage. The converter has a submodule, a resistance circuit breaker and an AC circuit breaker. The method comprises the following steps:

[0022] 1) In the uncontrolled charging stage, all full-bridge submodules are locked, all resistor circuit breakers are disconnected, and all AC circuit breakers are closed for grid connection. At this time, all full-bridge submodules are charged; when the voltage of the full-bridge submodule is stable and reaches the steady-state voltage value When the charging time is 10 seconds, it enters the controlled charging stage;

[0023] 2) In the controlled charging stage, all resistor circuit breakers are further closed to increase the DC modulation index of the converter to M dc Adjust to 0 to obtain the average voltage of all full-bridge submodules in the fault-through MMC in real time And the average voltage command value of the submodule , making the average voltage track the submodule average voltage command value at the current moment, obtaining the AC side output voltage command of each phase unit, obtaining the number of full-bridge submodules required to be put into each phase unit based on the AC side output voltage and the DC modulation index, and then determining the full-bridge submodules required to be put into use through the voltage balancing algorithm and charging them; repeatedly obtaining the AC side output voltage command of each phase unit to obtain the number of full-bridge submodules required to be put into use for each phase unit at different moments, and then determining the full-bridge submodules required to be put into use at different moments and charging them; after the voltages of all full-bridge submodules reach the preset rated values, adjusting the DC modulation index to 1, and completing the pre-charging process.

[0024] In step 1) of the present invention, the steady-state voltage value is the theoretical maximum value that the capacitance of the full-bridge submodule can reach in the uncontrolled charging stage. The calculation formula for the steady-state voltage value is:

[0025] ;

[0026] in, is the steady-state voltage value; is the amplitude of the AC phase voltage at the grid connection point; is the total number of submodules (full-bridge submodules) in one bridge arm.

[0027] In step 2) of the present invention, at the initial moment of the controlled charging stage, the average voltage is the average voltage value of all full-bridge submodules whose voltage reaches the steady-state voltage value at the end of the uncontrolled charging stage.

[0028] In step 2) of the present invention, the voltage balancing algorithm is to sort the voltages of all full-bridge sub-modules in the same bridge arm. After the voltage balancing algorithm determines, if the full-bridge sub-modules put into operation will undergo a discharge process, a corresponding number of full-bridge sub-modules will be selected and put into operation in accordance with the order of the voltages of the full-bridge sub-modules from high to low; if the full-bridge sub-modules put into operation will undergo a charging process, a corresponding number of full-bridge sub-modules will be selected and put into operation in accordance with the order of the voltages of the full-bridge sub-modules from low to high.

[0029] like Figure 2 As shown in the figure, in the controlled charging stage, the AC side output voltage command of each phase unit is obtained by using the classic control structure of MMC. The average voltage of all full-bridge submodules in the fault-through MMC is calculated by using the submodule average voltage controller. The outer loop of the submodule average voltage controller is the submodule average voltage outer loop, and the submodule average voltage outer loop is based on the average voltage As feedback value, control feedback value tracking submodule average voltage command value The PI regulator outputs the current inner loop command. .

[0030] The inner loop of the submodule average voltage controller is a current inner loop with grid-connected point voltage feedforward and current decoupling. The three-phase current i a 、i b 、i c The dq axis current i is obtained by dq transformation d 、i q , the inner current loop controls the current i d Tracking current inner loop command , and control the current i q Track the first dq axis command ,in, =0; three-phase voltage u at the grid connection point sa 、u sb 、u sc The dq axis voltage u is obtained by dq transformation sd 、u sq , dq axis voltage u sd 、u sq As voltage output feedforward, the current inner loop finally outputs the second dq axis command , , the second dq axis instruction , After dq transformation, the AC side output voltage command of each phase unit is obtained , , ; is the AC side output voltage instruction of the a-phase unit; is the AC side output voltage instruction of the b-phase unit; is the AC side output voltage command of the c-phase unit. Figure 2 The specific process of dq conversion is omitted, but the process of dq conversion is common knowledge in the art.

[0031] like Figure 3 and Figure 4 As shown in Figure 1, the bridge arm voltage output range of the fault-through MMC is shown. p Indicates the output voltage of the upper bridge arm, u n Represents the output voltage of the lower bridge arm; U dc is the DC port voltage; is the angular velocity; t is the time. Figure 3 Corresponding to M dc=1 when the bridge arm voltage output range, Figure 4 Corresponding to M dc =0. By adopting a suitable DC modulation index M dc , the center axis of the maximum output waveform of the upper and lower bridge arm voltage can be changed from 0.5U dc Down to 0V, that is, Figure 3 The waveform is converted to Figure 4 The dotted waveform is shown. dc =0 corresponds to the maximum bridge arm voltage output range, M dc Changing from 1 to 0, the output range is doubled, which will prevent excessive charging surge current from being generated during the charging process due to insufficient output capacity of the AC side of the modular multilevel converter. Excessive surge will increase the risk of converter damage and reduce its operating life.

[0032] In step 2) of the present invention, the calculation formula for obtaining the number of full-bridge submodules required to be put into each phase unit is:

[0033] ;

[0034] Among them, round() represents the rounding function; p represents the upper bridge arm; n represents the lower bridge arm; j represents the phase unit, which is a, b or c respectively; Indicates the number of full-bridge submodules that need to be invested in the upper bridge arm in the j-phase unit; Indicates the number of full-bridge submodules that need to be put into use in the lower bridge arm in the j-phase unit; is the AC side output voltage instruction of the j-phase unit; is the average voltage of all submodules in the converter; is the total number of full-bridge sub-modules in one bridge arm.

[0035] when When it is greater than 0, it means that the corresponding upper bridge arm needs to be invested A submodule with positive output; when When it is less than 0, it means that the corresponding upper bridge arm needs to be invested A submodule with negative output level; when When it is greater than 0, it means that the corresponding lower bridge arm needs to be invested A submodule with positive output; when When it is less than 0, it means that the corresponding lower bridge arm needs to be invested A submodule with negative output.

[0036] In order to demonstrate the implementation effect of the AC side precharging method based on fault-ride-through MMC proposed in the present invention, a simulation model was built in MATLAB / Simulink simulation software for verification. Table 1 shows the parameter values ​​of the implementation case. The following parameters specify the specific values ​​of the component parameters and related voltages in the simulation, Figure 5 The simulation results are established under this parameter.

[0037] Table 1

[0038]

[0039] Figure 5 The simulation process of AC pre-charging corresponding to the fault-through MMC using the method of the present invention is demonstrated. At 0.1s, all full-bridge sub-modules are locked, the AC circuit breaker is closed, the resistor circuit breaker is disconnected, the current limiting resistor is connected, and the uncontrolled charging stage begins. The theoretical value of the sub-module capacitor voltage corresponding to the uncontrolled charging stage is 371V, and the simulation result is 371V, which is in line with theoretical expectations; at 0.5s, the controlled charging stage begins, the current limiting resistor is removed, and the control loop is opened, and the sub-module average voltage command value Taking the steady-state value of the uncontrolled charging stage as the initial value, it rises at a certain slope, and at this time satisfies the DC modulation index M dc =0; all full-bridge submodules of the fault-through MMC complete the charging process in 1.6s, that is, the voltages of all submodules reach the preset rated values; after charging is completed, the DC modulation index M is restored dc =1, the pre-charging process is completed. The simulation results verify the excellent effect of the voltage regulation method proposed in the present invention.

[0040] In summary, the present invention does not require additional pre-charging strategies and can directly complete the effective charging process of the fault-ride-through MMC through a control loop. The controlled charging process maintains accurate tracking of given instructions, is highly controllable, safe and reliable, and has important application value.

[0041] The above-mentioned embodiments only express several implementation modes 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 those of ordinary skill in the art, 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 pre-charging startup method for a fault-ride-through modular multi-level converter, characterized in that: The method is divided into an uncontrolled charging stage and a controlled charging stage. The fault-ride-through modular multi-level converter has a submodule, a resistor circuit breaker and an AC circuit breaker. The method comprises the following steps: 1) In the uncontrolled charging stage, disconnect all resistance circuit breakers, lock all submodules and close all AC circuit breakers to connect to the grid, and charge all submodules; when the voltage of the submodule is stable and reaches the steady-state voltage value, enter the controlled charging stage; 2) In the controlled charging stage, all resistance circuit breakers are further closed, the DC modulation index of the fault-ride-through modular multi-level converter is adjusted to 0, and the AC side output voltage command of each phase unit is repeatedly obtained to obtain the number of sub-modules that need to be put into operation for each phase unit at different times, and then the sub-modules that need to be put into operation at different times are determined and charged; when the voltages of all sub-modules reach the preset rated values, the DC modulation index is adjusted to 1 to complete the pre-charging process.

2. The pre-charging startup method of a fault-ride-through modular multi-level converter according to claim 1, characterized in that: The fault-ride-through modular multilevel converter includes three phase units, each of which is connected in parallel with each other; each phase unit is divided into two complementary bridge arms, an upper arm and a lower arm, each bridge arm is composed of a plurality of sub-modules, a bridge arm impedance and a bridge arm inductor connected in series; in each phase unit, the two bridge arm inductors of the upper and lower bridge arms are connected, and the connection point serves as the AC side output terminal of the phase unit; each AC output terminal is connected to a current limiting resistor, a resistor circuit breaker and an AC circuit breaker, the AC side output terminal is connected in series with the current limiting resistor, the resistor circuit breaker is connected in parallel with the current limiting resistor, and the AC circuit breaker is connected in series with the current limiting resistor.

3. The pre-charging startup method of a fault-ride-through modular multi-level converter according to claim 2, characterized in that: The submodule is a full-bridge submodule.

4. The pre-charging startup method of a fault-ride-through modular multi-level converter according to claim 2, characterized in that: In step 1), the calculation formula of the steady-state voltage value is: ； in, is the steady-state voltage value; is the amplitude of the AC phase voltage at the grid connection point; is the total number of submodules in a bridge arm.

5. The pre-charging startup method of a fault-ride-through modular multi-level converter according to claim 2, characterized in that: In step 2), the step of obtaining the AC side output voltage command of each phase unit includes: Obtaining the average voltage of all submodules in the fault-ride-through modular multilevel converter and the average voltage command value of the submodule at the current moment, so that the obtained average voltage tracks the average voltage command value of the submodule at the current moment, and obtaining the AC side output voltage command of each phase unit at the current moment; Wherein, when the AC side output voltage instruction is obtained for the first time, the average voltage is the average voltage value of all submodules whose voltage reaches the steady-state voltage value at the end of the uncontrolled charging stage.

6. The pre-charging startup method of a fault-ride-through modular multi-level converter according to claim 5, characterized in that: In step 2), the submodule average voltage command value changes in real time and increases gradually; the submodule average voltage command value at the initial moment is a steady-state voltage value, and the maximum value of the submodule average voltage command value is the rated value.

7. The pre-charging startup method of a fault-ride-through modular multi-level converter according to claim 6, characterized in that: The step of making the obtained average voltage track the average voltage command value of the submodule at the current moment to obtain the AC side output voltage command of each phase unit at the current moment comprises: The submodule average voltage controller is used to control the average voltage to track the submodule average voltage command value at the current moment, and the current inner loop command is obtained through the PI adjustment method. ; The three-phase current i at the grid connection point at the current moment a 、i b 、i c The dq axis current i is obtained by dq transformation d 、i q , control current i d Tracking current inner loop command , and control the current i q Track the first dq axis command ,in, =0; three-phase voltage u at the grid connection point sa 、u sb 、u sc The dq axis voltage u is obtained by dq transformation sd 、u sq , based on the dq axis voltage u sd 、u sq Output the second dq axis command , , the second dq axis instruction , After dq transformation, the AC side output voltage command of each phase unit at the current moment is obtained , , .

8. The pre-charging startup method of a fault-ride-through modular multi-level converter according to claim 7, characterized in that: In step 2), the number of submodules required to be put into use in each phase unit is obtained based on the AC side output voltage instruction and the DC modulation index; the calculation formula for the number of submodules required to be put into use in each phase unit is: ； Among them, round() represents the rounding function; p represents the upper bridge arm; n represents the lower bridge arm; j represents the phase unit, which is a, b or c respectively; M dc is the DC modulation degree; Indicates the number of submodules that need to be put into use in the upper bridge arm in the j-phase unit; Indicates the number of submodules that need to be put into use in the lower bridge arm in the j-phase unit; is the AC side output voltage instruction of the j-phase unit; is the average voltage of all submodules in the converter; is the total number of submodules in a bridge arm; when When it is greater than 0, it means that the corresponding upper bridge arm needs to be invested A submodule with positive output; when When it is less than 0, it means that the corresponding upper bridge arm needs to be invested A submodule with negative output level; when When it is greater than 0, it means that the corresponding lower bridge arm needs to be invested A submodule with positive output; when When it is less than 0, it means that the corresponding lower bridge arm needs to be invested A submodule with negative output.

9. The pre-charging startup method of a fault-ride-through modular multi-level converter according to claim 1, characterized in that: In step 2), the determination of the submodules that need to be put into operation at different times is implemented based on a voltage balancing algorithm.

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