CN104821711B - A kind of modular multi-level flexible direct-current transmission converter starts method - Google Patents

Abstract

The invention relates to a method for starting a modularized multi-level flexible direct current transmission converter, which comprises the following steps: A. The DC side poles of each converter station are connected, the sub-modules of each station are locked, and at the same time, the active station closes the AC switch, and enters the Controlled charging stage; B. After the sub-module voltage of each converter station is higher than the corresponding charging threshold voltage, the controllable voltage equalization charging method is respectively used to enter the controllable charging stage, and the active station cuts off the soft start resistance of the AC side; C. Each converter station adopts the closed-loop voltage equalization charging method according to the AC and DC side voltage conditions. When the voltage of its sub-modules reaches the rated level and is stable, they are respectively unlocked and enter the unlocking operation stage. By adding a closed-loop charging method after the controllable charging, the cutting number can be adjusted in real time according to the current sub-module voltage, and the sub-module voltage can be stabilized at the rated value through closed-loop control, avoiding the electrical shock during unlocking operation.

Description

A start-up method for a modular multi-level flexible direct current transmission converter

technical field

The invention relates to a method for starting a modularized multilevel flexible direct current transmission converter.

Background technique

Multi-terminal flexible DC transmission can realize multi-source power supply and multi-drop power receiving, which is a flexible, reliable and fast power transmission method in the power system. The modular multilevel converter has gradually become the development trend of multi-terminal flexible DC transmission due to its advantages of high output voltage waveform quality, low switching loss, easy expansion and strong fault ride-through capability.

The successful completion of the start-up process is the premise and basis for the normal operation of the multi-terminal system. A suitable start-up strategy should be able to coordinate and configure the start-up sequence of each converter in the multi-terminal system, suppress the electrical shock caused by their mutual influence, and avoid start-up failure. However, the MMC-MTDC DC capacitors are scattered in each sub-module, and its start-up not only needs to consider the balanced charging of the sub-module capacitors inside the converter, but also needs to consider the influence of DC coupling between converter stations on the MMC start-up process.

Therefore, it is particularly important to study the coordinated start-up control strategy of the MMC-MTDC system to solve the two key issues of sub-module pre-charging voltage equalization in the converter station and orderly unlocking between converter stations.

However, at present, there are few researches on the coordinated activation of MMC-MTDC systems at home and abroad. In terms of pre-charging and voltage equalization of the converter sub-modules of the multi-terminal system, some scholars have proposed that the sub-modules can be charged in a controlled stage through double closed-loop control. The AC soft-start resistor of the converter station with a power supply on the side (referred to as the active station) has been put into use, which increases the demand for the average power of the soft-start resistor, and will cause a drop in DC voltage when unlocking; some scholars have designed a carrier phase-shifting The start-up method in the controllable stage is complex, but the control implementation is complicated; some scholars have proposed a semi-blocking charging method, which can charge the sub-module to the rated value, but have not fully considered and studied the pre-charging method of the converter. In terms of orderly unlocking between converter stations, the existing research mainly focuses on the start-up strategy of the active station, or the orderly unlocking scheme of the DC side charging of the converter, and does not involve the hybrid charging of the AC and DC sides in the multi-terminal system, and No in-depth research has been done on the orderly unlocking between multi-converter stations in multi-terminal systems.

The Chinese patent application "A Startup Method for Modular Multilevel Flexible DC Transmission Converter" with application number 201210462977.6 discloses a solution. The main steps of the method are: during the start-up process, sort the sub-modules on the bridge arm, cut off some sub-modules with higher voltage, and charge the remaining sub-modules; and constantly repeat the above process until charging to a stable state .

This method has the following defects:

First, the above-mentioned charging method is a controllable charging method, which only removes a fixed number (theoretical removal number) of sub-modules. However, in actual engineering, due to the influence of line and sub-module loss, stray parameters and other factors, the sub-module voltage obtained by the theoretical cut-off number control will deviate from the rated value during charging steady state, and the equivalent sub-module voltage obtained by the current sub-module voltage is The AC voltage on the valve side is different from the actual AC voltage, and the lack of a sufficiently large damping element in the line will cause a large inrush current.

Second, in the early stage of pre-charging, the voltage of the sub-module is low, and the control power supply of the sub-module that adopts the self-powered mode cannot work, so the sub-module can only be locked and uncontrolled charging. According to the difference between the AC and DC side power supplies of the converter, the converter has different charging circuits, and the voltage of the power supply for charging the bridge arm or phase unit under each circuit is different, resulting in different steady-state voltages of the sub-modules, which also determines the charging threshold. The voltage is different. If the charging threshold voltage is set randomly, it will cause some stations to enter the controllable charging stage too early, too slow, or even unable to enter the controllable charging stage, which will cause a large electrical shock, increase the start-up time and even cause start-up failure. At the same time, if the charging mode is not distinguished in the controllable stage, and the cut-off number is set randomly, it will cause different charging of the sub-modules, fail to meet the charging requirements, and even cause overvoltage damage to the sub-modules.

Third, in the multi-terminal system, there is one and only one station for constant DC voltage control. The operation of the multi-terminal system requires a stable DC voltage. If a converter station is unlocked and operated before the fixed DC voltage control station, its injection or extraction of active power into the DC line will cause the DC voltage to increase or decrease, which is not conducive to operation control, and even cause system failure.

Contents of the invention

The purpose of the present invention is to provide a method for starting a modularized multi-level flexible direct current transmission converter to solve the above-mentioned problem 1, the problem that a large inrush current will be generated during unlocking operation.

To achieve the above object, the solution of the present invention includes:

A method for starting a modular multilevel flexible direct current transmission converter, comprising the following steps:

A. The DC side poles of each converter station are connected, the sub-modules of each station are locked, and the AC switch of the active station is closed at the same time, entering the uncontrolled charging stage;

B. After the sub-module voltage of each converter station is higher than the corresponding charging threshold voltage, the controllable voltage equalization charging method is respectively used to enter the controllable charging stage. At the same time, the active station cuts off the soft-start resistance of the AC side;

C. Each converter station adopts the closed-loop voltage equalization charging method according to the AC and DC side voltage conditions. When the voltage of its sub-modules reaches the rated level and is stable, they are respectively unlocked and enter the unlocking operation stage.

Further, in the above step C, when the AC and DC side voltages of the converter station whose station control mode is constant DC voltage control are stable, the station is put into the closed-loop voltage equalization charging method; when the voltage of its sub-module reaches the rated and stable, The station unlocks the pulse, and starts the DC voltage control with the slope controller, and enters the unlocking operation stage;

When the DC side voltage of the remaining stations is stable at the rated value, the closed-loop voltage equalization charging method is respectively put into use. When the voltage of the sub-modules reaches the rated value and is stable, they are respectively unlocked and enter the unlocking operation stage.

Further, in the stage of uncontrolled charging, according to the AC and DC side power supply conditions, the AC side pre-charging, DC side pre-charging or AC-DC side hybrid pre-charging are respectively put into use:

In the pre-charging mode on the AC side, the highest line voltage of the MMC is u _ij (i,j=a,b,c,i≠j), and the freewheeling diodes of the lower bridge arm of phase i and the upper bridge arm of phase j are turned on , to charge its sub-modules;

In the DC side pre-charging mode, the MMC DC voltage u _pn charges all sub-modules of the three phases a, b, and c at the same time;

In the hybrid pre-charging mode on the AC and DC sides, the highest line voltage u _ij of the MMC and the DC voltage u _pn charge the i-phase lower bridge arm and j-phase upper bridge arm sub-modules together, and the other bridge arms have no current.

Further, without controlling the charging stage, judge the peak value u _l of the AC valve side line voltage, and compare it with u _pn to determine the pre-charging method:

When u _l ≥ u _pn , it is the AC side pre-charging mode;

When u _l =0 and u _pn ≠0, it is the DC side pre-charging mode;

When 0<u _l <u _pn , it is the AC and DC side hybrid pre-charging mode.

Further, the charging threshold voltage is higher than the power-on voltage for reliable power-on of the sub-module drive power supply, and for active stations, the charging threshold voltage is lower than u _l /N; for passive stations, the charging threshold The voltage is lower than u _pn /2N; there are N sub-modules on one bridge arm, and the peak voltage of the AC valve side line is u _l .

Further, the controllable voltage equalization charging method: each control cycle, in the AC side pre-charging mode or the AC-DC side hybrid pre-charging mode, N sub-modules in the MMC bridge arm participating in the charging, or the MMC in the DC side pre-charging mode The 2N sub-modules of the phase unit are sorted according to the voltage, and the lower tube IGBT of the m sub-modules with the highest trigger voltage is turned on to make it in the cut-off state, while the rest of the sub-modules remain in the locked state; the cut-off number m _ref determined by the controllable voltage equalization charging method is calculated as follows , where round is a rounding function, and the rated voltage of the MMC sub-module is U _sm :

Further, the closed-loop voltage equalization charging method is based on the controllable voltage equalization charging method, and adjusts the number of sub-modules removed according to the closed-loop control result; the closed-loop control uses the rated voltage of the sub-module as input, and the actual voltage of the sub-module as feedback, and outputs The amount to adjust the number of submodule cuts.

Furthermore, the deviation Δu _sm between the rated voltage U _sm of the MMC sub-module and the actual value u _sm passes through a hysteresis comparator with a threshold voltage of ±ΔU _sm to obtain the adjustment amount Δm of the number of cuts, and passes Δm through an initial value of the controllable average The cut-off number m _ref determined by the piezoelectric charging method, the integral regulator whose time constant is τ, and the output is rounded to obtain the final cut-off number; the control logic of the hysteresis comparator is:

1) When Δusm≥ΔUsm, Δm=1;

2) When Δusm≤-ΔUsm, Δm=-1;

3) Otherwise, Δm remains unchanged.

By adding a closed-loop charging method after the controllable charging, the cutting number can be adjusted in real time according to the current sub-module voltage, and the sub-module voltage can be stabilized at the rated value through closed-loop control, avoiding the electrical shock during unlocking operation.

In the uncontrolled charging stage, the charging mode of each converter can be specified, and the corresponding charging threshold voltage can be obtained according to different modes, which is conducive to the timely conversion of the charging mode of the converter. At the same time, according to the difference between the charging mode and the station control mode in the uncontrolled charging stage, the corresponding theoretical cut-off number is calculated in the controlled charging stage, which can ensure that each converter sub-module is charged to the vicinity of the rated value, and realize reliable and effective start-up control .

The constant DC voltage control station is first unlocked to stabilize the DC bus voltage and ensure the stable operation of the multi-terminal system. Moreover, in the late stage of controllable charging, the stability of the AC and DC side voltages of the constant DC voltage control station means that the charging of the submodules of this station is in a steady state, and the rated and stable DC side voltage of other stations means that the voltage charging of the submodules of other stations is in a stable state. At this time, the closed-loop charging control is put into use, and the voltage of the sub-module is closer to the rated value, so the adjustment of the cut-off number is small, which can avoid the unstable charging of the sub-module caused by its large fluctuation.

Description of drawings

Fig. 1 is the schematic diagram of the three-terminal MMC-MTDC system of embodiment 1;

Fig. 2 is the MMC precharging mode identification flowchart of embodiment 1;

Figures 3-a, 3-b, and 3-c are schematic diagrams of the pre-charging circuit under the three pre-charging modes in the uncontrolled charging stage of embodiment 1;

Fig. 4 is the schematic diagram of the MMC closed-loop voltage equalizing charging control of embodiment 1;

5 is a schematic diagram of the MMC-MTDC system coordinated start-up process in Embodiment 1; ①～⑤ are the jump conditions for the start-up process; ①The stations are started; ②The lowest sub-module voltage is higher than the charging threshold voltage U _t ; ③AC and DC side voltages All are stable; ④The DC voltage is rated and stable; ⑤The sub-module is charged to the rated and stable;

FIG. 6 is a simulation waveform of coordinated startup of the three-terminal MMC-MTDC system in Embodiment 1. FIG.

detailed description

In the following several embodiments, the MMC-MTDC system shown in FIG. 1 is taken as an example for illustration. Figure 1 is a three-terminal MMC-MTDC system. MMC1 and MMC3 are defined as active stations, which adopt constant DC voltage control and constant AC power control respectively, and MMC2 is a passive station, which adopts constant AC voltage control. Certainly, the method of the present invention is not only applicable to a three-terminal system, but also can be directly applied to a multi-terminal system with more than three terminals. In a multi-terminal system, one and only one station is controlled by a constant DC voltage.

As shown in Figure 3, MMC1, MMC2, and MMC3 all contain 6 bridge arms, and a single bridge arm contains N half-bridge sub-modules with a rated voltage of U _sm , the peak voltage of the AC valve side line is u _l , and the DC bus voltage is u _pn , rated at U _dc . The parameters are shown in Table 1.

Table 1

The half-bridge sub-module has three types: input (upper IGBT is turned on, lower IGBT is turned off), cut-off (upper IGBT is turned off, lower IGBT is turned on) and latching (both upper and lower IGBTs are turned off) working status.

In addition to the half-bridge sub-module, as other implementation manners, the MMC may also adopt a structure such as an H-bridge topology.

Example 1

A method for starting a modularized multi-level flexible direct current transmission converter, including the following steps:

A. The DC side poles of each converter station are connected, the sub-modules of each station are locked, and the AC switch of the active station is closed at the same time, entering the uncontrolled charging stage;

B. After the sub-module voltage of each converter station is higher than the corresponding charging threshold voltage, the controllable voltage equalization charging method is respectively used to enter the controllable charging stage. At the same time, the active station cuts off the soft-start resistance of the AC side;

C. When the station control mode is constant DC voltage control, after the AC and DC side voltages of the converter station are stable, the station is put into closed-loop voltage equalization charging method; when the sub-module voltage reaches the rated and stable, the station unlocks the pulse, and Start the DC voltage control with slope controller and enter the unlocking operation stage;

D. When the DC side voltage of the remaining stations is stable at the rated value, the closed-loop voltage equalization charging method is used respectively. When the voltage of the sub-modules reaches the rated value and is stable, they are respectively unlocked and enter the unlocking operation stage.

From the perspective of the whole process, each converter station successively goes through the uncontrolled charging stage, the controlled charging stage and the closed-loop voltage equalizing charging until the unlocking operation; and the station control mode is the constant DC voltage control first enters the unlocking operation stage. The principle and working process of each of the above steps will be introduced in detail below.

In the uncontrolled charging stage described in step A, the MMC is respectively put into AC side pre-charging, DC side pre-charging or AC-DC side hybrid pre-charging according to the AC and DC power supply conditions. The specific identification process of these three pre-charging methods is shown in Figure 2 Shown: In each control cycle, judge the size of u _l , and compare it with u _pn , so as to determine the pre-charging method:

When u _l ≥ u _pn , it is the AC side pre-charging mode;

When u _l =0 and u _pn ≠0, it is the DC side pre-charging mode;

When 0<u _l <u _pn , it is the AC and DC side hybrid pre-charging mode.

In the pre-charging mode of the AC side, the highest line voltage of the MMC is u _ij (i,j=a,b,c,i≠j), then the anti-parallel diode of the lower tube of the upper bridge arm of phase i and the lower bridge arm of phase j is turned on, so that the DC bus voltage is u _ij , and the freewheeling diodes of the upper transistors of the sub-modules of the lower bridge arm of phase i and the upper bridge arm of phase j are turned on to charge their sub-modules, and the charging of other phase capacitors is similar.

In the DC side pre-charging mode, the MMC DC voltage u _pn charges all sub-modules of the three phases a, b, and c at the same time.

In the hybrid pre-charging mode on the AC and DC sides, the highest line voltage u _ij of the MMC and the DC voltage u _pn charge the i-phase lower bridge arm and j-phase upper bridge arm sub-modules together, and the other bridge arms have no current.

In the steady state of the uncontrolled charging phase, according to the calculation of the charging form shown in Figure 3, the voltages of the sub-modules in the three charging modes are as follows, and all of them are less than the rated value:

Specific to the three-terminal system in Figure 1, since the peak value of the side-line voltage u _l1 of the MMC1 AC valve = 290kV, which is higher than the peak value of the side-line voltage of the MMC3 AC valve u _l3 = 288kV, the DC bus voltage in the uncontrolled charging stage is u _pn = u _l1 , when After the MMC1 is unlocked and running, the DC bus voltage is U _dc =400kV. Combined with the MMC pre-charging mode identification process in the coordinated start control strategy of the present invention shown in Figure 2, MMC1 is the AC side pre-charging mode, MMC2 is the DC side pre-charging mode, and MMC3 is the AC-DC side hybrid pre-charging mode. Before MMC1 is unlocked, since the DC voltage is not much different from the voltage on the AC side of MMC3, the mixed charging phenomenon of MMC3 will not be too obvious.

The pre-charging circuits of MMC1, MMC2 and MMC3 in the uncontrolled charging phase are shown in Figure 3-a, 3-b and 3-c respectively. In the figure, it is assumed that u _ab is the current maximum line voltage of each station.

In Figure 3-a, it can be seen from the conduction conditions of the freewheeling diodes of the power module that the freewheeling diodes of the sub-modules of the upper bridge arm of phase a and the lower bridge arm of phase b are conducted, so that the DC bus voltage is the peak u of u _{ab l1} , at the same time, the freewheeling diodes of the upper tubes of the sub-modules of the lower bridge arm of phase a and the upper bridge arm of phase b are turned on to charge the capacitors of their sub-modules, and the charging conditions of the capacitors of other phases are similar.

In Figure 3-b, the DC bus voltage u _l1 charges all the sub-modules of the three phase units of the MMC at the same time.

In Figure 3-c, the DC bus voltage u _l1 is connected in series with the MMC AC valve side line voltage u _ab in the same direction, charging the capacitors of the sub-modules of the lower bridge arm of phase a and the upper bridge arm of phase b, and at this time the upper bridge arm of phase a and the b There is no current flow in the lower bridge arm.

In the steady state of the uncontrolled charging stage, the voltages of the three sub-modules are: u _sm1 = u _l1 /N = 0.67pu, u _sm2 = u _l1 /2N = 0.34pu, u _sm3 = (u _l1 +u _l2 )/2N = 0.67pu, none of them can reach the rated value, and the voltage of MMC2 is about half of that of MMC1 and MMC3.

In the controllable charging stage described in step B, the voltage of the sub-module (where the voltage of the sub-module is the lowest voltage of the sub-module, as other implementations, such as the average voltage) is higher than the charging threshold voltage, and the controllable voltage equalization charging method is respectively put into use, Enter the stage of controlled charging.

The charging threshold voltage is used to distinguish between the MMC uncontrolled charging stage and the controlled charging stage. For active stations, it should be slightly lower than u _l /N, and for passive stations, it should be slightly lower than u _pn /2N, and it should also be higher than The power-on voltage that enables the sub-module drive power to power on reliably.

The purpose of the controllable voltage equalization charging method is to realize the voltage increase of the sub-modules and ensure the voltage balance among the sub-modules. By cutting off a part of the sub-modules, the number of sub-module capacitors connected in series to the charging circuit is reduced. At the same time, in order to realize the voltage equalization of the sub-modules, each Under the control cycle, select the part of sub-modules with higher voltage in the loop to cut off. The specific method is: in each control cycle, under the AC side pre-charging mode or the AC-DC side hybrid pre-charging mode, N sub-modules in the MMC bridge arm participating in the charging (see Figure 3-a, Figure 3-c), or DC The 2N sub-modules of the MMC phase unit in the side pre-charging mode (see Figure 3-b, the phase unit is the corresponding upper and lower bridge arms) are sorted according to the voltage, and the m sub-modules with the highest trigger voltage are turned on. cut off state, while the rest of the sub-modules remain locked. The cut-off number m gradually increases from zero to a theoretical value m _ref , and the calculation method of the theoretical cut-off number m _ref is as follows, where round is a rounding function:

The key to the method of controllable voltage equalization charging lies in the setting of the number of cuts. As other implementations, a method that does not distinguish between pre-charging methods as in the document in the background technology (application number 201210462977.6) or other cut-off methods can also be used.

Specific to the system in Figure 1, the charging threshold voltages of the two active stations MMC1 and MMC3 and the passive station MMC2 are set to 1000V and 530V respectively. Since MMC1 is a constant DC voltage control station, it will be unlocked and operated first. In order to charge the sub-modules to the rated value through the AC power supply, the theoretical removal number of bridge arm sub-modules of MMC1 is set to 89 according to its calculation formula. MMC2 and MMC3 have reached a stable DC bus voltage before unlocking, making u _pn = U _dc in Figure 3-b and Figure 3-c. Therefore, for MMC2 with passive side pre-charging mode, the number of phase unit sub-modules theoretically removed is 290, and for the MMC3 with AC and DC side hybrid pre-charging mode, the theoretical removal number of its bridge arm sub-module is 55.

The closed-loop voltage equalization charging method described in steps C and D is a controllable voltage equalization charging method based on cutting off a set number of sub-modules, and adjusting the number of sub-modules cut off according to the closed-loop control result; the closed-loop control takes the rated voltage of the sub-modules as input, and uses The actual voltage of the sub-module is feedback, and the output is the adjustment amount of the number of sub-modules cut off.

The specific strategy is: pass the deviation Δu _sm between the rated voltage U _sm of the MMC sub-module and the actual value u _sm through a hysteresis comparator with a threshold voltage of ±ΔU _sm to obtain the adjustment amount Δm of the number of cuts, and pass Δm through an initial value of m _ref , an integral regulator with a time constant of τ and round the output to get the final cut number. The hysteresis comparator control logic is:

1) When Δusm≥ΔUsm, Δm=1;

2) When Δusm≤-ΔUsm, Δm=-1;

3) Otherwise, Δm remains unchanged.

The time constant τ of the integral regulator can take a value between tens of milliseconds and several seconds, such as τ=1. The hysteresis threshold voltage can be approximated by the following formula:

The hysteresis threshold voltage is related to the pre-charge method. The time constant τ of the integral regulator can take values between tens of milliseconds and several seconds. The threshold voltage of the hysteresis comparator can be as 10V.

The above uncontrolled charging phase, controllable charging phase and closed-loop charging phase are independent of each other. Moreover, each process is composed of several control cycles, similar to the uncontrolled charging method shown in Figure 2, where comparison, judgment, and execution are performed in each control cycle.

In this embodiment, by adding a closed-loop charging method after the controllable charging, the cut-off number can be adjusted in real time according to the current sub-module voltage, and the sub-module voltage can be stabilized at the rated value through closed-loop control, avoiding electrical shock during unlocking operation.

In the uncontrolled charging stage, the charging mode of each converter can be specified, and the corresponding charging threshold voltage can be obtained according to different modes, which is conducive to the timely conversion of the charging mode of the converter. At the same time, according to the different charging modes in the uncontrolled charging stage, the corresponding theoretical cut-off number is calculated in the controlled charging stage, which can ensure that each converter sub-module is charged to near the rated value, and realize reliable and effective start-up control.

The constant DC voltage control station is first unlocked to stabilize the DC bus voltage and ensure the stable operation of the multi-terminal system. Moreover, in the late stage of controllable charging, the stability of the AC and DC side voltages of the constant DC voltage control station means that the charging of the submodules of this station is in a steady state, and the rated and stable DC side voltage of other stations means that the voltage charging of the submodules of other stations is in a stable state. At this time, the closed-loop charging control is put into use, and the voltage of the sub-module is closer to the rated value, so the adjustment of the cut-off number is small, which can avoid the unstable charging of the sub-module caused by its large fluctuation.

According to the coordinated start-up control strategy of this embodiment shown in Figure 5, the key time point of the simulation is set as follows: at 0s, the three stations are started at the same time, the DC switches of each station are closed, the pulses of the submodules of the three stations are blocked, and the AC switches of MMC1 and MMC3 are closed at the same time. It enters the uncontrolled charging stage; at 10s, MMC1 has charged its sub-modules to the rated value through two modes of controllable voltage equalization charging and closed-loop voltage equalization charging, so MMC1 is unlocked for DC voltage ramp control; at 13s, MMC2 and MMC3 The sub-module voltage has reached the rated value under the action of the rated DC voltage and the closed-loop voltage equalization charging method, so MMC2 and MMC3 are unlocked; at 14s, the command slope of MMC2 and MMC3 is increased to make it enter the normal operating state. The simulation results are shown in Figure 6, and all the quantities in the figure are per unit values, and the sub-module voltage is selected as one sub-module for each of the upper and lower bridge arms of phase a.

It can be seen from Figure 6 that in the stage of uncontrolled charging, MMC1 in the AC side pre-charging mode is similar to MMC3 in the hybrid pre-charging mode, and the charging value is about twice that of MMC2 in the DC side pre-charging mode, which is consistent with theoretical analysis. unanimous. When the sub-module voltages of MMC1 and MMC3 reach the threshold voltage of 1000V and the sub-module voltage of MMC2 reaches the threshold voltage of 530V, the three stations automatically enter the controllable charging stage, and at the same time, MMC1 and MMC3 cut off the soft-start resistance. After entering the controllable charging stage, the three stations gradually cut off the sub-modules under the control of the controllable voltage equalization charging method and the closed-loop voltage equalization charging method, the charging rate increases, the voltage of the MMC1 sub-module rises to the rated value, and the voltage of the MMC2 and MMC3 sub-modules Because the DC bus voltage did not reach the rated value, they were stable at 0.71pu and 0.83pu respectively. At 10s, MMC1 is unlocked, and then the DC bus voltage rises with the command ramp, and finally stabilizes to the rated value. At the same time, the voltage of MMC2 and MMC3 sub-modules rises accordingly, and reaches the rated value under the excitation of DC voltage and the action of the closed-loop voltage equalization charging strategy . At 13s, MMC2 and MMC3 are unlocked and run, and at 14s, they start ramping the AC voltage and AC power until they reach the set value.

From the simulation results, the sequence coordination of the three converter stations during the start-up process is good, and the sub-modules can be balancedly charged according to the plan, and the voltage and current impact during the start-up process can be guaranteed to meet the requirements. It can be seen that the MMC-MTDC system coordinated startup control strategy designed by the present invention is effective and feasible.

The MMC-MTDC system coordinated start-up control strategy proposed in this embodiment can automatically adapt to various pre-charging methods of MMC and charge the voltage of sub-modules to the rated value, laying the foundation for smooth unlocking of MMC, and can unlock in an orderly manner through multi-terminal system stations The scheme and the DC voltage ramp control method can effectively suppress the electrical shock during the start-up process, and realize the fast and stable start-up of the multi-terminal system.

Example 2

A method for starting a modularized multi-level flexible direct current transmission converter, including the following steps:

A. Entering the stage of uncontrolled charging: the uncontrolled charging process adopts the uncontrolled charging method of the prior art, that is, different charging modes are not set according to the difference of the AC and DC side power sources of the converter.

B. When a certain voltage threshold voltage is reached, the controllable voltage equalization charging method is used to enter the controllable charging stage. For each sub-module, the above voltage threshold voltage value is the same.

C. When the station control mode is constant DC voltage control, after the AC and DC side voltages of the converter station are stable, the station is put into closed-loop voltage equalization charging method; when the sub-module voltage reaches the rated and stable, it enters the unlocking operation stage;

D. When the DC side voltage of the remaining stations is stable at the rated value, the closed-loop voltage equalization charging method is used respectively. When the voltage of the sub-modules reaches the rated value and is stable, they are respectively unlocked and enter the unlocking operation stage.

In the closed-loop voltage equalization charging method, a hysteresis comparator is used. Since the pre-charging mode is not distinguished, ΔU _sm is set to a constant value. Or replace the relatively strong hysteresis with a PI regulator.

According to the simulation results, this method is also feasible. According to the current sub-module voltage, the cut-off number is adjusted in real time, and the sub-module voltage is stabilized at the rated value through closed-loop control, which avoids the electrical shock during unlocking operation.

Example 3

In this embodiment, it also goes through the uncontrolled charging stage, the controllable charging stage and the closed-loop charging stage. The flow station decides to use the closed-loop voltage equalization charging method according to its own AC and DC side conditions until it is unlocked. According to the simulation results, this method can basically stabilize the sub-module voltage at the rated value through closed-loop control, but it is easy to cause system instability.

Three specific embodiments of the present invention have been given above, but the present invention is not limited to the described embodiments. Under the idea given by the present invention, the technical means in the above-mentioned embodiments are transformed, replaced, and modified in ways that are easy for those skilled in the art, and the functions played are basically the same as those of the corresponding technical means in the present invention. 1. The purpose of the invention realized is also basically the same, and the technical solution formed in this way is formed by fine-tuning the above-mentioned embodiments, and this technical solution still falls within the protection scope of the present invention.

Claims (9)

1. a kind of modular multi-level flexible direct-current transmission converter starts method, it is characterised in that as follows including step：

A, each current conversion station direct current side pole connection, each submodule locking of standing, while there is source station to close alternating-current switch, into not controlling charging Stage；

B, each current conversion station puts into controllable pressure charging respectively after its submodule voltage is higher than each self-corresponding charging threshold voltage Method, into the controllable charging stage, while there is source station excision AC is soft opens resistance；

C, each current conversion station puts into closed loop and presses charging method, when its submodule voltage reaches respectively according to AC and DC side voltage condition To after specified and stable, unlock respectively, into the unblock operation phase.

2. a kind of modular multi-level flexible direct-current transmission converter according to claim 1 starts method, its feature exists In,

In above-mentioned steps C, after prosecutor formula of standing is stable for the current conversion station AC and DC side voltage of constant DC voltage control, the station Input closed loop presses charging method；After its submodule voltage reaches specified and stabilization, station unblock pulse, and start with oblique The DC voltage control of rate controller, into the unblock operation phase；

After remaining each station DC-side Voltage Stabilization is in rated value, closed loop is put into respectively and presses charging method, when its submodule electricity After pressure reaches specified and stable, unlock respectively, into the unblock operation phase.

3. a kind of modular multi-level flexible direct-current transmission converter according to claim 1 starts method, its feature exists In not controlling the charging stage, according to AC and DC side power conditions, put into AC precharge, DC side precharge respectively or hand over DC side mixing precharge：

AC preliminary filling electrically under, MMC highests line voltage is u_ij, wherein i, j=a, b, c, i ≠ j；Under i phases on bridge arm and j phases Bridge arm submodule upper tube fly-wheel diode turns on, and is charged for its submodule；

DC side preliminary filling electrically under, MMC DC voltages u_pnCharged simultaneously for all submodules of a, b, c three-phase；

Alternating current-direct current side mixing preliminary filling electrically under, MMC highest line voltages u_ijWith DC voltage u_pnIt is bridge arm and j under i phases together Bridge arm submodule charges in phase, remaining bridge arm then no current.

4. a kind of modular multi-level flexible direct-current transmission converter according to claim 3 starts method, its feature exists In not controlling the charging stage, judge to exchange valve side line voltage peak u_lSize, and and u_pnCompare, so that it is determined that precharge side Formula：

Work as u_l≥u_pnWhen, for AC preliminary filling electrically；

Work as u_l=0 and u_pnWhen ≠ 0, for DC side preliminary filling electrically；

When 0<u_l<u_pnWhen, for alternating current-direct current side mixing preliminary filling electrically.

5. a kind of modular multi-level flexible direct-current transmission converter according to claim 4 starts method, its feature exists In, the charging threshold voltage higher than make submodule driving power can top electric upper piezoelectric voltage, it is described and for there is source station Charging threshold voltage is less than u_l/N；To passive station, the charging threshold voltage is less than u_pn/2N；There is N number of submodule on one bridge arm Block, exchange valve side line voltage peak is u_l。

6. a kind of modular multi-level flexible direct-current transmission converter according to claim 5 starts method, its feature exists In controllable to press charging method：Each controlling cycle, to AC preliminary filling electrically or alternating current-direct current side mixing preliminary filling electrically Under, participate in charging MMC bridge arms in N number of submodule, or the 2N submodule of MMC facies units of DC side preliminary filling electrically according to Voltage sorts, and the m submodule down tube IGBT conducting of trigger voltage highest is at excision state, while remaining submodule is protected Hold blocking；The controllable excision number m for pressing charging method to determine_refComputational methods are as follows, and wherein round takes to round up Integral function, the rated value of DC bus-bar voltage is U_dc, MMC submodules rated voltage is U_sm：

A kind of 7. modular multi-level flexible direct-current transmission converter startup side according to any one of claim 1-6 Method, it is characterised in that the closed loop press charging method be based on it is described it is controllable press charging method, according to closed-loop control result Adjust submodule excision number；Closed-loop control is using submodule rated voltage as input, using submodule virtual voltage as feedback, output Module cuts off number adjustment amount.

8. a kind of modular multi-level flexible direct-current transmission converter according to claim 7 starts method, its feature exists In MMC submodule rated voltages U_smWith actual value u_smDeviation delta u_smIt is ± Δ U by a threshold voltage_smStagnant ring compare Device, obtain cutting off number adjustment amount Δ m, the excision number m that Δ m is determined by an initial value for controllable pressure charging method_ref, when Between constant be τ integral controller and to output round, obtain finally cutting off number；The hysteresis comparator control logic is：

1) as Δ usm >=Δ Usm, Δ m=1；

2) as Δ usm≤- Δ Usm, Δ m=-1；

3) otherwise, Δ m is constant.

9. a kind of modular multi-level flexible direct-current transmission converter according to claim 8 starts method, its feature exists In,