CN103532418A - SVG submodule capacitor pre-charging strategy based on MMC - Google Patents

Abstract

Aiming at the problems of cumbersome process and complex structure of the current multilevel converter power unit capacitor precharging method, the present invention relates to a novel MMC (Modular Multilevel Converter)-based SVG (Static Var Generator) power unit capacitor precharging strategy. The method only needs to add a set of current-limiting resistors R and a set of resistance cut-off switches S in hardware structure, and the current-limiting resistors are connected in series between the inverter and the AC power grid, and the hardware structure is simple. The pre-charging strategy adopts the double closed-loop control mode of voltage and current. After the system is started, the sub-module voltage

Track the given current through the double closed loop of voltage loop and current loop

The active power is absorbed from the grid, so that the inverter works in the constant power boost mode during the charging phase, and the charging trajectory is smooth. After the charging is completed, the voltage loop regulator desaturates, and the current limiting resistor is cut off at the same time, and the system enters the normal working mode. This strategy completes the pre-charging of the power unit without adding additional hardware circuits and control algorithms, and the charging rate is controllable.

Description

A Capacitor Precharging Strategy for SVG Sub-module Based on MMC

technical field

The patent of the present invention relates to a floating capacitor precharging system and method, in particular to an MMC-based SVG power unit capacitor precharging strategy.

Background technique

In recent years, the domestic power industry has developed rapidly, and multi-level power conversion technology has emerged as the times require. The start-up or restart process of a high-voltage large-capacity converter is generally accompanied by a huge transient energy impact, and its power switching elements need to withstand a huge current transient stress.

Modular Multilevel Converter (MMC) is a kind of multilevel converter that has attracted much attention in recent years in inverter applications such as high-voltage and high-power drives that need to output energy from the common DC bus to the AC side. Inverter topology, which uses multiple sub-modules cascaded to achieve the purpose of high voltage and high power. By controlling the input and removal of each sub-module, the output waveform of the converter can be close to a sine wave, with better harmonic characteristics.

The MMC topology is shown in Figure 1. The three-phase MMC consists of three bridge arms. Each bridge arm is composed of N sub-modules and a reactor L connected in series. Each bridge arm is directly connected to the common DC bus. The device has many power modules. When starting up, the MMC needs to be pre-charged to ensure that the capacitor voltage of each sub-module can quickly reach the rated voltage.

In "Proceedings of the Chinese Society for Electrical Engineering, 2009, 29 (30), pp. 1-6", "New Modular Multi-level VSC Sub-module Capacitance Parameters and Voltage Equalization Strategy" proposed a "gradual The specific implementation method of the "module charging" pre-charging method is: select an external DC voltage source approximately equal to the rated voltage of the sub-module, connect it across the two sides of the MMC's busbar, and first send a shutdown signal to S1 of 2N sub-modules in each phase, except for those to be charged Outside the electronic module, send a turn-on signal to S2 of the remaining 2N-1 sub-modules. When the capacitor voltage of the sub-module is equal to the rated voltage, it will be fully charged. S1 of the module sends a shutdown signal to start the pre-charging process of the next sub-module, and so on. After 2N times of pre-charging, the MMC pre-charging is completed. Its advantage is that the transient processing energy level in the pre-charging process is low, and it is suitable for occasions such as laboratories where the available DC power supply is limited. The disadvantage is that when the number of MMC power units is large, this method is likely to cause a deviation in the DC capacitor voltage of the first charging and the second charging power unit, which will affect the output characteristics of the MMC in severe cases. At the same time, due to the need for an additional auxiliary DC power supply, engineering The application will increase the complexity of the system hardware.

Publication No.: CN102170140A Patent "A Start-Up Method for Modular Multilevel Converter Flexible DC Transmission System" proposes a MMC start-up method for flexible DC power transmission. The capacitor is charged by controlled rectification. After the capacitor voltage is stable, the switch tube is unlocked, the current limiting resistor is bypassed, and the system is directly put into closed-loop control of the capacitor voltage. The specific process is divided into three stages: in the first stage, all switching tubes are blocked first, the bypass switch of the current limiting resistor is turned on, and the capacitors of the two converter station MMC sub-modules are uncontrolled rectified and charged. After the capacitor voltage is stable, enter the second stage In the second stage, unlock the switch signal of the converter station MMC2, and make it enter the closed-loop control of the capacitor voltage. After the capacitor voltage of the MMC2 sub-module is stable, enter the third stage, close the current limiting resistor bypass switch, and unlock the switch of the converter station MMC1 signal, and make it enter the capacitor voltage closed-loop control, after the capacitor voltage of the sub-modules of the two converter stations stabilizes, the charging ends. However, this method still needs to continuously detect the capacitor voltage value, and at the beginning of the third stage, the sum of the capacitor voltages of all sub-modules of each phase bridge arm is equal to the peak value of the AC side voltage, which is less than the rated voltage U _dc of the MMC during normal operation, bypassing After the current limiting resistor is removed, the system directly performs closed-loop capacitor voltage control, which will cause a large inrush current and cause potential safety hazards.

Xu Zheng, Tu Qingrui and others' "Three-phase Modular Multilevel Converter Startup Method Without Auxiliary DC Power Supply" (Patent Publication No.: CN101795057A) proposed a three-phase A self-excited charging method for a modular multi-level converter, which uses an AC power source instead of a DC power source, and controls the switch state of the sub-module by detecting the polarity of the bridge arm current and the capacitor voltage of the sub-module to complete the charging of the sub-module of the converter . The specific implementation process is as follows: connect the AC power supply to the converter through the current-limiting resistor, send an off signal to the upper switching device S1 of all sub-modules, and send an on-signal to the lower switching device S2 of all sub-modules, detect the current of each bridge arm, and when the current When it is consistent with the charging direction, the switch device S2 of the sub-module to be charged is turned off, and the sub-module is charged. When the current is opposite to the charging direction, keep the switching device under the sub-module in the off state, and the capacitor voltage of the sub-module is maintained, and so on. When the capacitor voltage of the sub-module reaches the rated value, turn on the switching device S2 under the sub-module, the After the charging of the sub-module is completed, the next sub-module can be charged continuously, and the above steps are repeated until the capacitor voltages of all the sub-modules of the bridge arm reach the rated value, and the capacitors of the converter sub-modules are charged. However, in the charging process of each bridge arm of the converter, only a single module is put into charging each time, and the charging time of the converter is doubled. In addition, the capacitor voltage value and the current direction of the bridge arm need to be detected during the capacitor charging process, and the control is relatively complicated.

Therefore, the introduction of a reasonable pre-charging strategy is of great practical significance for the promotion of MMC technology.

Contents of the invention

Aiming at the problems existing in the above precharging methods, the present invention proposes a novel MMC-based capacitor precharging strategy for the sub-module of the SVG bridge arm.

The structure of the pre-charging model is simple, and only a current-limiting module I is added to the hardware. The current-limiting module I includes a charging current-limiting resistor R and a resistor cut-off switch S. One end of the current-limiting module I is connected to the output end of the inverter, and the other One end is connected to the grid side voltage. Its control circuit adopts double closed-loop control of current loop II and voltage loop III. After the system is started, it reaches the limit value of the output of the PI regulator. The inverter output current is controlled by the PR regulator to track the given current so that the inverter absorbs active power from the grid. After the charging is completed, the PI regulator desaturates, and the current limiting resistor is cut off at the same time, and the system starts to work normally.

Compared with the prior art, the present invention has the advantages of:

1) The entire process of the new MMC-based SVG sub-module capacitor pre-charging strategy is automatically completed by voltage loop III and current loop II, which can realize a natural transition from pre-charging to steady-state operation.

2) The new MMC-based SVG sub-module capacitor pre-charging strategy can realize simultaneous charging of multiple sub-modules, and the charging trajectory is smooth.

3) The new MMC-based SVG sub-module capacitor pre-charging strategy can easily adjust the pre-charging rate by adjusting the output limit of the voltage outer loop controller, and has higher flexibility.

4) The new MMC-based SVG sub-module capacitor pre-charging strategy only needs to be slightly changed under the normal compensation strategy without adding additional control algorithms.

Description of drawings

Figure 1 shows the main circuit of SVG based on MMC.

Fig. 2 is a functional block diagram of SVG pre-charging based on MMC.

Figure 3 The average voltage of sub-modules and the average voltage charging trajectory of sub-modules of each phase

Figure 4 single-phase sub-module capacitor voltage charging trace

In the figure: Ⅰ is the current limiting module; Ⅱ is the current loop; Ⅲ is the voltage loop; Ⅳ is the sub-module capacitor voltage control module.

Detailed ways

为MMC中所有子模块平均电压，为子模块给定电压，V_dcx为所有子模块电容电压实际值，△V_dcp为相间能量控制输出三相控制量，△V_dcx为各子模块电容电压控制量，PI为电压环调节器，PR为电流环调节器，

为FBD电流检测运算中有功电导，为无功电导，

为电流环给定，i_f为SVG补偿输出电流。The multi-level MMC-based bridge arm capacitor precharging strategy and its working principle of the present invention will be further described below with reference to FIG. 1 and FIG. 2 . As shown in FIG. 1 , the structure model of the multi-level MMC-based bridge arm capacitor pre-charging strategy in the present invention only adds a charging current-limiting resistor and a resistor cut-off switch on the original AC output side, and the structure is simple. Its control principle is shown in Figure 2, in the figure

is the average voltage of all sub-modules in the MMC, is the given voltage for sub-modules, V _dcx is the actual value of the capacitor voltage of all sub-modules, △V _dcp is the three-phase control value of the phase-to-phase energy control output, △V _dcx is the control value of the capacitor voltage of each sub-module, PI is the voltage loop regulator, PR is the current loop regulator,

is the active conductance in the FBD current detection operation, is the reactive conductance,

Given for the current loop, _if is the SVG compensation output current.

直流母线电压控制器PI很快饱和，△G_p达到PI输出的限幅值，△G_p叠加至FBD电流检测环节中的有功轴，电流环控制逆变器输出电流跟踪给定电流

逆变器从电网吸收有功功率，各功率单元的总储能增加，而逆变器内部由相间能量控制与子模块电容电压控制将逆变器吸收的能量均匀分配到各功率单元当中，各子模块电容电压逐渐升高，当时，PI调节器退保和，系统正常工作，整个预充电环节自动完成，且无需额外的程序或设备。The initial moment of system startup

The PI of the DC bus voltage controller saturates quickly, △G _p reaches the limit value of PI output, △G _p is superimposed on the active axis in the FBD current detection link, and the current loop controls the inverter output current to track the given current

The inverter absorbs active power from the grid, and the total energy storage of each power unit increases, and the energy absorbed by the inverter is evenly distributed to each power unit by the phase-to-phase energy control and sub-module capacitor voltage control inside the inverter. The module capacitor voltage gradually increases, when When the PI regulator is withdrawn and the system works normally, the entire pre-charging link is automatically completed without additional procedures or equipment.

即逆变器仅和电网进行有功能量的交换，且能量交换的幅度可由PI输出的限幅值大小来控制，当直流母线电压超过给定值时，直流母线电压控制器退饱和，此时恢复电流检测环节的

轴，系统切换到补偿模式，预充电环节结束。逆变器的出口侧串联限流电阻，限制逆变器在输出电压较低时的冲击电流，在预充电结束时通过闭合S开关切除限流电阻，系统进入正常的工作模式。At the beginning of pre-charging, before the output line voltage of the inverter is lower than the power line voltage, the SVG cannot inject reactive power compensation current to the grid. In order to ensure a smooth charging curve, during the pre-charging stage, the

That is, the inverter only exchanges active energy with the grid, and the magnitude of the energy exchange can be controlled by the limit value of the PI output. When the DC bus voltage exceeds a given value, the DC bus voltage controller desaturates. At this time recovery current sense link of the

axis, the system switches to compensation mode, and the pre-charging link ends. The outlet side of the inverter is connected in series with a current-limiting resistor to limit the inrush current of the inverter when the output voltage is low. At the end of pre-charging, the current-limiting resistor is cut off by closing the S switch, and the system enters the normal working mode.

The pre-determined hardware device in the above pre-charging process is only the pre-charging resistor, and its resistance value can be selected according to the maximum current allowed by the inverter. The output voltage of the device is zero, and the system is equivalent to a short circuit. Therefore, if the maximum allowable current is determined, the approximate calculation formula of the precharge resistance can be obtained as

$\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; pcm</span>}}}$

In the formula, U _s is the effective value of the phase voltage of the AC power supply, and I _pcm is the maximum current allowed to flow into the inverter during the pre-charging process.

Fig. 3 shows the average voltage of the three-phase sub-modules and the charging track of the average voltage of each phase sub-module of the modular multilevel converter of the present invention.

FIG. 4 shows the charging track of the capacitor voltage of the single-phase sub-module of the modular multilevel converter of the present invention.

Through the analysis of the sub-module capacitor pre-charging strategy, it can be concluded that the entire charging process is automatically completed by the voltage loop and the current loop, and the transition from the end of pre-charging to normal operation is natural, and the capacitor voltage charging trajectory is smooth.

Claims (3)

1. the novel SVG submodule electric capacity precharge strategy based on MMC, it is characterized in that: this precharge model structure is simple, on hardware, only increased a current limliting module I, current limliting module I comprises charging current limiter resistance R and resistance excision switch S, current limliting module is serially connected with between inverter output and AC network, adopt electric current loop II and Voltage loop III double circle controling mode, by the total energy storage of Voltage loop control inverter DC side, obey given, regulate inverter and AC to exchange meritorious size and Orientation, and the meritorious upper limit is determined by the output violent change of Voltage loop pi regulator, output current by current loop control inverter is obeyed given.

2. the novel SVG submodule electric capacity precharge strategy based on MMC of narrating according to claim 1, is characterized in that: in pre-charge process, Voltage loop pi regulator is saturated, is output as amplitude limit value Δ G _p, by Δ G _pbe superimposed to the meritorious axle in FBD current detecting link, testing result is given as electric current loop, by two closed loop co-controlling SVG, works in the permanent power charging stage.

3. the novel SVG submodule electric capacity precharge strategy based on MMC of narrating according to claim 1, is characterized in that: inverter inside is controlled with power cell capacitance voltage control IV and combined each power cell that is assigned to of the energy even of inverter absorption by alternate energy.

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