CN103199729B - A kind of modular multi-level converter submodule grouping Staircase wave method - Google Patents

Abstract

The invention provides a step wave modulation method for grouping sub-modules of a modular multi-level converter, which includes the following steps: dividing the sub-modules in the commutation chain into N sub-module groups; rounding the reference voltages of the sub-module groups to obtain The rounded reference staircase wave voltage V _{stair_wave} ; distribute the trigger pulse to the sub-modules inside the sub-module group; perform voltage equalization among the sub-module groups. In the present invention, the sub-modules are divided into N sub-module groups, and each sub-module group is regarded as a controllable voltage source. Step wave modulation is used in the sub-module groups, and the N sub-module groups use rounding corrections, which can achieve similar wave shift For the purpose of the phase, the appropriate voltage equalization control method is used for voltage stabilization control between the sub-module groups, which simplifies the complexity of the ladder wave modulation, and greatly reduces the requirements for the software and hardware of the modulation algorithm and the voltage equalization control algorithm.

Description

A Step Wave Modulation Method for Sub-module Grouping of Modular Multilevel Converter

technical field

The invention belongs to the technical field of power electronics, and in particular relates to a step wave modulation method for sub-module grouping of a modular multilevel converter.

Background technique

The traditional thyristor-based DC transmission system uses phase-controlled rectification to convert three-phase AC power into six-pulse or twelve-pulse DC power, but this DC power transmission system needs to absorb a large amount of reactive power, especially when the AC side is faulty case. The DC transmission system based on the voltage source converter (VSC-HVDC) is equivalent to a controllable voltage source, which can operate in four quadrants and realize the independent adjustment of active and reactive power on the AC side, which is of great significance for the formation of DC power grids. Since the modular multilevel converter adopts a modular design, each sub-module contains a large capacitor to clamp the voltage, and its voltage level and capacity can be expanded by connecting sub-modules in series, so this converter is a A very promising converter. But when this kind of converter is applied to high-voltage and large-capacity occasions, the number of sub-modules that need to be connected in series is very large, which makes the distribution of trigger pulses difficult, and the realization of the secondary system also becomes difficult or even unfeasible. Therefore, when the modular multilevel converter is used in high-voltage and large-capacity applications, it is necessary to reconsider the group control of many sub-modules to reduce the difficulty of trigger pulse distribution and make the secondary system easier to implement.

Contents of the invention

In order to overcome the above-mentioned deficiencies in the prior art, the present invention provides a step wave modulation method for sub-module grouping of modular multilevel converters, which divides sub-modules into N sub-module groups, and each sub-module group is regarded as a controllable voltage source , the sub-module group adopts ladder wave modulation, and the N sub-module groups use rounding corrections, which can achieve the purpose similar to carrier phase shifting. The sub-module groups adopt appropriate voltage equalization control mode for voltage stabilization control, which simplifies the ladder The complexity of wave modulation greatly reduces the requirements for software and hardware of modulation algorithm and voltage equalization control algorithm.

In order to realize the above-mentioned purpose of the invention, the present invention takes the following technical solutions:

A step wave modulation method for grouping sub-modules of a modular multilevel converter is provided, and the method includes the following steps:

Step 1: Divide the sub-modules in the commutation chain into N sub-module groups;

Step 2: Round the reference voltage of the sub-module group to obtain the rounded reference staircase wave voltage V _{stair_wave} ;

Step 3: Assign trigger pulses to the sub-modules inside the sub-module group;

Step 4: Perform pressure equalization among sub-module groups.

In the step 1, the sub-module group is a controllable voltage source, and the control signal of the controllable voltage source is the reference voltage V _ref of the sub-module group.

In said step 2, the reference staircase wave voltage V _{stair_wave} after rounding is expressed as

${\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; stairs</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; waves</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; floor</span>}<span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; ref</span>}}}{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; sm</span>}}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&delta;</span>}}_{\mathrm{<span\; class="notranslate">\; k</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

Wherein, V _ref is the reference voltage of the sub-module group, V _sm is the average voltage of the sub-module group, the floor function is a rounding function of negative infinity direction, δ _k is the rounding correction value of the sub-module group, δ _k =(2k -1)/2N+m _k , where k=1, 2, ... N; m _k is an integer, and The average value of the rounding correction amount δ _k of the sub-module group is 0.5, namely:

Described step 3 comprises the following steps:

Step 3-1: Determine whether the direction of the bridge arm current is positive or negative;

Step 3-2: Switch on or off the sub-module according to the direction of the bridge arm current.

Step 3-3: Determine whether the number of sub-modules that have been invested is consistent with the number of sub-modules that need to be invested. are re-consistent.

In the step 3-2, if the direction of the bridge arm current is positive, the bridge arm current charges the capacitance of the sub-module that has been put in, and the process of putting in or removing the sub-module is as follows:

A) If a sub-module needs to be put into operation, then find out the sub-module with a lower voltage among the sub-modules that have not been put into use, and put it into use;

B) If it is necessary to cut off the sub-module, then find out the sub-module with higher voltage among the sub-modules not put into it, and cut it off.

In the process of bridge arm current charging the capacitance of the sub-modules that have been put in, if the voltage of the sub-module with higher capacitance voltage among the sub-modules that has been put in is higher than the voltage of the sub-module with lower capacitance voltage among the sub-modules that are not put into use, Then cut off the sub-modules with higher capacitance voltage among the sub-modules that have been put in, and put in the sub-modules with lower capacitance voltage among the sub-modules that have not been put into use.

In the step 3-2, if the direction of the bridge arm current is negative, the bridge arm current discharges the capacitance of the sub-module that has been put in, and the process of putting in or removing the sub-module is as follows:

A) If a submodule needs to be put into operation, then find out the submodule with higher voltage among the submodules that have not been put into use, and put it into use;

B) If the sub-module needs to be removed, find out the sub-module with the lower voltage among the sub-modules that are not put into use, and remove it.

In the process of bridge arm current discharging the capacitance of the sub-modules that have been put in, if the sub-module with a lower voltage in the sub-modules that has been put in is lower than the capacitor voltage of the sub-module with a higher voltage in the sub-modules that are not put in, then Cut off the sub-modules with lower capacitance voltage among the sub-modules that have been put in, and put in the sub-modules with higher capacitance voltage among the sub-modules that have not been put into use.

In the step 4, the sub-module groups perform pressure equalization according to the following pressure equalization control method:

Mode 1: The rounding correction amount δ _k of N sub-module groups is used cyclically among the N sub-module groups to eliminate the voltage imbalance of the sub-module groups caused by the difference in rounding correction values;

Method 2: Sorting the average voltage V _sm of the sub-module groups, when the modular multilevel converter is working in the inverter state, the rounding correction value δ _k of the sub-module groups is assigned to the voltage in order from large to small From small to large sub-module groups, the sub-module group with low average capacitor voltage is charged more; when the modular multilevel converter is working in the rectification state, the rounding correction value δ _k of the sub-module group is changed from large to The smaller ones are assigned to the sub-module groups with voltage from large to small in turn, and the sub-module groups with higher average capacitor voltage discharge more;

Method 3: According to the difference between the average voltage V _sm of the sub-module group and the average voltage of the commutation chain, a DC component or an AC component is superimposed on the voltage modulation wave of the sub-module group, and the sub-module group with a higher average voltage is discharged. The submodule group with the lower average voltage is charged to equalize the voltage of the submodule group.

Compared with prior art, the beneficial effect of the present invention is:

1. The sub-module group is regarded as an independent controllable voltage source, and the step wave modulation is adopted inside the sub-module group, and the coupling between the sub-module groups is greatly reduced, thus greatly reducing the requirements for the software and hardware of the modulation algorithm and the control algorithm;

2. Using different rounding corrections between the sub-module groups can achieve the purpose of phase shifting similar to the carrier wave, so that the harmonics of the sub-module groups cancel each other out, so that the voltage waveform output by the commutation chain is closer to a sine wave;

3. Adopting an appropriate voltage equalization control method between the sub-modules can keep the voltage between the sub-module groups balanced, thereby ensuring the normal operation of the converter.

Description of drawings

Figure 1 is a schematic diagram of the step wave modulation method for grouping sub-modules of a modular multilevel converter;

Fig. 2 is a structural diagram of the modular multilevel converter in the sub-module grouping ladder wave modulation method of the modular multilevel converter.

detailed description

The present invention will be described in further detail below in conjunction with the accompanying drawings.

A step wave modulation method for grouping sub-modules of a modular multilevel converter is provided, and the method includes the following steps:

Step 1: Divide the sub-modules in the commutation chain into N sub-module groups;

Step 2: Round the reference voltage of the sub-module group to obtain the rounded reference staircase wave voltage V _{stair_wave} ;

Step 3: Assign trigger pulses to the sub-modules inside the sub-module group;

Step 4: Perform pressure equalization among sub-module groups.

In the step 1, the sub-module group is a controllable voltage source, and the control signal of the controllable voltage source is the reference voltage V _ref of the sub-module group.

In the step 2, the reference staircase wave voltage _{Vstair_wave} after rounding is expressed as

${\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; stairs</span>}<span\; class="notranslate">\; \_</span>\mathrm{<span\; class="notranslate">\; waves</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; floor</span>}<span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; ref</span>}}}{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; sm</span>}}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&delta;</span>}}_{\mathrm{<span\; class="notranslate">\; k</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

Wherein, V _ref is the reference voltage of the sub-module group, V _sm is the average voltage of the sub-module group, the floor function is a rounding function of negative infinity direction, δ _k is the rounding correction value of the sub-module group, δ _k =(2k -1)/2N+m _k , where k=1, 2, ... N; m _k is an integer, and The average value of the rounding correction amount δ _k of the sub-module group is 0.5, namely:

Described step 3 comprises the following steps:

Step 3-1: Determine whether the direction of the bridge arm current is positive or negative;

Step 3-2: Switch on or off the sub-module according to the direction of the bridge arm current.

Step 3-3: Determine whether the number of sub-modules that have been invested is consistent with the number of sub-modules that need to be invested. are re-consistent.

In the step 3-2, if the direction of the bridge arm current is positive, the bridge arm current charges the capacitance of the sub-module that has been put in, and the process of putting in or removing the sub-module is as follows:

A) If a sub-module needs to be put into operation, then find out the sub-module with a lower voltage among the sub-modules that have not been put into use, and put it into use;

B) If it is necessary to cut off the sub-module, then find out the sub-module with higher voltage among the sub-modules not put into it, and cut it off.

In the process of bridge arm current charging the capacitance of the sub-modules that have been put in, if the voltage of the sub-module with higher capacitance voltage among the sub-modules that has been put in is higher than the voltage of the sub-module with lower capacitance voltage among the sub-modules that are not put into use, Then cut off the sub-modules with higher capacitance voltage among the sub-modules that have been put in, and put in the sub-modules with low capacitance voltage among the sub-modules that have not been put into use.

In the step 3-2, if the direction of the bridge arm current is negative, the bridge arm current discharges the capacitance of the sub-module that has been put in, and the process of putting in or removing the sub-module is as follows:

A) If a submodule needs to be put into operation, then find out the submodule with higher voltage among the submodules that have not been put into use, and put it into use;

B) If the sub-module needs to be removed, find out the sub-module with the lower voltage among the sub-modules that are not put into use, and remove it.

In the process of bridge arm current discharging the capacitors of the sub-modules that have been put in, if the sub-module with a lower voltage in the sub-modules that has been put in is lower than the capacitor voltage of the sub-module with a higher voltage in the sub-modules that are not put in, then Cut off the sub-modules with lower capacitance voltage among the sub-modules that have been put in, and put in the sub-modules with higher capacitance voltage among the sub-modules that have not been put into use.

In the step 4, the sub-module groups perform pressure equalization according to the following pressure equalization control method:

Mode 1: The rounding correction amount δ _k of N sub-module groups is used cyclically among the N sub-module groups to eliminate the voltage imbalance of the sub-module groups caused by the difference in rounding correction values;

Method 2: Sorting the average voltage V _sm of the sub-module groups, when the modular multilevel converter is working in the inverter state, the rounding correction value δ _k of the sub-module groups is assigned to the voltage in order from large to small From small to large sub-module groups, the sub-module group with low average capacitor voltage is charged more; when the modular multilevel converter is working in the rectification state, the rounding correction value δ _k of the sub-module group is changed from large to The smaller ones are assigned to the sub-module groups with voltage from large to small in turn, and the sub-module groups with higher average capacitor voltage discharge more;

Method 3: According to the difference between the average voltage V _sm of the sub-module group and the average voltage of the commutation chain, a DC component or an AC component is superimposed on the voltage modulation wave of the sub-module group, and the sub-module group with a higher average voltage is discharged. The submodule group with the lower average voltage is charged to equalize the voltage of the submodule group.

Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: the present invention can still be Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention shall be covered by the scope of the claims of the present invention.

Claims (8)

1. a modular multi-level converter submodule grouping Staircase wave method, is characterized in that: said method comprising the steps of:

Step 1: the submodule in change of current chain is divided into N number of submodule group;

Step 2: rounded by the reference voltage of submodule group, obtains the reference staircase voltage V after rounding _{stair_wave};

Step 3: distribute trigger impulse to the submodule of submodule group inside;

Step 4: all press between submodule group;

In described step 2, the reference staircase voltage V after rounding _{stair_wave}be expressed as

$V_{\mathrm{stair}\_\mathrm{wave}}=\mathrm{floor}(\frac{V_{\mathrm{ref}}}{V_{\mathrm{sm}}}+{\mathrm{\&delta;}}_k)---\left(1\right)$

Wherein, V _refthe reference voltage of submodule group, V _smbe the average voltage of submodule group, floor function is the bracket function of negative infinity, δ _kbe submodule group round correction, δ _k=(2k-1)/2N+m _k, wherein k=1,2 ... N; m _kfor integer, and submodule group round correction δ _kmean value be 0.5, that is:

2. modular multi-level converter submodule grouping Staircase wave method according to claim 1, it is characterized in that: in described step 1, submodule group is controllable voltage source, and the control signal of controllable voltage source is the reference voltage V of submodule group _ref.

3. modular multi-level converter submodule grouping Staircase wave method according to claim 1, is characterized in that: described step 3 comprises the following steps:

Step 3-1: judge that the direction of bridge arm current is positive direction or negative direction;

Step 3-2: the direction according to bridge arm current is dropped into or excision submodule F.F. row;

Step 3-3: judge that whether the submodule number dropped into is consistent with the submodule number needing to drop into, if inconsistent, then the submodule of the submodule difference number that the submodule needing excision or input to drop into and needs drop into, makes the two again consistent.

4. modular multi-level converter submodule grouping Staircase wave method according to claim 3, it is characterized in that: in described step 3-2, if the direction of bridge arm current is positive direction, then the electric capacity of bridge arm current to the submodule dropped into charges, and drops into or excision process is to submodule:

A) if need submodule be dropped into, then find out and do not drop into the lower submodule of voltage in submodule, dropped into;

B) if need submodule be excised, then find out and do not drop into the higher submodule of voltage in submodule, excised.

5. modular multi-level converter submodule grouping Staircase wave method according to claim 4, it is characterized in that: in the process that the electric capacity of bridge arm current to the submodule dropped into charges, if the submodule that in the submodule dropped into, capacitance voltage is higher is than the voltage height Δ u not dropping into the lower submodule of capacitance voltage in submodule, then submodule higher for capacitance voltage in the submodule dropped into is excised, the lower submodule of capacitance voltage in submodule will do not dropped into simultaneously and drop into.

6. modular multi-level converter submodule grouping Staircase wave method according to claim 3, it is characterized in that: in described step 3-2, if the direction of bridge arm current is negative direction, then the electric capacity of bridge arm current to the submodule dropped into discharges, and drops into or excision process is to submodule:

A) if need submodule be dropped into, then find out and do not drop into the higher submodule of voltage in submodule, dropped into;

B) if need submodule be excised, then find out and do not drop into the lower submodule of voltage in submodule, excised.

7. modular multi-level converter submodule grouping Staircase wave method according to claim 6, it is characterized in that: in the process that the electric capacity of bridge arm current to the submodule dropped into discharges, if the submodule that in the submodule dropped into, voltage is lower Δ u lower than the capacitance voltage not dropping into the higher submodule of voltage in submodule, then submodule lower for capacitance voltage in the submodule dropped into is excised, the higher submodule of capacitance voltage in submodule will do not dropped into simultaneously and drop into.

8. modular multi-level converter submodule grouping Staircase wave method according to claim 1, is characterized in that: in described step 4, all presses between submodule group according to following Pressure and Control mode:

Mode 1:N submodule group round correction δ _krecycle between N number of submodule group, the voltage of the submodule group caused with the difference eliminated owing to rounding correction is unbalanced;

Mode 2: to the average voltage V of submodule group _smsort, when modular multi-level converter is operated in inverter mode, submodule group rounded correction δ _kbe assigned to voltage submodule group from small to large from big to small successively, the submodule group making capacitor averaging voltage low charging is more; When modular multi-level converter is operated in rectification state, submodule group rounded correction δ _kbe assigned to voltage submodule group from big to small from big to small successively, the submodule group electric discharge that capacitor averaging voltage is high is more;

Mode 3: according to the average voltage V of submodule group _smwith the difference of change of current chain average voltage, DC component or alternating current component is superposed in the voltage modulated ripple of submodule group, the submodule group higher to average voltage is discharged, and the submodule group lower to average voltage is charged, to make submodule group electric voltage equalization.