CN103746585B - Based on the multi-electrical level inverter neutral-point voltage balance method of hybrid modulation - Google Patents

Abstract

The invention discloses a kind of multi-electrical level inverter neutral-point voltage balance method based on hybrid modulation.Step is: in each control cycle, the sampling unit of digital processing control module detects the electric capacity instantaneous voltage between the mid point of multi-electrical level inverter DC bus and positive pole, electric capacity instantaneous voltage between the mid point of DC bus and negative pole, and determine the deviation of these two electric capacity instantaneous voltages, the work of SVPWM or DPWM control mode is selected through pulsewidth modulation strategy switch unit, control signal is exported by SVPWM/DPWM controlled processing unit, the operating state of multi-electrical level inverter every phase brachium pontis switching tube is controlled through drive circuit output pwm signal, control multi-electrical level inverter neutral point voltage balance simultaneously.Control method of the present invention has and reduces switching frequency, reduces switching loss, and output voltage, current harmonics are little, and output waveform quality is high and be convenient to the advantages such as Digital Realization, have great engineer applied and are worth.

Description

Based on the multi-electrical level inverter neutral-point voltage balance method of hybrid modulation

Technical field

The invention belongs to the control technology field in Technics of Power Electronic Conversion technology, particularly a kind of multi-electrical level inverter neutral-point voltage balance method based on hybrid modulation.

Background technology

The control method of traditional two-level inverter is simple, realize easily, but it is high also to there is output harmonic wave aberration rate, and it is large that switching tube bears voltage stress, inefficient shortcoming.Multi-electrical level inverter has, the advantage such as equivalent switching frequency high and output waveform harmonic wave little low to the voltage withstand class of switching device, is thus used widely in the high-power translation circuit of mesohigh.There is plurality of level to produce to export in phase voltage, in T-shaped or NPC type inverter topology, all must use two dc-link capacitances.Ideally, each capacitance voltage is the half of DC bus-bar voltage, but these two kinds of topologys of actual conditions all can exist problem and the mid-point voltage imbalance of a general character.When two capacitance voltages of DC side are unequal, will increase the voltage stress of inverter leg breaker in middle pipe, there is the pulsation (as triple-frequency harmonics) of low frequency in output voltage, increases the harmonic wave of system output voltage, electric current, affect output waveform quality.

The unbalanced reason of mid-point voltage is caused to mainly contain:

1) due to actual capacitance capacitance deviation that the error of manufacture technics causes;

2) characteristic of switching tube is inconsistent;

3) three-phase imbalance runs.

In order to solve mid-point voltage imbalance problem, Chinese scholars proposes a lot of method, mainly contains two kinds of methods: one is based on sinusoidal pulse width modulation (SPWM) technology, maintains neutral point voltage balance by injecting zero-sequence component; Another kind is based on space vector pulse width modulation (SVPWM) technology, controls mid-point voltage by regulating the action time of redundancy small vector.In SPWM mode, mainly through injecting zero-sequence component to maintain neutral point voltage balance in three-phase modulations ripple.But the computing of residual voltage needs skill, be generally the method by " estimate-verify-revise ", operand is large and method is complicated.Method conventional in SVPWM mode is the dispensed factor, is determined the relativity time of often pair of redundancy small vector by distribution factor.Different sectors, in different triangles, the formula that distribution factor calculates is different; Which process is complicated, and operand is large.

Summary of the invention

The object of the present invention is to provide that a kind of real-time is good, processing procedure simply and easily realize the digitized multi-electrical level inverter neutral-point voltage balance method based on hybrid modulation.

The technical solution realizing the object of the invention is: a kind of multi-electrical level inverter neutral-point voltage balance method based on hybrid modulation, in each control cycle, the sampling unit of digital processing control module detects the electric capacity instantaneous voltage between the mid point of multi-electrical level inverter DC bus and positive pole, electric capacity instantaneous voltage between the mid point of DC bus and negative pole, and determine the deviation of these two electric capacity instantaneous voltages, the work of SVPWM or DPWM control mode is selected through pulsewidth modulation strategy switch unit, control signal is exported by SVPWM/DPWM controlled processing unit, the operating state of multi-electrical level inverter every phase brachium pontis switching tube is controlled through drive circuit output pwm signal, control multi-electrical level inverter neutral point voltage balance simultaneously, concrete grammar comprises the following steps:

Step 1, sampling unit are sampled three-phase voltage signal that electric capacity instantaneous voltage between the DC bus-bar voltage of multi-electrical level inverter, electric capacity instantaneous voltage between the mid point of DC bus and positive pole, the mid point of DC bus and negative pole, multi-electrical level inverter export and the three-phase current signal that multi-electrical level inverter exports respectively;

Step 2, pulse-width modulation strategy switch unit, according to the deviation signal of two electric capacity instantaneous voltages described in step 1, select SVPWM control mode or the work of DPWM control mode;

Step 3, SVPWM/DPWM controlled processing unit, according to the control mode selected in the three-phase voltage signal of sampling unit output in step 1 and three-phase current signal, DC bus-bar voltage signal and step 2, determine the control signal of every phase brachium pontis switching tube of multi-electrical level inverter in current switch period;

The switch controlled signal that SVPWM/DPWM controlled processing unit exports is distributed to every each switching tube of phase brachium pontis of multi-electrical level inverter by step 4, drive circuit, controls operating state and the neutral point voltage balance of multi-electrical level inverter.

Compared with prior art, remarkable advantage of the present invention is: the hybrid modulation stratgy that (1) adopts space vector modulation (SVPWM) to combine with discontinuous modula tion (DPWM) mode realizes the neutral point voltage balance of multi-electrical level inverter, traditional SVPWM stability contorting and the little advantage of output voltage current harmonics can be played, when mid-point voltage occurs unbalanced, DPWM can be embodied and controls effectively to realize neutral-point voltage balance, reduces system equivalent switching frequency thus effect of minimizing switching loss again; (2) inductive current ripple reduces, and harmonic wave is little, output waveform quality is higher; (3) there is the function realizing inverter neutral-point voltage balance, and switching loss can be reduced to improve conversion efficiency.

Accompanying drawing explanation

Fig. 1 is the structure drawing of device of the multi-electrical level inverter neutral-point voltage balance method that the present invention is based on hybrid modulation.

Fig. 2 is T-shaped and NPC type three-level inverter topology structure chart.

Fig. 3 is 3 level space vector distribution map.

Fig. 4 is for the first little triangle vector sequence in sector second and threephase load connection status circuit diagram in Fig. 3.

The vector corresponding diagram of tradition SVPWM method and the inventive method when Fig. 5 is uneven for the mid-point voltage of the first little triangle in sector second in Fig. 3, wherein (a) is traditional SVPWM method, and (b) is the inventive method.

Fig. 6 is the flow chart of the multi-electrical level inverter neutral-point voltage balance method that the present invention is based on hybrid modulation.

Fig. 7 is the oscillogram of DC bus-bar voltage up and down added when 0.02s in embodiment before and after control mode of the present invention.

Embodiment

The present invention adopts and realizes neutral point voltage balance in conjunction with space vector modulation (SVPWM) and discontinuous modula tion (DPWM) control mode, according to electric capacity instantaneous voltage, whether deviation selects pulse width modulation controlled mode to closed loop control method, namely, when neutral point voltage balance, the work of SVPWM control mode is selected; When mid-point voltage is uneven, select the work of DPWM control mode.The control signal of every phase switching tube of multi-electrical level inverter in current control period is determined under corresponding pulse width modulation controlled mode.Control the operating state of every phase of multi-electrical level inverter through drive circuit thus realize realizing neutral point voltage balance by the time of control output low and high level.

Composition graphs 1, the present invention is based on the multi-electrical level inverter neutral-point voltage balance method of hybrid modulation, in each control cycle, the sampling unit of digital processing control module detects the electric capacity instantaneous voltage between the mid point of multi-electrical level inverter DC bus and positive pole, electric capacity instantaneous voltage between the mid point of DC bus and negative pole, and determine the deviation of these two electric capacity instantaneous voltages, the work of SVPWM or DPWM control mode is selected through pulsewidth modulation strategy switch unit, control signal is exported by SVPWM/DPWM controlled processing unit, the operating state of multi-electrical level inverter every phase brachium pontis switching tube is controlled through drive circuit output pwm signal, control multi-electrical level inverter neutral point voltage balance simultaneously, concrete grammar comprises the following steps:

Step 1, sampling unit are sampled three-phase voltage signal that electric capacity instantaneous voltage between the DC bus-bar voltage of multi-electrical level inverter, electric capacity instantaneous voltage between the mid point of DC bus and positive pole, the mid point of DC bus and negative pole, multi-electrical level inverter export and the three-phase current signal that multi-electrical level inverter exports respectively.

Step 2, pulse-width modulation strategy switch unit, according to the deviation signal of two electric capacity instantaneous voltages described in step 1, select SVPWM control mode or the work of DPWM control mode.The standard of described selection SVPWM control mode or the work of DPWM control mode is:

Judge DC bus mid-point voltage variation delta V _nPwhether in error range: if | Δ V _nP| < Verro, then select the work of SVPWM control mode; Otherwise select the work of DPWM control mode, wherein DC bus mid-point voltage variation delta V _nPfor:

${\mathrm{\&Delta;V}}_{\mathrm{NP}}=-\frac{(V_{c1}-V_{c2})}{2}$

In formula, V _c1for electric capacity instantaneous voltage, V between the positive pole of DC bus and mid point _c2for the electric capacity instantaneous voltage between the mid point of DC bus and negative pole, the DC bus-bar voltage that Verro is less than 0.05 times.

The work of described DPWM control mode specifically comprises the following steps:

Determine a, b, c three-phase action moment t of multi-electrical level inverter _aon, t _bon, t _con, and judge DC bus mid-point voltage variation delta V _nPwhether be less than 0:

If Δ V _nP< 0, all deducts action moment minimum value t in three-phase by the action moment of every for gained phase _onmin;

If Δ V _nP> 0, all added the action moment of every for gained phase wherein T _srepresent the switch periods of multi-electrical level inverter, t _onmax=max{t _aon, t _bon, t _con, t _onmin=min{t _aon, t _bon, t _con.

Step 3, SVPWM/DPWM controlled processing unit, according to the control mode selected in the three-phase voltage signal of sampling unit output in step 1 and three-phase current signal, DC bus-bar voltage signal and step 2, determine the control signal of every each switching tube of phase brachium pontis of multi-electrical level inverter in current switch period;

The switch controlled signal that SVPWM/DPWM controlled processing unit exports is distributed to every each switching tube of phase brachium pontis of multi-electrical level inverter by step 4, drive circuit, controls operating state and the neutral point voltage balance of multi-electrical level inverter.Described digital processing control module is realized by STM32F407 chip.

Below for three-level inverter, elaborate the specific embodiment of the invention.

Fig. 2 (a) and (b) are T-shaped and NPC type three-level inverter topology structure chart, can draw mid point current i from figure _owith the current i flowing through upper and lower dc-link capacitance ₁, i ₂relation as expression formula (1), with label direction in scheming for positive direction.

i _o＝i ₁-i ₂（1）

By capacitance current and its both end voltage relation, can by i ₁, i ₂be expressed as:

$\begin{array}{c}{i}_1=C_1\frac{{\mathrm{dV}}_{c1}}{\mathrm{dt}}\\ i_2=C_2\frac{{\mathrm{dV}}_{c2}}{\mathrm{dt}}\end{array}---\left(2\right)$

Make upper and lower dc-link capacitance C ₁, C ₂capacitance is equal, i.e. C ₁=C ₂=C, (2) formula is substituted into (1) formula, and can obtain mid point current expression is:

$i_o=C\frac{d(V_{c1}-V_{c2})}{\mathrm{dt}}---\left(3\right)$

The table 1 three-level inverter on off state table of comparisons

Table 1 is three-level inverter on off state table of comparisons when being ideally neutral point voltage balance; Often kind of corresponding a kind of output voltage of on off state.Wherein S _xn(X=a, b, c; N=1,2,3,4) switching tube of the inverter bridge leg respectively in corresponding diagram 2, V _dcrepresent DC bus-bar voltage, DC bus mid point O is with reference to zero-potential point, when neutral point voltage balance, and the voltage variety Δ V of mid point O _nP=0, dc-link capacitance C ₁, C ₂voltage when mid-point voltage is uneven and offset from zero current potential, dc-link capacitance C ₁, C ₂voltage unequal time, mid-point voltage variation delta V _nPexpression formula be

${\mathrm{\&Delta;V}}_{\mathrm{NP}}=-\frac{(V_{c1}-V_{c2})}{2}---\left(4\right)$

By formula (3), (4), can show that the relation of mid point electric current and mid-point voltage variable quantity can be expressed as:

${\mathrm{\&Delta;V}}_{\mathrm{NP}}=\&Integral;-\frac{1}{2C}{i}_o\mathrm{dt}---\left(5\right)$

When the mid point sense of current is that namely positive direction flows out mid point, mid-point voltage variation delta V _nP<0, so mid-point voltage reduces.Otherwise the mid point sense of current is that namely negative direction flows to mid point, mid-point voltage variation delta V _nP>0, so mid-point voltage raises.And mid point current i _oagain with the load phase current i being connected to mid point _a, i _b, i _crelevant.Three-level inverter often has three kinds of level states mutually, make the output level state of every phase voltage by $S_j=\left\{\begin{array}{c}1\\ 0\\ -1\end{array}\right.(j=a,b,c)$ Represent, 1 represents P-state, and 0 represents O state, and-1 represents N state.The expression formula of mid point electric current is:

i _o＝(1-|S _a|)·i _a+(1-|S _b|)·i _b+(1-|S _c|)·i _c=i _a+i _b+i _c-(|S _a|·i _a+|S _b|·i _b+|S _c|·i _c)（6）

For three-phase three-wire system inverter system, three-phase output current meets expression formula:

i _a+i _b+i _c=0（7）

Bring (7) formula into (6) formula and can obtain mid point current i _owith threephase load current i _a, i _b, i _cthe available following expression of relation represent:

i _o=-(|S _a|·i _a+|S _b|·i _b+|S _c|·i _c)（8）

Under 27 kinds of vector states, mid point current i _owith threephase load current i _a, i _b, i _cthe visible table 2 of physical relationship.

Mid point electric current and load current comparison table under table 227 kind of vector state

(8) formula is substituted into (5) formula, and the pass that can obtain mid-point voltage variable quantity and load phase current is:

${\mathrm{\&Delta;V}}_{\mathrm{NP}}=\&Integral;\frac{1}{2C}\left(\right|S_a|\&CenterDot;i_a+|S_b|\&CenterDot;i_b+|S_c|\&CenterDot;i_c)\mathrm{dt}---\left(9\right)$

As shown in Figure 3, in three-level inverter, what SVPWM vector control adopted usually is seven segmentation vector sequences, and each sequence for the vector that starts, is termination vector with negative small vector (positive small vector) with negative small vector (positive small vector).If for little DELTA vectors district, first sector second, its seven segmentations vector sequence is: [OON]-[PON]-[PPN]-[PPO]-[PPN]-[PON]-[OON]. threephase load connection status corresponding to each vector is as shown in Figure 4.Fig. 4 (c) is the load connection status of vector sequence [PPO] correspondence, and a, b phase load is connected to positive bus-bar end, and c is connected to mid point, under this sequence state, mid point electric current is the c phase current flowing into mid point, upper Alma Mater capacitor charging, as the above analysis mid-point voltage variation delta V _nP>0, so mid-point voltage raises; Fig. 4 (d) is the load connection status of vector sequence [OON] correspondence, and a, b phase load is connected to mid point, and c is connected to negative busbar end, under this sequence state, mid point electric current is a, b phase current sum flowing out mid point, Down Highway capacitor discharge, as the above analysis mid-point voltage variation delta V _nP<0, so mid-point voltage reduces; Fig. 4 (a) is the load connection status of vector sequence [PPN] correspondence, and a, b phase load is connected to positive bus-bar end, and c is connected to negative busbar end, and with mid point without direct electrical connection, under this sequence state, load phase current alignment voltage is without impact; Fig. 4 (b) is the load connection status of vector sequence [PON] correspondence, and a phase load is connected to positive bus-bar end, and b is connected to mid point, c is connected to negative busbar end, under this sequence state, mid point electric current is the uncertain b phase current in direction, so the impact of alignment voltage is also uncertain.As can be seen from Figure 3, positive small vector [PPO], negative small vector [OON] represent same vector, claim these two vectors to be redundancy small vector.The known redundancy of above-mentioned analysis positive small vector alignment voltage has rising effect, and redundancy is born small vector alignment voltage and had reducing effect.Regulate the positive small vector of redundancy and redundancy to bear the action time of small vector, namely reach the function regulating mid-point voltage.Working method of the present invention: when mid-point voltage is on the low side, action time redundancy being born small vector changes into by the positive small vector effect of redundancy, as shown in Figure 5, in this control cycle, OON state is all replaced with the effect of POO state action time, and a phase load is connected to positive bus-bar voltage end always, output voltage is clamped at positive bus-bar voltage P current potential, a phase brachium pontis switching tube is failure to actuate, and realizes the object, this working method and discontinuous PWM (DPWM) mode that promote mid-point voltage.Otherwise mid-point voltage is higher, the action time of positive for redundancy small vector is changed into and bears small vector effect by redundancy.

As shown in Figure 5, can find out that the essential distinction of DPWM working method and the SVPWM mode adopted when mid-point voltage is uneven in the present invention is that the positive and negative small vector of redundancy does not occur in pairs, the state of irredundant negative small vector OON in DPWM mode when mid-point voltage is on the low side, t in figure _jon(j=a, b, c) is every phase action moment.

When mid-point voltage is uneven, if do not taked any measure to regulate mid-point voltage, still adopting SVPWM mode, there is deviation by continuing in mid-point voltage, this variation delta V _nPcan be expressed as with the relation of the action time of vector each under which: the action time of large vector (PPN) is 2 (t _con-t _bon); The action time of middle vector (PON) is 2 (t _bon-t _aon); The action time of the positive small vector of redundancy (PPO) is the action time that redundancy bears small vector (OON) is 2t _aon; What the positive small vector of redundancy and negative small vector represented on the 3 level space vector distribution map shown in Fig. 3 is same vector, and the action time of this vector is threephase load current i can be made _a, i _b, i _cbe definite value in this control cycle, so (9) formula can discretely turn to:

$=\frac{1}{C}[(-i_b)\&CenterDot;(t_{\mathrm{bon}}-t_{\mathrm{aon}})+i_c(t_{\mathrm{aon}}+t_{\mathrm{con}}-\frac{T_s}{2})]$

Load current i in formula (10) _c<0, i _b>0, t _bon-t _aon> 0, so Δ V _nP<0, unbalance of neutral-point voltage is more serious; If in this control cycle, adopt DPWM mode alignment voltage to regulate, every phase action moment need redefine according to the following formula:

t _aon′＝t _aon-t _onmin

t _bon′＝t _bon-t _onmin（11）

t _con′＝t _con-t _onmin

T in formula _onmin=min (t _aon, t _bon, t _con), t in this little DELTA vectors district, first sector second _onmin=min (t _aon, t _bon, t _con)=t _aon, again formula (11) can be write as following expression:

t _aon′＝0

t _bon′＝t _bon-t _aon（12）

t _con′＝t _con-t _aon

Under this DPWM control mode, action time of vector [OON] is zero see Fig. 5, and a phase operate time is zero, and namely the switching tube of a phase brachium pontis is failure to actuate in this control cycle.The action time of each vector is: the action time of large vector (PPN) is 2 (t _con'-t _bon')=2 [(t _con-t _onmin)-(t _bon-t _onmin)]=2 (t _con-t _bon); The action time of middle vector (PON) is 2 (t _bon'-t _aon')=2 [(t _bon-t _onmin)-(t _aon-t _onmin)]=2 (t _bon-t _aon); The action time of the positive small vector of redundancy (PPO) is $2(\frac{T_s}{2}-t_{\mathrm{con}}^{\&prime;})=2[\frac{T_s}{2}-(t_{\mathrm{con}}-t_{\mathrm{on}\min })]=2(\frac{T_s}{2}-t_{\mathrm{con}}+t_{\mathrm{aon}});$ The action time that redundancy bears small vector (OON) is 0.The pass of mid-point voltage variable quantity and vector and action time thereof is:

$=\frac{1}{C}[(-i_b)\&CenterDot;(t_{\mathrm{bon}}-t_{\mathrm{aon}})-i_c(\frac{T_s}{2}{-t}_{\mathrm{con}}+t_{\mathrm{aon}})]$

Load current i in formula _c<0, i _b>0, t _bon-t _aon> 0, (-i _b) (t _bon-t _aon) <0, comparison expression (10), (13), known Δ V _nP< Δ V _nP', but due to | i _b|, | i _c| size is uncertain, so Δ V _nP' symbol and uncertain, but to it is evident that: after have employed this DPWM mode, mid-point voltage regulates toward desired value.If in this control cycle, occur overregulating, i.e. Δ V _nP' >0, mid-point voltage is higher, then next control cycle still needs to adopt DPWM mode alignment voltage to regulate, and need redefine the three-phase action moment by following formula:

$t_{\mathrm{aon}}^{\&prime;}=t_{\mathrm{aon}}+\frac{T_s}{2}-t_{\mathrm{on}\max }$

$t_{\mathrm{bon}}^{\&prime;}=t_{\mathrm{bon}}+\frac{T_s}{2}-t_{\mathrm{on}\max }---\left(14\right)$

$t_{\mathrm{con}}^{\&prime;}=t_{\mathrm{con}}+\frac{T_s}{2}-t_{\mathrm{on}\max }$

T in formula _onmax=max (t _aon, t _bon, t _con), determine new every phase action moment, produce the control signal of every phase switching tube, thus realize the object regulating mid-point voltage, specific implementation process is consistent with mid-point voltage situation on the low side, repeats no more.

What the positive and negative small vector of redundancy represented is same vector, no matter be adopt DPWM mode or SVPWM mode, the action time of this vector is constant, and the action time of large vector, middle vector is also constant, so the action effect of resultant vector in this control cycle is also constant, namely three-phase output voltage is constant.So adopt DPWM mode when mid-point voltage is uneven, when not affecting three-phase output voltage quality, the object realizing neutral point voltage balance but can be reached.

To sum up, no matter adopt SVPWM mode or DPWM mode, do not affect output voltage, but can reach the object of balance mid-point voltage.If mid-point voltage is on the low side, then the action moment of every phase is all deducted action moment minimum value t in three-phase _onminotherwise, then the action moment of every phase is added until neutral point voltage balance.

The present invention proposes a kind of multi-electrical level inverter neutral-point voltage balance method based on hybrid modulation, and Fig. 6 is the flow chart that closed loop control method is specifically implemented, and detailed step is as follows:

(1) sampling unit is sampled three-phase voltage signal that electric capacity instantaneous voltage between the DC bus-bar voltage of multi-electrical level inverter, electric capacity instantaneous voltage between the mid point of DC bus and positive pole, the mid point of DC bus and negative pole, multi-electrical level inverter export and the three-phase current signal that multi-electrical level inverter exports respectively, enters (2);

(2) DC bus mid-point voltage variation delta V is judged _nPwhether meet | Δ V _nP| < Verro, namely close to balance point, wherein Verro is definite value, if meet above-mentioned Rule of judgment, then enters (3), otherwise enters (4);

(3) SVPWM controlled processing unit, determines a, b, c three-phase action moment t of multi-electrical level inverter _aon, t _bon, t _con, jump to (5);

(4) DPWM controlled processing unit, determines a, b, c three-phase action moment t of multi-electrical level inverter _aon, t _bon, t _conjudge whether DC bus mid-point voltage variable quantity meets Δ V _nP<0; If meet Rule of judgment, is all deducted action moment minimum value t in three-phase the action moment of every for gained phase _onmin; Otherwise the action moment of every for gained phase is all added wherein T _srepresent the switch periods of multi-electrical level inverter, t _onmax=max{t _aon, t _bon, t _con, t _onmin=min{t _aon, t _bon, t _con, enter (5);

(5) by the action moment of every phase, determine every phase switch controlled signal, distribute to each switching tube of multi-electrical level inverter every phase brachium pontis through drive circuit, control operating state and the neutral point voltage balance of multi-electrical level inverter; Be back to afterwards (1), enter the circulation of next switch periods.

Embodiment 1

For checking the present invention proposes a kind of feasibility of the multi-electrical level inverter neutral-point voltage balance method based on hybrid modulation, the Simulink instrument in MATLAB is utilized to build three-level inverter circuit, direct current exports three-phase voltage by tri-level circuit inversion after clamping capacitance, exports smooth sinusoidal voltage waveform through EMI filter circuit.Electric parameter in simulation process is arranged as following table:

Fig. 7 is the dc-link capacitance C under above-mentioned electric parameter is arranged ₁, C ₂voltage V _c1, V _c2simulation waveform.Control mode of the present invention is added in the 0.02s moment, when not adding control mode of the present invention, dc-link capacitance C ₁, C ₂voltage V _c1, V _c2average value about 30V, mid-point voltage is seriously uneven, after the 0.02s moment adds control mode of the present invention, can find out that system enters stable state after about 0.0045s, dc-link capacitance C ₁, C ₂voltage V _c1, V _c2the difference of mean value within 2V, realize neutral point voltage balance, the multi-electrical level inverter neutral-point voltage balance method control effects based on hybrid modulation that visible the present invention proposes is remarkable.

The hybrid modulation stratgy that the inventive method adopts SVPWM and DPWM mode to combine realizes the neutral point voltage balance of multi-electrical level inverter.System in neutral point voltage balance situation according to the control mode work of SVPWM; Mid-point voltage is switched to the work of DPWM control mode time uneven.The method only needs the electric capacity instantaneous voltage between the mid point of electric capacity instantaneous voltage between the mid point of real-time sampling multi-electrical level inverter DC bus and positive pole, DC bus and negative pole, and determine the deviation of these two electric capacity instantaneous voltages, SVPWM control mode or DPWM control mode is selected through pulsewidth modulation strategy switch unit, SVPWM/DPWM controlled processing unit produces pulse-width modulation (PWM) signal according to selected control mode, control the every phase switch tube working status of multi-electrical level inverter through drive circuit, control the neutral point voltage balance of multi-electrical level inverter simultaneously.Relative to the single control method of tradition, the method can play traditional SVPWM stability contorting and the little advantage of output voltage current harmonics, when mid-point voltage occurs unbalanced, DPWM can be embodied and controls effectively to realize neutral-point voltage balance, reduces system equivalent switching frequency thus effect of minimizing switching loss again.Control method of the present invention does not increase control algolithm complexity, there is real-time good, computational process simply and easily realize, have and reduce switching frequency, minimizing switching loss, output voltage, current harmonics are little, output waveform quality is high and be convenient to the advantages such as Digital Realization, has great engineer applied and is worth.

Claims (3)

1. the multi-electrical level inverter neutral-point voltage balance method based on hybrid modulation, it is characterized in that, in each control cycle, the sampling unit of digital processing control module detects the electric capacity instantaneous voltage between the mid point of multi-electrical level inverter DC bus and positive pole, electric capacity instantaneous voltage between the mid point of DC bus and negative pole, and determine the deviation of these two electric capacity instantaneous voltages, the work of SVPWM or DPWM control mode is selected through pulsewidth modulation strategy switch unit, control signal is exported by SVPWM/DPWM controlled processing unit, the operating state of multi-electrical level inverter every phase brachium pontis switching tube is controlled through drive circuit output pwm signal, control multi-electrical level inverter neutral point voltage balance simultaneously, concrete grammar comprises the following steps:

Step 1, sampling unit are sampled three-phase voltage signal that electric capacity instantaneous voltage between the DC bus-bar voltage of multi-electrical level inverter, electric capacity instantaneous voltage between the mid point of DC bus and positive pole, the mid point of DC bus and negative pole, multi-electrical level inverter export and the three-phase current signal that multi-electrical level inverter exports respectively;

Step 2, pulse-width modulation strategy switch unit are according to the deviation of two electric capacity instantaneous voltages described in step 1, and select SVPWM control mode or the work of DPWM control mode, the standard of described selection SVPWM control mode or the work of DPWM control mode is:

Judge DC bus mid-point voltage variation delta V _nPwhether in error range: if | Δ V _nP| < Verro, then select the work of SVPWM control mode; Otherwise select the work of DPWM control mode, wherein DC bus mid-point voltage variation delta V _nPfor:

${\mathrm{\&Delta;V}}_{NP}=-\frac{(V_{c1}-V_{c2})}{2}$

In formula, V _c1for electric capacity instantaneous voltage, V between the positive pole of DC bus and mid point _c2for the electric capacity instantaneous voltage between the mid point of DC bus and negative pole, the DC bus-bar voltage that Verro is less than 0.05 times;

Step 3, SVPWM/DPWM controlled processing unit, according to the control mode selected in the three-phase voltage signal of sampling unit output in step 1 and three-phase current signal, DC bus-bar voltage and step 2, determine the control signal of every each switching tube of phase brachium pontis of multi-electrical level inverter in current switch period;

The switch controlled signal that SVPWM/DPWM controlled processing unit exports is distributed to every each switching tube of phase brachium pontis of multi-electrical level inverter by step 4, drive circuit, controls operating state and the neutral point voltage balance of multi-electrical level inverter.

2. the multi-electrical level inverter neutral-point voltage balance method based on hybrid modulation according to claim 1, it is characterized in that, described digital processing control module is realized by STM32F407 chip.

3. the multi-electrical level inverter neutral-point voltage balance method based on hybrid modulation according to claim 1, it is characterized in that, the work of described DPWM control mode specifically comprises the following steps:

Determine a, b, c three-phase action moment t of multi-electrical level inverter _aon, t _bon, t _con, and judge DC bus mid-point voltage variation delta V _nPwhether be less than 0:

If Δ V _nP< 0, all deducts action moment minimum value t in three-phase by the action moment of every for gained phase _onmin;

If Δ V _nP> 0, all added the action moment of every for gained phase wherein T _srepresent the switch periods of multi-electrical level inverter, t _onmax=max{t _aon, t _bon, t _con, t _onmin=min{t _aon, t _bon, t _con.