CN103746585A - Hybrid modulation-based control method for mid-point voltage balance of multilevel inverter - Google Patents

Abstract

The invention discloses a hybrid modulation-based control method for the mid-point voltage balance of a multilevel inverter. The method comprises the following steps: detecting a capacitance transient voltage between the midpoint and the anode and a capacitance transient voltage between the midpoint and the cathode of a direct-current bus of the multilevel inverter by a sampling unit of a digital processing control module within each control period, and determining the bias of the two capacitance transient voltages; selecting an SVPWM (space vector pulse width modulation) or DPWM (digital pulse width modulation) control mode through a pulse width modulation strategy switching unit to work; outputting a control signal through an SVPWM/DPWM control processing unit; outputting a PWM signal through a drive circuit to control the working condition of each phase bridge arm switch tube of the multilevel inverter, and meanwhile, controlling the mid-point voltage balance of the multilevel inverter. The control method disclosed by the invention has the advantages that the switching frequency is reduced, the switching loss is reduced, output voltage and current harmonics are small, the output waveform quality is high, digital realization is convenient, and the like, and has an important engineering application value.

Description

Multi-electrical level inverter neutral point voltage balance control method based on 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 control method based on hybrid modulation.

Background technology

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

Cause that the unbalanced reason of mid-point voltage mainly contains:

1) the actual capacitance capacitance deviation causing due to the error of technique manufacture;

2) characteristic of switching tube is inconsistent;

3) three-phase imbalance operation.

In order to solve mid-point voltage imbalance problem, Chinese scholars has proposed a lot of methods, mainly contains two kinds of methods: a kind of is based on sinusoidal pulse width modulation (SPWM) technology, by injecting zero-sequence component, maintains neutral point voltage balance; Another kind is based on space vector pulse width modulation (SVPWM) technology, by regulating, controls mid-point voltage the action time of redundancy small vector.In SPWM mode, mainly by inject zero-sequence component in three-phase modulations ripple, maintain neutral point voltage balance.But the computing of residual voltage needs skill, be generally that operand is large and method is complicated by the method for " estimating-verification-correction ".In SVPWM mode, conventional method is the dispensed factor, is determined the relativity time of every pair of redundancy small vector by distribution factor.Different sectors, in different triangles, the formula that distribution factor calculates is different; Which is processed 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 realizes the digitized multi-electrical level inverter neutral point voltage balance control method based on hybrid modulation.

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

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

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

Step 3, SVPWM/DPWM controlled processing unit, according to the control mode of selecting 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, are determined the control signal of every phase brachium pontis switching tube of multi-electrical level inverter in current switch periods;

Every each switching tube of phase brachium pontis of multi-electrical level inverter distributed to the switch controlled signal of SVPWM/DPWM controlled processing unit output 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 strategy that (1) adopts space vector modulation (SVPWM) to combine with discontinuous modulation (DPWM) mode is realized the neutral point voltage balance of multi-electrical level inverter, can bring into play the stable control of traditional SVPWM and the little advantage of output voltage current harmonics, can occur unbalanced time at mid-point voltage again, thereby embody DPWM, control and effectively realize neutral point voltage balance and control, reduce the effect that system equivalent switching frequency reduces switching loss; (2) inductive current ripple reduces, and harmonic wave is little, output waveform quality is higher; (3) have and realize the function that inverter neutral point voltage balance is controlled, and can reduce switching loss and improve conversion efficiency.

Accompanying drawing explanation

Fig. 1 is the structure drawing of device that the present invention is based on the multi-electrical level inverter neutral point voltage balance control method of 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 that to take the first little triangle in sector second in Fig. 3 be example vector sequence and threephase load connection status circuit diagram.

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

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

Fig. 7 is the oscillogram of DC bus-bar voltage up and down that adds control mode of the present invention front and back in embodiment when 0.02s.

Embodiment

The present invention adopts and realizes neutral point voltage balance in conjunction with space vector modulation (SVPWM) and discontinuous modulation (DPWM) control mode, according to electric capacity instantaneous voltage, whether deviation is selected pulse width modulation controlled mode to closed loop control method, when neutral point voltage balance, select the work of SVPWM control mode; When mid-point voltage is uneven, select the work of DPWM control mode.Under corresponding pulse width modulation controlled mode, determine the control signal of every phase switching tube of multi-electrical level inverter in current control cycle.Thereby the time that the operating state realization of controlling every phase of multi-electrical level inverter through drive circuit is exported low and high level by control is realized neutral point voltage balance.

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

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

Step 2, pulse-width modulation strategy switch unit, according to the deviation signal of two electric capacity instantaneous voltages described in step 1, are selected 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:

Judgement DC bus mid-point voltage variation delta V _nPwhether in error range: if | Δ V _nP| < Verro, selects 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 the positive pole of DC bus and electric capacity instantaneous voltage, V between mid point _c2for the mid point of DC bus and the electric capacity instantaneous voltage between 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, the 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, and the action of the every phase of gained is all deducted to action moment minimum value t in three-phase constantly _{on min};

If Δ V _nP> 0, and the action of the every phase of gained is all added constantly

t wherein _sthe switch periods that represents multi-electrical level inverter, t _{on max}=max{t _aon, t _bon, t _con, t _{on min}=min{t _aon, t _bon, t _con.

Step 3, SVPWM/DPWM controlled processing unit, according to the control mode of selecting 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, are determined the control signal of every each switching tube of phase brachium pontis of multi-electrical level inverter in current switch periods;

Every each switching tube of phase brachium pontis of multi-electrical level inverter distributed to the switch controlled signal of SVPWM/DPWM controlled processing unit output 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.

Three-level inverter take below as example, elaborate the specific embodiment of the invention.

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

i _o＝i ₁-i ₂ （1）

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

$\begin{array}{c}{\mathrm{<figure-callout\; id="1"\; label="i"\; filenames="HDA0000455301100000022.png,HDA0000455301100000031.png"\; state="\{\{state\}\}">i</figure-callout>}}_1={\mathrm{<figure-callout\; id="1"\; label="C"\; filenames="HDA0000455301100000022.png,HDA0000455301100000031.png"\; state="\{\{state\}\}">C</figure-callout>}}_1\frac{{\mathrm{<figure-callout\; id="1"\; label="dV\; c"\; filenames="HDA0000455301100000022.png,HDA0000455301100000031.png"\; state="\{\{state\}\}">dV</figure-callout>}}_c}{\mathrm{<figure-callout\; id="2"\; label="dt\; i"\; filenames="HDA0000455301100000012.png"\; state="\{\{state\}\}">dt</figure-callout>}}& \\ i_{}{}_2={\mathrm{<figure-callout\; id="2"\; label="C"\; filenames="HDA0000455301100000012.png"\; state="\{\{state\}\}">C</figure-callout>}}_2\frac{{\mathrm{<figure-callout\; id="2"\; label="dV\; c"\; filenames="HDA0000455301100000012.png"\; state="\{\{state\}\}">dV</figure-callout>}}_c}{\mathrm{dt}}\end{array}---\left(2\right)$

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

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

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

Table 1 is the three-level inverter on off state table of comparisons when being ideally neutral point voltage balance; Every kind of corresponding a kind of output voltage of on off state.S wherein _xn(X=a, b, c; N=1,2,3,4) switching tube of the inverter bridge leg in corresponding diagram 2 respectively, 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

offset from zero current potential when mid-point voltage is uneven, dc-link capacitance C ₁, C ₂voltage when unequal, 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 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 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 that is connected to mid point _a, i _b, i _crelevant.Three-level inverter is every three kinds of level states mutually, the output level state that makes 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 _cphysical relationship can be in Table 2.

Mid point electric current and load current comparison table under 27 kinds of vector states of table 2

By (8) formula substitution (5) formula, 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 conventionally is seven segmentation vector sequences, and each sequence is born small vector (positive small vector) as termination vector to bear small vector (positive small vector) for the vector that starts, take.If take little triangle vector district, first sector second is example, 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 that vector sequence [PPO] is corresponding, and a, b phase load are connected to positive bus-bar end, and c is connected to mid point, under this sequence state, mid point electric current is for flowing into the c phase current of 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 that vector sequence [OON] is corresponding, and a, b phase load are connected to mid point, and c is connected to negative busbar end, under this sequence state, mid point electric current is for flowing out a, the b phase current sum of 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 that vector sequence [PPN] is corresponding, and a, b phase load are connected to positive bus-bar end, and c is connected to negative busbar end, and without direct electrical connection, under this sequence state, load phase current alignment voltage is without impact with mid point; Fig. 4 (b) is the load connection status that vector sequence [PON] is corresponding, 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 of 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 that these two vectors are redundancy small vector.The positive small vector alignment of the known redundancy of above-mentioned analysis voltage has rising effect, and the negative small vector alignment voltage of redundancy has reducing effect.Regulate the action time of the negative small vector of the positive small vector of redundancy and redundancy, reach the function that regulates mid-point voltage.Working method of the present invention: when mid-point voltage is on the low side, changed into by the positive small vector effect of redundancy the action time of the negative small vector of redundancy, as shown in Figure 5, in this control cycle, OON state is all replaced with to the effect of POO state action time, 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 that promotes mid-point voltage, and this working method is discontinuous pulse-width modulation (DPWM) mode.Otherwise mid-point voltage is higher, will change into the action time of the positive small vector of redundancy by the negative small vector effect of redundancy.

As shown in Figure 5, can find out in the present invention that the DPWM working method adopting is the appearance in pairs of the positive and negative small vector of redundancy with the essential distinction of SVPWM mode when mid-point voltage is uneven, 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 moment of action mutually.

When mid-point voltage is uneven, if do not taked any measure to regulate mid-point voltage, still adopt SVPWM mode, mid-point voltage will continue to occur deviation, this variation delta V _nPcan be expressed as with the relation of action time of each vector under which: be 2 (t the action time of large vector (PPN) _con-t _bon); Be 2 (t the action time of middle vector (PON) _bon-t _aon); Be the action time of the positive small vector of redundancy (PPO)

be 2t the action time that redundancy is born small vector (OON) _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 be the action time of this vector

can make threephase load current i _a, i _b, i _cin this control cycle, be definite value, 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 action mutually constantly need redefine according to the following formula:

t _aon′＝t _aon-t _{on min}

t _bon′＝t _bon-t _{on min} （11）

t _con′＝t _con-t _{on min}

T in formula _{on min}=min (t _aon, t _bon, t _con), t in this little triangle vector district, first sector second _{on min}=min (t _aon, t _bon, t _con)=t _aon, again formula (11) can be write as to following expression:

t _aon′＝0

t _bon′＝t _bon-t _aon （12）

t _con′＝t _con-t _aon

Under this DPWM control mode, be zero to see Fig. 5 the action time of vector [OON], and a phase operate time is zero, and the switching tube of a phase brachium pontis is failure to actuate in this control cycle.Be the action time of each vector: be 2 (t the action time of large vector (PPN) _con'-t _bon')=2[(t _con-t _{on min})-(t _bon-t _{on min})]=2 (t _con-t _bon); Be 2 (t the action time of middle vector (PON) _bon'-t _aon')=2[(t _bon-t _{on min})-(t _aon-t _{on min})]=2 (t _bon-t _aon); Be the action time of the positive small vector of redundancy (PPO) $2(\frac{T_s}{2}-t_{\mathrm{con}}^{\&prime;})=2[\frac{T_s}{2}-(t_{\mathrm{con}}-t_{\mathrm{on}\min })]=2(\frac{T_s}{2}-t_{\mathrm{con}}+t_{\mathrm{aon}});$ Be 0 the action time that redundancy is born small vector (OON).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 uncertain, but it is evident that: after having adopted 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, and next control cycle still needs to adopt DPWM mode alignment voltage to regulate, and need redefine three-phase action constantly 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 _{on max}=max (t _aon, t _bon, t _con), determine new every action mutually constantly, produce the control signal of every phase switching tube, thereby realize the object that regulates 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 to 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, three-phase output voltage is constant.So adopt DPWM mode when mid-point voltage is uneven, in the situation that not affecting three-phase output voltage quality, but can reaches the object that realizes neutral point voltage balance.

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, the action of every phase is all deducted to action moment minimum value t in three-phase constantly _{on min}otherwise,, the action of every phase is added constantly

until neutral point voltage balance.

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

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

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

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

(4) DPWM controlled processing unit, a, b, the c three-phase action of determining multi-electrical level inverter is t constantly _aon, t _bon, t _conjudge whether DC bus mid-point voltage variable quantity meets Δ V _nP<0; If meet Rule of judgment, the action of the every phase of gained is all deducted to action moment minimum value t in three-phase constantly _{on min}; Otherwise the action of the every phase of gained is all added constantly

t wherein _sthe switch periods that represents multi-electrical level inverter, t _{on max}=max{t _aon, t _bon, t _con, t _{on min}=min{t _aon, t _bon, t _con, enter (5);

(5) by the action moment of every phase, determine every phase switch controlled signal, through drive circuit, distribute to every each switching tube of phase brachium pontis of multi-electrical level inverter, 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 verifying that the present invention proposes a kind of feasibility of the multi-electrical level inverter neutral point voltage balance control method based on hybrid modulation, utilize the Simulink instrument in MATLAB to build tri-level inversion circuit, direct current by tri-level circuit inversion output three-phase voltage, is exported smooth sinusoidal voltage waveform through EMI filter circuit after clamping capacitance.Electric parameter setting in simulation process is as following table:

Fig. 7 is the dc-link capacitance C under above-mentioned electric parameter arranges ₁, C ₂voltage V _c1, V _c2simulation waveform.At 0.02s, constantly add control mode of the present invention, when not adding control mode of the present invention, dc-link capacitance C ₁, C ₂voltage V _c1, V _c2mean value differ about 30V, mid-point voltage is seriously uneven, after 0.02s adds control mode of the present invention constantly, 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 in 2V, realize neutral point voltage balance, it is remarkable that the multi-electrical level inverter neutral point voltage balance control method based on hybrid modulation that visible the present invention proposes is controlled effect.

The hybrid modulation strategy that the inventive method adopts SVPWM to combine with DPWM mode is realized 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 when uneven.The method only needs the mid point of real-time sampling multi-electrical level inverter DC bus and the electric capacity instantaneous voltage between positive pole, the mid point of DC bus and the electric capacity instantaneous voltage between negative pole, and the deviation of definite these two electric capacity instantaneous voltages, through pulsewidth modulation strategy switch unit, select SVPWM control mode or DPWM control mode, SVPWM/DPWM controlled processing unit produces pulse-width modulation (PWM) signal according to selected control mode, through drive circuit, control the every phase switch tube working status of multi-electrical level inverter, control the neutral point voltage balance of multi-electrical level inverter simultaneously.With respect to the single control method of tradition, the method can be brought into play the stable control of traditional SVPWM and the little advantage of output voltage current harmonics, can occur unbalanced time at mid-point voltage again, thereby embody DPWM, control and effectively realize neutral point voltage balance and control, reduce the effect that system equivalent switching frequency reduces switching loss.Control method of the present invention does not increase control algolithm complexity, there is real-time good, computational process simply and easily realizes, there is the switching frequency of reduction, reduce 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 engineering using value.

Claims (4)

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

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

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

Step 3, SVPWM/DPWM controlled processing unit, according to the control mode of selecting 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, are determined the control signal of every each switching tube of phase brachium pontis of multi-electrical level inverter in current switch periods;

Every each switching tube of phase brachium pontis of multi-electrical level inverter distributed to the switch controlled signal of SVPWM/DPWM controlled processing unit output 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 control method based on hybrid modulation according to claim 1, is characterized in that, selects the standard of SVPWM control mode or the work of DPWM control mode to be described in step 2:

Judgement DC bus mid-point voltage variation delta V _nPwhether in error range: if | Δ V _nP| < Verro, selects 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 the positive pole of DC bus and electric capacity instantaneous voltage, V between mid point _c2for the mid point of DC bus and the electric capacity instantaneous voltage between negative pole, the DC bus-bar voltage that Verro is less than 0.05 times.

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

4. the multi-electrical level inverter neutral point voltage balance control method based on hybrid modulation according to claim 2, is characterized in that, the work of described DPWM control mode specifically comprises the following steps:

Determine a, b, the 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, and the action of the every phase of gained is all deducted to action moment minimum value t in three-phase constantly _onmin; If Δ V _nP> 0, and the action of the every phase of gained is all added constantly

t wherein _sthe switch periods that represents multi-electrical level inverter, t _{on max}=max{t _aon, t _bon, t _con, t _{on min}=min{t _aon, t _bon, t _con.

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