CN104065291A - System and method for controlling neutral point voltage balance with low frequency oscillation suppression function - Google Patents

Abstract

The invention discloses a system and a method for controlling neutral point voltage balance with a low frequency oscillation suppression function. The system comprises a multi-level inverter and a digital processing control module, wherein the digital processing control module comprises a sampling unit, a closed-loop control unit and a digital pulse width modulation (DPWM) unit; the output end of the DPWM unit is connected to each switching tube of the bridge arm of each phase of the multi-level inverter. The method comprises the following steps: in each switching period, the sampling unit acquires a direct-current bus voltage signal of the multi-level inverter, and a three-phase voltage signal and a three-phase current signal both output by the multi-level inverter, respectively; the acquired signals are processed by the closed-loop control unit to obtain three-phase modulating waves; the three-phase modulating waves are processed by the DPWM unit to obtain a pulse width modulation control signal which is used for controlling the working state of the switching tube of each phase of the multi-level inverter by virtue of a drive circuit. The system and the method are capable of suppressing the low frequency oscillation of the neutral point voltage of the direct-current bus of the multi-level inverter and realizing the neutral point voltage balance.

Description

Neutral point voltage balance control system and the method with low-frequency oscillation inhibit feature

Technical field

The invention belongs to the control technology field in Technics of Power Electronic Conversion technology, particularly a kind of neutral point voltage balance control system and method with low-frequency oscillation inhibit feature.

Background technology

In recent years, multi-electrical level inverter has been widely used in high-power conversion occasion in high pressure; Its circuit topological structure and control method become current study hotspot.Compare two-level inverter, multi-electrical level inverter has the plurality of advantages such as switch tube voltage stress is low, electromagnetic noise is little, harmonic wave of output voltage aberration rate is low.

The topology of multi-electrical level inverter can be divided three classes substantially: diode-clamped, cascade connection type, striding capacitance type, wherein most widely used is Diode-clamped inverter.Take three-level inverter as example, and three level Diode-clamped inverters can be divided into neutral point clamp type (NPC) inverter and T-shaped inverter.Ideally, in these two kinds of topologys, DC bus-bar voltage is divided equally by upper and lower dc-link capacitance; But in practical application, because a variety of causes makes upper and lower dc-link capacitance voltage, occur fluctuation and the upper and lower unequal situation of dc-link capacitance voltage, mid-point voltage is uneven.Mid-point voltage energy imbalance can cause the dangerous operation of power switch pipe, and output voltage waveforms quality reduces, and affects system and normally work when serious.

The control method of multi-electrical level inverter mainly contains: sinusoidal pulse width modulation (SPWM), space vector pulse width modulation (SVPWM), triple-frequency harmonics inject pulse-width modulation (THIPWM), discontinuous pulse-width modulation (DPWM) etc.At present, about neutral point voltage balance control problem, there is many research both at home and abroad.In above-mentioned pulse duration modulation method, DPWM modulator approach has the loss of the inverter switching device of reduction, improve the advantages such as inversion efficiency and receive much concern, several typical DPWM modulator approaches have at present: output voltage is each 60 ° of DPWM1 methods that interval switch is failure to actuate at power frequency positive-negative half-cycle peak value place; The DPWMMIN method that output voltage is failure to actuate in 120 ° of intervals of power frequency negative half period; (being referred to as DPWMx) such as DPWMMAX methods that output voltage is failure to actuate in 120 ° of intervals of power frequency positive half period.For example: document 1 (Wang Xinyu, He Yingjie, Liu Jinjun. inject zero-sequence component SPWM modulation three-level inverter DC side neutral point voltage balance control mechanism [J]. electrotechnics journal, 2011,26 (5): 71-77.) by injecting 6 times of time zero-sequence components, realize mid-point voltage control to three-phase modulations ripple control command voltage, but the method is comparatively complicated, it is controlled based on a power frequency period, compare with the method for controlling based on switch periods, regulate overlong time, document 2 (Song Wenxiang, Chen Guocheng, Deng. a kind of 3 level space vector modulator approach and realization [J] thereof with midpoint potential equilibrium function. Chinese electrical engineering, 2006, 26 (12): 96-100.) on the basis of traditional seven segmentation SVPWM control methods that do not possess neutral point voltage balance regulatory function, a kind of combination ten syllogic are proposed, nine segmentations, the new SVPWM control method alignment voltage of seven segmentation hybrid modulation is controlled, it is by stages choice of modulation mode in a power frequency period, wherein under ten syllogic modulation systems, in each switch periods, switching tube moves 12 times, under nine segmentation modulation systems, each switch periods switching tube action is 8 times, therefore seven more traditional segmentation SVPWM control methods of the average switching loss of this compound modulation increase greatly.Document 3 (Cheng Shanmei, king is light, Xia Litao, Wan Shuyun. discontinuous space vector pulse width modulation strategy study [J], servo techniques, 2002,36 (4): 37-40.) mainly study several discontinuous space vector pulse width modulation strategies, for a certain action mutually in certain certain area coverage in power frequency period, thereby reduce switching loss, but this control method do not have the ability that neutral point voltage balance is controlled.

In sum, traditional SVPWM, DPWMx, SPWM control method generally do not have the ability that neutral point voltage balance regulates, and traditional DPWMx method a certain section of continuum in power frequency period keeps certain phase switching tube to be failure to actuate, therefore can reduce switching loss, but in the switch region of being failure to actuate, inverter outputting inductance current ripples increases, longer in the time in the region of being failure to actuate, ripple is larger, certainly will affect output current harmonics characteristic.

Summary of the invention

The object of the present invention is to provide a kind of control method is simple, be easy to Digital Implementation neutral point voltage balance control system and the method with low-frequency oscillation inhibit feature.

The technical solution that realizes the object of the invention is: a kind of neutral point voltage balance control system with low-frequency oscillation inhibit feature, comprise multi-electrical level inverter and digital processing control module, wherein digital processing control module comprises sampling unit, Closed Loop Control Unit and the DPWM pwm unit connecting in turn; Described sampling unit gathers respectively the DC bus-bar voltage signal of multi-electrical level inverter, the three-phase current signal of the three-phase voltage signal of multi-electrical level inverter output, multi-electrical level inverter output, and the output of DPWM pwm unit accesses every each switching tube of phase brachium pontis of multi-electrical level inverter by drive circuit;

In each switch periods, the sampling unit of digital processing control module gathers respectively the DC bus-bar voltage signal of multi-electrical level inverter, the three-phase current signal of the three-phase voltage signal of multi-electrical level inverter output, multi-electrical level inverter output; The signal collecting is processed and is obtained three-phase modulations ripple V through Closed Loop Control Unit _a, V _b, V _c; Three-phase modulations ripple is processed and to be obtained pulse-width modulation (PWM) control signal through DPWM pwm unit, this pwm control signal can suppress multi-electrical level inverter DC bus mid-point voltage low-frequency oscillation, realize neutral point voltage balance; Described pwm control signal is controlled the operating state of the every phase switching tube of multi-electrical level inverter through drive circuit.

A neutral point voltage balance control method with low-frequency oscillation inhibit feature, the method, comprises the following steps than the multi-electrical level inverter in situation for low modulation:

Step 1, in each switch periods, the sampling unit of digital processing control module gathers respectively the DC bus-bar voltage signal of multi-electrical level inverter, the three-phase current signal of the three-phase voltage signal of multi-electrical level inverter output, multi-electrical level inverter output;

Step 2, Closed Loop Control Unit carry out proportion integration differentiation adjusting according to the signal collecting in step 1 and base reference signal, output three-phase modulations ripple V _a, V _b, V _c;

Step 3, DPWM pwm unit are to the three-phase modulations ripple V obtaining in step 2 _a, V _b, V _cregulate, acquisition can suppress multi-electrical level inverter DC bus mid-point voltage low-frequency oscillation, realize the pwm control signal of neutral point voltage balance;

Step 4, drive circuit are distributed to every each switching tube of phase brachium pontis of multi-electrical level inverter by gained pwm control signal in step 3, control the operating state of multi-electrical level inverter, suppress low-frequency oscillation simultaneously, regulate neutral point voltage balance.

Compared with prior art, its remarkable advantage is in the present invention: (1) has adopted a kind of DPWM control method, is applied to low modulation than the multi-electrical level inverter in situation, effectively reduces switching loss; (2) this DPWM control method has realized low modulation than the neutral point voltage balance of the multi-electrical level inverter in situation, and eliminates mid-point voltage low-frequency oscillation, has overcome the large shortcoming of outputting inductance current ripples; (3) this control method, without increasing extra hardware circuit, has reduced output voltage, current harmonics, improved output waveform quality, and control method is simple, real-time is good, is convenient to Digital Realization.

Accompanying drawing explanation

Fig. 1 is the structure chart that the present invention has the neutral point voltage balance control system of low-frequency oscillation inhibit feature.

Fig. 2 is three-level inverter topology structure chart, and wherein (a) is T-shaped, is (b) NPC type.

Fig. 3 is the changing trend diagram of mid point electric current and voltage variety in inversion situation, and wherein (a) is traditional SPWM control method, is (b) DPWM control method of the present invention.

Fig. 4 is the corresponding relation figure of three-phase modulations ripple and space vector distribution map, and wherein (a) is three-phase modulations ripple, is (b) space vector distribution map.

Fig. 5 is that in take in Fig. 4 (b), little triangle 1 region is example, mid point electric current and the output level state diagram of tradition SPWM control method and DPWM control method of the present invention, wherein (a) is that traditional SPWM control method, (b) are DPWM control method of the present invention.

Fig. 6 is the voltage oscillogram of dc-link capacitance up and down under traditional SPWM method control, DPWM method of the present invention control in embodiment, and wherein (a) is traditional SPWM control method, (b) is DPWM control method of the present invention.

Embodiment

Below in conjunction with drawings and the specific embodiments, the present invention is described in further detail.

The present invention has neutral point voltage balance control system and the method for low-frequency oscillation inhibit feature, modulation separately respectively in odd number, even number switch periods, and half period and the modulation separately respectively of later half cycle before in each switch periods, positive small vector of front half period (negative small vector) was equated with the action time of negative small vector of later half cycle (positive small vector), therefore in a switch periods, mid point current average is zero, has realized neutral point voltage balance and has not had low-frequency oscillation.

In conjunction with Fig. 1, the present invention has the neutral point voltage balance control system of low-frequency oscillation inhibit feature, comprise multi-electrical level inverter and digital processing control module, wherein digital processing control module comprises sampling unit, Closed Loop Control Unit and the DPWM pwm unit connecting in turn; Described sampling unit gathers respectively the DC bus-bar voltage signal of multi-electrical level inverter, the three-phase current signal of the three-phase voltage signal of multi-electrical level inverter output, multi-electrical level inverter output, and the output of DPWM pwm unit accesses every each switching tube of phase brachium pontis of multi-electrical level inverter by drive circuit; Described digital processing control module is STM32F407 chip.

In each switch periods, the sampling unit of digital processing control module gathers respectively the DC bus-bar voltage signal of multi-electrical level inverter, the three-phase current signal of the three-phase voltage signal of multi-electrical level inverter output, multi-electrical level inverter output; The signal collecting is processed and is obtained three-phase modulations ripple V through Closed Loop Control Unit _a, V _b, V _c; Three-phase modulations ripple is processed and to be obtained pulse-width modulation (PWM) control signal through DPWM pwm unit, this pwm control signal can suppress multi-electrical level inverter DC bus mid-point voltage low-frequency oscillation, realize neutral point voltage balance; Described pwm control signal is controlled the operating state of the every phase switching tube of multi-electrical level inverter through drive circuit.

A neutral point voltage balance control method with low-frequency oscillation inhibit feature, the method, comprises the following steps than the multi-electrical level inverter in situation for low modulation:

Step 1, in each switch periods, the sampling unit of digital processing control module gathers respectively the DC bus-bar voltage signal of multi-electrical level inverter, the three-phase current signal of the three-phase voltage signal of multi-electrical level inverter output, multi-electrical level inverter output;

Step 2, Closed Loop Control Unit carry out proportion integration differentiation adjusting according to the signal collecting in step 1 and base reference signal, output three-phase modulations ripple V _a, V _b, V _c;

Step 3, DPWM pwm unit are to the three-phase modulations ripple V obtaining in step 2 _a, V _b, V _cregulate, acquisition can suppress multi-electrical level inverter DC bus mid-point voltage low-frequency oscillation, realize the PWM control signal of neutral point voltage balance, concrete steps are:

(1) in each power frequency period, switch periods is counted:

In odd number switch periods, this switch periods is divided into the front half period and the later half cycle modulates respectively: in the front half period, three-phase modulations ripple V _a, V _b, V _cv all superposes _offset=-V _max, V wherein _max=max{V _a, V _b, V _cand V _max>0, for a phase of inverter output instantaneous voltage maximum, makes its output bridge arm voltage be clamped at reference to zero potential; Second half is interim, three-phase modulations ripple V _a, V _b, V _cv all superposes _offset=-V _min, V _min=min{V _a, V _b, V _cand V _min<0, for a phase of inverter output instantaneous voltage minimum, makes its output bridge arm voltage be clamped at reference to zero potential;

In even number switch periods, in the front half period, three-phase modulations ripple V _a, V _b, V _cv all superposes _offset=-V _min, for a phase of inverter output instantaneous voltage minimum, make its output bridge arm voltage be clamped at reference to zero potential; Second half is interim, three-phase modulations ripple V _a, V _b, V _cv all superposes _offset=-V _max, for a phase of inverter output instantaneous voltage maximum, make its output bridge arm voltage be clamped at reference to zero potential;

(2) the new three-phase modulations ripple of gained and triangular wave are handed over and are cut, acquisition can suppress multi-electrical level inverter DC bus mid-point voltage low-frequency oscillation, realize the pwm control signal of neutral point voltage balance.

Step 4, drive circuit are distributed to every each switching tube of phase brachium pontis of multi-electrical level inverter by gained pwm control signal in step 3, control the operating state of multi-electrical level inverter, suppress low-frequency oscillation simultaneously, regulate neutral point voltage balance.

Preferably, described low modulation is that modulation ratio m meets following formula than situation:

$0<m\&le;\frac{\sqrt{3}}{3}$

Three-level inverter take below as example, elaborate the invention process process.

(1) the uneven principle analysis of mid-point voltage under traditional control method:

If represent three kinds of states of inverter leg output level with " P ", " 0 ", " N ", corresponding output bridge arm voltage and on off state are in Table 1.

Table 1 three-level inverter brachium pontis output voltage and on off state reference table;

S wherein _xn(X=a, b, c; N=1,2,3,4) switching tube of each brachium pontis of inverter in corresponding diagram 2 respectively, V _dcrepresent DC bus-bar voltage.When neutral point voltage balance, the upper and lower equal component voltage of dc-link capacitance, define DC bus mid point O in this situation and be with reference to zero-potential point.Make Δ V _nPfor the voltage of DC bus mid point O is with respect to reference to zero-potential point variable quantity.Therefore,, when neutral point voltage balance, the voltage of mid point O is with respect to reference to zero-potential point variation delta V _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, the voltage of mid point O is with respect to reference to zero-potential point variation delta V _nPexpression formula be:

$\mathrm{\&Delta;}{V}_{\mathrm{NP}}=-\frac{(V_{c1}-V_{c2})}{2}---\left(1\right)$

Fig. 2 is T-shaped and NPC type three-level inverter topology, in a switch periods, makes mid point current instantaneous value i _o(t) mean value is i _o.For easy analysis, approximate think in a switch periods, i _o(t)=i _o.As shown in Figure 2, a switch periods T _sin, Δ V _nPwith i _opass be:

$\mathrm{\&Delta;}{V}_{\mathrm{NP}}={\&Integral;}_0^{T_s}-\frac{1}{2C}{i}_o\left(t\right)\mathrm{dt}=-\frac{1}{2C}{i}_o{T}_s---\left(2\right)$

Wherein, C is dc-link capacitance C ₁, C ₂capacitance.

The mid point current average i of a switch periods _oexpression formula be:

i _o＝-(D _a·i _a+D _b·i _b+D _c·i _c) (3)

I wherein _a, i _b, i _cfor inverter three-phase output load current, can be expressed as:

i _a＝I _msin(ωt-Φ)

i _b＝I _msin(ωt-2π/3-Φ) (4)

i _c＝I _msin(ωt+2π/3-Φ)

D wherein _a, D _b, D _cfor the duty ratio of each switch periods of A, B, C three-phase, traditional SPWM control method of take is example, can be expressed as:

D _a＝m|sin(ωt)|

D _b＝m|sin(ωt-2π/3)| (5)

D _c＝m|sin(ωt+2π/3)|

In formula (5), m is modulation ratio; Φ is power-factor angle; I _mfor threephase load current amplitude; ω is power frequency angular frequency.

By formula (3), (4), (5), can obtain i _oexpression formula to be write as matrix form as follows:

$i_o=-\left[\begin{array}{ccc}m\left|\sin \left(\mathrm{\&omega;t}\right)\right|& m\left|\sin (\mathrm{\&omega;t}-2\mathrm{\&pi;}/3)\right|& m\left|\sin (\mathrm{\&pi;t}+2\mathrm{\&pi;}/3)\right|\end{array}\right]\&CenterDot;\left[\begin{array}{c}{I}_m\sin (\mathrm{\&omega;t}-\mathrm{\&Phi;})\\ I_m\sin (\mathrm{\&omega;t}-2\mathrm{\&pi;}/3-\mathrm{\&Phi;})\\ I_m\sin (\mathrm{\&omega;t}+2\mathrm{\&pi;}/3-\mathrm{\&Phi;})\end{array}\right]---\left(6\right)$

By formula (2), (6), can obtain Δ V _nPexpression formula:

${\mathrm{\&Delta;V}}_{\mathrm{NP}}=\frac{1}{2C}{T}_S\&CenterDot;\left[\begin{array}{ccc}m\left|\sin \left(\mathrm{\&omega;t}\right)\right|& m\left|\sin (\mathrm{\&omega;t}-2\mathrm{\&pi;}/3)\right|& m\left|\sin (\mathrm{\&omega;t}+2\mathrm{\&pi;}/3)\right|\end{array}\right]\&CenterDot;\left[\begin{array}{c}{I}_m\sin (\mathrm{\&omega;t}-\mathrm{\&Phi;})\\ I_m\sin (\mathrm{\&omega;t}-2\mathrm{\&pi;}/3-\mathrm{\&Phi;})\\ I_m\sin (\mathrm{\&omega;t}+2\mathrm{\&pi;}/3-\mathrm{\&Phi;})\end{array}\right]---\left(7\right)$

Set modulation ratio m=0.5; Load is resistive load Φ=0; Threephase load current amplitude and switch periods are set as unit quantity, power frequency angular frequency=100 π.By formula (6), (7), can make respectively i ₀with time t, Δ V _nPwith the waveform of time t, as shown in Fig. 3 (a).

From Fig. 3 (a), can find out, under traditional control method, i _owith Δ V _nPwork frequency fluctuation with three times, shows as upper and lower dc-link capacitance voltage and fluctuates with frequency tripling, be i.e. i _o≠ 0, Δ V _nP≠ 0, mid-point voltage departs from reference to zero-potential point, has mid-point voltage imbalance problem.

(2) under control method of the present invention, mid-point voltage control principle is analyzed

The neutral point voltage balance control method with low-frequency oscillation inhibit feature proposed by the invention, the method is a kind of DPWM control method.Under DPWM of the present invention controls, the duty ratio D of each switch periods of A, B, C three-phase _a, D _b, D _cbe expressed as:

$\begin{array}{c}{D}_a=\frac{1}{2}m\left[\right|\sin \left(\mathrm{\&omega;t}\right)-\max \left(\mathrm{\&omega;t}\right)|+|\sin \left(\mathrm{\&omega;t}\right)-\min \left(\mathrm{\&omega;t}\right)\left|\right]\\ D_b=\frac{1}{2}m\left[\right|\sin (\mathrm{\&omega;t}-2\mathrm{\&pi;}/3)-\max \left(\mathrm{\&omega;t}\right)|+|\sin (\mathrm{\&omega;t}-2\mathrm{\&pi;}/3)-\min \left(\mathrm{\&omega;t}\right)\left|\right]\\ D_c=\frac{1}{2}m\left[\right|\sin (\mathrm{\&omega;t}+2\mathrm{\&pi;}/3)-\max \left(\mathrm{\&omega;t}\right)|+|\sin (\mathrm{\&omega;t}+2\mathrm{\&pi;}/3)-\min \left(\mathrm{\&omega;t}\right)\left|\right]\end{array}---\left(8\right)$

In formula $\begin{array}{c}\max \left(\mathrm{\&omega;t}\right)=\max (\sin \left(\mathrm{\&omega;t}\right),\sin (\mathrm{\&omega;t}-2\mathrm{\&pi;}/3),\sin (\mathrm{\&omega;t}+2\mathrm{\&pi;}/3))\\ \min \left(\mathrm{\&omega;t}\right)=\min (\sin \left(\mathrm{\&omega;t}\right),\sin (\mathrm{\&omega;t}-2\mathrm{\&pi;}/3),\sin (\mathrm{\&omega;t}+2\mathrm{\&pi;}/3))\end{array}$

For guaranteeing that three-phase modulations wave amplitude after stack is no more than the i.e. ovennodulation not of triangular wave amplitude, need satisfy condition:

m|max(ωt)-min(ωt)|≤1

So modulation ratio need meet be applied to low modulation than the multi-electrical level inverter in situation.

By formula (2), (3), (4), (8), can obtain mid point current average i in the next switch periods of control method of the present invention _owith mid-point voltage with respect to the voltage variety △ V with reference to zero-potential point _nPexpression formula.For with traditional SPWM control method comparative analysis i _owith △ V _nPthe Changing Pattern of t in time, under the conditions such as identical load, modulation ratio, switching frequency and power frequency period, i ₀with time t, △ V _nPwith the waveform of time t as shown in Fig. 3 (b).Can find out the mean value i of mid point electric current in any one switch periods _o=0, relatively with reference to zero potential, the variable quantity △ V of mid-point voltage _nP=0.Theory analysis proof the inventive method has realized neutral point voltage balance control, and does not have low-frequency oscillation.

The neutral point voltage balance control method with low-frequency oscillation inhibit feature that the present invention proposes, is applied to low modulation than the multi-electrical level inverter in situation.Be illustrated in figure 4 the corresponding relation of three-phase modulations ripple and space vector distribution map, low modulation than time, if three-phase modulations ripple is positioned at the I region shown in Fig. 4 (a), be reference voltage vector V _refdrop on interior little triangle 1 region shown in Fig. 4 (b), in traditional SPWM control method and the present invention, the control method process of DPWM as shown in Figure 5.In traditional SPWM control method, in each switch periods, can not guarantee that equate to the action time of paired positive small vector, negative small vector, as shown in Fig. 5 (a); DPWM control method modulation separately respectively in odd number, even number switch periods in the present invention, as shown in Fig. 5 (b), modulator approach in each switch periods is: a switch periods is divided into front half period and later half cycle, and in front half period and the modulation separately respectively of later half cycle, equated the action time of positive small vector of front half period (negative small vector) with the action time of negative small vector of later half cycle (positive small vector), thereby guarantee that mid point current average is zero in a switch periods, neutral point voltage balance, and there is not low-frequency oscillation.If take switch periods as unit quantity, its positive and negative small vector concrete action time is in Table 2.

The traditional SPWM of table 2, the positive and negative small vector of DPWM method of the present invention action time

In table 2, d _s1for V in Fig. 4 (b) ₁the action time of vector, d _s2for V ₂the action time of vector, d' _s1for at V ₁half of negative small vector ONN action time in vector action time

As shown in Fig. 5 (b), suppose that previous switch periods is odd number switch periods, an adjacent rear switch periods is even number switch periods:

(1), in odd number switch periods, this switch periods is divided into the front half period and the later half cycle modulates respectively: in the front half period, three-phase modulations ripple V _a, V _b, V _cv all superposes _offset=-V _max(V _max>0), V wherein _max=max{V _a, V _b, V _c, for a phase of inverter output instantaneous voltage maximum, make its output bridge arm voltage be clamped at reference to zero potential; Second half is interim, three-phase modulations ripple V _a, V _b, V _cv all superposes _offset=-V _min(V _min<0), V _min=min{V _a, V _b, V _c, for a phase of inverter output instantaneous voltage minimum, make its output bridge arm voltage be clamped at reference to zero potential.

(2) in even number switch periods, in the front half period, three-phase modulations ripple V _a, V _b, V _cv all superposes _offset=-V _min, for a phase of inverter output instantaneous voltage minimum, make its output bridge arm voltage be clamped at reference to zero potential.Second half is interim, three-phase modulations ripple V _a, V _b, V _cv all superposes _offset=-V _max, for a phase of inverter output instantaneous voltage maximum, make its output bridge arm voltage be clamped at reference to zero potential.

Under same switch frequency condition, traditional DPWMx, SVPWM, SPWM control method on-off times in a switch periods is respectively: 4,6,6 times; If DPWM control method of the present invention is not distinguished odevity switch periods and is modulated, although can realize neutral point voltage balance, in a switch periods, amount to action 8 times, on-off times increases on the contrary, and switching loss is increased.The inventive method adopts modulation separately respectively in odevity switch periods, has realized neutral point voltage balance, and has not had low-frequency oscillation; Single switch in the cycle on-off times add up to: 6 times.Therefore, adopt control method of the present invention, not only can realize neutral point voltage balance and suppress low-frequency oscillation, and without increasing on-off times, not increase switching loss.

Embodiment 1

For checking, the present invention proposes a kind of feasibility and validity with the neutral point voltage balance control method of low-frequency oscillation inhibit feature, utilizes the Simulink instrument in MATLAB to build respectively the tri-level inversion circuit under the control of SPWM method, the tri-level inversion circuit under DPWM method of the present invention control.Electric parameter setting in simulation process is as following table 3:

Electric parameter setting in table 3 simulation process

Fig. 6 is the dc-link capacitance C under two kinds of methods are controlled ₁, C ₂voltage V _c1, V _c2simulation waveform.Fig. 6 (a) is the simulation waveform of dc-link capacitance voltage under traditional SPWM method, dc-link capacitance C ₁, C ₂voltage V _c1, V _c2fundamental frequency vibration with three times is consistent with above-mentioned theory analysis; Fig. 6 (b) is the simulation waveform of dc-link capacitance voltage under DPWM method of the present invention, dc-link capacitance C ₁, C ₂voltage V _c1, V _c2be stabilized in mid-point voltage 300V left and right, analyze consistent with above-mentioned theory.It is remarkable that a kind of neutral point voltage balance control method with low-frequency oscillation inhibit feature that the present invention proposes is controlled effect.

In sum, the present invention has neutral point voltage balance control system and the method thereof of low-frequency oscillation inhibit feature, be applied to low modulation than the multi-electrical level inverter in situation, this control method was modulated respectively in front half period and the later half cycle of each switch periods, no matter be odd number switch periods or even number switch periods, in the front half period, equated the action time of positive small vector (negative small vector) with the action time of the interim negative small vector of second half (positive small vector), so it is zero that this DPWM method guarantees each switch periods mid point current average, realize neutral point voltage balance, and suppressed low-frequency oscillation.Different from traditional SVPWM, DPWMx, SPWM control method, this DPWM control method has realized the control of balance mid-point voltage; And overcome that low-frequency oscillation under traditional DPWMx method, inductive current ripple are large, mid-point voltage is controlled the shortcomings such as difficulty.This control method, without increasing extra hardware circuit, has output waveform harmonic content low, and real-time is good, and control method is simple, is convenient to the advantages such as Digital Realization.

Claims (5)

1. a neutral point voltage balance control system with low-frequency oscillation inhibit feature, it is characterized in that, comprise multi-electrical level inverter and digital processing control module, wherein digital processing control module comprises sampling unit, Closed Loop Control Unit and the DPWM pwm unit connecting in turn; Described sampling unit gathers respectively the DC bus-bar voltage signal of multi-electrical level inverter, the three-phase current signal of the three-phase voltage signal of multi-electrical level inverter output, multi-electrical level inverter output, and the output of DPWM pwm unit accesses every each switching tube of phase brachium pontis of multi-electrical level inverter by drive circuit;

In each switch periods, the sampling unit of digital processing control module gathers respectively the DC bus-bar voltage signal of multi-electrical level inverter, the three-phase current signal of the three-phase voltage signal of multi-electrical level inverter output, multi-electrical level inverter output; The signal collecting is processed and is obtained three-phase modulations ripple V through Closed Loop Control Unit _a, V _b, V _c; Three-phase modulations ripple is processed and to be obtained pulse-width modulation control signal through DPWM pwm unit, this pulse-width modulation control signal can suppress multi-electrical level inverter DC bus mid-point voltage low-frequency oscillation, realize neutral point voltage balance; Described pulse-width modulation control signal is controlled the operating state of the every phase switching tube of multi-electrical level inverter through drive circuit.

2. the neutral point voltage balance control system with low-frequency oscillation inhibit feature according to claim 1, is characterized in that, described digital processing control module is STM32F407 chip.

3. a neutral point voltage balance control method with low-frequency oscillation inhibit feature, is characterized in that, the method, comprises the following steps than the multi-electrical level inverter in situation for low modulation:

Step 1, in each switch periods, the sampling unit of digital processing control module gathers respectively the DC bus-bar voltage signal of multi-electrical level inverter, the three-phase current signal of the three-phase voltage signal of multi-electrical level inverter output, multi-electrical level inverter output;

Step 2, Closed Loop Control Unit carry out proportion integration differentiation adjusting according to the signal collecting in step 1 and base reference signal, output three-phase modulations ripple V _a, V _b, V _c;

Step 3, DPWM pwm unit are to the three-phase modulations ripple V obtaining in step 2 _a, V _b, V _cregulate, acquisition can suppress multi-electrical level inverter DC bus mid-point voltage low-frequency oscillation, realize the pulse-width modulation control signal of neutral point voltage balance;

Step 4, drive circuit are distributed to every each switching tube of phase brachium pontis of multi-electrical level inverter by gained pulse-width modulation control signal in step 3, control the operating state of multi-electrical level inverter, suppress low-frequency oscillation simultaneously, regulate neutral point voltage balance.

4. the neutral point voltage balance control method with low-frequency oscillation inhibit feature according to claim 3, is characterized in that, described low modulation is that modulation ratio m meets following formula than situation:

$0<m\&le;\frac{\sqrt{3}}{3}.$

5. the neutral point voltage balance control method with low-frequency oscillation inhibit feature according to claim 3, is characterized in that, the pwm unit of DPWM described in step 3 is to the three-phase modulations ripple V obtaining in step 2 _a, V _b, V _cregulate, acquisition can suppress multi-electrical level inverter DC bus mid-point voltage low-frequency oscillation, realize the pulse-width modulation control signal of neutral point voltage balance, concrete steps are:

(1) in each power frequency period, switch periods is counted:

In odd number switch periods, this switch periods is divided into the front half period and the later half cycle modulates respectively: in the front half period, three-phase modulations ripple V _a, V _b, V _cv all superposes _offset=-V _max, V wherein _max=max{V _a, V _b, V _cand V _max>0, for a phase of inverter output instantaneous voltage maximum, makes its output bridge arm voltage be clamped at reference to zero potential; Second half is interim, three-phase modulations ripple V _a, V _b, V _cv all superposes _offset=-V _min, V _min=min{V _a, V _b, V _cand V _min<0, for a phase of inverter output instantaneous voltage minimum, makes its output bridge arm voltage be clamped at reference to zero potential;

In even number switch periods, in the front half period, three-phase modulations ripple V _a, V _b, V _cv all superposes _offset=-V _min, for a phase of inverter output instantaneous voltage minimum, make its output bridge arm voltage be clamped at reference to zero potential; Second half is interim, three-phase modulations ripple V _a, V _b, V _cv all superposes _offset=-V _max, for a phase of inverter output instantaneous voltage maximum, make its output bridge arm voltage be clamped at reference to zero potential;

(2) the new three-phase modulations ripple of gained and triangular wave are handed over and are cut, acquisition can suppress multi-electrical level inverter DC bus mid-point voltage low-frequency oscillation, realize the pulse-width modulation control signal of neutral point voltage balance.