CN105006980A - Three-level NPC current transformer PWM control strategy based on carrier wave overlapping - Google Patents

Abstract

The invention discloses a three-level NPC current transformer PWM control strategy based on carrier wave overlapping. A voltage-type three-level midpoint clamping current transformer is employed, and the control strategy is enforced concretely according to the following steps: first, a carrier wave overlapping mode is selected; second, a carrier wave overlapping scope is determined; third, a three-phase modulating wave is determined; fourth, a switch signal of each switch tube of each phase is obtained through comparison of the modulating wave to the carrier wave, on and off of each switch tube is controlled through the switch signal, and therefore the control process is finished. The strategy reduces direct current capacitor voltage fluctuation of a three-level NPC current transformer system caused by carrier overlapping through closed loop control. The method is advantaged by simple implementation and good application effects.

Description

Based on the three level NPC current transformer PWM control strategy that carrier wave is overlapping

Technical field

The invention belongs to electric and electronic technical field, relate to based on the overlapping three level NPC current transformer PWM control strategy of carrier wave.

Background technology

Three level neutral-point-clamped (Neutral Point Clamped and NPC) current transformer (main circuit topology figure as shown in Figure 1) has that power output is large, output waveform THD is little, device voltage stress and many-sided advantage such as system EMI is low, is thus widely used in high-power occasion.In three-level current transformer, modulation strategy is a key technology of three level NPC current transformer, is born so far from three level NPC current transformer, and scholars just never stop the research of its modulation strategy.Through the research of two more than ten years, at present existing multiple three level modulation strategy comes out, but be wherein most widely used remain carrier modulation strategy (Carrier-based PWM strategy).

Be three level NPC current transformer convention carrier modulation strategy schematic diagram as shown in Figures 2 and 3, Fig. 2 and Fig. 3 U _rxfor x (x=a, b, c) phase modulating wave, U _c1and U _c2for x phase two carrier waves.Owing to there is not public part between two carrier wave regions, this carrier modulation strategy is called as the stacked modulation strategy of three level carrier wave, according to the phase relation in the stacked strategy of carrier wave between two carrier waves, stacked for carrier wave strategy can be divided into following two kinds again:

The a stacked strategy of carrier wave that the phase place of the stacked modulation strategy of () carrier wave homophase (Phase disposition pulse width modulation, PD-PWM)-upper and lower two carrier waves is identical, specifically as shown in Figure 2.

The b stacked strategy of carrier wave that the phase place of the anti-phase stacked modulation strategy of () carrier wave (Phase opposition disposition pulse widthmodulation, POD-PWM)-upper and lower two carrier waves is contrary, specifically as shown in Figure 3.

In recent years, the phase relation that scholars find not just between two carrier waves is the control freedom degree of carrier modulation strategy, offset relationship between carrier wave also can control the performance of carrier modulation strategy as one degree of freedom, consider that the modulation strategy of carrier phase and skew is called the overlapping modulation of carrier wave (Carrieroverlapping pulse width modulation simultaneously, COPWM) strategy, specifically as shown in Figure 4 and Figure 5, wherein U _rx, U _c1and U _c2for x phase modulating wave and two triangular carriers, V is the perunit value (V>1) of carrier widths in the overlapping method of carrier wave, according to the phase relation in the overlapping strategy of carrier wave between two carrier waves, also overlapping for carrier wave strategy can be divided into carrier wave with overlapping mutually modulation strategy (COPWM-A) and the anti-phase stacked modulation strategy (COPWM-B) of carrier wave.

Further research finds, the overlapping strategy of carrier wave can improve the direct voltage utilance of three level NPC converter system to a certain extent, and the overlapping strategy of carrier wave makes DC capacitor voltage fluctuation slightly reduce.

Summary of the invention

The object of the present invention is to provide based on the overlapping three level NPC current transformer PWM control strategy of carrier wave, this strategy can reduce the three level NPC current transformer DC capacitor voltage fluctuation caused by the overlapping method of carrier wave.

The technical solution adopted in the present invention is, based on the three level NPC current transformer PWM control strategy that carrier wave is overlapping, adopts voltage-type three level neutral-point-clamped current transformer, specifically implements according to following steps;

Step 1, selects carrier wave overlap mode;

Step 2, determines carrier wave overlapping ranges;

Step 3, determines three-phase modulations ripple;

Step 4, modulating wave and carrier wave ratio comparatively, obtain every mutually each switching tube switching signal, control turning on and off of each switching tube, thus complete control procedure with switching signal.

Feature of the present invention is also,

Carrier wave overlap mode is selected to be specially in step 1:

A) when system AC is three-phase three-line system, adopt carrier wave with overlapping mutually method;

B) when system AC be single phase system or three-phase four-wire system time, adopt carrier wave anti-phase overlapping method.

Determine in step 2 that carrier wave overlapping ranges is specially;

$\begin{array}{c}{U}_{az}\left(t\right)=A\sin \left({\mathrm{\ω}}_{s}t\right)+\underset{n=3,5,\mathrm{7...}}{\overset{\mathrm{\∞}}{\Σ}}B\sin \left({\mathrm{n\ω}}_{s}t\right)+\underset{m=1,3,\mathrm{5...}}{\overset{\mathrm{\∞}}{\Σ}}C\cos \left({\mathrm{m\ω}}_{c}t\right)\\ \underset{m=1}{\overset{\mathrm{\∞}}{\Σ}}\underset{n=\±1}{\overset{\±\mathrm{\∞}}{\Σ}}\{D\cos ({\mathrm{m\ω}}_{c}t+{\mathrm{n\ω}}_{s}t)+E\sin ({\mathrm{m\ω}}_{c}t+{\mathrm{n\ω}}_{s}t)\}\end{array}---\left(1\right)$

In formula (1), ω _sfor modulating wave angular frequency, ω _cfor carrier angular frequencies, A is fundamental voltage amplitude, and B is modulating wave harmonic amplitude, and C is carrier wave harmonic amplitude, D and E is sideband harmonic amplitude,

Wherein, to output performance have the greatest impact for fundamental voltage amplitude A and modulating wave harmonic amplitude B, its expression formula is respectively:

$A=\frac{{\mathrm{MU}}_{dc}}{\mathrm{\π}V}\[\frac{\mathrm{\π}}{2}+\arcsin \left(\frac{V-1}{M}\right)+\left(\frac{V-1}{M}\right)\sqrt{1-{\left(\frac{V-1}{M}\right)}^2}\]---\left(2\right)$

$\begin{array}{c}B=\frac{2{\mathrm{MU}}_{dc}\sqrt{1-{\left(\frac{V-1}{M}\right)}^2}}{\mathrm{\π}V(n^2-1)}\sin \[n\left(\arcsin \right(\frac{V-1}{M}))\]-\\ \frac{2{\mathrm{nU}}_{dc}(V-1)}{\mathrm{\π}(n^2-1)V}\cos \[n\left(\arcsin \right(\frac{V-1}{M}))\]+\frac{2U_{dc}(V-1)}{n\mathrm{\π}V}\cos \[n\left(\arcsin \right(\frac{V-1}{M}))\]\end{array}---\left(3\right)$

In formula (2) and (3), U _dcfor DC capacitor total voltage, M is modulating wave fundamental voltage amplitude perunit value,

From formula (2) and (3), V≤1+M;

Setting V≤1.5, accordingly, obtain the expression formula determining V:

$\left\{\begin{array}{c}1<V\&le;M+1\\ 1<V\&le;1.5\end{array}\right.---\left(4\right)$

In formula (4), V is perunit value.

The determination of step 3 modulating wave is specially:

A) setting three-phase modulations ripple U time initially _rxfor standard sine wave;

B) three-phase modulations ripple U is calculated _rxfor the mid point electric current caused during standard sine wave;

Three level NPC current transformer DC side mid point electric current is made up of three parts, as shown in formula (5), and wherein i _oxrepresent the mid point electric current caused by x phase current, x=a, b or c, i _oxwith phase current i _xas shown in formula (6), d _x2and d _x1represent the real-time duty ratio of x phase 2 pipe and 1 pipe respectively,

i _o＝i _oa+i _ob+i _oc(5)

i _ox＝(d _x2-d _x1)×i _x(x＝a,b,c) (6)

X phase 1 is managed and the real-time duty ratio of 2 pipes is:

$\left\{\begin{array}{c}{d}_{x1}=\left\{\begin{array}{c}\frac{U_{rx}+V-1}{V}(\mathrm{\&omega;}t\&Element;\&lsqb;0,x_1\&rsqb;\&cup;\&lsqb;x_2,2\mathrm{\&pi;}\&rsqb;)\\ 0(\mathrm{\&omega;}t\&Element;\&lsqb;x_1,x_2\&rsqb;)\end{array}\right.\\ d_{x2}=\left\{\begin{array}{c}1(\mathrm{\&omega;}t\&Element;\&lsqb;x_3,x_4\&rsqb;)\\ \frac{U_{rx}+1}{V}(\mathrm{\&omega;}t\&Element;\&lsqb;0,x_3\&rsqb;\&cup;\&lsqb;x_4,2\mathrm{\&pi;}\&rsqb;)\end{array}\right.\end{array}\right.---\left(7\right)$

In formula (7), x ₁, x ₂and x ₃, x ₄represent respectively x phase modulating wave with upper carrier wave and lower carrier wave the crossing moment in a power frequency period, x=a, a phase modulating wave with upper carrier wave and lower carrier wave its expression formula of crossing moment in a power frequency period as shown in formula (8):

$\left\{\begin{array}{c}{a}_1=\mathrm{\&pi;}+arcsin\&lsqb;(V-1)/M\&rsqb;\\ a_2=2\mathrm{\&pi;}-\arcsin \&lsqb;(V-1)/M\&rsqb;\\ a_3=\arcsin \&lsqb;(V-1)/M\&rsqb;\\ a_4=\mathrm{\&pi;}-\arcsin \&lsqb;(V-1)/M\&rsqb;\end{array}\right.---\left(8\right)$

X=b, b phase modulating wave with upper carrier wave and lower carrier wave its expression formula of crossing moment in a power frequency period as shown in formula (9):

$\left\{\begin{array}{c}{b}_1=2\mathrm{\&pi;}/3-arcsin\&lsqb;(V-1)/M\&rsqb;\\ b_2=5\mathrm{\&pi;}/3+arcsin\&lsqb;(V-1)/M\&rsqb;\\ b_3=2\mathrm{\&pi;}/3+arcsin\&lsqb;(V-1)/M\&rsqb;\\ b_4=5\mathrm{\&pi;}/3-arcsin\&lsqb;(V-1)/M\&rsqb;\end{array}\right.---\left(9\right)$

X=c, c phase modulating wave with upper carrier wave and lower carrier wave its expression formula of crossing moment in a power frequency period as shown in formula (10):

$\left\{\begin{array}{c}{c}_1=\mathrm{\&pi;}/3+arcsin\&lsqb;(V-1)/M\&rsqb;\\ c_2=4\mathrm{\&pi;}/3-\arcsin \&lsqb;(V-1)/M\&rsqb;\\ c_3=\mathrm{\&pi;}/3-arcsin\&lsqb;(V-1)/M\&rsqb;\\ c_4=4\mathrm{\&pi;}/3+\arcsin \&lsqb;(V-1)/M\&rsqb;\end{array}\right.---\left(10\right)$

Formula (7)-(10) are brought into the mid point current i that formula (6) obtains being caused by x phase load electric current _oxexpression formula:

$i_{ox}=\{\begin{array}{c}\frac{1-abs\left(U_{rx}\right)}{V}\&times;i_x(V-1\&le;|U_{rx}|\&le;1)\\ \frac{2-V}{V}\&times;i_x\left(\right|U_{rx}|\&le;V-1)\end{array}---\left(11\right)$

(11) are substituted into (5) and according to the fluctuation relation of mid point electric current and DC capacitor voltage:

$\mathrm{\&Delta;}u=u_{c1}-u_{c2}=\{\begin{array}{c}\&Sigma;\frac{1-abs\left(U_{rx}\right)}{CV}\&times;i_x\mathrm{\&Delta;}T(V-1\&le;|U_{rx}|\&le;1)\\ \&Sigma;\frac{2-V}{V}\&times;i_x\left(\right|U_{rx}|\&le;V-1)\end{array}---\left(12\right)$

In formula (12), the fluctuation of mid point electric current and DC capacitor voltage is closed and is △ u is DC capacitor voltage fluctuation situation, and △ T is switch periods, and C is DC capacitor voltage;

C) build Control loop, be defined as suppressing DC capacitor voltage to fluctuate the zero-sequence component U that need superpose in modulating wave _o,

Compared with 0 by the △ u calculated in b), its result is by obtaining the zero-sequence component U that need superpose in modulating wave after PI controller _o,

D) by U _owith a) in given sinusoidal modulation wave U _rxbe added, x=a, b, c, obtain three-phase modulations ripple U _ao, U _bo, U _co.

Step 4 is specially:

The three-phase modulations ripple U that step 3 is obtained _ao, U _bo, U _corespectively with upper and lower carrier wave U _c2and U _c1relatively, if certain phase modulating wave is greater than carrier wave U _c2, then first, second switching tube top-down of this phase is open-minded, and the 3rd, the 4th switching tube turns off, if certain phase modulating wave is less than carrier wave U _c2be greater than modulated ripple U _c1, then second, third switching tube top-down of this phase is open-minded, and the first, the 4th switching tube turns off, if certain phase modulating wave is less than modulated ripple U _c1, then top-down 3rd, the 4th switching tube of this phase is open-minded, and first, second switching tube turns off.

The invention has the beneficial effects as follows: the present invention is based on the three level NPC current transformer PWM control strategy that carrier wave is overlapping, this strategy reduces to be fluctuated by the overlapping three level NPC converter system DC capacitor voltage caused of carrier wave by closed-loop control, the method realizes simple, and effect is good.

Accompanying drawing explanation

Fig. 1 is three level NPC main circuit of converter topological diagram;

Fig. 2 is the stacked modulation strategy schematic diagram of three level carrier wave homophase;

Fig. 3 is the anti-phase stacked modulation strategy schematic diagram of three level carrier wave;

Fig. 4 is that three level carrier wave is with overlapping mutually modulation strategy schematic diagram;

Fig. 5 is the anti-phase overlapping modulation strategy schematic diagram of three level carrier wave;

Fig. 6 is that three level carrier wave is with overlapping mutually method a phase 4 switching tube oscillograms;

Fig. 7 the present invention is based on the overlapping method zero-sequence component U of carrier wave in the overlapping three level NPC current transformer PWM control strategy of carrier wave _odetermination Method And Principle figure;

Fig. 8 is the analogous diagram that the present invention is based on the overlapping three level NPC current transformer PWM control strategy output voltage pulse of carrier wave;

Fig. 9 is the analogous diagram that the present invention is based on the overlapping three level NPC current transformer PWM control strategy DC capacitor voltage fluctuation of carrier wave.

Embodiment

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

The invention provides based on the overlapping three level NPC current transformer PWM control strategy of carrier wave, adopt voltage-type three level neutral-point-clamped current transformer as shown in Figure 1, (if three-level inverter structure, then three-phase alternating current part is load to comprise three-phase alternating current part; If the device such as three-level rectifier, static reacance generator, then three-phase alternating current part is that alternating current source adds three-phase AC flat wave reactor), three level DC side External connecting part is (if three-level inverter structure, then DC side External connecting part is direct voltage source, this DC source can be practical power, also can be the DC source obtained by AC power rectification; If three-level rectifier structure, then DC side External connecting part is load; If three level static reacance generator, then DC side is without External connecting part), voltage-type three level neutral-point-clamped main circuit of converter part, voltage sensor, current sensor, AD conversion chip and digital processing unit, wherein, voltage sensor senses three-phase alternating current portion voltage and each capacitance voltage of DC side, current sensor detect each phase current of AC, voltage sensor is connected with digital processing unit by AD conversion chip with current sensor, and digital processing unit and corresponding drive circuit control the switch of each power device in three-level current transformer;

Specifically implement according to following steps;

Step 1, selects carrier wave overlap mode;

Research shows, in two kinds of overlapping strategies of carrier wave, carrier wave is little with the output line voltage THD overlapping mutually strategy, and the output phase voltage THD of the anti-phase overlapping strategy of carrier wave is little.According to this theoretical foundation, carrier wave overlap mode is selected to be specially:

A) when system AC is three-phase three-line system, adopt carrier wave with overlapping mutually method;

B) when system AC be single phase system or three-phase four-wire system time, adopt carrier wave anti-phase overlapping method;

Step 2, determines carrier wave overlapping ranges;

Determine that carrier wave overlapping ranges is specially;

$\begin{array}{c}{U}_{az}\left(t\right)=A\sin \left({\mathrm{\&omega;}}_{s}t\right)+\underset{n=3,5,\mathrm{7...}}{\overset{\mathrm{\&infin;}}{\&Sigma;}}B\sin \left({\mathrm{n\&omega;}}_{s}t\right)+\underset{m=1,3,\mathrm{5...}}{\overset{\mathrm{\&infin;}}{\&Sigma;}}C\cos \left({\mathrm{m\&omega;}}_{c}t\right)\\ +\underset{m=1}{\overset{\mathrm{\&infin;}}{\&Sigma;}}\underset{n=\&PlusMinus;1}{\overset{\&PlusMinus;\mathrm{\&infin;}}{\&Sigma;}}\{D\cos ({\mathrm{m\&omega;}}_{c}t+{\mathrm{n\&omega;}}_{s}t)+E\sin ({\mathrm{m\&omega;}}_{c}t+{\mathrm{n\&omega;}}_{s}t)\}\end{array}---\left(1\right)$

In formula (1), ω _sfor modulating wave angular frequency, ω _cfor carrier angular frequencies, A is fundamental voltage amplitude, and B is modulating wave harmonic amplitude, and C is carrier wave harmonic amplitude, D and E is sideband harmonic amplitude,

Wherein, to output performance have the greatest impact for fundamental voltage amplitude A and modulating wave harmonic amplitude B, its expression formula is respectively:

$A=\frac{{\mathrm{MU}}_{dc}}{\mathrm{\&pi;}V}\&lsqb;\frac{\mathrm{\&pi;}}{2}+arcsin\left(\frac{V-1}{M}\right)+\left(\frac{V-1}{M}\right)\sqrt{1-{\left(\frac{V-1}{M}\right)}^2}\&rsqb;---\left(2\right)$

$\begin{array}{c}B=\frac{2{\mathrm{MU}}_{dc}\sqrt{1-{\left(\frac{V-1}{M}\right)}^2}}{\mathrm{\&pi;}V(n^2-1)}\sin \&lsqb;n\left(\arcsin \right(\frac{V-1}{M}))\&rsqb;-\\ \frac{2{\mathrm{nU}}_{dc}(V-1)}{\mathrm{\&pi;}(n^2-1)V}\cos \&lsqb;n\left(\arcsin \right(\frac{V-1}{M}))\&rsqb;+\frac{2U_{dc}(V-1)}{n\mathrm{\&pi;}V}\cos \&lsqb;n\left(\arcsin \right(\frac{V-1}{M}))\&rsqb;\end{array}---\left(3\right)$

In formula (2) and (3), U _dcfor DC capacitor total voltage, M is modulating wave fundamental voltage amplitude perunit value,

From formula (2) and (3), V≤1+M;

In reality, V larger then direct voltage utilance is higher, but in output waveform, low-order harmonic content also raises, so need concrete analysis, in order to reduce the low-order harmonic content in output waveform in the present invention, setting V≤1.5, accordingly, obtain the expression formula determining V:

$\left\{\begin{array}{c}1<V\&le;M+1\\ 1<V\&le;1.5\end{array}\right.---\left(4\right)$

In formula (4), V is perunit value;

Step 3, determines three-phase modulations ripple;

The determination of modulating wave is specially:

A) setting three-phase modulations ripple U time initially _rxfor standard sine wave;

B) three-phase modulations ripple U is calculated _rxfor the mid point electric current caused during standard sine wave;

Research shows: DC capacitor voltage fluctuation is proportional to mid point current fluctuation, therefore causes the fluctuation of mid point electric current to calculate the impact of the method on capacitance voltage by calculating the overlapping method of carrier wave,

Three level NPC current transformer DC side mid point electric current is made up of three parts, as shown in formula (5), and wherein i _oxrepresent the mid point electric current caused by x phase current, x=a, b or c, i _oxwith phase current i _xas shown in formula (6), d _x2and d _x1represent the real-time duty ratio of x phase 2 pipe and 1 pipe respectively,

i _o＝i _oa+i _ob+i _oc(5)

i _ox＝(d _x2-d _x1)×i _x(x＝a,b,c) (6)

X phase 1 is managed and the real-time duty ratio of 2 pipes is: as Fig. 6 gives the oscillogram of a phase four switching tubes:

$\left\{\begin{array}{c}{d}_{x1}=\left\{\begin{array}{c}\frac{U_{rx}+V-1}{V}(\mathrm{\&omega;}t\&Element;\&lsqb;0,x_1\&rsqb;\&cup;\&lsqb;x_2,2\mathrm{\&pi;}\&rsqb;)\\ 0(\mathrm{\&omega;}t\&Element;\&lsqb;x_1,x_2\&rsqb;)\end{array}\right.\\ d_{x2}=\left\{\begin{array}{c}1(\mathrm{\&omega;}t\&Element;\&lsqb;x_3,x_4\&rsqb;)\\ \frac{U_{rx}+1}{V}(\mathrm{\&omega;}t\&Element;\&lsqb;0,x_3\&rsqb;\&cup;\&lsqb;x_4,2\mathrm{\&pi;}\&rsqb;)\end{array}\right.\end{array}\right.---\left(7\right)$

In formula (7), x ₁, x ₂and x ₃, x ₄represent respectively x phase modulating wave with upper carrier wave and lower carrier wave the crossing moment in a power frequency period, x=a, a phase modulating wave with upper carrier wave and lower carrier wave its expression formula of crossing moment in a power frequency period as shown in formula (8):

$\left\{\begin{array}{c}{a}_1=\mathrm{\&pi;}+arcsin\&lsqb;(V-1)/M\&rsqb;\\ a_2=2\mathrm{\&pi;}-\arcsin \&lsqb;(V-1)/M\&rsqb;\\ a_3=\arcsin \&lsqb;(V-1)/M\&rsqb;\\ a_4=\mathrm{\&pi;}-\arcsin \&lsqb;(V-1)/M\&rsqb;\end{array}\right.---\left(8\right)$

X=b, b phase modulating wave with upper carrier wave and lower carrier wave its expression formula of crossing moment in a power frequency period as shown in formula (9):

$\left\{\begin{array}{c}{b}_1=2\mathrm{\&pi;}/3-arcsin\&lsqb;(V-1)/M\&rsqb;\\ b_2=5\mathrm{\&pi;}/3+arcsin\&lsqb;(V-1)/M\&rsqb;\\ b_3=2\mathrm{\&pi;}/3+arcsin\&lsqb;(V-1)/M\&rsqb;\\ b_4=5\mathrm{\&pi;}/3-arcsin\&lsqb;(V-1)/M\&rsqb;\end{array}\right.---\left(9\right)$

X=c, c phase modulating wave with upper carrier wave and lower carrier wave its expression formula of crossing moment in a power frequency period as shown in formula (10):

$\left\{\begin{array}{c}{c}_1=\mathrm{\&pi;}/3+arcsin\&lsqb;(V-1)/M\&rsqb;\\ c_2=4\mathrm{\&pi;}/3-\arcsin \&lsqb;(V-1)/M\&rsqb;\\ c_3=\mathrm{\&pi;}/3-arcsin\&lsqb;(V-1)/M\&rsqb;\\ c_4=4\mathrm{\&pi;}/3+\arcsin \&lsqb;(V-1)/M\&rsqb;\end{array}\right.---\left(10\right)$

Formula (7)-(10) are brought into the mid point current i that formula (6) obtains being caused by x phase load electric current _oxexpression formula:

$i_{ox}=\{\begin{array}{c}\frac{1-abs\left(U_{rx}\right)}{V}\&times;i_x(V-1\&le;|U_{rx}|\&le;1)\\ \frac{2-V}{V}\&times;i_x\left(\right|U_{rx}|\&le;V-1)\end{array}---\left(11\right)$

(11) are substituted into (5) and according to the fluctuation relation of mid point electric current and DC capacitor voltage:

$\mathrm{\&Delta;}u=u_{c1}-u_{c2}=\{\begin{array}{c}\&Sigma;\frac{1-abs\left(U_{rx}\right)}{CV}\&times;i_x\mathrm{\&Delta;}T(V-1\&le;|U_{rx}|\&le;1)\\ \&Sigma;\frac{2-V}{V}\&times;i_x\left(\right|U_{rx}|\&le;V-1)\end{array}---\left(12\right)$

In formula (12), the fluctuation of mid point electric current and DC capacitor voltage is closed and is △ u is DC capacitor voltage fluctuation situation, and △ T is switch periods, and C is DC capacitor voltage;

C) build Control loop, be defined as suppressing DC capacitor voltage to fluctuate the zero-sequence component U that need superpose in modulating wave _o,

Compared with 0 by the △ u calculated in b), as shown in Figure 7, its result is by obtaining the zero-sequence component U that need superpose in modulating wave after PI controller _o;

D) by U _owith a) in given sinusoidal modulation wave U _rxbe added, x=a, b, c, obtain three-phase modulations ripple U _ao, U _bo, U _co;

Step 4, modulating wave and carrier wave ratio comparatively, obtain every mutually each switching tube switching signal, control turning on and off of each switching tube, thus complete control procedure with switching signal;

Step 4 is specially:

The three-phase modulations ripple U that step 3 is obtained _ao, U _bo, U _corespectively with upper and lower carrier wave U _c2and U _c1relatively, if certain phase modulating wave is greater than carrier wave U _c2, then first, second switching tube top-down of this phase is open-minded, and the 3rd, the 4th switching tube turns off, if certain phase modulating wave is less than carrier wave U _c2be greater than modulated ripple U _c1, then second, third switching tube top-down of this phase is open-minded, and the first, the 4th switching tube turns off, if certain phase modulating wave is less than modulated ripple U _c1, then top-down 3rd, the 4th switching tube of this phase is open-minded, and first, second switching tube turns off.

Emulate system shown in Figure 1 at Matlab/Simulink software, setting basic parameter is as shown in table 1:

Table 1 three level NPC converter simulation parameter

Utilize Matlab/Simulink software, according to simulation parameter in table 1, obtain simulation result figure as shown in Figure 8 and Figure 9; Wherein Fig. 8 is the analogous diagram that the present invention is based on the overlapping three level NPC current transformer PWM control strategy output voltage pulse of carrier wave, and Fig. 9 is the analogous diagram that the present invention is based on the overlapping three level NPC current transformer PWM control strategy DC capacitor voltage fluctuation of carrier wave.From Fig. 8 and Fig. 9, method of the present invention can realize PWM preferably, and meanwhile, owing to introducing closed-loop control, the present invention can reduce to be fluctuated by the overlapping three level NPC converter system DC capacitor voltage caused of carrier wave.

Claims (5)

1., based on the three level NPC current transformer PWM control strategy that carrier wave is overlapping, adopt voltage-type three level neutral-point-clamped current transformer, it is characterized in that, specifically implement according to following steps;

Step 1, selects carrier wave overlap mode;

Step 2, determines carrier wave overlapping ranges;

Step 3, determines three-phase modulations ripple;

Step 4, modulating wave and carrier wave ratio comparatively, obtain every mutually each switching tube switching signal, control turning on and off of each switching tube, thus complete control procedure with switching signal.

2. the three level NPC current transformer PWM control strategy overlapping based on carrier wave according to claim 1, is characterized in that, select carrier wave overlap mode to be specially in described step 1:

A) when system AC is three-phase three-line system, adopt carrier wave with overlapping mutually method;

B) when system AC be single phase system or three-phase four-wire system time, adopt carrier wave anti-phase overlapping method.

3. the three level NPC current transformer PWM control strategy overlapping based on carrier wave according to claim 1, is characterized in that, determine that carrier wave overlapping ranges is specially in described step 2;

$\begin{array}{c}{U}_{az}\left(t\right)=A\sin \left({\mathrm{\&omega;}}_{s}t\right)+\underset{n=3,5,7...}{\overset{\mathrm{\&infin;}}{\mathrm{\&Sigma;}}}B\sin \left({\mathrm{n\&omega;}}_{s}t\right)+\underset{n=1,3,5...}{\overset{\mathrm{\&infin;}}{\mathrm{\&Sigma;}}}C\cos \left({\mathrm{m\&omega;}}_{c}t\right)\\ +\underset{m=1}{\overset{\mathrm{\&infin;}}{\mathrm{\&Sigma;}}}\underset{n=\&PlusMinus;1}{\overset{\&PlusMinus;\mathrm{\&infin;}}{\mathrm{\&Sigma;}}}\{D\cos ({\mathrm{m\&omega;}}_{c}t+{\mathrm{n\&omega;}}_{s}t)+E\sin ({\mathrm{m\&omega;}}_{c}t+{\mathrm{n\&omega;}}_{s}t)\}\end{array}---\left(1\right)$

In formula (1), ω _sfor modulating wave angular frequency, ω _cfor carrier angular frequencies, A is fundamental voltage amplitude, and B is modulating wave harmonic amplitude, and C is carrier wave harmonic amplitude, D and E is sideband harmonic amplitude,

Wherein, to output performance have the greatest impact for fundamental voltage amplitude A and modulating wave harmonic amplitude B, its expression formula is respectively:

$A=\frac{{\mathrm{MU}}_{dc}}{\mathrm{\&pi;}V}\&lsqb;\frac{\mathrm{\&pi;}}{2}+arcsin\left(\frac{V-1}{M}\right)+\left(\frac{V-1}{M}\right)\sqrt{1-{\left(\frac{V-1}{M}\right)}^2}\&rsqb;\text{---}\left(2\right)$

$\begin{array}{c}\text{B=}\frac{{\text{2MU}}_{dc}\sqrt{1-{\left(\frac{V-1}{M}\right)}^2}}{\mathrm{\&pi;}V(n^2-1)}\sin \&lsqb;n\left(\arcsin \right(\frac{V-1}{M}))\&rsqb;-\\ \frac{2{\mathrm{nU}}_{dc}(V-1)}{\mathrm{\&pi;}(n^2-1)V}\cos \&lsqb;n\left(\arcsin \right(\frac{V-1}{M}))\&rsqb;+\frac{2U_{dc}(V-1)}{n\mathrm{\&pi;}V}\cos \&lsqb;n\left(\arcsin \right(\frac{V-1}{M}))\&rsqb;\end{array}---\left(3\right)$

In formula (2) and (3), U _dcfor DC capacitor total voltage, M is modulating wave fundamental voltage amplitude perunit value,

From formula (2) and (3), V≤1+M;

Setting V≤1.5, accordingly, obtain the expression formula determining V:

$\left\{\begin{array}{c}1<V\&le;M+1\\ 1<V\&le;1.5\end{array}\right.---\left(4\right)$

In formula (4), V is perunit value.

4. the three level NPC current transformer PWM control strategy overlapping based on carrier wave according to claim 1, it is characterized in that, the determination of described step 3 modulating wave is specially:

A) setting three-phase modulations ripple U time initially _rxfor standard sine wave;

B) three-phase modulations ripple U is calculated _rxfor the mid point electric current caused during standard sine wave;

Three level NPC current transformer DC side mid point electric current is made up of three parts, as shown in formula (5), and wherein i _oxrepresent the mid point electric current caused by x phase current, x=a, b or c, i _oxwith phase current i _xas shown in formula (6), d _x2and d _x1represent the real-time duty ratio of x phase 2 pipe and 1 pipe respectively,

i _o＝i _oa+i _ob+i _oc(5)

i _ox＝(d _x2-d _x1)×i _x(x＝a,b,c) (6)

X phase 1 is managed and the real-time duty ratio of 2 pipes is:

$\left\{\begin{array}{c}{d}_{x1}=\left\{\begin{array}{c}\frac{U_{rx}+V-1}{V}(\mathrm{\&omega;}t\&Element;\&lsqb;0,x_1\&rsqb;\&lsqb;x_2,2\mathrm{\&pi;}\&rsqb;)\\ 0(\mathrm{\&omega;}t\&Element;\&lsqb;x_1,x_2\&rsqb;)\end{array}\right.\\ d_{x2}=\left\{\begin{array}{c}1(\mathrm{\&omega;}t\&Element;\&lsqb;x_3,x_4\&rsqb;)\\ \frac{U_{rx}+1}{V}(\mathrm{\&omega;}t\&Element;\&lsqb;0,x_3\&rsqb;\&cup;\&lsqb;x_4,2\mathrm{\&pi;}\&rsqb;)\end{array}\right.\end{array}\right.---\left(7\right)$

In formula (7), x ₁, x ₂and x ₃, x ₄represent respectively x phase modulating wave with upper carrier wave and lower carrier wave the crossing moment in a power frequency period, x=a, a phase modulating wave with upper carrier wave and lower carrier wave its expression formula of crossing moment in a power frequency period as shown in formula (8):

$\left\{\begin{array}{c}{a}_1=\mathrm{\&pi;}+arcsin\&lsqb;(V-1)/M\&rsqb;\\ a_2=2\mathrm{\&pi;}-\arcsin \&lsqb;(V-1)/M\&rsqb;\\ a_3=arcsin\&lsqb;(V-1)/M\&rsqb;\\ a_4=\mathrm{\&pi;}-\arcsin \&lsqb;(V-1)/M\&rsqb;\end{array}\right.---\left(8\right)$

X=b, b phase modulating wave with upper carrier wave and lower carrier wave its expression formula of crossing moment in a power frequency period as shown in formula (9):

$\left\{\begin{array}{c}{b}_1=2\mathrm{\&pi;}/3-arcsin\&lsqb;(V-1)/M\&rsqb;\\ b_2=5\mathrm{\&pi;}/3+\arcsin \&lsqb;(V-1)/M\&rsqb;\\ b_3=2\mathrm{\&pi;}/3+arcsin\&lsqb;(V-1)/M\&rsqb;\\ b_4=5\mathrm{\&pi;}/3-\arcsin \&lsqb;(V-1)/M\&rsqb;\end{array}\right.---\left(9\right)$

X=c, c phase modulating wave with upper carrier wave and lower carrier wave its expression formula of crossing moment in a power frequency period as shown in formula (10):

$\left\{\begin{array}{c}{c}_1=\mathrm{\&pi;}/3+arcsin\&lsqb;(V-1)/M\&rsqb;\\ c_2=4\mathrm{\&pi;}/3-\arcsin \&lsqb;(V-1)/M\&rsqb;\\ c_3=\mathrm{\&pi;}/3-arcsin\&lsqb;(V-1)/M\&rsqb;\\ c_4=4\mathrm{\&pi;}/3+\arcsin \&lsqb;(V-1)/M\&rsqb;\end{array}\right.---\left(10\right)$

Formula (7)-(10) are brought into the mid point current i that formula (6) obtains being caused by x phase load electric current _oxexpression formula:

$i_{os}=\left\{\begin{array}{c}\frac{1-abs\left(U_{rx}\right)}{V}\&times;i_x(V-1\&le;|U_{rx}|\&le;1)\\ \frac{2-V}{V}\&times;i_x\left(\right|U_{rx}|\&le;V-1)\end{array}\right.---\left(11\right)$

(11) are substituted into (5) and according to the fluctuation relation of mid point electric current and DC capacitor voltage:

$\mathrm{\&Delta;}u=u_{c1}-u_{c2}=\{\begin{array}{c}\mathrm{\&Sigma;}\frac{1-abs\left(U_{rx}\right)}{CV}\&times;i_x\mathrm{\&Delta;}T(V-1\&le;|U_{rx}|\&le;1)\\ \mathrm{\&Sigma;}\frac{2-V}{V}\&times;i_x\left(\right|U_{rx}|\&le;V-1)\end{array}---\left(12\right)$

In formula (12), the fluctuation of mid point electric current and DC capacitor voltage is closed and is △ u is DC capacitor voltage fluctuation situation, and △ T is switch periods, and C is DC capacitor voltage;

C) build Control loop, be defined as suppressing DC capacitor voltage to fluctuate the zero-sequence component U that need superpose in modulating wave _o,

Compared with 0 by the △ u calculated in b), its result is by obtaining the zero-sequence component U that need superpose in modulating wave after PI controller _o,

D) by U _owith a) in given sinusoidal modulation wave U _rxbe added, x=a, b, c, obtain three-phase modulations ripple U _ao, U _bo, U _co.

5. the three level NPC current transformer PWM control strategy overlapping based on carrier wave according to claim 1, it is characterized in that, described step 4 is specially:

The three-phase modulations ripple U that step 3 is obtained _ao, U _bo, U _corespectively with upper and lower carrier wave U _c2and U _c1relatively, if certain phase modulating wave is greater than carrier wave U _c2, then first, second switching tube top-down of this phase is open-minded, and the 3rd, the 4th switching tube turns off, if certain phase modulating wave is less than carrier wave U _c2be greater than modulated ripple U _c1, then second, third switching tube top-down of this phase is open-minded, and the first, the 4th switching tube turns off, if certain phase modulating wave is less than modulated ripple U _c1, then top-down 3rd, the 4th switching tube of this phase is open-minded, and first, second switching tube turns off.