CN101594045A - A kind of specific harmonic elimination method of modularization multi-level converter - Google Patents

Abstract

The present invention proposes a kind of specific harmonic elimination method of novel modularized multilevel converter.The present invention is applied to three-phase modular multilevel inverter with specific harmonic elimination method, by the switching angle of choose reasonable submodule, improves its harmonic wave of output voltage characteristic, improves waveform quality, reaches littler harmonic wave of output voltage aberration rate.Use at engineering simultaneously, proposed the burst pulse optimisation strategy, to improve its engineering adaptability.

Description

A kind of specific harmonic elimination method of modularization multi-level converter

Technical field

The invention belongs to the flexible power transmission and distribution of electric power system, power electronics, Electric Traction, motor-driven and custom power technology field, be specifically related to a kind of selective harmonic elimination field of modularization multi-level converter.

Background technology

Multilevel converter is more welcome in recent years a kind of high-power electric and electronic converter, and successful Application at a lot of industrial circles, as Static Var Compensator, Electric Traction, FACTS, high voltage direct current transmission and active filter etc.The tradition multilevel converter has multiple different structure, and common have diode-clamped (NPC), striding capacitance type (FLC) and a Cascade H bridge type (CHC).Modularization multi-level converter (Modular Multilevel Converter, be called for short MMC) as a kind of new multilevel converter, had the advantage that traditional multilevel converter switching frequency is low, switching loss is little, high-frequency noise is low, voltage change ratio is little concurrently.Simultaneously, compare with FLC type multilevel converter with the NPC type, degree of modularity height, switching device quantity can not present non-linear growth with the increase of level number, and structural design is simple; Compare with the CHC type, power can two-way flow, and switching device has also reduced one times.

The modulation strategy that the tradition multilevel converter can adopt has multiple, as sinusoidal pulse width modulation (Pulse WidthModulation is called for short PWM), space vector pulse width modulation (SVPWM) and selective harmonic elimination pulse-width modulation (SHEPWM).And SHEPWM is more welcome a kind of, and it can eliminate the appointment subharmonic, reduces the voltage current waveform aberration rate.

Summary of the invention

In order to make that the distortion of MMC output voltage waveforms is littler, waveform quality is better, is necessary to study the method that SHEPWM is applied to MMC.

At modularization multi-level converter MMC topological structure, the present invention proposes the specific harmonic elimination method that is applicable to that this is topological.This method can be optimized MMC harmonic wave of output voltage characteristic under lower switching frequency, improve waveform quality.

The every mutually upper and lower half-bridge arm of modularization multi-level converter MMC all is to be formed by several submodules (Submodule) cascade independently, and each submodule can independently be controlled and cut-offs.Utilize this advantage, the switching angle of each submodule of choose reasonable to eliminate specific subharmonic in the output voltage, reduces wave distortion, improves waveform quality.Concrete grammar is as follows:

By in brachium pontis output staircase voltage, the selective harmonic elimination angle being set, to eliminate the harmonic wave of specific times.Because the MMC output voltage has 1/4 periodic symmetry, only need the switching angle in 1/4 cycle of consideration to find the solution.When if every phase half-bridge arm adopts 2N submodule to constitute required many level staircase waveform output voltage, output state changes only N submodule, and a demand that is to say is separated N interior switching angle of 1/4 cycle and got final product.

In three-phase MMC, after certain phase selective harmonic elimination angle was determined, passing through mutual wrong 120 ° and obtaining of other two-phase do not contain the frequency tripling harmonic wave like this, thereby can eliminate more high order harmonic components in the output line voltage.Concrete steps are as follows:

(1) draws the fourier progression expanding method expression formula of three-phase MMC output voltage by theory analysis, comprise the switching angle of N submodule in the expression formula;

(2) make that fundametal compoment equals desired value in the fourier progression expanding method expression formula, N-1 specific order harmonic components equals 0, thereby obtains N non-linear transcendental equation;

(3) determine the first-harmonic desired value of MMC output voltage, calculate N switching angle of half-bridge arm correspondence this moment;

(4) with the switching angle in the step (3) as initial value, the non-linear transcendental equation in the solution procedure (2) obtains the initial value of burst pulse Optimization Model;

(5) the selective harmonic elimination angle that step (4) is obtained is carried out burst pulse optimization, finally obtains can be used for N switch angle of engineering.

Description of drawings

The present invention is further described below in conjunction with accompanying drawing.

Fig. 1 is the circuit diagram according to three-phase MMC of the present invention;

Fig. 2 is according to the single brachium pontis output voltage waveforms of three-phase MMC of the present invention schematic diagram;

Fig. 3 is the computational methods schematic diagram according to selective harmonic elimination angle initial value of the present invention;

Fig. 4 is according to of the present invention when N=5, MMC output voltage spectrogram.

Embodiment

Be preferred embodiment of the present invention below, the technical scheme that the present invention realizes be described further below in conjunction with this accompanying drawing.

Be the circuit diagram according to three-phase MMC of the present invention as shown in Figure 1, modularization multi-level converter MMC is made of 6 half-bridge arms of three-phase, and every phase half-bridge arm is formed by 2N sub-module cascade.Every phase output voltage is:

v _an＝N·V _dc-v _au or (1)

v _an＝-N·V _dc+v _al

Specific harmonic elimination method is provided with breach by the ad-hoc location at brachium pontis output staircase voltage, eliminates the harmonic wave of specific times.These positions are exactly usually said selective harmonic elimination angle, and these angles are generally all determined by finding the solution non-linear transcendental equation group then, generally find the solution with numerical method.Because the MMC output voltage has 1/4 periodic symmetry, only need the switching angle in 1/4 cycle of consideration to find the solution.When if every phase half-bridge arm adopts 2N submodule to constitute required many level staircase waveform output voltage, output state changes only N submodule, and a demand that is to say is separated N interior switching angle of 1/4 cycle and got final product.

Modularization multi-level converter MMC output voltage waveforms as shown in Figure 2.To the waveform of Fig. 2 form of a fourier progression expanding method accepted way of doing sth (2), and make fundametal compoment equal the first-harmonic desired value, the harmonic wave of required elimination equals 0, thereby constitutes N independently nonlinear equation.Because wrong 120 ° mutually of each brachium pontis switching angles of three-phase MMC, the frequency tripling harmonic wave is cancelled out each other in the line voltage.

$v\left(\mathrm{\ωt}\right)=\underset{n=\mathrm{1,3,5},\·\·\·}{\overset{\∞}{\mathrm{\Σ}}}\left[\frac{4V_{\mathrm{dc}}}{\mathrm{\π}}\underset{k=1}{\overset{S}{\mathrm{\Σ}}}\cos \left(n{\mathrm{\θ}}_k\right)\right]\·\frac{\sin \left(\mathrm{n\ωt}\right)}{n}---\left(2\right)$

Wherein, V _DcBe the submodule capacitance voltage; N is the number of selective harmonic elimination angle in 1/4 cycle; θ _kBe k selective harmonic elimination angle, and k=1,2 ..., N; ω is a fundamental frequency angular speed.

Is example to eliminate N-1 than low-order harmonic, and the highest harmonic elimination number of times can reach the 3N-2 subharmonic.It is as follows to write out the non-linear transcendental equation group of finding the solution switching angle by fourier series:

$\frac{4V_{\mathrm{dc}}}{\mathrm{\π}}[\cos \left({\mathrm{\θ}}_1\right)+\cos \left({\mathrm{\θ}}_2\right)+\·\·\·+\cos \left({\mathrm{\θ}}_N\right)]=V_{\mathrm{ref}}$

cos(5θ ₁)+cos(5θ ₂)+…+cos(5θ _N)＝0

cos(7θ ₁)+cos(7θ ₂)+…+cos(7θ _N)＝0 (3)

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cos(hθ ₁)+cos(hθ ₂)+…+cos(hθ _N)＝0

Wherein, V _DcBe the submodule capacitance voltage; V _RefFirst-harmonic desired value for the MMC output voltage; θ _kBe k selective harmonic elimination angle, and k=1,2 ..., N; The harmonic number that h eliminates for appointment, and h=1,5,7,11,13 ..., 3N-2.

For three-phase MMC, constitute total N unknown number of equation group of switching angle, after definite first-harmonic desired value, can select to eliminate N-1 low-order harmonic component.For every phase half-bridge arm the situation of 10 submodules is arranged, the switching angle number of degrees are 5, and the harmonic number of the highest elimination is 3N-2=13 time.

Thereby finding the solution of 5 non-linear transcendental equations of the question resolves itself into.Choose initial value arbitrarily, carry out iterative computation and show, algorithm is difficult for convergence.Suitable initial value must be selected,, reference wave voltage u as shown in Figure 3 can be determined according to MMC first-harmonic desired value _Ref(ω t).When the output level v of modularization multi-level converter (t) is k+1 by the k saltus step, make shade 1 among Fig. 3 equate with 2 area, can obtain the initial value θ of k+1 selective harmonic elimination angle _K+1Each level jumping moment is carried out similar calculating, just can obtain the initial value of iteration.Example proves: carry out iterative computation with this initial value, algorithm is rapid convergence very.

Consider that switching device all has certain turning on and off the time among the MMC, when turning on and off, all need the regular hour, and the loss meeting of turn-on and turn-off process makes the switching device heating, will cause the switching device scaling loss when heat accumulation effect acquires a certain degree.Thereby the minimum pulse width of MMC, i.e. burst pulse have been determined according to the time of turning on and off of switching device.And following optimization method has been proposed at burst pulse:

(1) calculates burst pulse=(service time+turn-off time) * 360 °/20ms;

(2), then omit the switching angle that constitutes burst pulse if the width of burst pulse is not more than 1/2 of minimum pulse width;

(3) if the width of burst pulse greater than 1/2 of minimum pulse width, then the broadening burst pulse is to minimum pulse width.

The switching angle that obtains by burst pulse optimization can be directly used in engineering, and can eliminate 3N-2 time and all be limited in the low-down level with interior harmonic wave.

By Fig. 4 when the N=5, MMC output voltage spectrogram as can be seen, under the situation of 2N=10,13 times and all very little with interior harmonic content.

The principle of the invention is simple, is easy to realize.When switching frequency is hundred hertz, optimized the harmonic characterisitic of MMC output voltage well, reduced total percent harmonic distortion of line voltage.

The present invention is illustrated according to preferred embodiment, should be appreciated that but not limit the present invention in any form of the foregoing description, and all employings are equal to the technical scheme that form obtained of replacement or equivalent transformation, all drop in protection scope of the present invention.

Claims (3)

1, a kind of specific harmonic elimination method of three-phase modular multilevel inverter, it is characterized in that this method is by being provided with the selective harmonic elimination angle in brachium pontis output staircase voltage, to eliminate the harmonic wave of specific times, because output voltage has 1/4 periodic symmetry, only need the switching angle in 1/4 cycle of consideration to find the solution, when every phase half-bridge arm has 2N submodule, in 1/4 cycle, have only N submodule action, that is to say and have only N switching angle in 1/4 cycle, eliminate N-1 specific subharmonic except that the frequency tripling harmonic wave, wherein N is a natural number;

In three-phase modular multilevel inverter, after certain mutually specific harmonic elimination angle was determined, the selective harmonic elimination angle of other two-phase obtained by mutual wrong 120 °, does not contain the frequency tripling harmonic wave like this in the output line voltage, thereby can eliminate more high order harmonic components, concrete steps are as follows:

(1) draw the fourier progression expanding method expression formula of three-phase modular multilevel inverter MMC output voltage by theory analysis, comprise the switching angle of N submodule in the expression formula:

$v\left(\mathrm{\ωt}\right)=\underset{n=\mathrm{1,3,5},\·\·\·}{\overset{\∞}{\mathrm{\Σ}}}\left[\frac{4V_{\mathrm{dc}}}{\mathrm{\π}}\underset{k=1}{\overset{N}{\mathrm{\Σ}}}\cos \left({\mathrm{n\θ}}_k\right)\right]\·\frac{\sin \left(\mathrm{n\ωt}\right)}{n}$

Wherein, V _DcBe the submodule capacitance voltage; N is the number of selective harmonic elimination angle in 1/4 cycle; θ _kBe k selective harmonic elimination angle, and k=1,2 ..., N; ω is a fundamental frequency angular speed;

(2) make that fundametal compoment equals the first-harmonic desired value in the fourier progression expanding method expression formula, N-1 specific order harmonic components equals 0, thereby obtains N non-linear transcendental equation:

$\frac{{4V}_{\mathrm{dc}}}{\mathrm{\π}}[\cos \left({\mathrm{\θ}}_1\right)+\cos \left({\mathrm{\θ}}_2\right)+\·\·\·+\cos \left({\mathrm{\θ}}_N\right)]=V_{\mathrm{ref}}$

cos(5θ ₁)+cos(5θ ₂)+…+cos(5θ _N)＝0

cos(7θ ₁)+cos(7θ ₂)+…+cos(7θ _N)＝0

.

.

.

cos(hθ ₁)+cos(hθ ₂)+…+cos(hθ _N)＝0

Wherein, V _DcBe the submodule capacitance voltage; V _RefFirst-harmonic desired value for modularization multi-level converter MMC output voltage; θ _kBe k selective harmonic elimination angle, and k=1,2 ..., N; H is for specifying the harmonic number of eliminating;

It should be noted that N-1 the specific subharmonic of need eliminating can be 5,7,11,13,17 ..., 3N-2 ... subharmonic, promptly the value of h can be the N-1 in the above-mentioned harmonic number;

(3) the first-harmonic desired value of determination module multilevel converter MMC output voltage is calculated N switching angle of half-bridge arm correspondence at this moment;

(4) with the switching angle in the step (3) as initial value, the non-linear transcendental equation in the solution procedure (2) obtains the initial value of burst pulse Optimization Model;

(5) the selective harmonic elimination angle that step (4) is obtained is carried out burst pulse optimization, finally obtains can be used for N switch angle of engineering.

2, the method for claim 1 has proposed following further optimization method at the described burst pulse in the step (4):

(1) calculates burst pulse=(service time+turn-off time) * 360 °/20ms;

(2), then omit the switching angle that constitutes burst pulse if the width of burst pulse is not more than 1/2 of minimum pulse width;

(3) if the width of burst pulse greater than 1/2 of minimum pulse width, then the broadening burst pulse is to minimum pulse width.

3, as the described method of claim 1-2, with described brachium pontis output staircase voltage fourier progression expanding method, and make fundametal compoment equal the first-harmonic desired value, the harmonic component of required elimination equals 0, thereby constitutes N independently nonlinear equation.

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