CN105048847B - A kind of current transformer neutral-point voltage balance method based on SHEPWM - Google Patents

A kind of current transformer neutral-point voltage balance method based on SHEPWM Download PDF

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CN105048847B
CN105048847B CN201510496007.1A CN201510496007A CN105048847B CN 105048847 B CN105048847 B CN 105048847B CN 201510496007 A CN201510496007 A CN 201510496007A CN 105048847 B CN105048847 B CN 105048847B
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胡存刚
马大俊
张云雷
王群京
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Anhui University
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Abstract

The present invention relates to a kind of current transformer neutral-point voltage balance method based on SHEPWM, comprise the following steps：Switching angle quantity and modulation degree are determined according to control system performance requirement；Establish more level neutral-point-clamped type current transformer particular harmonics and eliminate control method mathematical modeling, try to achieve switching angle during corresponding modulation degree；Vector quantization analysis is carried out to the switch angle tried to achieve；Analyze influence of the different SHEPWM switch angles to current transformer mid-point voltage；Quantitatively calculate unbalance of neutral-point voltage amount corresponding to different SHEPWM switch angles；The neutral-point voltage balance of current transformer is realized using the real-time switching to SHEPWM switching angles.The control method of the present invention can effectively control neutral point voltage balance while specific low-order harmonic is eliminated, and greatly improve output waveform, reduce total harmonic distortion, and switching loss is low, and algorithm complex is low.

Description

A kind of current transformer neutral-point voltage balance method based on SHEPWM
Technical field
The present invention relates to the modulator approach of Multilevel Inverters, more particularly to a kind of current transformer midpoint electricity based on SHEPWM Press balance control method.
Technical background
The particular harmonic of more level neutral-point-clamped type current transformers eliminates optimum choice of the control method by switching time, just The pwm voltage waveform of locality control current transformer, make humorous in the absence of some specific (low orders) in the voltage of inverter output Ripple, the switching frequency of power device is reduced, reduce the switching loss of power device.
Although the particular harmonic of more level neutral-point-clamped type current transformers, which eliminates control method, can effectively eliminate some spies Fixed low-order harmonic, the switching loss of power device is reduced, but can not effectively control the balance of mid-point voltage.For more The problem of level neutral-point-clamped type current transformer neutral point voltage balance, there are many control strategies to be suggested, be broadly divided into two kinds, i.e., it is hard Part control strategy and software controlling strategies.Hardware control strategy typically will additionally increase power supply or power device and accumulator Part, so as to cause the increase of system bulk, cost improves, and the efficiency of system and stability reduce, therefore application is few；Phase Than under, while software controlling strategies save the volume and cost of system, there is very high stability, but it adds software The complexity of algorithm.With the fast development of High Performance DSP, complicated control algolithm becomes more and more easily to realize, therefore, base Increasing concern is obtained in the neutral-point voltage balance method of control algolithm.At present, the midpoint based on control algolithm Voltage balancing control method is Space Vector Modulation Strategy mostly, but can not be effective while neutral point voltage balance is controlled Eliminate specific low-order harmonic.According to literature search, the neutral-point voltage balance strategy based on SHEPWM, this hair are had no at present The control strategy of bright proposition can effectively control more level neutral-point-clamped type current transformers on the basis of specific low-order harmonic is eliminated Neutral point voltage balance.
The content of the invention
The purpose of the present invention is while specific low-order harmonic is eliminated, there is provided one kind can efficiently control more level midpoints The method of clamper type current transformer neutral point voltage balance, concrete technical scheme are as follows：
A kind of current transformer neutral-point voltage balance method based on SHEPWM, comprises the following steps：
Step S1：Switching angle quantity and modulation degree are determined according to control system performance requirement；
Step S2：Establish more level neutral-point-clamped type current transformer particular harmonics and eliminate control method mathematical modeling, according to step Rapid S1 obtains switching angle when switching angle quantity and modulation degree try to achieve corresponding modulation degree by optimized algorithm；
Step S3：The output of SHEPWM three-phases is considered as the space vector collection consistent with SVPWM, to what is tried to achieve in step S2 Switching angle carries out vector quantization analysis；
Step S4：According to step S3 analysis result, SHEPWM output waveforms are considered as space vector, analyze different open Close influence of the angle to more level neutral-point-clamped type current transformer mid-point voltages；
Step S5：Quantitatively calculate more level neutral-point-clamped current transformer unbalance of neutral-point voltage amounts corresponding to different switching angles；
Step S6：According to step S5 result of calculation, more level neutral-point-clampeds are realized using the real-time switching to switching angle The neutral-point voltage balance of current transformer.
Further, the step S6 comprises the following steps：
Step S61：The magnitude of voltage of DC bus capacitor is gathered in real time using voltage collection circuit, by the voltage of DC bus capacitor Value subtracts reference voltage level (half of DC voltage) and obtains the deviant of current sample time mid-point voltage, by the midpoint The deviant of voltage is fed back in dsp chip；
Step S62：In dsp, using step S61 value of feedback calculate mid-point voltage in current sample period it is total partially Shifting amount；
Step S63：The total drift amount of mid-point voltage, selects next week in the current sample period obtained according to step S62 The switching angle of phase.
It is further preferred that the current transformer neutral-point voltage balance method based on SHEPWM, comprises the following steps：
Step S1：Determine that switching angle quantity is 4 and modulation degree is 1.0 according to control system performance requirement；
Step S2：Establish more level neutral-point-clamped type current transformer particular harmonics and eliminate control method mathematical modeling, pass through something lost Propagation algorithm tries to achieve switching angle when modulation degree is 1.0；
Step S3：The output of SHEPWM three-phases is considered as the space vector collection consistent with SVPWM, to what is tried to achieve in step S2 Switching angle carries out vector quantization analysis；
Step S4：According to step S3 analysis result, SHEPWM output waveforms are considered as space vector, analyze different open Close influence of the angle to more level neutral-point-clamped type current transformer mid-point voltages；
Step S5：Quantitatively calculate more level neutral-point-clamped current transformer unbalance of neutral-point voltage amounts corresponding to different switching angles； Represent that mid-point voltage allows the maximum upper limit of skew with H, Δ Uc represents unbalance of neutral-point voltage amount, by different switching angle roots It is divided into four groups according to the influence of alignment voltage, (1) group switching angle makes Δ Uc>H, (2) group switching angle make 0<ΔUc<H, the (3) group switching angle makes-H<ΔUc<0, (4) group switching angle makes Δ Uc<-H；
Step S6：Comprise the following steps：
Step S61：The magnitude of voltage of DC bus capacitor is gathered in real time using voltage collection circuit, by the voltage of DC bus capacitor Value subtracts reference voltage level (half of DC voltage) and obtains the deviant of current sample time mid-point voltage, by the midpoint The deviant of voltage is fed back in dsp chip；
Step S62：In dsp, using step S61 value of feedback calculate mid-point voltage in current sample period it is total partially Shifting amount；
Step S63：The total drift amount of mid-point voltage, selects next week in the current sample period obtained according to step S62 The switching angle of phase, if the total drift amount of mid-point voltage is more than H in current sample period, (4) are selected within next cycle Group switching angle；If the total drift amount of mid-point voltage is less than H more than zero in current sample period, selected within next action period Select (3) group switching angle；If the total drift amount of mid-point voltage is more than-H less than zero in current sample period, in next effect Selection (2) group switching angle in cycle；If the total drift amount of mid-point voltage is less than-H in current sample period, in next work (1) is selected to organize switching angle with the cycle.
Further, more level neutral-point-clamped type current transformers are three level neutral-point-clamped type current transformers, in three level The application of point clamper type current transformer is relatively wide more typical.
Further, when generating pwm signal according to Multilevel Inverters control law, dead time is added.Can be effectively A bridge arm caused by postponing effect is avoided not complete switch off, and another bridge arm is in the conduction state, avoids straight-through damage Module.
The beneficial effects of the present invention are：
(1) particular harmonic of more level neutral-point-clamped type current transformers is eliminated control method with by control method of the invention The point balance of voltage combines, and is eliminating specific low-order harmonic simultaneously, can effectively control neutral point voltage balance, greatly improve output Waveform, reduce total harmonic distortion.
(2) present invention uses SHEPWM control algolithms, for the SPWM and SVPWM that compares, has lower switch Frequency, so as to which the switching loss of power device greatly reduce.
(3) present invention is compared to using the neutral point voltage balance of SHEPWM methods control multi-level frequency conversion device and is used SPWM For the method for SVPWM control neutral point voltage balances, the complexity of algorithm is effectively reduced.
Brief description of the drawings
Fig. 1, the present invention control method flow chart
Fig. 2, three level SVPWM polar plots
Fig. 3, embodiments of the invention three-phase voltage output waveform
In figure：P represents π
Embodiment
By taking three level neutral-point-clamped type current transformers as an example, to the present invention SHEPWM neutral-point voltage balance methods do into The explanation of one step.
Current transformer neutral-point voltage balance method based on SHEPWM, comprises the following steps：
Step S1：Switching angle quantity and modulation degree are determined according to control system performance requirement；Switch is chosen in the present embodiment Angle is 4, modulation degree 1.
Step S2：Establish more level neutral-point-clamped type current transformer particular harmonics and eliminate control method mathematical modeling,
U in formuladFor DC side supply voltage, fundamental modulation degreeN by taking switching angle number in 1/4 cycle, Switching angle should meet condition：M is the number of contained harmonic wave；Usm(M) it is M The reference voltage of subharmonic；
The present invention takes N=4, and exemplified by modulation degree m=1.0, four groups of switching angles are tried to achieve by genetic algorithm；
It is as shown in table 1 to obtain four groups of switching angles.
Switching angle during table 1m=1.0
Disaggregation α1 α2 α3 α4
① 4.10424 7.33147 17.4061 87.6462
② 14.2251 63.3489 67.8868 83.5792
③ 19.1006 46.5388 53.5809 85.4508
④ 47.099 57.6101 65.293 90.000
Step S3：The output of SHEPWM three-phases is considered as the space vector collection consistent with SVPWM, to what is tried to achieve in step S2 Switching angle carries out vector quantization analysis；The essential connection between SHEPWM and space vector pulse width modulation (SVPWM) is studied, will The output of SHEPWM three-phases is considered as the space vector collection consistent with SVPWM, and vector quantization is carried out to the threephase switch angle solved.This hair It is bright (to be noted with α 1=14 °, α 2=63 °, α 3=67 °, α 4=83 °：Simple to discuss, the data behind decimal point are cast out) exemplified by enter Row illustrates that three-phase voltage output waveform under the disaggregation is as shown in figure 3, according to the difference of output phase voltage, by three-level current transformer Output be divided into three state P (positive output), O (zero output), N (negative output).Under SHEPWM controls, each moment has it Corresponding output state.Three-phase output when to make a phases phase be θ is respectively a (θ), b (θ), c (θ).SHEPWM c phases switch shape State can be exported 120 ° of delay by a phases and be obtained, such as formula (2).
C (θ)=a (+120 ° of θ) (2)
Because SHEPWM waveforms are on 180 ° of odd symmetrys, on 90 ° of even symmetrys, i.e.,：
A (180 ° of-θ)=a (θ) (3)
A (360 ° of-θ)=- a (θ) (4)
According to formula (2), (3), (4), calculate c (+60 ° of θ) and obtain：
By formula (4) if can be seen that the output state of the known a certain angle of a phases, the angle can be obtained according to the equation and is prolonged In the output state of c phases after slow 60 °.
Similarly, formula (6), (7) can be obtained by the odd-even symmetry relation of output voltage wave in b, c phase：
A (+60 ° of θ)=- b (θ) (6)
B (+60 ° of θ)=- c (θ) (7)
Convolution (5), (6), (7), if in known SHEPWM a certain moment three-phase output state, can be obtained according to formula To the three-phase output state postponed after 60 °.
Three level SVPWM polar plots are as shown in Fig. 2 the new arrow that certain vector obtains with 60 ° of its rotate counterclockwise as seen from the figure Amount also meets that formula (5), (6), (7), therefore SHEPWM and SVPWM have contacting for essence, and SVPWM A great Qu correspond to SHEPWM 90 ° of output waveform are to 150 ° of regions, and other regions are by that analogy.So one 60 ° need to be only taken to study switching angle vector quantization Result.0 ° -60 ° of SHEPWM waveforms can be seen as different space vectors, as shown in table 2.
The vector quantization result of 0 ° -60 ° of 2 SHEPWM waveforms of table
Step S4：According to step S3 analysis result, SHEPWM output waveforms are considered as space vector, analyze different open Close influence of the angle to more level neutral-point-clamped type current transformer mid-point voltages；Analysis is understood based on more than, three-phase SHEPWM output waves Shape can be seen as space vector, including：Zero vector, positive small vector, bear small vector, middle vector, big vector.If define inverter On off state be：Sx=[Sa,Sb,Sc], then each phase voltage is expressed as
In formula：
Corresponding midpoint electric current inp(t) expression formula is：
inp(t)=- (| Sa(t)|ia(t)+|Sb(t)|ib(t)+|Sc(t)|ic(t)) (10)
I in formulaa(t),ib(t),ic(t) it is respectively three-phase output current.Corresponding midpoint electric current feelings during different vector effects Condition is as shown in table 3, and alignment electric current does not influence when wherein zero vector and big vector act on.
Corresponding midpoint electric current when 3 different vectors of table act on
Positive small vector inp(t) Negative small vector inp(t) Middle vector inp(t)
POO -ia(t) ONN ia(t) PON ib(t)
PPO ic(t) OON -ic(t) OPN ia(t)
OPO -ib(t) NON ib(t) NPO ic(t)
OPP ia(t) NOO -ia(t) NOP ib(t)
OOP -ic(t) NNO ic(t) ONP ia(t)
POP ib(t) ONO -ib(t) PNO ic(t)
The direction of rated current outflow current transformer is just, the expression formula for defining unbalance of neutral-point voltage amount is as follows：
Unp(t)=UC2(t)-UC1(t) (11)
When midpoint electric current is positive, electric current flows out current transformer, and electric capacity discharges under DC side, and mid-point voltage reduces；Together Reason, when midpoint electric current is bears, electric current flows into current transformer, and electric capacity charges under DC side, mid-point voltage rise.And then obtain The influence of alignment voltage is as shown in table 4 during different vector effects, and wherein "+" represents to raise mid-point voltage, and "-" is represented in making Point voltage reduces, and zero vector and big vector alignment voltage do not influence.
The influence of alignment voltage when 4 different vectors of table act on
Illustrate the influence of switching angle alignment voltage exemplified by 0 ° -60 ° of SHEPWM waveforms in Fig. 3.Wherein, small vector ONP, POP, PNO reduce mid-point voltage, and total action time is 24 °；Small vector OOP, POO raise mid-point voltage, total Action time is 8 °；Big vector PNP alignment voltages do not influence.Because make the action time of the vector of mid-point voltage reduction big In the action time for making the elevated vector of mid-point voltage, so mid-point voltage will reduce in 0 ° -60 °.By the analysis of step 2. As a result understand, it is only necessary to analyze the influence of each vector alignment voltage in 0 ° -60 ° it may determine that going out this group of switching angle alignment The influence of voltage, so α 1=14.2251 °, α 2=63.3489 °, α 3=67.8868 °, α 4=83.5792 ° this group switches Angle reduces mid-point voltage, and other switching angles can be made with similar analysis and understand that some switching angles make mid-point voltage liter Height, some reduce mid-point voltage.
Step S5：Quantitatively calculate more level neutral-point-clamped current transformer unbalance of neutral-point voltage amounts corresponding to different switching angles；
Defining three-phase output current is：
If inp1, inp2, inp3Respectively when a phases, middle electricity is electric out of 0 ° to 120 °, 120 ° to 240 °, 240 ° to 360 ° periods The total drift amount of stream is then to have：
Wherein x (θ), y (θ), z (θ) definition are：
Derive inp1And inp2Between relation it is as follows：
I can similarly be obtainednp3=inp1, so the total drift amount of middle electric current is in a cycle：
It is possible thereby to the total drift amount for deriving mid-point voltage in a cycle is：
In formula, C2For capacitor's capacity on the downside of DC side, Unp(t) it is t unbalance of neutral-point voltage value, can be with according to (17) formula Calculate the total drift amount of different switching angles mid-point voltage in a cycle.The present invention using four groups of switching angles in table 5 as Example illustrates the different degrees of influence of different switching angle alignment voltage, and wherein H is allowed by mid-point voltage in the maximum of skew Limit ,-H are allowed the greatest lower bound of skew by mid-point voltage.Δ Uc represent unbalance of neutral-point voltage amount, by different switching angles according to The influence of alignment voltage is divided into four groups, and (1) group switching angle makes Δ Uc>H, (2) group switching angle make 0<ΔUc<H, (3) Group switching angle makes-H<ΔUc<0, (4) group switching angle makes Δ Uc<-H；
The influence of the different switching angle alignment voltage of table 5
Step S6：Comprise the following steps：
Step S61：The magnitude of voltage of DC bus capacitor is gathered in real time using voltage collection circuit, by the voltage of DC bus capacitor Value subtracts reference voltage level (half of DC voltage) and obtains the deviant of current sample time mid-point voltage, by the midpoint The deviant of voltage is fed back in dsp chip；
Step S62：In dsp, using step S61 value of feedback calculate mid-point voltage in current sample period it is total partially Shifting amount；
Step S63：The flow chart of control method is as shown in figure 1, the DC bus capacitor collected with voltage collection circuit Magnitude of voltage subtracts reference voltage level (half of DC voltage), is the deviant of the sampling instant mid-point voltage, is allowed to anti- It is fed in dsp chip, calculates the total drift amount of mid-point voltage in current sample period.
The neutral point voltage balance of switching switching angle control Multilevel Inverters in real time is illustrated how by taking table 5 as an example.It is if current When the total drift amount of mid-point voltage is more than H in sampling period, then (4) group switching angle is selected within next action period；If work as When the total drift amount of mid-point voltage is less than H more than zero in the preceding sampling period, then (3) group switch is selected within next action period Angle；If the total drift amount of mid-point voltage is more than-H and is less than zero in current sample period, the is selected within next action period (2) switching angle is organized；If the total drift amount of mid-point voltage is less than-H in current sample period, selected within next action period (1) organizes switching angle.When generating pwm signal according to Multilevel Inverters control law, dead time is added.

Claims (4)

  1. A kind of 1. current transformer neutral-point voltage balance method based on SHEPWM, it is characterised in that comprise the following steps：
    Step S1：Switching angle number N and fundamental modulation degree m are determined according to control system performance requirement；
    Step S2：Establish more level neutral-point-clamped type current transformer particular harmonics and eliminate control method mathematical modeling,
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    U in formuladFor DC side supply voltage, fundamental modulation degreeN is switching angle number, and switching angle should meet condition：M is the number of contained harmonic wave；Usm(M) it is the reference voltage of M subharmonic；
    Switching angle number N and fundamental modulation degree m are obtained according to step S1, when trying to achieve corresponding fundamental modulation degree m by optimized algorithm Switching angle；
    Step S3：The output of SHEPWM three-phases is considered as the space vector collection consistent with SVPWM, to the switch tried to achieve in step S2 Angle carries out vector quantization analysis；
    Step S4：According to step S3 analysis result, SHEPWM output waveforms are considered as space vector, analyze different switching angles Influence to more level neutral-point-clamped type current transformer mid-point voltages；
    Step S5：Quantitatively calculate more level neutral-point-clamped current transformer mid-point voltage total drift amounts corresponding to different switching angles；
    Wherein △ UcFor current transformer mid-point voltage total drift amount；inp1For the offset of a phases period midpoint electric current from 0 ° to 120 °, C2 For capacitor's capacity on the downside of DC side, Unp(t) it is t unbalance of neutral-point voltage value, inp_totalFor midpoint electricity in a switch periods Flow total drift amount；ia(θ), ib(θ), ic(θ) is respectively three-phase output current value；X (θ), y (θ), z (θ) definition are：
    Step S6：According to step S5 result of calculation, more level neutral-point-clamped unsteady flows are realized using the real-time switching to switching angle The neutral-point voltage balance of device.
  2. A kind of 2. current transformer neutral-point voltage balance method based on SHEPWM as claimed in claim 1, it is characterised in that The step S6 comprises the following steps：
    Step S61：Gather the magnitude of voltage of DC bus capacitor in real time using voltage collection circuit, the magnitude of voltage of DC bus capacitor is subtracted Go reference voltage to be worth to the deviant of current sample time mid-point voltage, the deviant of the mid-point voltage is fed back into DSP In chip, the reference voltage level is DC side supply voltage UdHalf；
    Step S62：In dsp, the total drift of mid-point voltage in current sample period is calculated using step S61 value of feedback Amount；
    Step S63：The total drift amount of mid-point voltage, selects next cycle in the current sample period obtained according to step S62 Switching angle.
  3. A kind of 3. current transformer neutral-point voltage balance method based on SHEPWM as claimed in claim 1, it is characterised in that： More level neutral-point-clamped type current transformers are three level neutral-point-clamped type current transformers.
  4. 4. a kind of current transformer neutral-point voltage balance method based on SHEPWM as any one of claim 1-3, It is characterized in that：When generating pwm signal according to Multilevel Inverters control law, dead time is added.
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