CN111525825A - Carrier modulation-based neutral point voltage balance control method for three-level converter - Google Patents
Carrier modulation-based neutral point voltage balance control method for three-level converter Download PDFInfo
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- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
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- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
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Abstract
A carrier modulation-based neutral point voltage balance control method for a three-level converter. For the case that a three-level converter uses carrier modulation as a modulation strategy, when a midpoint voltage oscillation value delta U exceeds a limit value, the control method firstly determines a voltage U corresponding to a redundant small vectoroAnd current ioThen comparing absolute values of amplitudes of three-phase modulation waves, defining the modulation wave corresponding to the maximum value as maxphase, the modulation wave corresponding to the intermediate value as midphase, and the modulation wave corresponding to the minimum value as minphase, detecting delta U × Uo×ioAccording to the values of maxphase, midamble and minphase, the zero sequence voltage V is obtained by calculationneu(ii) a Will VneuAnd superposing the three-phase modulated wave to control the neutral point voltage to restore balance again. The control method can control the neutral point voltage balance of the three-level converter based on the carrier modulation, does not need to use a PI (proportional-integral) controller and does not change the polarity of a modulation wave, and has the advantages of convenience in realization and simplicity in calculation.
Description
Technical Field
The invention relates to a neutral point voltage balance control method, in particular to a neutral point voltage balance control method of a three-level converter based on carrier modulation.
Background
The main circuit topology of a three-level Neutral-Point-Clamped (NPC) converter is shown in fig. 1. Compared with the traditional two-level converter, the three-level converter has the advantages of high output power, small voltage stress of a switching device, low harmonic distortion rate of output line voltage and the like; compared with multilevel topologies such as a cascade H-bridge, the three-level converter has the advantages of simple circuit structure, convenience in back-to-back operation and the like. Therefore, the three-level converter is widely applied to the fields of wind power generation, reactive power compensation of an electric power system, locomotive traction and the like.
The neutral point voltage balance control is a key problem of the three-level NPC converter. The document "a general diode-clamped converter carrier modulation voltage-sharing strategy based on predictive control" (tred, gujiu, cycloindac, etc. [ J ] new electrical energy technology, 2018,37(4):24-34.) indicates that unbalanced midpoint voltage causes distortion of output voltage of a three-level NPC converter and increase of voltage stress of a switching device, and an effective midpoint voltage balancing control method must be designed to ensure safe and reliable operation of the three-level NPC converter.
The commonly used midpoint voltage balance control methods of the three-level converter can be divided into two types: the method comprises a midpoint voltage balance control method based on Space Vector modulation (SVPWM) and a midpoint voltage balance control method based on carrier modulation. The literature, "NPC three-level converter midpoint potential feedback control based on zero sequence injection" (xi luo yao, jinxinmin, wu schoolwisdom, et al. [ J ]. report of electrical engineering, 2012,27(12): 117-.
In the two types of point voltage balance control methods, the specific definition and classification of the redundant small vectors are as follows:
defining that the three level states of the three-level converter from high to low output are P, O, N, the voltage space vector of the three-level converter can be summarized in fig. 2. The voltage space vectors can be classified into zero vectors, small vectors, medium vectors and large vectors according to the amplitude values of the voltage space vectors, the small vectors at the same phase angle can be further classified into P-type small vectors and N-type small vectors according to the output three-phase level state, and the P-type small vectors and the N-type small vectors are mutually redundant small vectors. The classification of each voltage space vector is summarized in table 1.
In the literature, "study of neutral point potential control strategy of three-level ANPC converter" (Zhang Bo, Gejiu, Wangxiao Xin, et al. [ J ] new electrical energy technology, 2016,35(8):1-7.), the three-level converter is specifically designed based on the neutral point voltage balance control formula of carrier modulation, and table 1 shows the voltage space vector types of the three-level converter
The method is carried out. The method can effectively control the midpoint voltage balance of the three-level converter, but has the following two defects:
1) a PI controller needs to be used, and thus PI parameters need to be designed. When the PI parameter design is not precise enough, the method has poor control effect on the midpoint voltage balance. Under different working conditions, PI parameter values may be different and need to be redesigned;
2) the modulated wave after the zero sequence voltage is superposed has polarity change, namely the modulated wave after the zero sequence voltage is superposed can have a negative value in the positive half cycle of the original modulated wave or a positive value in the negative half cycle of the original modulated wave. This can cause serious distortion of the output voltage and current, and may cause two-level jump of the phase voltage, which causes a safety hazard.
Disclosure of Invention
In order to overcome the defects of the traditional neutral point voltage balance control method based on carrier modulation, the invention provides a neutral point voltage balance control method of a three-level converter based on carrier modulation. The invention does not need to use a PI controller, and the zero sequence voltage used does not change the polarity of the modulation wave. The method can effectively control the neutral point voltage balance of the three-level converter based on the carrier modulation, and has the advantages of convenient realization, simple calculation and high reliability.
Aiming at the condition that the three-level converter uses carrier modulation as a modulation strategy, when the oscillation value delta U of the midpoint voltage exceeds a limit value, the midpoint voltage balance control method of the three-level converter based on the carrier modulation firstly determines the voltage U corresponding to the small redundant vectoroAnd current ioThen comparing the absolute value of the amplitude of the three-phase modulation wave, defining the modulation wave corresponding to the maximum value of the absolute value of the amplitude of the three-phase modulation wave as maxphase, the modulation wave corresponding to the middle value of the absolute value of the amplitude of the three-phase modulation wave as midphase, and the modulation wave corresponding to the minimum value of the absolute value of the amplitude of the three-phase modulation wave as minphase, detecting delta U × Uo×ioAccording to the values of maxphase, midamble and minphase, the zero sequence voltage V is obtained by calculationneu(ii) a Zero sequence voltage VneuAnd superposing the three-phase modulated wave to control the neutral point voltage to restore balance again.
The invention relates to a carrier modulation-based neutral point voltage balance control method of a three-level converter, which comprises the following steps:
1. when the oscillation value delta U of the midpoint voltage exceeds a limit value, determining the voltage U corresponding to the redundant small vectoroAnd current io；
When the oscillation value delta U of the midpoint voltage exceeds a limit value, the control method firstly determines the voltage U corresponding to the redundant small vectoroAnd current io(ii) a Defining A-phase modulation wave and current as U respectivelyaAnd iaThe B-phase modulation wave and the current are respectively UbAnd ibThe C-phase modulation wave and the current are respectively UcAnd icVoltage U corresponding to redundant small vectoroAnd current ioThe determination method specifically comprises the following steps:
1) when U is turneda＞0、Ub＜0、Uc< 0, or when Ua＜0、Ub＞0、UcWhen the voltage is more than 0, the voltage U corresponding to the redundant small vectoro＝UaCorresponding current io＝ia；
2) When U is turneda＞0、Ub＜0、Uc> 0, or when Ua＜0、Ub＞0、UcWhen less than 0, the voltage U corresponding to the redundant small vectoro＝UbCorresponding current io＝ib；
3) When U is turneda＞0、Ub＞0、Uc< 0, or when Ua＜0、Ub＜0、UcWhen the voltage is more than 0, the voltage U corresponding to the redundant small vectoro＝UcCorresponding current io＝ic。
Wherein, UaIs a modulated wave of A phase, iaFor phase A current, UbModulated wave of B phase, ibFor phase B current, UcModulated wave of C phase, icThe phase C current.
2. Judging a modulation wave maxphase corresponding to the maximum value of the amplitude absolute value of the three-phase modulation wave, a modulation wave midphase corresponding to the middle value of the amplitude absolute value of the three-phase modulation wave and a modulation wave minphase corresponding to the minimum value of the amplitude absolute value of the three-phase modulation wave;
the control method of the invention defines the modulation wave corresponding to the maximum value of the amplitude absolute value of the three-phase modulation wave as maxphase, the modulation wave corresponding to the middle value of the amplitude absolute value of the three-phase modulation wave as midphase and the modulation wave corresponding to the minimum value of the amplitude absolute value of the three-phase modulation wave as minphase by comparing the amplitude absolute values of the three-phase modulation wave; the specific judgment method of maxphase, midphase and minphase is as follows:
1) for maxphase, when abs (U)a)≥abs(Ub) And abs (U)a)≥abs(Uc) When, let maxphase ═ Ua(ii) a When abs (U)b)≥abs(Ua) And abs (U)b)≥abs(Uc) When, let maxphase ═ Ub(ii) a When abs (U)c)≥abs(Ua) And abs (U)c)≥abs(Ub) When, let maxphase ═ Uc；
2) For minphase, when abs (U)a)≤abs(Ub) And abs (U)a)≤abs(Uc) When the minphase is equal to Ua(ii) a When abs (U)b)≤abs(Ua) And abs (U)b)≤abs(Uc) When the minphase is equal to Ub(ii) a When abs (U)c)≤abs(Ua) And abs (U)c)≤abs(Ub) When the minphase is equal to Uc；
3) For midphase, when UaNot equal maxphase and UaWhen not equal to minphase, let midphase be Ua(ii) a When U is turnedbNot equal maxphase and UbWhen not equal to minphase, let midphase be Ub(ii) a When U is turnedcNot equal maxphase and UcWhen not equal to minphase, let midphase be Uc；
In the above judging method, Ua、Ub、UcThe modulated waves of the A phase, the B phase and the C phase are respectively represented, and abs represents an absolute value.
3. Detecting the product direction of the voltage and the current corresponding to the midpoint voltage oscillation value and the redundant small vector;
the control method needs to detect delta U × Uo×ioIn which Δ U is the oscillation value of the midpoint voltage, UoFor redundant small vector-corresponding voltages, ioDefining neuflag as delta U × Uo×ioDirection of (1), Δ U × Uo×ioThe direction detection method comprises the following steps:
for a three-level inverter, when (Δ U × U)o×io) When the value is more than or equal to 0, the neuflag is made to be 1, and when the value is delta U × Uo×io) If the sum is less than 0, making neuflag equal to-1;
for a three-level rectifier, when (Δ U × U)o×io) When the value is more than or equal to 0, the neuflag is made to be-1, and when the value is delta U × Uo×io) If < 0, let neuflag equal 1.
4. Calculating to obtain a zero sequence voltage Vneu；
Invention controlThe method calculates and obtains the zero sequence voltage V according to the values of maxphase, midamble and minphaseneuAnd the maxphase, the midamble and the minphase are modulation waves corresponding to the maximum value, the intermediate value and the minimum value of the absolute value of the amplitude of the three-phase modulation wave respectively.
Zero sequence voltage VneuThe calculation method of (2) is as follows:
when the oscillation value delta U of the midpoint voltage is within the limit value range, the zero sequence voltage V is enabledneu＝0；
When the oscillation value delta U of the midpoint voltage exceeds a limit value, the zero sequence voltage VneuThe method comprises the following specific steps:
when neuflag is 1 and maxphase is more than 0, zero sequence voltage VneuThe calculation method comprises the following steps:
1) for the cases where minphase > 0 and midphase > 0, let Vneu＝1-maxphase；
2) For the case of minphase > 0 and midphase < 0, if (1-maxphase) < (-midphase), let Vneu1-maxphase. If (1-maxphase) is not less than (-midamble), let Vneu＝-midphase；
3) For the case of minphase < 0, let V be if (1-maxphase) < (-minphase)neu1-maxphase. If (1-maxphase) is not less than (-minphase), let Vneu＝-minphase；
When neuflag is 1 and maxphase is less than or equal to 0, zero sequence voltage VneuThe calculation method comprises the following steps:
1) for the cases where minphase < 0 and midphase < 0, let Vneu＝-minphase；
2) For the case of minphase < 0 and midphase > 0, if (1-midphase) < (-minphase), let Vneu1-midamble. If (1-midamble) ≥ minus (midamble), let Vneu＝-minphase；
3) For the cases of minphase > 0 and midphase > 0, if (1-midphase) < (-maxphase), let Vneu1-midamble. If (1-midamble) ≥ maxphase, let Vneu＝-maxphase；
4) For the case of minphase > 0 and midphase < 0, if (1-minphase) < (-midphase), let V beneu1-minor. If (1-minor) ≥("middle), order Vneu＝-midphase；
When neuflag is-1 and maxphase is > 0, zero sequence voltage VneuThe calculation method comprises the following steps:
1) for the cases where minphase > 0 and midphase > 0, let Vneu＝-minphase；
2) For the case where the minphase is > 0 and the midphase is < 0, let V be equal to or greater than (-1-midphase)neu1-midamble. If (-1-midamble) < (-minphase), let Vneu＝-minphase；
3) For the cases where minphase < 0 and midphase < 0, let V be if (-1-midphase) ≥ maxphaseneu1-midamble. If (-1-midamble) < (-maxphase), let Vneu＝-maxphase；
4) For the case where the minphase is < 0 and the midphase is > 0, let V be equal to or greater than (-1-minphase) (-midphase)neuIs-1-minor. If (1-minphase) < (-midamble), let Vneu＝-midphase；
When neuflag is-1 and maxphase is less than 0, zero sequence voltage VneuThe calculation method comprises the following steps:
1) for the cases where minphase < 0 and midphase < 0, let Vneu＝-1-maxphase；
2) For the case where minphase < 0 and midphase > 0, let V be equal to or greater than (-1-maxphase)neu1-maxphase. If (-1-maxphase) < (-midamble), let Vneu＝-midphase；
3) For the case of minphase > 0, let V be equal to or greater than (-1-maxphase)neu1-maxphase. If (-1-maxphase) < (-minphase), let Vneu＝-minphase；
In the above calculation method, neuflag represents Δ U × Uo×ioIn which Δ U is the oscillation value of the midpoint voltage, UoAnd ioThe voltage and the current corresponding to the redundant small vector are respectively, and maxphase, midphase and minphase are respectively a modulation wave corresponding to the maximum value of the absolute value of the amplitude of the three-phase modulation wave, a modulation wave corresponding to the intermediate value and a modulation wave corresponding to the minimum value.
5. Zero sequence voltage VneuSuperposing to a three-phase modulation wave;
the control method of the invention is realized by converting the zero sequence voltage VneuSuperposing the three-phase modulated wave to control the neutral point voltage to restore balance again; zero sequence voltage VneuThe specific method of superimposing the three-phase modulated wave is as follows:
in the above formula, Ua、Ub、UcRespectively representing modulated waves of A, B and C phases, VneuIs zero sequence voltage, Ua *、Ub *、Uc *Respectively representing the modulation waves of the A phase, the B phase and the C phase after the zero sequence voltage is superposed.
Drawings
FIG. 1 is a main circuit topology diagram of a three-level NPC converter;
FIG. 2 is a voltage space vector diagram of a three-level converter;
FIG. 3 is a graph showing the amplitude variation of a three-phase modulated wave in one fundamental period;
fig. 4a and 4b are current paths of a three-level converter corresponding to the POO, wherein fig. 4a is a current path of a three-level rectifier corresponding to the POO, and fig. 4b is a current path of a three-level inverter corresponding to the POO;
fig. 5a and 5b are ONN corresponding three-level current transformer current paths; fig. 5a is a current path of a three-level rectifier corresponding to ONN, and fig. 5b is a current path of a three-level inverter corresponding to ONN;
FIG. 6 is a schematic diagram showing the influence of the mobile modulation wave on the action time of the redundant small vector in the phase angle range of 60 degrees to 120 degrees;
FIG. 7 is a flowchart illustrating an implementation of a method for controlling the midpoint voltage balance of a three-level converter based on carrier modulation according to the present invention;
FIG. 8 shows the DC-side voltage and output phase voltage and current variations under the DC-side upper end capacitor initial voltage of 50V and lower end capacitor initial voltage of 150V without the midpoint voltage balance control method;
fig. 9 shows the initial voltage of the upper end capacitor of 50V and the initial voltage of the lower end capacitor of 150V on the dc side of the embodiment, and the change of the voltage and current of the dc side and the output phase in the conventional carrier-based midpoint voltage balance control method;
fig. 10a and 10b show the initial voltage of the upper end capacitor of 50V and the initial voltage of the lower end capacitor of 150V on the dc side according to the embodiment of the present invention; fig. 10a shows the change of the voltage and output phase voltage and current on the dc side, and fig. 10b shows the phase voltage, modulation wave and carrier wave;
fig. 11 shows the initial voltage of the upper end capacitor 150V and the initial voltage of the lower end capacitor 50V on the dc side according to the embodiment of the present invention, and the voltage and current changes of the dc side and the output phase in the method for controlling the voltage balance at the midpoint.
Detailed Description
The invention is further described with reference to the following figures and detailed description.
Aiming at the condition that the three-level converter uses carrier modulation as a modulation strategy, when the oscillation value delta U of the midpoint voltage exceeds a limit value, the midpoint voltage balance control method of the three-level converter based on the carrier modulation firstly determines the voltage U corresponding to the small redundant vectoroAnd current ioThen comparing the absolute value of the amplitude of the three-phase modulation wave, defining the modulation wave corresponding to the maximum value of the absolute value of the amplitude of the three-phase modulation wave as maxphase, the modulation wave corresponding to the middle value of the absolute value of the amplitude of the three-phase modulation wave as midphase, and the modulation wave corresponding to the minimum value of the absolute value of the amplitude of the three-phase modulation wave as minphase, detecting delta U × Uo×ioAccording to the values of maxphase, midamble and minphase, the zero sequence voltage V is obtained by calculationneu(ii) a Zero sequence voltage VneuAnd superposing the three-phase modulated wave to control the neutral point voltage to restore balance again.
The invention relates to a carrier modulation-based neutral point voltage balance control method of a three-level converter, which comprises the following steps:
1. when the oscillation value delta U of the midpoint voltage exceeds a limit value, determining the voltage U corresponding to the redundant small vectoroAnd current io；
When the midpoint potential deviation exceeds a limit value, the control method firstly determines the voltage U corresponding to the redundant small vectoroAnd current io. In each space vector of the three-level converter shown in fig. 2, only the small vector and the medium vector generate the current flowing into/out of the midpoint, so that only the small vector and the medium vector affect the midpoint voltage balance of the three-level converter. The directions of the midpoint currents corresponding to the two redundant small vectors at the same phase angle are opposite, and the effects of the midpoint currents on the midpoint voltage are also opposite. Thus, the midpoint voltage rebalancing can be controlled by redistributing the action times of the two redundant small vectors.
Defining A-phase modulation wave as UaPhase A current is iaThe B-phase modulated wave is UbPhase B current is ibThe C-phase modulation wave is UcPhase C current is icAccording to FIG. 2, the redundant small vectors in different phase angle regions and their corresponding voltages U can be combinedoAnd current ioSummarized in table 2.
TABLE 2 redundant Small vectors in different phase Angle regions and their corresponding voltages UoAnd current io
Three-phase modulated wave Ua、Ub、UcThe amplitude variation within one fundamental period is as in fig. 3. Analysis of FIG. 3 reveals that:
1) for the phase angle region of 0-60 DEG, there is Ua＞0、Ub＜0、Uc＞0；
2) For the phase angle region of 60-120 DEG, U is arrangeda＞0、Ub＜0、Uc＜0；
3) For the phase angle region of 120-180 DEG, U is arrangeda＞0、Ub＞0、Uc＜0；
4) For the phase angle region of 180-240 DEG, U is arrangeda＜0、Ub＞0、Uc＜0；
5) For the phase angle region of 240-300 DEG, U is arrangeda＜0、Ub＞0、Uc＞0；
6) For the phase angle region of 300-360 degrees, there is Ua＜0、Ub＜0、Uc＞0。
The above conclusion, in combination with table 2, can obtain the voltage U corresponding to the redundant small vectoroAnd current ioThe specific determination method comprises the following steps:
1) when U is turneda＞0、Ub＜0、Uc< 0, or when Ua＜0、Ub＞0、UcWhen the voltage is more than 0, the voltage U corresponding to the redundant small vectoro＝UaCorresponding current io＝ia；
2) When U is turneda＞0、Ub＜0、Uc> 0, or when Ua＜0、Ub＞0、UcWhen less than 0, the voltage U corresponding to the redundant small vectoro＝UbCorresponding current io＝ib；
3) When U is turneda＞0、Ub＞0、Uc< 0, or when Ua＜0、Ub＜0、UcWhen the voltage is more than 0, the voltage U corresponding to the redundant small vectoro＝UcCorresponding current io＝ic。
2. Judging a modulation wave maxphase corresponding to the maximum value of the amplitude absolute value of the three-phase modulation wave, a modulation wave midphase corresponding to the middle value of the amplitude absolute value of the three-phase modulation wave and a modulation wave minphase corresponding to the minimum value of the amplitude absolute value of the three-phase modulation wave;
the control method of the invention compares the absolute value of the amplitude of the three-phase modulation wave, and defines the modulation wave corresponding to the maximum value as maxphase, the modulation wave corresponding to the intermediate value as midphase and the modulation wave corresponding to the minimum value as minphase. The maxphase, midphase and minphase values are further used to calculate the zero sequence voltage.
3. Detecting the product direction of the voltage and the current corresponding to the midpoint voltage oscillation value and the redundant small vector;
the control method needs to detect delta U × Uo×ioIn which Δ U is the oscillation value of the midpoint voltage, UoFor redundant small vector-corresponding voltages, ioDefining neuflag as delta U × Uo×ioThe neuflag determines the direction of the zero sequence voltage, and the concrete proofs are as follows:
the carrier modulation nature is the same as SVPWM, and SVPWM is two redundant small vectors with the same action time in the beginning vector and the ending vector of each sampling period. The action time of two redundant small vectors can be equivalently adjusted by superposing a specific zero sequence voltage on a three-phase modulation wave modulated by a carrier wave, so that the neutral point voltage balance is adjusted.
In order to determine the direction of zero sequence voltage used by carrier modulation when the midpoint voltage is unbalanced, firstly, the change principle of the action time of the redundant small vector in each phase angle region when the midpoint voltage is unbalanced is analyzed. Definition C represents the DC side capacitance, Udc1And Udc2The voltages of the upper end and the lower end of the DC side of the three-level converter respectively, and delta U is equal to Udc1-Udc2Is the midpoint voltage oscillation value. Taking the phase angle region of 60 degrees to 120 degrees as an example, the redundant small vectors are POO and ONN. Wherein, the current path of the three-level rectifier corresponding to the POO is shown in fig. 4a, and the current path of the three-level inverter corresponding to the POO is shown in fig. 4 b; ONN for a three-level rectifier, and ONN for a three-level inverter, are shown in fig. 5a and 5b, respectively.
Comparing fig. 4a and fig. 5a, for a three-level rectifier, when the small acting vector is POO, there is Udc1＝∫iadt/C, and when the small vector is ONN, there is Udc2＝∫iadt/C。
Comparing fig. 4b and fig. 5b, for a three-level inverter, when the small acting vector is POO, there is Udc1＝∫-iadt/C, and when the small vector is ONN, there is Udc2＝∫-iadt/C。
The following conclusions can be drawn:
1) the effect of the same small vector on the midpoint voltage is opposite for a three-level rectifier and a three-level inverter. Such as POO, when iaIf < 0, it will cause U of the three-level rectifierdc1Decrease, but cause U of the three-level inverterdc1And (4) rising. Therefore, under the same condition, the change directions of the action time of the redundant small vectors are opposite for the three-level rectifier and the three-level inverter.
2) Taking a three-level inverter as an example, the influence of the action time of the small adjustment redundancy vector on the midpoint voltage is analyzed. When Δ U > 0, U should be madedc2Rising to maintain midpoint voltage balance. At this time if iaGreater than 0, increasing POO action time can make Udc2Rising; if iaLess than 0, reduced POO action time can make Udc2And (4) rising. When Δ U is less than 0, U should be set to bedc1Rising to maintain midpoint voltage balance. At this time if i a0, increasing ONN action time can make Udc1Rising; if ia< 0, reducing ONN action time allows Udc1And (4) rising. The +/-definitions respectively represent the action time of the corresponding redundant small vectors which should be increased/decreased, the change principle of the action time of the two redundant small vectors in the other 60-degree phase angle region when the midpoint voltage is unbalanced is analyzed on the same principle, and the results are summarized in table 3.
TABLE 3 principle of variation of action time of each redundant small vector when the point voltage is unbalanced
The principle of the time variation of the redundant small vectors of the three-level rectifier is opposite to that of table 3.
On the basis of determining the principle of action time change of the two redundant small vectors, the influence of the mobile modulation wave on the action time of the redundant small vectors is further analyzed. Fig. 6 is a schematic diagram showing the influence of the mobile modulation wave on the action time of the redundant small vector in the phase angle region of 60 degrees to 120 degrees. As can be seen from fig. 6, shifting up the modulated wave increases the P-type small vector operating time and decreases the N-type small vector operating time; and the modulated wave is moved downwards to correspondingly increase the action time of the N-type small vector and reduce the action time of the P-type small vector.
The specific determination method of the zero sequence voltage direction of the three-level inverter can be obtained by combining the analysis with table 3:
1) for the phase angle regions of 0 degree to 60 degrees, 120 degrees to 180 degrees and 240 degrees to 300 degrees, when the angle is delta U × ioWhen the voltage is more than 0, the action time of the P-type small vector is decreased, the action time of the N-type small vector is increased, the corresponding modulation wave moves downwards, the direction of the zero sequence voltage is negative, and when the value is delta U × ioWhen the zero sequence voltage direction is less than 0, the action time of the N-type small vector is reduced, the action time of the P-type small vector is increased, the corresponding modulation wave moves upwards, and the zero sequence voltage direction is positive.
2) For the phase angle regions of 60 degrees to 120 degrees, 180 degrees to 240 degrees and 300 degrees to 360 degrees, when the angle is delta U × ioWhen the voltage is more than 0, the action time of the N-type small vector is reduced, the action time of the P-type small vector is increased, the corresponding modulation wave moves upwards, the direction of the zero sequence voltage is positive, and when the value is delta U × ioWhen the zero sequence voltage direction is less than 0, the action time of the P type small vector is reduced, the action time of the N type small vector is increased, the corresponding modulation wave moves downwards, and the zero sequence voltage direction is negative.
Phase angle regions of 0-60 degrees, 120-180 degrees and 240-300 degrees, corresponding to UoLess than 0; phase angle regions of 60 degrees to 120 degrees, 180 degrees to 240 degrees and 300 degrees to 360 degrees, corresponding to UoIs greater than 0, when the value is delta U × io×UoWhen the voltage is more than 0, the action time of the N-type small vector is reduced, the action time of the P-type small vector is increased, the corresponding modulation wave moves upwards, the direction of the zero sequence voltage is positive, and when the value is delta U × io×UoWhen the zero sequence voltage direction is less than 0, the action time of the P type small vector is reduced, the action time of the N type small vector is increased, the corresponding modulation wave moves downwards, and the zero sequence voltage direction is negative.
Defining neuflag as delta U × Uo×ioBased on the above analysis, it can be known that the value of neuflag directly determines the direction of the zero sequence voltage Δ U × Uo×ioThe specific detection method of (2) is as follows:
for a three-level inverter, when (Δ U × U)o×io) When the value is more than or equal to 0, the neuflag is made to be 1, the corresponding modulation wave moves upwards, the direction of the zero sequence voltage is positive, and when the value is equal to (delta U × U)o×io) When the zero sequence voltage is less than 0, the neuflag is made to be-1, the corresponding modulation wave moves downwards, and the direction of the zero sequence voltage isNegative;
for a three-level rectifier, when (Δ U × U)o×io) When the value is more than or equal to 0, the neuflag is made to be-1, the corresponding modulation wave is shifted downwards, the direction of the zero sequence voltage is negative, and when the value is (delta U × U)o×io) When the voltage is less than 0, the neuflag is made to be 1, the corresponding modulation wave moves upwards, and the direction of the zero sequence voltage is positive.
4. Calculating to obtain a zero sequence voltage Vneu；
The control method of the invention calculates and obtains the zero sequence voltage V further according to the values of maxphase, midphase and minphase on the basis of determining the direction of the zero sequence voltageneuThe amplitude absolute value of the three-phase modulation wave is a maximum value, a middle value and a minimum value of the amplitude absolute value of the three-phase modulation wave; obtaining the zero sequence voltage V by using maxphase, midamble and minphase calculationneuThe principle of (1) is to ensure that the polarity of the modulation wave is unchanged, i.e. to ensure the superposed zero sequence voltage VneuThe latter modulated wave will not have negative value in the positive half cycle of the original modulated wave or positive value in the negative half cycle of the original modulated wave. For this reason, the following two cases are classified:
firstly, when the modulation wave should be shifted upwards and the zero sequence voltage direction is positive, namely, when neuflag is 1;
when maxphase is greater than 0, zero sequence voltage VneuThe calculation method comprises the following steps:
1) when the minphase is greater than 0 and the midphase is greater than 0, it is ensured that maxphase does not exceed 1, so that V isneu＝1-maxphase；
2) When minphase > 0 and midphase < 0, if (1-maxphase) < (-midphase), then it should be ensured that maxphase does not exceed 1, so let Vneu1-maxphase. If (1-maxphase) is not less than (-midamble), then it should be ensured that midamble does not exceed 0, so that V isneu＝-midphase；
3) When minphase is less than 0, if (1-maxphase) < (-minphase), then maxphase should be guaranteed not to exceed 1, so as to let Vneu1-maxphase. If (1-maxphase) is not less than (-minphase), then it should be ensured that minphase does not exceed 0, let Vneu＝-minphase；
When maxphase is less than or equal to 0, zero sequence voltage VneuIs calculated byThe method comprises the following steps:
1) when the minphase is less than 0 and the midphase is less than 0, it is ensured that the minphase does not exceed 0, so that V isneu＝-minphase；
2) If (1-midamble) < (-minphase) when minphase < 0 and midamble > 0, then it is ensured that midamble does not exceed 1, so Vneu1-midamble. If (1-midamble) ≥ minus (minphase), then it should be ensured that minphase does not exceed 0, so Vneu＝-minphase；
3) If (1-midamble) < (-maxphase) when minphase > 0 and midamble > 0, then it should be ensured that midamble does not exceed 1, so that V isneu1-midamble. If (1-midamble) ≥ maxphase, then it should be ensured that maxphase does not exceed 0, so let Vneu＝-maxphase；
4) When the minphase is greater than 0 and the midphase is less than 0, if (1-minphase) < (-midphase), then it should be ensured that the minphase does not exceed 1, so that V isneu1-minor. If (1-minor) is not less than (-minor), then it should be ensured that the median does not exceed 0, so that V isneu＝-midphase。
Secondly, when the modulation wave is to be shifted upwards and the zero sequence voltage direction is positive, namely neuflag is-1;
when maxphase is greater than 0, zero sequence voltage VneuThe calculation method comprises the following steps:
1) when the minphase is greater than 0 and the midphase is greater than 0, it is ensured that the minphase is not less than 0, so Vneu＝-minphase；
2) When the minphase is greater than 0 and the midphase is less than 0, if (-1-midphase) is greater than or equal to (-minphase), then it should be ensured that the midphase is not less than-1, so let Vneu1-midamble. If (-1-midamble) < (-minphase), then it should be ensured that minphase is not less than 0, so let Vneu＝-minphase；
3) When the minphase is less than 0 and the midphase is less than 0, if (-1-midphase) is not less than (-maxphase), then it should be ensured that the midphase is not less than-1, so let Vneu1-midamble. If (-1-midamble) < (-maxphase), then it should be ensured that maxphase is not less than 0, so let Vneu＝-maxphase；
4) If (-1-minphase) when minphase < 0 and midphase > 0More than or equal to (-midamble), at which time it should be ensured that the minphase is not less than-1, so that V isneuIs-1-minor. If (1-minor) < (-minor), then it should be ensured that the median is not less than 0, so let Vneu＝-midphase；
When maxphase is less than 0, zero sequence voltage VneuThe calculation method comprises the following steps:
1) when the minphase is less than 0 and the midphase is less than 0, it is ensured that maxphase is not less than-1, so that Vneu＝-1-maxphase；
2) When minphase is less than 0 and midphase is greater than 0, if (-1-maxphase) is greater than or equal to (-midphase), then it should be ensured that maxphase is not less than-1, so let Vneu1-maxphase. If (-1-maxphase) < (-midamble), then it should be ensured that midamble is not less than 0, so let Vneu＝-midphase；
3) When the minphase is greater than 0, if (-1-maxphase) is greater than or equal to (-minphase), then it should be ensured that maxphase is not less than-1, so that V isneu1-maxphase. If (-1-maxphase) < (-minphase), then it should be ensured that minphase is not less than 0, so let Vneu＝-minphase；
In the above calculation method, neuflag represents Δ U × Uo×ioIn which Δ U is the oscillation value of the midpoint voltage, UoAnd ioThe voltage and the current corresponding to the redundant small vector are respectively.
5. Zero sequence voltage VneuSuperposing to a three-phase modulation wave;
the control method realizes the neutral point voltage balance control of the three-level converter based on the carrier modulation. On the basis of obtaining the zero sequence voltage by calculation, the control method controls the three-phase modulation wave to move up or down by superposing the zero sequence voltage on the three-phase modulation wave, thereby equivalently controlling the action time of the redundant small vector and realizing the balance control of the midpoint voltage of the three-level converter.
The implementation flow of the method for controlling the neutral point voltage balance of the three-level converter based on the carrier modulation is shown in fig. 7.
The neutral point voltage balance control method of the three-level converter based on the carrier modulation can directly calculate and obtain zero sequence voltage without using a PI (proportional integral) controller, and the used zero sequence voltage can not change the polarity of a modulation wave. The invention can effectively control the midpoint voltage balance of the three-level converter based on the carrier modulation, and has the advantages of convenient realization, simple calculation and high reliability.
The following examples are provided to illustrate the effects of the present invention.
According to the embodiment of the invention, a three-level NPC inverter model is built by means of PSIM software, and the effectiveness of the three-level converter midpoint voltage balance control method based on carrier modulation provided by the invention is verified by utilizing simulation. The simulation conditions were as follows:
carrier modulation is used as a modulation strategy of the three-level NPC inverter, the simulation step size is 4us, the fundamental frequency is 50Hz, the carrier frequency is 750Hz, the modulation ratio is 0.7, the output load is a resistor of 5 omega and is connected with an inductor of 10mH in series, the voltage of a direct current side is 200V, and the oscillation limit value of the midpoint voltage is set to be 10V.
Fig. 8 shows the dc side voltage and output phase voltage and current changes in the dc side voltage and output phase voltage and current without adding the midpoint voltage balance control method, according to the embodiment, with the dc side upper end capacitor initial voltage of 50V and the lower end capacitor initial voltage of 150V. As can be seen from fig. 8, when the three-level NPC inverter has the midpoint voltage imbalance problem, if the midpoint voltage balance control method is not used, the upper-end voltage and the lower-end voltage on the dc side will always have a deviation, which causes the switching devices of half bridge arms to bear a large voltage stress, thereby affecting the safe operation of the three-level NPC inverter.
Fig. 9 shows the initial voltage of the upper end capacitor 50V and the initial voltage of the lower end capacitor 150V on the dc side of the embodiment, and the change of the voltage and the current of the dc side and the output phase in the conventional carrier-based midpoint voltage balance control method. Analyzing fig. 9, under the effect of the traditional carrier-based midpoint voltage balance control method, the voltages at the upper end and the lower end on the dc side of the three-level NPC inverter gradually tend to be balanced. However, the traditional carrier-based midpoint voltage balance control method can cause the polarity of a modulation wave to change, so that the phase voltage outputs a negative level in a positive half cycle, the output current is seriously distorted, and the performance of the three-level NPC inverter is influenced.
Fig. 10a and 10b show the initial voltage of the upper end capacitor of 50V and the initial voltage of the lower end capacitor of 150V on the dc side according to the embodiment of the present invention; fig. 10a shows changes in the dc side voltage, the output phase voltage, and the current, and fig. 10b shows the phase voltage, the modulation wave, and the carrier wave. As can be seen from fig. 10a, the point voltage balance control method of the present invention can gradually reduce the deviation value between the dc-side upper voltage and the dc-side lower voltage to the limit value. In addition, under the action of the midpoint voltage balance control method, the polarity of the output phase voltage cannot change, the output phase voltage cannot output a negative level in the positive half cycle of the phase voltage, and the distortion of the output current is small in the midpoint voltage regulation process, so that the defects of the traditional midpoint voltage balance control method based on the carrier wave are overcome. Analyzing fig. 10b, the point voltage balance control method of the present invention is implemented based on carrier modulation, and directly obtains the PWM signal of each switching device according to the comparison result between the modulation wave and the carrier, so that the calculation is simple and easy to implement.
Fig. 11 shows the initial voltage of the upper end capacitor 150V and the initial voltage of the lower end capacitor 50V on the dc side according to the embodiment of the present invention, and the voltage and current changes of the dc side and the output phase in the method for controlling the voltage balance at the midpoint. Comparing fig. 10 and fig. 11, in the case that the initial voltage of the capacitor at the upper end of the dc side is greater than the initial voltage of the capacitor at the lower end or the initial voltage of the capacitor at the upper end of the dc side is less than the initial voltage of the capacitor at the lower end, the method for controlling the midpoint voltage balance in the present invention can gradually reduce the deviation value between the voltage at the upper end and the voltage at the lower end of the dc side to the limit value, thereby controlling the midpoint voltage to restore the balance again.
As shown in fig. 8 to fig. 11, the results of the embodiments verify the effectiveness of the method for controlling the midpoint voltage balance of the three-level converter based on carrier modulation according to the present invention. The invention can effectively control the neutral point voltage to restore balance again no matter whether the oscillation direction of the neutral point voltage is positive or negative, thereby improving the reliability of the three-level converter. The invention can directly calculate and obtain the zero sequence voltage without using a PI controller, and overcomes the defects of the traditional carrier-based midpoint voltage balance control method. Under the action of the invention, the polarity of the output phase voltage is not changed, and the current distortion is smaller in the middle point voltage balance adjustment process. In addition, the PWM signals of the switching devices are obtained directly according to the comparison result of the modulation wave and the carrier wave, the calculation is simple, and the realization is convenient.
Claims (6)
1. A three-level converter midpoint voltage balance control method based on carrier modulation is characterized in that aiming at the condition that the three-level converter uses the carrier modulation as a modulation strategy, when a midpoint voltage oscillation value delta U exceeds a limit value, a voltage U corresponding to a small redundant vector is firstly determinedoAnd current ioThen comparing the absolute values of the amplitudes of the three-phase modulation waves, defining the modulation wave corresponding to the maximum value of the absolute values of the amplitudes of the three-phase modulation waves as maxphase, the modulation wave corresponding to the middle value of the absolute values of the amplitudes of the three-phase modulation waves as midphase, and the modulation wave corresponding to the minimum value of the absolute values of the amplitudes of the three-phase modulation waves as minphase, and detecting the amplitude of the three-phase modulation waves by detecting delta U × Uo×ioAccording to the values of maxphase, midamble and minphase, the zero sequence voltage V is obtained by calculationneu(ii) a Zero sequence voltage VneuAnd superposing the three-phase modulated wave to control the neutral point voltage to restore balance.
2. The method as claimed in claim 1, wherein the voltage U corresponding to the small redundant vector is determinedoAnd current ioThe method comprises the following steps:
1) when U is turneda＞0、Ub＜0、Uc< 0, or when Ua＜0、Ub＞0、UcWhen the voltage is more than 0, the voltage U corresponding to the redundant small vectoro＝UaCorresponding current io＝ia；
2) When U is turneda＞0、Ub＜0、Uc> 0, or when Ua＜0、Ub＞0、UcWhen less than 0, the voltage U corresponding to the redundant small vectoro＝UbCorresponding current io＝ib；
3) When U is turneda＞0、Ub＞0、Uc< 0, or whenUa＜0、Ub＜0、UcWhen the voltage is more than 0, the voltage U corresponding to the redundant small vectoro＝UcCorresponding current io＝ic；
Wherein, UaIs a modulated wave of A phase, iaFor phase A current, UbModulated wave of B phase, ibFor phase B current, UcModulated wave of C phase, icThe phase C current.
3. The method for controlling the neutral-point voltage balance of a three-level converter based on carrier modulation according to claim 1, wherein the method for determining the modulation wave maxphase corresponding to the maximum absolute value of the amplitude of the three-phase modulation wave, the modulation wave midphase corresponding to the intermediate absolute value of the amplitude of the three-phase modulation wave, and the modulation wave minphase corresponding to the minimum absolute value of the amplitude of the three-phase modulation wave is as follows:
1) for the modulation wave maxphase corresponding to the maximum value of the amplitude absolute value of the three-phase modulation wave, as abs (U)a)≥abs(Ub) And abs (U)a)≥abs(Uc) When, let maxphase ═ Ua(ii) a When abs (U)b)≥abs(Ua) And abs (U)b)≥abs(Uc) When, let maxphase ═ Ub(ii) a When abs (U)c)≥abs(Ua) And abs (U)c)≥abs(Ub) When, let maxphase ═ Uc；
2) For a modulation wave minphase corresponding to the minimum value of the amplitude absolute values of the three-phase modulation wave, when abs (U)a)≤abs(Ub) And abs (U)a)≤abs(Uc) When the minphase is equal to Ua(ii) a When abs (U)b)≤abs(Ua) And abs (U)b)≤abs(Uc) When the minphase is equal to Ub(ii) a When abs (U)c)≤abs(Ua) And abs (U)c)≤abs(Ub) When the minphase is equal to Uc；
3) For modulation wave midphase corresponding to the intermediate value of the amplitude absolute value of the three-phase modulation wave, when U isaNot equal maxphase and UaWhen not equal to minphase, let midphase be Ua(ii) a When U is turnedbNot equal maxphase and Ub≠minphaWhen se, let midamble be Ub(ii) a When U is turnedcNot equal maxphase and UcWhen not equal to minphase, let midphase be Uc；
In the above judging method, Ua、Ub、UcThe modulated waves of the A phase, the B phase and the C phase are respectively represented, and abs represents an absolute value.
4. The method as claimed in claim 1, wherein neuflag is defined to represent Δ U × Uo×ioDetects Δ U × Uo×ioThe method of orientation is as follows:
for a three-level inverter, when (Δ U × U)o×io) When the value is more than or equal to 0, the neuflag is made to be 1, and when the value is delta U × Uo×io) If the sum is less than 0, making neuflag equal to-1;
for a three-level rectifier, when (Δ U × U)o×io) When the value is more than or equal to 0, the neuflag is made to be-1, and when the value is delta U × Uo×io) When the value is less than 0, the neuflag is made to be 1;
wherein, Delta U is the oscillation value of the midpoint voltage, UoFor redundant small vector-corresponding voltages, ioThe current corresponding to the redundant small vector is the neuflag of delta U × Uo×ioIn the direction of (a).
5. The carrier modulation-based neutral point voltage balance control method for three-level converter according to claim 1, wherein the zero sequence voltage V is zero sequence voltageneuThe calculation method of (2) is as follows:
when the oscillation value delta U of the midpoint voltage is within the limit value range, the zero sequence voltage V is enabledneu＝0；
When the oscillation value delta U of the midpoint voltage exceeds a limit value, the zero sequence voltage VneuThe method comprises the following specific steps:
when neuflag is 1 and maxphase is more than 0, zero sequence voltage VneuThe calculation method comprises the following steps:
1) for the cases where minphase > 0 and midphase > 0, let Vneu＝1-maxphase；
2) For minphase > 0 and midpCase of hase < 0, if (1-maxphase) < (-midamble), let Vneu1-maxphase. If (1-maxphase) is not less than (-midamble), let Vneu＝-midphase；
3) For the case of minphase < 0, let V be if (1-maxphase) < (-minphase)neu1-maxphase. If (1-maxphase) is not less than (-minphase), let Vneu＝-minphase；
When neuflag is 1 and maxphase is less than or equal to 0, zero sequence voltage VneuThe calculation method comprises the following steps:
1) for the cases where minphase < 0 and midphase < 0, let Vneu＝-minphase；
2) For the case of minphase < 0 and midphase > 0, if (1-midphase) < (-minphase), let Vneu1-midamble. If (1-midamble) ≥ minus (midamble), let Vneu＝-minphase；
3) For the cases of minphase > 0 and midphase > 0, if (1-midphase) < (-maxphase), let Vneu1-midamble. If (1-midamble) ≥ maxphase, let Vneu＝-maxphase；
4) For the case of minphase > 0 and midphase < 0, if (1-minphase) < (-midphase), let V beneu1-minor. If (1-minor) ≥ midamble, let Vneu＝-midphase；
When neuflag is-1 and maxphase is > 0, zero sequence voltage VneuThe calculation method comprises the following steps:
1) for the cases where minphase > 0 and midphase > 0, let Vneu＝-minphase；
2) For the case where the minphase is > 0 and the midphase is < 0, let V be equal to or greater than (-1-midphase)neu1-midamble. If (-1-midamble) < (-minphase), let Vneu＝-minphase；
3) For the cases where minphase < 0 and midphase < 0, let V be if (-1-midphase) ≥ maxphaseneu1-midamble. If (-1-midamble) < (-maxphase), let Vneu＝-maxphase；
4) For the case where the minphase is < 0 and the midphase is > 0, let V be equal to or greater than (-1-minphase) (-midphase)neuIs-1-minor. If (1-minphase) < (-mi)dphase), let Vneu＝-midphase；
When neuflag is-1 and maxphase is less than 0, zero sequence voltage VneuThe calculation method comprises the following steps:
1) for the cases where minphase < 0 and midphase < 0, let Vneu＝-1-maxphase；
2) For the case where minphase < 0 and midphase > 0, let V be equal to or greater than (-1-maxphase)neu1-maxphase. If (-1-maxphase) < (-midamble), let Vneu＝-midphase；
3) For the case of minphase > 0, let V be equal to or greater than (-1-maxphase)neu1-maxphase. If (-1-maxphase) < (-minphase), let Vneu＝-minphase；
In the above calculation method, neuflag represents Δ U × Uo×ioIn which Δ U is the oscillation value of the midpoint voltage, UoAnd ioThe voltage and the current corresponding to the redundant small vector are respectively, and maxphase, midphase and minphase are respectively a modulation wave corresponding to the maximum value of the absolute value of the amplitude of the three-phase modulation wave, a modulation wave corresponding to the intermediate value and a modulation wave corresponding to the minimum value.
6. The carrier modulation based neutral point voltage balance control method of three-level converter according to claim 1, wherein the control method is to apply a zero sequence voltage VneuSuperposing the three-phase modulated wave to control the neutral point voltage to restore balance again, and the specific method comprises the following steps:
in the above formula, Ua、Ub、UcRespectively representing modulated waves of A, B and C phases, VneuIs zero sequence voltage, Ua *、Ub *、Uc *Respectively representing the modulation waves of the A phase, the B phase and the C phase after the zero sequence voltage is superposed.
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