CN111082688B - A carrier-inverting stacked PWM midpoint potential balance control method - Google Patents

Abstract

A carrier reverse phase laminated PWM midpoint potential balance control method. Aiming at the condition that a three-level converter uses carrier inversion lamination PWM as a modulation strategy, when the midpoint potential deviation exceeds a limit value, firstly determining a middle vector used by the carrier inversion lamination PWM in a current phase angle region; then detecting the midpoint potential deviation value and the modulation wave and midpoint current corresponding to the medium vector, judging the product direction of the three, and sending the product direction to a PI controller to obtain a medium vector time adjustment factor delta Uneu; and controlling the action time of the medium vector by superposing the medium vector time adjustment factor delta Uneu on a modulation wave corresponding to the medium vector, thereby controlling the neutral point potential balance under the action of carrier inversion lamination PWM. The control method can control the midpoint potential deviation under the action of the carrier reverse phase laminated PWM method within a limited range, and improves the reliability of the three-level converter.

Description

A carrier-inverting stacked PWM midpoint potential balance control method

technical field

The invention relates to a midpoint potential balance control method, in particular to a carrier inversion stacked PWM midpoint potential balance control method.

Background technique

A typical three-level converter is a three-level neutral point clamp (Neutral Point Clamped, NPC) converter, and its main circuit topology is shown in Figure 1. Compared with the traditional two-level converter, the three-level NPC converter can output three different level states by controlling the turn-on and turn-off of each switching device of the three-phase bridge arm. Compared with the cascaded H-bridge multi-level topology, it has the advantages of simple circuit structure and easy back-to-back operation. Based on the above advantages, three-level NPC converters are currently widely used in speed regulation occasions of medium and high voltage and high power motors.

The common mode voltage is the zero-sequence voltage with high frequency and high amplitude between the neutral point of the load and the reference potential point. Literature "A Voltage Source Inverter Model Prediction Method for Common Mode Voltage Suppression Based on Optimal Voltage Vector Selection" (Guo Leilei, Jin Nan, Shen Yongpeng [J]. Journal of Electrotechnical Technology, 2018, 33(6): 1347-2355. ) pointed out that when the three-level converter is used in the speed regulation of medium-high-voltage and high-power motors, the common-mode voltage will have an adverse effect on the bearing life and insulation performance of the traction motor and cause electromagnetic interference. In order to prolong the life of the traction motor, it is necessary to try to reduce the common mode voltage of the three-level converter.

Carrier inversion stacked PWM is a modulation strategy based on carrier comparison. It compares two triangular carriers with the same frequency, the same phase, and the same amplitude and opposite amplitude with the three-phase modulation wave to generate the corresponding PWM control signal. This controls the turn-on or turn-off of the power switching devices of each bridge arm of the three-level converter. The principle of carrier inversion stacked PWM is simple and easy to implement. The schematic diagram of its specific principle is shown in Figure 2.

The document "Pulse width modulation for power converters: principles and practice" (Holmes D.G, Lipo T A[M]. New York: IEEE Press, 2003.) pointed out that compared with traditional SPWM and SVPWM, the carrier inversion stacking PWM can convert the three-level The common-mode voltage value of the converter is reduced from one-third of the DC voltage value to one-sixth. Therefore, the carrier inversion stacked PWM is more in line with the requirements of the three-level converter to reduce the common mode voltage.

In addition to reducing the common-mode voltage, the three-level converter also needs to pay attention to maintaining the balance of the mid-point potential, that is, to maintain the deviation between the upper and lower voltages of the DC side of the three-level converter within a limited range. The unbalanced mid-point potential will generate low-order harmonics in the output voltage and cause the power devices of a half-side bridge arm of the three-level converter to withstand excessive voltage, which will endanger the safety of operation. Therefore, measures must be taken to ensure the three-level converter. The neutral point potential of the converter is balanced.

Define the DC side voltage of the three-level converter as 2E, the three level states of its output from high to low are P, O, N respectively, and the corresponding output voltages are E, 0, -E respectively, then the three power The individual voltage space vectors of the flat-converter are summarized in Figure 3. Each voltage space vector in FIG. 3 can be classified into zero vector, P-type small vector, N-type small vector, medium vector and large vector according to its magnitude and corresponding level state. Among them, the P-type small vector and the N-type small vector at the same phase angle are redundant small vectors to each other. Table 1 summarizes the specific classification of each voltage space vector of the three-level converter.

Table 1 Three-level converter voltage space vector types

The existing midpoint potential balance control method mainly relies on the principle that the P-type small vector and the N-type small vector at the same phase angle correspond to the opposite direction of the midpoint current. Then increase the redundant small vector action time that is beneficial to the balance of the neutral point potential, so as to control the upper and lower voltages of the DC side to restore the balance. The literature "Hybrid three-level midpoint potential balance control strategy" (Zhou Jinghua, [J]. Chinese Journal of Electrical Engineering, 2013, 33(24): 82-89.) introduces the principle of this method in detail.

Fig. 4 is the three-phase voltage waveform of the carrier inversion stacked PWM under the carrier ratio of 12. From the analysis, it can be seen that the equivalent voltage space vector sequence of carrier inversion stacked PWM in the phase angle range of 0 degrees to 60 degrees is PNP→ONP→ONO→OOO→ONO→ONP→PNP, PNP→PNO→ONO→OOO→ONO →PNO→PNN, which only uses the small vector ONO in each carrier cycle, and does not use the corresponding redundant small vector POP. Since the carrier-inverting stacked PWM uses only one small vector in each carrier cycle, it cannot use the existing method of redistributing the action time of redundant small vectors to control the midpoint potential balance.

Carrier inversion stacked PWM can effectively reduce the common-mode voltage of the three-level converter, but it cannot use the existing method of redistributing redundant small vector action time to control the midpoint potential balance. In order to better apply the carrier inversion stacked PWM in the three-level converter, it is necessary to design a mid-point potential balance control method suitable for the carrier inversion stacked PWM.

SUMMARY OF THE INVENTION

In order to overcome the disadvantage that the carrier anti-phase stacked PWM cannot use the existing method of redistributing redundant small vector action time to control the midpoint potential balance, the present invention proposes a carrier reversed stacked PWM midpoint potential balance control method. The invention can control the mid-point potential deviation under the action of the carrier inversion stacked PWM within a limited range, thereby improving the reliability of the three-level converter when the carrier inversion stacked PWM is used as a modulation strategy.

The carrier anti-phase stacked PWM mid-point potential balance control method of the present invention is aimed at the case where the three-level converter uses the carrier anti-phase stacked PWM as the modulation strategy. When the mid-point potential deviation exceeds the limit value, first determine the carrier in the current phase angle area. Invert the mid-vector used by the stacked PWM; then detect the mid-point potential deviation value and the modulation wave and mid-point current corresponding to the mid-vector, determine the product direction of the three, and send it to the PI controller to obtain the mid-vector time adjustment factor ΔUneu; The mid-vector time adjustment factor ΔUneu is superimposed on the modulation wave corresponding to the mid-vector to control the action time of the mid-vector, so as to control the mid-point potential balance under the action of the carrier anti-phase stacked PWM.

When the mid-point potential deviation exceeds the limit value, the carrier anti-phase stacked PWM mid-point potential balance control method of the present invention is specifically as follows:

1. Determine the neutral vector used by the carrier inversion stacked PWM in the current phase angle area;

In the control method of the present invention, when the mid-point potential deviation exceeds the limit value, firstly determine the mid-vector used by the carrier inversion stacked PWM in the current phase angle area; define the three level states of the three-level converter output from high to low respectively For P, O, N, in different phase angle regions, the neutral vectors used by the carrier inversion stacked PWM are as follows:

1) For the phase angle region of 30 degrees to 90 degrees, the neutral vector used by the carrier inversion stacked PWM is PNO;

2) For the phase angle area of 90 degrees to 150 degrees, the neutral vector used by the carrier inversion stacked PWM is PON;

3) For the phase angle region of 150 degrees to 210 degrees, the neutral vector used by the carrier inversion stacked PWM is OPN;

4) For the phase angle region of 210 degrees to 270 degrees, the neutral vector used by the carrier inversion stacked PWM is NPO;

5) For the phase angle region of 270 degrees to 330 degrees, the neutral vector used by the carrier inversion stacked PWM is NOP;

6) For the phase angle region of 330 degrees to 30 degrees, the neutral vector used by the carrier inversion stacked PWM is ONP.

2. Detect the modulation wave and midpoint current corresponding to the vector in the detection;

The control method of the present invention judges the product direction of the three by detecting the midpoint potential deviation value and the modulation wave and midpoint current corresponding to the midpoint vector, and sends it to the PI controller to obtain the midpoint vector time adjustment factor ΔUneu; the modulation corresponding to the midpoint vector is defined. The wave and mid-point current are U _x and i _x respectively. The modulation wave and mid-point current detection method corresponding to the mid-vector is as follows:

1) When the vector is PNO or NPO, U _x =U _mc , i _x = _ic ;

2) When the vector is PON or NOP, U _x =U _mb , i _x =i _b ;

3) When the vector is OPN or ONP, there are U _x =U _ma , i _{x =} ia ;

In the above detection method, U _ma , U _mb , and U _mc represent the modulated waves of the A-phase, B-phase, and _C -phase, respectively, and i _a , _ib , and ic are the currents of the A-phase, B-phase, and C-phase, respectively. The three-phase modulated waves U _ma , U _mb and U _mc are defined as follows:

In the definition of U _ma , U _mb and U _mc , U _a , U _b , and U _c represent the sine waves of A-phase, B-phase, and C-phase, respectively, and U ₀ is a zero-sequence component. U _a , U _b , U _c and U ₀ are defined as follows:

In the definition of U _a , U _b , U _c and U ₀ , M is the per-unit modulated wave amplitude, ω _b is the angular frequency, and U _max and U _min represent the maximum value of U _a , U _b , and U _c respectively. value and minimum value.

3. Detect the midpoint potential deviation value;

The control method of the present invention determines the product direction of the three by detecting the midpoint potential deviation value and the modulation wave corresponding to the midpoint vector and the midpoint current, and sends it to the PI controller to obtain the midpoint vector time adjustment factor ΔUneu; wherein, the midpoint potential deviation The value is detected as follows:

ΔU=U _dc1 -U _dc2

In the above formula, ΔU represents the midpoint potential deviation value, U _dc1 is the upper voltage of the DC side of the three-level converter, and U _dc2 is the lower voltage of the DC side of the three-level converter.

4. Add the mid-vector time adjustment factor ΔUneu to the modulation wave corresponding to the mid-vector;

The control method of the invention controls the action time of the mid-vector by superimposing the mid-vector time adjustment factor ΔUneu on the modulation wave corresponding to the mid-vector, so as to control the balance of the mid-point potential under the action of the carrier anti-phase stacked PWM; the mid-vector time adjustment factor The specific way of superimposing ΔUneu to the mid-vector corresponding to the modulated wave is as follows:

1) For the phase angle regions of 30 degrees to 90 degrees and 210 degrees to 270 degrees, there are U _ma =U _ma , U _mb =U _mb , U _mc =U _mc +ΔUneu;

2) For the phase angle regions of 90 degrees to 150 degrees and 270 degrees to 330 degrees, there are U _ma =U _ma , U _mb =U _mb +ΔUneu, U _mc =U _mc ;

3) For the phase angle regions of 150 degrees to 210 degrees and 330 degrees to 30 degrees, there are U _ma =U _ma +ΔUneu, U _mb =U _mb , U _mc =U _mc ;

In the above manner, U _ma , U _mb , and U _mc represent the modulated waves of the A-phase, the B-phase, and the C-phase, respectively, and ΔUneu is the time adjustment factor of the medium vector.

5. The range of the modulation wave corresponding to the mid-vector should be limited to -1 to 1;

The control method of the present invention controls the action time of the mid-vector by superimposing the mid-vector time adjustment factor ΔUneu on the modulation wave corresponding to the mid-vector, so as to control the mid-point potential balance under the action of the carrier anti-phase stacked PWM; wherein, the superimposed mid-vector time The range of the modulated wave corresponding to the neutral vector after adjusting the factor ΔUneu should be limited to -1 to 1. The specific limiting method is as follows:

1) For 30 degrees to 90 degrees, 210 degrees to 270 degrees phase angle area, there are:

2) For 90 degrees to 150 degrees, 270 degrees to 330 degrees phase angle area, there are:

3) For 150 degrees to 210 degrees, 330 degrees to 30 degrees phase angle area, there are:

In the above limiting method, U _ma , U _mb , and U _mc represent the modulated waves of the A-phase, the B-phase, and the C-phase, respectively.

6. When the midpoint potential deviation value of the three-level converter is within the limited range, set the time adjustment factor of the mid-vector ΔUneu=0;

The control method of the present invention is only put into use when the midpoint potential deviation exceeds the limit value. When the midpoint potential deviation value of the three-level converter is within a limited range, ΔUneu=0 is set, and the modulation wave is not adjusted. ΔUneu is the mid-vector time adjustment factor.

Description of drawings

Fig. 1 topology diagram of three-level NPC converter;

Figure 2 Schematic diagram of carrier inversion stacked PWM;

Figure 3. The voltage space vector diagram of the three-level converter;

Figure 4. The three-phase voltage under the action of the carrier anti-phase stacked PWM in the phase angle range of 0 degrees to 60 degrees under the carrier ratio of 12;

Figure 5 is a schematic diagram of the influence of the upward or downward movement of the neutral vector corresponding to the modulation wave used by the carrier inversion stacked PWM on the action time of the neutral vector in the area of the phase angle of 90 degrees to 120 degrees;

Fig. 6 is a schematic diagram of the influence of the upward or downward movement of the neutral vector corresponding to the modulation wave used by the carrier anti-phase stacked PWM on the action time of the neutral vector in the area of the phase angle of 120 degrees to 150 degrees;

FIG. 7 is a concrete schematic diagram of a carrier anti-phase stacked PWM midpoint potential balance control method of the present invention;

FIG. 8 shows the variation of the upper and lower voltages of the DC side without adding the neutral point potential balance protection measure in the embodiment with a fundamental frequency of 50 Hz and a fixed modulation ratio of 0.8;

FIG. 9 shows the results of the embodiment under the action of the mid-point potential balance control method of the present invention with a fundamental frequency of 50 Hz and a fixed modulation ratio of 0.8; wherein, FIG. 9 a shows the changes of the upper and lower voltages of the DC side, and FIG. 9 b shows the The comparison between the midpoint potential deviation value and the limit value;

Figure 10 shows the variation of the upper and lower voltages of the DC side without adding the neutral-point potential balance protection measure, with the fundamental frequency of 50 Hz, the modulation ratio being 0.1 to 1, and the cyclic variation of the embodiment;

Fig. 11 is the result of the embodiment under the action of the mid-point potential balance control method of the present invention with the fundamental frequency of 50 Hz and the modulation ratio varying from 0.1 to 1; Fig. 11a is the variation of the upper and lower voltages of the DC side, Fig. 11b is the comparison of the midpoint potential deviation value and the limit value; Fig. 11c is the modulation ratio of cyclic variation.

Detailed ways

The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

The carrier anti-phase stacked PWM mid-point potential balance control method of the present invention is aimed at the case where the three-level converter uses the carrier anti-phase stacked PWM as the modulation strategy. When the mid-point potential deviation exceeds the limit value, first determine the carrier in the current phase angle area. Invert the mid-vector used by the stacked PWM; then detect the mid-point potential deviation value and the modulation wave and mid-point current corresponding to the mid-vector, determine the product direction of the three, and send it to the PI controller to obtain the mid-vector time adjustment factor ΔUneu; The mid-vector time adjustment factor ΔUneu is superimposed on the modulation wave corresponding to the mid-vector to control the action time of the mid-vector, so as to control the mid-point potential balance under the action of the carrier anti-phase stacked PWM.

The carrier anti-phase stacked PWM midpoint potential balance control method of the present invention is specifically as follows:

First, judge whether the mid-point potential deviation exceeds the limit value, if so, execute the following steps, if not, set the mid-vector time adjustment factor ΔUneu=0.

When the midpoint potential deviation exceeds the limit value, the execution steps of the control method of the present invention are as follows:

1. Determine the neutral vector used by the carrier inversion stacked PWM in the current phase angle area;

In the control method of the present invention, when the mid-point potential deviation exceeds the limit value, firstly, the mid-vector used by the carrier inversion stacked PWM in the current phase angle region is determined. The reasons for determining the vector in use for carrier-inverting cascaded PWM are as follows:

If the product of the current flowing into the midpoint and the time is not equal to the product of the current flowing out of the midpoint and the time in a sampling period, it will cause the charging voltage and discharging voltage of the upper and lower capacitors on the DC side to be unequal, resulting in the difference in the midpoint potential. balance. Only the small vector and the middle vector will generate the midpoint current, that is, only the small vector and the middle vector will affect the midpoint potential, so the balance of the midpoint potential can be controlled by adjusting the action time of the small vector or the middle vector.

It can be seen from Figure 4 that the equivalent voltage space vector sequence of the carrier inversion stacked PWM in one sampling period is PNP→ONP→ONO→OOO. Suppose the sampling period is T, and the action times of PNP, ONP, ONO and OOO in one sampling period are T _L , T _M , T _S and T _Z respectively, and it can be known from the volt-second equivalent principle:

In formula (1), m represents the modulation ratio, θ represents the phase angle, T is the sampling period, T _L is the action time of the large vector PNP, T _M is the action time of the medium vector ONP, and T _S is the action time of the small vector ONO , T _Z is the action time of zero vector OOO. From the analysis of formula (1), it can be known that with the increase of the modulation ratio m, the action time of the medium vector and the large vector will increase, and the action time of the zero vector and the small vector will decrease.

The modulation ratio is proportional to the fundamental amplitude of the output line voltage of the three-level converter, so the midpoint current will gradually increase with the increase of the modulation ratio. Based on the above conclusions, it can be seen that for the carrier inversion stacked PWM, the influence of the medium vector on the midpoint potential is greater than that of the small vector. Therefore, it is possible to control the neutral point potential balance under the action of the carrier reversed-phase stacked PWM by controlling the action time of the neutral vector corresponding to the carrier reversed-phase stacked PWM.

In different phase angle regions, the mid-vectors used by the carrier inversion stacked PWM are as follows:

1) For the phase angle region of 0 degrees to 30 degrees, the carrier inversion stacked PWM equivalent voltage space vector sequence is PNP→ONP→ONO→OOO, and the corresponding neutral vector used is ONP;

2) For the phase angle region of 30 degrees to 60 degrees, the sequence of the equivalent voltage space vector of the carrier inversion stacked PWM is PNP→PNO→ONO→OOO, and the corresponding neutral vector used is PNO;

3) For the phase angle region of 60 degrees to 90 degrees, the sequence of the carrier inversion stacked PWM equivalent voltage space vector sequence is PNN→PNO→POO→OOO, and the corresponding neutral vector used is PNO;

4) For the phase angle area of 90 degrees to 120 degrees, the sequence of the equivalent voltage space vector of the carrier inversion stacked PWM is PNN→PON→POO→OOO, and the corresponding neutral vector used is PON;

5) For the phase angle region of 120 degrees to 150 degrees, the sequence of the equivalent voltage space vector of the carrier inversion stacked PWM is PPN→PON→OON→OOO, and the corresponding neutral vector used is PON;

6) For the phase angle area of 150 degrees to 180 degrees, the sequence of the equivalent voltage space vector of the carrier inversion stacked PWM is PPN→OPN→OON→OOO, and the corresponding neutral vector used is OPN;

7) For the phase angle region of 180 degrees to 210 degrees, the carrier inversion stacked PWM equivalent voltage space vector sequence is NPN→OPN→OPO→OOO, and the corresponding neutral vector used is OPN;

8) For the phase angle region of 210 degrees to 240 degrees, the sequence of the equivalent voltage space vector of the carrier inversion stacked PWM is NPN→NPO→OPO→OOO, and the corresponding neutral vector used is NPO;

9) For the phase angle region of 240 degrees to 270 degrees, the sequence of the equivalent voltage space vector of the carrier inversion stacked PWM is NPP→NPO→NOO→OOO, and the corresponding neutral vector used is NPO;

10) For the phase angle region of 270 degrees to 300 degrees, the carrier inversion stacked PWM equivalent voltage space vector sequence is NPP→NOP→NOO→OOO, and the corresponding neutral vector used is NOP;

11) For the phase angle region of 300 degrees to 330 degrees, the carrier inversion stacked PWM equivalent voltage space vector sequence is NNP→NOP→OOP→OOO, and the corresponding neutral vector used is NOP;

12) For the phase angle region of 330 degrees to 360 degrees, the sequence of the equivalent voltage space vector of the carrier inversion stacked PWM is NNP→ONP→OOP→OOO, and the corresponding neutral vector used is ONP.

2. Detect the modulation wave and midpoint current corresponding to the vector in the detection;

The control method of the present invention judges the product direction of the three by detecting the midpoint potential deviation value and the modulation wave corresponding to the midpoint vector and the midpoint current, and sends it to the PI controller to obtain the mid-vector time adjustment factor ΔUneu. The modulation wave and the midpoint current corresponding to the vector in the definition are U _x and i _x respectively, and the detection method of the modulation wave and the midpoint current corresponding to the vector in the control method of the present invention is as follows:

1) For the phase angle region of 30 degrees to 90 degrees, the neutral vector used by the carrier anti-phase stacked PWM is PNO, and the inflow and outflow midpoints correspond to the _C -phase, with U _x =U _mc , i _x =ic ;

2) For the phase angle region of 90 degrees to 150 degrees, the neutral vector used by the carrier anti-phase stacked PWM is PON, and the inflow and outflow midpoints correspond to the B-phase, with U _x =U _mb , i _x =i _b ;

3) For the 150 degree to 210 degree phase angle region, the mid-vector used by the carrier inversion stacked PWM is OPN, and the inflow and outflow midpoints correspond to the A-phase, with U _x =U _ma , i _{x =} ia ;

4) For the phase angle region of 210 degrees to 270 degrees, the neutral vector used by the carrier anti-phase stacked PWM is NPO, and the inflow and outflow midpoints correspond to the _C phase, with U _x =U _mc , i _x =ic ;

5) For the phase angle region of 270 degrees to 330 degrees, the neutral vector used by the carrier anti-phase stacked PWM is NOP, and the inflow and outflow midpoints correspond to the B phase, with U _x =U _mb , i _x =i _b ;

6) For the phase angle region of 330 degrees to 30 degrees, the neutral vector used by the carrier inversion stacked PWM is ONP, and the inflow and outflow midpoints correspond to the A phase, with U _x =U _ma , i _{x =} ia .

In the above detection method, U _ma , U _mb , and U _mc represent the modulated waves of the A-phase, B-phase, and _C -phase, respectively, and i _a , _ib , and ic are the currents of the A-phase, B-phase, and C-phase, respectively. In order to improve the utilization rate of DC voltage under the action of carrier anti-phase stacked PWM, the three-phase modulation waves U _ma , U _mb and U _mc of carrier anti-phase stacked PWM are superimposed by three-phase sine waves U _a , U _b , U _c with a specific zero sequence. The component U ₀ is obtained, which is specifically defined as follows:

In the definitions of U _ma , U _mb and U _mc , U _a , U _b , and U _c represent the sine waves of A-phase, B-phase, and C-phase, respectively, U ₀ is the zero-sequence component, and M is the per-unit modulation amplitude value, ω _b is the angular frequency, and U _max and U _min represent the maximum and minimum values of U _a , U _b , and U _c , respectively.

3. Detect the midpoint potential deviation value;

The control method of the present invention judges the product direction of the three by detecting the midpoint potential deviation value and the modulation wave corresponding to the midpoint vector and the midpoint current, and sends it to the PI controller to obtain the mid-vector time adjustment factor ΔUneu. On the basis of detecting the modulated wave and the midpoint current corresponding to the vector in the carrier-reversed stacked PWM equivalent medium, it is necessary to further detect the midpoint potential deviation value. The specific detection method is as follows:

ΔU=U _dc1 -U _dc2 (3)

In the above formula, ΔU represents the midpoint potential deviation value, U _dc1 is the upper voltage of the DC side of the three-level converter, and U _dc2 is the lower voltage of the DC side of the three-level converter.

4. Add the mid-vector time adjustment factor ΔUneu to the modulation wave corresponding to the mid-vector;

The control method of the present invention controls the action time of the mid-vector by superimposing the mid-vector time adjustment factor ΔUneu on the modulating wave corresponding to the mid-vector, so as to control the mid-point potential balance under the action of the carrier inverse-phase stacked PWM. The principle of using the mid-vector time adjustment factor ΔUneu to control the balance of the mid-point potential is as follows:

For the phase angle region of 30 degrees to 90 degrees, the mid-vector used by the carrier-inverting stacked PWM is PNO, and the corresponding mid-point current is the _C -phase current ic . When the midpoint potential deviation value ΔU<0, in order to restore the midpoint potential balance, the upper end voltage U _dc1 on the DC side of the three-level converter should be increased. At this time, if _ic > 0, increasing the action time of PNO can make U _dc1 rise; if _ic <0, reducing the action time of PNO can make U _dc1 rise; when the mid-point potential deviation value ΔU > 0, the mid-point potential is restored Balance, should make U _dc1 drop. At this time, if _ic > 0, reducing PNO action time can make U _dc1 decrease; if _ic <0, increasing PNO action time can make U _dc1 decrease;

For the phase angle region of 90 degrees to 150 degrees, the mid-vector used by the carrier-inverting stacked PWM is PON, and the corresponding mid-point current is the B-phase current _ib . When the midpoint potential deviation value ΔU<0, in order to restore the midpoint potential balance, the upper end voltage U _dc1 on the DC side of the three-level converter should be increased. At this time, if i _b > 0, increasing the PON action time can make U _dc1 rise; if i _b <0, reducing the PON action time can make U _dc1 rise; when the mid-point potential deviation value ΔU > 0, the mid-point potential is restored Balance, should make U _dc1 drop. At this time, if i _b >0, reducing the PON action time can make U _dc1 decrease; if i _b <0, increasing the PON action time can make U _dc1 decrease;

For the phase angle region of 150 degrees to 210 degrees, the mid-vector used by the carrier-inverting stacked PWM is OPN, and the corresponding mid-point current is the A-phase current i _a . When the midpoint potential deviation value ΔU<0, in order to restore the midpoint potential balance, the upper end voltage U _dc1 on the DC side of the three-level converter should be increased. At this time, if i _a >0, increasing the OPN action time can make U _dc1 rise; if i _a <0, reducing the OPN action time can make U _dc1 rise; when the midpoint potential deviation value ΔU>0, the midpoint potential is restored Balance, should make U _dc1 drop. At this time, if i _a >0, reducing OPN action time can make U _dc1 decrease; if i _a <0, increasing OPN action time can make U _dc1 decrease;

For the phase angle region of 210 degrees to 270 degrees, the neutral vector used by the carrier inversion stacked PWM is NPO, and the corresponding mid-point current is the _C -phase current ic . When the midpoint potential deviation value ΔU<0, in order to restore the midpoint potential balance, the upper end voltage U _dc1 on the DC side of the three-level converter should be increased. At this time, if _ic >0, increasing the NPO action time can make U _dc1 rise; if _ic <0, reducing the NPO action time can make U _dc1 rise; when the mid-point potential deviation value ΔU>0, the mid-point potential is restored Balance, should make U _dc1 drop. At this time, if _ic > 0, reducing NPO action time can make U _dc1 decrease; if _ic <0, increasing NPO action time can make U _dc1 decrease;

For the phase angle region of 270 degrees to 330 degrees, the mid-vector used by the carrier-inverting stacked PWM is NOP, and the corresponding mid-point current is the B-phase current _ib . When the midpoint potential deviation value ΔU<0, in order to restore the midpoint potential balance, the upper end voltage U _dc1 on the DC side of the three-level converter should be increased. At this time, if i _b > 0, increasing the NOP action time can make U _dc1 rise; if i _b <0, reducing the NOP action time can make U _dc1 rise; when the mid-point potential deviation value ΔU > 0, the mid-point potential is restored Balance, should make U _dc1 drop. At this time, if i _b >0, reducing the NOP action time can make U _dc1 decrease; if i _b <0, increasing the NOP action time can make U _dc1 decrease;

For the phase angle region of 330 degrees to 30 degrees, the mid-vector used by the carrier-inverting stacked PWM is ONP, and the corresponding mid-point current is the A-phase current i _a . When the midpoint potential deviation value ΔU<0, in order to restore the midpoint potential balance, the upper end voltage U _dc1 on the DC side of the three-level converter should be increased. At this time, if i _a >0, increasing ONP action time can make U _dc1 rise; if i _a <0, reducing ONP action time can make U _dc1 rise; when the midpoint potential deviation value ΔU>0, the midpoint potential is restored Balance, should make U _dc1 drop. At this time, if i _a >0, decreasing ONP action time can make U _dc1 decrease; if i _a <0, increasing ONP action time can make U _dc1 decrease.

Summarizing the above analysis, it can be concluded that:

For any phase angle region, when the product of the mid-point potential deviation value ΔU and the mid-point current i _x is less than zero, increasing the action time of the neutral vector corresponding to the carrier anti-phase stacked PWM is more conducive to the restoration of the mid-point potential balance; and when ΔU and i _x When the product of , is greater than zero, reducing the action time of the mid-vector corresponding to the carrier-inverted stacked PWM is more conducive to restoring the balance of the mid-point potential.

Further analyze the effect of moving the mid-vector corresponding modulation wave up or down on the action time of the mid-vector. Taking the area of phase angle from 90 degrees to 120 degrees as an example, the neutral vector used by the carrier inversion stacked PWM is _PON . The schematic diagram of the effect of the action time is shown in Figure 5: moving the modulated wave up will lead to an increase in the action time of the medium vector, and moving the modulated wave down will cause the action time of the medium vector to decrease. Taking the area of phase angle from 120 degrees to 150 degrees as an example, the neutral vector used by the carrier anti-phase stacked PWM is PON, and the neutral vector corresponds to the modulated wave U _mb >0. The schematic diagram of the effect of the action time is shown in Figure 6: moving the modulated wave up will lead to an increase in the action time of the medium vector, and moving the modulated wave down will lead to a decrease in the action time of the medium vector.

The analysis of the same idea shows that for the phase angle regions of 60 degrees to 120 degrees, 180 degrees to 240 degrees, and 300 degrees to 360 degrees, the value of the modulated wave corresponding to the median vector is less than zero. At this time, moving the modulated wave upward will cause the median vector The increase of the action time, the downward movement will lead to the decrease of the action time of the medium vector; for the phase angle regions of 0° to 60°, 120° to 180°, and 240° to 300°, the value of the modulated wave corresponding to the medium vector is greater than zero. When the modulating wave is moved upward, the action time of the middle vector will decrease, and moving down will cause the action time of the middle vector to increase.

Adding a positive value to the modulation wave corresponding to the mid-vector will cause the modulation wave to move up, and superimposing a negative value will cause the modulation wave to move down. Combining all the above analysis, the following conclusions can be drawn:

For any phase angle region, when the product of the mid-point potential deviation value ΔU, the mid-point current i _x , and the modulating wave U _mx corresponding to the mid-point vector is less than zero, it is more beneficial to move the modulating wave corresponding to the vector in the carrier inverse stacked PWM down. When the product of the mid-point potential deviation value ΔU, the mid-point current i _x , and the modulation wave U _mx corresponding to the mid-point vector is greater than zero, it is more conducive to the mid- The point potential returns to equilibrium.

Based on the above conclusions, the product direction of the mid-point potential deviation value, the modulation wave corresponding to the mid-point vector, and the mid-point current is sent to the PI controller to obtain the mid-vector time adjustment factor ΔUneu, and the mid-vector time adjustment factor ΔUneu is superimposed on the mid-vector corresponding to The modulating wave comes up to control the action time of the mid-vector, and the mid-point potential balance can be controlled under the action of the carrier anti-phase stacked PWM.

5. The range of the modulation wave corresponding to the mid-vector should be limited to -1 to 1;

The control method of the invention controls the action time of the mid-vector by superimposing the mid-vector time adjustment factor ΔUneu on the modulating wave corresponding to the mid-vector, so as to control the mid-point potential balance under the action of the carrier anti-phase laminated PWM; the carrier anti-phase laminated PWM is based on The three-phase modulated wave is compared with the triangular carrier to obtain the PWM signal of each power device. Since the value of the triangular carrier ranges from -1 to 1, in order to prevent overmodulation, the range of the modulated wave corresponding to the mid-vector after superimposing the mid-vector time adjustment factor ΔUneu should be Limited to -1 to 1.

6. When the midpoint potential deviation value of the three-level converter is within the limited range, set the time adjustment factor of the mid-vector ΔUneu=0;

The control method of the present invention is only put into use when the midpoint potential deviation exceeds the limit value. When the mid-point potential deviation value of the three-level converter is within a limited range, in order to improve the harmonic performance, the mid-vector time adjustment factor ΔUneu=0 is set, and the modulation wave is not adjusted.

The specific principle of the carrier anti-phase stacked PWM midpoint potential balance control method of the present invention is shown in FIG. 7 .

The invention overcomes the defect that the carrier anti-phase stacked PWM cannot use the existing method of redistributing redundant small vector action time to control the midpoint potential balance. A mid-point potential balance control method suitable for carrier inversion stacked PWM. The control method of the invention can control the mid-point potential deviation under the action of the carrier anti-phase stacked PWM within a limited range, thereby improving the reliability of the three-level converter when the carrier anti-phase stacked PWM is used as a modulation strategy.

The implementation effect of the present invention will be described below in conjunction with the embodiments.

The embodiment of the present invention builds a three-level converter model by means of PSIM software, and uses simulation to verify the effectiveness of a carrier-inverting stacked PWM midpoint potential balance control method proposed by the present invention. The simulation conditions are as follows:

The modulation strategy used by the three-level converter is carrier inversion stacked PWM, the simulation step size is 10us, the sampling frequency is 1200Hz, the carrier frequency is 750Hz, the three-phase output side is connected to a 1Ω resistor in series with a 5mH inductor, and the DC side voltage is 500V, where , the initial value of the upper end voltage of the DC side is 400V, the initial value of the lower end voltage is 100V, and the limit value of the midpoint potential deviation is set to 30V.

FIG. 8 shows the variation of the upper and lower voltages of the DC side when the fundamental frequency of the embodiment is 50 Hz, the modulation ratio is fixed at 0.8, and the neutral point potential balance protection measure is not added. It can be seen from Fig. 8 that when the three-level converter uses the carrier inversion stacked PWM as the modulation strategy, when the mid-point potential imbalance problem occurs under a fixed modulation ratio, if the mid-point potential balance control method is not added, the mid-point potential will not be balanced. The balance problem will always exist, which will adversely affect the safe and reliable operation of the three-level converter.

FIG. 9 shows the results of the embodiment under the action of the mid-point potential balance control method of the present invention with a fundamental frequency of 50 Hz and a fixed modulation ratio of 0.8; wherein, FIG. 9 a shows the changes of the upper and lower voltages of the DC side, and FIG. 9 b shows the The comparison of the midpoint potential deviation value and the limit value. The results of the embodiment shown in FIG. 9 show that when the three-level converter uses the carrier inversion stacked PWM as the modulation strategy, when the problem of midpoint potential imbalance occurs under a fixed modulation ratio, the use of the midpoint potential balance control method of the present invention can make The deviation value of the upper and lower voltages on the DC bus is reduced to within the limit value, so that the neutral point potential is restored to balance.

FIG. 10 shows the variation of the upper and lower voltages of the DC side without adding the neutral point potential balance protection measure in the embodiment with a fundamental frequency of 50 Hz and a cyclic change of the modulation ratio from 0.1 to 1. It can be seen from Figure 10 that when the three-level converter uses the carrier inversion stacked PWM as the modulation strategy, when the midpoint potential imbalance problem occurs under the cyclically changing modulation ratio, if the midpoint potential balance control method is not added, the midpoint potential balance control method The potential unbalance problem will always exist, which will adversely affect the safe and reliable operation of the three-level converter.

Fig. 11 is the result of the embodiment under the action of the mid-point potential balance control method of the present invention with the fundamental frequency of 50 Hz and the modulation ratio varying from 0.1 to 1; Fig. 11a is the variation of the upper and lower voltages of the DC side, Fig. 11b is the comparison of the midpoint potential deviation value and the limit value; Fig. 11c is the modulation ratio of cyclic variation. The results of the embodiment shown in Fig. 11 show that when the three-level converter uses the carrier inversion stacking PWM as the modulation strategy, when the mid-point potential imbalance problem occurs under the cyclically changing modulation ratio, the mid-point potential balance control method of the present invention is used, The deviation value of the upper and lower voltages of the DC bus can be reduced to within the limit value, so that the neutral point potential can be restored to balance.

As shown in FIG. 8 to FIG. 11 , the results of the embodiments verify the effectiveness of a method for controlling the midpoint potential balance of the carrier-inverting stacked PWM of the present invention. In view of the case where the three-level converter uses the carrier inversion stacked PWM as the modulation strategy, the present invention can effectively control the midpoint potential to restore the balance regardless of whether the modulation ratio changes. The invention overcomes the defect that the carrier anti-phase stacked PWM cannot use the existing method of redistributing redundant small vector action time to control the mid-point potential balance, and can control the mid-point potential deviation under the action of the carrier anti-phase stacked PWM within a limited range , thereby improving the reliability of the three-level converter when the carrier inversion stacked PWM is used as the modulation strategy.

Claims (8)

1. A carrier reverse phase laminated PWM midpoint potential balance control method is characterized in that: aiming at the condition that a three-level converter uses carrier inversion lamination PWM as a modulation strategy, when the midpoint potential deviation exceeds a limit value, firstly determining a middle vector used by the carrier inversion lamination PWM in a current phase angle region; then detecting the midpoint potential deviation value delta U and the modulation wave U corresponding to the medium vector_xMidpoint current i corresponding to the middle vector_xModulated wave U corresponding to vector in calculation_xMidpoint current i corresponding to the middle vector_xThe product of the intermediate point potential deviation value delta U and the intermediate point potential deviation value delta U is obtained, the direction of the product of the three is judged, the direction of the product is input into a PI controller, and the output of the PI controller is the intermediate vector time adjustment factor delta Uneu; and controlling the action time of the medium vector by superposing the medium vector time adjustment factor delta Uneu on a modulation wave corresponding to the medium vector, thereby controlling the neutral point potential balance under the action of carrier inversion lamination PWM.

2. The carrier inversion lamination PWM midpoint potential balance control method according to claim 1, wherein: when the midpoint potential deviation exceeds a limit value, firstly determining a middle vector used by carrier inversion lamination PWM in a current phase angle area; defining three level states of the three-level converter from high to low output to be P, O, N respectively, in different phase angle regions, the medium vectors used by the carrier phase inversion lamination PWM are respectively as follows:

1) for a phase angle region of 30 degrees to 90 degrees, the medium vector used by the carrier inversion lamination PWM is PNO;

2) for a phase angle region of 90 degrees to 150 degrees, a middle vector used by the carrier inversion lamination PWM is PON;

3) for the phase angle region of 150 degrees to 210 degrees, the middle vector used by the carrier phase inversion lamination PWM is OPN;

4) for a phase angle region from 210 degrees to 270 degrees, the middle vector used by the carrier inversion lamination PWM is NPO;

5) for a phase angle region of 270 degrees to 330 degrees, the middle vector used by the carrier inversion lamination PWM is NOP;

6) for the 330 degree to 30 degree phase angle region, the medium vector used by the carrier inversion stacked PWM is ONP.

3. The carrier inversion lamination PWM midpoint potential balance control method according to claim 1, wherein: the method for detecting the modulation wave and the midpoint current corresponding to the medium vector comprises the following steps:

1) when the middle vector is PNO or NPO, there is U_x＝U_mc，i_x＝i_c；

2) When the middle vector is PON or NOP, there is U_x＝U_mb，i_x＝i_b；

3) When the middle vector is OPN or ONP, there is U_x＝U_ma，i_x＝i_a；

In the above detection method, U_ma、U_mb、U_mcModulated waves i representing A, B and C phases, respectively_a、i_b、i_cThe currents of the A phase, the B phase and the C phase are respectively.

4. The carrier inversion lamination PWM midpoint potential balance control method according to claim 3, wherein: modulated wave U_ma、U_mbAnd U_mcThe definition is as follows:

to U_ma、U_mbAnd U_mcIn definition, U_a、U_b、U_cRepresents sine waves of A phase, B phase and C phase, U₀Is a zero sequence component; u shape_a、U_b、U_cAnd U₀Is defined as follows:

to U_a、U_b、U_cAnd U₀In the definition, M is a labelAmplitude, omega, of the modulated wave after unitary_bIs angular frequency, U_maxAnd U_minRespectively represents U_a、U_b、U_cMaximum and minimum values of (a).

5. The carrier inversion lamination PWM midpoint potential balance control method according to claim 3, wherein: the detection method of the midpoint potential deviation value comprises the following steps:

ΔU＝U_dc1-U_dc2

in the above formula, Δ U represents a midpoint potential deviation value, U_dc1For the upper voltage, U, of the DC side of the three-level converter_dc2The voltage is the lower end voltage of the direct current side of the three-level converter.

6. The carrier inversion lamination PWM midpoint potential balance control method according to claim 1, wherein: the control method controls the action time of the medium vector by superposing the medium vector time adjustment factor delta Uneu to the modulation wave corresponding to the medium vector, thereby controlling the neutral point potential balance under the action of carrier reverse phase laminated PWM; the specific way of superimposing the medium vector time adjustment factor Δ unneu to the medium vector corresponding modulation wave is as follows:

1) for the phase angle regions of 30 degrees to 90 degrees, 210 degrees to 270 degrees, there is U_ma＝U_ma，U_mb＝U_mb，U_mc＝U_mc+ΔUneu；

2) For phase angle regions of 90 degrees to 150 degrees, 270 degrees to 330 degrees, there is U_ma＝U_ma，U_mb＝U_mb+ΔUneu，U_mc＝U_mc；

3) For the phase angle regions of 150 degrees to 210 degrees and 330 degrees to 30 degrees, there is U_ma＝U_ma+ΔUneu，U_mb＝U_mb，U_mc＝U_mc；

In the above mode, U_ma、U_mb、U_mcThe modulation waves respectively represent A phase, B phase and C phase, and the delta Uneu is a medium vector time adjustment factor.

7. The carrier inversion lamination PWM midpoint potential balance control method according to claim 6, wherein: the range of the modulation wave corresponding to the intermediate vector after the intermediate vector time adjustment factor Δ unneu is defined as-1 to 1, and the specific defining method is as follows:

1) for the 30 degree to 90 degree, 210 degree to 270 degree phase angle regions, there are:

2) for the phase angle regions of 90 degrees to 150 degrees, 270 degrees to 330 degrees, there are:

3) for the phase angle region of 150 degrees to 210 degrees, 330 degrees to 30 degrees, there are:

in the above limiting method, U_ma、U_mb、U_mcThe modulated waves of the A phase, B phase and C phase are represented respectively.

8. The carrier inversion lamination PWM midpoint potential balance control method according to claim 1, wherein: the control method is only put into use when the midpoint potential deviation exceeds a limit value, when the midpoint potential deviation value of the three-level converter is within a limit range, delta Uneu is made to be 0, and the modulation wave is not adjusted; Δ unneu is the medium vector time adjustment factor.

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