WO2015010225A1

WO2015010225A1 - Control method for neutral-point potential balance on dc side in npc three-level structure

Info

Publication number: WO2015010225A1
Application number: PCT/CN2013/001464
Authority: WO
Inventors: 徐西昌; 陈桥梁; 倪嘉
Original assignee: 西安龙腾新能源科技发展有限公司
Priority date: 2013-07-24
Filing date: 2013-11-29
Publication date: 2015-01-29
Also published as: CN103414366A; CN103414366B

Abstract

A control method for neutral-point potential balance on the DC side in an NPC three-level structure, comprising: taking the size of a difference value between half of a bus voltage on the DC side of a photovoltaic inverter and a neutral-point voltage; acquiring, via a PI regulator, the value of a current which needs to be injected into a neutral point within a current switching cycle; and calculating an allocation ratio of a redundant small vector within one switching cycle using the action time of the redundant small vector of each sector in spatial vector pulse width modulation as a modulation means. The control method eliminates a DC bias and a low frequency fluctuation of neutral-point potential on the DC side of a diode-clamped three-level inverter circuit under an existing hardware condition, thereby realizing neutral-point balance.

Description

Control method for DC side midpoint potential balance in NPC three-level structure

The invention relates to a control method for a midpoint potential fluctuation of a DC side of a diode clamp type (NPC) three-level inverter circuit, and particularly relates to a control method for a DC side midpoint potential balance in an NPC three-level structure.

Background technique

Many PV grid-connected inverters use a midpoint clamp type three-level inverter circuit, and the input side uses capacitor voltage division, which can add a level to the inverter output, so that the PWM waveform of the inverter output is more traditional. The sinusoidal waveform of the voltage of the three-phase grid is closer. Since the current at the midpoint of the DC side flows in and out, it is easy to cause an imbalance in the midpoint potential, which has a very adverse effect on the performance and safety of the inverter. Therefore, the midpoint potential imbalance must be eliminated by appropriate control. At present, software control algorithms are widely studied. Some control the midpoint potential within a certain range by controlling the midpoint hysteresis. This method is simple and easy, but does not completely eliminate the low frequency fluctuation of the midpoint potential. It has also been shown that the midpoint current can be balanced by injecting zero-sequence voltage to achieve a midpoint potential balance. This method can completely eliminate the low-frequency fluctuation of the midpoint potential and achieve better control effects, but injecting zero sequence. The voltage control algorithm is relatively complex and not easy to apply in engineering, and the zero-sequence voltage calculation is inaccurate near the zero-crossing point of each phase modulation voltage. .

Summary of the invention

The object of the present invention is to provide a control method for the DC side midpoint potential balance in an NPC three-level structure, which can eliminate the DC offset and low frequency fluctuation of the diode clamp type three-level midpoint potential under the existing hardware conditions. To achieve a midpoint balance, the control algorithm can be made simple and accurate.

In order to solve the above technical problems, the technical solution adopted by the present invention:

The control method of the DC side midpoint potential balance in the NPC three-level structure takes the difference between the DC side bus voltage of the PV inverter and the midpoint voltage, and the current switching period needs to be injected into the midpoint through the PI regulator. The current value is used as a modulation means by the action time of the redundant small vector of each sector in the space vector pulse width modulation (SVPWM), and the distribution ratio of the redundant small vector in one switching period is quantitatively calculated, so that the midpoint current is maintained. Point potential balance is given; if there is no DC offset at the midpoint potential, it is only necessary to control the midpoint current to 0 in 1⁄2 switching cycle to maintain the midpoint potential balance and eliminate the low frequency of the three-level midpoint potential. fluctuation.

The above-mentioned NPC three-level structure control method for the DC side midpoint potential balance,

The specific steps for determining the space vector pulse width modulation (SVPWM) for each sector are as follows: When the PV inverter is running, the control chip obtains the three-phase inductor current 4, , and through the sampling circuit, and calculates the action time of the small vector and the medium vector respectively as t, and the current corresponding to the midpoint when they act. They are 4, 4 and ^ respectively.

(1) If the reference voltage vector is in the small triangle 1 (modulation degree ^« ≤ 0.5), if / _a i _Vl + 4 > 0, then select

^: as the adjustment redundancy small vector, otherwise as the adjustment redundancy small vector;

(2) If the reference voltage vector is in the small triangle 3 (modulation degree 0.5 <1), if

^-/, ^ _τ >ο, then select as the adjustment redundancy small vector, otherwise select as the adjustment redundancy small vector.

In the small triangle 1 zone: say ^; as the voltage vector sequence to adjust the redundant small vector - book

0 just -> ΟΟΝ -> 000-> POO ->000 ->OON - ONN

Jtl Jt2 J to Ju J to Jt2 |tl

-ia ic ia ic -ia

As a voltage vector sequence for adjusting redundant small vectors:

OON -> OOO -> POO-> PPO -> POO ->000 -> OON

Jt2 tO Jtl t2 J l J tO |t2

Ic ia -ic la ίς

Compared with the prior art, the beneficial effects of the invention:

The invention can eliminate the DC offset and low frequency fluctuation of the NPC type three-level midpoint potential without changing the existing hardware, finally realize the midpoint balance, improve the output current quality, and the control algorithm is simple and easy, There is a case of inaccurate calculation; the proposed redundant small vector selection criterion fully exploits the adjustment ability of the neutral point potential of the redundant small vector when the power factor is not 1, and maximizes the balance of the midpoint 3⁄4 position. .

DRAWINGS

1 is a circuit topology structure used in the present invention;

Figure 2 is a three-level inverter output vector space diagram;

Figure 3 is the first sector of the three-level inverter output vector space diagram;

Figure 4 shows the three-phase inductor current waveform (power factor of 1) corresponding to the vector space map of the three-level SVPWM from the first sector to the sixth sector.

Figure 5 is the simulation waveform of the three-phase voltage modulation wave and the redundant small vector distribution coefficient t with the small vector action time as the criterion when the inductor current leads the reference voltage by 20°. Instruction manual

Figure 6 is the simulation waveform of the three-phase voltage modulated wave and the redundant small vector partition coefficient with the small vector injected charge as the criterion when the inductor current leads the reference voltage by 20°;

Figure 7 shows the simulation waveform of the three-phase voltage modulated wave and the redundant small vector partition coefficient A with the small vector action time as the criterion when w = 0.8 and the inductor current leads the reference voltage by 20°.

Figure 8 is the simulation waveform of the three-phase voltage modulated wave and the redundant small vector partition coefficient A when the inductor current exceeds the reference voltage by 20°.

Figure 9 is a simulation waveform of the modulation wave, inductor current and midpoint voltage of the photovoltaic inverter switched to the control method of the present invention at 0.1 s. detailed description

The invention will be described in detail below with reference to the drawings and specific embodiments.

Referring to Figure 1, the front stage of the photovoltaic grid-connected inverter is a two-way parallel boost circuit, and the latter stage is a diode midpoint clamp type three-level inverter circuit.

Referring to Figure 2, when the NPC three-level circuit is operating, each bridge arm has three operating states: P, 0, N, if there are a total of 3 ³ = 27 operating states in the three-phase system. In the coordinate system, each working state corresponds to a voltage vector, and all 27 voltage vectors are put together to form a spatial voltage vector distribution map of a three-level circuit.

Referring to Fig. 3, the three-level inverter circuit usually adopts the SVPWM control algorithm, and the combination of the level states that the inverter can output is represented by a vector diagram, and the duty ratio of each phase modulated wave is obtained by a calculation method. The figure shows the SVPWM in the first sector under the classical sector division method. According to the recent three vector principle and the central symmetric seven-segment SVPWM modulation principle, the characteristics of the output voltage distortion of the multilevel circuit can be fully utilized while making The switching losses of the circuit are minimized.

Referring to FIG. 4, the vector space map of the three-level SVPWM has a three-phase inductor current waveform corresponding to the first sector to the sixth sector (the power factor is 1). In the interval (in the first sector), the action time corresponding to the vector is decreasing, and the absolute value of the current contributed to the midpoint | _a | is decreasing, the action time corresponding to the vector is increasing, and the current contributing to the midpoint is absolute. The value is also increasing. Therefore, it is inevitable to find a redundant small vector capable of injecting a charge to the midpoint at each switching cycle, thereby achieving a balance of the midpoint potential. If the phase difference between the inductor current and the reference voltage is limited to within ±30°, the midpoint current constant caused by the redundant small vector in the same sector can be guaranteed, and the absolute value of the midpoint current caused by the medium vector is the smallest. It is conducive to the simplification of the control algorithm and achieve better control effects.

Referring to Figure 5, the system simulation model is built. When / « == 0.4, the inductor current leads the reference voltage by 20°, the small vector action time is used as the criterion, and the three-phase electric pressure modulation wave appears to be hopping. The residual small vector partition coefficient A is also limited to the limit value ±1.

Referring to Figure 6, in the case of « = 0.4, the inductor current leads the reference voltage by 20°, if a small vector is used to inject more charge Less criterion is that it can eliminate the jump caused by the adjustment of the three-phase voltage modulation wave from t to the limit, and still has the ability to keep the midpoint current still zero, and reduce the harmonic distortion rate of the output current.

Referring to Figure 7, in the case of / « = 0.8, the inductor current leads the reference voltage by 20°, the small vector action time is used as the criterion, the three-phase voltage modulation wave appears to be hopping, and the redundant small vector distribution coefficient ^: It is limited to a limit value of ±1.

Referring to Figure 8, when w = 0.8, the inductor current leads the reference voltage by 20°, if the small vector injection charge is used as a criterion, the jump of the three-phase voltage modulation wave caused by the limit can be eliminated. The point current remains at zero, reducing the harmonic distortion of the output current.

Referring to Figure 9, the reference voltage - inductor current - midpoint voltage simulation waveform of the present invention is applied at 0.1 s, the DC bus voltage is 650 V, and the midpoint balance voltage is 325 V.

Embodiment - Referring to Figure 1, after the photovoltaic inverter is connected, the control chip will obtain the three-phase inductor current through the sampling circuit, and calculate the effect of the small vector ^;, and the medium vector that can affect the midpoint potential. The times are respectively, ^ and, when they act, the currents corresponding to the midpoints are ^, 4 and . You can do the following, see Figure 3:

If _W ≤ 0.5, the reference voltage is in the first sector small triangle 1:

(1) If ^^ + >0, select as the adjustment redundancy small vector, and calculate the distribution coefficient of redundant small vector *= ⁷ ^ ^Λ2 according to the current required at the midpoint;

(2) t _Vl +l _£ t _Vz ≤ , then choose as the adjustment redundancy small vector, and calculate the distribution coefficient ⁴ ^H_l V ^a l' of the redundant small vector according to the current required at the midpoint. If 0.5 < _W < 1, the reference voltage is within the first sector small triangle 3:

(1) If ^^ + ₂ -厶,>0, select as the adjustment redundancy small vector, and calculate the distribution coefficient of the redundant small vector according to the current required at the midpoint.

(2) If /^^^ + z- » 1⁄2 ₇ ≤ 0, select ^; as the adjustment redundancy small vector, calculate the distribution coefficient of the redundant small vector according to the current / of the midpoint = small triangle 2 and There is only one pair of redundant small vectors in the 4 area, and there is no selection problem.

The judgment basis and distribution coefficient t of the combination of the A and C triangle regions of each sector are as follows:

1. According to the above method, the judgment method of the switching mode of the A and C triangle regions of the first sector is as follows Bright

Table 1 Criterion of vector synthesis method for A and C triangle regions in the first sector

The distribution coefficient of each area ^ is as follows:

Table 2 Smaller vector allocation ratio of the first sector

2. According to the above method, the judgment method of the switching mode of the Α and C triangle regions of the second sector is as follows: Table 3 Criterion of the vector synthesis mode of the A and C triangle regions in the second sector Description

The distribution coefficient of each region ^: as follows:

Table 4 Small-area allocation ratio of the second sector

3. According to the above method, the judgment method of the A and C triangle region switching synthesis mode of the III sector is as follows: Table 5 A and C triangle region switching vector synthesis mode criterion in the third sector Description

The distribution coefficient of each area ^ is as follows:

Table 6 Small-area allocation ratio of the third sector

4. According to the above method, the judgment method of the A and C triangle region switching synthesis mode of the IV sector is as follows: Table 7 A and C triangle region switching vector synthesis mode criterion in the IV sector Description

The distribution coefficient t of each region is as follows - Table 8 The smaller vector allocation ratio of the iv sector

5. According to the above method, the judgment method of the A and C triangle region switching synthesis mode in the V zone is as follows: Table 9 A and C triangle region switching vector synthesis mode criterion in the sector Description

Distribution coefficient of each region A: as follows:

Table 10 V-sector small vector allocation ratio

6. According to the above method, the judgment method of the A and C triangle region switching synthesis mode of the first sector is as follows: Table 11 A and C triangle region switching vector synthesis mode criterion in the VI sector Triangle region synthesis mode switching criterion

A1

A2

C1

The distribution coefficient A of each area of C2 is as follows:

Table 12 Small VI distribution ratio of the sixth sector

Book

Claims

claims

1. The control method of the DC side midpoint potential balance in the NPC three-level structure is characterized by: taking the difference between half of the DC side bus voltage of the photovoltaic inverter and the midpoint voltage, and obtaining the current switching voltage through the PI regulator The current value that needs to be injected into the midpoint of the cycle, using the action time of the redundant small vectors in each sector in the space vector pulse width modulation as the modulation method, quantitatively calculates the distribution ratio of the redundant small vectors in a switching cycle, so that the midpoint The current follows the given setting to maintain the balance of the midpoint potential; if there is no DC bias in the midpoint potential, it only needs to control the midpoint current to 0 in each switching cycle to maintain the balance of the midpoint potential and eliminate the three-level Low-frequency fluctuations in point potential.

2. The control method for DC side midpoint potential balance in the NPC three-level structure according to claim 1, characterized in that: the specific steps for judging each sector of the space vector pulse width modulation are as follows:

When the photovoltaic inverter is running, the control chip will obtain the three-phase inductor current, , , through the sampling circuit, and calculate the action time of the small vector, and the middle vector as , and ^ ₇ respectively. The current corresponding to the midpoint when they act 4, 4 and ^ respectively;

(1) If the reference voltage vector is within the small triangle A, the modulation degree w ≤ 0.5, if / _a i + 4 i _y2 >0, then it is selected as the small redundant vector for adjustment, otherwise it is selected as the small vector for redundant adjustment;

(2) If the reference voltage vector is within the small triangle C, the modulation degree is 0.5< _W <1, if / _a ^ + 4 - / > 0, then select ^: as the adjustment redundant small vector, otherwise select as the adjustment redundant small vector vector;

In area A of the small triangle:

Take as the voltage vector sequence for adjusting redundant small vectors: onu -> OON -> OOO->PO0 ->ooo ->OON ->OH

Iti Jt2 to J i to |t2 tl

-ia ic ί ic -ia

Take as the voltage vector sequence for adjusting redundant small vectors:

OON ->000 -> POO-> PPO ->POO ->000 ->OON

|t2 «J ^tl [ t2 |U tO Jt2

3. The control method for DC side midpoint potential balance in the NPC three-level structure according to claim 2, characterized in that: according to the above method, the basis for judging the switching synthesis mode of the A and C triangle areas of the I sector is as follows : Table 1 Criteria for switching vector synthesis methods of triangle areas A and C in sector I Claims

The allocation coefficient A of each area is as follows - Table 2 Small vector allocation ratio of sector I

4. The control method for the DC side midpoint potential balance in the NPC three-level structure according to claim 2, characterized in that: according to the above method, the judgment basis of the A and 'C triangle area switching synthesis mode of the II sector As follows: Table 3 Criteria for switching vector synthesis methods of triangle areas A and C in sector Π Claims

The distribution coefficients for each region are as follows:

Table 4 Sector II small vector allocation ratio

5. The control method for DC side midpoint potential balance in the NPC three-level structure according to claim 2, characterized in that - according to the above method, the basis for judging the switching synthesis mode of the A and C triangular areas of sector III is as follows : Table 5 Criteria for switching vector synthesis methods of triangle areas A and C in sector III Claim PCT/CN2013/001464

The distribution coefficient A of each region is as follows:

Table 6 Small vector allocation ratio of sector III

Triangle with child points K comparison query

A1

A2

B

C1

C2

Dk-

6. The control method for DC side midpoint potential balance in the NPC three-level structure according to claim 2, characterized in that: according to the above method, the basis for judging the switching synthesis mode of the A and C triangular areas of sector IV is as follows : Table 7 Criteria for switching vector synthesis methods of triangle areas A and C in sector IV Claim PCT/CN2013/001464

The allocation coefficients of each area are as follows - Table 8 Sector IV small vector allocation ratio

7. The control method for the DC side midpoint potential balance in the NPC three-level structure according to claim 2, characterized in that: according to the above method, the basis for judging the switching synthesis mode of the A and C triangular areas in the V area is as follows: Table 9 Criteria for switching vector synthesis methods of triangle areas A and C in the sector Claim PCT/CN2013/001464

The distribution coefficients for each region are as follows:

Table 10 Small vector allocation ratio of sector V

8. The control method for the DC side midpoint potential balance in the NPC three-level structure according to claim 2, characterized in that: according to the above method, the basis for judging the switching synthesis mode of the A and C triangle areas of the VI sector is as follows : Table 11 Criteria for switching vector synthesis methods of triangle areas A and C in sector VI Claim PCT/CN2013/001464

The distribution coefficient t of each region is as follows:

Table 12 Small vector allocation ratio of sector VI