CN105245123A - One-dimensional modulation common-mode current suppression technology for three-phase neutral point-clamped three-level inverter - Google Patents

Abstract

The invention discloses a one-dimensional modulation common-mode current suppression technology for a three-phase midpoint clamp three-level inverter, which makes the three bridge arms of the non-isolated three-phase midpoint clamp three-level inverter work separately In the three vector states of 0, 1 and 2 or all three bridge arms work in the 1 vector state, the common mode voltage is guaranteed to be equal to half of the DC bus voltage, thereby effectively suppressing the common mode in the non-isolated photovoltaic grid-connected power generation system mode current. Compared with the existing common-mode current suppression technology, the one-dimensional modulation common-mode current suppression technology does not require any additional hardware facilities in the photovoltaic power generation system, thereby reducing the cost of the system, improving the energy conversion efficiency, and improving the one-dimensional modulation strategy algorithm It is simple, fast in operation, easy to implement, meets the needs of renewable energy and new energy power generation technologies, and is suitable for non-isolated photovoltaic grid-connected power generation systems without transformers.

Description

One-Dimensional Modulation Common Mode Current Suppression Technology for Three-phase Midpoint Clamped Three-level Inverter

technical field

The present invention relates to common-mode current suppression technology in a non-isolated photovoltaic grid-connected power generation system, in particular to a three-phase neutral point clamped (Neutralpointclamped—NPC) three-level inverter one-dimensional modulation (1DM) common-mode current suppression technology.

Background technique

The non-isolated photovoltaic grid-connected power generation method without transformers has quickly gained the attention of scientific researchers and industrial circles in various countries due to its absolute advantages of high change efficiency, small size, light weight and low cost, and has been obtained in some European countries. application. However, because there is no transformer as isolation, photovoltaic cells, photovoltaic inverters and the grid form a common-mode loop through the parasitic capacitance of photovoltaic cells to the ground; the common-mode voltage in the common-mode loop is constantly changing, causing the capacitance and inductance in the common-mode loop to charge and discharge. Thus, a large common-mode current is generated in the common-mode loop. High-frequency common-mode current will cause serious conduction and radiation interference to surrounding equipment, increase grid-connected current harmonics and system loss, and even endanger equipment and personal safety.

At present, methods for suppressing the common-mode current can be roughly divided into three types: the first method is branch shunting, that is, reducing the common-mode current by increasing the common-mode current branch. In this method, two capacitors are connected in parallel at both ends of the photovoltaic cell, and then the midpoint of the capacitor is connected to the midpoint of the power grid; in this way, in the common mode circuit, the parasitic capacitance of the photovoltaic cell to ground and the capacitance at both ends of the photovoltaic cell are connected in parallel. Large, the capacitance voltage fluctuation is relatively small, which plays the role of clamping the common mode voltage, so as to achieve the purpose of suppressing the common mode current. However, in practical applications, the midpoint of the DC side is connected to the midpoint of the power grid through the ground, and there must be a ground impedance in the connection line; the existence of the ground impedance will cause fluctuations in the voltage at both ends of the parasitic capacitance of the photovoltaic cell to the ground, which will also cause relatively large Large common mode current, so the branch shunt method needs to be further improved. The second method is to increase the impedance of the common-mode loop. When the amplitude of the common-mode voltage in the common-mode loop is constant, increasing the impedance of the common-mode loop can reduce the amplitude of the common-mode current to a certain extent and achieve the suppression of the common-mode voltage. current purpose. However, this method can only suppress the common-mode current, and generally requires a relatively large inductor in series in the common-mode loop to have a better effect of suppressing the common-mode current. The third is to reduce the common-mode voltage or keep the common-mode voltage constant. The existence of the common-mode voltage is the root cause of the common-mode current. If the common-mode voltage can be reduced or kept constant, it can be well suppressed effect of common-mode currents. The current methods to reduce the common-mode voltage or keep the common-mode voltage constant are two strategies to improve the topology of the inverter and improve the modulation technology. To change the topology of the inverter, active switches need to be added, and the system cost increases. Improved modulation techniques require no additional hardware.

At present, photovoltaic grid-connected inverters are mainly voltage-type two-level inverters, but with the continuous maturity of photovoltaic power generation technology and the continuous increase of photovoltaic installed capacity, domestic and foreign related enterprises and research institutes are researching based on multiple Level photovoltaic grid-connected inverters, especially three-level inverters have been more and more used in the field of photovoltaic power generation. However, in the existing research results of suppressing common-mode current, it is very rare to directly clamp the three-level inverter for the three-phase neutral point. The invention provides a common-mode current suppression method based on a one-dimensional modulation strategy for the inverter.

Contents of the invention

The purpose of the present invention is to effectively suppress the common mode current in the non-isolated photovoltaic system, and eliminate the threat of the common mode current to the system and personal safety. Based on the three-phase neutral point clamped three-level photovoltaic inverter (Neutralpointclamped—NPC), the present invention proposes a one-dimensional modulation common-mode current suppression technology, which can realize the common-mode voltage without adding any hardware Constant, effective rejection of common-mode currents. In order to solve the above-mentioned technical problems, the present invention is achieved through the following technical solutions:

A three-phase neutral-point clamped three-level inverter (NPC) one-dimensional modulation (1DM) common mode current suppression technology, which includes the following specific steps:

For each phase bridge arm of a three-phase neutral point clamped three-level inverter (NPC), define its switching state as V, then V has three values: 0, the bridge arm output terminal is connected to the negative terminal of the DC bus; 1, The output end of the bridge arm is connected to the midpoint of the DC bus; 2. The output end of the bridge arm is connected to the positive end of the DC bus; when the switching states V _A , V _B and V _C of the three-phase bridge arm satisfy the formula (1),

V _A +V _B +V _C =3(1)

Make the three bridge arms of the non-isolated three-phase neutral point clamped three-level inverter NPC work in three vector states of 0, 1 and 2 respectively, or all three bridge arms work in the 1 vector state; ensure the common mode voltage Constantly equal to one-half of the DC voltage source, thereby effectively suppressing the common mode current of the system;

Step 1: In a fundamental wave cycle, divide a fundamental wave cycle into six sectors according to the positive and negative of the three-phase modulation wave, and operate separately. In the I sector, the A-phase modulation wave is greater than zero, the B-phase modulation wave is less than zero, and the C-phase modulation wave is less than zero, phase A modulation wave of sector II is greater than zero, phase B modulation wave is greater than zero, phase C modulation wave is less than zero, sector III sector A phase modulation wave is less than zero, phase B modulation wave is greater than zero, and phase C modulation wave is less than zero Zero, phase A modulation wave of sector IV is less than zero, phase B modulation wave is greater than zero, phase C modulation wave is greater than zero, phase A modulation wave of sector V is less than zero, phase B modulation wave is less than zero, phase C modulation wave is greater than zero, Phase A modulation wave in sector VI is greater than zero, phase B modulation wave is less than zero, and phase C modulation wave is greater than zero; the sequence of vector action in sector I is 111—201—210—111, and the sequence of vector action in sector II is 111— 210—120—111, the order of vector action in sector III is 111—120—021—111, the order of action of vector in sector IV is 111—021—012—111, and the order of action of vector in sector V is 111—012— 102-111. The order of the vector action in the VI sector is 111-102-201-111; the positive and negative signs of the three-phase modulation wave in each sector are unchanged;

Step 2: Determine the vector state and its action time according to the instantaneous value of the three-phase modulation wave in the time domain. In each sector, first obtain the voltage vectors of phase A and phase B and their action time, where a _x is the standard The unitized reference output voltage; a _xi is the integer part of the unitized reference output voltage; V _refx is the reference output voltage (V); V _Maxx is the maximum value of the output phase voltage (V); Voltage difference (V); floor function of rounding down; S _x1 and S _x2 are adjacent voltage vectors; t _x1 and t _x2 are the action time (s) of corresponding voltage vectors;

Step 3: Reasonably arrange the voltage vectors of the three phases according to the formula V _A +V _B +V _C =3, and derive the voltage vectors of the three phases at any time and their action time to ensure that the common-mode voltage is constant; according to the vector states of the three bridge arms Determine the vector state of phase C based on the principle that the sum is 3, and finally combine the voltage vectors of the three phases reasonably so that the overall output state of the three phases is only neutral vector and zero vector 111; in sectors I, III, IV and VI, phase A The positive and negative signs of Phase B and Phase B are different; the positive and negative values of Phase A and Phase B in sectors II and V are the same;

Step 4: One-dimensional modulation without dead zone can better suppress the common mode, sample the three-phase inductor current, and then use the current signal of each phase to do AND logic with the second switch and the third switch in the corresponding bridge arm to obtain a new first The second switch and the third switch trigger the signal, so since there is no complementary relationship between the first switch and the third switch, and the second switch and the fourth switch, no dead zone can be added.

Owing to adopting above-mentioned technical scheme, the present invention has such beneficial effect compared with prior art:

The improved one-dimensional modulation common-mode current suppression technology of the present invention does not require any hardware facilities to be added to the photovoltaic power generation system, thereby reducing the cost of the system and improving the energy conversion efficiency. Only by improving the modulation strategy can the common-mode voltage be kept constant, reaching purpose of suppressing common-mode currents. At the same time, the modulation algorithm of the present invention is simple, the calculation speed is fast, and it is easy to implement, meets the technical requirements of power generation of renewable energy and new energy, and is suitable for non-isolated photovoltaic grid-connected power generation systems without transformers.

Description of drawings

Figure 1 is a non-isolated NPC three-level inverter photovoltaic grid-connected system diagram;

Figure 2 is a sector division diagram of the improved one-dimensional modulation technology;

Figure 3 is a schematic diagram of one-dimensional modulation;

Fig. 4 is a vector action sequence diagram of one-dimensional modulation common mode current technology;

Fig. 5 is a trigger signal of a switching device with a dead zone in phase A;

Fig. 6 is a trigger signal of a switching device without a dead zone in phase A;

Figure 7 is a waveform diagram of the experimental results.

detailed description

Below in conjunction with accompanying drawing and specific embodiment the present invention is described in further detail:

A three-phase neutral-point clamped three-level inverter one-dimensional modulation common-mode current suppression technology, the detailed description of which is as follows:

Figure 1 shows a non-isolated photovoltaic grid-connected power generation system based on a three-phase neutral-point clamped three-level inverter. When working in a 1-vector state, even if the sum of the vector states of the three bridge arms is 3, the common-mode voltage can be kept equal to half of the DC voltage source, thereby effectively suppressing the common-mode current of the system.

Referring to Figure 2, a fundamental cycle can be divided into six sectors according to the sign of the three-phase modulation wave within a fundamental wave cycle, and the sign of the three-phase modulation wave in each sector is constant, so It is convenient to determine the vector states of the three bridge arms.

On this basis, the vector state and its action time are determined according to the instantaneous value of the three-phase modulation wave in the time domain. In each sector, first determine the voltage vectors of phase A and phase B and their action time, refer to the 1DM modulation principle shown in Figure 3, and then determine the voltage of phase C according to the principle that the sum of the vector states of the three bridge arms is 3 Vector state, finally combine the voltage vectors of the three phases reasonably, so that the overall output state of the three phases is only the neutral vector and zero vector 111; in sectors I, III, IV and VI, the positive and negative signs of phase A and phase B are different ; In sectors II and V, the positive and negative values of phase A and phase B are the same, so take sectors I and II as examples for analysis.

The three-phase modulation wave is defined as: V _refa =m·V _Max cosθ, V _refb =m·V _Max cos(θ-2π/3), V _refcc =m·V _Max cos(θ+2π/3).

According to the 1DM modulation principle: a _a =(m·V _Max cosθ+V _Max )/E _p . In the NPC three-level inverter, E _p =V _Max , so a _a =1+mcosθ. When cosθ>0, a _ai =1, r _a =mcosθ, S _a1 =1, S _a2 =2, t _{a1 =} (1-mcosθ)·T _s =(1-m|cosθ|)·T _s , t _a2 = mcosθ·T _s =m|cosθ|·T _s ; cosθ<0, a _ai =0, r _a =1+mcosθ, S _a1 =0, S _a2 =1, t _a1 =-mcosθ·T _s = m|cosθ|·T _s , t _a2 =(1+mcosθ)·T _s =(1−m|cosθ|)·T _s .

From the above derivation, the voltage vector and action time of the three phases at any moment can be obtained, as shown in Table 1:

Table 11 DM vector action time

In region I, phase A t _c2 =m|cos(θ+2π/3)|·T _s is positive, and phase B is negative, so t _c2 =(1-m|cos(θ+2π/3) |)·T _s Phase A has two voltage vectors of 1 and 2, and phase B has two voltage vectors of 0 and 1. In area II, both phase A and phase B are positive, so both phase A and phase B have two voltage vectors of 1 and 2.

In sector I, V _refa >0, V _refb <0 and |V _refa |>|V _refb |, the action time of A-phase voltage vector 1 and B-phase voltage vector 1 in sector I are respectively: t _a1 = (1-m|cosθ|)·T _s , t _{b2 =} (1-m|cos(θ-2π/3)|)·T _s ; then t _a1 -t _b2 =(1-m|cosθ|)· T _s -(1-m|cos(θ-2π/3)|)·T _s =m|cos(θ-2π/3)·T _s -m|cosθ|·T _s =m·T _s ( |cos(θ-2π/3)|-|cosθ|)<0. Therefore, it can be seen that t _a1 <t _b2 , that is, the action time of the A-phase voltage vector 1 is shorter than the action time of the B-phase voltage vector 1 . Since t _a1 +t _a2 =T _s , t _b1 +t _b2 =T _s . Therefore, t _a2 >t _b1 , that is, the action time of phase A voltage vector 2 is greater than the action time of phase B voltage vector 0.

Since the action time of the A-phase voltage vector 1 is shorter than the action time of the B-phase voltage vector 1, the voltage vector 111 can be applied within the time t _a1 . However, the action time of the A-phase voltage vector 2 is longer than the action time of the B-phase voltage vector 0, and the vector 201 can be acted on within the time t _b1 . Then the action time of the voltage vector 210 is: t ₂₁₀ =T _s -t _a1 -t _b1 =T _s -(T _s -t _a2 )-t _b1 =t _a2 -t _b1 =T _s -t _a1 -(T _s -t _b2 )=t _b2 -t _a1 . In order to make the voltage vector 111 as the initial vector, the voltage vector 111 is first applied within the time t _a1 /2, then the voltage vector 201 is applied within the time t _b1 , then the voltage vector 210 is applied during the time t ₂₁₀ , and finally the voltage vector 210 is applied within the time t ₁₁ /2 A voltage vector 111 is applied for 2 hours.

In sector II, the reference voltages of both phases A and B are positive, and the voltage vectors of the two phases are switched between voltage vectors 1 and 2. The action time of A-phase voltage vector 1 is: t _a1 =(1-m|cosθ|)·T _s ; the action time of B-phase voltage vector 2 is: t _b2 =m|cos(θ-2π/3)|· T _s ; while V _refa +V _refb +V _refc =0 is satisfied in the three-phase symmetrical system; then t _a1 -t _b2 =(1-m|cosθ|)·T _s -m|cos(θ-2π /3)|·T _s =1-(mcosθ+mcos(θ-2π/3))=1+mcos(θ+2π/3)>0.

It can be seen that in sector II, the action time of A-phase voltage vector 1 is always longer than that of B-phase voltage vector 2, and at the same time, the action time of A-phase voltage vector 2 is always shorter than that of B-phase voltage vector 1. Therefore, the voltage vector 120 can be applied within the time of t _b2 , and the voltage vector 210 can be applied within the time of t _a2 , so the action time of the voltage vector 111 can be obtained as: t ₁₁₁ =T _s -t _a2 -t _b2 =T _s - (T _s -t _a1 )-t _b2 =t _a1 -t _b2 =T _s -t _a2 -(T _s -t _b1 )=t _b1 -t _a2 . In order to use the voltage vector 111 as the initial vector, the voltage vector 111 is first applied within the time t ₁₁₁ /2, then the voltage vectors 210 and 120 are applied within the time t _a2 and t _b2 respectively, and finally the voltage is applied within the time t ₁₁₁ /2 The vector is 111, which can ensure that the start vector and the end vector of any sector are both 111, so there will be no vector jump when switching sectors.

Referring to Fig. 4, I, II, III, IV, V and VI sector vector action sequence.

Referring to FIG. 5 , the trigger signal waveform of the phase A bridge arm switching device has a dead zone. When the driving signal has a dead zone, the common-mode voltage of the system is not equal to half of the DC bus voltage during the dead zone, so that the common-mode voltage will have a peak during the dead zone, resulting in an increase in the common-mode current.

Referring to FIG. 6 , the switching device of the bridge arm of phase A does not have a dead zone trigger signal waveform. The one-dimensional modulation without dead zone can better suppress the common mode. The specific implementation method is to sample the three-phase inductor current, and then use the current signal of each phase to perform AND logic with the second switch and the third switch in the corresponding bridge arm to obtain a new The second switch and the third switch trigger the signal, so since there is no complementary relationship between the first switch and the third switch, and between the second switch and the fourth switch, no dead zone may be added.

Referring to Fig. 7, A-phase voltage waveform, line voltage V _ab waveform, filtered A-phase voltage and current waveform, common-mode voltage and common-mode current waveform. Common mode voltage experimental parameters: DC input voltage E=200V, load at R=20Ω, LC filter (L=3mH, C=14uF), switching frequency f _s =5kHz, modulation ratio m=0.8, simulated photovoltaic cell to ground Parasitic capacitance C _G-PV = 220nF.

Claims (1)

1. a three-phase neutral-point-clamped three-level inverter one-dimensional modulation common mode current suppression technology, is characterized in that its content comprises following concrete steps:

For every phase brachium pontis of three-phase neutral-point-clamped three-level inverter, defining its on off state is V, then V has three kinds of values: 0, and brachium pontis output connects DC bus negative terminal; 1, brachium pontis output connects DC bus mid point; 2, brachium pontis output connects DC bus anode; As the on off state V of three-phase brachium pontis _a, V _band V _cwhen meeting formula (1),

V _A+V _B+V _C＝3(1)

Make three brachium pontis of non-isolated three-phase neutral-point-clamped three-level inverter NPC be operated in 0,1 and 2 three kind of vector state respectively, or three brachium pontis are all operated in 1 vector state; Ensure that common-mode voltage is constantly equal to 1/2nd of direct voltage source, thus the common mode current of effective suppression system;

Step 1: be divided into six sectors to operate respectively a primitive period according to three-phase modulations ripple is positive and negative within a primitive period, I sector A phase modulating wave is greater than zero, B phase modulating wave is less than zero, C phase modulating wave is less than zero, II sector A phase modulating wave is greater than zero, B phase modulating wave is greater than zero, C phase modulating wave is less than zero, III sector A phase modulating wave is less than zero, B phase modulating wave is greater than zero, C phase modulating wave is less than zero, IV sector A phase modulating wave is less than zero, B phase modulating wave is greater than zero, C phase modulating wave is greater than zero, V sector A phase modulating wave is less than zero, B phase modulating wave is less than zero, C phase modulating wave is greater than zero, VI sector A phase modulating wave is greater than zero, B phase modulating wave is less than zero, C phase modulating wave is greater than zero, be 111-201-210-111 in I sector vector sequence of operation, be 111-210-120-111 in II sector vector sequence of operation, be 111-120-021-111 in III sector vector sequence of operation, be 111-021-012-111 in IV sector vector sequence of operation, be 111-012-102-111 in V sector vector sequence of operation, be 111-102-201-111 in VI sector vector sequence of operation, the sign symbol of three-phase modulations ripple is constant in each sector,

Step 2: according to three-phase modulations ripple in the instantaneous value determination vector state of time domain and action time thereof, ask for voltage vector and the action time thereof of A phase and B phase in each sector first respectively, wherein a _xfor standardization reference output voltage; a _xifor the integer part of standardization reference output voltage; V _refxfor reference output voltage; V _maxxfor exporting phase voltage maximum; E exports the voltage difference between adjacent levels; The downward bracket function of floor; S _x1, S _x2for neighboring voltage vector; t _x1, t _x2for the action time of relevant voltage vector;

Step 3: according to formula V _a+ V _b+ V _cthe voltage vector of=3 Rational Arrangement three-phases, derivation three-phase voltage vector at any time and ensure that common-mode voltage is constant action time; Determine the vector state of C phase according to the vector state sum of three brachium pontis principle that is 3, the voltage vector of last reasonable combination three-phase, makes the output state of three-phase entirety only have middle vector zero vector 111; In sector I, III, IV and VI, the sign symbol of A phase and B phase is different; In II and V of sector, the positive and negative values of A phase and B phase is identical;

Step 4: common mode can be suppressed better without dead band one-dimensional modulation, sampling three-phase inductive current, then make and logic of second switch and the 3rd switch in the current signal of each phase and corresponding brachium pontis, obtain new second switch and the 3rd switch triggering signal, like this because the first switch and the 3rd switch, second switch and the 4th switch do not exist complementary relationship, therefore dead band can not be added.