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

One-dimensional modulation common-mode current suppression technology for three-phase neutral point-clamped three-level inverter Download PDF

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CN105245123A
CN105245123A CN201510675309.5A CN201510675309A CN105245123A CN 105245123 A CN105245123 A CN 105245123A CN 201510675309 A CN201510675309 A CN 201510675309A CN 105245123 A CN105245123 A CN 105245123A
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王立乔
桑霖霖
郭小强
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Yanshan University
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Abstract

The invention discloses a one-dimensional modulation common-mode current suppression technology for a three-phase neutral point-clamped three-level inverter. Three bridge arms of a non-isolated three-phase neutral point-clamped three-level inverter respectively work in three vector states 0, 1 and 2 or the three bridge arms all work in the vector state 1; and the condition that common-mode voltage is constantly equal to 1/2 of direct-current bus voltage is ensured, so that the common-mode current in a non-isolated grid-connected photovoltaic power generation system is effectively suppressed. Compared with an existing common-mode current suppression technology, according to the one-dimensional modulation common-mode current suppression technology, no hardware facility is added to the photovoltaic power generation system, so that the cost of the system is reduced; the energy conversion efficiency is improved; the improved one-dimensional modulation strategy is simple in algorithm, high in operation speed and convenient to achieve; and the one-dimensional modulation common-mode current suppression technology conforms to the requirements of renewable energy source and new energy power generation technologies, and is suitable for the non-isolated grid-connected photovoltaic power generation system which does not include a transformer.

Description

Three-phase neutral-point-clamped three-level inverter one-dimensional modulation common mode current suppression technology
Technical field
The present invention relates to the common mode current suppression technology in non-isolated grid-connected electricity generation system, particularly relate to a kind of three-phase neutral-point-clamped (Neutralpointclamped---NPC) three-level inverter one-dimensional modulation (1DM) common mode current suppression technology.
Background technology
Non-isolated parallel network power generation mode not containing transformer relies on the absolute predominance that Self-variation efficiency is high, volume is little, lightweight and cost is low, obtain rapidly the attention of various countries scientific research personnel and the concern of industrial quarters, be applied in part European countries at present.But because there is no transformer as isolation, photovoltaic cell, photovoltaic DC-to-AC converter and electrical network by photovoltaic cell over the ground parasitic capacitance define common mode loop; In common mode loop, common-mode voltage constantly changes, and causes electric capacity in common mode loop, inductance discharge and recharge, thus produce larger common mode current in common mode loop.The common mode current of high frequency can cause serious conduction and radiated interference, increase grid-connected current harmonic wave and system loss to surrounding devices, even jeopardizes equipment and personal safety.
The method of current suppression common mode current can be divided into three kinds substantially: the first is the method for branch road shunting, and the method namely by increasing common mode current branch road reduces common mode current.The method is two electric capacity in parallel at photovoltaic cell two ends, then connect electric capacity mid point and electrical network mid point; Like this in common mode loop photovoltaic cell parasitic capacitance and photovoltaic cell two ends electric capacity are in parallel over the ground; because photovoltaic cell shunt capacitance is larger; voltage fluctuation of capacitor is smaller, so just serves the effect of clamper common-mode voltage, thus reaches the object suppressing common mode current.But in actual applications, DC side mid point is connected with electrical network mid point by the earth, the earth impedance in connecting line, must be had; The earth impedance existence make photovoltaic cell over the ground parasitic capacitance two ends voltage produce fluctuation, also can produce larger common mode current thus, therefore branch road shunting method be further improved.The second is the method increasing common mode impedance loop, common-mode voltage variation amplitude is certain in common mode loop, increases common mode impedance loop and can reduce common mode current amplitude to a certain extent, reach the object suppressing common mode current.But this method can only play the effect suppressing common mode current, and generally need in common mode loop, seal in the effect that larger inductance just can have suppression common mode current preferably.The third reduces common-mode voltage or keeps common-mode voltage constant, and the existence of common-mode voltage is the basic reason producing common mode current, if can reduce common-mode voltage or keep the constant effect that well can be suppressed common mode current of common-mode voltage.Current reduction common-mode voltage or keep the constant method of common-mode voltage to be improve the topological structure of inverter and improve two kinds of strategies of modulation technique.Change inverter topology, need to increase active switch, system cost rises.Improve modulation technique and then need not add any hardware.
Current photovoltaic combining inverter is based on voltage-type two-level inverter, but along with the continuous maturation of photovoltaic power generation technology, the continuous increase of photovoltaic installed capacity, domestic and international relevant enterprise and scientific research institutions are all at the photovoltaic combining inverter of research based on many level, and particularly three-level inverter is more and more applied in field of photovoltaic power generation.And in the existing achievement in research suppressing common mode current, also very rare directly for three-phase neutral-point-clamped three-level inverter.The present invention is directed to this inverter and provide a kind of common mode current suppressing method based on one-dimensional modulation strategy.
Summary of the invention
The object of the invention is the common mode current in order to effectively suppress in non-isolated photovoltaic system, eliminate the threat of common mode current to system and personal safety.The present invention is based on three-phase neutral-point-clamped three-level photovoltaic inverter (Neutralpointclamped---NPC), propose a kind of one-dimensional modulation common mode current suppression technology, under the condition not increasing any hardware, can common-mode voltage be realized constant, effectively suppress common mode current.In order to solve the technical problem of above-mentioned existence, the present invention is achieved by the following technical solutions:
A kind of three-phase neutral-point-clamped three-level inverter (NPC) one-dimensional modulation (1DM) common mode current suppression technology, its content comprises following concrete steps:
For every phase brachium pontis of three-phase neutral-point-clamped three-level inverter (NPC), 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); V maxxfor exporting phase voltage maximum (V); E exports the voltage difference (V) between adjacent levels; The downward bracket function of floor; S x1, S x2for neighboring voltage vector; t x1, t x2for the action time (s) 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.
Owing to adopting technique scheme, the present invention compared with prior art has such beneficial effect:
Improvement one-dimensional modulation common mode current suppression technology of the present invention, do not need photovoltaic generating system to increase any hardware facility, thus reduce the cost of system, improve effciency of energy transfer, only need improvement modulation strategy just can realize common-mode voltage constant, reach the object suppressing common mode current.Meanwhile, modulation algorithm of the present invention is simple, fast operation, is easy to realize, meets the generation technology needs of regenerative resource and new forms of energy, is applicable to not containing the non-isolated grid-connected electricity generation system of transformer.
Accompanying drawing explanation
Fig. 1 is non-isolated NPC three-level inverter photovoltaic parallel in system figure;
Fig. 2 improves one-dimensional modulation technology se ctor partition figure;
Fig. 3 is one-dimensional modulation schematic diagram;
Fig. 4 is one-dimensional modulation common mode current technology vector action sequence diagram;
Fig. 5 is A facies tract dead band switching device triggering signal;
Fig. 6 is that A phase is not with dead band switching device triggering signal;
Fig. 7 is experimental result oscillogram.
Embodiment
Below in conjunction with accompanying drawing and embodiment, the present invention is described in further detail:
A kind of three-phase neutral-point-clamped three-level inverter one-dimensional modulation common mode current suppression technology, its detailed description is as follows:
It is the non-isolated grid-connected electricity generation system based on three-phase neutral-point-clamped three-level inverter shown in Fig. 1, when each brachium pontis be operated in respectively 0,1 and 2 three kind of vector state or three brachium pontis be operated in 1 vector state time, even if the vector state sum of three brachium pontis is 3 common-mode voltage also can be made to be constantly equal to 1/2nd of direct voltage source, thus the common mode current of effective suppression system.
See Fig. 2, according to three-phase modulations ripple is positive and negative, a primitive period can be divided into six sectors within a primitive period, the sign symbol of three-phase modulations ripple is constant in each sector, is convenient to the determination of three brachium pontis vector states like this.
On this basis, according to three-phase modulations ripple in the instantaneous value determination vector state of time domain and action time thereof.First voltage vector and the action time thereof of A phase and B phase is determined in each sector, 1DM modulation principle shown in Figure 3, then the principle being 3 according to the vector state sum of three brachium pontis determines the vector state of C phase, 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, therefore analyzes for sector I and II.
Definition three-phase modulations ripple is: V refa=mV maxcos θ, V refb=mV maxcos (θ-2 π/3), V refcc=mV maxcos (θ+2 π/3).
According to 1DM modulation principle: a a=(mV maxcos θ+V max)/E p.E in NPC three-level inverter p=V max, therefore a a=1+mcos θ.As cos θ >0, a ai=1, r a=mcos θ, S a1=1, S a2=2, t a1=(1-mcos θ) T s=(1-m|cos θ |) T s, t a2=mcos θ T s=m|cos θ | T s; Cos θ <0, a ai=0, r a=1+mcos θ, S a1=0, S a2=1, t a1=-mcos θ T s=m|cos θ | T s, t a2=(1+mcos θ) T s=(1-m|cos θ |) T s.
Three-phase voltage vector at any time and action time thereof can be obtained by above-mentioned derivation, as shown in table 1:
Table 11DM vector action time
A phase t in the I of region c2=m|cos (θ+2 π/3) | T sfor being negative value, therefore t on the occasion of, B phase c2=(1-m|cos (θ+2 π/3) |) T sa phase has 1 and 2 two kind of voltage vector, and B phase has 0 and 1 two kind of voltage vector.In region il A phase and B phase be on the occasion of, so A phase and B phase all have 1 and 2 two kind of voltage vector.
V in the I of sector refa>0, V refb<0 and | V refa| >|V refb|, in the I of sector, the action time of A phase voltage vector 1 and B phase voltage vector 1 is respectively: t a1=(1-m|cos θ |) T s, t b2=(1-m|cos (θ-2 π/3) |) T s; Then t a1-t b2=(1-m|cos θ |) T s-(1-m|cos (θ-2 π/3) |) T s=m|cos (θ-2 π/3) T s-m|cos θ | T s=mT s(| cos (θ-2 π/3) |-| cos θ |) <0.So known t a1<t b2, be namely less than the action time of B phase voltage vector 1 action time of A phase voltage vector 1.Due to t a1+ t a2=T s, t b1+ t b2=T s.So t a2>t b1, be namely greater than the action time of B phase voltage vector 0 action time of A phase voltage vector 2.
Be less than the action time of B phase voltage vector 1 action time due to A phase voltage vector 1, can at t a1applied voltage vector 111 in time.And be greater than the action time of B phase voltage vector 0 action time of A phase voltage vector 2, can at t b1effect vector 201 in time.Then the action time of 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 voltage vector 111 as start vector, first at t a1applied voltage vector 111, then t in/2 times b1applied voltage vector 201 in time, next at t 210applied voltage vector 210 in time, finally at t 111applied voltage vector 111 in/2 times.
In the II of sector the reference voltage of A, B two-phase be on the occasion of, then the voltage vector of two-phase all between voltage vector 1 and 2 switch.The action time of A phase voltage vector 1 is: t a1=(1-m|cos θ |) T s; The action time of B phase voltage vector 2 is: t b2=m|cos (θ-2 π/3) | T s; And V is met in three-phase symmetrical system refa+ V refb+ V refc=0; So can t be obtained a1-t b2=(1-m|cos θ |) T s-m|cos (θ-2 π/3) | T s=1-(mcos θ+mcos (θ-2 π/3))=1+mcos (θ+2 π/3) >0.
It can thus be appreciated that in the II of sector, be greater than the action time of B phase voltage vector 2 action time of A phase voltage vector 1 all the time, simultaneously the action time being less than B phase voltage vector 1 action time all the time of A phase voltage vector 2.So can at t b2time in applied voltage vector 120, at t a2time in applied voltage vector 210, so the action time that can obtain voltage vector 111 be: 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 make voltage vector 111 as start vector, first at t 111applied voltage vector 111 in/2 times, then respectively at t a2, t b2applied voltage vector 210 and 120 in time, finally at t 111applied voltage vector 111 in/2 times, can ensure to be 111 in any sector start vector and end vector like this, therefore can not again sector switch time produce vector leap.
See Fig. 4, I, II, III, IV, V and VI sector vector sequence of operation.
See Fig. 5, A phase brachium pontis switching device band dead band triggering signal waveform.When drive singal is with dead band, in Dead Time, the common-mode voltage of system is not equal to 1/2nd of DC bus-bar voltage, and in Dead Time, common-mode voltage can produce spike like this, causes common mode current to increase.
Dead band triggering signal waveform is not with see Fig. 6, A phase brachium pontis switching device.Better common mode can be suppressed without dead band one-dimensional modulation, specific implementation method is 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.
See Fig. 7, A phase voltage waveform, line voltage V aba phase voltage and current waveform, common-mode voltage and common mode current waveform after waveform, filtering.Common-mode voltage experiment parameter: DC input voitage E=200V, load at R=20 Ω, LC filter (L=3mH, C=14uF), switching frequency f s=5kHz, modulation ratio m=0.8, simulation photovoltaic cell parasitic capacitance C over the ground 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.
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