CN103346688B - Controlling method of improving current quality of grid-connected inverter during low-load operation - Google Patents

Abstract

The invention discloses a controlling method of improving current quality of a grid-connected inverter when operated in low load. The controlling method of improving the current quality of the grid-connected inverter when operated in low load achieves judgment of the operating mode of the grid-connected inverter according to the difference of the ratio of the actual output power and rated power of the inverter. When the ratio ranges from 0.6 to 1, the grid-connected inverter is in the normal working mode, and the classic voltage current double closed-loop PI controlling is adopted; when the ratio ranges from 0.3 to 0.6, the grid-connected inverter is in the mild low-load working mode, the classic voltage current double closed-loop PI controlling is adopted at the moment, and software measurements are added, wherein the software measurements comprise improving the switch frequency by one time, enabling an inner ring PI adjuster to be merged into a repetition controller and the like; when the ratio is less than 0.3, the grid-connected inverter is in the severe low load working mode, at the moment hardware measurements are added on the basis of the mild low load controlling method, the hardware measurements comprise starting a controlling relay and enabling a bridge-arm side addition inductor to be merged. The controlling method of improving the current quality of the grid-connected inverter when operated in low load ensures grid current when the grid-connected inverter runs under low load, can reduce the quantity of the added inductor, and saves cost.

Description

A kind of control method improving current quality under the low load operation of combining inverter

Technical field

The present invention relates to a kind of control method improving current quality under the low load operation of combining inverter, belong to power industry technical field.

Background technology

Pursuing today of low-carbon (LC) society, generation of electricity by new energy technology, as wind power generation, photovoltaic generation and fuel cell power generation etc. are more and more subject to people's attention, also increases day by day to the concern of the inversion transformation technique that generates electricity by way of merging two or more grid systems.Combining inverter is as the key component of generation of electricity by new energy technology, and its design is carried out according to nominal rating capacity usually, and these designs mainly comprise the type selecting of filter parameter, capacity, the type selecting of switching tube, the setting of switching frequency, the type selecting of transducer, Control System Design etc.But due to fluctuation and the randomness of solar energy and wind energy, combining inverter usually runs at non-rated operation point, even often works in low loaded-up condition.Therefore, inverter injects the current harmonics of electrical network when low load operation is also an important index.

Summary of the invention

For combining inverter ropy inherent shortcoming of grid-connected current when low load operation, the invention provides a kind of control method improving current quality under the low load operation of combining inverter, combining inverter is made to ensure the quality of grid-connected current when low load operation, meet grid-connected requirement, and adopt the control method of software and hardware combining can reduce the amount of additional inductor, cost-saving.

For achieving the above object, the present invention adopts following technical scheme:

Improve a control method for current quality under the low load operation of combining inverter, the topological structure of the combining inverter with LCL filter that this control method relates to comprises load load, DC side filter capacitor C _dc, three-phase full-bridge inverting circuit, LCL filter, DC side filter capacitor C _dcbe connected in parallel on the two ends of load load, DC side filter capacitor C _dctwo ends be connected with two inputs of three-phase full-bridge inverting circuit respectively, the three-phase output end of three-phase full-bridge inverting circuit is connected with the three-phase input end one_to_one corresponding of LCL filter, it is characterized in that: the three-phase output end of LCL filter respectively with brachium pontis side additional inductor L _e1be connected with three phase network again after series connection, described each brachium pontis side additional inductor L _e1all with corresponding control relay K _l1in parallel;

This control method is according to the real output P of inverter and rated power P _nthe difference of ratio λ, complete the judgement of mode of operation residing for inverter, and the entirety of carrying out control strategy switches;

Pattern one: as real output P and the rated power P of inverter _nratio λ when 0.6 ~ 1 scope, judge inverter be in normal mode of operation, now described control relay K _l1closed, adopt classical voltage and current double closed-loop PI to control, described classical voltage and current double closed-loop PI controls to comprise outer voltage and current inner loop control, and outer voltage and current inner loop control all to be controlled by pi regulator;

Pattern two: as real output P and the rated power P of inverter _nratio λ in 0.3 ~ 0.6 scope time, judge inverter be in slight low load operation pattern, now described control relay K _l1closed, on the basis that voltage and current double closed-loop PI classical described in retained-mode one controls, increase software measure; Described software measure comprises improving on inverter switching frequency, current PI adjuster in classical voltage and current double closed-loop PI controls and is incorporated to repetitive controller, namely realizes the common adjustment of internal circular current with current PI adjuster and repetitive controller combination regulation mode;

Pattern three: as real output P and the rated power P of inverter _nratio λ below 0.3 time, judge that inverter is in serious low load operation pattern, on the basis of the control method described in retained-mode two and software measure, increase hardware measure, described hardware measure comprises opens described control relay K _l1, brachium pontis side additional inductor L _e1put into operation.

(2) control strategy in pattern one performs according to following steps:

Step 1: gather DC side filter capacitor C _dcon voltage U _dc, brachium pontis side current i _a, i _b, i _cand line voltage E _a, E _b, E _c, by line voltage E _a, E _b, E _cobtain electric network voltage phase angle γ through phase-locked loop pll is phase-locked;

Step 2: the given U of DC voltage _dcrefwith DC side filter capacitor C _dcon voltage U _dcregulate through voltage PI regulator after subtracting each other, the output of voltage PI regulator is as the given i of current inner loop active current _dref;

Step 3: the brachium pontis side current i of collection _a, i _b, i _cutilize the active current feedback quantity i of electric network voltage phase angle γ under abc-dq coordinate transform becomes synchronous rotating frame dq _dwith reactive current feedback quantity i _q, the given i of active current _drefwith active current feedback quantity i _dsubtract each other, the given i of reactive current _qrefwith reactive current feedback quantity i _qsubtract each other, two subtract each other after output variable regulate through current PI adjuster; The given i of described reactive current _qref0 is taken as when unity power factor control;

Step 4: the output of current PI adjuster utilizes the controlled quentity controlled variable U of electric network voltage phase angle γ under dq-abc coordinate transform becomes three-phase static coordinate system abc _sa, U _sb, U _sc, the controlled quentity controlled variable U under three-phase static coordinate system abc _sa, U _sb, U _scinverter switching device signal S is generated through SPWM modulation _abccontrol three-phase full-bridge inverter exports;

Step 5: closing control relay K _l1switch, brachium pontis side additional inductor L _e1short circuit.

(3) control strategy in pattern two performs according to following steps:

Step 1: gather DC side filter capacitor C _dcon voltage U _dc, brachium pontis side current i _a, i _b, i _cand line voltage E _a, E _b, E _c, by line voltage E _a, E _b, E _cobtain electric network voltage phase angle γ through phase-locked loop pll is phase-locked;

Step 2: the given U of DC voltage _dcrefwith DC side filter capacitor C _dcon voltage U _dcregulate through voltage PI regulator after subtracting each other, the output of voltage PI regulator is as the given i of current inner loop active current _dref;

Step 3: the brachium pontis side current i of collection _a, i _b, i _cutilize the active current feedback quantity i of electric network voltage phase angle γ under abc-dq coordinate transform becomes synchronous rotating frame dq _dwith reactive current feedback quantity i _q, the given i of active current _drefwith active current feedback quantity i _dsubtract each other, the given i of reactive current _qrefwith reactive current feedback quantity i _qsubtract each other, two subtract each other after output variable regulate through current PI adjuster is in parallel with repetitive controller; The given i of described reactive current _qref0 is taken as when unity power factor control;

Step 4: current PI adjuster is in parallel with repetitive controller regulates the rear value exported to utilize the controlled quentity controlled variable U of electric network voltage phase angle γ under dq-abc coordinate transform becomes three-phase static coordinate system abc _sa, U _sb, U _sc, the controlled quentity controlled variable U under three-phase static coordinate system abc _sa, U _sb, U _scinverter switching device signal S is generated through SPWM modulation _abccontrol three-phase full-bridge inverter exports, and wherein the switching frequency of inverter brings up to 2 times of nominal switching frequency;

Step 5: closing control relay K _l1switch, brachium pontis side additional inductor L _e1short circuit.

(4) control strategy in pattern three performs according to following steps:

Step 1: gather DC side filter capacitor C _dcon voltage U _dc, brachium pontis side current i _a, i _b, i _cand line voltage E _a, E _b, E _c, by line voltage E _a, E _b, E _cobtain electric network voltage phase angle γ through phase-locked loop pll is phase-locked;

Step 2: the given U of DC voltage _dcrefwith DC side filter capacitor C _dcon voltage U _dcregulate through voltage PI regulator after subtracting each other, the output of voltage PI regulator is as the given i of current inner loop active current _dref;

Step 3: the brachium pontis side current i of collection _a, i _b, i _cutilize the active current feedback quantity i of electric network voltage phase angle γ under abc-dq coordinate transform becomes synchronous rotating frame dq _dwith reactive current feedback quantity i _q, the given i of active current _drefwith active current feedback quantity i _dsubtract each other, the given i of reactive current _qrefwith reactive current feedback quantity i _qsubtract each other, two subtract each other after output variable regulate through current PI adjuster is in parallel with repetitive controller; The given i of described reactive current _qref0 is taken as when unity power factor control;

Step 4: current PI adjuster is in parallel with repetitive controller regulates the rear value exported to utilize the controlled quentity controlled variable U of electric network voltage phase angle γ under dq-abc coordinate transform becomes three-phase static coordinate system abc _sa, U _sb, U _sc, the controlled quentity controlled variable U under three-phase static coordinate system abc _sa, U _sb, U _scinverter switching device signal S is generated through SPWM modulation _abccontrol three-phase full-bridge inverter exports, and wherein the switching frequency of inverter brings up to 2 times of nominal switching frequency;

Step 5: open control relay K _l1switch, brachium pontis side additional inductor L _e1put into operation.

Owing to have employed technique scheme, the present invention can make combining inverter ensure the quality of grid-connected current when low load operation, and adopts the control method of software and hardware combining can reduce the amount of additional inductor, cost-saving.

Accompanying drawing illustrates:

The determining program flow chart of Fig. 1 mode of operation of the present invention.

Fig. 2 normal mode of operation control structure of the present invention schematic diagram.

Fig. 3 the present invention is slight low load operation mode control architecture schematic diagram.

The serious low load operation mode control architecture schematic diagram of Fig. 4 the present invention.

Fig. 5 is based on the three-phase grid-connected inverter of LCL filter.

The relation curve of Fig. 6 system real output and current on line side THD.

The change of net side output current THD during the change of Fig. 7 switching frequency.

The change of current on line side THD when Fig. 8 brachium pontis side inductance and the inductance change of net side.

Embodiment

Below in conjunction with Figure of description, the present invention is made a concrete analysis of.

First, topological structure visible Fig. 2, Fig. 3, the Fig. 4 of the combining inverter with LCL filter of relating to of this control method.This topological structure comprises load load, DC side filter capacitor C _dc, three-phase full-bridge inverting circuit, LCL filter, DC side filter capacitor C _dcbe connected in parallel on the two ends of load load, DC side filter capacitor C _dctwo ends be connected with two inputs of three-phase full-bridge inverting circuit respectively, the three-phase output end of three-phase full-bridge inverting circuit is connected with the three-phase input end one_to_one corresponding of LCL filter.The three-phase output end of LCL filter respectively with brachium pontis side additional inductor L _e1be connected with three phase network again after series connection, described each brachium pontis side additional inductor L _e1all with corresponding control relay K _l1in parallel.

(1) according to the difference of the ratio of inverter real output and rated power, the judgement of residing mode of operation is completed.

Figure 1 shows that the determining program flow chart of mode of operation, e _a, e _b, e _cfor the line voltage gathered, i _a, i _b, i _cfor the brachium pontis side output current gathered, operate to example with unity power factor, then P=e _a* i _a+ e _b* i _b+ e _c* i _cfor inverter real output.P _nfor rated power, then λ=P/P _nfor the ratio of inverter real output and rated power.According to the difference of λ value, be divided into different mode of operations.When λ >=0.6, be in normal mode of operation; As 0.3 < λ < 0.6, be in slight low load operation pattern; When λ≤0.3, be in serious low load operation pattern.

(2) when inverter is in normal mode of operation, its control structure schematic diagram as shown in Figure 2.As seen from Figure 2 during this mode of operation, control relay K switch _l1closed, brachium pontis side additional inductor L _e1be shorted, the control method that this mode of operation adopts is that classical voltage and current double closed-loop PI controls.Described classical voltage and current double closed-loop PI controls to comprise outer voltage and current inner loop control, and outer voltage and current inner loop control all to be controlled by pi regulator.

The rate-determining steps that so-called classical voltage and current double closed-loop PI controls is as follows:

One: gather DC side filter capacitor C _dcon voltage U _dc, brachium pontis side current i _a, i _b, i _cand line voltage E _a, E _b, E _c, by line voltage E _a, E _b, E _cobtain electric network voltage phase angle γ through phase-locked loop pll is phase-locked.

Two: the given U of DC voltage _dcrefwith DC side filter capacitor C _dcon voltage U _dcregulate through voltage PI regulator after subtracting each other, the output of voltage PI regulator is as the given i of current inner loop active current _dref.

Three: the brachium pontis side current i of collection _a, i _b, i _cutilize the active current feedback quantity i of electric network voltage phase angle γ under abc-dq coordinate transform becomes synchronous rotating frame dq _dwith reactive current feedback quantity i _q, the given i of active current _drefwith active current feedback quantity i _dsubtract each other, the given i of reactive current _qrefwith reactive current feedback quantity i _qsubtract each other, two subtract each other after output variable regulate through current PI adjuster; The given i of described reactive current _qref0 is taken as when unity power factor control.

Four: the output of current PI adjuster utilizes the controlled quentity controlled variable U of electric network voltage phase angle γ under dq-abc coordinate transform becomes three-phase static coordinate system abc _sa, U _sb, U _sc, the controlled quentity controlled variable U under three-phase static coordinate system abc _sa, U _sb, U _scinverter switching device signal S is generated through SPWM modulation _abccontrol three-phase full-bridge inverter exports.

Five: closing control relay K _l1switch, brachium pontis side additional inductor L _e1be shorted.

(3) when inverter is in slight low load operation pattern, its control structure schematic diagram as shown in Figure 3.As seen from Figure 3 during this mode of operation, control relay K switch _l1closed, brachium pontis side additional inductor L _e1be shorted.

The method that this mode of operation adopts is on the basis retaining above-mentioned classical voltage and current double closed-loop PI control, increases following software measure:

A: improve inverter switching frequency.In this enforcement, inverter switching frequency controls 2 times at nominal switching frequency.

Figure 5 shows that the three-phase grid-connected inverter based on LCL filter, obtain the expression formula of current on line side THD according to the definition of THD:

${\mathrm{THD}}_g=\frac{1}{6\sqrt{2}}\frac{E^2}{f_{\mathrm{sw}}{L}_{i}P(4{\mathrm{\&pi;}}^2{f}_{\mathrm{sw}}^2{L}_{g}C-1)}\sqrt{\frac{3}{2}-\frac{4\sqrt{3}m}{\mathrm{\&pi;}}+\frac{9m^2}{8}}---\left(1\right)$

Wherein: P is grid-connected system real output, E is grid line voltage effective value, and m is modulation factor, f _swfor switching frequency (sample frequency is equal with switching frequency), L _ifor brachium pontis side inductance value, L _gfor net side inductance value, C is filtering capacitance.

Figure 6 shows that system real output and current on line side THD _grelation curve (wherein abscissa represents system real output, and ordinate represents current on line side THD _g), can find out, along with the reduction of power output, current on line side THD _galmost exponentially form rises.For usual grid-connected inverting system, in nominal load situation, current on line side THD _ggenerally be not more than 3%, under considering worst situation, in 3%.

From formula (1), current on line side THD _gbe inversely proportional to system power, therefore when systematic steady state operates in the rated load of 0.6 times, current on line side THD _gto 5% be risen to.Therefore time below the rated load that systematic steady state operates in 0.6 times, current on line side THD _g5% will be greater than and not meet grid-connected requirement.As can be seen here, when system is when low carrying row (below the rated load of 0.6 times), need to take appropriate measures and improve current on line side THD _g.

From current on line side THD _gcomputing formula in can find out, improve switching frequency and change filter parameter can reduce current on line side THD _g.

Usual change filter parameter is more complicated, therefore first-selected raising switching frequency, as can be seen from formula (1), and switching frequency and current on line side THD _galmost become 3 power inverse relations.Current on line side THD when Figure 7 shows that switching frequency changes _g(wherein abscissa represents system switching frequency to the situation of change, and ordinate represents current on line side THD _g), as can be seen from the figure, current on line side THD _gvery responsive to the change of switching frequency, improve switching frequency and can effectively reduce current on line side THD _g.

Switching frequency is determined by Systematical control, and in theory along with the reduction of system output power, switching frequency improves smoothly and can make at almost total power section output current THD _gconstant.But this must add numerically controlled complexity, special in power fluctuation, switching frequency also fluctuates very large, even causes the instability of whole system.

Consider the duration problem of above situation and program computation, here switching frequency unsmooth change, and be only when low carrying row, switching frequency is double, and the sample frequency of former control is constant.

B: be incorporated to repetitive controller when the inner ring PI adjustment in voltage and current double closed-loop PI controls and current PI regulate, namely realize the common adjustment of internal circular current with current PI adjuster and repetitive controller combination regulation mode.

The raising thing followed of actual breaker in middle frequency is the aggravation of the impact in dead band.Therefore, the basis that switching frequency improves needs to add some dead area compensations or harmonics restraint strategy, if Repetitive controller is to reduce the THD of output current _g, improve output current quality.

To sum up, under this pattern, rate-determining steps is as follows:

One: gather DC side filter capacitor C _dcon voltage U _dc, brachium pontis side current i _a, i _b, i _cand line voltage E _a, E _b, E _c, by line voltage E _a, E _b, E _cobtain electric network voltage phase angle γ through phase-locked loop pll is phase-locked.

Two: the given U of DC voltage _dcrefdeduct DC side filter capacitor C _dcon voltage U _dcregulate by voltage PI regulator, the output of voltage PI regulator is as the given i of current inner loop active current _dref.

Three: the brachium pontis side current i of collection _a, i _b, i _cutilize the active current feedback quantity i of electric network voltage phase angle γ under abc-dq coordinate transform becomes synchronous rotating frame dq _dwith reactive current feedback quantity i _q, the given i of active current _drefwith active current feedback quantity i _dsubtract each other, the given i of reactive current _qrefwith reactive current feedback quantity i _qsubtract each other, two subtract each other after output variable regulate through current PI adjuster is in parallel with repetitive controller; The given i of described reactive current _qref0 is taken as when unity power factor control.

Four: current PI adjuster is in parallel with repetitive controller regulates the rear value exported to utilize the controlled quentity controlled variable U of electric network voltage phase angle γ under dq-abc coordinate transform becomes three-phase static coordinate system abc _sa, U _sb, U _sc, the controlled quentity controlled variable U under three-phase static coordinate system abc _sa, U _sb, U _scinverter switching device signal S is generated through SPWM modulation _abccontrol three-phase full-bridge inverter exports, and wherein the switching frequency of inverter brings up to 2 times of nominal switching frequency;

Five: closing control relay K _l1switch, brachium pontis side additional inductor L _e1be shorted.

(4) when inverter is in serious low load operation pattern, its control structure schematic diagram as shown in Figure 4.As seen from Figure 4, during this mode of operation, open control relay K _l1switch, brachium pontis side additional inductor L _e1put into operation.

The method that this mode of operation adopts remains on the basis of above-mentioned slight low load operation pattern, and increase hardware measure, the hardware measure adopted in the present embodiment opens control relay K exactly _l1, be incorporated to brachium pontis side additional inductor L _e1.

When system power declines further, switching frequency can not unrestrictedly increase.Supposing can by electric current THD by improving switching frequency _gdrop to original 0.5 times, then, when the rated load of 0.6 times, switching frequency doubles, grid-connected current THD _gdrop to 2.5% by original 5%, when system power drop to further systematic steady state operate in the rated load of 0.3 times time, grid-connected current is elevated to 5% again, and now switching frequency can not improve further, therefore needs to take the Hardware Method changing filter parameter.

To choose brachium pontis inductance and net side inductance as object, and remain unchanged (situation one) at filter capacitor respectively and keep LCL resonance frequency constant, suitably adjusting filter capacitor C(situation two) analyze in two kinds of situations.

Current on line side THD when Figure 8 shows that brachium pontis side inductance and net side inductance change _gchange, in figure, abscissa and ordinate are respectively brachium pontis inductance and net side inductance change pu value, and Z coordinate represents corresponding current on line side THD _g.Beneath face represents situation one; Higher curved surface represents situation two.As can be seen from the figure, iff the single filter parameter of change, then change brachium pontis inductance and can reduce more current on line side THD while increasing less inductance _g.Certainly, if in order to obtain optimal current on line side THD _g, then while brachium pontis inductance increases, want suitable allotment net side inductance L g and filter capacitor C, but too complexity is not considered.

As can be seen here, when system power drops to steady operation further when the rated load of 0.3 times, brachium pontis side inductance can be improved and reduce current on line side THD further _g.The inductance value be incorporated to is identical with filter total inductance amount, but its rated current can obtain lower, and therefore price is more cheap.

To sum up, under this pattern, rate-determining steps is as follows:

One: gather DC side filter capacitor C _dcon voltage U _dc, brachium pontis side current i _a, i _b, i _cand line voltage E _a, E _b, E _c, by line voltage E _a, E _b, E _cobtain electric network voltage phase angle γ through phase-locked loop pll is phase-locked.

Two: the given U of DC voltage _dcrefwith DC side filter capacitor C _dcon voltage U _dcregulate through voltage PI regulator after subtracting each other, the output of voltage PI regulator is as the given i of current inner loop active current _dref.

Three: the brachium pontis side current i of collection _a, i _b, i _cutilize the active current feedback quantity i of electric network voltage phase angle γ under abc-dq coordinate transform becomes synchronous rotating frame dq _dwith reactive current feedback quantity i _q, the given i of active current _drefwith active current feedback quantity i _dsubtract each other, the given i of reactive current _qrefwith reactive current feedback quantity i _qsubtract each other, two subtract each other after output variable regulate through current PI adjuster is in parallel with repetitive controller; The given i of described reactive current _qref0 is taken as when unity power factor control.

Four: current PI adjuster is in parallel with repetitive controller regulates the rear value exported to utilize the controlled quentity controlled variable U of electric network voltage phase angle γ under dq-abc coordinate transform becomes three-phase static coordinate system abc _sa, U _sb, U _sc, the controlled quentity controlled variable U under three-phase static coordinate system abc _sa, U _sb, U _scinverter switching device signal S is generated through SPWM modulation _abccontrol three-phase full-bridge inverter exports, and wherein switching frequency brings up to 2 times of nominal switching frequency;

Five: open control relay K _l1switch, brachium pontis side additional inductor L _e1put into operation.

As fully visible, the present invention can make combining inverter ensure the quality of grid-connected current when low load operation, and adopts the control method of software and hardware combining can reduce the amount of additional inductor, cost-saving.

Claims (4)

1. improve a control method for current quality under the low load operation of combining inverter, the topological structure of the combining inverter with LCL filter that this control method relates to comprises load load, DC side filter capacitor C _dc, three-phase full-bridge inverting circuit, LCL filter, DC side filter capacitor C _dcbe connected in parallel on the two ends of load load, DC side filter capacitor C _dctwo ends be connected with two inputs of three-phase full-bridge inverting circuit respectively, the three-phase output end of three-phase full-bridge inverting circuit is connected with the three-phase input end one_to_one corresponding of LCL filter, it is characterized in that: the three-phase output end of LCL filter respectively with brachium pontis side additional inductor L _e1be connected with three phase network again after series connection, each brachium pontis side additional inductor L _e1all with corresponding control relay K _l1in parallel;

This control method is according to the real output P of inverter and rated power P _nthe difference of ratio λ, complete the judgement of mode of operation residing for inverter, and the entirety of carrying out control strategy switches;

Pattern one: as real output P and the rated power P of inverter _nratio λ when 0.6 ~ 1 scope, judge inverter be in normal mode of operation, now described control relay K _l1closed, adopt classical voltage and current double closed-loop PI to control, described classical voltage and current double closed-loop PI controls to comprise outer voltage and current inner loop control, and outer voltage and current inner loop control all to be controlled by pi regulator;

Pattern two: as real output P and the rated power P of inverter _nratio λ in 0.3 ~ 0.6 scope time, judge inverter be in slight low load operation pattern, now described control relay K _l1closed, on the basis that voltage and current double closed-loop PI classical described in retained-mode one controls, increase software measure; Described software measure comprises improving on inverter switching frequency, current PI adjuster in classical voltage and current double closed-loop PI controls and is incorporated to repetitive controller, namely realizes the common adjustment of internal circular current with current PI adjuster and repetitive controller combination regulation mode;

Pattern three: as real output P and the rated power P of inverter _nratio λ below 0.3 time, judge that inverter is in serious low load operation pattern, on the basis of the control method described in retained-mode two and software measure, increase hardware measure, described hardware measure comprises opens described control relay K _l1, brachium pontis side additional inductor L _e1put into operation.

2. a kind of control method improving current quality under the low load operation of combining inverter according to claim 1, is characterized in that: the control strategy in described pattern one performs according to following steps:

Step 1: gather DC side filter capacitor C _dcon voltage U _dc, brachium pontis side current i _a, i _b, i _cand line voltage E _a, E _b, E _c, by line voltage E _a, E _b, E _cobtain electric network voltage phase angle γ through phase-locked loop pll is phase-locked;

Step 2: the given U of DC voltage _dcrefwith DC side filter capacitor C _dcon voltage U _dcregulate through voltage PI regulator after subtracting each other, the output of voltage PI regulator is as the given i of current inner loop active current _dref;

Step 3: the brachium pontis side current i of collection _a, i _b, i _cutilize the active current feedback quantity i of electric network voltage phase angle γ under abc-dq coordinate transform becomes synchronous rotating frame dq _dwith reactive current feedback quantity i _q, the given i of active current _drefwith active current feedback quantity i _dsubtract each other, the given i of reactive current _qrefwith reactive current feedback quantity i _q

Subtract each other, two subtract each other after output variable regulate through current PI adjuster; The given i of described reactive current _qref0 is taken as when unity power factor control;

Step 4: the output of current PI adjuster utilizes the controlled quentity controlled variable U of electric network voltage phase angle γ under dq-abc coordinate transform becomes three-phase static coordinate system abc _sa, U _sb, U _sc, the controlled quentity controlled variable U under three-phase static coordinate system abc _sa, U _sb, U _scinverter switching device signal S is generated through SPWM modulation _abccontrol three-phase full-bridge inverter exports;

Step 5: closing control relay K _l1switch, brachium pontis side additional inductor L _e1short circuit.

3. a kind of control method improving current quality under the low load operation of combining inverter according to claim 1, is characterized in that: the control strategy in described pattern two performs according to following steps:

Step 1: gather DC side filter capacitor C _dcon voltage U _dc, brachium pontis side current i _a, i _b, i _cand line voltage E _a, E _b, E _c, by line voltage E _a, E _b, E _cobtain electric network voltage phase angle γ through phase-locked loop pll is phase-locked;

Step 2: the given U of DC voltage _dcrefwith DC side filter capacitor C _dcon voltage U _dcregulate through voltage PI regulator after subtracting each other, the output of voltage PI regulator is as the given i of current inner loop active current _dref;

Step 3: the brachium pontis side current i of collection _a, i _b, i _cutilize the active current feedback quantity i of electric network voltage phase angle γ under abc-dq coordinate transform becomes synchronous rotating frame dq _dwith reactive current feedback quantity i _q, the given i of active current _drefwith active current feedback quantity i _dsubtract each other, the given i of reactive current _qrefwith reactive current feedback quantity i _qsubtract each other, two subtract each other after output variable regulate through current PI adjuster is in parallel with repetitive controller; The given i of described reactive current _qref0 is taken as when unity power factor control;

Step 4: current PI adjuster is in parallel with repetitive controller regulates the rear value exported to utilize the controlled quentity controlled variable U of electric network voltage phase angle γ under dq-abc coordinate transform becomes three-phase static coordinate system abc _sa, U _sb, U _sc, the controlled quentity controlled variable U under three-phase static coordinate system abc _sa, U _sb, U _scinverter switching device signal S is generated through SPWM modulation _abccontrol three-phase full-bridge inverter exports, and wherein the switching frequency of inverter brings up to 2 times of nominal switching frequency;

Step 5: closing control relay K _l1switch, brachium pontis side additional inductor L _e1short circuit.

4. a kind of control method improving current quality under the low load operation of combining inverter according to claim 1, is characterized in that: the control strategy in described pattern three performs according to following steps:

Step 1: gather DC side filter capacitor C _dcon voltage U _dc, brachium pontis side current i _a, i _b, i _cand line voltage E _a, E _b, E _c, by line voltage E _a, E _b, E _cobtain electric network voltage phase angle γ through phase-locked loop pll is phase-locked;

Step 2: the given U of DC voltage _dcrefwith DC side filter capacitor C _dcon voltage U _dcregulate through voltage PI regulator after subtracting each other, the output of voltage PI regulator is as the given i of current inner loop active current _dref;

Step 3: the brachium pontis side current i of collection _a, i _b, i _cutilize the active current feedback quantity i of electric network voltage phase angle γ under abc-dq coordinate transform becomes synchronous rotating frame dq _dwith reactive current feedback quantity i _q, the given i of active current _drefwith active current feedback quantity i _dsubtract each other, the given i of reactive current _qrefwith reactive current feedback quantity i _qsubtract each other, two subtract each other after output variable regulate through current PI adjuster is in parallel with repetitive controller; The given i of described reactive current _qref0 is taken as when unity power factor control;

Step 4: current PI adjuster is in parallel with repetitive controller regulates the rear value exported to utilize the controlled quentity controlled variable U of electric network voltage phase angle γ under dq-abc coordinate transform becomes three-phase static coordinate system abc _sa, U _sb, U _sc, the controlled quentity controlled variable U under three-phase static coordinate system abc _sa, U _sb, U _scinverter switching device signal S is generated through SPWM modulation _abccontrol three-phase full-bridge inverter exports, and wherein the switching frequency of inverter brings up to 2 times of nominal switching frequency;

Step 5: open control relay K _l1switch, brachium pontis side additional inductor L _e1put into operation.