CN104601028A - Neutral-point voltage control system and method for parameter on-line tuning - Google Patents

Abstract

The invention discloses a neutral-point voltage control system for parameter on-line tuning and a neutral-point voltage control method for parameter on-line tuning. The neutral-point voltage control system comprises a three-level inverter and a digital processing control module, wherein the digital processing control module comprises a sampling unit, a digital phase-locked loop unit, an output loop-closed control unit, a PI parameter online tuning unit, a neutral-point voltage PI control unit and an SVPWM (Space Vector Pulse Width Modulation) vector control unit; the output end of the SVPWM vector control unit is connected with each phase bridge arm of the three-level inverter through a driving circuit. The direct-current bias of the neutral-point voltage and the neutral-point voltage fluctuation can be restrained by the system and the method, so that the neutral-point voltage balance control can be realized.

Description

The mid-point voltage control system of parameter on-line tuning and method

Technical field

The invention belongs to the control technology field in Technics of Power Electronic Conversion technology, particularly a kind of mid-point voltage control system of parameter on-line tuning and method.

Background technology

Along with developing rapidly of power electronic device, the power electronic equipment of high-power and Alternating Current Governor System are widely used in fields such as steel-making steel rolling industrial system, electrochemical industry system, railway ship power systems.These power electronic equipments all have the features such as control overflow is high, power is large, electric pressure is high, system complex.There is many restrictions such as the voltage stress height that switching loss is high, electromagnetic interference large, switching tube bears and make two-level inverter be difficult to meet these application scenarios in two-level inverter.Compared with two-level inverter, multi-electrical level inverter output level number is many, the staircase waveform platform exponent number obtained is many, thus the sine wave that is more near the mark, its harmonic components is fewer, the realization appearing as high-power system of multi-electrical level inverter provides one and takes into account high-performance and jumbo possibility, and its circuit topological structure and control method become current study hotspot.

The topology of multi-electrical level inverter can be divided three classes substantially: neutral point clamp type, cascade connection type, striding capacitance type, wherein most widely used is neutral point clamp type inverter.For three-level inverter, three level neutral point clamp type inverter can be divided into diode-clamped (NPC) inverter and T-shaped inverter.Ideally, in these two kinds of topologys, DC bus-bar voltage is divided equally by upper and lower dc-link capacitance; But in practical application, because a variety of causes makes upper and lower dc-link capacitance unbalanced-voltage-division weigh, namely mid-point voltage is uneven, and its form of expression can be divided into: (1) mid-point voltage direct current biasing; (2) mid-point voltage fluctuation.Mid-point voltage energy imbalance can cause that output waveform is of poor quality, the dangerous operation of power switch pipe, serious time influential system normally work.So neutral-point voltage balance problem is the research emphasis of three-level inverter always.

For this problem, Chinese scholars proposes the method for many solution neutral point voltage balance problems, and conventional method mainly divides the method for for a change hardware topology and the implement software method of regulable control algorithm.The method changing hardware topology has: (1) multiple DC source method; DC bus derided capacitors is replaced by DC source; (2) additional firmware circuit method.The advantage of hardware approach is that control effects is not subject to the operating mode of inverter and the impact of load variations, but all can increase hardware cost.Therefore, current most study neutral-point voltage balance method is the implement software method based on control algolithm, such as: (1) carries out the research of neutral point voltage balance based on space vector modulation; (2) research of neutral point voltage balance is carried out based on carrier modulation.The realization of first method is mainly by realizing neutral point voltage balance the action time redistributing the positive and negative small vector of redundancy.The realization of second method mainly realizes neutral point voltage balance by adding zero-sequence component in modulating wave.Such as: document 1 (Gong Bo, Cheng Shanmei, Qin Yi. based on the three level neutral-point voltage balance strategy [J] of carrier wave. electrotechnics journal, 2013, 28 (6): 172-177.) and patent 1 (Cheng Shanmei, Gong Bo, peaceful blog article, Liu Jiang. a kind of NPC type three-level inverter mid-point voltage control method based on carrier wave amplitude shift: China, CN201210140269.0.2012-09-19.) a kind of SPWM control program based on carrier wave luffing is proposed, change carrier amplitude in real time by sampling mid-point voltage to realize eliminating mid-point voltage direct current biasing, but the object that the method controls is the mean value of mid-point voltage unbalanced component at a power frequency period, therefore for the fluctuation of mid-point voltage without control effects.

Summary of the invention

The object of the present invention is to provide a kind of control method is simple, be easy to the parameter on-line tuning of Digital Implementation mid-point voltage control system and method.

The technical solution realizing the object of the invention is: a kind of mid-point voltage control system of parameter on-line tuning, comprises three-level inverter and digital processing control module, wherein:

Described digital processing control module comprises sampling unit, digital servo-control ring element, output closed-loop control unit, PI parameter on-line tuning unit, mid-point voltage PI control unit and SVPWM vector control unit;

Described sampling unit gathers DC bus-bar voltage and DC bus mid-point voltage signal, the three-phase voltage signal of three-level inverter output, the three-phase current signal of three-level inverter output of three-level inverter respectively;

The output of described SVPWM vector control unit is through every each switching tube of phase brachium pontis of drive circuit access three-level inverter;

In each switch periods, the sampling unit of digital processing control module gathers DC bus-bar voltage and DC bus mid-point voltage signal, the three-phase voltage signal of three-level inverter output, the three-phase current signal of three-level inverter output of three-level inverter respectively, obtains three-phase with reference to phase voltage signal through the process of output closed-loop control unit;

The phase current signal that the phase voltage that inverter that sampling unit collects exports, inverter export obtains phase voltage, the phase angle difference of phase current and power-factor angle through digital servo-control ring element

The power-factor angle that digital servo-control ring element obtains the three-phase current signal that sampling unit obtains obtains PI parameter through PI parameter on-line tuning cell processing;

The DC bus-bar voltage that the PI parameter that PI parameter on-line tuning unit obtains, sampling unit obtain and DC bus mid-point voltage signal, the process of output closed-loop control unit obtain three-phase with reference to phase voltage signal V _a, V _b, V _cnew three-phase is obtained with reference to phase voltage signal V through mid-point voltage PI control unit _a', V _b', V _c', this three-phase obtains pulse-width modulation control signal with reference to phase voltage signal through the process of SVPWM vector control unit;

Described pulse-width signal normally works through drive circuit control control three-level inverter, and achieves the biased suppression effective in mid-point voltage fluctuation of alignment voltage DC, thus controls neutral point voltage balance.

According to improvement of the present invention, a kind of mid-point voltage control method of the parameter on-line tuning based on aforementioned mid-point voltage control system is also proposed, the method is in the PI parameter of each power frequency period according to current three-phase current amplitude, power-factor angle on-line tuning mid-point voltage PI controller, the regulated quantity that the DC bus-bar voltage that sampling unit obtains and DC bus mid-point voltage signal obtain three-phase reference phase voltage after mid-point voltage PI controller corrects regulates three-phase with reference to phase voltage signal in real time, thus reach the object regulating mid-point voltage, comprise the following steps:

Step 1, in each switch periods, the three-phase current signal that the three-phase voltage signal that the sampling unit of digital processing control module gathers the DC bus-bar voltage of three-level inverter and DC bus mid-point voltage signal respectively, three-level inverter exports, three-level inverter export;

Step 2, output closed-loop control unit carry out proportion integration differentiation adjustment according to the signal collected in step 1 and base reference signal, export three-phase with reference to phase voltage signal V _a, V _b, V _c;

Step 3, digital servo-control ring element process the phase voltage collected in step 1, phase current signal and obtain phase voltage, the phase angle difference of phase current and power-factor angle

Step 4, PI parameter on-line tuning unit process the PI parameter obtaining mid-point voltage PI control unit to the power factor angle signal obtained in the three-phase current signal collected in step 1 and step 3;

Step 5, mid-point voltage PI control unit to the three-phase obtained in the DC bus-bar voltage collected in step 1 and DC bus mid-point voltage signal, step 2 with reference to phase voltage signal V _a, V _b, V _cprocess with the PI parameter obtained in step 4, obtain dynamic adjustments amount and be superimposed to three-phase with reference on phase voltage signal, obtain new three-phase with reference to phase voltage signal V _a', V _b', V _c';

The new three-phase reference phase voltage signal V that step 6, SVPWM vector control unit will obtain in step 5 _a', V _b', V _c' carry out vector calculus, obtain the pulse-width modulation control signal that can suppress mid-point voltage direct current biasing and mid-point voltage fluctuation;

Gained pulse-width modulation control signal in step 6 is distributed to every each switching tube of phase brachium pontis of three-level inverter by step 7, drive circuit, control the operating state of three-level inverter, suppress mid-point voltage direct current biasing and mid-point voltage fluctuation simultaneously, thus regulate neutral point voltage balance.

Compared with prior art, its remarkable advantage is in the present invention:

(1) dynamic response of this control method is fast; When the condition of work such as load changing or chugging changes, the PI parameter that PI parameter on-line tuning unit can be adjusted out suitable according to current system health in real time, thus effectively control neutral point voltage balance;

(2) when occurring that mid-point voltage is uneven, this control method can eliminate rapidly mid-point voltage direct current biasing component, effectively suppresses mid-point voltage fluctuation;

(3) this control method decreases output voltage, current harmonics, improves output waveform quality, and control method is simple, real-time is good, is convenient to Digital Realization.

Accompanying drawing explanation

Fig. 1 is the structure chart of the mid-point voltage control system of parameter on-line tuning of the present invention.

Fig. 2 a-2b is NPC type and T-shaped three-level inverter topology structure chart respectively.

Fig. 3 is SVPWM space vector distribution map.

Fig. 4 is the A equivalent modulating wave of SVPWM method.

After Fig. 5 is standardization process, the difference variation diagram of the dc-link capacitance voltage before and after controlling.

Fig. 6 is the simplified block diagram that mid-point voltage PI controls.

Fig. 7 is the flow chart of the mid-point voltage control method of the parameter on-line tuning that the present invention proposes.

Fig. 8 is DC side bus capacitor voltage V under control method of the present invention in embodiment _c1, V _c2oscillogram, wherein (a) is resistive load situation, and (b) is resistance sense loading condition.

Embodiment

Below in conjunction with drawings and the specific embodiments, the present invention is described in further detail.

The mid-point voltage control system of parameter on-line tuning of the present invention and method, Real-time Collection DC bus-bar voltage and DC bus mid-point voltage signal obtain three-phase with reference to phase voltage V through PI controller _a, V _b, V _cregulated quantity; New three-phase after superposition regulated quantity is with reference to phase voltage V _a', V _b', V _c' through the computing of SVPWM vector control, obtain can suppress mid-point voltage direct current biasing and mid-point voltage fluctuation pulse-width modulation control signal, thus realize mid-point voltage balance control.

Composition graphs 1, the mid-point voltage control system of parameter on-line tuning of the present invention, comprise three-level inverter and digital processing control module, digital processing control module comprises sampling unit, digital servo-control ring element, output closed-loop control unit, PI parameter on-line tuning unit, mid-point voltage PI control unit and SVPWM vector control unit; Described sampling unit gathers DC bus-bar voltage and DC bus mid-point voltage signal, the three-phase voltage signal of three-level inverter output, the three-phase current signal of three-level inverter output of three-level inverter respectively, and the output of SVPWM vector control unit is through every each switching tube of phase brachium pontis of drive circuit access three-level inverter.

In each switch periods, the sampling unit of digital processing control module gathers DC bus-bar voltage and DC bus mid-point voltage signal, the three-phase voltage signal of three-level inverter output, the three-phase current signal of three-level inverter output of three-level inverter respectively, obtains three-phase with reference to phase voltage signal through the process of output closed-loop control unit; The phase current signal that the phase voltage that inverter that sampling unit collects exports, inverter export obtains phase voltage, the phase angle difference of phase current and power-factor angle through digital servo-control ring element the power-factor angle that digital servo-control ring element obtains the three-phase current signal that sampling unit obtains obtains PI parameter through PI parameter on-line tuning cell processing; The DC bus-bar voltage that the PI parameter that PI parameter on-line tuning unit obtains, sampling unit obtain and DC bus mid-point voltage signal, the process of output closed-loop control unit obtain three-phase with reference to phase voltage signal V _a, V _b, V _cnew three-phase is obtained with reference to phase voltage signal V through mid-point voltage PI control unit _a', V _b', V _c', this three-phase obtains pulse-width modulation control signal with reference to phase voltage signal through the process of SVPWM vector control unit.Described pulse-width signal normally works through drive circuit control control three-level inverter, and achieves the biased suppression effective in mid-point voltage fluctuation of alignment voltage DC, thus controls neutral point voltage balance.

Preferably, described digital processing control module is STM32F407 chip.

Shown in composition graphs 1, Fig. 7, the disclosure also proposes a kind of mid-point voltage control method of parameter on-line tuning, the method is in the PI parameter of each power frequency period according to current three-phase current amplitude, power-factor angle on-line tuning mid-point voltage PI controller, the regulated quantity that the DC bus-bar voltage that sampling unit obtains and DC bus mid-point voltage signal obtain three-phase reference phase voltage after mid-point voltage PI controller corrects regulates three-phase with reference to phase voltage signal in real time, thus reach the object regulating mid-point voltage, comprise the following steps:

Step 1, in each switch periods, the three-phase current signal that the three-phase voltage signal that the sampling unit of digital processing control module gathers the DC bus-bar voltage of three-level inverter and DC bus mid-point voltage signal respectively, three-level inverter exports, three-level inverter export;

Step 2, output closed-loop control unit carry out proportion integration differentiation adjustment according to the signal collected in step 1 and base reference signal, export three-phase with reference to phase voltage signal V _a, V _b, V _c;

Step 3, digital servo-control ring element process the phase voltage collected in step 1, phase current signal and obtain phase voltage, the phase angle difference of phase current and power-factor angle

Step 4, PI parameter on-line tuning unit process the PI parameter obtaining mid-point voltage PI control unit to the power factor angle signal obtained in the three-phase current signal collected in step 1 and step 3, concrete steps are:

(4.1) according to the three-phase current determination current amplitude I that inverter in step 1 exports _m;

(4.2) according to current amplitude I _mwith the power-factor angle of step 3 gained determine the PI parameter of current switch period: proportionality coefficient k _{p (N)}, integration time constant T _(N), specific as follows:

In formula: m is modulation ratio; C is DC bus capacitor capacitance; ω _cfor cut-off frequency, ω _cbe typically chosen in the switching frequency f of 0.1 times _splace.Just proportionality coefficient k can be determined according to above formula after determining cut-off frequency _pvalue.The integration time constant T of PI controller is corner frequency ω _sinverse, in order to ensure at cut-off frequency ω _cthere are enough phase margins at place, corner frequency ω _scompare cut-off frequency ω _cthe minimum value that may obtain is little.

Corner frequency ω _s, cut-off frequency ω _ccan Rational choice according to the actual requirements.

(4.3) the PI parameter k of current switch period is compared _{p (N)}, T _(N)with the PI parameter k of a upper switch periods _{p (N-1)}, T _(N-1)whether equal, if unequal, perform (4.4) step; If equal, then perform (4.5) step;

(4.4) to PI parameter k _p, T composes new value k _{p (N)}, T _(N).

(4.5) PI parameter k _p, T is constant.

Step 5, mid-point voltage PI control unit to the three-phase obtained in the DC bus-bar voltage collected in step 1 and DC bus mid-point voltage signal, step 2 with reference to phase voltage signal V _a, V _b, V _cprocess with the PI parameter obtained in step 4, obtain dynamic adjustments amount and be superimposed to three-phase with reference on phase voltage signal, obtain new three-phase with reference to phase voltage signal V _a', V _b', V _c'; The deterministic process of regulated quantity △ V is as follows:

$\mathrm{\ΔV}=k_p\mathrm{\Δ}{V}_c+\frac{k_p}{T}{\∫}_0^t\mathrm{\Δ}{V}_c\mathrm{dt}$

In formula, △ V _cfor the difference of DC side two bus capacitor voltages.

The new three-phase reference phase voltage signal V that step 6, SVPWM vector control unit will obtain in step 5 _a', V _b', V _c' carry out vector calculus, obtain the pulse-width modulation control signal that can suppress mid-point voltage direct current biasing and mid-point voltage fluctuation;

Gained pulse-width modulation control signal in step 6 is distributed to every each switching tube of phase brachium pontis of three-level inverter by step 7, drive circuit, control the operating state of three-level inverter, suppress mid-point voltage direct current biasing and mid-point voltage fluctuation simultaneously, thus regulate neutral point voltage balance.

Below for T-shaped and NPC type three-level inverter, elaborate the invention process process.

(1) the uneven principle analysis of mid-point voltage under traditional SVPWM method

Fig. 2 a-2b is be T-shaped and NPC type three-level inverter topology respectively, and every phase brachium pontis has four switching tube S _x1, S _x2, S _x3, S _x4(x=a, b, c).Inverter every phase brachium pontis switching tube S is represented respectively with letter " P ", " 0 ", " N " _x1, S _x2, S _x3, S _x4three kinds of operating states and three kinds of brachium pontis output voltages.Ideally, the detailed corresponding relation of brachium pontis output voltage and on off state is in table 1.

Table 1 three-level inverter brachium pontis output level and on off state reference table

In table, " √ " represents that switching tube is open-minded, and "×" represents that switching tube turns off, V _dcrepresent DC bus-bar voltage.Often have three kinds of operating states mutually, so three-phase then has 27 (3 ³) plant operating state.With a vector in a kind of operating state corresponding SVPWM space vector distribution map, then 27 kinds of corresponding 27 vectors of operating state.SVPWM space vector distribution map as shown in Figure 3.Observe in Fig. 3 these 27 vectors known and can be divided into four classes according to mould length: 6 mould length are 2V _dcthe large vector of/3; 6 mould length are middle vector; 12 mould length are V _dcthe small vector of/3; 3 mould length is the zero vector of zero.When only having middle vector small vector effect in these 27 vectors, DC bus mid point is connected with load, and now mid point electric current is certain the phase load electric current being connected to mid point, and the transient expression formula of mid point electric current is:

i _o＝(1-|S _a|)i _a+(1-|S _b|)i _b+(1-|S _c|)i _c(1)

S in formula _x(x=a, b, c) is the function about inverter three kinds of operating states:

$S_x=\left\{\begin{array}{c}1\\ 0\\ -1\end{array}\right.---\left(2\right)$

1 representative " P " state, 0 representative " 0 " state ,-1 representative " N " state.Usually using the mean value of mid point electric current in switch periods as its instantaneous value:

i _o＝D _aoi _a+D _boi _b+D _coi _c(3)

Wherein D _ao, D _bo, D _cobe a switch periods T _sin time, threephase load is connected to the duty ratio of mid point.

Shown in Fig. 2 a, 2b, with the direction of mid point outflow of bus current for positive direction, the difference △ V of DC side two bus capacitor voltages _cwith the pass of mid point electric current be:

$\mathrm{\Δ}{V}_c=V_{c1}-V_{c2}=\frac{T_s}{C}{i}_o---\left(4\right)$

In formula, C is dc-link capacitance C ₁, C ₂capacitance, T _sfor switch periods, V _c1for electric capacity instantaneous voltage, V between the positive pole of DC bus and mid point _c2for the electric capacity instantaneous voltage between the mid point of DC bus and negative pole.Therefore, when neutral point voltage balance, △ V _c=0.Formula (3) is brought in formula (4) △ V can be obtained _cexpression formula be:

$\mathrm{\Δ}{V}_c=\frac{T_s}{C}(D_{\mathrm{ao}}{i}_a+D_{\mathrm{bo}}{i}_b+D_{\mathrm{co}}{i}_c)---\left(5\right)$

Wherein i _a, i _b, i _cfor inverter three-phase output load current, can be expressed as:

I in formula (6) _mfor the amplitude of load current, for load power factor angle, ω is power frequency angular frequency.When adopting SVPWM method, in order to can from modulating wave angle analysis D _ao, D _bo, D _coconcrete expression formula, SVPWM modulation is equivalent to the multi-carrier modulation scheme adding specific zero-sequence component in standard sine modulating wave, then for A phase, the equivalent modulating wave expression formula of SVPWM is:

$V_a=\left\{\begin{array}{cc}\sqrt{3}m\sin \left(\mathrm{\&omega;t}\right)& 0\&le;\mathrm{\&omega;t}<\frac{\mathrm{\&pi;}}{6},\frac{5\mathrm{\&pi;}}{6}\&le;\mathrm{\&omega;t}<\frac{7\mathrm{\&pi;}}{6},\frac{11\mathrm{\&pi;}}{6}\&le;\mathrm{\&omega;t}\&le;2\mathrm{\&pi;}\\ m\sin (\mathrm{\&omega;t}+\frac{\mathrm{\&pi;}}{6})& \frac{\mathrm{\&pi;}}{6}\&le;\mathrm{\&omega;t}<\frac{\mathrm{\&pi;}}{2},\frac{7\mathrm{\&pi;}}{6}\&le;\mathrm{\&omega;t}<\frac{3\mathrm{\&pi;}}{2}\\ m\sin (\mathrm{\&omega;t}-\frac{\mathrm{\&pi;}}{6})& \frac{\mathrm{\&pi;}}{2}\&le;\mathrm{\&omega;t}<\frac{5\mathrm{\&pi;}}{6},\frac{3\mathrm{\&pi;}}{2}\&le;\mathrm{\&omega;t}\&le;\frac{11\mathrm{\&pi;}}{6}\end{array}\right.---\left(7\right)$

In formula (7), m is modulation ratio.V _afigure as shown in Figure 4, is the saddle ripple of standard.Being not difficult to write out is a switch periods T _sin time, A phase load is connected to the duty ratio D of mid point _aoexpression formula such as formula shown in (8), the expression formula of all the other two-phases only differs 2 π/3 with A phase in phase place.

$D_{\mathrm{ao}}=\left\{\begin{array}{cc}1-\sqrt{3}m\sin \left(\mathrm{\&omega;t}\right)& 0\&le;\mathrm{\&omega;t}<\frac{\mathrm{\&pi;}}{6},\frac{5\mathrm{\&pi;}}{6}\&le;\mathrm{\&omega;t}<\frac{7\mathrm{\&pi;}}{6},\frac{11\mathrm{\&pi;}}{6}\&le;\mathrm{\&omega;t}\&le;2\mathrm{\&pi;}\\ 1-m\sin (\mathrm{\&omega;t}+\frac{\mathrm{\&pi;}}{6})& \frac{\mathrm{\&pi;}}{6}\&le;\mathrm{\&omega;t}<\frac{\mathrm{\&pi;}}{2},\frac{7\mathrm{\&pi;}}{6}\&le;\mathrm{\&omega;t}<\frac{3\mathrm{\&pi;}}{2}\\ 1-m\sin (\mathrm{\&omega;t}-\frac{\mathrm{\&pi;}}{6})& \frac{\mathrm{\&pi;}}{2}\&le;\mathrm{\&omega;t}<\frac{5\mathrm{\&pi;}}{6},\frac{3\mathrm{\&pi;}}{2}\&le;\mathrm{\&omega;t}\&le;\frac{11\mathrm{\&pi;}}{6}\end{array}\right.---\left(8\right)$

Get m=0.8, by T _s, C, I _mstandardization process, by D _ao, D _bo, D _coexpression formula substitute into the difference △ V making DC side two bus capacitor voltages in formula (5) _cwith the tendency chart that ω t changes, as shown in Figure 5, △ V _cwith 3 times of work frequency fluctuations.

(2) under control method of the present invention, mid-point voltage control principle is analyzed

The difference of capacitance voltage and the difference of reference value are superimposed to three limits as regulated quantity and obtain the PWM ripple with mid-point voltage control ability with reference on phase voltage signal after PI controller, and the simplified block diagram that mid-point voltage PI controls as shown in Figure 6.

In figure, G (s) is the transfer function of PI controller, and expression formula is:

$G\left(s\right)=k_p\left(\frac{1+\mathrm{sT}}{\mathrm{sT}}\right)---\left(9\right)$

In figure, Gc (s) is ssystem transfer function, by the difference △ V of Kirchhoff's current law (KCL) and dc-link capacitance voltage _cwith mid point current i _orelation known, transfer function G in Fig. 6 _cs the expression formula of () is:

$\left\{\begin{array}{c}C\frac{\mathrm{d\&Delta;}{V}_c}{\mathrm{dt}}=-\frac{i_o}{2}\\ G_C\left(s\right)=-\frac{I_o}{2}\end{array}\right.---\left(10\right)$

Wherein I _obe the mean value of a power frequency period mid point electric current:

$I_o=-\frac{1}{2\mathrm{\&pi;}}{\&Integral;}_0^{2\mathrm{\&pi;}}(V_a{i}_a+V_b{i}_b+V_c{i}_c)\mathrm{d\&omega;t}---\left(11\right)$

By formula (6), formula (7) substitute into formula (11) known, I _oexpression formula be:

Substitution formula (10) obtains transfer function G _cs the expression formula of () is:

System open loop transfer function after PI controller corrects is:

G _openthe cut-off frequency ω of (s) _cbe by part determines, cut-off frequency ω _cexpression formula be:

Cut-off frequency ω _cbe typically chosen in the switching frequency f of 0.1 times _splace, if order get m=0.8, by T _s, C, I _mstandardization process, is not difficult to try to achieve the scale parameter k of PI controller by formula (15) _p=0.91.The integration time constant T of PI controller is corner frequency ω _sinverse, in order to ensure at cut-off frequency ω _cthere are enough phase margins at place, corner frequency ω _smust than cut-off frequency ω _cthe minimum value that may obtain is little, so corner frequency ω herein _s=2 π 50rad/s, so integration time constant T=0.02s.The each power frequency period of PI parameter on-line tuning unit judges whether working state of system changes, if change, upgrade the parameter of PI controller, can ensure like this under system different operating state, mid-point voltage control effects is optimum.

The difference △ V of DC side two bus capacitor voltages _cafter PI controller, adjusted amount △ V obtains:

$\mathrm{\&Delta;V}=k_p\mathrm{\&Delta;}{V}_c+\frac{k_p}{T}{\&Integral;}_0^t\mathrm{\&Delta;}{V}_c\mathrm{dt}---\left(16\right)$

Regulated quantity △ V is superimposed to three-phase reference phase voltage V in formula (7) _a, V _b, V _con obtain new three-phase with reference to phase voltage V _a', V _b', V _c':

$\left\{\begin{array}{c}{V}_a^{\&prime;}=V_a+\mathrm{\&Delta;V}\\ V_b^{\&prime;}=V_b+\mathrm{\&Delta;V}\\ V_c^{\&prime;}=V_c+\mathrm{\&Delta;V}\end{array}\right.---\left(17\right)$

Three-phase " 0 " state duty ratio after correction is:

$\begin{array}{c}{D}_{\mathrm{ao}}^{\&prime;}=\left\{\begin{array}{cc}1-V_a-\mathrm{\&Delta;V}& V_a+\mathrm{\&Delta;V}\&GreaterEqual;0\\ 1+V_a+\mathrm{\&Delta;V}& V_a+\mathrm{\&Delta;V}<0\end{array}\right.\\ D_{\mathrm{bo}}^{\&prime;}=\left\{\begin{array}{cc}1-V_b-\mathrm{\&Delta;V}& V_b+\mathrm{\&Delta;V}\&GreaterEqual;0\\ 1+V_b+\mathrm{\&Delta;V}& V_b+\mathrm{\&Delta;V}<0\end{array}\right.\\ D_{\mathrm{co}}^{\&prime;}=\left\{\begin{array}{cc}1-V_c-\mathrm{\&Delta;V}& V_c+\mathrm{\&Delta;V}\&GreaterEqual;0\\ 1+V_c+\mathrm{\&Delta;V}& V_c+\mathrm{\&Delta;V}<0\end{array}\right.\end{array}---\left(18\right)$

Three-phase " 0 " state duty ratio D' after PI is corrected _ao, D ' _bo, D' _coa switch periods T after substitution formula (5) obtains correcting _sthe difference △ V ' of dc-link capacitance voltage in time _c:

$\mathrm{\&Delta;}{V}_c^{\&prime;}=\frac{T_s}{2C}(D_{\mathrm{ao}}^{\&prime;}{i}_a+D_{\mathrm{bo}}^{\&prime;}{i}_b+D_{\mathrm{co}}^{\&prime;}{i}_c)---\left(19\right)$

Get m=0.8, by T _s, C, I _mstandardization process, proportionality coefficient k _p=0.91, the △ V ' after correction _cwith the tendency chart that ω t changes, as shown in Figure 5.As seen from the figure, after PI corrects, △ V ' _camplitude be close to zero, so the fluctuation amplitude of mid-point voltage is almost nil.

Should be noted in adjustment process, three-phase new in formula (17) is with reference to phase voltage V _a', V _b', V _c' following constraints need be met

|V _x'|≤1(x＝a,b,c) (20)

The flow chart of the mid-point voltage control method of the parameter on-line tuning that the present invention proposes as shown in Figure 7.Specific implementation process is as follows:

(1) three-phase current signal that the three-phase voltage signal that the DC bus-bar voltage of three-level inverter of sampling and DC bus mid-point voltage signal, three-level inverter export, three-level inverter export, enters (2);

(2) output closed-loop control unit carries out proportion integration differentiation adjustment according to the signal collected in (1) and base reference signal, exports three-phase with reference to phase voltage signal V _a, V _b, V _c, enter (3);

(3) mid-point voltage PI control unit is to dc-link capacitance voltage difference △ V _ccarry out PI computing and obtain three-phase with reference to phase voltage signal V _a, V _b, V _cregulated quantity △ V:

$\mathrm{\&Delta;V}=k_p\mathrm{\&Delta;}{V}_c+\frac{k_p}{T}{\&Integral;}_0^t\mathrm{\&Delta;}{V}_c\mathrm{dt}$

V _a, V _b, V _csuperposition regulated quantity △ V obtains new three-phase with reference to phase voltage signal V _a', V _b', V _c', enter (4);

(4) the new three-phase that obtains according to (3) of SVPWM vector control unit is with reference to phase voltage signal V _a', V _b', V _c' determine the control signal of three-level inverter every phase brachium pontis switching tube in the vector sequence of the symmetrical output forms of seven segmentation vectors and current switch period, distribute to each switching tube of three-level inverter every phase brachium pontis through drive circuit, control operating state and the neutral point voltage balance of three-level inverter.

PI parameter k in (3) step _p, T PI parameter on-line tuning unit realize, specific implementation process is as follows:

(3.1) the A phase voltage that three-level inverter of sampling exports and three-phase current signal determination current amplitude I _m, and power-factor angle enter (3.2);

(3.2) the PI parameter of current switch period is determined: proportionality coefficient k _{p (N)}, integration time constant T _(N), specific as follows:

In formula: m is modulation ratio; C is DC bus capacitor capacitance; ω _cfor cut-off frequency, ω _cbe typically chosen in the switching frequency f of 0.1 times _splace.Just proportionality coefficient k can be determined according to above formula after determining cut-off frequency _pvalue.The integration time constant T of PI controller is corner frequency ω _sinverse, in order to ensure at cut-off frequency ω _cthere are enough phase margins at place, corner frequency ω _smust than cut-off frequency ω _cthe minimum value that may obtain is little, according to the actual requirements Rational choice;

(3.3) the PI parameter k of current switch period is judged _{p (N)}, T _(N)with the PI parameter k of a upper switch periods _{p (N-1)}, T _(N-1)whether equal, if unequal, perform (3.4) step; If equal, then perform (3.5) step;

(3.4) to PI parameter k _p, T composes new value k _{p (N)}, T _(N);

(3.5) PI parameter k _p, T is constant.

To sum up, under tradition SVPWM method there is the fluctuation of 3 times of work frequencies in mid-point voltage, and biased the fluctuation with mid-point voltage of parameter alignment voltage DC that the mid-point voltage control system of a kind of parameter on-line tuning that the present invention proposes and method can choose suitable PI controller under different operating mode controls, realize neutral point voltage balance.

For feasibility and the validity of the mid-point voltage control method of a kind of parameter on-line tuning of checking the present invention proposition, the Simulink instrument in MATLAB is utilized to build three-level inverter circuit.Electric parameter in simulation process is arranged as following table:

Fig. 8 a-8b gives the C of the dc-link capacitance under above-mentioned electric parameter is arranged ₁, C ₂voltage V _c1, V _c2simulation waveform.For when there is serious imbalance in checking, the control effects of the method that the present invention carries when namely electric capacity voltage phase difference is very large up and down.Up Highway UHW electric capacity C ₁the resistance of 2150 Ω in parallel, makes the inconsistent mid-point voltage of upper and lower electric capacity equiva lent impedance there is direct current biasing and fluctuation.When adopting traditional SVPWM method as shown in Figure 8 before 0.3s, because the inconsistent of circuit parameter causes mid-point voltage substantial deviation reference value, comprise larger direct current biasing with three times of power frequency fluctuations.0.3s switch to the present invention put forward the mid-point voltage control method of parameter on-line tuning, Fig. 8 a is in resistive load situation, reaches eliminate mid-point voltage direct current biasing through the regulating time of about 5ms, and the stable state suppressing mid-point voltage to fluctuate; Fig. 8 b is under resistance sense loading condition, and the regulating time through about 5ms reaches elimination mid-point voltage direct current biasing, and the stable state suppressing mid-point voltage to fluctuate.The mid-point voltage control method control effects of the parameter on-line tuning that simulating, verifying the present invention proposes is remarkable.

In sum, the mid-point voltage control system of parameter on-line tuning of the present invention and method, be applied to three-level inverter, this control method is in the parameter of each power frequency period according to current three-phase current amplitude, power-factor angle on-line tuning PI controller, if working state of system changes, then upgrade the parameter of PI controller, can ensure like this under system different operating state, mid-point voltage control effects is optimum.Error signal between the difference of the dc-link capacitance voltage collected in each switch periods and reference value obtains the regulated quantity for being superimposed to three-phase reference voltage after PI controller, controls neutral point voltage balance in real time.This control method dynamic response is fast, effectively can suppress three-level inverter DC bus mid-point voltage direct current biasing and fluctuation, overcome the shortcoming that outputting inductance current ripples is large in different loads situation; This control method has that output waveform harmonic content is low, real-time good, control method is simple, be convenient to Digital Realization advantage.

Claims (6)

1. a mid-point voltage control system for parameter on-line tuning, is characterized in that, comprises three-level inverter and digital processing control module, wherein:

Described digital processing control module comprises sampling unit, digital servo-control ring element, output closed-loop control unit, PI parameter on-line tuning unit, mid-point voltage PI control unit and SVPWM vector control unit;

Described sampling unit gathers DC bus-bar voltage and DC bus mid-point voltage signal, the three-phase voltage signal of three-level inverter output, the three-phase current signal of three-level inverter output of three-level inverter respectively;

The output of described SVPWM vector control unit is through every each switching tube of phase brachium pontis of drive circuit access three-level inverter;

In each switch periods, the sampling unit of described digital processing control module gathers DC bus-bar voltage and DC bus mid-point voltage signal, the three-phase voltage signal of three-level inverter output, the three-phase current signal of three-level inverter output of three-level inverter respectively, obtains three-phase with reference to phase voltage signal through the process of output closed-loop control unit;

The phase current signal that the phase voltage that three-level inverter that described sampling unit collects exports, three-level inverter export obtains phase voltage, the phase angle difference of phase current and power-factor angle through digital servo-control ring element

The power-factor angle that described digital servo-control ring element obtains the three-phase current signal that sampling unit obtains obtains PI parameter through PI parameter on-line tuning cell processing;

The DC bus-bar voltage that the PI parameter that described PI parameter on-line tuning unit obtains, sampling unit obtain and DC bus mid-point voltage signal, the process of output closed-loop control unit obtain three-phase with reference to phase voltage signal V _a, V _b, V _cnew three-phase is obtained with reference to phase voltage signal V ' through mid-point voltage PI control unit _a, V ' _b, V ' _c, this three-phase obtains pulse-width modulation control signal with reference to phase voltage signal through the process of SVPWM vector control unit;

Described pulse-width signal controls three-level inverter through drive circuit and normally works, and realizes alignment voltage DC and is biased the suppression with mid-point voltage fluctuation, thus control neutral point voltage balance.

2. the mid-point voltage control system of parameter on-line tuning according to claim 1, is characterized in that, described digital processing control module is STM32F407 chip.

3. the mid-point voltage control method based on the parameter on-line tuning of the mid-point voltage control system of parameter on-line tuning described in claim 1, it is characterized in that, three-phase current amplitude, power-factor angle that the method exports according to three-level inverter at each power frequency period information, on-line tuning goes out the PI parameter of mid-point voltage PI controller, DC bus-bar voltage and DC bus mid-point voltage is gathered in each switch periods, the regulated quantity obtaining three-phase reference phase voltage after mid-point voltage PI controller corrects regulates three-phase with reference to phase voltage signal in real time, thus reach the object regulating neutral point voltage balance, its specific implementation comprises the following steps:

Step 1, in each switch periods, the three-phase current signal that the three-phase voltage signal that the sampling unit of digital processing control module gathers the DC bus-bar voltage of three-level inverter and DC bus mid-point voltage signal respectively, three-level inverter exports, three-level inverter export;

Step 2, output closed-loop control unit carry out proportion integration differentiation adjustment according to the signal collected in step 1 and base reference signal, export three-phase with reference to phase voltage signal V _a, V _b, V _c;

Step 3, digital servo-control ring element process the phase voltage collected in step 1, phase current signal and obtain phase voltage, the phase angle difference of phase current and power-factor angle;

Step 4, PI parameter on-line tuning unit process the PI parameter obtaining mid-point voltage PI control unit to the power factor angle signal obtained in the three-phase current signal collected in step 1 and step 3;

Step 5, mid-point voltage PI control unit to the three-phase obtained in the DC bus-bar voltage collected in step 1 and DC bus mid-point voltage signal, step 2 with reference to phase voltage signal V _a, V _b, V _cprocess with the PI parameter obtained in step 4, obtain dynamic adjustments amount and be superimposed to three-phase with reference on phase voltage signal, obtain new three-phase with reference to phase voltage signal V ' _a, V ' _b, V ' _c;

The new three-phase reference phase voltage signal V ' that step 6, SVPWM vector control unit will obtain in step 5 _a, V ' _b, V ' _ccarry out vector calculus, obtain the pulse-width modulation control signal that can suppress mid-point voltage direct current biasing and mid-point voltage fluctuation; And

Gained pulse-width modulation control signal in step 6 is distributed to every each switching tube of phase brachium pontis of three-level inverter by step 7, drive circuit, control the operating state of three-level inverter, suppress mid-point voltage direct current biasing and mid-point voltage fluctuation simultaneously, thus regulate neutral point voltage balance.

4. the mid-point voltage control method of parameter on-line tuning according to claim 3, it is characterized in that, PI parameter on-line tuning unit described in step 4 carries out process to the power factor angle signal obtained in the three-phase current signal collected in step 1 and step 3 and obtains PI parameter, and concrete steps are:

(4.1) according to the three-phase current determination current amplitude I that inverter in step 1 exports _m;

(4.2) according to current amplitude I _mwith the power-factor angle of step 3 gained determine the PI parameter of current switch period: proportionality coefficient k _{p (N)}, integration time constant T _(N);

(4.3) the PI parameter k of current switch period is compared _{p (N)}, T _(N)with the PI parameter k of a upper switch periods _{p (N-1)}, T _(N-1)whether equal, if unequal, perform (4.4) step; If equal, then perform (4.5) step;

(4.4) to PI parameter k _p, T composes new value k _{p (N)}, T _(N);

(4.5) PI parameter k is kept _p, T is constant.

5. the mid-point voltage control method of parameter on-line tuning according to claim 4, is characterized in that, according to current amplitude I described in (4.2) _mwith the power-factor angle of step 3 gained determine the PI parameter of current switch period, specific as follows:

In formula: m is modulation ratio; C is DC bus capacitor capacitance; ω _cfor cut-off frequency, ω _cselect the switching frequency f at 0.1 times _splace, k _pfor proportionality coefficient, the integration time constant T of PI controller is corner frequency ω _sinverse, in order to ensure at cut-off frequency ω _cthere are enough phase margins at place, corner frequency ω _sbe less than cut-off frequency ω _cthe minimum value that may obtain.

6. the mid-point voltage control method of parameter on-line tuning according to claim 3, it is characterized in that, the PI of mid-point voltage described in step 5 control unit to the three-phase obtained in the DC bus-bar voltage collected in step 1 and DC bus mid-point voltage signal, step 2 with reference to phase voltage signal V _a, V _b, V _cprocess with the PI parameter obtained in step 4, obtain dynamic adjustments amount Δ V and be superimposed to three-phase with reference on phase voltage signal, obtain new three-phase with reference to phase voltage signal V ' _a, V ' _b, V ' _c, the deterministic process of regulated quantity Δ V is as follows:

$\mathrm{\&Delta;V}=k_p{\mathrm{\&Delta;V}}_c+\frac{k_p}{T}{\&Integral;}_0^t{\mathrm{\&Delta;V}}_c\mathrm{dt}$

In formula, Δ V _cfor the difference of DC side two bus capacitor voltages.