CN104485687A - Control method for PI (power to loop) resonance of photovoltaic grid-connected inverter based on switching of CCM (continuous current mode) and DCM (discontinuous current mode) - Google Patents

Abstract

The invention relates to a control method for PI (power to loop) resonance of a photovoltaic grid-connected inverter based on switching of CCM (continuous current mode) and DCM (discontinuous current mode). When the absolute value of grid-connected voltage Vo outputted by the photovoltaic inverter is higher than or equal to boundary voltage Vbo, the CCM control method is adopted; when the absolute value of the grid-connected voltage Vo outputted by the photovoltaic inverter is lower than the boundary voltage Vbo, the DCM control method is adopted. Under different working modes, the occupation space ratio of an alternating current small signal is adjusted by a PI resonance controller, so as to obtain the total occupation space ratio. According to the total occupation space ratio, the corresponding PWM (pulse width modulation) wave is generated to control the action of a pre-stage MOS (metal oxide semiconductor) tube, so as to enable the photovoltaic inverter to realize grid-connected output. The control method has the advantages that the harmonics of grid-connected current flowing through a zero point can be effectively eliminated, and the grid-connecting efficiency and grid-connecting quality of the inverter are improved.

Description

Based on the photovoltaic combining inverter PI resonance control method that continuous current mode and discontinuous conduct mode switch

Technical field

The invention belongs to new forms of energy electric control field, especially a kind of photovoltaic combining inverter PI resonance control method.

Background technology

Grid-connected current control technology mainly contains traditional control technology and the control technology based on modern control theory.Wherein traditional grid-connected current control technology mainly contains PI control, Hysteresis control, Repetitive controller, predictive-current control, monocyclic control method etc.And mainly contain fuzzy control, Sliding mode variable structure control, adaptive control etc. based on the grid-connected current control technology of modern control theory.For existing Miniature inverter product, due to will cost factor be considered, substantially be all adopt have algorithm simple, realize that cost is low, the PI controller of good stability, but PI controls to there are two problems: be difficult to realize the DAZ gene of reference sine wave and poor antijamming capability.Therefore propose ratio resonance (PR) to control, it can produce enough large gain at fixed frequency place, realizes DAZ gene.But due to the restriction of analogue system component parameter precision and digital system precision, ratio resonance (PR) controller not easily realizes, and when mains frequency produces skew, the harmonic wave that just cannot effectively suppress electrical network to produce.Therefore, on the basis that ratio resonance controls, propose a kind of accurate ratio resonance (PR) controller being easy to realize, both the high-gain of ratio resonance (PR) controller can have been kept, the impact of mains frequency skew on inverter output current can also be effectively reduced simultaneously, but accurate ratio resonance resonant controller can not suppress the DC component of prime input current, then this patent proposes PI resonance control method.

Photovoltaic DC-to-AC converter has three kinds of mode of operations, discontinuous conduct mode (DCM), electric current critical continuous conduction mode (BCM), continuous current mode (CCM).During model study at present for photovoltaic DC-to-AC converter, normally allow inverter work in one mode.Due under single-mode time, when input power is lower, grid connection efficiency is lower, and the harmonic wave at zero passage place can be larger.

Summary of the invention

In order to overcome existing photovoltaic DC-to-AC converter single-mode mode when input power is lower, the deficiency that grid connection efficiency is lower, the harmonic wave at zero passage place can be larger, the invention provides a kind of photovoltaic combining inverter PI resonance control method switched based on continuous current mode and discontinuous conduct mode effectively improving the grid connection efficiency of inverter, the harmonic wave of reduction grid-connected current.

The technical solution adopted for the present invention to solve the technical problems is:

Based on the photovoltaic combining inverter PI resonance control method that continuous current mode and discontinuous conduct mode switch, described control method comprises the following steps:

(1) photovoltaic input voltage V is detected _pv, compare with reference voltage 30V, judge that whether photovoltaic input voltage is higher than 30V, if higher than 30V, then perform step (2), otherwise treat machine testing, until photovoltaic input voltage is higher than 30V;

(2) inverter output voltage V is detected _o, and compare V _owith boundary voltage V _bosize, wherein boundary voltage V _bofor critical voltage when CCM pattern and DCM pattern switch; Work as inverter output voltage | V _o| higher than or equal boundary voltage V _botime, inverter work in ccm mode, as | V _o| lower than boundary voltage V _botime, inverter work is in dcm mode;

(3) the stable state duty ratio D of the correspondence under DCM and CCM pattern is calculated;

(4) by filter circuit, filtering process is carried out to flyback transformer primary current, detect after filtering after primary current i, and with reference current i _prido differential comparison and obtain Δ i=i-i _pri;

(5) adopt PI resonant controller to carry out computing to Δ i, calculate ac small signal duty ratio d;

(6) by superimposed for ac small signal duty ratio d and stable state duty ratio D, the duty ratio needed for drive circuit is obtained controller is according to duty ratio send corresponding PWM ripple and control prime mos pipe S _maction, thus the value changing the former limit input current i of flyback transformer;

(7) judge whether Δ i is less than ε, if Δ i≤ε, then perform step (8); If Δ i> is ε, then return and perform step (4), ε is the precision of current deviation;

(8) control photovoltaic inverter grid-connected to export, return step (1) afterwards.

Further, in described step (2), V _bopass through formulae discovery obtains.

Further again, in described step (4), described reference current is according to grid-connected voltage V _grid, photovoltaic input voltage U _m, photovoltaic input current I _mcalculation of parameter draws, i.e. reference current

Further, in described step (5), by by PI and quasi resonant control is in series is used for the PI resonance control and compensation device that flyback combining inverter realizes grid-connected output, its transfer function G (s) is:

$G\left(s\right)=G_{\mathrm{pi}}\left(s\right)G_c\left(s\right)=K\frac{\frac{1}{S_z}s+1}{s}(K_p+\frac{2K_R\mathrm{\ωs}/Q}{s^2+\mathrm{\ωs}+/Q+{\left(2\mathrm{\ω}\right)}^2})$

Wherein, G _pis () represents the transfer function of PI controller, G _cs () represents the transfer function of quasi resonant control, K _pbe the gain of resonant controller, Q is bandwidth factor, ω is resonance frequency, K _rfor resonance coefficient.

Technical conceive of the present invention is: the control mode adopting two kinds of mode of operations to switch, and effectively can improve the grid connection efficiency of inverter, reduces the harmonic wave of grid-connected current.

Beneficial effect of the present invention is mainly manifested in: can solve the problem of the grid-connected output of Miniature inverter in the harmonic distortion at zero passage place, the impact of mains frequency skew on inverter output current can be effectively reduced simultaneously, suppress the DC component of prime input current, improve the grid connection efficiency of inverter, reduce the THD value of grid-connected current.

Accompanying drawing explanation

Fig. 1 is the program control flow chart based on PI resonance control algolithm of the present invention.

Fig. 2 is miniature photovoltaic DC-to-AC converter fundamental diagram of the present invention.

Fig. 3 is the operating current figure of miniature photovoltaic DC-to-AC converter of the present invention under continuous current mode (CCM).

Fig. 4 is the operating current figure of miniature photovoltaic DC-to-AC converter of the present invention under discontinuous conduct mode (DCM).

Fig. 5 is the bode figure of PI resonant controller of the present invention.

Fig. 6 is the bode figure of resonant controller.

Embodiment

Below in conjunction with accompanying drawing, the invention will be further described.

With reference to Fig. 1 ~ Fig. 6, the photovoltaic combining inverter PI resonance control method that a kind of continuous current mode and discontinuous conduct mode switch, rate-determining steps is as follows:

Detect photovoltaic input voltage V _pv, setting reference voltage V _stfor 30V, photovoltaic input voltage V _pvwith reference voltage V _stcompare, judge photovoltaic input voltage V _pvwhether higher than 30V, if higher than 30V, then controller can normally work, otherwise treats machine testing, until photovoltaic input voltage V _pvhigher than 30V.

Detect inverter output voltage V _o, and compare V _owith boundary voltage V _bosize, wherein boundary voltage V _bofor critical voltage when CCM pattern and DCM pattern switch. f is switching frequency, and n is turn ratio, L _mbe transformer inductance, R is electrical network equiva lent impedance, V _pvit is photovoltaic input voltage.

When | V _o| < V _botime, in dcm mode, corresponding duty ratio is D in miniature photovoltaic DC-to-AC converter work _dCM, when | V _o|>=V _botime, in ccm mode, corresponding duty ratio is D in miniature photovoltaic DC-to-AC converter work _cCM.

Use State-space Averaging Principle can solve the duty ratio steady state solution obtained under CCM pattern and DCM pattern, be shown below

$D_{\mathrm{CCM}}=\frac{V_o}{V_o+n{V}_{\mathrm{pv}}};$

$D_{\mathrm{DCM}}=2\sqrt{\frac{L_m{I}_{\mathrm{in}}f}{V_{\mathrm{pv}}}}\left|\sin \left(\mathrm{\&omega;t}\right)\right|.$

Wherein V _othe grid-connected output voltage of Miniature inverter, L _mtransformer inductance, I _inbe the effective value of photovoltaic input current, ω is the angular frequency of grid-connected voltage.

Reference current is according to grid-connected voltage V _grid, photovoltaic input voltage U _m, photovoltaic input current I _mdraw etc. calculation of parameter, reference current carry out filtering process by RC filter circuit to flyback transformer primary current, the transfer function of filter circuit is primary current i after detecting after filtering, by filtered primary current i and reference current i _pricompare, then can obtain Δ i=i-i _pri.Adopt PI resonant controller to carry out computing to Δ i, calculate ac small signal duty ratio d.I.e. d=Δ i*G (s).

By superimposed for ac small signal duty ratio d and stable state duty ratio D, obtain the duty ratio needed for drive circuit namely controller is according to duty ratio send corresponding PWM ripple and control prime mos pipe S _maction, thus the former limit input current i changing flyback transformer _invalue, control circuit is as shown in Figure 2

According to i and i _pridifference DELTA i whether be less than ε to start PI resonant controller.If Δ i=i-i _pri> ε, then illustrate total duty ratio undesirable, then need to return the primary current i detecting flyback transformer again, and adjust total duty ratio if Δ i≤ε, the primary current of flyback transformer within the range of permission, then can make miniature photovoltaic inverter output current reach grid-connected requirement.

When miniature photovoltaic DC-to-AC converter work in ccm mode time, as shown in Figure 3, when miniature photovoltaic DC-to-AC converter is operated in dcm mode, operating current is as shown in Figure 4 for operating current.

In the present invention, the concrete steps of PI resonance control design case are as follows:

The controller that engineering is extensively used is PI controller, and it has two parameters to determine, proportionality coefficient and integral coefficient, and it controls transfer function:

$G_{\mathrm{pi}}=k_p+k_i\frac{1}{s}$

In formula, k _pfor proportionality coefficient, k _ifor integral coefficient.

Wherein, the effect of proportion adjustment is the deviation of reaction system in proportion, and system is once occur deviation, and proportion adjustment produces regulating action immediately to reduce deviation.Proportional action is large, can accelerate to regulate, and reduces error, but excessive ratio, the stability of system is declined, even causes the instability of system.Autocatalytic reset action makes system eliminate steady-state error, improves without margin.Because there is error, integral adjustment just carries out, until by error concealment, integral adjustment stops.The power of integral action depends on the constant Ti of the time of integration, and Ti is less, and the effect of integration is stronger.Otherwise Ti is larger, and then integral action is more weak, add integral adjustment and the stability of a system can be made to decline, dynamic response is slack-off.Integral action and proportional action are combined, and just constitute PI controller.

Because the present invention discusses by zero point of transfer function and gain, therefore, need above formula to be converted into formula form: $G_{\mathrm{pi}}\left(s\right)=k_p+k_i\frac{1}{s}=K\frac{\frac{1}{S_z}s+1}{s}$

In formula, K=k _ifor the gain of PI controller, S _z=k _p/ k _ifor the zero point of PI controller.

The basis that PI controls is introduced a second order concussion link again can function to achieve the objective sin ²the DAZ gene of (ω t).But there will be the problems such as the DAZ gene that is difficult to realize reference sine wave and poor antijamming capability owing to introducing second order concussion link, therefore, need to improve, by 2 ω second order concussion link ²/ (s ²+ 4 ω ²) be converted to quasi resonant control:

$G_c\left(s\right)=K_p+\frac{2K_R\mathrm{\&omega;s}/Q}{s^2+\mathrm{\&omega;s}/Q+{\left(2\mathrm{\&omega;}\right)}^2}$

In formula, K _rfor the voltage amplification factor at resonance frequency omega place, K _pit is the gain of resonant controller; Q is bandwidth factor, determines the bandwidth of resonant controller.

So far, by by PI and quasi resonant control is in series is used for the PI resonance control and compensation device that flyback combining inverter realizes grid-connected output, its transfer function is:

$G\left(s\right)=G_{\mathrm{pi}}\left(s\right)G_c\left(s\right)=K\frac{\frac{1}{S_z}s+1}{s}(K_p+\frac{2K_R\mathrm{\&omega;s}/Q}{s^2+\mathrm{\&omega;s}+/Q+{\left(2\mathrm{\&omega;}\right)}^2})$

Four parameter K of quasi resonant control in formula _p, bandwidth factor Q, resonance frequency omega, resonance coefficient K _rimpact be amplitude-frequency characteristic and the phase-frequency characteristic at 100rad/s resonance frequency place, be specially: K _prelevant with steady-state behaviour and interference free performance, K _plarger, steady-state gain is larger, and resonance peak is less, and cut-off frequency is higher; Bandwidth factor Q determines resonance frequency omega place bandwidth, and Q is larger, and bandwidth is larger; Resonance coefficient K _rdetermine the gain of resonance frequency omega place, K _rlarger, resonance spikes is larger, and Phase margin is less.Fig. 5 is the bode figure of PI resonant controller, Fig. 6 is that the bode of resonant controller schemes.As can be known from Fig. 5, the magnitude margin bandwidth of PI resonant controller at 100rad/s place becomes large, and the change of phase margin is relatively slow; As can be known from Fig. 6, resonant controller bandwidth in 100rad/s place magnitude margin is narrow, and phase margin change is violent.Therefore PI resonance algorithm is compared resonance control algolithm and is more prone to realize, and becomes large due to PI resonant controller bandwidth range, when mains frequency produces skew, also can realize the suppression to mains by harmonics, reduce the THD value of grid-connected current.

In the present embodiment, as shown in Figure 2, this inverter adopts the topological structure of inverse-excitation type conversion to the fundamental diagram of miniature photovoltaic DC-to-AC converter, and described photovoltaic DC-to-AC converter comprises photovoltaic cell PV, decoupling capacitor C _pv, prime power switch tube S _m, flyback transformer TR, secondary output diode D and output capacitance C _o, described decoupling capacitor C _pvin parallel with photovoltaic cell PV, one end of described photovoltaic cell PV is connected with one end, former limit of flyback transformer TR, the former limit other end of described flyback transformer TR and described prime power switch tube S _msource electrode connect, described prime power switch tube S _mdrain electrode be connected with the other end of described photovoltaic cell PV, described prime power switch tube S _mgrid be connected with PWM generator, described prime power switch tube S _mdrain electrode be connected with filter, described filter with for comparing current i and reference current i _pridifference DELTA i whether be less than the precision ε of current deviation current comparator connect, the output of described current comparator is connected with the input of PI resonant controller, and the output of described PI resonant controller is connected with described PWM generator; One end of the secondary of described flyback transformer TR is connected with the positive pole of described secondary output diode D, the other end of the negative pole of described secondary output diode D, the secondary of described flyback transformer TR respectively with output capacitance C _otwo ends connect, described output capacitance C _otwo ends be connected with full bridge inverter, the output of described full bridge inverter is connected with filter circuit, and the output of described filter circuit is the output of inverter.

The basic functional principle of inverse-excitation type photovoltaic miniature grid-connected inverter is that the direct current that prime photovoltaic cell exports generates half-sinusoid after switching tube Sm modulates, then the full-bridge circuit be made up of 4 IGBT carries out line commutation, then flows to electrical network after device filtering after filtering and carries out grid-connected.And the general principle of control system first detects prime input current i _in, after filtering, obtain current i and reference current i _prirelatively, Δ i can be obtained, then through the adjustment of PI resonance, eventually pass PWM wave producer, pwm signal is sent to mos pipe S _mend.

Claims (4)

1., based on the photovoltaic combining inverter PI resonance control method that continuous current mode and discontinuous conduct mode switch, it is characterized in that: described control method comprises the following steps:

(1) photovoltaic input voltage V is detected _pv, compare with reference voltage 30V, judge that whether photovoltaic input voltage is higher than 30V, if higher than 30V, then perform step (2), otherwise treat machine testing, until photovoltaic input voltage is higher than 30V;

(2) inverter output voltage V is detected _o, and compare V _owith boundary voltage V _bosize, wherein boundary voltage V _bofor critical voltage when CCM pattern and DCM pattern switch; Work as inverter output voltage | V _o| higher than or equal boundary voltage V _botime, inverter work in ccm mode, as | V _o| lower than boundary voltage V _botime, inverter work is in dcm mode;

(3) the stable state duty ratio D of the correspondence under DCM and CCM pattern is calculated;

(4) by filter circuit, filtering process is carried out to flyback transformer primary current, detect after filtering after primary current i, and with reference current i _prido differential comparison and obtain Δ i=i-i _pri;

(5) adopt PI resonant controller to carry out computing to Δ i, calculate ac small signal duty ratio d;

(6) by superimposed for ac small signal duty ratio d and stable state duty ratio D, the duty ratio needed for drive circuit is obtained controller is according to duty ratio send corresponding PWM ripple and control prime mos pipe S _maction, thus the value changing the former limit input current i of flyback transformer;

(7) judge whether Δ i is less than ε, if Δ i≤ε, then perform step (8); If Δ i> is ε, then return and perform step (4), ε is the precision of current deviation;

(8) control photovoltaic inverter grid-connected to export, return step (1) afterwards.

2., as claimed in claim 1 based on the inverter PI resonance control method that continuous current mode and discontinuous conduct mode switch, it is characterized in that: in described step (2), V _bopass through formulae discovery obtains, and wherein, f is switching frequency, and n is turn ratio, L _mbe transformer inductance, R is electrical network equiva lent impedance, V _pvit is photovoltaic input voltage.

3., as claimed in claim 1 or 2 based on the inverter PI resonance control method that continuous current mode and discontinuous conduct mode switch, it is characterized in that: in described step (4), described reference current is according to grid-connected voltage V _grid, photovoltaic input voltage U _m, photovoltaic input current I _mcalculation of parameter draws, i.e. reference current $i_{\mathrm{pri}}=\frac{\left|V_{\mathrm{grid}}\right|}{220*\sqrt{2}}{I}_m.$

4. as claimed in claim 1 or 2 based on the inverter PI resonance control method that continuous current mode and discontinuous conduct mode switch, it is characterized in that: in described step (5), by by PI and quasi resonant control is in series is used for the PI resonance control and compensation device that flyback combining inverter realizes grid-connected output, its transfer function G (s) is:

$G\left(s\right)=G_{\mathrm{pi}}\left(s\right)G_c\left(s\right)=K\frac{\frac{1}{S_z}s+1}{s}(K_p+\frac{{2K}_R\mathrm{\&omega;s}/Q}{s^2+\mathrm{\&omega;s}/Q+{\left(2\mathrm{\&omega;}\right)}^2})$

Wherein, G _pis () represents the transfer function of PI controller, G _cs () represents the transfer function of quasi resonant control, K _pbe the gain of resonant controller, Q is bandwidth factor, ω is resonance frequency, K _rfor resonance coefficient.