CN105006839A - Weak power grid source load impedance model analysis method of grid-connected power generation system - Google Patents

Abstract

The invention relates to a weak power grid source load impedance model analysis method of a grid-connected power generation system, and relates to the technical field of power grids. The technical problem that the existing method is not suitable for a weak power grid is solved. The method comprises the steps that a grid source load impedance model is defined for the grid-connected power generation system, wherein the model is combined with grid-connected inverter common coupling point voltage, alternating current power grid line impedance, alternating current power grid load impedance, alternating current power grid equivalent voltage, alternating current power grid equivalent current source and alternating current power grid access current of the grid-connected power generation system; the assumption that the load of an alternating current power grid is a resistive inductive load and the line impedance of the alternating current power grid is resistive inductive line impedance is carried out, so that an open-loop transfer function and a closed-loop transfer function of the grid-connected power generation system under double-loop control are acquired; and according to the acquired open-loop transfer function and the closed-loop transfer function, the grid-connected power generation system is analyzed. The method provided by the invention is suitable for analyzing the grid-connected power generation system under the weak power grid.

Description

The light current net net source lotus impedance model analytical method of grid-connected system

Technical field

The present invention relates to electrical network and technology, particularly relates to a kind of technology of light current net net source lotus impedance model analytical method of grid-connected system.

Background technology

Along with popularizing gradually of new forms of energy, the application of combining inverter in solar grid-connected electricity generation system or wind-driven grid-connected generating is more and more extensive.When electrical network mounting transformer more, leakage inductance and line impedance larger time, electrical network more and more presents light current net characteristic, now the research of combining inverter and electrical network reciprocal effect, then seem particularly important.

The current research emphasis for light current net is the reciprocal effect utilizing traditional Impedance Analysis to analyze now system.When three-phase power grid voltage and load are all symmetrical, in a phase situation, the circuit topological structure of combining inverter as shown in Figure 1, the L in Fig. 1 ₁, L ₂, C is respectively inverter side inductance, net side inductance, the filter capacitor of the LCL filter in grid-connected system, u _pccfor the combining inverter point of common coupling voltage of grid-connected system, Z _gridfor the AC network line impedance of grid-connected system, Z _loadfor the AC network load impedance of grid-connected system, U _dcfor the voltage of direct current network, U _sfor the AC network equivalent voltage of grid-connected system, power subsystem is expressed as the circuit after equivalent circuit as shown in Figure 4, combining inverter can regard current source I as _sparallel output impedance Z _inv, AC network can regard ideal voltage source U as _gseries AC power network line impedance Z _grid, can obtain grid-connected current expression formula is:

$I_g\left(s\right)=\[I_s\left(s\right)-\frac{U_g\left(s\right)}{Z_{inv}\left(s\right)}\]\×H\left(s\right)$

$H\left(s\right)=\frac{1}{1+\frac{Z_{grid}\left(s\right)}{Z_{inv}\left(s\right)}}$

When supposing there is no combining inverter, Network Voltage Stability; When electric network impedance is zero, inverter stable output, now the stability of output current depends on H (s), and H (s) is similar to the system closed loop transfer function, that has negative feedback control, and wherein forward gain is 1, and feedback oscillator is Z _grid(s)/Z _invs (), if electric network impedance and inverter are published books, impedanoe ratio meets Nyquist's stability criterion, then combining inverter can stable operation under this grid conditions, and the stability margin of grid-connected system also can by Z _grid(s)/Z _invs the nyquist plot of () characterizes.

Utilize traditional Impedance Analysis, the characteristic equation of H (s) can be obtained, characteristic equation can calculate the stable PI controller Kp parameter area of grid-connected system of sening as an envoy to thus, all parameters within the scope of this can ensure the stable of system, but and not all parameter can make system have good control effects.The grid-connected job stability criterion of the combining inverter that traditional Impedance Analysis provides is: the ratio between electric network impedance and inverter impedance need meet Nyquist criterion.The advantage of the method is the accurate parameters not needing combining inverter, and ad hoc approach (particular harmonic injection method) just can be utilized to obtain the output impedance of combining inverter, and then launches research to system.But the shortcoming of the method is: the phase margin 1) utilizing the ratio of electric network impedance and inverter output impedance to obtain in Impedance Analysis, be not the real phase margin of system, but a kind ofly characterize; Because phase margin directly reflects the damping size of system, now the damping degree of system cannot accurately define; 2) do not relate to magnitude margin in Impedance Analysis, and magnitude margin is also very important for the excellent degree of evaluating system, thus causes the one-sidedness of Impedance Analysis; 3) cut-off frequency of system cannot be embodied in Impedance Analysis; Although the amplitude-frequency characteristic intersection point of electric network impedance and inverter equivalent output impedance is defined as handing-over frequency, its physical significance is also uncertain; 4) utilize Impedance Analysis can transform grid-connected system, thus make system meet certain stable condition; But for some physical quantitys of system itself, well can not be studied by Impedance Analysis, therefore need have the Mathematical Modeling of more refinement to process system to whole grid-connected system.

In view of above reason, under weak grid conditions, utilize traditional Impedance Analysis can not carry out comprehensively careful analysis to grid-connected system, when the physical quantity therefore in grid-connected system changes, cannot assess the impact that it causes accurately.

Summary of the invention

For the defect existed in above-mentioned prior art, technical problem to be solved by this invention is to provide a kind ofly can carry out the light current net net source lotus impedance model analytical method of the grid-connected system of comprehensively careful analysis by the grid-connected system off the net to light current.

In order to solve the problems of the technologies described above, the light current net net source lotus impedance model analytical method of a kind of grid-connected system provided by the present invention, it is characterized in that, concrete steps are as follows:

1) for grid-connected system defines a net source lotus impedance model, concrete model formula is:

$\left\{\begin{array}{c}{u}_{pcc}-U_s=Z_{grid}{I}_s\left(s\right)\\ u_{pcc}=Z_{load}{I}_{load}\left(s\right)\\ I_g\left(s\right)-I_s\left(s\right)=I_{load}\left(s\right)\end{array}\right.$

Wherein, u _pccfor the combining inverter point of common coupling voltage of grid-connected system, Z _gridfor the AC network line impedance of grid-connected system, Z _loadfor the AC network load impedance of grid-connected system, U _sfor the AC network equivalent voltage of grid-connected system, I _ss AC network equivalent current source that () is grid-connected system, I _gs AC network networking electric current that () is grid-connected system;

2) suppose that the load of AC network is for resistance inductive load, the line impedance of AC network is resistance sense line impedance, according to the net source lotus impedance model that step 1 defines, the open-loop transfer function of acquisition grid-connected system under double-loop control, closed loop transfer function, are respectively:

$G_o\left(s\right)=\frac{G\left(s\right)(Z_{load}+Z_g)K_c{K}_{pwm}}{A_1{s}^3+A_2{s}^2+A_{3}s+Z_g{Z}_{load}}$

$G_{c1}\left(s\right)=\frac{G\left(s\right)(Z_{load}+Z_g)K_c{K}_{pwm}}{A_1{s}^3+A_2{s}^2+A_{3}s+Z_g{Z}_{load}+G\left(s\right)(Z_{load}+Z_g)K_c{K}_{pwm}}$

A ₁＝L ₁L ₂(Z _g+Z _load)C

A ₂＝(L ₁L ₂Z _load+L ₂Z _gK _cK _pwm+L ₂Z _loadK _cK _pwm)C

A ₃＝(Z _load+Z _g)(L ₁+L ₂)

Wherein, G _os () is the open-loop transfer function of grid-connected system under double-loop control, G _cls () is the closed loop transfer function, of grid-connected system under double-loop control, the outer ring controller that G (s) is grid-connected system, Z _gthe resistance sense line impedance of AC network, K _cfor ring controller in grid-connected system, K _pwmfor the inverter bridge equivalence amplification coefficient of grid-connected system, L ₁for the inverter side inductance of the LCL filter in grid-connected system, L ₂for the net side inductance of the LCL filter in grid-connected system, C is the filter capacitor of the LCL filter in grid-connected system;

3) according to open-loop transfer function, closed loop transfer function, that step 2 obtains, grid-connected system is analyzed.

Further, in step 2, when the outer ring controller of grid-connected system adopts PI controller, suppose that the load of AC network is purely resistive load, the line impedance of AC network is pure inductive circuit impedance, by grid-connected system outside ring controller adopt PI controller time open-loop transfer function be optimized for:

$G_{pi}\left(s\right)=\frac{K_p{K}_c{K}_{pwm}{L}_g{s}^2+\left(K_i{L}_g+K_{p}R\right)K_c{K}_{pwm}s+K_i{\mathrm{RK}}_c{K}_{pwm}}{L_1{L}_2{L}_g{\mathrm{Cs}}^5+B_1{s}^4+B_2{s}^3+B_3{s}^2}$

B ₁＝[L _g(L ₂K _cK _pwm+L ₁R)+L ₁L ₂R]C

B ₂＝L _g(L ₁+L ₂+RCK _cK _pwm)+L ₂RCK _cK _pwm

B ₃＝(L ₁+L ₂+L _g)R

Wherein, G _pi(s) for grid-connected system outside ring controller adopt PI controller time open-loop transfer function, K _pfor the proportionality coefficient of PI controller, K _ifor the integral coefficient of PI controller, L _gfor the pure inductive circuit impedance of AC network, R is the purely resistive load of AC network.

The light current net net source lotus impedance model analytical method of grid-connected system provided by the invention, from the angle of control system, the electric network impedance consider needs off the net for light current emphatically and point of common coupling load are brought in the control system of combining inverter and are gone research, thus the whole Mathematical Modeling of the load established from combining inverter control system to grid side and electric network impedance, thus when accurate study load and electric network impedance change more, concrete impact is produced on system; Electric network impedance and load are brought in control system and considers, also can affect the constant of choosing of combining inverter control system middle controller parameter, the system stability nargin that time thus, system transter obtains and cut-off frequency are system performance index truly, its size is the authentic representative excellent degree of systematic function then, can carry out comprehensively careful analysis by the grid-connected system off the net to light current; Further, the load-impedance model proposed, when the control strategy of combining inverter changes, still can be suitable for, thus possess in light current versatility off the net.

Accompanying drawing explanation

Fig. 1 is the combining inverter circuit topological structure figure of AC network in a phase situation;

Fig. 2 is the Mathematical Modeling block diagram of the light current net net source lotus impedance model analytical method of the grid-connected system of the embodiment of the present invention;

Fig. 3 is the block diagram of the another kind of version of the Part2 part shown in Fig. 2;

Fig. 4 is the equivalent circuit figure of the combining inverter circuit topological structure of AC network in a phase situation.

Embodiment

Illustrate below in conjunction with accompanying drawing and embodiments of the invention are described in further detail; but the present embodiment is not limited to the present invention; every employing analog structure of the present invention and similar change thereof, all should list protection scope of the present invention in, the pause mark in the present invention all represent and relation.

As shown in Figure 1, the light current net net source lotus impedance model analytical method of a kind of grid-connected system that the embodiment of the present invention provides, it is characterized in that, concrete steps are as follows:

1) according to KVL (Kirchhoff's second law) and KCL (Kirchhoff's current law (KCL)), for grid-connected system defines a net source lotus impedance model, concrete model formula is:

$\left\{\begin{array}{c}{u}_{pcc}-U_s=Z_{grid}{I}_s\left(s\right)\\ u_{pcc}=Z_{load}{I}_{load}\left(s\right)\\ I_g\left(s\right)-I_s\left(s\right)=I_{load}\left(s\right)\end{array}\right.$

Wherein, u _pccfor the combining inverter point of common coupling voltage of grid-connected system, Z _gridfor the AC network line impedance of grid-connected system, Z _loadfor the AC network load impedance of grid-connected system, U _sfor the AC network equivalent voltage of grid-connected system, I _ss AC network equivalent current source that () is grid-connected system, I _gs AC network networking electric current that () is grid-connected system;

2) suppose that the load of AC network is for resistance inductive load, the line impedance of AC network is resistance sense line impedance, according to the net source lotus impedance model that step 1 defines, the open-loop transfer function of acquisition grid-connected system under double-loop control, closed loop transfer function, are respectively:

$G_o\left(s\right)=\frac{G\left(s\right)(Z_{load}+Z_g)K_c{K}_{pwm}}{A_1{s}^3+A_2{s}^2+A_{3}s+Z_g{Z}_{load}}$

$G_{c1}\left(s\right)=\frac{G\left(s\right)(Z_{load}+Z_g)K_c{K}_{pwm}}{A_1{s}^3+A_2{s}^2+A_{3}s+Z_g{Z}_{load}+G\left(s\right)(Z_{load}+Z_g)K_c{K}_{pwm}}$

A ₁＝L ₁L ₂(Z _g+Z _load)C

A ₂＝(L ₁L ₂Z _load+L ₂Z _gK _cK _pwm+L ₂Z _loadK _cK _pwm)C

A ₃＝(Z _load+Z _g)(L ₁+L ₂)

Wherein, G _os () is the open-loop transfer function of grid-connected system under double-loop control, G _cls () is the closed loop transfer function, of grid-connected system under double-loop control, the outer ring controller that G (s) is grid-connected system, Z _gthe resistance sense line impedance of AC network, K _cfor ring controller in grid-connected system, K _pwmfor the inverter bridge equivalence amplification coefficient of grid-connected system, L ₁for the inverter side inductance of the LCL filter in grid-connected system, L ₂for the net side inductance of the LCL filter in grid-connected system, C is the filter capacitor of the LCL filter in grid-connected system;

Further, when the outer ring controller of grid-connected system adopts PI controller, suppose that the load of AC network is purely resistive load, the line impedance of AC network is pure inductive circuit impedance, by grid-connected system outside ring controller adopt PI controller time open-loop transfer function be optimized for:

$G_{pi}\left(s\right)=\frac{K_p{K}_c{K}_{pwm}{L}_g{s}^2+\left(K_i{L}_g+K_{p}R\right)K_c{K}_{pwm}s+K_i{\mathrm{RK}}_c{K}_{pwm}}{L_1{L}_2{L}_g{\mathrm{Cs}}^5+B_1{s}^4+B_2{s}^3+B_3{s}^2}$

B ₁＝[L _g(L ₂K _cK _pwm+L ₁R)+L ₁L ₂R]C

B ₂＝L _g(L ₁+L ₂+RCK _cK _pwm)+L ₂RCK _cKpwm

B ₃＝(L ₁+L ₂+L _g)R

Wherein, G _pi(s) for grid-connected system outside ring controller adopt PI controller time open-loop transfer function, K _pfor the proportionality coefficient of PI controller, K _ifor the integral coefficient of PI controller, L _gfor the pure inductive circuit impedance of AC network, R is the purely resistive load of AC network.

3) according to open-loop transfer function, closed loop transfer function, that step 2 obtains, grid-connected system is analyzed.

Fig. 2 is the Mathematical Modeling block diagram of the combining inverter circuit topological structure shown in Fig. 1, when adopting different control strategy, control ring and feedback variable may change to some extent, but Part2 part is the same, because the circuit relationships of light current local load off the net and electric network impedance is determined, therefore be general in light current Part2 off the net part, namely system transter is identical, and Fig. 3 is the block diagram of another kind of version of the Part2 part in Fig. 2;

The Part2 part in Fig. 2 can be obtained according to the net source lotus impedance model of definition, can by common point connect load and electric network impedance is concluded in the research of combining inverter control system, ssystem transfer function can be obtained thus, according to ssystem transfer function, the stability margin when electric network impedance change and load variations and cut-off frequency can be analyzed, thus judge steady-state behaviour and the dynamic property of system, can also according to certain performance requirement, when known electrical network impedance magnitude within the specific limits, utilize classical control theory, be met the controller parameter of control performance requirement, thus make the performance of whole grid-connected system reach good control effects,

Adopt the model method of net source lotus, the complete grid-connected system Mathematical Modeling that light current is off the net can accurately be described, thus study wherein variable (on the impact that system produces when such as electric network impedance or load variations) better, mainly comprise the impact of absolute on system and relative stability.

When electric network impedance value is larger, when controller parameter is constant, can there is larger distortion in the output grid-connected current of system.Now, adopting traditional Impedance Analysis to analyze, system Phase margin Pm is now comparatively large and system is still in stable state, and utilize the method for the embodiment of the present invention further can analyze the open-loop cut-off frequency of system very close to power frequency 50Hz, and there is consistency due to open-loop cut-off frequency and closed loop cut-off frequency, now the closed loop cut-off frequency of closed-loop system is also less, thus have impact on system's transient response greatly, cause output well can not follow the tracks of input.Therefore, according to the system open loop transfer function that the method for the embodiment of the present invention obtains, the controller parameter scope corresponding when open-loop cut-off frequency setting is greater than certain value can be determined by open-loop transfer function.In like manner, when the Proportional coefficient K of PI controller _pvalue when becoming large, system magnitude margin can reduce, and setting magnitude margin minimum value, then can obtain K _phigher limit.

The Mathematical Modeling of point of common coupling load and electric network impedance joins in the block diagram of combining inverter control system by the embodiment of the present invention, thus obtains the Mathematical Modeling being different from traditional combining inverter control system.Adding, based on KCL and KVL of load and electric network impedance model.The control of LCL type three-phase grid-connected inverter, adopts two-phase rest frame, thus can ensure that the independence of two axles controls, outer shroud adopts grid-connected current feedback, controller adopts PI or PR controller, and inner ring adopts capacitor current feedback, the general adoption rate controller of controller.Adopt double-loop control object to be increase system damping, thus suppress the resonance peak of LCL, improve the stability of system.

Traditional three-phase grid-connected inverter Mathematical Modeling, contains only LCL filter and control strategy, not load and electric network impedance, the stability range of LCL type three-phase grid-connected inverter is limited, namely the Selecting parameter of outer ring controller PI or PR must guarantee system stability within the specific limits, determine the parameter area making system stability, adopt Routh Criterion to ask for the characteristic equation of grid-connected system.

The embodiment of the present invention can be off the net at light current, no matter how control strategy for inverter changes, comprise adopted different filter as L-type or double mode under LC type, because the circuit relationships of load and electric network impedance is fixing, therefore the weak network system under being applicable to various control strategy, possess stronger versatility; Comprehensively can assess the excellent degree of system, thus take appropriate measures the characteristic improving system; The Mathematical Modeling that the embodiment of the present invention is set up is the real theoretical formulation of light current grid-connected system off the net, instead of the improvement on original Impedance Analysis, the stability margin of therefore obtained system and cut-off frequency are system performance index truly, fabulous and comprehensively describe system.And the embodiment of the present invention can obtain the open-loop transfer function of grid-connected system, the cut-off frequency by design system that thus can be artificial and magnitude margin, thus obtain ensureing the controller parameter scope of system under certain open-loop cut-off frequency and magnitude margin, under the prerequisite ensureing system stability, guarantee that system is within the scope of certain stability margin, and ensure that the cut-off frequency of system and magnitude margin size meet the requirements, finally obtain a controller parameter scope that systematic function can be made comparatively excellent.

Claims (2)

1. a light current net net source lotus impedance model analytical method for grid-connected system, it is characterized in that, concrete steps are as follows:

1) for grid-connected system defines a net source lotus impedance model, concrete model formula is:

$\left\{\begin{array}{c}{u}_{pcc}-U_s=Z_{grid}{I}_s\left(s\right)\\ u_{pcc}=Z_{load}{I}_{load}\left(s\right)\\ I_g\left(s\right)-I_s\left(s\right)=I_{load}\left(s\right)\end{array}\right.$

Wherein, u _pccfor the combining inverter point of common coupling voltage of grid-connected system, Z _gridfor the AC network line impedance of grid-connected system, Z _loadfor the AC network load impedance of grid-connected system, U _sfor the AC network equivalent voltage of grid-connected system, I _ss AC network equivalent current source that () is grid-connected system, I _gs AC network networking electric current that () is grid-connected system;

2) suppose that the load of AC network is for resistance inductive load, the line impedance of AC network is resistance sense line impedance, according to the net source lotus impedance model that step 1 defines, the open-loop transfer function of acquisition grid-connected system under double-loop control, closed loop transfer function, are respectively:

$G_o\left(s\right)=\frac{G\left(s\right)(Z_{load}+Z_g)K_c{K}_{pwm}}{A_1{s}^3+A_2{s}^2+A_{3}s+Z_g{Z}_{load}}$

$G_{c1}\left(s\right)=\frac{G\left(s\right)(Z_{load}+Z_g)K_c{K}_{pwm}}{A_1{s}^3+A_2{s}^2+A_{3}s+Z_g{Z}_{load}+G\left(s\right)(Z_{load}+Z_g)K_c{K}_{pwm}}$

A ₁＝L ₁L ₂(Z _g+Z _load)C

A ₂＝(L ₁L ₂Z _load+L ₂Z _gK _cK _pwm+L ₂Z _loadK _cK _pwm)C

A ₃＝(Z _load+Z _g)(L ₁+L ₂)

Wherein, G _os () is the open-loop transfer function of grid-connected system under double-loop control, G _cls () is the closed loop transfer function, of grid-connected system under double-loop control, the outer ring controller that G (s) is grid-connected system, Z _gthe resistance sense line impedance of AC network, K _cfor ring controller in grid-connected system, K _pwmfor the inverter bridge equivalence amplification coefficient of grid-connected system, L ₁for the inverter side inductance of the LCL filter in grid-connected system, L ₂for the net side inductance of the LCL filter in grid-connected system, C is the filter capacitor of the LCL filter in grid-connected system;

3) according to open-loop transfer function, closed loop transfer function, that step 2 obtains, grid-connected system is analyzed.

2. the light current net net source lotus impedance model analytical method of grid-connected system according to claim 1, it is characterized in that, in step 2, when the outer ring controller of grid-connected system adopts PI controller, suppose that the load of AC network is purely resistive load, the line impedance of AC network is pure inductive circuit impedance, by grid-connected system outside ring controller adopt PI controller time open-loop transfer function be optimized for:

$G_{pi}\left(s\right)=\frac{K_p{K}_c{K}_{pwm}{L}_g{s}^2+\left(K_i{L}_g+K_{p}R\right)K_c{K}_{pwm}s+K_i{\mathrm{RK}}_c{K}_{pwm}}{L_1{L}_2{L}_g{\mathrm{Cs}}^5+B_1{s}^4+B_2{s}^3+B_3{s}^2}$

B ₁＝[L _g(L ₂K _cK _pwm+L ₁R)+L ₁L ₂R]C

B ₂＝L _g(L ₁+L ₂+RCK _cK _pwm)+L ₂RCK _cK _pwm

B ₃＝(L ₁+L ₂+L _g)R

Wherein, G _pi(s) for grid-connected system outside ring controller adopt PI controller time open-loop transfer function, K _pfor the proportionality coefficient of PI controller, K _ifor the integral coefficient of PI controller, L _gfor the pure inductive circuit impedance of AC network, R is the purely resistive load of AC network.