CN105006839B - 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 analysis method of grid-connected system

Technical field

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

Background technology

With the gradually popularization of new forms of energy, combining inverter is in solar grid-connected electricity generation system or wind-driven grid-connected generating In application it is more and more extensive.When the transformator of electrical network mounting is more, when leakage inductance and larger line impedance, electrical network is increasingly presented Light current net characteristic, now combining inverter and the research of electrical network reciprocal effect, then be particularly important.

It is analyzing the interaction of now system using traditional Impedance Analysis currently for the research emphasis of light current net Affect.In three-phase power grid voltage and symmetrical load, the circuit topological structure of combining inverter such as Fig. 1 institutes in the case of a phase Show, the L in Fig. 1₁、L₂, C be respectively the inverter side inductance of LCL filter in grid-connected system, net side inductance, filtered electrical Hold, u_pccFor the combining inverter point of common coupling voltage of grid-connected system, Z_gridFor the alternating current netting twine of grid-connected system Roadlock resists, Z_loadFor the AC network load impedance of grid-connected system, U_dcFor the voltage of direct current network, U_sTo generate electricity by way of merging two or more grid systems it is Power subsystem is expressed as the circuit after equivalent circuit as shown in figure 4, parallel network reverse by the AC network equivalent voltage of system Device 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, grid-connected current expression formula is obtained is：

When hypothesis does not have combining inverter, Network Voltage Stability；When electric network impedance is zero, inverter output is stable, this When output current stability depend on H (s), H (s) similar to a system closed loop transfer function for possessing negative feedback control, 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 under the grid conditions can stable operation, the stability margin of grid-connected system Can be by Z_grid(s)/Z_invS the nyquist plot of () is characterized.

Using traditional Impedance Analysis, the characteristic equation of H (s) can be obtained, thus characteristic equation can be calculated and be sent as an envoy to The stable PI controller Kp parameter areas of grid-connected system, all parameters in the range of this ensure that stablizing for system, but It is and not all parameter can makes system have good control effect.The combining inverter that traditional Impedance Analysis are given is simultaneously Net job stability criterion is：Ratio between electric network impedance and inverter impedance need to meet Nyquist criterion.The method Advantage is and do not need the accurate parameters of combining inverter, just obtains grid-connected using ad hoc approach (particular harmonic injection method) The output impedance of inverter, and then system is launched to study.But the shortcoming of the method is：1) using electricity in Impedance Analysis Phase margin obtained by the ratio of net impedance and inverter output impedance, is not the real phase margin of system, but one Plant and characterize；As phase margin directly reflects the damping size of system, now the damping degree of system accurately cannot be defined；2) Magnitude margin is not related in Impedance Analysis, and magnitude margin is also extremely important for the excellent degree of assessment system , so as to cause the one-sidedness of Impedance Analysis；3) cut-off frequency of system cannot be embodied in Impedance Analysis；Although electrical network hinders The amplitude-frequency characteristic intersection point of anti-and inverter equivalent output impedance is defined as joining frequency, but its physical significance uncertain；4) profit Grid-connected system can be transformed with Impedance Analysis, so that system meets certain steady-state conditionss；But for system Some physical quantitys of itself, can not be studied well by Impedance Analysis, it is therefore desirable to which whole grid-connected system is had more The mathematical model of refinement is processing to system.

For these reasons, under weak grid conditions, can not be to grid-connected system using traditional Impedance Analysis Carry out comprehensively careful analysis, therefore the physical quantity in grid-connected system is when changing, it is impossible to accurately which is caused Impact be estimated.

The content of the invention

For defect present in above-mentioned prior art, the technical problem to be solved is to provide one kind can be to weak Grid-connected system under electrical network carries out the light current net net source lotus impedance model point of the grid-connected system of comprehensively careful analysis Analysis method.

In order to solve above-mentioned technical problem, a kind of light current net net source lotus impedance of grid-connected system provided by the present invention Modal analysis approaches, which, it is characterised in that comprise the following steps that：

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

Wherein, u_pccFor the combining inverter point of common coupling voltage of grid-connected system, Z_gridFor grid-connected system AC network line impedance, Z_loadFor the AC network load impedance of grid-connected system, U_sFor the alternating current of grid-connected system Net equivalent voltage, I_sThe AC network equivalent current source of (s) for grid-connected system, I_gThe alternating current of (s) for grid-connected system Net networking electric current；

2) load of AC network is assumed to hinder inductive load, the line impedance of AC network is resistance sense line impedance, According to the net source lotus impedance model of step 1 definition, open-loop transfer function of the grid-connected system under double -loop control, closed loop are obtained Transmission function is respectively：

A₁=L₁L₂(Z_g+Z_load)C

A₂=(L₁L₂Z_load+L₂Z_gK_cK_pwm+L₂Z_loadK_cK_pwm)C

A₃=(Z_load+Z_g)(L₁+L₂)

Wherein, G_oThe open-loop transfer function of (s) for grid-connected system under double -loop control, G_clS () to generate electricity by way of merging two or more grid systems is Closed loop transfer function of the system under double -loop control, outer ring controllers of the G (s) for grid-connected system, Z_gIt is the resistance of AC network Inductive circuit impedance, K_cFor the interior ring controller of grid-connected system, K_pwmFor the equivalent amplification system of inverter bridge of grid-connected system Number, L₁For the inverter side inductance of the LCL filter in grid-connected system, L₂For the LCL filter in grid-connected system Net side inductance, filter capacitors of the C for the LCL filter in grid-connected system；

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

Further, in step 2, when the outer ring controller of grid-connected system adopts PI controllers, it is assumed that alternating current The load of net is purely resistive load, and the line impedance of AC network is pure inductive circuit impedance, by grid-connected system in outer shroud Controller is optimized for using open-loop transfer function during PI controllers：

B₁=[L_g(L₂K_cK_pwm+L₁R)+L₁L₂R]C

B₂=L_g(L₁+L₂+RCK_cK_pwm)+L₂RCK_cK_pwm

B₃=(L₁+L₂+L_g)R

Wherein, G_piThe open-loop transfer function of (s) for grid-connected system when outer ring controller adopts PI controllers, K_pFor The proportionality coefficient of PI controllers, K_iFor the integral coefficient of PI controllers, L_gFor the pure inductive circuit impedance of AC network, R is exchange The purely resistive load of electrical network.

The light current net net source lotus impedance model analysis method of the grid-connected system that the present invention is provided, from the angle of control system Degree, brings the electric network impedance for needing to consider emphatically light current off the net and point of common coupling load the control system of combining inverter into In go research, so as to establish the whole mathematical modulo of load from combining inverter control system to grid side and electric network impedance Type, so as to more accurate research load and electric network impedance produce concrete impact when changing to system；By electric network impedance and load Consider in bringing control system into, also the selection constant of controller parameter in combining inverter control system can be affected, thus When system transter obtained by system stability nargin and cut-off frequency be system performance indications truly, its The size then authentic representative excellent degree of systematic function, can the grid-connected system off the net to light current carry out careful point comprehensively Analysis；Also, load-the impedance model for being proposed, when the control strategy of combining inverter changes, can still be suitable for, so as to Possess in light current versatility off the net.

Description of the drawings

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

Fig. 2 is the mathematical modulo of the light current net net source lotus impedance model analysis method of the grid-connected system of the embodiment of the present invention Type block diagram；

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

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

Specific embodiment

Embodiments of the invention are described in further detail below in conjunction with description of the drawings, but the present embodiment is not used to limit The system present invention, every analog structure and its similar change using the present invention, all should list protection scope of the present invention in, the present invention In pause mark represent the relation of sum.

As shown in figure 1, a kind of light current net net source lotus impedance model of grid-connected system provided by the embodiment of the present invention Analysis method, it is characterised in that comprise the following steps that：

1) according to KVL (Kirchhoff's second law) and KCL (Kirchhoff's current law (KCL)), it is grid-connected system definition One is netted source lotus impedance model, and concrete model formula is：

Wherein, u_pccFor the combining inverter point of common coupling voltage of grid-connected system, Z_gridFor grid-connected system AC network line impedance, Z_loadFor the AC network load impedance of grid-connected system, U_sFor the alternating current of grid-connected system Net equivalent voltage, I_sThe AC network equivalent current source of (s) for grid-connected system, I_gThe alternating current of (s) for grid-connected system Net networking electric current；

2) load of AC network is assumed to hinder inductive load, the line impedance of AC network is resistance sense line impedance, According to the net source lotus impedance model of step 1 definition, open-loop transfer function of the grid-connected system under double -loop control, closed loop are obtained Transmission function is respectively：

A₁=L₁L₂(Z_g+Z_load)C

A₂=(L₁L₂Z_load+L₂Z_gK_cK_pwm+L₂Z_loadK_cK_pwm)C

A₃=(Z_load+Z_g)(L₁+L₂)

Wherein, G_oThe open-loop transfer function of (s) for grid-connected system under double -loop control, G_clS () to generate electricity by way of merging two or more grid systems is Closed loop transfer function of the system under double -loop control, outer ring controllers of the G (s) for grid-connected system, Z_gIt is the resistance of AC network Inductive circuit impedance, K_cFor the interior ring controller of grid-connected system, K_pwmFor the equivalent amplification system of inverter bridge of grid-connected system Number, L₁For the inverter side inductance of the LCL filter in grid-connected system, L₂For the LCL filter in grid-connected system Net side inductance, filter capacitors of the C for the LCL filter in grid-connected system；

Further, when the outer ring controller of grid-connected system adopts PI controllers, it is assumed that the load of AC network is Purely resistive load, the line impedance of AC network is pure inductive circuit impedance, and grid-connected system is adopted in outer ring controller Open-loop transfer function during PI controllers is optimized for：

B₁=[L_g(L₂K_cK_pwm+L₁R)+L₁L₂R]C

B₂=L_g(L₁+L₂+RCK_cK_pwm)+L₂RCK_cKpwm

B₃=(L₁+L₂+L_g)R

Wherein, G_piThe open-loop transfer function of (s) for grid-connected system when outer ring controller adopts PI controllers, K_pFor The proportionality coefficient of PI controllers, K_iFor the integral coefficient of PI controllers, L_gFor the pure inductive circuit impedance of AC network, R is exchange The purely resistive load of electrical network.

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

Fig. 2 is the mathematical model block diagram of the combining inverter circuit topological structure shown in Fig. 1, when using different control strategies When, control ring and feedback variable may be changed, but Part2 parts be it is the same because light current local load off the net and What the circuit relationships of electric network impedance were to determine, therefore be general in light current Part2 parts off the net, i.e., system transter is Identical, Fig. 3 are the block diagrams of another kind of version of the Part2 parts in Fig. 2；

Part2 parts in Fig. 2 are obtained according to the net source lotus impedance model of definition, can be by common point connect load and electricity Net impedance is concluded in the research of combining inverter control system, it is hereby achieved that ssystem transfer function, passes according to system Delivery function, can analyze the stability margin and cut-off frequency when electric network impedance changes and loads change, so as to judge system Steady-state behaviour and dynamic property, can be with according to certain performance requirement, in known electrical network impedance magnitude within the specific limits In the case of, using classical control theory, the controller parameter of control performance requirement is met, so that entirely generate electricity by way of merging two or more grid systems being The performance of system reaches preferable control effect；

Using the model method of net source lotus, light current complete grid-connected system mathematical model off the net can accurately be described, so as to Wherein variable (impact that such as electric network impedance or load are produced to system when changing) is preferably studied, is mainly included to system The impact of absolute and relative stability.

When electric network impedance value is larger, in the case where controller parameter is constant, the output grid-connected current of system can be sent out The larger distortion of life.Now, system Phase margin Pm now can only be analyzed using traditional Impedance Analysis larger and be System is still in steady statue, and the open loop cutoff frequency of system can be further analyzed using the method for the embodiment of the present invention Rate very close to power frequency 50Hz, and as open-loop cut-off frequency and closed loop cut-off frequency have concordance, now closed loop system Closed loop cut-off frequency it is also less, the system's transient response so as to strong influence causes output track well defeated Enter.Therefore, the system open loop transmission function that method according to embodiments of the present invention is obtained, can determine that to work as by open-loop transfer function and opens The setting of ring cut-off frequency is more than controller parameter scope corresponding during certain value.In the same manner, when the Proportional coefficient K of PI controllers_p Value become it is big when, system magnitude margin can reduce, set magnitude margin minima, then can obtain K_pHigher limit.

The mathematical model of point of common coupling load and electric network impedance is added to combining inverter control by the embodiment of the present invention In the block diagram of system, so as to obtain the mathematical model different from traditional combining inverter control system.Load and electric network impedance mould The addition of type, is based on KCL and KVL.The control of LCL type three-phase grid-connected inverter, using two-phase rest frame, so as to can To ensure the independent control of two axles, outer shroud adopts grid-connected current feedback, controller to adopt PI or PR controllers, and internal ring is using electricity Capacitance current feeds back, the general adoption rate controller of controller.System damping is to increase using double -loop control purpose, so as to suppress The resonance peak of LCL, improves the stability of system.

Traditional three-phase grid-connected inverter mathematical model, contains only LCL filter and control strategy, load and Electric network impedance, the stability range of LCL type three-phase grid-connected inverter is limited, i.e. the parameter selection of outer ring controller PI or PR must System stability must be just can guarantee that within the specific limits, determine makes the parameter area of system stability, the feature side to grid-connected system Cheng Caiyong Routh Criterions are asking for.

The embodiment of the present invention can be off the net in light current, no matter how control strategy for inverter to change, including using different Wave filter such as L-type or it is double mode under LC types, as the circuit relationships of load and electric network impedance are fixed, therefore be applied to each The weak network system under control strategy is planted, possesses stronger versatility；The excellent degree of system can comprehensively be assessed, Improve the characteristic of system so as to take appropriate measures；The mathematical model set up by the embodiment of the present invention is that light current is off the net grid-connected The real theoretical formulation of system, rather than the improvement on original Impedance Analysis, therefore the stability margin of resulting system It is system performance indications truly with cut-off frequency, it is fabulous and comprehensively describe system.And the embodiment of the present invention The open-loop transfer function of grid-connected system can be obtained, it is possible thereby to the artificial cut-off frequency by design system and amplitude Nargin, so as to obtain ensure that controller parameter scope of the system under certain open-loop cut-off frequency and magnitude margin, is protecting On the premise of card system stability, it is ensured that system is in the range of certain stability margin, and ensures the cut-off frequency and amplitude of system Nargin size meets the requirements, and finally gives a controller parameter scope that systematic function can be made more excellent.

Claims (2)

1. the light current net net source lotus impedance model analysis method of a kind of grid-connected system, it is characterised in that comprise the following steps that：

1) a net source lotus impedance model is defined for grid-connected system, 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 exchange of grid-connected system Power network line impedance, Z_loadFor the AC network load impedance of grid-connected system, U_sAC network for grid-connected system etc. Effect voltage, I_sThe AC network equivalent current source of (s) for grid-connected system, I_gS () enters for the AC network of grid-connected system Net electric current；

2) load of AC network is assumed to hinder inductive load, the line impedance of AC network is resistance sense line impedance, according to The net source lotus impedance model of step 1 definition, obtains open-loop transfer function of the grid-connected system under double -loop control, closed loop transmission Function is respectively：

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

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

A₁=L₁L₂(Z_g+Z_load)C

A₂=(L₁L₂Z_load+L₂Z_gK_cK_pwm+L₂Z_loadK_cK_pwm)C

A₃=(Z_load+Z_g)(L₁+L₂)

Wherein, G_oThe open-loop transfer function of (s) for grid-connected system under double -loop control, G_clS () exists for grid-connected system Closed loop transfer function under double -loop control, outer ring controllers of the G (s) for grid-connected system, Z_gIt is the resistance sense of AC network Line impedance, K_cFor the interior ring controller of grid-connected system, K_pwmFor the equivalent amplification coefficient of inverter bridge of grid-connected system, L₁ For the inverter side inductance of the LCL filter in grid-connected system, L₂For the net side of the LCL filter in grid-connected system Inductance, filter capacitors of the C for the LCL filter in grid-connected system；

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

2. the light current net net source lotus impedance model analysis method of grid-connected system according to claim 1, its feature exist In in step 2, when the outer ring controller of grid-connected system adopts PI controllers, it is assumed that the load of AC network is pure resistance Property load, the line impedance of AC network is pure inductive circuit impedance, by grid-connected system in outer ring controller using PI controls Open-loop transfer function during device processed is optimized for：

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

B₁=[L_g(L₂K_cK_pwm+L₁R)+L₁L₂R]C

B₂=L_g(L₁+L₂+RCK_cK_pwm)+L₂RCK_cK_pwm

B₃=(L₁+L₂+L_g)R

Wherein, G_piThe open-loop transfer function of (s) for grid-connected system when outer ring controller adopts PI controllers, K_pControl for PI The proportionality coefficient of device processed, K_iFor the integral coefficient of PI controllers, L_gFor the pure inductive circuit impedance of AC network, R is AC network Purely resistive load.