CN109494787B - Grid-connected capacity limit calculation method for photovoltaic multi-feed-in power system - Google Patents
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Abstract
The invention discloses a grid-connected capacity limit calculation method for a photovoltaic multi-feed-in power system. Aiming at the photovoltaic multi-feed-in power system, calculating to obtain an extended admittance matrix, and obtaining a generalized short-circuit ratio and a generalized short-circuit ratio critical value of the photovoltaic multi-feed-in power system; comparing the absolute value of the difference between the generalized short-circuit ratio and the generalized short-circuit ratio critical value to meet a given threshold, calculating the sensitivity of the generalized short-circuit ratio of the photovoltaic multi-feed power system to the rated capacity of each photovoltaic device, and finely adjusting and up-regulating the capacity of the photovoltaic devices; and recalculating the sensitivity, adjusting the rated capacity of the fan equipment again according to the sensitivity, and continuously performing iterative processing to obtain the capacity limit. The method provided by the invention has the advantages of clear calculation thought and simple process, can obtain the maximum value of the grid-connected capacity of the photovoltaic multi-feed-in power system under the condition of small interference stability margin, provides guidance for new construction and extension of the photovoltaic power station from the aspect of small interference stability, and is beneficial to the comprehensiveness of planning and design consideration of the power system.
Description
Technical Field
The invention belongs to the technical field of new energy planning design, relates to a power system capacity calculation method, and particularly relates to a grid-connected capacity limit calculation method for a photovoltaic multi-feed-in power system.
Background
With the increase of global energy crisis and the increasing environmental protection situation, new energy resource units represented by photovoltaic are increasing in the electric power system, and become the trend of future energy development. But different from the large-scale new energy access local balance and consumption in Europe and America, the energy load distribution in China is unbalanced, areas with rich photovoltaic resources are located in the northwest and far away from the load center in the east, the requirement of large-scale remote transmission of electric energy exists for a long time, and the high-proportion multi-photovoltaic cluster grid connection is an important trend for future development of the photovoltaic power generation industry. Researches show that after increasingly large photovoltaic grid-connected capacity is accessed, a power system presents obvious power electronization characteristics and is easy to generate the problem of small interference stability, and the grid-connected scale of a new energy unit is restricted
At present, photovoltaic capacity limit research is mostly started from the static stability angle, and all indexes of a power grid are guaranteed to meet requirements. In practical engineering, for a power electronic multi-feed system, the problem of small interference stability is easier to occur compared with the problem of static voltage, and the practical planning needs to be guided by obtaining the photovoltaic grid-connected capacity limit from the angle of small interference stability.
Disclosure of Invention
According to the index, the invention provides a grid-connected capacity limit calculation method of a photovoltaic multi-feed-in power system based on a generalized short-circuit ratio, the calculation idea is clear, the process is simple, the maximum value of the grid-connected capacity of the photovoltaic multi-feed-in power system is explored under a certain small interference stability margin condition, guidance is provided for new construction and extension of a new energy power station from the small interference stability angle, and comprehensiveness of planning design consideration of the power system is facilitated.
The technical scheme of the invention comprises the following steps:
1) for grid-connected capacity sum SB∑And calculating to obtain an extended admittance matrix J by the photovoltaic multi-feed-in power system with determined photovoltaic inner and outer loop PI control parameters, alternating current network topology and power grid impedanceeqAccording to the extended admittance matrix JeqObtaining a generalized short-circuit ratio gSCR of the photovoltaic multi-feed-in power system through eigenvalue decomposition; obtaining a generalized short-circuit ratio critical value gSCR corresponding to the small interference stability margin according to a preset known small interference stability margin (as a target), namely a damping ratio of a dominant characteristic root of a linearization model of the photovoltaic multi-feed power systemref；
2) Comparing generalized short-circuit ratio gSCR with generalized short-circuit ratio critical value gSCRrefCalculating the absolute value of the difference to satisfy a given threshold valueThe sensitivity of the generalized short-circuit ratio gSCR relative to the rated capacity of each photovoltaic device is used for carrying out capacity fine adjustment and up-regulation on the photovoltaic devices, and the sum S of the current grid-connected capacityB∑On the basis, a unit is also added, the generalized short-circuit ratio gSCR is recalculated after adjustment, and the adjusted grid-connected capacity sum S is recordedB∑And a generalized short-circuit ratio gsscr;
recalculating the sensitivity of the generalized short-circuit ratio gSCR relative to the rated capacity of each feed-in point photovoltaic equipment by adopting the mode of step 1), adjusting the rated capacity of the photovoltaic equipment again according to the sensitivity, and continuously executing the process until the generalized short-circuit ratio gSCR and the generalized short-circuit ratio critical value gSCRrefThe absolute value of the difference meets a given threshold value, and at the moment, the grid-connected capacity of the system reaches the limit of the grid-connected capacity, so that the finally obtained grid-connected capacity sum SB∑And as the capacity limit of the photovoltaic multi-feed-in power system, the grid-connected capacity limit calculation of the photovoltaic multi-feed-in power system is completed.
The step 1) is specifically as follows:
1.1) obtaining gSCR by presetting known small interference stability margin according to the corresponding relation of the preset small interference stability margin and the generalized short-circuit ratioref；
1.2) obtaining an equivalent admittance matrix B from the alternating current network topology and the power grid impedance;
1.3) forming a capacity diagonal matrix S from the rated capacities of the photovoltaic devicesBObtaining an extended admittance matrix and a minimum eigenvalue of the extended admittance matrix according to the following formula, wherein the minimum eigenvalue is a generalized short-circuit ratio gSCR of the photovoltaic multi-feed-in power system;
gSCR＝minλ(Jeq)。
the step 2) is specifically as follows:
2.1) comparing the calculated generalized short-circuit ratio gSCR with a generalized short-circuit ratio threshold value gSCRrefThe absolute value of the difference between the values is related to a deviation threshold rho which is 0.01 XgSCRrefIf the absolute value of the difference is less than the deviation threshold rhoOr step 2.5) if the maximum iteration times are reached, or step 2.2) if the maximum iteration times are not reached;
according to the mechanical characteristics of the photovoltaic unit, the photovoltaic cell panel does not have mechanical elements, the photovoltaic unit has electrical signal transmission and rapid response, the fluctuation of illumination can directly cause the change of output power, and further the quick change of the system stability is caused, and the deviation threshold is taken as a generalized short-circuit ratio critical value gSCRrefI.e. wherein the deviation threshold is taken to be p 0.01 × g scrref。
2.2) calculating the sensitivity of the generalized short-circuit ratio gSCR of the photovoltaic multi-feed power system to the rated capacity of each photovoltaic device;
2.3) comparing the sensitivities obtained by the calculation in the step 2.2), and recording a feed-in branch k corresponding to the minimum value of the absolute value of the sensitivities;
2.4) considering that the stability of the photovoltaic unit changes rapidly, the rated capacity of the photovoltaic equipment of the feed-in branch k corresponding to the sensitivity with the minimum absolute value is adjusted up by one unit h, and the conservative estimation is that h is 0.001 × SBk，SBkThe rated capacity of the photovoltaic equipment represented as the kth feed-in branch is the rated capacity of the photovoltaic equipment corresponding to the sensitivity with the minimum absolute value, and the sum S of the current grid-connected capacityB∑On the basis, a unit h is also added, the generalized short-circuit ratio gSCR is recalculated after adjustment, and the step 2.1 is returned;
2.5) continuously repeating the steps 2.1) to 2.4), and outputting the final grid-connected capacity sum S of iterationB∑And the grid-connected capacity limit of the photovoltaic multi-feed-in power system.
In the step 2) or the step 2.2), the following formula is adopted to calculate the sensitivity of the generalized short-circuit ratio gSCR of the photovoltaic multi-feed power system with respect to the rated capacity of each photovoltaic device
Wherein psikRepresenting an extended admittance matrix JeqThe k-th element, phi, of the left eigenvector psi of the eigenvalue gSCRmRepresenting an extended admittance matrix JeqThe mth element of the right eigenvector phi of the eigenvalue gSCR, n being the total number of photovoltaic devices fed in, BkmRepresenting the element of the mth column of the kth row in the equivalence admittance matrix B, SBkThe rated capacity of the kth photovoltaic device is shown, and k represents the ordinal number of the photovoltaic device, namely the ordinal number of the feeding branch.
The sensitivity of the generalized short-circuit ratio gSCR of the photovoltaic multi-feed power system with respect to the rated capacity of each photovoltaic device is less than zero, specifically:
let psi, phi be the extended admittance matrix J respectivelyeqLeft and right eigenvectors corresponding to eigenvalues gSCR, i.e.
ψJeq＝gSCRψ
Jeqφ＝gSCRφ
And considering the equivalent admittance matrix as a symmetric matrix, obtaining:
wherein S isBRepresenting the rated capacity of the photovoltaic device.
And deducing the generalized short-circuit ratio gSCR of the photovoltaic multi-feed-in power system according to the rated capacity S of each photovoltaic device by combining the left eigenvector and the right eigenvectorBThe sensitivity of (a) is:
in the formula, phikIs an extended admittance matrix JeqThe kth element of the right eigenvector phi corresponding to the eigenvalue gsrc.
In the step 1), according to the extended admittance matrix JeqThe generalized short-circuit ratio gSCR of the photovoltaic multi-feed-in power system is obtained through eigenvalue decomposition, and the generalized short-circuit ratio gSCR specifically comprises the following steps:
capacity diagonal matrix S formed by rated capacities of photovoltaic devicesBBy alternating current network topology andobtaining an equivalent admittance matrix B and an expansion admittance matrix J from the power grid impedanceeqThe calculation is as follows:
wherein B is expressed as an equivalent admittance matrix of the photovoltaic multi-feed-in power system;
by making a pair JeqDecomposing the characteristic value to obtain the characteristic value lambda1,λ2,…,λnAnd the minimum characteristic value is used as the generalized short-circuit ratio gSCR of the photovoltaic multi-feed power system.
In the step 2), when calculating the sensitivity of the generalized short-circuit ratio gSCR of the photovoltaic multi-feed power system with respect to the rated capacity of each photovoltaic device, taking the damping ratio of the dominant characteristic root of the photovoltaic multi-feed power system not less than a certain specific value as a constraint condition, and establishing the following constraint condition:
σ(SB)≥σref
in the formula, σ (S)B) Representing the satisfaction of the rated capacity of each photovoltaic device by a matrix SBDominant characteristic root damping ratio, sigma, of photovoltaic multi-feed power systemrefRepresenting a damping ratio of a dominant characteristic root of a photovoltaic multi-feed-in power system corresponding to a preset known small interference stability margin;
after the generalized short circuit ratio index is introduced to represent the small interference stability of the system, the constraint condition is expressed as:
gSCR(SB)≥gSCRref
wherein gSCR (S)B) Representing the satisfaction of the rated capacity of each photovoltaic device by a matrix SBThe generalized short-circuit ratio of the photovoltaic multi-feed power system.
The invention considers the planning and design problem of photovoltaic power stations in engineering practice, a certain photovoltaic power station has a plurality of photovoltaic devices of the same type, and how to plan the capacity of each photovoltaic device is realized under the condition of the known small interference stability margin of the system, namely the critical damping ratio of the dominant characteristic root of the system, so that the sum of the system capacity reaches the maximum value.
Aiming at a photovoltaic multi-feed-in power system, an extended admittance matrix of the system can be obtained from information such as feed-in capacity, a grid structure and the like; according to the eigenvalue of the extended admittance matrix, the photovoltaic multi-feed-in system can be equivalent to a plurality of photovoltaic single-feed-in systems with the same small interference stability; the minimum eigenvalue of the extended admittance matrix is the generalized short-circuit ratio of the photovoltaic multi-feed-in power system, the sensitivity of the generalized short-circuit ratio on the rated capacity of each photovoltaic device can be obtained by combining the eigenvalue and the eigenvector property, and the grid-connected capacity limit of the photovoltaic multi-feed-in power system is calculated according to the sensitivity.
The invention has the beneficial effects that:
the capacity limit of the photovoltaic power station under the large-scale grid-connected condition is obtained from the angle of small interference stability based on the generalized short circuit ratio index of the photovoltaic multi-feed-in power system, guidance is provided for newly building and expanding the photovoltaic power station, comprehensiveness of planning and designing consideration of the power system is facilitated, and the problem of small interference stability can be effectively reduced in practical engineering application.
The method is clear in calculation thought and simple in process, aims to explore the maximum value of the grid-connected capacity of the photovoltaic multi-feed-in power system under a certain small interference stability margin condition, provides guidance for building and expanding the photovoltaic power station from the small interference stability angle, and has excellent effect and application value for the photovoltaic unit.
Drawings
Fig. 1 is an equivalent circuit diagram of a photovoltaic multi-feed power system in simulation verification according to an embodiment of the present invention.
Fig. 2 is a control block diagram of a photovoltaic device in simulation verification according to an embodiment of the present invention.
Fig. 3 is a schematic flow chart of the grid-connected capacity limit calculation method of the present invention.
Fig. 4 is a system dominant feature root trajectory in a capacity limit calculation process under scenario 1 in simulation verification according to an embodiment of the present invention.
Fig. 5 is a system dominant feature root trajectory in a capacity limit calculation process under scenario 2 in simulation verification according to an embodiment of the present invention.
Detailed Description
The invention is described in further detail below with reference to the figures and the embodiments.
The specific examples of the implementation of the complete method according to the inventive content of the present application are as follows:
a photovoltaic five-feed power system is built in Matlab/Simulink software, as shown in FIG. 1. The photovoltaic plant in a five-feed system is a typical voltage-sourced inverter. The outer ring of the inverter adopts constant direct current voltage UdcControl, as shown in fig. 2. The physical meanings of the variables in FIG. 2 are shown in Table 1 below:
TABLE 1UdcVariable corresponding table for control inverter
UdcThe parameter values for controlling the inverter variables are shown in table 2 below:
table 2 parameter values of inverter variables in simulation verification of the embodiment
| DC side injection power Pin/p.u. | 1 |
| DC capacitor voltage reference value Udcref/p.u. | 1 |
| Inner loop q-axis current reference value Icqref/p.u. | 0 |
| Direct current voltage control PI link parameter Kpdc | 0.5 |
| Direct current voltage control PI link parameter Kidc | 8 |
| Current inner loop PI link parameter Kpi | 0.2 |
| Current inner loop PI |
10 |
| Phase-locked loop PI link parameter Kppll | 2.5 |
| Phase-locked loop PI link parameter Kipll | 3200 |
| Filter inductance Lf/p.u. | 0.05 |
| DC side capacitor Cdc/p.u. | 0.12 |
The ac network parameters of the photovoltaic five-feed system are shown in table 3 below:
table 3 ac grid parameters in simulation verification of the embodiment
| Line impedance Z10/p.u. | 0.15 |
| Line impedance Z20/p.u. | 0.05 |
| Line impedance Z30/p.u. | 0.1 |
| Line impedance Z40/p.u. | 0.1 |
| Line impedance Z50/p.u. | 0.1 |
| Line impedance Z12/p.u. | 0.2 |
| Line impedance Z13/p.u. | 0.25 |
| Line impedance Z14/p.u. | 0.25 |
| Line impedance Z15/p.u. | 0.25 |
| Line impedance Z23/p.u. | 0.2 |
| Line impedance Z24/p.u. | 0.2 |
| Line impedance Z25/p.u. | 0.2 |
| Line impedance Z34/p.u. | 0.2 |
| Line impedance Z35/p.u. | 0.2 |
| Line impedance Z45/p.u. | 0.2 |
Given photovoltaic five-feed system rated capacity sum S∑11p.u., and the rated capacity of the five-feed system is SB1.5,1.5,5,1.5,1.5, 1.5) the system generalized short circuit ratio gsrc is 3.963. If the power grid requires that the small interference stability margin of the system meets the condition that the damping ratio of the dominant characteristic root is more than 0.04, calculating to obtain the corresponding gSCR minimum value gSCRref3.817. Optimization calculations were performed using the flowchart shown in fig. 3, where the sensitivity deviation threshold ρ was set to 0.0001, the maximum number of iterations was set to 10000, and the device rated capacity fine-tuning step h was set to 0.001 p.u.. The result shows that the generalized short-circuit ratio gSCR of the photovoltaic five-feed power system is reduced from an initial value of 3.963 to a value 3.817 meeting the constraint condition, and the sum S of the grid-connected capacity of the photovoltaic equipment of the five-feed power systemB∑12.443p.u., each feed point capacity allocation corresponding to the limit capacity is S'B＝diag(1.5,2.943,5,1.5,1.5)。
For the photovoltaic five-feed-in system, the following two situations are considered, and a method for increasing the grid-connected capacity of each feed-in point in an equal proportion is selected for comparison:
scenario 1: when the generalized short-circuit ratio reaches gSCR after the system is expandedrefAnd in time, the damping ratios of the dominant characteristic roots reach 0.04, and the requirement of small interference stability margin is met. The capacity is expanded by adopting an equal proportion method to be 1.267p.u., which is less than the expanded capacity 1.443p.u. based on the generalized short-circuit ratio sensitivity method. Under both methods, as capacity increases,the variation locus of the system feature root is shown in fig. 4 (note that the dominant feature roots are all conjugate complex roots in the extension process, and only the roots with positive/negative imaginary parts are selected for the description of the drawing).
Scenario 2: when the capacity reaches S after the system is expandedB∑max12.443p.u., expanding the capacity allocation of each feed-in point as S ″, by using equal proportion methodB3The generalized short-circuit ratio can only reach gsrc 3.496, and the dominant characteristic root damping ratio can only reach 0.031, which cannot meet the requirement of small interference stability margin like the method proposed herein. Under the two methods, as the capacity increases, the variation locus of the characteristic root of the system is shown in fig. 5 (note: the dominant characteristic roots are conjugate complex roots in the extension process, and only the roots with positive/negative absolute values are selected for drawing and explanation).
The capacity limit of the photovoltaic power station under the large-scale grid-connected condition is obtained from the angle of small interference stability based on the generalized short circuit ratio index of the photovoltaic multi-feed-in power system, accurate guidance can be provided for newly building and expanding the photovoltaic power station, comprehensiveness of planning, designing and considering of the power system is facilitated, the problem of small interference stability can be effectively reduced in practical engineering application, and the technology has remarkable technical effect.
The foregoing detailed description is intended to illustrate and not limit the invention, which is intended to be within the spirit and scope of the appended claims, and any changes and modifications that fall within the true spirit and scope of the invention are intended to be covered by the following claims.
Claims (3)
1. A grid-connected capacity limit calculation method for a photovoltaic multi-feed-in power system is characterized by comprising the following steps:
1) for grid-connected capacity sum SB∑And calculating to obtain an extended admittance matrix J by the photovoltaic multi-feed-in power system with determined photovoltaic inner and outer loop PI control parameters, alternating current network topology and power grid impedanceeqAccording to the extended admittance matrix JeqObtaining a generalized short-circuit ratio gSCR of the photovoltaic multi-feed-in power system through eigenvalue decomposition; obtaining the generalized sense corresponding to the small interference stability margin according to the preset known small interference stability marginShort circuit ratio threshold value gSCRref；
2) Comparing generalized short-circuit ratio gSCR with generalized short-circuit ratio critical value gSCRrefThe absolute value of the difference satisfies a given threshold value, the sensitivity of the generalized short-circuit ratio gSCR of the photovoltaic multi-feed power system on the rated capacity of each photovoltaic device is calculated, the capacity of the photovoltaic devices is finely adjusted and adjusted, and the sum S of the current grid-connected capacity is obtainedB∑On the basis, a unit is also added, the generalized short-circuit ratio gSCR is recalculated after adjustment, and the adjusted grid-connected capacity sum S is recordedB∑And a generalized short-circuit ratio gsscr; recalculating the sensitivity of the generalized short-circuit ratio gSCR relative to the rated capacity of the photovoltaic equipment at each feed-in point by adopting the mode of step 1), adjusting the rated capacity of the photovoltaic equipment again according to the sensitivity, and continuously executing the flow of the step 2) until the generalized short-circuit ratio gSCR and the critical value of the generalized short-circuit ratio gSCRrefThe absolute value of the difference satisfies a given threshold to obtain the final grid-connected capacity sum SB∑As capacity limit of photovoltaic multi-feed power system;
the step 2) is specifically as follows:
2.1) comparing the calculated generalized short-circuit ratio gSCR with a generalized short-circuit ratio threshold value gSCRrefThe absolute value of the difference between the values is related to a deviation threshold rho which is 0.01 XgSCRrefIf the absolute value of the difference is smaller than the deviation threshold rho or the maximum iteration number is reached, the step 2.5) is carried out, otherwise, the step 2.2) is carried out;
2.2) calculating the sensitivity of the generalized short-circuit ratio gSCR of the photovoltaic multi-feed power system to the rated capacity of each photovoltaic device;
2.3) comparing the sensitivities obtained by the calculation in the step 2.2), and recording a feed-in branch k corresponding to the minimum value of the absolute value of the sensitivities;
2.4) the photovoltaic installation rated capacity of the feed-in branch k corresponding to the sensitivity with the smallest absolute value is adjusted up by one unit h, and h is taken as 0.001 × SBk，SBkRated capacity of photovoltaic equipment represented as k-th feeding branch and sum of current grid-connected capacity SB∑On the basis, a unit h is also added, the generalized short-circuit ratio gSCR is recalculated after adjustment, and the step 2.1 is returned;
2.5) continuously repeating the steps 2.1) to 2.4), and outputting the final grid-connected capacity sum S of iterationB∑The grid-connected capacity limit of the photovoltaic multi-feed-in power system is obtained;
in the step 2) or the step 2.2), the following formula is adopted to calculate the sensitivity of the generalized short-circuit ratio gSCR of the photovoltaic multi-feed power system with respect to the rated capacity of each photovoltaic device
Wherein psikRepresenting an extended admittance matrix JeqThe k-th element, phi, of the left eigenvector psi of the eigenvalue gSCRmRepresenting an extended admittance matrix JeqThe mth element of the right eigenvector phi of the eigenvalue gSCR, n being the total number of photovoltaic devices fed in, BkmRepresenting the element of the mth column of the kth row in the equivalence admittance matrix B, SBkThe rated capacity of the kth photovoltaic device is shown, and k represents the ordinal number of the photovoltaic device.
2. The grid-connected capacity limit calculation method of the photovoltaic multi-feed-in power system according to claim 1, characterized in that: in the step 1), according to the extended admittance matrix JeqThe generalized short-circuit ratio gSCR of the photovoltaic multi-feed-in power system is obtained through eigenvalue decomposition, and the generalized short-circuit ratio gSCR specifically comprises the following steps:
capacity diagonal matrix S formed by rated capacities of photovoltaic devicesBObtaining an equivalent admittance matrix B from the AC network topology and the grid impedance, and expanding an admittance matrix JeqThe calculation is as follows:
wherein B is expressed as an equivalent admittance matrix of the photovoltaic multi-feed-in power system;
by pairsJeqDecomposing the characteristic value to obtain the characteristic value lambda1,λ2,…,λnAnd the minimum characteristic value is used as the generalized short-circuit ratio gSCR of the photovoltaic multi-feed power system.
3. The grid-connected capacity limit calculation method of the photovoltaic multi-feed-in power system according to claim 1, characterized in that: in the step 2), when calculating the sensitivity of the generalized short-circuit ratio gSCR of the photovoltaic multi-feed power system with respect to the rated capacity of each photovoltaic device, establishing the following constraint conditions:
σ(SB)≥σref
in the formula, σ (S)B) Representing the satisfaction of the rated capacity of each photovoltaic device by a matrix SBDominant characteristic root damping ratio, sigma, of photovoltaic multi-feed power systemrefRepresenting a damping ratio of a dominant characteristic root of a photovoltaic multi-feed-in power system corresponding to a preset known small interference stability margin;
after the generalized short circuit ratio index is introduced to represent the small interference stability of the system, the constraint condition is expressed as:
gSCR(SB)≥gSCRref
wherein gSCR (S)B) Representing the satisfaction of the rated capacity of each photovoltaic device by a matrix SBThe generalized short-circuit ratio of the photovoltaic multi-feed power system.
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