CN109713661B - Method for analyzing influence of wind power plant access on multi-machine system fault limit removal time - Google Patents
Method for analyzing influence of wind power plant access on multi-machine system fault limit removal time Download PDFInfo
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Abstract
The invention relates to an analysis method for influences of wind power plant access on multi-machine system fault limit clearing time, which is based on a transient energy function theory and starts from an original multi-machine system, and adopts a first integration method to construct a transient energy function containing the wind power plant multi-machine system; on the basis, the incidence relation between the critical energy value of the system and the transient output power of the wind power plant is deduced, the change of the critical energy borne by the system caused by the change of the network topology structure due to DFIG access is quantitatively analyzed, and the influence of the wind power plant access on the stability margin of the system is reflected from the critical energy; analyzing the change of a system state variable and an energy accumulation process according to the influence of the DFIG on the equivalent electromagnetic power of the system during the fault period, and giving an influence mode of wind power plant access on the transient energy accumulation speed of the system; the fault limit clearing time is calculated by the accumulation time from the transient energy to the critical energy of the system.
Description
Technical Field
The invention belongs to the field of analysis of influence of new energy access on transient stability of a power system, and particularly relates to analysis of influence of wind power plant access on fault limit clearing time of the power multi-machine system from a transient energy function on the power multi-machine system, so that influence on transient stability of the power multi-machine system is refracted.
Background
The continuous expansion of clean energy demand and the increasing maturity of wind power generation technology have led to the vigorous development and application of wind power generation worldwide. However, the access of large-scale wind farms brings many disadvantages while providing more energy supplies to the grid. For example, the access of a large proportion of wind power often changes the power flow distribution and the topological structure of the system, thereby affecting the total bearing capacity of the system for faults and increasing the possibility of system instability during the fault.
In order to enable a power system to operate safely under a high wind power ratio, a wind turbine generator is required to have a ride-through operation capability of withstanding a grid fault and keeping a grid-off state, in particular a Low Voltage Ride Through (LVRT) capability [1] . When the system is greatly disturbed, the transient response process of the wind driven generator is greatly different from that of the synchronous generator, and the transient stability of the power system is affected by the change of the running state of the wind driven generator differently from that of the synchronous generator. Different LVRT schemes can enable the fans to change the output active power and reactive power in the transient process to influence the mutual synchronism of power angles between synchronous machines, and the phenomenon is used for the large-capacity wind power with centralized accessThe field must not be ignored. Therefore, when the transient stability of the electric power multi-machine system including the wind power plant is researched, the change of the wind power plant access to the system limit stability margin needs to be analyzed, the influence of the power output characteristic of a fan during low voltage ride through on the relative motion of a power angle of a synchronous machine needs to be analyzed, and the influence relation on the system fault limit removal time is further explored.
The research related to the invention at present mainly aims at a single-machine infinite system or an equivalent double-machine system [2-3] The influence of wind power plant access on the transient stability of the power angle of the system is rarely and directly analyzed from the actual power multi-machine system, and the influence relation of the actual access condition of the wind power plant in the multi-machine system on the transient stability of the system is difficult to consider; many relevant researches give general rules through simulation analysis [5-7] The conclusion is not convincing due to the lack of theoretical support. In the research currently in question [2,9-10] Most of the wind power plants fail to comprehensively consider the mechanism that the wind power plant access comprehensively influences the stability of the transient power angle of the power system in the critical energy and energy accumulation process of the system, and the influence is only analyzed from a single angle, so that the conclusion is one-sidedness; in addition, some researches consider that the energy margin of the whole system is changed after the wind power plant is connected with the electric power multi-machine system, and the energy limit born by the wind turbine in the transient process is increased on the basis of the energy margin of the original system [4] The influence of fan access on the network topology structure is not considered, and the conclusion is limited. Therefore, the influence rule of the wind power plant access on the system stability margin and the influence rule of the external characteristics of the fan power during the fault period on the energy accumulation process need to be analyzed quantitatively from the power multi-machine system, so that the change of the system fault limit removal time is obtained, and the analysis of the influence of the wind power plant access on the transient stability limit of the power multi-machine system is realized.
Reference to the literature
[1] Technical specification (survey opinion draft) of accessing a wind power plant into a power system is S/OL, http:// www.docin.com/p-115458480.html.
[2] The method comprises the following steps of A, a new head, an Kingweisheng, a Chiyongning and the like, the fault behavior of the double-fed wind turbine generator and the influence on the transient stability of the power system [ J ], the automation of the power system, 2015, 39 (10): 16-21.
[3] Influence of large-scale wind power centralized access on transient power angle stability of a power system (I): theoretical basis [ J ] chinese electro-mechanical engineering, 2015, 35 (15): 3832-3842.
[4]Chowdhury MA,ShenW,Nasser H,et al.Transient stability ofpower system integrated with doubly fed induction generator wind farms[J].IET Renewable Power Generation,2015,9(2):184-194.
[5] Zhangmingming, create a source, luzhou, study of transient stability of a sending-end system power grid containing high-permeability wind power [ J ] power grid technology, 2013, 37 (3): 740-745.
[6] Liu Si Wei, Li Heng Yin, Zhou Ming. doubly-fed wind generator group influence pattern analysis [ J ] to grid technology, 2016, 40 (2): 471 to 476
[7]MeegahapolaL，Flynn D，Littler T.Transient stability analysis ofapower system with high wind penetration[C].Universities Power Engineering 43International Conference，Italy：IEEE，2008：1～5
[8] Dunli. influence of wind power integration on safety and stability of a power system [ D ]. denna: university of Shandong, 2013.
[9] The influence of the access of the doubly-fed wind turbine on the power angle stability of the power system [ J ] is the power grid technology, 2013, 37 (12): 3399 to 3405
[10] Mechanism analysis of influence of large-scale wind power access on system power angle stability [ J ] reported in china motor engineering, 2017, 37 (5): 1324-1331
Disclosure of Invention
The invention provides a method for analyzing the influence of wind power plant access on the fault limit removal time of a multi-machine system. The technical scheme is as follows:
an analysis method for influences of wind power plant access on multi-machine system fault limit clearing time is characterized by starting from an original multi-machine system on the basis of a transient energy function theory and constructing a transient energy function containing the multi-machine system of a wind power plant by adopting a first integration method; on the basis, the incidence relation between the critical energy value of the system and the transient output power of the wind power plant is deduced, the change of the critical energy borne by the system caused by the change of the network topology structure due to DFIG access is quantitatively analyzed, and the influence of the wind power plant access on the stability margin of the system is reflected from the critical energy; analyzing the change of a system state variable and an energy accumulation process according to the influence of the DFIG on the equivalent electromagnetic power of the system during the fault period, and giving an influence mode of wind power plant access on the transient energy accumulation speed of the system; calculating fault limit removal time through accumulation time from transient energy to system critical energy so as to provide an index reflecting the degree of influence on the transient stability of the system and realize analysis on the degree of influence of wind power plant access on the transient stability of the system;
the method specifically comprises the following steps:
the method comprises the following steps: establishing mathematical model of multi-machine system comprising wind power plant
The method comprises the following steps that n synchronous generators are arranged in a multi-machine system, all the synchronous generators are divided into two machine groups according to a complementary group inertia center transformation theory, namely a leading group S and a remaining group A, and the rotor angles and the change conditions of the synchronous generators in the same machine group in the transient process are the same, so that the synchronous generator model of the system under the mode of the two machine groups is obtained:
wherein, theta S And theta A Rotor angles of the synchronizers in the S cluster and the A cluster relative to the COI are respectively set; p mi 、P mj And P ei 、P ej Mechanical power and electromagnetic power of synchronous machines in the S machine group and the A machine group respectively; p COI Acceleration power that is the center of system inertia; m i And M j Representing inertia time constants of each synchronous machine in the S machine group and the A machine group;
contracting the node admittance matrix of the multi-machine system, quantifying the influence of DFIG on the system into the variation of the electromagnetic power of each synchronous machine, setting p synchronous machines in the S machine group, and dividing all nodes into three types: the system comprises an S cluster synchronizer, an A cluster synchronizer and a common node R in a network, wherein the S cluster synchronizer is connected with the common node R; after the DFIG is accessed, adding a parallel branch at a grid-connected point of the DFIG in a network, and setting a port node of the DFIG as W;
performing contraction processing and correction on a node voltage equation of the multi-machine system, and eliminating R type nodes in the equation; the DFIG external power characteristic is equivalent to the parallel grounding admittance of the W node, the W node is eliminated, only the potential node in the synchronous machine is reserved, the information contained in the node W is kneaded into a new admittance matrix, a correction matrix with delta Y as a system admittance matrix is obtained, and the influence relation of the DFIG on the equivalent electrical distance between the nodes of the synchronous machine after the DFIG is accessed is reflected:
wherein, Δ G ij And Δ B ij The correction values of conductance and susceptance among nodes in a system after the DFIG is connected are represented, and when i is equal to j, the correction values of self conductance and self susceptance of the node i are represented; when i is not equal to j, the mutual conductance and the mutual susceptance between the nodes i and j are represented;
step two: constructing a transient energy function containing a wind power plant multi-machine system by adopting a first integration method;
for a system motion equation set divided into two machine group modes, accumulating rotor motion equations of all synchronous machines in the same machine group to simplify observation dimensionality of the system into angle difference between a unified power angle and COI of the two machine groups, adding two summation equations obtained by the two machine groups to solve the first integral, and obtaining a transient energy function expression of the system containing DFIG under the COI coordinate as follows:
wherein the content of the first and second substances,
and
respectively, the state variables M of the S cluster and the A cluster synchronous machine at the stable balance point S And M A Respectively representing the sum of inertia time constants of all synchronous machines of the S machine group and the A machine group,
and
respectively representing the angle difference of rotor angles of the S cluster and the A cluster synchronous machine relative to the COI of the system when the system reaches a stable balance point after a fault; the expressions for the remaining variables are:
step three: the critical energy value of the system is obtained, and the influence of wind power plant access on the stability margin of the system is quantitatively analyzed:
the equivalent system equation of motion is obtained as follows:
wherein, P m.eq Mechanical power for an equivalent system; p e.eq Electromagnetic power for an equivalent system; theta SA For the equivalent system power angle, the expressions of the parameters are respectively:
the system can be calculated by the formula (5) to operate at a stable equilibrium point
And unstable equilibrium point
The work angles are respectively as follows:
assuming that the COI of the system remains unchanged during the fault duration and during the transient after the fault is removed and the rotor angle at the unstable equilibrium point of each synchronous machine is considered to be complementary to the rotor angle at the stable equilibrium point, the unstable equilibrium point is set as
The transient energy is used as the critical energy of the system, and the following can be obtained:
in the formula (I), the compound is shown in the specification,
and
respectively showing the rotor positions of the synchronous machines in the S group and the A group relative to the COI at the initial fault time;
the parameters delta G and delta B respectively represent the change amounts of self-admittance and mutual admittance between potential nodes in the synchronous machine after the DFIG is accessed, namely conductance and susceptance elements in a system correction matrix delta Y represented by the formula (2); the method has the advantages that the contraction analysis of the system node admittance matrix and the construction of the transient energy function of the DFIG system are utilized, so that the quantitative analysis of the influence of the access of the DFIG on the system critical energy is realized;
step four: quantitatively analyzing the influence of DFIG power external characteristics on an energy accumulation process during a fault:
for a system with a given critical energy, the degree of the change of the power angle of the system along with the time is used for judging,
the calculation formula of the system power angle is as follows:
system equivalent electromagnetic power P e.eq As a function of network parameters, P e.eq The larger the value, θ SA And omega SA The smaller the variation amount of (c); the power angle variation reflects the degree of deviation from stability of the system, and the larger the equivalent electromagnetic power of the system is, the longer the time for the power angle of the system to increase to a critical instability state is, which indicates that the transient energy accumulation process is slower, namely the system is more stable;
the active and reactive output of the DFIG in the crossing period causes different degrees of influence on equivalent electrical connection between the synchronous machines, and the influence is reflected as the influence of the node self-admittance and the mutual admittance of the synchronous machines;
step five: analyzing the influence relation of DFIG access on system fault limit removal time based on a transient energy function method;
substituting the average electromagnetic power of the cluster S and the cluster A into the system motion equation during the fault period to respectively calculate the state variable (theta) of the synchronous machine of each cluster i ,ω i ) An approximate relationship of the variables over time; substituting into the energy function expression (3), obtaining a univariate function expression V (t) of the transient energy accumulated in the system fault period with respect to time, and solving an equation V (t) which is V cr Calculating to obtain a limit removal time index reflecting the stability degree of the system, and realizing the analysis of the influence of wind power access on the limit removal time of system faults, V cr Is the critical energy of the system after DFIG access.
The analysis method based on the transient energy function can realize quantitative analysis of the influence on the transient stability of the multi-machine system comprising the wind power plant, and after the wind power plant is analyzed to be connected into the electric power multi-machine system, the transient power angle stability of the system is influenced mainly by changing the external characteristics of the network topology structure and the output power during the fault period, and the influence is essentially reflected in the change of the equivalent electromagnetic power of the multi-machine system synchronous machine caused by DFIG equivalent admittance. And theoretically, a quantitative relation between the change of the electromagnetic power and the output power characteristic of the DFIG during the access and fault of the DFIG by an equivalent admittance model is deduced, and the quantitative influence of the access of the wind power plant on the limit energy value (namely the stability margin) of the whole system and the energy accumulation process is further analyzed and obtained on the basis of an energy function method. The scheme considers the consideration of the system stability margin and the energy accumulation process, integrates the two aspects to obtain the influence factors of the system fault limit removal time, realizes the quantitative analysis of the influence, and provides an available technical scheme and a theoretical basis for the transient stability analysis of a multi-machine system comprising a wind power plant.
Drawings
FIG. 1 schematic diagram of an augmented DFIG-containing network
FIG. 2 is a schematic flow chart of an analysis method of the present embodiment
FIG. 3 schematic diagram of a DFIG-containing three-machine system
DFIG equivalent conductance comparison during fault in FIG. 4
DFIG equivalent susceptance comparison during fault in FIG. 5
Figure 6 Limit ablation time for three-machine System under scheme A
Figure 7 Limit ablation time for three-machine System under scenario B
FIG. 8 shows the power angle simulation result of the three-machine system without DFIG
FIG. 9 shows the power angle simulation result of the three-machine system when the scheme A is adopted
FIG. 10 shows the power angle simulation result of a three-machine system when scheme B is adopted
Detailed Description
The fault limit removal time is an important parameter reflecting the stability limit of the power system, and the influence relation of the wind power plant on the system fault limit removal time is explored by researching the influence of the wind power plant access on the maximum fault bearing capacity and the transient energy accumulation speed of the power system, so that a quantitative analysis way is provided for the influence of the wind power plant on the transient stability of the multi-machine system, and the practical requirement of safety analysis of the new energy power system is met.
The invention provides a method for analyzing the influence of wind power plant access on the fault limit removal time of a multi-machine system, and aims to realize quantitative research on the influence degree of the wind power plant access on the transient stability of the multi-machine system. Based on a transient energy function theory, starting from an original multi-machine system, constructing a transient energy function containing the wind power plant multi-machine system by adopting a first integration method; on the basis, the incidence relation between the critical energy value of the system and the transient output power of the wind power plant is deduced, the change of the critical energy borne by the system caused by the change of the network topology structure due to DFIG access can be quantitatively analyzed, and the influence of the wind power plant access on the stability margin of the system is reflected from the critical energy; meanwhile, the change of the system state variable and the energy accumulation process are analyzed according to the influence of the DFIG on the equivalent electromagnetic power of the system during the fault period, and an influence mode of wind power plant access on the transient energy accumulation speed of the system is given. Further, fault limit removal time can be calculated through accumulation time from transient energy to system critical energy, so that indexes which reflect the influence on the system transient stability degree are given, and the influence degree of wind power plant access on the system transient stability is analyzed.
The method comprises the following steps:
the method comprises the following steps: establishing mathematical model of multi-machine system comprising wind power plant
Consider a multiple synchronous machine system, assuming that there are n synchronous generators in the system. According to the complementary group inertia center transformation theory, all synchronous machines in the system can be divided into two clusters, namely a leading group S and a remaining group A. In order to simplify the analysis, assuming that the rotor angle and the variation condition of each synchronous machine in the same cluster are the same in the transient process, the synchronous generator model of the system under the two cluster modes is obtained as follows:
wherein, theta S And theta A Rotor angles of the synchronizers in the S cluster and the A cluster relative to the COI are respectively set; p mi 、P mj And P ei 、P ej Mechanical power and electromagnetic power of synchronous machines in the S machine group and the A machine group respectively; p COI Acceleration power that is the center of system inertia; m is a group of i And M j And the inertia time constants of the synchronous machines in the S machine group and the A machine group are shown.
Further, the system node admittance matrix is shrunk, and the influence of the DFIG on the system is quantized into the change of the electromagnetic power of each synchronous machine. If p synchronizers are provided in the S cluster, and if the grouping manner of the synchronizers in the system is known, all nodes in the system can be divided into three categories: the system comprises an internal potential node of the S cluster synchronous machine, an internal potential node of the A cluster synchronous machine and a common node R in the network. After the DFIG is connected, a parallel branch is added at the grid-connected point of the DFIG in the network. If the port node of the DFIG is W, the system network structure is as shown in fig. 1.
Performing contraction processing and correction on a node voltage equation of the system, and eliminating R type nodes in the equation; the external power characteristic of the DFIG can be equivalent to the parallel grounding admittance of the W node, the W node is eliminated, only the potential node in the synchronous machine is reserved, the information contained in the node W is kneaded into a new admittance matrix, and a correction matrix with delta Y as a system admittance matrix is obtained, which reflects the influence relationship of the DFIG on the equivalent electrical distance between the nodes of the synchronous machine after the DFIG is accessed:
wherein, Δ G ij And Δ B ij The correction values of conductance and susceptance among nodes in a system after the DFIG is connected are represented, and when i is equal to j, the correction values of self conductance and self susceptance of the node i are represented; when i ≠ j, it represents the mutual conductance and mutual susceptance between nodes i and j.
Step two: starting from multiple machines, a first integration method is adopted to construct a transient energy function containing a wind power plant multiple machine system.
The TEF describes the transient energy of the power system at different times during and after the fault is cleared, and is a function of the transient energy accumulated by the system, which is generated by the fault and developed gradually during the fault duration. The invention adopts a first integration method to construct the energy function of a multi-machine system, because the first integration represents the inherent property of the system in each running state. Therefore, the transient energy of the system of differential equations can be represented by the first integral of the system of differential equations.
For a system motion equation set divided into two machine group modes, the rotor motion equations of all synchronous machines in the same machine group are accumulated, so that the observation dimensionality of the system is simplified into the angle difference between the respective unified power angle and the COI of the two machine groups. And adding two summation equations obtained by the two machine groups, and solving the first integral of the two summation equations to obtain a transient energy function expression of the system containing the DFIG under the COI coordinate as follows:
wherein the content of the first and second substances,
and
respectively, the state variables M of the S cluster and the A cluster synchronous machine at the stable balance point S And M A Respectively representing the sum of inertia time constants of all synchronous machines of the S machine group and the A machine group,
and
respectively representing the angle difference of rotor angles of the S cluster and the A cluster synchronous machine relative to the COI of the system when the system reaches a stable balance point after a fault; the expressions for the remaining variables are:
step three: and solving a critical energy value of the system, and quantitatively analyzing the influence of wind power plant access on the stability margin of the system.
Further obtaining an equivalent system motion equation as follows:
wherein, P m.eq Mechanical power for an equivalent system; p e.eq Electricity for equivalent systemMagnetic power; theta SA Is the equivalent system power angle. The expressions of the parameters are respectively:
when the system is in a steady state, each synchronous machine rotor keeps rotating at a constant speed, and the angular acceleration of the rotor is zero. Therefore, the system can be operated at a stable equilibrium point calculated by equation (5)
And unstable equilibrium point
The work angles are respectively as follows:
therefore, due to the fact that the DFIG is connected, network parameters are changed, mechanical power and electromagnetic power of an equivalent system are indirectly changed, a stable operation point of the system is caused to move, and safety stability margin of the system is affected.
Assuming that the COI of the system remains unchanged during the fault duration and during the transient after fault removal and that the rotor angle at the unstable equilibrium point of each synchronous machine can be approximately considered to be complementary to the rotor angle at the stable equilibrium point, the unstable equilibrium point is used
The transient energy of (b) is used as the critical energy of the system, and can be obtained as follows:
in the formula (I), the compound is shown in the specification,
and
the rotor positions of the respective synchronous machines in the S group and the a group with respect to the COI at the failure initial time are shown.
Therefore, the Δ G and Δ B parameters respectively represent the changes of self-admittance and mutual admittance between potential nodes in the synchronous machine after the DFIG is connected, that is, conductance and susceptance elements in the system correction matrix Δ Y represented by the formula (2); obviously, the addition of the DFIG changes the topology structure of the original system network, so that the critical energy of the system changes, that is, the stability margin of the system is changed, which inevitably affects the transient stability of the system. Therefore, quantitative analysis of influence of DFIG access on system critical energy is realized by utilizing contraction analysis of a system node admittance matrix and the structure of a transient energy function of a system containing DFIG.
Step four: and quantitatively analyzing the influence of the DFIG power external characteristics on the energy accumulation process during the fault.
Further, the impact of the output power of the DFIG on the system state variables during the fault is analyzed. For a system with a given critical energy, when different LVRT schemes are adopted in the DFIG, the power output characteristics of the DFIG are different, and for the analysis of the transient stability influence degree, the analysis can be determined by the degree of the time-dependent change of the power angle of the system.
The calculation formula of the system power angle is as follows:
as can be seen from the observation formula (6), the equivalent electromagnetic power P of the system e.eq As a function of network parameters. And from the formula (9), P e.eq The larger the value, θ SA And omega SA The smaller the amount of change in. The power angle variation reflects the degree of deviation from stability of the system, so that the larger the equivalent electromagnetic power of the system is, the longer the time for the power angle of the system to increase to a critical instability state is, which indicates that the transient energy accumulation process is slower, namely, the system is more stable.
Active and reactive output pair synchronization during DFIG ride-throughEquivalent electrical connections between machines cause influences of different degrees, which are reflected as the influence of self-admittance and mutual admittance of the nodes of the synchronous machines. As can be seen from the definition of the parameters C and D by equation (6), C mainly reflects the conductance parameter and D mainly reflects the susceptance parameter. P due to different DFIG output characteristics under different LVRT schemes e.eq The values of (A) are different. Therefore, the equivalent electromagnetic power P can be obtained according to the system e.eq And (4) analyzing the quality of the transient stability of the system when the DFIG adopts different LVRT schemes in the fault period. By P e.eq The value of the parameter P is used as a criterion of the energy accumulation speed, and the P is calculated according to the influence of the DFIG output power under different LVRT schemes in specific multi-computer examples on the values of the parameter C and the parameter D e.eq And comparing the numerical values to indirectly analyze the quality of the transient stability of the system under different LVRT schemes.
Step five: and analyzing the influence relation of DFIG access on system fault limit removal time based on a transient energy function method.
Furthermore, the average electromagnetic power of the cluster S and the cluster A during the fault period is substituted into the system motion equation to respectively calculate the state variable (theta) of the synchronous machine of each cluster i ,ω i ) Approximate relationship of the variables over time. And substituting the energy function expression (3) to obtain a univariate function expression V (t) of the transient energy accumulated during the system fault period with respect to time. And then solving the equation V (t) V cr And calculating to obtain a limit removal time index reflecting the stability degree of the system, and realizing the analysis of the influence of wind power access on the limit removal time of the system fault.
In this way, the influence of the DFIG access on the energy margin of the system and the energy accumulation process during the fault is quantified by the transient energy function method into two indexes, namely, the critical energy value and the change of the system state variable. The power angle of the system reflects the speed of transient energy accumulation, the critical energy value reflects the stability margin of the system, and the conclusion of whether the system is unstable or not can be obtained by comparing the energy accumulated by the system in the transient process with the critical energy of the system. In practice, a curve of the transient energy value accumulated by the system during the fault period can be plotted by a mapping method, and the intersection point of the curve and the critical energy value is the fault limit cutting time corresponding to the system.
The invention is further illustrated by the following examples.
FIG. 3 is a three-machine nine-node system including a DFIG. Rated powers of the synchronizers SG1, SG2 and SG3 are 247.5MW, 192MW and 128MW respectively; the DFIG branch is connected with a node 7 and has the rated power of 75 MW; the three-phase symmetric fault is set at the node 8, and the fault starts from 0.2 s; the loads are all of constant impedance type; the system reference capacity SB is 100MVA, and the reference voltage UB is a rated voltage for each voltage class. In this example, the dominant clustering pattern of the known system divides the syncers SG2 and SG3 into a leading cluster S and SG1 into a remaining cluster a.
(1) Impact of DFIG access on critical energy
For the case that the DFIG is not accessed, the correction matrix Δ Y of the system may be regarded as 0. The critical energy of the DFIG-free three-machine system is V cr 0 ＝2.1516。
And calculating according to a formula, wherein after the DFIG is accessed, the correction matrix of the system in a steady state is as follows:
then, the critical energy of the system after DFIG access in the example is calculated to be V cr ＝4.0813。
Is observed to have V cr >V cr 0 It is shown that the access of the DFIG in this example increases the critical energy of the system, so that the maximum capability of maintaining transient stability is improved to some extent.
(2) Taking different LVRT strategies as an example, the influence of DFIG output power characteristics on the energy accumulation process during the fault period is analyzed
In order to analyze the influence of different LVRT schemes on the transient stability of the system, theoretical analysis and simulation verification are respectively carried out by adopting a stator series reactance comprehensive LVRT strategy (recorded as scheme A) of DFIG and a traditional crowbar LVRT strategy (recorded as scheme B) on the basis of the critical energy values obtained in the previous step of the embodiment. The main difference between the DFIG output characteristics under the two LVRT strategies is: the reactive power support meeting the LVRT requirement can be provided for the system by controlling the DFIG rotor side converter in the mode that the stator is connected with the reactor in series; and the rotor side converter of the DFIG is locked in a crowbar mode and cannot provide effective exciting current, so that the DFIG is in an asynchronous operation state, and reactive power needs to be absorbed from a system during the crossing.
And obtaining the equivalent ground admittance of the DFIG according to the output active power and reactive power of the DFIG and the voltage value of the outlet node of the DFIG during the system fault duration within 0.2-0.5 s. The equivalent conductance and susceptance of DFIG using the two different LVRT schemes are shown in fig. 4 and 5, respectively.
It is seen from the figure that due to the characteristic of electromechanical decoupling of the DFIG, the electrical quantity of the DFIG tends to be stable after a short rapid change in the initial stage of the fault, and the value is basically kept unchanged. Thus, for ease of analysis, the present example uses an average of the conductance and susceptance from 0.25s to 0.5s to reflect the equivalent admittance of the DFIG. The DFIG equivalent admittance values under the two LVRT schemes are:
further, the calculated equivalent system electromagnetic power parameters reflecting the influence of the DFIG power characteristics on the stability under different LVRT schemes are respectively as follows:
therefore, the equivalent electromagnetic power of the system is higher when the DFIG adopts the scheme A than the scheme B. Therefore, according to theoretical analysis, the degree of the change of the system power angle with time under the scheme A is smaller, which indicates that the scheme A is more favorable for the transient stability of the system than the scheme B.
Substituting the formula (12) into the formula (9) to obtain the relation of the system state variable changing along with time; in the energy function expression shown in formula (3), the limit cut-off time of the DFIG under the two LVRT schemes is obtained by mapping, which is CCTA 1.245s and CCTB 0.604s, respectively, as shown in fig. 6 and fig. 7. The calculation results are consistent with the theoretical analysis described above.
Fig. 8, 9 and 10 show the simulation results of the power angle of the system after the fault in three cases of no DFIG, with DFIG and adopting scheme a, and with DFIG and adopting scheme B.
The known system fails at 0.2s, so it can be seen from the figure that the limit cut-off time of the system is: the time is between 0.55s and 0.56s without grid-connected DFIG; the time for adopting the scheme A is between 1.40s and 1.41s for DFIG, and the time for adopting the scheme A is between 0.67s and 0.68s for DFIG, which is closer to the calculation result.
According to the simulation result, the following results are obtained: in the example, the DFIG adopts a scheme A which has better transient stability than a scheme B, and the inference obtained by theoretical calculation is verified; the limiting cut-off times for the systems under both crossing schemes are greater than for the systems without the DFIG, indicating that the DFIG under both schemes is beneficial to the transient stability of the system during crossing.
The conclusion shows that the transient stability of the system containing the DFIG is correct and effective by combining the conversion of the external power characteristics of the DFIG into the change of the electrical distance between the nodes of the synchronous machine and analyzing the transient stability of the system containing the DFIG by adopting a transient energy function method.
Claims (1)
1. A method for analyzing the influence of wind power plant access on the fault limit removal time of a multi-machine system is based on a transient energy function theory, starts with an original multi-machine system, and adopts a first integration method to construct a transient energy function containing the multi-machine system of the wind power plant; on the basis, the incidence relation between the critical energy value of the system and the transient output power of the wind power plant is deduced, the change of the critical energy borne by the system caused by the change of the network topology structure due to DFIG access is quantitatively analyzed, and the influence of the wind power plant access on the stability margin of the system is reflected from the critical energy; analyzing the change of a system state variable and an energy accumulation process according to the influence of the DFIG on the equivalent electromagnetic power of the system during the fault period, and giving an influence mode of wind power plant access on the transient energy accumulation speed of the system; calculating fault limit removal time by accumulation time from the transient energy to the critical energy of the system so as to provide an index which reflects the degree of influence on the transient stability of the system and realize analysis of the degree of influence of wind power plant access on the transient stability of the system; the method comprises the following steps:
the method comprises the following steps: establishing mathematical model of multi-machine system comprising wind power plant
The method comprises the following steps that n synchronous generators are arranged in a multi-machine system, all the synchronous generators are divided into two machine groups according to a complementary group inertia center transformation theory, namely a leading group S and a remaining group A, and the rotor angles and the change conditions of the synchronous generators in the same machine group in the transient process are the same, so that the synchronous generator model of the system under the mode of the two machine groups is obtained:
wherein, theta S And theta A Rotor angles of the synchronizers in the S cluster and the A cluster relative to the COI are respectively set; p mi 、P mj And P ei 、P ej Mechanical power and electromagnetic power of synchronous machines in the S machine group and the A machine group respectively; p COI Acceleration power that is the center of system inertia; m i And M j Representing inertia time constants of each synchronous machine in the S machine group and the A machine group;
contracting the node admittance matrix of the multi-machine system, quantifying the influence of DFIG on the system into the variation of the electromagnetic power of each synchronous machine, setting p synchronous machines in the S machine group, and dividing all nodes into three types: the system comprises an S cluster synchronizer, an A cluster synchronizer and a common node R in a network, wherein the S cluster synchronizer is connected with the common node R; after the DFIG is accessed, adding a parallel branch at a grid-connected point of the DFIG in a network, and setting a port node of the DFIG as W;
performing contraction processing and correction on a node voltage equation of the multi-machine system, and eliminating R type nodes in the equation; the DFIG external power characteristic is equivalent to the parallel grounding admittance of the W node, the W node is eliminated, only the potential node in the synchronous machine is reserved, the information contained in the node W is kneaded into a new admittance matrix, a correction matrix with delta Y as a system admittance matrix is obtained, and the influence relation of the DFIG on the equivalent electrical distance between the nodes of the synchronous machine after the DFIG is accessed is reflected:
wherein, Δ G ij And Δ B ij The correction values of conductance and susceptance among nodes in a system after the DFIG is connected are represented, and when i is equal to j, the correction values of self conductance and self susceptance of the node i are represented; when i is not equal to j, the mutual conductance and the mutual susceptance between the nodes i and j are represented;
step two: constructing a transient energy function containing a wind power plant multi-machine system by adopting a first integration method;
for a system motion equation set divided into two machine group modes, accumulating rotor motion equations of all synchronous machines in the same machine group to simplify observation dimensionality of the system into angle difference between a unified power angle and COI of the two machine groups, adding two summation equations obtained by the two machine groups to solve the first integral, and obtaining a transient energy function expression of the system containing DFIG under the COI coordinate as follows:
wherein the content of the first and second substances,
and
respectively, the state variables M of the S cluster and the A cluster synchronous machine at the stable balance point S And M A Respectively representing the sum of inertia time constants of all synchronous machines of the S machine group and the A machine group; the expressions for the remaining variables are:
step three: the critical energy value of the system is obtained, and the influence of wind power plant access on the stability margin of the system is quantitatively analyzed:
the equivalent system equation of motion is obtained as follows:
wherein, P m.eq Mechanical power for an equivalent system; p e.eq Electromagnetic power for an equivalent system; theta SA For the equivalent system power angle, the expressions of the parameters are respectively:
the system can be calculated by the formula (5) to operate at a stable equilibrium point
And unstable equilibrium point
The work angles are respectively as follows:
if the COI of the system is kept unchanged during the fault duration and during the transient process after the fault is removed and the rotor angle at the unstable balance point of each synchronous machine is considered to be complementary to the rotor angle at the stable balance point, the unstable balance point is defined as the unstable balance point
The transient energy of (b) is used as the critical energy of the system, and can be obtained as follows:
in the formula (I), the compound is shown in the specification,
and
respectively showing the rotor positions of the synchronous machines in the S group and the A group relative to the COI at the initial fault time;
the delta G and delta B parameters respectively represent the change amounts of self-admittance and mutual admittance between potential nodes in the synchronous machine after the DFIG is connected, namely conductance and susceptance elements in a system correction matrix delta Y represented by the formula (2); the method has the advantages that the contraction analysis of the system node admittance matrix and the construction of the transient energy function of the DFIG system are utilized, so that the quantitative analysis of the influence of the access of the DFIG on the system critical energy is realized;
step four: quantitatively analyzing the influence of DFIG power external characteristics on an energy accumulation process during a fault:
for a system with a given critical energy, the degree of the change of the power angle of the system along with the time is used for judging,
the calculation formula of the system power angle is as follows:
system equivalent electromagnetic power P e.eq As a function of network parameters, P e.eq The larger the value, θ SA And omega SA The smaller the variation amount of (c); the power angle variation reflects the degree of deviation from stability of the system, and the larger the equivalent electromagnetic power of the system is, the longer the time for the power angle of the system to increase to a critical instability state is, which indicates that the transient energy accumulation process is slower, namely the system is more stable;
the active and reactive output of the DFIG in the crossing period causes different degrees of influence on equivalent electrical connection between the synchronous machines, and the influence is reflected as the influence of the node self-admittance and the mutual admittance of the synchronous machines;
step five: DFIG access pair analysis system based on transient energy function methodThe influence relation of fault limit cutting time; substituting the average electromagnetic power of the cluster S and the cluster A into the system motion equation during the fault period to respectively calculate the state variable (theta) of the synchronous machine of each cluster i ,ω i ) An approximate relationship of the variables over time; substituting into the energy function expression (3), obtaining a univariate function expression V (t) of the transient energy accumulated in the system fault period with respect to time, and solving an equation V (t) which is V cr Calculating to obtain a limit removal time index reflecting the stability degree of the system, and realizing the analysis of the influence of wind power access on the limit removal time of system faults, V cr Is the critical energy of the system after DFIG access.
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