CN110707762A - Multi-energy complementary alternating current-direct current hybrid micro-grid load flow feasible region calculation method - Google Patents

Multi-energy complementary alternating current-direct current hybrid micro-grid load flow feasible region calculation method Download PDF

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CN110707762A
CN110707762A CN201910967623.9A CN201910967623A CN110707762A CN 110707762 A CN110707762 A CN 110707762A CN 201910967623 A CN201910967623 A CN 201910967623A CN 110707762 A CN110707762 A CN 110707762A
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power
droop
microgrid
direct current
formula
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CN110707762B (en
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于光远
庞磊
韩蓓
李国杰
汪可友
张庆
王志磊
尹爱辉
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State Grid Corp of China SGCC
Jinan Power Supply Co of State Grid Shandong Electric Power Co Ltd
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Shanghai Jiaotong University
Jinan Power Supply Co of State Grid Shandong Electric Power Co Ltd
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J5/00—Circuit arrangements for transfer of electric power between ac networks and dc networks
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/04—Circuit arrangements for ac mains or ac distribution networks for connecting networks of the same frequency but supplied from different sources
    • H02J3/06—Controlling transfer of power between connected networks; Controlling sharing of load between connected networks

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Abstract

The invention provides a multi-energy complementary alternating current-direct current hybrid micro-grid load flow feasible region calculation method, which comprises the following steps: s1, establishing a multi-energy complementary alternating current and direct current hybrid micro-grid load flow calculation model; s2, calculating a power flow feasible region of the two-dimensional droop coefficient injected into the space on the basis of the multi-energy complementary alternating current-direct current hybrid micro-grid power flow calculation model established in the S1; and S3, on the basis that the two-dimensional droop coefficients obtained through calculation in S2 are injected into the feasible region of the power flow in the space, increasing the dimensionality of the parameters, and traversing the parameter space layer by layer to obtain the feasible region of the power flow in the high-dimensional droop coefficient injection space. A terminal for performing the above method is also provided. The method greatly improves the speed of solving the voltage stabilization critical point, and has high calculation efficiency; efficient power flow feasible domain calculation is provided for analysis of the droop coefficient; and a reference is provided for stable operation of the multi-energy complementary alternating current-direct current hybrid micro-grid.

Description

Multi-energy complementary alternating current-direct current hybrid micro-grid load flow feasible region calculation method
Technical Field
The invention relates to the technical field of alternating current and direct current hybrid micro-grids, in particular to a multi-energy complementary alternating current and direct current hybrid micro-grid load flow feasible region calculation method.
Background
Distributed Generators (DGs) and loads in the microgrid are various, and the alternating-current and direct-current hybrid microgrid has the advantages of an alternating-current microgrid and a direct-current microgrid at the same time, so that the operation stability can be improved by optimizing inverter control parameters, and the development prospect is wide.
In an island operation mode, the droop coefficient of a distributed power inverter in an alternating current-direct current hybrid micro-grid influences the current feasible region of a system, and the traditional droop coefficient is determined by the capacity, the frequency and the voltage regulation range of the inverter and does not fully consider the influence. The droop coefficient of the inverter is closely related to the static voltage stability of the system, so that the analysis of the droop coefficient is very important from the perspective of the feasible region of the system power flow.
The bifurcation theory is a powerful tool for analyzing the feasible region of the system power Flow, and utilizes a Continuous Power Flow (CPF) method to obtain a saddle node bifurcation point of the system.
Through search, the following results are found:
zambroni De Souza A C, Santos M, Castila M, et al, Voltage security in AC microprocessors a Power flow-based approach-controlling generators [ J ] IET Recewable Power Generation,2015,9(8):954-960. A CPF algorithm for droop control DG networking is given, however, because it is very time consuming to find the trend feasible region.
The optimization model [ J ] of the rapid search of the static voltage stability domain boundary of the power system, the electrotechnical report, 2018,33(17): 4167) 4179, the Square ston, the Chenghao loyal, the Xuzhongdong, and the like, the method [ J ] of solving the nearest voltage stability critical point based on the curved surface quadratic standard approximation, 2016,40(2):69-76.DOI:10.7500/AEPS20150423009, respectively proposes the optimization model of the rapid search of the power system current feasible domain boundary, but the optimization model of the rapid search of the current feasible domain boundary still has the following problems: the influence of the droop coefficient under the inverter control mode and the corresponding control mode on the boundary of the feasible region of the power flow is not considered in the model; meanwhile, the droop coefficients considered by the model are only one-dimensional, and mutual influence among the droop coefficients cannot be reflected.
At present, no explanation or report of the similar technology of the invention is found, and similar data at home and abroad are not collected.
Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to provide a multi-energy complementary alternating current and direct current hybrid microgrid power flow feasible region calculation method.
The invention is realized by the following technical scheme.
According to one aspect of the invention, a method for calculating a power flow feasible region of a multi-energy complementary alternating-current and direct-current hybrid micro-grid is provided, and the method comprises the following steps:
s1, establishing a multi-energy complementary alternating current and direct current hybrid micro-grid load flow calculation model;
s2, calculating a power flow feasible region of the two-dimensional droop coefficient injected into the space on the basis of the multi-energy complementary alternating current-direct current hybrid micro-grid power flow calculation model established in the S1;
and S3, on the basis that the two-dimensional droop coefficients obtained through calculation in S2 are injected into the feasible region of the power flow in the space, increasing the dimensionality of the parameters, and traversing the parameter space layer by layer to obtain the feasible region of the power flow in the high-dimensional droop coefficient injection space.
Preferably, the multi-energy complementary ac/dc hybrid microgrid comprises: the AC sub-microgrid and the DC sub-microgrid are connected through an interconnection converter controlled by per unit droop.
Wherein: the ac sub-microgrid comprises: wind driven generator, energy storage battery and various loads; the direct current sub-microgrid comprises: renewable energy sources such as photovoltaic cells and wind driven generators, and loads such as electric vehicle charging stations.
Preferably, the S1, includes the following sub-steps:
s1.1, establishing an inverter interface power supply model, comprising:
when the multi-energy complementary alternating current-direct current hybrid micro-grid works in an island state, the traditional droop control node equation in the alternating current sub-micro-grid is as follows:
Figure BDA0002231013190000021
in the formula: m ispi、nqiActive power and reactive power droop coefficients of the AC sub-microgrid are respectively; pGi、QGiRespectively the active power and the reactive power of the DG flowing into the AC sub-microgrid at the node i; pG0i、QG0iRated active power and rated reactive power of the inverter at the node i are respectively set; omega0、U0iRespectively is the no-load angular frequency and the no-load voltage amplitude at the node i; omega, UiThe operation frequency of the alternating current-direct current hybrid micro-grid and the voltage amplitude of the node i are respectively;
by adopting an improved power coupling droop control strategy, a node load flow calculation model for controlling the power distribution of the low-voltage microgrid under the condition that a line has a resistance-inductance characteristic is as follows:
Figure BDA0002231013190000031
in the formula: r ═ R/X is the resistance-to-inductance ratio;
a node power flow calculation model adopting active-voltage (P-U) droop control in the direct current sub-microgrid:
Figure BDA0002231013190000032
in the formula: m ispjThe active droop coefficient is a direct current sub-microgrid; pGjThe active power flowing into the direct current sub-microgrid is the node j; u shape0jIs the no-load output voltage of node j; u shapejOperating the voltage amplitude for node j;
the droop coefficients of the AC sub-microgrid and the DC sub-microgrid meet the following constraints:
Figure BDA0002231013190000033
in the formula: vmax、VminThe maximum value and the minimum value of the node voltage are respectively; omegamax、ωminThe frequency limit of the alternating current-direct current hybrid micro-grid is respectively the upper limit and the lower limit; pG,min、PG,max、QG,min、QG,maxRespectively the upper limit and the lower limit of active power and the upper limit and the lower limit of reactive power;
s1.2, establishing a load and line model, comprising:
determining the influence of the terminal voltage and the frequency of a load point under a static load state:
Figure BDA0002231013190000034
in the formula: pLi、QLi、P0i、Q0iRespectively setting the active power and the reactive power of a load node i under a set frequency, and the active power and the reactive power of actual work; a. thepi、BpiThe active power coefficient of the load is proportional to the voltage quadratic power and the voltage first power respectively, Aqi、BqiThe load reactive power coefficient, C, being proportional to the voltage squared and squaredpi、CqiLoad active power and reactive power independent of voltage amplitudeA coefficient; k is a radical ofpf,i、kqf,iStatic frequency characteristic coefficients of load active power and reactive power respectively; omega and omega1Respectively setting the steady-state angular frequency and the set angular frequency of the AC sub-microgrid;
determining the frequency response of the line impedance parameter:
Figure BDA0002231013190000041
in the formula: r is0、x0、b0Resistance, reactance and susceptance of the power transmission line under the reference frequency; r isi、xi、biResistance, reactance and susceptance of the power transmission line under the actual operation condition;
s1.3, constructing a converter model between the overpass and the DC sub-microgrid, comprising:
and adopting a per-unit droop control strategy to per-unit the frequency at the alternating current side and the voltage at the direct current side respectively, so that droop curves of the alternating current sub-microgrid and the direct current sub-microgrid are unified under the same coordinate system:
Figure BDA0002231013190000042
in the formula:
Figure BDA0002231013190000043
ωmax、ωminthe per-unit value of the angular frequency of the alternating-current and direct-current hybrid micro-grid, and the maximum per-unit value and the minimum per-unit value of the frequency of normal operation of the alternating-current sub-micro-grid are respectively;
Figure BDA0002231013190000044
the per unit value is the voltage of a terminal node connected with the DC sub-microgrid by the interconnected converter; u shapedc、Udc,max、Udc,minRespectively representing the actual voltage value, the theoretical voltage maximum value and the theoretical voltage minimum value of the terminal node;
after the per unit processing of the formula (7), the active power passing through the interconnected converters is as follows:
Figure BDA0002231013190000045
in the formula: alpha is alphapActive droop coefficients for interconnected converters; when P is presentILCWhen the voltage is more than 0, the active power flows from the direct current sub-microgrid to the alternating current sub-microgrid through the interconnected converters, and meanwhile, the reactive power Q is injectedILC：
Figure BDA0002231013190000046
In the formula: u shapeACOutputting an actual voltage value for a terminal node connected with the AC sub-microgrid by the interconnected converter; u shape0Controlling a reference voltage for droop; alpha is alphaqThe reactive droop coefficient of the interconnected converters;
s1.4, establishing an alternating current-direct current hybrid micro-grid unified power flow calculation model, which comprises the following steps:
the balance equation of the active power and the reactive power of the droop control nodes in the AC sub-microgrid is as follows:
in the formula: pi、QiRespectively injecting active power and reactive power into the node i;
the active power equation of the droop control node in the direct current sub-microgrid is as follows:
fP_DC,j＝PGj+PILC-PLj-Pj＝0 (31)
in the formula: pjActive power injected for node j;
and (3) combining the equations (10) and (11) to obtain a unified power flow equation set of the alternating-current and direct-current hybrid micro-grid:
F(x)＝0 x∈Rn(32)
in the formula: x ═ θ, ω, Uac,Udc]N is the total number of variables to be solved; theta is the voltage phase angle of each node; u shapedcVoltage amplitude vectors of all nodes in a direct current region; u shapeacVoltage amplitude vectors of all nodes in the alternating current area are obtained;
and obtaining an AC/DC hybrid micro-grid unified power flow equation set which is an AC/DC hybrid micro-grid unified power flow calculation model.
Preferably, the S2, includes the following sub-steps:
s2.1, calculating the continuous power flow of the alternating current-direct current hybrid micro-grid, comprising the following steps:
calculating the distance from the running point to the boundary point of the power flow feasible region of the load along a specific growth direction, and characterizing DG and the power growth of the load by a parameter lambda, then:
Figure BDA0002231013190000051
in the formula: λ ═ 0 corresponds to the basic DG output and load level; k is DG and the power increasing direction of the load; lambda [ alpha ]crIs the maximum value of the load margin;
selecting M groups of power increasing directions, substituting the formula (13) into the formula (12) to obtain an alternating current-direct current hybrid microgrid CPF equation containing a droop coefficient and a load power increasing coefficient:
G(x,λ,Coe)＝0 x＝[θ,ω,Uac,Udc](34)
in the formula: x is a state variable, CoeIs a droop coefficient vector of the inverter;
the critical condition of the static voltage stability of the alternating current-direct current hybrid microgrid is that a Jacobian matrix of a formula (14) is singular, and a CPF equation of the alternating current-direct current hybrid microgrid begins to be solved, namely saddle node bifurcation occurs:
det(J(x))＝0 (35)
wherein J (x) is: a jacobian matrix of formula (14);
s2.2, solving the power flow feasible domain boundary containing the droop coefficient, comprising the following steps:
s2.2.1, initial point calculation:
constructing a two-dimensional droop coefficient injection space, solving a saddle node bifurcation point corresponding to a droop coefficient in a cell by using a traditional continuous power flow method, and taking the load stability margin of the solved saddle node bifurcation point as an initial value for solving the saddle node bifurcation point at the adjacent position;
s2.2.2, estimating links, establishing a prediction equation about the saddle node bifurcation point:
finding boundaries sigma in the feasible region of the power flow at the position of the initial valuecoeSag coefficient C along the curved surfaceoe_iTangent vector of changing direction:
Figure BDA0002231013190000061
in the formula: e.g. of the typepIs a unit vector in which the other elements except the p-th element is 1 are all 0; and performing quadratic fitting on a curved surface formed by corresponding saddle node bifurcation points in the initial droop coefficient interval:
Figure BDA0002231013190000062
in the formula: r, s and t are respectively a square term coefficient, a quadratic term coefficient and a primary term coefficient of the fitting curved surface, and D is a constant term;
solving the prediction equation shown in the formula (18) by using the formula (17), and selecting a proper step length along the direction of the tangent vector to obtain the prediction value of the next SNB point:
Figure BDA0002231013190000063
in the formula: sigma is the step length, and epsilon is the penalty term of prediction;
s2.2.3, correction of step size control and penalty term:
adopting a self-adaptive step size control method, wherein each iteration step size is as follows:
Figure BDA0002231013190000064
in the formula: a. b, c and d are parameters of an exponential function in a control step calculation formula (19), wherein a is an exponential function coefficient, b is an exponential function variable coefficient, c is a constant of a power in the exponential function, d is a constant in the exponential function, and the parameters are calculated according to alternating current and direct current mixingActual conditions of the micro-grid are given; | K ∞max＝|dλ/dCoe|maxThe rate of change of the load margin with respect to the droop coefficient;
for any set of selected droop coefficients (C)oe_i,Coe_j) Is estimated by the parameter space grid points (C) adjacent theretooe_i-1,Coe_j) And (C)oe_i,Coe_j-1) Corresponding saddle node bifurcation point SNBiAnd SNBjThe estimated value along the change direction of the respective parameters is determined:
Figure BDA0002231013190000065
i.e. take the minimum value from the two estimated values;
s2.2.4, correction link:
continuously increasing load margin on the basis of the estimated value, and performing continuous power flow calculation until a new droop coefficient (C) is reachedoe_i,Coe_j) SNB ofijPoint by point;
s2.2.5, loop iteration:
and taking the solution of the correction link as an initial value of the load margin corresponding to the next group of droop coefficients, returning to S2.2.1 for the next iteration until the parameter space is traversed, and drawing the feasible region boundary of the tidal current under the two-dimensional droop coefficient space.
Preferably, ε is a constant greater than 1.
Preferably, the S3, includes the following sub-steps:
s3.1, solving the section of the three-dimensional droop coefficient injected into the space trend feasible region, comprising the following steps: on the basis of injecting the two-dimensional droop coefficient into the space trend feasible region in the S2, estimating along the increasing direction of the third droop coefficient, calculating to obtain the value of the third droop coefficient fixed at the position after the estimated point, and searching the trend feasible region boundary under the two-dimensional droop coefficient injection space by adopting the method in the S2;
s3.2, constructing a three-dimensional droop coefficient injection space trend feasible region, comprising: and after the searching of the boundary of the feasible region of the plane trend at the position of the first pre-estimated value of the third droop coefficient is finished, pre-estimating on the basis, and repeating the step S3.1 until the parameter space of the third droop coefficient is traversed. And finally, obtaining a power flow feasible region boundary of the three-dimensional droop coefficient injected into the space, wherein the power flow feasible region boundary is the inside of a curved shell envelope surface in the three-dimensional space.
According to a second aspect of the present invention, there is provided a terminal comprising a memory, a processor and a computer program stored on the memory and operable on the processor, wherein the processor, when executing the computer program, is operable to perform any of the methods described above.
Compared with the prior art, the invention has the following beneficial effects:
1. compared with the traditional calculation method, the multi-energy complementary alternating current-direct current hybrid micro-grid load flow feasible region calculation method greatly improves the speed of obtaining the voltage stability critical point, and is high in calculation efficiency.
2. The multi-energy complementary alternating current-direct current hybrid micro-grid load flow feasible region calculation method provided by the invention provides efficient load flow feasible region calculation for analysis of a plurality of droop coefficients.
3. The multi-energy complementary alternating current-direct current hybrid micro-grid power flow feasible region calculation method provided by the invention fully considers the influence on the power flow feasible region caused by the mutual coupling of the droop coefficients of a plurality of inverters.
4. The method for calculating the feasible tidal current domain of the multi-energy complementary alternating current-direct current hybrid micro-grid provides reference for stable operation of the multi-energy complementary alternating current-direct current hybrid micro-grid.
Drawings
Other features, objects and advantages of the invention will become more apparent upon reading of the detailed description of non-limiting embodiments with reference to the following drawings:
FIG. 1 is a schematic diagram of a typical structure of a multi-energy complementary AC/DC hybrid micro-grid;
fig. 2 is a schematic diagram of a per-unit droop control strategy adopted by the ac-dc interconnected converter in the embodiment of the present invention;
FIG. 3 is a schematic view of load margins under different sets of active droop coefficients in an embodiment of the present disclosure;
FIG. 4 is a schematic diagram of an algorithm for solving a boundary of a feasible region of a power flow including two-dimensional droop coefficients according to an embodiment of the present invention;
FIG. 5 is a flow chart illustrating a process for determining feasible region boundaries when three parameters are changed according to an embodiment of the present invention;
fig. 6 is a schematic diagram of an algorithm for solving the boundary of the feasible region of the power flow including the three-dimensional droop coefficient according to an embodiment of the present invention.
Detailed Description
The following examples illustrate the invention in detail: the embodiment is implemented on the premise of the technical scheme of the invention, and a detailed implementation mode and a specific operation process are given. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention.
The embodiment of the invention provides
According to one aspect of the invention, a method for calculating a power flow feasible region of a multi-energy complementary alternating-current and direct-current hybrid micro-grid is provided, and the method comprises the following steps:
s1, establishing a multi-energy complementary alternating current and direct current hybrid micro-grid load flow calculation model;
and S2, calculating a power flow feasible region of the two-dimensional droop coefficient injected into the space on the basis of the multi-energy complementary alternating current-direct current hybrid micro-grid power flow calculation model established in the S1.
And S3, on the basis that the two-dimensional droop coefficients obtained through calculation in S2 are injected into the feasible region of the power flow in the space, increasing the dimensionality of the parameters, and traversing layer by layer to obtain the feasible region of the power flow in the high-dimensional droop coefficient injection space.
Embodiments of the invention are described in further detail below with reference to the accompanying drawings.
S1, establishing an alternating current-direct current hybrid micro-grid unified power flow calculation model;
as shown in fig. 1, the multi-energy complementary ac/dc hybrid microgrid structure includes: the AC sub-microgrid part and the DC sub-microgrid part are connected through an interconnection converter adopting per-unit droop control, wherein the AC sub-microgrid comprises a wind driven generator, an energy storage battery and various loads, and the DC sub-microgrid comprises renewable energy sources such as a photovoltaic battery and a wind driven generator and further comprises loads such as an electric vehicle charging station.
When the inverter operates in an island state, the voltage of a power grid is firstly ensured to be stable and the power is instantly balanced, so that all the inverters adopt droop control.
S1.1, establishing an inverter interface power supply model
When the alternating current-direct current hybrid micro-grid works in an island state, the traditional droop control node equation in the alternating current sub-micro-grid is as follows:
Figure BDA0002231013190000091
in the formula: m ispi、nqiActive power and reactive power droop coefficients of the AC sub-microgrid are respectively; pGi、QGiRespectively the active power and the reactive power of the DG flowing into the AC sub-microgrid at the node i; pG0i、QG0iRated active power and rated reactive power of the inverter at the node i are respectively set; omega0、U0iRespectively is the no-load angular frequency and the no-load voltage amplitude at the node i; omega, UiThe operation frequency of the alternating current-direct current hybrid micro-grid and the voltage amplitude of the node i are respectively;
the alternating current sub-microgrid has a lower voltage level, the ratio of the line resistance to the inductive reactance is larger than that of a high-voltage transmission line, and the conventional inductive droop control cannot meet the requirement of low-voltage microgrid control. Therefore, an improved power coupling droop control strategy (refer to Eajal A, Abdelwave M A, El-Saadany E F, et al. A. a. uneffected upper to the power flow analysis of AC/DC hybrid semiconductors [ J ]. IEEE Transactions on Sustainable Energy Energy,2016,7(3): 1145:. 1158. and Pentium, Kilina, Liyufeng. low-voltage microgrid three-phase inverter power coupling droop control strategy [ J ]. power automation equipment, 2014,34(03):28-33.) is adopted, and the control strategy realizes flexible and effective control on low-voltage microgrid power distribution under the condition that lines have inductance resistance characteristics:
Figure BDA0002231013190000092
in the formula: and R is R/X is the inductance resistance ratio.
A node load flow calculation model adopting active power-voltage (P-U) droop control in the direct current sub-microgrid is as follows:
Figure BDA0002231013190000093
in the formula: m ispjThe active droop coefficient is a direct current sub-microgrid; pGjThe active power flowing into the direct current sub-microgrid is the node j; u shape0jIs the no-load output voltage of node j; u shapejOperating the voltage amplitude for node j;
the droop coefficients of the AC sub-microgrid and the DC sub-microgrid meet the following constraints:
Figure BDA0002231013190000094
in the formula: vmax、VminThe maximum value and the minimum value of the node voltage are respectively; omegamax、ωminThe frequency limit of the alternating current-direct current hybrid micro-grid is respectively the upper limit and the lower limit; pG,min、PG,max、QG,min、QG,maxThe upper limit and the lower limit of active power and the upper limit and the lower limit of reactive power are respectively.
S1.2, establishing a load and line model
Usually, a load model in load flow calculation is a static load model under power frequency, and when the load model operates in an island mode, the frequency of the alternating-current sub-microgrid generally cannot be stabilized at the power frequency, so that the static load model needs to consider the influence of the voltage and the frequency at a load point:
Figure BDA0002231013190000101
in the formula: pLi、QLi、P0i、Q0iRespectively the active power of the load node i under the set frequency and the actual workReactive power; a. thepi、Bpi、CpiAnd Aqi、Bqi、CqiActive power coefficient and reactive power coefficient respectively; k is a radical ofpf,i、kqf,iStatic frequency characteristic parameters of the load; omega and omega1The steady-state angular frequency and the set angular frequency of the alternating current sub-microgrid are respectively.
When constructing the interchange direct current hybrid micro-grid admittance matrix, the frequency response of the line impedance parameters also needs to be considered:
Figure BDA0002231013190000102
in the formula: r is0、x0、b0Resistance, reactance and susceptance of the power transmission line under the reference frequency; r isi、xi、biThe resistance, reactance and susceptance of the transmission line under the actual operation condition.
S1.3, establishing a converter model between overpasses and direct current sub-microgrid
The interconnected converter is a bridge for communicating the alternating current sub-microgrid with the direct current sub-microgrid and has the key functions of maintaining the stable bus voltage and the balanced power. In this embodiment, the AC-dc interconnected converter adopts a per-unit droop control strategy (refer to Peyghami S, mokhhiri H, Blaabjerg f. autonomous Operation of a Hybrid AC/DCMicrogrid with Multiple interconnection Converters [ J ]. IEEE Transactions on SmartGrid,2018,9(6):6480-6488.DOI:10.1109/tsg.2017.2713941), which is to per-unit the AC-side frequency and the dc-side voltage respectively, so that the droop curves of the AC-dc microgrid and the dc-dc microgrid can be uniformly analyzed in the same coordinate system, as shown in fig. 2.
In the formula:
Figure BDA0002231013190000104
ωmax、ωminper unit value of angular frequency of AC/DC hybrid micro-grid and AC sub-micro-grid positiveThe frequency maximum per unit value and the frequency minimum per unit value which are frequently operated;
Figure BDA0002231013190000105
the per unit value is the voltage of a terminal node connected with the DC sub-microgrid by the interconnected converter; u shapedc、Udc,max、Udc,minThe actual voltage value, the theoretical voltage maximum value and the theoretical voltage minimum value of the terminal node are respectively.
After the per unit processing of the formula (7), the active power passing through the interconnected converters is as follows:
in the formula: alpha is alphapThe active droop coefficient of the interconnected converters. When P is presentILCWhen the voltage is more than 0, the active power flows from the direct current sub-microgrid to the alternating current sub-microgrid through the interconnected converters, and meanwhile, the reactive power Q is injectedILC：
Figure BDA0002231013190000112
In the formula: u shapeACOutputting an actual voltage value for a terminal node connected with the AC sub-microgrid by the interconnected converter; u shape0Controlling a reference voltage for droop; alpha is alphaqAnd the reactive droop coefficient of the interconnected converters.
S1.4, establishing a unified power flow calculation model of the alternating current-direct current hybrid micro-grid
The active power and reactive power balance equation of the droop control node in the AC sub-microgrid is as follows:
Figure BDA0002231013190000113
in the formula: pi、QiAnd injecting active power and reactive power into the node i.
Similarly, the power equation of the droop control node in the direct current sub-microgrid is as follows:
fP_DC,j＝PGj+PILC-PLj-Pj＝0 (51)
in the formula: pjActive power is injected for node j.
When the output power of the droop node DG exceeds the limit, the droop node DG is automatically converted into a PQ node, and the PQ node is substituted into a power equation again for calculation. The equations are combined to obtain a unified power flow equation set of the AC/DC hybrid micro-grid:
F(x)＝0 x∈Rn(52)
in the formula: x ═ θ, ω, Uac,Udc]N is the total number of variables to be solved; theta is the voltage phase angle of each node; u shapedcVoltage amplitude vectors of all nodes in a direct current region; u shapeacAnd voltage amplitude vectors of all nodes in the alternating current region.
Because no loose node exists in the alternating current-direct current hybrid micro-grid which totally adopts droop control, and the load also changes along with the frequency and the voltage, the Jacobian matrix is easy to be singular when the formula (12) is solved. An improved trust domain algorithm (refer to Wan Sn, Fanwangleing, Du Zheng Chun, modern electric power system analysis [ M ]. scientific Press, 2003:284- & Fan J, Lu N.on the modified grid region algorithm for non-linear equations [ J ]. Optimization methods and Software,2015,30(3):478- & 491,) can ensure global approximate third-order convergence under the condition that a Jacobian matrix of a nonlinear equation set is close to singularity, and can also accelerate the calculation precision and speed, so the algorithm is adopted in the embodiment to calculate the unified power flow equation set.
S2, calculating a power flow feasible region containing a two-dimensional droop coefficient on the basis of the unified power flow calculation model of the alternating current-direct current hybrid micro-grid established in the S1;
s2.1, alternating current-direct current hybrid micro-grid continuous load flow calculation
The continuous power flow method can solve the problem of convergence when the alternating current-direct current hybrid micro-grid is close to a stable limit operation state, so that the distance from an operation point to a power flow feasible region boundary point along a specific increasing direction of a load is calculated, and the capacity that the alternating current-direct current hybrid micro-grid can bear a newly added load is reflected. The parameter λ is used to characterize DG and the increase of load power, and there are:
in the formula: λ ═ 0 corresponds to the basic DG output and load level; k is a DG and a load power increasing direction, M representative power increasing directions are selected in the embodiment, and the formula (13) is taken into the formula (12), so as to obtain a CPF equation of the microgrid, which contains a droop coefficient and a load increasing coefficient:
G(x,λ,Coe)＝0 x＝[ω,θ,UAC,UDC](54)
in the formula: x is a state variable, CoeIs the droop coefficient vector of the inverter. The critical condition of the static voltage stabilization of the alternating-current and direct-current hybrid microgrid is that a jacobian matrix of formula (14) is singular, a tidal current equation starts to be solved, namely Saddle Node Bifurcation (SNB) occurs:
det(J(x))＝0 (55)
s2.2, solving the boundary of the feasible region of the power flow containing the droop coefficient
To obtain the boundary of the feasible region containing the parameter trend, the droop coefficient space needs to be traversed, and each time the load is gradually increased from the initial point to the saddle node bifurcation point, as shown in fig. 3:
in fig. 3, only three sets of droop coefficients and two sets of load margins obtained in the representative load increasing direction are selected, the load factors are calculated from zero, and the load stability margin information of the nearby position obtained in the previous step cannot be utilized, so that an algorithm for rapidly searching the feasible region boundary is provided as shown in fig. 4. As described in detail below.
1) Initial point calculation:
and constructing a two-dimensional droop coefficient injection space, solving a saddle node bifurcation point corresponding to a droop coefficient in a cell by using a traditional continuous power flow method, and taking the load stability margin of the saddle node bifurcation point obtained as an initial value for solving the saddle node bifurcation point at the adjacent position, thereby reducing the calculated amount and improving the efficiency.
2) And (4) estimating links:
finding boundaries sigma in the feasible region of the power flow at the position of the initial valuecoeSag coefficient C along the curved surfaceoe_iTangent vector of changing direction:
Figure BDA0002231013190000122
in the formula: e.g. of the typepIs a unit vector, and the other elements except the p-th element is 1 are 0. And performing quadratic fitting on a curved surface formed by corresponding saddle node bifurcation points in the initial droop coefficient interval:
Figure BDA0002231013190000131
in the formula: r, s and t are respectively a square term coefficient, a quadratic term coefficient and a primary term coefficient of the fitting curved surface, and D is a constant term.
Solving the prediction equation shown in the formula (18) by using the formula (17), and selecting a proper step length along the direction of the tangent vector to obtain the prediction value of the next SNB point:
in the formula: σ is the step size and ε is the penalty term for the prediction.
3) Step size control and penalty term correction:
adopting a self-adaptive step size control method, wherein each iteration step size is as follows:
Figure BDA0002231013190000133
in the formula: a. b, c and d are parameters of an exponential function in a control step length calculation formula (19) respectively, wherein a is an exponential function coefficient, b is an exponential function variable coefficient, c is a constant of a power in the exponential function, d is a constant in the exponential function, and the parameters are given according to the actual condition of the alternating current-direct current hybrid micro-grid; | K ∞max＝|dλ/dCoe|maxThe rate of change of the load margin with respect to the droop coefficient;
for any set of selected droop coefficients (C)oe_i,Coe_j) Is estimated by the parameter space grid points (C) adjacent theretooe_i-1,Coe_j) And (C)oe_i,Coe_j-1) Corresponding saddle node bifurcation point SNBiAnd SNBjThe estimated value along the change direction of the respective parameters is determined:
i.e. taking the minimum value from the two estimated values.
4) And a correction link:
continuously increasing load margin on the basis of the estimated value, and performing continuous power flow calculation until a new droop coefficient (C) is reachedoe_i,Coe_j) SNB ofijTo a point.
5) And (3) loop iteration:
and (3) taking the solution of the correction link as an initial value of the load margin corresponding to the next group of parameters, returning to the step 1) for the next iteration, and drawing the feasible region boundary of the power flow containing the control parameters until the parameter space is traversed.
And S3, on the basis that the two-dimensional droop coefficients obtained through calculation in S2 are injected into the feasible region of the power flow in the space, increasing the dimensionality of the parameters, and traversing layer by layer to obtain the feasible region of the power flow in the high-dimensional droop coefficient injection space. FIG. 5 is a flow chart for simultaneously examining the change of three droop coefficients; fig. 6 shows a three-dimensional droop coefficient trend feasible region boundary search diagram.
S3.1, solving the section of the three-dimensional droop coefficient injected into the space trend feasible region, comprising the following steps: based on the two-dimensional droop coefficients injected into the space power flow feasible region in S2, as shown in FIG. 6, a third droop coefficient C is addedoe_kThe growth direction of the C-shaped section is estimated, and after the estimated point is obtained through calculation, the C-shaped section is fixed at the positionoe_kThe method in S2 is used to search the boundary of the feasible region of the power flow under the two-dimensional droop coefficient injection space.
S3.2, constructing a three-dimensional droop coefficient injection space trend feasible region, comprising: and after the searching of the boundary of the feasible region of the plane trend at the position of the first pre-estimated value of the third droop coefficient is finished, pre-estimating on the basis, and repeating the step S3.1 until the parameter space of the third droop coefficient is traversed. And finally, obtaining a power flow feasible region boundary of the three-dimensional droop coefficient injected into the space, wherein the power flow feasible region boundary is inside the curved shell envelope surface in the three-dimensional space in the figure 6.
The algorithm can simultaneously consider the load margin when the three droop coefficients change, and the like when the number of the parameters is more.
In the embodiment, a continuous power flow estimation and correction strategy is used for reference, the defect that the SNB point is calculated from the initial load state is overcome, a large amount of calculation time is saved, and therefore the feasible region rapid calculation of the parameter power flow is realized.
Based on the method for calculating the feasible region of the power flow of the multi-energy complementary alternating-current and direct-current hybrid micro-grid provided by the embodiment of the invention, the embodiment of the invention also provides a terminal, and the terminal comprises: comprising a memory, a processor and a computer program stored on the memory and operable on the processor, the processor being operable to perform the method of any one of the preceding claims when executing the computer program.
According to the multi-energy complementary alternating current and direct current hybrid microgrid power flow feasible region calculation method provided by the embodiment of the invention, firstly, a multi-energy complementary alternating current and direct current hybrid microgrid power flow calculation model is established; calculating a load flow feasible region containing a droop coefficient on the basis of the multi-energy complementary alternating current-direct current hybrid micro-grid load flow calculation model established in the S1; . The terminal provided by the above embodiment of the present invention can be used to execute the above method. The method greatly improves the speed of solving the voltage stabilization critical point, and has high calculation efficiency; the coupling influence of the droop coefficients of the inverters on the feasible region of the power flow can be considered at the same time; and a reference is provided for stable operation of the multi-energy complementary alternating current-direct current hybrid micro-grid.
The foregoing description of specific embodiments of the present invention has been presented. It is to be understood that the present invention is not limited to the specific embodiments described above, and that various changes and modifications may be made by one skilled in the art within the scope of the appended claims without departing from the spirit of the invention.

Claims (7)

1. A multi-energy complementary alternating current-direct current hybrid micro-grid power flow feasible region calculation method is characterized by comprising the following steps:
s1, establishing a multi-energy complementary alternating current and direct current hybrid micro-grid load flow calculation model;
s2, calculating a power flow feasible region of the two-dimensional droop coefficient injected into the space on the basis of the multi-energy complementary alternating current-direct current hybrid micro-grid power flow calculation model established in the S1;
and S3, on the basis that the two-dimensional droop coefficients obtained through calculation in S2 are injected into the feasible region of the power flow in the space, increasing the dimensionality of the parameters, and traversing the parameter space layer by layer to obtain the feasible region of the power flow in the high-dimensional droop coefficient injection space.
2. The method for calculating the power flow feasible region of the multi-energy complementary alternating current-direct current hybrid microgrid according to claim 1, wherein the multi-energy complementary alternating current-direct current hybrid microgrid comprises: the AC sub-microgrid and the DC sub-microgrid are connected through an interconnection converter controlled by per unit droop.
3. The method for calculating the power flow feasible region of the multi-energy complementary alternating current-direct current hybrid microgrid according to claim 2, characterized in that the step S1 includes the following sub-steps:
s1.1, establishing an inverter interface power supply model, comprising:
when the multi-energy complementary alternating current-direct current hybrid micro-grid works in an island state, the traditional droop control node equation in the alternating current sub-micro-grid is as follows:
Figure FDA0002231013180000011
in the formula: m ispi、nqiActive power and reactive power droop coefficients of the AC sub-microgrid are respectively; pGi、QGiRespectively the active power and the reactive power of the DG flowing into the AC sub-microgrid at the node i; pG0i、QG0iRated active power and rated reactive power of the inverter at the node i are respectively set; omega0、U0iRespectively is the no-load angular frequency and the no-load voltage amplitude at the node i; omega, UiRespectively AC-DC hybrid micro-gridThe operating frequency and the voltage amplitude of the node i;
by adopting an improved power coupling droop control strategy, a node load flow calculation model for controlling the power distribution of the low-voltage microgrid under the condition that a line has a resistance-inductance characteristic is as follows:
Figure FDA0002231013180000012
in the formula: r ═ R/X is the resistance-to-inductance ratio;
a node power flow calculation model adopting active-voltage (P-U) droop control in the direct current sub-microgrid:
Figure FDA0002231013180000021
in the formula: m ispjThe active droop coefficient is a direct current sub-microgrid; pGjThe active power flowing into the direct current sub-microgrid is the node j; u shape0jIs the no-load output voltage of node j; u shapejOperating the voltage amplitude for node j;
the droop coefficients of the AC sub-microgrid and the DC sub-microgrid meet the following constraints:
Figure FDA0002231013180000022
in the formula: vmax、VminThe maximum value and the minimum value of the node voltage are respectively; omegamax、ωminThe frequency limit of the alternating current-direct current hybrid micro-grid is respectively the upper limit and the lower limit; pG,min、PG,max、QG,min、QG,maxRespectively the upper limit and the lower limit of active power and the upper limit and the lower limit of reactive power;
s1.2, establishing a load and line model, comprising:
determining the influence of the terminal voltage and the frequency of a load point under a static load state:
Figure FDA0002231013180000023
in the formula: pLi、QLi、P0i、Q0iRespectively setting the active power and the reactive power of a load node i under a set frequency, and the active power and the reactive power of actual work; a. thepi、BpiThe active power coefficient of the load is proportional to the voltage quadratic power and the voltage first power respectively, Aqi、BqiThe load reactive power coefficient, C, being proportional to the voltage squared and squaredpi、CqiLoad active power and reactive power coefficients which are irrelevant to the voltage amplitude are respectively; k is a radical ofpf,i、kqf,iStatic frequency characteristic coefficients of load active power and reactive power respectively; omega and omega1Respectively setting the steady-state angular frequency and the set angular frequency of the AC sub-microgrid;
determining the frequency response of the line impedance parameter:
Figure FDA0002231013180000024
in the formula: r is0、x0、b0Resistance, reactance and susceptance of the power transmission line under the reference frequency; r isi、xi、biResistance, reactance and susceptance of the power transmission line under the actual operation condition;
s1.3, constructing a converter model between the overpass and the DC sub-microgrid, comprising:
and adopting a per-unit droop control strategy to per-unit the frequency at the alternating current side and the voltage at the direct current side respectively, so that droop curves of the alternating current sub-microgrid and the direct current sub-microgrid are unified under the same coordinate system:
Figure FDA0002231013180000031
in the formula:
Figure FDA0002231013180000032
ωmax、ωminthe per unit value of the angular frequency of the alternating current-direct current hybrid micro-grid and the maximum per unit and the minimum per unit of the frequency of the normal operation of the alternating current sub-micro-grid are respectivelyA value;
Figure FDA0002231013180000033
the per unit value is the voltage of a terminal node connected with the DC sub-microgrid by the interconnected converter; u shapedc、Udc,max、Udc,minRespectively representing the actual voltage value, the theoretical voltage maximum value and the theoretical voltage minimum value of the terminal node;
after the per unit processing of the formula (7), the active power passing through the interconnected converters is as follows:
Figure FDA0002231013180000034
in the formula: alpha is alphapActive droop coefficients for interconnected converters; when P is presentILCWhen the voltage is more than 0, the active power flows from the direct current sub-microgrid to the alternating current sub-microgrid through the interconnected converters, and meanwhile, the reactive power Q is injectedILC：
Figure FDA0002231013180000035
In the formula: u shapeACOutputting an actual voltage value for a terminal node connected with the AC sub-microgrid by the interconnected converter; u shape0Controlling a reference voltage for droop; alpha is alphaqThe reactive droop coefficient of the interconnected converters;
s1.4, establishing an alternating current-direct current hybrid micro-grid unified power flow calculation model, which comprises the following steps:
the balance equation of the active power and the reactive power of the droop control nodes in the AC sub-microgrid is as follows:
Figure FDA0002231013180000036
in the formula: pi、QiRespectively injecting active power and reactive power into the node i;
the active power equation of the droop control node in the direct current sub-microgrid is as follows:
fP_DC,j＝PGj+PILC-PLj-Pj＝0 (11)
in the formula: pjActive power injected for node j;
and (3) combining the equations (10) and (11) to obtain a unified power flow equation set of the alternating-current and direct-current hybrid micro-grid:
F(x)＝0 x∈Rn(12)
in the formula: x ═ θ, ω, Uac,Udc]N is the total number of variables to be solved; theta is the voltage phase angle of each node; u shapedcVoltage amplitude vectors of all nodes in a direct current region; u shapeacVoltage amplitude vectors of all nodes in the alternating current area are obtained;
and obtaining an AC/DC hybrid micro-grid unified power flow equation set which is an AC/DC hybrid micro-grid unified power flow calculation model.
4. The method for calculating the power flow feasible region of the multi-energy complementary AC/DC hybrid microgrid according to claim 3, characterized in that the step S2 includes the following sub-steps:
s2.1, calculating the continuous power flow of the alternating current-direct current hybrid micro-grid, comprising the following steps:
calculating the distance from the running point to the boundary point of the power flow feasible region of the load along a specific growth direction, and characterizing DG and the power growth of the load by a parameter lambda, then:
Figure FDA0002231013180000041
in the formula: λ ═ 0 corresponds to the basic DG output and load level; k is DG and the power increasing direction of the load; lambda [ alpha ]crIs the maximum value of the load margin;
selecting M groups of power increasing directions, substituting the formula (13) into the formula (12) to obtain an alternating current-direct current hybrid microgrid CPF equation containing a droop coefficient and a load power increasing coefficient:
G(x,λ,Coe)＝0 x＝[θ,ω,Uac,Udc](14)
in the formula: x is a state variable, CoeIs a droop coefficient vector of the inverter;
the critical condition of the static voltage stability of the alternating current-direct current hybrid microgrid is that a Jacobian matrix of a formula (14) is singular, and a CPF equation of the alternating current-direct current hybrid microgrid begins to be solved, namely saddle node bifurcation occurs:
det(J(x))＝0 (15)
wherein J (x) is: a jacobian matrix of formula (14);
s2.2, solving the power flow feasible domain boundary containing the droop coefficient, comprising the following steps:
s2.2.1, initial point calculation:
constructing a two-dimensional droop coefficient injection space, solving a saddle node bifurcation point corresponding to a droop coefficient in a cell by using a traditional continuous power flow method, and taking the load stability margin of the solved saddle node bifurcation point as an initial value for solving the saddle node bifurcation point at the adjacent position;
s2.2.2, estimating links, establishing a prediction equation about the saddle node bifurcation point:
finding boundaries sigma in the feasible region of the power flow at the position of the initial valuecoeSag coefficient C along the curved surfaceoe_iTangent vector of changing direction:
Figure FDA0002231013180000051
in the formula: e.g. of the typepIs a unit vector in which the other elements except the p-th element is 1 are all 0; and performing quadratic fitting on a curved surface formed by corresponding saddle node bifurcation points in the initial droop coefficient interval:
Figure FDA0002231013180000052
in the formula: r, s and t are respectively a square term coefficient, a quadratic term coefficient and a primary term coefficient of the fitting curved surface, and D is a constant term;
solving the prediction equation shown in the formula (18) by using the formula (17), and selecting a proper step length along the direction of the tangent vector to obtain the prediction value of the next SNB point:
Figure FDA0002231013180000053
in the formula: sigma is the step length, and epsilon is the penalty term of prediction;
s2.2.3, correction of step size control and penalty term:
adopting a self-adaptive step size control method, wherein each iteration step size is as follows:
Figure FDA0002231013180000054
in the formula: a. b, c and d are parameters of an exponential function in a control step length calculation formula (19) respectively, wherein a is an exponential function coefficient, b is an exponential function variable coefficient, c is a constant of a power in the exponential function, d is a constant in the exponential function, and the parameters are given according to the actual condition of the alternating current-direct current hybrid micro-grid; | K ∞max＝|dλ/dCoe|maxThe rate of change of the load margin with respect to the droop coefficient;
for any set of selected droop coefficients (C)oe_i,Coe_j) Is estimated by the parameter space grid points (C) adjacent theretooe_i-1,Coe_j) And (C)oe_i,Coe_j-1) Corresponding saddle node bifurcation point SNBiAnd SNBjThe estimated value along the change direction of the respective parameters is determined:
Figure FDA0002231013180000055
i.e. take the minimum value from the two estimated values;
s2.2.4, correction link:
continuously increasing load margin on the basis of the estimated value, and performing continuous power flow calculation until a new droop coefficient (C) is reachedoe_i,Coe_j) SNB ofijPoint by point;
s2.2.5, loop iteration:
and taking the solution of the correction link as an initial value of the load margin corresponding to the next group of droop coefficients, returning to S2.2.1 for the next iteration until the parameter space is traversed, and drawing the feasible region boundary of the tidal current under the two-dimensional droop coefficient space.
5. The method for calculating the power flow feasible region of the multi-energy complementary alternating current-direct current hybrid micro-grid according to claim 4, wherein epsilon is a constant greater than 1.
6. The method for calculating the power flow feasible region of the multi-energy complementary AC/DC hybrid microgrid according to claim 4, characterized in that the step S3 includes the following sub-steps:
s3.1, solving the section of the three-dimensional droop coefficient injected into the space trend feasible region, comprising the following steps: on the basis of injecting the two-dimensional droop coefficient into the space trend feasible region in the S2, estimating along the increasing direction of the third droop coefficient, calculating to obtain the value of the third droop coefficient fixed at the position after the estimated point, and searching the trend feasible region boundary under the two-dimensional droop coefficient injection space by adopting the method in the S2;
s3.2, constructing a three-dimensional droop coefficient injection space trend feasible region, comprising: and after the searching of the boundary of the feasible region of the plane trend at the position of the first pre-estimated value of the third droop coefficient is finished, pre-estimating on the basis, and repeating the step S3.1 until the parameter space of the third droop coefficient is traversed. And finally, obtaining a power flow feasible region boundary of the three-dimensional droop coefficient injected into the space, wherein the power flow feasible region boundary is the inside of a curved shell envelope surface in the three-dimensional space.
7. A terminal comprising a memory, a processor and a computer program stored on the memory and operable on the processor, wherein the computer program, when executed by the processor, is operable to perform the method of any of claims 1 to 6.
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