CN115495892A - Stability analysis modeling method and device for low-voltage transformer area flexible interconnection system - Google Patents

Stability analysis modeling method and device for low-voltage transformer area flexible interconnection system Download PDF

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CN115495892A
CN115495892A CN202211079831.3A CN202211079831A CN115495892A CN 115495892 A CN115495892 A CN 115495892A CN 202211079831 A CN202211079831 A CN 202211079831A CN 115495892 A CN115495892 A CN 115495892A
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voltage
equivalent
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徐旖旎
熊雄
陈蕾
季宇
张颖
刘海涛
吴鸣
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China Online Shanghai Energy Internet Research Institute Co ltd
State Grid Corp of China SGCC
State Grid Zhejiang Electric Power Co Ltd
State Grid Jiangsu Electric Power Co Ltd
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China Online Shanghai Energy Internet Research Institute Co ltd
State Grid Corp of China SGCC
State Grid Zhejiang Electric Power Co Ltd
State Grid Jiangsu 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
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    • H02J1/102—Parallel operation of dc sources being switching converters
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Abstract

The invention discloses a stability analysis modeling method and a device for a low-voltage transformer area flexible interconnection system, which comprise the following steps: establishing a small signal model of the direct-current voltage control unit and a small signal model of the power control unit according to the control capabilities and characteristics of the direct-current voltage control unit, the photovoltaic and the energy storage in the low-voltage transformer area flexible interconnection system; performing equivalent reduced-order modeling on the small signal model of the direct-current voltage control unit and the small signal model of the power control unit to obtain an equipment small signal model; and operating any one grid-connected converter in the low-voltage transformer area flexible interconnection system in a constant direct-current voltage mode, operating the rest grid-connected converters in a power control mode, and forming an equivalent circuit reduction model for analyzing the stability of the low-voltage transformer area flexible interconnection system based on the small signal model. The method has the advantages that a low-voltage flexible interconnection system model of multi-converter grid connection is effectively simplified, the difficulty in analyzing the stability of small signals of the interconnection system is reduced, and the research on operation control strategies is effectively supported.

Description

Stability analysis modeling method and device for low-voltage transformer area flexible interconnection system
Technical Field
The invention relates to the technical field of low-voltage alternating current and direct current hybrid power distribution, in particular to a stability analysis modeling method and device for a low-voltage transformer area flexible interconnection system.
Background
The method is a novel scheme which aims at the challenges that the power supply capacity of a low-voltage alternating-current distribution network needs to be improved urgently and the power supply quality needs to be improved urgently. However, scenes and projects for realizing load balance and supply-demand interaction of a power distribution area by adopting flexible direct current technology interconnection at present are not popularized in a large area, and a set of complete equivalent impedance stability analysis modeling method is not formed.
When multiple converters run in parallel on the same direct current bus, mutual coupling action exists between the converters, and a large number of constant power loads with negative resistance characteristics are connected into an interconnection system, so that the stability problem of the interconnection system can be caused. The existing research results mostly refer to high-voltage direct-current transmission and medium-voltage direct-current distribution, and are developed aiming at the coordination control strategy of a multi-terminal converter station and a medium-voltage multi-port energy router. The problem to be solved at present is to effectively simplify a low-voltage flexible interconnection system model of multi-converter grid connection and deeply analyze the equivalent impedance stability of the system based on the model.
Disclosure of Invention
In order to solve the above problems, the present invention provides a stability analysis modeling method for a flexible interconnection system in a low-voltage transformer area, including:
establishing a small signal model of the direct-current voltage control unit and a small signal model of the power control unit according to the control capabilities and characteristics of the direct-current voltage control unit, the photovoltaic and the energy storage in the low-voltage transformer area flexible interconnection system;
performing equivalent reduced-order modeling on the small signal model of the direct-current voltage control unit and the small signal model of the power control unit to obtain an equipment small signal model;
and any one grid-connected converter in the low-voltage transformer area flexible interconnection system operates in a constant direct-current voltage mode, the rest grid-connected converters operate in a power control mode, and an equivalent circuit reduction model for stability analysis of the low-voltage transformer area flexible interconnection system is formed on the basis of the equipment small signal model.
Further, according to the control ability and the characteristics of transverter, photovoltaic and energy storage in the flexible interconnected system of low-voltage transformer district, establish the small-signal model of direct current voltage control unit, include:
dividing a direct current voltage control unit into a droop control loop, a direct current voltage control loop and a current control loop;
ignoring the current control loop according to the fact that the bandwidth of the current control loop is larger than that of the direct-current voltage control loop;
defining mu as the input/output current conversion coefficient of the DC voltage control unit, and satisfying mu =1.5U ac /U dc Wherein U is ac Is the AC sidePhase voltage amplitude, U dc For the voltage value of the direct current bus, a mathematical model of the direct current voltage control unit is obtained as follows:
Figure BDA0003832510380000021
the following variable substitutions are made for each part of the above formula:
Figure BDA0003832510380000022
in the formula,. DELTA.U ref Is an equivalent voltage source, Z s,0 Representing the equivalent output impedance, Z, of the DC voltage control unit without considering droop control s,droop Is the equivalent impedance associated with droop control;
the two modes can obtain a small signal model of the direct current voltage control unit which is formed by connecting an equivalent voltage source and an equivalent output impedance in series;
equivalent voltage source delta U ref Series impedance Z s,0 Becomes an equivalent current source delta U ref /Z s,0 Parallel impedance Z s,0 In the form that when the DC voltage control adopts PI control structure, the equivalent impedance Z s,0 Can be specifically expressed as:
Figure BDA0003832510380000023
the equivalent impedance Z in the above formula can be found by circuit theory analysis s,0 Essentially in the form of a parallel connection of a resistor, an inductor and a capacitor, namely:
Figure BDA0003832510380000024
comparing the control parameters in the two known DC voltage control units in the equivalent output impedance model Z s,0 In (1), namely:
Figure BDA0003832510380000025
an equivalent impedance model of the dc voltage control unit is derived therefrom.
Further, the small-signal model of the power control unit determines the constant power load P according to the constant power characteristic of the power control unit cpl Has the following resistance characteristics:
Figure BDA0003832510380000031
in the formula of U bus And P cpl The voltage of the direct current bus at the steady-state working point and the power value of the constant power unit are taken into consideration by taking the flowing direct current bus as the positive direction and taking the voltage stabilizing capacitor C at the input end into consideration p The equivalent impedance model of the power control unit is a parallel equivalent impedance model Z of a resistor and a capacitor p,0 A description is given.
The invention also provides a stability analysis modeling device for the low-voltage transformer area flexible interconnection system, which comprises:
the small signal model establishing unit is used for establishing a small signal model of the direct current voltage control unit and a small signal model of the power control unit according to the control capability and the characteristics of the direct current voltage control unit, the photovoltaic and the energy storage in the low-voltage transformer area flexible interconnection system;
the equipment small signal model obtaining unit is used for carrying out equivalent reduced order modeling on a small signal model of the direct current voltage control unit and a small signal model of the power control unit to obtain an equipment small signal model;
and the equivalent circuit step-down model forming unit is used for operating any one grid-connected converter in the low-voltage transformer area flexible interconnection system in a constant direct-current voltage mode, operating the rest grid-connected converters in a power control mode, and forming an equivalent circuit step-down model for analyzing the stability of the low-voltage transformer area flexible interconnection system based on the equipment small signal model.
Further, the small signal model establishing unit includes:
the dividing subunit is used for dividing the direct-current voltage control unit into a droop control loop, a direct-current voltage control loop and a current control loop;
the bandwidth comparison subunit is used for neglecting the current control loop according to the condition that the bandwidth of the current control loop is greater than that of the direct-current voltage control loop;
a mathematical model obtaining subunit for defining μ as an input/output current conversion coefficient of the DC voltage control unit, and satisfying μ =1.5U ac /U dc Wherein U is ac Is the amplitude of the AC side phase voltage, U dc For the dc bus voltage value, the mathematical model of the dc voltage control unit is obtained as follows:
Figure BDA0003832510380000032
the following variable substitutions are made for each part of the above formula:
Figure BDA0003832510380000033
in the formula, Δ U ref Is an equivalent voltage source, Z s,0 Representing the equivalent output impedance of the dc voltage control unit without considering droop control, Z s,droop Is the equivalent impedance associated with droop control;
the equivalent submodule is used for obtaining an equivalent circuit model of the direct-current voltage control unit formed by connecting an equivalent voltage source and an equivalent output impedance in series by the two modes;
an equivalent impedance expression subunit for converting an equivalent voltage source Δ U ref Series impedance Z s,0 Becomes an equivalent current source delta U ref /Z s,0 Parallel impedance Z s,0 In the form that when the DC voltage control adopts PI control structure, the equivalent impedance Z s,0 Can be specifically expressed as:
Figure BDA0003832510380000041
the equivalent impedance Z in the above formula can be found by circuit theory analysis s,0 Essentially in the form of a parallel connection of a resistor, an inductor and a capacitor, namely:
Figure BDA0003832510380000042
an equivalent impedance model obtaining subunit for comparing the control parameters in the two known DC voltage control units with the equivalent output impedance model Z s,0 In (1), namely:
Figure BDA0003832510380000043
an equivalent impedance model of the dc voltage control unit is derived therefrom.
Further, the method also comprises the following steps:
an equivalent impedance model description unit for determining a constant power load P according to the constant power characteristic of the power control unit cpl Has the following resistance characteristics:
Figure BDA0003832510380000044
in the formula of U bus And P cpl The voltage of the direct current bus at the steady-state working point and the power value of the constant power unit are taken into consideration by taking the flowing direct current bus as the positive direction and taking the voltage stabilizing capacitor C at the input end into consideration p The equivalent impedance model of the power control unit is a parallel equivalent impedance model Z of a resistor and a capacitor p,0 A description is given.
The invention provides a stability analysis modeling method and device for a low-voltage transformer area flexible interconnection system, aiming at the low-voltage transformer area flexible interconnection system of multi-converter grid connection, the invention fully considers the control capability and characteristics of factors such as multi-converter, photovoltaic and energy storage, etc., draws up the control mode of each converter in the system, establishes a small signal model aiming at the corresponding control mode, and is used for simplifying the stability analysis difficulty of the equivalent impedance of the system.
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FIG. 1 is a schematic flow chart of a stability analysis modeling method for a flexible interconnection system of a low-voltage transformer area according to the present invention;
FIG. 2 is a flexible interconnection system of a grid-connected platform area of a multi-converter, which is related to the invention;
FIG. 3 is a general model of a system network for grid connection of multiple converters according to the present invention;
FIG. 4 is a DC voltage control unit control strategy to which the present invention relates;
FIG. 5 is a transfer function of a DC voltage control unit according to the present invention;
FIG. 6 is a DC voltage control unit reduced order model according to the present invention;
FIG. 7 is a power control unit reduced order model to which the present invention relates;
FIG. 8 is an equivalent step-down model of a low-voltage transformer area flexible interconnection system of the multi-converter grid-connected system according to the present invention;
FIG. 9 shows the simulation results of the equivalent impedance model and the detailed model of the ring topology system according to the present invention;
FIG. 10 shows the simulation results of the equivalent impedance model and the detailed model of the non-ring topology system according to the present invention;
FIG. 11 is a schematic structural diagram for modeling stability analysis of a flexible interconnection system in a low-voltage transformer area according to the present invention.
Detailed Description
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. This invention may be embodied in many different forms than those herein set forth and should be readily appreciated by those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.
Fig. 1 is a schematic flow chart of a stability analysis modeling method for a low-pressure platform area flexible interconnection system provided by the invention, and the method provided by the invention is described in detail below with reference to fig. 1.
In the present invention, the dc voltage control unit and the power control unit refer to that the inverter can be used as the dc voltage control unit if the inverter operates in the dc voltage stabilization mode, and can be used as the power control unit if the inverter operates in the constant power mode.
Step S101, establishing a small signal model of a direct current voltage control unit and a small signal model of a power control unit according to the control capabilities and characteristics of the direct current voltage control unit, photovoltaic and energy storage in the low-voltage transformer area flexible interconnection system.
The stability analysis modeling method is mainly used for a low-voltage transformer area flexible interconnection system of multi-converter grid connection. Fig. 2 shows a low-voltage distributed flexible interconnection system, which includes a typical low-voltage distribution area, an AC load, a bidirectional AC/DC converter, a DC bus, and a DC-side analog load, and a photovoltaic power unit, and the specific operations can be divided into a ring network operation and a ring-release operation, and the steps and effectiveness of the present invention are described by taking the topology of the system as an example.
Firstly, a general model of a multi-converter interconnected system structure is determined, the multi-converter interconnected system structure is shown in fig. 3, and M voltage control units and N-M power control units are arranged in the system and are connected with each other through an N-terminal direct current power transmission network. The voltage control unit changes the DC reference voltage u of the current conversion unit through constant voltage control or voltage droop control ref (ii) a The power control unit can control and change the reference power p of the converter unit injected into the DC network ref (the per unit value is expressed as the reference current i ref ). According to the control parameter u refp (p＝1～M)、i refa And (q = M + 1-N), and the stability of the power transmission system under the interference of small disturbance is judged by solving the direct-current voltage and power change condition of the whole system.
And then, establishing a small signal model of the direct-current voltage control unit and a small signal model of the power control unit according to the control capabilities and characteristics of the direct-current voltage control unit, the photovoltaic and the energy storage in the low-voltage transformer area flexible interconnection system.
The specific control strategy of the dc voltage control unit is shown in fig. 4, and can be specifically divided into a droop control loop, a dc voltage control loop, and a current control loop. In droop control loop I ref Is a DC current reference value, i o,dc For the DC voltage control unit to output a DC current, G d (s) represents a droop control link, which can be generally described as:
G d (s)＝R d /(1+T lpf s) (1)
in the formula R d For sag factor, R d When the value is 0, the control is expressed as constant direct current voltage control; t is lpf Is a low pass filter time constant.
The direct current voltage control loop makes difference with the actual direct current voltage through the direct current voltage reference value and then passes through a direct current voltage control link G udc (s) deriving a reference current i for the current control loop sref 。G udc When(s) is general proportional integral control (PI control), it can be described as:
G udc (s)＝k p +k i /s (2)
in the formula k p And k i Proportional and integral coefficients, respectively.
On the basis of fig. 4, a small-signal model of the dc voltage control unit can be derived. Because the bandwidth of the current control loop is far larger than the bandwidth of the direct-current voltage control, the control dynamics of the current loop can be ignored when a system direct-current voltage control time scale equivalent impedance model is established. Based on this assumption, the mathematical model of the dc voltage control unit in fig. 4 can be described by a transfer function block diagram as shown in fig. 5.
Defining mu as the input/output current conversion coefficient of the DC voltage control unit, and satisfying mu =1.5U ac /U dc Wherein U is ac Is the amplitude of the AC side phase voltage, U dc The value of the direct current bus voltage is obtained.
Based on the above analysis, the mathematical model of the dc voltage control unit is obtained as follows:
Figure BDA0003832510380000071
and (4) carrying out variable substitution on each part in the formula (3) as follows:
Figure BDA0003832510380000072
in the formula, delta U ref Is an equivalent voltage source, Z s,0 Representing the equivalent output impedance of the dc voltage control unit without considering droop control, Z s,droop Is the equivalent impedance associated with droop control.
The equivalent circuit model of the dc voltage control unit, which is formed by connecting an equivalent voltage source and an equivalent output impedance in series, can be obtained from equations (3) and (4), and the circuit structure is a basic model for stability analysis based on an impedance method.
Performing the norton equivalent of FIG. 6 (a) yields FIG. 6 (b), the equivalent voltage source Δ U ref Series impedance Z s,0 Becomes equivalent current source delta U ref /Z s,0 Parallel impedance Z s,0 Forms thereof. When the DC voltage control adopts PI control, the equivalent impedance Z s,0 Can be specifically expressed as:
Figure BDA0003832510380000073
the equivalent impedance Z in the formula (5) can be found by circuit theory analysis s,0 It is essentially a parallel form of resistance, inductance and capacitance, namely:
Figure BDA0003832510380000074
comparing the formula (5) and the formula (6) shows that the control parameters such as proportion, integral and differential in the DC voltage control unit are in the equivalent output impedance model Z s,0 In (1), namely:
Figure BDA0003832510380000075
the equivalent impedance model of the dc voltage control system, i.e. the small signal model of the dc voltage control unit, is obtained from this, and is the structure of fig. 6 (c), which is more definite than the physical meaning of fig. 6 (b).
Next, an equivalent impedance model of the power control unit is established, and since the power control unit has a constant power operation characteristic, the equivalent impedance model is equivalent to the model shown in fig. 7 (a).
Near steady state operating point, constant power load P cpl Has the following resistance characteristics:
Figure BDA0003832510380000081
in the formula of U bus And P cpl The voltage of the direct current bus and the power value of the constant power unit at the steady-state working point are obtained, and the flowing direct current bus is taken as the positive direction. Considering input terminal voltage stabilizing capacitor C p The power control unit stability analysis model can be a parallel equivalent impedance model Z of a resistor and a capacitor p,0 Described, as shown in fig. 6 (b).
And S102, performing equivalent reduced-order modeling on the small signal model of the direct-current voltage control unit and the small signal model of the power control unit to obtain an equipment small signal model.
Taking the topology of the multi-converter grid-connected flexible interconnection system shown in fig. 2 as an example, four 10kV/380V distribution transformers are interconnected through four AC/DC transformers at the AC outlet side. Pseudo bipolar wiring at the direct current side, wherein the voltage grade is +/-375V, and the interelectrode voltage is 750V; when the dotted lines in the figure are not connected, the direct current side non-ring network operates; when the dotted lines are connected in the figure, the direct current side branch ring network operates. The direct current side is connected with one path of energy storage, one path of photovoltaic power generation unit and one path of direct current load. The technical parameters of the four-zone flexible interconnection system are shown in table 1.
TABLE 1 four-terminal distributed flexible interconnection system technical parameters
Figure BDA0003832510380000082
In a normal operation state, the working modes of the converters in the four-zone flexible interconnection system are shown in table 2:
TABLE 2 operating modes of the converters in normal operating conditions
Figure BDA0003832510380000083
By combining the equivalent impedance models of the dc voltage control unit and the power control unit, equivalent circuit reduced models of the interconnection system under the ring network topology and the non-ring network topology can be obtained as shown in fig. 8 (a) and (b), respectively.
Step S103, operating any one grid-connected converter in the low-voltage transformer area flexible interconnection system in a constant direct-current voltage mode, operating the rest grid-connected converters in a power control mode, and forming an equivalent circuit order reduction model for stability analysis of the low-voltage transformer area flexible interconnection system based on the equipment small signal model.
Aiming at a low-voltage transformer area flexible interconnection system with a multi-converter grid-connected structure, one grid-connected converter is set to operate in a constant direct-current voltage mode, other grid-connected converters operate in a power control mode, an interconnection system equivalent circuit reduced-order model can be formed based on an equipment small-signal model, and the model is equivalent to a power electronic full-order model through simulation verification, so that the difficulty in analyzing the stability of the system can be effectively reduced.
And finally, verifying the effectiveness of the equivalent circuit-based reduced order model by comparing the established equivalent impedance model with a detailed model. The verification method comprises the following steps:
1) Looped network topology verification
Description of working conditions: the initial output power of the power control ends #1 to #3 is respectively 200kW, -100kW, -100kW (flowing into a direct current network), the power of a resistive direct current load is 200kW, the output power of the energy storage device is-200 kW, the photovoltaic output is 200kW, and the output of the energy storage device is changed from-200 MW to-350 MW in the 1.5 th second. The comparison result of the equivalent impedance model and the detailed Thevenin equivalent model established in the steps 1-4 is shown as 9, wherein 'udc 4 detail' in the figure represents the simulation result of the detailed power electronic switch model, and 'udc 4 small' in the figure represents the simulation result of the equivalent impedance model. According to the diagram, when the energy storage device generates power disturbance, the dynamic state and the steady state of the simulation result of the equivalent impedance model and the detailed power electronic switch model are almost consistent, and the effectiveness of the equivalent impedance model is verified.
2) Non-ring network topology verification
And (3) description of working conditions: the initial output power of the power control ends #1 to #3 is respectively 200kW, -100kW, -100kW (flowing into a direct current network), the power of a resistive direct current load is 200kW, the output power of the energy storage device is-200 kW, the photovoltaic output is 200kW, and the output of the energy storage device is changed from-200 MW to-350 MW in the 1.5 th second. The comparison result of the equivalent impedance model and the detailed Thevenin equivalent model provided herein is shown in FIG. 10, wherein "udc4 detail" in the figure represents the simulation result of the detailed power electronic switch model, and "udc4 small" in the figure represents the simulation result of the equivalent impedance model. According to the diagram, when the energy storage device generates power disturbance, the dynamic state and the steady state of the simulation result of the equivalent impedance model and the detailed power electronic switch model are almost consistent, and the effectiveness of the equivalent impedance model is verified.
Based on the same inventive concept, the present invention also provides a stability analysis modeling apparatus 200 for a flexible interconnection system of a low-voltage transformer area, which is characterized by comprising:
the small signal model establishing unit 210 is used for establishing a small signal model of the direct current voltage control unit and a small signal model of the power control unit according to the control capabilities and characteristics of the direct current voltage control unit, photovoltaic and energy storage in the low-voltage transformer area flexible interconnection system;
an equipment small signal model obtaining unit 220, configured to perform equivalent reduced-order modeling on the small signal model of the dc voltage control unit and the small signal model of the power control unit to obtain an equipment small signal model;
and the equivalent circuit step-down model forming unit 230 is configured to operate any one of the grid-connected converters in the low-voltage transformer area flexible interconnection system in a constant direct-current voltage mode, operate the remaining grid-connected converters in a power control mode, and form an equivalent circuit step-down model for stability analysis of the low-voltage transformer area flexible interconnection system based on the device small-signal model.
Further, the small signal model establishing unit includes:
the dividing subunit is used for dividing the direct-current voltage control unit into a droop control loop, a direct-current voltage control loop and a current control loop;
the bandwidth comparison subunit is used for neglecting the current control loop according to the fact that the bandwidth of the current control loop is larger than that of the direct-current voltage control loop;
a mathematical model obtaining subunit for defining μ as an input/output current conversion coefficient of the DC voltage control unit, and satisfying μ =1.5U ac /U dc Wherein U is ac Is the amplitude of the AC side phase voltage, U dc For the dc bus voltage value, the mathematical model of the dc voltage control unit is obtained as follows:
Figure BDA0003832510380000101
the following variable substitutions are made for each part of the above formula:
Figure BDA0003832510380000102
in the formula, Δ U ref Is an equivalent voltage source, Z s,0 Representing the equivalent output impedance, Z, of the DC voltage control unit without considering droop control s,droop Is the equivalent impedance associated with droop control;
the equivalent submodule is used for obtaining an equivalent circuit model of the direct-current voltage control unit formed by connecting an equivalent voltage source and an equivalent output impedance in series by the two modes;
an equivalent impedance expression subunit for converting an equivalent voltage source Δ U ref Series impedance Z s,0 Becomes an equivalent current source delta U ref /Z s,0 Parallel impedance Z s,0 In the form that when the DC voltage control adopts PI control structure, the equivalent impedance Z s,0 Can be specifically expressed as:
Figure BDA0003832510380000111
the equivalent impedance Z in the above formula can be known by circuit theory analysis s,0 Essentially in the form of a parallel connection of a resistor, an inductor and a capacitor, namely:
Figure BDA0003832510380000112
a small signal model obtaining subunit for comparing the control parameters in the two known DC voltage control units with the equivalent output impedance model Z s,0 In (1), namely:
Figure BDA0003832510380000113
an equivalent impedance model of the dc voltage control unit is derived therefrom.
Further, the method also comprises the following steps:
an equivalent impedance model description unit for determining a constant power load P according to the constant power characteristics of the power control unit cpl Has the following resistance characteristics:
Figure BDA0003832510380000114
in the formula of U bus And P cpl The voltage of the direct current bus at the steady-state working point and the power value of the constant power unit are taken into consideration by taking the flowing direct current bus as the positive direction and taking the voltage-stabilizing capacitor C at the input end into consideration p The equivalent impedance model of the power control unit is a parallel equivalent impedance model Z of a resistor and a capacitor p,0 A description is given.
Compared with the prior art, the stability analysis modeling method and device for the low-voltage transformer area flexible interconnection system provided by the invention are used for aiming at the low-voltage transformer area flexible interconnection system of the multi-converter grid-connected system, the control capability and the characteristics of the factors such as the multi-converter, the photovoltaic and the energy storage are fully considered, the control mode of each converter in the system is drawn up, the small signal model is established aiming at the corresponding control mode to simplify the stability analysis difficulty of the equivalent impedance of the system, and the effectiveness of the reduced model is effectively verified through the simulation comparison of the reduced model and the system power electronic full-order model, so that the operation control strategy research of the low-voltage transformer area flexible interconnection system can be effectively supported.
Finally, it should be noted that: although the present invention has been described in detail with reference to the foregoing embodiments, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the spirit and scope of the invention.

Claims (6)

1. A stability analysis modeling method for a low-voltage transformer area flexible interconnection system is characterized by comprising the following steps:
establishing a small signal model of the direct-current voltage control unit and a small signal model of the power control unit according to the control capabilities and characteristics of the direct-current voltage control unit, the photovoltaic and the energy storage in the low-voltage transformer area flexible interconnection system;
carrying out equivalent reduced-order modeling on the small-signal model of the direct-current voltage control unit and the small-signal model of the power control unit to obtain a small-signal model of the equipment;
and any one grid-connected converter in the low-voltage transformer area flexible interconnection system operates in a constant direct-current voltage mode, the rest grid-connected converters operate in a power control mode, and an equivalent circuit reduction model for stability analysis of the low-voltage transformer area flexible interconnection system is formed on the basis of the equipment small signal model.
2. The method of claim 1, wherein the establishing of the small-signal model of the DC voltage control unit according to the control capability and characteristics of the inverter, the photovoltaic and the energy storage in the low-voltage transformer area flexible interconnection system comprises:
dividing a direct current voltage control unit into a droop control loop, a direct current voltage control loop and a current control loop;
ignoring the current control loop according to the fact that the bandwidth of the current control loop is larger than that of the direct-current voltage control loop;
defining mu as the input/output current conversion coefficient of the DC voltage control unit, and satisfying mu =1.5U ac /U dc Wherein U is ac Is the amplitude of the AC side phase voltage, U dc For the dc bus voltage value, the mathematical model of the dc voltage control unit is obtained as follows:
Figure FDA0003832510370000011
the following variable substitutions are made for each part of the above formula:
Figure FDA0003832510370000012
in the formula, Δ U ref Is an equivalent voltage source, Z s,0 Representing the equivalent output impedance of the dc voltage control unit without considering droop control, Z s,droop Is the equivalent impedance associated with droop control;
the two modes can obtain a small signal model of the direct current voltage control unit which is formed by connecting an equivalent voltage source and an equivalent output impedance in series;
equivalent voltage source delta U ref Series impedance Z s,0 Becomes an equivalent current source delta U ref /Z s,0 Parallel impedance Z s,0 In the form of equivalent impedance Z when the DC voltage control adopts PI control structure s,0 Can be specifically expressed as:
Figure FDA0003832510370000021
from circuit theory analysis, it can be known thatUpper type medium effective impedance Z s,0 Essentially in the form of a parallel connection of a resistor, an inductor and a capacitor, namely:
Figure FDA0003832510370000022
comparing the control parameters in the two known DC voltage control units in the equivalent output impedance model Z s,0 In (1), namely:
Figure FDA0003832510370000023
an equivalent impedance model of the dc voltage control unit is derived therefrom.
3. The method of claim 1, wherein the small signal model of the power control unit determines the constant power load P according to a constant power characteristic of the power control unit cpl Has the following resistance characteristics:
Figure FDA0003832510370000024
in the formula of U bus And P cpl The voltage of the direct current bus at the steady-state working point and the power value of the constant power unit are taken into consideration by taking the flowing direct current bus as the positive direction and taking the voltage stabilizing capacitor C at the input end into consideration p The equivalent impedance model of the power control unit is a parallel equivalent impedance model Z of a resistor and a capacitor p,0 A description is given.
4. A stability analysis modeling apparatus for a low-pressure platform area flexible interconnection system, comprising:
the small signal model establishing unit is used for establishing a small signal model of the direct current voltage control unit and a small signal model of the power control unit according to the control capabilities and characteristics of the direct current voltage control unit, photovoltaic and energy storage in the low-voltage transformer area flexible interconnection system;
the equipment small signal model obtaining unit is used for carrying out equivalent reduced order modeling on a small signal model of the direct current voltage control unit and a small signal model of the power control unit to obtain an equipment small signal model;
and the equivalent circuit step-down model forming unit is used for operating any one grid-connected converter in the low-voltage transformer area flexible interconnection system in a constant direct-current voltage mode, operating the rest grid-connected converters in a power control mode, and forming an equivalent circuit step-down model for analyzing the stability of the low-voltage transformer area flexible interconnection system based on the equipment small signal model.
5. The apparatus of claim 4, wherein the small signal model building unit comprises:
the dividing subunit is used for dividing the direct-current voltage control unit into a droop control loop, a direct-current voltage control loop and a current control loop;
the bandwidth comparison subunit is used for neglecting the current control loop according to the condition that the bandwidth of the current control loop is greater than that of the direct-current voltage control loop;
a mathematical model obtaining subunit for defining μ as an input/output current conversion coefficient of the DC voltage control unit, and satisfying μ =1.5U ac /U dc Wherein U is ac Is the amplitude of the AC side phase voltage, U dc For the dc bus voltage value, the mathematical model of the dc voltage control unit is obtained as follows:
Figure FDA0003832510370000031
the following variable substitutions are made for each part of the above formula:
Figure FDA0003832510370000032
in the formula, Δ U ref Is an equivalent voltage source, Z s,0 Is shown without considering sagEquivalent output impedance of the DC voltage control unit under control, Z s,droop Is the equivalent impedance associated with droop control;
the equivalent submodule is used for obtaining an equivalent circuit model of the direct-current voltage control unit formed by connecting an equivalent voltage source and an equivalent output impedance in series by the two modes;
an equivalent impedance expression subunit for converting an equivalent voltage source Δ U ref Series impedance Z s,0 Becomes an equivalent current source delta U ref /Z s,0 Parallel impedance Z s,0 In the form that when the DC voltage control adopts PI control structure, the equivalent impedance Z s,0 Can be specifically expressed as:
Figure FDA0003832510370000033
the equivalent impedance Z in the above formula can be known by circuit theory analysis s,0 Essentially in the form of a parallel connection of a resistor, an inductor and a capacitor, namely:
Figure FDA0003832510370000034
an equivalent impedance model obtaining subunit for comparing the control parameters in the two-type known DC voltage control unit with the equivalent output impedance model Z s,0 In (1), namely:
Figure FDA0003832510370000035
an equivalent impedance model of the dc voltage control unit is derived therefrom.
6. The apparatus of claim 4, further comprising:
an equivalent impedance model description unit for determining a constant power load P according to the constant power characteristic of the power control unit cpl Has the following resistance characteristics:
Figure FDA0003832510370000041
in the formula of U bus And P cpl The voltage of the direct current bus at the steady-state working point and the power value of the constant power unit are taken into consideration by taking the flowing direct current bus as the positive direction and taking the voltage stabilizing capacitor C at the input end into consideration p The equivalent impedance model of the power control unit is a parallel equivalent impedance model Z of a resistor and a capacitor p,0 A description is given.
CN202211079831.3A 2022-09-05 2022-09-05 Stability analysis modeling method and device for low-voltage transformer area flexible interconnection system Pending CN115495892A (en)

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN116613807A (en) * 2023-07-20 2023-08-18 国网上海市电力公司 Transient stability evaluation method and system for flexible interconnection system of power distribution area

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN116613807A (en) * 2023-07-20 2023-08-18 国网上海市电力公司 Transient stability evaluation method and system for flexible interconnection system of power distribution area
CN116613807B (en) * 2023-07-20 2023-10-03 国网上海市电力公司 Transient stability evaluation method and system for flexible interconnection system of power distribution area

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