CN114297874A - Method and system for determining capacitance value of frequency conversion valve submodule capacitor for flexible low-frequency power transmission - Google Patents

Method and system for determining capacitance value of frequency conversion valve submodule capacitor for flexible low-frequency power transmission Download PDF

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CN114297874A
CN114297874A CN202210003227.6A CN202210003227A CN114297874A CN 114297874 A CN114297874 A CN 114297874A CN 202210003227 A CN202210003227 A CN 202210003227A CN 114297874 A CN114297874 A CN 114297874A
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power
frequency
value
capacitance
fluctuation rate
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裘鹏
倪晓军
陆翌
许烽
宣佳卓
林进钿
丁超
郑眉
谢浩铠
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Electric Power Research Institute of State Grid Zhejiang Electric Power Co Ltd
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Abstract

The invention discloses a method and a system for determining capacitance values of frequency conversion valve sub-modules for flexible low-frequency power transmission. The method adopts the technical scheme that: establishing a mathematical model of the relation between the capacitance value of the frequency conversion valve submodule capacitor and the fluctuation rate of the capacitor voltage for flexible low-frequency power transmission; selecting an initial value of capacitance values of sub-modules of a frequency conversion valve, traversing each power point determined by a power circle/power circle cluster of the flexible low-frequency power transmission system according to the established mathematical model, and determining the fluctuation rate of capacitance voltage of each power point; and iterating the capacitance capacity value according to a preset capacitance voltage fluctuation rate target value until the capacitance voltage fluctuation rate of each power point determined by the power circle/power circle cluster of the flexible low-frequency power transmission system is less than or equal to the capacitance voltage fluctuation rate target value, and obtaining a proper margin coefficient to obtain the capacitance capacity value of the frequency conversion valve submodule. The method determines the final design value of the capacitance value of the sub-module capacitor by iterating the capacitance value of the sub-module capacitor, and can be used for guiding engineering design and construction.

Description

Method and system for determining capacitance value of frequency conversion valve submodule capacitor for flexible low-frequency power transmission
Technical Field
The invention belongs to the technical field of power transmission and distribution of a power system, and particularly relates to a method and a system for determining capacitance values of frequency conversion valve sub-modules for flexible low-frequency power transmission.
Background
With the development of loads and the further development of resources, large-capacity and long-distance electric energy transmission is necessary, however, due to the influence of line capacity rise effect and transmission loss, large-capacity and long-distance effective transmission of electric energy cannot be realized even if high-voltage grade is adopted for power frequency alternating current. After the 50 s of the 20 th century, the direct current transmission technology suitable for long-distance large-capacity transmission has been developed again. However, in both the aspects of voltage level conversion and fault current disconnection, because the technologies of the direct current transformer and the direct current breaker are not mature, and the equipment investment is huge, direct current networking is still difficult.
Flexible low-frequency power transmission is a novel alternating-current power transmission technology, and the power transmission frequency of the flexible low-frequency power transmission is between the power frequency and the direct current. Due to the fact that flexible low-frequency power transmission is combined with the technical characteristics of power frequency and direct-current power transmission, the flexible low-frequency power transmission system has wide application prospect in the fields of urban power grid interconnection, new energy grid connection, long-distance power supply and the like. Particularly, in the application occasions of wind power transmission in medium and far seas, the flexible low-frequency power transmission technology provides a new means for economically and efficiently transmitting the wind power in the medium and far seas. The offshore wind power transmission mainly comprises 3 schemes of power frequency alternating current, flexible direct current and flexible low-frequency alternating current. The power frequency alternating current scheme is limited by a submarine cable capacitance effect and is only suitable for offshore small-scale wind power output; the flexible direct current scheme requires the construction of an offshore converter platform, and faces technical challenges of light weight, miniaturization, high reliability and the like of a converter station. The advantages of flexible low-frequency power transmission are mainly reflected in that: 1) compared with a power frequency alternating current scheme, the system has longer effective power transmission distance and larger transmission capacity; 2) the fan can directly generate low-frequency power and boost the power to be output without building an offshore converter station, so that the construction, operation and maintenance cost is effectively reduced; 3) a multi-terminal system can be constructed by using an alternating current switch and a transformer, and has stronger networking capability and lower networking cost compared with flexible direct current. Based on the advantages, the grid-connected consumption of the wind power plant at the middle and far seas has technical and economic advantages compared with power frequency alternating current and flexible direct current by adopting a flexible low-frequency power transmission scheme.
So far, most of the published documents basically only study the topology, modeling and control strategies, etc. of the low frequency power transmission system, and there is little research on the selection of flexible low frequency power transmission parameters. In order to further determine technical parameters of the flexible low-frequency transmission frequency conversion valve, research needs to be conducted on a design method of capacitance values of sub-modules of the frequency conversion valve.
Disclosure of Invention
The invention aims to provide a method and a system for determining a capacitance value of a submodule capacitor of a frequency conversion valve for flexible low-frequency power transmission, which are used for determining the capacitance value of the submodule by deducing a mathematical theoretical model of a modular multi-level matrix frequency conversion valve (M3C) and combining a low-frequency system operation power circle/power circle cluster.
Therefore, the invention adopts a technical scheme that: the method for determining the capacitance value of the frequency conversion valve submodule capacitor for flexible low-frequency power transmission comprises the following steps:
establishing a mathematical model of the relation between the capacitance value of the frequency conversion valve submodule capacitor and the fluctuation rate of the capacitor voltage for flexible low-frequency power transmission;
selecting an initial value of capacitance values of sub-modules of a frequency conversion valve, traversing each power point determined by a power circle/power circle cluster of the flexible low-frequency power transmission system according to the established mathematical model, and determining the fluctuation rate of capacitance voltage of each power point;
and iterating the capacitance capacity value according to a preset capacitance voltage fluctuation rate target value until the capacitance voltage fluctuation rate of each power point determined by the power circle/power circle cluster of the flexible low-frequency power transmission system is less than or equal to the capacitance voltage fluctuation rate target value, and obtaining a proper margin coefficient to obtain the capacitance capacity value of the frequency conversion valve submodule.
Further, in the mathematical model, the calculation method of the maximum value of the fluctuation rate of the capacitance voltage of a single power point is as follows:
Figure BDA0003455645350000021
wherein, ω is1At industrial frequency angular frequency, omega2For low angular frequencies, Δ u (2 ω)1t) is the frequency-doubled component of power frequency 2, Deltau (2 omega)2t) is the 2-fold frequency component of the low frequency, Δ u (ω)1t+ω2t) is the frequency and component of the power frequency and low frequency, Deltau (omega)1t-ω2T) is the frequency difference component of the power frequency and the low frequency, T is the period after the power frequency component and the low frequency component are superposed, UCNThe submodule capacitor voltage nominal operation value.
Further, the calculation method of each fluctuation frequency component is as follows:
Figure BDA0003455645350000022
Figure BDA0003455645350000023
Δu(ω1t+ω2t)＝f1(ω1t+ω2t)+f2(ω1t+ω2t)+f3(ω1t+ω2t)，
Figure BDA0003455645350000031
Figure BDA0003455645350000032
Figure BDA0003455645350000033
Δu(ω1t-ω2t)＝f1(ω1t-ω2t)+f2(ω1t-ω2t)+f3(ω1t-ω2t)，
Figure BDA0003455645350000034
Figure BDA0003455645350000035
Figure BDA0003455645350000036
Figure BDA0003455645350000037
wherein, P1、Qs1、S1、Vs1、X1Respectively the active power, the reactive power, the apparent power, the bus voltage and the equivalent impedance of the frequency conversion valve at the power frequency bus2、Qs2、S2、Vs2、X2Respectively active power, reactive power, apparent power, bus voltage and equivalent impedance of the frequency conversion valve at a low-frequency bus; n is the number of the single bridge arm sub-modules; c is capacitance value of the capacitor; delta is the initial phase angle difference of the alternating-current voltage of the buses at the power frequency side and the low frequency side of the frequency conversion valve.
Furthermore, the horizontal and vertical coordinates of the power circle/power circle cluster are respectively the active power and the reactive power of the power frequency bus, and the power circle/power circle cluster is determined according to the frequency conversion valve parameters, the voltage and the current of the power frequency side bus, and the voltage and the current of the low-frequency side bus; the power circle/power circle cluster is a series of sub-power circles (i.e. the power circle cluster), or a power circle formed by intersecting and collecting a series of sub-power circles (i.e. the power circle).
Further, the sub-power circle is formed by a straight line x ═ P1min、x＝P1maxAnd each power point P1Upper boundary value Q of reactive power under conditionss1pAnd a lower boundary value Qs1nFormed closed figure, P1∈[P1min,P1max]Wherein Q iss1pAnd Qs1nThe calculation method comprises the following steps:
Qs1p＝max(Qs11p，Qs12p，Qs14p，Qs15p)，
Qs1n＝max(Qs11n，Qs12n，Qs13n，Qs14n，Qs15n)，
wherein, P1minAnd P1maxThe maximum value and the minimum value of the active power of the power frequency bus are specified for design; qs11p、Qs11nRespectively an upper boundary value and a lower boundary value of the reactive power under the constraint of the modulation degree; qs12p、Qs12nThe upper boundary value and the lower boundary value of the reactive power under the transformer capacity constraint are respectively; qs13nRespectively the lower boundary values of the reactive power under the valve side voltage constraint; qs14p、Qs14nRespectively an upper boundary value and a lower boundary value of the reactive power under the current constraint of the valve side; qs15p、Qs15nThe upper boundary value and the lower boundary value of the reactive power under the constraint of the bridge arm current are respectively.
Further, the iterative process proceeds according to the following steps:
step 1: obtaining the initial value C of capacitance value of capacitor0Maximum value epsilon of capacitance voltage fluctuation rate of each power operating point on power circle/power circle cluster under parameter condition0max；
Step 2: if epsilon0maxGreater than the target value epsilon of the fluctuation rate of the capacitor voltagerefIncreasing the capacitance value of the capacitor with the step length of deltaC until the mth iteration to obtain the capacitance value C of the capacitormThe maximum value epsilon of the fluctuation rate of the capacitance voltage is obtainedmaxLess than or equal to the target value epsilon of the fluctuation rate of the capacitor voltageref(ii) a If epsilon0maxLess than the target value epsilon of the fluctuation rate of the capacitor voltagerefThen reduce the electricityCapacitance value, up to the nth capacitance value CnCapacitance voltage fluctuation rate maximum value epsilon obtained through iterationmaxGreater than or equal to the target value epsilon of the fluctuation rate of the capacitor voltageref(ii) a If epsilon0maxEqual to the target value epsilon of the fluctuation rate of the capacitor voltagerefThen stopping iteration;
and step 3: if epsilon0maxGreater than the target value epsilon of the fluctuation rate of the capacitor voltagerefThen C ismThe minimum design value for meeting the requirement of the fluctuation rate of the capacitor voltage; if epsilon0maxLess than the target value epsilon of the fluctuation rate of the capacitor voltagerefThen C isnThe minimum design value for meeting the requirement of the fluctuation rate of the capacitor voltage; if epsilon0maxEqual to the target value epsilon of the fluctuation rate of the capacitor voltagerefThen C is0The minimum design value for meeting the requirement of the fluctuation rate of the capacitor voltage;
and 4, step 4: on the basis of the step 3, multiplying the minimum design value meeting the requirement of the fluctuation rate of the capacitance voltage by a proper margin coefficient k to obtain the final design value of the capacitance value as kCm、kCnOr kC0。
Further, the reference range of the margin coefficient k is more than or equal to 1.1 and less than or equal to 1.3, or determined according to electromagnetic transient simulation.
The other technical scheme adopted by the invention is as follows: flexible low frequency transmission is with trading frequency valve submodule piece capacitance capacity value determination system that includes:
a mathematical model construction unit: establishing a mathematical model of the relation between the capacitance value of the frequency conversion valve submodule capacitor and the fluctuation rate of the capacitor voltage for flexible low-frequency power transmission;
capacitance voltage fluctuation ratio determination unit: selecting an initial value of capacitance values of sub-modules of a frequency conversion valve, traversing each power point determined by a power circle/power circle cluster of the flexible low-frequency power transmission system according to the established mathematical model, and determining the fluctuation rate of capacitance voltage of each power point;
a capacitance value acquisition unit: and iterating the capacitance capacity value according to a preset capacitance voltage fluctuation rate target value until the capacitance voltage fluctuation rate of each power point determined by the power circle/power circle cluster of the flexible low-frequency power transmission system is less than or equal to the capacitance voltage fluctuation rate target value, and obtaining a proper margin coefficient to obtain the capacitance capacity value of the frequency conversion valve submodule.
Based on the technical scheme, the invention has the following beneficial technical effects: for a flexible low-frequency power transmission system, a mathematical model of a relation between a sub-module capacitance value and a capacitance voltage fluctuation rate is deduced for a modular multi-level matrix frequency conversion valve, the capacitance voltage fluctuation rate of all power operating points in a power circle/power circle cluster range of the frequency conversion valve is not larger than a target value through iteration of the sub-module capacitance value, and a final design value of the sub-module capacitance value is determined according to the target value, so that the flexible low-frequency power transmission system can be used for guiding engineering design and construction.
Drawings
Fig. 1 is a schematic diagram of a single-ended frequency conversion station of a conventional flexible low-frequency power transmission M3C;
fig. 2 is an equivalent circuit diagram of a conventional single-ended frequency conversion station of flexible low-frequency power transmission M3C;
FIG. 3 is a flow chart of a method for determining capacitance values of sub-modules of a frequency conversion valve for flexible low-frequency power transmission according to the invention;
FIG. 4 is a flow chart of capacitance iteration of the capacitor of the present invention;
fig. 5 is a structural diagram of a capacitance value determination system of a frequency conversion valve submodule for flexible low-frequency power transmission.
Detailed Description
In order to more specifically describe the present invention, the following detailed description of the embodiments of the present invention is provided with reference to the accompanying drawings.
Fig. 1 is a schematic diagram of a single-ended frequency conversion station of a conventional flexible low-frequency power transmission M3C. The flexible low-frequency power transmission single-ended system mainly comprises a power frequency bus, a power frequency boosting transformer (namely, a power frequency transformer in the figure), an M3C frequency conversion valve (namely, an AC frequency conversion valve in the figure), a low-frequency voltage reducing transformer (namely, a low-frequency transformer in the figure) and a low-frequency bus. The M3C frequency conversion valve is composed of 9 bridge arms, and each bridge arm is composed of N full-bridge sub-modules in cascade connection.
Fig. 2 is an equivalent circuit diagram of a conventional single-ended frequency conversion station of the flexible low-frequency power transmission M3C. Equivalent impedance X of power frequency valve side1Comprising a leakage reactance X of a power frequency transformerT1Equivalent value X on power frequency side of reactance of frequency conversion valve bridge armL1(ii) a Low frequency valve side equivalent impedanceX2Comprising a low-frequency transformer leakage reactance XT2Equivalent value X on low-frequency side of reactance of frequency conversion valve bridge armL2。
According to different engineering requirements, the power frequency transformer or the low frequency transformer may not be configured, and if not, the leakage reactance of the transformer in fig. 2 is 0.
Fig. 3 is a flowchart of a method for determining a capacitance value of a submodule capacitor of a frequency conversion valve for flexible low-frequency power transmission according to the present invention, and the following describes an implementation of designing a capacitance value of a submodule capacitor with reference to the flowchart.
Firstly, a mathematical model of the relation between the capacitance value of the frequency conversion valve submodule for flexible low-frequency power transmission and the fluctuation rate of the capacitance voltage is established.
The method for calculating the maximum value of the fluctuation rate of the capacitor voltage of a single power point comprises the following steps:
Figure BDA0003455645350000061
wherein, ω is1At industrial frequency angular frequency, omega2For low angular frequencies, Δ u (2 ω)1t) is the frequency-doubled component of power frequency 2, Deltau (2 omega)2t) is the 2-fold frequency component of the low frequency, Δ u (ω)1t+ω2t) is the frequency and component of the power frequency and low frequency, Deltau (omega)1t-ω2T) is the frequency difference component of the power frequency and the low frequency, T is the period after the power frequency component and the low frequency component are superposed, UCNThe submodule capacitor voltage nominal operation value.
The calculation method of each fluctuation frequency component comprises the following steps:
Figure BDA0003455645350000062
Figure BDA0003455645350000063
Δu(ω1t+ω2t)＝f1(ω1t+ω2t)+f2(ω1t+ω2t)+f3(ω1t+ω2t)，
Figure BDA0003455645350000064
Figure BDA0003455645350000065
Figure BDA0003455645350000066
Δu(ω1t-ω2t)＝f1(ω1t-ω2t)+f2(ω1t-ω2t)+f3(ω1t-ω2t)，
Figure BDA0003455645350000067
Figure BDA0003455645350000071
Figure BDA0003455645350000072
Figure BDA0003455645350000073
wherein, P1、Qs1、S1、Vs1、X1Respectively the active power, the reactive power, the apparent power, the bus voltage and the equivalent impedance of the frequency conversion valve at the power frequency bus2、Qs2、S2、Vs2、X2Respectively active power, reactive power, apparent power, bus voltage and equivalent impedance of the frequency conversion valve at a low-frequency bus; n is the number of the single bridge arm sub-modules; c is electricityA capacitance value; delta is the initial phase angle difference of alternating-current voltage on the power frequency side and the low frequency side of the frequency conversion valve.
Then, selecting an initial value of capacitance value of a frequency conversion valve submodule capacitor, and determining all power points (P) of the power circle/power circle cluster of the flexible low-frequency power transmission system according to the established mathematical modeli，Qi) Traversing to determine initial value epsilon of fluctuation rate of capacitance-capacitance voltage of all power points0(Pi，Qi). The horizontal and vertical coordinates of the power circle/power circle cluster are respectively the active power and the reactive power of the power frequency bus, the power circle/power circle cluster is determined according to the frequency conversion valve parameters, the voltage and the current of the power frequency side bus, the voltage and the current of the low frequency side bus, and the power circle/power circle cluster is a series of sub-power circles or a power circle formed by combining a series of sub-power circles.
The sub-power circle is formed by a straight line x ═ P1min、x＝P1maxAnd each power point P1Upper boundary value Q of reactive power under conditionss1pAnd a lower boundary value Qs1nFormed closed figure, P1∈[P1min,P1max]Wherein Q iss1pAnd Qs1nThe calculation method comprises the following steps:
Qs1p＝max(Qs11p，Qs12p，Qs14p，Qs15p)，
Qs1n＝max(Qs11n，Qs12n，Qs13n，Qs14n，Qs15n)，
wherein, P1minAnd P1maxThe maximum value and the minimum value of the active power of the power frequency bus are specified for design; qs11p、Qs11nRespectively an upper boundary value and a lower boundary value of the reactive power under the constraint of the modulation degree; qs12p、Qs12nThe upper boundary value and the lower boundary value of the reactive power under the transformer capacity constraint are respectively; qs13nRespectively the lower boundary values of the reactive power under the valve side voltage constraint; qs14p、Qs14nRespectively an upper boundary value and a lower boundary value of the reactive power under the current constraint of the valve side; qs15p、Qs15nThe upper boundary value and the lower boundary value of the reactive power under the constraint of the bridge arm current are respectively.
And finally, iterating the capacitance value according to a preset capacitance voltage fluctuation rate target value until the capacitance voltage fluctuation rate of each power point determined by the power circle/power circle cluster of the flexible low-frequency power transmission system is smaller than or equal to the capacitance voltage fluctuation rate target value, and obtaining a proper margin coefficient to obtain the capacitance value of the frequency conversion valve submodule.
Fig. 4 is a flow chart of capacitance value iteration of the capacitor, and the iteration process is performed according to the following steps:
step 1: obtaining the initial value C of capacitance value of capacitor0Maximum value epsilon of capacitance voltage fluctuation rate of all power operating points on power circle/power circle cluster under parameter condition0max＝max[ε0(Pi，Qi)]。
Step 2: if epsilon0maxGreater than the target value epsilon of the fluctuation rate of the capacitor voltagerefIncreasing the capacitance value of the capacitor with the step length of deltaC until the mth iteration to obtain the capacitance value C of the capacitormThe maximum value epsilon of the fluctuation rate of the capacitance voltage is obtainedmaxLess than or equal to the target value epsilon of the fluctuation rate of the capacitor voltageref(ii) a If epsilon0maxLess than the target value epsilon of the fluctuation rate of the capacitor voltagerefDecreasing the capacitance until the nth capacitance value C is obtainednCapacitance voltage fluctuation rate maximum value epsilon obtained through iterationmaxGreater than or equal to the target value epsilon of the fluctuation rate of the capacitor voltageref(ii) a If epsilon0maxEqual to the target value epsilon of the fluctuation rate of the capacitor voltagerefThen no iteration is needed.
And step 3: if epsilon0maxGreater than the target value epsilon of the fluctuation rate of the capacitor voltagerefThen C ismThe minimum design value for meeting the requirement of the fluctuation rate of the capacitor voltage; if epsilon0maxLess than the target value epsilon of the fluctuation rate of the capacitor voltagerefThen C isnThe minimum design value for meeting the requirement of the fluctuation rate of the capacitor voltage; if epsilon0maxEqual to the target value epsilon of the fluctuation rate of the capacitor voltagerefThen C is0The minimum design value required by the fluctuation ratio of the capacitance voltage is met.
And 4, step 4: on the basis of the step 3, multiplying the minimum design value meeting the requirement of the fluctuation rate of the capacitance voltage by a certain margin coefficient k to obtain the capacitance value of the capacitorFinal design value of kCm、kCnOr kC0。
The reference range of the margin coefficient k is more than or equal to 1.1 and less than or equal to 1.3, or determined according to electromagnetic transient simulation.
Example 2
The embodiment provides a system for determining a capacitance value of a frequency conversion valve submodule for flexible low-frequency power transmission, which is composed of a mathematical model building unit, a capacitance voltage fluctuation rate determining unit and a capacitance value obtaining unit, as shown in fig. 5.
A mathematical model construction unit: and establishing a mathematical model of the relation between the capacitance value of the frequency conversion valve submodule capacitor and the fluctuation rate of the capacitor voltage for flexible low-frequency power transmission.
Capacitance voltage fluctuation ratio determination unit: and selecting an initial value of the capacitance value of the sub-module of the frequency conversion valve, traversing each power point determined by the power circle/power circle cluster of the flexible low-frequency power transmission system according to the established mathematical model, and determining the fluctuation rate of the capacitance voltage of each power point.
A capacitance value acquisition unit: and iterating the capacitance capacity value according to a preset capacitance voltage fluctuation rate target value until the capacitance voltage fluctuation rate of each power point determined by the power circle/power circle cluster of the flexible low-frequency power transmission system is less than or equal to the capacitance voltage fluctuation rate target value, and obtaining a proper margin coefficient to obtain the capacitance capacity value of the frequency conversion valve submodule.
In the mathematical model building unit, the calculation method of the maximum value of the fluctuation rate of the capacitor voltage at a single power point comprises the following steps:
Figure BDA0003455645350000091
wherein, ω is1At industrial frequency angular frequency, omega2For low angular frequencies, Δ u (2 ω)1t) is the frequency-doubled component of power frequency 2, Deltau (2 omega)2t) is the 2-fold frequency component of the low frequency, Δ u (ω)1t+ω2t) is the frequency and component of the power frequency and low frequency, Deltau (omega)1t-ω2T) is the frequency difference component of the power frequency and the low frequency, T is the period after the power frequency component and the low frequency component are superposed, UCNThe submodule capacitor voltage nominal operation value.
The calculation method of each fluctuation frequency component comprises the following steps:
Figure BDA0003455645350000092
Figure BDA0003455645350000093
Δu(ω1t+ω2t)＝f1(ω1t+ω2t)+f2(ω1t+ω2t)+f3(ω1t+ω2t)，
Figure BDA0003455645350000094
Figure BDA0003455645350000095
Figure BDA0003455645350000096
Δu(ω1t-ω2t)＝f1(ω1t-ω2t)+f2(ω1t-ω2t)+f3(ω1t-ω2t)，
Figure BDA0003455645350000097
Figure BDA0003455645350000098
Figure BDA0003455645350000101
Figure BDA0003455645350000102
wherein, P1、Qs1、S1、Vs1、X1Respectively the active power, the reactive power, the apparent power, the bus voltage and the equivalent impedance of the frequency conversion valve at the power frequency bus2、Qs2、S2、Vs2、X2Respectively active power, reactive power, apparent power, bus voltage and equivalent impedance of the frequency conversion valve at a low-frequency bus; n is the number of the single bridge arm sub-modules; c is capacitance value of the capacitor; delta is the initial phase angle difference of the alternating-current voltage of the buses at the power frequency side and the low frequency side of the frequency conversion valve.
In the capacitance voltage fluctuation rate determining unit, the horizontal and vertical coordinates of the power circle/power circle cluster and the horizontal and vertical coordinates of the power circle/power circle cluster are respectively the active power and the reactive power of the power frequency bus, and the power circle/power circle cluster is determined according to the frequency conversion valve parameters, the voltage and the current of the power frequency side bus, and the voltage and the current of the low frequency side bus; the power circle/power circle cluster is a series of sub-power circles or a power circle formed by intersecting and collecting a series of sub-power circles.
Specifically, the sub-power circle is defined by a straight line x ═ P1min、x＝P1maxAnd each power point P1Upper boundary value Q of reactive power under conditionss1pAnd a lower boundary value Qs1nFormed closed figure, P1∈[P1min,P1max]Wherein Q iss1pAnd Qs1nThe calculation method comprises the following steps:
Qs1p＝max(Qs11p，Qs12p，Qs14p，Qs15p)，
Qs1n＝max(Qs11n，Qs12n，Qs13n，Qs14n，Qs15n)，
wherein, P1minAnd P1maxThe maximum value and the minimum value of the active power of the power frequency bus are specified for design; qs11p、Qs11nAre respectively noneThe upper and lower boundary values of the power under the constraint of the modulation degree; qs12p、Qs12nThe upper boundary value and the lower boundary value of the reactive power under the transformer capacity constraint are respectively; qs13nRespectively the lower boundary values of the reactive power under the valve side voltage constraint; qs14p、Qs14nRespectively an upper boundary value and a lower boundary value of the reactive power under the current constraint of the valve side; qs15p、Qs15nThe upper boundary value and the lower boundary value of the reactive power under the constraint of the bridge arm current are respectively.
In the capacitance value obtaining unit, the iterative process is performed according to the following steps, as shown in fig. 4:
step 1: obtaining the initial value C of capacitance value of capacitor0Maximum value epsilon of capacitance voltage fluctuation rate of each power operating point on power circle/power circle cluster under parameter condition0max；
Step 2: if epsilon0maxGreater than the target value epsilon of the fluctuation rate of the capacitor voltagerefIncreasing the capacitance value of the capacitor with the step length of deltaC until the mth iteration to obtain the capacitance value C of the capacitormThe maximum value epsilon of the fluctuation rate of the capacitance voltage is obtainedmaxLess than or equal to the target value epsilon of the fluctuation rate of the capacitor voltageref(ii) a If epsilon0maxLess than the target value epsilon of the fluctuation rate of the capacitor voltagerefDecreasing the capacitance until the nth capacitance value C is obtainednCapacitance voltage fluctuation rate maximum value epsilon obtained through iterationmaxGreater than or equal to the target value epsilon of the fluctuation rate of the capacitor voltageref(ii) a If epsilon0maxEqual to the target value epsilon of the fluctuation rate of the capacitor voltagerefThen stopping iteration;
and step 3: if epsilon0maxGreater than the target value epsilon of the fluctuation rate of the capacitor voltagerefThen C ismThe minimum design value for meeting the requirement of the fluctuation rate of the capacitor voltage; if epsilon0maxLess than the target value epsilon of the fluctuation rate of the capacitor voltagerefThen C isnThe minimum design value for meeting the requirement of the fluctuation rate of the capacitor voltage; if epsilon0maxEqual to the target value epsilon of the fluctuation rate of the capacitor voltagerefThen C is0The minimum design value for meeting the requirement of the fluctuation rate of the capacitor voltage;
and 4, step 4: on the basis of step 3, the capacity electricity will be satisfiedMultiplying the minimum design value required by the pressure fluctuation rate by a proper margin coefficient k to obtain the final design value of the capacitance value of the capacitor, namely kCm、kCnOr kC0。
The reference range of the margin coefficient k is more than or equal to 1.1 and less than or equal to 1.3, or determined according to electromagnetic transient simulation.
The principles and embodiments of the present invention have been explained by applying specific examples, and the above descriptions of the embodiments are only used to help understanding the method and the core idea of the present invention. It should be noted that, for those skilled in the art, it is possible to make various improvements and modifications to the present invention without departing from the principle of the present invention, and those improvements and modifications also fall within the scope of the claims of the present invention.

Claims (10)

1. The method for determining the capacitance value of the frequency conversion valve submodule capacitor for flexible low-frequency power transmission is characterized by comprising the following steps of:
establishing a mathematical model of the relation between the capacitance value of the frequency conversion valve submodule capacitor and the fluctuation rate of the capacitor voltage for flexible low-frequency power transmission;
selecting an initial value of capacitance values of sub-modules of a frequency conversion valve, traversing each power point determined by a power circle/power circle cluster of the flexible low-frequency power transmission system according to the established mathematical model, and determining the fluctuation rate of capacitance voltage of each power point;
and iterating the capacitance capacity value according to a preset capacitance voltage fluctuation rate target value until the capacitance voltage fluctuation rate of each power point determined by the power circle/power circle cluster of the flexible low-frequency power transmission system is less than or equal to the capacitance voltage fluctuation rate target value, and obtaining a proper margin coefficient to obtain the capacitance capacity value of the frequency conversion valve submodule.
2. The method for determining the capacitance and capacitance value of the frequency conversion valve submodule for flexible low-frequency power transmission according to claim 1, wherein in the mathematical model, the method for calculating the maximum value of the fluctuation rate of the capacitance voltage of a single power point comprises the following steps:
Figure FDA0003455645340000011
wherein, ω is1At industrial frequency angular frequency, omega2For low angular frequencies, Δ u (2 ω)1t) is the frequency-doubled component of power frequency 2, Deltau (2 omega)2t) is the 2-fold frequency component of the low frequency, Δ u (ω)1t+ω2t) is the frequency and component of the power frequency and low frequency, Deltau (omega)1t-ω2T) is the frequency difference component of the power frequency and the low frequency, T is the period after the power frequency component and the low frequency component are superposed, UCNThe submodule capacitor voltage nominal operation value.
3. The method for determining the capacitance value of the frequency conversion valve submodule for flexible low-frequency power transmission according to claim 2, wherein the method for calculating each fluctuation frequency component comprises the following steps:
Figure FDA0003455645340000014
Figure FDA0003455645340000012
Δu(ω1t+ω2t)＝f1(ω1t+ω2t)+f2(ω1t+ω2t)+f3(ω1t+ω2t)，
Figure FDA0003455645340000013
Figure FDA0003455645340000021
Figure FDA0003455645340000022
Δu(ω1t-ω2t)＝f1(ω1t-ω2t)+f2(ω1t-ω2t)+f3(ω1t-ω2t)，
Figure FDA0003455645340000023
Figure FDA0003455645340000024
Figure FDA0003455645340000025
Figure FDA0003455645340000026
wherein, P1、Qs1、S1、Vs1、X1Respectively the active power, the reactive power, the apparent power, the bus voltage and the equivalent impedance of the frequency conversion valve at the power frequency bus2、Qs2、S2、Vs2、X2Respectively active power, reactive power, apparent power, bus voltage and equivalent impedance of the frequency conversion valve at a low-frequency bus; n is the number of the single bridge arm sub-modules; c is capacitance value of the capacitor; delta is the initial phase angle difference of the alternating-current voltage of the buses at the power frequency side and the low frequency side of the frequency conversion valve.
4. The method for determining the capacitance value of the frequency conversion valve submodule for flexible low-frequency power transmission according to any one of claims 1-3, wherein the horizontal and vertical coordinates of the power circle/power circle cluster are the active power and the reactive power of a power frequency bus respectively, and the power circle/power circle cluster is determined according to the frequency conversion valve parameters, the power frequency side bus voltage and the power flow, and the low frequency side bus voltage and the power flow; the power circle/power circle cluster is a series of sub-power circles or a power circle formed by intersecting and collecting a series of sub-power circles.
5. The method for determining the capacitance capacity value of the frequency conversion valve submodule for flexible low-frequency power transmission according to claim 4, wherein the sub-power circle is formed by a straight line x-P1min、x＝P1maxAnd each power point P1Upper boundary value Q of reactive power under conditionss1pAnd a lower boundary value Qs1nFormed closed figure, P1∈[P1min,P1max]Wherein Q iss1pAnd Qs1nThe calculation method comprises the following steps:
Qs1p＝max(Qs11p，Qs12p，Qs14p，Qs15p)，
Qs1n＝max(Qs11n，Qs12n，Qs13n，Qs14n，Qs15n)，
wherein, P1minAnd P1maxThe maximum value and the minimum value of the active power of the power frequency bus are specified for design; qs11p、Qs11nRespectively an upper boundary value and a lower boundary value of the reactive power under the constraint of the modulation degree; qs12p、Qs12nThe upper boundary value and the lower boundary value of the reactive power under the transformer capacity constraint are respectively; qs13nRespectively the lower boundary values of the reactive power under the valve side voltage constraint; qs14p、Qs14nRespectively an upper boundary value and a lower boundary value of the reactive power under the current constraint of the valve side; qs15p、Qs15nThe upper boundary value and the lower boundary value of the reactive power under the constraint of the bridge arm current are respectively.
6. The method for determining the capacitance capacity value of the frequency conversion valve submodule for flexible low-frequency power transmission according to any one of claims 1-3, wherein the iteration process is carried out according to the following steps:
step 1: obtaining the initial value C of capacitance value of capacitor0Maximum value epsilon of capacitance voltage fluctuation rate of each power operating point on power circle/power circle cluster under parameter condition0max；
Step 2: if epsilon0maxIs greater thanTarget value epsilon of fluctuation ratio of capacitor voltagerefIncreasing the capacitance value of the capacitor with the step length of deltaC until the mth iteration to obtain the capacitance value C of the capacitormThe maximum value epsilon of the fluctuation rate of the capacitance voltage is obtainedmaxLess than or equal to the target value epsilon of the fluctuation rate of the capacitor voltageref(ii) a If epsilon0maxLess than the target value epsilon of the fluctuation rate of the capacitor voltagerefDecreasing the capacitance until the nth capacitance value C is obtainednCapacitance voltage fluctuation rate maximum value epsilon obtained through iterationmaxGreater than or equal to the target value epsilon of the fluctuation rate of the capacitor voltageref(ii) a If epsilon0maxEqual to the target value epsilon of the fluctuation rate of the capacitor voltagerefThen stopping iteration;
and step 3: if epsilon0maxGreater than the target value epsilon of the fluctuation rate of the capacitor voltagerefThen C ismThe minimum design value for meeting the requirement of the fluctuation rate of the capacitor voltage; if epsilon0maxLess than the target value epsilon of the fluctuation rate of the capacitor voltagerefThen C isnThe minimum design value for meeting the requirement of the fluctuation rate of the capacitor voltage; if epsilon0maxEqual to the target value epsilon of the fluctuation rate of the capacitor voltagerefThen C is0The minimum design value for meeting the requirement of the fluctuation rate of the capacitor voltage;
and 4, step 4: on the basis of the step 3, multiplying the minimum design value meeting the requirement of the fluctuation rate of the capacitance voltage by a proper margin coefficient k to obtain the final design value of the capacitance value as kCm、kCnOr kC0。
7. The method for determining the capacitance capacity value of the frequency conversion valve submodule for flexible low-frequency power transmission according to any one of claims 1-3, wherein the margin coefficient k is within a reference range of 1.1 ≦ k ≦ 1.3, or determined according to electromagnetic transient simulation.
8. Flexible low frequency transmission is with trading frequency valve submodule piece electric capacity value determination system that it features in, includes:
a mathematical model construction unit: establishing a mathematical model of the relation between the capacitance value of the frequency conversion valve submodule capacitor and the fluctuation rate of the capacitor voltage for flexible low-frequency power transmission;
capacitance voltage fluctuation ratio determination unit: selecting an initial value of capacitance values of sub-modules of a frequency conversion valve, traversing each power point determined by a power circle/power circle cluster of the flexible low-frequency power transmission system according to the established mathematical model, and determining the fluctuation rate of capacitance voltage of each power point;
a capacitance value acquisition unit: and iterating the capacitance capacity value according to a preset capacitance voltage fluctuation rate target value until the capacitance voltage fluctuation rate of each power point determined by the power circle/power circle cluster of the flexible low-frequency power transmission system is less than or equal to the capacitance voltage fluctuation rate target value, and obtaining a proper margin coefficient to obtain the capacitance capacity value of the frequency conversion valve submodule.
9. The system for determining the capacitance and capacitance value of the frequency conversion valve submodule for flexible low-frequency power transmission according to claim 8, wherein in the mathematical model, the method for calculating the maximum value of the fluctuation rate of the capacitance voltage at a single power point comprises the following steps:
Figure FDA0003455645340000041
wherein, ω is1At industrial frequency angular frequency, omega2For low angular frequencies, Δ u (2 ω)1t) is the frequency-doubled component of power frequency 2, Deltau (2 omega)2t) is the 2-fold frequency component of the low frequency, Δ u (ω)1t+ω2t) is the frequency and component of the power frequency and low frequency, Deltau (omega)1t-ω2T) is the frequency difference component of the power frequency and the low frequency, T is the period after the power frequency component and the low frequency component are superposed, UCNThe submodule capacitor voltage nominal operation value.
10. The system for determining the capacitance capacity value of the frequency conversion valve submodule for flexible low-frequency power transmission according to claim 8, wherein in the capacitance capacity value obtaining unit, an iterative process is performed according to the following steps:
step 1: obtaining the initial value C of capacitance value of capacitor0Maximum value epsilon of capacitance voltage fluctuation rate of each power operating point on power circle/power circle cluster under parameter condition0max；
Step 2: if epsilon0maxGreater than the target value epsilon of the fluctuation rate of the capacitor voltagerefIncreasing the capacitance value of the capacitor with the step length of deltaC until the mth iteration to obtain the capacitance value C of the capacitormThe maximum value epsilon of the fluctuation rate of the capacitance voltage is obtainedmaxLess than or equal to the target value epsilon of the fluctuation rate of the capacitor voltageref(ii) a If epsilon0maxLess than the target value epsilon of the fluctuation rate of the capacitor voltagerefDecreasing the capacitance until the nth capacitance value C is obtainednCapacitance voltage fluctuation rate maximum value epsilon obtained through iterationmaxGreater than or equal to the target value epsilon of the fluctuation rate of the capacitor voltageref(ii) a If epsilon0maxEqual to the target value epsilon of the fluctuation rate of the capacitor voltagerefThen stopping iteration;
and step 3: if epsilon0maxGreater than the target value epsilon of the fluctuation rate of the capacitor voltagerefThen C ismThe minimum design value for meeting the requirement of the fluctuation rate of the capacitor voltage; if epsilon0maxLess than the target value epsilon of the fluctuation rate of the capacitor voltagerefThen C isnThe minimum design value for meeting the requirement of the fluctuation rate of the capacitor voltage; if epsilon0maxEqual to the target value epsilon of the fluctuation rate of the capacitor voltagerefThen C is0The minimum design value for meeting the requirement of the fluctuation rate of the capacitor voltage;
and 4, step 4: on the basis of the step 3, multiplying the minimum design value meeting the requirement of the fluctuation rate of the capacitance voltage by a proper margin coefficient k to obtain the final design value of the capacitance value as kCm、kCnOr kC0。
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