CN114297874B - 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 PDFInfo
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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 the 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
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 grade conversion and fault current breaking, direct current networking is still difficult because direct current transformers and direct current circuit breakers are not mature in technology and equipment investment is huge.
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 the technical challenges of light weight, miniaturization, high reliability and the like of a converter station are faced. The advantages of flexible low-frequency power transmission are mainly reflected in that: 1) Compared with a power frequency alternating current scheme, the power transmission 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, a capacitance value design method of a submodule of the frequency conversion valve needs to be researched.
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 capacitor by deducing a mathematical theoretical model and combining a low-frequency system operation power circle/power circle cluster aiming at a modular multi-level matrix frequency conversion valve (M3C).
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:
wherein, ω is 1 At industrial frequency angular frequency, omega 2 At a low angular frequency，Δu(2ω 1 t) is the frequency-2 multiplication component of power frequency, delta u (2 omega) 2 t) is the low frequency 2-fold frequency component, Δ u (ω) 1 t+ω 2 t) is the frequency and component of the power frequency and low frequency, delta u (omega) 1 t-ω 2 T) 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, U CN And the sub-module capacitor voltage rated operation value.
Further, the calculation method of each fluctuation frequency component is as follows:
Δu(ω 1 t+ω 2 t)＝f 1 (ω 1 t+ω 2 t)+f 2 (ω 1 t+ω 2 t)+f 3 (ω 1 t+ω 2 t)，
Δu(ω 1 t-ω 2 t)＝f 1 (ω 1 t-ω 2 t)+f 2 (ω 1 t-ω 2 t)+f 3 (ω 1 t-ω 2 t)，
wherein, P 1 、Q s1 、S 1 、V s1 、X 1 Respectively active power, reactive power, apparent power, bus voltage and equivalent impedance of the frequency conversion valve at a power frequency bus 2 、Q s2 、S 2 、V s2 、X 2 Respectively 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 the 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 abscissa and ordinate 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 = P 1min 、x＝P 1max And each power point P 1 Upper boundary value Q of reactive power under the condition s1p And a lower boundary value Q s1n Formed closed figure, P 1 ∈[P 1min ,P 1max ]Wherein Q is s1p And Q s1n The calculation method comprises the following steps:
Q s1p ＝max(Q s11p ，Q s12p ，Q s14p ，Q s15p )，
Q s1n ＝max(Q s11n ，Q s12n ，Q s13n ，Q s14n ，Q s15n )，
wherein, P 1min And P 1max The maximum value and the minimum value of the active power of the power frequency bus are designed; q s11p 、Q s11n Respectively an upper boundary value and a lower boundary value of the reactive power under the constraint of the modulation degree; q s12p 、Q s12n Respectively an upper boundary value and a lower boundary value of the reactive power under the transformer capacity constraint; q s13n Respectively the lower boundary values of the reactive power under the valve side voltage constraint; q s14p 、Q s14n Respectively an upper boundary value and a lower boundary value of the reactive power under the current constraint of the valve side; q s15p 、Q s15n The 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 capacitor 0 Maximum value epsilon of capacitance voltage fluctuation rate of each power operating point on power circle/power circle cluster under parameter condition 0max ；
Step 2: if epsilon 0max Greater than the target value epsilon of the fluctuation ratio of the capacitor voltage ref Increasing the capacitance value of the capacitor with the step length of deltaC until the mth iteration to obtain the capacitance value C of the capacitor m The maximum value epsilon of the fluctuation rate of the capacitance voltage is obtained max Less than or equal to the target value epsilon of the fluctuation rate of the capacitor voltage ref (ii) a If epsilon 0max Less than the target value epsilon of the fluctuation rate of the capacitor voltage ref Decreasing the capacitance value of the capacitor until the nth capacitance value C is obtained n Capacitance voltage fluctuation rate maximum value epsilon obtained through iteration max Is greater than or equal to the target value epsilon of the fluctuation rate of the capacitor voltage ref (ii) a If epsilon 0max Equal to the target value epsilon of the fluctuation ratio of the capacitor voltage ref Then stopping iteration;
and 3, step 3: if epsilon 0max Greater than the target value epsilon of the fluctuation ratio of the capacitor voltage ref Then C is m The minimum design value for meeting the requirement of the fluctuation rate of the capacitor voltage; if epsilon 0max Less than the target value epsilon of the fluctuation rate of the capacitor voltage ref Then C is n The minimum design value for meeting the requirement of the fluctuation rate of the capacitor voltage; if epsilon 0max Equal to the target value epsilon of the fluctuation ratio of the capacitor voltage ref Then C is 0 The 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 kC m 、kC n Or kC 0 。
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 transmission system, a mathematical model of the 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 sub-module capacitance value is iterated, the capacitance voltage fluctuation rates of all power operation points in the range of a power circle/power circle cluster of the frequency conversion valve are not larger than a target value, a final design value of the sub-module capacitance value is determined according to the variation, and the design can be used for guiding engineering design and construction.
Drawings
Fig. 1 is a schematic diagram of a conventional flexible low-frequency power transmission M3C single-ended frequency conversion station;
fig. 2 is an equivalent circuit diagram of a conventional flexible low-frequency power transmission M3C single-ended frequency conversion station;
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 is made with reference to the accompanying drawings.
Fig. 1 is a schematic diagram of a conventional flexible low-frequency transmission M3C single-ended frequency conversion station. 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 formed by cascading N full-bridge sub-modules.
Fig. 2 is an equivalent circuit diagram of a conventional flexible low-frequency power transmission M3C single-ended frequency conversion station. Equivalent impedance X of power frequency valve side 1 Comprising a leakage reactance X of a power frequency transformer T1 Equivalent value X on power frequency side of reactance of frequency conversion valve bridge arm L1 (ii) a Low frequency valve side equivalent impedance X 2 Comprising a low-frequency transformer leakage reactance X T2 Equivalent value X on low-frequency side of reactance of frequency conversion valve bridge arm L2 。
According to different engineering requirements, the industrial 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:
wherein, ω is 1 At industrial frequency angular frequency, omega 2 For low angular frequencies, Δ u (2 ω) 1 t) is the frequency-doubled component of power frequency 2, deltau (2 omega) 2 t) is the low frequency 2-fold frequency component, Δ u (ω) 1 t+ω 2 t) is the frequency and component of the power frequency and low frequency, delta u (omega) 1 t-ω 2 T) 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, U CN The submodule capacitor voltage nominal operation value.
The calculation method of each fluctuation frequency component comprises the following steps:
Δu(ω 1 t+ω 2 t)＝f 1 (ω 1 t+ω 2 t)+f 2 (ω 1 t+ω 2 t)+f 3 (ω 1 t+ω 2 t)，
Δu(ω 1 t-ω 2 t)＝f 1 (ω 1 t-ω 2 t)+f 2 (ω 1 t-ω 2 t)+f 3 (ω 1 t-ω 2 t)，
wherein, P 1 、Q s1 、S 1 、V s1 、X 1 Respectively 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 bus 2 、Q s2 、S 2 、V s2 、X 2 Respectively 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 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 model i ，Q i ) Traversing to determine initial values epsilon of voltage fluctuation rates of capacitors and capacitors at all power points 0 (P i ，Q i ). 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/powerThe circle cluster is determined according to frequency conversion valve parameters, power frequency side bus voltage and power flow, and low frequency side bus voltage and power flow, and the power circle/power circle cluster is a series of sub-power circles or a power circle formed by intersecting and integrating a series of sub-power circles.
The sub-power circle is formed by a straight line x = P 1min 、x＝P 1max And each power point P 1 Upper boundary value Q of reactive power under the condition s1p And a lower boundary value Q s1n Formed closed figure, P 1 ∈[P 1min ,P 1max ]Wherein Q is s1p And Q s1n The calculation method comprises the following steps:
Q s1p ＝max(Q s11p ，Q s12p ，Q s14p ，Q s15p )，
Q s1n ＝max(Q s11n ，Q s12n ，Q s13n ，Q s14n ，Q s15n )，
wherein, P 1min And P 1max The maximum value and the minimum value of the active power of the power frequency bus are specified for design; q s11p 、Q s11n Respectively an upper boundary value and a lower boundary value of the reactive power under the constraint of the modulation degree; q s12p 、Q s12n The upper boundary value and the lower boundary value of the reactive power under the transformer capacity constraint are respectively; q s13n Respectively the lower boundary values of the reactive power under the valve side voltage constraint; q s14p 、Q s14n Respectively an upper boundary value and a lower boundary value of the reactive power under the valve side current constraint; q s15p 、Q s15n The upper and lower boundary values of the reactive power under the constraint of 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 0 Work under parametric conditionsMaximum value epsilon of capacitance voltage fluctuation rate of all power operating points on rate circle/power circle cluster 0max ＝max[ε 0 (P i ，Q i )]。
Step 2: if epsilon 0max Greater than the target value epsilon of the fluctuation rate of the capacitor voltage ref Increasing the capacitance value of the capacitor with the step length of deltaC until the mth iteration to obtain the capacitance value C of the capacitor m The maximum value epsilon of the fluctuation rate of the capacitor voltage is obtained max Less than or equal to the target value epsilon of the fluctuation rate of the capacitor voltage ref (ii) a If epsilon 0max Less than the target value epsilon of the fluctuation ratio of the capacitor voltage ref Decreasing the capacitance until the nth capacitance value C is obtained n Maximum value epsilon of fluctuation rate of capacitor voltage obtained by iteration max Greater than or equal to the target value epsilon of the fluctuation rate of the capacitor voltage ref (ii) a If epsilon 0max Equal to the target value epsilon of the fluctuation rate of the capacitor voltage ref Then no iteration is needed.
And 3, step 3: if epsilon 0max Greater than the target value epsilon of the fluctuation rate of the capacitor voltage ref Then C is m The minimum design value for meeting the requirement of the fluctuation ratio of the capacitor voltage; if epsilon 0max Less than the target value epsilon of the fluctuation ratio of the capacitor voltage ref Then C is n The minimum design value for meeting the requirement of the fluctuation ratio of the capacitor voltage; if epsilon 0max Equal to the target value epsilon of the fluctuation rate of the capacitor voltage ref Then C is 0 The 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 capacitor voltage by a certain margin coefficient k to obtain the final design value kC of the capacitance value of the capacitor m 、kC n Or kC 0 。
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:
wherein, ω is 1 At industrial frequency angular frequency, omega 2 For low angular frequencies, Δ u (2 ω) 1 t) is the frequency-doubled component of power frequency 2, deltau (2 omega) 2 t) is the low frequency 2-fold frequency component, Δ u (ω) 1 t+ω 2 t) is the frequency and component of the power frequency and low frequency, delta u (omega) 1 t-ω 2 T) 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, U CN The submodule capacitor voltage nominal operation value.
The calculation method of each fluctuation frequency component comprises the following steps:
Δu(ω 1 t+ω 2 t)＝f 1 (ω 1 t+ω 2 t)+f 2 (ω 1 t+ω 2 t)+f 3 (ω 1 t+ω 2 t)，
Δu(ω 1 t-ω 2 t)＝f 1 (ω 1 t-ω 2 t)+f 2 (ω 1 t-ω 2 t)+f 3 (ω 1 t-ω 2 t)，
wherein, P 1 、Q s1 、S 1 、V s1 、X 1 Respectively active power, reactive power, apparent power, bus voltage and equivalent impedance of the frequency conversion valve at a power frequency bus 2 、Q s2 、S 2 、V s2 、X 2 Respectively 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 abscissa and ordinate 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 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.
Specifically, the sub-power circle is defined by a straight line x = P 1min 、x＝P 1max And each power point P 1 Upper boundary value Q of reactive power under the condition s1p And a lower boundary value Q s1n Formed closed figure, P 1 ∈[P 1min ,P 1max ]Wherein Q is s1p And Q s1n The calculation method comprises the following steps:
Q s1p ＝max(Q s11p ，Q s12p ，Q s14p ，Q s15p )，
Q s1n ＝max(Q s11n ，Q s12n ，Q s13n ，Q s14n ，Q s15n )，
wherein, P 1min And P 1max The maximum value and the minimum value of the active power of the power frequency bus are designed; q s11p 、Q s11n Respectively an upper boundary value and a lower boundary value of the reactive power under the constraint of the modulation degree; q s12p 、Q s12n The upper boundary value and the lower boundary value of the reactive power under the transformer capacity constraint are respectively; q s13n Respectively the lower boundary values of the reactive power under the valve side voltage constraint; q s14p 、Q s14n Respectively an upper boundary value and a lower boundary value of the reactive power under the current constraint of the valve side; q s15p 、Q s15n The upper and lower boundary values of the reactive power under the constraint of 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 0 Maximum value epsilon of capacitance voltage fluctuation rate of each power operating point on power circle/power circle cluster under parameter condition 0max ；
Step 2: if epsilon 0max Greater than the target value epsilon of the fluctuation rate of the capacitor voltage ref Increasing the capacitance value of the capacitor with the step length of deltaC until the mth iteration to obtain the capacitance value C of the capacitor m The maximum value epsilon of the fluctuation rate of the capacitance voltage is obtained max Less than or equal to the target value epsilon of the fluctuation rate of the capacitor voltage ref (ii) a If epsilon 0max Less than the target value epsilon of the fluctuation ratio of the capacitor voltage ref Decreasing the capacitance until the nth capacitance value C is obtained n Maximum value epsilon of fluctuation rate of capacitor voltage obtained by iteration max Greater than or equal to the target value epsilon of the fluctuation rate of the capacitor voltage ref (ii) a If epsilon 0max Equal to the target value epsilon of the fluctuation rate of the capacitor voltage ref Then stopping iteration;
and step 3: if epsilon 0max Greater than the target value epsilon of the fluctuation rate of the capacitor voltage ref Then C is m The minimum design value for meeting the requirement of the fluctuation rate of the capacitor voltage; if epsilon 0max Less than the target value epsilon of the fluctuation ratio of the capacitor voltage ref Then C is n The minimum design value for meeting the requirement of the fluctuation rate of the capacitor voltage; if epsilon 0max Equal to the target value epsilon of the fluctuation rate of the capacitor voltage ref Then C is 0 The minimum design value for meeting the requirement of the fluctuation ratio 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 kC m 、kC n Or kC 0 。
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 principle and the embodiment of the present invention are explained by applying a specific example, and the above description of the embodiment is 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 the 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:
wherein, ω is 1 At industrial frequency angular frequency, omega 2 For low angular frequencies, Δ u (2 ω) 1 t) is the frequency-doubled component of power frequency 2, deltau (2 omega) 2 t) is the 2-fold frequency component of the low frequency, Δ u (ω) 1 t+ω 2 t) is the frequency and component of the power frequency and low frequency, deltau (omega) 1 t-ω 2 T) 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, U CN And the sub-module capacitor voltage rated operation value.
3. The method for determining the capacitance capacity 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 is as follows:
Δu(ω 1 t+ω 2 t)＝f 1 (ω 1 t+ω 2 t)+f 2 (ω 1 t+ω 2 t)+f 3 (ω 1 t+ω 2 t)，
Δu(ω 1 t-ω 2 t)＝f 1 (ω 1 t-ω 2 t)+f 2 (ω 1 t-ω 2 t)+f 3 (ω 1 t-ω 2 t)，
wherein, P 1 、Q s1 、S 1 、V s1 、X 1 Respectively 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 bus 2 、Q s2 、S 2 、V s2 、X 2 Respectively 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 value of the frequency conversion valve submodule for flexible low-frequency power transmission according to claim 4, wherein the sub-power circle is defined by a straight line x = P 1min 、x＝P 1max And each power point P 1 Upper boundary value Q of reactive power under conditions s1p And a lower boundary value Q s1n Formed closed figure, P 1 ∈[P 1min ,P 1max ]Wherein Q is s1p And Q s1n The calculation method comprises the following steps:
Q s1p ＝max(Q s11p ，Q s12p ，Q s14p ，Q s15p )，
Q s1n ＝max(Q s11n ，Q s12n ，Q s13n ，Q s14n ，Q s15n )，
wherein, P 1min And P 1max The maximum value and the minimum value of the active power of the power frequency bus are specified for design; q s11p 、Q s11n Respectively an upper boundary value and a lower boundary value of the reactive power under the constraint of the modulation degree; q s12p 、Q s12n The upper boundary value and the lower boundary value of the reactive power under the transformer capacity constraint are respectively; q s13n Respectively the lower boundary values of the reactive power under the valve side voltage constraint; q s14p 、Q s14n Respectively an upper boundary value and a lower boundary value of the reactive power under the valve side current constraint; q s15p 、Q s15n The 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 capacitor 0 Maximum value epsilon of capacitance voltage fluctuation rate of each power operating point on power circle/power circle cluster under parameter condition 0max ；
And 2, step: if epsilon 0max Greater than the target value epsilon of the fluctuation rate of the capacitor voltage ref Increasing the capacitance value of the capacitor with the step length of deltaC until the mth iteration to obtain the capacitance value C of the capacitor m The maximum value epsilon of the fluctuation rate of the capacitor voltage is obtained max Less than or equal to the target value epsilon of the fluctuation rate of the capacitor voltage ref (ii) a If epsilon 0max Less than the target value epsilon of the fluctuation rate of the capacitor voltage ref Decreasing the capacitance until the nth capacitance value C is obtained n Capacitance voltage fluctuation rate maximum value epsilon obtained through iteration max Greater than or equal to the target value epsilon of the fluctuation rate of the capacitor voltage ref (ii) a If epsilon 0max Equal to the target value epsilon of the fluctuation ratio of the capacitor voltage ref Stopping iteration;
and 3, step 3: if epsilon 0max Greater than the target value epsilon of the fluctuation ratio of the capacitor voltage ref Then C is m The minimum design value for meeting the requirement of the fluctuation rate of the capacitor voltage; if epsilon 0max Less than the target value epsilon of the fluctuation rate of the capacitor voltage ref Then C is n The minimum design value for meeting the requirement of the fluctuation rate of the capacitor voltage; if epsilon 0max Equal to the target value epsilon of the fluctuation rate of the capacitor voltage ref Then C is 0 The 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 kC m 、kC n Or kC 0 。
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 to 3, wherein a margin coefficient k is within a reference range of 1.1-1.3 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 the 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 the capacitance capacity value of the frequency conversion valve submodule by taking a proper margin coefficient.
9. 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 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:
wherein, ω is 1 At industrial frequency angular frequency, omega 2 For low angular frequencies, Δ u (2 ω) 1 t) is the frequency-2 multiplication component of power frequency, delta u (2 omega) 2 t) is the 2-fold frequency component of the low frequency, Δ u (ω) 1 t+ω 2 t) is the frequency and component of the power frequency and low frequency, deltau (omega) 1 t-ω 2 T) 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, U CN The 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 0 Maximum value epsilon of capacitance voltage fluctuation rate of each power operating point on power circle/power circle cluster under parameter condition 0max ；
Step 2: if epsilon 0max Greater than the target value epsilon of the fluctuation rate of the capacitor voltage ref Increasing the capacitance value of the capacitor with the step length of deltaC until the mth iteration to obtain the capacitance value C of the capacitor m The maximum value epsilon of the fluctuation rate of the capacitance voltage is obtained max Less than or equal to the target value epsilon of the fluctuation rate of the capacitor voltage ref (ii) a If epsilon 0max Less than the target value epsilon of the fluctuation rate of the capacitor voltage ref Decreasing the capacitance until the nth capacitance value C is obtained n Maximum value epsilon of fluctuation rate of capacitor voltage obtained by iteration max Greater than or equal to the capacitor voltage fluctuationTarget rate value epsilon ref (ii) a If epsilon 0max Equal to the target value epsilon of the fluctuation rate of the capacitor voltage ref Then stopping iteration;
and step 3: if epsilon 0max Greater than the target value epsilon of the fluctuation rate of the capacitor voltage ref Then C is m The minimum design value for meeting the requirement of the fluctuation rate of the capacitor voltage; if epsilon 0max Less than the target value epsilon of the fluctuation rate of the capacitor voltage ref Then C is n The minimum design value for meeting the requirement of the fluctuation rate of the capacitor voltage; if epsilon 0max Equal to the target value epsilon of the fluctuation ratio of the capacitor voltage ref Then C is 0 The 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 kC m 、kC n Or kC 0 。
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