CN115663876A - Method and system for designing main loop parameters of hybrid cascade extra-high voltage direct current system - Google Patents
Method and system for designing main loop parameters of hybrid cascade extra-high voltage direct current system Download PDFInfo
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Abstract
The invention relates to the field of direct-current power transmission, and provides a method and a system for designing main loop parameters of a hybrid cascade extra-high voltage direct-current system, wherein the method comprises the following steps: determining the operation mode of the hybrid cascade extra-high voltage direct current system; determining a basic control mode of a receiving-end VSC and an LCC, a voltage distribution mode of the VSC and the LCC and a current distribution mode among the VSCs, and constructing a steady-state mathematical model of the hybrid cascade system under the obtained constraint equation; calculating the port direct-current voltage variation range of the VSC in each operation mode, and designing the modulation ratio range in each operation mode; solving a PQ operation interval of the VSC based on a constraint condition of the PQ operation interval of the VSC, and determining a gear range; determining the maximum operation power of the hybrid cascade extra-high voltage direct current system in the operation mode; and solving a steady-state mathematical model of the hybrid cascade system based on the maximum operating power and VSC reactive power of the system to obtain steady-state operating parameters of the system.
Description
Technical Field
The invention relates to a method and a system for designing main loop parameters of a hybrid cascade extra-high voltage direct current system, and relates to the field of direct current power transmission.
Background
In order to realize long-distance large-capacity power transmission and multi-drop power supply and solve the problem of reduction of the receiving end multi-feed-in short circuit ratio, a hybrid cascade extra-high voltage direct current power transmission technology can be adopted, namely a technical scheme of cascade connection of a conventional direct current converter and a plurality of flexible direct current converters.
The purpose of the main loop parameter design of the hybrid cascade extra-high voltage direct current system is to determine the key parameters of direct current main equipment, to determine the steady-state operation parameters of the direct current system under various operation conditions, to be the premise and the basis of the construction of the hybrid cascade direct current system, and to directly determine the transient steady-state operation performance of the system.
The prior art does not relate to the problems of how to design parameters of a hybrid cascade extra-high voltage direct-current transmission main loop, complete the selection of main equipment parameters of a hybrid cascade system, obtain steady-state operation characteristics and the like.
Disclosure of Invention
In view of the above problems, an object of the present invention is to provide a method, a system, a device and a medium for designing main loop parameters of a hybrid cascaded extra-high voltage dc system, which can determine parameters of a dc main device and steady-state operation parameters.
In order to solve the technical problems, the invention adopts the technical scheme that:
in a first aspect, the invention provides a method for designing parameters of a main loop of a hybrid cascade extra-high voltage direct current system, comprising the following steps:
determining the operation mode of the hybrid cascade extra-high voltage direct current system;
determining a basic control mode of a receiving-end VSC and an LCC, a voltage distribution mode of the VSC and the LCC and a current distribution mode among the VSCs, and constructing a steady-state mathematical model of the hybrid cascade system under the obtained constraint equation;
calculating the port direct-current voltage variation range of the VSC in each operation mode, and designing the modulation ratio range in each operation mode;
solving a PQ operation interval of the VSC based on a constraint condition of the PQ operation interval of the VSC, and determining a gear range required to be configured by the flexible converter;
determining the maximum operation power of the hybrid cascade extra-high voltage direct current system in the operation mode;
and solving a steady-state mathematical model of the hybrid cascade system based on the maximum operating power and VSC reactive power of the system to obtain steady-state operating parameters of the system.
Further, the voltage distribution mode of the VSC and the LCC includes:
the VSC and the LCC are distributed in a mode of equal direct current voltage, and the corresponding constraint equation isU dcvsc =U dclcc ，U dcvsc AndU dclcc the direct current voltages of the VSC and the LCC are respectively port direct current voltages;
the VSC does not consider the fluctuation of direct current voltage, and the LCC bears the line voltage drop, and the corresponding constraint equation isU dcvsc =U setvsc ，U dcvsc Is the port dc voltage of the VSC,U setvsc is a set value.
Further, the current distribution mode among the VSCs includes:
the VSCs are distributed in an equal current mode, and then the constraint equation is as follows:I vsc1 =I vsc2 =...I vscn =I dc /n，I vsc1 ，...I vscn the dc currents of the 1 st to nth VSCs respectively,I dc is the total current flowing into the VSC valve pack;
and independently appointing the running power among the VSCs, and then the constraint equation is as follows:I vsc1 =P vsc1 /U setvsc ，I vsc2 =P vsc2 /U setvsc ， ... I vscn =I dc -I vsc1 -I vsc2... I vsc（n-1） ，P vsc1 andP vsc2 is the active power of VSC1 and VSC 2.
Further, the method for determining the modulation ratio range in each operation mode comprises the following steps:
calculating the minimum port voltage of the flexible-straight line in the operation mode according to the principle that the line resistance is maximumU dcinit ；
Traversing the working conditions of VSC power transition under various faults, calculating the port voltage after VSC power transition under each working condition, and determining the minimum port voltage under each working conditionU dcfinal ；
Determining a maximum modulation ratio that ensures that the VSC output voltage is not over-modulatedM absmax Calculating the maximum modulation ratio in each operation modeM max ，M max =U dcfinal ×M absmax /U dcinit ；
Designing the minimum modulation ratio according to the number of levels capable of meeting the harmonic requirement of the systemM min 。
Further, solving the PQ operation interval of the VSC includes:
a. determining limiting conditions of a PQ operation interval of the VSC, wherein the limiting conditions comprise a port direct-current voltage change range, a modulation ratio range, an alternating-current voltage change range, bridge arm current limitation and transformer capacity limitation of the VSC;
b. based on constraint conditions, solving a PQ operation interval, wherein the PQ operation interval is an intersection of different direct-current voltage and alternating-current voltage combinations;
c. judging whether the power interval meets the set requirement of the system on power exchange, if so, determining a PQ operation interval and a VCS gear, and if not, entering d;
d. if insufficient reactive power is sent out or the bridge arm current causes power limitation, the negative gear configuration is increased; and if the absorption reactive power is insufficient, increasing the positive gear configuration and entering the step b.
Further, the PQ operation interval is designed according to the union of PQ operation intervals under all gears, when a single gear cannot meet the system requirement, the configuration range of the gears is gradually increased to increase the PQ operation interval until the gear configuration meets the requirement of the system on the exchange capacity of the VSC under the operation mode, or the gear configuration is increased until the exchange power of the VSC and the system cannot be increased, namely the PQ operation interval under the operation mode is obtained, and the union of gears obtained under the operation modes is the gear designed by the VSC.
Further, a method of increasing a range of gear configurations, comprising: when the current limitation of the bridge arm causes insufficient absorption or emission of active power or reactive power, the negative gear configuration is increased, and when the modulation ratio limitation causes insufficient absorption of reactive power, the positive gear configuration is increased; when the modulation ratio limitation results in insufficient reactive power being emitted, the negative gear configuration is increased.
In a second aspect, the present invention provides a system for designing parameters of a main loop of a hybrid cascade extra-high voltage dc system, including:
the operation mode determining unit is configured to determine the operation mode of the hybrid cascade extra-high voltage direct current system;
the model building unit is configured to determine basic control modes of the VSC and the LCC at the receiving end, voltage distribution modes of the VSC and the LCC and current distribution modes among the VSCs, and build a steady-state mathematical model of the hybrid cascade system under a set constraint equation;
the parameter determination unit is configured to calculate a port direct-current voltage variation range of the VSC under each operation mode and design a modulation ratio range under each operation mode; solving a PQ operation interval of the VSC based on a constraint condition of the PQ operation interval of the VSC, and determining a gear range required to be configured by the flexible direct current converter; determining the maximum operation power of the hybrid cascade extra-high voltage direct current system in the operation mode;
and the model calculation unit is configured to solve a steady-state mathematical model of the hybrid cascade system based on the system operating power and the VSC reactive power to obtain steady-state operating parameters of the system.
In a third aspect, the present invention provides an electronic device, including computer program instructions, where the program instructions, when executed by a processor, are configured to implement the method for designing the main loop parameters of the hybrid cascaded extra-high voltage dc system.
In a fourth aspect, the present invention provides a computer-readable storage medium, where computer program instructions are stored on the computer-readable storage medium, where the program instructions, when executed by a processor, are configured to implement the method for designing the main loop parameters of the hybrid cascaded extra-high voltage direct current system.
Due to the adoption of the technical scheme, the invention has the following characteristics:
1. the steady-state mathematical model of the hybrid cascade system is constructed, and the steady-state mathematical model of the hybrid cascade system is solved within the determined system operation power and VSC reactive power to obtain the steady-state operation parameters of the system, so that the design scheme of the main loop provided by the invention can ensure that the system still has full-power operation capability after a single flexible direct current converter exits, the forced capability unavailability of the hybrid cascade system is reduced to 0.375% from 0.5% of conventional direct current, and the reliability of the system is effectively improved.
2. The invention provides a differential design method for modulation ratios of different operation modes for the first time, and effectively solves the problem of power band-switching limitation of a hybrid cascade system caused by insufficient voltage-reducing operation capacity of VSC.
In conclusion, the invention firstly provides a main loop design scheme of the hybrid cascade direct current system, fills the blank of the prior art, and can be widely applied to the main loop parameters of the hybrid cascade extra-high voltage direct current system.
Drawings
Various other advantages and benefits will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiments. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention. Like reference numerals refer to like parts throughout the drawings. In the drawings:
FIG. 1 is an overall flow chart of parameter design of a hybrid cascade extra-high voltage direct current main loop according to an embodiment of the invention.
Fig. 2 is a flow chart of a hybrid cascade extra-high voltage direct current VSC modulation ratio range design according to the embodiment of the invention.
Fig. 3 is a flow chart of a hybrid cascade extra-high voltage direct current VSC operating interval and gear design according to the embodiment of the present invention.
Fig. 4 is a topology structure diagram of a hybrid cascade extra-high voltage dc system according to an embodiment of the present invention.
Fig. 5 is a block diagram of an electronic device according to an embodiment of the invention.
Detailed Description
It is to be understood that the terminology used herein is for the purpose of describing particular example embodiments only, and is not intended to be limiting. As used herein, the singular forms "a", "an" and "the" may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms "comprises," "comprising," "includes," "including," and "having" are inclusive and therefore specify the presence of stated features, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order described or illustrated, unless specifically identified as an order of performance. It should also be understood that additional or alternative steps may be used.
For convenience of description, spatially relative terms, such as "inner", "outer", "lower", "upper", and the like, may be used herein to describe one element or feature's relationship to another element or feature as illustrated in the figures. Such spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures.
The method for designing the parameters of the main loop of the hybrid cascade extra-high voltage direct current is greatly different from the conventional direct current and flexible direct current projects, the operation capability of the hybrid cascade extra-high voltage direct current is limited by various factors such as a conventional direct current converter valve, a flexible direct current converter valve, the number of flexible direct current operations, the fault ride-through capability of a system and the like, and the system operation capability under hundreds of modes considering different flexible direct current operation numbers needs to be obtained by integrating the transient steady state operation characteristics of the system. In addition, the flexible direct current firstly shows wide-range fluctuation of direct current voltage from 0.7 to 1pu, the design of a power operation interval of the flexible direct current needs to mainly consider the factor of the wide-range fluctuation of the direct current voltage, and a single active power-reactive power diagram (PQ diagram) is difficult to adapt to various operation modes. Meanwhile, the configuration of the voltage regulating switch of the flexible direct current is also the key point of the design of main loop parameters, and the flexible direct current voltage regulating switch needs to be designed together with a modulation ratio to ensure the operation capacity of the direct current voltage under the large-range fluctuation.
Based on the above design thought, the method for designing the main loop parameters of the hybrid cascade extra-high voltage direct current system provided by this embodiment includes: determining the operation mode of the hybrid cascade extra-high voltage direct current system; determining a basic control mode of a receiving-end VSC and an LCC, a voltage distribution mode of the VSC and the LCC and a current distribution mode among the VSCs, and constructing a steady-state mathematical model of the hybrid cascade system under the obtained constraint equation; calculating the port direct-current voltage variation range of the VSC in each operation mode, and designing the modulation ratio range in each operation mode; solving a PQ operation interval of the VSC based on a constraint condition of the PQ operation interval of the VSC, and determining a gear range required to be configured by the flexible converter; determining the maximum operation power of the hybrid cascade extra-high voltage direct current system in the operation mode; and solving a steady-state mathematical model of the hybrid cascade system in the determined system operating power and VSC reactive power to obtain steady-state operating parameters of the system.
Exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. While exemplary embodiments of the invention are shown in the drawings, it should be understood that the invention can be embodied in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.
As shown in fig. 1, the method for designing parameters of a main loop of a hybrid cascade extra-high voltage dc system provided in this embodiment includes:
s1, determining an operation mode of the hybrid cascade extra-high voltage direct current system according to system requirements and topological characteristics of the hybrid cascade extra-high voltage direct current.
In this embodiment, the operation modes of the hybrid cascade extra-high voltage direct current are classified, and firstly, according to a conventional operation mode classification method of the extra-high voltage direct current, combinations of bipolar/monopolar earth/monopolar metal, a double valve group/single valve group, and a valve group are considered, wherein 45 kinds of VSC (voltage source converter) valve groups are considered in an integrated manner, the operation numbers of VSCs with different positive and negative poles and the operated VSC valves are further considered, and the operation modes included in each subclass can be determined in a combined manner.
Furthermore, when the VSC and the LCC (power grid phase-change converter) are subjected to power flow reversal, the voltage can not be reversed and the current can not be reversed respectively, so that the hybrid cascade extra-high voltage direct current is determined by the topological characteristic of the hybrid cascade without considering power reversal. The method comprises the steps that a power mutual-aid mode among a plurality of VSCs is configured according to system requirements, namely, part of VSCs operate in a rectification state, part of VSCs operate in an inversion state, and the VSCs in the inversion operation receive power of sending ends LCCs and the VSCs in the rectification state. The step-down operation is realized by switching to half-voltage operation, and the step-down operation of a double-valve group is avoided by normal and flexible direct asymmetric step-down, so that the configuration and the action frequency of the tap switch are reduced, the equipment safety risk caused by frequent action of the tap switch is greatly reduced, and the problem of serious limitation of power transfer caused by LCC withdrawal during asymmetric step-down is avoided.
And S2, determining a basic control mode of the VSC and the LCC at the receiving end, a voltage distribution mode of the VSC and the LCC and a current distribution mode among the VSCs according to the topological structure of the hybrid cascade, and setting up a constraint equation to construct a steady-state mathematical model of the hybrid cascade system.
In this embodiment, the voltage distribution manners of the VSC and the LCC include two types:
the first mode is as follows: the VSC and the LCC are distributed in a mode of equal direct current voltage, and the corresponding constraint equation isU dcvsc =U dclcc ，U dcvsc AndU dclcc the direct current voltages of the VSC and the LCC are respectively port direct current voltages;
the second mode is as follows: the VSC does not consider the fluctuation of direct-current voltage, and the LCC bears the line voltage drop, and the corresponding constraint equation isU dcvsc =U setvsc ，U dcvsc Is the port dc voltage of the VSC,U setvsc is a set value。
Under the first mode, the fluctuation range of the voltage of the port of the VSC is large, and the VSC needs to be configured with more gears. In the second mode, the fluctuation of the port voltage of the LCC is 2 times of that of the original port voltage, the LCC gears are configured more, and under the mode that the voltage drop is larger due to the return of unipolar metal and the like, when the gear is adjusted to the limit, the turn-off angle needs to be increased, the converter valve is increased in a reactive mode, and due to the fact that the LCC and the VSC operate asymmetrically, if the LCC exits due to failure, the voltage difference of the transmitting end and the receiving end is huge, and the power transfer capacity is severely limited. According to the system-friendly principle, the first mode is adopted when the VSC gear configuration is not limited.
In this embodiment, the current distribution manner among the VSCs includes two types:
the first mode is as follows: the VSCs are distributed in an equal current mode, and then the constraint equation is as follows:I vsc1 =I vsc2 =...I vscn =I dc /n，I vsc1 ，...I vscn the direct currents of the 1 st to nth VSCs respectively,I dc is the total current flowing into the VSC valve pack;
the second mode is as follows: and independently appointing the running power among the VSCs, and then the constraint equation is as follows:I vsc1 =P vsc1 /U setvsc ，I vsc2 =P vsc2 /U setvsc ， ... I vscn =I dc -I vsc1 -I vsc2... I vsc（n-1） ，P vsc1 andP vsc2 is the active power of the VSC1 and VSC 2.
And S3, solving the port direct-current voltage variation range of the VSC under each operation mode according to the maximum line resistance, the maximum operation current, the minimum line resistance and the minimum operation current, calculating the VSC voltage speed reduction requirement meeting various belt-turning power requirements under each operation mode and designing the modulation ratio range under each operation mode.
In this embodiment, the range of the dc voltage variation of the VSC port under each operation mode is determined to be [ [ solution ] ]U dcmin 、U dcmax ]。
In this embodiment, as shown in fig. 2, the method for determining the modulation ratio range in each operation mode is as follows:
calculating the minimum port voltage of the flexible-straight line in the operation mode according to the principle that the line resistance is maximumU dcinit ；
Traversing the working conditions of the VSC power under various faults, calculating the port voltage after the VSC power is transferred under various working conditions, and determining the minimum port voltage under various working conditionsU dcfinal ；
Determining a maximum modulation ratio that ensures that the VSC output voltage is not over-modulatedM absmax Calculating the maximum modulation ratio in each operation modeM max ，M max =U dcfinal ×M absmax /U dcinit Wherein, in the process,U dcfinal for the post-fault dc voltage under the operating condition of maximum voltage drop caused by the belt-turning power,U dcinit for the direct current voltage before the fault under the working condition of the maximum voltage drop caused by the belt turning power,M absmax in order to ensure the maximum modulation ratio of the flexible direct current output voltage without over modulation, if third harmonic injection is adopted, the maximum modulation ratio is generally between 1.05 and 1.15.
Designing the minimum modulation ratio according to the number of levels capable of meeting the harmonic requirement of the systemM min Generally, it is 0.7 to 0.85.
Further, different modulation ratio ranges can be respectively designed for each operation mode, in order to simplify a control protection strategy, the 45 operation modes can be further divided into 2 to 3 types according to the modulation ratio range required by power transfer, and a unified modulation ratio range is determined by the intersection of the modulation ratio ranges meeting the power transfer requirements of all the operation modes.
And S4, based on the determined port direct-current voltage and direct current of the LCC and the VSC, respectively and independently calculating equipment parameters such as the converter transformer rated voltage, rated capacity and short-circuit impedance of the LCC and the VSC according to the main equipment parameter selection method of the LCC and the VSC.
S5, solving a PQ operation interval of the VSC based on a constraint condition of the PQ operation interval of the VSC, and determining a gear range required to be configured by the flexible converter;
in this embodiment, as shown in fig. 3, solving the PQ operation interval of VSC includes:
s51, determining limiting conditions of a PQ operation interval of the VSC, wherein the limiting conditions comprise a port direct-current voltage change range, a modulation ratio range, an alternating-current voltage change range, bridge arm current limitation, transformer capacity limitation and other limiting conditions of the VSC;
s52, solving a PQ operation interval based on constraint conditions, wherein the PQ operation interval is an intersection under different direct-current voltage and alternating-current voltage combinations;
s53, judging whether the power interval meets the set requirement of the system on power exchange, if so, determining a PQ operation interval and a VCS gear, and if not, entering S54;
s54, if insufficient reactive power is generated or the bridge arm current causes power limitation, increasing negative gear configuration; if the absorption reactive power is insufficient, the positive gear allocation is increased, and the process proceeds to step S52.
Further, the PQ operation interval is designed according to the union of the PQ operation intervals at all gears, and when a single gear cannot meet system requirements, the configuration range of the gears is gradually increased to increase the PQ operation interval. And obtaining a PQ operation interval under the operation mode until the gear configuration meets the requirement of the system on the exchange capacity of the VSC under the operation mode, or increasing the gear and cannot increase the exchange power of the VSC and the system, wherein the union of the gears obtained under the operation modes is the gear designed by the VSC.
Further, different PQ operation intervals can be designed for each operation mode, in order to simplify a control protection strategy, 45 operation modes can be further divided into 2 to 3 types, each type provides a unified PQ operation interval meeting all the operation modes in the type, and each unified PQ operation interval is the intersection of all the operation mode PQ operation intervals in the type.
Further, the method for increasing the configuration range of the gear is as follows: when the current limitation of the bridge arm causes insufficient absorption or emission of active power or reactive power, the negative gear configuration is increased, and when the modulation ratio limitation causes insufficient absorption of reactive power, the positive gear configuration is increased; when the modulation ratio limitation results in insufficient reactive power being emitted, the negative gear configuration is increased.
S6, determining the maximum operation power corresponding to the VSC according to the PQ operation intervalP vscmax1 Calculating the maximum operation power of the corresponding rectification side under the maximum operation power, namely the maximum operation power of the systemP max1 And the calculation is performed according to the minimum line resistance.
S7, establishing an electromagnetic transient simulation model of the hybrid cascade system based on PSCAD/EMTDC software, and calculating the maximum system operating power meeting the AC fault ride-through requirement under each operating mode through simulation scanningP max2 During calculation, the system power is set as the system rated value, the overvoltage of the converter valve and the energy of the controllable self-recovery energy dissipater during the fault of the alternating current system are calculated in a simulation mode, and if the overvoltage and the energy of the controllable self-recovery energy dissipater are not out of limit, the maximum power is obtainedP max2 I.e. the rated power of the system, otherwise it is stepped downP max2 Until the requirements of converter valve overpressure and energy dissipater energy can be met.
In this embodiment, a detailed controllable self-recovery energy dissipater model is established in the PSCAD/EMTDC software, and the principle of meeting the fault ride-through requirement of the ac system is as follows: the module voltage does not exceed the overvoltage protection fixed value of the converter valve in unlocking operation, the bridge arm current does not reach the protection fixed value, and the energy of the energy dissipater is within the maximum design energy of the energy dissipater. Wherein, when the energy absorber arrester is of big characteristic and small characteristic, simulation needs to be respectively carried out.
S8, determining the maximum operation power of the system under the operation modeP max =max(P max1 ，P max2 ). If the maximum operating power is less than the rated operating power of the system, power limiting treatment is required, or the maximum operating power is further increased by adopting an IGBT device with larger current capacity, increasing the number of modules, increasing the capacitance of the modules and the likeP max1 AndP max2 until the system power requirements are met.
And S9, solving a steady mathematical model of the constructed hybrid cascade system in the finally determined system running power and VSC reactive power, and calculating the running characteristics of the running power from 0.1pu to 1pu in each running mode to form steady running parameters of the system.
In this embodiment, in each operation mode, a newton-raphson method is used to solve the steady-state mathematical model.
The application of the method for designing the main loop parameters of the hybrid cascade extra-high voltage direct current system provided by the invention is explained in detail through a specific embodiment.
As shown in fig. 4, the sending end of the hybrid cascaded extra-high voltage direct current transmission system adopts a conventional extra-high voltage direct current topology, and each pole is formed by cascading 2 twelve-pulse conventional direct current converters; the receiving end adopts a mixed cascade extra-high voltage direct current topology, each pole is formed by cascading a high-voltage end (namely 800kV to 400kV) twelve-pulse conventional direct current converter and a plurality of (shown in the figure as 3) flexible direct current converters with low-voltage ends (400 kV to neutral lines) connected in parallel, the flexible direct current converters adopt half-bridge type modular multilevel converters, and the receiving end conventional direct current converters and each flexible direct current converter are fed into different alternating current buses.
The invention is further described in detail by taking the design of main loop parameters of a +/-800 kV/8000MW receiving end single LCC and 3 VSC hybrid cascade extra-high voltage direct current transmission system as a specific embodiment.
1. Firstly, determining the operation mode of the hybrid cascade extra-high voltage direct current system according to the system requirements and the topological characteristics of the hybrid cascade extra-high voltage direct current.
Specifically, the operation modes of the hybrid cascade extra-high voltage direct current are divided into 7 categories and 45 categories according to the conventional extra-high voltage direct current, the operation number of VSCs with different anodes and cathodes is considered to be 1, 2 or 3, and the operation modes included in each category are determined in a combined mode to be 621 in total.
2. And determining a basic control mode, a voltage distribution mode and a current distribution mode of the VSC and the LCC of the receiving end according to the topological structure of the hybrid cascade, and constructing a steady-state mathematical model of the hybrid cascade system.
Specifically, a sending end LCC adopts constant direct current control, a receiving end LCC adopts constant direct current voltage control, 1 of 3 VSCs of a receiving end adopts constant direct current voltage control, and 2 adopt constant active power control.
And determining the voltage distribution mode of the VSC and the LCC, for example, adopting a high-low voltage equalization mode.
The current distribution between the VSCs is determined, for example in terms of 3 VSC current sharing.
And then obtaining a constraint equation as follows:I dc =I set ，U dcvsc =U dclcc =U set ，I vsc1 =I vsc2 =I vsc3 and finally, constructing a steady-state mathematical model of the hybrid cascade system:
in the formula (I), the compound is shown in the specification,U dR andU dI for the inter-terminal voltage of the single inverter of the transmitting and receiving LCC,R eq is the equivalent resistance of the dc loop,U di0R andU di0I the ideal no-load dc voltage for each 6-pulse converter on the rectifying side and the inverting side respectively,U di0NR andU di0NI the ideal no-load dc voltage rating for each 6-pulse converter on the rectifying side and the inverting side respectively,αis a trigger angle of a rectification side, gamma is a turn-off angle of an inversion side,d xR andd xI the inductance voltage drop per-unit values of the rectifying side and the inverting side are respectively,d rR andd rI the per-unit values of the resistance voltage drop on the rectifying side and the inverting side respectively,I dN for the purpose of a dc current rating, the dc current rating,U T is an inherent voltage drop of a 6 pulse inverter,P vscn andQ vscn the active power and the reactive power of the nth VSC,M n for the modulation ratio of the nth VSC,U sn andU cn the ac bus voltage and the inverter output voltage of the nth VSC,Xis the equivalent reactance between the ac bus of the VSC and the converter,δand n is the phase angle difference between the alternating-current bus voltage of the nth VSC and the output voltage of the converter.
3. Solving the port direct-current voltage variation range of the VSC under each operation mode according to the maximum line resistance, the maximum operation current, the minimum line resistance and the minimum operation current respectively, and designing the maximum modulation ratio under each operation modeM max Minimum modulation ratioM min 。
In this embodiment, the rated resistance is 10 Ω, the maximum resistance is 12 Ω, the minimum line resistance is 8 Ω, the maximum operating current is 5kA, and the minimum short-circuit current is 0.5kA. In the mode, under various belt-transferring working conditions, the working condition of the maximum voltage drop possible in flexible and direct mode is that the single valve group exits, and the voltage of the VSC needs to be reduced to 280kV from 340kV at the moment. Considering that the maximum modulation ratio that ensures no modulation is 1.05, the maximum modulation ratio in this manner is further determined to be 0.86.
4. And based on the determined port direct-current voltage and direct current of the LCC and the VSC, respectively and independently calculating equipment parameters such as the converter transformer rated voltage, rated capacity, short-circuit impedance and the like of the LCC and the VSC according to the method for selecting the main equipment parameters of the LCC and the VSC.
In this embodiment, under a rated working condition, the port direct-current voltage of the sending-end LCC is 400kV, and the direct-current is 5kA; the port direct-current voltage of the receiving end LCC is 375kV, and the direct-current is 5kA; the port direct-current voltage of the receiving-end VSC is 375kV, the direct current is 5kA, and basic equipment parameters of the LCC and the VSC are calculated according to a traditional method.
5. Based on the restrictions of the VSC such as the port direct-current voltage change range, the modulation ratio range, the alternating-current voltage change range, the bridge arm current limit, the transformer capacity limit and the like, the PQ operation interval of the VSC is solved.
The PQ operation interval is designed according to the union of the PQ operation intervals of all gears, and when a single gear cannot meet the system requirement, the configuration range of the gears is gradually increased to increase the PQ operation interval. And obtaining a PQ operation interval under the operation mode until the gear configuration meets the requirement of the system on the exchange capacity of the VSC under the operation mode or the gear is added and the exchange power of the VSC and the system can not be increased, wherein the union of the gears obtained under the operation modes is the gear designed by the VSC.
Determining the maximum operating power of the VSC as follows: when the VSC valve group is operated at full voltage and the current is symmetrical to the other pole current, the maximum operating power of a single VSC (single pole) is 1000MW; when the VSC valve group is operated at full voltage and is asymmetric to the current of the other pole or the VSC valve group is operated at half voltage and is symmetric to the current of the other pole, the maximum operating power of a single VSC (single pole) is 960MW; when the VSC valve group is operated at half pressure and is asymmetric with the other pole, the maximum operating power of a single VSC (single pole) is 800MW. The VSC should be designed to have + 24-6 gears.
6. Then, the maximum operation power corresponding to the VSC is determined according to the PQ operation intervalP vscmax1 Calculating the maximum operation power of the corresponding rectification side under the maximum operation power, namely the maximum operation power of the systemP max1 And the calculation is performed according to the minimum line resistance.
In the embodiment, when the number of the VSCs is 3, the maximum operating power of the system is the same as that of the conventional direct current; when the number of the VSCs is 2, the maximum operation power of the system is the same as that of the conventional direct current in most modes, only one condition is limited, namely the single-pole half-voltage operation containing the VSCs is realized, and the maximum operation power of the system is 1730MW at the moment.
7. Establishing a PSCAD simulation model of the hybrid cascade system, and calculating the maximum operation power of the system meeting the AC fault ride-through requirement under each operation mode by simulation scanningP max2 In this embodiment, by optimally configuring the parameters of the dissipaters,P max2 the method is consistent with the conventional direct current engineering.
8. Determining the maximum operation power of the system in the operation modeP max =max(P max1 ，P max2 ). And limiting the maximum operation power for the limited monopole half-voltage operation mode with the VSC operation number being 2.
9. And in the finally determined system running power and VSC reactive power, solving the constructed steady-state mathematical model of the hybrid cascade system, and calculating the running characteristics of the running power from 0.1pu to 1pu in each running mode to form steady-state running parameters of the system. Wherein, tables 1 and 2 show the steady state operation parameter tables in the bipolar full voltage 1+3 operation mode in this embodiment.
TABLE 1 Normal part
TABLE 2 Flexible parts
Example two: correspondingly, the embodiment provides a parameter design system for a main loop of a hybrid cascade extra-high voltage direct current system. The system provided by this embodiment can implement the method for designing the main loop parameters of the hybrid cascade extra-high voltage direct current system in the first embodiment, and the system can be implemented by software, hardware or a combination of software and hardware. For convenience of description, the present embodiment is described with the functions divided into various units, which are described separately. Of course, the functions of the units may be implemented in one or more of the same software and/or hardware when implemented. For example, the system may comprise integrated or separate functional modules or units to perform the corresponding steps in the method of an embodiment. Because the system of the embodiment is basically similar to the method embodiment, the description process of the embodiment is relatively simple, and relevant points can be referred to part of the description of the embodiment one.
Specifically, the system for designing parameters of a main loop of a hybrid cascaded extra-high voltage direct current system provided by this embodiment includes:
the operation mode determining unit is configured to determine the operation mode of the hybrid cascade extra-high voltage direct current system;
the model building unit is configured to determine a basic control mode of the receiving-end VSC and the receiving-end LCC, a voltage distribution mode of the VSC and the LCC and a current distribution mode among the VSCs, and build a steady-state mathematical model of the hybrid cascade system under a set constraint equation;
the parameter determination unit is configured to calculate a port direct-current voltage variation range of the VSC under each operation mode and design a modulation ratio range under each operation mode; solving a PQ operation interval of the VSC based on a constraint condition of the PQ operation interval of the VSC, and determining a gear range required to be configured by the flexible direct current converter; determining the maximum operation power of the hybrid cascade extra-high voltage direct current system in the operation mode;
and the model calculation unit is configured to solve a steady-state mathematical model of the hybrid cascade system based on the system operating power and the VSC reactive power to obtain steady-state operating parameters of the system.
Example three: the present embodiment provides an electronic device corresponding to the method for designing the main loop parameters of the hybrid cascaded extra-high voltage dc system provided in the first embodiment, where the electronic device may be an electronic device for a client, such as a mobile phone, a notebook computer, a tablet computer, a desktop computer, and the like, to execute the method of the first embodiment.
As shown in fig. 5, the electronic device includes a processor, a memory, a communication interface, and a bus, and the processor, the memory, and the communication interface are connected by the bus to complete communication therebetween. The bus may be an Industry Standard Architecture (ISA) bus, a Peripheral Component Interconnect (PCI) bus, an Extended ISA (Extended Industry Standard Component) bus, or the like. The memory stores a computer program capable of running on the processor, and the processor executes the method for designing the parameters of the main loop of the hybrid cascade extra-high voltage direct current system provided by the embodiment when running the computer program. Those skilled in the art will appreciate that the architecture shown in fig. 5 is merely a block diagram of some of the structures associated with the disclosed aspects and is not intended to limit the computing devices to which the disclosed aspects may be applied, and that a particular computing device may include more or less components than those shown, or may combine certain components, or have a different arrangement of components.
In some implementations, the logic instructions in the memory may be implemented in software functional units and stored in a computer readable storage medium when sold or used as a stand-alone product. Based on such understanding, the technical solution of the present application or portions thereof that substantially contribute to the prior art may be embodied in the form of a software product stored in a storage medium and including instructions for causing a computer device (which may be a personal computer, a server, or a network device) to execute all or part of the steps of the method according to the embodiments of the present application. And the aforementioned storage medium includes: a U disk, a removable hard disk, a Read-Only Memory (ROM), a Random Access Memory (RAM), an optical disk, and various other media capable of storing program codes.
In other implementations, the processor may be various general-purpose processors such as a Central Processing Unit (CPU), a Digital Signal Processor (DSP), and the like, and is not limited herein.
Example four: the method for designing the main loop parameters of the hybrid cascaded extra-high voltage dc system according to this embodiment may be embodied as a computer program product, and the computer program product may include a computer readable storage medium on which computer readable program instructions for executing the method for designing the main loop parameters of the hybrid cascaded extra-high voltage dc system according to this embodiment are loaded.
The computer readable storage medium may be a tangible device that retains and stores instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any combination of the foregoing.
The embodiments in the present specification are described in a progressive manner, and the same and similar parts among the embodiments are referred to each other, and each embodiment focuses on the differences from the other embodiments. In particular, for the system embodiment, since it is substantially similar to the method embodiment, the description is simple, and for the relevant points, reference may be made to the partial description of the method embodiment. In the description herein, references to the description of "one embodiment," "some implementations," or the like, mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of an embodiment of the specification. In this specification, the schematic representations of the terms used above are not necessarily intended to refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Furthermore, various embodiments or examples and features of different embodiments or examples described in this specification can be combined and combined by one skilled in the art without contradiction.
The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the application. It will be understood that each flow and/or block of the flow diagrams and/or block diagrams, and combinations of flows and/or blocks in the flow diagrams and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, and not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.
Claims (10)
1. A method for designing parameters of a main loop of a hybrid cascade extra-high voltage direct current system is characterized by comprising the following steps:
determining the operation mode of the hybrid cascade extra-high voltage direct current system;
determining a basic control mode of a receiving-end VSC and an LCC, a voltage distribution mode of the VSC and the LCC and a current distribution mode among the VSCs, and constructing a steady-state mathematical model of the hybrid cascade system under the obtained constraint equation;
calculating the port direct-current voltage variation range of the VSC in each operation mode, and designing the modulation ratio range in each operation mode;
solving a PQ operation interval of the VSC based on a constraint condition of the PQ operation interval of the VSC, and determining a gear range required to be configured by the flexible direct current converter;
determining the maximum operation power of the hybrid cascade extra-high voltage direct current system in the operation mode;
and solving a steady-state mathematical model of the hybrid cascade system based on the maximum operating power and VSC reactive power of the system to obtain steady-state operating parameters of the system.
2. The method for designing the main loop parameters of the hybrid cascade extra-high voltage direct current system according to claim 1, wherein the voltage distribution mode of the VSC and the LCC comprises the following steps:
the VSC and the LCC are distributed in a mode of equal direct current voltage, and then the corresponding constraint equation isU dcvsc =U dclcc ，U dcvsc AndU dclcc the direct current voltages of the ports of the VSC and the LCC are respectively;
the VSC does not consider the fluctuation of direct current voltage, and the LCC bears the line voltage drop, and the corresponding constraint equation isU dcvsc =U setvsc ，U dcvsc Is the port dc voltage of the VSC,U setvsc is a set value.
3. The method for designing the main loop parameters of the hybrid cascade extra-high voltage direct current system according to claim 2, wherein the current distribution mode among the VSCs comprises:
the VSCs are distributed in an equal current mode, and then the constraint equation is as follows:I vsc1 =I vsc2 =...I vscn =I dc /n，I vsc1 ，...I vscn the dc currents of the 1 st to nth VSCs respectively,I dc is the total current flowing into the VSC valve pack;
and independently appointing the running power among the VSCs, and then the constraint equation is as follows:I vsc1 =P vsc1 /U setvsc ，I vsc2 =P vsc2 /U setvsc ， ... I vscn =I dc -I vsc1 -I vsc2... I vsc（n-1） ，P vsc1 、P vsc2… is the active power of VSC1 and VSC 2.
4. The method for designing the main loop parameters of the hybrid cascade extra-high voltage direct current system according to claim 1, wherein the method for determining the modulation ratio range in each operation mode comprises the following steps:
calculating the minimum port voltage of the flexible and straight line in the operation mode according to the principle of maximum line resistanceU dcinit ；
Traversing the working conditions of the VSC power under various faults, calculating the port voltage after the VSC power is transferred under various working conditions, and determining the minimum port voltage under various working conditionsU dcfinal ；
Determining a maximum modulation ratio that ensures that the VSC output voltage is not over-modulatedM absmax Calculating the maximum modulation ratio in each operation modeM max =U dcfinal ×M absmax /U dcinit ；
Designing the minimum modulation ratio according to the number of levels capable of meeting the harmonic requirement of the systemM min 。
5. The method for designing parameters of a main loop of a hybrid cascaded extra-high voltage direct current system according to claim 1, wherein solving a PQ operation interval of VSC comprises:
a. determining limiting conditions of a PQ operation interval of the VSC, wherein the limiting conditions comprise a port direct-current voltage change range, a modulation ratio range, an alternating-current voltage change range, bridge arm current limitation and transformer capacity limitation of the VSC;
b. based on constraint conditions, solving a PQ operation interval, wherein the PQ operation interval takes an intersection under different direct-current voltage and alternating-current voltage combinations;
c. judging whether the power interval meets the set requirement of the system on power exchange, if so, determining a PQ operation interval and a VCS gear, and if not, entering d;
d. if insufficient reactive power is sent out or the bridge arm current causes power limitation, the negative gear configuration is increased; and if the absorption reactive power is insufficient, increasing the positive gear configuration and entering the step b.
6. The method for designing the main loop parameters of the hybrid cascaded extra-high voltage direct current system according to claim 5, wherein the PQ operation interval is designed according to a union of PQ operation intervals under all gears, when a single gear cannot meet system requirements, the configuration range of the gears is gradually increased to increase the PQ operation interval until the gear configuration meets the requirements of the system on the VSC exchange capacity under the operation mode, or when the gear cannot increase the exchange power of the VSC and the system any more, the PQ operation interval under the operation mode is obtained, and the union of the gears obtained under the operation modes is the gear designed by the VSC.
7. The method for designing the main loop parameters of the hybrid cascade extra-high voltage direct current system according to claim 5 or 6, wherein the method for increasing the configuration range of the gears comprises the following steps: when the current limitation of a bridge arm causes insufficient active power or reactive power to be absorbed or emitted, the configuration of negative gears is increased; when the modulation ratio limitation causes insufficient absorption reactive power, increasing positive gear configuration; when the modulation ratio limitation results in insufficient reactive power being emitted, the negative gear configuration is increased.
8. A hybrid cascade extra-high voltage direct current system main loop parameter design system is characterized by comprising:
the operation mode determining unit is configured to determine the operation mode of the hybrid cascade extra-high voltage direct current system;
the model building unit is configured to determine a basic control mode of the receiving-end VSC and the receiving-end LCC, a voltage distribution mode of the VSC and the LCC and a current distribution mode among the VSCs, and build a steady-state mathematical model of the hybrid cascade system under a set constraint equation;
the parameter determination unit is configured to calculate a port direct-current voltage variation range of the VSC under each operation mode and design a modulation ratio range under each operation mode; solving a PQ operation interval of the VSC based on a constraint condition of the PQ operation interval of the VSC, and determining a gear range required to be configured by the flexible converter; determining the maximum operation power of the hybrid cascade extra-high voltage direct current system in the operation mode;
and the model calculation unit is configured to solve a steady-state mathematical model of the hybrid cascade system based on the maximum operating power and VSC reactive power of the system to obtain steady-state operating parameters of the system.
9. An electronic device comprising computer program instructions, wherein the program instructions, when executed by a processor, are configured to implement the method for designing the main loop parameters of the hybrid cascaded extra-high voltage direct current system according to any one of claims 1 to 7.
10. A computer readable storage medium, wherein the computer readable storage medium has stored thereon computer program instructions, wherein the program instructions, when executed by a processor, are configured to implement the method for designing the main loop parameters of the hybrid cascaded extra-high voltage dc system according to any one of claims 1 to 7.
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