CN114944767A - Design of topological structure of LCC-MMC hybrid direct-current ice melting device - Google Patents
Design of topological structure of LCC-MMC hybrid direct-current ice melting device Download PDFInfo
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- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/02—Conversion of ac power input into dc power output without possibility of reversal
- H02M7/04—Conversion of ac power input into dc power output without possibility of reversal by static converters
- H02M7/12—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/21—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/217—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
- H02M7/2173—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only in a biphase or polyphase circuit arrangement
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- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
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- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/02—Conversion of ac power input into dc power output without possibility of reversal
- H02M7/04—Conversion of ac power input into dc power output without possibility of reversal by static converters
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Abstract
The invention relates to a design of a topological structure of an LCC-MMC hybrid direct-current ice melting device, belonging to the technical field of direct-current ice melting. Firstly, a hybrid direct-current ice melting device which is formed by a power grid phase-change converter and a modular multilevel converter is provided, wherein the MMC converter adopts a hybrid structure of full-bridge and half-bridge sub-modules; secondly, according to different line lengths and icing working condition requirements, a switching scheme and a coordination control strategy of the working mode of the direct-current ice melting device are designed, and a multiplexing function mode for improving the utilization rate of the device is designed. The invention improves the topological structure of the mixed ice melting technology to solve the problems of lowest power limitation, current interruption and relatively large harmonic ratio when small current is provided in the operation process of the LCC ice melting device, and simultaneously completes harmonic filtering and reactive support when the LCC ice melting is performed by utilizing the switching of the MMC working state.
Description
Technical Field
The invention relates to the technical field of flexible direct current transmission, in particular to a direct current ice melting device structure, and especially relates to a design of a topological structure of an LCC-MMC hybrid direct current ice melting device.
Background
Along with the acceleration of the modernization process of our country, the dependence degree of each community on electric energy gradually rises, the safe and stable operation of an electric power system also has higher requirements, and the ice coating of a power transmission line is one of important natural disasters threatening the safe and stable operation of a power grid. In the year 2005, 12 months, four states in the united states suffered from ice storm, and about 68.3 ten thousand users were affected by a plurality of areas with broken rods and large-area power supply interruption; in 1 month of 2008, 4 times of large-scale rain and snow weather successively appear in south China, power supply interruption of part of provinces and even grid paralysis occur, and the population in disaster exceeds 1 hundred million; in 11 months in 2020, when the Jilin province encounters a rain, snow and ice disaster, 325 power supply lines in the Changchun region are unstable in voltage and frequently trip, and 33.5 universal household power failure is caused. With the continuous acceleration of the power electronics process of the power system, the influence caused by ice and snow disasters is more severe, and in order to resist the damage of ice and snow, various countries in the world develop researches on ice prevention, ice removal and ice melting. The direct-current ice melting device is small in power supply capacity, high in ice melting speed and good in ice melting effect, is widely applied to occasions with excessive ice coating on the power transmission line, and becomes a standard configuration of an area easy to ice coating.
The LCC (Line Committed Converter) type direct-current ice melting device has the advantages of wide direct-current side adjusting range, high reliability, good economy and the like, and has the defects that a large amount of reactive power is consumed for ice melting, a large amount of harmonic waves are generated on an alternating-current side, and the electric energy quality of a system is influenced; the technology has the limitation of lowest operation power, and when the current on the direct current side is less than a certain value, the current interruption phenomenon can occur, and the current interruption can generate great overvoltage on a converter transformer and a smoothing reactor; the reactive compensation and filtering devices required to be configured for the LCC type direct-current ice melting device can increase the whole floor area of the ice melting device. The MMC (Modular Multilevel Converter) type direct current ice melting device has no harmonic problem in alternating current side Multilevel output, has the advantages of small occupied area, light weight, high operation efficiency, zero rise voltage/rise current and the like, and has the defects that the ice melting technology is relatively complex, the device cost is high, and the ice melting capacity is limited to a certain extent by the influence of IGBT devices. Therefore, designing a highly controllable, economical and efficient ice melting device is still a key technical problem to be solved.
Disclosure of Invention
The invention aims to provide a topological structure of an LCC-MMC hybrid direct-current ice melting device, which solves the problems of the existing direct-current ice melting technology. The invention designs a hybrid direct-current ice melting device which is formed by a power grid phase-change converter and a modular multilevel converter together, wherein the LCC converter adopts a rectifying structure formed by twelve pulsating thyristors, and the MMC converter adopts a mixed structure of a full-bridge submodule and a half-bridge submodule in a ratio of 1: 1. According to different line lengths and icing working condition requirements, a switching scheme and a coordination control strategy of the working mode of the direct-current ice melting device are designed; meanwhile, a multiplexing function mode for improving the utilization rate of the device is designed. Finally, a complete LCC-MMC mixed type direct current deicing device topological structure and a control scheme are formed, and the structure can be applied to the deicing process of the icing conductor of the power system.
The above object of the present invention is achieved by the following technical solutions:
the method comprises the following steps that firstly, a power grid commutation converter and a modular multilevel converter are adopted to form the topological structure of the LCC-MMC hybrid direct-current ice melting device; secondly, changing the control mode of the device according to different working state requirements of the line, and effectively deicing the icing line; and finally, aiming at the harmonic problem of the LCC in the ice melting process, designing an MMC harmonic current control strategy to counteract harmonic components fed into a power grid. The method comprises the following steps:
step (1), a topological structure of an LCC-MMC hybrid direct-current deicing device;
step (2), analyzing the working state requirement of the LCC-MMC mixed type direct current ice melting device;
step (3), LCC ice melting harmonic characteristic analysis and harmonic suppression;
and (4) determining the working mode and the control mode of the LCC-MMC under the ice melting state.
The topological structure of the LCC-MMC hybrid direct-current ice melting device in the step (1) adopts a hybrid direct-current ice melting device which is jointly composed of a power grid commutated converter (LCC) and a Modular Multilevel Converter (MMC), wherein the LCC power grid commutated converter adopts a rectifying structure composed of twelve pulsating thyristors, and the MMC modular multilevel converter adopts a hybrid structure of a full-bridge submodule and a half-bridge submodule 1: 1.
And (3) analyzing the working state requirement of the LCC-MMC mixed type direct current ice melting device in the step (2), and changing a control mode according to different requirements of a line so as to change the working state of the ice melting device.
And (4) analyzing the LCC ice melting harmonic characteristic and harmonic suppression in the step (3), analyzing the harmonic current frequency at the alternating current side when the LCC ice melting device melts ice, and suppressing harmonic waves through a harmonic current control strategy.
The current of the secondary side Y-side winding of the three-winding transformer corresponds to the current of the primary side winding
i A1 ＝i a1 (3-2)
In the formula: i.e. i A1 The current of the secondary side Y-side winding corresponds to the current of the primary side winding; i all right angle a1 Is the current of the secondary side Y side winding.
The secondary side delta side winding current of the three-winding transformer corresponds to the primary side winding current of the three-winding transformer
In the formula: i all right angle A2 The current of the secondary side delta side winding corresponds to the current of the primary side winding; i is d Is direct current; n is the harmonic order, n is 6k + -1, k is 1, 2, 3 …
Therefore, the current at the AC outlet when the LCC melts ice can be expressed as
In the formula: i.e. i A Is the total current of the primary winding.
And (5) determining the working mode and the control mode of the LCC-MMC in the ice melting state in the step (4), determining the working mode according to different ice melting currents required under different ice coating conditions, and adopting a corresponding control strategy.
The output dc voltage should satisfy a wide range of regulation conditions in view of differences in wire specifications and ice coating conditions. When the required ice melting capacity is small, only a mixed MMC ice melting mode is adopted for ice melting; when the required ice-melting capacity is large, an LCC ice-melting + MMC _ STATCOM mode is adopted for ice melting, and the current at the alternating current side of the hybrid MMC contains a current component with the polarity opposite to that of the harmonic wave through a harmonic wave current control process so as to offset the harmonic wave component fed into a power grid. And in the ice melting process, completing the balanced ice melting of the three-phase line by matching with corresponding switching operation.
The invention has the beneficial effects that: the problems of lowest power limitation in the operation process of the LCC type ice melting device, current interruption when small current is provided and relatively large proportion of harmonic waves are solved, harmonic filtering and reactive support during LCC ice melting are completed by switching of the MMC working state, meanwhile, the utilization rate of the device is improved by switching of the LCC-MMC mixed type direct current ice melting device working state, and the device has the advantages of reactive compensation, harmonic suppression, high ice melting current and the like, and is high in practicability.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the invention and do not constitute a limitation of the invention.
FIG. 1 is a topological structure diagram of an LCC-MMC hybrid ice melting device of the present invention;
FIG. 2 is a schematic diagram of a twelve-pulse thyristor-type ice melting topology of the present invention;
FIG. 3 is a hybrid MMC ice melting topology structure diagram of the present invention;
FIG. 4 is a schematic diagram of the hybrid MMC ice melting control of the present invention;
FIG. 5 is a schematic diagram of the thyristor-type ice-melting control of the present invention;
FIG. 6 is a schematic diagram of the harmonic current suppression (11 th harmonic current control) for the hybrid MMC of the present invention;
FIG. 7 is a schematic diagram of the harmonic current suppression of the hybrid MMC of the present invention (13 th harmonic current control);
FIG. 8 is a logic diagram of the device operation in the ice-melting state according to the present invention.
Detailed Description
The details of the present invention and its embodiments are further described below with reference to the accompanying drawings.
Referring to fig. 1 to 8, in the design of the topology structure of the LCC-MMC hybrid dc ice melting device of the present invention, firstly, a hybrid dc ice melting device is proposed, which is composed of a Line Commutated Converter (LCC) and a Modular Multilevel Converter (MMC), wherein the MMC adopts a hybrid structure of a full-bridge and a half-bridge sub-module; secondly, according to different line lengths and icing working condition requirements, a switching scheme and a coordination control strategy of the working mode of the direct-current ice melting device are designed, and a multiplexing function mode for improving the utilization rate of the device is designed. The topological structure of the mixed ice melting technology is improved to solve the problems of minimum power limitation, current interruption and relatively large harmonic ratio when small current is provided in the operation process of the LCC ice melting device, and meanwhile, harmonic filtering and reactive support are completed when the LCC ice melting device melts ice by switching the working state of the MMC.
1. A topological structure of an LCC-MMC hybrid direct-current ice melting device adopts a hybrid direct-current ice melting device which is formed by a power grid commutation converter and a modular multilevel converter together, wherein the LCC converter adopts a rectifying structure formed by twelve pulsating thyristors, and the MMC converter adopts a hybrid structure of a full-bridge submodule and a half-bridge submodule.
The topological structure of the LCC-MMC hybrid ice melting device is shown in figure 1, the structure combines the advantages of two direct-current ice melting technologies, and the ice melting defect caused by the structure of the structure is avoided. For the LCC type ice melting part, a twelve-pulse thyristor controllable rectifying circuit which is common in engineering is adopted, as shown in figure 2. Compared with a six-pulse rectifying circuit, the structure reduces the harmonic content on the alternating current side. For the mixed MMC ice melting part, a structure that the ratio of a full-bridge submodule to a half-bridge submodule in each half bridge arm is 1:1 is adopted, the structure realizes the wide-range continuous adjustment of output voltage and simultaneously reduces the number of the full-bridge submodules in the device, the number of required devices is reduced by 25%, and the economy of the device is improved by about 20%. As shown in fig. 3. When the hybrid MMC works normally, the external characteristics of the hybrid MMC and the half-bridge MMC are approximately consistent, but the hybrid MMC has the capability of reducing direct-current voltage to operate due to the fact that the full-bridge submodule is adopted, and wide-range adjustment of the direct-current voltage from zero to a rated value is achieved.
2. And the working state requirement analysis of the LCC-MMC hybrid direct-current ice melting device changes a control mode according to different requirements of a line, so that the working state of the ice melting device is changed.
When the power transmission line is iced, when the needed ice-melting current and the capacity are judged to be in the capacity range provided by the MMC, the MMC in the application device melts ice at the moment, multi-level output at the AC side is realized, the problem of harmonic interference does not exist, a filter does not need to be additionally arranged, the operation is simple, and the operation efficiency is high; when the ice-melting current and the capacity needed are judged to be larger than the capacity range which can be provided by the MMC, the LCC part in the application device melts ice at the moment, the limitation of the lowest running power of the LCC is avoided, corresponding harmonic output and reactive power consumption inevitably exist when the LCC is applied to melt ice, the LCC can be converted into a STATCOM mode by changing the control mode of the MMC part in the ice-melting device, corresponding harmonic filtering and reactive support are provided for the LCC to melt ice, and the purpose of improving the quality of electric energy of a network side is achieved. At this time, the device operates in an ice-melting state;
when the ice melting device is in a non-ice period, two parts in the LCC-MMC mixed type direct current ice melting device can respectively provide reactive support for a power system. The LCC part enables a direct current side to be short-circuited through an inductor through a disconnecting link, and adjusts a trigger angle to realize adjustment of reactive power within a certain range; and the MMC part performs reactive compensation on the power system by changing a reactive power reference value. At the moment, the device operates in a reactive compensation state;
when the system is overhauled, the open-close state of the disconnecting link can be switched, the positive bus and the negative bus on the direct current side are in short circuit, and whether rated direct current is output or not is tested. When the twelve-pulse thyristor-type direct-current ice melting device is subjected to a zero-power test, only the thyristor converter device is put into the device, the MMC converter is opened, and the direct-current side output current of the thyristor-type direct-current ice melting device is adjusted to reach a rated value; and similarly, the zero-power test of the MMC ice melting device can be completed only by inputting the MMC current converter. At this time, the apparatus is operated in a maintenance state.
3. And (5) analyzing the ice melting harmonic characteristics of the LCC.
As shown in figure 2, the LCC type ice melting part adopts a twelve-pulse thyristor controllable rectifier circuit, a three-winding transformer consists of a primary winding and two secondary windings, and the transformation ratio of the transformer is
The two secondary windings are respectively connected with a group of six-pulse rectifying devices, and taking phase a current as an example, the current of the secondary Y-side winding can be represented as follows:
in the formula: i.e. i a1 Is the current of the secondary side Y side winding; i is d Is direct current; n is the harmonic order, where n is 6k ± 1, k is 1, 2, 3 …. According to the relation between the primary winding and the secondary winding, the primary current i A1 Can be expressed as
i A1 ＝i a1 (3-2)
In the formula: i.e. i A1 The current of the secondary side Y-side winding corresponds to the current of the primary side winding.
The connection of the secondary winding makes the phase difference of the power supply in the two groups of six-pulse rectifying circuits 30 DEG, so that i at the moment a2 Can be expressed as
In the formula: i all right angle a2 Is the current of the secondary side delta side winding.
For the primary side current i A2 Secondary side current i a2 Should lead 30 deg. for the positive sequence component and should lag 30 deg. for the negative sequence component, i.e. should lead 30 deg.
In the formula: i all right angle A2 Is the minor side delta sideThe winding current corresponds to the current of the primary winding.
Thus the primary side current i A Can be expressed as
In the formula: i all right angle A Is the total current of the primary winding.
Through the analysis, the fact that when the LCC melts ice, the two six-pulsation thyristor rectifying devices respectively generate 6k +/-1 times of characteristic subharmonics at the AC outlet side of the secondary side of the transformer, and finally characteristic harmonics with the harmonic frequency of 12k +/-1 exist on the primary side of the AC side through superposition of the two parts of harmonics can be obtained, so that the harmonics needing to be filtered by the MMC device are 12k +/-1 times of harmonics with higher content.
4. And determining the working mode and the control mode of the LCC-MMC under the ice melting state, determining the working mode according to the difference of the ice melting current required under different ice coating conditions, and adopting a corresponding control strategy.
And according to the ice melting requirement, when the required ice melting current and the capacity are judged to be within the range which can be provided by the mixed MMC, the ice is melted by adopting a mixed MMC ice melting mode only. Considering the difference between the specification of the wire and the icing condition, the output direct current voltage needs to meet the wide-range regulation condition. The control of the hybrid MMC can be structurally divided into inner-loop control for solving the problem of rapid tracking of current and outer-loop control for determining i d 、i q The magnitude of the reference value. The inner and outer ring control block diagram of the hybrid MMC is shown in fig. 4, and since the half-bridge submodule cannot provide a negative level, when the voltage on the dc side decreases, the full-bridge submodule operates in a negative level state. The voltage-sharing modulation method of the mixed MMC adopts a minimum level approximation modulation strategy to determine the number of sub-modules needing to be conducted in a bridge arm of the mixed MMC within corresponding time.
And judging according to the ice melting guide rule, and when the required ice melting current and ice melting capacity exceed the ice melting capacity which can be provided by the hybrid MMC, selecting an LCC ice melting + MMC _ STATCOM mode for melting ice. In practical engineering, corresponding switching strategy is configured to complete three-phase line balanceAnd de-icing, wherein the direct current de-icing current is determined by line parameters, icing thickness, meteorological parameters and the like, and the de-icing current requirements of different overhead transmission lines under different icing thicknesses and meteorological environments are met by controlling the direct current. The ice melting current is determined and then used as a reference value I of the direct current dcref Reference value I of direct current dcref With the actual value of the direct current I dc The deviation is subjected to PI control to obtain a trigger signal of the thyristor, and direct current is controlled, and the control principle is shown in figure 5. The control mode has good tracking performance on the ice melting current, and the PI control parameters are properly adjusted, so that the corresponding speed can be increased, and the overshoot phenomenon is avoided.
For the harmonic current fed into the power grid in the LCC ice melting process, the current at the alternating current side of the hybrid MMC contains a current component with the polarity opposite to that of the harmonic through the harmonic current control process so as to offset the harmonic component fed into the power grid and achieve the purpose of comprehensive control of electric energy. As shown in fig. 6 and 7, since the 12k-1 th harmonic is a negative sequence component and the 12k +1 th harmonic is a positive sequence component, the control when considering the MMC filter is slightly different.
To sum up, a logic block diagram of the LCC-MMC hybrid dc ice melting device operating in the ice melting state is shown in fig. 8.
The above description is only a preferred example of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, improvement and the like of the present invention shall be included in the protection scope of the present invention.
Claims (5)
1. The utility model provides a design of LCC-MMC mixed type direct current ice-melt device topological structure which characterized in that: the method comprises the following steps:
step (1), a topological structure of an LCC-MMC hybrid direct-current ice melting device;
step (2), analyzing the working state requirement of the LCC-MMC mixed type direct current ice melting device;
step (3), LCC ice melting harmonic characteristic analysis and harmonic suppression;
and (4) determining the working mode and the control mode of the LCC-MMC in the ice melting state.
2. The design of the LCC-MMC hybrid DC thawing device topology of claim 1, characterized in that: the topological structure of the LCC-MMC hybrid direct-current ice melting device in the step (1) adopts a hybrid direct-current ice melting device which is formed by a power grid commutation converter LCC and a modular multilevel converter MMC together, wherein the LCC power grid commutation converter adopts a rectifying structure formed by twelve pulsating thyristors, and the MMC modular multilevel converter adopts a mixed structure of a full-bridge submodule and a half-bridge submodule 1: 1.
3. The design of the LCC-MMC hybrid DC deicing device topology structure of claim 1, characterized in that: and (3) analyzing the working state requirement of the LCC-MMC mixed type direct current ice melting device in the step (2), and changing a control mode according to different requirements of a line so as to change the working state of the ice melting device.
4. The design of the LCC-MMC hybrid DC thawing device topology of claim 1, characterized in that: analyzing the LCC ice melting harmonic characteristic and harmonic suppression in the step (3), analyzing the harmonic current frequency at the alternating current side when the LCC ice melting device melts ice, and suppressing harmonic waves through a harmonic current control strategy;
the current of the secondary side Y-side winding of the three-winding transformer corresponds to the current of the primary side winding
i A1 ＝i a1 (3-2)
In the formula: i.e. i A1 The current of the secondary side Y-side winding corresponds to the current of the primary side winding; i.e. i a1 Is the current of the secondary side Y side winding;
the secondary side delta side winding current of the three-winding transformer corresponds to the primary side winding current of the three-winding transformer
In the formula: i.e. i A2 For secondary side delta side winding currentA current corresponding to the primary winding; i is d Is direct current; n is the harmonic order, n is 6k + -1, k is 1, 2, 3 …
Therefore, the current at the AC outlet when the LCC melts ice can be expressed as
In the formula: i.e. i A Is the total current of the primary winding.
5. The design of the LCC-MMC hybrid DC thawing device topology of claim 1, characterized in that: determining the working mode and the control mode of the LCC-MMC in the ice melting state in the step (4), determining the working mode according to the difference of the ice melting current required under different ice coating conditions, and adopting a corresponding control strategy;
considering the difference between the specification of the lead and the icing condition, the output direct current voltage meets the wide-range regulation condition; when the required ice melting capacity is small, only a mixed MMC ice melting mode is adopted for melting ice; when the required ice-melting capacity is large, an LCC ice-melting + MMC _ STATCOM mode is adopted for ice melting, and the current at the alternating current side of the hybrid MMC contains a current component with the polarity opposite to that of the harmonic wave through a harmonic wave current control process so as to offset the harmonic wave component fed into a power grid; and in the ice melting process, corresponding switching operation is matched to complete the balanced ice melting of the three-phase line.
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