CN113193526A - Low-frequency alternating-current ice melting method based on back-to-back half-bridge MMC device - Google Patents
Low-frequency alternating-current ice melting method based on back-to-back half-bridge MMC device Download PDFInfo
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- CN113193526A CN113193526A CN202110483539.7A CN202110483539A CN113193526A CN 113193526 A CN113193526 A CN 113193526A CN 202110483539 A CN202110483539 A CN 202110483539A CN 113193526 A CN113193526 A CN 113193526A
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02G—INSTALLATION OF ELECTRIC CABLES OR LINES, OR OF COMBINED OPTICAL AND ELECTRIC CABLES OR LINES
- H02G7/00—Overhead installations of electric lines or cables
- H02G7/16—Devices for removing snow or ice from lines or cables
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/02—Circuit arrangements for ac mains or ac distribution networks using a single network for simultaneous distribution of power at different frequencies; using a single network for simultaneous distribution of ac power and of dc power
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/12—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/388—Islanding, i.e. disconnection of local power supply from the network
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
- H02J3/50—Controlling the sharing of the out-of-phase component
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/483—Converters with outputs that each can have more than two voltages levels
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/501—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode sinusoidal output voltages being obtained by the combination of several pulse-voltages having different amplitude and width
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/53—Conversion of dc power input into ac 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
- H02M7/537—Conversion of dc power input into ac 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, e.g. single switched pulse inverters
- H02M7/5387—Conversion of dc power input into ac 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, e.g. single switched pulse inverters in a bridge configuration
- H02M7/53871—Conversion of dc power input into ac 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, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
- Y02E40/10—Flexible AC transmission systems [FACTS]
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Abstract
The invention discloses a low-frequency alternating current ice melting method based on a back-to-back half-bridge MMC device, wherein two sets of dynamic reactive generators based on a half-bridge MMC are connected to a 10kV bus of a transformer substation and are used for reactive compensation at ordinary times; when the transmission line is frozen, the ice melting power supply is used; the dynamic reactive power generators of the half-bridge MMC are respectively called an MMC-STATCOM set A and an MMC-STATCOM set B; when ice needs to be melted, the alternating current side of the MMC-STATCOM set B is disconnected with a power grid, an alternating current transmission line to be melted is connected to the alternating current side, and the direct current side is connected in parallel with the direct current side of the MMC-STATCOM set A; the MMC-STATCOM of the B set converts the control mode from a reactive generator mode to a low-frequency alternating-current power supply control mode to realize ice melting; the problem that the ice melting device in the prior art needs a large amount of work during operation and maintenance is solved; high cost and the like.
Description
Technical Field
The invention belongs to the technical field of ice melting of power transmission lines, and particularly relates to a low-frequency alternating-current ice melting method based on a back-to-back half-bridge MMC device.
Technical Field
In order to prevent the power grid fault caused by ice disaster, ice melting devices based on various principles and technologies, such as six-pulse-wave ice melting devices and twelve-pulse-wave direct current based on SVC, appear in the prior art one after another, play a great role in the ice-resistant and electricity-conserving period, and achieve good effects. At present, the more SVC-based ice melting devices are applied, and can be used as SVC in normal times except for melting ice in a few times in winter, so that the asset interest rate is improved. However, since the thyristor is a half-controlled device, the ac current distortion rate is large, and an FC or a fixed capacitor needs to be configured, and the number of switches involved in topology switching is large, and the workload required for operation and maintenance is large. With the maturation of the STATCOM technology, in order to further improve the performance of an ice melting system, a direct current ice melting device based on the STATCOM is provided, and continuous adjustment of direct current ice melting voltage and ice melting current is realized through neutral point offset control and carrier phase shift. However, the ice-melting method based on the MMC-STATCOM needs a full-bridge unit submodule to adjust the ice-melting voltage and current, so the manufacturing cost is high.
Disclosure of Invention
The technical problem to be solved by the invention is as follows: the low-frequency alternating current ice melting method based on the back-to-back half-bridge MMC device is provided, and aims to solve the problems that an ice melting device in the prior art is based on a thyristor of a half-controlled device, the alternating current distortion rate is large, an FC or a fixed capacitor needs to be configured, the number of switches involved in topology switching is large, and the workload required in operation and maintenance is large; the ice melting method based on the MMC-STATCOM has the technical problems that the ice melting voltage and current can be adjusted only by a full-bridge unit submodule, the manufacturing cost is high and the like.
The technical scheme of the invention is as follows:
a low-frequency alternating current ice melting method based on a back-to-back half-bridge MMC device is characterized in that two sets of dynamic reactive generators based on a half-bridge MMC are connected to a 10kV bus of a transformer substation and are used for reactive compensation at ordinary times; when the transmission line is frozen, the ice melting power supply is used; the dynamic reactive power generators of the half-bridge MMC are respectively called an MMC-STATCOM set A and an MMC-STATCOM set B; when ice needs to be melted, the alternating current side of the MMC-STATCOM set B is disconnected with a power grid, an alternating current transmission line to be melted is connected to the alternating current side, the direct current side is connected in parallel with the direct current side of the MMC-STATCOM set A, the MMC-STATCOM set A serves as a reactive power supply, and meanwhile, the voltage of a direct current bus is kept stable; and the B set of MMC-STATCOM converts the control mode from the reactive generator mode to the low-frequency alternating-current power supply control mode to realize ice melting.
The A set of MMC-STATCOM is used for controlling the direct current bus voltage, the B set of MMC-STATCOM works in an island state, and the alternating current side is connected with an ice melting circuit.
The method for realizing ice melting comprises the following steps: the impedance of the ice melting line is adjusted by adjusting the output frequency of the alternating current side of the MMC-STATCOM set B, so that the ice melting current is continuously adjusted; the frequency adjustment range is 300Hz to 5 Hz.
The set B of MMC-STATCOM is divided into a valve control controller and a main control controller, wherein the main control controller comprises two control modes, namely a reactive generator mode and a low-frequency alternating current control mode, and the low-frequency alternating current control mode is as follows: firstly, an angular frequency value w is given, the angular frequency w integrates the current actual time to obtain an angle wt, the time t is the system time of a CPU (central processing unit), the wt is subjected to sine value to obtain the current value of a real-time sine wave as an a-phase sine wave, and meanwhile, a phase-shifting process is carried out by 120 degrees and 240 degrees to respectively obtain a b-phase sine wave and a c-phase sine wave; and then the a-phase sine wave, the B-phase sine wave and the c-phase sine wave are used as reference waveforms to be sent to a valve controller, and the B-set MMC-STATCOM is driven to operate.
The control flow of the low-frequency alternating current control mode comprises the following steps:
6, disconnecting the ice melting line after ice melting is finished; switching to a to-be-connected mode;
and 8, switching to an independent reactive power compensation mode after grid connection.
The parallel operation mode in the step 1 means that the B set of MMC-STATCOM can not continuously operate according to a conventional reactive compensation control strategy, and if the operation mode is not changed, the control system conflicts with the A set of MMC-STATCOM to cause direct-current voltage instability; the parallel operation mode is a fixed active mode and a fixed reactive mode, and the active given value is set to be 0; in addition, before step 1 is transferred to step 2, the reactive power setting is adjusted from the original value to 0.
And 2, in the frequency modulation operation mode, the B set of MMC-STATCOM is in an island operation mode.
Adjusting the frequency according to the ice melting current in the step 5, and calculating according to a formula 1:
in the formula (1), omega is the angular frequency (rad/s) output by the B set of MMC-STATCOM, u is the output alternating voltage (V), I is the ice melting current (A), s is the length (km) of the ice melting line, and L is the inductance (H) of the ice melting line per kilometer.
The control method of the main control comprises the following steps:
step 401: calculating line impedance according to the starting current;
step 402: calculating the angular frequency according to the line impedance and using the angular frequency as a given angular frequency value omega;
step 403: starting an integrator, and taking a controller hardware clock as a time variable;
step 404: taking a sine function as an a-phase sine wave reference signal;
step 405: preparing an ice melting line and connecting the ice melting line with the ice melting line;
step 406: phase shifting 120 degrees and 240 degrees to respectively obtain b-phase sinusoidal reference waves and c-phase sinusoidal reference waves;
step 407: the reference waveform is sent to a valve controller;
step 408: and (5) sequencing and pressure-equalizing operation of the valve banks.
The invention has the beneficial effects that:
according to the invention, a low-frequency alternating-current power supply is formed by back-to-back STATCOMs based on MMC, and the ice melting line impedance is changed by frequency adjustment, so that the ice melting current is adjusted. Because adopt half-bridge MMC device, theoretically make the cost greatly reduced towards the MMC device of reactive compensation demand, only be half of full-bridge MMC device. The whole set of device has both a reactive compensation function and an ice melting function, so that the function reuse is realized, and the equipment investment benefit is improved. When the current is regulated, the inductive characteristic of the circuit is fully utilized, the impedance of the inductive element is small at low frequency and large at high frequency, and the impedance regulation is realized by adopting frequency regulation instead of the traditional voltage regulation. By the technology, the ice melting method and the ice melting device which are high in cost performance and easy to control are realized; the problems that the prior ice melting device is based on a thyristor of a semi-controlled device, the alternating current distortion rate is large, an FC or a fixed capacitor needs to be configured, the number of switches involved in topology switching is large, and the workload required in operation and maintenance is large are solved; the ice melting method based on the MMC-STATCOM has the technical problems that the ice melting voltage and current can be adjusted only by a full-bridge unit submodule, the manufacturing cost is high and the like.
Description of the drawings:
FIG. 1 is a schematic diagram of a connection mode of an MMC-STATCOM set A and an MMC-STATCOM set B for reactive compensation in an embodiment of the present invention;
FIG. 2 is a schematic diagram illustrating a connection mode of the MMC-STATCOM set A and the MMC-STATCOM set B during ice melting according to the embodiment of the present invention;
FIG. 3 is a control flow chart of the MMC-STATCOM set in the embodiment of the invention during ice melting;
FIG. 4 is a schematic diagram of a control strategy of the MMC-STATCOM master controller of the present invention;
fig. 5 is a schematic diagram illustrating an implementation effect of the present invention.
The specific implementation mode is as follows:
the invention aims to realize the dual functions of dynamic reactive power compensation of a transformer substation and ice melting of a transmission line, adjust the line impedance, achieve the effect of direct current ice melting by adopting a low-frequency alternating current mode and overcome various defects of the direct current ice melting.
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in further detail below.
Fig. 1 shows a wiring manner of an MMC-STATCOM set a and an MMC-STATCOM set B used for reactive power compensation at ordinary times according to an embodiment of the present invention. The implementation premise of the invention is that the transformer substation is provided with two sets of half-bridge STATCOMs, the common function of the two sets of STATCOMs is dynamic reactive compensation, the content of the common function is not the content of the invention, and the invention is a new function and a new technology which are expanded beyond the common function. In the aspect of a wiring mode, the difference from the conventional function is that two switches K4 and K5 are added, after the two switches are closed, the MMC-STATCOM in the A set and the MMC-STATCOM in the B set form a back-to-back converter, the MMC-STATCOM in the A set is used for maintaining direct-current voltage, meanwhile, reactive power can still be compensated, and the MMC-STATCOM in the B set serves as an ice melting power supply.
FIG. 2 shows the connection mode of the MMC-STATCOM set A and the MMC-STATCOM set B in the embodiment of the invention when the MMC-STATCOM set A and the MMC-STATCOM set B are used for melting ice. When the power transmission line needs to melt ice, the MMC-STATCOM of the set A keeps the original reactive compensation operation mode unchanged, the MMC-STATCOM of the set B and the MMC-STATCOM of the set A share the direct current bus by closing the switches K4 and K5, and at the moment, as the two sets of MMC-STATCOMs have the function of maintaining the voltage of the direct current bus, the control system may be unstable, so that the control mode of the MMC-STATCOM of the set B needs to be switched to the virtual synchronous machine mode to ensure the stability of the direct current voltage. For the power transmission line to be de-iced, the switch of the substation on the opposite side of the line is reversed, the three-phase power transmission line is in short circuit on the opposite side, the local side is disconnected with K1, and is connected with K3, so that the power transmission line becomes the energy consumption load of the B set of MMC-STATCOM, the three-phase current is controlled to meet the de-icing requirement, and de-icing is realized.
FIG. 3 is a control flow of the MMC-STATCOM set B in the ice melting process according to the embodiment of the present invention, which includes the following steps:
step 301: switching the MMC-STATCOM of the B set to be in a parallel operation mode 2, and closing K4 and K5;
step 302: switching the MMC-STATCOM of the B set to a frequency modulation operation mode 3, and switching off K1;
step 303: adjusting the frequency to 300 Hz;
step 304: preparing an ice melting line, and closing K3;
step 305: adjusting the frequency according to the ice melting current;
step 306: after ice melting is finished, switching off K3, and switching to a grid-connected mode 4;
step 307: after ice melting is finished, switching off K4 and K5, and switching to a grid-connected mode 4;
step 308: after grid connection, K3 is switched off, and the mode is switched to the independent reactive compensation mode 1;
the parallel operation mode 2 in step 301 means that the MMC-STATCOM in the B set cannot continue to operate according to the conventional reactive compensation control strategy, because if the operation mode is not changed, the control system conflicts with the MMC-STATCOM in the a set, which causes the dc voltage to be unstable. The parallel operation mode 2 is a fixed active mode and a fixed reactive mode, the reactive given value is the same as the reactive given value of the parallel operation mode, and the active given value is set to be 0. In addition, before the transition from step 301 to step 302, the reactive power setting needs to be adjusted from the original value to 0.
The frequency modulation operation mode 3 in step 302 means that after the K2 is turned on, the B set of MMC-STATCOM is in an island operation mode, and power supply preparation is made for ice melting.
The adjusting of the frequency according to the ice-melting current in step 305 means that different line models and different line lengths all have an influence on the ice-melting current. The current frequency is calculated according to the following formula:
in the formula (1), omega is the angular frequency (rad/s) output by the B set of MMC-STATCOM, u is the output alternating voltage (V), I is the ice melting current (A), s is the length (km) of the ice melting line, and L is the inductance (H) of the ice melting line per kilometer.
FIG. 4 is a control strategy of the MMC-STATCOM master controller of the embodiment of the present invention, which includes the following steps:
step 401: calculating line impedance according to the starting current;
step 402: calculating the angular frequency according to the line impedance and using the angular frequency as a given angular frequency value omega;
step 403: starting an integrator, and taking a controller hardware clock as a time variable;
step 404: taking a sine function as an a-phase sine wave reference signal;
step 405: preparing an ice melting line, and closing K3;
step 406: phase shifting 120 degrees and 240 degrees to respectively obtain b-phase sinusoidal reference waves and c-phase sinusoidal reference waves;
step 407: the reference waveform is sent to a valve controller;
step 408: and (5) sequencing and pressure-equalizing operation of the valve banks.
The step 403 of taking the controller hardware clock as the time variable means that for the MMC-STATCOM master controller of the set B, the CPU real-time clock used by the hardware is taken as the time variable, so that the sinusoidal reference wave same as the given value can be obtained.
The reference waveform in step 407 is sent to the valve controller, which means that the valve controller only receives the reference waveform of the main controller, and the control strategy of the valve controller is still the same as that used in the dynamic reactive power compensation mode, and the control code of the valve controller does not need to be written additionally.
Fig. 5 shows an implementation effect of a low-frequency ac ice-melting method based on a half-bridge MMC device according to an embodiment of the present invention, where before t ═ 5s in fig. 5, a B set MMC-STATCOM high-frequency operation state is set, and at this time, 300Hz is used (for example purposes only), an ice-melting line shows a large impedance, and a line current is small and is in a preparation stage before ice-melting. When the output frequency of the B set of MMC-STATCOM is adjusted to be gradually reduced (only the final frequency is shown in figure 5), when the frequency reaches 5Hz, the line presents very small impedance, the line current is very large, and the ice melting requirement is met.
The low-frequency alternating current ice melting method based on the half-bridge MMC device has the following effects:
(1) compared with a full-bridge MMC-STATCOM, the half-bridge MMC-STATCOM is low in cost and easy to achieve.
(2) And the step-by-step smooth regulation of the ice melting current can be realized by adopting alternating current frequency regulation, and the control is easy.
(3) For the half-bridge MMC-STATCOM of the transformer substation, the ice melting function is added, and the equipment benefit is improved.
The embodiments in the present description are described in a progressive manner, each embodiment focuses on differences from other embodiments, and the same and similar parts among the embodiments are referred to each other. For the system disclosed by the embodiment, the description is relatively simple because the system corresponds to the method disclosed by the embodiment, and the relevant points can be referred to the method part for description.
Claims (9)
1. A low-frequency alternating current ice melting method based on a back-to-back half-bridge MMC device is characterized by comprising the following steps: two sets of dynamic reactive generators based on half-bridge MMC are connected to a 10kV bus of a transformer substation and are used for reactive compensation at ordinary times; when the transmission line is frozen, the ice melting power supply is used; the dynamic reactive power generators of the half-bridge MMC are respectively called an MMC-STATCOM set A and an MMC-STATCOM set B; when ice needs to be melted, the alternating current side of the MMC-STATCOM set B is disconnected with a power grid, an alternating current transmission line to be melted is connected to the alternating current side, the direct current side is connected in parallel with the direct current side of the MMC-STATCOM set A, the MMC-STATCOM set A serves as a reactive power supply, and meanwhile, the voltage of a direct current bus is kept stable; and the B set of MMC-STATCOM converts the control mode from the reactive generator mode to the low-frequency alternating-current power supply control mode to realize ice melting.
2. The low-frequency alternating-current deicing method based on the back-to-back half-bridge MMC device according to claim 1, characterized in that: the A set of MMC-STATCOM is used for controlling the direct current bus voltage, the B set of MMC-STATCOM works in an island state, and the alternating current side is connected with an ice melting circuit.
3. The low-frequency alternating-current deicing method based on the back-to-back half-bridge MMC device according to claim 1, characterized in that: the method for realizing ice melting comprises the following steps: the impedance of the ice melting line is adjusted by adjusting the output frequency of the alternating current side of the MMC-STATCOM set B, so that the ice melting current is continuously adjusted; the frequency adjustment range is 300Hz to 5 Hz.
4. The low-frequency alternating-current deicing method based on the back-to-back half-bridge MMC device according to claim 1, characterized in that: the set B of MMC-STATCOM is divided into a valve control controller and a main control controller, wherein the main control controller comprises two control modes, namely a reactive generator mode and a low-frequency alternating current control mode, and the low-frequency alternating current control mode is as follows: firstly, an angular frequency value w is given, the angular frequency w integrates the current actual time to obtain an angle wt, the time t is the system time of a CPU (central processing unit), the wt is subjected to sine value to obtain the current value of a real-time sine wave as an a-phase sine wave, and meanwhile, a phase-shifting process is carried out by 120 degrees and 240 degrees to respectively obtain a b-phase sine wave and a c-phase sine wave; and then the a-phase sine wave, the B-phase sine wave and the c-phase sine wave are used as reference waveforms to be sent to a valve controller, and the B-set MMC-STATCOM is driven to operate.
5. The low-frequency alternating-current deicing method based on the back-to-back half-bridge MMC device according to claim 4, characterized in that: the control flow of the low-frequency alternating current control mode comprises the following steps:
step 1, connecting the MMC-STATCOM set B and the MMC-STATCOM set A in parallel; the MMC-STATCOM of the B set is in a parallel operation mode;
step 2, disconnecting the MMC-STATCOM set B from the bus; the MMC-STATCOM set B is in a frequency modulation operation mode,
step 3, adjusting the frequency to 300 Hz;
step 4, preparing the ice melting line and connecting the ice melting line;
step 5, adjusting the frequency according to the ice melting current;
6, disconnecting the ice melting line after ice melting is finished; switching to a to-be-connected mode;
step 7, after the ice melting is finished, disconnecting the parallel connection with the MMC-STATCOM set A; switching to a to-be-connected mode;
and 8, switching to an independent reactive power compensation mode after grid connection.
6. The low-frequency alternating-current deicing method based on the back-to-back half-bridge MMC device according to claim 5, characterized in that: the parallel operation mode in the step 1 means that the B set of MMC-STATCOM can not continuously operate according to a conventional reactive compensation control strategy, and if the operation mode is not changed, the control system conflicts with the A set of MMC-STATCOM to cause direct-current voltage instability; the parallel operation mode is a fixed active mode and a fixed reactive mode, and the active given value is set to be 0; in addition, before step 1 is transferred to step 2, the reactive power setting is adjusted from the original value to 0.
7. The low-frequency alternating-current deicing method based on the back-to-back half-bridge MMC device according to claim 5, characterized in that: and 2, in the frequency modulation operation mode, the B set of MMC-STATCOM is in an island operation mode.
8. The low-frequency alternating-current deicing method based on the back-to-back half-bridge MMC device according to claim 5, characterized in that:
adjusting the frequency according to the ice melting current in the step 5, and calculating according to a formula 1:
in the formula (1), omega is the angular frequency (rad/s) output by the B set of MMC-STATCOM, u is the output alternating voltage (V), I is the ice melting current (A), s is the length (km) of the ice melting line, and L is the inductance (H) of the ice melting line per kilometer.
9. The low-frequency alternating-current deicing method based on the back-to-back half-bridge MMC device according to claim 4, characterized in that: the control method of the main control comprises the following steps:
step 401: calculating line impedance according to the starting current;
step 402: calculating the angular frequency according to the line impedance and using the angular frequency as a given angular frequency value omega;
step 403: starting an integrator, and taking a controller hardware clock as a time variable;
step 404: taking a sine function as an a-phase sine wave reference signal;
step 405: preparing an ice melting line and connecting the ice melting line with the ice melting line;
step 406: phase shifting 120 degrees and 240 degrees to respectively obtain b-phase sinusoidal reference waves and c-phase sinusoidal reference waves;
step 407: the reference waveform is sent to a valve controller;
step 408: and (5) sequencing and pressure-equalizing operation of the valve banks.
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