CN116961020A - Wind power multi-terminal flexible direct system frequency adjustment method based on self-adaptive reference power - Google Patents

Wind power multi-terminal flexible direct system frequency adjustment method based on self-adaptive reference power Download PDF

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Publication number
CN116961020A
CN116961020A CN202310676974.0A CN202310676974A CN116961020A CN 116961020 A CN116961020 A CN 116961020A CN 202310676974 A CN202310676974 A CN 202310676974A CN 116961020 A CN116961020 A CN 116961020A
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frequency
power
direct
deviation
converter station
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赵平
高亨孝
倪世杰
黄宇昕
贾浩森
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China Three Gorges University CTGU
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China Three Gorges University CTGU
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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
    • H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
    • H02J3/241—The oscillation concerning frequency
    • 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
    • H02J3/002—Flicker reduction, e.g. compensation of flicker introduced by non-linear load
    • 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
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • 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
    • 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
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H02J2003/365—Reducing harmonics or oscillations in HVDC
    • 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
    • H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
    • H02J2300/20—The dispersed energy generation being of renewable origin
    • H02J2300/28—The renewable source being wind energy
    • 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
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/70—Wind energy
    • Y02E10/76—Power conversion electric or electronic aspects

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Physics & Mathematics (AREA)
  • Nonlinear Science (AREA)
  • Control Of Eletrric Generators (AREA)

Abstract

A wind power multi-terminal flexible direct system frequency adjustment method based on self-adaptive reference power belongs to the field of power transmission and distribution of power systems. When the frequency disturbance occurs to the on-shore power grid, the disturbance degree is judged through the direct-current voltage, and when the direct-current voltage is smaller than a set threshold value, the normal side converter station adjusts the self-adaptive reference power according to the direct-current voltage deviation and the side frequency deviation. When the direct current voltage exceeds a set threshold value, the frequency reference value is adjusted by the transmitting-end converter station according to the deviation of the direct current voltage, and the wind turbine generator performs comprehensive inertia control according to the sensed frequency change. The virtual inertia control is adopted to enable other converter stations to participate in frequency control, and meanwhile, the virtual inertia coefficient is adaptively adjusted according to the direct current voltage deviation and the disturbance side frequency deviation, so that the direct current voltage deviation is reduced. The invention can reduce the deviation of direct current voltage while reducing frequency disturbance, and has less influence on the frequency of the normal side alternating current power grid.

Description

Wind power multi-terminal flexible direct system frequency adjustment method based on self-adaptive reference power
Technical Field
The invention belongs to the field of power transmission and distribution of power systems, and particularly relates to a frequency modulation method of a wind power multi-terminal flexible direct current system based on self-adaptive reference power.
Background
In recent years, offshore wind power gradually becomes an important component of renewable energy, and the characteristics of abundant wind resources, high available hours, no occupation of land and suitability for large-scale development are widely paid attention to. With the development of offshore wind power technology, more and more offshore wind power stations are connected into a power grid, the requirement is difficult to meet by adopting the traditional point-to-point double-end direct current power transmission technology, and the multi-terminal direct current power transmission system is used for offshore wind power grid connection and becomes a domestic and foreign trend. However, due to the decoupling effect of the flexible system, the ac systems to which the multi-terminal flexible system is connected lose the ability to mutually support when a disturbance occurs in the single ac grid on shore.
In order to solve the problem of grid-connected frequency stability of wind power through a multi-terminal flexible direct-current system, frequency control is generally added on the basis of direct-current voltage sag control of a receiving-end converter station, and a power reference value increment caused by frequency deviation of an alternating-current system is overlapped on an MMC initial power reference value, so that frequency change of a single alternating-current system is transmitted to the whole network through direct-current voltage, and unbalanced power is commonly consumed by other converter stations. In order to enable the additional frequency control to be adjusted in real time according to the running state of the system, the conventional droop coefficient is improved in the adaptive droop control of the VSC-MTDC connected with the low inertia system, so that a better frequency modulation effect is achieved, however, the dynamic direct current droop coefficient can influence the running of the system, and the safe running of the system can be threatened when serious.
Accordingly, the inventors now provide a frequency control method that does not change the droop coefficient of the direct current, ensuring safe operation of the system.
Disclosure of Invention
The invention aims to provide a frequency modulation method of a wind power multi-terminal flexible direct current system based on self-adaptive reference power, which can avoid the damage to the system caused by overlarge direct current voltage offset due to additional frequency control during the frequency disturbance of the offshore wind power multi-terminal flexible direct current system on the shore and improve the frequency stability of a non-disturbance side alternating current system.
In order to solve the technical problems, the invention adopts the following technical scheme:
the wind power multi-terminal flexible direct system frequency adjustment method based on self-adaptive reference power is applied to a wind power multi-terminal flexible direct grid-connected system, the wind power multi-terminal flexible direct grid-connected system comprises a wind power plant WPP, a sending end converter station MMC4, three receiving end converter stations are respectively a receiving end converter station MMC1, a receiving end converter station MMC2 and a receiving end converter station MMC3, and three alternating current power grids are respectively an alternating current power grid S1, an alternating current power grid S2 and an alternating current power grid S3; wind turbine generator system WPP inserts end current conversion station MMC4 through two sets of step-up transformers, and end current conversion station MMC4 is through the electric energy conversion that the rectification will carry into direct current, carries the receiving end current conversion station MMC1, receiving end current conversion station MMC2 and receiving end current conversion station MMC3 through direct current cable again with the electric energy, and receiving end current conversion station MMC1, receiving end current conversion station MMC2 and receiving end current conversion station MMC3 are with the electric energy conversion that transmits into alternating current, carry to AC grid S1, AC grid S2 and AC grid S3. The frequency modulation method of the system comprises adaptive reference power control of a receiving end converter station, control of a transmitting end converter station and comprehensive inertia control of a wind turbine generator.
When the controller of the receiving-end converter station works, the following steps are adopted:
step 1-1) taking frequency deviation as input, converting the frequency deviation into direct-current voltage deviation, and superposing an output voltage signal on a system direct-current voltage reference value, making a difference with actual measured power, and accessing the voltage input port of a sagging controller of a receiving-end converter station;
step 1-2) correcting the reference power of the converter station controller, and comparing the corrected value with the actual measured power to be used as a power input port of the converter station droop controller;
step 1-3) the power input end of the sag controller of the receiving end converter station passes through a sag coefficient K V Converting power signal into voltage signal, and convertingAnd adding the voltage input end signals of the current stations, and outputting control signals through the PI controller to control the operation of the receiving end current converter stations.
In step 1-1), the frequency signal is converted into a direct current voltage signal through virtual inertial control, and the relationship between the direct current voltage and the frequency is expressed as:
in U dc,ref For a set reference DC voltage, C eq Is the equivalent capacitance of the converter station, S MMC For rated capacity of the system, U dc0 For the initial DC voltage of the system, f 0 For nominal frequency, H MMC Is a virtual inertia coefficient, which is defined as follows:
wherein DeltaU dc,max And DeltaU dc,min As the maximum deviation and the minimum deviation of the DC voltage, deltaU dc And Δf is the measured DC voltage deviation and measured frequency deviation, H MMC0 Mu is an adjustment coefficient for the set initial virtual inertia coefficient.
In step 1-2), the reference power of the converter is modified, and the adaptive reference power is set as follows:
in the middle ofFor adaptive power reference value, < >>For the initial power reference value, P is the actual power, beta is the compensation coefficient, and the compensation coefficient is set as
Wherein alpha is an adjustment factor, K * The adjustable power margin factor for the converter station is specifically:
wherein P is max And P min Maximum and minimum active power for each converter station.
The controller of the transmitting converter station MMC4 adopts the following steps in operation:
step 2-1), taking the detected direct-current voltage deviation as input of a transmitting-end converter station, converting the direct-current voltage deviation into a frequency signal through a proportionality coefficient, and controlling the reference frequency of the wind turbine generator;
step 2-2) the wind turbine generator system passes through a primary frequency modulation coefficient K according to the input frequency signal d And an inertia coefficient K i And generating a control signal, and accessing the control signal to the output end of the wind turbine generator controller.
In step 2-1), the transmitting-end converter station converts the detected dc voltage deviation into a frequency signal by a scaling factor, where the scaling factor is set as:
wherein N is dc Is a proportionality coefficient, f ref For fan side reference frequency, U dc Is the actual DC voltage.
In step 2-2), the wind turbine generator sets adjust wind power output power according to the frequency deviation, and the output signal can be expressed as:
wherein ΔP is the adjusted power offset, K d Is one ofSecondary frequency modulation coefficient, K i Is an inertial control coefficient.
When the wind power system works, the following steps are adopted:
step 1: each receiving end converter station calculates an initial virtual inertia coefficient H according to the rated capacity of the converter station and the energy storage of the direct current capacitor of the converter station under the rated voltage MMC,0 The method comprises the steps of carrying out a first treatment on the surface of the The offshore wind turbine sets an inertia control coefficient K according to the rated rotation speed and rated capacity i Setting a frequency modulation trigger threshold T and a direct-current voltage safety constraint delta U T ；
Step 2: when the frequency of the on-shore alternating current power grid changes, detecting the real-time frequency change of each converter station, judging whether the frequency deviation delta f exceeds a threshold T, if so, entering a step 3, and if not, continuing to detect the frequency change without starting frequency modulation control;
step 3: when the frequency deviation is larger than the set threshold value, the detected frequency deviation is coupled to the direct-current voltage through virtual inertia control, the power support of the receiving-end converter station is caused under the action of droop control, and meanwhile the direct-current voltage deviation delta U is detected dc According to Δf and ΔU dc Dynamically adjusting the virtual inertia coefficient to ensure that the direct-current voltage is maintained within a safe range;
step 4: each receiving end converter station sets an adaptive reference power adjustment value delta P according to the respective adjustable power margin and frequency deviation ref.new Power reference value added to droop controlThe frequency stability of the power grid at the normal side is ensured in the process of participating in the frequency modulation function by the converter stations at the receiving ends;
step 5: detecting the direct-current voltage of the system, and judging whether the deviation of the direct-current voltage is larger than delta U T (5%); if yes, enter step 6; if not, the frequency of the wind power multi-terminal flexible-direct system is restored to be stable, and the control is ended;
step 6: when the receiving end alternating current system generates larger disturbance, the direct current voltage offset exceeds the set safety constraint delta U T (5%) wind power starting frequency modulation control, and the transmitting end converter station passes throughDetecting DC voltage offset DeltaU dc Acquiring the frequency change of a power grid, and modifying the frequency reference value f of the offshore alternating current system according to the frequency change ref Wind power adjusts output power through the detected frequency deviation and additional comprehensive inertia control;
step 7: after wind power participates in frequency modulation, the method enters step 5 to continuously detect the direct-current voltage change until the direct-current voltage deviation is smaller than the set safety constraint delta U T I.e. the frequency is restored to be stable and the control is ended.
The invention also comprises a frequency adjusting method of the wind power multi-terminal flexible direct system, which comprises the following steps:
step 1: each receiving end converter station calculates an initial virtual inertia coefficient H according to the rated capacity of the converter station and the energy storage of the direct current capacitor of the converter station under the rated voltage MMC,0 The method comprises the steps of carrying out a first treatment on the surface of the The offshore wind turbine sets an inertia control coefficient K according to the rated rotation speed and rated capacity i Setting a frequency modulation trigger threshold T and a direct-current voltage safety constraint delta U T ；
Step 2: when the frequency of the on-shore alternating current power grid changes, detecting the real-time frequency change of each converter station, judging whether the frequency deviation delta f exceeds a threshold T, if so, entering a step 3, and if not, continuing to detect the frequency change without starting frequency modulation control;
step 3: when the frequency deviation is larger than the set threshold value, the detected frequency deviation is coupled to the direct-current voltage through virtual inertia control, the power support of the receiving-end converter station is caused under the action of droop control, and meanwhile the direct-current voltage deviation delta U is detected dc According to Δf and ΔU dc Dynamically adjusting the virtual inertia coefficient to ensure that the direct-current voltage is maintained within a safe range;
step 4: each receiving end converter station sets an adaptive reference power adjustment value delta P according to the respective adjustable power margin and frequency deviation ref.new Power reference value added to droop controlThe frequency stability of the power grid at the normal side is ensured in the process of participating in the frequency modulation function by the converter stations at the receiving ends;
step 5: detecting the direct-current voltage of the system, and judging whether the deviation of the direct-current voltage is larger than delta U T (5%); if yes, enter step 6; if not, the frequency of the wind power multi-terminal flexible-direct system is restored to be stable, and the control is ended;
step 6: when the receiving end alternating current system generates larger disturbance, the direct current voltage offset exceeds the set safety constraint delta U T (5%) wind power starts frequency modulation control, and the transmitting end converter station detects the DC voltage offset delta U dc Acquiring the frequency change of a power grid, and modifying the frequency reference value f of the offshore alternating current system according to the frequency change ref Wind power adjusts output power through the detected frequency deviation and additional comprehensive inertia control;
step 7: after wind power participates in frequency modulation, the method enters step 5 to continuously detect the direct-current voltage change until the direct-current voltage deviation is smaller than the set safety constraint delta U T I.e. the frequency is restored to be stable and the control is ended.
The method can avoid the damage to the system caused by overlarge DC voltage deviation caused by additional frequency control during the frequency disturbance of the offshore wind power multi-terminal flexible DC system on the shore, and improves the frequency stability of the non-disturbance side AC system.
Compared with the prior art, the invention has the following technical effects:
(1) The wind power multi-terminal flexible direct system frequency adjustment method based on the self-adaptive reference power constructed by the invention utilizes virtual inertia control to reflect the frequency change to the system direct current voltage, and adjusts the virtual inertia coefficient H according to the direct current voltage deviation and the frequency deviation MMC The MMC-MTDC system can fully play the frequency modulation function and ensure the stability of direct current voltage;
(2) The direct current droop control based on the self-adaptive reference power adjusts the reference power according to the adjustable power margin and the frequency deviation of each converter station, and reasonably distributes unbalanced power. The normal side alternating current system is ensured to provide power support for the fault side converter station while the frequency stability of the normal side alternating current system is ensured.
Drawings
The invention is further illustrated by the following examples in conjunction with the accompanying drawings:
FIG. 1 is a schematic diagram of a wind power grid-connected structure through MMC-MTDC;
FIG. 2 is a flow chart of a wind power frequency modulation method based on self-adaptive reference power through an MMC-MTDC grid-connected system provided by the embodiment of the invention;
fig. 3 is a block diagram of adaptive virtual inertial control of a receiving-end converter station according to the present invention;
FIG. 4 is a block diagram of the DC voltage droop control based on adaptive reference power provided by the present invention;
fig. 5 is a diagram of a control method of an offshore end converter station according to the present invention;
FIG. 6 is a diagram of a comprehensive inertia control method of an offshore wind turbine provided by the invention;
FIG. 7 is a diagram of simulation results of DC voltage of the system when the load of the on-shore receiving end AC power grid is suddenly increased by 40 MW;
figure 8 is a graph of simulation results of the frequency of the receiving ac power grid when the load of the receiving ac power grid on shore provided by the invention increases by 40MW,
FIG. 9 is a graph of the simulation result of active power of the receiving end power grid when the load of the receiving end AC power grid on shore is suddenly increased by 40 MW;
fig. 10 is a graph of simulation results of direct current voltage of the flexible direct current system when the load of the on-shore receiving end alternating current power grid is suddenly reduced by 40 MW;
FIG. 11 is a graph of simulation results of the frequency of the receiving AC power grid when the load of the receiving AC power grid on shore is suddenly reduced by 40 MW;
FIG. 12 is a graph of the simulation result of active power of the receiving end power grid when the load of the receiving end AC power grid on shore provided by the invention is suddenly reduced by 40 MW;
fig. 13 is a graph of simulation results of direct current voltage of the flexible-direct system when the receiving end converter station on a certain side of the shore is in the out-of-service state;
fig. 14 is a diagram of simulation results of frequency of a receiving ac power grid when a receiving converter station on a certain side on shore is in a back-out operation;
fig. 15 is a diagram of simulation results of active power output by a fan when a receiving end converter station on a certain side of the shore is in a back-out operation;
Detailed Description
The wind power multi-terminal flexible direct system frequency adjustment method based on self-adaptive reference power is applied to a wind power multi-terminal flexible direct grid-connected system, the wind power multi-terminal flexible direct grid-connected system comprises a wind power plant WPP, a sending end converter station MMC4, three receiving end converter stations are respectively a receiving end converter station MMC1, a receiving end converter station MMC2 and a receiving end converter station MMC3, and three alternating current power grids are respectively an alternating current power grid S1, an alternating current power grid S2 and an alternating current power grid S3; wind turbine generator system WPP inserts end current conversion station MMC4 through two sets of step-up transformers, and end current conversion station MMC4 is through the electric energy conversion that the rectification will carry into direct current, carries the receiving end current conversion station MMC1, receiving end current conversion station MMC2 and receiving end current conversion station MMC3 through direct current cable again with the electric energy, and receiving end current conversion station MMC1, receiving end current conversion station MMC2 and receiving end current conversion station MMC3 are with the electric energy conversion that transmits into alternating current, carry to AC grid S1, AC grid S2 and AC grid S3. The frequency modulation method of the system comprises adaptive reference power control of a receiving end converter station, control of a transmitting end converter station and comprehensive inertia control of a wind turbine generator.
When the controller of the receiving-end converter station works, the following steps are adopted:
step 1-1) taking frequency deviation as input, converting the frequency deviation into direct-current voltage deviation, and superposing an output voltage signal on a system direct-current voltage reference value, making a difference with actual measured power, and accessing the voltage input port of a sagging controller of a receiving-end converter station;
step 1-2) correcting the reference power of the converter station controller, and comparing the corrected value with the actual measured power to be used as a power input port of the converter station droop controller;
step 1-3) the power input end of the sag controller of the receiving end converter station passes through a sag coefficient K V And converting the power signal into a voltage signal, adding the voltage signal with a voltage input end signal of the converter station, and outputting a control signal through a PI controller to control the operation of the receiving end converter station.
In step 1-1), the frequency signal is converted into a direct current voltage signal through virtual inertial control, and the relationship between the direct current voltage and the frequency is expressed as:
in U dc,ref For a set reference DC voltage, C eq Is the equivalent capacitance of the converter station, S MMC For rated capacity of the system, U dc0 For the initial DC voltage of the system, f 0 For nominal frequency, H MMC Is a virtual inertia coefficient, which is defined as follows:
wherein DeltaU dc,max And DeltaU dc,min As the maximum deviation and the minimum deviation of the DC voltage, deltaU dc And Δf is the measured DC voltage deviation and measured frequency deviation, H MMC0 Mu is an adjustment coefficient for the set initial virtual inertia coefficient.
In step 1-2), the reference power of the converter is modified, and the adaptive reference power is set as follows:
in the middle ofFor adaptive power reference value, < >>For the initial power reference value, P is the actual power, beta is the compensation coefficient, and the compensation coefficient is set as
Wherein alpha is an adjustment factor, K * The adjustable power margin factor for the converter station is specifically:
wherein P is max And P min Maximum and minimum active power for each converter station.
The controller of the transmitting converter station MMC4 adopts the following steps in operation:
step 2-1), taking the detected direct-current voltage deviation as input of a transmitting-end converter station, converting the direct-current voltage deviation into a frequency signal through a proportionality coefficient, and controlling the reference frequency of the wind turbine generator;
step 2-2) the wind turbine generator system passes through a primary frequency modulation coefficient K according to the input frequency signal d And an inertia coefficient K i And generating a control signal, and accessing the control signal to the output end of the wind turbine generator controller.
In step 2-1), the transmitting-end converter station converts the detected dc voltage deviation into a frequency signal by a scaling factor, where the scaling factor is set as:
wherein N is dc Is a proportionality coefficient, f ref For fan side reference frequency, U dc Is the actual DC voltage.
In step 2-2), the wind turbine generator sets adjust wind power output power according to the frequency deviation, and the output signal can be expressed as:
wherein ΔP is the adjusted power offset, K d For primary frequency modulation factor, K i Is an inertial control coefficient.
The wind power is operated by the multi-terminal flexible direct grid-connected system by the following steps:
step 1: each receiving end converter station calculates an initial virtual inertia coefficient H according to the rated capacity of the converter station and the energy storage of the direct current capacitor of the converter station under the rated voltage MMC,0 The method comprises the steps of carrying out a first treatment on the surface of the The offshore wind turbine sets an inertia control coefficient K according to the rated rotation speed and rated capacity i Setting a frequency modulation trigger threshold T and a direct-current voltage safety constraint delta U T ；
Step 2: when the frequency of the on-shore alternating current power grid changes, detecting the real-time frequency change of each converter station, judging whether the frequency deviation delta f exceeds a threshold T, if so, entering a step 3, and if not, continuing to detect the frequency change without starting frequency modulation control;
step 3: when the frequency deviation is larger than the set threshold value, the detected frequency deviation is coupled to the direct-current voltage through virtual inertia control, the power support of the receiving-end converter station is caused under the action of droop control, and meanwhile the direct-current voltage deviation delta U is detected dc According to Δf and ΔU dc Dynamically adjusting the virtual inertia coefficient to ensure that the direct-current voltage is maintained within a safe range;
step 4: each receiving end converter station sets an adaptive reference power adjustment value delta P according to the respective adjustable power margin and frequency deviation ref.new Power reference value added to droop controlThe frequency stability of the power grid at the normal side is ensured in the process of participating in the frequency modulation function by the converter stations at the receiving ends;
step 5: detecting the direct-current voltage of the system, and judging whether the deviation of the direct-current voltage is larger than delta U T (5%); if yes, enter step 6; if not, the frequency of the wind power multi-terminal flexible-direct system is restored to be stable, and the control is ended;
step 6: when the receiving end alternating current system generates larger disturbance, the direct current voltage offset exceeds the set safety constraint delta U T (5%) wind power starts frequency modulation control, and the transmitting end converter station detects the DC voltage offset delta U dc Acquiring the frequency change of a power grid, and modifying the frequency reference value f of the offshore alternating current system according to the frequency change ref Wind power is generated by detecting a frequency deviation,adjusting the output power of the power amplifier through additional integrated inertial control;
step 7: after wind power participates in frequency modulation, the method enters step 5 to continuously detect the direct-current voltage change until the direct-current voltage deviation is smaller than the set safety constraint delta U T I.e. the frequency is restored to be stable and the control is ended.
The invention also discloses a frequency adjusting method of the wind power multi-terminal flexible direct system, which is characterized by comprising the following steps of:
step 1: each receiving end converter station calculates an initial virtual inertia coefficient H according to the rated capacity of the converter station and the energy storage of the direct current capacitor of the converter station under the rated voltage MMC,0 The method comprises the steps of carrying out a first treatment on the surface of the The offshore wind turbine sets an inertia control coefficient K according to the rated rotation speed and rated capacity i Setting a frequency modulation trigger threshold T and a direct-current voltage safety constraint delta U T ；
Step 2: when the frequency of the on-shore alternating current power grid changes, detecting the real-time frequency change of each converter station, judging whether the frequency deviation delta f exceeds a threshold T, if so, entering a step 3, and if not, continuing to detect the frequency change without starting frequency modulation control;
step 3: when the frequency deviation is larger than the set threshold value, the detected frequency deviation is coupled to the direct-current voltage through virtual inertia control, the power support of the receiving-end converter station is caused under the action of droop control, and meanwhile the direct-current voltage deviation delta U is detected dc According to Δf and ΔU dc Dynamically adjusting the virtual inertia coefficient to ensure that the direct-current voltage is maintained within a safe range;
step 4: each receiving end converter station sets an adaptive reference power adjustment value delta P according to the respective adjustable power margin and frequency deviation ref.new Power reference value added to droop controlThe frequency stability of the power grid at the normal side is ensured in the process of participating in the frequency modulation function by the converter stations at the receiving ends;
step 5: detecting the direct-current voltage of the system, and judging whether the deviation of the direct-current voltage is larger than delta U T (5%); if yes, enter step 6; if not, wind power multiple endsThe frequency of the flexible-straight system is restored to be stable, and the control is finished;
step 6: when the receiving end alternating current system generates larger disturbance, the direct current voltage offset exceeds the set safety constraint delta U T (5%) wind power starts frequency modulation control, and the transmitting end converter station detects the DC voltage offset delta U dc Acquiring the frequency change of a power grid, and modifying the frequency reference value f of the offshore alternating current system according to the frequency change ref Wind power adjusts output power through the detected frequency deviation and additional comprehensive inertia control;
step 7: after wind power participates in frequency modulation, the method enters step 5 to continuously detect the direct-current voltage change until the direct-current voltage deviation is smaller than the set safety constraint delta U T I.e. the frequency is restored to be stable and the control is ended.
In order to facilitate a better understanding of the present invention by those of ordinary skill in the art, the following is further described:
fig. 1 is a schematic diagram of a wind power grid-connected structure through MMC-MTDC, and mainly comprises an offshore wind power plant, an offshore transmitting-end converter station, an onshore receiving-end converter station and an alternating current power grid.
FIG. 2 is a flow chart of a frequency modulation method of the offshore wind power multi-terminal flexible direct grid-connected system based on self-adaptive reference power. The process specifically comprises the following steps:
step 1: each receiving end converter station calculates an initial virtual inertia coefficient H according to the rated capacity of the converter station and the energy storage of the direct current capacitor of the converter station under the rated voltage MMC,0 The method comprises the steps of carrying out a first treatment on the surface of the The offshore wind turbine sets an inertia control coefficient K according to the rated rotation speed and rated capacity i Setting a frequency modulation trigger threshold T and a direct-current voltage safety constraint delta U T ；
Step 2: when the frequency of the on-shore alternating current power grid changes, detecting the real-time frequency change of each converter station, judging whether the frequency deviation delta f exceeds a threshold T, if so, entering a step 3, and if not, continuing to detect the frequency change without starting frequency modulation control;
step 3: when the frequency deviation is larger than the set threshold value, the detected frequency deviation is coupled to the direct current voltage through virtual inertia control, and under the action of droop control, the power support of the receiving end converter station is caused, and the direct current is detected simultaneouslyVoltage deviation deltau dc According to Δf and ΔU dc Dynamically adjusting the virtual inertia coefficient to ensure that the direct-current voltage is maintained within a safe range;
step 4: each receiving end converter station sets an adaptive reference power adjustment value delta P according to the respective adjustable power margin and frequency deviation ref.new Power reference value added to droop controlThe frequency stability of the power grid at the normal side is ensured in the process of participating in the frequency modulation function by the converter stations at the receiving ends;
step 5: detecting the direct-current voltage of the system, and judging whether the deviation of the direct-current voltage is larger than delta U T (5%); if yes, enter step 6; if not, the frequency of the wind power multi-terminal flexible-direct system is restored to be stable, and the control is ended;
step 6: when the receiving end alternating current system generates larger disturbance, the direct current voltage offset exceeds the set safety constraint delta U T (5%) wind power starts frequency modulation control, and the transmitting end converter station detects the DC voltage offset delta U dc Acquiring the frequency change of a power grid, and modifying the frequency reference value f of the offshore alternating current system according to the frequency change ref Wind power adjusts output power through the detected frequency deviation and additional comprehensive inertia control;
step 7: after wind power participates in frequency modulation, the method enters step 5 to continuously detect the direct-current voltage change until the direct-current voltage deviation is smaller than the set safety constraint delta U T I.e. the frequency is restored to be stable and the control is ended.
Fig. 3 is a block diagram of adaptive virtual inertial control of a receiving-end converter station according to the present invention, where in a conventional power system, a principle of participation of a synchronous generator in grid frequency modulation may be represented by a rotor motion equation:
wherein: h is the inertia coefficient of the synchronous generator, P m And P e Mechanical power sum of synchronous generatorsElectromagnetic power, f 0 Is the rated frequency.
In an MMC-MTDC system, for a single converter station, unbalanced power of input and output of the single converter station can be compensated by charging and discharging of a direct-current capacitor of the converter station, and a dynamic equation is as follows:
wherein: c (C) eq Is the equivalent capacitance of the converter station, S MMC For rated capacity of the system, U dc For system DC voltage, P in And P out Is the input power and the output power of the converter station.
And (3) building a relationship between direct-current side voltage and alternating-current measurement frequency according to a synchronous generator rotor motion equation:
wherein H is MMC Is the virtual inertia coefficient of the converter station.
Integrating operation is carried out on two ends of the (4) at the same time and the integrated operation is brought into the initial direct current voltage U of the system dc0 And rated frequency f 0 Reference dc voltage can be obtained:
wherein H is MMC The definition is as follows:
wherein DeltaU dc,max And DeltaU dc,min As the maximum deviation and the minimum deviation of the DC voltage, deltaU dc And Δf is the measured DC voltage deviation and measured frequency deviation, H MMC0 Mu is an adjustment coefficient for the set initial virtual inertia coefficient.
Fig. 4 is a block diagram of dc droop control based on adaptive reference power, which may be expressed as:
in the middle ofFor adaptive power reference value, < >>And P is the measured power, and β is the compensation coefficient.
When the alternating current system is disturbed, the larger the frequency change is, the larger the direct current voltage deviation is caused. To prevent the dc voltage from crossing the limit, the power to be compensated increases, and the compensation coefficient is:
wherein alpha is an adjustment factor, the adjustable power margin of each converter station is considered, the adjustable margin is introduced into the self-adaptive adjustment power, and the adjustable margin factor of each converter station is defined as
Bringing (7) (8) into (6) the proposed adaptive reference power is
As can be seen from the equation (9), the larger the dc voltage deviation is, the larger the compensation coefficient is, and if the dc deviation tends to be 0, the reference power is not compensated, and meanwhile, the adaptive reference power set by the equation helps to improve the distribution of unbalanced power after the system is disturbed, so that the system with small frequency deviation bears unbalanced power.
Fig. 5 is a diagram of a control method of an offshore end converter station according to the present invention. The wind field current measuring station adopts U/F control to multiply the deviation between the DC voltage and the rated value by a certain coefficient N dc The frequency change converted into an alternating current system is expressed as follows:
wherein N is dc Is a proportionality coefficient, f ref For fan side reference frequency, U dc Is the actual DC voltage.
FIG. 6 is a diagram of a method for controlling the integrated inertia of the offshore wind turbine.
The wind turbine generator system senses the frequency change, a frequency adjusting module is added on the basis of original control, primary frequency modulation and inertia control of the wind turbine generator system are carried out according to actual fluctuation of the power grid frequency, and the expression is as follows:
wherein ΔP is the adjusted power offset, K d For primary frequency modulation factor, K i Is an inertial control coefficient.
In order to verify the effectiveness of the method, a simulation model of wind power grid connection through an MMC-MTDC system shown in figure 1 is built in PSCAD/EMTDC. Wherein, each parameter is shown in table 1.
TABLE 1
Working condition 1: at t=4.5S, the ac grid S1 load increases by 40MW.
As can be seen from fig. 7, the dc voltage deviation is within 5%, the offshore wind farm does not participate in frequency modulation, and the S2 and S3 side converter stations adjust the output power to participate in frequency modulation. Compared with the traditional control method, the control strategy reduces the DC voltage deviation.
As can be seen from fig. 8 and 9, the frequency of the ac system S3 is greatly affected when participating in the frequency modulation process. Compared with the traditional control strategy, the provided strategy reduces the frequency deviation of the normal side S3 alternating current system and simultaneously improves the frequency minimum point of the disturbance side alternating current system S1 by adjusting the active power of the S2 and S3 side converter stations.
Working condition 2: at t=4.5S, the ac grid S1 load is reduced by 40MW.
As can be seen from fig. 10, the dc voltage deviation is within 5%, the offshore wind farm does not participate in frequency modulation, and the S2 and S3 side converter stations adjust the output power to participate in frequency modulation. Compared with the traditional control method, the control strategy reduces the DC voltage deviation.
As can be seen from fig. 11 and 12, the frequency of the ac system S3 is greatly affected when participating in the frequency modulation process. Compared with the traditional control strategy, the proposed strategy reduces the frequency deviation of the normal-side S3 alternating current system and simultaneously reduces the frequency peak value of the disturbance-side alternating current system S1 by adjusting the active power of the S2 and S3 side converter stations.
Working condition 3: at t=4.5 s, the converter station MMC1 is taken out of operation.
As can be seen from fig. 13, the dc voltage deviation exceeds 5%, the wind turbine participates in frequency modulation, the output power of the wind turbine is reduced, and unbalanced power in the system is reduced.
As can be seen from fig. 14 and 15, the wind turbine generator participates in frequency modulation to effectively reduce the frequency deviation at the normal side, and simultaneously reduce the dc voltage deviation, thereby ensuring the safe operation of the system.

Claims (8)

1. The wind power multi-terminal flexible direct system frequency adjustment method based on the self-adaptive reference power is characterized by being applied to a wind power multi-terminal flexible direct grid-connected system, wherein the wind power multi-terminal flexible direct grid-connected system comprises a wind power multi-terminal flexible direct grid-connected system, the wind power multi-terminal flexible direct grid-connected system comprises a wind power plant WPP, a transmitting end converter station MMC4, three receiving end converter stations are respectively a receiving end converter station MMC1, a receiving end converter station MMC2 and a receiving end converter station MMC3, and three alternating current power grids are respectively an alternating current power grid S1, an alternating current power grid S2 and an alternating current power grid S3; wind turbine generator system WPP inserts end current conversion station MMC4 through two sets of step-up transformers, and end current conversion station MMC4 is through the electric energy conversion that the rectification will carry into direct current, carries the receiving end current conversion station MMC1, receiving end current conversion station MMC2 and receiving end current conversion station MMC3 through direct current cable again with the electric energy, and receiving end current conversion station MMC1, receiving end current conversion station MMC2 and receiving end current conversion station MMC3 are with the electric energy conversion that transmits into alternating current, carry to AC grid S1, AC grid S2 and AC grid S3.
2. A method according to claim 1, characterized in that the controller of the receiving converter station, in operation, takes the steps of:
step 1-1) taking frequency deviation as input, converting the frequency deviation into direct-current voltage deviation, and superposing an output voltage signal on a system direct-current voltage reference value, making a difference with actual measured power, and accessing the voltage input port of a sagging controller of a receiving-end converter station;
step 1-2) correcting the reference power of the converter station controller, and comparing the corrected value with the actual measured power to be used as a power input port of the converter station droop controller;
step 1-3) the power input end of the sag controller of the receiving end converter station passes through a sag coefficient K V And converting the power signal into a voltage signal, adding the voltage signal with a voltage input end signal of the converter station, and outputting a control signal through a PI controller to control the operation of the receiving end converter station.
3. The method according to claim 2, wherein in step 1-1), the frequency signal is converted into a dc voltage signal by virtual inertial control, wherein the dc voltage versus frequency relationship is expressed as:
in U dc,ref For a set reference DC voltage, C eq Is the equivalent capacitance of the converter station, S MMC Is a systemRated capacity of U dc0 For the initial DC voltage of the system, f 0 Is rated frequency, f is measured frequency, H MMC Is a virtual inertia coefficient, which is defined as follows:
wherein DeltaU dc,max And DeltaU dc,min As the maximum deviation and the minimum deviation of the DC voltage, deltaU dc And Δf is the measured DC voltage deviation and measured frequency deviation, H MMC0 Mu is an adjustment coefficient for the set initial virtual inertia coefficient.
4. A method according to claim 2, characterized in that in step 1-2) the reference power of the converter is modified, and that the adaptive reference power is set as:
in the middle ofFor adaptive power reference value, < >>For the initial power reference value, P is the measured power, β is the compensation coefficient, and the compensation coefficient is set as follows:
wherein alpha is an adjustment factor, K * The adjustable power margin factor for the converter station is specifically:
wherein P is max And P min Maximum and minimum active power for each converter station.
5. A method according to claim 1, characterized in that the controller of the terminal converter station MMC4 is operative to take the steps of:
step 2-1), taking the detected direct-current voltage deviation as input of a transmitting-end converter station, converting the direct-current voltage deviation into a frequency signal through a proportionality coefficient, and controlling the reference frequency of the wind turbine generator;
step 2-2) the wind turbine generator system passes through a primary frequency modulation coefficient K according to the input frequency signal d And an inertia coefficient K i And generating a control signal, and accessing the control signal to the output end of the wind turbine generator controller.
6. The method according to claim 5, wherein in step 2-1), the converter station at the transmitting end converts the detected dc voltage deviation into a frequency signal by a scaling factor, the scaling factor being set to:
wherein N is dc Is a proportionality coefficient, f ref For fan side reference frequency, U dc Is the actual DC voltage.
7. The method according to claim 5, wherein in step 2-2), the wind power generation unit adjusts the wind power output power according to the frequency deviation, and the output signal thereof can be expressed as:
wherein ΔP is the adjusted power offset, K d For primary frequency modulation factor, K i Is an inertial control coefficient.
8. The frequency adjusting method of the wind power multi-terminal flexible direct system is characterized by comprising the following steps of:
step 1: each receiving end converter station calculates an initial virtual inertia coefficient H according to the rated capacity of the converter station and the energy storage of the direct current capacitor of the converter station under the rated voltage MMC,0 The method comprises the steps of carrying out a first treatment on the surface of the The offshore wind turbine sets an inertia control coefficient K according to the rated rotation speed and rated capacity i Setting a frequency modulation trigger threshold T and a direct-current voltage safety constraint delta U T ；
Step 2: when the frequency of the on-shore alternating current power grid changes, detecting the real-time frequency change of each converter station, judging whether the frequency deviation delta f exceeds a threshold T, if so, entering a step 3, and if not, continuing to detect the frequency change without starting frequency modulation control;
step 3: when the frequency deviation is larger than the set threshold value, the detected frequency deviation is coupled to the direct-current voltage through virtual inertia control, the power support of the receiving-end converter station is caused under the action of droop control, and meanwhile the direct-current voltage deviation delta U is detected dc According to Δf and ΔU dc Dynamically adjusting the virtual inertia coefficient to ensure that the direct-current voltage is maintained within a safe range;
step 4: each receiving end converter station sets an adaptive reference power adjustment value delta P according to the respective adjustable power margin and frequency deviation ref.new Power reference value added to droop controlThe frequency stability of the power grid at the normal side is ensured in the process of participating in the frequency modulation function by the converter stations at the receiving ends;
step 5: detecting the direct-current voltage of the system, and judging whether the deviation of the direct-current voltage is larger than delta U T (5%); if yes, enter step 6; if not, the frequency of the wind power multi-terminal flexible-direct system is restored to be stable, and the control is ended;
step 6: when the receiving end alternating current system generates larger disturbance, the direct current voltage offset exceeds the set safety constraint delta U T (5%) wind power starting frequency modulation control, feedingThe end converter station detects the DC voltage offset delta U dc Acquiring the frequency change of a power grid, and modifying the frequency reference value f of the offshore alternating current system according to the frequency change ref Wind power adjusts output power through the detected frequency deviation and additional comprehensive inertia control;
step 7: after wind power participates in frequency modulation, the method enters step 5 to continuously detect the direct-current voltage change until the direct-current voltage deviation is smaller than the set safety constraint delta U T I.e. the frequency is restored to be stable and the control is ended.
CN202310676974.0A 2023-06-08 2023-06-08 Wind power multi-terminal flexible direct system frequency adjustment method based on self-adaptive reference power Pending CN116961020A (en)

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