CN109038658B - Open sea wind power flexible direct current sending-out system and onshore alternating current single-phase earth fault ride-through method - Google Patents

Open sea wind power flexible direct current sending-out system and onshore alternating current single-phase earth fault ride-through method Download PDF

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CN109038658B
CN109038658B CN201810821016.7A CN201810821016A CN109038658B CN 109038658 B CN109038658 B CN 109038658B CN 201810821016 A CN201810821016 A CN 201810821016A CN 109038658 B CN109038658 B CN 109038658B
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current
alternating
mmc
voltage
phase
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CN109038658A (en
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雷宪章
杨杰
吴亚楠
王晓宇
袁艺嘉
刘亚丽
胡家兵
何震
林磊
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Huazhong University of Science and Technology
Electric Power Research Institute of State Grid Shandong Electric Power Co Ltd
Global Energy Interconnection Research Institute
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Huazhong University of Science and Technology
Electric Power Research Institute of State Grid Shandong Electric Power Co Ltd
Global Energy Interconnection Research Institute
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    • H02J3/386
    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02JCIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00Circuit arrangements for ac mains or ac distribution networks
    • H02J3/24Arrangements for preventing or reducing oscillations of power in networks
    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02JCIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00Circuit arrangements for ac mains or ac distribution networks
    • H02J3/26Arrangements for eliminating or reducing asymmetry in polyphase networks
    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02JCIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/46Controlling of the sharing of output between the generators, converters, or transformers
    • YGENERAL 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
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02EREDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00Energy generation through renewable energy sources
    • Y02E10/70Wind energy
    • Y02E10/76Power conversion electric or electronic aspects
    • YGENERAL 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
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02EREDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/50Arrangements for eliminating or reducing asymmetry in polyphase networks

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Abstract

The invention discloses an open sea wind power flexible direct current sending-out system and a land alternating current single-phase earth fault ride-through method, wherein the method comprises the following steps: s1 detecting the electric quantity of the alternating current side of the position of the first MMC converter in real time, S2 judging whether a land alternating current single-phase ground fault occurs according to the electric quantity of the alternating current side, if yes, turning to S3, and if not, returning to S1; s3 executing (a), (B), and (C) simultaneously; (A) suppressing an alternating negative sequence current generated during an onshore alternating single-phase ground fault by superimposing a negative sequence reference voltage in a differential mode component of a second MMC converter leg reference voltage; (B) setting the amplitude of the AC positive sequence current control command to be k times of a rated value to eliminate the overcurrent during the AC single-phase earth fault ride-through; (C) and zero-sequence double-frequency suppression voltage is superposed in the common-mode component of the bridge arm reference voltage of the second MMC converter to suppress the zero-sequence double-frequency fluctuation of the direct-current voltage generated during the onshore alternating-current single-phase ground fault.

Description

Open sea wind power flexible direct current sending-out system and onshore alternating current single-phase earth fault ride-through method
Technical Field
The invention belongs to the field of alternating current fault protection of an offshore wind power flexible direct current sending-out system, and particularly relates to an open-sea wind power flexible direct current sending-out system and a onshore alternating current single-phase earth fault ride-through method.
Background
Wind power generation is one of the most mature and scale development conditions of new energy power generation. Because wind power resources on the sea are rich and are closer to a load center than onshore wind power, offshore wind power is vigorously developed in China in the regions of the east south coast and the Bohai Bay in recent years. In theory, a High Voltage Alternating Current (HVAC) transmission technology and a High Voltage Direct Current (HVDC) transmission technology can be adopted for offshore wind power integration. According to related researches, a high-voltage alternating-current transmission mode can be considered under the conditions that the rated capacity of a wind power plant is less than 400MW and the offshore distance is less than 70km, but the following problems exist under the power transmission requirements of open sea and large capacity: (1) the same active power is transmitted, and the construction cost and the power loss of the alternating current transmission line are increased faster than those of the direct current transmission line; (2) the capacitance effect of the submarine cable can generate a large amount of reactive power, the effective load capacity of the cable is reduced, the voltage of a power grid is raised, and reactive compensation in the middle of the submarine power transmission cable is difficult to perform. Therefore, the large-scale open sea wind power flexible direct current transmission technology is highly concerned by academia and industry.
The onshore alternating-current transmission line generally adopts an overhead line, and various types of alternating-current faults (including single-phase earth faults, two-phase short-circuit faults and the like) cannot be avoided. Because the sending end converter of the open sea wind power flexible direct current sending system is connected with 100% renewable energy, once various types of alternating current faults occur in the onshore alternating current power transmission network, serious over-current damage is caused to the receiving end converter, a power transmission channel of the sending end converter is blocked, and the continuous output of the wind turbine generator cannot be discharged, so that the alternating current feeder of the fan generates overvoltage. At present, the main ideas of the academic and industrial circles for dealing with the above problems are: on one hand, the receiving end converter executes a corresponding fault ride-through strategy according to the fault type and the fault depth; on the other hand, the wind turbine generator set is put into the energy discharging device to consume redundant output. However, the above processing concept requires additional modification of the energy leakage device of each fan of the offshore wind farm, and the price and cost are high. Secondly, onshore alternating current fault protection of the open sea wind power through the flexible direct current sending system is a global problem, and the processing idea lacks coordination and coordination of a sending end system and a receiving end system.
In conclusion, the existing protection idea is difficult to effectively pass through the onshore alternating current fault of the flexible direct current sending-out system of the open-sea wind power plant, and further improvement and optimization space exists.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide an open-sea wind power flexible direct current sending-out system and a land alternating current single-phase earth fault ride-through method.
The invention provides a flexible direct current sending system for open sea wind power, which comprises: the system comprises a plurality of offshore wind power plants, a first MMC current converter, a second MMC current converter and an energy leakage device; a plurality of offshore wind power plants are connected to the first MMC current converter through alternating current feeders; the first MMC current converter and the second MMC current converter are connected through a positive and negative direct current transmission cable; the energy leakage device is arranged between the offshore wind farm and the first MMC current converter and used for dissipating the continuous active power output of the wind farm which is difficult to send out through the direct current transmission line during the onshore alternating current fault, and the overvoltage caused by the energy accumulation of the alternating current feeder line is avoided.
When the onshore alternating-current single-phase ground fault occurs, alternating negative-sequence current generated during the onshore alternating-current single-phase ground fault is restrained by superposing negative-sequence reference voltage in a differential mode component of bridge arm reference voltage of the second MMC converter; an amplitude limiting link is set for the alternating current positive sequence current control instruction to eliminate the overcurrent during the crossing of the alternating current single-phase earth fault; and zero-sequence double-frequency suppression voltage is superposed in the common-mode component of the bridge arm reference voltage of the second MMC converter to suppress the zero-sequence double-frequency fluctuation of the direct-current voltage generated during the onshore alternating-current single-phase ground fault.
When the alternating current feeder line of the wind power plant has overvoltage, the conduction duty ratio of an IGBT device in the energy discharging device is determined according to the detected amplitude of the alternating phase voltage and the rated direct current of the alternating phase voltage, and the energy discharging device is controlled to work and dissipate the overvoltage according to the duty ratio.
The invention also provides a onshore alternating-current single-phase earth fault ride-through method based on the open-sea wind power flexible direct-current sending system, which comprises the following steps:
s1: detecting the electric quantity of the alternating current side at the position of the first MMC converter in real time,
s2: judging whether a land AC single-phase earth fault occurs according to the AC side electric quantity, if so, switching to step S3, otherwise, returning to step S1;
s3: simultaneously performing the following step (A), step (B) and step (C):
the step (A) is as follows: suppressing an alternating negative sequence current generated during an onshore alternating single-phase ground fault by superimposing a negative sequence reference voltage in a differential mode component of a second MMC converter leg reference voltage;
the step (B) is as follows: controlling an alternating positive sequence current command id +*、iq +*Is set to k times the rated value to eliminate over-current during ac single-phase ground fault ride-through;
the step (C) is as follows: and zero-sequence double-frequency suppression voltage is superposed in the common-mode component of the bridge arm reference voltage of the second MMC converter to suppress the zero-sequence double-frequency fluctuation of the direct-current voltage generated during the onshore alternating-current single-phase ground fault.
Further, after step S3, the method further includes:
s4: detecting the amplitude of the alternating-current side phase voltage at the position of the energy discharging device in real time;
s5: judging whether overvoltage occurs to the alternating current feeder line of the wind power plant according to the phase voltage amplitude, if so, turning to the step S6, otherwise, returning to the step S4;
s6: and obtaining the conduction duty ratio of an IGBT device in the energy discharging device according to the phase voltage amplitude and the alternating-current phase voltage rated value, and controlling the energy discharging device to work according to the duty ratio to dissipate the overvoltage.
Generally, compared with the prior art, the above technical solution conceived by the present invention has the following beneficial effects:
(1) during the single-phase earth fault of land interchange, through installing the earial drainage device between first MMC transverter and wind-powered electricity generation field, and exert active control to the input and the excision of earial drainage device, during the trouble, can the at utmost transmit active power under the prerequisite of maintaining the alternating current busbar voltage.
(2) By applying an amplitude limiting link to the positive sequence current instruction of the second MMC converter, the overcurrent of the MMC converter during the onshore alternating current single-phase earth fault can be effectively avoided.
(3) Through applying direct current zero sequence fluctuation suppression to the second MMC transverter, the influence of the second MMC transverter on the safe operation of the first MMC transverter can be effectively avoided.
Drawings
Fig. 1 is a schematic structural diagram of the open sea wind power flexible direct current delivery system of the present invention.
Fig. 2 is a schematic diagram of an MMC converter, in which the submodules are half-bridge structures.
Fig. 3 is a negative sequence current control block diagram of a second MMC converter of the present invention, in which, (a) shows a negative sequence current controller configuration block diagram, and (b) shows a negative sequence current response block diagram.
Fig. 4 is a zero sequence double frequency fluctuation suppression control block diagram of the dc side of the second MMC converter of the present invention.
Fig. 5 is a chopper control block diagram of an ac side energy discharge resistor of the first MMC converter of the present invention.
FIG. 6 is a simulation diagram of the flexible direct current sending system of the open sea wind farm in the embodiment of the invention under the condition of the onshore alternating current single-phase ground fault. The MMC converter comprises a first MMC converter body, a second MMC converter body, a first direct current bus, a second direct current bus, a third direct current bus, a fourth direct current bus.
FIG. 7 is a simulation diagram of the flexible direct current sending system of the open sea wind farm for carrying out onshore alternating current single-phase ground fault ride-through in the embodiment of the invention. The method comprises the following steps of (a) a graph of the change of three-phase alternating current voltage of a second MMC converter along with time, (b) a graph of the change of the three-phase alternating current of the second MMC converter along with time, (c) a graph of the change of direct current voltage along with time, (d) a graph of the change of positive and negative direct current bus current along with time, (e) a graph of the change of active power of a first MMC converter along with time, (f) a graph of the change of the active power of a first MMC converter along with time, (g) duty ratio switching of an energy leakage device IGBT, (h) partial enlarged image of a region 1 in the duty ratio switching.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
The invention relates to a method for realizing onshore alternating current fault ride-through of an offshore wind power flexible direct current sending system by the coordination of a receiving-end modular Multilevel converter MMC (modular Multilevel converter) and a sending-end energy discharge resistor; the invention can solve the problems that the coordination of the transmitting end and the receiving end is not considered in the existing onshore alternating current fault protection strategy, and the energy discharge of the fault transient wind field output needs to depend on the energy discharge device of the fan. According to the invention, the energy release device is arranged between the wind power plant and the first MMC converter, and the ground AC fault ride-through is realized through the coordination and coordination between the sending end energy release device and the second MMC converter. During the fault period, the negative sequence current and fault transient overcurrent caused by the alternating current single-phase earth fault can be eliminated on the premise of not sacrificing the controllability of the converter. Meanwhile, through the chopping control of the sending end alternating current energy discharging device, the transient overvoltage of the alternating current feeder line of the wind power plant during the fault period can be effectively inhibited. During the single-phase earth fault of onshore interchange, the flexible direct current of open sea wind-powered electricity generation sends out the system and can furthest maintain power transmission.
As shown in fig. 1, the open sea wind power flexible dc output system provided by the present invention comprises: the system comprises a plurality of offshore wind power plants, a first MMC current converter, a second MMC current converter and an energy leakage device; the offshore wind power plants are connected to the first MMC current converter through alternating current feeders; the first MMC current converter and the second MMC current converter are connected through a positive and negative direct current transmission cable; the energy release device is installed between an offshore wind farm and a first MMC and is used for dissipating continuous active power output of the wind farm which is difficult to send out through a direct current transmission line during the onshore alternating current fault, and overvoltage caused by energy accumulation of an alternating current feeder line is avoided.
The first MMC current converter and the second MMC current converter have the same structure, as shown in fig. 2, each bridge arm of the MMC current converter can be formed by connecting single type half-bridge or full-bridge sub-modules in series or formed by connecting different types of sub-modules in series; the alternating current port of the MMC converter is grounded through a star-shaped reactance device, wherein the neutral point of the star-shaped reactance device is grounded through a resistor; the MMC converter comprises A, B, C three phases, each phase comprises an upper bridge arm and a lower bridge arm, and each bridge arm can be formed by connecting single-type half-bridge or full-bridge submodules in series or different-type submodules in series. And during normal work, the second MMC is used for determining the direct-current voltage, and the first MMC is used for determining the amplitude and the frequency of the alternating-current voltage.
The energy leakage device is arranged on an alternating current bus between the first MMC converter and the open sea wind power plant and is formed by connecting star-connected resistors in series with an IGBT device.
In the embodiment of the invention, the ride-through control of the second MMC converter in the open-sea wind power flexible direct current sending system during the land alternating current single-phase earth fault and the chopping control of the first MMC alternating current side energy discharging device during the fault are specifically described as follows:
as shown in fig. 3, the ride-through control of the second MMC converter during a land ac single-phase ground fault comprises the following steps:
(1) the second MMC converter continuously detects the electric quantity of the alternating current side at the position, whether a land alternating current single-phase earth fault occurs is judged according to the detected electric quantity, if yes, the step (2) is sequentially executed, and if not, the detection is continued; wherein the electrical quantities comprise an alternating voltage and an alternating current;
(2) after the second MMC converter detects the single-phase earth fault of the onshore alternating current, the following steps (A), (B) and (C) are simultaneously carried out:
the step (A) is as follows: adjusting the reference voltage of the bridge arm of the second MMC converter, and superposing a negative sequence reference voltage e in the differential mode component of the reference voltagej -*(j ═ a, b, c) to suppress ac negative-sequence current generated during a land ac single-phase ground fault; during the onshore alternating current fault period, the three-phase voltages of the alternating current power grid are not symmetrical to each other, negative sequence voltage is generated, and negative sequence current is excited. By superposing the negative sequence reference voltage in the bridge arm reference voltage, the negative sequence voltage in the alternating current power grid can be offset, so that three-phase negative sequence current in the power grid current can be eliminated.
Wherein a negative sequence reference voltage e is superposed in the differential mode component of the bridge arm reference voltagej -*(j ═ a, bc) can be calculated from the following equation:
Figure BDA0001741393920000071
Figure BDA0001741393920000072
wherein e isd -*、eq -*Respectively, negative sequence reference voltage ej -*A dq-transformed d-axis component and a q-axis component. i.e. ivd -、ivq -The d-axis component and the q-axis component of the detected alternating negative sequence current after dq conversion are respectively used. u. ofsd -、udq -D-axis component and q-axis component of the detected alternating negative sequence voltage after dq transformation are respectively. i.e. ivd -*、ivq -*The d-axis component and the q-axis component, respectively, of the alternating negative sequence current command are set to zero. KpAnd KIRespectively, proportional parameters and integral parameters of the PI controller. Omega is the angular frequency of the alternating current power grid, L is the equivalent reactance of the alternating current side of the MMC, and the magnitude of L is one half of the reactance value of the bridge arm.
The step (B) is: for AC positive sequence current control command id +*、iq +*And setting an amplitude limiting link, and selecting an amplitude limiting value command to be k times of a rated value according to the overload capacity of the AC line and engineering experience so as to eliminate the overcurrent phenomenon during the AC single-phase earth fault ride-through. k is generally selected to be 1.5 to 1.8. During the single-phase earth fault of interchange, the positive sequence voltage of alternating-current network appears falling for the alternating-current side power transmission ability of second MMC transverter is restricted, for further maintaining the power balance relation of direct-current side input and alternating-current side output, and its direct current voltage controller constantly increases positive sequence current instruction, improves alternating-current side power transmission ability, however this has led to exchanging the overcurrent phenomenon. The alternating current overcurrent phenomenon can be eliminated by adding an amplitude limiting link at the positive sequence current instruction to limit the continuous increase of the positive sequence current instruction. And the surplus active power continuously fed into the direct current side of the second MMC converter can be dissipated through chopping control of a subsequent energy discharging device.
Wherein, the AC positive sequence current control command id +*、iq +*The clipping value of (d) can be determined according to the following equation:
Figure BDA0001741393920000073
wherein the subscript "max" denotes the clipping value, i+ d_rated、i+ q_ratedRespectively, representing the ac positive sequence current rating.
The step (C) is as follows: adjusting the reference voltage of a bridge arm of a second MMC converter, and superposing zero-sequence double frequency suppression voltage in the common-mode component of the reference voltage
Figure BDA0001741393920000081
So as to restrain the zero-sequence double-frequency fluctuation of the direct current voltage generated during the earth alternating current single-phase earth fault.
As shown in fig. 4, zero sequence double frequency suppression voltage
Figure BDA0001741393920000082
Can be determined according to the following formula:
Figure BDA0001741393920000083
Figure BDA0001741393920000084
wherein u ispa、upb、upcRespectively output voltage u of an MMC three-phase upper bridge arm submodulena、unb、uncAnd the output voltages of the MMC three-phase lower bridge arm sub-modules are respectively. u. ofavgThe average value of six bridge arm voltages of the MMC is obtained; omegacTwice the angular frequency of the ac grid; k0Is the proportionality coefficient of the controller; xi is a damping ratio; s is the laplace operator.
As shown in fig. 5, in the embodiment of the present invention, the chopping control of the ac-side energy discharging device of the first MMC converter in the open-sea wind power flexible dc transmission system during the fault period is specifically described as follows:
(1) the alternating current side energy discharging device of the first MMC converter continuously detects the alternating current side phase voltage amplitude of the position, whether overvoltage occurs to an alternating current feeder line of the wind power plant is judged according to the detected alternating current phase voltage amplitude, if yes, the step (2) is sequentially executed, and if not, the detection is continued;
(2) and determining the conduction duty ratio d of the IGBT device in the energy discharging device according to the detected amplitude of the alternating-current phase voltage and the rated direct current of the alternating-current phase voltage.
The on duty ratio d of the IGBT device in the energy leakage device can be calculated by the following formula:
Figure BDA0001741393920000085
d=Kp_ac(Vm_ref-Vm)+KI_ac∫(Vm_ref-Vm)dt
wherein, Va、Vb、VcRespectively representing three-phase voltage to ground of the alternating current bus of the first MMC converter; vmIs the phase voltage amplitude; vm_refTaking 1.1 times of a rated value of the phase voltage as a reference value of the phase voltage according to engineering experience; kp_acIndicating a proportional parameter of the controller, KI_acRepresenting the integral parameters of the controller.
The invention also provides a onshore alternating-current single-phase earth fault ride-through method based on the open-sea wind power flexible direct-current sending system, which comprises the following steps:
s1: detecting the electric quantity of the alternating current side at the position of the first MMC converter in real time,
s2: judging whether a land AC single-phase earth fault occurs according to the AC side electric quantity, if so, switching to step S3, otherwise, returning to step S1;
s3: simultaneously performing the following step (A), step (B) and step (C):
step (A): suppressing an alternating negative sequence current generated during an onshore alternating single-phase ground fault by superimposing a negative sequence reference voltage in a differential mode component of a second MMC converter leg reference voltage;
step (B): controlling an alternating positive sequence current command id +*、iq +*Is set to k times the rated value to eliminate over-current during ac single-phase ground fault ride-through;
step (C): and zero-sequence double-frequency suppression voltage is superposed in the common-mode component of the bridge arm reference voltage of the second MMC converter to suppress the zero-sequence double-frequency fluctuation of the direct-current voltage generated during the onshore alternating-current single-phase ground fault.
In the embodiment of the invention, in the step 2(A) of ride-through control of the second MMC converter during the land AC single-phase earth fault, the negative-sequence reference voltage e superposed in the differential mode component of the bridge arm reference voltagej -*(j ═ a, b c) can be calculated from the following equation:
Figure BDA0001741393920000091
Figure BDA0001741393920000092
wherein e isd -*、eq -*Respectively, negative sequence reference voltage ej -*A dq-transformed d-axis component and a q-axis component. i.e. ivd -、ivq -The d-axis component and the q-axis component of the detected alternating negative sequence current after dq conversion are respectively used. u. ofsd -、udq -D-axis component and q-axis component of the detected alternating negative sequence voltage after dq transformation are respectively. i.e. ivd -*、ivq -*The d-axis component and the q-axis component, respectively, of the alternating negative sequence current command are set to zero. KpAnd KIRespectively, proportional parameters and integral parameters of the PI controller. Omega is the angular frequency of the alternating current power grid, L is the equivalent reactance of the alternating current side of the MMC, and the magnitude of L is one half of the reactance value of the bridge arm.
In the embodiment of the invention, in the step 2(B) of traversing control of the second MMC converter during the land AC single-phase earth fault, the AC positive sequence current control command id +*、iq +*Limit of (2)The amplitude may be determined according to the following equation:
Figure BDA0001741393920000101
wherein the subscript "max" denotes the clipping value, i+ d_rated、i+ q_ratedRespectively, representing the ac positive sequence current rating.
In the embodiment of the invention, in the step 2(C) of controlling the second MMC converter to pass through during the land ac single-phase ground fault, the zero-sequence double frequency suppression voltage
Figure BDA0001741393920000102
Can be determined according to the following formula:
Figure BDA0001741393920000103
wherein u ispa、upb、upcRespectively output voltage u of an MMC converter three-phase upper bridge arm submodulena、unb、uncAnd the output voltages of the MMC three-phase lower bridge arm sub-modules are respectively. u. ofavgThe average value of six bridge arm voltages of the MMC is obtained; omegacTwice the angular frequency of the ac grid; k0Is the proportionality coefficient of the controller; xi is a damping ratio; s is the laplace operator.
In the embodiment of the invention, the single-phase earth fault occurs in the onshore alternating-current power grid, the positive sequence voltage of the power grid drops, and the amplitude limiting control is performed on the alternating-current instruction by the second MMC converter during the fault period, so that the power transmission capability of the second MMC converter is limited. On the other hand, the active power output of the wind farm remains unchanged, so that part of the active power cannot be transmitted to the onshore ac power grid through the second MMC, and is continuously accumulated at the first MMC converter, causing overvoltage to occur on the ac feeder, and after step S3, further comprising:
s4: detecting the amplitude of the alternating-current side phase voltage at the position of the energy discharging device in real time;
s5: judging whether overvoltage occurs to the alternating current feeder line of the wind power plant according to the phase voltage amplitude, if so, turning to the step S6, otherwise, returning to the step S4;
s6: and obtaining the conduction duty ratio of an IGBT device in the energy discharging device according to the phase voltage amplitude and the alternating-current phase voltage rated value, and controlling the energy discharging device to work according to the duty ratio to dissipate the overvoltage.
Wherein the value of k can be 1.5-1.8.
In the embodiment of the invention, in the chopping control of the ac-side energy discharging device of the first MMC converter during a fault, the on-duty ratio d of the IGBT device in the energy discharging device can be calculated by the following formula:
Figure BDA0001741393920000111
d=Kp_ac(Vm_ref-Vm)+KI_ac∫(Vm_ref-Vm) dt; wherein, Va、Vb、VcRespectively representing three-phase voltage to ground of the alternating current bus of the first MMC converter; vmIs the phase voltage amplitude; vm_refTaking 1.1 times of a rated value of the phase voltage as a reference value of the phase voltage according to engineering experience; kp_acIndicating a proportional parameter of the controller, KI_acRepresenting the integral parameters of the controller.
In order to make those skilled in the art better understand the present invention, the onshore ac single-phase ground fault ride-through method of the open-sea wind power flexible dc delivery system according to the present invention is described in detail below with reference to specific embodiments.
In this embodiment, a flexible direct current sending system at two ends of a wind farm in the open sea is taken as an example, and the converter adopts a half-bridge type MMC structure, as shown in fig. 6. The effective value of the alternating current line voltage is 350kV, the direct current bus voltage is +/-320 kV, and each bridge arm comprises 320 half-bridge submodules. The capacitance of the MMC sub-module is 7.424mF, and the bridge arm inductance is 124 mH. The star-shaped grounding reactance is 3H, and the grounding resistance is 2000 omega. During normal operation, direct current voltage is confirmed to the second MMC, and alternating current voltage amplitude and frequency, traditional active power 1000MW are confirmed to first MMC transverter. Suppose a land ac line has a C-phase to ground fault.
After detecting the onshore alternating current single-phase ground fault, the second MMC converter is switched to a fault ride-through control mode: putting negative sequence current control to restrain the alternating negative sequence current; the positive sequence current instruction carries out amplitude limiting according to 1.5 times of a rated value, and overcurrent of the second MMC converter is prevented; inputting direct-current double-frequency zero-sequence fluctuation suppression control to eliminate the influence of the second MMC converter on the safe and stable operation of the first MMC converter; the energy leakage device on the first MMC current converter side is switched to chopping control, and active power output of the wind power plant is effectively consumed.
Assume a single pole ground fault occurs at 1.5 s. Fig. 6 shows a simulation result of fault characteristics of the open-sea wind power flexible direct current transmission system under the alternating current C-phase grounding fault. Therefore, after the alternating current single-phase earth fault occurs, the direct current voltage, the direct current and the active power transmitted by the second MMC current converter fluctuate twice frequency. This is due to the fact that the negative sequence potential of the ac grid excites the negative sequence current of the converter after a three-phase asymmetric fault on the ac side. This negative sequence current reacts with the positive sequence internal potential of the converter causing double frequency fluctuations in the ac instantaneous active power before introducing no additional control strategy. Considering the balance of ac and dc power, the dc side power also has double frequency fluctuation, resulting in double frequency fluctuation of dc voltage and dc current, as shown in fig. 6(c), (d), (e). On the other hand, since the wind farm maintains rated active power output all the time during the fault period, and the second MMC does not introduce an alternating current limiting link, the drop of the positive sequence potential of the alternating current grid causes the overcurrent of the alternating current, as shown in fig. 6 (b).
The fault ride-through simulation result of the open-sea wind power flexible direct current sending system under the fault that the alternating current C is connected with the ground is shown in fig. 7. Based on the knowledge of the single-phase earth fault of alternating current, after the negative sequence current control/direct current double frequency fluctuation control is introduced, the direct current voltage, the direct current double frequency fluctuation and the alternating negative sequence current of the system are effectively inhibited, as shown in fig. 7(b), (c) and (d). However, as shown in fig. 7(e), the control of the negative sequence current does not eliminate the double frequency fluctuation of the transmission power of the second MMC converter. This is because negative-sequence current control inevitably brings about negative-sequence internal potential. The negative sequence internal potential acts with the alternating positive sequence current and also causes double-frequency fluctuation of instantaneous active power. On the other hand, a current loop current limiting link is introduced in consideration of the requirement of alternating current limiting of the second MMC converter. The chopping control of the energy release resistor is enabled to prevent the phenomenon that overvoltage stress occurs on direct current voltage due to the fact that the output of the wind power plant is not matched with the power transmission capability of the MMC converter. Fig. 7(g) and (h) show the duty ratio of the energy release resistor switching, and the duty ratio of the energy release resistor switching is about 0.2. Fig. 7(f) shows the active power of the first MMC converter. Therefore, after the chopping control of the energy discharge resistor is enabled, the mean value of the active power input to the second MMC converter by the wind power plant is effectively reduced, and therefore overvoltage on the direct current side is avoided.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims (4)

1. The utility model provides a flexible direct current of open sea wind-powered electricity generation send out system which characterized in that includes: the system comprises a plurality of offshore wind power plants, a first MMC current converter, a second MMC current converter and an energy leakage device;
a plurality of offshore wind power plants are connected to the first MMC current converter through alternating current feeders;
the first MMC current converter and the second MMC current converter are connected through a positive and negative direct current transmission cable; the first MMC converter is used for determining direct-current voltage; the second MMC converter is used for determining the amplitude and the frequency of the alternating voltage; the first MMC converter and the second MMC converter have the same structure, and each bridge arm of the first MMC converter and the second MMC converter is formed by connecting half-bridge or full-bridge submodules of a single type in series or connecting submodules of different types in series;
the energy release device is arranged on an alternating current bus between the offshore wind farm and the first MMC converter and used for dissipating continuous active power output of the wind farm which is difficult to be sent out through a direct current transmission line during the onshore alternating current fault period, and overvoltage caused by energy accumulation of an alternating current feeder line is avoided; the energy leakage device is formed by connecting resistance series IGBT devices in star connection;
when the alternating current feeder line of the wind power plant has overvoltage, determining the conduction duty ratio of an IGBT device in the energy release device according to the detected amplitude of the alternating phase voltage and the rated value of the alternating phase voltage, and controlling the energy release device to work and dissipate the overvoltage according to the duty ratio; the on duty ratio d of the IGBT device in the energy leakage device is calculated by the following formula:
Figure FDA0002918977830000011
d=Kp_ac(Vm_ref-Vm)+KI_ac∫(Vm_ref-Vm)dt
wherein, Va、Vb、VcRespectively representing three-phase voltage to ground of the alternating current bus of the first MMC converter; vmIs the phase voltage amplitude; vm_refIs a phase voltage reference value; kp_acIndicating a proportional parameter of the controller, KI_acRepresents an integration parameter of the controller;
when the onshore alternating-current single-phase ground fault occurs, alternating negative-sequence current generated during the onshore alternating-current single-phase ground fault is restrained by superposing negative-sequence reference voltage in a differential mode component of bridge arm reference voltage of the second MMC converter; an amplitude limiting link is set for the alternating current positive sequence current control instruction to eliminate the overcurrent during the crossing of the alternating current single-phase earth fault; zero-sequence double-frequency suppression voltage is superposed in a common-mode component of bridge arm reference voltage of a second MMC converter to suppress zero-sequence double-frequency fluctuation of direct-current voltage generated during the onshore alternating-current single-phase ground fault;
the zero-sequence double frequency suppression voltage udiff_ref 0Determined according to the following formula:
Figure FDA0002918977830000021
Figure FDA0002918977830000022
wherein u ispa、upb、upcRespectively MMC three-phase upper bridge armSubmodule output voltage una、unb、uncRespectively outputting voltages for the MMC three-phase lower bridge arm sub-modules; u. ofavgThe average value of six bridge arm voltages of the MMC is obtained; omegacTwice the angular frequency of the ac grid; k0Is the proportionality coefficient of the controller; xi is a damping ratio; s is the laplace operator.
2. A land alternating current single-phase ground fault ride-through method of an open sea wind power flexible direct current output system based on claim 1 is characterized by comprising the following steps:
s1: detecting the electric quantity of the alternating current side at the position of the first MMC converter in real time,
s2: judging whether a land AC single-phase earth fault occurs according to the AC side electric quantity, if so, switching to step S3, otherwise, returning to step S1;
s3: simultaneously performing the following step (A), step (B) and step (C):
the step (A) is as follows: suppressing an alternating negative sequence current generated during an onshore alternating single-phase ground fault by superimposing a negative sequence reference voltage in a differential mode component of a second MMC converter leg reference voltage;
the step (B) is as follows: setting the amplitude of the AC positive sequence current control command to be k times of a rated value to eliminate the overcurrent during the AC single-phase earth fault ride-through;
the step (C) is as follows: and zero-sequence double-frequency suppression voltage is superposed in the common-mode component of the bridge arm reference voltage of the second MMC converter to suppress the zero-sequence double-frequency fluctuation of the direct-current voltage generated during the onshore alternating-current single-phase ground fault.
3. The method for overland ac single-phase ground fault ride-through according to claim 2, further comprising, after step S3:
s4: detecting the amplitude of the alternating-current side phase voltage at the position of the energy discharging device in real time;
s5: judging whether overvoltage occurs to the alternating current feeder line of the wind power plant according to the phase voltage amplitude, if so, turning to the step S6, otherwise, returning to the step S4;
s6: and obtaining the conduction duty ratio of an IGBT device in the energy discharging device according to the phase voltage amplitude and the alternating-current phase voltage rated value, and controlling the energy discharging device to work according to the duty ratio to dissipate the overvoltage.
4. A method as claimed in claim 2 or 3, wherein k is 1.5-1.8.
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