CN107565590B - Hybrid high-voltage direct-current power transmission system suitable for wind power transmission - Google Patents

Hybrid high-voltage direct-current power transmission system suitable for wind power transmission Download PDF

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CN107565590B
CN107565590B CN201710797852.1A CN201710797852A CN107565590B CN 107565590 B CN107565590 B CN 107565590B CN 201710797852 A CN201710797852 A CN 201710797852A CN 107565590 B CN107565590 B CN 107565590B
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converter
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CN107565590A (en
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韩平平
陈凌琦
丁明
张炎
张海天
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Hefei University of Technology
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Hefei University of Technology
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    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/70—Wind energy
    • Y02E10/76—Power conversion electric or electronic aspects
    • 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
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

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Abstract

The invention discloses a hybrid high-voltage direct-current transmission system suitable for wind power delivery, wherein a three-phase infinite voltage source is connected with a rectification side converter station at a transmission end of a transmission system, the rectification side converter station comprises a converter transformer and a modular multilevel converter MMC with a half-bridge MMC submodule, the converter MMC in the rectification side converter station is connected with a bipolar direct-current line, hybrid high-voltage direct-current circuit breakers are respectively connected in series on the bipolar direct-current line, the bipolar direct-current line is connected with an inversion side converter station, the inversion side converter station comprises a converter transformer and a power grid phase-change converter LCC, the inversion side converter station is connected with a receiving end alternating-current system, and the receiving end alternating-current system adopts a three-phase infinite power grid; the system can not only resist the influence of the overvoltage of the alternating current bus caused by direct current locking on the accessed wind power, but also automatically clear the fault of the direct current side.

Description

Hybrid high-voltage direct-current power transmission system suitable for wind power transmission
Technical Field
The invention relates to a hybrid high-voltage direct-current transmission system suitable for wind power delivery, and belongs to the technical field of high-voltage direct-current transmission.
Background
Because wind power resources are difficult to effectively absorb in the local, wind power is a research hotspot through high-voltage direct-current transmission. The traditional line communated converter based high voltage direct current transmission system (LCC-HVDC) is widely applied due to the advantages of fixed tidal current direction, large system transmission capacity, fast and controllable active power and the like. However, the LCC converter station needs to consume a large amount of reactive power during normal operation, and a large amount of area is occupied by equipment such as a filter for compensating reactive power. Particularly, when the direct current of a power transmission system is locked, the filter is difficult to cut off in a short time, redundant reactive power can rush into an alternating current system to cause overvoltage, and the stable operation of a wind turbine generator is threatened. On the other hand, in a special situation such as offshore wind power island delivery, the LCC needs an alternating current system with certain intensity to provide phase-change voltage for the LCC, and an independently operated wind farm is weak in intensity, difficult to establish stable alternating current voltage, and therefore cannot be connected with the LCC independently.
High voltage direct current transmission systems (modular multilevel converted HVDC, MMC-HVDC) based on modular multilevel converters are more and more concerned because of the advantages that the modular multilevel converted HVDC can be self-phase-converted without support, a reactive compensation device is not needed, and active and reactive independent control is not needed, and the like, and are very suitable for wind power transmission. However, the MMC-HVDC system is expensive to manufacture, and cannot self-clean the dc fault like the LCC, and the overcurrent caused by the dc fault can damage devices such as the IGBT. Aiming at the price problem, the hybrid direct current transmission which is comprehensively designed by combining the advantages of mature LCC technology, low cost and good MMC regulation performance becomes a good solution. At the present stage, an LCC-MMC hybrid power transmission model of a rectification side LCC and an inversion side MMC is mainly adopted. However, the topology sending end has the problems that the LCC cannot be directly connected with the wind power, the direct current voltage response speed of the inverter side MMC is low, and the alternating current fault of the rectifier side easily causes the cut-off of the direct current.
For the direct current fault processing of the MMC side, two modes of configuring a circuit breaker at the alternating current side and adopting a current converter with direct current fault ride-through capability are provided. The AC side circuit breaker is a mechanical switch, so that the action speed is low, and the suppression of fault current and the recovery of an AC/DC system are not facilitated; the modular multilevel converter (full bridge sub-module based MMC, C-MMC) based on the clamping bi-sub module reduces the number of devices, but has parallel coupling structurally, and bridge arm capacitors have two different charging states in series and parallel after the IGBT is locked.
The invention content is as follows:
the invention aims to solve the problems in the prior art, provides a hybrid high-voltage direct-current power transmission system suitable for wind power delivery, and aims to solve the problem that the existing LCC-HVDC cannot stabilize the voltage of a sending-end alternating-current bus during direct-current blocking and solve the problem that an MMC side cannot automatically clear faults on a direct-current side.
The technical scheme adopted by the invention for solving the technical problems is as follows:
the invention is suitable for the hybrid high-voltage direct-current transmission system for wind power delivery, and is characterized in that:
the utility model discloses a power transmission system, including transmission system's send end, rectification side converter station, bipolar direct current circuit, contravariant side converter station includes receiving end converter transformer and electric wire netting commutation converter LCC, and the receiving end AC system is connected to the contravariant side converter station, receiving end AC system is three-phase infinity electric wire netting.
The invention is suitable for a hybrid high-voltage direct-current transmission system for wind power transmission, and is also characterized in that a sending end converter transformer with a filtering function in a rectifying side converter station adopts an Ynd connection method, the network side of the sending end converter transformer adopts Y-type wiring, the valve side adopts △ -type wiring, one end of the sending end converter transformer is connected with a sending end power grid through an alternating-current circuit breaker, the other end of the sending end converter transformer is connected with a converter MMC through a starting circuit, the starting circuit is provided with a bypass circuit breaker and a starting resistor which are mutually connected in parallel, the sending end alternating-current system charges a capacitor in an MMC submodule through the starting resistor during starting, the starting resistor is short-circuited through the bypass circuit breaker to reduce circuit loss when starting is finished, the MMC comprises three groups of upper bridge arms and three groups of lower bridge arms, the three groups of upper bridge arms are connected with an anode direct-current.
The invention is suitable for the hybrid high-voltage direct-current transmission system for wind power transmission, and is characterized in that: each bridge arm in the converter MMC is formed by connecting 76 submodules in series and connecting 1 valve inductor in series, and an IGBT tube T in each submodule1Collector electrode of andIGBT tube T2Is connected with the emitting electrode of the capacitor C, and one end of the capacitor C is connected with the IGBT tube T1Is connected with the other end of the capacitor C and the IGBT tube T2Is connected with the IGBT tube T1Inverse parallel connected with diode D1And the IGBT tube T2Inverse parallel connected with diode D2。
The invention is suitable for the hybrid high-voltage direct-current transmission system for wind power transmission, and is characterized in that: the hybrid high-voltage direct-current circuit breaker is formed by connecting a bypass branch and a main circuit breaker in parallel;
the bypass branch is formed by connecting an ultra-fast mechanical switch and a current transfer switch in series;
the main circuit breaker is formed by connecting a plurality of switch units in series, each switch unit is provided with a plurality of IGBT (insulated gate bipolar transistor) tubes and anti-parallel diodes which are connected in series in a forward and reverse direction, and each switch unit is independently provided with a lightning rod for absorbing energy;
the invention is suitable for a hybrid high-voltage direct-current transmission system for wind power transmission and is also characterized in that the power grid commutation converter LCC is provided with two groups of LCC sub-circuits, each LCC sub-circuit is connected with a hybrid high-voltage direct-current circuit breaker through a direct-current reactor, a receiving end converter transformer is connected with an alternating-current bus of a receiving end alternating-current system, a twelve-pulse converter is arranged between the direct-current reactor and the receiving end converter transformer, the valve side of the receiving end converter transformer adopts △ type wiring, the network side adopts Y type wiring, an alternating-current filter and a reactive power compensation device are arranged on the alternating-current bus connected with the receiving end converter transformer, the alternating-current filter is connected with the reactive power compensation device in parallel and is grounded, and the twelve-pulse converter is formed by connecting two six-pulse converters.
The invention is suitable for the hybrid high-voltage direct-current transmission system for wind power transmission, and is characterized in that: the power grid commutation converter LCC adopts a constant direct current I for a direct current transmission systemdcAnd a fixed extinction angle gamma, wherein the direct current I is fixeddcThe control mode controls the current of the direct current line at the inversion side, and the constant arc-extinguishing angle gamma control mode controls the arc-extinguishing angle of the inverter; the modular multilevel converter MMC adopts a constant direct current voltage U for a direct current transmission systemdcAnd a constant AC voltage UacIn which the DC voltage U is constantdcControl mode controls the DC line voltage on the rectification side, the constant AC voltage UacThe control mode controls the voltage of the alternating-current bus at the transmitting end.
The invention is suitable for the hybrid high-voltage direct-current transmission system for wind power transmission, and is characterized in that: the on-off sequence strategy of the hybrid high-voltage direct-current circuit breaker is as follows:
0～t1: rated current I when system is in normal operationNThe current flows through the bypass branch circuit, the main circuit breaker is in an off state, and the current flowing through the main circuit breaker is 0;
t1～t2: after the direct current line fails, the current flowing through the bypass branch circuit rises; t is t2At the moment, the fault current reaches the set value IlimObtaining a switching-on/off instruction of the current transfer switch, switching off the current transfer switch, triggering and conducting an IGBT tube in the main breaker at the same time, and starting to transfer fault current to the main breaker;
t2～t3: the ultra-fast mechanical switch restores the turn-off capability along with the reduction of the current flowing through the ultra-fast mechanical switch; t is t3At the moment, the current flowing through the current transfer switch is reduced to 0, and the ultra-fast mechanical switch is switched off;
t3～t4: the current through the main breaker continues to rise, t4At the moment, the fault current reaches the limit ImainObtaining a main breaker switching-off instruction, triggering and switching off an IGBT tube in the main breaker, and transferring fault current to an energy absorption branch consisting of a lightning rod;
t4～t5: the fault current gradually decays to 0, t5And the isolating switch is disconnected at any moment, so that fault isolation is realized.
The invention is suitable for the hybrid high-voltage direct-current transmission system for wind power transmission, and is characterized in that: arranging a direct current breaker control system for realizing the on-off sequence strategy of the hybrid high-voltage direct current breaker; the direct current circuit breaker control system sets the on-off time instructions of a current transfer switch and a main circuit breaker in the circuit breaker in advance according to fault characteristics, the on-off time instructions of the current transfer switch and the main circuit breaker come from direct current side transient fault analysis and setting calculation of an on-off time sequence strategy of a hybrid high-voltage direct current circuit breaker, and the method specifically comprises the following steps:
step 1, performing single-stage earth fault analysis and double-pole short circuit fault analysis on a hybrid high-voltage direct-current power transmission system respectively, evaluating the overcurrent degree caused by faults, and determining the setting calculation of the on-off time sequence strategy of the hybrid high-voltage direct-current circuit breaker;
step 2, aiming at single-stage grounding fault analysis, short-term bias of a direct current bus and alternating current side voltage can be caused, the short-term bias can be recovered quickly along with the end of the fault, and the direct current can generate small-amplitude oscillation and can be recovered quickly due to short-term discharge of a line to ground capacitor, so that extra protective measures are not needed; aiming at bipolar short-circuit fault analysis, firstly, equating the whole fault loop; equating a resistor, an inductor and a capacitor in the whole fault loop to obtain an initial current condition of the inductor and an initial voltage condition of the capacitor, and obtaining a function expression of the fault current with respect to time according to a discharge theory of a second-order circuit;
and 3, inputting a function of the fault current with respect to time into a curve fitting tool for curve fitting, adding the respective cut-off current limit values of the current transfer switch and the main circuit breaker into a vertical axis to obtain an intersection point time corresponding to an intersection point of a fault current fitting curve, and taking the intersection point time as a time instruction of the direct current circuit breaker cut-off time sequence strategy control system.
Compared with the prior art, the invention has the beneficial effects that:
1. compared with an LCC-MMC hybrid power transmission model in the prior art, the invention adopts a topological structure of the LCC at the inversion side of the MMC at the rectification side, so that a sending end can be directly connected with a wind power plant without providing a commutation voltage for the wind power plant, the alternating current bus voltage at the sending end can be stabilized through a reactive voltage control system of the MMC when a direct current system is subjected to direct current locking, and the grid disconnection accident of a fan caused by overvoltage is effectively prevented.
2. Compared with the MMC-HVDC system in the prior art, the topological structure of the rectifying side MMC and the inverting side LCC is adopted, so that the economy is better, when a direct-current line fails, extra protective measures are not needed to be taken by the rectifying side MMC and the inverting side LCC, the LCC can cut off the failure by locking the converter of the LCC, and only the protective measures are needed to be taken by the MMC side.
3. Because the hybrid high-voltage direct-current circuit breaker is additionally arranged on the direct-current line at the rectifying side, compared with the two existing modes of cutting off the direct-current side fault, the system has higher cutting-off speed, better inhibiting effect on the fault current, better economical efficiency and avoiding excessive power electronic devices.
4. According to the method, the direct-current side fault analysis and the direct-current breaker sequential control strategy are combined to obtain the on-off time instructions of all control units of the direct-current breaker in advance, and compared with the method that the fault current is switched on and off by directly setting the on-off current limit criterion of all switch units, the problem that the fault is not timely cut off due to time delay can be avoided, and the protection is more rapid.
Drawings
FIG. 1 is a diagram of the topology of the system of the present invention;
FIG. 2 is a topological structure diagram of a converter MMC in the present invention;
FIG. 3 is a topological structure diagram of an MMC sub-module in the present invention;
FIG. 4 is a topological diagram of a hybrid high-voltage DC circuit breaker according to the present invention;
FIG. 5 is a structural topological diagram of a twelve-pulse converter in a power grid commutation converter LCC;
FIG. 6 is a structure diagram of a LCC control system of the power grid commutation converter;
FIG. 7 is a diagram of a MMC control system according to the present invention;
FIG. 8 is a timing diagram illustrating the opening of the hybrid DC circuit breaker;
FIG. 9 is a MMC sub-module capacitance injection short-circuit current loop of a first stage of bipolar short-circuit fault;
FIG. 10 is a bipolar short circuit second stage converter station injection short circuit current loop;
FIG. 11 is a first stage discharge circuit after a bipolar short fault;
FIG. 12 is a single-ended single-phase equivalent circuit of a first stage after a bipolar short-circuit fault;
FIG. 13 is a second stage discharge circuit after a bipolar short fault;
FIG. 14 is a single-ended single-phase equivalent circuit at a second stage after a bipolar short-circuit fault;
FIG. 15 is a simulation of the rectified side AC bus voltage during bipolar latching of the DC power transmission system;
FIG. 16 is a single-stage ground fault DC current simulation diagram;
FIG. 17 is a single-stage ground fault DC voltage simulation diagram;
fig. 18 is a single-stage ground fault a ac voltage simulation diagram;
fig. 19 is a single-stage ground fault a ac current simulation diagram;
FIG. 20 is a DC current simulation diagram for a bipolar short circuit fault;
FIG. 21 is a DC voltage simulation diagram for a bipolar short circuit fault;
FIG. 22 is a graph showing a bipolar short-circuit fault a AC voltage simulation;
FIG. 23 is a graph of a bipolar short-circuit fault a AC current simulation;
FIG. 24 is a plot of the DC current peaks fitted according to the fault analysis theory;
FIG. 25 is a plot of the time at which the fitted DC current reaches a peak;
fig. 26 is a fitted dc breaker switching element opening timing diagram;
FIG. 27 is a DC line fault current diagram at the open end of a DC circuit breaker;
Detailed Description
Fig. 1 shows a topology structure of a hybrid high-voltage direct-current power transmission model suitable for wind power transmission, which includes a sending-end alternating-current system, a sending-end converter transformer, a rectifying-side MMC converter station, a bipolar direct-current line, a hybrid high-voltage direct-current circuit breaker, positive and negative grounding capacitors, an inverting-side LCC converter station, a receiving-end converter transformer, a receiving-end alternating-current bus reactive power compensation device, and a receiving-end alternating-current system.
The hybrid high-voltage direct-current power transmission system suitable for wind power delivery in the embodiment is set as follows:
as shown in fig. 1, a three-phase infinite voltage source is connected to a rectification side converter station at a transmission end of a power transmission system, the rectification side converter station comprises a modular multilevel converter MMC comprising a transmission end converter transformer and a half-bridge MMC submodule, the converter MMC in the rectification side converter station is connected with a bipolar direct current circuit, a hybrid high-voltage direct current circuit breaker is respectively connected to the bipolar direct current circuit in series, the bipolar direct current circuit is connected with an inversion side converter station, the inversion side converter station comprises a receiving end converter transformer and a power grid phase-change converter LCC, the inversion side converter station is connected with a receiving end alternating current system, and the receiving end alternating current system is a three-phase infinite power grid.
The method includes that Ynd connection is adopted for a sending end converter transformer with a filtering function in a rectifying side converter station, Y-type wiring is adopted for a network side of the sending end converter transformer, △ type wiring is adopted for a valve side of the sending end converter transformer, one end of the sending end converter transformer is connected with a sending end power grid through an alternating current breaker, the other end of the sending end converter transformer is connected with a converter MMC through a starting circuit, the starting circuit is provided with a bypass breaker and a starting resistor which are connected in parallel, in the starting process, a sending end alternating current system charges capacitors in MMC submodules through the starting resistor, when the starting process is finished, the starting resistor is short-circuited through the bypass breaker to reduce circuit loss, the converter is provided with three groups of upper bridge arms and three groups of lower bridge arms, the three groups of upper bridge arms are connected with an anode direct current circuit, the three groups of lower bridge arms are connected with a cathode direct current circuit, the three groups of lower bridge arms are connected with a rectifier circuit, a converter MMC can directly connect with a phase-changing voltage for the sending end converter MMC, when the direct current system has a direct current locking, the converter side, the converter can also can better cut off the alternating current circuit of the converter system through a reactive voltage control system, the converter of the sending end converter can effectively prevent a fan from being cut off a high-voltage of a high-voltage direct current converter from being caused by overvoltage, the high-voltage direct current converter system, the high-current converter system from being blocked, the high-voltage.
Fig. 2 and 3 show a converter MMC topology and an MMC submodule topology, each bridge arm includes 76 sub-modules and 1 valve inductor, the converter MMC includes 6 bridge arms in this embodiment, each bridge arm of the converter MMC is formed by connecting 76 sub-modules and 1 valve inductor in series, the MMC submodule includes 2 IGBT tubes, 2 diodes and a capacitor, the IGBT tube T in the MMC submodule1Collector electrode and IGBT tube T2Is connected with the emitting electrode of the capacitor C, and one end of the capacitor C is connected with the IGBT tube T1Is connected with the other end of the capacitor C and the IGBT tube T2Is connected with the IGBT tube T1Inverse parallel connected with diode D1And IGBT tube T2Inverse parallel connected with diode D2。
As shown in fig. 4, the hybrid high-voltage dc circuit breaker in this embodiment is formed by connecting a bypass branch and a main circuit breaker in parallel, wherein the bypass branch is formed by connecting an ultrafast mechanical switch and a current transfer switch in series, the main circuit breaker is formed by connecting a plurality of switch units in series, each switch unit is provided with a plurality of forward and reverse series connected IGBT tubes and anti-parallel connected diodes, and each switch unit is independently provided with a lightning rod for absorbing energy. The hybrid high-voltage direct-current circuit breaker integrates the characteristics of stable operation, strong loading capacity, small on-state loss, accurate and controllable switching time of the solid-state direct-current circuit breaker, high reliability and long service life of the mechanical direct-current circuit breaker, and can reliably clear direct-current side faults of the MMC terminal on the basis of meeting the level of wind power delivery voltage.
Fig. 5 shows a structural topological diagram of a twelve-pulse converter in a power grid commutation converter LCC, in the embodiment, the power grid commutation converter LCC has two sets of LCC sub-circuits, each LCC sub-circuit is connected with a hybrid high-voltage direct-current circuit breaker through a direct-current reactor, a receiving-end converter transformer is connected with an alternating-current bus of a receiving-end alternating-current system, the twelve-pulse converter is arranged between the direct-current reactor and the receiving-end converter transformer, a valve side of the receiving-end converter transformer adopts △ type wiring, a network side adopts Y type wiring, an alternating-current filter and a reactive power compensation device are arranged on the alternating-current bus connected with the receiving-end converter transformer, the alternating-current filter is connected with the reactive power compensation device in parallel and grounded, and the twelve-pulse converter is formed by.
As shown in fig. 6 and 7, the LCC of the grid commutated converter in this embodiment applies a constant dc current I to the dc power transmission systemdcAnd a fixed extinction angle gamma, wherein the direct current I is fixeddcThe control mode controls the current of the direct current line at the inversion side, and the constant arc-extinguishing angle gamma control mode controls the arc-extinguishing angle of the inverter; modular multilevel converter MMC adopts constant direct current voltage U for direct current transmission systemdcAnd a constant AC voltage UacIn which the DC voltage U is constantdcControl mode controls the DC line voltage on the rectification side, the constant AC voltage UacThe control mode controls the voltage of the alternating-current bus at the transmitting end.
The grid commutation converter LCC shown in fig. 6 regulates the direct current I through the PI controllerdcAnd the arc-quenching angle gamma, a controllable trigger angle α is output, wherein the reference value of the DC current is from a low-voltage current-limiting link VDCOL, and the low-voltage current-limiting link VDCOL can limit the DC command when the DC voltage drops to a specified value, thereby reducing the possibility of commutation failure and promoting the fault recovery of the DC system, the fixed AC bus voltage U shown in FIG. 7acThe control mode of the Modular Multilevel Converter (MMC) can stabilize the voltage of a sending end alternating current bus, and an MMC control system comprises system level control, converter level control and valve level control from high to low; wherein the system level control receives an active power reference quantity and a reactive power reference quantity; active control includes constant active power P control and constant DC voltage UdcControl and constant frequency f control; the reactive control comprises constant reactive power Q control and constant alternating voltage UacControlling; the top layer system level control is determined by the system operation requirement, and the bottom layer valve level control outputs the pulse signal of the converter according to the reference voltage instruction; the converter level control is realized by controlling the active current i injected into the power griddAnd a reactive current iqIndependent decoupling control of system-level control quantity is realized, and the control output quantity is the reference voltage of the AC port of the converter; the main application of the hybrid high-voltage direct-current power transmission system in the embodiment is to access and send out the wind power at the sending end, so that the hybrid high-voltage direct-current power transmission system is used for sending an alternating-current bus at the sending endThe voltage is stabilized by using a constant AC bus voltage UacA control mode; meanwhile, in an MMC-HVDC system, only one MMC with and without two-end converter station control must be at a constant direct current voltage UdcIn the control mode, only the rectification side of the system adopts an MMC current converter, and the rectification side adopts a fixed UdcDecide UacA combined control mode; compared with indirect current control, the direct current control adopted in the embodiment can quickly track transient change of the MMC-HVDC, the current response speed is high, and the operation process of the MMC-HVDC is less influenced by transient state or fault state.
In this embodiment, a hybrid high-voltage dc circuit breaker is provided to solve the problem of too high fault current caused by the double-pole short-circuit fault of the dc line on the MMC side, as shown in fig. 8, a switching-off timing diagram of the hybrid high-voltage dc circuit breaker is shown, and the switching-off timing diagram is as follows:
0～t1: rated current I when system is in normal operationNThe current flows through the bypass branch circuit, the main circuit breaker is in an off state, and the current flowing through the main circuit breaker is 0;
t1～t2: after the direct current line fails, the current flowing through the bypass branch circuit rises; t is t2At the moment, the fault current reaches the set value IlimObtaining a switching-on/off instruction of the current transfer switch, switching off the current transfer switch, triggering and conducting an IGBT tube in the main breaker at the same time, and starting to transfer fault current to the main breaker;
t2～t3: the ultra-fast mechanical switch restores the turn-off capability along with the reduction of the current flowing through the ultra-fast mechanical switch; t is t3At the moment, the current flowing through the current transfer switch is reduced to 0, and the ultra-fast mechanical switch is switched off;
t3～t4: the current through the main breaker continues to rise, t4At the moment, the fault current reaches the limit ImainObtaining a main breaker switching-off instruction, triggering and switching off an IGBT tube in the main breaker, and transferring fault current to an energy absorption branch consisting of a lightning rod;
t4～t5: the fault current gradually decays to 0, t5And the isolating switch is disconnected at any moment, so that fault isolation is realized.
In specific implementation, a direct current breaker control system is arranged, and the direct current breaker control system is used for giving on-off time instructions to each switch unit, including the on-off time instructions t of the current transfer switch2And a main breaker on-off time instruction t4Therefore, the on-off sequence strategy of the hybrid high-voltage direct-current circuit breaker is realized; the direct current circuit breaker control system sets the on-off time instructions of a current transfer switch and a main circuit breaker in the circuit breaker in advance according to fault characteristics, the on-off time instructions of the current transfer switch and the main circuit breaker come from direct current side transient fault analysis and setting calculation of an on-off time sequence strategy of a hybrid high-voltage direct current circuit breaker, and the method specifically comprises the following steps:
step 1, performing single-stage earth fault analysis and double-pole short circuit fault analysis on a hybrid high-voltage direct-current power transmission system respectively, evaluating the overcurrent degree caused by faults, and determining the setting calculation of the on-off time sequence strategy of the hybrid high-voltage direct-current circuit breaker;
step 2, aiming at single-stage grounding fault analysis, short-term bias of a direct current bus and alternating current side voltage can be caused, the short-term bias can be recovered quickly along with the end of the fault, and the direct current can generate small-amplitude oscillation and can be recovered quickly due to short-term discharge of a line to ground capacitor, so that extra protective measures are not needed; aiming at bipolar short-circuit fault analysis, firstly, equating the whole fault loop; equating a resistor, an inductor and a capacitor in the whole fault loop to obtain an initial current condition of the inductor and an initial voltage condition of the capacitor, and obtaining a function expression of the fault current with respect to time according to a discharge theory of a second-order circuit;
and 3, inputting a function of the fault current with respect to time into a curve fitting tool for curve fitting, adding the respective cut-off current limit values of the current transfer switch and the main circuit breaker into a vertical axis to obtain an intersection point time corresponding to an intersection point of a fault current fitting curve, and taking the intersection point time as a time instruction of the direct current circuit breaker cut-off time sequence strategy control system.
The direct current side fault analysis and the direct current breaker time sequence control strategy are combined to obtain the on-off time instructions of the control units of the direct current breaker in advance, compared with the method that the fault current is switched on and off by directly setting the on-off current limit criterion of the switch units, the problem that the fault is cut off in time due to time delay can be avoided, and the protection is more rapid.
Aiming at the single-stage ground fault analysis in the step 2, taking the positive ground fault of the power transmission system as an example, after the fault occurs, the potential reference point of the direct current side is changed, and the voltage of the positive electrode is set to be UpNegative pole voltage is UnAt this time UpReduced to 0, UnIncrease to ground voltage to Udc(ii) a As shown in fig. 1, a valve side winding of the transformer at the MMC end adopts triangular wiring, so that a current loop cannot be formed between a grounding wire and an alternating current side, and therefore, grounding current cannot occur theoretically; the MMC direct-current side bipolar direct-current lines are grounded through a large resistor respectively and are approximately open-circuit, and each module capacitor does not have a discharge path with a fault grounding point; in practical engineering, the direct current line discharges through the grounding point due to the capacitance effect on the ground, and the grounding currents of the positive electrode and the negative electrode gradually attenuate to 0 along with the completion of the rapid discharge. The inverter side LCC adopts constant current control and can quickly lock the converter station when short-circuit fault occurs on the direct current side, so that the contribution of the inverter side LCC to the fault current can be ignored.
On the AC side, taking phase a as an example, the loop voltage in steady state operation satisfies the formula (1):
Figure BDA0001400789090000081
ua1and ua2A phase upper and lower bridge arm voltages, UsaFor a cross current side output voltage, UpAnd UnSince the bridge arm capacitance does not participate in discharging, the upper and lower bridge arm voltages are kept unchanged, so that the output voltage at the AC side is biased, ua1Sum of decrease ua2The rising amplitude of the voltage is a single-stage voltage value in normal operation, and the valve side alternating current is not influenced because a discharge loop is not generated with the alternating current side. As long as the voltage resistance and the insulation performance of the line and the alternating current equipment are good and the fault is cut off in a short time, the system can safely and stably operate; if the fault can not be cut off quickly,the system can also be kept in single-stage or reduced-pressure operation without taking additional protective measures.
For the bipolar short-circuit fault in step 2, the protection device triggers the MMC to latch within a period of time after the bipolar short-circuit fault occurs, so the transient process after the fault can be divided into two stages before and after the converter station latches, and the influence of the inverter-side LCC is still not considered here, and the reason is the same as the single-stage ground fault analysis.
As shown in fig. 9 and 10, in this embodiment, the fault current before locking mainly comes from the discharge of the capacitor of the inputted sub-module through the IGBT T1 at the upper part and the inductive freewheeling discharge, and the fault current after locking comes from the discharge of the ac power grid through the anti-parallel diode and the inductive freewheeling discharge, and the capacitors of the sub-modules which are not inputted are omitted in the figures.
FIG. 11 shows the first stage discharge loop after a double-pole short-circuit fault, where the grid resistance and inductance on the AC side are RSAnd LS，LVFor a single bridge arm valve inductance per phase. The resistance and the inductance of a direct current line between each stage of converter station and a fault point are respectively RLAnd LL. The fault current of the direct current line is the sum of the discharge currents of the three groups of bridge arms, and the capacitors of the three-phase upper and lower bridge arms and the valve inductors are connected in parallel, so that the equivalent inductor L is as follows: l ═ L (2/3) LV+2LLThe equivalent resistance R is: r is 2RL+RCOWherein R isCOThe contact resistance at bipolar short circuit. Because all the sub-module capacitors of the upper and lower bridge arms participate in discharging along with the input and the removal before the locking, the sub-modules in each bridge arm are equivalently connected in series, and the bridge arms are equivalently connected in parallel, the equivalent capacitor C is as follows: c ═ 6CSN, wherein CSThe number of the capacitors is the unit submodule capacitance of the converter, and n is the number of the MMC single-bridge-arm submodules. The capacitor voltage and the inductor current are known at the moment of fault, and R is less than R in the system
Figure BDA0001400789090000091
The discharge loop is thus a second order underdamped oscillatory discharge circuit of known initial conditions.
As shown in fig. 12, the initial condition of the single-ended single-phase equivalent circuit of the first stage after the bipolar short-circuit fault is as follows:
Figure BDA0001400789090000092
UC(0+) And UC(0—) The capacitor voltage before the single bridge arm fault and the capacitor voltage after the fault are I (0)+) And I (0)—) Respectively before and after the faultdcIs a single-stage DC voltage, I1For the direct current line current in normal operation, the fault loop current I can be obtained according to a current calculation formula of the oscillation discharge process of the second-order circuitfAs shown in formula (3):
Figure BDA0001400789090000093
capacitor voltage UCAs shown in formula (4):
Figure BDA0001400789090000094
in the formula:
Figure BDA0001400789090000095
the fault current shown in formula (3) is an oscillation damping function, then IfThe point at which t starts to make the derivative function 0 first from 0 after t is derived is the time when the discharge circuit reaches the peak value of the discharge current, and the formula (5) is shown as follows:
Figure BDA0001400789090000096
the time t cannot be directly solved for the equation (5), and the left part of the equation (5) is taken as a function of the time t, and an abscissa corresponding to the first intersection point of the left part and the horizontal axis is found by fitting a curve, that is, the time when the discharge current reaches the peak value.
As shown in fig. 13, the second stage discharging loop after the bipolar short-circuit fault exists in this embodiment, and there are two discharging loops, namely, a current loop fed by an anti-parallel diode in the ac system and a reactance follow current loop.
As shown in fig. 14, the single-ended single-phase equivalent circuit of the second stage after the bipolar short-circuit fault, the ac system is fed into the current loop through the anti-parallel diode, and forms a reactance follow current loop; setting the initial value of the inductive current after the converter is locked as I2The voltage of the AC system is
Figure BDA0001400789090000101
Wherein ω issFor the angular frequency of AC system, the upper bridge arm current I at this stage2up(t) and lower arm Current I2down(t) is represented by the formula (6):
Figure BDA0001400789090000102
Figure BDA0001400789090000103
when the freewheeling current decays to 0, the bridge arm current reverses. And after the stable state is reached, the bridge arm current is subjected to direct current bias.
Typically the protection device has triggered the ac circuit breaker to trip without the second phase fault current reaching steady state, whereas the first phase fault current is much higher than the second phase and reaches peak before the MMC latches.
Fig. 15 shows a simulation comparison graph of the voltages of the rectified side ac bus in the PSCAD and the hybrid dc transmission system in bipolar blocking. The locking time is set to 4s, U1 and U2 are voltage simulation of the LCC direct-current power transmission system in a locking state without cutting off and cutting off a filter respectively, U3 is voltage simulation of the MMC-LCC hybrid direct-current power transmission system, and the reference value is 320 kV. It can be seen from fig. 15 that the peak value of the ac voltage of the LCC dc transmission system exceeds 1.6p.u regardless of whether the filter is cut off, and even if the steady state value after the filter is cut off is about 1.3p.u, if wind power is connected, the safe operation of the LCC dc transmission system is threatened. And the transient state peak value of alternating voltage in the hybrid direct current transmission system is controlled to be 1.1p.u, and is quickly compensated to be 1.0p.u by a reactive power control system, and if wind power is connected, the safe and stable operation of the hybrid direct current transmission system can be ensured without being disconnected. It can thus be shown that the power transmission system has an advantage over LCC-HVDC in stabilizing the wind access point voltage.
Fig. 16 is a direct current simulation diagram of a single-stage ground fault, fig. 17 is a direct current voltage simulation diagram of a single-stage ground fault, fig. 18 is an a-phase alternating current voltage simulation diagram of a single-stage ground fault, and fig. 19 is an a-phase alternating current simulation diagram of a single-stage ground fault. The fault trigger time is set to 5 seconds and the fault clearing time is set to 5.1 seconds. As can be seen from the simulation diagram: a ground fault causes only a short bias of the dc bus and the ac side voltage and recovers quickly as the fault ends. The dc current will oscillate slightly and recover quickly due to the short-term discharge of the line-to-ground capacitance, so that no additional protective measures have to be taken.
Fig. 20 is a dc current simulation diagram of a bipolar short-circuit fault, fig. 21 is a dc voltage simulation diagram of a bipolar short-circuit fault, fig. 22 is an a-phase ac voltage simulation diagram of a bipolar short-circuit fault, and fig. 23 is an a-phase ac current simulation diagram of a bipolar short-circuit fault. The fault time is set to 5s, the MMC locking time is set to 5.01s, and the tripping time of the alternating current circuit breaker is set to 5.1 s. The fault current peak value of the direct current line occurs at 5.0057s, which is faster than the time 5.01s when the converter is locked; the peak reached 32.85 kA. A fault current curve obtained by combining the formula (3), the formula (5) and the table 1 and performing calculation through curve fitting is shown in fig. 24, and a fault current peak reaching time is shown in fig. 25, wherein the direct-current side fault current is further derived from the bridge arm current formula, so that the curve reflects the change trend of the bridge arm current. For dc line fault currents, only the portion of the fitted curve from time 0 to the first peak time is of interest.
Table 1 shows the parameters required for the bipolar short calculation as follows:
Figure BDA0001400789090000111
the peak value of the obtained direct-current fault current shown in fig. 24 is 35.32kA, and the time required for reaching the peak value shown in fig. 25 is 0.0053s, which are consistent with the actual results, and the error comes from the fact that the equivalent circuit ignores the discharge time of the capacitor, so that the obtained current peak value is slightly higher than the actual value, and the time for reaching the peak value is slightly earlier than the actual value. The direct current of the system in steady-state operation is 1.8kA as a reference, the actual fault current peak value reaches 18.25p.u, which far exceeds the standard that the maximum value of the system in safe operation is 5p.u, and the system safety is seriously threatened, so that a breaker must be additionally arranged to remove faults.
The theory of the connection formula (3) related to the on-off timing sequence of the dc circuit breaker and the intersection point obtained by curve fitting are shown in fig. 26, in which the current transfer switch is instructed to turn off at the moment t20.00013s, main breaker open time command t4At 0.00158s, the calculated time is input into the open-close control system of the direct-current circuit breaker, and the fault current of the direct-current line under the open-close of the direct-current circuit breaker is obtained through simulation and is shown in fig. 27. The fault current can be effectively controlled below the cutoff main branch cutoff current limit value of 9kA, namely 5p.u, the whole cutoff process is within 5ms, the cutoff requirement of the direct-current circuit breaker and the system overcurrent limit value are met, and the calculation method is proved to be capable of obtaining the fault current for effectively inhibiting the direct-current side bipolar short-circuit fault.

Claims (5)

1. The utility model provides a mixed high voltage direct current transmission system suitable for wind-powered electricity generation is sent outward which characterized by:
the power transmission system comprises a power transmission system and a power transmission system, wherein a transmission end of the power transmission system is a three-phase infinite voltage source and is connected with a rectification side converter station, the rectification side converter station comprises a transmission end converter transformer and a modular multilevel converter MMC of a half-bridge MMC submodule, the converter MMC in the rectification side converter station is connected with a bipolar direct current circuit, a hybrid high-voltage direct current circuit breaker is respectively connected in series on the bipolar direct current circuit, the bipolar direct current circuit is connected with an inversion side converter station, the inversion side converter station comprises a receiving end converter transformer and a power grid phase-change converter LCC, the inversion side converter station is connected with a receiving end alternating current system, and the receiving end alternating current system is a three-phase infinite;
the hybrid high-voltage direct-current circuit breaker is formed by connecting a bypass branch and a main circuit breaker in parallel; the bypass branch is formed by connecting an ultra-fast mechanical switch and a current transfer switch in series; the main circuit breaker is formed by connecting a plurality of switch units in series, each switch unit is provided with a plurality of IGBT (insulated gate bipolar transistor) tubes and anti-parallel diodes which are connected in series in a forward and reverse direction, and each switch unit is independently provided with a lightning rod for absorbing energy;
the on-off sequence strategy of the hybrid high-voltage direct-current circuit breaker is as follows:
0～t1: rated current I when system is in normal operationNThe current flows through the bypass branch circuit, the main circuit breaker is in an off state, and the current flowing through the main circuit breaker is 0;
t1～t2: after the direct current line fails, the current flowing through the bypass branch circuit rises; t is t2At the moment, the fault current reaches the set value IlimObtaining a switching-on/off instruction of the current transfer switch, switching off the current transfer switch, triggering and conducting an IGBT tube in the main breaker at the same time, and starting to transfer fault current to the main breaker;
t2～t3: the ultra-fast mechanical switch restores the turn-off capability along with the reduction of the current flowing through the ultra-fast mechanical switch; t is t3At the moment, the current flowing through the current transfer switch is reduced to 0, and the ultra-fast mechanical switch is switched off;
t3～t4: the current through the main breaker continues to rise, t4At the moment, the fault current reaches the limit ImainObtaining a main breaker switching-off instruction, triggering and switching off an IGBT tube in the main breaker, and transferring fault current to an energy absorption branch consisting of a lightning rod;
t4～t5: the fault current gradually decays to 0, t5The isolating switch is switched off at any moment to realize fault isolation;
arranging a direct current breaker control system for realizing the on-off sequence strategy of the hybrid high-voltage direct current breaker; the direct current circuit breaker control system sets the on-off time instructions of a current transfer switch and a main circuit breaker in the circuit breaker in advance according to fault characteristics, the on-off time instructions of the current transfer switch and the main circuit breaker come from direct current side transient fault analysis and setting calculation of an on-off time sequence strategy of a hybrid high-voltage direct current circuit breaker, and the method specifically comprises the following steps:
step 1, performing single-stage earth fault analysis and double-pole short circuit fault analysis on a hybrid high-voltage direct-current power transmission system respectively, evaluating the overcurrent degree caused by faults, and determining the setting calculation of the on-off time sequence strategy of the hybrid high-voltage direct-current circuit breaker;
step 2, aiming at single-stage grounding fault analysis, short-term bias of a direct current bus and alternating current side voltage can be caused, the short-term bias can be recovered quickly along with the end of the fault, and the direct current can generate small-amplitude oscillation and can be recovered quickly due to short-term discharge of a line to ground capacitor, so that extra protective measures are not needed; aiming at bipolar short-circuit fault analysis, firstly, equating the whole fault loop; equating a resistor, an inductor and a capacitor in the whole fault loop to obtain an initial current condition of the inductor and an initial voltage condition of the capacitor, and obtaining a function expression of the fault current with respect to time according to a discharge theory of a second-order circuit;
and 3, inputting a function of the fault current with respect to time into a curve fitting tool for curve fitting, adding the respective cut-off current limit values of the current transfer switch and the main circuit breaker into a vertical axis to obtain an intersection point time corresponding to an intersection point of a fault current fitting curve, and taking the intersection point time as a time instruction of the direct current circuit breaker cut-off time sequence strategy control system.
2. The hybrid high-voltage direct-current transmission system suitable for wind power delivery according to claim 1, wherein a delivery end converter transformer with a filtering function in the rectifying side converter station is connected Ynd, a grid side of the delivery end converter transformer is connected in a Y-shaped mode, a valve side of the delivery end converter transformer is connected in a △ mode, one end of the delivery end converter transformer is connected with a delivery end power grid through an alternating-current circuit breaker, the other end of the delivery end converter transformer is connected with a converter MMC through a starting circuit, the starting circuit is provided with a bypass circuit breaker and a starting resistor which are connected in parallel, the delivery end alternating-current system charges a capacitor in a submodule through the starting resistor MMC during starting, the starting resistor is short-circuited through the bypass circuit breaker to reduce circuit loss after starting, the converter MMC is provided with three groups of upper bridge arms and three groups of lower bridge arms, the three groups of upper bridge arms are connected with an anode direct-.
3. The hybrid high-voltage direct current transmission system suitable for wind power delivery according to claim 2, characterized in that: each bridge arm in the converter MMC is formed by connecting 76 submodules in series and connecting 1 valve inductor in series, and an IGBT tube T in each submodule1Collector electrode and IGBT tube T2Is connected with the emitting electrode of the capacitor C, and one end of the capacitor C is connected with the IGBT tube T1Is connected with the other end of the capacitor C and the IGBT tube T2Is connected with the IGBT tube T1Inverse parallel connected with diode D1And the IGBT tube T2Inverse parallel connected with diode D2。
4. The hybrid high-voltage direct-current transmission system suitable for wind power delivery according to claim 1 is characterized in that the power grid commutation converter LCC is provided with two groups of LCC sub-circuits, each LCC sub-circuit is connected with the hybrid high-voltage direct-current circuit breaker through a direct-current reactor, a receiving-end converter transformer is connected with an alternating-current bus of a receiving-end alternating-current system, a twelve-pulse converter is arranged between the direct-current reactor and the receiving-end converter transformer, a valve side of the receiving-end converter transformer is connected through △ type wiring, a grid side of the receiving-end converter transformer is connected through Y type wiring, an alternating-current filter and a reactive power compensation device are arranged on the alternating-current bus connected with the receiving-end converter transformer, the alternating-current filter is connected with the reactive power compensation device in parallel and is grounded, and the twelve.
5. The hybrid high-voltage direct current transmission system suitable for wind power delivery according to claim 1, characterized in that: the power grid commutation converter LCC adopts a constant direct current I for a direct current transmission systemdcAnd a fixed extinction angle gamma, wherein the direct current I is fixeddcThe control mode controls the current of the direct current line at the inversion side, and the constant arc-extinguishing angle gamma control mode controls the arc-extinguishing angle of the inverter; the modular multilevel converter MMC adopts a constant direct current voltage U for a direct current transmission systemdcAnd a constant AC voltage UacThe combined control mode of (a) and (b),wherein the DC voltage U is fixeddcControl mode controls the DC line voltage on the rectification side, the constant AC voltage UacThe control mode controls the voltage of the alternating-current bus at the transmitting end.
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