CN117060406B - Operation method of flexible direct current transmission system based on OWT-DMMC - Google Patents

Operation method of flexible direct current transmission system based on OWT-DMMC Download PDF

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CN117060406B
CN117060406B CN202311315904.9A CN202311315904A CN117060406B CN 117060406 B CN117060406 B CN 117060406B CN 202311315904 A CN202311315904 A CN 202311315904A CN 117060406 B CN117060406 B CN 117060406B
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mmc
direct current
current
voltage
dmmc
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CN117060406A (en
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马俊鹏
张岩
王顺亮
焦宁
刘天琪
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Sichuan University
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Sichuan University
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/001—Methods to deal with contingencies, e.g. abnormalities, faults or failures
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/007—Arrangements for selectively connecting the load or loads to one or several among a plurality of power lines or power sources
    • H02J3/0073—Arrangements for selectively connecting the load or loads to one or several among a plurality of power lines or power sources for providing alternative feeding paths between load and source when the main path fails, e.g. transformers, busbars
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/04—Circuit arrangements for ac mains or ac distribution networks for connecting networks of the same frequency but supplied from different sources
    • H02J3/06—Controlling transfer of power between connected networks; Controlling sharing of load between connected networks
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
    • H02J2203/10—Power transmission or distribution systems management focussing at grid-level, e.g. load flow analysis, node profile computation, meshed network optimisation, active network management or spinning reserve management
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
    • H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]
    • 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]

Abstract

The invention relates to the technical field of flexible direct current transmission, and discloses an operation method of a flexible direct current transmission system based on OWT-DMMC, wherein in the OWT-DMMC, a three-phase winding on the secondary side of a transformer is opened and is respectively connected with two identical half-bridge MMCs (MMC-I) and an in-phase alternating current port of MMC-II after being connected with a line inductor L in series; the primary side of the transformer is connected with the network side through an alternating current circuit; one of the transmitting end converter station and the receiving end converter station controls the direct current voltage level, and the other converter station controls the direct current line to transmit active power; the invention can realize flexible adjustment of the power flow of different DC ports, and in addition, redundant DC circuit loops exist, so that after DC faults occur, fault ride-through can be realized, and power transmission is not interrupted. The invention provides a power transmission structure and an operation mode with great advantages for a flexible direct current power transmission system.

Description

Operation method of flexible direct current transmission system based on OWT-DMMC
Technical Field
The invention relates to the technical field of flexible direct current transmission, in particular to an operation method of a flexible direct current transmission system based on OWT-DMMC.
Background
In order to cope with global fossil energy crisis, environmental and climate change problems, in recent years, renewable clean energy power generation modes represented by solar energy and wind energy have been rapidly developed and gradually replace traditional thermal power generation modes. However, the large-scale new energy power generation mode brings new problems of safe and stable operation and energy consumption. The modularized multi-level converter (modular multilevel converter, MMC) has the advantages of modularized design, good expansibility, low switching frequency of a single device, good harmonic performance and the like, and has been widely applied to the field of high-voltagedirect current (HVDC) as a converter. The traditional MMC has no direct-current fault processing capability, and after a short-circuit fault occurs on the direct-current side, the MMC converter cannot prevent an alternating-current power grid from feeding short-circuit current into a direct-current line, so that a semiconductor device of a converter valve is burnt out due to overcurrent, and the whole power transmission system stops running. Therefore, the research on the method for clearing the direct current fault of the MMC type high-voltage direct current transmission system has important engineering significance. In addition, the dc line current of the multi-terminal interconnected dc network is not fully controllable, which may cause overload of a part of the lines, and usually, a power flow control device is additionally embedded in each dc line to regulate the dc current.
The use of dc breakers (DC circuit breaker, DCCB) to cut short-circuited dc lines is the most straightforward method. However, the current DCCB is limited in turn-off time and limited dispersion capacity, and cannot be applied in certain high voltage transmission projects, and further research and optimization are required for the performance and cost of the DCCB to be applied in large-scale business in the high voltage transmission projects. The improvement of the submodule enables the MMC to have the capability of isolating direct current faults to become a current research hot spot. In recent years, many improved submodules with a direct-current fault clearing capability have been proposed, such as a clamp type double submodule, a self-blocking submodule and a reverse blocking submodule. However, the use of these improved sub-modules adds significant device manufacturing costs and control complexity.
In summary, the existing solutions cannot achieve both performance and economy. This limits to a certain extent the development and application of MMCs to the field of high voltage direct current transmission systems and direct current transmission and distribution.
Disclosure of Invention
Aiming at the problems, the invention aims to provide an operation method of a flexible direct current transmission system based on OWT-DMMC, which can flexibly adjust the power of a direct current port to enable the whole direct current line of a ring direct current network to be controllable; and the system has the capability of direct current fault ride-through, and can quickly realize fault isolation and not interrupt power transmission after the direct current fault occurs. The technical proposal is as follows:
the running method of the flexible direct current transmission system based on the OWT-DMMC is characterized in that the OWT-DMMC is an open winding transformer type double-module multi-level converter, in the OWT-DMMC, a three-phase winding on the secondary side of the transformer is opened, and is respectively connected with two identical half-bridge MMCs (MMC-I) and an in-phase alternating current port of MMC-II after being connected with a line inductance L in series; the primary side of the transformer is connected with the network side through an alternating current circuit;
the operation method comprises the following steps:
step 1: according to the structural characteristics of the MMC, a mathematical model and a dynamic characteristic equation of the OWT-DMMC are obtained:
upper bridge arm currenti pj With current of lower bridge armi nj Is expressed as:
（1）
wherein,i nj1 andi nj2 respectively isjLower leg currents of phases MMC-I and MMC-II,i pj1 andi pj2 respectively isjUpper leg currents of phases MMC-I and MMC-II,j=a, b, c phases;i zj1 andi zj2 the average currents of the upper bridge arm and the lower bridge arm of the MMC-I and the MMC-II respectively,i sj for MMC-I and MMC-II connectionsjPhase current;
the dynamic characteristic equation of OWT-DMMC is led out by kirchhoff's voltage law and expressed as:
（2）
wherein,V sj is the alternating voltage at the secondary side of the transformer,L s is the equivalent inductance of the secondary side line of the transformer,R s the equivalent resistance of the circuit;u nj1 andu nj2 the lower bridge arm output voltages of MMC-I and MMC-II respectively,L arm is bridge arm equivalent inductance;
defining differential mode voltages between two MMC corresponding phase bridge arms as follows:u jdiff =u nj1 –u nj2 ；
the equivalent inductance is defined as:L eq =L arm +L s ；
the final simplified equation of the dynamic characteristic equation is obtained by the symmetry of the structure:
（3）
the alternating-current side voltage of the OWT-DMMC is determined by the differential mode voltage of the lower bridge arm or the upper bridge arm of the MMC at the two ends;
step 2: determining the operation mode of a flexible direct current transmission system based on OWT-DMMC:
one of the transmitting end converter station and the receiving end converter station controls the direct current voltage level, and the other converter station controls the direct current line to transmit active power;
the outer ring controller generates a reference value of the inner ring current by setting an active class reference and a reactive class referencei dqref The inner loop current controller generates a current transformer modulation wave;
differential mode voltageu jdiff In a rotation vector coordinate systemdqThe under axis is expressed as:
（4）
wherein,u _dq1 andu _dq2 the voltage vectors which are respectively required to be generated by the MMC-I and the MMC-II;
the apparent powers of MMC-I and MMC-II are calculated as follows:
（5）
wherein,P 1 andQ 1 active power and reactive power respectively of the MMC-I input,P 2 andQ 2 active power and reactive power input by MMC-II respectively;representative ofi dq Is used for the conjugation of (a),i dq for three-phase AC line currentdqAn under-shaft form;jis an imaginary number;
satisfy the differential mode voltageu diffj On the premise of unchanged vector, regulating the voltage vectors of the MMC-I and the MMC-II to realize power flow control; if the power reference values of MMC-I and MMC-II are respectivelyP refdc1 AndP refdc2 setting a scale factorKIs thatP refdc1 ／（P refdc1 +P refdc2 ) And obtaining MMC-I and MMC-II modulation voltage vectors:
（6）
modulating voltage vectors by MMC-I and MMC-IIM dq1 And (3) withM dq2 Regulating the flow of power between MMC-I and MMC-II.
Further, the method for realizing the direct current fault ride through is also included, and specifically comprises the following steps:
step a: when the direct current short circuit fault occurs, the three-phase discharging circuit is equivalent to a simplified discharging circuit, which is an RLC series circuit with initial conditions, and the equivalent parameters are as follows:
（7）
wherein,L arm and (3) withR arm Respectively representing the equivalent inductance and the equivalent resistance of the bridge arm,L dc and (3) withR dc Respectively representing the inductance and the resistance of the short-circuit path of the direct current line,Nthe number of sub-modules put into each phase,Ca sub-module capacitor;C 1 、L 1 andR 1 the equivalent capacitance, the equivalent inductance and the equivalent resistance of the simplified discharge circuit are respectively;
step b: calculating the direct current after the direct current short circuit fault occurs:
assuming that a DC short-circuit fault occurst＝t 0 At the moment, the SM capacitor voltage isU c0 And the direct current isI dc0 The direct current obtained by the RLC circuit is as follows:
（8）
（9）
wherein,U dc0 the capacitor voltage is equivalent for all sub-modules,ωis natural the angular frequency of the light emitted by the light source,δin order for the attenuation factor to be a factor,ω 0 in order to oscillate the angular frequency of the discharge current,αis the initial phase;
step c: when the direct current short circuit fault is detected, the fault side MMC submodule is immediately bypassed, the normal MMC on the other side is used for supporting the power grid voltage, the OWT-DMMC is converted into a conventional MMC structure, and the power exchange between the alternating current side and the direct current side is continuously completed through the normal side MMC.
Further, in the step b, when the DC current after the DC short-circuit fault is calculated, if the bypass operation of the submodule occurst＝t 1 At the moment, the direct current fault currenti dc Reach toI dc1 The current loop is converted into a zero input RL loop, the current is gradually attenuated, and then the direct current expression is:
（10）
in the formula (i),I dc1 to the maximum value of direct currentI dc1 。
Compared with the prior art, the invention has the beneficial effects that: the flexible direct current transmission structure and the running method provided by the invention can realize flexible adjustment of the power flow of different direct current ports, and in addition, redundant direct current circuit loops exist, so that after a direct current fault occurs, fault ride-through can be realized, and power transmission is not interrupted, thereby being a very advantageous power transmission structure and running mode.
Drawings
Fig. 1 is a schematic topology diagram of an open winding transformer type dual-modular multilevel converter according to an embodiment of the present invention.
Fig. 2 is a schematic diagram of a peer-to-peer power transmission system based on OWT-DMMC according to an embodiment of the present invention.
Fig. 3 is a detailed circuit schematic diagram of an open winding transformer type dual-module multi-level converter according to an embodiment of the present invention.
Fig. 4 is a block diagram of MMC outer ring control provided in an embodiment of the invention.
Fig. 5 is a schematic diagram of inner loop control in complex vector form according to an embodiment of the present invention.
Fig. 6 is a schematic diagram of an equivalent circuit of a sub-module discharge stage according to an embodiment of the present invention.
Fig. 7 is a schematic diagram of clearing a dc fault according to an embodiment of the present invention.
Fig. 8 is a simulation diagram of a power flow variation provided in an embodiment of the present invention.
Fig. 9 (a) is a dc fault ride-through simulation diagram provided by an embodiment of the present invention: a time domain plot of dc voltage changes.
Fig. 9 (b) is a dc fault ride-through simulation diagram provided by an embodiment of the present invention: a time domain plot of dc line current variation.
Detailed Description
The invention will now be described in further detail with reference to the drawings and to specific examples.
As shown in fig. 1 and fig. 2, the flexible dc transmission structure provided in this embodiment and using an open-winding transformer type dual-module multi-level converter (open-winding transformer based dual modular multilevel converters, OWT-DMMC) is shown in fig. 1, which is a topological schematic diagram of the open-winding transformer type dual-module multi-level converter. The three-phase windings on the secondary side of the transformer are opened and are respectively connected with the same-phase alternating current ports of two identical half-bridge MMCs (MMC-I and MMC-II) after being connected with the line inductance L in series. The primary side of the transformer is connected with the network side through an alternating current line, and the primary side windings can be in star connection or delta connection.
The open winding transformer type double-module multi-level converter can be applied to a three-terminal or multi-terminal flexible direct current transmission system. Fig. 2 shows a point-to-point power transmission system based on OWT-DMMC, through which AC1 and AC2 complete power transmission. The StationA and the StationB are converter stations formed by OWT-DMMC, and the direct current ports of the converter stations are connected through a double-circuit direct current transmission line.
The novel power transmission structure of the invention has similar functions as the traditional double-end bipolar MMC type power transmission system, but has obvious performance advantages, and the operation mode is described in detail below.
The invention relates to a flexible direct current transmission system operation mode based on an open winding transformer type double-module multi-level converter, which comprises the following steps:
(1) Mathematical model of open winding transformer type double-module multi-level converter
The open winding transformer type double-module multi-level converter is based on the MMC structure, according to the structural characteristics of the MMC, the mathematical model and dynamic characteristic equation of the OWT-DMMC can be obtained, a detailed circuit schematic diagram (shown in figure 3),i dc1 ，i dc2 the direct current flows out of the two direct current ports respectively;i pa1 ，i na1 （i pa2 ，i na2 ) A phase a upper bridge arm current and a phase a lower bridge arm current of MMC-I (MMC-II) respectively;u pa1 ，u na1 （u pa2 ，u na2 ) A phase a upper bridge arm voltage and a phase a lower bridge arm voltage of MMC-I (MMC-II) respectively;i sa 、i sb 、i sc is three-phase alternating current, upper bridge arm currenti pj With current of lower bridge armi nj Can be expressed by the formula (1):
（1）；
in the above, the average current of the upper bridge arm and the lower bridge arm is defined as the bridge arm currenti zj Satisfy the formula (2)The current of each phase of the sub-module comprises alternating-current side phase current, direct-current loop current and the circulation in a three-phase bridge arm, and the three-phase bridge arm is symmetrical, so that the direct-current of each phase is one third of the direct-current loop current:
（2）；
the dynamic characteristic equation of the dominant OWT-DMMC is led out by kirchhoff voltage law and expressed as:
（3）；
defining the phase difference mode voltage between two MMC corresponding bridge arms asu diffj =u nj1 –u nj2 Definition of equivalent inductanceL eq =L arm +L s From the symmetry of the structure, the final simplified equation can be obtained as:
（4）。
from the above analysis, it can be seen that the mathematical model of OWMMC is similar to the conventional MMC, in which the difference is half of the differential mode voltage between the upper and lower legs of each phase, and the ac side voltage of OWT-DMMC is determined by the differential mode voltage of the lower leg (or the upper leg) of the MMC at both ends.
(2) Operation mode and function of flexible direct current transmission system based on open winding transformer type double-module multi-level converter
The flexible direct current transmission system based on the open winding transformer type double-module multi-level converter shown in fig. 2 is a double-end point-to-point transmission system, a converter station StationA is a transmitting end rectifying station, and a converter station StationB is a receiving end inversion station. The transmitting end converter station controls the size of the transmitted active power, and the receiving end converter station controls the direct current voltage.
Outer ringThe controller is as shown in figure 4 of the drawings,U dcref 、P sref 、Q sref 、U gmref respectively the reference values of DC average voltage, active power, reactive power and AC network voltage amplitude,U dc1 andU dc2 is a direct current voltage at two ends,P s andQ s for the active and reactive power of the system,U gm is the actual sampled value of the ac grid voltage amplitude. The outer ring controller generates an active reference value of the inner ring current by setting an active reference (power or direct current voltage) and a reactive reference (reactive power and alternating current network side voltage amplitude)i dref And reactive reference valuei qref The inner loop current controller generates a current transformer modulated wave.
Differential mode voltageu diffj In a rotation vector coordinate systemdqThe under axis can be expressed as:
（5）；
wherein,u _dq1 ，u _dq2 the voltage vectors that each of MMC-I and MMC-II needs to generate are respectively. The apparent powers of MMC-I and MMC-II can then be calculated as:
（6）；
wherein,P 1 andQ 1 active power and reactive power respectively of the MMC-I input,P 2 andQ 2 active power and reactive power input by MMC-II respectively;representative ofi dq Conjugation of (2);jis imaginary.
On the premise of satisfying the formula (5) and keeping the differential mode voltage vector unchanged, the MMC-I and the MMC-II are adjustedThe voltage vector can realize the current control, if the power reference values of MMC-I and MMC-II areP refdc1 AndP refdc2 setting a scale factorKIs thatP refdc1 ／（P refdc1 +P refdc2 ) And obtaining MMC-I and MMC-II modulation voltage vectors:
（7）；
modulating voltage vectors by MMC-I and MMC-IIM dq1 And (3) withM dq2 Regulating the flow of power between MMC-I and MMC-II.
Regulating power flow by voltage vector allocation as shown in fig. 5M αβ1 M αβ2 Is MMC-I and MMC-II inαβThe modulation voltage vector under the axis,θis the grid voltage phase calculated by the phase-locked loop. The power reference value of MMC-I and MMC-III is increased from 0.5pu to 0.75pu, and the power transmitted by the other DC loop is decreased from 0.5pu to 0.25pu. The corresponding simulated waveforms are shown in fig. 8.
In addition, the topological structure can realize direct current fault ride-through. Firstly, analyzing the dynamic response of the circuit structure after the fault occurs, discharging the speed of the submodule to a fault point when the direct current fault occurs, and increasing the direct current. The number of submodules put into each phase is alwaysNAnd following a voltage balance modulation strategy, an equivalent three-phase discharge circuit is shown in figure 6,L arm and (3) withR arm Representing the equivalent inductance and resistance of the bridge arm,L dc and (3) withR dc The equivalent parameters in the right-hand simplified circuit diagram representing the inductance and resistance of the dc link short-circuit path are:
（8）；
the simplified discharge circuit is an RLC series circuit with initial conditions. Assuming that the fault occurst＝t 0 At the moment, SM capacitor voltage isU c0 And the direct current isI dc0 The RLC circuit can solve the dc current as follows:
（9）；
（10）。
when a direct current short circuit fault is detected, the fault side MMC sub-module is immediately bypassed, the equivalent voltage output of the fault side sub-module is 0, fault current is gradually attenuated, the normal MMC on the other side supports grid voltage, the OWT-DMMC is converted into a conventional MMC structure, and power exchange between the alternating current side and the direct current side is continuously completed through the sound side MMC. The system architecture at this time is shown in fig. 7. If the sub-module bypass action occurst＝t 1 At the moment, the direct current fault currenti dc Reach toI dc1 The current loop is converted into a zero input RL loop, the current is gradually attenuated, and then the direct current expression is:
（11）；
as can be seen from equation (11), the DC current reaches a maximum when a fault is detected and an action is takenI dc1 And then the direct current is reduced to zero finally, so that the direct current fault current of the system is cleared, and the safety of the fault side power device is effectively protected.
The direct current fault ride-through process of the double-circuit direct current line point-to-point power transmission system based on OWT-DMMC is described below by taking the example that the direct current line between the MMC-I and the MMC-III has short circuit fault and transferring all rated power transmission to the direct current line between the MMC-II and the MMC-IV. Upon detection of a fault current greater than the set point, both converter station a and converter station B will change their modes of operation. All sub-modules of MMC-I and MMC-III will be bypassed and power delivery stopped, while MMC-II and MMC-IV continue to be used to regulate power transfer between AC and DC grids and to provide voltage support for their adjacent AC grids to ensure the stability of the AC grid. The double-end bipolar flexible direct current transmission system adopting the structure greatly improves the power supply reliability, and the direct current fault ride-through function is quick and effective.
After the fault is relieved, only the MMC-I and the MMC-III are required to be put into operation again, and as the voltages of the submodules of the MMC-I and the MMC-III can be regarded as being maintained at the rated value and the bypass is relieved according to the running quantity under the stable working condition, the bypass submodules can be put into operation gradually while the reference power is increased gradually, so that the direct current voltage is gradually increased to the rated value, and the simulation waveforms of the direct current fault ride-through process of the double-circuit point-to-point power transmission system based on the OWT-DMMC structure are shown in fig. 9 (a) and 9 (b).

Claims (3)

1. The operation method of the flexible direct current transmission system based on the OWT-DMMC is characterized in that in the OWT-DMMC, a three-phase winding on the secondary side of a transformer is opened, and is connected with two identical half-bridge MMCs (MMC-I) and MMC-II) respectively after being connected in series with a line inductance L; the primary side of the transformer is connected with the network side through an alternating current circuit;
the operation method comprises the following steps:
step 1: according to the structural characteristics of the MMC, a mathematical model and a dynamic characteristic equation of the OWT-DMMC are obtained:
upper bridge arm currenti pj With current of lower bridge armi nj Is expressed as:
（1）；
wherein,i nj1 andi nj2 respectively isjLower leg currents of phases MMC-I and MMC-II,i pj1 andi pj2 respectively isjUpper leg currents of phases MMC-I and MMC-II,j=a, b, c phases;i zj1 andi zj2 the average currents of the upper bridge arm and the lower bridge arm of the MMC-I and the MMC-II respectively,i sj for MMC-I and MMC-II connectionsjPhase current;
the dynamic characteristic equation of OWT-DMMC is led out by kirchhoff's voltage law and expressed as:
（2）；
wherein,V sj is the alternating voltage at the secondary side of the transformer,L s is the equivalent inductance of the secondary side line of the transformer,R s the equivalent resistance of the circuit;u nj1 andu nj2 the lower bridge arm output voltages of MMC-I and MMC-II respectively,L arm is bridge arm equivalent inductance;
defining differential mode voltages between two MMC corresponding phase bridge arms as follows:u jdiff = u nj1 –u nj2 ；
the equivalent inductance is defined as:L eq =L arm +L s ；
the final simplified equation of the dynamic characteristic equation is obtained by the symmetry of the structure:
（3）；
the alternating-current side voltage of the OWT-DMMC is determined by the differential mode voltage of the lower bridge arm or the upper bridge arm of the MMC at the two ends;
step 2: determining the operation mode of a flexible direct current transmission system based on OWT-DMMC:
one of the transmitting end converter station and the receiving end converter station controls the direct current voltage level, and the other converter station controls the direct current line to transmit active power;
outer ring controllerOver-setting the reference value of the active class reference and the reactive class reference to generate the inner loop currenti dqref The inner loop current controller generates a current transformer modulation wave;
differential mode voltageu jdiff In a rotation vector coordinate systemdqThe under axis is expressed as:
（4）；
wherein,u _dq1 andu _dq2 the voltage vectors which are respectively required to be generated by the MMC-I and the MMC-II;
the apparent powers of MMC-I and MMC-II are calculated as follows:
（5）；
wherein,P 1 andQ 1 active power and reactive power respectively of the MMC-I input,P 2 andQ 2 active power and reactive power input by MMC-II respectively;representative ofi dq Is used for the conjugation of (a),i dq for three-phase AC line currentdqAn under-shaft form;jis an imaginary number;
satisfy the differential mode voltageu diffj On the premise of unchanged vector, regulating the voltage vectors of the MMC-I and the MMC-II to realize power flow control; if the power reference values of MMC-I and MMC-II are respectivelyP refdc1 AndP refdc2 setting a scale factorKIs thatP refdc1 ／（P refdc1 +P refdc2 ) And obtaining MMC-I and MMC-II modulation voltage vectors:
（6）；
modulating voltage vectors by MMC-I and MMC-IIM dq1 And (3) withM dq2 Regulating the flow of power between MMC-I and MMC-II.
2. The method for operating a flexible direct current transmission system based on OWT-DMMC according to claim 1, further comprising a method for implementing direct current fault ride through, specifically:
step a: when the direct current short circuit fault occurs, the three-phase discharging circuit is equivalent to a simplified discharging circuit, which is an RLC series circuit with initial conditions, and the equivalent parameters are as follows:
（7）；
wherein,L arm and (3) withR arm Respectively representing the equivalent inductance and the equivalent resistance of the bridge arm,L dc and (3) withR dc Respectively representing the inductance and the resistance of the short-circuit path of the direct current line,Nthe number of sub-modules put into each phase,Ca sub-module capacitor;C 1 、L 1 andR 1 the equivalent capacitance, the equivalent inductance and the equivalent resistance of the simplified discharge circuit are respectively;
step b: calculating the direct current after the direct current short circuit fault occurs:
assuming that a DC short-circuit fault occurst＝t 0 At the moment, the SM capacitor voltage isU c0 And the direct current isI dc0 The direct current obtained by the RLC circuit is as follows:
（8）；
（9）；
wherein,U dc0 the capacitor voltage is equivalent for all sub-modules,ωis natural the angular frequency of the light emitted by the light source,δin order for the attenuation factor to be a factor,ω 0 in order to oscillate the angular frequency of the discharge current,αis the initial phase;
step c: when the direct current short circuit fault is detected, the fault side MMC submodule is immediately bypassed, the normal MMC on the other side is used for supporting the power grid voltage, the OWT-DMMC is converted into a conventional MMC structure, and the power exchange between the alternating current side and the direct current side is continuously completed through the normal side MMC.
3. The method according to claim 2, wherein, when the dc current after the dc short-circuit fault is calculated in the step b, if the sub-module bypass occurs in the following mannert＝t 1 At the moment, the direct current fault currenti dc Reach toI dc1 The current loop is converted into a zero input RL loop, the current is gradually attenuated, and then the direct current expression is:
（10）；
in the formula (i),I dc1 to the maximum value of direct currentI dc1 。
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CN116154747A (en) * 2021-11-23 2023-05-23 南京南瑞继保电气有限公司 Modularized multi-level voltage regulating device for DC power transmission system transformer, control method and control system
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