A method for suppressing circulating current in parallel MMC after a series converter hybrid DC line fault

Abstract

The present invention relates to the control field of hybrid DC transmission system, and discloses a method for suppressing multi-MMC parallel circulating current after a series converter type hybrid DC line fault, comprising: during the phase shift period after the DC line fault, the three parallel converters MMC1, MMC2, and MMC3 on the inverter side of the LCC-MMC series hybrid DC transmission system are switched from a master-slave control mode to a fixed DC current control for converter MMC1, and droop control for converter MMC2 and converter MMC3, and the DC current command values ​​of converter MMC2 and converter MMC3 are set to half of the DC current command value in steady state; after the fault is repaired, the converters MMC1, MMC2, and MMC3 are switched back to the master-slave control mode after a fixed deionization time t. The present invention can well control the DC current of each MMC during the recovery process to ensure the rapid and stable recovery of the system.

Description

Technical Field

[0001] The invention relates to the control field of a hybrid direct current transmission system, and in particular to a method for suppressing MMC parallel circulating current after a series converter type hybrid direct current line fails. Background Art

[0002] China's Baihetan-Jiangsu DC transmission project will use a hybrid DC transmission system with a line commutated converter (LCC) and a modular multilevel converter (MMC) in series. In the LCC-MMC series hybrid DC transmission system, the DC line is long and the operating environment is harsh, so its failure rate is relatively high compared with other transmission equipment. A DC line fault will threaten the safe and stable operation of the system, causing serious consequences such as interruption of system power transmission and even system shutdown. For LCC-HVDC systems, fault clearing and recovery are usually achieved through DC line fault recovery sequences (DFRS), that is, through phase shifting and de-ionization. For MMC-HVDC systems, the methods for fault clearing and recovery are 1) reclosing of AC and DC circuit breakers; 2) using sub-modules with fault clearing capabilities; 3) adding auxiliary circuits. However, the AC circuit breaker has too long an opening time to meet the requirements for rapid clearing and recovery of DC faults. Existing DC circuit breakers are limited by their breaking capacity, so the method of reclosing AC and DC circuit breakers is difficult to apply to the LCC-MMC series hybrid DC transmission system. Submodules with fault clearing capabilities generally use a combination of half-bridge and full-bridge submodules, while the LCC-MMC series hybrid DC transmission system uses a half-bridge submodule. The method of adding an auxiliary circuit is to add an auxiliary circuit to the half-bridge submodule, isolate the AC system, change the path of the fault current, and thus achieve the clearing and recovery of the DC fault. However, this method will bring about the problem of more complex control systems and increased costs, so it is not suitable for the LCC-MMC series hybrid DC transmission system.

[0003] For the DC line fault of the LCC-MMC series hybrid DC transmission system, the three MMCs connected in parallel on the inverter side cannot feed current to the fault point due to the unidirectional conductivity of the LCC on the inverter side. Therefore, the system layer can still refer to the fault clearing and recovery strategy of the LCC-HVDC system. The specific fault clearing and recovery strategy is: 1) After detecting the DC line fault, the LCC on the rectifier side forces phase shifting, and the LCC on the inverter side maintains the original control state. The MMC still adopts master-slave control, that is, MMC1 adopts fixed DC voltage control, and MMC2 and MMC3 adopt fixed active power control; 2) Then the system restarts after a certain de-ionization time. However, after the DC line fault, the MMCs connected in parallel on the inverter side do not exchange current with the DC side, but the three MMCs can form a closed loop. If MMC2 and MMC3 still use fixed active power control in the fault state and need to absorb current, MMC1 enters the rectification state to provide current to MMC2 and MMC3. At this time, MMC1 will have an overcurrent problem. In addition, during the system restart process, the DC system and MMC1 jointly provide DC current to MMC2 and MMC3, so the DC current of MMC2 and MMC3 increases greatly, resulting in DC current out of control. Therefore, the existing MMC master-slave control method cannot reasonably distribute current during the DC line fault and recovery process, resulting in the current out of control phenomenon of MMC. Summary of the invention

[0004] The present invention provides a method for suppressing MMC parallel circulating current after a series converter type hybrid DC line fault, which can effectively distribute the DC current between MMCs during the DC line fault clearing and recovery process, thereby suppressing the overcurrent fluctuation of the MMC during the phase shift period and realizing rapid and stable recovery of the system.

[0005] The present invention is achieved through the following technical solutions:

[0006] The method for suppressing MMC parallel circulating current after a series converter type hybrid DC line fault comprises:

[0007] S1. After the control and protection system detects a DC line fault, during the phase shift period after the DC line fault, the three parallel converters MMC1, MMC2 and MMC3 on the inverter side of the LCC-MMC series hybrid DC transmission system are switched from the master-slave control mode to the fixed DC current control of converter MMC1 and the droop control of converters MMC2 and MMC3. The current command values ​​of converters MMC2 and MMC3 are set to half of the DC current in the steady state. At the same time, the current command value of converter MMC1 is determined according to the changes of converters MMC2 and MMC3, so as to achieve the circulating current suppression of the three parallel MMCs during the DC line fault and fault recovery period.

[0008] S2. After the fault is repaired, the converters MMC1, MMC2 and MMC3 are switched back to the master-slave control mode after a fixed de-ionization time t.

[0009] As an optimization, the specific implementation steps of S1 are:

[0010] S1.1, respectively measuring the first relevant parameters at the AC side busbars of converters MMC1, MMC2 and MMC3 on the low voltage side of the inverter station in the LCC-MMC series hybrid DC transmission system and the second relevant parameters at the DC outlets of converters MMC2 and MMC3;

[0011] S1.2, based on the first related parameter and the second related parameter, respectively calculate the DC current per unit value I of the DC side of the converter MMC1, the converter MMC2, and the converter MMC3 1dPu ,I 2dPu ,I 3dPu , DC current command value per unit value I 1refpu ,I 2refpu ,I 3refpu , and the per-unit value of the DC side voltage command value of the MMC converter U MMCrefpu .

[0012] S1.3, when the control and protection system detects a DC line fault, during the phase shift period after the DC line fault, based on the parameter values ​​obtained in S1.2, the DC current deviation per unit value ΔI of the converter MMC1, converter MMC2, and converter MMC3 are calculated respectively. 1pu , ΔI 2pu , ΔI 3pu , and the DC current deviation per unit value ΔI 1pu , ΔI 1pu , ΔI 2pu The command values ​​I of the inner loop decoupling control of the converter MMC1, the converter MMC2 and the converter MMC3 are automatically input into the PI link to obtain the command values ​​I of the inner loop decoupling control of the converter MMC1, the converter MMC2 and the converter MMC3 dcref1 ,I dcref2 and I dcref3 , to distribute the DC current between converters MMC1, MMC2 and MMC3.

[0013] As an optimization, the specific steps of S1.2 are:

[0014] S1.2.1. Set the DC current command values ​​of the DC side of converter MMC1, converter MMC2, and converter MMC3 to I MMC1ref ,I MMC2ref and I MMC3ref , and set the DC side voltage command value U of converter MMC2 and converter MMC3 at the same timeMMCref ;

[0015] S1.2.2, set the DC current command value I MMC1ref ,I MMC2ref and I MMC3ref Divide by the reference DC current I of the converter MMC1 1base , the reference DC current I of the converter MMC2 2base , the reference DC current I of the converter MMC3 3base , respectively obtain the DC current command value per unit value I of converter MMC1, converter MMC2, and converter MMC3 1refpu ,I 2refpu ,I 3refpu ;

[0016] S1.2.3, set the DC side voltage command value U MMCref Divide by the reference value U of the DC side voltage of the MMC converter dcbase Get the DC side voltage command value U of the MMC converter MMCrefpu .

[0017] S1.2.4, the three-phase voltage U1 (including U 1a , U 1b , U 1b ), U2 (including U 2a , U 2b , U 2b ), U3 (including U 3a , U 3b , U 3b ) and three-phase current I1 (including I 1a ,I 1b ,I 1b ), I2 (including I 2a ,I 2b ,I 2b ), I3 (including I 3a ,I 3b ,I 3b ) divided by the reference value U of the AC side voltage and current base ,I base , to obtain the three-phase voltage per unit value U at the AC side bus of the converter MMC1 1pu (U 1apu , U 1bpu , U 1cpu ), three-phase current per unit value I 1pu (I 1apu ,I 1bpu ,I 1cpu ), based on the three-phase voltage per unit value U at the AC side bus of the converter MMC2 2pu (U 2apu , U2bpu , U 2cpu ), three-phase current per unit value I 2pu (I 2apu ,I 2bpu ,I 2cpu ) and the three-phase voltage per unit value U at the AC side bus of the converter MMC3 3pu (U 3apu , U 3bpu , U 3cpu ) and the three-phase current per unit value I 3pu (I 3apu ,I 3bpu ,I 3cpu );

[0018] S1.2.5, DC output voltage U MMC2 , DC output voltage U MMC3 Divide by the reference value U of the DC side voltage of the MMC converter dcbase , to obtain the DC side voltage per unit value U based on the converter MMC2 MMC2pu and the DC side voltage per unit value U based on the converter MMC3 MMC3pu ;

[0019] S1.2.6, the three-phase voltage standard value U 1pu (U 1apu , U 1bpu , U 1cpu ), U 2pu (U 2apu , U 2bpu , U 2cpu ), U 3pu (U 3apu , U 3bpu , U 3cpu ) and the three-phase current per unit value I 1pu (I 1apu ,I 1bpu ,I 1cpu ), I 2pu (I 2apu ,I 2bpu ,I 2cpu ), I 3pu (I 3apu ,I 3bpu ,I 3cpu ) are transformed by Clark to obtain the three-phase voltage per unit value U 1pu , U 2pu , U 3pu and I 1pu ,I 2pu ,I 3pu The α and β components are denoted as U 1αPu , U 1βPu ,I1αPu ,I 1βPu , U 2αPu , U 2βPu ,I 2αPu ,I 2βPu , U 3αPu , U 3βPu ,I 3αPu ,I 3βPu ;

[0020] S1.2.7, the three-phase voltage standard value U 1pu , U 2pu , U 3pu The α, β components and the per unit value of the three-phase current I 1pu ,I 2pu ,I 3pu The α and β components are multiplied and added together and then divided by the DC side voltage command value U MMCrefpu Get the DC current per unit value I 1dPu , DC current per unit value I 2dPu , DC current per unit value I 3dPu ,in,

[0021] I 1dPu =(U 1αPu *I 1αPu +U 1βPu *I 1βPu ) / U MMCrefpu ;

[0022] I 2dPu =(U 2αPu *I 2αPu +U 2βPu *I 2βPu ) / U MMCrefpu ;

[0023] I 3dPu =(U 3αPu *I 3αPu +U 3βPu *I 3βPu ) / U MMCrefpu ;

[0024] As an optimization, the specific implementation steps of S1.3 include:

[0025] S1.3.1. When the control and protection system detects a DC line fault, the DC current command values ​​of converters MMC1, MMC2, and MMC3 are marked as I 1refpu ,I 2refpu ,I 3refpu With DC current per unit value I 1dPu ,I 2dPu ,I 3dPuThe DC current deviation per unit value ΔI of the converter MMC1, converter MMC2, and converter MMC3 is obtained by making a difference. 1pu , ΔI 2pu , ΔI 3pu At the same time, the DC side voltage U of converter MMC2 and converter MMC3 are respectively MMC2pu , U MMC3pu and the DC side voltage command value per unit value U of the MMC converter MMCrefpu After the difference is made, the voltage deviation per unit value ΔU is obtained 2pu , ΔU 3pu ;

[0026] S1.3.2, set the DC current deviation of the converter MMC1 to the unit value ΔI 1pu Automatically input to the PI link to obtain the input command value I of the inner loop decoupling control of the converter MMC1 dcref1 , I dcref1 =(K p1 *ΔI 1pu *U MMCrefpu +K i1 *∫ΔI 1pu *U MMCrefpu dt), where K p1 , K i1 are the proportional coefficient and integral coefficient of the converter MMC1 in the PI link respectively;

[0027] S1.3.3, set the DC current deviation per unit value ΔI of converter MMC2 and converter MMC3 2pu , ΔI 3pu Multiply by U MMCrefpu Then multiply by the droop ratio coefficient K of the converter MMC2 d2 , the droop proportional coefficient K of the converter MMC3 d3 The voltage deviation per unit value ΔU between the converter MMC2 and the converter MMC3 is 2pu , voltage deviation per unit value ΔU 3pu After adding, we get the intermediate quantity ΔD2 and the intermediate quantity ΔD3;

[0028] S1.3.4. Input the intermediate quantities ΔD2 and ΔD3 into the PI link to obtain the input command value I of the inner loop decoupling control of the converter MMC2 and the converter MMC3. dcref2 and I dcref3 , I dcref2 =K p2 *ΔD2+K i2 *∫ΔD2dt,I dcref3 =K p3 *ΔD3+K i3 *∫ΔD3dt; where K p2 , Ki2 and K p3 , K i3 They are respectively the proportional coefficient and the integral coefficient of the converter MMC2 and the converter MMC3 in the PI link.

[0029] As an optimization, in S1.2.1, the DC current command value I of the converter MMC2 MMC2ref , the DC current command value I of the converter MMC3 MMC3ref are respectively half of the DC current command values ​​of the converter MMC2 and the converter MMC3 in steady state. The DC current command value of the converter MMC1 is: I MMC1ref =I MMC2ref +I MMC3ref -K d2 *(U MMCref -U MMC1 )-K d3 *(U MMCref -U MMC1 ).

[0030] As an optimization, in step S1.3.3, the droop proportional coefficient K of the converter MMC2 d2 for:

[0031]

[0032] Among them, I MMC2 is the DC current of the converter MMC2 when the inverter side receives the fault signal, I MMC2ref is the DC current command value, U MMCref is the DC side voltage command value of the MMC converter, U MMC2 It is the terminal voltage of converter MMC2 when the inverter side receives the fault signal.

[0033] As an optimization, in step S1.3.3, the droop proportional coefficient K of the converter MMC3 d3 for:

[0034]

[0035] Among them, I MMC3 is the DC current of the converter MMC3 when the inverter side receives the fault signal, I MMC3ref is the DC current command value of the converter MMC3, U MMCref is the DC side voltage command value of the MMC converter, U MMC3 It is the terminal voltage of converter MMC3 when the inverter side receives the fault signal.

[0036] As an optimization, the port voltage reference value U of the MMC converter MMCref It is 400kV.

[0037] As an optimization, the deionization time t is set to 0.15 seconds.

[0038] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0039] 1. The present invention can handle the problem of excessive circulating current between MMCs causing overcurrent in MMC during phase shift after a DC line fault in an LCC-MMC series hybrid DC transmission system, and can significantly reduce the circulating current between MMCs and maintain the current of the MMC at the command value.

[0040] 2. The present invention can handle the overcurrent problem caused by MMC out of control due to a large current increase in the MMC during the recovery period after a DC line fault. After the LCC releases the phase shift and restores the current transmission, the MMC current fluctuates less during the recovery period and there is no overcurrent phenomenon. In addition, during the fault recovery process, the MMC current can be restored to a normal operating state.

[0041] 3. Through simulation, it can be found that in the present invention, under the conditions of equal and unequal distribution of the DC current of the MMC controller, the circulating current between stations is significantly suppressed during the phase shift period, and the MMC current can be restored to the command value during the recovery process. Therefore, good control effect is achieved in both cases. In addition, the present invention does not require additional equipment, reduces the purchase and operation costs of the system, and is easy to promote. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, other relevant drawings can be obtained based on these drawings without creative work. In the drawings:

[0043] Figure 1 It is the overall structure diagram of the LCC-MMC series hybrid DC transmission system in the implementation case when the DC fault occurs;

[0044] Figure 2 This is the AC and DC side structure diagram of MMC;

[0045] Figure 3 This is the control block diagram of the droop control strategy;

[0046] Figure 4 This is the system current diagram when DC fault occurs when the DC current of three MMC converters is evenly divided;

[0047] Figure 5 This is the system current diagram during a DC fault when the DC current of the three MMC converters is not evenly distributed. DETAILED DESCRIPTION

[0048] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments and drawings. The exemplary embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0049] Example

[0050] like Figure 1-3 As shown, the present invention proposes a method for suppressing MMC parallel circulating current after a series converter type hybrid DC line fault based on droop control, reasonably distributes the DC current of the MMC during the DC line fault clearing and recovery process to suppress the fluctuation of the MMC current, realizes rapid and stable recovery of the system, takes into account the anti-interference ability and rapid recovery of the system, and enhances the anti-interference ability of the LCC-MMC series hybrid DC transmission system, which specifically includes:

[0051] S1. After the control and protection system detects a DC line fault, during the phase shift period after the DC line fault, the three parallel converters MMC1, MMC2 and MMC3 on the inverter side of the LCC-MMC series hybrid DC transmission system are switched from the master-slave control mode to the fixed DC current control of converter MMC1 and the droop control of converters MMC2 and MMC3. The current command values ​​of converters MMC2 and MMC3 are set to half of the DC current in the steady state. At the same time, the current command value of converter MMC1 is determined according to the changes of converters MMC2 and MMC3, so as to achieve the circulating current suppression of the three parallel MMCs during the DC line fault and fault recovery period.

[0052] S2. After the fault is repaired, the converters MMC1, MMC2 and MMC3 are switched back to the master-slave control mode after a fixed de-ionization time t.

[0053] In this embodiment, the specific implementation steps of S1 are:

[0054] S1.1, respectively measuring the first relevant parameters at the AC side busbars of converters MMC1, MMC2 and MMC3 on the low voltage side of the inverter station in the LCC-MMC series hybrid DC transmission system and the second relevant parameters at the DC outlets of converters MMC2 and MMC3;

[0055] Converter MMC1, converter MMC2 and converter MMC3 are three parallel modular multilevel converters on the low voltage side of the inverter side of the LCC-MMC series hybrid DC transmission system.

[0056] In this embodiment, the first related parameters include the three-phase voltage U1 (including U 1a , U 1b , U 1b ), three-phase current I1 (including I 1a ,I 1b ,I 1b ), the three-phase voltage U2 (including U 2a , U 2b , U 2b ), three-phase current I2 (including I 2a ,I 2b ,I 2b ), the three-phase voltage U3 (including U 3a , U 3b , U 3b ), three-phase current I3 (including I 3a ,I 3b ,I 3b ), the second related parameter includes the DC output voltage U of the converter MMC2 MMC2 , DC output voltage U of converter MMC3 MMC3 .

[0057] S1.2, based on the first related parameter and the second related parameter, respectively calculate the DC current per unit value I of the DC side of the converter MMC1, the converter MMC2, and the converter MMC3 1dPu ,I 2dPu ,I 3dPu , DC current command value per unit value I 1refpu ,I 2refpu ,I 3refpu , and the per-unit value of the DC side voltage command value of the MMC converter U MMCrefpu .

[0058] In this embodiment, the specific steps of S1.2 are:

[0059] S1.2.1. Set the DC current command values ​​of the DC side of converter MMC1, converter MMC2, and converter MMC3 to I MMC1ref ,I MMC2ref and I MMC3ref (directly set a specific value), and at the same time set the DC side voltage command value U of converter MMC2 and converter MMC3 MMCref ;

[0060] S1.2.2, set the DC current command value I MMC1ref ,I MMC2ref and I MMC3refDivide by the reference DC current I of the converter MMC1 1base , the reference DC current I of the converter MMC2 2base , the reference DC current I of the converter MMC3 3base , respectively obtain the DC current command value per unit value I of converter MMC1, converter MMC2, and converter MMC3 1refpu ,I 2refpu ,I 3refpu ;

[0061] S1.2.3, set the DC side voltage command value U MMCref Divide by the reference value U of the DC side voltage of the MMC converter dcbase Get the DC side voltage command value U of the MMC converter MMCrefpu .

[0062] S1.2.4, the three-phase voltage U1 (including U 1a , U 1b , U 1b ), U2 (including U 2a , U 2b , U 2b ), U3 (including U 3a , U 3b , U 3b ) and three-phase current I1 (including I 1a ,I 1b ,I 1b ), I2 (including I 2a ,I 2b ,I 2b ), I3 (including I 3a ,I 3b ,I 3b ) divided by the reference value U of the AC side voltage and current base ,I base , to obtain the three-phase voltage per unit value U at the AC side bus of the converter MMC1 1pu (U 1apu , U 1bpu , U 1cpu ), three-phase current per unit value I 1pu (I 1apu ,I 1bpu ,I 1cpu ), based on the three-phase voltage per unit value U at the AC side bus of the converter MMC2 2pu (U 2apu , U 2bpu , U 2cpu ), three-phase current per unit value I 2pu (I 2apu ,I 2bpu ,I 2cpu) and the three-phase voltage per unit value U at the AC side bus of the converter MMC3 3pu (U 3apu , U 3bpu , U 3cpu ) and the three-phase current per unit value I 3pu (I 3apu ,I 3bpu ,I 3cpu );

[0063] S1.2.5, DC output voltage U MMC2 , DC output voltage U MMC3 Divide by the reference value U of the DC side voltage of the MMC converter dcbase (the reference values ​​of the DC side voltages of the two converters are equal) to obtain the DC side voltage standard value U based on the converter MMC2 respectively MMC2pu and the DC side voltage per unit value U based on the converter MMC3 MMC3pu ;

[0064] S1.2.6, the three-phase voltage standard value U 1pu (U 1apu , U 1bpu , U 1cpu ), U 2pu (U 2apu , U 2bpu , U 2cpu ), U 3pu (U 3apu , U 3bpu , U 3cpu ) and the three-phase current per unit value I 1pu (I 1apu ,I 1bpu ,I 1cpu ), I 2pu (I 2apu ,I 2bpu ,I 2cpu ), I 3pu (I 3apu ,I 3bpu ,I 3cpu ) are transformed by Clark to obtain the three-phase voltage per unit value U 1pu , U 2pu , U 3pu and I 1pu ,I 2pu ,I 3pu The α and β components are denoted as U 1αPu , U 1βPu ,I 1αPu ,I 1βPu , U 2αPu , U 2βPu ,I 2αPu ,I2βPu , U 3αPu , U 3βPu ,I 3αPu ,I 3βPu ;

[0065] S1.2.7, the three-phase voltage standard value U 1pu , U 2pu , U 3pu The α, β components and the per unit value of the three-phase current I 1pu ,I 2pu ,I 3pu The α and β components are multiplied and added together and then divided by the DC side voltage command value U MMCrefpu Get the DC current per unit value I 1dPu , DC current per unit value I 2dPu , DC current per unit value I 3dPu ,in,

[0066] I 1dPu =(U 1αPu *I 1αPu +U 1βPu *I 1βPu ) / U MMCrefpu ;

[0067] I 2dPu =(U 2αPu *I 2αPu +U 2βPu *I 2βPu ) / U MMCrefpu ;

[0068] I 3dPu =(U 3αPu *I 3αPu +U 3βPu *I 3βPu ) / U MMCrefpu ;

[0069] S1.3, when the control and protection system detects a DC line fault, during the phase shift period after the DC line fault, based on the parameter values ​​obtained in S1.2, the DC current deviation per unit value ΔI of the converter MMC1, converter MMC2, and converter MMC3 are calculated respectively. 1pu , ΔI 2pu , ΔI 3pu The DC current deviation per unit values ​​ΔI1pu, ΔI1pu, and ΔI2pu are automatically input into the PI link (the signal is obtained by the sensor of the control and protection device and then input into the control system) to obtain the command value I of the inner loop decoupling control of the converter MMC1, the converter MMC2, and the converter MMC3 respectively. dcref1 ,I dcref2 and I dcref3, to distribute the DC current between the converters MMC1, MMC2 and MMC3. The current of the MMC is distributed by presetting the command value in advance.

[0070] In this embodiment, the specific implementation steps of S1.3 include:

[0071] S1.3.1. When the control and protection system detects a DC line fault, the DC current command values ​​of converters MMC1, MMC2, and MMC3 are marked as I 1refpu ,I 2refpu ,I 3refpu With DC current per unit value I 1dPu ,I 2dPu ,I 3dPu The DC current deviation per unit value ΔI of the converter MMC1, converter MMC2, and converter MMC3 is obtained by making a difference. 1pu , ΔI 2pu , ΔI 3pu At the same time, the DC side voltage U of converter MMC2 and converter MMC3 are respectively MMC2pu , U MMC3pu and the DC side voltage command value per unit value U of the MMC converter MMCrefpu After the difference is made, the voltage deviation per unit value ΔU is obtained 2pu , ΔU 3pu ;

[0072] S1.3.2, set the DC current deviation of the converter MMC1 to the unit value ΔI 1pu Automatically input to the PI link to obtain the input command value I of the inner loop decoupling control of the converter MMC1 dcref1 , I dcref1 =(K p1 *ΔI 1pu *U MMCrefpu +K i1 *∫ΔI 1pu *U MMCrefpu dt), where K p1 , K i1 They are respectively the proportional coefficient and integral coefficient of the converter MMC1 in the PI link (the proportional coefficient and integral coefficient are empirical values ​​obtained during debugging and can be directly set here);

[0073] S1.3.3, set the DC current deviation per unit value ΔI of converter MMC2 and converter MMC3 2pu , ΔI 3pu Multiply by U MMCrefpu Then multiply by the droop ratio coefficient K of the converter MMC2 d2 , the droop proportional coefficient K of the converter MMC3 d3The voltage deviation per unit value ΔU between the converter MMC2 and the converter MMC3 is 2pu , voltage deviation per unit value ΔU 3pu After adding, we get the intermediate quantity ΔD2 and the intermediate quantity ΔD3;

[0074] S1.3.4. Input the intermediate quantities ΔD2 and ΔD3 into the PI link to obtain the input command value I of the inner loop decoupling control of the converter MMC2 and the converter MMC3. dcref2 and I dcref3 , I dcref2 =K p2 *ΔD2+K i2 *∫ΔD2dt,I dcref3 =K p3 *ΔD3+K i3 *∫ΔD3dt; where K p2 , K i2 and K p3 , K i3 They are respectively the proportional coefficient and the integral coefficient of the converter MMC2 and the converter MMC3 in the PI link.

[0075] In this embodiment, the droop proportional coefficient K of the converter MMC2 is d2 for:

[0076]

[0077] Among them, I MMC2 is the DC current of the converter MMC2 when the inverter side receives the fault signal, I MMC2ref is the DC current command value, U MMCref is the DC side voltage command value of the MMC converter, U MMC2 It is the terminal voltage of converter MMC2 when the inverter side receives the fault signal.

[0078] In this embodiment, the droop proportional coefficient K of the converter MMC3 is d3 for:

[0079]

[0080] Among them, I MMC3 is the DC current of the converter MMC3 when the inverter side receives the fault signal, I MMC3ref is the DC current command value of the converter MMC3, U MMCref is the DC side voltage command value of the MMC converter, U MMC3 It is the terminal voltage of converter MMC3 when the inverter side receives the fault signal.

[0081] Next, the reliability of the control strategy of the LCC-MMC series hybrid DC transmission system under the conditions of MMC evenly sharing the DC current and unequally sharing the DC current during DC faults is simulated and verified.

[0082] The main parameters of the LCC-MMC series hybrid DC transmission system are shown in Table 1.

[0083] Table 1

[0084]

[0085]

[0086] The present invention verifies the reliability of the multi-terminal system control method by simulating two different working conditions.

[0087] 1) Under the condition of MMC DC current equalization, the current of each part of the system during the DC fault clearing and recovery process is simulated and verified, such as Figure 4 As shown. During the phase shift period, the circulating current between MMCs stations was small, and the currents of converters MMC1, MMC2, and MMC3 were maintained at 1.66kA, 0.83kA, and 0.83kA, respectively, without overcurrent. After the LCC released the phase shift and restored the current transmission, there was no overcurrent in the current of each MMC, and the peak value of the MMC current during the recovery process after the fault was 1.85kA. The current recovered stably after the fault disappeared and basically returned to a stable state after 2.4s, with good control performance.

[0088] 2) Simulate and verify the current at various locations in the system during the DC fault recovery process when the MMC DC current is not evenly distributed, such as Figure 5 As shown. During the phase shift, the circulating current between MMCs stations was small, and the currents of converters MMC1, MMC2, and MMC3 were maintained at 1.5kA, 0.75kA, and 0.75kA, respectively, without overcurrent. After the LCC released the phase shift and restored the current transmission, there was no overcurrent in the current of each MMC. The peak value of the MMC current during the recovery process after the fault was 2.13kA. The current recovered stably after the fault disappeared and basically returned to a stable state after 2.4s, with good control performance.

[0089] It can be seen that the method for suppressing the MMC parallel circulating current after a series converter type hybrid DC line fault proposed in the present invention can realize the reasonable distribution of the MMC DC current during the DC fault clearing and recovery process of the LCC-MMC series hybrid DC transmission system, thereby suppressing the fluctuation of the MMC current, realizing the rapid and stable recovery of the system, and enhancing the anti-interference ability of the LCC-MMC series hybrid DC transmission system.

[0090] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. MMC parallel circulating current suppression method after series converter hybrid DC line fault, It is characterized in that include: S1. After the control and protection system detects a DC line fault, during the phase shift period after the DC line fault, the three parallel converters MMC1, MMC2 and MMC3 on the inverter side of the LCC-MMC series hybrid DC transmission system are switched from the master-slave control mode to the fixed DC current control of converter MMC1 and the droop control of converters MMC2 and MMC3. The current command values ​​of converters MMC2 and MMC3 are set to half of the DC current in the steady state. At the same time, the current command value of converter MMC1 is determined according to the changes of converters MMC2 and MMC3, so as to achieve the circulating current suppression of the three parallel MMCs during the DC line fault and fault recovery period. The specific implementation steps of S1 are: S1.1, respectively measuring the first relevant parameters at the AC side busbars of converters MMC1, MMC2 and MMC3 on the low voltage side of the inverter station in the LCC-MMC series hybrid DC transmission system and the second relevant parameters at the DC outlets of converters MMC2 and MMC3; The first related parameters include the three-phase voltage U at the AC side bus of the converter MMC1 1 , three-phase current I 1 , the three-phase voltage U at the AC side bus of the converter MMC2 2 , three-phase current I 2 , the three-phase voltage U at the AC side bus of the converter MMC3 3 , three-phase current I 3 The second related parameter includes the DC output voltage U of the converter MMC2. MMC2 , DC output voltage U of converter MMC3 MMC3 ; S1.2, based on the first related parameter and the second related parameter, respectively calculate the DC current per unit value I of the DC side of the converter MMC1, the converter MMC2, and the converter MMC3 1dPu ,I 2dPu ,I 3dPu , DC current command value per unit value I 1refpu ,I 2refpu ,I 3refpu , and the per-unit value of the DC side voltage command value of the MMC converter U MMCrefpu ; S1.3, when the control and protection system detects a DC line fault, during the phase shift period after the DC line fault, based on the parameter values ​​obtained in S1.2, the DC current deviation per unit value ΔI of the converter MMC1, converter MMC2, and converter MMC3 are calculated respectively. 1pu , ΔI 2pu , ΔI 3pu , and the DC current deviation per unit value ΔI 1pu , ΔI 1pu , ΔI 2pu The command values ​​I of the inner loop decoupling control of the converter MMC1, the converter MMC2 and the converter MMC3 are automatically input into the PI link to obtain the command values ​​I of the inner loop decoupling control of the converter MMC1, the converter MMC2 and the converter MMC3 dcref1 ,I dcref2 and I dcref3 , to distribute the DC current between the converter MMC1, the converter MMC2, and the converter MMC3; S2. After the fault is repaired, the converters MMC1, MMC2 and MMC3 are switched back to the master-slave control mode after a fixed de-ionization time t.

2. The method for suppressing MMC parallel circulating current after a series converter type hybrid DC line fault according to claim 1, It is characterized in that The specific steps of S1.2 are: S1.2.

1. Set the DC current command values ​​of the DC side of converter MMC1, converter MMC2, and converter MMC3 to I MMC1ref ,I MMC2ref and I MMC3ref , and set the DC side voltage command value U of converter MMC2 and converter MMC3 at the same time MMCref ; S1.2.2, set the DC current command value I MMC1ref ,I MMC2ref and I MMC3ref Divide by the reference DC current I of the converter MMC1 1base , the reference DC current I of the converter MMC2 2base , the reference DC current I of the converter MMC3 3base , respectively obtain the DC current command value per unit value I of converter MMC1, converter MMC2, and converter MMC3 1refpu ,I 2refpu ,I 3refpu ; S1.2.3, set the DC side voltage command value U MMCref Divide by the reference value U of the DC side voltage of the MMC converter dcbase Get the DC side voltage command value U of the MMC converter MMCrefpu； S1.2.4, the three-phase voltage U 1 , U 2 , U 3 and three-phase current I 1 , I 2 , I 3 Divide by the reference value U of the AC side voltage and current base ,I base , to obtain the three-phase voltage per unit value U at the AC side bus of the converter MMC1 1pu , three-phase current per unit value I 1pu , based on the three-phase voltage per unit value U at the AC side bus of the converter MMC2 2pu , three-phase current per unit value I 2pu and the three-phase voltage per unit value U at the AC side bus of the converter MMC3 3pu And the three-phase current per unit value I 3pu ; S1.2.5, DC output voltage U MMC2 , DC output voltage U MMC3 Divide by the reference value U of the DC side voltage of the MMC converter dcbase , to obtain the DC side voltage per unit value U based on the converter MMC2 MMC2pu and the DC side voltage per unit value U based on the converter MMC3 MMC3pu ; S1.2.6, the three-phase voltage standard value U 1pu , U 2pu , U 3pu And the three-phase current per unit value I 1pu ,I 2pu ,I 3pu After Clark transformation, the three-phase voltage standard value U is obtained. 1pu , U 2pu , U 3pu and I 1pu ,I 2pu ,I 3pu The α and β components are denoted as U 1αPu , U 1βPu ,I 1αPu ,I 1βPu , U 2αPu , U 2βPu ,I 2αPu ,I 2βPu , U 3αPu , U 3βPu ,I 3αPu ,I 3βPu ; S1.2.7, the three-phase voltage standard value U 1pu , U 2pu , U 3pu The α, β components and the per unit value of the three-phase current I 1pu ,I 2pu ,I 3pu The α and β components are multiplied and added together and then divided by the DC side voltage command value U MMCrefpu Get the DC current per unit value I 1dPu , DC current per unit value I 2dPu , DC current per unit value I 3dPu ,in, AND 1dPu =(U 1αPu *AND 1αPu +U 1βPu *AND 1βPu ) / IN MMCrefpu ; AND 2dPu =(U 2αPu *AND 2αPu +U 2βPu *AND 2βPu ) / IN MMCrefpu ; AND 3dPu =(U 3αPu *AND 3αPu +U 3βPu *AND 3βPu ) / IN MMCrefpu。 3. The method for suppressing MMC parallel circulating current after a series converter type hybrid DC line fault according to claim 2, It is characterized in that The specific implementation steps of S1.3 include: S1.3.

1. When the control and protection system detects a DC line fault, the DC current command values ​​of converters MMC1, MMC2, and MMC3 are marked as I 1refpu ,I 2refpu ,I 3refpu With DC current per unit value I 1dPu ,I 2dPu ,I 3dPu The DC current deviation per unit value ΔI of the converter MMC1, converter MMC2, and converter MMC3 is obtained by making a difference. 1pu , ΔI 2pu , ΔI 3pu At the same time, the DC side voltage U of converter MMC2 and converter MMC3 are respectively MMC2pu , U MMC3pu and the DC side voltage command value per unit value U of the MMC converter MMCrefpu After the difference is made, the voltage deviation per unit value ΔU is obtained 2pu , ΔU 3pu ; S1.3.2, set the DC current deviation of the converter MMC1 to the unit value ΔI 1pu Automatically input to the PI link to obtain the input command value I of the inner loop decoupling control of the converter MMC1 dcref1 , I dcref1 =(K p1 *ΔI 1pu *U MMCrefpu +K i1 *∫ΔI 1pu *U MMCrefpu dt), where K p1 , K i1 are the proportional coefficient and integral coefficient of the converter MMC1 in the PI link respectively; S1.3.3, set the DC current deviation per unit value ΔI of converter MMC2 and converter MMC3 2pu , ΔI 3pu Multiply by U MMCrefpu Then multiply by the droop ratio coefficient K of the converter MMC2 d2 , the droop proportional coefficient K of the converter MMC3 d3 The voltage deviation per unit value ΔU between the converter MMC2 and the converter MMC3 is 2pu , voltage deviation per unit value ΔU 3pu After adding, we get the intermediate value ΔD 2 and the intermediate amount ΔD 3 ; S1.3.4, the intermediate amount ΔD 2 and the intermediate amount ΔD 3 The PI link is input to obtain the input command value I of the inner loop decoupling control of the converter MMC2 and the converter MMC3 respectively. dcref2 and I dcref3 , I dcref2 =K p2 *ΔD 2 +K i2 *∫ΔD 2 dt,I dcref3 =K p3 *ΔD 3 +K i3 *∫ΔD 3 dt; where K p2 , K i2 and K p3 , K i3 They are respectively the proportional coefficient and the integral coefficient of the converter MMC2 and the converter MMC3 in the PI link.

4. The method for suppressing MMC parallel circulating current after a series converter type hybrid DC line fault according to claim 3, It is characterized in that In S1.2.1, the DC current command value I of the converter MMC2 MMC2ref , the DC current command value I of the converter MMC3 MMC3ref are respectively half of the DC current command values ​​of the converter MMC2 and the converter MMC3 in steady state. The DC current command value of the converter MMC1 is: I MMC1ref =I MMC2ref +I MMC3ref -K d2 *(U MMCref -U MMC1 )-K d3 *(U MMCref -U MMC1 ).

5. The method for suppressing MMC parallel circulating current after a series converter type hybrid DC line fault according to claim 4, It is characterized in that In step S1.3.3, the droop proportional coefficient K d2 for: Among them, I MMC2 is the DC current of the converter MMC2 when the inverter side receives the fault signal, I MMC2ref is the DC current command value, U MMCref is the DC side voltage command value of the MMC converter, U MMC2 It is the terminal voltage of converter MMC2 when the inverter side receives the fault signal.

6. The method for suppressing MMC parallel circulating current after a series converter type hybrid DC line fault according to claim 5, It is characterized in that In step S1.3.3, the droop proportional coefficient K d3 for: Among them, I MMC3 is the DC current of the converter MMC3 when the inverter side receives the fault signal, I MMC3ref is the DC current command value of the converter MMC3, U MMCref is the DC side voltage command value of the MMC converter, U MMC3 It is the terminal voltage of the converter MMC3 when the inverter side receives the fault signal.

7. The method for suppressing MMC parallel circulating current after a series converter type hybrid DC line fault according to claim 6, It is characterized in that The port voltage reference value U of MMC converter MMCref It is 400kV.

8. The method for suppressing MMC parallel circulating current after a series converter hybrid DC line fault according to claim 1, It is characterized in that The deionization time t is set to 0.15 seconds.

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