A method for suppressing continuous commutation failure in hybrid double-fed DC transmission system

Abstract

The present invention relates to a method for suppressing continuous commutation failure of a hybrid double-fed DC power transmission system, comprising the following steps: a 、u b 、u c After Fourier transform and harmonic detection, U nmax and φ n , will U nmax and φ n Substitute the cut-off angle γ taking harmonics into account n The calculation formula of γ is n , based on the γ n , according to the cut-off angle setting value γ ref Calculate the cut-off angle setting value γ ref ; γ ref The measured value of the turn-off angle γ mes The AC voltage compensation ΔU is obtained through the proportional integral link; according to whether the AC system fault occurs and γ ref With γ mes Whether the difference is greater than 0, the fault judgment module Fault or controls whether the AC voltage compensation amount ΔU is put into the reactive power control link. Compared with the prior art, the present invention has the advantages of controlling the input value of reactive power on the MMC side and reducing the number of continuous commutation failures of LCC-HVDC.

Description

Technical Field

[0001] The invention relates to the field of direct current power transmission, and in particular to a method for suppressing continuous commutation failure of a hybrid double-feed direct current power transmission system. Background Art

[0002] Nowadays, LCC-HVDC is widely used in power transmission systems due to its own advantages. Since its converter uses semi-controlled devices, thyristors, for commutation, and the commutation voltage comes from the receiving end AC busbar, commutation failure usually occurs when the AC system fails. When commutation failure occurs in LCC, the DC current of the transmission system rises rapidly and the DC voltage drops rapidly, which will damage the thyristors. When the AC system fails seriously, it will cause continuous commutation failures and even DC blocking, which is not conducive to the safe and stable operation of the power grid. The converter of MMC-HVDC is a fully controlled device and will not fail to commutate. Therefore, it can supply power to weak AC systems with low AC system strength. MMC-HVDC can also independently decouple active and reactive power, and does not require compensation equipment for reactive compensation. Due to the high degree of modeling of MMC-HVDC subunits, it can adapt to various transmission line voltages and transmission capacities according to the number of converter power subunits. Relying on the advantages of MMC-HVDC itself, it complements LCC-HVDC to form a multi-input hybrid DC transmission system.

[0003] However, when the AC system at the receiving end of such a hybrid DC transmission system fails seriously, the DC loops will be coupled. At this time, if the conventional control strategy is adopted for the MMC-HVDC in the hybrid multi-feed DC system, the system will have continuous commutation failure, especially the continuous commutation failure caused by the AC bus voltage waveform distortion on the LCC-HVDC inverter side in the hybrid double-feed DC system. Therefore, in view of the problem of continuous commutation failure in the hybrid double-feed DC including MMC-HVDC, from the perspective of harmonics, the present invention provides a method for suppressing continuous commutation failure in a hybrid double-feed DC transmission system. Summary of the invention

[0004] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and provide a method for suppressing continuous commutation failure of a hybrid double-fed DC transmission system.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] The present invention provides a method for suppressing continuous commutation failure of a hybrid double-fed direct current transmission system, comprising the following steps:

[0007] Step 1: Convert the AC bus three-phase voltage u a 、u b 、uc After Fourier transform and harmonic detection, U nmax and Will U nmax and Substitute the cut-off angle γ taking harmonics into account n The calculation formula of γ is n , based on the γ n , according to the cut-off angle setting value γ ref Calculation formula, get the cut-off angle setting value γ ref ;

[0008] Step 2: Add γ ref The measured value of the turn-off angle γ mes Make a difference and obtain the AC voltage compensation ΔU through the proportional integral link;

[0009] Step 3: When an AC system fault occurs, γ ref With γ mes The difference is greater than 0. Based on this signal, the fault judgment module Fault or value is 1, and the AC voltage compensation amount ΔU is put into the reactive power control link of the outer loop of the flexible DC converter;

[0010] When the fault does not occur, γ ref With γ mes The difference is less than 0. Based on this signal, the fault judgment module Faultor value is 0 and the AC voltage compensation amount ΔU is 0.

[0011] Furthermore, the cut-off angle γ taking harmonics into account is n The calculation formula is expressed as follows:

[0012]

[0013] Where n is the harmonic order, is the harmonic phase angle, U L is the fundamental frequency voltage amplitude, U n is the AC voltage amplitude, γ min is the minimum turn-off angle.

[0014] Furthermore, the cut-off angle setting value γ ref The calculation formula is expressed as follows:

[0015]

[0016] Where, U is the AC bus line voltage, U L is the fundamental frequency voltage amplitude, U n is the AC voltage amplitude, γ min is the minimum cut-off angle, γ n is the cut-off angle taking harmonics into account.

[0017] Furthermore, the fault judgment module Fault or judges the signal through a delay link after the fault occurs.

[0018] Furthermore, the delay time length of the delay link is 2ms.

[0019] Furthermore, the AC voltage compensation amount ΔU is less than a set upper limit value.

[0020] Furthermore, the upper limit value is 350 Mvar.

[0021] Furthermore, the current reference value i output by the outer loop controller dref 、i qref Adjust the current component i of the d and q axes d 、i q , get the MMC-HVDC commutation bus d, q axis voltage reference value u dref and u qref .

[0022] In a second aspect, the present invention provides a hybrid double-fed direct current transmission system, comprising MMC-HVDC and LCC-HVDC, wherein the MMC-HVDC and the LCC-HVDC are fed into the same AC system via the same busbar, and the system is used to implement any one of the methods described above.

[0023] The third invention provides a storage medium having a program stored thereon, wherein the program implements the steps of any of the above-mentioned methods when executed.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1. The present invention designs a commutation failure suppression method for a hybrid double-fed direct current transmission system based on minimum shutdown area taking into account harmonics. According to the shutdown angle adjustment strategy invested in the MMC-HVDC outer loop reactive power control, the input value of the reactive power on the MMC side is controlled to reduce the number of continuous commutation failures of LCC-HVDC.

[0026] 2. In a weak AC system, the suppression method provided by the present invention can allow MMC-HVDC to output more reactive power during a fault, thereby improving the reactive support capacity of the receiving-end AC system. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram of the steps of the present invention;

[0028] Figure 2 It is a structural diagram of the hybrid double-feed DC power transmission system of the present invention;

[0029] Figure 3 It is a principle diagram of the suppression method based on the minimum cut-off area of ​​the present invention;

[0030] Figure 4 The single-phase grounding fault L f =0.5H waveform comparison of each parameter;

[0031] Figure 5 The three-phase grounding fault L f =0.5H waveform comparison of each parameter;

[0032] Figure 6 It is a comparison diagram of the failure probability of continuous phase change according to the present invention as the SCR changes. DETAILED DESCRIPTION

[0033] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0034] Example 1

[0035] In addition, this embodiment also provides a hybrid double-fed DC transmission system continuous commutation failure suppression strategy method, such as Figure 1 As shown, the method comprises the following steps:

[0036] Step 1: Convert the AC bus three-phase voltage u a 、u b 、u c After Fourier transform and harmonic detection, U nmax and Among them, U nmax is the maximum value of the nth harmonic content in each phase of the three phases, which is substituted into the cut-off angle γ taking into account the harmonics. n The calculation formula of γ is n , based on the γ n , according to the cut-off angle setting value γ ref Calculation formula, get the cut-off angle setting value γ ref .

[0037] Specifically, the cut-off angle γ taking into account the harmonics n The calculation formula is expressed as follows:

[0038]

[0039] Where n is the harmonic order, is the harmonic phase angle, U L is the fundamental frequency voltage amplitude, U n is the AC voltage amplitude, γ min is the minimum turn-off angle.

[0040] Cut-off angle setting value γ ref The calculation formula is expressed as follows:

[0041]

[0042] Where, U is the AC bus line voltage, U L is the fundamental frequency voltage amplitude, U n is the AC voltage amplitude, γ min is the minimum cut-off angle, γ n is the cut-off angle taking harmonics into account.

[0043] Step 2: Add γ ref The measured value of the turn-off angle γ mes The AC voltage compensation ΔU is obtained by the proportional integral (PI) link. Since the PI link value has upper and lower limits, no harmonics are generated under steady-state conditions. ref With γ mes If the difference is less than 0, the output compensation value is 0. When a fault occurs, γ ref With γ mes If the difference is greater than 0, a certain voltage compensation value ΔU is output according to the PI link. Since the maximum reactive power value is set in the VSC converter, the reactive power compensated by ΔU cannot exceed this value, and the maximum reactive power is 350Mvar.

[0044] Step 3: If Figure 3 As shown in Figure 2, when an AC system fault occurs, γ ref With γ mes The difference is greater than 0. Based on this signal, the fault judgment module Fault or value is 1, and the AC voltage compensation ΔU is put into the reactive control link of the outer loop of the flexible DC converter through the fault judgment module Fault or to start the control strategy. The fault judgment module Fault or judges that the signal is completed through the delay link after the fault occurs, and the delay time is 2ms.

[0045] When the fault does not occur, γ ref With γ mes The difference is less than 0, the fault judgment module Fault or value is 0, the AC voltage compensation amount ΔU is 0, and there is no need to start the control strategy.

[0046] like Figure 3 As shown, in this embodiment, the current reference value i output by the outer loop controller can also be dref 、i qref Adjust the current component i of the d and q axes d 、i q, get the MMC-HVDC commutation bus d, q axis voltage reference value u dref and u qre f .

[0047] In addition, this embodiment provides a hybrid double-feed direct current transmission system, including MMC-HVDC and LCC-HVDC, such as Figure 2 As shown, the MMC-HVDC and LCC-HVDC are fed into the same AC system via the same busbar, and the system is used to implement the above-mentioned method.

[0048] Specifically, one end of the LCC-HVDC is connected to the AC1 AC system, the AC1 AC system is connected to the converter transformer T1 via the resistor Z1, connected to the converter transformer T2 via two reverse semi-automatic thyristors, and finally merged into the AC3 AC system via the resistor Z3.

[0049] One end of the MMC-HVDC is connected to the AC2 AC system, which is connected to the converter transformer T3 via resistor Z2, connected to the converter transformer T4 via two IGBTs with self-shutdown capability, and finally merged into the AC3 AC system via resistor Z3.

[0050] Among them, Z1, Z2, and Z3 are impedances of equal value.

[0051] This embodiment verifies the effectiveness of the continuous commutation failure suppression strategy method proposed in this embodiment through simulation analysis. This embodiment simulates and verifies the proposed control strategy in PSCAD, and builds a hybrid double-fed DC transmission model. Among them, LCC-HVDC is a CIGRE standard test model with a rated transmission power of 1000MW and a DC voltage of 500kV. The rated transmission power of MMC-HVDC is 700MW and the DC voltage is 500kV. Both are fed into the inverter-side weak AC system AC3 under the same CIGRE model, and its short-circuit ratio (SCR) is set to 2.5. The specific parameters of the MMC-HVDC system are shown in the following table.

[0052]

[0053] Table 1 Parameters of hybrid double-feed DC transmission system

[0054] In addition, three comparative cases are set up, and the case descriptions are as follows:

[0055] Case 1: Conventional control strategy.

[0056] Case 2: A transient reactive power coordinated control strategy with a fixed minimum shutdown area that ignores harmonics is used, where the minimum shutdown angle is set to 7 degrees.

[0057] Case 3: Apply the transient reactive power coordinated control strategy with minimum shutdown area taking harmonics into account.

[0058] The fault conditions of three cases are: single-phase short-circuit grounding fault, starting from 3s, lasting 0.2s, and fault inductance 0.5H. The figure below compares the system response of different cases under this fault condition.

[0059] like Figure 4 As shown, it is a single-phase grounding fault L f =0.5H waveform comparison of various parameters, among which, Figure 4 (a) is a line graph showing the change of the shut-off angle over time for the three cases. Figure 4 (b), (c) and (d) are line graphs showing the changes of DC current, DC voltage and harmonic voltage distortion rate over time for the three cases respectively. Figure 4 (e) is a line graph showing the reactive power of the MMC inverter side changing with time for the three cases. Figure 4 (f) is a line graph of the MMC active power variation over time for the three cases. Figure 4 It can be seen that the first commutation failure occurred within 3 to 3.1 seconds, and the second commutation failure occurred in case 1 at 3.1 to 3.2 seconds. The control strategy of case 2 failed to prevent the occurrence of commutation failure, which was caused by insufficient reactive power compensation. In addition, before the optimization during the fault period, the output reactive power of cases 1 and 2 was not as much as that of case 3, and the active power jitter amplitude was not large. To a certain extent, the harmonic content was reduced, and the second turn-off angle did not drop to 0. It can be seen that after the optimization, no subsequent commutation failure occurred during the fault recovery period. Compared with cases 2 and 1, the DC current and voltage of case 3 were improved.

[0060] Fault condition 2: three-phase short-circuit grounding fault, with a fault inductance of 0.5H, a fault start time of 3 seconds, and a duration of 0.2 seconds. The system response of the three cases under this fault condition is compared as follows: Figure 5 shown.

[0061] like Figure 5 As shown, it is a three-phase grounding fault L f =0.5H Waveform comparison of each parameter, Figure 5 (a) is a line graph showing the change of the shut-off angle over time for the three cases. Figure 5 (b), (c) and (d) are line graphs showing the changes of DC current, DC voltage and harmonic voltage distortion rate over time for the three cases respectively. Figure 5 (e) is a line graph showing the reactive power of the MMC inverter side changing with time for the three cases. Figure 5 (f) is a line graph of the MMC active power variation over time for the three cases. Figure 5It can be seen that when a short-circuit fault occurs, the hybrid DC system fails to commutate for the first time, and the harmonic content does not decrease. The second commutation failure occurs at 3.1-3.2s. In case 1, basically no reactive power is generated, in case 2, 250Mvar of reactive power is generated, and in case 3, 330Mvar of reactive power is generated. Compared with case 1 and case 2, in case 3, the turn-off angle is not less than the minimum value, the harmonic content is reduced, the active power fluctuation is small, and more reactive power can be compensated, which increases the AC bus voltage during the fault and keeps it stable.

[0062] Similarly, single-phase and three-phase short-circuit fault conditions with ground inductance ranging from 0.01H to 0.7H can be set in the hybrid doubly-fed DC system, and the probability of continuous commutation failure under these 70 conditions can be counted. Different short-circuit ratios can be used to simulate the adaptability comparison of Case 1, Case 2 and Case 3 under different AC system strengths.

[0063] like Figure 6 As shown, Figure 6 This is a case comparison chart under different SCRs, where: Figure 6 (a) is the curve of the single-phase continuous commutation failure probability changing with SCR. Figure 6 (b) is a graph showing the probability of three-phase continuous commutation failure changing with SCR. Figure 6 It can be seen that for extremely weak AC systems, all three cases will lead to continuous commutation failures under different fault conditions. With the increase of SCR and the improvement of system strength, Case 3 can reduce the probability of continuous commutation failures compared to Case 1 and Case 2.

[0064] In general, the present invention aims at the problem of continuous commutation failure caused by the AC bus voltage waveform distortion on the LCC-HVDC inverter side in the hybrid double-fed DC system, and designs a commutation failure suppression method for the hybrid double-fed DC transmission system based on the minimum shutdown area taking into account harmonics. According to the shutdown angle adjustment strategy invested in the MMC-HVDC outer loop reactive power control, the input value of the reactive power on the MMC side is controlled, so that the number of continuous commutation failures of the LCC-HVDC is reduced.

[0065] When the system does not require reactive power compensation, the risk of continuous commutation failure in LCC-HVDC can be reduced by adjusting the AC voltage of MMC-HVDC based on the cut-off area of ​​harmonics.

[0066] The harmonic cut-off angle setting value is considered in the minimum cut-off area. Since the cut-off angle setting value is included in the MMC-HVDC outer loop reactive control, the MMC can supply sufficient reactive power to the receiving AC system even when a system failure occurs. The designed strategy can be implemented when the system SCR is large enough, and has no effect when the system SCR is small.

[0067] Example 2

[0068] This embodiment also provides a storage medium on which a program is stored, and when the program is executed, the method provided in the above embodiment is implemented. The storage medium includes: a USB flash drive, a mobile hard disk, a read-only memory (ROM), a random access memory (RAM), a magnetic disk or an optical disk, and other media that can store program codes.

[0069] The preferred specific embodiments of the present invention are described in detail above. It should be understood that a person skilled in the art can make many modifications and changes based on the concept of the present invention without creative work. Therefore, any technical solution that can be obtained by a person skilled in the art through logical analysis, reasoning or limited experiments based on the concept of the present invention on the basis of the prior art should be within the scope of protection determined by the claims.

Claims

1. A method for suppressing continuous commutation failure in a hybrid double-fed direct current transmission system, characterized in that: The following steps are involved: Step 1: Convert the AC bus three-phase voltage u a 、u b 、u c After Fourier transform and harmonic detection, U nmax and Will U nmax and Substitute the cut-off angle γ taking harmonics into account n The calculation formula of γ is n , based on the γ n , according to the cut-off angle setting value γ ref Calculation formula, get the cut-off angle setting value γ ref ; The cut-off angle γ considering harmonics n The calculation formula is expressed as follows: Where n is the harmonic order, is the harmonic phase angle, U L is the fundamental frequency voltage amplitude, U n is the AC voltage amplitude, γ min is the minimum cut-off angle; The cut-off angle setting value γ ref The calculation formula is expressed as follows: Where, U is the AC bus line voltage, U L is the fundamental frequency voltage amplitude, U n is the AC voltage amplitude, γ min is the minimum cut-off angle, γ n is the cut-off angle taking harmonics into account; Step 2: Add γ ref The measured value of the turn-off angle γ mes Make a difference and obtain the AC voltage compensation ΔU through the proportional integral link; Step 3: When an AC system fault occurs, γ ref With γ mes The difference is greater than 0. Based on this signal, the fault judgment module Fault or value is 1, and the AC voltage compensation amount ΔU is put into the reactive power control link of the outer loop of the flexible DC converter; When the fault does not occur, γ ref With γ mes The difference is less than 0. Based on this signal, the fault judgment module Fault or value is 0, and the AC voltage compensation amount ΔU is 0; The fault judgment module Fault or judges that the signal is completed through the delay link after the fault occurs; The AC voltage compensation amount ΔU is less than a set upper limit value.

2. A method for suppressing continuous commutation failure in a hybrid double-fed direct current transmission system according to claim 1, characterized in that: The delay time length of the delay link is 2ms.

3. A method for suppressing continuous commutation failure in a hybrid double-fed direct current transmission system according to claim 1, characterized in that: The upper limit value is 350 Mvar.

4. A method for suppressing continuous commutation failure in a hybrid double-fed direct current transmission system according to claim 1, characterized in that: The current reference value i output by the outer loop controller dref 、i qref Adjust the current component i of the d and q axes d 、i q , get the MMC-HVDC commutation bus d, q axis voltage reference value u dref and u qref .

5. A hybrid double-feed HVDC transmission system, comprising MMC-HVDC and LCC-HVDC, wherein: The MMC-HVDC and LCC-HVDC are fed into the same AC system via the same busbar, wherein the system is used to implement the method according to any one of claims 1 to 4.

6. A storage medium having a program stored thereon, characterized in that: When the program is executed, the steps of the method according to any one of claims 1 to 4 are implemented.

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