A method for selecting MMC AC bus voltage for overload operation requirements

Abstract

The present invention relates to a method for selecting an MMC AC bus voltage for overload operation requirements, and belongs to the technical field of flexible DC converter station parameter selection. The present invention comprises: according to the rated operating conditions of the MMC, obtaining a bus voltage that satisfies the modulation signal margin of not less than 0 and is the maximum under all operating conditions; U s.lim , that is, the limit bus voltage that is not over-modulated under rated operation of the MMC; based on U s.lim , calculate the maximum AC bus voltage that satisfies the modulation signal margin of not less than 0 when the zero-sequence signal is injected under all overload conditions, that is, the MMC AC bus voltage facing the overload operation requirements is obtained U s.opt The present invention can select the AC bus voltage for MMCs with different parameters, so as to meet the requirements of stable overload operation and ensure that the output current and loss are small.

Description

Technical Field

[0001] The invention relates to a method for selecting an MMC alternating current bus voltage facing an overload operation requirement, and belongs to the technical field of flexible direct current converter station parameter selection. Background Art

[0002] As a new type of voltage source converter topology, Modular Multilevel Converter (MMC) has the advantages of modular structure, flexible voltage level control, high output waveform quality, and low switching loss. Based on its unique advantages, MMC has been widely used in high-voltage DC transmission, DC distribution network, energy storage and other fields. At the same time, it can improve the efficiency and reliability of the power system, and has a wide range of application scenarios in promoting the large-scale development and utilization of renewable energy. In actual engineering application scenarios, when a short circuit or other fault occurs in the system, the converter will be in an overloaded operating state. In order to ensure the stable operation of the power system, MMC is required to have a certain overload operation capability. When MMC is in an overloaded operating state, the modulation signal may be overmodulated, resulting in the output being unable to stabilize at the preset waveform, which in turn causes the MMC to be unable to operate stably and collapse.

[0003] However, the existing MMC AC bus voltage selection has the following defects: it cannot take into account the contradiction between over-modulation risk and operating efficiency. In the case of system failure, the MMC may not be able to stably operate under overload; it does not consider the influence of AC bus voltage and zero-sequence signal on the modulation signal during overload operation; under overload operation requirements, the selection of too small AC bus voltage will increase the MMC phase current and its device coupling current, increase device loss, and reduce operating life. Therefore, the traditional AC bus voltage selection cannot meet the overload operation requirements of MMC. Summary of the invention

[0004] In order to solve the above problems, the present invention provides an MMC AC bus voltage selection method for overload operation requirements, which can select AC bus voltage for MMCs with different parameters to meet their overload stable operation and ensure small output current and loss.

[0005] The technical solution of the present invention is as follows:

[0006] A method for selecting an MMC AC bus voltage for overload operation requirements comprises the following steps:

[0007] S1: Based on the rated operating conditions of the MMC, obtain the bus voltage that satisfies the modulation signal margin not less than 0 and the maximum under all conditions U s.lim , that is, the limit bus voltage that is not over-modulated under rated operation of the MMC;

[0008] S2: Based on U s.lim , calculate the maximum AC bus voltage that satisfies the modulation signal margin of not less than 0 when the zero-sequence signal is injected under all overload conditions, that is, the MMC AC bus voltage facing the overload operation requirements is obtained U s.opt .

[0009] Preferably, step S1 comprises:

[0010] S11: In the rated operating range of MMC, the input power factor angle = 0, get the rated operating power of the converter S rated ,in S rated It is about the power factor angle The one-variable function of is expressed as formula (1);

[0011] (1);

[0012] S12: The and S rated Input formula (2) and solve to obtain the modulation signal margin under the current rated operating conditions. M MMC AC bus voltage amplitude equal to 0 U s , where there are two AC bus voltage amplitudes under a rated operating condition U s.small and U s.big , select a larger AC bus voltage amplitude U s.big As U s Output;

[0013] (2);

[0014] S13: Store the power factor angle in step S12 The output of U s ;

[0015] S14: Determine the power factor angle in step S13 Is it satisfied? If not, then Iterate and then convert the iterated power factor angle Enter formula (2) and repeat steps S11 to S13 until the condition is satisfied. ;

[0016] S15: stored in each iteration U s form Collection, Output The maximum value in the set is denoted by U s.lim .

[0017] Preferably, step S2 comprises:

[0018] S21: Input U s = U s.lim and = 0, the apparent overload power of the MMC converter is calculated by formula (3): S ol ;

[0019] (3);

[0020] S22: Based on U s , power factor angle And overload apparent power S ol , calculate the modulation signal when injecting zero-sequence signal by modeling or simulation method S ap ( t ), and substitute it into formula (4) and (5) to calculate the modulation signal margin under MMC overload operation M ;

[0021] (4);

[0022] (5);

[0023] S23: Determine the modulation signal margin under MMC overload operation M Is it satisfied? M >0, if not, then Iterate, and then iterate U s Re-enter and repeat steps S21 to S22 until the M >0;

[0024] S24: Determine the power factor angle in S23 Is it satisfied? If not, then Iterate and then convert the iterated power factor angle Enter formula (2) and repeat steps S21 to S23 until the condition is satisfied. ;

[0025] S25: When satisfy , output the current U s , record the MMC AC bus voltage for overload operation requirements U s.opt =U s .

[0026] Preferably, in steps S14 and S24, = π / 36.

[0027] Preferably, in step S23, U step =1kV.

[0028] For any details not provided in the present invention, please refer to the prior art.

[0029] The beneficial effects of the present invention are:

[0030] The MMC AC bus voltage selection method for overload operation requirements of the present invention is an AC bus voltage that accurately meets the MMC overload operation requirements, taking into account the conflicting issues between overmodulation risk and operation efficiency. The voltage can ensure that the MMC injects a zero-sequence signal during overload operation without overmodulation and operates stably, and achieves the effect of reducing losses during MMC overload operation.

[0031] The present invention can select the AC bus voltage for the MMC under different parameters and different overload requirements, thereby improving the overload operation capacity of the MMC and reducing the overload operation loss, thereby increasing the operating life of the converter station.

[0032] The present invention is of great significance for the selection of AC bus voltage, loss economic calculation, and converter station transformer tap change under the overload operation requirements of the flexible DC converter station. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The drawings in the specification, which constitute a part of the present application, are used to provide further understanding of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute improper limitations on the present application.

[0034] Figure 1 It is a schematic diagram of the process of solving the limit bus voltage without over-modulation under rated operation of MMC;

[0035] Figure 2 The schematic diagram of the process flow of solving the MMC AC bus voltage for overload operation requirements;

[0036] Figure 3This is a schematic diagram of the normal and overload operation domains of the MMC;

[0037] Figure 4 For the working condition, U s =The waveform of the modulation signal when 215kV injects zero-sequence signal;

[0038] Figure 5 For the working condition, U s =A phase current waveform when 215kV zero sequence signal is injected;

[0039] Figure 6 For the working condition, U s = The waveform of the modulation signal when 180kV zero-sequence signal is injected;

[0040] Figure 7 For the working condition, U s =A phase current waveform when 180kV zero sequence signal is injected;

[0041] Figure 8 For working condition 2, U s = Waveform diagram of 215kV injected zero-sequence signal modulation signal;

[0042] Fig. 9 For the following working conditions, the traditional method requires U s = Waveform diagram of 230kV injected zero-sequence signal modulation signal. DETAILED DESCRIPTION

[0043] In order to enable those skilled in the art to better understand the technical solutions in this specification, the technical solutions in the embodiments of the present invention are clearly and completely described below in conjunction with the drawings in the implementation of this specification, but are not limited to this. Anything not fully described in the present invention shall be based on the conventional technology in the art.

[0044] Example 1

[0045] A method for selecting an MMC AC bus voltage for overload operation requirements comprises the following steps:

[0046] S1: Based on the rated operating conditions of the MMC, obtain the bus voltage that satisfies the modulation signal margin not less than 0 and the maximum under all conditions U s.lim , that is, the limit bus voltage that is not over-modulated under rated operation of the MMC; Figure 1 As shown, step S1 specifically includes:

[0047] S11: In the rated operating range of MMC, the input power factor angle = 0, get the rated operating power of the converter S rated ,in S rated It is about the power factor angle The one-variable function of is expressed as formula (1);

[0048] (1);

[0049] Each MMC operation domain is different, resulting in f s () There is no specific expression for the same. However, as long as the MMC operating domain is determined, its rated operating power S rated It is about the power factor angle A one-variable function of, that is, given any MMC operating domain, given the power factor angle , then the only corresponding rated operating power can be obtained S rated .like Figure 3 As shown in the figure, the dotted line shows the rated operating range of the MMC. = At 0 o'clock, we can get S rated =1500MW, , ; When input = When π / 2, we can get S rated =600MVar, , .

[0050] S12: The and S rated Input formula (2) and solve to obtain the modulation signal margin under the current rated operating conditions. M MMC AC bus voltage amplitude equal to 0 U s , where there are two AC bus voltage amplitudes under a rated operating condition U s.small and U s.big , select a larger AC bus voltage amplitude U s.big As U s Output;

[0051] (2);

[0052] Modulation signal margin M is the power factor angle , Rated apparent power S rated , AC bus voltage amplitude U s Function, no specific expression, when input S rated ， ，U s When the rated working condition and U s Modulation signal margin under M , can be calculated by existing technology (such as through modeling or simulation), so the modulation signal margin can be M Considered as power factor angle , Rated apparent power S rated , AC bus voltage amplitude U s function.

[0053] if M >0, it means that the MMC is not over-modulated and can operate stably; if M< 0, it means that the MMC is overmodulated and cannot run stably. M =0 is the critical point of whether MMC is over-modulated.

[0054] S13: Store the power factor angle in step S12 The output of U s ;

[0055] S14: Determine the power factor angle in step S13 Is it satisfied? If not, then Iterate and then convert the iterated power factor angle Enter formula (2) and repeat steps S11 to S13 until the condition is satisfied. ;

[0056] It can take any value within 0~2π. The smaller the value, the more iterations, the more accurate the calculation but the longer the calculation time. In this embodiment, The initial input is 0, = π / 36, then a total of 2π / Each iteration calculates Can be calculatedU s ;

[0057] S15: stored in each iteration U s form Collection, Output The maximum value in the set is denoted by U s.lim .

[0058] For example =0, storage U s =2.33e5Enter Gather, now =0 is not satisfied, then iterate = π / 36, calculate this time U s =2.4e5 enters the set, until = 2π, that is, to find ∈[0,2π] for all rated operating powers U s Deposit In collection. Stored in the collection U s The number depends on Size, such as =π / 36, a total of 2π / =72 U s .

[0059] S2: Based on U s.lim , calculate the maximum AC bus voltage that satisfies the modulation signal margin of not less than 0 when the zero-sequence signal is injected under all overload conditions, that is, the MMC AC bus voltage facing the overload operation requirements is obtained U s.opt .like Figure 2 As shown, step S2 includes:

[0060] S21: Input U s = U s.lim and = 0, the apparent overload power of the MMC converter is calculated by formula (3): S ol ;

[0061] (3);

[0062] Same as formula (1), f ol () No specific expression, for example, S ol =α·S rated , α is the overload factor. If the MMC needs to have an overload capacity of 1.2 times, then S ol =1.2 S rated If the MMC needs to have an overload capacity of 1.3 times, then S ol =1.3 S rated .

[0063] S22: Based on U s , power factor angle And overload apparent power S ol , calculate the modulation signal when the zero-sequence signal is injected by existing methods such as modeling or simulation methods S ap ( t ), and substitute it into formula (4) and (5) to calculate the modulation signal margin under MMC overload operation M ;

[0064] (4);

[0065] (5);

[0066] Margin for modulation signal M The definition of is the upper margin of the modulation signal within the time period M up With upper margin M low Taking the upper bridge arm of phase A as an example, M up and M low As shown in formula (4). S ap (t) is the modulation signal of the upper bridge arm of phase A, which can be obtained by the following formula: ,in A 0 , A 1 , A 2 and A 3 Respectively represent the amplitudes of DC, 1st, 2nd and 3rd harmonic components; θ1 , θ 2 , θ 3 Indicates the phase angles corresponding to the 1st, 2nd and 3rd harmonic components; ω is the fundamental angular frequency;

[0067] S23: Determine the modulation signal margin under MMC overload operation M Is it satisfied? M >0, if not, then Iterate, and then iterate U s Re-enter and repeat steps S21 to S22 until the M >0;

[0068] U step The smaller the value of is, the more iterations there are, the more accurate the calculation is, but the longer the calculation time is; the larger the value is, the fewer iterations there are, the faster the calculation is, but the lower the calculation accuracy is. U step =1kV.

[0069] S24: Determine the power factor angle in S23 Is it satisfied? If not, then Iterate and then convert the iterated power factor angle Enter formula (2) and repeat steps S21 to S23 until the condition is satisfied. ;

[0070] S25: When satisfy , output the current U s , record the MMC AC bus voltage for overload operation requirements U s.opt =U s .

[0071] The following example verifies the method for selecting the MMC AC bus voltage for overload operation requirements in Example 1. The main circuit parameters of the MMC in Example 1 are shown in Table 1.

[0072] Table 1 MMC main circuit parameters

[0073]

[0074] MMC rated operating range S rated It is about the power factor angle Function, input MMC power factor angle = 0, the MMC rated power under this power factor angle is calculated S rated =1500MW. The rated operating domain of the MMC in the example is as follows Figure 3 As shown in the dashed box. = 0, S rated =1500MW, modulation signal margin M =0 U s = 233 kV Iteration Repeat the calculation until ≥ 2π, and finally the modulation signal margin of the rated operating domain under each power factor angle is obtained M =0 ∈[0, 2π] set, take the maximum value in the set U s.lim =260kV.

[0075] enter = 0, U s = 260kV, MMC overload operation area S ol It is about the power factor angle Assuming that the MMC needs to have an overload capacity of 1.3 times; = Taking 0 as an example, the MMC overload power under this power factor angle is calculated. The overload operation domain of the MMC in the example is as follows: Figure 3 When calculating the injected zero-sequence signal, = 0, U s = Modulation signal margin at 260kV M= 0.1616, but ≤ 2π, iteration Repeat the calculation until = 0.116, S ol = 1510.2e6, the modulation signal margin M< 0, iterative calculation Us = 250kV, = 0.116, U s = Modulation signal margin at 250kV M> 0, but ≤ 2π, right and U s Repeat the above operation to iterate and calculate again = 0.3805, at this power factor angle, the MMC overload power S ol =2100.2MW, iterative calculation to U s = 215kV, modulation signal margin M> 0. Continuous input U s = 215kV, iterative calculation to = 2π, the modulation signal margin always satisfies M> 0. Get the optimal AC bus voltage of MMC for overload operation requirements U s.opt = 215kV.

[0076] Note that working condition 1 is S ol =2100.2MW, = 0.3805, working condition 2 S ol =2100.2MW, = 2.761, where P=1950MW, Q=780MW in working condition 1, By formula =arctan(P / Q)can be obtained;

[0077] Figure 4 It's the working condition. U s = 215kV injected zero sequence signal amplitude A 3=0.1097, phase angle θ The waveform of the modulated signal when 3=4.5209. Figure 4 In the five cycles shown, the minimum value of the modulation signal margin is 0.00425, which satisfies M>0. At this time, the A phase current waveform is as follows Figure 5 As shown, its peak I p =6697A.

[0078] Figure 6 It's the working condition. U s = 180kV injected zero sequence signal amplitude A 3=0.1087, phase angle θ The waveform of the modulated signal when 3=4.8127. Figure 6 In the five cycles shown, the minimum value of the modulation signal margin is 0.0371, which satisfies M >0. At this time, the A phase current waveform is as follows Figure 7 As shown, its peak I p =7797A. Figure 5 , Figure 7 It can be seen that compared U s = 215kV, if U s Although it can meet the normal operation requirements of MMC when it is overloaded if it continues to decrease, when the converter transmits the same power, the phase current and its device coupling current are relatively large, the thermal effect of the current is large, the loss is large, and the current stress of devices such as IGBT / capacitor is large, which will have an adverse effect on the life of the converter device and the operating stability.

[0079] Figure 8 This is working condition 2. U s = 215kV injected zero sequence signal amplitude A 3=0.1096, phase angle θ The waveform of the modulated signal when 3=1.7621. Figure 8 In the five cycles shown, the minimum value of the modulation signal margin is 0.0026, which satisfies M >0.

[0080] MMC AC bus voltage for overload operation requirements calculated by example U s It can take into account both the stable operation and operating efficiency of MMC during overload, reduce operating losses during overload, increase the life of converter components, and ensure the safety and stability of the converter during overload operation.

[0081] The purpose of the present invention is to select an MMC AC bus voltage as large as possible to ensure that the MMC can operate stably without overmodulation during overload operation, and to ensure that the MMC phase current and its device coupling current are small, thereby reducing device losses, lowering the rated current requirements for devices such as IGBTs and capacitors, and extending the operating life of the MMC. The two operating conditions are extreme conditions where overmodulation is possible, proving that the selected AC bus voltage can still meet the requirement for stable operation without overmodulation under extreme conditions. The traditional method for selecting AC bus voltage only considers the rated operating conditions, but does not consider the overload operation requirements. Fig. 9 For the following working conditions, the traditional method requires U s = Waveform diagram of the 230kV injected zero-sequence signal modulation signal. The MMC modulation signal is over-modulated and cannot operate stably under overload conditions.

[0082] In the example, three verifications were conducted for two extreme working conditions: ① Working condition 1, U s = 215kV, solve the modulation signal waveform ( Figure 4 ) and A phase current ( Figure 5 );②Working condition 1, U s = 180kV, solve the modulation signal waveform ( Figure 6 ) and A phase current ( Figure 7 );③Working condition 2, U s = 215kV, solve the modulation signal waveform ( Figure 8 ). Figure 4 and Figure 8 The minimum values ​​of the modulation signal margin shown in the figure are 0.00425 and 0.0026, respectively, both greater than 0. This shows that the AC bus voltage selected by this method can enable the MMC to operate stably when facing a 1.3 times overload operation requirement (this example selects the two extreme conditions that are most prone to overmodulation. If the two extreme conditions can meet the requirement of stable operation of the MMC when overloaded, then all conditions can be met).

[0083] Compare Figure 4 and Figure 6 Discover, U s = At 215kV, the modulation signal margin is 0.00425 ( Figure 4 ), U s = At 180kV, the modulation signal margin is 0.0371 ( Figure 6 ), are greater than 0. This means that if the AC bus voltage is finally selectedU s = At 215kV, the voltage is further reduced to U s = 180kV can satisfy the requirement that MMC can operate stably without overmodulation when operating at 1.3 times overload, and the modulation signal margin increases with the decrease of AC bus voltage (which is conducive to stable overload operation).

[0084] But comparing the two U s The peak phase current under Figure 5 Phase A current peak value I p =6697A( U s = 215kV), Figure 7 Phase A current peak value I p =7797A( U s = 180kV). In MMC, the bridge arm current and the current flowing through IGBT / capacitor are coupled to the phase current. When the phase current peak increases, the bridge arm current and the current flowing through IGBT / capacitor increase, increasing the thermal effect of the device, increasing the operating loss of MMC, and reducing the economic efficiency of operation.

[0085] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for selecting MMC AC bus voltage for overload operation requirements, characterized in that: The steps include: S1: According to the rated operating conditions of the MMC, the maximum AC bus voltage U that satisfies the modulation signal margin of not less than 0 under all conditions is obtained. s.lim ; S2: Based on U s.lim , calculate the maximum AC bus voltage that satisfies the modulation signal margin of not less than 0 when the zero-sequence signal is injected under all overload conditions, that is, the MMC AC bus voltage U for overload operation requirements is obtained. s.opt ; Step S1 includes: S11: In the rated operating range of MMC, the input power factor angle Get the rated operating power S of the converter rated , where S rated It is about the power factor angle A univariate function of ; S12: The With S rated Input formula (2) and solve to obtain the MMC AC bus voltage amplitude U that satisfies the modulation signal margin M equal to 0 under the current rated operating conditions. s , where there are two AC bus voltage amplitudes U under a rated operating condition s.small and U s.big , select a larger AC bus voltage amplitude U s.big As U s Output; S13: Store the power factor angle in step S12 The output of U s ; S14: Determine the power factor angle in step S13 Is it satisfied? If not, then Iterate and then convert the iterated power factor angle Enter formula (2) and repeat steps S11 to S13 until the condition is satisfied. S15: U stored in each iteration s form Collection, Output The maximum value in the set, denoted as U s.lim ; Step S2 includes: S21: Input U s =U s.lim and The apparent overload power S of the MMC converter is calculated by formula (3): ol ; S22: Based on U s , power factor angle And overload apparent power S ol , calculate the modulation signal S when the zero-sequence signal is injected by modeling or simulation methods ap (t), and substitute it into formula (4) and (5) to calculate the modulation signal margin M under MMC overload operation; M=min{M up ,M low }(5); S23: Determine whether the modulation signal margin M under MMC overload operation satisfies M>0. If not, set U s =U s -U step Iterate and then use the iterated U s Re-enter and repeat steps S21 to S22 until M>0 is satisfied; S24: Determine the power factor angle in S23 Is it satisfied? If not, then Iterate and then convert the iterated power factor angle Enter formula (2) and repeat steps S21 to S23 until the condition is satisfied. S25: When satisfy When U s , record the MMC AC bus voltage U for overload operation requirements s.opt =U s .

2. The MMC AC bus voltage selection method for overload operation requirements according to claim 1 is characterized in that: In steps S14 and S24, 3. The MMC AC bus voltage selection method for overload operation requirements according to claim 2 is characterized in that: In step S23, U step =1kV.

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