Submodule redundancy adjusting system of flexible low-frequency power transmission system

Abstract

The invention discloses a sub-module redundancy adjusting system of a flexible low-frequency power transmission system. The sub-module redundancy adjusting system comprises bridge arm redundancy sub-modules of a modular multi-level matrix type alternating current converter and an on-load tap-changer arranged on a second frequency transformer. By adjusting the gears of the on-load tap-changer, the number of the fault sub-modules is larger than that of the redundant sub-modules, the modular multi-level matrix type alternating current converter is not locked, and the flexible low-frequency power transmission system continues to operate. According to the method, the bridge arm redundancy sub-module of the modular multilevel matrix type alternating current converter and the on-load tap-changer of the second frequency transformer are combined, the operation modulation degree of the alternating current converter under the condition that the bridge arm sub-module breaks down is limited within an allowable range through coordination control, and safe and reliable operation of the flexible low-frequency power transmission system is maintained.

Description

Technical Field

[0001] The invention belongs to the technical field of power transmission and distribution of electric power systems, and in particular to a submodule redundant regulation system of a flexible low-frequency power transmission system. Background Art

[0002] Traditional industrial frequency AC transmission technology is difficult to meet the needs of modern power systems for long-distance, large-capacity power transmission. DC transmission technology can break through the transmission capacity limitations of AC transmission technology, but the manufacturing technology of DC transmission equipment such as DC transformers and DC circuit breakers is not yet mature, the investment cost remains high, and it is difficult for DC transmission systems to achieve networking and operation. Flexible low-frequency transmission technology reduces the operating frequency of transmission lines through AC converters to increase the transmission capacity of AC systems. At the same time, the manufacturing difficulty of low-frequency AC transmission equipment such as low-frequency transformers and circuit breakers is significantly lower than that of DC transmission equipment, which is conducive to the construction of low-frequency power grids.

[0003] In the flexible low-frequency transmission system, the AC converter is responsible for completing the frequency coupling and power transfer between the power grid and the low-frequency transmission system, and is the core equipment of the flexible low-frequency system. The modular multilevel matrix AC converter is the AC converter with the greatest engineering application potential at present. Compared with other AC converter topologies, the modular multilevel matrix AC converter has technical advantages such as high modularity, good output harmonic characteristics, and fast and independent adjustment of reactive power. Among them, the high degree of modularity is an important reason why the modular multilevel matrix AC converter is suitable for high-voltage and large-capacity transmission scenarios. The modular multilevel matrix converter bridge arm composed of a large number of sub-modules in series can avoid the switch inconsistency problem faced by directly connecting power electronic devices in series. On the other hand, when individual sub-modules fail and stop operating, the redundant sub-modules can be put into use to maintain the continuous and stable operation of the modular multilevel matrix AC converter.

[0004] So far, the vast majority of published literature on flexible low-frequency transmission technology has mainly focused on system topology construction and steady-state control strategies, and there are few studies on the sub-module redundant regulation system of flexible low-frequency transmission systems. Proposing a sub-module redundant regulation system for a flexible low-frequency transmission system is an important part of improving the construction of a flexible low-frequency transmission system, and is of great significance for improving the power supply reliability of the flexible low-frequency transmission system, so it is necessary to conduct research. Summary of the invention

[0005] The purpose of the present invention is to provide a submodule redundancy adjustment system for a flexible low-frequency power transmission system, which combines a modular multi-level matrix AC switch bridge arm redundancy submodule and a transformer on-load tap changer to achieve stable and safe operation of the flexible low-frequency power transmission system bridge arm submodule failure through coordinated control.

[0006] To this end, the present invention adopts the following technical solution: a submodule redundant adjustment system of a flexible low-frequency power transmission system, the contents of which are as follows: The flexible low-frequency power transmission system includes a first-frequency AC power grid, a second-frequency AC power grid and an AC frequency conversion station. The first-frequency AC power grid is connected to the second-frequency AC power grid through the AC frequency conversion station, and the operating frequency of the first-frequency AC power grid is higher than that of the second-frequency AC power grid; the AC-AC frequency conversion station includes a modular multi-level matrix AC converter, a first frequency transformer and a second frequency transformer; the modular multi-level matrix AC converter is connected to the first-frequency AC power grid through the first frequency transformer, and is connected to the second-frequency AC power grid through the second frequency transformer; The submodule redundancy adjustment system comprises a bridge arm redundant submodule of a modular multi-level matrix AC switch and an on-load tap changer arranged on a second frequency transformer; by adjusting the gear position of the on-load tap changer, the number of faulty submodules is made greater than the number of redundant submodules, the modular multi-level matrix AC switch is not locked, and the flexible low-frequency power transmission system continues to operate.

[0007] Furthermore, the modular multi-level matrix AC switch adopts a second frequency AC grid side AC voltage amplitude and frequency control strategy so that the voltage amplitude and voltage frequency on the second frequency AC grid side of the AC switch remain constant.

[0008] Furthermore, when the on-load tap changer is arranged on the second frequency AC grid side winding of the second frequency transformer, at least one positive tap is arranged; when the on-load tap changer is arranged on the modular multi-level matrix AC converter side winding of the second frequency transformer, at least one negative tap is arranged.

[0009] Furthermore, for the on-load tap changer, when the voltage on the second frequency AC grid side of the second frequency transformer remains at a rated value, a peak voltage change on the AC converter side corresponding to a tap adjustment of one gear is at least greater than the rated voltage of one arm submodule.

[0010] Furthermore, the submodule redundant regulation system sets the operation modulation degree start value M s , when the modular multilevel matrix AC converter operates at a modulation level not exceeding M s When the tap of the second frequency transformer is at the rated position; when the modular multi-level matrix AC converter operates with a modulation index greater than M sWhen the tap of the second frequency transformer on the second frequency AC grid side is turned in a positive direction, or the tap of the second frequency transformer on the AC converter side is turned in a negative direction, until the operation modulation index of the modular multi-level matrix AC converter does not exceed the maximum operation modulation index. M max .

[0011] Furthermore, the submodule redundancy adjustment system sets a starting value for the number of submodules with single bridge arm failures. N s , when the number of faulty submodules in each bridge arm does not exceed N s When the tap of the second frequency transformer is at the rated position; when the number of faulty submodules in any bridge arm is greater than N s When the tap of the second frequency transformer located on the second frequency AC power grid side is turned in a positive direction, or the tap of the second frequency transformer located on the AC converter side is turned in a negative direction.

[0012] Furthermore, the number of gear adjustment N d Calculated according to the following formula: , in, N f Indicates the maximum number of faulty submodules in a single bridge arm; k 1 represents the conversion coefficient between the number of single-bridge arm fault submodules and the number of second frequency transformer tap positions.

[0013] Furthermore, the submodule redundancy adjustment system is provided with a comprehensive adjustment coefficient obtained by combining the operation modulation degree of the modular multi-level matrix AC switch and the number of single-arm fault submodules. X And the corresponding startup value X s ;when X No more than X s When the tap of the second frequency transformer is at the rated position; when X Greater than X s When the tap of the second frequency transformer on the second frequency AC grid side is turned in the positive direction, or the tap of the second frequency transformer on the AC converter side is turned in the negative direction, until the comprehensive adjustment coefficient X Less than or equal to the maximum allowed value X max .

[0014] Furthermore, the comprehensive adjustment coefficient X is obtained by weighted calculation as shown in the following formula: ,

[0015] in, M Indicates the modulation degree of the AC converter operation. a represents the first weighting coefficient, b represents the second weighting coefficient.

[0016] Furthermore, the submodule redundancy adjustment system sets an upper limit on the number of submodules with single bridge arm failures. N Lim The lower limit of the output AC voltage amplitude of the AC grid side of the second frequency AC converter U Low , when the number of faulty submodules in a single bridge arm exceeds N Lim Or the AC voltage amplitude output from the AC grid side of the AC converter at the second frequency is lower than U Low When the AC converter is locked and stopped.

[0017] Based on the above technical solution, the present invention has the following beneficial technical effects: the present invention adopts a submodule redundancy adjustment system combining a modular multi-level matrix AC converter bridge arm redundancy submodule and a second frequency transformer on-load tap changer, and realizes stable and safe operation of a flexible low-frequency power transmission system bridge arm submodule failure through coordinated control. The submodule redundancy adjustment system of the present invention is convenient to configure and simple to implement, providing an important reference for actual engineering design. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 A schematic diagram of the topological structure of the flexible low-frequency power transmission system of the present invention; Figure 2 A control flow chart of the modulation degree of the submodule redundant regulation system of the present invention; Figure 3 A control flow chart of the number of faulty submodules of the submodule redundancy adjustment system of the present invention; Figure 4 It is a control flow chart of the comprehensive adjustment coefficient of the submodule redundant adjustment system of the present invention; Figure 5 It is an equivalent circuit diagram of the submodule redundant adjustment system in the application example of the present invention. DETAILED DESCRIPTION

[0019] In order to describe the present invention more specifically, the technical solution of the present invention is described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0020] Figure 1The figure is a schematic diagram of the topological structure of the flexible low-frequency power transmission system. The flexible low-frequency power transmission system includes a first-frequency AC power grid, a second-frequency AC power grid and an AC-to-AC switching station. Among them, the operating frequency of the first-frequency AC power grid is higher than that of the second-frequency AC power grid; the first-frequency AC power grid and the second-frequency AC power grid are connected through the AC-to-AC switching station. The AC-to-AC switching station includes a modular multi-level matrix AC converter, a first frequency transformer and a second frequency transformer. The modular multi-level matrix AC converter is connected to the first-frequency AC power grid through the first frequency transformer; the modular multi-level matrix AC converter is connected to the second-frequency AC power grid through the second frequency transformer.

[0021] The submodule redundancy adjustment system includes a bridge arm redundant submodule of a modular multi-level matrix AC converter and an on-load tap changer arranged on a second frequency transformer; by adjusting the on-load tap changer gear, the number of faulty submodules is greater than the number of redundant submodules, the modular multi-level matrix AC converter is not locked, and the flexible low-frequency power transmission system continues to operate. The modular multi-level matrix AC converter adopts the AC voltage amplitude and frequency control strategy on the second frequency AC grid side: according to the voltage amplitude command and voltage frequency command issued by the dispatcher, the voltage amplitude and voltage frequency on the second frequency AC grid side of the AC converter are kept constant through voltage and current dual-loop decoupling control.

[0022] The second frequency transformer adopts an on-load tap changer. When the on-load tap changer is set on the second frequency AC grid side winding of the second frequency transformer, at least one level of positive tap is set; when the on-load tap changer is set on the AC converter side winding of the second frequency transformer, at least one level of negative tap is set. When the voltage on the second frequency AC grid side of the second frequency transformer is maintained at the rated value, the peak value change of the AC converter side voltage corresponding to the tap adjustment of one level should be greater than the rated voltage of at least one submodule.

[0023] The submodule redundancy adjustment system sets the upper limit of the number of submodules with single bridge arm failure N Lim The lower limit of the output AC voltage amplitude of the AC grid side of the second frequency AC converter U Low The submodule redundancy adjustment system is set by combining the modular multi-level matrix AC switch operation modulation index and the number of single-arm fault submodules to obtain the redundant system comprehensive adjustment coefficient. X And the corresponding startup value X s .

[0024] like Figure 2 As shown in the figure, when the number of faulty submodules in a single bridge arm exceeds the upper limit N LimOr the AC voltage amplitude output by the AC grid side of the second frequency AC converter is lower than the lower limit U Low If the above conditions are not met, it is determined whether the operation modulation index of the modular multi-level matrix AC converter exceeds the operation modulation index start value. M s ; When the operating modulation index of the modular multi-level matrix AC converter does not exceed the operating modulation index start value M s When the tap of the second frequency transformer is at the rated position, that is, the tap position remains unchanged; when the modular multi-level matrix AC converter is operated at a modulation index greater than M s When the second frequency transformer is located at the second frequency AC grid side, the tap of the second frequency transformer is turned in the positive direction, or the tap of the second frequency transformer is turned in the negative direction on the AC converter side, until the modulation index of the modular multi-level matrix AC converter does not exceed the maximum operation modulation index. M max .

[0025] like Figure 3 As shown in the figure, when the number of faulty submodules in a single bridge arm exceeds the upper limit N Lim Or the AC voltage amplitude output by the AC grid side of the second frequency AC converter is lower than the lower limit U Low If the above conditions are not met, it is determined whether the number of faulty submodules in a single bridge arm exceeds the start value. N s ; When the number of faulty submodules in a single bridge arm does not exceed N s When the tap of the second frequency transformer is at the rated position, that is, the tap position remains unchanged; when the number of faulty submodules in a single bridge arm is greater than N s When the number of single-arm submodule faults is changed compared with the previous one, it is determined whether the number of faults in the single-arm submodule has changed. If there is a change, the number of gear adjustment of the second frequency transformer tap is obtained, so that the tap of the second frequency transformer located on the second frequency AC grid side winding is positively dialed, or the tap of the second frequency transformer located on the AC converter side winding is negatively dialed; the gear adjustment number N d Obtained by looking up the table based on the number of faulty submodules. In particular, it can be calculated using the following formula N d : , in, N f Indicates the maximum number of faulty submodules in a single bridge arm; k1 represents the conversion coefficient between the number of single-bridge arm fault submodules and the number of second frequency transformer tap positions.

[0026] like Figure 4 As shown in the figure, when the number of faulty submodules in a single bridge arm exceeds the upper limit N Lim Or the AC voltage amplitude output by the AC grid side of the second frequency AC converter is lower than the lower limit U Low If the above conditions are not met, the comprehensive adjustment coefficient is determined. X Whether it exceeds the corresponding start value X s ; When the comprehensive adjustment coefficient X Not exceeding the start value X s When , the tap of the second frequency transformer is at the rated position, that is, the tap position remains unchanged; when X Greater than X s When the tap of the second frequency transformer on the second frequency AC grid side is turned in a positive direction, or the tap of the second frequency transformer on the AC converter side is turned in a negative direction, until the redundant system comprehensive adjustment coefficient of the modular multi-level matrix AC converter is X Less than or equal to the maximum allowed value X max In particular, the comprehensive adjustment coefficient X can be obtained by weighted calculation as shown in the following formula: , in, M Indicates the modulation degree of the AC converter operation. a represents the first weighting coefficient, b represents the second weighting coefficient.

[0027] Application Examples The AC voltage amplitude U of the second frequency AC grid side of the modular multi-level matrix AC converter s The voltage is set to 220kV, the frequency is set to 20Hz, the active power P of the new energy transmission system is 300MW, the reactive power Q is 0, the capacity of the second frequency transformer S is 330MVA, and the rated transformation ratio U is T1 / U T2 64kV / 220kV, U T1 is the rated voltage of the second transformer at the second frequency AC grid side, U T2 is the rated voltage on the AC converter side of the second transformer, and the short-circuit impedance percentage is U kThe number of submodules N in each bridge arm of the modular multi-level matrix AC switch is 64, of which the number of effective submodules N1 is 58, the number of redundant submodules N2 is 6, and the rated operating voltage U CN is 2.15kV, and the bridge arm inductance L is 15mH. At the same time, it is assumed that the voltage U on the first frequency AC grid side of the modular multi-level matrix AC converter is V1 Keep the effective value of 64kV unchanged.

[0028] The second frequency transformer tap is set on the second frequency AC power grid side. In addition to the rated transformation ratio, there are two tap adjustment gears, and the percentage of each gear is 7.5%.

[0029] The equivalent circuit of the submodule redundant regulation system is as follows: Figure 5 shown.

[0030] Calculate the leakage reactance X of the second frequency transformer T for:

[0031] Calculate the bridge arm reactance X L for:

[0032] Then the equivalent reactance X is:

[0033] According to the equivalent circuit and circuit theory, the voltage U on the second frequency AC grid side of the modular multi-level matrix AC converter can be calculated. V2 for:

[0034] The modulation index M at this time is:

[0035] Submodule redundancy adjustment system setting operation adjustment degree start value M s Set to 0.95, the maximum modulation index M max If it is set to 0.97, as the number of submodule failures increases, the relevant parameters change as shown in the following table.

[0036]

[0037] It can be seen from the above table that after adopting the technical solution of the present invention, under the condition that the submodule redundant regulation system does not stop operating, the submodule redundant regulation system allows the number of faulty submodules to be increased from 6 to 16, which greatly improves the reliability of the operation of the submodule redundant regulation system.

[0038] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the technical solutions described in the above embodiments may still be modified, or some of the technical features may be replaced by equivalents. Such modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention, and should be included in the protection scope of the present invention.

Claims

1. A submodule redundant regulation system for a flexible low-frequency power transmission system, characterized in that: The flexible low-frequency power transmission system includes a first-frequency AC power grid, a second-frequency AC power grid and an AC frequency conversion station. The first-frequency AC power grid is connected to the second-frequency AC power grid through the AC frequency conversion station, and the operating frequency of the first-frequency AC power grid is higher than that of the second-frequency AC power grid; the AC-AC frequency conversion station includes a modular multi-level matrix AC converter, a first frequency transformer and a second frequency transformer; the modular multi-level matrix AC converter is connected to the first-frequency AC power grid through the first frequency transformer, and is connected to the second-frequency AC power grid through the second frequency transformer; The submodule redundancy adjustment system comprises a bridge arm redundant submodule of a modular multi-level matrix AC switch and an on-load tap changer arranged on a second frequency transformer; by adjusting the gear position of the on-load tap changer, the number of faulty submodules is made greater than the number of redundant submodules, the modular multi-level matrix AC switch is not locked, and the flexible low-frequency power transmission system continues to operate.

2. The submodule redundant adjustment system of the flexible low-frequency power transmission system according to claim 1, characterized in that: The modular multi-level matrix AC switch adopts a second frequency AC grid side AC voltage amplitude and frequency control strategy.

3. The submodule redundant adjustment system of the flexible low-frequency power transmission system according to claim 1, characterized in that: When the on-load tap changer is arranged on the second frequency AC grid side winding of the second frequency transformer, at least one positive tap is arranged; when the on-load tap changer is arranged on the AC converter side winding of the second frequency transformer, at least one negative tap is arranged.

4. The submodule redundant adjustment system of the flexible low-frequency power transmission system according to claim 3, characterized in that: For the on-load tap changer, when the voltage on the second frequency AC grid side of the second frequency transformer remains at the rated value, the peak voltage change on the AC converter side corresponding to the tap adjustment by one gear is at least greater than the rated voltage of one bridge arm submodule.

5. The submodule redundant adjustment system of the flexible low-frequency power transmission system according to claim 3, characterized in that: The submodule redundant regulation system sets the operation modulation degree start value M s , when the modular multilevel matrix AC converter operates at a modulation index not exceeding M s When the tap of the second frequency transformer is at the rated position; when the modular multi-level matrix AC converter operates with a modulation index greater than M s When the tap of the second frequency transformer on the second frequency AC grid side is turned in a positive direction, or the tap of the second frequency transformer on the AC converter side is turned in a negative direction, until the operation modulation index of the modular multi-level matrix AC converter does not exceed the maximum operation modulation index. M max .

6. The submodule redundant adjustment system of the flexible low-frequency power transmission system according to claim 3, characterized in that: The submodule redundancy adjustment system sets the starting value of the number of submodules with single bridge arm failure N s , when the number of faulty submodules in each bridge arm does not exceed N s When the tap of the second frequency transformer is at the rated position; when the number of faulty submodules in any bridge arm is greater than N s When the tap of the second frequency transformer located on the second frequency AC power grid side is turned in a positive direction, or the tap of the second frequency transformer located on the AC converter side is turned in a negative direction.

7. The submodule redundancy adjustment system of the flexible low-frequency power transmission system according to claim 6, characterized in that: Number of gear adjustment N d Calculated according to the following formula: ， in, N f Indicates the maximum number of faulty submodules in a single bridge arm; k 1 represents the conversion coefficient between the number of single-bridge arm fault submodules and the number of second frequency transformer tap positions.

8. The submodule redundant adjustment system of the flexible low-frequency power transmission system according to claim 5 or 6, characterized in that: The submodule redundancy adjustment system is set to obtain a comprehensive adjustment coefficient obtained by combining the modular multi-level matrix AC switch operation modulation degree and the number of single bridge arm fault submodules. X And the corresponding startup value X s ;when X No more than X s When the tap of the second frequency transformer is at the rated position; when X Greater than X s When the tap of the second frequency transformer on the second frequency AC grid side is turned in the positive direction, or the tap of the second frequency transformer on the AC converter side is turned in the negative direction, until the comprehensive adjustment coefficient X Less than or equal to the maximum allowed value X max .

9. The submodule redundancy adjustment system of the flexible low-frequency power transmission system according to claim 8, characterized in that: The comprehensive adjustment coefficient X is obtained by weighted calculation as shown in the following formula: ， in, N f Indicates the maximum number of faulty submodules in a single bridge arm. M Indicates the modulation degree of the AC converter operation. a represents the first weighting coefficient, b represents the second weighting coefficient.

10. The submodule redundancy adjustment system of the flexible low-frequency power transmission system according to claim 5 or 6, characterized in that: The submodule redundancy adjustment system sets an upper limit on the number of submodules with single bridge arm failures N Lim The lower limit of the output AC voltage amplitude of the AC grid side of the second frequency AC converter U Low , when the number of faulty submodules in a single bridge arm exceeds N Lim Or the AC voltage amplitude output by the AC converter at the second frequency AC grid side is lower than U Low When the AC converter is locked and stopped.

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