A method and system for improving the operational reliability of a flexible low-frequency power transmission system

Abstract

The present invention discloses a method and system for improving the operational reliability of a flexible low-frequency power transmission system. The method for improving the operational reliability of the present invention comprises: when the number of faulty submodules in a single bridge arm of a modular multi-level matrix converter is greater than the number of redundant submodules, adjusting the reactive power on the power frequency grid side, the reactive power on the low-frequency grid side or the AC voltage amplitude on the low-frequency side, the submodule capacitor voltage and the active power of the new energy power generation system of the modular multi-level matrix converter, so that the modular multi-level matrix converter is not locked and maintains the voltage stability on its low-frequency grid side. The flexible low-frequency power transmission system can continue to operate, significantly improving the reliability of the flexible low-frequency power transmission system. The present invention enables the flexible low-frequency power transmission system to continue to operate when the number of faulty submodules in a single bridge arm of M3C exceeds the number of redundant submodules, thereby improving the operational reliability of the flexible low-frequency power transmission system.

Description

Technical Field

[0001] This invention belongs to the field of power system transmission and distribution technology, specifically a method and system for improving the operational reliability of a flexible low-frequency power transmission system. Background Technology

[0002] Traditional power frequency AC transmission technology cannot meet the demands of modern power systems for long-distance, high-capacity power transmission. While DC transmission technology can overcome the capacity limitations of AC transmission, the manufacturing technology for DC transmission equipment such as DC transformers and DC circuit breakers is not yet mature, resulting in high investment costs and hindering the grid-connected operation of DC transmission systems. Flexible low-frequency transmission technology, through modular multilevel matrix converters (M3C), reduces the operating frequency of transmission lines to increase the transmission capacity of AC systems. Furthermore, 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 beneficial for building low-frequency power grids.

[0003] In flexible low-frequency transmission systems, the M3C (Multi-Channel Converter) is responsible for frequency coupling and power transfer between the power frequency grid and the low-frequency transmission system, making it a core component of the flexible low-frequency system. The M3C is currently the most promising AC-AC converter topology for engineering applications. Compared to other topologies, the M3C boasts advantages such as high modularity, excellent output harmonic characteristics, and rapid independent reactive power adjustment. Its high modularity is a key reason why the M3C is suitable for high-voltage, high-capacity transmission scenarios. The M3C bridge arm, composed of numerous series-connected submodules, avoids the switching inconsistency problems encountered with directly series-connected power electronic devices. Furthermore, when individual submodules fail, redundant submodules can be deployed to maintain the continuous and stable operation of the M3C.

[0004] However, when the number of faulty submodules in a single M3C bridge arm exceeds the number of redundant submodules, there is currently no effective control or handling method to enable the flexible low-frequency transmission system to continue operating under such circumstances. Summary of the Invention

[0005] The technical problem to be solved by this invention is to overcome the defects of the prior art and provide a method and system for improving the operational reliability of a flexible low-frequency transmission system. When the number of faulty submodules in a single arm of the M3C exceeds the number of redundant submodules, this invention combines comprehensive measures of reactive power regulation on the M3C power grid side, reactive power regulation on the M3C low-frequency grid side (or M3C submodule capacitor voltage regulation), AC voltage amplitude regulation on the M3C low-frequency side, and active power regulation of the new energy power generation system. This ensures that the M3C is not blocked, maintains voltage stability on the low-frequency grid side, and enables the flexible low-frequency transmission system to continue operating, thereby improving the operational reliability of the flexible low-frequency transmission system.

[0006] Therefore, the present invention adopts the following technical solution.

[0007] In a first aspect, the present invention provides a method for improving the operational reliability of a flexible low-frequency power transmission system, comprising:

[0008] When the number of faulty submodules in a single arm of a modular multilevel matrix converter exceeds the number of redundant submodules, the reactive power on the power grid side, the reactive power on the low-frequency grid side, or the AC voltage amplitude on the low-frequency side, the capacitor voltage of the submodules, and the active power of the new energy power generation system of the modular multilevel matrix converter are adjusted to prevent the modular multilevel matrix converter from being blocked and to maintain the voltage stability on its low-frequency grid side, so that the flexible low-frequency transmission system can continue to operate.

[0009] Furthermore, the specific steps of the adjustment are as follows:

[0010] Step 1) When the modulation index of the modular multilevel matrix converter exceeds the starting value, reduce the power factor of the modular multilevel matrix converter on the power grid side to the minimum allowable value; Step 2) Based on Step 1), if the modulation index continues to increase, reduce the power factor of the modular multilevel matrix converter on the low-frequency grid side to the minimum allowable value, or reduce the AC voltage amplitude of the modular multilevel matrix converter on the low-frequency grid side to the minimum allowable value; Step 3) Based on Step 2), if the modulation index continues to increase, increase the capacitor voltage value of all sub-modules until the maximum safe operating value; Step 4) Based on Step 3), if the modulation index continues to increase, adjust the low-voltage load shedding system of the new energy power generation system, disconnect some new energy power generation equipment, and reduce the active power of the low-frequency transformer on the low-frequency grid side until the active power of the new energy power generation system reaches the minimum allowable value.

[0011] Further, step 1) specifically includes: when the modulation index of the modular multilevel matrix converter exceeds the starting value, adjusting the reactive power command on the power frequency grid side of the modular multilevel matrix converter so that the power factor on the power frequency grid side of the power frequency transformer is no longer 1 and begins to decrease until the power factor on the power frequency grid side of the power frequency transformer reaches the minimum allowable value.

[0012] Furthermore, step 2) specifically includes: when the power factor of the power frequency transformer on the power frequency grid side reaches the minimum allowable value and the modulation index of the modular multilevel matrix converter is still increasing, if the modular multilevel matrix converter adopts a constant active power and constant reactive power control strategy on the low frequency grid side, the reactive power command of the modular multilevel matrix converter on the low frequency grid side is adjusted so that the power factor of the low frequency transformer on the low frequency grid side is no longer 1 and begins to decrease until the power factor of the low frequency transformer on the low frequency grid side reaches the minimum allowable value.

[0013] Furthermore, step 3) specifically includes: when the power factor of the low-frequency transformer on the low-frequency grid side reaches the minimum allowable value and the modulation index of the modular multilevel matrix converter is still increasing, adjust the capacitor voltage values ​​of all sub-modules so that the capacitor voltage values ​​of the sub-modules are greater than their rated values, until the capacitor voltage values ​​of the sub-modules reach the maximum safe operating value.

[0014] Furthermore, step 2) specifically includes: when the power factor of the power frequency transformer on the power frequency grid side reaches the minimum allowable value and the modulation index of the modular multilevel matrix converter is still increasing, if the modular multilevel matrix converter adopts a control strategy of constant AC voltage amplitude and constant AC voltage frequency on the low frequency grid side, the AC voltage amplitude command of the modular multilevel matrix converter on the low frequency grid side is adjusted so that the AC voltage amplitude of the low frequency transformer on the low frequency grid side is no longer at the rated value, and the voltage on the low frequency grid side begins to decrease until the AC voltage amplitude of the low frequency transformer on the low frequency grid side reaches the minimum allowable value.

[0015] Furthermore, step 3) specifically includes: when the voltage amplitude on the low-frequency grid side of the low-frequency transformer reaches the minimum allowable value and the modulation index of the modular multilevel matrix converter is still increasing, adjust the capacitor voltage values ​​of all sub-modules so that the capacitor voltage values ​​of the sub-modules are greater than their rated values, until the capacitor voltage values ​​of the sub-modules reach the maximum safe operating value.

[0016] Furthermore, step 4) specifically includes: when the voltage value of the submodule capacitor reaches the maximum safe operating value and the modulation index of the modular multilevel matrix converter is still increasing, adjust the low-voltage load shedding system of the new energy power generation system, disconnect some new energy power generation equipment, and reduce the active power on the low-frequency grid side of the low-frequency transformer until the active power of the new energy power generation system reaches the minimum allowable value.

[0017] Furthermore, when the modulation index of the modular multilevel matrix converter exceeds the upper limit of the modulation index, the modular multilevel matrix converter is locked out, and the flexible low-frequency transmission system is shut down.

[0018] Secondly, the present invention provides a system for improving the operational reliability of a flexible low-frequency power transmission system, used to implement the above-mentioned method for improving the operational reliability of a flexible low-frequency power transmission system; the operational reliability improvement system includes a first reactive power adjustment unit, a second reactive power adjustment unit, a voltage amplitude adjustment unit, a submodule capacitor voltage adjustment unit, and an active power adjustment unit;

[0019] When the number of faulty submodules in a single arm of a modular multilevel matrix converter exceeds the number of redundant submodules, the reactive power on the power grid side of the modular multilevel matrix converter is reduced to the minimum allowable value through the first reactive power regulation unit, the reactive power on the low-frequency grid side is reduced to the minimum allowable value through the second reactive power regulation unit, or the AC voltage amplitude on the low-frequency side is reduced to the minimum allowable value through the voltage amplitude regulation unit, the submodule capacitor voltage is increased to the maximum safe operating value through the submodule capacitor voltage regulation unit, and the low-voltage load shedding system of the new energy power generation system is regulated through the active power regulation unit to disconnect some new energy power generation equipment and reduce the active power on the low-frequency grid side of the low-frequency transformer until the active power of the new energy power generation system reaches the minimum allowable value, so that the modular multilevel matrix converter does not block and maintains the voltage stability on its low-frequency grid side.

[0020] Compared with the prior art, the present invention has the following beneficial technical effects: When the number of faulty submodules in a single bridge arm of M3C exceeds the number of redundant submodules, the present invention combines comprehensive measures such as reactive power regulation on the M3C power frequency grid side, reactive power regulation on the M3C low frequency grid side (or M3C submodule capacitor voltage regulation), AC voltage amplitude regulation on the M3C low frequency side, and active power regulation of the new energy power generation system to ensure that M3C is not blocked, maintain voltage stability on the low frequency grid side, realize the continued operation of the flexible low frequency transmission system, and improve the operational reliability of the flexible low frequency transmission system. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the topology of the flexible low-frequency power transmission system of the present invention;

[0022] Figure 2 This is a flowchart of a method for improving the operational reliability of a flexible low-frequency power transmission system according to the present invention;

[0023] Figure 3 This is an equivalent circuit diagram of the power frequency side of the flexible low-frequency power transmission system of the present invention;

[0024] Figure 4 This is an equivalent circuit diagram of the low-frequency side of the flexible low-frequency power transmission system of the present invention;

[0025] Figure 5 This is a diagram illustrating the composition of a flexible low-frequency power transmission system reliability improvement system according to the present invention. Detailed Implementation

[0026] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0027] Example 1

[0028] This embodiment is a method for improving the operational reliability of a flexible low-frequency power transmission system.

[0029] Figure 1 This is a schematic diagram of the topology of a flexible low-frequency transmission system. The flexible low-frequency transmission system includes a power frequency grid, a low-frequency grid, and an AC / DC exchange station. The power frequency grid operates at a higher frequency than the low-frequency grid; the power frequency grid and the low-frequency grid are connected through the AC / DC exchange station. The AC / DC exchange station includes an M3C (modulator-distributor), a power frequency transformer, and a low-frequency transformer; one side of the M3C is connected to the power frequency grid via the power frequency transformer, and the other side of the M3C is connected to the low-frequency grid via the low-frequency transformer.

[0030] The aforementioned method for improving the reliability of flexible low-frequency power transmission systems involves adjusting the reactive power on the power grid side, the reactive power on the low-frequency grid side, or the AC voltage amplitude on the low-frequency side, the capacitor voltage of the sub-modules, and the active power of the new energy power generation system of the modular multilevel matrix converter when the number of faulty sub-modules in a single bridge arm of the modular multilevel matrix converter exceeds the number of redundant sub-modules. This ensures that the modular multilevel matrix converter does not lock out and maintains voltage stability on its low-frequency grid side, enabling the flexible low-frequency power transmission system to continue operating.

[0031] like Figure 2 As shown, the specific steps of the adjustment are as follows:

[0032] Step 1) When the modulation index of the modular multilevel matrix converter exceeds the starting value, reduce the power factor of the modular multilevel matrix converter on the power grid side to the minimum allowable value; Step 2) Based on Step 1), if the modulation index continues to increase, reduce the power factor of the modular multilevel matrix converter on the low-frequency grid side to the minimum allowable value, or reduce the AC voltage amplitude of the modular multilevel matrix converter on the low-frequency grid side to the minimum allowable value; Step 3) Based on Step 2), if the modulation index continues to increase, increase the capacitor voltage value of all sub-modules until the maximum safe operating value; Step 4) Based on Step 3), if the modulation index continues to increase, adjust the low-voltage load shedding system of the new energy power generation system, disconnect some new energy power generation equipment, and reduce the active power of the low-frequency transformer on the low-frequency grid side until the active power of the new energy power generation system reaches the minimum allowable value.

[0033] The turns ratio of a power frequency transformer is K T1 The turns ratio of the low-frequency transformer is K T2 They respectively satisfy:

[0034]

[0035] in, U g1N It is the voltage of the power frequency transformer on the power frequency grid side.U g1 The rated value, U v1N It is the voltage of the power frequency transformer on the M3C side. U v1 The rated value, U g2N It is the voltage of the low-frequency transformer on the low-frequency grid side. U g2 The rated value, U v2N It is the voltage of the low-frequency transformer on the M3C side. U v2 The rated value.

[0036] The M3C outputs AC voltage on both the power frequency grid side and the low frequency grid side, and its operating regulation is... M Defined as:

[0037]

[0038] in, U cap It is the capacitor voltage of the submodule. N sm1 This refers to the number of valid submodules within a bridge arm, i.e., the total number of submodules. N sm Subtract the number of redundant submodules N sm2 .

[0039] The specific details of step 1) are as follows:

[0040] When the number of faulty submodules in a single bridge arm of an M3C is greater than the number of redundant submodules N sm2 At that time, the number of effective submodules N sm1 The modulation of M3C begins to decrease. M Start increasing. Set the M3C modulation limit. M Lim When M3C's modulation M Exceed M Lim When M3C is blocked, the flexible low-frequency transmission system shuts down. Typically, the upper limit of M3C modulation is... M Lim Slightly less than 1.

[0041] when M > M s At the same time, adjust the reactive power command on the M3C power frequency grid side to improve the power factor of the power frequency transformer on the power frequency grid side. No longer 1, Start decreasing until the power factor on the power frequency grid side of the power frequency transformer is reduced. Reach the minimum allowed value.

[0042] Figure 3 and Figure 4 It is the equivalent circuit of a flexible low-frequency transmission system, with a power factor angle. The angle between the mains voltage and current is defined as the angle between the mains voltage and current. Typically, the control objective in the M3C mains voltage side capacitor voltage control mode is to make... =0, at this time U v1 satisfy:

[0043]

[0044] in, X T1 It is the short-circuit reactance of the low-frequency transformer referred to the power frequency grid side. I g1 It is the effective value of the low-frequency grid current.

[0045] When adjusting When >0, theoretically U v1 The minimum value is:

[0046]

[0047] Considering the minimum power factor constraint of low-frequency power grids, i.e., the maximum power factor angle... Restrictions, U v1 The minimum possible value is:

[0048]

[0049] Voltage on the power frequency grid side U g1 The voltage on the M3C power grid side is generally considered constant, therefore... U v1 The minimum adjustable value is:

[0050]

[0051] Typically, the power factor on the power frequency grid side The minimum allowed value is not less than 0.95 (i.e. The maximum allowed value is no greater than arccos0.95.

[0052] The specific details of step 2) are as follows:

[0053] When the power factor on the power frequency grid side When the minimum allowed value is reached, and the modulation of M3C is...M The power factor is still increasing. If the M3C adopts a constant active power and constant reactive power control strategy on the low-frequency grid side, adjusting the reactive power command on the low-frequency grid side of the M3C will improve the power factor of the low-frequency transformer on the low-frequency grid side. No longer 1, It begins to decrease until the power factor on the low-frequency grid side of the low-frequency transformer is reduced. Reach the minimum allowed value.

[0054] Power factor angle The angle between the low-frequency grid voltage and current is defined as . Typically, the control objective of the M3C's low-frequency grid-side power control mode is to make . =0, at this time U v2 satisfy:

[0055]

[0056] in, X T2 It is the short-circuit reactance of the low-frequency transformer referred to the low-frequency power grid side. I g2 It is the effective value of the low-frequency grid current.

[0057] When adjusting When >0, theoretically U v2 The minimum value is:

[0058]

[0059] Considering the minimum power factor constraint of low-frequency power grids, i.e., the maximum power factor angle... Restrictions, U v2 The minimum possible value is:

[0060]

[0061] When the voltage of the low-frequency grid is established by the M3C, the M3C adopts a control strategy of constant AC voltage amplitude and constant AC voltage frequency on the low-frequency grid side. The reactive power on the low-frequency grid side is determined by the renewable energy generation system, and the M3C cannot adjust the power factor. When the voltage of the low-frequency grid is established by the new energy power generation system, the M3C adopts a constant active power and constant reactive power control strategy on the low-frequency grid side, and the voltage on the low-frequency grid side... U g2 The voltage on the M3C power grid side is generally considered constant, therefore... U v2 The minimum adjustable value is:

[0062]

[0063] Typically, the power factor on the low-frequency grid side The minimum allowed value is not less than 0.95 (i.e. The maximum allowed value is no greater than arccos0.95.

[0064] The specific details of step 3) are as follows:

[0065] When the power factor of the power frequency transformer on the power frequency grid side When the minimum allowed value is reached, and the modulation of M3C is... M The voltage is still increasing. If the M3C adopts a control strategy of constant AC voltage amplitude and constant AC voltage frequency on the low-frequency grid side, adjusting the AC voltage amplitude command on the low-frequency grid side of the M3C will make the voltage amplitude of the low-frequency transformer on the low-frequency grid side... No longer the rated value , U g2 Start decreasing until the voltage amplitude on the low-frequency grid side of the low-frequency transformer is reduced. Reaching the minimum permissible value. Typically, the AC voltage amplitude on the low-frequency grid side... The minimum value is not lower than .

[0066] When the power factor of the low-frequency transformer on the low-frequency grid side When the minimum allowable value is reached, or when the voltage amplitude on the low-frequency grid side of the low-frequency transformer... When the minimum allowed value is reached, and the modulation of M3C is... M It's still increasing; adjusting the capacitor voltage values ​​of all sub-modules. U cap , making U cap Greater than its rated value U capN Until the submodule capacitor voltage reaches the maximum safe operating value. U capmax .

[0067] The specific details of step 4) are as follows:

[0068] When the submodule capacitor voltage reaches the maximum safe operating value, and the M3C modulation... M The pressure is still increasing, adjusting the low-voltage load shedding system of the renewable energy power generation system, disconnecting some renewable energy power generation equipment, and reducing the active power on the low-frequency grid side. P 2. Until the active power of the new energy power generation system reaches the minimum allowable value. P 2min Typically, the minimum allowable active power of a new energy power generation system is... P 2minNot lower than the rated value P 2N 70%.

[0069] The above-mentioned method for improving the operational reliability of flexible low-frequency power transmission systems is applied in the following ways:

[0070] M3C AC voltage rating on the power grid side U g1N The rated AC voltage on the low-frequency grid side is set at 220kV and 50Hz. U g2N The voltage is set at 220kV, and the frequency is set at 20Hz. Rated active power output of the new energy power generation system. P 2N The rated reactive power is 300MW. Q 2N The value is 0. The M3C employs a constant AC voltage amplitude and frequency control mode on the low-frequency grid side. Power frequency transformer capacity. S T1N 330MVA, rated transformer ratio U g1N / U v1N For 220kV / 64kV, the percentage of short-circuit impedance U k1 It is 15%. Low-frequency transformer capacity. S T2N 330MVA, rated transformer ratio U g2N / U v2N For 220kV / 64kV, the percentage of short-circuit impedance U k2 It is 15%. The number of submodules per M3C bridge arm. N sm There are 64, of which the number of effective submodules is... N sm1 There are 58 redundant submodules. N sm2 There are 6 submodules, each with a rated operating voltage. U capN 2.15kV, bridge arm inductance L arm The voltage is 15mH. Meanwhile, it is assumed that the voltage on the M3C's power grid side is... U g1 The effective value remains unchanged at 220kV.

[0071] Setting the power factor on the power frequency grid side To achieve the minimum allowable value of 0.95, set the power factor on the low-frequency grid side. To achieve the minimum allowable value of 0.95, set the output AC voltage amplitude on the low-frequency grid side. The minimum value is Set the capacitor voltage of the submodule U cap The maximum value is 2.4kV, and the allowable active power of the new energy low-voltage load shedding system is set. P The minimum value of 2 is 210MW.

[0072] The leakage reactance of the power frequency transformer was calculated. X T2 and leakage reactance of low-frequency transformers X T2 for:

[0073]

[0074] Assuming the cumulative transmission loss of the M3C and transformer is 3%, the current on the power grid side is calculated. I g1 and the current on the low-frequency grid side I g2 for:

[0075]

[0076] Based on equivalent circuit and circuit theory, the voltage on the M3C power grid side at this time can be calculated. U v1 and the voltage on the low-frequency grid side of M3C U v2 for:

[0077]

[0078] The adjustment system at this time M for:

[0079]

[0080] Set the modulation trigger value M s Set to 0.9, maximum modulation index. M Lim If the value is set to 0.95, the relevant parameters will change as the number of submodule failures increases, as shown in Table 1 below.

[0081] Table 1. Parameters varying with the number of submodule failures

[0082]

[0083] As shown in Table 1, after adopting the operational reliability improvement method of the present invention, the number of faulty sub-modules allowed to be increased from 12 to 22 under the condition that the flexible low-frequency power transmission system is not interrupted, which greatly improves the operational reliability of the flexible low-frequency power transmission system.

[0084] Example 2

[0085] This embodiment provides a system for improving the operational reliability of a flexible low-frequency power transmission system, used to implement the method for improving the operational reliability of a flexible low-frequency power transmission system described in Embodiment 1. It includes a first reactive power regulation unit, a second reactive power regulation unit, a voltage amplitude regulation unit, a submodule capacitor voltage regulation unit, and an active power regulation unit, such as... Figure 5 As shown.

[0086] When the number of faulty submodules in a single arm of a modular multilevel matrix converter exceeds the number of redundant submodules, the reactive power on the power grid side of the modular multilevel matrix converter is reduced to the minimum allowable value through the first reactive power regulation unit, the reactive power on the low-frequency grid side is reduced to the minimum allowable value through the second reactive power regulation unit, or the AC voltage amplitude on the low-frequency side is reduced to the minimum allowable value through the voltage amplitude regulation unit, the submodule capacitor voltage is increased to the maximum safe operating value through the submodule capacitor voltage regulation unit, and the low-voltage load shedding system of the new energy power generation system is regulated through the active power regulation unit to disconnect some new energy power generation equipment and reduce the active power on the low-frequency grid side of the low-frequency transformer until the active power of the new energy power generation system reaches the minimum allowable value, so that the modular multilevel matrix converter does not block and maintains the voltage stability on its low-frequency grid side.

[0087] The adjustment content of the first reactive power adjustment unit is the same as step 1 of embodiment 1, and will not be repeated here; the adjustment content of the second reactive power adjustment unit and the voltage amplitude adjustment unit is the same as step 2 of embodiment 1, and will not be repeated here; the adjustment content of the submodule capacitor voltage adjustment unit is the same as step 3 of embodiment 1, and will not be repeated here; the adjustment content of the active power adjustment unit is the same as step 4 of embodiment 1, and will not be repeated here.

[0088] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included within the protection scope of the present invention.

Claims

1. A method for improving the operational reliability of a flexible low-frequency power transmission system, characterized in that, When the number of faulty submodules in a single bridge arm of a modular multilevel matrix converter is greater than the number of redundant submodules, the reactive power on the power grid side, the reactive power on the low-frequency grid side, or the AC voltage amplitude on the low-frequency side, the capacitor voltage of the submodule, and the active power of the new energy power generation system of the modular multilevel matrix converter are adjusted so that the modular multilevel matrix converter is not blocked and its voltage on the low-frequency grid side remains stable. The specific steps for adjustment are as follows: Step 1) When the modulation index of the modular multilevel matrix converter exceeds the starting value, reduce the power factor of the modular multilevel matrix converter on the power grid side to the minimum allowable value; Step 2) Based on Step 1), if the modulation index continues to increase, reduce the power factor of the modular multilevel matrix converter on the low-frequency grid side to the minimum allowable value, or reduce the AC voltage amplitude of the modular multilevel matrix converter on the low-frequency grid side to the minimum allowable value; Step 3) Based on Step 2), if the modulation index continues to increase, increase the capacitor voltage value of all sub-modules until the maximum safe operating value; Step 4) Based on Step 3), if the modulation index continues to increase, adjust the low-voltage load shedding system of the new energy power generation system, disconnect some new energy power generation equipment, and reduce the active power of the low-frequency transformer on the low-frequency grid side until the active power of the new energy power generation system reaches the minimum allowable value.

2. The method for improving the operational reliability of a flexible low-frequency power transmission system according to claim 1, characterized in that, Step 1) specifically includes: when the modulation index of the modular multilevel matrix converter exceeds the starting value, the reactive power command of the power frequency grid side of the modular multilevel matrix converter is adjusted so that the power factor of the power frequency transformer on the power frequency grid side is no longer 1 and begins to decrease until the power factor of the power frequency transformer on the power frequency grid side reaches the minimum allowable value.

3. The method for improving the operational reliability of a flexible low-frequency power transmission system according to claim 2, characterized in that, Step 2) specifically includes: when the power factor of the power frequency transformer on the power frequency grid side reaches the minimum allowable value and the modulation index of the modular multilevel matrix converter is still increasing, if the modular multilevel matrix converter adopts a constant active power and constant reactive power control strategy on the low frequency grid side, the reactive power command of the modular multilevel matrix converter on the low frequency grid side is adjusted so that the power factor of the low frequency transformer on the low frequency grid side is no longer 1 and begins to decrease until the power factor of the low frequency transformer on the low frequency grid side reaches the minimum allowable value.

4. The method for improving the operational reliability of a flexible low-frequency power transmission system according to claim 3, characterized in that, Step 3) includes the following: when the power factor of the low-frequency transformer on the low-frequency grid side reaches the minimum allowable value and the modulation index of the modular multilevel matrix converter is still increasing, adjust the capacitor voltage values ​​of all sub-modules so that the capacitor voltage values ​​of the sub-modules are greater than their rated values, until the capacitor voltage values ​​of the sub-modules reach the maximum safe operating value.

5. The method for improving the operational reliability of a flexible low-frequency power transmission system according to claim 2, characterized in that, Step 2) specifically includes: when the power factor of the power frequency transformer on the power frequency grid side reaches the minimum allowable value and the modulation index of the modular multilevel matrix converter is still increasing, if the modular multilevel matrix converter adopts a control strategy of constant AC voltage amplitude and constant AC voltage frequency on the low frequency grid side, the AC voltage amplitude command of the modular multilevel matrix converter on the low frequency grid side is adjusted so that the AC voltage amplitude of the low frequency transformer on the low frequency grid side is no longer at the rated value, and the voltage on the low frequency grid side begins to decrease until the AC voltage amplitude of the low frequency transformer on the low frequency grid side reaches the minimum allowable value.

6. The method for improving the operational reliability of a flexible low-frequency power transmission system according to claim 5, characterized in that, Step 3) includes the following: when the voltage amplitude of the low-frequency transformer on the low-frequency grid side reaches the minimum allowable value and the modulation index of the modular multilevel matrix converter is still increasing, adjust the capacitor voltage values ​​of all sub-modules so that the capacitor voltage values ​​of the sub-modules are greater than their rated values, until the capacitor voltage values ​​of the sub-modules reach the maximum safe operating value.

7. The method for improving the operational reliability of a flexible low-frequency power transmission system according to claim 4 or 6, characterized in that, Step 4) includes the following: when the voltage value of the submodule capacitor reaches the maximum safe operating value and the modulation index of the modular multilevel matrix converter is still increasing, adjust the low-voltage load shedding system of the new energy power generation system, disconnect some new energy power generation equipment, and reduce the active power on the low-frequency grid side of the low-frequency transformer until the active power of the new energy power generation system reaches the minimum allowable value.

8. The method for improving the operational reliability of a flexible low-frequency power transmission system according to claim 1, characterized in that, When the modulation index of the modular multilevel matrix converter exceeds the upper limit of the modulation index, the modular multilevel matrix converter is locked, and the flexible low-frequency transmission system is shut down.

9. A system for improving the operational reliability of a flexible low-frequency power transmission system, used to implement the method for improving the operational reliability of a flexible low-frequency power transmission system as described in any one of claims 1-8; characterized in that, It includes a first reactive power regulation unit, a second reactive power regulation unit, a voltage amplitude regulation unit, a sub-module capacitor voltage regulation unit, and an active power regulation unit; When the number of faulty submodules in a single arm of a modular multilevel matrix converter exceeds the number of redundant submodules, the reactive power on the power grid side of the modular multilevel matrix converter is reduced to the minimum allowable value through the first reactive power regulation unit, the reactive power on the low-frequency grid side is reduced to the minimum allowable value through the second reactive power regulation unit, or the AC voltage amplitude on the low-frequency side is reduced to the minimum allowable value through the voltage amplitude regulation unit, the submodule capacitor voltage is increased to the maximum safe operating value through the submodule capacitor voltage regulation unit, and the low-voltage load shedding system of the new energy power generation system is regulated through the active power regulation unit to disconnect some new energy power generation equipment and reduce the active power on the low-frequency grid side of the low-frequency transformer until the active power of the new energy power generation system reaches the minimum allowable value, so that the modular multilevel matrix converter does not block and maintains the voltage stability on its low-frequency grid side.

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