A method for suppressing oscillations in MMC-HVDC direct current transmission systems

Abstract

An embodiment of the present invention relates to an oscillation suppression control method for an MMC-HVDC direct current transmission system, comprising the following steps: simulating the system based on system parameters; obtaining the power oscillation range and oscillation frequency of the direct current transmission system according to the simulation results, and calculating the direct current current oscillation range of the direct current transmission system based on the power oscillation range and oscillation frequency; collecting the currents of the six bridge arms of the MMC; calculating the direct current of the direct current transmission system based on the currents of the six bridge arms of the MMC; selecting a switching frequency optimization holding factor based on the relationship between the direct current and the direct current oscillation range; and performing voltage balancing sorting on the submodules based on the capacitor voltage of the submodule and the selected switching frequency optimization holding factor. The technical solution of the embodiment of the present invention implements different switching frequency optimization holding factors at different power points of the direct current transmission system, reduces the degree of capacitor voltage divergence of the submodules under low power conditions, and thus solves the oscillation problem of the direct current transmission system under low power conditions.

Description

Technical Field

[0001] The embodiments of the present invention relate to the fields of flexible power transmission and distribution and power electronics technology of power systems, and in particular to an oscillation suppression control method for an MMC-HVDC direct current transmission system. Background Art

[0002] With the development of fully controlled power electronic devices and the application of power electronics technology in power systems, high-voltage direct current (HVDC) transmission technology based on voltage-source converters (VSC-HVDC) is gaining increasing attention. The modular multilevel converter (MMC), a type of voltage-source converter used in flexible direct current (HVDC) transmission systems, is widely used in flexible HVDC transmission and renewable energy access systems due to its significant advantages.

[0003] Each bridge arm of the MMC-HVDC direct current transmission system is composed of a large number of cascaded sub-modules. The project adopts the nearest level approximation modulation method and constantly sorts the voltages of the capacitors of the numerous sub-modules to realize the number of sub-modules put into and removed in each bridge arm. The output voltage of the AC and DC sides approaches the required modulation voltage, achieving stable operation of the system and balancing the sub-module capacitor voltages.

[0004] The main voltage balancing method of the MMC-HVDC direct current transmission system is to select the sub-module with low capacitor voltage according to the order of the sub-module capacitor voltage and the direction of the bridge arm current when the bridge arm current is charging current, and select the sub-module with high capacitor voltage when the bridge arm current is discharging current, so as to achieve capacitor voltage balance.

[0005] When sorting the submodule capacitor voltages, MMC usually introduces a holding factor to increase the probability that a submodule that was in the on-state at the previous moment will still be in the on-state at the next moment, that is, to reduce the submodule switching frequency. Generally, the holding factor is slightly greater than 1.

[0006] During the operation of an MMC-HVDC direct current transmission system, there is a certain degree of divergence in the submodule voltage, resulting in inconsistent DC voltages between converters. This is especially true in back-to-back systems. Since the DC side has almost no impedance or very low impedance, inconsistent DC voltages have a greater impact under low-power conditions, which will further lead to DC side current oscillations. Summary of the Invention

[0007] Based on the above-mentioned situation of the prior art, the purpose of the embodiments of the present invention is to provide an oscillation suppression control method for an MMC-HVDC direct current transmission system, which realizes the execution of different switching frequency optimization holding factors at different power points of the direct current transmission system, reduces the degree of voltage divergence of the sub-module capacitors at low power, and thus solves the oscillation problem of the direct current transmission system at low power.

[0008] To achieve the above object, according to one aspect of the present invention, a method for controlling oscillation suppression in an MMC-HVDC direct current transmission system is provided, comprising the steps of:

[0009] S1. Simulate the system based on system parameters;

[0010] S2. Obtaining a power oscillation range and an oscillation frequency of the DC power transmission system according to the simulation results, and calculating a DC current oscillation range of the DC power transmission system according to the power oscillation range and the oscillation frequency;

[0011] S3, collecting the six bridge arm currents of the MMC;

[0012] S4. Calculating the DC current of the DC transmission system according to the currents of the six bridge arms of the MMC;

[0013] S5. Selecting a switching frequency optimization holding factor based on the relationship between the DC current and the DC current oscillation range;

[0014] S6. Sorting the submodules according to the capacitor voltages of the submodules and the selected switching frequency optimization holding factors.

[0015] 1. The control method according to claim 1, further comprising the steps of:

[0016] S7. Send pulse instructions to the submodules according to the sorting results.

[0017] Furthermore, in step S1, based on the MMC-HVDC direct current transmission system parameters and converter valve parameters, the system is simulated using a converter valve voltage balancing sequencing algorithm and a submodule switching frequency optimization algorithm.

[0018] Furthermore, in step S5, selecting a switching frequency optimization holding factor according to the relationship between the DC current and the DC current oscillation interval includes:

[0019] When the DC current is greater than or equal to the maximum value of the DC current oscillation interval, the switching frequency optimization holding factor is selected as the first optimization holding factor F1;

[0020] When the DC current is less than the maximum value of the DC current oscillation interval, the switching frequency optimization holding factor is selected as the second optimization holding factor F2;

[0021] Among them, F1>F2.

[0022] Furthermore, in step S6, the submodules are sorted according to the capacitor voltages of the submodules and the selected switching frequency optimization holding factors, including:

[0023] If the current is in the charging direction, N sub-modules with lower capacitor voltages need to be put into use. The voltage of the module in the cut-off state at the previous moment is multiplied by the switching frequency optimization holding factor, and then sorted.

[0024] If the current is in the discharge direction, the submodules that were in the on state at the previous moment are multiplied by the switching frequency optimization holding factor and then sorted.

[0025] Furthermore, in step S7, a submodule driving pulse instruction is generated according to the switching frequency optimization holding factor, the number of submodules to be switched on and / or off, and the capacitor voltage sorting result, and is sent to the submodule for switching.

[0026] Furthermore, the method further comprises the steps of:

[0027] The switching frequencies of the sub-modules when different switching frequency optimization holding factors are selected under different DC power levels are simulated and calculated respectively, and the switching frequency tolerance of the sub-modules under different DC power levels is verified.

[0028] In summary, an embodiment of the present invention provides an oscillation suppression control method for an MMC-HVDC direct current transmission system, comprising the following steps: simulating the system based on system parameters; obtaining the power oscillation range and oscillation frequency of the direct current transmission system based on the simulation results, and calculating the direct current current oscillation range of the direct current transmission system based on the power oscillation range and oscillation frequency; collecting the currents of the six bridge arms of the MMC; calculating the direct current of the direct current transmission system based on the currents of the six bridge arms of the MMC; selecting a switching frequency optimization holding factor based on the relationship between the direct current and the direct current oscillation range; and performing voltage balancing sorting on the submodules based on the capacitor voltage of the submodule and the selected switching frequency optimization holding factor. The technical solution of the embodiment of the present invention calculates the direct current based on the bridge arm current, and dynamically adjusts the switching frequency optimization holding factor of the submodule in the valve control system based on the current at the oscillation point of the direct current transmission system, thereby implementing different switching frequency optimization holding factors at different power points of the direct current transmission system, reducing the degree of submodule capacitor voltage divergence at low power, and thus solving the oscillation problem of the direct current transmission system at low power. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a flow chart of an oscillation suppression control method for an MMC-HVDC direct current transmission system according to an embodiment of the present invention;

[0030] Figure 2 It is a schematic diagram of the waveform related to the DC power rising process;

[0031] Figure 3 It is the simulation verification waveform of the switching frequency optimization holding factor dynamic adjustment logic and the sub-module voltage balancing sorting logic. DETAILED DESCRIPTION

[0032] To make the objectives, technical solutions, and advantages of the present invention more clearly understood, the present invention will be further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings. It should be understood that these descriptions are merely exemplary and are not intended to limit the scope of the present invention. In addition, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessary confusion of the concepts of the present invention.

[0033] The technical solutions of the embodiments of the present invention are described in detail below with reference to the accompanying drawings. The embodiments of the present invention provide an oscillation suppression control method for an MMC-HVDC direct current transmission system. The topology of the MMC-HVDC direct current transmission system may also include hybrid flexible direct current transmission, and the submodules are not limited to half-bridge submodules but also include full-bridge and clamped submodules. The power switching devices in the submodules are not limited to IGBTs but also include common power switching devices such as MOSFETs and IGCTs. Figure 1 The flowchart of the method is shown in FIG. , and the method comprises the following steps:

[0034] S1. Simulate the system based on system parameters. In step S1, the system can be simulated and modeled based on MMC-HVDC system parameters and converter valve parameters using a converter valve voltage balancing sequencing algorithm and a submodule switching frequency optimization algorithm (holding factor method).

[0035] S2. Determine the power oscillation range and oscillation frequency of the DC power transmission system based on the simulation results, and calculate the DC current oscillation range of the DC power transmission system based on the power oscillation range and oscillation frequency. The DC current oscillation range of the DC power transmission system can be determined by utilizing the relationship between DC power, DC current, and DC voltage.

[0036] S3. Collect the six bridge arm currents of the MMC. Since the valve control system receives the six bridge arm currents of the MMC transmitted by the measurement system or the pole control system (ie, the valve control system only receives the bridge arm currents), the six bridge arm currents of the MMC are directly collected here.

[0037] S4. Calculate the DC current of the DC transmission system based on the currents of the six bridge arms of the MMC. The DC current of the DC transmission system can be obtained by summing the currents of the six bridge arms and dividing the sum by 2.

[0038] According to certain optional embodiments, simulations are performed to calculate the switching frequencies of the submodules when different switching frequency optimization retention factors are selected at different DC power levels, and the submodule switching frequency tolerance is verified at different DC power levels. When the DC transmission system is operating at rated power, to reduce the losses of the MMC submodule switching components, the submodule switching frequency is generally controlled within 150 Hz. In this case, the submodule switching frequency optimization retention factor typically takes a larger value, such as the first optimization retention factor F1 (typically 1.04).

[0039] At the same hold factor, the maximum submodule voltage difference is essentially the same. However, at low power, the submodule voltage fluctuation range is smaller and changes more slowly. Therefore, the submodule divergence increases, the hold time increases, and the submodule voltage consistency deteriorates. At the same power, the smaller the optimized hold factor, the smaller the submodule voltage difference, which relatively reduces the divergence and improves the consistency.

[0040] In MMC-HVDC systems, especially back-to-back DC systems, the high switching frequency optimization retention factor submodules exhibit high divergence at low power levels, resulting in a lack of impedance on the DC side and a high risk of DC system oscillation. Simulations verify oscillation at different switching frequencies and select an appropriate switching frequency optimization retention factor that meets the submodule switching frequency tolerance requirements. This optimization retention factor, for example, the second optimization retention factor F2, is typically a small value (typically 1.006).

[0041] S5. Select a switching frequency optimization holding factor based on the relationship between the DC current and the DC current oscillation range. The selection steps are as follows: When the DC current is ≥ the maximum value of the DC current oscillation range, the switching frequency optimization holding factor is selected as the first optimization holding factor F1; when the DC current is < the maximum value of the DC current oscillation range, the switching frequency optimization holding factor is selected as the second optimization holding factor F2; where F1>F2. For example, based on the DC current oscillation range [x1, x2], the following logic is set in the valve control system: when the DC current calculated based on the bridge arm current is ≥ the maximum value x2 of the DC current oscillation range, the switching frequency optimization holding factor in the valve control is selected as the first optimization holding factor F1; when the DC current is < the maximum value x2 of the DC current oscillation range, the switching frequency optimization holding factor in the valve control is selected as the second optimization holding factor F2.

[0042] S6. Sort the submodules by voltage balancing based on their capacitor voltages and the selected switching frequency optimization retention factors. Sorting can be performed according to the following steps: If the current is in the charging direction, N submodules with lower capacitor voltages need to be put into operation. Multiply the voltage of the module in the cut-off state at the previous moment by the retention factor before sorting. If the current is in the discharging direction, multiply the submodule in the put-on state at the previous moment by the retention factor before sorting.

[0043] S7. Send pulse instructions to the submodules based on the sorting results. Generate submodule drive pulse instructions based on the switching frequency optimization holding factor, the number of submodules to be put into operation and / or removed, and the capacitor voltage sorting results, and send them to the submodules for switching: when the current is positive, send an input instruction to the submodule with low voltage, and a removal instruction to the submodule with high voltage; when the current is negative, send a removal instruction to the submodule with high voltage, and a input instruction to the submodule with low voltage. The following is an example of a specific embodiment (taking the DC system oscillation of a certain DC back-to-back interconnected project as an example).

[0044] A specific implementation plan is given through RTDS simulation. First, the MMC-HVDC back-to-back DC transmission system operates normally. After unlocking, the DC power ramps up at a rate of 1 pu / min.

[0045] The waveforms related to the DC power rising process are as follows: Figure 2 As shown in the figure, the DC power oscillation range is [60MW-350MW], which is converted into a DC current oscillation range of [70A-400A], and the oscillation frequency is mainly 33-34Hz.

[0046] The valve control system receives the six-bridge arm current transmitted by the pole control system in real time through the IEC60044-8 communication protocol, and obtains the DC current through calculation processing.

[0047] The valve control system sets the judgment logic. According to the DC current oscillation range, when the DC current is ≥500A, the valve control sets the switching frequency optimization holding factor to 1.04.

[0048] When the DC current is less than 500A, the valve control setting switching frequency optimization holding factor is 1.006.

[0049] When the switching frequency optimization holding factor is reduced, the submodule switching frequency will increase. The submodule switching frequency is checked when the switching frequency optimization holding factor is 1.006.

[0050] Simulation verification shows that when the DC current is 500A and the DC power is 450MW, the average switching frequency of the submodule is at most 350Hz when the switching frequency optimization factor is 1.006, which meets the switching frequency tolerance capability of the submodule.

[0051] The submodules perform voltage balancing sorting. If the current is in the charging direction, N submodules with lower voltages need to be put into use. At this time, the voltage of the module in the cut-off state at the previous moment is multiplied by the holding factor (according to the holding factor set by the valve control) and then sorted.

[0052] When the current is in the discharge direction, the submodules that were in the input state at the previous moment can be multiplied by the holding factor (according to the holding factor set by the valve control) and then sorted.

[0053] According to the voltage-sharing sorting results of the submodules based on the switching frequency optimization holding factor and the valve control modulation instructions, the submodule control pulses are generated, and the submodules execute the above control pulses;

[0054] According to the above switching frequency optimization and dynamic adjustment logic of the holding factor and the sub-module voltage sorting logic, simulation verification is carried out to obtain Figure 3 As shown in the waveform, after unlocking, the DC power ramps up at a rate of 1 pu / min, and power reversal is performed; the power oscillation range disappears and the oscillation is suppressed.

[0055] In summary, an embodiment of the present invention relates to an oscillation suppression control method for an MMC-HVDC direct current transmission system, comprising the following steps: simulating the system based on system parameters; obtaining the power oscillation range and oscillation frequency of the direct current transmission system based on the simulation results, and calculating the direct current current oscillation range of the direct current transmission system based on the power oscillation range and oscillation frequency; collecting the currents of the six bridge arms of the MMC; calculating the direct current of the direct current transmission system based on the currents of the six bridge arms of the MMC; selecting a switching frequency optimization holding factor based on the relationship between the direct current and the direct current oscillation range; and performing voltage balancing sorting on the submodules based on the capacitor voltage of the submodule and the selected switching frequency optimization holding factor. The technical solution of the embodiment of the present invention calculates the direct current based on the bridge arm current, and dynamically adjusts the switching frequency optimization holding factor of the submodule in the valve control system based on the current at the oscillation point of the direct current transmission system, thereby implementing different switching frequency optimization holding factors at different power points of the direct current transmission system, reducing the degree of submodule capacitor voltage divergence at low power, and thus solving the oscillation problem of the direct current transmission system at low power.

[0056] It should be understood that the above-described specific embodiments of the present invention are merely illustrative or illustrative of the principles of the present invention and do not constitute limitations of the present invention. Therefore, any modifications, equivalent substitutions, improvements, etc. made without departing from the spirit and scope of the present invention should be included within the scope of protection of the present invention. In addition, the appended claims are intended to cover all variations and modifications that fall within the scope and metes and bounds of the appended claims, or equivalents thereof.

Claims

1. A method for controlling oscillation suppression of an MMC-HVDC direct current transmission system, characterized in that: Including steps: S1. Simulate the system based on system parameters; S2. Obtaining a power oscillation range and an oscillation frequency of the DC power transmission system according to the simulation results, and calculating a DC current oscillation range of the DC power transmission system according to the power oscillation range and the oscillation frequency; S3, collecting the six bridge arm currents of the MMC; S4. Calculating the DC current of the DC transmission system according to the currents of the six bridge arms of the MMC; S5. Selecting a switching frequency optimization holding factor according to a relationship between the DC current and the DC current oscillation interval, including: when the DC current is greater than or equal to the maximum value of the DC current oscillation interval, selecting the switching frequency optimization holding factor as the first optimization holding factor F1; When the DC current is less than the maximum value of the DC current oscillation interval, the switching frequency optimization holding factor is selected as the second optimization holding factor F2; Among them, F1>F2; S6. Sorting the submodules according to the capacitor voltages of the submodules and the selected switching frequency optimization holding factors.

2. The control method according to claim 1, characterized in that: Also includes the steps: S7. Send pulse instructions to the submodules according to the sorting results.

3. The control method according to claim 2, characterized in that: In step S1, based on the MMC-HVDC direct current transmission system parameters and converter valve parameters, the system is simulated using a converter valve voltage balancing sequencing algorithm and a submodule switching frequency optimization algorithm.

4. The control method according to claim 3, characterized in that: In step S6, the submodules are sorted according to the capacitor voltages of the submodules and the selected switching frequency optimization holding factors, including: If the current is in the charging direction, N sub-modules with lower capacitor voltages need to be put into use. The voltage of the module in the cut-off state at the previous moment is multiplied by the switching frequency optimization holding factor, and then sorted. If the current is in the discharge direction, the submodules that were in the on state at the previous moment are multiplied by the switching frequency optimization holding factor and then sorted.

5. The control method according to claim 4, characterized in that: In step S7, a submodule driving pulse instruction is generated according to the switching frequency optimization holding factor, the number of submodules to be switched on and / or off, and the capacitor voltage sorting result, and is sent to the submodule for switching.

6. The control method according to claim 5, characterized in that: Also includes the steps: The switching frequencies of the sub-modules when different switching frequency optimization holding factors are selected under different DC power levels are simulated and calculated respectively, and the switching frequency tolerance of the sub-modules under different DC power levels is verified.

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