A parameter design method of a modular alternating support current converter

Abstract

This invention provides a parameter design method for a modular alternating support converter, relating to the field of power electronics technology. The modular alternating support converter includes three phase units; each phase unit includes an upper arm and a lower arm connected in series; each arm includes multiple cascaded sub-modules; the two ends of the phase unit serve as the positive and negative terminals on the DC side, and the midpoint is connected to an AC filter inductor on the AC side; a smoothing reactor and a DC filter inductor are connected in series at the positive terminal of the DC side; the number of sub-modules, the capacitance of the sub-module capacitors, and the AC and DC filter inductors in the arm are all designed according to corresponding conditions. This modular alternating support converter has good dynamic performance in current control, which helps ensure fault ride-through capability and equipment safety. It can provide a certain margin while ensuring the arm's ability to withstand DC voltage, and ensures the stability of the control current and the current-limiting capability under transient DC faults, thus helping to reduce potential safety hazards.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a parameter design method for a modular alternating support converter. Background Technology

[0002] With the continuous rise in global energy demand and the increasing interconnectivity of power systems, efficient long-distance power transmission technologies such as flexible DC transmission are playing an increasingly important role in energy transmission and consumption. As the core equipment of flexible DC transmission systems, the performance of the converter directly determines the efficiency and reliability of power transmission.

[0003] Among various converter topologies, the Modular Multilevel Converter (MMC) has become the mainstream topology choice for medium- and high-voltage, high-power applications due to its modular structure, excellent output waveform quality, and good scalability. To ensure the stable operation of the converter under actual conditions, the parameter design system of the existing MMC has been perfected, and its design content covers key parameters such as the number of modules, capacitor capacity, and bridge arm reactance.

[0004] While the parameter design methods for traditional MMC and its derivative topologies are relatively mature, their direct application to Modular Alternating Support Converters (MASCs) presents fundamental incompatibilities and significant engineering limitations. Existing design methods primarily rely on the assumption that the three-phase arms of a traditional MMC continuously participate in energy exchange throughout the entire cycle, using power ripple and voltage ripple constraints to determine capacitor values. However, the MASC topology employs an alternating support mechanism, with the connection between the arms and the DC side switching periodically, fundamentally altering the energy flow path. Based on the energy fluctuation characteristics of MASCs, their submodule energy storage requirements are significantly lower than those of traditional MMCs. Therefore, ignoring the low energy fluctuation characteristics and using traditional MMC methods for MASC parameter design will result in capacitor values ​​in the selected submodules being far greater than actual requirements, leading to a bulky and expensive converter that masks the lightweight technological advantages of this topology.

[0005] Furthermore, in the traditional MMC parameter system, the arm reactor is a key component for suppressing circulating current and limiting DC fault current, and its inductance directly affects the AC-side equivalent impedance and DC-side fault response. However, the MASC topology typically eliminates the arm reactor and has no interphase circulating current. Current technology lacks parameter compensation design for this type of armless inductor-free structure. This leads to impedance matching failure on the AC side, affecting the steady-state accuracy and dynamic response of the control. Moreover, it causes the power system to lose its ability to limit the rate of current rise under DC short-circuit faults, posing a serious safety hazard. Summary of the Invention

[0006] To address the aforementioned technical problems in the existing technology, this invention aims to provide a parameter design method for a modular alternating support converter. With appropriate parameter constraints, this method aims to reduce the converter size while ensuring the basic functions of the converter, guaranteeing the stability of the control current and the current limiting capability under transient DC faults, and reducing potential safety hazards.

[0007] Specifically, the technical solution is as follows: a parameter design method for a modular alternating support converter, comprising three phase units;

[0008] The phase unit includes an upper bridge arm and a lower bridge arm connected in series; each bridge arm includes cascaded... Each submodule;

[0009] The two ends of the phase unit serve as the positive and negative terminals on the DC side, and the midpoint is connected to the AC filter inductor on the AC side. A smoothing reactor and a DC filter inductor are connected in series on the positive terminal of the DC side. ;

[0010] Parameter design method, including the number of submodules in the bridge arm The design method, formula is as follows:

[0011] ;

[0012] In the formula, This represents the redundancy coefficient of the submodule. DC side voltage This is the rated voltage of the capacitor in the submodule.

[0013] Preferably, the submodule is a half-bridge submodule.

[0014] Preferably, the submodule is a full-bridge submodule.

[0015] Preferably, the redundancy factor The value range is from 1.1 to 1.3.

[0016] Furthermore, it also includes the design method for the capacitance value of the capacitors in the sub-module, the formula of which is as follows:

[0017] ;

[0018] In the formula, This refers to the capacitance value of the submodule capacitor. This represents the peak value of the energy fluctuation in the submodule. Ripple coefficient;

[0019] in,

[0020] ;

[0021] ;

[0022] ;

[0023] In the formula, to for Mutually The enable output period of the bridge arm, for Mutually Bridge arm voltage, for Mutually Bridge arm current, For the upper bridge arm, For the lower bridge arm, This represents the maximum voltage across the submodule capacitor. This represents the minimum voltage across the submodule capacitor; where, .

[0024] Furthermore, it also includes AC filter inductors. The design method, formula is as follows:

[0025] ;

[0026] In the formula, To consider only the minimum filter inductance required to filter out high-frequency harmonics, This is the reference value for the bridge arm inductance of the MMC.

[0027] Furthermore, it also includes DC filter inductors. The design method, formula is as follows:

[0028] .

[0029] Compared to existing technologies, the technical solution provided by this invention transfers the equivalent impedance of the MASC bridge arm inductor on the AC side to the AC output by setting an AC filter inductor, thus ensuring the dynamic performance of current control. The smoothing reactor and DC filter inductor can limit the fault current rise rate, which helps to ensure fault ride-through capability and equipment safety. The number of sub-modules in the bridge arm is designed based on the redundancy factor, which can leave a certain margin while ensuring the bridge arm's ability to withstand DC voltage, and ensure the stability of the control current and the current limiting capability under transient DC faults, which helps to reduce potential safety hazards. In addition, compared with traditional MMC, the MASC constrained by the number of sub-modules is smaller in size, generates less heat during operation, and its small size makes it easier to arrange and set up with other electrical components in space. Attached Figure Description

[0030] Figure 1This is a schematic diagram of the topology of MASC in one embodiment of the present invention.

[0031] Figure 2 This is a schematic diagram of the energy fluctuation of MASC in one embodiment of the present invention.

[0032] Figure 3 This is a schematic diagram illustrating the peak energy difference between the sub-modules of MASC and traditional MMC in one embodiment of the present invention.

[0033] Figure 4 This is an equivalent discharge circuit diagram of the instantaneous occurrence of a DC inter-electrode fault in one embodiment of the present invention. Detailed Implementation

[0034] The technical solutions provided by the present invention will be further described in detail below with reference to the embodiments and accompanying drawings.

[0035] like Figure 1 As shown, in the MASC provided in this embodiment, each phase unit is divided into an upper bridge arm and a lower bridge arm, for a total of 6 bridge arms. Each bridge arm consists of... It consists of several sub-modules (SMs) connected in series. Sub-modules can be composed of half-bridge sub-modules, full-bridge sub-modules, or full-half-bridge sub-modules, etc. Figure 1 middle, DC side voltage This is the DC side current. for AC phase voltage on the output side of the phase bridge arm, for Voltage of the upper bridge arm, for Current in the upper bridge arm, for Voltage of the lower bridge arm, for Current in the lower bridge arm, For smoothing reactors, For AC filter inductors, It is a DC filter inductor. For the communication side Phase-to-phase voltage, For the communication side Phase current. The two ends of the phase unit serve as the positive and negative terminals of the DC side, and the midpoint is connected to the AC filter inductor on the AC side; a smoothing reactor and a DC filter inductor are connected in series at the positive terminal of the DC side.

[0036] Define the communication side Phase voltage and Phase current, the formula is as follows:

[0037] ;

[0038] ;

[0039] ;

[0040] In the formula, The amplitude of the AC voltage. The amplitude of the alternating current. for The initial phase of the phase voltage, It is the power frequency angular frequency. The power factor angle.

[0041] 1. Submodule Quantity Parameter Design Method

[0042] In MMC, each bridge arm should be able to withstand the full DC-side voltage. Furthermore, a certain margin is reserved. The principle for determining the number of MASC bridge arm submodules is similar to that of MMC bridge arm submodules, but there are differences. Due to the different operating modes, additional redundancy needs to be set, and the redundancy coefficient of the submodule is defined as... The number of submodules in the bridge arm can be obtained. The design method, formula is as follows:

[0043] ;

[0044] In the formula, This is the rated voltage of the submodule capacitor.

[0045] To ensure reliable switch-off of the bridge arm, considering factors such as converter cost and size, the redundancy factor is... It can be set to 1.1 to 1.3.

[0046] 2. Submodule Quantity Parameter Design Method

[0047] The minimum capacitance value of the submodule capacitor is related to the maximum energy fluctuation within the fundamental cycle. The voltage fluctuation of the MASC capacitor varies under different operating conditions, and the scenario with the maximum voltage fluctuation needs to be considered when taking submodule capacitor parameters into account. However, excessively increasing the capacitance will significantly increase the size and cost of the converter. Therefore, it is necessary to calculate the energy fluctuation of the MASC and determine the required minimum capacitance value based on the acceptable maximum voltage fluctuation. The MASC can also be calculated using a method similar to that of the traditional MMC.

[0048] The submodule conserves energy within the power frequency cycle; therefore, the peak-to-peak value of the submodule capacitor voltage depends on the instantaneous peak-to-peak value of the energy. Let the maximum energy fluctuation of the submodule be... The formula is as follows:

[0049] ;

[0050] In the formula, E Max Peak energy E Min This is the energy valley value. The equivalent capacitance of the cascaded submodules. This represents the maximum voltage across the submodule capacitor. This is the minimum voltage across the submodule capacitor.

[0051] The voltage fluctuation of the submodule capacitor is typically 5% of the rated value. To calculate the fluctuation range of the capacitor voltage deviating from its DC component, a ripple factor is used. This indicates the amplitude of the fluctuation component and the rated voltage of the submodule capacitor. The ratio is expressed by the following formula:

[0052] ;

[0053] The rated voltage of the submodule capacitor can be expressed as:

[0054] ;

[0055] Therefore, rewriting the maximum energy fluctuation The formula is as follows:

[0056] .

[0057] Given a limited capacitance value, choose the smallest possible capacitor to meet the ripple factor requirements. The limit requirements are as follows. In summary, the capacitance value of a single submodule capacitor can be obtained. The selection formula is as follows:

[0058] ;

[0059] In the formula, This represents the peak value of the energy fluctuation of the submodule. Since the energy fluctuation of the submodules in the bridge arm is a piecewise function, it is difficult to directly determine the peak value. Since the analytical solution is not available, a numerical method is used to solve it, as shown in the following formula:

[0060] ;

[0061] In the formula, to for Mutually The enable output period of the bridge arm, for Mutually Bridge arm voltage, for Mutually Bridge arm current, For the upper bridge arm, For the lower bridge arm, among which, .

[0062] Based on the above formula, the energy fluctuation waveform of the MASC bridge arm under arbitrary modulation and power factor angles can be obtained. For example... Figure 2 The figure shows the waveforms of MASC energy fluctuations at different power factor angles when the modulation index M=0.9. Among them, This represents the maximum energy difference of MASC within the fundamental period under this operating condition.

[0063] like Figure 3 The figure shows the peak energy difference distribution between the submodules of the traditional MMC and MASC at different power factor angles when the modulation index M=0.9. As can be seen from the figure, the peak energy difference of the submodules in MASC is significantly lower than that of the traditional MMC. At the rated operating point (power factor angle...) Under these conditions, the peak energy difference between each level of the MMC interconnect submodules Peak energy difference between MASC and each level of interconnected submodules They are respectively:

[0064] ; ;

[0065] As can be seen from the formula, the peak energy difference of the submodule is positively correlated with the capacitance value of the submodule. Therefore, compared with the traditional MMC, the capacitance requirement of MASC can be reduced by about 40.7%, thus significantly reducing the volume.

[0066] 3. AC and DC filter inductor values

[0067] Compared to a conventional MMC, the MASC topology removes the bridge arm inductor structurally. To ensure that the MASC presents the same AC equivalent impedance as the MMC at the grid connection point, thus guaranteeing the same current control dynamic performance, the equivalent impedance of the MMC bridge arm inductor on the AC side needs to be transferred to the MASC's AC output. Since the AC current flows through both the upper and lower bridge arms simultaneously, the bridge arm inductors are connected in parallel with the AC current, and their equivalent impedance is half that of a single bridge arm inductor. Therefore, the AC filter inductor of the MASC... In addition to the standard filtering requirements, an extra inductance equivalent to 0.5 times the MMC bridge arm inductance should be added. The design formula is as follows:

[0068] ;

[0069] In the formula, This is the minimum filter inductance required to filter out high-frequency harmonics (typically 0.02 pu). This is a reference value for the bridge arm inductance of the MMC (typically 0.1~0.2 pu).

[0070] Unlike AC-side inductors, which are primarily based on the equivalent impedance matching principle during steady-state operation, DC filter inductors... The design primarily depends on the current-limiting capability under transient DC faults. In a conventional HB-MMC (Half-Bridge Modular Multilevel Converter), the bridge arm inductance reference value is... In fact, it performs the dual function of suppressing internal circulating current (steady state) and limiting the rate of rise of DC fault current (transient state). For example... Figure 4 As shown, especially at the instant of an inter-pole short-circuit fault on the DC side, the three phase units of the HB-MMC conduct simultaneously, relying on the parallel effect of the three-phase bridge arms and the smoothing reactor to jointly limit the fault current. Among these, The total voltage of the upper and lower bridge arms. This represents the fault current. Due to the three-phase parallel connection, the equivalent current-suppressing inductance of the bridge arm system to the DC fault circuit is 2 / 3 times the bridge arm inductance reference value. For the MASC topology, because its structure eliminates the bridge arm inductor and differs from the HB-MMC's three-phase shared fault current mechanism, MASC exhibits single-phase support characteristics during a DC fault. This means that there are no inductor components within the topology that can limit the rate of rise of the fault current. Therefore, the DC filter inductor of MASC... It needs to be independently operated with the smoothing reactor. The MASC (Multi-Inductor Controller) provides full functionality to limit the rate of rise of fault current. To ensure that the MASC possesses the same fault ride-through capability and equipment safety as the conventional HB-MMC (Hybrid HB-MMC), the equivalent total inductance of the DC fault loop must be identical for both. This requires the MASC's DC filter inductance to be... The inductance of the three-phase parallel bridge arm in the HB-MMC is fully compensated. Then, the DC filter inductor in the MASC... The formula for obtaining the value is as follows:

[0071] .

[0072] Based on this design principle, the fault current limiting capability lost by MASC due to the removal of bridge arm inductance is effectively compensated, and the current rise rate is limited to the same safe range as conventional MMC under DC fault conditions.

[0073] Compared to existing technologies, the technical solution provided by this invention transfers the equivalent impedance of the MASC bridge arm inductor on the AC side to the AC output by setting an AC filter inductor, thus ensuring the dynamic performance of current control. The setting of smoothing reactor and DC filter inductor can limit the fault current rise rate, which helps to ensure fault ride-through capability and equipment safety. The number of sub-modules in the bridge arm is designed based on the redundancy coefficient, which can leave a certain margin while ensuring the bridge arm's ability to withstand DC voltage, and ensure the stability of control current and current limiting capability under transient DC faults, which helps to reduce potential safety hazards. In addition, compared with traditional MMC, the MASC constrained by the number of sub-modules is smaller in size, generates less heat during operation, and its small size makes it easier to arrange and set up with other electrical components in space.

[0074] Furthermore, compared to traditional MMC, the capacitance required for the submodule capacitor in the bridge arm is significantly reduced, which can greatly reduce the size of the MASC, thus reducing construction costs and heat generation, and facilitating spatial arrangement. The constraint on the AC filter inductor helps to ensure the dynamic performance of the MASC's current control; the constraint on the DC filter inductor helps to limit the fault current rise rate, ensuring fault ride-through capability and equipment safety.

Claims

1. A parameter design method for a modular alternating support converter, characterized in that, Includes three phase units; The phase unit includes an upper bridge arm and a lower bridge arm connected in series; each bridge arm includes cascaded... Each submodule; The two ends of the phase unit serve as the positive and negative terminals on the DC side, and the midpoint is connected to the AC filter inductor on the AC side. A smoothing reactor and a DC filter inductor are connected in series on the positive terminal of the DC side. ; The parameter design method includes the number of sub-modules in the bridge arm. The design method is as follows: ； In the formula, This represents the redundancy coefficient of the submodule. DC side voltage This is the rated voltage of the capacitor in the submodule.

2. The parameter design method for a modular alternating support converter according to claim 1, characterized in that, The submodule is a half-bridge submodule.

3. The parameter design method for a modular alternating support converter according to claim 1, characterized in that, The submodule is a full-bridge submodule.

4. The parameter design method for a modular alternating support converter according to claim 1, characterized in that, Redundancy coefficient The value range is from 1.1 to 1.

3.

5. A parameter design method for a modular alternating support converter according to any one of claims 2 to 4, characterized in that, It also includes the design method for the capacitance value of capacitors in the submodule, the formula of which is as follows: ； In the formula, This refers to the capacitance value of the submodule capacitor. This represents the peak value of the energy fluctuation in the submodule. Ripple coefficient; in, ； ； ； In the formula, to for Mutually The enable output period of the bridge arm, for Mutually Bridge arm voltage, for Mutually Bridge arm current, For the upper bridge arm, For the lower bridge arm, This represents the maximum voltage across the submodule capacitor. This represents the minimum voltage across the capacitor in the submodule; where, .

6. The parameter design method for a modular alternating support converter according to claim 1, characterized in that, Also includes AC filter inductors The design method is as follows: ； In the formula, To consider only the minimum filter inductance required to filter out high-frequency harmonics, This is the reference value for the bridge arm inductance of the MMC.

7. The parameter design method for a modular alternating support converter according to claim 1 or 6, characterized in that, Also includes DC filter inductors The design method is as follows: 。

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