CN118036534A - Hybrid bridge arm multiplexing MMC simulation model construction method and simulation method - Google Patents

Hybrid bridge arm multiplexing MMC simulation model construction method and simulation method Download PDF

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CN118036534A
CN118036534A CN202410440341.4A CN202410440341A CN118036534A CN 118036534 A CN118036534 A CN 118036534A CN 202410440341 A CN202410440341 A CN 202410440341A CN 118036534 A CN118036534 A CN 118036534A
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bridge arm
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mmc
current
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CN118036534B (en
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王琛
余欣洋
王毅
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North China Electric Power University
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Abstract

The invention belongs to the technical field of converters, and provides a method for constructing a hybrid bridge arm multiplexing MMC simulation model and a simulation method, wherein a flywheel diode and a sub-module locking switch are introduced while the bridge arm output voltage is equivalent, and the proposed multi-state average value model considers the switching state and the locking state of a converter sub-module, so that the simulation operation efficiency is obviously improved while the transient characteristics of HAM-MMC during normal operation and fault ride-through are accurately reflected. The invention is used as a high-efficiency converter modeling research method, has practical value in researching a plurality of operation conditions such as HAM-MMC normal operation, fault ride-through, charging start and the like, can be popularized and applied to different simulation platforms and even real-time simulation systems, and can remarkably reduce the modeling complexity and operation calculation amount of a large-scale HAM-MMC simulation system.

Description

Hybrid bridge arm multiplexing MMC simulation model construction method and simulation method
Technical Field
The invention belongs to the technical field of converters, and particularly relates to a hybrid bridge arm multiplexing MMC simulation model construction method and a simulation method.
Background
The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.
By virtue of the advantages of high output voltage quality, easiness in expansion, redundancy fault-tolerant operation capability and the like, the modularized multi-level converter (modular multilevel converter, MMC) has been widely applied to the flexible direct-current transmission technology. With the continuous improvement of voltage class and capacity, the MMC conversion platform has high cost and large volume, and the defects are increasingly highlighted. The publication number CN112152496A discloses a bridge arm multiplexing MMC (AM-MMC), which performs time division multiplexing by dividing multiplexing bridge arms, and the AM-MMC can reduce the assembly number of sub-modules by at least 25% compared with the MMC with the same voltage level, thereby realizing light weight.
In order to provide the AM-MMC with dc short-circuit current blocking capability, literature Wang Chen, tao Jianye, wang Yi, etc. the topology and fault ride through strategy study of the half-bridge-full-bridge submodule hybrid bridge arm multiplexing MMC [ J ] chinese motor engineering journal, 2022, 42 (22): 8297-8309. Two hybrid AM-MMC (hybrid AM-MMC, HAM-MMC) types, FHF and HFH, HAM-MMC, which are composed of half-bridge submodules (half bridge submodule, HBSM) and full-bridge submodules (full bridge submodule, FBSM), are proposed.
The inventor finds that the operation characteristics of the FHF type HAM-MMC and the HFH type HAM-MMC can be researched by using an electromagnetic transient simulation tool, but for the HAM-MMC actually applied in engineering, the number of submodules contained in a bridge arm of the HAM-MMC is huge, the operation efficiency of a simulation model of the HAM-MMC is low due to a complex circuit structure and a large amount of data operation requirements, and the working characteristics of the submodules in a locking state cannot be reflected by the conventional AM-MMC average circuit model.
Disclosure of Invention
In order to solve at least one technical problem in the background art, the invention provides a hybrid bridge arm multiplexing MMC simulation model construction method and a simulation method, and provides a high-efficiency circuit modeling method with the capability of reflecting the locking state of a sub-module, which can accelerate the simulation operation speed on the basis of accurately retaining the characteristics of HAM-MMC.
In order to achieve the above purpose, the present invention adopts the following technical scheme:
The first aspect of the invention provides a method for constructing a hybrid bridge arm multiplexing MMC simulation model, which comprises the following steps:
The switching process of the submodules of the converter is equivalent under the normal running condition, and the mathematical relationship among bridge arm reference voltage, output voltage and current and capacitance voltage is obtained;
considering current paths of HBSM and FBSM under the locking condition respectively, combining mathematical relations of bridge arm reference voltage, output voltage current and capacitance voltage, introducing a freewheel diode and a submodule locking switch while carrying out equivalence on the bridge arm output voltage, and designing a single bridge arm average model formed by HBSM or FBSM;
based on a single bridge arm average model formed by HBSM or FBSM, fusing the multiplexing mode of the bridge arm into the average input proportion of the bridge arm, and establishing a corresponding multi-state average model of FHF type mixed bridge arm multiplexing MMC or HFH type mixed bridge arm multiplexing MMC.
Further, the step of performing equivalence on the switching process of the submodule of the converter under the normal operation condition to obtain the mathematical relationship among the bridge arm reference voltage, the output voltage current and the capacitor voltage comprises the following steps:
Calculating to obtain the number of the submodules opened at any moment of the bridge arm according to the reference voltage of the bridge arm and the rated voltage of the submodule capacitor;
Defining the ratio of the number of the sub-modules of the bridge arm which are opened at any moment to the number of the sub-modules of the bridge arm as the input proportion of the sub-modules of the bridge arm;
Continuously numbering sub-modules in the bridge arm from 1 to N/2, wherein the number of the sub-module which is put into at any moment belongs to a set L 0, the number of the sub-module which is bypassed belongs to a set L 1, and listing a first-order differential equation set by combining the relation among the capacitance value of the single sub-module, the capacitance voltage of the kth sub-module and the bridge arm current;
and summing N/2 equations corresponding to the first-order differential equation set, and combining the input proportion of the submodules to obtain the output voltage of the bridge arm under the condition that the capacitance voltages of all the submodules in the bridge arm are completely the same.
Further, the mathematical relationship among the bridge arm reference voltage, the output voltage current and the capacitor voltage is as follows:
，
，
Wherein, C is the capacitance value of a single submodule, each bridge arm comprises N/2 series submodules, m is the input proportion of the submodules in the bridge arm, u c is the sum of the capacitance voltages of all the submodules in the bridge arm, i is the bridge arm current, Time, d is the differential operator.
Further, the current paths of HBSM and FBSM in the locked condition are respectively:
When HBSM is locked, the discharging path of the submodule capacitor is blocked by the diode D 1, when the current i is larger than 0, the current charges the submodule capacitor through the diode D 1, and when the current i is smaller than 0, the submodule capacitor is bypassed by the diode D 2, and the capacitor cannot be charged;
When the FBSM is latched, diodes D 1,D2,D3 and D 4 form a full bridge rectifier circuit, the submodule capacitor can be charged regardless of whether the current i is less than 0 or greater than 0, and the discharge path of the capacitor is blocked.
Further, in combination with the mathematical relationship among the bridge arm reference voltage, the output voltage current and the capacitor voltage, the flywheel diode and the sub-module latching switch are introduced while the bridge arm output voltage is equivalent, and the design HBSM forms an equivalent circuit of the bridge arm, including:
based on the fluctuation condition of the capacitor voltage u c when the bridge arm output voltage mu c is represented by a controlled voltage source and the controlled current source with the current mi charges the capacitor with the capacitance value of (2C)/N;
Diode D 1、D2 and latching switches S 1 and S 2 are introduced into an equivalent controlled power supply loop of HBSM bridge arms;
When the converter works normally and the submodules are in a switching state, the switches S 1 and S 2 are closed, at the moment, the diode D 1、D4 is bypassed, the diodes D 2 and D 3 cannot be conducted due to bearing back pressure, a controlled voltage source is connected to the circuit to reflect the switching state of the submodules in the bridge arm, and the capacitor can be charged or discharged under the action of the controlled current source;
When the bridge arm submodules are locked, the switches S 1 and S 2 are turned off, the proportionality coefficient m is fixed to be 1, the bridge arm port voltage u is equal to the controlled voltage source voltage mu c only when the bridge arm current i is larger than 0, the bridge arm port voltage u is 0 in other cases, the diode D 4 blocks the discharging path of the capacitor, meanwhile, the diode D 3 provides a path for the negative current of the controlled current source, and the controlled current source mi can charge the capacitor only when the value is positive.
Further, in combination with the mathematical relationship among the bridge arm reference voltage, the output voltage current and the capacitor voltage, a freewheel diode and a sub-module latching switch are introduced while the bridge arm output voltage is equivalent, and the FBSM is designed to form an equivalent circuit of the bridge arm, including:
based on the fluctuation condition of the capacitor voltage u c when the bridge arm output voltage mu c is represented by a controlled voltage source and the controlled current source with the current mi charges the capacitor with the capacitance value of (2C)/N;
The equivalent controlled power supply loop of the FBSM bridge arm is introduced with 8 diodes D 1-D8 and locking switches S 1,S2,S3 and S 4;
When the converter works normally and the bridge arm submodules are in a switching state, the switches S 1、S2、S3 and S 4 are closed, at the moment, the diodes D 1、D4、D5 and D 8 are bypassed, the diodes D 2、D3、D6 and D 7 bear back pressure and cannot be conducted, a controlled voltage source is connected to a circuit, and the capacitor is charged or discharged under the action of a controlled current source;
When the bridge arm submodule is locked, the switches S 1、S2、S3 and S 4 are disconnected, the proportionality coefficient m is fixed to be 1, the diodes D 1、D2、D3 and D 4 form a full-bridge rectifying circuit, when the bridge arm current i is positive, the bridge arm port voltage u is equal to the controlled voltage source voltage mu c, otherwise, the bridge arm port voltage u is equal to the opposite number of mu c, the diodes D 5、D6、D7 and D 8 form the full-bridge rectifying circuit, the discharging loop of the capacitor is blocked, and the current mi charges the capacitor.
Further, the single bridge arm average model based on HBSM or FBSM is used for fusing the multiplexing mode of the bridge arm into the average input proportion of the bridge arm, and the establishment of the multi-state average model of the FHF type mixed bridge arm multiplexing MMC or the multi-state average model of the HFH type mixed bridge arm multiplexing MMC comprises the following steps:
combining bridge arm output voltages in a single bridge arm average model formed by HBSM or FBSM to obtain a voltage loop equation in an upper bridge arm multiplexing mode and a voltage loop equation in a lower bridge arm multiplexing mode;
The average input proportion of the multiplexing bridge arm is equivalent to the linear combination relation of the multiplexing average input proportion on the multiplexing bridge arm and the multiplexing average input proportion under the multiplexing bridge arm according to the multiplexing mode;
And the voltage loop equation and the linear combination relation are used for obtaining a multi-state average model of the FHF type mixed bridge arm multiplexing MMC or a multi-state average model of the HFH type mixed bridge arm multiplexing MMC.
Further, the linear combination relationship is:
，
，
，
Wherein m am is the average input ratio of the multiplexing bridge arm, m amu is the average input ratio of multiplexing on the multiplexing bridge arm, and m aml is the average input ratio of multiplexing under the multiplexing bridge arm.
The second aspect of the invention provides a hybrid bridge arm multiplexing MMC simulation method, which is applied to a multi-state average model of FHF hybrid bridge arm multiplexing MMC, and comprises the following steps:
determining an initial capacitance voltage value;
Judging whether the FHF type mixed bridge arm multiplexing MMC submodule is in a locking state, if so, completely switching off locking switches in a multi-state average value model of the FHF type mixed bridge arm multiplexing MMC, and fixing the average input proportion of each bridge arm after a multiplexing mode instruction is acquired; if not, closing all the locking switches in the FHF mixed bridge arm multiplexing MMC multi-state average model, acquiring a level control reference voltage and multiplexing mode instructions, and determining the average input proportion of each bridge arm according to the level control reference voltage, the rated voltage of the submodule capacitor and the number of the series submodules of the bridge arm;
determining the upper multiplexing average input proportion and the lower multiplexing average input proportion of the multiplexing bridge arm according to the average input proportion and the multiplexing mode instruction of the multiplexing bridge arm;
Based on the obtained average input proportion of the upper bridge arm and the lower bridge arm, the multiplexing average input proportion of the multiplexing bridge arm and the lower multiplexing average input proportion, the voltage and current values of the controlled source are combined, and the voltage and current of the multi-state average model of the FHF type mixed bridge arm multiplexing MMC are simulated and calculated.
The third aspect of the present invention provides a hybrid bridge arm multiplexing MMC simulation method, which is applied to a multi-state average model of an HFH hybrid bridge arm multiplexing MMC, and includes the following steps:
determining an initial capacitance voltage value;
Judging whether the HFH type mixed bridge arm multiplexing MMC submodule is in a locking state, if so, completely switching off locking switches in a multi-state average value model of the HFH type mixed bridge arm multiplexing MMC, and fixing the average input proportion of each bridge arm after a multiplexing mode instruction is acquired; otherwise, closing all locking switches in the HFH type mixed bridge arm multiplexing MMC multi-state average value model, acquiring a level control reference voltage and a multiplexing mode instruction, and determining the average input proportion of each bridge arm according to the level control reference voltage, the rated voltage of a submodule capacitor and the number of series submodules of the bridge arm;
determining the upper multiplexing average input proportion and the lower multiplexing average input proportion of the multiplexing bridge arm according to the average input proportion and the multiplexing mode instruction of the multiplexing bridge arm;
And based on the obtained average input proportion of the upper bridge arm and the lower bridge arm, the multiplexing average input proportion of the multiplexing bridge arm and the lower multiplexing average input proportion, and combining the voltage and current values of the controlled source, carrying out simulation calculation on the voltage and current of the multi-state average model of the HFH type mixed bridge arm multiplexing MMC.
Compared with the prior art, the invention has the beneficial effects that:
The invention builds a multi-state average model of FHF type and HFH type HAM-MMC, carries out multi-state equivalence on a single bridge arm by analyzing the working characteristics of HBSM and FBSM during normal switching and locking, eliminates the bridge arm switching process by analyzing the alternate conduction characteristics of HAM-MMC, and finally designs and discloses the use flow of the multi-state average model. Compared with the traditional AM-MMC average value model, the multi-state average value model provided by the invention can accurately reflect the states of the submodule during normal input cutting and locking while improving the simulation efficiency to the greatest extent, has the characteristics of high simplicity degree, simple locking/unlocking state switching and the like, has practical value when researching various operation working conditions such as HAM-MMC normal operation, fault ride-through, charging starting and the like, can be popularized and applied to different simulation platforms and even real-time simulation systems, and can remarkably reduce the modeling complexity and operation calculation amount of a large-scale HAM-MMC simulation system.
Additional aspects of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the invention.
Fig. 1 is a topology of HBSM and FBSM provided by an embodiment of the present invention;
FIG. 2 shows an FHF-type HAM-MMC according to an embodiment of the present invention;
FIG. 3 is a HFH-HAM-MMC provided by an embodiment of the present invention;
Fig. 4 is a multi-state equivalent circuit of HBSM bridge arms provided by an embodiment of the present invention;
Fig. 5 is a multi-state equivalent circuit of an FBSM bridge arm provided by an embodiment of the present invention;
FIG. 6 is a multi-state average model of FHF type HAM-MMC provided by an embodiment of the present invention;
FIG. 7 is a multi-state average model of HFH-HAM-MMC provided by an embodiment of the present invention;
FIG. 8 is a simulation flow of a multi-state average model of HAM-MMC provided by an embodiment of the present invention;
FIG. 9 is a single-ended HAM-MMC simulation model structure provided by an embodiment of the present invention;
FIG. 10 is a comparison of transient characteristics of an average value model and a switch model of FHF-type HAM-MMC provided by an embodiment of the present invention; in fig. 10, (a) is direct current response comparison, and in fig. 10, (b) is capacitance voltage and response comparison of an a-phase upper bridge arm submodule;
FIG. 11 is a comparison of transient characteristics of an average value model and a switch model of an HFH-type HAM-MMC provided by an embodiment of the present invention; in fig. 11, (a) is a direct current response comparison, and in fig. 11, (b) is an a-phase multiplexing arm submodule capacitor voltage and response comparison.
Detailed Description
The invention will be further described with reference to the drawings and examples.
It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the invention. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments according to the present invention. As used herein, the singular is also intended to include the plural unless the context clearly indicates otherwise, and furthermore, it is to be understood that the terms "comprises" and/or "comprising" when used in this specification are taken to specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof.
The invention provides a multi-state average value model comprising FHF type HAM and HFH type HAM, wherein a freewheeling diode and a sub-module locking switch are introduced while the output voltage of a bridge arm is equivalent, and the multi-state average value model considers the switching state and the locking state of a converter sub-module, so that the simulation operation efficiency is obviously improved while the transient characteristics of the HAM-MMC during normal operation and fault ride-through are accurately reflected. The invention has practical value when researching a plurality of operation conditions such as HAM-MMC normal operation, fault ride-through, charging start and the like as a high-efficiency converter modeling research method, and the obtained results can be popularized and applied to different simulation platforms and even real-time simulation systems, so that the modeling complexity and operation calculation amount of a large-scale HAM-MMC simulation system can be remarkably reduced.
Example 1
The embodiment provides a method for constructing a hybrid bridge arm multiplexing MMC simulation model, which comprises the following steps:
Step 1: the switching process of the submodules of the converter is equivalent under the normal running condition, and the mathematical relationship among bridge arm reference voltage, output voltage and current and capacitance voltage is obtained;
Step 2: considering current paths of HBSM and FBSM under the locking condition respectively, combining mathematical relations of bridge arm reference voltage, output voltage current and capacitance voltage, introducing a freewheel diode and a submodule locking switch while carrying out equivalence on the bridge arm output voltage, and designing a single bridge arm average model formed by HBSM or FBSM;
Step 3: based on a single bridge arm average model formed by HBSM or FBSM, fusing the multiplexing mode of the bridge arm into the average input proportion of the bridge arm, and establishing a corresponding multi-state average model of FHF type mixed bridge arm multiplexing MMC or HFH type mixed bridge arm multiplexing MMC.
Through the technical scheme, the flywheel diode and the sub-module locking switch are introduced while the bridge arm output voltage is equivalent, and the switching state and the locking state of the converter sub-module are considered by the multi-state average value model, so that the simulation operation efficiency is obviously improved while the transient characteristics of the HAM-MMC during normal operation and fault ride-through are accurately reflected.
As shown in fig. 1, which shows the topological structures of HBSM and FBSM, the basic functions of the two sub-modules can be equivalent to each other when the converter is operating normally, and the sub-module capacitor C can be put into the circuit for charging and discharging or bypass;
the FHF-HAM-MMC topology is shown in figure 2, wherein the FHF-HAM-MMC is configured with FBSM on the upper and lower bridge arms and multiplexed with bridge arm configuration HBSM;
HFH-type HAM-MMC topology as shown in FIG. 3, HFH-type HAM-MMC is configured HBSM on the upper and lower arms, and FBSM is configured on the multiplexing arm.
Each phase unit is connected to an alternating current power grid through upper and lower bridge arm change-over switches K j1 and K j2 (j=a, b, c) with complementary on-off states, and when K j1 is conducted and K j2 is disconnected, the phase units are in a lower bridge arm multiplexing mode; when K j1 is off and K j2 is on, the phase unit is in the upper bridge arm multiplexing mode. The change-over switch has various circuit design structures, but the nature is equivalent to an ideal controlled switch capable of blocking bidirectional current flow.
Each phase unit of the AM-MMC is composed of an upper bridge arm u, a multiplexing bridge arm m, a lower bridge arm L, two bridge arm inductances L and two bridge arm switches K j1、Kj2, each bridge arm contains N/2 series sub-modules, where N can be calculated by the following formula:
（1），
Wherein: u dc is the direct current rated voltage, U cN is the rated voltage of the submodule capacitor, and both are constants; to ensure that the number of submodules turned on per phase element is constant, U cN typically takes a voltage value that causes N to be configured as an even number. By configuring the fault current limiting sub-module, the AM-MMC can have direct current short circuit current blocking capability.
Each bridge arm of the HAM-MMC is composed of N/2 HBSM or FBSM connected in series, and under different modulation methods, the reference voltages obtained by each bridge arm in the phase unit may be different, but the ac valve side voltages of the synthesized output are the same.
The switching process of the submodules is equivalent under the normal running condition of the converter, and if the reference voltage of a certain bridge arm is u ref, the number n of the submodules which are opened at any time of the bridge arm can be expressed as:
（2），
Where round (x) is a rounding function and the value returned is the integer nearest to x. The ratio of the number n of the submodules opened at any moment to the number of the bridge arm submodules is defined as an input proportion m in, namely:
（3），
under the condition that overmodulation is not considered, m in represents N/2+1 discrete values in [0,1], and the change trend of m in is approximately continuous in consideration of the fact that the number of bridge arm submodules in actual engineering is large, and for further simplification, the round function in the formula (3) is omitted to obtain an average input proportion m as follows:
（4），
the input ratio of the bridge arm submodules can be approximately represented by m in the equation (4).
Continuously numbering submodules in the bridge arm from 1 to N/2, wherein the submodule numbers input at any moment belong to a set L 0, the submodule numbers bypassed belong to a set L 1, and if the current passing through the bridge arm is i, the following first-order differential equation set can be listed:
（5），
Wherein, C is the capacitance value of a single submodule, u ck (k=1, 2, …, N/2) is the capacitance voltage of the kth submodule, and i is the bridge arm current. Summing the corresponding N/2 equations in the formula (5) and combining the expression of the input proportion m in the formula (4) to obtain
(6)，
Where u c is the sum of the capacitance voltages of all the sub-modules in the bridge arm.
Dividing both sides of the formula (6) by N/2 at the same time to obtain:
(7)，
If the effect of balancing the capacitance and the voltage of the submodules in the bridge arm is considered to be ideal, that is, the capacitance and the voltage u ck of all the submodules in the bridge arm are completely the same, the output voltage u of the bridge arm can be expressed as
(8)，
The formulas (7) and (8) establish mathematical relations among bridge arm reference voltage, output voltage current and capacitance voltage, namely, the switching operation of submodule switching is eliminated, N/2 submodules in the bridge arm are regarded as a whole, and the formulas are universal for the bridge arm formed by HBSM or FBSM in normal operation.
However, when the sub-modules are required to be in a locking state during the starting or failure of the converter, the bridge arms formed by the sub-modules of different types show different transient characteristics due to different charge and discharge paths of HBSM and the FBSM in the locking state, and the current paths of HBSM and the FBSM in the locking state need to be considered respectively.
In the locked state, the controllable switching devices in the submodules are all locked, and current can be conducted only by the diodes, as shown in fig. 1, when HBSM is locked, a discharging path of the submodule capacitor is blocked by the diode D 1, when the current i is greater than 0, the current charges the submodule capacitor through the diode D 1, and when the current i is less than 0, the submodule capacitor is bypassed by the diode D 2, and the capacitor cannot be charged; when the FBSM is latched, diodes D 1,D2,D3 and D 4 form a full bridge rectifier circuit, the submodule capacitor can be charged regardless of whether the current i is less than 0 or greater than 0, and the discharge path of the capacitor is blocked. According to the principle, equivalent circuits of HBSM or FBSM constituent bridge arms can be respectively designed.
HBSM constitute an equivalent circuit of the bridge arm as shown in fig. 4. The bridge arm output voltage mu c represented by equation (8) is represented by a controlled voltage source, and equation (7) represents the fluctuation of the capacitor voltage u c when the capacitor with the capacitance value (2C)/N is charged by the controlled current source with the current mi. On the basis of the above, a diode D 1、D2 and latching switches S 1 and S 2 are introduced in the equivalent controlled power supply loop of HBSM bridge arms.
When the converter works normally and the submodules are in a switching state, the switches S 1 and S 2 are closed, at the moment, the diode D 1、D4 is bypassed, the diodes D 2 and D 3 cannot be conducted due to bearing back pressure, a controlled voltage source is connected to the circuit to reflect the switching state of the submodules in the bridge arm, and the capacitor can be charged or discharged under the action of the controlled current source; when the bridge arm sub-module is locked, the switches S 1 and S 2 are turned off, the proportionality coefficient m is fixed to be 1, the bridge arm port voltage u is equal to the controlled voltage source voltage mu c only when the bridge arm current i is larger than 0, the bridge arm port voltage u is 0 in other cases, the diode D 4 blocks the discharging path of the capacitor, meanwhile, the diode D 3 provides a path for the negative current of the controlled current source, and the controlled current source mi can charge the capacitor only when the value is positive.
An equivalent circuit of the FBSM constituting the bridge arm is shown in fig. 5. The definition of the equivalent capacitance of the controlled voltage source and the controlled current source is completely consistent with the value and the equivalent circuit of the HBSM bridge arm.
On the basis of the above, 8 diodes D 1、D2、…、D8 and blocking switches S 1,S2,S3 and S 4 are introduced in the equivalent controlled power supply loop of the FBSM bridge arm. When the converter works normally and the submodules are in a switching state, the switches S 1、S2、S3 and S 4 are closed, at the moment, the diodes D 1、D4、D5 and D 8 are bypassed, the diodes D 2、D3、D6 and D 7 cannot be conducted due to bearing back pressure, a controlled voltage source is connected into a circuit so as to reflect the switching state of the submodules in the bridge arm, and the capacitor can be charged or discharged under the action of the controlled current source; when the converter fails or starts, i.e. the submodule is locked, the switches S 1、S2、S3 and S 4 are opened, the proportionality coefficient m is fixed to be 1, the diodes D 1、D2、D3 and D 4 form a full-bridge rectifying circuit, when the bridge arm current i is positive, u is equal to the controlled voltage source voltage mu c, and conversely u is equal to the opposite number of mu c, the diodes D 5、D6、D7 and D 8 also form the full-bridge rectifying circuit, the discharging loop of the capacitor is blocked, and the current mi can only charge the capacitor regardless of the positive and negative directions.
Based on the established single bridge arm average model formed by HBSM or FBSM, a multi-state average model of the three-phase HAM-MMC can be established. Let m xj (x=a, b, c, j=u, m, l) be the average input ratio of the x-phase j bridge arm, u cxj (x=a, b, c, j=u, m, l) be the sum of the capacitance voltages of all the sub-modules of the x-phase j bridge arm, i xj (x=a, b, c, j=u, m, l) be the current of the x-phase j bridge arm, and take the a-phase as an example, the voltage loop equation in the upper bridge arm multiplexing mode can be listed as:
(9)，
Similarly, in the lower bridge arm multiplexing mode, the voltage loop equation is
(10)，
In the formulas (9) and (10), U dc is a direct current bus voltage value, and L is a bridge arm inductance value.
When the HAM-MMC works, multiplexing bridge arms switch multiplexing modes every half power frequency period, namely the multiplexing bridge arms and the upper bridge arms are equivalent to the multiplexing of the upper bridge arms in half power frequency period to output voltage in series; and multiplexing the bridge arm and the lower bridge arm to output voltage in series in a half power frequency period of multiplexing of the lower bridge arm. In this way, the average input proportion m am of the multiplexing bridge arm can be equivalently regarded as the linear combination of the multiplexing average input proportion m amu on the multiplexing bridge arm and the multiplexing average input proportion m aml under the multiplexing bridge arm according to the multiplexing mode, namely:
(11)，
M amu and m aml in formula (11) can be defined as respectively in multiplexing mode
(12)，
(13)，
Combining equations (11), (12) and (13), HAM-MMC half-cycle voltage loop equations (9) and (10) can be equivalent to a set of equations, namely:
(14)，
Equation (14) eliminates arm switching operation of HAM-MMC by embodying the multiplexing mode in the average input ratio. And similarly, the capacitance-voltage differential equation of the formula (7) is expanded to three bridge arms, so that the following can be obtained:
(15)，
wherein C E is equivalent capacitance, and the value is (2C)/N.
And combining the equation (14), the equation (15) and the bridge arm multi-state equivalent method, wherein the multi-state average model of FHF type HAM-MMC and HFH type HAM-MMC is shown in the figures 6 and 7 respectively, and the multi-state average model of B, C two phases can be obtained by replacing a in the subscript letter of the A phase variable in the figures 6 and 7 with b or c.
Considering that the states of the submodules are generally uniform during locking or normal switching, the labels and states of the locking switches S of each bridge arm are designed to be the same reference number, after a multi-state average model circuit of the HAM-MMC is built according to system parameters, the initial value u cxj of the capacitance voltage sum of the submodules of the bridge arm needs to be determined firstly, when the converter works normally, namely the submodules are in the normal switching state, all the locking switches S are closed, the average input coefficient m xj of each bridge arm can be determined by a formula (4), and m xmu and m xml (x=a, b, c) are determined according to formulas (12) and (13); similarly, when the submodules of the converter are in a locked state, all the locking switches S are turned off, the average input coefficient m xj of each bridge arm is fixed to be 1, and then m xmu and m xml are determined according to formulas (12) and (13). In summary, the flow of efficient simulation using the proposed multi-state average model of HAM-MMC is shown in fig. 8.
In order to verify the effectiveness of the proposed multi-state average model of the HAM-MMC, a single-ended HAM-MMC average and a switch model as shown in fig. 9 were respectively built based on MATLAB/Simulink, and the characteristics of the single-ended HAM-MMC average and the switch model in the normal switching state and the direct-current fault blocking state were compared and verified, and the simulated parameters are shown in the following table 1.
TABLE 1 HAM MMC simulation parameters
Setting a direct-current voltage source to be put into the state when the simulation starts, enabling the HAM-MMC to be in a normal running state when the simulation starts, putting the short-circuit resistor R s into the state when the simulation starts for 1.49 seconds, cutting off the voltage source at the same time, locking the submodule when the simulation starts for 1.5 seconds, and enabling the converter to be in a direct-current fault clearing state. Fig. 10 shows a comparison of transient characteristics of the FHF HAM-MMC average model and the switching model, in which (a) in fig. 10 compares transient responses of the dc current, the short circuit current is cleared to 0 after the inverter is blocked for 0.003 seconds, and the current responses of the average model and the switching model are substantially identical; in fig. 10 (b), the sum of the capacitance voltages of the submodules of the upper bridge arm of the a phase is compared, the change trend of the capacitance voltages of the two models before and after the locking of the converter is basically the same, and the capacitance voltage is charged to 6.1kV from 4.9kV when the locking is performed for 1.5 seconds.
FIG. 11 is a graph showing transient characteristics of the HFH-type HAM-MMC average model and the switch model, where (a) in FIG. 11 compares transient responses of DC current, short circuit current is cleared to 0 after 0.005 seconds of converter blocking, and current responses of the average model and the switch model are substantially identical; in fig. 11 (b), the sum of the capacitance voltages of the submodules of the a-phase multiplexing bridge arm is compared, the change trend of the capacitance voltages of the two models before and after the locking of the converter is basically the same, and the capacitance voltage is charged to 6.9kV from 4.9kV when the current is locked for 1.5 seconds.
From the simulation comparison results, the HAM-MMC average value model has multi-state simulation capability, and can accurately keep transient response of normal operation and locking state of the converter.
Example two
The embodiment provides a mixed bridge arm multiplexing MMC simulation method, which is applied to a multi-state average model of FHF mixed bridge arm multiplexing MMC, and comprises the following steps:
determining an initial capacitance voltage value;
Judging whether the FHF type mixed bridge arm multiplexing MMC submodule is in a locking state, if so, completely switching off locking switches in a multi-state average value model of the FHF type mixed bridge arm multiplexing MMC, and fixing the average input proportion of each bridge arm after a multiplexing mode instruction is acquired; if not, closing all the locking switches in the FHF mixed bridge arm multiplexing MMC multi-state average model, acquiring a level control reference voltage and multiplexing mode instructions, and determining the average input proportion of each bridge arm according to the level control reference voltage, the rated voltage of the submodule capacitor and the number of the series submodules of the bridge arm;
determining the upper multiplexing average input proportion and the lower multiplexing average input proportion of the multiplexing bridge arm according to the average input proportion and the multiplexing mode instruction of the multiplexing bridge arm;
Based on the obtained average input proportion of the upper bridge arm and the lower bridge arm, the multiplexing average input proportion of the multiplexing bridge arm and the lower multiplexing average input proportion, the voltage and current values of the controlled source are combined, and the voltage and current of the multi-state average model of the FHF type mixed bridge arm multiplexing MMC are simulated and calculated.
Example III
The embodiment provides a mixed bridge arm multiplexing MMC simulation method, which is applied to a multi-state average model of an HFH-type mixed bridge arm multiplexing MMC, and comprises the following steps:
determining an initial capacitance voltage value;
Judging whether the HFH type mixed bridge arm multiplexing MMC submodule is in a locking state, if so, completely switching off locking switches in a multi-state average value model of the HFH type mixed bridge arm multiplexing MMC, and fixing the average input proportion of each bridge arm after a multiplexing mode instruction is acquired; otherwise, closing all locking switches in the HFH type mixed bridge arm multiplexing MMC multi-state average value model, acquiring a level control reference voltage and a multiplexing mode instruction, and determining the average input proportion of each bridge arm according to the level control reference voltage, the rated voltage of a submodule capacitor and the number of series submodules of the bridge arm;
determining the upper multiplexing average input proportion and the lower multiplexing average input proportion of the multiplexing bridge arm according to the average input proportion and the multiplexing mode instruction of the multiplexing bridge arm;
And based on the obtained average input proportion of the upper bridge arm and the lower bridge arm, the multiplexing average input proportion of the multiplexing bridge arm and the lower multiplexing average input proportion, and combining the voltage and current values of the controlled source, carrying out simulation calculation on the voltage and current of the multi-state average model of the HFH type mixed bridge arm multiplexing MMC.
The above description is only of the preferred embodiments of the present invention and is not intended to limit the present invention, but various modifications and variations can be made to the present invention by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (10)

1. The method for constructing the mixed bridge arm multiplexing MMC simulation model is characterized by comprising the following steps of:
The switching process of the submodules of the converter is equivalent under the normal running condition, and the mathematical relationship among bridge arm reference voltage, output voltage and current and capacitance voltage is obtained;
considering current paths of HBSM and FBSM under the locking condition respectively, combining mathematical relations of bridge arm reference voltage, output voltage current and capacitance voltage, introducing a freewheel diode and a submodule locking switch while carrying out equivalence on the bridge arm output voltage, and designing a single bridge arm average model formed by HBSM or FBSM;
based on a single bridge arm average model formed by HBSM or FBSM, fusing the multiplexing mode of the bridge arm into the average input proportion of the bridge arm, and establishing a corresponding multi-state average model of FHF type mixed bridge arm multiplexing MMC or HFH type mixed bridge arm multiplexing MMC.
2. The method for constructing the hybrid bridge arm multiplexing MMC simulation model according to claim 1, wherein the step of performing the equivalent switching process of the submodule under the normal operation condition of the converter to obtain the mathematical relationship among the bridge arm reference voltage, the output voltage and the current and the capacitance voltage comprises the following steps:
Calculating to obtain the number of the submodules opened at any moment of the bridge arm according to the reference voltage of the bridge arm and the rated voltage of the submodule capacitor;
Defining the ratio of the number of the sub-modules of the bridge arm which are opened at any moment to the number of the sub-modules of the bridge arm as the input proportion of the sub-modules of the bridge arm;
Continuously numbering sub-modules in the bridge arm from 1 to N/2, wherein the number of the sub-module which is put into at any moment belongs to a set L 0, the number of the sub-module which is bypassed belongs to a set L 1, and listing a first-order differential equation set by combining the relation among the capacitance value of the single sub-module, the capacitance voltage of the kth sub-module and the bridge arm current;
and summing N/2 equations corresponding to the first-order differential equation set, and combining the input proportion of the submodules to obtain the output voltage of the bridge arm under the condition that the capacitance voltages of all the submodules in the bridge arm are completely the same.
3. The method for constructing the hybrid bridge arm multiplexing MMC simulation model according to claim 1 is characterized in that the mathematical relationship among the bridge arm reference voltage, the output voltage current and the capacitance voltage is:
，
，
Wherein, C is the capacitance value of a single submodule, each bridge arm comprises N/2 series submodules, m is the input proportion of the submodules in the bridge arm, u c is the sum of the capacitance voltages of all the submodules in the bridge arm, i is the bridge arm current, Time, d is the differential operator.
4. The method for constructing the mixed bridge arm multiplexing MMC simulation model according to claim 1, wherein the current paths of HBSM and the FBSM under the locking condition are respectively as follows:
When HBSM is locked, the discharging path of the submodule capacitor is blocked by the diode D 1, when the current i is larger than 0, the current charges the submodule capacitor through the diode D 1, and when the current i is smaller than 0, the submodule capacitor is bypassed by the diode D 2, and the capacitor cannot be charged;
When the FBSM is latched, diodes D 1,D2,D3 and D 4 form a full bridge rectifier circuit, the submodule capacitor can be charged regardless of whether the current i is less than 0 or greater than 0, and the discharge path of the capacitor is blocked.
5. The method for constructing the mixed bridge arm multiplexing MMC simulation model according to claim 2, wherein the method is characterized in that a flywheel diode and a submodule latching switch are introduced while equivalent is carried out on the bridge arm output voltage by combining mathematical relations of bridge arm reference voltage, output voltage current and capacitance voltage, and an equivalent circuit of the bridge arm is formed by designing HBSM, and the method comprises the following steps:
based on the fluctuation condition of the capacitor voltage u c when the bridge arm output voltage mu c is represented by a controlled voltage source and the controlled current source with the current mi charges the capacitor with the capacitance value of (2C)/N;
Diode D 1、D2 and latching switches S 1 and S 2 are introduced into an equivalent controlled power supply loop of HBSM bridge arms;
When the converter works normally and the submodules are in a switching state, the switches S 1 and S 2 are closed, at the moment, the diode D 1、D4 is bypassed, the diodes D 2 and D 3 cannot be conducted due to bearing back pressure, a controlled voltage source is connected to the circuit to reflect the switching state of the submodules in the bridge arm, and the capacitor can be charged or discharged under the action of the controlled current source;
When the bridge arm submodules are locked, the switches S 1 and S 2 are turned off, the proportionality coefficient m is fixed to be 1, the bridge arm port voltage u is equal to the controlled voltage source voltage mu c only when the bridge arm current i is larger than 0, the bridge arm port voltage u is 0 in other cases, the diode D 4 blocks the discharging path of the capacitor, meanwhile, the diode D 3 provides a path for the negative current of the controlled current source, and the controlled current source mi can charge the capacitor only when the value is positive.
6. The method for constructing the mixed bridge arm multiplexing MMC simulation model according to claim 2, wherein the mathematical relations of the bridge arm reference voltage, the output voltage current and the capacitance voltage are combined, the flywheel diode and the sub-module locking switch are introduced while the bridge arm output voltage is equivalent, and the FBSM is designed to form an equivalent circuit of the bridge arm, comprising:
based on the fluctuation condition of the capacitor voltage u c when the bridge arm output voltage mu c is represented by a controlled voltage source and the controlled current source with the current mi charges the capacitor with the capacitance value of (2C)/N;
The equivalent controlled power supply loop of the FBSM bridge arm is introduced with 8 diodes D 1-D8 and locking switches S 1,S2,S3 and S 4;
When the converter works normally and the bridge arm submodules are in a switching state, the switches S 1、S2、S3 and S 4 are closed, at the moment, the diodes D 1、D4、D5 and D 8 are bypassed, the diodes D 2、D3、D6 and D 7 bear back pressure and cannot be conducted, a controlled voltage source is connected to a circuit, and the capacitor is charged or discharged under the action of a controlled current source;
When the bridge arm submodule is locked, the switches S 1、S2、S3 and S 4 are disconnected, the proportionality coefficient m is fixed to be 1, the diodes D 1、D2、D3 and D 4 form a full-bridge rectifying circuit, when the bridge arm current i is positive, the bridge arm port voltage u is equal to the controlled voltage source voltage mu c, otherwise, the bridge arm port voltage u is equal to the opposite number of mu c, the diodes D 5、D6、D7 and D 8 form the full-bridge rectifying circuit, the discharging loop of the capacitor is blocked, and the current mi charges the capacitor.
7. The method for constructing the hybrid bridge arm multiplexing MMC simulation model according to claim 1, wherein the method for constructing the hybrid bridge arm multiplexing MMC multi-state average model based on the single bridge arm average model formed by HBSM or FBSM, which fuses the multiplexing mode of the bridge arm to the average input ratio of the bridge arm, comprises the steps of:
combining bridge arm output voltages in a single bridge arm average model formed by HBSM or FBSM to obtain a voltage loop equation in an upper bridge arm multiplexing mode and a voltage loop equation in a lower bridge arm multiplexing mode;
The average input proportion of the multiplexing bridge arm is equivalent to the linear combination relation of the multiplexing average input proportion on the multiplexing bridge arm and the multiplexing average input proportion under the multiplexing bridge arm according to the multiplexing mode;
And the voltage loop equation and the linear combination relation are used for obtaining a multi-state average model of the FHF type mixed bridge arm multiplexing MMC or a multi-state average model of the HFH type mixed bridge arm multiplexing MMC.
8. The method for constructing the hybrid bridge arm multiplexing MMC simulation model of claim 7, wherein the linear combination relationship is:
，
，
，
Wherein m am is the average input ratio of the multiplexing bridge arm, m amu is the average input ratio of multiplexing on the multiplexing bridge arm, and m aml is the average input ratio of multiplexing under the multiplexing bridge arm.
9. The mixed bridge arm multiplexing MMC simulation method is applied to a multi-state average model of FHF type mixed bridge arm multiplexing MMC, and is characterized by comprising the following steps:
determining an initial capacitance voltage value;
Judging whether the FHF type mixed bridge arm multiplexing MMC submodule is in a locking state, if so, completely switching off locking switches in a multi-state average value model of the FHF type mixed bridge arm multiplexing MMC, and fixing the average input proportion of each bridge arm after a multiplexing mode instruction is acquired; if not, closing all the locking switches in the FHF mixed bridge arm multiplexing MMC multi-state average model, acquiring a level control reference voltage and multiplexing mode instructions, and determining the average input proportion of each bridge arm according to the level control reference voltage, the rated voltage of the submodule capacitor and the number of the series submodules of the bridge arm;
determining the upper multiplexing average input proportion and the lower multiplexing average input proportion of the multiplexing bridge arm according to the average input proportion and the multiplexing mode instruction of the multiplexing bridge arm;
Based on the obtained average input proportion of the upper bridge arm and the lower bridge arm, the multiplexing average input proportion of the multiplexing bridge arm and the lower multiplexing average input proportion, the voltage and current values of the controlled source are combined, and the voltage and current of the multi-state average model of the FHF type mixed bridge arm multiplexing MMC are simulated and calculated.
10. The mixed bridge arm multiplexing MMC simulation method is applied to a multi-state average model of the HFH-type mixed bridge arm multiplexing MMC, and is characterized by comprising the following steps of:
determining an initial capacitance voltage value;
Judging whether the HFH type mixed bridge arm multiplexing MMC submodule is in a locking state, if so, completely switching off locking switches in a multi-state average value model of the HFH type mixed bridge arm multiplexing MMC, and fixing the average input proportion of each bridge arm after a multiplexing mode instruction is acquired; otherwise, closing all locking switches in the HFH type mixed bridge arm multiplexing MMC multi-state average value model, acquiring a level control reference voltage and a multiplexing mode instruction, and determining the average input proportion of each bridge arm according to the level control reference voltage, the rated voltage of a submodule capacitor and the number of series submodules of the bridge arm;
determining the upper multiplexing average input proportion and the lower multiplexing average input proportion of the multiplexing bridge arm according to the average input proportion and the multiplexing mode instruction of the multiplexing bridge arm;
And based on the obtained average input proportion of the upper bridge arm and the lower bridge arm, the multiplexing average input proportion of the multiplexing bridge arm and the lower multiplexing average input proportion, and combining the voltage and current values of the controlled source, carrying out simulation calculation on the voltage and current of the multi-state average model of the HFH type mixed bridge arm multiplexing MMC.
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