CN108736700B - MMC submodule capacitor voltage static balance control method in off-network inverter circuit starting process - Google Patents

MMC submodule capacitor voltage static balance control method in off-network inverter circuit starting process Download PDF

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CN108736700B
CN108736700B CN201810613615.XA CN201810613615A CN108736700B CN 108736700 B CN108736700 B CN 108736700B CN 201810613615 A CN201810613615 A CN 201810613615A CN 108736700 B CN108736700 B CN 108736700B
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mmc
modules
charging
mmc sub
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CN108736700A (en
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孙向东
解飞飞
任碧莹
陈桂涛
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Xian University of Technology
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M1/00—Details of apparatus for conversion
    • H02M1/36—Means for starting or stopping converters
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
    • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
    • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/53—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M7/537—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
    • H02M7/5375—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters with special starting equipment

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Abstract

The invention discloses a static voltage balance control method for MMC sub-modules in starting of an off-network inverter circuit. The alternating current side of the inverter circuit is an off-grid load, namely when the capacitor of the MMC sub-module is precharged, the charging can be completed only through the bus voltage on the direct current side. The method comprises the following steps: firstly, carrying out uncontrolled pre-charging on all MMC sub-modules for a first preset time length; then, sequencing all the MMC sub-modules according to the capacitance voltage, and carrying out controllable pre-charging on all the sub-modules for a second preset time length according to a certain rule; and finally, switching the system to a normal inversion working condition, and finishing the whole starting of the system. By the method, the problems that the voltage of the capacitor is inconsistent when pre-charging is finished due to inconsistent nominal capacitor parameters of the MMC sub-modules, so that a large surge current can appear on the direct current side when the inverter circuit is put into normal operation, the safety of an MMC system is influenced, the voltage balance control of the capacitor of the MMC sub-modules in the normal operation process is not facilitated and the like are mainly solved.

Description

MMC submodule capacitor voltage static balance control method in off-network inverter circuit starting process
Technical Field
The invention relates to the technical field of power electronics, in particular to a static voltage balance control method for an MMC sub-module capacitor in starting of an off-grid inverter circuit.
Background
In recent years, Modular Multilevel Converters (MMCs) have received wide attention from scholars and engineers at home and abroad because of their advantages of high modularity, strong expansibility, high output electric energy quality, low voltage harmonic content, low voltage distortion rate, and the like, and particularly in the field of High Voltage Direct Current (HVDC), the MMC has been studied in abundance, and is also applied in some demonstration projects at home and abroad. In addition, with the continuous and deep research on the inverter circuit with the MMC structure, the inverter circuit has reference significance for possible application in power quality management, high-power variable-frequency speed regulation systems, energy storage systems and the like in the future.
Before an inverter circuit based on a Modular Multilevel Converter (MMC) structure normally operates, the problem of static voltage balance of capacitance of a submodule of the MMC in a pre-starting process needs to be solved. At present, the conditions of the off-grid inverter circuit with the MMC structure are not considered in the pre-starting process: because the capacitor parameters of the sub-modules have errors (the general error value is not more than +/-20 percent) and the problem of inconsistent capacitor voltage of the MMC sub-modules when pre-charging is completed is caused, when the inverter circuit is put into normal operation, a larger surge current still can appear on the direct current side, so that the safety of the MMC system is influenced, and the voltage balance control of the capacitor in the MMC sub-modules in the normal operation process is not facilitated.
Disclosure of Invention
In view of this, an object of the present invention is to provide a static balancing control method for the capacitance voltage of an MMC submodule during starting of an off-network inverter circuit, so as to solve the problem of inconsistent capacitance voltage of the MMC submodule when pre-charging is completed due to a capacitance parameter error of the MMC submodule, so as to avoid a problem that a large surge current occurs on a dc side when the inverter circuit is put into normal operation, thereby avoiding the problems of affecting the safety of an MMC system and being unfavorable for voltage balancing control of the capacitance of the MMC submodule during normal operation.
In order to achieve the above purpose, the embodiment of the invention adopts the following technical scheme:
a static voltage balance control method for MMC sub-module capacitors in starting of an off-grid inverter circuit comprises the following steps:
the method comprises the following steps: controlling a switching tube (VT1) in each MMC submodule in the three-phase inverter circuit to be switched on and a switching tube (VT2) to be switched off so as to enable all capacitors (C) to be in a charging state;
step two: disconnecting the DC switch to pass the DC bus voltage through a current limiting resistor (R)0) The control method comprises the steps of applying the control method to all capacitors (C), disconnecting an alternating current switch to cut off connection with an alternating current load, and simultaneously carrying out uncontrolled pre-charging on the capacitors (C) of all MMC sub-modules, wherein the uncontrolled pre-charging time is a first preset time length;
step three: the DC bus voltage is connected in and passes through a current limiting resistor (R)0) The method comprises the following steps of carrying out controllable pre-charging on a capacitor (C) in a three-phase inverter circuit, wherein the time of the controllable pre-charging is a second preset time length, and the method comprises the following substeps:
the method comprises the steps that current voltage values of capacitors (C) of all MMC sub-modules are collected through a voltage sensor, all MMC sub-modules of each phase are sorted from small to large according to the corresponding current voltage values and are divided into two groups, wherein the number of all capacitors (C) of any phase circuit in a three-phase inverter circuit is 2N, N MMC sub-modules with small current voltage values are a low voltage group, and N MMC sub-modules with large current voltage values are a high voltage group;
controlling the MMC sub-modules of the low-voltage group to be in an input state, enabling the corresponding capacitors (C) to be in a charging state, namely switching tubes (VT1) of all MMC sub-modules in the low-voltage group are switched on and switching tubes (VT2) are switched off, controlling the MMC sub-modules of the high-voltage group to be in a cut-off state, cutting off the corresponding capacitors (C) for charging, namely switching tubes (VT1) of all MMC sub-modules in the high-voltage group are switched off and switching tubes (VT2) are switched on;
the DC bus voltage is connected in and passes through a current limiting resistor (R)0) Charging capacitors (C) of N MMC sub-modules of a low-voltage group of each phase in a three-phase inverter circuit;
repeatedly executing the sub-steps according to a certain period or frequency so that the current voltage values of the capacitors (C) of all the MMC sub-modules after the controllable pre-charging for a second preset time length belong to a preset range, wherein the preset range is
Figure BDA0001696150910000031
UdcIs DC bus voltage, is voltage fluctuation coefficient of capacitor, and is 0<<1；
Step four: controlling the switching tubes (VT1) of all MMC sub-modules to be switched on and the switching tubes (VT2) to be switched off so as to enable all capacitors (C) to be in a charging state;
step five: closing the DC and AC switches to cut off the current limiting resistor (R)0) And connected to an ac load;
step six: and starting an inversion working condition program to enable the system to enter a normal working state, and finishing the whole starting of the system.
In a preferred option of the embodiment of the present invention, in the method for controlling static balancing of capacitance and voltage of an MMC sub-module during starting of the off-network inverter circuit, a formula for calculating a first preset time duration includes:
Figure BDA0001696150910000032
Figure BDA0001696150910000033
Figure BDA0001696150910000034
Figure BDA0001696150910000035
(initial conditions were:
Figure BDA0001696150910000036
wherein is taking
Figure BDA0001696150910000037
t0For the first preset time length to be calculated, the time constant is tauref＝R0·Ceq_refEquivalent capacitance of Ceq_refT is the response time, the voltage response of the uncontrolled precharge equivalent circuit is
Figure BDA0001696150910000038
The current response is
Figure BDA0001696150910000039
The voltage response of the capacitor of the MMC sub-module is
Figure BDA00016961509100000310
CrefIs the nominal value of the capacitance.
In a preferred option of the embodiment of the present invention, in the method for controlling static balancing of capacitance and voltage of the MMC sub-module during starting of the off-network inverter circuit, the formula for calculating the second preset time duration includes:
Figure BDA0001696150910000041
Figure BDA0001696150910000042
Figure BDA0001696150910000043
Figure BDA0001696150910000044
wherein is taking
Figure BDA0001696150910000045
The initial voltage value of the capacitor of the MMC submodule is taken as
Figure BDA0001696150910000046
t-t0For a second predetermined time period, t, to be calculated0Is a first preset duration, CrefIs the nominal value of the capacitance, t0To not control the time when charging is completed, t is the response time, and the equivalent capacitance is C'eqTime constant τ ═ R0·C”eqThe voltage response is
Figure BDA0001696150910000047
The current response is
Figure BDA0001696150910000048
The capacitance-voltage response of the MMC sub-module is uc”(t)。
According to the static voltage balance control method for the MMC sub-module capacitor in the starting of the off-network inverter circuit, the capacitor is charged into an uncontrolled pre-charging stage and a controlled pre-charging stage, and charging control is performed according to the current voltage value of the capacitor in the controlled pre-charging stage, so that the problem that the voltage of each capacitor is inconsistent after pre-charging is completed due to the fact that the parameters of each capacitor are inconsistent can be avoided, the problem that a large surge current can occur on a direct current side when an inverter control circuit performs inversion work is avoided, and the problems that the safety of an MMC system is influenced, the voltage balance control of the capacitor of the MMC sub-module in a normal operation process is not facilitated and the like are further avoided. In addition, the control algorithm in the method is simple and easy to realize.
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in detail below.
Drawings
FIG. 1 is a topological diagram of a main circuit of an inverter circuit start control system based on MMC;
FIG. 2 is an equivalent circuit of the submodule capacitor of the MMC system in an uncontrolled pre-charging stage, (a) is the equivalent circuit of the pre-charging process; (b) is an RC first-order zero state response circuit;
FIG. 3 is an equivalent circuit of the submodule capacitor of the MMC system in a controllable pre-charging stage, (a) is the equivalent circuit of the pre-charging process; (b) is an RC first-order full response circuit;
FIG. 4 is a MMC-based inverter circuit phase A all submodule capacitor voltage waveform;
FIG. 5 is a DC side current waveform for an inverter circuit based on MMC;
FIG. 6 is an MMC-based inverter circuit AC side load A phase voltage waveform;
fig. 7 is a phase a current waveform of an ac side load of an inverter circuit based on MMC.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. The components of embodiments of the present invention generally described and illustrated in the figures herein may be arranged and designed in a wide variety of different configurations.
Thus, the following detailed description of the embodiments of the present invention, presented in the figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of selected embodiments of the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
It should be noted that: like reference numbers and letters refer to like items in the following figures, and thus, once an item is defined in one figure, it need not be further defined and explained in subsequent figures. In the description of the present invention, the terms "first", "second", and the like are used only for distinguishing the description, and are not intended to be construed as only or implying relative importance.
In the description of the present invention, unless otherwise expressly specified or limited, the terms "disposed," "connected," and "connected" are to be construed broadly, e.g., as meaning fixedly connected, detachably connected, or integrally connected; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meanings of the above terms in the present invention can be understood in specific cases to those skilled in the art.
As shown in fig. 1, an embodiment of the present invention provides a main circuit topology diagram of an off-network inverter charging control system based on an MMC structure. Wherein, each phase of the three-phase inverter circuit consists of an upper bridge arm and a lower bridge arm, and each bridge arm is provided with N MMC sub-modules SMijA bridge arm inductor Larm。
Each MMC sub-module SMijThe half-bridge structure is formed by connecting two switching tubes VT1 and VT2 in series, connecting a power diode VD1 and VD2 in parallel in an anti-parallel mode respectively, and then connecting a capacitor C in parallel. The DC side of the three-phase inverter circuit is connected with the DC bus voltage through a DC contact KM1 of a DC switch, and the DC contact KM1 is a normally open contact and is connected in parallel with a current limiting resistor R0. And, three-phase inverterThe alternating current side of the circuit is connected with an alternating current load (equivalent resistance R and equivalent inductance L) through three alternating current contacts KM2 of the alternating current switch, and the three alternating current contacts KM2 are normally open contacts.
In a three-phase inverter circuit, MMC sub-module SMijThere are two working states, which are: an input state and an excision state. In the on state, the switching tube VT1 is turned on and the switching tube VT2 is turned off. In the cut-off state, the switching tube VT1 is turned off and the switching tube VT2 is turned on.
The embodiment of the invention provides a static voltage balance control method for MMC sub-module capacitors in starting of an off-network inverter circuit, which comprises the following steps:
the method comprises the following steps: controlling a switching tube VT1 in each MMC submodule in the three-phase inverter circuit to be switched on and a switching tube VT2 to be switched off so as to enable all capacitors C to be in a charging state;
step two: disconnecting the DC switch to pass the DC bus voltage through the current limiting resistor R0The method comprises the steps that the method is applied to all capacitors C, the alternating current switch is disconnected to cut off the connection with an alternating current load, so that the capacitors C of all MMC sub-modules are subjected to uncontrolled pre-charging simultaneously, wherein the uncontrolled pre-charging time is a first preset time length;
step three: connected to the DC bus voltage via a current limiting resistor R0The method comprises the following steps of carrying out controllable pre-charging on a capacitor C in a three-phase inverter circuit, wherein the controllable pre-charging time is a second preset time length, and the method comprises the following substeps:
the method comprises the steps that current voltage values of capacitors C of all MMC sub-modules are collected through a voltage sensor, all MMC sub-modules of each phase are sorted from small to large according to the corresponding current voltage values and are divided into two groups, wherein the number of all capacitors C of any phase circuit in a three-phase inverter circuit is 2N, N MMC sub-modules with small current voltage values are a low voltage group, and N MMC sub-modules with large current voltage values are a high voltage group;
controlling the MMC sub-modules of the low-voltage group to be in an input state, enabling the corresponding capacitors C to be in a charging state, namely switching tubes VT1 of all MMC sub-modules in the low-voltage group are switched on, switching tubes VT2 are switched off, controlling the MMC sub-modules of the high-voltage group to be in a cut-off state, cutting off the corresponding capacitors C to be charged, namely switching tubes VT1 of all MMC sub-modules in the high-voltage group are switched off, and switching tubes VT2 are switched on;
connected to the DC bus voltage via a current limiting resistor R0Charging capacitors C of N MMC sub-modules of a low-voltage group of each phase in a three-phase inverter circuit;
repeatedly executing the sub-steps according to a certain period or frequency so that the current voltage values of the capacitors C of all the MMC sub-modules after the controllable pre-charging for a second preset time length belong to a preset range, wherein the preset range is
Figure BDA0001696150910000071
UdcIs DC bus voltage, is voltage fluctuation coefficient of capacitor, and is 0<<1；
Step four: controlling the switching tubes VT1 of all MMC sub-modules to be switched on and the switching tube VT2 to be switched off so as to enable each capacitor C to be in a charging state;
step five: closing the DC and AC switches to cut off the current limiting resistor R0And connected to an ac load;
step six: and starting an inversion working condition program to enable the system to enter a normal working state, and finishing the whole starting of the system.
In the above steps, step one and step two are uncontrolled precharge stages. Wherein the uncontrolled precharge time (i.e. the first preset time duration) can be obtained by calculation. In detail, in the uncontrolled precharge phase, all MMC submodules are in the on state, that is, 2N capacitors of each phase are charged simultaneously, as shown in step one. According to this operation, an equivalent circuit during charging can be established as shown in fig. 2(a), and the capacitance value of each MMC sub-module is CijTo simplify the analysis, the bridge arm inductance L is ignoredarmThe total equivalent capacitance of FIG. 2(a) is CeqThe equivalent capacitance of each phase is CjThe calculation formula is as follows:
Ceq＝∑Cj(j＝a,b,c) (1)
Figure BDA0001696150910000081
Cij＝Cref±ΔCij(i＝1,2,3,…,2N；j＝a,b,c) (3)
wherein, CrefIs the nominal value of the capacitance, Δ CijAnd j is a phase of the three-phase inverter circuit, and j is a, b and c.
According to the equivalent capacitance, the charging process can be equivalent to an RC first-order zero state response circuit, as shown in FIG. 2(b), the voltage response of which is
Figure BDA0001696150910000082
The current response is
Figure BDA0001696150910000083
Wherein, UdcFor dc bus voltage, the time constant τ ═ R0·CeqThe time constant of the RC first-order circuit, and t is the response time. At this time, the accurate solving of the current of each phase of the MMC system is difficult, so that the capacitance voltage of each MMC submodule is difficult to solve, and the following simplification processing is performed.
Because the controllable pre-charging stage is directly switched to when the uncontrolled pre-charging process is finished, the controllable pre-charging stage does not directly participate in the switching of the normal inversion of the system, and the charging time is short, the safety performance of the system cannot be influenced, therefore, the charging time can be selected to simplify calculation, and a relatively reasonable uncontrolled pre-charging time is selected. As can be seen from the above, the uncontrolled precharge process can be equivalent to an RC first-order zero state response circuit. Therefore, the capacitance values of all the MMC sub-modules can be taken as the nominal value, and the pre-charging time which is closer to the true value is calculated. In this case, the time constant is denoted as τref＝R0·Ceq_refEquivalent capacitance of Ceq_refThe voltage response is
Figure BDA0001696150910000084
The current response is
Figure BDA0001696150910000085
The voltage response of the capacitor is
Figure BDA0001696150910000091
The specific response calculation formula is as follows:
Figure BDA0001696150910000092
Figure BDA0001696150910000093
Figure BDA0001696150910000094
Figure BDA0001696150910000095
(initial conditions were:
Figure BDA0001696150910000096
through the analysis and calculation, the first preset time duration, namely the duration of the uncontrolled precharge in the first step and the second step can be obtained, and the duration can enable the current voltage value of each capacitor to be close to or equal to U after the uncontrolled precharge is completeddc/2N。
In the above steps, step three is a controllable precharge stage, and the duration is a controllable precharge duration (i.e. a second preset duration). After a plurality of times of controllable pre-charging, the voltage value of the capacitor of each MMC sub-module is close to Udc/N。
The second preset time length can be obtained through calculation. In detail, as can be seen from the third step, each phase in the three-phase inverter circuit has 2N MMC submodules participating in the sequencing, but only N MMC submodules (three-phase MMC submodules are charged simultaneously, and 3N submodules are provided) are put into the charging state at any time to perform charging, so that the voltage of the capacitor in the charging state is relatively good in consistency. And, for MThe more the MC sub-modules are sequenced, the better the voltage consistency of each capacitor is. According to this working process, an equivalent circuit during pre-charging can be established as shown in fig. 3(a), and the capacitance value of each MMC sub-module is CijThe value of which is related only to the capacitance of N of the 2N MMC sub-modules to simplify the analysis, the bridge arm inductance L was ignoredarmThe influence of (c). The total equivalent capacitance of FIG. 3(a) is C'eqEquivalent capacitance per phase is C'jThe calculation formula is as follows:
C'eq＝∑C'j(j＝a,b,c) (8)
Figure BDA0001696150910000101
Cij＝Cref±ΔCij(i＝1,2,3,…,N；j＝a,b,c) (10)
where N is the number of submodules being charged.
According to the equivalent capacitance, the charging process can be equivalent to an RC first-order full response circuit, as shown in FIG. 3(b), the voltage response of which is
Figure BDA0001696150910000102
The current response is
Figure BDA0001696150910000103
Initial voltage value of capacitor
Figure BDA0001696150910000104
Wherein, UdcIs the DC bus voltage, t0To not control the time when charging is complete, the time constant τ' ═ R0·C'eqThe time constant of the RC first-order circuit, and t is the response time.
As can be seen from the above, the charging process can be equivalent to an RC first-order full response circuit, so that the equivalent capacitor voltage is obtained
Figure BDA0001696150910000105
Charging near DC side voltage UdcThe length of charging time is determined by time constantτ and the degree of proximity of the two. In order to quickly calculate the capacitance voltage of each MMC sub-module and ensure that the time for charging the capacitances of all the MMC sub-modules is sufficient, analysis is further simplified, and the maximum limit value of the capacitance values of all the MMC sub-modules is 1.2C ″refThe initial value of the capacitance voltage of the MMC sub-module is taken as
Figure BDA0001696150910000106
At this time, the time constant of the RC first-order full response increases, and is denoted as τ ″ ═ R0·C”eqThe equivalent capacitance is C'eqThe voltage response is
Figure BDA0001696150910000107
The current response is
Figure BDA0001696150910000108
The capacitance-voltage response of the MMC sub-module is uc”(t), the specific response calculation formula is as follows:
Figure BDA0001696150910000109
Figure BDA00016961509100001010
Figure BDA00016961509100001011
Figure BDA0001696150910000111
according to the above formula, the longer the charging time is, the closer the capacitance voltage of the MMC sub-module is to the voltage during normal operation, but in practice, the charging time cannot be too long. According to the analysis of the MMC system during normal operation, the capacitance of the MMC submodule is always in two states of charging and discharging, so that the capacitance voltage is in a fluctuation state, and the fluctuation coefficient of the capacitance voltage is (0)<<1) The value is in the system main circuitThe parameter design is determined. The fluctuation range of the capacitance and voltage of the MMC sub-module is
Figure BDA0001696150910000112
Therefore, the voltage on the capacitor of the MMC sub-module should at least reach
Figure BDA0001696150910000113
Defining m as a charging time coefficient, and setting t-t0M τ ", the capacitance voltage of the MMC submodule is
Figure BDA0001696150910000114
Then it can be further obtained that:
m＝-ln2 (15)
therefore, it is ensured that the capacitive charging time for each phase of N MMC sub-modules (three phase MMC sub-modules charging simultaneously, for a total of 3N sub-modules) should be larger than m τ ". τ "and current limiting resistor R0Selecting and equivalent capacitor C'e'qIt is related. When the value of the capacitance C' of the MMC sub-module is fixed, R0The larger the value of (d), the larger the value of tau ″, the capacitive voltage of the MMC submodule reaching
Figure BDA0001696150910000115
The longer the time required, the less the current spike on the dc side of the capacitance of the MMC submodule during the pre-charge phase. When the value of tau' is constant, the smaller the value of tau ", the larger the value of m, and the capacitance voltage of the MMC sub-module reaches to
Figure BDA0001696150910000116
The longer the required time is, the closer the capacitance voltage of the MMC sub-module is to Udcand/N, the smaller the peak value of the surge current when the MMC system is switched from the pre-charging stage to the normal inversion. According to the size of the direct current bus voltage and the specific number of MMC sub-modules of each phase, the proper current limiting resistor R is selected in a comprehensive consideration mode0And m value, the capacitance voltage of the MMC sub-module is close to U in the shortest possible charging timedc/N。
Alternatively, the period or frequency of dividing the high voltage group and the low voltage group in step three may be set according to the actual application requirements. And the period or frequency determines the switching frequency of the switching tube in the pre-charging stage, so that the switching frequency of the switching tube in the pre-charging stage does not exceed the switching frequency of the switching tube under the normal inversion working condition for the convenience of designing a radiator of the switching tube.
In the process of carrying out controllable pre-charging, if the requirements on the capacitance voltage of the MMC sub-module and the ripple waves on the direct current side of the three-phase inverter circuit are low, the frequency of the sorting division can be low, so that the power loss of the switching tube in the pre-charging stage is reduced, and the calculated amount of a controller for carrying out control work is reduced.
Further, the present embodiment also provides an application example, in which the dc bus voltage U isdc1000V, current limiting resistor R 050 Ω bridge arm inductance Larm5mH, nominal value of capacitance CrefWhen the ripple coefficient of the capacitor voltage is equal to or less than 5%, and the limit value of the error value Δ C is ± 20% of the nominal value, the value range of the capacitor is as follows: 1.6mF is less than or equal to (C)ref+ Δ C) is less than or equal to 2.4mF, the capacitance values of all MMC submodules are randomly selected in the range, the number N of each bridge arm submodule is 4, the alternating load resistance R is 50 Ω, the alternating load inductance L is 15mH, the frequency of the MMC submodule in the starting stage according to the capacitor voltage sequence is 100Hz, when the MMC system is switched to the normal inversion working condition, the carrier phase-shifting modulation strategy is adopted for control (of course, the carrier stacking modulation strategy and the nearest level approximation modulation strategy are also applicable), and the voltage waveform on the load side is the output result of five levels.
According to the parameter calculation, when the uncontrolled pre-charging is finished, the capacitance voltage of the MMC sub-module is at the reference voltage Uref_NAround 125V. When the controllable pre-charging is completed, the voltage is close to the reference voltage Uref_2N250V. According to the parameter simulation, the fluctuation range of the capacitance and voltage of the MMC submodule in the normal inversion working condition is as follows: u is more than or equal to 245Vcref255V or less, the fluctuation coefficient of the capacitance voltage of the MMC sub-module is 2%, the ripple coefficient of the capacitance voltage of the MMC sub-module is met with the requirement that the ripple coefficient of the capacitance voltage of the MMC sub-module is 5%, and the charging time coefficient can be calculatedAnd m is 3.219. Under the condition that the capacitance of the MMC submodule has errors, in an uncontrolled pre-charging stage, the charging time constant tau of the MMC system at the moment can be calculated according to the analysisrefAnd when the uncontrolled pre-charging is finished, the capacitance voltage of the MMC sub-module reaches 0.95Uref_NTime t required00.12 s. In the controllable pre-charging stage, the charging time constant τ ″, which is 0.09s, of the MMC system can be calculated according to the analysis, and when the controllable pre-charging is completed, the time t-t required for completing the charging of the N MMC sub-modules (three-phase MMC sub-modules are charged simultaneously, and 3N sub-modules are used in total) is obtained0And 0.290s, the time required for charging all MMC sub-modules is 2 (t-t)0) 0.58 s. The longer the charging time is, the closer the capacitance voltage of the MMC sub-module is to the UdcAnd N, the fluctuation of direct current side current, bridge arm surge current and capacitance voltage of the MMC module is smaller when the MMC system is switched to a normal inversion working condition from a pre-charging stage. Meanwhile, considering that the switching-on and switching-off of the switching tube also need a certain time, the capacitance charging time of the MMC sub-module in the controllable pre-charging stage is taken as 0.68s, the total charging time of the MMC system is t ═ 0.8s, and the MMC system is switched at 0.8s to enter the normal inversion working condition to operate.
In summary, according to the static voltage balance control method for the MMC submodule in the off-network inverter circuit pre-starting process, the capacitor charging is divided into the uncontrolled pre-charging stage and the controllable pre-charging stage, and the charging control is performed according to the current voltage value of the capacitor in the controllable pre-charging stage, so that the problem that the voltage of each capacitor is inconsistent after the pre-charging is completed due to inconsistent parameters of each capacitor can be avoided, the problem that the inverter control circuit generates large surge current due to the direct current side during the inversion operation is avoided, and the problems that the MMC affects the safety of the system and is not beneficial to the voltage balance control of the capacitor of the submodule MMC in the normal operation process are avoided. In addition, the control algorithm in the method is simple and easy to realize.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (3)

1. A static voltage balance control method for MMC sub-module capacitors in starting of an off-grid inverter circuit is characterized by comprising the following steps:
the method comprises the following steps: controlling a switching tube VT1 in each MMC submodule in the three-phase inverter circuit to be switched on and a switching tube VT2 to be switched off so as to enable all capacitors (C) to be in a charging state;
step two: disconnecting the DC switch to pass the DC bus voltage through the current limiting resistor R0The control method comprises the steps of applying the control method to all capacitors (C), disconnecting an alternating current switch to cut off connection with an alternating current load, and simultaneously carrying out uncontrolled pre-charging on the capacitors (C) of all MMC sub-modules, wherein the uncontrolled pre-charging time is a first preset time length;
step three: connected to the DC bus voltage via a current limiting resistor R0The method comprises the following steps of carrying out controllable pre-charging on a capacitor (C) in a three-phase inverter circuit, wherein the time of the controllable pre-charging is a second preset time length, and the method comprises the following substeps:
the method comprises the steps that current voltage values of capacitors (C) of all MMC sub-modules are collected through a voltage sensor, all MMC sub-modules of each phase are sorted from small to large according to the corresponding current voltage values and are divided into two groups, wherein the number of all capacitors (C) of any phase circuit in a three-phase inverter circuit is 2N, N MMC sub-modules with small current voltage values are a low voltage group, and N MMC sub-modules with large current voltage values are a high voltage group;
controlling the MMC sub-modules of the low-voltage group to be in an input state, enabling the corresponding capacitors (C) to be in a charging state, namely switching tubes VT1 of all MMC sub-modules in the low-voltage group are switched on, switching tubes VT2 are switched off, controlling the MMC sub-modules of the high-voltage group to be in a cutting-off state, cutting off the corresponding capacitors (C) to be charged, namely switching tubes VT1 of all MMC sub-modules in the high-voltage group are switched off, and switching tubes VT2 are switched on;
connected to the DC bus voltage via a current limiting resistor R0Low voltage for each phase of three-phase inverter circuitThe capacitors (C) of the N MMC sub-modules of the group are charged;
repeatedly executing the sub-steps according to a certain period or frequency so that the current voltage values of the capacitors (C) of all the MMC sub-modules after the controllable pre-charging for a second preset time length belong to a preset range, wherein the preset range is
Figure FDA0002444535750000021
UdcIs DC bus voltage, is voltage fluctuation coefficient of capacitor, and is 0<<1；
Step four: controlling the switch tubes VT1 of all MMC sub-modules to be switched on and the switch tube VT2 to be switched off so as to enable all capacitors (C) to be in a charging state;
step five: closing the DC and AC switches to cut off the current limiting resistor R0And connected to an ac load;
step six: and starting an inversion working condition program to enable the system to enter a normal working state, and finishing the whole starting of the system.
2. The MMC sub-module capacitor voltage static balance control method in the starting of off-network inverter circuit of claim 1, wherein the formula for calculating the first preset duration comprises:
Figure FDA0002444535750000022
Figure FDA0002444535750000023
Figure FDA0002444535750000024
Figure FDA0002444535750000025
(initial conditions were:
Figure FDA0002444535750000026
)；
wherein is taking
Figure FDA0002444535750000027
t0For the first preset time length to be calculated, the time constant is tauref＝R0·Ceq_refEquivalent capacitance of Ceq_refT is the response time, the voltage response of the uncontrolled precharge equivalent circuit is
Figure FDA0002444535750000028
The current response is
Figure FDA0002444535750000029
The voltage response of the capacitor of the MMC sub-module is
Figure FDA00024445357500000210
CrefIs the nominal value of the capacitance.
3. The MMC sub-module capacitor voltage static balance control method in the starting of off-network inverter circuit of claim 1, wherein the formula for calculating the second preset duration comprises:
Figure FDA0002444535750000031
Figure FDA0002444535750000032
Figure FDA0002444535750000033
Figure FDA0002444535750000034
wherein is taking
Figure FDA0002444535750000035
The initial voltage value of the capacitor of the MMC submodule is taken as
Figure FDA0002444535750000036
t-t0For a second predetermined time period, t, to be calculated0Is a first preset duration, CrefIs the nominal value of the capacitance, t0To not control the time when charging is completed, t is the response time, and the equivalent capacitance is C'eqTime constant of
Figure FDA0002444535750000037
The voltage response is
Figure FDA0002444535750000038
The current response is
Figure FDA0002444535750000039
The capacitance-voltage response of the MMC sub-module is uc”(t)。
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