CN115395809A - MMC adaptive phase power balance control method and system - Google Patents
MMC adaptive phase power balance control method and system Download PDFInfo
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- CN115395809A CN115395809A CN202211026915.0A CN202211026915A CN115395809A CN 115395809 A CN115395809 A CN 115395809A CN 202211026915 A CN202211026915 A CN 202211026915A CN 115395809 A CN115395809 A CN 115395809A
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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
- 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
- H02M7/483—Converters with outputs that each can have more than two voltages levels
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
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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
- H02M1/32—Means for protecting converters other than automatic disconnection
- H02M1/325—Means for protecting converters other than automatic disconnection with means for allowing continuous operation despite a fault, i.e. fault tolerant converters
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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
- 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
- H02M7/5387—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 in a bridge configuration
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
- Y02E40/50—Arrangements for eliminating or reducing asymmetry in polyphase networks
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Abstract
The invention discloses a MMC self-adaptive phase power balance control method and a system, belonging to the technical field of modular multilevel converters and comprising the following steps: the method comprises the steps of determining the modulation state of an MMC in real time based on the modulation ratio of each phase voltage of the MMC, taking the AC voltage overmodulation of the MMC as a boundary, integrating balanced unbalanced power of each phase of the MMC in a critical overmodulation state, and updating distribution coefficients in real time, so that each phase of unbalanced power is adaptively and reasonably distributed to two control links of zero-sequence voltage injection and direct-current circulation control, the redistribution of three-phase power of a bridge arm is realized through the unified control of the two links of zero-sequence voltage injection and direct-current circulation control, the influence of asymmetry of an AC power grid on the bridge arm power is eliminated, the problem of overmodulation of the AC voltage is effectively prevented, the balanced control of the phase power can be effectively realized, and the operation reliability of the MMC is greatly improved.
Description
Technical Field
The invention belongs to the technical field of modular multilevel converters, and particularly relates to an MMC adaptive phase power balance control method and system.
Background
The Modular Multilevel Converter (MMC) has the characteristics of low loss, easiness in expansion, convenience in redundant fault-tolerant design and the like, and is widely applied to high-power systems such as high-voltage direct-current power transmission, offshore wind power and flexible alternating-current power transmission. In actual operation, under the influence of factors such as asymmetric faults or uneven line impedance, the phenomenon that the voltage of the power grid on the alternating current side is asymmetric occurs sometimes. The unbalanced power of three phases can be generated when the alternating current side voltage is asymmetric, so that the problems of uneven heating and the like caused by unbalanced bridge arm current and asymmetric phase power are solved, the safe and stable operation of systems such as flexible direct current transmission is threatened, and therefore, the MMC phase power balance is realized under the condition of asymmetric power grid voltage, and the key for guaranteeing the uninterrupted operation of the MMC is realized.
The traditional phase power balance control method is usually used for carrying out balance control on the phase power of the MMC based on zero sequence voltage injection, however, the zero sequence voltage injection can increase the alternating voltage, in the balance control process, if the zero sequence voltage injection is carried out without limitation, the problem of overmodulation of the MMC can be caused, further, the three-phase current is distorted, and the balance control on the phase power cannot be effectively realized.
Disclosure of Invention
Aiming at the defects or improvement requirements in the prior art, the invention provides an MMC adaptive phase power balance control method and system, which are used for solving the technical problems that overmodulation risks exist in the prior art and balance control of phase power cannot be effectively realized.
In order to achieve the above object, the present invention provides an MMC adaptive phase power equalization control method, which comprises the following steps:
s1, respectively carrying out PI control on the difference value of the capacitance voltage average value of each phase of the MMC and the total capacitance voltage average value of a three-phase submodule to obtain unbalanced power of each phase;
s2, pair distribution coefficient k m Carrying out initialization;
s3, respectively carrying out unbalanced power on each phase according to a distribution coefficient k m Distributing to obtain first unbalanced power and second unbalanced power of each phase; respectively injecting zero-sequence voltage into the alternating-current voltage reference value of the corresponding bridge arm based on the first unbalanced power of each phase; dividing the second unbalanced power of each phase by the MMC direct-current bus voltage to obtain a direct-current circulating current reference value corresponding to the bridge arm;
s4, respectively carrying out capacitance and voltage balance control on sub-modules in the bridge arms on the basis of alternating current voltage reference values of the bridge arms, and simultaneously carrying out inner-ring bridge arm current control on the basis of direct current circulation reference values of the bridge arms;
s5, integrating MMC each-phase alternating current by taking MMC alternating voltage overmodulation as a boundaryUnbalanced power that can be balanced with voltage in critical overmodulation state, to distribution coefficient k m And updating, and turning to the step S3 to continuously and evenly control the MMC phase power.
Further preferably, the first unbalanced power of the j-th phase is: delta P j0 ＝k m ×ΔP j (ii) a The second unbalanced power of the j-th phase is: delta P jdc ＝(1-k m )×ΔP j ；
Wherein, Δ P j Unbalanced power of j phase; j = a, b, c; k is a radical of formula m ∈[0,1](ii) a When k is m If =1, the unbalanced power of each phase is distributed to obtain only the first unbalanced power; when k is m =0, the unbalanced power of each phase is distributed to obtain only the second unbalanced power.
Further preferably, the partition coefficient k m Updating is carried out through the following formula:
k m ＝min(k j )
wherein k is j The power distribution coefficient of the j-th phase is specifically as follows:
U 0 and
are respectively when k m The amplitude and the phase angle of zero sequence voltage required to be injected when the MMC three-phase power is equal to 1; u shape j Is the MMC j-th alternating voltage amplitude; u shape dc The direct current voltage is MMC bus direct current voltage; m is j Is MMC j phase voltage modulation ratio.
Further preferably, the MMC j-th phase voltage modulation ratio m j Comprises the following steps:
further preferably, by judging the modulation ratio m of the j-th phase j Critical injection quantity U of zero sequence voltage of =1 hour j0b And k is m Injection quantity U of zero sequence voltage in time of =1 0 To determine the j-th phaseWhether overmodulation occurs; the method specifically comprises the following steps: when U is turned 0 ≤U j0b When no overmodulation occurs in the j phase, the MMC phase voltage modulation ratio m of the j phase is the same j Less than or equal to 1; when U is turned 0 ＞U j0b When the j phase is overmodulating, the MMC phase voltage modulation ratio m of j phase appears j Is more than 1; wherein, the first and the second end of the pipe are connected with each other,
further preferably, the MMC phase j capacitance voltage average value
Obtaining the result after filtering the average value of the capacitor voltage of each submodule of the j-th phase of the MMC through a double frequency trap; wherein j = a, b, c.
Further preferably, the three-phase submodule overall capacitance voltage average value
Comprises the following steps:
wherein the content of the first and second substances,
and the average value of the capacitance voltage of the j phase of the MMC.
In a second aspect, the present invention provides an MMC adaptive phase power balancing control system, including: the MMC adaptive phase power equalization control method comprises a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to execute the MMC adaptive phase power equalization control method provided by the first aspect of the invention.
In a third aspect, the present invention provides a modular multilevel converter, each phase of which comprises an upper bridge arm and a lower bridge arm; when the modular multilevel converter is subjected to phase power balance control, the MMC self-adaptive phase power balance control method provided by the first aspect is adopted.
In a fourth aspect, the present invention further provides a computer-readable storage medium, which includes a stored computer program, where the computer program, when executed by a processor, controls a device in which the storage medium is located to execute the MMC adaptive phase power equalization control method according to the first aspect of the present invention.
Generally, by the above technical solution conceived by the present invention, the following beneficial effects can be obtained:
1. the invention provides an MMC self-adaptive phase power balance control method, which is characterized in that the modulation state of an MMC is determined in real time based on the modulation ratio of each phase of voltage of the MMC, the AC voltage of the MMC is used as a boundary, balanced unbalanced power which can be balanced when each AC voltage of the MMC is in a critical overmodulation state is synthesized, and a distribution coefficient is updated in real time, so that each phase of unbalanced power is self-adaptively and reasonably distributed to two control links of zero-sequence voltage injection and direct current circulation control, the redistribution of three-phase power of a bridge arm is realized by the unified control of the two links of zero-sequence voltage injection and direct current circulation regulation, the influence of asymmetry of an AC power grid on the power of the bridge arm is eliminated, the problem of overmodulation of the AC voltage is effectively prevented, and the balanced control of the phase power can be effectively realized.
2. The MMC self-adaptive phase power balance control method provided by the invention self-adaptively distributes unbalanced power by detecting the modulation ratio, and independently adopts a zero sequence voltage injection strategy when the MMC overmodulation cannot be caused by only injecting zero sequence voltage balanced phase power; when the phase power is balanced only by injecting zero sequence voltage, two regulation strategies of zero sequence voltage injection and direct current circulation regulation are adopted to balance the phase power, the method is suitable for various operation working conditions, the problem of unbalance of the bridge arm power of the MMC under the condition of asymmetrical alternating current side is solved, the influence of asymmetrical alternating current power grid on the bridge arm power is eliminated, and the reliability of uninterrupted operation of the MMC under the condition of asymmetrical alternating current power grid fault is greatly improved.
Drawings
Fig. 1 is a schematic structural diagram of adaptive phase power balancing control of an MMC provided in embodiment 1 of the present invention;
fig. 2 is a schematic diagram of an MMC simulation system with YD transformer according to embodiment 1 of the present invention;
fig. 3 is a schematic diagram of a calculation process of a dc circulating reference value provided in embodiment 1 of the present invention;
fig. 4 is a block diagram of calculating and distributing unbalanced power according to embodiment 1 of the present invention;
fig. 5 is a schematic diagram of a process of performing zero-sequence voltage injection on an ac voltage reference value of a corresponding bridge arm based on a first unbalanced power of each phase according to embodiment 1 of the present invention;
fig. 6 is a block diagram of overmodulation boundary determination and power distribution coefficient update control according to embodiment 1 of the present invention;
fig. 7 is a simulation waveform diagram of each major electrical quantity of an MMC under a fault when a conventional phase power balancing method based on zero-sequence voltage injection is adopted according to embodiment 1 of the present invention; wherein, (a) is an alternating current voltage simulation oscillogram of the MMC under the fault when the traditional phase power balancing method based on zero sequence voltage injection is adopted; (b) When a traditional phase power balancing method based on zero sequence voltage injection is adopted, an alternating current simulation oscillogram of an MMC under a fault is obtained; (c) When a traditional phase power balancing method based on zero sequence voltage injection is adopted, a circulating current simulation oscillogram of an MMC under a fault is obtained; (d) When a traditional zero sequence voltage injection-based phase power balancing method is adopted, a sub-module capacitor voltage simulation oscillogram of the MMC under a fault is obtained; (e) When a traditional phase power balancing method based on zero sequence voltage injection is adopted, a zero sequence voltage simulation oscillogram of an MMC under a fault is obtained; (f) The method is a simulation oscillogram of the MMC voltage modulation ratio under the fault when a traditional phase power balancing method based on zero sequence voltage injection is adopted;
fig. 8 is a simulation waveform diagram of each major electrical quantity of an MMC under a fault when the adaptive phase power balancing method provided by the present invention is adopted in embodiment 1 of the present invention; wherein, (a) is the simulation oscillogram of alternating voltage of MMC under the trouble when adopting the adaptive phase power equalization method that the invention provides; (b) When the adaptive phase power balancing method provided by the invention is adopted, the alternating current simulation oscillogram of the MMC under the fault is obtained; (c) When the adaptive phase power balancing method provided by the invention is adopted, a circulating current simulation oscillogram of the MMC under a fault is obtained; (d) When the adaptive phase power balancing method provided by the invention is adopted, a sub-module capacitor voltage simulation oscillogram of the MMC under a fault is obtained; (e) When the adaptive phase power balancing method provided by the invention is adopted, the zero sequence voltage simulation oscillogram of the MMC under the fault is obtained; (f) The invention provides a simulation oscillogram of the MMC voltage modulation ratio under the fault when the self-adaptive phase power balancing method provided by the invention is adopted.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
Examples 1,
An MMC adaptive phase power balancing control method, as shown in fig. 1, includes the following steps:
s1, respectively carrying out PI control on the difference value of the capacitance voltage average value of each phase of the MMC and the total capacitance voltage average value of a three-phase submodule to obtain unbalanced power of each phase;
specifically, as shown in fig. 2, a schematic diagram of an MMC simulation system with YD transformer is shown. Each phase is divided into an upper bridge arm and a lower bridge arm, and each bridge arm comprises N sub-modules. Each submodule is formed by connecting two IGBTs in parallel with a capacitor. In the figure, L and R are respectively bridge arm inductance, equivalent resistance and U dc 、i dc MMC direct current bus voltage and current respectively; u. of pj 、u nj J-phase upper and lower bridge arm voltages respectively; i.e. i pj 、i nj J-phase upper and lower bridge arm currents respectively; u. of j 、i j The method comprises the steps of respectively obtaining MMC alternating-current side grid-connected voltage and current; wherein j = a, b, c. The variable reference directions are shown in fig. 2.
In an alternative embodiment, the MMC phase j has an average value of the capacitor voltage
For MMC phase j sub-module capacitor voltageThe average value is filtered by a frequency doubling trap to obtain a result, specifically:
wherein j = a, b, c; s is a laplace variable; omega 2 Is the central angular frequency of the double frequency trap; τ is a damping coefficient; u. of Cpji The capacitance voltage of the ith sub-module of the j-phase upper bridge arm is obtained; u. of Cnji And the capacitance voltage of the ith sub-module of the j-phase lower bridge arm.
Further, the overall capacitance voltage average value of the three-phase sub-modules
Comprises the following steps:
further, the unbalanced power of the j-th phase is:
wherein, K leg_p And K leg_i Proportional coefficients and integral coefficients of PI links are respectively; s is the Laplace variable.
S2, pair distribution coefficient k m Carrying out initialization;
s3, respectively carrying out unbalanced power on each phase according to a distribution coefficient k m Distributing to obtain first unbalanced power and second unbalanced power of each phase; respectively injecting zero-sequence voltage into the alternating-current voltage reference value of the corresponding bridge arm based on the first unbalanced power of each phase; dividing the second unbalanced power of each phase by the MMC direct-current bus voltage to obtain a direct-current circulating current reference value corresponding to the bridge arm; the direct current circulation reference value (namely the adjustment quantity of the j-th phase direct current circulation) of the j-th phase bridge arm is as follows: i.e. i jdc_cir ＝ΔP jdc /U dc (ii) a Specifically, in an alternative embodiment, Δ P is calculated jdc /U dc Then further limiting the amplitude of the obtained result to obtain the final direct currentThe calculation process of the circulation reference value and the direct-current circulation reference value is shown in fig. 3. It should be noted that the clipping element is not necessary, and whether clipping is required or not is determined according to actual situations.
Specifically, a block diagram of calculating and distributing the unbalanced power is shown in fig. 4, and each phase of unbalanced power is distributed to a zero sequence voltage injection link and a direct current circulation control link respectively. The j-th phase first unbalanced power input by the zero-sequence voltage injection link is as follows: delta P j0 ＝k m ×ΔP j (ii) a The second unbalanced power of the j phase input by the direct current circulation control link is as follows: delta P jdc ＝(1-k m )×ΔP j (ii) a In the formula,. DELTA.P j Unbalanced power of j phase; j = a, b, c; k is a radical of m ∈[0,1](ii) a When k is m When =1, the unbalanced power of each phase is distributed to obtain only the first unbalanced power thereof; when k is m Where =0, the unbalanced power of each phase is distributed to obtain only its second unbalanced power.
Specifically, a process of performing zero-sequence voltage injection on the ac voltage reference value of the corresponding bridge arm based on the first unbalanced power of each phase is shown in fig. 5, and the specific process is as follows:
converting a three-phase alternating current through Park into a dq coordinate system by adopting the following formula;
wherein, I d 、I q D-axis and q-axis components of the alternating current respectively; omega is angular frequency; i is the amplitude of the alternating current, and the amplitudes of the three-phase alternating current are the same;
the phase angle of a alternating current, the phase angles of b alternating current and c alternating current are sequentially lagged by 120 degrees, and the formula is as follows
To indicate.
Obtaining the delta P which only generates the first unbalanced power based on the relation between the zero sequence voltage and the phase power j0 Zero sequence voltage regulation component of time
And
the method specifically comprises the following steps:
obtaining the zero sequence voltage signal to be injected according to the zero sequence voltage regulation component and the phase-locked loop signal
The method specifically comprises the following steps: will be provided with
Respectively multiplying sin (ω t) and cos (ω t), and adding to obtain the zero sequence voltage signal to be injected
Namely that
And then injecting the obtained zero sequence voltage signal into an alternating voltage reference value to perform subsequent capacitor voltage balance control of sub-modules in the bridge arm.
S4, respectively carrying out capacitance and voltage balance control on sub-modules in the bridge arms on the basis of alternating current voltage reference values of the bridge arms, and simultaneously carrying out inner-ring bridge arm current control on the basis of direct current circulation reference values of the bridge arms;
specifically, respectively carrying out capacitance-voltage balance control on sub-modules in bridge arms on the basis of alternating voltage reference values after zero-sequence voltage injection of each bridge arm; and meanwhile, inputting the direct current circulating current reference value into a corresponding inner ring bridge arm current control link to control the inner ring bridge arm current.
S5, alternating current with MMCThe voltage overmodulation is taken as a boundary, unbalanced power which can be balanced by zero-sequence voltage injection when each phase alternating current voltage of the integrated MMC is in a critical overmodulation state is distributed to a distribution coefficient k m And updating, and turning to the step S3 to continuously and evenly control the MMC phase power.
It should be noted that, although both the direct current circulating current regulation and the zero sequence voltage injection control strategy can perform phase power balance control, compared with zero sequence voltage injection, only performing direct current circulating current regulation may cause asymmetry of bridge arm currents, so that phase current stresses of each phase are unequal, and a risk of damaging safe and stable operation of a power system exists; in the process, the unbalanced power is adaptively distributed by detecting the modulation ratio, and a zero sequence voltage injection strategy is independently adopted when the MMC overmodulation cannot be caused by injecting the zero sequence voltage balanced phase power only; when the phase power is balanced only by injecting zero sequence voltage, the phase power is balanced by adopting two control strategies of zero sequence voltage injection and direct current circulation regulation.
On the premise of guaranteeing safe and stable operation of the power system, system capacity is utilized to the maximum, overmodulation of the system under zero sequence voltage injection is taken as a boundary, and power distribution coefficients are dynamically updated. Specifically, the distribution coefficient is updated by the following formula:
k m ＝min(k j )
wherein k is j The power distribution coefficient of the j-th phase is specifically as follows:
U 0 and
are respectively when k m When the phase angle is not less than 1, calculating the amplitude and phase angle of zero-sequence voltage required to be injected when MMC three-phase power is equal, specifically based on the relation between the zero-sequence voltage and the phase power; u shape j Is the MMC j-th alternating voltage amplitude; u shape dc The direct current voltage is MMC bus direct current voltage; m is j Is MMC j phase voltage modulation ratio.
It should be noted that, it is assumed that the injection amplitude of the zero sequence voltage is U 0 Phase angle of
When the system is in use, the unbalanced power delta P of the system is completely compensated; at zero sequence voltage injection amplitude of U 0b Phase angle of
The system unbalanced power is compensated k m Δ P, wherein k m As power division coefficient, k m ∈[0,1]Then, there are:
in the formula (I), the compound is shown in the specification,
thus, the method can obtain the result,
U 0b ＝k m U 0
that is, the phase angle of the injected zero sequence voltage is only related to the fault characteristic and is related to the power distribution coefficient k under the same fault condition m Is irrelevant. At equilibrium k m At delta P, the amplitude of the injected zero sequence voltage is k at which the total unbalanced power delta P is balanced m And (4) doubling. Based on the method, the output upper limit of the maximum utilization system can be solved, namely the power distribution coefficient k when the MMC is positioned at the overmodulation boundary m 。
When the over-modulation of the system is caused by injecting zero-sequence voltage complete balance phase power, the zero-sequence voltage amplitude phase angles are respectively U 0 、
And the modulation ratio of the overmodulation phase is m j . At equilibrium k j When delta P is reached, the maximum modulation ratio of the phase is just made to be 1, and the phase angles of the amplitude of the injected zero sequence voltage are respectively k according to the formula j U 0 、
Then according to the modulation ratio calculation formula, the modulation ratio expression obtained after injecting the zero sequence voltage is as follows:
wherein, when j = a, r =0; j = b, r = -1; and j = c, r =1. Further, the following can be obtained:
when multi-phase over-modulation exists, the power distribution coefficient is selected to meet the requirement that the modulation ratio of each phase is less than or equal to 1, so that the power distribution coefficient k is selected m Comprises the following steps:
k m ＝min(k a ,k b ,k c )
further, in an optional embodiment, the voltage modulation ratio of each phase of the MMC may be calculated by a voltage modulation ratio calculation formula; specifically, the MMC j-th phase voltage modulation ratio m j The ratio of the amplitude of the alternating current voltage to the voltage of the direct current bus is specifically as follows:
when m is j When the current phase is more than 1, judging that overmodulation occurs in the j phase; when m is j And when the current value is less than or equal to 1, judging that the modulation does not occur in the j phase.
In another alternative embodiment, the modulation ratio m of the j-th phase can be determined j Critical injection quantity U of zero sequence voltage of =1 hour j0b And k m Injection quantity U of zero sequence voltage in time of =1 0 To determine whether overmodulation occurs in the j-th phase. Specifically, when U is 0 ≤U j0b When no overmodulation occurs in the j phase, the MMC phase voltage modulation ratio m of the j phase is the same j Less than or equal to 1; when U is turned 0 ＞U j0b When the j phase is overmodulating, the MMC phase voltage modulation ratio m of j phase appears j Is more than 1; wherein the content of the first and second substances,
note that U is 0 The injection of (2) will cause the modulation ratio to increase, U j0b Is the modulation ratio m of the j-th phase j Critical injection amount of zero sequence voltage at time of =1, so if the injected zero sequence voltage exceeds U under one fault j0b This critical value will be overmodulating, and the decision method directly puts U on the boundary where overmodulation will be caused by injecting the control zero sequence voltage 0 To modulation ratio m j The connection is clearer.
Specifically, fig. 6 shows a block diagram of overmodulation boundary determination and power distribution coefficient update control.
In order to verify the effectiveness of the MMC adaptive phase power balance control method provided by the invention, an MMC simulation system with an YD transformer type shown in FIG. 2 is built on a Matlab/Simulink simulation platform, and model parameters are shown in Table 1.
TABLE 1
Fig. 7 and 8 are simulation waveforms of major electrical quantities of an MMC under a fault when the conventional zero-sequence voltage injection-based phase power balancing method and the adaptive phase power balancing method of the present invention are respectively adopted. And C-phase band transition resistance ground faults occur on the high-voltage side of the transformer when 3.2s is set, and the transition resistance is 2 omega.
As shown in fig. 7, before 3.2s, the MMC alternating voltage current is three symmetric, and the injected zero sequence voltage component is 0. The sub-module capacitor voltage is stabilized at about 2000V, and the MMC can normally run in a grid-connected mode. At 3.2s, a C-phase metallic ground fault occurred on the high-voltage side. As shown in fig. 7 (a), the voltage waveform of the low-voltage side phase of the transformer is constant, the amplitude of the a-phase voltage is constant, the amplitude of the B-phase voltage and the amplitude of the C-phase voltage are both dropped, and the amplitude of the alternating current is increased to ensure that the transmission power is constant. As shown in (f) of fig. 7, when the phase power is balanced by using the conventional zero sequence voltage injection-based method, the system is overmodulating, which affects the normal operation of the MMC, the output ac current of the MMC is distorted as shown in (b) of fig. 7, and the sub-module capacitor voltage is greatly fluctuated as shown in (d) of fig. 7. Simulation results show that the MMC can be overmodulatied by a traditional phase power balance control strategy based on zero sequence voltage injection, and safe and stable operation of the MMC is affected. And when the MMC overmodulation, the method fails, and the balance of phase power cannot be realized, so that the output current of an alternating current side is distorted, and the power supply quality is reduced.
As shown in fig. 8, the MMC can also operate normally and stably before 3.2 s. After the C-phase zone transition resistance ground fault occurs in 3.2s, the control system adjusts the unbalanced power distribution coefficient in a self-adaptive mode, and meanwhile, the zero sequence voltage injection and direct current circulation are adopted to adjust and control the balanced phase power. As shown in (f) of fig. 8, on the premise of controlling the B-phase modulation ratio to be maintained at 1, part of the power is balanced by injecting the zero-sequence voltage shown in (f) of fig. 8, and the rest of the asymmetric power is balanced by the dc circulating current regulation link shown in (c) of fig. 8. As shown in (b) of fig. 8, the MMC outputs an alternating current of three symmetry. Simulation results show that the adaptive phase power balance control strategy provided by the invention can automatically adjust the power distribution coefficient according to the fault condition, and realize phase power balance through the cooperation of zero sequence voltage injection and direct current circulation regulation links. The problems of overmodulation, current distortion and the like caused by the traditional zero sequence voltage injection method are avoided. The safe and stable operation of the MMC system is facilitated.
In conclusion, according to the invention, according to actual fault working condition information, by self-adaptively adjusting two control links of zero sequence voltage and direct current circulation, the redistribution of the three-phase power of the bridge arm is realized, the problem of the unbalance of the bridge arm power of the MMC under the condition of asymmetrical alternating current side is solved, the influence of the asymmetry of an alternating current power grid on the bridge arm power is eliminated, and the operation reliability of the MMC is improved. Compared with the traditional phase power balancing strategy, the method has wider application range, and avoids the problems of voltage overmodulation, current distortion and the like possibly occurring in the traditional control strategy.
Examples 2,
An MMC adaptive phase power equalization control system, comprising: the MMC adaptive phase power balance control method comprises a memory and a processor, wherein the memory stores a computer program, and the processor executes the MMC adaptive phase power balance control method provided by the embodiment 1 of the invention when executing the computer program.
The related technical scheme is the same as embodiment 1, and is not described herein.
Examples 3,
A modular multilevel converter, each phase of which comprises an upper bridge arm and a lower bridge arm; when the modular multilevel converter is subjected to phase power balance control, the MMC adaptive phase power balance control method provided by the embodiment 1 of the invention is adopted.
The related technical scheme is the same as embodiment 1, and is not described herein.
Examples 4,
A computer-readable storage medium, which includes a stored computer program, wherein when the computer program is executed by a processor, a device in which the storage medium is located is controlled to execute the MMC adaptive phase power equalization control method provided in embodiment 1 of the present invention.
The related technical scheme is the same as embodiment 1, and is not described herein.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (10)
1. An MMC adaptive phase power equalization control method is characterized by comprising the following steps:
s1, respectively carrying out PI control on the difference value of the capacitance voltage average value of each phase of the MMC and the total capacitance voltage average value of the three-phase submodule to obtain the unbalanced power of each phase;
s2, a pair of distribution coefficients k m Carrying out initialization;
s3, respectively carrying out unbalanced power on each phase according to the distribution coefficient k m Distributing to obtain first unbalanced power and second unbalanced power of each phase; respectively injecting zero-sequence voltage into the alternating-current voltage reference value of the corresponding bridge arm based on the first unbalanced power of each phase; dividing the second unbalanced power of each phase by the MMC direct-current bus voltage to obtain a direct-current circulating current reference value corresponding to the bridge arm;
s4, respectively carrying out capacitance and voltage balance control on sub-modules in the bridge arms on the basis of alternating current voltage reference values of the bridge arms, and simultaneously carrying out inner-ring bridge arm current control on the basis of direct current circulation reference values of the bridge arms;
s5, integrating unbalanced power which can be balanced when each phase alternating voltage of the MMC is in a critical overmodulation state by taking the MMC alternating voltage overmodulation as a boundary, and distributing coefficient k m And (4) updating, and turning to the step S3 to continuously balance and control the MMC phase power.
2. The MMC adaptive phase power balancing control method of claim 1, wherein the first unbalanced power of the j-th phase is: delta P j0 ＝k m ×ΔP j (ii) a The second unbalanced power of the j-th phase is: delta P jdc ＝(1-k m )×ΔP j ；
Wherein, Δ P j Unbalanced power of j phase; j = a, b, c; k is a radical of m ∈[0,1](ii) a When k is m When =1The unbalanced power of each phase is distributed to obtain only the first unbalanced power of each phase; when k is m Where =0, the unbalanced power of each phase is distributed to obtain only its second unbalanced power.
3. The MMC adaptive phase power balancing control method of claim 2, wherein the partition coefficient k m Updating is carried out through the following formula:
k m ＝min(k j )
wherein k is j The power distribution coefficient of the j-th phase is specifically as follows:
U 0 and
are respectively when k m The amplitude and the phase angle of zero sequence voltage required to be injected when the MMC three-phase power is equal to 1; u shape j Is the MMC j-th alternating voltage amplitude; u shape dc The direct current voltage is MMC bus direct current voltage; m is j The j-th phase voltage modulation ratio is MMC.
4. The MMC adaptive phase power balancing control method of claim 3, wherein the MMC j-th phase voltage modulation ratio m j Comprises the following steps:
5. the MMC adaptive phase power balancing control method of claim 3, wherein the modulation ratio m of the j-th phase is determined j Critical injection quantity U of zero sequence voltage of =1 hour j0b And k m Injection quantity U of zero sequence voltage in time of =1 0 Judging whether overmodulation occurs in the j phase; the method specifically comprises the following steps: when U is turned 0 ≤U j0b When no overmodulation occurs in the j phase, the MMC phase voltage modulation ratio m of the j phase is the same j Less than or equal to 1; when U is turned 0 ＞U j0b When the j phase is overmodulating, the MMC phase voltage modulation ratio m of j phase appears j Is more than 1; wherein the content of the first and second substances,
6. the MMC adaptive phase power balance control method of any one of claims 1 to 5, wherein a capacitor voltage average value U 'of a j-th phase of MMC' Cj Obtaining the result after filtering the average value of the capacitor voltage of each submodule of the j-th phase of the MMC through a double frequency trap; wherein j = a, b, c.
7. The MMC adaptive phase power balancing control method of any of claims 1-5, wherein the three-phase sub-module total capacitance voltage average
Comprises the following steps:
wherein the content of the first and second substances,
and the average value of the capacitance voltage of the j phase of the MMC.
8. An MMC adaptive phase power equalization control system, comprising: a memory storing a computer program and a processor executing the computer program to perform the MMC adaptive phase power balancing control method of any of claims 1-7.
9. A modular multilevel converter, each phase of which comprises an upper bridge arm and a lower bridge arm, is characterized in that when the modular multilevel converter is subjected to phase power balance control, the MMC self-adaptive phase power balance control method of any one of claims 1 to 7 is adopted.
10. A computer-readable storage medium, comprising a stored computer program, wherein the computer program, when executed by a processor, controls a device in which the storage medium is located to perform the MMC adaptive phase power balancing control method of any one of claims 1-7.
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| CN116938022B (en) * | 2023-09-18 | 2023-12-15 | 国网湖北省电力有限公司 | MMC type energy converter fault control method, device, system and medium |
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