CN112701895B - Improved control strategy of MMC (modular multilevel converter) during single-phase earth fault based on model prediction - Google Patents

Improved control strategy of MMC (modular multilevel converter) during single-phase earth fault based on model prediction Download PDF

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CN112701895B
CN112701895B CN202110112024.6A CN202110112024A CN112701895B CN 112701895 B CN112701895 B CN 112701895B CN 202110112024 A CN202110112024 A CN 202110112024A CN 112701895 B CN112701895 B CN 112701895B
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bridge arm
mmc
alternating current
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sequence
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CN112701895A (en
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郑曦
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Zheng Xi
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XUZHOU TRANSMISSION AND DISTRIBUTION CO Ltd
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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
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Abstract

The invention discloses an improved control strategy of an MMC during single-phase earth fault based on model prediction, which separates positive sequence, negative sequence and zero sequence of a control quantity and a sampling quantity of the MMC, establishes a prediction model of the MMC on an alpha-beta coordinate system according to a control target, predicts the number of conducted submodules of upper and lower bridge arms of each phase of the MMC by using a value function on the basis of the prediction model, and finally determines the on-off state of each submodule by combining a sorting voltage-sharing algorithm. The invention realizes the control of positive sequence, negative sequence and zero sequence alternating current and bridge arm circulation when the single-phase earth fault of the power grid occurs, does not need various control loops, simplifies the control structure, simultaneously does not need a PI controller, omits a fussy parameter setting process, and improves the dynamic response capability of the system compared with the traditional PI control.

Description

Improved control strategy of MMC (modular multilevel converter) during single-phase earth fault based on model prediction
Technical Field
The invention relates to a control method for power grid faults in the field of multi-level power electronic converters, in particular to an improved control strategy for an MMC (modular multilevel converter) during single-phase earth faults based on model prediction.
Background
The direct current transmission technology based on the voltage source type converter is called as flexible direct current transmission technology in China, and has the characteristics of no need of phase-changing voltage, independent control of active power and reactive power output, capability of supplying power to a passive network and the like due to the adoption of a fully-controlled switching device, has wide application prospects in the fields of new energy power generation grid connection, urban power grid capacity increasing transformation, isolated load power supply of offshore platforms and the like, and the Modular Multilevel Converter (MMC) becomes a preferred technical scheme of the current flexible direct current transmission engineering due to the advantages of modular design, low switching frequency, good harmonic performance and the like. For a flexible direct current transmission system, when the converter transformer adopts different wiring groups, the influence degree of the single-phase earth fault of a power grid on the converter is different. The zero sequence current can form circulation in the triangle winding, the triangle winding is adopted on the transformer valve side to isolate the zero sequence current, and the Y is adopted on the network side0Y with windings shaped to ensure reliable grounding of the system0The/delta transformer wiring group is widely applied. However, in the high voltage direct current transmission system, the voltage level of the valve side of the converter transformer is often very high, which is obviously different from that of the low voltage side of the traditional transformer, so that the adoption of the triangular winding on the valve side of the transformer can put higher requirements on the insulation fit of the system.
Y0/Y0The wiring group is used as another feasible wiring mode of the converter transformer, and natural neutral points exist on two sides of the transformer, so that the reliable grounding and insulation matching design of a system is facilitated. By Y0/Y0When the zero sequence current on the valve side is too large, the blocking of the MMC due to overcurrent may be caused, but the zero sequence current is compared with the zero sequence current on the valve side, so that the blocking of the MMC is avoided0A/delta connection group capable of enhancing the work of the MMC converter by inhibiting the zero sequence current on the network sideRate delivery capability. There are 3 main control objectives for MMC: alternating current, bridge arm circulation and submodule capacitor voltage, the control of positive sequence, negative sequence and zero sequence alternating current fault current, bridge arm circulation and submodule capacitor voltage is mainly realized by PI closed loop control in the conventional power grid ground fault control strategy, more control loops are adopted, and the control complexity is high. Meanwhile, as the number of PI controllers is large, the setting of parameters is difficult, and great burden is increased for debugging personnel.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides an improved control strategy of an MMC in single-phase earth fault based on model prediction, which utilizes a value function to realize the control of positive sequence, negative sequence and zero sequence alternating current, bridge arm circulation and sub-module capacitor voltage of the MMC in single-phase earth fault of a power grid, does not need various control loops, simplifies the control, does not need a PI controller at the same time, and omits a fussy parameter setting process.
In order to achieve the above object, the present invention provides an improved control strategy for an MMC during a single-phase ground fault based on model prediction, which separates a positive sequence, a negative sequence and a zero sequence of a control quantity and a sampling quantity of the MMC, establishes a prediction model for the MMC on an α - β coordinate system according to a control target, predicts the number of submodules, which are conducted by upper and lower bridge arms, of each phase of the MMC by using a cost function on the basis, and finally determines the on-off state of each submodule by combining a sorting voltage-sharing algorithm, and specifically comprises the following steps:
s1: and (3) carrying out positive sequence, negative sequence and zero sequence separation on the control quantity and the sampling quantity of the MMC at the moment k, wherein the separation process is as follows: the control quantity at the moment k is the bridge arm voltage of the upper bridge arm and the lower bridge arm of the MMC, and symbols u are respectively usedpj(k) And unj(k) And j represents three phases of a, b and c of the power grid voltage. u. ofpj(k) And unj(k) The number of submodules conducted by the upper and lower bridge arms of the MMC is determined, and the submodules can be expressed as follows:
Figure GDA0003450236810000031
wherein n ispj(k) And nnj(k) The number of submodules for conducting the upper bridge arm and the lower bridge arm of the MMC at the moment k is 0-N, N is the number of the submodules of each bridge arm of the MMC, and U is the number of the submodules of each bridge arm of the MMCcRated voltage of each sub-module capacitor of MMC, and Uc＝Udcand/N. Based on the voltage u, the AC equivalent output voltage u of the MMC at the moment kj(k) Can use upj(k) And unj(k) Expressed as:
Figure GDA0003450236810000032
the sampling quantity at the moment k is the bridge arm current i of the upper bridge arm and the lower bridge arm of the MMC respectivelypj(k) And inj(k) And the network voltage usj(k) Based on this, the AC current i at time k of MMCsj(k) And bridge arm circulation izj(k) Available bridge arm current ipj(k) And inj(k) Expressed as:
Figure GDA0003450236810000033
at the network voltage usj(k) For example, the separation steps of the positive sequence, the negative sequence and the zero sequence components of the control quantity and the sampling quantity at the time k are as follows:
first, u is transformed by means of Clarke coordinatessj(k) Converting the a-b-c coordinate system into an alpha-beta coordinate system by the following method:
Figure GDA0003450236810000041
wherein u issα(k) And usβ(k) Are each usj(k) The components on the alpha axis and the beta axis, and then the positive sequence component and the negative sequence component can be obtained by using a signal delay method, specifically:
Figure GDA0003450236810000042
wherein the content of the first and second substances,
Figure GDA0003450236810000043
and
Figure GDA0003450236810000044
are each usα(k) The positive and negative sequence components of (a),
Figure GDA0003450236810000045
and
Figure GDA0003450236810000046
are each usβ(k) Positive and negative sequence components of (a), qusα(k) Representing a component usα(k) Delay 90 °, qusβ(k) Representing a component usβ(k) Delayed by 90. u. ofsj(k) Zero sequence component
Figure GDA0003450236810000047
The acquisition mode is as follows:
Figure GDA0003450236810000048
similarly, an AC equivalent output voltage u can be obtainedj(k) Positive and negative sequence components in the alpha and beta axes
Figure GDA0003450236810000049
And
Figure GDA00034502368100000410
and zero sequence component u0(k) Alternating current isj(k) Positive and negative sequence components in the alpha and beta axes
Figure GDA00034502368100000411
And
Figure GDA00034502368100000412
and zero sequence component
Figure GDA00034502368100000413
Bridge armCirculation izj(k) Component i in the alpha and beta axeszα(k)、izβ(k)；
S2: establishing a prediction model of the MMC on an alpha-beta coordinate system according to the control target of the MMC in S1, and calculating the predicted value of each control target at the k +1 moment, wherein the specific process is as follows: according to the first-order forward eulerian method, the prediction model of the positive sequence alternating current in the α - β coordinate system can be expressed as:
Figure GDA0003450236810000051
wherein L isdIs an equivalent alternating current inductance of MMC, Ld＝L/2+LsL is bridge arm inductance, LsIs an alternating current inductance. RdEquivalent alternating current resistance, R, for MMCd＝R/2+RsR is bridge arm resistance, RsIs an alternating current resistance. T issIs the sampling frequency.
Figure GDA0003450236810000052
And respectively, the predicted values of the positive sequence alternating current at the moment k +1 on an alpha-beta coordinate system. Likewise, the prediction model of negative sequence alternating current in the α - β coordinate system can be expressed as:
Figure GDA0003450236810000053
wherein the content of the first and second substances,
Figure GDA0003450236810000054
and respectively predicting the negative sequence alternating current at the moment k +1 in an alpha-beta coordinate system. The prediction model of zero sequence alternating current can be expressed as:
Figure GDA0003450236810000055
wherein the content of the first and second substances,
Figure GDA0003450236810000056
is the time of k +1And (5) predicting the zero sequence alternating current. The prediction model of the bridge arm loop current in the alpha-beta coordinate system can be expressed as follows:
Figure GDA0003450236810000057
wherein izα(k+1)、izβ(k +1) are predicted values of the bridge arm circulating current at the time k +1 in an alpha-beta coordinate system, and the control quantity u at the time k in S1 is obtainedpj(k) And unj(k) Substituting the various possible values into the prediction equation of the control target to obtain all possible prediction values of the control target at the moment of k + 1;
s3: establishing corresponding value functions and system value functions for each control target at the k +1 moment in S2, and solving the number of submodules to be conducted by each bridge arm, wherein the specific process is as follows: establishing a cost function f of the positive sequence alternating currents pComprises the following steps:
Figure GDA0003450236810000062
wherein isαref、isβrefThe given values of the positive sequence alternating current at the moment k +1 on an alpha-beta coordinate system can be obtained through an outer ring PI control ring. Value function f of negative sequence alternating currents NComprises the following steps:
Figure GDA0003450236810000064
in order to suppress the negative-sequence alternating current, given values of the negative-sequence alternating current on the α - β coordinate system are all set to 0. Value function f of zero sequence alternating currents 0Comprises the following steps:
Figure GDA0003450236810000066
also, to suppress the zero sequence alternating current, the set value of the zero sequence alternating current is setIs 0. Value function f of bridge arm circulationzComprises the following steps:
fz＝(0-izα(k+1))+(0-izβ(k+1))
in order to suppress the bridge arm circulating current, given values of the bridge arm circulating current on an alpha-beta coordinate system are all set to be 0. In order to comprehensively control the positive sequence alternating current, the negative sequence alternating current, the zero sequence alternating current and the bridge arm circulation, a value function f of the system needs to be established, and the form of the value function f is as follows:
Figure GDA0003450236810000071
wherein the content of the first and second substances,
Figure GDA0003450236810000072
and λzRespectively positive sequence alternating current, negative sequence alternating current, zero sequence alternating current and weight coefficients of bridge arm circulation, the proportion of different control targets in a system cost function can be changed by adjusting the weight coefficients, the predicted value of each control target at the moment of k +1, which is obtained at S2, is substituted into the cost function, the switch state with the minimum value of the system cost function f is the optimal switch state, and the optimal switch state is defined as [ N ]pj，Nnj]，NpjThe number of submodules which are connected to the jth upper bridge arm is NnjThe number of submodules which are conducted by the jth phase lower bridge arm is shown;
s4: balancing capacitor voltage of each submodule in the S3 and determining the input state of each submodule by using a sorting voltage-sharing algorithm, wherein the specific process is as follows: performing balance control on the bridge arm capacitor voltage by using a sequencing algorithm, and calculating the optimal switching state [ N ] according to the bridge arm current direction of the converter and the optimal switching state obtained in the step S3pj，Nnj]And determining the switching condition of each submodule, wherein the specific method comprises the following steps:
s400: determining the switch state of each submodule of an upper bridge arm, and defining the bridge arm current of the upper bridge arm and the lower bridge arm of the j-th phase of the MMC as ipjAnd inj": if ipj>0, sequencing the capacitor voltages of the N sub-modules of the upper bridge arm of the j phase in an ascending order, and selecting the sub-modules with small capacitor voltagesNpjThe submodules work in the input state, and the other submodules work in the cut-off state; if ipjLess than or equal to 0, sorting the capacitance voltages of the N sub-modules of the upper bridge arm of the j phase in a descending order, and selecting N with large capacitance voltagepjAnd the submodules enable the submodules to work in the input state, and the other submodules work in the cut-off state.
S401: determining the switching state of each submodule of a lower bridge arm: if inj>0, sequencing the capacitor voltages of the N sub-modules of the upper bridge arm of the j phase in an ascending order, and selecting N with small capacitor voltagenjThe submodules work in the input state, and the other submodules work in the cut-off state; if injLess than or equal to 0, sorting the capacitance voltages of the N sub-modules of the upper bridge arm of the j phase in a descending order, and selecting N with large capacitance voltagenjAnd the submodules enable the submodules to work in the input state, and the other submodules work in the cut-off state.
Compared with the prior art, the method and the device realize the control of the MMC positive sequence, the MMC negative sequence, the MMC zero sequence and the bridge arm circulation when the single-phase earth fault of the power grid occurs, do not need various control loops, simplify the control structure, simultaneously do not need a PI controller, omit the complicated parameter setting process, and improve the dynamic response capability of the system compared with the traditional PI control.
Drawings
FIG. 1 is a flow chart of the present invention;
FIG. 2 is a main circuit topology of the modular multilevel converter of the present invention;
FIG. 3 is a sub-module circuit topology of the present invention;
fig. 4 is a system control block diagram of the present invention.
Detailed Description
The invention will be further explained with reference to the drawings.
As shown in fig. 1, the present embodiment provides an improved control strategy of an MMC during a single-phase ground fault based on model prediction, which separates a positive sequence, a negative sequence and a zero sequence of a control quantity and a sampling quantity of the MMC, establishes a prediction model of the MMC on an α - β coordinate system according to a control target, predicts the number of submodules, which are conducted by upper and lower bridge arms of each phase of the MMC, of each phase by using a cost function on the basis, and finally determines the on-off state of each submodule by combining a sorting voltage-sharing algorithm, and specifically includes the following steps:
s1: and (3) carrying out positive sequence, negative sequence and zero sequence separation on the control quantity and the sampling quantity of the MMC at the moment k, wherein the separation process is as follows: the control quantity at the moment k is the bridge arm voltage of the upper bridge arm and the lower bridge arm of the MMC, and symbols u are respectively usedpj(k) And unj(k) And j represents three phases of a, b and c of the power grid voltage. u. ofpj(k) And unj(k) The number of submodules conducted by the upper and lower bridge arms of the MMC is determined, and the submodules can be expressed as follows:
Figure GDA0003450236810000091
wherein n ispj(k) And nnj(k) The number of submodules for conducting the upper bridge arm and the lower bridge arm of the MMC at the moment k is 0-N, N is the number of the submodules of each bridge arm of the MMC, and U is the number of the submodules of each bridge arm of the MMCcRated voltage of each sub-module capacitor of MMC, and Uc＝Udcand/N. Based on the voltage u, the AC equivalent output voltage u of the MMC at the moment kj(k) Can use upj(k) And unj(k) Expressed as:
Figure GDA0003450236810000092
the sampling quantity at the moment k is the bridge arm current i of the upper bridge arm and the lower bridge arm of the MMC respectivelypj(k) And inj(k) And the network voltage usj(k) Based on this, the AC current i at time k of MMCsj(k) And bridge arm circulation izj(k) Available bridge arm current ipj(k) And inj(k) Expressed as:
Figure GDA0003450236810000093
at the network voltage usj(k) For example, the separation steps of the positive sequence, the negative sequence and the zero sequence components of the control quantity and the sampling quantity at the time k are as follows:
first, u is transformed by means of Clarke coordinatessj(k) Converting the a-b-c coordinate system into an alpha-beta coordinate system by the following method:
Figure GDA0003450236810000101
wherein u issα(k) And usβ(k) Are each usj(k) The components on the alpha axis and the beta axis, and then the positive sequence component and the negative sequence component can be obtained by using a signal delay method, specifically:
Figure GDA0003450236810000102
wherein the content of the first and second substances,
Figure GDA0003450236810000103
and
Figure GDA0003450236810000104
are each usα(k) The positive and negative sequence components of (a),
Figure GDA0003450236810000105
and
Figure GDA0003450236810000106
are each usβ(k) Positive and negative sequence components of (a), qusα(k) Representing a component usα(k) Delay 90 °, qusβ(k) Representing a component usβ(k) Delayed by 90. u. ofsj(k) Zero sequence component
Figure GDA0003450236810000107
The acquisition mode is as follows:
Figure GDA0003450236810000108
similarly, an AC equivalent output voltage u can be obtainedj(k) Positive and negative sequence components in the alpha and beta axes
Figure GDA0003450236810000109
And
Figure GDA00034502368100001010
and zero sequence component u0(k) Alternating current isj(k) Positive and negative sequence components in the alpha and beta axes
Figure GDA00034502368100001011
And
Figure GDA00034502368100001012
and zero sequence component
Figure GDA00034502368100001013
Bridge arm circulation izj(k) Component i in the alpha and beta axeszα(k)、izβ(k)；
S2: establishing a prediction model of the MMC on an alpha-beta coordinate system according to the control target of the MMC in S1, and calculating the predicted value of each control target at the k +1 moment, wherein the specific process is as follows: according to the first-order forward eulerian method, the prediction model of the positive sequence alternating current in the α - β coordinate system can be expressed as:
Figure GDA0003450236810000111
wherein L isdIs an equivalent alternating current inductance of MMC, Ld＝L/2+LsL is bridge arm inductance, LsIs an alternating current inductance. RdEquivalent alternating current resistance, R, for MMCd＝R/2+RsR is bridge arm resistance, RsIs an alternating current resistance. T issIs the sampling frequency.
Figure GDA0003450236810000112
And respectively, the predicted values of the positive sequence alternating current at the moment k +1 on an alpha-beta coordinate system. Likewise, the prediction model of negative sequence alternating current in the α - β coordinate system can be expressed as:
Figure GDA0003450236810000113
wherein the content of the first and second substances,
Figure GDA0003450236810000114
and respectively predicting the negative sequence alternating current at the moment k +1 in an alpha-beta coordinate system. The prediction model of zero sequence alternating current can be expressed as:
Figure GDA0003450236810000115
wherein the content of the first and second substances,
Figure GDA0003450236810000116
the method is a predicted value of the zero-sequence alternating current at the moment of k + 1. The prediction model of the bridge arm loop current in the alpha-beta coordinate system can be expressed as follows:
Figure GDA0003450236810000121
wherein izα(k+1)、izβ(k +1) are predicted values of the bridge arm circulating current at the time k +1 in an alpha-beta coordinate system, and the control quantity u at the time k in S1 is obtainedpj(k) And unj(k) Substituting the various possible values into the prediction equation of the control target to obtain all possible prediction values of the control target at the moment of k + 1;
s3: establishing corresponding value functions and system value functions for each control target at the k +1 moment in S2, and solving the number of submodules to be conducted by each bridge arm, wherein the specific process is as follows: establishing a cost function f of the positive sequence alternating currents pComprises the following steps:
Figure GDA0003450236810000123
wherein isαref、isβrefRespectively, the given value of the positive sequence alternating current at the moment k +1 on an alpha-beta coordinate system can pass throughAnd obtaining an outer ring PI control ring. Value function f of negative sequence alternating currents NComprises the following steps:
Figure GDA0003450236810000125
in order to suppress the negative-sequence alternating current, given values of the negative-sequence alternating current on the α - β coordinate system are all set to 0. Value function f of zero sequence alternating currents 0Comprises the following steps:
Figure GDA0003450236810000127
also, to suppress the zero-sequence alternating current, the given value of the zero-sequence alternating current is set to 0. Value function f of bridge arm circulationzComprises the following steps:
fz＝(0-izα(k+1))+(0-izβ(k+1))
in order to suppress the bridge arm circulating current, given values of the bridge arm circulating current on an alpha-beta coordinate system are all set to be 0. In order to comprehensively control the positive sequence alternating current, the negative sequence alternating current, the zero sequence alternating current and the bridge arm circulation, a value function f of the system needs to be established, and the form of the value function f is as follows:
Figure GDA0003450236810000131
wherein the content of the first and second substances,
Figure GDA0003450236810000132
and λzRespectively positive sequence alternating current, negative sequence alternating current, zero sequence alternating current and weight coefficients of bridge arm circulation, the proportion of different control targets in a system cost function can be changed by adjusting the weight coefficients, the predicted value of each control target at the moment of k +1, which is obtained at S2, is substituted into the cost function, the switch state with the minimum value of the system cost function f is the optimal switch state, and the optimal switch state is defined as [ N ]pj，Nnj]，NpjThe upper arm of the j-th phase should be conductedNumber of submodules, NnjThe number of submodules which are conducted by the jth phase lower bridge arm is shown;
s4: balancing capacitor voltage of each submodule in the S3 and determining the input state of each submodule by using a sorting voltage-sharing algorithm, wherein the specific process is as follows: performing balance control on the bridge arm capacitor voltage by using a sequencing algorithm, and calculating the optimal switching state [ N ] according to the bridge arm current direction of the converter and the optimal switching state obtained in the step S3pj，Nnj]And determining the switching condition of each submodule, wherein the specific method comprises the following steps:
s400: determining the switch state of each submodule of an upper bridge arm, and defining the bridge arm current of the upper bridge arm and the lower bridge arm of the j-th phase of the MMC as ipjAnd inj": if ipj>0, sequencing the capacitor voltages of the N sub-modules of the upper bridge arm of the j phase in an ascending order, and selecting N with small capacitor voltagepjThe submodules work in the input state, and the other submodules work in the cut-off state; if ipjLess than or equal to 0, sorting the capacitance voltages of the N sub-modules of the upper bridge arm of the j phase in a descending order, and selecting N with large capacitance voltagepjAnd the submodules enable the submodules to work in the input state, and the other submodules work in the cut-off state.
S401: determining the switching state of each submodule of a lower bridge arm: if inj>0, sequencing the capacitor voltages of the N sub-modules of the upper bridge arm of the j phase in an ascending order, and selecting N with small capacitor voltagenjThe submodules work in the input state, and the other submodules work in the cut-off state; if injLess than or equal to 0, sorting the capacitance voltages of the N sub-modules of the upper bridge arm of the j phase in a descending order, and selecting N with large capacitance voltagenjAnd the submodules enable the submodules to work in the input state, and the other submodules work in the cut-off state.
As shown in FIG. 2, a main circuit of a Modular Multilevel Converter (MMC) applying the improved control strategy comprises a three-phase circuit a, a three-phase circuit b and a three-phase circuit c, wherein each phase comprises an upper bridge arm, a lower bridge arm, a reactor L and a resistor R which are connected in series, and the upper bridge arm comprises N sub-modules (SM)p1-SMpN) The lower bridge arm comprises N sub-modules (SM)n1-SMnN)。LsIs an alternating current inductance, RsIs an alternating current resistance usjFor three-phase mains voltage, j ═ a, b, c, UdcIs a dc voltage.
Each phase of the MMC consists of 2N submodules, as shown in fig. 3, diodes D1 and D2 in the submodules are anti-parallel diodes of power switches VT1 and VT2, respectively; c is a DC capacitor with a voltage uc(ii) a The power switches VT1 and VT2 are connected in series and then connected in parallel with the dc capacitor C, A, B is the input and output ends of the sub-module, as shown in fig. 2, the upper bridge arm and the lower bridge arm are each formed by connecting N sub-modules in series, that is, the output end B of the previous sub-module is connected to the input end a of the next sub-module. Upper bridge arm top sub-module SMp1The input end A of the bridge arm is connected to the positive pole of a direct current power supply, and the lowest sub-module SM of the lower bridge armnNThe output end B of the DC power supply is connected to the cathode of the DC power supply.
For convenience of description, first, an operating state of the MMC during normal operation is described, where the operating state of the MMC during normal operation is as shown in fig. 3, when the power switch VT1 is turned on and the power switch VT2 is turned off, a current charges the capacitor C through the diode D1, or discharges the capacitor C through the power switch VT1, and a sub-module output voltage is + ucCalled the input state; when the power switch VT1 is turned off and the power switch VT2 is turned on, the current passes through the power switch VT2 or the diode D2, the capacitor C is always in the bypass state, the voltage thereof does not change, the sub-module output voltage is 0, which is called the cut-off state,
the influence of the bridge arm current direction on the sub-module capacitance voltage is illustrated by taking the MMC phase a as an example, as shown in FIG. 2, the upper bridge arm current ipaAnd lower arm current inaAll the positive directions of (1) are downward. When upper bridge arm current ipaAnd lower arm current inaWhen the value of the voltage is greater than 0, the sub-module capacitor C is charged for charging current, and the capacitor voltage is increased; when upper bridge arm current ipaAnd lower arm current inaWhen the value of (d) is less than 0, the sub-module capacitor C will discharge, and the capacitor voltage will decrease, for a discharge current.

Claims (2)

1. The improved control strategy of the MMC during the single-phase earth fault based on model prediction is characterized in that positive sequence, negative sequence and zero sequence separation are carried out on the control quantity and the sampling quantity of the MMC, a prediction model of the MMC is established on an alpha-beta coordinate system according to a control target, the number of sub-modules conducted by upper and lower bridge arms of each phase of the MMC is predicted by using a value function on the basis, and finally the on-off state of each sub-module is determined by combining with a sorting voltage-sharing algorithm;
the method comprises the following specific steps:
s1: firstly, carrying out positive sequence, negative sequence and zero sequence separation on the control quantity and the sampling quantity of the MMC at the k moment, wherein the separation process is as follows:
the control quantity at the moment k is the bridge arm voltage of the upper bridge arm and the lower bridge arm of the MMC, and symbols u are respectively usedpj(k) And unj(k) The expression, j ═ a, b, c, represents the three phases of the grid voltage a, b, c; u. ofpj(k) And unj(k) The number of submodules conducted by the upper and lower bridge arms of the MMC is determined, and the submodules can be expressed as follows:
Figure FDA0003526190520000011
wherein n ispj(k) And nnj(k) The number of submodules for conducting the upper bridge arm and the lower bridge arm of the MMC at the moment k is 0-N, N is the number of the submodules of each bridge arm of the MMC, and U is the number of the submodules of each bridge arm of the MMCcRated voltage of each sub-module capacitor of MMC, and Uc＝Udc/N，UdcBased on the AC equivalent output voltage u of MMC at k momentj(k) Can use upj(k) And unj(k) Expressed as:
Figure FDA0003526190520000012
the sampling quantity at the moment k is the bridge arm current i of the upper bridge arm and the lower bridge arm of the MMC respectivelypj(k) And inj(k) And the network voltage usj(k) Based on this, the AC current i at time k of MMCsj(k) And bridge arm circulation izj(k) Available bridge arm current ipj(k) And inj(k) Expressed as:
Figure FDA0003526190520000021
at the network voltage usj(k) For example, the separation steps of the positive sequence, the negative sequence and the zero sequence components of the control quantity and the sampling quantity at the time k are as follows:
first, u is transformed by means of Clarke coordinatessj(k) Converting the a-b-c coordinate system into an alpha-beta coordinate system by the following method:
Figure FDA0003526190520000022
wherein u issα(k) And usβ(k) Are each usj(k) The components on the alpha axis and the beta axis, and then the positive sequence component and the negative sequence component can be obtained by using a signal delay method, specifically:
Figure FDA0003526190520000023
wherein the content of the first and second substances,
Figure FDA0003526190520000024
and
Figure FDA0003526190520000025
are each usα(k) The positive and negative sequence components of (a),
Figure FDA0003526190520000026
and
Figure FDA0003526190520000027
are each usβ(k) Positive and negative sequence components of (a), qusα(k) Representing a component usα(k) Delay 90 °, qusβ(k) Representing a component usβ(k) Retardation of 90 °; u. ofsj(k) Zero sequence component
Figure FDA0003526190520000028
The acquisition mode is as follows:
Figure FDA0003526190520000031
similarly, an AC equivalent output voltage u can be obtainedj(k) Positive and negative sequence components in the alpha and beta axes
Figure FDA0003526190520000032
And
Figure FDA0003526190520000033
and zero sequence component u0(k) Alternating current isj(k) Positive and negative sequence components in the alpha and beta axes
Figure FDA0003526190520000034
And
Figure FDA0003526190520000035
and zero sequence component
Figure FDA0003526190520000036
Bridge arm circulation izj(k) Positive and negative sequence components i in the alpha and beta axeszα(k)、izβ(k)；
S2: establishing a prediction model of the MMC on an alpha-beta coordinate system according to the control target of the MMC in S1, and calculating the predicted value of each control target at the k +1 moment, wherein the specific process is as follows:
according to the first-order forward eulerian method, the prediction model of the positive sequence alternating current in the α - β coordinate system can be expressed as:
Figure FDA0003526190520000037
wherein L isdIs an equivalent alternating current inductance of MMC, Ld＝L/2+LsL is bridge arm inductance, LsIs an alternating currentFeeling; rdEquivalent alternating current resistance, R, for MMCd＝R/2+RsR is bridge arm resistance, RsIs an alternating current resistor; t issIs the sampling frequency;
Figure FDA0003526190520000038
respectively predicting values of the positive sequence alternating current at the moment k +1 in an alpha-beta coordinate system; likewise, the prediction model of negative sequence alternating current in the α - β coordinate system can be expressed as:
Figure FDA0003526190520000039
wherein the content of the first and second substances,
Figure FDA00035261905200000310
respectively predicting values of negative sequence alternating current at the moment k +1 in an alpha-beta coordinate system; the prediction model of zero sequence alternating current can be expressed as:
Figure FDA0003526190520000041
wherein the content of the first and second substances,
Figure FDA0003526190520000042
is a predicted value of the zero sequence alternating current at the moment of k + 1; the prediction model of the bridge arm loop current in the alpha-beta coordinate system can be expressed as follows:
Figure FDA0003526190520000043
wherein izα(k+1)、izβ(k +1) are predicted values of the bridge arm circulation at the moment of k +1 in an alpha-beta coordinate system respectively; by controlling the amount u at the time k in S1pj(k) And unj(k) Substituting the various possible values into the prediction equation of the control target to obtain all possible prediction values of the control target at the moment of k + 1;
s3: establishing corresponding value functions and system value functions for each control target at the k +1 moment in S2, and solving the number of submodules to be conducted by each bridge arm, wherein the specific process is as follows:
establishing a cost function f of the positive sequence alternating currents pComprises the following steps:
Figure FDA0003526190520000044
wherein isαref、isβrefRespectively setting values of the positive sequence alternating current at the moment k +1 on an alpha-beta coordinate system, wherein the values can be obtained through an outer ring PI control ring; value function f of negative sequence alternating currents NComprises the following steps:
Figure FDA0003526190520000045
in order to suppress the negative sequence alternating current, given values of the negative sequence alternating current on the α - β coordinate system are all set to 0; value function f of zero sequence alternating currents 0Comprises the following steps:
Figure FDA0003526190520000051
likewise, to suppress the zero-sequence alternating current, the given value of the zero-sequence alternating current is set to 0; value function f of bridge arm circulationzComprises the following steps:
fz＝(0-izα(k+1))+(0-izβ(k+1))
in order to inhibit the bridge arm circulation, setting all given values of the bridge arm circulation on an alpha-beta coordinate system as 0; in order to comprehensively control the positive sequence alternating current, the negative sequence alternating current, the zero sequence alternating current and the bridge arm circulation, a value function f of the system needs to be established, and the form of the value function f is as follows:
Figure FDA0003526190520000052
wherein the content of the first and second substances,
Figure FDA0003526190520000053
and λzThe weight coefficients of the positive sequence alternating current, the negative sequence alternating current, the zero sequence alternating current and the bridge arm circulation are respectively, and the proportions of different control targets in a system value function can be changed by adjusting the sizes of the weight coefficients; substituting the predicted values of the control targets at the time k +1, which are obtained in S2, into the cost function, so that the switching state with the smallest value of the system cost function f is the optimal switching state, and defining the optimal switching state as [ N ]pj，Nnj]，NpjThe number of submodules which are connected to the jth upper bridge arm is NnjThe number of submodules which are conducted by the jth phase lower bridge arm is shown;
s4: and balancing the capacitor voltage of each submodule in the S3 and determining the input state of each submodule in the S3 by utilizing a sequencing voltage-sharing algorithm.
2. The MMC improved control strategy based on model prediction of claim 1, wherein the balancing of the capacitor voltage of each submodule and the determination of the input state of each submodule in S4 are as follows:
performing balance control on the bridge arm capacitor voltage by using a sequencing algorithm, and calculating the optimal switching state [ N ] according to the bridge arm current direction of the converter and the optimal switching state S3pj，Nnj]And determining the switching condition of each submodule, wherein the specific method comprises the following steps:
s400: determining the switch state of each submodule of an upper bridge arm, and defining the bridge arm current of the upper bridge arm and the lower bridge arm of the j-th phase of the MMC as ipjAnd inj: if ipj>0, sequencing the capacitor voltages of the N sub-modules of the upper bridge arm of the j phase in an ascending order, and selecting N with small capacitor voltagepjThe submodules work in the input state, and the other submodules work in the cut-off state; if ipjLess than or equal to 0, sorting the capacitance voltages of the N sub-modules of the upper bridge arm of the j phase in a descending order, and selecting N with large capacitance voltagepjSub-moduleTo make them work in the input state and other submodules work in the cut-off state;
s401: determining the switching state of each submodule of a lower bridge arm: if inj>0, sequencing the capacitor voltages of the N sub-modules of the upper bridge arm of the j phase in an ascending order, and selecting N with small capacitor voltagenjThe submodules work in the input state, and the other submodules work in the cut-off state; if injLess than or equal to 0, sorting the capacitance voltages of the N sub-modules of the upper bridge arm of the j phase in a descending order, and selecting N with large capacitance voltagenjAnd the submodules enable the submodules to work in the input state, and the other submodules work in the cut-off state.
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