阅读说明：本技术 一种mmc换流阀调制策略的选择方法 (Selection method of modulation strategy of MMC converter valve ) 是由 荣飞 尹章涛 饶宏 黄守道 周保荣 马河涛 于 2017-02-28 设计创作，主要内容包括：本发明公开了一种MMC换流阀调制策略的选择方法,在一定的容量和直流电压条件下,首先求解出一个工频周期内采用载波移相调制得到的换流阀总损耗,然后求解出一个工频周期内采用最近电平逼近调制得到的换流阀总损耗,最后令两种调制方式下的损耗相等,求解出MMC换流阀桥臂模块个数。当实际应用中的子模块个数大于或等于求解得到的N值时,采用最近电平逼近的调制方式；当实际应用中的子模块个数小于求解得到的N值时,采用载波移相的调制方式。本发明所述的调制策略的选择方法,有利于减小MMC换流阀系统的损耗,降低运行成本。(The invention discloses a selection method of a modulation strategy of an MMC converter valve, which comprises the steps of firstly solving the total loss of the converter valve obtained by adopting carrier phase shift modulation in a power frequency period under the condition of certain capacity and direct-current voltage, then solving the total loss of the converter valve obtained by adopting nearest level approximation modulation in the power frequency period, and finally making the losses under the two modulation modes equal to each other and solving the number of bridge arm modules of the MMC converter valve. When the number of the sub-modules in practical application is larger than or equal to the N value obtained by solving, adopting a modulation mode of nearest level approximation; and when the number of the sub-modules in practical application is smaller than the solved N value, a modulation mode of carrier phase shift is adopted. The selection method of the modulation strategy is beneficial to reducing the loss of the MMC converter valve system and reducing the running cost.)

1. A method for selecting a modulation strategy of an MMC converter valve is characterized in that the MMC converter valve adopts a three-phase six-bridge-arm symmetrical inversion structure, each phase comprises an upper bridge arm and a lower bridge arm, each bridge arm comprises N identical SM submodules and a bridge arm inductor which are mutually connected in series, and a phase line is led out from a connecting line of the upper bridge arm inductor and the lower bridge arm inductor; the method for selecting the modulation strategy is characterized by comprising the following steps of:

step 1, calculating total loss P of the MMC converter valve obtained by adopting a carrier phase shift modulation mode in a power frequency period_cps；

Step 2, calculating the total loss P of the MMC converter valve obtained by adopting the nearest level approximation modulation mode in a power frequency period_nlm；

Step 3, comparing the losses of the two, and determining a proper modulation strategy;

make the total system loss equal under two modulation modes, i.e. P_cps-P_nlmSolving the value N when the value is 0;

when the number of SM submodules in practical application is larger than the N value obtained by solving, adopting a modulation mode of nearest level approximation; when the number of SM submodules in practical application is smaller than the solved N value, a modulation mode of carrier phase shift is adopted; and when the number of the sub-modules in practical application is equal to the N value obtained by solving, considering the cost of the control system and adopting a modulation mode of nearest level approximation.

2. The method for selecting the modulation strategy of the MMC converter valve according to claim 1, wherein the step 1 comprises the following steps:

(1.1) calculating the average current and the effective current flowing through each device in a power frequency period:

flow through T in one power frequency period₁The average current and the effective current of (a) are respectively:

flow through T in one power frequency period₂The average current and the effective current of (a) are respectively:

current through D in a power frequency cycle₁The average current and the effective current of (a) are respectively:

current through D in a power frequency cycle₂The average current and the effective current of (a) are respectively:

wherein T is a power frequency period, m is a modulation ratio, omega is a fundamental wave angular frequency, T represents time, k represents the kth SM submodule and takes the values of 1,2, … and N; f. of_cIs carrier frequency, S is rated power of the MMC converter valve,is a power factor angle, U, of an MMC converter valve in a rated state_dcFor the direct-current side voltage of the MMC converter valve, the integral time t₁、t₂And t₃Calculated by the following formula:

(1.2) calculating the on-state loss and the dynamic loss generated by the kth SM submodule:

wherein, U_CEFor bias of the turn-on voltage of the IGBT tube, r_CEIs its corresponding on-resistance; u shape_fIs the on-voltage bias of the diode, r_fIs its corresponding on-resistance; e_{T_ref}The sum of single turn-on loss and single turn-off loss of the IGBT is obtained; e_{D_ref}Single reverse recovery losses for the diode; u shape_CE、r_CE、U_f、r_f、、E_{T_ref}And E_{D_ref}All can be obtained from the specification of IGBT manufacturers; u shape_refAnd I_refMeasurement by IGBT manufacturers E_{T_ref}Collector-emitter voltage and collector current;

(1.3) calculating the total loss of the MMC converter valve:

3. the method for selecting the modulation strategy of the MMC converter valve according to claim 2, wherein the step 2 comprises the following steps:

(2.1) calculating the on-state loss:

wherein n is_apNumber of submodules put into the bridge arm on phase a, i_apRepresenting the upper arm current of phase a, P_{T_con}(i_ap) And P_{T_con}(i_ap) To relate to i_apIs obtained by the following equation:

in the above formula, round () is a rounding function;

(2.2) calculating the necessary switching losses:

wherein, a₁、b₁And c₁A quadratic fitting coefficient of IGBT turn-off loss; a is₂、b₂And c₂Quadratic fitting coefficients are obtained for the IGBT turn-on loss; a is₃、b₃And c₃A second-order fitting coefficient of the reverse recovery loss of the diode; a is₁、b₁、c₁、a₂、b₂、c₂、a₃、b₃And c₃Obtained from the specification of IGBT manufacturers; k is a radical of₁、k₂And k₃The cut-off voltage correction coefficients, which are the turn-off loss of the IGBT, the turn-on loss of the IGBT and the reverse recovery loss of the diode, are calculated by the following formula,

(2.3) calculating additional switching losses:

wherein C represents the capacitance size of the submodule, Deltau is the maximum deviation allowed by the capacitor voltage under the voltage-sharing control and is determined by a user, f is the control frequency, I_mRepresents the maximum value of the a-phase output current, I_maxThese two values are calculated by the following equation for the maximum value of the bridge arm current:

(2.4) calculating the total loss of the converter valve under the recent level approximation modulation strategy:

P_nlm＝6(P_con+P_sw1+P_sw2)。

4. the method for selecting the modulation strategy of an MMC converter valve according to claim 3, characterized in that in step (2.2), a₁、b₁、c₁For the IGBT turn-off loss quadratic fitting coefficient, the curve of' typical collector current-turn-off loss at 125 ℃ in the specification of IGBT manufacturers is obtained by adopting a quadratic curve fitting mode, a₁Is the constant term coefficient in the fitting method, b₁Is the coefficient of a first order term, c₁Is the coefficient of the quadratic term; a is₂、b₂、c₂For the quadratic fitting coefficient of the IGBT opening loss, the quadratic fitting coefficient is obtained by adopting a quadratic curve fitting mode to a typical collector current-opening loss curve at 125 ℃ in the specification of IGBT manufacturers, a₂Is the constant term coefficient in the fitting method, b₂Is the coefficient of a first order term, c₂Is the coefficient of the quadratic term; a is₃、b₃、c₃The secondary fitting coefficient of the reverse recovery loss of the diode is obtained by adopting a secondary curve fitting mode to a typical on-state current-reverse recovery loss curve at 125 ℃ in the specification of IGBT manufacturers, and a₃Is the constant term coefficient in the fitting method, b₃Is the coefficient of a first order term, c₃Is the coefficient of the quadratic term.

5. The MMC converter valve modulation strategy of claim 4, wherein a₁Is 378.2, b₁Is 4.025, c₁Is 0.00006071, a₂Is 684.4, b₂Is 3.059, c₂0.0006558; u shape_fIs 1.079V, r_fIs 0.001109 omega, a₃Is 644.2, b₃Is 3.103, c₃Is-0.0007948.

6. The MMC converter valve modulation strategy of any of claims 2-5, wherein S is 2MVA, U_dc10kV, m is 0.95, omega is 50Hz, T is 0.02s,

Technical Field

The invention belongs to the field of flexible direct current transmission, and particularly relates to the aspects of modulation strategies and loss calculation of a modular multilevel converter.

Background

With the development of power electronic technology, Modular Multilevel (MMC) converter valves have greatly facilitated the development of high-voltage direct-current transmission technology. But high loss is a major limiting factor in high power transmission.

Each bridge arm of the Modular Multilevel (MMC) converter valve is connected with a plurality of identical submodules in series, output multilevel can be realized by controlling the conduction and the closing of the submodules, the harmonic content of output waveforms is low, a filter does not need to be additionally arranged in a power system, and the cost is greatly reduced. Losses in the MMC converter valve system are mainly generated by IGBTs and diodes in submodules, a common modulation strategy comprises carrier phase shift modulation and nearest level approximation modulation, and losses of the MMC converter valve system under different modulation strategies are different. And the proper modulation strategy can greatly reduce the running cost of the system.

Although the carrier frequency of each sub-module is fixed in the carrier phase-shift modulation mode, a plurality of PI regulators are needed, so that the complexity of a control system is high, and PI setting is difficult, so that the recent level approximation modulation is often adopted in the situation that the number of bridge arm sub-modules is large at present. However, under recent level approaching modulation, sub-module voltage-sharing control enables the average switching frequency of the sub-modules to increase along with the increase of the number of bridge arm sub-modules, and therefore system loss rises sharply. The carrier phase shift modulation method does not have the problem. The two modulation modes have advantages and disadvantages, so that a selection method of the modulation strategy of the MMC converter valve needs to be designed, so that the system loss is reduced, and the operation cost of the system is reduced.

Disclosure of Invention

The invention aims to solve the problem that in order to overcome the defects of the prior art, the invention provides a method for selecting a modulation strategy of an MMC converter valve, and the modulation strategy with the minimum loss under a specific system parameter is selected.

In order to achieve the above object, the invention adopts the technical scheme that:

a method for selecting a modulation strategy of an MMC converter valve is characterized in that the MMC converter valve adopts a three-phase six-bridge-arm symmetrical inversion structure, each phase comprises an upper bridge arm and a lower bridge arm, each bridge arm comprises N identical SM submodules and a bridge arm inductor which are mutually connected in series, and a phase line is led out from a connecting line of the upper bridge arm inductor and the lower bridge arm inductor; each SM submodule comprises two IGBT tubes T₁And T₂Two diodes D₁And D₂And a capacitor C; wherein D is₁And D₂Are respectively connected in reverse parallel at T₁And T₂Upper, T₁The collector of the capacitor C is connected with the anode of the capacitor C, the emitter is used as the positive port of the SM submodule, T₂The collector electrode of the capacitor C is connected to the positive port, the emitter electrode is used as the negative port of the SM submodule, the negative electrode of the capacitor C is connected to the negative port, and T₁And T₂The grid of the grid receives a control signal; under the condition of certain capacity and direct-current voltage, the selection method of the modulation strategy comprises the following steps:

step 1, calculating the total loss P of the MMC converter valve obtained by adopting a carrier phase shift modulation mode in a power frequency period_cps；

Step 2, calculating the total loss P of the MMC converter valve obtained by adopting the nearest level approximation modulation mode in a power frequency period_nlm；

Step 3, comparing the losses of the two, and determining a proper modulation strategy;

make the total loss of the MMC converter valves under two modulation modes equal, namely P_cps-P_nlmSolving the number N of SM submodules in each bridge arm of the MMC converter valve when the number is 0;

when the number of SM submodules in practical application is larger than the N value obtained by solving, adopting a modulation mode of nearest level approximation; when the number of SM submodules in practical application is smaller than the solved N value, a modulation mode of carrier phase shift is adopted; and when the number of the sub-modules in practical application is equal to the N value obtained by solving, considering the cost of the control system and adopting a modulation mode of nearest level approximation.

Further, the step 1 specifically includes the following steps:

(1.1) calculating the average current and the effective current flowing through each device in a power frequency period:

flow through T in one power frequency period₁The average current and the effective current of (a) are respectively:

flow through T in one power frequency period₂The average current and the effective current of (a) are respectively:

current through D in a power frequency cycle₁The average current and the effective current of (a) are respectively:

current through D in a power frequency cycle₂The average current and the effective current of (a) are respectively:

wherein T is the power frequency period, m is the modulation ratio, omega is the fundamental angular frequency, T represents time, and k represents the kthThe SM submodule takes the value of 1,2, …, N; f. of_cIs carrier frequency, S is rated power of the MMC converter valve,

is a power factor angle, U, of an MMC converter valve in a rated state_dcFor the direct-current side voltage of the MMC converter valve, the integral time t₁、t₂And t₃Can be calculated by the following formula:

(1.2) calculating the on-state loss and the dynamic loss generated by the kth SM submodule:

wherein, U_CEFor bias of the turn-on voltage of the IGBT tube, r_CEIs its corresponding on-resistance; u shape_fIs the on-voltage bias of the diode, r_fIs its corresponding on-resistance; e_{T_ref}The sum of single turn-on loss and single turn-off loss of the IGBT is obtained; e_{D_ref}Single reverse recovery losses for the diode; u shape_CE、r_CE、U_f、r_f、、E_{T_ref}And E_{D_ref}All can be obtained from the specification of IGBT manufacturers; u shape_refAnd I_refMeasurement by IGBT manufacturers E_{T_ref}Collector-emitter voltage and collector current;

(1.3) calculating the total loss of the MMC converter valve:

further, the step 1 specifically includes the following steps:

(2.1) calculating the on-state loss:

wherein n is_apNumber of submodules put into the bridge arm on phase a, i_apRepresenting the upper arm current of phase a, P_{T_con}(i_ap) And P_{T_con}(i_ap) To relate to i_apCan be obtained by the following equations:

in the above formula, round () is a rounding function;

(2.2) calculating the necessary switching losses:

wherein, a₁、b₁、c₁A quadratic fitting coefficient of IGBT turn-off loss; a is₂、b₂、c₂Quadratic fitting coefficients are obtained for the IGBT turn-on loss; a is₃、b₃、c₃A second-order fitting coefficient of the reverse recovery loss of the diode; k is a radical of₁、k₂And k₃Turn-off loss of IGBT, turn-on loss of IGBT and diode respectivelyThe cutoff voltage correction factor for reverse recovery loss can be calculated using the following equation,

(2.3) calculating additional switching losses:

wherein C represents the capacitance size of the submodule, Deltau is the maximum deviation allowed by the capacitor voltage under the voltage-sharing control and is determined by a user, f is the control frequency, I_mRepresents the maximum value of the a-phase output current, I_maxThese two values are calculated by the following equation for the maximum value of the bridge arm current:

(2.4) calculating the total loss of the converter valve under the recent level approximation modulation strategy:

P_nlm＝6(P_con+P_sw1+P_sw2)。

further, in the step (2.2), a₁、b₁、c₁For the IGBT turn-off loss quadratic fitting coefficient, the curve of' typical collector current-turn-off loss at 125 ℃ in the specification of IGBT manufacturers is obtained by adopting a quadratic curve fitting mode, a₁Is the constant term coefficient in the fitting method, b₁Is the coefficient of a first order term, c₁Is the coefficient of the quadratic term; a is₂、b₂、c₂For the quadratic fitting coefficient of the IGBT opening loss, the quadratic fitting coefficient is obtained by adopting a quadratic curve fitting mode to a typical collector current-opening loss curve at 125 ℃ in the specification of IGBT manufacturers, a₂Is the constant term coefficient in the fitting method, b₂Is the coefficient of a first order term, c₂Is the coefficient of the quadratic term; a is₃、b₃、c₃The secondary fitting coefficient of the reverse recovery loss of the diode is obtained by adopting a secondary curve fitting mode to a typical on-state current-reverse recovery loss curve at 125 ℃ in the specification of IGBT manufacturers, and a₃Is the constant term coefficient in the fitting method, b₃Is the coefficient of a first order term, c₃Is the coefficient of the quadratic term.

Further, a₁Is 378.2, b₁Is 4.025, c₁Is 0.00006071, a₂Is 684.4, b₂Is 3.059, c₂0.0006558; u shape_fIs 1.079V, r_fIs 0.001109 omega, a₃Is 644.2, b₃Is 3.103, c₃Is-0.0007948.

Further, S is 2MVA, U_dc10kV, m is 0.95, omega is 50Hz, T is 0.02s,

is 0, f_cIs 200Hz, C is 50mF, Δ U is 100V, U_refIs 2800V, I_ref1200A, the type of IGBT tube is Infineon-FZ1200R45HL, E_{T_ref}Is 10600mJ, E_{D_ref}3200 mJ; u shape_CEIs 1.342V, r_CEIs 0.00126 Ω.

The invention has the beneficial effects that:

the invention takes low system loss as a target, and obtains an optimal modulation strategy of MMC under specific capacity and voltage level by calculating the number of SM submodules corresponding to the equal loss in two modulation modes of carrier phase shift modulation and nearest level approximation modulation; the invention provides basis for selecting a modulation mode with lower system loss in practical application, thereby reducing the loss of the MMC converter valve and reducing the operation cost of the MMC converter valve.

Drawings

FIG. 1 is a structure diagram of an MMC converter valve

FIG. 2 shows a waveform of the current of the upper bridge arm in phase a

FIG. 3 is a relationship between loss and the number of sub-modules in two modulation modes

Detailed Description

In order to make the technical problems, technical solutions and advantageous effects solved by the present invention more apparent, the present invention is further described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.