阅读说明：本技术 故障下抑制mmc子模块电容电压波动的二倍频环流注入方法 (Double-frequency circulating current injection method for inhibiting capacitance voltage fluctuation of MMC sub-module under fault ) 是由 许建中 邓伟成 于 2021-02-03 设计创作，主要内容包括：本发明公开了一种故障下抑制MMC子模块电容电压波动的二倍频环流注入方法,包括：以发生交流单相故障且考虑二倍频环流注入的子模块电容电压的基频和二倍频波动分量作为抑制目标,建立优化目标函数；以目标函数最小化作为目标进行全局优化,获得最优的二倍频环流注入的幅值和相位；根据最优的二倍频环流注入的幅值和相位生成参考信号,结合调制算法实现二倍频环流的注入。该方法可以有效降低故障期间子模块电容电压波动,并减少MMC子模块在故障期间所承受的最大电压。(The invention discloses a double-frequency circulating current injection method for inhibiting the fluctuation of capacitance and voltage of an MMC sub-module under a fault, which comprises the following steps: establishing an optimized objective function by taking the fundamental frequency and the double-frequency fluctuation component of the sub-module capacitor voltage which generates the alternating-current single-phase fault and considers the double-frequency circulating current injection as the inhibition objective; performing global optimization by taking the objective function minimization as an objective to obtain the optimal amplitude and phase of the double frequency circulation injection; and generating a reference signal according to the amplitude and the phase of the optimal double frequency circulation injection, and combining a modulation algorithm to realize the injection of the double frequency circulation. The method can effectively reduce the fluctuation of the sub-module capacitor voltage during the fault period, and reduce the maximum voltage born by the MMC sub-module during the fault period.)

1. A double-frequency circulating current injection method for inhibiting the fluctuation of capacitance and voltage of an MMC sub-module under a fault is characterized by comprising the following steps:

establishing an optimized objective function by taking the fundamental frequency and the double-frequency fluctuation component of the sub-module capacitor voltage which generates the alternating-current single-phase fault and considers the double-frequency circulating current injection as the inhibition objective;

performing global optimization by taking the objective function minimization as an objective to obtain the optimal amplitude and phase of the double frequency circulation injection;

and generating a reference signal according to the amplitude and the phase of the obtained optimal double frequency circulation injection, and combining a modulation algorithm to realize the injection of the double frequency circulation.

2. The double-frequency circulating current injection method for inhibiting MMC sub-module capacitor voltage fluctuation under the fault according to claim 1, wherein before establishing the optimized objective function, an expression of the sub-module capacitor voltage considering double-frequency circulating current injection under the alternating current single-phase ground fault is calculated, and the expression comprises the following steps:

when an alternating-current single-phase earth fault occurs, the switching functions of the three phases are as follows:

wherein t is time, and omega is fundamental angular frequency; s_{au_f}、S_{bu_f}、S_{cu_f}Respectively representing the switching functions of a phase, b phase and c phase after the fault; m represents a modulation ratio of 2U_m/U_dc；N₀Is a direct current voltage U_dcAnd submodule reference voltage U_cQuotient of (1), i.e. U_dc/U_c(ii) a N is the number of bridge arm submodules;

when an alternating-current single-phase earth fault occurs, the current of an upper bridge arm of three phases is as follows:

wherein, I_mIs the amplitude of the alternating current,is an initial phase angle, I₂Showing the magnitude of the double frequency circulating injection,representing the phase angle, i, of the double frequency circulating injection_{au_f}、i_{bu_f}、i_{cu_f}Respectively representing a, b and c three-phase upper bridge arm currents after the fault;

defining a double frequency circulating injection coefficient k₂Comprises the following steps:

when an alternating current single-phase earth fault occurs, a three-phase switching function and a three-phase upper bridge arm current are combined to obtain a three-phase upper bridge arm submodule capacitor voltage containing double frequency circulating current injection after the fault:

wherein u is_{cau_f}、u_{cbu_f}、u_{ccu_f}And respectively representing the capacitance and voltage of the bridge arm sub-modules on the a phase, the b phase and the C phase after the fault, wherein C represents the capacitance value of the bridge arm sub-modules.

3. The method for injecting double-frequency circulating current for suppressing the capacitance voltage fluctuation of the MMC sub-module under the fault according to claim 2, wherein the establishing of the optimized objective function by taking the fundamental frequency and the double-frequency fluctuation component of the sub-module capacitance voltage of the double-frequency circulating current injection when the alternating current single-phase fault occurs as the suppression target comprises:

according to the three-phase upper bridge arm submodule capacitor voltage containing double-frequency circulating current injection after the fault, obtaining fundamental frequency and double-frequency fluctuation of the capacitor voltage of the fault upper bridge arm submodule when the alternating-current single-phase fault occurs, and further obtaining a fluctuation amplitude;

obtaining fundamental frequency and double frequency fluctuation of capacitor voltage of non-fault two-phase upper bridge arm sub-modules in the same mode, and further obtaining fluctuation amplitude;

establishing a target function according to the obtained fundamental frequency of the capacitor voltage of the three-phase bridge arm submodule and the fluctuation amplitude of the double frequency fluctuation

Wherein the content of the first and second substances,|u_{cau_f1}i and I u_{cau_f2}I is the fluctuation amplitude of the fundamental frequency and the double frequency fluctuation of the capacitor voltage of the bridge arm sub-module on the phase a respectively; | u_{cbu_f1}I and I u_{cbu_f2}I is respectively the fundamental frequency of the capacitor voltage of the bridge arm submodule on the b phase and the fluctuation amplitude of the double frequency fluctuation; | u_{ccu_f1}I and I u_{ccu_f2}And l is the fluctuation amplitude of the fundamental frequency and the double frequency fluctuation of the capacitor voltage of the bridge arm submodule on the c phase respectively, and k represents the weight coefficient of the secondary fluctuation of the capacitor voltage.

4. The double-frequency circulating current injection method for inhibiting the capacitance and voltage fluctuation of the MMC sub-modules under the fault according to claim 3, wherein if the fault phase is a phase a, the fundamental frequency and double-frequency fluctuation of the capacitance and voltage of the bridge arm sub-modules on the phase a are as follows:

wherein u is_{cau_f1}And u_{cau_f2}Respectively the fundamental frequency and the double frequency fluctuation of the capacitor voltage of the bridge arm submodule on the phase a;

for u is paired_{cau_f1}And u_{cau_f2}Performing an identity transform to obtain:

u_{cau_f1}＝A_{cau_f1}cosωt+B_{cau_f1}sinωt

u_{cau_f2}＝A_{cau_f2} cos2ωt+B_{cau_f2} sin2ωt

obtaining u from the identity transformation result_{cau_f1}And u_{cau_f2}The fluctuation amplitude is:

5. the double-frequency circulating current injection method for inhibiting the capacitance voltage fluctuation of the MMC sub-modules under the fault according to claim 3 or 4, wherein if the fault phase is a phase a, the fundamental frequency and double-frequency fluctuation of the capacitance voltage of the bridge arm sub-modules on the phase B are as follows:

wherein u is_{cbu_f1}And u_{cbu_f2}Fundamental frequency and double frequency fluctuation of the capacitor voltage of the bridge arm sub-module on the B phase

For u is paired_{cbu_f1}And u_{cbu_f2}Performing an identity transform to obtain:

u_{cbu_f1}＝A_{cbu_f1} cosωt+B_{cbu_f1} sinωt

u_{cbu_f2}＝A_{cbu_f2} cos2ωt+B_{cbu_f2} sin2ωt

obtaining u from the identity transformation result_{cbu_f1}And u_{cbu_f2}The fluctuation amplitude is:

6. the double-frequency circulating current injection method for inhibiting the capacitance voltage fluctuation of the MMC sub-modules under the fault according to claim 3 or 4, wherein if the fault phase is a phase a, the fundamental frequency and double-frequency fluctuation of the capacitance voltage of the bridge arm sub-modules on the phase C are as follows:

wherein u is_{ccu_f1}And u_{ccu_f2}Respectively the fundamental frequency and the double frequency fluctuation of the capacitor voltage of the bridge arm submodule on the C phase;

for u is paired_{ccu_f1}And u_{ccu_f2}Performing an identity transform to obtain:

obtaining u from the identity transformation result_{ccu_f1}And u_{ccu_f2}The fluctuation amplitude is:

7. the double-frequency circulating current injection method for inhibiting the capacitance-voltage fluctuation of the MMC sub-module under the fault according to claim 1, wherein the reference signal is generated according to the amplitude and the phase of double-frequency circulating current injection, and the injection of the double-frequency circulating current is realized by combining a modulation algorithm, and comprises the following steps:

generating a reference signal according to the amplitude and the phase of the double frequency circulating current injection and combining a reference signal generator;

the reference signal obtains a double frequency circulation modulation voltage signal through a double frequency circulation injection controller, and finally injection of the double frequency circulation is realized by changing a modulation wave of a modulation algorithm.

Technical Field

The invention relates to the technical field of flexible direct current transmission, in particular to a double-frequency circulating current injection method for inhibiting the fluctuation of capacitance and voltage of an MMC sub-module under a fault.

Background

Flexible direct current transmission (VSC-HVDC) is a new generation of direct current transmission technology following alternating current transmission, conventional direct current transmission. The flexible direct current transmission technology has the characteristics of independent active and reactive power adjustment, strong weak power grid access and low voltage ride through capability, low alternating current filtering, reactive power compensation requirements and the like. The Modular Multilevel Converter (MMC) has the characteristics of flexible control, module expansibility, low switching frequency, low harmonic content and the like. At present, MMC structures are adopted in Shanghai south-Virginia flexible direct current engineering, Nanao three-terminal flexible direct current engineering, Zhoushan five-terminal flexible direct current engineering, Xiamen +/-320 kV flexible direct current demonstration engineering and Zhang Bei direct current power grid engineering which are built in China.

Ac single-phase earth faults are the most common and frequent type of fault occurring in electrical power systems. When the alternating-current single-phase earth fault occurs, the dynamic characteristic of the MMC can be greatly changed, the fluctuation of the sub-module capacitor voltage can be inevitably increased, and the overvoltage of a device can be possibly generated, so that the power electronic device is damaged. Therefore, a method for restraining the fluctuation of the capacitance and voltage of the MMC sub-module under the AC single-phase earth fault is explored, the reliability of the MMC is improved, and the method has important engineering significance.

Disclosure of Invention

The invention aims to provide a double-frequency circulating current injection method for inhibiting the fluctuation of the capacitance voltage of an MMC sub-module under a fault, which can effectively reduce the fluctuation of the capacitance voltage of the sub-module during the fault and reduce the maximum voltage borne by the MMC sub-module during the fault.

The purpose of the invention is realized by the following technical scheme:

a double-frequency circulating current injection method for inhibiting capacitance and voltage fluctuation of an MMC sub-module under a fault comprises the following steps:

establishing an optimized objective function by taking the fundamental frequency and the double-frequency fluctuation component of the sub-module capacitor voltage which generates the alternating-current single-phase fault and considers the double-frequency circulating current injection as the inhibition objective;

performing global optimization by taking the objective function minimization as an objective to obtain the optimal amplitude and phase of the double frequency circulation injection;

and generating a reference signal according to the amplitude and the phase of the obtained optimal double frequency circulation injection, and combining a modulation algorithm to realize the injection of the double frequency circulation.

It can be seen from the above technical solutions that the fundamental frequency and the double frequency ripple component of the voltage are used as the suppression target, the amplitude and phase of the injected double frequency circulating current are optimally designed, and finally the calculated double frequency circulating current is injected into the three phases by changing the modulation wave signal. The method can effectively reduce the fluctuation of the sub-module capacitor voltage during the fault period, and reduce the maximum voltage born by the MMC sub-module during the fault period.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on the drawings without creative efforts.

Fig. 1 is a flowchart of a double-frequency circulating current injection method for suppressing the voltage fluctuation of a capacitor of an MMC sub-module in case of a fault according to an embodiment of the present invention;

fig. 2 is a block diagram of the overall system control of the double frequency circulating current injection method for suppressing the voltage fluctuation of the capacitance of the MMC submodule under a fault according to the embodiment of the present invention.

Detailed Description

The technical solutions in the embodiments of the present invention are clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments of the present invention without making any creative effort, shall fall within the protection scope of the present invention.

The embodiment of the invention provides a double-frequency circulating current injection method for inhibiting the fluctuation of capacitance and voltage of an MMC sub-module under a fault, wherein a flow chart and an overall system control block diagram are shown in figures 1-2, and the method mainly comprises the following steps:

1. and establishing an optimized objective function by taking the fundamental frequency and the double-frequency fluctuation component of the sub-module capacitor voltage which generates the alternating-current single-phase fault and considers the double-frequency circulating current injection as the inhibition objective.

In the embodiment of the invention, firstly, an expression of submodule capacitor voltage considering double frequency circulation injection under the condition of alternating current single-phase earth fault is calculated, and then an optimized objective function is established.

1) And calculating an expression of the submodule capacitor voltage considering double frequency circulation injection under the AC single-phase earth fault.

When an alternating-current single-phase earth fault occurs (taking a-phase single-phase earth as an example), the switching function of three phases is as follows:

wherein t is time, and omega is fundamental angular frequency; s_{au_f}、S_{bu_f}、S_{cu_f}Respectively representing the switching functions of a phase, b phase and c phase after the fault; m represents a modulation ratio of 2U_m/U_dc，U_mIs the amplitude of the alternating voltage; n is a radical of₀Is a direct current voltage U_dcAnd submodule reference voltage U_cQuotient of (1), i.e. U_dc/U_c(ii) a And N is the number of the submodules of the bridge arm.

Those skilled in the art can understand that the MMC is composed of six bridge arms, i.e., above a phase, below a phase, above b phase, below b phase, above c phase, and below c phase, each bridge arm further includes N sub-modules, and the sub-module is the minimum unit.

When an alternating-current single-phase ground fault occurs, because a fault phase cannot transmit power, the transmission of 1/3 power is often reduced in consideration of reduced bridge arm current stress, and at the moment, the upper bridge arm currents of three phases are:

wherein, I_mIs the amplitude of the alternating current,is an initial phase angle, I₂Showing the magnitude of the double frequency circulating injection,representing the phase angle, i, of the double frequency circulating injection_{au_f}、i_{bu_f}、i_{cu_f}Respectively representing a, b and c three-phase upper bridge arm currents after the fault;

to simplify the representation, a frequency doubling circulating injection coefficient k is defined₂Comprises the following steps:

when an alternating current single-phase earth fault occurs, a three-phase switching function and a three-phase upper bridge arm current are combined to obtain a three-phase upper bridge arm submodule capacitor voltage containing double frequency circulating current injection after the fault:

wherein u is_{cau_f}、u_{cbu_f}、u_{ccu_f}And respectively representing the capacitance and voltage of the bridge arm sub-modules on the a phase, the b phase and the C phase after the fault, wherein C represents the capacitance value of the bridge arm sub-modules.

2) And establishing an optimized objective function by taking the fundamental frequency and the double-frequency fluctuation component of the sub-module capacitor voltage which generates the alternating-current single-phase fault and considers the double-frequency circulating current injection as the inhibition objective. The main process is as follows:

A. and obtaining fundamental frequency and double frequency fluctuation of the capacitance voltage of the bridge arm submodule on the fault phase when the alternating current single-phase fault occurs according to the capacitance voltage of the three-phase upper bridge arm submodule containing double frequency circulation injection after the fault, thereby obtaining the fluctuation amplitude.

If the fault phase is a phase a, the fundamental frequency and double frequency fluctuation of the bridge arm submodule capacitor voltage on the phase a are as follows:

wherein u is_{cau_f1}And u_{cau_f2}Respectively the fundamental frequency and the double frequency fluctuation of the capacitor voltage of the bridge arm submodule on the phase a;

for u is paired_{cau_f1}And u_{cau_f2}Performing an identity transform to obtain:

the A, B parameters referred to herein for the different subscripts are intermediate parameters.

Obtaining u from the identity transformation result_{cau_f1}And u_{cau_f2}The fluctuation amplitude is:

B. and obtaining fundamental frequency and double frequency fluctuation of the capacitor voltage of the non-fault two-phase upper bridge arm submodule by adopting the same mode, and further obtaining the fluctuation amplitude.

The fundamental frequency and double frequency fluctuation of the capacitor voltage of the bridge arm submodule on the B phase are as follows:

wherein u is_{cbu_f1}And u_{cbu_f2}Fundamental frequency and double frequency fluctuation of the capacitor voltage of the bridge arm sub-module on the B phase

For u is paired_{cbu_f1}And u_{cbu_f2}Performing an identity transform to obtain:

obtaining u from the identity transformation result_{cbu_f1}And u_{cbu_f2}The fluctuation amplitude is:

the fundamental frequency and double frequency fluctuation of the capacitor voltage of the bridge arm submodule on the C phase are as follows:

wherein u is_{ccu_f1}And u_{ccu_f2}Respectively the fundamental frequency and the double frequency fluctuation of the capacitor voltage of the bridge arm submodule on the C phase;

for u is paired_{ccu_f1}And u_{ccu_f2}Performing an identity transform to obtain:

obtaining u from the identity transformation result_{ccu_f1}And u_{ccu_f2}The fluctuation amplitude is:

C. establishing a target function according to the obtained fundamental frequency of the capacitor voltage of the three-phase bridge arm submodule and the fluctuation amplitude of the double frequency fluctuation

Wherein the content of the first and second substances,|u_{cau_f1}i and I u_{cau_f2}I is the fluctuation amplitude of the fundamental frequency and the double frequency fluctuation of the capacitor voltage of the bridge arm sub-module on the phase a respectively; | u_{cbu_f1}I and I u_{cbu_f2}I is respectively the fundamental frequency of the capacitor voltage of the bridge arm submodule on the b phase and the fluctuation amplitude of the double frequency fluctuation; | u_{ccu_f1}I and I u_{ccu_f2}And l is the fundamental frequency of the capacitor voltage of the bridge arm submodule on the c phase and the fluctuation amplitude of the double frequency fluctuation respectively.

k represents a weight coefficient of the secondary fluctuation of the capacitor voltage. Since the influence of the double frequency voltage fluctuation on the overall voltage fluctuation amplitude is smaller, the k value is set to 0.5 here. It should be noted that the value of k is not fixed, and can be adjusted appropriately according to actual conditions.

2. And performing global optimization by taking the objective function minimization as an objective to obtain the optimal amplitude and phase of the double frequency circulation injection.

In the embodiment of the invention, global optimization is carried out by taking objective function minimization as a target, and k is determined according to different modulation ratios m₂、Optimal values of the two variables. It should be noted that the present invention does not limit the optimization method, and the user can select the optimization method according to the actual situation.

It will be appreciated by those skilled in the art that since the objective function is based on the capacitor voltage fluctuation, its minimized solution can achieve a minimization of the capacitor voltage fluctuation, i.e. if the amplitude and phase of the optimal solution are injected, the objective can be achieved, in particular, by designing the parameter k₂，Can realize suppression of the fluctuation of the capacitor voltage.

3. And generating a reference signal according to the amplitude and the phase of the optimal double frequency circulation injection, and combining a modulation algorithm to realize the injection of the double frequency circulation.

In the embodiment of the invention, the optimal amplitude and phase of double frequency circulating current injection are obtained, namely k is obtained₂、An optimal value, after which a reference signal i can be generated in conjunction with a reference signal generator_{2fd_ref}And i_{2fq_ref}(ii) a Wherein i_{2fd_ref}Reference signal, i, for d-axis double frequency circulating injection controller_{2fq_ref}Injecting a reference signal of the controller for q-axis double frequency circulation; the reference signal is injected into the controller through the double frequency circulation to obtain a double frequency circulation modulation voltage signal U_{cirj_ref}Finally, the injection of the double frequency circulation is realized by changing the modulation wave of the modulation algorithm.

In the control block diagram of the whole system shown in fig. 2, the positive sequence and negative sequence double-loop controllers in the upper dotted line frame are the classic control scheme adopted in the current fault; wherein, T_abc/dqThe module is a dq transformation module; i.e. i_{d_refp}And i_{q_refp}Respectively generating positive sequence differential mode voltage reference values e for positive sequence dq axis current reference values through dq conversion_{j_refp}；i_{d_refn}And i_{q_refn}Respectively negative sequence dq axis current reference values, and generating a negative sequence differential mode voltage reference value e through dq conversion_{j_refn}；I₂Andshowing the double frequency circulating current magnitude and phase. The lower dotted line frame shows the working process of the reference signal generator and the double frequency circulation injection controller, wherein the PI module is a proportional integral link; l is_armIs the bridge arm reactance value; t is_dq/abcThe module is a dq inverse transformation module; i.e. i_cira、i_cirb、i_circRespectively measuring the a phase, the b phase and the c phase double frequency circulation; i.e. i_2fdAnd i_2fqD-axis double frequency circulation current and q-axis double frequency circulation current i_{2fd_ref}And i_{2fq_ref}Is a corresponding reference value; u. of_{cird_ref}And u_{cirq_ref}D-axis double-frequency circulating current modulation voltage and q-axis double-frequency circulating current modulation voltage are respectively generated into a double-frequency circulating current modulation voltage signal U through dq inverse transformation_{cirj_ref}。

Through the above description of the embodiments, it is clear to those skilled in the art that the above embodiments can be implemented by software, and can also be implemented by software plus a necessary general hardware platform. With this understanding, the technical solutions of the embodiments can be embodied in the form of a software product, which can be stored in a non-volatile storage medium (which can be a CD-ROM, a usb disk, a removable hard disk, etc.), and includes several instructions for enabling a computer device (which can be a personal computer, a server, or a network device, etc.) to execute the methods according to the embodiments of the present invention.

The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.