CN115021319A

CN115021319A - Networking and island operation control method and system of modular multilevel converter

Info

Publication number: CN115021319A
Application number: CN202210760872.2A
Authority: CN
Inventors: 刘黎; 蔡旭; 俞恩科; 詹志雄; 饶友元; 杨仁炘
Original assignee: Shanghai Jiaotong University; Zhoushan Power Supply Co of State Grid Zhejiang Electric Power Co Ltd
Current assignee: Shanghai Jiaotong University; Zhoushan Power Supply Co of State Grid Zhejiang Electric Power Co Ltd
Priority date: 2022-06-29
Filing date: 2022-06-29
Publication date: 2022-09-06

Abstract

The invention provides a networking and island operation control method and system of a modular multilevel converter, which work in a constant active power-reactive power control mode during networking operation and work in a constant alternating current frequency-alternating current voltage control mode during island operation, and can realize seamless switching of two operation modes. And on the alternating current side, the output current of the current converter is limited in the networking-island switching process by adopting the self-adaptive virtual admittance and the current inner ring. On the direct current side, double closed-loop control of active power-direct current is designed, and seamless switching of networking and island modes is achieved in a control loop by utilizing amplitude limiting links and feedforward of total energy of submodules. Under the control strategy, the modularized multi-level converter can realize seamless switching of networking and island modes without adding an additional networking/island detection link, the switching process is very smooth, and stable operation of an island power system in the switching process can be ensured.

Description

Networking and island operation control method and system of modular multilevel converter

Technical Field

The invention relates to the technical field of control of a flexible direct current power transmission grid-connected system, in particular to a networking and island operation control method and system of a modular multilevel converter.

Background

Alternating current-direct current hybrid power supply system to island power system, as shown in fig. 1, mainly by the island load, the new forms of energy station, exchange submarine cable, exchange main network and gentle direct system (including the gentle direct current conversion station of island side, the gentle direct current conversion station of main network side and direct current submarine cable) constitute, exchange main network side and see off alternating current conversion by gentle direct current conversion station with alternating current conversion to direct current, the gentle direct current conversion station of rethread island side converts to the interchange and supplies power to island electric wire netting, exchange main network and island still have an interchange submarine cable to be connected simultaneously, exchange main network also can supply power to island electric wire netting through this submarine cable simultaneously, after exchanging the submarine cable because of the fault disconnection, the island electric wire netting will only be supplied power by gentle direct system.

In the system, a main network side converter station mainly works in a constant direct current voltage control mode to establish and maintain the voltage of direct current transmission. The island side converter station has two working modes, when the alternating current submarine cable is not disconnected, the island side flexible direct current converter station is called to work in a networking mode, and after the alternating current submarine cable is disconnected, the island side flexible direct current converter station is called to work in an island mode. In the networking mode, the island side flexible direct current converter station needs to adopt P-Q control, tracks the phase of a power grid through a phase-locked loop, and regulates active power and reactive power provided for an island system through d/Q axis decoupling and voltage-current double closed-loop control. In an island mode, an island side flexible direct current converter station needs to adopt V-f control, the amplitude and the frequency of alternating current side voltage are kept constant, and voltage of an island power system is established.

When an alternating current submarine cable between an island power grid and an alternating current main grid is interrupted accidentally due to sudden failure, the flexible direct current converter station at the island side is required to switch the working modes, and the documents of the prior art are searched and found out:

"Lingwaijia, Sun Weizhen, Zhang Jinglong, Dong Yunlong. Zhoushan multi-terminal flexible direct current transmission demonstration project typical operation mode analysis [ J ]. power grid technology, 2016, 40 (06): 1751-. However, the control strategy depends on the detection speed of the control and protection system, and the control and protection system is easy to cause the oscillation of the island power system when the detection is delayed.

"liu sheng, xu zheng, tang g, hua wen, zhang jing MMC-HVDC networking and island operation state transition strategy [ J ]. china electro-mechanical engineering report, 2015, 35 (09): 2152-: 1914-1916.

Chinese patent application CN111130141A discloses a switching controller for flexible dc converter station networking and island operation, which comprises: the system comprises an information acquisition module and an operation mode judgment module; the information acquisition module is used for acquiring the operation information of the flexible direct current converter station, the networking operation controller and the island operation controller and sending the operation information to the operation mode judgment module; and the operation mode judgment module is used for receiving the operation information sent by the information acquisition module and sending an operation instruction to the networking operation controller and the island operation controller according to the operation information, so that the problem that the prior art has no effective control on the networking and island operation switching of the flexible direct current converter station is solved. However, the controller still has the following problems:

the networking-island conversion of the controller needs to switch the controller, the switching process needs to be judged by collecting operation information, both the information collecting process and the judging process can cause the time delay of the switching process, and when the planned extranet-island conversion occurs (if an alternating current circuit fails), the switching time delay easily causes the oscillation of a system.

No description or report of the similar technology to the invention is found at present, and similar data at home and abroad are not collected yet.

Disclosure of Invention

Aiming at the defects in the prior art, the invention provides a networking and island operation control method and system of a modular multilevel converter.

The invention is realized by the following technical scheme.

According to one aspect of the invention, a networking and island operation control method of a modular multilevel converter is provided, and comprises the following steps:

on the ac side of the modular multilevel converter:

calculating the sum of sub-module capacitance energy, making a difference between the sum of the sub-module capacitance energy and the total rated energy of the sub-module capacitance, performing per unit on the difference, multiplying the difference by a set proportion, adding the obtained result and the rated frequency to obtain a reference value of the output alternating current frequency of the modular multilevel converter, and integrating the reference value of the output alternating current frequency to obtain a reference phase; the measured value of the reactive power is subtracted from a preset reference value, and a reference value of the output alternating voltage is obtained after a proportion link;

carrying out park transformation on the voltage and the current of the grid connection point under the reference phase to obtain measured values of the voltage and the current of the d/q axis; comparing the obtained d-axis voltage measurement value with the reference value of the output alternating voltage, comparing the obtained q-axis voltage measurement value with 0, and obtaining the reference value of d/q-axis current after passing through a self-adaptive virtual admittance link and an annular current limiting link; comparing the obtained measured value of the d/q-axis current with the reference value of the d/q-axis current, and obtaining the alternating voltage d/q-axis reference value of the modular multilevel converter through a proportional-integral link; obtaining a reference value of the three-phase alternating voltage of the modular multilevel converter after the d/q axis reference value of the alternating voltage is subjected to park inverse transformation under the reference phase;

on the dc side of the modular multilevel converter:

calculating a given value of the direct current side current, making a difference with a measured value of the direct current side current, and passing the difference through an amplitude limiting link; the sum of the sub-module capacitor energy is subjected to per unit and then is subjected to difference with the total rated energy of the sub-module capacitor, the obtained difference is added to the output of the amplitude limiting link, and a proportional-integral link is performed to obtain a reference value of the direct current voltage;

and adding or subtracting half of the reference value of the direct-current voltage and half of the reference value of the three-phase alternating-current voltage respectively to obtain output reference voltages of six bridge arms, modulating the output reference voltages to obtain a switching signal of each bridge arm of the modular multilevel converter, and finishing the operation control of the modular multilevel converter.

In the operation control method:

the energy sum of the sub-module capacitors and the total rated energy of the sub-module capacitors are differentiated, the differentiated value is subjected to per unit and then multiplied by a set proportion to replace the function of a phase-locked loop in grid connection, so that the modular multilevel converter and a power grid are automatically kept synchronous, a reference phase is automatically generated in the modular multilevel converter in the networking-island switching process, the switching of an operation controller is not needed, and the modular multilevel converter can share one set of operation controller in the networking and island states;

when the network is operated, the amplitude limiting link does not work; under the island operation, through input amplitude limiting link, the power of modularization multilevel converter input according to the power demand automatically regulated direct current side of alternating current side to make the submodule piece electric capacity energy sum of modularization multilevel converter maintain within the safe range of the total rated energy of submodule piece electric capacity, satisfy the demand to the island power supply.

Optionally, the calculating a sum of sub-module capacitance energies, making a difference between the sum of the sub-module capacitance energies and a total rated energy of a sub-module capacitance, performing per unit on the difference, multiplying the difference by a set proportion, adding an obtained result and a rated frequency to obtain a reference value of an output ac frequency of the modular multilevel converter, and integrating the reference value of the output ac frequency to obtain a reference phase includes:

ΔW _total ＝6N _total C _SM U _SMnom ΔU _SMav (13)

in the formula, N _total The total number of the submodules of the single bridge arm of the submodules of the modular multilevel converter, C _SM Is a capacitance value of a submodule capacitor, U, of a modular multilevel converter _SMnom For sub-module capacitor voltage rating, U _SMav Is the sub-module capacitor voltage average value, Δ U _SMav For deviation of sub-module capacitor voltage from nominal value, Δ W _total Is the difference between the sum of the sub-module capacitance energies and the total rated energy of the sub-module capacitance, U _SMav Is obtained by the following formula:

according to Δ W _total Obtaining the output frequency omega of the modularized multi-level converter _mmc Comprises the following steps:

in the formula, H _v Is a virtual coefficient of inertia, S _nom Rated capacity, omega, of modular multilevel converter _nom The output frequency omega obtained for the rated frequency of the modularized multi-level converter _mmc The reference value of the output alternating current frequency is obtained; the set proportion is as follows: 1/(2H) _v )；

Output frequency omega of the modular multilevel converter _mmc After an amplitude limiting link, the phase theta of the internal potential of the converter is obtained through integration _mmc ：

θ _mmc ＝∫ω _mmc (16)

The obtained phase theta of the internal potential _mmc Namely the reference phase;

through the step of generating the reference phase, the reference phase is automatically generated inside the modular multilevel converter, and the modular multilevel converter automatically synchronizes the phase of the power grid in a networking state; under an island state, the modular multilevel converter automatically maintains the output alternating current frequency at 0.99 omega without control switching _nom To 1.01 omega _nom In the meantime, stable operation of the island power grid is ensured;

the step of obtaining the reference value of the output alternating voltage after a proportional control link by subtracting the measured value of the reactive power from a preset reference value comprises the following steps:

after the difference is made between the measured value of the reactive power and the given value, the amplitude of the internal potential of the modular multilevel converter is obtained through a proportional control link:

E-E _nom ＝K _Q (Q _mmc -Q _ref ) (17)

wherein E is the magnitude of the internal potential, E _nom For internal potential amplitude ratings, K _Q For reactive control of proportional parameters, Q _mmc For reactive power, Q, output by the inverter _ref Setting the reactive power of the converter, wherein the obtained amplitude E of the internal potential is a reference value of the output alternating voltage;

through the step of generating the reference value of the output alternating voltage, the reactive power output by the modular multilevel converter is related to the amplitude of the output alternating voltage thereof in a networking state, so that the modular multilevel converter controls the output reactive power; in an island state, control switching is not needed, so that the modular multilevel converter controls output voltage.

Optionally, the performing a park change on the grid-connected point voltage and the current in the reference phase to obtain a measured value of the d/q axis voltage and the current includes:

measuring three-phase voltage u of grid-connected point of modular multilevel converter _abc And current i _abc Performing park transformation based on the reference phase to obtain a voltage measurement value u of d/q axis _dq And a current measurement value i _dq ；

The step of comparing the obtained d-axis voltage measurement value with the reference value of the output alternating voltage, comparing the obtained q-axis voltage measurement value with 0, and obtaining the reference value of the d/q-axis current through a self-adaptive virtual admittance link comprises the following steps:

the obtained d-axis voltage measured value u _d Making difference with reference value E of output AC voltage to obtain q-axis voltage measured value u _q Making a difference with 0, and obtaining a reference value of the d/q axis current through a self-adaptive virtual admittance link; wherein:

the self-adaptive virtual admittance link is used for limiting output current in the networking-island switching process and comprises a virtual resistor R _v And a virtual reactance L _v Obtaining the expected value i of the d/q axis current ^* _d And i ^* _q Comprises the following steps:

in the formula, s is Laplace operator;

calculating the expected value i of the d/q axis current ^* _d And i ^* _q Inputting an annular current limiting link to obtain a reference value i of the d/q axis current _dref And i _qref Comprises the following steps:

in the formula i _lim The amplitude limit of the output current of the converter is determined according to the tolerance capability of a switching device used by the converter, and the output current is generally 1.2-1.5 times of rated current;

the step of comparing the obtained measured value of the d/q-axis current with the reference value of the d/q-axis current and obtaining the reference value of the d/q-axis alternating voltage of the modular multilevel converter through a proportional-integral link comprises the following steps:

converting the reference value i of the d/q axis current _dref And i _qref Respectively with measured values i of said d/q-axis currents _d And i _q Performing subtraction, and adding power grid voltage feedforward and decoupling terms after passing through a proportional-integral controller to obtain an alternating voltage d/q axis reference value u of the modular multilevel converter _dref And u _qref Comprises the following steps:

in the formula, k _p Is a proportional controller parameter, k _i For integral controller parameters, s is the Laplace operator, ω _nom The frequency is rated frequency of the modular multilevel converter, and L is converter grid-connected inductance;

through the steps, under the condition that the filter capacitor is not arranged on the AC side of the converter, the voltage on the AC side is stably controlled, the reference value of the AC current is generated, the amplitude of the output AC current is limited, and the overcurrent problem of the modular multilevel converter in the networking-island conversion process is avoided.

Optionally, the given value of the current at the direct current side is calculated, the difference is made with the measured value of the current at the direct current side, and the difference is processed by an amplitude limiting link; the sum of the sub-module capacitor energy is subjected to per-unit treatment and then is subjected to difference with the total rated energy of the sub-module capacitor, the obtained difference value is added to the output of the amplitude limiting link, and a proportional-integral link is further carried out to obtain a reference value of the direct-current voltage; the method comprises the following steps:

a preset reference value P of active power _ref And the measured value P _mmc Making difference, and obtaining given value i of direct current side current after a proportion-integration link _dcref ：

In the formula, k _pP Is a proportional controller parameter, k _iP Is an integral controller parameter, s is a laplacian operator;

setting the given value i of the direct current side current _dcref With measured value i of the direct side current _dc Making difference, and obtaining current error i after a limiting link _e ：

In the formula i _max Is the maximum value of the DC error i _min Is the minimum value of the direct current error;

the sum of the sub-module capacitor energy is subjected to per unit to be differenced with the total rated energy of the sub-module capacitor, and the obtained sub-module capacitor energy deviation is added to the output of the amplitude limiting link, namely the current error i _e And finally, a direct current voltage measured value u outside the direct current side current limiting reactance of the modular multilevel converter is subjected to a proportional-integral link _dco Adding to obtain a reference value of the direct-current voltage:

in the formula, k _pdc Is a proportional controller parameter, k _idc For integrating the controller parameter, Δ W _total Is the deviation of the sum of the sub-module capacitance energies from the total rated energy of the sub-module capacitance, W _totalnom A rated value of the total rated energy of the sub-module capacitor;

when the modular multilevel converter is operated in a networking mode, the deviation between the measured value of the direct current side current and the given value of the direct current side current does not reach the amplitude limit, at the moment, the control of the direct current side of the modular multilevel converter is active power-direct current double closed-loop control, the active power output by the modular multilevel converter can be stably controlled, and meanwhile, the sum of sub-module capacitor energy is kept unchanged;

when the modular multilevel converter operates in an island, an active power outer ring is saturated, the deviation between the measured value of the direct current side current and the given value of the direct current side current reaches amplitude limiting, the control of the direct current side of the modular multilevel converter is single closed-loop control of sub-module capacitance energy deviation, the sum of the sub-module capacitance energy is maintained in a safety range, the direct current side power and the input/output power of the alternating current side are automatically kept in balance, and control switching is not needed.

Optionally, the i _max Is taken to be 0.1, i _min The value of (a) is-0.1.

Optionally, the safety range is: the sum of the sub-module capacitance energies is maintained within +/-10% of the total rated energy of the sub-module capacitance.

Optionally, the adding or subtracting a half of the reference value of the direct-current voltage to a half of the reference value of the three-phase alternating-current voltage to obtain output reference voltages of six bridge arms, and modulating the output reference voltages to obtain a switching signal of each bridge arm of the modular multilevel converter, so as to complete operation control of the modular multilevel converter, includes:

according to the obtained reference value u of the three-phase alternating voltage _a 、u _b And u _c And obtaining a reference value u of the DC voltage _dcref And calculating reference output voltages of six bridge arms of the receiving end MMC:

in the formula u _pa 、u _pb 、u _pc 、u _na 、u _nb 、u _nc Respectively an A-phase upper bridge arm and a B-phase upper bridge armReference output voltages of the C-phase upper bridge arm, the A-phase lower bridge arm, the B-phase lower bridge arm and the C-phase lower bridge arm; u. of _cira 、u _cirb 、u _circ The output voltages controlled by the double frequency loop are respectively used for controlling the double frequency loop in the modular multilevel converter;

outputting the reference output voltage u of the A-phase upper bridge arm, the B-phase upper bridge arm, the C-phase upper bridge arm, the A-phase lower bridge arm, the B-phase lower bridge arm and the C-phase lower bridge arm _pa 、u _pb 、u _pc 、u _na 、u _nb And u _nc And after the latest level modulation, switching signals of each submodule in each bridge arm of the modular multilevel converter are obtained, and the operation control of the modular multilevel converter is completed.

According to another aspect of the present invention, there is provided a networking and islanding operation control system of a modular multilevel converter, comprising: the self-synchronizing control module works on the alternating current side of the modular multilevel converter, the current inner ring module based on the self-adaptive virtual admittance, and the voltage decoupling control module and the modulation module work on the direct current side of the modular multilevel converter; wherein:

the alternating current side self-synchronizing control module is used for calculating the sum of sub-module capacitance energy, making a difference between the sum of the sub-module capacitance energy and the total rated energy of the sub-module capacitance, performing per unit on the difference, multiplying the difference by a set proportion, adding the obtained result and the rated frequency to obtain a reference value of the output alternating current frequency of the modular multilevel converter, and integrating the reference value of the output alternating current frequency to obtain a reference phase; the measured value of the reactive power is subtracted from a preset reference value, and a reference value of the output alternating voltage is obtained after a proportion link;

the current inner loop module based on the self-adaptive virtual admittance is used for carrying out park transformation on the voltage and the current of the grid-connected point under the reference phase to obtain the measured values of the voltage and the current of the d/q axis; comparing the obtained d-axis voltage measurement value with the reference value of the output alternating voltage, comparing the obtained q-axis voltage measurement value with 0, and obtaining the reference value of d/q-axis current after passing through a self-adaptive virtual admittance link and an annular current limiting link; comparing the obtained measured value of the d/q-axis current with the reference value of the d/q-axis current, and obtaining the alternating voltage d/q-axis reference value of the modular multilevel converter through a proportional-integral link; obtaining a reference value of the three-phase alternating voltage of the modular multilevel converter after the d/q axis reference value of the alternating voltage is subjected to park inverse transformation under the reference phase;

the direct current side voltage decoupling control module is used for calculating a given value of direct current side current, making a difference with a measured value of the direct current side current, and enabling the difference value to pass through an amplitude limiting link; the sum of the sub-module capacitor energy is subjected to per-unit treatment and then is subjected to difference with the total rated energy of the sub-module capacitor, the obtained difference value is added to the output of the amplitude limiting link, and a proportional-integral link is further carried out to obtain a reference value of the direct-current voltage;

the modulation module is used for adding or subtracting a half of the reference value of the direct-current voltage and a half of the reference value of the three-phase alternating-current voltage respectively to obtain output reference voltages of six bridge arms, modulating the output reference voltages to obtain a switching signal of each bridge arm of the modular multilevel converter, and completing operation control of the modular multilevel converter.

Due to the adoption of the technical scheme, compared with the prior art, the invention has the following beneficial effects:

the networking and island operation control method and system of the modular multilevel converter provided by the invention work in a constant active power-reactive power control mode during networking operation and work in a constant alternating current frequency-alternating voltage control mode during island operation, and can realize seamless switching of two operation modes.

The networking and island operation control method and system of the modular multilevel converter provided by the invention can accurately control the active power and the reactive power of the grid-connected network when networking is realized, and can accurately control the alternating current frequency of an island power grid when the island is isolated, compared with the existing seamless switching control strategy, the control performance is more excellent.

The networking and island operation control method and system of the modular multilevel converter provided by the invention adopt the self-adaptive virtual admittance link to be matched with the annular current limiting link, are more suitable for the modular multilevel converter without a filter capacitor and a voltage reference point at an alternating current side, and the addition of the self-adaptive virtual admittance link can ensure that the networking-island switching process is very smooth, can ensure that the voltage and the frequency of an island power system are stable in the switching process and after the switching, and further realize the stable operation of the island power system after the switching.

The invention provides a networking and island operation control method and system of a modular multilevel converter, which is a networking-island seamless switching control strategy applied to the modular multilevel converter in an alternating current-direct current hybrid power supply system.

The networking and island operation control method and system of the modular multilevel converter do not need to acquire information in the networking and island switching process, do not need to judge the operation mode, do not need to modify a controller, and can realize the switching process in a full-automatic manner.

Drawings

Other features, objects and advantages of the invention will become more apparent upon reading of the detailed description of non-limiting embodiments with reference to the following drawings:

fig. 1 is a basic topological diagram of an AC/DC power supply system of an offshore island.

Fig. 2 is a schematic control diagram of an island-side modular multilevel converter station according to an embodiment of the present invention;

FIG. 3 is a waveform diagram illustrating simulation of response to a network-island operation switchover process according to an embodiment of the present invention; wherein, (a) is direct current bus voltage, (b) is alternating current bus voltage, (c) is alternating current frequency, (d) is alternating current, (e) is output active power, and (f) is output reactive power.

Detailed Description

The present invention will be described in detail with reference to specific examples. The following examples will assist those skilled in the art in further understanding the invention, but are not intended to limit the invention in any way. It should be noted that variations and modifications can be made by persons skilled in the art without departing from the concept of the invention. All falling within the scope of the present invention.

An embodiment of the present invention provides a networking and islanding operation control method for a modular multilevel converter, which may include the following steps:

on the ac side of the modular multilevel converter:

calculating the sum of the sub-module capacitor energies, making a difference between the sum of the sub-module capacitor energies and the total rated energy of the sub-module capacitors, performing per unit on the difference, multiplying the difference by a set proportion, adding the obtained result and the rated frequency to obtain a reference value of the output alternating current frequency of the modular multilevel converter, and integrating the reference value of the output alternating current frequency to obtain a reference phase; the measured value of the reactive power is subtracted from a preset reference value, and a reference value of the output alternating voltage is obtained after a proportion link;

carrying out park transformation on the voltage and the current of the grid connection point under a reference phase to obtain measured values of the voltage and the current of a d/q axis; comparing the obtained d-axis voltage measurement value with a reference value of an output alternating voltage, comparing the obtained q-axis voltage measurement value with 0, and obtaining a reference value of d/q-axis current after passing through a self-adaptive virtual admittance link and an annular current limiting link; comparing the obtained measured value of the d/q-axis current with the reference value of the d/q-axis current, and obtaining the alternating voltage d/q-axis reference value of the modular multilevel converter through a proportional-integral link; carrying out park inverse transformation on the reference value of the d/q axis of the alternating voltage under a reference phase to obtain a reference value of the three-phase alternating voltage of the modular multilevel converter;

on the dc side of the modular multilevel converter:

calculating a given value of the direct current side current, making a difference with a measured value of the direct current side current, and passing the difference through an amplitude limiting link; the sum of the sub-module capacitor energy is subjected to per-unit treatment and then is subjected to difference with the total rated energy of the sub-module capacitor, the obtained difference value is added to the output of an amplitude limiting link, and a proportional-integral link is further carried out to obtain a reference value of the direct current voltage;

and respectively adding or subtracting half of the reference value of the direct-current voltage and half of the reference value of the three-phase alternating-current voltage to obtain output reference voltages of six bridge arms, modulating the output reference voltages to obtain a switching signal of each bridge arm of the modular multilevel converter, and finishing the operation control of the modular multilevel converter.

In the operation control method:

the sum of the sub-module capacitor energy and the total rated energy of the sub-module capacitors are differentiated, the difference is subjected to per unit processing and then multiplied by a set proportion, and the function of a phase-locked loop in grid connection is replaced, so that the modular multilevel converter and a power grid are automatically kept synchronous, a reference phase is automatically generated in the modular multilevel converter in the networking-island switching process, switching of an operation controller is not needed, and the modular multilevel converter can share one operation controller in the networking and island states;

when the network is operated, the amplitude limiting link does not work; in island operation, the modular multilevel converter automatically adjusts the power input by the direct current side according to the power requirement of the alternating current side by putting into an amplitude limiting link, and the sum of the sub-module capacitor energy of the modular multilevel converter is kept within the safety range of the total rated energy of the sub-module capacitor, so that the requirement on island power supply is met.

Further:

the direct current side of the modularized multi-level converter adopts double closed loop control of a power outer loop and a power inner loop, an amplitude limiting link is added between double loops, the amplitude limiting link does not work when the modularized multi-level converter is connected with a network, the converter works in a constant active power-constant reactive power control mode at the moment, the amplitude limiting link can be automatically put into use when the modularized multi-level converter is in island operation, the converter can automatically adjust the power input by the direct current side according to the power requirement of the alternating current side at the moment, the sum of the sub-module capacitor energy of the converter is kept within +/-10% of the total rated energy of the sub-module capacitor, the converter is embodied in a constant alternating current voltage-constant alternating current frequency working mode, and the control structure is not changed in the network-island switching process, namely control switching is not needed.

The method is characterized in that a self-synchronization mode based on sub-module capacitor energy is adopted on the alternating current side of the modular multilevel converter, the difference between the output alternating current frequency and the rated frequency is set as the difference between the sum of the sub-module capacitor energy and the total rated energy of the sub-module capacitor multiplied by a fixed proportion, the function of a phase-locked loop can be replaced during grid connection, the converter is automatically kept synchronous with a power grid, after the converter is converted into an island operation, the output frequency can be automatically limited within +/-1 percent due to the fact that the sum of the sub-module capacitor energy of the converter is limited within +/-10 percent of the total rated energy of the sub-module capacitor, the requirement for power supply of the island can be met, and the control structure is not changed in the switching process, namely control switching is not needed.

In a preferred embodiment, calculating the sum of the sub-module capacitance energies, making a difference between the sum of the sub-module capacitance energies and the total rated energy of the sub-module capacitance, performing per unit on the difference, multiplying the difference by a set proportion, adding the obtained result and the rated frequency to obtain a reference value of the output alternating current frequency of the modular multilevel converter, and integrating the reference value of the output alternating current frequency to obtain a reference phase, includes:

ΔW _total ＝6N _total C _SM U _SMnom ΔU _SMav (25)

in the formula, N _total The total number of the submodules of the single bridge arm of the submodules of the modular multilevel converter, C _SM Is a capacitance value, U, of a submodule capacitor of a modular multilevel converter _SMnom For sub-module capacitor voltage rating, U _SMav Is the sub-module capacitor voltage average value, Δ U _SMav For deviation of sub-module capacitor voltage from nominal value, Δ W _total Is the difference between the sum of the sub-module capacitance energies and the total rated energy of the sub-module capacitance, U _SMav Is obtained by the following formula:

Output frequency omega of modular multilevel converter _mmc After an amplitude limiting link, the phase theta of the internal potential of the converter is obtained through integration _mmc ：

θ _mmc ＝∫ω _mmc (28)

The phase θ of the obtained internal potential _mmc Namely the reference phase;

the measured value of the reactive power is differed from a preset reference value, and after a proportional control link, the reference value of the output alternating voltage is obtained, which comprises the following steps:

E-E _nom ＝K _Q (Q _mmc -Q _ref ) (29)

wherein E is the magnitude of the internal potential, E _nom For internal potential amplitude ratings, K _Q For controlling the ratio in reactive modeParameter, Q _mmc For reactive power, Q, output by the inverter _ref Setting the reactive power of the converter, wherein the obtained amplitude E of the internal potential is a reference value of the output alternating voltage;

In a preferred embodiment, the park point voltage and current are subjected to park change in a reference phase to obtain d/q axis voltage and current measurement values, including:

Comparing the obtained d-axis voltage measurement value with a reference value of output alternating voltage, comparing the obtained q-axis voltage measurement value with 0, and obtaining a reference value of d/q-axis current through a self-adaptive virtual admittance link, wherein the method comprises the following steps:

in the formula, s is a Laplace operator;

the expected value i of the d/q axis current ^* _d And i ^* _q Inputting an annular current limiting link to obtain a reference value i of the d/q axis current _dref And i _qref Comprises the following steps:

in the formula i _lim The output current amplitude limit of the converter is determined according to the tolerance capability of a switching device used by the converter, and is generally 1.2-1.5 times of rated current;

comparing the obtained measured value of the d/q axis current with the reference value of the d/q axis current, and obtaining the alternating voltage d/q axis reference value of the modular multilevel converter through a proportional-integral link, wherein the method comprises the following steps:

reference value i of d/q axis current _dref And i _qref Respectively with measured values of d/q-axis current i _d And i _q Performing difference, adding a power grid voltage feedforward and decoupling term after passing through a proportional-integral controller to obtain an alternating voltage d/q axis reference value u of the modular multilevel converter _dref And u _qref Comprises the following steps:

In a preferred embodiment, a given value of the direct current side current is calculated, a difference is made between the given value and a measured value of the direct current side current, and the difference value passes through an amplitude limiting link; the sum of the sub-module capacitor energy is subjected to per-unit treatment and then is subjected to difference with the total rated energy of the sub-module capacitor, the obtained difference value is added to the output of an amplitude limiting link, and a proportional-integral link is further carried out to obtain a reference value of the direct current voltage; the method comprises the following steps:

setting a given value i of the direct current side current _dcref With measured value i of the direct side current _dc Making difference, and obtaining current error i after a limiting link _e ：

the sum of the sub-module capacitor energy is subjected to per unit to be differenced with the total rated energy of the sub-module capacitor, and the obtained sub-module capacitor energy deviation is added to the output of an amplitude limiting link, namely a current error i _e And finally, a direct current voltage measured value u outside the direct current side current limiting reactance of the modular multilevel converter is subjected to a proportional-integral link _dco Adding to obtain a reference value of the direct-current voltage:

in the formula, k _pdc Is a proportional controller parameter, k _idc For integrating the controller parameter, Δ W _total The deviation of the sum of the sub-module capacitance energies from the total rated energy of the sub-module capacitance, W _totalnom A rated value of the total rated energy of the sub-module capacitor;

when the modularized multi-level converter is operated in a networking mode, the deviation between the measured value of the direct current side current and the given value of the direct current side current does not reach the amplitude limit, at the moment, the control of the direct current side of the modularized multi-level converter is active power-direct current double closed-loop control, the active power output by the modularized multi-level converter can be stably controlled, and meanwhile, the sum of sub-module capacitor energy is kept unchanged;

when the island operates, an active power outer ring is saturated, the deviation between the measured value of the direct current side current and the given value of the direct current side current reaches amplitude limiting, the control of the direct current side of the modular multilevel converter is single closed-loop control of sub-module capacitance energy deviation, the sum of sub-module capacitance energy is maintained in a safety range, the direct current side power and the power input/output by the alternating current side are automatically kept in balance, and control switching is not needed.

In a preferred embodiment, i _max Is taken to be 0.1, i _min The value of (a) is-0.1.

In a preferred embodiment, the safety ranges are: the sum of the sub-module capacitance energies is maintained within +/-10% of the total rated energy of the sub-module capacitance.

In a preferred embodiment, adding or subtracting a half of a reference value of a direct-current voltage to or from a half of a reference value of a three-phase alternating-current voltage to obtain output reference voltages of six bridge arms, modulating the output reference voltages to obtain a switching signal of each bridge arm of the modular multilevel converter, and completing operation control of the modular multilevel converter, the method includes:

according to the reference value u of the obtained three-phase alternating voltage _a 、u _b And u _c And obtaining a reference value u of the DC voltage _dcref And calculating reference output voltages of six bridge arms of the receiving end MMC:

in the formula u _pa 、u _pb 、u _pc 、u _na 、u _nb 、u _nc Reference output voltages of an A-phase upper bridge arm, a B-phase upper bridge arm, a C-phase upper bridge arm, an A-phase lower bridge arm, a B-phase lower bridge arm and a C-phase lower bridge arm are respectively set; u. u _cira 、u _cirb 、u _circ The output voltages controlled by the double frequency loop are respectively used for controlling the double frequency loop in the modular multilevel converter;

outputting reference output voltages u of an A-phase upper bridge arm, a B-phase upper bridge arm, a C-phase upper bridge arm, an A-phase lower bridge arm, a B-phase lower bridge arm and a C-phase lower bridge arm _pa 、u _pb 、u _pc 、u _na 、u _nb And u _nc And after the latest level modulation, switching signals of each submodule in each bridge arm of the modular multilevel converter are obtained, and the operation control of the modular multilevel converter is completed.

The networking and island operation control method of the modular multilevel converter provided by the embodiment of the invention is applied to seamless switching control of the modular multilevel converter in an alternating current-direct current hybrid power supply system between networking and island, the modular multilevel converter works in a constant active power-reactive power control mode during networking operation, works in a constant alternating current frequency-alternating current voltage control mode during island operation, and can realize seamless switching of two operation modes. The control method is divided into two parts, the total energy of submodules (namely the sum of capacitance energy of the submodules) of the modular multilevel converter is utilized to simulate a synchronous generator rotor on an alternating current side, and the output current of the converter is limited in the networking-island switching process by adopting a self-adaptive virtual admittance (a self-adaptive virtual admittance link) and a current inner ring (an annular current limiting link). On the direct current side, the characteristic that the modularized multi-level converter can quickly adjust direct-current voltage by changing the number of input sub-modules is utilized, active power-direct current double closed-loop control is designed, and seamless switching of networking and island modes is achieved by utilizing an amplitude limiting link and the total energy of the sub-modules in a control loop. Under the control method, the modular multilevel converter can realize seamless switching between the networking mode and the island mode without adding an additional networking/island detection link, and the switching process is very smooth, so that the stable operation of an island power system in the switching process can be ensured.

In some embodiments of the invention:

the nominal frequency is typically 50 Hz.

When the network is operated, the deviation between a direct current measured value and a given value does not reach the amplitude limit, at the moment, the control of the direct current side of the converter is embodied as active power-direct current double closed-loop control, the active power output by the converter can be stably controlled, and meanwhile, the energy of the sub-modules is maintained unchanged. When the island operates, an active power outer ring is saturated, the deviation of a direct current measured value and a given value reaches amplitude limiting, the direct current side of the converter is represented as single closed-loop control of sub-module energy deviation, the sub-module energy is maintained in a safety range, the direct current side power keeps balance with the input/output power of the alternating current side automatically, and an additional switching strategy is not needed. Since the ratio of the energy deviation of the submodules to the rated energy is equal to i _max Or i _min The sub-module capacitance energy can not exceed or fall below the rated value (namely the total rated energy of the sub-module capacitance) generally in engineering, so i _max Typically 0.1, i _min Typically, it is-0.1.

The technical solutions provided by the above embodiments of the present invention are further described below with reference to the accompanying drawings.

As shown in fig. 2, the control strategy of the control method provided by the above embodiment of the present invention may include four parts: the method comprises the steps of AC side self-synchronization control, current inner loop control based on self-adaptive virtual admittance, DC side voltage decoupling control and modulation control.

Wherein:

in the ac-side self-synchronization control:

calculating and linearizing the difference between the total energy sum and the rated value of the submodules:

ΔW _total ＝6N _total C _SM U _SMnom ΔU _SMav (37)

in the formula, N _total The total number of the submodules of the single bridge arm of the submodules of the modular multilevel converter, C _SM Sub-module capacitor of modular multilevel converterValue U _SMnom For sub-module capacitor voltage rating, U _SMav Is the mean value of the sub-module capacitor voltage, U _SMav Can be obtained from the following formula:

according to Δ W _total Obtaining the output frequency omega of the converter _mmc Comprises the following steps:

wherein H _v Is a virtual coefficient of inertia, S _nom For converter rated capacity, omega _nom The inverter rated frequency is typically 50 Hz.

ω _mmc Through an amplitude limiting link (the lower limit of the amplitude limiting link is set to be 0.99 omega) _nom The upper limit is set to 1.01 ω _nom ) The phase theta of the potential in the converter can be obtained by post-integration _mmc ：

θ _mmc ＝∫ω _mmc (40)

The amplitude limiting link comprises: if the input value is less than the upper limit, the output is equal to the input, the input value is greater than the upper limit, the output is equal to the upper limit, the input value is greater than the upper limit, the output is equal to the input, the input value is less than the lower limit, and the output is equal to the lower limit.

Then, after the difference is made between the measured value of the reactive power and the given value, the amplitude of the internal potential of the converter is obtained through a droop link:

E-E _nom ＝K _Q (Q _mmc -Q _ref ) (41)

in the networking state, the method for generating the internal potential phase can help the converter to automatically synchronize the grid phase, such as: when the frequency of the power grid is reduced, the power angle of the converter is increased, the output active power is increased, the total energy of the submodules is reduced, and the output alternating current frequency of the converter is reduced until the frequency is the same as the frequency of the power grid. In an island state, the phase generation method can be used without switchingAutomatically maintaining the output frequency at 0.99 omega _nom To 1.01 omega _nom And meanwhile, stable operation of the island power grid is ensured.

In a networking state, reactive power output by the modular multilevel converter is related to the amplitude of output alternating voltage of the modular multilevel converter, the method for generating the internal potential amplitude can help the converter to control the output reactive power, and in an island state, the converter can help the converter to control the output voltage without control switching.

Through the self-synchronizing control of the alternating current side, the following functions can be realized:

in the traditional method, the networking-island operation state needs to be detected and judged, and different controllers are selected according to the networking state or the island state, wherein the reference phase is realized by detecting the voltage of a power grid, and the voltage of the power grid is not used for detection under the island operation, so that the controllers need to be switched.

The amplitude limiting link does not work when the network is in operation, the amplitude limiting link is automatically put into operation when the island is in operation, the total energy of the submodules of the converter is limited to +/-10%, the alternating current frequency is limited to +/-1%, and the converter is automatically switched from a constant active power mode to a constant alternating current voltage-constant alternating current frequency mode.

In the current inner loop control based on the adaptive virtual admittance:

measuring converter grid-connected point three-phase voltage and current u _abc Based on the internal potential phase θ _mmc Carrying out Park conversion to obtain the voltage u of d/q axis _dq And current i _dq D-axis voltage u _d Difference with internal potential amplitude E, q-axis voltage u _q From 0, through a virtual admittance procedure, the virtual admittance including a virtual resistance R _v And a virtual reactance L _v And obtaining a desired value i of the d/q axis current ^* _d And i ^* _q ：

Then i is put ^* _d And i ^* _q Inputting an annular current limiting link to finally obtain a reference value i of the d/q axis current _dref And i _qref ：

Then i is put _dref And i _qref Are respectively connected with i _d And i _q Performing difference, adding power grid voltage feedforward and decoupling terms after passing through a proportional-integral controller to obtain an output alternating voltage reference value u of the modular multilevel converter _dref And u _qref ：

Wherein k is _p Is a proportional controller parameter, k _i Is the integral controller parameter, s is Laplace operator, L is converter grid-connected inductance, and finally u is paired _dref And u _qref At theta _mmc Carrying out Park inverse transformation to obtain a three-phase alternating voltage reference value u _abc 。

The method can stably control the voltage of the alternating current side under the condition that the filtering capacitor is not arranged on the alternating current side of the converter, generate the reference value of the alternating current, and limit the amplitude of the output alternating current, so that the converter cannot have the overcurrent problem in the networking-island conversion process.

In the direct current side voltage decoupling control:

reference value P of active power _ref And the measured value P _mmc Making a difference, and obtaining a given value i of the direct current side current after a proportion-integration link _dcref ：

Wherein k is _pP Is a proportional controller parameter, k _iP To integrate the controller parameters, i _dcref With measured value i of direct current _dc Making difference, and obtaining current error i after a limiting link _e ：

In the formula i _max Is the maximum value of the DC error, i _min Is the minimum value of the dc current error. The total energy of the sub-module is unified and then is subtracted from the rated value, and the difference is added to the current error i _e And finally, a direct current voltage measured value u outside the direct current side current limiting reactance of the modular multilevel converter is subjected to a proportional-integral link _dco Adding to obtain a reference value of the direct-current voltage:

in the formula k _pdc Is a proportional controller parameter, k _idc Is an integral controller parameter.

When the network is operated, the deviation of a direct current measured value and a given value does not reach the amplitude limit, at the moment, the control of the direct current side of the converter is embodied as active power-direct current double closed-loop control, the active power output by the converter can be stably controlled, and meanwhile, the energy of the sub-modules is maintained unchanged. When the island operates, an active power outer ring is saturated, the deviation of a direct current measured value and a given value reaches an amplitude limit, the direct current side of the converter is embodied as single closed-loop control of sub-module energy deviation, the sub-module energy is maintained in a safety range, the direct current side power keeps balance with the power input/output of the alternating current side automatically, and an extra switching strategy is not needed. Since the ratio of the energy deviation of the submodules to the rated energy is equal to i _max Or i _min The capacitance energy of the submodule cannot exceed or fall below 10% of the rated value in engineering, so i _max Typically 0.1, i _min Generally, it is taken at-0.1.

In the modulation control:

according to the reference value u of the AC three-phase voltage of the current converter obtained in the inner loop control _a 、u _b And u _c d.C. side reference voltage u of the sum converter _dcref And calculating the reference output voltage of six bridge arms of a receiving end MMC:

u _pa 、u _pb 、u _pc 、u _na 、u _nb 、u _nc and the reference output voltages of the phase A upper bridge arm, the phase B upper bridge arm, the phase C upper bridge arm, the phase A lower bridge arm, the phase B lower bridge arm and the phase C lower bridge arm are respectively. u. of _cira 、u _cirb 、u _circ Is the output voltage of the double frequency circulation control and is used for controlling the double frequency circulation in the converter.

Then u is put _pa 、u _pb 、u _pc 、u _na 、u _nb 、u _nc After the latest level modulation, switching signals of each submodule in each bridge arm of the modular multilevel converter can be obtained, and the operation control of the modular multilevel converter is completed.

The control method provided by the above embodiment of the present invention is subjected to simulation verification.

The simulation verification is implemented through PSCAD/EMTDC software, a simulation model is built based on an island AC/DC power supply system in fig. 1, an AC main network adopts voltage source equivalence, an island power grid adopts single load equivalence, loads comprise 60MW active power and 54Mvar inductive reactive power, the access point short-circuit impedance of a converter station at an island side is 0.072H, and other parameters are shown in table 1.

TABLE 1 simulation System parameters

Working condition one: initial active power given setting of island side converter is 2And 0MW, setting the reactive power to be 10Mvar, and disconnecting the AC submarine cable between the island power grid and the AC main grid when t is 3 s.

Fig. 3 shows simulation results, where (a) in fig. 3 is dc system voltage, fig. 3 (b) and (c) are ac voltage and frequency of an islanding system, respectively, (d) in fig. 3 is output current of the islanding-side modular multilevel converter, and fig. 3 (e) and (f) are output active power and reactive power of the islanding-side modular multilevel converter, respectively. Therefore, by adopting the control method provided by the invention, the island side converter can accurately control the active power and the reactive power to be the same as the instruction values when the network is in operation, the voltage and the frequency of the island system can be kept constant when the island is in operation, the impact is very small in the process of converting the network into the island, and the stable operation of the island system can be ensured.

An embodiment of the present invention provides a networking and islanding operation control system for a modular multilevel converter, including: the self-synchronizing control module works on the alternating current side of the modular multilevel converter, the current inner ring module based on self-adaptive virtual admittance, and the voltage decoupling control module and the modulation module work on the direct current side of the modular multilevel converter; wherein:

the AC side self-synchronizing control module is used for calculating the sum of the sub-module capacitor energies, making a difference between the sub-module capacitor energy sum and the total rated energy of the sub-module capacitor, performing per-unit on the difference, multiplying the difference by a set proportion, adding the obtained result and the rated frequency to obtain a reference value of the output AC frequency of the modular multilevel converter, and integrating the reference value of the output AC frequency to obtain a reference phase; the measured value of the reactive power is subtracted from a preset reference value, and a reference value of the output alternating voltage is obtained after a proportion link;

the current inner loop module based on the self-adaptive virtual admittance is used for carrying out park transformation on the voltage and the current of the grid-connected point under a reference phase to obtain the measured values of the voltage and the current of the d/q axis; comparing the obtained d-axis voltage measurement value with a reference value of an output alternating voltage, comparing the obtained q-axis voltage measurement value with 0, and obtaining a reference value of d/q-axis current after passing through a self-adaptive virtual admittance link and an annular current limiting link; comparing the obtained measured value of the d/q-axis current with the reference value of the d/q-axis current, and obtaining the alternating voltage d/q-axis reference value of the modular multilevel converter through a proportional-integral link; carrying out park inverse transformation on the reference value of the d/q axis of the alternating voltage under a reference phase to obtain a reference value of the three-phase alternating voltage of the modular multilevel converter;

the direct current side voltage decoupling control module is used for calculating a given value of direct current side current, making a difference with a measured value of the direct current side current, and enabling the difference value to pass through an amplitude limiting link; the sum of the sub-module capacitor energy is subjected to per-unit treatment and then is subjected to difference with the total rated energy of the sub-module capacitor, the obtained difference value is added to the output of an amplitude limiting link, and a proportional-integral link is further carried out to obtain a reference value of the direct current voltage;

and the modulation module is used for adding or subtracting half of the reference value of the direct-current voltage and half of the reference value of the three-phase alternating-current voltage respectively to obtain output reference voltages of six bridge arms, modulating the output reference voltages to obtain a switching signal of each bridge arm of the modular multilevel converter, and finishing the operation control of the modular multilevel converter.

In the control system provided by this embodiment, the operational relationship between the modules is as shown in fig. 2, where: the output of the AC side self-synchronizing control module is transmitted to the current inner ring module of the self-adaptive virtual admittance to generate a reference value of the output voltage of the AC side of the converter, the DC voltage decoupling control module generates a reference value of the output voltage of the DC side of the converter, the reference value of the output voltage of the AC side and the reference value of the output voltage of the DC side of the converter are input into the modulation module, and finally a control signal of the converter is generated.

It should be noted that, the steps in the method provided by the present invention may be implemented by using corresponding modules, devices, units, and the like in the system, and those skilled in the art may implement the composition of the system by referring to the technical solution of the method, that is, the embodiment in the method may be understood as a preferred example for constructing the system, and will not be described herein again.

The networking and island operation control method and system of the modular multilevel converter provided by the embodiment of the invention are applied to networking-island seamless switching control of the modular multilevel converter in an alternating current-direct current hybrid power supply system, work in a constant active power-reactive power control mode during networking operation, work in a constant alternating current frequency-alternating current voltage control mode during island operation, and can realize seamless switching of two operation modes. And on the alternating current side, the output current of the current converter is limited in the networking-island switching process by adopting the self-adaptive virtual admittance and the current inner ring. On the direct current side, the seamless switching of the networking mode and the island mode is realized by designing the double closed-loop control of active power-direct current and utilizing the feedforward of an amplitude limiting link and the total energy of a submodule in a control loop. Under the control method, the modular multilevel converter can realize seamless switching between the networking mode and the island mode without adding an additional networking/island detection link, and the switching process is very smooth, so that the stable operation of an island power system in the switching process can be ensured.

The foregoing description of specific embodiments of the present invention has been presented. It is to be understood that the present invention is not limited to the specific embodiments described above, and that various changes or modifications may be made by one skilled in the art within the scope of the appended claims without departing from the spirit of the invention.

Claims

1. A networking and island operation control method of a modular multilevel converter is characterized by comprising the following steps:

on the ac side of the modular multilevel converter:

on the dc side of the modular multilevel converter:

calculating a given value of the direct current side current, making a difference with a measured value of the direct current side current, and passing the difference through an amplitude limiting link; the sum of the sub-module capacitor energy is subjected to per-unit treatment and then is subjected to difference with the total rated energy of the sub-module capacitor, the obtained difference value is added to the output of the amplitude limiting link, and a proportional-integral link is further carried out to obtain a reference value of the direct-current voltage;

and adding or subtracting half of the reference value of the direct current voltage and half of the reference value of the three-phase alternating current voltage respectively to obtain output reference voltages of six bridge arms, modulating the output reference voltages to obtain a switching signal of each bridge arm of the modular multilevel converter, and finishing the operation control of the modular multilevel converter.

2. The networking and island operation control method of the modular multilevel converter according to claim 1, wherein the calculating the sum of the sub-module capacitance energies, differentiating the sum of the sub-module capacitance energies from the total rated energy of the sub-module capacitance, and multiplying the difference by a set proportion after per unit, adding the obtained result with the rated frequency to be the reference value of the output alternating current frequency of the modular multilevel converter, and integrating the reference value of the output alternating current frequency to be the reference phase comprises:

ΔW _total ＝6N _total C _SM U _SMnom ΔU _SMav (1)

according to Δ W _total Obtaining the output frequency omega of the modular multilevel converter _mmc Comprises the following steps:

θ _mmc ＝∫ω _mmc (4)

through the step of generating the reference phase, the reference phase is automatically generated inside the modular multilevel converter, and the modular multilevel converter automatically synchronizes the phase of the power grid in a networking state; in an island state, control switching is not needed, and the modularization is moreThe level converter automatically maintains the output AC frequency at 0.99 omega _nom To 1.01 omega _nom In the meantime, stable operation of the island power grid is ensured;

E-E _nom ＝K _Q (Q _mmc -Q _ref ) (5)

3. A method according to claim 1, wherein the parker changing the grid-connected point voltage and current in the reference phase to obtain the measured values of d/q axis voltage and current comprises:

in the formula, s is Laplace operator;

the reference value i of the d/q axis current is measured _dref And i _qref Respectively with measured values i of said d/q-axis currents _d And i _q Making difference, after a proportional-integral controller, adding power network voltage feedforward and decoupling terms to obtain modular multilevel converterReference value u of alternating voltage d/q axis of current transformer _dref And u _qref Comprises the following steps:

through the steps, under the condition that the filter capacitor is not arranged on the AC side of the converter, the voltage on the AC side is stably controlled, the reference value of the AC current is generated, and the amplitude of the output AC current is limited.

4. A method according to claim 1, wherein the given value of the dc side current is calculated and subtracted from the measured value of the dc side current, and the difference is passed through a limiting element; the sum of the sub-module capacitor energy is subjected to per unit and then is subjected to difference with the total rated energy of the sub-module capacitor, the obtained difference is added to the output of the amplitude limiting link, and a proportional-integral link is performed to obtain a reference value of the direct current voltage; the method comprises the following steps:

a preset reference value P of active power _ref And the measured value P _mmc Making a difference, and obtaining a given value i of the direct current side current after a proportion-integration link _dcref ：

during networking operation, the deviation between the measured value of the direct-current side current and the given value of the direct-current side current does not reach the amplitude limit, at the moment, the control of the direct-current side of the modular multilevel converter is active power-direct current double closed-loop control, the active power output by the modular multilevel converter can be stably controlled, and meanwhile, the sum of the sub-module capacitor energy is kept unchanged;

5. Networking and island operation control method of modular multilevel converter according to claim 4, characterized in that i is _max Is taken to be 0.1, i _min Is-0.1.

6. The networking and island operation control method of the modular multilevel converter according to claim 4, wherein the safety range is as follows: the sum of the sub-module capacitance energies is maintained within +/-10% of the total rated energy of the sub-module capacitance.

7. The networking and island operation control method of the modular multilevel converter according to claim 1, wherein the step of adding or subtracting a half of the reference value of the dc voltage to or from a half of the reference value of the three-phase ac voltage to obtain output reference voltages of six bridge arms, and modulating the output reference voltages to obtain switching signals of each bridge arm of the modular multilevel converter to complete operation control of the modular multilevel converter comprises:

in the formula u _pa 、u _pb 、u _pc 、u _na 、u _nb 、u _nc Reference output voltages of an A-phase upper bridge arm, a B-phase upper bridge arm, a C-phase upper bridge arm, an A-phase lower bridge arm, a B-phase lower bridge arm and a C-phase lower bridge arm are respectively set; u. of _cira 、u _cirb 、u _circ The output voltages controlled by the double frequency loop are respectively used for controlling the double frequency loop in the modular multilevel converter;

8. A networking and island operation control system of a modular multilevel converter is characterized by comprising: the self-synchronizing control module works on the alternating current side of the modular multilevel converter, the current inner ring module based on the self-adaptive virtual admittance, and the voltage decoupling control module and the modulation module work on the direct current side of the modular multilevel converter; wherein: