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CN108832657B - Control method for virtual synchronous motor of alternating current-direct current hybrid microgrid bidirectional power converter - Google Patents

Control method for virtual synchronous motor of alternating current-direct current hybrid microgrid bidirectional power converter Download PDF

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CN108832657B
CN108832657B CN201810651726.XA CN201810651726A CN108832657B CN 108832657 B CN108832657 B CN 108832657B CN 201810651726 A CN201810651726 A CN 201810651726A CN 108832657 B CN108832657 B CN 108832657B
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voltage
power
frequency
power converter
bidirectional power
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CN108832657A (en
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秦文萍
李峰
韩肖清
王鹏
贾燕冰
孟润泉
任春光
王祺
周一雄
刘龙峰
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Taiyuan University of Technology
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for AC mains or AC distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for AC mains or AC distribution networks
    • H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for AC mains or AC distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
    • H02J3/48—Controlling the sharing of the in-phase component
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for AC mains or AC distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
    • H02J3/50—Controlling the sharing of the out-of-phase component
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
    • H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]

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Abstract

本发明公开了一种交直流混合微电网双向功率变换器虚拟同步电机控制方法，涉及交直流混合微电网中虚拟同步电机控制领域。本发明的双向功率变换器为由IGBT开关管构成的三相电压源型PWM变换器，交流侧经过LC滤波器和线路阻抗Z ac 连接至交流母线，直流侧经过直流电容C dc 和线路阻抗Z dc 连接至直流母线；分别通过频率控制单元、虚拟励磁控制单元和双向功率传输控制单元，实现了能量的双向传输，通过直接控制交流频率和直流电压，维持交、直流子网负荷均衡，充分利用交、直流子网各种类型电源和负荷的吞吐特性为交流频率和直流电压提供惯性，使双向功率变换器在稳态下呈现下垂特性，并通过建立双向功率变换器闭环系统小信号模型给出关键参数设计方法。

Figure 201810651726

The invention discloses a virtual synchronous motor control method for a bidirectional power converter of an AC-DC hybrid microgrid, and relates to the field of virtual synchronous motor control in an AC-DC hybrid microgrid. The bidirectional power converter of the present invention is a three - phase voltage source type PWM converter composed of IGBT switch tubes . The dc is connected to the DC bus; the frequency control unit, the virtual excitation control unit and the bidirectional power transmission control unit are respectively used to realize the bidirectional transmission of energy. By directly controlling the AC frequency and DC voltage, the load balance of the AC and DC sub-networks is maintained, and the full use of The throughput characteristics of various types of power sources and loads in the AC and DC sub-networks provide inertia for the AC frequency and DC voltage, so that the bidirectional power converter exhibits droop characteristics in a steady state. Key parameter design method.

Figure 201810651726

Description

Control method for virtual synchronous motor of alternating current-direct current hybrid microgrid bidirectional power converter
Technical Field
The invention relates to the field of control over virtual synchronous motors of alternating-current and direct-current hybrid micro-grids, in particular to a control method for a virtual synchronous motor of a bidirectional power converter of an alternating-current and direct-current hybrid micro-grid, which is suitable for balancing loads of the alternating-current and direct-current hybrid micro-grid and enables the bidirectional power converter to show dynamic frequency regulation characteristics similar to those of a synchronous motor.
Background
The improvement of the permeability of the distributed power supply provides a new challenge for the operation and control of the power system, and the micro-grid technology integrating various renewable energy sources, energy storage and loads can effectively avoid the advantages of uncertainty of output of the distributed new energy sources, influence of load randomness on the power system and the like as the micro-grid technology can be used as a self-operation and self-management unit of the power system, so that the micro-grid technology becomes an important component of a future smart grid. The micro-grid is divided into an alternating current micro-grid, a direct current micro-grid and an alternating current and direct current hybrid micro-grid according to different types of buses. With the increasing of direct current type distributed power sources and loads, an alternating current-direct current hybrid microgrid formed by interconnecting a direct current microgrid and an existing alternating current distribution network can provide support for an existing alternating current system, saves power electronic conversion devices required by a direct current type element access system, reduces energy loss, and improves the economy and reliability of the whole system.
In the alternating current-direct current hybrid micro-grid, a bidirectional power converter is connected with an alternating current bus and a direct current bus to control energy exchange between an alternating current sub-network and a direct current sub-network, and the bidirectional power converter plays an extremely important role in maintaining stable operation of the hybrid micro-grid. The inventor proposes a normalized bidirectional droop control method, which performs normalized calculation on the alternating current sub-network frequency and the direct current sub-network voltage, and controls the power flowing through the bidirectional power converter by comparing the normalized values of the two, so that the alternating current sub-network and the direct current sub-network bear loads in a balanced manner. However, the droop control response speed is very fast, inertia is basically not provided when the droop control system is connected to a large power grid, and when the permeability of the distributed power supply is gradually increased, the rotational inertia of the whole power grid is reduced, so that the system stability is not facilitated. Aiming at the problem of low droop control rotational inertia, a student provides virtual synchronous motor control, and by simulating a traditional synchronous motor rotor motion equation, a power electronic converter has inertia, improves the dynamic response of micro-grid alternating-current frequency, improves the stability of a system, and enables a distributed power supply to be accessed into a traditional power grid more friendly. A learner compares the grid-connected inverter with the synchronous generator, finds that the grid-connected inverter and the synchronous generator have one-to-one correspondence, and the grid-connected inverter and the synchronous generator can be physically equivalent to each other, so that a virtual synchronous generator control strategy of the grid-connected inverter is designed; some researchers have proposed a virtual synchronous motor control strategy to control the fast charging of the electric vehicle so that the rectifier has inertia and damping characteristics. The two kinds of control make the power electronic converter device have inertia, and can reduce the influence of indirect fluctuation of the distributed power supply on a power grid, but the two kinds of control only consider a single power flow direction and can only be applied to the power electronic converter in a specific occasion. The learner provides a virtual motor control strategy for a bidirectional power converter by combining with the normalized bidirectional droop control, the control strategy can provide inertia for alternating current frequency and reduce the coupling influence of rapid power fluctuation of an alternating current sub-network and a direct current sub-network, but the control strategy belongs to current type control, does not have the capacity of autonomous frequency modulation and voltage regulation, and cannot provide voltage and frequency support for a hybrid micro-grid during the operation of an island. The learners propose a layered control strategy, which utilizes an energy storage system of a lower layer direct current sub-network to provide inertia for alternating current frequency of a bidirectional power converter, and the method has the advantages of single inertia source, small inertia parameter selection range, unstable operation area and need to design an additional system stabilizer. Therefore, in the ac/dc hybrid micro-grid, the existing related control technology of the bidirectional power converter still has many defects, and the bidirectional power converter needs a control method which considers the rotational inertia and enables the bidirectional power converter to have the frequency modulation and voltage regulation capabilities.
Disclosure of Invention
The invention provides a virtual synchronous motor control method for a bidirectional power converter of an alternating-current and direct-current hybrid microgrid, aiming at solving the problems that due to the fact that droop control is adopted in the alternating-current and direct-current hybrid microgrid, inertia is small, damping is low, an inertia source is single, current type control does not have frequency modulation and voltage regulation capabilities, and voltage and frequency support cannot be provided in an island.
The invention is realized by the following technical scheme: a control method for a virtual synchronous motor of a bidirectional power converter of an AC/DC hybrid microgrid comprises the bidirectional power converter, wherein the bidirectional power converter is a three-phase voltage source type PWM converter formed by IGBT switching tubes, and the AC side of the bidirectional power converter passes through an LC filter and line impedance ZacConnected to an AC bus, the DC side of which passes through a DC capacitor CdcAnd line impedance ZdcThe bidirectional power converter is connected to a direct current bus and is controlled by a control unit; the control unit comprises a frequency control unit, a virtual excitation control unit and a bidirectional power transmission control unit; the frequency control unit and the virtual excitation control unit respectively generate an alternating-current side phase angle delta and a voltage reference value E of the control loop, the alternating-current side phase angle delta generates a three-phase sine wave with a phase difference of 120 degrees after passing through a sine function, and the three-phase sine wave is multiplied by the voltage reference value E to obtain a three-phase sine reference voltage EabcReference voltage eabcThe three-phase sinusoidal modulation wave is generated as an input signal of voltage-current double closed-loop control after control and regulation, PWM modulation is carried out, and finally a PWM signal is generated to control the bidirectional power converter.
The invention relates to a virtual synchronous motor control method of an alternating current-direct current hybrid microgrid bidirectional power converter, which is characterized in that a main control component is a three-phase voltage source type PWM converter consisting of IGBT (insulated gate bipolar transistor) switching tubes, and an alternating current side passes through an LC (inductor-capacitor) filter (the parasitic resistance of a filter inductor L is R)L) And line impedance ZacConnected to an AC bus, the DC side of which passes through a DC capacitor CdcAnd line impedance ZdcConnected to the DC bus, the bidirectional power converter is controlled by a control unit, the control unit comprises a bidirectional power transmission control unit, a frequency control unit and a virtual excitation controlMaking a unit three part; the frequency control unit generates a voltage reference value E of the control loop, the virtual excitation control unit generates an alternating-current side phase angle delta of the control loop, the alternating-current side phase angle delta generates a three-phase sine wave with a phase difference of 120 degrees after passing through a sine function, and the three-phase sine wave is multiplied by the voltage reference value E to obtain a three-phase sine reference voltage EabcReference voltage eabcThe three-phase sinusoidal modulation wave is generated as an input signal of voltage-current double closed-loop control after control and regulation, PWM modulation is carried out, and finally a PWM signal is generated to control the bidirectional power converter. The bidirectional power converter is physically equivalent to a synchronous motor, the synchronous motor is divided into a synchronous generator and a synchronous motor according to different energy conversion processes, and the main difference of the synchronous generator and the synchronous motor is the phase angle difference delta between excitation electromotive force and terminal voltage from a voltage equationG，δG>0 time is the generator, deltaG<Motor at 0, deltaGWill change the power flow direction of the synchronous machine, delta and deltaGCorresponding; the frequency control unit controls the phase angle δ on the ac side, and controls the power flow direction in the bidirectional power converter.
In the control unit, the frequency control unit, the virtual excitation control unit and the bidirectional power transmission control unit have the following control methods:
a. a frequency control unit:
the mechanical equation of motion of a conventional synchronous motor is:
Figure BDA0001705015100000031
in the formula: j is the moment of inertia, D is the damping coefficient, omega is the virtual rotor angular frequency, omegaNFor nominal angular frequency, P, of the systemm、PeMechanical power and electromagnetic power respectively; when the converter is controlled by a virtual synchronous motor, the effect of a damping winding is not considered, and the damping coefficient and the droop coefficient k of primary frequency modulation are taken into considerationωEqual, i.e.:
Figure BDA0001705015100000032
by detecting active power PeRelative to PmIs equal to Pm-Pe＝-△PeAnd the control of the angular frequency omega is realized. In the hybrid micro-grid, the active power change can cause the change of alternating current frequency and direct current voltage, so the change of the direct current voltage can reflect the active power change quantity delta P of the hybrid micro-grid. From the direct current side, the active power change Δ P in the dynamic process is composed of two parts: active power variation quantity delta P caused by direct current side droop controlinBelongs to steady state power variation; instantaneous charge and discharge power Delta P of direct current capacitorCThe method belongs to dynamic power fluctuation, and the two parts respectively correspond to the steady-state variable quantity and the instantaneous variable rate of the direct-current voltage:
Figure BDA0001705015100000033
in the formula: k is a radical ofuIs the direct current droop coefficient; u. ofdcIs the actual value of the DC voltage, UdcNIs a DC voltage rating; cdcIs the capacity of the direct current capacitor.
After the direct-current voltage is measured, the power variation quantity delta P in the hybrid micro-grid can be obtained through calculation, and then the frequency is controlled, so that the control equation of the virtual synchronous motor of the bidirectional power converter in the hybrid micro-grid is as follows:
Figure BDA0001705015100000041
the virtual synchronous motor control equation of the bidirectional power converter introduces direct current voltage and a reference value thereof into a control link, can realize direct control of the direct current voltage while controlling alternating current frequency, belongs to voltage type control, enables the bidirectional power converter to have frequency modulation and voltage regulation functions, and can provide voltage and frequency support for a micro-grid operated in an island.
In the hybrid microgrid, the capacity of distributed power supplies in alternating current subnetworks and direct current subnetworks is limited, and the provided inertia is limited, but the hybrid microgrid has the advantages that the alternating current subnetworks and the direct current subnetworks comprise various types of distributed power supplies, energy storage and loads, energy storage systems in the alternating current subnetworks and the direct current subnetworks can provide inertia for alternating current frequency, and in the dynamic adjustment process, throughput characteristics of other power supplies (such as alternating current synchronous generators, micro gas turbines and the like) and loads along with frequency and voltage changes can also provide part of inertia. The virtual synchronous motor control equation of the alternating current-direct current hybrid micro-grid bidirectional power converter not only is a control equation of a hybrid micro-grid, but also reflects the instantaneous power balance relationship of the hybrid micro-grid after the virtual motor control is added.
During the dynamic regulation, the change of frequency will cause the power of the AC sub-network to change, Δ Pac＝kω(ω-ωN) The system comprises an alternating current sub-network traditional synchronous generator primary frequency modulation, load power frequency characteristics and charge and discharge power of controllable stored energy when the frequency changes;
the change of the DC voltage will cause the change of the power of the DC sub-networkdc＝ku(udc-UdcN)And DC capacitor rapid charging and discharging power CdcUdcN(dudc/dt)，△PdcThe method comprises the steps of including the throughput power of a direct current sub-network energy storage unit and a load when the voltage fluctuates; therefore, the inertia source in the hybrid micro-grid is not only the direct-current side energy storage system, but also the alternating-current and direct-current sub-network power variation can provide inertia for the alternating-current frequency, and the inertia source is enlarged.
When the system reaches steady state, the AC frequency and DC voltage are stable, J omegaN(d ω/dt) ═ 0 and CdcUdcN(dudcAnd dt) is 0, the control equation of the virtual synchronous motor of the bidirectional power converter becomes:
kω(ω-ωN)＝ku(udc-UdcN)
when the stable state is reached, the hybrid micro-grid shows a droop characteristic, and the alternating current sub-network and the direct current sub-network can bear the active power variation in a balanced manner;
b. a virtual excitation control unit:
the traditional synchronous generator changes reactive power through an excitation adjusting device, adjusts electromotive force and then adjusts terminal voltage, a virtual synchronous motor controls and simulates virtual excitation control of the traditional synchronous generator, the virtual excitation control controls voltage through controlling reactive power, namely, reactive-voltage droop control, and a control equation of the traditional synchronous generator is as follows:
E＝Eref+kq(Qref-Q)
wherein E is the virtual potential effective value; erefThe effective value of no-load potential; k is a radical ofqIs the reactive-voltage droop coefficient; qrefAnd Q are the reactive power reference value and the actual value, respectively.
c. Bidirectional power transmission control unit:
setting up
Figure BDA0001705015100000051
And
Figure BDA0001705015100000052
respectively outputting voltage e to the middle points of AC side bridge arms of the bidirectional power converterabcVoltage u of filter capacitorabcAnd the current i flowing through the bidirectional power converterabcOf (2) synthetic phasors of
Figure BDA0001705015100000053
For reference phasors, an alternating-current side voltage equation of the bidirectional power converter can be obtained:
Figure BDA0001705015100000054
in the formula, XLIs the reactance value, X, of the AC filter inductorLThe method comprises the following steps that (1) omega L is obtained, and omega is an alternating current angular frequency, namely a virtual rotor angular frequency of a bidirectional power converter;
the power flowing from the dc sub-network to the ac sub-network through the bidirectional power converter is:
Figure BDA0001705015100000055
where δ is the voltage phasor
Figure BDA0001705015100000056
And
Figure BDA0001705015100000057
phase angle difference of (1), RLParasitic resistances, usually R, of the AC filter inductancesL<<XLThus, the bidirectional power converter power transfer characteristic is approximated as:
Figure BDA0001705015100000058
the power transmission direction of the bidirectional power converter can be controlled by controlling the positive and negative of the phase angle delta, and the bidirectional flow of power is realized.
The voltage equation and the power transmission characteristic of the bidirectional power converter are similar to those of the synchronous motor and the output power, have one-to-one correspondence relationship, and can be subjected to physical equivalence. Compared with the traditional synchronous motor, the three-phase PWM converter has higher flexibility, so that the four-quadrant operation of the converter can be realized by controlling the phase angle difference delta, and the bidirectional flow of power is controlled: when delta > 0, the bidirectional power converter operates in a virtual synchronous generator mode, i.e. an inverter mode, with power flowing from the DC sub-network to the AC sub-network, PtIs positive; when delta < 0, the bidirectional power converter operates in a virtual synchronous motor mode, i.e. a commutation mode, with power flowing from the ac sub-network to the dc sub-network, PtIs negative; when delta is 0, no power flows in the bidirectional power converter, no power is exchanged between the AC and DC sub-networks, and P ist＝0。
When the AC side phase angle delta passes through a sine function, a three-phase sine wave with a phase difference of 120 degrees is generated and multiplied by a voltage reference value E to obtain a three-phase sine reference voltage EabcReference voltage eabcThe three-phase sinusoidal modulation wave is generated as an input signal of voltage-current double closed-loop control after control and regulation, PWM modulation is carried out, and finally a PWM signal is generated to control a bidirectional power converter。
Compared with the prior art, the invention has the following beneficial effects:
1) the bidirectional power converter has the steady-state characteristic of droop control and the dynamic characteristic similar to a synchronous motor, and the bidirectional flow of energy is realized;
2) direct control of direct current voltage and alternating current frequency can be achieved simultaneously, the bidirectional power converter can be used as a voltage source to adjust frequency and voltage, and alternating current and direct current loads in the hybrid micro-grid are balanced;
3) in the dynamic adjustment process, the throughput characteristics of power supplies and loads in the AC and DC subnets are fully utilized to provide inertia for the AC frequency, the inertia is not provided by energy storage and a DC capacitor in the DC subnetwork, and the inertia source is enlarged. Meanwhile, the direct-current voltage can have inertia;
4) therefore, the dynamic response of the alternating current frequency and the direct current voltage has inertia and does not influence each other, and the coupling influence of power change is reduced.
Drawings
Fig. 1 is a typical structure diagram of an ac/dc hybrid microgrid.
Fig. 2 is a structural diagram of a bidirectional power converter in an ac/dc hybrid microgrid.
FIG. 3 is a block diagram of a bidirectional power converter virtual synchronous motor control.
Fig. 4 is a small-signal model diagram of a closed-loop system of a bidirectional power converter.
Fig. 5 is a root trace diagram of a closed loop system of the bidirectional power converter during J change.
FIG. 6 is kωAnd (3) a root trace diagram of a closed loop system of the bidirectional power converter during variation.
FIG. 7 is kuAnd (3) a root trace diagram of a closed loop system of the bidirectional power converter during variation.
FIG. 8 is CdcAnd (3) a root trace diagram of a closed loop system of the bidirectional power converter during variation.
Fig. 9 is a response diagram of the inverter mode of the bidirectional power converter when J is 3.
Fig. 10 is a response diagram of the inverter mode of the bidirectional power converter when J is 6.
Fig. 11 is a response diagram of the bidirectional power converter in the rectification mode when J is 3.
Fig. 12 is a response diagram of the bidirectional power converter in the rectification mode when J is 6.
Fig. 13 is a diagram of a bidirectional power converter mode switching response.
Fig. 14 is a (partially enlarged) graph of the ac frequency and the dc voltage response when the dc capacitance changes.
Detailed Description
The present invention is further illustrated by the following specific examples.
The embodiment is mainly used for a bidirectional power converter in an alternating current-direct current hybrid micro-grid, as shown in fig. 1, in the alternating current-direct current hybrid micro-grid, an alternating current sub-network and a direct current sub-network are connected through one or more bidirectional power converters, the alternating current sub-network is connected with a large power grid through a circuit breaker at a public connection point, and the circuit breaker is disconnected during island operation. In the AC and DC sub-networks, different types of distributed power supplies, energy storage devices and local loads are respectively connected to an AC bus and a DC bus through corresponding power electronic converters.
A control method for a virtual synchronous motor of a bidirectional power converter of an AC/DC hybrid microgrid comprises the bidirectional power converter, the structure of the bidirectional power converter is shown in figure 2, the bidirectional power converter is a three-phase voltage source type PWM converter formed by IGBT switching tubes, and the AC side of the bidirectional power converter passes through an LC filter and line impedance ZacConnected to an AC bus, the DC side of which passes through a DC capacitor CdcAnd line impedance ZdcThe bidirectional power converter is connected to a direct current bus and is controlled by a control unit; the control unit comprises a frequency control unit, a virtual excitation control unit and a bidirectional power transmission control unit, and a control block diagram is shown in fig. 3; the frequency control unit and the virtual excitation control unit respectively generate an alternating-current side phase angle delta and a voltage reference value E of the control loop, the alternating-current side phase angle delta generates a three-phase sine wave with a phase difference of 120 degrees after passing through a sine function, and the three-phase sine wave is multiplied by the voltage reference value E to obtain a three-phase sine reference voltage EabcReference voltage eabcAs the input signal of voltage-current double closed-loop control, the three-phase sine modulation wave is generated after control and regulationAnd performing PWM modulation, and finally generating a PWM signal to control the bidirectional power converter.
In the control unit, the frequency control unit, the virtual excitation control unit and the bidirectional power transmission control unit have the following control methods:
a. a frequency control unit:
the mechanical equation of motion of a conventional synchronous motor is:
Figure BDA0001705015100000071
in the formula: j is the moment of inertia, D is the damping coefficient, omega is the virtual rotor angular frequency, omegaNFor nominal angular frequency, P, of the systemm、PeMechanical power and electromagnetic power respectively; when the influence of the damping winding is ignored, the damping coefficient and the droop coefficient k of the primary frequency modulationωEqual, i.e.:
Figure BDA0001705015100000081
by detecting active power PeRelative to PmIs equal to Pm-Pe＝-△PeAnd the control on the angular frequency omega is realized, and in the hybrid micro-grid, the active power change can simultaneously cause the conversion of the alternating current frequency and the direct current voltage, so that the direct current voltage change can reflect the active power conversion quantity delta P of the hybrid micro-grid. From the direct current side, the active power change Δ P in the dynamic process is composed of two parts: active power variation quantity delta P caused by direct current side droop controlinBelongs to steady state power variation; instantaneous charge and discharge power Delta P of direct current capacitorCThe method belongs to dynamic power fluctuation, and the two parts respectively correspond to the steady-state variable quantity and the instantaneous variable rate of the direct-current voltage:
Figure BDA0001705015100000082
in the formula: k is a radical ofuIs a DC droop systemCounting; u. ofdcIs the actual value of the DC voltage, UdcNIs a DC voltage rating; cdcIs the direct current capacitance;
after the direct-current voltage is measured, the power variation quantity delta P in the hybrid micro-grid can be obtained through calculation, and then the frequency is controlled, so that the control equation of the virtual synchronous motor of the bidirectional power converter in the hybrid micro-grid is as follows:
Figure BDA0001705015100000083
during the dynamic regulation, the change of frequency will cause the power of the AC sub-network to change, Δ Pac＝kω(ω-ωN) The system comprises an alternating current sub-network traditional synchronous generator primary frequency modulation, load power frequency characteristics and charge and discharge power of controllable stored energy when the frequency changes;
the change of the DC voltage will cause the change of the power of the DC sub-networkdc＝ku(udc-UdcN)And DC capacitor rapid charging and discharging power CdcUdcN(dudc/dt)，△PdcThe method comprises the steps of including the throughput power of a direct current sub-network energy storage unit and a load when the voltage fluctuates;
in the dynamic regulation process, the power variation of the alternating current sub-network and the direct current sub-network can provide inertia for the alternating current frequency;
when the system reaches steady state, the AC frequency and DC voltage are stable, J omegaN(d ω/dt) ═ 0 and CdcUdcN(dudcAnd dt) is 0, the control equation of the virtual synchronous motor of the bidirectional power converter becomes:
kω(ω-ωN)＝ku(udc-UdcN)
when the stable state is reached, the hybrid micro-grid shows a droop characteristic, and the alternating current sub-network and the direct current sub-network can bear the active power variation in a balanced manner;
b. a virtual excitation control unit:
the virtual excitation control controls the voltage by controlling the reactive power, namely, the reactive-voltage droop control, and the control equation is as follows:
E＝Eref+kq(Qref-Q)
wherein E is the virtual potential effective value; erefThe effective value of no-load potential; k is a radical ofqIs the reactive-voltage droop coefficient; qrefAnd Q are the reactive power reference value and the actual value, respectively.
c. Bidirectional power transmission control unit:
setting up
Figure BDA0001705015100000091
And
Figure BDA0001705015100000092
respectively outputting voltage e to the middle points of AC side bridge arms of the bidirectional power converterabcVoltage u of filter capacitorabcAnd the current i flowing through the bidirectional power converterabcOf (2) synthetic phasors of
Figure BDA0001705015100000093
For reference phasors, an alternating-current side voltage equation of the bidirectional power converter can be obtained:
Figure BDA0001705015100000094
in the formula, XLIs the reactance value, X, of the AC filter inductorLThe method comprises the following steps that (1) omega L is obtained, and omega is an alternating current angular frequency, namely a virtual rotor angular frequency of a bidirectional power converter;
the power flowing from the dc sub-network to the ac sub-network through the bidirectional power converter is:
Figure BDA0001705015100000095
where δ is the voltage phasor
Figure BDA0001705015100000096
And
Figure BDA0001705015100000097
phase angle difference of (1), RLParasitic resistances, usually R, of the AC filter inductancesL<<XLThus, the bidirectional power converter power transfer characteristic is approximated as:
Figure BDA0001705015100000098
the power transmission direction of the bidirectional power converter can be controlled by controlling the positive and negative of the phase angle delta, and the bidirectional flow of power is realized: when delta > 0, the bidirectional power converter operates in a virtual synchronous generator mode, i.e. an inverter mode, with power flowing from the DC sub-network to the AC sub-network, PtIs positive;
when delta < 0, the bidirectional power converter operates in a virtual synchronous motor mode, i.e. a commutation mode, with power flowing from the ac sub-network to the dc sub-network, PtIs negative;
when delta is 0, no power flows in the bidirectional power converter, no power is exchanged between the AC and DC sub-networks, and P ist＝0。
In the control unit, control parameters are determined by establishing a small signal model of the bidirectional power converter of the AC/DC hybrid microgrid:
model of small signal
a. The small-signal model of the bidirectional power transfer characteristic of the bidirectional power converter is:
Figure BDA0001705015100000101
in the formula, E0、δ0Steady state operation values of electromotive force E and phase delta respectively, wherein delta represents the variation of each variable;
at steady state operation, delta0Is approximately 0, satisfies KPδKQE＞＞KPEKQδThe small-signal model of the bidirectional power transmission characteristic is simplified as follows:
Figure BDA0001705015100000102
b. the small signal model for frequency control is:
Figure BDA0001705015100000103
in the formula, s is Laplace operator;
c. the small signal model of the virtual excitation control unit is as follows:
ΔE＝-kqΔQ；
d. in an actual circuit, the active power change of the hybrid micro-grid causes the direct-current voltage change:
Figure BDA0001705015100000104
Pin-PN＝-ku(udc-UdcN)
in the formula, PinFor the power flowing from the DC sub-network to the DC capacitor, PoutFor the power of the DC capacitor flowing to the bidirectional power converter, PCFor charging and discharging power of DC capacitor, PNA reference power for droop control;
e. the small-signal model of the change of the direct-current voltage caused by the change of the active power of the hybrid micro-grid is as follows:
CdcUdcNsΔudc＝ΔPin-ΔPout
ΔPin＝-kuΔudc
according to the bidirectional power transmission characteristic, the frequency control, the virtual excitation control and the small signal model of the direct-current voltage change of the bidirectional power converter, as shown in fig. 4, a closed-loop transfer function of the virtual synchronous motor control closed-loop system of the alternating-current and direct-current hybrid micro-grid bidirectional power converter is obtained:
Figure BDA0001705015100000111
on the premise of ensuring the stable operation of the system (the characteristic roots of the system are all positioned on the left half plane) and considering the droop characteristic, the key parameter J, k of the system is gradually increasedω，、kuAnd CdcAnd obtaining a root track of the closed-loop system, and observing the change of the stability of the system according to the root track to provide a stability basis for the selection of the key parameters.
In fig. 5, the moment of inertia J increases, the dominant pole of the root track of the system approaches the virtual axis, the adjustment time increases, the damping ratio decreases, the overshoot increases, and the system stability becomes worse. In FIG. 6, kωAnd increasing, gradually changing the dominant pole of the root track of the system from a conjugate complex root into two different negative real roots, gradually changing the system from under damping to over damping, enhancing the stability of the system, and reducing overshoot, wherein one negative real root is gradually far away from the virtual axis, the other negative real root is gradually close to the virtual axis, and the closer the negative real root is to the virtual axis, the longer the adjustment time and the transition process time of the system are. K in FIG. 7uIncreasing, changing the leading pole of the system root track from conjugate complex root to two different negative root, changing the system from underdamping to over-damping, enhancing the system stability, reducing overshoot, and reducing overshoot due to one negative root along with kuThe increase gradually approaches the imaginary axis, and the system settling time and transition time increase. In FIG. 8, CdcAnd the leading pole of the root track of the system is close to the virtual axis, the adjustment time is increased, the damping ratio is reduced, the overshoot is increased, and the stability of the system is poor. Therefore, the moment of inertia J and the DC capacitance C are increaseddcWill degrade system stability; increase the droop coefficient kωAnd kuThe stability of the system will be improved but the settling time and the transition time of the system will be extended. When the parameter design is carried out, two aspects of system stability and transient process response time need to be considered comprehensively.
According to the bidirectional power transmission characteristic, the frequency control, the virtual excitation control and the small signal model of the direct current voltage change of the bidirectional power converter, the dynamic response equations of the alternating current frequency and the direct current voltage are respectively as follows:
Figure BDA0001705015100000112
Figure BDA0001705015100000113
in the formula, k1＝-1/kω，T1＝JωN/kω；k2＝-1/ku，T2＝CdcUdcN/ku。
The dynamic response equation of the alternating current frequency and the direct current voltage shows that transfer functions among the frequency variation, the direct current voltage variation and the active power variation are all first-order inertia links. When active power changes, the change of alternating current frequency and direct current voltage has a transition time, the abrupt change of the alternating current frequency and the direct current voltage can be restrained, the dynamic response of the alternating current frequency and the direct current voltage has certain inertia, and the inertia time constant T can be used in the transition time1And T2And (4) showing. Inertia time constant T of AC frequency1And control parameters J and kωCorrelation; inertia time constant T of DC voltage2And a DC capacitor CdcAnd a control parameter kuAnd (4) correlating. When the active power changes, the dynamic response of the alternating current frequency and the direct current voltage is determined by respective control parameters and does not influence each other, so that the coupling influence when the active power changes is reduced. k is a radical of1And k2And determining the offset of the alternating current frequency and the direct current voltage in the steady state, and indicating that the hybrid micro-grid presents droop characteristics in the steady state.
② parameter determination
From the small signal model, kωAnd kuChanges in (b) will change system stability and transient time, but these two parameters are determined primarily by the ac frequency and dc voltage offset range. Generally stipulate, in steady state, Δ ωmax＝1％ωN(ii) a The fluctuation range of the DC bus voltage is within 5 percent of the rated value, namely delta udc-max＝5％UdcN。
When the virtual motor control is not adopted, the direct current capacitor mainly has the functions of buffering energy exchange at the alternating current side and the direct current side, stabilizing direct current voltage and inhibiting harmonic voltage at the direct current side, so that the selection principle is mainly selected according to the requirements of steady-state voltage following performance and dynamic anti-interference performance. In the virtual motor control, the dc capacitor can be increased appropriately to provide a part of inertia for the ac frequency and the dc voltage, but the stability of the whole closed-loop system is reduced due to the excessive dc capacitor, so the dc capacitor is not suitable to be too large.
The moment of inertia J can be adjusted to T according to an alternating current frequency dynamic response equation1Is selected according to the requirements. According to the small signal model analysis, the moment of inertia J is not too large; in addition, the determination of J should be selected according to the backup capacity (energy storage configuration, rotating backup, etc.) in the hybrid microgrid.
Fig. 9 and 10 are response diagrams of the inverter mode of the bidirectional power converter, comparing the inertia moment J to 3 and the inertia moment J to 6, showing the ac frequency f and the dc voltage u of the bidirectional power converterdcAnd a transmission power PtThe response characteristic of (c). The AC load is increased by 1000W when the simulation time is 8s, 2s, and the AC load is decreased by 1000W when the simulation time is 5 s. The system reaches the new steady state over 0.6s and 1.2s, respectively. In the dynamic regulation process, the direct current voltage is rapidly reduced by 5V (reduced by 2.5%) along with the active power, and the alternating current frequency is reduced by 0.05Hz (reduced by 0.1%) through the transition process. When J is 6, the inertia time constant of the ac frequency is large, the ac frequency transition process time is long, and the inertia is large. In a steady state, the bidirectional power converter has active power when 500W flows through the bidirectional power converter, the power flows from the direct current side to the alternating current side, the bidirectional power converter works in an inversion mode, the alternating current frequency and the direct current voltage are respectively reduced by 0.05Hz and 5V, droop characteristics are shown, and the alternating current sub-network and the direct current sub-network can bear loads in a balanced mode.
Fig. 11 and 12 are response diagrams of the commutation pattern of the bidirectional power converter comparing the ac frequency f and the dc voltage u of the bidirectional power converter when the inertia moment J is 3 and J is 6dcAnd a transmission power PtThe response characteristic of (c). The DC load is increased by 1000W when the simulation time is 8s, 2s, and the DC load is reduced by 1000W when the simulation time is 5 s. The system reaches the new steady state over 0.6s and 1.2s, respectively. In the dynamic regulation process, the direct current voltage is rapidly reduced by 5V (reduced) along with the active powerBy 2.5%), the ac frequency was reduced by 0.05Hz (by 0.1%) through the transition process. When J is 6, the inertia time constant of the ac frequency is large, the ac frequency transition process time is long, and the inertia is large. In a steady state, the bidirectional power converter has active power flowing through-500W, the power flows from the alternating current side to the direct current side, the bidirectional power converter works in a rectification mode, the alternating current frequency and the direct current voltage are respectively reduced by 0.05Hz and 5V, droop characteristics are shown, and the alternating current sub-network and the direct current sub-network can bear load in a balanced mode.
FIG. 13 is a graph of the switching response of the bi-directional power converter illustrating the AC frequency f and DC voltage u of the bi-directional power converter with a moment of inertia J of 6dcAnd a transmission power PtThe response characteristic of (c). And when the simulation time is 8s and 0-2 s, the alternating current load is increased by 300W, the alternating current sub-network is overloaded, the bidirectional power converter works in an inversion mode, and the alternating current sub-network and the direct current sub-network respectively bear 150W of alternating current load. And when 2-5 s, the direct current load is increased by 800W, at the moment, the direct current sub-network is overloaded, the bidirectional power converter works in a rectification mode, the alternating current side and the direct current side respectively bear 550W loads, the alternating current sub-network bears 300W alternating current loads and 250W direct current loads, the direct current sub-network bears 550W direct current loads, and-250W active power flows through the bidirectional power converter and runs in the rectification mode. And (5-8 s), cutting off the AC/DC load increment, and recovering the system to the rated operation state. In the switching process, the direct current voltage changes rapidly, the alternating current frequency reaches a steady state through a relatively slow transition process, and the switching circuit has inertia. And in a steady state, the AC/DC sub-network has a droop characteristic, and the load is borne in a balanced manner.
FIG. 14 is a graph of the AC frequency and DC voltage response (partial amplification) with the DC capacitance varying, J ═ 6, versus Cdc1000. mu.F and CdcThe dynamic response of the system ac frequency and dc voltage at 2000 muf. CdcWhen the voltage is increased, the inertia time constant of the direct current voltage is increased, and the change is relatively slow; cdcThe inertia time constant of the alternating current frequency is not influenced by the increase of the voltage, and the dynamic response of the alternating current frequency is not changed. The change of the inertia time constant of the direct current voltage does not influence the inertia time constant of the alternating current frequency. The alternating current frequency determines delta, the delta directly regulates the active power, and the active power is changed when the inertia time constant of the alternating current frequency changesThe dynamic response of the direct current voltage is changed, but the change is caused by the delta P in the formula and is independent of the inertia time constant of the direct current voltage, and the inertia time constant of the alternating current frequency does not influence the inertia time constant of the direct current voltage when changing. In summary, the inertia time constants of the alternating current frequency and the direct current voltage are decoupled from each other and do not affect each other.

Claims (1)

1.一种交直流混合微电网双向功率变换器虚拟同步电机控制方法，包括双向功率变换器，所述双向功率变换器为由IGBT开关管构成的三相电压源型PWM变换器，交流侧经过LC滤波器和线路阻抗Zac连接至交流母线，直流侧经过直流电容Cdc和线路阻抗Zdc连接至直流母线，其特征在于：所述双向功率变换器由控制单元所控制；所述控制单元包括频率控制单元、虚拟励磁控制单元和双向功率传输控制单元三部分；频率控制单元和虚拟励磁控制单元分别产生控制回路的交流侧相角δ和电压参考值E，交流侧相角δ经过正弦函数后，生成相位相差120°的三相正弦波，与电压参考值E相乘后，得到三相正弦的参考电压eabc，参考电压eabc作为电压-电流双闭环控制的输入信号，经过控制调节后生成三相正弦调制波，进行PWM调制，最终生成PWM信号控制双向功率变换器；1. An AC-DC hybrid microgrid bidirectional power converter virtual synchronous motor control method, comprising a bidirectional power converter, the bidirectional power converter is a three-phase voltage source type PWM converter composed of IGBT switching tubes, and the AC side passes through The LC filter and the line impedance Z ac are connected to the AC bus, and the DC side is connected to the DC bus through the DC capacitor C dc and the line impedance Z dc , characterized in that: the bidirectional power converter is controlled by a control unit; the control unit It includes three parts: frequency control unit, virtual excitation control unit and bidirectional power transmission control unit; the frequency control unit and virtual excitation control unit respectively generate the AC side phase angle δ and the voltage reference value E of the control loop, and the AC side phase angle δ passes through the sine function Then, generate a three-phase sine wave with a phase difference of 120°, and multiply it with the voltage reference value E to obtain the three-phase sine reference voltage e abc , which is used as the input signal of the voltage-current double closed-loop control, and is adjusted by control. Then generate a three-phase sinusoidal modulation wave, perform PWM modulation, and finally generate a PWM signal to control the bidirectional power converter; 所述控制单元中，频率控制单元、虚拟励磁控制单元和双向功率传输控制单元三部分的控制方法如下：In the control unit, the control methods of the frequency control unit, the virtual excitation control unit and the bidirectional power transmission control unit are as follows: a.频率控制单元：a. Frequency control unit: 传统同步电机的机械运动方程为：The mechanical motion equation of the traditional synchronous motor is:
Figure FDA0002888509550000011
Figure FDA0002888509550000011
 式中：J为转动惯量，D为阻尼系数，ω为虚拟转子角频率，ωN为系统额定角频率，Pm、Pe分别为机械功率和电磁功率；忽略阻尼绕组影响，阻尼系数与一次调频的下垂系数kω相等，即：In the formula: J is the moment of inertia, D is the damping coefficient, ω is the virtual rotor angular frequency, ω N is the rated angular frequency of the system, P m and P e are the mechanical power and the electromagnetic power respectively; ignoring the influence of the damping winding, the damping coefficient is the same as the primary The droop coefficient k ω of frequency modulation is equal, namely:
Figure FDA0002888509550000012
Figure FDA0002888509550000012
 通过检测有功功率Pe相对于Pm的瞬时变化量△P＝Pm-Pe＝-△Pe，实现对角频率ω的控制，混合微电网上，有功功率变化会同时引起交流频率和直流电压变化，因此，通过直流电压变化来反映混合微电网有功功率的变化量△P，从直流侧看，动态过程中有功功率变化△P由两部分组成：直流侧下垂控制引起的有功功率变化量△Pin，属于稳态功率变化量；直流电容瞬时充放电功率△PC，属于动态功率波动，这两部分分别对应直流电压的稳态变化量和瞬时变化率：By detecting the instantaneous change of active power P e relative to P m ΔP=P m -P e =-ΔP e , the control of the diagonal frequency ω is realized. Therefore, the change of the active power of the hybrid microgrid is reflected by the change of the DC voltage. From the perspective of the DC side, the change of the active power in the dynamic process is composed of two parts: the change of the active power caused by the droop control of the DC side. ΔP in , which belongs to the steady-state power variation; DC capacitor instantaneous charge and discharge power ΔP C , which belongs to the dynamic power fluctuation. These two parts correspond to the steady-state variation and instantaneous variation rate of the DC voltage respectively:
Figure FDA0002888509550000021
Figure FDA0002888509550000021
 式中：ku为直流下垂系数；udc为直流电压实际值，UdcN为直流电压额定值；Cdc为直流电容容量；In the formula: ku is the DC droop coefficient; u dc is the actual value of the DC voltage, U dcN is the rated value of the DC voltage; C dc is the DC capacitance capacity; 测量直流电压后，经过计算即可得到混合微电网中功率变化量△P，进而实现对频率的控制，因此，混合微电网中双向功率变换器的虚拟同步电机控制方程为：After measuring the DC voltage, the power variation ΔP in the hybrid microgrid can be obtained through calculation, and then the frequency control can be realized. Therefore, the control equation of the virtual synchronous motor of the bidirectional power converter in the hybrid microgrid is:
Figure FDA0002888509550000022
Figure FDA0002888509550000022
 在动态调节过程中，频率的变化将引起交流子网的功率发生变化，△Pac＝kω(ω-ωN)，其中包含交流子网传统同步发电机一次调频、负荷功频特性以及可控储能在频率变化时的充放电功率；In the process of dynamic adjustment, the change of frequency will cause the power of the AC sub-network to change, ΔP ac =k ω (ω-ω N ), which includes the primary frequency regulation of the traditional synchronous generator of the AC sub-network, the load power frequency characteristics and the possible Controlling the charge and discharge power of the energy storage when the frequency changes; 直流电压变化将引起直流子网功率变化△Pdc＝ku(udc-UdcN)和直流电容快速充放电功率CdcUdcN(dudc/dt)，△Pdc中包括直流子网储能单元和负荷在电压波动时的吞吐功率；The DC voltage change will cause the DC sub-grid power change ΔP dc = ku ( u dc -U dcN) and the DC capacitor fast charge and discharge power C dc U dcN (du dc /dt), ΔP dc includes the DC sub-grid storage The throughput power of the energy unit and the load when the voltage fluctuates; 上述动态调节过程中，交、直流子网功率变化量均可为交流频率提供惯性；In the above dynamic adjustment process, the AC and DC sub-network power changes can provide inertia for the AC frequency; 当系统达到稳态时，交流频率和直流电压稳定，JωN(dω/dt)＝0且CdcUdcN(dudc/dt)＝0，双向功率变换器虚拟同步电机的控制方程变为：When the system reaches a steady state, the AC frequency and DC voltage are stable, Jω N (dω/dt) = 0 and C dc U dcN (du dc /dt) = 0, the control equation of the bidirectional power converter virtual synchronous motor becomes: kω(ω-ωN)＝ku(udc-UdcN)k ω (ω-ω N )=k u (u dc -U dcN ) 在达到稳态时，混合微电网表现为下垂特性，交、直流子网能够均衡承担有功功率变化量；When reaching a steady state, the hybrid microgrid exhibits a drooping characteristic, and the AC and DC sub-grids can equally bear the variation of active power; b.虚拟励磁控制单元：b. Virtual excitation control unit: 虚拟励磁控制，通过控制无功功率对电压进行控制，即无功-电压下垂控制，其控制方程为：Virtual excitation control controls voltage by controlling reactive power, that is, reactive power-voltage droop control, and its control equation is: E＝Eref+kq(Qref-Q)E=E ref +k q (Q ref -Q) 式中，E为虚拟电势有效值；Eref为空载电势有效值；kq为无功-电压下垂系数；Qref和Q分别为无功功率参考值与实际值；In the formula, E is the virtual potential effective value; E ref is the no-load potential effective value; k q is the reactive power-voltage droop coefficient; Q ref and Q are the reference value and actual value of reactive power, respectively; c.双向功率传输控制单元：c. Bidirectional power transfer control unit: 设定
Figure FDA0002888509550000031
和
Figure FDA0002888509550000032
分别为双向功率变换器交流侧桥臂中点输出电压eabc、滤波电容电压uabc和流经双向功率变换器的电流iabc的合成相量，以
Figure FDA0002888509550000033
为参考相量，可得双向功率变换器交流侧电压方程：set up
Figure FDA0002888509550000031
and
Figure FDA0002888509550000032
are the synthetic phasor of the midpoint output voltage e abc of the AC side bridge arm of the bidirectional power converter, the filter capacitor voltage u abc and the current i abc flowing through the bidirectional power converter, and
Figure FDA0002888509550000033
As a reference phasor, the AC side voltage equation of the bidirectional power converter can be obtained:
Figure FDA0002888509550000034
Figure FDA0002888509550000034
 式中，XL为交流滤波电感的电抗值，XL＝ωL，ω为交流角频率，即为双向功率变换器虚拟转子角频率；In the formula, XL is the reactance value of the AC filter inductor, XL = ωL, and ω is the AC angular frequency, which is the virtual rotor angular frequency of the bidirectional power converter; 直流子网通过双向功率变换器流向交流子网的功率为：The power flowing from the DC sub-network to the AC sub-network through the bidirectional power converter is:
Figure FDA0002888509550000035
Figure FDA0002888509550000035
 式中，δ是电压相量
Figure FDA0002888509550000036
与
Figure FDA0002888509550000037
的相角差，RL为交流滤波电感的寄生电阻，RL<<XL，因此双向功率变换器功率传输特性近似为：where δ is the voltage phasor
Figure FDA0002888509550000036
and
Figure FDA0002888509550000037
The phase angle difference of , RL is the parasitic resistance of the AC filter inductor, RL << XL , so the power transfer characteristic of the bidirectional power converter is approximately:
Figure FDA0002888509550000038
Figure FDA0002888509550000038
 通过控制相角δ的正负即可控制双向功率变换器的功率传输方向，实现功率的双向流动：By controlling the positive and negative of the phase angle δ, the power transmission direction of the bidirectional power converter can be controlled to realize the bidirectional flow of power: 当δ＞0时，双向功率变换器运行于虚拟同步发电机模式，即逆变模式，功率从直流子网流向交流子网，Pt为正；When δ>0, the bidirectional power converter operates in the virtual synchronous generator mode, that is, the inverter mode, the power flows from the DC sub-network to the AC sub-network, and P t is positive; 当δ＜0时，双向功率变换器运行于虚拟同步电动机模式，即整流模式，功率从交流子网流向直流子网，Pt为负；When δ<0, the bidirectional power converter operates in the virtual synchronous motor mode, that is, the rectification mode, the power flows from the AC sub-network to the DC sub-network, and P t is negative; 当δ＝0时，双向功率变换器中没有功率流动，交、直流子网之间没有功率交换，Pt＝0。When δ=0, there is no power flow in the bidirectional power converter, no power exchange between the AC and DC sub-networks, and P t =0.
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