CN112350362A - Direct-current oscillation suppression method for flexible direct-current power distribution system based on superconducting energy storage - Google Patents

Direct-current oscillation suppression method for flexible direct-current power distribution system based on superconducting energy storage Download PDF

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CN112350362A
CN112350362A CN202011184794.3A CN202011184794A CN112350362A CN 112350362 A CN112350362 A CN 112350362A CN 202011184794 A CN202011184794 A CN 202011184794A CN 112350362 A CN112350362 A CN 112350362A
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direct
distribution system
power distribution
smes
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马文忠
丁安敏
田洪英
赵雨
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China University of Petroleum East China
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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
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • 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
    • H02J1/00—Circuit arrangements for dc mains or dc distribution networks
    • H02J1/10—Parallel operation of dc sources
    • 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
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H02J2003/365—Reducing harmonics or oscillations in HVDC
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/10—Flexible AC transmission systems [FACTS]
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/60—Superconducting electric elements or equipment; Power systems integrating superconducting elements or equipment
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

Abstract

The invention discloses a method for suppressing direct current oscillation of a flexible direct current power distribution system based on superconducting energy storage, which comprises the following steps of: firstly, establishing a flexible direct-current power distribution system, and connecting a superconducting magnetic energy storage device in parallel with a direct-current power grid of the system; secondly, in order to improve the control precision, the superconducting magnetic energy storage device adopts dead-beat prediction control to realize quick and accurate tracking of the reference quantity; thirdly, controlling the insulated gate bipolar transistor S through a DC/DC converter in the superconducting magnetic energy storage device1、S2The superconducting magnet is charged and discharged, and the absorption of the oscillation power of the power distribution system is completed; fourthly, when the system stably runs, the superconducting magnetic energy storage device is in a standby mode and does not influence the system; and fifthly, when the power distribution system is subjected to small interference, direct current oscillation occurs, and the system stability becomes poor or even is unstable, the superconducting magnetic energy storage device is put into operation. The invention can change the current of the flexible direct current distribution system without changingThe direct current oscillation of the system is inhibited under the condition of the original operation mode of the device, and the stability of the system is improved.

Description

Direct-current oscillation suppression method for flexible direct-current power distribution system based on superconducting energy storage
Technical Field
The invention relates to the field of flexible direct current transmission, in particular to a flexible direct current distribution direct current oscillation suppression method based on superconducting energy storage, which combines the technical characteristics of superconducting energy storage to ensure that a system can stably operate.
Background
New energy sources represented by wind energy and photovoltaic are becoming important choices for sustainable development of energy sources. The power distribution system based on the flexible direct current technology of the modular multilevel converter has the advantages of flexible structure, capability of realizing independent control of active power and reactive power, low harmonic content of the output alternating voltage of the converter and the like, can effectively solve the problems of the alternating current power distribution system in the aspects of power supply reliability, operation efficiency, electric energy quality, distributed power consumption and the like, and is more suitable for the development of modern power distribution systems.
The converters connected with the flexible direct current distribution system and the alternating current system are controlled by constant power, the power converters have the characteristic of constant power load, and can generate negative resistance action, reduce system damping, and make the dynamic characteristic of the system poor and even cause system oscillation instability. Meanwhile, the equivalent resistance of devices such as a direct current reactor, a direct current circuit and a direct current capacitor in a direct current power grid is small, and external factors such as trigger delay and communication delay of a converter valve switching device can cause system oscillation. In consideration of the influence of the above factors on the stable operation of the system, effective measures need to be taken to suppress the system oscillation and ensure the safe and stable operation of the power distribution system.
The control method for improving the stability of the flexible direct current power distribution system can be generally divided into two levels of a power electronic device level and a system control level. The control of the power electronic device level is mainly researched under the condition that the system is in a steady state or generates small disturbance, and the stability of the flexible direct current power distribution system is improved through a multi-time scale robust stability control strategy. The system level control generally optimizes the control parameters and structure of the system, so as to improve the system damping, such as a virtual impedance method, a virtual inertia method, an active damping compensation method, and the like.
The energy storage device is also an effective way for improving the safety and stability of the power grid. The existing power storage technology mainly comprises storage battery energy storage, super capacitor energy storage, superconducting magnetic energy storage, water storage energy storage, flywheel energy storage and the like. The superconducting magnetic energy storage device SMES has the characteristics of high response speed, high efficiency, strong anti-interference capability and the like, so that the superconducting magnetic energy storage device SMES not only can be used for inhibiting low-frequency power oscillation of a power grid and improving the voltage and frequency characteristics of the power grid, but also can be used for adjusting a power factor and realizing dynamic management of the power grid. SMES has unique advantages in the control of the stability of power systems, and has been applied to the suppression of power system resonance, new energy generation fluctuations, and the like.
Disclosure of Invention
In view of this, the invention provides a method for suppressing direct current oscillation of a flexible direct current power distribution system based on superconducting energy storage, and SMES adopts dead-beat prediction control, so that the response accuracy of an SMES device is improved. SMES is controlled by an insulated gate bipolar transistor S1、S2The self running mode is switched by switching on and off, the power balance in a power distribution system is maintained by reasonably controlling the charging and discharging of the superconducting magnet in the SMES, and the direct current oscillation is restrained. The invention can inhibit the system direct current oscillation without changing the original operation mode of the system converter, improves the stability of the system, ensures the safe and stable operation of the system, and has rapid response and better dynamic performance.
In order to achieve the purpose, the invention adopts the following technical scheme:
a direct current oscillation suppression method of a flexible direct current distribution system based on superconducting energy storage comprises the following steps:
step A: a flexible direct-current power distribution system containing SMES is built and mainly comprises a flexible interconnection device, an alternating-current and direct-current load, a direct-current microgrid and other equipment, the power distribution system is connected with an alternating-current power grid through an MMC current converter, and a phase-locked loop link is added into the system to provide phase reference for a control system orientation common point.
And B: the SMES adopts dead-beat predictive control to predict the variable at the (k + 2) th moment, so that the reference quantity can be quickly and accurately tracked;
and C: controlling an insulated gate bipolar transistor S by a DC/DC converter in SMES1、S2The charging and the discharging of the superconducting magnet are realized.
Step D: when the power distribution system stably runs, the SMES is in a standby mode and does not influence the system;
step E: when a power distribution system is subjected to small interference, direct current oscillation occurs, and the stability of the system is poor or even unstable, the SMES is put into operation according to specific conditions.
In the step A, a converter control structure connected with an alternating current power grid in the flexible direct current power distribution system mainly comprises an outer ring controller and an inner ring controller. The inner ring control links of the two converter stations adopt current control, and in the outer ring control link, MMC (modular multilevel converter)1The outer ring controller adopts constant DC voltage and constant reactive power control, MMC2The outer loop controller adopts constant active power and constant reactive power for control.
In the step A, the MMC current converter is provided with 6 bridge arms, and each bridge arm is formed by connecting 1 reactor and N sub-modules in series. The MMC circuit is highly modularized, the number of submodules of the converter can be increased or decreased, the requirements of different power and voltage levels can be met, the integrated design is convenient to realize, and the cost is saved. The DC/DC converter in SMES comprises 2 fully-controlled devices, namely an insulated gate bipolar transistor S1、S 22 power diodes D1、D2. The DC/DC converter can work in two quadrants, and the flowing direction of current in the superconducting magnet can be ensured to be unchanged no matter the converter works in a rectification state or an inversion state.
In the step a, it is required to determine whether the flexible dc power distribution system is operating in a stable state, and whether oscillation occurs on the dc side of the system. If the power distribution system runs normally, the SMES is in a standby mode and does not exchange power with the system; if the distribution system generates direct current oscillation, the dynamic characteristic of the system is poor, and even the system is unstable, the SMES is put into operation as required.
In the step a, the main reason that the flexible direct-current power distribution system generates direct-current oscillation is that the power distribution system contains converters controlled by constant power, and the constant-power load characteristics of the converters can generate a negative resistance effect, so that the damping of the system is reduced, the dynamic characteristics of the system are poor, and even the system oscillation is unstable. Meanwhile, the equivalent resistance of devices such as a direct current reactor, a direct current circuit and a direct current capacitor in a direct current power grid is small, and external factors such as trigger delay and communication delay of a converter valve switching device can cause system oscillation.
In the step B, the SMES adopts dead-beat prediction control, and the state of the system at the next sampling moment is predicted by combining a system discrete state mathematical model, so that the control performance of the SMES device is greatly improved.
Further, the above-mentioned dead-beat control strategy applied to SMES includes:
step B1: deducing a mathematical model of the SMES in a working state;
step B2: discretizing the mathematical model of the SMES to obtain an SMES discretization mathematical model;
step B3: predicting the state of the system at the next sampling moment by combining a system discrete state mathematical model, and calculating the control quantity at the next moment by taking the state that the instruction at the current moment is equal to the state at the next moment as a target to obtain a traditional dead-beat prediction model;
step B4: and (4) predicting the state at the k +2 moment by considering the influence of control delay to obtain a new dead-beat prediction mathematical model, thereby deducing the expression of the duty ratio at the k +1 moment.
In the step C, the insulated gate bipolar transistor S1、S2The switch belongs to a full-control device, can control the on and off of the device through a control signal, and has the advantages of high switching speed, strong current capacity and good thermal stability. Two switching devices S of a DC/DC converter in SMES are regulated according to the actual needs of a power distribution system1、S2The on-off of the superconducting magnet realizes the chopping control of the charging and discharging current in the superconducting magnet, thereby absorbing the unbalanced power in the system and inhibiting the direct current oscillation.
In the step D, when the power distribution system stably runs, S of the DC/DC converter in the SMES1Opening, S2Off, DC current at S1And D1The SMES is in standby mode and does not exchange power with the system.
In step E, when the power distribution system generates dc oscillation, the SMES device has two operating modes: a charge mode and a discharge mode. The step E comprises the following steps:
step E1: when SMES is operating in charging mode, S1、S2The superconducting magnet is charged to absorb the redundant energy of the direct current distribution system, and the mathematical model is
Figure BDA0002751113300000021
Step E2: when SMES is operating in discharge mode, S1、S2Simultaneously turning off the superconducting magnet to enable the energy of the superconducting magnet to pass through D1And D2The loop flows into the direct current grid. The SEMS device consumes the unbalanced power of the system through charging and discharging, inhibits the oscillation of the system and improves the stability of the system, and the mathematical model is
Figure BDA0002751113300000022
Compared with the prior art, the invention has the beneficial effects that:
1. the direct current power distribution system is connected with an alternating current power grid through the MMC converter, the MMC circuit is highly modularized, the requirements of different power and voltage levels can be met by increasing and decreasing the number of sub-modules connected into the converter, the integrated design is convenient to achieve, the project period is shortened, and the cost is saved.
2. According to the invention, the SMES inhibits the direct-current oscillation of the direct-current power distribution system, the SMES can absorb the oscillation power of the system and inhibit the direct-current oscillation of the system under the condition of not changing the original control strategy of a system converter, and the system has the advantages of high response speed, high efficiency and strong anti-interference capability;
3. the SMES adopts dead-beat predictive control, can quickly respond when a system is disturbed, shortens the adjusting time, reduces the overshoot of the waveform, and has good control precision and dynamic performance.
5. In the invention, when the direct current power distribution system normally operates, the SMES device is in a standby mode, and the SMES device and the system are not influenced mutually; when the direct current power distribution system generates direct current oscillation, the SMES device is put into a working state to complete direct current oscillation suppression of the direct current power distribution and transmission system, absorb oscillation power in the direct current power distribution system and ensure safe and stable operation of the system;
6. in the whole view, the invention has obvious effects on inhibiting the direct current oscillation of a direct current distribution system and improving the system stability.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below.
FIG. 1 is a flow chart of a method for suppressing DC oscillation in a flexible DC power distribution system based on superconducting energy storage;
FIG. 2 is a flexible DC power distribution system including SMES;
FIG. 3 is a topological structure diagram of an MMC;
FIG. 4 is a diagram of the topology of SMES;
FIG. 5 is a block diagram of an SMES control strategy based on dead-beat predictive control;
fig. 6 is a diagram of an operation mode of the SMES, where fig. 6(a) shows the SMES in a standby mode, fig. 6(b) shows the SMES in a charge mode, and fig. 6(c) shows the SMES in a discharge mode.
Detailed Description
The invention is described in further detail below with reference to the figures and specific embodiments, specific structural and functional details disclosed herein are merely for purposes of describing example embodiments.
It should be apparent that the described exemplary embodiments are only a few embodiments of the invention, and not all embodiments. Other embodiments, which are encompassed by the present invention and all modifications, equivalents, and alternatives falling within the scope of the present disclosure are within the scope of the present invention as defined by the appended claims.
Fig. 1 provides a flowchart of a method for suppressing dc oscillation of a flexible dc power distribution system based on superconducting energy storage, which includes the following specific steps:
step 1: firstly, a flexible direct-current power distribution system needs to be built, the power distribution system adopts a two-end power supply topological structure, the flexible direct-current power distribution system comprises a plurality of devices such as a flexible interconnection device, an alternating-current/direct-current load and a direct-current microgrid, and the devices exchange power with a direct-current network through a converter. The direct current system is connected with the alternating current system through the MMC converter station, and a phase-locked loop link is added into the system to provide a phase reference for a control system orientation common point.
Step 2: and (3) judging whether the direct current power distribution system operates in a stable state or not on the basis of the step 1, and judging whether oscillation is generated on the direct current side of the system or not. If the direct current power distribution system normally operates, the SMES device is in a standby mode and does not affect the system; if direct current oscillation occurs in a direct current distribution system, the dynamic characteristic of the system is deteriorated, and even the system is unstable, the SMES device is put into operation as required.
And step 3: on the basis of the step 2, the SMES device adopts dead-beat prediction control, and calculates the control quantity at the next moment by sampling the state quantity and the instruction value of the system at the current moment, predicting the system state at the next sampling moment of the system by combining a system discrete state mathematical model, and taking the instruction that the state at the next moment is equal to the current moment as a target. In order to eliminate the influence of control delay, the control strategy also predicts the state of the system at the k +2 th moment, and greatly improves the control performance of the SMES device.
And 4, step 4: on the basis of step 2, two switching devices S of the DC/DC converter in the SMES are regulated according to the actual needs of the power grid1、S2The on and off of the superconducting magnet realizes the chopping control of the charging and discharging current in the superconducting magnet.
And 5: on the basis of step 4, when the direct current distribution system stably operates, S in the DC/DC converter1Opening, S2Off, DC current at S1And D1The SMES device and the direct current distribution system have no energy exchange and do not influence each other.
Step 6: on the basis of step 4, when the direct current power distribution system generates direct current oscillation, the SMES passes through S1、S2There are two modes of operation: a charge mode and a discharge mode.
And 7: on the basis of step 6, when the SMES operates in the charging mode, S1、S2Meanwhile, the superconducting magnet is conducted and charged to absorb redundant energy of the system; when the SMES device operates in the discharging mode, S1、S2Simultaneously turn off to enable the energy of the superconducting magnet to pass through the power diode D1And D2The loop flows into the direct current grid. The SEMS device consumes the unbalanced power of the system through charging and discharging, inhibits the system oscillation and improves the system stability.
And 8: after step 7, the dc power distribution system operates safely and stably in the original control mode, and the SMES device returns to the standby mode.
Fig. 2 shows a structure of a flexible dc power distribution system including SMES according to an embodiment of the present invention. The flexible direct current power distribution system adopts a topological structure with two ends for power supply, and the direct current power grid voltage of the system is 10 kV. And a plurality of devices such as the flexible interconnection device, the alternating current and direct current load, the direct current microgrid and the like exchange power with the direct current network through the converter.
The flexible direct current power distribution system comprises two MMC converter stations, wherein the MMC converter stations1Using constant DC voltage control, MMC2And the AC sides of the converter stations are connected with an AC power grid by adopting constant active power control. The SMES comprises a DC/DC converter and a superconducting magnet, and stable operation of the system is ensured by switching different working modes. An electromagnetic transient model of the system shown in fig. 2 is built in Smlink, and direct current oscillation of a direct current power distribution system is excited by changing transmission power in the system.
As shown in fig. 3, a topology structure diagram of a converter MMC in a flexible dc distribution dc oscillation suppression method based on superconducting energy storage is provided. The point O represents a zero potential reference point, one converter has 6 bridge arms, and each bridge arm is provided with a reactor L0And N sub-modules are connected in series, and an upper bridge arm and a lower bridge arm of each phase are combined together to form a phase unit. The bridge arm reactors in the MMC can restrain interphase circulating currents caused by the fact that instantaneous values of direct-current voltages of bridge arms of all phases are not equal completely, meanwhile, impact currents when a direct-current bus fails can be effectively restrained, and reliability of the system is improved. In the right red dotted box of fig. 3 is the topology of an MMC sub-module. VT1、VT2Representative insulationGate bipolar transistor, VD1、VD2Represents an antiparallel diode, C0Is a DC side capacitor, ucoIs the voltage across the capacitor, ismIs the current flowing into the submodule. Each submodule is provided with a connecting port for connecting a main circuit and a topology in series, and the MMC supports the voltage of a direct current bus through the direct current side capacitor voltage of each submodule.
As shown in fig. 4, a topology structure diagram of an SMES device in a method for suppressing dc oscillation of a flexible dc power distribution system based on superconducting energy storage is provided. In FIG. 4, S1、S2Is an insulated gate bipolar transistor, D1、D2Is a power diode, LcoilIs a superconducting magnet, icoilFor unidirectional current flow through the superconducting magnet, C is a DC capacitor, ucThe terminal voltage of the capacitor C. The DC/DC converter can work in two quadrants, and the current can be ensured to flow in the superconducting magnet in a constant direction no matter the converter works in a rectification state or an inversion state, and a closed loop of the superconducting magnet is ensured. Adjusting two switching devices S according to the actual needs of the power grid1、S2The on and off of the superconducting magnet realizes the chopping control of the charging and discharging current in the superconducting magnet.
As shown in fig. 5, a block diagram of a control strategy of an SMES device in a superconducting energy storage based flexible dc power distribution system dc oscillation suppression method is provided. The SMES device adopts a control strategy of dead-beat prediction, and calculates the sampling value of the current converter at the next moment according to the sampling value of the current moment and the discretization mathematical model of the controlled object in each switching period, so that the controlled quantity can track the command value in one beat. In order to eliminate errors caused by control delay as much as possible, variables at the k +2 th moment in the dead-beat control algorithm are predicted, and variables at the approximate k +1 moment are obtained by using the difference of the variables at two adjacent moments.
As shown in fig. 6, an operation mode diagram of SMES in a method for suppressing dc oscillation of a flexible dc power distribution system based on a superconducting energy storage device is provided. When the DC/DC converter charges and discharges the superconducting magnet, the capacitor voltage ucStable, neglecting in-lineSwitching losses and magnet losses.
Fig. 6(a) shows the SMES in standby mode. When the system stably operates, the SMES is in a standby mode and does not exchange power with the direct-current power distribution system, so that the influence on the normal operation of the system is avoided, and S at the moment1Opening, S2Off, DC current at S1And D1Is circulated through the loop. Magnet current i when switch losses are neglectedcoilInvariable, superconducting magnet LcoilThe energy in (a) is unchanged.
When the system is oscillated by small interference, the operation mode of the SMES is shown in fig. 6(b), (c).
FIG. 6(b) shows the SMES charging mode, where S1、S2And simultaneously, the superconducting magnet is switched on, the current of the magnet rises, the magnet absorbs the redundant energy of the direct current distribution system, the energy in the superconducting magnet is increased, and the mathematical expression is that
Figure BDA0002751113300000051
FIG. 6(c) shows the SMES discharge mode, in which S is present1、S2Is always in an off state, and the energy of the superconducting magnet passes through D1And D2The circuit flows into a direct current power grid, the current flowing through the superconducting magnet is reduced, the energy in the superconducting magnet is reduced, and the mathematical model expression is
Figure BDA0002751113300000052
In general, the voltage versus current relationship of a superconducting magnet may be expressed as
Figure BDA0002751113300000053
In the expression, icoil,0Is the initial current of the superconducting magnet, VcoilIs the voltage across the superconducting magnet.
The exchange power of an SMES device with the system can be expressed as
Pcoil＝Vcoilicoil
It should be noted that the above-mentioned embodiments are only preferred embodiments of the present invention, and the general method defined herein can be implemented in other embodiments without departing from the scope of the present invention. Therefore, any minor changes or modifications to the present invention without departing from the spirit of the present invention shall fall within the protection scope of the present invention.

Claims (8)

1. A direct-current oscillation suppression method for a flexible direct-current power distribution system based on superconducting energy storage is characterized in that the power distribution system adopts a two-end power supply topological structure, and a plurality of devices such as a flexible interconnection device, an alternating-current/direct-current load, a direct-current micro-grid and the like exchange power with a direct-current network through a converter. The direct current power grid is connected with an alternating current system through a converter station MMC, and the MMC meets the requirements of different power and voltage grades by increasing and decreasing the number of submodules accessed into a converter. The superconducting magnetic energy storage device SMES based on dead beat prediction control is connected in parallel to a direct current power distribution system, a DC/DC converter in the SMES can work in two quadrants to control an insulated gate bipolar transistor S1、S2To control the operation mode of the SMES, and to realize charging and discharging of the superconducting magnet, so as to eliminate unbalanced power in the system, wherein the suppressing method comprises:
step A: a flexible direct-current power distribution system containing SMES is built and mainly comprises a flexible interconnection device, an alternating-current and direct-current load, a direct-current microgrid and other equipment, the power distribution system is connected with an alternating-current power grid through an MMC current converter, and a phase-locked loop link is added into the system to provide phase reference for a control system orientation common point.
And B: the SMES adopts dead-beat predictive control to predict the variable at the (k + 2) th moment, so that the reference quantity can be quickly and accurately tracked;
and C: controlling an insulated gate bipolar transistor S by a DC/DC converter in SMES1、S2The charging and the discharging of the superconducting magnet are realized.
Step D: when the power distribution system stably runs, the SMES is in a standby mode and does not influence the system;
step E: when a power distribution system is subjected to small interference, direct current oscillation occurs, and the stability of the system is poor or even unstable, the SMES is put into operation according to specific conditions.
2. The method for suppressing direct current oscillation of a flexible direct current power distribution system based on superconducting energy storage according to claim 1, wherein in the step a, a converter control structure connected with an alternating current power grid in the flexible direct current power distribution system mainly comprises an outer loop controller and an inner loop controller. The inner ring control links of the two converter stations adopt current control, and in the outer ring control link, MMC (modular multilevel converter)1The outer ring controller adopts constant DC voltage and constant reactive power control, MMC2The outer loop controller adopts constant active power and constant reactive power for control.
3. The method for suppressing direct-current oscillation of a flexible direct-current power distribution system based on superconducting energy storage according to claim 1, wherein in the step a, it is determined whether the flexible direct-current power distribution system is operated in a stable state, and whether oscillation is generated on a direct-current side of the system. If the power distribution system runs normally, the SMES device is in a standby mode and does not exchange power with the system; if the distribution system generates direct current oscillation, the dynamic characteristic of the system is poor, and even the system is unstable, the SMES is put into operation as required.
4. The method for suppressing direct-current oscillation of a flexible direct-current power distribution system based on superconducting energy storage according to claim 1, wherein in the step a, the main reason that the flexible direct-current power distribution system generates direct-current oscillation is that power distribution systems include constant-power controlled converters, and the constant-power load characteristics of the converters generate a negative resistance effect, so that system damping is reduced, and dynamic characteristics of the system are deteriorated, even the system oscillation is unstable. Meanwhile, the equivalent resistance of devices such as a direct current reactor, a direct current circuit and a direct current capacitor in a direct current power grid is small, and external factors such as trigger delay and communication delay of a converter valve switching device can cause system oscillation.
5. The method for suppressing direct-current oscillation of a flexible direct-current power distribution system based on superconducting energy storage according to claim 1, wherein in the step B, the SMES adopts dead-beat prediction control, the state of the system at the next sampling time is predicted by sampling the state quantity and the command value of the power distribution system at the current time and combining a system discrete state mathematical model, and the control quantity at the next time is calculated by taking the state at the current time equal to the state at the next time as a target. In order to eliminate the influence of control delay, the control strategy also predicts the state of the system at the (k + 2) th moment, and the accuracy of SMES control is greatly improved.
6. The method for suppressing the direct-current oscillation of the flexible direct-current power distribution system based on the superconducting energy storage as claimed in claim 1, wherein in the step C, an insulated gate bipolar transistor S is adopted1、S2The switch belongs to a full-control device, can control the on and off of the device through a control signal, and has the advantages of high switching speed, strong current capacity and good thermal stability. Two switching devices S of a DC/DC converter in SMES are regulated according to the actual needs of a power distribution system1、S2The on-off of the superconducting magnet realizes the chopping control of the charging and discharging current in the superconducting magnet, thereby absorbing the unbalanced power in the system and inhibiting the direct current oscillation.
7. The method as claimed in claim 1, wherein the step D is performed by S of a DC/DC converter in SMES when the power distribution system is in stable operation1Opening, S2Off, DC current at S1And D1The SMES is in standby mode and does not exchange power with the system.
8. The method for suppressing direct-current oscillation of a flexible direct-current power distribution system based on superconducting energy storage according to claim 1, wherein in the step E, when the direct-current oscillation occurs in the power distribution system, the SMES device has two operation modes: a charge mode and a discharge mode. The step E comprises the following steps:
e1: when SMES is operating in charging mode, S1、S2The superconducting magnet is charged to absorb the redundant energy of the direct current distribution system, and the mathematical model is
Figure FDA0002751113290000021
E2: when SMES is operating in discharge mode, S1、S2Simultaneously turning off the superconducting magnet to enable the energy of the superconducting magnet to pass through D1And D2The loop flows into the direct current grid. The SEMS device consumes the unbalanced power of the system through charging and discharging, inhibits the oscillation of the system and improves the stability of the system, and the mathematical model is
Figure FDA0002751113290000022
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