CN113315123A - Back-to-back flexible loop closing switch state switching method - Google Patents
Back-to-back flexible loop closing switch state switching method Download PDFInfo
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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
- H02J3/001—Methods to deal with contingencies, e.g. abnormalities, faults or failures
- H02J3/00125—Transmission line or load transient problems, e.g. overvoltage, resonance or self-excitation of inductive loads
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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
- H02J3/04—Circuit arrangements for AC mains or AC distribution networks for connecting networks of the same frequency but supplied from different sources
- H02J3/06—Controlling transfer of power between connected networks; Controlling sharing of load between connected networks
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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
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M5/00—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases
- H02M5/40—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into DC
- H02M5/42—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into DC by static converters
- H02M5/44—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into DC by static converters using discharge tubes or semiconductor devices to convert the intermediate DC into AC
- H02M5/453—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into DC by static converters using discharge tubes or semiconductor devices to convert the intermediate DC into AC using devices of a triode or transistor type requiring continuous application of a control signal
- H02M5/458—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into DC by static converters using discharge tubes or semiconductor devices to convert the intermediate DC into AC using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
- H02M5/4585—Conversion of AC power input into AC power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into DC by static converters using discharge tubes or semiconductor devices to convert the intermediate DC into AC using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only having a rectifier with controlled elements
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- 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]
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Abstract
本发明公开一种背靠背柔性合环开关状态切换方法，包括以下步骤：1)若该侧电网发生故障时，变流器与故障侧电网脱离从而工作在电压源模式，即为Vf模式；如果该侧电网恢复，该侧变流器由Vf模式经过与电网预同步后切换为PQ模式，运行于UdcQ控制模式下的变流器不需要变动；2)若运行于UdcQ控制模式下的变流器侧电网失电，此时该侧应由UdcQ模式切换至Vf模式；而另一侧变流器应由PQ模式切换至UdcQ模式，待到电网恢复后，重新预同步并网。采用柔性切换策略，会大幅度降低强制切换下交流侧电压和电流的冲击量，减小对交直流侧设备的影响，具有一定的实际应用价值。
The invention discloses a back-to-back flexible loop closing switch state switching method, comprising the following steps: 1) if the grid on the side is faulty, the converter is separated from the grid on the fault side so as to work in the voltage source mode, namely the Vf mode; When the side power grid is restored, the converter on this side is switched from Vf mode to PQ mode after pre-synchronizing with the grid, and the converter running in U dc Q control mode does not need to be changed; 2) If it operates in U dc Q control mode The power grid on the side of the converter is lost, at this time, this side should be switched from U dc Q mode to Vf mode; while the other side of the converter should be switched from PQ mode to U dc Q mode. Synchronous grid connection. Using the flexible switching strategy will greatly reduce the impact of the AC side voltage and current under forced switching, and reduce the impact on the AC and DC side equipment, which has certain practical application value.
Description
Technical Field
The invention belongs to the field of control strategies of power electronic devices of a power distribution network, and relates to a back-to-back flexible loop-closing switch state switching method.
Background
With the wide grid connection of various distributed new energy, the flexible access of electric vehicles and the high-quality service and flexible interaction of user sides, the development of intelligent power distribution systems oriented to future comprehensive energy systems has become a wide consensus of countries in the world. Especially, at the end of a power system, in the power distribution network layer directly facing power consumers, along with the continuous abundance of regulation and control means and power supply types, the power supply quality of key core loads is guaranteed by improving the utilization rate of a power supply system and the operation performance of the power distribution network, and the power distribution network becomes an important concern of an intelligent power distribution network.
The development of the intelligent power distribution network is mainly focused on the aspect of a secondary system and limited by the problems of short-circuit capacity, an electromagnetic ring network and the like, and a primary system of the power distribution network is still forced to adopt a power supply mode of closed-loop design and open-loop operation. The traditional distribution network is mainly regulated and controlled by means of transformer tap adjustment, conventional switch on-off control and the like, the regulation and control speed is low, the flexibility is insufficient, and the problems of switching operation, loop closing current impact, safety, reliability and the like often exist. The flexible loop closing switch is used as a novel device to replace a traditional interconnection switch. The power supply device can accurately control active power and reactive power of feeders on two sides connected with the power supply device, realizes asynchronous interconnection of power grids on two sides of alternating current, can realize quick on-off control, and prevents propagation of fault problems. The introduction of the flexible closed-loop switch thoroughly changes the power supply mode of the traditional power distribution network closed-loop design and open-loop operation, avoids potential safety hazards caused by switch displacement, greatly improves the real-time performance and the rapidity of power distribution network control, and brings a great deal of benefits for the operation of the power distribution network.
The traditional power distribution network still adopts the circuit breaker as the interconnection switch in a large number, can not realize closing the ring operation, can't guarantee the power supply continuity of interconnection switch department key load.
Disclosure of Invention
In order to solve the problems in the prior art, the invention provides a back-to-back flexible loop-closing switch state switching method, which considers the power supply continuity of key loads on two sides, can realize dual-power supply for the key loads on the two sides, and greatly improves the power supply reliability.
The technical scheme adopted by the invention is as follows:
a back-to-back flexible loop closing switch state switching method comprises the following steps:
when the flexible closed-loop switch operates, the voltage of the middle direct current link is kept constant, and the converter on the side of the strong power grid is in a constant direct current side voltage control mode, namely UdcA Q mode; the converter on the other side works in a current source mode, namely a PQ mode, in a grid-connected state;
1) if the converter side power grid operating in the PQ control mode has a fault, the converter is separated from the fault side power grid so as to work in a voltage source mode, namely a Vf mode; if the side power grid is recovered, the side converter is switched into a PQ mode after being pre-synchronized with the power grid from a Vf mode, and operates in a U modedcThe converter in the Q control mode does not need to be changed;
2) if operating in UdcWhen the power grid on the converter side in the Q control mode is out of power, the side is controlled by UdcSwitching the Q mode to the Vf mode; while the other side converter should be switched from PQ mode to U modedcAnd in the Q mode, after the power grid is recovered, pre-synchronizing and grid-connecting again.
As a further improvement of the invention, the current inner loop control part of the flexible loop closing switch working in three control modes is the same, and specifically comprises the following steps:
the reference value given by the inner ring controller needs to be recorded in real time inside the converter; when different modes are switched, the state value recorded at the previous moment is endowed with the state of the outer ring controller at the next moment, so that the controller outputs smooth transition before and after, and the jump of the given reference value of the current inner ring in the switching process is reduced;
when the fault isolation process runs smoothly, the phase of the power grid needs to be detected and tracked in real time, and when a fault occurs, the fault-side converter continues to run on the basis of the phase value recorded at the last moment, so that the continuity of the phase in the switching process of the converter controller is ensured.
As a further improvement of the invention, when the power grids on both sides of the flexible closed-loop switch are normal, the heavy-current grid-side converter is required to be in a U statedcQ control mode, the other side is in PQ mode; the controllers of the converters on the two sides simultaneously record the current inner ring given reference value and the grid voltage phase value of the controllers; the method specifically comprises the following steps:
1) at the moment of power failure of a power grid at the converter side operating in a PQ mode, the Vf controller starts to operate on the basis of the recorded state value, and the phases of the converters are superposed on the basis of the phases of the power grid before power failure, so that smooth transition from a current source to a voltage source is ensured; operating simultaneously in UdcThe Q-mode converter does not need to be switched;
2) run in UdcAt the moment of power failure of a power grid at the converter side in the Q mode, the Vf controller starts to operate on the basis of the recorded state values, the phases of the converters are overlapped on the basis of the phases of the power grid before power failure, and the converter at the other side is switched to U from the PQ mode on the basis of the recorded state valuesdcA Q mode for supporting a DC side voltage;
3) when the fault is cleared and the power grid is recovered, the converter running in the Vf mode is ready to perform presynchronization operation; after the presynchronization is finished and the power grid is connected, switching to a required mode according to the requirement, wherein a side converter is required to be in a U statedcAnd (3) controlling the mode by Q.
As a further improvement of the invention, after the power grid is recovered, the converter needs to be connected to the power grid again, and the fault-side converter needs to be switched from a Vf mode to a grid-connected operation mode; firstly, presynchronizing the amplitude, frequency and phase of the output voltage at the AC side of the converter operating in a Vf mode;
the presynchronization of the amplitude is achieved by means of a voltage regulator and the presynchronization of the frequency and phase is achieved by means of a modified phase-locked loop.
As a further improvement of the present invention, switching the Vf mode to the grid-connected operation mode specifically includes:
when the Vf control mode is switched to the grid-connected operation mode, grid-connected presynchronization is firstly carried out, a frequency-phase controller and a voltage amplitude controller are switched in, and a presynchronization unit is started;
the output voltage, the frequency and the phase are spontaneously adjusted through the quick pre-synchronization unit, and when the frequency, the amplitude and the phase of the output voltage of the alternating current side of the converter are consistent with those of the voltage of the power grid side, pre-synchronization starting is completed; closing a circuit breaker between the flexible switch and the power grid, and enabling the converter to transit from a pre-synchronous starting mode to a grid-connected control mode;
and when the output phase difference of the phase regulator and the output voltage difference of the voltage regulator are detected to be regulated to be within an acceptable range, the pre-synchronization loop is cut off, and the converter completely operates in a grid-connected control mode.
As a further improvement of the present invention, the method for synchronously following and adjusting the reference value of the inner loop current in three control modes comprises:
in a dq synchronous rotating coordinate system, when the grid voltage is oriented, the active power and the reactive power output by the converter are indirectly controlled by controlling d-axis current and q-axis current, and the method specifically comprises the following steps:
in the formula, P and Q respectively represent active power and reactive power output by the single-side converter, udAnd uqRespectively representing two components, i, of the grid-side voltage in dq synchronous rotation coordinate systemdAnd iqRespectively representing two components of the three-phase alternating current in a dq synchronous rotation coordinate system; the current transformers on two sides of the flexible loop closing switch need to record reference values of the current inner loop at any time, namely the reference values areidAnd iq。
As a further improvement of the present invention, when the system is operating stably, the intermediate dc-side capacitor voltage should be kept constant, that is, the sum of the output powers of the two converters should be kept to be 0, and the dc-side voltage is controlled to be a constant value to ensure the balance of the active power, specifically:
P1＝udci1
P2＝udci2
in the formula, P1And P2Respectively representing the active power, u, output by two converters on both sidesdcValue of the DC voltage, i, of the intermediate DC link1And i2The two converters respectively flow current on the direct current side.
Compared with the prior art, the invention has the following advantages:
the inner ring structures of the control loops of the flexible loop closing switch working in three control modes are the same, so that the key for ensuring smooth switching is to ensure that the given reference value of the current inner ring cannot be suddenly changed at the moment of switching the control modes, thereby reducing the impact generated in the switching process and having faster dynamic response. In addition, a pre-synchronization unit is added to realize synchronization of all links before grid connection, so that the voltage of an alternating current side output by a converter is consistent with the voltage of a power grid during grid connection, the impact of current during switching can be greatly reduced, the influence on various devices on a direct current side is reduced, and the method has certain practical value.
Drawings
The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. In addition, the shapes, the proportional sizes, and the like of the respective members in the drawings are merely schematic for facilitating the understanding of the present invention, and do not specifically limit the shapes, the proportional sizes, and the like of the respective members of the present invention. Those skilled in the art, having the benefit of the teachings of this invention, may choose from the various possible shapes and proportional sizes to implement the invention as a matter of case. In the drawings:
FIG. 1 is a two-level converter back-to-back connection topology;
FIG. 2 shows basic control modes of the flexible loop closing switch;
FIG. 3 is a basic control block diagram of a flexible loop closing switch;
FIG. 4 is a block diagram of state switching control in three control modes;
FIG. 5 is a pre-synchronization flow diagram;
FIG. 6 is a block diagram of frequency-phase pre-synchronization control;
FIG. 7 is a diagram of amplitude presynchronization control;
FIG. 8 is a plot of voltage current waveforms for PQ mode switching to Vf mode when soft switching is not employed;
FIG. 9 is a plot of voltage and current waveforms for PQ mode switching to Vf mode with soft switching;
FIG. 10U without Flexible switchingdcThe voltage and current waveform diagram of the mode from Q to PQ;
FIG. 11U with flexible switchingdcThe voltage and current waveform diagram of the mode from Q to PQ;
FIG. 12 is a voltage-current waveform diagram without pre-synchronization grid connection;
fig. 13 is a voltage-current waveform diagram of the pre-synchronization grid connection.
Detailed Description
In order to make those skilled in the art better understand the technical solution of the present invention, the technical solution in the embodiment of the present invention will be described below in detail and completely with reference to the drawings in the embodiment of the present invention, and it is obvious that the described embodiment is only a part of the embodiment of the present invention, and not a whole embodiment. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, shall fall within the scope of protection of the present invention.
It will be understood that when an element is referred to as being "disposed on" another element, it can be directly on the other element or intervening elements may also be present. When an element is referred to as being "connected" to another element, it can be directly connected to the other element or intervening elements may also be present. The terms "vertical," "horizontal," "left," "right," and the like as used herein are for illustrative purposes only and do not denote a single embodiment.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.
The flexible loop closing switch is a power electronic device used for replacing a traditional interconnection switch, can accurately control active power and reactive power transmitted between feeders connected to two sides of the flexible loop closing switch, and provides certain voltage reactive power support. When the system breaks down, the fault can be quickly isolated, so that the fault influence range is reduced, and the fault can be merged into the power grid again after being eliminated to realize flexible power supply.
When the open circuit fault occurs in the AC system on one side, the power loss of all loads under the 380V bus is caused. Therefore, the converter close to the fault side should be quickly switched from the current operation control mode to the Vf control mode, so as to ensure the timely recovery of the lost electrical network. If no switching measure is taken after the fault, the voltage of the power-off side drops, the current changes rapidly, and various devices in the system are seriously damaged, and meanwhile, the flexible loop-closing switch must ensure that the converter on one side operates in a U statedcAnd a Q control mode for maintaining the DC voltage constant.
A back-to-back flexible loop closing switch state switching method comprises the following steps:
the flexible loop closing switch is a power electronic converter used for connecting two alternating current systems, and can realize asynchronous interconnection and fault isolation between the two alternating current systems. When the flexible closed-loop switch operates, in order to keep the system stably operating, the voltage of the intermediate direct current link must be kept constant, so that the converter on the side of the strong power grid is fixedDC side voltage control mode, namely UdcA Q mode; the converter on the other side works in a current source mode, namely a PQ mode, in a grid-connected state; 1) if the grid on the side has a fault, the converter is separated from the grid on the fault side so as to work in a voltage source mode, namely a Vf mode. If the side power grid is recovered, the side converter is switched into a PQ mode after being pre-synchronized with the power grid from a Vf mode, and operates in a U modedcThe converter in the Q control mode does not need to be changed; 2) if operating in UdcWhen the power grid on the converter side in the Q control mode is out of power, the side is controlled by UdcThe Q mode is switched to the Vf mode, the voltage and the frequency of the power loss region are continuously supported, and the converter on the other side is switched to the U mode from the PQ modedcAnd (4) a Q mode. And after the power grid is recovered, pre-synchronizing and connecting the power grid again. In summary, the converter state switching is at UdcAnd switching among three modes of Q, PQ and Vf. If one side of the grid is in power loss, the converter on the side needs to be switched to a Vf control mode, and meanwhile, the other side is ensured to operate in a U statedcAnd (3) controlling the mode by Q. If the fault side power grid is recovered, the side converter needs to be pre-synchronized and then is connected to the power grid, and a control strategy after the power grid is connected to the power grid is selected according to requirements.
S100: the current inner loop control part of the flexible loop closing switch working in three control modes is the same. 1) On the basis, in order to ensure smooth switching, the change rate of the reference value given by the inner ring controller in the switching process should be reduced. Therefore, the reference value given by the inner ring controller needs to be recorded in real time inside the converter. When different modes are switched, the state value recorded at the previous moment is endowed to the state of the outer ring controller at the next moment, so that the controller outputs smooth transition before and after, and the jump of the given reference value of the current inner ring in the switching process is reduced. 2) When a fault occurs, the fault-side converter continues to operate on the basis of the phase value recorded at the last moment, and the continuity of the phase in the switching process of the converter controller is ensured.
And S200, after the power grid is recovered, the converter needs to be connected into the power grid again, and the fault-side converter needs to be switched into a grid-connected operation mode from a Vf mode. In order to realize smooth transition between states, a pre-synchronization unit is required to be added, and the synchronization of amplitude, frequency and phase before grid connection is realized. Presynchronization of amplitude can be achieved by means of a voltage regulator, and presynchronization of frequency and phase can be achieved by means of a modified phase-locked loop.
As a further improvement of the invention, the given difference between the two inner loops of the controller during the switching process should be reduced, and the continuity of the phase during the switching process of the converter should be ensured.
As a further improvement of the present invention, S100 specifically includes:
when the power grids on two sides of the flexible closed-loop switch are normal, the heavy-current grid side converter is in a U statedcQ control mode, the other side is in PQ mode. And the controllers of the converters on both sides record the current inner ring given reference value and the grid voltage phase value of the controllers.
1) At the moment of power failure of a power grid at the converter side operating in the PQ mode, the Vf controller starts to operate on the basis of the recorded state values, and phases of the converters are superposed on the basis of the phases of the power grid before power failure, so that smooth transition from a current source to a voltage source is ensured. Operating simultaneously in UdcThe Q-mode converter does not require switching.
2) Run in UdcAt the moment of power failure of a power grid at the converter side in the Q mode, the Vf controller starts to operate on the basis of the recorded state values, the phases of the converters are overlapped on the basis of the phases of the power grid before power failure, and the converter at the other side is switched to U from the PQ mode on the basis of the recorded state valuesdcAnd a Q mode for supporting the DC voltage.
3) When the fault is cleared and the grid is restored, the converter operating in Vf mode should be ready for presynchronization operation. After the presynchronization is finished and the power grid is connected, switching to a required mode according to the requirement, wherein a side converter is required to be in a U statedcAnd (3) controlling the mode by Q.
As a further improvement of the present invention, when the Vf control mode is switched to the grid-connected operation mode, it is necessary to pre-synchronize the amplitude, frequency, and phase of the output voltage at the ac side of the converter operating in the Vf mode, and access the converter to the power grid after meeting the error requirement.
As a further improvement of the present invention, S200 specifically includes:
when the Vf control mode is switched to the grid-connected operation mode, grid-connected presynchronization is firstly carried out, a frequency-phase controller and a voltage amplitude controller are switched in, and a presynchronization unit is started;
the output voltage, the frequency and the phase are spontaneously adjusted through the quick pre-synchronization unit, and when the frequency, the amplitude and the phase of the output voltage of the alternating current side of the converter are consistent with those of the voltage of the power grid side, pre-synchronization starting is completed; closing a circuit breaker between the flexible switch and the power grid, and enabling the converter to transit from a pre-synchronous starting mode to a grid-connected control mode;
and when the output phase difference of the phase regulator and the output voltage difference of the voltage regulator are detected to be regulated to be within an acceptable range, the pre-synchronization loop is cut off, and the converter completely operates in a grid-connected control mode.
As a further improvement of the present invention, the method for synchronously following and adjusting the reference value of the inner loop current in three control modes comprises:
in a dq synchronous rotating coordinate system, when the grid voltage is oriented, the active power and the reactive power output by the converter are indirectly controlled by controlling d-axis current and q-axis current, and the method specifically comprises the following steps:
in the formula, P and Q respectively represent active power and reactive power output by the single-side converter, udAnd uqRespectively representing two components, i, of the grid-side voltage in dq synchronous rotation coordinate systemdAnd iqRespectively representing two components of a three-phase alternating current in a dq synchronous rotating coordinate system. The current transformers on two sides of the flexible loop closing switch need to record reference values of the current inner loop at any time, namely idAnd iq。
As shown in fig. 1, when the system is in stable operation, the intermediate dc-side capacitor voltage should be kept constant, that is, the sum of the output powers of the two converters should be kept to be 0, and the dc-side voltage is controlled to be a constant value to ensure the balance of the active power, specifically:
P1＝udci1
P2＝udci2
in the formula, P1And P2Respectively representing the active power, u, output by two converters on both sidesdcValue of the DC voltage, i, of the intermediate DC link1And i2The two converters respectively flow current on the direct current side.
The high-voltage grid-side converter must be kept at UdcAnd Q control mode, so as to stabilize the constant voltage of the direct current side. The control mode of the converter on the other side is changed along with the change of the operation mode of the power grid. In a grid-connected operation mode, the converter on the other side works in a PQ mode, and the power flow on the two sides of the feeder line can be flexibly controlled; 1) when the side power grid fails, the converter is separated from the fault side power grid to work in an island state, so that the PQ mode is converted into a Vf mode, stable voltage and frequency support is provided for a load, and the converter on the other side does not need to be switched; 2) if operating in UdcThe converter side power grid in the Q control mode loses power, and the converter is controlled to be powered by UdcThe Q mode is switched to Vf mode, and the other side converter should be switched from PQ mode to U modedcAnd (4) a Q mode.
The present invention is described in detail below:
the invention provides a back-to-back flexible loop-closing switch state switching method, which mainly comprises the following two processes:
the basic control strategy of the double-end converter of the flexible loop closing switch is shown in fig. 2, and the specific control block diagram is shown in fig. 3.
The first process is as follows: the flexible closed-loop switch is switched from a PQ mode to a Vf mode
When flexible closed-loop switch both sides electric wire nettingWhen the current transformer is normal, the heavy current network side current transformer operates in UdcAnd the Q mode and the other side converter operate in a PQ mode. When the power grid running on the PQ mode side loses power and needs to be switched to the Vf control mode, the output value of the Vf control outer ring is in a state before the previous controller exits, so that the output of the controller is smoothly transited before and after, and the given jump of the current inner ring in the switching process is reduced. Meanwhile, when the smooth switching of the fault isolation process is ensured, the phase of the power grid needs to be detected in real time, and the continuity of the phase in the control switching process of the converter is ensured, so that the converter on the fault side of the flexible loop closing switch can continue to operate on the basis of the phase before the power grid loses power, as shown in fig. 4.
And a second process: the flexible loop closing switch is composed of a UdcQ mode is switched to Vf mode
When the power grids on two sides of the flexible closed-loop switch are normal, the heavy-current grid side converter operates in UdcAnd the Q mode and the other side converter operate in a PQ mode. If operating in UdcAfter the power grid on the Q mode side loses power, the converter on the side is required to be switched to a Vf control mode on the basis of a current inner loop given reference value recorded before power loss, and phase information required by a control loop is accumulated on the basis of the power grid phase recorded before power loss. While the other side converter is switched from PQ mode to U modedcQ mode to maintain the dc side voltage constant as shown in fig. 4.
The third process: the flexible closed-loop switch is pre-synchronized by Vf mode and then connected to the power grid
After the power grid is recovered, the flexible loop closing switch needs to be connected into the power grid again, and the fault side converter needs to be switched from the Vf mode to the PQ mode. In order to realize smooth transition between states, a pre-synchronization unit is added here to realize the synchronization of amplitude, frequency and phase before grid connection. Pre-synchronization of frequency and phase can be achieved by a modified phase-locked loop, as shown in fig. 6; pre-synchronization of the amplitude may be tracked by a voltage regulator in the voltage source controller, as shown in FIG. 7.
The output voltage, the frequency and the phase are spontaneously adjusted through the rapid pre-synchronization unit, when the output voltage at the alternating current side of the converter and the voltage frequency, the amplitude and the phase difference at the power grid side are within an acceptable range, the breaker is closed, the flexible loop closing switch is connected to the power grid, and the pre-synchronization unit is cut off at the same time, and a flow chart is shown in fig. 5.
Example 1:
simulation analysis is carried out on the basis of a distribution network with a 380V voltage class by taking flexible loop-closing switches (2) and (1) in the figure 2 as backgrounds. Flexible loop closing switch left side converter operating in UdcAnd a Q control strategy, wherein the right converter operates in a PQ control mode. If a fault occurs, the right converter still operates in the PQ mode, and a waveform inconsistent with the standard grid voltage is generated on the power loss side, as shown in fig. 8. And a flexible switching strategy is adopted, and the Vf mode is switched to meet the grid voltage standard, as shown in fig. 9.
Example 2:
simulation analysis is carried out on the basis of a distribution network with a 380V voltage class by taking flexible loop-closing switches in figures 2(1) and 2(3) as backgrounds. When the power grid is normal, the left converter operates in UdcQ-mode, right converter operating in PQ-mode. After the left side power grid loses power, if the left and right side converters do not operate, the left side will generate a waveform inconsistent with the power grid voltage, as shown in fig. 10. Left hand to Vf mode and right hand to UdcIn Q-mode, the left side produces the same waveform as the standard grid voltage, as shown in fig. 11.
Example 3:
in the example 1 scenario, the right converter switches from Vf mode to PQ mode. A pre-synchronization process is performed first, as shown in fig. 5. After the presynchronization process is completed, the connection switch between the flexible loop closing switch and the power grid can be closed. If flexible switching is not adopted in the whole process, a large impact current can be generated in the right-side alternating current part, as shown in fig. 12. By adopting a flexible switching strategy, the instantaneous impact of alternating current can be reduced, and the influence on equipment is reduced, as shown in fig. 13.
In a word, the invention discloses a state switching method of a back-to-back flexible loop closing switch, wherein the flexible loop closing switch is a power electronic converter used for connecting two alternating current systems, and can realize asynchronous interconnection and fault isolation between the two alternating current systems. In order to maintain the system during operation of the flexible loop-closing switchThe voltage of the middle direct current link is required to be kept constant, so that the converter on the side of the strong power grid is in a constant direct current side voltage control mode, namely UdcA Q mode; the converter on the other side works in a current source mode, namely a PQ mode, in a grid-connected state; 1) if the grid on the side has a fault, the converter is separated from the grid on the fault side so as to work in a voltage source mode, namely a Vf mode. If the side power grid is recovered, the side converter is switched into a PQ mode after being pre-synchronized with the power grid from a Vf mode, and operates in a U modedcThe converter in the Q control mode does not need to be changed; 2) if operating in UdcWhen the power grid on the converter side in the Q control mode is out of power, the side is controlled by UdcThe Q mode is switched to the Vf mode, the voltage and the frequency of the power loss region are continuously supported, and the converter on the other side is switched to the U mode from the PQ modedcAnd (4) a Q mode. And after the power grid is recovered, pre-synchronizing and connecting the power grid again. In summary, the converter state switching is at UdcThe switching is carried out among three modes of Q, PQ and Vf, wherein one side is necessarily positioned at UdcAnd (3) controlling the mode by Q. By adopting a flexible switching strategy, the impact quantity of voltage and current on the AC side under forced switching can be greatly reduced, the influence on equipment on the AC side and the DC side is reduced, and the flexible switching method has certain practical application value.
It should be noted that, in the description of the present invention, the terms "first", "second", and the like are used for descriptive purposes only and for distinguishing similar objects, and no precedence between the two is considered as indicating or implying relative importance. In addition, in the description of the present invention, "a plurality" means two or more unless otherwise specified.
It is to be understood that the above description is intended to be illustrative, and not restrictive. Many embodiments and many applications other than the examples provided would be apparent to those of skill in the art upon reading the above description. The scope of the present teachings should, therefore, be determined not with reference to the above description, but should instead be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. The disclosures of all articles and references, including patent applications and publications, are hereby incorporated by reference for all purposes. The omission in the foregoing claims of any aspect of subject matter that is disclosed herein is not intended to forego such subject matter, nor should the applicant consider that such subject matter is not considered part of the disclosed subject matter.
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