CN114709825B - Dynamic lightning protection method and system for alternating current-direct current power distribution network based on power electronic transformer - Google Patents

Dynamic lightning protection method and system for alternating current-direct current power distribution network based on power electronic transformer Download PDF

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CN114709825B
CN114709825B CN202210628207.8A CN202210628207A CN114709825B CN 114709825 B CN114709825 B CN 114709825B CN 202210628207 A CN202210628207 A CN 202210628207A CN 114709825 B CN114709825 B CN 114709825B
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power
pet
distribution network
direct current
load
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CN114709825A (en
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石旭江
童充
潘晓明
詹若培
吴堃铭
周瑜
谢智敏
龚烈锋
华夏
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Suzhou Power Supply Co of State Grid Jiangsu Electric Power Co Ltd
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Suzhou Power Supply Co of State Grid Jiangsu Electric Power Co Ltd
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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
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
    • H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
    • 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
    • H02J3/003—Load forecast, e.g. methods or systems for forecasting future load demand
    • 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
    • H02J3/004—Generation forecast, e.g. methods or systems for forecasting future energy generation
    • 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
    • H02J3/007—Arrangements for selectively connecting the load or loads to one or several among a plurality of power lines or power sources
    • H02J3/0073—Arrangements for selectively connecting the load or loads to one or several among a plurality of power lines or power sources for providing alternative feeding paths between load and source when the main path fails, e.g. transformers, busbars
    • 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
    • H02J3/02—Circuit arrangements for ac mains or ac distribution networks using a single network for simultaneous distribution of power at different frequencies; using a single network for simultaneous distribution of ac power and of dc power
    • 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
    • H02J3/381—Dispersed generators
    • 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
    • H02J5/00—Circuit arrangements for transfer of electric power between ac networks and dc networks
    • 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
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/22—Conversion of dc power input into dc power output with intermediate conversion into ac
    • H02M3/24—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
    • H02M3/28—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac
    • H02M3/325—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal
    • H02M3/335—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/3353—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having at least two simultaneously operating switches on the input side, e.g. "double forward" or "double (switched) flyback" converter
    • 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
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/22—Conversion of dc power input into dc power output with intermediate conversion into ac
    • H02M3/24—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
    • H02M3/28—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac
    • H02M3/325—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal
    • H02M3/335—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/33569—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements
    • H02M3/33576—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements having at least one active switching element at the secondary side of an isolation transformer
    • H02M3/33584—Bidirectional converters
    • 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
    • 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
    • H02J2203/10—Power transmission or distribution systems management focussing at grid-level, e.g. load flow analysis, node profile computation, meshed network optimisation, active network management or spinning reserve management
    • 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]
    • 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
    • H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
    • H02J2300/20—The dispersed energy generation being of renewable origin
    • 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]

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  • Power Engineering (AREA)
  • Supply And Distribution Of Alternating Current (AREA)

Abstract

The invention discloses a dynamic lightning protection method and a dynamic lightning protection system for an alternating current-direct current power distribution network based on a power electronic transformer, wherein the method comprises the following steps: constructing an expected lightning fault set; establishing an uncertainty scene set of renewable energy sources and loads and carrying out scene reduction; carrying out load flow calculation on the alternating current and direct current hybrid power distribution network containing PET; establishing a static safety evaluation index; establishing a power flow transfer optimization model under the condition of considering the N-1 line fault caused by lightning by taking the minimum static safety evaluation index as a target function; and performing alternating current and direct current optimal power flow calculation to realize dynamic lightning protection of the alternating current and direct current hybrid power distribution network. The method is based on thunder prediction information, an optimized scheduling strategy is formulated in a day-to-day stage, the tidal current coordination among the AC and DC networks is realized through PET, the influence of thunder on the operation of the AC and DC hybrid power distribution network is reduced, the method is suitable for an application scene considering distributed renewable energy access, and the uncertainty problem caused by renewable energy output fluctuation can be solved.

Description

Dynamic lightning protection method and system for alternating current-direct current power distribution network based on power electronic transformer
Technical Field
The invention belongs to the technical field of lightning protection of power distribution networks, and relates to a dynamic lightning protection method and system for an alternating current-direct current power distribution network based on a power electronic transformer.
Background
With the development of distributed power sources and loads, the limitation of the acceptance capacity of the traditional power distribution network is gradually highlighted. Meanwhile, a new energy revolution characterized by the utilization of renewable energy is being developed, and the permeability of renewable energy in power distribution networks is increasing. The intelligent power distribution network which combines the direct current technology with the traditional power distribution network and has bidirectional power flow better conforms to the development prospect of the future power distribution network. Compared with the traditional alternating current distribution network, the alternating current and direct current hybrid distribution network has the advantage of providing plug-in interfaces for the distributed generators. The alternating current and direct current hybrid power distribution network further improves the power flow controllability, the power quality and the power transmission capacity of the system, and the alternating current and direct current hybrid power distribution network is proved to be an effective technical means for coping with system uncertainty. The Power Electronic Transformer (PET) has flexible power flow controllability and is expected to become a key interface device of an alternating current-direct current hybrid power distribution network.
With the acceleration of the urbanization process and the continuous expansion of the scale of the power distribution network, the safety and reliability of the power distribution network as a key link for connecting the power grid and users are more and more emphasized. In recent years, extreme weather is frequent, and power system fault tripping caused by lightning is increased. However, distribution networks have lower insulation levels and are more sensitive to lightning strikes than transmission networks. Therefore, the lightning protection performance of the distribution network must be improved to reduce the occurrence of faults caused by lightning and improve the safety and reliability of power supply.
Traditional lightning protection measures for power distribution networks are focused on protecting single equipment, and fault tripping rate of the power distribution networks in lightning stroke is reduced by enhancing lightning stroke resistance of power distribution lines and the equipment. The traditional method can reduce lightning harm to a certain extent, but is difficult to completely eliminate the loss of lightning strikes to the power distribution network, and cannot completely avoid the line tripping condition caused by lightning, so that the power distribution reliability of the power distribution network is seriously influenced because the whole load of the downstream of a power distribution line is lost due to the loss of the power distribution line. Meanwhile, the topological complexity degree and the equipment quantity of the power distribution network are considered, and the lightning protection measures of single equipment of the power distribution network, such as the lightning arrester is transformed and upgraded, so that considerable manpower and material resources are consumed, and the economical efficiency of the lightning protection measures is to be improved.
Disclosure of Invention
In order to overcome the defects in the prior art, the invention aims to provide a dynamic lightning protection method and system for an alternating current-direct current power distribution network based on a power electronic transformer.
In order to achieve the above purpose, the invention adopts the following technical scheme:
the dynamic lightning protection method for the alternating current-direct current power distribution network based on the power electronic transformer comprises the following steps of:
step 1, constructing an expected lightning fault set of an alternating current-direct current hybrid power distribution network containing a power electronic transformer PET by adopting N-1 line faults caused by lightning;
step 2, establishing an initial power prediction error uncertainty scene set representing the probability distribution of renewable energy and load power prediction errors based on a multi-scene analysis technology, and carrying out scene reduction;
step 3, carrying out load flow calculation on the alternating current and direct current hybrid power distribution network containing the PET based on the expected lightning fault set and the uncertainty scene set;
step 4, establishing a static safety assessment index capable of reflecting the influence of lightning on the AC/DC hybrid power distribution network;
step 5, establishing a power flow transfer optimization model under the condition of considering the N-1 line fault caused by lightning by taking the minimum static safety evaluation index as an objective function;
and step 6, combining the power grid load flow calculation result, performing alternating current and direct current optimal load flow calculation based on a load flow transfer optimization model, and realizing dynamic lightning protection of the alternating current and direct current hybrid power distribution network.
The invention further comprises the following preferred embodiments:
preferably, in step 1, the expected lightning fault set of the ac/dc hybrid power distribution network is divided into an N-1 line fault caused by the lightning of the ac network and an expected lightning fault set under the N-1 line fault caused by the lightning of the dc network, specifically:
the method comprises the steps that lightning faults are simulated to be disconnection problems of corresponding accident equipment, namely, any disconnection of all N lines of a system is taken as an expected fault, and an expected fault set of an alternating current and direct current hybrid power distribution network is further divided into a line N-1 fault set of an alternating current network and a line N-1 fault set of a direct current network; will be firstiThe fault occurring in a line is recorded asE i The probability of failure is recorded as
Figure 137584DEST_PATH_IMAGE001
。
Preferably, step 2 specifically comprises:
step 2.1, generating an initial power prediction error scene set of renewable energy sources and loads by adopting a Latin hypercube sampling method;
and 2.2, carrying out scene reduction on the initial power prediction error scene set by adopting a synchronous back substitution method.
Preferably, step 2.1 specifically comprises:
step 2.1.1, fitting historical sample data to obtain a power prediction error probability distribution function of the renewable energy sources and the load;
step 2.1.2, equally dividing the probability distribution function of the power prediction error intoNsAn equal probability interval;
step 2.1.3, extracting random numbers in each probability intervalp o (o=1,2,…,Ns)；
Step 2.1.4, inverse transformation is carried out on the probability distribution function of the power prediction error to obtain a prediction error sample value in a corresponding probability intervalu o Also known as scenes
Figure DEST_PATH_IMAGE002
And forming an initial power prediction error scene set.
Preferably, step 2.2 specifically comprises:
step 2.2.1, set iteration number
Figure 305261DEST_PATH_IMAGE003
Setting deleted scene setJ=J (0) Is an empty set;
step 2.2.2, calculate
Figure DEST_PATH_IMAGE004
Scenes to be deleted by the sub-iteration
Figure 598970DEST_PATH_IMAGE005
To make
Figure DEST_PATH_IMAGE006
Is composed of
Figure 219307DEST_PATH_IMAGE007
The probability distance represented by the following equation takes a minimum value:
Figure DEST_PATH_IMAGE008
in the formula:
Figure 886524DEST_PATH_IMAGE009
as a scene
Figure DEST_PATH_IMAGE010
The probability of (d);
vnumbering scenes;
Figure 908707DEST_PATH_IMAGE011
to replace
Figure DEST_PATH_IMAGE012
The scene (c);
step 2.2.3, delete scene
Figure 373317DEST_PATH_IMAGE013
Let us order
Figure DEST_PATH_IMAGE014
，
Figure 497262DEST_PATH_IMAGE015
Deleted scenes
Figure DEST_PATH_IMAGE016
Using the scene closest to its Euclidean distanceu v Instead of, simultaneously combining scenesu o Probability of adding to the sceneu v So that the sum of the probabilities of all scenes is always 1;
sceneu v Probability of (2)π v Become into
Figure 220368DEST_PATH_IMAGE017
：
Figure DEST_PATH_IMAGE018
Step 2.2.4, judging whether the number of the deleted scenes reaches a set value, if not, returning to the step 2.2.2, otherwise, enabling the deleted scenes to reach the set valueJ=J a() And the scene reduction is finished.
Preferably, step 3.1, establishing a PET power flow calculation model according to the topological structure of the PET;
step 3.2, establishing a load flow calculation model of the alternating current network and the direct current network;
and 3.3, writing a power flow calculation equation set by the power flow calculation model in the steps 3.1 and 3.2 in combination with the control mode of each port, and alternately and iteratively calculating the power flow of the alternating-current and direct-current hybrid power distribution network containing the power electronic transformer until convergence.
Preferably, the topology structure of the PET in step 3.1 specifically includes a medium voltage transformation module, an isolation module, and a low voltage transformation module;
the medium-voltage conversion module comprises a medium-voltage alternating current module and a medium-voltage direct current module;
the medium-voltage alternating current module converts medium-voltage alternating current of power frequency into medium-voltage direct current and is composed of VSC, and the medium-voltage direct current module is connected with a medium-voltage direct current bus inside the PET;
the isolation module adopts a QAB converter to realize a DC/DC conversion function, uses a four-winding high-frequency transformer to realize the electrical isolation among ports, and controls the voltage of each internal direct-current bus by adjusting the transformation ratio of each winding of HFT;
the low-voltage conversion module comprises a low-voltage alternating current module and a low-voltage direct current module;
the low-voltage AC module inverts the voltage of the connected internal DC bus into low-voltage AC required by a network connected with the module, and the low-voltage DC module is connected with the PET internal low-voltage DC bus;
according to the structure, the PET external network is connected to the QAB converter through the corresponding port and the conversion module, and the QAB converter is used as a core element of the PET to realize the functions of power regulation and energy bidirectional exchange;
therefore, a PET power flow calculation model is established from an alternating current port, a direct current port and a QAB converter.
Preferably, in step 3.3, when the power flow of the alternating current and direct current hybrid power distribution network containing the PET is calculated, the condition of power flow distribution in the network is calculated, and the internal state variable and the power flow distribution of the PET are solved to provide a control reference value of the PET;
during specific calculation, the PET power flow calculation model plays a coupling role between an alternating current system and a direct current system, when the alternating current network power flow and the direct current network power flow are respectively solved, nodes of all ports connected with the network are equivalent according to a control mode of a power electronic transformer, power interaction is realized between the alternating current system and the direct current system through the alternating current port and the direct current port of the PET, and iterative variables in the calculation process are as follows:
selecting an interactive variable of a PET medium-voltage alternating-current port and a medium-voltage alternating-current distribution network as a global iterative quantity of the alternating-current and direct-current hybrid power distribution network load flow calculation when the first timekAlternating current network state variable, PET state variable, direct current network state variable, global iteration quantity and the second stage after sub-global iterationk-1 global iteration result difference is less than allowable errorε r In time, the load flow calculation result of the alternate iteration can be output; otherwise, substituting the global iteration variable intokThe solution continues in +1 global iterations.
Preferably, in step 4, the slave mineThe safety level of the operation of the power grid is comprehensively measured from two angles of uncertainty and severity of the occurrence of the electric fault, and indexes are evaluated according to static safety
Figure 847789DEST_PATH_IMAGE019
The method embodies static safety, and establishes the following static safety evaluation indexes:
Figure DEST_PATH_IMAGE020
in the formula:tis time;
E i represents the firstiLightning failure of a strip line;
Figure 263727DEST_PATH_IMAGE021
、
Figure DEST_PATH_IMAGE022
respectively a PET transmission power out-of-limit risk index and a loss load risk index;
Figure 364317DEST_PATH_IMAGE023
、
Figure DEST_PATH_IMAGE024
respectively corresponding weight coefficients of the PET transmission power out-of-limit risk index and the loss load risk index;
Figure 625535DEST_PATH_IMAGE025
for lightning faults
Figure DEST_PATH_IMAGE026
Time of day
Figure 107463DEST_PATH_IMAGE027
Causing the consequence of PET port out-of-limit
Figure DEST_PATH_IMAGE028
The severity of (d);
Figure 445034DEST_PATH_IMAGE029
for lightning faultsE i Time of daytCause the consequence of load loss
Figure DEST_PATH_IMAGE030
The severity of (d);
Figure 261680DEST_PATH_IMAGE031
for lightning faultsE i The probability of (c).
Preferably, the lightning failure
Figure DEST_PATH_IMAGE032
Time of day
Figure 77321DEST_PATH_IMAGE033
Severity caused by PET Port violation
Figure DEST_PATH_IMAGE034
Comprises the following steps:
Figure 928602DEST_PATH_IMAGE035
Figure DEST_PATH_IMAGE036
Figure 168566DEST_PATH_IMAGE037
Figure DEST_PATH_IMAGE038
in the formula:
Figure 223241DEST_PATH_IMAGE039
is a time of day
Figure DEST_PATH_IMAGE040
The PET port out-of-limit loss expectation value;
w PET (t,s) As a scenesTime of daytLoss value due to the out-of-limit of the lower PET port;
san uncertainty scene;
Figure 91840DEST_PATH_IMAGE041
the total number of scenes in the uncertainty scene set;
Figure DEST_PATH_IMAGE042
as a scenesThe occurrence probability of (2);
z is a PET number;
nthe total number of PET is;
Figure 548360DEST_PATH_IMAGE043
numbering PET ports;
Figure DEST_PATH_IMAGE044
、
Figure 211423DEST_PATH_IMAGE045
、
Figure DEST_PATH_IMAGE046
respectively of the z-th PET
Figure 18973DEST_PATH_IMAGE047
Active power, reactive power and maximum apparent power transmitted by the signal port.
Preferably, the lightning failureE i Time of daytSeverity of loss of load
Figure DEST_PATH_IMAGE048
Comprises the following steps:
Figure 160104DEST_PATH_IMAGE049
Figure DEST_PATH_IMAGE050
Figure 468201DEST_PATH_IMAGE051
Figure DEST_PATH_IMAGE052
in the formula:
Figure 52897DEST_PATH_IMAGE053
is a time of day
Figure DEST_PATH_IMAGE054
Expected load loss rate of (1);
w Load (t,s) As a scenesTime of daytThe load loss rate of the AC/DC distribution network with the load priority is considered;
e. f represents a load number;
N AC,loss 、N DC,loss the sum of the numbers of the loads which cannot be recovered and the loads which are not recovered of the alternating current and the direct current respectively;
N AC,load 、N DC,load the total load number of the AC network and the DC network before the fault is respectively;
Figure 331432DEST_PATH_IMAGE055
、
Figure DEST_PATH_IMAGE056
are respectively scenessTime of daytThe load quantity of the lower load node e which cannot be recovered and the total load quantity of the load node e are calculated;
Figure 292566DEST_PATH_IMAGE057
、
Figure DEST_PATH_IMAGE058
the important grade factors of the loads e and f are respectively;
Figure 707367DEST_PATH_IMAGE059
as an index for the removal of the load,
Figure DEST_PATH_IMAGE060
representing that the load is fully restored at that point,
Figure 462964DEST_PATH_IMAGE061
representing the point where the load is totally removed.
Preferably, in step 5, the alternating current and direct current hybrid power distribution network is usedtUncertainty of lightning faultE i Establishing a power flow transfer optimization model under the condition of considering the N-1 line fault caused by lightning for the objective function with the minimum static safety evaluation index;
the target function expression is:
Figure DEST_PATH_IMAGE062
the constraint conditions comprise constraint conditions of various flexible resources and power flow constraint conditions of the power distribution network.
Preferably, in step 6, the voltage of each node and the capacity of each line in the alternating current/direct current power distribution network are not out of limit through three active management modes, namely a power flow control function of the PET, active and reactive control of a distributed power supply in the system and load shedding, so that the model is optimized according to power flow transferThe type is found inTIn the course of one evaluation period of time,Mthe dynamic lightning protection of the alternating current and direct current hybrid power distribution network is realized by considering the tidal current distribution situation of the alternating current and direct current power distribution network under faults.
The invention also provides a dynamic lightning protection system of the alternating current-direct current power distribution network based on the power electronic transformer, and the system is used for realizing the dynamic lightning protection method of the alternating current-direct current power distribution network based on the power electronic transformer.
Compared with the prior art, the invention has the beneficial effects that:
according to the method, the power flow control capability of the power electronic transformer PET is utilized, the whole alternating current-direct current hybrid power distribution network is used as a whole to implement an active lightning protection strategy, and when a lightning stroke fault occurs in an alternating current circuit, power flow transfer is carried out through a direct current network with the PET as a coupling element, so that power is supplied to a downstream load of the fault circuit; when the direct current line has lightning stroke fault, the power flow is transferred through the alternating current network.
The method is based on the thunder prediction information, an optimized scheduling strategy is formulated in the day stage, the power flow mutual aid among the AC and DC networks is realized through PET, and the influence of thunder on the operation of the AC and DC hybrid power distribution network is reduced.
The method is suitable for application scenes considering distributed renewable energy access, and can solve the problem of uncertainty caused by renewable energy output fluctuation.
Drawings
FIG. 1 is a flow chart of a dynamic lightning protection method for an AC/DC distribution network based on a power electronic transformer according to the invention;
FIG. 2 is a schematic diagram of an embodiment of a dynamic lightning protection method for an AC/DC distribution network based on a power electronic transformer according to the present invention;
FIG. 3 is a schematic diagram of a power electronic transformer based on a four-active-bridge converter according to an embodiment of the present invention;
FIG. 4 is a schematic diagram of a VSC equivalent topology in an embodiment of the present invention;
fig. 5 is a schematic diagram of a structure of an ac/dc hybrid power distribution network in an embodiment of the present invention.
Detailed Description
The invention is further described below with reference to the accompanying drawings. The following examples are only for illustrating the technical solutions of the present invention more clearly, and the protection scope of the present invention is not limited thereby.
As shown in fig. 1-2, the present invention provides a dynamic lightning protection method for an ac/dc distribution network based on a power electronic transformer, which, in a preferred but non-limiting embodiment of the present invention, comprises the following steps 1-6:
step 1, constructing an expected lightning fault set: an expected lightning fault set of an alternating current-direct current hybrid power distribution network containing a power electronic transformer is constructed by adopting N-1 line faults caused by lightning.
The problem of cutting off of corresponding accident line is simulated to the prediction trouble that will be triggered by the thunder and lightning by the prediction thunder and lightning trouble set structure of alternating current-direct current hybrid power distribution network, adopts N-1 circuit trouble to construct prediction thunder and lightning trouble set promptly, further can divide into the prediction thunder and lightning trouble set under the N-1 circuit trouble that alternating current network thunder and lightning leaded to and the N-1 circuit trouble that direct current network thunder and lightning leaded to, concrete:
the lightning fault is simulated as the disconnection problem of corresponding accident equipment, namely any disconnected one of all N lines of the system is taken as an expected fault, and the expected fault set of the alternating current and direct current hybrid power distribution network can be further divided into a line N-1 fault set of an alternating current network and a line N-1 fault set of a direct current network. The fault of the ith line is recorded asE i The probability of failure is recorded as
Figure 963216DEST_PATH_IMAGE063
. Namely, it isE i For line N-1 faults due to lightning, i.e. secondiOpen failure of the bar line.
Elements in the lightning fault set are expected to be the open fault of any AC line and the open fault of any DC line, such as: if the system has 10 ac lines and 10 dc lines, the expected failure set is: sequentially disconnecting 10 ac lines + sequentially disconnecting 10 dc lines, 20 expected faults were formed.
Step 2, performing uncertainty analysis on renewable energy sources and loads, and establishing an initial power prediction error uncertainty scene set representing renewable energy source and load power prediction error probability distribution based on a multi-scene analysis technology and performing scene reduction;
the high permeability of renewable energy in the ac-dc hybrid power distribution network increases the uncertainty from the power generation side of the distribution network, creating a double superposition effect of power generation side fluctuations and load side uncertainty. The multi-scenario analysis technology is an effective method for solving the random optimization problem, and the method simulates the possible scenarios, so that the uncertainty variable in the model is converted into a plurality of deterministic scenario problems, and the difficulty of modeling and solving is reduced.
The multi-scene analysis technology comprises two main steps of scene generation and scene reduction, the larger the number of scene sets used for simulation is, the more accurate the obtained result can be, but the calculation load can be increased, so that the original scene set needs to be reduced through probability distance to obtain an approximate subset meeting the requirement, the probability distance between the scene set reserved after reduction and the original scene set is shortest, and the calculation load is reduced while certain reliability is kept.
Further preferably, step 2 specifically includes:
step 2.1, generating an initial power prediction error scene set of renewable energy sources and loads by adopting a Latin hypercube sampling method; the method has the advantages of high sampling precision and high sampling efficiency. The power prediction error probability distribution function of the renewable energy sources and the load can be obtained by fitting historical sample data.
Latin hypercube sampling is a layered sampling method, the overall distribution of random variables can be reflected by sampling values, and the power of renewable energy sources and loads cannot be accurately predicted, so that power prediction errors exist.
The prediction error is generally considered to conform to a certain probability distribution function (the probability distribution function can be obtained by fitting historical sample data, that is, the following step 2.1.1. for quantitative analysis, a large enough sample set is generated by a latin hypercube sampling method to characterize the probability distribution.
The following steps 2.1.2-2.1.4 are implementation steps of latin hypercube sampling.
Step 2.1.1, fitting historical sample data to obtain a power prediction error probability distribution function of the renewable energy sources and the load;
after step 2.1.1 obtains 1) a power prediction error probability distribution function of renewable energy sources and 2) a power prediction error probability distribution function of loads, step 2.1.2-2.1.4 are respectively executed for 1) and 2).
Step 2.1.2, equally dividing the probability distribution function of the power prediction error intoNsAn equal probability interval;
Nsthe larger the simulated prediction error distribution, the better the effect, but at the same time the amount of calculation needed increases proportionally to the calculation time, and therefore the value depends on the specific application requirements.
Step 2.1.3, extracting random numbers in each probability intervalp o (o=1,2,…,Ns)；
Step 2.1.4, inverse transformation is carried out on the probability distribution function of the power prediction error to obtain a prediction error sample value in a corresponding probability intervalu o Also known as scenes
Figure DEST_PATH_IMAGE064
Forming an initial power prediction error scene set;
and 2.2, carrying out scene reduction on the initial power prediction error scene set by adopting a synchronous back substitution method.
Step 2.2.1, set iteration number
Figure 193952DEST_PATH_IMAGE065
Setting deleted scene setJ=J (0) Is an empty set;
step 2.2.2, calculate
Figure DEST_PATH_IMAGE066
Scenes to be deleted by the sub-iteration
Figure 728839DEST_PATH_IMAGE067
To make
Figure DEST_PATH_IMAGE068
Is composed of
Figure 389758DEST_PATH_IMAGE069
The probability distance represented by the following equation takes a minimum value:
Figure DEST_PATH_IMAGE070
in the formula:
Figure 377306DEST_PATH_IMAGE071
as a scene
Figure DEST_PATH_IMAGE072
The probability of (d);
vnumbering scenes;
Figure 414663DEST_PATH_IMAGE073
to replace
Figure DEST_PATH_IMAGE074
The scene (c);
step 2.2.3, delete scene
Figure 69636DEST_PATH_IMAGE075
Let us order
Figure DEST_PATH_IMAGE076
，
Figure 167036DEST_PATH_IMAGE077
Deleted scenes
Figure DEST_PATH_IMAGE078
Using the scene closest to its Euclidean distanceu v Instead of, simultaneously combining scenesu o Probability of adding to the sceneu v So that the sum of the probabilities of all scenes is always 1;
sceneu v Probability of (2)π v Become into
Figure 641879DEST_PATH_IMAGE079
：
Figure DEST_PATH_IMAGE080
Step 2.2.4, judging whether the number of the deleted scenes reaches a set value, if not, returning to the step 2.2.2, otherwise, enabling the deleted scenes to reach the set valueJ=J a() And the scene reduction is finished.
And 3, calculating the load flow of the alternating-current and direct-current hybrid power distribution network containing the power electronic transformer aiming at the expected lightning fault set and the uncertainty scene set.
And 3, alternating current and direct current optimal power flow calculation is carried out in the step 6, the static safety assessment index in the step 4 can be calculated only when the power flow calculation result of the network exists, and a target function (the static safety assessment index is minimum) is realized in the optimal power flow calculation.
Step 3.1, establishing a load flow calculation model of the power electronic transformer according to the topological structure of the power electronic transformer;
a multi-port power electronic transformer based on a four-active-bridge converter structure is adopted as a networking device of an alternating current and direct current hybrid power distribution network, and the topological structure of the multi-port power electronic transformer is shown in figure 3. The power electronic transformer with the structure has the advantage of realizing multi-type power supply integration by using the least DC/DC conversion links, can avoid redundant intermediate electric energy conversion links, and has higher power density.
The topology shown in fig. 3 includes a medium voltage transformation module, an isolation module, and a low voltage transformation module.
The medium-voltage conversion module comprises a medium-voltage alternating current module and a medium-voltage direct current module;
the medium-voltage alternating current module is used for converting medium-voltage alternating current of power frequency into medium-voltage direct current and is composed of a Voltage Source Converter (VSC), and the medium-voltage direct current module is connected with a medium-voltage direct current bus inside the PET;
the QAB converter is used as an isolation module in a power electronic transformer to realize a DC/DC conversion function, a four-winding high-frequency transformer (HFT) is used for realizing the electrical isolation among ports, and the voltage of each internal direct-current bus is controlled by adjusting the transformation ratio of each winding of the HFT;
the low-voltage conversion module comprises a low-voltage alternating current module and a low-voltage direct current module;
the low-voltage AC module inverts the voltage of the connected internal DC bus into low-voltage AC required by a network connected with the module, and the low-voltage DC module is connected with the internal low-voltage DC bus of the PET.
The structure shows that the PET external network is connected to the QAB converter through the corresponding port and the electric energy conversion module, and the QAB converter is used as a core element of the PET to realize the functions of power regulation and energy bidirectional exchange.
For a four-active-bridge converter in a power electronic transformer, the active power flowing between any two ports is controlled by phase-shifting the square-wave voltage generated by its corresponding full-bridge module, wherein:
active power flowing from port x to port y
Figure 214419DEST_PATH_IMAGE081
The calculation formula is as follows:
Figure DEST_PATH_IMAGE082
in the formula (I), the compound is shown in the specification,
Figure 458318DEST_PATH_IMAGE083
、
Figure DEST_PATH_IMAGE084
are respectively and portxAnd portyPhase shift angle of the connected converters;
Figure 726620DEST_PATH_IMAGE085
is a portxAnd portyThe difference in the phase shift angle of the converter;
Figure DEST_PATH_IMAGE086
、
Figure 688759DEST_PATH_IMAGE087
are respectively portsxAnd portyTaking the direct-current voltage of the port No. 1 as a direct-current voltage per unit value corresponding to a reference value;
Figure DEST_PATH_IMAGE088
is the switching frequency;
Figure 67919DEST_PATH_IMAGE089
is a portxAnd portyEquivalent inductance between.
Further preferably, the VSC is used as an element in PET for realizing ac/dc power conversion, and the equivalent topology thereof is shown in fig. 4. The AC network passes through the filter in turn
Figure DEST_PATH_IMAGE090
（
Figure 431904DEST_PATH_IMAGE091
Filter reactance), bridge arm reactors
Figure DEST_PATH_IMAGE092
（
Figure 136686DEST_PATH_IMAGE093
For the impedance of the bridge-arm reactors,
Figure DEST_PATH_IMAGE094
in the form of a resistor, the resistance of the resistor,
Figure 333925DEST_PATH_IMAGE095
reactance) and the VSC converter to the PET internal dc bus. Setting AC networkA node voltage of
Figure DEST_PATH_IMAGE096
（
Figure 500464DEST_PATH_IMAGE097
、
Figure DEST_PATH_IMAGE098
Amplitude and phase angle, respectively), the VSC AC side voltage is
Figure 469688DEST_PATH_IMAGE099
，
Figure DEST_PATH_IMAGE100
Is composed of
Figure 594639DEST_PATH_IMAGE101
Hysteresis
Figure DEST_PATH_IMAGE102
The phase angle of (d); apparent power injected into the port by the AC network is
Figure 16524DEST_PATH_IMAGE103
（
Figure DEST_PATH_IMAGE104
、
Figure 252334DEST_PATH_IMAGE105
Active and reactive power injected into the port for ac networks, respectively) and the power injected into the VSC is
Figure DEST_PATH_IMAGE106
（
Figure 76064DEST_PATH_IMAGE107
、
Figure DEST_PATH_IMAGE108
Active and reactive power injected into VSC, respectively).
Taking the power flow direction in fig. 4 as positive, we can obtain:
Figure 106337DEST_PATH_IMAGE109
Figure DEST_PATH_IMAGE110
Figure 12589DEST_PATH_IMAGE111
reducing the internal loss of the PET to all ports for unified equivalence, and injecting active power of the direct current bus by the VSC
Figure DEST_PATH_IMAGE112
And
Figure 520931DEST_PATH_IMAGE113
are equal. Setting the DC bus voltage as
Figure DEST_PATH_IMAGE114
The VSC injects a DC bus current of
Figure 464747DEST_PATH_IMAGE115
The following can be obtained:
Figure DEST_PATH_IMAGE116
a power flow calculation model of the power electronic transformer is given from an alternating current port, a direct current port and a QAB converter.
(1) AC port
Control mode in connection with PET forkFor the second global iteration, the known variables of the load flow calculation of the ac port can be divided into the following two cases:
(a) according to the current calculation result of the iteration PET external alternating current system, the known variable is the port injection active power
Figure 665921DEST_PATH_IMAGE117
And reactive power
Figure DEST_PATH_IMAGE118
；
(b) According to the current flow calculation result of the external alternating current system of the iteration PET and the QAB converter, the known variable is VSC direct current side active power
Figure 62398DEST_PATH_IMAGE119
And AC port injecting reactive power
Figure DEST_PATH_IMAGE120
。
Variable to be solved of alternating current port under various control modes
Figure 640010DEST_PATH_IMAGE121
Comprises the following steps:
Figure DEST_PATH_IMAGE122
Figure 438333DEST_PATH_IMAGE123
and
Figure DEST_PATH_IMAGE124
voltage and lag angle of the VSC alternating current side are respectively;
to solve for the above variables, the two cases of the known variables can be separately column-written with respect to
Figure 75988DEST_PATH_IMAGE125
、
Figure DEST_PATH_IMAGE126
、
Figure 956832DEST_PATH_IMAGE127
Power flow correction equation of
Figure DEST_PATH_IMAGE128
、
Figure 72555DEST_PATH_IMAGE129
、
Figure DEST_PATH_IMAGE130
The superscript, which represents the number of iterations, is shown in the following two equationskTemporarily omitted.
Figure 990964DEST_PATH_IMAGE131
Figure DEST_PATH_IMAGE132
Wherein the content of the first and second substances,
Figure 284673DEST_PATH_IMAGE133
、
Figure DEST_PATH_IMAGE134
、
Figure 639431DEST_PATH_IMAGE135
variables to be solved which are respectively AC ports
Figure DEST_PATH_IMAGE136
Active power and reactive power are injected into the alternating current port and active power injected into the direct current bus by the VSC;
Figure 309578DEST_PATH_IMAGE137
is the power loss at the AC port, and can be expressed as the amplitude of the current injected into the AC port
Figure DEST_PATH_IMAGE138
In the form of a quadratic function of (a),
Figure 66181DEST_PATH_IMAGE139
can be expressed as：
Figure DEST_PATH_IMAGE140
(2) DC port
For direct current portpIn other words, the known variable iskPort injection active power of sub-global iterationP pport-DC And the voltage of the connected direct current network node, according to the known variable, the current amplitude of the direct current port can be obtained according to the circuit relation
Figure 551300DEST_PATH_IMAGE141
Further obtaining the power loss of the DC port
Figure DEST_PATH_IMAGE142
. The exchange power between the DC port and the QAB converter in this iteration can be obtained by referring to the following formula
Figure 924512DEST_PATH_IMAGE143
。
Figure DEST_PATH_IMAGE144
(3) QAB converter
In power flow calculations, a known variable of the QAB converter is the injected active power at each DC port except for the power balancing portP p The phase shift angle phi of each full-bridge converter under the condition that the variable to be solved corresponds to the required port power QAB . Therefore, the QAB converter needs to solve the power flow equation set
Figure 132771DEST_PATH_IMAGE145
Comprises the following steps:
Figure DEST_PATH_IMAGE146
in the formula:
Figure 9460DEST_PATH_IMAGE147
the power flow correction equations of the ports 2, 3 and 4 respectively;
Figure DEST_PATH_IMAGE148
the converter phase shift angles for ports 2, 3, 4, respectively.
Step 3.2, establishing a load flow calculation model of the alternating current network and the direct current network; wherein, the load flow calculation models of the alternating current network and the direct current network are the basic knowledge of the power system.
And 3.3, writing a power flow calculation equation set by the power flow calculation model in the steps 3.1 and 3.2 in combination with the control mode of each port, and alternately and iteratively calculating the power flow of the alternating-current and direct-current hybrid power distribution network containing the power electronic transformer until convergence. That is, 3 calculation models are obtained in the steps 3.1 and 3.2, and the iterative calculation is carried out alternately until the models are converged.
Based on the power electronic characteristics of PET, the PET-containing AC/DC hybrid power distribution network load flow calculation needs to solve internal state variables and load flow distribution of a power electronic transformer to provide a control reference value of the power electronic transformer besides the condition of calculating the load flow distribution in the network, so that the traditional load flow algorithm is not applicable any more.
The power flow calculation model of the power electronic transformer plays a role in coupling between an alternating current system and a direct current system, when the power flows of an alternating current network and a direct current network are respectively solved, nodes of all ports connected with the networks are equivalent according to the control mode of the power electronic transformer, and power interaction is achieved between the alternating current system and the direct current system through the alternating current ports and the direct current ports of the power electronic transformer.
Specifically, the iterative variables exchanged between the 3 calculation models are as follows:
selecting interactive variables of PET medium-voltage alternating-current port and medium-voltage alternating-current distribution networkU is ∠δ is AndQ is as a global iteration quantity of the load flow calculation of the AC/DC hybrid power distribution networkkAlternating current network state variables (alternating current node voltage, phase) after sub-global iterationAngle), PET state variables (U ic 、θ i 、Φ QAB ) Direct current network state variables (node voltage, power), global iteration quantity andk-1 global iteration result difference is less than allowable errorε r In time, the load flow calculation result of the alternate iteration can be output; otherwise, substituting the global iteration variable intokThe solution continues in +1 global iterations.
Step 4, establishing a static safety assessment index capable of reflecting the influence of lightning on the AC/DC hybrid power distribution network;
in an alternating current and direct current hybrid power distribution network, overvoltage and high power caused by lightning stroke can be actively adjusted and eliminated through various flexible resources (PET, energy storage and the like) in the system. Therefore, the operation safety after the lightning fault of the alternating current and direct current hybrid power distribution network mainly depends on the load loss condition, namely the load transfer condition after the fault needs to be analyzed.
Risk assessment is based on the probability of occurrence of a fault caused by thunder, the influence of the thunder fault on system operation is considered, then a corresponding static safety risk index is established, the probability and the severity of the fault are combined in a quantitative mode, the operation state of the system can be taken into consideration in the assessment process, and the risk assessment method is suitable for researching the influence of the thunder on the alternating current-direct current hybrid power distribution network.
The influence of lightning on the alternating current-direct current hybrid power distribution network is related to the risk that the power of PET (positron emission tomography) transmission is out of limit and the risk that a system loses load after a lightning fault occurs. The transmission power of the PET port and the load shedding amount of the system directly influence the power value of a corresponding node in the load flow calculation of the alternating current-direct current hybrid power distribution network.
Comprehensively considering the influence of the lightning on the AC/DC hybrid power distribution network, evaluating indexes of static safety by static safety
Figure 910551DEST_PATH_IMAGE149
The method comprises the following steps of establishing the following static safety assessment indexes of the lightning to the alternating current-direct current hybrid power distribution network:
Figure DEST_PATH_IMAGE150
in the formula:tis time;
E i representing a lightning fault;
Figure 239901DEST_PATH_IMAGE151
、
Figure DEST_PATH_IMAGE152
respectively a PET transmission power out-of-limit risk index and a loss load risk index;
Figure 251851DEST_PATH_IMAGE153
、
Figure DEST_PATH_IMAGE154
respectively corresponding weight coefficients of the PET transmission power out-of-limit risk index and the loss load risk index;
Figure 983046DEST_PATH_IMAGE155
for lightning faults
Figure DEST_PATH_IMAGE156
Time of day
Figure DEST_PATH_IMAGE157
Causing the consequence of PET port out-of-limit
Figure DEST_PATH_IMAGE158
The severity of (d);
Figure 114426DEST_PATH_IMAGE159
for lightning faultsE i Time of daytCause the consequence of load loss
Figure DEST_PATH_IMAGE160
The severity of (d);
Figure 681804DEST_PATH_IMAGE161
for lightning faultsE i The probability of (c).
The above-mentioned lightning failure
Figure DEST_PATH_IMAGE162
Time of day
Figure 497445DEST_PATH_IMAGE163
Severity caused by PET Port violation
Figure DEST_PATH_IMAGE164
Comprises the following steps:
Figure 83147DEST_PATH_IMAGE165
Figure DEST_PATH_IMAGE166
Figure 60461DEST_PATH_IMAGE167
Figure DEST_PATH_IMAGE168
in the formula:
Figure 364404DEST_PATH_IMAGE169
is a time of day
Figure DEST_PATH_IMAGE170
The PET port out-of-limit loss expectation value;
w PET (t,s) As a scenesTime of daytLoss value due to the out-of-limit of the lower PET port;
san uncertainty scene;
Figure 980805DEST_PATH_IMAGE171
the total number of scenes in the uncertainty scene set;
Figure DEST_PATH_IMAGE172
as a scenesThe occurrence probability of (2);
z is a PET number;
nthe total number of PET is;
Figure 421014DEST_PATH_IMAGE173
numbering PET ports;
Figure DEST_PATH_IMAGE174
、
Figure 834809DEST_PATH_IMAGE175
、
Figure DEST_PATH_IMAGE176
respectively of the z-th PET
Figure 360468DEST_PATH_IMAGE177
Active power, reactive power and maximum apparent power transmitted by the signal port.
The above-mentioned lightning failureE i Time of daytSeverity of lost load
Figure DEST_PATH_IMAGE178
Comprises the following steps:
Figure 517911DEST_PATH_IMAGE179
Figure DEST_PATH_IMAGE180
Figure 343785DEST_PATH_IMAGE181
Figure DEST_PATH_IMAGE182
in the formula:
Figure 662902DEST_PATH_IMAGE183
is a time of day
Figure DEST_PATH_IMAGE184
Expected load loss rate of (1);
w Load (t,s) As a scenesTime of daytThe load loss rate of the AC/DC distribution network with the load priority is considered;
e. f represents a load number;
N AC,loss 、N DC,loss the sum of the numbers of the loads which cannot be recovered and the loads which are not recovered of the alternating current and the direct current respectively;
N AC,load 、N DC,load the total load number of the AC network and the DC network before the fault is respectively;
Figure 941436DEST_PATH_IMAGE185
、
Figure DEST_PATH_IMAGE186
are respectively scenessTime of daytThe load quantity of the lower load node e which cannot be recovered and the total load quantity of the load node e are calculated;
Figure 634061DEST_PATH_IMAGE187
、
Figure DEST_PATH_IMAGE188
the important grade factors of the loads e and f are respectively;
Figure 48862DEST_PATH_IMAGE189
as an index for the removal of the load,
Figure DEST_PATH_IMAGE190
representing that the load is fully restored at this point,
Figure 538880DEST_PATH_IMAGE191
representing the point where the load is totally removed.
And 5, considering the establishment of a power flow transfer optimization model under the N-1 line fault caused by lightning.
And (3) making an intra-day dispatching plan of the operation of the alternating current and direct current power distribution network by adopting a dynamic lightning protection idea on the basis of real-time lightning prediction data. For an expected lightning fault set and an uncertainty scene set which are established, the influence of lightning faults on users is reduced to the greatest extent through the PET power flow transfer function, and various flexible resources in a network need to be coordinated and controlled, so that the operation problem of an alternating current and direct current power distribution network under the influence of lightning is essentially an optimization problem.
Time of AC/DC hybrid power distribution networktUncertainty of lightning faultE i Establishing a power flow transfer optimization model under the condition of considering the N-1 line fault caused by lightning for the objective function with the minimum static safety evaluation index;
the target function expression is:
Figure DEST_PATH_IMAGE192
the constraint conditions of the alternating current-direct current optimal power flow comprise constraint conditions of various flexible resources and power flow constraint conditions of the power distribution network. For example: the flexible resource is distributed renewable energy, and the constraint condition is that the output range does not exceed the maximum capacity; the power flow constraint conditions of the power distribution network comprise that node voltage and line current are not out of limit and the like.
And 6, combining the power grid load flow calculation result, and performing alternating current and direct current optimal load flow calculation based on a load flow transfer optimization model to realize dynamic lightning protection of the alternating current and direct current hybrid power distribution network.
By three active management modes of PET power flow control function, active and reactive control of distributed power supply in the system and load shedding, the voltage of each node and the capacity of each line in the AC/DC power distribution network are not out of limit, and the current of each node and the capacity of each line in the AC/DC power distribution network are obtained according to a power flow transfer optimization modelTIn each of the evaluation periods, the evaluation period,Nand the dynamic lightning protection of the AC/DC hybrid power distribution network is realized by predicting the power flow distribution condition of the AC/DC power distribution network under the fault.
The system is used for the dynamic lightning protection method of the alternating current-direct current power distribution network based on the power electronic transformer.
Examples
Taking the system shown in fig. 5 as an example, the system forms an ac/dc hybrid power distribution network based on three four-port PETs, and the system includes renewable energy sources.
According to fig. 2, first, the ac line N-1 fault set and the dc line N-1 fault set caused by lightning are established according to step 1, and a total of 29 fault sets are expected.
Next, according to step 2, based on historical prediction error data of renewable energy and load, an initial power prediction error uncertainty scene set capable of representing the prediction error probability distribution is generated and scene reduction is performed.
And further, receiving an opening signal of electric shock of a corresponding line breaker, determining a current expected fault (for example, if the line 10 breaker is opened, the current expected fault belongs to the opening of the line 10), reading in a topological structure of the system, establishing a corresponding alternating current-direct current power distribution network power flow calculation model according to the step 3, wherein the PET control mode needs to be determined according to the current network topological structure, and when a specific PET control variable and each flexible resource control variable (in the example, fan and photovoltaic output) take values, the influence of the line fault caused by thunder on the operation of the power distribution network can be minimized, solving the power flow transfer optimization model under the N-1 line fault caused by the thunder in the step 5 through optimal power flow calculation, wherein the obtained result is the thunder active protection method proposed to be implemented in the scheduling stage in the day.
If the lightning fault is predicted at an in-day stage without implementing a corresponding load transfer strategy, all loads downstream of the line 10 will lose power. By utilizing the dynamic lightning protection method, the control mode and the control variable of the PET are adjusted, the load transfer is realized by the direct current network by coordinating the active and reactive power output and the load shedding amount of the distributed power supply in the system, the tidal current complementation between the alternating current network and the direct current network is realized, and the aim of minimizing the static safety assessment index of the system in the step 4 (namely minimizing the influence of thunder on the operation of the power distribution network) is fulfilled.
The beneficial effects of the invention are that compared with the prior art:
the method utilizes the power flow control capability of the power electronic transformer PET, implements an active lightning protection strategy on the whole AC/DC hybrid power distribution network, and when a lightning stroke fault occurs in an AC circuit, carries out power flow transfer through a DC network with the PET as a coupling element to supply power to a downstream load of the fault circuit; when the direct current line has lightning stroke fault, the power flow is transferred through the alternating current network.
The method is based on the thunder prediction information, an optimized scheduling strategy is formulated in the day stage, the power flow mutual aid among the AC and DC networks is realized through PET, and the influence of thunder on the operation of the AC and DC hybrid power distribution network is reduced.
The method is suitable for application scenes considering distributed renewable energy access, and can solve the problem of uncertainty caused by renewable energy output fluctuation.
The present applicant has described and illustrated embodiments of the present invention in detail with reference to the accompanying drawings, but it should be understood by those skilled in the art that the above embodiments are merely preferred embodiments of the present invention, and the detailed description is only for the purpose of helping the reader to better understand the spirit of the present invention, and not for limiting the scope of the present invention, and on the contrary, any improvement or modification made based on the spirit of the present invention should fall within the scope of the present invention.

Claims (14)

1. A dynamic lightning protection method for an AC/DC power distribution network based on a power electronic transformer is characterized by comprising the following steps:
the method comprises the following steps:
step 1, constructing an expected lightning fault set of an alternating current-direct current power distribution network containing a power electronic transformer PET by adopting N-1 line faults caused by lightning;
step 2, establishing an initial power prediction error uncertainty scene set representing the probability distribution of renewable energy and load power prediction errors and carrying out scene reduction;
step 3, carrying out load flow calculation on the alternating current and direct current power distribution network containing the PET based on the expected lightning fault set and the uncertainty scene set;
step 4, establishing a static safety assessment index capable of reflecting the influence of lightning on the AC/DC power distribution network;
step 5, establishing a power flow transfer optimization model under the condition of considering the N-1 line fault caused by lightning by taking the minimum static safety evaluation index as an objective function;
and 6, combining the power grid load flow calculation result, and performing alternating current and direct current optimal load flow calculation based on a load flow transfer optimization model to realize dynamic lightning protection of the alternating current and direct current power distribution network.
2. The dynamic lightning protection method for the alternating current-direct current power distribution network based on the power electronic transformer is characterized by comprising the following steps of:
in the step 1, the expected lightning fault set of the AC/DC power distribution network is divided into an N-1 line fault caused by the lightning of the AC network and an expected lightning fault set under the N-1 line fault caused by the lightning of the DC network, and the method specifically comprises the following steps:
the method comprises the steps that lightning faults are simulated to be disconnection problems of corresponding accident equipment, namely, any disconnection of N lines of a system serves as an expected fault, and an expected fault set of an alternating current and direct current power distribution network is further divided into a line N-1 fault set of an alternating current network and a line N-1 fault set of a direct current network; the fault of the ith line is recorded as E i The probability of failure is recorded as P r (E i )。
3. The dynamic lightning protection method for the alternating current-direct current power distribution network based on the power electronic transformer is characterized by comprising the following steps of:
the step 2 specifically comprises the following steps:
step 2.1, generating an initial power prediction error scene set of renewable energy sources and loads by adopting a Latin hypercube sampling method;
and 2.2, carrying out scene reduction on the initial power prediction error scene set by adopting a synchronous back substitution method.
4. The dynamic lightning protection method for the AC/DC power distribution network based on the power electronic transformer as claimed in claim 3, wherein:
step 2.1 specifically comprises:
step 2.1.1, fitting historical sample data to obtain a power prediction error probability distribution function of the renewable energy sources and the load;
step 2.1.2, equally dividing the probability distribution function of the power prediction error into Ns equal probability intervals;
step 2.1.3, extracting random number p in each probability interval o Wherein o ═ 1,2, …, Ns;
step 2.1.4, inverse transformation is carried out on the probability distribution function of the power prediction error to obtain a prediction error sample value u in a corresponding probability interval o Also known as scene u o And forming an initial power prediction error scene set.
5. The dynamic lightning protection method for the AC/DC power distribution network based on the power electronic transformer as claimed in claim 3, wherein:
the step 2.2 specifically comprises the following steps:
step 2.2.1, set the iteration number a to 1, and set the deleted scene set J to J (0) Is an empty set;
step 2.2.2, calculate the scene u to be deleted for the a-th iteration o Let u be u o The probability distance represented by the following equation takes a minimum value:
Figure FDA0003746404000000021
in the formula:
Figure FDA0003746404000000022
π o as a scene u o The probability of (d);
v is a scene number;
u v to substitute u o The scene (c);
step 2.2.3, delete scene u o Let J (a) ＝J (a-1) ∪{u o A +1, deleted scene u o Using the scene u closest to its Euclidean distance v Instead, the scene u is simultaneously divided o Is added to the scene u v So that the sum of the probabilities of all scenes is always 1;
scene u v Probability of (n) v Is changed into pi v ′：
π v ′＝π o +π v
Step 2.2.4, judging whether the number of deleted scenes reaches a set value, if not, returning to step 2.2.2, otherwise, making J equal to J (a) And the scene reduction is finished.
6. The dynamic lightning protection method for the alternating current-direct current power distribution network based on the power electronic transformer is characterized by comprising the following steps of:
step 3.1, establishing a PET load flow calculation model according to the topological structure of the PET;
step 3.2, establishing a load flow calculation model of the alternating current network and the direct current network;
and 3.3, writing a power flow calculation equation set by the power flow calculation model in the steps 3.1 and 3.2 in combination with the control mode of each port, and alternately and iteratively calculating the power flow of the alternating-current and direct-current power distribution network containing the power electronic transformer until convergence.
7. The dynamic lightning protection method for the AC/DC power distribution network based on the power electronic transformer as claimed in claim 6, wherein:
3.1, the topological structure of the PET specifically comprises a medium-voltage conversion module, an isolation module and a low-voltage conversion module;
the medium-voltage conversion module comprises a medium-voltage alternating current module and a medium-voltage direct current module;
the medium-voltage alternating current module converts medium-voltage alternating current of power frequency into medium-voltage direct current and is composed of VSC, and the medium-voltage direct current module is connected with a medium-voltage direct current bus inside the PET;
the isolation module adopts a QAB converter to realize a DC/DC conversion function, uses a four-winding high-frequency transformer to realize the electrical isolation among ports, and controls the voltage of each internal direct-current bus by adjusting the transformation ratio of each winding of HFT;
the low-voltage conversion module comprises a low-voltage alternating current module and a low-voltage direct current module;
the low-voltage AC module inverts the voltage of the connected internal DC bus into low-voltage AC required by a network connected with the module, and the low-voltage DC module is connected with the PET internal low-voltage DC bus;
according to the structure, the PET external network is connected to the QAB converter through the corresponding port and the conversion module, and the QAB converter is used as a core element of the PET to realize the functions of power regulation and energy bidirectional exchange;
therefore, a PET power flow calculation model is established from an alternating current port, a direct current port and a QAB converter.
8. The dynamic lightning protection method for the AC/DC power distribution network based on the power electronic transformer according to claim 6, characterized in that:
step 3.3, when the power flow of the alternating current and direct current distribution network containing the PET is calculated, the power flow distribution condition in the network is calculated, and the internal state variable and the power flow distribution of the PET are solved to provide a control reference value of the PET;
during specific calculation, the PET power flow calculation model plays a coupling role between an alternating current system and a direct current system, when the alternating current network power flow and the direct current network power flow are respectively solved, nodes of all ports connected with the network are equivalent according to a control mode of a power electronic transformer, power interaction is realized between the alternating current system and the direct current system through the alternating current port and the direct current port of the PET, and iterative variables in the calculation process are as follows:
selecting an interactive variable of a PET medium-voltage alternating-current port and a medium-voltage alternating-current distribution network as a global iteration quantity of alternating-current and direct-current distribution network load flow calculation, and when the difference between an alternating-current network state variable, a PET state variable, a direct-current network state variable, the global iteration quantity and a k-1 time global iteration result after the k time global iteration is smaller than an allowable error epsilon r In the process, the load flow calculation result of the alternate iteration can be output; otherwise, substituting the global iteration variable into k +1 times of global iteration to continue solving.
9. The dynamic lightning protection method for the alternating current-direct current power distribution network based on the power electronic transformer is characterized by comprising the following steps of:
in step 4, the safety level of the operation of the power grid is comprehensively measured from the two aspects of uncertainty and severity of lightning fault occurrence, and the index R (E) is evaluated in a static safety manner i And t) embodying static safety, and establishing the following static safety evaluation indexes:
Figure FDA0003746404000000041
in the formula: t is time;
E i representing a lightning fault of the ith line;
Figure FDA0003746404000000042
respectively a PET transmission power out-of-limit risk index and a loss load risk index;
α PET 、α Load respectively corresponding weight coefficients of a PET transmission power out-of-limit risk index and a loss load risk index;
Figure FDA0003746404000000043
for lightning faults E i Consequence C of PET port out-of-limit caused by next time t PET The severity of (d);
Figure FDA0003746404000000044
for lightning faults E i The result of the loss of load at the next moment t C Load The severity of (d);
P r (E i ) For lightning faults E i The probability of (c).
10. The dynamic lightning protection method for the alternating current/direct current power distribution network based on the power electronic transformer is characterized by comprising the following steps of:
the lightning failure E i Severity of the lower time t caused by PET Port violation
Figure FDA0003746404000000045
Comprises the following steps:
Figure FDA0003746404000000051
Figure FDA0003746404000000052
Figure FDA0003746404000000053
Figure FDA0003746404000000054
in the formula:
Figure FDA0003746404000000055
the PET port out-of-limit loss expected value at the moment t;
w PET (t, s) is a scene s, and a loss value caused by the out-of-limit of the PET port at the moment t;
s is an uncertainty scene;
n s the total number of scenes in the uncertainty scene set;
π s is the occurrence probability of scene s;
z is a PET number;
n is the total number of PET;
m is a PET port number;
Figure FDA0003746404000000056
the active power, the reactive power and the maximum apparent power transmitted by the m port of the z PET under the scene s and the moment t are respectively.
11. The dynamic lightning protection method for the alternating current/direct current power distribution network based on the power electronic transformer is characterized by comprising the following steps of:
the lightning failure E i Severity of loss of load at time t
Figure FDA0003746404000000057
Comprises the following steps:
Figure FDA0003746404000000058
Figure FDA0003746404000000059
Figure FDA0003746404000000061
P loss,e (t,s)＝α e P load,e (t,s)
in the formula:
Figure FDA0003746404000000062
is the load loss rate expected value at time t;
w Load (t, s) is a scene s, and the load loss rate of the AC/DC power distribution network of the load priority is considered at the moment t;
e. f represents a load number;
N AC,loss 、N DC,loss the sum of the numbers of the loads which cannot be recovered and the loads which are not recovered of the alternating current and the direct current respectively;
N AC,load 、N DC,load the total load number of the AC network and the DC network before the fault is respectively;
P loss,e (t,s)、P load,e (t, s) are respectively a scene s, the load quantity of the load e which cannot be recovered at the moment t and the total load quantity of the load e;
γ e (0<γ e ≤1)、γ f (0<γ f less than or equal to 1) are respectively the important grade factors of the loads e and f;
α e (0≤α e less than or equal to 1) is taken as the index of load cutting.
12. The dynamic lightning protection method for the alternating current-direct current power distribution network based on the power electronic transformer is characterized by comprising the following steps of:
step 5, uncertainty lightning fault E is calculated according to the moment t of the AC/DC power distribution network i Static safety assessment index R (E) of i T) establishing a power flow transfer optimization model under the condition of considering the N-1 line fault caused by the lightning for the objective function at minimum;
the target function expression is:
f＝minR(E i ,t)
the constraint conditions comprise constraint conditions of various flexible resources and power flow constraint conditions of the power distribution network.
13. The dynamic lightning protection method for the alternating current-direct current power distribution network based on the power electronic transformer is characterized by comprising the following steps of:
and 6, ensuring that the voltages of all nodes and the capacity of all lines in the alternating current and direct current power distribution network are not out of limit through three active management modes of a power flow control function of PET, active and reactive control of a distributed power supply in the system and load shedding, and solving the power flow distribution conditions of the alternating current and direct current power distribution network under M expected faults in T evaluation periods according to a power flow transfer optimization model to realize dynamic lightning protection of the alternating current and direct current power distribution network.
14. Alternating current-direct current distribution network developments lightning protection system based on power electronic transformer, its characterized in that:
the system is used for realizing the dynamic lightning protection method for the alternating current-direct current power distribution network based on the power electronic transformer as claimed in any one of claims 1 to 13.
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