CN110601155B - Protection method of multi-terminal flexible extra-high voltage direct current transmission system - Google Patents
Protection method of multi-terminal flexible extra-high voltage direct current transmission system Download PDFInfo
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- CN110601155B CN110601155B CN201910801172.1A CN201910801172A CN110601155B CN 110601155 B CN110601155 B CN 110601155B CN 201910801172 A CN201910801172 A CN 201910801172A CN 110601155 B CN110601155 B CN 110601155B
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- 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
- H02H7/261—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 involving signal transmission between at least two stations
- H02H7/262—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 involving signal transmission between at least two stations involving transmissions of switching or blocking orders
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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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- 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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Abstract
The invention discloses a protection method of a multi-terminal flexible direct-current transmission system, which comprises a quick primary protection strategy and a backup protection strategy when a primary fails. The strategy is a non-unit protection strategy based on voltage and current, local position voltage and current signals are sampled, a short-circuit area is determined by combining the change rate of fault current along with time and k-means in machine learning, then threshold level and action time of forward fault action and backward fault action are judged, and when primary protection of a multi-terminal flexible direct-current transmission system fails, a proper direct-current circuit breaker can be rapidly distinguished, selected and triggered, so that short-circuit faults are eliminated in a shorter time, the peak value of short-circuit current is reduced, and the purpose of improving the safety and stability of a power grid system is achieved.
Description
Technical Field
The invention belongs to the field of power system protection, and particularly relates to a protection method of a multi-terminal flexible direct-current power transmission system.
Background
With the development of direct current transmission technology and the wide application of extra-high voltage alternating current and direct current technology, a large-scale multi-feed-in direct current transmission system and a receiving end system are formed in China at present. The multi-terminal direct current system has higher compatibility, stability, reliability and safety. However, when the ac fault of the receiving end system may cause the breakdown of the whole system, the dc transmission power is interrupted, and the safe and stable operation of the whole ac/dc system is threatened. The direct current fault protection of the multi-end flexible direct current system is one of key technologies for the development of the direct current system, and the main technical difficulties comprise reliable identification and rapid isolation of direct current faults. When a fault occurs, the protection system must accurately locate the fault and remove the fault. The protection system is composed of a primary protection system and a backup protection system. When a fault occurs, the primary protection immediately acts to remove the fault; when primary protection fails, backup protection must be initiated and the fault cleared. The current fault identification algorithm of primary protection failure adopted by the rapid backup protection is a linear identification method, and the fault current time limit is 3 ms. The algorithm has the disadvantage of easily causing false recognition, and is therefore limited in practical application.
In the prior art, the patent number CN106253240B is named as a single-end protection method for a multi-end flexible direct current power grid system based on boundary characteristics, and firstly, wavelet decomposition is adopted to extract high-frequency components in fault current; calculating high-frequency transient energy Ej of fault current and voltage drop VL on the smoothing reactor; then, judging the fault direction by utilizing the voltage drop VL on the smoothing reactor; if the positive direction has no fault, the fault protection process is exited; if the positive direction fault is judged, further judging whether the fault exists in the area by utilizing a high-frequency transient state energy Ej criterion; if Ej > Eset is satisfied, judging that the fault is in the area, and executing fault protection action; otherwise, judging that no fault exists in the area, and exiting the fault protection process. The method cannot quickly put back-up protection when primary protection action fails, so a strategy for quickly identifying faults in a primary protection area and triggering a proper direct-current circuit breaker is provided. When primary protection action fails, the strategy can quickly diagnose faults and select proper trigger time to trigger backup protection isolation faults, so that fault current is reduced.
Disclosure of Invention
The present invention is directed to solving the above problems of the prior art. The protection method of the multi-terminal flexible extra-high voltage direct current transmission system achieves faster protection action and avoids generation of large fault current. The technical scheme of the invention is as follows:
a protection method for a multi-terminal flexible extra-high voltage direct current transmission system comprises the following steps:
s1, calculating current and voltage threshold V according to the voltage and current signals of the line collected by the relay protection systemth、IthAnd rate of change of current with time
S2, rate of change of passing current with time
Judging whether the fault is a positive fault or a negative fault; if the positive direction is positive, the fault is a positive direction fault; if the fault is negative, the fault is a reverse fault; then, the acquired voltage and current are used as input quantities by a k-means method of machine learning, the input quantities are compared with a threshold value, and whether the fault is an in-zone fault or an out-of-zone fault is predicted;
s3, if the fault is an in-zone fault and a positive direction fault, checking the state of the primary protection circuit breaker, triggering a primary protection circuit breaker on-off signal, recording whether the fault is cleared or not, and exchanging fault state and circuit breaker action state information between relay protection systems of primary protection and backup protection;
and S4, if the fault is an in-zone fault and a reverse fault, triggering a circuit breaker of the backup protection to clear the fault according to the states of the primary protection system and the backup protection system if the fault is not removed.
And S5, if the fault is judged to be out of the area, the fact that no fault exists in the area is described, and the fault protection process is quitted.
Further, in step S1, the threshold voltage boundary calculation formula is: vth＝A×idc+ B, A and B are constants related to system parameters; the threshold current boundary calculation formula is: i isth＝1.25idc；idcIs the line current to be protected.
Further, the k-means classification method in the step 2 is divided into 2 types according to the measured voltage and current signals of the primary protection relay system and the backup protection relay system, namely, the primary protection action region and the backup protection action region, k in the k-means algorithm is 2, the measurement distance is selected as the minkowski distance, and the coefficient of the threshold voltage calculation formula is obtained.
Further, in step S3, the conditions for triggering the opening operation of the primary protective breaker are as follows:
and the line voltage is less than the threshold voltage, and the line current is greater than the threshold current, triggering the action signal.
Further, in step S3, the information exchange mechanism between the primary protection relay system and the backup protection relay system is: the initial state is 0, and when a certain level of protection acts, the state becomes 0. T ispp、TbpAction signals representing primary protection and backup protection, respectively, namely: if T ispp＝1,TbpIf the value is 0, the primary protection is acted; if T ispp＝0,TbpWhen the recorded fault is cleared, if the measured voltage is less than the line voltage, the fault is cleared, TFC＝1；TFCIndicating a fault condition of the line being protected, and if the measured voltage is greater than or equal to the line voltage, the fault has cleared, TFCWhen T is equal to 0, mixingFCThe status of (2) is returned to the backup protection system.
Further, in step S4, the backup protection operation conditions are: t isFC1, the line voltage is less than the threshold voltage, the line current is greater than the threshold current, and wait for an overshoot time of 0.2ms, then decide if Tpp＝1,TbpIf 1, then a backup protection action signal is issued.
The invention has the following advantages and beneficial effects:
the method provides a primary and local backup protection strategy aiming at multi-terminal flexible extra-high voltage direct current transmission, can realize high-speed fault identification in a primary protection area and select and trigger a proper circuit breaker, can quickly identify primary protection failure in a backup protection area, and triggers an action signal after waiting for 0.2ms overshoot time to quickly isolate faults. The method can reduce the fault current peak value and the voltage reduction level, and compared with the existing linear identification algorithm (3ms), the k-means method can quickly and accurately classify the boundaries of primary protection and backup protection. Meanwhile, the method only needs local system parameters, voltage and current signals of the installation position of the circuit breaker and is not influenced by the structural change of the system. The naive Bayes classification method can make high-accuracy classification only by a small amount of data, is insensitive to noise data, and can process real-time data and discrete data.
Drawings
FIG. 1 is a schematic diagram of the method implementation of the preferred embodiment of the present invention;
fig. 2 is a flow chart of a protection strategy for a multi-terminal flexible extra-high voltage direct current transmission system;
FIG. 3 is a flowchart of a k-means classification method according to an embodiment of the present invention.
FIG. 4 is a layout diagram of a single line (fig. a) and a bus bar protection area (fig. b) of the multi-terminal flexible DC system according to the embodiment of the present invention
FIG. 5 is a layout diagram of a linear bus bar protection area according to an embodiment of the present invention
FIG. 6 is a current-voltage waveform diagram illustrating a primary protection startup clear fault in an embodiment of the present invention
FIG. 7 is a current-voltage waveform diagram illustrating a backup protection startup clear fault in an embodiment of the present invention
Detailed Description
The technical solutions in the embodiments of the present invention will be described in detail and clearly with reference to the accompanying drawings. The described embodiments are only some of the embodiments of the present invention.
The technical scheme for solving the technical problems is as follows:
the invention provides a framework for determining threshold ranges and action time of primary protection and backup protection based on a machine learning k-means classifier, and fault types are determined and classified by measuring local voltage and current, so that rapid action of the backup protection is realized. Compared with the existing protection, the protection method realizes faster protection action and avoids the generation of large fault current. When the primary protection fails, the backup protection acts after being accelerated to 0.2ms, and meanwhile, the relay protection system implementing the protection scheme is not changed by the structure of the power transmission network.
In the embodiment of the invention, whether the fault area and the primary protection normally cut off the fault can be quickly identified, and the backup protection can be quickly started, so that the short-circuit current and voltage drop are reduced, the loss of a power grid is reduced, and the aim of improving the safety and stability of a power grid system is fulfilled.
The implementation of the invention is introduced based on a simulation case, the specific implementation flow chart is shown in figure 2, and the implementation steps are as follows:
(1) the single-line diagram of the multi-terminal flexible extra-high voltage direct current transmission network in the embodiment is shown in fig. 4(a), is a four-terminal modular multilevel converter direct current transmission network, is a bipolar transmission network, is provided with an annular network, and both ends of each line are provided with a series reactor and a hybrid direct current breaker. The lengths of the lines L24, L43, L31 and L21andL32 are 80km,80km,100km,100km and 150km, respectively. The voltage of the direct current line is controlled by terrestrial VSC power stations (MMC-1 and MMC-2) by means of P-Vdc voltage drop control and reactive power control. A fault occurs in line L31 between bus 1and bus 3. The protection area of the line is shown in fig. 4(b), and is divided into 2 areas: area 1 is a primary protection area, and area 2 is a secondary protection area. Relays R13, R12 and R1 measure the voltage current signals V and I, respectively, of the line. The parameters of the system are shown in tables 1-3:
TABLE 1 basic parameters of the System
TABLE 2PI parameters
TABLE 3 pi-type structural equivalent parameters of transmission lines
(2) The accurate prediction of the fault area of the line is the premise of reliable execution of the protection strategy, so that firstly, according to the parameters of the line, PSCAD software simulates and collects voltage and current data of the line, the collection frequency is 10KHz, and then the boundary of a forward fault and a backward fault is determined. Respectively assuming that a low-resistance grounding fault and a high-resistance grounding (10 omega) fault occur at different positions of a line 3, drawing the acquired voltage and current values on a plane, namely a V-I plane, and then dividing data into a region 1and a region 2 by adopting a K-means classification method to obtain a boundary line of the region and the region 2. The K-means classification method is shown in the flow chart of FIG. 3, line L31Failure F3Is shown in fig. 5, and has a threshold voltage expressed as Vth＝80iR(t)+281，iR(t)Representing the current (kA) measured by the relay.
(3) The faults at 8 different positions are respectively at L31On-line for testing primary securityAnd (4) protection and post-protection strategies, and the fault is assumed to occur in 1.4 ms. The primary protection detects the fault current and generates a circuit breaker open signal and changes TPP-13And TfcHas a value of 1. The detection and removal of the fault location is determined by the voltage current threshold, circuit breaker CB13 opens at 1.15ms, the fault current decreases to 0 after 2.375ms, indicating that the fault has cleared, the voltage has also recovered to greater than the threshold voltage (400kV), and then the value of Tvc returns to 0 the voltage current waveform of the primary protection clear fault and the state waveform of the R1 relay are shown in fig. 6.
(4) When the F3 fault is not cleared, i.e., the primary protection fails to clear the fault, backup protection should be enabled to trigger the breaker open signal. According to the method, after the backup protection waits for the overshoot time of 0.2ms, the backup protection is started, the on-off signals of the circuit breakers B12, B13 and B1 are triggered, and the fault is removed. I.e. 2.575ms after the fault occurred, at which time the value of Tvc remained 0, indicating that the fault was cleared, TBP-1And sending a signal, wherein the breaker is switched off after 3.575ms, and the fault current is reduced to 0 after 4.575 ms. The waveform of the current voltage is shown in fig. 7.
(5) Compared with a linear identification algorithm, the method can reduce the fault clearing time in primary protection from 4.6ms to 2.375 ms; the fault clearing time in the backup protection is reduced from 7.8ms to 4.575ms, which shows that the method has great application advantages.
The above examples are to be construed as merely illustrative and not limitative of the remainder of the disclosure. After reading the description of the invention, the skilled person can make various changes or modifications to the invention, and these equivalent changes and modifications also fall into the scope of the invention defined by the claims.
Claims (6)
1. A protection method for a multi-terminal flexible extra-high voltage direct current transmission system is characterized by comprising the following steps:
s1, calculating current and voltage threshold V according to the voltage and current signals of the line collected by the relay protection systemth、IthAnd current is at any timeRate of change therebetween
S2, rate of change of passing current with time
Judging whether the fault is a positive fault or a negative fault; if the positive direction is positive, the fault is a positive direction fault; if the fault is negative, the fault is a reverse fault; then, by using the acquired voltage and current as input quantities through a k-means method of machine learning, comparing the input quantities with a threshold value, and predicting whether the fault is an in-zone fault or an out-of-zone fault;
s3, if the fault is an in-zone fault and a positive direction fault, checking the state of the primary protection circuit breaker, triggering a primary protection circuit breaker on-off signal, recording whether the fault is cleared or not, and exchanging fault state and circuit breaker action state information between relay protection systems of primary protection and backup protection;
s4, if the fault is an intra-area fault and a reverse fault, triggering a backup protection circuit breaker to clear the fault according to the states of the primary protection system and the backup protection system if the fault is not removed;
and S5, if the fault is judged to be out of the area, the fact that no fault exists in the area is described, and the fault protection process is quitted.
2. The method according to claim 1, wherein in step S1, the calculation formula of the primary protection threshold voltage boundary of the intra-zone fault is as follows: vth＝A×idc+ B, A and B are constants related to system parameters; the threshold current boundary calculation formula is: i isth＝1.25idc；idcIs the line current to be protected.
3. The protection method for the multi-terminal flexible extra-high voltage direct-current power transmission system according to claim 2, wherein the k-means classification method in step S2 is divided into 2 classes according to the measured voltage and current signals of the primary protection relay system and the backup protection relay system, namely, a primary protection action region and a backup protection action region, k is 2 in the k-means algorithm, the measurement distance is selected as a minkowski distance, and a coefficient of a threshold voltage calculation formula is obtained.
4. The method according to claim 2, wherein in step S3, the conditions for triggering the opening action of the primary protection breaker are as follows:
and the line voltage is less than the threshold voltage, and the line current is greater than the threshold current, triggering the action signal.
5. The method according to claim 4, wherein in step S3, the information exchange mechanism of the primary protection relay system and the backup protection relay system is as follows: the initial states are all 0, and when a certain level of protection acts, the state is changed into 1; t ispp、TbpAction signals representing primary protection and backup protection, respectively, namely: if T ispp＝1,TbpIf the value is 0, the primary protection is acted; if T ispp＝0,TbpWhen the backup protection is activated and the fault is cleared, T is recordedFCIndicating a fault condition of the line being protected, and if the measured voltage is less than the line voltage, indicating that the fault is clear, TFC1 is ═ 1; if the measured voltage is greater than or equal to the line voltage, the fault has cleared, TFCWhen T is equal to 0, mixingFCThe status of (2) is returned to the backup protection system.
6. The method according to claim 5, wherein in step S4, the conditions of the backup protection action are as follows: t isFC1, the line voltage is less than the threshold voltage, the line current is greater than the threshold current,and waits for an overshoot time of 0.2ms and then determines if Tpp＝1,TbpIf 1, then a backup protection action signal is issued.
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| CN111786386B (en) * | 2020-07-15 | 2021-09-07 | 华北电力大学 | Control system and method for preventing direct current blocking based on transient energy method |
| CN112436494B (en) * | 2020-11-26 | 2023-03-31 | 云南电网有限责任公司电力科学研究院 | Circuit breaker fault backup protection method suitable for high-voltage direct-current power grid |
| CN113161983B (en) * | 2021-03-29 | 2022-06-03 | 重庆大学 | Power transmission line self-adaptive overload protection method considering dynamic thermal characteristics |
| CN117175494B (en) * | 2023-11-03 | 2024-02-06 | 昆明理工大学 | Adaptive reclosing method and system for flexible direct-current power grid |
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