CN115313324A - Single-ended quantity protection method suitable for multi-ended flexible direct current system - Google Patents
Single-ended quantity protection method suitable for multi-ended flexible direct current system Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
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Abstract
The invention relates to a single-ended quantity protection method suitable for a multi-ended flexible direct current system, which analyzes the fault characteristics of direct current line current and bridge arm current after a flexible direct current power grid direct current line fails, and provides a fault current direction criterion to select a pre-trip circuit breaker; setting a protection action time margin according to the action delay of the circuit breaker so as to determine reasonable protection action time through the locking time of the converter station; the method can realize accurate identification and rapid isolation of the fault line on the premise of ensuring that the converter station is not locked, does not depend on line boundary elements, and has strong transition resistance.
Description
Technical Field
The invention belongs to the technical field of electric power systems and automation, and particularly relates to a single-end quantity protection method suitable for a multi-end flexible direct current system.
Background
When a fault occurs in a direct current power grid, capacitors of sub-modules of each converter are discharged violently, fault current rises rapidly, and safety of relevant electrical equipment in the system is seriously affected. In order to rapidly cut off a fault line to ensure the safe operation of the system, the research on a rapid and reliable protection scheme of the flexible direct current transmission system is of great significance.
The main protection of the flexible direct-current power grid mostly selects a protection principle based on single-end electric quantity so as to meet the requirement of the main protection of the flexible direct-current power grid on protection rapidity. According to principle division, the existing single-end quantity protection methods comprise a voltage-current method, a traveling wave method, a boundary method and the like.
However, none of the above methods consider the problem of coordination between the circuit breaker operation and the converter station lockout, and the converter station lockout may still occur during fault isolation. When the converter station is put into operation again, the capacitor needs to be charged again, so that the rapid recovery of power supply of a power grid is not facilitated, and the power failure time is greatly prolonged.
Therefore, the invention analyzes the fault characteristics of the direct current line current and the bridge arm current after the direct current line of the flexible direct current power grid has a fault, and provides a single-end protection scheme of the flexible direct current power grid based on the locking time of the converter station on the basis of considering the action delay and the action protection time margin of the circuit breaker so as to realize the quick removal of the fault line before the converter station is locked.
Disclosure of Invention
The invention aims to overcome the defects of the prior art, provides a single-end quantity protection method suitable for a multi-end flexible direct current system, analyzes the fault characteristics of direct current line current and bridge arm current after a flexible direct current grid direct current line fails, and provides a fault current direction criterion to select a pre-trip circuit breaker; setting a protection action time margin according to the action delay of the circuit breaker so as to determine reasonable protection action time through the locking time of the converter station; the method can realize accurate identification and rapid isolation of the fault line on the premise of ensuring that the converter station is not locked, does not depend on line boundary elements, and has strong transition resistance.
The technical problem to be solved by the invention is realized by the following technical scheme:
a single-end quantity protection method suitable for a multi-end flexible direct current system is characterized by comprising the following steps: the method comprises the following steps:
s1, analyzing the directional characteristic of a fault current of a direct-current line and the fault characteristic of a bridge arm current after a fault occurs on a fault direct-current side of a flexible direct-current power grid;
(1) DC line fault current direction characteristic
The method comprises the steps that a four-terminal flexible direct current system model is used as a basis for analysis, and for a flexible direct current power grid of a true bipolar wiring line, no matter a bipolar short circuit, a pole-to-metal return line short circuit or a unipolar grounding short circuit fault occurs on a direct current side, a fault pole MMC can form a fault loop on the direct current side through a fault pole, the metal return line or the ground before a converter station is locked;
when a positive grounding fault or a bipolar short-circuit fault occurs on the line, the direction of the fault current variation on the positive line is from the bus to the line; when a negative earth fault or a bipolar short-circuit fault occurs on the line, the direction of the fault current variation on the negative line is from the line to the bus;
therefore, the positive direction of the fault current variation of the positive electrode line is from the bus to the line, and the positive direction of the fault current variation of the negative electrode line is from the line to the bus, so that the fault current variation of both sides of the fault line is positive no matter the positive electrode line or the negative electrode line has a fault;
in practical engineering, when the length difference of each section of the direct current power grid is not very large, the impedance difference of each section of the direct current power grid is not very large, and for any bus, if a fault occurs on one outgoing line of the bus, the situation that the directions of fault current variation on two outgoing lines of the bus are the same is avoided; for the converter station connected with the bus, the direction of the fault current variation on the fault line measured by the converter station is determined to be positive, and the direction of the fault current variation on the other line is determined to be negative; therefore, when the directions of the fault current variation quantities on the two outgoing lines of a certain bus are the same, the line connected with the bus is not necessarily a fault line;
(2) Bridge arm current fault characteristics
In a typical half-bridge MMC converter topological structure, a half-bridge MMC converter consists of three symmetrical phase units, each phase comprises an upper bridge arm and a lower bridge arm, and each bridge arm is formed by connecting a plurality of identical sub-modules and bridge arm reactors in series;
the MMC bridge arm current consists of three parts in normal operation, namely alternating current side current, direct current side current and bridge arm circulation current, and upper and lower bridge arm currents i jp 、i jn (j = a, b, c) may be respectively expressed as:
in the formula: I.C. A ac Is the amplitude of the AC side current, I cir Is the bridge arm circulating current amplitude;
after a fault, sub-module capacitors in each converter station quickly discharge to a fault point, so that fault current of a direct-current line rapidly rises, direct-current components of bridge arm current rapidly increase, and meanwhile, the equivalent voltage of a bridge arm is changed due to discharge of the sub-module capacitors, so that the alternating-current side injection current is changed;
after the fault of the direct current side occurs, the change of the current of each bridge arm of the converter station is mainly caused by the change of a direct current component and the change of an injection current of the alternating current side; the proportion of the bridge arm circulating current in the bridge arm current is smaller than that of the direct current and the alternating current, and the amplitude of the bridge arm circulating current is not changed greatly in a short time before and after a fault, so that the amplitude of the bridge arm circulating current can be approximately considered to be not changed in a few milliseconds after the fault occurs;
after a fault occurs on the direct current side, the rectifier station and the converter station show different fault characteristics: the amplitude values of the direct current and the alternating current of the rectifying station after the fault are continuously increased, and the amplitude values of the direct current and the alternating current of the inverter station after the fault are subjected to the processes of firstly reducing and then reversely increasing;
s2, aiming at the direct-current line fault generated by the multi-terminal flexible direct-current power grid, a single-terminal quantity protection scheme based on the converter station blocking time is provided;
firstly, selecting a pre-trip circuit breaker according to the direction of fault current variation; secondly, determining the action time of protection according to the locking time of the converter station, and on the basis of correctly selecting a pre-trip circuit breaker, completing the removal of a fault line by the cooperation of the action time of the protection of each section of line;
(1) Fault current direction criterion and pre-trip circuit breaker selection
Adopt the current variation volume to reflect fault current's change situation, its positive direction is selected and is related to with the electric current line polarity that flows through, and the current variation volume positive direction of positive pole circuit is for flowing to the circuit from the generating line promptly, and the current variation volume positive direction of negative pole circuit is for flowing to the generating line from the circuit, and the definition current rate of change is as follows:
in the formula: i.e. i k And i k-1 Representing adjacent current sample values within a sample interval Δ T;
defining a current variation direction index S which represents the fault current variation direction measured by the end part of the direct current circuit;
s =1 represents that the direction of the current variation is positive, when the result of expression (2) is greater than 0;
s = -1 represents that the direction of the current change amount is negative, and the result of the formula (2) is smaller than 0;
s =0 represents that no current change is detected, when the result of equation (2) equals 0;
considering the normal fluctuation condition of the current on the direct current line when the system normally operates, when the absolute value of the detected current change rate exceeds the setting value k set And in time, the direction index changes, and the following fault current change quantity direction criterion is provided:
by combining the analysis of the fault current characteristics of the direct current line, each converter station can select a pre-trip circuit breaker according to the fault current variation direction index S measured at the end part of the line, and the pre-trip circuit breaker selection criterion S im 、S in Are respectively circuit breakers CB im 、CB in And (3) extracting the selection criterion of the pre-trip circuit breaker shown in the formula (4) for each converter station according to the detected fault current variation direction index:
(2) Protection action time calculation based on converter station lockout time
On the basis of determining the pre-trip circuit breaker, the protection action time of each converter station depends on the locking time of the converter station, and on the premise of ensuring that the converter station is not locked, the protection action time is determined under the condition of considering the action delay and the action time margin of the circuit breaker, and the specific calculation method is shown as formula (5):
t act ＝t block -t cb -t yd (5)
in the formula: t is t block Is the lock-out time of the converter station;
t cb delaying the opening of the circuit breaker;
t yd a protection action time margin;
considering the action delay of the circuit breaker and the matching problem of the circuit breakers at two sides of the circuit, t is yd Is set to be and cb equality, the coordination of each section of protection can be ensured to the maximum extent;
considering that bridge arm current of the inverter station has different fault characteristics from those of the rectifier station, a unified processing method is provided for the inverter station, virtual locking time of the inverter station is defined, and the fault severity can be well reflected; assuming that the circulating current amplitude of the bridge arm is not changed after the fault occurs, respectively analyzing and processing the AC and DC components of the bridge arm current of the inverter station;
in order to enable the inverter station to show the effect of continuously increasing the received active power after the fault, the variable quantity of the alternating current after the fault is processed in a reverse way, namely the value after the fault is subtracted from the double normal operation value, and the expression of the direction change processing is shown as the formula (6):
i ac '＝2i ac0 -i ac (6)
in the formula: i.e. i ac ' is a virtual alternating current;
i ac0 alternating current for normal operation;
i ac actual alternating current after the fault;
the amplitude of the fault current on the virtual alternating current side of the inverter station obtained after the direction change processing after the fault is continuously increased and is consistent with the change trend of the current on the alternating current side of the rectifier station under the same initial current;
in order to make the direct current of the inverter station show the same effect of continuous increase as that of the rectifier station in consideration of the transmission condition of the direct-current side power, the direct-current side power converter is used for the i dc And i ac Similarly, the direction-changing processing expression is shown as formula (7):
i dc '＝2i dc0 -i dc (7)
in the formula: i.e. i dc ' is a modified direct current;
i dc0 the direct current is the direct current before the fault;
s3, predicting waveform fitting based on the mobile data window, and predicting the locking time of the convertor station in real time through fault data by the convertor station;
based on the analysis of the fault characteristics of the bridge arm currents, after the direct-current side fault occurs, the change of each bridge arm current is mainly caused by a direct-current component and an alternating-current component, the change of the bridge arm circulation current is small, namely I can be considered as cir Same as before the failure; thus to the direct current i dc And an alternating current I ac sin(ωt+θ 1 ) Respectively carrying out fitting prediction to obtain the current waveforms of the bridge arms after the faults;
after the direct current side fault occurs, direct current at the outlet of the converter station can be directly measured, so that for direct current components of bridge arm current, fitting prediction can be directly carried out according to direct current measurement values at the outlet of the converter station;
after the fault of the direct current side occurs, the change of the injection current of the alternating current side is not only the amplitude I ac Of frequency ω and phase θ 1 Some degree of variation may also occur; in the control link of the converter, alternating current is converted into d and q axis components to participate in control, and the change conditions of the d and q axis components are similar to the change conditions of direct current; therefore, in the fitting prediction of the alternating current, the measured alternating current can be converted into the form of the d-axis component and the q-axis component of the alternating current through park transformation, and then the fitting prediction of the alternating current is carried out on i d 、i q Respectively carrying out fitting prediction, and finally obtaining a fitting prediction result of the alternating current side current through park inverse transformation;
during the first period of time after the DC side fault occurs, the DC current i dc And d, q-axis components i of the alternating current d 、i q The slope of the linear gradient is not strictly linear increase, and is continuously changed;
a second-order polynomial fitting method is adopted to respectively carry out fitting prediction on a direct current component and an alternating current component in the fault current of the bridge arm, and because the change of the fault current acceleration rate has continuity, a mobile data window is adopted to carry out the fitting prediction on the fault current;
first according to the latest T nh Carrying out curve fitting on the sampled data of the window length to obtain an expression of fault current, and then obtaining a subsequent length t according to the expression yc The fault current value of (a); the time length t of the fault current waveform is predicted by combining a protection action time calculation formula provided by the formula (5) yc Is set to t yc ＝t cb +t yd I.e. the time length after the current moment is obtained by fitting prediction every time is t yc The bridge arm current value of (1); if the predicted current value reaches the converter station locking condition, a tripping signal is sent out immediately; and if the predicted waveform does not reach the converter station locking condition, not sending a trip signal, and performing next prediction after the next sampling data is obtained.
The invention has the advantages and beneficial effects that:
compared with the prior art, the single-terminal quantity protection method applicable to the multi-terminal flexible direct current system is characterized in that protection action time is calculated based on the locking time of the converter station, and further, each protection can selectively remove a fault line through the cooperation of the action time; in addition, the invention also fully considers the matching problem of the protection action and the converter station locking, ensures that the converter station locking condition can not occur during the fault isolation period, and is beneficial to the rapid recovery of the power supply of the power grid after the fault is cleared.
Drawings
FIG. 1 is a schematic diagram of a four-terminal DC grid structure;
FIG. 2 is a schematic diagram illustrating the direction of the fault current variation of the DC line under different fault types;
FIG. 3 is a schematic diagram of a half-bridge MMC converter topology;
fig. 4 is a schematic diagram of pre-trip circuit breaker selection criteria;
FIG. 5 is a schematic diagram of a fault current prediction principle based on a moving data window;
FIG. 6 is a comparison graph of predicted values and actual values of bridge arm fault currents;
fig. 7 is a flow chart of protection logic based on the lockout time of the converter station.
Detailed Description
The present invention is further illustrated by the following specific examples, which are intended to be illustrative, not limiting and are not intended to limit the scope of the invention.
The invention provides a single-end quantity protection method for a multi-end flexible direct current system aiming at a multi-end flexible direct current power grid constructed by a modular multi-level converter, analyzes the fault characteristics of direct current line current and bridge arm current after a direct current line of the flexible direct current power grid fails, and provides a fault current direction criterion to select a pre-trip circuit breaker; and setting a protection action time margin according to the action delay of the circuit breaker so as to determine reasonable protection action time through the locking time of the converter station. The method can realize accurate identification and rapid isolation of the fault line on the premise of ensuring that the converter station is not locked, does not depend on line boundary elements, and has strong transition resistance. The method comprises the following steps:
s1, analyzing the directional characteristic of a direct current line fault current and the fault characteristic of a bridge arm current after a fault occurs on a direct current side of a flexible direct current power grid fault
(1) DC line fault current direction characteristic
Based on the four-terminal flexible direct-current system model shown in fig. 1, for a flexible direct-current power grid with true bipolar wiring, no matter a bipolar short circuit, a pole-to-metal return line short circuit or a unipolar ground short circuit fault occurs on a direct-current side, a fault pole MMC can form a fault loop on the direct-current side through a fault pole, a metal return line or the ground before the converter station is locked.
After a fault occurs in the multi-end flexible direct current system adopting the true bipolar connection mode, the direction of the fault current variation on the direct current side line of the converter station at any end is shown in fig. 2. When the positive earth fault or the bipolar short-circuit fault occurs on the line, the direction of the fault current variation on the positive line is from the bus to the line; when a negative earth fault or a bipolar short-circuit fault occurs on the line, the direction of the fault current variation on the negative line is from the line to the bus. Therefore, the positive direction of the fault current variation of the positive line is from the bus to the line, and the positive direction of the fault current variation of the negative line is from the line to the bus, so that the fault current variations on both sides of the fault line are positive no matter whether the positive line or the negative line has a fault.
In practical engineering, when the length difference of each section of the direct current power grid is not very large, the impedance difference of each section of the direct current power grid is not very large. For any bus, if a fault occurs on one outgoing line of the bus, the situation that the directions of the fault current change quantities on the two outgoing lines of the bus are the same does not occur. For the converter station connected with the bus, the direction of the fault current variation measured on the fault line is determined to be positive, and the direction of the fault current variation measured on the other line is determined to be negative. Therefore, when the directions of the variation amounts of the fault currents on the two outgoing lines of a certain bus are the same, it can be considered that the line connected to the bus is not necessarily a fault line.
(2) Bridge arm current fault characteristic
A typical half-bridge MMC converter topology is shown in fig. 3, and the half-bridge MMC converter is composed of three symmetrical phase units, each phase includes an upper bridge arm and a lower bridge arm, and each bridge arm is formed by connecting a plurality of identical sub-modules and bridge arm reactors in series.
Research has shown that the MMC bridge arm current consists of three parts, namely, an alternating current side current, a direct current side current and a bridge arm circulating current, during normal operation. Upper and lower bridge arm current i jp 、i jn (j = a, b, c) can be respectively expressed as:
in the formula: I.C. A ac Is the amplitude of the AC side current, I cir Is the amplitude of the bridge arm circulating current.
After a fault, sub-module capacitors in each converter station are quickly discharged to a fault point, so that the fault current of a direct-current line is rapidly increased, and the direct-current component of the bridge arm current is rapidly increased. Meanwhile, the equivalent voltage of the bridge arm is changed due to the discharge of the sub-module capacitor, and further the injection current at the alternating current side is changed.
After a dc-side fault occurs, the change in the current of each arm of the converter station is mainly caused by the change in the dc component and the change in the injected ac-side current. The proportion of the bridge arm circulating current in the bridge arm current is small relative to the direct current and the alternating current, and the amplitude of the bridge arm circulating current is not changed greatly in a short time before and after a fault, so that the amplitude of the bridge arm circulating current can be approximately considered to be not changed in a few milliseconds after the fault occurs.
After a fault occurs on the direct current side, the rectifier station and the converter station show different fault characteristics: the amplitudes of the direct current and the alternating current of the rectifier station after the fault are continuously increased, and the amplitudes of the direct current and the alternating current of the inverter station after the fault are subjected to the processes of firstly reducing and then reversely increasing.
S2, aiming at direct-current line faults occurring in the multi-terminal flexible direct-current power grid, a single-terminal quantity protection scheme based on converter station blocking time is provided
Firstly, selecting a pre-trip circuit breaker according to the fault current variation direction; secondly, the action time of the protection is determined according to the locking time of the converter station. On the basis of correctly selecting the pre-trip circuit breaker, the protection of each section of circuit can complete the removal of the fault circuit by the cooperation of action time.
(1) Fault current direction criterion and pre-trip circuit breaker selection
The current change quantity is adopted to reflect the change condition of fault current, and the positive direction selection is related to the polarity of the current flowing through the line, namely, the positive direction of the current change quantity of the positive line flows from the bus to the line, and the positive direction of the current change quantity of the negative line flows from the line to the bus. The current change rate is defined as follows:
in the formula i k And i k-1 Representing adjacent current sample values within a sample interval deltat.
And defining a current change direction index S which represents the fault current change direction measured by the end part of the direct current circuit.
S =1 represents that the direction of the current variation is positive, when the result of expression (2) is greater than 0;
s = -1 represents that the direction of the current change amount is negative, and the result of the formula (2) is smaller than 0;
s =0 represents that no current change is detected, when the result of equation (2) is equal to 0.
Considering the normal fluctuation of the current on the direct current line when the system normally operates, when the absolute value of the detected current change rate exceeds a setting value k set And in time, the direction index changes, so the following fault current change quantity direction criterion is provided:
by combining the analysis of the characteristics of the fault current of the direct current line, each converter station can select the pre-trip circuit breaker according to the direction index S of the fault current variation measured at the end of the line, and a schematic diagram of the selection criterion of the pre-trip circuit breaker is shown in fig. 4. In FIG. 4, S im 、S in Are respectively a circuit breaker CB im 、CB in And (3) extracting the selection criterion of the pre-trip circuit breaker shown in the formula (4) for each converter station according to the detected fault current variation direction index:
(2) Protection action time calculation based on converter station lockout time
The protection action time of each converter station depends on the blocking time of the converter station on the basis of the determination of the pre-trip circuit breaker. Under the premise of ensuring that the converter station is not locked, the protection algorithm determines the protection action time under the condition of considering the action delay and the action time margin of the circuit breaker, and the specific calculation method is as shown in the formula (5):
t act ＝t block -t cb -t yd (5)
in the formula: t is t block For the blocking time of the converter station, t cb Delay for opening of circuit breaker, t yd To protect the action time margin.
Considering the action delay of the circuit breaker and the matching problem of the circuit breakers at two sides of the circuit, t is yd Is set to be equal to t cb Equal, the cooperation of each section of protection can be guaranteed to the greatest extent.
Considering that bridge arm current of the inverter station has different fault characteristics from those of the rectifier station, a following unification processing method is provided for the inverter station, virtual locking time of the inverter station is defined, and the fault severity can be well reflected. And (3) respectively analyzing and processing the AC and DC components of the bridge arm current of the inverter station on the assumption that the circulating current amplitude of the bridge arm is not changed after the fault occurs.
In order to enable the inverter station to show the effect of continuously increasing the received active power after the fault, the variable quantity of the alternating current after the fault is processed in a reverse way, namely the variable quantity is obtained by subtracting the value after the fault from twice of the normal operation value, and the expression of the direction change processing is shown as the formula (6):
i ac '＝2i ac0 -i ac (6)
in the formula: i.e. i ac Is a virtual alternating current i ac0 For normal operation with alternating current, i ac Is the actual ac current after the fault.
The amplitude of the fault current on the virtual alternating current side of the inverter station obtained after the direction change processing after the fault is continuously increased and is consistent with the change trend of the current on the alternating current side of the rectifier station under the same initial current.
In order to make the direct current of the inverter station show the same effect of continuous increase as that of the rectifier station in consideration of the transmission condition of the direct-current side power, the direct-current side power converter is used for the i dc And (ii) processing method ac Similarly, the direction-changing processing expression is shown as formula (7):
i dc '＝2i dc0 -i dc (7)
in the formula: i.e. i dc ' is a modified direct current, i dc0 Is direct current before failure.
S3, waveform fitting prediction based on the mobile data window is carried out, and the converter station locking time is predicted in real time through fault data by each converter station
Based on the analysis of the fault characteristics of the bridge arm currents, after the direct-current side fault occurs, the change of each bridge arm current is mainly caused by a direct-current component and an alternating-current component, the change of the bridge arm circulation current is small, and the Icir is considered to be the same as that before the fault. Therefore, the direct current idc and the alternating current Iacsin (ω t + θ 1) are respectively subjected to fitting prediction, and the bridge arm current waveform after the fault can be obtained.
After the direct current side fault occurs, the direct current at the outlet of the converter station can be directly measured, so that the direct current component of the bridge arm current can be directly subjected to fitting prediction according to the direct current measured value at the outlet of the converter station.
After the fault occurs on the direct current side, the change of the injection current on the alternating current side not only changes the amplitude Iac, but also changes the frequency ω and the phase θ 1 to a certain extent. In the control link of the converter, alternating current is converted into d and q axis components to participate in control, and the change conditions of the d and q axis components are similar to the change conditions of direct current. Therefore, in the fitting prediction of the alternating current, the measured alternating current can be converted into the forms of the components of the d and q axes of the alternating current through park transformation, then the id and iq are respectively subjected to fitting prediction, and finally the fitting prediction result of the alternating current side current is obtained through park inverse transformation.
During the first period of time after the dc-side fault occurs, the dc current idc and the d and q-axis components id and iq of the ac current do not increase exactly linearly, and their slopes change constantly. Therefore, a second-order polynomial fitting method is adopted to respectively carry out fitting prediction on the direct current component and the alternating current component in the fault current of the bridge arm. Due to the fact that the change of the fault current acceleration rate has consistency, the fault current fitting prediction is carried out by adopting the moving data window.
The basic principle of the method is that a basic data window used for prediction is adjusted in real time along with the updating of sampling data, the follow-up waveform fitting prediction is carried out according to fault current data in the latest data window, and then locking condition judgment is carried out to determine the action time of protection.
A schematic of the process is shown in figure 5. First according to the latest T nh Carrying out curve fitting on the sampled data of the window length to obtain an expression of fault current, and then obtaining a subsequent length t according to the expression yc The fault current value of (a). The protection action time calculation formula provided by the combination formula (5) predicts the fault current waveform time length t yc Is set to t yc ＝t cb +t yd I.e. the time length t after the current moment is obtained by fitting prediction every time yc The bridge arm current value of (1). If the predicted current value reaches the converter station locking condition, a tripping signal is sent out immediately; and if the predicted waveform does not reach the converter station locking condition, not sending a tripping signal, and performing next prediction after the next time sampling data is obtained.
As shown in fig. 6 a), b) and c), the predicted values of the bridge arm currents obtained by the prediction method based on the moving data window are compared with the real values at 3ms, 5ms and 7ms after the fault. It can be seen that the more the predicted time is close to the locking time, the more accurate the predicted value of the bridge arm current is. Compared with a method for predicting for a long time by adopting a fixed data window, the prediction method based on the mobile data window has higher precision, and can obtain more accurate locking time and protection action time. And when the fault current acceleration rate changes due to the breaker action and other conditions, the fault current prediction method based on the mobile data window can effectively predict the latest fault current acceleration rate.
The method for predicting the fault current waveform fitting based on the change data window is combined with the protection scheme based on the converter station blocking time and the method for processing the fault current waveform of the inverter station, so that a protection logic flow chart is obtained, which is shown in fig. 7.
Although the embodiments of the present invention and the accompanying drawings are disclosed for illustrative purposes, those skilled in the art will appreciate that: various substitutions, changes and modifications are possible without departing from the spirit and scope of the invention and the appended claims, and therefore the scope of the invention is not limited to the disclosure of the embodiments and the accompanying drawings.
Claims (1)
1. A single-ended quantity protection method suitable for a multi-ended flexible direct current system is characterized by comprising the following steps: the method comprises the following steps:
s1, analyzing the directional characteristic of a direct current line fault current and the fault characteristic of a bridge arm current after a fault occurs on a direct current side of a flexible direct current power grid fault;
(1) DC line fault current direction characteristic
The method comprises the steps that a four-terminal flexible direct current system model is used as a basis for analysis, and for a flexible direct current power grid of a true bipolar wiring line, no matter a bipolar short circuit, a pole-to-metal return line short circuit or a unipolar grounding short circuit fault occurs on a direct current side, a fault pole MMC can form a fault loop on the direct current side through a fault pole, the metal return line or the ground before a converter station is locked;
when a positive grounding fault or a bipolar short-circuit fault occurs on the line, the direction of the fault current variation on the positive line is from the bus to the line; when a negative earth fault or a bipolar short-circuit fault occurs on the line, the direction of the fault current variation on the negative line is from the line to the bus;
therefore, the positive direction of the fault current variation of the positive electrode line is from the bus to the line, and the positive direction of the fault current variation of the negative electrode line is from the line to the bus, so that the fault current variation of both sides of the fault line is positive no matter the positive electrode line or the negative electrode line has a fault;
in practical engineering, when the length difference of each section of the line of the direct-current power grid is not very large, the impedance difference of each section of the line is not very large, and for any bus, if a fault occurs on one outgoing line of the bus, the situation that the fault current variation directions on two outgoing lines of the bus are the same cannot occur; for the converter station connected with the bus, the direction of the fault current variation on the fault line measured by the converter station is determined to be positive, and the direction of the fault current variation on the other line is determined to be negative; therefore, when the directions of the fault current variation quantities on the two outgoing lines of a certain bus are the same, the line connected with the bus is not necessarily a fault line;
(2) Bridge arm current fault characteristic
In a typical half-bridge MMC converter topological structure, a half-bridge MMC converter consists of three symmetrical phase units, each phase comprises an upper bridge arm and a lower bridge arm, and each bridge arm is formed by connecting a plurality of identical sub-modules and bridge arm reactors in series;
the MMC bridge arm current consists of three parts, namely alternating current side current, direct current side current and bridge arm circulating current, and upper and lower bridge arm currents i jp 、i jn (j = a, b, c) may be respectively expressed as:
in the formula: i is ac Is the amplitude of the AC side current, I cir Is the bridge arm circulating current amplitude;
after a fault, sub-module capacitors in each converter station quickly discharge to a fault point, so that fault current of a direct-current line rapidly rises, direct-current components of bridge arm current rapidly increase, and meanwhile, the equivalent voltage of a bridge arm is changed due to discharge of the sub-module capacitors, so that the alternating-current side injection current is changed;
after the fault of the direct current side occurs, the change of the current of each bridge arm of the converter station is mainly caused by the change of a direct current component and the change of an injection current of the alternating current side; the proportion of the bridge arm circulating current in the bridge arm current is smaller than that of the direct current and the alternating current, and the amplitude of the bridge arm circulating current is not changed greatly in a short time before and after a fault, so that the amplitude of the bridge arm circulating current can be approximately considered to be not changed in a few milliseconds after the fault occurs;
after a fault occurs on the direct current side, the rectifier station and the converter station show different fault characteristics: the amplitude values of the direct current and the alternating current of the rectifying station after the fault are continuously increased, and the amplitude values of the direct current and the alternating current of the inverter station after the fault are subjected to the processes of firstly reducing and then reversely increasing;
s2, aiming at the direct-current line fault generated by the multi-terminal flexible direct-current power grid, a single-terminal quantity protection scheme based on the converter station blocking time is provided;
firstly, selecting a pre-trip circuit breaker according to the fault current variation direction; secondly, determining the action time of protection according to the locking time of the converter station, and on the basis of correctly selecting a pre-trip circuit breaker, protecting each section of circuit to complete the removal of a fault circuit by the cooperation of the action time;
(1) Fault current direction criterion and pre-trip circuit breaker selection
Adopt the change situation of current variation volume reaction fault current, its positive direction is selected and is related to with the circuit polarity that the electric current flows through, and the current variation volume positive direction of anodal circuit is from the bus flow direction circuit promptly, and the current variation volume positive direction of negative pole circuit is from the circuit flow direction bus, and the definition electric current rate of change is as follows:
in the formula: i.e. i k And i k-1 Representing adjacent current sample values within a sample interval Δ T;
defining a current variation direction index S which represents the fault current variation direction measured by the end part of the direct current circuit;
s =1 represents that the direction of the current variation is positive, and the result of equation (2) is greater than 0;
s = -1 represents that the direction of the current change amount is negative, and the result of the formula (2) is smaller than 0;
s =0 represents that no current change is detected, when the result of equation (2) equals 0;
considering the normal fluctuation condition of the current on the direct current line when the system normally operates, when the absolute value of the detected current change rate exceeds the setting value k set And in time, the direction index changes, and the following fault current change quantity direction criterion is provided:
by combining the analysis of the fault current characteristics of the direct current line, each converter station can select the pre-trip circuit breaker according to the fault current variation direction index S measured at the end part of the line, and the pre-trip circuit breaker selection criterion S im 、S in Are respectively a circuit breaker CB im 、CB in And (3) extracting the selection criterion of the pre-trip circuit breaker shown in the formula (4) for each converter station according to the detected fault current variation direction index:
(2) Protection action time calculation based on converter station lockout time
On the basis of determining the pre-tripping circuit breaker, the protection action time of each converter station depends on the locking time of the converter station, on the premise of ensuring that the converter stations are not locked, the protection action time is determined under the condition of considering the action delay and the action time margin of the circuit breaker, and the specific calculation method is as shown in formula (5):
t act ＝t block -t cb -t yd (5)
in the formula: t is t block Is the lock-out time of the converter station;
t cb delaying the opening of the circuit breaker;
t yd a protection action time margin;
considering the action delay of the circuit breaker and the matching problem of the circuit breakers at two sides of the circuit, t is yd Is set to be and cb equality, the coordination of each section of protection can be ensured to the maximum extent;
considering that bridge arm current of the inverter station has different fault characteristics from those of the rectifier station, a unified processing method is provided for the inverter station, virtual locking time of the inverter station is defined, and the fault severity can be well reflected; assuming that the circulating current amplitude of the bridge arm is not changed after the fault occurs, respectively analyzing and processing the AC and DC components of the bridge arm current of the inverter station;
in order to enable the inverter station to show the effect of continuously increasing the received active power after the fault, the variable quantity of the alternating current after the fault is processed in a reverse way, namely the value after the fault is subtracted from the double normal operation value, and the expression of the direction change processing is shown as the formula (6):
i ac '＝2i ac0 -i ac (6)
in the formula: i.e. i ac ' is a virtual alternating current;
i ac0 alternating current for normal operation;
i ac actual alternating current after the fault;
the amplitude of the fault current on the virtual alternating current side of the inverter station obtained after the direction change processing after the fault is continuously increased and is consistent with the change trend of the current on the alternating current side of the rectifier station under the same initial current;
in order to make the direct current of the inverter station show the same effect of continuous increase as that of the rectifier station in consideration of the transmission condition of the direct-current side power, the direct-current side power converter is used for the i dc And i ac Similarly, the direction-changing processing expression is shown as formula (7):
i dc '＝2i dc0 -i dc (7)
in the formula: i.e. i dc ' is a modified direct current;
i dc0 the direct current is the direct current before the fault;
s3, predicting waveform fitting based on the mobile data window, and predicting the locking time of the convertor station in real time through fault data by the convertor station;
based on the analysis of the fault characteristics of the bridge arm currents, after the direct-current side fault occurs, the change of each bridge arm current is mainly caused by a direct-current component and an alternating-current component, the change of the bridge arm circulation current is small, namely I can be considered as cir Same as before the failure; thus to the direct current i dc And an alternating current I ac sin(ωt+θ 1 ) Respectively carrying out fitting prediction to obtain the current waveforms of the bridge arms after the faults;
after the direct current side fault occurs, the direct current at the outlet of the converter station can be directly measured, so that the direct current component of the bridge arm current can be directly subjected to fitting prediction according to the direct current measurement value at the outlet of the converter station;
after the fault of the direct current side occurs, the change of the injection current of the alternating current side is not only the amplitude I ac Of frequency ω and phase θ 1 Some degree of variation may also occur; in the control link of the converter, alternating current is converted into d and q axis components to participate in control, and the change conditions of the d and q axis components are similar to the change conditions of direct current; therefore, in the fitting prediction of the alternating current, the measured alternating current can be converted into the form of the d-axis component and the q-axis component of the alternating current through park transformation, and then the fitting prediction of the alternating current is carried out on i d 、i q Respectively carrying out fitting prediction, and finally obtaining a fitting prediction result of the alternating current side current through park inverse transformation;
during the first period of time after the DC side fault occurs, the DC current i dc And d, q-axis components i of the alternating current d 、i q The slope of the linear gradient is not strictly linear increase, and is continuously changed;
a second-order polynomial fitting method is adopted to respectively carry out fitting prediction on a direct current component and an alternating current component in the fault current of the bridge arm, and because the change of the fault current acceleration rate has continuity, a mobile data window is adopted to carry out the fitting prediction on the fault current;
first according to the latest T nh Carrying out curve fitting on the sampled data of the window length to obtain an expression of fault current, and then obtaining a subsequent length t according to the expression yc The fault current value of (a); the time length t of the fault current waveform is predicted by combining a protection action time calculation formula provided by the formula (5) yc Is set to t yc ＝t cb +t yd I.e. the time length t after the current moment is obtained by fitting prediction every time yc The bridge arm current value of (1); if the predicted current value reaches the converter station locking condition, a tripping signal is sent out immediately; and if the predicted waveform does not reach the converter station locking condition, not sending a trip signal, and performing next prediction after the next sampling data is obtained.
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