CN115453265B - Island micro-grid fault transient protection method based on initial fault current waveform characteristics - Google Patents
Island micro-grid fault transient protection method based on initial fault current waveform characteristics Download PDFInfo
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Abstract
The island micro-grid fault transient protection method based on the initial fault current waveform characteristics comprises the steps of firstly sampling currents on two sides of a protected line in an island micro-grid system, then calculating a difference value between the currents on two sides at the current moment and the current at the moment corresponding to the previous power frequency period and a first-order change gradient sum of the currents, judging whether the micro-grid system has faults, if so, acquiring current data of the initial fault, sequentially calculating an expansion corrosion differential value, a bias coefficient and an energy value of the initial fault current, judging whether the product of the bias coefficients of the expansion corrosion differential values of the initial fault currents on two sides is larger than zero, if so, judging that the line is a fault line with micro sources connected on two sides and j phase is a fault phase, if not, judging that the line is a fault line with micro sources connected on only one side and j phase is a fault phase, and if not, judging that the line is a non-fault phase or a fault line of the fault line.
Description
Technical Field
The invention relates to the technical field of power systems, in particular to an island micro-grid fault transient protection method based on the initial fault current waveform characteristics.
Background
As an effective way and a way for solving the problem of reliable power supply of an offshore island, an island micro-grid has new energy consumption and independent operation capability, is far away from a land power supply center, and only depends on distributed micro-sources such as diesel, wind power, photovoltaic and the like in the micro-grid to supply power to a load. The special geographic position determines that island micro-grids can face severe environmental challenges such as lightning storm, low temperature and the like at any time, so that system short-circuit faults frequently occur. However, because island micro-grid lacks large grid support, has characteristics such as system inertia is weak, the damping is little, power electronic converter overcurrent ability is poor, and island micro-grid short circuit fault bearing ability is poor, and the problem that important equipment burns out easily appears even the system is unstability, consequently need the study quick reliable island micro-grid short circuit fault protection technique of urgency.
After the island micro-grid has a short-circuit fault, corresponding fault ride-through control strategies are started by inverter type micro-sources such as wind power and photovoltaic, the output current of the inverter is limited within 1.5-2 times of rated current, the switching influence of the inverter control strategies is considered, the inverter type micro-source fault transient response process contains rich fault information, the fault is usually within 5ms after the fault, and the fault transient information introduced by the inverter type micro-source can be utilized to realize rapid fault detection. The existing micro-grid protection method is mostly based on fault steady-state information such as current amplitude, phase and power direction, fault detection and discrimination can be completed usually only by at least one power frequency period, and the control of the amplitude and phase of the output current of the inverter by a fault ride-through control strategy can lead to the fact that the difference of internal and external fault steady-state characteristics of a micro-grid system area is smaller, so that the protection selectivity and rapidity requirements of the micro-grid system are hardly met. Therefore, the invention utilizes the transient state characteristics of the initial current of the fault to construct a protection criterion and provides the island micro-grid fault transient state protection method based on the waveform characteristics of the initial current of the fault.
Disclosure of Invention
The invention aims to provide an island micro-grid fault transient protection method based on initial fault current waveform characteristics, which can rapidly distinguish fault lines and fault phases.
In order to solve the technical problems, the invention adopts the following technical methods: an island micro-grid fault transient protection method based on initial fault current waveform characteristics comprises the following steps:
step S1, respectively defining two sides of a protected circuit in an island micro-grid system as an e side and an f side, respectively collecting current signals of one side of the protection device in real time by the protection device of the e side and the f side, storing current data in a power frequency period, and generating a current sampling value sequence i ej (k) And i fj (k) J represents a, b and c phases, and k represents a kth sampling value;
step S2, calculating the difference value delta i between the current phase current at the moment j of the protected line e side and the current phase current at the moment j corresponding to the previous power frequency period ej (k) And the difference delta i between the current phase current at the moment j of the f side and the current phase current at the moment j corresponding to the previous power frequency period fj (k)；
Step S3, calculating the current difference Deltai obtained in step S2 by using the following formula (1) ej (k) First order variation gradient of (2) and K ej (k) Current difference Δi fj (k) First order variation gradient of (2) and K fj (k) Judging the first-order change gradient and K ej (k)、K fj (k) Whether or not they are all greater than threshold value K set If not, go to step S1; if yes, the protection devices on the e side and the f side of the protected line in the island micro-grid system mark the current moment as the fault moment, and current data of the e side and the f side of the protected line in x power frequency periods after the fault is acquired;
wherein Q represents the number of sampling points;
step S4, calculating the differential value Deltaf of expansion operation of the protected line e-side current in x power frequency periods after the fault by using mathematical morphological expansion and corrosion operation and a formula (3) dile_j (n) and Corrosion operation differential valueΔf eroe_j (n) expansion operation differential value Δf of f-side current dilf_j (n) and the Corrosion operation differential value Δf erof_j (n) the mathematical morphological expansion and corrosion operation is defined as the following formula (2), and the differential value f of the expansion and corrosion operation of the current at the e side of the protected line in x power frequency periods after the fault is obtained according to the formula (4) grade_j (n) differential value f of expansion corrosion operation of f-side current gradf_j (n) finally calculating the differential value f of the expansion corrosion operation according to the formula (5) grade_j Skewness coefficient S of (n) fgrade_j Differential value f of expansion corrosion operation gradf_j Skewness coefficient S of (n) fgradf_j ；
Wherein f (n) represents an input signal, 0.ltoreq.n<N and N are the length of input signals, g (m) represents a structural element, and m is more than or equal to 0<M is the length of the structural element, N is greater than M, and the symbolRepresenting mathematical morphological dilation operations,/->Representing mathematical morphological erosion operations;
Δf dil (n)＝f dil (n)-f dil (n-1)；Δf ero (n)＝f ero (n)-f ero (n-1) (3)
wherein μ is the differential value f of the expansion corrosion operation grad An average value of (n),sigma is the differential value f of expansion corrosion operation grad (n) standard deviation, < >>
S5, calculating energy values of e-side and f-side currents of the protected line in x power frequency periods after faults according to the following formula (6);
wherein E is ej And E is fj Respectively representing the energy value of j-phase current at the e side of a protected line and the energy value of j-phase current at the f side of the protected line in x power frequency periods after faults;
step S6, starting the protection criterion #1, judging the differential value f of the expansion corrosion operation of the current at the e side of the protected line within 5ms after the fault grade_j Skewness coefficient S of (n) fgrade_j Differential value f of expansion corrosion operation of f-side current gradf_j Skewness coefficient S of (n) fgradf_j If the product of the two phases is larger than zero, judging that the protected circuit is a fault circuit with both sides connected with an inverter type micro source and the j phase is a fault phase; if the energy value of the current on the E side and the energy value of the current on the f side of the protected line within 5ms after the fault are judged by starting the protection criterion #2 if the energy value of the current on the E side and the energy value of the current on the f side are smaller than zero ej -E fj Whether or not is far greater than the energy threshold E set If yes, judging that the protected circuit is a fault circuit with one side connected with an inverter type micro source and j phases are fault phases; if not, the protected line is determined to be a non-faulty line or a non-faulty phase of a faulty line.
Step S7, the protection devices on the e side and the f side of the protected line in the island micro-grid system start protection measures according to the judging result in the step S6, specifically: if the protection criterion #1 is met in the step S6, judging that the protected circuit is a fault circuit with both sides connected with an inverter type micro source and j phases are fault phases, respectively sending tripping signals to the j-phase circuit breakers on one side of the protection circuit by the protection devices on the e side and the f side of the protection circuit to realize split-phase tripping; if the protection criterion #1 is not met but the protection criterion #2 is met in the step S6, the protected circuit is judged to be a fault circuit with one side connected with an inverter type micro source and the j phase is judged to be a fault phase, then the protection devices on the e side and the f side of the protected circuit respectively send tripping signals to the j-phase circuit breaker on one side of the protected circuit, and split-phase tripping is realized; if both protection criteria #1 and #2 in step S6 are not satisfied, it is determined that the protected line is a non-faulty line or a non-faulty phase of the faulty line, the protection device of the protected line will not send a trip signal to the j-phase circuit breaker thereof.
Further, in step S1, the sampling frequency of the protection devices on the e side and the f side of the protected line in the island micro-grid system is 10kHz, that is, 200 sampling points are collected within 20ms of one power frequency period.
Further, the x power frequency periods are one quarter of the power frequency period, namely 5ms.
Further, in step S3, the current difference Δi is calculated using equation (1) ej (k) First order variation gradient of (2) and K ej (k) Current difference Δi fj (k) First order variation gradient of (2) and K fj (k) The sampling point number Q is 10.
Still further, the threshold value K in step S3 set 0.01.
Preferably, the energy threshold E in step S6 set Is 10 times the minimum value of the current energy on both sides of the protected line within 5ms after the fault.
According to the characteristics that after the island micro-grid system has a short circuit fault, each inverter type micro-source outputs fault current with phase jump at the initial stage of the fault, the fault current flows through each feeder line feed-in fault point, and the phase jump directions of the fault initial currents at two sides of a fault line and a non-fault line are different, the island micro-grid fault transient protection method based on the waveform characteristics of the fault initial currents is provided. According to the method, the deviation coefficient of the differential value of the mathematical morphological dilation corrosion operation is used for representing the phase jump direction of the initial fault current, and meanwhile, the initial fault current energy value is used for representing the initial fault current amplitude condition, so that when the island micro-grid has a short-circuit fault, the protection devices on the two sides of the protected line can judge the fault line and the fault phase. The method only needs to exchange the deviation coefficient of the differential value of the initial fault current expansion corrosion operation on the two sides of the protected line and the initial fault current energy value, has small communication data volume and has lower requirements on communication bandwidth and speed. And after the micro-grid system breaks down, an inverter type micro-source starts a current limiting control strategy to conduct current suppression within 5ms after the fault, namely, the output current of the inverter type micro-source can generate a phase jump phenomenon within 5ms after the fault, so that the fault line and the fault phase can be rapidly judged after the fault occurs, and further, the protection devices on the two sides of the fault line can rapidly cut off the circuit breaker of the fault phase of the fault line so as to realize fault transient protection.
Drawings
FIG. 1 is a schematic diagram of a typical island micro-grid system;
FIG. 2 is a waveform of current across a line at the early stage of a fault in an island microgrid; (a) The two sides are connected with fault phase current waveforms at the two sides of a fault line when an inverter type micro source is connected; (b) Only one side is connected with fault phase current waveforms at two sides of a fault line when an inverter type micro source is connected; (c) a current waveform on both sides of the non-faulty line;
FIG. 3 is a differential value of the dilation-erosion operation of the initial fault current in the island micro-grid; (a) Expansion corrosion operation differential values of fault phase current waveforms on two sides of a fault line when the inverter type micro source is connected to two sides; (b) Only one side is connected with the differential value of the expansion corrosion operation of the fault phase current waveforms at the two sides of the fault line when the inverter type micro source is connected; (c) Expansion corrosion operation differential value of current waveforms at two sides of a non-fault line;
fig. 4 is a flowchart of an island micro-grid fault transient protection method based on the initial fault current waveform characteristics.
Detailed Description
The invention will be further described with reference to examples and drawings, to which reference is made, but which are not intended to limit the scope of the invention.
As shown in fig. 1, a typical island micro-grid system structure mainly comprises a photovoltaic power generation unit, a wind power generation unit, an energy storage system, a power load, a current measurement unit, a circuit breaker and other components, wherein a photovoltaic inverter adopts a PQ control strategy, the energy storage inverter adopts a constant frequency and constant voltage control strategy (Vf control), all inverter micro-sources are connected into a 10kV network through an interface transformer, the low-voltage side of the interface transformer is in a Y-type winding connection mode, so that power is conveniently supplied to a single-phase load, and the high-voltage side is in a delta-type winding connection mode.
For the photovoltaic inverter adopting the PQ control strategy, the photovoltaic inverter is in a unit power factor working state during normal operation, and the phase difference of output voltage and current of the photovoltaic inverter is 0 degrees. When the instantaneous value of any phase current is larger than the maximum allowable value (generally 1.2p.u. -2p.u.), the photovoltaic inverter starts a current limiting control strategy, reactive current and active current reference values of a current control loop are adjusted to reference values shown in (7) - (8), and the phase difference of output voltage and current of the photovoltaic inverter is between (0 DEG, 90 DEG).
Wherein i is * Ld And i * Lq Active and reactive current reference values, I, adjusted by limiting control strategy max At maximum allowable value, V of For outputting positive sequence voltage amplitude value, V for photovoltaic inverter after fault n Is the rated voltage value.
For the energy storage inverter adopting the Vf control strategy, the energy storage inverter is in a rated operation state during normal operation, the output voltage of the energy storage inverter can track the voltage reference value with zero error, and can be equivalently used as a constant voltage source with zero internal impedance. Because the line resistance of the 10kV micro-grid is larger, the phase difference of the output voltage and the output current of the energy storage inverter can be approximately considered to be equal to 0 degrees. When a short circuit fault occurs in a certain feeder line in the micro-grid, the system bus voltage drops, and when the instantaneous value of any one phase current is larger than the maximum allowable value (generally 1.2p.u. -2p.u.), the energy storage inverter starts a current limiting control strategy to switch the inverter from a Vf control mode to a current control mode, and the reactive current and the active current reference value of a current control loop are also shown in formulas (7) - (8), wherein the phase difference of the output voltage and the current of the energy storage inverter is between (0 DEG and 90 DEG).
After the island micro-grid system has short-circuit fault, each inverter type micro-source outputs fault current with phase jump at the initial stage of fault, and the fault current flows through each feeder line to feed into a fault point. For a fault line with both sides of the line connected with an inverter type micro source, the current directions of fault phases at both sides of the line are the same, namely the phase jump directions are the same; for a fault line with only one side of the line connected with an inverter type micro source, the phase jump directions of fault phase currents at two sides of the line are opposite, and the current at one side connected with the inverter type micro source is far greater than the current at the other side; the phase jump directions of the non-fault phase currents at the two sides of the fault line and the non-fault line are opposite, and the amplitudes of the currents at the two sides of the line are equal. The fault phase current waveforms at two sides of the fault line, only one side of the fault line and the non-fault line of which the two sides are connected with the inverter type micro source in the island micro grid are respectively shown in fig. 2 (a) - (c).
The invention provides an island micro-grid transient protection method based on the current waveform characteristics of the initial fault by utilizing the phase jump characteristic differences of the current waveforms of the initial fault line and the non-fault line, wherein the process is shown in fig. 4, and the specific steps are as follows:
step S1, two sides of a protected line in an island micro-grid system are respectively provided withProtection devices defined as an e side and an f side (because of the characteristics of a topological structure of a micro-grid system, bidirectional trend directions and the like, protection devices are arranged on both sides of a circuit in the existing micro-grid system) respectively collect current signals on one side of the protection devices in real time through a current measurement unit, store current data in a power frequency period and generate a current sampling value sequence i ej (k) And i fj (k) J represents a, b and c phases, k represents a kth sampling value, and the sampling frequency is 10kHz, namely 200 sampling points are acquired within 20ms of one power frequency period.
Step S2, calculating the difference value delta i between the current phase current at the moment j of the protected line e side and the current phase current at the moment j corresponding to the previous power frequency period ej (k) And the difference delta i between the current phase current at the moment j of the f side and the current phase current at the moment j corresponding to the previous power frequency period fj (k)。
Step S3, calculating the current difference Deltai obtained in step S2 by using the following formula (1) ej (k) First order variation gradient of (2) and K ej (k) Current difference Δi fj (k) First order variation gradient of (2) and K fj (k) Judging the first-order change gradient and K ej (k)、K fj (k) Whether or not they are all greater than threshold value K set If not, go to step S1; if yes, the protection devices on the e side and the f side of the protected line in the island micro-grid system mark the current moment as the fault moment, and current data of the e side and the f side of the protected line in 5ms after the fault is acquired. Current difference Δi in normal operating state ej (k)、Δi fj (k) All are zero, the first-order change gradient and K ej (k)、K fj (k) Is also equal to zero, and the micro-grid system is in fault moment delta i ej (k) And Δi fj (k) Sudden increase, K ej (k)、K fj (k) Greater than zero, thus the aforementioned threshold value K set Set to 0.01, avoiding the influence of measurement errors.
Where Q represents the number of sampling points, and is set to 10.
Step S4, expanding by mathematical morphologyAnd corrosion operation, and equation (3) calculates the differential value Deltaf of expansion operation of the protected line e-side current within 5ms after the fault dile_j (n) and the Corrosion operation differential value Δf eroe_j (n) expansion operation differential value Δf of f-side current dilf_j (n) and the Corrosion operation differential value Δf erof_j (n) the definition of mathematical morphological expansion and corrosion operation is as shown in the following formula (2), and the differential value f of expansion corrosion operation of the current at the e side of the protected line within 5ms after the fault is obtained according to the formula (4) grade_j (n) differential value f of expansion corrosion operation of f-side current gradf_j (n) finally calculating the differential value f of the expansion corrosion operation according to the formula (5) grade_j Skewness coefficient S of (n) fgrade_j Differential value f of expansion corrosion operation gradf_j Skewness coefficient S of (n) fgradf_j 。
Wherein f (n) represents an input signal, 0.ltoreq.n<N, N is the length of the input signal, here 50 sampling points, g (m) represents the structural element, 0.ltoreq.m<M is the length of the structural element, N is greater than M, and the symbolRepresents a mathematical morphological dilation operation and Θ represents a mathematical morphological erosion operation.
Δf dil (n)＝f dil (n)-f dil (n-1)；Δf ero (n)＝f ero (n)-f ero (n-1) (3)
Wherein μ is the differential value f of the expansion corrosion operation grad An average value of (n),sigma is the differential value f of expansion corrosion operation grad (n) standard deviation, < >>
Since the expansion and corrosion operations are the processes of expanding the boundary of the input signal to the outside and contracting the boundary of the input signal to the inside respectively, the differential operation of expansion and corrosion of the input signal can be used for edge detection, and the definition is shown as the formula (4), when deltaf dil (n) and Δf ero (n) when the values are all greater than zero, the differential value f of the expansion corrosion operation of the input signal grad (n) is equal to Δf dil (n) and Δf ero Subtracting the small value from the large value in (n), f grad (n) is positive, considering that the input signal is at a rising edge; when Deltaf dil (n) and Δf ero (n) each less than or equal to zero, the differential value f of the expansion corrosion operation of the input signal grad (n) is equal to Δf dil (n) and Δf ero Subtracting the large value from the small value in (n), f grad (n) is negative and the input signal is considered to be on a falling edge. As shown in fig. 3 (a) - (c), the differential values of the expansion corrosion operation corresponding to the fault line and the non-fault line can be obtained by the initial fault current passing formulas (2), (3) and (4), so that it can be seen that, for the fault line with the inverter type micro source connected to both sides of the line, the current phase jump directions of the fault phase current at both sides of the line are the same, and the distribution of the differential values of the expansion corrosion operation of the fault phase current waveforms at both sides of the line is biased to the negative half axis; for a fault line with only one side of the line connected with an inverter type micro source, the phase jump directions of fault phase currents at two sides of the line are opposite, the distribution conditions of expansion corrosion operation differential values of fault phase current waveforms at two sides of the line are opposite, one side of the fault line is biased to a positive half shaft, and the other side of the fault line is biased to a negative half shaft; for a non-fault line, no matter whether two sides are connected with micro sources or only one side is connected with micro sources, the current phase jump directions of two sides of the line are opposite, the expansion corrosion operation differential value distribution of the current waveform at one side of the line is biased to a positive half shaft, and the other side is biased to a negative half shaft. From this, we can know that the differential can be calculated by using the expansion corrosion of the currentThe value distribution situation judges the fault line and fault phase with micro source connected to the two sides of the line, and the calculation of the deviation coefficient can convert the positive and negative semi-axis distribution situation of the differential value (a group of data) of the expansion corrosion operation into a numerical value, the numerical value distribution deviates to the positive semi-axis, the deviation coefficient is larger than zero, the numerical value distribution deviates to the negative semi-axis, and the deviation coefficient is smaller than zero, so that the communication data quantity at the two sides of the line can be reduced, and the communication bandwidth requirement is reduced, therefore, after the differential value of the expansion corrosion operation of the line current is calculated in the step S4, the deviation coefficient of the differential value of the expansion corrosion operation is further calculated, and the method also becomes the basis of the subsequent structural protection criterion # 1. However, according to the analysis described in fig. 2 (a) - (c) and fig. 3 (a) - (c), we cannot distinguish "the fault line and the fault phase connected with the micro source only on one side of the line" and "the non-fault line connected with the micro source only on both sides" according to the differential value distribution condition of the expansion corrosion operation of the current, however, as shown in fig. 2 (a) - (c), for the fault line connected with the inverter type micro source only on one side of the line, the current phase jump directions of the fault phases on both sides of the line are opposite, and the current on the side connected with the inverter type micro source is far greater than the current on the other side; the phase jump directions of the non-fault phase currents at the two sides of the fault line and the non-fault line are opposite, and the amplitudes of the currents at the two sides of the line are equal. Accordingly, we can know that the two conditions of the differential value of the expansion corrosion operation of the current and the energy value of the current can be effectively partitioned, and for this purpose, step S5 is introduced, which provides a basis for constructing the protection criterion #2.
And S5, calculating the energy values of the e-side and f-side currents of the protected line within 5ms after the fault according to the following formula (6).
Wherein E is ej And E is fj The energy value of the j-phase current on the e side of the protected line and the energy value of the j-phase current on the f side of the protected line within 5ms after the fault are respectively represented.
And (5) integrating the steps S4 and S5, and pre-constructing a protection criterion #1 and a protection criterion #2 for distinguishing fault lines and fault phases of the island micro-grid.
Protection criterion #1: judging the differential value f of expansion corrosion operation of the current at the e side of the protected line within 5ms after the fault grade_j Skewness coefficient S of (n) fgrade_j Differential value f of expansion corrosion operation of f-side current gradf_j Skewness coefficient S of (n) fgradf_j If the product of the two phases is greater than zero, judging that the protected line is a fault line and the j phase is a fault phase; and if the protection criterion is smaller than zero, enabling the protection criterion #2.
Protection criterion #2: judging the difference (E) between the energy value of the E-side current and the energy value of the f-side current of the protected line within 5ms after the fault ej -E fj Whether or not is far greater than the energy threshold E set If yes, the protected circuit is judged to be a fault circuit and the j phase is judged to be a fault phase; if not, the protected line is determined to be a non-faulty line or a non-faulty phase of a faulty line.
Step S6, starting the protection criterion #1, judging the differential value f of the expansion corrosion operation of the current at the e side of the protected line within 5ms after the fault grade_j Skewness coefficient S of (n) fgrade_j Differential value f of expansion corrosion operation of f-side current gradf_j Skewness coefficient S of (n) fgradf_j If the product of the two phases is larger than zero, judging that the protected circuit is a fault circuit with both sides connected with an inverter type micro source and the j phase is a fault phase; if the energy value of the current on the E side and the energy value of the current on the f side of the protected line within 5ms after the fault are judged by starting the protection criterion #2 if the energy value of the current on the E side and the energy value of the current on the f side are smaller than zero ej -E fj Whether or not is far greater than the energy threshold E set If yes, judging that the protected circuit is a fault circuit with one side connected with an inverter type micro source and j phases are fault phases; if not, the protected line is determined to be a non-faulty line or a non-faulty phase of a faulty line. It is noted that the energy threshold E is used to avoid the influence of measurement errors, sampling synchronization errors and filter introduction errors set The value is 10 times of the minimum value of the initial current energy of faults at two sides of the line.
Step S7, the protection devices on the e side and the f side of the protected line in the island micro-grid system start protection measures according to the judging result in the step S6, specifically: if the protection criterion #1 is met in the step S6, judging that the protected circuit is a fault circuit with both sides connected with an inverter type micro source and j phases are fault phases, respectively sending tripping signals to the j-phase circuit breakers on one side of the protection circuit by the protection devices on the e side and the f side of the protection circuit to realize split-phase tripping; if the protection criterion #1 is not met but the protection criterion #2 is met in the step S6, the protected circuit is judged to be a fault circuit with one side connected with an inverter type micro source and the j phase is judged to be a fault phase, then the protection devices on the e side and the f side of the protected circuit respectively send tripping signals to the j-phase circuit breaker on one side of the protected circuit, and split-phase tripping is realized; if both protection criteria #1 and #2 in step S6 are not satisfied, it is determined that the protected line is a non-faulty line or a non-faulty phase of the faulty line, the protection device of the protected line will not send a trip signal to the j-phase circuit breaker thereof.
In summary, the protection devices on two sides of the protected line only need to exchange the deviation coefficient of the expansion corrosion operation differential value of the initial fault current and the initial fault current energy value, the communication data volume is small, and the requirements on the communication bandwidth and the speed are low. In addition, after the micro-grid system fails, the inverter type micro-source generally starts a current limiting control strategy to conduct current suppression within 5ms after the fault, namely, the output current of the inverter type micro-source can generate a phase jump phenomenon within 5ms after the fault, so that the fault line and fault phase can be judged within 5ms after the fault.
In order to verify the effectiveness of the invention, a 10kV island micro-grid system model shown in fig. 1 is built in PSCAD/EMTDC, the capacities of an inverter type micro-source photovoltaic 1, a photovoltaic 2 and a fan are all 500kW, the inverter adopts a PQ control strategy, the energy storage capacity is 500kVA, and the inverter adopts a Vf control strategy. The capacity of load 1 was (1+j0.03) MVA, and the capacities of load 2 and load 3 were (0.5+j0.015) MVA. All feeder lines are 2km in length, and the unit-length positive sequence impedance and zero sequence impedance of the line are respectively 0.265+j0.078 omega/km and 2.7+j0.318 omega/km.
In the micro-grid of fig. 1, when an AB interphase short-circuit fault occurs at the F1 position and the fault resistance is 1Ω, the morphological characteristic value and the energy value of the current within 5ms after the faults at the two sides of each feeder line and the discrimination results of the fault line and the fault phase are shown in table 1. As can be seen from the table, the differential value deviation coefficients of the expansion corrosion operation of the currents of the A phase and the B phase at two sides of the feeder line 23 meet the protection criterion #1, the current energy values of the A phase and the B phase meet the protection criterion #2, the differential value deviation coefficients of the expansion corrosion operation of the currents of the other two sides of the feeder line do not meet the protection criterion #1, and the current energy values at two sides do not meet the protection criterion #2, so that the method can accurately judge that the feeder line 23 is a fault line, the A phase and the B phase are fault phases, and the C phase is a non-fault phase.
TABLE 1
The foregoing embodiments are preferred embodiments of the present invention, and in addition, the present invention may be implemented in other ways, and any obvious substitution is within the scope of the present invention without departing from the concept of the present invention.
In order to facilitate understanding of the improvements of the present invention over the prior art, some of the figures and descriptions of the present invention have been simplified, and some other elements have been omitted from this document for clarity, as will be appreciated by those of ordinary skill in the art.
Claims (6)
1. The island micro-grid fault transient protection method based on the initial fault current waveform characteristics is characterized by comprising the following steps of:
step S1, respectively defining two sides of a protected circuit in an island micro-grid system as an e side and an f side, respectively collecting current signals of one side of the protection device in real time by the protection device of the e side and the f side, storing current data in a power frequency period, and generating a current sampling value sequence i ej (k) And i fj (k) J represents a, b and c phases, and k represents a kth sampling value;
step S2, calculating j-phase current and j-phase current at the current moment of the protected line e sideThe difference delta i of j-phase current at the corresponding moment of the previous power frequency period ej (k) And the difference delta i between the current phase current at the moment j of the f side and the current phase current at the moment j corresponding to the previous power frequency period fj (k)；
Step S3, calculating the current difference Deltai obtained in step S2 by using the following formula (1) ej (k) First order variation gradient of (2) and K ej (k) Current difference Δi fj (k) First order variation gradient of (2) and K fj (k) Judging the first-order change gradient and K ej (k)、K fj (k) Whether or not they are all greater than threshold value K set If not, go to step S1; if yes, the protection devices on the e side and the f side of the protected line in the island micro-grid system mark the current moment as the fault moment, and current data of the e side and the f side of the protected line in x power frequency periods after the fault is acquired;
wherein Q represents the number of sampling points;
step S4, calculating the differential value Deltaf of expansion operation of the protected line e-side current in x power frequency periods after the fault by using mathematical morphological expansion and corrosion operation and a formula (3) dile_j (n) and the Corrosion operation differential value Δf eroe_j (n) expansion operation differential value Δf of f-side current dilf_j (n) and the Corrosion operation differential value Δf erof_j (n) the mathematical morphological expansion and corrosion operation is defined as the following formula (2), and the differential value f of the expansion and corrosion operation of the current at the e side of the protected line in x power frequency periods after the fault is obtained according to the formula (4) grade_j (n) differential value f of expansion corrosion operation of f-side current gradf_j (n) finally calculating the differential value f of the expansion corrosion operation according to the formula (5) grade_j Skewness coefficient S of (n) fgrade_j Differential value f of expansion corrosion operation gradf_j Skewness coefficient S of (n) fgradf_j ；
Wherein f (n) represents an input signal, 0.ltoreq.n<N and N are the length of input signals, g (m) represents a structural element, and m is more than or equal to 0<M is the length of the structural element, N is greater than M, and the symbolRepresenting mathematical morphological dilation operations,/->Representing mathematical morphological erosion operations;
Δf dil (n)＝f dil (n)-f dil (n-1)；Δf ero (n)＝f ero (n)-f ero (n-1) (3)
wherein μ is the differential value f of the expansion corrosion operation grad An average value of (n),sigma is the differential value f of expansion corrosion operation grad (n) standard deviation, < >>
S5, calculating energy values of e-side and f-side currents of the protected line in x power frequency periods after faults according to the following formula (6);
wherein E is ej And E is fj Respectively representing the energy value of j-phase current at the e side of a protected line and the energy value of j-phase current at the f side of the protected line in x power frequency periods after faults;
step S6, starting the protection criterion #1, judging the differential value f of the expansion corrosion operation of the current at the e side of the protected line within 5ms after the fault grade_j Skewness coefficient S of (n) fgrade_j Differential value f of expansion corrosion operation of f-side current gradf_j Skewness coefficient S of (n) fgradf_j If the product of the two phases is larger than zero, judging that the protected circuit is a fault circuit with both sides connected with an inverter type micro source and the j phase is a fault phase; if the energy value of the current on the E side and the energy value of the current on the f side of the protected line within 5ms after the fault are judged by starting the protection criterion #2 if the energy value of the current on the E side and the energy value of the current on the f side are smaller than zero ej -E fj Whether or not is far greater than the energy threshold E set If yes, judging that the protected circuit is a fault circuit with one side connected with an inverter type micro source and j phases are fault phases; if not, judging that the protected line is a non-fault line or a non-fault phase of a fault line;
step S7, the protection devices on the e side and the f side of the protected line in the island micro-grid system start protection measures according to the judging result in the step S6, specifically: if the protected circuit is judged to be a fault circuit with both sides connected with an inverter type micro source and j phases are fault phases in the step S6, the protection devices on the e side and the f side of the protected circuit respectively send tripping signals to the j-phase circuit breakers on one side of the protected circuit to realize split-phase tripping; if the protected circuit is judged to be a fault circuit with one side connected with an inverter type micro source and the j phase is a fault phase in the step S6, the protection devices on the e side and the f side of the protected circuit respectively send tripping signals to the j-phase circuit breaker on one side of the protected circuit to realize split-phase tripping; if it is determined in step S6 that the protected line is a non-faulty line or a non-faulty phase of a faulty line, the protection device of the protected line does not send a trip signal to its j-phase breaker.
2. The island micro-grid fault transient protection method based on the initial fault current waveform characteristics of claim 1, wherein the method comprises the following steps of: in step S1, the sampling frequency of the protection devices on the e side and the f side of the protected line in the island micro-grid system is 10kHz, that is, 200 sampling points are collected within 20ms of one power frequency period.
3. The island micro-grid fault transient protection method based on the initial fault current waveform characteristics of claim 2, wherein the method comprises the following steps of: the x power frequency periods are one quarter of the power frequency period, namely 5ms.
4. The island micro-grid fault transient protection method based on the initial fault current waveform characteristics of claim 3, wherein the method comprises the following steps of: in step S3, the current difference Δi is calculated using equation (1) ej (k) First order variation gradient of (2) and K ej (k) Current difference Δi fj (k) First order variation gradient of (2) and K fj (k) The sampling point number Q is 10.
5. The island micro-grid fault transient protection method based on the initial fault current waveform characteristics of claim 4, wherein the method comprises the following steps: the threshold value K in step S3 set 0.01.
6. The island micro-grid fault transient protection method based on the initial fault current waveform characteristics of claim 5, wherein the method comprises the following steps: the energy threshold E in step S6 set Is 10 times the minimum value of the current energy on both sides of the protected line within 5ms after the fault.
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