CN110429572B - Rapid protection method for interelectrode fault of direct-current power distribution network - Google Patents
Rapid protection method for interelectrode fault of direct-current power distribution network Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H3/00—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection
- H02H3/006—Calibration or setting of parameters
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
- H02H7/268—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured for dc systems
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Abstract
The invention discloses a rapid protection method for interelectrode faults of a direct-current power distribution network, which solves inductance parameters of a line by using fault transient electric quantity and a line RL model and judges the fault line according to the magnitude of the inductance parameters; in order to further improve the sensitivity of the method, a small inductor is connected in series at the head end of each line, and the inductor voltage is used for replacing the differential in the RL model of the line, so that the truncation error when the differential replaces the differential is eliminated, the calculation amount when the inductance parameter of the line is calculated is reduced, the calculation process is simplified, and the requirement of the algorithm on the sampling rate is reduced.
Description
Technical Field
The invention belongs to the field of relay protection of power systems, and relates to an interpolar fault protection method for a flexible direct-current power distribution network.
Background
In recent years, with the continuous improvement of requirements of users on power supply requirements, power supply reliability, power quality and the like and the wide access of distributed power sources, energy storage devices and direct current loads, flexible direct current power distribution networks receive more and more attention and research and become important development directions of future power distribution networks. However, compared with an alternating current system, the flexible direct current distribution network has the characteristics of low inertia and low impedance, and once an interelectrode fault occurs in a direct current line, energy storage elements (mainly capacitance elements in each converter) distributed in the direct current distribution network can rapidly release energy to a fault point, so that the fault current reaches tens of thousands of amperes or even hundreds of kiloamperes within milliseconds, and the safe operation of power electronic devices and other electrical equipment is seriously threatened. Therefore, fault detection and isolation need to be completed reliably within milliseconds, which puts extremely high requirements on the quick action of relay protection of the medium-voltage direct-current distribution network.
The traditional single-end electric quantity protection (such as current protection, distance protection and the like) cannot effectively prevent the misoperation of the line protection caused by the fault of the head end of the next adjacent line, and the quick-acting section protection cannot realize full-line quick action, so that the protection is difficult to adapt to the high requirement of a medium-voltage flexible direct-current power distribution network on the protection speed. The current protection principle is simple, but because the direct current lines in the direct current distribution network are mostly cable lines, the line impedance is small, the distance is short, the difference between the tail end fault and the head end fault current of the line is relatively small, the protection range is necessarily reduced according to the setting principle of avoiding the tail end short circuit current of the line, and the sensitivity of the protection quick-acting section is influenced.
The current differential protection scheme of the direct current distribution network adopted at present (Zhaoyu Ming, Li tree Peng, research on fault location and protection configuration of flexible direct current distribution network based on MMC, protection and control 2015,43(22): 127-.
In the existing subway and ship direct-current power distribution system, current rise rate di/dt and current increment delta I protection are generally adopted as main protection of a traction system, but the protection utilizing electric quantity differentiation is adopted, the constant value of the protection depends on simulation calculation, a universal constant value setting method is lacked, and the capacity of enduring transition resistance is limited.
(Xianyong Feng, Li Qi, Jiuping pan.A Novel Fault Location Method and Algorithm for DC Distribution protection.IEEE Transactions on Industry Applications,2017,53(3): 1834-.
The method is simple, small in calculated amount and low in sampling frequency requirement, but cannot protect the whole length of the line, and is only suitable for the condition that the current-limiting reactor at the outlet of the converter can be used as local information, so that certain limitation exists.
Disclosure of Invention
The invention aims to provide a rapid protection method for interelectrode faults of a direct-current power distribution network, which aims to solve the technical problems; the invention can realize full-line quick action, has small calculated amount, easy realization and high accuracy.
In order to achieve the purpose, the invention adopts the following technical scheme:
a rapid protection method for interelectrode faults of a direct current distribution network comprises the following steps:
electricity utilizing boundary inductor added on positive and negative pole line of protective installationAnd replacing a differential term in a RL model of a line between the protection installation position and the fault point to obtain the inter-bus voltage u of the protection installation positiondcPositive boundary inductance voltage uLmarpNegative boundary inductance voltage uLmarnPositive electrode current i of linepAnd line negative current inThe relation between:
wherein: u. ofLmar＝uLmarp-uLmarm；idc＝ip-in；LmarThe inductance value of the positive and negative line boundary inductor at the installation position is protected; l isx、RxLine inductance and resistance between the boundary inductance and the fault point; rfIs a fault resistance;
wherein: n is the line inductance sequence from boundary inductance to fault point Lx(k)]The number of middle elements; l issetIs a setting value; krelIs a reliability factor;
further, LsetThe value of the inductance value is set according to the measured inductance of the protection of the line when the head end of the next adjacent line is hidden from fault, namely the setting value L of the protection of the lineset＝Krel(L+Lmar.next) L is the inductance of the line, Lmar.nextThe boundary inductance added for the next line protection installation is satisfied
The action is protected.
Further, in the above-mentioned case,
Lmar2is the boundary inductance of the head end of the next adjacent line.
Compared with the prior art, the invention has the following beneficial effects: the method comprises the steps of solving inductance parameters of a line by using fault transient electric quantity and a line RL model, and judging a fault line according to the inductance parameters; in order to further improve the sensitivity of the method, a small boundary inductor is connected in series at the head end of each line, and the inductor voltage is used for replacing the differential in the RL model of the line, so that the truncation error when the differential replaces the differential is eliminated, the calculation amount when the inductance parameter of the line is calculated is reduced, the calculation process is simplified, and the requirement of the algorithm on the sampling rate is reduced.
Drawings
FIG. 1 is a schematic diagram of a radial DC distribution network with a single-side power supply;
FIG. 2 is a R-L equivalent circuit diagram of a fault line;
FIG. 3 is an electromagnetic transient model of a DC power distribution network;
FIG. 4 is a flow chart of rapid protection of interpolar faults in a DC distribution network;
FIG. 5 shows the behavior of protection 5 (protection of line L5) and the relative error in measured inductance during a metallic fault at various points in line L5; where FIG. 5(a) is the average of the measured inductances; FIG. 5(b) is a calculation error of the measured inductance;
FIG. 6 shows the behavior of protection 5 (protection of line L5) and the relative error in measured inductance during a metallic fault at different points in line L6; where FIG. 6(a) is the average of the measured inductances; fig. 6(b) shows the calculation error of the measured inductance.
Detailed Description
The invention is described in further detail below with reference to the accompanying drawings:
to illustrate the method of the present invention, a single line diagram of a typical single-side power supply radial multi-stage power supply flexible dc distribution network shown in fig. 1 is shown. In order to realize single-end electric quantity full-line quick-action protection based on inductance parameter identification, a small-value inductor (L) is connected to the protection installation position of each direct current linemar) As boundary elements (as shown in the lines L1, L2, and L3 in fig. 1), the boundary inductance cannot affect the normal operation of the system, and can effectively distinguish the fault at the tail end of the current line from the fault at the head end of the next line.
When an inter-pole fault occurs in the line L1 in the network shown in fig. 1, the equivalent circuit of the line L1 is shown in fig. 2, in which: u. ofdcTo protect the interelectrode voltage at the head end of the line at the installation site; i.e. ip、inRespectively the positive and negative current of the circuit; u. ofLmarp、uLmarnThe voltage drop on the boundary inductance of the positive pole and the negative pole of the line can be obtained by additionally arranging a voltage transformer; l isxAnd RxLine resistance and inductance from boundary inductance to fault point, RfIs a fault resistance.
From the circuit shown in fig. 2, it is possible to obtain:
and in formula (1):
namely, the method comprises the following steps:
Lmarthe inductance value of the positive and negative line boundary inductor at the installation position is protected; in order to avoid the substitution error caused by substituting differential for differential in the process of parameter identification, u in the formula (3) is usedLmarp/LmarAnd uLmarn/LmarIn place of di in the formula (1)pDt and dinThe/dt union of the same terms can be obtained:
protection installation position inter-bus-bar voltage sampling value u utilizing any two adjacent sampling points in fault transient processdc(k) And udc(k +1) positive boundary inductance voltage sampling value uLmarp(k) And uLmarp(k +1), negative boundary electrodeVoltage sensing sampling value uLmarn(k) And uLmarn(k +1) line positive current sampling value ip(k) And ip(k +1) and line negative current sample value in(k) And in(k +1), the following system of equations can be found:
wherein: i.e. idc(k+1)＝ip(k+1)-in(k+1)，idc(k)＝ip(k)-in(k)，uLmar(k+1)＝uLmarp(k+1)-uLmarn(k+1)，uLmar(k)＝uLmarp(k)-uLmarn(k)。
Designing a cut-off frequency of fwOf i low pass filter, todc(k+1)、idc(k)、uLmar(k+1)、uLmar(k)、udc(k+1)、udc(k) Filtering to obtain i'dc(k+1)、i”dc(k)、u”Lmar(k+1)、u”Lmar(k)、u”dc(k+1)、u”dc(k) Substituting the value into the formula (5) and solving the value to obtain the L shown in the formula (6)xAnd RxThe numerical solution of (c). The reason why the low-pass filter is designed here is: the error exists between the actual model of the line and the RL model, and the cut-off frequency f of the low-pass filter is used for ensuring the accuracy of the algorithmwThe selection should be made according to the applicable frequency band of the RL model of the line. (Suonangle, King, Monty, et al. Rapid Direction elements were identified based on the parameters of the RL model. the university of Western Ann traffic, 2006,40(6): 689-. For a direct current distribution network, the line length is less than 100km, so the cut-off frequency of the filter is lower than 600Hz, and f is adopted in the inventionw＝300Hz。
A measured inductance sequence [ L ] can be obtained by using the sampling data in the fault transient process and combining the formula (6)x(k)]Then consider Lx(k) There is a calculation fluctuation, so the present invention uses the average value of the calculated inductance
Forming a protection criterion, wherein the specific criterion is shown as a formula (7), wherein: l issetAnd protecting the setting value.
To ensure the selectivity of the protection process proposed by the invention, LsetAnd (2) setting according to the measured inductance protected by the line when the fault of the head end of the next adjacent line is avoided, wherein the set inductance of the line meets the following conditions (taking a line L1 shown in FIG. 1 as an example):
Lset1＝Krel(LL1+Lmar2) (8)
in formula (8): l isL1Inductance of the entire line L1; l ismar2The boundary inductance is the head end of the next adjacent line; krelFor reliable coefficients, 0 < K since the protection in the text is under-protectionrelAnd (3) less than 1, the specific value of the method should take the error of the mutual inductor, the parameter measurement error and the error of the protection device into consideration.
Meanwhile, in order to ensure that the protection does not reject and the boundary inductance is effective when the tail end of the line has a fault, the setting value of the protection also needs to meet the following conditions:
Lset1>L1 (9)
the value lower limit of the boundary resistor added at the next adjacent line protection installation position can be solved by combining the vertical type (8) and the formula (9):
it should be noted that:boundary inductor L for protecting installationmarThe system should be slightly larger without affecting the normal condition of the system. For example, in the embodiment, when KrelTake 0.9, LL1At 2.8mH, L can be calculated from the formula (10)mar2Should be greater than 0.311, according to the above principle, L in the embodimentmar2The value is 1 mH.
As can be seen from the above analysis: the voltage on the boundary inductor is used for replacing the differential in the formula (1), so that the replacement error of replacing the differential by the differential is eliminated, the matrix operation when the least square method is used for solving the line parameters is avoided, the calculated amount is small, the process is simple, the transient data is used for identifying the fault, and the fault can be rapidly removed within milliseconds.
Fig. 4 is a specific implementation flow of the protection principle of the present invention.
Examples
The direct current distribution network electromagnetic transient model shown in fig. 1 is built in the PSCAD/EMTP, and the electromagnetic transient model is shown in fig. 3. In the model, each phase of an upper Bridge arm and a lower Bridge arm of an MMC are formed by connecting 10 Half-Bridge Sub-modules (HBSM) in series, the Modulation mode is Nearest Level approximation Modulation (NLM), and the control mode is constant direct current voltage and constant reactive power control; the line model is formed by connecting a plurality of pi models in series, wherein every 500m is equivalent to one pi model; the load and the adapter thereof are irrelevant to a 0-mode fault network of the direct-current distribution network, so that the load and the adapter thereof are equivalent to a constant resistance model; other parameters are shown in table 1.
TABLE 1 simulation model-related parameters
The sampling rate of the model is 20kHz, and one measuring inductance Lx can be calculated at every two sampling points. In consideration of the calculation fluctuation of the measured inductance, the measured inductance is solved by adopting data within 0.002s after the fault moment in the embodiment of the invention, and the average value of the solved inductance and the calculation fluctuation are used as the protection criterion. And if the measurement delay is taken into consideration (when the current rising rate is within 100A/mu s, the measurement time of the Hall sensor does not exceed 3 mu s), the algorithm delay (the algorithm delay does not exceed several mu s by adopting a high-speed processor) and the action time (about 0.1 ms) of the direct-current circuit breaker. Therefore, the protection principle of the invention can rapidly act within 5ms, and can meet the high requirement of the direct-current power distribution network on the protection speed.
In order to verify the accuracy and correctness of the protection principle, the simulation model and the simulation data thereof in the embodiment of the invention are used for verifying the accuracy and correctness of the protection principle.
In an embodiment, the interpolar faults of the line L5 and the line L6 at different positions and different fault resistances are first simulated, and the mean measured impedance at the L5 protection installation is compared with fault simulation data
The calculation of the fluctuation sigma 5 (see formula (11)) and the calculation of the relative error alpha are performed to verify the calculation accuracy of the protection principle of the present invention under different fault conditions, and the specific calculation results are shown in table 2.
The results shown in table 2 show that:
(1) when the method is used in a metallic fault (Rf is 0 omega), the calculation fluctuation and the relative error of the average measurement inductance are small; as the fault resistance increases, the average measured inductance decreases continuously, while the calculation ripple and calculation error increase.
(2) When the fault point is closer to the protection installation position, the calculation fluctuation and calculation error of the average measured inductance are greatly influenced by the fault resistance, especially when the head end of the line of the section is in fault through the fault resistance, the average measured inductance of the line protection of the section and the average measured inductance of the previous-stage line are both greatly reduced, and the steady-state overrunning phenomenon occurs when the average measured inductance of the previous-stage line protection meets the protection criterion.
The reason why the above phenomenon occurs is that: as the fault resistance increases, the error of the mathematical model created by the present invention increases, and therefore the value of the measured inductance calculated therefrom also deviates significantly.
However, since the protection principle of the present invention is based on the transient electrical quantity information, the operation time is fast, and the fault resistance at the initial stage of the fault is mainly the arc resistance, and the numerical value is small (close to zero), the rapid protection of the inter-electrode fault based on the fault transient electrical quantity can meet the requirement as long as the high accuracy can be ensured when the fault resistance is small, and the simulation result shows that: the protection principle of the invention guarantees a very high accuracy (calculation error less than < 10%) when the fault resistance is close to 0.
In the above situation, the present embodiment simulates the metallic inter-electrode fault at different fault positions of the lines L5 and L6, calculates the measured inductance of the protection 5 (protection of the line L5) by using the simulation data, and compares the calculated inductance with the setting value (the value is shown in table 3) to verify whether the protection principle according to the present invention can operate reliably, that is, the protection principle does not operate in the case of an internal fault of the line and does not operate erroneously in the case of a fault other than the line, and the specific simulation result is shown in fig. 5 and fig. 6. The values of the setting values of the protection criterion in the embodiment are shown in table 3.
TABLE 3 protection rating Table for L5 under different fault conditions
| Lmar5 | 1mH | Lset5 | 3.42mH |
| Lmar5 | 1mH | σset | 10％ |
| Krel | 0.85 | - | - |
As can be seen from the waveforms shown in fig. 5 and 6:
(1) according to the protection method, when any point of the line L5 fails, the average value of the measured inductance of the protection 5 (protection of the line L5) is smaller than the set inductance, and full-line quick action can be realized; and when any point on the next line L6 has a fault, the average value of the measured inductance of the protection 5 is larger than the set inductance, so that the fault does not occur when the area is out of order.
(2) When the line L5 and the next-stage line L6 have faults at different places, the calculation errors of the measured inductance at the protection 5 are within the range of +/-5% by using the fault transient electrical quantity without considering the fault resistance, so that the accuracy of the algorithm is ensured.
Claims (3)
1. A rapid protection method for interelectrode faults of a direct current distribution network is characterized by comprising the following steps:
step 1, series boundary inductors are additionally arranged in positive and negative circuits at the protection installation position of a direct-current power distribution network;
the differential term in the RL model of the line between the protection installation position and the fault point is replaced by the voltage of the boundary inductor additionally arranged on the positive pole line and the negative pole line of the protection installation position to obtainInter-bus voltage u to the protection installationdcPositive boundary inductance voltage uLmarpNegative boundary inductance voltage uLmarnPositive electrode current i of linepAnd line negative current inThe relation between:
wherein: u. ofLmar＝uLmarp-uLmarn；idc＝ip-in；LmarThe inductance value of the positive and negative line boundary inductor at the installation position is protected; l isx、RxLine inductance and resistance between the boundary inductance and the fault point; rfIs a fault resistance;
step 2, utilizing inter-bus voltage sampling values u of any two adjacent sampling points at protection installation positions in fault transient processdc(k) And udc(k +1) positive boundary inductance voltage sampling value uLmarp(k) And uLmarp(k +1) negative boundary inductance voltage sampling value uLmarn(k) And uLmarn(k +1) line positive current sampling value ip(k) And ip(k +1) and line negative current sample value in(k) And in(k +1) to obtain uLmar(k)＝uLmarp(k)-uLmarn(k)、uLmar(k+1)＝uLmarp(k+1)-uLmarn(k+1)，idc(k)＝ip(k)-in(k)、idc(k+1)＝ip(k+1)-in(k +1) and by clipping to a frequency fwLow pass filter pair udc(k)、uLmar(k)、idc(k) And udc(k+1)、uLmar(k+1)、idc(k +1) filtering to obtain u "dc(k)、u”Lmar(k)、i”dc(k) And u "dc(k+1)、u”Lmar(k+1)、i”dc(k +1) and finding the line inductance L from the boundary inductance to the fault pointx(k)：
Step 3, calculating a line inductance sequence [ L ] of a boundary inductance fault point by using the sampling data in the fault transient processx(k)]Of the element Lx(k) Calculating the line inductance from the boundary inductance to the protection installation position for any two adjacent sampling points, and then calculating the average value of all elements in the line inductance
And use
Forming a protection criterion:
wherein: n is the line inductance sequence from boundary inductance to fault point Lx(k)]The number of middle elements; l issetIs a setting value.
2. The method for rapidly protecting interpolar faults of direct current distribution network according to claim 1, wherein L issetThe value of the inductance value is set according to the measured inductance of the protection of the line when the head end of the next adjacent line is hidden from fault, namely the setting value L of the protection of the lineset＝Krel(L+Lmar.next) L is the inductance of the line, Lmar.nextThe boundary inductance added for the next line protection installation is satisfied
A time protection action; wherein, KrelIs a reliability factor.
3. The method for rapidly protecting interpolar faults of a direct current distribution network according to claim 2, wherein,
Lmar2is the boundary inductance of the head end of the next adjacent line.
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| US6392422B1 (en) * | 1997-06-17 | 2002-05-21 | Dip.-Ing. Walther Bender Gmbh & Co. Kg | Monitoring insulation and fault current in an A/C current network to provide load shutoff whenever differential current exceeds a certain response value |
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