CN110768221A - Adaptive reclosing method for overhead flexible direct-current power grid - Google Patents
Adaptive reclosing method for overhead flexible direct-current power grid Download PDFInfo
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- CN110768221A CN110768221A CN201911005346.XA CN201911005346A CN110768221A CN 110768221 A CN110768221 A CN 110768221A CN 201911005346 A CN201911005346 A CN 201911005346A CN 110768221 A CN110768221 A CN 110768221A
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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
- H02H7/265—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 making use of travelling wave theory
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H1/00—Details of emergency protective circuit arrangements
- H02H1/0007—Details of emergency protective circuit arrangements concerning the detecting means
- H02H1/003—Fault detection by injection of an auxiliary voltage
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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
-
- 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
- H02H7/28—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 meshed systems
Abstract
The invention discloses an adaptive reclosing method for an overhead flexible direct-current power grid, which belongs to the field of flexible direct-current power transmission and comprises the following steps: when a direct-current short-circuit fault signal is received, the direct-current breaker is opened; after dissociation is carried out, a part of sub-modules of a transfer branch of the direct-current circuit breaker are superposed; discharging the superposed partial sub-modules through a capacitor, and injecting voltage traveling waves with preset amplitudes into the fault line, wherein the direct-current voltage of the line has a first singular point; when the voltage traveling wave is reflected at a fault point or the tail end of a fault line, a second singular point of the line direct-current voltage is generated; detecting a singular point of the direct current voltage by utilizing wavelet transformation; and judging the fault property of the fault line according to the polarities of the wavelet transform modulus extreme values of the first singular point and the second singular point to perform self-adaptive reclosing. The invention avoids the problem of secondary impact caused by permanent fault of the reclosing of the direct current breaker and reduces the requirement of the flexible direct current power grid on the breaking capacity of the direct current breaker.
Description
Technical Field
The invention belongs to the field of flexible direct current transmission, and particularly relates to an adaptive reclosing method for an overhead flexible direct current power grid.
Background
With the increasing demand for renewable energy grid connection, a multi-terminal flexible direct-current power grid based on a Modular Multilevel Converter (MMC) technology becomes one of the development trends of the future smart power grid.
In consideration of power transmission distance, voltage class and cost, the flexible-direct system usually adopts an overhead line for large-scale power transmission. Overhead line faults are mostly transient faults. After fault isolation, the system should be restarted quickly to recover the power supply of the system, thereby improving the reliability of the system. At present, an automatic reclosing scheme is often adopted in engineering to restart faults. The method cannot judge the fault property, once the fault is superposed on the permanent fault, secondary damage is caused to the system, and the secondary opening of the direct current breaker further increases the breaking capacity requirement of the breaker. In order to improve the success rate of reclosure and the safety of a system, the fault property needs to be judged before the breaker is closed. If the judgment result is an instantaneous fault, the breaker should be switched on immediately; if the judgment result is a permanent fault, the locked circuit breakers do not coincide; this reclosing method by previously judging the nature of the fault is called "adaptive reclosing".
Aiming at the self-adaptive reclosing research of a direct current system, a fault property judgment method is provided by a few scholars. The literature, "Wangshuai, Bitianshu, Liwei, and the like," MMC-MTDC line bipolar permanent fault rapid identification method researches "to realize permanent fault identification by using the difference of positive and negative direct-current voltages. This method is only applicable to double short circuit faults. The document ' li bin, donkang, he jia, and the like ' discloses a fault property identification method of a true bipolar direct-current power transmission system ' for a unipolar ground fault, and provides a fault identification scheme based on fault pole residual voltage. If the residual voltage of the fault pole is larger than the setting value, the fault pole is judged to be a transient fault, and the breaker is required to be immediately superposed. None of the above methods takes into account the effect of fault resistance. The document "direct current adaptive reclosing method using full bridge MMC for injecting a characteristic signal" realizes the injection of an active signal by applying additional control to a full bridge type converter, however, the method is not suitable for a half bridge type converter topology. After the signal injection, the fault property is judged by using the propagation characteristics of the traveling wave. The above document does not make a detailed analysis of the propagation characteristics of the traveling wave.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide an adaptive reclosing method for an overhead flexible direct-current power grid, and aims to solve the problems that the existing automatic reclosing causes secondary damage to the overhead flexible direct-current power grid and needs to be opened again, so that the flexible direct-current power grid has high requirements on the breaking capacity of a direct-current breaker.
In order to achieve the purpose, the invention provides an adaptive reclosing method for an overhead flexible direct current power grid, which comprises the following steps:
(1) when a direct-current short-circuit fault signal is received, the direct-current breaker is opened;
(2) after dissociation is carried out, a part of sub-modules of a transfer branch of the direct-current circuit breaker are superposed;
(3) discharging the superposed partial sub-modules through a capacitor, and injecting voltage traveling waves with preset amplitudes into the fault line to generate a first singular point of the line direct-current voltage;
(4) when the voltage traveling wave is reflected at a fault point or the tail end of a fault line, a second singular point of the line direct-current voltage is generated;
(5) detecting a first singular point and a second singular point of the direct current voltage by utilizing wavelet transformation;
(6) and judging the fault property of the fault line according to the polarities of the wavelet transform modulus extreme values of the first singular point and the second singular point, and performing adaptive reclosing.
Preferably, the fault nature of the faulty line includes permanent faults and transient faults; if the polarities of the wavelet transformation mode extreme values of the first singular point and the second singular point are opposite before and after the voltage traveling wave is reflected, judging that the fault of the fault line is a permanent fault; otherwise, the fault of the fault line is judged to be a transient fault.
Preferably, the preset amplitude of the voltage travelling wave is Non·Uc(ii) a Wherein N isonIs a sub-dieThe number of blocks; u shapecThe discharge voltage of each sub-module capacitor;
preferably, step (4) is specifically:
if the voltage traveling wave is reflected at the fault point, the direct-current voltage of the line generates a second singular point with the polarity opposite to the wavelet transformation mode of the first singular point;
if the voltage travelling wave is reflected at the tail end of the fault line, the direct-current voltage of the line generates a second singular point with the same polarity as the wavelet transformation mode of the first singular point.
More specifically, it is explained as follows: if the line fault is a permanent fault, the voltage traveling wave is reflected at a fault point, and at the moment, a second singular point appears in the direct-current voltage of the line; the reflection coefficient at the fault point is negative, so the polarity of the reflected voltage is negative. The inherent traveling wave of the line is superposed with the negative voltage traveling wave, so that the direct-current voltage is rapidly reduced; therefore, the polarity of the wavelet transformation modulus of the second singular point and the first singular point of the direct-current voltage is opposite.
If the line fault is a transient fault, the voltage traveling wave is reflected at the tail end of the line, and at the moment, a second singular point appears in the direct-current voltage of the line; the reflection point has a positive reflection coefficient, so the polarity of the reflection voltage is positive. The positive voltage traveling wave is superposed on the inherent traveling wave of the line, so that the direct-current voltage quickly rises; therefore, the polarity of the wavelet transformation mode of the second singular point and the first singular point of the direct-current voltage is the same.
Preferably, the deionization time is 350 ms.
Preferably, the direct current breaker comprises a main branch, a transfer branch and an absorption branch;
the transfer branch circuit comprises a plurality of sub-modules, and each sub-module comprises a capacitor;
when the fault current is converted to the transfer branch circuit, the capacitor in the sub-module is charged; when freed, the coincident sub-module capacitances will discharge.
Preferably, the dc circuit breaker is a hybrid dc circuit breaker.
More specifically, the dc circuit breaker is an ABB typical hybrid dc circuit breaker or a cascaded full-bridge sub-module hybrid dc circuit breaker or a rectified hybrid dc circuit breaker.
Preferably, the method for performing adaptive reclosing according to the fault property comprises the following steps:
if the line fault is judged to be a permanent fault, the direct current breaker is locked, and the direct current line fault is waited to be repaired;
and if the line fault is judged to be a transient fault, a closing instruction is sent to the direct current breaker to carry out reclosing.
Through the technical scheme, compared with the prior art, the invention has the following beneficial effects:
(1) the invention provides an overhead flexible direct current power grid self-adaptive reclosing method, which is characterized in that singular point detection of direct current voltage is carried out by utilizing wavelet transformation, and the fault property of a fault line is judged according to the polarities of wavelet transformation module extreme values of a first singular point and a second singular point of the direct current voltage of the line before and after voltage traveling wave reflection, so that the problem of secondary impact caused by permanent fault of circuit breaker reclosing is avoided, the requirement of a flexible direct current power grid on the breaking capacity of a direct current circuit breaker is reduced, and the safety and the power supply reliability of flexible direct current are improved.
(2) According to the invention, when partial sub-modules in the transfer branch of the direct current breaker are superposed, the capacitance response of the buffer circuit in the transfer branch is fully considered, and compared with a scheme without considering the charge and discharge of the capacitor in the buffer circuit, the method better conforms to the real actual situation.
(3) The invention only controls the direct current breaker, and compared with the existing method for controlling the converter, the invention avoids the influence on a sound circuit and is more suitable for self-adaptive superposition under a direct current power grid.
Drawings
Fig. 1 illustrates an adaptive reclosing method for an overhead flexible direct current power grid according to the present invention;
FIG. 2 is a topology of an overhead flexible direct current power grid provided by the present invention;
fig. 3 is a schematic structural diagram of a hybrid high-voltage direct-current circuit breaker provided by the invention;
fig. 4 is a schematic diagram of voltage traveling wave injection when sub-modules of a transfer branch of a circuit breaker provided by the invention are partially overlapped;
FIG. 5 is a transient fault DC voltage waveform provided by the present invention;
FIG. 6 is a permanent fault DC voltage waveform at different positions provided by the present invention;
FIG. 7 is a schematic diagram of the extreme value of the wavelet transform modulus of the DC voltage under the transient fault provided by the present invention;
fig. 8 is a schematic diagram of a direct-current voltage wavelet transformation modulus extreme value under a permanent fault provided by the invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
As shown in fig. 1, the invention provides an adaptive reclosing method for an overhead flexible direct current power grid, which includes:
(1) when a direct-current short-circuit fault signal is received, the direct-current breaker is opened;
(2) after dissociation is carried out, a part of sub-modules of a transfer branch of the direct-current circuit breaker are superposed;
(3) discharging the superposed partial sub-modules through a capacitor, and injecting voltage traveling waves with preset amplitudes into the fault line to generate a first singular point of the line direct-current voltage;
(4) when the voltage traveling wave is reflected at a fault point or the tail end of a fault line, a second singular point of the line direct-current voltage is generated;
(5) detecting a first singular point and a second singular point of the direct current voltage by utilizing wavelet transformation;
(6) and judging the fault property of the fault line according to the polarities of the wavelet transform modulus extreme values of the first singular point and the second singular point, and performing adaptive reclosing.
Preferably, the fault nature of the faulty line includes permanent faults and transient faults; if the polarities of the wavelet transformation mode extreme values of the first singular point and the second singular point are opposite before and after the voltage traveling wave is reflected, judging that the fault of the fault line is a permanent fault; otherwise, the fault of the fault line is judged to be a transient fault.
Preferably, the preset amplitude of the voltage travelling wave is Non·Uc(ii) a Wherein N isonThe number of the submodules is; u shapecThe discharge voltage of each sub-module capacitor;
preferably, step (4) is specifically:
if the voltage traveling wave is reflected at the fault point, the direct-current voltage of the line generates a second singular point with the polarity opposite to the wavelet transformation mode of the first singular point;
if the voltage travelling wave is reflected at the tail end of the fault line, the direct-current voltage of the line generates a second singular point with the same polarity as the wavelet transformation mode of the first singular point.
More specifically, it is explained as follows: if the line fault is a permanent fault, the voltage traveling wave is reflected at a fault point, and at the moment, a second singular point appears in the direct-current voltage of the line; the reflection coefficient at the fault point is negative, so the polarity of the reflected voltage is negative. The inherent traveling wave of the line is superposed with the negative voltage traveling wave, so that the direct-current voltage is rapidly reduced; therefore, the polarity of the wavelet transformation modulus of the second singular point and the first singular point of the direct-current voltage is opposite.
If the line fault is a transient fault, the voltage traveling wave is reflected at the tail end of the line, and at the moment, a second singular point appears in the direct-current voltage of the line; the reflection coefficient of the reflection point is positive, so the polarity of the reflection voltage is positive. The positive voltage traveling wave is superposed on the inherent traveling wave of the line, so that the direct-current voltage quickly rises; therefore, the polarity of the wavelet transformation mode of the second singular point and the first singular point of the direct-current voltage is the same.
Preferably, the deionization time is 350 ms.
Preferably, the direct current breaker comprises a main branch, a transfer branch and an absorption branch;
the transfer branch circuit comprises a plurality of sub-modules, and each sub-module comprises a capacitor;
when the fault current is transferred to the transfer branch circuit, the sub-module capacitor is charged;
when freed, the coincident sub-module capacitors discharge.
Preferably, the dc circuit breaker is a hybrid dc circuit breaker, and more particularly, the hybrid dc circuit breaker is an ABB typical hybrid dc circuit breaker or a cascade full-bridge sub-module type hybrid dc circuit breaker or a rectification type hybrid dc circuit breaker.
Preferably, the method for performing adaptive reclosing according to the fault property comprises the following steps:
if the line fault is judged to be a permanent fault, the direct current breaker is locked, and the direct current line fault is waited to be repaired;
and if the line fault is judged to be a transient fault, sending a closing instruction to the direct current breaker to carry out reclosing.
Fig. 2 is a topological system of an overhead flexible direct current power grid provided by this embodiment, a sub-module of the system adopts a half-bridge type structure, and since a half-bridge type MMC does not have a fault self-clearing capability, high-voltage direct current circuit breakers need to be installed on two sides of a line for fault isolation. In order to limit the magnitude of fault current, current-limiting reactors are arranged on two sides of the circuit. The system is a pseudo-bipolar system, and the overhead line adopts a frequency dependence model.
Fig. 3 is a schematic structural diagram of a hybrid high-voltage dc circuit breaker according to this embodiment. The direct current breaker consists of a main branch, a transfer branch and an absorption branch; the transfer branch is formed by connecting a plurality of semiconductor submodules in series. Each submodule contains an Insulated Gate Bipolar Transistor (IGBT) and an antiparallel diode. In consideration of the dispersion of the turn-on of the IGBT, the sub-modules need to be connected with a buffer branch in parallel to prevent the IGBT from being broken down. When the direct current transformer normally operates, direct current flows through the main branch circuit so as to reduce on-state loss of the direct current main branch circuit. After the direct current fault occurs, the direct current fault current gradually commutates to the transfer branch circuit, and when the transfer branch circuit current is reduced to zero, the quick isolating switch is disconnected under the condition of zero current, and the time is consumed for about 2 ms. Then the IGBT in the main branch is switched off, the fault current is transferred to the absorption branch, and the lightning arrester consumes the fault energy. In the process, the sub-module buffer branch circuit capacitors C are charged to Uc. Assuming that the number of the total sub-modules of the conversion branch of the direct-current circuit breaker is N, the charging voltage of each sub-module capacitor is as follows: u shapec＝UdcNN; wherein, UdcNThe rated direct current voltage of the overhead flexible direct current power grid.
Fig. 4 is a schematic diagram of voltage traveling wave injection when sub-modules of a circuit breaker transfer branch circuit provided in this embodiment partially overlap. After the fault occurs and the direct current breaker is disconnected, after the free time of 350ms, the N of the transfer branch of the direct current breaker is superposedonAnd a sub-module. The superposed sub-module capacitor voltage 'capacitor C-resistor R-IGBT' discharges; according to the equivalent circuit diagram, the equivalent is to inject the amplitude N into the fault circuiton·UcOf the voltage travelling wave.
Fig. 5 is a transient fault dc voltage waveform provided by the present embodiment. The injected voltage traveling wave propagates forward of the faulty line, reflecting at the end of the faulty line. Since the reflection coefficient at the end of the faulty line is positive, a positive voltage traveling wave is superimposed on the line's inherent traveling wave, and the dc voltage rises rapidly.
Fig. 6 shows the permanent fault dc voltage waveforms at different positions according to the present embodiment. The injected voltage traveling wave propagates to the front of the fault line, and is reflected at the fault point. Because the equivalent reflection coefficient of the fault point is negative, a negative voltage traveling wave is superposed on the inherent traveling wave of the line, and the direct-current voltage is rapidly reduced.
Fig. 7 is a schematic diagram of a modulus value of a wavelet transform of a dc voltage under a transient fault according to this embodiment. Before injecting voltage traveling wave signals, the direct current voltage of the fault line is in a stable state after the fault. Injecting voltage traveling waves into the fault line, wherein the direct-current voltage of the line suddenly rises to generate a first singular point, and the extreme value of a wavelet transformation module of the first singular point is negative; the injected voltage traveling wave propagates to the front of the fault line, and is reflected at the tail end of the fault line, and the direct-current voltage of the line has a second singular point. Since the reflection coefficient is positive, the dc voltage rises rapidly, and the wavelet transform mode extreme value at the second singular point is negative. The polarities of the extreme values of the wavelet transformation moduli of the first singular point and the second singular point are the same, and therefore, the fault is discriminated as a transient fault.
Fig. 8 is a schematic diagram of a modulus value of a wavelet transform of a dc voltage under a permanent fault according to this embodiment. The fault occurs at the faulty line termination. The injected voltage traveling wave propagates to the front of a fault line, reflection occurs at a fault point, and a second singular point appears in line direct-current voltage. Since the reflection coefficient is negative, the dc voltage drops rapidly, and the wavelet transform mode extreme value at the second singular point is positive. The polarity of wavelet transform mode extreme values of the first singular point and the second singular point are opposite, so that the fault is judged to be a permanent fault.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (8)
1. A self-adaptive reclosing method for an overhead flexible direct current power grid is characterized by comprising the following steps:
(1) when a direct-current short-circuit fault signal is received, the direct-current breaker is opened;
(2) after dissociation is carried out, a part of sub-modules of a transfer branch of the direct-current circuit breaker are superposed;
(3) discharging the superposed partial sub-modules through a capacitor, and injecting voltage traveling waves with preset amplitudes into the fault line to generate a first singular point of the line direct-current voltage;
(4) when the voltage traveling wave is reflected at a fault point or the tail end of a fault line, a second singular point of the line direct-current voltage is generated;
(5) detecting a first singular point and a second singular point of the direct current voltage by utilizing wavelet transformation;
(6) and judging the fault property of the fault line according to the polarities of the wavelet transform modulus extreme values of the first singular point and the second singular point, and performing adaptive reclosing.
2. The adaptive reclosing method for an overhead flexible direct current power grid according to claim 1, wherein the fault nature of the fault line includes permanent faults and transient faults; if the polarities of the wavelet transform mode extreme values of the first singular point and the second singular point of the line direct-current voltage are opposite before and after the voltage traveling wave is reflected, judging that the fault of the fault line is a permanent fault; otherwise, the fault of the fault line is judged to be a transient fault.
3. The adaptive reclosing method for an overhead flexible direct current power grid according to claim 1 or 2, characterized in that the preset amplitude of the voltage traveling wave is Non·Uc；
Wherein N isonThe number of the submodules is; u shapecIs the discharge voltage of each sub-module capacitor.
4. The adaptive reclosing method for the overhead flexible direct current power grid according to claim 1, wherein the step (4) is specifically as follows:
if the voltage traveling wave is reflected at the fault point, the direct-current voltage of the line generates a second singular point with the polarity opposite to the wavelet transformation modulus of the first singular point;
and if the voltage traveling wave is reflected at the tail end of the fault line, the direct-current voltage of the line generates a second singular point with the same polarity as the wavelet transform mode of the first singular point.
5. The adaptive reclosing method for an overhead flexible direct current power grid according to claim 1, wherein the free time is 350 ms.
6. The adaptive reclosing method for the overhead flexible direct current power grid according to any one of claims 1 to 5, wherein the direct current breaker comprises a main branch, a transfer branch and an absorption branch;
the transfer branch circuit comprises a plurality of sub-modules, and each sub-module comprises a capacitor;
when fault current is transferred to the transfer branch, the capacitor is charged; when freed, the coincident sub-module capacitors discharge.
7. The adaptive reclosing method for an overhead flexible direct current power grid according to claim 6, wherein the direct current breaker is a hybrid direct current breaker.
8. The adaptive reclosing method for the overhead flexible direct current power grid according to claim 2, wherein the method for performing the adaptive reclosing according to the fault property comprises the following steps:
if the line fault is judged to be the permanent fault, the direct current breaker is locked, and the direct current line fault is waited to be repaired;
and if the line fault is judged to be the transient fault, sending a closing instruction to the direct current breaker to carry out reclosing.
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| CN112260236B (en) * | 2020-09-17 | 2022-07-19 | 湘潭大学 | Three-phase self-adaptive reclosing method and device |
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Application publication date: 20200207 |