WO2022068074A1 - 一种缓慢发展型永久性故障预警方法和系统 - Google Patents

一种缓慢发展型永久性故障预警方法和系统 Download PDF

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Publication number
WO2022068074A1
WO2022068074A1 PCT/CN2020/136201 CN2020136201W WO2022068074A1 WO 2022068074 A1 WO2022068074 A1 WO 2022068074A1 CN 2020136201 W CN2020136201 W CN 2020136201W WO 2022068074 A1 WO2022068074 A1 WO 2022068074A1
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fault
transient
insulation
early warning
insulation degradation
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English (en)
French (fr)
Inventor
石访
张恒旭
谢伟
方陈
张林林
华斌
刘舒
徐凯
朱征
时志雄
魏新迟
鲍伟
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Shandong University
State Grid Shanghai Electric Power Co Ltd
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Shandong University
State Grid Shanghai Electric Power Co Ltd
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/08—Locating faults in cables, transmission lines, or networks
    • G01R31/081—Locating faults in cables, transmission lines, or networks according to type of conductors
    • G01R31/086—Locating faults in cables, transmission lines, or networks according to type of conductors in power transmission or distribution networks, i.e. with interconnected conductors
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/08—Locating faults in cables, transmission lines, or networks
    • G01R31/088—Aspects of digital computing
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/12—Testing dielectric strength or breakdown voltage ; Testing or monitoring effectiveness or level of insulation, e.g. of a cable or of an apparatus, for example using partial discharge measurements; Electrostatic testing

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  • the present disclosure relates to a slowly developing permanent fault early warning method and system.
  • the distribution line directly connects the users, directly affects all aspects of life and production, and plays an extremely important role in the power system. Due to the responsibility of distributing electric energy, the distribution network has a large coverage, with multiple feeders, multiple branches, and multiple topologies. Due to the characteristics of change, many points and wide areas, the operating environment is relatively harsh, and it is very prone to faults. Statistics show that most line faults are transient faults.
  • the present disclosure proposes a slowly developing permanent fault early warning method and system, which acquires the waveforms of the transient fault recordings before the permanent fault occurs, and extracts the key feature quantities of the fault. Determine the degree of insulation deterioration of the line, and then issue an early warning according to the degree of insulation deterioration.
  • the first objective of the present disclosure is to propose a slow-developing permanent fault early warning method, including:
  • the key characteristic quantities include, the number of transient faults in a period of time, the interval between two transient faults, the transient zero-mode current amplitude, the fault duration and the low-band energy factor of the zero-sequence current.
  • the number of transient faults in a period of time is positively related to the degree of insulation deterioration; the interval between two transient faults is negatively related to the degree of insulation deterioration; the transient zero-mode current amplitude is positively related to the degree of insulation deterioration;
  • the fault duration is positively correlated with the degree of insulation deterioration; the low-frequency band energy factor of zero-sequence current is negatively correlated with the degree of insulation deterioration.
  • the number of transient faults occurring within a period of time, the interval between two transient faults, the transient zero-mode current amplitude and the fault duration are directly obtained from the acquired recorded waveforms.
  • the obtained recording waveform is decomposed into frequency bands to obtain the energy of each sub-band, and the low-frequency band energy factor of the zero-sequence current is obtained according to the ratio of the energy of the first sub-band to the sum of the energy of each sub-band.
  • the insulation degradation threshold is determined according to the actual operation experience of the specific distribution network.
  • the second purpose of the present disclosure is to propose a slow-developing permanent fault early warning system, including a detection module, a controller and an alarm model.
  • the detection module obtains the recorded waveform of the transient fault
  • the controller receives the data obtained by the detection module.
  • After recording the waveform extract the key feature quantity of the recorded waveform, calculate the insulation deterioration value of each transient fault according to the key feature quantity, accumulate the insulation deterioration value obtained many times, and obtain the accumulated insulation deterioration value.
  • an alarm message will be sent to the alarm module for alarm.
  • the key feature quantities extracted by the controller from the recorded waveform include, the number of transient faults in a period of time, the interval between two transient faults, the transient zero-mode current amplitude, the fault duration and Low-band energy factor of zero-sequence current.
  • the present disclosure utilizes the recorded waveforms of each transient fault before the occurrence of the permanent fault to obtain the key feature quantity of the fault, calculates the cumulative insulation deterioration value of the line through the extracted key feature quantity, and judges the degree of insulation deterioration according to the cumulative insulation deterioration value, And issued an early warning to realize the early warning of slow-developing permanent failures.
  • the present disclosure can complete the early warning of slowly developing permanent faults only by obtaining the transient fault recording waveform, without the need to additionally detect other fault features, and without the need to install a large number of additional other fault feature detection and processing devices, The operating cost is reduced, and it is more in line with the requirements of practical applications.
  • Fig. 1 is the fault early warning process of the slowly developing permanent fault early warning method of the present disclosure
  • Fig. 2 is the cumulative insulation deterioration value change curve of the present disclosure
  • Fig. 3 (a) is a spectrum energy distribution diagram of a slow-developing permanent fault transient fault 1 in Example 1;
  • Fig. 3(b) is a spectrum energy distribution diagram of a slow-developing permanent fault transient fault 2 in Example 1;
  • Fig. 3 (c) is a spectrum energy distribution diagram of a slow developing permanent fault transient fault 3 in Example 1;
  • Fig. 4 is a single-phase ground fault component mode network diagram
  • Fig. 5 is the equivalent circuit of instantaneous single-phase ground fault
  • FIG. 6 is a graph showing the variation of line parameters in different periods of insulation degradation.
  • the insulation of power equipment in distribution network presents a process of slow deterioration.
  • solid insulation such as cables and insulators, it is generally not broken down under operating voltage. It should be considered that the insulation may break down under the action of instantaneous overvoltage, but the duration of instantaneous overvoltage is not long, and after a few seconds will disappear, after which the grid will resume normal operation.
  • the medium and low voltage distribution network is a small current grounding system
  • the fault current is small and the arc suppression coil will suppress the instantaneous arc, even if the instantaneous shock may occur under the operating voltage
  • the arc will also be extinguished when the fault current crosses zero at high frequency.
  • the impurities and moisture at the fault point are burned and dried by the arc heating, the arc can be naturally extinguished without re-ignition.
  • the single-phase grounding fault has obvious arc grounding characteristics.
  • the insulation weak point may continue to be broken down and further damage the insulation, and the unstable arc grounding will gradually become metallic. Grounding develops and eventually a permanent single-phase grounding occurs. In this process, the number of instantaneous insulation breakdowns is generally increasing, and the time between two transient faults is also shortening.
  • L is the sum of the loop inductances of the two-line mode, which is approximately equal to the positive value of the line. Twice the sequence inductance value, R is the sum of the two-line mode loop resistance, approximately twice the positive sequence resistance value of the line, R tr represents the zero-sequence equivalent resistance, which is the equivalent value in the arc channel formed by the fault grounding point Transition resistance, C 0 is zero-mode capacitance, 3L L represents the inductance value of the arc suppression coil, uk represents the additional zero-sequence power supply voltage at the fault point, and the closing of the switch S is equivalent to the occurrence of an instantaneous ground fault.
  • the capacitance and inductance currents of the equivalent circuit are deduced with the help of Laplace transform and inverse transform, as shown in equations (1) and (2).
  • I Cm the amplitude of the capacitive current flowing through the single-phase ground fault point
  • I Cm U m ⁇ C 0
  • ⁇ – the attenuation coefficient of the transient capacitive current
  • I Lm – the inductor current amplitude in the loop – the initial phase angle of the faulted phase voltage
  • ⁇ L – the time constant of the inductance loop.
  • i C is the capacitor loop current
  • the first term in equation (1) is the transient capacitive current component, which has the property of high-frequency free oscillation. The first term will gradually decay, and finally there is only the steady-state capacitive current component in the final loop, which is the second term in equation (1).
  • i L is the inductor loop current. Since the loop has no capacitance and only contains inductance and resistance, the first term of transient inductor current in equation (2) is the decaying DC component, and the second term is the steady-state inductor current component.
  • the first term in formula (3) is the steady-state component of the total current. From the observation of the first term, it can be seen that the steady-state current is the difference between the steady-state components of the capacitor and the inductor current.
  • the composition of the total current steady-state component is single, and its nature is related to the compensation degree of the arc suppression coil. When the compensation degree is greater than 1, a small inductive fundamental current will flow through the head end of the fault line and the grounding point; when the compensation degree is less than 1, the The fault line and the grounding point flow a small capacitive fundamental current; when the compensation degree is equal to 1, the fundamental current of the fault line and the grounding point is zero. Therefore, the compensation effect of the arc suppression coil makes the steady-state component very weak, and more information about the instantaneous fault cannot be obtained from it.
  • the second term is the transient component of the total current, which is formed by the superposition of the transient components of the capacitor current and the inductor current.
  • the frequencies of the two are different, or even have a large difference, so they basically cannot cancel each other out, but are more likely to become larger.
  • the transient zero-mode current expression is:
  • the transient zero-mode current component can be expressed as:
  • the transient zero-mode current component does not contain the inductive component, but only the capacitive component of oscillation attenuation.
  • the free oscillation angular frequency ⁇ f of the transient current is determined by the oscillation frequency of the capacitive component. After considering the actual distribution network parameters, the calculation formula is:
  • the distribution line parameters R, L, C 0 can be approximately regarded as fixed, while the transition impedance has a larger variation range, so the free oscillation angular frequency of the transient current mainly depends on the transition.
  • the change of impedance, transition resistance will cause the change of the oscillation frequency of the transient current.
  • the increase of the transition resistance will move the spectral energy distribution of the fault signal to the direction of the low frequency band, that is, the transition resistance will increase, and the content of the high frequency band of the transient zero-mode current will be reduced.
  • the fault transient zero-mode current oscillation frequency ⁇ f of the overhead line is in the range of about 300Hz to 1500Hz.
  • the cable line has a much smaller inductance than the overhead line and the distributed capacitance is larger than that of the overhead line.
  • the fault transient zero-mode current oscillation frequency ⁇ f is generally in the range of 1500Hz to 3000Hz, so the value of the fault transient zero-mode current oscillation frequency ⁇ f is much larger than the power frequency ⁇ .
  • the amplitude of the fault transient zero-mode current is mainly determined by the ratio of ⁇ f / ⁇ , and the amplitude is proportional to the oscillation frequency.
  • the oscillation frequency ⁇ f is related to the transition resistance, so the transition resistance changes the oscillation characteristics at the same time. The magnitude of the transient zero-mode current, the larger the transition resistance, the smaller the transient zero-mode current amplitude.
  • the degree of insulation deterioration of the three-phase lines is not exactly the same, so the transient fault always occurs at the weakest point of the insulation, forming a single-phase grounding, so the transient faults are generally single-phase grounding faults.
  • the transient component in the zero-mode current mainly contains the capacitive component of oscillation attenuation and is much larger than the steady-state component, which is suitable as a data source for studying the insulation state of the line.
  • the transition resistance shows a downward trend. This change will cause a change in the oscillation frequency of the transient current and move the spectral energy distribution to the high frequency band.
  • the transition resistance changes the oscillation characteristics and also changes the transient state.
  • the magnitude of the zero-mode current increases the magnitude of the transient zero-mode current.
  • a slow-developing permanent fault early warning method including:
  • the transient zero-mode current amplitude reflects the grounding impedance of the fault point to a certain extent, and also indirectly reflects the degree of insulation deterioration, which is positively correlated with the degree of insulation deterioration.
  • the proportion of low-frequency band energy in the total energy is the low-frequency band energy factor of zero-sequence current.
  • the energy distribution of zero-sequence current in each frequency band It is related to the intensity of the instantaneous fault. The smaller the energy factor in the low frequency band of the zero-sequence current, the smaller the transition resistance and the more serious the insulation degradation, which is negatively correlated with the degree of insulation degradation.
  • the other four features can be directly calculated based on the transient fault recording data, while the low-band energy factor of the zero-sequence current needs to be decomposed into the recorded waveform before being calculated.
  • the zero-sequence current is decomposed with equal bandwidth by wavelet packet transform, that is, the signal is passed through a conjugate quadrature filter bank formed by a combination of high and low pass, and the signal of the upper layer is continuously subdivided into different frequency bands. Each time it acts on the signal, the sampling interval is doubled and the data points are halved.
  • the sampling frequency of the signal is f s
  • the Nyquist frequency is f s /2
  • the frequency band bandwidth after n-layer decomposition is f s /2/2
  • n f s /2 n+1
  • the jth sub-band is The frequency range can be expressed as:
  • the coefficient of the jth subband after decomposing the signal x k with n layers is: m is the number of coefficients in each sub-band, then the energy of each sub-band is:
  • the energy factor E Lk in the low frequency band of the zero-sequence current can be obtained as:
  • Each key feature quantity is constructed as an insulation degradation value function D according to its correlation with the degree of insulation degradation.
  • D is, the more serious the degree of insulation degradation is.
  • Cumulative degradation value of line k Defined as historical degradation value The sum of the degradation value D′ k caused by this transient fault is shown in formula (10):
  • the neural network method based on sample training can also obtain the mapping relationship between the key feature quantity and the degree of insulation degradation, and introduce weights to distinguish different key Contribution of the feature quantity to the degree of insulation deterioration.
  • the present disclosure analyzes the development law of permanent faults and the principle of transient grounding faults, and summarizes the characteristics of slowly developing permanent grounding faults.
  • the key feature set of the discharge waveform of the permanent fault is obtained, and the extracted key features reflect the insulation degradation process in the slowly developing permanent grounding fault, so as to predict the slowly developing permanent fault.
  • the early warning method of the present disclosure is designed based on the signal processing mechanism of the distribution network fault recording device, and does not have the problem of excessive increase in operating costs due to the need to install a large number of additional fault feature detection and processing devices. requirements for practical applications.
  • a slowly developing permanent fault early warning system including a detection module, a controller and an alarm model, the detection module obtains the recorded waveform of the transient fault, and the controller receives the recorded waveform obtained by the detection module Then, extract the key feature quantities of the recorded waveform, calculate the insulation degradation value of each transient fault according to the key feature quantity, and accumulate the insulation degradation values obtained for multiple times to obtain the cumulative insulation degradation value. When the cumulative insulation degradation value When the insulation deterioration threshold is exceeded, alarm information is sent to the alarm module for alarm.
  • the key features extracted by the controller from the recorded waveform including the number of transient faults in a period of time, the interval between two transient faults, the transient zero-mode current amplitude, the fault duration and the zero-sequence current Low frequency band energy factor.

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  • General Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Mathematical Physics (AREA)
  • Theoretical Computer Science (AREA)
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Abstract

一种缓慢发展型永久性故障预警方法和系统，方法包括：获取线路发生永久性故障前的各次瞬时性故障的录波波形；通过录波波形判断故障区段，并提取关键特征量；依据关键特征量计算各次瞬时性故障时的绝缘劣化值；将同一故障区段多次获得的绝缘劣化值进行累积，获取累积绝缘劣化值；当累积绝缘劣化值超过绝缘劣化阈值时，发出预警。获取永久故障发生前的各次瞬时性故障录波波形，并提取故障关键特征量，通过提取的关键特征量判断线路的绝缘劣化程度，进而根据绝缘劣化程度发出预警。

Description

一种缓慢发展型永久性故障预警方法和系统 技术领域
本公开涉及一种缓慢发展型永久性故障预警方法和系统。
背景技术
本部分的陈述仅仅是提供了与本公开相关的背景技术信息，不必然构成在先技术。
配电线路直接连接用户，直接影响着生活生产的各方面，在电力系统中有着极其重要的作用，由于承担着分配电能的职责，配电网络覆盖面较大，具有多馈线、多分支、拓扑多变、点多面广等特点，运行环境相对较恶劣，非常容易发生故障，统计数据表明大部分线路故障属于瞬时性故障。
对于一条配电线路来说，当单相瞬时接地故障多次发生在一相同位置时，每次故障都会在接地点释放能量，破坏接地点处的线路绝缘。尽管这种故障是暂时的，故障持续时间大多从几毫秒至数分钟不等，但每次故障发生后线路的绝缘都会产生不同程度的劣化，这样多次的积累会使该线路的绝缘劣化达到极限，最终在某次接地后集中爆发形成永久性故障，这种故障模式可总结为缓慢发展型永久性故障，缓慢发展型永久性故障模式也符合实际中很多的故障情景。
如何对缓慢发展型永久性故障进行预警，从而避免永久性故障的发生是极其重要的。
发明内容
本公开为了解决上述问题，提出了一种缓慢发展型永久性故障预警方法和系统，获取永久故障发生前的各次瞬时性故障录波波形，并提取故障关键特征量，通过提取的关键特征量判断线路的绝缘劣化程度，进而根据绝缘劣化程度发出预警。
本公开的第一目的是，提出一种缓慢发展型永久性故障预警方法，包括：
获取线路发生永久性故障前的各次瞬时性故障的录波波形；
通过录波波形判断故障区段，并提取关键特征量；
依据关键特征量计算各次瞬时性故障时的绝缘劣化值；
将同一故障区段多次获得的绝缘劣化值进行累积，获取累积绝缘劣化值；
当累积绝缘劣化值超过绝缘劣化阈值时，发出预警。
进一步的，关键特征量，包括，一段时间内瞬时性故障发生的次数、两次瞬时性故障之间的间隔、暂态零模电流幅值、故障持续时间和零序电流低频带能量因子。
进一步的，一段时间内瞬时性故障发生的次数与绝缘劣化程度呈正相关；两次瞬时性故障之间的间隔与绝缘劣化程度呈负相关；暂态零模电流幅值与绝缘劣化程度呈正相关；故障持续时间与与绝缘劣化程度呈正相关；零序电流低频带能量因子与绝缘劣化程度呈负相关。
进一步的，一段时间内瞬时性故障发生的次数、两次瞬时性故障之间的间隔、暂态零模电流幅值和故障持续时间，从获取的录波波形中直接获取。
进一步的，对获取的录波波形进行频带分解获得各子频带能量，根据第一个子频带能量与各子频带能量之和的比值获取零序电流低频带能量因子。
进一步的，计算各次瞬时性故障的绝缘劣化值时，考虑关键特征量与绝缘劣化程度的相关关系。
进一步的，绝缘劣化值越大说明绝缘劣化程度越严重。
进一步的，绝缘劣化阈值，根据具体配电网实际运行经验确定。
本公开的第二目的是，提出一种缓慢发展型永久性故障预警系统，包括，检测模块、控制器和报警模型，通过检测模块获取瞬时性故障的录波波形，控制器接收检测模块获取的录波波形后，提取录波波形的关键特征量，依据关键特征量计算各次瞬时性故障时的绝缘劣化值，将多次获得的绝缘劣化值进行累积，获取累积绝缘劣化值，当累积绝缘劣化值超过绝缘劣化阈值时，发送报警信息至报警模块进行报警。
进一步的，控制器从录波波形中提取的关键特征量，包括，一段时间内瞬时性故障发生的次数、两次瞬时性故障之间的间隔、暂态零模电流幅值、故障持续时间和零序电流低频带能量因子。
与现有技术相比，本公开的有益效果为：
1、本公开利用永久故障发生前的各次瞬时性故障录波波形，获取故障关键特征量，通过提取的关键特征量计算了线路的累积绝缘劣化值，根据累积绝缘劣化值判断绝缘劣化程度，并发出预警，实现了对缓慢发展型永久性故障的预警。
2、本公开仅获取瞬时性故障录波波形，即可完成对缓慢发展型永久性故障的预警，而无需额外对其他故障特征进行检测，也无需大量安装额外的其他故障特征检测和处理装置，降低了运营成本，更符合实际应用的要求。
附图说明
构成本申请的一部分的说明书附图用来提供对本申请的进一步理解，本申请的示意性实施例及其说明用于解释本申请，并不构成对本申请的不当限定。
图1为本公开缓慢发展型永久性故障预警方法的故障预警流程；
图2为本公开累积绝缘劣化值变化曲线；
图3(a)为实施例1中某缓慢发展型永久性故障瞬时故障1频谱能量分布图；
图3(b)为实施例1中某缓慢发展型永久性故障瞬时故障2频谱能量分布图；
图3(c)为实施例1中某缓慢发展型永久性故障瞬时故障3频谱能量分布图；
图4为单相接地故障分量模网络图；
图5为瞬时单相接地故障等值电路；
图6为绝缘劣化不同时期的线路参数变化图。
具体实施方式：
下面结合附图与实施例对本公开作进一步说明。
应该指出，以下详细说明都是例示性的，旨在对本申请提供进一步的说明。除非另有指明，本文使用的所有技术和科学术语具有与本申请所属技术领域的普通技术人员通常理解的相同含义。
需要注意的是，这里所使用的术语仅是为了描述具体实施方式，而非意图限制根据本申请的示例性实施方式。如在这里所使用的，除非上下文另外明确指出，否则单数形式也意图包括复数形式，此外，还应当理解的是，当在本说明书中使用术语“包含”和/或“包括”时，其指明存在特征、步骤、操作、器件、组件和/或它们的组合。
实施例1
对架空线路来说，其上设备众多，有较大概率发生瞬时接地故障，例如雷击、绝缘部件闪络、瞬时放电、树枝等异物短时触碰等都会导致配电线路瞬时接地故障的发生，根据实际故障统计，在某架空线为主的配电网中，20次永久性接地故障里有7次曾在故障前发生不同程度的瞬时故障，其中有2次在永久性故障前 的三天内分别发生了多达15次和16次的瞬时性接地，有1次在永久性故障前一小时内发生了7次瞬时性接地，再如某变电站，6次永久性接地故障前72小时内曾有瞬时接地现象发生，最多的有16次瞬时性故障发生。对电缆线路来说，现代电缆的绝缘层主要为交联聚乙烯，通常认为其绝缘水平较高且绝缘击穿不可逆，一旦击穿则不可恢复，在电缆故障的实际统计中，也发现了相当部分的瞬时故障，如电缆中间接头和终端的故障、电缆本体的瞬时击穿现象，在某电缆故障案例中，内部持续100分钟的永久性接地故障发生前，检测到有8次瞬时性接地故障发生。
可以看出对于一条配电线路来说，当单相瞬时接地故障多次发生在一相同位置时，每次故障都会在接地点释放能量，破坏接地点处的线路绝缘。尽管这种故障是暂时的，故障持续时间大多从几毫秒至数分钟不等，大多小于1s，但每次故障发生后线路的绝缘都会产生不同程度的劣化，这样多次的积累会使该线路的绝缘劣化达到极限，最终在某次接地后集中爆发形成永久性故障，这种故障模式可总结为缓慢发展型永久性接地故障，这种故障模式也符合实际中很多的故障情景。
随着以现代信息技术为基础的电网同步量测系统及调度自动化系统的发展，故障录波装置及配网侧PMU等高精度同步量测装置进一步覆盖配电网各重要节点，调度中心能够获取到的有统一同步时标的精确信息越来越多，早期绝缘放电的各电气量的实际变化波形也能以时间序列的形式得到记录，其中蕴含着反映系统运行状态的丰富信息。
因此对永久性故障发生前的各次瞬时性故障录波数据进行分析，找到故障早期特征，得到故障特征-时空分布响应特性，进而分析判断线路的绝缘水平，适时给出报警信号，对缓慢发展型永久性接地故障提出预警方法，从而避免永久故障的发生是极其重要的。
分析永久性故障总体发展规律可知，永久性故障的形成是一个缓慢发展的过程，在发生永久性故障前，往往会发生瞬时性故障接地，此时元件会有间歇性放电-击穿的物理演化过程。通过对电缆、架空线及绝缘子及等主要设备的绝缘老化及故障形成进行分析得到配电线路永久性故障的总体发展规律。
配电网电力设备在运行电压及热应力、外机械力、受潮等环境条件的长期作用下，其绝缘呈现出一个缓慢劣化的过程。对于电缆、绝缘子内部等固体绝缘来说，在运行电压下一般不会被击穿，需要考虑的是在瞬时过电压作用下绝缘可能击穿，但瞬时过电压持续时间不长，在几秒后便会消失，此后电网将恢复正常运行。对于电力设备的外绝缘、油绝缘和气体绝缘来说，由于中低压配电网为小电流接地系统，故障电流较小且消弧线圈会抑制瞬时电弧，即使在运行电压下可能发生瞬时性击穿，在故障电流高频过零点时电弧也会熄灭，再考虑到故障点杂质和水分被电弧加热而烧毁和烘干，电弧可自然熄灭不会重燃。劣化初期，单相接地故障有明显地电弧接地特征，当再次出现过电压及各种恶劣运行条件时，绝缘薄弱点可能被继续击穿并进一步损坏绝缘，不稳定地电弧接地也逐渐向金属性接地发展，最终发生永久性单相接地。在这一过程中，总体上瞬时绝缘击穿次数呈上升态势，两次瞬时性故障间隔时间也呈缩短趋势。
在对绝缘劣化形成的电树枝导致的绝缘电阻和分布电容变化规律研究的相关工作中还发现，劣化前期会出现分布电容波动并不断增加而劣化中后期会出现等效电阻逐渐减小的现象，电阻的下降和电容的增加与绝缘劣化形成水树枝的程度一致且绝缘的相对介电常数和绝缘电阻均呈现非线性变化，如图6所示。可以看出在线路绝缘的劣化过程中，一般伴有等效分布电容的增加和绝缘电阻的下降，即在缓慢发展型永久性接地故障的瞬时性故障发展阶段过渡电阻呈下降趋势。
配电网中其他各种故障大多均由单相接地故障发展而来，单相接地故障发生后，一是非故障相电压升高至线电压，在其绝缘薄弱点处单相故障很可能发展成为两相乃至三相故障，特别是间歇性电弧接地会使非故障相出现高达3.5p.u.的电弧接地过电压而造成跳闸，二是对于断路器等开关设备，还会因为接地电弧与非故障相接触导致两相及三相短路，造成线路跳闸使故障范围扩大。
分析瞬时性接地故障原理可知，我国配电网运行方式大部分为中性点经消弧线圈接地，在发生瞬时单相接地故障时，系统的过渡过程可通过在故障点附加一个零序电压源来等效，实际数据表明在谐振接地配电网中线路网络阻尼率，即故障接地点有功电流与容性电流之比很小，忽略接地点的有功电流不会影响分析结果，以此作为接下来分析的简化条件，在图4单相接地故障分量模网络图基础上 得到瞬时单相接地故障等值电路如图5所示，图中L是两线模回路电感之和，约等于线路正序电感值的两倍，R为两线模回路电阻的和，近似为线路正序电阻值的两倍，R tr表示零序等值电阻,为故障接地点所形成的电弧通道中的等效过渡电阻，C 0为零模电容，3L L表示消弧线圈电感值，u k表示故障点附加的的零序电源电压，开关S闭合相当于瞬时接地故障发生。根据动态电路分析理论，借助Laplace变换及反变换对等值电路的电容和电感电流进行推导，如式(1)、(2)所示。
Figure PCTCN2020136201-appb-000001
Figure PCTCN2020136201-appb-000002
式中，I Cm–单相接地故障点流过的容性电流幅值，I Cm＝U mωC 0；ω f–暂态电流自由震荡角频率；δ–暂态电容电流衰减系数；ω–电源电压角频率；I Lm–回路中电感电流幅值；
Figure PCTCN2020136201-appb-000003
–故障相电压初相角；τ L–电感回路时间常数。
其中i C为电容回路电流，式(1)中第一项为暂态电容电流分量，具有高频自由震荡性质，是由于回路中含有电感和电容而产生的，因为回路中同时还有电阻，第一项会逐渐衰减，最终回路中只有稳态电容电流分量，也就是式(1)中第二项。i L为电感回路电流，由于该回路没有电容仅含有电感和电阻，所以式(2)中第一项暂态电感电流为衰减直流分量，第二项为稳态电感电流分量。
两部分分量叠加可得到接地点故障总电流，如式(3)所示：
Figure PCTCN2020136201-appb-000004
式(3)中第一项为总电流的稳态分量，观察第一项可知，稳态电流为电容和电感电流稳态分量之差，两者角频率相同，均为电源电压角频率，所以总电流稳态分量的成分单一，其性质与消弧线圈补偿度有关，当补偿度大于1时，故障线路首端和接地点会流过较小的感性基波电流；补偿度小于1时，故障线路和接地点流过的是较小的容性基波电流；补偿度等于1时，故障线路和接地点基波电 流为零。因此消弧线圈的补偿作用使得稳态分量非常微弱，无法从中得到更多的有关瞬时故障的信息。
第二项为总电流暂态分量，是由电容电流和电感电流的暂态分量叠加而成，二者频率不同，甚至相差较大，所以它们基本不能相互抵消反而更有可能变大，零模电流的暂态性质和接地点过渡阻抗、线路参数之间相互影响、存在紧密联系，虽然并不能直接据此直接评估配电线路的绝缘状态，但是从这一关系可以发现，通过提取瞬时故障后零模电流的相关暂态特征量来跟线路绝缘状态进行关联是可行的，据此暂态零模电流被本文选取作为研究配电线路绝缘状态的特征量提取来源。
接下来对暂态零模电流进行具体分析，暂态零模电流表达式为：
Figure PCTCN2020136201-appb-000005
1)故障初始相角
多数情况下，配电网中线路某处因绝缘击穿而引起的瞬时性接地故障常发生于相电压接近于最大值的瞬间，即故障初始相角
Figure PCTCN2020136201-appb-000006
的时刻，此时暂态零模电流分量可表示为：
Figure PCTCN2020136201-appb-000007
即发生瞬时性接地故障时暂态零模电流分量中不包含感性分量，只有震荡衰减的容性分量。
2)过渡电阻与暂态电流振荡频率
暂态电流自由震荡角频率ω f是由容性分量的振荡频率决定，考虑实际配电网参数后，其计算公式为：
Figure PCTCN2020136201-appb-000008
对一个谐振接地配电网来说，配电线路参数R、L、C 0可以近似看作是固定的，而过渡阻抗的变化范围则较大，因而暂态电流自由震荡角频率主要取决于过渡阻抗，过渡电阻的变化会引起暂态电流振荡频率的变化。考虑到配电的实际参 数，过渡电阻的增大会使故障信号的频谱能量分布向低频带方向移动，即过渡电阻变大，暂态零模电流高频带的含量会变少。
3)过渡电阻与暂态零模电流幅值
在实际配电网中单相接地故障时，架空线路的故障暂态零模电流振荡频率ω f约在300Hz～1500Hz的范围内，电缆线路由于其电感远小于架空线路而分布电容大于架空线路，故障暂态零模电流振荡频率ω f范围一般为1500Hz～3000Hz，因此故障暂态零模电流振荡频率ω f的值远大于工频频率ω。故障暂态零模电流的幅值大小主要由ω f/ω的比值决定而且幅值与振荡频率成正比关系，振荡频率ω f与过渡电阻相关，因此过渡电阻在改变振荡特性的同时也改变了暂态零模电流的幅值大小，过渡电阻越大，暂态零模电流的幅值越小。
基于分别对永久性接地故障规律及瞬时性接地故障原理的分析，对缓慢发展型永久性接地故障的特点做出总结：
1)瞬时性接地故障一般为单相接地故障
实际配电网中，三相线路的绝缘劣化程度不会完全一致，因此瞬时性故障总是在绝缘最薄弱点发生，形成单相接地，因此瞬时性故障一般均为单相接地故障。
2)接近发生永久性故障时，瞬时性故障发生频率及持续时间呈上升趋势
当线路绝缘的劣化加重时，绝缘强度变的不容易恢复，在发生瞬时性故障时电弧燃烧的持续时间变长，且该线路变的更容易建立电弧，这又会进一步加剧绝缘劣化，这一过程不断发展最终绝缘完全损坏。接近发生永久性故障时，瞬时性故障发生频率及持续时间呈上升趋势。
3)故障暂态零模电流中蕴含着丰富的瞬时性接地有关信息
绝缘劣化而引起的瞬时性接地故障常发生于初始相角
Figure PCTCN2020136201-appb-000009
附近，零模电流中暂态分量主要含震荡衰减的容性分量且比稳态分量大得多，适合作为研究线路绝缘状态的数据来源。
4)过渡电阻引发暂态零模电流频谱变化和幅值变化
绝缘劣化过程中发生的故障，过渡电阻呈下降趋势，这一变化会引起暂态电流振荡频率的变化，使频谱能量分布向高频段移动，且过渡电阻在改变振荡特性的同时也改变了暂态零模电流的幅值大小，使暂态零模电流的幅值变大。
基于上述研究，在该实施例中，提出了一种缓慢发展型永久性故障预警方法， 包括：
(一)获取永久性故障前的各次瞬时性故障的录波波形；
(二)从获取的录波波形中定位故障区段，并提取关键特征量，关键特征量，包括，一段时间内瞬时性故障发生的次数、两次瞬时性故障之间的间隔、暂态零模电流幅值、故障持续时间和零序电流低频带能量因子；
a.一段时间内瞬时性故障发生的次数N k
设定一段固定时间长度的窗口，在窗口沿时间轨迹滑动的过程中，考察时间窗口内瞬时性故障发生次数，瞬时性故障发生越频繁，则线路绝缘受到的破坏越严重，距永久性接地故障发生也就越近，与绝缘劣化程度呈正相关。
b.两次故障之间的间隔T kit
考察第k条线路本次瞬时性故障与上一次瞬时性故障之间的时间间隔，与绝缘劣化程度呈负相关，在绝缘接地故障频率上升，线路绝缘破坏加重的过程中，该时间间隔会越来越短。
c.暂态零模电流幅值I k
暂态零模电流幅值一定程度上反映了故障点接地阻抗的大小，也间接反映了绝缘劣化的程度，与绝缘劣化程度呈正相关，暂态零模电流幅值越大，绝缘劣化程度越高，该次瞬时性故障越严重。
d.故障持续时间T kdt
故障持续时间越长，该次瞬时性接地故障的强度越大，则绝缘破坏越严重，线路绝缘水平下降程度越大，与绝缘劣化程度呈正相关。
e.零序电流低频带能量因子E Lk
定义低频带能量在总能量中的占比为零序电流低频带能量因子，线路绝缘劣化过程中，常伴随绝缘电阻的下降和等效分布电容的增加,零序电流的在各频带的能量分布与瞬时故障的强度有关，零序电流低频带能量因子越小，则过渡电阻越小，绝缘劣化越严重，与绝缘劣化程度呈负相关。
除零序电流低频带能量因子外，其他四个特征均可依据瞬时性故障录波数据直接计算，而零序电流低频带能量因子需先对录波波形进行频带分解后再计算。利用小波包变换对零序电流进行等宽频带分解，也就是让信号通过高低通组合形成的共轭正交滤波器组，不断将上一层的信号继续细分至不同的频段上，滤波器 每作用在信号上一次，采样间隔就增加一倍，数据点减半。若信号的采样频率为f s，则由采样定理，Nyquist频率为f s/2，n层分解后频段带宽为f s/2/2 n＝f s/2 n+1，第j个子频段的频率范围可以表示为：
Figure PCTCN2020136201-appb-000010
记信号x k进行n层分解后第j个子频段的系数为
Figure PCTCN2020136201-appb-000011
m为各子频带下的系数个数，则各子频带能量为：
Figure PCTCN2020136201-appb-000012
通过合理设置分解层数n，使工频50Hz出现在j＝1频段且避免三次、五次谐波和工频混叠，则可得零序电流低频带能量因子E Lk为：
Figure PCTCN2020136201-appb-000013
(三)依据关键特征量计算各次瞬时性故障时的绝缘劣化值，并将同一故障区段，多次获得的绝缘劣化值进行累积，获取累积绝缘劣化值；
将各关键特征量按其与绝缘劣化程度的相关关系构造为绝缘劣化值函数D，D越大说明绝缘劣化程度越严重。线路k的累积劣化值
Figure PCTCN2020136201-appb-000014
定义为历史劣化值
Figure PCTCN2020136201-appb-000015
与本次瞬时性故障造成的劣化值D′ k之和，如式(10)所示：
Figure PCTCN2020136201-appb-000016
Figure PCTCN2020136201-appb-000017
利用瞬时性故障的录波波形计算各特征量后更新绝缘劣化值函数，获得的累积绝缘劣化值变化曲线如图2所示。
本公开，除通过关键特征量与绝缘劣化程度间的相关性关系构造函数外，也可基于样本训练的神经网络方法获得关键特征量与绝缘劣化程度间的映射关系，并通过引入权重区别不同关键特征量对绝缘劣化程度的贡献。
(四)当累积绝缘劣化值超过绝缘劣化阈值时，发出预警，绝缘劣化阈值， 根据具体配电网实际运行经验确定。
在某缓慢发展型永久性故障案例中各次瞬时性故障频谱能量分布如图3(a)-3(c)所示，永久性故障之前曾发生过三次瞬时性故障。
本公开在分析永久性故障发展规律及瞬时性接地故障原理，对缓慢发展型永久性接地故障的特点做出总结的基础上，从瞬时性故障发生频率和瞬时性故障强度两方面提取了早期瞬时性故障放电波形的关键特征集，并用提取的关键特征量对缓慢发展型永久性接地故障中绝缘劣化过程进行了良好反映，从而预测了缓慢发展型永久性故障。
本公开的预警方法是基于配电网故障录波装置信号处理机制所设计的，不会出现因为需要大量安装额外的其他故障特征检测和处理装置带来的运营成本过多增加的问题，更符合于实际应用的要求。
实施例2
在该实施例中，提出一种缓慢发展型永久性故障预警系统，包括，检测模块、控制器和报警模型，检测模块获取瞬时性故障的录波波形，控制器接收检测模块获取的录波波形后，提取录波波形的关键特征量，依据关键特征量计算各次瞬时性故障时的绝缘劣化值，并将多次获得的绝缘劣化值进行累积，获取累积绝缘劣化值，当累积绝缘劣化值超过绝缘劣化阈值时，发送报警信息至报警模块进行报警。
控制器从录波波形中提取的关键特征量，包括，一段时间内瞬时性故障发生的次数、两次瞬时性故障之间的间隔、暂态零模电流幅值、故障持续时间和零序电流低频带能量因子。
以上仅为本申请的优选实施例而已，并不用于限制本申请，对于本领域的技术人员来说，本申请可以有各种更改和变化。凡在本申请的精神和原则之内，所作的任何修改、等同替换、改进等，均应包含在本申请的保护范围之内。
上述虽然结合附图对本公开的具体实施方式进行了描述，但并非对本公开保护范围的限制，所属领域技术人员应该明白，在本公开的技术方案的基础上，本领域技术人员不需要付出创造性劳动即可做出的各种修改或变形仍在本公开的保护范围以内。

Claims (10)

  1. 一种缓慢发展型永久性故障预警方法，其特征在于，包括：
    获取线路发生永久性故障前的各次瞬时性故障的录波波形；
    通过录波波形判断故障区段，并提取关键特征量；
    依据关键特征量计算各次瞬时性故障时的绝缘劣化值；
    将同一故障区段多次获得的绝缘劣化值进行累积，获取累积绝缘劣化值；
    当累积绝缘劣化值超过绝缘劣化阈值时，发出预警。
  2. 如权利要求1所述的一种缓慢发展型永久性故障预警方法，其特征在于，关键特征量，包括，一段时间内瞬时性故障发生的次数、两次瞬时性故障之间的间隔、暂态零模电流幅值、故障持续时间和零序电流低频带能量因子。
  3. 如权利要求2所述的一种缓慢发展型永久性故障预警方法，其特征在于，一段时间内瞬时性故障发生的次数与绝缘劣化程度呈正相关；两次瞬时性故障之间的间隔与绝缘劣化程度呈负相关；暂态零模电流幅值与绝缘劣化程度呈正相关；故障持续时间与与绝缘劣化程度呈正相关；零序电流低频带能量因子与绝缘劣化程度呈负相关。
  4. 如权利要求2所述的一种缓慢发展型永久性故障预警方法，其特征在于，一段时间内瞬时性故障发生的次数、两次瞬时性故障之间的间隔、暂态零模电流幅值和故障持续时间，从获取的录波波形中直接获取。
  5. 如权利要求2所述的一种缓慢发展型永久性故障预警方法，其特征在于，对获取的录波波形进行频带分解获得各子频带能量，根据第一个子频带能量与各子频带能量之和的比值获取零序电流低频带能量因子。
  6. 如权利要求1所述的一种缓慢发展型永久性故障预警方法，其特征在于，计算各次瞬时性故障的绝缘劣化值时，考虑关键特征量与绝缘劣化程度的相关关系。
  7. 如权利要求1所述的一种缓慢发展型永久性故障预警方法，其特征在于，绝缘劣化值越大说明绝缘劣化程度越严重。
  8. 如权利要求1所述的一种缓慢发展型永久性故障预警方法，其特征在于，绝缘劣化阈值，根据具体配电网实际运行经验确定。
  9. 一种缓慢发展型永久性故障预警系统，其特征在于，包括，检测模块、 控制器和报警模型，通过检测模块获取瞬时性故障的录波波形，控制器接收检测模块获取的录波波形后，提取录波波形的关键特征量，依据关键特征量计算各次瞬时性故障时的绝缘劣化值，将多次获得的绝缘劣化值进行累积，获取累积绝缘劣化值，当累积绝缘劣化值超过绝缘劣化阈值时，发送报警信息至报警模块进行报警。
  10. 如权利要求9所述的一种缓慢发展型永久性故障预警系统，其特征在于，所述控制器从录波波形中提取的关键特征量，包括，一段时间内瞬时性故障发生的次数、两次瞬时性故障之间的间隔、暂态零模电流幅值、故障持续时间和零序电流低频带能量因子。
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