Traveling wave protection method for power electronic transformer side line of medium-voltage direct-current power distribution network

Abstract

The invention provides a medium-voltage direct-current power distribution network power electronic transformer side line traveling wave protection method, and relates to the technical field of power system relay protection, and the method comprises the steps: taking a fault current overcurrent as a fault recognition criterion, and starting power electronic transformer side line traveling wave main protection based on overcurrent detection; and backward voltage traveling waves on the line are calculated, and the attenuation step characteristics of the backward voltage traveling waves are extracted by using wavelet transform, so that main protection determines whether to act according to whether the attenuation step characteristics exist or not. By adopting the scheme, the fault identification of the power electronic transformer side line is realized.

Description

Technical Field

[0001] The present application relates to the technical field of relay protection for power systems, and in particular to a method and system for traveling wave protection of power electronic transformer-side lines in a medium-voltage direct current distribution network. Background Art

[0002] A DC solid-state transformer (DCSST) is a power electronic transformer widely used in medium-voltage DC distribution systems. Its main function is to realize DC power conversion between different voltage levels.

[0003] DCSSTs typically utilize multiple modules in parallel or series to increase power capacity and efficiency, and rely on DC-side shunt capacitors for voltage stabilization. However, this results in the DCSST feeding fault current, dominated by capacitor discharge, into the fault point during a line fault, causing a rapid increase in current. Without DC reactors in the DC line, existing protection cannot quickly identify faults before the converter shuts down. Therefore, fast protection technology is needed to accurately identify faults. Summary of the Invention

[0004] The present application aims to solve one of the technical problems in the related art at least to a certain extent.

[0005] To this end, the first objective of this application is to propose a traveling wave protection method for a power electronic transformer-side line in a medium-voltage DC distribution network, thereby realizing fault identification of the power electronic transformer-side line.

[0006] The second objective of the present application is to provide a traveling wave protection system for power electronic transformer-side lines in a medium-voltage DC distribution network.

[0007] To achieve the above objectives, the first embodiment of the present application proposes a traveling wave protection method for a power electronic transformer-side line in a medium-voltage DC distribution network, comprising:

[0008] Taking the fault current overcurrent as the fault identification criterion, the DCST side line traveling wave main protection is activated based on the overcurrent detection;

[0009] The reverse voltage traveling wave on the line is calculated, and the attenuation step characteristics of the reverse voltage traveling wave are extracted using wavelet transform, so that the main protection determines whether to operate based on whether the attenuation step characteristics exist.

[0010] To achieve the above-mentioned objectives, a second embodiment of the present invention provides a traveling wave protection system for power electronic transformer-side lines in a medium-voltage DC distribution network, comprising:

[0011] The traveling wave protection module based on overcurrent detection is used to use the fault current overcurrent as the fault identification criterion and activate the DCST side line traveling wave main protection based on the overcurrent detection;

[0012] The traveling wave protection module based on reverse voltage traveling wave step is used to calculate the reverse voltage traveling wave on the line and extract the attenuation step characteristics of the reverse voltage traveling wave using wavelet transform, so that the main protection can determine whether to operate based on whether the attenuation step characteristics exist.

[0013] The traveling wave protection method and system for the power electronic transformer-side line of a medium-voltage DC distribution network in the embodiments of the present application adopt a binary wavelet transform algorithm with a scale of three to extract the traveling wave refraction characteristics of the adjacent converter side, thereby realizing the identification of in-zone and out-of-zone faults of the power electronic transformer-side line.

[0014] Additional aspects and advantages of the present application will be given in part in the description below, and in part will become apparent from the description below, or will be learned through practice of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the following description of the embodiments in conjunction with the accompanying drawings, in which:

[0016] Figure 1 This is the topology diagram of the medium voltage DC distribution network;

[0017] Figure 2 A flow chart of a traveling wave protection method for a power electronic transformer-side line in a medium-voltage DC distribution network provided in Example 1 of the present application;

[0018] Figure 3 This is a schematic diagram of the principle of binary wavelet transform according to an embodiment of the present application;

[0019] Figure 4 This is a schematic diagram of the non-exponential decay characteristics of an embodiment of the present application;

[0020] Figure 5 Schematic diagram of the exponential decay characteristics of an embodiment of the present application. DETAILED DESCRIPTION

[0021] The following describes in detail embodiments of the present application, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present application, and should not be construed as limiting the present application.

[0022] Figure 1 is the topology diagram of the medium voltage DC distribution network, such as Figure 1As shown, in a medium-voltage DC power distribution system, the reverse voltage traveling waves on both sides of the DCSST are affected by the refraction and reflection of the adjacent converter boundaries during propagation. According to the basic principles of signal processing, the step wave contains harmonic components of all frequencies, and the boundaries on the adjacent converter sides will exhibit different refractive indexes for the high and low frequency components in the initial traveling wave that approximates the step, thus causing the refracted waves on the adjacent converter sides to produce exponential attenuation characteristics in the time domain. Therefore, in the DCSST protection design, this application fully considers the differences in the traveling wave propagation characteristics on different sides.

[0023] The following describes a traveling wave protection method and system for a power electronic transformer-side line in a medium-voltage DC distribution network according to an embodiment of the present application with reference to the accompanying drawings.

[0024] Figure 2 This is a flow chart of a traveling wave protection method for a power electronic transformer-side line in a medium-voltage DC distribution network provided in Example 1 of the present application.

[0025] like Figure 2 As shown, the traveling wave protection method for the power electronic transformer side line of the medium voltage DC distribution network includes the following steps:

[0026] In step 201, the fault current overcurrent is used as a fault identification criterion, and the DCST side line traveling wave main protection is started based on the overcurrent detection.

[0027] In this embodiment, starting the traveling wave main protection of the DCST-side line based on overcurrent detection includes:

[0028] When a fault occurs, the DCSST side line will feed a large amount of current to the fault point. Therefore, the fault current overcurrent can be used as a fault identification criterion to prevent false start of protection.

[0029] i-i0>K setl I line,N

[0030] K setl is the starting criterion setting value of the DCST side line direction overcurrent, i0 is the line current measured 500μs before the current acquisition time, I line,N is the rated current of the line.

[0031] The overcurrent startup criterion of DCSST needs to take into account the startup sensitivity and reliability against high-frequency noise interference. When the line current on both sides of the DCSST is higher than the rated current, the protection starts:

[0032] K setl =1

[0033] Step 202 : Calculate the reverse voltage traveling wave on the line and extract the attenuation step feature of the reverse voltage traveling wave using wavelet transform, so that the main protection determines whether to operate according to whether the attenuation step feature exists.

[0034] In this embodiment, the reverse voltage traveling wave is calculated, and the attenuation step characteristics of the reverse voltage traveling wave are extracted using wavelet transform, including:

[0035]

[0036] Where x is the position, usually the distance from a point to a specific end of the line, t is the time, and u f (x, t) is the forward voltage traveling wave on the line, u b (x, t) is the reverse voltage traveling wave, Z c is the wave impedance of the line, u(x,t) is the voltage at point x on the line at time t, and i(x,t) is the current at point x on the line at time t;

[0037] In this embodiment, the wavelet transform is a signal time-frequency analysis method based on a variable window function. It can decompose the time domain signal into different scales, reflecting the signal details in different frequency domains. This embodiment uses the wavelet transform to identify: 1) the step of the reverse voltage traveling wave; 2) the exponential decay characteristics of the reverse voltage traveling wave.

[0038]

[0039] Where j = 1, 2, 3, ..., represents the scale of wavelet decomposition. Specifically, in this formula:

[0040] The first formula represents the decomposition of the j-th layer wavelet signal, represents the approximate coefficient of the jth layer, representing the main low-frequency characteristics of the signal at this layer; h(k) is the low-pass filter coefficient, used to extract the low-frequency part of the signal; is the approximate coefficient of the previous layer; Represents the process of filtering and downsampling the approximate coefficients of the previous layer.

[0041] The second formula represents the reconstruction of the j-th layer wavelet signal, represents the detail coefficient of the jth layer, representing the details and high-frequency characteristics of the signal at this layer; g(j) is the high-pass filter coefficient, which is used to extract the low-frequency part of the signal; is the approximate coefficient of the previous layer; kgkA2j-1dfn-2j-1k represents the process of filtering and upsampling the approximate coefficient of the previous layer.

[0042] In this embodiment, the modulus maximum of the wavelet transform is considered to extract the negative polarity step feature of the traveling wave, and the appropriate wavelet transform scale is selected by analyzing the frequency bands of the noise and step feature, specifically including:

[0043] An ideal step signal is a broadband function, but the system's random noise, due to the high sampling frequency, mostly manifests as high-frequency noise. At a sampling period of 1 MHz, according to the sampling theorem, the acquired signal contains signals up to 500 kHz. The exponential decay characteristic of the fault traveling wave can be approximated as a negative step superimposed on a positive triangular wave. Therefore, the result of its wavelet transform is a negative high-frequency signal at the moment of the fault and a continuous positive medium- and low-frequency signal after the step moment. The duration of the attenuation step's traveling wave depends on the line length from the fault point to the adjacent converter side, accounting for approximately 1 / 3 of the DCSST negative polarity step period. The lower limit of the frequency band for extracting the attenuation step characteristic is approximately 60 kHz. Therefore, it is necessary to select an appropriate wavelet transform scale and frequency band within the range of 60 kHz to 500 kHz.

[0044] Since the high-frequency signal bands extracted by wavelet transforms of different scales are different, the fault information that can be obtained is also different. Figure 3 Based on the wavelet transform principle, a dyadic wavelet transform with a scale of three can extract high-frequency signals in the 62.5kHz to 125kHz range, meeting the requirements of this embodiment for extracting the exponential decay characteristics of fault traveling waves. On the one hand, a dyadic wavelet transform with a scale of four can extract high-frequency signals in the 31.25kHz to 62.5kHz frequency band, where it is difficult to extract the exponential decay characteristics of traveling waves. On the other hand, compared to wavelet transforms with scales one and two, a wavelet transform with a scale of three can better avoid interference from high-frequency random noise.

[0045] Therefore, the binary wavelet transform with a scale of three is adopted as the fault feature extraction algorithm, and its modulus maximum can better characterize the exponential decay characteristics of the negative polarity step.

[0046] In this embodiment, if a negative traveling wave modulus maximum is detected followed by a continuous positive signal between 67.5kHz and 125kHz, the fault traveling wave is considered to have an exponential decay characteristic. For a negative modulus maximum, if three positive points are detected before the next negative signal appears, the traveling wave corresponding to that modulus maximum is considered to have an exponential decay characteristic.

[0047] In this embodiment, detection is started from the initial traveling wave head of the fault. If three consecutive reverse voltage traveling wave heads with exponential decay characteristics are detected, it is considered that an out-of-zone fault is measured.

[0048] M 指数衰减特征 →Out-of-area fault

[0049] Where M 指数衰减特征It is the criterion for extracting the exponential decay feature, and the exponential decay feature is extracted through the protection algorithm.

[0050] In this embodiment, the main protection determines whether to operate based on whether there is a decay step feature, including:

[0051] The fault characteristic of the DCSST-side line is primarily manifested in the absence of an exponential decay characteristic in the reverse voltage traveling wave received by the DCSST-side line of the faulty line. Because the initial fault traveling wave must be refracted by adjacent converters before reaching the non-faulty line, the reverse voltage traveling wave on the non-faulty line exhibits an exponential decay characteristic. Within the propagation range of the traveling wave, the DCSST-side line of the faulty line can measure a negative polarity step reverse voltage traveling wave, while the DCSST-side line of the non-faulty line can detect a negative polarity step reverse voltage traveling wave with an exponential decay characteristic. Figure 4 is a schematic diagram of non-exponential decay characteristics, Figure 5 Schematic diagram of exponential decay characteristics.

[0052] After accurately detecting a fault, an algorithm detects the exponential decay characteristics of the reverse voltage traveling wave. The difficulty in detecting line faults on the DCSST side lies in distinguishing between far-zone and out-of-zone faults. Accurately distinguishing far-zone and out-of-zone faults is difficult using only the initial traveling wave data from the fault. To ensure protection reliability, a delay strategy is implemented for both near-out-of-zone and far-in-zone faults. Further judgment is made by the near backup protection on the DCSST side of the line, with the delay time determined by the backup protection requirements. When the exponential decay characteristic conditions are met, the protection is delayed.

[0053] In order to implement the above embodiment, the present application also proposes a traveling wave protection system for power electronic transformer-side lines in a medium-voltage DC distribution network.

[0054] The traveling wave protection system for the power electronic transformer side of the medium voltage DC distribution network includes:

[0055] The traveling wave protection module based on overcurrent detection is used to use the fault current overcurrent as the fault identification criterion and activate the DCST side line traveling wave main protection based on the overcurrent detection;

[0056] The traveling wave protection module based on reverse voltage traveling wave step is used to calculate the reverse voltage traveling wave on the line and extract the attenuation step characteristics of the reverse voltage traveling wave using wavelet transform, so that the main protection can determine whether to operate based on whether the attenuation step characteristics exist.

[0057] Furthermore, in the embodiment of the present application, the overcurrent detection module is specifically configured to:

[0058] When a fault occurs, the DCSST side line feeds current to the fault point. The fault current is detected as overcurrent and the DCSST side line traveling wave main protection is activated. The fault current overcurrent is expressed as:

[0059] i-i0>K setl I line,N

[0060] Among them, K setl is the starting criterion setting value of the DCST side line direction overcurrent, i is the line current measured at the current acquisition moment, i0 is the reference line current measured before the current acquisition moment, I line,N is the rated current of the line.

[0061] Specifically, in the embodiment of the present application, the reverse voltage traveling wave on the line is calculated, and the attenuation step characteristics of the reverse voltage traveling wave are extracted using wavelet transform, including:

[0062] The reverse voltage traveling wave is expressed as:

[0063]

[0064] Where x is the position on the line, t is the time, and u f (x, t) is the forward voltage traveling wave on the line, u b (x, t) is the reverse voltage traveling wave, Z c is the wave impedance of the line, u(x,t) is the voltage at point x on the line at time t, and i(x,t) is the current at point x on the line at time t;

[0065] A scale-three binary wavelet transform is used as the fault feature extraction algorithm, and the modulus maximum of the wavelet transform is used to extract the negative polarity step feature and the exponential decay feature of the step of the reverse voltage traveling wave.

[0066] If a continuous positive high-frequency signal appears after the negative traveling wave modulus maximum is detected, the fault traveling wave is considered to have an exponential decay characteristic. The specific process includes:

[0067] For a negative polarity modulus maximum, if three positive polarity points are detected before the next negative polarity appears, it is considered that the reverse voltage traveling wave corresponding to the modulus maximum has an exponential decay characteristic.

[0068] Furthermore, in the embodiment of the present application, the main protection is configured to determine whether to operate according to whether a decay step feature exists, including:

[0069] When a fault occurs, within the range of traveling wave propagation, the DCSST side of the fault line detects a negative polarity step reverse voltage traveling wave without exponential decay characteristics, and the DCSST side of the non-fault line detects a negative polarity step reverse voltage traveling wave with exponential decay characteristics. Starting from the initial traveling wave head of the fault, if there are three consecutive reverse voltage traveling wave heads with exponential decay characteristics, it is considered that a fault has occurred and protection is delayed.

[0070] It should be noted that the above explanation of the embodiment of the traveling wave protection method for the power electronic transformer side line of the medium voltage DC distribution network is also applicable to the traveling wave protection system for the power electronic transformer side line of the medium voltage DC distribution network of this embodiment, and will not be repeated here.

[0071] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example" or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification and features of different embodiments or examples without contradiction.

[0072] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of such features. Throughout the description of this application, "plurality" means at least two, for example, two, three, etc., unless otherwise specifically defined.

[0073] Any process or method description in a flowchart or otherwise described herein may be understood to represent a module, segment or portion of code comprising one or more executable instructions for implementing the steps of a custom logical function or process, and the scope of the preferred embodiments of the present application includes alternative implementations in which functions may be performed out of the order shown or discussed, including performing functions in a substantially simultaneous manner or in the reverse order depending on the functions involved, which should be understood by those skilled in the art to which the embodiments of the present application belong.

[0074] The logic and / or steps represented in the flowcharts or otherwise described herein, for example, can be considered as a sequenced list of executable instructions for implementing the logical functions, and can be embodied in any computer-readable medium for use by, or in conjunction with, an instruction execution system, apparatus, or device (e.g., a computer-based system, a system including a processor, or other system that can fetch and execute instructions from an instruction execution system, apparatus, or device). For purposes of this specification, a "computer-readable medium" can be any device that can contain, store, communicate, propagate, or transport a program for use by, or in conjunction with, an instruction execution system, apparatus, or device. More specific examples (a non-exhaustive list) of computer-readable media include the following: an electrical connection with one or more wires (electronic devices), a portable computer disk cartridge (magnetic device), random access memory (RAM), read-only memory (ROM), erasable and programmable read-only memory (EPROM or flash memory), fiber optic devices, and a portable compact disc read-only memory (CDROM). Furthermore, the computer-readable medium may even be paper or other suitable medium on which the program is printed, since the program may be obtained electronically, for example, by optically scanning the paper or other medium and then editing, interpreting or processing it in another suitable manner if necessary, and then storing it in a computer memory.

[0075] It should be understood that various parts of the present application can be implemented using hardware, software, firmware, or a combination thereof. In the above embodiments, multiple steps or methods can be implemented using software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented using hardware, as in another embodiment, any one of the following technologies known in the art or a combination thereof can be used to implement: a discrete logic circuit having a logic gate circuit for implementing a logic function on a data signal, an application-specific integrated circuit having a suitable combination of logic gate circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), etc.

[0076] Those skilled in the art will understand that all or part of the steps in the method of the above embodiment can be completed by instructing related hardware through a program, and the program can be stored in a computer-readable storage medium. When the program is executed, it includes one or a combination of the steps of the method embodiment.

[0077] In addition, the functional units in the various embodiments of the present application may be integrated into a processing module, or each unit may exist physically separately, or two or more units may be integrated into a module. The above-mentioned integrated module may be implemented in the form of hardware or in the form of a software functional module. If the integrated module is implemented in the form of a software functional module and sold or used as an independent product, it may also be stored in a computer-readable storage medium.

[0078] The storage medium mentioned above may be a read-only memory, a magnetic disk, or an optical disk, etc. Although the embodiments of the present application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present application. Persons skilled in the art may make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present application.

Claims

1. A traveling wave protection method for power electronic transformer side lines in a medium voltage DC distribution network, characterized in that: include: Taking the fault current overcurrent as the fault identification criterion, the traveling wave main protection of the line on the power electronic transformer DCSST side is activated based on the overcurrent detection; The reverse voltage traveling wave on the line is calculated, and the attenuation step characteristics of the reverse voltage traveling wave are extracted using wavelet transform, so that the main protection determines whether to operate based on whether the attenuation step characteristics exist.

2. The method according to claim 1, wherein The method of using the fault current overcurrent as a fault identification criterion and starting the DCST-side line traveling wave main protection based on the overcurrent detection includes: When a fault occurs, the DCSST side line feeds current to the fault point. The fault current is detected as overcurrent and the DCSST side traveling wave main protection is activated. The fault current overcurrent is expressed as: i-i0>K setl I line,N Among them, K setl is the starting criterion setting value of the DCST side line direction overcurrent, i is the line current measured at the current acquisition moment, i0 is the reference line current measured before the current acquisition moment, I line,N is the rated current of the line.

3. The method according to claim 1, wherein The calculating of the reverse voltage traveling wave on the circuit and extracting the attenuation step characteristics of the reverse voltage traveling wave by wavelet transform include: The reverse voltage traveling wave is expressed as: Where x is the position on the line, t is the time, and u f (x, t) is the forward voltage traveling wave on the line, u b (x, t) is the reverse voltage traveling wave, Z c is the wave impedance of the line, u(x,t) is the voltage at point x on the line at time t, and i(x,t) is the current at point x on the line at time t; A scale-three binary wavelet transform is used as the fault feature extraction algorithm, and the modulus maximum of the wavelet transform is used to extract the negative polarity step feature and the exponential decay feature of the step of the reverse voltage traveling wave. If a continuous positive high-frequency signal appears after the negative traveling wave modulus maximum is detected, the fault traveling wave is considered to have an exponential decay characteristic. The specific process includes: For a negative polarity modulus maximum, if three positive polarity points are detected before the next negative polarity appears, it is considered that the reverse voltage traveling wave corresponding to the modulus maximum has an exponential decay characteristic.

4. The method according to claim 1, wherein The step of causing the main protection to determine whether to operate according to whether a decay step characteristic exists includes: When a fault occurs, within the range of traveling wave propagation, the DCSST side of the fault line detects a negative polarity step reverse voltage traveling wave without exponential decay characteristics, and the DCSST side of the non-fault line detects a negative polarity step reverse voltage traveling wave with exponential decay characteristics. Starting from the initial traveling wave head of the fault, if there are three consecutive reverse voltage traveling wave heads with exponential decay characteristics, it is considered that a fault has occurred and protection is delayed.

5. A traveling wave protection system for power electronic transformer side lines in a medium voltage DC distribution network, characterized in that: include: The traveling wave protection module based on overcurrent detection is used to use the fault current overcurrent as the fault identification criterion and start the DCST side line traveling wave main protection based on the overcurrent detection; The traveling wave protection module based on reverse voltage traveling wave step is used to calculate the reverse voltage traveling wave on the line and extract the attenuation step characteristics of the reverse voltage traveling wave using wavelet transform, so that the main protection can determine whether to operate based on whether the attenuation step characteristics exist.

6. The system according to claim 5, wherein: The overcurrent detection module is specifically used to: When a fault occurs, the DCSST side line feeds current to the fault point. The fault current is detected as overcurrent and the DCSST side line traveling wave main protection is activated. The fault current overcurrent is expressed as: i-i0>K setl I line,N Among them, K setl is the starting criterion setting value of the DCST side line direction overcurrent, i is the line current measured at the current acquisition moment, i0 is the reference line current measured before the current acquisition moment, I line,N is the rated current of the line.

7. The system according to claim 5, wherein: The calculating of the reverse voltage traveling wave on the circuit and extracting the attenuation step characteristics of the reverse voltage traveling wave by wavelet transform include: The reverse voltage traveling wave is expressed as: Where x is the position on the line, t is the time, and u f (x, t) is the forward voltage traveling wave on the line, u b (x, t) is the reverse voltage traveling wave, Z c is the wave impedance of the line, u(x,t) is the voltage at point x on the line at time t, and i(x,t) is the current at point x on the line at time t; A scale-three binary wavelet transform is used as the fault feature extraction algorithm, and the modulus maximum of the wavelet transform is used to extract the negative polarity step feature and the exponential decay feature of the step of the reverse voltage traveling wave. If a continuous positive high-frequency signal appears after the negative traveling wave modulus maximum is detected, the fault traveling wave is considered to have an exponential decay characteristic. The specific process includes: For a negative polarity modulus maximum, if three positive polarity points are detected before the next negative polarity appears, it is considered that the reverse voltage traveling wave corresponding to the modulus maximum has an exponential decay characteristic.

8. The system according to claim 5, wherein: The step of causing the main protection to determine whether to operate according to whether a decay step characteristic exists includes: When a fault occurs, within the range of traveling wave propagation, the DCSST side of the fault line detects a negative polarity step reverse voltage traveling wave without exponential decay characteristics, and the DCSST side of the non-fault line detects a negative polarity step reverse voltage traveling wave with exponential decay characteristics. Starting from the initial traveling wave head of the fault, if there are three consecutive reverse voltage traveling wave heads with exponential decay characteristics, it is considered that a fault has occurred and protection is delayed.

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