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The study of traveling wave method ranging in cable fault ranging began in the early 1940s. It is based on the traveling wave transmission theory to achieve fault location of transmission lines. The current wave method has become a research hotspot.
(1) The early traveling wave method can be divided into A, B, and C categories according to the principle of fault location:
1 A type fault distance measuring device is to use the fault wave to reach the bus end and reflect to the fault point, and then the time difference between the fault point and the bus end to reach the bus end to determine the fault point distance. However, this method does not solve the problem of distinguishing between the reflected wave at the fault point and the transmitted wave at the fault point of the reflected wave at the opposite bus end, so it is difficult to realize.
2 Type B fault distance measuring device is to record the time that the traveling wave generated by the fault point reaches the two ends of the line, and then realizes the distance measurement by means of the communication link. Since this type of distance measuring device uses the information of the first traveling wave arriving at the bus end after the occurrence of the fault, there is no problem of distinguishing between the reflected wave of the fault point and the transmitted wave of the reflected wave at the opposite bus end at the fault point. However, it requires communication at both ends of the line, and the time scales on both sides must be the same. This requires the use of GPS technology to achieve.
3 C type fault distance measuring device is to send high-voltage high-frequency or direct current pulse after the fault occurs, and to measure the distance from the device to the fault point according to the high-frequency pulse. This kind of distance measuring device has a simple principle and high precision, but it needs to add high-frequency pulse signal generators and other components, which is more expensive and complicated. In addition, the fault point reflection pulse at the time of ranging is often difficult to distinguish from the interference, and requires that the three phases of the transmission line have high frequency signal processing and carrier channel equipment.
Comparison of three types of ranging principle: Type A and Type C are single-end ranging and do not require communication at both ends of the line. They all need to be positioned according to the round trip time from the device installation point to the fault point. Therefore, they are also called echoes. Positioning method; and B-type ranging principle belongs to double-end communication, which requires double-end information. The A-type ranging principle and the B-type ranging principle are suitable for transient and persistent faults, while the C-type ranging principle is only applicable to persistent faults.
(2) Modern traveling wave method In a sense, the modern traveling wave method is the development of the early A-type traveling wave method. Since the mid-1960s, a lot of in-depth studies have been conducted on the transmission line traveling wave transmission theory proposed in 1926, and a great deal of work has been done in phase mode conversion, parameter frequency change, and transient numerical calculation, further deepening the Wave method ranging and understanding of many related factors.
1) Travelling wave correlation method The traveling wave correlation method is based on the principle that the waveform between the forward voltage traveling wave moving toward the fault point and the reverse voltage traveling wave returning from the fault point is similar, the polarity is reversed, and the time delay is Δt Corresponds to the time it takes for a traveling wave to travel back and forth between the bus and the point of failure. After correlating the two, the positive wave was inverted and delayed by Δt, and the correlation function showed a maximum value.
This method also has the problem of distinguishing between the reflected wave at the fault point and the transmitted wave at the fault point of the reflected wave at the opposite bus end. Due to the presence of transmitted waves on the opposite side in some fault conditions, they will overlap with the reflected waves that occur at the point of failure, which will bring great difficulties to correlation method ranging.
Low-voltage pulse reflection method works
1. Applications Low-voltage pulse reflection method (hereinafter referred to as low-voltage pulse method) is used to measure the low resistance, short circuit and open circuit faults of cables. According to statistics, such failures account for about 10% of cable faults. Low-voltage pulse method can also be used to measure the length of the cable, electromagnetic wave propagation speed in the cable, but also can be used to distinguish the middle of the cable, T-joint and terminal head.
2. Principle of Operation During the test, a low-voltage pulse is injected into the cable. The pulse propagates along the cable to impedance mismatch points, such as short-circuit points, fault points, and intermediate connectors. The pulse is reflected and sent back to the measuring point. The result is recorded by the instrument (Figure 3.1). ). The time difference between the transmitted pulse and the reflected pulse on the waveform is Δt, corresponding to the time when the pulse reciprocates once at the measuring point and the impedance mismatch point. Knowing the pulse velocity V in the cable, the distance of the impedance mismatch point can be calculated by the following equation. L=V•Δt?2 (3.1)
Figure 3.1 Low-voltage pulse reflection principle By identifying the polarity of the reflected pulse, the nature of the fault can be determined. The open circuit fault reflection pulse has the same polarity as the transmit pulse, and the short circuit fault reflection pulse has the opposite polarity to the transmit pulse.
Known from Equation 3.1, the wave velocity of the pulse in the cable is critical to accurately calculating the fault distance. When the wave velocity value of the cable is not known, it can be measured as follows. If the length of the cable under test is known, the wave velocity in the cable can be deduced from the time Δt between the transmitted pulse and the cable terminal reflection pulse:
V=2•L?Δt (3.2)
3. Selection of firing pulses
1) Pulse shape The voltage pulse used by the cable fault measuring instrument is generally rectangular, exponential, and bell-shaped (also called raised cosine). Since the rectangular pulse formation is relatively easy, it is applied more.
2) The pulse width pulse always has a certain time width. Assuming Ï„, the reflected pulse that arrives within the Ï„ time overlaps with the transmitted pulse and cannot be distinguished. Therefore, the fault point distance cannot be detected, and a blind spot appears. Assume that the pulse emission width is 0.5 s, the cable wave velocity is 160 m / s, and the measurement blind zone is 40 meters. The wider the pulse sent by the instrument, the larger the measurement blind zone. From the perspective of reducing blind spots, the transmission pulse width is narrower, but the narrower the pulse, the richer the high-frequency components it contains, and the high-frequency loss of the line, so that the amplitude of the reflected pulse is too small, and the distortion is severe. Distance measurement effect. In order to solve this problem, the pulse reflection instrument divides the pulse width into several ranges, and selects the pulse width according to the distance of the measuring distance. The farther the measuring distance is, the wider the pulse is.
2) High-frequency traveling wave method The high-frequency traveling wave method differs from other traveling wave methods in that it extracts high-frequency traveling wave components of voltage or current, and then performs digital signal processing, and then performs fault location according to the A-type traveling wave method. . This method successfully distinguishes between the reflected wave at the fault point and the transmitted wave at the fault point of the reflected wave at the opposite bus end based on the reflection characteristics of the bus end at high frequencies.
(3) Problems to be solved by using traveling wave method Ranging The reliability and accuracy of traveling wave method ranging are theoretically not affected by the line type, fault resistance, and both-side systems, but in practice, it is affected by many engineering factors. Constraints.
1) The digital simulation of the traveling wave signal acquisition shows that the line voltage and current of the primary fault on the line are very obvious and contain rich fault information, but they need to be measured by the transformer. The key is how to measure the traveling wave signal from the secondary side of the transformer in an economical and simple way. In general, the cutoff frequency of voltage and current transformers should be no less than 10khz to ensure that the signal is not excessively distorted. Capacitive voltage transformers (CVTs) for high voltage transmission lines obviously cannot meet the requirements. The distance measuring device using traveling wave generated by the fault can be used to share the measuring transformer with other line protection (such as distance protection), otherwise it is difficult to apply and popularize. In order to reach the accuracy of a tower (less than 1km), the secondary signal rise time should be within a few microseconds. Experimental studies have shown that transient response characteristics of current transformers (CTs) can satisfy such high response speeds.
Therefore, the traveling wave distance measuring device can share the current transformer with other protective devices, so it is easy to be popularized and used.
2) Uncertainties of traveling wave signals caused by faults Uncertainties of traveling wave signals generated by faults are mainly manifested in three aspects:
1 Uncertainty of the failure The uncertainty of the fault is mainly manifested in the angle of fault occurrence and the type of fault. The moment when the fault occurs is random, and it is related to the cause of the fault and the state of the line. At the same time, the types of failures are also different. They can be metal failures, or they can be short-circuit failures with varying resistances of different sizes.
2 Uncertainty of the bus wiring method The traveling wave distance measurement theory is based on the propagation and reflection of traveling waves, and the wiring on the bus bars is not fixed, which causes uncertainty in the arrival of travel waves to the bus bars. However, traveling wave ranging requires a sufficiently strong reflection on the bus side to be detected.
3 The nonlinearity of the circuit and other components of the system and the influence of the frequency dependent characteristics Due to the skin effect, the actual three-phase line has the loss and the parameter changes with frequency. In the system, the loss of the ground model parameters is large and the frequency dependent characteristics are serious, which makes the analysis of transient traveling wave signals complicated and difficult to describe accurately. Therefore, the line mode component is generally used for traveling wave ranging.
3 The identification of reflected wave at the fault point The correct identification of the reflected wave at the fault point is a key technical problem for accurate and reliable fault location. There are a lot of features on the line that are very similar to the reflected waves at the point of failure. Under normal operating conditions, the major interference is mainly from the operation of circuit breakers and disconnectors. Any of the above operations will produce drastic voltage changes. After a fault occurs, disturbances can also occur when traveling waves propagate along transmission lines. For example, the transposition point of the line and the crossing point of other lines will be interfered by the change of the wave impedance, which increases the difficulty of recognition. In addition to eliminating line interference, the key point of reflected wave recognition at the fault point is to distinguish whether the reflected wave is from the fault point or the line-to-end bus. The terminal equipment of the early traveling wave method was limited by the technical conditions at the time, and its structure and use were quite complicated, such as the synchronization device of the B type method, the high frequency and DC pulse generating device in the C type method, etc. The real-time automation in operation requires increasing the technical complexity and cost of traveling wave method ranging, which hinders the wider application of traveling wave method ranging.
The recording and processing of 4 traveling wave signals The transient traveling wave signal generated by the fault lasts only for a short period of time, after entering the steady state after multiple reflections, it is necessary to record useful transient traveling wave signals within a few milliseconds after the fault occurs. . In addition, in order to ensure that there is sufficient accuracy in distance measurement, in order to acquire high-frequency transient traveling waves, the sampling frequency should not be too low and should be in the order of hundred kilohertz.
In spite of this, the use of faulty traveling wave ranging is much easier than implementing relay protection. The purpose of using traveling wave protection is to obtain a high speed of action (less than 10ms). A key issue is how to distinguish faults from other causes, such as those caused by lightning strikes and system operation. There is no such problem of differentiation for ranging. Because it only provides system faults, the fault distance will be accurately given. By checking whether the protection is active, it is easy to know if the system has failed.
In short, the traveling wave method has many unique advantages in theory. It can be believed that these problems will eventually be solved with the development of the new traveling wave ranging method. The new traveling wave method has a very broad application prospect.
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